Applied Surface Science 应用表面科学
2017 年 1 月 1 日,第 391 卷,B 部分,第 72-123 页
A review on g-C3N4-based photocatalysts
一篇关于 g-C 3 N 4 基光催化剂的综述
Graphical abstract 图形摘要
The photocatalytic fundamentals, versatile properties, design strategies and potential applications of g-C3N4-based photocatalysts were systematically summarized and addressed.
系统总结和讨论了 g-C 3 N 4 基光催化剂的光催化基础、多功能性能、设计策略和潜在应用。
Keywords 关键词
碳氮(g-C 3 N 4 )复合光催化剂共催化剂人工光合作用 Z-方案异质结纳米碳
1. Introduction 1. 引言
Nowadays, among the various possibilities for exploring attractive sustainable energy sources and technologies, photocatalytic technology is considered as one of the most appealing and promising technologies to directly harvest, convert and store renewable solar energy for generating sustainable and green energy and a broad range of environmental applications. In 1972, the pioneering work of photoelectrochemical (PEC) H2 production from water splitting using a Pt-attached n-TiO2 cell was firstly reported by Fujishima and Honda [1]. Subsequently, Bard extended the basic principle of PEC water splitting system to the heterogeneous photocatalytic systems with illuminated semiconductor particles suspended in water as photocatalysts [2], [3], [4], [5]. Since then, the heterogeneous photocatalysis occurring on powdered semiconductors has been widely used in the different fields, such as water splitting [6], [7], [8], [9], [10], environmental remediation [9], [11], [12], [13], [14], [15], CO2 reduction [16], [17], [18], [19], [20], disinfection [21] and selective organic transformations[22], [23], [24]. It is noteworthy that there has been a growing interest in the use of semiconductors as photocatalysts for various applications (the red columns in Fig. 1a). In 2015, more than 5500 papers about the photocatalytic applications have been published, further indicating the high importance and tremendous research interests in heterogeneous photocatalysis. Clearly, the presence of efficient photocatalysts plays an essential role in determining the overall quantum efficiency of all these photocatalytic reaction systems. During the past 40 years, various available semiconductor materials such as TiO2, SrTiO3, CdS, BiVO4, Ta3N5, TaON, g-C3N4, Ag3PO4, and their nanostructured assemblies have been extensively employed as photocatalysts to directly harness solar energy for different redox reactions [7], [25], [26], [27], [28], [29], [30]. As the most widely employed “golden” photocatalyst, TiO2 has dominated the published work on heterogeneous photocatalysis owing to its chemical stability, high chemical inertness, nontoxicity, and low cost [31], [32], [33], [34], [35], [36], which accounts for three-fifths of all photocatalytic research (the green columns in Fig. 1a). However, the large bandgap of anatase TiO2 (3.2 eV) restricts the utilization of broad spectrum of solar light (only utilization of the ultraviolet (UV) light in the sunlight, accounting for only 4% of the incoming solar spectrum), thus leading to much lower quantum efficiencies in using the solar spectra. To enhance the photocatalytic efficiency of titania under visible light (about 43% of the sunlight), a variety of modification strategies, including doping, surface sensitization, nanostructuring, introducing defects or amorphous disorder layers, loading co-catalysts, coupling with carbon and other semiconductors, have been utilized to overcome the TiO2 materials-related issues and limitations, such as the control of the band gap, band structure, optical properties and available surface area for photo-induced reactions [7], [25], [34], [35], [36], [37], [38], [39], [40]. Up to now, no one robust and commercially available material could meet all requirements, such as high visible-light quantum efficiency, stability, safety and cheapness [7], [41]. Thus, to tackle these challenges, it is highly desirable to search for novel visible-light-driven semiconductor materials and further fabricate highly efficient systems/architectures for energy supply and environmental remediation.
当今,在探索吸引人的可持续能源来源和技术的各种可能性中,光催化技术被认为是直接收集、转换和储存可再生太阳能以产生可持续和绿色能源以及广泛应用于环境的一种最吸引人和有前途的技术之一。 1972 年,藤岛和本多首次报告了使用 Pt-附加 n-TiO1 电池从水分裂中生产 H 2 的光电化学(PEC)工作 [1]。随后,Bard 将 PEC 水分裂系统的基本原理扩展到了悬浮在水中的受照半导体颗粒作为光催化剂的异质光催化系统 [2],[3],[4],[5]。 从那时起,发生在粉状半导体上的异质光催化在不同领域被广泛应用,比如水分裂 [6],[7],[8],[9],[10],环境修复 [9],[11],[12],[13],[14],[15],CO 2 还原 [16],[17],[18],[19],[20],消毒 [21] 和选择性有机转化[22],[23],[24]。值得注意的是,人们对半导体作为光催化剂用于各种应用的兴趣不断增长 (图 1a 中的红色柱状图)。 2015 年,有超过 5500 篇关于光催化应用的论文发布,进一步表明了异质光催化的高重要性和巨大研究兴趣。 显然,高效的光催化剂在决定所有这些光催化反应系统的总量子效率中起着至关重要的作用。 在过去的 40 年里,各种可用的半导体材料,如 TiO 2 ,SrTiO 3 ,CdS,BiVO 4 ,Ta 3 N 5 ,TaON,g-C 3 N 4 ,Ag 3 PO 4 及其纳米结构组装体被广泛用作光催化剂,以直接利用太阳能进行不同的氧化还原反应[7], [25], [26], [27], [28], [29], [30]。作为最广泛应用的“黄金”光催化剂,TiO 2 由于其化学稳定性,高化学惰性,无毒性和低成本,主导了有关非均相光催化的已发表研究工作的三分之二[31], [32], [33], [34], [35], [36](如图 1a 中的绿色列)。然而,锐钛矿型 TiO 2 的大带隙(3.2eV)限制了对太阳光谱的广谱利用(仅利用太阳光中的紫外线(UV)光,占来自太阳光谱的仅 4%),从而导致利用太阳光谱的量子效率大大降低。为了增强钛白粉在可见光下(大约占阳光的 43%)的光催化效率,采用了各种改性策略,如掺杂,表面敏化,纳米结构化,引入缺陷或非晶失序层,载体协同催化剂,与碳和其他半导体的耦合,以克服与 TiO 2 材料相关的问题和限制,如带隙控制,带结构,光学性质,以及用于光诱导反应的有效表面积 [7], [25], [34], [35], [36], [37], [38], [39], [40]。到目前为止,还没有一种坚固且商业化可用的材料可以满足所有要求,如高可见光量子效率,稳定性,安全性和便宜性[7], [41]。 因此,为了应对这些挑战,迫切需要寻找新型可见光驱动的半导体材料,并进一步制造高效的能源供应和环境修复系统/结构。
The graphite-like carbon nitride (g-C3N4), as a metal-free polymer n-type semiconductor, possess many promising properties, such as unique electric, optical, structural and physiochemical properties, which make g-C3N4-based materials a new class of multifunctional nanoplatforms for electronic, catalytic and energy applications [42], [43]. Especially, g-C3N4-based photocatalysts have attracted increasing interest worldwide, since Wang and his coworkers first discovered the photocatalytic H2 and O2 evolution over C3N4 in 2009 [44]. Clearly, the annual number of citations for Wang’s pioneering paper published on Nature Materials in 2009 significantly increases every year (as shown in Fig. 1b). Thus, the g-C3N4-based nanostructures are emerging as ideal candidates for a variety of energy and environmental photocatalytic applications, such as photocatalytic water reduction and oxidation, degradation of pollutants and carbon dioxide reduction [27], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57]. More interestingly, as observed from Fig. 1b, although the annual number of publications about g-C3N4-based photocatalysts is much smaller than that about TiO2 photocatalysts, the publications of g-C3N4 photocatalysis present an obvious approach to those of graphene-based photocatalysis [26], [58]. More interestingly, the g-C3N4-based photocatalysts together with the graphene-based ones are significantly reducing the proportion of TiO2 photocatalysts (as shown in Fig. 1a). Absolutely, the g-C3N4, as the very exciting sustainable material, has become a shining star in the photocatalytic field.
石墨烯类碳氮(g-C 3 N 4 )作为一种无金属的聚合物 n 型半导体,具有许多有前途的性质,如独特的电学、光学、结构和物理化学性质,这使得基于 g-C 3 N 4 的材料成为电子、催化和能源应用的新型多功能纳米平台[42],[43]。特别是,基于 g-C 3 N 4 的光催化剂自 2009 年王等人首次发现 C 3 N 4 光催化产氢和氧演变以来,引起全球日益增长的兴趣[44]。显然,王在 2009 年发表在《自然材料》上的开创性论文的年引用次数每年都显著增加(如图 1b 所示)。因此,基于 g-C 3 N 4 的纳米结构正成为各种能源和环境光催化应用的理想候选者,例如光催化还原和氧化、污染物降解和二氧化碳还原[27],[45],[46],[47],[48],[49],[50],[51],[52],[53],[54],[55],[56],[57]。更有趣的是,如图 1b 所示,尽管关于基于 g-C 3 N 4 的光催化剂的年度发表量远小于关于 TiO 2 光催化剂的发表量,但关于 g-C 3 N 4 的光催化发表影响显著超过关于基于石墨烯的光催化[26],[58]。更有趣的是,基于 g-C 3 N 4 的光催化剂和基于石墨烯的光催化剂明显减少了 TiO 2 光催化剂的比例(如图 1a 所示)。毫无疑问,作为令人兴奋的可持续材料,g-C 3 N 4 已经成为光催化领域的明星。
Interestingly, as a novel metal-free polymeric semiconductor, g-C3N4 was quite different from most other semiconductors, which could also be readily utilized to form various highly tailorable hybrid photocatalysts with controllable compositions, sizes, thickness, pore structures, size distributions, and morphologies. Hence, it is of great interest to develop g-C3N4-based photocatalysts for various applications through suitable modification, which is still considered as a research topic of scientific and technological significance in the fields of energy and environmental chemistry. Importantly, many significant and major breakthroughs have been achieved in the synthesis and application of g-C3N4-based photocatalysts. In particular, many novel nanostructured g-C3N4-based photocatalysts, including 1D nanorods, 2D nanosheets and 3D hierarchical structures, have been extensively developed in the past several years due to their favorable absorption of solar radiation, efficient separation of charge carriers, high surface areas and exposed reactive sites.
有趣的是,作为一种新型的无金属聚合物半导体,g-C 3 N 4 与大多数其他半导体有很大不同,它也可以被方便地利用来形成各种高度可定制的杂化光催化剂,具有可控的组成、尺寸、厚度、孔结构、尺寸分布和形态。因此,通过适当的改性开发基于 g-C 3 N 4 的光催化剂对于各种应用是非常有趣的,仍然被认为是能源和环境化学领域科学和技术意义上的一个研究课题。值得注意的是,在 g-C 3 N 4 -based 光催化剂的合成和应用方面已取得了许多重要的突破。特别是在过去几年中,由于其对太阳辐射的有利吸收、有效的载流子分离、高比表面积和暴露的反应位点,许多新颖的纳米结构的 g-C 3 N 4 光催化剂,包括 1D 纳米棒、2D 纳米片和 3D 分层结构,已经得到了广泛的发展。
In fact, several excellent reviews are already available that focus on the synthesis and modification of g-C3N4-based photocatalysts and their applications in solving the energy and environmental issues [27], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [59]. However, only a handful of reviews have focused on the versatile properties and rational design of g-C3N4-based photocatalysts. Thus, it seems timely to offer a relatively comprehensive and fully updated review on the state-of-the-art advances of g-C3N4-based photocatalysts for heterogeneous photocatalysis. In the present Review, we devote our attention to fundamentals, versatile properties, rational design and potential applications of g-C3N4 photocatalysts. We believe that this review will not only promote the further developments of new g-C3N4-based materials and architectures with improved utilization of solar energy and photocatalytic efficiency, but also could therefore help to address the challenges for the widespread use of g-C3N4-based photocatalysts in the renewable and sustainable energy production and storage. It is also hoped that this review can provide some new ideas to develop new materials and architectures for the other sustainable energy-related fields such as solar cells [42], [43], [60], [61], light emitting devices [62], [63], fuel cells [64], [65], [66], [67], [68], [69], batteries [70], [71], [72], [73], [74], [75] and sensing devices [76], [77], [78], [79], [80], [81], [82].
事实上,已经有几篇优秀的评论专注于 g-C 3 N 4 基光催化剂的合成和改性,以及它们在解决能源和环境问题中的应用[27],[45],[46],[47],[48],[49],[50],[51],[52],[53],[54],[55],[56],[57],[59]。然而,只有少数评论专注于 g-C 3 N 4 基光催化剂的多功能性能和合理设计。因此,现在是时候就 g-C 3 N 4 基光催化剂的最新进展提供相对全面和完全更新的评论了。在本综述中,我们致力于 g-C 3 N 4 光催化剂的基本原理、多功能性能、合理设计和潜在应用。我们相信,这篇评论不仅将推动新型 g-C 3 N 4 材料和结构的进一步发展,提高太阳能利用和光催化效率,还有助于应对 g-C 3 N 4 光催化剂在可再生和可持续能源生产和储存中的普遍应用挑战。同时,希望这篇评论能为开发用于其他可持续能源相关领域(如太阳能电池[42],[43],[60],[61],发光装置[62],[63],燃料电池[64],[65],[66],[67],[68],[69],电池[70],[71],[72],[73],[74],[75]和传感装置[76],[77],[78],[79],[80],[81],[82])的新材料和结构提供一些新思路。
2. Fundamentals of g-C3N4-based photocatalysts
2. g-C 3 N 4 基光催化剂的基础知识
2.1. Mechanism of heterogeneous photocatalysis
2.1. 异质光催化机制
So far, the fundamental mechanism of heterogeneous photocatalysis has been well proposed, as shown in Fig. 2. Basically speaking, the heterogeneous photocatalysis involves seven key stages, which could be usually classified into four major processes: light harvesting (stage 1); charge excitation (stage 2); charge separation and transfer (stages 3, 4 and 5) and surface electrocatalytic reactions (stages 6 and 7). Firstly, it is known that the light harvesting process (stage 1) is strongly dependent on the surface morphology and structure of photocatalysts, which can usually be significantly improved through constructing the hierarchical macroporous or mesoporous architectures, due to more efficient utilization of light through its multiple reflections and scattering effects [83]. At this regard, the flat and smooth surface of 2D g-C3N4 is unfavorable for improving the light harvesting. Secondly, the charge excitation of a semiconductor is strongly associated with its unique electronic structures. Generally, an electron in the VB of the semiconductor could be thus excited to its CB under the light irradiation with energy higher than or equal to its band gap energy (Eg), leaving a positive hole in the VB. The band gap structures of several typical photocatalysts were summarized in Table 1. As observed in Table 1, as compared to TiO2, BiVO4 and WO3, g-C3N4 has the most negative CB level (−1.3 V vs NHE at pH 7) and a medium band gap (2.7 eV) [44], [84], facilitating its wide application in visible-light photocatalysis. Thus, to achieve more utilization of visible light, the band gap of g-C3N4 should be further narrowed by facile doping, defect and other possible sensitization strategies [7]. Thirdly, the unfavorable charge recombination in the bulk (stage 4) and on the surface (stage 5) of a semiconductor is detrimental to the charge separation and transfer (stage 3) to surface/interface active sites, which has been regarded as the decisive factor for determining the photocatalytic quantum efficiency. Usually, shortening the diffusion length of photo-generated charge carriers or constructing interfacial electric fields could efficiently reduce the recombination rates, thus substantially enhancing the photocatalytic activity [7], [85], [86]. Finally, it is clear that only energetic enough electrons and holes that migrate to the surface of the semiconductor without recombination can be trapped by the surface active sites or co-catalysts, and further stimulate the elctrocatalytic reduction (stage 6) and oxidation (stage 7) reactions of the reactants adsorbed on the semiconductor, respectively. It should be noted that the surface reactions possibly occur only when the reduction and oxidation potentials are more positive and negative than CB and VB levels, respectively. Some typical standard redox potentials have been listed in Table 2. Notably, almost all reactions in Table 2 exhibit the same linear pH dependence with a slope of −0.059 V, apart from E0 (O2/O2−) which is pH-independent [83], [87]. Furthermore, for the surface electrocatalytic reactions (surface charge utilization), the large onset overpotential and sluggish kinetics are two key factors limiting the surface photocatalytic efficiency of reduction and oxidation reactions. Principally, these two restrictive factors can be overcome by loading suitable co-catalysts (electrocatalysts) simultaneously [88]. More importantly, the co-catalysts (electro-catalysts) can play the additional roles in improving the photostability and charge separation of semiconductors [89]. The complicated co-catalyst effects will be thoroughly discussed in the section 4.7.
到目前为止,非均相光催化的基本机制已经得到了很好的提出,如图 2 所示。基本上来说,非均相光催化包括七个关键阶段,通常可分为四个主要过程:光收集(阶段 1);电荷激发(阶段 2);电荷分离和转移(阶段 3、4 和 5)以及表面电催化反应(阶段 6 和 7)。首先,众所周知,光收集过程(阶段 1)严重依赖于光催化剂的表面形态和结构,通常可以通过构建分级大孔或介孔结构来显着改善,因为这样可以通过多次反射和散射效应更有效地利用光线[83]。在这方面,2D g-C 3 N 4 的平坦光滑表面不利于改善光的收集。其次,半导体的电荷激发与其独特的电子结构密切相关。一般来说,在光照射能量高于或等于其带隙能量(E g )的情况下,半导体束带(VB)中的电子会被激发到其导带(CB),在 VB 中留下一个正空穴。几种典型光催化剂的带隙结构总结如表 1。从表 1 中可以看出,与 TiO 2 ,BiVO 4 和 WO 3 相比,g-C 3 N 4 的导带(CB)电位最负(pH7 时为−1.3V vs NHE)且带隙中等(2.7eV)[44],[84],有利于其在可见光光催化中的广泛应用。因此,为了更有效地利用可见光,g-C 3 N 4 的带隙应进一步通过简易掺杂、缺陷和其他可能的敏化策略来缩小[7]。 第三,半导体体内(第 4 阶段)和表面(第 5 阶段)的不利电荷复合对电荷分离和传输(第 3 阶段)到表面/界面活性位点有害,这被认为是决定光催化量子效率的决定性因素。通常,缩短光生电荷载体的扩散长度或构建界面电场可以有效减少复合速率,从而显著提高光催化活性[7],[85],[86]。最后,明显只有足够高能的电子和空穴迁移到半导体表面而没有复合,才能被表面活性位点或共催化剂捕获,并进一步刺激半导体上吸附的反应物的电催化还原(第 6 阶段)和氧化(第 7 阶段)反应。需要指出的是,表面反应可能仅发生在还原和氧化电位比导带(CB)和价带(VB)的水平更正和负时。表 2 中列出了一些典型的标准氧化还原电位。值得注意的是,表 2 中的几乎所有反应都表现出相同的线性 pH 依赖性,斜率为-0.059V,除了 pH-独立的 E 0 (O 2 /O 2 − )[83],[87]。此外,对于表面电催化反应(表面电荷利用),大的起始超电位和缓慢的动力学是限制还原和氧化反应的表面光催化效率的两个关键因素。原则上,这两个限制因素可以通过同时加载合适的共催化剂(电催化剂)来克服[88]。 更重要的是,辅助催化剂(电催化剂)可以在提高半导体的光稳定性和电荷分离方面发挥额外作用[89]。复杂的辅助催化剂效应将在第 4.7 节中彻底讨论。
Semiconductor 半导体 | Crystal structure 晶体结构 | Band Gap Structure (PH = 7,vs NHE) 带隙结构(pH=7,相对标准氢电极) | Ref. 参考文献 | ||
---|---|---|---|---|---|
Empty Cell | Empty Cell | CB | VB | Eg/eV E g /电子伏特 | Empty Cell |
TiO2 | Anatase 锐钛矿 | −0.5 | 2.7 | 3.2 | [90] |
Cu2O 氧化铜 2 | −1.16 | 0.85 | 2.0 | [91] | |
CdS | −0.9 | 1. 5 | 2.4 | [92] | |
g-C3N4 | −1.3 | 1.4 | 2.7 | [44], [84] | |
g-C3N4 | −1.53 -1.53 | 1.16 | 2.7 | [93]a | |
Ta3N5 | −0.75 -0.75 | 1.35 | 2.1 | [94] | |
TaON | −0.75 -0.75 | 1.75 | 2.5 | [94] | |
BiVO4 | −0.3 -0.3 | 2.1 | 2.4 | [95] | |
WO3 | −0.1 | 2.7 | 2.8 | [96] | |
Ag3PO4 | cubic 立方的 | 0.04 | 2.49 | 2.45 | [97] |
- a
Measurement by the valence band X-ray photoelectron spectroscopy (VB XPS) spectrum.
通过价带 X 射线光电子能谱(VB XPS)谱进行测量。
Reaction 反应 | E0(V) vs NHE at pH 0 E 0 (V)vs 标准氢电极在 pH 0 时 |
---|---|
2H+ + 2e−→H2 (g) 2H + +2e→H 2 (g) | 0 |
O2(g) + e− →O2− (aq) O 2 (g)+e→O 2 (aq) | −0.33 |
O2(g) + H+ + e− → HO2 (aq) | −0.046 |
O2(g) + 2H+ + 2e− → H2O2 (aq) | 0.695 |
2H2O (aq) + 4 h+ → O2 (g) + 4H + | 1.229 |
OH− + h+ → OH | 2.69 |
O3(g) + 2 H+ + 2 e− → O2 (g) + H2O O 3 (g) + 2 H + + 2 e → O 2 (g) + H 2 O | 2.075 |
CO2 + e− →CO2 CO 2 + e → CO 2 − | −1.9 |
2 CO2(g) + 2 H+ + 2 e− → HOOCCOOH(aq) 2 CO 2 (g)+2 H + +2 e→HOOCCOOH(aq) 2 CO 2 (气体)+2 H + +2 e→HOOCCOOH(溶液) | −0.481 |
CO2(g) + 2H+ + 2e− → HCOOH(aq) CO 2 (气体)+2H + +2e→HCOOH(aq) | −0.199 |
CO2(g) + 2 H+ + 2e− → CO(g) + H2O CO 2 (气体)+2 H + + 2e→CO(气体)+H 2 O | −0.11 |
CO2(g) + 4H+ + 4e− → C(s) + 2H2O CO 2 (气体)+4H + +4e→C(s)+2H 2 O | 0.206 |
CO2(g) + 4H+ + 4e− → HCHO(aq) + H2O CO 2 (气体)+4H + +4e→HCHO(水溶液)+H 2 O | −0.07 |
CO2(g) + 6H+ + 6e− → CH3OH(aq) + H2O CO 2 (气体)+6H + +6e→CH 3 OH(水溶液)+H 2 O | 0.03 |
CO2(g) + 8H+ + 8e− → CH4 (g) + 2H2O CO 2 (气体)+8H + +8e→CH 4 (气体)+2H 2 O | 0.169 |
2CO2(g) + 8H2O + 12e− → C2H4(g) + 12OH 2CO 2 (g) + 8H 2 O + 12e→C 2 H 4 (g) + 12OH− | 0.07 |
2CO2(g) + 9H2O + 12e− → C2H5OH(aq) + 12OH 2CO 2 (g) + 9H 2 O + 12e→C 2 H 5 OH(aq) + 12OH− | 0.08 |
3CO2(g) + 13H2O + 18e− → C3H7OH(aq) + 18OH 3CO 2 (g) + 13H 2 O + 18e→C 3 H 7 OH(aq) + 18OH− | 0.09 |
H2O2(aq) + H+ + e− → H2O + OH H 2 O 2 (aq) + H + + e→H 2 O + OH− | 1.14 |
HO2 + H+ + e− → H2O2(aq) HO 2 + H + + e→H 2 O 2 (aq) | 1.44 |
H2O2(aq) + 2H+ + 2e− → 2H2O | 1.763 |
However, it should be noteworthy that the photocatalytic quantum efficiency (ηc) is strongly determined by the cumulative effect of the efficiency in all four-step processes, including light harvesting efficiency (ηabs), charge separation efficiency (ηcs), charge migration and transport efficiency (ηcmt), and charge utilization efficiency (ηcu) for H2 generation. The relationship between them could be expressed according to Eq. (1) [7]:(1)ηc = ηabs × ηcs × ηcmt × ηcu
Therefore, to design highly efficient photocatalysts for various photocatalytic applications, all these typical four-step processes must be comprehensively considered and optimized. Despite the significant advances in heterogeneous photocatalysis, there are still many challenges related to the further enhancements of light harvesting (especially for the visible light region), charge carrier excitation, separation and utilization. In order to solve these key scientific problems, a variety of engineering modification strategies have been proposed and applied in improving the visible-light photocatalytic performances of heterogeneous semiconductor materials, such as band structure engineering, micro/nano engineering, bionic engineering, co-catalyst engineering, surface/interface engineering and their synergistic effects [7]. The detailed modification strategies and rational design on g-C3N4-based photocatalysts will be thoroughly discussed in section 4.
因此,要为各种光催化应用设计高效的光催化剂,必须全面考虑和优化所有这些典型的四步过程。尽管在异质光催化方面取得了显著进展,但仍然存在许多与进一步提高光收集效率(特别是对可见光区域)、电荷载体激发、分离和利用相关的挑战。为了解决这些关键的科学问题,已经提出并应用了各种工程改性策略,以改善异质半导体材料可见光光催化性能,例如带结构工程、微/纳米工程、仿生工程、共催化剂工程、表面/界面工程及其协同效应[7]。将在第 4 部分详细讨论 g-C 3 N 4 -基光催化剂的详细改性策略和合理设计。
2.2. Advantages and challenges of g-C3N4-based photocatalysts
2.2. 石墨氮化碳基光催化剂的优势和挑战
Based on the above mechanism analysis, it is clear that the band-gap and nano structures are crucial for their photocatalytic applications. The band gap structures of g-C3N4, as well as some standard potentials of typical redox reactions at pH 7 are illustrated in Fig. 3. Clearly, as shown in Fig. 3, g-C3N4 has a moderate band gap of 2.7 eV, corresponding to an optical wavelength of 460 nm, which makes it active under visible light. Considering thermodynamic losses and overpotentials in the photocatalytic process, the band gap of 2.7 eV accidentally lies in between 2 eV and 3.1 eV, thus achieving both the water-splitting with enough endothermic driving forces (much larger than 1.23 eV) and light absorption in the visible range (smaller than 3.1 eV) [7]. More importantly, g-C3N4 also has a suitable CB position for various reduction reactions. It is noted form Fig. 3 that the favorable level of top CB of g-C3N4 is much more negative than those of conventional inorganic semiconductor counterparts in Table 1 and the potentials of H2-evolution, CO2-reduction and O2-reduction reactions, suggesting that the photo-generated electrons in g-C3N4 possess a large thermodynamic driving force to reduce various kinds of small molecules, like H2O, CO2 and O2. As a consequence, the appropriate electronic band structures of g-C3N4 are favorable for its extensive applications in wide areas, such as photocatalytic water splitting, CO2 reduction, pollutant degradation, organic synthesis and disinfection.
根据上述机理分析,可以明显看出带隙和纳米结构对它们的光催化应用至关重要。如图 3 所示,g-C 3 N 4 的带隙结构以及在 pH7 下一些典型氧化还原反应的标准电位进行了说明。显然,如图 3 所示,g-C 3 N 4 具有 2.7eV 的中等带隙,对应于 460nm 的光波长,在可见光下活跃。考虑到光催化过程中的热力学损失和过电势,2.7eV 的带隙巧合地位于 2eV 和 3.1eV 之间,因此实现了足够的吸热驱动力(远大于 1.23eV)的水裂解和在可见光范围内的光吸收(小于 3.1eV)[7]。更重要的是,g-C 3 N 4 的导带位置也适合于各种还原反应。从图 3 可见,g-C 3 N 4 的有利顶部导带能级比表 1 中常规无机半导体对应物的更负值,以及 H 2 生成,CO 2 还原和 O 2 还原反应的电位,表明 g-C 3 N 4 中的光生电子具有大的热力学驱动力来还原各种小分子,如 H 2 O,CO 2 和 O 2 。因此,g-C 3 N 4 适当的电子能带结构有利于其在光催化水裂解、CO 2 还原、污染物降解、有机合成和消毒等广泛领域的广泛应用。
Apart from the simplest and straightforward advantage of suitable optical band gap and position, it is widely accepted to date that this metal-free g-C3N4 material also possesses a stacked 2D layered structure, in which the single-layer nitrogen heteroatom-substituted graphite nanosheets, formed through sp2 hybridization of C and N atoms, are bound by van der Waals forces, only (as shown in Fig. 4a) [99]. Ideally, condensed g-C3N4 consists of only two earth-abundant elements: C and N, with a C/N molar ratio of 0.75, suggesting that g-C3N4 could be readily fabricated at low cost. It also turned out that the g-C3N4 has the advantages of biocompatibility and nontoxicity. Surprisingly, the viability activity of HeLa cells could be maintained in the aqueous solution of g-C3N4 nanosheets with a concentration of up to 600 mg mL−1 [100]. Furthermore, g-C3N4 could be readily fabricated through the traditional thermal condensation of several low-cost N-rich organic solid precursors such as urea, thiourea, melamine, dicyandiamide, cyanamide, and guanidine hydrochlorid, at 500–600 °C in air or inert atmosphere (Fig. 4) [27], [101]. However, the disordered and defective g-C3N4 structures could be fabricated due to the incomplete removal of intermediates. Thus, the crystalline and condensed g-C3N4 can be readily prepared by other various fabrication strategies, including the ionothermal synthesis (molten salt strategy) [102], [103], [104], [105], [106], molecular self-assembly [107], [108], [109], [110], [111], microwave irradiation [112], [113] and ionic liquid strategy [114], [115], [116], [117] (Fig. 4).
除了适当的光学带隙和位置的最简单和直接的优势之外,到目前为止广泛认为这种无金属的 g-C 3 N 4 材料还具有堆叠的二维层状结构,其中通过 C 和 N 原子的 sp 2 杂化形成的单层氮杂原子替代的石墨纳米片,仅通过范德华力结合(如图 4a 所示)[99]。理想情况下,浓缩的 g-C 3 N 4 仅由两种丰富的地球元素组成:C 和 N,C/N 摩尔比为 0.75,这表明 g-C 3 N 4 可以低成本制备。同时,g-C 3 N 4 也具有生物相容性和无毒性的优点。令人惊讶的是,HeLa 细胞在浓度高达 600mgmL −1 的 g-C 3 N 4 纳米片水溶液中的活力活性仍然能够保持[100]。此外,g-C 3 N 4 可以通过尿素、硫脲、三聚氰胺、二氰胺、氰胺和盐酸胍等低成本富含 N 的有机固体前体的传统热凝聚在空气或惰性气氛中 500-600°C 中制备(如图 4)[27],[101]。然而,由于中间体的不完全去除,可能会制备出无序和有缺陷的 g-C 3 N 4 结构。因此,通过其他各种制备策略,包括离子熔盐合成(熔融盐策略)[102],[103],[104],[105],[106],分子自组装[107],[108],[109],[110],[111],微波辐射[112],[113]和离子液体策略[114],[115],[116],[117]可以容易地制备出结晶和浓缩的 g-C 3 N 4 (如图 4)。
In addition, the above three obvious advantages, as well as several other advantages of g-C3N4, including the rich surface properties, non-toxicity, abundance, and good stabilities, are summarized in Fig. 3, all of which give access to a wide variety of applications [48]. All these features principally already allow its direct use in sustainable chemistry as a multifunctional heterogeneous metal-free photocatalyst.
此外,g-C 3 N 4 的丰富表面性质、无毒性、丰富性和良好的稳定性等几个明显优势,以及图 3 中总结的其他几个优势,都使其可以广泛应用 [48]。所有这些特性基本上已经允许其作为多功能的无机负载型金属免费光催化剂直接用于可持续化学。
Unfortunately, the bulk g-C3N4 generally exhibits the low photocatalytic efficiency, due to some serious drawbacks of g-C3N4 material itself. Specifically, we will highlight several prominent challenges of g-C3N4 itself here: the high electron–hole recombination rate, insufficient visible absorption (below 460 nm), low surface area of g-C3N4 (∼10 m2/g, the high degree of condensation of the monomers) and small active sites for interfacial (photo)reactions, slow surface reaction kinetics, moderate oxidation ability, grain boundary effects and low charge mobility which disrupt the delocalization of electrons [51], [118]. Additionally, it should be noted from Fig. 3 that the photo-generated holes of C3N4 with moderate oxidation ability can only achieve oxygen evolution from water oxidation, instead of the formation of the nonselective hydroxyl radicals, OH. At this point, the g-C3N4-based photocatalysts seem to be a suitable candidate for selective photooxidation and related transformations of organic compounds in aqueous media, avoiding the direct mineralization to CO2 by the strong OH [119]. Thus, it is therefore highly desirable to lower the top level of VB of C3N4 to enhance its water oxidation power, as 4-electron water oxidation reaction oxidation to O2 is a more challenge half-reaction for water splitting. Although these prominent challenges of g-C3N4 itself greatly limit its photocatalytic performance enhancements, they also afford more opportunities to construct more efficient g-C3N4-based photocatalysts in the future studies.
不幸的是,由于 g-C 3 N 4 材料本身存在一些严重缺陷,普遍的大量 g-C 3 N 4 表现出低光催化效率。具体来说,这里将强调 g-C 3 N 4 本身存在的几个突出挑战:高电子-空穴复合速率,不足的可见光吸收(低于 460nm),g-C 3 N 4 的低比表面积(∼10m 2 /g,单体的高度缩聚)和界面(光)反应的活性位点较少,缓慢的表面反应动力学,适度的氧化能力,晶界效应和影响电子离域化的低电荷迁移率 [51],[118]。此外,从图 3 可以看出,具有适度氧化能力的 C 3 N 4 光生空穴仅能从水氧化反应中实现氧发生,而不是形成非选择性的羟基自由基, OH。在这一点上,g-C 3 N 4 基光催化剂似乎是选择性光氧化和水介质中有机化合物相关转化的合适候选者,避免了强 OH 直接矿化为 CO 2 [119]。因此,降低 C 3 N 4 最高自带带的价带能级以增强其水氧化能力,对于 4 电子水氧化反应氧化到 O 2 是更具挑战性的半反应,这是极为理想的。虽然 g-C 3 N 4 本身的这些突出挑战极大地限制了其光催化性能的提高,但也为未来研究中构建更高效的基于 g-C 3 N 4 的光催化剂提供了更多机会。
2.3. Design considerations of g-C3N4-based photocatalysts
2.3. 基于 g-C 3 N 4 的光催化剂设计考虑
For these above reasons, careful consideration must be given to the rational design of g-C3N4 for achieving the optimum photocatalytic performances. To avoid some of these drawbacks and maximize the photocatalytic efficiency, several modification strategies have been pursued to design highly efficient g-C3N4-based photocatalysts. Fig. 5 summarizes the design considerations of g-C3N4-based photocatalysts based on their detailed composition, structures and properties. More definitely speaking, hetero-junction construction [120], [121], dimensionality tuning (nano-templating [122], [123]) and nanocarbon loading have been widely applied in promoting the charge transfer, mobility and separation respectively. Furthermore, suitable co-catalyst loading [44], [124], [125], [126], [127] and defect control have been available in accelerating the surface reaction kinetics (charge utilization). In addition, pore texture tailoring, surface sensitization, and band-gap engineering (non-metal doping [128] and co-polymerization [84], [129], [130] strategies) were utilized to create highly mesoporous g-C3N4 with high surface area and to increase the light harvesting and visible absorption through the red-shift of its optical absorption edge, respectively. In future, it is expected more and more engineering modification strategies will be developed to improve the photocatalytic performances of g-C3N4-based photocatalysts. More importantly, all different photocatalytic stages such as light harvesting, charge excitation, charge transfer, mobility and separation, and surface charge utilization should be simultaneously considered and optimized [7]. In other words, the synergy and integration effect of these different strategies should be paid more attention. In the following sections, the versatile properties, design strategies and potential applications will be thoroughly summarized.
由于上述原因,必须仔细考虑合理设计 g-C 3 N 4 以实现最佳光催化性能。为了避免一些缺点并最大程度提高光催化效率,已经采用了几种改性策略来设计高效的基于 g-C 3 N 4 的光催化剂。图 5 总结了基于其详细组成、结构和性质的 g-C 3 N 4 基光催化剂的设计考虑。更确切地说,异质结构构建[120],[121],维度调控(纳米模板[122],[123])和纳碳负载已被广泛应用于促进电荷转移、迁移和分离。此外,适当的共催化剂负载[44],[124],[125],[126],[127]和缺陷控制可加速表面反应动力学(电荷利用)。此外,孔结构调节、表面敏化和带隙工程(非金属掺杂[128]和共聚[84],[129],[130]策略)被用于创建具有高比表面积的高介孔性 g-C 3 N 4 ,并通过光学吸收边缘的红移来增加光的收获和可见光吸收。未来,预计将会开发更多的工程改性策略来改善基于 g-C 3 N 4 的光催化剂的光催化性能。更重要的是,应同时考虑和优化不同的光催化阶段,如光的收获、电荷激发、电荷转移、迁移和分离以及表面电荷利用[7]。 换句话说,这些不同策略的协同作用和整合效应应该得到更多关注。在接下来的部分中,将对多功能性能、设计策略和潜在应用进行全面总结。
3. Versatile properties of g-C3N4-based photocatalysts
3. g-C 3 N 4 基光催化剂的多功能性质
As is known, the photocatalytic efficiency of g-C3N4-based photocatalysts is mainly governed by all parameters/properties of g-C3N4 itself, including crystal structural, surface physicochemical, stability, optical, adsorption, electrochemical, photoelectrochemical and electronic properties. Thus, a fundamental understanding and deterministic control of these chemical and structural factors will enable the scalable production of g-C3N4-based composite photocatalysts with the best photocatalytic behavior, which will be favorable for creating some robust g-C3N4-based material systems for practical photocatalytic applications and fundamental insights into photocatalytic enhancement mechanisms at the single-atom level.
众所周知,基于 g-C 3 N 4 的光催化剂的光催化效率主要由 g-C 3 N 4 本身的所有参数/性质所支配,包括晶体结构、表面物理化学性质、稳定性、光学、吸附、电化学、光电化学和电子性能。因此,对这些化学和结构因素的基本理解和确定性控制将使得能够大规模生产基于 g-C 3 N 4 的复合光催化剂,其具有最佳的光催化行为,这将有利于为实际光催化应用创建一些健壮的基于 g-C 3 N 4 的材料系统,并为单原子水平上的光催化增强机制提供基本见解。
3.1. Crystal structural properties
3.1 晶体结构性质
It is known that C3N4 possesses seven different phases, e.g., α-C3N4, β-C3N4, cubic C3N4, pseudocubic C3N4, g-h-triazine, g-h-heptazine and g-o-triazine, which exhibit the band gaps of 5.49, 4.85, 4.30, 4.13, 2.97, 2.88 and 0.93 eV, respectively, in terms of GW method (as shown in Table 3) [131]. Among them, the famous super hard β-C3N4 crystalline phase has been demonstrated to possess the similar hardness/low compressibility to that of diamond [132]. Except the pseudocubic and g-h-triazine phases, other five phases have indirect band gaps in their bulk structures [127], [131], [133]. As observed from Table 3, it is clear that g-h-triazine and g-h-heptazine phases exhibit the suitable band gaps of 2.97 and 2.88 eV for visible-light absorption, favoring their applications in different photocatalytic fields. More interestingly, g-C3N4 also shows the stacked 2D layered structure, as displayed in Fig. 6a. Furthermore, two different condensation states have been demonstrated as the primary building block in a single layer of g-C3N4 networks: s-triazine units (ring of C3N3; Fig. 6b) with a periodic array of single carbon vacancies; and tri-s-triazine/heptazine subunits (triring of C6N7; Fig. 6c) connected through planar tertiary amino groups with larger periodic vacancies in the lattice [134]. More importantly, it has been experimentally and theoretically demonstrated that the energetically favored tri-s-triazine-based g-C3N4 was 30 kJ mol−1 more stable than the triazine-based g-C3N4, adequately suggesting that tri-s-triazine is the most widely accepted basic unit for the g-C3N4 networks [131], [134], [135], [136].
众所周知,碳氮 C 3 N 4 具有七种不同的相,例如α-C 3 N 4 ,β-C 3 N 4 ,立方体 C 3 N 4 ,伪立方体 C 3 N 4 ,g-h-三唑啉,g-h-庚三唑啉和 g-o-三唑啉,它们分别展现出 5.49、4.85、4.30、4.13、2.97、2.88 和 0.93 电子伏特的带隙,根据 GW 方法计算(如表 3 所示)[131]。其中,著名的超硬β-C 3 N 4 晶相已被证明具有类似于金刚石的硬度/低压缩性[132]。除了伪立方体和 g-h-三唑啉相外,其他五相在其体块结构中具有间接带隙[127],[131],[133]。从表 3 中可以看出,g-h-三唑啉和 g-h-庚三唑啉相的适宜带隙分别为 2.97 和 2.88 电子伏特,有利于它们在不同光催化领域的应用。更有趣的是,g-C 3 N 4 也展现出叠加的二维层状结构,如图 6a 所示。此外,在 g-C 3 N 4 网络的单层中已经证明了两种不同的凝结状态作为基本构建块:s-三唑烷基单元(C 3 N 3 环;见图 6b)具有周期性的单个碳空位排列,以及三-s-三唑啉/庚三唑啉亚基(C 6 N 7 三环;见图 6c)通过平面三级氨基团连接,在晶格中具有较大周期性空位[134]。 更重要的是,实验证明和理论上证明了,在能量上更有利的三聚 s-三嗪基 g-C 3 N 4 比三嗪基 g-C 3 N 4 更稳定 30 kJ/mol,充分表明三聚 s-三嗪是 g-C 3 N 4 网络中被广泛接受的基本单元[131], [134], [135], [136]。
C3N4 phases C 3 N 4 相 | Space group 空间群 | Lattice parameter (Å) 晶格常数(Å) | Z | GW approximation band gap (eV) GW 近似带隙(eV) | local density approximation band gap (eV) 局域密度近似带隙(eV) |
---|---|---|---|---|---|
Alpha 阿尔法 | P31c (159) | a = 6.465, c = 4.709 | 4 | 5.49 | 3.76 |
Beta 贝塔 | P3 (143) | a = 6.406, c = 2.406 | 2 | 4.85 | 3.12 |
Cubic 立方的 | I-43d (220) | a = 5.411 | 2 | 4.30 | 2.87 |
Pseudocubic 假立方 | P–42 m (111) | a = 3.426 | 1 | 4.13 | 2.53 |
g-h-triazine g-h-三嗪 | P-6m2 (187) | a = 4.746, c = 6.586 | 2 | 2.97 | 1.16 |
g-h-heptazine | Cmc21 (36) | a = 7.083, b = 12.269, c = 6.871 | 4 | 2.88 | 0.89 |
g-o-triazine | P2 mm (25) | a = 4.147, b = 4.754, c = 6.474 | 2 | 0.93 | – |
It is known that XRD has been extensively employed to precisely measure the lattice constant and crystal structures. The typical experimental XRD pattern of bulky g-C3N4 powders have two distinct diffraction peaks located at 27.40° and 13.0° (as shown in Fig. 7b), which can be indexed as (002) and (100) diffraction planes for graphitic materials (JCPDS 87-1526) [44]. Clearly, the XRD results indicate that the g-C3N4 exhibits the flake-like structure with interplanar stacking distance of 0.325 nm revealed by (002) diffraction, which is similar to that of graphite with stacking distance of 0.34 nm [102], [137]. The distance of 0.681 nm for in-plane structural packing motif is slightly smaller than that of the tris-s-triazine units (ca. 0.73 nm), presumably due to the bending of 2D layered structures [44]. However, the g-C3N4 nanotubes exhibit one distinct XRD diffraction peak at 17.4° (corresponding to an interplanar separation of d = 0.49 nm, Fig. 7a), indicating the formation of the s-triazine units (with the theoretical value of d = 0.47 nm [127]) in g-C3N4 [138]. Consequently, the exact periodic units in each layer of g-C3N4 could be readily identified by the XRD peak associated with an in-plane structural packing motif.
众所周知,XRD 已被广泛应用于精确测量晶格常数和晶体结构。厚实的 g-C 3 N 4 粉末的典型实验 XRD 图样具有两个明显的衍射峰,分别位于 27.40°和 13.0°(如图 7b 所示),可被索引为石墨材料(JCPDS 87-1526)的(002)和(100)衍射面[44]。显然,XRD 结果表明 g-C 3 N 4 呈片状结构,间隙层间距为 0.325nm((002)衍射),类似于石墨的 0.34nm [102], [137]。平面内结构堆积基元的距离为 0.681nm,略小于三 s-三嗪单位的距离(约 0.73nm),可能是由于 2D 层状结构的弯曲[44]。然而,g-C 3 N 4 纳米管呈现一个明显的 XRD 衍射峰(对应间层间距 d=0.49nm,图 7a),表明在 g-C 3 N 4 中形成 s-三嗪单位(与理论值 d=0.47nm [127])[138]。因此,通过与平面内结构堆积基元相关的 XRD 峰,可以很容易地确定 g-C 3 N 4 每层中的周期单元。
3.2. Surface physicochemical properties
3.2 表面物理化学性质
It is known that a variety of surface defects on the surface of polymeric g-C3N4 material lead to the formation of multiple functionalities. Commonly, the basic primary and/or secondary amine groups (e.g., CNH2 and C2NH) on the terminating edges in the single layer of g-C3N4 (as shown in Fig. 8) could be created by a small quantity of hydrogen impurity, owing to the incomplete polycondensation [49], [101]. Therefore, it is not surprising that the g-C3N4 materials with surface defects and electron-rich properties also exhibit the unique nucleophilic character from basic surface functionalities (for the activation of CO2) or H-bonding motifs (as shown in Fig. 8), thus facilitating their more valuable applications in catalysis, as compared to the ideal and defect-free g-C3N4 [101]. Furthermore, it is easily understood that the abundant basic groups (NH, N, NH2 and NC) on the surface of g-C3N4 are beneficial for the removal of acidic toxic molecules through chemical adsorption based on electrostatic interactions [139]. Similar to the hydrophobic nature of the nanocarbon surface, hydrophobicity of g-C3N4 could lead to the formation of weakly interacted interfacial layer, thus significantly restricting the electron transport and separation and surface electrocatalytic reactions [61]. At this point, the hydrophilicity of g-C3N4 materials (with decreasing contact angle of water on their surface) could be improved through introducing oxygen-containing functional groups (hydroxyl and carboxyl) by means of chemical oxidation, thus greatly favoring their good dispersion in the aqueous solutions and further enhanced interfacial coupling and photocatalytic activities [140], [141], [142], [143], [144], [145].
众所周知,聚合物 g-C 3 N 4 材料表面的各种缺陷导致多种功能的形成。通常,g-C 3 N 4 单层中终止边缘上的基本一级和/或二级胺基团(例如,CNH 2 和 C 2 NH)可由少量氢杂质创建,这归因于不完全的缩聚作用[49],[101]。因此,g-C 3 N 4 材料具有表面缺陷和富电子性质,表现出独特的亲核特性,从而促进了其在催化中比理想和无缺陷的 g-C 3 N 4 更有价值的应用(如图 8 所示),如活化 CO 2 或氢键合基序(如图 8 所示)[101]。此外,可以轻松理解,g-C 3 N 4 表面丰富的碱性基团( NH , N , NH 2 和 N C )有助于通过电荷间的化学吸附去除酸性有毒分子[139]。类似于纳米碳表面的疏水性质,g-C 3 N 4 的疏水性可能导致弱相互作用的界面层形成,从而显著限制了电子传输和分离以及表面电催化反应[61]。 目前,通过化学氧化引入含氧功能团(羟基和羧基)可以提高 g-C 3 N 4 材料的亲水性(使水在其表面的接触角减小),从而极大地促进它们在水溶液中的良好分散,并进一步增强界面耦合和光催化活性[140],[141],[142],[143],[144],[145]。
Commonly, these functional groups and networks (CN) could be further identified by the Fourier transform infrared (FT-IR) spectra, X-ray photoelectron spectroscopy (XPS) measurements, Raman spectra, and Boehm titration analysis. Generally, the chemical composition and bonding information of g-C3N4 could be partially identified by FT-IR measurement. Fig. 9 depicts typical FTIR spectra of g-C3N4 powders obtained by heating melamine (MCN), thiourea (TCN), and urea (UCN) [146]. As shown, the main characteristic peaks observed in the region from 900 to 1700 cm−1 were usually assigned to stretching vibration signals of aromatic heptazine-derived repeating units, including the typical sp2 CN stretching modes and out-of-plane bending vibrations of the sp3 CN bonds [130], [147], [148], [149], [150], while the sharp absorption peak centered at approximately 810 cm−1 was attributed to the characteristic breathing mode of tri-s-triazine cycles [150], [151], [152]. Meanwhile, the absorption band at 883 cm−1 were indexed as the deformation mode of NH in amino groups [153], whereas the broadened peaks between 3000 and 3500 cm−1 were related to the stretching vibration [146], [149], [150], [153] of residual free NH in the bridging CNHC units and OH originated from physically adsorbed water species on g-C3N4 surface, respectively. In the recorded Raman spectra, several characteristic peaks of g-C3N4 can be observed at 1616, 1555, 1481, 1234, 751, 705, 543, and 479 cm−1, further confirming the vibration modes of CN heterocycles [151], [154]. It should be noted that the peak at 1234 cm−1, corresponding to the NC (sp2) bending vibration, exhibits significant blue shift (1250 cm−1 for 1-layer g-C3N4), due to the phonon confinement and strong quantum confinement effect [154]. Moreover, it has also been experimentally and theoretically demonstrated that the ratios of peak heights of 751–705 cm−1 (I751/I705) and 543–479 cm−1 (I543/I479), corresponding to layer–layer deformation vibrations or the correlation vibrations, obviously increased with decreasing the layer number of g-C3N4 [154]. Additionally, the nitrogen containing species can be further quantitatively analyzed by the element analysis and Boehm titration. For the element analysis, X-ray photoelectron spectroscopy (XPS) can not only reveal the atom ratio of carbon to nitrogen, but also identify the carbon and nitrogen species in g-C3N4. For example, the main peak at 288.2 eV in the high-resolution C 1s XPS spectra of the 1.0 wt% RGO/g-C3N4 sample, indicates the existence of the NCN2 coordination [151]. The N 1 s binding energies at about 398.6, 399.8, and 401.5 eV in the high-resolutionN1 s XPS spectra of the 1.0 wt% RGO/g-C3N4 sample can be assigned to sp2-hybridized nitrogen (CNC), tertiary nitrogen (N(C)3) and amino functional groups having a hydrogen atom (CNH), respectively [146], [151], [155], [156]. For Boehm titration analysis, it was found that the content of basic group per unit area of g-C3N4 generally decreased with increasing the calcination temperature [157]. In a word, the combination of Raman vibration properties, FTIR, XPS spectra and Boehm titration analysis can fully reveal the surface functional groups of g-C3N4 nanomaterials (Figs. 9,10 and 11).
通常,这些功能基团和网络(C N )可以通过傅里叶变换红外(FT-IR)光谱、X 射线光电子能谱(XPS)测量、拉曼光谱和 Boehm 滴定分析进一步确定。一般来说,通过 FT-IR 测量可以部分确定 g-C 3 N 4 的化学组成和键合信息。图 9 描述了通过加热三聚氰胺(MCN)、硫脲(TCN)和尿素(UCN)得到的 g-C 3 N 4 粉末的典型 FTIR 光谱[146]。如图所示,在 900 到 1700cm −1 范围内观察到的主要特征峰通常被指定为芳香基七氮杂环重复单元的拉伸振动信号,包括典型的 sp 2 C N 拉伸模式和 sp 3 C N 键的平面外弯曲振动[130],[147],[148],[149],[150],而约 810cm −1 处的尖锐吸收峰被归因为三对三嗪环的特征呼吸模式[150],[151],[152]。同时,883cm −1 处的吸收带被索引为氨基团中 N H 的变形模式[153],而在 3000 到 3500cm −1 之间的宽峰与 g-C 3 N 4 表面上物理吸附的水分子和 C NH C 单元中残留的游离 N H 的拉伸振动[146],[149],[150],[153]有关,分别。在记录的拉曼光谱中,可以观察到 g-C 3 N 4 的几个特征峰,分别位于 1616、1555、1481、1234、751、705、543 和 479cm −1 ,进一步确认了 CN 杂环的振动模式[151],[154]。 需要注意的是,1234 厘米处的峰值 −1 ,对应于 N C(sp 2 )弯曲振动,呈现明显的蓝移(1 层 g-C 3 N 4 为 1250 厘米 −1 ),这是由于声子约束和强量子约束效应[154]。此外,实验证明,在 751–705 厘米 −1 (I 751 /I 705 )和 543–479 厘米 −1 (I 543 /I 479 )的峰值高度比,对应于层间变形振动或相关振动,显著随着 g-C 3 N 4 层数减少而增加,理论上也得到了证实[154]。此外,可以通过元素分析和 Boehm 滴定进一步定量分析含氮物种。对于元素分析,X 射线光电子能谱(XPS)不仅可以揭示碳氮原子比,还可以确定 g-C 3 N 4 中的碳和氮物种。例如,在 1.0wt% RGO/g-C 3 N 4 样品的高分辨率 C1s XPS 光谱中的 288.2eV 主峰表明存在 N C N 2 配位[151]。在 1.0wt% RGO/g-C 3 N 4 样品的高分辨率 N1s XPS 光谱中,N1s 束缚能在约 398.6、399.8 和 401.5eV 处,分别对应于 sp 2 杂化氮(C N C)、三级氮(N (C) 3 )和具有氢原子的氨基官能团(C N H)[146],[151],[155],[156]。对于 Boehm 滴定分析,发现 g-C 3 N 4 单位面积的碱性基团含量通常随着焙烧温度的升高而减少[157]。 总之,拉曼振动性能、傅里叶变换红外光谱(FTIR)、X 射线光电子能谱(XPS)和 Boehm 滴定分析的结合可以充分揭示 g-C 3 N 4 纳米材料的表面功能团(图 9、10 和 11)。
Interestingly, the basic surface functionalities can be further evidenced by the isoelectric point (IEP) and the zeta potentials of g-C3N4 dispersions [146]. It is known that the IEP is an important physicochemical parameter of many compounds, such as oxides, sulfides, hydroxides, and nitrides, which has been widely used to estimate the surface charges of compound particles at various pH conditions. In general, the solid particles are positively charged and negatively charged at pH values below and above the IEP point, respectively. For example, Wang et al. found that the zeta-potential of bulk g-C3N4 dispersions in water was −47.4 mV, showing a negative surface charge, whereas, g-C3N4 exhibited a positive surface charge with a zeta-potential of +30 mV after the successful protonation in HCl solution [158]. Similarly, Yu and his coworkers experimentally demonstrated that the IEPs of TCN, MCN, and UCN samples are 4.4, 5.0 and 5.1, respectively, further confirming their basic and negatively charged surface. Thus, in the initial pH, the MCN, UCN and TCN samples exhibited the negative Zeta potentials of −17.0, −30.7 and −19.9 mV, respectively (as shown in Fig. 12) [146]. Thus, the surface protonation of g-C3N4 treated by the acid solution with pH below its IEPs has become a popular strategy to reverse the surface properties of g-C3N4 from negative to positive charge, facilitating the construction of composite materials through electrostatic interactions with negatively charged materials and the enormous enhancement in photocatalytic performance [138], [158], [159], [160], [161], [162]. It was believed that surface protonation modification could simultaneously achieve better dispersion, adjusted electronic band gaps, and increased surface area and ionic conductivity [158]. In the contrary, the surface modification of g-C3N4 via alkaline hydrothermal treatment (e.g. NaOH and ammonium hydroxide) can create more surface hydroxylation and rich surface H-bond network of g-C3N4 with the negatively charged surface, thus greatly enhancing the interfacial charge transfer, and increasing the specific surface area and pore volume [163], [164], [165]. Particularly, the H-bonding network can offer multiple channels and inmate interfaces for the proton transfer from water to the photo-excited electrons on g-C3N4 surface, stabilize the negatively charged intermediate and transition states, thus obviously promoting charge separation and photocatalytic H2 evolution [166].
有趣的是,基本表面功能可以通过石墨相氮化碳(g-C 3 N 4 )分散液的等电点(IEP)和 zeta 电位进一步证明[146]。众所周知,IEP 是许多化合物的重要理化参数,如氧化物、硫化物、氢氧化物和氮化物,已被广泛用于估计不同 pH 条件下化合物颗粒的表面电荷。一般来说,在 IEP 点以下和以上的 pH 值下,固体颗粒分别带正电荷和负电荷。例如,Wang 等发现水中 g-C 3 N 4 分散体的ζ-电位为-47.4mV,表明呈负表面电荷,而在 HCl 溶液中成功质子化后,g-C 3 N 4 表现出正表面电荷,ζ-电位为+30mV[158]。同样,于氏及其合作者实验证明了 TCN、MCN 和 UCN 样品的 IEP 分别为 4.4、5.0 和 5.1,进一步证实了它们的基础和负电荷表面。因此,在初始 pH 条件下,MCN、UCN 和 TCN 样品显示了分别为-17.0、-30.7 和-19.9mV 的负 zeta 电位(如图 12 所示)[146]。因此,将酸性溶液处理的 g-C 3 N 4 的表面质子化到其 IEP 以下的 pH 成为了将 g-C 3 N 4 的表面特性从负电荷转变为正电荷的一种常用策略,通过电荷作用促进与带负电荷材料的复合材料构建并显著增强光催化性能[138],[158],[159],[160],[161],[162]。 据信表面质子化修饰可以同时实现更好的分散性、调整的电子带隙,以及增加的表面积和离子导电性[158]。相反地,通过碱性水热处理(如 NaOH 和氢氧化铵)对 g-C 3 N 4 进行表面修饰可以产生更多的表面羟基化和富含负电荷表面的 g-C 3 N 4 的富氢键网络,从而极大地增强界面电荷转移,并增加特定表面积和孔体积[163],[164],[165]。特别地,氢键网络可以为从水向光生电子在 g-C 3 N 4 表面的质子转移提供多通道和孔隙界面,稳定负电荷中间体和过渡态,从而明显促进电荷分离和光催化 H 2 产生[166]。
3.3. Stability properties
3.3 稳定性质
The defect-rich and N-bridged tri-s-triazine-based g-C3N4 was found to be energetically favored relative to the other phases, which exhibits extraordinary thermal stability up to 600 °C [149], [167], [168], [169]. In air, over a period of months, the stable g-C3N4 only exhibited a slightly lighter color change, because of its strong water adsorption effects [167]. Fig. 13 gives the thermogravimetric-differential scanning calorimetry (TG-DSC) analysis for melamine and g-C3N4 prepared by heat polymerization of melamine at 520 °C in air [149], which clearly indicated that the formation and decomposition of g-C3N4 (stable up to 600 °C) involve a series of processes, e.g., the sublimation and thermal condensation of melamine (297–390 °C), de-ammonation process (545 °C) and further oxidation decomposition (630–750 °C) of g-C3N4 material [44], [149]. It has been observed that melem transforms into a g-C3N4 material by temperature-dependent X-ray powder diffractometry investigations above 560 °C [134]. The complete decomposition temperature of g-C3N4 ranging from 700 to 750 °C, is in good agreement with the Gillan’s report [167], [170]. These typical processes were also observed during the formation of g-C3N4 by using cyanamide as precursors (as shown in Fig. 14), except for the first formation of melamine through condensing the cyanamide precursors [101]. Notably, the thermal stability of g-C3N4 has been regarded to be the highest in organic materials, which can be obviously affected by the different polymerization degrees of g-C3N4 in different preparation methods [149], [167], [170], [171], [172], [173]. The high thermal stability of g-C3N4 polymeric semiconductor not only features its various applications, as a heterogeneous organic catalyst, at operating temperature below 500 °C, but also allows its easy removal by simply increasing the calcination temperature beyond 600 °C, thus favoring its utilization as confinement templates, structuring agents or nitrogen sources for synthesizing a refined carbon nanostructure or metal nitride nanostructures with continuously adjustable composition, such as TaON, Ta3N5, ternary aluminum gallium nitride and titanium vanadium nitride [55], [174], [175], [176], [177].
缺陷丰富且 N 桥三异噁唑基 g-C 3 N 4 相对于其他相具有更有利的能级,表现出高达 600°C 的非凡热稳定性 [149],[167],[168],[169]。在空气中,经过数月时间,稳定的 g-C 3 N 4 仅表现出轻微的颜色变化,这是由于其强大的吸水效应所致 [167]。图 13 给出了三聚氰胺和热聚合制备的 g-C 3 N 4 的热重-差示扫描量热分析 (TG-DSC) 分析结果,清楚地表明了 g-C 3 N 4 的形成和分解 (在 600°C 稳定) 涉及一系列过程,例如三聚氰胺的升华和热缩聚 (297–390°C)、脱氨化过程 (545°C) 和进一步的氧化分解 (630–750°C) [44],[149]。观察到马来酸腐转变为 g-C 3 N 4 材料,通过温度相关的 X 射线粉末衍射法研究表明在 560°C 以上 [134]。g-C 3 N 4 的完全分解温度范围为 700 到 750°C,与 Gillan 的报告 [167],[170] 相吻合 [149]。这些典型过程也在使用氰胺作为前体形成 g-C 3 N 4 时观察到(如图 14 所示),除了第一次通过凝结氰胺前体形成三聚氰胺 [101]。值得注意的是,g-C 3 N 4 的热稳定性被认为是有机材料中最高的,这显然受到不同制备方法中 g-C 3 N 4 的不同聚合度的影响[149],[167],[170],[171],[172],[173]。 g-C 3 N 4 聚合半导体具有很高的热稳定性,不仅能够在 500°C 以下的操作温度下作为非均相有机催化剂应用,而且通过简单增加焙烧温度至 600°C 以上即可轻松去除,有利于其作为限域模板、结构化剂或氮源,用于合成精制碳纳米结构或金属氮化物纳米结构,其成分可以连续调节,例如 TaON、Ta 3 N 5 、三元铝镓氮化物和钛钒氮化物[55], [174], [175], [176], [177]。
Furthermore, the g-C3N4 also exhibits superior chemical stability [167]. Similar to that of graphite, it has been demonstrated that the g-C3N4 with optimized van der Waals interactions between the single layers is insoluble in water, acid, base, and various kinds of organic solvents, including ethanol, toluene, diethyl ether and THF [55], [127], [133], [167]. However, notably, the molten alkali metal hydroxides and KMnO4 could lead to the hydroxolysis and the strong oxidation decomposition of intrinsic structures of the g-C3N4 materials, respectively [178]. In particular, the excellent acid stability and interesting protonation effects have been also further confirmed [178]. Commonly, the concentrated-acid treatment at room temperature could result in the formation of a non-transparent solution containing highly dispersed nanosheets without destroying the graphite-like structure of g-C3N4 [158], because the protonation effects could break both sheets and stacks from its defects, as well as fabricate a highly porous texture between the adjoining layers. Zhang et al. demonstrated that the relatively good-quality g-C3N4 thin films could be fabricated by conventional dip/disperse-coating techniques using a stable protonated g-C3N4 colloidal suspension [179]. More recently, the first liquid state NMR spectra and a lyotropic liquid crystal phase of g-C3N4 have been successfully observed through the highly protonated g-C3N4 thermodynamic solutions with concentration up to 300 mg mL−1, highlighting the promising applications of g-C3N4 solubilization technique in concentrated H2SO4 [180].
此外,g-C 3 N 4 也表现出卓越的化学稳定性[167]。与石墨类似,已经证明优化范德华相互作用的 g-C 3 N 4 在水、酸、碱和各种有机溶剂包括乙醇、甲苯、乙醚和 THF 中都不溶[55],[127],[133],[167]。然而,值得注意的是,熔融的碱金属氢氧化物和 KMnO 4 可能会导致 g-C 3 N 4 材料固有结构的水解和强氧化分解[178]。特别是,优异的耐酸性和有趣的质子化效果也进一步得到了确认[178]。通常,室温下的浓酸处理可能导致形成不透明的溶液,其中高度分散的纳米片层不会破坏 g-C 3 N 4 的类石墨结构,因为质子化效果可以破坏其缺陷处的片层和堆积,并在毗邻层之间形成高度多孔的纹理[158]。张等人证明,可通过传统的浸渍/分散涂覆技术使用稳定的质子化 g-C 3 N 4 胶体悬浮液制备相对优质的 g-C 3 N 4 薄膜[179]。最近,通过高度质子化 g-C 3 N 4 热力学溶液成功观察到了第一次液态 NMR 光谱和液晶相的 g-C 3 N 4 ,其中浓度高达 300mgmL −1 ,突显了 g-C 3 N 4 的溶解技术在浓硫酸中的应用前景[180]。
3.4. Electronic properties
3.4. 电子性质
It is well known that the suitable electronic properties play important roles in better photocatalysis. To look insight into its electronic structure, the density-functional-theory (DFT) calculations were applied in obtaining the detailed oxidation and reduction levels of valance and conduction bands. The results shown in Fig. 15 demonstrated that the highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO-LUMO) gaps of melem molecule, polymeric melon and an infinite sheet of a hypothetically, fully condensed g-C3N4 were 3.5, 2.6 and 2.1 eV, respectively [44]. Clearly, the calculated band gap of polymeric melon is very close to the experimentally measured medium-band gap of 2.7 eV. The wavefunction investigations of the valence band (Fig. 15b) and conduction band (Fig. 15c) are mainly derived from nitrogen pz orbitals and carbon pz orbitals, which serve as oxidation and reduction sites for O2 and H2 evolution reactions, respectively [44]. Recently, it was found that, compared to the underestimated band gaps of semiconductors and insulators calculated by density functional theory (DFT) with local density approximation (LDA), the band structures obtained by the GW approximation (larger than the LDA band gap by 1.73 eV) are much more close to the experimentally reported values [131].
众所周知,合适的电子性质在光催化中起着重要作用。为了深入了解其电子结构,采用密度泛函理论(DFT)计算来获得平面和导带的详细氧化和还原能级。图 15 所示的结果表明,三聚异隆酮分子,聚合物甜瓜和假设的完全凝结的 g-C 3 N 4 无限片的最高占据分子轨道/最低未占据分子轨道(HOMO-LUMO)间隙分别为 3.5、2.6 和 2.1eV [44]。显然,聚合物甜瓜的计算带隙非常接近实验测得的 2.7eV 的中等带隙。氮 p z 轨道和碳 p z 轨道分别主要用作 O 2 和 H 2 的氧化和还原位点。最近发现,与由局部密度近似(LDA)的密度泛函理论(DFT)计算低估的半导体和绝缘体带隙相比,通过 GW 近似方法得到的带结构(比 LDA 带隙大 1.73eV)更接近实验报告的值 [131]。
Besides the DFT calculations, the band edge positions of g-C3N4 materials can be determined by electrochemicalimpedance spectra (EIS), based on the Mott–Schottky (M–S) plot, and the valence band X-ray photoelectron spectra (VB-XPS), respectively [7]. Wang and his coworkers measured the flat band potentials of bulk g-C3N4 through the M–S method, indicating that its conduction band and valance band edges are located at −1.3 and 1.4 V vs NHE at pH 7, respectively [54], [84], [187]. Meanwhile, Yan et al. obtained the accurate conduction band and valance band edges of bulk g-C3N4 located at −1.53 and 1.16 V vs NHE at pH 7 by the VB-XPS mehod, respectively [93]. However, it is worthy of noting that these two results seem to be slightly contradictory. The main reason is probably that many researchers directly equate the flat-band potential with the conduction band potential. In fact, for an n-type semiconductor, the conduction band potential is more negative by about −0.1 or −0.2 V than the flat-band potential [7]. Considering the slight difference (−0.2 V) between the flat-band potential and the conduction band potential, there was complete agreement between these two experimental results. The band structures of different types of g-C3N4 samples obtained by these two methods have been summarized and listed in Fig. 16. As shown in Fig. 16, the doping of P and C can make the conduction and valance band edges of g-C3N4 more negative and positive, respectively, thereby facilitating the reduction and oxidation reactions. In future, it is expected that more accurate band positions of g-C3N4 could be available and applied in the design and development of highly efficient g-C3N4-based photocatalysts.
除了 DFT 计算之外,g-C 3 N 4 材料的带边位置还可以通过电化学阻抗谱(EIS)和基于莫特-肖特基(M-S)图以及价带 X 射线光电子能谱(VB-XPS)分别确定[7]。王等人通过 M-S 方法测量了块体 g-C 3 N 4 的平带位势,表明其导带和价带边分别位于 pH 7 下相对于 NHE 在-1.3 和 1.4V,[54],[84],[187]。与此同时,闫等人通过 VB-XPS 方法获得了块体 g-C 3 N 4 的准确导带和价带边,分别位于 pH 7 下相对于 NHE 在-1.53 和 1.16V,[93]。然而,值得注意的是,这两个结果似乎略有矛盾。主要原因可能是许多研究人员直接将平带位势等同于导带位势。实际上,对于 n 型半导体,导带位势要更负约-0.1 或-0.2V 左右[7]。考虑到平带位势和导带位势之间的轻微差异(-0.2V),这两个实验结果之间存在完全一致。通过这两种方法得到的不同类型 g-C 3 N 4 样品的带结构已总结并列于图 16 中。如图 16 所示,P 和 C 的掺杂可以使 g-C 3 N 4 的导带和价带边更负和更正,从而有利于还原和氧化反应。预计在未来,更准确的 g-C 3 N 4 带位置将能够被应用于高效 g-C 3 N 4 基光催化剂的设计和开发中。
In addition, the underlying electronic properties and charge carrier dynamics, including the microscopic dynamic process of the charge generation, recombination, separation, and transfer, play crucial roles in determining the photocatalytic performance [7], [58]. Thus, the in-depth understanding of the electrical properties and charge carrier dynamics is, therefore, fundamentally important to help us to design and construct the more efficient and stable g-C3N4-based composite photocatalysts. To date, many different advanced techniques, such as femtosecond transient absorption (TA) spectroscopy [188], [189], time-resolved fluorescence spectroscopy, transient photocurrent decay, Nyquist impedance plots and the transient photovoltage (TPV) technique [190], [191], [192], [193], [194], [195] have been available in studying the charge carrier dynamics of g-C3N4-based composite photocatalysts. For instance, Wang et. al measured the time-resolved PL spectrum of bulk g-C3N4 and revealed that the photoinduced charge carriers in bulk g-C3N4 showed a lifetime of ∼5 ns even at 298 K, indicating the fast recombination rate (Fig. 17a) [123]. The greatly suppressed PL signal of mpg-C3N4 further indicated that the surface terminal sites of mpg-C3N4 can promote the electron relocalization, thus accelerating the catalytic functions of mpg-C3N4 for surface redox reactions. Similarly, the isotype heterojunctions between g-C3N4 and S-doped g-C3N4 have been found to exhibit a matched band alignment, which can significantly promote the charge separation between them, thus resulting in prolonging the lifetime of photo-excited charge carriers by about 2.15 ns [121]. It is believed that the prolonged lifetime of photo-generated charge carriers could further increase their utilization efficiency in driving surface photoredox reactions. Recently, the TA spectra of g-C3N4 also revealed that the existence of silica templates can prolong the lifetime of excited charge carriers by about hundreds of picoseconds, thereby achieving the high photocatalytic activities (Fig. 17b) [188]. More recently, it was demonstrated that the charge separate efficiency in g-C3N4-based photocatalysts could be also revealed by the SPV measurement, as an advanced and facile technology. As shown in Fig. 18, the obviously increased SPV signal in the range of 300–450 nm could be achieved through loading Ni nanoparticles on g-C3N4 as co-catalysts, suggesting the greatly accelerated charge separation efficiency [192], [193].
此外,基本电子性质和电荷载流子动力学,包括电荷产生、复合、分离和转移的微观动力学过程,在确定光催化性能方面起着至关重要的作用[7],[58]。因此,深入理解电学性质和电荷载流子动力学对我们帮助设计和构建更高效稳定的 g-C 3 N 4 基复合光催化剂至关重要。迄今为止,已有许多不同的先进技术可用于研究 g-C 3 N 4 基复合光催化剂的电荷载流子动力学,如飞秒暂态吸收(TA)光谱[188],[189],时间分辨荧光光谱、瞬态光电流衰减、Nyquist 阻抗图以及瞬态光电压(TPV)技术[190],[191],[192],[193],[194],[195]。例如,王等人测量了体相 g-C 3 N 4 的时间分辨 PL 光谱,并揭示了即使在 298K 下,体相 g-C 3 N 4 中的光诱导电荷载流子的寿命约为∼5ns,表明了快速复合率(图 17a)[123]。mpg-C 3 N 4 的光致发光信号极大地被抑制,进一步表明 mpg-C 3 N 4 的表面末端位可以促进电子重定位,从而加速 mpg-C 3 N 4 表面氧化还原反应的催化功能。类似地,g-C 3 N 4 和 S 掺杂的 g-C 3 N 4 之间的异质结呈现出匹配的能带结构,这能够显著促进它们之间的电荷分离,从而使光激发电荷载流子的寿命延长约 2.15ns[121]。 据信,光生电荷载体的延长寿命可能进一步提高它们在驱动表面光氧化还原反应中的利用效率。最近,g-C 3 N 4 的 TA 光谱也表明,硅模板的存在可以将激发电荷载体的寿命延长数百皮秒,从而实现高光催化活性(图 17b)[188]。最近的研究表明,基于 g-C 3 N 4 的光催化剂的电荷分离效率也可以通过 SPV 测量来展现,作为一种先进且便捷的技术。如图 18 所示,通过在 g-C 3 N 4 上负载 Ni 纳米颗粒作为共催化剂,可以在 300-450nm 范围内明显增加 SPV 信号,表明电荷分离效率大大加快[192],[193]。
3.5. Optical properties 3.5. 光学性质
For various kinds of photochemistry-related applications of g-C3N4, the decisive optical properties, including Ultraviolet–visible (UV/Vis) absorption, photoluminescence (PL) and electrochemiluminescence (ECL), have been readily further revealed by means of the theoretical calculations or experimental characterizations [47], [55], [187]. The typical UV/Vis absorption spectrum of g-C3N4 prepared at different temperature were displayed in Fig. 19a [44]. Indeed, the absorption edge of conventional g-C3N4 shows an obvious red shift towards longer wavelengths with increasing condensation temperatures, indicating that the increasing polymerization degrees can achieve a decreasing bandgap [44], [196]. The results are also consistent with those obtained by the theoretical calculations. Furthermore, it can be seen that these two samples fabricated at 550 and 600 °C exhibit very similar strong bandgap absorption, with edges at approximately 450 nm. The band gap energy (Eg) can be further obtained according to the intercept of the tangents to the plots of (αhν)1/2 vs. photon energy [146], [148]. The bandgap of the condensed g-C3N4 prepared at 550 °C is estimated to be 2.7 eV from its UV–vis spectrum, in good agreement with previous studies [148]. In fact, the greyish yellow color of g-C3N4 can further confirmed the favorable medium band gap for visible light absorption, as observed in the inset of Fig. 19a. More interestingly, other modification strategies, such as doping by Fe, S, P, C, I, O and B atomics (as shown in [16]) and barbituric acid moleculares [84] can also lead to a redshift of the adsorption edges.
对于 g-C 3 N 4 的各种光化学相关应用,包括紫外–可见(UV/Vis)吸收、光致发光(PL)和电化学发光(ECL)等决定性光学特性,已经通过理论计算或实验表征进一步揭示 [47], [55], [187]。 g-C 3 N 4 在不同温度下制备的典型 UV/Vis 吸收光谱如图 19a 所示[44]。实际上,传统 g-C 3 N 4 的吸收边缘随着缩聚温度的增加明显向较长波长的红移,表明增加的聚合度可以实现能隙的降低 [44], [196]。该结果也与理论计算得出的结果一致。此外,可以看到在 550°C 和 600°C 制备的这两个样品具有非常相似的强带隙吸收,在大约 450nm 处有吸收边缘。带隙能量(E g )可以通过绘制(αhν) 1/2 vs. photon energy 图并取其切线的截距进一步获得 [146], [148]。根据其 UV–vis 光谱,可估算出在 550°C 制备的紧凑 g-C 3 N 4 的带隙为 2.7eV,与先前研究结果相符[148]。实际上,g-C 3 N 4 的灰黄色也进一步证实了其适合的可见光吸收带隙,在图 19a 的插图中观察到。更有趣的是,其他修饰策略,如 Fe,S,P,C,I,O 和 B 等原子的掺杂(如[16]中所示)和巴比妥酸分子[84]也会导致吸收边缘的红移。
Apart from UV/Vis absorption spectrum of g-C3N4, intensive investigation also focus on its PL and ECL spectrum, due to its semiconductor properties. Both PL and ECL spectrum of g-C3N4 were also displayed in Fig. 19b [78]. As observed in Fig. 19b (top figure), the room-temperature PL spectrum of g-C3N4 solid powder at λ = 365 nm, exhibited a strong blue emission band ranging from 400 to 650 nm, with a maximum peak of ca. 470 nm in the blue region. It is generally believed that the intensity of room-temperature PL signal is employed to directly and qualitatively elucidate the recombination rate of photo-generated electrons and holes in irradiated g-C3N4 [155], [156], [197], [198]. Commonly, lower peak intensities imply the improved charge trapping and efficiently transferring, thus further prolonging the lifetimes of charge carriers and facilitating the enhancements in photocatalytic activity [199], [200], [201]. It should be noted that the room-temperature PL spectrum of g-C3N4 nanosheets are sensitively and markedly determined by their condensation degree (or optical band gap), thickness and sizes [47], [100], [202], [203]. It was also observed in the bottom of Fig. 19b that the g-C3N4-modified electrode in 0.10 M K2SO4 and 3.0 mM K2S2O8 showed the slightly broader ECL spectrum, with a maximum peak at ca. 470 nm (2.6 eV), which matches closely with the room-temperature PL spectrum of g-C3N4 solid powder. The blue ECL emission from g-C3N4 is strong enough to be observed with naked eyes as shown in the inset of bottom Fig. 19b [78]. Although there are obvious differences between the excited state of g-C3N4 in ECL and the room-temperature PL spectrum, the similar maximum emission peak of both types of luminescence suggested that identical ECL emission is also attributed to the band gap luminescence [78]. The results clearly demonstrated that the g-C3N4 semiconductor could be also a new kind of efficient and promising luminophore for ECL sensing to achieve the sensitive and selective detection of trace metal ions, such as Cu2+. Therefore, it is naturally expected that the metal-free and non-toxic g-C3N4 could be extensively utilized as a multifunctional optical material for light emitting devices [62], [63], bioimaging, [100], [142], [204], [205] ECL sensing probe [76], [78], [206], [207] and fluorescent probes [208], [209], [210].
除了研究 g-C 3 N 4 的 UV/Vis 吸收光谱外,还重点关注其 PL 和 ECL 谱,这是由于其半导体特性。 g-C 3 N 4 的 PL 和 ECL 谱也显示在图 19b [78] 中。如图 19b(顶部图)所示,g-C 3 N 4 固体粉末的室温 PL 谱在 λ=365nm 时展现出强烈的蓝色发射带,范围从 400 到 650nm,具有约 470nm 的最大峰值在蓝色区域。一般认为室温 PL 信号的强度被用来直接和定性地阐明辐照的 g-C 3 N 4 中光生电子和空穴的复合速率 [155],[156],[197],[198]。通常来说,较低的峰值强度意味着改善了电荷捕获和有效传输,从而延长了电荷载体的寿命,并促进了光催化活性的提高 [199],[200],[201]。值得注意的是,g-C 3 N 4 纳米片的室温 PL 谱受它们的缩聚程度(或光学带隙)、厚度和尺寸的影响是敏感和显著的 [47],[100],[202],[203]。在图 19b 底部也观察到 g-C 3 N 4 修饰电极在 0.10MK 2 SO 4 和 3.0mMK 2 S 2 O 8 中展现出稍微宽广的 ECL 谱,最大峰值约为 470nm(2.6eV),这与 g-C 3 N 4 固体粉末的室温 PL 谱相匹配。从 g-C 3 N 4 的蓝色 ECL 发射足够强大,裸眼可见,如图 19b 底部的插图所示 [78]。 尽管 g-C 3 N 4 在电化学发光(ECL)和室温下的光致发光(PL)光谱的激发态之间存在明显差异,但两种发光类型的最大发射峰相似,表明相同的 ECL 发射也归因于带隙发光 [78]。结果清楚地表明 g-C 3 N 4 半导体也可以成为一种新的高效且有前景的发光物质,用于 ECL 传感,以实现对微量金属离子(如 Cu 2+ )的敏感和选择性检测。因此,可以自然期望无金属且无毒的 g-C 3 N 4 可以广泛应用于光发射器件 [62],[63],生物成像 [100],[142],[204],[205],ECL 传感探针 [76],[78],[206],[207]以及荧光探针 [208],[209],[210] 等多功能光学材料。
3.6. Adsorption properties
3.6. 吸附性能
Generally speaking, adsorption property of a given adsorbent is strongly dependent by both its porous microtexture and surface chemical property [211], [212], [213], [214], [215]. Similar to the 2D graphene or graphene oxide materials, a wide variety of targeted adsorbates can be adsorpted on the multiple different functional groups (e.g., amino groups) and defect sites on g-C3N4 through different types of interactions such as physical adsorption (π-π stacking interaction), electrostatic attraction, or chemical interaction (surface complexation or acid-base interactions) [216]. Due to the weaker π-π stacking interaction in the physical adsorption, the stronger electrostatic attraction and chemical interaction have been proposed to the improved adsorption properties of g-C3N4, which will be thoroughly discussed in this section.
通常来说,给定吸附剂的吸附性能在很大程度上取决于其多孔微观结构和表面化学性质 [211], [212], [213], [214], [215]。类似于二维石墨烯或氧化石墨烯材料,各种靶向吸附物可以通过不同类型的相互作用(如物理吸附(π-π堆积作用)、静电吸引力或化学相互作用(表面络合或酸碱相互作用))被吸附到 g-C 3 N 4 的多种不同功能团(如氨基团)和缺陷位点上 [216]。由于物理吸附中π-π堆积作用较弱,据提出了较强的静电吸引力和化学相互作用来改善 g-C 3 N 4 的吸附性能,这将在本节中进行详细讨论。
Based on the electrostatic attraction, it has been demonstrated that the selective photodecomposition of anionic methyl orange (MO) or cationic methyl violet (MV) and methylene blue (MB) could be achieved over positively or negatively charged TiO2-based semiconductors, respectively [217], [218], [219]. Similarly, the electrostatic attraction between the negatively charged g-C3N4 and positively charged adsorbate molecules, such as cationic MV and MB have been proposed to achieve the selective adsorption and photocatalysis in many studies. For example, Yu and his coworkers demonstrated that the negatively charged g-C3N4 particles exhibited extraordinaryly higher adsorption capacity towards a cationic MB dye than anionic MO dye in aqueous suspension [146]. Further results showed that the adsorption kinetics of MB on three different kinds of g-C3N4 (Fig. 20a) could be well depicted by a pseudo-second-order kinetic equation as follows [220]:(2)dqt/dt = k2(qe − qt) 2(3)t/qt = 1/ (k2qe2) + t/qewhere qt (mg g−1) is the adsorption amount at time t, and k2 (g mg−1 min−1) is the pseudo-second-order rate constant, and qe (mg g−1) is the maximum adsorption amount. Notably, it was also revealed that the adsorption kinetics of heavy metal cationic ions and perfluorooctane sulfonate over g-C3N4 also followed the pseudo-second-order kinetic model [157], [221]. Meanwhile, it can be also found that the Langmuir model, as compared to Freundlich model, could be used to better fit the adsorption isotherms of MB on three g-C3N4 samples (Fig. 20b), indicating the homogeneity of the adsorbent surface [146]. Differences in the adsorption activity of three samples towards MB can be attributed to the synergistic effect of surface area and zeta potential.
基于静电吸引力,已经证明在带正电或者带负电的 TiO 2 基半导体上能够实现对负离子甲基橙(MO)或者正离子甲基紫(MV)和亚甲基蓝(MB)的选择性光解反应[217],[218],[219]。同样地,许多研究提出了负电荷 g-C 3 N 4 与正电荷吸附分子(例如正离子 MV 和 MB)之间的静电吸引力用于实现选择性吸附和光催化。例如,于雨等人展示了负电荷 g-C 3 N 4 微粒在水相悬浮液中对正离子 MB 染料的吸附容量明显高于对负离子 MO 染料的吸附容量[146]。进一步的结果显示,在三种不同类型的 g-C 3 N 4 (见图 20a)上,MB 的吸附动力学可以很好地由伪二级动力学方程描述如下[220]:
其中 q t (mgg −1 )是时间 t 时的吸附量,k 2 (gmg −1 min −1 )是伪二级速率常数,q e (mgg −1 )是最大吸附量。需要注意的是,也发现重金属阳离子和全氟辛烷磺酸盐在 g-C 3 N 4 上的吸附动力学也符合伪二级动力学模型[157],[221]。同时,可以发现与 Frundlich 模型相比,Langmuir 模型能够更好地拟合三种 g-C 3 N 4 样品对 MB 的吸附等温线(见图 20b),表明吸附剂表面的均质性[146]。 三种样品对亚甲基蓝的吸附活性差异可以归因于比表面积和ζ电位的协同效应。
Apart from the electrostatic attraction, the chemical interaction (surface complexation or acid-base interactions) between g-C3N4 and targeted adsorbates has been widely applied in manipulating the adsorption properties of g-C3N4. Fifty years ago, Pearson firstly proposed the well-known hard and soft acids and bases (HSAB) principle [222], [223], stating that hard and soft acids will interact preferentially with hard and soft bases, respectively. So far, this principle has been widely employed to clarify the chemi-adsorption interactions between various kinds of adsorbents and adsorbates with different acidic and basic properties [224], [225]. According to the HSAB principle, it is clear that the acidic molecules such as CO2, H2S and NOx should be readily chemically bond to the basic nitrogen-containing groups of g-C3N4. Recently, Oh et al. demonstrated that the g-C3N4 functionalized porous reduced graphene oxide aerogel could achieve a large CO2 adsorption capacity (0.43 mmol g−1) and high CO2 selectivity against N2 under ambient conditions, and an easy regeneration of 98% adsorpted CO2 by simple pressure swing [226]. Importantly, the DFT calculations further revealed that the strong dipole interaction induced by electron-rich nitrogen at the microporous edges of g-C3N4 is the critical factor for achieving the high-capacity, regenerative and selective CO2 capture. It is highly desired that the chemoselectivity of g-C3N4 interactions for other gases could be also further tailored through effectively developing the porous structure and increasing the content of nitrogen-containing groups on the surface of g-C3N4 [227]. For example, Jia et al. demonstrated that a simple oxygen-atmosphere UV irradiation could create the sufficient acidic sites on hierarchically ordered macro-/mesostructured g-C3N4 films, such as COOH and N-oxide groups through replacing its surface basic nitrogen-containing groups, [228] thereby achieving the highly selective chemi-adsorption of basic molecules. In the contrary, Yu and his coworkers demonstrated that the loading of amine groups on g-C3N4 through monoethanolamine solution treatment can successfully achieved a 3.76-times enhancements in the adsorbed CO2 amount, as compared that of pristine g-C3N4 under ambient pressure and temperature (Fig. 21), owing to the combination effects of both physical and chemical adsorption of CO2 [227]. Importantly, it was also observed that the enhanced adsorption and activation (favoring the formation of nonlinear HCO3−) of CO2 molecules over amine-functionalized g-C3N4 were beneficial for the improvement of CO2 photoreduction efficiency and selective formation of CH4 [227]. More surprisingly, the CO2 adsorption capacity of g-C3N4 microspheres with 3D hierarchical pores and a much higher BET surface area (550 m2/g) could reach 2.90 and 0.97 mmol/g at 25 and 75 °C, respectively [229]. However, even so, the CO2 adsorption capacity is still obviously lower than those of famous “molecular basket” adsorbents (133 mg/g) [230], [231], [232], activated carbon (3.75 mmol/g) [233] and metal organic framework (3–5 mmol/g) [234], [235], [236], implying there are still ample room to enhance the CO2 adsorption capacity of g-C3N4 semiconductor photocatalysts. In this end, the g-C3N4 semiconductors with rich basic nitrogen-containing groups, high surface areas, porous structures and suitable band gaps seem to be very promising for the applications in the field of photocatalytic CO2 reduction, H2 evolution and the oxidation of NOx and H2S, because porous g-C3N4 could simultaneously serve as light-harvesting, charge-excitation, charge-transportation, adsorption (for acid CO2, H+, NOx and H2S molecules) and catalytic centers [237].
除了静电吸引力外,g-C 3 N 4 与目标吸附物之间的化学相互作用(表面络合或酸碱相互作用)在操控 g-C 3 N 4 的吸附性能中得到了广泛应用。五十年前,Pearson 首次提出了著名的硬硬酸碱和软软酸碱(HSAB)原理[222],[223],指出硬酸和软酸会分别与硬碱和软碱发生优先相互作用。到目前为止,该原理已被广泛应用于阐明各种吸附剂和具有不同酸性和碱性性质的吸附物之间的化学-吸附相互作用[224],[225]。根据 HSAB 原理,显然,如 CO 2 ,H 2 S 和 NO x 等酸性分子应很容易地与 g-C 3 N 4 的碱性含氮基团发生化学键合。最近,Oh 等人证明,g-C 3 N 4 功能化多孔还原石墨烯气凝胶可以在常温下实现较大的 CO 2 吸附容量(0.43mmol/g),并且对 N 2 具有较高的 CO 2 选择性,并可通过简单的压力摆动再生 98%吸附的 CO 2 [226]。重要的是,DFT 计算进一步揭示了由 g-C 3 N 4 微孔边缘的富电子氮诱导的强偶极相互作用是实现高容量、可再生和选择性 CO 2 捕获的关键因素。非常希望 g-C 3 N 4 相互作用对其他气体的化学选择性也能够通过有效地开发多孔结构并增加 g-C 3 N 4 表面含氮基团的含量而进一步定制。 例如,贾等人证明,简单的氧气大气紫外辐射能够在具有等级结构的宏观-/介观结构 g-C 3 N 4 薄膜上产生足够的酸性位点,例如通过取代其表面的碱性含氮基团形成 COOH 和 N-氧化物基团,[228]从而实现对碱性分子的高度选择性化学吸附。相反,余和他的同事证明,通过单乙醇胺溶液处理可在 g-C 3 N 4 上负载胺基团,成功地使吸附 CO 2 量增加了 3.76 倍,相比之下原始 g-C 3 N 4 在常压和温度下(图 21),这归功于 CO 2 的物理和化学吸附的综合效应[227]。重要的是,还观察到,在含氨基化的 g-C 3 N 4 上增强的 CO 2 吸附和活化(有利于非线性 HCO 3 − 的形成)对提高 CO 2 光还原效率和选择性生成 CH 4 有好处[227]。更令人惊讶的是,具有 3D 分级孔道和更高 BET 比表面积(550m 2 /g)的 g-C 3 N 4 微球在 25 和 75°C 分别能达到 2.90mmol/g 和 0.97mmol/g 的 CO 2 吸附量[229]。然而,即使如此,CO 3 吸附量仍然显然低于一些著名的“分子筐”吸附剂(133mg/g)[230], [231], [232],活性炭(3.75mmol/g)[233]和金属有机框架(3-5mmol/g)[234], [235], [236],这意味着仍然有很大的提高 g-C 3 N 4 半导体光催化剂 CO 2 吸附量的空间。 在这方面,富含碱性氮基团、具有高比表面积、多孔结构和适当带隙的 g-C 3 N 4 半导体似乎非常适用于光催化 CO 2 还原、H 2 生成以及 NO x 和 H 2 S 的氧化等应用领域,因为多孔的 g-C 3 N 4 可以同时作为光吸收、电荷激发、电荷输运、吸附(对于 CO 2 、H + 、NO x 和 H 2 S 分子)和催化中心 [237]。
3.7. Electrochemical properties
3.7 电化学性能
As shown in Fig. 2, the photocatalytic reduction and oxidation reactions on the surface of semiconductors are fundamentally electrocatalytic ones driven by the photo-generated electrons and positive holes, respectively. More interestingly, g-C3N4 semiconductor itself can also serve as the multifunctional electrocatalysts with higher activity than pure carbon [238], [239], which play significant roles in achieving the improved overall photocatalytic efficiency. Commonly, pyridinic N atoms of g-C3N4 with strong electron-accepting ability can serve as active sites for the electrochemical reactions, making it a potential metal-free electrocatalyst [55]. Unfortunately, the moderate conductivity and poor electron transfer ability of g-C3N4 greatly limit its electrochemical performances and applications in various electrocatalysis fields, such as O2 reduction reaction (ORR) for fuel cells [240] and H2 evolution reaction (HER) for water splitting [241]. Consequently, in the past several years, extensive research efforts have been devoted to the exploration of hybrid g-C3N4-based electrocatalysts with higher electroconductivity and more active sites, through several strategies, such as manipulating size, thickness and structure, coupling with various semimetal carbon materials and doping with heteroatoms [242].
如图 2 所示,半导体表面的光催化还原和氧化反应基本上是由光生电子和正空穴驱动的电催化反应。更有趣的是,g-C 3 N 4 半导体本身也可以作为多功能电催化剂,其活性比纯碳高[238],[239],在提高光催化效率方面发挥着重要作用。通常,具有较强电子受体能力的吡啶氮原子可以作为 g-C 3 N 4 的活性位点进行电化学反应,使其成为潜在的无金属电催化剂[55]。不幸的是,g-C 3 N 4 的中等电导率和较差的电子传输能力大大限制了它在各种电催化领域的电化学性能和应用,如燃料电池的氧还原反应(ORR)[240]和水分解的 H 2 演化反应(HER)[241]。因此,在过去几年里,广泛的研究工作已经致力于探索基于混合 g-C 3 N 4 的更高电导率和更多活性位点的电催化剂,通过几种策略,如调控尺寸、厚度和结构,与各种半金属碳材料的耦合和杂原子掺杂[242]。
It has been well accepted that surface electrocatalytic ORR over various heterogeneous semiconductors has been found to play key roles in determining the photocatalytic activity of pollutant degradation [20], [25], [243] and organic synthesis [244], [245]. However, so far, most of ORR co-catalysts loaded on the surface of semiconductors for photodegradation and photosynthesis are mainly constituted of noble metal elements, such as Pt [246], [247], [248], Au [249], [250] and Ag [251], [252] nanoparticles/clusters. Fortunately, it has been recently revealed that the N-doped nanpcarbon materials, such as N/S or B/N co-doped graphene [253], [254], N-doped graphene/porous carbon [255], Mn3O4/N-doped graphene [256] and N-doped graphene (carbon spheres, nanoyubes or nanocages) [257], [258], [259], [260], [261], exhibited excellent electrocatalytic ORR activities for direct methanol fuel cells (DMFC). More interestingly, it has been demonstrated that the electrocatalytic 4e− ORR activities of g-C3N4 could be significantly enhanced via the coupling with different kinds of conductive carbon support, owing to the improved electron accumulation and transfer on the surface of g-C3N4. Accordingly, the new-generation metal-free g-C3N4-carbon hybrid cathode electrocatalysts in DMFC, could achieve both the superior electrochemical ORR efficiency and the high CO/methanol tolerances [48]. So far, various kinds of nanostructured g-C3N4-carbon hybrids, such as g-C3N4@carbon [64], [69], 2D g-C3N4 nanosheet/1D carbon nanotube [65], 2D g-C3N4 nanosheets/graphene [66], [262], [263], [264], hollow mesoporous g-C3N4 nanosphere/3D graphene [265], graphene supported Co-g-C3N4 [266] and g-C3N4@cobalt oxide[267] have been extensively fabricated and demonstrated to be high-efficiency ORR electrocatalysts. For example, Qiao and coworkers theoretically revealed that the limited electron transfer ability of g-C3N4 leads to the low ORR catalytic activity of pure g-C3N4 through an unfavorable 2e− pathway (as shown in Fig. 22A) [69], whereas, the accelerated electron transfer and increased active sites in the nanoporous g-C3N4@carbon composite could achieve a nearly 100% of selectivity for 4e− ORR pathway in alkaline aqueous solution. The further experimental results confirmed that the g-C3N4@ordered mesoporous carbon (CMK-3) exhibited a considerably lower onset potential and comparatively higher ORR current density, as compared to those of g-C3N4 (m) and the g-C3N4/CMK-3 mixture electrodes (as shown in Fig. 22B). By the same way, the hybrid of g-C3N4 and conductive metal has also been found to significantly improve its the sluggish cathodic ORR in fuel cells [241], [268], [269]. At this point, it is highly desired that more and more metal-free g-C3N4/carbon and earth-abundant metal/g-C3N4 hybrid ORR electrocatalysts could be exploited and utilized as co-catalysts to greatly facilitate the photocatalytic activity of pollutant degradation and organic transformation.
有很多研究表明,各种异质半导体上的表面电催化 ORR 在决定污染物降解的光催化活性[20],[25],[243]和有机合成[244],[245]中起着关键作用。然而,到目前为止,大多数加载在半导体表面上的 ORR 辅催化剂主要由贵金属元素构成,比如 Pt [246],[247],[248],Au [249],[250]和 Ag [251],[252]纳米颗粒/团簇。幸运的是,最近揭示了 N 掺杂纳米碳材料,比如 N/S 或 B/N 共掺杂石墨烯[253],[254],N 掺杂石墨烯/多孔碳[255],Mn 3 O 4 /N 掺杂石墨烯[256]和 N 掺杂石墨烯(碳微球、纳米管或纳米笼)[257],[258],[259],[260],[261],展现出了优异的直接甲醇燃料电池(DMFC)的电催化 ORR 活性。更有趣的是,研究表明,g-C 3 N 4 的电催化 4e − ORR 活性通过与不同种类的导电碳支持物的耦合可以得到显著增强,因为 g-C 3 N 4 表面上电子的积聚和传递得到了改善。因此,在 DMFC 中,新一代无金属 g-C 3 N 4 -碳混合阴极电催化剂既能实现优越的电化学 ORR 效率,又能耐受高 CO/甲醇。[48] 到目前为止,各种纳米结构的 g-C 3 N 4 -碳杂化物,如 g-C 3 N 4 @碳 [64],[69],2D g-C 3 N 4 纳米片/1D 碳纳米管 [65],2D g-C 3 N 4 纳米片/石墨烯 [66],[262],[263],[264],中空介孔 g-C 3 N 4 纳米球/3D 石墨烯 [265],石墨烯支撑的 Co-g-C 3 N 4 [266]和 g-C 3 N 4 @氧化钴[267]已被广泛制备并证明是高效的 ORR 电催化剂。例如,乔及其同事从理论上揭示了 g-C 3 N 4 的有限电子传输能力导致纯 g-C 3 N 4 的低 ORR 催化活性,通过一个不利的 2e − 途径(如图 22A 所示)[69],而在纳米多孔 g-C 3 N 4 @碳复合材料中加速的电子转移和增加的活性位点能在碱性水溶液中实现近 100%的 4e − ORR 途径选择性。进一步的实验结果证实,g-C 3 N 4 @有序介孔碳(CMK-3)表现出较低的起始电位和相对较高的 ORR 电流密度,与 g-C 3 N 4 (m)和 g-C 3 N 4 /CMK-3 混合电极相比(如图 22B 所示)。同样,g-C 3 N 4 和导电金属的杂化也被发现显著改善了燃料电池中缓慢的阴极 ORR[241],[268],[269]。 目前,人们非常希望能够开发和利用更多无金属 g-C 3 N 4 /碳和丰富的地球金属/g-C 3 N 4 混合氧还原反应电催化剂作为协同催化剂,大大促进光催化活性,用于污染物降解和有机物转化。
Similar to the ORR, the thermodynamically uphill HER, as a central reaction in the electrochemical water splitting, always requires suitable catalysts with high electrocatalytic activity and durability to accelerate the sluggish kinetics. In contrast to most of Pt-free electrocatalysts, the best-known heterogeneous Pt/C composite has proven to be the most effective HER electrocatalyst, due to their high exchange current density at low overpotentials, chemical inertness, versatility, high conductivity, and resistance to oxidation [270]. However, high cost and low abundance of the noble metal Pt dramatically restricted its practical widespread applications. Although the cheap and earth-abundant transition metals, such as Fe, Co, Ni, W, Mo and their molecular derivatives, as Pt’s alternatives, have been found to display outstanding electrocatalytic HER activity, their low stability in acidic and basic media are unfavorable for the long-term operation [88], [124], [242], [271], [272], [273], [274], [275]. In this regard, metal-free g-C3N4/conductive carbon hybrids beyond metals have also shown great promise as attractive HER electrocatalysts for water splitting reactions due to their earth abundance, tunable molecular structures, the unique advantages to easily fabricated a variety of nanostructures and strongly tolerance acid/alkaline environments [239]. The theoretical calculations revealed that the strong electronic coupling between g-C3N4 and graphene could significantly improve electron conductivity and optical absorption of g-C3N4, thus leading to the greatly promoted charge separation and transfer at the graphene/g-C3N4 interface [276]. Subsequently, Qiao and co-workers further experimentally verified that the as-constructed multilayered g-C3N4 nanodomains on nitrogen doped ultrathin graphene sheets (C3N4@NG) exhibited superior HER activity, achieving a 10-mA cm−2 HER current density at an overpotential of ∼240 mV (as shown in Fig. 23a) [239]. From the viewpoint of thermodynamics, compared to too strong and weak chemical adsorption of H* on g-C3N4 and N-graphene with the Gibbs free-energy of −0.54 and 0.57 eV (Fig. 23b), respectively, the C3N4@NG hybrid with the Gibbs free-energy of about 0.19 eV, exhibits a mediated adsorption–desorption behavior, facilitating the overall HER kinetics. In additional, as displayed in the volcano plot (Fig. 23c), the HER activity of metal-free C3N4@NG is very close to those of famous MoS2electrocatalysts. It was also demonstrated that the broken N–3C bonds at the edge of g-C3N4 lead to the formation of defect sites, pyridinic nitrogens, which could act as the stable H* adsorption sites and electrocatalytic HER active sites [239]. Inspired by this interesting work, Qu and co-workers developed a more active 3D porous HER elctrocatalyst based on the hybrid of 1D g-C3N4 nanoribbons and 2D graphene sheets, exhibiting a current density of 10 mA cm−2 at an overpotential of ∼207 mV, in 0.5 M H2SO4 solution [277]. It is believed that the high activity was attributed to the increased proton binding sites on 1D g-C3N4 nanoribbons, close contact between g-C3N4 and graphene, and unique 3D porous networks for improved mass transfer and diffusion. More recently, the supramolecular Cu-doped g-C3N4, as a biomimetic HER electrocatalyst, has been also demonstrated to show a high current density of 10 mA cm−2 at a low overpotential of 0.39 V in acidic media [278]. Thus, developing the novel non-noble-metal g-C3N4-based HER electrocatalysts with high stability and activity is still a promising direction in the near future.
类似于 ORR,热力学上向上的 HER 作为电化学水分解中的中心反应,始终需要具有高电催化活性和耐久性的合适催化剂来加快缓慢的动力学反应。与大多数非铂电催化剂相比,已知最有效的非均相 Pt/C 复合材料被证明是最有效的 HER 电催化剂,因为它们在低过电位时具有高交换电流密度、化学稳定性、多功能性、高导电性和对氧化的抵抗力[270]。然而,贵金属铂的高成本和低丰度极大地限制了其实际广泛应用。虽然廉价和丰富的过渡金属,如 Fe、Co、Ni、W、Mo 及其分子衍生物,作为铂的替代品,已被发现具有出色的电催化 HER 活性,但它们在酸性和碱性介质中的低稳定性不利于长期运行[88],[124],[242],[271],[272],[273],[274],[275]。在这方面,无金属 g-C 3 N 4 /导电碳杂化物已显示出极具吸引力的 HER 电催化剂的巨大潜力,用于水分解反应,因为它们丰富的地球资源、可调节的分子结构、轻松制备各种纳米结构和强大的耐酸碱环境[239]。理论计算表明,g-C 3 N 4 和石墨烯之间的强电子耦合可以显著提高 g-C 3 N 4 的电子导电性和光吸收能力,从而在石墨烯/g-C 3 N 4 界面上大大促进电荷分离和传输[276]。 随后,乔等人进一步实验证实,氮掺杂的超薄石墨烯表面构建的多层 g-C 3 N 4 纳米颗粒(C 3 N 4 @NG)表现出优越的 HER 活性,在过电位约为 240mV 时达到 10mAcm −2 的 HER 电流密度(如图 23a 所示)[239]。从热力学的角度来看,与 g-C 3 N 4 和氮掺杂石墨烯上 H * 的化学吸附能力过强和过弱相比,它们的 Gibbs 自由能分别为−0.54 和 0.57eV(如图 23b 所示),C 3 N 4 @NG 混合物的 Gibbs 自由能约为 0.19eV,表现出一个介导的吸附-脱附行为,有利于整体的 HER 动力学。此外,如火山图(图 23c)所示,无金属的 C 3 N 4 @NG 的 HER 活性与著名的 MoS 2 电催化剂非常接近。还有证据表明,g-C 3 N 4 边缘的断裂 N–3C 键导致了缺陷位点、吡啶型氮的形成,可作为稳定的 H*吸附位点和电催化 HER 活性位点[239]。受到这一有趣研究的启发,屈等人开发了基于 1D g-C 3 N 4 纳米带和 2D 石墨烯片杂化的更活跃的 3D 多孔 HER 电催化剂,其在 0.5M H 2 SO 4 溶液中过电位约为 207mV 时,展现出 10mAcm −2 的电流密度[277]。人们相信该高活性是由于 1D g-C 3 N 4 纳米带上的增加质子结合位点、g-C 3 N 4 与石墨烯之间的密切接触,以及独特的 3D 多孔网络用于改善质量传递和扩散。 最近,超分子 Cu 掺杂的 g-C 3 N 4 ,作为一种仿生 HER 电催化剂,也被证明在酸性介质中表现出较高的 10mAcm −2 的电流密度,在低过电位 0.39V 时。[278]因此,开发具有高稳定性和活性的新型非贵金属 g-C 3 N 4 -基 HER 电催化剂仍然是一个有前途的方向。
3.8. Photoelectrochemical properties
3.8. 光电化学性能
Besides direct utilization as electrocatalysts, g-C3N4 has proven to be a promising photoelectrode candidate for solar energy conversion in the PEC cells, due to its superior chemical and thermal stability and suitable electronic band structure. Initially, Zhang and Antonietti firstly observed the maximum cathodic photocurrent response (up to ca. 50 mA cm−2 at −0.3 V, IPCE ∼ 3% at 420 nm) of bulk mpg-C3N4 film in KCl aqueous solution containing Fe(II) ions, under visible light (λ > 420 nm, 150 W Xe lamp) [181]. The flat band potential (Efb) of different g-C3N4 semiconductors can also be further estimated from the onset photocurrent potential. More importantly, a tripled maximum photocurrent was also observed for the binary composite film of mpg-C3N4 and standard Degussa P25 TiO2 (as the electron-transport channel) under the same conditions. However, the unfavorable factors of bulk g-C3N4 film in this study, such as larger domain sizes, grain boundary defects and textural effects, implying there is still ample room to further optimize the g-C3N4 photoelectrodes. Similarly, the weak transient photocurrent response of different bulk or modified g-C3N4 solids was extensively confirmed in other works [84], [114], [123], which is generally used to identify the enhanced photocatalytic activity and charge separation as an auxiliary tool. To further improved the PEC properties of g-C3N4 films and extend their applications, various fabrication methods and modification strategies of g-C3N4 films have been widely developed [43]. For example, Zhang et al. achievedan almost 3-fold higher cathodic PEC activity for hydrogen evolution from water than that of the pure g-C3N4 through simultaneously fabricating a sponge-like structure and incorporating active carbon-dopant sites, under simulated solar-irradiation [279]. They attributed the enhanced activity to the increased charge mobility, surface area, mass transfer, active sites and π-conjugated structure. Furthermore, the g-C3N4-based composite photoelectrode films such as g-C3N4/CuInS2 [280], TiO2/g-C3N4 [281], [282], [283], CdS/g-C3N4 [284], g-C3N4/WO3 [285], [286], [287], g-C3N4/N-doped graphene/NiFe-layered double hydroxide [288], Fe2O3/g-C3N4 [289], and g-C3N4/MoS2 [290], also exhibited significantly enhanced PEC activity for hydrogen evolution or water oxidation, due to the promoted charge separation, enhanced visible-light absorption, accelerated surface reaction kinetics and suppressed photocorrosion. For example, Feng and coworkers demonstrated that the as-fabricated WO3 nanosheet Array/g-C3N4/CoOx layered heterojunction photoanode exhibited a photocurrent density of 3.61 mA cm−2 at 1.6 V vs. NHE (as shown in Fig. 24a) [285]. It is suggested that the increased light-harvesting ability, unique 3D nanostructures with 2D layered nano-junctions, accelerated water oxidation kinetics, and excellent charge transfer and separation are the possible reasons for the enhanced PEC water-oxidation activity (as shown in Fig. 24b). Additionally, the g-C3N4-based films have also been widely applied in the PEC degradation of organic pollutants [291], [292], [293], [294], [295], [296]. Typically, Zhu and coworkers demonstrated that the g-C3N4 film could achieve the removal of 89.3% of the total organic carbon (TOC) under a 2.5 V bias, which was 2.4 times higher than that of photocatalytic degradation [293]. It is believed that the dramatic enhancement in activity can be attributed to the promoted activity of electrocatalytic (EC) oxidation, improved charge separation, and increased reactive radical species, such as OH and O2−, due to the combination effects of photocatalysis and electrocatalysis. Thus, in the near future, it is naturally expected that the g-C3N4-based films can be widely used in more and more PEC fields, and their activity should be further improved through better balancing electronic structures (e.g. band-gap and redox ability), stability, change-carrier mobility and active sites, surface area.
除了作为电催化剂的直接利用之外,g-C 3 N 4 已经被证明是光电化学电池中太阳能转化的有前途的光电极候选材料,由于其优越的化学和热稳定性以及适当的电子能带结构。最初,张和安东尼蒂首次观察到在含 Fe(II)离子的 KCl 水溶液中,可见光条件下(λ> 420nm,150W Xe 灯)mpg-C 3 N 4 薄膜的最大阴极光电流响应(最高可达 50mAcm −2 ,在−0.3V 时,IPCE∼3%,在 420nm 处)[181]。不同 g-C 3 N 4 半导体的平带电位(E fb )也可以通过光电流起始电位进一步估算。更重要的是,在相同条件下,mpg-C 3 N 4 和标准 Degussa P25 TiO 2 的二元复合薄膜也观察到了光电流的三倍最大值。但是,在本研究中,大块 g-C 3 N 4 薄膜的不利因素,如更大的结晶颗粒大小,晶界缺陷和纹理效应,意味着仍然有足够的空间来进一步优化 g-C 3 N 4 光电极。同样,在其他作品中广泛证实了不同大块或改性的 g-C 3 N 4 固体的弱瞬态光电流响应,这通常用于作为辅助工具来识别增强光催化活性和电荷分离。为了进一步改善 g-C 3 N 4 膜的 PEC 性能并拓展其应用,已经广泛发展了各种 g-C 3 N 4 薄膜的制备方法和改性策略[43]。例如,张等。 通过同时制备海绵状结构和结合活性碳掺杂位点,在模拟太阳辐射条件下,实现了对水的阴极光电化学(PEC)活性几乎是纯 g-C 3 N 4 的 3 倍。他们将增强的活性归因于增加的电荷迁移性、表面积、质量传递、活性位点和π-共轭结构。此外,基于 g-C 3 N 4 的复合光电极膜,例如 g-C 3 N 4 /CuInS 2 [280],TiO 2 /g-C 3 N 4 [281],[282],[283],CdS/g-C 3 N 4 [284],g-C 3 N 4 /WO 3 [285],[286],[287],g-C 3 N 4 /N 掺杂石墨烯/NiFe 层状双氢氧化物 [288],Fe 2 O 3 /g-C 3 N 4 [289]和 g-C 3 N 4 /MoS 2 [290],也显著提高了用于水解氢或水氧化的 PEC 活性,原因在于促进了电荷分离、增强的可见光吸收、加速的表面反应动力学并抑制了光腐蚀。
例如,冯及其同事证明,制备的 WO 3 纳米片阵列/g-C 3 N 4 /CoO x 层状异质结光阳极在 1.6V vs.NHE(如图 24a 所示)条件下表现出 3.61mAcm −2 的光电流密度 [285]。有人认为增强的光收集能力、具有 2D 层状纳米结构的独特 3D 纳米结构、加速的水氧化动力学以及优秀的电荷转移和分离是增强 PEC 水氧化活性的可能原因(如图 24b 所示)。 另外,基于 g-C 3 N 4 的薄膜也被广泛应用于光电化学(PEC)降解有机污染物[291],[292],[293],[294],[295],[296]。典型地,朱等人展示了 g-C 3 N 4 薄膜在 2.5V 偏压下可以去除 89.3%的总有机碳(TOC),比光催化降解高出 2.4 倍[293]。人们认为,活性显著提高可归因于促进了电催化(EC)氧化活性、改善了电荷分离和增加了活性自由基物种(如 OH 和 O 2 − )的组合效应光催化和电催化。因此,在不久的将来,自然期望基于 g-C 3 N 4 的薄膜可以广泛应用于更多 PEC 领域,并且通过更好地平衡电子结构(如带隙和氧化还原能力)、稳定性、载流子迁移率和活性位点、表面积来进一步提高它们的活性。
4. Design strategies of g-C3N4-based photocatalysts
4. g-C 3 N 4 基光催化剂的设计策略
Although the multi-function properties endow g-C3N4 a bright future in the various kinds of photocatalytic applications, low quantum efficiency limits its practical utilization in a large scale. To date, tremendous efforts have been made to improve the photocatalytic efficiency of g-C3N4 through different design strategies, including band-gap engineering, defect control, pore texture tailoring, dimensionality tuning, surface sensitization, heterojunction construction, co-catalyst and nanocarbon loading, which will be systematically discussed in this section.
尽管多功能性使得 g-C 3 N 4 在各种光催化应用中拥有光明的前景,但低量子效率限制了其在大规模实际应用中的利用。迄今为止,已经付出了巨大努力来改善 g-C 3 N 4 的光催化效率,采用了不同的设计策略,包括带隙调控、缺陷控制、孔结构调节、维度调控、表面敏化、异质结构构筑、共催化剂和纳米碳加载,这些将在本节中进行系统讨论。
4.1. Band-gap engineering
4.1. 带隙工程
In general, it is known that the ideal bandgap of a semiconductor should be ∼2.0 eV, which could harvest a variety of visible light to generate sufficient electrons and holes with strong driving forces for photocatalytic redox reactions [7], [10]. However, the 2.7 eV bandgap of g-C3N4 make it only utilize the solar light with wavelength below 460 nm. Thus, in order to further enhance the light harvesting ability of g-C3N4, various band-gap engineering strategies, including atom-level (foreign metal and non-metal elements) and molecular-level (copolymerization) doping, have been widely exploited and demonstrated to achieve the enhanced photocatalytic performance [27], [54], which will be summarized in Fig. 25 and discussed in this section.
一般而言,人们知道半导体的理想带隙应该是∼2.0eV,这样可以收集各种可见光以产生足够的电子和空穴,具有强烈的驱动力用于光催化氧化还原反应[7],[10]。然而,对于 g-C 3 N 4 的 2.7eV 带隙,只能利用波长低于 460nm 的太阳光。因此,为了进一步增强 g-C 3 N 4 的光吸收能力,已广泛开发并证明了各种带隙工程策略,包括原子级(外来金属和非金属元素)和分子级(共聚物化)掺杂,以实现增强的光催化性能[27],[54],这将在图 25 中总结,并在本节讨论。
As shown in Fig. 25, two possible kinds of cation doping, namely, cave doping and interlayer doping have been observed. Their detailed doping mechanism was shown in Fig. 26. On the one hand, the metal ions (Mn+) can be incorporated into the large caves (the triangular pores) between the connected triazine structures in the plane of g-C3N4 (as shown in Fig. 26a), through the strong coordination interactions between them and negatively charged nitrogen atoms, thus achieving the so-called cave doping [297]. The previous studies revealed that the transition metal ions, such as Fe3+, Zn2+, Mn3+, Co3+, Ni3+ and Cu2+ can be doped into the large caves of g-C3N4 [245], [298], [299], [300], [301], [302], [303]. The DFT calculations demonstrated that the cave doping of Pt and Pd atoms could effectively improve the carrier mobility, narrow the bandgap or optical gap, and enhance the light absorption, which are favorable for photocatalytic reactions [304]. More interestingly, it was revealed that the cave doping of alkali-metal ions such as Li+, Na+, and K+ will induce the un-uniform spatial charge distribution in different intercalated regions, increase the free carrier concentration, improve charge transport and separation rate [106]. Recently, Zhu and coworkers demonstrated that the K+ doping could decrease the VB level of g-C3N4, thus resulting in enhanced separation and immigration of photo-generated carriers under visible light [305]. More recently, Dong and coworkers revealed that K atoms could achieve an interlayer doping (as shown in Fig. 26b), instead of the cave doping of Na atoms in g-C3N4 [306]. It is believed that K atoms can bridge the two adjacent g-C3N4 layers, which lead to the narrowed band gap, extended π conjugated systems, and positive-shifted valence band position, thus achieving the increased visible-light harvesting, efficient charge separation, and strong oxidation capability, respectively. In contrary, despite of the increased in-planar electron density and visible-light absorption, the cave doping of Na atoms still exhibits high recombination rate of carriers in the g-C3N4 planes, thus resulting in the reduced photocatalytic performance [306]. This work might provide new insights into the deep understanding on the metal doping of g-C3N4 and the design of electronically optimized layered photocatalysts for enhanced solar energy conversion.
Apart from metal doping, the non-metal doping of g-C3N4 has been majorly realized through the chemically substituted doping. As displayed in Fig. 16, almost all the non-metal doping, such as S [118], [182], [307], [308], [309], [310], [311], P [183], [312], [313], [314], [315], [316], B [114], [317], [318], [319], [320], O [184], C [185], and I [128], [186], could narrow the bandgap of g-C3N4 and enhance its light harvesting capability. In general, the C self-doping can substitute the bridging N atoms [185], whereas the O [184], [321], S [307], [309], [322] and I [128], [186] doping could achieve the replacement of N atoms in the aromatic triazine rings (as shown in Fig. 27). Interestingly, the doping of these different elements can promote the delocalization of the π-conjugated electrons, which is fundamentally important for improving the conductivity, mobility and separation of photo-generated electrons, thus greatly enhancing the photocatalytic performances of doped g-C3N4. In the contrary, the substituted doping of P [183], [315], [316], [323], [324], [325], [326] and B [327], [328] atoms preferentially occur on the C atoms, thus leading to the formation of strong Lewis acid sites (P+) on the basic surface (from amine or imine groups) of g-C3N4, due to the intrinsic polarization of P–N bond and delocalization of one extra lone electron in electron-rich P atom [324]. Most recently, Qiao’s group demonstrated that the P-doping of porous g-C3N4 nanosheets can drastically narrow the intrinsic band gap from 2.98 to 2.66 eV and promote the photo-excitation of electrons from the VB of P-doped g-C3N4, due to the formation of vacant midgap states below the CB minimum of g-C3N4 through the hybridization of C 2s2p, N 2s2p and P 3s3p, thus enhancing visible light absorption [326]. Meanwhile, it was also demonstrated that the (NH4)2HPO4 as phosphorus precursor could achieve the cave doping (in the interstitial sites, as shown in Fig. 27). Furthermore, it is well known that S atoms have been found to preferentially substitute N atoms with a larger electronegativity (3.04), thus leading to the decreased VB/CB levels and band gap [118], [182], [199], [307], [310], [329]. Nevertheless, as special cases, in situ sulfur and boron doping of g-C3N4 has also been found to replace the C and N atoms in the rings, respectively [182], [330]. In addition, the F doping (NH4F as a cheap fluorine source) can achieve the formation of the CF bonding in g-C3N4 (as shown in Fig. 27), thus lowering the electronic band gaps [331]. However, it should be point out that excessive doping of nonmetal and metal is found to be detrimental to enhance the photocatalysis, because the more defects can also act as the recombination centers of electron–hole pairs. In future, the co-doping of different metals and/or nonmetals, such as Fe/P [332] S/Co/O [333], S/P [334], P/O [335], K/Na [336] and C/Fe [337] deserves more attention, due to their positive synergetic effects on the visible-light absorption and photocatalytic properties.
In addition, copolymerization at molecular level was also widely employed to strongly enhance the photocatalytic activity of g-C3N4, via simultaneously modulating its band gap, electronic structures and physical and chemical properties. Commonly, it is believed that the copolymerization modification with structure-matching aromatic compounds or organic additives could increase the desired delocalization of π-conjugated electrons and improve the intrinsic drawbacks in g-C3N4, thus maximizing the photochemical activities [54], [338], [339], [340], [341]. For example, Wang and co-workers demonstrated that the tunable bandgaps of tri-striazine-based g-C3N4 ranging from 2.67 to 1.58 eV could be obtained (as shown in Fig. 28a) through the copolymerization of dicyandiamide (monomer) and different amounts of barbituric acid (BA, comonomer) [84]. More interestingly, 2D g-C3N4 nanosheets fabricated by the one-pot condensation of urea and electron-rich thiophene co-monomers could achieve the highest quantum efficiency of 8.8% at 420 nm for H2 generation, (Fig. 28b) owing to the narrowed band gap, improved electron migration and through the formation of surface dyadic structures [129], [342]. In contrary, the molecular doping by an electron-deficient pyromellitic dianhydride could thereby enhance the strong photooxidation capability of g-C3N4, due to greatly decreased both the CB and VB positions [343]. To sum up, as a unique bottom-up way for tailoring the bandgap of g-C3N4, the copolymerization approach provides more opportunities for designing highly effective polymeric photocatalysts with desired electrical properties and band gap through incorporating structure-matching organic moleculars, which also provides insights into the mechanism of heterogeneous photocatalysis of organic semiconductors at molecular levels.
4.2. Defect control
It is widely accepted that a high degree of crystallinity is advantageous for enhancing the photocatalytic redox reactions, as compared to the negative roles of defects as charge-recombination sites [7]. For example, ionothermal synthesis [102], [103], [105], [108], [344], [345] thermal polymerization in ammonia [346] and microwave-assisted heating synthesis [112], [113] have been broadly employed to achieve the highly crystalline g-C3N4 for efficient photocatalytic hydrogen evolution. It is believed that the enhanced crystallinity of g-C3N4 could improve the charge-carrier mobility and separation, thus leading to significantly improved photoactivity. However, more recently, as an effective strategy, the creation of nitrogen vacancies [347] or amorphous structures [348] in g-C3N4, has been extensively demonstrated to improve the visible-light activity, owing to the efficiently extended absorption edge and promoted lifetime of charge carriers. For example, Liu and co-workers demonstrated the high-temperature Ar-atmosphere treatment of bulk g-C3N4 at 620 °C could disrupt the weak interactions of hydrogen bonds and van der Waals forces, and destroy the long-range order in crystalline g-C3N4 structures, thus fabricating the amorphous g-C3N4 with the short-range order (as shown in the inset of Fig. 29a) [348]. Such structure disorder changes in amorphous g-C3N4 could effectively narrow its bandgap from 2.7 to 1.9 eV, corresponding to an obvious red-shift of absorption edge from 460 to 682 nm (as shown in Fig. 29a). The further valence band XPS analysis reveals that the levels of VB and CB could be reduced by 0.31 and 0.61 eV, respectively, without affecting the thermodynamic requirements for O2 evolution and water reduction (as shown in Fig. 29b). These results could open up new ways to develop visible light-driven amorphous or defective g-C3N4 photocatalysts.
Apart from Ar-atmosphere heat treatment, the high-temperature treatment of pristine g-C3N4 in a H2, NH3 or vacuum atmosphere could also create nitrogen or carbon vacancies in carbon nitrides, thus achieving the narrowed band gap and improved photocatalytic performances [347], [349], [350], [351]. For example, a novel g-C3N4 photocatalyst with N-vacancy structures and a bandgap of 2.03 eV could be fabricated by heating the melon in a H2 atmosphere [347], which exhibits promising photocatalytic activities towards generating OH radicals and decomposing the organic pollutant Rhodamine B. It is believed that the nitrogen-vacancy defects could greatly widen visible light absorption range and suppress the unexpected fast recombination of photo-excited carriers, thus achieving the improved photoactivity. Similarly, it was also demonstrated that the introduction of hydrogenated defects in g-C3N4 nanosheets could greatly enhance the photocatalytic hydrogen evolution [349], [352]. Furthermore, a simple thermal treatment under an NH3 atmosphere can not only develop highly porous g-C3N4 nanosheets with plenary carbon vacancies through etching their lattice carbon sites by the reactive radicals from the NH3 decomposition [350], [351], but also can enhance the surface area, porosity and crystallinity of condensed g-C3N4, due to the greatly reduced N defects in the π-conjugated network [346]. More interestingly, through employing both DFT and molecular dynamics calculations, Wu et al. indicated that the defect within g-C3N4 played a key role in the adsorption and dissociation of water, whereas, water does not dissociate on the perfect g-C3N4 sheet (as shown in Fig. 30) [353]. However, it should be noted that the excessive nitrogen vacancies as the recombination centers could be also harmful for the photocatalysis [354]. To demonstrate this point, Osterloh and co-workers found that surface structure defects in g-C3N4, with energy levels at +0.97 V and −0.38 V (vs.NHE), limit visible light driven hydrogen evolution and photovoltage [191]. More interestingly, it was also demonstrated that the vacuum heat-treatment at 500 °C could obtain the highest photoactivity for H2 evolution due to the increased content of the tri-s-triazine phase and suitable N defects in the tri-s-triazine ring building blocks [355], which are similar to the previous report about the vacuum-treated titanium dioxide [356]. Consequently, the controlled defect concentration in g-C3N4 is crucial for achieving the ideal photoactivity.
4.3. Pore texture tailoring
Another attractive design strategy is to tailor the porous structures/texture of g-C3N4 materials, which can significantly increase their exposed surface area and accessible channels(porosity) and active sites in g-C3N4, thus facilitating the molecular mass transfer/transport, charge migration and separation, surface reactions and light harvesting [83]. All these advantageous features can benefit the enhancement of photocatalytic efficiency. So far, a variety of highly porous g-C3N4 with diverse nanoarchitectures and morphology have been widely fabricated through several typical pathways, such as hard templating (nanocasting), soft templating (self-assembly along the structure directing agents), self-templating (supramolecular self-assembly) and template-free methods [27], [48], [54], [357], which have been thoroughly summarized in Table 4. The detailed comparison and discussion between them will be highlighted in this section.
Morphology | Templates | Precursor | Band gap [eV] | BJH pore size (nm) | Pore volume (cm3 g−1) | BET surface area [m2 g−1] | Ref. (year) |
---|---|---|---|---|---|---|---|
Hard templating methods | |||||||
Nanotubes | Porous anodic Al2O3 (AAO) membranes | EDA, CTC | [358] (2009) | ||||
Nanorods | CA | 2.9 | 25 | [359] (2011) | |||
Ordered mesoporous | Ordered Mesoporous SBA-15 | CA | 2.74 | 5.3 | 0.34 | 239 | [122] (2009) |
Mesoporous | 12-nm SiO2 particles | CA | 2.7 | 373 | [123] (2009) | ||
8.3 | 0.41 | 126 | |||||
Ordered mesoporous | SBA-15 pre-treated with 1 M HCl | CA | 3.4 | 0.49 | 517 | [360] (2013) | |
Hierarchical mesostructures | Mesostructured cellular silica foams | EDA, CTC | 4/43 | 0.9 | 550 | [229] (2010) | |
Inverse opal structures | Uniform-sized silica nanospheres | CA | 20 | 0.79 | 230 | [361] (2011) | |
70 | 1.7 | 140 | |||||
Mesoporous nanorods | SBA-15 nanorods | CA | 3.9 | 110–200 | [362] (2012) | ||
Ordered mesoporous | SBA-15 | ATC | 2.78 | 5.3 | 0.34 | 239 | [170] (2011) |
Mesoporous | 12-nm SiO2 particles | ATC | 0.77 | 176 | [363] (2012) | ||
Hollow nanospheres | Monodisperse silica@ mesoporous silica | CA | 2.9 | 79 | [364] (2012) | ||
Helical rodlike | Chiral mesoporous silica | CA | 2.75 | 3.8/10.7 | 56 | [365] (2014) | |
Porous nanorod | Chiral silica nanorods | CA | 2.7 | 52 | [366] (2014) | ||
Mesoporous sphere | 7 nm colloidal silica particles | Urea | 7 | 224 | [367] (2012) | ||
Porous composite | Graphene oxide sheets | MA | 2.7 | 17.6 | 0.09 | 26.6 | [151] (2011) |
g-C3N4@CMK-3 composite | CMK-3 mesoporous carbon | CA | 3.0 | 0.49 | 623 | [69] (2011) | |
Cubic mesoporous | Ordered mesoporous silica KIT-6 | CA | 3.6 | 0.4 | 208 | [368] (2010) | |
Nanosheet-based nanospheres | KCC-1 silica spheres | CA | 2.86 | 3.8 | 0.4 | 160 | [369] (2014) |
Porous | CaCO3 | DCDA | 38.6 | [370] (2015) | |||
Porous | ZnCl2 | DCDA | 2.89 | 0.08 | 46 | [371] (2015) | |
Macroscopic 3D Porous monolith | MA sponge | Urea | 0.76 | 78 | [372] (2015) | ||
Soft templating methods | |||||||
Sponge-like mesopore | Ionic liquids(BmimBF4) | DCDA | 0.32 | 444 | [114] (2010) | ||
Porous nanosheets | BmimBF4 | Urea | 25.0 23.4 | 0.40 0.51 | 73 90 | [373] (2014) | |
BMIM-PF6 | DCDA | [312] (2010) | |||||
Nanoporous | BmimDCN | DCDA | 5.6 | 0.179 | 81 | [374] (2010) | |
Worm-like porous | Pluronic P123 | MA | 1.55 | 90 | [375] (2012) | ||
Nanoporous | Pluronic P123 | DCDA | 0.128 | 299 | [374] (2010) | ||
Nanoporous | Triton X-100 | DCDA | 3.8 | 0.284 | 116 | [374] (2010) | |
Bimodal mesoporous | Triton X-100 | MA, GA | 3.8/10–40 | [376] (2011) | |||
Nanoporous | Triton X-100 | MA sulfate | 2.47–2.57 | 50–135 | [377] (2014) | ||
Bubble (urea) | DCDA | 60 | [378] (2014) | ||||
Nanoporous | Bubble (thiourea) | DCDA | 2.72 | 3.7 | 46.4 | [379] (2013) | |
Porous | Bubble (water vapor) | Urea | 18.2 | 0.321 | 69.6 | [380] (2012) | |
Sponge-like | Ammonium alginate or gelatin | DCDA | 2.49 | 63 | [279] (2013) | ||
Mesopore | Bubble (sublimed sulfur) | MA | 50 | 46 | [381] (2015) | ||
Mesopore | Bubble (sucrose) | MA | 13.2 | 0.355 | 121 | [382] (2015) | |
Honeycomb-like | Bubble (water) | urea | 2.65 | 0.68 | 106 | [383] (2015) | |
Diatom-structure | Diatomite | CA | 5 | [384] (2013) | |||
Self-templating (Supramolecular self-assembly) methods | |||||||
Hollow box | Self-assembly | CAA/MA | 45 | [110] (2013) | |||
Hollow spheres | Self-assembly | MA/CAA | 30–40 | 0.4 | 77 | [107] (2013) | |
Spherical particles | Self-assembly | MA/CAA | 2.79 | 0.3 | 66 | [385] (2013) | |
Hollow tube-like | Self-assembly | CAA/MA | 41 | [386] (2015) | |||
Hollow to wormlike | Self-assembly | MA, urea, CAA | 2.73 | 12.84 | 0.31 | 97.4 | [387] (2014) |
Fiber-type/sheet-like | Self-assembly | CA, MA, DPT | 2.25 | 75 ± 5 | [111] (2014) | ||
Roll-like | Self-assembly | CAA, MA, BA | 60–70 | [388] (2014) | |||
Crystalline | Self-assembly | MA, TAP | 119 | [108] (2014) | |||
Porous | Self-assembly | CAA/MA | 2.7 | 77 | [389] (2014) | ||
Template-free methods | |||||||
Nanobelts | MA | [390] (2011) | |||||
Porous | MA hydrochloride | 2.83 | 69 | [391] (2012) | |||
Porous | DCDA | 2.72 | 15.8 | 0.50 | 201 | [392] (2013) | |
Hierarchical structure | MA | 2.74 | 35.6 | [393] (2014) | |||
Nanoporous | Ball milling/hydrothermal method | MA | 2.75 | 0.15 | 30.9 | [394] (2015) | |
Nanosheets | urea | 2.78 | 1.41 | 288 | [153] (2013) | ||
Seaweed-like architecture | Freezing assistant assembly | DCDA | <20 | 130 | [395] (2015) | ||
Monolayer mesoporous | Freezing assistant assembly/exfoliation | DCDA | 2.75 | <20 | 331 | [396] (2016) | |
Porous microspheres | Solvothermal in acetonitrile (200 °C) | CAC/MA | 2.42 | 8.5 | [397] (2015) | ||
Nanosheets | Urea, Ph4BNa | 2.83 | 16.2 | 0.62 | 144 | [328] (2013) | |
Nanorod-network superstructures | Solvothermal in acetonitrile (180 °C) | CAC/MA | 1.92 | 30 | [398] (2012) |
BmimBF4: 1-butyl-3methylimidazolium tetrafluoroborate; BMIM-PF6: 1-butyl3-methylimidazolium hexafluorophosphate; BmimDCN Ph4BNa: sodium tetraphenylboron; CA: cyanamide; MA: melamine; DCDA: Dicyandiamide; CAA: cyanuric acid; CAC: Cyanuric chloride; DPT: 2,4-diamino-6-phenyl-1,3,5-triazine; BA: barbituric acid; ATC: ammonium thiocyanate; EDA: ethylenediamine; CTC: carbon tetrachloride; GA: glutaraldehyde, TAP: 2,4,6-triaminopyrimidine.
As observed in Table 4, it is clear that the hard templating (nanocasting) strategy is deemed to be one of the most simple and effective methods to construct mesoporous g-C3N4 photocatalysts with superior high surface area (up to 517–623 m2/g) [69], [229], [360]. In theory, various kinds of macro/mesoporous materials with super high surface area can be employed as hard templates to construct porous g-C3N4. To date, various kinds of hard templates such as porous anodic Al2O3 [358], [359], CaCO3 [370], graphene oxide nanosheets, [151] CMK-3 mesoporous carbon [69], mesoporous silica (nanospheres, [123], [361], [363] foams, [229] SBA-15, [122], [170], [362] chiral silica, [365], [366] silica KIT-6, [368] and KCC-1 [369]) have been available in developing highly porous g-C3N4. Absolutely, the mesoporous silica materials have been demonstrated to be the most widely used hard templates. Unfortunately, the trapped air and the weak-acid walls of mesopores in the silica templates greatly prevent the infiltration and fast mass diffusion of basic organic precursor molecules into them, thus leading to the incomplete utilization of their porous structures and the limited enhancement in the surface area of porous g-C3N4 [59]. Thus, to maximize the roles of porous silica templates, Zhang et al. demonstrated that the combined strategy of dilute HCl pretreatment of SBA-15 and sonication-vacuum insertion could increase the surface area of mesoporous g-C3N4 up to 517 m2 g−1, due to the improved surface reactivity of the silica and removed trapped air [360]. However, the hazardous agents for removing the silica templates, such as NH4F or HF, are harmful for environment, g-C3N4 itself or other materials in a g-C3N4-based composite photocatalysts, restricting the practical applications of hard-templating strategy in a large scale. Thus, it is expected that more and more easy-removal or non-removable hard templates, such as CaCO3, Al2O3, Fe2O3 and various nanocarbons, should be further developed and applied in the fabricating the highly porous g-C3N4 with different nanostructures [69].
The “greener” soft-template route is also an interesting strategy to avoid various kinds of unfavorable factors aforementioned for the hard-templating methods, which has also witnessed great advances in developing porous g-C3N4 micro-and nanostructures. In general, the amphiphilic organic molecules are easy to form self-assembly micelles with different structures in solution, which can function as soft templates (structure directing agents) to induce the growth of precursors around them and further form expected composite structures [54], [357]. Clearly, so far, various soft templates, such as ionic liquids [114], [373], Pluronic P123, [374], [375] Triton X-100 [374], [376], [377], bubble [378], [379], [380], [381], [383] and biomolecules [279], [382], [384] have been widely utilized in the fabrication of porous g-C3N4 photocatalysts. For example, Zhang et al. demonstrated that the mixture of 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4, as a soft template) and dicyandiamide could achieve the B/F-co-doped mesoporous g-C3N4 with a surface area of 444 m2 g−1 and a large total pore volume (0.32 m2 g−1) [114]. Similarly, the nanoporous g-C3N4 fabricated by using Pluronic P123 block polymers as a soft template, could exhibit a high surface area of 299 m2 g−1 [374]. However, the obvious disadvantages of ionic liquids and polymers, such as the high cost, unexpected carbon residue and insolubility in water, greatly limit their extensive practical applications in a large scale. At this regard, the use of bubbles of water vapor as soft templates seems more promising in synthesis of porous g-C3N4, due to the absence of impurities and post treatments [380], [383]. Also, the interesting biomolecules as soft templates are highly desirable in the future studies [279], [382], [384].
In addition, much attention has also been paid to the supramolecular self-assembly and template-free methods. For the supramolecular self-assembly strategies, it has been demonstrated that the formation of hydrogen-bonded supramolecular assemblies (or complex) between melamine precursores and triazine derivatives plays key roles in determining the different nanostructured morphologies of porous g-C3N4 materials [385], [386], [387], [388]. For the template-free route, the (hydro) solvothermal [328], [394], [397], [398] and freezing assistant assembly [395], [396] methods have been successfully exploited to fabricat porous g-C3N4 materials. However, compared to the hard-templating methods, the surface area of porous g-C3N4 prepared by these two strategies is still much smaller. More importantly, there are only limited precursors which could form ordered and stable supramolecular aggregates in a solvent, based on noncovalent interactions (e.g. hydrogen bonding). Accordingly, it is expected that these two appealing approaches could be finely controlled and combined with other strategies, such as an infrared heating process [399] or a simple reflux method [400], to design and fabricate highly efficient porous g-C3N4-based semiconductors with high surface area and unique nanostructures in a low-temperature solution in future research [54]. In addition, it is also expected that the two strategies of in situ template-sacrificial dissolution [401] and chemically induced self-transformation [217], [402], [403], [404], [405], [406], [407], [408] could be applied in developing highly porous g-C3N4-based semiconductors.
4.4. Dimensionality tuning
Generally, compared with the bulk counterparts, nanostructured g-C3N4 semiconductors with unique dimensions and configurations could exhibit several obvious advantages for solar photocatalysis, such as the higher surface area, shorter charge migration length, higher solubility and tunable electronic structure [37]. Detailedly speaking, the charge carriers generated in the ultrathin g-C3N4 nanosheets can readily reach their surface for redox reactions through the very short paths, as compared to traditional 3D bulk g-C3N4 semiconductors, thus achieving the rapid charge separation. More importantly, through controlling the layer number without changing the atomic structure, the energy band structure of g-C3N4 could be effectively tailored due to the quantum confinement effects, thus leading to improved activity and selectivity for various reactions, such as CO2 reduction [409] and O2 reduction [410]. In addition, the nanostructured g-C3N4 with different dimensions also exhibited significantly enhanced opened-up surface areas and highly exposed active sites, thereby greatly facilitating the photocatalytic enhancements. All these advanced features endow the nanostructured g-C3N4 with attractive structure-dependent, morphology-dependent and thickness-dependent applications ranging from photocatalysis to other emerging fields. Thus, as a simple way, tuning physical dimensions of g-C3N4 has become a popular strategy to manipulate the optical, electrical, and redox properties, thus achieving the desired catalytic activity, selectivity, and long-term stability. So far, nanostructured g-C3N4 with various different dimensionality, such as 0D quantum dots [411], [412], [413], [414], 1D nanowires/nanorods/nanotubes [138], [277], [359], [395], [400], 2D nanosheets [100], [415], [416], 3D hierarchical structures [393], [417], [418], [419] have been widely exploited and applied in the photocatalysis [55]. Among them, the 2D ultrathin g-C3N4 nanosheets have proven to be more promising for various photocatalytic applications [27], [52], [56], whose fabrication strategies will be highlighted in this section.
Generally, the free-standing ultrathin g-C3N4 nanosheets could be obtained via two distinct synthetic strategies, including the top-down exfoliation of layered bulk g-C3N4 materials and bottom-up assembly of precursors (molecular building blocks) in a 2D manner [52], [56]. Typically, these two fabrication strategies can be further classified into five detailed categories: ultrasonication-assisted liquid exfoliation, chemical exfoliation, thermal oxidation etching, combined and other approaches. In order to facilitate further comparison, various different fabrication methods for g-C3N4 nanosheets have been thoroughly summarized in Table 5. As observed in Table 5, it is clear that the 2D single-layer and few-layer g-C3N4 nanosheets could exhibit much higher surface areas in the range from 50 to 384 m2 g−1, which are several times larger than that of bulk layered g-C3N4 (10 m2 g−1), thus significantly favoring the photocatalytic enhancement.
Bulk g-C3N4 | Solvents, bulk powder/solvent (mg/ml), time (h) | Concentration (mg/mL)/Yield (%) | Thickess (nm)/Layer numbers | Band gap (eV)/surface area (m2 g−1) | Ref. (year) |
---|---|---|---|---|---|
Ultrasonication-assisted liquid exfoliation | |||||
MA (P, 600 °C) | Water, 100/100, 16 h | 0.15/ | 2.5/7 | 2.70/ | [100] (2013) |
Poly(triazine imide) (I) | Water, 2/1, 15 h | 0.2/ | 1–2/3–6 | [103] (2014) | |
CA (P, 530 °C) | Water, 200/200, 24 h | ∼1.8/5–6 | 2.6/ | [410] (2013) | |
MA (P, 600 °C) | Water, 50/50, 10 h | /14.5 | 1.2/4 | [299], [420](2013) | |
MA (P, 600 °C) | Water, 50/50, 10 h | /14.5 | 1.0/3 | [421] (2013) | |
MA (P, 600 °C) | Water, 100/200, 16 h | /8.6 | [422] (2013) | ||
MA (P, 550 °C) | Water, 50/50, 2 h | 1.2/<5 | [423] (2014) | ||
MA (P, 550 °C) | Water/organic solvents, 500/150, 10 h | 3/ | 0.38/1 | 2.79/59.4 | [424] (2015) |
Commercial g-C3N4 (Carbodeon Ltd) | Isopropanol, 30/10, 10 h | 2/ < 9 | 2.65/384 | [416] (2013) | |
CA (P, 550 °C) | Ethanol, 50/50, 2 h | 2–3/5–8 | 2.73/112.5 | [425] (2014) | |
DCDA (P, 600 °C) | 1,3-butanediol, 60/25, 24 h | 0.35/ | 0.9–2.1/3–6 | 2.79/32.54 | [426] (2014) |
MA (P, 550 °C) | 30 wt% isopropanol + water, 4/1, 10 h | 2/6 | 2.70/ | [427] (2014) | |
MA (P, 600 °C) | Water, 100/200, 16 h | /8.6 | [422] (2013) | ||
DCDA (P, 520 °C) | DMF, 50/200, 2h(80 °C)+ melamine | 2–3/6–9 | 2.75/116.76 | [428] (2015) | |
Commercial g-C3N4 (Carbodeon Ltd) | Ethanolamine/1,3-butanediol/3-pyridinemethanol,30/10,4h | 1/ | 7 | [429] (2015) | |
Chemical exfoliation | |||||
MA (P, 550 °C) | Liquid ammonia-assisted lithiation | /85% | 2.5/8 | 2.78/22.5 | [430] (2014) |
MA + LiCl(P, 380 + 550 °C) | The intercalation of LiCl ions and the liquid exfoliation in water | 2–3/6–9 | 2.82/186 | [431] (2016) | |
DCDA (P, 550 °C) | H2SO4 (98 wt%) treatment for 8 h, then rapid exfoliation | /30% | 0.4/1 | 2.92/205.8 | [178] (2013) |
MA (P, 550 °C) | H2SO4 (98 wt%), 1000/15, 15 min | /70% | 2.5 | 2.93/86.29 | [432] (2015) |
DCDA (P, 550 °C) | H2SO4 (98 wt%) | 300/ | /1 | 3.28/ | [180] (2015) |
DCDA (P, 550 °C) | HCl treatment for 1 h, then exfoliation for 2 h | /25–30% | 2–4/6–10 | 2.75/305 | [82] (2014) |
MA (P, 550 °C) | 6 M HCl, 320/80, hydrothermal at 110 °C for 5 h | /109.3 | [433] (2014) | ||
MA (P, 550 °C) | 0.1 M NaOH, 1000/90, hydrothermal at 150 °C for 18 h | /65 | [164] (2013) | ||
MA (P, 520 °C) | HCl treatment for 1 h, then exfoliation for 2 h | /25–30% | 9.0/30 | 3.42/ | [82] (2014) |
MA (P, 600 °C) | HNO3 (63 wt%) treatment for 8 h, then sonicated exfoliation | 2.7/179.5 | [434] (2015) | ||
Thermal oxidation etching | |||||
DCDA (P, 550 °C) | Air, 500 °C, 2 h | /<6% | 1.62–2.62/4–7 | 2.97/306 | [415] (2012) |
DCDA(P, 550 °C) | Air, 500 °C, 2 h | /<6% | 1.62–2.62/4–7 | 2.97306 | [409] (2014) |
Urea (P, 550 °C) | Air, 550 °C, 2 h | /8.0 | 16/ | 2.86/151 | [435] (2015) |
MA (P, 550 °C) | Air, 500 °C, 2 h | 3.06/165.66 | [436] (2014) | ||
MA (P, 550 °C) | Air, 500 °C, 2 h | 5–8/10–20 | 2.91/122.6 | [326] (2015) | |
MA (P, 520 + 540 °C) | Air, 540 °C, 2 h | /0.35% | 2.93/210 | [437] (2015) | |
MA (P, 500 + 520 °C) | Air, 520 °C, 4 h | 2.7/8 | 2.82/153.32 | [438] (2016) | |
MA (P, 500 + 550 °C) | Air, 500 °C, 2 h | 1.9/6 | 2.97/150.1 | [439] (2014) | |
MA (P, 600 °C) | Air, 500 °C, 4 h | 0.9/3 | [440] (2015) | ||
DCDA (P, 520 °C) | Air, 400 °C, 4 h | 2.89/ | [441] (2013) | ||
Commercial g-C3N4 (Carbodeon Ltd) | H2, 400 °C, 4 h | 2/6 | /260 | [442] (2015) | |
DCDA(P, 550 °C) | NH3, 510 °C, 1 h | 20/50 | 2.95/196 | [350] (2015) | |
Mixed DCDA/NH4Cl | Air, 550 °C, 2 h | 3.1/8 | 2.83/52.9 | [443] (2014) | |
Mixed DCDA/NH4Cl | Air, 550 °C, 4 h | 1.0/3 | 2.77/77.7 | [444] (2015) | |
Mixed MA/KCl | Air, 550 °C, 4 h | 1.5–6.3 | 2.71/ | [445] (2014) | |
Mixed MA/KBH4 | Air, 550 °C, 4 h | 1.5/4 | [446] (2014) | ||
Guanidinium cyanurate | Air, 550 °C, 4 h | 0.6–1.5/1–3 | 2.87/133.8 | [447] (2015) | |
Guanidinium chloride | Air, 600 °C, 4 h | 2.44/109.9 | [448] (2014) | ||
DCDA (P, 600 °C) | Air, 350 °C, 2 h (NH4Cl intercalation, N2) | 2–3/6–9 | 2.85/30.1 | [449] (2014) | |
Combined approaches | |||||
MA (P, 520 °C) | (Air, 550 °C, 3 h)+ (isopropanol, 10/100, 8 h) | 0.5/1 | [450] (2014) | ||
MA (P, 500 + 530 °C) | (Air, 550 °C, 3 h)+ (methanol, 100/100, 4 h) | 0.4–0.5/1 | [451] (2014) | ||
MA (P, 500 + 530 °C) | (Air, 550 °C, 200 min)+ (methanol, 100/100, 4 h) | 0.8–1.2/2–3 | 3.03/ | [452] (2015) | |
MA (P, 520 °C) | (Air, 550 °C, 3 h)+ (isopropanol, 8 h) | 0.5/1 | 3.0/380 | [453] (2014) | |
MA (P, 550 °C) | (Air, 550 °C)+ (H2SO4 + HNO3, 100/40, 5 h) | 2.95/109.30 | [454] (2014) | ||
MA (P, 520 °C) | (Air, 550 °C, 3 h)+ (0.5 M HCl, 20/200, 8 h) | 0.6/1 | [455] (2015) | ||
DCDA (P, 550 °C) | (H2SO4 (98 wt%), 2000/50, 2 h) + (water, 1 h) | 5–10/15–20 | 3.0/140 | [456] (2015) | |
DCDA/2-aminobenzonitrile (P, 550 °C) | (5 M HNO3, 400/80, 20 h/under reflux)+ (water, 4 h) | 0.6–4/2–10 | 2.89/ | [457] (2016) | |
Urea (P, 600 °C) | (H2SO4 + HNO3, 1000/200, 4 h)+ (water, 30/30, 2 h) | /10.5 | [458] (2016) | ||
MA (P, 550 °C) | (75 wt% H2SO4, 300/30, 2 h 130 °C) + (water + 300/200,2 h) | 2.5/7 | 2.88/80 | [459] (2015) | |
MA (P, 550 °C) | (H2SO4 (98 wt%), 1000/10, 8 h) + (N2, 550 °C, 2 h) | 0.8–1.4/3–4 | 2.79/54.3 | [460] (2016) | |
Other approaches | |||||
MA (P, 550 °C) | Ball milling method | /1–2 | 2.98/ | [461] (2015) | |
MA and carbon fibre | Microwave heating approach | 1.6/5 | 2.88/239 | [462] (2016) |
P, pyrolysis (or thermal polycondensation); DMF, dimethylformamide.
Inspired by the formation of graphene/metal dichalcogenide/double hydroxide nanosheets by liquid exfoliation of layered bulk counterparts [463], [464], [465], [466], [467], [468], [469], it is highly expected that the mono- or few-layer C3N4 nanosheets could be obtained through a simple liquid exfoliation of bulk layered g-C3N4 by sonication. Clearly, the well matched surface energy between a liquid solvent and g-C3N4 (115 mJ m−2) could effectively reduce their enthalpy of mixing, thus leading to the enhanced exfoliation efficiency of bulk g-C3N4 into 2D nanosheets [426], [465]. Thus, the ultrasonication treatments in various solvents with proper surface energy, such as water (102 mJ m−2), methanol, ethanol, N-methyl-pyrrolidone (NMP), 1-isopropanol(IPA), dimethyl formamide(DMF), acetone, acetonitrile, 1,4-dioxane and their mixtures, have been applied in overcoming the weak van der Waals forces between the two adjacent g-C3N4 layers and successfully exfoliating the bulk layered g-C3N4 into 2D ultrathin nanosheets [100], [103], [178], [416], [425], [426]. For example, Xie and coworkers successfully prepared the ultrathin g-C3N4 nanosheets (0.15 mg/mL) with a size distribution ranging from 70 to 160 nm and a height of ∼2.5 nm (about 7 layers) by a “green” liquid exfoliation route from bulk g-C3N4 in water for the first time (as shown in Fig. 31a and b) [100]. To further enhance the dispersion concentration of exfoliated mono-layer or few-layer g-C3N4 nanosheets, the better solvent effects of organic and mixed solvents have been utilized to exfoliate the bulk C3N4 materials [424], [429]. For example, the exfoliation of commercial g-C3N4 in the mixed solution (ethanolamine, 1,3-butanediol and 3-pyridinemethanol) could obtain the 7-layer g-C3N4 nanosheets with a concentration of 1 mg/mL [429]. In another example, the single layer of g-C3N4 nanosheets with a concentration of 3 mg/mL have been achieved through the effective exfoliation in the mixed water/organic solvents [424]. However, some typical problems, including the use of organic solvents and additives, long ultrasonication exfoliation time and low yield, still need to be further overcome. Thus, the liquid ammonia-assisted lithiation and the intercalation of LiCl ions have been exploited to achieve the large-scale exfoliation of bulk g-C3N4 materials [430], [431]. Especailly, the 8-layer-thick O-doped g-C3N4 nanosheets could be fabricated by this liquid ammonia-assisted lithiation method in a large scale (10 g) and high yield (85%), under mild conditions [430]. More fortunately, it was demonstrated that the chemical exfoliation in acid or alkaline solution could not only obtain amphoteric single-layer g-C3N4 nanosheets with the negatively charged carboxyl and positively charged −NH2/NH3+ groups, but also efficiently reduce the ultrasonication-treatment time from more than 10 h to 2 h [82], [178], [432], [470]. For example, Xu et al. successfully obtained single layer of g-C3N4 nanosheets with a thickness of 0.4 nm by means of the rapid exothermic effect of the intercalated concentrated H2SO4 (98%) dissolving in deionized water [178]. Following this report, Tong et al. developed an interesting high-throughput method to rapidly fabricate g-C3N4 nanosheets through combining the oxidation, protonation and heating effects of concentrated H2SO4 [432]. The g-C3N4 nanosheets with a thickness of 2.5 nm and different degree of exfoliation could be easily achieved though directly adding controlled amount of H2O into a bulk g-C3N4 suspension using the concentrated H2SO4 as solvent. This facile acid-exfoliation method with low cost and controlled exfoliation degree would open up new opportunities for the large-scale fabrication and extensive application of g-C3N4 nanosheets.
Although bulk g-C3N4 could be also easily thermally exfoliated into 2 nm-thickness nanosheets (around 6–7 layers) through the direct oxidation “etching” [415], the poor yield (<6%) and significant interface defects, thus leading to the significantly reduced photoability and photoactivity. Motivated by the successful thermal exfoliation of the NH4Cl-intercalation g-C3N4 materials [449], Wu and coworkers recently developed a facile one-step dicyandiamide-blowing method with NH4Cl as the gas template to achieve the scalable fabrication of high quality 2D g-C3N4 nanosheets [443]. As shown in Fig. 32, it is believed that the gases (NH3 and HCl) released from NH4Cl during heating directly achieved the fast exfoliation of bulk g-C3N4 into the crinkly 8-layer g-C3N4 nanosheets (with a thickness of 3.1 nm and a high band gap of 2.83 eV). Other methods such as thermal exfoliation of C3N4-based intercalation compound [449], microwave heating [462] and ball milling [461] have also been successfully applied in obtaining 2D ultrathin C3N4 nanosheets. More interestingly, the perfect combination of liquid, chemical exfoliation and thermal oxidation etching could fully explore their potentials and achieve the low-cost, simple, fast and scalable synthesis of 2D g-C3N4 nanosheets [450], [451], [453], [455], [457], [460], which deserves more attention in the near future.
In addition to developing new fabrication methods of g-C3N4 nanosheets, more efforts should be also devote to narrowing their larger band gaps (0.1–0.2 eV higher than that of bulk g-C3N4) and promoting the fast utilization of photo-generated charge carriers migrated to the surface of g-C3N4 nanosheets. As effective strategies, doping [84], [118], [128], [328], [471], introducing nitrogen vacancies [347], [472], sensitization [473], copolymerization [130], [474], [475] and hybridization with other semiconductors or co-catalysts could be employed to further achieve the 600 nm or near-infrared (NIR) photocatalysis of g-C3N4 nanosheets in aqueous solution [52], [476], [477], [478].
4.5. Surface sensitization
In general, there are five typical strategies to increase the visible-light absorbance of wide band gap semiconductors: band-gap engineering (impurity doping and solid solution), defect control, surface plasmon resonance (SPR) effect, sensitization by dye and quantum dot [16]. Although the aforementioned two strategies of band-gap engineering and defect control can partially extend their visible-light absorption, the moderate band gaps (∼2.7 eV) of g-C3N4-based semiconductors are still the main bottlenecks affecting the highly effective generation of photo-generated charge carriers, which thereby play the crucial roles in determining the visible-light photocatalytic performances of g-C3N4-based semiconductors. Thus, other three strategies, including loading plasmonic metals, sensitization by quantum dots (QDs) and organic dyes (the corresponding mechanisms shown in Fig. 33), have been also widely applied in enhancing the visible-light absorbance of g-C3N4-based semiconductors, which will be thoroughly discussed in this section.
Firstly, the famous SPR effects of noble metals, such as Au [77], [200], [479], [480], [481], [482] and Ag [433], [455], [479], [483], [484], [485], [486], [487], [488], [489], [490], [491], have been widely employed to improve the visible-light absorbance and charge separation of g-C3N4. In general, it is well accepted that the deposited plasmonic metals could function as electron sink, reduction co-catalyst and photosensitizers to enhance the visible-light absorption of a given semiconductor [493]. For example, Bai et al. fabricated the core–shell nanostructured Ag@C3N4 photocatalysts through the simple methanol-reflux treatment of g-C3N4 nanosheets deposited by Ag nanoparticles (as shown in Fig. 34) [488]. The combination of LSPR effect of Ag nanoparticles and their hybrid effect with C3N4 could achieve 1.8- and 30-time enhancements in the photocatalytic MB degradation and H2 evolution, respectively. In another example, Wei et al. constructed the type-II 2D-1D C3N4/TiO2 hybrid nanofibers and further decorated plasmonic noble metal nanoparticles (Au, Ag, or Pt) with sizes from 5 to 10 nm on them [479]. The resulting SPR sensitized heterostructures could achieve highly efficient photocatalytic H2 evolution due to the simultaneous implementation of improved light absorption, charge separation and utilization. In future, it is expected that the plasmonic alloys [494], [495], Cu [496], [497], [498] and Bi [499], [500], [501] nanoparticles could be deposited onto g-C3N4 to boost their visible-light photocatalytic activity.
Secondly, the quantum dots modified g-C3N4 photocatalytic systems are still very interesting and promising [502], [503], [504], [505]. For example, Ge et al. first demonstrated that the deposition of 30 wt% CdS QDs onto the bulk g-C3N4 could achieve a 9-fold enhancement in the visible-light photocatalytic H2-evolution activity, due to the increased the absorbance of visible light and promoted charge separation [502]. Since then, CdS QDs have been widely used to improve the visible-light activity of bulk g-C3N4 for various kinds of applications [284], [405], [502], [503], [506], [507], [508]. In future, it is expected that more efficient CdS/g-C3N4 composite photocatalysts with earth-abundant co-catalysts should be further exploited.
Thirdly, various kinds of low-cost organic dyes, such as magnesium phthalocyanine (MgPc) [509], zinc phthalocyanine [510], [511], [512], [513], [514], [515], Xanthene [516], Erythrosin B (ErB) [473], [517] and Eosin Y (EY) [518], have been readily coupled with different nanostructured g-C3N4 to fabricate the highly efficient organic semiconductor heterojunctions. For example, Domen and his coworkers deposited an organic MgPc dye (with a band gap of 1.8 eV) on the Pt/mpg-C3N4 composite semiconductors and achieve the enhanced photocatalytic H2 evolution under long-wavelength irradiation (>600 nm) [509]. The results indicated that the monolayer dye could achieve the highest photocatalytic H2-evolution performance due to promoted charge generation, transfer and utilization, whereas excess thickness of the dye layer will cover the co-catalyst sites, thus reducing the photocatalytic activity [509]. Similarly, Lu and coworkers successfully demonstrated that the sensitization of mesoporous g-C3N4 with a EY dye could achieve an H2-evolution AQE of 19.4% under 550 nm irradiation [519]. It is suggested that the high surface area and nanoporous structure of mpg-C3N4 are greatly favorable for deposition of EY molecules on its surface, thus promoting the significantly increased and extended light harvesting in the visible-light response region and further improving H2-evolution activity. More surprisingly, Xu and coworkers demonstrated that the deposition of ErB dye onto Pt/g-C3N4 sample exhibited a remarkably enhanced H2 evolution rate (652.5 or 162.5 μmol h−1) from an aqueous solution of TEOA under visible light irradiation (λ > 420 nm or > 550 nm), with an AQY of 33.4% at 460 nm [473]. The resulting ternary Pt/g-C3N4/ErB photocatalyst also showed the stability and good recyclability, remaining 90% of the activity after 5 runs [473]. Most recently, it has been demonstrated that the promising earth-abundant Co(OH)2 and MoS2 could be utilized as co-catalysts to boost the photocatlytic H2-evolution activity over these dye-sensitized g-C3N4 photocatalysts [517], [520], [521]. More interestingly, the g-C3N4-based photocatalytic systems co-sensitized by two organic dyes or inorganic photosensitizers (plasmonic metals and QDs) are also highly desirable in future studies [510], [512], [521], [522]. However, the apparent quantum effciency and stabilities of these systems are still needed to be further enhanced.
4.6. Heterojunction construction
Enhancing photocatalytic activity of g-C3N4-based semiconductors could be also achieved through constructing semiconductor heterojunctions, which could induce the band bending and the formation of internal electrical field, thus significantly boosting the efficient spatial charge separation [7]. In general, according to the semiconductors’ energy bands and Femi levels of metal co-catalysts, semiconductor heterojunctions can be divided into four types: Schottky junction (Fig. 35A), Type I (Fig. 35B), Type II (Fig. 35C) and Type III (Fig. 35D) heterojunctions. Clearly, only Schottky junction and Type II heterojunctions can significantly promote the fast spatial separation of electrons and holes, thus retarding their recombination and prolonging their lifetime. Thus, for photocatalysis, the construction of Schottky junction and Type II alignment should be highly desired. The Schottky junction will be discussed in the next section. Here, we will focus on the Type II g-C3N4-based heterojunctions. As observed the CB and VB levels of different semiconductors in Table 1 and Fig. 16, it is easily found that several commonly used semiconductors, such as TiO2 [523], Cu2O [524], [525], [526], ZnO [291], [527], [528], [529], [530], [531], [532], [533], [534], [535], [536], [537], WO3 [286], [538], [539], [540], [541], [542], [543], [544], BiVO4 [545], [546], [547], [548], [549], [550], [551], (BiO)2CO3 [552], Ag3PO4 [553], [554], [555], [556], [557], [558], [559], [560], CdS [197], [284], [405], [502], [503], [506], [508], [561], [562], [563], [564], [565], [566], BiOX [567], [568], [569], [570], [571], [572], [573], [574], [575], Bi2WO6 [552], [576], [577], [578], [579], [580], [581], [582], [583], Fe2O3 [115], [289], [584], [585], [586], [587] and different types of g-C3N4 [121], [588] can be combined with g-C3N4 to construct the Type II heterojunctions and achieve the efficient charge separation. Here, the CdS/g-C3N4 and the g-C3N4 isotype heterojunctions will be discussed.
Besides the above mentioned systems of CdS quantum dots/g-C3N4 [502], [503], [561], various kinds of other nanostructured combinations, such as core/shell CdS@g-C3N4 Nanowires [284], [562], g-C3N4/Au/CdS Z-scheme [589], [590], CdS nanoparticles/2D g-C3N4 nanosheets [563] and CdS nanorods/g-C3N4 nanosheets [197], [562], [591], [592] have been also successfully constructed to obtain the highly efficient and stable composite photocatalysts. Among them, the core/shell heterojunctions seem to be more promising, due to the suppressed CdS photocorrosion and the optimized intimate interface contact. For example, Zhang et al. demonstrated that the as-constructed novel CdS/2 wt% g-C3N4 core/shell nanowires could achieve an optimal photocatalytic activity of up to 4152 μmol h−1 g−1 [562]. It is believed that the well-matched Type II g-C3N4/CdS heterojunctions could achieve the positive synergic effect of accelerating the separation of charge carriers and inhibiting the CdS corrosion (as shown in Fig. 36a), thus greatly enhancing the photocatalytic activity and photostability [562]. Interestingly, based on the slight difference in their electronic band structures (Fig. 36b), Wang and coworkers et al. firstly demonstrated that the as-prepared Type II isotype heterojunctions of g-C3N4/S-mediated g-C3N4 (S-g-C3N4) exhibited the matched band gaps and efficiently promoted charge separation of the band offsets (Fig. 36b), thus significantly enhancing photocatalytic activity for H2 evolution [121]. The isotype heterostructure is similar to that of phase junction in the formation mechanism, which provides new opportunities to construct buried layered junctions in the various copolymerized g-C3N4-based composites with improved charge photon-excitation and charge separation [54]. Similarly, Dong and coworkers in situ constructed a novel Type II layered g-C3N4/g-C3N4 metal-free isotype heterojunction with enhanced photocatalytic activity for the removal of NO in air [588]. The results further confirmed the key roles of the Type II isotype heterojunction in achieving the efficient charge separation and transfer across the heterojunction interface as well as prolonged lifetime of charge carriers.
As observed in Figs. 35 b and 36, for the favorable Type II heterojunction systems, the photocatalytic redox reactions mainly occur on the surface of semiconductor with lower CB and VB edges, implying the weaker reduction and oxidation ability (driving forces) in this kind of heterojunction-type photocatalytic system. In contrary, as observed in Fig. 37, for the all-solid-state Z-scheme photocatalytic system with the Ohmic-contact interfaces, their photocatalytic activities are majorly dependent on the surface properties of semiconductor with higher CB and VB edges, thus leading to the strong redox ability and enhanced photocatalysis [593]. In nature, photosynthesis of plants generally proceeds according to the so-called Z-scheme photocatalytic process, in which two isolated reactions of water oxidation and CO2 reduction are linked together through the redox mediators [593]. Mimicking the natural photosynthesis process, the artificial Z-scheme semiconductor heterojunctions have been successfully proposed and constructed by combining two different semiconductors through liquid-state or all-solid-state mediators. Each semiconductor in the Z-scheme is only responsible for one (oxidation or reduction) reaction, thus achieving the extremely extended visible-light absorption, strengthened redox ability, improved photostability, charge-separation and photocatalytic efficiency [594], [595]. In this regard, the all-solid-state g-C3N4-based Z-scheme photocatalytic systems seem to exhibit many advantages and great potential in practical applications in photocatalytic fields. Commonly, the artificial heterogeneous all-solid-state g-C3N4-based Z-scheme photocatalytic systems could overcome the weak oxidation ability and decrease the reduction capacity of single-g-C3N4 and g-C3N4-based heterojunctions, respectively, and simultaneously fulfill a wide absorption range, long-term stability, high charge-separation efficiency and strong redox ability [523], [596], [597].
Typically, two kinds of all-solid-state g-C3N4-based Z-scheme photocatalytic systems with and without mediators are displayed in Fig. 37a and b, respectively. For the rational design of these two kinds of all-solid-state g-C3N4-based Z-scheme photocatalytic systems, the achievement of the intimate Ohmic contact should be the most important point, which could be obtained through introducing the conductors, such as conductive carbon and metals, or fabricating the perfect interfacial contact. For example, Yu and coworkers, for the first time, constructed a direct g-C3N4–TiO2 Z-scheme photocatalyst without an electron mediator by a facile calcination route, which could achieve the intimate interfacial contact [523]. The results showed that the as-prepared Z-scheme photocatalysts was highly dependent on the g-C3N4 content. Ideally, the surface of the TiO2 nanoparticles should be partially covered by the g-C3N4 nanoparticles, which are favorable for the formation of a g-C3N4-TiO2 Z-scheme photocatalytic system (see Fig. 38a and b). In contrary, the excessively high contents of g-C3N4 can not only lead to the complete cover of TiO2 surface, which could decrease the charge carrier excitation of TiO2, but also increase the recombination rate of photo-generated electrons and holes on g-C3N4 (see Fig. 38c). Similar Z-scheme systems between g-C3N4 and TiO2 have also been observed in other reports [598], [599]. Some other direct g-C3N4-based Z-scheme photocatalysts, such as Bi2O3/g-C3N4 [600], ZnO/g-C3N4 [601], [602], BiVO4/g-C3N4 [603], g-C3N4/Bi2MoO6 [604], WO3/g-C3N4 [605], [606], g-C3N4/Ag3PO4 [607], [608], [609], BiOCl-g-C3N4 [610], Bi2WO6/g-C3N4 [505] have also been available. Additionally, some typical electron mediators, such as nanocarbon [611], Au [590], [612], RGO [613] and Bi [614] could be used to construct the indirect g-C3N4-based Z-scheme photocatalysts. In future, improvements in the morphology of semiconductors and interfacial coupling should be deeply and continually investigated to search for highly effective g-C3N4-based Z-scheme photocatalysts for practical applications [598], [615], [616], [617], [618]. More importantly, the direct or indirect experimental evidence for supporting the proposed Z-scheme charge-transfer mechanism should be provided as far as possible. In fact, the technologies such as radicals (O2− and OH) trapping experiments, metal deposition and double-beam photoacoustic (DB-PA) spectroscopy have been employed to reveal the real Z-scheme charge-transfer mechanism [523], [534], [601], [608], [619], [620].
In addition, it should be also noted that the interfacial contact/coupling performances could be further improved by several strategies, such as, increasing the contact areas, improving the tightness of interfaces [621], [622], [623], [624] and introducing the highly-conductive interfacial mediator [625], [626], [627]. Clearly, larger contact area can provide sufficient charge transfer and trapping channels for achieving their fast separation. As compared with other types of composite pohotocatalysts with 1D (i.e. 0D/1D and 1D/1D) or 2D (i.e. 0D/2D and 1D/2D) contact interfaces, the unique 2D–2D layered nano-junctions possess the much larger contact surface between the two adjacent sheets (as shown in Fig. 39), thus favoring more efficient interfacial charge separation and photo-activity enhancement [476]. More fortunately, g-C3N4 itself possesses a unique 2D layered structure, which holds great promise for potential applications in constructing 2D layered composite photocatalysts. In 2011, Xiang et al. first constructed graphene/g-C3N4 composite photocatalysts with the larger 2D-2D coupling interfaces (as shown in Fig. 40a) [151], demonstrating a more than 3.07-time enhancement of photocatalytic H2-evolution activity (using Pt and methanol as cocatalyst and sacrificial agent, respectively). Subsequently, a series of 2D g-C3N4-based layered heterojunctions (e.g. MoS2 [120], [290], [292], [628], [629], [630], [631], [632], [633], SnS2 [634], [635], [636], WS2 [637], [638], graphene [482], [639], SnNb2O6 [640], WO3 [287], BiOBr [641], layered double hydroxide [288], [642], [643], Bi4O5I2 [644] and Bi2O2CO3 [645]) have been widely fabricated for different photocatalytic applications. Among them, g-C3N4/MoS2 2D-2D coupling systems have attracted much attention since the first report about concept of layered nanojunctions by Hou et al. in 2003 (as shown in Fig. 40b) [120]. It is believed that 2D layered MoS2 can function as co-catalysts [646], stable semiconductor sensitizers [647] or electron trapper [648], [649] in these systems. More interestingly, the layered triple-nanojunctions in the ternary g-C3N4 nanosheets/N-doped graphene/MoS2 have also been successfully fabricated, which has been shown to significantly improve the photocatalytic activity of MB oxidation and Cr(VI) reduction through the synergistic effects of multiple 2D-2D coupling interfaces [650]. These results highlighted that the unique 2D–2D layered nano-junctions with larger contact area are more promising for the fast separation of photo-excited charge carriers across the interfaces with respect to the 0D–2D and 1D–2D coupling systems. More interestingly, the coupling of g-C3N4 nanosheets and semiconductor nanosheets with exposed facets seems to be more promising in various kinds of photocatalytic applications, due to the diversified synergy effects [418], [427], [559], [572], [641], [651], [652], [653], [654], [655], [656]. In future, it is expected that the 2D–2D interface coupling performances could be further enhanced through improving the tightness of interfaces and introducing the interfacial mediator [476], [638].
4.7. Co-catalyst loading
The level of the conduction bands of g-C3N4 is −1.3 V (vs. NHE), which is more negative than the reduction potentials of CO2/CO(−0.51 V), H +/H2 (0.41 V), and O2/O2− (0.33 V), respectively (As shown in Fig. 41). Thermodynamically, the photo-generated electrons on the CB of g-C3N4 have much stronger driving force (or over-potentials) for these three typical kinds of reduction reactions, as compared to those in TiO2 [28], [657], [658]. However, the obvious structure defects in bulk g-C3N4 generally lead to their fast recombination with the photo-excited holes. More importantly, the photocatalytic H2 evolution and CO2 reduction are typical up-hill reactions, thus resulting in the sluggish kinetics on the surface of bulk g-C3N4. Fortunately, these disadvantageous factors could be simultaneously overcome by loading suitable reduction co-catalysts onto the surface of g-C3N4, which could lower the reaction activation energy (or electrochemical overpotentials), improve the charge separation and transport, increase stability of photocatalyts, and accelerate the sluggish reaction kinetics of various surface reduction reactions, thus greatly enhancing the photocatlytic activity [88], [124]. Essentially speaking, the single-electron or multi-electron O2-reduction reactions (as shown in Table 2) are of significant importance for photocatalytic degradation [83], [410], [657], selective organic transformations [119], [244] and disinfection [659], [660]. More interestingly, the co-catalysts can also achieve the selective photoreduction products of CO2 [16], [661]. In addition, it is also necessary to deposit suitable hole co-catalysts to accelerate the difficult water oxidation reactions, due to the lower overpotential of g-C3N4 for water oxidation (0.59 V), as well as the inherent challenges of four-electron water oxidation [7], [662]. Thus, it is obvious that all these reduction and oxidation co-catalysts play decisive roles in achieving highly efficient and selective photocatalytic reactions. These four types of co-catalysts over g-C3N4 (as shown in Fig. 41), namely, H2-evolution, CO2-reduction, O2-reduction and H2O-oxidation co-catalysts, are summarized in Table 6, which will be compared and discussed in this section in detail.
Roles/functions of co-catalysts | Noble-metal co-catalysts | Earth-abundant metal co-catalysts | Metal-free and Hybrid co-catalysts (nano carbons) |
---|---|---|---|
H2 evolution | Metal: Pt [663], single-atom Pt [664] Sulfides: Ag2S [665] Plasmonic: Au [666], [667], Ag [479], [488] Bimetallic: PtCo [193], AuPd [194] Co-loading: Au/Pt [668], Au/PtO [200] | Ni(OH)2 [126], [669], WS2 [637], [638], Cu(OH)2 [521], MoS2 [328], [517], [628], [632], NiSx [125], [197], [670], [671], [672], [673], NiOx, Cu [674], Ni [675], [676], Ni(dmgH) 2 [677], hollow Zn0.30Co2.70S4 [678], [Ni (TEOA)2]Cl2 [679], Co0.04Mo0.96S2 [651] | Graphene [151], carbon black/NiS [156], carbon QDs [680], CNTs [681], CNTs/NiS [198], C60, carbon fiber [682], ZIF-8 derived carbon [683], acetylene black/Ni(OH)2[155] |
Hole co-catalysts | RuO2, [44], [133] | CoOx [285], [684], Co(OH)2 [685], layered double hydroxide (LDH) [288], [686] Co(II) ions [687], Co3O4 [688], [689], Co-Pi [690] | Graphene/LDH [288] |
CO2 reduction | Pt [691], [692], [693], single-atom (Pd/Pt) [694], ruthenium complex [695], [696], [697], [698], [699], [700] | Layered double hydroxide [642] | Phosphate [701], graphene [160], [702], [703], [704], carbon [705], CNTs [706], S-doped porous carbon [707], UiO-66 (zirconium-based MOFs) [708], Co-ZIF-9 (cobalt-based MOFs)[709] |
O2 reduction (degradation) | Pt, Au [590], Ag [433], [483], [485], [488], [506], [710], [711], [712], [713], [714], [715], [716], [717], [718], [719], [720], [721], [722], [723], Ag2O [724], [725], [726], [727], [728], Pd [729], [730], [731], [732], bimetallic Au/Pt [481], Ag Quantum Cluster[733] | NiO [734], MoS2 [292], [599], [629], [630], [735], Fe(III) [736], H3BO3 [737], Phosphate [738], [739], Fe (III)/Co(III) [245] | Graphene [220], [244], [739], [740], [741], [742], [743], [744], carbon QDs [745], [746], CNTs [747], biochar [748], PANI [749], [750], [751], C60 [752], [753] graphene/Ag [710], [754], CNTs/Au [755], ordered mesoporous carbon [756], [757] |
Similar to those co-catalysts on TiO2 [25], these different co-catalysts over g-C3N4-based photocatalysts can be divided into three categories (as shown in Table 6): noble-metal co-catalysts, earth-abundant metal co-catalysts, metal-free and hybrid co-catalysts (nano carbons). The nano carbons as co-catalysts will be highlighted in the next section. Here, we will focus on the noble-metal and earth-abundant metal co-catalysts, especially for the latter. As observed from Table 5, it is clear that the more efforts have been devoted to the developments of H2-evolution and O2-reduction co-catalysts. On the one hand, the noble-metal co-catalysts (e.g., single-atom [664], bimetallic [193], [194], [481] and co-loading [200], [668]) seem to be more promising for both photocatalytic H2-evolution and O2-reduction reactions, which deserve more attention in the future studies. On the other hand, for earth-abundant metal co-catalysts, the 2D layered co-catalysts, such as MoS2 [292], [328], [517], [599], [628], [629], [630], [632], [735], Ni(OH)2 [126], [669], WS2 [637], [638], Cu(OH)2 [521], and NiSx [125], [197], [670], [671], [672], [673], have become the shinning stars in the fields of photocatalytic H2 generation and degradation of pollutants (for O2 reduction), which will be still the prescriptive research topics in these fields. In addition, the fabrication of nanostructured hybrid co-catalysts [155], [156], [198], [651], [678] and development of molecular clusters (such as Fe(III)/Co(III) [245], [736], [758]) and amorphous co-catalysts[759] might become an attractive direction in the near future.
Relatively speaking, there are only limited reports about the photocatalytic water oxidation over g-C3N4-based photocatalysts modified by Co-based molecules or compounds, [285], [684], [685], [687], [688], [689] RuO2 [44], [133] or LDH [288], [686]. In this regard, other water-oxidation co-catalysts such as cubic Co complex [760], [761], Nocera cobalt phosphate (CoPi) [762], [763], IrO2 [764], NiOx [765], [766], [767], or g-C3N4/graphene [262], should be paid more attention in future studies. Similarly, so far, only few co-catalysts on g-C3N4, e.g., Pt-based, [691], [692], [693], [694] ruthenium complex [695], [696], [697], [698], [699], [700], LDH [642], Phosphate [701], MOFs [708], [709] and carbon [160], [702], [703], [704], [705], [706], [707], have been available for selective photocatalytic CO2 reduction. Thus, in future, it is expected that more and more earth-abundant CO2-reduction co-catalysts with high activity and selectivity could be developed and utilized in the production of solar fuels from the CO2 photoreduction.
4.8. Nanocarbon loading
So far, various kinds of carbon materials, including CNTs [147], [198], [747], [755], [768], graphene [151], [160], [288], [650], [702], [703], [769], [770], [771], [772], [773], C60 [752], [753], carbon quantum dots [745], [774], [775], [776], [777], carbon fibers [682], activated carbon, carbon black [156], [778], [779], acetylene black [155], etc. have been widely coupled with g-C3N4 to fabricated the g-C3N4/carbon hybrid materials [780]. The essential reasons can be attributed to the promoted charge separation by the as-formed carbon-based Schottky-junction between g-C3N4 and the highly conductive nano carbon materials and the enhanced adsorption performances from narrowing band gap due to the carbon doping [25]. More specifically, coupling with various carbon rich materials with g-C3N4 not only compensate the disadvantages of individual semiconductor materials, but also induce the interesting synergetic effects, like supporting material, increasing adsorption and active sites, electron acceptor and transport channel, cocatalyst, photosensitization, photocatalyst, and band gap narrowing effect (as shown in Fig. 42) [781]. Here, we will highlight the hybrids of g-C3N4/graphene, which have been the most widely investigated g-C3N4/carbon composite photocatalysts.
Compared to the 0D and 1D carbon materials, graphene, a sp2-hybridized 2D carbon nanosheet, exhibits a much higher optical transmittance, conductivity (∼5000 W m−1 K−1), electron mobility (200,000 cm2 V−1 s−1), theoretical specific surface area (∼2600 m2 g−1) and more suitable work function (4.42 eV) for H2 evolution [17], [658], [782], [783], [784], [785], [786], [787]. Thus, the combination of g-C3N4 and graphene has been widely shown to be one of promising strategies to favor the charge transfer and inhibit the charge recombination process, thereby leading to an enhanced photocatalytic activity for H2 production. As displayed in Fig. 40a, the 2D-2D coupling interface in the g-C3N4/graphene hybrids could achieve much larger interfacial coupling areas and more efficient charge separation, as compared to the 0D-2D and 1D-2D hybrids. For example, the doping of g-C3N4 by graphene could be achieved through formation of COC covalent bonding or π-π stacking interactions, both of which can effectively narrow the band gap of g-C3N4, thus leading to the enhanced visible-light photocatalytic activity [702], [788], [789], [790]. Recently, based on the state-of-the-art hybrid DFT, Xu et al. systematically investigated the interaction between the g-C3N4 and RGO sheets [773]. It was demonstrated that the appropriate O concentration plays a crucial role in altering the direct gap to indirect one. Most importantly, a higher O concentration could achieve a type-II, staggered band alignment at the g-C3N4-RGO interface, leading to the high hydrogen-evolution activity over O atoms (as active sites) in the RGO (Fig. 43) [773]. The findings pave the way for developing RGO-based composites for photocatalytic applications. At this point, the semiconductor properties of RGO are more promising in constructing composite photocatalysts [740], which deserves more attention in future studies. In addition, doping or co-doping of RGO materials with heteroatoms has also been demonstrated to exhibit the significantly enhanced electrocatalytic performances [791], which are also highly expected to be utilized in constructing g-C3N4/doped RGO composite photocatalysts.
5. Potential applications of g-C3N4-based composite photocatalysts
5.1. Photocatalytic water splitting
Since the pioneering works by Honda and Fujishima in 1972 [1], various heterogeneous photocatalysts have been widely applied in the attracted photocatalytic hydrogen production from water reduction [7]. Normally, photocatalytic water splitting systems can be divided into half-reaction water splitting (for H2 and O2) and overall water splitting systems [7]. Interestingly, g-C3N4 has been extensively applied in these two systems to boost their photocatalytic activity for water splitting. Table 7 summarizes the photocatalytic activities of g-C3N4 based photocatalysts for H2 generation on various conditions, including the amount of photocatalysts, the sacrificial reagents, H2 generation rate, and the corresponding quantum efficiency in this review. As shown in Table 7, co-catalysts and sacrificial reagents are crucial for achieving the highly efficient photocatalytic H2 evolution, which will be discussed in detail.
photocatalyst | structure | synthetic method | co-catal./mass ratio | mass (g) | light source | incident light | aqueous reaction | co-catal./activity (μmol h−1) | QY (%) | reference (year) |
---|---|---|---|---|---|---|---|---|---|---|
Ni@/g-C3N4 | particles/lamellar | solvothermal method | Ni/10 wt.% | 0.05 | 500 W Xe lamp | quartz reactor | 10 vol.% TEOA | Ni/8.41 | [192] (2015) | |
NiS/g-C3N4 | nanoparticles/lamellar | hydrothermal method | NiS/1.25 wt.% | 0.1 | 300 W Xe lamp, λ > 420 nm | Pyrex reactor | 15 vol.% TEOA | NiS/46 | 1.9 (440 nm) | [125] (2013) |
NiS/g-C3N4 | nanoparticles/lamellar | in stiu ion-exchange method | NiS/1.5 mol% | 0.1 | 300 W Xe lamp, λ > 420 nm | 10 vol.% TEOA | NiS/44.77 | [670] (2014) | ||
NiS/e-C3N4 | ultrathin nanosheets/nanoparticles | liquid exfoliation-hydrothermal method | NiS/1.0 wt.% | 0.05 | 150 W Xe lamp, λ > 400 nm | quartz reactor | 10 vol.% TEOA | NiS/4.2 | [671] (2015) | |
Ni(OH)2/g-C3N4 | nanoparticles/lamellar | precipitation method | Ni(OH)2/0.5 wt.% | 0.05 | 350 W Xe arc lamp, λ > 400 nm | Pyrex reactor | 10 vol.% TEOA | Ni(OH)2/7.6 | 1.1 (420 nm) | [126] (2013) |
Ni/NiO/g-C3N4 | core-shell/lamellar | in situ immersion method | Ni/NiO/2 wt.% | 0.05 | 350 W Xe lamp, λ > 420 nm | Pyrex reactor | 10 vol.% TEOA | Ni/NiO/10 | [792] (2015) | |
NiS/CNT/mpg-C3N4 | nanoparticles/nanotubes/lamellar | sol-gel-precipitation method | NiS/1 wt.% | 0.05 | 300 W Xe lamp, λ ≥ 420 nm | quartz reactor | 10 vol.% TEOA | NiS/26.05 | [198] (2015) | |
NiS/CB/g-C3N4 | nanoparticles/lamellar | physical mixing-chemical deposition | CB/0.5 wt.%, NiS/1.0 wt.% | 0.05 | 300 W Xe lamp, λ ≥ 420 nm | quartz reactor | 15 vol.% TEOA | CB, NiS/49.6 | [156] (2015) | |
Ni(dmgH)2/g-C3N4 | sub-mircowires/lamellar | chemical deposition | Ni(dmgH)2/3.5 wt.% | 0.005 | 300 W Xe lamp, λ > 420 nm | quartz reactor | 15 vol.% TEOA | Ni(dmgH)2/1.18 | [677] (2014) | |
g-C3N4 | ultrathin nanosheets | co-polymerization-surface activation-exfoliation | Pt/3 wt.% | 0.05 | 300 W Xe arc lamp, λ > 420 nm | Pyrex reactor | 10 vol.% TEOA | Pt/28.55 | [793] (2016) | |
pm-g-C3N4 | porous | sintering | Pt/3 wt.% | 0.1 | 300 W Xe lamp, λ > 420 nm | 10 vol.% TEOA | Pt/41.7 | [159] (2017) | ||
g-C3N4 | microsphere | solvothermal method | Pt/3 wt.% | 300 W Xe lamp, λ > 420 nm | 15 vol.% TEOA | Pt/1.80 | 1.62 (420 nm) | [397] (2015) | ||
g-C3N4 | lamellar | heating acetic acid treat melamine | Pt/1 wt.% | 0.1 | 300 W Xe lamp, λ > 420 nm | Pyrex reactor | 10 vol.% TEOA | Pt/34 | [794] (2015) | |
g-C3N4 | flower-like nanorods | recrystallization method | Pt/3 wt.% | 0.05 | 300 W Xe lamp, λ > 420 nm | quartz reactor | 10 vol.% TEOA | Pt/261.8 | [795] (2015) | |
quasi-2D-C3N4 | lamellar | melamine-assisted exfoliation method | Pt/0.6 wt.% | 0.1 | 300 W Xe lamp, λ > 420 nm | 10 vol.% TEOA | Pt/89.28 | [428] (2015) | ||
mg-C3N4 | mesoporous | calcinating-dissolving method | Pt/3 wt.% | 0.05 | 300 W xenon lamp, λ > 400 nm | Pyrex reactor | 10 vol.% TEOA | Pt/272 | [796] (2015) | |
CNIC | nanotubes | molten salt | Pt/3 wt.% | 0.1 | 300 W Xe lamp, λ > 420 nm | Pyrex reactor | 10 vol.% TEOA | Pt/502 | 21.2 (420 nm) | [106] (2013) |
CNT/g-C3N4 | nanotubes/lamellar | calcinating method | CNT/2 wt.%, Pt/1.2 wt.% | 0.1 | 300 W Xe arc lamp, λ > 420 nm | 10 vol.% TEOA | Pt/39.4 | [147] (2014) | ||
MVNTs/g-C3N4 | nanotubes/particles | calcinating method | MVNTs/2.0 wt.% | 0.1 | 300 W Xe arc lamp, λ > 400 nm | quartz reactor | 25 vol.% CH3OH | MVNTs/1.15 | [681] (2012) | |
C/g-C3N4 | fiber/lamellar | electrospinning and calcinations method | Pt/1.0 wt.% | 0.05 | 350 W Xe arc lamp, λ > 420 nm | Pyrex reactor | 10 vol.% TEOA | Pt/54 | [682] (2015) | |
C/g-C3N4 | nanoparticles/rectangular nanotube | molten salt method | carbon black/0.5 wt.%, Pt/3 wt.% | 0.1 | visible light, λ > 420 nm | quartz reactor | 25 vol.% CH3OH | C, Pt/69.8 | [779] (2014) | |
C-dots/g-C3N4 | dots/lamellar | mixing-calcinating method | C-dots/0.25 wt.% | 0.05 | 4 × UV-LEDs (3 W, λ = 420 nm) | Pyrex reactor | 25 vol.% CH3OH | 8.6 | [797] (2015) | |
CQDs/g-C3N4 | quantum dots/lamellar | hydrothermal method | CQDs, Pt/3 wt.% | 0.05 | 400 W Xe lamp, λ > 420 nm | Pyrex reactor | 10 vol.% TEOA | Pt/5.805 | [680] (2016) | |
C-dots/g-C3N4 | nanodots/ultrathin nanosheets | hydrothermal method | C-dots/0.2 wt.%, Pt/0.2 wt.% | 0.05 | 300 W Xe lamp, λ > 420 nm | 5 vol.% CH3OH | Pt/88.1 | [777] (2016) | ||
C-ZIF/g-C3N4 | nanoparticles/lamellar | thermal condensation | C-ZIF/1 wt.% | 0.1 | 300 W Xe lamp, λ > 400 nm | 0 vol.% TEOA | C-ZIF/32.58 | [683] (2016) | ||
N/g-C3N4 | nanosheet | calcinating method | N-doped,Pt/3 wt.% | 0.05 | 300 W Xe lamp, λ > 420 nm | Pyrex reactor | 10 vol.% TEOA | Pt/64 | [798] (2016) | |
C3N4+x | lamellar | co-thermal condensation | N-doped,Pt/3 wt.% | 0.08 | 300 W Xe lamp, λ > 400 nm | 10 vol.% TEOA | Pt/44.28 | [799] (2015) | ||
P/g-C3N4 | flowers of in-plane mespores | calcinating method | P-doped, Pt/3 wt.% | 0.05 | 300 W Xe arc lamp, λ > 400 nm | Pyrex reactor | 10 vol.% TEOA | Pt/104.1 | [315] (2015) | |
P/g-C3N4 | lamellar | copolymerization | P-doped | 0.1 | 300 W Xe lamp, λ > 420 nm | Pyrex reactor | 10 vol.% TEOA | Pt/50.6 | [324] (2015) | |
Br/g-C3N4 | lamellar | calcinating method | Br-doped, Pt/3 wt.% | 300 W Xe lamp, λ ≥ 420 nm | 10 vol.% TEOA | Pt/48 | [800] (2016) | |||
g-C3N4 | nanosheets have a porous network | calcinating method | O-doped, Pt/3 wt.% | 0.05 | 300 W Xe lamp, λ > 420 nm | Pyrex reactor | 10 vol.% TEOA | Pt/60.2 | 7.8 (420 nm) | [321] (2015) |
S/g-C3N4 | lamellar | calcinating method | S-doped, Pt/1 wt.% | 0.1 | 300 W Xe arc lamp, λ > 400 nm | quartz reactor | 25 vol.% CH3OH | Pt/12.16 | [307] (2013) | |
K-g-C3N4 | lamellar | KCl-template method | K, Pt/0.5 wt.% | 0.01 | 300 W Xe lamp, λ > 420 nm | Pyrex reactor | 10 vol.% TEOA | Pt/102.8 | [445] (2014) | |
Zn/g-C3N4 | lamellar | calcinating method | Zn-doped, Pt/0.5 wt.% | 0.2 | 200 W Xe arc lamp, λ ≥ 420 nm | Pyrex reactor | 18.5 vol.% CH3OH | Pt/59.5 | 3.2 (420 nm) | [300] (2011) |
Co-Pi/g-C3N4 | nanoparticles/aggregated sheets | in situ photodepositions | Co–Pi, Pt/1 wt.% | 0.1 | 300 W Xe arc lamp, λ > 400 nm | quartz reactor | 25 vol.% CH3OH 0.05 M AgNO3 | Pt/19.48 | [690] (2013) | |
Au/g-C3N4 | nanoparticles/lamellar | deposition-precipitation method | Au/1 wt.% | 0.02 | 125 W Hg lamp, λ > 400 nm | 10 vol.% TEOA | Au/177.4 | [666] (2014) | ||
AuPd/g-C3N4 | nanoparticles/lamellar | chemical reduction-calcinating method | AuPd/0.5 wt.% | 0.05 | 300 W Xe arc lamp, λ ≥ 400 nm | quartz reactor | 10 vol.% TEOA | AuPd/16.3 | [194] (2015) | |
Pt-Au@ g-C3N4 | lamellar | photodeposition method | Pt, Au | 0.08 | 300 W Xe lamp, λ > 420 nm | Pyrex reactor | 6.2 vol.% TEOA | Pt-Au/17 | [350] (2015) | |
Au/PtO/g-C3N4 | nanoparticles/lamellar | photodeposition method | Au/0.33 wt.%, Pt/0.40 wt.% | 0.05 | 300 W Xe arc lamp, λ > 400 nm | Pyrex reactor | 25 vol.% CH3OH | Au/PtO/16.9 | [200] (2016) | |
CsTaWO6/Au/g-C3N4 | gathered block/nanoparticles/lamellar | impregnaton method | Au/0.5 wt.% | 0.05 | 300 W Xe lamp, | quartz reactor | 20 vol.% CH3OH | Au/0.458 | [667] (2015) | |
Cd0.5Zn0.5S/g-C3N4 | nanoparticles/lamellar | hydrothermal method | 0.1 | Xe lamp, λ > 420 nm | Pyrex reactor | 0.35 M Na2S and 0.25 M Na2SO3 | 20.8 mL/h | 37 (425 nm) | [801] (2015) | |
Cd0.2Zn0.8S/g-C3N4 | nanoparticles/lamellar | hydrothermal method | 0.05 | 300 W Xe lamp, λ > 420 nm | Pyrex reactor | 0.1 M Na2S and 0.1 M Na2SO3 | 208.8 | [802] (2015) | ||
CdLa2S4/mpg-C3N4 | particles/lamellar | hydrothermal method | 0.05 | 300 W Xe lamp, λ > 420 nm | quartz reactor | 0.1 M Na2S and 0.5 M Na2SO3 | 299.24 | 7.1 (420 nm) | [803] (2016) | |
CdS QDs/g-C3N4 | QDs/lamellar | in situ hydrothermal method | Pt/0.5 wt.% | 0.005 | 300 W Xe lamp, λ > 420 nm | quartz reactor | 0.1 M L-ascorbic acid | Pt/22.47 | 8 (420 nm) | [503] (2013) |
CdS QDs/g-C3N4 | QDs/lamellar | chemical impregnation method | Pt/1.0 wt.% | 0.1 | 300 W Xe lamp, λ > 400 nm | quartz reactor | 25% vol.% CH3OH | Pt/17.27 | [502] (2012) | |
CdS QDs/g-C3N4 | QDs/hollow | Pt/3 wt.% | 0.02 | 300 W Xe lamp, λ > 420 nm | Pyrex reactor | 10 vol.% TEOA | Pt/601 | [804] (2015) | ||
g-C3N4/CdS | particles/particles | in situ self-transformation method | Pt/1.0 wt.% | 0.05 | 350 W Xe arc lamp, λ≥420 nm | Pyrex reactor | 0.35 M Na2S and 0.25 M Na2SO3 | Pt/265.15 | 3.16 (420 nm) | [348] (2015) |
CdS/g-C3N4 | core/cell nanowires | solvothermal-chemisorption method | Pt/0.6 wt.% | 0.05 | 350 W Xe arc lamp, λ≥420 nm | Pyrex reactor | 0.35 M Na2S and 0.25 M Na2SO3 | Pt/207.6 | 4.3 (420 nm) | [562] (2013) |
NiS/CdS/g-C3N4 | nanoparticles/nanorodes/lamellar | in situ hydrothermal method | NiS/9 wt.% | 0.05 | 300 W Xe lamp, λ≥420 nm | quartz reactor | 10 vol.% TEOA | NiS/128.2 | [197] (2015) | |
Ni(OH)2/CdS/g-C3N4 | core/shell nanorodes | hydrothermal method | Ni(OH)2/4.76 | 0.001 | 300 W Xe lamp, λ > 420 nm | quartz reactor | 0.25 M Na2S and 0.35 M Na2SO3 | Ni(OH)2/115.18 | 16.7 (450 nm) | [669] (2016) |
MoS2/g-C3N4 | flower-like/lamellar | in situ light-assisted method | MoS2/2.89 wt.% | 0.01 | 300 W Xe lamp, λ > 400 nm | Pyrex reactor | 10 vol.% TEOA | MoS2/2.52 | [632] (2015) | |
MoS2/g-C3N4 | nanoparticles/lamellar | mixing-calcinating method | MoS2/0.5 wt.%, Pt/1.0 wt.% | 0.1 | 300 W Xe arc lamp, λ > 400 nm | quartz reactor | 25 vol.% CH3OH | MoS2, Pt/23.1 | [628] (2013) | |
MoS2/CN-Py | flower-like/lamellar | in suit solvothermal method | 0.05 | 300 W Xe lamp, λ > 420 nm | Pyrex reactor | 10 vol.% TEOA | 25 | [805] (2016) | ||
ZnS/g-C3N4 | microsphere/lamellar | precipitation method | 0.05 | 4 × UV-LEDs (3 W, λ = 420 nm) | Pyrex reactor | 25 vol.% CH3OH | 9.7 | [806] (2014) | ||
WS2/g-C3N4 | slabs/porous sheet-like | gas-solid reaction | WS2/0.01 wt.% | 0.05 | 300 W Xe lamp, λ > 420 nm | Pyrex reactor | 25 vol.% CH3OH | WS2/5.05 | [638] (2015) | |
WS2/mpg-C3N4 | mesoporous nanosheets | impregnation-sulfidation method | WS2/0.3 wt.% | 0.05 | 300 W Xe lamp, λ > 420 nm | Pyrex reactor | 10 vol.% TEOA | WS2/6.12 | [637] (2014) | |
CaIn2S4/g-C3N4 | nanoplate-lamellar | hydrothermal method | Pt/1.0 wt.% | 0.05 | 4 × UV-LEDs (3 W, λ = 420 nm) | Pyrex reactor | 0.5 M Na2S and 0.5 M Na2SO3 | Pt/5.1 | [807] (2015) | |
Cu/g-C3N4 | nanoparticles/lamellar | H2 reduction | Cu | 0.05 | Xe lamp, λ > 400 nm | 25 vol.% CH3OH | Cu/1.025 | 0.35 (420 nm) | [674] (2016) | |
CuO/g-C3N4 | mono-dispersed sphere/lamellar | wet impregnation-calcination method | Pt/1.0 wt.% | 0.1 | 300 W Xe lamp, λ > 420 nm | 10 vol.% TEOA | Pt/93.7 | [808] (2016) | ||
Cu(OH)2/g-C3N4 | clusters/spherical particles | precipitation method | Cu(OH)2/0.34mol% | 0.1 | 300 W Xe arc lamp, λ > 400 nm | Pyrex reactor | 25 vol.% CH3OH | Cu(OH)2/4.87 | [809] (2014) | |
CuO2/g-C3N4 | nanoparticles/lamellar | in situ reduction method | Pt/3 wt.% | 0.1 | 300 W Xe lamp, λ > 420 nm | Pyrex reactor | 10 vol.% TEOA | Pt/24.13 | [525] (2014) | |
CuO2@g-C3N4 | core@shell octahedra | solvothermal and chemisorption method | Pt/3 wt.% | 0.3 | 300 W Xe lamp, λ > 420 nm | Pyrex reactor | 10 vol.% TEOA | Pt/79.5 | [802] (2015) | |
g-C3N4/InVO4 | lamellar/nanoparticles | hydrothermal method | Pt/0.6 wt.% | 0.05 | 300 W Xe arc lamp, λ > 420 nm | 20 vol.% CH3OH | Pt/10.6 | 4.9 (420 nm) | [351] (2015) | |
ZnFe2O4/g-C3N4 | flakes/lamellar | calcinating method | Pt/1 wt.% | 0.1 | 300 W Xe arc lamp, λ > 430 nm | Pyrex reactor | 10 vol.% TEOA | Pt/20 | [439] (2014) | |
CuFe2O4/g-C3N4 | nanoparticles/lamellar | calcinating method | Pt/3 wt.% | 0.1 | 300 W Xe lamp, 680 nm >λ > 420 nm | quartz reactor | 10 vol.% TEOA | Pt/76 | [810] (2016) | |
FeOX/g-C3N4 | granular-like | calcinating method | Pt/3 wt.%, FeOX(20 g urea/100 mg ferrocene) | 0.1 | 300 W Xe lamp, 780 nm > λ > 420 nm | quartz reactor | 10 vol.% TEOA | FeOX, Pt/108 | [811] (2016) | |
N-CeOx/g-C3N4 | nanoparticles/lamellar | one-pot annealing method | Pt/1 wt.% | 0.05 | 300 W Xe lamp, λ > 420 nm | Pyrex reactor | 10 vol.% TEOA | Pt/14.62 | [812] (2015) | |
Ag2O/g-C3N4 | nanaosheets | hydrothermal method | 0.01 | 300 W Xe lamp, λ > 420 nm | Pyrex reactor | 10 vol.% TEOA | 33.04 | [813] (2015) | ||
Ag2S/g-C3N4 | particle/mesopores sheets | precipitation method | 0.05 | 4 × UV-LEDs (3 W, λ = 420 nm) | Pyrex reactor | 20 vol.% CH3OH | 10 | [665] (2014) | ||
TiO2/g-C3N4 | yolk-shell spheres/lamellar | solvothermal method | 0.05 | 4 × UV-LEDs (3 W, λ = 420 nm) | Pyrex reactor | 25 vol.% CH3OH | 5.6 | [814] (2016) | ||
g-C3N4/B-TiO2 | two removed the top of the pyramids | solvothermal method | 0.1 | 300 W Xe arc lamp, λ > 400 nm | Pyrex reactor | 25 vol.% CH3OH | 47.3 | [815] (2016) | ||
CoTiO3/g-C3N4 | rods/lamellar | a facial in situ growth method | Pt/3 wt.% | 0.02 | 300 W Xe lamp | quartz reactor | 10 vol.% C2H5OH | Pt/11.7 | 38.4 (365 nm) | [816] (2016) |
C, N–TiO2/g-C3N4 | nanoparticles/ultrathin nanosheets | one-pot solvothermal method | 0.1 | 300 W Xe lamp, λ > 400 nm | 10 vol.% TEOA | 3.92 | [817] (2015) | |||
N,S–TiO2/g-C3N4 | semi-spherical nanoparticles/lamellar | in situ thermal induced polymerization method | 0.05 | 125 W medium pressure Hg lamp, λ ≥ 400 nm | 10 vol.% TEOA | 317 | [818] (2015) | |||
g-C3N4/Pt–TiO2 | nanoparticles/nanoparticles/lamellar | chemical adsorption-calcinating method | 1% wt.%Pt/TiO2 | 0.1 | 300 W Xe lamp, λ ≥ 420 nm | Pyrex reactor | 10 vol.% TEOA | Pt/178 | [819] (2012) | |
g-C3N4/MoS2/TiO2 | lamellar/nanosheets/nanorodes | hydrothermal method | 0.1 | 300 W Xe arc lamp, λ > 400 nm | Pyrex reactor | 25 vol.% CH3OH | 125 | [820] (2016) | ||
C3N4/rGO/WO3 | nanosheets/nanosheets/nanoparticles | hydrothermal method | Pt/1 wt.% | 0.02 | 250 W iron doped metal halid UV–vis lamp (λ > 420 nm) | Pyrex reactor | deionized water | Pt/2.84 | 0.9 (420 nm) | [821] (2015) |
W18O49/g-C3N4 | nanowires/lamellar | solvothermal method | Pt/3 wt.% | 0.005 | 300 W Xe arc lamp, λ > 420 nm | quartz reactor | 10 vol.% TEOA | Pt/3.69 | 1.79 (400 nm) | [822] (2016) |
Co-Pi: cobalt-phosphate; MVNTS: multi-walled carbon nanotubes; py:pyridine; CNIC: carbon nitride intercalation compound; C-ZIF: ZIF-8 derived.
As observed in Table 7, it is clear that the high H2-evolution activity over nanostructured g-C3N4-based semiconductors is generally obtained via loading the shape-dependent noble-metal Pt nanoparticles as co-catalysts [315], [321], [348], [445], [562], [809], [819], [823]. In the pioneering work, Wang and co-workers found that the loading 3 wt% Pt as co-catalysts on g-C3N4 could achieve the H2-evolvtion amount of 770 μmol after 72 h in the TEOA aqueous solution, whereas the bare g-C3N4 exhibits the negligible H2-production activity. This is due to the rapid recombination of CB electrons and VB holes, and the large H2-evolution overpotential over pure g-C3N4. Subsquently, various engineering strategies, highlighted in the section 4, have been employed to enhance the H2 evolution activity over the g-C3N4/Pt systems [369], [395], [396]. For example, Wang and coworkers demonstrated that nanospherical g-C3N4 frameworks, with interconnecting nanosheets and highly open-up spherical surfaces with sharp edges, achieved an AQY of 9.6% at 420 nm for H2 evolution [369]. More recently, Cao et al. also demonstrated that the spherical Pt nanoparticles on g-C3N4 exhibiter much better H2-evolution activity than those of octahedral and cubic Pt nanoparticles on g-C3N4, due to its favorable exposed facets, disparity and adsorption energies induced by the spherical shape (as shown in Fig. 44) [823]. More interestingly, it was also demonstrated that the single-atom Pt co-catalyst on g-C3N4 exhibited a 8.6-fold enhancement in photocatalytic H2-evolution activity, as compared to that of Pt nanoparticles-loaded g-C3N4, due to the maximized atom efficiency and improved surface trap states [664]. Unfortunately, their high cost and low stability significantly limit their practical application in a large scale. Therefore, design and preparation of advanced alternative co-catalysts with excellent performance and low cost are of great importance for the development of these photocatalytic systems. Accordingly, the earth-abundant first-row transition metal electrocatalysts, including Fe-, Co-, Ni- and Cu-based co-catalysts have been of increasing interest in both electrocatalysis and photocatalysis in the recent years [124], [272], [273], [274], [824]. Importantly, loading suitable earth-abundant co-catalysts onto the g-C3N4 provides a facile and strategy to construct real robust g-C3N4-based H2-evolution photocatalytic systems. Here, we will highlight the unique Ni-based co-catalysts for enhancing the activity of the g-C3N4 semiconductor.
To date, various kinds of Ni-based co-catalysts, including Ni [192], [675], [676], [825], Ni(OH)x [126], [155], [669], NiSx [125], [156], [197], [198], [670], [672], [673], [Ni(TEOA)2] Cl2 [679], NiOx [734], [792], and Ni(dmgH)2 [677], [826] have been widely employed as co-catalysts to accelerated the photocatalytic activity of the g-C3N4 for different applications [272]. For example, Yu et al. demonstrated that the as-fabricated 0.5 mol% Ni(OH)2-modified g-C3N4 composite photocatalysts exhibited the highest H2-production rate of 7.6 μmol h−1 (with an AQE of 1.1% at 420 nm, as shown in Fig. 45a), approaching that of optimal 1.0 wt% Pt/g-C3N4 (8.2 μmol h−1), due to the promoted charge separation and surface reaction rate induced by the loading of Ni(OH)2 (as shown in Fig. 45b) [126]. This work perfectly highlighted the promising utilization of low cost Ni(OH)2 as a substitute for noble metals (such as Pt) in the photocatalytic H2 production for g-C3N4. Following this work, Li and co-workers recently demonstrated that the dual electron co-catalysts of robust acetylene black and Ni(OH)2 could significantly enhance the photocatalytic H2-evolution activity over g-C3N4/Ni (OH)2 hybrid systems. When inserting 0.5% acetylene black into the interface regions between Ni(OH)2 and g-C3N4, a 3.31 time enhancement in H2-evolution activity can be thus achieved. It is suggested that the enhanced activity can be attributed to the effectively promoted separation of photo-generated electron-hole pairs and enhanced the following H2-evolution kinetics [155]. Especially, various cheap nanoconbons should be fully integrated with robust co-catalysts and g-C3N4 to maximize the functions of co-catalyst and g-C3N4 semiconductor.
In addition, several other highly efficient ternary g-C3N4-based photocatalysts have also been developed recently, For instance, Li and co-workers confirmed that the CdS photosensitizer and NiS co-catalysts in ternary g-C3N4-CdS-NiS composites play key roles in boosting the H2-generation acticity of g-C3N4 under visible-light illumination [197]. The highest H2-production activity of 2563 μmolg−1 h−1 is about 1528 times as high as that of the pure g-C3N4 (Fig. 46). In another work, when inserting carbon black into the interfaces between g-C3N4 and NiS, the average H2-evolution rate of g-C3N4/0.5%CB/1.5%NiS can reach 992 μmol g−1 h−1, which is about 2.51-fold higher than that of g-C3N4/1.5%NiS with a photocatalytic H2 production rate of 395 μmol g−1 h−1 (Fig. 47) [156]. In another paper by Li and co-workers, mpg-C3N4/CNT/NiS was synthesized via the sol-gel method and the direct precipitation process [198]. The results show that the mpg-C3N4/CNT/NiS composite exhibits the highest H2-evolution rate of 521 μmol g−1 h−1, which is about 148 times as high as that of mpg-C3N4/CNT. These results highlighted that the ternary hybrid should be a promising direction for constructing highly efficient earth-abundant g-C3N4-based composite H2-evolution photocatalysts.
However, there are few attempt reported to reveal the nature and structural features of the Ni co-catalyst as well as the possible structural and bonding situation occurring at the co-catalyst active sites under light irradiation [676]. To achieve this aim, in situ EPR measurements (Fig. 48a) confirmed that a continuous increase of the Ni0 signal was detected (though, not all Ni2+ species were reduced to Ni0 at the same time), indicating the Ni0 acts as the HER active sites during the photocatalysis (Fig. 48b) [676]. For other Ni species, although the electrochemical performances, including polarization curves, time dependence of the current density, and electrochemical impedance spectroscopy of g-C3N4 have been widely determined [197], the accurate catalytic mechanism still needs to be uncovered in future. Therefore, both the deep mechanism studies and continuous efforts in developing new co-catalysts are highly expected, to achieve the rational design of highly efficient Ni-based co-catalysts modified g-C3N4 photocatalysts for practical applications.
Finally, it should be pointed out that the oxidation half reaction of g-C3N4 should be paid more attention, which is crucial for improving the photocatalytic H2-evolution activity and deeply understand the underlying mechanism for water splitting [444], [685], [686], [687]. Clearly, constructing the practical g-C3N4-based overall water splitting systems is still very challenging. Recently, Kang and coworkers demonstrated that the metal-free carbon nanodots–carbon nitride nanocomposite exhibited the impressive performance for photocatalytic solar water splitting [774]. The measured quantum efficiencies of 16% (420), 6.29% (580 nm), and 4.42% (600 nm), correspond to an overall solar energy conversion efficiency of 2.0%. It was verified that carbon dots-C3N4 catalyzes water splitting to hydrogen and oxygen via the stepwise two-electron/two-electron two-step pathway under visible light irradiation. The composite nature of the catalyst provides sufficient proximity between the H2O2 generation sites on the C3N4 surface and the carbon dots so that H2O2 decomposition and O2 generation in the second stage become efficient [774]. This work demonstrated that the control of oxidation half reaction played key roles in achieving the overall water splitting. More recently, Wang and coworkers discovered the irradiated g-C3N4 loaded by Pt, PtOx, and CoOx as redox cocatalysts, can split pure water without the use of sacrificial reagents, while pure g-C3N4 is virtually inactive for overall water splitting by photocatalysis [827]. In addition, the liquid state Z-scheme systems, including the g-C3N4 (3 wt% Pt)/WO3 (0.5 wt% Pt)/NaI (5 mM) and g-C3N4 (3 wt% Pt)/BiVO4/FeCl2(2 mM), have proven to achieve the overall water splitting [828]. However, in these systems, the Pt and PtOx are still the noble metal. Thus, it is clear that developments of low-cost kinetic promoters for both H2 and O2 evolution are still the major bottleneck for achieving the practical H2 evolution or overall water splitting on g-C3N4.
5.2. Photocatalytic degradation of pollutants
With rapid growth of population and accelerating industrialization, the environmental contamination has become a major threat to public health all over the world. Since the first report on heterogeneous photocatalytic remediation of environmental pollutants (CN− in water) on titania by Frank and Bard in 1977 [829], the heterogeneous photocatalysis has been widely used in widespread environmental purification such as air and water purification [309], [830], [831], [832], [833], [834], [835], [836], [837], [838], [839]. Furthermore, a variety of ways for increasing the photodecomposition efficiency of pollutants over g-C3N4-based semiconductors have been exploited [9], [11], [20], [840], which have been summarized in Table 8.
Photocatalyst | Structure | Synthetic method | Co-catal./optimized mass ratio | Mass (g) | Light source | Targe pollutant/concentration/volume | Degradation time/efficiency | Kapp [10−2 min−1] | Main active species | Ref. (year) |
---|---|---|---|---|---|---|---|---|---|---|
In2O3/g-C3N4 | particles/lamellar | calcinating method | 0.1 | 300 W halogen tungsten lamp, λ > ≥ 400 nm | RhB/1 × 10−5 M/100 mL | 40 min/99% | O2−, OH | [841] (2014) | ||
TiO2–In2O3@g-C3N4 | nanoparticles/nanoparticles- lamellar | solvothermal method | 0.08 | 350 W xenon arc lamp, λ > 420 nm | RhB/0.01 g L−1/80 mL | 60 min/100% | 4.6 | OH | [797] (2015) | |
CaIn2S4/g-C3N4 | nanoplate/lamellar | hydrothermal method | 0.05 | 500 W tungsten lamp, λ > 400 nm | MO/0.01 g L−1/100 mL | 120 min/90% | O2−, h+ | [807] (2015) | ||
SiO2/g-C3N4 | core-shell nanosphere | calcinating method | 0.07 | 300 W xenon lamp, λ ≥ 400 nm | RhB/0.01 g L−1/70 mL | 150 min/94.3% | OH, h+ | [842] (2015) | ||
g-C3N4/TiO2 | lamellar/particles | sol-gel method | 0.1 | 500 W xenon lamp, λ > 420 nm | MB/0.01 g L−1/100 mL | 360 min/92% | h+ | [843] (2016) | ||
TiO2/g-C3N4 | core-cell | self-assembly method | 0.05 | 500 W xenon lamp, UV–vis light | RhB/10 ppm/200 mL | 80 min/82% | 4.4 | [844] (2015) | ||
TiO2/g-C3N4 | mesoporous spheres/lamellar | melt-infiltrating-calcinating method | 0.05 | 500 W xenon lamp, λ > 400 nm | RhB/0.01 g L−1/50 mL | 140 min/100% | [845] (2016) | |||
TiO2/g-C3N4 | yolk-shell spheres/lamellar | solvothermal method | 0.01 | 350 W xenon arc lamp, λ > 420 nm | RhB/0.01 g L−1/80 mL | 150 min/99.3% | 33.5 | O2−, h+ | [814] (2016) | |
g-C3N4/TiO2 | lamellar/nanoparticles | calcinating method | 0.1 | 2 × 150 W tungsten lamp, λ > 420 nm | ciprofloxacin//0.01 g L−1/100 mL | 60 min/95% | OH, h+, e− | [846] (2015) | ||
g-C3N4/TiO2 | lamellar/roundish particles | sol-gel method | 0.04 | 400 W halide lamp, λ > 400 nm | RhB/0.01 g L−1/40 mL | 150 min/100% | h+ | [436] (2014) | ||
g-C3N4/TiO2 | hydrothermal-calcination method | 0.03 | 300 W xenon lamp, λ > 420 nm | acyclovir/10 ppm/100 mL | 90 min/100% | 1.57 | h+ | [847] (2016) | ||
g-C3N4/F-TiO2 | lamellar | hydrothermal method | 0.1 | 50 W LED light, λ = 410 nm | MB/0.01 g L−1/100 mL | 60 min/89% | 3.74 | [655] (2014) | ||
g-C3N4/Ag/TiO2 | lamellar-particles/microspheres | photodeposition-Physical mixing method | Ag/2 wt.% | 0.03 | 300 W xenon lamp, λ > 420 nm | MO/0.0135 g L−1/30 mL phenol/0.0166 g L−1/30 mL | 240 min/94% 240 min/96% | [848] (2014) | ||
K-Na/g-C3N4 | lamellar | calcinating method | K-Na co-doped | 0.05 | 250 W high-pressure sodium lamp, 420 < λ < 800 nm | RhB/1 × 10−5 M/200 mL | 120 min/90% | 0.86 | O2−, OH | [336] (2015) |
mg-C3N4 | mesopores | calcinating-dissolving method | 0.05 | 300 W xenon lamp, λ > 420 nm | RhB/0.005 g L−1/100 mL | 20 min/95% | [849] (2015) | |||
g-C3N4 | lamellar | thermal condensation | 0.02 | 300 W xenon lamp, λ > 420 nm | 2,4,6-TCP/10−4M/20 mL | 180 min/100% | 1.17 | O2−, OOH | [850] (2013) | |
mg-C3N4 | mesoporous | calcinating dissolving method | 0.05 | 300 W xenon lamp, λ > 400 nm | MO/0.01 g L−1/100 mL | 120 min/90% | 15.13 | [796] (2015) | ||
C60/mg-C3N4 | particles/lamellar | calcinating method | C60/0.03 wt.% | 0.025 | 500 W xenon lamp, λ > 420 nm | MB/1 × 10−5 M/50 mL phenol/5 × 10−6 M/50 mL | 300 min/100% 300 min/46% | 1.73 0.15 | OH, h+ | [753] (2014) |
CDs/g-C3N4 | dots/lamellar | mixing-calcinating method | carbon dots/0.5 wt.% | 0.05 | 300 W xenon lamp, λ > 420 nm | phenol/0.01 g L−1/50 mL | 200 min/100% | [851] (2016) | ||
C-dots/g-C3N4 | dots/lamellar | mixing-calcinating method | C-dots/0.25 wt.% | 0.1 | 3 W LED lamp, λ = 365 ± 5 nm | RhB/1 × 10−5 M/100 mL | 60 min/84% | 1.3 | O2− | [745] (2016) |
Pd/mpg-C3N4 | particles/mesoporous | chemical reduction | Pd-doped/1.5 wt.% | 350 W xenon lamp, λ > 420 nm | BPA/0.02 g L−1/50 mL | 360 min/100% | 1 | O2− | [729] (2013) | |
Pt/C3N4 | nanoparticles/nanotubes | hydrothermal method | Pt/2 wt.% | 0.1 | 300 W xenon lamp, λ > 420 nm | PCP/0.02 g L−1/100 mL | 420 min/98% | [852] (2015) | ||
Au/Pt/g-C3N4 | particles/lamellar | photodeposition method | Au/2 wt.%, Pt/0.5 wt.% | 0.1 | 500 W xenon lamp, λ > 400 nm | TC-HCl/0.02 g L−1/100 mL | 180 min/93% | 42.86 | [481] (2015) | |
Au/g-C3N4/α-Fe2O3 | particles/lamellar/rhombohedral-like | hydrothermal and ultrasonication method | Au | 0.05 | halogen lamp | RhB/0.004 g L−1/250 mL | 3.16 | [853] (2015) | ||
ZnO@mpg-C3N4 | core-shell | physically mixing method | 0.025 0.05 | 500 W xenon lamp, λ > 420 nm 500 W xenon lamp, λ = 254 nm | MB/1 × 10−5 M/50 mL MB/1 × 10−5 M/100 mL | 150 min/100% – | 2.05 13.5 | – | [533] (2014) | |
ZnO/g-C3N4 | flowerlike/lamellar | hydrothermal method | 0.02 | mild UV light | MB/1 × 10−6 M/30 mL | 90 min/98.3% | [854] (2015) | |||
g-C3N4/ZnO/AgCl | lamellar/particles | precipitation and refluxing method | 0.1 | 50 W LED lamp | RhB/2.5 × 10−5M/250 mL | 270 min/99% | 1.54 | OH | [535] (2015) | |
Zn2SnO4/g-C3N4 | particles/lamellar | mixing-calcinating method | 0.2 | 300 W xenon lamp, 420 < λ < 800 nm | RhB/0.01 g L−1/100 mL | 120 min/97% | 3.8 | O2−, h+ | [855] (2015) | |
V2O5/g-C3N4 | particles/lamellar | mixing-calcinating method | 0.05 | 250 W xenon lamp, λ > 420 nm | RhB/0.01 g L−1/100 mL | 60 min/95.5% | 4.91 | ·O2−, h+ | [616] (2016) | |
g-C3N4/V2O5 | lamellar/nanoparticles | one-pot method | 0.05 | 200 W xenon lamp | RhB/0.01 g L−1/50 mL | 80 min/100% | [856] (2015) | |||
GdVO4/g-C3N4 | small particles/big particles | mixing-calcinating method | 0.3 | 350 W xenon lamp, λ > 420 nm | RhB/0.01 g L−1/300 mL | 120 min/96.7% | 4.34 | O2−, h+ | [857] (2013) | |
g-C3N4/SmVO4 | lamellar/aggregated particles | mixing-calcinating method | 0.3 | 350 W xenon lamp, λ > 420 nm | RhB/0.01 g L−1/300 mL | 120 min/100% | 3.45 | [858] (2013) | ||
g-C3N4/SnO2 | lamellar/nanoparticles | mixing-calcinating method | 0.1 | 300 W xenon lamp, λ > 420 nm | MO/10 ppm/100 mL | 180 min/73% | 0.78 | [859] (2014) | ||
SnNb2O6/g-C3N4 | nanosheets/lamellar | mixing-calcinating method | 0.02 | 500 W tungsten light lamp, λ > 420 nm | MB/0.01 g L−1/80 mL | 240 min/99% | 1.73 | O2−, h+ | [640] (2016) | |
Ag/g-C3N4 | particles/lamellar | photodeposition method | Ag/2 wt.% | 0.1 | 300 W xenon lamp, 400 < λ < 680 nm | MO/0.01 g L−1/100 mL PNP/0.01 g L−1/100 mL | 120 min/100% 120 min/100% | – O2−, h+ | [489] (2013) | |
AgI@ g-C3N4 | core-shell | precipitation method | 0.1 | 250 W halide lamp, λ > 420 nm | RhB/0.01 g L−1/100 mL | 120 min/96% | 2.52 | O2−, h+, OH | [860] (2015) | |
AgCl@pg-C3N4 | nanoparticles/lamellar | modified precipitation method | 0.2 | 300 W xenon lamp, λ > 420 nm | atrazine/100 ppm/500 mL | 60 min/99% | [861] (2015) | |||
Ag3VO4/g-C3N4 | particles/lamellar | deposition-precipitation method | 0.05 | 400 W metal halide lamp, λ > 420 nm | RhB/0.005 g L−1/100 mL | 100 min/99.5% | 5.56 | O2−, h+, OH | [862] (2015) | |
g-C3N4/Ag2O | particles/lamellar | precipitation method | 0.02 | 300 W xenon lamp, λ ≥ 400 nm | MO/0.02 g L−1/50 mL phenol/0.02 g L−1/50 mL | 30 min/90% 180 min/82% | [728] (2013) | |||
Ag2O/g-C3N4 | nanoparticles/lamellar | coprecipitation method | 0.1 | 300 W xenon lamp, λ ≥ 400 nm | RhB/0.005 g L−1/100 mL | 40 min/100% | 10.53 | O2−, h+ | [726] (2014) | |
Ag@g-C3N4 | core-shell | refluxing method | Ag/0.5 wt.% | 0.025 | 500 W xenon arc lamp, λ > 420 nm | MB/1 × 10−5 M/50 mL | 1.4 | h+ | [488] (2014) | |
Ag@AgCl/g-C3N4 | nanoparticles/lamellar | in situ ion exchange method | 0.025 | 300 W xenon lamp, 400 < λ < 700 nm | RhB/0.01 g L−1/100 mL | 30 min/100% | 19.54 | O2−, h+ | [433] (2014) | |
AgVO3/g-C3N4 | nanoribbons/ultrathin nanosheets | in situ hydrothermal method | 0.05 | 500 W xenon lamp, λ > 420 nm | BF/0.02 g L−1/50 mL Bisphenol/0.02 g L−1/50 mL | 90 min/100% 90 min/85% | 5.52 2.08 | OH, h+ OH, h+ | [863] (2015) | |
Ag3PO4/g-C3N4 | spherical particles/lamellar | chemisorptions method | 0.02 | 500 W xenon lamp, λ > 400 nm | MO/0.02 g L−1/40 mL | 60 min/100% | 4 | h+ | [864] (2014) | |
Ag/Fe3O4/g-C3N4 | particles/nanoclusters/lamellar | hydrothermal-photodeposition method | Ag/3 wt.%, Fe3O4/9 wt.% | 0.05 | 300 W xenon lamp, λ > 420 nm | tetracycline/0.02 g L−1/100 mL | 90 min/88% | 2.18 | O2−, h+ | [865] (2016) |
g-C3N4/CdS | lamellar/nanotube | precipitation method | 0.08 | 500 W xenon lamp, λ > 420 nm | MB/0.025 g L−1/200 mL | 180 min/90.45% | OH, h+ | [506] (2014) | ||
CdS QDs/npg-C3N4 | QDs/lamellar | mixing-calcinating method | 0.1 | 500 W tungsten light lamp, λ > 400 nm | RhB/0.01 g L−1/100 mL | 90 min/88.2% | O2−, h+ | [866] (2016) | ||
CdWO4/g-C3N4 | nanorods/lamellar | mixing-calcinating method | 0.05 | 500 W xenon lamp, λ ≥ 400 nm | RhB/1 × 10−5 M/50 mL | 240 min/46% | 0.27 | OH, h+ | [867] (2015) | |
Cd0.2Zn0.8S/g-C3N4 | nanoparticles/lamellar | hydrothermal method | 0.05 | 500 W xenon lamp, λ > 420 nm | RhB/0.01 g L−1/50 mL phenol/0.01 g L−1/50 mL | 80 min/95.8% 180 min/76.1% | [802] (2015) | |||
BiPO4/mg-C3N4 | nanorods/mesoporous | in situ method | 0.1 | 300 W xenon arc lamp, λ > 420 nm | MO/0.02 g L−1/100 mL | 120 min/100% | 0.77 | [868] (2014) | ||
BiOCl/C3N4 | flowerlike/amorphous | solvothermal method | 0.2 | 300 W xenon arc lamp, λ > 400 nm | MO/0.01 g L−1/500 mL | 80 min/95% | h+ | [869] (2013) | ||
g-C3N4/BiOBr | lamellar/nanosheets | deposition-precipitation method | 0.02 0.04 | 400 W halogen lamp, λ > 400 nm | RhB/0.06 g L−1/40 mL 2,4-DCP/0.01 g L−1/40 mL | 100 min/98% 180 min/80% | 4.01 3.91 | h+ OH | [870] (2013) | |
BiOxIy/g-C3N4 | square thin-plates/lamellar | controlled hydrothermal | 0.01 | 100 W xenon arc lamp | CV/10 ppm/100 mL | 36 h/99% | 0.283 | O2− | [871] (2016) | |
g-C3N4/Bi2O2CO3 | lamellar/plates | mixing-calcinating method | 0.05 | 1000 W xenon lamp, λ > 420 nm | RhB/1 × 10−5 M/50 mL | 240 min/56% | 17 | O2−, h+ | [872] (2014) | |
g-C3N4/Bi2WO6 | lamellar/nanosheets | hydrothermal method | 0.4 | 300 W xenon lamp, λ > 400 nm | MO/0.01 g L−1/200 mL | 180 min/94.82% | 1.66 | [580] (2014) | ||
Bi2WO6 QDS/g-C3N4 | QDS-lamellar | in situ method | 0.05 | 300 W xenon lamp, λ ≥ 400 nm | RhB/0.01 g L−1/50 mL | 30 min/100% | 16.8 | O2−, h+, OH | [505] (2015) | |
g-C3N4/Bi2WO6 | agglomeration | mixing-calcinating method | 0.15 | 500 W xenon lamp, λ > 420 nm | MO/0.01 g L−1/50 mL | 180 min/99.9% | 3.66 | [576] (2011) | ||
g-C3N4/Bi2MoO6 | lamellar-hollow sphere | hydrothermal method | 0.02 | 400 W metal halide lamp, λ ≥ 420 nm | RhB/0.01 g L−1/50 mL | 70 min/98% | 5.15 | O2− | [873] (2014) | |
g-C3N4/Bi2WO6 | lamellar-flakelike | hydrothermal method | 0.1 | 400 W xenon lamp, λ > 400 nm | MO/0.005 g L−1/100 mL 2,4-DCP/0.02 g L−1/100 mL | 120 min/93% 300 min/92% | – 0.856 | O2− | [874] (2013) | |
g-C3N4/Bi4Ti3O12 | lamellar/particles | ball milling | 2 g L−1 | 500 W xenon lamp, λ > 420 nm | acid orange II/0.05 g L−1 | 4.1 | O2−, h+ | [875] (2016) | ||
HSbO3/g-C3N4 | small particles/big particles | mixing-calcinating method | 0.3 | 400 W halogen-tungsten lamp, λ > 420 nm | RhB/0.015 g L−1/100 mL | 240 min/90% | 1.41 | O2− | [876] (2015) | |
CuTCPP/g-C3N4 | rodlike/lamellar | ethanol dispersion method | 0.025 | 500 W xenon lamp, λ > 420 nm | phenol/5 ppm/50 mL | 0.04 | O2−, h+ | [877] (2015) | ||
g-C3N4/CuOx | lamellar/nanoparticles | mixed solvent-thermal method | 0.01 | 350 W xenon lamp | MO/0.02 g L−1/50 mL | 70 min/83.3% | 2.05 | O2−, h+ | [878] (2016) | |
β-Fe2O3/g-C3N4 | nanoparticles | in situ growth strategy method | 0.2 | 300 W xenon arc lamp, λ > 420 nm | MO/0.01 g L−1/160 mL | 240 min/86% | 0.88 | O2−, OH | [587] (2016) | |
[WO4]2−/g-C3N4 | lamellar | calcinating method | [WO4]2—doped | 0.1 | 300 W xenon lamp, λ > 420 nm | RhB/10 ppm/100 mL | 90 min/87% | 2.2 | O2−, OH | [879] (2015) |
WO3/g-C3N4 | nanorods/lamellar | mixing-calcinating method | 0.05 | 500 W tungsten lamp | RhB/0.005 g L−1/100 mL | 90 min/91% | 2.61 | [544] (2015) | ||
H3PW12O40/C3N4 NTs | particles/tubular | hydrothermal method | 0.2 | 300 W xenon lamp, λ > 420 nm | MO/0.01 g L−1/100 mL DEP/0.01 g L−1/100 mL | 240 min/99% 1440 min/85% | [880] (2014) | |||
g-C3N4/MoO3 | lamellar/broader particles | mixing-calcinating method | 0.1 | 300 W xenon lamp, λ > 400 nm | MB/0.01 g L−1/100 mL | 180 min/93% | 1.47 | [881] (2013) | ||
g-C3N4/Bi2MoO6 | lamellar/nanoparticles | solvothermal method | 0.03 | 50 W LED light, λ = 410 nm | MB/0.01 g L−1/30 mL | 40 min/90% | 6.88 | [604] (2015) | ||
MoS2/g-C3N4 | nanosheets/lamellar | impregnation and calcinating method | 0.04 | 300 W xenon lamp, λ > 400 nm | RhB/0.01 g L−1/50 mL MO/0.01 g L−1/50 mL | 20 min/96% 180 min/95% | 15.2 1.61 | h+ h+ | [633] (2016) | |
Ce/g-C3N4 | lamellar | calcinating method | Ce-doped | 0.05 | 250 W high-pressure sodium lamp, 400 < λ < 800 nm | RhB/10 ppm/200 mL | 120 min/90% | 1.55 | O2− | [882] (2015) |
CoO4/g-C3N4 | lamellar | mixing and heating method | CoO4-doped/0.2 wt.% | 0.1 | 250 W xenon lamp, λ > 420 nm | MO/0.01 g L−1/100 mL | 120 min/100% | O2− | [688] (2014) | |
NiO/g-C3N4 | nanoparticles/lamellar | calcinating method | 0.05 | 500 W xenon lamp, λ > 420 nm | MB/0.005 g L−1/100 mL | 40 min/100% | 5.1 | [734] (2014) |
As observed in Table 8, constructing the g-C3N4-based semiconductor heterojunction and loading suitable O2-reduction co-catalysts are the general two strategies to achieve the improved photocatalytic degradation activity, which will be discussed in this section. It is clear that the OH radicals in g-C3N4-based photocatalysts mainly originated from multi-electron O2 reduction reactions driven by photo-generated electrons on the CB of g-C3N4, because the photo-generated holes exhibited much negative potentials (1.4 V) than that of OH/OH− (+2.29 V, vs NHE, pH = 7), leading to the failure in driving the oxidation reaction of adsorbed OH− groups to OH radicals [83], [410]. Thus, for improving the photocatalytic activity of g-C3N4, more efforts have been devoted to strengthening the decisive O2-reduction reactions.
On the one hand, the photocatalytic degradation of gas-phase pollutants over g-C3N4-based photocatalysts has been extensively investigated, such as NOx [144], [883], [884], formaldehyde [523], acetaldehyde [539], [885] and so forth. For example, Dong and coworkers deposited the monodispersed plasmonic Ag nanoparticles onto g-C3N4 nanosheets to extend visible-light absorption, increase the generation of O2− and enhance the charge separation, thus achieving the enhanced the photocatalytic activity of g-C3N4 nanosheets towards oxidation of NO to final products [884]. Katsumata et al. demonstrated that WO3/g-C3N4 heterojunction photocatalysts showed a 1.4 times enhancement in photodegradation of acetaldehyde gas, as compared to pristine g-C3N4 [539]. In another paper by Yu et al., a direct TiO2/g-C3N4 Z-scheme photocatalyst without an electron mediator (as shown in Fig. 38) exhibited a high photocatalytic performance in the oxidation decomposition of formaldehyde in air [523]. In these studies, the enhancement in photoactivity was primarily accredited to the improved transfer and separation of photogenerated charge carriers and promoted O2 reduction. A mechanically mixed g-C3N4 and TiO2 sample with similar content did not remarkably improve the conversion of NOx, thus confirming that the interaction between g-C3N4 and P25 is vital for the enhanced activity. EPR measurements once again indicated that O2− was the main active species involved in the oxidation of NO under both visible and UV light irradiation [886]. In addition, it should be noted that the adsorption of gas-phase pollutants on the g-C3N4 should be carefully optimized to achieve the ideal photocatalytic degradation efficiency.
On the other hand, the g-C3N4-based photocatalysts have been widely used in the photocatalytic degradation of liquid-phase pollutants, such as MB [319], [506], [734], [843], [887], [888], [889], [890], [891], [892], [893], MO [314], [330], [576], [580], [617], [688], [894], [895], [896], [897], [898], RhB [324], [330], [742], [857], [858], [872], [899], [900], [901], [902], [903], [904], [905], [906] and so forth. As observed in Table 8, in pure g-C3N4-based visible-light systems, the two main reactive species, O2− and h+ species, are generally involved in the degradation of pollutants. Interestingly, besides the aforementioned two species, the presence of OH radicals in Ag/g-C3N4 systems further provides the direct evidence for its increased catalytic performance [489]. The further enhanced photocatalytic activity was observed over the Z-scheme Ag@AgBr/g-C3N4 plasmonic photocatalyst. It is believed that the Z-scheme system retained the photoinduced electrons and hole with strong reduction and oxidation power in the CB of g-C3N4 and VB of AgBr, respectively, thus achieving the high efficient photodegradation of MB. During the photocatalysis, the generated Br0 atoms and superoxide radicals with high oxidizing capabilities are favorable for the further enhancement in the degradation reaction [486].
Furthermore, it is also noted that the g-C3N4-based heterojunctions are widely used in the photocatalytic degradation, whereas, few systems were further loaded by co-catalysts. For those systems loaded by co-catalysts, the expensive Ag- and Pt-based co-catalysts are widely chosen. At this point, the atomically dispersed noble-metal co-catalysts with much stronger metal-g-C3N4 interactions are considerably promising for the applications of photodegradation [664], [694], [907]. Therefore, it is expected that more and more earth-abundant co-catalysts and other modification strategies can be applied in the photocatalyctic degradation of liquid-phase pollutants. The homogeneous molecular systems [908] and metal-ion clusters (such as Fe(III) and Cu (II)) [909], [910], [911], [912], [913], [914], [915], [916] with the maximum atom utilization efficiency are highly appealing as co-catalysts for applications in the photodegradation of pollutants over g-C3N4-based photocatalysts. What’s more, the deep investigation on the degradation mechanism is also extremely expected. Similar to TiO2-based semiconductors, particular attention should be focused on the investigations on the tunable photocatalytic selectivity of g-C3N4-based photocatalysts towards decomposition of pollutants with positive/negative charge carriers through precisely controlling the surface charge properties of g-C3N4 [217], [218], [219]. In addition, the interesting photocatalytic degradation of antibiotics over g-C3N4-based multi-junctions and deep mechanisms also deserve more attention in the near future [481], [744], [917], [918], [919].
5.3. Photocatalytic carbon dioxide reduction
From the viewpoint of development of sustainable energy, conversion of the rapidly increasing greenhouse gases to valuable energy-bearing compounds (such as CO, methane, and methanol) using solar energy would be one of the best solutions to overcome both serious problems of the global warming and shortages of fossil fuels. Therefore, since the first demonstration of photocatalytic CO2 reduction by Inoue and co-workers in 1979 [920], significant advancements have been made in exploiting efficient and feasible semiconductors for reduction of carbon dioxide with water during last two decades [16], [18], [921], [922], [923], [924], [925]. Among these semiconductor materials, the g-C3N4-based photocatalysts have attracted an increasing interesting in selective photocatalytic conversion of CO2 to hydrocarbons or chemicls, due to its excellent stability, sufficiently negative CB levels, innocuity and low price [17], [19], [658]. Significant progresses were summarized in Table 9, which will thoroughly discussed in this section.
Material | Morphologies and microstructures | Synthesis method | Cocatalyst | Light source | Mass [g]/systems/Volume [mL] | Selective products (activity) [μM h−1 g−1] | Ref. (year) |
---|---|---|---|---|---|---|---|
Gas-solid systems for CO2 photoreduction | |||||||
Ag3PO4/g-C3N4 | nanoparticles/nanosheets | direct thermolysis in-situ deposition | 300 W Xe lamp, λ ≥ 420 nm | / | CO(44) CH3OH(8.5) CH4(2) C2H5OH(1.1) | [608] (2015) | |
g-C3N4/Bi2WO6 | Bi2WO6 nanoflakes/thin layer g-C3N4 nanosheets | in situ hydrothermal approach | 300 W Xe lamp, λ ≥ 420 nm | 0.1/CO2 and H2O vapor/500 mL | CO(5.19) | [926] (2015) | |
m-CeO2/g-C3N4 | mesoporous | hard-template route | 300 W of Xenon-arc lamp | 0.05/CO2 and H2O vapor/500 mL | CO(10.16) CH4(13.88) | [927] (2016) | |
g-C3N4/KNbO3 | g-C3N4 nanosheets/layered structure KNbO3 | ultrasonic dispersion followed by heat treatment method | 300 W Xe lamp, λ≥ 420 nm | 0.1/CO2 and H2O vapor | CH4(2.5) | [693] (2015) | |
SnO2-x/g-C3N4 | SnO2-x nanoparticles/g-C3N4 sheets are | directly calcining simple calcination of g-C3N4 and Sn6O4 (OH) 4 | 500 W Xe lamp | 0.02/CO2 and H2O vapor | CO(19.2) CH4(1.4) CH3OH(3.1) | [928] (2015) | |
g-C3N4/NaNbO3 | NaNbO3 nanowires/g-C3N4 nanosheets | calcination | 300 W xenon arc lamp λ > 420 nm | 0.05/CO2 and H2O vapor/230 mL | CH4(6.4) | [929] (2014) | |
g-C3N4-N-TiO2 | TiO2 nanoparticles/g-C3N4 nanosheets | in situ synthesized by thermal treatment | / | 300 W xenon arc lamp | 0.1/CO2 and H2O vapor/780 mL | CO(12.25) | [930] (2014) |
Mo-doped g-C3N4 | worm-like mesostructures | simple pyrolysis method | / | 300 W Hg lamp | 0.1/CO2 and H2O vapor/2700 mL | CO(111) CH4(15.4) | [931] (2016) |
C3N4–MCF | sponge-like structure | hard-template synthesis | / | / | / | CH3CHO(8) | [932] (2015) |
B4C/C3N4 | B4C particles/g-C3N4 nanosheets | solvent evaporation method | 0.8%wt Pt | 300 W Xenon short arc lamp 405 nm < λ < 723 nm | 0.006/CO2 and H2O vapor/100 mL | CH4(0.85) | [933] (2016) |
Pt/g-C3N4 | nanoparticles/lamellar | chemical reduction process in ethylene glycol | 2 wt% Pt | 15 W energy-saving daylight bulb | CO2 and H2O vapor | CH4(1.302) | [692] (2015) |
S-doped g-C3N4 | layered structures contain many irregular pore sizes | simply calcinating thiourea | 1 wt% Pt | 300-W simulated solar Xe arc lamp | 0.1 g/CO2 and H2O vapor/200 mL | CH3CHO(0.37) | [199] (2015) |
ZnO/g-C3N4 | highly mesoporous | a simple impregnation method | 500 W Xe lamp | 0.01/CO2 and H2O vapor/132 mL | CO(39) CH3OH(10) CH4(4) C2H5OH(1.5) | [536] (2015) | |
g-C3N4/ZnO | ZnO microcrystals/g-C3N4 nanosheets | a one-step facile calcination method | 300 W simulated solar Xe arc lamp | 0.1/CO2 and H2O vapor/200 mL | CH3OH(0.6) | [220] (2015) | |
RGO/g-C3N4 | sandwich-Like Hybrid Nanosheets | electrostatic self-assembly construction of 2D/2D | 15 wt% RGO | 15 W energy-saving Daylight bulb | CO2 and H2O vapor | CH4(1.393) | [934] (2015) |
BiOI/g-C3N4 | BiOI particles/g-C3N4 nanosheets | in situ syntheized | 300 W xenon arc lamp (λ > 400 nm). | 0.1/CO2 and H2O vapor/180 mL | CO(3.44) CH4(0.2) | [614] (2016) | |
g-C3N4–Pt | two-dimensional lamellar structure and numerous randomly organized nanosheets | directly heating and Pt was deposited on g-C3N4 | 1 wt% Pt | 300 W simulated solar Xe arc lamp | 0.1/CO2 and H2O vapor/200 mL | CH4(0.3) HCHO(0.075) CH3OH(0.24) | [691] (2014) |
AgX/g-C3N4 (X = Cl and Br) | irregular spheres of AgX/g-C3N4 nanosheets | sonication-assisted deposition-precipitation approach | 500 W Xenon arc lamp, (λ > 400 nm) | 0.1/CO2 and H2O vapor | 30%AgBr/g-C3N4 CH4(1.092) | [935] (2016) | |
AgCl/C3N4 | AgCl nanoparticles/C3N4 nanosheet | in situ deposition–precipitation approach. | 15 W energy-saving daylight lamp | CO2 and H2O vapor | CH4(0.95) | [936] (2016) | |
GO-g-C3N4 | sandwich-like | a facile one-pot impregnation–thermal reduction strategy | 15 wt% GO | 15 W energy-saving daylight bulb | CO2 and H2O vapor | CH4(5.87) | [702] (2015) |
RGO/p-C3N4 | sandwich-like | a novel combined ultrasonic dispersion and electrostatic self-assembly strategy | 15 wt% rGO | 15 W energy-saving daylight lamp | 0.1 g/CO2 and H2O vapor | CH4(1.393) | [160] (2015) |
g-C3N4/ZnO | a direct Z-scheme mechanism | a one-step facile calcination method | 300 W simulated solar Xe arc lamp | 0.1 g/CO2 and H2O vapor/200 mL | CH3OH(0.6) | [601] (2015) | |
Suspension systems for CO2 photoreduction | |||||||
RuP/g-C3N4 | mesoporous structure | adsorption | RuP | 400 W high pressure Hg lamp with a NaNO2 solution filter | 0.005/CO2 and MeCN/TEOA mixture (4:1, v/v) 4 mL | HCOOH(7.8) | [697] (2016) |
2D hydroxyl-rich C3N4 nanosheets | thin nanosheets | thermal condensation of melamine | / | 300 W Xe lamp, λ > 420 nm | 80 mL CO2 saturated water solution | CH4(0.75) | [937] (2015) |
UiO-66/10%CNNS | ten layers CNNS/the UiO-66 microspheres | facile electrostatic self-assembly method | / | 300 W xenon arclamp, 400 nm< λ < 800 nm | 5 mL CO2 saturated of solution (MeCN/TEOA 4:1)/330 mL | CO(9.9) | [708] (2015) |
amine-functionalized g-C3N4 | two-dimensional lamellar and porous structure with anomalous shape | simple thermal condensation amine functionalization | / | 300 W Xe arc lamp | 0.1/CO2 saturated water solution/200 mL | CH4(0.34) CH3OH(0.28) | [227] (2015) |
WO3/ g-C3N4(P-CW) | highly dispersed WO3 particles/g-C3N4 particles | planetary mill photodeposition method | 0.5%wt Au, 0.5%wt Ag | light-emitting diode (LED), λ = 435 nm | 0.003/CO2 and H2O | Ag/g-C3N4 + WO3: CH3OH(24.05) Au/g-C3N4 + WO3 CH3OH(34.02) | [620] (2014) |
CNU–BAX | dense and stacked particles and sheets | a facile one-pot chemical condensation of urea | / | 300 W Xe lamp, λ > 420 nm | 0.03/CO2 saturated solution of CoCl2, 2,2-bipyridine, triethanolamine | CO(469) | [938] (2015) |
CdS/g-C3N4 | nanospheres/flaky morphology | polycondensation and hydrothermal methods | / | 250 W lamp, λ = 365 nm | 0.02/CO2 saturated methanol solution/20 mL | HCOOH (1352.07) | [939] (2016) |
On the one hand, various strategies such as loading co-catalysts and nanocarbons, doping, constructing Z-scheme and heterojunction, have been widely used to enhance the photocatalytic activity for CO2 reduction [704], [940], [941]. For example, Yu et al. demonstrated that the Pt content showed a significant influence on both the activity and selectivity of g-C3N4 for photocatalytic reduction of CO2 into CH4, CH3OH and HCHO (Fig. 49a) [691]. It is believed that the Pt cocatalyst not only facilitates the interfacial electron transfer from g-C3N4 to Pt NPs (as shown in Fig. 49b), but also lower the overpotential for the CO2 reduction. More recently, Maeda and his coworkers fabricated the Ru complex decorated g-C3N4 photocatalysts through the continuous stirring of a methanol solution containing two materials at room temperature overnight [695]. The resulting heterogeneous photocatalyst systems could achieve the highest apparent quantum yield of 5.7% at 400 nm for the photocatalytic reduction of CO2 into formic acid under visible-light irradiation (as shown in Fig. 50) [695]. Surprisingly, the ternary hybrid of plasmonic Ag nanoparticles and g-C3N4/binuclear Ru(II) complex could achieve a very high turnover number of > 33,000 with a high selectivity of 87–99% for HCOOH production, due to the combination effects of plasmonic Ag and Z-Scheme charge transfer (as shown in Fig. 51) [700]. These are the best values that have been reported for heterogeneous photocatalysts for CO2 reduction under visible-light irradiation to date. The present study clearly highlighted the great potential of complex molecular co-catalyst on carbon nitride in photocatalytic CO2 reduction under visible light. Thus, it is expected that the multi coupling of complex molecular co-catalyst, g-C3N4 and other photosensitizers may provide exciting opportunities for promising CO2 photoreduction over g-C3N4-based photocatalysts. In addition, it is also noted from Table 9 that there are few earth-abundant co-catalysts reported to accelerate the CO2 photoreduction over g-C3N4-based photocatalysts, which should be urgently developed in the near future. At this point, the nano-carbons, such as RGO and CNTs, are highly expected to coupling with the g-C3N4 to obtain highly efficient metal-free g-C3N4-based photocatalysts for CO2 photoreduction [934].
Besides co-catalysts, doping and nanostructured heterojunction were also extensively used to enhance the visible light absorption and photocatalytic CO2 reduction activity of g-C3N4-based photocatalysts. Wang et al. fabricated sulfur-doped g-C3N4 photocatalysts by employing thiourea as the sulfur precursor for the reduction of CO2 to CH3OH [199]. The DFT studies confirmed that the electrons can be easily excited from the VB to the impurity state, and then to the CB of sulfur-doped g-C3N4 owing to the impurity sulfur doping (as shown in Fig. 52a), which induced additional electrons, resulting in the spin polarization. As the band gap was narrowed from 2.7 to 2.63 eV, the light absorption was broadened in the sulfur-doped g-C3N4, generating more electrons and holes under the light irradiation. Thus, the CH3OH yield (1.12 μmol g−1) was 1.5 times higher than that the unmodified g-C3N4 (0.81 μmol g−1) (as shown in Fig. 52b). In another example, Yu et al. constructed a binary g-C3N4/ZnO photocatalyst with an intimate contact interface via a one-step facile calcination method [220]. The results showed that the as-prepared g-C3N4/ZnO photocatalytic system exhibited enhanced photocatalytic activity for CO 2 reduction by a factor of 2.3 compared with pure g-C3N4 (as shown in Fig. 53a). The better performances of the g-C3N4/ZnO binary composite photocatalytic system could be well explained by the direct Z-scheme mechanism rather than the conventional heterojunction-type mechanism (as shown in Fig. 53b and c), which was achieved due to the highly efficient ZnO-to-g-C3N4 electron transfer occurring at the intimate contact interface between the g-C3N4 phase and ZnO phase. This work highlighted that the rational construction of direct Z-scheme g-C3N4-based photocatalytic system without an electron mediator should be promising strategy for the applications in the photocatalytic CO2 reduction.
On the other hand, the product selectivity of photocatalytic CO2 reduction should be also a major consideration in designing semiconductor photocatalysts [942]. For example, Liu and coworkers obtained the g-C3N4 nanosheets by the thermal delamination of bulk g-C3N4 in air. It was shown that g-C3N4 nanosheets with a band gap of 2.97 eV yielded the major product of CH4, whereas bulk g-C3N4 with a smaller band gap of 2.77 eV formed the main product of CH3CHO (Fig. 54a) [409]. This elucidated that the nanosheets had a larger band gap by 0.2 eV, leading to a lower VB edge by 80 meV and a higher CB edge by 120 meV. Therefore, the nanosheets provided a larger thermodynamic diving force for the hole and electron transfer by means of a greater difference in energy level between redox potentials of the reactants and band edges (Fig. 54b). This indirectly led to a larger proportion of long-lived charge carriers for the nanosheets in contrast to the bulk. As a consequence, the formation of CH4 was more favorable due to rapid transfer of photoexcited electrons in the nanosheets to the intermediate species.
However, it is worth mentioning that the current apparent quantum efficiency is still low for the commercial applications. Thus, there is still ample room to further improve the CO2 photoreduction in g-C3N4-based photocatalytic system. Furthermore, the exact reaction mechanism for the CO2 photoreduction should be paid more attention. In this regard, various strategies to enhance the CO2 adsorption of porous absorbents could be employed to improve the adsorption and activation of CO2 on g-C3N4-based photocatalysts [943], [944], [945]. Especially, the selectivity of CO2 photoreduction should be deeply investigated for each system. In fact, it is strongly suggested that all products from the CO2 conversion requires should be measured, in gas and liquid phase for suspension systems. To well understand the photocatalytic reaction steps, DFT can be used to identify the activation state of CO2. In addition, the isotopic labeling analysis using 13CO2 as the reactant is nacessary to confirm the obtained prodcuts stemmed from the photofixation of CO2 in order to exclude the possibility of photodissociation of the organic impurities or even carbon-containing catalysts.
5.4. Photocatalytic selective organic transformations
Recently, the robust metal-free g-C3N4-based photocatalysts have been shown to have great potential for selective organic transformation under mild conditions, including oxidation of aromatic compounds [395], [507], [946], [947], [948], [949], [950], [951], [952], [953], [954], [955], [956], photo catalytic esterification of benzaldehyde and alcohol [957], oxidative cleavage of the carbon–carbon bond of α-hydroxy ketones [958] and allylic oxidation [959]. For example, Wang and coworkers demonstrated that direct oxidation of benzene to phenol with H2O2 catalyzed by porous Fe-g-C3N4 could be achieved, in both the presence and absence of visible light irradiation [950]. By taking advantage of the photocatalytic functions of g-C3N4, the yield of the phenol synthesis can be markedly improved. Furthermore, the same research group demonstrated that the metal-free graphene sheet/g-C3N4 nanocomposite could achieve the selective photocatalytic oxidation of cyclohexane to cyclohexanone through the superoxide radical anion (O2−) induced from the activation of O2 (as shown in Fig. 55), highlighting the key roles of O2-reduction reaction in the photocatalytic selective organic transformations [244]. In another report, Zhang et al. [956] revealed that a 38% conversion of benzene to phenol with 97% selectivity could be achieved using FeCl3-modified mesoporous carbon nitride as a visible-light photocatalyst to activate H2O2. Li et al. [960] developed a Mott–Schottky photocatalyst consisting of mesoporous carbon nitride with Pd nanoparticles. The efficient electron transfer from the g-C3N4 to the Pd resulted in a high photocatalytic activity and selectivity for the room-temperature CC bond formation by coupling aryl halides with different coupling partners. More recently, Yin and coworkers demonstrated that an acid-base bifunctional P-doped g-C3N4 photocatalysts could achieve the cycloaddition reactions, due to the synergetic effect of acid (halide anions) and basic sites for ring opening of epoxide and adsorption/activation of CO2 [325]. In future, it is expected that various kinds of multifunctional g-C3N4 composite photocatalysts could be widely used in the selective photocatalytic organic transformations, which are hardly proceed by traditionary thermal catalysis. Also, the deep mechanism investigation is highly desired.
5.5. Photocatalytic disinfection
As a nontoxic, efficient, and stable method, photocatalytic disinfection has been shown to be superior in comparison with traditional water disinfection methods, including chlorination, ozone, and ultraviolet (UV), have some disadvantages [659]. Commonly, the toxic metals in the efficient metal-based photocatalysts for bacterial inactivation is unfavorable for “green” water disinfection [660]. At this point, the robust and non-toxic metal-free g-C3N4 seems to be more promising in water disinfection. However, limited studies have concentrated on the visible-light-induced photocatalytic inactivation of bacteria over g-C3N4 [450], [659], [660], [961]. For example, Huang et al. demonstrated that metal-free robust g-C3N4 photocatalyst exhibit antibacterial activity for the inactivation of Escherichia coli K-12 (E. coli) under visible light irradiation [961]. Especially, a novel heterojunction related to g-C3N4 was synthesized via cowrapping the RGO and g-C3N4 (CN) sheets on α-sulfur (α-S8) by Wang and his coworkers. The results indicated that the visible-light-driven photocatalytic activities of this system for bacterial inactivation was significantly improved, which will also change with the shells arrangement evolution of RGO and CN. The enhanced activities can be ascribed to the electrons or holes migration in the system as shown in Fig. 56 [660]. In addition, Zhao et al. indicated that the atomic single layer g-C3N4 with the thickness of 0.5 nm exhibited performance of photocatalytic disinfection for inactivation of Escherichia coli, due to low charge transfer resistance and efficient charge separation [450]. More recently, An and coworkers demonstrated that the g-C3N4/TiO2 hybrid photocatalyst, comprised of micron-sized TiO2 spheres wrapped with lamellar g-C3N4, exhibited significantly enhanced photocatalytic activity for the inactivation of Escherichia coli K-12, due to improved light absorption and the effective charge separation [659].
6. Conclusions and future prospects
In summary, this review highlights the advantages, versatile properties, design strategies and potential application of robust g-C3N4-based composite photocatalysts. Obviously, g-C3N4 has proven to be one of the most promising candidates suitable for designing and fabricating advanced composite photocatalysts for various applications. Therefore, there is little doubt that the explosive growth of g-C3N4-based composite photocatalysts will continue to accelerate in the near future. To date, although considerable progress has been achieved in the recent years, there are still many challenges to rationally fabricate the highly efficient g-C3N4-based photocatalysts towards various applications and deeply understand the underlying enhancement mechanism of composite photocatalysts by g-C3N4. There are still many open issues and opportunities for further research effort. Accordingly, more studies are also needed to make full use of the outstanding structural and electronic properties of g-C3N4 in the composite photocatalysts.
On the one hand, versatile properties of g-C3N4-based photocatalysts are still needed to be explored carefully. Since the highly effective and stable g-C3N4-based photocatalysts with narrowed band gaps are difficult to obtain, the design and development of conjugated narrow-band polymer might provide alternative ideas for boosting the advancements of photocatalysis based on the organic semiconductors [962], [963], [964], [965], [966], [967], [968]. Furthermore, the accurate control of surface defects and facile scale preparation methods of g-C3N4 nanosheets are highly desired. The advanced electrocatalysts and photoelectrocatalysts based g-C3N4 should be exploited as an important research community for extending the applications of g-C3N4-based photocatalysts. Among various design strategies, the dimensionality tuning, pore texture tailoring, heterojunction construction (especially for Z-scheme construction), co-catalyst and nanocarbon loading seem to be more promising in developing practical g-C3N4-based photocatalysts. Typically, Z-scheme construction is more interesting and promising than the traditional heterojunction. At this point, more investigations should be paid to this strategy. Absolutely, the developments of earth-abundant co-catalysts are still a hard task in these photocatalytic fields. The magical nanocarbons will play the irreplaceable roles in constructing highly efficient g-C3N4-based photocatalysts for all the time.
In addition, the applications of g-C3N4-based photocatalysts are mainly focused on the H2 evolution and degradation of pollutants. However, the photocatalytic CO2 reduction over g-C3N4-based photocatalysts become more and more attractive in the past three years. The basic nature of the g-C3N4 surface determines its bright future in the fields of CO2 reduction. Furthermore, work on the O2 evolution from the other half-reaction of H2O splitting should gain more attention in the near future, which involves in both the water splitting and CO2 reduction. Additionally, the exact reaction mechanism, particularly the CO2 reduction using g-C3N4-based photocatalysts, still remains doubtful and unresolved to date. The deep studying of reaction pathways is crucial for revealing the photocatalytic enhancement fundamental and further rationally design the highly efficient g-C3N4-based photocatalysts in the future. Furthermore, some key issues that account for the high photocatalytic activity, i.e. optical absorption, electronic band structure, and charge transfer dynamics, should be exhaustively investigated to gain theoretical insights by means of both computational (first-principles DFT) and experimental simulations. In terms of experimental work, reactant adsorption sites, charge transfer dynamics, and molecular orbitals should also be deeply researched. The joint efforts by researchers from various fields and countries must bring one and one exciting time for g-C3N4-based photocatalysts.
Acknowledgments
Li would like to thank Industry and Research Collaborative Innovation Major Projects of Guangzhou (201508020098), NSFC (20906034) and the State Key Laboratory of Advanced Technology for Material Synthesis and Processing (Wuhan University of Technology) (2015-KF-7) for their support. X. Chen would like to thank the College of Arts and Sciences, University of Missouri—Kansas City and University of Missouri Research Board for their financial support.
References
- [1]Nature, 238 (1972), pp. 37-38
- [2]J. Am. Chem. Soc., 100 (1978), pp. 4317-4318
- [3]J. Photochem., 10 (1979), pp. 59-75
- [4]Science, 207 (1980), pp. 139-144
- [5]J. Phys. Chem., 86 (1982), pp. 172-177
- [6]Acc. Chem. Res., 28 (1995), pp. 141-145
- [7]J. Mater. Chem. A, 3 (2015), pp. 2485-2534
- [8]Chem. Rev., 110 (2010), pp. 6503-6570
- [9]Chem. Soc. Rev., 39 (2010), pp. 4206-4219
- [10]Chem. Soc. Rev., 38 (2009), pp. 253-278
- [11]Chem. Rev., 95 (1995), pp. 69-96
- [12]J. Mater. Chem., 19 (2009), pp. 5089-5121
- [13]Appl. Catal. B: Environ., 125 (2012), pp. 331-349
- [14]Energy Environ. Sci., 7 (2014), pp. 2831-2867
- [15]Water. Res., 44 (2010), pp. 2997-3027
- [16]Sci. China Mater., 57 (2014), pp. 70-100
- [17]J. Phys. Chem. Lett., 6 (2015), pp. 4244-4251
- [18]Adv. Mater., 26 (2014), pp. 4607-4626
- [19]Mater. Horiz., 2 (2015), pp. 261-278
- [20]J. Photochem. Photobiol. C, 15 (2013), pp. 1-20
- [21]Catal. Today, 147 (2009), pp. 1-59
- [22]J. Photochem. Photobiol. C, 9 (2008), pp. 157-170
- [23]Angew. Chem. Int. Ed., 52 (2013), pp. 812-847
- [24]Chem. Soc. Rev., 43 (2014), pp. 473-486
- [25]Chin. J. Catal., 36 (2015), pp. 2049-2070
- [26]Adv. Energy Mater., 5 (2015), p. 1500010
- [27]Adv. Mater., 27 (2015), pp. 2150-2176
- [28]J. Phys. Chem. Lett., 5 (2014), pp. 2101-2107
- [29]Chem. Rev., 95 (1995), pp. 735-758
- [30]Adv. Mater., 24 (2012), pp. 229-251
- [31]J. Photochem. Photobiol. C, 1 (2000), pp. 1-21
- [32]J. Photochem. Photobiol. C, 13 (2012), pp. 169-189
- [33]Chem. Rev., 107 (2007), pp. 2891-2959
- [34]Science, 293 (2001), pp. 269-271
- [35]Science, 331 (2011), pp. 746-750
- [36]Science, 297 (2002), pp. 2243-2245
- [37]Chem. Soc. Rev., 41 (2012), pp. 7909-7937
- [38]Chem. Rev., 114 (2014), pp. 9890-9918
- [39]Chem. Rev., 114 (2014), pp. 9987-10043
- [40]Chem. Soc. Rev., 44 (2015), pp. 1861-1885
- [41]J. Am. Chem. Soc., 132 (2010), pp. 7559-7567
- [42]J. Power Sources, 274 (2015), pp. 77-84
- [43]J. Am. Chem. Soc., 136 (2014), pp. 13486-13489
- [44]Nat. Mater., 8 (2009), pp. 76-80
- [45]J. Photochem. Photobiol. C, 20 (2014), pp. 33-50
- [46]Chem. Soc. Rev., 42 (2013), pp. 6593-6604
- [47]Angew. Chem. Int. Ed., 51 (2012), pp. 68-89
- [48]Energy Environ. Sci., 5 (2012), pp. 6717-6731
- [49]ACS Appl. Mater. Interfaces, 6 (2014), pp. 16449-16465
- [50]Appl. Surf. Sci., 358 (2015), pp. 15-27
- [51]Catal. Sci. Technol., 5 (2015), pp. 5048-5061
- [52]Energy Environ. Sci., 8 (2015), pp. 3092-3108
- [53]Nanoscale, 7 (2015), pp. 15-37
- [54]Angew. Chem. Int. Ed., 54 (2015), pp. 12868-12884
- [55]Chem. Soc. Rev., 45 (2016), pp. 2308-2326
- [56]J. Mater. Chem. A, 3 (2015), pp. 23642-23652
- [57]Green Chem., 17 (2015), pp. 715-736
- [58]Chem. Rev., 115 (2015), pp. 10307-10377
- [59]Chem. Rev., 116 (2016), pp. 7159-7329
- [60]Adv. Funct. Mater., 26 (2016), pp. 1719-1728
- [61]J. Mater. Chem. A, 4 (2016), pp. 8000-8004
- [62]Jpn. J. Appl. Phys., 47 (2008), p. 7842
- [63]Appl. Phys. Lett., 82 (2003), pp. 4017-4019
- [64]Angew. Chem. Int. Ed., 51 (2012), pp. 3892-3896
- [65]Angew. Chem. Int. Ed., 53 (2014), pp. 7281-7285
- [66]Angew. Chem. Int. Ed., 50 (2011), pp. 5339-5343
- [67]Chem. Commun., 52 (2016), pp. 1725-1728
- [68]Chem. Commun., 51 (2015), pp. 12399-12402
- [69]J. Am. Chem. Soc., 133 (2011), pp. 20116-20119
- [70]ACS Appl. Mater. Interfaces, 7 (2015), pp. 10823-10827
- [71]Energy Environ. Sci., 8 (2015), pp. 2664-2667
- [72]Nano Energy, 8 (2014), pp. 157-164
- [73]Nanoscale, 6 (2014), pp. 12555-12564
- [74]Sci. Rep. U. K., 6 (2016), p. 24314
- [75]Small, 11 (2015), pp. 2817-2824
- [76]ACS Appl. Mater. Interfaces, 7 (2015), pp. 23672-23678
- [77]ACS Appl. Mater. Interfaces, 7 (2015), pp. 8330-8338
- [78]Anal. Chem., 84 (2012), pp. 4754-4759
- [79]Biosens. Bioelectron., 68 (2015), pp. 210-217
- [80]Chem. Commun., 50 (2014), pp. 15415-15418
- [81]J. Mater. Chem. B, 3 (2015), pp. 1289-1300
- [82]Small, 10 (2014), pp. 2382-2389
- [83]Chem. Soc. Rev., 45 (2016), pp. 2603-2636
- [84]Angew. Chem. Int. Ed., 49 (2010), pp. 441-444
- [85]Heterogeneous PhotocatalysisJ.C. Colmenares, Y.-J. Xu (Eds.), Springer, Berlin Heidelberg (2016), pp. 175-210
- [86]Photocatalysis: ApplicationsG.L.P.D. Dionysios Dionysiou, Jinhua Ye, Jenny Schneider, Detlef Bahnemann (Eds.), The Royal Society of Chemistry (2016), pp. 255-302
- [87]Biochem. J., 253 (1988), pp. 287-289
- [88]Accounts. Chem. Res., 46 (2013), pp. 1900-1909
- [89]Angew. Chem. Int. Ed., 54 (2015), pp. 3047-3051
- [90]J. Am. Chem. Soc., 105 (1983), pp. 27-31
- [91]Chem. Commun. (1999), pp. 1069-1070
- [92]J. Phys. Chem., 89 (1985), pp. 5676-5681
- [93]Dalton Trans., 39 (2010), pp. 1488-1491
- [94]J. Phys. Chem. B, 107 (2003), pp. 1798-1803
- [95]J. Phys. Chem. C, 112 (2008), pp. 548-554
- [96]J. Electrochem. Soc., 124 (1977), pp. 215-224
- [97]Nat. Mater., 9 (2010), pp. 559-564
- [98]Standard Potentials in Aqueous SolutionCRC Press (1985)
- [99]ChemSusChem, 8 (2015), pp. 931-946
- [100]J. Am. Chem. Soc., 135 (2013), pp. 18-21
- [101]J. Mater. Chem., 18 (2008), pp. 4893-4908
- [102]Chem. Eur. J., 14 (2008), pp. 8177-8182
- [103]J. Am. Chem. Soc., 136 (2014), pp. 1730-1733
- [104]J. Mater. Chem. A, 3 (2015), pp. 24232-24236
- [105]Angew. Chem. Int. Ed., 53 (2014), pp. 7450-7455
- [106]Phys. Chem. Chem. Phys., 15 (2013), pp. 18077-18084
- [107]Adv. Funct. Mater., 23 (2013), pp. 3661-3667
- [108]Angew. Chem. Int. Ed., 53 (2014), pp. 11001-11005
- [109]Angew. Chem. Int. Ed., 53 (2014), pp. 3654-3658
- [110]J. Am. Chem. Soc., 135 (2013), pp. 7118-7121
- [111]Langmuir, 30 (2014), pp. 447-451
- [112]Green Chem., 16 (2014), pp. 4663-4668
- [113]J. Mater. Chem. A, 3 (2015), pp. 10205-10208
- [114]Angew. Chem. Int. Ed., 49 (2010), pp. 3356-3359
- [115]J. Power Sources, 245 (2014), pp. 866-874
- [116]RSC Adv., 3 (2013), pp. 19624-19631
- [117]J. Mater. Chem. A, 2 (2014), pp. 5340-5351
- [118]Energy Environ. Sci., 4 (2011), pp. 675-678
- [119]J. Am. Chem. Soc., 132 (2010), pp. 16299-16301
- [120]Angew. Chem. Int. Ed., 52 (2013), pp. 3621-3625
- [121]Angew. Chem. Int. Ed., 51 (2012), pp. 10145-10149
- [122]Chem. Mater., 21 (2009), pp. 4093-4095
- [123]J. Am. Chem. Soc., 131 (2009), pp. 1680-1681
- [124]Chem. Soc. Rev., 43 (2014), pp. 7787-7812
- [125]ChemSusChem, 6 (2013), pp. 2263-2268
- [126]Catal. Sci. Technol., 3 (2013), pp. 1782-1789
- [127]Science, 271 (1996), pp. 53-55
- [128]Adv. Mater., 26 (2014), pp. 805-809
- [129]Energy Environ. Sci., 7 (2014), pp. 1902-1906
- [130]Angew. Chem. Int. Ed., 51 (2012), pp. 3183-3187
- [131]Int. J. Hydrogen Energy, 37 (2012), pp. 11072-11080
- [132]Science, 245 (1989), pp. 841-842
- [133]J. Phys. Chem. C, 113 (2009), pp. 4940-4947
- [134]J. Am. Chem. Soc., 125 (2003), pp. 10288-10300
- [135]J. Phys. Chem. B, 111 (2007), pp. 10671-10680
- [136]New J. Chem., 26 (2002), pp. 508-512
- [137]Chem. Mater., 19 (2007), pp. 4396-4404
- [138]Nanoscale, 4 (2012), pp. 3687-3692
- [139]J. Mater. Chem., 20 (2010), pp. 10801-10803
- [140]Chem. Eur. J., 22 (2016), pp. 5142-5145
- [141]Nano Lett., 15 (2015), pp. 5137-5142
- [142]Chem. Eur. J., 21 (2015), pp. 6241-6246
- [143]J. Mater. Chem. A, 2 (2014), pp. 19145-19149
- [144]RSC Adv., 4 (2014), pp. 5553-5560
- [145]Phys. Chem. Chem. Phys., 17 (2015), pp. 3309-3315
- [146]Appl. Surf. Sci., 344 (2015), pp. 188-195
- [147]Phys. Chem. Chem. Phys., 16 (2014), pp. 8106-8113
- [148]J. Mater. Chem., 21 (2011), pp. 15171-15174
- [149]Langmuir, 25 (2009), pp. 10397-10401
- [150]ACS Catal., 2 (2012), pp. 940-948
- [151]J. Phys. Chem. C, 115 (2011), pp. 7355-7363
- [152]Phys. Chem. Chem. Phys., 11 (2009), pp. 2730-2740
- [153]J. Colloid Interface Sci., 401 (2013), pp. 70-79
- [154]Carbon, 80 (2014), pp. 213-221
- [155]RSC Adv., 6 (2016), pp. 31497-31506
- [156]Appl. Surf. Sci., 358 (2015), pp. 204-212
- [157]RSC Adv., 3 (2013), pp. 22480-22489
- [158]J. Am. Chem. Soc., 131 (2009), p. 50
- [159]Appl. Surf. Sci., 295 (2014), pp. 253-259
- [160]Nano Energy, 13 (2015), pp. 757-770
- [161]ACS Catal., 5 (2015), pp. 6973-6979
- [162]RSC Adv., 5 (2015), pp. 26281-26290
- [163]Int. J. Hydrogen Energy, 41 (2016), pp. 3888-3895
- [164]J. Mater. Chem. A, 1 (2013), pp. 6489-6496
- [165]J. Hazard. Mater., 300 (2015), pp. 598-606
- [166]J. Mater. Chem. A, 1 (2013), pp. 14089-14096
- [167]Chem. Mater., 12 (2000), pp. 3906-3912
- [168]J. Mater. Chem., 12 (2002), pp. 2463-2469
- [169]J. Phys. Chem. B, 114 (2010), pp. 9429-9434
- [170]J. Mater. Chem., 21 (2011), pp. 13032-13039
- [171]J. Mater. Chem., 11 (2001), pp. 474-478
- [172]Nanoscale, 7 (2015), pp. 12343-12350
- [173]Catal. Sci. Technol., 4 (2014), pp. 3235-3243
- [174]Adv. Mater., 19 (2007), pp. 264-267
- [175]Angew. Chem. Int. Ed., 51 (2012), pp. 961-965
- [176]Angew. Chem. Int. Ed., 52 (2013), pp. 4572-4576
- [177]ACS Nano, 2 (2008), pp. 2489-2496
- [178]J. Mater. Chem. A, 1 (2013), pp. 14766-14772
- [179]Angew. Chem. Int. Ed., 54 (2015), pp. 6297-6301
- [180]J. Am. Chem. Soc., 137 (2015), pp. 2179-2182
- [181]Chem. Asian. J., 5 (2010), pp. 1307-1311
- [182]J. Mater. Chem., 22 (2012), pp. 15006-15012
- [183]Angew. Chem. Int. Ed., 55 (2016), pp. 1830-1834
- [184]Chem. Commun., 48 (2012), pp. 12017-12019
- [185]Chem. Commun., 48 (2012), pp. 6178-6180
- [186]J. Mater. Chem. A, 3 (2015), pp. 4612-4619
- [187]ACS Catal., 2 (2012), pp. 1596-1606
- [188]Chem. Mater., 27 (2015), pp. 8237-8247
- [189]RSC Adv., 5 (2015), pp. 5725-5734
- [190]Appl. Phys. Lett., 99 (2011), p. 084105
- [191]J. Mater. Chem. A, 2 (2014), pp. 20338-20344
- [192]Phys. Chem. Chem. Phys., 17 (2015), pp. 29899-29905
- [193]J. Mater. Chem. A, 3 (2015), pp. 23274-23282
- [194]Carbon, 92 (2015), pp. 31-40
- [195]Appl. Catal. A-Gen., 510 (2016), pp. 161-170
- [196]J. Phys.: Condens. Matter, 24 (2012), p. 162201
- [197]J. Mater. Chem. A, 3 (2015), pp. 18244-18255
- [198]Dalton Trans., 44 (2015), pp. 18260-18269
- [199]Appl. Catal. B: Environ., 176–177 (2015), pp. 44-52
- [200]J. Colloid Interface Sci., 461 (2016), pp. 56-63
- [201]J. Phys. Chem. C, 120 (2016), pp. 265-273
- [202]Sci. Rep. U. K., 3 (2013), p. 1943
- [203]ACS Nano, 9 (2015), pp. 12480-12487
- [204]J. Mater. Chem. B, 2 (2014), pp. 5768-5774
- [205]J. Mater. Chem. C, 3 (2015), pp. 73-78
- [206]Nanoscale, 5 (2013), pp. 225-230
- [207]Biosens. Bioelectron., 73 (2015), pp. 7-12
- [208]Nanoscale, 6 (2014), pp. 4157-4162
- [209]Nanoscale, 6 (2014), pp. 1890-1895
- [210]Catal. Sci. Technol., 3 (2013), pp. 1027-1035
- [211]Adsorpt. Sci. Technol., 24 (2006), pp. 363-374
- [212]Appl. Therm. Eng., 27 (2007), pp. 869-876
- [213]J. Chem. Eng. Data, 55 (2010), pp. 5729-5732
- [214]Kinet. Catal., 51 (2010), pp. 754-761
- [215]J. Chem. Eng. Data, 55 (2010), pp. 3164-3169
- [216]RSC Adv., 4 (2014), pp. 3823-3851
- [217]J. Am. Chem. Soc., 132 (2010), pp. 11914-11916
- [218]Chem. Commun., 47 (2011), pp. 4532-4534
- [219]Nanoscale, 4 (2012), pp. 3193-3200
- [220]Appl. Surf. Sci., 358 (2015), pp. 425-435
- [221]J. Colloid Interface Sci., 456 (2015), pp. 7-14
- [222]J. Am. Chem. Soc., 85 (1963), pp. 3533-3539
- [223]J. Am. Chem. Soc., 105 (1983), pp. 7512-7516
- [224]Chem. Eng. J., 132 (2007), pp. 233-239
- [225]Appl. Surf. Sci., 228 (2004), pp. 84-92
- [226]ACS Nano, 9 (2015), pp. 9148-9157
- [227]Appl. Surf. Sci., 358 (2015), pp. 350-355
- [228]Chem. Commun., 50 (2014), pp. 5976-5979
- [229]Nano Res., 3 (2010), pp. 632-642
- [230]Microporous Mesoporous Mater., 62 (2003), pp. 29-45
- [231]Energy Fuel, 16 (2002), pp. 1463-1469
- [232]J. Am. Chem. Soc., 131 (2009), pp. 5777-5783
- [233]Chem. Eng. J., 160 (2010), pp. 571-577
- [234]Chem. Commun., 49 (2013), pp. 653-661
- [235]Langmuir, 28 (2012), pp. 12122-12133
- [236]Energy Fuel, 25 (2011), pp. 835-842
- [237]ACS Appl. Mater. Interfaces, 7 (2015), pp. 8166-8175
- [238]J. Phys. Chem. C, 113 (2009), pp. 20148-20151
- [239]Nat. Commun., 5 (2014), p. 3783
- [240]ACS Appl. Mater. Interfaces, 7 (2015), pp. 19626-19634
- [241]J. Mater. Chem. A, 3 (2015), pp. 23120-23135
- [242]Chem. Soc. Rev., 45 (2016), pp. 3039-3052
- [243]J. Phys. Chem. C, 118 (2014), pp. 12077-12086
- [244]J. Am. Chem. Soc., 133 (2011), pp. 8074-8077
- [245]ChemSusChem, 4 (2011), pp. 274-281
- [246]J. Mol. Catal. A: Chem., 390 (2014), pp. 7-13
- [247]Chin. J. Catal., 34 (2013), pp. 385-390
- [248]Appl. Surf. Sci., 319 (2014), pp. 21-28
- [249]J. Colloid Interface Sci., 334 (2009), pp. 58-64
- [250]J. Phys. Chem. Lett., 4 (2013), pp. 2847-2852
- [251]Appl. Catal. B: Environ., 60 (2005), pp. 211-221
- [252]J. Hazard. Mater., 177 (2010), pp. 971-977
- [253]Angew. Chem. Int. Ed., 51 (2012), pp. 11496-11500
- [254]Angew. Chem. Int. Ed., 52 (2013), pp. 3110-3116
- [255]Adv. Mater., 25 (2013), pp. 6226-6231
- [256]Adv. Funct. Mater., 24 (2014), pp. 2072-2078
- [257]J. Mater. Chem. A, 2 (2014), pp. 18139-18146
- [258]Adv. Mater., 24 (2012), pp. 5593-5597
- [259]Chin. J. Catal., 34 (2013), pp. 2138-2145
- [260]Science, 323 (2009), pp. 760-764
- [261]ACS Nano, 4 (2010), pp. 1321-1326
- [262]ChemSusChem, 7 (2014), pp. 2125-2130
- [263]ACS Appl. Mater. Interfaces, 6 (2014), pp. 1011-1017
- [264]ChemCatChem, 7 (2015), pp. 3873-3880
- [265]J. Power Sources, 272 (2014), pp. 696-702
- [266]Langmuir, 29 (2013), pp. 3821-3828
- [267]J. Mater. Chem. A, 1 (2013), pp. 10538-10545
- [268]Int. J. Hydrogen Energy, 39 (2014), pp. 2828-2841
- [269]ChemElectroChem, 1 (2014), pp. 1359-1369
- [270]Chem. Rev., 110 (2010), pp. 6446-6473
- [271]Acc. Chem. Res., 46 (2013), pp. 2355-2364
- [272]Appl. Surf. Sci., 351 (2015), pp. 779-793
- [273]Energy Environ. Sci., 7 (2014), pp. 3519-3542
- [274]Energy Environ. Sci., 5 (2012), pp. 6012-6021
- [275]Chem. Soc. Rev., 44 (2015), pp. 5148-5180
- [276]J. Am. Chem. Soc., 134 (2012), pp. 4393-4397
- [277]Angew. Chem. Int. Ed., 53 (2014), pp. 13934-13939
- [278]Appl. Surf. Sci., 357 (2015), pp. 221-228
- [279]Sci. Rep. U. K., 3 (2013), p. 2163
- [280]J. Mater. Chem. A, 1 (2013), pp. 6407-6415
- [281]J. Phys. Chem. C, 119 (2015), pp. 20283-20292
- [282]Phys. Chem. Chem. Phys., 17 (2015), pp. 17887-17893
- [283]Chem. Commun., 47 (2011), pp. 10323-10325
- [284]RSC Adv., 5 (2015), pp. 14074-14080
- [285]Adv. Mater., 26 (2014), pp. 5043-5049
- [286]RSC Adv., 5 (2015), pp. 69753-69760
- [287]Phys. Chem. Chem. Phys., 18 (2016), pp. 10255-10261
- [288]Nano Lett., 16 (2016), pp. 2268-2277
- [289]Int. J. Hydrogen Energy, 39 (2014), pp. 9105-9113
- [290]ACS Appl. Mater. Interfaces, 8 (2016), pp. 5280-5289
- [291]Appl. Surf. Sci., 358 (2015), pp. 296-303
- [292]Chem. Eur. J., 22 (2016), pp. 4764-4773
- [293]Appl. Catal. B: Environ., 180 (2016), pp. 324-329
- [294]Electrochim. Acta, 71 (2012), pp. 10-16
- [295]Energy Technol., 3 (2015), pp. 982-988
- [296]RSC Adv., 5 (2015), pp. 101552-101562
- [297]Dalton Trans., 43 (2014), pp. 8178-8183
- [298]Adv. Mater., 21 (2009), pp. 1609-1612
- [299]Nanoscale, 5 (2013), pp. 11604-11609
- [300]Sci. Technol. Adv. Mater., 12 (2011), p. 034401
- [301]RSC Adv., 4 (2014), pp. 24863-24869
- [302]J. Mater. Chem. A, 2 (2014), pp. 6772-6780
- [303]J. Phys. Chem. C, 120 (2016), pp. 56-63
- [304]ACS Catal., 1 (2011), pp. 99-104
- [305]Appl. Catal. B: Environ., 164 (2015), pp. 77-81
- [306]ACS Catal., 6 (2016), pp. 2462-2472
- [307]Mater. Res. Bull., 48 (2013), pp. 3919-3925
- [308]J. Phys. Condens. Matter, 25 (2013), p. 085507
- [309]Int. J. Hydrogen Energy, 39 (2014), pp. 15373-15379
- [310]ACS Appl. Mater. Interfaces, 6 (2014), pp. 4321-4328
- [311]Mater. Lett., 149 (2015), pp. 50-53
- [312]J. Am. Chem. Soc., 132 (2010), pp. 6294-6295
- [313]Mater. Res. Bull., 48 (2013), pp. 3485-3491
- [314]RSC Adv., 5 (2015), pp. 68728-68735
- [315]ACS Appl. Mater. Interfaces, 7 (2015), pp. 16850-16856
- [316]J. Alloy Compd., 672 (2016), pp. 271-276
- [317]Appl. Surf. Sci., 360 (2016), pp. 1016-1022
- [318]ACS Appl. Mater. Interfaces, 7 (2015), pp. 18450-18459
- [319]RSC Adv., 6 (2016), pp. 25568-25576
- [320]J. Phys. Chem. C, 120 (2016), pp. 98-107
- [321]Nano Energy, 12 (2015), pp. 646-656
- [322]J. Phys. Chem. C, 116 (2012), pp. 23485-23493
- [323]RSC Adv., 4 (2014), pp. 21657-21663
- [324]J. Mater. Chem. A, 3 (2015), pp. 3862-3867
- [325]Carbon, 100 (2016), pp. 81-89
- [326]Energy Environ. Sci., 8 (2015), pp. 3708-3717
- [327]Chem. Sci., 2 (2011), pp. 446-450
- [328]Angew. Chem. Int. Ed., 52 (2013), pp. 1735-1738
- [329]J. Mater. Chem. A, 3 (2015), pp. 1841-1846
- [330]Langmuir, 26 (2010), pp. 3894-3901
- [331]Chem. Mater., 22 (2010), pp. 5119-5121
- [332]Appl. Surf. Sci., 311 (2014), pp. 164-171
- [333]RSC Adv., 5 (2015), pp. 79585-79592
- [334]Dalton Trans., 44 (2015), pp. 20889-20897
- [335]Appl. Surf. Sci., 357 (2015), pp. 131-138
- [336]Appl. Surf. Sci., 332 (2015), pp. 625-630
- [337]Chem. Eur. J., 20 (2014), pp. 9805-9812
- [338]ACS Catal., 5 (2015), pp. 5008-5015
- [339]ACS Appl. Mater. Interfaces, 7 (2015), pp. 5497-5505
- [340]Chem. Commun., 51 (2015), pp. 9706-9709
- [341]Appl. Catal. B: Environ., 182 (2016), pp. 68-73
- [342]J. Catal., 310 (2014), pp. 24-30
- [343]ACS Catal., 3 (2013), pp. 912-919
- [344]Macromol. Rapid Commun., 34 (2013), pp. 850-854
- [345]Energy Environ. Sci., 8 (2015), pp. 3345-3353
- [346]Appl. Catal. B: Environ., 181 (2016), pp. 413-419
- [347]Adv. Mater., 26 (2014), pp. 8046-8052
- [348]Adv. Mater., 27 (2015), pp. 4572-4577
- [349]J. Phys. Chem. C, 119 (2015), pp. 14938-14946
- [350]Adv. Funct. Mater., 25 (2015), pp. 6885-6892
- [351]Nanoscale, 7 (2015), pp. 18887-18890
- [352]Chem. Mater., 27 (2015), pp. 4930-4933
- [353]Appl. Surf. Sci., 358 (2015), pp. 363-369
- [354]ACS Catal., 5 (2015), pp. 3058-3066
- [355]J. Mater. Chem. A (2014)
- [356]Catal. Today, 225 (2013), pp. 2-9
- [357]J. Mater. Chem. A, 3 (2015), pp. 14081-14092
- [358]J. Phys. Chem. C, 113 (2009), pp. 8668-8672
- [359]Chem. Mater., 23 (2011), pp. 4344-4348
- [360]Adv. Funct. Mater., 23 (2013), pp. 3008-3014
- [361]Chem. Asian J., 6 (2011), pp. 103-109
- [362]Chem. Sci., 3 (2012), pp. 2170-2174
- [363]Catal. Sci. Technol., 2 (2012), pp. 1396-1402
- [364]Nat. Commun., 3 (2012), p. 1139
- [365]Angew. Chem. Int. Ed., 53 (2014), pp. 11926-11930
- [366]ACS Appl. Mater. Interfaces, 6 (2014), pp. 8434-8440
- [367]Chem. Asian J., 7 (2012), pp. 2139-2144
- [368]Angew. Chem. Int. Ed., 49 (2010), pp. 9706-9710
- [369]Adv. Mater., 26 (2014), pp. 4121-4126
- [370]J. Mater. Chem. A, 3 (2015), pp. 5126-5131
- [371]J. Colloid Interface Sci., 451 (2015), pp. 108-116
- [372]Adv. Mater., 27 (2015), pp. 4634-4639
- [373]ChemSusChem, 7 (2014), pp. 1547-1550
- [374]ChemSusChem, 3 (2010), pp. 435-439
- [375]Chem. Commun., 48 (2012), pp. 3430-3432
- [376]J. Mater. Chem., 21 (2011), pp. 3890-3894
- [377]RSC Adv., 4 (2014), pp. 61877-61883
- [378]Appl. Catal. B: Environ., 147 (2014), pp. 229-235
- [379]Langmuir, 29 (2013), pp. 10566-10572
- [380]Nanoscale, 4 (2012), pp. 5300-5303
- [381]Chem. Commun., 51 (2015), pp. 425-427
- [382]Chem. Commun., 51 (2015), pp. 16244-16246
- [383]Nanoscale, 7 (2015), pp. 2471-2479
- [384]Energy Environ. Sci., 6 (2013), pp. 1486-1493
- [385]Angew. Chem. Int. Ed., 52 (2013), pp. 11083-11087
- [386]ChemCatChem, 7 (2015), pp. 2826-2830
- [387]ChemCatChem, 6 (2014), pp. 3419-3425
- [388]Chem. Mater., 26 (2014), pp. 5812-5818
- [389]ACS Appl. Mater. Interfaces, 6 (2014), pp. 16481-16486
- [390]Appl. Phys. A, 105 (2011), pp. 161-166
- [391]J. Mater. Chem., 22 (2012), pp. 1160-1166
- [392]RSC Adv., 3 (2013), pp. 9465-9469
- [393]Mater. Lett., 118 (2014), pp. 208-211
- [394]J. Alloy Compd., 628 (2015), pp. 372-378
- [395]Appl. Catal. B: Environ., 165 (2015), pp. 511-518
- [396]ACS Nano, 10 (2016), pp. 2745-2751
- [397]Appl. Catal. B: Environ., 165 (2015), pp. 503-510
- [398]Angew. Chem. Int. Ed., 51 (2012), pp. 11814-11818
- [399]ACS Appl. Mater. Interfaces, 7 (2015), pp. 25162-25170
- [400]J. Phys. Chem. C, 117 (2013), pp. 9952-9961
- [401]Cryst. Growth Des., 8 (2008), pp. 930-934
- [402]Adv. Funct. Mater., 16 (2006), pp. 2035-2041
- [403]J. Catal., 249 (2007), pp. 59-66
- [404]Small, 4 (2008), pp. 87-91
- [405]Appl. Surf. Sci., 358 (2015), pp. 385-392
- [406]J. Phys. Chem. C, 113 (2009), pp. 10712-10717
- [407]J. Phys. Chem. C, 112 (2008), pp. 2050-2057
- [408]J. Hazard. Mater., 301 (2016), pp. 522-530
- [409]Chem. Commun., 50 (2014), pp. 10837-10840
- [410]J. Phys. Chem. Lett., 6 (2015), pp. 958-963
- [411]J. Mater. Chem., 22 (2012), pp. 21832-21837
- [412]Anal. Chem., 86 (2014), pp. 4528-4535
- [413]Chem. Commun., 50 (2014), pp. 10148-10150
- [414]Chem. Soc. Rev., 45 (2016), pp. 2239-2262
- [415]Adv. Funct. Mater., 22 (2012), pp. 4763-4770
- [416]Adv. Mater., 25 (2013), pp. 2452-2456
- [417]J. Mater. Sci., 51 (2016), pp. 6998-7007
- [418]Int. J. Hydrogen Energy, 41 (2016), pp. 3877-3887
- [419]Sci. Rep. U. K., 6 (2016)
- [420]Nanoscale, 5 (2013), pp. 8921-8924
- [421]Anal. Chem., 85 (2013), pp. 5595-5599
- [422]ACS Appl. Mater. Interfaces, 5 (2013), pp. 6815-6819
- [423]Chem. Sci., 5 (2014), pp. 3946-3951
- [424]Appl. Catal. B: Environ., 163 (2015), pp. 135-142
- [425]Nanoscale, 6 (2014), pp. 14984-14990
- [426]J. Mater. Chem. A, 2 (2014), pp. 2563-2570
- [427]Chem. Commun., 50 (2014), pp. 6094-6097
- [428]J. Mater. Chem. A, 3 (2015), pp. 22404-22412
- [429]ACS Appl. Mater. Interfaces, 7 (2015), pp. 24032-24045
- [430]RSC Adv., 4 (2014), pp. 32690-32697
- [431]Appl. Catal. B: Environ., 190 (2016), pp. 93-102
- [432]RSC Adv., 5 (2015), pp. 88149-88153
- [433]ACS Appl. Mater. Interfaces, 6 (2014), pp. 22116-22125
- [434]Appl. Surf. Sci., 358 (2015), pp. 246-251
- [435]Appl. Surf. Sci., 358 (2015), pp. 393-403
- [436]Appl. Surf. Sci., 311 (2014), pp. 574-581
- [437]RSC Adv., 5 (2015), pp. 49317-49325
- [438]J. Hazard. Mater., 313 (2016), pp. 219-228
- [439]Rsc. Adv., 4 (2014), pp. 28519-28528
- [440]ACS Appl. Mater. Interfaces, 7 (2015), pp. 543-552
- [441]RSC Adv., 3 (2013), pp. 22269-22279
- [442]J. Mater. Chem. A, 3 (2015), pp. 24237-24244
- [443]J. Mater. Chem. A, 2 (2014), pp. 18924-18928
- [444]Angew. Chem. Int. Ed., 54 (2015), pp. 13561-13565
- [445]ChemSusChem, 7 (2014), pp. 2654-2658
- [446]Analyst, 139 (2014), pp. 5065-5068
- [447]RSC Adv., 5 (2015), pp. 14027-14033
- [448]Catal. Sci. Technol., 4 (2014), pp. 4258-4264
- [449]Nanoscale, 6 (2014), pp. 1406-1415
- [450]Appl. Catal. B: Environ., 152 (2014), pp. 46-50
- [451]RSC Adv., 4 (2014), pp. 624-628
- [452]RSC Adv., 5 (2015), pp. 15052-15058
- [453]Environ. Sci. Technol., 48 (2014), pp. 11984-11990
- [454]Appl. Catal. B: Environ., 187 (2016), pp. 144-153
- [455]Appl. Catal. B: Environ., 165 (2015), pp. 335-343
- [456]Nanoscale, 7 (2015), pp. 10334-10339
- [457]Chem. Commun., 52 (2016), pp. 453-456
- [458]Microchim. Acta, 183 (2016), pp. 773-780
- [459]Phys. Chem. Chem. Phys., 17 (2015), pp. 23532-23537
- [460]J. Colloid Interface Sci., 468 (2016), pp. 211-219
- [461]RSC Adv., 5 (2015), pp. 56239-56243
- [462]Chem. Commun., 52 (2016), pp. 3396-3399
- [463]Science, 340 (2013), p. 1226419
- [464]Nat. Nanotechnol., 3 (2008), pp. 563-568
- [465]Science, 331 (2011), pp. 568-571
- [466]Nat. Mater., 12 (2013), pp. 850-855
- [467]Nat. Commun., 5 (2014), p. 4477
- [468]Appl. Surf. Sci., 300 (2014), pp. 159-164
- [469]Appl. Surf. Sci., 359 (2015), pp. 868-874
- [470]Chem. Commun., 51 (2015), pp. 7176-7179
- [471]J. Am. Chem. Soc., 132 (2010), pp. 11642-11648
- [472]J. Phys. Chem. C, 116 (2012), pp. 11013-11018
- [473]Catal. Sci. Technol., 3 (2013), pp. 1703-1711
- [474]Angew. Chem. Int. Ed., 53 (2014), pp. 9240-9245
- [475]J. Am. Chem. Soc., 137 (2014), pp. 1064-1072
- [476]Chem. Commun., 50 (2014), pp. 10768-10777
- [477]Angew. Chem. Int. Ed., 54 (2015), pp. 7230-7232
- [478]Mater. Lett., 146 (2015), pp. 87-90
- [479]Nanoscale, 8 (2016), pp. 11034-11043
- [480]Mater. Res. Bull., 75 (2016), pp. 51-58
- [481]ACS Appl. Mater. Interfaces, 7 (2015), pp. 9630-9637
- [482]RSC Adv., 5 (2015), pp. 88249-88257
- [483]Mater. Chem. Phys., 177 (2016), pp. 529-537
- [484]Catal. Today, 258 (2015), pp. 41-48
- [485]Ceram. Int., 40 (2014), pp. 9293-9301
- [486]J. Hazard. Mater., 271 (2014), pp. 150-159
- [487]Mater. Lett., 126 (2014), pp. 5-8
- [488]Appl. Catal. B: Environ., 147 (2014), pp. 82-91
- [489]Appl. Catal. B: Environ., 142 (2013), pp. 828-837
- [490]ChemCatChem, 5 (2013), pp. 2343-2351
- [491]Colloid Surf. A, 436 (2013), pp. 474-483
- [493]Appl. Surf. Sci., 360 (2016), pp. 601-622
- [494]J. Phys. Chem. C, 114 (2010), pp. 19263-19269
- [495]K.K.M., B.K., N.G., S.B., V.A., Appl. Catal. B: Environ. 199 (2016) 282–291.
- [496]Mater. Res. Bull., 68 (2015), pp. 203-209
- [497]J. Alloy Compd., 656 (2016), pp. 685-688
- [498]Appl. Surf. Sci., 366 (2016), pp. 120-128
- [499]ACS Appl. Mater. Interfaces, 7 (2015), pp. 27925-27933
- [500]Phys. Chem. Chem. Phys., 17 (2015), pp. 10383-10390
- [501]ACS Catal., 4 (2014), pp. 4341-4350
- [502]J. Phys. Chem. C, 116 (2012), pp. 13708-13714
- [503]Int. J. Hydrogen Energy, 38 (2013), pp. 1258-1266
- [504]Mater. Res. Bull., 55 (2014), pp. 212-215
- [505]Appl. Surf. Sci., 355 (2015), pp. 379-387
- [506]Appl. Surf. Sci., 295 (2014), pp. 164-172
- [507]Appl. Catal. B: Environ., 158 (2014), pp. 382-390
- [508]Chin. J. Catal., 36 (2015), pp. 372-379
- [509]Phys. Chem. Chem. Phys., 12 (2010), pp. 13020-13025
- [510]ACS Catal., 5 (2015), pp. 504-510
- [511]Phys. Chem. Chem. Phys., 16 (2014), pp. 4106-4114
- [512]J. Power Sources, 298 (2015), pp. 30-37
- [513]RSC Adv., 4 (2014), pp. 58226-58230
- [514]ACS Catal., 4 (2014), pp. 162-170
- [515]Catal. Sci. Technol., 4 (2014), pp. 3251-3260
- [516]ACS Appl. Mater. Interfaces, 7 (2015), pp. 21868-21874
- [517]Int. J. Hydrogen Energy, 40 (2015), pp. 353-362
- [518]Phys. Chem. Chem. Phys., 15 (2013), pp. 7657-7665
- [519]J. Phys. Chem. C, 116 (2012), pp. 19644-19652
- [520]Chem. Commun., 51 (2015), pp. 13496-13499
- [521]Appl. Catal. B: Environ., 188 (2016), pp. 56-64
- [522]Nanoscale, 8 (2016), pp. 2249-2259
- [523]Phys. Chem. Chem. Phys., 15 (2013), pp. 16883-16890
- [524]Appl. Surf. Sci., 351 (2015), pp. 1146-1154
- [525]Appl. Catal. B: Environ., 152 (2014), pp. 335-341
- [526]Mater. Lett., 158 (2015), pp. 278-281
- [527]Energy Environ. Sci., 4 (2011), pp. 2922-2929
- [528]Chem. Eng. J., 209 (2012), pp. 386-393
- [529]Dalton Trans., 41 (2012), pp. 6756-6763
- [530]J. Mol. Catal. A: Chem., 368 (2013), pp. 9-15
- [531]RSC Adv., 4 (2014), pp. 45397-45406
- [532]Dalton Trans., 43 (2014), pp. 13105-13114
- [533]Appl. Catal. B: Environ., 147 (2014), pp. 554-561
- [534]Nanoscale, 6 (2014), pp. 4830-4842
- [535]Appl. Surf. Sci., 358 (2015), pp. 261-269
- [536]Appl. Catal. B: Environ., 168 (2015), pp. 1-8
- [537]J. Hazard. Mater., 299 (2015), pp. 462-470
- [538]Dalton Trans., 42 (2013), pp. 8606-8616
- [539]J. Hazard. Mater., 260 (2013), pp. 475-482
- [540]RSC Adv., 3 (2013), pp. 13646-13650
- [541]Chem. Soc., 35 (2014), pp. 1794-1798
- [542]New J. Chem., 38 (2014), pp. 5462-5469
- [543]Ceram. Int., 40 (2014), pp. 11963-11969
- [544]Ceram. Int., 41 (2015), pp. 5600-5606
- [545]Small, 10 (2014), pp. 2783-2790
- [546]J. Sol-Gel. Sci. Technol., 72 (2014), pp. 443-454
- [547]Acta Phys. Chim. Sin., 31 (2015), pp. 1145-1152
- [548]J. Alloy Compd., 626 (2015), pp. 401-409
- [549]Phys. Chem. Chem. Phys., 17 (2015), pp. 10218-10226
- [550]Chem. Mater., 28 (2016), pp. 1318-1324
- [551]Appl. Catal. B: Environ., 192 (2016), pp. 72-79
- [552]Appl. Surf. Sci., 365 (2016), pp. 314-335
- [553]J. Mater. Chem. A, 1 (2013), pp. 5333-5340
- [554]Chem. Eng. J., 228 (2013), pp. 435-441
- [555]Mater. Res. Bull., 51 (2014), pp. 432-437
- [556]J. Colloid Interface Sci., 417 (2014), pp. 115-120
- [557]Chin. J. Catal., 35 (2014), pp. 78-84
- [558]Phys. Chem. Chem. Phys., 17 (2015), pp. 11577-11585
- [559]RSC Adv., 5 (2015), pp. 91979-91987
- [560]Appl. Catal. B: Environ., 183 (2016), pp. 133-141
- [561]J. Mater. Chem. A, 1 (2013), pp. 3083-3090
- [562]ACS Appl. Mater. Interfaces, 5 (2013), pp. 10317-10324
- [563]Phys. Chem. Chem. Phys., 16 (2014), pp. 21280-21288
- [564]J. Mater. Sci., 50 (2015), pp. 3057-3064
- [565]J. Phys. Chem. C, 119 (2015), pp. 28417-28423
- [566]Eur. J. Inorg. Chem, 2015 (2015), pp. 1744-1751
- [567]J. Colloid Interface Sci., 416 (2014), pp. 212-219
- [568]Colloid Surf. A, 461 (2014), pp. 202-211
- [569]Dalton Trans., 43 (2014), pp. 12026-12036
- [570]Talanta, 132 (2015), pp. 871-876
- [571]Dalton Trans., 44 (2015), pp. 795-803
- [572]Nanoscale, 7 (2015), pp. 18971-18983
- [573]J. Colloid Interface Sci., 442 (2015), pp. 97-102
- [574]J. Inorg. Organomet. Polym. Mater., 26 (2016), pp. 91-99
- [575]J. Mater. Sci., 51 (2016), pp. 4769-4777
- [576]Appl. Catal. B: Environ., 108 (2011), pp. 100-107
- [577]L. Ge, C. Han, J. Liu, in: H.B. Ji, Y. Chen, S.Z. Chen (Eds.), Advanced Materials and Processes Ii, Pts 1–3, 2012, pp. 957–960.
- [578]J. Mater. Chem., 22 (2012), pp. 11568-11573
- [579]Chin. J. Inorg. Chem., 29 (2013), pp. 2057-2064
- [580]Ceram. Int., 40 (2014), pp. 9077-9086
- [581]RSC Adv., 5 (2015), pp. 99339-99346
- [582]J. Mater. Res., 31 (2016), pp. 713-720
- [583]Appl. Surf. Sci., 355 (2015), pp. 939-958
- [584]RSC Adv., 4 (2014), pp. 38222-38229
- [585]Appl. Surf. Sci., 358 (2015), pp. 181-187
- [586]RSC Adv., 5 (2015), pp. 95727-95735
- [587]Appl. Catal. B: Environ., 187 (2016), pp. 171-180
- [588]ACS Appl. Mater. Interfaces, 5 (2013), pp. 11392-11401
- [589]APL Mater., 3 (2015), p. 104410
- [590]Appl. Catal. B: Environ., 168 (2015), pp. 465-471
- [591]J. Mater. Sci., 51 (2016), pp. 893-902
- [592]J. Taiwan Inst. Chem. Eng., 60 (2016), pp. 643-650
- [593]Adv. Mater., 26 (2014), pp. 4920-4935
- [594]ACS Catal., 3 (2013), pp. 1486-1503
- [595]Nat. Mater., 15 (2016), p. 611
- [596]J. Phys. Chem. Solids, 72 (2011), pp. 1104-1109
- [597]Catal. Commun., 12 (2011), pp. 794-797
- [598]Chem. Eng. J., 281 (2015), pp. 549-565
- [599]RSC Adv., 6 (2016), pp. 10487-10497
- [600]J. Hazard. Mater., 280 (2014), pp. 713-722
- [601]J. Mater. Chem. A, 3 (2015), pp. 19936-19947
- [602]Chin. J. Catal., 36 (2015), pp. 2135-2144
- [603]Dalton Trans., 44 (2015), pp. 4297-4307
- [604]Appl. Surf. Sci., 358 (2015), pp. 377-384
- [605]RSC Adv., 4 (2014), pp. 21405-21409
- [606]Mater. Chem. Phys., 149 (2015), pp. 512-521
- [607]Ind. Eng. Chem. Res., 53 (2014), pp. 8018-8025
- [608]Environ. Sci. Technol., 49 (2015), pp. 649-656
- [609]Chem. Eur. J., 20 (2014), pp. 17590-17596
- [610]RSC Adv., 4 (2014), pp. 19456-19461
- [611]Chem. Commun., 51 (2015), pp. 17144-17147
- [612]RSC Adv., 6 (2016), pp. 28263-28269
- [613]Chem. Eng. J., 290 (2016), pp. 136-146
- [614]ACS Appl. Mater. Interfaces, 8 (2016), pp. 3765-3775
- [615]ACS Appl. Mater. Interfaces, 7 (2015), pp. 15285-15293
- [616]Appl. Catal. B: Environ., 180 (2016), pp. 663-673
- [617]RSC Adv., 4 (2014), pp. 13610-13619
- [618]J. Mater. Chem. A, 3 (2015), pp. 11006-11013
- [619]ChemSusChem, 8 (2015), pp. 1350-1358
- [620]J. CO2 Util., 6 (2014), pp. 17-25
- [621]Energy Environ. Sci., 7 (2014), pp. 791-796
- [622]Nanoscale, 6 (2014), pp. 14950-14961
- [623]J. Mater. Chem. A, 2 (2014), pp. 9380-9389
- [624]Nat. Mater., 13 (2014), pp. 1076-1078
- [625]ACS Nano, 8 (2014), pp. 623-633
- [626]J. Mater. Chem. A, 2 (2014), pp. 19156-19166
- [627]Adv. Mater., 26 (2014), p. 5689
- [628]Int. J. Hydrogen Energy, 38 (2013), pp. 6960-6969
- [629]Langmuir, 30 (2014), pp. 8965-8972
- [630]Catal. Commun., 49 (2014), pp. 63-67
- [631]Mater. Charact., 87 (2014), pp. 70-73
- [632]J. Mater. Chem. A, 3 (2015), pp. 7375-7381
- [633]Appl. Surf. Sci., 364 (2016), pp. 694-702
- [634]RSC Adv., 4 (2014), pp. 31019-31027
- [635]Appl. Catal. B: Environ., 163 (2015), pp. 298-305
- [636]RSC Adv., 6 (2016), pp. 10802-10809
- [637]Appl. Catal. B: Environ., 156 (2014), pp. 122-127
- [638]Appl. Surf. Sci., 358 (2015), pp. 196-203
- [639]RSC Adv., 6 (2016), pp. 25695-25702
- [640]Appl. Catal. B: Environ., 183 (2016), pp. 113-123
- [641]Appl. Catal. B: Environ., 142 (2013), pp. 1-7
- [642]ChemCatChem, 6 (2014), pp. 2315-2321
- [643]Electrochim. Acta, 186 (2015), pp. 292-301
- [644]RSC Adv., 6 (2016), pp. 10895-10903
- [645]RSC Adv., 5 (2015), pp. 42736-42743
- [646]J. Am. Chem. Soc., 134 (2012), pp. 6575-6578
- [647]Langmuir, 20 (2004), pp. 5865-5869
- [648]Catal. Sci. Technol., 4 (2014), pp. 2650-2657
- [649]Nano Res., 8 (2015), pp. 175-183
- [650]Adv. Mater., 25 (2013), pp. 6291-6297
- [651]RSC Adv., 6 (2016), pp. 23709-23717
- [652]Appl. Surf. Sci., 358 (2015), pp. 223-230
- [653]Appl. Catal. B: Environ., 164 (2015), pp. 420-427
- [654]J. Hazard. Mater., 268 (2014), pp. 216-223
- [655]Appl. Catal. B: Environ., 156 (2014), pp. 331-340
- [656]Nano Res., 9 (2016), pp. 3-27
- [657]Appl. Surf. Sci., 358 (2015), pp. 28-45
- [658]Energy Storage Mater., 3 (2016), pp. 24-35
- [659]Water. Res., 86 (2015), pp. 17-24
- [660]Environ. Sci. Technol., 47 (2013), pp. 8724-8732
- [661]Chem. Commun., 52 (2016), pp. 35-59
- [662]Phys. Chem. Chem. Phys., 16 (2014), pp. 6810-6826
- [663]Phys. Chem. Chem. Phys., 17 (2015), pp. 13929-13936
- [664]Adv. Mater., 28 (2016), pp. 2427-2431
- [665]Dalton Trans., 43 (2014), pp. 4878-4885
- [666]ChemCatChem, 6 (2014), pp. 1453-1462
- [667]Appl. Surf. Sci., 358 (2015), pp. 252-260
- [668]Appl. Surf. Sci., 358 (2015), pp. 304-312
- [669]Nanoscale, 8 (2016), pp. 4748-4756
- [670]J. Phys. Chem. C, 118 (2014), pp. 7801-7807
- [671]Phys. Chem. Chem. Phys., 17 (2015), pp. 17355-17361
- [672]RSC Adv., 4 (2014), pp. 60873-60877
- [673]RSC Adv., 4 (2014), pp. 6127-6132
- [674]RSC Adv., 6 (2016), pp. 34633-34640
- [675]Phys. Chem. Chem. Phys., 17 (2015), pp. 24086-24091
- [676]Chem. Commun., 52 (2016), pp. 104-107
- [677]Appl. Surf. Sci., 319 (2014), pp. 344-349
- [678]J. Am. Chem. Soc., 138 (2016), pp. 1359-1365
- [679]ChemSusChem, 5 (2012), pp. 2133-2138
- [680]Appl. Surf. Sci., 375 (2016), pp. 110-117
- [681]Appl. Catal. B: Environ., 117 (2012), pp. 268-274
- [682]Appl. Surf. Sci., 358 (2015), pp. 287-295
- [683]J. Mater. Chem. A, 4 (2016), pp. 3822-3827
- [684]ACS Appl. Mater. Interfaces, 8 (2016), pp. 2287-2296
- [685]ACS Catal., 5 (2014), pp. 941-947
- [686]J. Mater. Chem. A, 3 (2015), pp. 18622-18635
- [687]Small, 11 (2015), pp. 1215-1221
- [688]Appl. Catal. B: Environ., 147 (2014), pp. 546-553
- [689]Chem. Sci., 3 (2012), pp. 443-446
- [690]Appl. Catal. B: Environ., 142 (2013), pp. 414-422
- [691]Phys. Chem. Chem. Phys., 16 (2014), pp. 11492-11501
- [692]Dalton Trans., 44 (2015), pp. 1249-1257
- [693]RSC Adv., 5 (2015), pp. 93615-93622
- [694]J. Am. Chem. Soc., 138 (2016), pp. 6292-6297
- [695]Angew. Chem. Int. Ed., 54 (2015), pp. 2406-2409
- [696]Acs Appl. Mater. Interfaces, 8 (2016), pp. 6011-6018
- [697]Chem. Lett., 45 (2016), pp. 182-184
- [698]ChemCatChem, 7 (2015), pp. 1422-1423
- [699]Chem. Commun., 49 (2013), pp. 10127-10129
- [700]J. Am. Chem. Soc., 138 (2016), pp. 5159-5170
- [701]Chem. Eng. J. (2016)
- [702]Chem. Commun., 51 (2015), pp. 858-861
- [703]Adv. Mater., 27 (2015), pp. 6906-6913
- [704]J. Mater. Chem. A, 2 (2014), pp. 3407-3416
- [705]ACS Appl. Mater. Interfaces, 8 (2016), pp. 17212-17219
- [706]Chem. Eur. J. (2016), 10.1002/chem.201601674
- [707]ChemSusChem, 9 (2016), pp. 795-799
- [708]Adv. Funct. Mater., 25 (2015), pp. 5360-5367
- [709]Phys. Chem. Chem. Phys., 16 (2014), pp. 14656-14660
- [710]RSC Adv., 5 (2015), pp. 15993-15999
- [711]ACS Appl. Mater. Interfaces, 8 (2016), pp. 2617-2627
- [712]Nanoscale, 7 (2015), pp. 13723-13733
- [713]Ceram. Int., 42 (2016), pp. 5575-5581
- [714]RSC Adv., 5 (2015), pp. 56136-56144
- [715]RSC Adv., 5 (2015), pp. 58738-58745
- [716]RSC Adv., 5 (2015), pp. 56913-56921
- [717]ACS Sustain. Chem. Eng., 3 (2015), pp. 269-276
- [718]Appl. Surf. Sci., 358 (2015), pp. 213-222
- [719]Mater. Lett., 145 (2015), pp. 167-170
- [720]ChemCatChem, 6 (2014), pp. 3409-3418
- [721]Appl. Catal. B: Environ., 144 (2014), pp. 622-630
- [722]Mater. Res. Bull., 48 (2013), pp. 3873-3880
- [723]Appl. Catal. A: Gen., 409 (2011), pp. 215-222
- [724]Mater. Lett., 142 (2015), pp. 15-18
- [725]RSC Adv., 5 (2015), pp. 40000-40006
- [726]Catal. Sci. Technol., 4 (2014), pp. 758-765
- [727]Ind. Eng. Chem. Res., 53 (2014), pp. 17645-17653
- [728]ACS Appl. Mater. Interfaces, 5 (2013), pp. 12533-12540
- [729]Appl. Catal. B: Environ., 142 (2013), pp. 553-560
- [730]RSC Adv., 3 (2013), pp. 10973-10982
- [731]Appl. Catal. B: Environ., 184 (2016), pp. 174-181
- [732]J. Phys. Chem. C, 120 (2016), pp. 14467-14473
- [733]Chem. Eur. J., 21 (2015), pp. 9126-9132
- [734]RSC Adv., 4 (2014), pp. 22491-22496
- [735]Chem. Eng. J., 289 (2016), pp. 306-318
- [736]Chemosphere, 148 (2016), pp. 34-40
- [737]Appl. Surf. Sci., 358 (2015), pp. 240-245
- [738]Chem. Commun., 50 (2014), pp. 1999-2001
- [739]CrystEngComm, 16 (2014), pp. 1287-1295
- [740]Appl. Catal. B: Environ., 164 (2015), pp. 380-388
- [741]Dalton Trans., 43 (2014), pp. 12514-12527
- [742]Chin. J. Catal., 36 (2015), pp. 1009-1016
- [743]Dalton Trans., 44 (2015), pp. 11223-11234
- [744]RSC Adv., 6 (2016), pp. 61162-61174
- [745]Appl. Catal. B: Environ., 185 (2016), pp. 225-232
- [746]Appl. Surf. Sci., 370 (2016), pp. 514-521
- [747]Dalton Trans., 42 (2013), pp. 7604-7613
- [748]Appl. Surf. Sci., 358 (2015), pp. 231-239
- [749]J. Polym. Mater., 32 (2015), pp. 411-422
- [750]Acta Phys. Chim. Sin., 31 (2015), pp. 764-770
- [751]J. Inorg. Mater., 30 (2015), pp. 1018-1024
- [752]Dalton Trans., 43 (2014), pp. 982-989
- [753]Appl. Catal. B: Environ., 152 (2014), pp. 262-270
- [754]J. Mater. Chem. A, 3 (2015), pp. 10119-10126
- [755]RSC Adv., 5 (2015), pp. 24281-24292
- [756]Chem. Lett., 44 (2015), pp. 348-350
- [757]Dalton Trans., 43 (2014), pp. 7236-7244
- [758]J. Mol. Catal. A: Chem., 391 (2014), pp. 92-98
- [759]Appl. Catal. B: Environ., 193 (2016), pp. 217-225
- [760]Chem. Commun., 52 (2016), pp. 3050-3053
- [761]J. Catal., 338 (2016), pp. 168-173
- [762]J. Phys. Chem. C, 116 (2012), pp. 5082-5089
- [763]Science, 321 (2008), pp. 1072-1075
- [764]Angew. Chem. Int. Ed., 52 (2013), pp. 11252-11256
- [765]Green Chem., 18 (2016), pp. 944-950
- [766]Science, 344 (2014), pp. 1005-1009
- [767]Science, 342 (2013), pp. 836-840
- [768]RSC Adv., 5 (2015), pp. 65303-65307
- [769]J. Mater. Chem., 22 (2012), pp. 2721-2726
- [770]Dalton Trans., 43 (2014), pp. 6295-6299
- [771]Phys. Chem. Chem. Phys., 16 (2014), pp. 4230-4235
- [772]J. Colloid Interface Sci., 436 (2014), pp. 29-36
- [773]Chem. Mater., 27 (2015), pp. 1612-1621
- [774]Science, 347 (2015), pp. 970-974
- [775]Phys. Chem. Chem. Phys., 18 (2016), pp. 1050-1058
- [776]Phys. Chem. Chem. Phys., 17 (2015), pp. 31140-31144
- [777]Appl. Catal. B: Environ., 193 (2016), pp. 248-258
- [778]Chem. Soc., 36 (2015), pp. 2527-2533
- [779]Dalton Trans., 43 (2014), pp. 12013-12017
- [780]Inorg. Chem. Front., 3 (2016), pp. 336-347
- [781]J. Photochem. Photobiol. C, 27 (2016), pp. 72-99
- [782]Phys. Rev. B, 66 (2002), p. 033408
- [783]Chem. Soc. Rev., 41 (2012), pp. 782-796
- [784]J. Phys. Chem. Lett., 4 (2013), pp. 753-759
- [785]Adv. Mater., 25 (2013), pp. 3820-3839
- [786]Science, 347 (2015), p. 1246501
- [787]Angew. Chem. Int. Ed., 54 (2015), pp. 11350-11366
- [788]Energy Environ. Sci., 4 (2011), pp. 4517-4521
- [789]Small, 9 (2013), pp. 3336-3344
- [790]J. Colloid Interface Sci., 431 (2014), pp. 42-49
- [791]Appl. Surf. Sci., 358 (2015), pp. 2-14
- [792]ChemCatChem, 7 (2015), pp. 2864-2870
- [793]J. Mater. Chem. A, 4 (2016), pp. 2445-2452
- [794]Appl. Surf. Sci., 354 (2015), pp. 196-200
- [795]RSC Adv., 5 (2015), pp. 102700-102706
- [796]ACS Sustain. Chem. Eng., 3 (2015), pp. 3412-3419
- [797]Appl. Catal. B: Environ., 170 (2015), pp. 195-205
- [798]Carbon, 99 (2016), pp. 111-117
- [799]J. Mater. Chem. A, 3 (2015), pp. 13819-13826
- [800]Appl. Catal. B: Environ., 192 (2016), pp. 116-125
- [801]Int. J. Hydrogen Energy, 40 (2015), pp. 7546-7552
- [802]Dalton Trans., 44 (2015), pp. 14368-14375
- [803]Appl. Catal. B: Environ., 192 (2016), pp. 234-241
- [804]Appl. Catal. B: Environ., 179 (2015), pp. 479-488
- [805]Appl. Catal. B: Environ., 190 (2016), pp. 36-43
- [806]RSC Adv., 4 (2014), pp. 62223-62229
- [807]ACS Appl. Mater. Interfaces, 7 (2015), pp. 19234-19242
- [808]Mater. Lett., 178 (2016), pp. 308-311
- [809]Mater. Chem. Phys., 143 (2014), pp. 1462-1468
- [810]RSC Adv., 6 (2016), pp. 18990-18995
- [811]Carbon, 101 (2016), pp. 62-70
- [812]Green Chem., 17 (2015), pp. 509-517
- [813]J. Mater. Chem. A, 3 (2015), pp. 15710-15714
- [814]J. Mater. Chem. A, 4 (2016), pp. 1806-1818
- [815]Int. J. Hydrogen Energy, 41 (2016), pp. 7292-7300
- [816]ACS Appl. Mater. Interfaces, 8 (2016), pp. 13879-13889
- [817]RSC Adv., 5 (2015), pp. 101214-101220
- [818]Phys. Chem. Chem. Phys., 17 (2015), pp. 8070-8077
- [819]Phys. Chem. Chem. Phys., 14 (2012), pp. 16745-16752
- [820]Mater. Res. Bull., 76 (2016), pp. 79-84
- [821]Catal. Sci. Technol., 5 (2015), pp. 3416-3422
- [822]J. Mol. Catal. A: Chem., 418 (2016), pp. 95-102
- [823]Phys. Chem. Chem. Phys., 18 (2016), pp. 19457-19463
- [824]Energy Environ. Sci., 6 (2013), pp. 3553-3558
- [825]J. Mol. Catal. A: Chem., 398 (2015), pp. 268-274
- [826]Chem. Commun., 50 (2014), pp. 1754-1756
- [827]Chem. Sci., 7 (2016), pp. 3062-3066
- [828]J. Am. Chem. Soc., 136 (2014), pp. 12568-12571
- [829]J. Am. Chem. Soc., 99 (1977), pp. 303-304
- [830]Appl. Catal. B: Environ., 94 (2010), pp. 295-302
- [831]J. Phys. Chem. C, 114 (2010), pp. 19378-19385
- [832]Appl. Catal. B: Environ., 69 (2007), pp. 171-180
- [833]J. Mater. Chem. A, 1 (2013), pp. 10727-10735
- [834]J. Phys. Chem. C, 113 (2009), pp. 16394-16401
- [835]J. Mater. Chem., 21 (2011), pp. 1049-1057
- [836]Phys. Chem. Chem. Phys., 17 (2015), pp. 15339-15347
- [837]Environ. Sci. Technol., 42 (2008), pp. 4902-4907
- [838]Phys. Chem. Chem. Phys., 13 (2011), pp. 4853-4861
- [839]J. Phys. Chem. C, 115 (2011), pp. 7339-7346
- [840]J. Photochem. Photobiol. C, 6 (2005), pp. 186-205
- [841]Appl. Surf. Sci., 301 (2014), pp. 428-435
- [842]Appl. Surf. Sci., 357 (2015), pp. 346-355
- [843]Adv. Powder Technol., 27 (2016), pp. 330-337
- [844]Chem. Soc., 36 (2015), pp. 333-339
- [845]Appl. Catal. B: Environ., 188 (2016), pp. 342-350
- [846]Appl. Surf. Sci., 329 (2015), pp. 17-22
- [847]Appl. Catal. B: Environ., 180 (2016), pp. 726-732
- [848]ACS Appl. Mater. Interfaces, 6 (2014), pp. 14405-14414
- [849]Catal. Commun., 59 (2015), pp. 131-135
- [850]Chem. Eng. J., 218 (2013), pp. 183-190
- [851]Appl. Catal. B: Environ., 180 (2016), pp. 656-662
- [852]Appl. Catal. B: Environ., 165 (2015), pp. 428-437
- [853]Appl. Catal. B: Environ., 176 (2015), pp. 654-666
- [854]Ceram. Int., 41 (2015), pp. 12923-12929
- [855]J. Hazard. Mater., 292 (2015), pp. 79-89
- [856]Appl. Surf. Sci., 358 (2015), pp. 188-195
- [857]Chem. Eng. J., 215 (2013), pp. 721-730
- [858]Appl. Catal. B: Environ., 129 (2013), pp. 255-263
- [859]Chem. Eng. J., 246 (2014), pp. 277-286
- [860]Appl. Surf. Sci., 358 (2015), pp. 319-327
- [861]Ceram. Int., 41 (2015), pp. 1197-1204
- [862]Appl. Surf. Sci., 324 (2015), pp. 324-331
- [863]Appl. Catal. A: Gen., 501 (2015), pp. 74-82
- [864]Appl. Surf. Sci., 289 (2014), pp. 394-399
- [865]Appl. Catal. B: Environ., 182 (2016), pp. 115-122
- [866]Catal. Today, 264 (2016), pp. 250-256
- [867]Appl. Surf. Sci., 358 (2015), pp. 343-349
- [868]Chem. Eng. J., 241 (2014), pp. 344-351
- [869]Chem. Eng. J., 234 (2013), pp. 361-371
- [870]Superlattice Microstruct., 76 (2014), pp. 90-104
- [871]RSC Adv., 6 (2016), pp. 33478-33491
- [872]Appl. Surf. Sci., 322 (2014), pp. 249-254
- [873]Appl. Catal. B: Environ., 160 (2014), pp. 89-97
- [874]ACS Appl. Mater. Interfaces, 5 (2013), pp. 7079-7085
- [875]Appl. Catal. B: Environ., 192 (2016), pp. 57-71
- [876]Chem. Eng. J., 270 (2015), pp. 405-410
- [877]Appl. Catal. B: Environ., 166 (2015), pp. 366-373
- [878]RSC Adv., 6 (2016), pp. 39774-39783
- [879]Appl. Catal. B: Environ., 176 (2015), pp. 91-98
- [880]Appl. Catal. B: Environ., 156 (2014), pp. 141-152
- [881]Appl. Surf. Sci., 283 (2013), pp. 25-32
- [882]Chem. Soc., 36 (2015), pp. 17-23
- [883]RSC Adv., 5 (2015), pp. 90750-90756
- [884]Appl. Surf. Sci., 358 (2015), pp. 356-362
- [885]Appl. Catal. B: Environ., 150 (2014), pp. 479-485
- [886]Appl. Catal. B: Environ., 184 (2016), pp. 28-34
- [887]Appl. Surf. Sci., 280 (2013), pp. 967-974
- [888]Appl. Surf. Sci., 361 (2016), pp. 251-258
- [889]Appl. Surf. Sci., 347 (2015), pp. 602-609
- [890]J. Chem. N. Y., 2013 (2013), pp. 1-5
- [891]J. Mater. Res., 29 (2014), pp. 2473-2482
- [892]Optoelectron. Adv. Mater., 5 (2011), pp. 648-650
- [893]Rare Met., 30 (2011), pp. 276-279
- [894]Catal. Commun., 70 (2015), pp. 17-20
- [895]Eur. J. Inorg. Chem. (2015), pp. 4108-4115
- [896]J. Alloy Compd., 622 (2015), pp. 1098-1104
- [897]RSC Adv., 5 (2015), pp. 76963-76972
- [898]RSC Adv., 4 (2014), pp. 35144-35148
- [899]Appl. Surf. Sci., 358 (2015), pp. 336-342
- [900]J. Mol. Catal. A: Chem., 345 (2011), pp. 90-95
- [901]Mater. Sci. Semicond. Process., 35 (2015), pp. 45-54
- [902]RSC Adv., 6 (2016), pp. 34334-34341
- [903]RSC Adv., 4 (2014), pp. 40029-40035
- [904]Appl. Surf. Sci., 358 (2015), pp. 313-318
- [905]RSC Adv., 3 (2013), pp. 20862-20868
- [906]RSC Adv., 5 (2015), pp. 24944-24952
- [907]Science, 352 (2016), pp. 797-801
- [908]Nat. Commun., 6 (2015), p. 8647
- [909]Appl. Catal. B: Environ., 160 (2014), pp. 658-665
- [910]Appl. Catal. B: Environ., 187 (2016), pp. 163-170
- [911]Appl. Catal. B: Environ., 144 (2014), pp. 75-82
- [912]J. Phys. Chem. C, 120 (2016), pp. 3722-3730
- [913]J. Phys. Chem. C, 114 (2010), pp. 16481-16487
- [914]Appl. Surf. Sci., 351 (2015), pp. 66-73
- [915]J. Phys. Chem. C, 118 (2014), pp. 8891-8898
- [916]Appl. Catal. B: Environ., 183 (2016), pp. 231-241
- [917]Appl. Catal. B: Environ., 134 (2013), pp. 83-92
- [918]Appl. Catal. B: Environ., 144 (2014), pp. 369-378
- [919]Appl. Surf. Sci., 353 (2015), pp. 391-399
- [920]Nature, 277 (1979), pp. 637-638
- [921]Angew. Chem. Int. Ed., 52 (2013), pp. 7372-7408
- [922]Coord. Chem. Rev., 257 (2012), pp. 171-186
- [923]ACS Nano, 4 (2010), pp. 1259-1278
- [924]Energy Environ. Sci., 9 (2016), pp. 2177-2196
- [925]Appl. Surf. Sci., 342 (2015), pp. 154-167
- [926]J. Mater. Chem. A, 3 (2015), pp. 5189-5196
- [927]Nano Energy, 19 (2016), pp. 145-155
- [928]Sol. Energy Mater. Sol. Cells, 137 (2015), pp. 175-184
- [929]ACS Catal., 4 (2014), pp. 3637-3643
- [930]Appl. Catal. B: Environ., 158–159 (2014), pp. 20-29
- [931]Catal. Commun., 74 (2016), pp. 75-79
- [932]Mat. Sci. Eng. B: Adv., 202 (2015), pp. 1-7
- [933]J. Colloid Interface Sci., 464 (2016), pp. 89-95
- [934]ACS Appl. Mater. Interfaces, 7 (2015), pp. 25693-25701
- [935]Appl. Catal. B: Environ., 180 (2016), pp. 530-543
- [936]Catal. Sci. Technol., 6 (2016), pp. 744-754
- [937]RSC Adv., 5 (2015), pp. 33254-33261
- [938]Appl. Catal. B: Environ., 179 (2015), pp. 1-8
- [939]Chem. Phys. Lett., 651 (2016), pp. 127-132
- [940]Dalton Trans., 43 (2014), pp. 9158-9165
- [941]J. Am. Chem. Soc., 136 (2014), pp. 8839-8842
- [942]Chem. Commun., 51 (2015), pp. 7950-7953
- [943]J. Colloid Interface Sci., 401 (2013), pp. 34-39
- [944]RSC Adv., 5 (2015), pp. 7066-7073
- [945]RSC Adv., 2 (2012), pp. 6784-6791
- [946]Angew. Chem. Int. Ed., 50 (2011), pp. 657-660
- [947]ChemSusChem, 8 (2015), pp. 3459-3464
- [948]Ind. Eng. Chem. Res., 51 (2012), pp. 9510-9514
- [949]Appl. Catal. B: Environ., 142 (2013), pp. 487-493
- [950]J. Am. Chem. Soc., 131 (2009), pp. 11658-11659
- [951]ChemCatChem, 5 (2013), pp. 192-200
- [952]Chin. J. Catal., 36 (2015), pp. 2030-2035
- [953]Chin. J. Catal., 33 (2012), pp. 1347-1353
- [954]Appl. Catal. B: Environ., 152 (2014), pp. 383-389
- [955]ChemSusChem, 7 (2014), pp. 738-742
- [956]RSC Adv., 3 (2013), pp. 5121-5126
- [957]Angew. Chem. Int. Ed., 51 (2012), pp. 9512-9516
- [958]Appl. Catal. A: Gen., 468 (2013), pp. 184-189
- [959]Adv. Synth. Catal., 353 (2011), pp. 1447-1451
- [960]Sci. Rep. U. K., 3 (2013)
- [961]Chem. Commun., 50 (2014), pp. 4338-4340
- [962]ACS Catal., 6 (2016), pp. 3594-3599
- [963]J. Am. Chem. Soc., 138 (2016), pp. 2868-2876
- [964]Nat. Mater., 14 (2015), pp. 505-511
- [965]Angew. Chem. Int. Ed., 55 (2016), pp. 1824-1828
- [966]Chem. Commun., 50 (2014), pp. 8177-8180
- [967]J. Mater. Chem. A, 3 (2015), pp. 16064-16071
- [968]J. Am. Chem. Soc., 137 (2015), pp. 15338-15341
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