1. It is well known that precise control of the morphology of semiconductor photocatalysts plays an important role in improving their physical and chemical properties and the performance of photocatalytic systems (Xie et al., 2021; Xuehua Wang et al., 2021; Cheng et al., 2022; Mingming Liu et al., 2022; Wang et al., 2022; Xia Liu et al., 2022). The photocatalytic performance of ZnIn₂S₄ photocatalyst can be significantly improved by morphological adjustment, which can be attributed to the following four factors: 1) the specific surface area of ZnIn₂S₄ can be increased; 2) it promotes mass transfer and light capture; 3) it is beneficial to expose more active sites on the surface that can participate in redox reactions; 4) it shortens the migration distance and accelerates the migration speed of photogenerated carriers. Therefore, researchers have explored different morphologies of ZnIn₂S₄ as a photocatalyst to improve its photocatalytic performance, including nanosheets, nanoflowers, nanowires, nanorods, nanorings, and nanotubes (Guan et al., 2018; Xu et al., 2021).

Element doping can extend the scope of light absorption, add catalytic sites of a photocatalyst, and adjust the hydrogen adsorption and desorption characteristics (Ida et al., 2018; Quan Zhang et al., 2021; Hou et al., 2022). By introducing donor/acceptor energy levels into the doped ions in semiconductors, the concentration and energy distribution of carriers near the conduction band/valence band edge can be adjusted to improve the electron transition behavior. Therefore, with the rapid development of research on photocatalyst modification of ZnIn₂S₄, many researchers are committed to introducing cations or anions into ZnIn₂S₄.

Cationic doping caused Fermi levels to pass through the conduction band, giving the material metallic properties, thus improving the conductivity and photocarrier migration ability. For example, Qiu et al. (Pan et al., 2021; Qiu et al., 2021; Shi et al., 2022) introduced nickel ions into the ZnIn₂S₄ lattice by the solvothermal method and prepared Ni-doped ZnIn₂S₄ nanosheets with few layers. The photocatalytic activity of ZnIn₂S₄ nanosheets was about seven times higher than that of pure ZnIn₂S₄ nanosheets. Theoretical calculations show that Ni ions are preferentially embedded in the zinc rather than indium sites in tetrahedrons, which induced a narrower band gap, higher electronic conductivity, and more charge carriers that can participate in the hydrogen evolution process. More importantly, Ni dopants can subtly change the electronic structure of the S site and achieve the optimal free energy of hydrogen adsorption on Ni by fine-tuning the S−H bond energy. Therefore, the Ni doping in ZnIn₂S₄ nanoparticles can prolong the lifetime of the photoexcited charge and then enhance the activity of photocatalytic hydrogen evolution. Therefore, the doping of metal cations in the photocatalyst can improve the light absorption range and contribute to the enhancement of photocatalytic activity. On the contrary, anion (O, N, and P, etc.) doping can regulate the valence band to promote the migration of holes and adjust the conduction band to enhance the reduction ability of photogenerated electrons (Goswami et al., 2021; Shuqu Zhang et al., 2021).

Defects engineering is applied to photocatalysts to improve the separation efficiency of photocarriers. The introduced defects can be used as a center to capture photocarriers and prevent their recombination, thus improving charge separation and exposing more active sites. Vacancies in photocatalysts are typical point defects, which play an important role in improving photocatalytic performance due to their regulation of physicochemical and photoelectrochemical properties such as photocarrier migration, light absorption, surface acidity and alkalinity, surface active sites, adsorption properties, solubility properties, and electronic structure. In recent years, with the increasing interest in ZnIn₂S₄ photocatalysts, the studies on vacancy engineering based on ZnIn₂S₄ (sulfur, zinc, and indium vacancies) are also increasing (Lee et al., 2019; Pengfei Wang et al., 2019).

Yu Liu et al. (2022) proposed preparing ZnIn₂S₄ microspheres with S vacancy defects through solvothermal and low-temperature hydrogenation reduction strategies. Due to the formation of S vacancy defects on the surface, the band gap of ZnIn₂S₄ microspheres was reduced to 2.38 eV, which has good visible light response activity. Experimental results and density functional theory calculations showed that the surface S vacancy caused by the surface field potential difference promotes the spatial separation of electrons and holes, thus improving the performance of the photocatalyst and greatly deepening the surface defects engineering understanding of how to affect the separation of light raw charge and to find other efficient and stable metal sulfide photocatalysts which provides a new train of thought. Tai and Zhou, (2021) used reactive ion etching to generate Zn vacancies in ZnIn₂S₄ particles. With the increase of Zn vacancy concentration, the band gap of ZnIn₂S₄ decreases from 2.17 to 2.06 eV. Under the optimum Zn vacancy concentration, the photocatalytic hydrogen evolution rate of ZnIn₂S₄ is 2.7 times higher than that of pure ZnIn₂S₄, and the photocatalytic process of ZnIn₂S₄ is stable without any degradation through cyclic experiments, showing good stability. The existence of Zn vacancies reduces the charge carrier transfer resistance, improves the charge separation rate, and prolongs the emission decay life (He et al., 2019).

In the sandwich ZnIn₂S₄ stacking structure, Zn or S atoms exposed to the surface are easily desorbed, resulting in Zn or S defects. However, due to steric hindrance, the atoms located in the middle layer of a sandwich structure are difficult to be removed, so it is difficult to form in-layer defects. Zn or S defects promote the directional migration of photogenerated electrons but have little effect on hole regulation. Luan et al, (2022) successfully prepared ultra-thin ZnIn₂S₄ nanosheets with an abundant [InS]₆ intermediate layer and perfect [InS]₄ and [ZnS]₄ surface layer by controlling the crystal growth of ZnIn₂S₄ with the rapid heating and hydrothermal method. The in vacancy induces the redistribution of orbitals near the maximum value of the valence band, separates the oxidation and reduction sites on both sides of the ultra-thin ZnIn₂S₄ nanosheet with in vacancy, and increases the density of states between the valence band and the conduction band. The electrons around indium vacancy are delocalized, which is conducive to the interlayer charge transfer and improves the conductivity of the ZnIn₂S₄ nanosheet.

In order to overcome the inherent shortcomings of the high electron–hole recombination rate and low utilization rate of a single unmodified semiconductor photocatalyst, the construction of heterojunction by coupling two semiconductor materials is generally considered to be an effective strategy (Yang et al., 2020; Liu et al., 2021; Xu et al., 2022). The construction of heterojunction with a suitable band position will form a potential gradient between the heterogeneous interfaces to promote the separation and transfer of photocarriers and can also enhance the optical capture performance (Zhao et al., 2019; Mu et al., 2020; Zhang et al., 2020; Zuo et al., 2020). According to the band orientation and carrier transfer path, the structure of heterojunction photocatalyst can be divided into many types, including Type−I (transboundary state photocatalyst), Type−II (alternate state photocatalyst), Z-scheme (alternate state photocatalyst), and Mott−Schottky type (Yang et al., 2018; Li et al., 2019; Hu et al., 2020). In recent years, various heterostructures based on ZnIn₂S₄ have been successfully constructed, and their photocatalytic properties in energy and environmental applications have been studied (Zhenfei Yang et al., 2021; Chen et al., 2022).

The Type 1 heterojunction is a kind of semiconductor heterojunction in which the valence band and conduction band of one semiconductor are located between the valence band and conduction band of the other one. Under the irradiation of incident light, the conduction band to the electronic from high to low conduction band direction and the hole from low to high with direction, in the process of a photocatalytic oxidation–reduction reaction, will be two semiconductor materials to bring to the lower conduction band and a high price (Chao et al., 2021b; Longlu Wang et al., 2021). Different from Type−I heterojunction, Type−II heterojunction is formed by a staggered conduction band and valence band of two semiconductor materials (Jiang et al., 2019; Yan et al., 2019). The movement direction of charge carriers and redox reaction sites of Type−I heterojunction is the same as that of Type−I heterojunction. Due to structural differences, Type−II heterojunction can effectively promote the separation of photogenerated carriers and inhibit their recombination, and the energy conversion efficiency is significantly improved (Zizhong Zhang et al., 2018; Zhao et al., 2021). Although Type−II heterojunction photocatalysts exhibit good photocatalytic performance, such high photocatalytic performance sacrifices the redox ability of charge carriers, so the reduced driving force may not smoothly drive the specific photocatalytic reaction. Due to the well matching of the electronic band structure of the two semiconductor materials, the Z-scheme heterojunction keeps the electrons at a more negative potential and the holes at a corrected potential, resulting in a strong redox ability (Sabbah et al., 2022; Su et al., 2022).