

Recent status and future perspectives of ZnIn2S4 for energy conversion and environmental remediation
English
Recent status and future perspectives of ZnIn2S4 for energy conversion and environmental remediation
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Key words:
- ZnIn2S4
- / Photocatalysis
- / Energy conversion
- / Environmental remediation
- / Hydrogen evolution
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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
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
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-Had 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-Hads 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.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.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 Vs-M-ZnIn2S4 were easy to transfer to MoS2QDs and participated in hydrogen production. It was found that 2MoS2QDs@Vs-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-VIn. (h) Photocatalytic hydrogen evolution activities of pristine-ZIS and ZIS-VIn. Reprinted with permission [76]. Copyright 2022, Elsevier.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 Vs synergistically modulate charge separation with Vs being an electron trap and Cu acting as a hole trap. Besides, regulated by Vs 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-VIn 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-VIn, 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 Sv-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 Ti3C2Tx/ZnIn2S4 heterostructure to enhance hydrogen evolution. With S vacancies as the electron trap and Ti3C2Tx as the electron acceptor, the as-prepared material shortened carrier transfer distance and thus made it easier for electrons transferring to Ti3C2Tx 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.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 HxZIS-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.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 HxZIS 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 HxZIS-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, VZn-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.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.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. Ct 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.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 3 (a) The adsorption free energy of H of different neighboring S atoms and S-Hads 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.
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.
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-VIn. (h) Photocatalytic hydrogen evolution activities of pristine-ZIS and ZIS-VIn. Reprinted with permission [76]. Copyright 2022, Elsevier.
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.
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 HxZIS-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.
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.
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.
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. Ct 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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