Molecular engineering of carbon nitrides for overall photosynthesis of H<sub>2</sub>O<sub>2</sub>

Molecular engineering of carbon nitrides for overall photosynthesis of H₂O₂

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Jin Ma, Xiaoxiao Peng, Zhixin Zhou^*, Yanfei Shen^*, Yuanjian Zhang^*

Chinese Chemical Letters | 2023, 34(12) : 108784

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Chinese Chemical Letters | 2023, 34(12): 108784

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Molecular engineering of carbon nitrides for overall photosynthesis of H₂O₂

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Jin Ma, Xiaoxiao Peng, Zhixin Zhou^*, Yanfei Shen^*, Yuanjian Zhang^*

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Jiangsu Engineering Laboratory of Smart Carbon-Rich Materials and Device, Jiangsu Province Hi-Tech Key Laboratory for Bio-Medical Research, School of Chemistry and Chemical Engineering, Medical School, Southeast University, Nanjing 211189, China

Published: 2023-12-15 doi: 10.1016/j.cclet.2023.108784

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Abstract

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Artificial photocatalysis offers a promising strategy to sustainably produce hydrogen peroxide (H₂O₂) that is one of the most valuable multifunctional chemicals. Among various photocatalysts, polymeric carbon nitride (pCN) has drawn continuous attention in non-sacrificial H₂O₂ production. However, the poor activity of half reactions, i.e., the oxygen reduction reaction (ORR) and water oxidation reaction (WOR), greatly restricts the efficiency of photocatalytic H₂O₂ production. In this highlight, we discuss the significant advances in molecular engineering of carbon nitrides for H₂O₂ photosynthesis and the importance of the deep understanding of the photocatalysis process for rational design and reaction pathways of organic conjugated polymers to address the growing H₂O₂ demand. Furthermore, we summarize the emerging applications of photocatalytic H₂O₂ productions beyond energy and environment.

Key words

Carbon nitrides / Photocatalysts / H₂O₂ production / Oxygen reduction reaction / Water oxidation reaction

Cite this Article

Jin Ma, Xiaoxiao Peng, Zhixin Zhou, Yanfei Shen, Yuanjian Zhang. Molecular engineering of carbon nitrides for overall photosynthesis of H₂O₂[J]. Chinese Chemical Letters, 2023 , 34 (12) : 108784 - . DOI: 10.1016/j.cclet.2023.108784

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Hydrogen peroxide (H₂O₂) is one of the most valuable multifunctional chemicals with widespread applications. Currently, the well-established industrial anthraquinone-based method for H₂O₂ production is energy-intensive and environmentally unfriendly [1]. In addition to eco-friendly and economical electrochemical methods [2], artificial photosynthetic H₂O₂ generation is also considered as a clean and energy-saving approach that has attracted significant interest. Substantial efforts have been devoted to developing new photocatalysts with high efficiency, good selectivity, and stability [3]. Visible-light-responsive organic polymers [4], such as polymeric carbon nitrides, have been widely investigated as potential catalysts for H₂O₂ photosynthesis because of their high physical and chemical stability, low cost, and adjustable electronic bandgaps.

In 2014, Shiraishi et al. first reported that graphitic carbon nitride (g-C₃N₄) could produce H₂O₂ under photoirradiation using alcohol as an electronic donor [5]. Inspired by this seminal work, numerous polymeric carbon nitride-based photocatalysts have emerged in recent years for the photocatalytic production of H₂O₂, especially for solar-driven H₂O₂ production without any sacrificial agents [6]. However, most carbon nitride materials cannot produce effective holes or form active surface sites to directly oxidize H₂O into H₂O₂, and tend to undergo the thermodynamically favorable 4e⁻ WOR process [7]. Molecular engineering of carbon nitrides, including heteroatom doping (e.g., P or Sb) [8], modifying heteromolecules (e.g., pyromellitic diimide or biphenyl diimide) [9], surface defect engineering (e.g., C≡N groups and N vacancies) [10], and modulating triazine/heptazine based frameworks (e.g., by spatially separated redox centers) [11,12], plays a key role in the formation of architectures with great topologies [13]. Indeed, some substantial significant advances have been made to improve the non-sacrificial ORR and WOR efficiency for H₂O₂ photosynthesis; however, the efficiency and stability of current state-of-the-art photocatalysts are still low.

Each half-reaction includes different reaction pathways determined by the electron transfer number (Fig. 1a) [14-16]. For ORR half-reaction, the direct two-electron process exhibits a thermodynamically more favorable energy level (+0.695 V vs. NHE) and the indirect stepwise single-electron reduction firstly reduce O₂ to form hydroperoxyl radicals (·OOH, −0.05 V vs. NHE) which then couple with protons to generate H₂O₂ (+1.44 V vs. NHE). For the WOR half-reaction, H₂O can be directly oxidized by two holes to form H₂O₂ (+1.76 V vs. NHE), and the stepwise single-electron WOR process requires high concentrations of hydroxyl radicals (·OH, +2.73 V vs. NHE) to form the desired H₂O₂. Theoretically, based on the combination of different ORR and WOR pathways, there are four different pathways for the overall photosynthesis of H₂O₂: 2e⁻ ORR and 4e⁻ WOR pathways [8], stepwise 1e⁻ ORR and 4e⁻ WOR pathways [10], stepwise 1e⁻ ORR and 1e⁻ WOR pathways [17], and 2e⁻ ORR and 2e⁻ WOR pathways [11,12,18,19], which provide potent guidance for the design of photocatalytic processes (Fig. 1b) [14].

The creation of heteroatom-doped photocatalysts is an essential strategy in the pursuit of advanced photocatalytic processes. In an elegant study, Ohno et al. reported a Sb single-atom carbon nitride photocatalyst (Sb-SAPC) for non-sacrificial photocatalytic H₂O₂ synthesis under ambient conditions [8]. The introduction of highly active and selective Sb sites for the 2e⁻ ORR to consume the O₂ generated from 4e⁻ WOR in the photocatalytic system offers a high yield and selectivity for H₂O₂ photosynthesis (Figs. 2a and b), with an apparent quantum yield of 17.6% at 420 nm (Fig. 2c), together with a solar-to-chemical conversion (SCC) efficiency of 0.61% (Fig. 2d) for H₂O₂ synthesis. The d¹⁰ electronic configuration of Sb single-atom sites produces a new O—O stretching mode in the Sb—OOH species with an end-on adsorption configuration appearing under photo-irradiation, which reduces the possibility of O—O bond breaking and enhances the trapping of photogenerated electrons for the 2e⁻ ORR. Moreover, these sites are energetically unfavorable for the side hydrogen evolution reactions. In contrast, the photogenerated holes were concentrated at the N atoms in the s-heptazine moiety next to the Sb site to catalyze the 4e⁻ WOR. Both the 2e⁻ ORR and 4e⁻ WOR pathways formed a prime overall photosynthesis of H₂O₂ (Fig. 2e). Further examples of Ga- and Co-doped carbon nitrides also proved that the single atoms in carbon nitride certainly tailor the electronic structure and induce charge redistribution. It was supposed to not only promote the separation/transfer of charge carriers and the absorption/activation of O₂ in the 2e⁻ ORR reaction but also effectively enhance the 2e⁻/4e⁻ WOR reaction [19,20].

In addition to heteroatom doping, incorporating heteromolecules, such as aromatic diimides, an important class of n-type organic semiconductors with high electron mobility and stability, also positively adjusts the electronic structure and shifts the oxidation and reduction potentials, owing to their high electron affinity [9]. It has been demonstrated that the VB position of g-C₃N₄ is not thermodynamically feasible for the concurrent WOR. The introduction of pyromellitic diimide (PDI, Fig. 3a) and biphenyl diimide (BDI, Fig. 3b) into the g-C₃N₄ structure can generally positively shift the CB and VB positions and improve the thermodynamic driving force for catalyzing the O₂ evolution reaction (Fig. 3c). Figs. 3d and e show the reaction pathways for the overall photosynthesis of H₂O₂ via the coupling of the 2e⁻ ORR pathway with the 4e⁻ WOR pathway on g-C₃N₄/PDI and g-C₃N₄/BDI. Accordingly, g-C₃N₄/PDI and g-C₃N₄/BDI exhibited considerable H₂O₂ production rates without sacrificial agents in a pure water system, and the average SCC efficiencies of the optimized g-C₃N₄/PDI and g-C₃N₄/BDI reached 0.10% and 0.13%, respectively (Fig. 3f).

Furthermore, engineering defects on a photocatalyst surface is a universal and powerful method for improving the photocatalytic conversion efficiency. Recently, Zheng et al. sequentially introduced the –C≡N groups and N vacancies into g-C₃N₄ (N_v—C≡N—CN, Fig. 4a) [10]. The synergistic effect of the dual defect sites enhanced the visible-light absorption (Fig. 4b) and carrier separation capabilities (Fig. 4c). Besides, it has been verified that N vacancies can effectively adsorb and activate O₂ (Fig. 4d), while the C≡N groups enhance the adsorption of H⁺ (Fig. 4e), which synergistically promotes the formation and hydrogenation of OOH* to H₂O₂ (Fig. 4f). Accordingly, the N_v—C≡N—CN group achieved overall photosynthesis of H₂O₂ in pure water via concurrent stepwise single-electron ORR and four-electron WOR pathways (Fig. 4g). Meanwhile, the N_v—C≡N—CN photocatalyst exhibited 8.5 times H₂O₂ production rates higher than pristine bulk carbon nitrides without sacrificial agents, leading to a significant SCC efficiency of 0.23% (Fig. 4h).

Expect for the above-mentioned engineering of g-C₃N₄, the modulation of triazine/heptazine-based frameworks has great potential for the photocatalytic synthesis of H₂O₂. Rationally designed monomers for pre-assembly and precise regioselective coupling polymerization play key roles in the formation of architectures with separated redox centers. Very recently, Zhang et al. modulated a triazine-based framework with spatially separated redox centers and reported well-defined C₅N₂ with a conjugated C═N linkage to photosynthesize H₂O₂ from H₂O without sacrificial agents under hypoxic conditions (Fig. 5a) [12]. Compared with the tertiary N linkage in g-C₃N₄ breaking the conjugation over the heptazine rings, the conjugated linkers were redistributed in the C₅N₂ units, resulting in the strengthened delocalization of π-electrons, which downshifted the CB and VB positions. C₅N₂ with unusually low conduction ( > 0 V vs. NHE) and valence band positions promoted realistic selectivity in thermodynamics and high activity in kinetics for overall H₂O₂ photosynthesis through synergistic 2e⁻ ORR and 2/4e⁻ WOR (Fig. 5b), and C₅N₂ demonstrated the highest H₂O₂ photocatalytic activity (698 µmol L⁻¹ h⁻¹) under hypoxic conditions among the polymeric photocatalysts (Fig. 5c). For the first time, C₅N₂ was successfully applied to hypoxic/normoxic tumor photodynamic therapy/chemodynamic therapy (PDT/CDT) (Fig. 5d) and exhibited competitive performance under hypoxic and normoxic conditions (Fig. 5e). These results indicate that C₅N₂ offers an opportunity to potentially cut the cost of H₂O₂ production in a large scale and opens new avenues for highly efficient and selective H₂O₂ photosynthesis, particularly under hypoxic conditions in biomedical therapy systems.

Although previous studies have been devoted to enhancing the SCC efficiency toward solar-driven H₂O₂ production via the design of polymer photocatalysts, the efficiency of non-sacrificial H₂O₂ generation has not yet fulfilled the industrial requirements. Thus, the technical and economic feasibility of H₂O₂ photosynthesis should be rigorously analyzed. Apart from the wide use of H₂O₂ in the energy field owing to its high energy density, its strong oxidation power, and endogenous nature may achieve a potential application in environmental and biomedical fields, such as coupling solar-driven H₂O₂ production with dye degradation [18], conversion of organic molecules [21], bacterial disinfection [22], and cancer therapy [12], which could potentially lower the cost of large-scale production of H₂O₂.

In summary, we highlight the significant advances in H₂O₂ photosynthesis in the molecular engineering of carbon nitrides, a deep understanding of the photocatalysis process for the rational design and reaction pathways of carbon nitrides, and the technical and economical emerging applications of classical examples to address the growing H₂O₂ demand. The high energy density, strong oxidation power, and endogenous nature of H₂O₂ make it potentially applicable in energy, environmental, and biomedical fields by coupling solar-powered technologies for H₂O₂ production with carbon nitride. Furthermore, other potential catalysts, such as organic conjugated polymer resorcinol formaldehyde resins with a low bandgap, also exhibit outstanding efficiency without sacrificial reagents for the overall photosynthesis of H₂O₂ [3,23].

Declaration of competing interest

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

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This research was supported by the National Natural Science Foundation of China (Nos. 22174014 and 22074015).

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Year 2023 volume 34 Issue 12

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doi: 10.1016/j.cclet.2023.108784

Receive Date：2023-04-18
Online Date：2025-11-21
Published：2023-12-15

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Received：2023-04-18
Revised：2023-06-07
Accepted：2023-07-05

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表12种不同金属材料的力学参数

科 Family	属数 Number of genus	种数 Number of species	占总种数比例 Percentage of total species (%)	属 Genus	种数 Number of species	占总种数比例 Percentage of total species (%)
鹅膏菌科Amanitaceae	2	11	5.26	鹅膏菌属 Amanita	10	4.78
小菇科 Mycenaceae	2	12	5.74	丝盖伞属 Inocybe	5	2.39
多孔菌科 Polyporaceae	8	14	6.70	蜡蘑属 Laccaria	5	2.39
红菇科 Russulaceae	3	23	11.00	小皮伞属 Marasmius	6	2.87
				小菇属 Mycena	11	5.26
				光柄菇属 Pluteus	5	2.39
				红菇属 Russula	17	8.13
				栓菌属 Trametes	5	2.39

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Fig. 1. (a) Reaction scheme for photocatalytic H₂O₂ production and the corresponding energy diagrams for the ORR and WOR pathways. (b) Four classical combinations of ORR and WOR pathways for the overall photosynthesis of H₂O₂. Reprint with permission [14,15]. Copyright 2022, ACS.

Fig. 2. (a) Schematic structures of O₂ adsorption on metal surface. (b) ORR mechanism on a metal particle (top) and isolated atomic site (bottom). (c) Action spectra of PCN, PCN_Na15, and Sb-SAPC15 towards H₂O₂ production in phosphate buffer solution. (d) Solar-to-chemical conversion efficiencies of PCN, PCN_Na15, and Sb-SAPC15 under AM 1.5, illumination in a phosphate buffer solution. (e) Reaction mechanism of CN with Sb sites for photocatalytic H₂O₂ production. The white, gray, blue, red and magenta spheres refer to hydrogen, carbon, nitrogen, oxygen and Sb atoms, respectively. Reprint with permission [8]. Copyright 2021, Nature.

Fig. 3. (a) Molecular structure of g-C₃N₄/PDI. (b) Molecular structure of g-C₃N₄/BDI. (c) Electronic band structures of g-C₃N₄, g-C₃N₄/PDI, and g-C₃N₄/BDI. (d) Mechanism of g-C₃N₄/PDI for photocatalytic H₂O₂ production in pure water. (e) Mechanism of g-C₃N₄/BDI for photocatalytic H₂O₂ production in pure water system. (f) Change in the amount of H₂O₂ formed and SCC efficiency under simulated AM1.5 G sunlight (1 sun) with time. Reprint with permission [9]. Copyright 2016, ACS.

Fig. 4. (a) Defect structure in g-C₃N₄ heptazine units. Inset: C≡N groups and N vacancies. (b) UV–Vis DRS of all samples. Inset: Photographs of PCN and N_v—C≡N—CN. (c) Photoluminescence (PL) spectra of all samples. (d) O₂ adsorption energy and (e) H⁺ adsorption energy on g-C₃N₄ with different defect models. (f) Free energy profiles for photocatalytic H₂O₂ evolution reactions over PCN, C≡N—CN, N_v—CN, and N_v—C≡N—CN. (g) Photocatalytic H₂O₂ generation rates of N_v—C≡N—CN under different reaction gasses or sacrificial agents. (h) Comparison of photocatalytic H₂O₂ generation activity of different samples under the same conditions in a pure water system. Reprint with permission [10]. Copyright 2022, RSC.

Fig. 5. (a) Conjugated linker formation in the synthesis of C₅N₂ (5) from melamine (1) and p-phthalaldehyde (2) via Schiff base reactions. (b) Bandgap position of C₅N₂ and mechanism of photocatalytic H₂O₂ production by C₅N₂. Inset: C═N linkage in C₅N₂. (c) Summary of the photocatalytic activity for H₂O₂ production in a hypoxic microenvironment using different metal-free polymers in the literature. (d) Therapeutic processes using C₅N₂ to generate ·OH via the Fenton reaction under light irradiation in hypoxic environments. (e) Cell viability assay of C₅N₂−Fe−NS-treated 4T1 cells under light irradiation in hypoxic and normoxic environments. Reprint with permission [12]. Copyright 2022, Wiley.

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