Near-Infrared Light Driven ZnIn2S4-Based Photocatalysts for Environmental and Energy Applications: Progress and Perspectives

Zinc indium sulfide (ZnIn2S4), as a significant visible-light-responsive photocatalyst, has become a research hotspot to tackle energy demand and environmental issues owing to its excellent properties of high stability, easy fabrication, and remarkable catalytic activity. However, its drawbacks, including low utilization of solar light and fast photoinduced charge carriers, limit its applications. Promoting the response for near-infrared (NIR) light (~52% solar light) of ZnIn2S4-based photocatalysts is the primary challenge to overcome. In this review, various modulation strategies of ZnIn2S4 have been described, which include hybrid with narrow optical gap materials, bandgap engineering, up-conversion materials, and surface plasmon materials for enhanced NIR photocatalytic performance in the applications of hydrogen evolution, pollutants purification, and CO2 reduction. In addition, the synthesis methods and mechanisms of NIR light-driven ZnIn2S4-based photocatalysts are summarized. Finally, this review presents perspectives for future development of efficient NIR photon conversion of ZnIn2S4-based photocatalysts.

Although ZnIn 2 S 4 exhibits excellent photocatalytic activity in the UV-visible light range, it has poor photocatalytic activity in near-infrared light (NIR) [44,45] due to its low response towards NIR. The solar spectrum consists of ultraviolet (UV, 300-400 nm), visible (400-700 nm), and near-infrared (NIR, 700-2500 nm) regions, which account for of 5%, 43%, and 52%, respectively [44][45][46]. Therefore, the NIR light absorption of ZnIn 2 S 4 -based should be extended to improve solar light utilization, and the fabrication of NIR-responsive ZnIn 2 S 4 photocatalysts is the crucial approach to promote its photocatalytic efficiency for further practical applications. There are three mechanisms to promote utilization of near-infrared light by ZnIn 2 S 4 : (i) constructing narrow bandgap semiconductors such as Ag 2 S [47], Ag 2 O [48], and Bi 2 WO 6 [49] , etc. to absorb NIR light directly to generate electron hole carriers for photocatalytic redox reactions; (ii) coupling with up-conversion materials such as carbon quantum dots (CQDs), Cu 2 (OH)PO 4 , etc., to convert NIR light to high-energy UV-visible light, thus promoting the NIR photocatalytic activity; and (iii) the photothermal effect, in which photothermal composites convert NIR light to thermal energy and boost photocatalytic activity [27,33,[50][51][52]. In order to extend the NIR light absorption range and sperate photoinduced charge carriers, several strategies have been applied to fabricate ZnIn 2 S 4 -based NIR light-driven photocatalysts, including bandgap engineering, construction of narrow optical gap materials, up-conversion, and plasmonic materials [53][54][55][56].
Herein, recent progress in NIR-driven ZnIn 2 S 4 photocatalysts for the application of hydrogen evolution, pollutant removal, and CO 2 reduction is overviewed. Firstly, different construction strategies of ZnIn 2 S 4 -based photocatalysts to allow full utilization of NIR in the solar spectrum are comprehensively summarized, including hybrid with narrow optical gap materials, bandgap engineering, up-conversion materials, and plasmonic materials. Focus then turns to the applications of photocatalytic water splitting, organic pollutants removal and CO 2 reduction. Recent research into strategies and applications of NIR-driven ZnIn 2 S 4 -based photocatalysis is summarized in the table below (Table 1). In addition, this review discusses the mechanisms of ZnIn 2 S 4 -based NIR light-driven photocatalysts in the respective photocatalytic systems. Finally, current challenges and perspectives for the future development of efficient NIR-responsive ZnIn 2 S 4 -based photocatalysts are put forward.

Hybrid with Narrow Optical Gap Materials
Narrow optical gap materials (NOGMs, e.g., Ag 2 O, CuInS 2 and black phosphorus) have an inherent response to visible-NIR light. However, the rapid recombination of photoinduced carriers and band positions limits their application in photocatalysis. Therefore, the use of traditional narrow bandgap semiconductors by itself is inefficient and impractical for photocatalytic utilization. Constructing binary or ternary heterostructures photocatalysts consequently served as a potential candidate for achieving distinguished photocatalytic performance. In recent years, more attention has focused on coupling ZnIn 2 S 4 with NOGMs for NIR light-driven photocatalysis as a result of enhanced NIR-driven photoactivity and photo-generated charge carriers.
Metal chalcogenides exhibit a broad-band absorption property in the visible to NIR range with a narrow bandgap. For instance, Xu et al. [57] fabricated AgIn 5 S 8 /ZnIn 2 S 4 heteromicrospheres with visible-NIR absorption characteristics in the range of 200-810 nm synthesized by a partial cation exchange method. TEM (transmission electron microscopy, (Figure 1a)) and HRTEM (high-resolution transmission electron microscopy, (Figure 1b)) show that AgIn 5 S 8 flake-like structures were distributed on the surface of ZnIn 2 S 4 , proving the formation of a heterojunction. The absorption edge of heterostructure shows the redshift from 593 nm to 810 nm compared to pristine ZnIn 2 S 4 , and the Rhodamine B (RhB) photodegradation rate reached 99.2% within 50 min. Additionally, the thickness of AgIn 5 S 8 could be a crucial factor affecting the photodegradation efficiency from the joint results of photodegradation and X-ray photoelectron spectroscopy (XPS) analysis [57]. In another study, Yu et al. [53] prepared ZnIn 2 S 4 @CuInS 2 microflowers through a two-step hydrothermal approach, and HRTEM images (Figure 1c,d) displayed that CuInS 2 wrapped on the surface of ZnIn 2 S 4 , which indicated the successful integration between CuInS 2 and ZnIn 2 S 4 . The absorption region was extended from visible light to the NIR region owing to the good construction of CuInS 2 and ZnIn 2 S 4 . Moreover, further characteristics such as the photocurrent and chemical impedance spectroscopy, etc., led to the generation of more photoinduced electrons and a higher performance of charge transfer. Consequently, the ZnIn 2 S 4 integrated with 0.05% CuInS 2 exhibited a remarkable hydrogen evolution rate of 1168 µmol g −1 under λ > 420 nm light irradiation. The mechanism (Figure 1e) demonstrated that the remarkable photocatalytic activity was contributed to by the built-in electric field between ZnIn 2 S 4 and CuInS 2 , which created a larger interface contact area and thus promoted charge separation and transfer. Moreover, a slight decrease in hydrogen evolution photoactivity was observed in the recycling reactions due to the photocorrosion effect of the hole consumption [64,65] and stability of the ZnIn 2 S 4 @CuInS 2 materials was confirmed by SEM and TEM carried out before and after the reactions.
Research on hybrid with narrow optical gap materials is in its early stages, and combined strategies should be considered to balance the benefits of broad solar light absorption by NOGMs due to its narrow bandgap, and its drawbacks of fast photoinduced carrier recombination. Moreover, the innovative design of ZnIn 2 S 4 -based photocatalysts could avoid their photocorrosion and promote their recycling and stability performance.  [53]. (e) Schematic diagrams of the formation of p−n junction and proposed charge s aration process in the ZnIn2S4@CuInS2 core−shell photocatalysts. Reprinted with permission fr Ref. [53]. Copyright 2020, American Chemical Society.
Research on hybrid with narrow optical gap materials is in its early stages, and co bined strategies should be considered to balance the benefits of broad solar light abso tion by NOGMs due to its narrow bandgap, and its drawbacks of fast photoinduced c rier recombination. Moreover, the innovative design of ZnIn2S4-based photocataly could avoid their photocorrosion and promote their recycling and stability performan

Bandgap Engineering
In addition to incorporating NIR-spectrum response semiconductors, bandgap en neering (BGE) is a promising strategy for fabricating NIR-responsive ZnIn2S4. The int duction of atom doping, defects, and disorders is a common approach applied to mod the ZnIn2S4 electronic structure and composition stoichiometry and thus enhance the lig utilization to the NIR range. However, only a few cases of bandgap engineering of ZnIn have been reported for construction of NIR-responsive photocatalysts. For instance, H et al. reported the formation of Zn-defective ZnIn2S4-Laponite heterostructures due Mg 2+ leached from Laponite in the synthesis process ( Figure 2a); leached Mg 2+ replaced Zn in ZnIn2S4 owing to their similar ionic radii, which led to the formation of Zn defe in ZnIn2S4. Due to this engineering defect, the absorption range of the ZnIn2S4-Lapon samples was broadened from UV to NIR at approximately 800 nm (Figure 2b). Furth more, higher charge separation efficiency and greater photoinduced charge were o served by using transient photocurrent measurements and electrochemical impedan spectroscopy (EIS) methods. Eventually, the photocatalytic degradation of methyl oran (MO) by ZnIn2S4-Laponite was 3.3 times more efficient than pure ZnIn2S4, which could attributed to the formation of Zn vacancies in ZnIn2S4 acting as electron capture cent and therefore promoting the photoinduced charge separation [54].
To date, beyond introducing defects, other bandgap engineering methods such atom doping and disorders, etc., on ZIS have not yet been explored. These bandgap en  [53]. (e) Schematic diagrams of the formation of p-n junction and proposed charge separation process in the ZnIn 2 S 4 @CuInS 2 core-shell photocatalysts. Reprinted with permission from Ref. [53]. Copyright 2020, American Chemical Society.

Bandgap Engineering
In addition to incorporating NIR-spectrum response semiconductors, bandgap engineering (BGE) is a promising strategy for fabricating NIR-responsive ZnIn 2 S 4 . The introduction of atom doping, defects, and disorders is a common approach applied to modify the ZnIn 2 S 4 electronic structure and composition stoichiometry and thus enhance the light utilization to the NIR range. However, only a few cases of bandgap engineering of ZnIn 2 S 4 have been reported for construction of NIR-responsive photocatalysts. For instance, Hill et al. reported the formation of Zn-defective ZnIn 2 S 4 -Laponite heterostructures due to Mg 2+ leached from Laponite in the synthesis process ( Figure 2a); leached Mg 2+ replaced the Zn in ZnIn 2 S 4 owing to their similar ionic radii, which led to the formation of Zn defects in ZnIn 2 S 4 . Due to this engineering defect, the absorption range of the ZnIn 2 S 4 -Laponite samples was broadened from UV to NIR at approximately 800 nm ( Figure 2b). Furthermore, higher charge separation efficiency and greater photoinduced charge were observed by using transient photocurrent measurements and electrochemical impedance spectroscopy (EIS) methods. Eventually, the photocatalytic degradation of methyl orange (MO) by ZnIn 2 S 4 -Laponite was 3.3 times more efficient than pure ZnIn 2 S 4 , which could be attributed to the formation of Zn vacancies in ZnIn 2 S 4 acting as electron capture centers and therefore promoting the photoinduced charge separation [54].
To date, beyond introducing defects, other bandgap engineering methods such as atom doping and disorders, etc., on ZIS have not yet been explored. These bandgap engineering approaches could be feasible ways to enhance the absorption of NIR light and therefore promote photocatalytic efficiency.

Up-Conversion Materials
Up-conversion is an anti-Stokes luminescence process that derives from sensitized triplet-triplet annihilation, converting low-energy pump photons to higher-energy emission (UV or visible light photons). The up-conversion process makes it feasible to exploit the solar spectrum owing to the conversion of NIR light to UV or visible light. Therefore, constructing ZnIn2S4-based up-conversion materials is a promising approach to developing NIR-responsive photocatalysts. Among up-conversion materials, lanthanide (Ln) and carbon dots (CDs) are the most attractive to trigger NIR light-driven ZnIn2S4-based photocatalysts.
The up-conversion system utilizes the solar spectrum by the following mechanisms: excited-state absorption (ESA), photon avalanche (PA), energy migration-mediated UC (EMU), and energy transfer up-conversion (ETU) ( Figure 3). Specifically, the ETU system includes successive energy transfer (SET), cross-relaxation (CR), and cooperative up-conversion (CU). Lanthanide (Ln)-based UC materials generally include two types of ions as the donor and acceptor, respectively; therefore the ion composition and matrix could be prominent factors for their efficiency. Host matrices such as NaYF4, NaGdF4, and CaF2 are commonly used, as well as ion compositions of Yb/Er, Yb/Tm, and Yb/Ho. Moreover, direct Ln ion doping and heterostructure development are common strategies to develop Lanthanide materials with ZnIn2S4. Shi and Guo et al. [55]have successfully prepared a three-dimensional NaYF: Yb 3+ /Tm 3+ @ZnIn2S4 (NYF@ZIS) material through a facile hydrothermal and water-bathing process, successfully leading to ZnIn2S4 coating onto the surface of hexagonal prisms (Figure 4a-f). NYF@ZIS exhibited intensive optical absorption ranging from 650 to 1000 nm, giving the property of utilizing NIR light. The UC process is ascribed to the energy transfer between Yb 3+ and Tm 3+ ions, and eventually, ZnIn2S4 was excited through the ET process from the up-conversion luminescence agent of NaYF:Yb 3+ /Tm 3+ . Furthermore, the separation of photogenerated charge carriers was promoted by the construction of ZnIn2S4 and NaYF:Yb 3+ /Tm 3+ . In this way, the application of H2 evolution for NaYF:Yb 3+ /Tm 3+ @ZnIn2S4 was tested under NIR light (λ > 800 nm) irradiation and showed an evolution rate of 17.81 μmol g −1 h −1 . Coupling up-conversion nanoparticles (UCNPs, NaYF:Yb/Tm) enhanced the photocatalytic performance of ZnIn2S4 according to Li and Idris et al. [58], in which UCNPs absorbed NIR photons and emitted UV-visible photons to excite ZnIn2S4. Ultimately, the photoinduced energy transfer gave rise to remarkable CO and CH4 production rates of 1500 and 220 nmol g −1 h −1 under NIRlight illumination. Overall, the Ln doping promoted ZnIn2S4 photocatalytic performance by not only increasing available high-energy photons with the absorption range through up-conversion luminescence approaches but also by morphology modification for efficient separation and migration of photoinduced charge carriers.

Up-Conversion Materials
Up-conversion is an anti-Stokes luminescence process that derives from sensitized triplet-triplet annihilation, converting low-energy pump photons to higher-energy emission (UV or visible light photons). The up-conversion process makes it feasible to exploit the solar spectrum owing to the conversion of NIR light to UV or visible light. Therefore, constructing ZnIn 2 S 4 -based up-conversion materials is a promising approach to developing NIR-responsive photocatalysts. Among up-conversion materials, lanthanide (Ln) and carbon dots (CDs) are the most attractive to trigger NIR light-driven ZnIn 2 S 4 -based photocatalysts.
The up-conversion system utilizes the solar spectrum by the following mechanisms: excited-state absorption (ESA), photon avalanche (PA), energy migration-mediated UC (EMU), and energy transfer up-conversion (ETU) (Figure 3). Specifically, the ETU system includes successive energy transfer (SET), cross-relaxation (CR), and cooperative up-conversion (CU). Lanthanide (Ln)-based UC materials generally include two types of ions as the donor and acceptor, respectively; therefore the ion composition and matrix could be prominent factors for their efficiency. Host matrices such as NaYF 4 , NaGdF 4 , and CaF 2 are commonly used, as well as ion compositions of Yb/Er, Yb/Tm, and Yb/Ho. Moreover, direct Ln ion doping and heterostructure development are common strategies to develop Lanthanide materials with ZnIn 2 S 4 . Shi and Guo et al. [55] have successfully prepared a three-dimensional NaYF: Yb 3+ /Tm 3+ @ZnIn 2 S 4 (NYF@ZIS) material through a facile hydrothermal and water-bathing process, successfully leading to ZnIn 2 S 4 coating onto the surface of hexagonal prisms (Figure 4a-f). NYF@ZIS exhibited intensive optical absorption ranging from 650 to 1000 nm, giving the property of utilizing NIR light. The UC process is ascribed to the energy transfer between Yb 3+ and Tm 3+ ions, and eventually, ZnIn 2 S 4 was excited through the ET process from the up-conversion luminescence agent of NaYF:Yb 3+ /Tm 3+ . Furthermore, the separation of photogenerated charge carriers was promoted by the construction of ZnIn 2 S 4 and NaYF:Yb 3+ /Tm 3+ . In this way, the application of H 2 evolution for NaYF:Yb 3+ /Tm 3+ @ZnIn 2 S 4 was tested under NIR light (λ > 800 nm) irradiation and showed an evolution rate of 17.81 µmol g −1 h −1 . Coupling up-conversion nanoparticles (UCNPs, NaYF:Yb/Tm) enhanced the photocatalytic performance of ZnIn 2 S 4 according to Li and Idris et al. [58], in which UCNPs absorbed NIR photons and emitted UV-visible photons to excite ZnIn 2 S 4 . Ultimately, the photoinduced energy transfer gave rise to remarkable CO and CH 4 production rates of 1500 and 220 nmol g −1 h −1 under NIRlight illumination. Overall, the Ln doping promoted ZnIn 2 S 4 photocatalytic performance by not only increasing available high-energy photons with the absorption range through up-conversion luminescence approaches but also by morphology modification for efficient separation and migration of photoinduced charge carriers. Molecules 2023, 28, x FOR PEER REVIEW 6 of 17     Besides Ln-based ZnIn 2 S 4 construction, carbon dots (CDs) are another up-conversion material for efficient sunlight utilization, which possesses properties of low toxicity, up-and down-conversion fluorescent, good electron transfer, and excellent photostability [67][68][69][70]. Xu et al. [59] constructed carbon quantum dots doped ZnIn 2 S 4 nanoflowers (ZIS/CQDs) with a wide light response range through a simple in situ hydrothermal synthesis approach (Figure 5a). It demonstrated remarkably expanded absorption attributed to its up-converted mechanism, and the suppression of photoexcited charge recombination owing to the CQDs' performance of photon harvest and transportation. Additionally, the doping content of CQDs also affected the photocatalytic removal efficiency of tetracycline hydrochloride (TCH), and the highest degradation rate reached almost three times higher than pure ZnIn 2 S 4 irradiated under NIR light.
the reaction mechanisms in CQDs/ZnIn2S4/BiOCl, CQDs absorbed NIR photons and emitted visible light photons to excite ZnIn2S4, resulting in electron migration from the valence band to the conduction band. On the one hand, the electrons migrated to CQDs reacting with oxygen in the water to produce reactive •O2 − and finally reacted with antibiotic compounds. On the other hand, holes in VB of ZnIn2S4 transferred to BiOCl and degraded pollutants (Figure 5b). The enhanced photocatalytic performance was attributed to the van der Waals forces between ZnIn2S4 and BiOCl, as well as the introduction of CQDs, which acted as electron acceptors, extended the light absorption edge to NIR and finally boosted photoinduced charge migration due to its up-conversion luminescence ability. Additionally, several approaches including electron spin-resonance spectroscopy (ESR), active species trapping experiments, and high-performance liquid chromatography-tandem mass spectrometry (LC-MS/MS), were carried out to elucidate the photocatalytic degradation pathway of antibiotics (TC, CIP, and OTC) and revealed that the reactive species (•O2 − , OH, and holes) played important roles in the degradation processes.
In general, although the up-conversion strategy is a promising way to promote ZnIn2S4 photocatalytic efficiency, it requires further modification for practical application. Based on the mechanism of up-conversion, improving the light overlap of Ln-based materials or carbon quantum dots emitted and ZnIn2S4 absorbed promotes the enhancement of solar energy utilization. Therefore, up-conversion materials with wide light absorption and photostability are needed for higher photocatalytic efficiency.  CQDs promoted the photocatalytic degradation performance of CQDs/ZnIn 2 S 4 /BiOCl for antibiotic removal at visible and NIR regions, as reported by Lu et al. [60], in which CQDs modified 2D BiOCl nanosheets/2D layered flower-like ZnIn 2 S 4 with van der Waals (VDW) force in between. The as-prepared CQDs/ZnIn 2 S 4 /BiOCl heterostructures exhibited noteworthy degradation rates that were 42.5, 2.75, and 1.94 more times efficient than those of pristine BiOCl, ZnIn 2 S 4 , and ZnIn 2 S 4 /BiOCl photocatalysts, respectively. For the reaction mechanisms in CQDs/ZnIn 2 S 4 /BiOCl, CQDs absorbed NIR photons and emitted visible light photons to excite ZnIn 2 S 4 , resulting in electron migration from the valence band to the conduction band. On the one hand, the electrons migrated to CQDs reacting with oxygen in the water to produce reactive •O 2 − and finally reacted with antibiotic compounds. On the other hand, holes in VB of ZnIn 2 S 4 transferred to BiOCl and degraded pollutants (Figure 5b). The enhanced photocatalytic performance was attributed to the van der Waals forces between ZnIn 2 S 4 and BiOCl, as well as the introduction of CQDs, which acted as electron acceptors, extended the light absorption edge to NIR and finally boosted photoinduced charge migration due to its up-conversion luminescence ability. Additionally, several approaches including electron spin-resonance spectroscopy (ESR), active species trapping experiments, and high-performance liquid chromatography-tandem mass spectrometry (LC-MS/MS), were carried out to elucidate the photocatalytic degradation pathway of antibiotics (TC, CIP, and OTC) and revealed that the reactive species (•O 2 − , OH, and holes) played important roles in the degradation processes.
In general, although the up-conversion strategy is a promising way to promote ZnIn 2 S 4 photocatalytic efficiency, it requires further modification for practical application. Based on the mechanism of up-conversion, improving the light overlap of Ln-based materials or carbon quantum dots emitted and ZnIn 2 S 4 absorbed promotes the enhancement of solar energy utilization. Therefore, up-conversion materials with wide light absorption and photostability are needed for higher photocatalytic efficiency.

Surface Plasmon Resonance
Surface plasmon resonances (SPR) or localized surface plasmon resonances (LSPR) is defined as the electron coherent oscillations in metal nanoparticles in response to visible-NIR light irradiation (Figure 6a) [71,72]. On the one hand, the SPR effect triggered by plasmonic metals promotes the photocatalytic ability by absorbing a broader light range, and therefore, the process is affected by the size, shape, and dielectric environment of nanoparticles. Both light scattering and light concentration are essential to the absorption edge extension of photocatalysts by increasing the optical path owing to light trapping or photonic effects (Figure 6b) [72]. On the other hand, direct electron injection energy transfer (DET) and plasmon-induced energy transfer (PIRET) systems are constructed by integrating nanoparticles with semiconductors' heterostructure to extend the light-response region, as well as accelerating charge carrier transfer in between (Figure 6b) [71,73]. Noble nanoparticles, such as Au, Ag, and Pt, are commonly introduced to form heterostructures resulting in the enhancement of ZnIn 2 S 4 photocatalytic activity. Yin et al. fabricated assembling coreshell Au@Pt nanoparticles on 3D ZnIn 2 S 4 microsphere composites by following multiple steps, first by using citrate reduction and solvothermal methods to form Au nanoparticles and ZnIn 2 S 4 , respectively, and obtain Au@Pt nanoparticles via electrostatic interaction force and eventually collect composites attributed to the electrostatic force and assist the precursors (Figure 6c) [63]. As shown in Figure 6c, the absorption edge of Au@Pt/ZnIn 2 S 4 has been extended from the visible to the NIR light region compared with pure ZnIn 2 S 4 and Au@Pt since the Au@Pt serves as the secondary light source by concentrating photons to ZnIn 2 S 4 , thus generating an EM field with higher energy than therefore the process is affected by the size, shape, and dielectric environment of nanoparticles surrounding light [74][75][76], which further promotes the utilization of light spectrum. Moreover, the electron hole separation efficiency is accelerated because electrons from both ZnIn 2 S 4 and Au transfer to Pt nanoparticles according to the energy-band scheme (Figure 6d,e). Benefiting from these mechanisms above, Au@Pt/ZnIn 2 S 4 experiences 10 times greater hydrogen evolution efficiency compared with pure ZnIn 2 S 4 under vis-NIR light irradiation.
In addition to noble nanoparticles, nonmetallic semiconductors show a plasmonic effect owing to the properties of element doping or lattice vacancies such as Cu 2-x S, MoO 3-x , WO 3-X, and W 18 O 49 [77][78][79][80][81]. Among them, W 18 O 19 has strong absorption in the visible-NIR wavelengths ( Figure 7a) and thus is a promising candidate for developing ZnIn 2 S 4 NIR hybrid photocatalysts as a result of sufficient vacancy on its surface [62]. For example, Zhang et al. [62] prepared the W 18 (Figure 7h) with the pump light at 400 nm, the ZnIn 2 S 4 image exhibits a higher concentration of short-lived electrons than that of W 18 O 49 /ZnIn 2 S 4 at the signal range from 550 to 750 nm, which indicates that more photoinduced electrons recombine with holes in the ZnIn 2 S 4 than W 18 O 49 /ZnIn 2 S 4 . In this way, it is concluded that in the W 18 O 49 /ZnIn 2 S 4 heterostructure, the recombination of electrons and holes in ZnIn 2 S 4 is suppressed, and thus, the transfer efficiency of photoexcited charge carriers is accelerated due to the Z-scheme construction of W 18 O 49 and ZnIn 2 S 4 . In addition to noble nanoparticles, nonmetallic semiconductors show a plasmonic effect owing to the properties of element doping or lattice vacancies such as Cu2-xS, MoO3x, WO3-X, and W18O49 [77][78][79][80][81]. Among them, W18O19 has strong absorption in the visible-NIR wavelengths (Figure 7a) and thus is a promising candidate for developing ZnIn2S4 NIR hybrid photocatalysts as a result of sufficient vacancy on its surface [62]. For example, Zhang et al. [62] prepared the W18O49/ZnIn2S4 composites (Figure 7b,c) with a full-spectrum sunlight response with excellent hydrogen evolution under UV and NIR light illumination (Figure 7d). The mechanisms of W18O49/ZnIn2S4 for hydrogen evolution were proposed as shown in Figure 7e,f. The DFT calculation is conducted to prove that W 5+ -W 5+ pairs are the prerequisite for the LSPR effect of W18O49. Multiple additional approaches, such as ultrafast transient absorption spectroscopy (TA) and ESR, are applied to investigate the mechanisms, and the results show that W18O49 captures two photons to excite ZnIn2S4, which provides more opportunity for ZnIn2S4 to utilize solar spectrum. Moreover, as in the ultrafast transient absorption spectroscopy (TA) measurement, in the plots of pure ZnIn2S4 (Figure 7g) and W18O49/ZnIn2S4 (Figure 7h) with the pump light at 400 nm, the ZnIn2S4 image exhibits a higher concentration of short-lived electrons than that of W18O49/ZnIn2S4 at the signal range from 550 to 750 nm, which indicates that more photoinduced electrons recombine with holes in the ZnIn2S4 than W18O49/ZnIn2S4. In this way, it is Apart from W 18 O 49 , WO 3-X with lattice vacancy also has a surface plasmon resonance (SPR) effect owing to its collective oscillations of free carriers. Ni et al. [56] fabricated WO 3-X /ZnIn 2 S 4 heterojunctions via a straightforward scaled-up process for H 2 production photoactivity. The interaction of WO 3-X with ZnIn 2 S 4 is beneficial for expanding the absorption light from UV-visible to NIR light range, which gives the photocatalysts capability to generate H 2 by splitting water throughout the full solar spectrum. WO 3-X /ZnIn 2 S 4 photocatalysts exhibit a remarkable H 2 production rate of 20,957 µmol h −1 g −1 , which is 2.7 times higher than that of pure ZnIn 2 S 4 under simulated sunlight illumination. Together with the results of the EIS (Figure 8a) and PL measurements (Figure 8b), the WO 3-X /ZnIn 2 S 4 -with a smaller resistance curve and weaker PL emission than pure ZnIn 2 S 4 -indicates more efficient electron transfer and charge separation. Additionally, the proposed mechanism ( Figure 8c) reveals that after absorbing UV-vis-NIR light, electrons in the VB of WO 3-X and ZnIn 2 S 4 migrate to their respective CB, then electrons and holes in the VB of ZnIn 2 S 4 combine together as a result of the electron transfer from CB of WO 3-X . The H 2 production eventually occurs in the CB of ZnIn 2 S 4 . The G H* value of the WO 3-X /ZnIn 2 S 4 hybrid calculated by the DFT method further confirms the mechanism (Figure 8d). concluded that in the W18O49/ZnIn2S4 heterostructure, the recombination of electrons and holes in ZnIn2S4 is suppressed, and thus, the transfer efficiency of photoexcited charge carriers is accelerated due to the Z-scheme construction of W18O49 and ZnIn2S4. Apart from W18O49, WO3-X with lattice vacancy also has a surface plasmon resonance (SPR) effect owing to its collective oscillations of free carriers. Ni et al. [56] fabricated WO3-X/ZnIn2S4 heterojunctions via a straightforward scaled-up process for H2 production photoactivity. The interaction of WO3-X with ZnIn2S4 is beneficial for expanding the absorption light from UV-visible to NIR light range, which gives the photocatalysts capability to generate H2 by splitting water throughout the full solar spectrum. WO3−X/ZnIn2S4 photocatalysts exhibit a remarkable H2 production rate of 20,957 μmol h −1 g −1 , which is 2.7 times higher than that of pure ZnIn2S4 under simulated sunlight illumination. Together with the results of the EIS (Figure 8a) and PL measurements (Figure 8b), the WO3−X/ZnIn2S4-with a smaller resistance curve and weaker PL emission than pure ZnIn2S4-indicates more efficient electron transfer and charge separation. Additionally, the proposed mechanism ( Figure 8c) reveals that after absorbing UV-vis-NIR light, electrons in the VB of WO3-X and ZnIn2S4 migrate to their respective CB, then electrons and holes in the VB of ZnIn2S4 combine together as a result of the electron transfer from CB of WO3−X. The H2 production eventually occurs in the CB of ZnIn2S4. The △GH* value of the WO3−X/ZnIn2S4 hybrid calculated by the DFT method further confirms the mechanism (Figure 8d). In another study, Zhou et al. [61] synthesized a novel photocatalyst of K3PW12O40@ZnIn2S4 and further loaded Ag2S quantum dots via the hydrothermal method followed by a cation exchange process, construction a dodecahedral K3PW12O40 capped with flower-like ZnIn2S4, and finally by distribution of Ag2S dots on the surface ( Figure  9a). The introduction of Ag2S quantum dots led to a red shift from visible light to NIR light with higher optical absorption intensity (Figure 9b) compared with pristine ZnIn2S4 and K3PW12, respectively. It displayed high photocatalytic performance and photostability In another study, Zhou et al. [61] synthesized a novel photocatalyst of K 3 PW 12 O 40 @ZnIn 2 S 4 and further loaded Ag 2 S quantum dots via the hydrothermal method followed by a cation exchange process, construction a dodecahedral K 3 PW 12 O 40 capped with flower-like ZnIn 2 S 4 , and finally by distribution of Ag 2 S dots on the surface (Figure 9a). The introduction of Ag 2 S quantum dots led to a red shift from visible light to NIR light with higher optical absorption intensity (Figure 9b) compared with pristine ZnIn 2 S 4 and K 3 PW 12 , respectively. It displayed high photocatalytic performance and photostability both for tetracycline hydrochloride and H 2 production activity, notably with an optimal photodegradation rate of 99% (Figure 9c) and hydrogen production rate of 2.1 mmol g −1 h −1 . Overall, the boosted NIR photoactivity was attributed to the SPR effect of Ag 2 S quantum dots (Figure 9d), which have extensive absorption of NIR light. Furthermore, the unique structure of K 3 PW 12 O 40 @ZnIn 2 S 4 /Ag 2 S provided more efficient light scattering and reactive reaction sites and prolonged the lifetime of photoinduced charge carriers.

Conclusions and Prospects
In recent years, ZnIn2S4 has come to be regarded as a promising photocatalyst due to its excellent properties of low cost, facile synthesis, and environmental harmlessness. Benefiting from these features, it has been widely applied in organic pollutant removal, hydrogen evolution, CO2 reduction, etc. Nevertheless, its photocatalytic efficiency is far from satisfactory due to its limited NIR light absorption, and thus, the fabrication of ZnIn2S4based NIR photocatalysts is highly desired to enhance NIR light utilization. The present review summarizes strategies for promoting the extension of ZnIn2S4 response into the NIR range, as well as an overview of their fabrication methods, mechanisms for reactions, and applications. Strategies including hybrid with narrow optical gap materials, bandgap Figure 9. (a) Schematic illustration of K 3 PW 12 O 40 @ZnIn 2 S 4 /Ag 2 S preparation, hollow dodecahedral K 3 PW 12 O 40 , ZnIn 2 S 4 and Ag 2 S were named HD-KPW, ZIS and AS. (b) UV-Vis-NIR diffuse reflectance spectra and (c) tetracycline hydrochloride photodegradation of K 3 PW 12 O 4 , ZnIn 2 S 4 , Ag 2 S, K 3 PW 12 O 40 @ZnIn 2 S 4 , and K 3 PW 12 O 40 @ZnIn 2 S 4 /Ag 2 S, respectively. (d) Proposed mechanism of K 3 PW 12 O 40 @ZnIn 2 S 4 /Ag 2 S heterojunctions for pollutant removal and hydrogen production. Reprinted with permission from Ref. [61]. Copyright 2022, Royal Society of Chemistry.

Conclusions and Prospects
In recent years, ZnIn 2 S 4 has come to be regarded as a promising photocatalyst due to its excellent properties of low cost, facile synthesis, and environmental harmlessness. Benefiting from these features, it has been widely applied in organic pollutant removal, hydrogen evolution, CO 2 reduction, etc. Nevertheless, its photocatalytic efficiency is far from satisfactory due to its limited NIR light absorption, and thus, the fabrication of ZnIn 2 S 4based NIR photocatalysts is highly desired to enhance NIR light utilization. The present review summarizes strategies for promoting the extension of ZnIn 2 S 4 response into the NIR range, as well as an overview of their fabrication methods, mechanisms for reactions, and applications. Strategies including hybrid with narrow optical gap materials, bandgap engineering, up-conversion materials, and surface plasmon resonance are overviewed to extend ZnIn 2 S 4 's NIR light absorption and boost its solar energy efficiency. In addition, the photocatalytic applications of NIR-driven ZnIn 2 S 4 -based photocatalysts have been reported in the field of hydrogen production, pollutant removal, and CO 2 reduction.
Despite the promising results that have been accomplished, there remain several primary challenges to overcome. (1) The NIR absorption range of ZnIn 2 S 4 -based photocatalytic systems should be further expanded to make better utilization of solar energy. More research on constructing ZnIn 2 S 4 with up-conversion or SPR materials is needed to enhance the absorption range. (2) Photosensitizers can absorb NIR photons to generate electrons, and if combined with ZnIn 2 S 4 -based photocatalysts, could further promote the NIR utilization of the ZnIn 2 S 4 -hybrid photosystems. However, no fabrications and applications of photosensitizers/ZnIn 2 S 4 composites have yet been reported. More efforts could be devoted to this photocatalysis direction. (3) The light absorption intensity of the NIR ZnIn 2 S 4 -based photocatalysts is relatively inadequate, which results in low efficiency from solar to chemical energy. Therefore, modifications to the structure, morphology, and composition of SPR materials should be explored, which could enhance the NIR intensity by concentrating light and extending the absorption edge. (4) The mechanism of ZnIn 2 S 4 -based photosystems for NIR absorption is unclear; further approaches such as DFT calculations and advanced technics are strongly recommended to investigate the NIR absorption principles. (5) Future lab-scale and practical research on constructing easysynthesis and environmentally friendly NIR ZnIn 2 S 4 -based photocatalysts is required with the aim to apply the photocatalysis technology to industrial applications.

Conflicts of Interest:
The authors declare no conflict of interest.