ZnIn₂S₄-based heterostructure photocatalysts for solar energy conversion: a comprehensive review

Abstract

Solar energy driven photocatalysis is a promising technology to solve the urgent energy and environmental problems, while its practical applications are heavily limited by the lack of ideal photocatalytic materials. ZnIn₂S₄ has recently attracted extensive attention as a visible-light-responsive photocatalyst due to its prominent advantages of simple synthesis, excellent stability, a wide light absorption range, and an appropriate band structure. However, the photocatalytic performance of pristine ZnIn₂S₄ can hardly meet the requirement of practical applications. In this context, various ZnIn₂S₄-based heterostructures have been developed with improved photocatalytic performances. This comprehensive review focuses on the recent progress regarding ZnIn₂S₄-based heterostructures for energy conversion applications. First, the fundamental physicochemical properties of ZnIn₂S₄ were introduced, including the crystal structure, optical properties, synthesis methods and modification strategies. Then, the ZnIn₂S₄-based heterostructure photocatalysts were classified based on their different charge transfer mechanisms, and the representative heterostructure systems were categorically introduced. The widespread applications of ZnIn₂S₄-based heterostructures for solar fuel synthesis mainly include H₂ production, CO₂ reduction, N₂ fixation, etc. Finally, the current challenges and perspectives of ZnIn₂S₄-based heterostructure photocatalysts were also discussed. This review aims to highlight the recent advancements and challenges of ZnIn₂S₄-based heterostructures, and further provides an instructive direction and foresight on the design of high-performance ZnIn₂S₄-based photocatalysts for solar energy conversion and storage.

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Zhao Jing

Zhao Jing received his Bachelor's degree from the Qingdao University of Science and Technology in 2022. He's now pursuing his Master’s degree at the East China University of Science and Technology. His research interests focus on the design of novel photo(electro)catalysts for solar energy conversion.

Qiang Wang

Qiang Wang is currently an associate professor at the Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences. He received his PhD degree from the East China University of Science and Technology in 2019, and then worked as a postdoctoral fellow with Professor Jianlin Shi at the Shanghai Institute of Ceramics, Chinese Academy of Sciences. His research focuses on photo(electro)catalytic solar fuel production and environmental remediation.

Chen-Ming Fan

Chenming Fan is currently a postdoctoral fellow with associate professor Pengyi Tang at the Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences. He received his PhD degree from the East China University of Science and Technology in 2024. His research focuses on molten-salt electrochemistry and electrocatalysis.

Xiao-Fan Yang

Xiaofan Yang received his Bachelor's degree from the East China University of Science and Technology in 2022. He's now pursuing his PhD degree at the East China University of Science and Technology under the supervision of Prof. Bing Li. His research interest focuses on photoelectrochemical solar fuel production.

Peng-Yi Tang

Peng-Yi Tang received BS degree in chemical engineering and technology from Hunan Normal University (2010) and MS degree in physical chemistry from Lanzhou University (2013). Then, he obtained a PhD degree in materials science from Universitat Autònoma de Barcelona (2018). In 2019–2021, he worked at the Ernst Ruska-Centre (ER-C) for Microscopy and Spectroscopy, Forschungszentrum Jülich as a Humboldt postdoc. Since 2021, he has been working as an associate professor at the Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences. His research interests include solar fuel materials, such as photoelectrochemcial water splitting, carbon dioxide electrochemcial reduction, and material microstructure etc.

Bing Li

Bing Li is currently a professor of materials science and engineering at the East China University of Science and Technology. She received her PhD degree (2001) from Northeastern University, and then she joined the faculty of the East China University of Science and Technology as an associate professor in 2002. She was previously affiliated as a visiting scientist at MIT (2006–2007). Her research interests include novel electrochemical technologies, energy storage and conversion, electrocatalysis and photocatalysis.

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1 Introduction

The emergence and fast development of industrial civilization is heavily dependent on the extensive consumption of fossil fuels, which results in escalating energy crises and environmental challenges. Therefore, the demand for clean and sustainable energy sources is growing rapidly. In this context, the efficient conversion of solar energy into renewable fuels and chemical feedstocks is emerging as a promising approach to solve global energy and environmental problems. As an important way of harnessing and storing solar energy, photocatalysis technology combines photochemistry and catalysis, establishing a close link between light and catalysts to initiate or accelerate chemical transformation reactions.¹ This technology has broad application prospects, as it not only enables efficient energy conversion and utilization but also transforms harmful substances into harmless resources, facilitating environmental restoration and purification.^2–4 In 1972, Fujishima and Honda discovered the phenomenon that a photo-irradiated n-type semiconductor TiO₂ electrode could decompose water to produce H₂.^5,6 After decades of development, there is still great interest in TiO₂ photocatalytic technology, especially in the field of solar energy conversion, including photocatalytic water splitting, industrial waste degradation, CO₂ reduction, and biomass conversion.^7–12 Nevertheless, the TiO₂ photocatalyst has a relatively wide energy bandgap (E_g > 3.0 eV), which means that about only 5% of the solar spectrum can be utilized, greatly limiting its solar photocatalytic efficiency.^13–15

In recent years, a large number of photocatalytic materials have been intensively investigated. Among them, sulfide semiconductors have attracted particular attention due to their unique photocatalytic performance and excellent stability.^16–18 Depending on the type of elements involved, sulfide semiconductors are generally classified as binary, ternary, and quaternary.^19,20 Compared to sulfides like CdS and PdS, ZnIn₂S₄(ZIS) exhibits remarkable photo-stability, environmental friendliness, and human compatibility.²¹ ZIS is a direct bandgap semiconductor (E_g = 2.4–2.6 eV), which is suitable for both water reduction and oxidation.²² It possesses various advantages such as visible light absorption, excellent optoelectronic properties, and tunable bandgaps. However, the photocatalytic activity of pristine ZIS is limited by rapid electron–hole recombination and the shortage of catalytically active sites.²³ The construction of ZIS-based heterostructure photocatalysts has been extensively reported to significantly improve the spatial separation of charge carriers. Based on the charge transfer mechanism, ZIS-based heterostructure photocatalysts can be classified into four types: conventional heterostructures (type-I and type-II), p–n heterostructures, Z-scheme heterostructures and S-scheme heterostructures.²⁴ Up to now, ZIS-based heterostructures for photocatalytic H₂ production,^25–28 N₂ fixation,^29,30 CO₂ reduction^31–33 and H₂O₂ photosynthesis^34,35 have attracted increasing attention. The corresponding reaction mechanisms and thermodynamic requirements are illustrated in Fig. 2 a–c.^32,36,37 In light of the rapid advancements being made in the field of ZIS photocatalysts, it is imperative to provide a comprehensive overview of the achievements and future prospects of this research area. An examination of the developmental timeline of ZIS (Fig. 1) reveals that ZIS heterostructures and their applications are currently a focal point of research endeavors. However, a paucity of comprehensive summaries and overviews of this subject is evident in the extant literature. This review article provides an overview of the research progress of ZIS, including its crystal structure and preparation techniques. It also delves into various heterostructure strategies that intensify the photocatalytic performance of ZIS-based composite materials and their practical applications. Additionally, the review discusses the main challenges currently faced by these materials and the key issues that need to be addressed in future research. The aim of this review is to provide comprehensive insight into ZIS-based heterostructure photocatalysts and their corresponding solar fuel application.

Fig. 1 Schematic diagram of the development history of ZIS.

Fig. 2 (a) Schematic illustration of the three fundamental processes in photocatalytic H₂O₂ production (potential vs. SHE at pH = 0).³⁷ Reproduced with permission from ref. 37. Copyright 2023 Elsevier. (b) Redox potentials (vs. SHE at pH = 0) of typical N₂ fixation reactions.³⁶ Reproduced with permission from ref. 36. Copyright 2021 Elsevier. (c) Schematic illustration of photocatalytic CO₂ reduction on a semiconductor photocatalyst coloaded with reduction and oxidation cocatalysts for solar fuel production (potential vs. SHE at pH = 7).³² Reprinted with permission from ref. 32. Copyright 2018 John Wiley and Sons.

2 Fundamental properties of ZIS

2.1. Crystal and electronic structures

The differences in the crystal structure affect the bandgap, charge separation, and migration efficiency, thus influencing the photocatalytic activity. Typically, there are three crystal structures of ZIS: the hexagonal phase,³⁸ cubic phase³⁹ and rhombohedral phase,⁴⁰ as depicted in Fig. 3a. Currently, the cubic and hexagonal phases are the main types of ZIS photocatalysts being studied.⁴³ In terms of synthesis, the hexagonal phase of ZIS can be prepared at lower temperatures,⁴⁴ while it can be converted to the cubic phase under high-temperature and high-pressure conditions.⁴⁵ The utilization of different metal salt precursors can realize the controlled synthesis of the ZIS crystal phase. For instance, the employment of metal nitrates has been demonstrated to facilitate the synthesis of cubic-phase ZIS.⁴⁶ Furthermore, by using different metal precursors and modulating the pH value of the solution, the mutual transformation between the hexagonal and cubic phases can be achieved.⁴⁷ In terms of the crystal structure, cubic-phase ZIS has an ABC stacking type, where Zn and In atoms are coordinated in tetrahedral and octahedral arrangements, forming a closely-packed structure. On the other hand, hexagonal phase ZIS has an ABAB stacking type, forming S–Zn–S–In–S–In–S layers.⁴⁸ At the edge of the (110) plane, the sulfur atoms of hexagonal phase ZIS exhibit excellent H adsorption properties and outstanding resistance to photocorrosion. This structure endows hexagonal-phase ZIS with different characteristics from the cubic phase in terms of photocurrent density and photoluminescence intensity. Additionally, DFT calculations further indicate that the band structure of hexagonal phase ZIS is more dispersed, implying higher electron mobility for the hexagonal phase.⁴⁹ Overall, hexagonal phase ZIS exhibits greater band dispersion, smaller effective mass, and higher migration capability compared to cubic phase ZIS, making it more suitable for photocatalytic reactions.⁵⁰ Therefore, the synthesis of hexagonal phase ZIS has been widely investigated, particularly in the application of photocatalytic H₂ production.⁴⁸ Besides the hexagonal and cubic phases, few studies have shown that rhombohedral phase ZIS exhibits a narrower bandgap, more negative conduction band (CB) potential, and higher efficiency of photo-generated charge carrier separation.⁴⁰ A number of effective methodologies have been formulated for the purpose of synthesizing the rhombohedral phase of ZIS. Such a methodology involves either annealing hexagonal-phase ZIS at temperatures exceeding 600 °C,^48,51 or conducting solid-phase reactions at 700 °C.⁵² Additionally, the binary flux-assisted method can also be used to prepare rhombohedral phase ZIS.⁵³ Nevertheless, there are still few investigations about the relationship between carrier dynamics in the crystal structure and photocatalytic activity.⁴⁹ Therefore, further research on the influence of crystal carrier dynamics on chemical reactions is expected to be conducive to understanding the unique phenomena associated with different crystal structures in photocatalysis. For instance, highly exposed crystal facets can offer more effective catalytic sites, while different crystal orientations can provide unique electric fields or structural features that are favorable for charge-carrier separation.⁵⁴

Fig. 3 (a) From left to right, crystal structures of hexagonal, cubic and rhombohedral ZIS. (b) Calculated density of states (DOS) for pristine ZIS structures. The Fermi level is set to 0 eV.⁴¹ Reproduced with permission from ref. 41. Copyright 2023 Elsevier. The band structure of (c) cubic ZIS and hexagonal phase ZIS calculated by DFT with the exchange–correlation functional approximated by GGA, GGA + U, mBJ-GGA, and mBJ-GGA + U. The VBM is set to 0 eV and indicated by a dotted horizontal line for easy visualization.⁴² Reproduced with permission from ref. 42. Copyright 2019 John Wiley and Sons.

By using DFT calculations, the electronic band structure of ZIS can be theoretically evaluated. As shown in Fig. 3b, the valence band (VB) of ZIS contains hybridized S 3p and Zn 3d orbitals, whereas the CB consists of S 3p and In 5s orbitals. For semiconductor materials, appropriate light excitation can cause electron transitions from the VB to the CB, forming electron–hole pairs. Zhang et al. compared and discussed the effects of the Hubbard U term and the mBJ potential on the prediction of electronic states and band gaps.⁴²Fig. 3c and d display the theoretical band structures of cubic and hexagonal phase ZIS using various exchange–correlation functionals, including GGA, GGA + U, mBJ-GGA, and mBJ-GGA + U. The inclusion of the Hubbard U correction was necessary to accurately address the strong “on-site” electron correlation of the Zn 3d orbital. The incorporation of the mBJ potential significantly enhanced the bandgap, suggesting that the mBJ potential could provide accurate bandgap estimations for both cubic and hexagonal phase ZIS. The bandgaps obtained from mBJ-GGA + U calculations for cubic and hexagonal phase ZIS were 2.59 eV and 0.916 eV, respectively, and were in good agreement with experimental data. In order to effectively increase the electron transfer rate of ZIS and modulate its band structure, various strategies have been developed, such as doping, and heterostructure and defect engineering.^55–57 First, cation/anion substitution into a semiconductor introduces donor–acceptor energy levels within the bandgap, which can adjust the concentration and energy distribution of carriers near the CB/VB edge, thereby improving charge transfer dynamics.⁵⁷ Additionally, by forming heterostructures, the separation efficiency of photo-generated electron–hole pairs and catalytic activity can be effectively improved. Moreover, defect engineering, including sulfur vacancies,⁵⁸ metal doping⁵⁹ and oxide modification,⁶⁰ has been shown to increase the surface-active sites and charge transfer channels, further enhancing the photocatalytic performance.

2.2. Synthesis methods

The physicochemical properties of ZIS nanomaterials are critically governed by multiple structural parameters, particularly particle size, crystallinity, defect density, morphological characteristics, and specific surface area. These key parameters exhibit strong correlations with the employed synthesis techniques and their corresponding process conditions.²² Notably, morphology engineering has been demonstrated to effectively modulate the band structure of ZIS nanomaterials through quantum confinement effects and surface state modifications, thereby influencing the separation efficiency of photogenerated charge carriers.⁶¹ To achieve precise control over these critical parameters, various advanced synthetic strategies have been developed, including but not limited to hydrothermal synthesis, solvothermal processing, microwave-assisted preparation, and template-directed growth approaches. This review systematically elaborates on these state-of-the-art fabrication methodologies with particular emphasis on their underlying mechanisms, while presenting representative case studies that highlight successful applications in photocatalytic and energy-related fields.

2.2.1. Hydrothermal synthesis. The hydrothermal synthesis method employs aqueous-phase reactions between water vapor and solid precursors in an aqueous solution to enable controlled nanoparticle formation.⁶² This technique offers distinct advantages in tailoring nanoparticle dimensions and morphology,⁶³ while simultaneously ensuring high crystallinity in the synthesized powders.⁶⁴ These characteristics establish hydrothermal synthesis as a preferred strategy for fabricating ZIS nanomaterials.⁶⁵

Lei et al. successfully synthesized ZIS via a facile hydrothermal approach and further utilized it as an efficient visible-light-driven photocatalyst for hydrogen evolution through water splitting.⁶⁶ Their findings highlighted ZIS's promising potential for solar-driven H₂ production. Notably, critical synthesis parameters including the reaction temperature and pH value were found to significantly influence the crystalline structure, morphological features, optical characteristics, and photocatalytic performance of ZIS products.⁶⁷ To systematically optimize synthesis conditions, Shen et al. developed ZIS photocatalysts with tunable morphologies and crystallinity through solvothermal/hydrothermal approaches using water, methanol, and ethylene glycol as mediating solvents. Comparative analysis revealed that water-mediated ZIS exhibited superior photocatalytic stability and corrosion resistance, achieving H₂ production rates substantially exceeding those of their methanol- and ethylene glycol-derived counterparts.⁶⁸ Recently, ZIS nanosheets were directly grown on the surface of TiO₂ hollow spheres through hydrothermal synthesis, forming Z-scheme heterostructure composite materials with enhanced charge separation efficiency.⁶⁹ Meanwhile, Xin et al. synthesized oxygen-doped ZIS (D-O-ZIS) via a multi-step hydrothermal strategy: (1) thermal-induced atomic migration generated distorted edge structures (D-ZIS), followed by (2) O₂ plasma treatment to introduce cationic-site oxygen doping. The introduced structural distortion disrupted atomic periodicity, thereby facilitating enhanced overall water splitting efficiency (Fig. 4a). In the modified S1–S2–O configuration of D-O-ZIS, the complementary electronic states between electron-enriched S1 and electron-depleted S2 sites collectively optimized adsorption/desorption energetics for both H₂ and O₂ intermediates. Morphological characterization via SEM/TEM revealed a hierarchical nanoflower architecture assembled from interconnected nanosheets (Fig. 4b and c). High-resolution TEM (HR-TEM) imaging identified lattice spacings of 0.32 nm, consistent with the (102) crystallographic plane of hexagonal ZIS (Fig. 4d–f). Comparative analysis demonstrated edge-shell distortions in D-ZIS and D-O-ZIS (yellow highlighted regions, Fig. 4d and e), with thicknesses of 1.3 ± 0.3 nm and 2.2 ± 0.2 nm, respectively (Fig. 4g). Elemental mapping confirmed the uniform spatial distribution of Zn, In, S, and O across D-O-ZIS (Fig. 4h), while SAED patterns indicated crystalline lattice distortions at the edges of modified samples relative to pristine ZIS (Fig. 4i). Atomic-scale HAADF-STEM profiles further localized Zn vacancies predominantly at the edges of D-ZIS/D-O-ZIS, with oxygen dopants in D-O-ZIS concentrated at peripheral regions (Fig. 4j).⁷⁰

Fig. 4 (a) Schematic synthesis routes of ZIS, D-ZIS, and D-O-ZIS. Atomic color codes: S (yellow), Zn (blue), In (orange), and O (green). Insets highlight edge structural variations across the samples. (b) SEM and (c) TEM micrographs of D-O-ZIS exhibiting hierarchical architecture. (d) HR-TEM image of D-O-ZIS with magnified lattice fringes (0.32 nm spacing); arrows denote edge distortion zones, while yellow dashed outlines demarcate ∼2.2 nm surface shells. (e) Comparative HR-TEM image of D-ZIS showing ∼1.3 nm distorted edge shells (yellow dashed). (f) Pristine ZIS HR-TEM image lacking detectable edge shells (yellow dashed). (g) Line-scan profiles quantifying shell thickness gradients from the edge to the core (referenced to panels d–f). (h) EDX elemental maps confirming homogeneous Zn/In/S/O distribution in D-O-ZIS (scale: 500 nm). (i) SAED pattern of D-O-ZIS edge domains revealing lattice disorder (scale: 5 nm⁻¹). (j) HAADF-STEM cross-sectional elemental scans delineating Zn vacancy localization and oxygen dopant distribution in D-ZIS/D-O-ZIS.⁷⁰ Reprinted with permission from ref. 70. Copyright 2024 Springer Nature.

2.2.2. Solvothermal method. Solvothermal synthesis is a technique that uses a closed pressure vessel to synthesize nanomaterials with water or other organic solvents (such as methanol, ethanol, and polyols) under high-pressure conditions. By heating solvents beyond their boiling points, this method enables the production of monodisperse nanocrystals with high crystallinity, representing a significant improvement compared to conventional oil-bath approaches in terms of crystallinity control and size distribution uniformity.⁷¹ Recent advancements demonstrated its versatility in constructing complex ZIS-based architectures. For instance, a representative case involved the in situ growth of ultrathin g-C₃N₄ nanosheets and NiS nanoparticles on ZIS surfaces through one-step solvothermal processing. The resultant ternary NiS/ZIS/ultrathin-g-C₃N₄ composite established dual charge-transfer pathways, achieving a 4.8-fold enhancement in photocatalytic hydrogen evolution compared to pristine ZIS.⁷² Furthermore, Pan et al. successfully prepared oxygen-doped ultrathin ZIS nanosheets via single-pot solvothermal synthesis, demonstrating exceptional CO₂ reduction performance with 92% selectivity for CH₄ production.⁵⁶ In solvothermal methods, the solvent has a significant impact on the crystal structure and morphology. For instance, it was discovered that ZIS synthesized through a solvothermal method in a water–ethanol mixed solvent demonstrates excellent H₂ production performance. The increased crystal facets generated during the mixed solvent solvothermal process greatly enhanced the photocatalytic reactivity.⁷³

2.2.3. Microwave-assisted method. A. V. Hippel first proposed the fundamental understanding of the interaction between macroscopic microwaves and matter, as well as the applications of this technology.⁷⁴ The core principle of the microwave-assisted method lies in the synergistic interaction between thermal and non-thermal effects, which facilitates precise control over energy transfer mechanisms.⁷⁵ The application of this technology not only significantly reduces energy consumption and processing time but also utilizes microwave energy to generate unique internal heating phenomena, enabling the development and utilization of new chemical substances. Compared with traditional heating methods, the microwave-assisted method can boost heating effects, enhancing overall production quality.^75–77 Furthermore, the microwave-assisted method offers higher efficiency and precision, reducing unnecessary waste and losses, thus bringing about extensive prospects in various fields. In recent years, it has been found that the pH medium significantly affects the catalytic activity when synthesizing ZIS/g-C₃N₄ through a microwave-assisted method.⁷⁸ In another study, Zhou et al. synthesized Ni and In co-doped ZIS photocatalysts via a microwave-assisted solvothermal method. In doping at tetrahedrally coordinated Zn sites enhanced the electron delocalization effect around In sites, thereby reducing the electron potential well along the z-axis direction. Simultaneously, Ni doping at tetrahedrally coordinated Zn sites decreased the negative charge density of S sites, optimized the hydrogen adsorption/desorption equilibrium, and consequently significantly enhanced the photocatalytic activity.⁷⁹

Above all, this study further elaborates on the influence of three primary synthesis methods—hydrothermal, solvothermal, and microwave-assisted approaches—on the long-term stability of ZIS-based heterojunction photocatalysts. Hydrothermal/solvothermal methods synthesize materials through solution reactions at high temperature and pressure with slow reaction kinetics, while the microwave-assisted method employs microwave heating for rapid reactions. These two approaches differentially influence the material structure, thereby impacting long-term stability. For instance, ZIS/QDs composites with moderate lattice strain synthesized via the solvothermal method demonstrate exceptional hydrogen production stability under simulated sunlight (144 hours), exhibiting an average decay rate of merely 0.201% per hour.⁸⁰ Furthermore, Su et al. comprehensively compared the morphology, performance, and long-term stability of ZIS materials synthesized by hydrothermal (H-ZIS) and microwave-assisted (M-ZIS) methods. Both H-ZIS/Pt and M-ZIS/Pt exhibited remarkable photocatalytic stability: the former accumulated 3319 μmol H₂ over 92 hours, while the latter generated 2554.5 μmol H₂ over 188 hours.⁸¹ This suggests that while materials synthesized via the microwave-assisted method may exhibit inferior photocatalytic performance compared to those prepared by hydrothermal and solvothermal methods, they demonstrate superior long-term stability.

2.2.4. Other synthesis methods. Other methods for synthesizing ZIS include template synthesis, spray pyrolysis, and chemical vapor deposition. For example, employing TEOA as a structure template, a series of ZIS compounds were synthesized. The results indicated that the ZIS synthesized in TEOA solution had higher activity than those synthesized in pure water.⁸² Besides, spray pyrolysis has been successfully employed for preparing ZIS thin films with controlled grain dimensions (120–200 nm), revealing cubic spinel crystallinity and an optical bandgap of 2.12 eV. Moreover, the chemical vapor deposition technique can also be used to synthesize ZIS nanosheets with uniform morphology and high purity.⁸³ Overall, different synthesis methods have their own advantages and disadvantages. The choice of an appropriate method depends on the desired film properties, preparation conditions, and practical application requirements.

2.3. Modification strategies

Owing to the unsatisfactory photocatalytic performance of pure ZIS, various effective strategies have been proposed for further performance improvement:⁸⁴

2.3.1. Morphology and structure design. The photocatalytic efficacy of ZIS is intrinsically governed by its nano-architecture, where dimensionality control enables precise modulation of quantum confinement effects and surface energetics. Through advanced synthetic protocols, bulk ZIS can be transformed into 0D (nanoparticles and quantum dots), 1D (nanotubes, nanowires, and nanorods), 2D (nanosheets), and 3D (flowers and microspheres) hierarchical structures. Compared with the bulk structure, nanosheets have a larger accessible surface area and shorter pathways for charge transfer to the surface.⁸⁵ Therefore, 2D ZIS nanosheets and 3D hierarchical structures assembled by nanosheets have advantages in photocatalysis. Chen et al. reported a spatially distributed heterostructure composed of hollow g-C₃N₄ microtubes (T-CN) and ZIS nanosheets. The ZIS nanosheets were vertically arranged on the outer and inner surfaces of T-CN, forming a layered core/shell structure, which showed remarkable performance for photocatalytic CO₂ reduction.⁸⁵

2.3.2. Element doping. Element doping, a strategic modification technique involving the incorporation of foreign elements into host materials, serves as an effective approach to tailor structural characteristics and physicochemical properties of photocatalysts, ultimately enhancing their photocatalytic activity and operational stability. This methodology primarily exerts influence through three fundamental mechanisms: lattice structure modulation, surface-active site engineering, and electronic structure optimization.⁸⁶ Based on elemental classification, doping strategies can be categorized into metallic and non-metallic approaches. A representative example demonstrates that Mo-doped ZIS exhibited significantly enhanced hydrogen evolution performance. This improvement originates from dual synergistic effects: (1) the unique three-dimensional hollow microsphere architecture, self-assembled from interconnected nanosheets, provided exceptional light-harvesting capabilities while maximizing active site exposure; (2) the Mo doping strategy induced substantial electronic structure modifications, including enhanced surface hydrophilicity, a broadened spectral response range, accelerated charge carrier dynamics, and reduced hydrogen evolution overpotential.⁸⁷ In addition, non-metal doping has also been considered a cost-effective strategy to improve the photocatalytic performance. For instance, using NaH₂PO₄ as a precursor, P-doped ZIS nanosheets were prepared through a hydrothermal method. It was found that P substitution in the ZIS lattice significantly modulates its electronic configuration. The optimized P-ZIS photocatalyst exhibited excellent photocatalytic CO generation efficiency under visible-light irradiation.⁸⁸

2.3.3. Defect engineering. Defect engineering in photocatalysis refers to the technique of improving photocatalytic performance by controlling and manipulating defects or vacancies in the catalyst.⁸⁹ Defects in materials can be classified into three types: point defects, line defects, and bulk defects according to their atomic structure. Among them, point defects are the most common type of defects, including vacancies (such as oxygen vacancies, metal vacancies, etc.), interstitial atoms (atoms occupying crystal sites that are normally empty), and impurity atoms (foreign atoms incorporated into the crystal). Point defects can affect the electronic structure and energy band structure of materials, thereby influencing their photocatalytic performance (Fig. 5a–c).^22,92–94 Recently, the synthesis of the CdS@ZIS heterostructure with sulfur vacancies has been reported; by adjusting the relative content of sulfur vacancies in ZIS, the light absorption of the catalyst could be controlled.⁹⁵ Wang et al. rationally constructed a Z-scheme heterostructure consisting of ZIS with abundant sulfur vacancies and MoSe₂. The addition of N₂H₄·H₂O was a crucial step in the formation of sulfur vacancies and coordinatively unsaturated sulfur atoms. The sulfur vacancies helped intensify light absorption and promoted the separation of photogenerated charge carriers, while the coordinatively unsaturated sulfur atoms provided anchoring sites for Mo atoms. This further facilitated the formation of Mo–S bonds and the in situ growth of MoSe₂ on the S_v-ZIS surface as shown in Fig. 5d.⁹¹

Fig. 5 A diagrammatic representation of point defects includes (a) vacancies, (b) substitutional doping, and (c) interstitial doping.⁹⁰ Reprinted with permission from ref. 90. Copyright 2024 John Wiley and Sons. (d) Schematic presentation of the synthetic route of S_v-ZIS and S_v-ZIS/MoSe₂ heterostructure.⁹¹ Reprinted with permission from ref. 91. Copyright 2021 Springer Nature.

2.3.4. Surface modification. Surface modification involves introducing external materials or compounds to the catalyst surface to alter its surface properties and structure, thereby enhancing photocatalytic activity and stability. Surface modification can be achieved through physical methods such as plasma treatment, atomic layer deposition and chemical vapor deposition, or chemical methods such as solution impregnation, the sol–gel process and in situ synthesis.⁹⁶ The Ni-modified ZIS composite material (Ni: ZIS) was synthesized using a photodeposition method. The Ni: ZIS composite not only improved charge transfer but also had a synergistic effect on α-H extraction.²³

2.3.5. Cocatalyst deposition. Cocatalyst deposition, a pivotal surface modification technique in photocatalysis engineering, serves to optimize reaction kinetics through dual mechanisms: enhancing charge carrier management and tailoring active surface sites, thereby synergistically boosting both quantum efficiency and long-term operational stability.⁹⁷ This strategy proves particularly valuable in addressing two fundamental limitations of semiconductor photocatalysts-rapid charge recombination and insufficient catalytic activation energy. A paradigm of this approach was demonstrated by Wang et al. through the strategic integration of NiO as a cocatalyst to form a heterostructure with ZIS, significantly improving the charge carrier separation.⁹⁸

2.3.6. Heterostructure construction. Heterostructures have prominent advantages in photocatalysis, including efficient charge separation, abundant active sites, tunable optoelectronic properties, and improved catalyst stability.⁹⁹ By rationally constructing heterostructures, the efficiency and performance of photocatalytic reactions can be improved. Therefore, many heterostructures based on ZIS have been designed to boost the photocatalytic performance of ZIS.

3 ZIS-based heterostructures

A heterostructure, as a special type of p–n junction in semiconductors, typically consists of several distinct semiconductor materials. In such configurations, a unique electric field arises at the interface due to the differences between the constituent materials, which modifies the electronic energy band structure.^100,101 To ensure proper functionality, it is necessary to maintain lattice continuity in the heterostructure at the interface between different materials, which requires crystal structure matching of the surfaces in contact. In recent years, the construction of ZIS-based heterostructure photocatalysts has received growing attention and become a research hotspot due to their significantly improved spatial charge carrier separation efficiency and photocatalytic performance.¹⁰² Heterostructure ZIS-based photocatalysts can be classified into conventional heterostructures (type-I and type-II), p–n heterostructures, Z-scheme heterostructures, S-scheme heterostructures, and heterostructure systems based on cocatalyst deposition.²⁴ The charge transfer pathways of various heterostructures are shown in Fig. 6:

Fig. 6 Schematic illustrating electron–hole pair transfer pathways in heterostructure configurations (RP: reduction photocatalyst; OP: oxidation photocatalyst), where PC 1 and PC 2 denote distinct photocatalysts.²⁴ Reproduced with permission from ref. 24. Copyright 2023 Elsevier.

3.1. Type-II heterostructures

In type-II heterostructures, the unique staggered band gap structure of the two semiconductors can enable highly efficient charge transfer. During the photoexcitation process, the photogenerated charge carriers in type-II heterostructures can effectively improve the separation efficiency of photogenerated electrons and holes through a ‘bidirectional transfer pathway’.¹⁰³ Furthermore, the presence of an internal electric field (IEF) significantly prolongs the lifetime of photogenerated electrons. Typically, photon-excited electrons in the CB tend to recombine with holes in the VB in an independent ZIS system, which hinders photocatalytic reactions (as shown in Fig. 7). The utilization of a type-II heterostructure with staggered band structures is a promising strategy to effectively achieve spatial separation of electrons and holes and reduce recombination. Therefore, designing appropriate band structures for ZIS-based type-II heterostructures is of fundamental importance. Xu et al.'s theoretical investigation showed that Al doping introduced a potential difference between the Al-ZIS and ZIS regions, causing a shift in the band structure (as shown in Fig. 7). The formation of a type-II heterostructure through aluminum doping on Al-CuS/ZIS interfaces facilitated efficient charge carrier separation and directional transport. In this configuration, photogenerated electrons spontaneously transferred from the conduction band of phase II to phase I, while holes migrated inversely from the valence band of phase I to phase II. This charge redistribution established complementary polarization fields, with ZIS domains acquiring positive charges and Al-modified regions accumulating negative charges. The interfacial charge differential induced a p–n heterostructure accompanied by a space-charge layer and built-in potential, which synergistically drove charge separation through band bending effects. Concurrently, the Schottky barrier formed at ZIS/CuS interfaces enables Fermi level equilibration via electron injection from ZIS to CuS. This metallic-semiconductor junction served as an electron reservoir, effectively suppressing charge recombination through interfacial energy alignment. Structural characterization confirmed that the integrated architecture of Al-CuS/ZIS optimized surface-active site accessibility while maintaining robust interfacial connections. This hierarchical design enhanced both oxidation and reduction kinetics through spatial separation of redox centers, leading to significantly improved photocatalytic efficiency. The cooperative effects of multiple charge transfer pathways (type-II heterostructure, Schottky junction, and polarization field) collectively contributed to superior carrier management and catalytic performance.¹⁰⁴ In a related study, Li et al. employed a novel approach to in situ grow two-dimensional ZIS nanosheets on the surface of one-dimensional defective CdS nanorods, forming an atomic-level CdS/ZIS heterostructure. This configuration benefited from the type-II heterostructures, which facilitate efficient charge separation and transfer. Additionally, theoretical calculations further verified the increased effects of charge transfer and electronic coupling, evidencing the smart design of atomic-level 1D/2D heterostructures for efficient solar-driven energy conversion.¹⁰⁵ Despite the widespread investigation and notable progress inspired by the theoretically favorable charge separation mechanism in type-II heterostructures, critical limitations emerge upon systematic evaluation across multiple dimensions. Thermochemically, a fundamental paradox arises where enhanced charge separation efficiency paradoxically diminishes the system's redox potential – a critical parameter governing photocatalytic activity, as evidenced by recent thermodynamic analyses.¹⁰⁶ Kinetically, the inherent coulombic repulsion between like-charged carriers creates an intrinsic energy barrier that impedes further directional migration of photogenerated electrons and holes. Furthermore, energy utilization analysis reveals an under-exploitation of the inherent potential difference between constituent materials' conduction and valence bands, which could otherwise be strategically harnessed to optimize photocatalytic processes.¹⁰⁷ In summary, although the charge transfer mode of type-II heterostructures is highly regarded in theoretical contexts, it is subject to limitations with regard to the sustained separation of photogenerated electrons and holes. Furthermore, this transfer mode has been shown to reduce the overall redox capacity and energy utilization efficiency of heterostructures.¹⁰⁶

Fig. 7 A schematic depiction of electron transfer at the interface of the Al-CuS/ZIS heterostructure.¹⁰⁴ Reprinted with permission from ref. 104. Copyright 2023 Elsevier.

3.2. p–n Heterostructures

A p–n junction is a structure composed of p-type and n-type semiconductors, where the predominant charge carriers in the p-type semiconductor are positively charged holes, whereas those in the n-type semiconductor are negatively charged free electrons.¹⁰⁸ Typically, the formation of a p–n heterostructure can generate an IEF due to the difference in the Fermi level and work function between the p-type and n-type semiconductors, which provides electrostatic forces for charge transfer modulation.¹⁰⁹ ZIS is a classic n-type semiconductor that can form a p–n heterostructure when paired with a p-type semiconductor. When ZIS and the p-type semiconductor are combined, photogenerated electrons are transferred from ZIS to the p-type semiconductor, leaving behind positive charges in ZIS. Simultaneously, holes migrate from the p-type semiconductor to ZIS, resulting in negative charge accumulation in the p-type region. This charge separation establishes an internal electric field at the interface. This exchange continues until the Fermi level of the system equilibrates, establishing an IEF at the interface. Upon exposure to light, photogenerated electrons move from the CB of the p-type semiconductor to the CB of n-type ZIS, and photogenerated holes move in the opposite direction. This efficient charge separation can significantly boost the photocatalytic performance. Constructing p–n heterostructures has been proved to be an effective strategy for enhancing photocatalytic performance. For example, Wang et al. introduced the charge transfer mechanism in the p–n heterostructure of CuCo₂S₄ (CCS) and ZIS. The photocatalytic mechanism was governed by Fermi level disparities between semiconductor components, establishing an IEF that promoted directional charge transport and suppressed electron–hole recombination. Under photoexcitation, electrons migrated from the CB of CCS to that of ZIS, while holes followed a reverse pathway from ZIS to CCS. This spatial charge separation enabled distinct redox processes: electrons accumulated on ZIS drive proton reduction (2H⁺ + 2e⁻ → H₂) and superoxide radical (·O₂⁻) formation, whereas holes localized on CCS mediate hydroxyl radical (·OH)-assisted oxidation of ofloxacin. The engineered p–n heterostructure achieved dual functionality by decoupling hydrogen evolution and pollutant degradation into spatially separated reaction zones, leveraging the synergistic interplay between charge dynamics and catalytic activity to simultaneously enhance H₂ production efficiency and pharmaceutical contaminant removal.¹¹⁰ In another study, a CoFe₂O₄/ZIS p–n heterostructure photocatalyst was synthesized using ultrasonic calcination, where CoFe₂O₄ nanoparticles were attached to ZIS nanosheets. The IEF effectively separated and transferred photogenerated electrons and holes, enhancing H₂ production efficiency.¹¹¹ It was generally believed that the IEF based on p–n heterostructures could effectively regulate the migration pathways of photogenerated charge carriers, and the band structure of semiconductors also played a crucial role in charge transfer processes. Although there is no consensus on which is more critical, researchers still believe that the interaction between the band structure and IEF can significantly increase the photocatalytic performance of semiconductors. Previous studies have shown that constructing p–n heterostructures is an effective approach to improve the photocatalytic performance of semiconductors, but the understanding of the corresponding reaction mechanism is still not yet sufficiently deep. In order to gain a deeper understanding, in situ XPS and AFM can be used to indirectly study the IEF. In short, the charge density differences at the interface between n-type and p-type semiconductors caused by the IEF can be observed through in situ XPS and AFM analysis. Constructing p–n heterostructures as one of the important modification strategies to improve photocatalytic activity still deserves further systematic exploration.¹⁰⁹

3.3. Z-scheme heterostructures

In comparison with the type-II heterostructure, Z-scheme heterostructures have the capacity to resolve the contradiction between high carrier separation efficiency and strong redox ability. Bard et al. proposed a traditional Z-scheme system, which included appropriate redox couples such as Fe³⁺/Fe²⁺, IO³⁻/I⁻, and I³⁻/I⁻.¹¹² However, the application of this scheme is limited by the solution phase, and efficient charge transfer seems to be challenging in such a system. Considering that other charge transfer pathways may be more advantageous, a solid-state Z-scheme is proposed. In the solid-state Z-scheme, selecting suitable charge transfer paths is crucial for achieving efficient photocatalytic reactions, but their charge transfer relies on conductors such as metal nanoparticles of gold, silver, and copper. Therefore, direct Z-scheme photocatalysts have emerged, which do not involve any intermediates, either redox couples or conductors, making it the most common heterostructure structure at present.¹⁰⁷ In 2013, Yu et al. introduced the concept of a direct Z-scheme mechanism over a g-C₃N₄/TiO₂ heterostructure.¹¹³ Recently, Qi et al. developed a flower-like Z-scheme heterostructure that combines cobalt porphyrin ([meso-tetra(4-sulfonatophenyl) porphyrin], CoTPPS) and ZIS. The interfacial electronic configuration of ZIS@CoTPPS was quantitatively characterized through work function analysis and DFT simulations. Fig. 8a and b reveal a substantial work function disparity, driving spontaneous electron migration from CoTPPS to ZIS until Fermi level equilibration. DFT-calculated charge density differences (Fig. 8c) visualized interfacial polarization, with yellow/blue isosurfaces corresponding to electron accumulation/depletion zones. Post-contact charge redistribution (Fig. 8d) established an IEF that governed directional carrier transport. Under illumination, photoexcited electrons in ZIS's CB and holes in its VB followed a Z-scheme transfer pathway, optimizing redox potential retention for catalytic reduction. Contact potential difference (ΔCPD) measurements (Fig. 8e) demonstrated distinct photo response behaviors: individual CoTPPS and ZIS exhibited elevated surface potentials under light, whereas the ZIS@CoTPPS heterojunction showed reduced ΔCPD, indicative of suppressed charge recombination mediated by the IEF. In situ ATR-FTIR spectroscopy (Fig. 8f) tracked reaction intermediates, revealing time-dependent growth of *COOH (1755, 1516 cm⁻¹) and CO₂-related species (1600, 1303 cm⁻¹), with a characteristic CO vibration (1720 cm⁻¹) confirming catalytic CO production. DFT simulations (Fig. 8g) identified CO desorption as the rate-determining step in ZIS-mediated photocatalysis, providing mechanistic insights into the system's kinetic constraints. This multi-technique investigation elucidated the synergistic interfacial coupling and reaction dynamics governing the heterostructure's photocatalytic performance.¹¹⁴ In another study, a novel donor–acceptor Covalent Organic Polymer (COP) was introduced, and a COP-ZIS heterostructure was constructed through an in situ condensation reaction; the COP-ZIS heterostructure with Pt exhibits optimized H₂ production performance.⁵⁵ In summary, the direct Z-scheme heterostructure has shown great potential in improving the efficiency, stability, and selectivity of photocatalysis. While direct Z-scheme photocatalysts are still under development, fully harnessing their potential presents considerable challenges. Consequently, future advancements in this area should concentrate on several critical aspects. Firstly, the optimization of the physical and chemical characteristics of individual semiconductors within direct Z-scheme photocatalysts is essential. Secondly, it is crucial to achieve precise management of the interface between the two semiconductors used in direct Z-scheme systems. Enhancing the contact interface can significantly improve photocatalytic efficiency. Thirdly, the creation of ternary or multi-component photocatalytic systems could lead to further elevations in both the performance and selectivity of direct Z-scheme photocatalysts. For instance, strategically depositing appropriate oxidation and reduction cocatalysts on their respective sites within the direct Z-scheme photocatalysts could boost both the separation efficiency of electrons and holes, and the overall photocatalytic activity.

Fig. 8 (a) The electronic structure and reaction dynamics of the ZIS@CoTPPS heterostructure were systematically investigated: (a and b) electrostatic potential distributions of CoTPPS and ZIS precursors, (c) charge density difference analysis (yellow/blue: electron accumulation/depletion) at the heterointerface, (d) proposed interfacial charge transfer pathway, (e) work function modulation evidenced by ΔCPD measurements, (f) in situ FTIR spectroscopy identifying critical adsorption sites, and (g) a reduced reaction energy barrier via the *C intermediate on ZIS@CoTPPS (purple) versus ZIS (orange).¹¹⁴ Reprinted with permission from ref. 114. Copyright 2024 Elsevier.

3.4. S-scheme heterostructures

Based on different band structures, photocatalysts can be categorized into reduction photocatalysts (RP) and oxidation photocatalysts (OP). Among these, the S-scheme heterostructure is a unique structure composed of RP and OP with staggered band structures. Despite the similarity to the conventional type-II heterostructure, the charge transfer pathway in the S-scheme heterostructure is, in essence, dissimilar. In a conventional type-II heterostructure, photogenerated electrons and holes accumulate in the CB of OP and the VB of RP, respectively, leading to weaker redox reaction capability. Conversely, in an S-scheme heterostructure, strong photogenerated electrons and holes are retained in the CB of RP and the VB of OP, respectively, thus avoiding the ineffective recombination of photogenerated charge carriers. This configuration introduces higher redox potentials, thereby enhancing the performance of the catalyst.^115–118 It is noteworthy that the charge separation and recombination dynamics are influenced by different band bending structures (e.g. n–n junctions, p–p junctions, p–n junctions, and n–p junctions). Nevertheless, the design of S-scheme heterostructures aims to maximize the overall redox capabilities of the system, regardless of the specific types of OP and RP.¹⁰⁶ In 2019, Yu et al. first proposed a novel concept of S-scheme heterostructures, which highlighted significant differences in charge transfer mechanisms between S-scheme and traditional heterostructures, and discussed their limitations and future development directions.¹⁰⁷ Recently, a novel construction strategy based on the thermal solubility characteristics of MoO₃ for the in situ preparation of Mo-doped ZIS S-scheme heterostructure photocatalysts was reported. The performance improvement could be mainly attributed to two effects brought by Mo doping: broadening the light absorption range and forming Mo–S species. The formation of Mo–S species significantly increased the free electron concentration near the Fermi level and reduced the free energy change for the H₂ evolution reaction.¹¹⁹ Additionally, Zhu et al. employed a simple in situ hydrothermal method to encapsulate Mo-modified ZIS nanosheets on the surface of NiTiO₃ microrods, producing multifunctional Mo-modified ZIS-encapsulated NTO microrods (Mo-ZIS@NTO). It exhibited excellent performance, and the amount of H₂ generated displays a steady increasing trend with increasing time, with Mo_1.4-ZIS@NTO giving the highest value of 70.28 mmol g⁻¹. It also achieved a hydrogen evolution rate of 14.06 mmol g⁻¹ h⁻¹ and an apparent quantum efficiency of 44.1% at 420 nm.¹²⁰ To further investigate the influence of the interface and lattice matching degree on S-scheme heterostructures, it was found that the intermediate facets of the S-scheme heterostructure ZnIn₂S₄–Zn₂In₂S₅, which resemble a bone joint-like structure, serve as a platform for complexation sites. The interfacial architecture induced an IEF that regulated ordered charge recombination pathways at heterointerfaces, contrasting with random recombination processes (Fig. 9a). HR-TEM analysis (Fig. 9b) resolved distinct (104) lattice spacings of 0.29 nm (hexagonal ZnIn₂S₄) and 0.32 nm (Zn₂In₂S₅), with interfacial regions exhibiting lattice distortion and edge dislocations associated with their (006) planes. Molecular dynamics simulations (Fig. 9c) demonstrated that 13° lattice rotation (α) enabled atomic registry between ZnIn₂S₄ and Zn₂In₂S₅ (006) planes, where interfacial strain drove atomic rearrangement through bond length adaptation and symmetry mismatch compensation. This mechanical interlocking created semi-coherent interfaces, as evidenced by atomic-resolution HAADF-STEM mapping (Fig. 9d and e) that showed continuous atomic connectivity with periodic strain modulation. Fig. 9f to h illustrate that the observed signals at 1.81 and 1.84 Å⁻¹ in ZIS-2 and ZIS-R correspond to the Zn–S bonds, with the signal in ZIS-2 being weaker than that in ZIS-R. This difference is due to lattice distortion caused by interfacial compression in ZIS-2, highlighting the existence of a unique intermediate structure.¹²¹ Despite the considerable advancements in S-scheme heterostructures, there remains a substantial potential for enhancing their efficiency. For example, refining S-scheme heterostructure systems may involve the deployment of specific oxidation and reduction cocatalysts on the surfaces of OP and RP to modify their Fermi levels, thereby increasing the Fermi level discrepancy between them. Moreover, integrating MOFs (metal–organic frameworks) and COFs (covalent organic frameworks) with S-scheme heterostructures is highly promising due to their extremely high specific surface area, tunable cavities, and modifiable light absorption capabilities. The cooperative interactions between MOF or COF-based materials and S-scheme heterostructures can considerably boost the photocatalytic efficiency. Adjusting the band structure is equally critical for the effectiveness of S-scheme heterostructures. Altering the band structure through heteroatom doping, vacancy introduction, or nanostructure creation can influence the overall performance of these heterostructures. Precise manipulation of these factors can greatly boost their photocatalytic capabilities.^106,107

Fig. 9 (a) Comparative analysis of charge migration pathways in S-scheme heterojunction architectures, (b) HR-TEM imaging of ZIS-2, paired with (c) crystallographic models (blue/orange spheres: Zn₂In₂S₅/ZnIn₂S₄ atomic configurations), (d and e) atomic-scale HAADF-STEM imaging resolves the phase boundaries in ZIS-2, and (f) R-space Zn K-edge EXAFS spectra for ZnS, ZIS-2, and ZIS-R, accompanied by Morlet wavelet transforms of k³-weighted spectra for ZIS-R (g) and ZIS-2 (h).¹²¹ Reprinted with permission from ref. 121. Copyright 2024 American Chemical Society.

4 Solar fuel applications

Photocatalytic technology is regarded as a promising clean energy strategy due to its high efficiency, economical efficiency, and low environmental impact. In recent years, ZIS-based nanomaterials have demonstrated significant potential for application in environmental remediation, energy production, and energy storage. In the following section, the applications of ZIS-based heterostructure photocatalysts in water splitting, CO₂ reduction, N₂ fixation, and H₂O₂ synthesis will be systematically introduced.

4.1. Water splitting

In recent years, solar water splitting has emerged as a primary method for the storage of solar energy, with the potential to produce hydrogen fuel that is characterized by its cleanliness. Despite the extensive use of numerous semiconductors for this purpose, ZIS is regarded as one of the most promising photocatalysts due to its exceptional performance. Ida et al. emphasized that the various dimensions of nanomaterials are of significance with regard to the time and energy required for water splitting reactions.¹²² H₂ production via water splitting requires the transfer of four electrons during the reduction of water molecules. The excitation of a four-photon effect on the surface of a nanocrystal typically requires a minimum duration of 4 ms and sufficient energy input. However, this requirement starkly conflicts with the carrier lifetime of approximately 1 μs—the latter being three orders of magnitude shorter than the former. An even more critical challenge arises from the practical conditions of sunlight: its photon flux density is around 2000 μmol s⁻¹ m⁻², which implies that within the ultrashort temporal window of carrier survival (1 μs), the number of photons captured per unit area falls far below the threshold required for the four-photon process. These dual constraints of temporal scale and energy supply create a significant bottleneck for effectively driving water-splitting reactions under natural sunlight.¹²³ To shorten the carrier travel distance and reduce recombination, adopting 2D nanomaterial structures could better meet these requirements. The layered structure has been demonstrated to facilitate the light absorption process even at lower flux densities, thereby ensuring that the generated charge carriers need to move shorter distances. Lei et al. first reported the H₂ production effect by using a ZIS semiconductor in photocatalytic water splitting, but the H₂ production rate was only 257 μmol g⁻¹ h⁻¹.⁶⁶ Constructing heterostructures has now become the mainstream method to improve the PEC performance of ZIS photocatalysts for water splitting. For example, a ternary MoS₂/ZIS/graphene quantum dot (GQD) heterostructure was synthesized recently, and the results showed that the photocatalytic H₂ production rate of the MoS₂/ZIS/GQD heterostructure reached 21.63 mmol g⁻¹ h⁻¹ and a total amount of 89.1 mmol g⁻¹ within 4 h, which is 36.7 times and 33.8 times higher than that of pure ZIS.¹²⁴ Furthermore, Wang et al. designed a hollow core–shell structure composed of FeNi₂S₄@ZIS (FNS@ZIS) featuring an S-scheme heterostructure for photocatalytic hydrogen evolution enhanced by photothermal effects. Remarkably, the FNS core exhibited outstanding photothermal properties, generating heat under visible light exposure, which effectively mitigates thermal loss in the reaction system. This led to an increase in the local temperature of the photocatalyst with accelerating charge migration. Moreover, the S-scheme heterostructure, formed through in situ growth, possessed a tightly bonded interface that facilitates the separation and transfer of charge carriers, resulting in a high redox potential. Fig. 10a and b show the surface electrostatic potential of the ZIS (102) nanosheet shell and hollow FNS (311) core. Measured work functions were 5.20 eV for ZIS and 6.07 eV for FNS, suggesting free electron migration from ZIS to FNS upon contact. Band gaps from density of states calculations were 2.52 eV for ZIS and 1.75 eV for FNS. The theoretical potentials for the CB and VB of ZIS and FNS are presented in Fig. 10c and d. The charge density distributions shown in Fig. 10e and f backed up the assumption that at the heterostructure interface, electrons moved from ZIS to FNS, leading to FNS gaining holes. This led to the creation of an IEF between the two materials. Fig. 10g presents the electron localization function mapping of the hollow core–shell FNS@ZIS S – scheme heterostructure, which revealed the interactions between ZIS and FNS. The photothermal effect, IEF and Coulomb attraction cause the electrons in the CB of FNS to recombine with the holes in the VB of ZIS, enhancing charge separation efficiency and maintaining the redox capability of photogenerated charges, and the photocatalytic water splitting ability of the hollow core–shell FNS@ZIS S-scheme heterostructure has been enhanced.¹²⁵ To date, many efforts have been made to improve the photocatalytic H₂ production performance of ZIS heterostructures (Table 1). However, research into their stability has been relatively limited and clearly lagged behind the needs for practical applications. Researchers typically infer the corrosion mechanisms from the results and develop in situ characterization techniques to reveal the actual corrosion processes in real reaction environments, which is crucial for exploring new methods to achieve long-term stability of ZIS heterostructures. Overall, as a promising photocatalytic material for large-scale applications, the activity and stability of ZIS heterostructures still require further improvement.¹⁵⁵

Fig. 10 (a) Planar-averaged electrostatic potential profiles and work function comparisons for (a) ZIS and (b) FNS, (c) electronic DOS spectra of (c) ZIS and (d) FNS, (e and f) charge density difference distributions at the FNS/ZIS interface post-contact: (e) 3D spatial mapping and (f) planar cross-sectional analysis, and (g) electron localization function visualization of the hollow core–shell FNS@ZIS S-scheme heterojunction.¹²⁵ Reprinted with permission from ref. 125. Copyright 2024 John Wiley and Sons.

Table 1 Typical ZIS-based heterostructures for photocatalytic H₂ production

Photocatalyst	Light source	Heterostructure	Cocatalyst	Sacrificial agent	H2 yield (mmol g^−1 h^−1)	AQY (%)	Stability (h)	Ref.
ZIS/g-C3N4	300 W Xe lamp	Type-I	1 wt% Pt	TEOA	4.854	—	>5	126
ZIS/g-C3N4	300 W Xe lamp (>400 nm)	Type-I	—	TEOA	8.601	0.92%@420 nm	>24	127
ZIS/g-C3N4	300 W Xe lamp	Type-I	NiS	TEOA	5.02	30.5%@420 nm	—	72
ZIS/CaTiO3	300 W Xe lamp (>420 nm)	Type-I	—	Na2S/Na2SO3	22.186	—	>12	128
ZIS/Mo2C	300 W Xe lamp AM 1.5G	Type-I	—	TEOA	40.93	71.6%@420 nm	—	129
ZIS/CdS	300 W Xe lamp	Type-II	—	TEOA	5.8	0.48%@420 nm	>35	130
ZIS/MoS2	300 W Xe lamp (>420 nm)	Type-II	—	Lactic acid	4.288	3.85%@420 nm	>16	131
ZIS/CuInS2	150 W Xe lamp (>420 nm)	Type-II	rGO	Na2S/Na2SO3	33.75	—	>15	132
ZIS/g-C3N4	300 W Xe lamp (>420 nm)	Type-II	H2Cl6Pt	TEOA	1.63	—	>8	133
ZIS/WS2	150 W Xe lamp	Type-II	—	Na2S/Na2SO3	29.3	21%@400 nm	—	134
Au@UiO66@ZIS	300 W Xe lamp (>420 nm)	Type-II	—	Na2S/Na2SO3	39.16	16.8%@420 nm	—	135
ZIS/NiTPP	300 W Xe lamp (>420 nm)	Type-II	—	TEOA	28.8	64.0%@400 nm	>20	136
Ti3C2@TiO2/ZIS	300 W Xe lamp	Type-II	M@TiO2	Na2S/Na2SO3	1.867	—	>16	137
CoFe2O4/ZIS	250 W Xe lamp (>420 nm)	p–n	CFO NPs	TEOA	16	5%@420 nm	>25	138
Ag2O/ZIS	300 W Xe lamp	p–n	3 wt% Pt	TEOA	9.337	0.9%@420 nm	—	139
MoS2/ZIS	300 W Xe lamp	p–n	—	Lactic acid	151.42	20.88%@365 nm	—	140
CuS/ZIS	300 W Xe lamp (>400 nm)	p–n	—	Na2S/Na2SO3	7.91	2.52%@420 nm	>20	44
FeWO4/ZIS	300 W Xe lamp	p–n	—	Na2S/Na2SO3	3.531	—	>18	141
CdS/MoS2/ZIS	300 W Xe lamp (>420 nm)	Z-scheme	—	Lactic acid	2.1	—	>24	142
ZIS/Cu3P	300 W Xe lamp (>420 nm)	Z-scheme	Cu3P	Na2S/Na2SO3	2.56	22.3%@420 nm	>20	143
ZIS/WO3	300 W Xe lamp (>420 nm)	Z-scheme	—	TEOA	3.9	4.5%@420 nm	—	144
ZIS/NiTiO3	3 × 300 W LED (>420 nm)	Z-scheme	—	TEOA	4.43	4.39%@450 nm	>10	145
HxMoO3@ ZIS	300 W Xe lamp (>420 nm)	Z-scheme	—	TEOA	5.9	32.95%@420 nm	>20	146
ZIS/H2Ta2O6	300 W Xe lamp AM 1.5G	Z-scheme	1 wt% Pt	TEOA	3.217	5.2%@420 nm	>30	147
ZIS/CdS	300 W Xe lamp (>420 nm)	Z-scheme	—	Na2S/Na2SO3	7.4	—	>20	148
ZIS/g-C3N4	300 W Xe lamp (>400 nm)	Z-scheme	Pt	Na2S/Na2SO3	11.914	—	—	149
ZIS/Co3O4	300 W Xe lamp (>400 nm)	S-scheme	C/MoS2	TEOA	6.7	11%@420 nm	—	150
ZIS/CdS/Pd	225 W Xe lamp	S-scheme	PdS	Na2S/Na2SO3	19.19	—	—	151
ZIS/Sv-ZIS	300 W Xe lamp (>420 nm)	S-scheme	—	TEOA	2.912	19.8%@420 nm	>12	152
ZIS/CeO2	UV light (3 W) (>420 nm)	S-scheme	—	Na2S/Na2SO3	1.38	7.6%@420 nm	>4	153
MoO3@Mo-ZIS	300 W Xe lamp (>420 nm)	S-scheme	—	TEOA	5.5	19.2%@365 nm	>25	119
S-CN/ZIS	Visible light (>420 nm)	S-scheme	—	Na2S/Na2SO3	19.25	—	>10	154

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4.2. CO₂ reduction

Photocatalytic CO₂ reduction is a clean, economical, and environmentally friendly method that converts CO₂ into hydrocarbon fuels using solar energy. This strategy involves not only the reduction half-reaction of CO₂ but also the oxidation half-reaction of H₂O, forming a carbon-neutral cycle and providing a practical solution to global energy and environmental issues. For example, Inoue et al. successfully converted CO₂ into small quantities of chemical fuels, including HCOOH, HCHO, and CH₃OH, using various semiconductor materials such as TiO₂, ZnO, and CdS.¹⁵⁶ The photocatalytic CO₂ reduction reaction primarily includes three key steps: first, the generation of electron–hole pairs in the semiconductor under light irradiation; next, the separation and migration of these electron–hole pairs to the semiconductor surface; and finally, the oxidation of H₂O and the reduction of CO₂ on the semiconductor surface.^157,158 This process demonstrates how solar energy could be utilized to convert CO₂ into useful chemicals, providing a new direction for sustainable energy. Designing suitable heterostructures is crucial for improving the efficiency and selectivity of CO₂ reduction. The key role of heterostructures is to provide an interface for effective charge transfer between photogenerated electrons and catalysts. Furthermore, the energy band structure and level tuning of heterostructures could also regulate the separation efficiency of electrons and holes, enhancing the efficiency of the CO₂ reduction reaction.

Sabbah et al. reported the design and synthesis of a direct Z-scheme photocatalyst system, which was the first reported on direct Z-scheme ZnS/ZIS nanocomposites.¹⁵⁹ In addition to Z-scheme heterostructures, S-scheme heterostructures could also effectively promote the separation of photogenerated carriers and improve the CO₂ reduction capability of photocatalysts. For instance, Li et al. introduced a novel photocatalyst that features a core–shell structure comprising an S-bridged covalent triazine framework (SCTF) as the core and a ZIS shell. This design aimed to simultaneously achieve carbon dioxide reduction and selective furfural synthesis at distinct active sites. Fig. 11a illustrates how the SCTF/ZIS core–shell photocatalyst enables photocatalytic CO₂ reduction and selective alcohol-to-aldehyde conversion through spatially separated sites for reduction and oxidation. Electrons could move from the ZIS shell to the SCTF core due to favorable band alignment. The presence of pyridine nitrogen atoms in the SCTF enhances the adsorption of CO₂, which in turn reduces the energy barrier for the formation of *COOH and promotes the production of *CO, while furfuryl alcohol oxidation occurs on the ZIS shell, producing furfural through a dehydrogenation process. This system optimizes the use of electrons and holes for efficient photocatalysis. In situ Kelvin probe force microscopy revealed the distribution of photo-generated electrons and holes, and by detecting the ΔCPD before and after illumination, the charges transfer pathways could be estimated. In the ΔCPD images of the SCTF sphere (Fig. 11b and c), linear scanning results show a negative ΔCPD and indicate electron accumulation (Fig. 11d). In the case of SCTF/ZIS-0.2, the positive ΔCPD under light illumination indicates hole accumulation on ZIS, with electrons transferring from ZIS to the SCTF, which is consistent with the XPS results (Fig. 11e–g). The formation of an SCTF/ZIS S-scheme heterostructure as well as charge transfer dynamics are depicted in Fig. 11h. The accumulation of electrons in the conduction band of the SCTF enhances the reduction activity of CO₂. Time-resolved photoluminescence (TRPL) shows that the average charge lifetime of SCTF/ZIS-0.2 is shorter than that of ZIS and the SCTF, with a higher charge transfer rate constant, indicating an enhancement in the charge carrier separation efficiency (Fig. 11i).¹⁶⁰ In a separate study, a straightforward “topological atomic extraction” method was reported that selectively removes zinc from the ultrathin half-unit cell ZIS (HZIS), resulting in a 1.60 nanometer-thick in-plane In₂O₃/HZIS S-scheme heterostructure. This innovative structure demonstrated enhanced charge separation capabilities, making it an effective photocatalyst for carbon dioxide reduction reactions based on ZIS, with a significant increase in CO generation rates, increasing from 6.8 times to 128 times. The redistribution of charge leads to the formation of a local electric field at the heterostructure interface, facilitating the separation of charge carriers within the S-scheme photocatalytic system. This process provides the HZIS with long-lived carriers, thus effectively enhancing the performance of CRR.¹⁶¹ Additionally, Zhang et al. developed a ZnO@ZIS heterostructure with the objective of enhancing photocatalytic activity. Based on the literature and FT-IR test results, the photocatalytic reduction process of CO₂ could be inferred as follows:¹⁶²

ZnO@ZIS + hv → ZnO (h⁺) + ZIS (e⁻) (1)

H₂O + h⁺ → ˙OH + H⁺ (2)

CO₂ + H₂O → H₂CO₃ (3)

H₂CO₃ → H⁺ + HCO₃⁻ (4)

HCO₃⁻ → H⁺ + CO₃²⁻ (5)

COOH* + H⁺ + e⁻ → CO* + H₂O (8)

CO* → CO (9)

CO* + H⁺ + e⁻ → CHO* (10)

CHO* + 2H⁺ + 2e⁻ → CH₃O* (11)

Fig. 11 (a) Schematic representation depicting the cooperative photocatalytic processes of CO₂ photoreduction and FFA oxidation occurring on the SCTF/ZIS heterostructure. Comparative surface potential analyses of pristine SCTF and optimized SCTF/ZIS-0.2 composites under different conditions: (b and e) ΔCPD mapping in dark environments, (c and f) corresponding photovoltage distribution under visible spectrum illumination (λ ≥ 420 nm), (d and g) quantitative potential variation profiles derived from differential ΔCPD measurements, (h) energy band alignment diagram elucidating interfacial charge transfer pathways between SCTF and ZIS components, (i) comparative TRPL decay curves for the SCTF, ZIS, and their optimized composite (SCTF/ZIS-0.2) recorded under 365 nm pulsed laser excitation.¹⁶⁰ Reproduced with permission from ref. 160. Copyright 2024 John Wiley and Sons.

The intrinsic narrow bandgap of ZIS endows it with pronounced solar spectral absorption. Constructing heterostructures with complementary semiconductors facilitates optimized band alignment and interfacial charge transfer dynamics, thereby synergistically elevating the photoconversion efficiency via enhanced light harvesting and suppressed carrier recombination (Table 2).¹⁶⁰ The formation of heterostructures also creates more active sites at the interface, which are imperative for the CO₂ reduction reaction. These additional active sites have the capacity to enhance reaction rates and catalytic performance. Additionally, the CO₂ reduction pathway is complex (potentially producing CO, CH₄, CH₃OH, etc.), and achieving high selectivity for a single product requires precise design of active sites, such as through single-atom doping.^182–184 In summary, ZIS heterostructures not only improve photocatalytic reaction efficiency but also promote system stability and durability for CO₂ reduction, demonstrating significant application potential in addressing global climate change.³²

Table 2 Typical ZIS-based heterostructures for photocatalytic CO₂ reduction

Photocatalyst	Light source	Heterostructure	Main products	Performance (μmol g^−1 h^−1)	Stability (h)	Ref.
ZIS/g-C3N4	300 W Xe lamp (>420 nm)	Type-I	CO	7368.7	>8	163
ZIS/CdS	300 W Xe lamp (>420 nm)	Type-II	CO	3340	>35	130
ZIS/PCN	300 W Xe lamp	Type-II	CO	44.6	>40	164
ZIS/g-C3N4	300 W Xe lamp (>420 nm)	Type-II	CO, CH4	13.33, 2.89 respectively	>18	165
ZIS/g-C3N4	300 W Xe lamp	Type-II	CH4, CO	3.66, 4.7 respectively	>100	166
ZIS/Au/CdS	300 W Xe lamp	Type-II	CO	63.07	>16	167
CuS@ZIS	300 W Xe lamp (>400 nm)	p–n	CH4	43.6	>16	168
ZIS/TiO2	300 W Xe lamp	Z-scheme	CH4	1.135	—	169
ZIS/ZnS	300 W halogen lamp	Z-scheme	CH3CHO, CH3OH	61.27, 0.228 respectively	>24	170
ZIS/Bi2WO6	300 W Xe lamp	Z-scheme	CH4	4.89	>22	171
ZIS/BiVO4	300 W Xe lamp	Z-scheme	CO	4.75	>5	172
Co9S8@ZIS/CdS	300 W Xe lamp	Z-scheme	CO	82.10	>20	173
CdIn2S4/ZIS	300 W Xe lamp (>400 nm)	Z-scheme	CO	1194.5	>16	174
ZIS@Ni(OH)2/NiO	300 W Xe lamp (>420 nm)	Z-scheme	CH4	33.44	>24	175
ZIS/g-C3N4	300 W Xe lamp	S-scheme	CO	883	>9	176
ZnO@ZIS	300 W Xe lamp	S-scheme	CO, CH4	39.76, 3.92 respectively	>16	177
TiO2@ZIS	Full spectrum lamp	S-scheme	CO, CH3OH, CH4	18.32	>9	178
ZIS/g-C3N4	300 W Xe lamp	S-scheme	CO	50.8	>16	179
g-C3N4/CuFe2O4/ZIS	350 W Xe lamp	S-scheme	CH4	267.4	>24	180
MoO3−x@ZIS	300 W Xe lamp (>420 nm)	S-scheme	CO, CH4	4.65, 28.3 respectively	>15	181
Sv-ZnS/ZIS	300 W Xe lamp (>420 nm)	S-scheme	CO	2075.7	>12	152

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4.3. N₂ fixation

Plants cannot directly utilize free N₂ from the atmosphere; it must be converted into compound forms before absorption, a process known as N₂ fixation. With the continuous growth of the global population, there is an increasing demand for sustainable agriculture, which in turn is driving the requirement for nitrogen fertilisers.¹⁸⁵ In order to address this issue, humankind has invented artificial N₂ fixation techniques. The most common commercial method is the Haber process. This process marks a significant advancement in human history. However, in the Haber process, ammonia is typically produced at high pressure (100–200 atm) and high temperature (300–500 °C), making the method neither environmentally friendly nor energy efficient.¹⁸⁶ In contrast, photocatalytic N₂ fixation technology exhibits unique advantages. It relies on solar energy instead of fossil fuels, significantly reducing the consumption of traditional energy sources and carbon dioxide emissions. In 1977, Schrauzer et al. first reported the ability of Fe-doped TiO₂ for photocatalytic N₂ fixation; however, the efficiency of N₂ fixation was suboptimal.¹⁸⁷ To improve this efficiency, researchers have focused on developing new photocatalysts and heterostructure strategies. For instance, Zhang et al. synthesized a direct Z-scheme heterostructure BiVO₄/ZIS (BVO/S_v-ZIS) rich in sulfur vacancies via a solvothermal method. As shown in Fig. 12, the heterostructure provided the necessary thermodynamic conditions and photogenerated electrons for photocatalytic ammonia synthesis. Sulfur vacancies served as a bridge between the heterostructure and the nitrogen reduction reaction, connecting N₂ for chemisorption while capturing electrons from the heterostructure. This process altered the local electronic structure, allowing electrons to be efficiently transferred to nitrogen, thereby promoting the photocatalytic nitrogen reduction reaction.¹⁸⁸ Moreover, for efficient N₂ fixation, ZIS/2D BiOCl nanosheet heterostructures were synthesized to enhance the fixation process. Specifically, the chemically adsorbed nitrogen molecules at the photocatalyst/solution interface were activated by photogenerated electrons, breaking the N≡N bond to generate the coupling proton-containing N₂H* intermediate (eqn (15)), which ultimately formed (eqn (16)). then dissolved in water and reacts with water molecules to generate NH₄⁺ (eqn (17)).²⁹

ZIS/BiOCl + hν → ZIS/BiOCl (e⁻ + h⁺) (14)

e⁻ + H⁺ + N₂ → N₂H* (15)

Fig. 12 Photocatalytic ammonia synthesis schematic diagram.¹⁸⁸ Reproduced with permission from ref. 188. Copyright 2022 Elsevier.

ZIS heterostructures exhibit both high efficiency and tunability in the field of photocatalytic N₂ fixation. Leveraging S-scheme/Z-scheme heterostructure design, defect engineering, and photothermal synergy offers the potential to overcome existing bottlenecks. Future research should focus on in situ characterization of interfacial charge transport mechanisms, development of low-cost materials, and construction of multi-technology integrated systems to realize the practical application of solar-driven N₂ fixation technology.

4.4. H₂O₂ synthesis

H₂O₂ has garnered significant attention due to its cost-effectiveness, low environmental toxicity, and eco-friendly nature. It is a versatile oxidizing agent and reactive oxygen species. It finds extensive applications in industrial organic synthesis, advanced oxidation processes for wastewater remediation, and pulp/paper manufacturing.¹⁸⁹ However, current industrial-scale production predominantly relies on the anthraquinone auto-oxidation method, a multi-step energy-intensive process plagued by high operational complexity, substantial economic costs, and generation of hazardous byproducts.¹⁹⁰ Although the direct reaction between H₂ and O₂ to synthesize H₂O₂ is thermodynamically feasible, this approach entails inherent risks of explosive hazards and catalyst poisoning due to uncontrolled radical chain reactions. Furthermore, the storage and transportation of concentrated H₂O₂ solutions impose substantial economic burdens and safety challenges stemming from their intrinsic thermal instability.¹⁹¹ To address these limitations, recent advances highlight ZIS-based heterostructure photocatalysts as promising candidates for on-site H₂O₂ production via solar-driven oxygen reduction reactions.¹⁹² Recently, Ruan et al. proposed an iso-elemental heterostructure catalyst formed by integrating Zn₃In₂S₆ nanoflowers with ZnIn₂S₄ nanosheets. The architecture reduced contact energy barriers and enhanced lattice matching, facilitating effective separation of electron–hole pairs and extending visible light absorption, which was crucial for H₂O₂ synthesis. Fig. 13a illustrates that the catalyst architecture enhanced electron–hole pair separation, extends visible light absorption, and created more active sites for the oxygen reduction and water oxidation reactions, which were also essential for H₂O₂ synthesis, while Fig. 13b shows that minimizing lattice mismatch improved energy level alignment for efficient charge carrier dynamics. Theoretical analysis in Fig. 13c established a type-II charge transfer pathway, confirming effective spatial separation and transfer of photogenerated carriers in Iso-ZIS for H₂O₂ synthesis. Additionally, the feasibility and superiority of this configuration were validated by replacing ZnIn₂S₄ with other non-iso-elemental catalysts like CdIn₂S₄, TiO₂, and CdS.¹⁹³ In another study on interfacial coherency and lattice matching, researchers prepared a compact ZnIn₂S₄–CdIn₂S₄ (ZIS–CIS) heterostructure that was rich in sulfur defects. DFT calculations validated the contribution of interfacial coherency to O₂ adsorption. As shown in Fig. 13d and e, O₂ polarization was weak in RZCIS (randomly mixed ZIS and CIS), but increased in ZCIS-3 (ZIS : CIS ratio of 1 : 1), indicating enhanced molecular polarization and activation. Furthermore, this work provided an exhaustive exploration of the reaction mechanism of H₂O₂. In situ FTIR experiments revealed that as simulation time increased, the characteristic water vapor absorbance peaks (3642 and 1638 cm⁻¹) exhibited progressive intensity attenuation during illumination, potentially attributed to competing photocatalytic O₂ reduction or H₂O oxidation pathways. Notably, the emergence of new vibrational signatures at 1389 cm⁻¹ (assigned to H–O–H bending in H₂O₂) and 1460 cm⁻¹ (associated with adsorbed *OOH intermediates) provided spectroscopic evidence for O₂-to-H₂O₂ conversion through a sequential pathway involving superoxide radical formation (Fig. 13f). Combined with DFT simulations (Fig. 13g), the reaction mechanism was proposed to initiate with O₂ activation via electron transfer (O₂ + e⁻ → ·O₂⁻), identified as the kinetically limiting step, followed by *OOH protonation to yield H₂O₂. When ZCIS-3 was light-activated (Fig. 13h), electrons preferentially migrated from CIS to ZIS, prolonging electron–hole separation. Surface polarization enhanced molecular activation of O₂, leading to the generation of superoxide radicals (·O₂⁻) that reacted with protons to form H₂O₂.¹⁹⁴ Liu et al. further engineered a Z-scheme core–shell photocatalytic system through interfacial assembly of two-dimensional ZIS nanosheets onto a defective iron-based metal–organic framework (DNM-88B), achieving synergistic in situ H₂O₂ synthesis-utilization. Under illumination, photoinduced electrons sequentially participated in two redox cycles: (1) Fe³⁺ reduction to Fe²⁺, which triggered Fenton-like reactions with H₂O₂ to produce ·OH for pollutant mineralization; (2) the oxygen reduction reaction occurred at the ZIS CB, which was thermodynamically favorable for ·O₂⁻ generation via single-electron transfer (O₂ + e⁻ → ·O₂⁻), with subsequent ·O₂⁻ disproportionation contributing to H₂O₂ accumulation. Crucially, the coordinatively unsaturated Fe sites (Fe-CUS) in DNM-88B modulated the Fe²⁺/Fe³⁺ redox couple, lowering activation barriers for both H₂O₂ formation (through enhanced O₂ adsorption) and its catalytic conversion (via accelerated Fe²⁺ regeneration). This dual functionality established DNM-88B as the pivotal component governing the cascade photocatalytic-Fenton processes. The reaction equation was as follows:¹⁹⁵

DNM88B@ZIS + hv → DNM88B@ZIS (e⁻ + h⁺) (18)

e⁻ + O₂ → ·O₂⁻ (19)

·O₂⁻ + 2H⁺ → H₂O₂ (20)

Fe³⁺ + H₂O₂ → Fe²⁺ + ·OOH + H⁺ (21)

Fe²⁺ + H₂O₂ → Fe³⁺ + ·OH + OH⁻ (22)

·OH/·O²⁻/h⁺ + pollutants → CO₂ + H₂O (23)

Fig. 13 (a and b) Schematic representation of carrier transfer at the iso-elemental interface of ZnIn₂S₄/Zn₃In₂S₆ and (c) band structure diagram of the iso-elemental heterostructure ZnIn₂S₄/Zn₃In₂S₆.¹⁹³ Reproduced with permission from ref. 193. Copyright 2024 John Wiley and Sons. Comparison of charge densities for O₂ adsorption in (d) RZCIS and (e) ZCIS-3, (f) in situ FTIR spectra of ZCIS-3, (g) comparison of Gibbs free energies for H₂O₂ formation from various materials, and (h) diagram illustrating the photocatalytic generation of H₂O₂.¹⁹⁴ Reproduced with permission from ref. 194. Copyright 2024 American Chemical Society.

Typical ZIS-based heterostructure photocatalysts for H₂O₂ synthesis are briefly summarized in Table 3; although the ZIS-based heterostructure photocatalytic H₂O₂ production technology has many advantages, it still faces several challenges and limitations in practical applications. Issues such as the energy conversion efficiency of photocatalysts need further research and resolution. For example, effectively utilizing the Pauling and Yeager configurations of O₂ adsorption could enhance electron utilization efficiency and H₂O₂ selectivity.^211–213 Additionally, improvement measures should include optimizing reaction conditions and processes to reduce production costs.²¹⁴ These efforts will make the technology more mature and reliable, and hold promise for future industrial-scale production.

Table 3 Typical ZIS-based heterostructures for photocatalytic H₂O₂ synthesis

Photocatalyst	Light source	Heterostructure	H2O2 yield (μmol g^−1 h^−1)	AQY (%)	Stability (h)	Ref.
ZIS/g-C3N4	Xe lamp AM 1.5G	Type-II	798	11.73%@400 nm	>20	196
ZIS/polyimide	300 W Xe lamp (>420 nm)	Type-II	411.07	0.081%@400 nm	>10	197
ZIS/g-C3N4	300 W Xe lamp AM 1.5G	Type-II	2770	—	>15	198
ZIS/Zn3In2S6	300 W Xe lamp	Type-II	1408.2	—	—	193
Ag-CdS1−x@ZIS	300 W Xe lamp (>420 nm)	Z-scheme	1183.7	—	>24	199
CdS@ZIS	300 W Xe lamp (>400 nm)	Z-scheme	604.8	—	>12	200
ZIS/CdIn2S4	Xe lamp (>420 nm)	Z-scheme	843.02	2.99%@420 nm	>5	194
ZIS/PBN	300 W Xe lamp	Z-scheme	491.28	—	—	201
BiVO4@ZIS	300 W Xe lamp (>420 nm)	Z-scheme	1585.99	—	>12.5	202
ZIS/UiO66-NH2	300 W Xe lamp (>400 nm)	Z-scheme	850	—	>8	203
ZIS/ZIF-8	300 W Xe lamp (>420 nm)	Z-scheme	742.7	2.45% @420 nm	>10	204
ZIS/Bi2S3	300 W Xe lamp (400–780 nm)	Z-scheme	1223	1.77%@400 nm	>8	205
ZIS@ZnO	300 W Xe lamp	S-scheme	928	—	>4	206
TiO2 NT/ZIS	Xe lamp	S-scheme	19 560	—	>1.5	207
ZIS/Bi2Sn2O7	300 W Xe lamp (>420 nm)	S-scheme	2542	19.8%@420 nm	>25	208
ZIS/ZnO	300 W LED lamp (>380 nm)	S-scheme	897.6	16.6%@400 nm	>10	209
Ag-ZIS/C-In2O3	300 W Xe lamp (>420 nm)	S-scheme	2420	6.65%@420 nm	>14	210

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Based on the comprehensive comparison summarized in the tables and discussions above, the performance advantages and limitations of various heterostructures in practical applications have been further analyzed. Type-I heterojunctions promote charge separation through interfacial electric fields in H₂ production, but their reduction capability is limited, which may similarly hinder their performance in other applications due to insufficiently negative CB positions. Type-II heterostructures facilitate spatial carrier separation via band alignment, yet charge accumulation may reduce their reduction capability, imposing higher stability requirements in applications. The IEF of p–n junctions enhances charge separation, but their complex fabrication and stability issues may limit their versatility across multiple applications. In contrast, Z-scheme and S-scheme heterostructures preserve stronger redox capabilities, making them potentially more advantageous in fields such as CO₂ reduction and H₂O₂ synthesis, where both high CB and VB positions are required.²¹⁵ However, challenges like dependence on cocatalysts and intricate interface engineering remain. Furthermore, the suitability of each heterostructure must be evaluated in the context of specific reaction mechanisms. For instance, N₂ fixation, which involves multi-electron transfer and high reduction potentials to convert N₂ to NH₃, may favor Z-scheme systems due to their retained strong reduction ability. Similarly, H₂O₂ synthesis, which requires both O₂ reduction and H₂O oxidation, demands precise band alignment and efficient charge separation. Meanwhile, CO₂ reduction necessitates finely tuned band positions to achieve selective product formation.

5 Conclusions, perspectives, and outlook

ZIS-based heterostructure photocatalysts exhibit notable potential attributed to their superior light absorption characteristics and efficient charge separation capabilities. This review article systematically explores the advancements in ZIS-based heterostructure photocatalysts for applications in solar energy conversion. Firstly, we briefly introduce the crystal structure, optical properties, phase transitions, and synthetic methods of ZIS. Secondly, we review several typical ZIS-based heterostructure photocatalysts, including type-II, p–n type, Z-scheme and S-scheme, briefly explaining the mechanisms of heterostructure formation and identification methods. Lastly, we provide a detailed and comprehensive explanation of the applications of ZIS heterostructures for water splitting, CO₂ reduction, N₂ fixation, and H₂O₂ synthesis, offering theoretical foundations and research directions for subsequent work. Although inspiring achievements have been made for catalyst design and enhancement of application performance, there are still numerous opportunities and challenges (Fig. 14):

Fig. 14 Schematic overview of the future development outlook for ZIS heterostructure photocatalysts.^216–219 Reproduced with permission from ref. 216. Copyright 2013 Elsevier. Reproduced with permission from ref. 217. Copyright 2023 John Wiley and Sons. Reproduced with permission from ref. 218. Copyright 2024 John Wiley and Sons. Reproduced with permission from ref. 219. Copyright 2024 American Chemical Society.

(1) ZIS-based heterostructures exhibit excellent photocatalytic properties but face multiple challenges in large-scale production, including the maturity of synthesis control technologies, high energy consumption, costs, and environmental issues. Currently, ZIS is most commonly synthesized using hydrothermal or solvothermal techniques. The hydrothermal method is generally considered relatively eco-friendly due to its mild reaction conditions and minimal solvent usage. Although adjusting reaction conditions can produce ZIS with various shapes, precise control over its morphology and thickness remains unachieved. A low-temperature water bath may consume less energy, and plasma treatment could serve as a green surface modification technology that eliminates the need for harmful chemicals.^220,221 Additionally, the use of non-precious metal cocatalysts like MoS₂ and Ni-CNT as alternatives to precious metals like Pt is both environmentally friendly and cost-effective, making it significant for further practical applicaiton.²²² It is noteworthy that effectively utilizing the conduction and valence band edges of ZIS heterostructure catalysts to simultaneously facilitate reactions, such as converting 5-hydroxymethylfurfural into high-value-added chemicals while co-producing hydrogen, can achieve a “kill two birds with one stone” approach to resource utilization.^219,221 With technological advancements, more efficient and environmentally friendly production methods are expected to be developed. By adopting new synthesis strategies, there is hope for its widespread application in the fields of energy and the environment, with a promising future outlook.

(2) The operating mechanism of ZIS-based heterogeneous structure photocatalysts, particularly the pathways of charge transfer, remains a focal point of research. Different research teams have proposed various charge transfer models for the same photocatalytic system, but the specific reasons for these discrepancies are still unclear. To gain a deeper understanding of this issue, researchers must continue to utilize advanced characterization techniques and theoretical methods, such as time-dependent density functional theory (TDDFT) calculations, in conjunction with current artificial intelligence technologies, to predict optimal doping/composite combinations through high-throughput computations.^223,224 These tools can help precisely elucidate the details of charge movement, thereby allowing for a more accurate assessment and validation of different charge transfer pathways, ultimately enhancing the design and efficiency of photocatalysts.

(3) In practical applications, ZIS may degrade in performance and stability due to photocorrosion under long-term exposure to visible light.²²⁵ To mitigate the effects of photocorrosion on ZIS in its applications, several improvement measures can be adopted. Firstly, by doping ZIS materials or forming heterostructures with other materials, the structural and photochemical stability can be enhanced.^95,226 Secondly, applying surface protective coatings like SiO₂ can reduce the direct erosion of ZIS by environmental factors. Additionally, optimizing optical design and controlling the temperature and humidity of the usage environment can effectively prolong the lifespan of the material and enhance its resistance to photocorrosion. It is worth noting that supramolecular self-assembly technology provides an innovative approach for enhancing the stability of photocatalysts through a triadic strategy of structural protection, dynamic repair, and electronic synergy.²¹⁷ These comprehensive measures will help improve the performance and stability of ZIS in applications such as optoelectronic devices.²²⁷

(4) The recovery of ZIS catalysts is an important research area that involves efficient resource utilization and environmental protection. In the context of catalytic reactions, ZIS catalysts have been demonstrated to exert a highly effective influence on the chemical processes concerned. However, it is imperative to implement appropriate recovery and reuse methods following their utilization. By optimizing recovery techniques, not only can catalyst waste be minimized, but production costs can also be reduced, thereby improving overall economic efficiency. Up to now, extensive studies have demonstrated that combining catalysts with magnetic materials can improve the recovery rate of the catalysts.^216,228 Wang et al. pointed out that in the CoFe₂O₄/ZIS system, CoFe₂O₄ imparts strong magnetism to the composite material, allowing for a recovery rate of over 99% within 5 seconds under an external magnetic field.²²⁸ In addition, compared to traditional batch reactors, the core of plate reactor technology lies in an “integrated catalyst–substrate” and “removable regeneration”. Immobilized loading, modular design, and optimized mass transfer can significantly enhance catalyst recovery efficiency and stability.^229,230 Moreover, employing green chemistry methods for the recovery and regeneration of catalysts contributes to minimizing environmental impact and promoting sustainable development.²³¹

Data availability

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

Author contributions

Zhao Jing: conceptualization, investigation, visualization, writing – original draft. Qiang Wang: conceptualization, writing – review & editing, funding acquisition. Chenming Fan: visualization. Xiaofan Yang: investigation. Pengyi Tang: supervision, writing – review & editing, funding acquisition. Bing Li: supervision, writing – review & editing, funding acquisition.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (no. 52204323 and 52074130) and the Hundred Talents Program (B) of the Chinese Academy of Sciences (no. E2XBRD1001).

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