Hierarchical Ternary Sulﬁdes as Effective Photocatalyst for Hydrogen Generation Through Water Splitting: A Review on the Performance of ZnIn 2 S 4

: One of the major aspects and advantages of solar energy conversion is the photocatalytic hydrogen generation using semiconductor materials for an eco-friendly technology. Designing a low-cost efﬁcient material to overcome limited light absorption as well as rapid recombination of photogenerated charge carriers is essential to achieve considerable hydrogen generation. In recent years, sulﬁde based semiconductors have attracted scientiﬁc research interest due to their excellent solar response and narrow band gap. The present review focuses on the recent approaches in the development of hierarchical ternary sulﬁde based photocatalysts with a special focus on ZnIn 2 S 4 . We also observe how the electronic structure of ZnIn 2 S 4 is beneﬁcial for water splitting and the various strategies involved for improving the material efﬁciency for photocatalytic hydrogen generation. The review places emphasis on the latest advancement/new insights on ZnIn 2 S 4 being used as an efﬁcient material for hydrogen generation through photocatalytic water splitting. Recent progress on essential aspects which govern light absorption, charge separation and transport are also discussed in detail. the H 2 generation of µ mol g − 1 h − 1 . The results reveal that the superior activity of the heterostructure is due to the positive synergetic effect between MoS 2 and graphene, where MoS 2 and graphene act as H 2 evolution reaction


Introduction
Depletion of fossil fuels and deterioration of energy supply demands a better solution for sustainable energy, driving extensive research on the use of Hydrogen (H 2 ) as an alternate energy source. The conversion of sunlight and water, two major resources of energy on earth, into H 2 has been advocated as an ideal goal in providing a clean and green energy system without compromising environmental safety. The utilization of solar energy to dissociate water into H 2 and oxygen (O 2 ) through photocatalysis assisted by a semiconductor photocatalyst is gaining momentum among various ways of H 2 production. With the pioneering work from Honda and Fujishima on the splitting of water using single-crystal TiO 2 [1,2], many revolutionary ideas have contributed towards developing an efficient photocatalyst for the effective utilization of solar energy and high H 2 yield in absence of any carbonaceous by-products [3][4][5][6]. The mechanism of photo splitting of water involves absorption of radiation with an energy greater than the band-gap of the photocatalyst. It mainly consists of two half-reactions: oxidation of water to form O 2 and the reduction of protons to form H 2 [7][8][9][10][11][12][13][14][15] These redox reactions can be initiated only when the positions of the conduction band (CB) is more negative than H 2 evolution potential (0V vs Normal Hydrogen Electrode (NHE)) and the valence band (VB) is more positive than the water oxidation potential (+1.23 V Vs NHE) [16]. For photo-assisted water splitting, oxide semiconductors continue to hold a reputation due to their ease of availability, low cost and photostability. Various oxide semiconductors, especially oxides consisting of metal cations (Ga 3+ , In 3+ , Ge 4+ , Sn 4+ , Sb 5+ ) with d 10 configuration and metal cations (Ti 4+ , Zr 4+ , Nb 5+ , Ta 5+ , W 6+ ) with d 0 configuration have been reported for good photocatalytic activity when assisted by co-catalyst like RuO 2 [17][18][19][20][21][22][23]. Perovskite structured metal oxides like SrTiO 3 and KTaO 3 can split water without an external bias in powdered form and show enhanced results when combined with NiO or Rh cocatalyst [24,25]. However, since their valence band comprises of deep 2p oxygen orbital, their O2p levels lie at a potential of 3 eV making the effective band-gap (E g ) to fall in the UV region (4% of the available spectra). For the efficient utilization of the solar spectrum, the band-gap of a photocatalyst should be in the visible region having E g < 3 eV. This cultivates the need of material with a narrow band-gap having suitable valence band positions. In this regard, metal sulfides are known for their shallow valence band which consists of S3p levels at more negative potential when compared to O2p levels in metal oxides thereby narrowing the bandgap energy [26]. The suitable band edge positions in CdS with narrow band-gap energy~2.4 eV makes it one of the most studied materials for solar water splitting application. Regardless of the benefits, CdS experiences photocorrosion that greatly eradicates its practical application. Sulfide ions present on the surface of CdS get easily oxidized by photogenerated holes to form solid sulfur which is irreversible [27]. Nevertheless, the development of a photocatalytic corrosion-resistant, visible light-absorbing material with enhanced photoactivity is the doorway to effectuate the cycle of converting sunlight to hydrogen. Hence in view of the prospective applications, ternary semiconductor chalcogenides with hierarchical structures are emerging with greater scope in photocatalysis [28]. Numerous investigations have claimed that the ternary metal sulfides (ZnIn 2 S 4 , CdIn 2 S 4 , CaIn 2 S 4 , MgIn 2 S 4 , etc) with AB 2 X 4 (where A = Zn, Cd, Mg, Ca, B = Ga, In and C = S) structure could be a new class of potential visible-light active photocatalysts due to their appreciable chemical stability and optical band gaps. These ternary sulphide compounds with AB 2 X 4 structure have been explored for photo-assisted decomposition of water as listed in Tables 1 and 2. Among the ternary metal sulfides, ZnIn 2 S 4 is the only member of the family with a layered structure with potential applications in charge storage, opto-electronics and photoconduction [29]. In the hexagonal layered structure of ZnIn 2 S 4 , the atoms are arranged in layers along the c-axis. Zn atoms are tetrahedrally bonded to the S atoms (Zn-S 4 ) while the In atoms have two different environments, a tetrahedral arrangement with four S atoms (In-S 4 ) and an octahedral arrangement with 6 S atoms (In-S 6 ). This layered arrangement of atoms is responsible for the improved photocatalytic performance of ZnIn 2 S 4 [30][31][32]. It has also been shown that the ZnIn 2 S 4 structure initially shows a resistivity drop during the exposure to light creating electron/hole pairs. The doubly negative charged surface traps the holes leading to an enhanced free-electron transport to the surface [33]. It is also known that a photocatalyst with a p-block metal ion having d 10 configuration exhibits good photocatalytic performance during water decomposition [18]. Hence, the presence of In 3+ metal ions with d 10 configuration in ZnIn 2 S 4 could be promising for good photocatalytic activity for water decomposition. This review with detailed information on photocatalytic activity of various ternary semiconductor sulfide chalcogenides with hierarchical structures will give a bird's eye view on their performance with a special emphasis on ZnIn 2 S 4 .

Crystalline Structures of Sulfide Based Photocatalysts
When a metal or semi-metal cation is combined with a sulfur anion, it forms a metal sulfide compound with a stoichiometry of M x S y (MS, M 2 S, M 3 S 4 , MS 2 ). Bi-metal sulfides are formed in a similar approach with a stoichiometry of A 1-x B x S y , where x and y are integers [57]. The occupation of metal and sulfur atoms in metal sulfide are arrangements of a close-packed system with four notable structures (Figure 1a). In the first case, we have a symmetrical sodium chloride (NaCl) structure type with each ion occupying an octahedron position with six nearest neighbors. It is called the pyrite structure when the crystal comprises two sulfide ions in each octahedron position [57][58][59]. Then we have the sphalerite structure where the metal ions are bounded by six oppositely charged ions positioned tetrahedrally [60]. The next important structure type is called the fluorite, in which every metal cation is surrounded by eight anions and every anion, in turn, is surrounded by four cations. If this symmetry is reversed with every metal cation surrounded by four anions and every anion, in turn, is surrounded by eight cations, then it is called the anti-fluorite structure [57,61]. Another metal sulfide category has a cubic spinel structure possessing a stoichiometry of AB 2 S 4 , where A site is occupied by divalent metal ion (such as Mg, Zn, Fe, Cu etc) arranged tetragonally, while B site is occupied by trivalent metal ions (such as In, Cr, Ti, Co) arranged octahedrally [57]. The oxidation state adopted by sulfur is usually 2 − and hence to maintain the valence symmetry in the AB 2 S 4 structure, the divalent A-site is occupied by cation with 2+ oxidation state and trivalent B-site is occupied by cation with 3+ oxidation state with the structure possibility of A 2+ B 3+ S 4 2− [57]. These structures are called ternary metal chalcogenides. From among the family of AB 2 S 4 semiconductors, ZnIn 2 S 4 is the only member with a layered structure (see Figure 1b) [62]. It is a potentially visible-light-responsive photocatalyst reported with three different polymorphs, including, cubic, hexagonal and rhombohedral phase [63]. ZnIn 2 S 4 displays less toxicity when compared to metal sulfides such as CdS and Sb 2 S 3 while exhibiting similar optical properties which portrays the advantage of utilizing ZnIn 2 S 4 in environmental remediation applications. Additionally, ZnIn 2 S 4 can also be beneficial over ZnS with its narrow bandgap value reported between 2.06 eV-2.85 eV [63]. The splendid physical and chemical properties of ZnIn 2 S 4 have attracted immense attention in various applications including H 2 generation [37][38][39][40][41][42][43][44][45][46][47][48][49], CO 2 reduction [64][65][66], environmental remediation [40] under visible light irradiation. The layer structured semiconductor ZnIn 2 S 4 exhibiting cubic polymorph contains ABC stacking of S atoms with tetrahedral and octahedral coordination of Zn and In atoms respectively, while the hexagonal phase consists of ABABA stacking of S atoms with Zn; half of the In atoms are coordinated tetragonally and the other half In atoms are coordinated octahedrally. The rhombohedral phase exhibits a strong Zn-S and In-S bond in the layer with a weaker S-S bond, with every S atom corresponding to a different layer [66]. The band structure of ZnIn 2 S 4 has also been explored theoretically on the basis of Density Functional Theory (DFT) [55,64,67,68]. Studies suggest that ZnIn 2 S 4 is a direct bandgap semiconductor as both VB and CB of ZnIn 2 S 4 lie on G point of thee Brillouin zone [55]. Valence band consists mainly of S3p and Zn3d orbitals and the conduction band consists of hybridized In5s5p and S3p orbitals. Under photo illumination, electrons would transfer from the valence band to the conduction band leaving behind the photogenerated holes which are beneficial for photo-assisted water splitting [55].

Electronic Structure Beneficial for Water Splitting
Photocatalytic water splitting is an energetically uphill reaction (Gibbs energy = 237 kJ/mol) which involves positive energy change and multiple electron transfer similar to naturally occurring photosynthesis. Hence, photocatalytic water splitting is often referred to as "artificial photosynthesis" [69]. Theoretically, a material possessing a minimum bandgap of 1.23 eV is suitable for photochemical water splitting, as explained in Figure 2 [16]. Despite the fact that metal oxide photocatalysts are easily available, metal sulfides have attracted a lot of attention for their high absorbance in the 'hole controlled photocatalysis'. Notably, the photocatalytic activity depends on the quantity of photon absorption. In this regard, high absorbance materials like sulfides show better photocatalytic activity [69,70]. Ionic character in the range of 20-30% is essential for the increased photocatalytic activity of a catalytic material [70]. High ionic character is much needed for water adsorption on the catalyst surface in spite of the fact that the ionic character is responsive to surface corrosion. The majority of the oxide materials provide 50% ionic character attributed to their high electronegativity difference while sulfides also perfectly match this basic requirement [70]. A drawback with the oxide materials is that they face hydrogen embrittlement (metal hydride formation inside the lattice) due to the high affinity of hydrogen with oxygen. This leads to the reduction in catalyst durability in photocatalytic water splitting reactions.
Conversely, the rate of hydrogen embrittlement on the sulfide surface is comparatively lesser than the oxide surface [70].

Photocatalytic Hydrogen Evolution by ZnIn 2 S 4
ZnIn 2 S 4 , a ternary compound of the AB 2 X 4 family has been identified as a visible light photocatalyst with desirable band energy positions to split water photocatalytically. To achieve the same, various experimental synthesis techniques have been explored. Bai et al., [34] synthesized a series of flower-like ZnIn 2 S 4 through surfactant-assisted hydrothermal method where the pH level of reactant played a major role in providing a maximum yield of H 2 about 1545 µmol g −1 h −1 . Photostable ZnIn 2 S 4 [45] could produce H 2 through water reduction for at least up to 150 h. Chaudari et al., [32] have reported excellent photocatalytic activity of ZnIn 2 S 4 for the production of H 2 through the splitting of H 2 S. The quantity of the surfactant (triethylamine), solvent and the synthesis temperature both have a huge impact on the morphology of the product as well as the rate of H 2 production [45,50]. ZnIn 2 S 4 synthesized at 160 • C at pH 1 showed 34.3% of apparent quantum yield. Among a series of surfactants used, a sample synthesized with cetyltrimethylammonium bromide (CTAB) is noted to have a larger d(006) space value [47]. It is claimed that larger d(006) space could promote higher separation of photogenerated charge carriers thereby enhancing the photocatalytic performance. Synthesis of ZnIn 2 S 4 through thermal sulfidation at different temperatures provides clear insights into the phase change of the material from cubic to rhombohedral with respect to changes in the synthesis temperature [51]. This study provides a precise understanding of the effect of phase change on optical and photocatalytic properties of ZnIn 2 S 4 .

Strategies for Enhancing Photocatalytic Performance
In a typical photo-assisted water splitting process, a photocatalyst is excited with energy greater than or equal to the band-gap value to generate electron-hole pairs. These photogenerated charge carriers would migrate to the surface of the photocatalyst and react with the organic pollutant. Photogenerated electrons would reduce H + to H 2 whilst the holes would be consumed by the sacrificial agent added to the system. Though its bandgap lies in the visible region, ZnIn 2 S 4 suffers from recombination of charge carriers as well as poor migration [63]. In order to improve the photocatalytic performance of ZnIn 2 S 4 , several approaches have been made including the formation of heterostructure with a suitable photocatalyst, doping with metal ions, surface modification, control of morphology etc. These modifications aid in improving the surface and optical properties of ZnIn 2 S 4 and escalating migration of charge carriers with lower recombination rate resulting in enhanced photocatalytic activity.

Heterostructure Formation
A heterostructured photocatalyst is advantageous in the sense that it assists in extending the light absorption range and accelerates charge transfer. Building a heterojunction between ZnIn 2 S 4 and other suitable semiconductors could facilitate band alignment which could suppress the recombination rate leading to enhanced photocatalytic performance. Lin et al., (2018) designed a heterojunction between g-C 3 N 4 /ZnIn 2 S 4 achieving in-situ growth of ZnIn 2 S 4 nano leaves on the surface of g-C 3 N 4 nanosheets ( Figure 3) via a one-step surfactant-assisted solvothermal method [37].  [52]. Experimental results suggest that 3% MWCNT embedded in the interior of ZnIn 2 S 4 microspheres shows a maximum H 2 production rate of 684 µmol h −1 [52]. Fan et al., (2010) hydrothermally synthesized ZnIn 2 S 4 and deposited it over electrospun poly(HFBA-co-MAA)/PVDF fibers [53]. Here Poly(HFBA-co-MAA) is referred to as Hexafluorobutylacrylate-co-methacrylic acid and PVDF is referred to polyvinylidene fluoride. It is suggested that the use of polymercarriers has the advantages of being easy recycle, flexibility, excellent weatherability and large surface area. The growth of ZnIn 2 S 4 microspheres on polymer surface (organic carrier) aids in photocorrosion resistance and enhances photocatalytic performance. ZnIn 2 S 4 /fluoropolymer fiber composites were able to produce 9.1 mL/h even when recycled up to three runs. On the other hand, Li et al., (2010) synthesized series of ZnS coated ZnIn 2 S 4 via facile solvothermal synthesis using methanol as the solvent [28]. Among the samples synthesized with different mol% of ZnS loading, the sample with 17% ZnS loading showed better photocatalytic activity. In their investigation, glucose is used as a hole scavenger and acts as an electron donor to inhibit photocorrosion on the catalyst surface. Thus H 2 production is improved by preventing photocorrosion and recombination of electrons and holes at the semiconductor surface. The superior photocatalytic activity was noted in the presence of glucose and went up to 103 µmol with an irradiation for 10 h whereas the value was just 16 µmol in the absence of glucose.

Doping as a Strategy to Enhance Photocatalysis
Doping is generally considered a common strategy in boosting up the spectral response of a semiconductor. Commonly, doping an element into a semiconductor could narrow the bandgap and enhance the light-absorption ability [63,68]. Owing to the special electronic configuration of rare earth elements (REEs) with vacant f orbital, doping of rare-earth ions has attracted research interest in the field of photocatalysis. Fien et al., (2014), has doped a series of RE ions (La 3+ , Ce 3+ , Er 3+ , Gd 3+ , Y 3+ ) into the lattice of ZnIn 2 S 4 [38]. In their study, the photocatalytic H 2 production efficiency was observed to increase in the order of La-ZnIn 2 S 4 >Ce-ZnIn 2 S 4 >Er-ZnIn 2 S 4 >Gd-ZnIn 2 S 4 >Y-ZnIn 2 S 4 post modification of ZnIn 2 S 4 with RE ions. The decreasing number of electrons in the rare-earth 4f shell was consistent with the increased photocatalytic activity. REEs existed as RE 2 O 3 oxide and modified the lattice of ZnIn 2 S 4 , increased its surface area and introduced defects on the catalyst surface, thus inhibiting recombination of photogenerated charge carriers. ZnIn 2 S 4 lattice was also modified with La 3+ for H 2 evolution activity [39]. Compared to pure ZnIn 2 S 4 , the H 2 evolution could be increased by 141.6% for 1 wt% La-doped samples. Zhu et al., (2017) on the other hand introduced reduced graphene oxide (RGO) and La into ZnIn 2 S 4 lattice [43] and used Pt as co-catalyst for H 2 evolution. Among a series of samples synthesized by them, 1.0 Pt/1.0RGO/1.0 La-ZnIn 2 S 4 gave the highest productivity of 2255 µmol g −1 h −1 . It was claimed that RGO could transfer the photoexcited electrons which are easily captured by the surface defects produced by La modification on ZnIn 2 S 4 to generate H 2 . Shen et al., (2012) modified ZnIn 2 S 4 with series of alkaline earth (AE) metals (Ca, Ba, Sr) [49]. Among the AEmodified samples, only Ca incorporated samples show higher photocatalytic activity than pure ZnIn 2 S 4 , while the other two samples perform similar to bare samples. The variation in the photocatalytic performance was analyzed on the basis of UV-Vis spectroscopy and photoluminesce spectra. From UV-Vis absorption spectra it was clear that all samples exhibited the same profile with an intense absorption edge at 500 nm post-incorporation of AE. However, photoluminescence (PL) show decreased emission intensities in the order of ZnIn 2 S 4 >Ca-ZnIn 2 S 4 >Sr-ZnIn 2 S 4 >Ba-ZnIn 2 S 4 . It is expected that the introduction of AE ions could produce surface defects acting as trap centers for electrons, improving the charge separation and leading to a higher photocatalytic performance by Ba-ZnIn 2 S 4 sample. However, only Ca-ZnIn 2 S 4 exhibits better performance. This contradiction is explained by the defect emission peaks in the PL spectra at about 400-450 nm. Quenching in the native defect peaks relates to more non-radiative recombination in the system. Hence, both Ba and Sr doped samples possess fewer electrons and holes involved in photocatalysis thereby showing decreased efficiency in H 2 evolution [49]. An enhancement in photocatalytic activity post-incorporation of different transition metal (Cr, Mn, Fe, Co) ions into the lattice of ZnIn 2 S 4 [54] is also investigated. The effect of Mn, Cr, Fe and Co doping on photocatalytic activity is analyzed on the basis of band structure and photoluminescence properties. The enhanced performance of the Mn-doped sample is attributed to the increased number of electrons and holes for photocatalysis induced by Mn doping. However, the decreased photocatalytic activity for Fe, Cr, Co-doped samples is attributed to the impurity levels created in the band-gap region which act as nonradiative recombination centers for photogenerated electrons and holes.  analyzed the photocatalytic activity of Cu-doped ZnIn 2 S 4 for H 2 evolution application [55]. Incorporating Cu in ZnIn 2 S 4 lattice increased hydrogen evolution up to 151.5 µmol h −1 under visible light irradiation. Similar behavior could be observed upon Ni incorporation (Jing et al., (2010)) Ni 2+ existed in NiS state post doping with its energy level lying close to the valence band of S3p orbital (see Figure 4) and acts as trapping sites for photogenerated holes while getting oxidized to Ni 3+ state. Due to the instability of Ni 3+ , it reverts back to Ni 2+ state by releasing a hole. Thus, the shallow trapping of holes can extend the lifetime of the charge carrier separation and promote enhancement in the photocatalytic activity [56].

Morphology and Porosity
Shape, size and structure of a material have a strong correlation with its physical and chemical properties. Thus tuning the shape and size of semiconductor photocatalyst could be a practical approach in controlling their photocatalytic activity. A simple solution chemistry route has been proposed by Gou et al., (2005) for the shape-controlled synthesis of ZnIn 2 S 4 of various dimensions [31]. With the combination of hydrothermal, solvothermal and surfactant template techniques well-defined morphology of ZnIn 2 S 4 could be achieved. This includes the synthesis of materials with various morphologies such as nanotubes, nanowires, nanoribbons, microspheres etc. For the nanoribbons, the solvothermal route has been used with pyridine as the solvent with a synthesis temperature >180 • C, while for the nanotubes, the synthesis temperature was lowered to <160 • C. Microspheres of ZnIn 2 S 4 composed of irregular sheets were obtained when the solvent is replaced with water. The as-synthesized nano and microstructures exhibit modified optical properties with strong absorption from the UV to the visible range. Chaudhari et al., (2011) has proposed a detailed mechanism to control the microsphere shaped morphology of ZnIn 2 S 4 by varying the solvent concentration (see Figure 5) [32]. In their study, (TEA), the sample exhibited marigold like morphology with curved petals in absence of triethylamine (TEA), while the sample with 0.005 mol of TEA possessed marigold morphology with increased puffiness. On further increasing the concentration of TEA to 0.01 mol, the entire flower-like morphology was drastically suppressed to smaller plates. It is proposed that the excess TEA could increase the bonding strength between TEA and surface atoms of ZnIn 2 S 4 easily breaking down the Van der Waals force exerted between the petals. This accounts for the formation of nanoplates instead of micron-sized flowers. With TEA~0.015 mol accelerated growth of unidimensional structures, like nanostrips, could also be achieved. Among ZnIn 2 S 4 samples synthesized with various morphologies, H 2 evolution was higher (about 5287 µmol h −1 ) in the case of the sample prepared with 0.01 mol of TEA with flower-like morphology. Lin et al., (2018) designed a heterojunction between 2D g-C 3 N 4 nanosheets and 2D ZnIn 2 S 4 nano leaves via facile surfactant-assisted solvothermal method for enhanced photo-induced H 2 generation [37]. From the as-synthesized bulk g-C 3 N 4 , thinner g-C 3 N 4 nanosheets are exfoliated through the thermal oxidation process. These exfoliated nanosheets can help in constraining the vertical movement of charge carriers and facilitate recombination resistance [36]. ZnIn 2 S 4 nano leaves were extracted from bulk ZnIn 2 S 4 microspheres with the help of trisodium citrate dihydrate. These structures mimicking leaves from nature are expected to be beneficial for charge separation and promotion of improved photocatalytic H 2 generation. A combination of these two 2D structures, could create effective heterojunction with high-speed charge transportation and migration with enhanced photocatalytic activity. The effort of Tian et al., (2014) in modifying ZnIn 2 S 4 with REEs had significant influence on the morphology and porous nature of the material. Unmodified ZnIn 2 S 4 possessed gully-ball like spherical structures with collapsed nanosheets. When Y 3+ was added, the surface of ZnIn 2 S 4 was partially open with pores ranging between 0.3 µm to 0.5 µm. The addition of Gd 3+ could open up the surface with many porous sheets to a given rose-like structure, while the addition of Er 3+ and Ce 3+ opened up the entire surface with a reduced gap possessing porous nanosheets ranging from 0.1 µm to 0.2 µm. A regular morphology with a fully opened sphere was observed with the addition of La 3+ . This shows that the addition of rare earth ions onto the surface of ZnIn 2 S 4 significantly modifies the surface with more regular, stabilized textures arresting agglomeration and maintaining mesopores. Their findings can be related to the fact that the addition of REEs promote-opening of porous structures with a decreased gap between nanosheets, increased surface area and pore volume providing better photocatalytic activity than pure ZnIn 2 S 4 [38]. Solvent mediated synthesis of ZnIn 2 S 4 by Shen et al., (2008) provides new insights on the effect of solvents on the morphology, crystallinity and photocatalytic properties of ZnIn 2 S 4 [50]. Samples synthesized with H 2 O as solvent presented a flowering-cherry-sphere-like structure with numerous petals. MeOH mediated synthesis resulted in smaller compact spheres with reduced petal length and thickness. Under ethylene glycol mediated condition, the sample did not show flower-like structure but presented clusters of irregular sheets. These results suggested that different organic solvents would hinder the growth of ZnIn 2 S 4 and affect crystallinity. It was observed that the aqueous mediated sample showed regular morphology with good crystallinity and the highest photocatalytic activity among all the samples.

Role of Sacrificial Agent
Sacrificial agents play a prominent role in photocatalytic H 2 production reaction. Many reports suggest that the efficiency of a photocatalyst also relies on the nature of the sacrificial agent. In case of sulfide photocatalyst, amines and sulfide/sulfite-based sacrificial agents can get easily adsorbed on the surface of the catalyst and consume holes when compared to alcohols and sugar. The use of alcohols and sugars produces a neutral pH medium while amines and mixtures of sulfide and sulfite produce a high alkaline medium which is preferential for efficient H 2 production [71,72]. Li et al., (2010) in their work have used biomass glucose as an electron donor. From the viewpoint of renewable sources, they claim that glucose from cellulose or starch, when used as an electron donor, is far better when compared to S 2− obtained from sulfide/sulfite mixtures. Here, the H 2 production experiment with ZnS/ZnIn 2 S 4 has been conducted with and without the usage of glucose as an electron donor. In the absence of glucose, it is noted that the H 2 production is saturated after a certain time whereas, a substantial increase in the rate of H 2 production is noted in the presence of glucose. In absence of glucose, S 2− generated from ZnS photocatalyst acts as a sacrificial agent for H 2 generation. In the presence of glucose, a proportional increase of H 2 with respect to light irradiation was observed. This is attributed to the fact that glucose acts as a direct hole scavenger inhibiting photo corrosion leading to an enhancement in photoactivity [28]. Similarly, several other hole scavengers such as KOH, triethylamine, lactic acid have also been used for H 2 production with sulfide-based photocatalysts [32,35,37,42].

Compounds with AB 2 X 4 Structure Other than ZnIn 2 S 4
Several multi-metal sulfide photocatalysts have been reported for water splitting applications using various sacrificial agents (see Figure 6). It is mainly attributed to the electronic properties of sulfide materials having conduction band composed of d, s, p orbitals and valence band having S3p orbitals which are negative compared to 2p orbitals of oxide materials. Hence sulfide materials possess band positions negative enough than oxides to reduce water to hydrogen. In addition, their narrow bandgap helps to cover the maximum of the solar spectrum [73]. Table 2 summarizes various such sulfide photocatalysts with AB 2 X 4 structure (besides ZnIn 2 S 4 ).  To enhance the performance of sulphide photocataysts, several techniques such as, doping, creating heterojunctions, introducing sacrificial agents and co-catalysts have been followed. CaIn 2 S 4 has been proposed as an efficient photocatalyst for H 2 generation by Ding et al., (2013) and a modification of CaIn 2 S 4 with g-C 3 N 4 has been proposed by Jiang et al., (2015) [74,79]. Jiang et al., (2015) elucidates the enhanced photocatalytic performance of CaIn 2 S 4 /g-C 3 N 4 heterostructure through H 2 production and degradation of textile dye methyl orange. Two-dimensional g-C 3 N 4 /cubic CaIn 2 S 4 based heterojunctions provide interfacial contact which promotes charge separation to facilitate enhanced photoactivity. Pt co-catalyst assisted H 2 production with 30% CaIn 2 S 4 /g-C 3 N 4 nanocomposite resulted in H 2 evolution rate of 102 µmol g −1 h −1 (three times higher than that of pristine CaIn 2 S 4 ) [79]. Kale et al., (2006) synthesized CdIn 2 S 4 with fine marigold-like morphology through aqueous-mediated hydrothermal method and two-dimensional nanotubes morphology with the diameter of 25 nm, through methanol-mediated solvothermal process. In the H 2 evolution reaction, a quantum yield of 16.8% was achieved in the case of marigold-like morphology while 17.1% was achieved for CdIn 2 S 4 with nanotube morphology [81]. NiS 2 nanoparticles were deposited onto CdLa 2 S 4 nanocrystals as co-catalyst for the enhancement of photocatalytic activity. The NiS 2 loaded sample resulted in significant enhancement for H 2 production under visible light irradiation. Compared to the pristine CdLa 2 S 4 , 2 wt% NiS 2 loading sample exhibited three times higher H 2 production rate up to 2.5 mmol g −1 h −1 [82]. Interesting morphologies like self-assembled nanohexagon flowers, nanoprisms and nanowires for CdLa 2 S 4 were demonstrated through facile hydrothermal synthesis by varying the reaction medium with water and methanol. A wide variation in morphologies attained from highly crystalline 3D nanoprisms to 1D nanowires showed the influence of the reaction medium during synthesis (Figure 7). The optical band gap of bare nanoprisms, nanowires, nanohexagon flowers and nanoplates of CdLa 2 S 4 range from 2.1 eV to 2.3 eV and are active under the visible region of the solar spectrum. CdLa 2 S 4 with 3D prism morphology generated maximum amount of H 2 up to 2552 mmol h −1 g −1 [75].
To enhance the performance of sulphide photocataysts, several techniques such as, doping, creating heterojunctions, introducing sacrificial agents and co-catalysts have been followed. CaIn2S4 has been proposed as an efficient photocatalyst for H2 generation by Ding et al., (2013) and a modification of CaIn2S4 with g-C3N4 has been proposed by Jiang et al., (2015) [74,79]. Jiang et al., (2015) elucidates the enhanced photocatalytic performance of CaIn2S4/g-C3N4 heterostructure through H2 production and degradation of textile dye methyl orange. Two-dimensional g-C3N4/cubic CaIn2S4 based heterojunctions provide interfacial contact which promotes charge separation to facilitate enhanced photo-activity. Pt co-catalyst assisted H2 production with 30% CaIn2S4/g-C3N4 nanocomposite resulted in H2 evolution rate of 102 μmol g −1 h −1 (three times higher than that of pristine CaIn2S4) [79]. Kale et al., (2006) synthesized CdIn2S4 with fine marigold-like morphology through aqueous-mediated hydrothermal method and two-dimensional nanotubes morphology with the diameter of 25 nm, through methanol-mediated solvothermal process. In the H2 evolution reaction, a quantum yield of 16.8% was achieved in the case of marigold-like morphology while 17.1% was achieved for CdIn2S4 with nanotube morphology [81]. NiS2 nanoparticles were deposited onto CdLa2S4 nanocrystals as co-catalyst for the enhancement of photocatalytic activity. The NiS2 loaded sample resulted in significant enhancement for H2 production under visible light irradiation. Compared to the pristine CdLa2S4, 2 wt% NiS2 loading sample exhibited three times higher H2 production rate up to 2.5 mmol g −1 h −1 [82]. Interesting morphologies like self-assembled nanohexagon flowers, nanoprisms and nanowires for CdLa2S4 were demonstrated through facile hydrothermal synthesis by varying the reaction medium with water and methanol. A wide variation in morphologies attained from highly crystalline 3D nanoprisms to 1D nanowires showed the influence of the reaction medium during synthesis (Figure 7). The optical band gap of bare nanoprisms, nanowires, nanohexagon flowers and nanoplates of CdLa2S4 range from 2.1 eV to 2.3 eV and are active under the visible region of the solar spectrum. CdLa2S4 with 3D prism morphology generated maximum amount of H2 up to 2552 mmol h −1 g −1 [75]. CdS nanocrystals incorporated CdLa2S4 microspheres with 0.4 wt% of Pt as cocatalyst showed a high H2-production rate of 2.25 mmol h −1 [80]. Magnesium-based CdS nanocrystals incorporated CdLa 2 S 4 microspheres with 0.4 wt% of Pt as co-catalyst showed a high H 2 -production rate of 2.25 mmol h −1 [80]. Magnesium-based chalcogenide photocatalyst MgIn 2 S 4 , on the other hand, when integrated with polyaniline (PANI) proves to be an efficient H 2 generating photocatalyst [78]. PANI/MgIn 2 S 4 nanoflower photocatalysts with 1% PANI loading synthesized through facile chemisorption method exhibits a decent H 2 evolution of 200.8 µmol g −1 h −1 under visible light irradiation. Chauhan et al., (2019) reported co-catalyst free CuCo 2 S 4 nanosheets as a promising semiconductor photocatalyst for water splitting reactions under visible light irradiation. A simple hydrothermal route was adapted for the synthesis of nanosheets, exhibiting an appropriate band-gap of 2.24 eV. A quantum yield of 2.48% was achieved for the photo-catalytically active CuCo 2 S 4 nanosheets under visible light and it exhibited excellent weight-normalized photoactivity generating H 2 at the rate of~25,900 µmol g −1 h −1 . CuCo 2 S 4 nanosheets have been considered important due to the extraordinary long-term operational stability up to 12 h study time without using any co-catalyst [70,71].

Conclusions
It is observed from the review that, tremendous efforts have been put up in exploring sulfide photocatalysts and improving their efficiency in the past several decades. It is necessary for an ideal photocatalyst to have higher efficiency with excellent solar spectrum response to be applicable at the industrial level. REEs semiconductors are known for their narrow band gaps and superior photoresponse. Moreover, many promising strategies for enhancing the performance of the catalyst by introducing foreign elements, integrating suitable semiconductors to form heterojunctions, etc are being explored. At this stage, it is necessary to address the key issue of enhancing the H 2 production efficiency of sulfur-based photocatalysts. Intensive investigations need to be carried out in improving the behavior of photogenerated electrons and holes which could be affected by crystallinity, surface states, defects and morphology of the semiconductor. The lack of extensive experiments for scaling-up the efficiency of sulfides to promote laboratory research to industrial-scale needs to be overcome through systematic research. Among various sulfide catalysts, ZnIn 2 S 4 is an emerging ternary metal chalcogenide photocatalyst with excellent features such as good crystallinity, porous hierarchical morphology, optical properties, easy fabrication and eco-friendly nature. Despite encouraging properties, ZnIn 2 S 4 based compounds as a photocatalyst face many challenges. However, with its certain superior and moldable properties can be considered as a promising material for photo-assisted applications in the future.