Research Progress of ZnIn 2 S 4 -Based Catalysts for Photocatalytic Overall Water Splitting

: Photocatalytic overall water splitting in solar–chemical energy conversion can effectively mitigate environmental pollution and resource depletion. Stable ternary metal indium zinc sulﬁde (ZnIn 2 S 4 ) is considered one of the ideal materials for photocatalytic overall water splitting due to its unique electronic and optical properties, as well as suitable conduction and valence band positions for suitable photocatalytic overall water splitting, and it has attracted widespread researcher interest. Herein, we ﬁrst brieﬂy describe the mechanism of photocatalytic overall water splitting, and then introduce the properties of ZnIn 2 S 4 including crystal structure, energy band structure, as well as the main synthetic methods and morphology. Subsequently, we systematically summarize the research progress of ZnIn 2 S 4 -based photocatalysts to achieve overall water splitting through modiﬁcation methods such as defect engineering, heterostructure construction, and co-catalyst loading. Finally, we provide insights into the prospects and challenges for the overall water splitting of ZnIn 2 S 4


Introduction
The world is currently suffering from environmental pollution and resource depletion, with energy issues looming large. According to relevant studies, the annual global consumption of energy is equivalent to the solar energy reaching the Earth's surface every hour; therefore, solar energy as an abundant, non-polluting natural resource has replaced the traditional fuel fossil as a research hotspot [1]. However, solar energy has limitations such as intermittency and low density, so an effective storage method is needed to make efficient use of solar energy [2]. Since 1972, when it was reported that TiO 2 semiconductors could produce hydrogen and oxygen when irradiated by ultraviolet light, photocatalysis, which uses solar energy to convert it into storable chemical energy, has attracted extensive research [3].
Hydrogen, as a clean, high-energy-density solar fuel, is the ideal energy carrier. Since most photocatalytic hydrogen production studies require the use of sacrificial agents to achieve this, photocatalytic overall water splitting is considered a low-cost, ideal method for converting solar energy into hydrogen energy [4][5][6]. The photocatalytic overall water splitting process is based on three fundamental photocatalytic processes: photocatalyst absorption of photons to generate electron-hole pairs, photogenerated charge transfer and separation, and surface redox reactions. A variety of semiconductor catalysts such as metal oxides, metal sulfides, and nitrides are currently used in the field of photocatalytic overall water splitting [7][8][9][10][11][12]. Among them, metal sulfides have the advantages of good charge transfer ability, suitable energy band structure for overall water splitting, and excellent

Mechanisms of Photocatalytic Overall Water Splitting
Photocatalytic overall water splitting consists of two half-reactions, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Theoretically, in order to achieve overall water splitting, the semiconductor band gap should be no less than 1.23 eV under standard conditions, the potential at the bottom of the conduction band should be less than 0 eV (H2/H + = 0 eV vs. NHE, pH = 0), and that at the top of the valence band should be greater than 1.23 eV (H2O/O2 = 1.23 eV vs. NHE, pH = 0) [27]. The redox potential of water is all located within the band gap of the photocatalyst and photocatalytic overall water splitting is thermodynamically feasible. However, photocatalytic overall water splitting is an uphill reaction requiring additional energy to promote water splitting, which is a thermodynamically unfavorable process (G > 0); therefore, hydrogen and oxygen are prone to the reverse reaction and H2O reformation, which severely inhibits the photocatalytic water splitting activity [28].
Semiconductor-based photocatalysts for photocatalytic overall water splitting are based on three basic processes of photocatalysis: under solar irradiation with an energy greater than the band gap of the photocatalyst, photogenerated electrons are excited to leap to the conduction band and photogenerated holes remain in the valence band; photogenerated charges migrate separately to the semiconductor reaction site; and un-recombined photogenerated electrons and holes undergo redox reactions of water at the catalyst surface [29]. From the kinetic point of view, the recombination of photogenerated carriers is much faster than their redox reactions at the surface. The Coulomb force constraints between photogenerated charges and high interfacial potential barriers during charge transfer lead to rapid photogenerated carrier recombination and low utilization efficiency, which severely limit photocatalytic activity [30].
In addition, the range of solar energy utilization affects the photocatalytic activity. According to relevant research reports, the UV content of natural sunlight is less than 3%, the visible content is less than 40%, and the near-infrared occupies about 50% of the sunlight, while photocatalytic materials capture light basically in the UV and visible region, with a low efficiency of solar energy utilization [31,32]. The overall photocatalytic water splitting activity is limited by the low light collection capacity of the catalyst, the rate of photogenerated charge separation and migration, and the surface oxidation reaction [33]. Therefore, researchers have adopted corresponding modification strategies to prepare photocatalysts with high activity and high solar energy utilization efficiency. The stable ternary metal sulfide ZnIn2S4 is one of the ideal materials for photocatalytic overall water splitting due to its advantages. As shown in Scheme 2, the ZnIn2S4-based photocatalysts have thermodynamically suitable conduction and valence band positions for photocatalytic water splitting. However, single-component photocatalytic water splitting is difficult to achieve due to the overall low charge utilization and solar utilization as well as photocorrosion phenomena. Therefore, modification strategies such as the doping of Scheme 1. Modification strategies for ZnIn 2 S 4 -based photocatalysts in photocatalytic overall water splitting are briefly described.

Mechanisms of Photocatalytic Overall Water Splitting
Photocatalytic overall water splitting consists of two half-reactions, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Theoretically, in order to achieve overall water splitting, the semiconductor band gap should be no less than 1.23 eV under standard conditions, the potential at the bottom of the conduction band should be less than 0 eV (H 2 /H + = 0 eV vs. NHE, pH = 0), and that at the top of the valence band should be greater than 1.23 eV (H 2 O/O 2 = 1.23 eV vs. NHE, pH = 0) [27]. The redox potential of water is all located within the band gap of the photocatalyst and photocatalytic overall water splitting is thermodynamically feasible. However, photocatalytic overall water splitting is an uphill reaction requiring additional energy to promote water splitting, which is a thermodynamically unfavorable process (G > 0); therefore, hydrogen and oxygen are prone to the reverse reaction and H 2 O reformation, which severely inhibits the photocatalytic water splitting activity [28].
Semiconductor-based photocatalysts for photocatalytic overall water splitting are based on three basic processes of photocatalysis: under solar irradiation with an energy greater than the band gap of the photocatalyst, photogenerated electrons are excited to leap to the conduction band and photogenerated holes remain in the valence band; photogenerated charges migrate separately to the semiconductor reaction site; and un-recombined photogenerated electrons and holes undergo redox reactions of water at the catalyst surface [29]. From the kinetic point of view, the recombination of photogenerated carriers is much faster than their redox reactions at the surface. The Coulomb force constraints between photogenerated charges and high interfacial potential barriers during charge transfer lead to rapid photogenerated carrier recombination and low utilization efficiency, which severely limit photocatalytic activity [30].
In addition, the range of solar energy utilization affects the photocatalytic activity. According to relevant research reports, the UV content of natural sunlight is less than 3%, the visible content is less than 40%, and the near-infrared occupies about 50% of the sunlight, while photocatalytic materials capture light basically in the UV and visible region, with a low efficiency of solar energy utilization [31,32]. The overall photocatalytic water splitting activity is limited by the low light collection capacity of the catalyst, the rate of photogenerated charge separation and migration, and the surface oxidation reaction [33]. Therefore, researchers have adopted corresponding modification strategies to prepare photocatalysts with high activity and high solar energy utilization efficiency. The stable ternary metal sulfide ZnIn 2 S 4 is one of the ideal materials for photocatalytic overall water splitting due to its advantages. As shown in Scheme 2, the ZnIn 2 S 4 -based photocatalysts have thermodynamically suitable conduction and valence band positions for photocatalytic water splitting. However, single-component photocatalytic water splitting is difficult to achieve due to the overall low charge utilization and solar utilization as well as photocorrosion phenomena. Therefore, modification strategies such as the doping of heteroatoms, formation of defects, construction of heterojunctions, and loading of co-catalysis were adopted to enhance the ZnIn 2 S 4 -based photocatalytic performance.

Crystal and Energy Band Structures
The ternary metal sulfide ZnIn2S4 has three crystal phase structures: cubic, hexagonal, and rhombic. As shown in Figure 1a, the cubic ZnIn2S4 has an ABC stack in which the Zn atom is tetrahedrally coordinated to the S atom and the In atom is octahedrally coordinated to the S atom [34]. As shown in Figure 1b, the hexagonal ZnIn2S4 has a layered structure in which the atoms are repeatedly stacked in the order S-Zn-S-In-S-In-S. In the hexagonal crystal phase, the Zn atoms and half of the In atoms form a tetrahedral coordination with the S atoms, and the remaining In atoms form an octahedral coordination with the S atoms [35]. As shown in Figure 1c, the crystal structure of rhombic ZnIn2S4 is similar to that of hexagonal ZnIn2S4, consisting of a sandwich layer with one octahedron and two tetrahedra. Rhombic ZnIn2S4 differs from hexagonal ZnIn2S4 in that the Zn atoms and half of the In atoms are mixed in the tetrahedral sites [36]. Different crystalline phases have different properties, the cubic phase has thermoelectric properties, the hexagonal phase has photoconductivity, and the rhombic phase has good charge transfer ability. In addition, the researchers found that the photocatalytic activity of ZnIn2S4 can be effectively enhanced by changing the crystalline phase. For example, in 2011, Shen et al. synthesized ZnIn2S4 with different crystalline phase structures by high-temperature thermal sulfide treatment of metal oxide precursors [37]. They performed thermal sulfidation reactions on Zn-In mixed oxide precursors under an H2S atmosphere and synthesized cubic ZnIn2S4 and gradually transformed it into rhombohedra when the thermal sulfidation temperature was increased from 400 to 800 °C. The rhombohedral ZnIn2S4 has a good charge transfer ability and light absorption, enhancing the photocatalytic hydrogen production activity under sacrificial agents. In addition, ZnIn2S4 can be used to transform the crystalline phase by using different metal precursors and changing the reaction temperature to prepare more active crystalline phases and structures for photocatalytic applications. Density functional theory (DFT) has been widely used in semiconductor materials to study the electronic structure of materials. DFT provides theoretical insight into the electronic energy band structure of cubic, hexagonal, and rhombic ZnIn2S4, and all three crystalline phases are direct band gap semiconductors. Although the respective band gaps of the cubic, hexagonal, and rhombic crystalline phases are known from theoretical calculations, the actual band gaps deviate from the calculated results due to the limitations of the local density approximation (LDA) functional [38]. Scheme 2. Schematic diagram of ZnIn 2 S 4 -based photocatalyst photocatalytic overall water splitting.

Crystal and Energy Band Structures
The ternary metal sulfide ZnIn 2 S 4 has three crystal phase structures: cubic, hexagonal, and rhombic. As shown in Figure 1a, the cubic ZnIn 2 S 4 has an ABC stack in which the Zn atom is tetrahedrally coordinated to the S atom and the In atom is octahedrally coordinated to the S atom [34]. As shown in Figure 1b, the hexagonal ZnIn 2 S 4 has a layered structure in which the atoms are repeatedly stacked in the order S-Zn-S-In-S-In-S. In the hexagonal crystal phase, the Zn atoms and half of the In atoms form a tetrahedral coordination with the S atoms, and the remaining In atoms form an octahedral coordination with the S atoms [35]. As shown in Figure 1c, the crystal structure of rhombic ZnIn 2 S 4 is similar to that of hexagonal ZnIn 2 S 4 , consisting of a sandwich layer with one octahedron and two tetrahedra. Rhombic ZnIn 2 S 4 differs from hexagonal ZnIn 2 S 4 in that the Zn atoms and half of the In atoms are mixed in the tetrahedral sites [36]. Different crystalline phases have different properties, the cubic phase has thermoelectric properties, the hexagonal phase has photoconductivity, and the rhombic phase has good charge transfer ability. In addition, the researchers found that the photocatalytic activity of ZnIn 2 S 4 can be effectively enhanced by changing the crystalline phase. For example, in 2011, Shen et al. synthesized ZnIn 2 S 4 with different crystalline phase structures by high-temperature thermal sulfide treatment of metal oxide precursors [37]. They performed thermal sulfidation reactions on Zn-In mixed oxide precursors under an H 2 S atmosphere and synthesized cubic ZnIn 2 S 4 and gradually transformed it into rhombohedra when the thermal sulfidation temperature was increased from 400 to 800 • C. The rhombohedral ZnIn 2 S 4 has a good charge transfer ability and light absorption, enhancing the photocatalytic hydrogen production activity under sacrificial agents. In addition, ZnIn 2 S 4 can be used to transform the crystalline phase by using different metal precursors and changing the reaction temperature to prepare more active crystalline phases and structures for photocatalytic applications. Density functional theory (DFT) has been widely used in semiconductor materials to study the electronic structure of materials. DFT provides theoretical insight into the electronic energy band structure of cubic, hexagonal, and rhombic ZnIn 2 S 4 , and all three crystalline phases are direct band gap semiconductors. Although the respective band gaps of the cubic, hexagonal, and rhombic crystalline phases are known from theoretical calculations, the actual band gaps deviate from the calculated results due to the limitations of the local density approximation (LDA) functional [38].

Synthetic Methods and Morphology of ZnIn2S4
The conformation and structure of ZnIn2S4, a stable ternary metal sulfide semiconductor, significantly affect the photocatalytic activity. Among them, ZnIn2S4 with different morphological structures such as zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) plays an important role in the solar-chemical energy conversion process.
Zero-dimensional ZnIn2S4 is mainly available in quantum dots (QD). Quantum dots, as a hot research material in the energy environment, have the advantages of easy synthesis, abundant surface sites, and controllable size, and due to the quantum confinement effect, their light collection range can be adjusted to the near-infrared region, which significantly improves the solar light utilization effect [39]. Peng et al. prepared size-tunable ZnIn2S4 quantum dots using oleylamine as the ligand and uncoordinated octadecene as the solvent [40]. Due to the different effects of reaction temperature in controlling the nucleation and growth process of nanocrystals thermodynamically and kinetically, ZnIn2S4 nanocrystals with sizes ranging from 2.1 to 10.1 nm were synthesized by varying the reaction temperature between 140 °C and 210 °C. Experiments have shown that ZnIn2S4 nanocrystals with small size and annealed to remove the capping agent are highly active in the degradation of methylene orange. Currently, 1D ZnIn2S4 is mainly available as nanowires, nanotubes, and nanoribbons, but most 1D ZnIn2S4 synthesis requires the use of templates, so most of the synthesized morphologies are 2D and 3D ZnIn2S4 [41,42]. The majority of ZnIn2S4 syntheses are hexagonal due to the advantages of good stability, simple preparation, and high activity of the hexagonal crystalline phase ZnIn2S4. It is easy for the hexagonal ZnIn2S4 layered structure to form a 2D nanosheet morphology; especially, the ultra-thin 2D nanosheets have the advantages of short photogenerated carrier migration distance, large specific surface area, and abundant surface-active sites, which significantly improve the photocatalytic activity of ZnIn2S4-based materials [43]. In 2018, Zhang et al. prepared ultrathin ZnIn2S4 nanosheets (Vs-M-ZnIn2S4) rich in S-vacancies by exfoliating large blocks of ZnIn2S4 synthesized using lithium intercalation [44]. Then, MoS2 quantum dots (MoS2QDs) were grown in the S-vacancy region of Vs-M-ZnIn2S4 induced by S-vacancies in one of the Zn cuts to synthesize atomic-level heterojunction MoS2QDs@Vs-M-ZnIn2S4. Unlike ZnIn2S4, the S-vacancies in MoS2QDs@Vs-M-ZnIn2S4 act as electron traps, enriching electrons in the Zn plane and transferring them to MoS2QDs via Zn-S bonds, preventing the rapid recombination of photogenerated charges due to vertical electron transport and improving the hydrogen production activity of the photocatalyst.
However, due to the high surface energy of the individual nanosheets, ZnIn2S4 tends to aggregate to form 3D microspheres during growth, and both the specific configuration of its synthesis and the increased specific surface area and active sites enhance the photocatalytic activity of the catalyst. Shen et al. synthesized persimmon-layered ZnIn2S4 photocatalysts by using an oleylamine (OA)-assisted solvent method, adding tetrahydrofuran  The conformation and structure of ZnIn 2 S 4 , a stable ternary metal sulfide semiconductor, significantly affect the photocatalytic activity. Among them, ZnIn 2 S 4 with different morphological structures such as zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) plays an important role in the solarchemical energy conversion process.
Zero-dimensional ZnIn 2 S 4 is mainly available in quantum dots (QD). Quantum dots, as a hot research material in the energy environment, have the advantages of easy synthesis, abundant surface sites, and controllable size, and due to the quantum confinement effect, their light collection range can be adjusted to the near-infrared region, which significantly improves the solar light utilization effect [39]. Peng et al. prepared size-tunable ZnIn 2 S 4 quantum dots using oleylamine as the ligand and uncoordinated octadecene as the solvent [40]. Due to the different effects of reaction temperature in controlling the nucleation and growth process of nanocrystals thermodynamically and kinetically, ZnIn 2 S 4 nanocrystals with sizes ranging from 2.1 to 10.1 nm were synthesized by varying the reaction temperature between 140 • C and 210 • C. Experiments have shown that ZnIn 2 S 4 nanocrystals with small size and annealed to remove the capping agent are highly active in the degradation of methylene orange. Currently, 1D ZnIn 2 S 4 is mainly available as nanowires, nanotubes, and nanoribbons, but most 1D ZnIn 2 S 4 synthesis requires the use of templates, so most of the synthesized morphologies are 2D and 3D ZnIn 2 S 4 [41,42]. The majority of ZnIn 2 S 4 syntheses are hexagonal due to the advantages of good stability, simple preparation, and high activity of the hexagonal crystalline phase ZnIn 2 S 4 . It is easy for the hexagonal ZnIn 2 S 4 layered structure to form a 2D nanosheet morphology; especially, the ultra-thin 2D nanosheets have the advantages of short photogenerated carrier migration distance, large specific surface area, and abundant surface-active sites, which significantly improve the photocatalytic activity of ZnIn 2 S 4 -based materials [43]. In 2018, Zhang et al. prepared ultrathin ZnIn 2 S 4 nanosheets (Vs-M-ZnIn 2 S 4 ) rich in S-vacancies by exfoliating large blocks of ZnIn 2 S 4 synthesized using lithium intercalation [44]. Then, MoS 2 quantum dots (MoS 2 QDs) were grown in the S-vacancy region of Vs-M-ZnIn 2 S 4 induced by S-vacancies in one of the Zn cuts to synthesize atomic-level heterojunction MoS 2 QDs@Vs-M-ZnIn 2 S 4 . Unlike ZnIn 2 S 4 , the S-vacancies in MoS 2 QDs@Vs-M-ZnIn 2 S 4 act as electron traps, enriching electrons in the Zn plane and transferring them to MoS 2 QDs via Zn-S bonds, preventing the rapid recombination of photogenerated charges due to vertical electron transport and improving the hydrogen production activity of the photocatalyst.
However, due to the high surface energy of the individual nanosheets, ZnIn 2 S 4 tends to aggregate to form 3D microspheres during growth, and both the specific configuration of its synthesis and the increased specific surface area and active sites enhance the photocatalytic activity of the catalyst. Shen et al. synthesized persimmon-layered ZnIn 2 S 4 photocatalysts by using an oleylamine (OA)-assisted solvent method, adding tetrahydrofuran (THF) solution to form a hexagonal structure and OA selectively adsorbed on the ZnIn 2 S 4 hexagon to form nanoplates, which then self-assembled to form persimmon [45]. The material has excellent photocatalytic hydrogen production activity, with the best catalyst achieving 220.45 mmol h −1 after 3% Pt loading. Some of the ZnIn 2 S 4 photocatalysts synthesized in current research, such as marigold and peony-flower-like, as well as rose and marigold-like, are 3D microspheres [46,47]. Furthermore, hollow structured materials with their rich surface-active sites, short charge migration paths, and good light collection capabilities have attracted the interest of a wide range of researchers [48]. In 2014, Warule et al. synthesized hollow marigold-like ZnIn 2 S 4 materials for photocatalytic hydrogen production by a surfactant-assisted hydrothermal method [49]. The hollow marigold-like nanoparticles have a higher specific surface area and more active sites, exhibiting better hydrogen production activity. As most photocatalytic reactions occur on the surface of the material, the morphology and configuration of the material have an important influence on the photocatalytic activity. The synthesis of materials with high specific surfaces, abundant active sites, and unique configurations is very important for improving the performance of ZnIn 2 S 4 . We provide a brief summary of the various morphologies of ZnIn 2 S 4 in Table 1. Hydrothermal method. Unlike other synthesis methods that use templates and special equipment, the hydrothermal synthesis of ZnIn 2 S 4 is relatively gentle and simple. The reaction temperature, the type of reaction precursor, the pH of the reaction system, the use of surfactants, and the type of surfactant all influence the synthesis, morphology, and crystallinity of the ZnIn 2 S 4 -based catalysts. In 2006, Guo et al. prepared ZnIn 2 S 4 solid and hollow microsphere structures hydrothermally under cetyltrimethylammonium bromide (CTAB) and ethylene glycol (PEG) as surfactants, respectively [52]. Two different ZnIn 2 S 4 crystalline phase materials were hydrothermally synthesized by the addition of different metal precursors by Chen et al. in 2012 [53]. The cubic phase was synthesized when metal nitrates were added as precursors, whereas a thermodynamically stable hexagonal phase was prepared when metal chlorides containing electronegative low-chloride ions were used as precursors. In addition, Warule et al. in 2014 synthesized hollow marigold-like and rose-shaped ZnIn 2 S 4 photocatalysts by a surfactant-assisted hydrothermal method using polyvinylpyrrolidone (PVP) and diethylamine (DEA), respectively [49]. Studies have shown that varying the concentration of PVP (100 ppm-300 ppm) results in the synthesis of different forms ranging from twisted to hollow marigold-like. In 2022, Yin et al. synthesized ZnIn 2 S 4 with different assembled microstructures for dehydrogenation treatment by a one-step hydrothermal method and investigated the effect of different solvents on the structure of ZnIn 2 S 4 [54]. The experimental results showed that the solvents were water, ethanol, and ethylene glycol in accordance with which smooth surface-petal-like, relatively rough micro-disk-like and thin nanosheets were synthesized. In 2022, Zou et al. hydrothermally prepared ZnIn 2 S 4 materials with different crystalline phases by adjusting the pH of the system using oxalic acid as a chelating agent [55]. The cubic phase of ZnIn 2 S 4 was prepared without the addition of oxalic acid and hydrothermal preparation after oxalic acid modification transformed the original cubic phase of ZnIn 2 S 4 into a hexagonal phase.
Solvothermal method. Like hydrothermal methods, solvothermal synthesis is relatively simple and is widely used in the synthesis of materials for the energy environment. The nature of the solvent such as alkalinity, viscosity, and type of solvent play an important role in the preparation of ZnIn 2 S 4 materials in terms of their morphology. Guo et al. in 2006 found that reaction temperature has a significant effect on ZnIn 2 S 4 morphology [52]. They prepared the catalysts by varying the reaction temperature solvothermally using pyridine as the solvent; when the reaction temperature was between 120 and 160 • C, ZnIn 2 S 4 grew in the (002) direction to form nanoribbons, and when the reaction temperature was 180 • C and higher, the material formed nanotubular shapes. In addition to the reaction temperature, the type of solvent has an equally important influence on the synthetic microstructure of ZnIn 2 S 4 . In 2008, Shen et al. used water, methanol, and ethylene glycol as solvents, with the first two synthesizing cherry-shaped microspheres and ethylene glycol as the solvent to synthesize micro-clustered clusters [56]. The results show that the catalysts synthesized with water as the solvent are more crystalline and stable and have the best hydrogen production efficiency. Furthermore, Su et al. in 2016 used water, ethanol, methanol, as well as ethylene glycol as solvent precursors for the synthesis of ZnIn 2 S 4 by a solvothermal method to investigate the effect of different types of solvents on the selective oxidation activity of aromatic alcohols [57]. The experimental structures showed that the best photocatalytic performance of ZnIn 2 S 4 was prepared using ethanol as the solvent, and that the difference in performance between the different solvents was mainly due to the degree of exposure of the basic crystalline surface, resulting in different exposures of the special surface.
Other synthesis methods. In addition to hydrothermal and solvothermal methods, microwave-assisted methods, thermal sulfur methods, chemical vapor deposition, and spray pyrolysis are also applied to the synthesis of ZnIn 2 S 4 in different forms and structures [58][59][60]. In 2012, Huang et al. synthesized ZnIn 2 S 4 in micro-spherical form by a simple spray pyrolysis method [61]. They first synthesized the solution by magnetic stirring, then atomized the solution and fed it into a tubular reactor for reaction to produce spherical particles. In addition, Pop et al. in 2022 prepared ZnIn 2 S 4 photocatalysts by the microwaveassisted oil bath production method to investigate the effect of ZnIn 2 S 4 materials with different zinc concentrations and reaction temperatures on methyl orange adsorption [62]. The experimental results show that disordered cubic ZnIn 2 S 4 is synthesized at 160 • C and hexagonal phase ZnIn 2 S 4 structures are synthesized at 180 • C. Moreover, the morphology changes from nanoparticles to nanoflower, hollow-microsphere structures as the zinc concentration and reaction temperature increase. Although there are numerous methods to synthesize ZnIn 2 S 4 , hydrothermal and solvothermal methods are widely used in the synthesis of ZnIn 2 S 4 -based photocatalysts due to the advantages of the simple synthesis process and low cost.
Since most catalytic reactions occur on the surface of catalysts, the structure and morphology of materials have important effects on their properties. Therefore, we briefly describe representative morphologies in ZnIn 2 S 4 materials as well as hydrothermal and solvothermal synthesis methods. It is hoped that this will provide some reference for the preparation of ZnIn 2 S 4 photocatalytic overall water splitting morphologies using simple synthetic methods.

Defect Engineering
Defect engineering not only modulates the electronic structure, provides abundant active sites, and improves the efficiency of charge separation and migration of materials, but also changes the morphology and interfacial reactions of materials and plays an important role in the catalytic reactions of materials [63]. Therefore, defect engineering is often used in photocatalytic systems to modulate material properties, among which metal sulfides have received a lot of attention because of their tunable electronic structure. Doping and vacancies, which are point defects in defect engineering, have been applied to modulate material properties in sulfide catalysts such as ZnIn 2 S 4 . Su et al. synthesized Al 3+ -doped strontium calcium titanate photocatalysts (Al-STO) for photocatalytic overall water splitting by polymerization complexation [64]. The doping of low-valence metal cations effectively promotes the migration rate of photogenerated carriers, and the appropriate oxygen vacancies on the surface facilitate the adsorption of water molecules and hydroxyl groups to promote the reduction reaction. The reduction in intrinsic Ti 3+ defects effectively promotes the separation and migration efficiency of photogenerated carriers in concert with the oxygen vacancies on the surface to improve the photocatalytic activity of strontium-titanate-based photocatalysts. The best material-2% Al-STO-achieved hydrogen yields of 1.256 mmol h −1 and oxygen yields of 0.692 mmol h −1 under a loaded co-catalyst, with an apparent quantum efficiency (AQE) of up to 55.46% at 365 nm for the composite. It follows that the introduction of defects in semiconductor photocatalysts is a promising method of modification.

Doping Strategy
Doping strategies are modification methods that introduce impurity atoms, such as metals and non-metals, into the structure of photocatalytic materials to form defects to alter the material properties, mainly through both doping and ion exchange [65,66]. Material doping with heteroatoms has the advantages of narrowing the band gap of the composite, enhancing light absorption, changing the morphological structure of the material, and increasing the separation and migration rates of photogenerated carriers, and is therefore considered to be an effective method for effectively increasing the activity of sulfide photocatalysts [67]. Alkaline earth metals such as Ca and Ba, rare-earth metals such as La and Y, transition metals such as Ag, Co, and Ni, as well as non-metals such as N and O have been employed in ZnIn 2 S 4 , and good photocatalytic activity has been achieved [68].

Metal Doping
Pan et al. in 2021 reported Ag-ZnIn 2 S 4 composite photocatalytic materials with dual defects in Ag doping and nanopores prepared by cation exchange between 2D ZnIn 2 S 4 monolayers and Ag, achieving ZnIn 2 S 4 photocatalytic overall water splitting without cocatalysts and sacrificial agents under visible light irradiation [69]. The experimental results show that Ag doping can effectively narrow the band gap, improve the light collection capacity of the catalyst, and promote photogenerated charge separation. The hydrogen yield of the composites was 56.6 µmol g −1 h −1 and the oxygen yield was 29.1 µmol g −1 h −1 . The Ag-ZnIn 2 S 4 composite has good photocatalytic activity, as shown in Figure 2a; the Ag element doping redistributes the charge; the positively charged enriched Ag adsorbs water molecules and promotes the oxygen production reaction; and the sulfur atoms suspended on the nanopores promote the hydrogen production reaction. In addition, the ultrathin 2D ZnIn 2 S 4 monolayer narrows the migration distance and increases the photogenerated charge separation and migration rate. They also prepared Cu-ZnIn 2 S 4 with a similar structure to Ag-ZnIn 2 S 4 by the same method. The experimental results show that Cu doping has a stronger charge migration rate and higher hydrogen production activity, but as shown in Figure 2b, Cu doping does not enable photocatalytic all-water decomposition, which confirms that Ag doping is important for the Ag-ZnIn 2 S 4 materials to achieve photocatalytic all-water decomposition.
pended on the nanopores promote the hydrogen production reaction. In addition, the ultrathin 2D ZnIn2S4 monolayer narrows the migration distance and increases the photogenerated charge separation and migration rate. They also prepared Cu-ZnIn2S4 with a similar structure to Ag-ZnIn2S4 by the same method. The experimental results show that Cu doping has a stronger charge migration rate and higher hydrogen production activity, but as shown in Figure 2b, Cu doping does not enable photocatalytic all-water decomposition, which confirms that Ag doping is important for the Ag-ZnIn2S4 materials to achieve photocatalytic all-water decomposition. In addition, in 2022, Sun et al. used a magnetic stirring one-step solvothermal method to synthesize in situ interstitial zinc-doped ultrathin ZnIn2S4 nanosheets (dZni-ZnIn2S4) and to study the photocatalytic performance of the materials for photocatalytic overall water splitting without any auxiliary agent under visible-light irradiation [70]. The Zn In addition, in 2022, Sun et al. used a magnetic stirring one-step solvothermal method to synthesize in situ interstitial zinc-doped ultrathin ZnIn 2 S 4 nanosheets (dZni-ZnIn 2 S 4 ) and to study the photocatalytic performance of the materials for photocatalytic overall water splitting without any auxiliary agent under visible-light irradiation [70]. The Zn doping not only produces a static potential difference, but also forms a short range of disordered structures with abundant active sites. As shown in Figure 2e, the Zn doping leads to broadening and weakening of the ZnIn 2 S 4 (006) peak and disruption of interlayer stacking, and widens the material nanosheet spacing, forming a short-range disordered structure. The differential charge density map (Figure 2c,d) reveals that, unlike the original ZnIn 2 S 4 with uniform charge density distribution, dZni-ZnIn 2 S 4 leads to charge redistribution with positive charge enrichment at the Zn site, lowering the water oxidation potential barrier and increasing the electron density at the sulfur site, which facilitates H + adsorption reduction and improves the photocatalytic overall water splitting activity. In addition, the ultrathin nanosheets formed by the dZni-ZnIn 2 S 4 material shorten the migration distance and effectively promote photogenerated carrier separation, achieving a hydrogen yield of 42.8 µmol g −1 h −1 and an oxygen yield of 19.1 µmol g −1 h −1 .
Subsequently, Sun et al. in 2022 also used a magnetic stirring one-step solvothermal method to synthesize Al-ZnIn 2 S 4 composites and investigate their hydrogen and oxygen production properties under visible-light irradiation [71]. Al-ZnIn 2 S 4 has a rich mesoporous structure that maximizes the exposure of active sites and enhances photocatalytic activity. The Al doping also produces an expanded layer spacing that induces a domain electrostatic potential difference, effectively promoting photogenerated carrier separation. Al doping redistributes the charge, enriching the Al sites with positive charge, increasing the electron density around the S sites, promoting redox reactions in the material, reducing photocorrosion, and improving stability. Due to these characteristics, the material achieves efficient photocatalytic overall water splitting without any auxiliary agent, with a hydrogen yield of 77.  [72]. The calcined ZnIn 2 S 4 improves light absorption, and the surface-rich S-vacancies extend the photogenerated charge lifetime to inhibit recombination but do not increase the carrier density. The increase in electron paramagnetic resonance (EPR) signal after calcination indicates that calcination introduces S-vacancies, which increases and then decreases with increasing calcination time, and that the surface oxygen doping gradually fills in. After air calcination, the Fourier-transform infrared (FTIR) spectrum shows SO 4 2− bidentate characteristic peaks, further demonstrating the synergistic effect of the material between the oxygen doping and the S-vacancies. The synergistic effect of S-vacancies and oxygen doping on the material surface enhances photogenerated carrier lifetimes and concentrations and improves the photocatalytic overall water splitting activity. The materials are loaded with Pt/Cr co-catalyst and show remarkable photocatalytic activity with a hydrogen yield of 270.2 µmol g −1 h −1 and an oxygen yield of 130.0 µmol g −1 h −1 , which are 13 times higher than those of the pristine ZnIn 2 S 4 .
In summary, compared with the undoped ZnIn 2 S 4 -based material, the material doping element can effectively reduce the band gap of the composite material, improve the material's light collection capacity, change the morphological structure of the material, modulate the electronic structure of the material, and increase the photogenerated carrier separation and migration rate. Although the role of doping elements in photocatalysis remains in doubt due to their possible negative effects as photogenerated charge recombination centers, doping strategies do play an important role in modifying ZnIn 2 S 4 -based photocatalytic materials.

Vacancy Introduction
Vacancies are material lattice defects caused by the lack of cations or anions, which is mainly divided into cationic and anionic types [73]. He et al. prepared single-atom sulfur vacancy CdS nanorods (Sv-CdS NRs) with a spin-polarized electric field by the hydrothermal method combined with anaerobic heating to achieve photocatalytic overall water splitting without any co-catalyst [74]. Single-atom sulfur vacancies are introduced to induce spin-polarization properties in the CdS system, providing a strong facilitating electric field that enhances photogenerated carrier separation and migration rates. Sv-CdS NRs significantly improves the photocatalytic activity of the material, and the best material, Sv-CdS-2 NRs, was able to achieve a hydrogen evolution performance of 363.8 µmol g −1 h −1 and an oxygen evolution performance of 181.9 µmol g −1 h −1 . In 2022, a rhombic ZnIn 2 S 4 -800 material rich in S-vacancies was reported by Jing et al. [75]. They designed and prepared a rhombic crystal phase and sulfur-vacancy-rich ZnIn 2 S 4 -800 material by calcining hexagonal ZnIn 2 S 4 at high temperature under a nitrogen atmosphere, unlike the previous harsh preparation conditions, which changed the crystal phase of ZnIn 2 S 4 . As shown in Figure 3a, X-ray diffraction (XRD) patterns of ZnIn 2 S 4 before and after calcination show that high-temperature calcination under a nitrogen atmosphere has modified the crystal phase and structural morphology of ZnIn 2 S 4 . Characterization by electron paramagnetic resonance (Figure 3b), UV-diffuse reflectance spectroscopy (Figure 3c), and the energy band structure (Figure 3d) confirms that the introduction of sulfur vacancies into ZnIn 2 S 4 by calcination has narrowed the band gap and improved the light gathering capacity of the material. Appropriate S-vacancies as electron capture sites to increase the photogenerated carrier density inhibit recombination and promote hydrogen production reactions, and the calcined material has a smaller average mass of electrons and holes, accelerating the rate of photogenerated charge migration. The material loaded with Pt/Cr co-catalyst achieves hydrogen yields of 68.0 µmol g −1 h −1 and oxygen yields of 31.0 µmol g −1 h −1 in pure water under visible-light irradiation.
ZnIn2S4 by calcination has narrowed the band gap and improved the light gathering capacity of the material. Appropriate S-vacancies as electron capture sites to increase the photogenerated carrier density inhibit recombination and promote hydrogen production reactions, and the calcined material has a smaller average mass of electrons and holes, accelerating the rate of photogenerated charge migration. The material loaded with Pt/Cr co-catalyst achieves hydrogen yields of 68.0 µmol g −1 h −1 and oxygen yields of 31.0 µmol g −1 h −1 in pure water under visible-light irradiation. In summary, the introduction of appropriate vacancies in ZnIn2S4-based photocatalytic materials can narrow the band gap and improve visible-light absorption, increase the charge density of photogenerated carriers and the rate of migration of photogenerated charge separation, and regulate the electronic structure of the material, providing an abundance of surface-active sites and effectively modulating the crystal and energy band In summary, the introduction of appropriate vacancies in ZnIn 2 S 4 -based photocatalytic materials can narrow the band gap and improve visible-light absorption, increase the charge density of photogenerated carriers and the rate of migration of photogenerated charge separation, and regulate the electronic structure of the material, providing an abundance of surface-active sites and effectively modulating the crystal and energy band structure of the material, thus effectively improving the photocatalytic activity of the material.

Construction of Heterogeneous Junctions
The construction of heterojunctions from two or more semiconductors is a better modification strategy due to the strong Coulombic forces on the photogenerated charges of a single semiconductor, which are prone to recombination and result in poor photocatalytic activity. The construction of heterojunctions not only improves the photoresponse range of the material, provides a larger specific surface area, exposes more active sites, and modulates the catalyst to form unique structures, but also increases the photogenerated carrier separation and migration rates [76,77]. Depending on the energy band structure of the semiconductor and the photogenerated carrier transfer path, heterojunctions can be classified into six categories: type I heterojunction, type II heterojunction, Schottky junction, p-n heterojunction, Z-scheme heterojunction, and S-scheme heterojunction. The construction of a heterojunction by coupling ZnIn 2 S 4 with another semiconductor is one of the effective modification methods to overcome the disadvantages of ZnIn 2 S 4 -based photocatalysts such as slow photogenerated charge separation and migration, and easy recombination [78,79]. Therefore, it is important to design and prepare heterojunctions with an appropriate energy band structure and well-matched geometry. In recent years, the photocatalytic performance of heterojunction materials constructed from metal oxides, carbon nitride, and ZnIn 2 S 4 has been improved to varying degrees and applied in the field of environmental energy [80][81][82][83][84]. Currently, the most widely used in photocatalytic overall water splitting are Z-scheme heterojunctions and Schottky heterojunctions.

Z-Scheme Heterojunction
Most semiconductor materials are unable to achieve one-step photo-excited overall water splitting, due to drawbacks such as the lack of a suitable energy band structure and severe photogenerated carrier complexation. In recent years, the use of two-step photoexcitation, coupling two or more semiconductors to construct a Z-scheme heterojunction, has been widely used to improve the photocatalytic activity of ZnIn 2 S 4 -based photocatalysts with good results [85,86]. There are two main types of Z-scheme heterojunctions: (1) Indirect Z-scheme heterojunctions in which the two semiconductors are not in contact and the charge is transferred through a charge transfer medium such as Au and graphene; (2) a direct Z-scheme heterojunction in which two semiconductors come into contact, generating an electric field at the inner boundary due to differences in the work function and Fermi energy levels, accelerating the separation of photogenerated carriers [87,88]. The construction of Z-scheme heterojunctions facilitates the spatial separation of photogenerated charges, inhibits recombination, and maintains the high redox capacity of the material, effectively improving photocatalytic activity.

Indirect Z-Scheme Heterojunction
In 2017, Zhong et al. reported an indirect Z-scheme heterojunction material, ZnIn 2 S 4 /RGO/ BMO, in which ZnIn 2 S 4 acts as a hydrogen-depleting photocatalyst, Bi 2 MoO 6 (BMO) acts as an oxygen-depleting photocatalyst, and reduced graphene oxide (RGO) acts as an electron mediator (Figure 4a) [89]. They prepared BMO and ZnIn 2 S 4 by solvent thermal and hydrothermal methods, respectively, and then loaded Pt and CoOx co-catalysts onto ZnIn 2 S 4 and BMO, respectively, to achieve photocatalytic overall water splitting without sacrificial agents. The construction of Z-scheme heterojunctions and the appropriate number of electron-mediated RGO can effectively increase the photogenerated charge separation and migration rates of the catalysts and improve the photocatalytic activity of the composites.   In 2018, Yang et al. reported a sea-urchin-shaped ZnIn 2 S 4 -based catalyst (ZnIn 2 S 4 -Au-TiO 2 ), achieving a hydrogen yield of 186.3 µmol g −1 h −1 and an oxygen yield of 66.3 µmol g −1 h −1 without co-catalysts and sacrificial agents [90]. They first synthesized TiO 2 microspheres, then synthesized Au-TiO 2 by chemically depositing Au nanoparticles on the TiO 2 surface, and finally prepared the indirect Z-scheme heterojunction material ZnIn 2 S 4 -Au-TiO 2 by direct deposition of ZnIn 2 S 4 on the Au-TiO 2 surface via a solvothermal method (Figure 4b). The Z-scheme heterojunction can effectively promote the electron-hole pair separation efficiency, and the ultra-thin ZnIn 2 S 4 nanosheets increase the specific surface area of the material and provide more active sites. As shown in Figure 4c, the ultravioletvisible (UV-Vis) diffuse reflectance spectra show an increase in the visible absorption capacity of the material due to the surface plasmon resonance of the gold nanoparticles (Au NPs). The composite material promotes the separation of photogenerated carriers and increases the light collection capacity of the material, thus effectively improving the photocatalytic activity of the material.
In 2021, Geng et al. reported a Pt-ZnIn 2 S 4 /RGO/Co 3 O 4 -BiVO 4 (110) photocatalyst [92]. They first prepared BiVO 4 by the solvothermal method, RGO/Co 3 O 4 -BiVO 4 (110) by photo-deposition and reduction, and Pt-ZnIn 2 S 4 by the hydrothermal method and photodeposition. Finally, they used a self-assembly method to prepare Pt-ZnIn 2 S 4 /RGO/Co 3 O 4 -BiVO 4 (110) Z-scheme photocatalysts for photocatalytic overall water splitting without sacrificial agents. The decagonal BiVO 4 can accumulate holes and electrons on the (110) and (040) sides and top surfaces, respectively, allowing effective spatial separation of photogenerated carriers, so the decagonal BiVO 4 with the (110) surface can be used as an oxygen production photocatalyst. ZnIn 2 S 4 was used as a hydrogen production photocatalyst and RGO as an electron mediator to construct Z-scheme heterojunctions with BiVO 4 (110). The Z-scheme heterojunction effectively increases the rate of photogenerated charge separation and migration, and the appropriate amount of RGO electron mediators and Pt/Co 3 O 4 co-catalyst loading promotes electron transfer. The material achieves a hydrogen yield of 24.5 µmol g −1 h −1 and an oxygen yield of 11.9 µmol g −1 h −1 under visible-light irradiation in pure water.

Direct Z-Scheme Heterojunction
In 2019, Ding et al. synthesized PtS-ZnIn 2 S 4 /WO 3 -MnO 2 direct Z-scheme heterojunctions, achieving a hydrogen yield of 38.8 µmol g −1 h −1 and an oxygen yield of 15.7 µmol g −1 h −1 [91]. They used a hydrothermal method to synthesize orthorhombic WO 3 •H 2 O nanoplates, which were dehydrated by solvent heat treatment to transform the orthorhombic phase WO 3 to the hexagonal phase. The self-assembly of hexagonal ZnIn 2 S 4 nanosheets on the surface of hexagonal WO 3 nanorods to prepare ZnIn 2 S 4 /WO 3 , Pt, and MnO 2 were selectively deposited on ZnIn 2 S 4 and WO 3 to prepare materials for photocatalytic overall water splitting under pure water without sacrificial agents. Self-assembled ZnIn 2 S 4 on the surface of WO 3 nanorods forms an ohmic contact at the interface and a Z-scheme heterojunction between two semiconductor-matched energy band structures (Figure 4d). The construction of Z-scheme heterojunctions effectively promotes the effective spatial separation of photogenerated charges and maintains the high redox capacity of the original semiconductor, resulting in materials with excellent photocatalytic activity.
In 2020, Zhao et al. prepared BiVO 4 @ZnIn 2 S 4 /Ti 3 C 2 MXene quantum dots (BV@ZIS/TC QDs) with layered core-shell structures by in situ growth combined with a two-step solvothermal strategy [93]. They first synthesized BiVO 4 microspheres by the hydrothermal method, on which ZnIn 2 S 4 nanosheets were grown in situ to form a hierarchical core-shell structure (Figure 5a). A solid contact surface and matching energy band structure was ensured between two semiconductors to construct the Z-scheme heterojunction, and then it was loaded with Ti 3 C 2 MXene QDs (TC QDs) as a co-catalyst to synthesize composite material for photocatalytic overall water splitting. Material electron transfer pathways are characterized, BiVO 4 @ZnIn 2 S 4 (BV@ZIS) constructs space-charge-separated all-solid z-structures, and co-catalyst TC QDs form Schottky barriers between the interfaces, ef-fectively promoting photogenerated carrier separation and migration rates (Figure 5b). The unique layered core-shell structure of the material increases light utilization, shortens the charge diffusion distance, increases more active sites, and effectively improves the photocatalytic activity. The material achieves a hydrogen yield of 102.67 µmol g −1 h −1 and an oxygen yield of 50.83 µmol g −1 h −1 under visible-light irradiation. method, on which ZnIn2S4 nanosheets were grown in situ to form a hierarchical core-shell structure (Figure 5a). A solid contact surface and matching energy band structure was ensured between two semiconductors to construct the Z-scheme heterojunction, and then it was loaded with Ti3C2 MXene QDs (TC QDs) as a co-catalyst to synthesize composite material for photocatalytic overall water splitting. Material electron transfer pathways are characterized, BiVO4@ZnIn2S4 (BV@ZIS) constructs space-charge-separated all-solid zstructures, and co-catalyst TC QDs form Schottky barriers between the interfaces, effectively promoting photogenerated carrier separation and migration rates (Figure 5b). The unique layered core-shell structure of the material increases light utilization, shortens the charge diffusion distance, increases more active sites, and effectively improves the photocatalytic activity. The material achieves a hydrogen yield of 102.67 µmol g −1 h −1 and an oxygen yield of 50.83 µmol g −1 h −1 under visible-light irradiation. In 2021, Wang et al. reported a Z-scheme heterojunction ZIS-WO/C-wood (Sv-ZnIn 2 S 4 -Ov-WO 3 /C-wood) catalyst photothermal integrated system for photocatalytic overall water splitting [94]. They first synthesized sulfur-deficient Pt/Sv-ZnIn 2 S 4 (Pt/ZIS) by hydrothermal and reduction methods, followed by oxygen-deficient WO 3 (WO) by solvothermal and calcination methods, and CoOx/WO by loading CoOx. Finally, Pt/ZIS and CoOx/WO were dispersed in water to form a solution and spin-coated onto C-wood, and vacuumassisted heat treatment was applied to construct a photocatalyst photothermal integration system for photocatalytic overall water splitting. ZIS and WO construct Z-scheme heterojunctions, forming a built-in electric field that effectively improves the efficiency of photogenerated charge separation and migration. The conductive material C-wood can act as an electron transfer medium, facilitating electron transfer, and can also use the photothermal effect to convert liquid water to water vapor, transforming the three-phase into a solid/gas two-phase and lowering the carrier recombination and photocatalytic reaction barrier. The photothermal-assisted Z-scheme heterojunction materials effectively increase the photocatalytic activity, achieving a hydrogen yield of 169.2 µmol g −1 h −1 and an oxygen yield of 82.5 µmol g −1 h −1 .
Zuo et al. synthesized TiO 2 -ZnIn 2 S 4 nanoflowers (TNZIS) by integrating ultrathin TiO 2 nanosheets into ZnIn 2 S 4 growth solutions in 2021 to construct direct Z-scheme heterojunctions for photocatalytic overall water splitting under co-catalyst-free conditions (Figure 6a) [95]. TiO 2 nanosheets (TiO 2 NSs) construct direct Z-scheme heterojunctions with ZnIn 2 S 4 to enhance the rate of photogenerated charge separation and inhibit recombination. TiO 2 has a larger work function and smaller Fermi energy level than ZnIn 2 S 4 (Figure 6c), causing electrons from ZnIn 2 S 4 to tend to flow into TiO 2 to balance the Fermi energy level and bend the energy band, forming a (−)TiO 2 /(+)ZnIn 2 S 4 built-in electric field at the heterojunction interface (Figure 6b,d) and promoting direct Z-scheme heterojunction formation. The introduction of TiO 2 NSs can effectively inhibit the aggregation of ZnIn 2 S 4 materials and increase the specific surface area and active sites of the materials, thus ef-fectively improving the photocatalytic activity. The best material, TNZIS-50, produces 214.9 µmol g −1 h −1 of hydrogen and 81.7 µmol g −1 h −1 of oxygen.
( Figure 6a) [95]. TiO2 nanosheets (TiO2 NSs) construct direct Z-scheme heterojunctions with ZnIn2S4 to enhance the rate of photogenerated charge separation and inhibit recombination. TiO2 has a larger work function and smaller Fermi energy level than ZnIn2S4 (Figure 6c), causing electrons from ZnIn2S4 to tend to flow into TiO2 to balance the Fermi energy level and bend the energy band, forming a (−)TiO2/(+)ZnIn2S4 built-in electric field at the heterojunction interface (Figure 6b,d) and promoting direct Z-scheme heterojunction formation. The introduction of TiO2 NSs can effectively inhibit the aggregation of ZnIn2S4 materials and increase the specific surface area and active sites of the materials, thus effectively improving the photocatalytic activity. The best material, TNZIS-50, produces 214.9 µmol g −1 h −1 of hydrogen and 81.7 µmol g −1 h −1 of oxygen. In 2022, Zhang et al. designed and prepared a direct Z-scheme heterojunction BiFeO 3 /ZnIn 2 S 4 with ferroelectric polarization and internal electric field synergy by a two-step solvothermal method to achieve photocatalytic overall water splitting without a co-catalyst [96]. They prepared BiFeO 3 by the solvothermal method and then grew ultrathin ZnIn 2 S 4 nanosheets on BiFeO 3 polyhedral particles to prepare BiFeO 3 /ZnIn 2 S 4 . Chalcogenide ABO 3 -type ferroelectric semiconductors BiFeO 3 and ZnIn 2 S 4 construct Z-scheme heterojunctions. Due to the different Fermi energy levels of BiFeO 3 and ZnIn 2 S 4 forming a built-in electric field, the ferroelectric polarization and internal electric field effectively promote photogenerated carrier migration and separation efficiency. The increased specific surface area and rich pore distribution of the material provide more catalytic active sites, thus effectively increasing the photocatalyst activity and achieving a hydrogen yield of 87.3 µmol g −1 h −1 and an oxygen yield of 42.3 µmol g −1 h −1 .
Yang et al. reported a direct Z-scheme heterojunction BiOBr/ZnIn 2 S 4 (BOB/ZIS) with atomic contact surfaces [97]. They constructed direct Z-scheme heterojunctions with atomic contact surfaces by in situ growth of ZnIn 2 S 4 nanosheets on the surface of BiOBr nanosheets using a solvothermal method to achieve photocatalytic overall water splitting without sacrificial agents and co-catalysts ( Figure 7a). As shown in Figure 7b, the disappearance of the Bi-O peak in X-ray photoelectron spectroscopy (XPS) O 1s and the appearance of Bi-S stretching vibrations in the composite at 1116 cm −1 in the FTIR spectrum of BiOBr prove (Figure 7c) that the introduction of TAA into the material during the growth of ZnIn 2 S 4 breaks the Bi-O bond and forms a Bi-S bond at the heterojunction interface, creating an interface with atomic-level seamlessness. Direct Z-scheme heterojunctions with atomic interfacial connections are effective in increasing the photogenerated charge transfer rate, achieving hydrogen yields of 628 µmol g −1 h −1 and oxygen yields of 304 µmol g −1 h −1 under visible-light irradiation in pure water.
pearance of the Bi-O peak in X-ray photoelectron spectroscopy (XPS) O 1s and the appearance of Bi-S stretching vibrations in the composite at 1116 cm −1 in the FTIR spectrum of BiOBr prove (Figure 7c) that the introduction of TAA into the material during the growth of ZnIn2S4 breaks the Bi-O bond and forms a Bi-S bond at the heterojunction interface, creating an interface with atomic-level seamlessness. Direct Z-scheme heterojunctions with atomic interfacial connections are effective in increasing the photogenerated charge transfer rate, achieving hydrogen yields of 628 µmol g −1 h −1 and oxygen yields of 304 µmol g −1 h −1 under visible-light irradiation in pure water.  [98]. They first synthesized the highcrystalline perylene-dicarboximide supramolecule (HC-PDI) with a highly ordered crystal structure and efficient water oxidation activity by nucleophilic addition and self-assembly, and then synthesized organic-inorganic hybrids by lateral epitaxial growth of ZnIn 2 S 4 nanosheets on highly crystalline HC-PDI nanorods ( Figure 7d). As ZnIn 2 S 4 and HC-PDI have different work functions and Fermi energy levels (Figure 7e), electrons in ZnIn 2 S 4 will spontaneously transfer to HC-PDI, creating an electron-consuming layer at the internal interface to generate energy band bending and built-in electric fields, building direct Z-scheme heterojunctions. As shown in Figure 7f, the differential charge density between HC-PDI and ZnIn 2 S 4 can reveal the charge transfer path. The strong covalent coupling between ZnIn 2 S 4 and HC-PDI provides a fast channel for the charge, and the Z-scheme heterojunction effectively promotes the photogenerated charge separation rate. The proper lateral epitaxial growth of ZnIn 2 S 4 gives the composite a layered dendritic structure, which facilitates the improvement of the specific surface area, pore size, pore volume, and visible light collection capacity of the material.
In 2023, Zou et al. constructed Z-scheme heterojunctions with ZnIn 2 S 4 based on the excellent oxidation activity of InVO 4 metal oxides [99]. They first synthesized InVO 4 nanosheets by the hydrothermal method under acidic conditions, and then formed layered InVO 4 @ZnIn 2 S 4 (InVZ) heterojunctions by the in situ growth of ZnIn 2 S 4 on them by the magnetic stirring reflux oil bath method. The InVO 4 work function is greater than that of ZnIn 2 S 4 , causing electrons in ZnIn 2 S 4 to spontaneously enter InVO 4 , resulting in the bending of the interfacial energy band within the heterojunction and the formation of (−)InVO 4 @(+)ZnIn 2 S 4 . The built-in electric field promotes the construction of Z-scheme heterojunctions. The material semiconductor types and the corresponding conduction and valence band positions were demonstrated by characterization of the Mott-Schottky and Tauc energy band gaps of InVO 4 and ZnIn 2 S 4 . The construction of InVO 4 @ZnIn 2 S 4 Z-scheme heterojunctions is further confirmed by their interleaved energy band structures. The Z-heterojunction of InVZ effectively promotes the separation and migration of photogenerated carriers and maintains the high redox activity of the catalyst. The composite material has a larger surface area and pore volume, providing more active sites, thus effectively improving the overall photocatalytic overall water splitting activity of the material. The optimum material InVZ-90 achieves a hydrogen yield of 153.3 µmol g −1 h −1 and an oxygen yield of 76.9 µmol g −1 h −1 under visible light without any co-catalyst.

Schottky Junctions
The Schottky junction is an interface between a metal and a semiconductor. As the metal and semiconductor have different escape work and Fermi energy levels, the Fermi energy level shifts when the metal and semiconductor come into contact until the Fermi energy level equilibrates [100,101]. When the semiconductor is an n-type semiconductor and the metal escape work is greater than the semiconductor Fermi energy level, a Schottky barrier will form at the interface, limiting the flow of electrons from the semiconductor to the metal, inhibiting photogenerated carrier compounding, and effectively improving photocatalytic activity.
In 2021, Cai et al. synthesized a yolk-shell ZnIn 2 S 4 -based photocatalyst NiCo 2 S 4 /ZnIn 2 S 4 / Co 3 O 4 for photocatalytic overall water splitting without sacrificial agents [102]. They first prepared nickel-cobalt-based metal-organic framework (MOF) materials as precursors by the solvothermal method, synthesized yolk-shell NiCo 2 S 4 (NCS) with semi-metallic properties by sulfidation reaction and heat treatment, then grew ZnIn 2 S 4 nanosheets in situ on the surface of NCS by low-temperature solvothermal method, and finally decorated Co 3 O 4 nanoparticles to prepare photocatalysts (Figure 8a). The characteristic peak of NiCo-glyceric acid disappeared and shifted to NiCo 2 S 4 when vulcanized at different vulcanization temperatures for 8 h. The precursors were vulcanized at 150, 180, and 210 • C for 8 h to form ball-in-ball hollow spheres, yolk-shell hollow spheres, and a single hollow sphere, respectively, with the yolk-shell structure providing more active sites and improving the photocatalytic activity of the material. As shown in Figure 8b,c, based on the energy band diagrams of ZnIn 2 S 4 and NCS as well as the energy band diagrams after contact, it is shown that the semi-metallic NCS forms a Schottky-specific heterojunction with ZnIn 2 S 4 to facilitate charge transfer. The photocatalyst has a unique yolk-shell structure that locates reduction and oxidation sites on the inner and outer surfaces of ZnIn 2 S 4 , respectively, allowing for directional charge separation, inhibition of inverse reactions, and providing more active sites.
Catalysts 2023, 13, x FOR PEER REVIEW 18 of 27 nanosheets in situ on the surface of NCS by low-temperature solvothermal method, and finally decorated Co3O4 nanoparticles to prepare photocatalysts (Figure 8a). The characteristic peak of NiCo-glyceric acid disappeared and shifted to NiCo2S4 when vulcanized at different vulcanization temperatures for 8 h. The precursors were vulcanized at 150, 180, and 210 °C for 8 h to form ball-in-ball hollow spheres, yolk-shell hollow spheres, and a single hollow sphere, respectively, with the yolk-shell structure providing more active sites and improving the photocatalytic activity of the material. As shown in Figure 8b,c, based on the energy band diagrams of ZnIn2S4 and NCS as well as the energy band diagrams after contact, it is shown that the semi-metallic NCS forms a Schottky-specific heterojunction with ZnIn2S4 to facilitate charge transfer. The photocatalyst has a unique yolkshell structure that locates reduction and oxidation sites on the inner and outer surfaces of ZnIn2S4, respectively, allowing for directional charge separation, inhibition of inverse reactions, and providing more active sites. In 2022, Liu et al. designed and prepared a sandwich structure material Nb4C3Tx MXene@ZnIn2S4-OH by in situ growth and peroxide plasma post-treatment [103]. They first prepared uniformly dispersed accordion-like multilayer Nb4C3Tx MXenes by selective chemical etching, then ultrathin ZnIn2S4 was epitaxially grown on their surface to In 2022, Liu et al. designed and prepared a sandwich structure material Nb 4 C 3 Tx MXene@ZnIn 2 S 4 -OH by in situ growth and peroxide plasma post-treatment [103]. They first prepared uniformly dispersed accordion-like multilayer Nb 4 C 3 Tx MXenes by selective chemical etching, then ultrathin ZnIn 2 S 4 was epitaxially grown on their surface to synthesize the sandwich structure Nb 4 C 3 Tx MXene@ ZnIn 2 S 4 at the double heterojunction interface (Figure 9a), followed by a peroxy plasma technique to generate many hydroxyl functional groups to obtain photocatalysts for photocatalytic monolithic water splitting in the absence of sacrificial agents. The work function of ZnIn 2 S 4 and Nb 4 C 3 Tx MXene shows that the transfer of electrons from ZnIn 2 S 4 to Nb 4 C 3 Tx MXene at the interface causes the ZnIn 2 S 4 energy band to rise, holes remain in ZnIn 2 S 4 , and the Nb 4 C 3 Tx MXene@ZnIn 2 S 4 photocatalyst forms a Schottky junction (Figure 9b,c). The photocatalyst promotes photogenerated carrier transfer and has a larger surface area and pore size and pore volume for rapid adsorption of water molecules. In addition, the OH functional group on the surface of ZnIn 2 S 4 collects photogenerated holes, and the unique eggshell-type structure of the composite material allows the generated H 2 and O 2 to be distributed internally and externally, achieving spatial separation of photogenerated carriers and suppressing the inverse reaction. The photocatalyst achieves a hydrogen yield of 53.

Loaded Co-Catalyst
Photocatalytic overall water splitting is mainly based on three basic processes of photocatalysis. The low overall photogenerated charge separation and migration efficiency and the severe compounding of photogenerated charges limit the catalytic photocatalytic activity due to the Coulombic force constraint and the high potential barrier of the transfer process, as well as the unfavorable thermodynamic process of water generation of H2 and O2. In this regard, the loading of co-catalysis on semiconductors is a preferable strategy. When a semiconductor is loaded with a co-catalyst, photogenerated electrons and photogenerated holes migrate to the co-catalyst for reduction-oxidation reactions, effectively facilitating photogenerated charge separation and migration. In addition, loaded co-catalysts have the advantage of reducing the activation energy, inhibiting photo-corrosion, and providing an abundance of surface reaction sites [104,105]. Unlike modification strategies such as doping and the construction of heterojunctions, catalysts loaded with cocatalysts can usually be synthesized using photo-deposition.
Suitable co-catalysts are available for both the hydrogen and oxygen production halves of the photocatalytic overall water splitting. The main hydrogen-dissolving co-catalysts are noble metals such as Pt and Au, transition metal monomers such as Co and Ni, transition metal sulfides such as NiP and MoS2, and phosphides. The main oxygen-dis-

Loaded Co-Catalyst
Photocatalytic overall water splitting is mainly based on three basic processes of photocatalysis. The low overall photogenerated charge separation and migration efficiency and the severe compounding of photogenerated charges limit the catalytic photocatalytic activity due to the Coulombic force constraint and the high potential barrier of the transfer process, as well as the unfavorable thermodynamic process of water generation of H 2 and O 2 . In this regard, the loading of co-catalysis on semiconductors is a preferable strategy. When a semiconductor is loaded with a co-catalyst, photogenerated electrons and photogenerated holes migrate to the co-catalyst for reduction-oxidation reactions, effectively facilitating photogenerated charge separation and migration. In addition, loaded co-catalysts have the advantage of reducing the activation energy, inhibiting photo-corrosion, and providing an abundance of surface reaction sites [104,105]. Unlike modification strategies such as doping and the construction of heterojunctions, catalysts loaded with co-catalysts can usually be synthesized using photo-deposition.
Suitable co-catalysts are available for both the hydrogen and oxygen production halves of the photocatalytic overall water splitting. The main hydrogen-dissolving co-catalysts are noble metals such as Pt and Au, transition metal monomers such as Co and Ni, transition metal sulfides such as NiP and MoS 2 , and phosphides. The main oxygen-dissolving cocatalysts are noble metal oxides such as RuO 2 and IrO 2 , and transition metal oxides such as CoOx [106]. In photocatalytic overall water splitting systems, we usually load hydrogenand oxygen-precipitating co-catalysts onto the catalyst separately to reduce the catalyst surface potential barrier, improve the efficiency of photogenerated charge separation and migration, and better achieve photocatalytic overall water splitting. For example, CdS loaded with Pt and Ru complexes as hydrogen and oxygen production co-catalysts, Mgdoped BaTaO 2 N loaded with Cr 2 O 3 /(Na)Rh and IrO 2 as hydrogen and oxygen production co-catalysts, and Ge 3 N 4 loaded with spatially separated mixed cathode and anode cocatalysts, and CoOx-Mo 2 N all enable photocatalytic monolithic water splitting [107,108].
The photocatalytic activity can be influenced by adjusting the amount of co-catalyst loading, the type of co-catalyst, the shape of the co-catalyst, etc. In 2023, Jing et al. prepared spatially separated double-coordinated co-catalyst ZnIn 2 S 4 composites [109]. They first prepared the hexagonal crystalline phase ZnIn 2 S 4 by the hydrothermal method, and then prepared ZnIn 2 S 4 -Pt-Cr and ZnIn 2 S 4 -Rh-Cr by photo-deposition with Pt-Cr and Rh-Cr co-catalysts loaded on the surface of ZnIn 2 S 4 . The elements Pt and Rh are used as water reduction sites and Cr as water oxidation sites to promote hydrogen and oxygen production reactions, respectively, for overall water splitting. The energy-dispersive X-ray spectroscopy (EDS)-scanned elemental signals of Rh, Cr, and Pt elements at different positions to probe the loading position of the co-catalysts showed that: Pt and Cr elements had consistent signal changes and were hybrids; Rh and Cr elements had different signal changes and achieved spatial separation. In addition, X-ray photoelectron spectroscopy (XPS) demonstrated the formation of Rh-S bonds in the ZnIn 2 S 4 -Rh-Cr material, facilitating the spatial separation of the co-catalyst. The materials loaded with double-assisted catalysts can effectively improve the separation and migration efficiency of photogenerated carriers, inhibit photo-corrosion of the materials, and improve the photocatalytic performance of the materials. The photocatalytic overall water splitting performance of the loaded spatially separated dual-assisted catalyst ZnIn 2 S 4 -Rh-Cr composite was twice that of the ZnIn 2 S 4 -Pt-Cr material, achieving hydrogen yields of 5.9 µmol h −1 and oxygen yields of 2.9 µmol h −1 at AM 1.5 G. As shown in Table 2, in recent years, researchers have achieved overall water splitting of ZnIn 2 S 4 -based photocatalysts through modification strategies such as doping, vacancies, construction of heterojunctions, and loading of co-catalysts.

Conclusions and Outlook
In a two-carbon context, the use of clean, renewable solar energy is an ideal solution to current energy and environmental problems. Photocatalytic overall water splitting, which produces clean, high-energy-density hydrogen without sacrificial agents, is the ideal, low-cost method for solar-chemical energy conversion in a variety of solar applications. Compared with oxides and nitrides, ZnIn 2 S 4 as a ternary metal sulfide has the advantages of tunable band gap, satisfying the energy band of photocatalytic overall water splitting, strong photostability, and easy preparation, which makes it a more ideal material for photocatalytic overall water splitting. In this study, we systematically review recent advances in ZnIn 2 S 4 -based photocatalysts for photocatalytic overall water splitting in solar-chemical energy conversion. This study mainly introduces the basic principles of its photocatalytic overall water splitting, the properties of the ZnIn 2 S 4 photocatalyst, including its crystal configuration and energy band structure, the main synthesis methods, and morphology. It also reviews the research progress of photocatalytic overall water splitting of ZnIn 2 S 4based photocatalytic materials, including modification strategies such as elemental doping, vacancy defects, the construction of heterojunctions, and the loading of co-catalysts.
Although good progress has been made in the current study of ZnIn 2 S 4 -based photocatalysts for water splitting, there are still some limitations of ZnIn 2 S 4 -based photocatalysts for water splitting that limit their wide application. The overall photogenerated charge separation and utilization efficiency are low due to the Coulombic force constraint between the photogenerated electrons and holes and the interfacial potential barriers between charge transfers, which are much higher than the photogenerated electrons and holes that they separate and migrate to the redox sites to participate in the reaction. Furthermore, although ZnIn 2 S 4 has a good light collection capacity, it is mainly responsive to visible light and does not utilize the near-infrared light, which is nearly 50% of solar energy, and the inevitable existence of ZnIn 2 S 4 -based photocatalysts as metal sulfides with their photo-corrosive fluxes affects their photocatalytic overall water splitting activity. Therefore, more efforts are still needed to make ZnIn 2 S 4 -based photocatalysts better for use in photocatalytic overall water splitting systems: (1) Building active crystal surfaces. Very few studies have systematically addressed the relationship between the active crystal plane of ZnIn 2 S 4 photocatalysts and the overall water splitting activity of photocatalysis. Usually, photocatalytic reactions occur on the surface of the material, so the specific surface area of the material and the exposed crystalline surface all affect the catalyst activity. Particularly due to the anisotropy of crystals, different crystal faces have different structures and atomic arrangements and usually exhibit different properties. It is important to clarify the relationship between different crystal types and their different crystal faces and photocatalytic activity, and to explore which crystal exposures alter the conduction/valence band position of the material and enhance electron reduction/hole oxidation, and which crystal exposures accumulate photogenerated charges/holes. Improving the photogenerated charge separation and migration efficiency and reducing photo-corrosion can be achieved by modulating the crystalline surface of ZnIn 2 S 4 -based photocatalysts. Therefore, if the relationship between the different crystalline facets of ZnIn 2 S 4 and photocatalytic activity can be clarified, we can better design photocatalytic materials with higher activity and significantly improve their photocatalytic overall water splitting performance.
(2) Photothermal assistance for broad spectrum utilization. Most current applications for photocatalytic monolithic water splitting ZnIn 2 S 4 -based materials respond to visible light and largely fail to utilize near-infrared light, which accounts for approximately 50% of sunlight; therefore, expanding the efficiency of light utilization is essential to achieve high-efficiency photocatalytic overall water splitting. Due to the inherent nature of ZnIn 2 S 4 materials, most current ZnIn 2 S 4 -based photocatalytic overall water splitting systems employ very limited modification strategies such as doping, vacancies, and the construction of heterojunctions to extend the photo-response range. The combination of photothermal assistance with ZnIn 2 S 4 -based photocatalysts is a very promising strategy for enhancing overall water splitting due to the advantages of photothermal catalysis in terms of wider spectral utilization, enhanced free carrier concentration, and promotion of photogenerated carrier separation. Further efforts are still needed to explore the design of photothermal-assisted ZnIn 2 S 4 -based photocatalysts to achieve broad-spectrum photocatalytic total hydrolysis and the development of low-sink cost materials to replace expensive precious metals.
(3) Morphological adjustments. By adjusting the morphology of ZnIn 2 S 4 -based photocatalytic materials, we can increase the specific surface area, pore size, and volume, and increase the active site to improve the light collection capacity, such as hollow structures for broadband absorption and ultra-thin nanosheets to shorten the photogenerated charge migration efficiency. The current sandwich structure and yolk-shell structure in ZnIn 2 S 4 -based photocatalytic overall water splitting applications have spatially separated water redox sites, allowing for directional separation of photogenerated charges, improved photocatalytic activity, as well as a layered core-shell structure to enhance incident light scattering and reflection effects and improve light utilization efficiency. Although the current modified morphologies are all very good at improving the overall photocatalytic overall water splitting activity, there is still room for improvement. Further research is needed to determine whether some of the complex morphological synthesis steps and materials used can be simplified and cost-effective, whether the prepared materials are stable, and whether morphological adjustments can be made to the ZnIn 2 S 4 native catalyst to achieve single-component photocatalytic overall water splitting.