Highly Efficient Photocatalytic H₂ Evolution from Water using Visible Light and Structure-Controlled Graphitic Carbon Nitride ^†
利用可见光和结构可控的石墨碳氮化物 ^† 高效光催化水分解H ₂

Inorganic semiconductors have been at the forefront of photocatalytic water splitting for the synthesis of renewable fuels ever since the discovery of the phenomenon in the late 20th century.¹ Advances towards an efficient photocatalyst suitable for hydrogen and oxygen evolution under visible light notably include: metal and nonmetal mixed oxides,² sulfides and nitrides,³ doped perovskites,⁴ and nitrided pyrochlores.⁵ These successes have drawn considerable interest in the field, but are still far from meeting industrial requirements of efficiency and stability. The major challenge is to develop a highly efficient, low-cost, and robust photocatalyst that can successfully serve practical needs. A stable, organic photocatalyst, graphitic carbon nitride (g-C₃N₄), was found to show sufficient redox power to dissociate water under visible light in a suspension (see Figure S1 in the Supporting Information).⁶ However, the latest documented quantum yields for H₂ production from water with g-C₃N₄ (excluding dye-sensitized systems) do not exceed 4 %,⁶, ⁷ which is still unsatisfactory for industrial applications.⁸ Graphitic carbon nitride is composed of extremely abundant elements and is nontoxic with proven stability, both thermally and in solutions of pH 1–14. Therefore, if an effective and facile strategy is devised to improve its energy-conversion efficiency, it can meet all the aforementioned three requirements for a practical photocatalyst. Although there has been extensive research on g-C₃N₄-based photocatalysts,^7f,^7h,⁷ⁱ little attention has been paid to the effects of the protonation and polymerization of pristine g-C₃N₄.
自世纪发现光催化分解水合成可再生燃料以来，无机半导体一直处于光催化分解水合成可再生燃料的最前沿。 ¹ 在适用于可见光下的氢和氧析出的高效光催化剂方面的进展值得注意地包括：金属和非金属混合氧化物、 ² 硫化物和氮化物、 ³ 掺杂的钙钛矿、 ⁴ 和氮化的焦磷酸盐。 ⁵ 这些成功在该领域引起了相当大的兴趣，但仍然远远不能满足工业对效率和稳定性的要求。主要的挑战是开发一种高效，低成本，鲁棒的光催化剂，可以成功地满足实际需求。发现一种稳定的有机光催化剂，即石墨碳氮化物（g-C ₃ N ₄ ），在可见光下显示出足够的氧化还原能力，可使悬浮液中的水解离（参见支持信息中的图S1）。 然而，最新记录的从具有g-C ₃ N ₄ 的水生产H ₂ 的量子产率（不包括染料敏化体系）不超过4%、 ⁶ 、 ⁷ ，这对于工业应用仍然是不令人满意的。 ⁸ 石墨碳氮化物由极其丰富的元素组成，并且无毒，在热和pH 1-14的溶液中都具有经证实的稳定性。因此，如果设计一种有效和简便的策略来提高其能量转换效率，它可以满足实用光催化剂的上述三个要求。尽管已经对g-C ₃ N ₄ 基光催化剂进行了广泛的研究，但很少关注原始g-C ₃ N ₄ 的质子化和聚合的影响。

Herein, we report a g-C₃N₄ photocatalyst, synthesized from low-cost and abundant urea, that shows an extremely high quantum yield of 26.5 % under visible light: nearly an order of magnitude higher than the previous record of approximately 4 % for hydrogen evolution with g-C₃N₄.⁷ To the best of our knowledge, this value is much greater than that of most semiconductor photocatalysts, except sulfides (i.e. CdS and ZnS), which are highly efficient but inherently unstable.¹ More importantly, we have determined a protonation mechanism from both experimental and computational findings to explain the extraordinary photocatalytic activity of g-C₃N₄ derived from a tailored polymerization route.
在此，我们报道了由低成本和丰富的尿素合成的g-C ₃ N ₄ 光催化剂，其在可见光下显示出26.5%的极高量子产率：比先前用g-C ₃ N ₄ 析氢的约4%的记录高近一个数量级。 ⁷ 据我们所知，该值比大多数半导体光催化剂的值大得多，除了硫化物（即CdS和ZnS），它们是高效的，但本质上不稳定。 ¹ 更重要的是，我们已经确定了质子化机制，从实验和计算结果来解释非凡的光催化活性的g-C ₃ N ₄ 来自定制的聚合路线。

For comparison, samples of g-C₃N₄ were successfully prepared by using different precursors (urea, dicyandiamide (DCDA), and thiourea) under identical conditions and characterized by X-ray diffraction (XRD), attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, transmission electron microscopy (TEM), and UV/Vis and Raman spectroscopy (see Figures S2 and S3 in the Supporting Information for details of characterization and analysis). Graphitic carbon nitride synthesized from different precursors (600 °C, 5 °C min⁻¹ ramp rate) was tested for hydrogen evolution in an aqueous sacrificial solution containing triethanolamine (TEOA) at room temperature and atmospheric pressure, in a procedure similar to a previously reported method.⁶, ^7f The fully optimized results are shown in Figure 1 and further summarized in Table 1. The urea-derived g-C₃N₄ exhibited superior hydrogen evolution in comparison to either the widely used DCDA- or thiourea-derived g-C₃N₄ under both full arc and visible-light irradiation (Figure 1 a,b). The urea-derived g-C₃N₄ evolved hydrogen at approximately 20 000 μmol h⁻¹ g⁻¹, 15 times faster than DCDA-derived and 8 times higher than thiourea-derived g-C₃N₄, as reflected in the turnover number (TON, over a platinum cocatalyst; see the Supporting Information for the calculation): Urea-derived g-C₃N₄ had a TON of 641.1, which is much higher than that of the other samples.
为了比较，通过使用不同的前体成功地制备了g-C ₃ N ₄ 样品（尿素，双氰胺（DCDA），和硫脲）在相同的条件下，并通过X射线衍射（XRD），衰减全反射傅里叶变换红外光谱（FTIR），（ATR-FTIR）光谱，透射电子显微镜（TEM），以及UV/维斯和拉曼光谱（有关表征和分析的详细信息，请参见支持信息中的图S2和S3）。以类似于先前报道的方法的程序，在室温和大气压下，在含有三乙醇胺（TEOA）的牺牲水溶液中测试由不同前体（600 ℃，5 ℃ min ⁻¹ 升温速率）合成的石墨碳氮化物的析氢。 ⁶ 、 ^7f 完全优化的结果如图1所示，并在表1中进一步总结。 与广泛使用的DCDA-或硫脲-衍生的g-C ₃ N ₄ 相比，在全电弧和可见光照射下，脲-衍生的g-C ₃ N ₄ 表现出上级析氢（图1a，b）。 尿素衍生的g-C ₃ N ₄ 以约20 000 μmol h ⁻¹ g ⁻¹ 放出氢，比DCDA衍生的快15倍，比硫脲衍生的g-C ₃ N ₄ 高8倍，如周转数所反映的（TON，在铂助催化剂上;参见计算的支持信息）：尿素衍生的g-C ₃ N ₄ 具有641.1的TON，其远高于其它样品的TON。

Even under irradiation with visible light (λ≥395 nm, Figure 1 b), the urea-derived g-C₃N₄ evolved H₂ at 3300 μmol h⁻¹ g⁻¹, more than 10 times faster than the DCDA-derived g-C₃N₄ at 300 μmol h⁻¹ g⁻¹ and nearly 7 times faster than thiourea-derived g-C₃N₄ at 500 μmol h⁻¹ g⁻¹. It may be tempting to conclude that the activity difference is due to differences in surface area; however, the urea-derived g-C₃N₄ has a specific surface area (SSA) only 3.4 times greater than that of DCDA-derived g-C₃N₄ and 2.4 times that of the sample of thiourea-derived g-C₃N₄ (see Figure S4). Meanwhile, the activity was over 15 times and 8 times higher than that of the samples synthesized by using DCDA and thiourea, respectively. Interestingly, urea-derived g-C₃N₄ calcined at 550 °C showed a surface area of 83.5 m² g⁻¹, double that of the sample calcined at 600 °C. However, the activity of the former sample is only a third that of the latter (see Figure S5 a). Zhang et al.^9b prepared g-C₃N₄ at 550 °C (3 h) from a urea precursor and reported a hydrogen-evolution rate (HER) of 625 μmol h⁻¹ g⁻¹, which is similar to that of the sample prepared in this study. The difference in activity is due to the shorter calcination time employed during sample preparation, which probably affects the level of polymerization of the compound. Curiously, despite being able to absorb less light, the urea-derived carbon nitride still outperforms both DCDA- and thiourea-derived counterparts (Table 1). Therefore, the activity cannot be directly attributed to either surface area or optical absorption.
即使在可见光照射下（λ≥395 nm，图1 b），尿素衍生的g-C ₃ N ₄ 在3300 μmol h ⁻¹ g ⁻¹ 下释放H ₂ ，比DCDA衍生的g-C ₃ N ₄ 在300 μmol h ⁻¹ g ⁻¹ 下快10倍以上，比硫脲衍生的g-C ₃ N ₄ 在500 μmol h ⁻¹ g ⁻¹ 下快近7倍。 可能很容易得出结论，活性差异是由于表面积的差异;然而，尿素衍生的g-C ₃ N ₄ 的比表面积（SSA）仅为DCDA衍生的g-C ₃ N ₄ 的3.4倍，为硫脲衍生的g-C ₃ N ₄ 样品的2.4倍（参见图S4）。同时，该催化剂的活性分别比用DCDA和硫脲合成的催化剂提高了15倍和8倍以上。有趣的是，在550 °C下煅烧的尿素衍生的g-C ₃ N ₄ 显示出83.5m2 ² g ⁻¹ 的表面积，是在600 °C下煅烧的样品的表面积的两倍。 然而，前者样品的活性仅为后者的三分之一（见图S5 a）。Zhang等人 ^9b 在550 °C（3 h）下由尿素前体制备g-C ₃ N ₄ ，并报道了625 μmol h ⁻¹ g ⁻¹ 的析氢速率（HER），其与本研究中制备的样品相似。活性的差异是由于样品制备过程中采用的煅烧时间较短，这可能影响化合物的聚合水平。奇怪的是，尽管能够吸收较少的光，但脲衍生的碳氮化物仍然优于DCDA衍生和硫脲衍生的对应物（表1）。因此，活性不能直接归因于表面积或光吸收。

Extensive tests were carried out on the urea-derived sample owing to its excellent performance in evolving hydrogen from water. The synthesis parameters were tailored to augment hydrogen production from water, including the reaction temperature, ramp rate, cocatalyst species, and loading of the cocatalyst (see Figure S5).
由于尿素衍生的样品在从水中析氢方面的优异性能，对其进行了广泛的测试。调整合成参数以增加由水的氢气生产，包括反应温度、升温速率、助催化剂物质和助催化剂的负载量（参见图S5）。 

X-ray photoelectron spectroscopy (XPS) was undertaken to accurately determine the specific bonding and structure of the samples. In all samples, the typical C 1s and N 1s peaks were observed, as in previous studies.⁹ A residual O 1s peak, probably due to calcination in air, was also present. XPS spectra of the three samples (prepared from urea, thiourea, and DCDA) are shown in Figures S6 (N 1s XPS) and S7 (C 1s and O 1s XPS spectra) of the Supporting Information. The C 1s spectra show CC, CNC, and a trace amount of CO bonding at 285.1, 288.2, and 289.2 eV, respectively. The N 1s spectra can be fitted to elucidate four separate signals, and provides a better idea of the bonding structure, since carbon spectra are susceptible to contamination. The N 1s core levels at 398.7, 399.7, and 400.9 eV correspond to sp² CNC, sp³ HN[C]₃, and CNH_x (amino functional groups), respectively.¹⁰ The weak peak at 404.4 eV can be attributed to terminal nitrate groups, charging effects, or π excitations.^9a, ¹¹ Hybridized sp³ nitrogen not only has three chemical bonds to carbon, but because of hybridization, can also be bonded to hydrogen perpendicular to the direction of the graphitic layer.
进行X射线光电子能谱（XPS）以准确地确定样品的特定键合和结构。在所有样品中，观察到典型的C 1 s和N 1 s峰，与以前的研究一样。 ⁹ 还存在残余O 1 s峰，可能是由于在空气中煅烧。三种样品（由尿素、硫脲和DCDA制备）的XPS光谱见支持信息的图S6（N 1 s XPS）和S7（C 1 s和O 1 s XPS光谱）。C 1 s光谱分别在285.1、288.2和289.2 eV处显示C <$C、C <$N <$C和微量的C <$O键合。N 1 s光谱可以拟合以阐明四个独立的信号，并提供了一个更好的想法的键合结构，因为碳光谱容易受到污染。在398.7、399.7和400.9 eV处的N 1 s核心能级分别对应于sp ² C <$N <$C、sp ³ H <$N <$[C] ₃ 和C <$NH _x （氨基官能团）。 ¹⁰ 在404处的弱峰。4 eV可以归因于末端硝酸根基团、充电效应或π激发。 ^9a 、 ¹¹ 杂化sp ³ 氮不仅与碳有三个化学键，而且由于杂化，还可以与垂直于石墨层方向的氢键合。

Figure 2 shows the distinctive trend in bonding ratios versus activity between samples (see also Table S1). The ratio of sp² CNC bonds to the sum of sp³ HN[C]₃ and CNH_x bonds (the latter represents the total amount of protons) is 2.83 in urea, 2.7 for thiourea, and only 2.31 in DCDA. As it is part of the heptazine ring, linked by a double and a single bond to two opposing carbon atoms, the sp²-bonded nitrogen atom is the principle participant that contributes to band-gap absorption and therefore is an extremely important part of the structure. Both hybridized sp³ nitrogen atoms and surface functional amino groups (CNH_x) are also key features when considering bulk and surface properties. Along with CNH_x bonding, graphitic carbon nitride possesses a positively charged, acidic surface, as confirmed by ζ-potential measurements (see Figure S8). Elemental analysis (EA) further confirmed the trend shown in Figure 2: As bulk H (atom %) increases, the HER per SSA (μmol m⁻² h⁻¹) decreases (see Figure S9). From these results and in combination with XPS analysis, we could conclude that a lower proton concentration leads to a larger HER.
图2显示了样品之间结合率与活性的独特趋势（另见表S1）。sp ² C N C键与sp ³ H N [C] ₃ 和C NH _x 键之和（后者代表质子的总量）的比率在尿素中为2.83，硫脲为2.7，而在DCDA中仅为2.31。由于它是七嗪环的一部分，通过双键和单键连接到两个相对的碳原子上，sp ² 键合的氮原子是有助于带隙吸收的主要参与者，因此是结构的极其重要的部分。当考虑本体和表面性质时，杂化的sp ³ 氮原子和表面官能氨基（C = NH _x ）也是关键特征。沿着C = NH _x 键，石墨氮化碳具有带正电荷的酸性表面，如通过电势测量所证实的（参见图S8）。 元素分析（EA）进一步证实了图2所示的趋势：随着本体H（原子%）增加，每SSA的HER（μmol m ⁻² h ⁻¹ ）降低（见图S9）。从这些结果并结合XPS分析，我们可以得出结论，较低的质子浓度导致较大的HER。

The increase in proton concentration (from both sp³ nitrogen atoms and CNH_x) most likely stems from the route of condensation, and even though all three samples were synthesized under identical conditions, the extent of polymerization varied owing to the different precursors, which is consistent with the trends in XRD and FTIR measurements (see Figure S2). In particular, only urea-derived g-C₃N₄ loses hydrogen in the form of formaldehyde, owing to the presence of oxygen in the precursor (see Figures S10 and S11 for thermogravimetic analysis–differential scanning calorimetry–mass spectroscopy analysis). This trend not only applies to different precursors, but urea-derived g-C₃N₄ synthesized at different temperatures (550, 650 °C) also follows suit (Figure 2; see also Table S1): As protonation increases, activity decreases. Therefore, both the precursors and the synthetic parameters can control the protonation and polymerization, thus leading to varying activity. A small change in the preparation method can have a huge impact on photocatalytic hydrogen production (such as a change from 550 to 600 or 650 °C; see Figure S5 a and ramp-rate changes in Figure S5 b).
质子浓度的增加（来自sp ³ 氮原子和C = NH _x ）最可能源于缩合途径，并且即使所有三个样品都是在相同条件下合成的，聚合程度也由于不同的前体而变化，这与XRD和FTIR测量的趋势一致（参见图S2）。特别地，由于前体中存在氧，仅尿素衍生的g-C ₃ N ₄ 以甲醛的形式损失氢（参见图S10和S11的热重分析-差示扫描量热-质谱分析）。这种趋势不仅适用于不同的前体，而且在不同温度（550、650 °C）下合成的尿素衍生的g-C ₃ N ₄ 也遵循这种趋势（图2;也参见表S1）：随着质子化增加，活性降低。因此，前体和合成参数都可以控制质子化和聚合，从而导致不同的活性。 制备方法的微小变化可能对光催化制氢产生巨大影响（例如从550 ℃变化到600或650 ℃;参见图S5 a和图S5 B中的斜坡速率变化）。

To determine exactly why polymerization and protonation status influences H₂-production rates, we modeled protonation by DFT simulations using periodic supercells. Time-dependent DFT (TDDFT) simulations were performed on cluster models to determine the effects of hydrogen on excited-state properties. The density of states (DOS) is shown in Figure 3 c. It can be clearly seen that the conduction-band edge (CBE) of the protonated system is shifted down in energy (towards more positive values with respect to the normal hydrogen electrode, NHE) by 0.34 eV. This shift significantly modifies the electrochemical properties, as it provides a lower overpotential for reduction reactions, as also shown in the UV/Vis absorption spectra (see Figure S2 c). The reason behind the drop in the position of the CBE can be clearly seen in the site-decomposed DOS (see discussion in the Supporting Information and Figure S12). The effects of protonation on excited-state properties of a molecular model were also calculated. The lowest energy vibrationally stable structure involves strong distortions from planarity of all three heptazine rings. The onset of optical absorption on protonated g-C₃N₄ occurs at a lower energy (more positive with respect to the NHE) than for deprotonated g-C₃N₄. Indeed, two absorption peaks of the C₁₈N₂₈H₁₃ model occur at lower energies than that of the initial absorption peak of the C₁₈N₂₈H₁₂ model, in qualitative agreement with the DFT DOS in Figure 3. This result verifies our DFT-based electronic-structure analysis with TDDFT.
为了确切地确定为什么聚合和质子化状态影响H ₂ -生产率，我们使用周期性超晶胞通过DFT模拟来建模质子化。用含时密度泛函（TDDFT）方法对团簇模型进行了模拟，以确定氢原子对激发态性质的影响。态密度（DOS）如图3 c所示。 可以清楚地看出，质子化系统的导带边缘（CBE）在能量上向下移动（相对于正常氢电极NHE朝向更正值）0.34eV。这种变化显著改变了电化学性质，因为它为还原反应提供了较低的过电位，如UV/维斯吸收光谱所示（见图S2 c）。CBE位置下降的原因可以在现场分解的DOS中清楚地看到（参见支持信息和图S12中的讨论）。 计算了质子化对分子模型激发态性质的影响。最低能量的振动稳定结构涉及所有三个heptazine环的平面性的强烈扭曲。质子化的g-C ₃ N ₄ 上的光吸收的开始发生在比去质子化的g-C ₃ N ₄ 更低的能量（相对于NHE更正）处。实际上，C ₁₈ N ₂₈ H ₁₃ 模型的两个吸收峰出现在比C ₁₈ N ₂₈ H ₁₂ 模型的初始吸收峰更低的能量处，与图3中的DFT DOS定性一致。这一结果验证了我们基于DFT的电子结构分析与TDDFT。

We also plotted the distribution of the lowest-energy exciton for both carbon nitride models (see Figure S13 b,c). For deprotonated C₃N₄, the exciton is distributed relatively homogeneously over the cluster, with transitions from occupied N p_z orbitals to empty C p_z* orbitals. For protonated C₃N₄, the exciton is more heterogeneous; the photohole on the protonated heptazine ring and the photoelectron are distributed evenly on the other two heptazine rings. Although there is a better spatial separation between photohole and photoelectron, both charge carriers are more localized around the central N3 site, and thus are not as available to participate in the photochemical reactions. Moreover, this localization will act to increase the exciton-recombination rate, thus hindering the efficient utilization of charge carriers.
我们还绘制了两种氮化碳模型的最低能量激子的分布（见图S13 b，c）。对于去质子化的C ₃ N ₄ ，激子相对均匀地分布在团簇上，从占据的N p _z 轨道跃迁到空的C p _z * 轨道。对于质子化的C ₃ N ₄ ，激子更加不均匀，质子化的七嗪环上的光空穴和光电子均匀地分布在另外两个七嗪环上。虽然光空穴和光电子之间存在更好的空间分离，但两种电荷载流子都更局限于中心N3位点周围，因此不能参与光化学反应。此外，这种局部化将起到增加激子复合速率的作用，从而阻碍电荷载流子的有效利用。

A relatively small change in protonation has a two-pronged detrimental effect on the reduction ability of g-C₃N₄. Protonation significantly reduces the reduction potential and also localizes excitons around a central nitrogen N3 site, thus hindering migration to active sites. In this material, protonation is essentially controlled through the degree of polymerization, but also coupled with the degree of condensation. Graphitic carbon nitride, if “underpolymerized” (at low temperatures), has incomplete heptazine coupling, which results in excess hydrogen-passivating N3c′ nitrogen sites, thus hampering activity. If “overpolymerized” (at about 650 °C), the structure of g-C₃N₄ tends to overly condense into buckled multilayered crystals of reduced surface area; as a result, the density of active sites is reduced, and photocatalytic ability is adversely affected. Moreover, the buckling also distorts the sp² planar geometry, thus leading to charge-trapping states at nitrogen sites and hence reduced activity.
质子化的相对小的变化对g-C ₃ N ₄ 的还原能力具有两方面的不利影响。质子化显著降低还原电位，并且还将激子定位在中心氮N3位点周围，从而阻碍迁移到活性位点。在这种材料中，质子化基本上通过聚合度来控制，但也与缩合度相结合。石墨碳氮化物如果“欠聚合”（在低温下），则具有不完全的七嗪偶联，这导致过量的氢钝化N3 c ′氮位点，从而阻碍活性。如果“过度聚合”（在约650 °C下），g-C ₃ N ₄ 的结构倾向于过度缩合成表面积减小的屈曲多层晶体;结果，活性位点的密度减小，并且光催化能力受到不利影响。 此外，屈曲也扭曲的sp ² 平面几何形状，从而导致在氮位点的电荷捕获状态，从而降低活性。

As mentioned previously, apart from the need for a photocatalyst to be cheap and robust, it must exhibit a high quantum yield for hydrogen production from water if it is to be considered commercially viable. Compounds that traditionally exhibit high efficiencies either suffer from instability (e.g. sulfides¹³) or are made of relatively expensive metals (e.g. GaAs–GaInP₂¹⁴). The cheap and stable urea-derived g-C₃N₄ in this study has a peak internal quantum yield of 28.4 % at 365 nm (Figure 4 a). Even under irradiation with visible light at λ=400 nm, the quantum yield is 26.5 %, nearly an order of magnitude greater than the highest reported (3.75 % at 420 nm,^7f obtained by liquid exfoliation). To ensure the reliability of our measurement, we examined as a reference a benchmark cyanamide-derived g-C₃N₄, which showed comparable activity (Table 2; the small difference is due to the use of a 395 nm long-pass filter instead of a 420 nm filter). As proposed previously, the huge enhancement in hydrogen-evolution rate of our urea-derived sample (3327.5 vs. 142.3 μmol h⁻¹ g⁻¹) can be attributed to a lower protonation status and the condensation state. Recently, a facile synthetic method for three-dimensional porous g-C₃N₄ was introduced by using aggregates of melamine and cyanuric acid (MCA) co-crystals in dimethyl sulfoxide (DMSO, sample denoted MCA_DMSO) as precursors.¹² It was reported that the quantum yield of g-C₃N₄_MCA_DMSO at λ=420 nm was 2.3 % (Table 2), much higher than that of melamine-derived bulk g-C₃N₄ (0.26 %) under the same conditions. We repeated this study and observed very similar morphologies and optical properties to those reported (see Figure S14). Correspondingly, a similar quantum yield of 3.1 % at λ=400 nm was obtained (Table 2; the difference is due to the wavelength of the band-pass filter). Since MCA_DMSO is another oxygen-containing precursor, the rise in the quantum yield as compared to that of a melamine sample further supports our proposed protonation mechanism. Furthermore, the reason why our optimized urea-derived g-C₃N₄ is more than 10 times more efficient than MCA_DMSO g-C₃N₄, in terms of quantum yield at 400 nm, given the very similar specific surface area of these two samples, is because of the much higher oxygen concentration in the urea precursor, which helps to passivate protonation sites and polymerize g-C₃N₄ without structural instability/buckling.
如前所述，除了需要光催化剂便宜和耐用之外，如果要认为其在商业上是可行的，则其必须表现出用于从水制氢的高量子产率。传统上表现出高效率的化合物或者遭受不稳定性（例如硫化物 ¹³ ）或者由相对昂贵的金属制成（例如GaAs-GaInP ₂ ¹⁴ ）。本研究中廉价且稳定的尿素衍生的g-C ₃ N ₄ 在365 nm处具有28.4%的峰值内量子产率（图4a）。 即使在λ=400 nm的可见光照射下，量子产率也为26.5%，几乎比报道的最高值（420 nm下的3.75%，通过液体剥离获得的 ^7f ）大一个数量级。为了确保我们的测量的可靠性，我们检查了作为基准的氰胺衍生的g-C ₃ N ₄ ，其显示出相当的活性（表2;小的差异是由于使用395 nm长通滤光片而不是420 nm滤光片）。 如前所述，我们的尿素衍生样品的析氢速率的巨大增强（3327.5对142.3 μmol h ⁻¹ g ⁻¹ ）可以归因于较低的质子化状态和缩合状态。近年来，以三聚氰胺和三聚氰酸（MCA）共晶在二甲基亚砜（DMSO）中的聚集体为前驱体，合成了三维多孔g-C ₃ N ₄ 。据报道，g-C ₃ N ₄ _MCA_DMSO在λ=420 nm处的量子产率为2.3%（表2），远高于相同条件下三聚氰胺衍生的本体g-C ₃ N ₄ 的量子产率（0.26%）。我们重复了该研究，并观察到与报告的形态和光学性质非常相似的形态和光学性质（见图S14）。相应地，在λ=400 nm处获得3.1%的类似量子产率（表2;差异是由于带通滤波器的波长）。 由于MCA_DMSO是另一种含氧前体，与三聚氰胺样品相比，量子产率的上升进一步支持了我们提出的质子化机制。此外，在400 nm处的量子产率方面，我们的优化的脲衍生的g-C ₃ N ₄ 比MCA_DMSO g-C ₃ N ₄ 有效10倍以上的原因是，考虑到这两种样品非常相似的比表面积，这是因为脲前体中的氧浓度高得多，这有助于钝化质子化位点和无结构不稳定性/屈曲的P-C ₃ N ₄ 。

The stability of the optimized photocatalyst was also tested in an extended experiment (Figure 4 b). The high activity was reproducible, and the material showed excellent stability during a period of 30 h. Attributes for a high quantum yield commonly include good absorption, efficient charge separation, and rapid carrier transfer to the surface for redox reactions. Even though the overall band gap of urea-based carbon nitride is larger than that of both the DCDA- and thiourea-derived carbon nitrides, it produces more hydrogen; therefore, the bulk absorption and band gap are not the determining factor in overall activity. Owing to the fewer protons in g-C₃N₄ (urea), the band-gap and therefore the overpotential is larger, which also results in better separation of charge and consequently causes better migration of charger carriers to active sites. The trend of the proton concentration and the trend of polymerization for the sample set are in very good agreement with the corresponding activity, which contributes to charge transport. Therefore, for the first time, it has been demonstrated that the protonation and degree of polymerization determines hydrogen-evolution rates from water, whereby the surface area plays a minor role.
还在扩展实验中测试了优化的光催化剂的稳定性（图4 b）。 高活性是可再现的，并且材料在30 h的时间段内显示出优异的稳定性。高量子产率的属性通常包括良好的吸收、有效的电荷分离和快速的载流子转移到表面以进行氧化还原反应。尽管脲基碳氮化物的总带隙大于DCDA和硫脲衍生的碳氮化物，但它产生更多的氢;因此，体吸收和带隙不是总活性的决定因素。由于g-C ₃ N ₄ （尿素）中的质子较少，带隙较大，因此过电位较大，这也导致电荷更好地分离，从而导致电荷载体更好地迁移到活性位点。 样品组的质子浓度趋势和聚合趋势与相应的活性非常一致，这有助于电荷传输。因此，第一次证明了质子化和聚合度决定了从水中析氢的速率，而表面积起次要作用。

To conclude, we have presented a novel strategy for the production of a structure-controlled graphitic carbon nitride, which acts as a highly efficient photocatalyst for hydrogen synthesis from water by using solar energy. The internal quantum yield in the visible region is 26.5 %, nearly an order of magnitude higher than reported previously. Under full-arc irradiation, the optimized g-C₃N₄ can be tailored to produce approximately 20 000 μmol g⁻¹ of hydrogen per hour from water. The optimized g-C₃N₄ photocatalyst is very stable. It exhibited a near-linear profile of H₂ production from water for 30 h and resulted in a TON of over 641 in a 6 h test under irradiation with a 300 W Xe lamp. Data from XPS, FTIR spectroscopy, ζ-potential measurements, and XRD show that both the protonation status and the degree of polymerization can influence the g-C₃N₄ hydrogen-evolution rate. In other words, it has been experimentally proven that as the degree of polymerization increases and the protonation status is reduced, the hydrogen-evolution rate can be significantly enhanced. By using two different lines of computational evidence (DFT and TDDFT), we showed that this enhancement is because of a shift in the position of the conduction-band edge, thus increasing the overpotential for reduction reactions at the surface. There is a significant shift in the CBE position even for limited proton concentrations that are less than experimental values. Furthermore, excess protonation localizes photoelectrons at non-active redox sites. It was also shown that both a high degree of polymerization and a low level of protonation can be achieved by means of an oxygen-containing precursor, such as urea in this study, as further verified by another recently reported oxygen-containing g-C₃N₄ precursor.¹²
总之，我们提出了一种新的策略，用于生产结构控制的石墨氮化碳，它作为一种高效的光催化剂，用于利用太阳能从水中合成氢。在可见光区的内量子产率为26.5%，比以前报道的高近一个数量级。在全弧辐照下，优化后的g-C ₃ N ₄ 每小时可从水中产生约20000 μ molg ⁻¹ 氢气。优化后的g-C ₃ N ₄ 光催化剂稳定性较好。它表现出从水中产生H ₂ 30小时的近线性曲线，并在用300 W白炽灯照射的6小时测试中导致超过641的TON。XPS、FTIR、X-射线衍射（XRD）和X-射线光电子能谱（XPS）的数据表明，聚合物的质子化状态和聚合度都会影响g-C ₃ N ₄ 的析氢速率。 换句话说，已经实验证明，随着聚合度的增加和质子化状态的降低，可以显著提高析氢速率。通过使用两种不同的线的计算证据（DFT和TDDFT），我们表明，这种增强是因为在导带边缘的位置的移动，从而增加了在表面的还原反应的过电位。有一个显着的变化，CBE的位置，即使是有限的质子浓度小于实验值。此外，过量的质子化将光电子定位在非活性氧化还原位点。还表明，高聚合度和低水平的质子化可以通过含氧前体（例如本研究中的脲）来实现，如通过另一种最近报道的含氧g-C ₃ N ₄ 前体进一步验证的。 ¹²