English

• 1. Introduction

  H₂O₂ is an important efficient and versatile oxidizer and disinfectant, which has been widely used in chemical [1-5], medical [6-10], energy [11-15], and environmental fields [16-20] since its first synthesis by in 1818. Currently, commercially available H₂O₂ is mainly produced by electrolysis [21-25], conventional anthraquinone method [26-30], alcohol oxidation [31-35], cathodic reduction [36-40], and direct hydroxide oxidation [41-45]. Among them, anthraquinone method is the main method for industrial production of H₂O₂, which can produce H₂O₂ in large quantities, and its process is relatively stable and not easy to produce side reactions, but the process method is energy-consuming and costly, and there are also certain safety risks. Hence, there exists an imperative and pressing need to develop technologies for the safe, efficient, and environmentally benign production of H₂O₂. In this context, the deployment of photocatalytic and photoelectrocatalytic technology for the direct production of H₂O₂ from renewable energy sources has emerged as a highly significant research avenue [46-50]. Over the preceding decade, a significant annual escalation in both the quantity of scholarly publications and the frequency of citations pertaining to the photocatalytic and photoelectrocatalytic H₂O₂ production has been observed (Fig. S1 in Supporting information).

  The key to photocatalytic and photoelectrocatalytic technology is to develop high-efficiency and stable photocatalysts. The photocatalyst and photoelectrocatalyst can capture photons from the sun and generate electron-hole pairs, which causes oxidation and reduction reactions [51-55]. H₂ generation [56-60], N₂ fixation [61-65], antibiotic degradation [66-70] are the key research content of today's society. The photocatalytic H₂O₂ production necessitates the utilization of solar energy as a direct energy source, precluding the need for any intermediate energy conversion processes [71-75]. The photocatalytic process primarily entails the absorption of photons, the subsequent separation of photogenerated charge carriers, and the interfacial transfer of these carriers to facilitate the requisite reactions on the semiconductor photocatalyst surface [76-79]. Since the inaugural instance of employing ZnO for the H₂O₂ production [80], there has been a surge of research interest over the past few decades, focusing on the development of high-performance catalysts for H₂O₂ production under illumination conditions. The development and engineering of highly active and selective photocatalysts, achieved through the modulation of their intrinsic properties, have yielded a plethora of remarkable outcomes. Despite significant advancements, the solar-to-chemical conversion (SCC) efficiency of the photocatalysts remains unsatisfactory, with a maximum efficiency of only 10.1% [81]. Furthermore, the pinnacle of the apparent quantum efficiency (AQE) for non-sacrificial H₂O₂ production has reached 1.19% [82]. The principal challenges that constrain the efficiency of H₂O₂ production pertain to the limited light absorption range, swift charge recombination, sluggish surface kinetics for water oxidation, and diminished selectivity for the reduction of O₂ [83-85].

  Photocatalysts and photoelectrocatalysts are generally in powder form, belonging to the nano-scale, dispersed in water, which is difficult to be recycled and reutilized, creating unnecessary waste in the experimental process. This realization has spurred researchers to explore more efficacious strategies aimed at enhancing the catalytic efficiency and optimizing the utilization of photocatalysts [86-88]. After continuous exploration, research workers found that the catalyst can be fixed on the substrate, and by applying an electric field, the photogenerated charges can be separated with greater efficiency under the synergistic effect of the internal and external electric fields, and the photogenerated electrons can be introduced into the counter electrode through the external circuit, weakening the photocatalytic catalyst charge compounding situation, and then an electrochemistry-assisted photocatalytic technology was developed [89-91], photoelectrocatalytic technology [92-95]. When light fields are introduced into conventional electrocatalytic techniques, the electronic properties of specific electrocatalysts are susceptible to disturbances, such as electron transfer, energy band bending, Fermi energy levels, and intermediate desorption energies, which can significantly alter the intrinsic pathways and performance of catalysis. Compared with monoelectrocatalysis, photoelectrocatalysis has higher energy efficiency and potential to activate small molecules at low overpotentials. Photoelectrochemical reactions are photomagnetization processes that generate photogenerated electrons-holes on semiconductor surfaces in contact with an electrolyte [96-100]. The electrons and holes are segregated on the catalyst surface, facilitating the reaction of electrons with oxygen to form H₂O₂, while holes react with water in a concurrent process. Photoelectrochemical (PEC) cells are capable of spatially segregating the electrolyte via two distinct half-channels, thereby mitigating the decomposition of H₂O₂. In addition, photo/electrocatalytic can also be used to degrade HCHO in indoor air [101].

  Among the various modification methods, the creation of a suitable heterojunction structure was considered an effective strategy. Shen et al. [102] successfully engineered a high-low junction photocatalyst by integrating BiOBr with Bi₂S₃, characterized by an extensive interfacial contact area. This integration was achieved through a synergistic approach combining the molten salt method with an ion exchange strategy. Specifically, Bi₂S₃ was epitaxially grown on the surface of BiOBr via a high-temperature molten salt reaction, resulting in an intimate interface between the two materials. The deep VB position of BiOBr, coupled with the narrow band gap of Bi₂S₃, induces an intrinsic internal electric field (IEF) and band bending at the heterojunction. This configuration effectively facilitates the separation and transfer of photogenerated charge carriers, thereby enhancing the photocatalytic performance. Moreover, BiOBr/Bi₂S₃ retains a high oxidation potential, which significantly boosts its photocatalytic oxidation activity. Interestingly, in the molten state, the close binding between BiOBr and Bi₂S₃, facilitated by the ion-exchange strategy, promotes the in-situ H₂O₂ production, further augmenting the photocatalytic efficiency. Mechanisms such as Z-scheme and S-scheme are the most common ones used in the literature to characterize charge transfer in heterojunction structures. The concept of Z-scheme heterojunction was initially proposed by Bard in 1979 and has since been widely adopted in the field of photocatalysis. Cheng et al. [103] synthesized Z-scheme Ag/ZnFe₂O₄-Ag-Ag₃PO₄ composites to produce H₂O₂ by two consecutive steps of single-electron oxygen reduction. In recent years, Z-scheme heterojunction was widely used for photocatalytic CO₂ reduction [104-108], H₂ generation [109-113], N₂ fixation [114-118] and H₂O₂ production [119-123].

  However, there is still some confusion about the mechanism of Z-scheme heterojunction. S-scheme heterojunction is composed of a reduced semiconductor and an oxidized semiconductor, with the oxidized component being capable of existing as either a p-type or an n-type semiconductor. The migration of photogenerated charge carriers is effectively facilitated by the IEF present at the interfaces of the disparate semiconductors, thereby sustaining a high redox potential [124-127]. Li et al. [128] employed a special self-assembly strategy to fabricate an innovative S-scheme composite by integrating sulfur-doping porous graphite carbon nitride (S-pCN) with WO_2.72. During the formation of the heterojunction, the electron-deficient state associated with surface oxygen vacancies on WO_2.72 was alleviated by the lone pair electrons of S and N elements present in S-pCN. By meticulously adjusting the composite ratios, an optimal number of surface oxygen vacancies were retained on WO_2.72, which in turn facilitates electron migration and the generation of free radicals. The IEF and band bending effects at the interface accelerate the transfer of photogenerated charges, thereby promoting the separation of less reactive photogenerated carriers and preserving those with higher redox potentials, specifically electrons (e⁻) and holes (h⁺). In recent years, S-scheme heterojunction has received unprecedented attention for their excellent photocatalytic activity. They are widely used for photocatalytic CO₂ reduction [129-133], H₂ generation [134-138], N₂ fixation [139-143], antibiotic degradation [144-148] and H₂O₂ production [149-153].

  As delineated in the abstract of the article depicted in Fig. 1, in this review, we commenced with a discussion of the mechanisms associated with Z-scheme and S-scheme heterojunctions within the realm of photocatalytic and photoelectrocatalytic H₂O₂ production. Subsequently, we introduced in detail the preparation strategies and characterization techniques employed for the development of Z-scheme or S-scheme photocatalysts and photoelectrocatalysts for the H₂O₂ production, and briefly described the multifunctional applications of such catalysts in the H₂O₂ production. Ultimately, the article concluded with a prospective outlook on future research directions and the unresolved issues that warrant attention.

  Figure 1

  

  Figure 1.  Summary of this review content.

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  2. Fundamentals of Z-scheme and S-scheme heterojunctions for photocatalytic and photoelectrocatalytic H₂O₂ production

  2.1 Basic principles of photocatalytic H₂O₂ production

  The H₂O₂ production from semiconductor photocatalysts driven by light irradiation encompasses a series of intricate and sequential reaction processes [154-157], as shown in Fig. 2. Initially, upon irradiation of the photocatalyst with light of sufficient energy, electrons within the valence band (VB) are excited and transition to the conduction band (CB), thereby generating holes (h⁺) in the VB. The photogenerated electrons, along with hydrogen ions, migrate toward the surface of the photocatalyst. During this migration, charge carriers are prone to recombine, resulting in the dissipation of energy as light or heat. The surviving carriers that reach the catalyst surface participate in reduction or oxidation reactions with molecular O₂ and aqueous H₂O, respectively, while a portion of them inevitably undergo recombination [158-160]. Ultimately, the spatially separated electrons and holes participate in surface redox reactions, primarily the oxygen reduction reaction (ORR) and the water oxidation reaction (WOR), thereby culminating in the H₂O₂ production [161-163]. Hence, to enhance the H₂O₂ production, it is imperative to retard the recombination of photogenerated e⁻ and h⁺, while concurrently ensuring that the photocatalytic system possesses a robust redox potential capable of facilitating H₂O₂ production. Typically, the photocatalytic H₂O₂ production is predominantly achieved through the proton-coupled two-electron ORR, which can also proceed either through a sequential one-electron pathway (Fig. 2, Eqs. 1 and 2) or via a coordinated two-electron pathway (Fig. 2, Eq. 3) [164-166].

  Figure 2

  

  Figure 2.  Schematic diagram of relevant mechanisms of the photocatalytic H₂O₂ production.

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  In the case of the former pathway, where H₂O₂ is synthesized via a two-step electron reduction coupled with a two-step hole oxidation process, the rate-determining step is the formation of the superoxide radical (^•O₂⁻) intermediate. This step necessitates a potential that is more negative than −0.33 V relative to the normal hydrogen electrode (NHE), as indicated in Eq. 1. Conversely, in the latter pathway, this intermediate formation is bypassed, as O₂ is directly reduced to H₂O₂ in a single, concerted step. Consequently, a synergistic two-electron pathway for ORR is chemically more favorable. Additionally, the oxidation of H₂O₂ via a two-electron WOR has been documented. Nevertheless, this process is fraught with challenges, primarily due to the high standard potential of +2.73 V vs. NHE, the sluggish kinetics involved, and the relatively low selectivity when compared to the O₂ evolution process from H₂O. Despite the significant potential of photocatalytic H₂O₂ production, the yield of H₂O₂ remains unsatisfactory, attributed to rapid recombination of photogenerated e⁻ and h⁺, as well as the relatively slow kinetics of the redox reactions involved. Furthermore, in the context of single-component photocatalysts, an inherent trade-off exists between redox capacity and the spectral range of light absorption. The construction of step-structured (S-scheme) heterojunction has emerged as an innovative strategy to mitigate these challenges [167-170].

  Two-step electron reduction channel (Eqs. 1–3):

  O2+e−→•O2−(−0.33V)

  (1)

  •O2−+H+→•OOH(−0.13V)

  (2)

  •OOH+H++e−→H2O2(−0.046V)

  (3)

  Two-step hole oxidation channel (Eq. 4):

  h++OH−→•OH+•OH→H2O2(2.29V)

  (4)

  Z-scheme photocatalysts have garnered significant attention within the realm of photocatalysis due to their unique electronic configurations and enhanced performance in various energy conversion and environmental remediation applications. These photocatalysts can be broadly categorized into two main types: direct and indirect Z-scheme photocatalysts. Direct Z-scheme photocatalysts consist of two semiconductor materials with overlapping bandgaps, which facilitate the transfer of photogenerated e⁻ and h⁺. In contrast, indirect Z-scheme photocatalysts utilize a redox mediator to bridge the electron transfer gap between the two semiconductors. The redox mediator serves as an electron acceptor for one semiconductor and an electron donor for the other, thereby establishing a cyclic electron transfer mechanism. This cyclic pathway enables sustained photocatalytic activity and enhances the rate of H₂O₂ production [171-175].

  The S-scheme was initially conceived as a remedial approach to address the inherent challenges associated with Type Ⅱ or Z-scheme photocatalysts [176-180], and has recently been successfully applied to a wide range of critical photocatalytic reactions, including H₂ release [181-185], CO₂ reduction [186-190], pollutant degradation [191-195], N₂ immobilization [196-200], and H₂O₂ production [201-205]. At the core, S-scheme systems are constituted by two intimately associated semiconductor photocatalysts with interdigitated energy band diagrams. The photocatalyst characterized by a higher conduction band minimum (CBM), valence band maximum (VBM), and Fermi energy level is designated as the reduced photocatalyst (RP), whereas its counterpart is referred to as the oxidized photocatalyst (OP). Owing to the higher Fermi energy level of the RP compared to that of the OP, free electrons spontaneously migrate from the RP to the OP upon intimate contact, thereby equilibrating the Fermi energy levels at the interface. This electron transfer results in a net positive charge on the RP side and a negative charge on the OP side at the interface, thereby establishing an IEF oriented from the RP to the OP. Upon light irradiation, when electron-hole pairs are generated within both photocatalysts, the IEF facilitates the migration of photogenerated electrons from the OP's CB to the RP's VB, where they recombine with the photogenerated holes. Concurrently, the CB electrons in the RP and VB holes in the OP migrate towards the surface of the photocatalysts to participate in catalytic reactions. These charge carriers, endowed with maximal redox capacity, provide a more robust thermodynamic driving force for photocatalytic H₂O₂ production. Consequently, the S-scheme heterojunction not only enhances the transfer and separation of charge carriers but also accelerates the redox reactions, culminating in the efficient photocatalytic H₂O₂ production [206-210].

  2.2 Fundamentals of the reaction mechanism of photoelectrocatalytic H₂O₂ production

  Photoelectrocatalytic H₂O₂ production is usually carried out by ORR or WOR, as shown in Fig. S2 (Supporting information) and the production of H₂O₂ by semiconductor photoelectrocatalytic materials, for example, involves the following processes. Initially, the photocathode within the photocatalytic system absorbs photons with energy exceeding its bandgap (E_g), thereby generating electron-hole pairs. Subsequently, the applied potential promotes the transfer of electrons to activated oxygen species, thereby inducing the spatial separation of photogenerated e⁻ and h⁺ pairs [211-213]. These charge carriers migrate to the CB and VB of the semiconductor, respectively, where they participate in a variety of redox reactions. It is worth noting that the light absorption capacity of a semiconductor is directly proportional to its band gap width; a larger band gap requires higher-energy photons to induce electronic excitation. Moreover, the energy band structure of a semiconductor is intricately connected to the redox reactions occurring on its surface. Therefore, in order to achieve efficient photocatalytic H₂O₂ production, the energy band positions of the applied semiconductors need to be coordinated to achieve high yields.

  There are multiple electron pathway reactions in the ORR process, such as one-electron reduction to ^•O₂⁻, two-electron reduction to H₂O₂, and four-electron reduction to H₂O. Kinetically, O₂ would be more inclined to undergo a four-electron reaction to produce H₂O. Currently, it is a widely accepted notion that H₂O₂ can be produced via either a two-step one-electron indirect reduction pathway or a one-step two-electron direct reduction pathway. Han et al. synthesized CoSe₂ with varying Se stoichiometry using a straightforward hydrothermal approach [214]. They induced a phase transition of CoSe₂ from the orthorhombic to the cubic phase, selectively for Se-rich and Se-deficient compositions, by incorporating NaBH₄. The resulting catalyst demonstrated exceptional activity, high selectivity for H₂O₂, and enduring stability during a two-electron acidic ORR in 0.1 mol/L HClO₄. The yield of H₂O₂, ascertained through a flow cell configuration, amounted to 115.92 mmol g_cat^-1 h^-1, thereby underscoring its prospective viability for deployment in wastewater treatment endeavors. This investigation markedly augmented the comprehension of the crucial function that catalyst crystal phase and defect engineering played in bolstering the catalytic efficacy of the two-electron ORR towards H₂O₂ production. Alternatively, solar synthesis of H₂O₂ can be via the 2e⁻ route from ORR, which is achieved by applying a photocatalytic system or a photocathode in a PEC system. In general, the efficacy of a photoelectrocatalytic system for H₂O₂ production is contingent upon several pivotal factors, including the morphology and electronic energy band structure of the catalyst utilized, in addition to the composition of the reaction solution (Eqs. 5–7) [215-217].

  O2+e−→•O2−E0=−0.33V

  (5)

  •O2−+2H+→H2O2E0=1.44V

  (6)

  O2+2H++2e−→H2O2E0=0.68V

  (7)

  The production of H₂O₂ by the photoelectrocatalytic WOR pathway is a two-electron process. The semiconductor photoanode is fixed in the middle of the electrolyte through an electrode fixture and is in full contact with the electrolyte. Upon the incidence of photons whose energy equals or surpasses the band gap of the photoanode, electrons within the VB of the semiconductor are promoted to the conduction band, thereby generating photogenerated holes within the VB. These photogenerated holes migrate to the surface of the photoanode, which possesses potent oxidizing capabilities, facilitating the oxidation of H₂O to H₂O₂ at the electrolyte-photoanode interface. Concurrently, the photogenerated electrons are conveyed to the counter electrode via the external circuit under the influence of an applied bias voltage, where they reduce H⁺ to H₂. H₂O₂ is produced at the photoanode through H₂O oxidation, however, the production and accumulation of H₂O₂ by H₂O oxidation needs a more positive than the production of O₂ from H₂O potential. Therefore, it does not occur readily (Eqs. 8–10) [218-220].

  2H2O+2h+→H2O2+2H+E0=1.78V

  (8)

  H2O+h+→•OH+H+E0=2.73V

  (9)

  •OH+•OH→H2O2E0=1.14V

  (10)

  3. Fabrication strategies of Z-scheme and S-scheme heterojunction for photocatalytic and photoelectrocatalytic H₂O₂ production

  The fabrication of Z-scheme and S-scheme heterojunction catalysts represents a crucial approach for the efficient photocatalytic and photoelectrocatalytic H₂O₂ production. A scientific and stable interfacial structure can effectively facilitate separation and migration of photogenerated charge carriers, thereby significantly improving the efficiency of H₂O₂ production via photocatalytic pathways. Consequently, reliable preparation strategies for heterojunctions are of particular importance. The successful preparation of Z-scheme and S-scheme heterojunction catalysts with high-quality interfacial morphology commonly employs several strategies, including in-situ growth, self-assembly, deposition, one-step calcination, ion exchange, Interlayer coordination, template method (Fig. S3 in Supporting information). The following discussion will delve into these preparation strategies in detail.

  3.1 In-situ growth method

  The in-situ growth method refers to a strategy whereby a second semiconductor nucleates and grows on the surface or interface of a pre-synthesized semiconductor under specific conditions, thereby achieving a close integration of the two semiconductors to form a heterojunction. The advantage of the in-situ growth strategy lies in its simple preparation process and the tight interfacial bonding of the formed heterojunction. However, its disadvantage is that the in-situ synthesis of multi-component heterojunctions may lead to uneven component distribution due to differences in reaction kinetics. Various successfully synthesized Z-scheme or S-scheme heterojunction catalysts for the photocatalytic H₂O₂ production, such as CdS/RF [46], TiO₂@BTTA [201], iCOF/BO [203], ZnO/PBD [221], WS₂/g-C₃N₄ [119], ZIS-Z/OCN [122], r-CNW [121], CdS/Bi₂WO₆ [153], BiOBr/WO₃ [120], have been fabricated using in-situ growth strategies, including methods like oil bath, photodepositing, hydrothermal and solvothermal.

  Specifically, Zhu et al. employed the oil bath method to fabricate a CdS/RF S-scheme heterojunction composite by in-situ growth of CdS nanoparticles on the surface of resorcinol-formaldehyde (RF) resin spheres (Fig. 3a) [46]. The Field-emission scanning electron microscopy (FESEM) images clearly demonstrated the uniform growth of CdS nanoparticles on the surface of RF spheres. It is noteworthy that during the synthesis of CdS/RF, sodium citrate was found to be an essential chelating agent for the formation of the core-shell structure. In the absence of sodium citrate, CdS nanoparticles were randomly distributed within the composite rather than forming a uniform shell on the surface of the RF spheres. The successfully synthesized CdS/RF S-scheme heterojunction composite achieved a H₂O₂ release rate of up to 801 µmol L^-1 h^-1 under simulated solar irradiation. Moreover, Xia et al. utilized the photodeposition method combined with the oil bath method to in-situ grow Bi₂O₃(BO) nanoparticles on the ionic covalent organic framework (iCOF) nanofibers, successfully constructing the iCOF/BO S-scheme heterojunction composite for photocatalytic H₂O₂ production via a dual-channel pathway (Fig. 3b) [203]. The FESEM, TEM, HRTEM, high-angle annular dark-field (HAADF) images, and the corresponding elemental maps substantiated the uniform distribution of BO nanoparticles on the surface of iCOF nanofibers, providing further evidence for the successful preparation of the iCOF/BO composite. Under oxygen-saturated and visible light irradiation conditions, the synthesized iCOF/BO S-scheme heterojunction composite exhibits a H₂O₂ production efficiency of 9.76 mmol g^-1 h^-1 in pure water. Recently, Yu et al. placed a mixture containing a certain amount of TiO₂ nanofibers, acetic acid, acetonitrile, 4,4′,4′′-(1,3,5-triazine-2,4,6-triyl)triphenylamine (TA), and 1,3,5-benzenetricarbaldehyde (BT) after ultrasonication into a Pyrex tube, filled it with N₂, and maintained it at room temperature for 72 h [201]. They successfully grew a covalent organic framework (labeled as BTTA) synthesized through a Schiff base condensation reaction in situ on the surface of TiO₂ nanofibers (TiO₂ NFs), resulting in the preparation of a TiO₂ @BTTA S-scheme heterojunction composite material (Fig. 3c). The FESEM and high-resolution transmission electron microscopy (HRTEM) images, along with energy-dispersive X-ray (EDX) spectra, substantiated the uniform growth of BTTA on the surface of TiO₂ NFs. The porosity and encapsulation capability of BTTA endowed the composite material with a wealth of active sites and superior light absorption performance. The resulting TiO₂ @BTTA composite material achieved a H₂O₂ release rate of 740 µmol L^-1 h^-1 and a 96% conversion rate of furfural alcohol under light irradiation.

  Figure 3

  

  Figure 3.  (a) Preparation route of CdS/RF composite. Copied with permission [46]. Copyright 2023, Elsevier. (b) The synthesis process for iCOF/BO composite. Copied with permission [203]. Copyright 2024, Editorial Office of Acta Physico-Chimica Sinica. (c) Synthetic procedure of TiO₂@BTTA composites. Copied with permission [201]. Copyright 2023, Elsevier.

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  The solvothermal or hydrothermal method is one of the commonly used approaches to implement the in-situ growth strategy for constructing S-scheme or Z-scheme heterojunction catalysts for photocatalytic H₂O₂ production. The hydrothermal method is a process in which, within a sealed pressure vessel, using water as the solvent, a high-temperature and high-pressure reaction environment is created by heating and pressurizing, enabling substances that are usually insoluble or sparingly soluble to dissolve and recrystallize. For instance, Wu et al. employed the Suzuki-Miyaura reaction and the hydrothermal method [221]. With 1,6-bis(pyrene)-benzodithiophene serving as the electron donor and 2,6-dibromobenzodithiophene-4,8-dione as the electron acceptor, 1,6-bis(pyrene)-benzodithiophene dione (PBD) was synthesized, and ZnO nanoparticles were in-situ grown on the PBD substrate, successfully fabricating the ZnO/PBD S-scheme heterojunction composite (Fig. 4a). Through transmission electron microscopy (TEM) and HRTEM images, it can be clearly observed that the ZnO nanoparticles, as highly crystalline single crystals, were uniformly distributed on the amorphous PBD nanosheets. Under oxygen-rich and illumination conditions, ZnO/PBD has a H₂O₂ production efficiency of 4.07 mmol g^-1 h^-1 in 10 vol% methanol solution. The solvothermal method is a material preparation method developed on the basis of the hydrothermal method. It mainly uses organic solvents or non-aqueous media as the reaction medium and promotes chemical reactions by controlling the temperature and pressure in a closed system. For instance, Li et al. utilized the solvothermal method to control the growth of WS₂ on S-doped graphitic carbon nitride (SCN) and obtained excellent interfacial characteristics by means of horizontal implantation, thus preparing the WS₂/SCN Z-scheme heterojunction composite (Fig. 4b) [119]. TEM and HRTEM images clearly showed that WS₂ nanosheets grew horizontally and uniformly on the SCN nanosheets, demonstrating the successful construction of the WS₂/SCN heterojunction. Under light illumination, the successfully prepared WS₂/SCN Z-scheme heterojunction composite exhibited a H₂O₂ production rate of up to 5216 µmol L^-1 h^-1 in 10 vol% isopropanol solution. Moreover, Liu et al. utilized the solvothermal method to in-situ grow oxygen-doped graphitic carbon nitride (OCN) on Zn vacancy-containing ZnIn₂S₄ (ZIS-Z) flower-like microspheres, successfully fabricating a 2D/2D ZIS-Z/OCN Z-scheme composite (Fig. 4c) [122]. The SEM, atomic force microscopy (AFM), TEM, and HRTEM images clearly displayed that the ultrathin nanosheets OCN with a porous structure were highly dispersed on ZIS-Z with a flower-like microsphere structure composed of numerous ultrathin, smooth, and intertwined nanosheets. These observations further substantiated the intimate contact between ZIS-Z and OCN and provided comprehensive validation for the epitaxial formation of the ultrathin 2D/2D heterojunction in the ZIS-Z/OCN composite. Under visible light irradiation for 80 min, the removal rate of chlortetracycline hydrochloride (CTC) and the production rate of H₂O₂ by ZIS-Z/OCN were 88.43% and 168.67 µmol/L, respectively.

  Figure 4

  

  Figure 4.  (a) Synthesis route of ZnO/PBD. Copied with permission [221]. Copyright 2024, Editorial Office of Acta Physico-Chimica Sinica. (b) Formation illustration of WS₂/S-g-C₃N₄ Z-scheme heterostructures. Copied with permission [119]. Copyright 2024, Elsevier. (c) Schematic representation of preparing ZIS-Z/OCN. Copied with permission [122]. Copyright 2023, Elsevier.

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  3.2 Self-assembly method

  The self-assembly method is an approach which enables two or more semiconductors to spontaneously arrange and assemble into a stable structure based on the interactions of non-covalent bonds. These non-covalent bond interactions in the preparation of Z-scheme or S-scheme heterojunction catalysts for H₂O₂ production can be mainly classified into electrostatic, π-π stacking, and hydrogen bonding interaction. Among them, the latter two interactions generally occur simultaneously to assist two semiconductors in self-assembling and thus closely combining to form a heterojunction. The advantage of the self-assembly strategy lies in its ability to achieve precise design of the interface, and the preparation conditions conform to the principle of environmental friendliness. However, it is also prone to the drawback of insufficient structural stability.

  Self-assembly guided by electrostatic interactions can be mainly divided into two categories. One is dominated by ionic bonding. For example, in Z-scheme or S-scheme heterojunction catalysts such as ZnO/COF(TpPa-Cl) [222], ZnSe QD/COF [151], OCN@CNT-x [223], the semiconductors with opposite charges are attracted to each other by electrostatic attraction, thus forming stable heterojunctions. The other is dominated by charge transfer. For instance, in Z-scheme or S-scheme heterojunction catalysts like 3DOM SCN/T [224], BP/BiOBr [225], Bi₄Ti₃O₁₂/g-C₃N₄ [226], charge transfer occurs between distinct semiconductors. This process not only facilitates the self-assembly of the composite but also results in the formation of stable heterojunctions.

  Electrostatic self-assembly dominated by ionic bonding usually requires adjusting the pH values of semiconductor solutions to achieve opposite charge properties between the two semiconductors. Specifically, Zhang et al. combined positively charged ZnO with negatively charged TpPa-Cl to prepare the ZnO/TpPa-Cl (ZT) S-scheme heterojunction catalyst (Fig. 5a) [222]. By measuring the zeta potentials of ZnO and TpPa-Cl respectively, the realization of the electrostatic self-assembly process dominated by ionic bonding was theoretically supported. At a pH value of 7, ZnO nanoparticles exhibited a positive zeta potential, whereas TpPa-Cl showed the opposite. Consequently, ZnO and TpPa-Cl could be combined together through electrostatic attraction, which was conducive to the formation of a stable heterojunction. Through TEM and HRTEM images, the close combination of ZnO and TpPa-Cl could be clearly observed, further demonstrating the successful preparation of the ZT heterojunction composite. Under oxygen-rich and simulated solar light irradiation conditions, ZT has the maximum H₂O₂ production efficiency of 2443 µmol g^-1 h^-1 in 10 vol% ethanol solution.

  Figure 5

  

  Figure 5.  (a) Schematic illustration of the formation of the ZT heterojunction. Copied with permission [222]. Copyright 2022, Elsevier. (b) Schematic representation of the synthesis process for 3DOM SCN/T. Copied with permission [224]. Copyright 2023, Elsevier. (c) Schematic depiction of the preparation process for Sol-BP/BiOBr composites. Copied with permission [225]. Copyright 2020, Elsevier.

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  The electrostatic self-assembly process dominated by charge transfer usually occurs under certain conditions. After two semiconductors are fully contacted through continuous stirring or ultrasonic treatment and other means, one semiconductor transfers electrons to the interface where it contacts the other semiconductor, thus driving the self-assembly process and forming a stable heterojunction. For example, Jiang et al. employed the electrostatic self-assembly strategy, which is primarily driven by charge transfer, to combine TiO₂ with three-dimensionally ordered macroporous sulfur-doped graphitic carbon nitride (3DOM SCN), thereby fabricating the 3DOM SCN/TiO₂ (SCN/T) S-scheme composite (Fig. 5b) [224]. Through comparing the differences in the binding energies of various elements in the XPS spectra between the single SCN substance and the SCN/T composite, it could be seen that when SCN and TiO₂ were fully contacted, electrons migrate from the interface of SCN to TiO₂, thus driving the self-assembly process and forming a stable heterojunction. Through SEM and TEM images, it could be clearly observed that TiO₂ nanoparticles were uniformly anchored on the SCN framework, further demonstrating the successful preparation of the SCN/T heterojunction composite. Under oxygen-rich and 300 W Xe lamp irradiation conditions, SCN/T has the maximum photocatalytic H₂O₂ production efficiency of 2128 µmol g^-1 h^-1 in deionized water. Furthermore, Li et al. also adopted the analogous strategy dominated by charge transfer to combine exfoliated black phosphorus (BP) nanosheets with BiOBr nanosheets possessing a relatively high Fermi level [225]. In particular, they proposed to use the ultrasonic-assisted solvothermal method instead of the conventional simple ultrasonic combination method to construct a novel layered BP/BiOBr S-scheme heterojunction with a more uniform and stable structure (Fig. 5c). By comparing the binding energies of various elements in the XPS spectra of pure BP, pure BiOBr and the BP/BiOBr composite, it could be found that when BP and BiOBr were in contact with each other, electrons migrate from the interface of BiOBr to BP, thus driving the self-assembly process and forming a stable heterojunction. Through the SEM and HRTEM images, the fine structures of the crystal planes and phase interfaces of the BP/BiOBr distinctive heterojunction could be clearly witnessed, further demonstrating the effective synthesis of the BP/BiOBr heterojunction. Under the conditions of oxygen-rich and irradiation with a 300 W Xe lamp (420 nm < λ < 780 nm), the efficiency of BP/BiOBr for producing H₂O₂ in pure water is 1.62 µmol L^-1 min^-1.

  π-π Stacking represents a unique spatial arrangement of aromatic compounds, denoting a weak interaction that frequently occurs between aromatic rings. This non-covalent interaction is as significant as hydrogen bonding. Besides the self-assembly driven by electrostatic interactions, π-π stacking or the synergistic effect of π-π stacking and hydrogen bonding usually also serves as a driving force to guide the self-assembly process of two semiconductors, facilitating the formation of stable heterojunctions at the semiconductor interfaces. For example, heterojunction catalysts such as PDI-Ala/S-C₃N₄ [227], CoPc/K/Na/PCN [228], Co-N-C/SA-PDI [229], PDI/g-C₃N₄ [230], were all driven by π-π stacking or the synergistic effect of π-π stacking and hydrogen bonding in the self-assembly process of two semiconductors, thus forming stable heterojunctions. In particular, Li et al. based on the fact that g-C₃N₄ has an aromatic heterocyclic structure and that N,N′-bis(propionic acid)-perylene-3,4,9,10-tetracarboxylic diimide (PDI-Ala) has a rigid planar structure with the characteristic of delocalized large π bonds, utilized π-π interactions to self-assemble PDI-Ala on the surface of sulfur-doped g-C₃N₄ (S-C₃N₄) nanosheets, thereby preparing PDI-Ala/S-C₃N₄ S-scheme composite (Fig. 6a) [227]. Through X-ray diffraction (XRD), infrared spectroscopy and DFT calculations, it was inferred that PDI-Ala self-assembles into supramolecules through lateral hydrogen bonds and longitudinal π-π stacking, while S-C₃N₄ and PDI-Ala were closely combined through π-π interactions and N—C bonds. Through TEM and elemental mapping, it was clearly observed that PDI-Ala is uniformly distributed on the porous two-dimensional S-C₃N₄ nanosheet structure, further demonstrating the successful preparation of the PDI-Ala/S-C₃N₄ heterojunction composite. Under the irradiation of a 300 W xenon lamp (420 nm < λ < 780 nm), the yield of H₂O₂ of the PDI-Ala/S-C₃N₄ in distilled water is 28.3 µmol g^-1 h^-1. Moreover, Liu et al. adopted a simple hydrothermal method. CoPc was self-assembled on the surface of alkali metal-doped g-C₃N₄ nanosheets (K/Na/PCN) through π-π stacking and hydrogen bonding interactions to prepare CoPc/K/Na/PCN Z-scheme composite (Fig. 6b) [228]. Based on the results of infrared (FTIR) spectroscopy, it could be analyzed and inferred that CoPc was combined with K/Na/PCN through non-covalent bonds, and the planar CoPc was introduced onto the surface of K/Na/PCN through π-π stacking and hydrogen bonding effected and covered the surface of K/Na/PCN nanosheets in the form of single or multiple molecular layers. As the amount of CoPc increased, the degree of self-aggregation of CoPc on the surface of K/Na/PCN also increased. Through the SEM and elemental mapping images, it can be clearly observed that CoPc uniformly and stably covers the surface of K/Na/PCN nanosheets, further demonstrating the successful preparation of CoPc/K/Na/PCN heterojunction composite. Under the conditions of oxygen-saturated adsorption and irradiation with a 60 W blue LED bulb, CoPc/K/Na/PCN can oxidize amines into imines in an ethanol solution with a certain amount of amines added, and simultaneously couple to produce H₂O₂ with a yield ranging from 3.27 mmol g^-1 h^-1 to 3.40 mmol g^-1 h^-1.

  Figure 6

  

  Figure 6.  (a) The fabrication process of PDI-Ala/S-C₃N₄. Copied with permission [227]. Copyright 2021, Editorial Office of Acta Physico-Chimica Sinica. (b) Synthetic pathways of CoPc/K/Na/PCN. Copied with permission [228]. Copyright 2024, Elsevier.

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  3.3 Deposition method

  The deposition method is one of the more common strategies for constructing Z-scheme or S-scheme heterojunction catalysts, apart from the in-situ growth and self-assembly methods. The deposition strategy generally refers to a method of preparing heterojunction composite materials by depositing one semiconductor directly onto the surface of another semiconductor in a certain way. This deposition process usually occurs in a solution. Depending on the differences in the environment during the solution stirring, we classify the deposition strategy into the in-situ deposition method and the chemical bath deposition method here. The in-situ deposition method usually involves the solution stirring process at room temperature, while the chemical bath deposition method usually conducts the solution stirring under the conditions of water bath or chemical bath heating. The advantage of the deposition strategy is its wide applicability and the potential for large-scale preparation of catalysts. However, it also has the disadvantage of a relatively long preparation process.

  Specifically, Meng et al. adopted the in-situ deposition strategy to deposit g-C₃N₄ nanosheets in situ onto the crystalline material (ZIF-L) formed by zinc atoms and the organic ligand 2-methylimidazole (2-MI) through coordination bonds, thus preparing ZIF-L/g-C₃N₄ Z-scheme heterojunction composite materials (ZCx) for the efficient production of H₂O₂ (Fig. 7a) [231]. Through the SEM and HRTEM images, it can be clearly observed that the fragmented and laminated g-C₃N₄ nanosheets were evenly distributed on the surface of ZIF-L with a smooth leaf-like morphology, and the two were closely combined, which demonstrates the successful preparation of the ZIF-L/g-C₃N₄ heterojunction composite. Under the irradiation condition with a 300 W xenon lamp, continuous ultrasonication of ZIF-L/g-C₃N₄ in ultrapure water can achieve a H₂O₂ yield of 1.45 mmol g^-1 h^-1.

  Figure 7

  

  Figure 7.  (a) Schematic representation of the fundamental synthetic protocol for ZCx. Copied with permission [231]. Copyright 2024, Elsevier. (b) Schematic illustration of the synthesis process for ZnO/ZnIn₂S₄. Copied with permission [232]. Copyright 2023, Elsevier. (c) Schematics for the synthesis of ZnO/CuInS₂ photocatalyst. Copied with permission [50]. Copyright 2024, Wiley-VCH.

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  Wu et al. adopted the electrospinning and chemical bath deposition methods to deposit ZnIn₂S₄ nanoparticles onto ZnO fibers, thus preparing a low-dimensional ZnO/ZnIn₂S₄ S-scheme photocatalyst for the efficient production of H₂O₂ (Fig. 7b) [232]. Through FESEM, TEM, HRTEM, HADDF images, as well as EDX elemental mapping results, it was clearly observed that several ZnIn₂S₄ nanoparticles were uniformly loaded on the surface of ZnO, and the composite exhibited a core-shell architecture, with ZnO constituting the core and ZnIn₂S₄ forming the shell. Concurrently, ZnIn₂S₄ nanoparticles were predominantly synthesized on the matrix of ZnO nanofibers, thereby evidencing the intimate contact between ZnO and ZnIn₂S₄ and further corroborating the successful fabrication of the ZnO/ZnIn₂S₄ heterojunction composite. Under the conditions of oxygen saturation and simulated sunlight irradiation, the yield of H₂O₂ of ZnO/ZnIn₂S₄ in 10 vol% methanol solution could reach 928 µmol g^-1 h^-1.

  Meng et al. prepared a plasmonic near-infrared-responsive ZnO/CuInS₂ S-scheme heterojunction photocatalyst for the photocatalytic oxidation of glycerol to produce H₂O₂ through an in-situ deposition strategy [50]. Specifically, a certain amount of ZnO nanocages were uniformly dispersed in n-hexane. Subsequently, a certain amount of CuInS₂ quantum dot (QD) solution was rapidly added under vigorous stirring. Finally, the ZnO/CuInS₂ quantum dot heterostructure was obtained by vacuum-drying (Fig. 7c). The FESEM, STEM, HRTEM, HADDF images, and EDX elemental mappings clearly showed that ZnO/CuInS₂ had a uniform particle size distribution, a distinct cubic morphology, and a hollow internal structure, further confirming the successful preparation of the composite. Upon the introduction of 10 mg of ZnO/CuInS₂ into a mixed solution comprising 30 mL of deionized water and 4.5 µL of glycerol, following the attainment of adsorption equilibrium of oxygen within the solution, the mixture was subjected to irradiation from a 300 W Xe lamp (wavelength range: 350–1100 nm), after 4 h, the glycerol oxidation rate of the ZnO/CuInS₂ composite reached a maximum of 0.27 mmol L^-1 h^-1, and correspondingly, the photocatalytic H₂O₂ production rate was 0.31 mmol L^-1 h^-1. In addition, Pradhan et al. [124] employed an in-situ deposition strategy and utilized a one-pot solvothermal method to in-situ deposit Bi₂MoO₆ nanosheets onto InVO₄/CeVO₄, thereby constructing a series of Bi₂MoO₆/InVO₄/CeVO₄ ternary S-scheme heterostructures for highly efficient photocatalytic production of H₂O₂ and H₂. Among them, the yield of H₂O₂ could reach 1700 µmol L^-1 g^-1 h^-1. The ternary heterostructure exhibited a unique morphology composed of Bi₂MoO₆ nanoplates, InVO₄ nanorods, and CeVO₄ nanosheets. Moreover, Xu et al. also prepared a graphitic carbon nitride/polyhedral oligomeric silsesquioxane PDI (p-CN/P-PDI) S-scheme composite through in-situ deposition strategy [55].

  3.4 Other methods

  Among the strategies for preparing heterojunction catalysts used in photocatalytic production of H₂O₂, in addition to the commonly used in-situ growth, self-assembly and deposition methods, several other preparation strategies have also been mentioned in some research papers, such as one-step calcination, ion exchange, interlayer coordination, template method and machine learning. These have provided novel and referable preparation ideas for researchers in the field of preparing Z-scheme or S-scheme heterojunction composites for photocatalytic production of H₂O₂. Herein, we will briefly introduce several other preparation strategies, aiming to provide references for researchers and help them explore better preparation strategies.

  Specifically, Zhou et al. [233] adopted a straightforward and expeditious one-step calcination approach. Initially, they homogeneously blended titanate powder with a urea solution of specified concentration and subjected the mixture to drying at a predetermined temperature. Subsequently, the dried material was directly subjected to high-temperature calcination, yielding mesoporous TiO_2-x/g-C₃N₄ S-scheme composite that were abundant in oxygen vacancies (Fig. 8a). Chen et al. [234] adopted the ion exchange strategy and prepared the hollow tubular bifunctional In₂S₃/MnIn₂S₄ S-scheme heterojunction photoelectrocatalyst by subjecting In₂S₃ to an exchange treatment with Mn²⁺. Specifically, In₂S₃ was thoroughly dispersed in ethanol, followed by the rapid addition of a Mn²⁺ ethanol solution of a specific concentration. The resultant mixture was stirred at 60 ℃ for 2 h, then filtered and washed with ethanol and deionized water. Finally, the product was dried under vacuum to obtain the In₂S₃/MnIn₂S₄ composite (Fig. 8b). Liu et al. [127] adopted the strategies of interlayer coordination and interlayer separation to prepare a series of graphitic carbon nitride/polyoxometalate (CN-HMoP) S-scheme heterojunction photocatalysts. Specifically, a predetermined quantity of phosphomolybdic acid was dissolved in a ternary solvent mixture composed of H₂O, ethanol, and DMF in a volumetric ratio of 3:2:1. Graphitic carbon nitride (CN) was added and then subjected to ultrasonic stirring for 2 h. After that, the mixture was introduced into a high-pressure reactor for a solvothermal reaction to prepare the final product CN-HMoP (Fig. 8c). Dai et al. [235] took the rod-shaped Bi-MOF named CAU-17 as a template and adopted a one-pot oil bath method to prepare the ZnIn₂S₄/Bi₂S₃ Z-scheme heterojunction photocatalyst. Specifically, a certain amount of CAU-17, InCl₃·4H₂O, ZnCl₂ and thioacetamide (TAA) were thoroughly mixed in deionized water. Following the adjustment of the mixed solution's pH to 2.5, the reaction was carried out in an oil bath at 80 ℃ for 6 h. Ultimately, the yellow precipitate was harvested via centrifugation, subsequently washed with pure water and absolute ethanol, and dried to yield ZnIn₂S₄/Bi₂S₃ (Fig. 8d).

  Figure 8

  

  Figure 8.  (a) The synthetic process of the TCNx sample. Copied with permission [233]. Copyright 2024, Elsevier. (b) The illustration to prepare In₂S₃/MnIn₂S₄ catalyst. Copied with permission [234]. Copyright 2024, Elsevier. (c) Schematic showing the synthesis process of CN/HMoP synthesis. Copied with permission [127]. Copyright 2024, Elsevier. (d) Creative synthetic method of ZnIn₂S₄/Bi₂S₃ hybrids in this work. Copied with permission [235]. Copyright 2024, Wiley-VCH.

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  In recent years, the continuous advancement of computer technology has given rise to a novel catalyst preparation strategy, namely the machine learning strategy. However, almost all the catalysts for producing H₂O₂ designed by the machine learning strategy are single-atom catalysts at present, and no heterojunction catalysts for producing H₂O₂ have been found to be designed yet. Herein, the process of designing catalysts for producing H₂O₂ by the machine learning strategy will be briefly introduced, aiming to provide researchers with new preparation ideas and explore the possibility of using the machine learning strategy to design heterojunction catalysts for producing H₂O₂ in the future. For example, Deng et al. [236] proposed an iterative machine learning (iML) approach, which can significantly diminish the required size of the training dataset. By incorporating spatial coordinate information as a feature, the objective of swiftly identifying the target catalyst with optimal catalytic activity from a vast array of single-atom catalysts can be realized. Concurrently, it has been established that RhN₂O₂A was an exemplary catalyst for photocatalytic H₂O₂ production, characterized by an exceedingly low overpotential of 0.013 V (Fig. 9a). Moreover, Zhang et al. [237] constructed five machine learning (ML) models, utilizing the adsorption free energies (ΔG(O^*)) of 149 single-atom catalysts (SACs) and the limiting potentials (U_L) of 31 SACs as the foundational data. Subsequently, descriptors capable of accurately describing SACs were obtained. Moreover, the two-electron oxygen reduction reaction (2e⁻ ORR) catalytic performances of 690 unknown SACs were well predicted, and four 2e⁻ ORR materials with relatively high selectivity and activity, namely Zn@Pc-N₃C₁, Au@Pd-N₄, Au@Pd-N₁C₃ and Au@Py-N₃C₁, were screened out. Finally, the U_Ls of these SACs were verified through DFT calculations, and all were found to exceed the standard values, thereby demonstrating the efficacy of the ML models (Fig. 9b).

  Figure 9

  

  Figure 9.  (a) Scheme of iML method. Copied with permission [236]. Copyright 2022, Elsevier. (b) Selection methodology for SACs exhibiting elevated selectivity and catalytic activity. Copied with permission [237]. Copyright 2023, Elsevier.

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  4. Characterization techniques of Z-scheme and S-scheme heterojunctions for photocatalytic and photoelectrocatalytic H₂O₂ production

  Characterization techniques on Z-scheme and S-scheme heterojunction catalysts for H₂O₂ production, such as X-ray diffraction (XRD), SEM, TEM, UV visible diffuse reflectance spectroscopy (UV–vis DRS), photoluminescence spectrum (PL), time-resolved photoluminescence spectrum (TRPL), electron paramagnetic resonance (EPR), a variety of photoelectrochemical testings, in-situ characterization techniques, density function theory (DFT) calculations and so on, especially, EPR, a variety of photoelectrochemical testings, in-situ characterization techniques and DFT calculations, provide significant guidance and basis for studying the charge transfer pathways and revealing the mechanisms of catalytic reactions. These characterizations also offer scientific research directions for the design of efficient Z-scheme and S-scheme heterojunction catalysts and the optimization of their performance. In this context, these characterization techniques are presented and discussed in detail.

  4.1 Electron paramagnetic resonance (EPR)

  In the field of photocatalytic H₂O₂ production, EPR (usually also known as ESR) testing, a technique that uses an electron paramagnetic resonance spectrometer to detect unpaired electrons in materials, is frequently employed to identify the types of free radicals and their relative quantities. This helps researchers infer the charge-transfer mechanisms and catalytic mechanisms of Z-scheme or S-scheme heterojunction composites during the catalytic process.

  For example, Li et al. [128] utilized ESR experiments to characterize the primary reactive species involved in the photocatalytic process of 0.5S-pCN/WO_2.72 S-scheme composite. As shown in Figs. 10a-c, under dark conditions, no characteristic peaks corresponding to DMPO-^∙O₂⁻ and DMPO-^∙OH were detected. However, upon light irradiation, distinct peaks for DMPO-^∙O₂⁻ and DMPO-^∙OH emerged, thereby confirming that ^∙O₂⁻ and ^∙OH were the predominant reactive species. In addition, the main reason for the insignificant change in the TEMPO-h⁺ signal was that some h⁺ reacted with H₂O molecules and hydroxyl ions on the material surface to form ^∙OH, thus enhancing the DMPO-^∙OH signal. Since only S-scheme can retain a high conduction band and a deep valence band potential, the results of the ESR test helped to confirm the inference that the 0.5S-pCN/WO_2.72 composite formed an S-scheme heterojunction (Fig. 10d). Moreover, Liu et al. [122] conducted a series of EPR tests on the ZIS-Z/OCN Z-scheme heterojunction composite to explore the mechanism of photocatalytic H₂O₂ production. As shown in Fig. 10e, the absence of detectable H₂O₂ in the photoreaction system devoid of O₂ and ZIS-Z/OCN underscores the indispensable roles of both O₂ and ZIS-Z/OCN in the photocatalytic H₂O₂ production. Analogously, the introduction of superoxide dismutase (SOD) into the reaction system resulted in the nondetection of H₂O₂. In contrast, a small amount of H₂O₂ was monitored in the absence of IPA. The aforementioned results suggested that the presence of ^∙O₂⁻ was a fundamental prerequisite for the photocatalytic H₂O₂ production, thus inferring that H₂O₂ was not generated through a one-step two-electron method. Based on the EPR results in Figs. 10f-h, it can be deduced that both ¹O₂ and ^∙O₂⁻ were intermediate products in photocatalytic H₂O₂ production. Meanwhile, there was an energy transfer process during the photocatalytic H₂O₂ production, but ¹O₂ was mainly produced through the charge-transfer route. Consequently, the pathway of photocatalytic H₂O₂ production by ZIS-Z/OCN was inferred.

  Figure 10

  

  Figure 10.  ESR detection for DMPO-^∙O₂⁻ (a), DMPO-^∙OH (b) and TEMPO-h⁺ (c) under dark and visible-light irradiation, schematic illustrations of WO_2.72 and 0.5S-pCN before and after contact, as well as the formation of IEF and band bending (d). Copied with permission [128]. Copyright 2023, Elsevier. The photocatalytic H₂O₂ production yield of 40ZIS-Z/OCN under various conditions (e), ESR spectra of DMPO-^∙OH (f), DMPO-^∙O₂⁻ (g) and TEMP-¹O₂ (h) for 40ZIS-Z/OCN under visible light irradiation. Copied with permission [122]. Copyright 2023, Elsevier. Transient photocurrent response (i), EIS analysis (j) and K-L curves (k) of ZnO, ZnIn₂S₄, and ZZS-20. Copied with permission [232]. Copyright 2023, Elsevier.

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  4.2 Photoelectrochemical testings

  There are numerous types of photoelectrochemical tests. Among them, electrochemical impedance spectroscopy (EIS) and transient photocurrent test (i-t) are frequently employed to assess and characterize the separation and migration efficiency of photogenerated e⁻ and h⁺ in Z-scheme or S-scheme heterojunction composites, thereby reflecting the photocatalytic activity of the materials. In addition, the slope of the curve in the linear sweep voltammetry (LSV) results can be calculated through the Koutecki-Levich (K-L) equation to estimate the number of electron transferred during the oxygen reduction reaction, thereby determining the reaction pathway during the photocatalytic H₂O₂ production.

  For example, Wu et al. [232] utilized i-t and EIS to evaluate the charge separation efficiency and transfer ability of the ZZS-20 S-scheme heterojunction composite. As shown in Figs. 10i and j, the photocurrent density of ZZS-20 was significantly elevated compared to that of ZnO and ZnIn₂S₄. This disparity attributed to the superior charge separation efficacy of the S-scheme heterojunction within the composite structure. Meanwhile, ZZS-20 exhibited the smallest arc radius, indicating that it had the strongest charge transfer ability. These findings collectively demonstrate that the construction of an S-scheme heterojunction was advantageous for enhancing the separation and transfer efficiency of photogenerated charges. Moreover, to elucidate the reaction pathways of different catalysts during the photocatalytic H₂O₂ production, rotational disk electrode tests were usually performed on materials to determine the number of electrons transferred (n) in ORR. Fig. 10k showed the slopes of LSV curves for ZnO, ZnIn₂S₄, and ZZS-20 calculated using K-L equation respectively. The calculated n value of ZZS-20 was 2.5, indicating a greater propensity for the two-electron transfer pathway in photocatalytic H₂O₂ production. Similarly, Jiang et al. [224] utilized the KL plot of SCN, TiO₂, and SCN/T9 at −1.0 V vs. Ag/AgCl, which indicated that the n value of SCN/T9 was 2.12, thus inferring that the H₂O₂ production on SCN/T9 predominantly proceeded via the direct two-electron ORR pathway.

  4.3 In-situ characterization techniques

  In-situ characterization techniques have been pivotal in advancing the study of photocatalytic H₂O₂ production. These methodologies have not only enabled researchers to attain a profound understanding of the reaction mechanisms but also undergirded the development of more efficacious and sustainable protocols for H₂O₂ production. Balakrishnan et al. [238] employed in-situ fourier transform infrared spectroscopy (FTIR) to elucidate the mechanistic pathway of photocatalytic H₂O₂ production within the photocatalytic system supported by OPACN hydrogel (Figs. 11a-c). Their findings confirmed that the formation of H₂O₂ via photocatalysis on OPACN hydrogel proceeded via a two-step-two-electron reduction process. Zan et al. [239] capitalized on the semiconductor property that an increase in electron binding energy implies electron loss, and conversely, a decrease signifies electron gain. By employing in-situ X-ray photoelectron spectroscopy (XPS) technology, they observed the differences in electron binding energies of elements C, N and Bi under both dark and illuminated conditions to determine the direction of electron flow in the CNQDs/BiOBr composite material. The analysis of Figs. 11d-f indicated that an S-scheme heterojunction was successfully synthesized within the CNQDs/BiOBr composite, thereby conferring it with enhanced photocatalytic performance for the production of H₂O₂. Li et al. [240] utilized 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a radical scavenger to perform in-situ ESR characterization technique on various samples, thereby delving deeper into the mechanistic insights of the oxygen reduction reaction for photocatalytic H₂O₂ production using Au/F-TiO₂ composite. As shown in Fig. 11g, the results indicated that superoxide radicals were formed by the initial combination of O₂ with electrons and protons within the photocatalytic reaction medium. The generated HO₂. radicals continued to react with one e⁻ and one H⁺, ultimately leading to the formation of H₂O₂. Consequently, H₂O₂ was progressively synthesized through a single-electron ORR on Au/F-TiO₂ composite. Guo et al. [241] dissected the complex process of H₂O₂ production over an innovative 0D/2D carbon dots-modified CN nanosheet heterojunction (CDS₁₀MCN) photocatalyst using in-situ diffuse infrared fourier transform spectroscopy (DRIFTS). As illustrated in Fig. 11h, the infrared spectra exhibited positive peaks corresponding to various intermediate species. Wang et al. [150] further verified the electron transfer mechanism between the PCN/MnS S-scheme heterojunction during the photocatalytic process by using in-situ Kelvin probe force microscopy (KPFM), which helped to deduce the generation mechanism of H₂O₂. As shown in the results of Figs. 11i-k, upon light irradiation, the surface potential of MnS exhibited a decrease, whereas the surface potential of PCN showed an increase. Therefore, under the action of the IEF, photoelectrons transfered from PCN to MnS, thereby augmenting the reduction capacity of the photogenerated electrons and consequently facilitating the photocatalytic H₂O₂ production.

  Figure 11

  

  Figure 11.  In-situ FTIR spectra of a series of OPACN composites during the photocatalytic H₂O₂ production (a) and an enlarged view of the FTIR peak (b, c). Copied with permission [238]. Copyright 2024, Elsevier. (d-f) In-situ XPS spectra of C, N and Bi in CNQDs/BiOBr-1.50%. Copied with permission [239]. Copyright 2023, Editorial Office of Acta Physico-Chimica Sinica. (g) In-situ ESR spectra of 0.1% Au/TiO₂ composites and pure TiO₂. Copied with permission [240], Copyright 2021, MDPI. (h) In-situ DRIFTS of CDs₁₀MCN collected in the H₂O₂ photocatalytic processes in the O₂ atmosphere. Copied with permission [241]. Copyright 2024, Wiley-VCH. In-situ KPFM potential diagram in dark (i), under light irradiation (j), and surface potential (k) of PCN(5)/MnS-D. Copied with permission [150]. Copyright 2023, Elsevier.

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  4.4 Density function theory (DFT)

  DFT calculations can be utilized to clarify the impact of catalyst selection on the photocatalytic production rates of H₂O₂. This analysis also permits a deeper understanding of the relationship between the surface properties of the catalyst and the dynamics of the catalytic reaction, among other aspects. Zhang et al. [242] demonstrated through DFT calculations that the reduction in photocatalytic H₂O₂ decomposition may be attributed to the suppression of H₂O₂ adsorption by the photocatalyst. As shown in Fig. 12a, the TiO₂ (101) surface exhibited the most pronounced adsorption affinity for H₂O₂. Conversely, as depicted in Fig. 12b, the maximum adsorption energy between H₂O₂ and MoSe₂ was more positive due to lower surface reactivity, hence the absolute adsorption energy of H₂O₂ on core-shell biphase (1T-2H)-MoSe₂/TiO₂ nanoribbons (NRAs) was significantly lower than that on TiO₂ NRAs. These findings indicated that the (1T-2H)-MoSe₂ on TiO₂ NRAs adsorbs less H₂O₂, thereby inhibiting the decomposition of H₂O₂. Guo et al. [241] employed DFT calculations to further elucidate the complex interplay between the electronic structures of CDs and CDs₁₀MCN and its impact on reaction kinetics. As illustrated in Figs. 12c and d, the integration of CDs significantly enhanced the negative charge density of the CDs₁₀MCN heterojunction, which in turn reduced the adsorption capacity for OOH^*. The results demonstrated that the adsorption efficiency of H₂O₂ can be enhanced via 2e⁻ ORR pathway. Li et al. [243] elucidated the influence of the local coordination environment of zinc sites on the catalytic activity for oxygen reduction to H₂O₂ through theoretical calculations. As depicted in Figs. 12e-k, they identified a robust electronic interaction between Zn and O atoms, with the adsorption free energy of O₂ on Zn-N₃O being lower compared to Zn-N₄. Furthermore, Zn-N₃O facilitated the accumulation of electrons in ^*OOH. In summary, adsorbed oxygen was more readily activated to ^*OOH on Zn-N₃O compared to Zn-N₄ and subsequently converted into H₂O₂.

  Figure 12

  

  Figure 12.  Optimal configurations of H₂O₂ adsorption on TiO₂ (101) (a) and 2H-MoSe₂ (103) planes (b). Copied with permission [242]. Copyright 2022, Elsevier. Optimized unit cells of states for CDs₁₀MCN (c) and free energy diagram of CDs₁₀MCN for photocatalytic H₂O₂ production (d). Copied with permission [241]. Copyright 2024, Wiley-VCH. (e-k) The theoretical calculation results regarding the Zn-N₄ and Zn-N₃O models. Copied with permission [243]. Copyright 2024, Wiley-VCH.

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  5. Multifunctional applications of Z-scheme and S-scheme heterojunctions for photocatalytic and photoelectrocatalytic H₂O₂ production

  Nowadays, Z-scheme or S-scheme heterojunction catalysts for photocatalytic and photoelectrocatalytic H₂O₂ production are quite common. However, with the in-depth development of research in the catalytic field, researchers no longer pursue the design of catalysts with only single-benefit. Therefore, an increasing number of multi-functional and multi-benefit catalysts have emerged, which are not limited to merely achieving the single function of photocatalytic or photoelectrocatalytic H₂O₂ production. For instance, the coupling of photocatalytic or photoelectrocatalytic H₂O₂ production with the degradation of pollutants, as well as heterojunction catalysts that can respectively achieve functions such as efficient H₂O₂ production, H₂ production, and nitrogen fixation. Here, we will outline some Z-scheme or S-scheme heterojunction catalysts for photocatalytic and photoelectrocatalytic H₂O₂ production with multi-functional applications, including the utilization of in-situ photocatalytic and photoelectrocatalytic H₂O₂ production for self-Fenton degradation.

  5.1 Z-scheme heterojunction catalysts for H₂O₂ production

  5.1.1   Z-scheme heterojunction catalysts for photocatalytic H₂O₂ production

  Specifically, Balakrishnan et al. [238] employed pectin as the foundational material and utilized distinct functionalized graphitic carbon nitrates (GCN), amino-rich GCN (ACN), oxidized GCN (OCN), along with phosphorylated GCN (PCN) through a wet impregnation method to effectively construct 3D OPACN hydrogel dual Z-scheme heterojunction catalyst. It is capable of facilitating the efficient degradation of tetracycline (TC), the photocatalytic H₂O₂ production and the fixation of N₂ under photocatalysis. Fig. 13a depicted the photoluminescence (PL) spectrum of GCN-based catalysts. Analysis of the spectrum revealed that the PL intensity of the catalysts was attenuated post-modification. Specifically, the OPACN hydrogel demonstrated the minimal PL intensity among the catalysts evaluated, suggesting that the incorporation of pectin with the OPACN has enhanced the separation efficiency of photogenerated charge carriers. The efficiency of photogenerated charge carrier separation for the GCN-based catalysts were further elucidated through electron impedance spectroscopy (EIS). As illustrated in Fig. 13b, the OPACN hydrogel exhibited the minimum arc radius, indicating that, within the spectrum of catalysts under assessment, it possessed the most outstanding charge-carrier separation efficiency. The Mott-Schottky (M-S) curves of ACN, OCN and PCN composite materials were shown in Fig. 13c. As can be observed from Fig. 13c, the M-S curves of all three samples exhibited positive slopes. This phenomenon served as an indication that they are all n-type semiconductors. Moreover, it was possible to estimate that the conduction band potentials of OCN, ACN and PCN were −0.907, −0.697 and −0.217 eV respectively. Additionally, the authors utilized the iodometric method to measure the content of photocatalytically H₂O₂ production. It was ultimately concluded that the OPACN hydrogel could photocatalytically generate 1204 µmol/L of H₂O₂ within a 60-min period (Fig. 13d). After 6 times of experiment (Fig. 13e), the production of H₂O₂ only decreased by 56 µmol/L. Ultimately, Fig. 13f showed the reaction mechanism of OPACN hydrogel photocatalytically degrading tetracycline, fixing nitrogen, and producing H₂O₂ under visible light. OPACN hydrogel suppressed the recombination of carriers by constructing dual Z-scheme heterojunction, ensuring higher carrier separation efficiency in the system. At the same time, by changing the band gap, it achieved better visible light utilization. In addition, in OPACN hydrogel, the effective interaction between pectin and OPACN through hydrogen bonds led to a higher electron transfer rate in the system and promoted the generation of superoxide radicals, making the OPACN system had strong photocatalytic activity and reusability.

  Figure 13

  

  Figure 13.  Photoluminescence spectra (a), EIS Nyquist plot (b), Mott-schottky plot (c) and the yield of H₂O₂ (d) for the catalysts. (e) The results of repeatability experiments on the production of H₂O₂ by the OPACN hydrogel. (f) The photocatalytic mechanism illustration of OPACN hydrogel. Copied with permission [238]. Copyright 2024, Elsevier.

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  Ji et al. [244] have engineered an innovative Z-scheme heterojunction photocatalyst consisting of cobalt-iron oxide and perylene diimide organic supramolecule (CoFeO/PDIsm). Within the photocatalytic-self-Fenton system, CoFeO/PDIsm utilized photogenerated h⁺ for the mineralization of organic pollutants, while the photogenerated e⁻ efficiently produced in-situ H₂O₂. As a result, CoFeO/PDIsm exhibited an excellent in-situ photocatalytic H₂O₂ production rate of 281.7 µmol g^-1 h^-1 (Fig. 14a) in the pollutant solution, and the corresponding total organic carbon (TOC) removal rate of ciprofloxacin (CIP) was 63.7% (Fig. 14b), which far exceeded that of existing photocatalysts. In addition, CoFeO/PDIsm exhibited stable and remarkable degradation capabilities for various organic pollutants (Fig. 14c). After multiple cycle experiments, CoFeO/PDIsm still exhibited stable photocatalytic performance. As shown in Fig. 14d, under certain experimental conditions, CoFeO/PDIsm exhibited the best degradation rate constant value (K) of 0.0445 min^-1 for CIP in the photocatalytic self-Fenton system under an air atmosphere. From this, it can be judged that the excellent degradation efficiency in the CoFeO/PDIsm system mainly comes from the in-situ H₂O₂ activated by Fe(Ⅱ). The findings presented in Fig. 14e indicated that, within the photocatalytic self-Fenton system, Fe³⁺ was efficiently reduced to Fe²⁺. This reduction further enhanced the utilization efficiency of H₂O₂, consequently leading to the generation of a substantial quantity of ^∙OH radicals. Specifically, in the photocatalytic self-Fenton system, CoFeO/PDIsm produced ^∙OH radicals at a high concentration, with a generation rate of 144.5 µmol L^-1 h^-1. This rate was 5.7-fold higher than that in the Fenton system, as shown in Fig. 14f. Moreover, CoFeO/PDIsm exhibited an outstanding ^∙OH conversion rate of 50%, significantly surpassing that achieved in the Fenton reaction (Fig. 14g). Thus, it was inferred that the high mineralization ability of CoFeO/PDIsm mainly stemmed from the rapid conversion of in-situ photocatalytic H₂O₂ production to ^∙OH in the photocatalytic self-Fenton system. Ultimately, as depicted in Fig. 14h, the potential mechanism of CIP degradation by CoFeO/PDIsm within the photocatalytic self-Fenton system was proposed. Upon excitation of CoFeO/PDIsm by visible light, the holes in the valence band of PDIsm underwent recombination with the electrons in the conduction band of CoFeO. Subsequently, electrons with enhanced reducing capacity were retained in the conduction band of PDIsm, which could be utilized for the in-situ photocatalytic H₂O₂ production. In the meantime, the h⁺ with higher oxidation ability in the VB of CoFeO combined with ^∙OH, which was utilized for the mineralization of CIP.

  Figure 14

  

  Figure 14.  (a) H₂O₂ production of the samples. (b) The comparison of TOC removal rate. (c) The TOC removal rate of various organic pollutants over CoFeO/PDIsm. (d) The rate constants of the reaction for the degradation of CIP under diverse systems. (e) The concentration of Fe²⁺ under diverse systems. (f) The concentration of radical ^∙OH under diverse systems (f). (g) ^∙OH transformation rate under diverse systems. (h) The postulated mechanism for the degradation reaction induced by CoFeO/PDIsm. Copied with permission [244]. Copyright 2023, Elsevier.

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  5.1.2   Z-scheme heterojunction catalysts for photoelectrocatalytic H₂O₂ production

  Feng et al. [245] synthesized a yolk-shell structured Z-scheme heterojunction composite, designated as YS-CuCo₂S₄@Cu₂O-NR, with an octahedral copper oxide core and a tubular CuCo₂S₄ shell, by adjusting the composition and morphology through a hydrothermal method. YS-CuCo₂S₄@Cu₂O-NR possesses upper-level photocatalytic activity, which can be synergized by spatial confinement effects to selective oxidation of benzyl alcohol (BA) to generate high value-added benzaldehyde (BAD)/degradation of multiple pollutants using Fenton-like reaction, with 80% BA conversion, 99% selectivity (Fig. 15a), 12 mmol L^-1 g^-1 yield to H₂O₂, and >90% degradation efficiency for multiple pollutants. Furthermore, the time-dependent conversion of BA as depicted in the HPLC chromatogram presented in Fig. 15b did not exhibit over-oxidation products. As illustrated in Fig. 15c, for the selective oxidation of BA by YS-CuCo₂S₄@Cu₂O-NR, the conversion and selectivity reached 79.4% and 99.2% respectively at 2.5 h. Additionally, the recyclability of YS-CuCo₂S₄@Cu₂O-NR was investigated through conducting four cyclic photocatalytic processes for BAD production, with each cycle lasting 2.5 h. After four cycles, it retained approximately 88% of its initial activity (Fig. 15d). To delve into the potential oxidation processes of BA, trapping experiments and ESR tests were carried out. As demonstrated in Fig. 15e, the results of the trapping experiments revealed that photogenerated holes and ^∙O₂⁻ radicals were of vital importance in the photocatalytic oxidation of BA. The DMPO-^∙O₂⁻ and DMPO-^∙C radicals were detected, and the characteristic peak signals augmented as time elapsed, suggesting that the ^∙C radical also played a pivotal role in the photocatalytic oxidation of BA (Fig. 15f).

  Figure 15

  

  Figure 15.  The selective photocatalytic oxidation of benzaldehyde over various samples (a), the HPLC chromatogram depicting benzaldehyde and the resultant benzoin dimethyl acetal generated within YS-CuCo₂S₄@Cu₂O-NR (b), time-dominated selective oxidation of benzaldehyde (c), the recyclability of YS-CuCo₂S₄@Cu₂O-NR during the oxidation of benzaldehyde accompanied by the production of H₂O₂ (d), EPR signals recorded for YS-CuCo₂S₄@ Cu₂O-NR under light illumination (e), DMPO-^∙C/^∙O²⁻ in benzaldehyde with YS-CuCo₂S₄@Cu₂O-NR (f). Copied with permission [245]. Copyright 2024, Elsevier.

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  Sun et al. [246] prepared a 3D hollow-sphere NiFe₂O₄@ZnFe₂O₄ (NF@ZF) Z-scheme heterojunction catalyst via the hydrothermal method. Subsequently, an asymmetric-wettability Janus photoelectrode was constructed by loading hydrophobic polytetrafluoroethylene (PTFE) on one face of three-dimensional porous graphite felt (GF) and NF@ZF on the opposite face. This Janus photoelectrode was characterized by a hydrophobic oxygen-storage layer and a hydrophilic catalyst layer. As illustrated in Fig. 16a, the synthesized Janus photocathode demonstrated remarkable photoelectric characteristics, featuring a low onset potential and a swift increase in current density. Moreover, the impact of electrode potential on the H₂O₂ yield was explored by quantitatively measuring the H₂O₂ concentration subsequent to 1 h of photoelectrochemical reduction of O₂. As presented in Fig. 16b, the H₂O₂ yield of the Janus photocathode with asymmetric wettability achieved a maximum of 255.9 mg L^-1 h^-1 at −0.7 V versus Ag/AgCl, accompanied by a Faraday efficiency (FEs) of 72% (Fig. 16c). To assess the stability of the Janus electrode, a long-term photoelectrochemical oxygen reduction reaction was conducted. After 9 h of testing, there was scarcely any alteration in the current (Fig. 16d). The H₂O₂ yield exhibited an approximately linear correlation with the catalytic duration, and the Faraday efficiency remained steadily within the range of 60%−80% (Fig. 16e). Additionally, the oxidation of arsenite (As(Ⅲ)) to arsenate (As(Ⅴ)) and the transformation/degradation capability of roxarsone (ROX) in the Janus‖Fe in-situ Fenton system were investigated. The results suggested that a potential of −0.7 V was adequate for the oxidation process, and the optimal pH value of the system was 3 (Fig. 16f). The degradation efficiency of ROX reached nearly 100% within 120 min, while 20 ppm of As(Ⅲ) was almost entirely oxidized to As(Ⅴ) within 45 min (Fig. 16g). Finally, the oxidation mechanism of As(Ⅲ) in the Janus‖Fe in-situ Fenton system was shown in Fig. 16h.

  Figure 16

  

  Figure 16.  LSV curves of different electrodes (a), the influence of different applied potentials on the production of H₂O₂ (b), H₂O₂ yields and FEs of different electrodes (c), stability experiment for the H₂O₂ production via PEC (d), H₂O₂ yield and FEs of the Janus electrode (e), the influence of different conditions on As(Ⅲ) oxidation and ROX degradation (f), both the conversion efficiency of As(Ⅲ) and the degradation efficiency of ROX over different systems (g), schematic illustration of the mechanism underlying the photoelectrochemical process (h). Copied with permission [246]. Copyright 2024, Elsevier.

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  5.2 S-scheme heterojunction catalysts for H₂O₂ production

  5.2.1   S-scheme heterojunction catalysts for photocatalytic H₂O₂ production

  Jing et al. [247] prepared an S-scheme heterojunction composite (MCN@CdS) by loading CdS nanoparticles onto square-tubular g-C₃N₄ (MCN) via the solvothermal method. Under visible-light irradiation, MCN@CdS exhibited excellent dual photocatalytic properties. It can not only photocatalytically reduce oxygen to produce H₂O₂, but also had a strong ability to photocatalytically degrade antibiotics. As shown in Figs. 17a and b, the H₂O₂ production rate of MCN@CdS can reach 0.95 mmol/L within 90 min, and the degradation rate of TC was as high as 99.5% within 120 min. After five cyclic experiments, each lasting for 90 min, the photocatalytic production of H₂O₂ by MCN@CdS decreased by only 0.1 mmol/L, indicating that MCN@CdS had a certain degree of stability. The larger rate constant (k value) of MCN@CdS also reflected its better photocatalytic performance (Fig. 17c). In addition, MCN@CdS exhibited the lowest emission intensity in Fig. 17d, indicating that MCN@CdS had the highest separation efficiency of photo-carriers. As shown by the results of time-resolved PL spectroscopy (TRPL) in Fig. 17e, the composite exhibited a longer carrier lifetime. Moreover, the superior photoelectric properties of MCN@CdS were verified in Figs. 17f and g. The results indicated that MCN@CdS was more conducive to charge transfer and had a higher photocurrent intensity under visible light. Finally, the mechanism of the coupling of H₂O₂ production and TC degradation by photocatalysis over MCN@CdS S-scheme heterojunction catalyst was presented in Fig. 17h.

  Figure 17

  

  Figure 17.  Photocatalytic H₂O₂ production by the samples (a), degradation performance (b) and the corresponding degradation kinetics (c) of TC by the samples, PL (d), TRPL (e) and EIS (f) spectra, along with photocurrent response (g) of different samples, mechanism diagram of the coupling of H₂O₂ production and TC degradation by photocatalysis of MCN@CdS S-scheme heterojunction composite (h). Copied with permission [247]. Copyright 2024, Elsevier.

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  Du et al. [248] constructed a photo-self-Fenton system based on resorcinol-formaldehyde resin/MIL-88A(Fe) (RF/88A) S-scheme composite. This system facilitates the in-situ photocatalytic H₂O₂ production when exposed to visible-light irradiation, which is then utilized for the degradation of TC and sterilization purposes. As shown in Fig. 18a, within 180 min, RF/88A can achieve a degradation efficiency of over 80% for TC upon visible-light irradiation. In addition, within the tests of antibacterial activity, 35%-RF/88A nearly eliminated 100% of both Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) in 60 min upon visible-light irradiation (Figs. 18b-e). After five consecutive repeated experiments, each lasting for 3 h, the efficiency of photocatalytic self-Fenton degradation of TC by 35%-RF/88A decreased by <10%, which proved that 35%-RF/88A exhibited substantial photocatalytic stability. Moreover, Sun et al. [249] constructed a photocatalysis-self-Fenton system built upon a 2D/2D Bi₂Fe₄O₉/ZnIn₂S₄ (BFO/ZIS) S-scheme catalyst, which was capable of achieving highly efficient degradation of antibiotic pollutants. As shown in Fig. 18f, under pure water conditions without additional oxygen supply and sacrificial agents, in 2 h, the H₂O₂ production efficiency via photocatalysis of BFO/ZIS approached 30 µmol/L. This is because after the formation of the heterostructure, the decrease in the photocatalytic H₂O₂ concentration in BFO/ZIS may be ascribed to the Fenton reaction that occurs between H₂O₂ and iron species throughout the process of in-situ photocatalytic H₂O₂ production, leading to the activation of H₂O₂ to generate ^∙OH radicals. The BFO/ZIS composite with the optimal performance achieved an excellent degradation efficiency of 88.8% for TC after 2 h of visible-light illumination (Fig. 18g). After four consecutive cyclic tests, each lasting for 2.5 h, BFO/ZIS still showed remarkable photocatalytic self-Fenton degradation efficiency for TC. Finally, the potential mechanism of antibiotic degradation by the photocatalysis-self-Fenton system was proposed in Fig. 18h. Specifically, in BFO/ZIS, ZIS promoted the in-situ production of H₂O₂ from deionized water through photocatalysis, while BFO provided Fe²⁺ as a Fenton reagent. The synergistic effect between them constituted a self-sufficient photocatalysis-self-Fenton system, which was employed for the degradation of antibiotics.

  Figure 18

  

  Figure 18.  TC degradation efficiency of diverse samples (a), the cell density of E. coli (b) and S. aureus (c) for diverse samples, the antibacterial performances of diverse samples for E. coli (d) and S. aureus (e). Copied with permission [248]. Copyright 2024, Elsevier. Concentration of H₂O₂ production curves for different samples (f), the TC degradation performance upon diverse conditions (g), possible mechanism for TC degradation of BFO/ZIS (h). Copied with permission [249]. Copyright 2024, Elsevier.

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  5.2.2   S-scheme heterojunction catalysts for photoelectrocatalytic H₂O₂ production

  Recently, Chen et al. [234] employed the ion exchange strategy to create a hollow tubular In₂S₃/MnIn₂S₄ S-scheme composite. Capable of triggering the ORR on the hydrophobic In₂S₃/MnIn₂S₄/PVDF/NF photo-anode and the ClOR on the hydrophilic In₂S₃/MnIn₂S₄/CP photo-cathode, the catalyst led to the build-up of practical concentrations of H₂O₂ and HClO, attaining levels as high as 2108 µmol/L and 28.5 mg/L respectively. Through the utilization of a dual-electrode co-photoelectrocatalytic system involving O₂ and Cl⁻, which replaced the oxygen evolution reaction (OER) with ClOR, H₂O₂ and HClO were generated simultaneously at a lower voltage in an H-type cell (Fig. 19a). In a single-chamber cell, the electrodes can trigger the activation of H₂O₂ and HClO, which then act in synergy to yield singlet oxygen (¹O₂) and numerous hydroxyl radicals (^∙OH). This combined action effectively leads to the breakdown of organic pollutants. Fig. 19b illustrated the linear sweep voltammetry (LSV) curves for the various systems. Compared to the PEC ORR||OER system, the PEC ORR||ClOR electrolysis demanded a relatively reduced potential, which effectively illustrated the energy-conservation benefit of supplanting OER with ClOR. As depicted in Fig. 19c, under the condition of 1.2 V Ag/AgCl, the accumulated concentrations of H₂O₂ and HClO within 2 h respectively reached 1.49 mmol/L and 17.3 mg/L. Fig. 19d demonstrated that the electrolytic cell coupled with both reactions exhibited good yields for both H₂O₂ and HClO, and the photolytic stability could be maintained for up to 8 h. Additionally, Fig. 19e presented the results of MB degradation for each system, with the H₂O₂-HClO coupled system exhibiting the strongest degradative capability. Combining the analysis of various characterization results, the authors have proposed the mechanism for the improved Fenton-like degradation by the H₂O₂-HClO coupled system when exposed to visible light (Fig. 19f). This work provides a synergistic strategy for the concurrent generation of H₂O₂ and HClO acid, offering an efficient solution for pollutant degradation in the realms of environmental and energy engineering. Finally, as depicted in Fig. 19g, the complete mechanism of In₂S₃/MnIn₂S₄ photoelectrocatalytic system for degrading organic pollutants through the production of H₂O₂/HClO was presented. In addition, Table S1 (Supporting information) summarized representative Z-scheme and S-scheme heterojunction catalysts for the production of H₂O₂ in the fields of photocatalysis [250-260].

  Figure 19

  

  Figure 19.  H₂O₂ production at the anode (a), LSV plots of In₂S₃/MnIn₂S₄/PVDF/NF||In₂S₃/MnIn₂S₄/CP (b), different potentials for the production of H₂O₂ and HClO (c), recyclability and stability of ORR||ClOR system (d), degradation efficiency of MB in different systems (e), mechanism of ^∙OH activation (f), mechanism diagram of organic pollutant degradation by H₂O₂/HClO production in In₂S₃/MnIn₂S₄ photoelectrocatalytic system (g). Copied with permission [234]. Copyright 2024, Elsevier.

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  6. Conclusion and outlook

  This review presents an exhaustive review of the mechanisms underpinning the photocatalytic and photoelectrocatalytic H₂O₂ production, utilizing Z-scheme and S-scheme heterojunction catalysts. Additionally, it discusses several research findings pertaining to the photocatalytic and photoelectrocatalytic H₂O₂ production. In particular, the resolution of H₂O₂ decomposition poses a significant challenge, and the development of effective strategies for H₂O₂ extraction and photocatalyst recovery is of paramount importance. Despite the notable advancements in Z-scheme and S-scheme catalysts for photocatalytic and photoelectrocatalytic H₂O₂ production in recent years, the quest for catalysts that exhibit high activity, selectivity, and stability continues to be a pivotal focus within the domains of photocatalytic and photoelectrocatalytic H₂O₂ production.

  The catalytic systems and their operational environments are also undergoing continuous evolution [261-265]. The fabrication of bifunctional catalysts capable of mediating both 2e⁻ ORR and 2e⁻ WOR presents expanded opportunities for the efficient photocatalytic and photoelectrocatalytic H₂O₂ production and the effective partitioning and exploitation of photogenerated electron-hole pairs. To augment the separation efficiency of these photogenerated charge carriers, the utilization of sacrificial agents is a prevalent strategy. However, these agents pose a significant environmental pollution risk. Consequently, there is a pressing need to prioritize the development of catalytic systems that can operate without the reliance on sacrificial agents, thereby mitigating their environmental impact. In addition to the use of pure water systems, the production of H₂O₂ from seawater is beginning to receive attention. The implementation of a seawater environment further alleviates the constraints on the preparatory conditions necessary for H₂O₂ production. H₂O₂ can be synthesized under non-extreme, nature-simulating conditions, which is undoubtedly a significant milestone in harnessing energy from natural sources and represents an exciting advancement in the field. However, it must be acknowledged that the current system is confined to laboratory-scale operations, and further efforts are imperative to translate this technology into practical industrial applications.

  In the realm of photocatalytic and photoelectrocatalytic H₂O₂ production, there exist several avenues of thought that warrant in-depth discussion and research [266-270]. Fig. 20 mainly summarizes the potential development directions of Z-scheme and S-scheme heterojunction materials in the field of photocatalytic and photoelectrocatalytic H₂O₂ production. First and foremost, enhancing the oxidation resistance and photocorrosion resistance of both the ion-exchange membrane and the catalyst is imperative for ensuring the long-term stability within an environment rich in high-concentration H₂O₂ [271-275]. This enhancement is pivotal in substantially improving the cyclic stability of the integral reaction system. Secondly, enhancing the mass transfer efficiency within the device is achievable through a meticulous redesign of the flow cell's channel dimensions and their spatial configuration. In parallel, the strategic placement of spacer groups at the interfacial region can further contribute to a significant improvement in mass transfer efficiency [276-280]. Thirdly, the decomposition of H₂O₂ is a critical factor that must be addressed. By inhibiting the decomposition of H₂O₂ during the production process and optimizing the collection methodology, the overall production efficiency of the system can be significantly enhanced [281-285]. Fourthly, beyond the photocatalytic and photoelectrocatalytic H₂O₂ production within a standalone system, integrating the H₂O₂ production reaction system with other processes, such as wastewater treatment, hydrogen generation, and organic synthesis, can construct a catalytic system with enhanced economic value. This approach aims to achieve a multifunctional and highly efficient H₂O₂ production reaction [286-290]. Fifthly, it is essential to explore superior pathways for the catalytic reaction aimed at enhancing the high activity and high selectivity in the photocatalytic and photoelectrocatalytic synthesis of H₂O₂ [291-294]. Sixthly, efforts should be directed towards immobilizing the catalyst on a membrane structure to facilitate the efficient production of H₂O₂. Concurrently, this approach enhances the catalyst's recyclability, thereby realizing a catalytic system that is both sustainable and facile in terms of recovery and reuse [295-297]. Seventhly, promote the process of implementing photocatalytic H₂O₂ production from the laboratory stage to the industrialization stage. Ultimately, it is anticipated that in the imminent future, the possibility of using machine learning strategies to design heterojunction catalysts for the photocatalytic and photoelectrocatalytic H₂O₂ production can be explored, so as to enhance the scientificity and efficiency of catalyst preparation [298-301].

  Figure 20

  

  Figure 20.  A schematic diagram of the research directions and development prospects of photocatalytic and photoelectrocatalytic H₂O₂ production.

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  Declaration of competing interest

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

  CRediT authorship contribution statement

  Xibao Li: Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation. Yiyang Wan: Writing – original draft, Visualization, Validation, Software, Resources, Methodology, Investigation, Formal analysis, Data curation. Fang Deng: Writing – original draft, Software, Resources, Methodology, Investigation, Formal analysis, Data curation. Yingtang Zhou: Writing – original draft, Software, Resources, Methodology, Investigation, Formal analysis, Data curation. Pinghua Chen: Writing – original draft, Validation, Software, Resources, Methodology, Investigation. Fan Dong: Writing – review & editing, Software, Resources, Methodology, Investigation, Funding acquisition. Jizhou Jiang: Writing – review & editing, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.

  Acknowledgments

  The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (Nos. 22262024, 52470078, 62004143), Jiangxi Province Academic and Technical Leader of Major Disciplines (No. 20232BCJ22008), the Key Project of Natural Science Foundation of Jiangxi Province (Nos. 20232ACB204007), Double Thousand Talent Plan of Jiangxi Province, the Natural Science Foundation of Jiangxi Province (No. 2022ACB203014), the Key R&D Program of Hubei Province (No. 2022BAA084), and the Innovation Project of Engineering Research Center of Phosphorus Resources Development and Utilization of Ministry of Education (No. LCX202404).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111418.

  
  
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  Figure 1  Summary of this review content.

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  Figure 2  Schematic diagram of relevant mechanisms of the photocatalytic H₂O₂ production.

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  Figure 3  (a) Preparation route of CdS/RF composite. Copied with permission [46]. Copyright 2023, Elsevier. (b) The synthesis process for iCOF/BO composite. Copied with permission [203]. Copyright 2024, Editorial Office of Acta Physico-Chimica Sinica. (c) Synthetic procedure of TiO₂@BTTA composites. Copied with permission [201]. Copyright 2023, Elsevier.

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  Figure 4  (a) Synthesis route of ZnO/PBD. Copied with permission [221]. Copyright 2024, Editorial Office of Acta Physico-Chimica Sinica. (b) Formation illustration of WS₂/S-g-C₃N₄ Z-scheme heterostructures. Copied with permission [119]. Copyright 2024, Elsevier. (c) Schematic representation of preparing ZIS-Z/OCN. Copied with permission [122]. Copyright 2023, Elsevier.

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  Figure 5  (a) Schematic illustration of the formation of the ZT heterojunction. Copied with permission [222]. Copyright 2022, Elsevier. (b) Schematic representation of the synthesis process for 3DOM SCN/T. Copied with permission [224]. Copyright 2023, Elsevier. (c) Schematic depiction of the preparation process for Sol-BP/BiOBr composites. Copied with permission [225]. Copyright 2020, Elsevier.

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  Figure 6  (a) The fabrication process of PDI-Ala/S-C₃N₄. Copied with permission [227]. Copyright 2021, Editorial Office of Acta Physico-Chimica Sinica. (b) Synthetic pathways of CoPc/K/Na/PCN. Copied with permission [228]. Copyright 2024, Elsevier.

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  Figure 7  (a) Schematic representation of the fundamental synthetic protocol for ZCx. Copied with permission [231]. Copyright 2024, Elsevier. (b) Schematic illustration of the synthesis process for ZnO/ZnIn₂S₄. Copied with permission [232]. Copyright 2023, Elsevier. (c) Schematics for the synthesis of ZnO/CuInS₂ photocatalyst. Copied with permission [50]. Copyright 2024, Wiley-VCH.

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  Figure 8  (a) The synthetic process of the TCNx sample. Copied with permission [233]. Copyright 2024, Elsevier. (b) The illustration to prepare In₂S₃/MnIn₂S₄ catalyst. Copied with permission [234]. Copyright 2024, Elsevier. (c) Schematic showing the synthesis process of CN/HMoP synthesis. Copied with permission [127]. Copyright 2024, Elsevier. (d) Creative synthetic method of ZnIn₂S₄/Bi₂S₃ hybrids in this work. Copied with permission [235]. Copyright 2024, Wiley-VCH.

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  Figure 9  (a) Scheme of iML method. Copied with permission [236]. Copyright 2022, Elsevier. (b) Selection methodology for SACs exhibiting elevated selectivity and catalytic activity. Copied with permission [237]. Copyright 2023, Elsevier.

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  Figure 10  ESR detection for DMPO-^∙O₂⁻ (a), DMPO-^∙OH (b) and TEMPO-h⁺ (c) under dark and visible-light irradiation, schematic illustrations of WO_2.72 and 0.5S-pCN before and after contact, as well as the formation of IEF and band bending (d). Copied with permission [128]. Copyright 2023, Elsevier. The photocatalytic H₂O₂ production yield of 40ZIS-Z/OCN under various conditions (e), ESR spectra of DMPO-^∙OH (f), DMPO-^∙O₂⁻ (g) and TEMP-¹O₂ (h) for 40ZIS-Z/OCN under visible light irradiation. Copied with permission [122]. Copyright 2023, Elsevier. Transient photocurrent response (i), EIS analysis (j) and K-L curves (k) of ZnO, ZnIn₂S₄, and ZZS-20. Copied with permission [232]. Copyright 2023, Elsevier.

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  Figure 11  In-situ FTIR spectra of a series of OPACN composites during the photocatalytic H₂O₂ production (a) and an enlarged view of the FTIR peak (b, c). Copied with permission [238]. Copyright 2024, Elsevier. (d-f) In-situ XPS spectra of C, N and Bi in CNQDs/BiOBr-1.50%. Copied with permission [239]. Copyright 2023, Editorial Office of Acta Physico-Chimica Sinica. (g) In-situ ESR spectra of 0.1% Au/TiO₂ composites and pure TiO₂. Copied with permission [240], Copyright 2021, MDPI. (h) In-situ DRIFTS of CDs₁₀MCN collected in the H₂O₂ photocatalytic processes in the O₂ atmosphere. Copied with permission [241]. Copyright 2024, Wiley-VCH. In-situ KPFM potential diagram in dark (i), under light irradiation (j), and surface potential (k) of PCN(5)/MnS-D. Copied with permission [150]. Copyright 2023, Elsevier.

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  Figure 12  Optimal configurations of H₂O₂ adsorption on TiO₂ (101) (a) and 2H-MoSe₂ (103) planes (b). Copied with permission [242]. Copyright 2022, Elsevier. Optimized unit cells of states for CDs₁₀MCN (c) and free energy diagram of CDs₁₀MCN for photocatalytic H₂O₂ production (d). Copied with permission [241]. Copyright 2024, Wiley-VCH. (e-k) The theoretical calculation results regarding the Zn-N₄ and Zn-N₃O models. Copied with permission [243]. Copyright 2024, Wiley-VCH.

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  Figure 13  Photoluminescence spectra (a), EIS Nyquist plot (b), Mott-schottky plot (c) and the yield of H₂O₂ (d) for the catalysts. (e) The results of repeatability experiments on the production of H₂O₂ by the OPACN hydrogel. (f) The photocatalytic mechanism illustration of OPACN hydrogel. Copied with permission [238]. Copyright 2024, Elsevier.

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  Figure 14  (a) H₂O₂ production of the samples. (b) The comparison of TOC removal rate. (c) The TOC removal rate of various organic pollutants over CoFeO/PDIsm. (d) The rate constants of the reaction for the degradation of CIP under diverse systems. (e) The concentration of Fe²⁺ under diverse systems. (f) The concentration of radical ^∙OH under diverse systems (f). (g) ^∙OH transformation rate under diverse systems. (h) The postulated mechanism for the degradation reaction induced by CoFeO/PDIsm. Copied with permission [244]. Copyright 2023, Elsevier.

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  Figure 15  The selective photocatalytic oxidation of benzaldehyde over various samples (a), the HPLC chromatogram depicting benzaldehyde and the resultant benzoin dimethyl acetal generated within YS-CuCo₂S₄@Cu₂O-NR (b), time-dominated selective oxidation of benzaldehyde (c), the recyclability of YS-CuCo₂S₄@Cu₂O-NR during the oxidation of benzaldehyde accompanied by the production of H₂O₂ (d), EPR signals recorded for YS-CuCo₂S₄@ Cu₂O-NR under light illumination (e), DMPO-^∙C/^∙O²⁻ in benzaldehyde with YS-CuCo₂S₄@Cu₂O-NR (f). Copied with permission [245]. Copyright 2024, Elsevier.

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  Figure 16  LSV curves of different electrodes (a), the influence of different applied potentials on the production of H₂O₂ (b), H₂O₂ yields and FEs of different electrodes (c), stability experiment for the H₂O₂ production via PEC (d), H₂O₂ yield and FEs of the Janus electrode (e), the influence of different conditions on As(Ⅲ) oxidation and ROX degradation (f), both the conversion efficiency of As(Ⅲ) and the degradation efficiency of ROX over different systems (g), schematic illustration of the mechanism underlying the photoelectrochemical process (h). Copied with permission [246]. Copyright 2024, Elsevier.

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  Figure 17  Photocatalytic H₂O₂ production by the samples (a), degradation performance (b) and the corresponding degradation kinetics (c) of TC by the samples, PL (d), TRPL (e) and EIS (f) spectra, along with photocurrent response (g) of different samples, mechanism diagram of the coupling of H₂O₂ production and TC degradation by photocatalysis of MCN@CdS S-scheme heterojunction composite (h). Copied with permission [247]. Copyright 2024, Elsevier.

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  Figure 18  TC degradation efficiency of diverse samples (a), the cell density of E. coli (b) and S. aureus (c) for diverse samples, the antibacterial performances of diverse samples for E. coli (d) and S. aureus (e). Copied with permission [248]. Copyright 2024, Elsevier. Concentration of H₂O₂ production curves for different samples (f), the TC degradation performance upon diverse conditions (g), possible mechanism for TC degradation of BFO/ZIS (h). Copied with permission [249]. Copyright 2024, Elsevier.

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  Figure 19  H₂O₂ production at the anode (a), LSV plots of In₂S₃/MnIn₂S₄/PVDF/NF||In₂S₃/MnIn₂S₄/CP (b), different potentials for the production of H₂O₂ and HClO (c), recyclability and stability of ORR||ClOR system (d), degradation efficiency of MB in different systems (e), mechanism of ^∙OH activation (f), mechanism diagram of organic pollutant degradation by H₂O₂/HClO production in In₂S₃/MnIn₂S₄ photoelectrocatalytic system (g). Copied with permission [234]. Copyright 2024, Elsevier.

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  Figure 20  A schematic diagram of the research directions and development prospects of photocatalytic and photoelectrocatalytic H₂O₂ production.

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