Elsevier

Applied Surface Science 应用表面科学

Volume 391, Part B, 1 January 2017, Pages 72-123
2017 年 1 月 1 日,第 391 卷,B 部分,第 72-123 页
Applied Surface Science

A review on g-C3N4-based photocatalysts
一篇关于 g-C 3 N 4 基光催化剂的综述

https://doi.org/10.1016/j.apsusc.2016.07.030Get rights and content 获取权限和内容

Highlights 亮点

  • The photocatalytic fundamentals of g-C3N4 were systematically summarized.
    对 g-C 3 N 4 的光催化基本原理进行了系统总结。

  • The versatile properties of g-C3N4 photocatalysts were highlighted.
    突出了 g-C 3 N 4 光催化剂的多功能特性。

  • The different design strategies of g-C3N4 photocatalysts were reviewed.
    对 g-C 3 N 4 光催化剂的不同设计策略进行了回顾。

  • The important photocatalytic applications of g-C3N4 were also addressed.
    也讨论了 g-C 3 N 4 的重要光催化应用。

Abstract 摘要

As one of the most appealing and attractive technologies, heterogeneous photocatalysis has been utilized to directly harvest, convert and store renewable solar energy for producing sustainable and green solar fuels and a broad range of environmental applications. Due to their unique physicochemical, optical and electrical properties, a wide variety of g-C3N4-based photocatalysts have been designed to drive various reduction and oxidation reactions under light irradiation with suitable wavelengths. In this review, we have systematically summarized the photocatalytic fundamentals of g-C3N4-based photocatalysts, including fundamental mechanism of heterogeneous photocatalysis, advantages, challenges and the design considerations of g-C3N4-based photocatalysts. The versatile properties of g-C3N4-based photocatalysts are highlighted, including their crystal structural, surface phisicochemical, stability, optical, adsorption, electrochemical, photoelectrochemical and electronic properties. Various design strategies are also thoroughly reviewed, including band-gap engineering, defect control, dimensionality tuning, pore texture tailoring, surface sensitization, heterojunction construction, co-catalyst and nanocarbon loading. Many important applications are also addressed, such as photocatalytic water splitting (H2 evolution and overall water splitting), degradation of pollutants, carbon dioxide reduction, selective organic transformations and disinfection. Through reviewing the important state-of-the-art advances on this topic, it may provide new opportunities for designing and constructing highly effective g-C3N4-based photocatalysts for various applications in photocatalysis and other related fields, such as solar cell, photoelectrocatalysis, electrocatalysis, lithium battery, supercapacitor, fuel cell and separation and purification.
作为最具吸引力和吸引力的技术之一,异质光催化已被用于直接收集、转换和储存可再生太阳能,以生产可持续和绿色太阳能燃料以及广泛的环境应用。由于它们独特的物理化学、光学和电学性质,设计了各种基于 g-C 3 N 4 的光催化剂,在适合波长的光照射下,驱动各种还原和氧化反应。在本综述中,系统总结了基于 g-C 3 N 4 的光催化剂的光催化基础,包括异质光催化的基本机理、优势、挑战以及 g-C 3 N 4 的设计考虑。突出了基于 g-C 3 N 4 的多功能性质,包括它们的晶体结构、表面物理化学、稳定性、光学、吸附、电化学、光电化学和电子性能。还全面审查了各种设计策略,包括带隙工程、缺陷控制、维度调控、孔结构调控、表面增敏、异质结构构建、共催化剂和纳米碳负载。还讨论了许多重要的应用,如光催化分解水(H 2 进化和整体分解水)、污染物降解、二氧化碳还原、选择性有机转化和消毒。 通过审查有关该主题的重要最新进展,可能为设计和构建高效的 g-C 3 N 4 基光催化剂在光催化及其他相关领域的各种应用,如太阳能电池、光电催化、电催化、锂电池、超级电容器、燃料电池及分离与纯化,提供新的机遇。

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。

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Keywords 关键词

Carbon nitride (g-C3N4)
Composite photocatalysts
Co-catalysts
Artificial photosynthesis
Z-scheme heterojunction
Nanocarbons

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。

Fig. 1
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Fig. 1. (a) The annual number of publications containing the word “graphene* (TiO2*, g-C3N4* or carbon nitride*)” in the title and “photocataly*” in the topic since 2009. (b) The annual number of citations for Wang’s pioneering paper published on Nature Materials in 2009. (Using Web of Science, date of search: Jul 2, 2016).
图 1. (a) 自 2009 年以来,标题中包含“石墨烯* (TiO 2 *, g-C 3 N 4 *或碳氮化物*)”并且主题为“光催化*”的出版物年发表数量。(b) 自 2009 年以来,Wang 在 2009 年在《自然材料》上发表的开创性论文的年引用次数。(使用 Web of Science,搜索日期:2016 年 7 月 2 日)。

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。

Fig. 2
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Fig. 2. The fundamental mechanism of heterogeneous photocatalysis. The typical stages: (1) light harvesting; (2) charge excitation; (3) charge separation and transfer; (4) bulk charge recombination; (5) surface charge recombination; (6) surface reduction reactions; and (7) surface oxidation reactions.
图 2. 异质光催化的基本机理。典型阶段:(1)光吸收;(2)电荷激发;(3)电荷分离和转移;(4)体相电荷复合;(5)表面电荷复合;(6)表面还原反应;和(7)表面氧化反应。

Table 1. Band gap structures of several typical photocatalysts.
表 1. 几种典型光催化剂的带隙结构。

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 CellEmpty CellCBVBEg/eV E g /电子伏特Empty Cell
TiO2Anatase 锐钛矿−0.52.73.2[90]
Cu2O 氧化铜 2 −1.160.852.0[91]
CdS−0.91. 52.4[92]
g-C3N4−1.31.42.7[44], [84]
g-C3N4−1.53 -1.531.162.7[93]a
Ta3N5−0.75 -0.751.352.1[94]
TaON−0.75 -0.751.752.5[94]
BiVO4−0.3 -0.32.12.4[95]
WO3−0.12.72.8[96]
Ag3PO4cubic 立方的0.042.492.45[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。

Table 2. Standard redox potentials for some typical species [83], [98]].
表 2. 一些典型物种的标准氧化还原电位[83],[98]。

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 → HO2radical dot (aq)−0.046
O2(g) + 2H+ + 2e → H2O2 (aq)0.695
2H2O (aq) + 4 h+ → O2 (g) + 4H +1.229
OH + h+ → radical dotOH2.69
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
HO2radical dot + H+ + e → H2O2(aq)
HO 2 radical dot + H + + e→H 2 O 2 (aq)
1.44
H2O2(aq) + 2H+ + 2e → 2H2O1.763

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。

Fig. 3
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Fig. 3. The redox potentials of the relevant reactions with respect to the estimated position of the g-C3N4 band edges at pH 7.
图 3. 在 pH 7 条件下,相关反应的氧化还原电位与估计的 g-C 3 N 4 带边位置相关。

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)。

Fig. 4
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Fig. 4. Fabrication strategies of g-C3N4 and N-rich precursors.
图 4. g-C 3 N 4 和富氮原料的制备策略。

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, radical dotOH. 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 radical dotOH [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, radical dot 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 radical dot 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。

Fig. 5
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Fig. 5. Design considerations of g-C3N4-based photocatalysts based on the different photocatalytic stages.
图 5. 基于不同光催化阶段的 g-C 3 N 4 基光催化剂的设计考虑。

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]。

Table 3. Structural parameters and total energies for different C3N4 phases [131].
表 3. 不同 C 3 N 4 相的结构参数和总能量 [131]。

C3N4 phases
C 3 N 4
Space group 空间群Lattice parameter (Å) 晶格常数(Å)ZGW 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 = 4.70945.493.76
Beta 贝塔P3 (143)a = 6.406, c = 2.40624.853.12
Cubic 立方的I-43d (220)a = 5.41124.302.87
Pseudocubic 假立方P–42 m (111)a = 3.42614.132.53
g-h-triazine g-h-三嗪P-6m2 (187)a = 4.746, c = 6.58622.971.16
g-h-heptazineCmc21 (36)a = 7.083, b = 12.269, c = 6.87142.880.89
g-o-triazineP2 mm (25)a = 4.147, b = 4.754, c = 6.47420.93
Fig. 6
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Fig. 6. The stacked 2D layered structure of g-C3N4 (a); structures of s-triazine (b) and tri-s-triazine (c) as the primary building blocks of g-C3N4.
图 6. g-C 3 N 4 的叠层 2D 结构(a);s-三嗪(b)和三 s-三嗪(c)作为 g-C 3 N 4 的主要构建模块的结构。

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。

Fig. 7
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Fig. 7. XRD patterns of (a) the tubular carbon nitride and (b) the bulky g-C3N4 synthesized by directly heating melamine at 520 °C for 2 h [138].
图 7. (a) 管状碳氮化物和(b) 直接在 520°C 加热三聚氰胺 2 小时合成的臃肿 g-C 3 N 4 的 X 射线衍射图样 [138]。

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 (single bondNHsingle bond, double bondNsingle bond, single bondNH2 and single bondNsingle bondCdouble bond) 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( single bond NH single bonddouble bond N single bondsingle bond NH 2single bond N single bond C double bond )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]。

Fig. 8
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Fig. 8. Multiple functional surface properties of polymeric g-C3N4 material with defects [101].
图 8. 具有缺陷的聚合物 g-C 3 N 4 材料的多功能表面性能 [101]。

Commonly, these functional groups and networks (Csingle bondNsingle bond) 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 Cdouble bondN stretching modes and out-of-plane bending vibrations of the sp3 Csingle bondN 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 Nsingle bondH 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 Nsingle bondH in the bridging Csingle bondNHsingle bondC units and Osingle bondH 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 Ndouble bondC (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 Ndouble bondCsingle bondN2 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 (Cdouble bondNsingle bondC), tertiary nitrogen (Nsingle bond(C)3) and amino functional groups having a hydrogen atom (Csingle bondNsingle bondH), 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 single bond N single bond )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 double bond N 拉伸模式和 sp 3 C single bond 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 single bond 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 single bond NH single bond C 单元中残留的游离 N single bond 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 double bond 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 double bond C single bond 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 double bond N single bond C)、三级氮(N single bond (C) 3 )和具有氢原子的氨基官能团(C single bond N single bond 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)。

Fig. 9
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Fig. 9. FTIR spectra of g-C3N4 powders obtained by heating melamine (MCN), thiourea (TCN), and urea (UCN) [146].
图 9. 通过加热三聚氰胺(MCN)、硫脲(TCN)和尿素(UCN)得到的 g-C 3 N 4 粉末的傅里叶红外光谱 [146]。

Fig. 10
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Fig. 10. Comparison between the Raman spectra of coplanar bulk and 1-layer g-C3N4 samples (780 nm laser) [154].
图 10. 比较共面体积和单层 g-C 3 N 4 样品的拉曼光谱(780nm 激光) [154]。

Fig. 11
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Fig. 11. High-resolution XPS spectra of C 1s (A) for the 1.0 wt% RGO/g-C3N4 sample (a) and GO (b) and N 1s (B) for the 1.0 wt% RGO/g-C3N4 sample [151].
图 11. 1.0wt% RGO/g-C 3 N 4 样品的高分辨率 XPS 光谱中的 C 1s(A)(a)和 GO(b),以及 N 1s(B)的图谱 [151]。

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]。

Fig. 12
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Fig. 12. Zeta potentials of MCN, TCN, and UCN powders as functions of the pH value of the suspensions (as shown in Fig. 12) [146].
图 12. MCN,TCN 和 UCN 粉末的 Zeta 电位随悬浮液的 pH 值变化的函数(如图 12 所示)[146]。

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]。

Fig. 13
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Fig. 13. The TG-DSC analysis for heating the melamine (a) and the g-C3N4 (b) obtained by heat polymerization of melamine at 520 °C [149].
图 13. 图示了通过 520°C 热聚合得到的热分析法对三聚氰胺(a)和 g-C 3 N 4 (b)的 TG-DSC 分析 [149]。

Fig. 14
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Fig. 14. Reaction path for the formation of g-C3N4 starting from cyanamide.
图 14. 由氰胺起始形成 g-C 3 N 4 的反应路径。

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]。

Fig. 15
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Fig. 15. Electronic structure of g-C3N4. (a) DFT band structure for g-C3N4 calculated along the Γ–X and Y–Γ directions. The potentials for H+ to H2 and H2O to O2 are displayed by the blue and red dashed lines, respectively; the Kohn–Sham orbitals for the valence band (b) and conduction band (c) of g-C3N4. The C, N and H atoms are gray, blue and white, respectively. The isodensity surfaces are drawn for a charge density of 0.01qe°A−3 [44]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
图 15. g-C 3 N 4 的电子结构。(a) 计算得到的沿Γ–X 和 Y–Γ方向的 g-C 3 N 4 的 DFT 能带结构。蓝色和红色虚线分别表示 H + 到 H 2 和 H 2 O 到 O 2 的电势;g-C 3 N 4 的 Kohn–Sham 轨道的价带(b)和导带(c)。C,N 和 H 原子分别用灰色、蓝色和白色表示。以 0.01q e °A −3 的电荷密度绘制等电密度表面[44]。(关于图中颜色的解释,请参阅本文的网络版。)

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。

Fig. 16
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Fig. 16. Schematic illustration of the band structures of different types of g-C3N4 samples: g-C3N4 [84], Fe–g-C3N4 [54], [181], S–g-C3N4 [182], P–g-C3N4 [183], O–g-C3N4 [184], C–g-C3N4 [185], I–g-C3N4 [186] and B–g-C3N4 [84]. VB-XPS: valence band X-ray photoelectron spectroscopy; MS: electrochemical analysis by Mott–Schottky plots.
图 16 g-C 3 N 4 样品不同类型的能带结构示意图:g-C 3 N 4 [84],Fe–g-C 3 N 4 [54],[181],S–g-C 3 N 4 [182],P–g-C 3 N 4 [183],O–g-C 3 N 4 [184],C–g-C 3 N 4 [185],I–g-C 3 N 4 [186] 和 B–g-C 3 N 4 [84]。VB-XPS:价带 X 射线光电子能谱;MS:Mott–Schottky 图的电化学分析。

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]。

Fig. 17
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Fig. 17. (a), time-resolved PL spectrum monitored at 525 nm under 420 nm excitation at 298 K for bulk g-C3N4 (black) and mpg-C3N4 (red) [123];(b), time-resolved PL spectra monitored at 480 nm under 420 nm excitation at 77 K for CN and CNS–CN [121]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
图 17. (a),在 298K 下以 420nm 激发下监测到 525nm 处的时分辨发光光谱,对比了块状 g-C 3 N 4 (黑色)和 mpg-C 3 N 4 (红色)[123];(b),在 77K 下以 420nm 激发下监测到 480nm 处的时分辨发光光谱,对比了 CN 和 CNS–CN [121]。(关于该图例中颜色的解释,请参阅本文的网络版本。)

Fig. 18
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Fig. 18. SPV of g-C3N4 (Ni0) and Ni@g-C3N4 (Ni10). The inset shows the schematic setup of SPV measurements [192].
图 18. g-C 3 N 4 (Ni0)和 Ni@g-C 3 N 4 (Ni10)的 SPV。插图显示了 SPV 测量的示意图[192]。

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。

Fig. 19
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Fig. 19. (a) UV/Vis absorption spectra of g-C3N4 prepared at different temperature. Inset: photograph of the photocatalyst [44]; (b) the room-temperature PL spectrum of g-C3N4 solid powder (λ = 365 nm, top figure) and the ECL spectrum of g-C3N4-modified electrode in 0.10 M K2SO4 and 3.0 mM K2S2O8 solution by cycling the potential between 0.00 and −1.30 V (vs. Ag/AgCl) with a scan rate of 100 mV/s and step potential of 1 mV (bottom figure) [78].
图 19。(a) 在不同温度制备的 g-C 3 N 4 的 UV/Vis 吸收光谱。插图:光催化剂的照片[44];(b) g-C 3 N 4 固体粉末的室温 PL 光谱(λ=365nm,顶部图)和 g-C 3 N 4 修饰电极在 0.10MK 2 SO 4 和 3.0mMK 2 S 2 O 8 溶液中的 ECL 光谱,通过以 100mV/s 的扫描速率和 1mV 的步进电位在 0.00 和−1.30V(对 Ag/AgCl)之间循环电位(底部图)[78]。

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。

Fig. 20
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Fig. 20. Adsorption kinetics (a) and adsorption isotherms (b) of methylene blue on MCN, TCN, and UCN [146].
图 20. 甲基蓝在 MCN,TCN 和 UCN 上的吸附动力学(a)和吸附等温线(b)[146]。

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]。

Fig. 21
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Fig. 21. (a) CO2-capture capacities of g-C3N4 (CN) and amine-functionalized g-C3N4 (3CN) [227]; (b) CO2 adsorption isotherms of the mesoporous g-C3N4 microspheres at 25 and 75 °C [229].
图 21. (a) g-C 3 N 4 (CN)和胺功能化的 g-C 3 N 4 (3CN)的 CO 2 -捕获容量 [227]; (b) g-C 3 N 4 介孔微球在 25 和 75°C 的 CO 2 吸附等温线 [229]。

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。

Fig. 22
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Fig. 22. (A), (a) Free energy plots of ORR on g-C3N4 with 0e, 2e, and 4e paths (corresponding to paths I, II, and III). (b–d), Schemes of ORR’s pathway on pristine g-C3N4 with 0e, 2e or 4e participation, respectively (red areas represent the active sites facilitating ORR). (B) ORR polarization curves for various electrocatalysts on rotating electrode at 1500 rpm in O2-saturated 0.1 M KOH solution [69]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
图 22 (A),(a) g-C 3 N 4 上 ORR 的自由能图,显示 0e 、2e 和 4e 路径(分别对应路径 I、II 和 III)。(b-d),分别展示了在原始 g-C 3 N 4 上进行 0e 、2e 或 4e 参与的 ORR 路径示意图(红色区域代表促进 ORR 的活性位点)。(B) 在 1500rpm 下在 O 2 -饱和的 0.1M KOH 溶液中旋转电极上各种电催化剂的 ORR 极化曲线[69]。(有关本图图例中颜色的解释,请参阅本文的网络版本。)

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。

Fig. 23
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Fig. 23. (a) The HER polarization curves of g-C3N4, NG, g-C3N4/NG mixture, 33 wt% of g-C3N4@NG and referenced 20% Pt/C smaples (electrolyte: 0.5 M H2SO4, scan rate: 5 mV s−1). (b) The calculated Gibbs free-energy for chemical adsorption of H* on three metal-free catalysts and Pt reference at the equilibrium potential. (c) Volcano plots of i0 as a function of the ΔGH* for the C3N4@NG (red triangle), various metals (open symbols) and a nanostructured MoS2 electrocatalyst (closed symbol) [239]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
图 23.(a) g-C 3 N 4 ,NG,g-C 3 N 4 /NG 混合物,33wt% g-C 3 N 4 @NG 和参考 20% Pt/C 样品的 HER 极化曲线(电解液:0.5M H 2 SO 4 ,扫描速率:5mVs −1 )。(b)在平衡电位下,三种无金属催化剂和 Pt 参考的 H * 化学吸附的计算吉布斯自由能。(c) i 0 的火山图谱作为ΔG H* 的函数,用于 C 3 N 4 @NG(红色三角形),各种金属(空心符号)和纳米结构 MoS 2 电催化剂(实心符号)[239]。 (有关本图图例中颜色的解释,请参阅本文的网页版。)

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。

Fig. 24
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Fig. 24. (a) Variation of photocurrent density versus applied voltage. Number lables (3), (2), and (1) data represent the hybrid 3D WO3/C3N4//CoOx, WO3/C3N4, and WO3, respectively. (b) Energy diagram and expected charge flow of WO3/C3N4 [285].
图 24 (a) 光电流密度随施加电压变化图。数字标签(3)、(2)和(1)数据分别代表混合 3D WO 3 /C 3 N 4 //CoO x ,WO 3 /C 3 N 4 和 WO 3 。(b) WO 3 /C 3 N 4 的能级图和预期电荷流[285]。

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。

Fig. 25
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Fig. 25. Summary of band-gap engineering for g-C3N4.

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.

Fig. 26
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Fig. 26. Two kinds of metal ion doping of g-C3N4 framework: (a) cave doping, the incorporation of metal ions (Mn +) through the coordination interactions, Color scheme: C, red; N, yellow [27]; (b) interlayer doping (the interlayer bridging pattern for K) [306]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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 Csingle bondF 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.

Fig. 27
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Fig. 27. Possible substituted sites of non-metal doping in the single layer of g-C3N4.

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.

Fig. 28
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Fig. 28. (a) UV–vis absoprtion spectra of g-C3N4 and CNBx (arrow direction, x = 0.05, 0.1, 0.2, 0.5, 1, 2), where x refers to the weight ration of barbituric acid [84]. (b) H2 evolution over different g-C3N4 copolymerized by urea and various monomers (3wt%Pt as co-catalyst) [129].

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.

Fig. 29
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Fig. 29. (a) UV–vis absorption spectra g-C3N4 (GCN) and amorphous g-C3N4 (ACN). Inset: Schematic of monolayer crystalline GCN and ACN; (b) The detailed band structures of GCN and CAN, as well as the redox potentials of water splitting [348].

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 radical dotOH 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.

Fig. 30
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Fig. 30. (a) Lateral view and (b) vertical view for the interactions between water and a layer of defect g-C3N4 sheet. Red, white, gray and white spheres represent O, H, C and N atoms, respectively. (c) Spatial distribution functions of O and H atoms projected onto the defect g-C3N4 sheet within the first layer [353]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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.

Table 4. Summarization of different fabrication methods for mesoporous g-C3N4.

MorphologyTemplatesPrecursorBand gap [eV]BJH pore size (nm)Pore volume (cm3 g−1)BET surface area [m2 g−1]Ref. (year)
Hard templating methods
NanotubesPorous anodic Al2O3 (AAO)
membranes
EDA, CTC[358] (2009)
NanorodsCA2.925[359] (2011)
Ordered mesoporousOrdered Mesoporous SBA-15CA2.745.30.34239[122] (2009)
Mesoporous12-nm SiO2 particlesCA2.7373[123] (2009)
8.30.41126
Ordered mesoporousSBA-15 pre-treated with 1 M HClCA3.40.49517[360] (2013)
Hierarchical mesostructuresMesostructured cellular silica foamsEDA, CTC4/430.9550[229] (2010)
Inverse opal structuresUniform-sized silica nanospheresCA200.79230[361] (2011)
701.7140
Mesoporous
nanorods
SBA-15 nanorodsCA3.9110–200[362] (2012)
Ordered mesoporousSBA-15ATC2.785.30.34239[170] (2011)
Mesoporous12-nm SiO2 particlesATC0.77176[363] (2012)
Hollow nanospheresMonodisperse silica@ mesoporous silicaCA2.979[364] (2012)
Helical rodlikeChiral mesoporous silicaCA2.753.8/10.756[365] (2014)
Porous nanorodChiral silica
nanorods
CA2.752[366] (2014)
Mesoporous
sphere
7 nm colloidal silica particlesUrea7224[367] (2012)
Porous compositeGraphene oxide sheetsMA2.717.60.0926.6[151] (2011)
g-C3N4@CMK-3 compositeCMK-3 mesoporous carbonCA3.00.49623[69] (2011)
Cubic mesoporousOrdered mesoporous silica KIT-6CA3.60.4208[368] (2010)
Nanosheet-based nanospheresKCC-1 silica spheresCA2.863.80.4160[369] (2014)
PorousCaCO3DCDA38.6[370] (2015)
PorousZnCl2DCDA2.890.0846[371] (2015)
Macroscopic 3D Porous monolithMA spongeUrea0.7678[372] (2015)

Soft templating methods
Sponge-like mesoporeIonic liquids(BmimBF4)DCDA0.32444[114] (2010)
Porous nanosheetsBmimBF4Urea25.0
23.4
0.40
0.51
73
90
[373] (2014)
BMIM-PF6DCDA[312] (2010)
NanoporousBmimDCNDCDA5.60.17981[374] (2010)
Worm-like
porous
Pluronic P123MA1.5590[375] (2012)
NanoporousPluronic P123DCDA0.128299[374] (2010)
NanoporousTriton X-100DCDA3.80.284116[374] (2010)
Bimodal mesoporousTriton X-100MA, GA3.8/10–40[376] (2011)
NanoporousTriton X-100MA sulfate2.47–2.5750–135[377] (2014)
Bubble (urea)DCDA60[378] (2014)
NanoporousBubble (thiourea)DCDA2.723.746.4[379] (2013)
PorousBubble (water vapor)Urea18.20.32169.6[380] (2012)
Sponge-likeAmmonium alginate or gelatinDCDA2.4963[279] (2013)
MesoporeBubble (sublimed sulfur)MA5046[381] (2015)
MesoporeBubble (sucrose)MA13.20.355121[382] (2015)
Honeycomb-likeBubble (water)urea2.650.68106[383] (2015)
Diatom-structureDiatomiteCA5[384] (2013)

Self-templating (Supramolecular self-assembly) methods
Hollow boxSelf-assemblyCAA/MA45[110] (2013)
Hollow spheresSelf-assemblyMA/CAA30–400.477[107] (2013)
Spherical particlesSelf-assemblyMA/CAA2.790.366[385] (2013)
Hollow tube-likeSelf-assemblyCAA/MA41[386] (2015)
Hollow to wormlikeSelf-assemblyMA, urea, CAA2.7312.840.3197.4[387] (2014)
Fiber-type/sheet-likeSelf-assemblyCA, MA, DPT2.2575 ± 5[111] (2014)
Roll-likeSelf-assemblyCAA, MA, BA60–70[388] (2014)
CrystallineSelf-assemblyMA, TAP119[108] (2014)
PorousSelf-assemblyCAA/MA2.777[389] (2014)

Template-free methods
NanobeltsMA[390] (2011)
PorousMA hydrochloride2.8369[391] (2012)
PorousDCDA2.7215.80.50201[392] (2013)
Hierarchical structureMA2.7435.6[393] (2014)
NanoporousBall milling/hydrothermal methodMA2.750.1530.9[394] (2015)
Nanosheetsurea2.781.41288[153] (2013)
Seaweed-like architectureFreezing assistant assemblyDCDA<20130[395] (2015)
Monolayer mesoporousFreezing assistant assembly/exfoliationDCDA2.75<20331[396] (2016)
Porous microspheresSolvothermal in acetonitrile (200 °C)CAC/MA2.428.5[397] (2015)
NanosheetsUrea, Ph4BNa2.8316.20.62144[328] (2013)
Nanorod-network superstructuresSolvothermal in acetonitrile (180 °C)CAC/MA1.9230[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.

Table 5. Summarization of different fabrication methods for g-C3N4 nanosheets.

Bulk g-C3N4Solvents, bulk powder/solvent (mg/ml), time (h)Concentration (mg/mL)/Yield (%)Thickess (nm)/Layer numbersBand gap (eV)/surface area (m2 g−1)Ref. (year)
Ultrasonication-assisted liquid exfoliation
MA (P, 600 °C)Water, 100/100, 16 h0.15/2.5/72.70/[100] (2013)
Poly(triazine imide) (I)Water, 2/1, 15 h0.2/1–2/3–6[103] (2014)
CA (P, 530 °C)Water, 200/200, 24 h∼1.8/5–62.6/[410] (2013)
MA (P, 600 °C)Water, 50/50, 10 h/14.51.2/4[299], [420](2013)
MA (P, 600 °C)Water, 50/50, 10 h/14.51.0/3[421] (2013)
MA (P, 600 °C)Water, 100/200, 16 h/8.6[422] (2013)
MA (P, 550 °C)Water, 50/50, 2 h1.2/<5[423] (2014)
MA (P, 550 °C)Water/organic solvents, 500/150, 10 h3/0.38/12.79/59.4[424] (2015)
Commercial g-C3N4 (Carbodeon Ltd)Isopropanol, 30/10, 10 h2/ < 92.65/384[416] (2013)
CA (P, 550 °C)Ethanol, 50/50, 2 h2–3/5–82.73/112.5[425] (2014)
DCDA (P, 600 °C)1,3-butanediol, 60/25, 24 h0.35/0.9–2.1/3–62.79/32.54[426] (2014)
MA (P, 550 °C)30 wt% isopropanol + water, 4/1, 10 h2/62.70/[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)+ melamine2–3/6–92.75/116.76[428] (2015)
Commercial g-C3N4 (Carbodeon Ltd)Ethanolamine/1,3-butanediol/3-pyridinemethanol,30/10,4h1/7[429] (2015)

Chemical exfoliation
MA (P, 550 °C)Liquid ammonia-assisted lithiation/85%2.5/82.78/22.5[430] (2014)
MA + LiCl(P, 380 + 550 °C)The intercalation of LiCl ions and the liquid exfoliation in water2–3/6–92.82/186[431] (2016)
DCDA (P, 550 °C)H2SO4 (98 wt%) treatment for 8 h, then rapid exfoliation/30%0.4/12.92/205.8[178] (2013)
MA (P, 550 °C)H2SO4 (98 wt%), 1000/15, 15 min/70%2.52.93/86.29[432] (2015)
DCDA (P, 550 °C)H2SO4 (98 wt%)300//13.28/[180] (2015)
DCDA (P, 550 °C)HCl treatment for 1 h, then exfoliation for 2 h/25–30%2–4/6–102.75/305[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%9.0/303.42/[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%1.62–2.62/4–72.97/306[415] (2012)
DCDA(P, 550 °C)Air, 500 °C, 2 h/<6%1.62–2.62/4–72.97306[409] (2014)
Urea (P, 550 °C)Air, 550 °C, 2 h/8.016/2.86/151[435] (2015)
MA (P, 550 °C)Air, 500 °C, 2 h3.06/165.66[436] (2014)
MA (P, 550 °C)Air, 500 °C, 2 h5–8/10–202.91/122.6[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 h2.7/82.82/153.32[438] (2016)
MA (P, 500 + 550 °C)Air, 500 °C, 2 h1.9/62.97/150.1[439] (2014)
MA (P, 600 °C)Air, 500 °C, 4 h0.9/3[440] (2015)
DCDA (P, 520 °C)Air, 400 °C, 4 h2.89/[441] (2013)
Commercial g-C3N4 (Carbodeon Ltd)H2, 400 °C, 4 h2/6/260[442] (2015)
DCDA(P, 550 °C)NH3, 510 °C, 1 h20/502.95/196[350] (2015)
Mixed DCDA/NH4ClAir, 550 °C, 2 h3.1/82.83/52.9[443] (2014)
Mixed DCDA/NH4ClAir, 550 °C, 4 h1.0/32.77/77.7[444] (2015)
Mixed MA/KClAir, 550 °C, 4 h1.5–6.32.71/[445] (2014)
Mixed MA/KBH4Air, 550 °C, 4 h1.5/4[446] (2014)
Guanidinium cyanurateAir, 550 °C, 4 h0.6–1.5/1–32.87/133.8[447] (2015)
Guanidinium chlorideAir, 600 °C, 4 h2.44/109.9[448] (2014)
DCDA (P, 600 °C)Air, 350 °C, 2 h (NH4Cl intercalation, N2)2–3/6–92.85/30.1[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)0.8–1.2/2–33.03/[452] (2015)
MA (P, 520 °C)(Air, 550 °C, 3 h)+ (isopropanol, 8 h)0.5/13.0/380[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)5–10/15–203.0/140[456] (2015)
DCDA/2-aminobenzonitrile (P, 550 °C)(5 M HNO3, 400/80, 20 h/under reflux)+ (water, 4 h)0.6–4/2–102.89/[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)2.5/72.88/80[459] (2015)
MA (P, 550 °C)(H2SO4 (98 wt%), 1000/10, 8 h) + (N2, 550 °C, 2 h)0.8–1.4/3–42.79/54.3[460] (2016)

Other approaches
MA (P, 550 °C)Ball milling method/1–22.98/[461] (2015)
MA and carbon fibreMicrowave heating approach1.6/52.88/239[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.

Fig. 31
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Fig. 31. (a) Schematic illustration for the synthesis process of ultrathin g-C3N4 nanosheets via liquid exfoliation. (b) AFM image of the synthetic g-C3N4 nanosheets. (d) The corresponding height image of two random nanosheets [100].

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.

Fig. 32
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Fig. 32. Schematic illustration for fabricting the ultrathin nanosheets of g-C3N4 through a dicyandiamide-blowing method [443].

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.

Fig. 33
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Fig. 33. Mechanisms for g-C3N4 photocatalysts sensitized by (a) plasmonic metals, (b) quantum dots (QDs), (c) organic dyes.

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.

Fig. 34
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Fig. 34. Synthetic route and charge-separation mechanism for C3N4 and core–shell nanostructured Ag@C3N4 photocatalysts under light irradiation [488].

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.

Fig. 35
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Fig. 35. Spatial charge-separation mechanisms for four different types of semiconductor heterojunctions: (A) Schottky junction, (B) Type I, (C) Type II, and (D) Type III heterojunctions.

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.

Fig. 36
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Fig. 36. Schematic illustration of spatial charge separation in CdS/g-C3N4 (a) and g-C3N4/S-g-C3N4(b).

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].

Fig. 37
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Fig. 37. Schematic illustration of spatial charge separation in the all-solid-state g-C3N4-based Z-scheme photocatalytic systems with (a) and without (b) mediators.

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 (radical dotO2 and radical dotOH) 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].

Fig. 38
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Fig. 38. Schematic illustration for the charge transfer and separation in g-C3N4-TiO2 Z-scheme photocatalysts under UV light irradiation [523].

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].

Fig. 39
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Fig. 39. Schematic illustration of 2D layered composites in comparison with other kinds of composites [476].

Fig. 40
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Fig. 40. Schematic illustration of 2D-2D coupling of g-C3N4/graphene (a) and g-C3N4/MoS2(b).

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/O2radical dot (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.

Fig. 41
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Fig. 41. Four kinds of co-catalysts over g-C3N4.

Table 6. Summary of various kinds of co-catalysts over g-C3N4-based photocatalysts.

Roles/functions of co-catalystsNoble-metal co-catalystsEarth-abundant metal co-catalystsMetal-free and Hybrid co-catalysts (nano carbons)
H2 evolutionMetal: 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-catalystsRuO2, [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 reductionPt [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.

Fig. 42
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Fig. 42. Roles of carbon materials in enhancing the performance of g-C3N4-based composite photocatalysts [781].

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 Csingle bondOsingle bondC 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.

Fig. 43
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Fig. 43. The proposed mechanism for photocatalytic water splitting over the g-C3N4(electron sink and H2-evolution site)/RGO-3 composite [773].

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.

Table 7. Summary of the photocatalytic H2 evolution on g-C3N4-based photocatalysts.

photocatalyststructuresynthetic methodco-catal./mass ratiomass (g)light sourceincident lightaqueous reactionco-catal./activity (μmol h−1)QY (%)reference (year)
Ni@/g-C3N4particles/lamellarsolvothermal methodNi/10 wt.%0.05500 W Xe lampquartz reactor10 vol.% TEOANi/8.41[192] (2015)
NiS/g-C3N4nanoparticles/lamellarhydrothermal methodNiS/1.25 wt.%0.1300 W Xe lamp, λ > 420 nmPyrex reactor15 vol.% TEOANiS/461.9 (440 nm)[125] (2013)
NiS/g-C3N4nanoparticles/lamellarin stiu ion-exchange methodNiS/1.5 mol%0.1300 W Xe lamp, λ > 420 nm10 vol.% TEOANiS/44.77[670] (2014)
NiS/e-C3N4ultrathin nanosheets/nanoparticlesliquid exfoliation-hydrothermal methodNiS/1.0 wt.%0.05150 W Xe lamp, λ > 400 nmquartz reactor10 vol.% TEOANiS/4.2[671] (2015)
Ni(OH)2/g-C3N4nanoparticles/lamellarprecipitation methodNi(OH)2/0.5 wt.%0.05350 W Xe arc lamp, λ > 400 nmPyrex reactor10 vol.% TEOANi(OH)2/7.61.1 (420 nm)[126] (2013)
Ni/NiO/g-C3N4core-shell/lamellarin situ immersion methodNi/NiO/2 wt.%0.05350 W Xe lamp, λ > 420 nmPyrex reactor10 vol.% TEOANi/NiO/10[792] (2015)
NiS/CNT/mpg-C3N4nanoparticles/nanotubes/lamellarsol-gel-precipitation methodNiS/1 wt.%0.05300 W Xe lamp, λ ≥ 420 nmquartz reactor10 vol.% TEOANiS/26.05[198] (2015)
NiS/CB/g-C3N4nanoparticles/lamellarphysical mixing-chemical depositionCB/0.5 wt.%, NiS/1.0 wt.%0.05300 W Xe lamp, λ ≥ 420 nmquartz reactor15 vol.% TEOACB, NiS/49.6[156] (2015)
Ni(dmgH)2/g-C3N4sub-mircowires/lamellarchemical depositionNi(dmgH)2/3.5 wt.%0.005300 W Xe lamp, λ > 420 nmquartz reactor15 vol.% TEOANi(dmgH)2/1.18[677] (2014)
g-C3N4ultrathin nanosheetsco-polymerization-surface activation-exfoliationPt/3 wt.%0.05300 W Xe arc lamp, λ > 420 nmPyrex reactor10 vol.% TEOAPt/28.55[793] (2016)
pm-g-C3N4poroussinteringPt/3 wt.%0.1300 W Xe lamp, λ > 420 nm10 vol.% TEOAPt/41.7[159] (2017)
g-C3N4microspheresolvothermal methodPt/3 wt.%300 W Xe lamp, λ > 420 nm15 vol.% TEOAPt/1.801.62 (420 nm)[397] (2015)
g-C3N4lamellarheating acetic acid treat melaminePt/1 wt.%0.1300 W Xe lamp, λ > 420 nmPyrex reactor10 vol.% TEOAPt/34[794]
(2015)
g-C3N4flower-like nanorodsrecrystallization methodPt/3 wt.%0.05300 W Xe lamp, λ > 420 nmquartz reactor10 vol.% TEOAPt/261.8[795]
(2015)
quasi-2D-C3N4lamellarmelamine-assisted exfoliation methodPt/0.6 wt.%0.1300 W Xe lamp, λ > 420 nm10 vol.% TEOAPt/89.28[428] (2015)
mg-C3N4mesoporouscalcinating-dissolving methodPt/3 wt.%0.05300 W xenon lamp, λ > 400 nmPyrex reactor10 vol.% TEOAPt/272[796] (2015)
CNICnanotubesmolten saltPt/3 wt.%0.1300 W Xe lamp, λ > 420 nmPyrex reactor10 vol.% TEOAPt/50221.2 (420 nm)[106] (2013)
CNT/g-C3N4nanotubes/lamellarcalcinating methodCNT/2 wt.%, Pt/1.2 wt.%0.1300 W Xe arc lamp, λ > 420 nm10 vol.% TEOAPt/39.4[147] (2014)
MVNTs/g-C3N4nanotubes/particlescalcinating methodMVNTs/2.0 wt.%0.1300 W Xe arc lamp, λ > 400 nmquartz reactor25 vol.% CH3OHMVNTs/1.15[681] (2012)
C/g-C3N4fiber/lamellarelectrospinning and calcinations methodPt/1.0 wt.%0.05350 W Xe arc lamp, λ > 420 nmPyrex reactor10 vol.% TEOAPt/54[682] (2015)
C/g-C3N4nanoparticles/rectangular nanotubemolten salt methodcarbon black/0.5 wt.%, Pt/3 wt.%0.1visible light, λ > 420 nmquartz reactor25 vol.% CH3OHC, Pt/69.8[779] (2014)
C-dots/g-C3N4dots/lamellarmixing-calcinating methodC-dots/0.25 wt.%0.054 × UV-LEDs (3 W, λ = 420 nm)Pyrex reactor25 vol.% CH3OH8.6[797] (2015)
CQDs/g-C3N4quantum dots/lamellarhydrothermal methodCQDs, Pt/3 wt.%0.05400 W Xe lamp, λ > 420 nmPyrex reactor10 vol.% TEOAPt/5.805[680] (2016)
C-dots/g-C3N4nanodots/ultrathin nanosheetshydrothermal methodC-dots/0.2 wt.%, Pt/0.2 wt.%0.05300 W Xe lamp, λ > 420 nm5 vol.% CH3OHPt/88.1[777]
(2016)
C-ZIF/g-C3N4nanoparticles/lamellarthermal condensationC-ZIF/1 wt.%0.1300 W Xe lamp, λ > 400 nm0 vol.% TEOAC-ZIF/32.58[683] (2016)
N/g-C3N4nanosheetcalcinating methodN-doped,Pt/3 wt.%0.05300 W Xe lamp, λ > 420 nmPyrex reactor10 vol.% TEOAPt/64[798] (2016)
C3N4+xlamellarco-thermal condensationN-doped,Pt/3 wt.%0.08300 W Xe lamp, λ > 400 nm10 vol.% TEOAPt/44.28[799] (2015)
P/g-C3N4flowers of in-plane mesporescalcinating methodP-doped, Pt/3 wt.%0.05300 W Xe arc lamp, λ > 400 nmPyrex reactor10 vol.% TEOAPt/104.1[315] (2015)
P/g-C3N4lamellarcopolymerizationP-doped0.1300 W Xe lamp, λ > 420 nmPyrex reactor10 vol.% TEOAPt/50.6[324]
(2015)
Br/g-C3N4lamellarcalcinating methodBr-doped, Pt/3 wt.%300 W Xe lamp, λ ≥ 420 nm10 vol.% TEOAPt/48[800] (2016)
g-C3N4nanosheets have a porous networkcalcinating methodO-doped, Pt/3 wt.%0.05300 W Xe lamp, λ > 420 nmPyrex reactor10 vol.% TEOAPt/60.27.8 (420 nm)[321] (2015)
S/g-C3N4lamellarcalcinating methodS-doped, Pt/1 wt.%0.1300 W Xe arc lamp, λ > 400 nmquartz reactor25 vol.% CH3OHPt/12.16[307] (2013)
K-g-C3N4lamellarKCl-template methodK, Pt/0.5 wt.%0.01300 W Xe lamp, λ > 420 nmPyrex reactor10 vol.% TEOAPt/102.8[445] (2014)
Zn/g-C3N4lamellarcalcinating methodZn-doped, Pt/0.5 wt.%0.2200 W Xe arc lamp, λ ≥ 420 nmPyrex reactor18.5 vol.% CH3OHPt/59.53.2 (420 nm)[300] (2011)
Co-Pi/g-C3N4nanoparticles/aggregated sheetsin situ photodepositionsCo–Pi, Pt/1 wt.%0.1300 W Xe arc lamp, λ > 400 nmquartz reactor25 vol.% CH3OH
0.05 M AgNO3
Pt/19.48[690] (2013)
Au/g-C3N4nanoparticles/lamellardeposition-precipitation methodAu/1 wt.%0.02125 W Hg lamp, λ > 400 nm10 vol.% TEOAAu/177.4[666] (2014)
AuPd/g-C3N4nanoparticles/lamellarchemical reduction-calcinating methodAuPd/0.5 wt.%0.05300 W Xe arc lamp, λ ≥ 400 nmquartz reactor10 vol.% TEOAAuPd/16.3[194] (2015)
Pt-Au@ g-C3N4lamellarphotodeposition methodPt, Au0.08300 W Xe lamp, λ > 420 nmPyrex reactor6.2 vol.% TEOAPt-Au/17[350] (2015)
Au/PtO/g-C3N4nanoparticles/lamellarphotodeposition methodAu/0.33 wt.%, Pt/0.40 wt.%0.05300 W Xe arc lamp, λ > 400 nmPyrex reactor25 vol.% CH3OHAu/PtO/16.9[200] (2016)
CsTaWO6/Au/g-C3N4gathered block/nanoparticles/lamellarimpregnaton methodAu/0.5 wt.%0.05300 W Xe lamp,quartz reactor20 vol.% CH3OHAu/0.458[667] (2015)
Cd0.5Zn0.5S/g-C3N4nanoparticles/lamellarhydrothermal method0.1Xe lamp, λ > 420 nmPyrex reactor0.35 M Na2S and 0.25 M Na2SO320.8 mL/h37 (425 nm)[801] (2015)
Cd0.2Zn0.8S/g-C3N4nanoparticles/lamellarhydrothermal method0.05300 W Xe lamp, λ > 420 nmPyrex reactor0.1 M Na2S and 0.1 M Na2SO3208.8[802]
(2015)
CdLa2S4/mpg-C3N4particles/lamellarhydrothermal method0.05300 W Xe lamp, λ > 420 nmquartz reactor0.1 M Na2S and 0.5 M Na2SO3299.247.1 (420 nm)[803]
(2016)
CdS QDs/g-C3N4QDs/lamellarin situ hydrothermal methodPt/0.5 wt.%0.005300 W Xe lamp, λ > 420 nmquartz reactor0.1 M L-ascorbic acidPt/22.478 (420 nm)[503] (2013)
CdS QDs/g-C3N4QDs/lamellarchemical impregnation methodPt/1.0 wt.%0.1300 W Xe lamp, λ > 400 nmquartz reactor25% vol.% CH3OHPt/17.27[502] (2012)
CdS QDs/g-C3N4QDs/hollowPt/3 wt.%0.02300 W Xe lamp, λ > 420 nmPyrex reactor10 vol.% TEOAPt/601[804] (2015)
g-C3N4/CdSparticles/particlesin situ self-transformation methodPt/1.0 wt.%0.05350 W Xe arc lamp, λ≥420 nmPyrex reactor0.35 M Na2S and 0.25 M Na2SO3Pt/265.153.16 (420 nm)[348] (2015)
CdS/g-C3N4core/cell nanowiressolvothermal-chemisorption methodPt/0.6 wt.%0.05350 W Xe arc lamp, λ≥420 nmPyrex reactor0.35 M Na2S and 0.25 M Na2SO3Pt/207.64.3 (420 nm)[562] (2013)
NiS/CdS/g-C3N4nanoparticles/nanorodes/lamellarin situ hydrothermal methodNiS/9 wt.%0.05300 W Xe lamp, λ≥420 nmquartz reactor10 vol.% TEOANiS/128.2[197] (2015)
Ni(OH)2/CdS/g-C3N4core/shell nanorodeshydrothermal methodNi(OH)2/4.760.001300 W Xe lamp, λ > 420 nmquartz reactor0.25 M Na2S and 0.35 M Na2SO3Ni(OH)2/115.1816.7
(450 nm)
[669]
(2016)
MoS2/g-C3N4flower-like/lamellarin situ light-assisted methodMoS2/2.89 wt.%0.01300 W Xe lamp, λ > 400 nmPyrex reactor10 vol.% TEOAMoS2/2.52[632] (2015)
MoS2/g-C3N4nanoparticles/lamellarmixing-calcinating methodMoS2/0.5 wt.%, Pt/1.0 wt.%0.1300 W Xe arc lamp, λ > 400 nmquartz reactor25 vol.% CH3OHMoS2, Pt/23.1[628] (2013)
MoS2/CN-Pyflower-like/lamellarin suit solvothermal method0.05300 W Xe lamp, λ > 420 nmPyrex reactor10 vol.% TEOA25[805] (2016)
ZnS/g-C3N4microsphere/lamellarprecipitation method0.054 × UV-LEDs (3 W, λ = 420 nm)Pyrex reactor25 vol.% CH3OH9.7[806] (2014)
WS2/g-C3N4slabs/porous sheet-likegas-solid reactionWS2/0.01 wt.%0.05300 W Xe lamp, λ > 420 nmPyrex reactor25 vol.% CH3OHWS2/5.05[638] (2015)
WS2/mpg-C3N4mesoporous nanosheetsimpregnation-sulfidation methodWS2/0.3 wt.%0.05300 W Xe lamp, λ > 420 nmPyrex reactor10 vol.% TEOAWS2/6.12[637] (2014)
CaIn2S4/g-C3N4nanoplate-lamellarhydrothermal methodPt/1.0 wt.%0.054 × UV-LEDs (3 W, λ = 420 nm)Pyrex reactor0.5 M Na2S and 0.5 M Na2SO3Pt/5.1[807] (2015)
Cu/g-C3N4nanoparticles/lamellarH2 reductionCu0.05Xe lamp, λ > 400 nm25 vol.% CH3OHCu/1.0250.35 (420 nm)[674]
(2016)
CuO/g-C3N4mono-dispersed sphere/lamellarwet impregnation-calcination methodPt/1.0 wt.%0.1300 W Xe lamp, λ > 420 nm10 vol.% TEOAPt/93.7[808] (2016)
Cu(OH)2/g-C3N4clusters/spherical particlesprecipitation methodCu(OH)2/0.34mol%0.1300 W Xe arc lamp, λ > 400 nmPyrex reactor25 vol.% CH3OHCu(OH)2/4.87[809] (2014)
CuO2/g-C3N4nanoparticles/lamellarin situ reduction methodPt/3 wt.%0.1300 W Xe lamp, λ > 420 nmPyrex reactor10 vol.% TEOAPt/24.13[525] (2014)
CuO2@g-C3N4core@shell octahedrasolvothermal and chemisorption methodPt/3 wt.%0.3300 W Xe lamp, λ > 420 nmPyrex reactor10 vol.% TEOAPt/79.5[802] (2015)
g-C3N4/InVO4lamellar/nanoparticleshydrothermal methodPt/0.6 wt.%0.05300 W Xe arc lamp, λ > 420 nm20 vol.% CH3OHPt/10.64.9 (420 nm)[351] (2015)
ZnFe2O4/g-C3N4flakes/lamellarcalcinating methodPt/1 wt.%0.1300 W Xe arc lamp, λ > 430 nmPyrex reactor10 vol.% TEOAPt/20[439] (2014)
CuFe2O4/g-C3N4nanoparticles/lamellarcalcinating methodPt/3 wt.%0.1300 W Xe lamp, 680 nm >λ > 420 nmquartz reactor10 vol.% TEOAPt/76[810] (2016)
FeOX/g-C3N4granular-likecalcinating methodPt/3 wt.%, FeOX(20 g urea/100 mg ferrocene)0.1300 W Xe lamp, 780 nm > λ > 420 nmquartz reactor10 vol.% TEOAFeOX, Pt/108[811] (2016)
N-CeOx/g-C3N4nanoparticles/lamellarone-pot annealing methodPt/1 wt.%0.05300 W Xe lamp, λ > 420 nmPyrex reactor10 vol.% TEOAPt/14.62[812] (2015)
Ag2O/g-C3N4nanaosheetshydrothermal method0.01300 W Xe lamp, λ > 420 nmPyrex reactor10 vol.% TEOA33.04[813] (2015)
Ag2S/g-C3N4particle/mesopores sheetsprecipitation method0.054 × UV-LEDs (3 W, λ = 420 nm)Pyrex reactor20 vol.% CH3OH10[665] (2014)
TiO2/g-C3N4yolk-shell spheres/lamellarsolvothermal method0.054 × UV-LEDs (3 W, λ = 420 nm)Pyrex reactor25 vol.% CH3OH5.6[814] (2016)
g-C3N4/B-TiO2two removed the top of the pyramidssolvothermal method0.1300 W Xe arc lamp, λ > 400 nmPyrex reactor25 vol.% CH3OH47.3[815] (2016)
CoTiO3/g-C3N4rods/lamellara facial in situ growth methodPt/3 wt.%0.02300 W Xe lampquartz reactor10 vol.% C2H5OHPt/11.738.4 (365 nm)[816] (2016)
C, N–TiO2/g-C3N4nanoparticles/ultrathin nanosheetsone-pot solvothermal method0.1300 W Xe lamp, λ > 400 nm10 vol.% TEOA3.92[817] (2015)
N,S–TiO2/g-C3N4semi-spherical nanoparticles/lamellarin situ thermal induced polymerization method0.05125 W medium pressure Hg lamp, λ ≥ 400 nm10 vol.% TEOA317[818]
(2015)
g-C3N4/Pt–TiO2nanoparticles/nanoparticles/lamellarchemical adsorption-calcinating method1% wt.%Pt/TiO20.1300 W Xe lamp, λ ≥ 420 nmPyrex reactor10 vol.% TEOAPt/178[819] (2012)
g-C3N4/MoS2/TiO2lamellar/nanosheets/nanorodeshydrothermal method0.1300 W Xe arc lamp, λ > 400 nmPyrex reactor25 vol.% CH3OH125[820]
(2016)
C3N4/rGO/WO3nanosheets/nanosheets/nanoparticleshydrothermal methodPt/1 wt.%0.02250 W iron doped metal halid UV–vis lamp (λ > 420 nm)Pyrex reactordeionized waterPt/2.840.9 (420 nm)[821] (2015)
W18O49/g-C3N4nanowires/lamellarsolvothermal methodPt/3 wt.%0.005300 W Xe arc lamp, λ > 420 nmquartz reactor10 vol.% TEOAPt/3.691.79 (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.

Fig. 44
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Fig. 44. The photocatalytic H2-evolution rate over g-C3N4 modified by Pt co-catalysts with different shapes [823].

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.

Fig. 45
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Fig. 45. (a) Comparison of the photocatalytic H2-production activity of the Nix (x, Ni(OH)2 to (g-C3N4 + Ni(OH)2) was 0, 0.1, 0.5, 1.0, 1.6, and 10 (mol%))and Pt-deposited g-C3N4 samples in triethanolamine aqueous solution. (b) Charge separation mechanisms in the Ni(OH)2/g-C3N4 system under visible light [126].

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.

Fig. 46
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Fig. 46. (A) Time courses and (B) the average rate of photocatalytic H2 evolution over the photocatalysts: (a) g-C3N4; (b) g-C3N4-CdS; (c) g-C3N4-9%NiS; (d) CdS-9%NiS; (e) g-C3N4-CdS-3%NiS; (f) g-C3N4-CdS-6%NiS; (g) g-C3N4-CdS-9%NiS; (h) g-C3N4-CdS-12%NiS; (i) g-C3N4-CdS-15%NiS [197].

Fig. 47
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Fig. 47. (A) The average rate of H2 evolution and (B) proposed charge transfer mechanisms in the g-C3N4/CB/NiS composite under visible light irradiation: A g-C3N4, B g-C3N4-0.5% CB, C g-C3N4-1.5% NiS, D g-C3N4-0.5%CB-1.5%NiS, E g-C3N4-1.0% CB-1.5% NiS, F g-C3N4-1.5% CB-1.5% NiS, G g-C3N4-1.5% NiS-0.5% CB [156].

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.

Fig. 48
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Fig. 48. (a) In situ EPR studies of a suspension of Cat-1 in TEOA solution under continuous visible-light irradiation with increasing the time. (b) Charge separation in sg-CN and the formation of Ni0 nanoparticles during photocatalysis [676].

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.

Table 8. Summary of the photocatalytic degradation of pollutants over g-C3N4-based photocatalysts.

PhotocatalystStructureSynthetic methodCo-catal./optimized mass ratioMass (g)Light sourceTarge pollutant/concentration/volumeDegradation time/efficiencyKapp [10−2 min−1]Main active speciesRef. (year)
In2O3/g-C3N4particles/lamellarcalcinating method0.1300 W halogen tungsten lamp, λ > ≥ 400 nmRhB/1 × 10−5 M/100 mL40 min/99%radical dotO2, radical dotOH[841] (2014)
TiO2–In2O3@g-C3N4nanoparticles/nanoparticles- lamellarsolvothermal method0.08350 W xenon arc lamp, λ > 420 nmRhB/0.01 g L−1/80 mL60 min/100%4.6radical dotOH[797] (2015)
CaIn2S4/g-C3N4nanoplate/lamellarhydrothermal method0.05500 W tungsten lamp, λ > 400 nmMO/0.01 g L−1/100 mL120 min/90%radical dotO2, h+[807] (2015)
SiO2/g-C3N4core-shell nanospherecalcinating method0.07300 W xenon lamp, λ ≥ 400 nmRhB/0.01 g L−1/70 mL150 min/94.3%radical dotOH, h+[842] (2015)
g-C3N4/TiO2lamellar/particlessol-gel method0.1500 W xenon lamp, λ > 420 nmMB/0.01 g L−1/100 mL360 min/92%h+[843] (2016)
TiO2/g-C3N4core-cellself-assembly method0.05500 W xenon lamp, UV–vis lightRhB/10 ppm/200 mL80 min/82%4.4[844] (2015)
TiO2/g-C3N4mesoporous spheres/lamellarmelt-infiltrating-calcinating method0.05500 W xenon lamp, λ > 400 nmRhB/0.01 g L−1/50 mL140 min/100%[845] (2016)
TiO2/g-C3N4yolk-shell spheres/lamellarsolvothermal method0.01350 W xenon arc lamp, λ > 420 nmRhB/0.01 g L−1/80 mL150 min/99.3%33.5radical dotO2, h+[814] (2016)
g-C3N4/TiO2lamellar/nanoparticlescalcinating method0.12 × 150 W tungsten lamp, λ > 420 nmciprofloxacin//0.01 g L−1/100 mL60 min/95%radical dotOH, h+, e[846] (2015)
g-C3N4/TiO2lamellar/roundish particlessol-gel method0.04400 W halide lamp, λ > 400 nmRhB/0.01 g L−1/40 mL150 min/100%h+[436] (2014)
g-C3N4/TiO2hydrothermal-calcination method0.03300 W xenon lamp, λ > 420 nmacyclovir/10 ppm/100 mL90 min/100%1.57h+[847]
(2016)
g-C3N4/F-TiO2lamellarhydrothermal method0.150 W LED light, λ = 410 nmMB/0.01 g L−1/100 mL60 min/89%3.74[655] (2014)
g-C3N4/Ag/TiO2lamellar-particles/microspheresphotodeposition-Physical mixing methodAg/2 wt.%0.03300 W xenon lamp, λ > 420 nmMO/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-C3N4lamellarcalcinating methodK-Na co-doped0.05250 W high-pressure sodium lamp, 420 < λ < 800 nmRhB/1 × 10−5 M/200 mL120 min/90%0.86radical dotO2, radical dotOH[336] (2015)
mg-C3N4mesoporescalcinating-dissolving method0.05300 W xenon lamp, λ > 420 nmRhB/0.005 g L−1/100 mL20 min/95%[849] (2015)
g-C3N4lamellarthermal condensation0.02300 W xenon lamp, λ > 420 nm2,4,6-TCP/10−4M/20 mL180 min/100%1.17radical dotO2, radical dotOOH[850] (2013)
mg-C3N4mesoporouscalcinating dissolving method0.05300 W xenon lamp, λ > 400 nmMO/0.01 g L−1/100 mL120 min/90%15.13[796] (2015)
C60/mg-C3N4particles/lamellarcalcinating methodC60/0.03 wt.%0.025500 W xenon lamp, λ > 420 nmMB/1 × 10−5 M/50 mL
phenol/5 × 10−6 M/50 mL
300 min/100%
300 min/46%
1.73
0.15
radical dotOH, h+[753] (2014)
CDs/g-C3N4dots/lamellarmixing-calcinating methodcarbon dots/0.5 wt.%0.05300 W xenon lamp, λ > 420 nmphenol/0.01 g L−1/50 mL200 min/100%[851] (2016)
C-dots/g-C3N4dots/lamellarmixing-calcinating methodC-dots/0.25 wt.%0.13 W LED lamp, λ = 365 ± 5 nmRhB/1 × 10−5 M/100 mL60 min/84%1.3radical dotO2[745] (2016)
Pd/mpg-C3N4particles/mesoporouschemical reductionPd-doped/1.5 wt.%350 W xenon lamp, λ > 420 nmBPA/0.02 g L−1/50 mL360 min/100%1radical dotO2[729] (2013)
Pt/C3N4nanoparticles/nanotubeshydrothermal methodPt/2 wt.%0.1300 W xenon lamp, λ > 420 nmPCP/0.02 g L−1/100 mL420 min/98%[852] (2015)
Au/Pt/g-C3N4particles/lamellarphotodeposition methodAu/2 wt.%, Pt/0.5 wt.%0.1500 W xenon lamp, λ > 400 nmTC-HCl/0.02 g L−1/100 mL180 min/93%42.86[481] (2015)
Au/g-C3N4/α-Fe2O3particles/lamellar/rhombohedral-likehydrothermal and ultrasonication methodAu0.05halogen lampRhB/0.004 g L−1/250 mL3.16[853] (2015)
ZnO@mpg-C3N4core-shellphysically mixing method0.025
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-C3N4flowerlike/lamellarhydrothermal method0.02mild UV lightMB/1 × 10−6 M/30 mL90 min/98.3%[854] (2015)
g-C3N4/ZnO/AgCllamellar/particlesprecipitation and refluxing method0.150 W LED lampRhB/2.5 × 10−5M/250 mL270 min/99%1.54radical dotOH[535]
(2015)
Zn2SnO4/g-C3N4particles/lamellarmixing-calcinating method0.2300 W xenon lamp, 420 < λ < 800 nmRhB/0.01 g L−1/100 mL120 min/97%3.8radical dotO2, h+[855] (2015)
V2O5/g-C3N4particles/lamellarmixing-calcinating method0.05250 W xenon lamp, λ > 420 nmRhB/0.01 g L−1/100 mL60 min/95.5%4.91·O2, h+[616] (2016)
g-C3N4/V2O5lamellar/nanoparticlesone-pot method0.05200 W xenon lampRhB/0.01 g L−1/50 mL80 min/100%[856]
(2015)
GdVO4/g-C3N4small particles/big particlesmixing-calcinating method0.3350 W xenon lamp, λ > 420 nmRhB/0.01 g L−1/300 mL120 min/96.7%4.34radical dotO2, h+[857] (2013)
g-C3N4/SmVO4lamellar/aggregated particlesmixing-calcinating method0.3350 W xenon lamp, λ > 420 nmRhB/0.01 g L−1/300 mL120 min/100%3.45[858] (2013)
g-C3N4/SnO2lamellar/nanoparticlesmixing-calcinating method0.1300 W xenon lamp, λ > 420 nmMO/10 ppm/100 mL180 min/73%0.78[859] (2014)
SnNb2O6/g-C3N4nanosheets/lamellarmixing-calcinating method0.02500 W tungsten light lamp, λ > 420 nmMB/0.01 g L−1/80 mL240 min/99%1.73radical dotO2, h+[640] (2016)
Ag/g-C3N4particles/lamellarphotodeposition methodAg/2 wt.%0.1300 W xenon lamp, 400 < λ < 680 nmMO/0.01 g L−1/100 mL
PNP/0.01 g L−1/100 mL
120 min/100%
120 min/100%

radical dotO2, h+
[489] (2013)
AgI@ g-C3N4core-shellprecipitation method0.1250 W halide lamp, λ > 420 nmRhB/0.01 g L−1/100 mL120 min/96%2.52radical dotO2, h+, radical dotOH[860] (2015)
AgCl@pg-C3N4nanoparticles/lamellarmodified precipitation method0.2300 W xenon lamp, λ > 420 nmatrazine/100 ppm/500 mL60 min/99%[861] (2015)
Ag3VO4/g-C3N4particles/lamellardeposition-precipitation method0.05400 W metal halide lamp, λ > 420 nmRhB/0.005 g L−1/100 mL100 min/99.5%5.56radical dotO2, h+, radical dotOH[862] (2015)
g-C3N4/Ag2Oparticles/lamellarprecipitation method0.02300 W xenon lamp, λ ≥ 400 nmMO/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-C3N4nanoparticles/lamellarcoprecipitation method0.1300 W xenon lamp, λ ≥ 400 nmRhB/0.005 g L−1/100 mL40 min/100%10.53radical dotO2, h+[726] (2014)
Ag@g-C3N4core-shellrefluxing methodAg/0.5 wt.%0.025500 W xenon arc lamp, λ > 420 nmMB/1 × 10−5 M/50 mL1.4h+[488] (2014)
Ag@AgCl/g-C3N4nanoparticles/lamellarin situ ion exchange method0.025300 W xenon lamp, 400 < λ < 700 nmRhB/0.01 g L−1/100 mL30 min/100%19.54radical dotO2, h+[433] (2014)
AgVO3/g-C3N4nanoribbons/ultrathin nanosheetsin situ hydrothermal method0.05500 W xenon lamp, λ > 420 nmBF/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
radical dotOH, h+
radical dotOH, h+
[863] (2015)
Ag3PO4/g-C3N4spherical particles/lamellarchemisorptions method0.02500 W xenon lamp, λ > 400 nmMO/0.02 g L−1/40 mL60 min/100%4h+[864] (2014)
Ag/Fe3O4/g-C3N4particles/nanoclusters/lamellarhydrothermal-photodeposition methodAg/3 wt.%, Fe3O4/9 wt.%0.05300 W xenon lamp, λ > 420 nmtetracycline/0.02 g L−1/100 mL90 min/88%2.18radical dotO2, h+[865] (2016)
g-C3N4/CdSlamellar/nanotubeprecipitation method0.08500 W xenon lamp, λ > 420 nmMB/0.025 g L−1/200 mL180 min/90.45%radical dotOH, h+[506] (2014)
CdS QDs/npg-C3N4QDs/lamellarmixing-calcinating method0.1500 W tungsten light lamp, λ > 400 nmRhB/0.01 g L−1/100 mL90 min/88.2%radical dotO2, h+[866] (2016)
CdWO4/g-C3N4nanorods/lamellarmixing-calcinating method0.05500 W xenon lamp, λ ≥ 400 nmRhB/1 × 10−5 M/50 mL240 min/46%0.27radical dotOH, h+[867] (2015)
Cd0.2Zn0.8S/g-C3N4nanoparticles/lamellarhydrothermal method0.05500 W xenon lamp, λ > 420 nmRhB/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-C3N4nanorods/mesoporousin situ method0.1300 W xenon arc lamp, λ > 420 nmMO/0.02 g L−1/100 mL120 min/100%0.77[868] (2014)
BiOCl/C3N4flowerlike/amorphoussolvothermal method0.2300 W xenon arc lamp, λ > 400 nmMO/0.01 g L−1/500 mL80 min/95%h+[869] (2013)
g-C3N4/BiOBrlamellar/nanosheetsdeposition-precipitation method0.02
0.04
400 W halogen lamp, λ > 400 nmRhB/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+
radical dotOH
[870] (2013)
BiOxIy/g-C3N4square thin-plates/lamellarcontrolled hydrothermal0.01100 W xenon arc lampCV/10 ppm/100 mL36 h/99%0.283radical dotO2[871]
(2016)
g-C3N4/Bi2O2CO3lamellar/platesmixing-calcinating method0.051000 W xenon lamp, λ > 420 nmRhB/1 × 10−5 M/50 mL240 min/56%17radical dotO2, h+[872] (2014)
g-C3N4/Bi2WO6lamellar/nanosheetshydrothermal method0.4300 W xenon lamp, λ > 400 nmMO/0.01 g L−1/200 mL180 min/94.82%1.66[580] (2014)
Bi2WO6 QDS/g-C3N4QDS-lamellarin situ method0.05300 W xenon lamp, λ ≥ 400 nmRhB/0.01 g L−1/50 mL30 min/100%16.8radical dotO2, h+, radical dotOH[505] (2015)
g-C3N4/Bi2WO6agglomerationmixing-calcinating method0.15500 W xenon lamp, λ > 420 nmMO/0.01 g L−1/50 mL180 min/99.9%3.66[576] (2011)
g-C3N4/Bi2MoO6lamellar-hollow spherehydrothermal method0.02400 W metal halide lamp, λ ≥ 420 nmRhB/0.01 g L−1/50 mL70 min/98%5.15radical dotO2[873] (2014)
g-C3N4/Bi2WO6lamellar-flakelikehydrothermal method0.1400 W xenon lamp, λ > 400 nmMO/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