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

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 (E_g), 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。

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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 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− → HO2 (aq)	−0.046
O2(g) + 2H+ + 2e− → H2O2 (aq)	0.695
2H2O (aq) + 4 h+ → O2 (g) + 4H +	1.229
OH− + h+ → OH	2.69
O3(g) + 2 H+ + 2 e− → O2 (g) +  H2O O 3 (g) + 2 H + + 2 e → O 2 (g) + H 2 O	2.075
CO2 + e− →CO2 CO 2 + e → CO 2 −	−1.9
2 CO2(g) + 2 H+ + 2 e− → HOOCCOOH(aq) 2 CO 2 (g)+2 H + +2 e→HOOCCOOH(aq) 2 CO 2 (气体)+2 H + +2 e→HOOCCOOH(溶液)	−0.481
CO2(g) + 2H+ + 2e− → HCOOH(aq) CO 2 (气体)+2H + +2e→HCOOH(aq)	−0.199
CO2(g) + 2 H+ + 2e− → CO(g) + H2O CO 2 (气体)+2 H + + 2e→CO(气体)+H 2 O	−0.11
CO2(g) + 4H+ + 4e− → C(s)  + 2H2O CO 2 (气体)+4H + +4e→C(s)+2H 2 O	0.206
CO2(g) + 4H+ + 4e− → HCHO(aq) + H2O CO 2 (气体)+4H + +4e→HCHO(水溶液)+H 2 O	−0.07
CO2(g) + 6H+ + 6e− → CH3OH(aq) + H2O CO 2 (气体)+6H + +6e→CH 3 OH(水溶液)+H 2 O	0.03
CO2(g) + 8H+ + 8e− → CH4 (g) + 2H2O CO 2 (气体)+8H + +8e→CH 4 (气体)+2H 2 O	0.169
2CO2(g) + 8H2O + 12e− → C2H4(g) + 12OH 2CO 2 (g) + 8H 2 O + 12e→C 2 H 4 (g) + 12OH−	0.07
2CO2(g) + 9H2O + 12e− → C2H5OH(aq) + 12OH 2CO 2 (g) + 9H 2 O + 12e→C 2 H 5 OH(aq) + 12OH−	0.08
3CO2(g) + 13H2O + 18e− → C3H7OH(aq) + 18OH 3CO 2 (g) + 13H 2 O + 18e→C 3 H 7 OH(aq) + 18OH−	0.09
H2O2(aq) + H+ + e− → H2O + OH H 2 O 2 (aq) + H + + e→H 2 O + OH−	1.14
HO2 + H+ + e− → H2O2(aq) HO 2 + H + + e→H 2 O 2 (aq)	1.44
H2O2(aq) + 2H+ + 2e− → 2H2O	1.763

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。

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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）。

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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。

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。

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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。

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]。

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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。

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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]。

It is known that a variety of surface defects on the surface of polymeric g-C₃N₄ material lead to the formation of multiple functionalities. Commonly, the basic primary and/or secondary amine groups (e.g., CNH₂ and C₂NH) on the terminating edges in the single layer of g-C₃N₄ (as shown in Fig. 8) could be created by a small quantity of hydrogen impurity, owing to the incomplete polycondensation [49], [101]. Therefore, it is not surprising that the g-C₃N₄ materials with surface defects and electron-rich properties also exhibit the unique nucleophilic character from basic surface functionalities (for the activation of CO₂) or H-bonding motifs (as shown in Fig. 8), thus facilitating their more valuable applications in catalysis, as compared to the ideal and defect-free g-C₃N₄ [101]. Furthermore, it is easily understood that the abundant basic groups (NH, N, NH₂ and NC) on the surface of g-C₃N₄ are beneficial for the removal of acidic toxic molecules through chemical adsorption based on electrostatic interactions [139]. Similar to the hydrophobic nature of the nanocarbon surface, hydrophobicity of g-C₃N₄ could lead to the formation of weakly interacted interfacial layer, thus significantly restricting the electron transport and separation and surface electrocatalytic reactions [61]. At this point, the hydrophilicity of g-C₃N₄ materials (with decreasing contact angle of water on their surface) could be improved through introducing oxygen-containing functional groups (hydroxyl and carboxyl) by means of chemical oxidation, thus greatly favoring their good dispersion in the aqueous solutions and further enhanced interfacial coupling and photocatalytic activities [140], [141], [142], [143], [144], [145].
众所周知，聚合物 g-C ₃ N ₄ 材料表面的各种缺陷导致多种功能的形成。通常，g-C ₃ N ₄ 单层中终止边缘上的基本一级和/或二级胺基团（例如，CNH ₂ 和 C ₂ NH）可由少量氢杂质创建，这归因于不完全的缩聚作用[49]，[101]。因此，g-C ₃ N ₄ 材料具有表面缺陷和富电子性质，表现出独特的亲核特性，从而促进了其在催化中比理想和无缺陷的 g-C ₃ N ₄ 更有价值的应用（如图 8 所示），如活化 CO ₂ 或氢键合基序（如图 8 所示）[101]。此外，可以轻松理解，g-C ₃ N ₄ 表面丰富的碱性基团（ NH ， N ， NH ₂ 和 N C ）有助于通过电荷间的化学吸附去除酸性有毒分子[139]。类似于纳米碳表面的疏水性质，g-C ₃ N ₄ 的疏水性可能导致弱相互作用的界面层形成，从而显著限制了电子传输和分离以及表面电催化反应[61]。 目前，通过化学氧化引入含氧功能团（羟基和羧基）可以提高 g-C ₃ N ₄ 材料的亲水性（使水在其表面的接触角减小），从而极大地促进它们在水溶液中的良好分散，并进一步增强界面耦合和光催化活性[140]，[141]，[142]，[143]，[144]，[145]。

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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 (CN) could be further identified by the Fourier transform infrared (FT-IR) spectra, X-ray photoelectron spectroscopy (XPS) measurements, Raman spectra, and Boehm titration analysis. Generally, the chemical composition and bonding information of g-C₃N₄ could be partially identified by FT-IR measurement. Fig. 9 depicts typical FTIR spectra of g-C₃N₄ powders obtained by heating melamine (MCN), thiourea (TCN), and urea (UCN) [146]. As shown, the main characteristic peaks observed in the region from 900 to 1700 cm⁻¹ were usually assigned to stretching vibration signals of aromatic heptazine-derived repeating units, including the typical sp² CN stretching modes and out-of-plane bending vibrations of the sp³ CN bonds [130], [147], [148], [149], [150], while the sharp absorption peak centered at approximately 810 cm⁻¹ was attributed to the characteristic breathing mode of tri-s-triazine cycles [150], [151], [152]. Meanwhile, the absorption band at 883 cm⁻¹ were indexed as the deformation mode of NH in amino groups [153], whereas the broadened peaks between 3000 and 3500 cm⁻¹ were related to the stretching vibration [146], [149], [150], [153] of residual free NH in the bridging CNHC units and OH originated from physically adsorbed water species on g-C₃N₄ surface, respectively. In the recorded Raman spectra, several characteristic peaks of g-C₃N₄ can be observed at 1616, 1555, 1481, 1234, 751, 705, 543, and 479 cm⁻¹, further confirming the vibration modes of CN heterocycles [151], [154]. It should be noted that the peak at 1234 cm⁻¹, corresponding to the NC (sp²) bending vibration, exhibits significant blue shift (1250 cm⁻¹ for 1-layer g-C₃N₄), due to the phonon confinement and strong quantum confinement effect [154]. Moreover, it has also been experimentally and theoretically demonstrated that the ratios of peak heights of 751–705 cm⁻¹ (I₇₅₁/I₇₀₅) and 543–479 cm⁻¹ (I₅₄₃/I₄₇₉), corresponding to layer–layer deformation vibrations or the correlation vibrations, obviously increased with decreasing the layer number of g-C₃N₄ [154]. Additionally, the nitrogen containing species can be further quantitatively analyzed by the element analysis and Boehm titration. For the element analysis, X-ray photoelectron spectroscopy (XPS) can not only reveal the atom ratio of carbon to nitrogen, but also identify the carbon and nitrogen species in g-C₃N₄. For example, the main peak at 288.2 eV in the high-resolution C 1s XPS spectra of the 1.0 wt% RGO/g-C₃N₄ sample, indicates the existence of the NCN₂ coordination [151]. The N 1 s binding energies at about 398.6, 399.8, and 401.5 eV in the high-resolutionN1 s XPS spectra of the 1.0 wt% RGO/g-C₃N₄ sample can be assigned to sp²-hybridized nitrogen (CNC), tertiary nitrogen (N(C)₃) and amino functional groups having a hydrogen atom (CNH), respectively [146], [151], [155], [156]. For Boehm titration analysis, it was found that the content of basic group per unit area of g-C₃N₄ generally decreased with increasing the calcination temperature [157]. In a word, the combination of Raman vibration properties, FTIR, XPS spectra and Boehm titration analysis can fully reveal the surface functional groups of g-C₃N₄ nanomaterials (Figs. 9,10 and 11).
通常，这些功能基团和网络（C N ）可以通过傅里叶变换红外（FT-IR）光谱、X 射线光电子能谱（XPS）测量、拉曼光谱和 Boehm 滴定分析进一步确定。一般来说，通过 FT-IR 测量可以部分确定 g-C ₃ N ₄ 的化学组成和键合信息。图 9 描述了通过加热三聚氰胺（MCN）、硫脲（TCN）和尿素（UCN）得到的 g-C ₃ N ₄ 粉末的典型 FTIR 光谱[146]。如图所示，在 900 到 1700cm ⁻¹ 范围内观察到的主要特征峰通常被指定为芳香基七氮杂环重复单元的拉伸振动信号，包括典型的 sp ² C N 拉伸模式和 sp ³ C N 键的平面外弯曲振动[130]，[147]，[148]，[149]，[150]，而约 810cm ⁻¹ 处的尖锐吸收峰被归因为三对三嗪环的特征呼吸模式[150]，[151]，[152]。同时，883cm ⁻¹ 处的吸收带被索引为氨基团中 N H 的变形模式[153]，而在 3000 到 3500cm ⁻¹ 之间的宽峰与 g-C ₃ N ₄ 表面上物理吸附的水分子和 C NH C 单元中残留的游离 N H 的拉伸振动[146]，[149]，[150]，[153]有关，分别。在记录的拉曼光谱中，可以观察到 g-C ₃ N ₄ 的几个特征峰，分别位于 1616、1555、1481、1234、751、705、543 和 479cm ⁻¹ ，进一步确认了 CN 杂环的振动模式[151]，[154]。 需要注意的是，1234 厘米处的峰值 ⁻¹ ，对应于 N C（sp ² ）弯曲振动，呈现明显的蓝移（1 层 g-C ₃ N ₄ 为 1250 厘米 ⁻¹ ），这是由于声子约束和强量子约束效应[154]。此外，实验证明，在 751–705 厘米 ⁻¹ （I ₇₅₁ /I ₇₀₅ ）和 543–479 厘米 ⁻¹ （I ₅₄₃ /I ₄₇₉ ）的峰值高度比，对应于层间变形振动或相关振动，显著随着 g-C ₃ N ₄ 层数减少而增加，理论上也得到了证实[154]。此外，可以通过元素分析和 Boehm 滴定进一步定量分析含氮物种。对于元素分析，X 射线光电子能谱（XPS）不仅可以揭示碳氮原子比，还可以确定 g-C ₃ N ₄ 中的碳和氮物种。例如，在 1.0wt% RGO/g-C ₃ N ₄ 样品的高分辨率 C1s XPS 光谱中的 288.2eV 主峰表明存在 N C N ₂ 配位[151]。在 1.0wt% RGO/g-C ₃ N ₄ 样品的高分辨率 N1s XPS 光谱中，N1s 束缚能在约 398.6、399.8 和 401.5eV 处，分别对应于 sp ² 杂化氮（C N C）、三级氮（N （C） ₃ ）和具有氢原子的氨基官能团（C N H）[146]，[151]，[155]，[156]。对于 Boehm 滴定分析，发现 g-C ₃ N ₄ 单位面积的碱性基团含量通常随着焙烧温度的升高而减少[157]。 总之，拉曼振动性能、傅里叶变换红外光谱（FTIR）、X 射线光电子能谱（XPS）和 Boehm 滴定分析的结合可以充分揭示 g-C ₃ N ₄ 纳米材料的表面功能团（图 9、10 和 11）。

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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]。

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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]。

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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]。

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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]。

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]。

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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]。

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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。

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 p_z orbitals and carbon p_z 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]。

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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 0.01q_e°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。

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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]。

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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。）

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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]。

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 (E_g) 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。

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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]。

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)dq_t/dt = k₂(q_e − q_t) ²(3)t/q_t = 1/ (k₂q_e²) + t/q_ewhere q_t (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 q_e (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。

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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 NO_x 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 NO_x and H₂S, because porous g-C₃N₄ could simultaneously serve as light-harvesting, charge-excitation, charge-transportation, adsorption (for acid CO₂, H⁺, NO_x 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]。

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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]。

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。

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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。

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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 ΔG_H* 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。)

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 (E_fb) 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/CoO_x 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。

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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//CoO_x, 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]。

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。

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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.

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Fig. 26. Two kinds of metal ion doping of g-C₃N₄ framework: (a) cave doping, the incorporation of metal ions (M^{n +}) 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 CF bonding in g-C₃N₄ (as shown in Fig. 27), thus lowering the electronic band gaps [331]. However, it should be point out that excessive doping of nonmetal and metal is found to be detrimental to enhance the photocatalysis, because the more defects can also act as the recombination centers of electron–hole pairs. In future, the co-doping of different metals and/or nonmetals, such as Fe/P [332] S/Co/O [333], S/P [334], P/O [335], K/Na [336] and C/Fe [337] deserves more attention, due to their positive synergetic effects on the visible-light absorption and photocatalytic properties.

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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.

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Fig. 28. (a) UV–vis absoprtion spectra of g-C₃N₄ and CNB_x (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].

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.

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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 OH radicals and decomposing the organic pollutant Rhodamine B. It is believed that the nitrogen-vacancy defects could greatly widen visible light absorption range and suppress the unexpected fast recombination of photo-excited carriers, thus achieving the improved photoactivity. Similarly, it was also demonstrated that the introduction of hydrogenated defects in g-C₃N₄ nanosheets could greatly enhance the photocatalytic hydrogen evolution [349], [352]. Furthermore, a simple thermal treatment under an NH₃ atmosphere can not only develop highly porous g-C₃N₄ nanosheets with plenary carbon vacancies through etching their lattice carbon sites by the reactive radicals from the NH₃ decomposition [350], [351], but also can enhance the surface area, porosity and crystallinity of condensed g-C₃N₄, due to the greatly reduced N defects in the π-conjugated network [346]. More interestingly, through employing both DFT and molecular dynamics calculations, Wu et al. indicated that the defect within g-C₃N₄ played a key role in the adsorption and dissociation of water, whereas, water does not dissociate on the perfect g-C₃N₄ sheet (as shown in Fig. 30) [353]. However, it should be noted that the excessive nitrogen vacancies as the recombination centers could be also harmful for the photocatalysis [354]. To demonstrate this point, Osterloh and co-workers found that surface structure defects in g-C₃N₄, with energy levels at +0.97 V and −0.38 V (vs.NHE), limit visible light driven hydrogen evolution and photovoltage [191]. More interestingly, it was also demonstrated that the vacuum heat-treatment at 500 °C could obtain the highest photoactivity for H₂ evolution due to the increased content of the tri-s-triazine phase and suitable N defects in the tri-s-triazine ring building blocks [355], which are similar to the previous report about the vacuum-treated titanium dioxide [356]. Consequently, the controlled defect concentration in g-C₃N₄ is crucial for achieving the ideal photoactivity.

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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.)

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.

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.

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.

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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.

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

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.

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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.

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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].

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.

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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.

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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].

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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 (O₂⁻ and OH) trapping experiments, metal deposition and double-beam photoacoustic (DB-PA) spectroscopy have been employed to reveal the real Z-scheme charge-transfer mechanism [523], [534], [601], [608], [619], [620].

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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].

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

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

The level of the conduction bands of g-C₃N₄ is −1.3 V (vs. NHE), which is more negative than the reduction potentials of CO₂/CO(−0.51 V), H ⁺/H₂ (0.41 V), and O₂/O₂⁻ (0.33 V), respectively (As shown in Fig. 41). Thermodynamically, the photo-generated electrons on the CB of g-C₃N₄ have much stronger driving force (or over-potentials) for these three typical kinds of reduction reactions, as compared to those in TiO₂ [28], [657], [658]. However, the obvious structure defects in bulk g-C₃N₄ generally lead to their fast recombination with the photo-excited holes. More importantly, the photocatalytic H₂ evolution and CO₂ reduction are typical up-hill reactions, thus resulting in the sluggish kinetics on the surface of bulk g-C₃N₄. Fortunately, these disadvantageous factors could be simultaneously overcome by loading suitable reduction co-catalysts onto the surface of g-C₃N₄, which could lower the reaction activation energy (or electrochemical overpotentials), improve the charge separation and transport, increase stability of photocatalyts, and accelerate the sluggish reaction kinetics of various surface reduction reactions, thus greatly enhancing the photocatlytic activity [88], [124]. Essentially speaking, the single-electron or multi-electron O₂-reduction reactions (as shown in Table 2) are of significant importance for photocatalytic degradation [83], [410], [657], selective organic transformations [119], [244] and disinfection [659], [660]. More interestingly, the co-catalysts can also achieve the selective photoreduction products of CO₂ [16], [661]. In addition, it is also necessary to deposit suitable hole co-catalysts to accelerate the difficult water oxidation reactions, due to the lower overpotential of g-C₃N₄ for water oxidation (0.59 V), as well as the inherent challenges of four-electron water oxidation [7], [662]. Thus, it is obvious that all these reduction and oxidation co-catalysts play decisive roles in achieving highly efficient and selective photocatalytic reactions. These four types of co-catalysts over g-C₃N₄ (as shown in Fig. 41), namely, H₂-evolution, CO₂-reduction, O₂-reduction and H₂O-oxidation co-catalysts, are summarized in Table 6, which will be compared and discussed in this section in detail.