Chalcogenides and Chalcogenide-Based Heterostructures as Photocatalysts for Water Splitting

Abstract

Chalcogenides are essential in the conversion of solar energy into hydrogen fuel due to their narrow band gap energy. Hydrogen fuel could resolve future energy crises by substituting carbon fuels owing to zero-emission carbon-free gas and its eco-friendliness. The fabrication of different metal chalcogenide-based photocatalysts with enhanced photocatalytic water splitting have been summarized in this review. Different modifications of these chalcogenides, including coupling with another semiconductor, metal loading, and doping, are fabricated with different synthetic routes that can remarkably improve the photo-exciton separation and have been extensively investigated for photocatalytic hydrogen generation. In this direction, this review is undertaken to provide an overview of the enhanced photocatalytic performance of the binary and ternary chalcogenide heterostructures and their mechanisms for hydrogen production under irradiation of light.

1. Introduction

Metal oxides and chalcogenides are semiconductors that possess conductivities between those of conductors and insulators and these materials are widely researched semiconductor materials. Due to their stability, resistance to photo-corrosion, non-toxicity, and inexpensive preparation, and the metal oxides such as TiO₂, ZnO, and SnO₂ are commonly applied as photocatalysts [1,2,3,4]. Despite these favorable characteristics, a major drawback of using these metal oxides as photocatalysts is their large band gap energies (≥3 eV) [5]. Such wide band gaps only enable them to absorb the ultraviolet portion of the solar spectrum (~4% of the total solar energy), which restricts widespread practical applications [2,3,4]. Many strategies have been devised to prepare metal oxides with narrower band gaps for improved harvesting of visible light [4,6,7]. However, the scope for band gap tuning by doping and other strategies is limited, and much larger decreases in band gap can be achieved by moving outside the oxide compositional space.

Chalcogenides, compounds comprising one or more electropositive elements and at least one chalcogen ion (S²⁻, Se²⁻, or Te²⁻) [8], are well known for their narrow band gap energies [9,10]. These compounds continue to attract attention due to their numerous desirable properties, including narrow band gap energy, low toxicity, biocompatibility, low cost, and facile synthesis [11,12]. The utilization of chalcogenides and chalcogenide-based semiconductor materials in photocatalytic applications has been reported widely, mainly because of their narrow band gap energies that enable efficient harvesting of visible light [13].

Owing to the devastating impact of conventional fossil fuel use on the environment and the growing demand for energy, an increasing number of countries, companies, and researchers are considering hydrogen (H₂) energy as a potential solution to the pressing environmental and sustainability problems associated with the current global energy system [14]. H₂ is an alternative source of energy because they are clean, non-toxic, eco-friendly, and renewable. Unlike other alternative energy sources, H₂ has the advantage of producing only water as a by-product upon consumption, which does not cause any pollution or release greenhouse gases [15]. Moreover, the potential clean and green energy source H₂ as a replacement for fossil fuels can be produced in large quantities using simple and low-cost methods [16]. However, H₂ in nature exists in combination with other elements, such as oxygen in water, thus, methods to obtain pure hydrogen should be considered. The production of H₂ employing visible light is one of the most promising approaches for sustainable energy and environmental problems.

Water splitting is a process whereby water molecules are separated into oxygen and hydrogen. The equation for the chemical reaction is as follows:

2H₂O → 2H₂ + O₂

Since pure water in nature does not absorb solar energy readily, a photocatalyst is required for the water-splitting process [17]. The reaction takes place in three steps as shown in Figure 1: step (1) to generate photoexcited electron-hole (e⁻/h⁺) pairs, the photocatalyst absorbs incoming photons with energies greater than the band gap of the material, step (2) the photogenerated carriers separate and migrate to the catalyst surface, and step (3) surface-adsorbed species (protons, H₂O molecules, and intermediates) are reduced to form hydrogen and oxidized to form oxygen by photogenerated electrons and holes, respectively [18]. According to Maeda and co-workers, the first and second steps depend on the electronic structure and properties of the semiconductor used. In general, high crystallinity promotes water-splitting activity because the density of defects acting as recombination centers is low [19]. In contrast, the third step occurs in the presence of a solid co-catalyst, which is usually a noble metal such as platinum or rhodium, or a metal oxide such as nickel oxide. The loading of the co-catalyst onto the photocatalyst surface provides active sites and lowers the activation energy required for H₂/O₂ gas evolution.

There are several factors affecting the photocatalytic H₂O splitting as shown in Figure 2. The solar-to-hydrogen conversion efficiency of a material is mainly based on (1) light absorption, (2) e⁻/h⁺ photogeneration, and (3) separation and migration of e⁻/h⁺. There are a few principles to keep in mind, including the band gap energy, stability, the use of sacrificial reagents, solution pH, band structure and alignment, effective mass, carrier transport, crystallinity, and surface area of the material used.

According to Maeda and co-workers, photocatalysts for water splitting under visible-light irradiation must satisfy two requirements; the band gap energy should fall in the range of 1.23 eV (the thermodynamic minimum) to 3.0 eV (the edge of the visible spectrum), and the conduction and valence band edges should align with the H⁺/H₂ and H₂O/O₂ redox potentials [19]. In practice, the lower end of the band gap range must be considerably larger than 1.23 eV, perhaps as high as ~2.4 eV, to account for entropic losses in the photocatalyst as well as concentration and kinetic over potential losses. Apart from that, the electron and hole-effective masses are among the key factors for efficient H₂O splitting. For a given mean free time between collisions, a smaller effective mass translates into larger carrier mobility and thus faster charge transport to the semiconductor–liquid interface. In general, the effective mass depends on the crystallographic direction, and therefore certain directions are more favorable for charge transport.

When selecting the material for H₂O splitting, the band edge position is crucial. Their VB should be more positive than the O₂ generation potential, whereas the CB should be more negative than the H₂ generation potential [20]. In addition to possessing a suitable band gap and band edge alignment, photocatalytic materials must have adequate light absorption in the visible region, which accounts for ~43% of the solar energy incident on the earth. The crystallinity and surface-active sites of the selected materials are also important for e⁻/h⁺ transfer and oxygen evolution reaction (OER)/hydrogen evolution reaction (HER), respectively. Materials with high crystallinity and few defects can reduce the e⁻/h⁺ recombination. The rate of OER/HER reactions should be larger than backward reactions, which is mostly depending on the number of active sites. Moreover, the material chosen should be stable either in acidic or basic electrolytes.

The present review provides an overview of recent literature on chalcogenides and chalcogenide-based heterostructures, with an emphasis on their synthesis and application for H₂ production by photocatalytic water splitting and the associated mechanisms. The main developments in binary and ternary chalcogenides of different metals including cadmium, copper, gallium, molybdenum, tin, titanium, vanadium, and zinc as well as their heterostructures are summarized. The relevant geometric structures and band structures of these chalcogenides as well as the key optical, electrical, and photocatalytic properties are also highlighted. Finally, the prospects of chalcogenide-based photocatalysts are discussed, and some general conclusions are drawn. The following sections cover the attributes of various chalcogenides and chalcogenide-based heterostructures that have recently been applied to photocatalytic water splitting [19,21].

2. General Synthesis Approaches of Chalcogenides

Although various synthesis methods of chalcogenides have been reported, synthesizing chalcogenides with high stability and to be photocatalytically active simultaneously is very challenging. Appropriate temperature, pressure, metal precursors, source of sulfur, and solvent combinations are important to synthesize these materials. Researchers have developed various synthesis routes for the fabrication of chalcogenides including hydrothermal [22], hot injection [23], solvothermal [24], and sol-gel [25]. Table 1 shows the different synthesis methods of chalcogenides. Gu et al. have synthesized AgInS₂ using a low-temperature liquid method and doped it with Mn²⁺ for H₂O splitting [26]. They yielded flower-like AgInS₂ spheres with sizes in the range of 200−800 nm. While the particles of Mn-doped AgInS₂ (1:100) are spherical with uniform particle sizes of about 300–400 nm. The calculated band gap energies of AgInS₂ and Mn-doped AgInS₂ (1:100) were found to be 1.63 and 1.52 eV, respectively. The H₂ production rate of AgInS₂ is 53 μmol h⁻¹g⁻¹. After Mn doping, the H₂ production rate of Mn-doped AgInS₂ (1:100) increases slightly, reaching 73 μmol h⁻¹g⁻¹. The hydrogen generation of Mn-doped AgInS₂ (1:100) was enhanced by loading multi-walled carbon nanotubes, reaching 105 μmol h⁻¹g⁻¹, which is ~2 times of AgInS₂ and ~1.4 times of Mn-doped AgInS₂ (1:100).

Hydrothermal involves a chemical reaction in an aqueous solution at elevated temperature and high pressure. For instance, Kannan et al. have hydrothermally synthesized Cd_0.5Zn_0.5S using Cu(NO₃)₂·3H₂O, Na₂WO₄·2H₂O and L-Cysteine as the starting materials at different temperatures (120, 140, 160, 180, and 200 °C) for 24 h [22]. Different morphologies, including flake-like and sponge-like examples of Cd_0.5Zn_0.5S, were observed. Moreover, they found that Cu₂WS₄ synthesized at the high temperature of 180 °C and exhibited excellent antibacterial activity against different bacterial strains. This shows that precise control over the reaction conditions, including pressure, time, pH value, concentration, and temperature are necessary for the successful synthesis of chalcogenides. In another study, Yang et al. have successfully fabricated NiS₂ using the hydrothermal method and Ni(NO₃)₃·6H₂O as Ni source and thioacetamide as the source of sulfur.

Solvothermal is similar to the hydrothermal method, but it uses a non-aqueous solvent (solvent other than water). For example, Wang et al. have used different Cd(CH₃COO)₂·2H₂O and sulfur powder ratios (1:1, 4:5, 2:3, 4:7, 1:2, and 1:3), precursor concentrations (1.5 and 6 mmol), temperatures (100, 140, 180, and 220 °C), and reaction times (4, 24, and 60 h) to yield CdS with different sizes and morphologies using solvothermal method [27]. The authors reported that the morphology, phase composition, and crystallinity of CdS were highly influenced by the precursor ratio, precursor concentration, temperature, and reaction time. The Cd concentration regulated the morphology of CdS morphology due to its effect on the formation of shape-determinant CdS nuclei. Nanorods, multipods, and triangular-like shape CdS nanocrystals were obtained with 1.5, 3, and 6 mmol of Cd, respectively. The arm diameter in CdS multipods increases from 10 to 60 nm with the sulfur concentration increasing when the cadmium concentration is maintained at 3 mmol.

Table 1. Different preparation methods of chalcogenides.

Synthesized Materials	Synthesis Method	Metal Precursor Used	Source of Sulfur	Solvent Used	Ref.
AgInS2	Low-temperature liquid method	AgNO3 In(NO3)3·xH2O	Thioglycollic acid Thioacetamide	Water	[26]
AgInS2	Microwave	AgNO3 In(NO3)3∙ 4.5H2O	Sulfur powder	Glycerol, Oleic acid, Oleylamine, 1-dodecanethiol and 1-octadecene	[28]
Cd0.5Zn0.5S	Hydrothermal	Cd(CH3COO)2·2H2O Zn(CH3COO)2·2H2O	Na2S·9H2O	Water	[29]
CdS	Solvothermal	Cd(CH3COO)2·2H2O	Sulfur powder	Dodecylamine	[27]
Cu2WS4	Hydrothermal	Cu(NO3)2· 3H2O Na2WO4 2H2O	L-Cysteine	Water	[22]
Cu2WSe4	Hot injection	CuCl2·2H2O WCl4	Se powder	Oleylamine	[23]
Cu2ZnSnS4	Hot injection	Cu(acac)2 Zn(OAc)2 ·2H2O Sn(OAc)4	1-dodecylthiol and tert-dodecylthiol	1-octadecene	[30]
CuCdS2	Solvothermal	Copper nitrate Cadmium acetate	Sodium thiosulphate pentahydrate	Ethylene glycol	[24]
CuS	Hydrothermal	Copper acetate dihydrate	Thiourea	Water	[31]
MoS2	One-pot liquid-phase reaction	(NH4)6Mo7O24	Na2S	Water	[32]
NiS2	Hydrothermal	Ni(NO3)3·6H2O	Thioacetamide	Water	[33]
VS2	Single-step chemical vapor deposition	VCl3	Sulphur powder	-	[34]
Zn0.5Cd0.5S	Combustion method	Zn(NO3)2⋅4H2O, Cd(NO3)2⋅6H2O	Thioacetamide	Water	[35]
ZnS	Co- precipitation	Zn(NO3)2	Na2S	Water	[36]

Table 1. Different preparation methods of chalcogenides.

Synthesized Materials	Synthesis Method	Metal Precursor Used	Source of Sulfur	Solvent Used	Ref.
AgInS2	Low-temperature liquid method	AgNO3 In(NO3)3·xH2O	Thioglycollic acid Thioacetamide	Water	[26]
AgInS2	Microwave	AgNO3 In(NO3)3∙ 4.5H2O	Sulfur powder	Glycerol, Oleic acid, Oleylamine, 1-dodecanethiol and 1-octadecene	[28]
Cd0.5Zn0.5S	Hydrothermal	Cd(CH3COO)2·2H2O Zn(CH3COO)2·2H2O	Na2S·9H2O	Water	[29]
CdS	Solvothermal	Cd(CH3COO)2·2H2O	Sulfur powder	Dodecylamine	[27]
Cu2WS4	Hydrothermal	Cu(NO3)2· 3H2O Na2WO4 2H2O	L-Cysteine	Water	[22]
Cu2WSe4	Hot injection	CuCl2·2H2O WCl4	Se powder	Oleylamine	[23]
Cu2ZnSnS4	Hot injection	Cu(acac)2 Zn(OAc)2 ·2H2O Sn(OAc)4	1-dodecylthiol and tert-dodecylthiol	1-octadecene	[30]
CuCdS2	Solvothermal	Copper nitrate Cadmium acetate	Sodium thiosulphate pentahydrate	Ethylene glycol	[24]
CuS	Hydrothermal	Copper acetate dihydrate	Thiourea	Water	[31]
MoS2	One-pot liquid-phase reaction	(NH4)6Mo7O24	Na2S	Water	[32]
NiS2	Hydrothermal	Ni(NO3)3·6H2O	Thioacetamide	Water	[33]
VS2	Single-step chemical vapor deposition	VCl3	Sulphur powder	-	[34]
Zn0.5Cd0.5S	Combustion method	Zn(NO3)2⋅4H2O, Cd(NO3)2⋅6H2O	Thioacetamide	Water	[35]
ZnS	Co- precipitation	Zn(NO3)2	Na2S	Water	[36]

Microwave-assisted synthesis is reported to be a clean, fast, and convenient synthetic route of chalcogenides. It works by applying microwave irradiation to chemical reactions which is based on efficient heating [5]. For example, Hu et al. have successfully synthesized AgInS₂ with tunable composition and optical properties using microwave-assisted synthesis [28]. In another study, the co-precipitation reaction between Na₂S solution and Zn(NO₃)₂ at room temperature produced ZnS [36]. The combustion method carried out by Tang et al. to produce Zn_0.5Cd_0.5S using Zn(NO₃)₂·4H₂O, Cd(NO₃)₂·6H₂O and thioacetamide [35]. Using water as the solvent, they heated the starting materials on the heating jacket until the gel-like precursor was formed, then the precursor was transferred into a Muffle furnace and heated at 300 °C for 15 min which resulted in the formation of yellow Zn_0.5Cd_0.5S solid. Chemical vapor deposition (CVD) is a technique that uses thermally induced chemical reactions at the surface of a heated substrate. For example, Gopalakrishnan et al. have deposited VS₂ on the surface of vertically aligned Si nanowires using the CVD method [34]. In the typical synthesis, they used VCl₃ and sulfur powder as the precursors of V and S, respectively.

Based on the findings, it can be observed that the properties of chalcogenides can be tuned by controlling the reaction parameters during synthesis. Therefore, the optimization of these parameters is of crucial importance to fabricate the desired properties of chalcogenides with enhanced H₂O splitting ability.

3. Binary Chalcogenides and Their Photocatalytic Water-Splitting Activities

H₂ is a clean fuel, and its usage can address many of the issues caused by using fossil fuels. It has several uses, as shown in Figure 3; it is widely used as a feedstock in the chemical industry to produce ammonia, methanol, and various fuels like diesel, gasoline, etc. It is also used as a transport fuel. It has several other applications in the production of metals and agricultural industries. A cost-effective and long-lasting chalcogenide-based photocatalysts can make the H₂ generating process more economical and suitable. The use of binary chalcogenides and their modifications (compounds consisting of only one chalcogen and one electropositive atom [10]) for photocatalytic water splitting will be discussed in the following subsections.

3.1. Cd-Based Chalcogenides

CdS and CdSe have both been used extensively in the fields of photocatalysis and photovoltaics, particularly in photoelectrochemical cells. Despite having a favorable bandgap for water splitting (~2.42 eV) [37,38] CdS is unstable in an aqueous solution under light irradiation due to photocorrosion. Therefore, in order to improve its photostability and photocatalytic activity, many studies have focused on synthesizing different morphologies of CdS together with the addition of co-catalysts and sacrificial agents [38]. Some of the different morphologies of CdS nanostructures that have been studied include hollow spheres [27], quasi-nanospheres [39], nanowires [39], and nanotubes [39], all of which can be synthesized by hydrothermal methods. Among these, CdS nanospheres were found to exhibit the highest H₂ evolution rate during photocatalytic testing in the presence of Na₂SO₃ and Na₂S hole scavengers.

In a recent study, one-dimensional CdS nanotubes displayed a remarkably high valence band edge compared with bulk CdS due to quantum confinement effects, and it was found that the photocatalytic efficiency of CdS nanotubes is higher than that of bulk CdS. Furthermore, the stabilized valence band, tubular structure, and strong p-d orbital hybridization led to a significant enhancement in photostability, making applications in aqueous solutions feasible [40].

In addition to the aforementioned studies focused on enhancing CdS through the preparation of different morphologies and sizes, another strategy to impart anti-photocorrosion properties to CdS is surface-modification with non-noble co-catalysts such as Ni, Ni₂P, and CuS-Ni_xP [41,42,43] as well as noble metals such as Pt and Au [44,45].

Another strategy to improve the photostability of CdS in an aqueous solution is to coat it with a thin layer of ZnO. Tso and co-workers have synthesized composite nanowires comprising CdS cores covered with thin ZnO shells, yielding CdS/ZnO core–shell nanowires [46]. These heterostructures provided improved stability compared with bare CdS nanowires and also led to reduced electron-hole recombination, causing an increase in H₂ evolution activity of over two orders of magnitude as shown in Figure 4. Investigations also revealed that nanowires with 10–30 nm-thick shells absorbed more incident light than nanowires with thinner ZnO shells and had a lower probability of electron-hole recombination, the combination of which led to higher H₂ evolution activity.

Several recent studies have examined photogenerated electron-hole pair separation in CdS/CuS nanocomposites [46]. Pan and co-workers found a high H₂ evolution rate of 561.7 μmol h⁻¹ during photocatalytic water splitting using a CdS/CuS nanocomposite [47]. In another work, a CdS/CuS nanostructured heterojunction was synthesized [48]. The prepared material was applied as a water-splitting photocatalyst and showed that the nanocomposites exhibit an enhanced rate of photocatalytic H₂ evolution under visible-light irradiation. It can be concluded that CuS as a non-noble catalyst can improve the stability of CdS for photocatalytic H₂ production in an aqueous solution. The enhancement in stability found in the CdS/CuS system can be explained by the transfer of holes from CdS to CuS, which inhibits the photocorrosion of CdS (Figure 5) [49].

Several authors have studied the factors affecting the performance of nanocrystalline CdSe quantum dots (QD) in photocatalytic water-splitting applications. Osterloh and co-workers showed a clear relationship between the extent of quantum size confinement and the photocatalytic water-splitting activity of suspended CdSe QDs. They found that the rate of H₂ evolution increased with QD size in nm as follows: 2.20 > 2.00 > 2.48 > 1.75 > 2.90 > 2.61 > 2.75 > 3.05 > 3.49 > 4.03 > 4.81. These results confirm that photocatalytic activity can be controlled by the extent of quantum confinement (Figure 6) [50].

Covalent organic frameworks (COFs) have drawn research interest owing to their stability, porous structure, large surface area, and tunable band structure [51]. It is an organic polymer arranged periodically with high crystallinity and high porosity, that is built from the integration of selected organic blocks via covalent bond linkage [52]. For instance, You et al. have synthesized COFs-p-phenylenediamine@CdS (COFs-Ph@CdS) using the hydrothermal method [53]. The authors reported that of the H₂ generation of bare CdS, COFs-Ph@CdS-x (x = 1, 2, 3, 4) photocatalysts were 16.4, 139.1,17.5, 37.0, and 15.2 μmol, respectively. The photocatalytic H₂ production activity of solid COFs-Ph@CdS-1 exhibited 29 and eight times higher compared to those of pristine COFs and CdS, respectively. The enhanced activity may be attributed to a stable coordination bond between Cd²⁺ ions and N atoms in COFs-Ph@CdS was formed, which served as a migrated channel for photo-generated carriers and their unique hollow structure that enables the production of more photo-generated electrons and holes and absorb light more efficiently.

High rates of photocatalytic H₂ production were demonstrated by Das and co-workers using a surface ligand that binds strongly to CdSe QDs via tridentate coordination [54]. These authors also reported the effects of surface stabilizing and solubilizing agent dynamics. In more recent work, the influence of ligand exchange on the photocatalytic activity of CdSe QDs in solar water-splitting cells was studied [55]. It was found that CdSe QDs bearing shorter ligands showed better photocatalytic activity when compared to pristine CdSe QDs in a photoelectrochemical cell. These results should assist with the design and optimization of photochemical energy conversion systems employing nanocrystalline CdSe QD photocatalysts.

In another study, Zhou et al. have successfully prepared peanut-chocolate-ball-like CdS/Cd for H₂ evolution [56]. They reported that CdS/Cd exhibited a high photocatalytic H₂ production of 95.40 lmol h⁻¹ which is about 32.3 times higher than bare CdS and displays exceptional photocatalytic stability over 205 h in comparison to CdS-based photocatalyst reported in previous studies. In recent studies, Li et al. 2% NiS/CdS present a high H₂ evolution rate of 18.9 mmol g⁻¹h⁻¹ in comparison to pristine CdS [57]. The improved activity may be attributed to the presence of an interface between NiS and CdS that efficiently promote the charge separation and NiS nanoparticles serve as highly active H₂ evolution sites.

3.2. Cu-Based Chalcogenides

Visible-light-active CuS/Au nanostructures were obtained by decorating CuS particles with Au nanoparticles, which were prepared by reducing HAuCl₄ on the CuS surface [58]. The as-synthesized nanostructures displayed excellent catalytic performance in electrochemical H₂ evolution tests, with good durability under acidic conditions. It was found that the CuS/Au nanostructured catalyst could produce a current density of 10 mA/cm² upon the application of only 0.179 V versus the reversible hydrogen electrode (RHE). The CuS/Au catalyst also proved to be an efficient photocatalyst for the degradation of organic pollutants under visible-light irradiation.

The coupling of CuS with metal oxide has been shown to enhance its photocatalytic activities. For instance, Chandra and co-workers have synthesized CuS/TiO₂ heterostructures with varying TiO₂ contents using a simple hydrothermal method for the photocatalytic evolution of H₂ [59]. The highest rate of visible-light photocatalytic H₂ production was 1262 μmol h⁻¹ g⁻¹, which is around 9.7 and 9.3 times faster in comparison to pristine TiO₂ nanospindles or CuS nanoflakes, respectively. The enhanced rate of H₂ evolution is attributable to increased light harvesting and more efficient charge separation when an optimum amount of CuS is deposited on TiO₂. Growing CuS nanoflakes on TiO₂ nanoparticles to form a CuS/TiO₂ heterostructure facilitates charge carrier separation at the heterostructure interface and thus enhances photocatalytic performance.

In another study, Dubale et al. fabricated nanocomposites comprising CuS and Cu₂O/CuO heterostructures with and without decoration by Pt nanoparticles using an in-situ growth method [60]. Cu₂O/CuO was prepared by straightforward anodization of Cu film, and a sputtering technique was used to deposit Pt nanoparticles onto the prepared Cu₂O/CuO/CuS photocathodes. The prepared heterostructures were found to be very promising and highly stable photocathodes for H₂ evolution under visible-light irradiation, with optimized Cu₂O/CuO/CuS photocathodes reaching photocurrent densities of up to 5.4 mA cm², some 2.5 times higher than that achieved by bare Cu₂O/CuO. Upon decorating the Cu₂O/CuO with both CuS and Pt, a further increase in photocurrent density to 5.7 mA/cm² was achieved due to the suppression of the charge carrier recombination.

Metal loading on the surface of Cu-based chalcogenides has been reported to improve the photocatalytic evolution of H₂. For instance, Ma and co-workers have reported that hydrothermally synthesized Au/CuSe/Pt nanoplates showed strong dual-plasmonic resonance and have been employed for photocatalytic H₂ generation under irradiation of visible and near-infrared [61]. The as-prepared Au/CuSe/Pt nanoplates exhibited outstanding H₂ generation activities of around 7.8 and 9.7 times those of Au/CuSe and Pt/CuSe composites, respectively. In the Au/CuSe/Pt system, light harvesting is enhanced by the plasmonic units (CuSe and Au), while Pt is mainly present as a co-catalyst promoting the H₂ evolution reaction as shown in Figure 7.

In a study conducted by Liang et al. CuS was deposited on the surface of CdS nanorods using the cation exchange method for generation of H₂ under the irradiation of light [62].

In a study by Hou et al., CuS has been used as a co-sensitizer along with CdS to improve photocatalytic H₂ production [63]. They decorated CdS–CuS on TiO₂ nanotube arrays using the hydrothermal method. The as-prepared material exhibited remarkably high photocatalytic H₂ evolution ability, and the photocatalytic H₂ production rate was about 62.02 μmol cm⁻²h⁻¹. In a different study, Mandari and co-workers synthesized a stable CuS/Ag₂O/g-C₃N₄ material using hydrothermal and precipitation methods [31]. The as-synthesized material displayed a remarkable photocatalytic H₂ production of 1752 μmol h⁻¹g⁻¹ which is considerably superior to those of the pristine CuS and g-C₃N₄. The enhanced H₂ production activity may be ascribed to the formation of heterojunctions which resulted in the increase in visible light harvested and suppressed the photogenerated charge carrier recombination.

3.3. Ga-Based Chalcogenides

Monolayers of GaS and GaSe have been studied as photocatalysts by several authors but their performance is limited by low optical absorption and inefficient electron-hole pair separation [64,65]. One strategy to overcome these limitations is the construction of van der Waals (vdW) heterostructures by pairing GaX (GaX, X = S, Se) with arsenene (two-dimensional arsenic), as demonstrated by Peng and co-workers using first-principle calculations [66]. Such GaX/As heterostructures possess band gaps and band alignment that satisfy the requirements for photocatalytic water splitting. In contrast to pristine GaX monolayers, the type of band gap in Se_0.5GaS_0.5/As and S_0.5GaSe_0.5/As changes favorably from indirect to direct as the interlayer distance is varied. Furthermore, these heterostructures exhibit transport anisotropy with high electron mobilities of up to ∼2000 cm² V⁻¹ s⁻¹, which facilitates photogenerated electron-hole pair separation and migration. All of the GaX/As heterostructures studied also show enhanced visible-light absorption beyond that of pristine GaX monolayers. It seems likely that the exceptional properties of GaX/As heterostructures will render them competitive with existing photocatalysts for solar-driven water splitting [66].

3.4. Mo-Based Chalcogenides

Molybdenum disulfide (MoS₂) is a two-dimensional (2D) material, and it tends to exhibit more remarkable photocatalytic properties than its bulk counterparts [67,68]. Li and co-workers have investigated the photocatalytic properties of pristine MoS₂ nanosheets and Ag-modified MoS₂ nanosheets. They found that the rate of photocatalytic H₂ production by Ag-modified MoS₂ nanosheets could reach 2695 μmol h⁻¹ g⁻¹. They also found that Ag-doped MoS₂ nanosheets exhibit superior stability and higher H₂ evolution efficiency compared with pristine MoS₂ nanosheets, which is mainly due to increased visible-light absorption with increasing Ag content (Figure 8) [69].

Photocorrosion-resistant material, MoS₂/Zn_0.5Cd_0.5S/g-C₃N₄, has been successfully synthesized by Tang et al. to produce H₂ under irradiation of visible light [35]. The authors reported that the as-prepared materials exhibited a maximum H₂ production rate of 4914 μmol g⁻¹h⁻¹ in Na₂S-Na₂SO₃ solutions which served as the sacrificial agent. The high evolution rate of H₂ was owing to the improved charge separation between the interfaces.

In another study, Yuan et al. designed 2D-2D MoS₂@Cu-ZnIn₂S₄ using the solvothermal method for photocatalytic production of H₂ [70]. The highest H₂ evolution rate of 5463 μmol g⁻¹h⁻¹ was observed for 6% MoS₂@Cu-ZnIn₂S₄ under irradiation of visible light which is 72 times higher than that of pristine Cu-ZnIn₂S₄. The excellent activity may be ascribed to the high visible light absorption property and abundant active sites for the H₂ evolution reaction to take place. Moreover, the production rate of H₂ remain unchanged after three cycles which suggested the high durability of the synthesized MoS₂@Cu-ZnIn₂S₄ for photocatalytic H₂ production.

An enhanced photocatalytic H₂ evolution was performed by Yin and co-workers with the maximum evolution rate of 872.3 mol h⁻¹ using MoS₂/C composites sensitized with Erythrosin B [32]. The enhanced performance was attributed to the efficient photo-generated charge transfer and separation between Erythrosin B and MoS₂ or C as well as the presence of more active sites for H₂ evolution. The photocatalytic evolution of H₂ could also be achieved through photoelectrochemical catalytic activities. For instance, Hassan and co-workers have reported the optimized MoS₂/GaN₂ exhibited significantly higher photoelectrochemical performance in comparison to the bare materials under the same visible conditions [71]. The as-prepared photoanode achieved efficient light harvesting with a photocurrent density of 5.2 mA cm⁻² which is 2.6 times higher than bare GaN. Moreover, MoS₂/GaN₂ exhibited enhanced applied-bias-photon-to-current conversion efficiency of 0.91%, while the reference GaN yielded an efficiency of 0.32%. The authors reported that the decrease in charge transfer resistance between the electrolyte and semiconductor interface and the improved separation charge carriers in the MoS₂/GaN heterostructures were all attributed to significant improvements in photocurrent density and efficiency.

MoS₂-based nanomaterials have been found to be an efficient visible light-responsive material for H₂ production. Based on the reported work, revealed that the surface modification of the MoS₂ could lead to an improved photocatalytic activity owing to the effective charge transfer and separation of photo-generated e⁻/h⁺ pairs.

New strategies have been developed to use MoSe₂-based nanosheets as photocatalysts for the H₂ evolution reaction. Zhao and co-workers achieved an enhancement in the photocatalytic activity of MoSe₂ by surface modification and doping with Ni and Co [72]. Their study found that Ni_0.15Mo_0.85Se₂ nanosheets exhibited the highest photocatalytic activity in both alkaline and acidic media. These results provide an attractive alternative to Pt-based catalysts.

3.5. Sn-Based Chalcogenides

Recently, it has been discovered that monolayers of the Sn-based mono- and dichalcogenides SnX and SnX₂ (X = S or Se) are promising candidates for photocatalytic water splitting. These materials exhibit very good visible light absorption, low exciton binding energies, and high carrier mobility. However, in their pure and strain-free state, the valence band edges of Sn-monochalcogenide monolayers are predicted to be too high in energy for effective water oxidation. This issue can potentially be overcome by crystal engineering to induce a moderate tensile strain, which is predicted to lower the valence band edge to a level suitable for water oxidation [21]. Li and co-workers reported that monolayers of both SnX and SnX₂ are expected to exhibit good optical absorption in the visible region of the solar spectrum, and the predicted sequence of visible light absorption strength is as follows: SnSe > SnSe₂ > SnS > SnS₂. Moreover, SnX and SnX₂ monolayers also have excellent carrier mobility, which results in the fast migration of photogenerated carriers to the surface. Hence, the possibility of rapid redox reactions on the surface of the photocatalyst.

In another study, Lei et al. successfully prepared SnS/g-C₃N₄ nanosheets using a facile ultrasonic and microwave heating approach, which formed intimate interfacial contact and a suitable energy band structure [73]. The optimized sample displayed enhanced photocatalytic H₂ evolution from H₂O with the aid of Pt as a co-catalyst in comparison to pure g-C₃N₄. The stability of the photocatalysts was significantly improved after being loaded with MoO₃ particles due to the formation of a Z-scheme heterojunction. Among all as-prepared samples, the 10% SnS/g-C₃N₄ exhibits the highest photocatalytic rate of 818.93 mmol h⁻¹g⁻¹ under AM1.5G irradiation, 2.90 times to pure g-C₃N₄, due to the matched energy band structure between g-C₃N₄ and SnS, which improves the separation efficiency of photo-generated carriers and hinders the recombination of hole-electron pairs. Additionally, SnS nanosheets have improved the light absorption efficiency of the prepared material and generated more catalytic active sites as well as shortening the carriers’ transport path as well.

Liu and co-workers have synthesized flower-like nanostructure SnS₂ with generated sulfur vacancies using a new simple strategy of ion exchange [74]. The morphology of the flower-like structure provides shorter carrier diffusion lengths and more surface-active sites. While, the presence of S vacancies helps introduce defect levels in SnS₂, which leads to a reduction in work function, eventually improving carrier density and separation efficiency. The well-engineered sample 5%Cu/SnS_2-x shows the highest photocatalytic activity with an H₂ yield of 1.37 mmol h⁻¹, which is about six times higher than that of pure SnS₂.

SnS₂/ZnIn₂S₄ composites were successfully fabricated by decorating SnS₂ nanosheets with ZnIn₂S₄ microspheres [75]. Geng and co-workers reported that the SnS₂/ZnIn₂S₄ composites showed superior photocatalytic properties for H₂ generation in comparison to pristine ZnIn₂S₄. Furthermore, a notable impact was observed on the photocatalytic activity of ZnIn₂S₄ when varying the mass ratio of SnS₂. The 2.5% SnS₂/ZnIn₂S₄ in particular exhibited the highest photocatalytic H₂ generation rate of 769 μmol g⁻¹h⁻¹, which was about 10.5 times the photocatalytic activity of pristine ZnIn₂S₄. The enhanced photocatalytic activity may be due to the formation of SnS₂/ZnIn₂S₄ heterojunction, which enabled highly active charge separation and transfers on the interface of SnS₂ and ZnIn₂S₄.

Li and co-workers have synthesized SnO₂/SnS₂ with excellent photocatalytic H₂ evolution performance under simulated light irradiation [76]. The material exhibited a high H₂ production rate of 50 μmol h⁻¹ which is 4.2 times higher than that of pure SnO₂ under the same condition. A different study by Mangiri et al. has successfully synthesized a stable CdS/MoS₂-SnS₂ for water splitting via the solvothermal method [77]. The as-prepared CdS/MoS₂-SnS₂ (6 wt%) exhibited the highest H₂ production rate of 185.36 mmol h⁻¹g⁻¹ which is much higher than pristine CdS (2.5 mmol h⁻¹g⁻¹) and 6 wt% of MoS₂-loaded CdS nanorods (123 mmol h⁻¹g⁻¹). The effective photocatalytic performance of MoS₂-SnS₂-loaded CdS may be ascribed to the ability of the material to harvest light effectively, better separation of e⁻/h⁺ pairs, formation of trapping sites, high active catalytic zones, migration of charge carriers towards the surface of a semiconductor, and suitable energy levels.

3.6. Ti-Based Chalcogenides

The tri-chalcogenide TiS₃ was found to be an effective photoanode for use in photoelectrochemical cells. Photocatalytic H₂ evolution rates of up to 1.80 ± 0.05 μmol min⁻¹ were obtained, corresponding to a photoconversion efficiency of ~7% [78]. Furthermore, the ternary compounds Ti_xNb_1-xS₃ (Nb-rich) and Nb_xTi_1−xS₃ (Ti-rich) were tested as photoanodes and compared with the corresponding binary trisulfides. The binary compounds were polycrystalline with a nanoribbon morphology and adopted the monoclinic TiS₃ (P21/m) and triclinic NbS₃ (P2) crystal structures, respectively. H₂ generation experiments revealed that the Ti-rich phase was a superior photocatalyst compared with the Nb-rich phase, exhibiting H₂ production rates of up to 2.2 ± 0.1 mol/min cm² [79]. Ti-based chalcogenides are not widely explored for their photocatalytic H₂ production activity. Therefore, further studies and optimizations are required.

3.7. V-Based Chalcogenides

Certain transition metals located on the first row of the periodic table can participate in the formation of tetra-chalcogenides thanks to their ability to adopt +4 oxidation states (Figure 9). Among these metals is vanadium, which can form the binary tetra-chalcogenide VS₄. Nanocomposites comprising VS₄ and graphene were evaluated for solar-driven water splitting by Guo and co-workers. Excellent performance was obtained due to the formation of C–S bonds with a π-conjugated structure, which assists with transferring electrons from the S 2p orbital to graphene [80]. The ternary tetra-chalcogenides Cu₃Nb_1−xV_xS₄ and Cu₃Ta_1−xV_xS₄, which exist as solid solutions and adopt the sulvanite structure, were successfully synthesized in a solid-state reaction by Ikeda and co-workers [81]. The band gaps of the synthesized materials were estimated to fall in the range of 1.6–1.7 eV, and the band structure of the Cu₃M_1−xV_xS₄ system varies with composition. It was found that for most compositions of solid solution, superior photocatalytic activity was achieved compared with the ternary Cu₃MS₄ (M = V, Nb, Ta) compounds [81].

Li et al. synthesized the VS₂@C₃N₄ heterojunction with S vacancies via the in situ supramolecular self-assembly method [82]. The as-synthesized material shows a remarkable synergistic H₂ production rate (9628 μmol g⁻¹h⁻¹) and wastewater degradation efficiency as well as stability, which is 16.0 times that of pristine C₃N₄. Based on the theoretical calculations and experiment findings, it shows that S vacancies have resulted in the formation of compact heterostructure and the reduction in the work function, which promotes interfacial carriers’ transfer and surface properties. The core–shell structure improves the stability of S vacancies.

In another study, Gopalakrishnan et al. synthesized a core–shell heterostructures photocathode consisting of VS₂ deposited on a silicon nanowire surface via single-step chemical vapor deposition for H₂ evolution under solar irradiation [34]. The core–shell nanowire heterostructure photocathode displayed an outstanding solar-assisted water reduction performance at pH∼7 over a pristine photocathode. The as-prepared material produces about 23 μmol cm⁻² h⁻¹ (at 0 V vs. RHE) of H₂ gas. Recently, Zhong et al. have doped Ni₃S₂ with nitrogen and coupled it with VS₂ using the hydrothermal method [83]. Based on their results, Ni₃S₂/VS₂ without nitrogen doping exhibited improved oxygen evolution reaction (OER) performance with an extremely low overpotential of 227 mV at 10 mA/cm² which may be attributed to the presence of high number of active sites and excellent interfaces. Moreover, Ni₃S₂/VS₂ with nitrogen doping shows high electrocatalytic hydrogen evolution reaction (HER) performance with a low HER overpotential (151 mV at 10 mA/cm²) and this is due to the presence of nitrogen doping that significantly improves the conductivity and increases the number of catalytic active sites.

3.8. Zn-Based Chalcogenides

Kurnia and co-workers have investigated the performance of ZnS thin films during photoelectrochemical H₂ generation [84]. They found that ZnS is an inexpensive photocatalyst that exhibits activity under visible-light irradiation without the addition of a co-catalyst. The ZnS thin films were estimated to have band gaps of ~2.4 eV, and photocurrent densities exceeding 1.5 mA/cm² were achieved under visible-light irradiation (λ ≥ 435 nm). These photocurrents are remarkably high for H₂ generation by undoped ZnS under visible-light irradiation.

A modification of ZnO nanosheets, using a thin layer of ZnS, was developed to enhance visible light absorption, leading to improved photoelectrochemical activity in water-splitting experiments [85,86]. Sánchez-Tovar and co-workers have prepared ZnO/ZnS heterostructures using anodization of Zn in glycerol/water/NH₄F electrolytes under controlled hydrodynamic conditions in the presence of various concentrations of Na₂S [85]. In another work, Zhang et al. synthesized ZnS/ZnO/ZnS sandwich nanosheets with an effective band gap of ~2.72 eV using thermal evaporation, and these materials resulted in potent visible-light photocatalytic activity [86]. These works show that coating ZnO with an ultrathin surface layer of ZnS is an interesting approach for enhancing its water-splitting activity under solar irradiation.

Arai and co-workers have synthesized hollow Cu-doped ZnS particles as visible-light photocatalysts for the decomposition of hydrogen sulfide under solar irradiation to generate H₂. The Cu-doped ZnS particles were synthesized starting from a ZnO precursor that was co-precipitated on Cu by exploiting the difference in reduction potential between Zn and Cu. The visible-light photoactivity of Cu-doped ZnS was six- and 130-times higher than the Cu-free “ZnS-shell” and pristine ZnS particles prepared using co-precipitation, respectively. A comparative study found that the “Cu-ZnS-shell” particles were highly active under visible-light irradiation (λ > 440 nm), and the rate of H₂ evolution was optimized with a Cu (2 wt%)/ZnS photocatalyst [36]. In another recent study, Cu/ZnS microspheres were prepared using a template-free microwave irradiation method for H₂ generation from an aqueous Na₂S solution under visible-light irradiation. The optimized H₂ evolution rate of 973.1 μmol h⁻¹ g⁻¹ was obtained for 2.0 mol% Cu-doped ZnS [87].

4. Ternary Chalcogenide Heterostructures for Water Splitting

Recently, ternary mixed-metal chalcogenides have shown excellent photocatalytic H₂ production activities, as shown in

Table 2. Photocatalytic H₂ production activities using different ternary chalcogenides.

Photocatalyst Used	Amount of Photocatalyst Used	Electrolyte/ Solvent Used	Morphology	H2 Produced	Ref.
MoS2/g-C3N4/ZnIn2S4	50 mg	90 mL water and 10 mL triethanolamine	3D flower-like nanospheres	ZnIn2S4: 904 μmol g−1h−1 MoS2/g-C3N4/ZnIn2S4 loaded with with 3 wt% g-C3N4 and 1.5 wt% MoS2: 6291 μmol g−1h−1	[90]
ZnIn2S4@ CuInS2	50 mg	0.25 mol/L Na2S and 0.35 mol/L Na2SO3 as a sacrificial agent in 100 mL of aqueous solution	Marigold-like spherical structure comprising numerous thin nanosheets	1168 μmol g−1	[91]
Ti3C2 MXene@ TiO2/CuInS2	10 mg	100 mL water solution containing 20% methanol as sacrificial agent	Layered structure decorated with small CuInS2 nanoparticles	356.27 μmol g−1h−1	[92]
ZnIn2S4-rGO-CuInS2	15 mg	50 mL of aqueous Na2S/Na2SO3 (10 vol%) solution	Dispersed marigold-like structured ZnIn2S4 and layer-structured CuInS2 on reduced graphene oxide sheets	2531 μmol/g after 5 h	[93]
CdS/CoMoS4	80 mg	8 mL of lactic acid was added into 80 mL of water	Nanorod structured	7.5 at.% CdS/ CoMoS4: 208 μmol h−1	[94]
Carbon nanotube modified Zn0.83Cd0.17S	1 mg	40 mL of an aqueous solution containing 0.1 M Na2S/0.02 M Na2SO3	Irregular	0.25 wt% carbon nanotube modified Zn0.83Cd0.17S: 5.41 mmol h−1g−1	[95]

. For instance, Zn_xIn₂S_3+x (x = 1–5) has attracted the attention of researchers as a new member of the semiconductor family. These compounds possess unique electronic structures, tunable optical properties, and, for certain compositions, band gaps suitable for visible-light absorption and energy bands that straddle the water redox potentials, making them ideal for use in photocatalysis and energy conversion reactions [88]. Wu and co-workers have successfully synthesized Zn_xIn₂S_3+x with x ranging from 1 to 5 [88]. Zn_xIn₂S_3+x samples display strong absorption of visible light due to excitation of the fundamental band gap transition, and the absorption edges of the samples move to a shorter wavelength as x is increased from 1 to 5 due to the widening of the band gap. Zn_xIn₂S_3+x samples have band gap energies ranging from 2.65 eV to 2.84 eV, depending on chemical composition. ZnIn₂S₄ exhibited the lowest electron-hole recombination rate among all the compositions studied. One of the contributing factors leading to this finding is the presence of fewer composition faults in the ZnIn₂S₄ structure compared with the other samples. The existence of composition faults results in additional energy barriers to be overcome by charge carriers during transit to the particle surface, which leads to increased recombination and lower photocatalytic reaction rate. Based on the photoelectrochemical experiments conducted, it was found that ZnIn₂S₄ can generate more photocurrent than the other compositions. Electrochemical impedance spectroscopy results also suggest that ZnIn₂S₄ has the lowest resistance to interfacial electron transfer among the compositions studied, suggesting that ZnIn₂S₄ is the most efficient at interfacial charge transfer and consistent with the photocurrent trend. ZnIn₂S₄ shows the highest photocatalytic activity, followed by Zn₂In₂S₅, Zn₃In₂S₆, Zn₄In₂S_7, and, lastly, Zn₄In₂S_8, with H₂ production rates of 2.93, 2.86, 2.32, 2.15, and 2.05 mmol h⁻¹g⁻¹, respectively. Overall, it can be concluded that ternary metal chalcogenides with the formula Zn_xIn₂S_3+x (x = 1–5) exhibit appropriate properties for use as photocatalysts, especially for water-splitting applications. In a different study, Fan et al. fabricated CuS@ZnIn₂S₄ hierarchical nanocages for H₂ evolution under visible light [89]. The presence of an interface between CuS and ZnIn₂S₄ improved the solar energy utilization and separation and transfer efficiency of photogenerated carriers. The as-prepared materials exhibited a photocatalytic H₂ evolution rate as high as 7910 μmol h⁻¹g⁻¹.

In another report by Hojamberdiev and co-workers, they have synthesized layered crystals of trigonal ZnIn₂S₄ by a flux method using a range of binary fluxes, including CaCl₂:InCl₃, SrCl₂:InCl₃, BaCl₂:InCl₃, NaCl:InCl₃, KCl:InCl₃, and CsCl:InCl₃, from waste containing ZnS from the mining industry [7]. Among them, KCl:InCl₃ was the most favorable for synthesizing phase-pure trigonal ZnIn₂S₄. The trigonal ZnIn₂S₄ crystals grown in this study using KCl:InCl₃ flux exhibited higher photocatalytic H₂ evolution activity (132 μmol h⁻¹) in comparison to previously reported hexagonal ZnIn₂S₄ crystals prepared using the hydrothermal method. The superior performance obtained using the binary flux method can be ascribed to higher crystallinity and decreased defect density. The existence of secondary phases (namely ZnS and In₂S₃) in ZnIn₂S₄ crystals grown with CaCl₂:InCl₃ and NaCl:InCl₃ fluxes were found to have a positive impact on the photocatalytic activity, leading to H₂ evolution rates of 232 and 188 μmol h⁻¹, respectively. The enhanced H₂ production activity may be ascribed to efficient charge separation and interfacial charge transfer. Moreover, Mn_0.5Cd_0.5S/carbon black/CuS have been fabricated for H₂ production using sonochemical loading, and the subsequent in-situ deposition routes [96]. Lv et al. reported that the optimized Mn_0.5Cd_0.5S/carbon black/CuS exhibited the highest H₂ production of 819.9 μmol h⁻¹, which is 4.79-, 2.08-, and 1.47-fold increments more than pristine Mn_0.5Cd_0.5S, Mn_0.5Cd_0.5S/0.5 carbon black and Mn_0.5Cd_0.5S/CuS, respectively.

Group-III chalcogenide monolayers adopting two-faced ‘Janus’ structures are reported to be efficient photocatalysts for solar-driven water splitting. Several research groups have theoretically investigated the geometric structure, chemical stability, and electronic and optical properties of Janus group-III chalcogenide monolayers with layered ternary (XMMX′, where X, X′ = S, Se, or Te and M = Ga or In) [97,98] or quaternary composition (XMM′X′, where M and M′ are Ga and In) [99,100]. Among the ternary Janus monolayers, SGa₂Te, SeGa₂Te, SIn₂Te, and SeIn₂Te are found to possess direct band gaps. Since direct gap semiconductors generally possess higher absorption coefficients than indirect gap semiconductors, these ternary group-III chalcogenide monolayers can potentially exhibit good light harvesting and efficient electron-hole pair generation [97]. However, most of these materials have not yet been synthesized, which will be an important next step in their photocatalytic evaluation. As reported by Bai and co-workers, group-III chalcogenide monolayers adopt a honeycomb-like geometric structure with a double M m (M = Ga or In) layer sandwiched between two chalcogen layers.

Depending on the combination of metal and chalcogen, both direct and indirect band gaps occur, with gap energies ranging from 1.54 eV to 2.91 eV. Although all these band gaps surpass the free energy change of the water-splitting reaction (1.23 eV per electron), photocatalytic water splitting is not guaranteed because the band edges must also straddle the water redox potentials. Figure 10 shows the calculated alignment between the band edges of the Janus XMMX′ monolayers and the water redox potentials. According to these calculations, all of the monolayers are capable of complete water splitting with the exception of SGa₂Te, which cannot drive the oxygen evolution reaction due to its valence band edge laying above the O₂/H₂O redox potential [97].

Although the pristine GaTe and InTe monolayers exhibit strong absorption at short wavelengths, they are inferior to the corresponding Janus monolayers at longer wavelengths. Among the Ga-based Janus chalcogenide monolayers, SeGa₂Te and SGa₂Te possess the strongest optical absorption, while SeIn₂Te and SIn₂Te exhibit the strongest absorption of all the Janus-type monolayers studied.

5. Prospects of Chalcogenides and Chalcogenide-Based Heterostructures

Even though steady progress has been made in utilizing chalcogenides as photocatalysts, a range of challenges still need to be addressed to promote the field, gain interest from other researchers, and achieve commercially exploitable results:

• In order to further boost the activity and stability of chalcogenides for water splitting, it is important to create precise methods for obtaining pure phase, active interface, exposed active surface, optimized electronic structure, and enhanced electronic conductivity.
• Recent research has shown that chalcogenides are very promising materials for H₂ evolution by photocatalytic water splitting. It is expected that, with further knowledge, controlled doping, surface engineering, and development of their performance can be further improved [101].
• Binary metal chalcogenides such as CdS and CdSe are unstable in acidic media and are also susceptible to photocorrosion. As such, potential replacements that are more stable in acidic media and that do not exhibit photocorrosion should be explored.
• Despite many recent studies on the use of ternary and quaternary chalcogenides as photocatalysts for H₂ production, the exact cause of photocorrosion in these materials is yet to be explored in detail and should be researched thoroughly in order to synthesize highly stable, multifunctional chalcogenides.
• Preparation of chalcogenide using low-cost methods while can produce large quantities of products also require more attention. Optimization of different parameters in the synthetic reactions such as precursors, temperature, pH, and reaction time should be studied for optimal yield to facilitate the production of these materials at a commercial scale.
• Detailed studies on extending the lifetime of photo-generated carriers and suppressing recombination are required to improve the photocatalytic activity of these materials for broader applications. Several approaches that could be employed include coupling with other semiconductors, loading of noble metals, and doping with metal or non-metal ions.

6. Conclusions

In summary, this review provides an overview of chalcogenide and chalcogenide-based heterostructures that have been extensively used for photocatalytic H₂O-splitting activities. This article summarizes the different modifications of chalcogenide materials that can improve their absorption of visible light ability, enhancing charge separation and reducing the recombination rate for water splitting. Moreover, this review also includes the mechanisms involved in the water splitting of binary, ternary, and chalcogenide-based heterostructures. Furthermore, combining different metal-based chalcogenides with other semiconductors has the potential to improve photocatalytic efficiency for generating H₂ and O₂ by water splitting.