Surface engineering strategies for particulate photocatalysts toward photocatalytic H₂O₂ production

Abstract

The photocatalytic production of hydrogen peroxide (H₂O₂) using particulate photocatalysts is a safe, sustainable and green process that requires only oxygen and water as feedstocks and solar energy as a power source. Surface engineering on particulate photocatalysts can significantly improve the performance of photocatalytic H₂O₂ production by optimizing light absorption, surface charge separation, and reaction pathways. To provide a comprehensive and systematic illustration of this topic, various surface engineering strategies are classified and elaborated in this review, which are mainly included in the following two aspects. The first one involves surface modification relating to crystal facets, surface vacancies, and surface functional groups. The second one focuses on surface composite strategies, including combination with metals, semiconductors, carbon nanomaterials, polyoxometalates or their derivatives, and organic compounds. Finally, the challenges and prospects in the surface engineering strategies for particulate photocatalysts for promoting photocatalytic H₂O₂ production are analyzed and discussed.

---

Wider impact

Photocatalytic H₂O₂ production has attracted extensive attention in recent years because it uses only H₂O and O₂ as source materials and can be operated under ambient conditions. In a typical photocatalyst system, the surface of a particulate semiconductor provides an important platform for the adsorption and activation of reactant molecules, the separation and transfer of surface charge carriers, the promotion of H₂O₂ formation and the prohibition of H₂O₂ decomposition. Therefore, innovative surface engineering strategies are summarized and illustrated in this review, aiming to provide a deep understanding of the structure-performance relationship and construct efficient photocatalytic H₂O₂ production systems. Specifically, surface engineering strategies, including surface modifications and surface composite strategies, are classified and elaborated. The challenges and prospects in the surface engineering strategies for particulate photocatalysts are further analyzed and summarized. This review is anticipated to offer valuable guidance for the design and fabrication of advanced particulate photocatalysts for efficient H₂O₂ synthesis.

1. Introduction

H₂O₂ has attracted considerable scientific interest since its synthesis was initiated in the eighteenth century and has been recognized as one of the 100 most important chemicals globally.^1–3 H₂O₂ has strong oxidation ability, and its decomposition generates water (H₂O) and oxygen (O₂) as the only by-products without any pollution; this makes it an ideal green oxidizer for environmental remediation and the chemical industry.⁴ Furthermore, owing to its high energy density, H₂O₂ has garnered significant attention in the field of energy conversion.^5,6 Compared with molecular hydrogen (H₂), H₂O₂ has demonstrated superiority in enabling liquid-phase storage and transport under ambient conditions. These characteristics render H₂O₂ as a promising alternative energy carrier, exhibiting distinct advantages over H₂ in energy storage and transport systems.^7,8

Currently, the most widely used method for industrial H₂O₂ production is the anthraquinone process. As shown in Fig. 1, this process comprises four sequential steps: (1) anthraquinones (e.g., 2-ethylanthraquinone) undergo hydrogenation with H₂, forming hydrogenated anthraquinones. (2) Hydrogenated derivatives are oxidized in the presence of O₂, yielding H₂O₂. (3) The generated H₂O₂ is extracted via H₂O to produce an aqueous solution. (4) The regenerated anthraquinones are recycled into subsequent hydrogenation and oxidation cycles, creating a closed-loop system.⁹ Although the process is widely used, the high energy consumption of the hydrogenation step and the associated organic waste discharge problem result in low overall energy efficiency and high environmental risks. These inherent limitations highlight the critical demand for eco-friendly and sustainable alternatives to H₂O₂ synthesis.¹⁰

Fig. 1 Schematic representation of the anthraquinone oxidation process. AQ, AHQ, THAHQ, and THAQ represent 2-alkylanthraquinone, 2-alkylanthrahydroquinone, 5,6,7,8-tetrahydroanthrahydroquinone, and 5,6,7,8-tetrahydroanthraquinone, respectively.

Amidst escalating global energy demands and resource shortages, the development of renewable energy sources, particularly solar energy, has become increasingly critical.¹¹ Photocatalysis has been verified to possess distinct advantages over conventional chemical methods, including operational simplicity, reduced energy demands, and inherent environmental compatibility, owing to its superiority in ambient operation and direct solar-to-chemical energy conversion.^12,13 This technology has demonstrated considerable potential in applications such as H₂O splitting, CO₂ reduction and H₂O₂ production, offering a promising pathway for energy supply and environmental protection.¹⁴ Photocatalytic H₂O₂ production represents a sustainable innovation, surpassing the energy-intensive multi-step anthraquinone process in industry.^15,16 The basic principle of photocatalytic H₂O₂ production is shown in Fig. 2(a).¹⁷

Fig. 2 (a) Schematic of the photocatalysis process. CB and VB represent the conduction band and valence band, respectively. (b) Energy diagram for oxygen reduction reaction (ORR) and water oxidation reaction (WOR) on a semiconductor photocatalyst for H₂O₂ production.

Recently, the photocatalytic synthesis of H₂O₂ has become a research hotspot. Various novel materials and surface modification strategies have been developed to improve photocatalytic H₂O₂ production performance, and many exciting progresses have been achieved. On the one hand, studies have demonstrated that the synthesis selectivity and efficiency depend critically on the component and band structure of the applied photocatalyst. The development of new photocatalysts, including TiO₂, C₃N₄, and covalent organic framework (COFs), has significantly enhanced both the H₂O₂ evolution rate and selectivity.^18,19 On the other hand, modifications of particulate photocatalysts have been explored to improve H₂O₂ generation efficiency, including molecular engineering, defect regulation, and element doping (e.g., nitrogen, sulfur, and phosphorus).^20–22 These approaches enhance photocatalytic performance by optimizing light absorption and charge utilization. Previous reviews have mainly focused on photocatalysts based on their preparation methods or reaction pathways.^2,4 However, to date, there are no reviews relating to the significant advancements in photocatalytic H₂O₂ production from the perspective of surface engineering strategies.

The surface of a particulate photocatalyst plays an important role in the transfer and utilization of charge carriers, adsorption and activation of reactant molecules, promotion of H₂O₂ formation and inhibition of H₂O₂ decomposition. Various surface engineering strategies have been developed to improve the photocatalytic H₂O₂ production performance, and many exciting progresses have been achieved recently. However, to date, no specific review has systematically covered this critical topic. To fill this gap, serial surface engineering strategies are classified and elaborated in this review to provide a comprehensive and systematic illustration of this topic. The surface engineering strategies for light-harvesting semiconductors cover surface modification strategies and surface compositing strategies. The former mainly involves crystal facet engineering, surface vacancy engineering and surface functional group regulation to tailor material properties, and the latter includes the integration of functional components, such as metals, semiconductors, carbon nanomaterials, polyoxometalates (POMs) and their derivatives, and organic compounds to enhance specific functionality. Therefore, the challenges and prospects in surface engineering strategies of particulate photocatalysts are also analysed and summarized, which is anticipated to offer valuable guidance for the design and fabrication of advanced particulate photocatalysts for efficient H₂O₂ synthesis.

2. Basics of photocatalytic H₂O₂ production

2.1 Oxygen reduction reaction (ORR) to produce H₂O₂

The ORR is an important step in the photocatalytic production of H₂O₂, where O₂ molecules are reduced to H₂O₂ by accepting electrons and protons. In the ORR for H₂O₂ production, two primary mechanisms exist: the two-electron direct reduction pathway and the superoxide radical (˙O₂⁻)-mediated pathway. In the two-electron direct reduction pathway, O₂ with the participation of a proton accepts two electrons to form H₂O₂ in one step. This reaction can be expressed as eqn (1):

O₂ + 2H⁺ + 2e⁻ → H₂O₂. (1)

In the ˙O₂⁻-mediated pathway, electron transfer occurs in two sequential steps. First, O₂ captures an electron to form a ˙O₂⁻ (eqn (2)). Then, the ˙O₂⁻ reacts with another electron and two protons to produce H₂O₂ (eqn (3)):

O₂ + e⁻ → ˙O₂⁻, (2)

˙O₂⁻ + 2H⁺ + e⁻ → H₂O₂. (3)

The key distinction between these two mechanisms is that the one-step two-electron pathway is easier to occur thermodynamically and has fewer side reactions.²³ In contrast, the two-step single electron pathway is more advantageous kinetically.

2.2 Water oxidation reaction (WOR) to produce H₂O₂

WOR is another common pathway to H₂O₂ production, primarily occurring through two mechanisms: the direct two-electron pathway and the hydroxyl radical (˙OH)-mediated pathway. In the direct two-electron pathway, H₂O molecules are oxidized, undergoing a two-electron transfer process to form H₂O₂ (eqn (4)):

2H₂O → H₂O₂ + 2H⁺ + 2e⁻. (4)

However, the ˙OH-mediated pathway involves ˙OH acting as an intermediate. Initially, H₂O molecules undergo oxidation, generating highly reactive ˙OH (eqn (5)). Subsequently, two ˙OH couple together to form H₂O₂ (eqn (6)):

H₂O → ˙OH + H⁺ + e⁻ (5)

2˙OH → H₂O₂. (6)

Owing to the high reactivity of ˙OH, some side reactions are prone to occur in this route. Although WOR has primarily been investigated for H₂O₂ production, H₂O₂ can only become the dominant product over appropriate photocatalysts under controlled reaction conditions. Efficient H₂O₂ generation requires photocatalysts that can activate H₂O molecules and precisely modulate the electron–proton transfer kinetics to stabilize the active intermediates.

2.3 Integrated dual-channel pathway (overall photocatalytic H₂O₂ evolution reaction)

Through rational design, H₂O₂ can be co-produced by a dual-channel pathway, including both ORR and WOR. The integration of ORR and WOR establishes a highly efficient H₂O₂ generation system, wherein ORR consumes electrons and protons, while WOR provides both electrons and protons via H₂O oxidation. This electron-proton coupling regulation ensures the atomic utilization of reactants (100%).²⁴ Through rational surface modification, the active sites and surface electronic structure can be optimized to enhance photocatalytic activity and selectivity. Additionally, the band structure should be elaborately tailored to fulfil the energy requirements of the ORR and WOR, as presented in Fig. 2.

2.4 Kinetics of H₂O₂ production and decomposition

The H₂O₂ production and decomposition kinetics jointly dominate the net accumulation efficiency of its photocatalytic system, and this dynamic balance directly determines the yield and stability in practical applications. The decomposition of H₂O₂ in the photocatalytic system mainly involves the following two mechanisms: (1) photocatalytic decomposition under ultraviolet (UV) light, where H₂O₂ absorbs UV light and decomposes into ˙OH and other reactive oxygen species;²⁵ (2) photocatalytic redox decomposition, where H₂O₂ interacts with the photogenerated electrons and holes on the surface of the photocatalyst, forming the reactive oxygen species that further accelerate the decomposition of H₂O₂. The formation and decomposition kinetics of H₂O₂ can be described using eqn (7):where K_f and K_d denote the rate constants of the formation (μmol L⁻¹ min⁻¹) and decomposition (min⁻¹) of H₂O₂, respectively. A larger K_f and a smaller K_d suggest higher activity toward H₂O₂ production.²⁶ By optimizing the surface electron density and active sites, and regulating the band structure, the H₂O₂ decomposition can be effectively suppressed, leading to improved accumulation concentration of H₂O₂.

2.5 Performance evaluation

The apparent quantum efficiency (AQE) is a key metric for evaluating the efficiency of photocatalytic H₂O₂ production.^27,28 It is defined as the ratio of the number of electrons and holes for producing H₂O₂ molecules to the number of incident photons at a specific wavelength. The amount of H₂O₂ produced during the reaction is quantified using chemical analysis methods (e.g., spectrophotometry and iodometry), while the number of incident photons refers to the total number of photons illuminating the photocatalyst surface during the reaction.

The solar-to-chemical energy conversion efficiency (SCC) of photocatalytic H₂O₂ production is a critical performance metric reflecting the ability of the system to convert solar energy into chemical energy. This efficiency is generally influenced by factors including light absorption, charge separation, and surface reaction kinetics.²⁹ It is typically calculated using the following eqn (8):

where ΔG represents the change in Gibbs free energy required for H₂O₂ production, typically 98.2 kJ mol⁻¹. R denotes the rate of H₂O₂ production per unit time (mol s⁻¹), A is the irradiation area (m²), and P is the solar irradiance per unit area (W m⁻²). When evaluating and comparing the H₂O₂ performance, both the AQE and SCC are two key indices.³⁰ These metrics assess the effectiveness and efficiency of photocatalytic H₂O₂ production from different perspectives, which can provide a more comprehensive evaluation of the photocatalytic efficiency and the practical application potential.

The fundamental photocatalytic H₂O₂ production process is that the semiconductor is photoexcited to produce electrons and holes that separate and migrate to the photocatalyst surface and participate in redox reactions with the adsorbed reactants. Given that photocatalytic reactions occur on the surface of the photocatalyst, the surface properties and functional components can play an important role in photocatalytic H₂O₂ production by affecting the charge utilization, reactant adsorption and activation, reaction route, H₂O₂ accumulation and decomposition. To address these challenges and develop efficient photocatalytic H₂O₂ production systems, this review focuses on surface engineering strategies based on light harvesting semiconductors in the following two aspects: (1) surface modification strategies, including crystal facet engineering, surface vacancy engineering and functional group tailoring, to precisely modulate the surface properties of semiconductors; (2) surface composite strategies involving the integration of functional components, such as metals, semiconductors, carbon nanomaterials, polyoxometalates (POMs) and their derivatives, as well as organic compounds. A schematic illustration of these surface engineering strategies for particulate photocatalysts for H₂O₂ production is summarized, as depicted in Fig. 3.

Fig. 3 Schematic of typical surface engineering strategies over particulate photocatalysts for promoted H₂O₂ production.

3. Surface modification strategies

The physicochemical properties of semiconductors vary with different surface structures, which in turn influence their photocatalytic H₂O₂ production performance by modulating key processes, such as light absorption, charge separation, reactant adsorption and activation, reaction pathways, and product desorption.³¹ In this section, the surface modification strategies of semiconductors, including crystal facet engineering, surface vacancy engineering, and functional group regulation, are systematically discussed, and their impact on photocatalytic H₂O₂ production is also discussed afterwards (Table 1).

Table 1 Summary of the surface modification strategies for particulate photocatalysts for H₂O₂ production

Strategy	Photocatalyst	Light source	H2O2 (μmol h^−1 g^−1)	Reaction solution	AQY^h (%)	Ref.
a MIL-125: Ti-based metal organic framework derived from Ti metal ions and carboxylate organic ligands.b Cv-g-C3N4: g-C3N4 with C vacancies.c DCN-15A: defected g-C3N4 with surface NH2 vacancy.d Bpt-CTF: covalent triazine frameworks with 1,3-bis(4-cyanophenyl)thiourea precursor.e SO3H-COF: covalent organic framework with sulfonic acid groups.f TBTN-COF: cyanide-based covalent organic framework combining 2,4,6-trimethylbenzene-1,3,5-tricarbonitrile.g BM-Au/TiO2: gold nanoparticle-loaded rutile TiO2 with a bimodal size distribution around 10.6 nm and 2.3 nm.h AQY: apparent quantum yield.
Crystal facet engineering	TiO2(001)	300 W Xe lamp	649.2	10 vol% isopropanol solution	—	32
	MIL-125^a	500 W Xe lamp, λ ≥ 400 nm	0.57	Water	—	33
	BiOCl(001)	300 W Xe lamp	3850	5 vol% formic acid solution	—	34
Surface vacancy engineering	Cv-g-C3N4^b	300 W Xe lamp, λ ≥ 420 nm	90	Water	—	35
	DCN-15A^c	AM1.5G, λ ≥ 420 nm	83.3	20 vol% isopropanol solution	—	36
	Defect ZrS3	AM 1.5G	2603	30 mL water with 1 mmol benzylamine	—	37
Surface functional group regulation	Bpt-CTF^d	300 W Xe lamp	6536.2	Water	8.6 (400 nm)	38
	SO3H-COF^e	300 W Xe lamp, λ ≥ 400 nm	497.1	10 vol% methanol solution	15 (400 nm)	39
	TBTN-COF^f	300 W Xe lamp, λ ≥ 420 nm	1376.6	Water	7.6 (420 nm)	40
	BM-Au/TiO2^g	300 W Xe lamp, λ ≥ 430 nm	35	Water	—	41

---

3.1 Crystal facet engineering

The surface structure of a certain crystal is determined by its specific crystal facets exposed, with each crystal facet exhibiting distinct atomic arrangements, electronic structures, and surface energies, which can affect photocatalytic H₂O₂ production performance in the following issues.⁴² (1) The arrangement and coordination of surface atoms affect the adsorption of molecules or ions (Fig. 4(a)). (2) The crystal orientations determine the separation of the photogenerated charge carriers (Fig. 4(b)). (3) The redox abilities of charge carriers depend on the surface electronic band structure, which relies on crystal facets (Fig. 4(c)). Therefore, the preparation of semiconductors exposed to suitable crystal facets is an effective way to regulate their photocatalytic H₂O₂ production performance. Crystal facet engineering is applicable to photocatalysts with anisotropic crystalline structures, where different crystallographic planes exhibit distinct photocatalytic performances. This approach is particularly effective for some oxide semiconductors (e.g., TiO₂, ZnO, and BiVO₄).

Fig. 4 Roles and applications of crystal facet engineering in photocatalytic H₂O₂ production. Schematics illustrating the important roles of facets: (a) reactant adsorption, (b) charge separation, and (c) redox ability due to the distinct band structure. (d) Free energy diagram of TiO₂ with different exposed facets for photocatalytic H₂O₂ production. Reproduced with permission.³² Copyright 2023, Elsevier. (e) Illustration of the spatial separation of h⁺/e⁻ between (001) and (111) facets on a Ti-based metal organic framework derived from Ti metal ions and carboxylate organic ligands. Reproduced with permission.³³ Copyright 2022, the Royal Society of Chemistry. (f) Schematic of the band structure of the BiOCl (001) and BiOCl (010) samples. Reproduced with permission.³⁴ Copyright 2021, the Royal Society of Chemistry.

The adsorption of reactant molecules is highly related to the exposed facet of the semiconductor. For example, Gao and co-workers reported that the anatase TiO₂(001) facet played a significant role in photocatalytic H₂O₂ production. The higher activity of the (001) facet than those of the (101) and (100) facets was attributed to the higher intrinsic activity of the active sites. Theoretical calculations indicated that the under-coordinated Ti atoms on the (001) surface served as strong O₂ adsorption sites (Fig. 4(d)). This study provided an insight into the relationship between the crystal facet structure and photocatalytic activity at the atomic scale.³² Besides, semiconductors with different exposed facets show distinct adsorption capacities of surface modifying agents. A large number of hydroxyl groups on the TiO₂(001) facet tended to bond with stearic acid (SA) molecules. The enhanced hydrophobicity with a higher adsorbing proportion of SA led to an increased accumulation of O₂ in the interfacial microenvironment, thereby enhancing the photocatalytic H₂O₂ generation rate.⁴³

Charge separation, another significant step in photocatalytic H₂O₂ evolution, can also be regulated by facet engineering. Pan's group found that meso-tetra (4-carboxyphenyl) porphyrin with a selectively exposed (400) facet could exhibit a higher photocatalytic H₂O₂ generation rate than that with an exposed (022) or (020) facet. The higher photocatalytic performance was attributed to the strong internal electric field on the (400) facet, which could effectively prohibit charge recombination.⁴⁴ In addition, the formation of facet junctions can affect the separation of photogenerated electrons and holes between two different facets. The oxidative sites are located on the (001) facet of a Ti-based metal organic framework (MOF) derived from Ti metal ions and carboxylate organic ligands (MIL-125) photocatalyst, while the reduction reaction occurs on its (111) facet. Therefore, such a formed (001)/(111) facet junction of MIL-125 could achieve the spatial separation of redox reaction sites, avoiding the possible recombination of charge carriers (Fig. 4(e)).³³

The energy band position of the semiconductor is an important index for determining the reaction pathway, which can be effectively tailored by crystal facet engineering. Xu and co-workers demonstrated that BiOCl with exposed (010) facets produced H₂O₂ via both the ORR and WOR, while only ORR could occur on BiOCl with the exposed (001) facet (Fig. 4(f)). This is because the conduction band minimum (CBM) of BiOCl with the exposed (001) (−1.28 V vs. NHE) or (010) (−1.03 V vs. NHE) facet is more negative than the oxidation potential of ˙O₂⁻/O₂ (−0.33 V vs. NHE). However, only holes in the valence band maximum (VBM) of BiOCl with the exposed (010) facet (2.19 V vs. NHE) can oxidize H₂O to ˙OH (1.99 V vs. NHE).³⁴ Crystal facet engineering enables the precise control over the redox ability of photocatalysts, thereby influencing the reaction pathway and efficiency of the photocatalytic H₂O₂ production.

3.2 Surface vacancy engineering

Surface vacancy, as a typical structural defect formed by atom deletion, further enriches the surface chemical environment of the photocatalyst.⁴⁵ Appropriate surface vacancy regulates the electronic structure of the photocatalyst, thereby affecting light absorption (Fig. 5(a)) and charge separation (Fig. 5(b)). In addition, the surface vacancy participates in the construction of the active site to regulate the reactant adsorption, reaction pathway, and product desorption (Fig. 5(c)). However, excessive vacancies can act as charge recombination centers, resulting in decreased photocatalytic activity.⁴⁶ Therefore, creating favorable vacancies on the material surface is important to improve the performance of photocatalytic H₂O₂ production.⁴⁷ Surface vacancy engineering is widely applied to modulate semiconductors, especially for g-C₃N₄, metal oxides, perovskite oxides, metal sulfides and oxyhalides. Herein, the roles of surface vacancy in photocatalytic H₂O₂ production performance are analyzed and discussed to offer an insight into the rational design of photocatalysts, hoping to further deepen the understanding of the relationship between surface chemical environment and photocatalytic performance.

Fig. 5 Roles and applications of the surface vacancy of semiconductors for photocatalytic H₂O₂ production. Schematics of the important roles of surface vacancy in tuning (a) light absorption, (b) charge separation, and (c) reactant adsorption. Density of states of (d) g-C₃N₄ and (e) defective g-C₃N₄ with N vacancies. (f) Schematic of mechanisms underlying the photoexcited dynamics involved in photocatalytic H₂O₂ evolution over g-C₃N₄ with N vacancies. Reproduced with permission.³⁶ Copyright 2018, Wiley-VCH. (g) The changed H₂O₂ generation pathway from single-electron reduction on g-C₃N₄ to two-electron reduction on g-C₃N₄ with C vacancies. Reproduced with permission.³⁵ Copyright 2016, Elsevier.

The introduced surface vacancy can modulate the electronic structure of photocatalysts, affecting their light absorption ability. Specifically, compared with the conduction and valence band states, weak bonding at vacancy sites reduces the splitting between bonding orbitals and antibonding orbitals, which facilitates the formation of distinct electronic states within the band. There are two primary mechanisms for the enhanced light absorption derived from such electronic states: (1) narrowing the bandgap to enlarge the light absorption wavelength range, and (2) acting as midgap states, which can accommodate the photogenerated electrons from the valence band, thereby enabling the absorption of photons with energies below the intrinsic bandgap threshold.⁴⁸ For instance, Ye's group introduced surface N vacancies in g-C₃N₄ (DCN) by a one-step photo-assisted route using a hydrazine as a reducing agent. Density functional theory calculations demonstrate that the DCN showed a narrower bandgap and a defect state within the band (Fig. 5(d) and (e)). The experimental results revealed that DCN exhibited broader and stronger absorption tails extending to 600 nm than g-C₃N₄. Consequently, the H₂O₂ production performance of DCN was 10.1 times higher than that of g-C₃N₄.³⁶ The surface vacancy plays a pivotal role in impacting light absorption to regulate photocatalytic performance by inducing midgap states and narrowing the bandgap.

Besides light absorption, surface vacancies play a crucial role in modulating charge separation. Surface vacancies can act as charge carrier trapping sites to regulate the separation of electrons and holes. For example, Tian and co-workers prepared defective ZrS₃ nanobelts with S₂²⁻ and S²⁻ vacancies and systematically investigated their effects on modulating charge carrier dynamics. Specifically, it was demonstrated that S₂²⁻ vacancies could prohibit charge carrier recombination, while S²⁻ vacancies could accelerate the extraction of the photogenerated holes towards the surface and promote electron transfer.³⁷ Shi and co-workers constructed defected g-C₃N₄ with surface NH₂ vacancy (DCN-15A) by photothermal treatment. The introduction of an N vacancy in g-C₃N₄ could lead to the formation of defect states, which facilitated the charge transfer process and prohibited the recombination of electrons and holes (Fig. 5(f)).³⁶

After the photogenerated electrons and holes transfer to the photocatalyst surface, the reaction between the reactants and the charge carriers occurs only when the reactants are effectively adsorbed on the active sites. The coordinatively unsaturated atoms at the vacancy sites promote the adsorption of reactant molecules. Zhang and co-workers induced N vacancies into g-C₃N₄ (Nv-CN). The calculated adsorption energy revealed that the O₂ adsorption energy on Nv-CN was lower than that on g-C₃N₄, indicating that the surface N vacancy sites were more favorable for the adsorption of O₂. The strong O₂ desorption peak and increased O–O bond length in Nv-CN further experimentally demonstrated that N vacancies promoted O₂ adsorption and activation.¹⁹ In addition, surface vacancies play a decisive role in modulating the reaction pathways by enhancing reactant adsorption and stabilizing key intermediates at defect sites. Wang's group observed that the photocatalytic H₂O₂ activity of g-C₃N₄ with C vacancies (Cv-g-C₃N₄) was 14-fold higher than that of the pristine g-C₃N₄. The experimental results demonstrated that C vacancies changed the ORR pathway from a two-step single-electron indirect reduction to a more efficient one-step two-electron direct reduction (Fig. 5(g)).³⁵ Therefore, the surface vacancy can affect the reactant adsorption and the reaction pathway, thus influencing the final efficiency of photocatalytic H₂O₂ generation.

3.3 Surface functional group regulation

Surface functional group modification involves the introduction or precise regulation of specific chemical groups on the photocatalyst surface to tailor its physicochemical properties and modulate its photocatalytic performance.^49–52 The modification of functional groups can mainly modulate the O₂/H₂O₂ adsorption, charge separation and surface reactions, thereby influencing the photocatalytic H₂O₂ production performance.^53–55 For example, polar functional group modification can affect the charge separation; electron-withdrawing functional group modification can regulate both processes of charge separation and O₂ adsorption; surface protonation provides a proton-rich environment for ORR; surface complexation or adsorption of cationic/anionic species can mediate the H₂O₂ adsorption and decomposition (Fig. 6(a)). The surface functional group modification strategy is effective for materials featuring abundant surface-active groups (e.g., –OH and –NH₂) or those amenable to directional chemical functionalization, such as g-C₃N₄, MOFs, COFs, and TiO₂.

Fig. 6 Roles and applications of the surface functional group modification in photocatalytic H₂O₂ production. (a) Schematics illustrating the roles of surface functional group modification in photocatalytic H₂O₂ production. (b) Rational synthesis of Bpu-CTF and Bpt-CTF photocatalysts featuring different charge separation and transfer abilities. The CTF represents covalent triazine frameworks; the Bpu represents 1,3-bis(4-cyanophenyl)urea precursor; and the Bpt represents 1,3-bis(4-cyanophenyl)thiourea precursor. Reproduced with permission.³⁸ Copyright 2022, Wiley-VCH. (c) Mechanism for photocatalytic H₂O₂ production on the surface of cyanide-based COF combining 2,4,6-trimethylbenzene-1,3,5-tricarbonitrile. Reproduced with permission.⁴⁰ Copyright 2024, Wiley-VCH. (d) A proposed mechanism of the photocatalytic ORR for H₂O₂ production based on Au/TiO₂-CO₃²⁻. Reproduced with permission.⁴¹ Copyright 2016, Wiley-VCH.

Polar functional groups create a built-in electric field (BEF) that enhances charge separation. The BEF forms dipoles that drive electrons and holes in opposite directions. Han's group introduced polar thiourea moieties onto covalent triazine frameworks (CTF) to construct the BEF, enhancing the charge separation (Fig. 6(b)).³⁸ Then, a series of triazine COF photocatalysts functionalized with oxygen groups of varying polarity was further prepared and analyzed. The results showed that the strongly polar sulfonic acid groups (–SO₃H) generated powerful BEF, boosting charge separation and H₂O₂ production.³⁹ The modification with electron-withdrawing functional groups can play dual functions in regulating both charge separation and O₂ adsorption. A representative example is the cyano group (–CN) that has been extensively employed for photocatalytic H₂O₂ production.⁵⁶ Zhou and co-workers designed a cyanide-based COF combining 2,4,6-trimethylbenzene-1,3,5-tricarbonitrile (TBTN-COF). The TBTN-COF exhibited more efficient charge separation and higher charge carrier availability than its counterpart without –CN. Besides, the –CN sites with concentrated electron density acted as O₂ adsorption sites, facilitating the formation of *OOH intermediates and a favorable ORR pathway (Fig. 6(c)).⁴⁰ In addition to –CN, the electron-withdrawing groups, such as sulfone group, hydroxyl group, vinyl group and benzothiadiazole units, have also been applied to regulate the charge separation and O₂ adsorption during photocatalytic H₂O₂ evolution.^57–60

Proton is another functional group for surface modification.^61–63 The protonated surface serves as a proton enrichment platform that can directly supply protons for ORR. The abundant proton facilitates the establishment of a new reaction equilibrium by accelerating the forward reaction while inhibiting the reverse reaction. Wen and co-workers reported a protonation and an O-doped g-C₃N₄ photocatalyst. The abundant protons on the surface can combine with the adsorbed O₂, thus promoting the formation of the OOH* intermediate and facilitating the H₂O₂ production.⁶⁴ Surface protonation is an effective strategy to provide protons for facilitating the H₂O₂ formation, avoiding the usage of an acid solution for the reaction.

In addition, the complexation and adsorption of ions have emerged as effective approaches to regulating the process of photocatalytic H₂O₂ production. The H₂O₂ decomposition occurs on the TiO₂ surface because H₂O₂ tends to complex with the surface Ti–OH group to form the Ti–OOH, which can be reduced by the photogenerated electrons. Maurino and co-workers found that photocatalytic H₂O₂ production over TiO₂ was undetectable; however, it reached a stable millimolar concentration after adding F⁻. They proposed that the competitive adsorption of F⁻ and H₂O₂ on the TiO₂ surface hindered the H₂O₂ adsorption and subsequent decomposition.⁶⁵ Teranishi and co-workers found that CO₃²⁻ ions chemisorbed on the TiO₂ surface could inhibit the H₂O₂ decomposition by suppressing a Ti–OH-mediated degradation pathway (Fig. 6(d)).⁴¹ By leveraging the interaction between photocatalyst surfaces and ions, the H₂O₂ decomposition can be finely regulated. The versatility of surface complexation and adsorption of cationic/anionic species highlights their potential for designing highly efficient and stable photocatalytic H₂O₂ production systems.

In this section, surface modification strategies, including crystal facet engineering, surface vacancy engineering, and functional group regulation, enable atomic/molecular-level regulation of the physicochemical properties of semiconductors, thereby optimizing the key steps in photocatalytic H₂O₂ synthesis. Crystal facet engineering and surface vacancy engineering can promote intrinsic activity through atomic-level structural optimization, but their applications are constrained by limited material varieties and challenging synthesis approaches. Crystal facet engineering is mainly applicable to inorganic semiconductors with anisotropic crystalline structures. However, the modulation of the surface vacancy depends highly on the controllability of the surface structure. The electronic structure and chemical microenvironment can be easily tuned by grafting the photocatalyst surface with specific functional groups. Functional group modification is mainly applicable to organic semiconductors, which offer greater diversity and flexibility but suffer from instability under specific conditions. Therefore, continuous exploration of surface modification strategies is necessary to develop efficient and stable photocatalysts. In addition, these strategies can be combined with surface composite strategies, as discussed in later sections, to collectively improve the H₂O₂ production efficiency.

4. Surface composite strategies

Surface composite strategies are another part of effective surface engineering, whose emphasis lies in the integration with other materials to achieve the function-oriented optimization of photocatalysts.^2,4,66,67 Distinct from surface modification approaches that focus on altering the intrinsic surface properties of photocatalysts, surface composite strategies can benefit from the advantages of both materials, thereby effectively alleviating the inherent limitations of photocatalyst systems with only a single material. In this section, the surface composite strategies are overviewed (Table 2).

Table 2 Summary of the surface composite strategies for particulate photocatalysts for H₂O₂ production

Strategy	Photocatalyst	Light source	H2O2 (μmol h^−1 g^−1)	Reaction solution	AQY^k (%)	Ref.
a C-W18O49/Ni-SA: Ni single-atom-decorated C-W18O49 photocatalyst.b CoSA/Py-CTF: Co single-atom-modified CTF photocatalyst.c Sb-SAPC: Sb single-atom decorated polymeric carbon nitride.d CN/Zn-MOF(lc): composite photocatalyst with g-C3N4 and Zn-based metal organic framework.e CuO/Cu(OH)2@CN: composite photocatalyst of CuO, Cu(OH)2 and g-C3N4.f α-Fe2O3/CQD@CN: composite photocatalyst of α-Fe2O3, carbon quantum dots and g-C3N4.g CQD-CTF: composite photocatalyst of carbon quantum dots and covalent triazine frameworks.h g-C3N4-PW11: composite photocatalyst of g-C3N4 and polyoxometalate cluster of [PW11O39]^7−.i g-C3N4-SiW11: composite photocatalyst of g-C3N4 and polyoxometalate cluster of [SiW11O39]^8−.j g-C3N4/BDI: electron-deficient biphenyl diimide units modified g-C3N4 photocatalyst.k AQY: apparent quantum yield.
Composite with metals	C-W18O49/Ni-SA^a	300 W Xe lamp, λ ≥ 420 nm	338.9	Water	28.51 (365 nm)	68
	CoSA/Py-CTF^b	300 W Xe lamp, λ ≥ 420 nm	1159.32	Water	13.2 (420 nm)	69
	TiO2/MoSx–Au	300 W Xe lamp	3044	10 vol% ethanol solution	7.2 (365 nm)	70
	AgPd/BiVO4/CoOx	LED lamp, λ ≥ 400 nm	1560	Water	5.8 (420 nm)	71
	Sb-SAPC^c	300 W Xe lamp, λ ≥ 420 nm	647	Water	17.6 (420 nm)	6
	CN/Zn-MOF(lc)^d	300 W Xe lamp, λ ≥ 400 nm	1912	20 vol% ethanol solution	—	72
Composite with semiconductors	ZnO/WO3	300 W Xe lamp	3.364	Water	0.17 (365 nm)	73
	CuO/Cu(OH)2/CN^e	300 W Xe lamp, λ ≥ 420 nm	677	Water	—	74
	In2S3/CdS	AM 1.5G	836	Water	—	75
Composite with carbon nanomaterials	Fe2O3/CQD@CN^f	300 W Xe lamp, λ ≥ 420 nm	69.3	Water	17.89 (420 nm)	76
	CQD-CTF^g	300 W Xe lamp, λ ≥ 420 nm	259	Water	1.03 (420 nm)	77
	rGO/TiO2	300 W Xe lamp, λ ≥ 320 nm	1500	5 vol% isopropanol solution	—	78
Composite with POMs or their derivatives	g-C3N4-PW11^h	300 W Xe lamp	3.5	Water	—	79
	g-C3N4-SiW11^i	AM 1.5G	178	5 vol% ethanol solution	6.5 (420 nm)	80
Composite with organic compounds	g-C3N4/BDI^j	AM 1.5G, λ ≥ 420 nm	28.3	Water	4.6 (420 nm)	81
	TiO2	AM 1.5G, λ ≥ 300 nm	490	Furfural alcohol solution	—	82

---

4.1 Composite with metals

Metals, usually employed as reduction cocatalysts, play a pivotal role in modulating the electronic structure of photocatalysts, influencing the activity and selectivity of photocatalytic H₂O₂ production. Researchers have developed various metal cocatalysts for photocatalytic H₂O₂ production via ORR, but the advancement of cocatalysts targeting WOR remains limited. Generally, noble metals, such as Au, Pd, Pt, and Ag, are commonly utilized as cocatalysts for photocatalytic H₂O₂ production via the route of ORR.^83–85 Considering the high cost of noble metals, alternatives based on non-noble metals, including Ni and Co, have also been explored. Metal cocatalyst modification is a widely applicable surface engineering strategy. However, its effective implementation crucially depends on lattice matching, electronic structure compatibility, and proper band alignment between the cocatalyst and the semiconductor.

4.1.1 Composite with metal nanoparticles. Metal nanoparticles as an efficient ORR cocatalyst can significantly accelerate the reaction kinetics process owing to their unique electronic structure. Based on their particle size, metal cocatalysts can be categorized into metal nanoparticles, metal clusters, and metal single atoms. When co-catalysts are deposited on the semiconductor, the efficiency of ORR is generally improved mainly by enhancing the electron separation and transfer, and lowering the activation energy or the overpotential of ORR. What is more, metal cocatalysts with distinct electronic and geometric configurations exhibit different affinities for adsorbates, resulting in distinct O₂ adsorption behaviors and H₂O₂ desorption processes, which determine the photocatalytic activity and selectivity. Generally speaking, the photocatalytic H₂O₂ performance can be significantly enhanced by precisely regulating the species, size, morphology, and dispersion of metal nanoparticle cocatalysts.^86,87

Au, Pd, Pt, Ag, and Ni nanoparticles have commonly been used as effective ORR co-catalysts for photocatalytic H₂O₂ production, among which Au nanoparticles are the most widely studied.⁸⁸ However, the weak O₂ adsorption ability on the Au cocatalyst, caused by its intrinsic electronic structure, limits its photocatalytic performance of H₂O₂ production. According to molecular orbital theory, the adsorption energy of the adsorbate is determined by the antibonding-orbital occupancy degree between the metal sites and adsorbates. Yu's group regulated the electronic structure of Au nanoparticles by introducing MoS_x as an electron mediator to optimize the O₂ adsorption.⁷⁰ For the TiO₂/MoS_x–Au photocatalyst, Au nanoparticles were selectively deposited onto MoS_x, which was subsequently anchored on TiO₂. The electron-deficient Au^δ+ active sites emerged after introducing MoS_x, stemming from directional electron transfer from Au nanoparticles to MoS_x, as shown in Fig. 7(a). This electronic redistribution suppressed the antibonding orbital occupancy of Au–O_ads and enhanced O₂ adsorption. Pd is another typical cocatalyst for photocatalytic H₂O₂ production, showing a stronger O₂ affinity than Au owing to the symmetry-adapted linear combination of Pd 4d orbitals and O 2p orbitals. Chu's group had precisely loaded Pd and CoO_x cocatalysts on the {010} and {110} facets of BiVO₄:Mo, respectively, using a photodeposition method.⁸⁹ The Pd cocatalyst served as an active site for O₂ adsorption and steered two-electron ORR for H₂O₂ production. Further experimental results proved that the selective deposition of Pd and CoO_x cocatalysts tailored the energetics of {010} and {110} facets, respectively, thus promoting charge separation and suppressing charge carrier recombination. Moreover, Xu's group loaded a series of metal nanoparticles onto the WO₃ photocatalyst and investigated their effects on the photocatalytic H₂O₂ production.⁹⁰ They found that the order of k_f values was Pd > Au > Pt > Ag, while the order of k_d values was Pt > Pd > Ag > Au. These results revealed the distinct photocatalytic characteristics of different metal cocatalysts.

Fig. 7 Application of particulate photocatalysts with metal cocatalysts for photocatalytic H₂O₂ production. (a) Schematic of electron-deficient Au^δ+ formation to reinforce Au–O_ads bond: (1) weak Au–O_ads bond on the Au surface, (2) the formation of electron-deficient Au^δ+ sites via MoS_x incorporation, and (3) strong Au–O_ads bond on MoS_x–Au surface. Reproduced with permission.⁷⁰ Copyright 2024, Nature Publishing Group. Energy-band diagrams for (b1) the Au/TiO₂, (b2) Ag/TiO₂, and (b3) AuAg/TiO₂ heterojunctions. E_vac, E_F, Φ_M, Φ_b, and χ denote the vacuum level, Fermi level, work function of metal, Schottky barrier height, and electron affinity of the TiO₂ conduction band, respectively. (b4) Mechanism for the photocatalytic production of H₂O₂ on the AuAg/TiO₂ photocatalyst. Reproduced with permission.¹³ Copyright 2012, American Chemical Society. (c) Schematic of the charge separation process on CoO_x/BiVO₄/(Ag/Pd). Reproduced with permission.⁷¹ Copyright 2022, Nature Publishing Group. (d) End-on O₂ adsorption type on an isolated atomic site. Reproduced with permission.⁶ Copyright 2021, Springer Nature Limited.

However, the photocatalytic H₂O₂ performance is only moderately improved for the photocatalysts with monometallic cocatalysts owing to the following aspects. First, the electronic structures of monometallic cocatalysts often mismatch with the diverse energy levels of photocatalysts, and high Schottky barriers impede electron transfer. Second, the current cocatalysts usually exhibit an electron configuration mismatch between the adsorbed O₂ and the metal sites, leading to either excessively strong or weak in O₂ adsorption. Third, some cocatalysts adsorb O₂ in a side-on O₂ adsorption configuration rather than the optimum end-on O₂ adsorption model, resulting in a broken O–O bond and decreased H₂O₂ selectivity. Alloying offers a promising approach to precisely modulating the electronic and geometric configurations, thereby regulating the photocatalytic H₂O₂ production process. Tsukamoto's group developed a bimetallic AuAg alloy cocatalyst for efficient photocatalytic H₂O₂ generation.¹³ On one hand, the alloying effect significantly promoted the ORR for H₂O₂ formation. As illustrated in Fig. 7(b1)–(b4), the optimal Schottky barrier height of the AuAg alloy facilitated the efficient electron transfer from TiO₂ to the AuAg alloy. Besides, the electron-rich Au sites were created to accelerate the ORR owing to the electronegativity difference between Au and Ag. On the other hand, this alloying strategy effectively suppressed the H₂O₂ decomposition. The construction of similar core–shell structured cocatalysts is another promising strategy for finely modulating the electronic configurations of monometallic cocatalysts. Chu's group constructed Ag/Pd core/shell cocatalyst to regulate the Schottky barrier between BiVO₄ and Pd, which is a major hurdle of the charge separation.⁷¹ The Ag core with a low work function served to reduce the Schottky barrier between the {010} facet of BiVO₄, while the Pd shell with a high work function could act as the active sites to drive the two-electron ORR (Fig. 7(c)). Overall, the interaction between two metals in the cocatalyst, whether in an alloy or core–shell configuration, can effectively regulate the charge transfer dynamics and O₂ adsorption ability by tailoring both electronic and geometric structures, thus affecting the final H₂O₂ evolution performance.

4.1.2 Composites with metal clusters. Metal cluster cocatalysts, composed of several to hundreds of atoms with sub-nanometer dimensions, can exhibit higher atomic utilization than the case of metal nanoparticle cocatalysts. The local coordination environment of the metal cluster cocatalyst greatly affects its activity and stability. For example, Guo's group confined Pd clusters to fluorinated COFs for photocatalytic H₂O₂ production.⁹¹ The introduction of fluorine not only lowered the d-band center but also strengthened the metal-support interaction between Pd clusters and COFs. Specifically, the lower d-band center weakened the binding energy between the intermediate oxygen species and Pd, facilitating H₂O₂ desorption from the Pd sites. Improved stability was also observed for the photocatalyst system mainly owing to the enhanced metal-support interaction between Pd clusters and COFs.

4.1.3 Composite with single-atom metals. Unlike a nanoparticle and cluster, a single atom can achieve 100% atomic utilization efficiency and provide a well-defined active site. Therefore, a diverse range of single-atom metal cocatalysts, including noble metals, transition metals, and main group metals, has been extensively investigated for photocatalytic H₂O₂ production.^92–94

The adsorbed O₂ molecules on the metal surface can generally be categorized into three types: Pauling-type (end-on), Griffiths-type (side-on), and Yeager-type (side-on). The end-on O₂ adsorption configuration minimizes O–O bond breaking, thereby promoting high selectivity for the two-electron ORR. The isolated single-atom site facilitates O₂ adsorption in an end-on type, thereby inhibiting O–O bond breaking and improving the activity and selectivity of the two-electron ORR. Ohno's group designed an Sb single-atom decorated polymeric carbon nitride (Sb/PCN) photocatalyst for H₂O₂ production in pure water under an O₂ atmosphere.⁶ The isolated Sb atomic sites adsorbed O₂ in the end-on type, which could inhibit the O–O bond breaking and achieve a favorable two-electron ORR for H₂O₂ production (Fig. 7(d)).

Besides, the electronic structure of single-atom cocatalysts can be easily regulated by modifying their physicochemical environment. Li and co-workers designed a PCN photocatalyst modified with a Zn single atom.⁹⁵ The coordination environment of Zn sites could be tailored by changing Zn salt anions. The O atoms in the acetate anion and the N atoms in g-C₃N₄ coordinated with Zn single-atoms, forming a Zn–N₃O moiety. The favorable electronic structure of the asymmetric Zn–N₃O moiety promoted O₂ adsorption and facilitated the formation of the *OOH intermediate, thus achieving a high photocatalytic H₂O₂ evolution performance. The complex and diverse coordination structures between single-atom species and supporting materials enable the precise modulation of d-band centers. Jin and co-workers synthesized Ni single-atom-decorated C-W₁₈O₄₉ photocatalyst (C-W₁₈O₄₉/Ni-SA) using an impregnation–calcination method.⁹⁶ The downshifted d-band center of Ni single-atom led to moderate O₂ adsorption, lowered reaction energy barrier for *OOH conversion, and favorable H₂O₂ desorption. Similarly, Yamashita's group developed a hafnium-based MOF with missing-linker defects and Ni single-atoms to enhance photocatalytic H₂O₂ production from H₂O and O₂.⁷² Ni single-atoms suppressed charge recombination, and the linker defects inhibited H₂O₂ decomposition. This cooperative effect enabled the H₂O₂ production activity to be 6.3 times higher than that of the pristine MOF photocatalyst.

Moreover, single-atom cocatalysts provide an ideal platform for identifying the active sites and exploring the structure–performance relationship. Li and co-workers developed a hybrid CN/Zn-MOF photocatalyst with Zn–O₄ sites, which is composed of a Zn single atom and four coordinated O atoms.⁷² The activated Zn single-atom sites facilitated the capture of photoelectrons and the formation of the *OOH intermediate. Zhu and co-workers synthesized a Co single-atom modified CTF photocatalyst (Co_SA/Py-CTF) using a ligand engineering strategy.⁶⁹ The Co single-atoms dispersed on Py-CTF substrates were predominantly in the Co–N₃ configuration. The experimental results revealed the dynamic reconstruction behavior of the Co single-atom active sites. Specifically, Co single atoms are dynamically detached from the Py-CTF interface following O₂ adsorption under light irradiation. These released atoms were combined with their adjacent counterparts to form transient Co atom pairs, which served as the actual active sites for ORR. This newly formed active site configuration facilitated a transition in O₂ adsorption from the Pauling-type to the Yeager-type geometry and enhanced electron transfer from Py-CTF to O₂. In summary, single-atom cocatalysts serve as an ideal platform for elucidating the dynamic evolution of active sites and uncovering the structure–performance relationship. The unique geometric structure and distinctive electronic properties of single-atom cocatalysts enable the achievement of specific O₂ adsorption configurations, which can facilitate the O₂ adsorption, intermediate evolution, and H₂O₂ desorption.

4.2 Composites with semiconductors

The surface modification of semiconductors by compositing with another semiconductor to form a heterojunction is a powerful strategy that can effectively regulate light absorbance and charge separation. It can be classified into several primary types based on the alignment of the energy levels of the two semiconductors (Fig. 8(a)–(c)), which are type I, type II, and type III. In the type I heterojunction, the band alignment is straddling, where the CBM and VBM levels of the wider bandgap semiconductor are more negative and more positive, respectively, than those of the narrower bandgap semiconductor. In this band alignment, charge carriers tend to accumulate in the narrower bandgap semiconductor, resulting in weaker redox ability and no improvement in charge separation. In the type III heterojunction, the VBM level of one semiconductor lies above the CBM level of the other semiconductor, resulting in no charge transfer between these two semiconductors. Both type I and type III heterojunctions show unfavorable charge transfer characteristics. In contrast, the type II heterojunction features a staggered band alignment in which the CBM and VBM levels of the two semiconductors are offset. This configuration enables the photogenerated electrons and holes to migrate to two different semiconductors, making the spatial separation of charge carriers feasible.^97,98

Fig. 8 Mechanisms of typical heterojunctions and their application for photocatalytic H₂O₂ production. Schematic energy diagrams of three different types of heterojunctions: (a) type I, (b) type II, and (c) type III. (d) Photocatalytic production of CO and H₂O₂ over NH₂-MIL-125-Ti/WO_3–x heterojunction photocatalyst. Reproduced with permission.⁷³ Copyright 2023, Elsevier. (e) The mechanism of photocatalytic H₂O₂ production over the CuO/Cu(OH)₂@g-C₃N₄ composite. Reproduced with permission.⁷⁴ Copyright 2024, Elsevier.

Besides those different types of heterojunctions, another different heterostructure of the Z-scheme has recently been extensively investigated. The original idea was first proposed in 1979 but was experimentally demonstrated in 2001 with the application of the photocatalytic overall water splitting reaction.^99,100 Z-scheme systems can be classified into several types according to the difference in mediators and system configurations: aqueous redox mediator in a powder suspension system, solid-state electron mediator in a powder suspension system, and solid-state electron mediator in a sheet system. It is noteworthy that two photocatalysts with well-matched band structures and surface configurations may form a Z-scheme mechanism directly, driving the overall photocatalytic reaction even without the electron mediator. The photocatalytic H₂O₂ production using a Z-scheme heterojunction photocatalyst is currently conducted following the above approach. In this case, the reduction reaction tends to occur on the semiconductor with a more negative CBM, whereas the oxidation reaction occurs on the other. Then, the remaining holes and electrons in the two semiconductors recombine via the interparticle physical contact or a solid-state electron mediator to complete the entire reaction cycle. Efficient charge recombination at the interface is achieved to enable more photogenerated charge carriers to participate in the corresponding redox reactions.¹⁰¹ Herein, the Z-scheme heterojunction without a solid-state electron mediator is similar to an S-scheme heterojunction.^102,103

Enhanced charge separation efficiency can be achieved by constructing effective heterojunction structures.^104,105 He and co-workers constructed a heterojunction photocatalyst composed of organic amine-constrained ions intercalated with carbon nitride and CdSe-diethylenetriamine (K⁺/I⁻-CN/CdSe-D) for H₂O₂ production.⁷⁵ The formation of C–Se bonds at the interface between K⁺/I⁻-CN and CdSe-D significantly improved the electron conductivity and enhanced the charge carrier transfer efficiency. Jiang and co-workers developed an NH₂-MIL-125-Ti/WO_3–x heterojunction photocatalyst to promote charge separation and transfer by regulating the intensity of the interfacial electric field, which could be increased by maximizing the Fermi level difference between WO_3–x and NH₂-MIL-125-Ti (Fig. 8(d)).⁷³ In addition, heterojunction photocatalysts demonstrate unique advantages in enabling dual-channel H₂O₂ synthesis. Generally, it is challenging for a single semiconductor to achieve dual-channel photocatalytic H₂O₂ production since rigorous redox ability is thermodynamically needed. It is possible to promote charge separation while preserving the strong redox abilities of semiconductors by constructing heterojunctions. For example, Qin and co-workers designed a CuO/Cu(OH)₂@g-C₃N₄ heterojunction photocatalyst for dual-channel H₂O₂ production.⁷⁴ In this system, g-C₃N₄ with the strong reduction ability enabled two-step one-electron ORR, while CuO with sufficient oxidation ability could drive the WOR (Fig. 8(e)).

4.3 Composite with carbon nanomaterials

Carbon nanomaterials formed by sp² or sp³ hybridization of carbon atoms are commonly used for compositing owing to their unique advantages, including high specific surface area, excellent electrical conductivity, tunable electronic structures, and remarkable chemical stability.¹⁰⁶ Therefore, various carbon nanomaterials, including carbon quantum dots (CQDs), carbon nanotubes (CNTs), and reduced graphene oxide (rGO), have been incorporated. Specifically, the integration of carbon nanomaterials with semiconductors offers a powerful strategy to extend light absorption, improve charge transfer, and promote the O₂ adsorption, leading to a boosted photocatalytic H₂O₂ evolution rate. Chen and co-workers developed a ternary photocatalyst of α-Fe₂O₃/CQD@g-C₃N₄ for H₂O₂ production.⁷⁶ The up-conversion fluorescence spectra revealed that the CQDs featured the highest fluorescence emission intensity at approximately 445 nm, while the light absorption extended into the near-infrared region on account of the up-conversion of the loaded CQDs. In addition, the enhanced charge separation and transfer can also be achieved by incorporating carbon nanomaterials owing to their high conductivity and exceptional electron capture and storage abilities. For instance, Yang and co-workers designed a CTF photocatalyst with CQDs embedded within its porous structure as an electron transport medium, leading to promoted charge separation and transfer efficiency.⁷⁷ Interestingly, carbon nanomaterials can also act as active sites for the two-electron ORR with high activity and selectivity. Choi's group designed a rGO/TiO₂ photocatalyst and demonstrated that rGO could act as an ORR cocatalyst.⁷⁸ Beyond boosting photocatalytic H₂O₂ production, carbon nanomaterials play a pivotal role in reinforcing the structural and chemical stability of photocatalysts. For example, Zhu's group prepared rGO-modified black phosphorus quantum dots (rGO@BPQDs) and anchored them on g-C₃N₄ via π–π interactions and self-trapping pore confinement effect.¹⁰⁷ The oxygen-bridged rGO encapsulation created a robust core–shell architecture that effectively suppressed BPQD degradation, enhancing both thermal stability and oxidation resistance by 3.2-fold compared to bare BPQDs. These findings establish carbon nanomaterials as indispensable stabilizers for designing composite photocatalysts that combine high H₂O₂ productivity with long-term operational durability.

4.4 Composites with POMs or their derivatives

POMs are metal-oxygen cluster compounds composed of oxygen and pre-transition metal ions (such as V, Mo, and W) in high oxidation states. POMs have abundant active sites owing to the abundant metal centers within their well-defined structures. Moreover, POMs can regulate the band structure and electron transfer efficiency of photocatalysts. Zhao and co-workers developed a novel g-C₃N₄-PW₁₁ hybrid photocatalyst by combining g-C₃N₄ and a polyoxometalate cluster of [PW₁₁O₃₉]⁷⁻ (PW₁₁) with an organic linker.⁷⁹ The covalent combination of PW₁₁ and g-C₃N₄ could positively shift the VBM level, which was favorable for WOR. In addition, the g-C₃N₄-PW₁₁ exhibited enhanced separation of charge carriers. They further prepared a hybrid photocatalyst of g-C₃N₄-SiW₁₁ through the covalent combination of g-C₃N₄ and the polyoxometalate cluster of [SiW₁₁O₃₉]⁸⁻ (SiW₁₁).⁸⁰ The introduction of SiW₁₁ enhanced the O₂ adsorption ability, which could facilitate the two-electron ORR for H₂O₂ production. In addition, considering that the POMs derived metal oxides can accept, transport and store electrons, they constructed a hybrid photocatalyst of g-C₃N₄ modified with POM-derived metal oxides by calcinating the precursors of g-C₃N₄ and POMs.¹⁰⁸ The enhanced photocatalytic H₂O₂ performance of g-C₃N₄-PWO was mainly attributed to the enhanced electron transfer efficiency and improved O₂ adsorption capacity. Therefore, it can be concluded that owing to the distinct structural and functional versatility of POMs and their derivatives, the corresponding composite photocatalysts can be further regulated to promote photocatalytic H₂O₂ production.

4.5 Composites with organic compounds

The primary advantage of using organic compounds to modify semiconductors lies in their versatility and flexibility owing to the diversity of organic compounds and available modification methods, such as chemical bonding, self-assembly, and physical adsorption. Thus, the photocatalytic H₂O₂ performance can be effectively regulated in terms of charge separation, reactant adsorption and activation. For example, Hirai's group designed an electron-deficient biphenyl diimide (BDI) unit-modified g-C₃N₄ photocatalyst and investigated its mechanism of photocatalytic H₂O₂ production in the presence of pure water and O₂.⁸¹ Experimental and theoretical calculation results revealed that the photogenerated electrons were localized on the melem units while holes were localized on the BDI units. Such spatial charge separation could effectively prohibit the recombination of charge carriers, leading to enhanced H₂O₂ production. Song's group designed an SA-modified TiO₂ photocatalyst for H₂O₂ evolution (Fig. 9(a)).⁴³ The SA modification increased the O₂ accessibility in the interfacial microenvironment. Organic compounds adsorbed or complexed on the photocatalyst's surface can also influence the photocatalytic H₂O₂ production. For example, Long's group investigated the effect of alcohol complexes on the photocatalyst surface for H₂O₂ production.⁸² The surface complexation of benzyl alcohol (BA) or furfural alcohol (FFA) on TiO₂ demonstrated the dual functionality of enhancing H₂O₂ evolution and inhibiting H₂O₂ decomposition (Fig. 9(b)). Taking FFA as an example, the strong interaction between TiO₂ surface and FFA induces a negative shift in the flat band potential, which is favorable for ORR. Besides, the formation of stable FFA complexes suppresses the H₂O₂ adsorption and subsequent decomposition. These synergistic effects collectively promote the H₂O₂ production efficiency.

Fig. 9 Surface composite of particulate photocatalysts with organic compounds for photocatalytic H₂O₂ production. (a) Scheme of H₂O₂ production on the stearic acid modified TiO₂ photocatalyst. Reproduced with permission.⁴³ Copyright 2023, American Chemical Society. (b) A two-electron ORR pathway for H₂O₂ formation on TiO₂ in the presence of benzyl alcohol (BA) and furfural alcohol (FFA). Reproduced with permission.⁸² Copyright 2020, Elsevier.

In this section, surface composite strategies, including the integration of light-absorbing semiconductors with metals, other semiconductors, carbon materials, POMs, and organic compounds, can significantly enhance photocatalytic H₂O₂ production performance. Semiconductors with insufficient surface active sites can be composited with metals or POMs to enhance intrinsic activity, yet the development of metals and POMs specifically for H₂O₂ synthesis remains limited. Semiconductors with poor charge transfer characteristics can be effectively alleviated by compositing with carbon nanomaterials or other semiconductors. Moreover, to achieve efficient dual-channel H₂O₂ synthesis, compositing with other semiconductors to construct a well-designed heterojunction structure is essential. Surface composition strategies should also be a key focus in future studies to facilitate the development of high-performance photocatalytic systems.

5. Summary and outlook

Based on the surface engineering strategies of particulate semiconductor photocatalysts, the recent progress in surface modification and composite strategies to achieve efficient and stable photocatalytic H₂O₂ production has been systematically summarized in this review, which is anticipated to offer valuable guidance for the design and fabrication of advanced photocatalysts for efficient solar H₂O₂ synthesis. To date, significant progress has been achieved in photocatalytic H₂O₂ production, especially in terms of the design and construction of efficient particulate photocatalysts with tailored surface modifications. However, the study of photocatalytic H₂O₂ production is still in the laboratory mainly because of the challenges of charge utilization and H₂O₂ decomposition. To further address these great challenges, the surface engineering strategies described here can provide a probable approach that is expected to focus on the following aspects.

(1) It is essential to deepen the understanding of reaction mechanisms and the structure–performance relationship, which can provide a theoretical basis for modifying particulate photocatalysts to promote H₂O₂ production. Herein, advanced in situ characterization techniques and theoretical calculations are of vital importance to clarify the process mechanisms. Advanced in situ characterization techniques, such as in situ environmental transmission electron microscope, operando X-ray absorption fine structure measurements, and in situ Fourier transform infrared spectroscopy, combined with theoretical calculations can reveal microstructural evolution, interfacial reactions, and intermediate dynamics.

(2) It is necessary to rapidly screen and fabricate complicated surface modifiers over particulate photocatalysts to achieve efficient H₂O₂ production. Traditionally, finding the optimal combinations of photocatalyst and surface engineering strategies through trial-and-error testing of numerous possibilities is time-consuming. Herein, emerging computational and data-driven tools for photocatalyst design are highly suggested. Machine learning revolutionizes photocatalyst design through data-driven predictive modeling, which uncovers structure–performance relationships across overall chemical spaces and is expected to achieve optimal surface properties of the photocatalyst. Data-driven high-throughput screening using reliable descriptors offers a faster alternative for screening the optimal combinations of photocatalyst and surface engineering strategies and identifying high-performance systems. Large language models can autonomously design, manage, and execute multi-step experiments for developing efficient H₂O₂ photocatalysts. Integrating the emerging computational and data-driven tools into lab workflows shows a strong potential to perform complex tasks with minimal human input, promising to revolutionize research efficiency and innovation.

(3) It is highly expected to achieve the continuous synthesis of high-concentration and high-purity H₂O₂ based on modified particulate photocatalysts. The significantly boosted H₂O₂ formation and remarkably prohibited H₂O₂ decomposition should be ensured to achieve high-concentration H₂O₂ synthesis, in which both the rational surface engineering of photocatalysts and the well-designed reaction system need to be developed. It is essential to avoid the use of sacrificial reagents and other additives through surface strategies to produce a high-purity H₂O₂ solution. To enhance the long-term stability and reusability of photocatalysts, it is necessary to immobilize particulate photocatalysts on the sheet substrate, in which effective surface strategies are essential to construct and stabilize the active sites. Furthermore, developing cost-effective surface engineering strategies with consideration of synthesis complexity and scalability for large-scale production is necessary for future real-world applications.

Photocatalytic H₂O₂ evolution using particulate photocatalysts has garnered significant attention as a scalable and cost-effective approach to sustainable H₂O₂ production. In recent decades, remarkable fundamental research progress in this field has been made, achieving a maximum H₂O₂ concentration of tens of millimoles per liter. This concentration is close to the level of the practical application; for example, a concentration of 1–3 wt% (294–890 mM) is used in H₂O₂ disinfectants. Therefore, it is anticipated that photocatalytic H₂O₂ evolution can be developed for future industrial applications. It is our sincere hope that this review will attract more researchers to boost this field and accelerate its development progress in both fundamental and application aspects.

Author contributions

Y. Z. and Z. C. conceived the review and wrote the manuscript. M. L., S. Y. and S. C. conceived the review, reviewed the manuscript, and revised it. All the authors have given approval to the final version of the article and have agreed to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Conflicts of interest

There are no conflicts to declare.

Data availability

No primary research results, software or code has been included and no new data were generated or analysed apart of this review.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (22272082, 22372001), the Funds for International Cooperation and Exchange of the National Natural Science Foundation of China (W2421036), Anhui Natural Science Foundation for Outstanding Young Scholars (2408085Y008), the Fundamental Research Funds for the Central Universities, Nankai University (63213098) and Starting Fund for Scientific Research of High-Level Talents, Anhui Agricultural University (rc382108).

Notes and references

1. P. Liu, T. Liang, Y. Li, Z. Zhang, Z. Li, J. Bian and L. Jing, Nat. Commun., 2024, 15, 7823.
2. X. Zeng, Y. Liu, X. Hu and X. Zhang, Green Chem., 2021, 23, 1466–1494.
3. X. Hu, X. Zeng, Y. Liu, J. Lu and X. Zhang, Nanoscale, 2020, 12, 16008–16027.
4. H. Li, W. Wang, K. Xu, B. Cheng, J. Xu and S. Cao, Chin. J. Catal., 2025, 72, 24–47.
5. N. Lewis, Science, 2007, 315, 798–801.
6. Z. Teng, Q. Zhang, H. Yang, K. Kato, W. Yang, Y. Lu, S. Liu, C. Wang, A. Yamakata, C. Su, B. Liu and T. Ohno, Nat. Catal., 2021, 4, 374–384.
7. G. Duca and S. Travin, Am. J. Phys. Chem., 2020, 9, 139371.
8. J. Chang, Q. Li, J. Shi, M. Zhang, L. Zhang, S. Li, Y. Chen, S. Li and Y. Lan, Angew. Chem., Int. Ed., 2023, 62, e202218868.
9. K. Zhang, J. Liu, L. Wang, B. Jin, X. Yang, S. Zhang and J. Park, J. Am. Chem. Soc., 2020, 142, 8641–8648.
10. C. Zhao, X. Wang, S. Ye and J. Liu, Sol. RRL, 2022, 6, 2200427.
11. R. Service, Science, 2005, 309, 548–551.
12. Z. Chen, C. Chu, D. Yao, Q. Li and S. Mao, Adv. Funct. Mater., 2024, 34, 2400506.
13. D. Tsukamoto, A. Shiro, Y. Shiraishi, Y. Sugano, S. Ichikawa, S. Tanaka and T. Hirai, ACS Catal., 2012, 2, 599–603.
14. Q. Tian, X. Zeng, C. Zhao, L. Jing, X. Zhang and J. Liu, Adv. Funct. Mater., 2023, 33, 2213173.
15. J. Ran, W. Guo, H. Wang, B. Zhu, J. Yu and S. Qiao, Adv. Mater., 2018, 30, 1800128.
16. Y. Shiraishi, T. Takii, T. Hagi, S. Mori, Y. Kofuji, Y. Kitagawa, S. Tanaka, S. Ichikawa and T. Hirai, Nat. Mater., 2019, 18, 985–993.
17. J. Sun, H. Jena, C. Krishnaraj, K. Rawat, S. Abednatanzi, J. Chakraborty, A. Laemont, W. Liu, H. Chen, Y. Liu, K. Leus, H. Vrielinck, V. Speybroeck and P. Voort, Angew. Chem., Int. Ed., 2023, 62, e202216719.
18. C. Zhang, L. Yuan, C. Liu, Z. Li, Y. Zou, X. Zhang, Y. Zhang, Z. Zhang, G. Wei and C. Yu, J. Am. Chem. Soc., 2023, 145, 7791–7799.
19. X. Zhang, P. Ma, C. Wang, L. Gan, X. Chen, P. Zhang, Y. Wang, H. Li, L. Wang, X. Zhou and K. Zheng, Energy Environ. Sci., 2022, 15, 830–842.
20. Z. Yong and T. Ma, Angew. Chem., Int. Ed., 2023, 62, e202308980.
21. Y. Li, D. Luan and X. Lou, Adv. Mater., 2024, 36, 2412386.
22. J. Yang, X. Zeng, M. Tebyetekerwa, Z. Wang, C. Bie, X. Sun, I. Marriam and X. Zhang, Adv. Energy Mater., 2024, 14, 2400740.
23. W. Yu, C. Hu, L. Bai, N. Tian, Y. Zhang and H. Huang, Nano Energy, 2022, 104, 107906.
24. C. Zhao, H. Li, Y. Yin, W. Tian, X. Yan, J. He, Z. Chen, S. Ye and J. Liu, Angew. Chem., Int. Ed., 2025, 64, e202420895.
25. J. Wang, Y. Zhang, S. Jiang, C. Sun and S. Song, Angew. Chem., Int. Ed., 2023, 62, e202307808.
26. T. Tian, Z. Wang, K. Li, H. Jin, Y. Tang, Y. Sun, P. Wan and Y. Chen, Materials, 2024, 17, 1748.
27. Z. Wang, C. Li and K. Domen, Chem. Soc. Rev., 2019, 48, 2109–2125.
28. D. Chen, W. Chen, Y. Wu, L. Wang, X. Wu, H. Xu and L. Chen, Angew. Chem., Int. Ed., 2023, 62, e202217479.
29. Y. Huang, M. Shen, H. Yan, Y. He, J. Xu, F. Zhu, X. Yang, Y. Ye and G. Ouyang, Nat. Commun., 2024, 15, 5406.
30. Y. Kofuji, Y. Isobe, Y. Shiraishi, H. Sakamoto, S. Tanaka, S. Ichikawa and T. Hirai, J. Am. Chem. Soc., 2016, 138, 10019–10025.
31. Y. Bo, C. Gao and Y. Xiong, Nanoscale, 2020, 12, 12196–12209.
32. Y. Gao, X. Zhang, C. Ban, R. Feng, J. Hou, J. Meng, G. Yang, C. Gao, L. Xia, P. Ma, K. Wang and X. Qu, Mater. Today Energy, 2024, 40, 101483.
33. Y. Zheng, H. Zhou, B. Zhou, J. Mao and Y. Zhao, Catal. Sci. Technol., 2022, 12, 969–975.
34. Y. Xu, H. Fu, L. Zhao, L. Jian, Q. Liang and X. Xiao, New J. Chem., 2021, 45, 3335–3342.
35. S. Li, G. Dong, R. Hailili, L. Yang, Y. Li, F. Wang, Y. Zeng and C. Wang, Appl. Catal., B, 2016, 190, 26–35.
36. L. Shi, L. Yang, W. Zhou, Y. Liu, L. Yin, X. Hai, H. Song and J. Ye, Small, 2018, 14, 1703142.
37. Z. Tian, C. Han, Y. Zhao, W. Dai, X. Lian, Y. Wang, Y. Zheng, Y. Shi, X. Pan, Z. Huang, H. Li and W. Chen, Nat. Commun., 2021, 12, 2039.
38. C. Wu, Z. Teng, C. Yang, F. Chen, H. Yang, L. Wang, H. Xu, B. Liu, G. Zheng and Q. Han, Adv. Mater., 2022, 34, 2110266.
39. L. Li, X. Lv, Y. Xue, H. Shao, G. Zheng and Q. Han, Angew. Chem., Int. Ed., 2024, 63, e202320218.
40. E. Zhou, F. Wang, X. Zhang, Y. Hui and Y. Wang, Angew. Chem., Int. Ed., 2024, 63, e202400999.
41. M. Teranishi, R. Hoshino, S. Naya and H. Tada, Angew. Chem., Int. Ed., 2016, 55, 12773–12777.
42. S. Bai, L. Wang, Z. Li and Y. Xiong, Adv. Sci., 2016, 4, 1600216.
43. Z. Chen, H. Chen, K. Wang, J. Chen, M. Li, Y. Wang, P. Tsiakaras and S. Song, ACS Catal., 2023, 13, 6497–6508.
44. Y. Shao, Y. Zhang, C. Chen, S. Dou, Y. Lou, Y. Dong, Y. Zhu and C. Pan, Chin. J. Catal., 2024, 61, 205–214.
45. Y. Wang, W. Yu, C. Wang, F. Chen, T. Ma and H. Huang, eScience, 2024, 4, 100228.
46. D. Maarisetty and S. S. Baral, J. Mater. Chem. A, 2020, 8, 18560–18604.
47. F. N. Habarugira, D. Yao, W. Miao, C. Chu, Z. Chen and S. Mao, Chin. Chem. Lett., 2024, 35, 109886.
48. S. Bai, N. Zhang, C. Gao and Y. Xiong, Nano Energy, 2018, 53, 296–336.
49. W. Chi, B. Liu, Y. Dong, J. Zhang, X. Sun, C. Pan, H. Zhao, Y. Ling and Y. Zhu, Appl. Catal., B, 2024, 355, 124077.
50. W. Li, B. Han, Y. Liu, J. Xu, H. He, G. Wang, J. Li, Y. Zhai, X. Zhu and Y. Zhu, Angew. Chem., Int. Ed., 2024, 64, e202421356.
51. X. Zhang, S. Cheng, C. Chen, X. Wen, J. Miao, B. Zhou, M. Long and L. Zhang, Nat. Commun., 2024, 15, 2649.
52. Z. Xie, X. Chen, W. Wang, X. Ke, X. Zhang, S. Wang, X. Wu, J. Yu and X. Wang, Angew. Chem., Int. Ed., 2024, 63, e202410179.
53. Y. Guo, Y. Dong, B. Liu, B. Ni, C. Pan, J. Zhang, H. Zhao, G. Wang and Y. Zhu, Adv. Funct. Mater., 2024, 34, 2402920.
54. Y. Lin, J. Zou, X. Wu, S. Tong, Q. Niu, S. He, S. Luo and C. Yang, Nano Lett., 2024, 24, 6302–6311.
55. K. Li, Q. Ge, Y. Liu, L. Wang, K. Gong, J. Liu, L. Xie, W. Wang, X. Ruan and L. Zhang, Energy Environ. Sci., 2023, 16, 1135–1145.
56. L. Chen, C. Chen, Z. Yang, S. Li, C. Chu and B. Chen, Adv. Funct. Mater., 2021, 31, 2105731.
57. Y. Luo, B. Zhang, C. Liu, D. Xia, X. Ou, Y. Cai, Y. Zhou, J. Jiang and B. Han, Angew. Chem., Int. Ed., 2023, 62, e202305355.
58. N. Li, C. Wang, W. Gao, W. Li, B. Li and X. Wang, Sep. Purif. Technol., 2025, 356, 129978.
59. H. Yu, F. Zhang, Q. Chen, P. Zhou, W. Xing, S. Wang, G. Zhang, Y. Jiang and X. Chen, Angew. Chem., Int. Ed., 2024, 63, e202402297.
60. S. Li, R. Ma, C. Tu, W. Zhang, R. Li, Y. Zhao and K. Zhang, Angew. Chem., Int. Ed., 2024, 64, e202421040.
61. Q. Zhu, L. Shi, Z. Li, G. Li and X. Xu, Angew. Chem., Int. Ed., 2024, 63, e202408041.
62. P. Dong, X. Xu, T. Wu, R. Luo, W. Kong, Z. Xu, S. Yuan, J. Zhou and J. Lei, Angew. Chem., Int. Ed., 2024, 63, e202405313.
63. R. Ma, L. Wang, H. Wang, Z. Liu, M. Xing, L. Zhu, X. Meng and F. Xiao, Appl. Catal., B, 2019, 244, 594–603.
64. D. Wen, Y. Su, J. Fang, D. Zheng, Y. Xu, S. Zhou, A. Meng, P. Han and C. Wong, Nano Energy, 2023, 117, 108917.
65. V. Maurino, C. Minero, G. Mariella and E. Pelizzetti, Chem. Commun., 2005, 2627–2629.
66. X. Sun, J. Yang, X. Zeng, L. Guo, C. Bie, Z. Wang, K. Sun, A. K. Sahu, M. Tebyetekerwa, T. E. Rufford and X. Zhang, Angew. Chem., Int. Ed., 2024, 63, e202414417.
67. L. Wang, J. Zhang, Y. Zhang, H. Yu, Y. Qu and J. Yu, Small, 2021, 18, 2104561.
68. C. Jin, H. Shen, J. Li, X. Guo, S. Rao, W. Yang, Q. Liu, Z. Sun and J. Yang, Nano Lett., 2024, 24, 14484–14492.
69. C. Zhu, Y. Yao, Q. Fang, S. Song, B. Chen and Y. Shen, ACS Catal., 2024, 14, 2847–2858.
70. X. Zhang, D. Gao, B. Zhu, B. Cheng, J. Yu and H. Yu, Nat. Commun., 2024, 15, 3212.
71. T. Liu, Z. Pan, K. Kato, J. J. M. Vequizo, R. Yanagi, X. Zheng, W. Yu, A. Yamakata, B. Chen, S. Hu, K. Katayama and C. Chu, Nat. Commun., 2022, 13, 7783.
72. Y. Li, Y. Guo, D. Luan, X. Gu and X. Lou, Angew. Chem., Int. Ed., 2023, 62, e202310847.
73. H. Jiang, L. Wang, X. Yu, L. Sun, J. Li, J. Yang and Q. Liu, Chem. Eng. J., 2023, 466, 143129.
74. L. Qin, P. She, P. Si, S. Liu, G. Dai and C. Pan, Environ. Res., 2025, 264, 120309.
75. J. Hu, B. Li, X. Li, T. Yang, X. Yang, J. Qu, Y. Cai, H. Yang and Z. Lin, Adv. Mater., 2024, 36, 2412070.
76. X. Chen, W. Zhang, L. Zhang, L. Feng, C. Zhang, J. Jiang, T. Yan and H. Wang, J. Mater. Chem. A, 2020, 8, 18816–18825.
77. Y. Yang, Q. Guo, Q. Li, L. Guo, H. Chu, L. Liao, X. Wang, Z. Li and W. Zhou, Adv. Funct. Mater., 2024, 34, 2400612.
78. G. Moon, W. Kim, A. D. Bokare, N. Sung and W. Choi, Energy Environ. Sci., 2014, 7, 4023–4028.
79. S. Zhao, X. Zhao, H. Zhang, J. Li and Y. Zhu, Nano Energy, 2017, 35, 405–414.
80. S. Zhao, X. Zhao, S. Ouyang and Y. Zhu, Catal. Sci. Technol., 2018, 8, 1686–1695.
81. Y. Kofuji, S. Ohkita, Y. Shiraishi, H. Sakamoto, S. Tanaka, S. Ichikawa and T. Hirai, ACS Catal., 2016, 6, 7021–7029.
82. J. Zhang, L. Zheng, F. Wang, C. Chen, H. Wu, S. A. K. Leghari and M. Long, Appl. Catal., B, 2020, 269, 118770.
83. Y. Pan, W. Si, Y. Li, H. Tan, J. Liang and F. Hou, Chin. Chem. Lett., 2024, 35, 109877.
84. J. Cai, J. Huang, S. Wang, J. Iocozzia, Z. Sun, J. Sun, Y. Yang, Y. Lai and Z. Lin, Adv. Mater., 2019, 31, 1806314.
85. R. Du, K. Xiao, B. Li, X. Han, C. Zhang, X. Wang, Y. Zuo, P. Guardia, J. Li, J. Chen, J. Arbiol and A. Cabot, Chem. Eng. J., 2022, 441, 135999.
86. J. Yang, D. Wang, H. Han and C. Li, Acc. Chem. Res., 2013, 46, 1900–1909.
87. H. Hirakawa, S. Shiota, Y. Shiraishi, H. Sakamoto, S. Ichikawa and T. Hirai, ACS Catal., 2016, 6, 4976–4982.
88. Z. Chen, D. Yao, C. Chu and S. Mao, Chem. Eng. J., 2023, 451, 138489.
89. T. Liu, Z. Pan, J. J. M. Vequizo, K. Kato, B. Wu, A. Yamakata, K. Katayama, B. Chen, C. Chu and K. Domen, Nat. Commun., 2022, 13, 1034.
90. Y. Wang, Y. Wang, J. Zhao, M. Chen, X. Huang and Y. Xu, Appl. Catal., B, 2021, 284, 119691.
91. Y. Liu, L. Li, H. Tan, N. Ye, Y. Gu, S. Zhao, S. Zhang, M. Luo and S. Guo, J. Am. Chem. Soc., 2023, 145, 19877–19884.
92. W. Cai, Y. Tanaka, X. Zhu and T. Ohno, Nano Res., 2024, 17, 7027–7038.
93. R. Zhang, H. Xu, Z. Huang, J. Zhang, L. Liu, Z. Ma, Z. Zhang, K. Wang, P. Liu, H. Liu and X. Zheng, Adv. Funct. Mater., 2024, 35, 2420504.
94. H. Tan, P. Zhou, M. Liu, Y. Gu, W. Chen, H. Guo, J. Zhang, K. Yin, Y. Zhou, C. Shang, Q. Zhang, L. Gu, N. Zhang, J. Ma, Z. Zheng, M. Luo and S. Guo, J. Am. Chem. Soc., 2024, 146, 31950–31960.
95. Y. Li, Y. Guo, G. Fan, D. Luan, X. Gu and X. Lou, Angew. Chem., Int. Ed., 2024, 63, e202317572.
96. Y. Kondo, K. Honda, Y. Kuwahara, K. Mori, H. Kobayashi and H. Yamashita, ACS Catal., 2022, 12, 14825–14835.
97. S. Chen, Y. Qi, C. Li, K. Domen and F. Zhang, Joule, 2018, 2, 2260–2288.
98. S. Chen, C. Li, K. Domen and F. Zhang, Joule, 2023, 7, 2445–2467.
99. R. Abe, K. Sayama, K. Domen and H. Arakawa, Chem. Phys. Lett., 2001, 344, 339–344.
100. A. J. Bard, J. Photochem., 1979, 10, 59–75.
101. J. Cheng, W. Wang, J. Zhang, S. Wan, B. Cheng, J. Yu and S. Cao, Angew. Chem., Int. Ed., 2024, 63, e202406310.
102. K. Meng, J. Zhang, B. Cheng, X. Ren, Z. Xia, F. Xu, L. Zhang and J. Yu, Adv. Mater., 2024, 36, 2406460.
103. L. Wang, J. Sun, B. Cheng, R. He and J. Yu, J. Phys. Chem. Lett., 2023, 14, 4803–4814.
104. X. Sun, L. Li, S. Jin, W. Shao, H. Wang, X. Zhang and Y. Xie, eScience, 2023, 3, 100095.
105. H. He, Z. Wang, J. Zhang, C. Shao, K. Dai and K. Fan, Adv. Funct. Mater., 2024, 34, 2315426.
106. D. Sundar, C. Liu, S. Anandan and J. Wu, Molecules, 2023, 28, 5383.
107. J. Xiong, X. Li, J. Huang, X. Gao, Z. Chen, J. Liu, H. Li, B. Kang, W. Yao and Y. Zhu, Appl. Catal., B, 2020, 266, 118602.
108. S. Zhao and X. Zhao, J. Catal., 2018, 366, 98–106.

---

Footnote

† These authors contributed equally to the work.

---