Recent advances in photocatalytic hydrogen evolution of AgIn5S8‐based photocatalysts

The development of semiconductor photocatalysts is of great significance for the realization of efficient photocatalytic hydrogen evolution (PHE). AgIn5S8, as an emerging ternary metal sulfide photocatalyst, possesses the advantages of suitable bandgap (1.7–2.0 eV), environment‐friendly elements, and strong photostability, which holds great potential to realize high‐efficiency PHE. Although AgIn5S8‐based photocatalysts have achieved promising research progresses, their PHE performances are still far below the level of commercial applications. In this review, the basic semiconductor properties of AgIn5S8 and PHE mechanism are first introduced in detail. Subsequently, the development process and PHE activities of AgIn5S8‐based photocatalysts are systematically summarized, mainly including morphology control, Schottky junction formation through cocatalyst loading, and construction of different types of heterojunctions. Finally, the current issues and the possible solutions of AgIn5S8‐based photocatalysts in future studies are presented.

3][4][5][6][7][8][9][10][11] It is quite pressing to explore the clean and renewable energy to solve the current energy issues.[23][24] Fujishima and Honda explored that the photoelectrochemical catalysis can be realized on the TiO 2 electrode, which pioneered the PHE technique by TiO 2 photocatalyst. [25]Nevertheless, the excessively wide band gap of TiO 2 makes it unsuitable for the absorption of visible light during the corresponding PHE process. [21,26]To this end, the development of narrow bandgap photocatalysts is of great importance to achieve efficient visible-light PHE.
36] Therefore, the PHE rate on CdS-based photocatalysts has achieved higher than 10 mmol g −1 h −1 . [33]However, the CdS-based photocatalysts generally suffer from the photocorrosion phenomenon, leading to a serious decrease in photostability.In this regard, researchers have attempted various efficient paths to overcome this issue.The most typical advanced CdS-based photocatalysts are dual-cocatalysts decorated rimous CdS spheres, [37] ternary heterojunction photocatalyst of CuWO 4 /CdS/carbon dots (CuWO 4 /CdS/CDs), [38] and S-scheme heterojunction photocatalyst of CdS/W 18 O 49 with oxygen vacancies, [39] which not only effectively hinder the photocorrosion phenomenon to a certain extent but also greatly enhance the charge separation.Apart from CdS, researchers have also explored other several BMS photocatalysts, like, MoS 2 , PbS, and ZnS, which have also been reviewed in detail by Zheng et al. [33] The authors highlighted the positive properties of them and also pointed out the issues to be addressed in the future researches.Therefore, the exploration of new kinds of MS photocatalysts is quite meaningful for the realization of effective visible-light PHE.
Although AgIn 5 S 8 holds impressive semiconductor properties and better application prospects in PHE, the corresponding development is still in an early research stage.With the combination of theoretical and experimental methods, researchers have formulated corresponding optimization strategies, aiming to enhance the hydrogen production rate of AgIn 5 S 8 -based photocatalysts.Nevertheless, the corresponding review of AgIn 5 S 8 -based photocatalysts in the application of PHE has been rarely reported.In this regard, we perform this review to provide a clear understanding and research direction of AgIn 5 S 8 -based photocatalysts.In this review, the crystal and electronic band structures of AgIn 5 S 8 are first introduced.Subsequently, the basic mechanism and charge transfer behaviors of the PHE are fully discussed and the properties of AgIn 5 S 8 to the PHE are highlighted.Afterwards, we systematically and comprehensively reviewed the development of AgIn 5 S 8 -based photocatalysts, which contain the morphology control, Schottky junction formation through cocatalyst loading, and construction of different types of heterojunctions.Finally, some issues need to be further solved and the perspective of AgIn 5 S 8 -based photocatalysts is pointed out and some possible solutions are pointed out.

| SEMICONDUCTOR PROPERTIES OF AgIn 5 S 8
The crystal structure and electronic band structure of AgIn 5 S 8 were studied by Zhang et al. in detail through the density functional theory (DFT) calculation, which clearly demonstrates the basic semiconductor properties of AgIn 5 S 8 . [58]In the Ag 4 In 20 S 32 (four AgIn 5 S 8 unit) supercell crystal structure (Figure 1A), four Ag and four In atoms occupy the tetrahedral coordination sites and the remaining 16 In atoms occupy the octahedral coordination sites.Furthermore, the tetrahedron of (Ag or In)S 4 and [In]S 6 octahedron are linked by edges and corners sharing.For AgIn 5 S 8 planes in (110) and (111), the corresponding images of electron localization function (ELF) are also illustrated (Figure 1B), in which the ELF value is an effective parameter to identify the lone-pair electrons and shell structures.In the limited value range of ELF values (from 0 to 1), the lower value (ELF = 0), middle value (ELF = 0.5), and upper value (ELF = 1) represent the perfect electron delocalization, electron-gas-like pair probability for bonding, and perfect electron localization, respectively.The weaker bonding between Ag and S and stronger between In and S can be demonstrated due to the higher ELF values between In and S, which is caused by the higher electronegativity difference of In─S.For the electronic band structure and density of states of AgIn 5 S 8 , the corresponding DFT-calculated results are shown in Figure 1C.Obviously, the calculated bandgap of AgIn 5 S 8 is 1.16 eV, which is not similar to the experimental values (1.7-2.0 eV) due to the bandgap underestimating by generalized gradient approximation functional. [59]In addition to the basic semiconductor properties, the optical properties of AgIn 5 S 8 that are beneficial for efficient PHE, including optical absorption coefficient, photoconductivity, and Raman scattering have been deeply studied in the literature.Moreover, researchers have also highlighted the thermoelectric performance (Figure 1D) [60] and phase transfer property under high pressure of AgIn 5 S 8 (Figure 1E), [61] which is meaningful for the wider STE applications.

| BASIC MECHANISM OF PHE
The typical PHE system mainly contains the components of photocatalyst, reactive medium, and sacrificial agent (Figure 1F).Generally, during the whole PHE process, the corresponding reaction system needs to be kept in a highspeed stirring, which can maximize the light absorption and avoid photocatalyst particles precipitation. [24]The charge transfer behaviors of semiconductor photocatalysts during PHE are also illustrated (Figure 1G).Specifically, the incident light irradiates the semiconductor photocatalyst.If the photon energy is higher than the bandgap, the electrons on the valence band (VB) can be excited and then transferred to the CB, resulting in the formation of photogenerated electron-hole pairs.Subsequently, the photogenerated carriers transferred from the bulk phase to the surface of the photocatalyst and participated in HER (photogenerated electrons) and oxygen evolution reaction (OER) (photogenerated holes), respectively. [58]Aiming to realize efficient PHE, two important basic semiconductor properties of the photocatalyst are CB potential and bandgap value.Specifically, the corresponding VB and CB potentials should cover the redox potential of O 2 /H 2 O (1.23 V) and the redox potential of H + /H 2 (0 V vs. NHE), respectively. [66]To this end, the bandgap of the semiconductor photocatalyst should be larger than 1.23 eV, aiming to provide an efficient driving force for HER via water splitting.Meanwhile, for the realization of visible-light PHE, the bandgap should also be narrower than 3.0 eV. [62]As expected, the experimental bandgap of AgIn 5 S 8 is 1.7-2.0eV, falling in the range of the above-mentioned 1.23-3.0eV.Moreover, the promising optical properties of AgIn 5 S 8 are further beneficial to achieve efficient visible-light PHE.
Apart from photoexcitation and formation of photogenerated carriers, another process that needs to be paid attention is the volume and surface recombination (Figure 1H), which occurs during the photogenerated carriers migration from the inner to the photocatalyst surface. [63]Typically, the final amount of photogenerated electrons migrating to the water/photocatalyst interface, greatly influences the hydrogen production rate.Therefore, any-electrons consuming process needs to be avoided during the PHE process for the realization of highefficiency hydrogen production.Similarly, the electronsgeneration process should be effectively preserved and optimized to maximize the hydrogen production.Thus, in the aspect of charge generation, charge separation, and charge recombination, the semiconductor photocatalyst should possess the following basic properties: (i) suitable bandgap to absorb incident light as much as possible, (ii) minimized scattering or reflection of light, and (iii) optimized structure to avoid the volume and surface charge recombination.Moreover, the semiconductor photocatalyst should also have high charge-generation efficiency by using the absorbed photons, rather than heat-, phonon-generation.Therefore, for a more efficient application of AgIn 5 S 8 photocatalyst in PHE, the corresponding optimization strategies should also be adopted, aiming to further enhance the photogenerated charges separation.

| AgIn 5 S 8 -BASED SEMICONDUCTOR PHOTOCATALYSTS IN PHE APPLICATION
For the realization of efficient visible-light PHE, the critical factor is the exploration of semiconductor photocatalyst with visible-light range bandgap and promising semiconductor properties. [66]The semiconductor photocatalyst of TiO 2 in photoelectrochemical catalysis pioneered the development of semiconductor photocatalyst. [25]Nevertheless, the corresponding PHE process can only be performed under UV light irradiation due to the wide bandgap of TiO 2 .Therefore, in the early development stage of semiconductor photocatalysts, the challenge is the realization of visible-light PHE.Compared with TiO 2 , MS photocatalysts generally own a narrow bandgap in the range of 1.23-3.0eV, which can effectively realize the visible-light PHE.In the exploration stage of MS photocatalysts, BMS photocatalysts are first researched and achieve great research progress due to the facile fabrication process and simple phase.With the deepening study, the BMS photocatalysts also have many issues that are difficult to solve in the current research stage. [33]Fortunately, the TMS photocatalysts can overcome some issues of BMS photocatalysts in the recent research stage due to the stronger photostability, more flexible compositions, lower toxics, and more tunable band structures, which have also been systematically reviewed by our group. [50]Specifically, the TMS photocatalyst of AgIn 5 S 8 possesses a suitable bandgap of 1.7-2.0eV, high light absorption coefficient, nontoxic elements, and high photostability, [67] which has a great potential to realize the efficient visible-light PHE.On the basis of the above advantages of AgIn 5 S 8 , more attention has been paid to develop the AgIn 5 S 8 -based photocatalysts in the PHE application.In this part, the recent advances of AgIn 5 S 8 -based photocatalysts are comprehensively and systematically summarized, including the bare AgIn 5 S 8 with morphology control, cocatalyst loading, and AgIn 5 S 8 -based heterojunction photocatalysts.

| Mechanism illustration of morphology control
For semiconductor photocatalysts, the optimization of morphology control can effectively enhance the STE efficiency, which has been deeply studied by Wang et al. [68] Specifically, the manganese oxides with the morphologies of flower-, rod-, wire-, and tube-like have been synthesized through the hydrothermal method and the catalytic degradation of pollutants has been studied.Compared with the tube-and rod-like α-MnO 2 , the flower-like spherical Mn 2 O 3 possesses more sufficient active sites and surface area, which highlights the optimization strategy of morphology control.Guided by this optimization strategy, several different morphologies of AgIn 5 S 8 photocatalysts have been explored for the PHE application and achieved promising research progress.

| AgIn 5 S 8 photocatalysts with different morphologies
In 2007, the AgIn 5 S 8 photocatalyst was fabricated by Chen and Ye using a simple two-step process of coprecipitation and heat treatment at 750°C and the PHE rate achieved 200 μmol g −1 h −1 , which pioneered the PHE application of AgIn 5 S 8 photocatalyst. [69]Since then, researchers have attempted to optimize the hydrogen production rate of AgIn 5 S 8 photocatalyst through the optimization strategy of morphology control.Typically, Song et al. fabricated the AgIn 5 S 8 single crystalline octahedrons with exposed (111)  facets by a facile hydrothermal process and realized the enhanced PHE performance (Figure 1I).In this study, the octahedron morphology control of AgIn 5 S 8 was determined by the accurately released S 2− of thioacetamide thermal decomposition. [64]Experimentally, the AgIn 5 S 8 photocatalyst with the morphology of regular octahedrons exhibited the highest hydrogen production rate (600 μmol g −1 h −1 ).Compared with other morphologies, the regular octahedrons of AgIn 5 S 8 possess lower defect density and higher crystallinity, which can effectively hinder the charge recombination.Meanwhile, the exposed (111) facet owns higher In atom density, which can be conducted as the active site to accelerate the migration of photogenerated charges due to the CB domination of In 5s5p orbitals.Hence, this study provides a novel strategy to enhance the hydrogen production rate of AgIn 5 S 8 photocatalyst through the strategy of morphology control.Moreover, Lai et al. also deeply studied the surface engineering of AgIn 5 S 8 crystals during the experimental process of chemical bath deposition (Figure 1J), which supply a further advanced route for the morphology control of AgIn 5 S 8 photocatalyst in PHE application. [65]p to now, in addition to the works mentioned above, the AgIn 5 S 8 photocatalysts with different morphologies and the applications in PHE are rarely studied.[72][73][74][75] Therefore, referring to the studies of AgIn 5 S 8 thin film photoanodes, researchers can pay more attention to fabricate the AgIn 5 S 8 photocatalysts with more kinds of morphologies, aiming to maximize the role of morphology control to greatly optimize the hydrogen production rate of AgIn 5 S 8 photocatalyst.

| Subsection summary
Although various promising morphologies have been synthesized to improve the PHE performance of MS photocatalysts, which can provide a guiding research path for AgIn 5 S 8 , there are still some intrinsic challenges that need to be further overcome.Specifically, the morphology multifarious is still not enough to meet the desired PHE application; some of the advanced morphologies need to be synthesized under complex experimental conditions; the working principle of some morphologies, especially the precise analysis of charge transfer behavior, has not been fully understood.Therefore, further research on morphology control needs to focus on broadening the species of morphologies, simplifying the experimental fabrication process, and deeper understanding of working principles, which is greatly meaningful to enhance the PHE performance of AgIn 5 S 8 photocatalyst.

| Mechanism illustration of cocatalyst loading
Generally, during the PHE process, the volume and surface charge recombination can be prevented by the loading of cocatalyst owing to the formation of the Schottky junction and the corresponding mechanism illustration is exhibited in Figure 2A. [30]Specifically, the Schottky junction is made up of a semiconductor photocatalyst and a metal-like material, in which a space-charge-separation region can be formed. [76]Therefore, the formation of the Schottky junction through cocatalyst loading can significantly promote the photogenerated carriers separation.[47] On the other hand, the high cost of the noble-metal cocatalyst also makes researchers exploring the low-cost non-noble-metal cocatalysts.More efficient and faster charge separation can be achieved after the combination of cocatalyst with a semiconductor photocatalyst.Moreover, after the cocatalyst loading, the formation of the Schottky junction and lower Fermi level (E F ) makes photogenerated electrons easily migrate from the semiconductor photocatalyst to the cocatalyst and directly participate in HER (Figure 2B). [76]Therefore, the prevention of photogenerated carriers recombination and enhancement of charge migration can effectively promote the HER.
F I G U R E 2 HER mechanism illustration of PHE process with (A) cocatalyst loading.Reproduced with the permission. [30]Copyright 2016, American Chemical Society.(B) Formation of Schottky junction.Reproduced with the permission. [76]Copyright 2020, KeAi.PHE mechanism and the corresponding charge transfer behavior of (C) MoS 2 -MoO 3−x /AgIn 5 S 8 .Reproduced with the permission. [77]Copyright 2018, Elsevier.(D) NiS/AgIn 5 S 8 .Reproduced with the permission. [78]Copyright 2020, American Chemical Society.CB, conduction band; HER, hydrogen evolution reaction; PHE, photocatalytic hydrogen evolution; VB, valence band.photocatalyst. [77]Specifically, as shown in Figure 2C Semiconductor quantum dots (QDs), with specific uniform size (2-10 nm) and uniform distribution, are favorable for more efficient PHE performance due to the extended light absorption range, quantum confinement effect, and electronic performance of size-dependent. [79]articularly, when the size of the nanoparticles is similar to the wavelength of the electron, the quantum confinement effect can be observed, thereby providing appropriate CB and VB, energy levels, potential walls, and bandgaps.In a semiconductor photocatalyst at the nanoscale, the confinement of photogenerated carriers largely depends on the Bohr radius, which is a typical property of the photocatalyst.This effect greatly depends on the properties of semiconductor materials and usually occurs in larger nanocrystals, that is, nanocrystals with a Bohr radius of 2.34 nm or about 10 nm. [80]Nevertheless, the small nano-size of QDs also easily suffers from the issues of particle agglomeration and charge recombination. [80]Taking the QDs-mediated precipitation process, Zhang et al. fabricated the 0D/2D composite photocatalyst with semiconductor photocatalyst of Zn-AgIn 5 S 8 QDs and cocatalyst of NiS nanosheets (NiS/Zn-AgIn 5 S 8 ). [78]Benefitting from the efficient charge migration and sufficient active sites of NiS cocatalyst, the NiS/Zn-AgIn 5 S 8 photocatalyst exhibited the hydrogen production rate and apparent quantum yield (AQY) of 5200 μmol g −1 h −1 , which is 11 folds than that of bare AgIn 5 S 8 QDs.In the active sites, the transfer of HER-participating photogenerated electrons is effectively enhanced by the cocatalyst loading of NiS, which is due to the formation of NiS ultrathin nanosheets during the QD-mediated precipitation process.This can greatly accelerate charge migration and supply sufficient active sites in the 0D/2D structure.At the same time, the lightshielding effect is also minimized by decreasing the NiS nanosheet thickness (Figure 2D).Therefore, this work not only developed the NiS cocatalyst for the PHE performance enhancement of AgIn 5 S 8 but also highlights the positive effect of 0D/2D structure.

| Subsection summary
The optimization strategy of cocatalyst loading via the formation of the Schottky junction greatly assists the separation of photogenerated electron-hole pairs, thereby enhancing the PHE performance of AgIn 5 S 8 photocatalyst.Nevertheless, the excess loading of cocatalyst can take the negative effects of decreasing light absorption range, shielding incident light, acting as the recombination center, and covering the active site, leading to a decrease in the PHE performance. [81]lthough various studies have focused on the effect of the loading amount of the cocatalyst, the best loading amount still needs to be deeply researched and controlled, aiming to maximize the positive effect of cocatalyst loading.Moreover, most of the cocatalysts, developed in AgIn 5 S 8 photocatalyst, are single cocatalyst, which only enhance the HER-participating efficiency of photogenerated electrons.Typically, the dual cocatalyst of WP/Co 3 O 4 has been successfully applicated on Zn x Cd 1−x S photocatalyst and realized efficient preliminary PHE-OWS, in which the WP and Co 3 O 4 served as the reduction cocatalyst and oxidation cocatalyst, respectively. [82]The synergistic effect of WP and Co 3 O 4 is not only beneficial for charge separation but also effectively enhances the surface redox ability, resulting in the realization of preliminary PHE-OWS.Therefore, in addition to the optimization of the loading amount of a single cocatalyst, the exploration of a dual cocatalyst is also a meaningful research direction for the development of AgIn 5 S 8 photocatalyst.

| Single photocatalyst
In addition to the surface and volume charge recombination mentioned in Section 2, another serious issue of a single photocatalyst is the narrow light absorption range and weak redox ability, which is caused by the selfcharge recombination.As shown in Figure 3A, there is a strong Coulombic force between photogenerated electrons and holes, which can rapidly haul the photogenerated electrons from CB to VB in nano-or pico-second.
Photocatalyst with the optimization strategy of heterojunction construction, containing two or more than two semiconductors, can effectively broaden the visible-light absorption range and promote the separation of photogenerated carriers to each semiconductor. [49]

| Heterojunction photocatalyst of type-II and Z-scheme
Up to now, for the AgIn 5 S 8 -based photocatalysts in PHE application, the most commonly used heterojunction types are type-II and Z-scheme heterojunction.The corresponding PHE mechanism and the charge transfer behaviors are demonstrated in detail (Figure 3B).Specifically, due to the potential difference of the CBs of the two semiconductors, the photogenerated electrons can directly migrate from the CB of semiconductor 1 to the CB of semiconductor 2 and participate in HER.Simultaneously, for the photogenerated holes, an opposite transfer route in the VBs from Semiconductor 2 to Semiconductor 1 is exhibited and directly participates in OER.Therefore, for a vivid and clear definition, the charge transfer behavior of type-II heterojunction is "double charge transfer route."Hence, the type-II heterojunction is greatly beneficial to enhance the charge separation. [83]However, the CB of Semiconductor 2 is more positive than that of Semiconductor 1; the VB of Semiconductor 1 is more negative than Semiconductor 2. Therefore, although more efficient charge separation is realized through the design of type-II heterojunction, the reducibility and oxidizability are both decreased.Moreover, in the CB and VB, electrostatic repulsions also hinder the migration of photogenerated electrons and holes.Aiming to further maximize the positive effect of heterojunction construction, the Z-scheme heterojunction was then developed.Specifically, under light excitation, the strongly reducibility photogenerated electrons in the CB of Semiconductor 1 and the strong oxidizability holes in the VB of Semiconductor 2 are both effectively preserved.At the same time, the weak redox photogenerated carriers between the two semiconductors directly recombine (Figure 3C).Therefore, both enhanced charge separation and preservation of redox ability can be realized in the Z-scheme heterojunction photocatalyst, which is also conducive to the realization of PHE via overall water splitting (PHE-OWS) in the thermodynamics aspect. [84]

| AgIn 5 S 8 -based heterojunction photocatalysts
To combine the conventional photocatalyst of TiO 2 and AgIn 5 S 8 , Li et al. used the one-pot hydrothermal method and fabricated the type-II heterojunction photocatalyst of AgIn 5 S 8 /TiO 2 .The hydrogen production rate of the acquired AgIn 5 S 8 /TiO 2 photocatalyst reached 850 μmol g −1 h −1 , which is almost eightfolds than the pure AgIn 5 S 8 under the same fabrication method. [85]On the basis of the electronegativity empirical equation and characterization of UV-Vis, the authors proposed the corresponding charge transfer behavior of AgIn 5 S 8 /TiO 2 photocatalyst under the light excitation (Figure 4A).The construction of type-II heterojunction, through combining AgIn 5 S 8 and TiO 2 , results in an enhanced separation of photogenerated electron-hole pairs and a synergistic effect.On this basis, the author further improved the hydrogen production rate of AgIn 5 S 8 /TiO 2 type-II heterojunction photocatalyst to 3.718 mmol g −1 h −1 by using F I G U R E 3 Charge transfer behaviors of (A) bare photocatalyst.Reproduced with the permission. [49]Copyright 2022, Wiley.Heterojunction photocatalysts of (B) type-II and (C) Z-scheme.Reproduced with the permission. [76]Copyright 2020, KeAi.CB, conduction band; HER, hydrogen evolution reaction; VB, valence band.the low-temperature water bath deposition method.(Figure 4B). [86]In this broaden work, it has been proved that the continued high-speed magnetic stirring during the synthesis process can realize a closer combination between AgIn 5 S 8 and TiO 2 , resulting in a more efficient charge separation and faster electron transition from AgIn 5 S 8 to TiO 2 .Therefore, this work not only emphasized the type-II heterojunction construction but also pointed out the importance of the selection of fabrication method, aiming to realize a further enhancement of the PHE performance.Recently, researchers have also attempted to combine AgIn 5 S 8 with other kinds of semiconductors, aiming to develop more advanced AgIn 5 S 8 -based type-II heterojunction photocatalysts and further realize the efficient PHE.The two representative photocatalysts are AgIn 5 S 8 /g-C 3 N 4 [87] and AgIn 5 S 8 / ZnIn 2 S 4 . [88]The corresponding PHE mechanism and charge transfer behaviors are shown in Figure 4C,D.
Moreover, in addition to the type-II heterojunction construction, other optimization strategies have also been applicated, like, the 0D/2D structure and QDs of AgIn 5 S 8 .Therefore, these works also highlighted the effective path to further improve the hydrogen production rate of AgIn 5 S 8 -based type-II heterojunction photocatalysts.
In recent years, the advanced AgIn 5 S 8 -based heterojunction photocatalysts with Z-scheme characteristics have also been explored, aiming to achieve more efficient PHE.Zhu et al. constructed the heterojunction photocatalyst of AgIn 5 S 8 /ZnS with the Z-scheme characteristic via a twostep hydrothermal method, in which the ZnS was introduced to the Zn vacancies. [89]As can be seen in Figure 5A, after the combination of AgIn 5 S 8 and ZnS, forming the Z-scheme heterojunction, an effective preservation of photogenerated electrons without the loss of reducibility is realized in the CB of AgIn 5 S 8 and directly F I G U R E 4 Recent AgIn 5 S 8 -based type-II heterojunction photocatalysts and the application in PHE.(A) AgIn 5 S 8 /TiO 2 by one-pot hydrothermal method.Reproduced with the permission. [85]Copyright 2013, American Chemical Society.(B) AgIn 5 S 8 /TiO 2 by lowtemperature water bath deposition method.Reproduced with the permission. [86]Copyright 2013, Elsevier.(C) AgIn 5 S 8 /g-C 3 N 4 .Reproduced with the permission. [87]Copyright 2019, Elsevier.(D) AgIn 5 S 8 /ZnIn 2 S 4 .Reproduced with the permission. [88]Copyright 2018, Elsevier.CB, conduction band; NHE, normal hydrogen electrode; VB, valence band.participated in HER.Moreover, owing to the strong interface interaction and intimate contact between the AgIn 5 S 8 and ZnS, photogenerated electrons in the defect level of ZnS and photogenerated holes in the VB of AgIn 5 S 8 can be directly recombined.Therefore, apart from the Z-scheme heterojunction construction, the introduction of Zn vacancies in ZnS can further accelerate the combination of photogenerated carriers with low redox ability.Later, Zhang et al. fabricated the Z-scheme Zn-AgIn 5 S 8 /α-Fe 2 O 3 heterojunction photocatalyst, possessing the morphology of QDs and structure of 0D/2D (Figure 5B). [90]In addition to the positive effects of Z-scheme heterojunction construction, an extremely close interface contact between Zn-AgIn 5 S 8 and α-Fe 2 O 3 can be achieved by electrostatic adsorption because of the zeta potential difference, resulting in a uniform dispersion of 0D Zn-AgIn 5 S 8 on α-Fe 2 O 3 and prevention of QDs agglomeration.Meanwhile, the 0D Zn-AgIn 5 S 8 QDs also effectively exert the positive effect of a short electron transmission path and sufficient specific surface area.Moreover, the high conductivity of α-Fe 2 O 3 nanosheets can greatly hinder the charge recombination in 0D Zn-AgIn 5 S 8 QDs because of the rapid leading out of the photogenerated charges.Finally, the Z-scheme 0D/2D Zn-AgIn 5 S 8 /α-Fe 2 O 3 heterojunction photocatalyst achieved the hydrogen production rate and AQY of 1700 μmol g −1 h −1 and 7.48% at 420 nm, respectively.Xu et al. also designed the AgIn 5 S 8 -based Z-scheme heterojunction photocatalyst of AgIn 5 S 8 /ZnO and took it in the application of photocatalytic decontamination of pharmaceutical pollutants (Figure 5C). [67]Although the authors did not study the corresponding application in PHE, it provided an important scientific design route of AgIn 5 S 8 -based Z-scheme heterojunction photocatalyst, which is greatly meaningful for further development and the preliminary realization of PHE-OWS.
transfer paths and extended light absorption range.However, guided by the mature researched MS photocatalysts, like, CdS, [91] Zn x Cd 1−x S, [29] and ZnIn 2 S 4 , [45] the intrinsic challenges still need to be solved and deeper efforts are required for the exploration of AgIn 5 S 8 -based heterojunction photocatalysts.First, the charges transfer modes in AgIn 5 S 8 -based heterojunction photocatalysts, including the trapping, migration, and recombination, are still not fully clear, which need to be deeply explored by using advanced characterization techniques.Second, some of the high-quality AgIn 5 S 8 -based heterojunction photocatalysts still need to be constructed under complexed and time-consuming conditions, which need to be solved by developing an efficient and facile fabrication process.Finally, more recently explored semiconductor materials with excellent semiconductor properties need to be combined with AgIn 5 S 8 to construct a more advanced AgIn 5 S 8 -based heterojunction photocatalyst, aiming to achieve further enhanced PHE performance.

| SUMMARY OF OPTIMIZATION STRATEGIES
The schematic timeline for the development of AgIn 5 S 8based photocatalyst in PHE applications across the years is summarized in Figure 6.For the whole PHE process of AgIn 5 S 8 photocatalyst, the final hydrogen production is greatly influenced by the amount of the HER-participating photogenerated electrons, hence the electron-consuming factor (recombination of photogenerated carriers) should be hindered through optimization strategies.For bare AgIn 5 S 8 , the volume/surface charge recombination generally occurs, which is also the universal phenomenon of semiconductor photocatalysts during the PHE process. [23]The morphology control of photocatalyst can supply more sufficient active sites and surface area, [92][93][94] which can enhance the PHE performance of AgIn 5 S 8 photocatalyst due to the hindering of volume/surface charge recombination to a certain extent.Nevertheless, the charge recombination caused by the strong Coulombic force between photogenerated electrons and holes cannot be avoided through the optimization strategy of morphology control.In this regard, the optimization strategies of cocatalyst loading and construction of heterojunction have been adopted, aiming to further enhance the PHE performance of AgIn 5 S 8 photocatalyst.Specifically, the Schottky junction can be formed through cocatalyst loading, in which the photogenerated electrons can effectively get rid of Coulombic force and directly migrate to the metal-like cocatalyst to participate HER.The construction of heterojunction can not only accelerate the charge separation and migration via different carrier migration paths but also extend the light-response range, which is another effective path to prevent the charge recombination caused by the Coulombic force.Especially, under the premise of highefficiency charge separation, the Z-scheme heterojunction can further preserve the photogenerated electrons and holes with strong redox ability, which has a great potential to realize PHE-OWS.
F I G U R E 6 Development of AgIn 5 S 8 in PHE applications across the years.PHE, photocatalytic hydrogen evolution.
AgIn 5 S 8 owns the semiconductor properties of environmentally friendly elements, suitable bandgap, and appropriate electronic band structures, which is conducive to achieve efficient visible-light PHE.During the past few years, AgIn 5 S 8 -based photocatalysts have been widely studied and achieved great research progress in PHE application, which is also comprehensively and systematically summarized in this review.The whole content mainly includes the basic mechanism and charge transfer behavior illustration of PHE, semiconductor properties of AgIn 5 S 8 , bare AgIn 5 S 8 photocatalyst, cocatalyst loading, and AgIn 5 S 8 -based photocatalysts through the optimization strategies of morphology control and heterojunction construction.The corresponding schematic illustration and PHE performances are exhibited in Figure 7 and Table 1.Despite great achievements that have been realized, it still has many issues that need to be resolved, aiming to further enhance the hydrogen production rate or realize the efficient PHE-OWS owing to some unresolved limitations of AgIn 5 S 8 in the current development stage.Typically, the AgIn 5 S 8 photocatalyst generally suffers from the photocorrosion phenomenon under visible-light irradiation, which is also a common issue of MS photocatalysts. [28]Specifically, the S 2− can be oxidized to sulfur during the PHE process due to the easy combination of S 2− and photogenerated holes in the VB of MS photocatalyst. [40]In this case, the hydrogen production rate and photostability of MS photocatalysts have decreased.Another challenge of AgIn 5 S 8 photocatalyst is the weak oxidability.Specifically, the negative CB potential of AgIn 5 S 8 photocatalyst makes a relatively strong driving force to perform HER.However, the VB potential is hard to effectively perform OER due to the narrow bandgap of AgIn 5 S 8 , [67] which is difficult to realize the efficient PHE-OWS.Therefore, deeper mechanism understanding and experimental investigations should be conducted, aiming to realize the universal application of AgIn 5 S 8 -based photocatalysts in PHE.The possible further solution can be concluded as the following aspects.nanowires, nanoflowers, and microclusters, [45,46,49] which have realized the efficient PHE performance enhancement.Therefore, the morphology control of these TMS photocatalysts provides a valuable and clear research direction for further development of AgIn 5 S 8 photocatalyst to broaden the diversity of morphology.Second, in the current stage, the positive effect of morphology control has been mainly proved through the experimental results.A deeper theoretical understanding, especially the corresponding charge transfer and migration behaviors, needs to be further studied.Moreover, in addition to the single optimization strategy of morphology control, further combination with the design of advanced structure can further effectively improve the hydrogen production rate, which has also been demonstrated in other mature BMS or TMS photocatalysts.The recently developed advanced structures mainly include metal-organic frameworks, [95] covalent-organic framework, [96] core-shell structure, [97] hollow structure, [98] and nanocages. [45]Therefore, the promising research direction of morphology and structure engineering is greatly meaningful for the further PHE enhancement of AgIn 5 S 8 photocatalyst.
(ii) Vacancy introduction.[101][102] However, the suitable amount of vacancies after optimization has a positive effect on the PHE performance because of the broaden light absorption range, enhanced dissociation properties, accelerated migration of photogenerated charges, and increased active sites. [103]Nevertheless, the optimization strategy of vacancy introduction in AgIn 5 S 8 photocatalyst has rarely been studied.As mentioned in Section 4.3.3, in the Z-scheme AgIn 5 S 8 /ZnS heterojunction photocatalyst, the vacancy introduction has been applied in ZnS. [89]Moreover, the deeply researched photocatalyst of ZnIn 2 S 4 with sulfur vacancies has also been applied in photocatalytic H 2 O 2 production. [104]Guided by these, future researches of AgIn 5 S 8 can pay more attention to vacancy introduction, including the Ag, In, and S vacancies, which is a key point to effectively improve the activity of PHE.(iii) Heterojunction construction and realization of PHE-OWS.To date, in the development process of application, researchers have mainly focused on the construction of type-II and Z-scheme heterojunctions for AgIn 5 S 8 -based heterojunction photocatalysts in the research of PHE.Although significant progresses have been achieved, most of them have not realized the preliminarily PHE-OWS and some essential issues still need to be overcome.First, the diversity of AgIn 5 S 8 -based heterojunction photocatalysts in PHE application is relatively low due to the rare study.][107][108] Therefore, the combination of AgIn 5 S 8 with various other kinds of suitable semiconductors and other heterojunction types of type-I heterojunction, p-n heterojunction, and recently developed S-scheme heterojunction need to be further widely studied, aiming to construct more advanced AgIn 5 S 8 -based heterojunction photocatalysts with better band alignment and achieve better PHE performance.Second, the current synthesis method of AgIn 5 S 8 -based heterojunction photocatalysts is complex.Simpler paths and more facile fabrication methods should be explored without reducing PHE activity.Finally, in view of PHE-OWS, [109] more comprehensive researches of AgIn 5 S 8 -based heterojunction photocatalysts needs to be performed, especially the advanced Z-scheme or S-scheme heterojunctions because of the effective preservation of strong redox ability photogenerated charges to perform PHE-OWS.(iv) Development of advanced synthesis method.In addition to the above-mentioned typical issues, another typical problem of AgIn 5 S 8 photocatalyst is the application of extra sulfur source in the synthesis process, like, thioacetamide, thiourea, Na 2 S, and so forth.During the fabrication process, the using of these extra sulfur sources not only makes the synthesis process more complex but also easy to introduce high-density sulfur vacancies, which can act as the nonradiative recombination center and are harmful to PHE performance. [35,103][112][113][114][115][116] The MS photocatalysts, synthesized by BDCA solution process with sulfur-rich feature, need not use any extra sulfur source, which can greatly reduce the fabrication complexity and decrease the sulfur vacancies density.Typically, the MS photocatalysts of phased junction CdS [35] and bournonite CuPbSbS 3 [48] photocatalysts have been successfully synthesized using this method and have achieved promising PHE performance.Guided by this, the BDCA solution process can be also applicated in the fabrication of AgIn 5 S 8 , aiming to improve the quality of AgIn 5 S 8 for further optimization strategies.(v) Broaden other allotropes of AgIn x S y in PHE application.Apart from AgIn 5 S 8 , AgInS 2 , as another allotrope of AgIn x S x , has also been developed in STE applications.[119] In the early stage of exploration, the AgInS 2 was mainly used in the STE technique of solar cell, [120][121][122][123] and until recently the AgInS 2 -based photocatalysts of type-II AgInS 2 /TiO 2 heterojunction photocatalysts were synthesized and applied in PHE or photocatalytic degradation. [118,124]In view of the semiconductor properties of AgInS 2 , future researches can pay more attention on the AgInS 2 , which can broaden the PHE application of AgIn x S y .(vi) Development of machine learning and in situ characterization.A better mechanism understanding and optimized process of semiconductor photocatalysts are greatly essential for the improvement of PHE performance.[127][128] Representatively, as the pioneered semiconductor photocatalyst in PHE application, the molecular dynamics simulation of TiO 2 with the assistance of machine learning has been deeply studied.Specifically, the anatase TiO 2 , water, and TiO 2 (001)-water interface could be predicted by the building of a deep learning neural networks model, which can effectively predict the atomic energy. [129]The fabrication of advanced TiO 2 -based photocatalysts can be dynamically assisted by the understanding of water dissociation on TiO 2 surface.Therefore, the machine learning of TiO 2 provides a guidance research path for the development of AgIn 5 S 8 photocatalyst.32][133] In summary, AgIn 5 S 8 possesses several excellent semiconductor properties that are beneficial for efficient visible-light PHE.In the past few years, promising research progresses have been achieved on AgIn 5 S 8based photocatalyst in PHE application and the corresponding optimization strategies have been systematically discussed in this review.In future researches, more valuable observations or commercial application levels of AgIn 5 S 8 -based photocatalyst can be realized with continued efforts by optimizing morphology, structure, vacancy, and heterojunction.

4. 2 . 2 |
AgIn 5 S 8 -based photocatalysts with cocatalyst loading Song et al. designed the Mo-based cocatalyst in which the composition is MoO 3−x nanoparticles and MoS 2 nanosheets (MoS 2 -MoO 3−x ) and realized the efficient PHE performance improvement of AgIn 5 S 8 photocatalyst.The MoS 2 nanosheets with a few layers structure can supply more sufficient surface active sites and the high conductivity of MoO 3−x nanoparticles with oxygen defects greatly accelerate the charge migration, resulting in a great PHE performance enhancement of AgIn 5 S 8 , in the MoS 2 -MoO 3−x /AgIn 5 S 8 photocatalyst, the high conductivity of MoO 3−x nanoparticles and sufficient surface active sites of MoS 2 nanosheets can play the synergetic effects due to the close neighborhood among MoS 2 , MoO 3−x , and AgIn 5 S 8 , which is greatly beneficial for the charge migration from AgIn 5 S 8 to MoS 2 nanosheets and/ or MoO 3−x nanoparticles.
(i) Morphology and structure engineering.In the development process of AgIn 5 S 8 photocatalyst for PHE application, despite some studies having focused on the morphology control, it still needs wider exploration and further optimization.First, other deeply studied TMS photocatalysts, like, CuInS 2 , Zn x Cd 1−x S, and ZnIn 2 S 4 , have widely explored various kinds of morphologies, like, nanospheres, nanododecahedrons, F I G U R E 7 Schematic illustration of AgIn 5 S 8 -based photocatalyst during the development process.
T A B L E 1 Note: (a) Synthesized by one-pot hydrothermal method.(b) Synthesized by low-temperature water bath deposition method.Abbreviations: AQY, apparent quantum yield; PHE, photocatalytic hydrogen evolution.ZHENG ET AL.