English

• 1. Introduction

  Nowadays, the rapid development of industrialization and worldwide population growth inevitably bring about severe environmental pollution and non-renewable energy shortage. Currently, photocatalytic technology has been generally considered as an effective and sustainable approach to solving these problems [1]. Generally, an ideal photocatalyst needs to have a suitable bandgap, wide light absorption range, high carrier separation efficiency and excellent physiochemical stability. In this aspect, transition metal compounds exhibit great potential in catalysis [2,3]. To date, researchers have developed numerous photocatalysts such as metal oxide (TiO₂ [4,5], WO₃ [6,7]), metal chalcogenides (ZnS [8], CdS [9], MoS₂ [10-12]), metal nitrides [13], and graphitic carbon nitride (g-C₃N₄ [14-16]). However, their photocatalytic performance is usually limited by unsuitable electronic structure and optical properties, which would lead to unwanted recombination of photo-generated electron-hole pairs and limited light harvesting capability. Therefore, researches on semiconductor photocatalysts with more desirable catalytic activity have always been the key to photocatalysis. For example, Zhou et al. [17,18] creatively constructed 3D-MoSSe/NiSe₂ and NiSe₂/Ni foam to produce hydrogen efficiently.

  Zinc indium sulfide (ZnIn₂S₄), a typical AB₂X₄ family semiconductor, has become a new hotspot in recent years. It has three major crystal structures of hexagonal, cubic and trigonal phase [19], with hexagonal ZnIn₂S₄ exhibiting the best photocatalytic behavior [20,21].

  Compared with conventional TiO₂ which only absorbs ultraviolet light [22] and has a broad bandgap of approximately 3.05 eV [5], ZnIn₂S₄ has superior light harvesting capability in the visible region [23] and a narrower bandgap. Take hexagonal ZnIn₂S₄ as an example, its conduction band and valence band position are −0.85 V vs. NHE and 1.56 V vs. NHE, respectively [24], which is suitable for many redox reactions such as photocatalytic water splitting where the reduction potential of H⁺/H₂ is 0 V vs. NHE (pH 0) and the oxidation potential of O₂/H₂O is 1.23 V vs. NHE (pH 0) [25]. Moreover, ZnIn₂S₄ is a bimetallic sulfide mainly obtained by incorporating zinc into indium sulfide [26] and has no toxicity in comparison with other monometallic sulfide such as CdS [27]. Besides, its stable S-Zn-S-In-S-In-S lamellar structure makes it easier to be modified and construct heterostructures with other materials [28-30], exhibiting great application potential in various fields such as photocatalytic water splitting [31,32], carbon dioxide reduction [26,33], pollutant removal [34,35], nitrogen fixation [36] and sensors [37]. As a result, it is of great significance to summarize the recent advances of ZnIn₂S₄-based photocatalysts.

  Herein, we first gave the concrete preparation process of ZnIn₂S₄ with diverse morphological structures. Then, considering the photocatalytic performance of pristine ZnIn₂S₄ is still far from satisfactory and cannot meet the actual demand due to its slow carrier transfer kinetics and adverse electron-hole recombination, various modulation strategies such as layer and size control, doping, vacancy engineering, co-catalyst loading and hetero-nanostructures were elucidated clearly. Thereafter, several important applications of Znln₂S₄ were summarized comprehensively. At last, we discussed the conclusion and give our personal outlook. We sincerely hope this review can provide some new ideas for researchers to further promote the development of this fascinating material.

  2. Preparation

  Till now, the common methods for preparation of ZnIn₂S₄ mainly include hydrothermal method [23], solvothermal method [38], spray pyrolysis method [39], chemical vapor deposition method [40], microwave heating method [41], etc. Among them, hydrothermal and solvothermal methods have been the most studied owing to their relative mild reactive condition and controllability.

  By virtue of regulating temperature, solvent and surfactant, ZnIn₂S₄ with diverse morphological structures can be obtained and exhibit excellent photocatalytic performance (Fig. 1). For example, as early as 2006, Gou et al. [42] proposed a shape-controlled synthesis of ZnIn₂S₄ and successfully synthesized ZnIn₂S₄ nanowires, nanotubes, nanoribbons and microspheres via a facile solvothermal and hydrothermal route. With zinc sulfate, indium chloride, and thioacetamide (TAA) as precursors and pyridine as the solvent, ZnIn₂S₄ nanotubes and nanoribbons were fabricated at temperatures above 180 ℃ and below 160 ℃, respectively. With the same reagent and water replacing pyridine, ZnIn₂S₄ solid and hollow microspheres were prepared in the presence of cationic surfactant cetyltrimethylammonium bromide (CTAB) and nonionic surfactant polyethylene glycol (PEG), respectively. Additionally, large quantities of ZnIn₂S₄ nanowires could also be transformed from ZnIn₂S₄ hollow microspheres samples after an ultrasonic dispersion treatment for over 40 min, when the spheres were almost entirely disintegrated. In 2013, Hu et al. [43] creatively developed an ion-induced strategy to prepare ZnIn₂S₄ nano-rings. First, NaInS₂ was transformed into Cu-doped NaInS₂ nanosheets (Cu-NaInS₂) and the subsequent Kirkendall effect led to the formation of Cu-NaInS₂ nanoring. It was found that the Cu species diffused outward faster than the In³⁺ that diffused inward, which led to the thinning in the center of Cu-NaInS₂ nanosheet and the formation of a hollow structure finally. Then the Cu-ZnIn₂S₄ was fabricated via ion exchange reaction between the Na⁺ of the as-prepared Cu-NaInS₂ nanoring and Zn²⁺. Their ingenious method provided a template for the preparation of quaternary alloys. The above-mentioned ZnIn₂S₄ with various morphological structures opens the door for preparing composite materials with diverse nanostructures and superior photocatalytic performance through increasing specific surface area and active sites, facilitating light-harvesting capacity, improving charge separation efficiency, etc.

  Figure 1

  

  Figure 1.  Schematic diagram of ZnIn₂S₄ with different morphologies.

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  3. Modulation strategies

  It is generally known that the photocatalytic activity of pristine ZnIn₂S₄ can be limited due to the rapid recombination of electron-hole pairs, sluggish reaction kinetics and poor light absorption capability. Hence, many regulation strategies have been proposed for the sake of achieving superior photocatalytic performance. Herein, we briefly classify these modulation strategies into layer and size control, vacancy engineering, exotic-atom doping, co-catalyst loading and nano-heterostructures (Fig. 2).

  Figure 2

  

  Figure 2.  Schematic illustration of different modulation strategies for ZnIn₂S₄.

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  3.1 Layer control & size control

  Compared with bulk catalyst, ultrathin two-dimensional photocatalysts have many advantages such as stackability, large specific surface area, improved charge migration rate, and high mechanical flexibility [25,44,45]. Therefore, the preparation of ultrathin ZnIn₂S₄ nanosheets via layer control has been extensively researched. In 2017, Yang et al. [46] proposed a self-surface charge exfoliation strategy which briefly involved a facile low-temperature refluxing process and a water-assisted exfoliation treatment by ultrasonication. The as-prepared ZnIn₂S₄ nanosheet with a thickness of 2.5 nm proved to be a single-unit-cell atomic layer and is easy to integrate with MoSe₂ to form a heterostructure because of electrostatic attraction, exhibiting an improved H₂ generation rate of 6454 mmol g⁻¹ h⁻¹. This method overcame the drawbacks of lattice mismatching and expensive cost brought by conventional epitaxial growth method and realized independent control of thickness and composition of layer design.

  Besides, the size of photocatalysts can determine structural or optical properties and influence photocatalytic process directly. Therefore, researches on preparing ZnIn₂S₄ quantum dots with controllable size are of great significance as well. For example, with oleylamine as the ligand and noncoordinating octadecene as the solvent, Peng et al. [47] successfully prepared ZnIn₂S₄ nanocrystals with tunable size from 2.1 nm to 9.2 nm by temperature control. It was found that the band gap of ZnIn₂S₄ increased from 2.35 eV to 3.28 eV with the size decreasing from 9.2 nm to 2.1 nm, which was ascribed to the size-dependent quantum confinement effect. Interestingly, ZnIn₂S₄ with a nanoplate structure can also be obtained by replacing the sulfur powder with thiourea as the sulfur source. So far, the accurate control of layer and size of ZnIn₂S₄ has not been realized and still needs further study.

  3.2 Heteroatom-doping

  Doping is an effective strategy to modulate physicochemical properties of ZnIn₂S₄ with thickness of atomic level, including extending light absorption range, improving electric conductivity, increasing active sites, adjusting band gap and facilitating the H adsorption and desorption, etc. Here we briefly divided the doping strategy into metal doping and nonmetal doping.

  3.2.1   Metal doping

  Till now, many metal elements such as Cu [48], Mo [49], La [50], Fe [51] have been utilized to modify pristine Znln₂S₄ to improve its photocatalytic performance. Generally, metal doping can regulate electronic structure, boost carrier separation and extend light absorption spectrum. For example, Qiu et al. [52] fabricated Ni-doped ZnIn₂S₄ nanosheets with Ni atom substituting Zn atom selectively. In Fig. 3a, the adsorption free energy of H of neighboring S atoms decreased clearly after Ni doping and the S-H_ad bonds were longer than before due to Ni incorporation, which facilitated the adsorption and desorption process of H atoms. Besides, the Ni-ZnIn₂S₄ exhibited a narrower band gap and an increased density of states (DOS) at valence band maximum (VBM), which contributed to more charge-carriers participating in the photocatalytic reaction (Fig. 3c). Therefore, the hydrogen evolution activity achieved 5.43 mmol g⁻¹ h⁻¹, which was almost 7 times as much as that of bare ZnIn₂S₄ (Fig. 3b).

  Figure 3

  

  Figure 3.  (a) The adsorption free energy of H of different neighboring S atoms and S-H_ads bond length before and after Ni doping. (b) Photocatalytic hydrogen evolution performance of ZnIn₂S₄ with different Ni doping quantities. (c) Schematic illustration of photocatalytic hydrogen production process of ZnIn₂S₄ and Ni-ZnIn₂S₄. Reprinted with permission [52]. Copyright 2021, Elsevier. (d) Schematic depiction of the dual-defect (Ag dopants and nanoholes) formed on ZnIn₂S₄ monolayers and the reactive mechanism of the dual-defect structures in photocatalytic overall water splitting. (e) Comparison of the photocatalytic H₂ and O₂ evolution behaviors of ZnIn₂S₄, Ag-ZnIn₂S₄, and Cu-ZnIn₂S₄ under visible light illumination (300 W xenon lamp, λ > 420 nm). Reprinted with permission [53]. Copyright 2021, American Chemical Society.

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  Moreover, Pan et al. [53] prepared Ag-doped ZnIn₂S₄ through a facile aqueous cation exchange reaction between Zn atom of ZnIn₂S₄ and Ag⁺, which led to the formation of dual defects (Ag dopant and nanoholes). The former boosted photocatalytic oxygen evolution reaction (OER) and the latter facilitated hydrogen evolution reaction in the meantime (Fig. 3d), successfully realizing an all-in-one photocatalytic water splitting (Fig. 3e).

  Of note, exotic atoms within photocatalysts can also serve as a recombination center, which is disadvantageous to photocatalytic reactions. For example, Wang et al. [54] prepared Cu-doped ZnIn₂S₄ with different doping amount to study atomic-level effect of doping on catalytic activity. They found that ZnIn₂S₄ with doping quantity of 0.5 wt% exhibited the optimal hydrogen evolution rate of 26.2 mmol h⁻¹ g⁻¹ while ZnIn₂S₄ with doping quantity of 3.6 wt% exhibited a worse photocatalytic performance of 0.9 mmol h⁻¹ g⁻¹, compared with pristine ZnIn₂S₄ (6.5 mmol h⁻¹ g⁻¹). On the one hand, suitable amount of Cu doping can improve light absorption, boost carrier separation and maintain stable coordination structure. On the other hand, excessive Cu doping can lead to the formation of structural distortion and upshift of valence band maximum, which would in turn increase the possibility of electron-hole pairs recombination and worsen photocatalytic behavior.

  3.2.2   Nonmetal doping

  According to the existing literature, C, O and N elements [55] are three major nonmetal dopants of ZnIn₂S₄ nanosheets because they can easily substitute the exposed S atom in the surface of ZnIn₂S₄. For example, Xu et al. [56] prepared carbon quantum dots doped ZnIn₂S₄ nanoflowers with extended light harvesting spectrum, which exhibited improved photocatalytic tetracycline hydrochloride degradation capability. Yang et al. [57] fabricated oxygen-doped ZnIn₂S₄ where O atoms selectively substituted S atoms in [ZnS]₄ tetrahedron and a tremendously boosted hydrogen evolution rate of 2120 µmol h⁻¹ g⁻¹ was obtained. Du et al. [58] synthesized N-doped ZnIn₂S₄ and successfully produced hydrogen at 11086 µmol g⁻¹ h⁻¹. Recently, it is reported that phosphorus element can serve also as dopant to improve photocatalytic performance of ZnIn₂S₄. Qin et al. [59] successfully prepared P-doped ZnIn₂S₄ via a simple hydrothermal method with NaH₂PO₄ as a precursor. Different from pure ZnIn₂S₄ microsphere, the as-prepared P-ZnIn₂S₄ existed in the form of dispersed ultrathin nanosheets (Figs. 4a and b), which provided enormous exposed specific sites and was beneficial to the CO₂ adsorption process. The optimal P-ZIS with ratio of 1:15 exhibits a CO generation rate of 476 µmol h⁻¹ g⁻¹, which was 2.5 times higher than that of pristine ZnIn₂S₄ (Fig. 4c). In addition, Chong et al. [60] fabricated phosphorus-doped ZIS with P atoms replaced S3 sites exhibiting the best stability (Fig. 4d). They also performed detailed Density Functional Theory (DFT) calculations to unravel the principle of doping effect on photocatalytic water splitting behavior. First, P doping can upshift the valence band maximum and reduce the bandgap of ZnIn₂S₄, thus enhancing the light harvesting capability and carrier separation efficiency. Second, owing to the remarkable charge density increase around S2 active sites which are favorable to hydrogen evolution reaction (Fig. 4e), H* adsorption was greatly improved and better hydrogen evolution reaction (HER) performance was achieved.

  Figure 4

  

  Figure 4.  SEM image of (a) ZIS and (b) P-ZIS. (c) The effect of ZIS/NaH₂PO₄ ratio on the photocatalytic CO₂ reduction. Reprinted with permission [59]. Copyright 2021, American Chemical Society. (d) Simulated crystal structure for pristine ZIS, and P-ZIS (SPD-3). (e) Free energy diagram for hydrogen evolution for each site on ZIS and P-ZIS. Reprinted with permission [60]. Copyright 2022, Springer Nature. To sum up, exotic-atom doping is undoubtedly a pivotal method to enhancing the catalytic activity of ZnIn₂S₄ nanosheets: (1) adjust band gap and extend light absorption range; (2) improve electric conductivity; (3) form defects and increase active sites; (4) facilitate reactant adsorption and desorption.

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  3.3 Co-catalyst loading

  To improve catalytic behavior of pristine ZnIn₂S₄, we can also load some co-catalysts on ZnIn₂S₄ such as metal atoms, metal sulfides and metal oxides.

  Some single atoms with unique electronic structure can greatly regulate absorption intensity and coordination structure of reactants and intermediates, thus promoting catalytic behavior. Metal atoms loaded on ZnIn₂S₄ usually possess localized surface plasmon resonance effect and can build Schottky barrier that facilitate light harvesting capability and charge separation, thus endowing ZnIn₂S₄ with superior catalytic performance. Pt, Au and Pd are three common metal atoms deposited on ZnIn₂S₄. Very recently, Shi et al. [61] immobilized Pt atom on hexagonal ZnIn₂S₄ nanosheets and successfully produced hydrogen at the rate of 17.5 mmol g⁻¹ h⁻¹, with generous H₂ bubbles being observed. That can be ascribed to the synergistic effect of decreased electron-hole recombination and a tip effect induced by Pt single sites. Wang et al. [62] rationally loaded Pd atoms on ZnIn₂S₄ via in situ icing-assisted photo-reduction method. In this case, the Pd atom was coordinated with three sulfur atoms. The introduction of Pd atoms induced charge polarization and provided more active sites. With electrons aggregating around Pd single atoms-ZnS layer and holes distributed to InS₂ layer, the as-prepared Pd-ZnIn₂S₄ can effectively realize dual-function of photocatalytic hydrogen evolution (11.1 mmol g⁻¹ h⁻¹) and oxidation of benzylamine to N-benzylidene benzylamine (10.2 mmol g⁻¹ h⁻¹). Feng et al. [63] constructed double-metal loaded Au-Pd/ZnIn₂S₄, which outperformed Pd/ZnIn₂S₄ and Au/ZnIn₂S₄ in the oxidation of aromatic alcohol. Moreover, Pan et al. [64] successfully anchored Ni atoms in ZnIn₂S₄ nanosheets with rich S vacancies. It was found that the incorporation of Ni atoms induced the formation of Ni-O-M (Zn/In) atomic interface, which is favorable for the charge transfer and carrier separation.

  Metal sulfides and metal oxides can also be loaded on ZnIn₂S₄, which actually forms an 0D-2D heterostructure. For example, Zhang et al. [65] ingeniously grew MoS₂ quantum dots at S vacancies on a Zn facet of monolayer ZnIn₂S₄ nanosheet, which is obtained via a facile bulk ZnIn₂S₄ exfoliation and lithium intercalation process. With S vacancies as the electron trap, photogenerated electrons were greatly prevented from transmitting from the Zn facet to In facet and recombing with the holes. And the ultrathin 2D structure also reduced the interlayer resistance and facilitated the charge transfer. Therefore, the photoexcited electrons of V_s-M-ZnIn₂S₄ were easy to transfer to MoS₂QDs and participated in hydrogen production. It was found that 2MoS₂QDs@V_s-M-ZnIn₂S₄ exhibited the optimal photocatalytic performance (6.884 mmol g⁻¹ h⁻¹) and excessive S vacancies can also serve as the recombination center and lower H₂ evolution rate. Notably, there exist some metal sulfides that exhibited the opposite effect. Shen et al. [66] and Lim et al. [67] found that Ag₂S, SnS, CuS and MoS₂ loading could boost catalytic activity while CoS, NiS or MnS loading hindered the photocatalytic activity of ZnIn₂S₄. Zhang et al. [28] loaded CeO₂ on ZnIn₂S₄ nanosheets through facile solvothermal and room temperature precipitation methods. The highest hydrogen generation rate of the composite reached 847.42 µmol g⁻¹ h⁻¹, which is 2.37 times higher than that of bare ZnIn₂S₄. Besides, metal oxides like NiO, IrO₂, MoO₂ [68] and ZnO [69] loaded on ZIS have been reported to be favorable to photocatalytic water splitting as well.

  Moreover, some other nanoparticles such as AgIn₅S₈ [70] and NiSe₂ [71] were also well prepared to modify ZnIn₂S₄ and boost photocatalysis.

  3.4 Vacancy engineering

  Atomic vacancy, a typical point defect, forms when some atom breaks away from the bondage of other atoms. Vacancies can improve photocatalytic activity through the following aspects: (1) serve as active site which can trap electrons or holes; (2) extend light absorption range; (3) adjust energy band position and electronic properties; (4) promote adsorption and desorption of intermediate. Zn, S and In are three major vacancies of ZnIn₂S₄-based composites.

  3.4.1   Zn vacancy

  Tai and Zhou [72] introduced Zn vacancies within hierarchical ZnIn₂S₄ nanoparticles via reactive ion etching technique (Fig. 5a). The as-obtained ZnIn₂S₄ had a narrower bandgap and improved cycling stability, thus exhibiting a 2.7-fold enhancement of hydrogen evolution efficiency. He et al. [73] also prepared 3D ZnIn₂S₄ nanospheres with rich Zn vacancies and conducted carbon dioxide reduction. Zn vacancies introduced a new defect level to capture photogenerated electrons, which facilitated the separation of electron-hole pairs. As shown in Fig. 5b, under light irradiation, electrons and holes are produced in conduction band and valence band, respectively. The photo-excited electrons are further transferred to the defect level instead of recombining with holes and activate CO₂ to generate intermediate CO²⁻ and CO. Eventually, with the synergistic effect of efficient charge separation, increased light response range and more active sites, an enhanced CO₂ reduction rate of 276.7 µmol g⁻¹ h⁻¹ was achieved (Fig. 5c).

  Figure 5

  

  Figure 5.  (a) The schematic illustration of Zn vacancy formation. Reprinted with permission [72]. Copyright 2021, Elsevier. (b) The reactive mechanism of CO₂ reduction for the ZnIn₂S₄ with Zn vacancies. (c) The photocatalytic performance of 3D-ZIS with Zn vacancies and bulk-ZIS, respectively. Reprinted with permission [73]. Copyright 2019, Wiley VCH. (d) Schematic diagram of the surface domain potential difference between adjacent microdomains induced by the surface S vacancy defects. Reprinted with permission [74]. Copyright 2022, Elsevier. (e) Schematic depiction of gradient H migration for H₂ evolution of Cu doped ZIS. (f) H₂ evolution activities of ZIS and xCu-ZIS (x = 1%-6%). Reprinted with permission [75]. Copyright 2021, American Chemical Society. (g) Schematic diagram for the photocatalytic hydrogen evolution of pristine-ZIS and ultrathin ZIS-V_In. (h) Photocatalytic hydrogen evolution activities of pristine-ZIS and ZIS-V_In. Reprinted with permission [76]. Copyright 2022, Elsevier.

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  3.4.2   S vacancy

  Liu et al. [74] prepared defective ZnIn₂S₄ microspheres with rich S vacancies via a solvothermal method and a low-temperature hydrogenation route. As shown in Fig. 5d, the introduction of S vacancies narrowed bandgap and induced surface domain potential difference which improved light harvest capability and promoted electrons to transfer from S vacancies to adjacent favorable domain and facilitated spatial electron-hole separation. Therefore, the defective ZnIn₂S₄ microspheres outperformed pristine ZnIn₂S₄ without S vacancies. Zhang et al. [75] introduced hetero-atom Cu elaborately via substituting the Zn atom of ZIS. Due to electron nonequilibrium and lattice mismatch caused by Cu confinement, a self-adapting S vacancy was induced because of Jahn-Teller distortions (Fig. 5e). Atomic Cu and V_s synergistically modulate charge separation with V_s being an electron trap and Cu acting as a hole trap. Besides, regulated by V_s and driven by photo-thermal effect, an efficient channel of gradient H migration was constructed. The optimum 5%Cu-ZlS nanosheets with S vacancy exhibit remarkable photo-catalytic HER performance compared with pristine ZnIn₂S₄ (Fig. 5f). Besides, Wang et al. [76] elaborately synthesized a sulfur-deficient ZnIn₂S₄/oxygen-deficient WO₃ composite with C-wood as a photothermal substrate. S and O vacancies improved carrier utilization and light absorption behavior, promoting ZnIn₂S₄ and WO₃ to produce hydrogen and oxygen, respectively. Moreover, C-wood as an electron bridge also induced photothermal effect which transformed liquid water into water vapor and was more conductive to water splitting. Under these synergistic effects, this photocatalytic system exhibited 169.2 and 82.5 µmol/h for H₂ and O₂ production, respectively.

  3.4.3   In vacancy

  Very recently, Luan et al. [77] innovatively prepared ultrathin ZnIn₂S₄ nanosheets with interlayer In vacancies via a heating-up hydrothermal process. According to the electron paramagnetic resonance (EPR) spectra, a greater peak intensity that confirms the presence of defects in ultra-ZIS-V_In nanosheets can be clearly observed.

  In Fig. 5g, for the pristine ZIS, electrons transferred to the surface [InS]₄ tetrahedron from interlayer [ZnS]₄ tetrahedron under light irradiation, while the holes were still generated in the interlayer [ZnS]₄ tetrahedron. In contrast, for the ultra-ZIS-V_In, photo-generated electrons were excited to the surface [InS]₄ tetrahedron and holes were generated on the surface [ZnS]₄ tetrahedron because the introduction of In vacancies led to the S 3p orbitals near the edge of valence band transferring from the interlayer [ZnS]₄ tetrahedron to the surface [ZnS]₄ tetrahedron. As a result, ZIS with In vacancies exhibited higher carrier separation efficiency and a fantastic HER performance of 13.4 mmol h⁻¹ g⁻¹, which is 8.9 folds higher than that of defect-free ZnIn₂S₄ (Fig. 5h). Generally, In vacancies are difficult to realize due to the steric hindrance caused by its location in the interlayer of ZnIn₂S₄ stacking structure.

  3.5 ZnIn₂S₄ hetero-nanostructures

  By integrating two materials in one, we can obtain many composites with fascinating properties, which are determined by the hetero-junctions formed between interfaces. Generally, hetero-junctions can be divided into type Ⅰ heterojunction (straddling bandgap), Type Ⅱ heterostructures, p-n heterojunction, Schottky junction and Z-scheme heterojunction [78]. According to the morphology of different materials, we concisely classified ZnIn₂S₄ hetero-nanostructures into 2D-2D, 1D-2D and 3D nanostructures.

  3.5.1   2D-2D stacking materials

  Feng et al. [79] synthesized 2D-2D MoSe₂/ZnIn₂S₄ via a facile a secondary hydrothermal route and the as-obtained composite delivered a HER performance of 1226 µmol g⁻¹ h⁻¹. Soon afterwards, Wang et al. [80] innovatively fabricated a 2D-2D S_v-ZnIn₂S₄/MoSe₂ heterostructure by adding monohydrate (N₂H₄·H₂O) in the process to form S vacancies which facilitated charge separation and could serve as anchoring sited for Mo atoms. Such structure followed a typical Z-scheme mechanism and exhibited an extraordinarily high hydrogen evolution rate of 63.21 mmol g⁻¹ h⁻¹. Su et al. [30] constructed Ti₃C₂T_x/ZnIn₂S₄ heterostructure to enhance hydrogen evolution. With S vacancies as the electron trap and Ti₃C₂T_x as the electron acceptor, the as-prepared material shortened carrier transfer distance and thus made it easier for electrons transferring to Ti₃C₂T_x and participating in HER. Moreover, the Schottky barrier which formed after contact also suppressed the recombination of electron-hole pairs. Finally, a high hydrogen evolution rate of 148.4 µmol/h was obtained. Additionally, Qin et al. [81] prepared a ZnIn₂S₄/g-C₃N₄ 2D/2D photocatalysts with abundant S vacancies via hydrothermal method with Zn(CH₃COO)₂·2H₂O, InCl₃·4H₂O and TAA dispersed ethanol solution. With the large contact area, ameliorated charge kinetics and S vacancies serving as the electron trap, a hydrogen evolution rate up to 6095.1 µmol g⁻¹ h⁻¹ was realized.

  3.5.2   1D-2D

  Generally, 1D-2D heterostructure refers to the growth of 2D materials on 1D nanostructures. As early as 2013, Xu et al. [29] fabricated CdS/ZnIn₂S₄ nano-heterostructure. The lattice mismatch and bond mismatch made ZnIn₂S₄ grow helically on the CdS nanowires, thus widening the surface area. In 2014, Liu et al. [82] prepared ZnIn₂S₄ nanosheets/TiO₂ nanorods heterostructure arrays for water splitting and the as-designed heterostructure improved current density and decreased onset potential. Recently, Li et al. [83] also constructed a ZnIn₂S₄/TiO₂ heterostructure in which 2D ZnIn₂S₄ were grown on TiO₂ nanofibers via a hydrothermal route. This composite exhibited optimal hydrogen production rate of 6.03 mmol h⁻¹ g⁻¹ with TEOA as sacrificial reagent. Under light irradiation, electrons of ZnIn₂S₄ bend the conduction band upward and electrons of TiO₂ downward, hence forming an internal electric field. With electrons on TiO₂ conduction band recombing with holes on ZnIn₂S₄ valence band, electrons on ZnIn₂S₄ conduction band simultaneously participated in HER, which followed a typical S-scheme heterostructure mechanism.

  3.5.3   3D core-shell nanostructure

  Generally, core-shell structure is a closed hollow structure mainly consisting of porous material. Such unique structure is favorable for charge transfer, carrier separation and absorption of reactants in shell [84]. For example, Wang et al. [85] fabricated Co₉S₈@ZnIn₂S₄ heterostructure cage which possessed large specific area and abundant active sites and delivered a hydrogen-production rate of 6250 µmol h⁻¹ g⁻¹. His team also successfully prepared Co/N-Doped graphitic carbon nanocages [86] for HER and realized a more efficient H₂ evolution rate of 11270 µmol h⁻¹ g⁻¹. Very recently, Fan et al. [87] creatively prepared Ce-doped ZnIn₂S₄ tetrakaidecahedron hollow nanocages via a facile hydrothermal route with Ce-based metal-organic-frameworks. The tetrakaidecahedron hollow structure and Ce dopant synergistically enlarged the surface area, facilitated light harvesting and improved carrier separation efficiency, therefore contributing to an optimal HER activity of 7.46 mmol g⁻¹ h⁻¹. Many other core-shell heterostructures have also been extensively prepared such as hollow Ti₃C₂@ZnIn₂S₄ [88], ZnIn₂S₄/Ta₃N₅ [89] and TiO₂@ZnIn₂S₄ hollow nanospheres [90]. However, despite so much merits above, the closed core-shell structure can also inhibit gas reactants diffusion and mass transfer, which goes against its catalytic activity. Therefore, there is still a long way to go for the design of an elaborate porous ultrathin structure.

  4. Applications

  As a promising photocatalyst with suitable band gap, ZnIn₂S₄ has important applications in various fields such as water splitting, value-added products production, pollutant removal, CO₂ reduction, nitrogen fixation. Fundamental principle of ZnIn₂S₄ photocatalysis is very simple. Under light irradiation, electrons are excited to the conduction band and holes are generated on the valence band. Then the photo-generated electrons can be transferred to participate in reduction reactions such as H₂ evolution, CO₂ reduction, and nitrogen fixation. Meanwhile, the photo-generated holes take part in oxidizing reactions such as O₂ production and oxidation of alcohol and benzene.

  4.1 H₂ production

  Many elaborate designs of ZnIn₂S₄-based photocatalysts for HER are constantly emerging these years. Chao and Zhang et al. [91] atomically anchored Mo sites on ZnIn₂S₄ hierarchical nanotubes (HNTs) through a novel in-situ NH₄⁺-etched strategy under mild condition (Fig. 6a). Benefited from the mesoporous structure of ZnIn₂S₄ HNTs and well-defined Mo-S₂O₁ sites with distinctive coordination configuration, the hollow Mo single-atoms anchored ZnIn₂S₄ (MoSA-ZIS) HNTs presented enhanced HER performance of 29.9 µmol g⁻¹ h⁻¹ compared with ZnIn₂S₄ NSs (5.98 µmol g⁻¹ h⁻¹) (Fig. 6b). Very recently, Su et al. [92] used ZnIn₂S₄ to wrap NiMoO₄ nanorods through a hydrothermal and solvothermal process (Fig. 6c). During synthesis process, Mo⁵⁺ was more suitable for doping due to its similar ionic radius to Zn²⁺ and Ni²⁺ was dissolved on ZnIn₂S₄ nanosheets. The as-prepared Mo-doped/Ni-supported ZnIn₂S₄ nanosheets can successfully realize an optimal hydrogen evolution rate of 5.14 mmol h⁻¹ g⁻¹ (Fig. 6d). That could be attributed to the synergistic effect of heterojunctions with Mo doping and metallic Ni supports, which extended the light absorption range, enhanced charge transport kinetic, and maintained strong redox capability.

  Figure 6

  

  Figure 6.  (a) Schematic diagram of synthetic process for the MoSA-ZIS HNTs photocatalyst. (b) The rates of photocatalytic H₂ evolution over MoO₃ NRs, ZnIn₂S₄ NSs and MoSA-ZIS HNTs. Reprinted with permission [91]. Copyright 2021, Springer. (c) Synthetic scheme of NMO@M-ZIS-N. (d) The photocatalytic HER rate of ZIS and NMO@M-ZIS-N with different NMO loading (TEOA as a sacrificial agent). Reprinted with permission [92]. Copyright 2022, The Royal Society of Chemistry. (e) Schematic illustration of the process of capturing the water molecules and a possible photocatalytic mechanism of the HER. (f) Hydrogen evolution activities of ZIS, ZIS@C and C@ZIS (λ ≥ 420 nm). Reprinted with permission [93]. Copyright 2021, American Chemical Society. (g) Schematic illustration for the synthesis of ZnIn₂S₄-MOFL. (h) Schematic illustration of photocatalytic mechanism of ZnIn₂S₄-MOFL. Reprinted with permission [94]. Copyright 2021, Elsevier.

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  Li et al. [93] synthesized a nano-confined ZnIn₂S₄@C photocatalyst where ultrathin ZnIn₂S₄ nanosheets were capsulated in microporous carbon nanocage (MCN) via impregnation and selective etching method.

  As shown in Fig. 6e, the hydroxyl functional groups on the outer carbon cages can facilitate the hydrophilicity of ZIS@C, contributing to the water molecules gathering around the external carbon shell. Then, water molecules could be adsorbed inside the nanocavity due to the nanoconfined effect and participated in hydrogen production reaction. Besides, a core-shell structure C@ZIS photocatalyst where the ZIS was coated on the outside of the carbon shell was also prepared for contrast, but its photocatalytic activity only delivered a moderate improvement (Fig. 6f). Zhang et al. [94] made use of metal-organic-framework-layers (MOFL) to modify hollow tubular Mo-S₂O₁ sites (Fig. 6g) and surprisingly high HER activity of 28.2 mmol g⁻¹ h⁻¹. In Fig. 6h, the photogenerated electrons of MOFL can easily recombined with holes of ZnIn₂S₄, leaving photoexcited electrons of ZnIn₂S₄ participating in HER with Pt assistance and the holes of MOFL serving as oxidizing agent, respectively.

  Recently, Du et al. [95] elaborately combined heterojunction effect, cocatalyst effect and photothermal effect in one and successfully prepared Mo₂C/Mo-S₂O₁ sites composite for efficient hydrogen generation. From Fig. 7a, we can clearly observe that the temperature increases as the reaction proceeds. The photothermal effect is especially important for the enhancement of hydrogen evolution rate (22.11 mmol g⁻¹ h⁻¹) among the three factors. As shown in Fig. 7b, it is easy for electrons and holes to migrate to the conduction band and valence band of Mo₂C and participate in redox reactions. In Fig. 7c, H₂ generation of both ZIS and MC-ZIS increases with the higher temperature and the latter is better than the former overall, which can be ascribed to Mo₂C modification.

  Figure 7

  

  Figure 7.  (a) Photothermal mapping images under visible irradiation in the photocatalytic process. (b) Schematic illustration of the band structure over 2D ZnIn₂S₄/amorphous Mo₂C nanoparticles for photocatalytic H₂ generation process. (c) H₂ evolution rates of ZIS and MC-ZIS-2 at different temperature. Reprinted with permission [95]. Copyright 2020, Elsevier. (d) Schematic illustration of hydrogenation process. (e) Photocatalytic H₂ production rate over BCHZIS per hour. (f) Comparison of photocatalytic H₂ production rate between H_xZIS-4h and BCHZIS. (g) Hydrogen evolution amount and the corresponding color change of the ZIS nanosheets. Reprinted with permission [32]. Copyright 2019, Elsevier. (h) FDTD simulation results of NCS/ZIS under wavelengths of 420 nm (left) and 750 nm (right). (i) The possible photocatalytic mechanism of NiCo₂S₄/ZnIn₂S₄. Reprinted with permission [96]. Copyright 2022, Elsevier.

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  Our group [32] creatively proposed a facile hydrogenation method via an in-situ UV-visible light irradiation for the first time. After 9-h illumination, the H_xZIS was vigorously stirred (1000 r/min, 12 h). The hydrogen evolution amount of bleached ZIS at each hour was illustrated by Fig. 7e. Notably, the color can be bleached within several hours and the bleached ZnIn₂S₄ (denoted as BCHZIS) exhibited an efficient and durable photocatalytic hydrogen production rate of 170 µmol/h compared with pristine H_xZIS-4h (about 134 µmol/h) thanks to lower charge-transfer resistance and shorter carrier diffusion pathway induced by H incorporation (Figs. 7d and f). The hydrogen production amount increased in the first 4 h and then decreased and an apparent color change of ZIS from yellow to black was observed (Fig. 7g). The reason for the initial activity decreased after 4 h can be ascribed to the excessive amount of doped H that narrowed the band gap and thus affected reduction ability.

  Moreover, Guo et al. [96] constructed an intimate contact between noble-metal-free NiCo₂S₄ (NCS) and ZnIn₂S₄ (ZIS) via a two-step solvothermal method and realized 6834.6 µmol g⁻¹ h⁻¹ compared with pure ZnIn₂S₄. The localized surface plasmon resonance (LSPR) effect induced by NiCo₂S₄ realized the synergy of near-infrared electric field enhancement and photothermal effect, the former facilitating the rapid transfer of photogenerated electrons and the later boosting the activation of H-OH bond in adsorbed water molecules. According to finite time domain (FDTD) simulated calculation (Fig. 7h), it is easy to observe that the electric field intensity was remarkably strengthened with the increase of incident light wavelength, which is favorable to the charge transfer from ZIS to NCS. A possible mechanism was also proposed in Fig. 7i. Under light irradiation, the photo-excited electrons of ZIS transferred to the conduction band of NCS and produce hydrogen. The Schottky barrier was also formed simultaneously and promote carrier separation.

  4.2 CO₂ reduction

  Carbon dioxide, a greenhouse gas with massive emission, has caused critical environmental pollution in the past few decades. Efficient reduction of CO₂ to value-added organic matter such as methane and formate is a feasible approach to alleviating this problem.

  Recently, Chi et al. [26] incorporated zinc in the synthesis of indium sulfide (In₂S₃), successfully obtaining Znln₂S₄ nanoflowers with hexagonal structure. The as-prepared Znln₂S₄ catalyst exhibits 99.3% Faradaic efficiency of CO₂ reduction to formate with outstanding stability at 300 mA/cm² over 60 h of continuous operation. In contrast, ln₂S₃ without Zn incorporation loses catalytic activity quickly under the same circumstances. Researchers found that Zn introduction improved the covalency of In-S bonds to a large extent, thus locking S catalytic sites which can activate H₂O to react with CO₂ and facilitate he formation of HCOO* intermediates. Besides, Jiao et al. [33] prepared one-unit-cell thick ZnIn₂S₄ with abundant Zn vacancies to facilitate CO₂ reduction to formate. On the basis of advantages brought by Zn vacancies such as increased charge density and decreased electrochemical impedance, V_Zn-ZnIn₂S₄ exhibited a CO₂ reduction rate of 33.2 µmol g⁻¹ h⁻¹ and excellent stability after 24 h. Wang et al. [97] constructed a ZnIn₂S₄-In₂O₃ hierarchical tubular heterostructure for promoting CO₂ reduction to CO through growing ZnIn₂S₄ on the outer and inner surfaces of hexagonal In₂O₃ nanotubes. Such hollow porous tubular heterostructure was conductive to charge transfer and light absorption and realized a CO production rate of 3075 µmol h⁻¹ g⁻¹).

  She et al. [98] prepared spike-like double yolk-shell structure TiO₂@ ZnIn₂S₄ for CO₂ reduction to syngas. As shown in Fig. 8a, under light irradiation, the photogenerated electrons of ZnIn₂S₄ could be transferred to the conduction band of TiO₂ and participated in the CO₂ reduction and hydrogen evolution reactions. Such an elaborate structure can provide more active sites, improve light absorption and reduce charge transfer path. Additionally, it was also found that the CO₂ reduction activity increased at first and then decreased with ZnIn₂S₄ loading amount increasing (Fig. 8b). The optimal reduction performance is 325.29 µmol/g and 227.18 µmol/g for CO and H₂, respectively. Han et al. [99] elaborately designed a ZnIn₂S₄/BiVO₄ hierarchical heterostructure by assembling ZnIn₂S₄ nanosheets on BiVO₄ decahedron (Fig. 8c) in a hydrothermal and solvothermal route. The as-prepared composite exhibited an enhanced photocatalytic activity for O₂, CO and CH₄ evolution compared with bare ZnIn₂S₄ and BiVO₄ (Fig. 8d).

  Figure 8

  

  Figure 8.  (a) Possible mechanism of CO₂ reduction with the TiO₂@ZnIn₂S₄ photocatalyst. (b) CO/H₂ generation rates of D-Y-TiO₂@ZnIn₂S₄ with different loading amounts of ZnIn₂S₄. Reprinted with permission [98]. Copyright 2022, The Royal Society of Chemistry. (c) Schematic illustration of the photocatalytic CO₂ reduction for ZnIn₂S₄/BiVO₄. (d) Comparison of photocatalytic activity over different ZnIn₂S₄/BiVO₄ samples. Reprinted with permission [99]. Copyright 2021, American Chemical Society. (e) Schematic depiction of mechanism of the pristine ZIS and O-doped ZIS samples. Reprinted with permission [100]. Copyright 2021, Elsevier. (f) Schematic diagram of lattice mismatch and strain effect between cubic ZnS and hexagonal ZIS phases in ZnS/ZIS. Reprinted with permission [101]. Copyright 2022, Elsevier.

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  Pan et al. [100] fabricated O-doped ZnIn₂S₄ and achieved a CO evolution rate of 1680 µmol h⁻¹ g⁻¹ under visible light irradiation, which can be ascribed to the upshift of conduction band (Fig. 8e) and enhanced reduction capability induced by O dopant. Sabbah et al. [101] constructed a ZnS/ZnIn₂S₄ heterostructure which followed a typical Z-scheme mechanism. As shown in Fig. 8f, the interface lattice mismatch led to a micro-strain, which resulted in electrons in the conduction band of ZnS recombing with holes in the valence band of ZIS. Therefore, the electrons of ZIS and holes of ZnS with better redox capability participated in CO₂ reduction and O₂ production reactions.

  4.3 Value-added products synthesis

  Oxidation of aromatic alcohols to aromatic aldehydes is a significant organic synthesis process. Heating and suitable catalysts are two prerequisites for reaction to proceed. However, the generation of some unwanted by-products and massive consumption of heating energy are two main problems in practical production. Recently, some ZnIn₂S₄-based photocatalysts have been reported to realize highly efficient and selective photocatalytic synthesis of aldehydes.

  For example, our group [102] elaborately prepared plasmonic W₁₈O₄₉/ZnIn₂S₄ photocatalyst to oxidize benzyl alcohol (BA) to benzaldehyde (BAD) selectively. Due to the LSPR effect of W₁₈O₄₉, hot electrons are generated and continuously inject into the CB of ZIS, further increasing the electron density and boosting molecule oxygen activation. Besides, photothermal effect brings a quick surface temperature increase (Fig. 9a), synergistically enhancing the reaction rate of BA selective oxidation. Under NIR light irradiation, both ZnIn₂S₄ and W₁₈O₄₉ can be photoexcited and generate electron-hole pairs. Then the transfer pathway of photoinduced carriers of WOZ heterojunctions follows the Z-scheme mechanism, leaving the highly reductive electrons on the conduction band of ZIS to activate the adsorbed O₂ on photocatalyst surface to produce ^·O₂⁻,which participates in the BA selective oxidation (Fig. 9d). The highest conversion efficiency of BA into BAD of WOZ-60 was 3.1 and 18.6 folds of pristine ZIS and W₁₈O₄₉, respectively. Similarly, Lu et al. [103] combined ZnIn₂S₄ nanosheets and hierarchical W₁₈O₄₉ micro-flowers, successfully realizing full-spectrum photocatalytic hydrogen evolution. Through density-functional theory calculation, they proposed that the localized electrons around periodic arranged W⁵⁺-W⁵⁺ pairs were the basis reason of LSPR of W₁₈O₄₉, which created hot electrons and injected into the conduction band of ZnIn₂S₄. Li and Peng et al. [104] decorated polypyrrole (PPy) nanotubes with ultrathin Znln₂S₄ nanosheets via in-situ growth method (Fig. 9b). S vacancy With PPy as the electron collector, the as-prepared optimal 10%-PPy@ZnIn₂S₄ composite exhibited the best hydrogen evolution rate of 1428 µmol g⁻¹ h⁻¹ and selective conversion efficiency (98.9%) of 1,4-benzenedimethanol (BDM) to 1,4-phthalaldehyde (PAD) simultaneously (Fig. 9c).

  Figure 9

  

  Figure 9.  (a) The infrared photothermal images of different photocatalysts. Measuring conditions: 300 W Xe light (λ>400 nm) with the light intensity of 0.5 W/cm². Reprinted with permission [102]. Copyright 2021, Elsevier. (b) FESEM images of 10%-PPy@ZIS. (c) Photocatalytic H₂ evolution and PAD production efficiency of 10%-PPy@ZIS. Reprinted with permission [104]. Copyright 2022, Elsevier. (d) Schematic illustration for the hot electron pathways of the photocatalytic process. Reprinted with permission [102]. Copyright 2021, Elsevier. (e) HRTEM image of MoS₂-ZIS. (f) Photocatalytic performance of prinstine ZIS and MoS₂-ZIS with different amounts of MoS₂. (g) Proposed reaction mechanism for dual reaction for furfuryl alcohol degradation and H₂ evolution over MoS₂-ZIS under visible light irradiation. Reprinted with permission [105]. Copyright 2021, Elsevier. (h) Photocatalytic deuteration of carbonyls to produce alcohols using D₂O as the sole deuterium source. (i) TEM images of D-ZIS. Reprinted with permission [106]. Copyright 2022, John Wiley & Sons, Ltd.

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  Tan et al. [105] prepared ZnIn₂S₄ microspheres with MoS₂ as cocatalyst via hydrothermal method and photo-deposition method. The HRTEM image (Fig. 9e) clearly confirmed the successful introduction of MoS₂. According to Fig. 9f, ZIS loaded with MoS₂ exhibited superior photocatalytic performance than bare ZnIn₂S₄ and the optimal sample is 1%-MoS₂-ZIS (68.8 µmol for H₂ evolution and 70.8 µmol for FAL generation). As shown in Fig. 9g, under visible irradiation, the photoexcited electrons on the conduction band of ZnIn₂S₄ were transferred to MoS₂ owing to its more positive Fermi level than ZnIn₂S₄, therefore facilitating carrier separation. Then, the electrons could easily react with H⁺ adsorbed on the MoS₂ surface to produce hydrogen and the holes participated in the oxidation of furfuryl alcohol (FOL) to obtain the value-added product furfural (FAL). Recently, Han et al. [106] prepared ultrathin 2D ZnIn₂S₄ with plentiful S vacancies assisted with surfactant modification for efficient photocatalytic deuteration of carbonyls to alcohols with D₂O as the deuterium source under mild reaction condition (Fig. 9h). Interestingly, the hexagonal moiré pattern resulting from interference between the crystalline lattices of the stacked nanosheets in D-ZIS could also be observed (Fig. 9i). As we know, a moiré pattern is generally formed when two periodic patterns are overlaid with a relative twist, therefore verifying the formation of ultrathin 2D structure. This research overcame the drawbacks of previous deuteration such as harsh reaction conditions, noble-metal catalysts, and costly deuterium sources.

  4.4 Pollutants degradation

  Nowadays, with the development of industrialization, the consequent contaminants have become an increasingly serious problem that troubles many countries all around the world. In recent years, the ZnIn₂S₄-based photocatalysts have been widely researched by scientific researchers, expected to mitigate the pollution issues.

  In 2021, Zhang et al. [107] elaborately designed hierarchical Z-scheme Ag₃PO₄@ZnIn₂S₄ nanoscoparium through decorating Ag₃PO₄ nanoparticles on the ZnIn₂S₄ nanoscopariums (Figs. 10a and b), greatly contributing to tetracycline degradation. The highest tetracycline removal efficiency can reach 92.3% under the optimal component conditions (Ag₃PO₄@ZnIn₂S₄-30%) and the system exhibits excellent durability after five successive runs. Additionally, they also detected the reactive oxygen species during the photocatalytic reaction, and found that both superoxide radical (^·O₂⁻) and hydroxyl radical (^·OH) were involved in the photocatalytic reaction, with ^·OH playing the leading role. As shown in Fig. 10c, electrons on the conduction band of Ag₃PO₄ recombined with holes on the valence band of ZnIn₂S₄, leaving electrons on the conduction band of ZnIn₂S₄ and holes on the valence band of Ag₃PO₄ participating in redox reaction more advantageously. Clearly, it followed a typical Z-scheme mechanism. Besides, Zhang et al. [35] successfully constructed a MoS₂QDs@ ZnIn₂S₄@RGO photocatalyst (Fig. 10d), which can not only purify pollutants such as rhodamine B (RhB), eosin Y (EY), fulvic acid (FA), methylene blue (MB) and p-nitrophenol (PNP), but also produce hydrogen simultaneously without noble metals as co-catalysts. It is worth noting that these dye molecules can also be photoexcited and provide electrons which were further transmitted to the conduction band of ZIS or RGO. That's why the concentration of organics decreased with prolonged irradiation time, whereas the hydrogen evolution still increased obviously (Fig. 10e). The mechanism is clearly depicted in Fig. 10f. With light irradiation, the photogenerated electrons of ZnIn₂S₄ were transferred to RGO (reduced grapheme oxide) and MoS₂QDs to promote hydrogen evolution reaction, while the photogenerated holes were used to oxidize and degrade dye molecules that were adsorbed to catalysts. Zhou et al. [34] synthesized Phosphorus-doped g-C₃N₄/ZnIn₂S₄ (PCN/ZIS) heterojunction photocatalyst via a solvothermal method. Improved by the synergistic effect of heterostructure and P doping (Fig. 10g), an enhanced tetracycline degradation activity of 0.0874/min was realized.

  Figure 10

  

  Figure 10.  (a) HRTEM image of Ag₃PO₄@ZnIn₂S₄. (b) Structural diagram of Ag₃PO₄@ZnIn₂S₄. (c) Photocatalytic mechanism for the degradation of tetracycline on Z-Scheme Ag₃PO₄@ZnIn₂S₄ photocatalyst. Reprinted with permission [107]. Copyright 2021, Elsevier. (d) SEM images of 5MoS₂QDs@ZnIn₂S₄@RGO1. (e) Simultaneous hydrogen production with PNP degradation. C_t and C₀ are the residue concentration and initial concentration (20 mg/L) in water, respectively. (f) Illustration of electron flow and energy conversion during organic pollutant degradation. Reprinted with permission [35]. Copyright 2017, Elsevier. (g) Schematic depiction of PCN/ZIS photocatalyst. Reprinted with permission [34]. Copyright 2021, Elsevier. (h) Reactive mechanism of photocatalytic H₂ production and simultaneous HCHO degradation of ZnIn₂S₄-NiO/BiVO₄. Reprinted with permission [108]. Copyright 2021, Elsevier.

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  Yang et al. [108] successfully constructed a Z-scheme ZnIn₂S₄-NiO/BiVO₄ hierarchical hetero-junction via assembling NiO₂ particles and ZnIn₂S₄ nanosheets on the surface successively, thereby achieving highly efficient photo-catalytic hydrogen production and formaldehyde degradation simultaneously. As shown in Fig. 10h, e⁻ on the CB of BiVO₄ are combined with the h⁺ on the VB of ZnIn₂S₄, which improves redox ability of the photo-catalytic system and promotes separation of photo-generated electron-hole pairs. O₂ concentration, pH and HCHO initial concentration are three main factors that affect catalytic performance. Under the optimal circumstance (80% of O₂, 13 of pH and 1.5 mol/L of HCHO initial concentration), the outstanding H₂ evolution activity and HCHO degradation activity are 2724.89 µmol/h and 17.00 mmol/h, respectively.

  4.5 N₂ fixation

  Although the nitrogen content in the atmosphere reaches 70%, the nitrogen molecule is very stable, and most organisms cannot directly use it. Therefore, it is of great significance to convert nitrogen in the atmosphere into nitrogen-containing compounds and obtain the nitrogen needed by organisms. ZnIn₂S₄ as an efficient photocatalyst has also shown promising behavior in nitrogen fixation. For example, Zang et al. [36] constructed 2D/2D ZnIn₂S₄/BiOCl heterostructure via in-situ solvothermal techniques to optimize N₂ fixation and phenol degradation. In detail, photoexcited electrons of ZnIn₂S₄ can activate N≡N bond and generate intermediates such as N₂H* and species. Finally, NH₃* was combined with H₂O molecules to form NH₄⁺. Besides, Chen et al. [109] fabricated a novel core-shell polyaniline@ZnIn₂S₄ heterostructure photocatalysts, with ZnIn₂S₄ nanoparticles growing directly on the surface of polyaniline (PANI) nanorods. Benefiting from the strong interfacial interactions, the samples exhibited an increased N₂ photo-fixation performance which is 10 times higher than that of pure ZnIn₂S₄.

  5. Conclusions and outlook

  ZnIn₂S₄ is an emerging material with great prospect in the field of photocatalysis and is expected to effectively improve environmental pollution and energy shortage problem. As a ternary chalcogenide semiconductor with a unique lamellar structure, ZnIn₂S₄ has various excellent characteristics such as nontoxicity, low cost, suitable energy band structure, desirable photocatalytic performance and good durability. However, photocatalytic performance of bare ZnIn₂S₄ is severely confined due to the rapid recombination of electron-hole pairs, sluggish carrier transfer kinetics and limited light absorption range. As a result, many scientific researches have been carried out to enhance photocatalytic performance of pristine ZnIn₂S₄. Firstly, element doping, which substitutes exotic atoms for some atoms in ZnIn₂S₄ or injects exotic atoms into crystal lattice of ZnIn₂S₄, can realize some favorable effects such as increased active sites, narrower band gap, improved electric conductivity and extended light harvest range. Second, vacancy engineering which mainly involves S, Zn and In vacancies and distortions is conductive to increasing charge density, prohibiting electron-hole recombination as well. Additionally, construction of heterostructure like 1D-2D, 2D-2D, and 3D can enlarge specific surface area and shorten charge diffusion path, thus improving charge utilization and photocatalytic performance. To date, by means of those modulation strategies to promote catalytic performance, researchers have realized various kinds of applications in different fields such as hydrogen evolution, CO₂ reduction, nitrogen fixation, value-added products and pollutant removal. Nevertheless, there are still some challenges and drawbacks in the research process. Herein, we sorted out our outlook as follows.

  (1) Currently, the most common method to preparing ZnIn₂S₄ is limited to hydrothermal or solvothermal approach. Although we can synthesize ZnIn₂S₄ with different morphology by changing reaction conditions, the precise control of shape and thickness of ZnIn₂S₄ is still not achieved. Therefore, a more facile and elaborate synthesis method need to be explored in order to realize accurate preparation of ZnIn₂S₄.

  (2) Theory should be combined with experiment more closely in order to unravel the in-depth functional mechanism during reactive process.

  (3) More advanced characterization methods and theoretical calculations such as in-situ testing should be introduced to shed light on the essential mechanism of photocatalysis.

  (4) Novel structures such as Moiré pattern [110] which results from interlacement between the crystalline lattices of ZnIn₂S₄ and specific interlayer stacking are still rarely studied, which are expected to be a hotspot in the future.

  (5) In a nutshell, ZnIn₂S₄ is a promising photocatalyst for energy conversion and environmental remediation. We genuinely hope that this review can provide some new insights into this fascinating material.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was financially supported by the National Funds for Distinguished Young Scientists (No. 61825503), the Natural Science of China (Nos. 51902101, 61775101 and 61804082), Natural Science of Jiangsu Province (No. BK20201381), and Science of Nanjing University of Posts and Telecommunications (No. NY219144).

  
  
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  Figure 1  Schematic diagram of ZnIn₂S₄ with different morphologies.

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  Figure 2  Schematic illustration of different modulation strategies for ZnIn₂S₄.

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  Figure 3  (a) The adsorption free energy of H of different neighboring S atoms and S-H_ads bond length before and after Ni doping. (b) Photocatalytic hydrogen evolution performance of ZnIn₂S₄ with different Ni doping quantities. (c) Schematic illustration of photocatalytic hydrogen production process of ZnIn₂S₄ and Ni-ZnIn₂S₄. Reprinted with permission [52]. Copyright 2021, Elsevier. (d) Schematic depiction of the dual-defect (Ag dopants and nanoholes) formed on ZnIn₂S₄ monolayers and the reactive mechanism of the dual-defect structures in photocatalytic overall water splitting. (e) Comparison of the photocatalytic H₂ and O₂ evolution behaviors of ZnIn₂S₄, Ag-ZnIn₂S₄, and Cu-ZnIn₂S₄ under visible light illumination (300 W xenon lamp, λ > 420 nm). Reprinted with permission [53]. Copyright 2021, American Chemical Society.

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  Figure 4  SEM image of (a) ZIS and (b) P-ZIS. (c) The effect of ZIS/NaH₂PO₄ ratio on the photocatalytic CO₂ reduction. Reprinted with permission [59]. Copyright 2021, American Chemical Society. (d) Simulated crystal structure for pristine ZIS, and P-ZIS (SPD-3). (e) Free energy diagram for hydrogen evolution for each site on ZIS and P-ZIS. Reprinted with permission [60]. Copyright 2022, Springer Nature. To sum up, exotic-atom doping is undoubtedly a pivotal method to enhancing the catalytic activity of ZnIn₂S₄ nanosheets: (1) adjust band gap and extend light absorption range; (2) improve electric conductivity; (3) form defects and increase active sites; (4) facilitate reactant adsorption and desorption.

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  Figure 5  (a) The schematic illustration of Zn vacancy formation. Reprinted with permission [72]. Copyright 2021, Elsevier. (b) The reactive mechanism of CO₂ reduction for the ZnIn₂S₄ with Zn vacancies. (c) The photocatalytic performance of 3D-ZIS with Zn vacancies and bulk-ZIS, respectively. Reprinted with permission [73]. Copyright 2019, Wiley VCH. (d) Schematic diagram of the surface domain potential difference between adjacent microdomains induced by the surface S vacancy defects. Reprinted with permission [74]. Copyright 2022, Elsevier. (e) Schematic depiction of gradient H migration for H₂ evolution of Cu doped ZIS. (f) H₂ evolution activities of ZIS and xCu-ZIS (x = 1%-6%). Reprinted with permission [75]. Copyright 2021, American Chemical Society. (g) Schematic diagram for the photocatalytic hydrogen evolution of pristine-ZIS and ultrathin ZIS-V_In. (h) Photocatalytic hydrogen evolution activities of pristine-ZIS and ZIS-V_In. Reprinted with permission [76]. Copyright 2022, Elsevier.

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  Figure 6  (a) Schematic diagram of synthetic process for the MoSA-ZIS HNTs photocatalyst. (b) The rates of photocatalytic H₂ evolution over MoO₃ NRs, ZnIn₂S₄ NSs and MoSA-ZIS HNTs. Reprinted with permission [91]. Copyright 2021, Springer. (c) Synthetic scheme of NMO@M-ZIS-N. (d) The photocatalytic HER rate of ZIS and NMO@M-ZIS-N with different NMO loading (TEOA as a sacrificial agent). Reprinted with permission [92]. Copyright 2022, The Royal Society of Chemistry. (e) Schematic illustration of the process of capturing the water molecules and a possible photocatalytic mechanism of the HER. (f) Hydrogen evolution activities of ZIS, ZIS@C and C@ZIS (λ ≥ 420 nm). Reprinted with permission [93]. Copyright 2021, American Chemical Society. (g) Schematic illustration for the synthesis of ZnIn₂S₄-MOFL. (h) Schematic illustration of photocatalytic mechanism of ZnIn₂S₄-MOFL. Reprinted with permission [94]. Copyright 2021, Elsevier.

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  Figure 7  (a) Photothermal mapping images under visible irradiation in the photocatalytic process. (b) Schematic illustration of the band structure over 2D ZnIn₂S₄/amorphous Mo₂C nanoparticles for photocatalytic H₂ generation process. (c) H₂ evolution rates of ZIS and MC-ZIS-2 at different temperature. Reprinted with permission [95]. Copyright 2020, Elsevier. (d) Schematic illustration of hydrogenation process. (e) Photocatalytic H₂ production rate over BCHZIS per hour. (f) Comparison of photocatalytic H₂ production rate between H_xZIS-4h and BCHZIS. (g) Hydrogen evolution amount and the corresponding color change of the ZIS nanosheets. Reprinted with permission [32]. Copyright 2019, Elsevier. (h) FDTD simulation results of NCS/ZIS under wavelengths of 420 nm (left) and 750 nm (right). (i) The possible photocatalytic mechanism of NiCo₂S₄/ZnIn₂S₄. Reprinted with permission [96]. Copyright 2022, Elsevier.

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  Figure 8  (a) Possible mechanism of CO₂ reduction with the TiO₂@ZnIn₂S₄ photocatalyst. (b) CO/H₂ generation rates of D-Y-TiO₂@ZnIn₂S₄ with different loading amounts of ZnIn₂S₄. Reprinted with permission [98]. Copyright 2022, The Royal Society of Chemistry. (c) Schematic illustration of the photocatalytic CO₂ reduction for ZnIn₂S₄/BiVO₄. (d) Comparison of photocatalytic activity over different ZnIn₂S₄/BiVO₄ samples. Reprinted with permission [99]. Copyright 2021, American Chemical Society. (e) Schematic depiction of mechanism of the pristine ZIS and O-doped ZIS samples. Reprinted with permission [100]. Copyright 2021, Elsevier. (f) Schematic diagram of lattice mismatch and strain effect between cubic ZnS and hexagonal ZIS phases in ZnS/ZIS. Reprinted with permission [101]. Copyright 2022, Elsevier.

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  Figure 9  (a) The infrared photothermal images of different photocatalysts. Measuring conditions: 300 W Xe light (λ>400 nm) with the light intensity of 0.5 W/cm². Reprinted with permission [102]. Copyright 2021, Elsevier. (b) FESEM images of 10%-PPy@ZIS. (c) Photocatalytic H₂ evolution and PAD production efficiency of 10%-PPy@ZIS. Reprinted with permission [104]. Copyright 2022, Elsevier. (d) Schematic illustration for the hot electron pathways of the photocatalytic process. Reprinted with permission [102]. Copyright 2021, Elsevier. (e) HRTEM image of MoS₂-ZIS. (f) Photocatalytic performance of prinstine ZIS and MoS₂-ZIS with different amounts of MoS₂. (g) Proposed reaction mechanism for dual reaction for furfuryl alcohol degradation and H₂ evolution over MoS₂-ZIS under visible light irradiation. Reprinted with permission [105]. Copyright 2021, Elsevier. (h) Photocatalytic deuteration of carbonyls to produce alcohols using D₂O as the sole deuterium source. (i) TEM images of D-ZIS. Reprinted with permission [106]. Copyright 2022, John Wiley & Sons, Ltd.

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  Figure 10  (a) HRTEM image of Ag₃PO₄@ZnIn₂S₄. (b) Structural diagram of Ag₃PO₄@ZnIn₂S₄. (c) Photocatalytic mechanism for the degradation of tetracycline on Z-Scheme Ag₃PO₄@ZnIn₂S₄ photocatalyst. Reprinted with permission [107]. Copyright 2021, Elsevier. (d) SEM images of 5MoS₂QDs@ZnIn₂S₄@RGO1. (e) Simultaneous hydrogen production with PNP degradation. C_t and C₀ are the residue concentration and initial concentration (20 mg/L) in water, respectively. (f) Illustration of electron flow and energy conversion during organic pollutant degradation. Reprinted with permission [35]. Copyright 2017, Elsevier. (g) Schematic depiction of PCN/ZIS photocatalyst. Reprinted with permission [34]. Copyright 2021, Elsevier. (h) Reactive mechanism of photocatalytic H₂ production and simultaneous HCHO degradation of ZnIn₂S₄-NiO/BiVO₄. Reprinted with permission [108]. Copyright 2021, Elsevier.

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