Research progress of heterogeneous photocatalyst for H2O2 production: A mini review
Jiaming Li , Na Xu , Yafei Zhang , Hongjun Dong , Chunmei Li
H2O2 is an environmentally friendly oxidant that has a wide range of applications in biomedicine [1], wastewater treatment [2] and chemical synthesis [3]. The traditional production of H2O2 is mainly based on the anthraquinone (AQ) oxidation process [4-6]. In this process, AQ process is hydrogenated using a nickel catalyst and then oxidized by air to produce H2O2 [7-9]. Subsequently, further extraction, purification, and distillation are required to obtain commercial H2O2. However, the AQ process is energy-intensive, polluting, and expensive, posing a significant challenge to the environmental and economic viability of H2O2 synthesis [10-12].
Semiconductor-based photocatalysis provides an economically viable method for harnessing solar energy, converting it into chemical energy, and storing it [13-18]. The generation of H2O2 by solar-driven photocatalytic O2 reduction and/or H2O oxidation is considered a viable strategy to replace the anthraquinone oxidation process in the future [19-21]. The studies on photocatalytic H2O2 production have increased significantly in recent years. In this photocatalytic process, the choice of catalyst is crucial [22-24]. However, the efficiency of photocatalytic H2O2 production is still unsatisfactory because of the rapid recombination of electron-hole pairs and slow reaction kinetics [25-28]. Therefore, it is imminent to search for feasible modification strategies to improve the photocatalytic activity of H2O2 production for future environmental and energy-related applications.
Over the past decade, researchers have tried many varieties of modification methods to have been developed to improve this activity of photocatalytic H2O2 production, including doping metallic or nonmetallic elements [29], vacancy engineering [30], surface engineering [31], nanoparticle deposition [32], heterostructure construction [33], combinations of two or more methods, and so on [34-36]. For example, Deng et al. controlled the polymerization of dopamine (DA) on the surface of CN nanosheets into polydopamine (PDA) and obtained a CN-PDA material rich in pyrrolidone nitrogen [37]. The CN-PDA is rich in pyrrolidinyl N atoms, which can be rapidly depleted by the protonation of superoxide radicals (·O2−) and converted to H2O2. The H2O2 generation efficiency of the CN-PDA material is 47 times higher than that of the normal CN material without adding any sacrificial agent. In another study, Yue et al. synthesized a covalent organic skeleton (COF) material with a cpt topology having a six-armed core (TBD-COF) [38]. Among them, TBD-COF was synthesized from a C3-symmetric molecule with six aldehyde groups, named 4′,4′″,4′″″-(1,3,5-triazine-2,4,6-triyl) tris(([1,1′-biphenyl]−3,5-diacetaldehyde) (TBD)) and C2-symmetric 2,2′-bipyridine-5,5′-diamine (BPY) as a building block synthesis. TBD-COF with cpt topology could drive H2O2 photosynthesis via O2-·O2−–H2O2, O2-·O2−−1O2nullH2O2, and H2O—H2O2 channels at a rate of 5448 µmol/L. The energy barriers of rate determination steps (RDS) in the three channels during H2O2 photosynthesis were effectively reduced by topological adjustment. These above strategies contributed to the efficiency and sustainability goals to varying degrees.
Notably, in the modification methods reported above, heterojunction materials have been shown to be one of the most promising approaches for the preparation of highly efficient photocatalysts [39-41]. For example, Cheng et al. synthesized covalently linked molecular heterojunctions via successive C—H activation and Knoevenagel polymerization reactions [42]. The electron-deficient benzodithiene-4,8‑dione (BTD) reacts with the electron-rich 4,4′-dibromobiphenyl (B) and then copolymerizes with the electron-rich p-phthalonitrile (CN) to form a molecular heterostructure (BTDBCN). This heterostructure showed excellent H2O2 synthesis ability under visible light and no sacrificial agent. The optimal photocatalytic H2O2 generation rate of the BTDBCN heterostructure reached 1920 µmol g−1 h−1, which was 2.2 and 11.6 times higher than that of the BTDB and BTDCN materials alone, respectively. In addition, Wei et al. constructed the S-scheme heterojunction Cs3PMo12/CC by introducing insoluble cerium phosphomolybdate (Cs3PMo12) into carbonized cellulose (CC) [43]. Cs3PMo12/CC achieved dual-channel H2O2 photosynthesis with H2O oxidation as well as O2 reduction in pure water. The reaction system was exposed to natural outdoor light, and 240 µmol/L H2O2 was successfully obtained. These excellent activities are due to the below unique advantage of heterostructure photocatalysts in photocatalytic H2O2 generation reactions [44]. (1) The enhanced separation efficiency of photogenerated charge carriers: Interfaces in heterostructures can create built-in electric fields that facilitate the separation of photogenerated electrons and holes, reducing recombination rates, which enhances the effective utilization of electrons and holes, thereby increasing H2O2 production efficiency [45]. (2) Band structure regulation: By designing appropriate heterostructures, the band positions can be optimized, allowing photogenerated electrons and holes to have suitable energy levels for oxygen reduction reaction (ORR) and water oxidation reaction (WOR), thereby increasing the rate of H2O2 production [6]. (3) The improved light absorption capability: heterostructures can combine different light-absorbing materials to extend the range of light absorption, particularly in the visible light region, which increases the generation of photogenerated carriers, thereby boosting H2O2 production efficiency [46]. (4) The promotion of heterogeneous reactions: interfaces in heterostructures can provide more active sites, promoting the adsorption and reaction of O2 and H2O on the surface of the photocatalyst, increasing the reaction rate and selectivity for H2O2 production. (5) The increased reaction selectivity: through interface effects and surface modifications, heterostructures can regulate reaction pathways to preferentially catalyze the production of H2O2 and reduce side reactions [11]. (6) The enhanced stability and photocorrosion resistance: heterostructures can improve the chemical and photostability of the photocatalyst and reduce photocorrosion inferior, which extended the catalyst's lifespan and maintained high H2O2 production efficiency [47]. Therefore, the heterostructured photocatalysts significantly enhance the efficiency and selectivity of H2O2 production through various synergistic mechanisms. However, despite obtaining significant research progress, a comprehensive review of heterojunctions in the field of photocatalytic generation of H2O2 is still lacking, and thus a systematic summary and analysis is necessary.
In this work, the potential applications of heterojunctions for the photocatalytic generation of H2O2 are reviewed and evaluated. Firstly, the photocatalytic reaction process of heterojunctions and the mechanism of photocatalytic production of H2O2 are briefly described, including the catalytic mechanisms of type-Ⅰ, type-Ⅱ, Z-scheme, S-scheme, and Schottky heterojunctions. The report summarizes the two main reaction pathways for photogeneration of H2O2, including ORR and WOR. Subsequently, the major recent research advances in this field are summarized. Through analyzing the structural design and performance enhancement methods of various heterojunction catalysts, their functions and mechanisms in improving the efficiency of H2O2 generation are explored. Finally, the current research problems and the future development direction are reasonable. The general overview based on heterojunction materials for photogeneration H2O2 is shown in Fig. 1 [48-53].
Photocatalytic technologies driven by solar energy have attracted much attention in the past decades and have been successfully applied in several research areas, such as water separation to produce H2 [54] and O2 [55], CO2 reduction [56], pollutant degradation [57], and H2O2 generation [58]. In photocatalyst-induced redox reactions, the photocatalytic activity is controlled by several key processes [15,59]. Firstly, the photocatalytic semiconductor material absorbs photons with energy exceeding its band gap, causing electrons in the valence band (VB) to jump to the conduction band (CB), thereby exciting the generation of electron (e−)-hole (h+) pairs [52,60,61]. Subsequently, the isolated photogenerated holes and electrons transfer to the photocatalyst surface and further participate in the reduction and oxidation processes on the photocatalyst surface [62-64].
However, photogenerated electrons and holes may recombine inside or on the surface of the photocatalyst, thus wasting energy without participating in the chemical reaction, which hinders the improvement of photocatalytic efficiency [65,66]. To overcome this problem, constructing heterojunction materials has become an effective strategy. Heterojunctions combine materials with different energy levels, providing lower energy levels to accommodate excited e− or h+ [67]. This structural design promotes the efficient separation and transfer of holes and electrons, thus significantly improving the efficiency of photocatalytic reactions [68]. Compared with other photocatalysts, heterojunction photocatalysts have significant advantages in H2O2 production. Firstly, heterojunction materials have high charge separation efficiency. The heterojunction structure promotes the separation of photogenerated electrons and holes through interfacial effects, which improves the photocatalytic efficiency by reducing the complexation of charge carriers. Moreover, the heterojunction is able to selectively promote the 2e− ORR reaction step and improve the selectivity of H2O2 generation by modulating the energy band structure. Besides, heterojunctions are usually classified into five types depending on the adjacent band structures and electron transfer mechanisms: Type-Ⅰ heterojunctions, type-Ⅱ heterojunctions, Z-scheme heterojunctions, S-scheme heterojunctions, and Schottky heterojunction [69].
As for type-Ⅰ heterojunction photocatalysts (Fig. 2a), the VB and CB positions of semiconductor A are lower and higher than those of semiconductor B, respectively [70]. However, the oxidation and reduction reactions occur on semiconductors with lower redox potentials, respectively, which may limit the redox capability of type-Ⅰ heterojunction photocatalysts. Additionally, electrons and holes eventually accumulate on the same semiconductor, which prevents the effective separation of electron-hole pairs and thus reduces the photocatalytic efficiency [71].
For the type-Ⅱ heterojunctions (Fig. 2b), photogenerated electrons and holes can be transferred to lower energy levels in diverse materials [72]. The potential difference between semiconductors A and B creates an electric field at the interface, which effectively drives the separation of photogenerated charge carriers. As a result, electron-hole pairs can be efficiently separated as well as participate in successive oxidation and reduction reactions [73]. Appropriate band overlap allows e− and h+ to accumulate in diverse materials, which is crucial for the design of effective heterojunction photocatalysts.
In the Z-scheme heterojunction, the band arrangement is akin to that of a type-Ⅱ heterojunction, but the electron transfer mechanism is very different [74]. As given in Fig. 2c, the Fermi level of semiconductor B is lower than that of semiconductor A. When semiconductors A and B are in contact, electrons migrate from semiconductor A to semiconductor B through the interface until the Fermi level reaches equilibrium. At this point, a built-in electric field is formed at the interface between the positively charged semiconductor A and the negatively charged semiconductor B. Under the influence of this built-in electric field, the photogenerated electrons in the CB of semiconductor B as well as photogenerated holes in the VB of semiconductor A recombine at the interface of the two photocatalysts [75]. In order to inhibit charge recombination, photogenerated electrons with weak reduction potentials and photogenerated holes with weak oxidation potentials are actively sacrificed, thereby retaining photogenerated carriers with strong redox capabilities. These efficient carriers subsequently drive the oxidation and reduction reactions, which significantly improve the overall performance of the photocatalyst [76].
The S-scheme heterojunction is a coupling of an oxidizing photocatalyst (OP) and a reducing photocatalyst (RP). The staggered distribution of the two semiconductors due to different work functions forms an S-scheme heterojunction via an interlocking mode (Fig. 2d). OP and RP are in close contact, resulting in the bending of the Fermi energy levels in the interfacial region until the Fermi energy levels of the two photocatalysts reach equilibrium. The unique structure of this heterojunction is based on the interleaved band positions between the two photocatalysts. Due to the macroscopic step-like shape of the charge transfer path, it has also become a step heterojunction. The construction of the S-scheme system can effectively balance the photogenerated charge carriers with the weak redox capacity, which enhances the overall redox driving force of the system [77-79].
Schottky heterojunction is an interfacial structure formed between a metal and a semiconductor material (Fig. 2e). When a semiconductor is in contact with a metal, because of the different work function (Φ) of the metal as well as the semiconductor, electrons flow from the semiconductor with lower Φ to the metal with higher Φ until their Fermi energy levels reach equilibrium [80]. At this point, a space charge layer forms in the semiconductor due to the free electron density limit, and the upward bending of the energy bands forms a Schottky barrier. The Schottky barrier can improve the photogenerated electron transfer efficiency of photocatalytic materials and improve the utilization efficiency of photogenerated carriers, which can enhance the photocatalytic activity obviously.
The whole process of photocatalytic generation of H2O2 can be described by the equation in Fig. 3. Fundamentally, the photoproduction of H2O2 consists of two complementary half-reactions: ORR and WOR [81-83]. When a photon is absorbed by the photocatalyst, electrons are excited from the VB to the CB, generating photo-induced e− and h+ in the CB and VB, respectively. These photogenic charge carriers then transfer to the photocatalyst surface and take part in a series of redox reactions to selectively generate H2O2 [84,85].
In the photocatalytic process of ORR, the generation of H2O2 can be achieved through a one-step two-electron (2e−) process and a two-step single-electron process [86]. Since photogenerated electrons in the CB can reduce oxygen, the CB energy level position is a key parameter for evaluating the ORR process [46,87,88]. For semiconductors with CB redox potentials ranging from −0.33 V to +0.68 V vs. NHE, H2O2 can be produced via a direct 2e− transfer pathway (Eq. 2). However, as the redox potential becomes positive, the thermodynamic driving force for the generation of H2O increases (Potential of O2/H2O = +1.23 V vs. NHE). This means that for photocatalysts with CB positions greater than +0.68 V vs. NHE, the synthesis of H2O2 becomes thermodynamically unfavourable in favour of the 4e− transfer process that generates H2O (Eq. 3) [89].
In a two-step single-electron transfer ORR, O2 is first reduced to ·O2− (Eq. 1). Subsequently, ·O2− is further converted to H2O2 (Eq. 5) [62]. Since the redox potential of O2/·O2− is −0.33 V vs. NHE, the CB position of the semiconductor needs to be lower than −0.33 V vs. NHE in order to drive the one-electron transfer ORR. This CB position also meets the thermodynamic requirements of the 2e− transfer ORR. While the single-electron transfer ORR is kinetically favourable, it requires a larger band gap to ensure thermodynamic viability, which restricts the light absorption ability of the photocatalyst [90,91]. In addition, ·O2− may react with photogenerated holes to form a single linear state of oxygen (1O2), which reduces the selectivity to H2O2 [92]. These factors impose limitations on the single electron transfer pathway in practical applications.
In addition, photogenerated holes have a strong oxidizing ability and can be induced to form H2O2 from water molecules by a 2e− transfer oxygen evolution process (Eq. 6) or to generate O2 by a 4e− transfer oxygen evolution process (Eq. 4) [93]. The 4e− transfer oxygen evolution reaction (OER) has been extensively studied as a half-reaction in water cracking [94]. Despite the extensive work, the 4e− transfer pathway inevitably faces the problem of sluggish reaction kinetics, leading to unsatisfactory performance [95]. Due to the added value of H2O2 and its superior reaction kinetics, the 2e− transfer method has received more and more attention in recent years. It is noteworthy that the redox potential of the 2e− transfer process is +1.76 V vs. NHE, which is higher than that of the 4e− transfer process (+1.23 V vs. NHE). This suggests that the formation of H2O2 is thermodynamically less favorable than that of OER and therefore more likely to generate O2, thus hindering the formation of H2O2 in WOR [96]. Therefore, the 2e− WOR pathway to generate H2O2 is thermodynamically more challenging than the OER process. The production of O2, rather than H2O2, is more likely to occur on the valence band where the photocatalytic reaction proceeds. This thermodynamically unfavorable condition limits the efficient generation of H2O2 in WOR, thus hindering further applications of the 2e− WOR pathway. Optimizing the full reaction pathway for H2O2 becomes more complicated due to the thermodynamically unfavorable 2e− WOR pathway. Photogenerated carriers are more inclined to participate in the side reactions of O2 generation (H2O oxidation or H2O2 oxidation), which may lead to further oxidation or decomposition of H2O2 in the reaction system. The thermodynamic unavoidableness of the 2e− WOR process results in the decrease of the selectivity of H2O2 generation and the exacerbation of the side reactions in the full pathway of H2O2 reaction, which limits the H2O2 generation to be optimized and applied. In addition, the single electron transfer path of WOR can generate ·OH, which is considered to be a highly reactive radical species in catalytic processes. Due to the higher redox potential (+2.73 V vs. NHE), the generation of ·OH from WOR is difficult to occur (Eq. 7).
In the photocatalytic H2O2 generation stage, many factors can also contribute to the inefficiency of H2O2 generation. Notably, the decomposition of H2O2 is often neglected. The decomposition of H2O2 often reduces the apparent H2O2 generation efficiency because H2O2 is oxidized and decomposed by h+. Meanwhile, unreasonable light intensity, ambient temperature, and solution pH can also lead to the rapid decomposition of H2O2. In addition, side reactions in the catalytic process can directly reduce the selectivity of photocatalytic generation of H2O2, such as the 4e− ORR reaction and the 2e− reduction of H+. Avoiding the ineffective decomposition of H2O2 is of great significance in improving the efficiency of the H2O2 accumulation process.
In conclusion, the mechanism of photocatalytic synthesis of H2O2 mainly involves 2e− ORR and 2e− WOR. The presence of competing reactions, including 4e− ORR and 4e− WOR, reduces the selectivity of H2O2 [97,98]. Ideally, photocatalysts should be able to perform both 2e− ORR and 2e− WOR to produce H2O2. Therefore, the development of a dual-pathway approach to H2O2 production is very important to fully utilize photogenerated electrons and holes.
The key to designing effective heterojunction interfacial effects is to fabricate high quality surfaces between semiconductors with suitable band structure [99,100]. Over the past decades, many methods have been developed for synthesizing heterojunction semiconductor materials [73,101]. Therefore, we summarize and classify these synthesis methods from the perspective of preparation strategies. Although our main focus is applied to H2O2 generated heterojunctions, other types of heterojunctions are also reviewed and discussed to broaden the understanding of general synthesis methods for heterojunctions. We hope that this cognitive framework will be effective in driving future research to improve existing production methods.
The impregnation method is a kind of simple and economical synthesis strategy. During the impregnation process, the substrate material is completely submerged in the precursor solution, so that a more uniform precursor distribution can be realized and a uniform heterojunction structure can be formed [102]. In addition, due to the low cost and simple steps of this method, the impregnation method has been successfully applied to the preparation of a variety of heterojunction photocatalysts, which have shown excellent performance in photocatalytic hydrogen production, degradation of organic pollutants, and photocatalytic synthesis of H2O2.
For example, Luo et al. used perovskite CsPbBr3 nanocrystals (CPB NCs) impregnated with lead-porphyrin MOF (Pb-TCPP) as a sacrificial matrix and prepared heterojunction CPB ⊆ Pb-TCPP with MOF structure via a simple impregnation method (Fig. 4a) [103]. S-Scheme heterojunctions were constructed with the help of ionic reactions between CsPbBr3 and Pb-TCPP. This strategy formed an intimate interface for the CPB NCs immobilized in the Pb-TCPP matrix, effectively preventing the recombination of the photogenerated electron-hole pairs. Furthermore, Tang et al. prepared novel nitrogen-sulfur co-doped graphene quantum dots (NSGDs) modified sea urchin-like TiO2 nanorods by impregnation method [104]. The TiO2 nanorods were coupled with NSGDs, and the resulting NSGDs/TiO2 composites formed S-scheme heterojunctions due to the work function difference. The simultaneous effect of the increased electron density around the Ti as well as the resulting internal electric field dramatically enhanced the light energy absorption and the separation of photogenerated carriers. The degradation of toluene using the NSGDs/TiO2 catalyst reached 99.8% within 6 h under visible light irradiation.
Typically, the substrate material is impregnated by the method into a solution containing the precursor, and then further processing, such as roasting, continues to form the desired heterojunction structure. Therefore, the impregnation method is usually coupled with other processes. For example, Zhang et al. constructed non-metallic type-Ⅱ heterojunctions H-g-C3N4/BPQDs via hydrothermal impregnation, high-temperature calcination, as well as ice-assisted ultrasonication using g-C3N4 and black phosphorus quantum dots (BPQDs) as original materials [105]. The H-g-C3N4 obtained based on this method has a porous structure as well as a high specific surface area, and the combination of the type-Ⅱ heterojunction effectively enhances the absorption of visible light. Another representative work is that Liu et al. first loaded CeCl3·7H2O and NiCl2·2H2O on chitosan by impregnation and then obtained the CeNCl-CeO2 heterojunction-modified Ni catalysts by high-temperature calcination under N2 atmosphere (CeNCl-CeO2/Ni/N—C) (Fig. 4b) [106]. The impregnation-calcination method successfully doped Ni and obtained CeNCl-CeO2 heterojunctions, which greatly improved the catalytic performance thanks to the strong electronic interactions between the heterostructures and Ni nanoparticles.
Overall, the impregnation method, as a traditional and effective synthesis strategy, has important applications in the preparation of heterojunction photocatalysts. However, the heterojunction properties obtained by this method are highly dependent on the selection and pretreatment of the substrate materials. For some special structures or highly demanding heterojunction materials, the impregnation method may not be able to meet their preparation needs. Therefore, other processes have been investigated and applied to compensate for the limitations of the impregnation method.
The solvothermal method has been extensively investigated in the synthesis of heterojunctions because of its ease of tuning the morphological characteristics of heterojunctions and its benefits in exposing crystalline surfaces [107]. It is reported that solvothermal methods are usually carried out at high temperatures and pressures to enhance the photocatalytic activity of heterojunction photocatalysts through disruption and recombination of binding bonds or formation of new defects. This method is able to precisely regulate the synthesis conditions to generate heterojunction materials with high crystallinity and specific structures, thus demonstrating significant advantages in photocatalytic applications. For example, Li et al. synthesized Bi@Ov-BiOBr/Cu3P via adding KBr and Bi(NO3)3⋅5H2O to Cu3P through a solvothermal method (Fig. 5a) [108]. The combination of Bi@OV-BiOBr and Cu3P gave type-Ⅰ high-low heterojunction composites. The rich oxygen vacancies (Ov) effectively improve the capacity of the catalyst to capture electrons, promote electron aggregation, and reduce the carrier recombination rate of the material. This was due to the high temperature and pressure conditions in the solvent heating process leading to the disruption of the Bi-O bond, which in turn led to the precipitation of Bi metal from the BiOBr lattice as well as the formation of Ov. Zhou et al. also obtained S-Scheme heterojunction materials (ZIS/BTO) by growing ZnIn2S4 nanosheets on lamellar Bi4Ti3O12 surfaces using the molten salt method and the low-temperature solvent-heating method [109]. Since the solvent heat treatment does not damage the structure of BTO, ZIS/BTO has the 2D structural features of BTO and integrates the advantages of S-Scheme heterojunction. ZIS/BTO not only has abundant active sites, but also the separation of photogenerated electrons is improved. Zhang et al. synthesized BiVO4/Sv- ZnIn2S4 (BVO/Sv-ZIS) core-shell direct Z-scheme heterojunctions with a large number of sulfur vacancies (Sv) by a solvothermal method [110]. With ZnIn2S4 nanosheets as the shell layer, the BiVO4 core is tightly wrapped. The Z-scheme heterojunction was successfully constructed because of the tight contact between the two phases.
Solvothermal methods have been used to construct new heterojunction materials, such as double-Z heterojunctions. For instance, Sepehrmansourie with co-workers successfully prepared a novel double-Z-scheme material, UiO-66/NH2−MIL-125/g-C3N4, by growing well-structured UiO-66-on-MIL-125 on g-C3N4 nanosheets using the solvent-thermal method [107]. During the synthesis process, NH2−MIL-125 was firstly prepared as the host MOF by the solvothermal method, followed by the growth of UiO-66 crystals on the surface of NH2−MIL-125 by the same method. Finally, g-C3N4 nanosheets were modified on the Zr-MOF-on-Ti-MOF surface to obtain the target material. All the morphological features of UiO-66 epitaxially grown on the sheet NH2−MIL-125 surface were preserved (Fig. 5b).
The advantage of the solvothermal method is the promotion of the product crystallization process under high temperature and high-pressure environment, which makes the heterojunction photocatalysts have high crystallinity. The morphology and size of the heterojunction can be precisely controlled by adjusting the parameters, such as solvent and temperature [111]. However, this method usually needs to be realized with the help of high-temperature and high-pressure equipment, and the preparation process is complicated, which still faces challenges in large-scale production.
As for the chemical deposition method, there are many obvious advantages. First, this method is very simple, green, fast, and non-polluting to the product. Secondly, it can deposit active substances uniformly on the surface of the substrate material, thus forming heterojunction structures with good interfacial bonding and high catalytic properties [112]. By controlling the deposition conditions, precise regulation of the thickness and composition of the deposited layer can be realized. This makes the chemical deposition method suitable for the preparation of high-quality heterojunction photocatalysts.
One of the representative works is that Xing et al. modified 2D Al2O3 nanosheets using PdO nanodots (PdO NDs//Al2O3 NSs) via a combination of template synthesis and photochemical deposition [113]. In the synthesis stage, graphene oxide was used as a sacrificial template for the synthesis of Al2O3 2D nanoparticles, which were adsorbed with Al3+ to obtain GO-Al and further annealed to obtain Al2O3 NSs powder. Then, Pd NDs were photochemically deposited on Al2O3 NSs, and PdO NDs//Al2O3 NSs were obtained after drying. PdO NDs//Al2O3 NSs heterojunctions with a large specific surface area showed excellent stability. Wu et al. used a combination of hydrothermal, chemical precipitation, and photochemical deposition methods to construct the Au/ZnWO4/CdS ternary system, and the novel heterojunction successfully attached CdS and Au nanoparticles to ZnWO4 nanorods with Ov (Fig. 6a) [114]. The efficient combination of ZnWO4 nanorods containing Ov with CdS and Au nanoparticles not only enhanced the light absorption of ZnWO4, but also the type-Ⅱ heterojunction between the two phases synergistically inhibited charge recombination with the metal bonding, which greatly enhanced the photocatalytic activity of the heterojunction. Gao et al. synthesized lamellar BiVO4 with higher facet exposure by adding TiCl3, and the BiVO4/Ag/CdS all-solid Z-scheme heterojunction was further constructed by photo-reduction of Ag and chemical deposition of CdS on the prepared BiVO4 (Fig. 6b) [115]. The enriched facets of BiVO4 lead to the formation of more BiVO4/Ag/CdS ternary structures, and thus BiVO4/Ag/CdS has a better photogenerated electron-hole pair separation efficiency than BiVO4/Ag/CdS.
However, the chemical deposition method has some shortcomings. First, the equipment and operational complexity are high, usually requiring special reaction devices and strict process control. Second, the method requires high requirements for the selection of deposition materials and surface treatment of the deposition substrate, which may limit the application of certain materials.
In-situ growth is the method that grows another material directly on the substrate material during the preparation of heterojunction photocatalysts. The advantage of this method is that by growing the new material directly on the substrate, close contact as well as excellent interfacial bonding between the two can be realized, hence improving the photocatalytic performance of the heterojunction. For instance, Liu et al. via in-situ growth of conducting MOF [Cu3(HITP)2] (HITP = 2,3,6,7,10,11-hexaiminotriphenylene) on hexagonal boron nitride (h-BN) nanosheets, successfully synthesized n-n heterojunction Cu3(HITP)2@h-BN containing Cu-N and B-N dual active sites (Fig. 7a) [116]. The construction of the n-n heterojunction and the Cu-N/B-N dual active sites effectively modulate the electronic structure of the dual active sites and the band gap energy near the Fermi energy level. The optimized Cu3(HITP)2@h-BN has excellent nitrogen reduction reaction activity with an NH3 yield of 146.2 µg h−1 mgcat−1. In addition, Ji et al. prepared a novel composite photocatalyst by in-situ continuous ion implantation method (NH2-UiO-66@BiOI) [117]. The formed compact heterojunction enhanced visible light absorption property and promoted its reactive performance during the photoreaction process.
This method is also capable of generating nanostructures with special morphology and high specific surface area, which helps to further enhance the performance of the photocatalyst. The representative work is that Xu et al. prepared a novel 2D TiO2/Ti3CN heterojunction semiconductor and used it as a photocathode for the reduction of carbon dioxide in a photoelectrocatalytic (PEC) CO2 reduction system (Fig. 7b) [118]. TiO2/Ti3CN heterojunctions (TOCN) were grown in-situ by addition of the morphology oriented reagent NaBF4. Subsequently, TOCN was deposited on FTO glass by electrophoretic deposition (EPD) to obtain photocathode TOCN-FTO, and Pd nanoparticles were deposited on the TOCN-FTO electrode to obtain Pd@TOCN-FTO electrode. The carefully tuned in-situ growth process provides sufficient surface-exposed active sites for reactant adsorption and redox reactions and shortens the ion transport distance to improve the PEC performance.
Overall, each of these synthesis methods has its own characteristics, and it is necessary to choose the appropriate method according to the specific needs in the practical application, and the combination of a variety of processes can optimize the performance and production efficiency of heterojunction photocatalysts. In addition, the development of simpler, low-cost, and efficient large-scale preparation techniques is also an important direction to enhance the potential of photocatalysts for practical applications.
It is well known that semiconductor materials play a central role in the photocatalytic production of H2O2 [119,120]. So far, a variety of functional materials have been developed for the preparation of photocatalysts with remarkable results [121]. Rapid recombination of light-induced charges and limited light absorption are the main inhibiting factors for photocatalysis [44]. The heterojunction can be formed by combining two or more semiconductors with interleaved bandgaps, thus effectively increasing light absorption and overcoming the bottleneck of low photocatalytic efficiency [122-124]. In addition, the well-designed band alignment can inhibit the rapid recombination of photoinduced carriers and promote effective interfacial charge transfer [125]. Based on the charge transfer mechanism, various heterojunctions, including type-Ⅰ, type-Ⅱ, Z-scheme, S-scheme, and Schottky heterojunction, have been constructed for different photocatalyst applications. Therefore, the design principles as well as recent advances of different types of heterojunctions for photocatalytic generation of H2O2 will be discussed in detail in this section.
Type-Ⅰ heterojunction consists of semiconductors with staggered energy levels [97]. Under light illumination, photogenerated electrons and holes are transferred to the CB, which has a lower energy level, and the VB, which has a higher energy level, where redox reactions take place [126]. The simple arrangement of energy bands in type-Ⅰ heterojunctions allows the concentration of photogenerated electrons and holes in the lower CB and higher VB energy levels, respectively, thus reducing their recombination probability [69]. In addition, the type-Ⅰ heterojunction has a relatively simple structure and a wide range of material choices, which can be optimized by combining different semiconductor materials [45].
As a photocatalyst with visible light activity, indium sulfide (In2S3) can effectively reduce O2 to ·O2− to generate H2O2 due to its suitable CB position and special electronic structure [65]. However, the poor photocatalytic activity of In2S3 is because of the rapid recombination of photogenerated electron-hole pairs. To address this problem, Chen et al. synthesized 2D/2D g-C3N4/In2S3 with a type-Ⅰ heterojunction structure by growing In2S3 nanosheets on g-C3N4 nanosheets via a hydrothermal method (Figs. 8a and b) [127]. In this heterojunction, photo-excited electrons from g-C3N4 are transferred to In2S3 to generate H2O2 via a two-step single-electron ORR pathway. Compared to pristine In2S3, the reduction of electrons in the In2S3 CB of the g-C3N4/In2S3 heterojunction is significantly enhanced, which improves the ability to generate ·O2− (Fig. 8c). These radicals are eventually reduced by photoelectrons to form H2O2. In addition, Li et al. designed a Type-Ⅰ phosphate-doped carbon nitride/oxygen-doped carbon nitride (P-C3N4/O—C3N4) heterojunction for the photocatalysis-self-Fenton reaction [128]. The H2O2 yield of P-C3N4/O—C3N4 in air-saturated solution is 179 µmol/L, which is 7.2, 2.5, 2.5, and 2.1 times faster than that of C3N4, P-C3N4, O—C3N4, and phospho-oxygen co-doped C3N4, respectively.
However, the fact that all charge carriers tend to accumulate in the more positive CB and the more negative VB after light absorption does not improve the charge carrier separation absolutely based on Type-Ⅰ heterojunction. Effective spatial separation plays a crucial role in significantly improving the photocatalytic H2O2 generation performance [129]. Therefore, other types of heterojunctions have been investigated to solve this problem.
As mentioned earlier, the conventional type-Ⅱ heterojunction system consists of two solid photocatalysts with interleaved band structures. They have similar charged positions and are tightly bound together to form a stable heterojunction structure [130]. Under sunlight irradiation, photoexcited electrons in the highly reducing CB flow into the CB of the neighboring low-reducing semiconductor, while photoexcited holes in the highly oxidizing VB diffuse into the VB of the neighboring low-oxidizing semiconductor. This charge transfer pathway greatly facilitates the spatial separation of electrons and holes, thus reducing their recombination possibility [49]. In addition, the internal electric field further promotes the separation and transfer of charge carriers.
Researchers have developed varieties of type-Ⅱ systems for photocatalytic production of H2O2 in recent years. A representative work is that Zhang et al. successfully distributed g-C3N4 coated on Co9S8 nanosheets via a two-step protonation and dip-coating method [131]. Type-Ⅱ heterojunction of protonated g-C3N4 coated Co9S8 semiconductors was successfully constructed (Fig. 9a). The photocatalytic system achieved a H2O2 yield of 2.17 mmol/L within 5 h in an alkaline medium containing pure oxygen and inorganic electron donors (Fig. 9b). The optimized photocatalyst achieved an apparent quantum yield (AQY) of 18.10% at 450 nm light and showed great reusability. The construction of type-Ⅱ heterojunction improved the electron and hole transport within the interface of g-C3N4 and Co9S8 substrates, which facilitated the separation of the photogenerated charge carrier. H2O2 was generated via 2e− ORR. Meanwhile, electrons in the CB of g-C3N4 acted as an ancillary pathway for the one-electron reduction of O2, which further enhanced the production of H2O2 (Fig. 9c).
Similarly, Liu et al. constructed a type-Ⅱ heterojunction material (BON—Ov/CN) consisting of g-C3N4 and Bi2O2(NO3)(OH) modified Ov (Fig. 9d). The successful construction of type-Ⅱ heterojunction greatly improved the photocatalytic activity of H2O2 [132]. The optimal BON—Ov/CN photocatalytic H2O2 yield was 706.2 µmol/L, which was 4.9 times higher than that of CN alone (Fig. 9e). The strong interfacial electric field formed by the potential difference between the two semiconductors effectively promoted the directional charge transfer at the interface (Fig. 9f). In summary, these findings provide experimental evidence for exploring the synergistic effect between heterojunction and surface engineering and provide a reference for designing efficient type-Ⅱ heterojunction structures for H2O2 photocatalysis. Type-Ⅱ heterojunction has great potential as a promising strategy.
The establishment of Type-Ⅱ heterojunctions accelerates the efficient separation of electrons and holes, which helps to break the kinetic reaction rate limitation of photocatalysts [133]. However, the conventional photocatalytic H2O2 generation process is also limited by the slow oxygen mass transfer rate in water, which restricts the further increase of H2O2 generation [134]. To address this challenge, Li et al. utilized heterojunction engineering to simultaneously regulate electron transfer and hydrophobicity [135]. 2D Zn3In2S6 nanosheets were grown on a 1D PCN-222 skeleton to form a hierarchical porous coordination network based on Zn3In2S6/porphyrin (Fig. 9g). The hydrophilicity-optimized Zn3In2S6/PCN system (302 mmol/L in 3 h) showed 1.7 and 107.7 times higher H2O2 yields than those of PCN-222 and Zn3In2S6, respectively (Fig. 9h). This is the first example of the use of heterojunction engineering to accelerate electron transfer and tune hydrophobicity to produce high concentrations of H2O2. The addition of Zn3In2S6 tuned the hydrophobicity of PCN-222, enhanced O2 adsorption, and optimized the reaction system. The hydrophobicity of Zn3In2S6 modulates the surface hydrophobicity of PCN-222 and enhances its ability to adsorb O2. This not only overcame the limitation of slow mass transfer of oxygen in water, but also achieved spontaneous separation and collection of H2O2.The addition of Zn3In2S6 tunes the hydrophobicity of PCN-222 and enhances the adsorption of O2 to optimise the reaction system. The hydrophobicity of Zn3In2S6 tunes the surface hydrophobicity of PCN-222 and enhances its ability to adsorb O2. This not only overcame the limitation of slow mass transfer of oxygen in water, but also achieved spontaneous separation and collection of H2O2.The in-situ growth of Zn3In2S6 maximised the electron transfer through the matched band structure in the ZIS/PCN heterojunction, and the O2 generated by energy transfer further promoted the photocatalytic activity (Fig. 9i). This study offers a new way to design effective photocatalysts for H2O2 production by simultaneously modulating electron transfer and hydrophobic properties through heterojunction engineering.
In summary, the type-Ⅱ heterojunction achieves effective separation of photoexcited electrons and holes by optimizing the band structure, and this heterojunction structure improves the overall performance of the photocatalyst.
In the previous section, we summarized various types of typical heterojunction structures and their catalytic mechanisms. Although these heterojunction photocatalysts can effectively improve the efficiency of electron-hole separation, the overall redox capacity of the composites is limited by the fact that the redox process occurs on semiconductors with different redox potentials [136]. To overcome these problems, Z-scheme heterojunctions have attracted more and more attention in recent years due to their unique structure interleaving the CB and VB energy levels of two semiconductor materials, which allows the separation and migration of photogenerated electrons and holes on different semiconductors [137]. This structure can effectively boost the photocatalytic activity and enhance the redox capability of the material.
With a suitable energy band structure as well as good chemical stability, g-C3N4 is one of the ideal materials for the construction of efficient Z-scheme heterojunctions [138]. In recent years, the development of g-C3N4-based Z-scheme heterojunction photocatalysts using different methods has been widely investigated, and significant progress has been made in enhancing the photocatalytic H2O2 generation performance [139]. For instance, Ai with co-workers synthesized g-C3N4 materials with different nitrogen vacancies through a facile calcination method and combined them with CeO2 nanotubes to form CeO2/CN heterojunction (Fig. 10a) [140]. The H2O2 yields of 2.01 and 0.78 mmol g−1 h−1 for Ar-CeO2/CN and NH3—CeO2/CN, respectively, which are 2.14 and 2.71 times higher than those of the corresponding bare g-C3N4 (Fig. 10b). Due to the formation of a Z-scheme charge transfer mechanism, the CeO2/CN heterojunction significantly improved H2O2 generation activity. Remarkably, CeO2/CN heterojunctions not only initiate H2O2 generation through the formation of ·O2− radicals via a single-step 2e- ORR pathway, but also allow the generation of H2O2 via the oxidation of H2O (Fig. 10c).
In addition, Z-scheme heterojunctions are also constructed via controlling the semiconductor growth mode and doping synergies, which can efficiently accelerate the charge separation [141]. For instance, Li et al. prepared sulfur-doped g-C3N4 nanosheets by controlling the growth of WS2 on g-C3N4 through a two-step high-temperature thermal polymerization method [142]. Subsequently, they synthesized WS2/S-g-C3N4 (WS2/SCN) Z-scheme heterojunction photocatalysts by solvothermal method and achieved an H2O2 generation rate of 5216 µmol g−1 h−1, which is 6.4 times higher than that of S-g-C3N4 samples. The internal electric field of the Z-scheme heterojunction enhances the generation of ·O2− during the ORR process, which further accepts electrons to generate H2O2, and these studies provide a feasible solution for the design of highly efficient heterojunction photocatalysts to increase the generation of H2O2. In conclusion, Z-scheme heterojunctions can not only efficiently separate photogenerated carriers but also enhance the redox capacity of the composites, which provides a new route for the efficient photocatalytic synthesis of H2O2.
By integrating photocatalysts and piezoelectric catalysts to create novel Z-scheme heterojunctions, it is possible to combine semiconductor photoelectric effects with piezoelectric properties [47]. In such systems, periodic ultrasound waves induce the formation of piezoelectrically polarized charges, which promote the bending of energy bands at the interface effectively. Photogenerated holes, or electrons, are attracted to the opposite polarization charge, promoting photocatalytic redox reactions. On this basis, Meng and his team integrated zeolitic imidazolate frameworks (ZIF) with g-C3N4 nanosheets to construct a ZIF-L/g-C3N4 piezoelectric Z-scheme heterojunction in a photocatalytic system with dual piezoelectric and photovoltaic properties [143]. After the introduction of ZIF-L foliated material, the dispersion of g-C3N4 flakes on the surface of ZIF-L tends to be uniform (Fig. 10d). The ZIF-L/g-C3N4 Z-scheme heterojunction (ZCx) significantly enhanced the H2O2 yield. The system takes advantage of the polarization potential formed inside the g-C3N4 nanosheets to induce an internal electric field, together with their good dispersion on the ZIF-L blades, which significantly improves the transfer and utilization efficiency of photogenerated electrons and holes. Among them, the highest H2O2 yield reached 1.45 mmol g−1 h−1 for the ZIF-L/g-C3N4, which was 4.8 and 9.1 times higher than that of g-C3N4 and ZIF-L without sacrificial agent, respectively (Fig. 10e). It was found that ZIF-L/g-C3N4 could change the H2O2 generation mechanism from single-channel to dual-channel, resulting in a significant increase in the H2O2 yield through the ORR and WOR pathways without using any sacrificial agents (Fig. 10f).
The dual Z-scheme heterojunctions system has a unique structural design and multi-channel charge transfer capability by introducing two heterojunction interfaces to form tight interfacial contacts and efficient charge separation and transfer pathways [144-146]. One of the representative works is that Gao et al. successfully developed the 2D/0D/2D g-C3N4 nanosheets/FeOOH quantum dots/ZnIn2S4 nanosheets (CNFeZn) double Z-scheme heterojunction system (DZSS) [147]. In this well-designed DZSS, the staggered energy band structures of 2D g-C3N4, 0D FeOOH, and 2D ZnIn2S4 form many space charge transport channels among the g-C3N4/FeOOH with FeOOH/ZnIn2S4 interfaces, which can significantly accelerate the separation as well as transfer of photogenerated electrons and holes. After 60 min of illumination, the H2O2 yield of DZSS reached about 301.19 mmol/L, which was 5.1 and 2.3 times higher than that of pristine g-C3N4 and ZnIn2S4, respectively. This study demonstrated that the dual Z-scheme heterojunction could not only significantly improve the photocatalytic yield of H2O2, but also showed its great potential for photocatalyst design. The dual Z-scheme heterojunction photocatalytic system provides new insights and technical support for the development of efficient photocatalysts for H2O2 production. This work provides a new perspective for the development of novel bifunctional catalysts for efficient H2O2 production. The application of Z-scheme heterojunction photocatalyst generation for H2O2 is exhibited in Table 1 [136,137,140,142-144, 147-155]. For catalysts designed to participate only in specific half-reactions, sacrificial agents are often employed to deplete the non-participating carrier species. These reagents serve to protect the photocatalyst from self-degradation or recombination of the photogenerated carriers and to direct the selectivity of the reaction towards the desired H2O2.To ensure equilibrium between the photogenerated carriers, an in-depth study of the full reaction pathway for the generation of H2O2 is an effective way to reduce the use of sacrificial agents. By comprehensively resolving the migration and reaction pathways of photogenerated e− and h⁺ in the photocatalytic process, it is possible to optimise the reaction conditions, avoid unwanted side-reactions, and reduce the reliance on sacrificial agents. In addition, coupling other semi-reactive processes to rationalise the consumption of excess e− or h⁺ has also been shown to be effective. This strategy not only improves the energy utilisation efficiency of the reaction, but also further reduces the need for additional sacrificial agents, thus enhancing the economic efficiency of the photocatalytic reaction.
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| Catalyst | H2O2 yield (µmol−1 h−1 g−1) |
Sacrificial agent | Catalyst dosage (g/L) |
AQY | Illumination | Ref. |
| OCN@In2S3 | 633 | / | 0.5 | / | Visible light | [136] |
| Mn3O4/Co9S8 | 1600 | / | 1 | 26.33% (450 nm) | 300 W Xe lamp |
[137] |
| CeO2/CN | 2010 | Isopropanol | 0.5 | / | Visible light | [140] |
| WS2/S-g-C3N4 | 5216 | Isopropanol | 0..2 | / | 300 W Xe lamp |
[142] |
| ZIF-L/g-C3N4 | 1450 | / | 0.5 | / | 300 W Xe lamp |
[143] |
| g-C3N4/TiO2 | 133.04 µmol/L | Ethanol | 1.5 cm2 films (in 5 mL) |
/ | 300 W Xe lamp |
[144] |
| g-C3N4/FeOOH/ZnIn2S4 | 6024 | Isopropanol | 0.4 | Visible light | [147] | |
| MnIn2S4/WO3 | 1188 | / | 0.5 | / | Simulated | [148] |
| A-TxCNx | 1132 | Ethanol | 0.1 | / | Visible light | [149] |
| TiO2/B-doped g-C3N4 | 550 | Ethanol | 1 | / | Visible light | [150] |
| CoWO4@Bi2WO6 | 85 | / | 1 | / | Visible light | [151] |
| CuBi2O4/MoS2 | 5828 | / | 1 | / | 150 W Xe lamp |
[152] |
| Bi2S3/Sb2S3 | 541.5 | Ethanol | 1 | / | 400 W metal halide lamp | [153] |
| O-doped g-C3N4/ZnIn2S4-Zn | 1400 | Isopropanol | 0.2 | 18.6% (400 nm) | Visible light | [154] |
| TiO2/B-doped g-C3N4 | 550 | Ethanol | 1 | / | Visible light | [155] |
According to the above summary, we can conclude that the design of Z-scheme heterojunctions with rational energy band structures for efficient photocatalytic production of H2O2 will be an important direction for future research. These studies not only demonstrate the potential of improving photocatalytic performance by optimizing the heterojunction structure and material combinations but also provide important theoretical basis and technical support for practical applications in the future.
Z-scheme heterojunctions combine two semiconductors with suitable energy band structures to form a "Z"-shaped energy band arrangement that spatially separates electrons and holes [156-158]. Building on this concept, Yu and colleagues proposed the concept of S-scheme heterojunctions in 2019 [159,160]. Briefly, an S-scheme heterojunction consists of an OP and a RP [161]. The OP has a more positive VB, while the RP has a more negative CB [162-164]. When the RP and OP are in contact, electrons are transferred from the RP to the OP until equilibrium is reached because of the difference in Fermi levels. The internal electron transfer generates an internal electric field at the interface, accompanied by energy band bending, which promotes the transfer of photogenerated charge carriers [165-167]. Compared with other types of heterojunctions, S-scheme heterojunctions enhance the separation of photogenerated charge carriers through internal electric field and energy band bending while maintaining the maximum redox potential of the photocatalytic system [168-170]. The strong interfacial electric field in S-scheme heterojunctions contributes to a more efficient separation of photogenerated electrons and holes and promotes the directional migration of electrons. Therefore, the research and development of novel S-scheme heterojunction photocatalysts have received increasing attention.
To date, characterization methods such as in-situ irradiated X-ray photoelectron spectroscopy (ISI-XPS) and electron paramagnetic resonance (EPR) tests have provided strong evidence for the construction of S-scheme heterojunctions [171]. Particularly when RP and OP are in contact, due to their different work functions, electrons are transferred from RP to OP until equilibrium is reached at the Fermi level [172]. Accordingly, if an S-scheme heterojunction is successfully constructed, the XPS spectrum of OP in the composite will shift to a lower binding energy, while the XPS spectrum of RP will shift to a higher binding energy. In addition, the S-scheme heterojunction maintains the maximum redox potential of the photocatalytic system, which can be demonstrated by the EPR test that detects the generation of ·OH and ·O2− radicals. In addition to experimental characterization, density functional theory (DFT) calculations of the difference in charge distributions are an effective theoretical validation method for S-scheme heterojunctions [173]. DFT calculations provide detailed information about charge distribution and electron transfer, which helps researchers understand the internal mechanism of the S-scheme heterojunctions and predict their performance.
One of the representative works is that Liu with co-workers successfully synthesized an S-scheme heterojunction layered porous ZnO/g-C3N4 photocatalyst via a two-step calcination method using ZIF-8 and urea as precursors [174]. Among them, a large number of ZnO NPs are evenly dispersed on the CN nanosheets (Fig. 11a). The S-scheme heterojunction photocatalyst exhibits significant H2O2 generation activities under light irradiation, which are 3.4 and 5.0 times higher than those of original g-C3N4 and ZnO, respectively (Fig. 11b). Kelvin probe force microscopy, ISI-XPS, and EPR verified the internal charge transfer and separation mechanism of S-scheme heterojunction. Under light radiation, electrons in both ZnO and CN are excited from VB to CB. Then, the formation of the internal electric field and the bending of the bands recombine the photoinduced electrons of ZnO with the photoinduced holes of CN, preserving the photoinduced holes of ZnO in VB and the photoinduced electrons of CN in CB (Fig. 11c). Besides, Zhang et al. synthesized an S-scheme heterojunction photocatalyst consisting of ultrathin g-C3N4 (U-CN) and polydopamine (PDA) using in situ self-polymerization to form C3N4/PDA (CNP) [175]. Fig. 11d shows the ultra-thin sheet structure of CNP. The as-prepared S-scheme heterojunction facilitated the charge separation as well as the transfer process. Its optimal photocatalyst exhibited an extremely high H2O2 yield of 3801.25 µmol g−1 h−1 under light-irradiation conditions, which was 2 and 11 times higher than that of pure U-CN and PDA, respectively (Fig. 11e). Under light irradiation, the electrons in U-CN will be in the highest occupied molecular orbital of PDA, while the photogenerated electrons in the lowest unoccupied molecular orbital of PDA and the holes in the VB of U-CN will be retained. These light-generated e− and h+ have stronger reducing and oxidizing abilities, respectively. The charge transfer pathway of CNP is shown to be an S-scheme mechanism, and the internal electric field of the S-scheme heterojunction can effectively improve the carrier transfer so that more photogenerated electrons will participate in the 2e− ORR. (Fig. 11f). In conclusion, the construction of S-type heterojunction improves the effective separation of photogenerated electrons and holes, which greatly enhances the efficiency and activity of the photocatalyst.
The unique electron transfer pathway of the S-scheme heterojunction system determines its strong redox capability, which has been extensively studied in the photocatalytic production of H2O2. However, its generally low response to the narrow range of the solar spectrum (λ ≤ 450 nm) often leads to low apparent quantum efficiency. Therefore, expanding the photoresponse range of the catalysts to improve the photon utilization efficiency is an effective strategy. For instance, Chai et al. combined the organic semiconductor zinc phthalocyanine (ZnPc) with polymer carbon nitride (PCN) via π-π interactions to form ZnPc/PCN S-scheme heterojunction photocatalysts with near-infrared absorption capability [176]. Fig. 12a shows the wave-dependent H2O2 generation of the ZnPc/PCN-1 photocatalytic system, confirming its near-infrared response. Moreover, under visible light irradiation, the H2O2 yield of ZnPc/PCN-1 (0.94 mmol L−1 h−1) was higher than that of ZnPc (0.01 mmol L−1 h−1) and PCN (0.69 mmol L−1 h−1) (Fig. 12b). It can be seen from the band structure of ZnPc/PCN that S-scheme heterojunction is formed between ZnPc and PCN. Under simulated sunlight irradiation, electrons in the valence band maximum (VBM) of ZnPc and PCN are excited to transition to the conduction band minimum (CBM). There are a large number of electrons and holes in CB of PCN and VB of ZnPc, respectively, to ensure that ZnPc/PCN-1 can maintain a high oxidation–reduction potential (Fig. 12c). This study demonstrates that the integration of broad-spectrum absorbing materials such as Metal phthalocyanine (MPcs) into S-scheme heterojunctions can effectively improve light utilization and photocatalytic activity, especially in the case of H2O2 production.
The construction of S-scheme heterojunctions is a promising strategy, according to the above reports. However, the unavoidable photocorrosion problem during photocatalytic reactions limits the application of metal composites in heterojunction structures [177]. For example, CdSe is a commonly used metal selenium in photocatalytic reactions, but it is prone to photocorrosion weakness, resulting in low catalytic activity and stability [178]. To address this problem, He et al. synthesized an organo-inorganic hybrid ion intercalated carbon nitride/cadmium selenide diethylenetriamine (K+/I−-CN/CdSe-D) S-scheme heterojunction by utilizing organic amine-constrained ion intercalation, which successfully suppressed the photocorrosion of cadmium selenide [179]. K+/I−-CN/CdSe-D can be used to inhibit CdSe photocorrosion via the C-Se bonding. The observation of the key intermediates ·O2− (1138 cm−1) and ·OOH* (1303 cm−1) with in- situ FTIR spectroscopy confirms the two-step one-electron ORR pathway for K+/I−-CN/CdSe-D catalyzed H2O2 generation (Fig. 12d). Among them, 40% K+/I−-CN/CdSe-D showed the H2O2 production rate of 2240.23 µmol h−1 g−1, which is higher than that of other prepared samples (Fig. 12e). After illumination, the photogenerated e− with weak redox ability on CB in K+/I−-CN migrates to VB in CdSe-D under the drive of the bending band and interfacial electric field. The recombination of generated e− with weakly generated h+ directly indicates the formation of S-scheme heterojunction (Fig. 12f). Such excellent activity is mainly because I− and K+ ions optimized the interfacial properties of the composite S-scheme heterojunction, facilitating the rapid separation as well as transfer of photogenerated carriers from the photocatalyst to the reaction site via C-Se bonds. The K+/I−-CN/CdSe-D heterojunction focuses on the dual modulation of C-Se bonds and ionic intercalation, which provides a new approach for the preparation of photocatalysts for the generation of H2O2.
By exploiting the synergistic effects of interfacial chemical bonding and ionic intercalation, S-scheme photocatalysts offer an effective strategy for controlling charge transfer and enhancing photocatalytic H2O2 performance [180-182]. In addition, efforts have been made to create more environmentally friendly and cost-effective S-scheme composite photocatalysts. By optimizing the synthesis process and material structure, the performance and stability of the photocatalysts can be further improved, leading to more efficient photocatalytic H2O2 production [183-185]. The application of the S-scheme heterojunction photocatalyst for H2O2 generation is shown in Table 2 [39,174-176,179,181,183,186-192].
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| Catalyst | H2O2 yield (µmol−1 h−1 g−1) | Sacrificial agent | Catalyst dosage (g/L) | AQY | Illumination | Ref. |
| CdS/K2 Ta2 O6 | 160.89 | / | 0.6 | / | Visible light | [39] |
| BiOI/g-C3N4/CoP | 2733 | Ethanol | 0.1 | / | Simulated | [174] |
| ZnO/g-C3N4 | 1360 | Ethanol | 0.4 | / | 300 W Xe lamp | [175] |
| NiO/C3N5 | 91.2 | Isopropanol | 0.2 | 5.94% (400 nm) | Visible light | [176] |
| ZnPc/PCN | 1870 | Ethanol | 1 | 1.11% (800 nm) | 300 W Xe lamp | [179] |
| MSCN-5 | 4863 | Ethanol | 0.4 | / | Visible light | [181] |
| C3N4/PDA | 3801 | Ethanol | 0.5 | 2.22% (400 nm) | Simulated | [183] |
| ZnO/WO3 | 6788 | Ethanol | 1 | / | 300 W Xe lamp | [186] |
| Sv-ZIS/CN | 1310. 18 | Isopropanol | 0.4 | / | Visible light | [187] |
| TiO2/In2S3 | 376 | Ethanol | 0.5 | / | 300 W Xe lamp | [188] |
| ZnO/COF | 2443 | Ethanol | 0.5 | / | 300 W Xe lamp | [189] |
| TiO2/PDA | 2200 | Ethanol | 0.5 | / | 300 W Xe lamp | [190] |
| In2O3/ZnIn2S4 | 5716 | Ethanol | 0.4 | / | Visible light | [191] |
| TiO2@RF | 66,600 | / | 0.67 | / | 300 W Xe lamp | [192] |
All in all, S-scheme heterojunctions not only show great potential in improving photocatalytic efficiency but also provide a new way to overcome the photocorrosion problem as well as to enhance the stability of photocatalysts by optimizing interfacial chemical bonding and ionic intercalation techniques. These research advances pave the way for the design and development of a new generation of efficient, stable, and environmentally friendly photocatalysts.
Schottky heterojunction consists of an interface between a metal and a semiconductor material. This structure is accompanied by the creation of a Schottky barrier. The Schottky barrier formed at the contact among the metal with the semiconductor effectively prevents electrons from flowing back to the semiconductor, which enables the structure to effectively separate photogenerated electrons and holes [182,193,194], which is widely used in the photocatalysis field due to their unique electronic properties.
For example, Yan et al. first reported the construction of a Schottky junction photocatalyst using a Bi/Bi2O3 core-shell structure as an integral unit for photocatalytic H2O2 production. As shown in Fig. 13a, the quasi-core-shell structure of Bi/Bi2O3 is distributed on the surface of g-C3N4. Among them, Bi nanoparticles are encased in a Bi2O3 film [52]. With adding no sacrificial agents, the Bi/Bi2O3@g-C3N4 Schottky junction photocatalyst with 2% Bi/Bi2O3 achieved an H2O2 production rate of 92.51 µmol L−1 h−1, which is 70.6 times higher than that of pure g-C3N4 (Fig. 13b). The introduction of metallic Bi/Bi2O3 not only facilitated the unidirectional interface transfer of excited g-C3N4 but also accelerated the second step of the single-electron reduction of ·O2− to H2O2, a key rate determining step (Fig. 13c). The Bi/Bi2O3@g-C3N4 Schottky junction photocatalyst has dual functions, providing a continuous and sufficient electron supply for multi-electron pathways involved in H2O2 production and improving the selectivity of the 2e− ORR. This work proposes a feasible approach for the application of Schottky junctions in the efficient solar-driven production of hydrogen peroxide in green reaction solutions. Besides, Wong et al. achieved efficient accumulation of H2O2 by modulating interfacial Schottky junctions using photopiezoelectric band bending [195]. The turn-bending band potential of the electron flow was controlled on a polarized photopiezoelectric BiFeO3 (BFO) coupled to the photocatalyst BiOCl/BiVO4 (BCV). Notably, the Schottky barrier height (SBH) formed in the BCV varies with the polarity of the BFO surface. Thus, with successive compressive and tensile strains, the dynamics of the Schottky barrier interface between the BCV and the polarized surface of the BFO change. By controlling the SBH at the heterojunction interface, the generation of H2O2 was selectively promoted to 178.7 mg g−1 h−1, which is higher than most previous studies.
In heterojunction materials used for H2O2 generation, the presence of Schottky junctions significantly improves the transfer efficiency of photogenerated electrons. The photogenerated electrons are rapidly transferred to the reaction interface through the Schottky junction to react with dissolved oxygen to produce H2O2. Therefore, Schottky heterojunctions have been further used in photo-Fenton reactions based on self-produced H2O2.
For example, Hu et al. proposed a Fe-protoporphyrin bridging strategy to construct a bionic oxygen-carrying system as well as Schottky junction by growing MIL-100(Fe) on Fe-protoporphyrin-modified MXene [196]. An optimized synthesis using ascorbic acid (AA) was carried out for Fe-doped MXene (MXFAA). Subsequently, protoporphyrin was introduced into MIL(Fe) and synthesized with MXFAA to obtain MXFAA/MIL(Fe)-POR. MXFAA/MIL(Fe)-POR has a thick multilayer structure with well-dispersed MIL(Fe) crystals on the MXFAA surface and interstitials (Fig. 13d). The introduced Schottky junction enables MXFAA/MIL(Fe)-POR to achieve H2O2 generation rates 5.8–12.0 times higher than those of MIL(Fe) (Fig. 13e). The Fe-protoporphyrin can simulate the behaviour of biomimetic hemin to trap oxygen, and the Schottky junction effectively facilitates the separation of carriers, thus greatly accelerating the conversion of dissolved O2 to H2O2. Due to the difference in the work function between MIL(Fe) and MXFAA, a portion of photoelectrons are migrated from the CB of MIL(Fe) to MXFAA. A space charge layer is formed on the MIL(Fe) side, which generates a Schottky barrier, effectively inhibiting the reverse flow of photoelectrons to MIL(Fe) and thus reducing the rate of photoelectron-hole complexation (Fig. 13f).
Therefore, Schottky junctions formed at the interface of semiconductors and metals are advantageous in constructing spatial photogenerated charge separation regions [197]. The Schottky barrier can drive electrons directly to the metal and prevent them from flowing back to the semiconductor, thus significantly suppressing the recombination of charge carriers and improving the photocatalytic activity [198].
In addition to the heterogeneous junctions mentioned above, there are other special heterojunctions used for the photocatalytic generation of H2O2. From the interface engineering point of view, the creation of p-n heterojunctions among p-type with n-type semiconductors has been shown to be an effective method for separating photogenerated charge carriers and for improving the photocatalytic activity of photocatalysts [119]. In p-n heterojunctions, the internal electric field generated between the two semiconductors promotes the efficient transfer of charge carriers. When a p-semiconductor as well as an n-semiconductor come into contact, free electrons transfer from the n-semiconductor across the interface to the p-semiconductor until their Fermi levels reach equilibrium [199]. As a result, an internal electric field is formed at the interface pointing from the n-type semiconductor to the p-type semiconductor. This causes the energy bands of the n-type semiconductor to bend upward and the energy bands of the p-type semiconductor to bend downward. This internal electric field greatly facilitates the separation of photogenerated electrons and holes, thus improving the photocatalytic efficiency [200].
Notably, the charge transfer pathway in p-n heterojunctions is akin to that of type-Ⅱ heterojunctions [201]. However, because of the large Fermi level difference among p-type semiconductors with n-type semiconductors, the internal electric field in p-n heterojunctions is typically stronger, leading to more efficient charge transfer and separation. Since the band-edge position of most p-type semiconductors does not cross the redox potential of water, there are few reports on the photocatalytic production of H2O2 using p-n junctions. One of the works is that Acharya et al. established a range of MIS/e-BCN p-n heterojunction photocatalysts (MSBCN) among p-type exfoliated B-doped g-C3N4 (p-e-BCN) with n-type MgIn2S4 (n-MIS) by an in situ hydrothermal method [202]. The presence of these two separate materials in MSBCN is demonstrated by the two different lattice spacings shown in Fig. 14a, corresponding to the (400) face of MIS and the (002) face of e-BCN. The p-n heterojunction formation enhanced the separation efficiency of photogenerated h+ and e− and retarded the electron-hole pair recombination rate. The optimal H2O2 yield of MSBCN-10 reached 2175 µmol h−1 g−1 after 2 h of irradiation with visible light under oxygen-saturated conditions (Fig. 14b). After the formation of p-n heterojunction among p-e BCN with n-MIS, the flat band potentials of MSBCN-10 composites show positive and negative changes, respectively (Fig. 14c). At this time, the CB potential of the heterojunction is more negative than the redox potential of O2 to generate H2O2, and the electrons on it can rapidly couple protons to reduce O2 to generate H2O2. Therefore, the construction of the p-n heterojunction realizes a double-charge generation path for H2O2. However, p-n heterojunctions are still less studied for photocatalytic production of H2O2 due to the fact that the band-edge position of most p-type semiconductors cannot cross the redox potential limit of water [203].
Moreover, heterojunction photocatalytic materials can combine the advantages of other types of heterojunctions and Schottky junctions, facilitating both reduction reactions on the CB and oxidation reactions on the VB, thereby synergistically boosting the photocatalytic production of H2O2. For instance, Yang with co-workers synthesized Schottky-functionalized Z-scheme heterojunction photocatalysts Ti3C2/g-C3N4/BiOCl (TC/g-CN/BOC) via a simple hydrothermal method [204]. Z-scheme heterojunction was formed between BiOCl and g-C3N4. Under illumination, e− and h+ are generated on the corresponding CB and VB, respectively. The majority of photogenerated electrons generated by BiOCl combine with holes of g-CN on the contact surface. The Schottky junction exists between g-C3N4 and Ti3C2. As a result, the electrons jumping to g-C3N4 CB are rapidly transferred to Ti3C2 and participate in the photocatalytic reduction reaction. The Schottky barrier formed between Ti3C2 and g-C3N4 makes it difficult for the electrons to return to the CB, which further hinders the electron-hole complexation. The synergistic effect of the dual built-in electric fields allowed TC/g-CN/BOC to exhibit outstanding photocatalytic H2O2 evolution performance under simulated sunlight (1275 µmol L−1 h−1). Experimental data demonstrated that the establishment of the Schottky barrier efficiently inhibited the recombination of photogenerated charge carriers, while the Z-scheme heterojunction facilitated interfacial electron transfer. This dual-channel photocatalyst not only provides a reference for constructing novel photocatalysts but also presents new ideas for the photocatalytic synthesis of H2O2.
All in all, it is summarized in this minireview that photocatalysts play an important role in the photocatalytic production of H2O2 by generating photogenerated electrons and holes through photoexcitation, thus converting solar energy into chemical energy. Heterojunctions, with reasonable energy band structure, strong interfacial electric field effect, and enhanced redox ability, have significant advantages in photocatalytic H2O2 synthesis. Firstly, the optical nature of heterojunction semiconductors gives them a broad spectral response. Carefully designed heterojunctions can utilize visible and even infrared range light, which improves the efficiency of the photocatalyst under natural light conditions and plays a key role in the photocatalytic reaction. Secondly, the heterojunction materials have superior redox capabilities compared to other types of photocatalysts. By constructing various heterostructures, the oxidation and reduction capabilities of the materials can be simultaneously enhanced to promote the target reaction. Finally, the multifunctional synergistic effect of heterojunctions allows the design of different composites to optimize catalytic activity and selectivity. Heterojunctions can combine the properties of different materials to further enhance the photocatalytic performance through synergistic effects, such as dual internal electric fields and multi-channel charge transfer pathways.
Therefore, in this minireview, we present the progress of heterojunctions in photocatalytic generation of H2O2. The photoreaction mechanisms and H2O2 photoproduction pathways of different types of heterojunctions in photocatalysis are discussed separately. Then, the progress of heterojunctions for photocatalytic production of H2O2 is comprehensively summarized. Heterojunction materials show great potential in photocatalytic reactions. However, there are still some drawbacks in the photocatalytic production of H2O2. For example, the selectivity of H2O2 production is low, and current studies still focus on the ORR pathway, and dual-pathway reactions without sacrificial agents are still rare. In addition, the practical application and commercialization of heterojunction photocatalysts require further substantial progress in the engineering of efficient heterojunction photocatalysts. Here, we list the major drawbacks and propose some improvements for the photocatalytic production of H2O2 from heterojunction materials (Fig. 15) [205-208].
(1) The development of simple and economical methods for the large-scale production of high-efficiency heterojunction photocatalysts is still a major challenge. Existing photocatalytic materials usually suffer from high costs and complicated preparation processes, so finding efficient heterojunction materials suitable for large-scale preparation is crucial for practical applications.
(2) The selectivity and efficiency of heterojunction-induced H2O2 generation still need to be improved. Most of the current photogenerated H2O2 studies focus on the ORR reaction pathway, and the key to improving the selectivity is to improve the selectivity of the 2e− ORR pathway rather than the 4e− ORR pathway. It is feasible to adjust the active sites through doping and surface engineering, design catalysts with suitable energy band structures, and modulate their conduction band potentials to promote the 2e− ORR pathway. Future studies should focus on optimizing the reaction conditions and designing new catalytic systems to improve the selectivity and yield of H2O2.
(3) Photocorrosion and stability over long time operation are the main problems faced by heterojunction photocatalysts. The development of photocatalytic materials with high stability by optimizing the catalyst structure and introducing stable material compositions is important for achieving efficient long-term operation of photocatalysts.
(4) With the aid of advanced tools such as DFT and in-situ characterization, the interfacial properties and electron transfer pathways of heterojunctions in the photocatalytic process can be further analyzed, which will help to optimize the design of the photocatalysts and enhance their performance. In-depth study of the reaction mechanism of photogeneration of H2O 2 remains a challenge. Combining these methods will provide new ideas for the development of efficient and stable heterojunction photocatalysts and promote the practical application of photocatalytic H2O2 production technology.
The research on heterojunction materials in photocatalytic production of H2O2 has progressed rapidly and shows a broad application prospect. Further development of efficient and economical photocatalysts will promote breakthroughs in this field. In practical applications, optimizing the selectivity and stability of heterojunction photocatalysts is a key problem. Through multidisciplinary collaboration, including the fields of materials science, chemistry, and computational simulation, the limitations of existing technologies can be overcome to enhance the efficiency and sustainability of photocatalytic H2O2 production.
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.
Jiaming Li: Writing – original draft, Resources, Data curation. Na Xu: Writing – review & editing, Supervision, Conceptualization. Yafei Zhang: Writing – review & editing, Funding acquisition. Hongjun Dong: Conceptualization. Chunmei Li: Writing – review & editing, Supervision, Funding acquisition, Conceptualization.
This work was supported by the National Natural Science Foundation of China (Nos. 52072153, 52202238), the Postdoctoral Science Foundation of China (No. 2021M690023), and the Zhenjiang Key R&D Programmes (No. SH2021021).
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Figure 1 Schematic diagram of photocatalytic production of H2O2 over heterojunction materials. The inserted figures are Copied with permission: [48]; Copyright 2023, Elsevier. [49]; Copyright 2022, Wiley. [50]; Copyright 2021, Elsevier. [51]; Copyright 2023, Elsevier. [52]; Copyright 2023, Royal Society of Chemistry. [53]; Copyright 2022, Elsevier.
Figure 2 Photogenerated carrier transfer routes of (a) Type-Ⅰ heterojunction; (b) Type-Ⅱ heterojunction; (c) Z-scheme heterojunction; (d) S-scheme heterojunction; (e) Schottky heterojunction.
Figure 8 (a) SEM images and (b) HRTEM images of the g-C3N4/In2S3–10%. (c) The photocatalytic H2O2 generation process of 2D/2D g-C3N4/In2S3 heterostructure under light irradiation condition. Reprinted with permission [127]. Copyright 2023, Elsevier.
Figure 9 (a) SEM images of PCN-5/CS. (b) Photocatalytic H2O2 generation performance and (c) charge transfer mechanism over PCN-5/CS. Reprinted with permission [131]. Copyright 2022, Elsevier. (d) SEM image of BON—Ov/CN. (e) Photocatalytic H2O2 generation performance. (f) Schematic diagram of H2O2 photocatalytic mechanism of BON—Ov/CN. Reprinted with permission [132]. Copyright 2024, Elsevier. (g) SEM image of ZIS/PCN. (h) Photocatalytic H2O2 generation performance. (i) Mechanism for H2O2 production over ZIS/PCN heterojunction. Reprinted with permission [135]. Copyright 2024, Elsevier.
Figure 10 (a) SEM image of the NH3—CeO2/CN. (b) Photocatalytic H2O2 generation performance. (c) Possible catalytic mechanism of the NH3—CeO2/CN catalyst. Reprinted with permission [140]. Copyright 2024, Elsevier. (d) SEM image of the g-C3N4/ZIF-L. (e) Photocatalytic H2O2 generation performance via g-C3N4, ZIF-L and the ZC50 heterojunction. (f) Schematic diagram of the piezo-photocatalytic synergistically promoting the H2O2 production. Reprinted with permission [143]. Copyright 2024, Elsevier.
Figure 11 (a) SEM image of the ZnO/g-C3N4 composite. (b) Photogeneration properties of H2O2 from ZnO/g-C3N4. (c) Work functions of ZnO and CN before contact, Internal electric field and band edge bending at the interface of ZnO and CN after contact and S-scheme charge-transfer mechanism between ZnO and CN under light irradiation. Reprinted with permission [174]. Copyright 2021, American Chemical Society. (d) SEM image of the C3N4/PDA photocatalyst. (e) Photogeneration properties of H2O2 from C3N4/PDA. (f) The schematic of the S-scheme photocatalytic mechanism over CNP-4 heterojunction. Reprinted with permission [175]. Copyright 2023, Wiley.
Figure 12 (a) Wavelength-dependent H2O2 yield rates on ZnPc/PCN. (b) Photocatalytic H2O2 production on MPcs/PCN (M = H, Ni, Co, Fe, Zn, and Mn). (c) The photocatalytic mechanism in ZnPc/PCN S-scheme heterojunction. Reprinted with permission [176]. Copyright 2024, Elsevier. (d) In-situ FTIR spectra of K+/I−-CN/CdSe-D in photocatalytic reaction. (e) Corresponding evolution rate of Photocatalytic H2O2 production over the respective catalysts. (f) Schematic diagram of the charge transfer and separation of K+/I−-CN/CdSe-D heterojunction. Reprinted with permission [179]. Copyright 2024, Wiley.
Figure 13 (a) TEM images of the Bi/Bi2O3@g-C3N4. (b) H2O2 generation properties of Bi/Bi2O3@g-C3N4. (c) Energy band diagram of the Bi/Bi2—O3@g-C3N4 heterojunction. Reprinted with permission [52]. Copyright 2023, Royal Society of Chemistry. (d) SEM images of the MXFAA/MIL(Fe)-POR. (e) H2O2 generation properties of MXFAA/MIL(Fe)-POR. (f) Photoreaction mechanism of MXFAA/MIL(Fe)-POR. Reprinted with permission [196]. Copyright 2022, Elsevier.
Figure 14 (a) HRTEM images of the MSBCN-10. (b) H2O2 generation properties of MSBCN-10. (c) p-n heterojunction double charge transfer pathway on MSBCN-10 photocatalysts. Reprinted with permission [202]. Copyright 2022, American Chemical Society.
Table 1. Application of Z-scheme heterojunction photocatalyst in H2O2.
| Catalyst | H2O2 yield (µmol−1 h−1 g−1) |
Sacrificial agent | Catalyst dosage (g/L) |
AQY | Illumination | Ref. |
| OCN@In2S3 | 633 | / | 0.5 | / | Visible light | [136] |
| Mn3O4/Co9S8 | 1600 | / | 1 | 26.33% (450 nm) | 300 W Xe lamp |
[137] |
| CeO2/CN | 2010 | Isopropanol | 0.5 | / | Visible light | [140] |
| WS2/S-g-C3N4 | 5216 | Isopropanol | 0..2 | / | 300 W Xe lamp |
[142] |
| ZIF-L/g-C3N4 | 1450 | / | 0.5 | / | 300 W Xe lamp |
[143] |
| g-C3N4/TiO2 | 133.04 µmol/L | Ethanol | 1.5 cm2 films (in 5 mL) |
/ | 300 W Xe lamp |
[144] |
| g-C3N4/FeOOH/ZnIn2S4 | 6024 | Isopropanol | 0.4 | Visible light | [147] | |
| MnIn2S4/WO3 | 1188 | / | 0.5 | / | Simulated | [148] |
| A-TxCNx | 1132 | Ethanol | 0.1 | / | Visible light | [149] |
| TiO2/B-doped g-C3N4 | 550 | Ethanol | 1 | / | Visible light | [150] |
| CoWO4@Bi2WO6 | 85 | / | 1 | / | Visible light | [151] |
| CuBi2O4/MoS2 | 5828 | / | 1 | / | 150 W Xe lamp |
[152] |
| Bi2S3/Sb2S3 | 541.5 | Ethanol | 1 | / | 400 W metal halide lamp | [153] |
| O-doped g-C3N4/ZnIn2S4-Zn | 1400 | Isopropanol | 0.2 | 18.6% (400 nm) | Visible light | [154] |
| TiO2/B-doped g-C3N4 | 550 | Ethanol | 1 | / | Visible light | [155] |
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Table 2. Application of S-scheme heterojunction photocatalyst for H2O2 generation.
| Catalyst | H2O2 yield (µmol−1 h−1 g−1) | Sacrificial agent | Catalyst dosage (g/L) | AQY | Illumination | Ref. |
| CdS/K2 Ta2 O6 | 160.89 | / | 0.6 | / | Visible light | [39] |
| BiOI/g-C3N4/CoP | 2733 | Ethanol | 0.1 | / | Simulated | [174] |
| ZnO/g-C3N4 | 1360 | Ethanol | 0.4 | / | 300 W Xe lamp | [175] |
| NiO/C3N5 | 91.2 | Isopropanol | 0.2 | 5.94% (400 nm) | Visible light | [176] |
| ZnPc/PCN | 1870 | Ethanol | 1 | 1.11% (800 nm) | 300 W Xe lamp | [179] |
| MSCN-5 | 4863 | Ethanol | 0.4 | / | Visible light | [181] |
| C3N4/PDA | 3801 | Ethanol | 0.5 | 2.22% (400 nm) | Simulated | [183] |
| ZnO/WO3 | 6788 | Ethanol | 1 | / | 300 W Xe lamp | [186] |
| Sv-ZIS/CN | 1310. 18 | Isopropanol | 0.4 | / | Visible light | [187] |
| TiO2/In2S3 | 376 | Ethanol | 0.5 | / | 300 W Xe lamp | [188] |
| ZnO/COF | 2443 | Ethanol | 0.5 | / | 300 W Xe lamp | [189] |
| TiO2/PDA | 2200 | Ethanol | 0.5 | / | 300 W Xe lamp | [190] |
| In2O3/ZnIn2S4 | 5716 | Ethanol | 0.4 | / | Visible light | [191] |
| TiO2@RF | 66,600 | / | 0.67 | / | 300 W Xe lamp | [192] |
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