Recent advances in photocatalytic overall production of hydrogen peroxide from metal-free photocatalysts

Recent advances in photocatalytic overall production of hydrogen peroxide from metal-free photocatalysts

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Congxu Wang^a^,^c, Xuan Xie^d, Feng Qiu^a, Lei Zhu^b, Imran Shakir^e, Yuxi Xu^c

Chinese Chemical Letters | 2026, 37(1) : 111604

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Chinese Chemical Letters | 2026, 37(1): 111604

• Review •

Recent advances in photocatalytic overall production of hydrogen peroxide from metal-free photocatalysts

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Congxu Wang^a^,^c, Xuan Xie^d, Feng Qiu^a, Lei Zhu^b, Imran Shakir^e, Yuxi Xu^c

Affiliations

^aSchool of Physics and Optoelectronic Engineering, Hangzhou Institute for Advanced Study, University of Chinese Academy of Sciences, Hangzhou 310024, China

^bSchool of Chemistry and Materials Science, Hubei Engineering Research Center for Key Technologies in Modern Paper and Sanitary Products Manufacturing, Hubei Engineering University, Xiaogan 432000, China

^cSchool of Engineering, Westlake University, Hangzhou 310024, China

^dSchool of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China

^eDepartment of Physics, Faculty of Science, Sustainability Research Center, Islamic University of Madinah, Madinah 42351, Saudi Arabia

About Author:

¹ These authors contributed equally in this work.

CRediT authorship contribution statement

Congxu Wang: Writing – original draft.

Xuan Xie: Writing – original draft.

Feng Qiu: Supervision.

Lei Zhu: Writing – original draft.

Imran Shakir: Writing – review & editing.

Yuxi Xu: Supervision, Project administration.

Published: 2026-01-15 doi: 10.1016/j.cclet.2025.111604

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Abstract

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Hydrogen peroxide (H₂O₂) has been recognized as a green and nonpolluting multifunctional oxidant with extensive applications in environmental protection, metal etching, textile printing and dyeing, chemical synthesis and food processing. However, over 90% of industrial H₂O₂ is currently produced through the multi-step anthraquinone oxidation process, which suffers from a process with some drawbacks such as complex, high-energy consumption, and toxic byproducts emissions. Compared to the traditional anthraquinone method, artificial photosynthesis of H₂O₂ using semiconductor photocatalysts has emerged as a sustainable alternative due to its use of water and O₂ as the clean reactants and sole energy as the driving force. In recent years, metal-free photocatalysts mainly including covalent organic frameworks (COFs), covalent triazine frameworks (CTFs) and carbon nitrile (g-C₃N₄) have garnered significant interest due to their superior thermal and chemical stability, diverse synthesis methods, tunable functionality, light weight nature and non-toxicity. These materials also exhibit adjustable band structure and unique photoelectric properties. Sustainable efforts have been made to advance metal-free photocatalysts for artificial photosynthesis of H₂O₂, however, a comprehensive summary of current research status on metal-free-based photocatalytic overall H₂O₂ production remain scarce. This review outlines recent process in overall H₂O₂ photosynthesis based on metal-free photocatalysts. First, we introduced the fundamental concepts of photocatalytic overall H₂O₂ production. Then, we analyze representative studies on photocatalytic overall H₂O₂ synthesis using metal-free materials. Finally, we discuss the challenges and future perspectives in this field to guide the design and synthesis of metal-free systems for H₂O₂ generation.

Key words

Photocatalysis / H₂O₂ generation / Metal-free photocatalysts / Tunable functionality / Overall synthesis

Cite this Article

Congxu Wang, Xuan Xie, Feng Qiu, Lei Zhu, Imran Shakir, Yuxi Xu. Recent advances in photocatalytic overall production of hydrogen peroxide from metal-free photocatalysts[J]. Chinese Chemical Letters, 2026 , 37 (1) : 111604 - . DOI: 10.1016/j.cclet.2025.111604

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1　Introduction

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The increasing of the global energy crisis and environmental pollution issues caused by excessive consumption of fossil fuel necessitate the development of low-carbon and clean energy technologies [1–8]. Hydrogen peroxide (H₂O₂) has gained much attention as a promising candidate to address these issues due to its green, environmentally friendly and sustainable nature [9–12]. H₂O₂ is irreplaceable in water treatment applications as it is a clean oxidant that decomposes into water and oxygen without producing harmful byproducts. Additionally, H₂O₂ is widely used in chemical synthesis, medicine, semiconductor manufacturing, the military, aerospace and other industries [13–15]. Moreover, it has been proposed an alternative energy carrier in fuel cells due to its safety and comparable power density as compressed hydrogen. Currently, over 90% of industrial H₂O₂ is produced via the traditional industrial anthraquinone (AQ) method [16,17]. However, this process involves multiples steps-hydrogenation, oxidation, extraction, purification and concentration-resulting in significant energy loss and the generation of toxic byproducts [18–20]. Furthermore, AQ method relies on precious metals catalysts, increasing the production costs. Therefore, developing an efficient, safe, green, and sustainable method for H₂O₂ preparation is an urgent task. Semiconductor based photocatalytic technology emerged as an economically viable method for utilizing solar energy to synthesize H₂O₂ by mimicking natural photosynthesis [21–23]. The sunlight-driven O₂ reduction and H₂O oxidation is considered a feasible strategy for replacing AQ oxidation process [24–28]. However, most photocatalytic H₂O₂ synthesis research and reviews primarily on the oxygen reduction reaction (ORR), while studies on the water oxidation reaction (WOR) remain limited [13,29,30]. This review focuses on photocatalytic overall H₂O₂ generation, which involves both O₂ reduction and H₂O oxidation. The choice of photocatalyst plays a crucial role in enhancing the efficiency of the photocatalytic process.

For a long time, the research on photocatalytic H₂O₂ synthesis has predominantly centered on inorganic photocatalysts, such as TiO₂, BiVO₄, WO₃, ZnS and CdS-based materials [4,29,31,32]. However, these materials suffer from inherent drawbacks, including a narrow absorption range, low sunlight utilization efficiency (particularly in the ultraviolet region), and limited variability, which restricts their widespread application in photocatalytic H₂O₂ generation field [9,10,14]. Various strategies-such as micro/nano structuring, surface defect engineering and heterojunctions formation-have been proposed to improve the optoelectronic properties of inorganic semiconductors. However, the efficiency of H₂O₂ production with these materials remain unsatisfactory.

Recently, metal-free organic polymers-including covalent organic frameworks (COFs), covalent triazine frameworks (CTFs), and carbon nitrile (g-C₃N₄) have attracted significant attention due to their unique optical and electrical properties [21,33–37]. These materials offer notable advantages, such as facile designability, tunable functionality, large specific surface area, and porosity, which enhance interaction between O₂/H₂O molecules and catalytic active centers, thereby improving the photocatalytic efficiency [38–41]. Moreover, the extended π-conjugation of organic polymers broadens the visible absorption range and promotes photoinduced charge separation, thereby suppressing electron-hole recombination and maximizing solar energy utilization [42–44]. In addition, the designable modular structure and versatile functionalities of organic polymers provides huge potential for the applications of photocatalysis in regulating photocatalyst composition, optoelectronic characteristics, and inter-facial properties at the atomic and molecular level [15,45,46]. So that conduction band (CB) and valence band (VB) potential of metal-free photocatalysts to simultaneously meet the requirements of redox reaction of O₂ and H₂O. These unique characteristics have received much attention and led to the rapid development of metal-free based photocatalysts with interesting structures features, but there is an urgent need to summarize and highlight the research status of metal-free photocatalytic systems for photocatalytic overall H₂O₂ generation. This review aims to summarize the process of metal-free organic photocatalysts in photocatalytic overall H₂O₂ generation and provides in-depth insight into the synthetic strategy of metal-free photocatalysts to improve photocatalysis performance (Fig. 1). First, the fundamental principles of photocatalytic overall H₂O₂ generation were explicated. Then, we discuss various strategies for designing and synthesizing metal-free based photocatalysts including g-C₃N₄, CTFs, and COF to enhance their photocatalytic performance. The reaction mechanism is also analyzed to promote a deeper understanding of photocatalytic overall H₂O₂ production. Additionally, we explore the structure-activity relationships that guide the development of high-performance metal-free photocatalytic systems. Ultimately, this review aims to provide readers with a comprehensive understanding of the critical role of metal-free photocatalysts in the overall H₂O₂ generation and offer strategic insights for further improving their photocatalytic performance.

2　Fundamental principles of photocatalytic H₂O₂ production

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Artificial photosynthesis on semiconductors can be divided four typically steps: light absorption inducing excitation formation, carrier separation, carrier transfer to surface of photocatalysts, and subsequent surface redox reactions (Fig. 2a) [37,47–53]. Together, these four steps determine the photocatalytic efficiency. Under illumination of light, semiconductor absorbs photons with energies greater than its band-gap, upon which generating photo-excited electrons and holes at the CB and VB, which subsequently migrate to the surface of photocatalyst for initiate the redox reaction [22,24,54,55]. To realize excellent performance, photocatalysts should keep efficient in all these four steps. In principle, photocatalysts with narrow band-gap can maximize the utilization of photons. On the other hand, to drive surface redox reactions, the minimum band-gap value should be satisfied, so that the conduction and valence band edges pass through the potential of the target redox reactions, respectively [23,26,40,56]. Therefore, when adjusting the band-gap of photocatalysts materials, the delicate balance between light absorption and band edges should be kept in mind. In addition, charge separation and transfer also play a vital role in determining the final photocatalytic performance.

As for photocatalytic overall H₂O₂ synthesis, it mainly includes ORR and WOR processes [13,30,57]. Fig. 2b showed the reaction energy barrier of ORR and WOR, vs. normal hydrogen electrode (NHE). The ORR reaction process contains two ways, which is direct one-step 2e^– ORR (ⅰ) and indirect two-step 1e^– ORR (ⅱ), respectively. And the one-step 2e^– ORR process exhibits a more favorable energy level than indirect two-step 1e^– ORR. The generated ·O₂^– from two-step 1e^– ORR not only has a more negative energy level (−0.33 V) than one-step 2e^– process but also is rather to involve in other redox reactions [4,9,10,13]. Therefore, the efficiency and selectivity of the two-step 1e^– pathway for producing H₂O₂ are much lower than one-step 2e^– ORR. Optimization the composition and structure of semiconductor photocatalysts to enhance the efficiency and selectivity of the one-step 2e^– ORR pathway is an effective guidance for photosynthesis H₂O₂. Similarly to the ORR process, the direct WOR also include several different reaction pathways, such as direct one-step 2e^– WOR, indirect one-step 2e^– WOR and 4e^– WOR processes [14]. Likewise, the direct WOR is thermodynamically more advantageous, while the indirect WOR is kinetically more favorable [21,37]. The direct one-step 2e^– WOR is carried out by oxidation photo-generated holes (ⅳ), with an oxidation potential of up to 1.76 V. Indirect two-step 1e^– WOR can be used for photosynthesis of H₂O₂ (ⅴ), but the production of H₂O₂ from the coupled ·OH induced by hole requires a high concentration of ·OH, which is almost not limited to increasing the yield of photosynthesis H₂O₂ [31,48]. Besides, the competitive reaction of converting H₂O to O₂ and H⁺ by a one-step 4e^– process (1.23 V, ⅵ) inevitably happens simultaneously, resulting in a reduced H₂O₂ formation efficiency and selectivity for semiconductor photocatalysts [31,49,50]. To achieve efficient photosynthesis H₂O₂ without sacrificing reagents, it is necessary to simultaneously carry out O₂ reduction and H₂O oxidation reactions. Obviously, ORR process is theoretically easier to achieve than WOR process for photocatalytic synthesis of H₂O₂. Therefore, how to realize high-efficient WOR process is the key to achieve photocatalytic overall H₂O₂ synthesis [30,58–60].

The generation intermediates during ORR and WOR process can be measured by different in situ experimental characterizations, including in-situ Raman and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) due to in situ characterization technology allows us to real-time observation the redox reaction process. Thus, in situ characterization is gradually developing to an advanced research strategy in materials, catalysis, and energy [4,34]. Additionally, the reaction barrier of ORR and WOR can be calculated by theoretical calculations. Currently, the theoretical calculations is a powerful method to clarify the reaction process in detail [2,11].

Based on the above result, metal-free photocatalysts have shown significant potential in the field of photocatalytic overall H₂O₂ production and have become an indispensable component in this field. Although some research on metal-free organic polymer for H₂O₂ generation has been reported, there is still a lack of related reviews to summarize the photocatalytic overall H₂O₂ generation of metal-free organic polymer for. Based on this, we will introduce the research process of representative metal-free organic polymer photocatalysts mainly including COFs, CTFs and g-C₃N₄ in overall H₂O₂ generation in recent years in the next section.

3　Photocatalytic overall H₂O₂ production by metal-free photocatalysts

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3.1　The design and synthesis of metal-free-based photocatalysts

Compared with traditional inorganic photocatalysts, organic polymers possess the simple synthesis methods, strong designability, and excellent response to visible light, therefore, it holds great promise in photocatalytic overall H₂O₂ production [61–67]. Based on the abundant synthesis methods of organic polymers, their pho-toelectrochemical properties can be highly modulate by adjusting their structure at molecular level [68–71]. Currently, the chemical design synthesis of organic photocatalysts is mainly divided into intrinsic molecular structure and heterostructure design [44,72–75]. The intrinsic molecular structure design mainly relies on deliberately selecting suitable building blocks and linkages for modulating photoelectric properties of the polymers, thereby improving photocatalytic efficiency. Metal-free-based photocatalysts can be designed to harvest wide range light when showing exceptional stability under various operational conditions. Single-component photocatalysts usually face a fundamental contradiction: a narrow bandgap enables wide solar absorption but exacerbates rapid electron-hole recombination due to strong coulomb interactions [76,77]. Therefore, it is necessary to design the photocatalyst to decrease the recombination rate of photo-excited carriers and widen the range of light absorption. Two types of semiconductors with different staggered band structures to form heterojunctions can not only broaden the absorption range of light, but also adjust the band-gap structure. To promote the separation and transfer of carriers, the construction of heterojunction is an effective way to solve the sunlight absorption and the electron-hole pairs recombination [71,78]. Of course, there are many other efficient strategies for photocatalytic overall H₂O₂ production, such as microenvironment regulation, spin state regulation as well as amorphous 2D layered materials. These strategies could effective regulation the electronic band structures, microtopograph and physicochemical properties of photocatalysts.

3.2　Photocatalytic overall H₂O₂ production by g-C₃N₄

g-C₃N₄ have shown promising in photocatalysis overall H₂O₂ filed due to its unique characteristics, such as band structure, easily manufactured, excellent chemical and thermal stability, and environmental friendliness [56,79–81]. The highly delocalized π-conjugated system is formed by sp² hybridization of C and N resulting in a typical semiconductor property of g-C₃N₄. It can be synthesized by simple sintering of nitrogen-rich precursors, such as melamine, cyan-amide, thiourea, and urea. The optical band gap of g-C₃N₄ at 2.7 V, with CB −1.3 V, which is more negative than the potential of O₂ reduction to H₂O₂. And the VB is located at +1.4 V, which meets the potential energy requirement of 4e^- H₂O oxidation. Thus, recent research has highlighted the potential of g-C₃N₄ in photocatalytic overall H₂O₂ generation from pure water and O₂ under visible light irradiation [60,80,82]. Table 1 summarizes some typical examples of photocatalytic overall H₂O₂ generation of recent g-C₃N₄-based photocatalysts [83–99]. In 2014, Hirai's group first reported photocatalytic overall H₂O₂ production using g-C₃N₄ (Fig. 3a) [83]. They found incorporating pyromellitic diimide (PDI) into g-C₃N₄ could positively shift the oxidation and reduction potentials due to the strong electron affinity of PDI. As shown in Fig. 3b, the CB and VB of g-C₃N₄/PDIx all become more positive after incorporate PDI, thus resulting in the enhanced photocatalytic water oxidation performance. Notably, g-C₃N₄/PDIx not only showed a decreased narrowed band gap for extended light response range but also had a deeper VB position than g-C₃N₄ which possessed enough thermodynamic driving force to enhance photocatalytic water oxidation ability. The apparent quantum yield (AQY) test was consistent with the energy band structure of g-C₃N₄/PDIx, indicating that the optimized band-gap structure could promote water oxidation and O₂ reduction. After that, various methods for preparing highly active g-C₃N₄ for photocatalytic overall H₂O₂ evolution have been developed, including construction of single-atom doped, heterostructures and functionalization. In this part, the research process on photocatalytic overall H₂O₂ generation over g-C₃N₄-based photocatalysts are systemically summarized. And we introduce the different strategies in details to review photocatalytic overall H₂O₂ production over g-C₃N₄.

3.2.1　Single-atom doping

Single-atom has high active centers, such that unique properties exist for the ideal design of new photocatalysts with high activities, low-cost and stability [100,101]. Therefore, given the photocatalytic overall H₂O₂ generation, some optimal design of single-atom doped have been prepared and display exceptional performance in overall H₂O₂ production. Guo's group designed Ga-doped g-C₃N₄ (CNIO-GaSA) with Ga-N₄ sites, where Ga³⁺ acts as a Lewis acid to polarize adsorbed O₂, facilitating 2e⁻ reduction to *OOH. Density functional theory (DFT) calculations revealed that the Ga-doped site lowers the *OOH formation barrier by 0.3 eV compared to pristine g-C₃N₄ [91].

In situ DRIFTS spectra confirmed the presence of *OOH intermediates, validating the dominant 2e⁻ ORR pathway. However, the sluggish water oxidation kinetics (4e⁻ WOR) limits the overall efficiency, suggesting the need for cocatalysts to improve O₂ evolution. The synthetic process of CNIO-GaSA was shown in Fig. 4a. It was prepared by combining Ga(NO₃)₃ and dicyandiamide as the precursor and silica opal as template with subsequent removal strategies. Partial density of states (PDOS) reveals the formation hybridized Ga 4p and N 2p states could enhance the conductivity of CN contribute to reduce the transfer resistance of charge carriers. The transmission electron microscopy (TEM) (Fig. 4b) image show that obtained CNIO-GaSA has the periodic pore structure with an average diameter of ~250 nm. CNIO-GaSA presents a significantly enhanced H₂O₂ production rate (368.5 µmol/h), which is about 4.0 and 7.8 times, respectively higher than that of CNIO and CN. In addition, CNIO-GaSA exhibits the highest catalytic activity compare to other previously reported photocatalysts. Significantly, CNIO-GaSA also shows the excellent 100% efficiency for kill bacteria, thereby offering promising potential practical application. Although the CNIO-GaSA shows excellent photocatalytic activity toward overall H₂O₂ generation, the Ga is rare metals, which is not fit for practical application. On this basis, Zhang's group reported a high-loading Ni single-atom photocatalyst for efficient overall H₂O₂ synthesis with an AQY and solar-to-chemical conversion (SCC) up to 10.9% and 0.82%, respectively (Fig. 4c) [90].

They found that Ni-N₃ sites transform into high-valent O_1-Ni-N₂ sites using different in-situ characterizations. And the PDOS of Ni confirm that Ni 3d orbitals contribute to both CB and VB, suggesting Ni single atoms exceedingly optimize the electronic structure. Extensive experiments and detailed calculations prove that the development of the sites structure decreases the energy barrier of *OOH and inhibits the O=O bond dissociation resulting in improved H₂O₂ production activity. As shown in the proposed synthesis process, the high-loading M-SAPs can be divided into modulating the microtopography and optimizing the loading process. PuCN showed a higher overall H₂O₂ generation rate (41.1 µmol L^-1 h^-1) than of BCN (16.5 µmol L^-1 h^-1) due to the porous ultra-thin structure improved the contact range of O₂. Apparently, the NiSAPs-PuCN showed highest H₂O₂ generation rate up to 342.2 µmol L^-1 h^-1, which was about 20.7 and 8.3 times higher than that of BCN and PuCN, respectively. Furthermore, the H₂O₂ generation activity of NiSAPs-PuCN can be improved to 640.1 µmol L^-1 h^-1 under AM 1.5 G irradiation. The AQY was tested owing to the AQY value is generally recognized as one of more reliable factors for assessing the photocatalytic property. Experiment demonstrated that the introduction of Ni active sites can greatly facilitate the O₂ adsorption compared to pure BCN (decreased from 1.1 eV to −0.83 eV). It is essential that format the core intermediate *OOH for overall H₂O₂ generation via 2e⁻ ORR. The calculation result suggest that improvement O₂ adsorption capacity can promote intermediate formation, thus improving the conversion efficiency of H₂O₂. Charge density difference calculation result showed the charge redistribution of NiSAPs-PuCN is more remarkable than BCN, suggesting the Ni sites can promote the *OOH formation. Liu's group reported atomically dispersed Au on potassium-incorporated g-C₃N₄ (AuKPCN) that could simultaneously improve photocatalytic production of ·OH and H₂O₂ (Fig. 5) [102]. Details experiments and theoretical calculations proved that the low-valent Au changed the materials' band gap structure to trap localized holes produced under photo-excitation. Fig. 5a showed the 1e⁻ WOR has the highest efficiency for ·OH production owing to one h⁺ can produce one ·OH. The in-situ Raman and DRIFTS testing result confirm that one-electron WOR could produce ·OH and unleash proton to accelerate ORR. The Au element has s orbital and d¹⁰ orbitals of Au element was filled by one electron and fully electron resulting in formation a highly concentrated hole under excited state (Fig. 5b). And the AuKPCN exhibited a remarkable AQY of around 85% at the wavelength range of 400–420 nm in different organic electron donor system (Fig. 5c). To further study the oxidation half reaction of AuKPCN, Ag⁺ and DMPO were added. The electron spin resonance (ESR) test result suggests the ·OH can be detected in the presence of AuKPCN without found O₂ and H₂O₂, indicating the WOR process can produce the ·OH over AuKPCN (Fig. 5d). Significantly, the generation amount of ·OH and H₂O₂ is close to 2:1 (Fig. 5e), which consistent with the theoretical proportion based on 1e⁻ WOR. As we have seen yet, AuKPCN showed the highest AQY, which was larger than previously reported photocatalysts for produce ·OH via 1e⁻ and 2e⁻ WOR. This outstanding work first reports on 1e⁻ WOR generation ·OH and released the proton for enhancing the H₂O₂ production rate. But the ·OH can dissociation H₂O₂ owing to the its strong oxidizing properties.

3.2.2　Heterojunction

As well known, the single-component photocatalyst often cannot satisfy all the requirements for photocatalytic overall H₂O₂ production due to the fast photo-generated charge recombination and insatiable redox capacity [103–107]. Therefore, to decrease the recombination rate of photo-excited carriers and raise redox capacity, it is necessary to reform the photocatalyst. The construction of heterojunction by combining two kinds of semiconductors with interleaving band structure can not only decrease the recombination rate of photo-excited electron and hole, but also adjust redox capacity for fulfillment different photocatalysis requirements. Fan's group reported a binary semiconductors photocatalytic system for photocatalytic overall H₂O₂ generation [94]. Extensive experiments and calculation revealed that heterojunction can enhance light absorption capabilities and reduce interlayer charge transfer distance. They used X-ray absorption fine structure and in situ XPS to study coordination environment and charge transfer mechanism of heterojunction. Fig. 6a illustrated the synthesis route for K⁺/I⁻-CN/CdSe-D heterojunction. The I⁻ and K⁺ ion intercalation into g-C₃N₄ was achieved by the assistance of KCl/KI salt. And the composite was formed by the in situ modified CdSe onto K⁺/I⁻-CN nanosheets using a microwave hydrothermal method. The photocatalytic overall H₂O₂ production activity of obtained samples was assessed under visible light irradiation. The photocatalytic performance of heterojunction surpassed that of the individual CdSe-D and K⁺/I⁻-CN. The 40% K⁺/I⁻-CN/CdSe-D exhibited the highest photocatalytic activity (448.63 µmol/L), which exceeded that most metal-free photocatalysts. They also tested the photocatalytic overall H₂O₂ generation with different mass loading. The results showed a significant linear relationship between catalyst amount and the yields of H₂O₂, further suggesting the reliability of the photocatalytic activity. Further the continuous irradiation experiment demonstrated excellent photocatalytic stability of heterojunction. Remarkably, the K⁺/I⁻-CN/CdSe-D not only exhibited the excellent photocatalytic stability, but also showed higher overall H₂O₂ generation rate than the other photocatalysts, which make it very promising photocatalyst considering its easy preparation. This study emphasizes the significance of interface chemical and ionic intercalation for enhancing the photocatalytic overall H₂O₂ generation. Tao et al. prepared g-C₃N₄ homojunction with functional surface for efficient photocatalytic overall H₂O₂ generation [92]. Kelvin probe force microscopy (KPFM) and calculations results suggest that surface hydroxyls of g-C₃N₄ can extract electron from the bulk to the surface. The photocatalytic overall H₂O₂ production performance of homojunction is assessed under visible light irradiation. As shown in Fig. 6b, homo CN showed a highest H₂O₂ yield (508.4 µmol/L), which was about 2.9 and 6.6 times higher than HD-CN and HR-CN respectively. To demonstrate the H₂O₂ synthesis route, photocatalytic H₂O₂ generation was tested under different measurement conditions for 2 h. As shown in Fig. 6c, 177 µmol/L of H₂O₂ yield was produced in pure water by homo-CN, while HD-CN and HR-CN almost have no H₂O₂ yield. When benzoquinone (BQ) added in the reaction system, all the samples show that H₂O₂ yields reduce sharply. This result suggests that obtained samples synthesize H₂O₂ via two step single-electron route. Electron paramagnetic resonance (EPR) characterization results further confirm that the two step single-electron process (Fig. 6d). The steady state PL and fluorescence lifetime measurement result proved that homojunction can improve charge separation and transfer (Figs. 6e and f). UV-vis diffuse reflectance spectra (UV-vis DRS) characterization revealed that homojunction extends the visible light absorption range that is beneficial to photocatalytic overall H₂O₂ production (Fig. 6g).

3.2.3　Functionalization

In addition to the reported heterojunction and single-atom doped strategies, functionalization and defects are also effective strategy for photocatalytic overall H₂O₂ generation. Ouyang's group reported an elaborately modified g-C₃N₄ (U-POCN and AQ/U-POCN) photocatalyst for overall H₂O₂ generation (Figs. 7a and b) [87]. The AQ/U-POCN exhibited the highest catalytic efficiency (75 µmol L⁻¹ h⁻¹) ever reported without using any additives. Calculations and experiments revealed that P, O-co-doping could improve the water oxidation ability of the g-C₃N₄, which was capable of oxidation H₂O to H₂O₂ directly. Similarly, Shi's group reported tartaric acid-modified g-C₃N₄ (TA-CN) by introducing hydrophilic hydroxyl groups into the g-C₃N₄ (Fig. 7c) [95]. They found π-electron-rich can effectively reduce exciton binding energy result in increasing the transfer rate of photo-excited electrons, enabling faster migration from the production site to the active sites, thereby enhancing photocatalytic overall H₂O₂ generation efficiency. Besides, the hydroxyl groups modulate reaction kinetics by enhancing the adsorption ability of intermediates by their hydrophilic intrinsic, which favors the overall reaction efficiency.

3.3　Photocatalytic overall H₂O₂ production by CTFs

CTFs are a class of organic porous materials formed by covalent bonding with a triazine ring as the structural unit, typically obtained from nitrogen-rich aromatic precursors by polymerization reaction (Fig. 8) [58]. While CTFs are a branch of the COFs family, their unique triazine ring structure endows them with exceptional physicochemical properties and remarkable stability [63,108,109]. In 2008, Kuhn et al. successfully obtained the first CTFs material (CTF-1) under ionothermal conditions [110]. This pioneering work not only established the foundation for the exploration and development of CTFs but also opened the door to their extensive applications across energy, catalysis and adsorption and separation.

In recent years, CTFs have gradually become a promising catalytic platform for the photocatalytic overall H₂O₂ generation by virtue of their unique photophysical properties [111,112]. Their advantages for overall H₂O₂ production can be attributed to several key factors, including (1) abundant N atoms, high specific surface area and ordered pore structure that can promote oxygen adsorption, activation, and transport; (2) structural tunability provides remarkable flexibility to enhance the selectivity and yield of H₂O₂ production; (3) unique π-conjugated structure in the triazine ring facilitates efficient long-range migration of photo-generated electrons; (4) highly stable C≡N bonds endow the catalyst with excellent durability under acidic, basic and complex reaction conditions. Nevertheless, CTFs still face some key challenges in the photocatalytic production of H₂O₂, such as the rapid recombination of photo-generated charge carriers, sluggish reaction kinetics, and the inherently high impedance and low electrical conductivity characteristic of organic semiconductors, which greatly limit the further enhancement of photocatalytic efficiency [113–115]. To address these challenges, recent research has introduced a range of innovative strategies, including the modulation of the framework structure, the introduction of heterojunction and the optimization of the electron transfer pathway, which provide new ideas and directions to achieve a breakthrough in the photocatalytic production of H₂O₂ by CTFs [116–118].

3.3.1　Introducing the functional groups

In 2019, Xu's group first reported photocatalytic overall H₂O₂ production using functionalized CTFs [30]. They found that introducing acetylene or diacetylene groups into CTFs (CTF-BPDCN, CTF-EDDBN, and CTF-BDDBN) can significantly enhance photocatalytic H₂O₂ production rate due to the reduced energy associated with OH* formation and thus enable a new 2e^- oxidation route toward overall H₂O₂ generation. As shown in Fig. 9a, CTFs with acetylene and diacetylene are prepared from their corresponding nitrile precursors. They tested the photocatalytic overall H₂O₂ generation performances of different samples in pure water under visible light irradiation. The average H₂O₂ generation rate of the CTF-BDDBN was calculated to be 79 µmol, which was about 2 and 3.8 times higher than that of CTF-BPDCN and CTF-EDDBN (Fig. 9b). Moreover, the SCC efficiency of the best-performing CTF-BDDBN is measured to be 0.14% (Fig. 9c), which was larger than previously reported organic photocatalysts. Further theoretical calculations revealed alkynyl groups can develop a new 2e⁻ oxidation route toward overall H₂O₂ production (Figs. 9d and e). After that, Han et al. developed a thiourea-functionalized CTF (Bpt-CTF) through a polarization engineering strategy, effectively addressing the sluggish kinetics of H₂O₂ production during 2e⁻ oxygen photoreduction (Fig. 9f) [119]. Photocatalytic performance tests showed that the Bpt-CTF greatly enhanced H₂O₂ production by 2e⁻ ORR from water and O₂. The H₂O₂ production rate reached 3268.1 µmol h⁻¹ g⁻¹ of Bpt-CTF without the use of any sacrificial agents or co-catalysts, which was more than an order of magnitude improvement compared to Dc-CTF. Mechanistic studies revealed that the superior photocatalytic activity of Bpt-CTF was attributed to the incorporation of the (thio)urea functional group, which greatly enhanced the polarization of the CTFs, thereby significantly promoting charge separation and optimizing the electron transfer dynamics. Previously reported on functionalized CTFs are widely limited by the amorphous or weakly crystalline structure. Generally, improved the crystallinity of organic materials can accelerate charge transport and separation in the skeleton, which is conducive to their photocatalytic application. In view of this, Xu's group successfully synthesized two high-crystalline CTFs with 1,1′-biphenyl-4,4′-dicarbonitrile (BP-CN) and 9H-fluorene-2,7-dicarbonitrile (FL-CN) as the building blocks (Figs. 10a and b) [37]. Notably, the exfoliated CTF-FL exhibited an exceptional H₂O₂ production rate of 5007 µmol g⁻¹ h⁻¹ from O₂ and pure water under sacrificial agent-free conditions, significantly surpassing all previously reported CTF-based photocatalysts and establishing itself among the most efficient metal-free H₂O₂ photocatalytic systems to date. And femtosecond-transient absorption (fs-TA) spectroscopy characterization results confirm the locked-in molecular architecture suppress rapid recombination of photoexcited carriers. Time-resolved photoluminescence (TRPL) analysis demonstrated spatially locked structure could extended π-electron delocalization and enhance strong intermolecular interaction (Fig. 10d). Structural characterization and theoretical calculations further reveal that the outstanding photocatalytic performance of CTF-FL stems from its spatially locked structure, which not only enhances the efficiency of separation and transfer of photo-excited charge-carriers, but also regulates the local electronic structure, leading to a shift of the WOR pathway to shift from the 4e⁻ to the 2e⁻ process, and ultimately achieves 100% atom utilization efficiency. And the overall H₂O₂ generation test was performed under AM 1.5 G one-sun illumination. Remarkably, the SCC efficiency of exfoliated CTF-FL reached up to 0.91%, which exceed the most metal-free photocatalysts ever reported (Fig. 10c). Total density of states (TDOS) results suggest agreement with the experimental results of band gaps for CTF-FL (2.41 eV) and CTF-BP (2.52 eV). The reaction barrier is calculated to further understand the photocatalytic overall H₂O₂ production mechanism. The calculation result suggests that the spatially locked can reduce the barrier of ORR and WOR resulting in boosting efficient overall H₂O₂ production (Figs. 10e and f).

3.3.2　Functionalization

Inspired by the stepwise energy- and charge transfer processes in natural photosynthesis, Zhang and co-worker designed and synthesized three CTFs with stepwise charge transfer functionality (asy-CTF, Ace-asy-CTF and Th-asy-CTF) (Figs. 11a and b) [120]. Without the presence of a sacrificial agent, the Ace-asy-CTF showed the highest photocatalytic overall H₂O₂ generation performance compare to asy-CTF and Th-asy-CTF. The AQY of Ace-asy-CTF was determined under different monochromatic wavelengths. The AQY of Ace-asy-CTF reached 4.4% at 420 nm wavelength. Furthermore, Ace-asy-CTF showed a SCC efficiency up to 0.48% under AM 1.5 G simulated sunlight illumination. The long-term recycling tests suggests that Ace-asy-CTF possessed excellent photocatalysis stability. In situ characterization and theoretical calculations revealed that the acetylene units in Ace-asy-CTF as key site for efficient charge separation and exciton transport through a stepwise charge transfer mechanism, thus enhancing the photocatalytic activity of the Ace-asy-CTF. The inherent high impedance and suboptimal electrical conductivity of CTFs still limit their charge transfer ability during photocatalysis. For this reason, Zhou et al. successfully prepared bonded CQD-CTF photocatalysts by embedding carbon quantum dots (CQDs) into the pores of CTFs (Fig. 11c) [55]. The optimized 0.5% CQD-CTF exhibited excellent H₂O₂ production activity in a pure water system without sacrificial agent, achieving a generation rate as high as 1036 µmol g⁻¹ h⁻¹, which was 4.6 times higher than that of the original CTF. Theoretical calculations and in situ characterization further elucidated the incorporation of CQDs not only improved the electrical conductivity of CTFs but also strengthened their proton affinity and oxidative capability, thereby markedly boosting the catalytic performance of 0.5% CQD-CTF in both the ORR and WOR.

3.3.3　Single-atom doped

In addition to the reported heterojunction and functionalization strategies, single-atom doped in CTFs for photocatalytic overall H₂O₂ production has also been extensive studied. Shen's group prepared a high-density Co single-atom-loaded pyridine modified CTF (CoSA/Py-CTF) for photocatalytic overall H₂O₂ generation (Fig. 12a) [121]. The obtained CoSA/Py-CTF exhibited a remarkable photocatalytic overall H₂O₂ evolution rate of 2898.3 µmol g⁻¹ h⁻¹ under visible light irradiation with an AQY and SCC up to 13.2% at 420 nm and 0.15% under AM 1.5, respectively, which exceed most metal-free photocatalysts ever reported. The FT-IR and XRD demonstrated the single-atom-loaded CTF was prepared successfully (Figs. 12b and c). This research provides perceptive observations into the intricate dynamic behavior and catalytic mechanism of SACs. After that, Shen's group further study photocatalytic overall H₂O₂ evolution using Ni single-atom doped CTFs (Fig. 12d) [122]. They found dual-pathway synergistically promoting photocatalytic overall H₂O₂ evolution by ORR and WOR. Dual active site CTFs loading single-atom Ni and pyridine N (d-CTF-Ni) presented excellent overall H₂O₂ evolution rate of 869.1 µmol g⁻¹ h⁻¹. This study offers a new insightful on the dual active site catalytic mechanism for photocatalytic overall H₂O₂ generation.

3.4　Photocatalytic overall H₂O₂ production by COFs

COFs are a classic representative of porous organic polymers with high crystallinity and periodic structures. The COFs usually are synthesized using reversible chemical reactions, such as boroxine, boronate-ester, imide, and imidazole, thus, leading to highly crystallinity [123–125]. In 2005, Yaghi's research group first successfully prepared COF-1 and COF-5 by reversible boronate-ester reaction (Fig. 13) [123]. Over the past few years, the COFs materials with different linkages develop rapidly due to their exactly structural characteristics and great potential in various field, such as energy storage and conversion, separation and photo/electron catalysis (Fig. 14) [126–132]. The COFs with semiconductor characteristics are one of the promising candidate materials for overall H₂O₂ production due to their large surface area, structural designability, high chemical stability, light weight, extensive π-conjugation, flexible synthetic strategy. The extensive π-conjugation in COFs extended covalent systems promotes the absorbance of visible light. The π-π stacking interaction of framework benefit the migration and transport of carriers, which can prevent the recombination of photo-excited carriers and maximize photocatalytic overall H₂O₂ production efficiency. The larger surface area can expose abundant active sites and promote contact between water and COFs frameworks. It is the synthesis link-ages modules provide rich options for design production with different photo-electric and physical properties for various application. Given these unique properties, COFs have shown satisfactory employment in photocatalytic overall important that H₂O₂ production, and many of them have exhibited better performance than inorganic semiconductors [133]. However, the COF's photocatalytic overall H₂O₂ production was not achieved until 2021, which is far behind other photocatalytic applications such as water splitting, CO₂ reduction, and organic degradationc [134–136]. In 2021, Yu's group first reported photocatalytic overall H₂O₂ production using triphenylbenzene-dimethoxyterephthaldehyde-COF (TPB-DMTP-COF) at gas–liquid–solid interface (Fig. 15a) [134].

The reported TPB-DMTP-COF can efficient reduce O₂ to H₂O₂ in a wide pH range and pure water condition (Fig. 15b). TPB-DMTP-COF showed a high H₂O₂ generation rate of 2882 µmol g⁻¹ h⁻¹ under visible light irradiation due to the high crystallinity and large specific surface area as high as 2747 m²/g (Fig. 15c). More importantly, a solar-to-chemical conversion of 0.76% is achieved for H₂O₂ generation surpassing most of the metal-free photocatalysts ever reported. This study provides fundamental insights into the reasonable designed synthesis of highly efficient metal-free photocatalysts for overall H₂O₂ generation by precise control over photocatalytic reaction pathways. Since then, the continuous efforts have been devoted to design COFs resulting in rapid development of COFs-based photocatalytic systems with various structure and properties preference through different strategies. In this section, the research advance on photocatalytic overall H₂O₂ generation over COFs and COF-based heterojunction for enhancing photocatalytic performance are systematic summarized and discussed in detail. Some typical examples of photocatalytic overall H₂O₂ generation by COFs are summarized in below Table 2 [13,18,19,43,57,134,137–147]. And we present the differently strategies, such as functional group modification, introducing donor-acceptor, and COFs nanohybrids, to review photocatalytic overall H₂O₂ production over COFs and its heterojunction.

3.4.1　Introducing functional groups

Introducing functional groups into organic semiconductors is well known as an effective strategy to endow semiconductors with unique characteristic [54,135,136,148–150]. Firstly, the well design functional groups can modulate the photo-electronic properties of COFs, enhance visible light response and improve the photo-excited carriers separation efficiency [148]. Additionally, the modified special groups can change the stability of COFs under different environment, holding the catalytic stability over long time testing. Importantly, incorporation the functional groups can optimize the efficiency of charges transfer, decrease the recombination of electron-hole pairs and improve the yield of H₂O₂ [75]. Hou and co-authors reported a simple and efficient strategy for the synthesis a class of COFs photocatalysts with multiple charge transfer channels by introducing dicyano-functional group (Figs. 16a-c) [151]. As a result, COF-2CN with multiple charge transfer channels exhibits the significantly higher H₂O₂ generation rate (1601 µmol g⁻¹ h⁻¹) than other samples (Fig. 16d). This suggests that dicyano-functional group in COFs can greatly improve the overall generation efficiency of H₂O₂. Moreover, the AQY and SCC efficiency of COF-2CN are determined to be 6.8% and 0.6% respectively under visible-light irradiation, which is significantly higher than other organic polymer (Fig. 16e). COF-2CN also presents excellent photocatalytic stability in long time test (Fig. 16f). And the COF-2CN also showed the excellent stability in different pH (from 3 to 11), suggesting COF-2CN can be used in different pH conditions. Importantly, COF-2CN exhibits similar photocatalytic production rates of H₂O₂ in different water environment compare to ultrapure water conditions (Fig. 16g). Fs-TA spectroscopy reveals that photo-excited carriers generated by COF-2CN can be dissociated more efficiently compare to COF-0CN and COF-1CN owing to COF-2CN possess four channels for charge transfer. To better understand the mechanism of dicyano group in increasing the charge transfer channels, DFT calculations were carried out. The results suggest that dicyano-functional group as electron acceptor in COF-2CN can accept more photo-excited electrons comparing with other samples, which is contribute to the rapidly charge transfer and resulting in efficient ORR process. In the 2e⁻ WOR process, the energy barrier of rate-determining step of COF-2CN is apparent lower than that by COF-0CN and COF-1CN (Fig. 16h), which suggests that the introduction of dicyano groups into COFs can possess superior H₂O dehydrogenation property. In ORR process, COF-2CN still exhibits the same result further suggesting dicyano groups can enhance photocatalytic over H₂O₂ production rate (Fig. 16i). This work not only open a new way to synthesize COF-based photocatalyst for efficient overall H₂O₂ photosynthesis by introducing the dicyano groups into COFs, but also develops a new way for the practical application of organic photocatalysts in H₂O₂ photosynthesis. The PDOS demonstrates that the C 2p orbital contributed by valence band top and conduction band bottom of all COFs, confirming the well π-delocalization for all COFs. The results are beneficial for the efficient charge transfer of ORR process. In his work, author tests the photocatalytic activity using differentwater quality (such as tap water, river water, and lake water), which contribute to practical application. But this work is deficient in the sustainable production of H₂O₂. Chen's group report vinyl modified COFs (DVA-COF) for improving the photocatalytic H₂O₂ generation efficiency (Fig. 17a) [152].

This research suggested that the anchored vinyl groups in DVA-COF not only extended the light response range but also improve carrier separation and transfer efficiency that facilitate H₂O₂ production rate by a 2e⁻ ORR pathway. Therefore, DVA-COF with vinyl group produced H₂O₂ much more (84.5 µmol/h) than PDA-COF (8.6 µmol/h), which is also superior to most reported COFs-based photocatalysts (Fig. 17b). Moreover, DFT calculations showed that the introduction of vinyl groups improves the O₂ adsorption and affects the catalytic efficiencies, which decreases the barrier energy of O₂ adsorption affinity toward O₂, thereby optimizing the photocatalytic activity of DVA-COF (Fig. 17c). EPR characterization results suggest ^•O₂^– is a significant intermediate in the photocatalytic overall H₂O₂ production process using PDA-COF. Additionally, Zhang et al. introduced kfto-form anthrahydroquinone into COFs (kf-AQ) by mechanochemical synthesis (Fig. 18a) [153]. Kfto-form anthraquinone containing COF showed efficient H₂O₂ photosynthesis in alkaline water, with a record H₂O₂ production rate of 4784 µmol h⁻¹ g⁻¹ under visible light irradiation without reagents. The keto-form structure in Kf-AQ can enhance the water adsorption through the formation of OH⁻ clusters with weakened hydrogen bonds according to the dehydrogenation of water and efficient H₂O₂ photosynthesis. Luo et al. reported a COFs with cyanide group for highly efficient overall H₂O₂ generation from air and water through photocatalytic ORR and WOR (Fig. 18b) [154]. Without using any sacrificial agent, the synthesized COF is found to enable a H₂O₂ production rate as high as 4742 µmol h⁻¹ g⁻¹ from water and air and O₂ utilization and the SCC up to 0.68, which is bigger than most reported COFs and other organic photocatalysts. This study inspires a rational design of efficiency COF photocatalyst for high-performance artificial photocatalysis. Shen's group designed and developed thioether-decorated triazine-based COF (TDB-COF) for efficient overall H₂O₂ photosynthesis. TDB-COF exhibits an excellent photocatalytic H₂O₂ production rate of 723.5 µmol h⁻¹ g⁻¹ without any sacrificial agents and presents remarkable cyclic stability (Fig. 18c) [155]. The modification of thioether groups extended the light absorption range of TDB-COF and narrowed its energy band gap. Experimental and theoretical analysis revealed that this method can not only effectively optimize the band structure and improve visible-light absorption for 2e⁻ WOR but also enhance the photo-excited carriers transfer and separate for 2e⁻ ORR process to generate H₂O₂, thus resulting ultimately boosting the overall H₂O₂ photosynthesis. This work provides deeply suggestions for the development of the functional COFs towards overall H₂O₂ photosynthesis.

3.4.2　D-A structures

Enhancing the photocatalytic efficiencies necessitates a key stage of separation and rapid migration of photo-excited carriers. D-A structures offer several advantages for photosynthesis owing to the construction of internal electric field that facilitate carrier separation and transfer, also wide light response ranges, enabling them to absorb a broad zone of the solar spectrum [142,156–158]. In addition, D-A system can effectively reduce the exciton binding energy based on previous theoretical studies and experiments. The COFs with various D-A building blocks can be selected to meet O₂ reduction and water oxidation band structures and energy band requirements. Therefore, the energy levels of D−A materials can be customized for optimal alignment with the redox potentials of photocatalytic overall H₂O₂ production. Further, COFs with the large specific surface area and high porosity can offer more active sites for O₂ adsorption, subsequently increase the local O₂ concentration of catalyst surface, and capture photo-excited electrons. To explore these properties, Mou and co-workers reported three kinds of azoles-linked COFs including thiazole-linked TZ-COF, oxazole-linked OZ-COF and imidazole-linked IZ-COF for photocatalytic overall H₂O₂ generation (Fig. 19a) [143]. They demonstrated that electron-rich pyrene building block server as electron donor and the azole linkage acts as an acceptor leading to the electron directional transfers from pyrene block to azole fragments. Additionally, light absorption, energy levels and photo-excited carrier transfer can be modulated easily by constructing D-π-A structure. TZ-COF displayed the highest SCC value of 0.036% and AQY of 0.6% at 475 nm among all of them due to the more accessible channels of charge transfer were constructed in TZ-COF via the donor-π-acceptor structure. Fs-TA spectroscopy was carried out to study photoexcited carrier dynamics. And the results demonstrated TZ-COF has a longer-lived excited state absorption decay than OZ-COF and IZ-COF, which is agreement with the high efficiency of photoexcited carrier separation and transfer. DFT calculations and partial density of states (PDOS) were carried out (Fig. 19b). The computational results suggested that the benzene ring fragment between pyrene unit and azole linkage can absorb O₂ efficient by donor-acceptor complex reaction, which combine the photo-excited electrons to generate *O₂. And it further integrates with the proton to generate *OOH intermediate for the formation of H₂O₂. Moreover, the O₂ can be generated by H₂O oxidation through photoinduced h⁺, which is benefit to liberation O₂ for H₂O₂ generation under the condition of low or no O₂ concentration. Zhai et al. presented a mixed-linker strategy to construct a donor-acceptor-acceptor (D–A–A) type COF photocatalyst, FS-OHOMeCOF (Fig. 19c) [159]. The FS-OHOMe-COF structure characteristics widen π–π conjugation that enhances charge mobility, while the introduction of sulfone groups not only as active sites facilitates the reactions rate between water and surface of catalyst but also increases stability through improved inter-layer forces. The test results of temperature-dependent PL spectroscopy demonstrated that electron donating TpOMe group in COFs can facilitate delocalized charge transfer. They investigated photocatalytic overall H₂O₂ generation for these samples, initially by suspending these COFs in pure water under visible light irradiation. The FS-OHOMe-COF possessed the highest overall H₂O₂ production rate than FS-OH—COF and FS-OMe-COF. The external quantum efficiency (EQE) and SCC was tested and determined to be 9.6% and 0.58%, which is higher than the most reported polymer photocatalysts. It was worth noting that the EQE and the SCC were superior to most metal-free photocatalysts ever reported which make it very appealing photocatalyst for photocatalytic overall H₂O₂ production. Liu et al. prepared another type of D-A COF with optimal intramolecular polarity by introducing phenyl functional groups as electron donors for modulating exciton separation and transfer to boost the direct photocatalytic overall H₂O₂ generation from water, air and sunlight (Fig. 20a) [43]. They find that weak intramolecular polarity in D-A COFs constrains excitons dissociation in D-A COFs was constrained by weak intramolecular polarity, yet excessive strong intramolecular polarity suppresses excitons formation and lowers photo-stability of COFs by weakened π-conjugated effect. The optimal intramolecular polarity in COFs (named COF-N32) can promote exciton formation and dissociation, resulting in the high and stable H₂O₂ yield with SCC efficiency of 0.31%, which is superior to solar-to-biomass efficiency by plants (~0.1%). Wang's group report benzotrithiophene (Btt)-based D-A COFs with spatially separated redox centers for the photocatalytic production of H₂O₂ from water and oxygen without any sacrificial agents (Fig. 20b) [142]. They revealed the concept of a uniport "atom spot-molecular area" by a D-A strategies in CTFs to directly elucidate the differences of the energy band levels, charge transfer direction, and O₂ adsorption. The photocatalytic overall H₂O₂ about higher than that of TpaBtt (252 µmol g⁻¹ h⁻¹) and TapbBtt (557 µmol g⁻¹ h⁻¹), also exceeded most of the previously reported metal-free photocatalysts. The efficiency of SCC was further tested under 1 sun illumination and measured to be 0.297%, which was larger than natural synthetic plants. Wang's group successfully synthesized cyanide-COFs (TBTN—COFs) including 2,4,6-trimethylbenzene-1,3,5-tricarbonitrile (TBTN) and benzotrithiophene-2,5,8-tricarbaldehyde (BTT) building blocks with water-affinity and charge-transfer (Fig. 20c) [160]. The TBTN—COF exhibits excellent overall H₂O₂ production with the highest H₂O₂ yield of 11,013 µmol g⁻¹ h⁻¹, which was about double higher than that of TMT-COF (6392.3 µmol g⁻¹ h⁻¹). The TBTN—COF shows a AQY value of 7.59% at 420 nm wavelength, which exceeded most reported COFs photocatalysts.

3.4.3　Heterojunction

COFs offer a diverse array of functional groups like hydroxyl, amino, and aldehyde for construction the heterojunction photocatalysts [161–163]. These groups, when combined with another semiconductor, can form stable heterojunctions photocatalysts that effectively separation electrons and holes. Huang's research group reported a heterostructure composed by covalent bonded COFs and perylene diimide polymers for photocatalytic overall H₂O₂ production from water and oxygen under visible light irradiation (Fig. 21a) [164]. Detail experimental and theoretical calculations analysis has revealed formation an interfacial electric field (IEF) between COFs and perylene diimide polymers, which creates an internal driving force to improve the separation rate of photo-excited electron and hole. Moreover, mechanistic studies suggest that the H₂O₂ generation accompany a simultaneous 2e⁻ ORR and 4e⁻ OER process. The photocatalytic overall H₂O₂ generation of the heterostructure has been tested in an O₂-saturated seawater. The H₂O₂ production is only 782, 641, and 996 µmol g⁻¹ h⁻¹ for single COFs and perylene diimide polymers, respectively. After formation the heterojunctions between COFs and perylene diimide polymers, the overall H₂O₂ production rate sharply raised to 3846 µmol g⁻¹ h⁻¹ with the suitable composition (Fig. 21b), suggesting the enhancement in photocatalytic overall H₂O₂ production rate owing to creation of IET. The AQE of heterostructure was tested and determined to be 3.24%, 4.52%, and 2.29% at the wavelength of 420, 435, and 460 nm have in seawater, respectively (Fig. 21c). The SCC efficiency of heterostructure was measured to be 0.24%, which is larger than the typical photosynthetic efficiency of plants. The heterostructure showed excellent photocatalytic stability with no obvious performance decay when it was continuously irradiated (Fig. 21d). This study provides a universal approach for rational design heterostructure to enhance the electron-hole separation rate and activity of catalytic centers simultaneously. Functional groups provided by COF can also form stable heterojunctions when combined with an inorganic semiconductor. Bi's group reported construction of heterojunctions by combing indium sulfide (In₂S₃) and COFs, namely TpMA, for photocatalytic overall H₂O₂ production (Fig. 21e) [104]. TpMA/In₂S₃ heterojunctions with different In₂S₃ loading amount were prepared via a hydrothermal method. The optical characteristics of samples were investigated in detailed by UV–vis DRS, electrochemical impedance spectroscopy (EIS) and photocurrent. The In₂S₃ showed decreased band gap of 1.97 V compared with TpMA (2.36 V). Therefore, the optical absorption properties of TpMA/In₂S₃ heterojunctions were accompanied by a red-shift in the absorption edge. This result suggests that the heterojunction could extend light absorption range. EIS and photocurrent results confirmed the heterojunctions strategy could apparently enhance the generation, separation and transportation of light-excited carriers. The single TpMA and In₂S₃ showed poorly activity, generating about 237.14 and 31.76 µmol/L H₂O₂, respectively. It is obvious that the heterojunction samples presented apparently higher activity than single samples. TpMA and heterojunctions showed decreased activity in dark or within an Argon atmosphere suggesting that O₂ is an important component in the photocatalytic overall H₂O₂ generation.

4　Outlook and perspective

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The photocatalytic overall H₂O₂ generation from H₂O and O₂ under sunlight irradiation is a promising way to alleviate energy crisis and environmental pollution. This review systemically summarizes recent advances in photocatalytic overall H₂O₂ production using metal-free photocatalysts, including g-C₃N₄, COFs and CTFs. Additionally, we discuss the primary research strategies for enhancing overall H₂O₂ generation. Despite recent progress, the photocatalytic overall H₂O₂ generation performance remains low, with the highest reported SCC efficiency reaching only 1.41%. The low photocatalytic activity can be attributed to the inherent drawbacks of metal-free photocatalysts, particularly their limited charge separation and transfer efficiency, insufficient thermodynamic driving force for water oxidation, and restricted visible-light absorption. To improve the efficiency of metal-free photocatalysts for overall H₂O₂ generation, the following key aspects should be addressed:

(1) Photoexcited charge separation and transfer serve as a crucial link between light absorption and surface redox reactions, significantly affecting SCC efficiency. Photogenerated charge carriers, including electrons and holes, must efficiently migrate to the photocatalyst surface to drive the WOR and ORR for H₂O₂ production. Metal-free photocatalysts typically generate Frenkel exciton with strong binding energies under light illumination, making charge separation relatively difficult. Therefore, reducing exciton binding energy is a promising strategy for improving carrier separation. Various approaches, including doping, heterojunction engineering, and the introduction of functional groups, have been explored to lower exciton binding energy in metal-free photocatalysts. Although these strategies have led to some reductions in exciton binding energy, the improvements remain insufficient for achieving significantly higher SCC efficiency. Therefore, further efforts should focus on developing new molecular structures to better regulate exciton binding energy. Efficient charge transfer is equally important for improving overall H₂O₂ generation. Current strategies aimed at enhancing the transfer rate of charge transfer strategies, such as reducing migration distance and doped, still fail to satisfy the practical application. Compared to inorganic semiconductors, organic photocatalyst generally contain substantial defects that promote charge recombination and hinder carrier mobility. To fundamentally improve charge transport efficiency, it is essential to minimize intrinsic defects in metal-free photocatalysts. In this regard, the development of innovative synthesis strategies to reduce defect density is urgently needed to enhance the overall performance of photocatalytic H₂O₂ generation.

(2) Novel strategies should be developed to extend the light response range of metal-free photocatalysts. Light absorption is the first step in photocatalytic reactions, and is a prerequisite for achieving subsequent reactions of photocatalytic overall H₂O₂. To extend the light absorption range, only a few methods have been developed, such as doped and heterojunctions. However, these strategies remain ineffective in significantly improve light absorption. For metal-free materials, enhancing crystallinity and conjugation can generally enhance improve light absorption and charge transport within the skeleton, which is beneficial for increasing the efficiency of SCC conversion process. Additionally, other strategies, such as introducing electron donor groups and photosensitizer into metal-free photocatalysts, should also be explored. The effectiveness of these approaches in enhancing light absorption has already been demonstrated in previous studies.

(3) Compared to ORR progress, the WOR reaction requires a higher redox potential to overcome thermodynamics barrier. Therefore, the design and development of new metal-free photocatalysts with sufficient thermodynamic driving force is crucial for achieving efficient overall H₂O₂ production. Various strategies have been reported for regulating the WOR thermodynamic driving force in metal-free photocatalysts, including single-atom doped, introducing acetylene and leveraging topological effects. Among them, most methods primarily focus on direct 2e⁻ WOR, which is thermodynamically more favorable than 1 e⁻ WOR. However, the 1e⁻ WOR exhibits superior kinetics compared to the 2e⁻ WOR due to the larger potential difference between the reaction and photo-excited holes. Additionally, on the oxidation side, the 1e⁻ WOR can release more protons from H₂O, significantly enhancing the hydrogenation rate during the 2e⁻ ORR process. Therefore, a promising strategy for achieving highly efficient photocatalytic overall H₂O₂ generation is the development of new metal-free photocatalysts with an efficient 1e⁻ WOR capability. Here we could study carefully the effective strategy from electrocatalysis and thermal catalysis to improve the WOR thermodynamic driving force of photoctalysis.

(4) Machine learning (ML) is an important area of artificial intelligence that produces different models and resolves various challenging issues. The ML has been widely used in electrocatalysis to help researchers better understand the mechanism of electrocatalytic reactions. In recent years, ML techniques also develop rapidly in accelerating the synthesis and discovery of efficient photocatalysts for H₂O₂ generation. Training model to predict active solar photocatalysts with available knowledge could subsequently lead toward rationally synthesizable photocatalysts for overall H₂O₂ generation. The combination of ML and photocatalysis overall H₂O₂ production learning might pave the road toward open of efficient catalyst screening platform significant benefiting photocatalysis H₂O₂ generation community. Thus, given a basic description about photocatalysis overall H₂O₂ production, such as light harvesting, photoexcited separation and transfer, redox reaction, and training them to discovery of efficient photocatalysts is a future crucial research paradigm.

(5) The poor stability of metal-free photocatalysts should be carefully addressed. To improve the stability of metal-free photocatalysts, the robust chemical bond should be considered, such as polyimide and polyetherether. And the improvement the crystallinity of metal-free photocatalysts is an also effective strategy to enhance the stability. Another feasible method is protection mechanisms, such as coating protective layer at the surface of metal-free photocatalysts.

In summary, significant challenges remain, and substantial efforts are needed to overcome the low SCC efficiency in overall H₂O₂ generation. Nevertheless, metal-free photocatalysts remain promising materials for efficient, green and sustainable overall H₂O₂ generation due to their unique physical chemistry characteristics. In recent years, remarkable progress has been made, with metal-free photocatalysts achieving an SCC of up to 1.41% for overall H₂O₂ production. It is therefore highly possible that the utilization of metal-free photocatalysts for overall H₂O₂ generation could help bridge the gap toward achieving industrial application.

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doi: 10.1016/j.cclet.2025.111604

Receive Date：2025-05-22
Online Date：2026-10-29
Published：2026-01-15

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Received：2025-05-22
Revised：2025-07-10
Accepted：2025-07-17

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^aSchool of Physics and Optoelectronic Engineering, Hangzhou Institute for Advanced Study, University of Chinese Academy of Sciences, Hangzhou 310024, China

^cSchool of Engineering, Westlake University, Hangzhou 310024, China

^dSchool of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China

^eDepartment of Physics, Faculty of Science, Sustainability Research Center, Islamic University of Madinah, Madinah 42351, Saudi Arabia

Corresponding:

E-mail addresses: drfqiu@ucas.edu.cn (F. Qiu)

Lei.zhu@hbeu.edu.cn (L. Zhu)

mshakir@iu.edu.sa (I. Shakir)

xuyuxi@westlake.edu.cn (Y. Xu)

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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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Table 1 Summary of the reported g-C₃N₄ based photocatalysts for photocatalytic overall H₂O₂ production.

Photocatalyst	SCC (%)	AQY (%)	Ref.
g-C₃N₄/PDI	–	–	[83]
g-CN-MI-40	–	–	[84]
Ni-CAT-CN60	0.1	0.96@420 nm	[85]
ORP/GCN	–	–	[86]
AQ/U-POCN	–	–	[87]
SCN5	–	–	[88]
CN/CML	–	–	[89]
NiSAPs-PuCN	0.82	10.9@420 nm	[90]
CNIO-GaSA	0.4	9.1@459 nm	[91]
homo-CN	0.19	25.7@420 nm	[92]
CoOx-NvCN	0.47	5.73@420 nm	[93]
40%K⁺/I⁻-CN/CdSe-D	–	–	[94]
TA-CN-3	0.34	1.7@420 nm	[95]
MOC-AuNP/g-C₃N₄	–	–	[96]
CN-DMAP-GLU	–	0.792@420 nm	[97]
pH-MCN	–	–	[98]
HCP-24	–	2.38@420 nm	[99]

Table 2 Some examples of photocatalytic H₂O₂ generation by COFs.

Photocatalyst	SCC (%)	AQY (%)	Ref.
TPB-DMTP	0.76	18.4@420 nm	[134]
TF50-COF	0.17	5.1@400 nm	[18]
COF-TfBpy	1.08	13.6@420 nm	[19]
TTF-BT-COF	0.49	11.9@420 nm	[137]
TD-COF	0.15	–	[138]
HEP-TAPT	0.65	15.35@420 nm	[139]
COF-N₃₂	0.31	6.2@459 nm	[43]
TpDz	0.62	11.9@420 nm	[140]
Py-Da-COF	0.09	4.5@420 nm	[141]
TaptBtt	0.296	–	[142]
TZ-COF	0.036	0.6@475 nm	[143]
Kf-AQ	0.7	15.8@400 nm	[131]
TB-COF	1.08	–	[144]
TPB-COF-OH	0.84	9.61@420 nm	[57]
DVA-COF	0.08	2.84@420 nm	[130]
COF-2CN	0.6	6.8@459 nm	[129]
TBD-COF	1.04	–	[145]
QH—HPTP-COF	1.41	–	[13]
TAH—COF	0.66	7.72@420 nm	[146]
DHAA	0.23	4.6@380 nm	[147]

Fig.1 Development of representative metal-free-based photocatalysts for photocatalytic overall H₂O₂ production.

Fig.2 Schematic of the processes involved in (a) photocatalytic overall H₂O₂ generation and (b) corresponding redox potentials.

Fig.4 (a) Synthetic process of CNIO-GaSA. (b) SEM image of CNIO-GaSA. Copied with permission [91]. Copyright 2022, Springer Nature. (c) Schematic diagram of the synthesis of high-loading MSAPs-PuCN. Copied with permission [90]. Copyright 2023, Springer Nature.

Fig.5 (a) Schematic diagram showing the intermediates in the multielectron-transfer processes of WOR. (b) Excited properties of group 11 elements at different chemical states. (c) Average AQY of AuKPCN for photocatalytic H₂O₂ with different sacrificial agent. (d) ESR spectra of AuKPCN recorded in 0.1 mol/L AgNO₃ solution. (e) Generation amount of ·OH and H₂O₂ in SA with saturated O₂. Copied with permission [102]. Copyright 2024, Springer Nature.

Fig.6 (a) Schematic diagram of the synthesis K⁺/I⁻-CN/CdSe-D. Copied with permission [94]. Copyright 2024, Wiley-VCH GmbH. (b) The H₂O₂ yield of HR-CN, HD-CN and homo-CN. (c) H₂O₂ yield of obtained samples under different conditions. (d) EPR signal of DMPO ^•O₂^- of homo-CN. (e) Steady-state photoluminescence spectra and (f) fluorescence lifetime decay curves. (g) UV–vis DRS and energy band gap plots. Copied with permission [92]. Copyright 2023, Elsevier.

Fig.7 Structures model of (a) U-POCN and (b) AQ/U-POCN. Copied with permission [87]. Copyright 2021, Wiley-VCH GmbH. (c) Schematic diagram of the synthesis of TA-CN sample by solution pyrolysis. Copied with permission [95]. Copyright 2024, Elsevier.

Fig.8 The molecular structure of CTFs.

Fig.9 (a) Scheme of the synthesis of samples from their precursors. (b) Typical time course of H₂O₂ production using different samples. (c) The SCC efficiencies measured under simulated AM1.5 G. Calculated free energy different active sites: (d) Triazine structure in all CTFs, (e) benzene group in all CTFs. Copied with permission [30]. Copyright 2019, Wiley-VCH GmbH. (f) Rational synthesis of samples with different charge separation and transfer ability. Copied with permission [119]. Copyright 2022, Wiley-VCH GmbH.

Fig.10 Schematic diagram for synthesizing of (a) CTF-BP and (b) CTF-FL. (c) Comparison of the SCC efficiency for CTF-NSs with the reported representative metal-free photocatalysts; (d) Time-resolved PL spectra of CTF-BP (pink) and CTF-FL (blue); Gibbs free energy diagrams of (e) ORR and (f) WOR steps on CTF-BP and CTF-FL. Copied with permission [37]. Copyright 2024, American Chemical Society.

Fig.11 (a) Natural photosynthesis charge separation processes. (b) Schematic illustration of the stepwise charge transfer within the CTFs. Copied with permission [120]. Copyright 2024, American Chemical Society. (c) Scheme of the synthetic route of CQD-CTFs. Copied with permission [55]. Copyright 2024, Wiley-VCH GmbH.

Fig.12 (a) Synthetic routes of CoSA/Tr-CTF and CoSA/Py-CTF. (b) FT-IR spectra and (c) XRD pattern of samples. Cpied with permission [121]. Copyright 2024, American Chemical Society. (d) The synthesis strategy and structure of d-CTF-Ni. Copied with permission [122]. Copyright 2024, Elsevier.

Fig.13 Structure model of COF-1 and COF-5.

Fig.14 The typical reactions types for COFs materials.

Fig.15 (a) Structure model of TPB-DMTP-COF. (b) Visible light-driven H₂O₂ production in pure water. (c) Photocatalysis stability testing of TPB-DMTP-COF. Copied with permission [134]. Copyright 2021, Wiley-VCH GmbH.

Fig.16 Chemical structure of COF-0CN (a), COF-1CN (b) and COF-2CN (c). (d) Time-dependent H₂O₂ photogeneration using visible light for fabricated samples. (e) The AQY of COF-2CN at selected wavelength. (f) Long-term testing of COF-2CN for H₂O₂ photosynthesis. (g) Photocatalytic H₂O₂ production by COF-2CN in ultrapure water, tap water, river water, lake water and sea water under visible-light irradiation. Calculated energy profile for oxidation of water into H₂O₂ (h) and reduction of oxygen into H₂O₂ (i) on three COFs at U = 0 V vs. SHE at pH 7. Copied with permission [151]. Copyright 2024, Wiley-VCH GmbH.

Fig.17 (a) Synthetic routes used to prepare samples. (b) Photocatalytic performance curves over obtained samples. (c) Free energy diagram of ORR pathways for PDA-COF (orange) and DVA-COF (blue). Copied with permission [30]. Copyright 2024, Wiley-VCH GmbH.

Fig.18 (a) a Schematic of Kf-AQ condensation. Reprinted with permission [153]. Copyright 2024, Springer Nature. (b) The synthetic route of cyanide-functionalized D-A-π-D ECUT-COF-50 and D-A-π-A ECUT-COF-51. Reprinted with permission [154]. Copyright 2025, Elsevier. (c) Chemical structure of TDB-COF. Reprinted with permission [155]. Copyright 2023, Elsevier.

Fig.19 Synthetic routes (a) of TZ-COF, OZ-COF and IZ-COF. (b) D-π-A model and differences of COFs (the inset is the local charge distribution). Copied with permission [143]. Copyright 2023, Wiley-VCH GmbH. (c)The synthetic routes and chemical structures of the different samples. Copied with permission [159]. Copyright 2024, Wiley-VCH GmbH.

Fig.20 The schematic illustration of the octupolar structure in different samples. Copied with permission [43]. Copyright 2023, Springer Nature. (b) Synthesis of TpaBtt, TapbBtt and TaptBtt. Copied with permission [137]. Copyright 2024, Springer Nature. (c) Synthetic routes of TBTN—COF and TMT-COF. Copied with permission [140]. Copyright 2024, Wiley-VCH GmbH.

Fig.21 (a) Schematic illustration the synthesis process of the UP-TP. (b) Photocatalytic overall H₂O₂ generation of various photocatalysts. (c) Wavelength-dependent AQE of H₂O₂ generation over UP-TP and the corresponding DRS spectrum. (d) Continuous H₂O₂ production over long-time testing. Copied with permission [164]. Copyright 2024, Wiley-VCH GmbH. (e) Schematic illustration of the synthesis process for TpMA/In₂S₃. Copied with permission [104]. Copyright 2024, Elsevier.

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