Hierarchical Ternary Sulfides as Effective Photocatalyst for Hydrogen Generation Through Water Splitting: A Review on the Performance of ZnIn₂S₄

Abstract

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 ZnIn₂S₄. We also observe how the electronic structure of ZnIn₂S₄ 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 ZnIn₂S₄ 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.

1. 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 (H₂) as an alternate energy source. The conversion of sunlight and water, two major resources of energy on earth, into H₂ 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 H₂ and oxygen (O₂) through photocatalysis assisted by a semiconductor photocatalyst is gaining momentum among various ways of H₂ production. With the pioneering work from Honda and Fujishima on the splitting of water using single-crystal TiO₂ [1,2], many revolutionary ideas have contributed towards developing an efficient photocatalyst for the effective utilization of solar energy and high H₂ 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 O₂ and the reduction of protons to form H₂ [7,8,9,10,11,12,13,14,15]

H₂O + h⁺_vb → 2H⁺ + ½ O₂ [Photooxidation of water]

(1)

2 H⁺ + 2 e⁻_cb → H₂ [Photoreduction of protons]

(2)

These redox reactions can be initiated only when the positions of the conduction band (CB) is more negative than H₂ 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 (Ga³⁺, In³⁺, Ge⁴⁺, Sn⁴⁺, Sb⁵⁺) with d¹⁰ configuration and metal cations (Ti⁴⁺, Zr⁴⁺, Nb⁵⁺, Ta⁵⁺, W⁶⁺) with d⁰ configuration have been reported for good photocatalytic activity when assisted by co-catalyst like RuO₂ [17,18,19,20,21,22,23]. Perovskite structured metal oxides like SrTiO₃ and KTaO₃ 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 (ZnIn₂S₄, CdIn₂S₄, CaIn₂S₄, MgIn₂S₄, etc) with AB₂X₄ (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 AB₂X₄ structure have been explored for photo-assisted decomposition of water as listed in

Table 1. Photocatalytic activity of ZnIn₂S₄-based material in H₂ generation with respective parameters.

No.	Photocatalyst	Synthesis Method	Morphology	Co-Catalyst Used	Light Source	Sacrificial Agent	Catalyst Mass Used (g)	Yield (μmol h−1)	Ref.
1.	ZnIn2S4	CPBr assisted Hydrothermal	Flower like microspheres	-	250 W Hg lamp	Na2SO3/Na2S	0.1	1544.8 (μmol g−1 h−1)	[34]
2.	ZnIn2S4/MoSe2	Electrostatic self-assembly	Uniform sheet like structure	-	300 W Xe lamp	Lactic acid	0.005	6454	[35]
3.	ZnIn2S4/g-C3N4	Hydrothermal	Hierarchical sheets		300 W Xe lamp	Triethanolamine	0.005	14.1	[36]
4.	g-C3N4/ZnIn2S4	Hydrothermal	Micron-sized bulk morphology with sheets	-	300 W Xe lamp	Triethanolamine	0.05	2780	[37]
5.	2 wt% Y-ZnIn2S4	Hydrothermal	Microflowers with irregular opening	-	350 W Xe lamp	Na2SO3/Na2S	0.2	155.9	[38]
6.	2 wt% Gd-ZnIn2S4	Hydrothermal	Rose like micro-clusters	-	350 W Xe lamp	Na2SO3/Na2S	0.2	163.1	[38]
7.	2 wt% Er-ZnIn2S4	Hydrothermal	Microflowers and sheets of smaller width	-	350 W Xe lamp	Na2SO3/Na2S	0.2	173.1	[38]
8.	2 wt% Ce-ZnIn2S4	Hydrothermal	Microflowers and sheets of smaller width	-	350 W Xe lamp	Na2SO3/Na2S	0.2	181.7	[38]
9.	2 wt% La-ZnIn2S4	Hydrothermal	Completely developed microflower	-	350 W Xe lamp	Na2SO3/Na2S	0.2	219.5	[38]
10.	1wt%La-ZnIn2S4	Hydrothermal	Self-organised microflowers	-	350 W Xe lamp	Na2SO3/Na2S	0.2	583.4 (μmol g−1 h−1)	[39]
11.	ZnIn2S4-MoS2	Microwave synthesis	Flower-like microflowers	-	150 W Xe lamp	Na2SO3/Na2S	0.1	111.6 (μmol g−1 h−1)	[40]
12.	CdxIn2S4-Zn1-xIn2S4	Hydrothermal	Particle	-	500 W Xe lamp	Na2SO3/Na2S	0.2	590 (μmol g−1 h−1)	[41]
13.	ZnIn2S4-MoS2	Wet Chemical method	Flake like structure	-	300 W Xe lamp	Triethanolamine	0.025	8898 (μmol g−1 h−1)	[42]
14.	1wt%La1%rGO-ZnIn2S4	Hydrothermal	Hollow spheres loaded with sheets	-	350 W Xe lam	Na2SO3/Na2S	0.2	2255 (μmol g−1 h−1)	[43]
15.	MoS2/graphene/ZnIn2S4	Hydrothermal	Flower like microstructure	-	300 W Xe lamp	Na2SO3/Na2S	0.05	4169	[44]
16.	ZnIn2S4	Hydrothermal	-	Pt	300 W Xe lamp	Na2SO3/Na2S	0.3	257 (μmol g−1 h−1)	[45]
17.	ZnIn2S4	Hydrothermal	Flower, nanoplates nanostrips	-	300 W Xe lamp	KOH(photodecomposition of H2S)	0.5	5287 (μmol g−1 h−1)	[32]
18.	ZnIn2S4	Hydrothermal	Floriated microspheres	Pt	300 W Xe lamp	Na2SO3/Na2S	0.1	8420 (μmol g−1 h−1)	[46]
19	ZnIn2S4	CTAB assisted Hydrothermal	Microspheres with petals	Pt	300 W Xe lamp	Na2SO3/Na2S	0.2	62.25 (μmol g−1 h−1)	[47]
20.	ZnIn2S4	Surfactant assisted Hydrothermal	Microspheres with petals	Pt	300 W Xe lamp	Na2SO3/Na2S	0.2	692 (μmol g−1 h−1)	[48]
21.	ZnIn2S4	microwave assisted Hydrothermal	Microclusters	Pt	300 W Xe lamp	Na2SO3/Na2S	0.1	69.2	[49]
22.	ZnIn2S4	Solvothermal	Flowering cherry sphere like structure	Pt	300 W Xe lamp	Na2SO3/Na2S	0.2	136.5 (μmol g−1 h−1)	[50]
23.	ZnIn2S4	Thermal Sulfidation	Irregular lumps in micro scalar size	Pt	300 W Xe lamp	Na2SO3/Na2S	0.2	~55 (μmol g−1 h−1)	[51]
24.	ZnIn2S4/MWCNT	Hydrothermal	Floriated microspheres	-	300 W Xe lamp	Na2SO3/Na2S	0.1	6840 (μmol g−1 h−1)	[52]
25.	ZnIn2S4/Fluoropolymer	Solvothermal/polymerisation	Particles and flowers	-	350 W Xe lamp	Na2SO3/Na2S	1.02	~398 (μmol g−1 h−1)	[53]
26.	ZnIn2S4/Transition metal loaded	CTAB assisted hydrothermal	Microspheres	Pt	300 W Solar simulator	Na2SO3/Na2S	0.05	4000 (μmol g−1 h−1)	[54]
27.	Cu-ZnIn2S4	Hydrothermal	Microsphere with petals/sheets	Pt	300 W Xe lamp	Na2SO3/Na2S	0.2	757.5 (μmol g−1 h−1)	[55]
28.	Ni-ZnIn2S4	Hydrothermal	Microsphere with petals/sheets	Pt	350 W Xe lamp	Na2SO3/Na2S	0.2	~45 (μmol g−1 h−1)	[56]
29.	ZnS-ZnIn2S4	Solvothermal	Microspheres	Pt	400 W metal halide lamp	Glucose	0.1	103 (μmol g−1 h−1)	[28]

and

Table 2. Other ternary metal sulfides and composites for photocatalytic H₂ generation with respective parameters.

S. No.	Photocatalys	Synthesis Method	Morphology	Co-Catalyst Used	Light Source	Sacrificial Agent	Catalyst Mass Used (g)	Yield (μmol h−1)	Ref.
1.	CaIn2S4	Hydrothermal	Irregular grains	-	300 W Xe lamp	-	0.05	30.92 (μmol g−1 h−1)	30.92 (μmol g−1 h−1)	[74]
2.	CdLa2S4	Hydrothermal	Nanoprism and nanowires	-	450 W Xe lamp	Methanol	00.01	2252	[75]
3.	MnIn2S4/gC3N4	Hydrothermal	Nanoflakes	-	300 W Xe lamp	Na2SO3/Na2S	0.03	200.8 (μmol g−1 h−1)	[76]
4.	MnIn2Se4	Hydrothermal	Nanosheets	CoSeO3	300 W Xe lamp	Na2SO3/Na2S	-	319 (μmol g−1 h−1)	[77]
5.	MgIn2S4/polyaniline	Solvothermal	Flower like microsphere with nanosheets	-	300 W Xe lamp	-	-	17.53 (μmol g−1 h−1)	[78]
6.	g-C3N4/CaIn2S4	Hydrothermal	Thin and irregular nanosheets with wrinkles	-	300 W Xe lamp	Na2SO3/Na2S	0.05	102	[79]
7.	1wt%CdS-CdLa2S4	Solvothermal	Submicrosphere consisting of nanoprism	-	300 W Xe lamp	Na2SO3/Na2S	0.2	500	[80]
8.	3wt%CdS-CdLa2S4	Solvothermal	Submicrosphere consisting of nanoprism	-	300 W Xe lam	Na2SO3/Na2S	0.2	2280	[80]
9.	5wt%CdS-CdLa2S4	Solvothermal	Submicrosphere consisting of nanoprism	-	300 W Xe lam	Na2SO3/Na2S	0.2	1420	[80]
10.	CdIn2S4	Hydrothermal	Microsphere with nanopetals and nanotubes	-	450 W Xe lamp	KOH (photodecomposition of H2S)	0.5	6960 (μmol g−1 h−1)	[81]
11.	CdIn2S4	Surfactant assisted hydrothermal	Microsphere with nanopetals	-	300 W Xe lamp	KOH (photodecomposition of H2S)	0.5	6476 (μmol g−1 h−1)	[81]
12.	CuCo2S4	Hydrothermal	2D nanosheet like morphology	-	250 W Mercury lamp	Na2SO3/Na2S	0.06	25900 (μmol g−1 h−1)	[70]

. Among the ternary metal sulfides, ZnIn₂S₄ 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 ZnIn₂S₄, the atoms are arranged in layers along the c-axis. Zn atoms are tetrahedrally bonded to the S atoms (Zn-S₄) while the In atoms have two different environments, a tetrahedral arrangement with four S atoms (In-S₄) and an octahedral arrangement with 6 S atoms (In-S₆). This layered arrangement of atoms is responsible for the improved photocatalytic performance of ZnIn₂S₄ [30,31,32]. It has also been shown that the ZnIn₂S₄ 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 d¹⁰ configuration exhibits good photocatalytic performance during water decomposition [18]. Hence, the presence of In³⁺ metal ions with d¹⁰ configuration in ZnIn₂S₄ 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 ZnIn₂S₄.

2. 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_xS_y (MS, M₂S, M₃S₄, MS₂). Bi-metal sulfides are formed in a similar approach with a stoichiometry of A_1-xB_xS_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 AB₂S₄, 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 AB₂S₄ 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 A²⁺B³⁺S₄²⁻ [57]. These structures are called ternary metal chalcogenides. From among the family of AB₂S₄ semiconductors, ZnIn₂S₄ 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]. ZnIn₂S₄ displays less toxicity when compared to metal sulfides such as CdS and Sb₂S₃ while exhibiting similar optical properties which portrays the advantage of utilizing ZnIn₂S₄ in environmental remediation applications. Additionally, ZnIn₂S₄ 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 ZnIn₂S₄ have attracted immense attention in various applications including H₂ generation [37,38,39,40,41,42,43,44,45,46,47,48,49], CO₂ reduction [64,65,66], environmental remediation [40] under visible light irradiation. The layer structured semiconductor ZnIn₂S₄ 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 ZnIn₂S₄ has also been explored theoretically on the basis of Density Functional Theory (DFT) [55,64,67,68]. Studies suggest that ZnIn₂S₄ is a direct bandgap semiconductor as both VB and CB of ZnIn₂S₄ 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].

3. 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].

4. Photocatalytic Hydrogen Evolution by ZnIn₂S₄

ZnIn₂S₄, a ternary compound of the AB₂X₄ 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 ZnIn₂S₄ through surfactant-assisted hydrothermal method where the pH level of reactant played a major role in providing a maximum yield of H₂ about 1545 μmol g⁻¹ h⁻¹. Photostable ZnIn₂S₄ [45] could produce H₂ through water reduction for at least up to 150 h. Chaudari et al., [32] have reported excellent photocatalytic activity of ZnIn₂S₄ for the production of H₂ through the splitting of H₂S. 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 H₂ production [45,50]. ZnIn₂S₄ 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 ZnIn₂S₄ 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 ZnIn₂S₄.

5. 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 H₂ whilst the holes would be consumed by the sacrificial agent added to the system. Though its bandgap lies in the visible region, ZnIn₂S₄ suffers from recombination of charge carriers as well as poor migration [63]. In order to improve the photocatalytic performance of ZnIn₂S₄, 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 ZnIn₂S₄ and escalating migration of charge carriers with lower recombination rate resulting in enhanced photocatalytic activity.

5.1. 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 ZnIn₂S₄ 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-C₃N₄/ZnIn₂S₄ achieving in-situ growth of ZnIn₂S₄ nano leaves on the surface of g-C₃N₄ nanosheets (Figure 3) via a one-step surfactant-assisted solvothermal method [37].

Pudkon et al., (2020) investigated the effect of coupling MoS₂ with ZnIn₂S₄ in H₂ generation. An improved H₂ generation of 200 μmol g⁻¹ h⁻¹ was observed with 40% loading of MoS₂ into ZnIn₂S₄ lattice due to the enhanced charge separation efficiency and transportation, as well as by charge recombination suppression at the contact interface between ZnIn₂S₄ and MoS₂ [40] Yuan et al., (2016) constructed MoS₂-graphene/ZnIn₂S₄ microarchitectures for the improved H₂ generation of 4167 μmol g⁻¹ h⁻¹. The results reveal that the superior activity of the heterostructure is due to the positive synergetic effect between MoS₂ and graphene, where MoS₂ and graphene act as H₂ evolution reaction catalyst and electron transfer bridge respectively [44]. Chai et al., (2012) fabricated a series of composite heterostructures between multiwall carbon nanotubes (MWCNT) and ZnIn₂S₄ via a facile hydrothermal method. It is envisaged that MWCNT electron-accepting and electron-transporting properties increase the life of electron-hole pairs generated by semiconductor thus helping in enhancing the photocatalytic property [52]. Experimental results suggest that 3% MWCNT embedded in the interior of ZnIn₂S₄ microspheres shows a maximum H₂ production rate of 684 μmol h⁻¹ [52]. Fan et al., (2010) hydrothermally synthesized ZnIn₂S₄ 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 polymer-carriers has the advantages of being easy recycle, flexibility, excellent weatherability and large surface area. The growth of ZnIn₂S₄ microspheres on polymer surface (organic carrier) aids in photocorrosion resistance and enhances photocatalytic performance. ZnIn₂S₄/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 ZnIn₂S₄ 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 H₂ 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.

5.2. 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 (La³⁺, Ce³⁺, Er³⁺, Gd³⁺, Y³⁺) into the lattice of ZnIn₂S₄ [38]. In their study, the photocatalytic H₂ production efficiency was observed to increase in the order of La-ZnIn₂S₄>Ce-ZnIn₂S₄>Er-ZnIn₂S₄>Gd-ZnIn₂S₄>Y-ZnIn₂S₄ post modification of ZnIn₂S₄ with RE ions. The decreasing number of electrons in the rare-earth 4f shell was consistent with the increased photocatalytic activity. REEs existed as RE₂O₃ oxide and modified the lattice of ZnIn₂S₄, increased its surface area and introduced defects on the catalyst surface, thus inhibiting recombination of photogenerated charge carriers. ZnIn₂S₄ lattice was also modified with La³⁺ for H₂ evolution activity [39]. Compared to pure ZnIn₂S₄, the H₂ 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 ZnIn₂S₄ lattice [43] and used Pt as co-catalyst for H₂ evolution. Among a series of samples synthesized by them, 1.0 Pt/1.0RGO/1.0 La-ZnIn₂S₄ 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 ZnIn₂S₄ to generate H₂. Shen et al., (2012) modified ZnIn₂S₄ with series of alkaline earth (AE) metals (Ca, Ba, Sr) [49]. Among the AE-modified samples, only Ca incorporated samples show higher photocatalytic activity than pure ZnIn₂S₄, 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 ZnIn₂S₄>Ca– ZnIn₂S₄>Sr– ZnIn₂S₄>Ba– ZnIn₂S₄. 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– ZnIn₂S₄ sample. However, only Ca- ZnIn₂S₄ 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 H₂ evolution [49]. An enhancement in photocatalytic activity post-incorporation of different transition metal (Cr, Mn, Fe, Co) ions into the lattice of ZnIn₂S₄ [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 non-radiative recombination centers for photogenerated electrons and holes. Shen et al., (2008) analyzed the photocatalytic activity of Cu-doped ZnIn₂S₄ for H₂ evolution application [55]. Incorporating Cu in ZnIn₂S₄ 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)) Ni²⁺ 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 Ni³⁺ state. Due to the instability of Ni³⁺, it reverts back to Ni²⁺ 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].

5.3. 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 ZnIn₂S₄ of various dimensions [31]. With the combination of hydrothermal, solvothermal and surfactant template techniques well-defined morphology of ZnIn₂S₄ 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 ZnIn₂S₄ 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 ZnIn₂S₄ 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 ZnIn₂S₄ 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 ZnIn₂S₄ samples synthesized with various morphologies, H₂ 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-C₃N₄ nanosheets and 2D ZnIn₂S₄ nano leaves via facile surfactant-assisted solvothermal method for enhanced photo-induced H₂ generation [37]. From the as-synthesized bulk g-C₃N₄, thinner g-C₃N₄ 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]. ZnIn₂S₄ nano leaves were extracted from bulk ZnIn₂S₄ 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 H₂ 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 ZnIn₂S₄ with REEs had significant influence on the morphology and porous nature of the material. Unmodified ZnIn₂S₄ possessed gully-ball like spherical structures with collapsed nanosheets. When Y³⁺ was added, the surface of ZnIn₂S₄ was partially open with pores ranging between 0.3 μm to 0.5 μm. The addition of Gd³⁺ could open up the surface with many porous sheets to a given rose-like structure, while the addition of Er³⁺ and Ce³⁺ 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 La³⁺. This shows that the addition of rare earth ions onto the surface of ZnIn₂S₄ 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 ZnIn₂S₄ [38]. Solvent mediated synthesis of ZnIn₂S₄ by Shen et al., (2008) provides new insights on the effect of solvents on the morphology, crystallinity and photocatalytic properties of ZnIn₂S₄ [50]. Samples synthesized with H₂O 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 ZnIn₂S₄ 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.

5.4. Role of Sacrificial Agent

Sacrificial agents play a prominent role in photocatalytic H₂ 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 H₂ 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 S²⁻ obtained from sulfide/sulfite mixtures. Here, the H₂ production experiment with ZnS/ZnIn₂S₄ has been conducted with and without the usage of glucose as an electron donor. In the absence of glucose, it is noted that the H₂ production is saturated after a certain time whereas, a substantial increase in the rate of H₂ production is noted in the presence of glucose. In absence of glucose, S²⁻ generated from ZnS photocatalyst acts as a sacrificial agent for H₂ generation. In the presence of glucose, a proportional increase of H₂ 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 H₂ production with sulfide-based photocatalysts [32,35,37,42].

6. Compounds with AB₂X₄ Structure Other than ZnIn₂S₄

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 AB₂X₄ structure (besides ZnIn₂S₄).

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⁻¹g⁻¹ [75].

CdS nanocrystals incorporated CdLa₂S₄ microspheres with 0.4 wt% of Pt as co-catalyst showed a high H₂-production rate of 2.25 mmol h⁻¹ [80]. Magnesium-based chalcogenide photocatalyst MgIn₂S₄, on the other hand, when integrated with polyaniline (PANI) proves to be an efficient H₂ generating photocatalyst [78]. PANI/MgIn₂S₄ nanoflower photocatalysts with 1% PANI loading synthesized through facile chemisorption method exhibits a decent H₂ evolution of 200.8 μmol g⁻¹ h⁻¹ under visible light irradiation. Chauhan et al., (2019) reported co-catalyst free CuCo₂S₄ 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 CuCo₂S₄ nanosheets under visible light and it exhibited excellent weight-normalized photoactivity generating H₂ at the rate of ~25,900 μmol g⁻¹ h⁻¹. CuCo₂S₄ 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].

7. 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 H₂ 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, ZnIn₂S₄ 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, ZnIn₂S₄ 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.