Design and architecture of ZnIn₂S₄ and ZnIn₂S₄-based hybrid materials for photocatalytic, electrocatalytic and photoelectrochemical hydrogen evolution

Abstract

Photocatalytic innovations are routinely employed in the production of hydrogen, remediation of environmental damage, lowering CO₂ emissions, and numerous additional critical disciplines because of their sustainability, ease of being implemented, and dependability on solar energy as a mandate source. ZnIn₂S₄, a ternary metal sulfide, has garnered considerable interest among visible-light-responsive photocatalysts due to its outstanding properties that include convenient synthesis, outstanding resilience, and controllable band configuration. However, its limited light-harvesting ability, rapid recombination of photogenerated charges, and low redox capacity remain significant limitations that hinder the optimization of the photocatalytic activity of ZnIn₂S₄ photocatalysts. These challenges can be addressed through the formation of S-scheme heterojunctions by integrating ZnIn₂S₄ and other semiconductors. Recently, various semiconductor photocatalysts, such as sulfur compounds (ZnS, CoS, and FeS₂), metal oxides (WO₃, TiO₂, and In₂O₃), and some organic compounds, have been combined with ZnIn₂S₄ to derive ZnIn₂S₄-based S-scheme heterojunctions to improve its catalytic performance. However, their implementation is limited by photogenerated carrier recombination and photocorrosion. These challenges can be addressed through the formation of S-scheme heterojunctions by integrating ZnIn₂S₄ with additional semiconductors; however, the photocatalytic activity of S-scheme heterojunctions still needs to be enhanced. To date, the extensive photocatalytic applications of ZnIn₂S₄-based S-scheme heterojunctions have been thoroughly demonstrated with specific examples, including H₂ production, CO₂ reduction, and environmental remediation. Currently, the modification of ZnIn₂S₄ through metal ion and non-metal doping has received limited attention. Consequently, investigations into the impact of the non-metallic doping of ZnIn₂S₄ on its properties can be extended. Herein, we outline the current challenges and critical issues related to ZnIn₂S₄ and its photocatalysts. Furthermore, we provide perspectives on future advancements and highlight various challenges associated with ZnIn₂S₄-based materials.

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1. Introduction

Hydrogen (H₂) has received recognition as an appropriate and development-friendly way to generate energy from renewable resources. Furthermore, the combustion of H₂ does not generate any CO₂ or NO emissions into the atmosphere. At present, environmental issues and energy scarcity are global challenges that threaten the advancement of human civilization. In this context, utilizing sustainable energy reserves for semiconductor-based catalysis is considered one of the most promising technologies to address these issues. In early 1972, Fujishima and Honda discovered that H₂ evolution could be accomplished via a photo-based electrochemical water-splitting process using TiO₂ and ultraviolet light.¹ Since then, the widespread application of semiconductor-based photocatalysis in water splitting,² reduction of CO₂,³ elimination of contaminants,^4–7 disinfection of microbes,⁸ and organic compound synthesis⁹ has been inspired by their groundbreaking work. However, despite this, the substantial band gap of TiO₂ of roughly 3.2 eV poses a challenge in meeting the demands of practical applications. In this case, the functionality of TiO₂-based devices is constrained to ultraviolet or near-ultraviolet light, inhibiting their capacity to effectively employ solar energy.¹⁰

Visible sunlight consists of 43% of the total energy radiated by the sun. Alternatively, ultraviolet light only consists of 4%. Consequently, significant research has been devoted to the development of visible-sunlight-activated, photosynthetic-based catalysts to facilitate the full use of solar energy. At present, a multitude of apparent light-irradiated photocatalyst designs have been executed or built by researchers. These designs include complex oxides, such as simple oxides Fe₂O₃,¹¹ and WO₃,¹² Ag₃PO₄,¹³ Bi₂MoO₆,¹⁴ and Bi₂WO₆,⁷ graphitic carbon nitride (g-C₃N₄),^8,15 metal chalcogenides (CdS),¹⁶ MIL-125 (Ti) derivatives,¹⁷ and TiO₂ derivatives (TiO_2−xN_x).¹⁸

Metal chalcogenides have been the topic of significant study among visible-light-response photosynthetic catalysts due to their unique optical and catalytic features, as well as their corresponding high degree of chemical equilibrium.¹⁹ Recently, there has been considerable interest in new tertiary metal chalcogenides.²⁰ Among them, ZnIn₂S₄ (ZIS) is an outstanding, apparent, light-sensitive, photo-based catalyst possessing unique outstanding characteristics. This material is composed of arranged structures and has the ability to display three distinct crystal polymorphs including cubic, hexagonal, and rhombohedral. Alternatively, it should be noted that all crystal polymorphs of ZnIn₂S₄ exhibit photocatalytic properties when subjected to visible light. The successful synthesis of ZnIn₂S₄ micro/nanostructures showing diverse morphological properties has been achieved owing to the organized framework of ZnIn₂S₄.

ZnIn₂S₄ is a member of the semiconductor family with a direct band gap, which can be tuned in the range of 2.06 to 2.85 eV. It operates as an ordinary photocatalyst susceptible to visible light (at about 570 nm).^21–24 Furthermore, compared to traditional metal sulfides such as Sb₂S₃,²⁵ ZnS and CdS,¹⁶ it exhibits comparable optical characteristics and seems to be less toxic, suggesting its potential for extensive implementation in environmental preservation.²¹ However, despite being less harmful to humans, ZnIn₂S₄ cannot respond to ultraviolet-visible light owing to its broad energy band gap. In contrast, it is a typical photocatalyst that can respond to visible light.^24,26 Furthermore, compared to ZnIn₂S₄, Zn₃In₂S₆,²⁷ and CuGaS₂,²⁶ it is relatively easy to generate due to the abundance of available raw materials and its straightforward chemical composition.²⁸ Furthermore, ZnIn₂S₄ exhibits significant chemical durability during the photocatalytic process.²⁹ As a result, ZnIn₂S₄ has attracted significant attention due to its interesting chemical and physical characteristics, in addition to its low cost, minimal toxic effects, and simple fabrication.^21,26,29 Due to the above-mentioned advantages, ZnIn₂S₄ has attracted considerable interest for a multitude of possible applications in diverse domains, including selective organic synthesis,¹⁷ evolution of H₂,^30–33 and reducing carbon dioxide (CO₂)^34–36 in the presence of visible light^37–39 and protecting the environment.^40–44 However, ZnIn₂S₄ is characterized by several drawbacks, including the rapid recombination of photogenerated electrons and holes, slow charge-carrier migration, and structural imperfections, which include stacking and displacement faults. Consequently, the charge-carrier lifetime and absorption capacities of ZnIn₂S₄ have been considered inadequate. In general, a catalytic process involves three processes, including the adsorption of electromagnetic radiation, the production and segmentation of electron and hole charges, and photocatalytic surface reactions.⁴⁵ Meanwhile, the photocatalytic ability of a substance is extremely susceptible to variations in its crystal structure, physical characteristics of its surface, and chemical composition.⁴⁴ Consequently, these drawbacks reduce the photocatalytic efficiency of ZnIn₂S₄. Thus, in an effort to overcome these constraints, research has been devoted to strengthening the photocatalytic capabilities of ZnIn₂S₄. The strategies for improving the control of its morphology and structure, together with surface alteration have been extensively discussed. Moreover, surface functionalization, element doping, heterojunction formation, modification with a co-catalyst, etc. have been successfully applied to enhance the photocatalytic activity of Znln₂S₄. Furthermore, the crystal structures and dimensions of semiconductors generally impact their optical characteristics, surface area, and charge transfer.

Controlling the shape and structure of micro- and nanostructures is also acknowledged to have the potential to enhance the functional applications of photocatalytic materials. Moreover, modifying their surface can inhibit the recombination of photogenerated electron–hole pairs, influence the dynamics of charge carriers, and alter their optical response capability. As a result of these approaches, the photosensitive characteristics and catalytic efficacy of ZnIn₂S₄ have been enhanced. For example, Chen et al.⁴⁶ performed a comparative examination of the photocatalytic efficacy of hexagonal ZnIn₂S₄ microscopic spheres and cubic zinc indium-sulfide (ZnIn₂S₄) nanoparticles with respect to dye degradation. Additionally, the photocatalyst properties of ZnIn₂S₄ doped with Cu were investigated by Shen et al.,⁴⁷ and they found a diminished band gap and higher absorption of radiation compared to that of unmodified ZnIn₂S₄. Cui et al.⁴⁸ successfully synthesized a ZnIn₂S₄/K₂La₂Ti₃O₁₀ heterostructure composite with considerably improved photocatalytic activity towards the evolution of hydrogen. Furthermore, a ZnIn₂S₄/carbon dot composite material was assembled by Shi et al., which showed exceptional capacity for the deterioration of methyl orange via photocatalysis.⁴⁰ Thus, it is imperative to publish a review concisely outlining the achievements and predicting the future development of ZnIn₂S₄. This review focuses on recent developments of ZnIn₂S₄, including its characteristics, synthesis, and photocatalytic applications related to the evolution of H₂, the removal of harmful substances, and reducing CO₂. Initially, the physical traits and chemical components of ZnIn₂S₄, such as its electronic, crystal, and optical features, as well as its shape and structure, are summarized. Subsequently, we deliberate on contemporary innovations in the synthesis of ZnIn₂S₄ and the underlying mechanisms for the formation of its micro- and nanostructures. Furthermore, a synopsis of the modifying strategies and photocatalytic applications of ZnIn₂S₄-based composites and ZnIn₂S₄ micro/nanostructures for the conversion of energy and pollutant removal is also provided. Finally, we highlight the current obstacles and pivotal concerns regarding ZnIn₂S₄ and its composites, which require further investigation in the future. It is expected that this comprehensive analysis will result in a greater understanding of ZnIn₂S₄.

2. Properties of ZnIn₂S₄

2.1. Crystal properties

The charge transmission path in ZnIn₂S₄ heterojunctions is consistent with the S-scheme concept. The preliminary results of density functional theory (DFT) calculations indicate that the Bader charge of the S atoms in ZnIn₂S₄ is optimized at 6.72e, as shown in Fig. 1(A). Layered ZnIn₂S₄ is a linear band gap semiconductor with a crystal structure that can exist as three distinct polymorphs, including cubic, hexagonal, and rhombohedral, as depicted in Fig. 1(B and C). ZnIn₂S₄ is also semiconductor member in the broad category of ternary metal chalcogenides. Its structure is comprised of multiple layers and adopts its universal formula, AB₂X₄, where A can be Zn, Cd, or Hg; B can be Al, Ga, or In; and X can be S, Se, or Te.⁵³

Fig. 1 (A) Optimal ZnIn₂S₄ structure. Reproduced from ref. 49 with permission from Elsevier, Copyright 2023. (B) Crystalline configuration of ZnIn₂S₄: (a) cubic, (b) hexagonal and (c) rhombohedral. Reproduced from ref. 50 with permission from Elsevier, Copyright 2022. (C) Crystalline structures of (a) hexagonal, (b) cubic and (c) rhombohedral ZnIn₂S₄. Reproduced from ref. 51 with permission from Elsevier, Copyright 2011. (D) Schematic depicting the crystalline structure of ZnIn₂S₄. Reproduced from ref. 52 with permission from Elsevier, Copyright 2023.

The rhombohedral and hexagonal ZnIn₂S₄ crystal structures are similar, as shown in Fig. 1(D), which are made up of two layers of tetrahedra and one layer of octahedra stacked on top of each other. Both half of the indium atoms and zinc establish tetrahedral coordination with four sulfur atoms in the hexagonal-phase P₆₃mc, whereas the other half of the indium atoms form octahedral coordination with six sulfur atoms. The heterogeneous pattern of distribution of Zn and half of In atoms in the tetrahedral positions is what distinguishes the rhombohedral phase R₃m from the hexagonal phase.

It can also take on three different crystalline forms, which are hexagonal, cubic, and rhombohedral. Its atomic configuration is composed of a series of S–In–S–Zn–S frameworks. The hexagonal polymorph of ZnIn₂S₄ demonstrates numerous polytypes, each containing, for example, ABCA or ABAB stacking patterns of S elements. Particularly, the S atoms exhibit tetrahedral coordination with Zn and half of the In atoms, whereas the remaining In atoms form an octahedral coordination. Two poorly bonded S layers are separated by a layer of vacancies produced by the layering of these bundles. As a result, a wide assortment of polytypes can be observed, extending from one pattern for each unit cell (1H polytype) to 24 patterns for each unit cell (24R polytype).⁵⁴ In regard to the cubic polymorph of ZnIn₂S₄, it is comprised an ABC stacking of S atoms and is a direct cubic spinel phase. The S atoms have tetrahedral coordination with the Zn atoms, while the In atoms have octahedral coordination. The rhombohedral polymorphism of ZnIn₂S₄ is described as a layered compound consisting of distinct layers that are connected by inadequate S–S bonding between the S atoms of each layer and dominant zinc-sulfide and indium-sulfide bonds within each phase. The S atoms are tetrahedrally coordinated around the Zn atoms, whereas the In atoms are octahedrally or tetrahedrally connected. The discourse surrounding hexagonal and cubic ZnIn₂S₄ is comparatively more prevalent than that of rhombohedral ZnIn₂S₄, as demonstrated by the extensive investigation of the former. In the case of solid materials, their properties can be tuned through phase transitions instead of introducing novel elements. This demonstrates significant technological utility and enables the extension of the properties of solid materials.⁵⁵ Thus, it is possible to convert 3R hexagonal ZnIn₂S₄ to cubic ZnIn₂S₄ by applying an elevated pressure of approximately 4 MPa and a temperature of roughly 400 °C. When subjected to vacuum or ambient pressure at a temperature surpassing 500 °C, cubic ZnIn₂S₄ has the ability to change into hexagonal ZnIn₂S₄. Shen's group noticed a transformation from cubic shape to rhombohedral ZnIn₂S₄ upon thermal sulfidation in the temperature range of 400 °C to 800 °C.⁵¹ Furthermore, it was shown by Chen et al.⁵⁶ that it is feasible to manufacture the cubic and hexagonal polymorphs of ZnIn₂S₄ based on the preferred orientation of the metals. When metal nitrates were utilized as the precursor molecules, ZnIn₂S₄ was obtained in the cubic phase; in contrast, hexagonal-phase ZnIn₂S₄ was generated when metal chlorides were used as the precursors. This suggests that the crystal structure is greatly affected by the metal ions. Variations in the crystal polymorphs of ZnIn₂S₄ give rise to different features. Previous investigations have documented the photoconductivity⁵⁷ and photoluminescence⁵⁸ of hexagonal ZnIn₂S₄ and thermal electricity⁵⁴ demonstrated by cubic ZnIn₂S₄. Additionally, both hexagonal and cubic ZnIn₂S₄ exhibit photocatalytic activity for the elimination of contaminants and the evolution of H₂.^23,46,56 However, greater efficiency has been reported for hexagonal ZnIn₂S₄. Although the published research on the rhombohedral polymorph of ZnIn₂S₄ is limited, Yang et al. and Shen et al. independently recognized its photocatalytic activity in the evolution of hydrogen (H₂) and provided appropriate photographic evidence of its reactivity under irradiation.^51,59

A heterophase ZnIn₂S₄ composite consisting of cubic quantum dots with hexagon-shaped tiny spheres was investigated by Wang et al.⁶⁰ The establishment of a heterophase junction prompted an enhancement in the operating effectiveness of hydrogen evolution. Each phase of ZnIn₂S₄ displays unique characteristics and offers a variety of scenarios. Its hexagonal polymorph is applied as a luminescent and conductive material, whereas its cube-shaped polymorph shows thermal properties. When exposed to ultraviolet light, all these different forms of ZnIn₂S₄ showed amazing photocatalytic properties for the degradation of contaminants and production of hydrogen gas.^61–65 The utility and functional potential of a nanomaterial depend on its exterior dimensions, form, and formulation. Numerous modifications associated with any of these attributes may culminate in the fabrication of nanomaterials that exhibit adequate characteristics for particular purposes. To date, nanostructures have been constructed using numerous processes, including hydrothermal synthesis, sol–gel manufacturing, ion doping, deposition techniques, precipitation-induced methods, chemically generated vapor accumulation, and fermentation.^66–69 In recent decades, the soft-template-assisted method has emerged as an established technique to obtain the desired attributes.^70,71 ZnIn₂S₄-based materials have been extensively implemented in photocatalysis, optoelectronics, and energy optimization given that they possess distinctive favorable characteristics, such as outstanding stability, superior light absorption, and tunable band gaps. However, they possesses distinct shortcomings. Generally, ZnIn₂S₄ experiences rapid charge-carrier recombination, limiting its photocatalytic efficiency.⁷² The accelerated recombination of photogenerated electrons and holes may occur before accomplishing the desired operations, reducing the overall efficiency of the material. Furthermore, its low quantum proficiency and insufficient quantum efficiency for certain responses make it unsuitable for widespread photocatalytic applications, such as water splitting and CO₂ reduction.

2.2. Morphological characteristics

It is common knowledge that the dimensions, morphological structures, and proportion of semiconductor materials determine their properties. Many studies have shown that ZnIn₂S₄ photocatalysts can be shaped in different ways and in multiple dimensions. Additionally, a short explanation of the distinct shapes of ZnIn₂S₄ is presented in the subsequent section. Numerous research groups worldwide have been working extremely diligently on the development of light-sensitive photocatalysts with robust redox characteristics. Consequently, several photocatalysts, including WO₃, graphitic carbon nitride (CN), Bi₂MoO₆, CdS, and BiVO₄, have been synthesized and implemented.^73–77 In this case, among the ternary sulfur compounds, ZnIn₂S₄ stands out owing to its extraordinary electromagnetic and optical properties.^23,78,79

ZnIn₂S₄ has been synthesized and prepared in various dimensions and shapes based on its lamellar crystalline structure. These structures include nanowires, tiny ribbons, and nanoparticles, which have a zero-dimensional structure; nanosheets, which are microspheres comprised of flakes and thin films; and flower-like, porous, and hollow microspheres, which have a three-dimensional structure (Fig. 2).

Fig. 2 (A) SEM image of flower-like microsphere morphology. Reproduced from ref. 80 with permission from Elsevier, Copyright 2013. (B) SEM images of porous sphere ZIS. Reproduced from ref. 81 with permission from Springer, Copyright 2019. (C) SEM image showing hydrangea-like spheres. Reproduced from ref. 82 with permission from Springer, Copyright 2019. (D) TEM image of a hollow marigold-like ZnIn₂S₄ sample. Reproduced from ref. 83 with permission from The Royal Society of Chemistry, Copyright 2014.

In general, the features of semiconductors can be enormously dictated by their dimensions and shapes. There are several reports demonstrating that ZnIn₂S₄ is sensitive to surface defects. However, the electron–hole pair recombination happening in surface imperfections could be inhibited in zero-dimensional ZnIn₂S₄ containing small quantum dots and nanoparticles due to their uniform distribution and confinement effect. Additionally, it is hypothesized that the expanded morphological features of one-dimensional ZnIn₂S₄ stimulate the dispersion of photoinduced electron–hole pairs. Concerning the dispute of two-dimensional ZnIn₂S₄ nanostructures, sheet-like ZnIn₂S₄ demonstrates a greater variety of poorly coordinated surface atoms and an expanded area of unique outer layer compared to bulk ZnIn₂S₄, which allows the accessibility to a greater quantity of active sites. Considering the two-dimensional nature of ZnIn₂S₄ thin films, their characteristics are easily recognized. Moreover, three-dimensional ZnIn₂S₄ structures are frequently found in the form of flowers or microspheres, increasing the overall number of active site, which is advantageous for practical applications.

Numerous synthetic techniques can be implemented for the preparation of ZnIn₂S₄ with diverse morphologies. As illustrated,^84,85 a thin film of ZnIn₂S₄ was produced using the spray pyrolysis and spin-coating techniques. In addition, ZnIn₂S₄ demonstrating different sizes and shapes was produced through hydrothermal and solvothermal processes. The solvothermal technique was utilized to create 3D-hierarchical, persimmon-like ZnIn₂S₄.⁷⁹ The hydrothermal synthesis of ZnIn₂S₄ nanoparticles²³ was also implemented. The synthesis of ZnIn₂S₄ photocatalysts enables detailed investigations into current innovations.

2.3. Optical and electronic characteristics

ZnIn₂S₄ demonstrates extraordinary optical and electronic characteristics, including an adjustable energy band, which can be tuned in the range of 2.06 to 2.85 eV.^24,29 The band-like structures of ZnIn₂S₄ materials can be computed by implementing density functional theory (DFT). Based on the displayed data in Fig. 3(A), the valence band peak and conductivity band peak agree with the G attribute of the Brillouin zone of the ZnIn₂S₄ crystal. This feature demonstrates that the crystal functions as a direct band gap semiconductor.⁴⁷ In general, approximation function constraints result in an underestimation of the direct band gap semiconductors. Therefore, the estimated band gaps of cubic and hexagonal ZnIn₂S₄ were determined to be 0.28 and 1.36 eV, respectively, which are less than the experimentally reported values.³⁴ Moreover, because of its band gap, ZnIn₂S₄ can be absorb visible-light radiation. Its broad shoulder and tail boundaries amplify electromagnetic radiation in the visible band (approximately 570 nm), rendering it an appealing and auspicious photocatalyst for the conversion and elimination of pollutants and production of energy. Anisotropic electrical conductivity is an additional feature of layered ZnIn₂S₄.

Fig. 3 (A) Density of phases of ZnIn₂S₄. Reproduced from ref. 86 with permission from The Royal Society of Chemistry, Copyright 2024. (B) Framework of the electronic energy bands of ZnIn₂S₄. Reproduced from ref. 87 with permission from Elsevier, Copyright 2024. (C) Band structure of ZnIn₂S₄.⁸⁸ (D) Er-doped ZnIn₂S₄/DOS. Reproduced from ref. 88 with permission from Elsevier, Copyright 2024.

Additionally, the band separation of RCu-ZIS reveals two mid-level energy states. The main state is close to the CBM and changes as the In and S orbitals combine at the RCu-ZIS level, which is the accepting region level of the S gaps.⁸⁹ The amalgamation of In, Cu, and S spheres generated a supplementary mid-gap state, which significantly expanded through the CBM and correlated with the donor metrics of clumps of In/Cu/S vacancies,^90,91 resulting in the formation and separation of photogenerated electrons, respectively.^92,93 Employing DFT calculations, the electrical configuration and charge transfer mechanisms of CS/ZIS were subsequently elucidated. The band gap of ZnIn₂S₄ was determined to be 1.692 eV, as shown by the energy level diagram in Fig. 3(B). To evaluate the effect of Er doping, researchers adopted the DFT+U approach to calculated the band gap of the model. Although the use of U-value correction is unlikely to completely prevent the band gap from being undervalued, implementing the same methodology for evaluating band gap variations for different doping mechanisms continues to be a reliable procedure. To clarify the different electronic-level modifications induced by Er doping, the band composition of the Er-doped system was computed in conjunction with the untreated ZnIn₂S₄, as shown in Fig. 3(C and D). The bandgap in pristine ZnIn₂S₄ is 0.57 eV, indicating that it is semiconducting. Its Fermi level is located at 2.89 eV, its valence band maximum (VBM) is 2.62 eV, and its conduction band minimum (CBM) is 3.19 eV. Owing to the strong impact of the 4f electrons in Er, the material transforms considerably upon the incorporation of Er, which can be detected. These electrons change the structure of the electrical setting by adding more energy levels and interacting with existing electronic arrangements in the valence and conduction bands. This appropriately diminished band gap in the doped system boosts the proficiency of photocatalytic responses.

The optically distinctive features of semiconductors, particularly those with a size on the nanometer scale, are predominantly determined by their fundamental electronic properties, which are precisely linked to their chemical and ionic composition, atomic arrangement, and physical dimensions.⁵⁶ Temperature, the form and dimensions of tiny particles, and the arrangement of ions in their surroundings all have an impact on the electrical configuration of semiconductors, greatly influencing the formation of their optical band gap.^56,94,95 For example, Chen et al.⁵⁶ performed a comparative study of the photocatalytic properties of both the cubic and hexagonal phases of ZnIn₂S₄ prepared hydrothermally. Their findings indicated that the absorption edges of the cubic and hexagonal ZnIn₂S₄ were 538 nm and 490 nm, while their band gaps were 2.5 eV and 2.3 eV, respectively. As the degree of crystal formation of zinc indium sulfide shifted from hexagonal shape to cubic, the absorption band was red-shifted, confirming the influence of crystallinity and indicating that cubic ZnIn₂S₄ has greater capacity to absorb light.

Additionally, the band gap transformation happened because of the quantum confinement phenomenon, which caused big changes in the emissions during photo-excitation. Therefore, materials exhibiting distinctive crystallite sizes exhibit different optical characteristics. Shen et al.⁹⁶ investigated the impact of crystallite size on the optical characteristics of ZnIn₂S₄ through the synthetic process of the material in various media. The results indicated that the crystallite dimensions of ZnIn₂S₄ synthesized in ethylene glycol (EG), methanol (MeOH), and water were 4.1, 5.4 and 6.6 nm, respectively. Furthermore, a noticeable variation in the ability to soak up water was observed in response to solvent variations. The absorption peaks observed at 500 nm, 525 nm, and 470 nm corresponded to ZnIn₂S₄–H₂O, ZnIn₂S₄–EG, and ZnIn₂S₄–MeOH, which possessed band gaps of 2.48 eV, 2.35 eV, and 2.64 eV, respectively. Regarding this investigation, it was possible to conclude that the reduction in crystalline size resulted in the creation of a narrower band gap and a blue-shift in the absorption edge. Furthermore, the crystallographic dimensions of ZnIn₂S₄ were modified by Peng et al. from 2.1 nm to 9.2 nm, resulting in band gaps that fluctuated between 2.35 eV and 3.28 eV.⁹⁷ Particularly noteworthy was the substantial blue-shift in the absorption edge when the size of the crystalline material was below 6 nm. Although the precise excited-state Bohr radius of ZnIn₂S₄ is not yet recognized, the phenomenon of quantum confinement has been detected in ZnIn₂S₄ nanocrystals as small as 6 nm.

3. Synthetic process of ZnIn₂S₄ and ZnIn₂S₄-based hybrid materials

The synthetic process of ZnIn₂S₄via the high-pressure Stock Barger technique was initiated in early 1969.⁹⁸ Subsequently, ZnIn₂S₄ was conventionally manufactured through chemical vapor transport, a process requiring high vacuum, extremely high temperatures, and a transportation agent.⁹⁹ For this purpose, to synthesize ZnIn₂S₄ micro/nanostructures displaying diverse morphological properties under moderate conditions, an abundance of synthetic approaches have been designed including solvothermal,^22,96,100 microwave-assisted,²¹ thermal sulfidation,⁵¹ ultrasonic,³⁴ spray pyrolysis,⁸⁵ spin-coating,⁸⁴ chemical vapor breakdown,⁵⁹ hot injection,^19,101 thermal vapor decomposition procedure,⁵⁹ and synthesis of colloidal substances.¹⁰² Regarding the analysis of ZnIn₂S₄ nanostructures, the hydrothermal method is the most prominent among these techniques. A brief overview of the synthetic methodologies that have been widely utilized and extensively investigated is provided in this section. ZnIn₂S₄ has been synthesized with an extensive variety of micro- and nanostructures through the adjustment of reaction parameters, such as surfactants, synthetic methods, precursor solution, pH level, reaction duration, and temperature. Additional information with regard to synthetic methodologies is elaborated on in the implementing section.

3.1. Hydrothermal procedure

In comparison to the traditional solid-state reactions, the hydrothermal procedure to produce micro- and nanostructured ZnIn₂S₄ is more convenient and beneficial. ZnIn₂S₄ has been successfully hydrothermally synthesized into many different types of morphologies, including hollow spheres,¹⁰³ nanoparticles,¹⁰² flower-shaped microspheres,¹⁰² and nanosheet coatings.^51,104 Lei et al.²³ first implemented the hydrothermal procedure for manufacturing ZnIn₂S₄ nanoparticles in 2003. The effective fabrication of cubic ZnIn₂S₄ nanoparticles measuring 20 nm in length was observed. Researchers frequently manufacture ZnIn₂S₄ using the hydrothermal synthesis technique, manipulating the necessary variables to create various morphological forms.^23,103,105 CTAB was used in another study to construct a ZIS with a floral pattern structure.⁶⁵ The researchers synthesized ZIS microflowers using a hydrothermal process, which allowed the modification of its lattice S-content. A mixture of 0.5 mmol of Zn(CH₃COO)₂·2H₂O and 1 mmol of In(NO₃)₃·xH₂O was added to water, together with different amounts of L-cysteine, which acted as the sulfur source. The mixture was stirrer for 30 min, and then poured into an autoclave and heated.¹⁰⁶ To obtain the final product, the resulting yellow solid was washed and dehydrated in an oven. It was found that alterations in appearance happened during the implementation of the hydrothermal treatment process.

Ln_xO_y and ZnIn₂S₄ photocatalysts were synthesized using the standard hydrothermal technique. After sonication for 15 min, a certain quantity of Ln_xO_y was distributed in ionized water. The aforementioned suspension was added dropwise to 0.5 mmol of Zn(CH₃COO)₂·2H₂O, 1 mmol of In(NO₃)₃·3H₂O, and 4 mmol of thioacetamide (TAA) diluted in ionized water. Following that, the mixture was magnetically stirred for one hour in total. Subsequently, the resulting suspension was heated at 160 °C for 24 h in a Teflon-lined stainless-steel autoclave. Afterwards, the byproducts were centrifuged multiple times using purified water and 100% ethanol, and then dried overnight and the Ln_xO_y/ZnIn₂S₄ (Ln_xO_y/ZIS) composite samples collected. As previously mentioned, purified ZnIn₂S₄ (ZIS) was produced; however, Ln_xO_y was not present. The procedure for the synthesis of the sample is shown in Fig. 4(A).

Fig. 4 (A) Diagram showing the steps involved in the synthesis of Ln_xO_y/ZIS photocatalysts. Reproduced from ref. 107 with permission from Elsevier, Copyright 2024. (B) Fiercely aggressive response mechanism that occurs during the hydrothermal analysis of ZnIn₂S₄ MFs is illustrated and shown schematically. Reproduced from ref. 106 with permission from Elsevier, Copyright 2021. (C) Diagrammatic representation of the steps involved in manufacturing the RP/ZIS composite. Reproduced from ref. 108 with permission from Elsevier, Copyright 2023. (D) Procedure suggested to explain the formation of the morphology of ZnIn₂S₄ microspheres. Reproduced from ref. 109 with permission from Elsevier, Copyright 2021.

S-containing organic compound precursors are beneficial for fabricating the lattice components of ZnIn₂S₄ micro/nanostructures given that they react with the water-based solution and generate H₂S during the hydrothermal procedure via the wet-chemical synthesis of metal sulfides. However, because of this process, theoretically, the stoichiometric addition of S and metallic precursors to the hydrothermal treatment solution cannot guarantee the synthesis of defect-free ZnIn₂S₄ micro/nanocrystals. Furthermore, from a thermodynamic perspective, the lattice S-vacancy defects in ZnIn₂S₄ micro/nanocrystals are more straightforward to develop in comparison to metal-vacancy defects. As illustrated in Fig. 4(B), the formation energies of S-vacancy defects in the exterior and interior of ZnIn₂S₄ are significantly less than that of metal Zn- or In-vacancy defects. A one-step hydrothermal procedure was implemented to produce an array of RP/ZIS composites, as shown in Fig. 4(C). The aforementioned purified RP was dissolved in 60 mL of deionized water. Subsequently, it was completely dispersed in a mixture comprised of TAA (4.0 mmol), InCl₃·4H₂O (2.0 mmol), and ZnCl₂ (1.0 mmol). The resulting solution was transferred to a Teflon-lined high-pressure autoclave at 180 °C and heated for 24 h after violent agitation for 30 min. After centrifuging to collect the yellow precipitate samples, they were washed separately with ethanol and deionized water three times. Finally, the RP/ZIS composites with various RP weight ratios (0.5 wt%, 1 wt%, 3 wt%, and 5 wt%) were vacuum-dried overnight at 60 °C. Comparatively, a similar experimental protocol was used to prepare purified ZnIn₂S₄ in the absence of RP.

Numerous research groups have investigated the development of ZnIn₂S₄ microspheres under a variety of synthetic parameters. For example, porous ZnIn₂S₄ microspheres were hydrothermally synthesized by Chen et al.¹¹⁰ without the involvement of surfactants. The process for the production of hexagonal porous ZnIn₂S₄ microspheres involved the combination of thioacetamide (TAA), ZnCl₂, and InCl₃·4H₂O. The above-mentioned microspheres possessed spherical superstructures resembling marigolds. Additionally, the microspheres possessed a mean diameter in the range of 3 to 7 μm and were comprised of a substantial quantity of nanosheets. The development mechanism of ZnIn₂S₄ microspheres was explained, as exemplified in Fig. 4(D).

The Oswald maturation process was responsible for the expansion of the microspheres. Additionally, the impact of the pH level of the solution containing the precursor on the development of microspheres was more comprehensively investigated. ZnIn₂S₄ demonstrated no evidence of aggregation into rule less dollops rather than microspheres when the pH was approximately 4. At pH 1, nanosheets accumulated, whereas microspheres developed at pH 2.5. Furthermore, the precipitation disappeared when the pH dropped below 0.5. The absence of precipitation can be attributed to the resolution potential of the TAA aqueous solution, which decomposes at a particular temperature in an acidic environment to produce H₂S.

On the contrary, organic surfactants have the capacity to function as templates or framework-directing agents throughout the process of structural formation. Regarding the formation of microspheres, Gou et al.²² synthesized ZnIn₂S₄ microspheres via the hydrothermal technique in the presence of polyethylene glycol (PEG) or cetyltrimethylammonium chloride (CTAB). The precursors were ZnSO₄·7H₂O, InCl₃·4H₂O, and thioacetamide (TAA). The CTAB-assisted miniature particles displayed extraordinary long-term stability, measuring an average size of 2 μm in diameter. Compared to this, the PEG-assisted ZnIn₂S₄ resembled a hollow sphere with a size in the range of 500 nm to 10 μm and comprised nanowires. Consequently, the mechanism for its formation was investigated. CTAB assembled into spherical micelles, directing the formation of ZnIn₂S₄ microspheres. This happened due to the polar hydrophilic centers and hydrophobic alkyl chains in CTAB. Subsequently, Zn²⁺, In³⁺, and TAA were incorporated in the micelles. Ultimately, the removal of CTAB induced the formation of ZnIn₂S₄ nuclei and crystal progression inside the micelles, culminating in the development of homogeneous and durable ZnIn₂S₄ microspheres. In contrast to the development of CTAB-assisted ZnIn₂S₄ microspheres, PEG has been used to generate hollow, sphere-like ZnIn₂S₄ nanowires. Despite the fact that PEG is capable of creating large chains in water-based solutions, the anisotropy and nucleation crystal development of ZnIn₂S₄ remained limited to long chains. Consequently, ZnIn₂S₄ nanowires were generated due to the anisotropic crystal growth. Consequently, the ZnIn₂S₄ tiny wires formed a cavity-network spherical framework by virtue of the hydrogen-bonding effect and the adaptability of the PEG chains. The hollow spheres of the nanowire were obtained following the decomposition of PEG. The surfactants frequently used in the synthesis of ZnIn₂S₄ include polyvinyl pyrrolidone (PVP), cetylpyridinium bromide (CPBr), triethylamine (TEA), polyethylene glycol (PEG) and cetyltrimethylammonium chloride (CTAB).

Furthermore, using a TEA-assisted hydrothermal process,¹⁰⁴ Chaudhari et al. manufactured ZnIn₂S₄ nanoplates and nanostrips that resembled rose and hollow marigold flowers, respectively. They observed that TEA exerted considerable effect on the shape and form of the final products. The development of marigold flower-shaped ZnIn₂S₄ occurred in the absence of surfactant. The petal thickness was determined to be 3–5 nm, and the dimensions of the flower varied in the range of 3–5 mm. The introduction of 0.005 mol TEA resulted in the formation of distorted flowers growing on nanoplates. The deformed flowers varied in size from 0.5 to 1 μm, whereas the size of the nanoplates varied in the range of 200 to 300 nm. Upon the addition of 0.01 mol TEA to the resulting solution, nanoplates and nanostrips that had been layered into bundles were observed. Furthermore, randomly grown, highly deformed flower-like structures resembling nanostrips were observed when the proportion of TEA was elevated to 0.015 mol. The differences in particle size and shape were hypothesized to originate from the combined influence of the van der Waals forces of attraction and the strength of the bonding coordination between TEA and the outermost atom layer of the ZnIn₂S₄ nanoparticles.

3.2. Solvothermal technique

The approach known as the solvothermal technique is currently gaining significant popularity in the design, synthesis, and advancement of ZnIn₂S₄ micro/nanostructures. The solvent properties such as pH level, coordination capacity, viscosity, surface tension, density, and dipole moment significantly impact the morphology of the generated products. Furthermore, the characteristics of the solution, such as pH, surface tension, viscosity, dipole moment, and hydrophobic properties, modulate the shape and structure of ZnIn₂S₄.^64,79,111 Solvothermal synthesis was used to produce ZnIn₂S₄ in a variety of morphologies, including nanotubes, nanoribbons,²² microspheres,¹¹¹ 3D-hierarchical persimmon-like structures,⁷⁹ micro/nanopeony,⁶⁴ and micro-clusters.⁹⁶ In this study, we demonstrate the facile photodeposition technique employed to synthesize ZIS-blended materials treated with nickel (Ni:ZIS). The prepared composites exhibited excellent activity in a photoredox dual reaction, specifically the transformation of benzyl alcohol (BA) to benzaldehyde (BAD) under ultraviolet (UV) radiation to generate H₂. Concurrently employing electrons and holes generated by sunlight can accomplish this.¹¹² In conjunction with expediting the dissociation of photoexcited electron–hole pairs, Ni may additionally serve as functional sites for the abstraction of H, which facilitates selectivity optimization.

Gou et al.²² demonstrated the utilization of the solvothermal technique to generate ZnIn₂S₄ nanoribbons and nanotubes. The solvent implemented was pyridine (Py), whereas the precursors utilized were ZnSO₄, InCl₃, and thioacetamide (TAA). The findings displayed that either the temperature or time of the reaction substantially triggered the morphology of the resulting products. Fig. 5(A)(a) demonstrates that ZnIn₂S₄ nanotubes were eventually generated by increasing the temperature to 180 °C and extending the duration of the reaction. As shown, ZnIn₂S₄ nanoribbons were produced within the temperature range of 120–160 °C. Consequently, extended reaction times and elevated temperatures may be advantageous for the establishment of tubular structures. The process of creating ZnIn₂S₄ nanoribbons and nanotubes additionally included nucleation and self-growth. Autogenously, ZnIn₂S₄ nuclei were generated during the chemical interaction of InCl₃, ZnSO₄, and thioacetamide (TAA). Subsequently, the atomic building block grew antagonistically and self-assembled into ZnIn₂S₄ lamellar membrane mesostructures. Under suitable conditions, the ZnIn₂S₄ multilayered composites ultimately developed into different shapes. Additionally, the formation of nanoribbons and nanotubes under similar synthetic conditions vanished when pyridine was substituted with alcohol, water, tetrahydrofuran, etc. Therefore, it can be observed that the solvent characteristics such as viscosity, pH, bonding interaction, surface tension, and dipole moment significantly influenced the process for the synthesis of ZnIn₂S₄.

Fig. 5 (A) (a) Illustration of structure-forming procedure of ZnIn₂S₄ nanoribbons and nanotubes. (b) Block diagram demonstrating the process by which ZnIn₂S₄ nanowires are formed utilizing PEG-6000 surfactant. Reproduced from ref. 113 with permission from Elsevier, Copyright 2022. (B) Schematic illustrating the route for the synthesis of PI/ZnIn₂S₄ heterojunction photocatalysts. Reproduced from ref. 114 with permission from Elsevier, Copyright 2024.

In addition, the combination of H₂O and PEG-6000 as a solvent and surface-active agent material permits the formation of prolonged chains of the nonionic surfactant PEG-6000 in water, respectively. These chain structures are comprised of a variety of active oxygen atoms, which demonstrate significant interactions with ionized metals. Fig. 5(A)(b) illustrates how restricted nucleation and crystal growth constrain the development of nanowires in ZnIn₂S₄ to the chains. This is primarily due to their asymmetrical crystal growth. Afterwards, the elimination of the PEG-6000 surfactant transformed the nanowires into cylindrical, hollow objects. After an extended period of ultrasonic dispersion, the ZnIn₂S₄ nanowires fully detached from the fibrous microspheres. The solvothermal approach was employed for manufacturing PI/ZnIn₂S₄ heterojunction photocatalysts, as demonstrated in Fig. 5(B). For comparison, a precise, identical procedure was implemented for generating ZnIn₂S₄ powder, eliminating the requirement for PI powdered form.

3.3. Microwave-assisted technique

The microwave-assisted technique is more effective than traditional heating, and microwave irradiation may substantially shorten the reaction time.¹¹⁵ Using microwaves to assist hydrothermal production,¹¹⁶ Chen et al.¹¹⁶ synthesized ZnIn₂S₄ microspheres in the shape of marigolds as an example. Each stage of the synthesis technique could be completed within minutes, required no templates, and was simple to perform. The obtained ZnIn₂S₄ demonstrated a spherical, marigold-like morphology, displaying an ordinary diameter of 2.0 μm. It was assembled from a substantial arrangement of nanosheets. Furthermore, an appearance of floccules between nanosheets was observed, which may possibly have been caused by the shortened reaction time or the flawed crystal.

The SEM and TEM techniques were employed for assessing the shape of the as-fabricated RGO/ZnIn₂S₄ nanocomposites. The SEM image of the representative 1.0 wt%-RGO/ZnIn₂S₄ nanocomposite, as shown in Fig. 6(A), indicated flower-like microspheres with diameters in the range of 300–350 nm. The researchers originally reported the formation of two-dimensional, sheet-on-sheet flake-like RGO/ZnIn₂S₄ nanocomposites via the standard solvothermal approach.¹¹⁹ In contrast, the microsphere morphology obtained from the microwave-assisted synthesized sample was substantially distinct. This suggests that the development of the flower-like microspheres is significantly influenced by the microwave heating procedure. The TEM image of the 1.0 wt%-RGO/ZnIn₂S₄ nanocomposite evident showed that the microspheres resembling flowers are made up of several interconnected ultrathin tiny sheets, as shown in Fig. 6(B). The hexagonal phase of ZnIn₂S₄ has a clear lattice-fringe spacing of 0.32 nm in the (1, 2) crystal plane, demonstrating the emergence of a well-defined hexagonal ZnIn₂S₄ crystal structure contained within microspheres. Additionally, the hexagonal ZnIn₂S₄ lattice-fringe integrates with the amorphous RGO area, suggesting that the hexagonal ZnIn₂S₄ and RGO nanosheets are in close proximity, as shown in Fig. 6(C). The atomic arrangements of the half-unit-cell ZnIn₂S₄ layer are illustrated by the HAADF-STEM result in Fig. 6(D and E). It is generally accepted that the dimensions and luminance of the In atom facilitate easy differentiation from Zn.^120–122 The enlarged picture in Fig. 6(F) makes it apparent that the In atoms partially replaced Zn positions. Typically, the six In atoms (bright spots) that make up a regular hexagon have six Zn atoms (dark spots) in the middle of the hexagon. Following In doping, the strain and In atoms (bright spots) displace the partly centered Zn atoms (black spots). Alternatively, because Zn and Ni have comparable atomic numbers, the Ni atoms and Zn atoms are identical. The Ni atom has the potential to substitute Zn atoms in NiIn–ZIS, owing to the identical electronic arrangement and atomic radius shared by Ni and Zn atoms. Moreover, the presence of crystal defects was demonstrated by the disorganized atomic structures observed in particular regions. Concisely, a compound called Ni, In, co-doped ZnIn₂S₄ was synthesized using the microwave-assisted solvothermal technique. In this compound, some of the Zn atoms were substituted with In and Ni atoms in the [ZnS]₄ layer. The electrical composition of NiIn–ZIS was modulated by the incorporation of In and Ni, which led to an enhancement in the photocatalytic activity of the catalyst.

Fig. 6 Images illustrating the morphology of 1.0 wt%-RGO/ZnIn₂S₄ nanocomposite: (A) SEM picture. (B) Slightly lower resolution TEM picture. (C) HRTEM picture. Reproduced from ref. 117 with permission from Elsevier, Copyright 2014. Microwave-assisted solvothermal method assessment of NiIn–ZnIn₂S₄. (D) HAADF-STEM pictures. (E and F) Magnified picture. Reproduced from ref. 118 with permission from Elsevier, Copyright 2022.

Furthermore, ZnIn₂S₄ submicrospheres with hierarchical porosity were generated by Hu et al.²¹ by employing a microwave-assisted solvothermal methodology utilizing (EG) as the solvent solution in the absence of templates, surfactants, or catalysts. It is widely recognized that the compound ethylene glycol is an outstanding microwaveable absorbent; subsequently, this characteristic may trigger reaction systems to heat up rapidly, thereby undoubtedly reducing the reaction time. The hierarchically porous ZnIn₂S₄ submicrospheres developed through a process involving both nucleation and self-assembly. Following the arrangement of ZnIn₂S₄ nuclei into mesostructured fibers due to the fundamental asymmetry of the hexagonal crystal structure, intermediary substances resembling fibers self-assembled into superstructures with a hierarchical porous architecture. Conversely, the quick development of porous ZnIn₂S₄ spheres may be facilitated by the microwave irradiation-induced local effects, which simultaneously stimulate the crystallization and self-assembling of processes occurring within a sealed solvothermal system.

3.4. Other synthetic procedures

Numerous other synthetic procedures, including thermal sulfidation,⁵¹ spray-pyrolysis technique,^85,123–125 ultrasonic technique,³⁴ chemical vapor deposition,⁵⁹ colloidal particle synthesis,¹⁰² and hot injection,^97,101 have been applied for the formation of ZnIn₂S₄ micro- and nanostructures. Yang et al.⁵⁹ found that by employing chemical vapor deposition, ZnIn₂S₄ nanoplates could be synthesized. The ultrasonic spray pyrolysis technique is an economically viable and adaptable technique for manufacturing delicate films and tiny particles. This technique is based on an aerosol procedure, as depicted in Fig. 7(A). Metallic salts dispersed in water-based solvents are utilized to generate metal, oxide, and composite nanostructures with precisely regulated shapes and chemical properties.¹²⁷ This procedure necessitates the application of an initiating solution.¹²⁸ This technique generates delicate films by atomizing the solution into minute particles, which are then transferred to a preheated substrate.¹²⁹ CdIn₂S₄ thin films were fabricated using the spray pyrolysis technique at a temperature of 350 °C on a glass substrate. The resulting composite exhibited a crystal structure with a size of 59 nm. These Cd-doped thin films are well suited for photodetection applications.

Fig. 7 (A) Schematic of the spray-pyrolysis method. Reproduced from ref. 126 with permission from Elsevier, Copyright 2023. (B) SEM pictures of deposited ZnIn₂S₄ films with varying concentrations of precursor thiourea. The thiourea is 10% excess of the stoichiometric ratio A. Reproduced from ref. 85 with permission from Elsevier, Copyright 2008.

The thin films of ZnIn₂S₄, as illustrated in Fig. 7(B), were synthesized using the spray pyrolysis technique⁸⁵ with the addition of ZnCl₂, InCl₃, and (NH₂)₂CS, based on the experiments performed by Li et al.⁸⁵ The application of the spray pyrolysis method is a straightforward, cost-effective, and secure solution for the formation of thin films. As the heated substrate, glass covered with indium-doped tin oxide (ITO) was utilized. All the coatings were applied at an initial substrate temperature of 400 °C. The obtain ZnIn₂S₄ films were uniform with a compact size, devoid of any discernible fractures or tiny holes. The approximate thickness of the coatings produced was 500 nm. Conversely, thin coatings of ZnIn₂S₄ in the hexagonal phase were produced via spin-coating by Xie et al.⁸⁴ The obtained ZnIn₂S₄ thin films were comprised of rod-shaped nanocrystals with a length and width of 26.5 nm and 13.3 nm, respective. Although solvothermal synthesis has been utilized to successfully generate ZnIn₂S₄ nanotubes and nanowires, Shi et al.²⁹ implemented the straightforward template-assisted technique for manufacturing the same materials. A permeable polycarbonate membrane with a diameter of 200 nm was implemented as the rigid template. In addition, the porous polycarbonate membrane exhibited the capability to simulate diversified ZnIn₂S₄ structures by accelerating the reaction duration and temperature. The circumference of the ZnIn₂S₄ nanotubes and ZnIn₂S₄ nanowires in their natural state was approximately 200 nm. In the template-assisted method, crystal development and nucleation were restricted to the apertures of the template. Heterogeneous nucleation occurred during the initial phases of the crystal growth owing to the decreased interfacial energy among the crystal nuclei and the pore walls compared to that between the crystal nuclei and solution. Subsequently, in situ crystal growth proceeded, prompting the establishment of ZnIn₂S₄ nanotubes. Increasing the duration of the process by increasing the temperature and time may stimulate the development of crystals in the interior surface of the pore boundaries that extend toward the center. Consequently, the gradual thickening of the tube wall resulted in the development of ZnIn₂S₄ nanowires. Furthermore, Chen et al. manufactured ZnIn₂S₄ nanosheets through ultrasonic liquid exfoliation, which was thought to be the only viable method for producing single-layered compounds.³⁴ Following its initial manufacturing by conventional hydrothermal means, bulk ZnIn₂S₄ was sonicated for five hours in an isopropanol solution, producing ZnIn₂S₄ nanosheets.

In the previous section, the prevalent recent synthetic techniques to generate ZnIn₂S₄ micro- and nanostructures were briefly discussed. The techniques for the synthesis of ZnIn₂S₄ micro/nanostructures can be classified into four groups depending on the solvents utilized, reaction parameters, and energy source. These four categories include the hydrothermal technique, microwave-assisted methodology, solvothermal method, and additional synthetic procedures. The hydrothermal method of manufacturing, which is the prevailing technique employed in the manufacturing of ZnIn₂S₄, was initially developed. This method is distinguished by its relative convenience, cost-effectiveness, and straightforward use. Micro/nanostructured ZnIn₂S₄ with numerous morphological characteristics has been manufactured efficiently by adjusting the reaction parameters, including reagents, temperature, pH, and surfactants, which is the second category. The solvothermal method was demonstrated, which is another widely employed strategy for the synthesis of ZnIn₂S₄. The properties of the organic solvent incorporated significantly influence the shape, size, and framework of the final ZnIn₂S₄. As an expansion of the hydrothermal and solvothermal techniques, the microwave-assisted methodology was demonstrated in third group. This synthetic method demonstrates a substantially accelerated reaction rate when microwave energy is utilized as an aid compared to the conventional thermal process.

The last section covered other important techniques used to prepare ZnIn₂S₄, in addition to the hydrothermal/solvothermal technique and microwave-assisted method. Upon evaluation of their attributes (energy source, reagent states, etc.), it was challenging to categorize each of these methods into the initial three separate categories. The effective creation of ZnIn₂S₄ micro/nanostructures with unique geometries, such as thin coatings and nanotubes, using these synthetic methods was a complementary approach. The reasons for the selection of a synthetic method are determined by factors such as energy consumption, cost, and accessibility. Additionally, the artificially generated environments and reaction constraints have a substantial influence on the shape and structure of ZnIn₂S₄. Therefore, selecting the appropriate synthetic methodology to produce ZnIn₂S₄ with the desired structure and morphology for various applications is significant.

4. Performance improvement strategies

ZnIn₂S₄ is a highly intriguing substance that can potentially be implemented in the fields of photocatalysis, optoelectronics, and energy preservation. It is believed that the structure and appearance of semiconductors may significantly influence their spectral characteristics and photocatalytic efficiency. Thus, determining the morphological and structural characteristics of ZnIn₂S₄ to enhance its photocatalytic efficiency for the evolution of hydrogen is an achievable strategy. Examples of micro- and nanostructures of ZnIn₂S₄ and their photocatalytic productivity in the manufacturing of hydrogen are presented in Table 1.

Table 1 Hydrogen generation of ZnIn₂S₄ and their productivity

Semiconductor	Synthesis method	Morphological characteristics	Co-catalyst modification	Sacrificial reagent	Illumination source (nm)	Bandgap (eV)	Optimum H2 generation (μmol g^−1 h^−1)	Superficial quantum yield (%)	Ref.
ZnIn2S4	Hydrothermal	Particles that resemble spheres	Pt	Na2S/Na2SO3	300 W xenon lamp λ > 420 nm	2.3	257	N/A	23
Mo-ZIS/Ni–Ni HCP (NMZ)	In situ assembly of ZnIn2S4 nanosheets solvothermal	Nanoflower-like structure	Pt	Triethanolamine	300 W Xe lamp λ ≥ 400 nm	2.63, 2.57, and 3.15 eV	26.7 mmol g^−1 h^−1	20%	130
ZIS/TiO2	Hydrothermal	Core–shell-like structure with nanoflower-like ZIS	N/A	Triethanolamine	UV light (λ > 400 nm) 300 W Xe lamp	3.06 eV	2.1515 mmol g^−1 h^−1	22.34%	131
						2.59 eV	65.40 m^2 g^−1
1D CdS/2D ZnIn2S4	Two-step solvothermal	Nanorod-like morphology	N/A	ZnCl2, InCl3 thioacetamide	Monochromatic light	2.35, 2.6 eV	26.4, 67.8, 54.6, and 45.2 m^2 g^−1	N/A	132
	In situ growth
Ti3C2 MXene@TiO2/ZnIn2S4	Two-step hydrothermal	Accordion-like layered structure	Titanium	N/A	300 W xenon (Xe) lamp	2.45 eV	1185.8 μmol g^−1 h^−1	N/A	133
						1.55 eV
HEMP/ZnIn2S4	Eutectic solvent	Petal-like morphology	Pt	Na2SO	300 W xenon (Xe) lamp	128.6 eV	4630.21 μmol h^−1 g^−1	10%	134
				Na2SO3		129.5 eV
ZnIn2S4	Hydrothermal	Microsphere	Pt	Na2S/Na2SO3	300 W xenon lamp λ > 430 nm	2.43	562.25	18.4%@420 nm	65
CoMoS4/ZnIn2S4 (CMS-ZIS)	In situ deposition	Flower-like microspheres	N/A	Na2SO4	UV light λ ≥ 420 nm	2.37 eV	3942.1 μmol g^−1 h^−1	8.23%	135
	Hydrothermal					1.59 eV
3-CPDs/ZIS	Hydrothermal	Micro spherical morphology	N/A	Na2S/Na2SO3	300 W Xe arc lamp (λ > 420 nm)	1.57 1.41 eV	133 μmol g^−1 h^−1	N/A	136
Cu–ZnIn2S4/WO3/WS2	Two-step hydrothermal	Flower-like microspheres	N/A	N/A	300 W Xe arc lamp (λ > 420 nm)	37 eV, 5.17 eV, 5.55 eV	93 032.29 μmol g^−1 h^−1	37.04%	137
ZnIn2S4/B–C3N4	Electrostatic self-assembly	A two-dimensional stacking structure made up of numerous thick, massive nanosheets	Melamine	Triethanolamine (TEOA)	Visible light	2.47 eV, 2.68 eV, 2.54 eV, 2.08 eV, 1.69 eV	0.876 mmol g^−1 h^−1	4.25%	138
BiOCl@ZIS	Oil-bath	Microplates	Pt	N/A	UV-light irradiation (λ > 420 nm)	2.20 eV	674 μmol g^−1 h^−1	N/A	139
						3.35 eV
ZnIn2S4	Solvothermal/hydrothermal	Flowering cherry-like microsphere	Pt	Na2S/Na2SO3	300 W, Xe lamp λ > 430 nm	2.36	136.5, aqueous	4.2%@420 nm	96
MoO3@Mo-ZIS	Solvothermal (EG, 120 °C, 6 hours)	Disordered behavior sheet-like structure assembles micro-flower structure	Pt	EG	300 W Xe lamp (λ > 420)	5500	19.2	N/A	140
ZnIn2S4	Hydrothermal	Rose-like micro clusters	Pt	Na2S/Na2SO3	300 W Xe lamp λ > 430 nm	2.51	611	11.9%@420 nm	100
Cu2−xS@ZnIn2S4	Hydrothermal (ethanol, 120 °C, 2 h)	N/A	Pt	N/A	300 W xenon (Xe) lamp	N/A	4653.4	16.9%	141
ZnIn2S4	Thermal sulfidation	Particles	Pt	Na2S/Na2SO3	300 W Xe lamp λ > 430 nm	2.5	N/A	N/A	51
TiO2/ZnIn2S4	Hydrothermal (water/ethanol, 160 °C, 12 h)	Petal-like ZnIn2S4 nanosheets	Pt (1%)	(TEOA), lactic acid, methanol, and Na2S/Na2SO3	300 W xenon (Xe) lamp	N/A	6030	10.5%	141
MoS2/ZnIn2S4	Solvothermal	Nanosheets	MoS2	Lactic acid	300 W xenon arc lamp λ > 420 nm	N/A	6884@420 nm	63.9%	142
ZnIn2S4/BPQDs	Electron acceptors	Nanoflower-shaped morphology	N/A	Na2SO4	Xenon lamp of 300 W	2.18, 2.01, eV	1207 μmol g^−1 h^−1	N/A	131
Ni(OH)2/NiIn2S4/ZnIn2S4 (NOH/NIS/ZIS)	In situ photochemical transition	N/A	N/A	TEOA	UV light 300 W Xe lamp	2.44, 1.52, 4.24 eV	5448.3 μmol h^−1 g^−1	10.6%	131
ZnIn2S4/BiFeO3	Ultrasonic-assisted calcination method	Hierarchically organized flower-like microspheres	N/A	Na2S/Na2SO3	300 W xenon lamp	2.59 1.86 eV	1568.4 μmol g^−1 h^−1	4.1% at 420 nm	143
I–ZnIn2S4	One-spot solvothermal	Flower-like microspheres	Pt	TEOA	300 W Xe lamp	2.24 eV, 2.17 eV	4.73 mmol g^−1 h^−1	29.56%	144
2D/2D ZnIn2S4/WO3	In situ growth	2D rectangular nanosheet structure	Au NPs	Na2S/Na2SO3	Visible light 300 W xenon lamp	4.58, 5.59 eV	2610.6 or 3566.3 μmol g^−1 h^−1	7.3- and 6.6-fold	145
	Solvothermal synthesis
CoMn2O4–ZnIn2S4	Solvothermal/in situ	CMO hollow microcubes	Ag/AgCl	TEOA	300 W xenon lamp	1.59, 2.68 eV	11.04 mmol h^−1 g^−1	N/A	146
ZnIn2S4−x–WO3−x	In situ hydrothermal	Hierarchical flower-like structure	N/A	N/A	Visible-light irradiation 300 W xenon lamp (λ > 420 nm)	2.55 2.26 eV	737.75 μmol g^−1 h^−1	N/A	145
CDs/Vs-ZIS	One-step hydrothermal	2D ultrathin nanosheet	N/A	TEOA	UV-light irradiation (λ ≥ 420 nm)	2.57 2.53 eV	5.93 mmol g^−1 h^−1	4.61%	146
		Morphology
ZnIn2S4	Hydrothermal	Porous microspheres	N/A	Na2S/Na2SO3	250 W Hg lamp	2.43	1544.8	N/A	25
Au–ZnIn2S4/NaTaO3	2-Step hydrothermal (DMF/glycerol, 180 °C, 10 h)	Floriform ZnIn2S4 microsphere	N/A	N/A	300 W xenon lamp	N/A	11 404	10.1%	147
ZnIn2S4	Hydrothermal	Marigold-like microspheres	Pt	N/A	400 W Hg lamp λ ≥ 420 nm	2.32	N/A	6.47% visible light	148
ZnIn2S4/TiO2	Microwave-assisted hydrothermal (water, 160 °C, 0.5 h)	N/A	Pt (1%)	N/A	300 W xenon lamp	N/A	8774	39%	149
In2S3–ZnIn2S4/Au	In2S3 nanorods, ion exchange reaction and photoreduction	Rod-like structure	Au	N/A	Visible-light irradiation	2.47 2.58 eV	12 369 μmol h^−1 g^−1	4.31% at 520 nm	150
CF@ZIS	One-pot hydrothermal	Microsphere morphology	N/A	N/A	Visible light	2.29 2.26 eV	278.9 μmol h^−1 g^−1	N/A	151
ZnIn2S4	Hydrothermal	Microsphere	Pt	Na2S/Na2SO3	350 W xenon lamp λ > 420 nm	2.3	363	N/A	152
ZnIn2S4/BiOBr	Solvothermal (water/glycerol, 80 °C, 2 h)	N/A	Pt (3%)	N/A	300 W Xe lamp (320 < λ < 780)	N/A	17 000	11.7%	153
MoS2/ZnIn2S4	Solvothermal	Microspheres	MoS2	Na2S/Na2SO3	300 W xenon lamp λ > 420 nm	N/A	3891.6 5 wt% MoS2/ZnIn2S4	N/A	142
ZnIn2S4	Hydrothermal	Microsphere	Pt	Na2S/Na2SO3	350 W Xe lamp λ > 420 nm	2.29	248.9	4.11%	154
RGO/ZnIn2S4	Microwave-assisted	Microspheres that resembling flowers	RGO	Lactic acid	300 W xenon lamp λ > 420 nm	N/A	2646 1.0 wt% RGO/ZnIn2S4	10	117
ZnIn2S4	Hydrothermal	Accumulative sheet	Pt	TEA	400 W	2.34	1690	N/A	155
ZnIn2S4	Solvothermal	Grains marigold-like microsphere	N/A	Na2S/Na2SO3	λ ≥ 420 nm 300 W xenon lamp λ > 400 nm	2.69	268 μmol h^−1	N/A	111
ZnIn2S4	Solvothermal	Persimmon-like particles	Pt	Na2S/Na2SO3	300 W Xe lamp λ > 420 nm	2.59	2204.5	13.2%@420 nm	79
ZnIn2S4/CDs/g-C3N4	Solvothermal (water/glycerol, 80 °C, 2 h)	Stacked sheet-on-sheet structure	Pt (1%)	N/A	300 W Xe lamp (λ > 420)	N/A	17 580	12.7	156
ZnIn2S4	Hydrothermal	Floriated microspheres	Pt	Na2S/Na2SO3	300 W xenon lamp λ ≥ 420 nm	2.1	8420	34.3%@420 nm	22
ZnIn2S4	Hot-injection	Flower-like sub microspheres	Pt	Na2S/Na2SO3	300 W Xe lamp λ > 420 nm	2.5	612	N/A	19

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Scholars have examined the interaction between the structure of crystals and their photocatalytic activity. For instance, Shen et al. investigated the effects of crystallinity.⁹⁶ In the case of the reaction under aqueous conditions, blossoming, cherry-like, tiny spheres of ZnIn₂S₄ were produced, whereas in organic solvents, micro-clusters of ZnIn₂S₄ were generated. It was discovered that the ZnIn₂S₄ microspheres resembling flowering cherries had a less dense structure and improved crystallinity. An investigation was conducted into the hydrogen generation efficiency of freshly prepared ZnIn₂S₄ photocatalysts loaded with 1.0 wt% Pt. The findings indicated that the hydrogen production rate of the flowering-cherry-shape microsphere ZnIn₂S₄ was 136.5 μmol g⁻¹ h⁻¹, which surpassed that by the miniature clustering ZnIn₂S₄. The enhanced crystalline structure of the former could potentially account for its optimal hydrogen evolution efficacy. Additionally, the impact of crystal structure parameters was investigated by Shen et al. using ZnIn₂S₄ prepared via the hydrothermal route using CTAB.⁶⁵ ZnIn₂S₄ synthesized using 9.6 mmol of CTAB produced hydrogen at the fastest rate (562.25 μmol g⁻¹ h⁻¹) when 1% Pt was added to it. Additionally, at 420 nm, its apparent quantum yield approached 18.4%. In addition to determining the crystalline structures, molecular structure also has an effect on photocatalytic activity. For example, Bai et al.²⁸ documented that the photocatalytic activity of porous ZnIn₂S₄ microspheres was outstanding compared to that of bulk ZnIn₂S₄ in the context of hydrogen evolution. The slit-shaped, permeable arrangement of the porous ZnIn₂S₄ microspheres was recognized, together with precise interfacial dimensions of 165.4 m² g. The favorable catalytic activity was the outcome of the void structure and the significant, precise surface region and highest volume ratio, which correspondingly hindered electron–hole recombination, while facilitating charge movement. The significance of the molecular makeup and structure of ZnIn₂S₄ in relation to its photocatalytic efficacy for hydrogen evolution can be deduced within this particular category. Enhanced crystallinity and appropriate crystal structure deformation may have an advantageous influence on photocatalytic activity. Furthermore, permeability and precise surface area are important indicators of photocatalytic performance.

In particular, the molecular shape and crystal structure of nanostructured semiconductors can have a big impact on how well they work as photocatalysts and in optoelectronics. In some cases, these factors may even enhance their physical and chemical properties. Nano- and microstructures of ZnIn₂S₄ have been made, which have shown varying levels of photocatalytic activity in the degradation of contaminants. The implications of crystal structures have been an ongoing area of research. Chen et al.⁵⁶ performed an investigation into the photocatalytic breakdown of methyl orange (MO) employing cubic and hexagonal ZnIn₂S₄, which were prepared hydrothermally. The results from this study demonstrated that cubic ZnIn₂S₄ facilitated the complete decomposition of MO within a three-hour duration, in contrast to hexagonal ZnIn₂S₄, which only caused 52% of MO to begin to degrade. This suggests that cubic ZnIn₂S₄ demonstrated a more advantageous photocatalytic degradation rate for MO removal compared with hexagonal ZnIn₂S₄. Furthermore, ZnIn₂S₄ nanocrystals and ZnIn₂S₄ nanoplates were synthesized by Peng et al.⁹⁷ employing 9-octadecenylamine as the ligand of choice and non-coordinating octadecene as the solvent. In an attempt to eliminate 9-octadecenylamine, which the researchers hypothesized could inhibit charge transfer between ZnIn₂S₄ nanocrystals, they heated the nanoparticles at 400 °C. The structure of the crystals of ZnIn₂S₄ nanoparticles was observed to be substantially smaller compared to that of ZnIn₂S₄ tiny plates. Additionally, the starting deterioration rate parameters for the unannealed ZnIn₂S₄ nanocrystals, treated ZnIn₂S₄ nanoplates, annealed ZnIn₂S₄ nanocrystals, and unannealed ZnIn₂S₄ nanoparticles were 0.35, 0.27, 0.63, and 0.89 h⁻¹, respectively. It is readily apparent that the annealed ZnIn₂S₄ exhibited improved photocatalytic activity for the deterioration of MO compared with the unannealed ZnIn₂S₄. Furthermore, the deterioration rate constant of the ZnIn₂S₄ nanocrystals was more substantial than that of the ZnIn₂S₄ nanoplates. Therefore, the higher level of photocatalytic dissociation of MO can be attributed to the extended area of asymmetric surface sites for coordination, as well as the splitting of photogenerated holes and electrons facilitated by the small crystal dimensions of the ZnIn₂S₄ nanocrystals and their superior outermost layer area to volume ratio.

With the possible exception of crystal structure, the influence of the morphology of photocatalysts on their photocatalytic performance is generally significant. For example, Chen et al.¹¹⁰ employed a thermal solution technique for producing porous ZnIn₂S₄ microspheres, and subsequently evaluated their photocatalytic efficiency in the decomposition of MO. The temporal fluctuations in MO concentration was detected by examining the variation in the maximum absorption at 464 nm via UV-vis spectroscopy. A photodegradation efficiency of approximately 100% was measured for 10 ppm MO upon exposure to 2.5 h of illumination in the presence of ZnIn₂S₄. Therefore, ZnIn₂S₄ with a porous structure demonstrated an increase in photocatalytic activity regarding visible light. In addition, ZnIn₂S₄ nanotubes and nanowires were prepared by Shi et al., and their catalytic capability was scrutinized in the degradation of MO.²⁹ Through the use of ZnIn₂S₄ nanotubes, MO underwent total decomposition following a 210 min irradiation period. During the same period, 76% of MO was diminished using ZnIn₂S₄ nanowires. Thus, the photocatalytic efficiency of ZnIn₂S₄ nanotubes was superior to that of ZnIn₂S₄ nanowires. Furthermore, the ZnIn₂S₄ nanowires and nanotubes possessed specific surface areas of 55.9 m² g and 78.3 m², respectively. The prominent absorption edge detected for pure ZIS at 530 nm can be ascribed to the fundamental band slit of ZnIn₂S₄. Purified NP revealed substantial absorption of radiation in the UV-visible light spectrum (200–800 nm).¹⁵⁷ As a result, it is feasible to hypothesize that the boosted photocatalytic effectiveness of ZnIn₂S₄ nanotubes can be attributed to their void configuration and an incredibly thin tubular exterior, which provided more active surface areas.

4.1. Element doping

Generally, element doping can be expected to reduce the band gap of materials, while improving their capability to capture light. Additionally, doping has the capacity to improve the interfacial transfer of charges, hinder the recombination of photogenerated electron–hole pairs, and incorporate additional inherent defects. Furthermore, the structure and appearance of semiconductors can be tuned through the incorporation of particular elements. Thus, tremendous efforts have been devoted to element doping in micro- and nanostructured ZnIn₂S₄ to its performance in photocatalytic hydrogen generation from water. The investigated ZnIn₂S₄ materials and their corresponding data are specified in Table 1.

XRD was employed to evaluate the phases and crystal structures of Co₃O₄, ZnIn₂S₄, and a series of Co₃O₄/ZnIn₂S₄ samples, as shown in Fig. 8(A). In the XRD pattern of Co₃O₄, the (1 1 1), (2 2 0), (3 1 1), (4 0 0), (5 1 1), and (4 4 0) crystal planes are represented by the peaks at 2θ values of 31.3°, 36.8°, 44.8°, 59.4°, and 65.2°, respectively, which are consistent with the data from the standard XRD card (JCPDS No. 74-2120). The (0 0 6), (1 0 2), (1 1 0), and (0 2 2) crystal planes of ZnIn₂S₄ are represented by the peaks at 2θ values of 21.6°, 27.7°, 47.2°, 52.4°, and 55.6°, which match that in the standard XRD card (JCPDS No. 65-2023) for hexagonal-phase ZnIn₂S₄. As anticipated, the distinctive XRD peaks of ZnIn₂S₄ and Co₃O₄ are apparent in the configurations of the produced heterojunction photocatalytic materials, revealing that the Co₃O₄/ZnIn₂S₄ photocatalysts are pure two-phase complexes, where no impurity peaks were noticed.

Fig. 8 (A) XRD patterns of ZnIn₂S₄, Co₃O₄, and Co₃O₄/ZnIn₂S₄ photocatalysts. Reproduced from ref. 158 with permission from Elsevier, Copyright 2023. (B) Spectra of 1.5% Pt/ZIS, 1% Pt/ZIS, 0.5% Pt/ZIS, and ZIS. Reproduced from ref. 159 with permission from Elsevier, Copyright 2022. (C) XRD design of the R-ZIS and H-ZIS. Reproduced from ref. 52 with permission from Elsevier, Copyright 2023. (D) XPS spectrum of photocatalysts: ZnIn₂S₄-800, 700, 600, 500, 400, 300. Reproduced from ref. 51 with permission from Elsevier, Copyright 2023.

Fig. 8(B) illustrates the XRD patterns for the ZnIn₂S₄ (or ZIS) and Pt/ZIS samples, showcasing multiple Pt loadings. The samples exhibit distinctive peak shapes indicating the hexagonal-phase ZnIn₂S₄ at angles of 21.6°, 27.7°, and 47.2°, which correspond to the crystal planes (003), (011), and (110) of the material. The distinctive peaks associated with Pt are non-discernible in the patterns, conceivably due to the diminutive dimensions of the Pt micron-sized particles. The XRD patterns of the prepared samples are displayed in Fig. 8(C). One of the H-ZIS with TAA precursors coincides with the hexagonal phase (JCPDS No. 65-2122), and the rhombohedral ZnIn₂S₄ (JCPDS No. 65-2023) can be correlated with the XRD pattern of R-ZIS with Na₂S precursor. The rhombohedral ZnIn₂S₄ can be conveniently synthesized harnessing the one-step hydrothermal treatment approach with Na₂S as the sulfur source, according to the preceding results. Fig. 8(D) displays the XRD patterns of the ZnIn₂S₄ samples (ZnIn₂S₄-T, T = 300–800 °C) produced in an H₂S environment at different temperatures. Only the diffraction pattern from the Zn–In oxide precursor was apparent when the ZnIn₂S₄ sample emerged by sulfurizing the oxide precursor at 300 °C. This suggests that the temperature was insufficient to convert the oxide precursor into ZnIn₂S₄. The typical cubic ZnIn₂S₄ reflection (ICSD-JCPDS card no. 00-048-1778, a = 10.62 Å) indicates that cubic ZnIn₂S₄ emerged when the sulfidation temperature exceeded 400 °C. Rhombohedral ZnIn₂S₄ (ICSD-JCPDS card no. 00-049-1562, a = 3.86 Å, c = 37.01 Å) crystallites formed together with cubic ZnIn₂S₄ when the sulfidation temperature increased to 500 °C. The diffraction peaks allocated to the cubic phase were less prominent as the sulfidation temperature increased from 500 °C to 700 °C, whereas the peaks assigned to the rhombohedral phase became noticeably more noticeable. At 800 °C, the oxide precursor sulfurized, revealing the rhombohedral ZnIn₂S₄ as the sole crystalline phase.

Researchers have attempted to alter the optics and electrical characteristics of ZnIn₂S₄ through the transition-metal ion doping process. An instance of this can be seen in the straightforward hydrothermal preparation of Cu-doped ZnIn₂S₄ samples by Shen et al.,⁴⁷ where Pt (1 wt%) was loaded on the photocatalysts as a co-catalyst. Jing et al.³¹ utilized a hydrothermal method to generate ZnIn₂S₄ microspheres enriched with Ni²⁺. ZnIn₂S₄ enriched with 0.3 wt% Ni achieved the largest hydrogen production rate, which was two-times that of ZnIn₂S₄. Compared to the surface, initially, the loaded Ni²⁺ ions permeated the ZnIn₂S₄ lattice, showing potential to boost its photocatalytic performance. The moderate trap sites created Ni²⁺ enabled effective electron and hole separation. In the hydrogen evolution reaction, rare earth-ion-doped ZnIn₂S₄ demonstrated enhanced photocatalytic activity independent of transition-metal ion modification. The photocatalytic efficiency of ZnIn₂S₄ substituted with Gd³⁺, La³⁺, Ce³⁺, Er³⁺, and Y³⁺ was examined by Tian et al.¹⁶⁰ Their observations revealed that the photocatalytic effectiveness for hydrogen evolution was boosted by 69%, 106%, 61%, 53%, and 46%, respectively, compared to the unmodified ZnIn₂S₄ (106.3 μmol g⁻¹ h⁻¹). The relatively small alteration in optical properties observed upon the introduction of rare earth ions in this investigation was a significant feature. Alternatively, the introduction of rare earth ions in ZnIn₂S₄ resulted in an increase in the number of defects that were capable of capturing photoinduced electrons and preventing the recombination of electron–hole pairs. Furthermore, supplementing ZnIn₂S₄ with rare earth ions decreased its crystal size and enhanced its structural characteristics, including its tiny pores and textural structures, which became more predictable and stable. Consequently, its photocatalytic activity was supplemented due to the synergistic influence among structure, morphology, and defects. Furthermore, it was determined by Tian et al.¹⁶¹ that La–ZnIn₂S₄ demonstrated superior photocatalytic activity. Consequently, they carried out an independent examination of its photocatalytic effectiveness. The hydrogen production rate by La–ZnIn₂S₄ was determined to be 2917 μmol g⁻¹ h⁻¹, surpassing that of pristine ZnIn₂S₄ by 2.4 times.

Shen et al.¹⁶² employed a microwave-assisted hydrothermal methodology to synthesize ZnIn₂S₄ photocatalysts that were substituted with alkaline-earth metals (Ca, Sr, and Ba). Experiments on the photocatalytic production of H₂ were conducted to gain a better understanding of the functionality of alkaline-earth metallic substances in their as-used condition. The research results demonstrated that Ca-doped ZnIn₂S₄ exhibited the most accelerated hydrogen evolution rate, surpassing that of the non-doped ZnIn₂S₄ by two times. Despite this, the photocatalytic activity of ZnIn₂S₄ was not considerably boosted by Sr- or Ba-doping. The incorporation of alkaline-earth metallic substances into ZnIn₂S₄ had no significant impact on its band structure. Thus, the enhancement in catalytic activity can be attributed to the suppressed charge recombination. Furthermore, the suppressed charge recombination can be attributed to the band transformation and the use of augmented non-radioactive recombination.

Yang et al.¹⁶³ portrayed the extraordinary catalytic activity of ZnIn₂S₄ doped with O, in addition to the impact of metal ion substitution on the evolution of hydrogen. The H₂ evolution rate by O-doped ZnIn₂S₄ was predicted to be 2120 μmol g⁻¹ h⁻¹, which is 4.5 times greater than that by pristine ZnIn₂S₄. The outstanding performance of hydrogen production was predominantly assigned to the replacement of oxygen atoms with sulfur atoms, which generated an increased number of charge carriers. Additionally, O-doped ZnIn₂S₄ displayed a band separation of 2.07 eV, which is considerably smaller compared to that of pure ZnIn₂S₄ (2.17 eV). It was discovered that the VB maximum energies of pure ZnIn₂S₄ and O-substituted ZnIn₂S₄ are 1.69 and 0.73 eV, respectively. Consequently, the CB of ZnIn₂S₄ doped with O is roughly 0.86 eV greater than that of pure ZnIn₂S₄. In comparison with pristine ZnIn₂S₄, the greater VB and the minimally displaced CB of O-doped ZnIn₂S₄ resulted in enhanced carrier mobility and photogenerated hole consumption. Furthermore, the enhanced reducing capability of O-doped ZnIn₂S₄ was indicated by the increased potential of its CB edge. As a consequence of the intended band arrangement for O-doped ZnIn₂S₄, its photocatalytic performance was enhanced. When exposed to ultraviolet (UV) radiation, the migration of holes (h⁺) and electrons (e⁻) happened. The submerged oxygen (O₂) captured the assembled CB band electrons, triggering the emergence of reactive superoxide radicals (˙O₂⁻).¹⁶⁴ The photoinduced holes (h⁺) accumulated at the VB may potentially be employed simultaneously for the generation of hydroxyl radicals (˙OH).^164,165 The acceptable correlation between the precise positions of the Zn, In, and S peaks and that documented in the literature confirms that the ZIS photocatalyst was successfully manufactured.^166–168

Rare earth components, transition metals, and alkaline-earth metal substances are some of the metal elements that have been used to modify ZnIn₂S₄ photocatalysts in recent research. The addition of certain metals to ZnIn₂S₄ may be helpful because they may lower the band gap and make it easier for light to pass through, which would make the photocatalytic process work better. On the contrary, they can operate as electron traps and produce favorable effects on the structure and appearance of the material, consequently significantly enhancing the photocatalytic activity of ZnIn₂S₄. However, metal modification continues to be associated with certain limitations. If the concentration of dopant ions surpasses a specific threshold, it can induce the recombination of photoinduced electrons and holes. Furthermore, regardless of the fact that non-metal doping has been scrutinized and demonstrated to enhance the photocatalytic performance of ZnIn₂S₄, its unique perspective requires greater consideration in future investigations. Transition-metal doping, comprised of Fe,¹⁶⁹ Cu,^47,170,171 and Mo,^172,173 can influence the optical characteristics of ZnIn₂S₄. ZIS treated with Fe was found to be better at breaking down 2,4- and 6-tribromophenol than defect-free ZIS.¹⁶⁹ According to Xing et al.,¹⁷² flower-shaped hollow microscopic particles were found in Mo-doped ZIS. These particles produced H₂ gas about 10 times faster than regular ZIS. Wang et al.¹⁷⁰ fabricated miniature sheets of ZIS modified with copper. The hydrogen production rate was 26.2 mmol g⁻¹ h⁻¹, regardless of the optical circumstances of doping. At an absorption wavelength of 420 nm, it demonstrated an exceptionally high quantum productivity of 4.76%. Also, copper-doped zinc indium sulfide (ZIS) was prepared using the conventional hydrothermal technique.⁴⁷ ZIS exhibits significant catalytic characteristics, which can be further modified using rare earth metals.^160,161,174 Tian et al.^160,161 reported the photocatalytic impact of zinc indium-sulfide (ZIS) doped with Ce³⁺, Gd³⁺, La³⁺, Er³⁺, and Y³⁺. A simple hydrothermal technique was used to prepare ZIS doped with various types of rare earth metals. Researchers have discovered that rare earth metallic substances are in an oxide state, indicating an overall reduction in their crystalline dimensions and particle size. Although rare earth metals have no significant impact on the optical characteristics of ZIS, they should contribute to boosting its porosity volume, nanoparticle distribution, corresponding electron movement, and exterior dimensions. Shen et al.¹⁶² created multiple types of ZIS photocatalysts treated by Sr²⁺, Ca²⁺, Ba²⁺, and Ca loading, which boosted the hydrogen-producing efficacy of ZIS by approximately two times that of the pristine ZIS.

Noble metals, by virtue of their reduced Fermi level, demonstrate favorable electronic conductivity and can function as electron traps, thereby facilitating the dissociation of photogenic carriers. Thus, co-catalysts comprised of Au,¹⁷⁵ Pt,⁶⁵ and Pd¹⁷⁶ are frequently employed in photocatalytic systems. On the outermost layer of ZnIn₂S₄, Li's group¹⁷⁷ doped a Pd nanoparticle layer measuring approximately 6 nm. The obtained photocatalyst was employed in alkylation induced by the illumination of amines and the alkylation of α-H of ketones under the coordinated effect of alcohol. As a culmination of the efficacy of the unsaturated palladium atoms, Pd–ZnIn₂S₄ exhibited amazing photocatalytic properties during the alkylation process. Au–Pd-co-modified ZnIn₂S₄ nanoplates were synthesized by Feng et al.¹⁷⁸ for catalyzing the oxidation of benzyl alcohol. In contrast to ZnIn₂S₄, the dissociation of photogenerated carriers was enhanced, and the absorption of electromagnetic radiation improved. Also, the surface adsorption capacity and interaction with O₂ were greatly improved owing to the participation of Au and Pd atoms, making the photocatalytic oxidation of benzyl alcohol more effective. When exposed to illumination, rare metals also exhibit exterior plasmon resonance. Because of this, the electrons migrate directly from the metal to the semiconductor transmission path, which makes the semiconductor better at reacting with light. The photocatalytic hydrogen production by Pt-loaded ZnIn₂S₄ was 11 times greater in magnitude than purified ZnIn₂S₄ under the ideal conditions.⁶⁵ However, as the quantity of Pt increased, the photocatalytic activity declined. This is due to the fact that an overabundance of Pt generates a shielding effect and serves as a point of attachment for photogenerated carriers. Therefore, it is crucial to carefully consider the loading and dispersion of noble metals when designing mixed systems with ZnIn₂S₄.

4.2. Hybrid nanocomposites

By manufacturing heterojunctions or co-catalyzing ZnIn₂S₄, the amplitude of light absorption can be expanded, and charge transmission may be augmented, leading to enhanced photocatalytic efficiency by ZnIn₂S₄-based composite materials. The efficiency of ZnIn₂S₄-based photocatalysts has been described in Table 1. In general, the utilization of co-catalysts results in a comprehensive enhancement in photocatalytic efficiency, including enhanced stability, selectivity, and physical activity. In photocatalytic processes, Au,¹⁷⁵ Pt,⁶⁵ and Pd¹⁷⁶ are frequently used as co-catalysts. Using ultraviolet (UV) radiation, we loaded palladium nanoparticles with an external span of approximately 6 nm on the outermost portion of ZIS. The beneficial capacity of Pd–ZIS in alkylation processes is attributed to the interaction of alcohols over zinc indium sulfide during palladium-based hydrogenation.¹⁷⁶ Certain types of metallic-free substances (graphene), noble sulfides, or metal oxides (MoS₂, NiS, CdS, PdS, etc.), and transition-metal sulfides or compounds (MoS₂, NiS, CdS, PdS, etc.) may serve as co-catalysts for assisting ZnIn₂S₄ in achieving high photocatalytic efficiency. The chemical transformation of ZnIn₂S₄ with a co-catalyst has been the topic of extensive research to date. Furthermore, dual co-catalyst systems, such as MoS₂/graphene, are currently being developed in an endeavor to strengthen the productivity of photocatalytic technologies.^32,179

The precious metal Pt is frequently utilized as a co-catalyst in water reductions catalyzed by ZnIn₂S₄. Pt demonstrates extraordinary conductivity and has the capability to accommodate additional active sites. Shen et al.⁶⁵ evaluated the photocatalytic efficiency of exposed ZnIn₂S₄ and ZnIn₂S₄ loaded with Pt. To evaluate the distinctive effects of Pt, the experiments were conducted utilizing microsphere-shaped ZnIn₂S₄⁻¹, which was manufactured using 1.8 mmol CTAB and subjected to a 1 h hydrothermal methodology. The most effective photocatalytic capacity was observed by the investigators for 1.0 wt% Pt-laden ZnIn₂S₄⁻¹, which was 11-times that of pristine ZnIn₂S₄⁻¹. As the platinum concentration increased to over 3.0 wt%, an overall decrease in photocatalysis activity was observed. An additional examination was performed with regard to the composition of Pt loading using 3D-hierarchical persimmon-like ZnIn₂S₄ fabricated by Shen et al.⁷⁹ The photocatalytic effectiveness of ZnIn₂S₄ mixed with 3.0 wt% Pt was the highest. It demonstrated a 15-fold additional boost in the rate of hydrogen production compared to pure ZnIn₂S₄. Nevertheless, an additional 5 wt% of Pt loading resulted in the minimum photocatalytic activity. Due to protective influence and saturated Pt, a reduction in photocatalytic activity occurred. The excessive Pt may generate photoexcited charges with additional recombination sites. Additionally, it is achievable to determine that the ideal input concentration of Pt fluctuates among ZnIn₂S₄ produced via different methodologies.

However, the restricted availability and high price of Pt prevent its widespread implementation. Alternatively, MoS₂ is a transition-metal sulfide characterized by a two-dimensional layered structure and high anisotropy.¹⁴² In ZnIn₂S₄ systems for the photocatalytic evolution of hydrogen, MoS₂ has been characterized as an effective complementary catalyst, owing to its distinguished physical and chemical properties and low price. For instance, Chen et al.¹⁸⁰ deposited MoS₂ onto ZnIn₂S₄ nanoparticles via a straightforward in situ photo-assisted decomposition technique. Through the accumulation of MoS₂ as an additional catalyst, the photocatalytic efficiency was substantially increased. Pt was loaded on ZnIn₂S₄ in a parallel experiment. The hydrogen evolution rate by 0.375 wt% MoS₂/ZnIn₂S₄ reached the maximum of 8.047 mmol g⁻¹ h⁻¹, whereas pristine ZnIn₂S₄ demonstrated a rate of 0.283 mmol g⁻¹ h⁻¹. In comparison, MoS₂/ZnIn₂S₄ displayed an H₂ production rate of 5.117 mmol g⁻¹ h⁻¹ when packed with Pt. This finding indicates that MoS₂/ZnIn₂S₄ displayed greater photocatalytic activity. The process of deposition of MoS₂ on the surface of ZnIn₂S₄ might hinder the recombination of electrons and holes, thus increasing the its catalytic activity. Furthermore, transition-metal sulfides are commonly employed as co-catalysts for the modification of ZnIn₂S₄. For example, Wei et al.¹⁸¹ discovered that NiS-loaded ZnIn₂S₄ boosted the photocatalytic hydrogen (H₂) generation velocity to 2094 mol g⁻¹ h⁻¹, which was 7.4-times greater than that of purified ZnIn₂S₄. Thus, it was hypothesized that nickel sulfide functioned as a competent co-catalyst and established a junction with ZnIn₂S₄, thus facilitating charge-carrier separation and charge transfer. Additionally, PdS was utilized as a co-catalyst in the manufacturing of ZnIn₂S₄/CdIn₂S₄ composite-based photocatalysts by Yu et al.¹⁸² The composite ZnIn₂S₄/CdIn₂S₄ preloaded with PdS displayed a photocatalytic yield that was 1.6 times greater than that of unloaded ZnIn₂S₄. It has been documented that graphene possesses beneficial characteristics as a co-catalyst in catalytic H₂ production, which include a stable composition, customizable surface properties, and high specific surface area.¹⁸³ Ye et al.¹¹⁹ manufactured an RGO/ZnIn₂S₄ composite by sequentially growing ZnIn₂S₄ nanosheets on RGO using a straightforward one-pot solvothermal technique. It was found that the H₂ production rate by 1.0 wt% RGO/ZnIn₂S₄ was 4.3 intervals larger than that of pristine ZnIn₂S₄. RGO behaved as a successful mediator and electron acceptor. The successful distinction of charges emitted by light was achieved through the migration of electrons from the CB of ZnIn₂S₄ to RGO. Carbon imperfections in RGO may also function as crucial sites for the evolution of hydrogen. Thus, the enhancement in catalytic performance was attributed to the incorporation of RGO. For the purpose of examining the implications of RGO loading in greater detail, Ye's research group implemented a rapid microwave-assisted technique for manufacturing flower-shaped RGO/ZnIn₂S₄ microspheres. Following that, their photocatalytic activity for hydrogen production was investigated.¹¹⁷ For comparison, they deposited 1.0 wt% platinum on ZnIn₂S₄. The maximum rate of hydrogen production of 2646 μmol g⁻¹ h⁻¹ was achieved using 1.0 wt% RGO/ZnIn₂S₄, which displayed outstanding photocatalytic capability in comparison to the 1.0 wt% Pt/ZnIn₂S₄ manufactured in this study. The improvement can be attributed to the synergistic effects generated by the remarkable properties of RGO, the enhanced conductivity of the RGO/ZnIn₂S₄ composite, and the stronger interaction between RGO and ZnIn₂S₄.

For the further modification of ZnIn₂S₄, carbon quantum dots (CQDs), CNTs, and graphene have been significantly employed. Chai et al.¹⁸⁴ employed hydrothermal synthesis to produce ZnIn₂S₄ composite materials comprised of multi-walled carbon nanotubes (MWCNTs). It was determined that the MWCNT/ZnIn₂S₄ composite demonstrated the maximum photocatalytic activity of 6840 μmol g⁻¹ h⁻¹, which is 1.5-times the rate of pristine ZnIn₂S₄. Furthermore, at 420 nm, the MWCNT/ZnIn₂S₄ composite material exhibited an extraordinarily high apparent quantum yield, which was 23.3%. The capability of MWCNTs to function as effective electron acceptors enabled the separation of photogenerated electron–hole pairs. Subsequently, the decreased recombination rate associated with photogenerated charges triggered the process of electron migration to the outermost layer of ZnIn₂S₄, where they reacted with H⁺ to generate hydrogen. Chen et al.¹⁸⁵ employed a two-step approach to fabricate carbon nanofibers (CNFs) with a hierarchical core–shell structure and ZnIn₂S₄ configuration. The CNF (15 wt%)/ZnIn₂S₄ composite resulted in the maximum hydrogen production rate of 3166 μmol g⁻¹ h⁻¹. This was 3.9 times higher than that of the undoped ZnIn₂S₄. Also, the quantum yield of 15 wt% CNFs/ZnIn₂S₄ was estimated to be 25.35%. The photocatalytic efficiency of ZnIn₂S₄ was enhanced due to its modification with carbon nanofibers (CNFs), which contributed to the increased separation of the photogenerated charge carriers and modulated its light absorbance, in conjunction with the formation of a core–shell hierarchical framework.

Gao et al.¹⁸⁶ used an impregnation method to prepare a new co-catalyst called Co(dmgH)₂pyCl (where dmgH is dimethylglyoxime and py is pyridine) on hexagonal ZnIn₂S₄. This co-catalyst did not contain any noble metals. It was found that Co(dmgH)₂pyCl, which was used as a co-catalyst during the photocatalytic procedure, significantly improved the photocatalytic activity of ZnIn₂S₄. ZnIn₂S₄ loaded with Pt functioned as the regulating group. The conclusions highlighted that ZnIn₂S₄ loaded with 3.0 wt% Co(dmgH)₂pyCl exhibited an H₂ production rate of 3840 μmol g⁻¹ h⁻¹, which was 6.5 times that by the pristine ZnIn₂S₄ and 2.5 times greater than that of 3.0 wt% Pt-loaded ZnIn₂S₄. Thus, it is obvious that the combination of noble metal-free materials exhibit potential in the domain of photocatalysis because of their reasonable price and ability to remarkably strengthen the activity in photocatalytic reactions.

This section summarized co-catalyst-modified ZnIn₂S₄ photocatalysts and their photocatalytic activity in the evolution of hydrogen. The expensive metal Pt is often employed as a complementary catalyst, and because of its very low Fermi level, it can conceivably function as an electron trap, which greatly improves the photocatalytic effectiveness of materials. However, despite this, it is essential to explore substitutes for Pt due to its limited availability and high cost. Consequently, various carbon materials and transition-metal sulfides (e.g., MoS₂ and NiS) have been incorporated into ZnIn₂S₄ for the purpose of significantly improving its photocatalytic activity. Materials made from carbon and transition metals, such as sulfur dioxide, are abundant and inexpensive in comparison to the precious metal Pt. Determining the ideal concentration of a co-catalyst is of great importance to prevent it from hindering the decomposition capacity of photocatalysts and acting as recombination sites for photogenerated charge carriers produced during co-catalyst modification. 2D MXene materials have been used as co-catalysts in photocatalysis for a few years now owing to their abundant functional groups, facilitating easy electron transport.^187–189 In 2020, Zuo et al.¹⁹⁰ successfully prepared sandwich-like hierarchical MXene-ZnIn₂S₄ heterostructures by embedding very thin ZnIn₂S₄ nanosheets on the two layers of Ti₃C₂TX MXene that were not in contact.

4.3. Transition-metal nanocomposites

Numerous ZnIn₂S₄-based composites have been established to boost the absorption of harmful substances, improve the photoresponse, and especially impede the recombination of photoinduced electron–hole pairs. These composites include heterojunctions with other semiconductors, such as TiO₂,^191–193 Fe₂O₃,¹⁹⁴ FeOOH,¹⁹⁴ CuO,¹⁹⁵ CdIn₂S₄,¹⁸³ ZnFe₂O₄,⁴² Ag₃PO₄,¹⁹⁶ g-C₃N₄,¹⁹⁵ UiO-66 (ref. 197) and WO_2.72/ZnIn₂S₄.¹⁹⁰ By implementing carbon materials such as carbon quantum dots¹⁷⁷ and graphene,¹⁹⁸ ZnIn₂S₄ can be transformed using organic polymers including polypyrrole,¹⁹⁹ polypyrrole doped with anthraquinone-2-sulfonate,²⁰⁰ and polyvinylidene difluoride.²⁰¹ ZnIn₂S₄-based composites have found extensive application in the removal of chemical contaminants due to their enhanced photocatalytic degradation capability.

4.4. Conductive organic polymer nanocomposites

Overall, conductive organic polymers demonstrate exceptional electrical conductivity, which might prove advantageous for charge transfer. As a consequence, inorganic–organic composite photocatalysts, which include polypyrrole and CdS, have been manufactured.²⁰² With regard to ZnIn₂S₄, multiple research groups have implemented this modification strategy. For example, 3D hierarchical ZnIn₂S₄/PVDF-poly(MMA-co-MAA) (ZnIn₂S₄/polymer) composites were produced by Peng et al.²⁰¹ by employing both electrospinning and hydrothermal techniques, where PVDF and MMA-co-MAA represent polyvinylidene difluoride and polymethyl methacrylate-co-methacrylic acid, respectively. The researchers examined the influence of ZnIn₂S₄/polymer and pure ZnIn₂S₄ on methyl orange (MO) deposition. The maximum photocatalytic efficacy was achieved with a ZnIn₂S₄/polymer nanocomposite concentration of 50 wt% ZnIn₂S₄. Upon exposure to radiation for 120 min, the polymer composite containing 50% ZnIn₂S₄ exhibited an enhanced photodegradation of 92% of MO. In contrast, the unmodified ZnIn₂S₄ powder only achieved 80% MO degradation within the same period. A ZnIn₂S₄/polymer composite was determined to have greater activated surface area and a greater number of active catalytic sites. Consequently, the ZnIn₂S₄/polymer composite exhibited enhanced photocatalytic capabilities. Furthermore, the elevated photocatalytic activity can be attributed to the instantaneous charge generation and successful separation of electron–hole pairs generated by ZnIn₂S₄ on the polymer, which also strengthened the photocatalyst surface. The findings demonstrated that the use of electrospun polymers to change the surface of ZnIn₂S₄ presents a compelling strategy for augmenting its photodegradation potential.

Furthermore, Gao et al. synthesized an integrated photocatalyst employing polypyrrole and ZnIn₂S₄. This catalyst could break down chloramphenicol when exposed to ultraviolet (UV) radiation.¹⁹⁹ Previous research investigations confirmed that polypyrrole (PPy) possesses a comprehensive π-conjugated electron system, which contributes to its remarkable stability and comparatively elevated mobility as a carrier.²⁰³ In 60 min, the solution with 4% polypyrrole/ZnIn₂S₄ broke down 30 mg L⁻¹ of chloramphenicol completely. In contrast, it took 120 min for pure ZnIn₂S₄ to break down chloramphenicol completely. Despite the narrow band gap of ZnIn₂S₄/polypyrrole, its absorbance was enhanced in the visible-light range. The expanded photocatalytic efficiency corresponded to the beneficial segregation of photoinduced electron–hole pairs and wide absorbance of light. In the investigation by Li et al.,²⁰⁴ they introduced diethanolamine (DI-EA), ethanolamine (EA), and triethanolamine (TRI-EA), respectively, into Pt–ZnIn₂S₄. These modifications substantially improved the photocatalytic efficiency of ZnIn₂S₄ in the context of hydrogen production and the deterioration of contaminants. In contrast to functioning as electron donors, surface modifiers can enhance the functionality of photocatalytic reactions through the absorption of reactants. In the investigation by Gao et al.,²⁰⁰ they evaluated how well ZnIn₂S₄ modified with polypyrrole and anthraquinone-2-sulfonate degraded contaminants in the presence of light. The anthraquinone-2-sulfonate-doped polypyrrole can generate O²⁻ by acting as an electron acceptor–donor intermediate. This could make ZnIn₂S₄ more effective at breaking down materials through photocatalysis. In the presence of visible light, it was demonstrated that ZnIn₂S₄ composite materials with a carbon coating showed improved ability to break down and absorb substances.¹⁷ Therefore, to enhance the photocatalytic performance of ZnIn₂S₄, it has been modified with numerous carbon materials, including carbon nanoparticles,⁴⁰ carbon,¹⁷ and graphene.⁴³

Graphene is a fascinating material for modification due to its extraordinary conductivity, high specific surface area, and customizable structure. Li et al.⁴³ prepared a ZnIn₂S₄ composite embedded in graphene (Gr) and investigated the photocatalytic capacity of ZnIn₂S₄@Gr microspheres for the decomposition of phenol in the presence of ultraviolet light. The findings demonstrated that the photocatalytic degradation constant rate for ZnIn₂S₄ at 2% Gr was 3.03 h⁻¹, which was an 8.4-fold improvement compared to pristine ZnIn₂S₄. The improved photodegradation activity can be attributed to the higher rate of charge-carrier separation, which is a result of the heterojunction formed between ZnIn₂S₄ and graphene. Furthermore, a composite of cross-linked ZnIn₂S₄ and reduced graphene oxide (rGO) was synthesized by Chen et al. and its photocatalytic productivity was examined in the degradation of 4-nitrophenol.²⁰⁵

It was found that introducing rGO in ZnIn₂S₄ enhanced its capacity to decompose 4-nitrophenol. Under the same experimental conditions, 97.8% of 4-nitrophenol could be decomposed after 360 min of illumination when the proportion of rGO in the composite was 1.5 wt%, whereas 77.1% of 4-nitrophenol was decomposed using pristine ZnIn₂S₄. The boosted catalytic activity can be attributed to the collective influence of numerous variables, particularly the reduced band gap and valence band, which was optimal, superior electron transport characteristics due to the formation of the Zn–O–C bond, and a comparatively large surface area. Besides graphene, the use of other carbon constituents can also improve the catalytic performance of ZnIn₂S₄ catalysts. Therefore, a carbon dot/ZnIn₂S₄ photocatalyst was manufactured by Shi et al. via the hydrothermal technique.⁴⁰ Subsequently, the photocatalytic capabilities of pure ZnIn₂S₄ and the optimal CDs/Zn₂S₄ composite were examined for the degradation of MO in the liquid phase. The maximum degradation value by the CDs/ZnIn₂S₄ hybrid combination at the optimal concentration was 0.09387 min⁻¹, which was 2.34-times higher than the degradation value by the standard ZnIn₂S₄. This can be attributed to the rapid charge transport from ZnIn₂S₄ to CDs owing to the close interfaces established between ZnIn₂S₄ and CDs. Furthermore, compared with pristine ZnIn₂S₄, the CDs/ZnIn₂S₄ composite displayed a red-shift light absorption, suggesting its potential for the enhanced photogeneration of charges when exposed to visible light. The implementation of CDs permitted the effective transformation of photoinduced electrons from the conduction band (CB) of ZnIn₂S₄ to the CDs. This prevented the recombination of photogenerated charges on the outermost layer of the photocatalyst, resulting in an outstanding photocatalytic performance.

4.5. Heterojunction construction

Heterojunctions are typically characterized as interactions of distinct semiconductors displaying different band structures, which might result in band bending. By employing the naturally occurring potential gradient between heterojunctions of semiconductors, the velocity of electron transfer can be improved, while simultaneously lowering the recombining rate of photoinduced charged species. As a consequence, numerous groups have attempted to establish heterojunctions between ZnIn₂S₄ and other semiconductors to increase their capacity for hydrogen evolution. For example, Mei et al.²⁰⁶ successfully investigated an In₂S₃/ZnIn₂S₄ heterojunction characterized by a photocatalytic H₂ evolution rate of 678 μmol g⁻¹ h⁻¹, surpassing that by both In₂S₃ and ZnIn₂S₄. Furthermore, a ZnIn₂S₄/CdIn₂S₄ composite photocatalyst was synthesized by Yu et al.¹⁸² using the hydrothermal technique. The electron transfer was substantially accelerated by the heterojunction, resulting in enhanced photocatalytic activity. The hydrogen production rate by the ZnIn₂S₄/CdIn₂S₄ complex catalyst was approximately 2.5-times higher than that by ZnIn₂S₄. Li et al.²⁰⁷ developed a ZnIn₂S₄/In(OH)₃ composite incorporating an innovative heterojunction. Considering the anisotropic electrical conductivity of multilayered ZnIn₂S₄, In(OH)₃ was coupled along the stacked planes at the outermost edges of ZnIn₂S₄ sheets to prevent resistance during electron transfer across the ZnIn₂S₄ layers. Furthermore, the blended material demonstrated an apparent quantum yield of 38.4% at 420 nm. As a result of the configuration of the novel heterojunction in the ZnIn₂S₄/In(OH)₃ composite, the transportation and segregation of charge carriers were more effective, contributing to the enhanced photocatalytic activity.

Together with studying heterojunctions, which are made up of two different semiconductors, tertiary heterostructure photocatalysts have also been investigated. Chen et al.¹⁸⁰ used the ionic layer adsorption-reaction procedure together with two sequential solvothermal techniques to fabricate a ternary heterostructure composite of CdS, ZnFe₂O₄, and ZnIn₂S₄. ZnIn₂S₄ nanosheet stereoscopic films were prepared and used to study the effect of ZnFe₂O₄ nanoparticles and CdS quantum dots. Their study showed that CdS/ZnFe₂O₄/ZnIn₂S₄ was more than three-times more active for photocatalytic hydrogen production than pure ZnIn₂S₄. CdS quantum dots and ZnFe₂O₄ nanoparticles have the potential to improve the light absorption characteristics and function as photosensitizers. Conversely, the presence of a distinguishable stereoscopic network containing pores enhanced the contact area between the reaction solution and composite film, thereby increasing the quantity of catalytically active areas and facilitating the photocatalytic process. Finally, increased photocatalytic activity and rapid separation of photogenerated electron–hole pairs were both caused by electron propagation across the junction of the heterojunction.

In one study, ascorbic acid was employed as a sacrificial agent during photocatalytic hydrogen evolution. The photocatalytic hydrogen generation rates of ZnIn₂S₄, BiYWO₆, and ZnIn₂S₄@BiYWO₆ composites with various mass ratios were evaluated and the findings are displayed in Fig. 9(A). Pure ZnIn₂S₄ exhibited a relatively low rate of hydrogen generation (only 71.43 μmol g h), which is most likely caused by intense photogenerated electron–hole pair recombination occurring in this single-component photocatalyst.²⁰⁹ Given that the hydrogen reduction potential (H⁺/H₂ = 0 eV) is substantially less positive than the conduction band minimum (ECB = 0.05 eV) of BiYWO₆, the hydrogen production rate of this compound is unquestionably zero. Additionally, when the content of BiYWO₆ was gradually increased in the ZnIn₂S₄@BiYWO₆ composites, the rate of hydrogen generation increased to the maximum value (5189.39 μmol g h for 10ZnIn₂S₄–BiYWO₆), which was 73-times greater than that of the unmodified ZnIn₂S₄.²¹⁰ The process of separating photogenerated electron–hole pairs was found to be substantially sped up by the heterojunction in the composites. Subsequently, as the mass ratio of BYW continued to increase, the rate at which hydrogen was generated began to decline. Ultimately, the ZnIn₂S₄@BiYWO₆ composites exhibited substantially higher photocatalytic activity than the unmodified ZnIn₂S₄, indicating that the heterojunctions generated in the composites considerably improve their photocatalytic performance.²¹¹

Fig. 9 (A) Hydrogen generation by heterojunction ZnIn₂S₄@BiYWO₆ photocatalysts with varying mass ratios. (B) Hydrogen manufacturing rates of 10ZIS–BYW in various sacrificial reagents. (C) Hydrogen production rates of in the process of cycling experiments, 10ZIS–BYW demonstrated outstanding stability. Reproduced from ref. 208 with permission from Elsevier, Copyright 2024.

Cycling studies were conducted to assess the stability of ZnIn₂S₄@BiYWO₆ in various photocatalytic procedures, with 10 ZnIn₂S₄–BiYWO₆ functioning as the photocatalyst. According to the data presented in Fig. 9(B), the rate of generating hydrogen declined gradually in the cycling investigations, with the highest decay rate representing merely 4.8%. This suggests that the ZnIn₂S₄@BiYWO₆ composites have excellent long-term stability and are capable of sustaining numerous cycles of reactions.²¹² Furthermore, the photocatalytic efficiency of ZnIn₂S₄@BiYWO₆ was examined with multiple sacrificial agents. Some common sacrificial agents, including a 5% solution of ascorbic acid, 5% solution of methanol, 5% solution of lactic acid, and 5% solution of glycerin, were selected for the photocatalytic procedure. All other conditions remained consistent during the experiment. According to the data presented in Fig. 9(C), the 10 ZnIn₂S₄–BiYWO₆ catalyst showed the highest level of catalytic efficiency when ascorbic acid was employed as the sacrificial agent. Ascorbic acid is very acidic, resulting in a high concentration of H⁺ ions in the solution. This abundance of H⁺ ions facilitates the process, making it much more effective.²¹³ Furthermore, Lin et al.²¹⁴ productively synthesized a composite composed of 2D/2D g-C₃N₄ nanosheets and ZnIn₂S₄ nanoleaf, exhibiting a hydrogen generation rate of 2780 μmol g⁻¹ h⁻¹. The researchers utilized a 2D/2D g-C₃N₄ nanosheet@ZnIn₂S₄ nanoleaf composition to fabricate a direct-contact heterojunction. The interfacial bridge demonstrated extensive intimate contact, with high-velocity charge transfer nanochannels facilitated by the direct heterojunction. The primary benefit of the developed heterojunction was the fact that it increased the positive impact of segregating photoinduced electrons and holes. Conversely, the upsurge in the charge transport in the nanochannels facilitated the effective migration of charge carriers. Thus, an enhanced photocatalytic performance was attained.

Fig. 10(A) illustrates the photocatalytic mechanism. As illustrated in this image, when illuminated by sunshine, the sensitizer EY molecule becomes activated into the unstable EY¹*, and ultimately transformed into EY³*. Simultaneously, EY³* will be swiftly reduced to EY⁻, and EY⁻ will ultimately acquire EY molecules after losing e⁻ as a result of its intrinsic instability. Both NiMoO₄ and CuMnO₂ will generate electrons (e⁻) upon exposure to light, and their electrons (e⁻) will migrate into the conduction band (CB). They will have positively charged holes (h⁺) in their valence band (VB) at this point. NiMoO₄ and CuMnO₂ have different band gaps, which suggests that the CuMnO₂/NiMoO₄ catalyst creates a type I heterojunction in its interface area. Furthermore, NiMoO₄ is considerably stronger than CuMnO₂ based on its ECB position. In this scenario, the electrons on NiMoO₄ will migrate to CuMnO₂ at the CB. Hydrogen is produced concurrently by the decay process between H⁺ and e⁻ during water splitting. The holes will move to CuMnO₂ because NiMoO₄ has a larger positive value than CuMnO₂. Consequently, TEOA will interact with the holes (h⁺) in the valence band of CuMnO₂. At this point, there will be a significant depletion of h⁺. Consequently, the CuMnO₂/NiMoO₄ composite catalyst exhibited boosted photocatalytic activity for water splitting owing to the separation and transport kinetics of photogenerated e⁻ and h⁺.

Fig. 10 (A) Photocatalytic pathway of type I heterojunction ZIS/NMO-3. Reproduced from ref. 215 with permission from Elsevier, Copyright 2024. (B) Illustration of the ZIS@NOH/NO Z-scheme heterojunction employed for photocatalytic CO₂ diminution propelled by sunlight. Reproduced from ref. 130 with permission from Elsevier, Copyright 2024. (C) Diagram showing photocatalytic degradation of CdSe under sunlight illumination. Reproduced from ref. 216 with permission from Elsevier, Copyright 2022. (D) Illustration of n-type Sv-ZnIn₂S₄ and p-type PdSe₂ heterojunction. Reproduced from ref. 217 with permission from Elsevier, Copyright 2023.

As depicted in Fig. 10(B), the photoinduced holes on the valence band of Ni(OH)₂ are proficient at merging with the photogenerated electrons in the conduction band of ZIS under the influence of light. This mechanism yields an important improvement in the dissociation performance of photogenerated carriers. In addition, the defects in ZIS and Ni(OH)₂ can make it harder for the photogenerated electrons and holes to recombine, which increases the catalytic action. Ultimately, the leftover electrons migrate to the co-catalyst, NiO, and play an integral role in the elimination process. The photothermal effect may also increase the temperature of the material to speed up the photogeneration of charge carriers, create new chemical bonds, which improves the product selectivity, lower the barrier for the catalytic process, and facilitate the adsorption and desorption of both chemical species. Ultimately, the ZIS@NOH/NO Z-scheme heterojunction achieved excellent selectivity and activity for photothermal-assisted photocatalytic carbon dioxide reduction. The development of the CdSe/ZnIn₂S₄ Z-scheme heterostructure and the charge transmission upon exposure to sunlight are shown in Fig. 10(C). The Fermi level (EF) and the energy band potential in n-type semiconductors are essentially equivalent. Therefore, CdSe has a greater EF than ZnIn₂S₄. The electrons have a tendency to migrate inevitably from CdSe to ZnIn₂S₄ until their Fermi levels match, according to XPS data. An internal electric field (IEF) forms at the CdSe/ZnIn₂S₄ interface as an effect of the band edge bending due to the charge rearrangement. Both ZnIn₂S₄ and CdSe QDs would be activated by photon absorption under solar light. This process entails the transport of electrons from the VB to the CB. Because of the influence of IEF and band bending, the electrons in the CB of ZnIn₂S₄ interact with the h⁺ in the VB of CdSe. Better redox-capable photoexcited carriers are retained by this technique. The ECB of CdSe is more negative, as shown in Fig. 10(C), than the O₂/·O₂⁻ potential (−0.33 V vs. NHE). Subsequently, O₂ is converted to ˙O₂⁻ by the CB of CdSe.

In contrast to conventional semiconductors, metal–organic frameworks (MOFs) incorporate organic ligands with metal units, allowing molecular-level modifications to photocatalysts. These modifications manifested interesting characteristics, including adjustable optical structures and porous structures.²¹⁸ Consequently, the ZnIn₂S₄@NH₂-MIL-125(Ti) nanomaterial was synthesized by Liu et al. employing the solvothermal technique.²¹⁹ Their investigation demonstrated that the ZnIn₂S₄@NH₂-MIL-125(Ti) composite, consisting of 40 wt% NH₂-MIL-125(Ti), displayed an H₂ generation rate that was 6.5-times higher than that of pristine ZnIn₂S₄. A heterojunction structure was generated through the reaction of ZnIn₂S₄ and NH₂-MIL-125 (Ti). The extraordinary charge-carrier segregation and transfer of electrons in the ZnIn₂S₄@NH₂-MIL-125(Ti) composite were facilitated by the electrically charged band structure and the internal electric field in the generated heterojunction, resulting in its superior photocatalytic efficiency. Furthermore, ZnIn₂S₄ modified with UiO-66 was synthesized and employed for the elimination of pollutants; this will be clarified in the subsequent section on pollution elimination. As an outcome, the manufacturing of heterojunctions composed of ZnIn₂S₄ and additional semiconductors is a prominent and highly optimal technique for improving the catalytic H₂ production efficiency. The optimal segregation of photogenerated charge carriers is generally facilitated by the fundamental potential gradient that exists between the semiconductor interfaces. Therefore, an extension in the useful lifespan of charge carriers can potentially improve the effectiveness of photocatalysis. Furthermore, various types of heterojunctions may be developed according to the differentiation of the band structures between semiconductors. The investigation and significance of constructing heterojunctions to enhance the photocatalytic performance on materials depend on the wide variety of semiconductors and heterojunctions.

The precision of study results can be confirmed by theoretical calculations, which may additionally assist with the design of novel materials and give further insight into the development of photocatalysts. The electron transport between ZIS-Vs and NiO has a conceptual basis owing to the calculation performed statistically utilizing the model displayed in Fig. 11(A). The work functions (Φ) of ZIS-Vs and NiO are 5.12 eV and 5.43 eV, respectively, as shown in Fig. 11(B), which were calculated using the following formula: Φ = E_vac − E_f, where E_f and E_vac represent the Fermi and vacuum energy levels, respectively.²²¹ Because the Φ of ZIS-V is superior to that of NiO, the E_f of ZIS-Vs exceeds that of NiO. Fig. 11(C) illustrates the potential transmission of charge between ZIS-Vs and NiO considering the previously mentioned observations. When the higher E_f (low Φ) ZIS-Vs and lower E_f (high Φ) NiO come into contact in the absence of light, the electrons in ZIS-Vs migrate spontaneously to NiO. The interface region of ZIS-Vs is positively charged as a result of the electron displacement, creating an electron-depleting section, and upward band bending occurs in ZIS-Vs. Conversely, the NiO band edge bends downward as a result of the electrons being acquired, making the NiO interface more negatively charged and enabling the establishment of an electron accumulation sheet. Consequently, at the interface between ZIS-Vs and NiO, a built-in electric field (IEF) is produced from ZIS-Vs to NiO. The aforementioned result is significantly supported by the computed charge density difference (Fig. 11(D)) of the geometric model (as shown) of the composite semiconductor. Photogenerated holes remain in the VB as a result of the photogenerated electrons of ZIS-Vs and NiO being transported from the VB to CB under Vis illumination. Subsequently, the photogenerated electrons of NiO are likely to combine with the photogenerated holes of ZIS-Vs, driven by IEF, band edge bending, and Coulomb contact. As a result, holes accumulate in the VB of NiO, and electrons cluster in the CB of ZIS-Vs. The sacrificial agent consumes the holes, while the electrons efficiently convert H⁺ to H₂. This approach helps boost the redox potential and restrict the spatial reproduction of electron–hole pairs on an individual semiconductor.

Fig. 11 (A) Schematic representation of ZIS-Vs and NiO. (B) Electrostatic potentials of NiO and ZIS-Vs. (C) Capacity for transmitting charges between NiO and ZIS-Vs. (D) Atomic structure models and distinct charge density diagrams of the ZIS-Vs and NiO samples analyzed both before and after exposure to ultraviolet radiation. Reproduced from ref. 220 with permission from Elsevier, Copyright 2024.

Initially, we examined the distinct levels of sulfur vacancy (Sv) in virgin ZnIn₂S₄ (also known as ZIS-X). In agreement with our assumption, ZIS with a moderate Sv content (ZIS-M) was found to be the optimal photocatalyst. Subsequently, this ideal ZIS-M was blended with various MXene co-catalyst ratios to formulate the MXene/ZIS Schottky heterojunction (ZMX). It was hypothesized that the moderate Sv levels in this configuration constitute advantageous electron entrapment. In an attempt to facilitate the successful differentiation and migration of photogenerated carriers, it was predicted that the co-catalyst MXene will conveniently transport photogenerated electrons to the surface. It is anticipated that the collaborative action of Sv and MXene co-catalysis will generate and convey an extensive variety of carriers, which could boost the efficacy of photocatalytic hydrogen generation. Accordingly, four different crystalline simulations of ZIS were conducted, each with a different amounts of Sv, to verify the catalyst design hypothesis (Fig. 12(A–D)). According to theoretical investigations, ZIS-M possesses a superior hydrogen adsorption-free energy. This conclusion is consistent with electron paramagnetic resonance (EPR) examination findings, further supporting the better hydrogen evolution performance of ZIS-M (Fig. 12(E) and (F)). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were applied to convey the shape and structure of ZMX (Fig. 12(G)). The MXene component is responsible for the reported lattice spacing of 0.23 nm, whereas ZIS corresponds to the spacing of 0.32 nm.^223,224

Fig. 12 (A–D) ZIS models without Sv and that with rich, moderate, and poor Sv content. (E) Computed hydrogen adsorption Gibbs free energies for ZIS with varying Sv contents (P, M, and R stand for poor, moderate, and rich Sv content, respectively). (F) ZIS and ZIS-X EPR spectra. (G) HRTEM image of ZMX. Reproduced from ref. 222 with permission from Wiley Online Library, Copyright 2024.

Subsequent research revealed that the IEF during heterojunction accelerates the band bending in the direction of efficient charge separation.²²⁵ Furthermore, it is well-established that the interface between two semiconductors exhibiting different Fermi levels forms an IEF, producing a higher IEF when their Fermi levels deviate considerably.²²⁶ In this instance, the density of states (DOS) was utilized to investigate the electrical band structures of TpPa-1, ZIS, and Sv-ZIS. The C 2p orbital is the primary constituent of the higher edge of VB and the lower segment of CB for TpPa-1, as seen in Fig. 13(A). According to Fig. 13(B), the S 3p orbital contributes to the VB maximum for ZIS, while the In 5s orbital contributes to the CB minimum of ZIS. With the notable exception of a smaller bandgap, the band structure development of Sv-ZIS is comparable to that of ZIS (Fig. 13(C)). The implications of the sulfur-vacancy in the heterojunction, specifically the IEF regulation and the interfacial charge transfer strategy, were eventually evaluated. The distinctions in charge density distribution, as shown in Fig. 13(D–G), suggest that charge buildup proceeds on the TpPa-1 side, whereas charge depletion arises on the ZIS and Sv-ZIS sides adjacent to the interface. This implies that an interfacial IEF originated with guidance from ZIS and Sv-ZIS to TpPa-1. In the ZIS/TpPa-1 contact, the interfacial transmission of charges quantity was calculated to be 0.03e. Particularly, although sulfur-vacancy modification arises, the charge transfer increases to 0.08e in Sv-ZIS/TpPa-1, implying the emergence of an expanded IEF, owing to the greater E_f difference between Sv-ZIS and TpPa-1. Overall, the improved IEF and S-scheme charge transfer interact in conjunction to guarantee more rapid photocarrier migration and separation, thereby boosting the photocatalytic efficiency.

Fig. 13 Computed density of states (DOS) for (A) TpPa-1 (001), (B) ZIS (311), (C) Sv-ZIS (311) and (D) Sv-ZIS (311)/TpPa-1 (001). Computed 3D charge density difference distribution of (E) ZIS (311)/TpPa-1 (001), (F) and Sv-ZIS (311)/TpPa-1 (001) and (G) with an isosurface of 1.5 × 10⁴ e Å⁻³. Reproduced from ref. 227 with permission from Elsevier, Copyright 2024.

5. Applications

As a consequence of its affordability, efficiency, and environmental friendliness, the strategy of photocatalysis exploiting materials made of semiconductors has generally been deemed to be a viable technique for converting pure sunlight energy into chemical energy. Throughout the course of the last 10 years, photocatalysts based on ZnIn₂S₄ have been extensively utilized in photocatalytic water dissociation for the objectives of CO₂ photoreduction, hydrogen evolution, and water pollution remediation. The remainder of this article will provide an explanation of their sophisticated utilization in the domain of ecological restoration and renewable energy from the sun.

5.1. Photocatalytic hydrogen evolution

The capacity of photocatalytic technology to transform materials with little additional value into high-value-added products is regarded as an exceptionally valuable utilization direction that satisfies the necessities of sustainable development to a substantial extent. The solar-driven photocatalytic analysis procedure is easy to understand and is performed at a low temperature, which is different from conventional thermal conversion, which requires difficult experimental circumstances.¹⁷ In the last few decades, scientists have used photocatalytic systems based on ZnIn₂S₄ to change chemicals selectively and make specific organic compounds. Wang et al.¹⁷³ synthesized imidazoles via the photocyclization of easily accessible amines at ambient temperature. The process was completed using sunlight to couple a C–C to a C–N bond, followed by a dehydrogenation process on the surface of an Mo–ZnIn₂S₄ heterogeneous photocatalyst. With the maximum average yield of 96%, a diverse array of amines was transformed into trisubstituted and tetrasubstituted imidazoles. Additionally, Hao's group²²⁸ produced 3,4-dihydroisoquinoline through the semi-dehydrogenation of 1,2,3,4-tetrahydroisoquinoline using the obtained MoS₂/ZnIn₂S₄, which demonstrated excellent photocatalytic activity. Also, a series of MS/ZnIn₂S₄ samples (MS = PtS and NiS) was shown to participate in this reaction, together with MoS₂/ZnIn₂S₄. These results suggest that ZnIn₂S₄ is indeed effective. These studies not only present a method that is easy, cheap, and good for the environment, but also show how powerful photocatalytic reactions can be in the domain of organic compound photocatalytic synthesis.

Fig. 14(A) and (B) compare the H₂ production output of 1% Pt/ZIS by thermocatalysis at 45 °C under dark conditions (TC), photocatalysis upon exposure to full spectrum light bombardment at 20 °C regulated by circulating water (PC), and photothermal catalysis under the full spectrum of radiation without temperature regulation (PTC). A photothermal synergistic catalytic phenomenon was highlighted by the PTC H₂ output rate (∼19.4 mmol g⁻¹ h⁻¹), which was considerably greater than the PC process (∼9.3 mmol g⁻¹ h⁻¹), TC process (∼0.05 mmol g⁻¹ h⁻¹), and their combined rate. Fig. 14(C) displays the influence of Pt accumulation on PTC generation of H₂ during a reaction time of 4 h. The results showed that as the Pt loading increased, the solar-to-hydrogen (STH) efficiency and the rate of H₂ production initially increased, and then declined. The highest STH efficiency (0.63%) and the greatest hydrogen generation rate (19.4 mmol g⁻¹ h⁻¹) were attained with a loading of 1 wt% Pt, which was roughly 15-times greater than that with the assistance of ZnIn₂S₄. By employing a 2 h reaction duration, the influence of common sacrificial agents on PTC H₂ generation was additionally investigated as a significant experimental component. The triethanolamine that was selected outperformed DL lactic acid and Na₂S/Na₂SO₃, as seen in Fig. 14(D).

Fig. 14 (A) Catalytic hydrogen generation curves over time. (B) Hydrogen production rate and solar-to-hydrogen (STH) efficiency after 4 h of reaction over 1% Pt/ZIS under varying conditions. (C) Influence of Pt loading on H₂ production rate and STH efficiency after 4 h of reaction over Pt/ZIS under photothermal catalytic (PTC) conditions. (D) Impact of sacrificial agents on H₂ output rate with a process duration of 2 h over 1% Pt/ZIS under PTC environments. Reproduced from ref. 159 with permission from Elsevier, Copyright 2022.

In this instance of photodecomposition of H₂S, an alkaline reaction system, is generally utilized, resulting in the transformation of H₂S to HS⁻. S₂²⁻ and H⁺ would be generated by the photogenerated pores decomposing HS⁻ during the photocatalytic process. The transformation of H⁺ to H₂ would happen in the presence of electrons. The photodecomposition of H₂S using ZnIn₂S₄ as a photocatalyst under visible light was first documented by Chaudhari et al.¹⁰⁴ ZnIn₂S₄ was precipitated by amplification of a straightforward surfactant-assisted (TEA-assisted) hydrothermal procedure. The most effective catalytic operation yielded a rate of approximately 10 574 μmol g⁻¹ h⁻¹ when the hydrogen generation quantity was considered. Further studies by Chaudhari et al.¹⁰³ utilized PVP and substituted DEA for TEA in the hydrothermal procedure. Without the surfactants, ZnIn₂S₄ displayed a photocatalytic hydrogen evolution rate of approximately 8022 mol g⁻¹ h⁻¹. The PVP-assisted ZnIn₂S₄ demonstrated a hydrogen production rate of approximately 8818 μmol g⁻¹ h⁻¹, whereas the DEA-assisted ZnIn₂S₄ displayed a photocatalytic performance with a rate of approximately 8682 μmol g⁻¹ h⁻¹. According to these results, we can assume that H₂S reduction represents a potentially fruitful strategy for the evolution of hydrogen. Field emission scanning electron microscopy (FESEM) was utilized to examine the structural features of the PVP-assisted ZnIn₂S₄ specimens containing different PVP amounts. The fluctuation in concentration significantly impacted the emergence of hollow marigold flower morphologies. According to the comprehensive assessment, prematurely deformed blooms were observed at a low dose of PVP (100 ppm), as shown in Fig. 15(A and B). By elevating the concentration of PVP to 200 ppm, hollow spheres with a radius of approximately 4–6 μm and a cavity of approximately 2.5 μm emerged. The fascinating result is inflated flowers with hollow chambers when the percentage of PVP was progressively increased to approximately 300 ppm. The petals of the flower were fully developed and clearly differentiated from each other. These hollow, highly permeable flowers function similar to the hollow structure of nanoparticles. Fig. 15(C) shows how multiple neatly packed nanosheets (petals) collaborate to develop a hierarchy of nanostructured hollow blossoms of marigold. A TEM investigation was conducted to investigate the hollow chamber and crystal form of the ZnIn₂S₄ sample, which had been assisted by 300 ppm PVP. The transmission electron microscopy (TEM) image in Fig. 15(D) validates the apparent existence of a hollow structure, complementing the outcomes of the scanning electron microscopy (SEM) results. The transmission electron microscopy (TEM) image revealed the existence of hollow microspheres at high magnification. These microspheres consist of hierarchical nanostructures, built using nanosheets. Remarkably, the extremely delicate hexagonal flower petals of ZnIn₂S₄ measuring approximately 1 μm in length were observed encircling the space inside. The research by Kale et al.²²⁹ looked into how well a ZnIn₂S₄/graphene composite can reduce H₂S using light. The ZnIn₂S₄ and graphene composite demonstrated an H₂ production rate of approximately 6365 μmol g⁻¹ h⁻¹.

Fig. 15 (A–C) FESEM images of the sample synthesized using PVP as a binding agent. (D) Specimen synthesized employing 300 ppm PVP as the binding agent utilizing images obtained from TEM. Reproduced from ref. 103 with permission from The Royal Society of Chemistry, Copyright 2014.

5.2. Photoelectrocatalytic H₂ evolution

The combustion of unsustainable energy sources generates exhaust fumes, which contribute to pollution in the atmosphere. In contrast, hydrogen is a pure and environmentally friendly energy source whose byproduct from its consumption is water. Therefore, hydrogen is considered a hopeful and prospective source of energy for the prospective development of modern society, given that it has the capability to alleviate the energy scarcity. Over the course of the past 10 years, photocatalytic water splitting employing solar light has been demonstrated to be an environmentally benign and exceptionally valuable method for producing clean hydrogen (H₂) fuel. This technique has the potential to significantly eliminate the energy shortage crisis.^230–238 As an outcome, a broad spectrum of semiconductor materials has been consistently evaluated throughout the development of hydrogen evolution photocatalysts. Throughout this investigation, it has been noticed that photocatalysts based on ZnIn₂S₄ demonstrate outstanding efficiency. To meet the requirement for semiconductors to facilitate hydrogen development, their conduction channels need to be negatively charged in contrast to the prospect of reducing H⁺ to H₂ (E_H⁺/H2 = 0 versus NHE at pH = 0). Most significantly, the conduction band edge potential of ZnIn₂S₄ is −1.18 eV, which is favorable for the formation of hydrogen.²³⁴ In addition, two distinct types of hydrogen reactions occur when ZnIn₂S₄ is employed as a photocatalyst for the visible-light production of hydrogen, i.e., H₂S reduction and water splitting.

Pristine CdIn₂S₄ reveals remarkable capacity to split water and create H₂,¹⁷ with a maximum average H₂ production rate of 886.32 μmol g⁻¹ h⁻¹. The mean percentage H₂ output of CdIn₂S₄/Z/C–X hybrids, generated through the inclusion of ZnO/Co₃O₄ nanopowder into the starting material solution of CdIn₂S₄, was in the range of 46.56–87.49%, which was greater than that by a comparable amount of CdIn₂S₄. ZnIn₂S₄ is an integral part of the structural family of sulfur spinel-like elements, MIn₂S₄ (M = Mn and Cd).⁵⁴ Furthermore, it is a quintessential ternary complex derivative whose band gap can be modified to match the visible-light range (2.06–2.85 eV).^13,239,240 ZnIn₂S₄ exhibits photocatalytic hydrogen production activity and chemically stable properties commensurate with this. Kazuhiko Maeda et al.²⁴¹ constructed a Z-scheme ZrO₂–TaON photocatalytic framework in an attempt to improve the performance of TaON in the visible-light photocatalytic production of H₂. Li et al.²⁴² The unprecedented Pt/t-ZrO₂/g-C₃N₄ photocatalyst was synthesized, exhibiting an efficient catalytic hydrogen production amount of 722.5 μmol g⁻¹ h⁻¹ and an ultraviolet (UV) energy conversion effectiveness of 0.215%. Zhang et al.²⁴³ synthesized the S-scheme g-C₃N₄/ZrO₂ heterojunction via conventional calcination, which compared to ZrO₂, exhibited superior photoelectrocatalytic HER efficiency.

Recently, numerous experiments have been accomplished on the use of metal carbides (Mo₂C), phosphides (Ni₂P, Ni1₂P₅), transition-metal hydroxides (Ni(OH)₂), and metal sulfides (MoS₂ and Co₉S₈) as co-catalysts that are mixed with ZnIn₂S₄ to make the process of hydrogen evolution more efficient. In a subsequent investigation, Zeng et al.²⁴⁴ implemented a simple solution-phase technique for producing ZnIn₂S₄/Ni1₂P₅ composite photocatalysts. The results of significant research have also shown that metal sulfides are very important and unique co-catalysts in the fabrication of photocatalysts for the production of hydrogen (H₂).^245–249 To improve the photocatalytic performance and balance, Ye and Wen, Wang et al., and other researchers reported the preparation of similar photocatalysts using analogous semiconductors.^250,251

Shi et al.²⁵² synthesized the MoS₂/ZnIn₂S₄ photocatalyst in an interesting attempt in the field of 2D/2D metal-sulfide photocatalyst production. In the photocatalytic experiment, the MoS₂ (1.5% Pt)/ZnIn₂S₄ sample exhibited a maximum hydrogen production rate of 8898 μmol g⁻¹ h⁻¹. This value exceeded that of pristine ZnIn₂S₄ by 16-times and increased by 2.5-times in comparison to 1.5% Pt-coated ZnIn₂S₄. By promoting electron transfer from the charge-carrier recombination site (CB) of ZnIn₂S₄ to MoS₂, the 2D/2D MoS₂/ZnIn₂S₄ composites showed significantly increased photocatalytic activity. In addition, Mo₂C is a cost-effective co-catalyst that demonstrates a Pt-like electrochemical structure, which accounts for its extraordinary electrical conductivity and abundance of active sites for the creation of hydrogen.^252,253 The manufacturing of Mo₂C–ZnIn₂S₄ composite photocatalysts by means of the insertion of a co-catalyst and heterojunction, and photothermal treatment was originally conceived and reported by Du et al.²⁵⁴ The Mo₂C–ZnIn₂S₄ composite displayed the most advantageous hydrogen fabrication rate of 22 110 μmol h⁻¹ g⁻¹, complemented by an exceedingly high AQY of 71.6% at 420 nm. This value is 3.4-times superior to that of pristine zinc indium sulfide (ZnIn₂S₄). Another effective and easy-to-find strategy for the stronger productivity of catalytic H₂ progression is to create the appropriate heterojunction by adding extra material for semiconductors. It is known that WS₂ has very high intrinsic electrical conductivity, which is one reason why it is widely used in the field of catalysis.^255–258 According to Pudkon et al.,²⁵⁹ the composite photocatalysts made of ZnIn₂S₄ and WS₂ worked better at separating photoinduced electron–hole pairs because of the charge transfer procedure in the type-II heterojunction.

The optimized ZnIn₂S₄/WS₂ heterojunction photocatalyst displayed efficient photocatalytic activity when subjected to UV-visible illumination. The hydrogen evolution rate by this catalyst was 293.3 μmol h⁻¹ g⁻¹, which is much larger than that of purified ZnIn₂S₄ and the physical properties of a mixture composed of ZnIn₂S₄ and WS₂. Additionally, Xiong et al.²⁶⁰ discovered in an additional investigation that the photocatalytic hydrogen generation efficacy of ZnIn₂S₄ can be substantially boosted throughout its interface with few-layer WS₂. The improved WS₂/ZnIn₂S₄ photocatalyst demonstrated extraordinary photocatalytic efficiency without the presence of any co-catalyst. It achieved an effective hydrogen manufacturing rate of 2550 μmol h⁻¹ g⁻¹ when subjected to UV-visible light, and the corresponding AQE of 3.2% at 420 nm. The reference sample consisted of a 1 wt% Pt/ZnIn₂S₄ heterojunction, which demonstrated an H₂ manufacturing amount of 930 μmol h⁻¹ g⁻¹. This observation indicates that WS₂, which is abundant, affordable, and free of noble metals, can potentially serve as an appropriate replacement for Pt. Making heterostructures by combining ZnIn₂S₄ with metal–organic frameworks (MOFs) is also a good idea because it can improve the photocatalytic ability by making it easier for photoinduced charge carriers to move around and separate.

Of particular significance, the MoS₂/ZIS and ZIS/GQDs H₂ production rates culminated at appropriate proportions in the MoS₂10/ZIS and ZIS/GQDs10 samples, exceeding 15.17 mmol h⁻¹ g⁻¹ and 10.57 mmol h⁻¹ g⁻¹ (Fig. 16(A) and (B)), respectively. The aggregation of zero-dimensional GQDs, resulting in the concealment of certain active areas on ZIS,²⁶² and the shadowing impact of excess 2D MoS₂, which diminishes the accessibility of light to the ZIS interface, are responsible for the decline in the hydrogen production activity with a further boost in GQDs or MoS₂. Compared to pristine ZIS, the MoS₂10/ZIS/GQDs10 heterojunction yielded H₂ at an output of 21.63 mmol h⁻¹ g⁻¹ and an aggregate content of 89.1 mmol g⁻¹ in 4 h (Fig. 16(C)), which are 37.3 and 33.8 times more substantial, respectively. The precise separation and distribution of photoinduced carriers attainable by the heterojunction structures are liable for these modifications.^{240,263–269} Furthermore, the MoS₂/ZIS/GQDs samples revealed excellent photocatalytic stability; after five continuous cycles, there was certainly no discernible decline in the formation of H₂ (Fig. 16(D)).

Fig. 16 (A) H₂ generation rate of MoS₂/ZIS samples with various concentrations of MoS₂, (B) H₂ generation rates of ZIS/GQDs samples with varying amounts of GQDs, and (C) H₂ evolution rate of MoS₂/ZIS/GQDs with distinct levels of MoS₂ and GQDs. (D) Recycling assessment of photocatalytic hydrogen development for the MoS₂10/ZIS/GQDs10 sample. Reproduced from ref. 261 with permission from Elsevier, Copyright 2024.

The primary explanation for poor ability of WO₃ to generate hydrogen and decompose RhB, as illustrated in Fig. 17(A and B), is that its CB potential is insufficient to meet the necessary requirements for hydrogen creation. ZIS, RZW, ZW-2, ZW-3, ZW-4, ZW-5, and ZW-6 all have different hydrogen outputs of 104.25, 35.50, 370.50, 400.00, 737.75, 490.50, and 437.75 μmol g⁻¹ h⁻¹, respectively. Considering a value approximately seven times higher compared to pure ZIS, the ZW-4 photocatalyst yielded the largest amount of hydrogen. Furthermore, as shown in Fig. 17(B), ZW-4 also demonstrated full decomposition activity, with a value greater than 99.99%. By employing 0.2 g L⁻¹ ZIS@CCS as the catalyst, researchers conducted gradient experiments with OFL concentrations of 10 mg L⁻¹, 20 mg L⁻¹, 50 mg L⁻¹, 100 mg L⁻¹, and 200 mg L⁻¹ to investigate the influence of OFL concentration on the development of hydrogen rate and the effectiveness of dye elimination. According to the results, ZIS@CCS was inclined to attain equilibrium and exhibited a diminished hydrogen generation rate at moderate OFL concentrations (10 mg L⁻¹, 20 mg L⁻¹, and 50 mg L⁻¹). It demonstrated a long-term enhancement impact on the generation of hydrogen at high doses (100 mg L⁻¹ and 200 mg L⁻¹), as shown in Fig. 17(C). Fig. 17(D) illustrates the photocatalytic formation of hydrogen by the CoAl-LDHs/ZnIn₂S₄ composite utilizing an Xe lamp equipped with a 420 nm long-pass filter, with the associated hydrogen evolution rates. The hydrogen manufacturing yields by all the CoAl-LDHs/ZnIn₂S₄ composite samples surpass that by CoAl-LDHs and ZnIn₂S₄ individually. The photocatalytic activity of ultrathin CoAl-LDHs/ZnIn₂S₄ was 9.7-times greater than that of CoAl-LDHs/ZnIn₂S₄ (bulk), confirming that the ultrathin structure facilitates the transmission of photogenerated charges, thus boosting the photocatalytic degradation capability. Among the samples, ultrathin CoAl-LDHs/ZnIn₂S₄ (1 : 2) displayed the highest photocatalytic H₂ generation activity (1563.64 μmol g⁻¹ h⁻¹), surpassing that of ZnIn₂S₄ by 4.6-times and CoAl-LDHs by 646-times.

Fig. 17 (A) H₂ production as a function of time for individual photocatalysts of WO₃, ZIS, RZW, ZW-2, ZW-3, ZW-4, ZW-5, ZW-6.¹⁴⁵ (B) Graph illustrating hydrogen evolution and degradation by WO₃, ZIS, RZW, ZW-2, ZW-3, ZW-4, ZW-5, ZW-6. Reproduced from ref. 145 with permission from Elsevier, Copyright 2024. (C) H₂ production as a function of time; 10 mg L⁻¹, 20 mg L⁻¹, 50 mg L⁻¹, 100 mg L⁻¹, 200 mg L⁻¹. Reproduced from ref. 270 with permission from Elsevier, Copyright 2024. (D) Rate of photocatalytic H₂ generation for ZnIn₂S₄ and x-CoAL-LDH composite. Reproduced from ref. 49 with permission from Elsevier, Copyright 2023.

5.3. Electrocatalytic water splitting for hydrogen evolution

On an enormous scale, water splitting to produce hydrogen has been recorded. For the purpose of carrying out the photocatalytic splitting of water for hydrogen generation, photocatalysts are typically stimulated by illumination, instantaneously producing electrons and voids. Following this, the photogenerated holes and electrons migrate towards the outermost layer of the photocatalyst. The reduction of H⁺ to H₂ would take place by means of the photogenerated electrons. Conversely, electron donors such as S²⁻/SO₃²⁻,^47,65 lactic acid,²⁷¹ glucose,²³⁴ triethanolamine,^40,155,180 and ethanol amines²⁰⁴ consume holes. To split water, in broad terms, it is admitted that the band gap of semiconductors must surpass 2 eV, considering the energy losses and the reduction potential of H⁺ to H₂.²⁷² It has been demonstrated that ZnIn₂S₄ generally exhibits an adjustable band gap energy in the range of 2.06–2.85 eV, a value that is highly complementary with water splitting for hydrogen generation.

The most significant feature of water splitting utilizing a finely ground photocatalyst is its lack of complexity, as illustrated in Fig. 18(A)(a). Direct sunlight illuminates photocatalyst particles dispersed in a water reservoir, effectively generating hydrogen. The requirement to separate the hydrogen (H₂) generated by the oxygen (O₂) in the technique of photocatalytic water splitting is a drawback. However, a Z-scheme photocatalyst can resolve this issue. Furthermore, the use of a powdered photocatalyst system provides significant benefits for the widespread implementation of solar water splitting due to its straightforward nature. Photocatalytic water splitting is a highly appealing reaction that has the potential to significantly upgrade green, viable chemical processes, tackle energy and environmental challenges, and ultimately lead to an energy source transformation. The semiconductor ZnIn₂S₄ photocatalyst possesses an appropriate crystalline structure and band structure, which is worthwhile for the development of incredibly effective PHE. To achieve the large-scale production of hydrogen, a comprehensive recognition of PHE and catalytic efficiency techniques is mandatory. Fig. 18(A)(b) presents the corresponding mechanism scenarios.

Fig. 18 (A)(a and b) Graphical representation of the mechanism of water dissociation for PHE employing a particle semiconductor photocatalyst. Demonstration of the PHE mechanism in a semiconductor photocatalyst. Reproduced from ref. 233 with permission from The Royal Society of Chemistry, Copyright 2009. (B) Photoanode of the photoelectrochemical water-splitting reaction is a charge transfer technique involving MWCNT/H–ZnIn₂S₄. Reproduced from ref. 273 with permission from Elsevier, Copyright 2023. (C) Visual representations of dZni-ZIS nanosheets employed in photocatalytic overall splitting of water propelled by solar energy. Reproduced from ref. 274 with permission from Elsevier, Copyright 2022. (D) Representation of the Cu/ZIS/TiO₂ (VO) photocatalytic entire water dissociation mechanism functioning under simulated solar conditions. Reproduced from ref. 275 with permission from Elsevier, Copyright 2024.

The conduction band (CB) edge potential of H–ZnIn₂S₄ (−0.55 V) is significantly lower than the Fermi value of MWCNTs, indicating the exceptional transmission of charged particles from H–ZnIn₂S₄ to MWCNTs. UV-light energizes the electrons in H–ZnIn₂S₄, promoting them to the conduction band (CB) and creating vacancies in the valence band (VB). The unoccupied states in the valence band (VB) of H–ZnIn₂S₄ are capable of participating in the oxygen evolution reaction (OER), while the electrons triggered by light in the conduction band (CB) of H–ZnIn₂S₄ can be transferred to multi-walled carbon nanotubes (MWCNTs). Subsequently, these electrons can conveniently escape towards the counter electrode through an external circuit and engage in the hydrogen evolution reaction (HER), as depicted in Fig. 18(B). Multi-walled carbon nanotubes (MWCNTs) demonstrate extraordinary electron transport characteristics as a function of their elevated electrical conductivity. MWCNTs, when connected with H–ZnIn₂S₄, operate as successful paths for transporting photogenerated electrons far away from the interface, thus restricting their recombination with holes. H–ZnIn₂S₄ is known for having the capacity to capture light and develop electron–hole pairs. The effective differentiation between electrical charges boosts the all-encompassing generation of the light current. The hypothesized mechanism for the photocatalytic overall separation of water by dZni-ZIS nanosheets is illustrated in Fig. 18(C). The dZni-ZIS nanosheets, featuring appropriate band gap configurations, are capable of gathering UV-vis light from sunshine and demonstrate adequate potentials for comprehensive water splitting. The electrons and holes generated by photoexcitation can be immediately segregated and conveyed to the reaction sites to commence redox reactions. The expanded interlayer spacing significantly diminishes the van der Waals interactions, facilitating the manifestation of monolayer characteristics to a degree, resulting in dZni-ZIS nanosheets with an increased relative surface region and interface-active sites. The mechanism for the photocatalytic reaction on Cu/ZIS/TiO₂(VO) was proposed (Fig. 18(D)). Under simulations including sunshine exposure, both TiO₂(VO) and Cu/ZIS may create photogenerated electrons, and afterwards relocate them to their corresponding conduction networks due to their acceptable band gaps.

The earliest documentation of ZnIn₂S₄ being employed for the photocatalytic production of hydrogen via water splitting was in 2003 by Lei et al.²³ The photocatalyst in the hydrogen generation reaction was a cubic-shaped sphere of Znln₂S₄. The exposed ZnIn₂S₄ generated hydrogen at a conventional average rate of 57 μmol g⁻¹ h⁻¹ upon loading Pt on the outermost layer of ZnIn₂S₄, and the maximum photocatalytic performance was reported to be 257 μmol g⁻¹ h⁻¹. Although ZnIn₂S₄ displays a band gap that is favorable for the production of hydrogen from water, this material is still constrained by its structural shortcomings, suboptimal separation probability, and inadequate migration potential of photogenerated charge carriers. As a result, numerous approaches have been established in an effort to boost the photocatalytic yield of ZnIn₂S₄, such as taking advantage of its structure and appearance and transforming its surface. By transforming the outermost layer and optical characteristics of semiconductors, increasing the charge transfer, and decreasing the generation of charged particles, these techniques can improve the effectiveness of photocatalytic systems.

Hydrogen generation depending on ultraviolet light exposure (λ > 400 nm) is utilized to evaluate the photocatalytic efficiency of materials. In a study, TEOA and 1 wt% Pt were selected as the sacrificial solution and co-catalyst, respectively. When the photocatalysis procedure was executed, the uninterrupted emission of H₂ was noticed throughout each catalyst, as seen in Fig. 19(A). Only a small concentration of H₂ (4.7 mmol g⁻¹ for CN and 6.0 mmol g⁻¹ for ZIS) was identified when CN or ZIS was employed alone, revealing their inadequate photocatalytic performance. Significantly more H₂ was generated upon doping CN with S, which can be attributed to its greater capacity to absorb light, as shown by the UV-vis spectral analysis. After the formation of the composite, ZIS@CN and ZIS@SCN produced more hydrogen than the ZIS and CN/SCN photocatalysts working alone. This shows that the heterojunction was successfully constructed. Fig. 19(B) displays the mean evolution of the hydrogen rate across multiple photocatalysts. Compared to the ZIS/CN photocatalysts, all the ZIS/SCN photocatalysts displayed considerably greater activity, with their H₂ evolution efficiency depending on the SCN ratio. In particular, the addition of SCN induced an increase in the generation of H₂ up to a maximum of 100 mg, after which the activity gradually decreased as the intake of SCN increased from 100 mg to 150 mg. It is expected that the overwhelming inclusion of SCN may cause the porous structures in ZIS to be blocked. Consequently, ZIS@100SCN exhibited the maximum H₂ yield rate of 9.3 mmol h⁻¹ g⁻¹, which was 1.87-times greater than that by ZIS@100 CN. Notably, ZIS@100 CN generated H₂ at an approximately 2.1-times and 1.66-times higher rate than purified ZIS and CN, whereas ZIS@100SCN generated H₂ roughly 3.1-times and 2.19-times higher than purified ZIS and SCN, respectively. Consequently, the ZIS@SCN heterojunction showed superior activity to ZIS@CN. Four subsequent experiments were executed to attempt to evaluate the reusability of ZIS@SCN. Following each run, the photocatalysts were collected and cleaned with distilled water. For recycling, the catalysts were subsequently dried at 60 °C. In the four cycles, the catalyst retained over 90% efficiency, demonstrating the strong stability of the ZIS@SCN catalyst (Fig. 19(C)). Subsequently, the photocatalyst was utilized with 1 wt% Pt as a co-catalyst for overall water splitting. Stable H₂ and O₂ were produced with a stoichiometric ratio of 2 : 1 for ZIS@SCN, as anticipated (Fig. 19(D)). In addition, in comparison to other described heterojunctions, it displayed greater photocatalytic activity in total water splitting.^277,278 Thus, the ZIS/SCN heterojunction shows excellent potential in water splitting based on all these observations.

Fig. 19 (A) Three times repeated photocatalytic H₂ evolution, (B) three times repeated hourly average H₂ generation, (C) variation in H₂ cyclic testing, and (D) ZIS@100SCN water splitting overall. Reproduced from ref. 276 with permission from Elsevier, Copyright 2024.

5.4. Pollutant elimination

Currently, many countries and regions worldwide are dealing with extreme problems related to environmental pollution caused by many different types of poisonous pollutants. Environmental contaminants, which include organic compounds, dyes, and heavy metals, have a tendency to be resistant to natural degradation processes and represent significant dangers to human health. Thus, the utilization of photocatalytic technology for eliminating these dangerous contaminants is both encouraging and environmentally advantageous due to its effectiveness in terms of cost, straightforward application of solar energy, the elimination of secondary pollution, and outstanding efficiency. In the last few decades, numerous assessments have been dedicated to the development of ZnIn₂S₄-based photocatalysts for environmental remediation. Dyes, which are prominent pollutants, are composed of –N double bonds and are distinguished by their resistance to both decomposition and elimination.^279,280 For example, Chen et al. created ZnIn₂S₄ microspheres that are similar to marigolds using a very straightforward hydrothermal technique.⁶³ As expected, the synthesized ZnIn₂S₄ could degrade Congo red (CR), methyl orange (MO), and rhodamine B (RhB) using visible light. Recently, it was established that a significant number of OH radicals participate in the photocatalytic decomposition of these colored substances. An additional investigation conducted by Shi et al. indicated that a ZnIn₂S₄ photocatalyst, which was co-catalyzed with carbon dots (CDs), displayed improved catalytic characteristics during its application in the decomposition of methyl blue (MB), MO, and RhB.⁴⁰ The optimized CDs/Znln₂S₄-blended substance might entirely break down MO in 40 min, and its continual deterioration was 2.34-times that by ZnIn₂S₄. In contrast, the catalytic decomposition rates of MB and RhB on CDs (5)/ZnIn₂S₄ were 63% and 92%, respectively, following 40 min of ultraviolet (UV) bombardment.

Photocatalysis is a scientific procedure for removing hazardous compounds from water contamination. This method is considerably better than traditional approaches simply because it avoids the generation of additional contamination and transforms poisonous substances into environmentally beneficial compounds.^281–283 This degradation activity is the result of manipulating the redox capacity to absorb photogenerated electrons.^284,285 Gao et al. synthesized a polyvinylidene fluoride (PVDF) compound with zinc sulfide (ZIS) using an unpredictable reactor and a photocatalytic membrane that comes with the reactor setup.²⁸⁶ The catalyst exhibited activity to detoxify 57% of the total organic carbon (TOC) during a 3 h reaction. They used the hydrothermal technique to build samarium (Sm)-doped ZIS and used the accumulated chemical base to successfully adsorb rhodamine B dye by applying –N(Et)₂ units.¹⁷⁴ Extensive environmental pollution has been caused by hazardous, harmful substances, and thus photocatalysis based on semiconductors is an exciting opportunity for the removal of this contamination.^287,288 ZnIn₂S₄ has demonstrated favorable efficiency in facilitating the removal of pollutants as a visible-light-responsive photocatalyst. For example, Hu et al.²¹ seemed to be the first to utilize ZnIn₂S₄ in the dissociation of methylene blue (MB), demonstrating its feasibility in the field of photocatalytic degradation under sunlight radiation. Nevertheless, the photocatalytic performance of ZnIn₂S₄ can be increased. As a consequence, a significant contribution has been allocated to boosting the photocatalytic degradation activity of ZnIn₂S₄. Several techniques for optimizing the photocatalytic activity of ZnIn₂S₄ in the elimination of pollutants are discussed in the following sections. These techniques include controlling its composition and morphology, doping elements and fabricating composites using ZnIn₂S₄. Furthermore, the application of ZnIn₂S₄-based composite materials and ZnIn₂S₄ micro/nanostructures for the eradication of pollutants is detailed in Table 2.

Table 2 Pollutant elimination using ZnIn₂S₄-based composites

Semiconductor	Synthesis technique	Morphological characteristics	Pollutants	Catalytic decomposition action	Propagation parameter for uncoated ZnIn2S4	Ref.
Fe–ZnIn2S4	Hydrothermal	Microspheres	2,4,6-Tribromophenol	0.436 min^−1 (k)	1.11	169
ZnIn2S4/CdIn2S4	In situ hydrothermal	Flowers like spheres	Atrazine	99.82%	73.78%	289
Tremella-like ZnIn2S4/graphene	Hydrothermal	Flower-like microsphere	Dopamine	0.001 Mm	N/A	290
Mo-ZIS/Ni–Ni HCP	Wet chemical solvothermal	Nanoflower-like structure	Hydrogen production process of ZIS	26.7 mmol g^−1 h^−1	N/A	130
ZIS/TiO2	Hydrothermal	Novel core–shell-like structured	Photocatalytic hydrogen evolution	65.40 m^2 g^−1, 1.42 μA cm^−2	2.15	131
ZnIn2S4@In(OH)3@CdS	ZIS hydrothermal growth and CdS CBD growth	Flower-like microsphere	H2 production CdS nanoparticles	1384 μmol g^−1 h^−1	N/A	291
ZrC@ZnIn2S4 (ZrC@ZIS)	Photothermal-assisted	The floral microspherical shape was achieved by stacking nanosheets	H2 production	32.87 μmol g^−1 h^−1	43.54%	292
PdS-decorated ZnIn2S4 (PdS-ZIS)	Facile hydrothermal and in situ deposition	Flower-like micro spherical	H2 evolution	4-MPD selectivity (>99%)	N/A	289
RP/ZnIn2S4	One-step hydrothermal	Flower spheroidal morphology	Antibiotic removal	85.1%	Z-type charge transfer mechanism	293
Zn0.01Co0.99Se2/ZnIn2S4	Hydrothermal	Rhombic dodecahedrons	H2 production	26.48 μmol g^−1 h^−1	Light absorption capacity	289
KNbO3/ZnIn2S4	Through a combination of hydrothermal, thermal therapy, and hard template procedures	Hollow KNbO3 spheres	Oxytetracycline hydrochloride (OTCH), Ciprofloxacin (CIP), and Rhodamine B (RhB) remediation	99.8%, 96.8% and 97.5%	N/A	289
				97.8%
ZnIn2S4/NiMoO4	Electrostatic self-assembly	Short rod-like NiMoO4 and micro flower-like spherical ZnIn2S4	Hydrogen evolution	173.09 μmol	Electron reduction ability of the outer layer of catalyst	294
				3461.8 μmol g^−1 h^−1
CNT–ZnIn2S4	One-step hydrothermal	Nanosheet-assembled flower structure	H2 evolution	1185 μmol g^−1 h^−1	Photoinduced charge-carrier separation	295
Co9S8/N,S–CNTs–ZnIn2S4 (CSCNs–ZIS)	S-doped carbon nanotubes	N/A	Hydrogen production	2409.2 μmol h^−1 g^−1 0.5 wt% Fe–ZnIn2S4	N/A	292
Sm–ZnIn2S4	Hydrothermal	Microspheres	RhB	0.5 wt% Fe–ZnIn2S4 0.057 min^−1 (k)	2.7	174
ZnIn2S4 NSs/SnO2 QDs/TiO2 NTs (ZST NTs)	In situ dipping, and hydrothermal	SnO2 QDs/TiO2 NTs	Implementing PCP for 316 SS that is utilized in aquatic applications	28 μA cm^−2 to 715 μA cm^−2	N/A	296
1D@2D/2D WO2.72/ZnIn2S4	Oil bath	Resembling a ball of flowers	Hydrogen evolution	8.12 mmol g^−1 h^−1	37.5%	297
1D cotton fiber@ZnIn2S4 (CF@ZIS)	One-pot hydrothermal	Microsphere shape originated from a large number of incredibly thin nanosheets	H2 production	278.9 μmol h^−1 g^−1	50%	151
ZnIn2S4/Ir composites	Hydrothermal	Flower-like morphology	Photocatalytic degradations of RhB and MO	0.0305 min^−1	3.6	298
				0.0285 min^−1	2.8
2D/2D CoAl-LDHs/ZnIn2S4	Electrostatic attraction	Nanosheet structures	Hydrogen production	1563.64 μmol g^−1 h^−1	N/A	49
0D/3D CoS2/ZnIn2S4	One-photon excitation path system, mild and facile water-bath	Micro-flower's structure	Hydrogen evolution	22.24 mmol h^−1 g^−1	14.9%	299
CDs/ZnIn2S4	Hydrothermal	Microspheres	Methyl orange	2% Sm–ZnIn2S4	2.34	40
				100% (40 min)
ZnIn2S4/Polymer	Hydrothermal	Nanosheets	Methyl orange	92% (120 min)	1.15	201
				ZnIn2S4 (50 wt%)/polymer
TiO2@ZnIn2S4	Hydrothermal	Core–shell	Methyl blue	91% (4 h)	1.2	191
Fe2O3–ZnIn2S4 FeOOH–ZnIn2S4	Hydrothermal	N/A	2,4,6-Tribromophenol	Fe2O3(10%)–ZnIn2S4 0.00861, 1.59 min^−1 (k)	N/A	194
			UV, visible	FeOOH(10%)–ZnIn2S4 0.00599, 1.52 min^−1 (k)
ZnIn2S4@Gr	Solvothermal	Microspheres	Phenol	0.00861, 1.59 min^−1 (k)	8.4	43
				3.03 h^−1 (k)
ZnIn2S4–graphene	Wet chemistry	Sheet-like structure	4-Nitroaniline	ZnIn2S4–3% GR	N/A	198
ZnIn2S4/rGO	Hydrothermal	Microspheres, nanosheets	4-Nitrophenol	97.8% (360 min) ZnIn2S4/rGO-1.5%	2	205
TiO2@ZnIn2S4	Hydrothermal	Core–shell	Methylene blue	90% (3 h)	1.3	195
CuO/ZnIn2S4	Impregnation–calcination	Microspheres		97% (3 h)	1.4
ZnIn2S4/g-C3N4	Hydrothermal	Nanoparticles nanosheets	Rhodamine B	100% (20 min) ZnIn2S4/g-C3N4-20	N/A	195
ZnIn2S4/CdIn2S4–CC	Hydrothermal	Nanosheets	Methylene blue	96% (50 min)	N/A	300
ZnIn2S4/g-C3N4	Hydrothermal	Flower-like particles	2,4-DichlorophenoXyacetic	ZnIn2S4/CdIn2S4 (20%)–CC 0.0129 min^−1 (k)	6.45	41
P–C3N4/ZnIn2S4	Solvothermal	On nanosheets	Acid 4-nitroaniline	20% ZnIn2S4/g-C3N4 99.4% (90 min) 20 wt% P–C3N4/ZnIn2S4	2.56	42
		Sphere-like structures
ZnFe2O4–ZnIn2S4	Solvothermal	Microspheres	4-Nitrophenol	1.25 h^−1 (k) 2.5 wt% ZnFe2O4–ZnIn2S4	16.4	42
ZnIn2S4/Bi3TaO7	Solvothermal (ethanol, 230 °C, 24 h)	N/A	Tetracycline	0.00775	2.3	301
ZnIn2S4/CdIn2S4	Chemical co-precipitation	Microspheres	Methyl orange	0.046 min^−1 (k)	2.22	302
CdS/ZnIn2S4/RGO	Solvothermal synthesis	Helical structure	Methylene green	100% (2 min) CdS/ZnIn2S4/RGO-10%	N/A	303
ZnIn2S4/Bi4Ti3O12	Solvothermal (water/glycerol, 80 °C, 4 h)	N/A	Tetracycline	0.02234	2.8	304
g-C3N4/Znln2S4	Hydrothermal	Microsphere	TC	100% (120 min)	22.8	305
				50 wt% g-C3N4/ZnIn2S4
ZnIn2S4/BiOCl/FeVO4	Liquid phase mixing	N/A	Rhodamine B	0.12415	13.4	306
AQS/PPy-ZnIn2S4	Hydrothermal	Particles	Tetracycline	N/A	2	200
PPy–ZnIn2S4	Hydrothermal	Microstructures	Methylene blue, CHL	N/A	N/A	199
Ag3PO4/g-C3N4/Znln2S4	Hydrothermal & deposition–precipitation	Sheet	Tetracycline	83% (60 min) ACZ-3	4.5	196
ZnIn2S4/UiO-66	Hydrothermal	Flower-like microspheres	Cr(IV)	min^−1 (k)	2.9	197

---

Fig. 20(A) provides a simplified illustration of the electron flow and decomposition mechanisms. Upon exposure to solar radiation, both Ta₃N₅ and ZnIn₂S₄ materials can effectively produce excited electrons in their conduction bands (CB). Nonetheless, when uncoated Ta₃N₅ behaves as a photocatalyst, the photogenerated electrons and holes are susceptible to interaction throughout the chain reaction phase. As a result, bare Ta₃N₅ demonstrates diminished photocatalytic degradation efficacy owing to the restricted involvement of photogenerated electron–hole pairs in the photocatalytic procedure. The superior photocatalytic property observed for TN@ZIS nanocomposites is due to the interface charge transfer between Ta₃N₅ nanorods and ZnIn₂S₄ tiny sheets. The interfacial charge transfer is enhanced by the establishment of a typical type-II heterojunction, featuring a compact interface, consistent with the concepts of band alignment. The photogenerated holes in Ta₃N₅ are instantly transferred to the valence band (VB) of ZnIn₂S₄, mitigating the significant recombination of electron–hole pairs. The holes generated in the valence band of ZnIn₂S₄ facilitate the oxidation of H₂O, culminating in the emergence of reactive oxygen species (ROS) such as ˙OH, which play an integral part in the course of decomposition. Considering the results mentioned previously, the potential type-II heterojunction mechanism for the photodegradation activity by ZnIn₂S₄/g-C₃N₄ is schematically presented in Fig. 20(B). The addition of BQ and IPA quenchers substantially lowered the quantity of photodegradation of SMX, demonstrating that O₂˙⁻ and OH˙ active components playing a predominant role in the catalytic reaction. Conversely, the incorporation of AO did not result in any significant decomposition, suggesting that the holes are not the primary reactive species responsible for the degradation of organic pollutants. Consequently, O₂˙⁻ and OH˙ radicals accounted for the considerable drug elimination upon exposure to visible-light radiation. The fundamental degradation mechanism is illustrated in Fig. 20(C). Enhancing the chemical reaction specificity of the reactant, while sustaining conversion is facilitated by the redox potential and band edge orientation of semiconductor photocatalysts in water. Reactive oxygen species (ROS) originate through the diminution of oxygen (O₂) in the conduction band by photoinduced electrons, while hydroxyl radicals (˙OH) are formed by the photoinduced holes, which have a considerable oxygen-dependent potential. It was shown by researchers, as depicted in Fig. 20(D), that reduction and desorption are the main ways that hexavalent chromium is completely removed by the ZS-1-packed structure of three-dimensional foam. More specifically, minimizing pollutants is critical for the entire water purification procedure. In the absence of light, operations such as complex interfacial assembly and interaction with electrostatic forces may trap a small percentage of Cr(VI) on the blended foams. Because the polysaccharide matrix has a hydrophilic surface, the electrons that are stimulated and released by the uniformly dispersed photocatalysts can migrate rapidly to Cr(VI), triggering its removal in solution (E(Cr(VI)/Cr(III) = 0.57 eV)).^311,312

Fig. 20 (A) Photocatalytic degradation of TC/MO using Ta₃N₅@ZnIn₂S₄. Reproduced from ref. 307 with permission from Elsevier, Copyright 2024. (B) Diagram depicting the procedure wherein the SMX pollutant is broken down using the ZnIn₂S₄/g-C₃N₄ catalyst. Reproduced from ref. 308 with permission from Elsevier, Copyright 2023. (C) Proposed photocatalytic pathway of heterojunction composites CB/ZnIn₂S₄ in the presence of sunlight. Reproduced from ref. 309 with permission from Elsevier, Copyright 2021. (D) Photocatalytic route of Cr VI to Cr III. Reproduced from ref. 310 with permission from Elsevier, Copyright 2023.

As a consequence of its abundance and consistent chemical characteristics, titanium dioxide (TiO₂) has been extensively implemented in the evolution of hydrogen and the decomposition of contaminants. However, the photocatalytic capabilities of TiO₂ are constrained by its large band gap, low dissociation rate of photoinduced carriers, and primary absorption of UV light.³¹³ By virtue of its 2D layered structure, compatible band gap, and superior reduction capacity of conduction band electrons, g-C₃N₄ is widely employed for hydrogen production, pollutant decomposition, and additional photo-oxidation reactions.^314–316 Substantial interfacial connections, special optical characteristics, and an appropriate band gap are all distinctive characteristics of the 2D ultrathin layered structure of ZnIn₂S₄.^317–319 ZnIn₂S₄ may better mitigate carrier recombination and couple with an extensive selection of appropriate semiconductors. Thus, an outstanding technique to achieve a reduced carrier recombination rate and enhanced photocatalytic performance is to implement a graphene aerogel as the carrier and integrate these three semiconductors to derive a ternary heterojunction graphene aerogel.

The photocatalytic performance of the synthesized photocatalyst was evaluated by employing TC as the target pollutant. The researchers conducted numerous recycling experiments to investigate the stability and reusability of the ZIS/MOF-525-3 photocatalyst, which showed the best overall performance in terms of pollutant degradation. The catalyst was merely purified and utilized after each degradation experiment, rinsed with pure water, heated in an oven set at 80 °C, and then the degradation experiment was performed. The catalyst maintained a decomposing efficiency of 84%, irrespective of a modest decline following the first cycle.³²⁰ The gradual decline in the adsorption and eliminating ability of the subsequent degradation assessments was triggered by the fact that the TC retained on the surface of the ZIS/MOF525-3 material was partially eliminated when the material was reconstructed. Nevertheless, the photocatalytic performance of ZIS/MOF525-3 did not change much over the course of five cycles, and the rate of degradation did not change much between the first and fourth cycles. According to the above-mentioned data, the ZIS/MOF525-3 photocatalyst was stable and suitable for several cycles.

5.5. Photocatalytic CO₂ reduction

The substantial reliance on non-renewable energy sources has significantly contributed to air pollution and climate issues, in addition to the energy crisis. A factor contributing to the current global climate change circumstances is the atmospheric accumulation of excessive carbon dioxide (CO₂). Although hydrogen is an acceptable substitute for fossil fuels for addressing energy-limited availability, the photocatalytic transformation of CO₂ into renewable energy or precious organic compounds is also an appealing way to resolve the problem of energy scarcity. With regard to the band gap and CB potential of Znln₂S₄, the photocatalytic reduction of CO₂ appears to be feasible. In recent times, numerous publications^13,120,321 have described ZnIn₂S₄ and its derivatives nanomaterials for the photoreduction of CO₂. Chen et al.³⁴ used ZnIn₂S₄ to turn CO₂ into formic acid in the presence of ultraviolet light. Consequently, one of the most desirable options is the photocatalytic exploitation of sunlight to convert carbon dioxide (CO₂) into energy-containing elements, which include methanol (CH₃OH), carbon monoxide (CO), and ethanol (C₂H₅OH). This technique not only reuses atmospheric CO₂ directly through the incorporation of direct sunlight energy but also complies with the requirement for renewable fuels.^231,322,323 In the last decade, the ZnIn₂S₄ photocatalyst has attracted consideration as a potentially practical option for the successful and specialized conversion of carbon dioxide to solar renewable energies by means of H₂O. Also, pure methanol was used as the solvent. Methanol is mostly used as both a reagent to react with the holes that are excited by light and a medium to absorb CO₂. Alternatively, methanol was used as a reactant in the esterification of the formic acid that was produced on the CB of Znln₂S₄. The observations demonstrated that the methyl formate production yield for hexagonal ZnIn₂S₄ was 762.36 μmol g⁻¹, whereas that for cubic ZnIn₂S₄ was 629.62 μmol g⁻¹. Furthermore, Mohamed et al.³⁵ established palladium as the outermost layer of ZnIn₂S₄ to improve its CO₂-reducing efficacy. The mechanism that developed when ZnIn₂S₄ was exposed to light photons and CO₂ was as follows: CO₂ + 4H₂O → CH₃OH + 3O₂. Consequently, the methanol concentration generated served as a metric for the amount of CO₂ reduced. It was determined that the photocatalytic effectiveness of the Pd/ZnIn₂S₄ nanocomposite was better than that of the untreated Znln₂S₄. The amount of methanol produced was investigated for the series of 0.5 wt% Pdphoto–oxidation reactionZnln₂S₄, 1.5 wt% Pd–ZnIn₂S₄ and 2.0 wt% Pd–ZnIn₂S₄. Among them, 1.5% Pd/ZnIn₂S₄ exhibited the best activity for reducing CO₂ because its band gap was only 2.20 eV, which is much smaller than that of other materials. In comparison, the unmodified ZnIn₂S₄ demonstrated a band gap of 2.78 eV. Furthermore, its UV-light absorbance increased from 460 to 569 nm. Consequently, the enhanced functioning of photocatalysis observed can be attributed to the synergistic impact resulting from the higher capacity for capturing light and the successful segregation of photogenerated electron–hole pairs. In addition, Jiao et al. used defect engineering to prepare the ZnIn₂S₄ photocatalyst, which led to the effective distribution of ZnIn₂S₄ with lots of zinc vacancies.¹²⁰ This study found that ZnIn₂S₄ with abundant zinc vacancies was better at using light to break down carbon dioxide than ZnIn₂S₄ with few zinc vacancies. It was suggested that the introduction of defects may function as trap states, capturing the electrons that are photoexcited, and consequently triggering the segregation of electron–hole pairs. Consequently, the enhanced capability for photocatalytic CO₂ reduction was attributed to the abundance of incorporated zinc vacancies. Based on the preceding analysis, researchers proposed a Z-scheme mechanism for clarifying the enhanced efficiency for the reduction of CO₂ by the ZIS/BTO photocatalyst (Fig. 21(A)). Upon exposure to visible-light stimulation, ZIS and BTO create both electrons and holes. The electrons in the conduction band of ZIS are successfully transmitted across the ZIS/BTO heterojunction interface, where they establish connections with holes in the valence band of BTO. Consequently, the higher-energy electrons in the conduction band of ZIS can facilitate the reduction of CO₂, in contrast to the higher-energy holes in the valence band of BTO, which are employed for H₂O oxidation. As a result, the direct Z-scheme heterojunction structure preserves elevated reduction and oxidation capacities, thus boosting the PCR effectiveness. Fig. 21(B) presents a synopsis of the mechanisms involved in the photocatalytic mitigation of CO₂. Following exposure to sunlight, the photogenerated electrons from Bi₂WO₆ and ZnIn₂S₄ migrate to the conduction band, while the holes endure in the valence band. The photogenerated electrons in the conduction band of Bi₂WO₆ recombine with holes in the valence band of ZnIn₂S₄, facilitated via an interface electric field. This Z-scheme heterojunction successfully segregates the photogenerated electrons and holes. The accumulating electrons in the conduction band of ZnIn₂S₄ show an aggressive reductive capacity, facilitating the conversion of CO₂ into CH₄, CO, and numerous other contaminants. TEOA depletes the valence band pores of Bi₂WO₆. The Z-scheme charge transfer mechanism is crucial in boosting the productivity for the photocatalytic mitigation of CO₂.

Fig. 21 (A) Diagram depicting the electron transmission process in the direct Z-scheme ZIS/BTO heterojunction structure in the photocatalytic reduction of CO₂. Reproduced from ref. 324 with permission from Elsevier, Copyright 2024. (B) Mechanism illustration of CO₂ elimination under sunlight on Bi₂WO₆/ZnIn₂S₄. Reproduced from ref. 325 with permission from Elsevier, Copyright 2024. (C) Mechanism illustration of CO₂ elimination and PhCH₂OH oxidation on IO/ZIS microtube. Reproduced from ref. 298 with permission from Elsevier, Copyright 2024. (D) Methodology suggested for the photocatalytic reduction of CO₂ on ZnIn₂S₄/TiO₂. Reproduced from ref. 326 with permission from Elsevier, Copyright 2023.

When ZIS and IO come into contact at the interface in an illuminated background, internal electrons will migrate from ZIS to IO based on the differences in their Fermi level and carrier concentration. ZIS loses its inherent electrons with a positive charge, causing upward band bending until the Fermi points of IO and ZIS reach equilibrium. As IO absorbs electrons that are negatively charged, the band bends downward. The positive and negative charges at the interface will cause an internal electric field to develop, pointing from ZIS to IO. PCCs, encompassing electrons and holes that are photogenerated, are created on the outermost portion of IO and ZIS when exposed to sunlight, as illustrated in Fig. 21(C). Photogenerated electrons may accumulate on the ZIS (outer surface) CB owing to the Coulomb attraction force and internal electromagnetic field connection. Their decline pathway involves the conversion of *CO₂ to *COOH intermediate, which is subsequently transformed into *CO. They have a considerable possibility of elimination. Fig. 21(D) illustrates that under visible light, both ZIS and TiO₂ are activated to generate electrons and holes, respectively. To promote the integration of holes in the EVB of ZIS, the electrons excited by sunlight in the ECB of TiO₂ rapidly travel to the EVB of ZIS. Finally, results in the generation of electrons in the conduction band of TiO₂ and holes in the valence band of ZIS. The electron-transfer system TC MXene effectively distinguishes photogenerated electrons in ZIS and holes in TiO₂. Electrons transition from the valence band to the conduction band in TiO₂ and ZIS due to the incorporation of the TC MXene in ZIS@TO/TC-10.

Recently published studies have included the cooperation of ZnIn₂S₄ with various additional semiconductors to enhance its photocatalytic carbon dioxide (CO₂) reduction capability, beyond that of noble metal modification, defect engineering, and structure control. Swift transmission of charge is conceivable, and the dissociation of photogenerated pairs of electrons and holes can potentially be enhanced by the establishment of a heterojunction between semiconductors. By using Bi₂S₃ as a sensitizer, Chen et al.³⁶ created a Bi₂S₃–ZnIn₂S₄ nanocomposite. Purified methanol was utilized for conducting the photocatalytic reaction, serving as both the medium of operation and reductant. Methyl formate generation was considered a measure for assessing the photocatalytic capacity. Some studies showed that the Bi₂S₃–ZnIn₂S₄ nanocomposite was better at using light to facilitate reactions than pure Znln₂S₄. The Bi₂S₃–Zn₂S₄ nanocomposite was observed to have a broader light absorption region extending from 600 nm to 800 nm. This observation suggests that the incorporation of Bi₂S₃ into the freshly prepared composite enhanced its light-capturing capability.

It was possible for photogenerated charge carriers to move quickly because of the close contact between Bi₂S₃ and Znln₂S₄. This made the photocatalytic system work better. Yang et al.³²¹ also made a Z-scheme heterostructure, Znln₂S₄/TiO₂, and tested how well it could use light to transform CO₂ into CH₄. The Znln₂S₄/TiO₂ compound made it much easier to make CH₄, about 39 times better than pristine Znln₂S₄. The heightened photocatalytic efficacy can be attributed to the beneficial segregation of photogenerated electron–hole pairs and the increased light absorption. Furthermore, Wang et al.³²⁷ fabricated a hierarchical heterostructure nanocomposite consisting of Znln₂S₄–In₂O₃, and the resultant photocatalyst demonstrated outstanding activity in the complete transformation of CO₂ to CO. The researchers believe that the expanded surface area contributed to the boosted photocatalytic success, more accessible active sites, and better movement and deterioration of charge carriers. The ZIS/TiO₂ photocatalysts, which follow a 3D Z-scheme, are very good at absorbing light, as shown by their ability to effectively reduce CO₂ and create valuable hydrocarbons. The peak methane (CH₄) production by 3D ZIS/TiO₂ was 1.135 μmol g⁻¹ h⁻¹, which is much larger than the average rate of production by the pure ZIS, which was 0.029 μmol g⁻¹ h⁻¹. Different catalysts, such as ZIS/N-graphene,³²⁸ polymeric C₃N₄/ZIS,³²⁹ g-C₃N₄/ZIS,³³⁰ ZIS/CNO,³³¹ and ZIS/ZnAlO_x,³³⁰ have been used in many studies to focus on the process of transforming CO₂ into CH₄, CO, and CH₃OH.

To illustrate the dual functionality of the photocatalysts, their contribution to CO₂ reduction was assessed in conjunction with H₂ generation. Despite the inclusion of sacrificial agents, the effectiveness of the photocatalytic reduction of CO₂ was evaluated in a gas–solid phase. The principal components (C₂H₄, CH₄, and CO) from the photocatalytic reduction of CO₂ by ZIS and Cu-ZIS are depicted in Fig. 22, where the corresponding time-yield curves are displayed in Fig. 22(A–C).³³²Fig. 22(D) illustrates the average rates for each product. The graphs manifest the manner in which the selectivity and rate of carbon reduction by Cu-ZIS fluctuate with an increase in the copper doping levels.³³² CO and CH₄ were the predominant products observed, which is consistent with insights from prior investigations. Occasionally, the emergence of C₂H₄, which requires electron transfer, became apparent.³²⁶ Higher copper doping levels significantly boosted the yield of C₂H₄, increasing from 0.4 μmol g⁻¹ h⁻¹ to 3.3 μmol g⁻¹ h⁻¹, which was roughly 8.25 times higher. This demonstrates that the catalyst derived from this investigation has a major influence on carbon reduction procedures. Additionally, using Cu7-ZIS, the rate of CH₄ nearly doubled from 2.8 μmol g⁻¹ h⁻¹ to 5.1 μmol g⁻¹ h⁻¹; meanwhile, the rate of CO more than quadrupled from 8.5 μmol g⁻¹ h⁻¹ to 35.9 μmol g⁻¹ h⁻¹. The most significant improvement was perceived in the C₂H₄ yield, despite the rate of CH₄ remaining comparatively constant as the degree of copper doping increasing. The promising potential of the proposed catalysts for carbon reduction applications is illustrated by these results, which highlight the superior efficacy and selectivity of Cu–ZIS catalysts in the photocatalytic degradation of CO₂.

Fig. 22 (A–D) CO₂ reduction of ZIS and Cu–ZIS photocatalytic time-yield graphs. (A) C₂H₄. (B) CH₄. (C) CO. (D) Mean reduction graph for each compound. Reproduced from ref. 332 with permission from Elsevier, Copyright 2024.

The outstanding performance of ZnIn₂S₄ in the transformation of energy has been demonstrated in investigations associated with the photocatalytic evolution of hydrogen.^333,334 Additionally, the band gap of ZnIn₂S₄ demonstrates that it may be useful in reducing CO₂ emissions. Photocatalytic CO₂ conversion to energy sources or other advantageous organic compounds is regarded as an additional solution to the energy shortage. There were not as many publications about ZnIn₂S₄-catalyzed CO₂ conversion as that for hydrogen evolution. However, the photocatalytic degradation of carbon dioxide (CO₂) is still a statistically significant area of research. Moreover, in water-based solutions, the reduction of CO₂ typically faces competition from the hydrogen generation process. Consequently, in the future, it is advisable to dedicate greater attention to CO₂ reduction through photocatalytic processes utilizing ZnIn₂S₄ and ZnIn₂S₄-based composites.

5.6. Purification of wastewater using photocatalysis

Methyl orange (MO) and rhodamine B (RhB), two commonly encountered dye pollutants present in wastewater, were specifically assigned for assessing the efficacy of photocatalysts in ameliorating wastewater. As illustrated in Fig. 23(A and B), the dye degradation efficiency with respect to RhB and MO maintained a similar trend of ZIS-10 > ZIS-15 > ZIS-5 > ZIS, which is consistent with the results for the chemical evolution of hydrogen. Among the photocatalysts, ZIS-10 exhibited the highest quasi-first-order rate constant for the degradation of RhB and MO, with estimated values of 0.86 min⁻¹ (Fig. 23(C)) and 0.037 min⁻¹ (Fig. 23(D)), respectively. Fig. 23(C and D) present a summary of the decay rate constants for RhB and MO degradation utilizing the ZIS-x photocatalysts, respectively. Considering ZIS, ZIS-5, and ZIS-15, the rate constant for RhB is 0.40, 0.49, and 0.54 min⁻¹, respectively. The rate constant for MO decomposition with ZIS, ZIS-5, and ZIS-15 is 0.015, 0.017, and 0.020 min⁻¹, respectively, maintaining a consistent pattern. Additionally, ZIS-10 exhibited outstanding stability throughout dye breakdown.³³⁶ After five cycles, there was hardly a noticeable reduction in the degradation rates, as seen in Fig. 23(E and F). EPR and active trapping techniques were implemented to further investigate the mechanisms for the decomposition of the dyes.

Fig. 23 (A) Decomposition performance for RhB and (B) degradation performance for MO for ZIS, ZIS-5, ZIS-10, and ZIS-15, respectively. The rate constant (K) was derived by deploying the first-order kinetic equation for the decomposition of RhB (C) and MO (D) in ZIS, ZIS-5, ZIS-10, and ZIS-15, respectively. (E) and (F) Cycle stability tests using ZIS-10 for the decomposition of RhB and MO upon sunlight exposure (λ ≥ 420 nm). Reproduced from ref. 335 with permission from Elsevier, Copyright 2024.

As is widely recognized, photogenerated holes (h⁺), ˙OH, and ˙O²⁻ are the three primary active species that may be implicated in photocatalytic decomposition.³³⁷ The formation of ˙O²⁻, h⁺, and ˙OH was successfully tracked using spin-trapping EPR procedures. Following the inclusion of DMPO, as illustrated in Fig. 24(A), a negligible EPR signal was observed in the dark; however, under light, clear EPR signals with an intensity ratio of 1 : 1 : 1 : 1 that correspond to the DMPO–˙O²⁻ adducts appeared, demonstrating the synthesis of ˙O²⁻.³³⁸ The TEMPO spin-trapping EPR supports the presence of h⁺. The EPR signal vanished upon exposure to radiation, confirming the formation of h⁺ under light,³³⁷ in contrast to the three-line ESR signal reported for the TEMPO–h⁺ adduct with a light intensity ratio of 1 : 1 : 1 (Fig. 24(B)) in the dark.

Fig. 24 (A)–(C) EPR spectra of ZIS-10 for DMPO–˙O²⁻, TEMPO–h⁺, and DMPO–˙OH adducts under light and darkness conditions, respectively. (D) Active species entrapment experiment with ZIS-10. Reproduced from ref. 335 with permission from Elsevier, Copyright 2024.

Furthermore, DMPO–˙OH conjugated with an intensity ratio of 1 : 2 : 2 : 1 (Fig. 24(C)) following illumination, in contrast with the minimal signal in the dark,³³⁹ implying the generation of ˙OH upon exposure to light. Considering RhB as the pollutant, active species entrapment experiments over ZIS-10 were conducted to further investigate the main active species for the degradation of dyes. When isopropyl alcohol (IPA) was included as the trapping agent for ˙OH, the degradation efficiency hardly diminished, as seen in Fig. 24(D), suggesting that ˙OH does not represent the main active species.³⁴⁰ Conversely, when TEOA was employed to capture h⁺, the deterioration rate dropped by 72.27%. Similarly, the Ar degradation rate dropped by 66.46% to stop the creation of O₂. These outcomes imply that the primary variables influencing dye degradation are h⁺ and ˙O²⁻.³⁴¹

5.7. Coupling of water splitting for hydrogen production with the oxidation of organic compounds

In the coupled mechanisms of photocatalytic water splitting for hydrogen generation and organic substance transformation, photothermal materials primarily function by capturing light energy and transforming it into thermal energy, thereby conveying the necessary thermodynamic conditions for the reaction to proceed. Photocatalytic water splitting for the generation of hydrogen entails the consumption of light energy to dissociate water into hydrogen and oxygen gases. In organic transformation reactions, photothermal substances can substantially elevate the temperature of the chemical reaction system, enhancing the speed of the reaction. Photothermal materials can create the appropriate temperature conditions to excite the reactants or speed up the chemical reaction process in the creation or breakdown of specific organic molecules. The electrons stimulated by sunlight in the solar-powered hydrogen fabrication procedure cause proton reduction, leading to H₂, a perfect half-reaction. However, the entire water-splitting process may be hampered or even blocked by the sluggish reaction kinetics associated with the oxidation half-reaction induced by holes. In the approach of producing hydrogen employing solar energy, water acts the electron acceptor. Organic sacrificial agents typically serve as hole scavengers to increase the rate of hydrogen evolution. Triethylamine, lactic acid, triethanolamine, sodium sulfite, and formic acid are a few prominent instances of substances employed.^342,343

Implementing defects, researchers prefer to establish a synergistic photothermal oxidation–reduction system. Typically, inorganic sacrificial agents operate merely as hole scavengers that accelerate the formation of H₂, not as proton suppliers. Therefore, it may be more appropriate to employ organic chemicals as sacrificial agents when developing a dual-function system. Through the cooperative functioning of the reaction system, this procedure may trigger the specific oxidation of organic molecules and discharge of H₂, aldehydes (acetaldehyde, formaldehyde, etc.), ketones, and acids (ascorbic acid, formic acid, etc.). Alcohols (ethanol, methanol, glycerol, etc.) are examples of organic sacrificial agents that are susceptible to high oxidation potentials. In the presence of light with a certain wavelength, these organic substances can be oxidized to their respective acids or ketones. For example, the frequently utilized methanol is first oxidized to formaldehyde, and subsequently oxidized to carbon dioxide or formic acid, as shown in eqn (1)–(3).³⁴⁴

CH₃OH → HCHO + H₂, ΔG = 64.1 kJ mol⁻¹ (1)

HCHO + H₂O →HCOOH + H₂, ΔG = 47.8 kJ mol⁻¹ (2)

HCOOH → CO₂ + H₂, (41 (2015) 1205–1216), ΔG = −95.8 kJ mol⁻¹ (3)

A dual-function oxidizing–reduction mechanism can be established that employs organic oxidation procedures rather than sacrificial reagents. This makes the high-value utilization of organic molecules and the production of hydrogen feasible via the splitting of water by photocatalytic means. This technique has important scientific implications. However, there are still limitations regarding the selectivity and productivity of dual-function photothermal catalysis. Eqn (1) and (2) demonstrate that while negative Gibbs free energies (eqn (3)) imply that the reactions thrive preferentially and that sacrificial agents are more susceptible to oxidizing the resulting CO₂, positive Gibbs free energy values signal that the reactions are not spontaneous. Researchers tackle these scenarios by developing and engineering effective photothermal catalysts, together with meticulously regulating the reaction conditions aimed at improving the performance and specificity of photothermal catalysis. The coupling reactions in photothermal catalysis are presently facilitated by enhancing the degree of separation and application of photogenerated electrons and holes, and ultimately by expanding the light absorption spectrum to promote effective reaction advancement.

The integrated oxidation process of benzyl alcohol and the generation of hydrogen serve for the evaluation of the photocatalytic capabilities of prepared samples. The photocatalytic efficiency of PZIS was successfully assessed. PZIS showed moderate activity for the synthesis of BAD (2.1 μmol h⁻¹) and the generation of H₂ (2.6 μmol h⁻¹), as illustrated in Fig. 25(A).³⁴⁵ Alternatively, with a BAD generation rate of 5.5 μmol h⁻¹ and evolution of H₂ rate of 7.9 μmol h⁻¹, VZIS revealed a comparatively more intense activity. These observations indicate that the chemisorption of benzyl alcohol is enhanced by incorporating abundant Zn vacancies. The performance for the dehydrogenation of benzyl alcohol was substantially improved by the inclusion of Ni/VZIS hybrid photocatalysts (Fig. 25(B)). The photocatalytic activities first surged, and subsequently declined with an increase in the loading amount of Ni. The optimum rates of H₂ and benzyl aldehyde formation were 81.7 μmol h⁻¹ and 73.7 μmol h⁻¹, respectively, using 1% Ni/VZIS. The degree of selectivity for benzyl aldehyde yielded 91%, and the associated benzyl alcohol rate of transformation was 81%. The performance varied slightly when the loading quantity of Ni was further increased; this might happen because Ni deposition depleted the base sites. The catalytic activity was substantially maintained for four cycles, according to the stability test using 1% Ni/VZIS, suggesting its good reusability (Fig. 25(C)). The conclusions from the control studies indicate that in instances where the reaction takes place in the absence of a catalyst or in the dark, barely any H₂ and BAD were generated. These results validate the effectiveness of the Ni/VZIS photocatalyst in the selective oxidation of benzyl alcohol to BAD and H₂.

Fig. 25 (A) Potential of photocatalysis to convert BA into BAD and H₂ over PZIS and VZIS. (B) Prepared samples for the photocatalytic conversion of BA into BAD and H₂. (C) Photocatalytic conversion of BA into BAD and H₂via recycling experiments of 1%Ni/ZIS. Reproduced from ref. 345 with permission from Elsevier, Copyright 2024.

5.8. Photothermal catalytic CO₂ reduction performance

The photocatalytic proficiency of the anchored photocatalyst was evaluated via the determination of CO₂ photoreduction in the context of H₂O, adopting multiple wavelengths of light for illumination. Generally, the photocatalytic rate is assessed per unit mass of catalyst employed; however, contemporary studies have highlighted the dearth of scientific rigor of this technique, which is ascribed to the impact of numerous variables, such as light intensity, reactor geometry, wavelength, and semiconductor surface region, on the photocatalytic velocity.³⁴⁶ Consequently, in the realm of heterogeneous materials, standards and procedures for assessing photocatalytic activity are being presented.³⁴⁷ In this study, velocity was assessed as a function of unit surface area, considering the constraints related to mass normalization and the unique attributes of immobilized catalysts. Fig. 26(A) shows the CO₂ reduction activities of pristine ZIS and ZIS/Cu_xO composites. It indicates that CO and CH₄ were the main catalytic products, with virtually no additional byproducts being detected in significant amounts. The pure ZIS sample showed comparatively modest activity for CO₂ reduction (176.7 μmol m⁻² h⁻¹ for CO and 182.6 μmol m⁻² h⁻¹ for CH₄) despite being subjected to simulated sunlight for 2 h.

Fig. 26 (A) Photocatalytic reduction of CO₂ to CH₄ and CO activities across multiple catalysts upon exposure to UV-vis-IR light irradiation. Production rates of (B) CO and (C) CH₄ over distinct catalysts under UV-vis, IR, and UV-vis-IR light irradiation. (D) Controlled studies for ZIS/Cu_xO-2 under varying experimental circumstances. Reproduced from ref. 348 with permission from Elsevier, Copyright 2025.

However, when Cu_xO nanoparticles were inserted into ZIS nanosheets, the photocatalytic efficiency for CO₂ reduction increased substantially. Additionally, the photocatalytic activity of the composites initially boosted before declining as the Cu_xO content increased, suggesting that the mass ratio of every element in the composite had an enormous effect on its catalytic efficiency. According to this phenomenon, the concentration of Cu_xO nanoparticles on the surface of ZIS nanosheets might boost the performance of photocatalytic processes through the inclusion of additional active sites once it exceeds a particular threshold. Excess Cu_xO nanoparticles would accumulate on the surface of the nanosheets as the proportion of nanoparticles surged. The procedure culminated in a degraded photocatalytic performance by lowering the light absorption by the photocatalyst and including more recombination sites, which hindered the effective separation of photogenerated carriers. Consequently, by varying the mass ratio of every element in the composite catalyst, the catalytic efficiency may be precisely controlled. With the ideal Cu_xO loading quantity, ZIS/Cu_xO-2 produced CO and CH₄ yields of 1515.8 and 1579.4 μmol m⁻² h⁻¹, which were significantly higher than that of pure ZIS by 7.5 and 7.6 times, respectively. The ZIS/Cu_xO-2-induced CO and CH₄ yields under UV-vis-IR irradiation, as displayed in Fig. 26(B and C), were substantially greater than that under UV-vis (948 and 923 μmol m⁻² h⁻¹) and IR (387 and 540 μmol m⁻² h⁻¹), respectively, suggesting that the synergistic effect of heat and light might be responsible for the enhanced photocatalytic reduction activities. The generation routes of CO and CH₄ in the ZIS/Cu_xO-2 photocatalytic system were explored under diverse experimental settings (Fig. 26(D)). In the dark, CO and CH₄ were undetectable in the chemical reaction system, signifying that ZIS/Cu_xO-2 demonstrated no catalytic activity in the absence of illumination. Regardless of irradiation, the mechanism failed to generate either CO or CH₄ in the absence of a catalyst. Thus, both light and photocatalyst are crucial in the mechanism of photocatalytic CO₂ reduction.³⁴⁹ To the best of our knowledge, there is currently a relatively limited amount of research on ZnIn₂S₄-based thin films for photocatalytic CO₂ reduction. Nonetheless, the modified sample demonstrated comparable or better photocatalytic efficiency for CO₂ reduction compared with recently published film photocatalysts.^350–355

6. Conclusion

ZnIn₂S₄ is an emerging ternary metal chalcogenide photocatalyst that possesses visible-light excitation capabilities. The exceptional research value of ZnIn₂S₄ originates essentially from its outstanding characteristics, including extraordinary structure and optical properties, facile fabrication, and protection of the environment, among others. The hydrothermal and solvothermal techniques, among others, have been implemented in recent years for the production of ZnIn₂S₄ micro- and nanostructures, including thin films, microspheres, nanoparticles, sheets, and filaments. ZnIn₂S₄ has been extensively applied in the field of hydrogen production, pollutant elimination, and CO₂ reduction owing to its remarkable characteristics. Despite displaying an encouraging response to obvious light, the photocatalytic efficiency and functions of ZnIn₂S₄ remain constrained due to its structural imperfections, limited light absorption area, and unsatisfactory separation efficacy of photogenerated charge carriers. In broad terms, three processes are involved in a catalytic process including photon absorption, generation and separation of electron–hole charges, and catalytic surface reactions. During a photocatalytic reaction, ZnIn₂S₄ can encourage the electrons on the VB to move to the CB by absorbing photons. This would leave more holes on the VB. After that, the photogenerated holes and electrons move towards the surface of ZnIn₂S₄, making it easier for catalytic surface reactions to happen. Alternatively, certain free radicals, such as hydroxyl radicals, may be created by the surface reaction and facilitate the photocatalytic process. Strengthening the capacity for the absorption of ZnIn₂S₄, reducing the band gap, and extending the lifetime of photogenerated charges are all potential methods for enhancing the photocatalytic effectiveness of this material. As a consequence, numerous methodologies have been implemented in an effort to enhance the photocatalytic activity of ZnIn₂S₄ while preserving its properties. Furthermore, surface modifications such as element doping, heterojunction formation, and S-scheme heterojunction modification have been found to be effective methodologies for increasing the photocatalytic activity of ZnIn₂S₄. Consequently, the emergence of heterojunctions between ZnIn₂S₄ and other semiconductors can concurrently alleviate the aforementioned limitations of the single-component ZnIn₂S₄ photocatalyst. Composites composed of ZnIn₂S₄ have demonstrated extraordinary photocatalytic performances. In conclusion, ZnIn₂S₄ micro/nanostructures and Znln₂S₄-based composites are anticipated to have significant activity for both energy transformation and ecological protection. Although expected beneficial results have been observed, the field of photocatalysis utilizing ZnIn₂S₄ and Znln₂S₄-based composites is still in its developing phase and encounters numerous obstacles.

The significance of the transfer of photogenerated electron–hole pairs in the photocatalytic process is universally recognized. Fortunately, numerous variables can influence the outcome of this procedure, including crystallinity, crystalline structure, surface surroundings, imperfections, and morphology. Despite this, there is a lack of comprehensive and methodical research regarding the complicated processes underlying the synergistic effects produced by these factors and their interrelationships. Although metal doping can improve the photocatalytic efficiency and accelerate the migration of charges, dopant ions can additionally be utilized as recombination sites for electrons and holes generated by the photocatalyst. Thus, to strengthen the photocatalysis performance, the surface can be changed by mixing it with semiconductors, carbon-based substances, organic components, and other similar substances. However, it is important to note that an overabundance of these modifications might impede the generation of energy and hinder catalytic mechanisms due to obscuring influences. According to the metal stoichiometry, the amount of metal in the composition of binary or ternary chalcogenide semiconductors has an enormous effect on their band gap. However, the exact procedure by which the metal ratio affects surface defects, certain surface regions, and aperture volume has yet to be established. Throughout photocatalytic hydrogen production, sacrificial reagents are consistently introduced into the solution to prevent photocorrosion. Nonetheless, the consumption of the sacrificial agent is non-reversible, and resupplying it throughout the reaction is expensive. Thus, investigating an alternative methodology to safeguard photocatalysts against photocorrosion is a vital requirement. More research is needed to determine the influence of defects in ZnIn₂S₄ on its electronic properties and how doping changes these properties. The investigation of industrial-scale photocatalytic processes, such as hydrogen evolution, pollutant removal, and CO₂ reduction, remains scarce.

7. Perspectives

From our perspective, there are countless possible directions in which future photocatalytic exploration involving ZnIn₂S₄ and ZnIn₂S₄-based composites merits additional investigation. In comparison to metal ion doping, non-metal doping for the modification of ZnIn₂S₄ has received less attention. As a result, research into the effects of non-metallic modifications on the properties of ZnIn₂S₄ should be broadened. Furthermore, the simultaneous doping of two or three types of elements merits consideration, given that this doping technique frequently maximizes properties by combining the advantages of individual doping agents. Accurate doping strategies that modify the HOMO and LUMO should be attempted to improve the oxidation and reduction capabilities, respectively. Additional investigation into the construction of heterojunctions should be conducted because of their potential to significantly enhance the transmission of charges and hinder the recombination of photogenerated electron–hole pairs. Furthermore, it is reasonable to concurrently implement element doping and heterojunction formation to modify ZnIn₂S₄. The sensitization process for dyes may represent an innovative strategy for enhancing its photocatalytic performance. The modification of organic chelating ligands has the possibility to yield research advantages. For example, one possible way is to combine different types of dopants and make nanostructures with different shapes, likely nanorods, nanosheets, nanowires, nanotubes, and nanoribbons. It will be helpful to learn more about the processes that affect the formation and optical properties of ZnIn₂S₄ and its composites.

ZnIn₂S₄, a type of n-type semiconductor bimetallic sulfide material, is recognized globally to be one of the best prospects for the conversion of CO₂ using solar power owing to its extraordinary light and chemical resistance in photocatalytic research, together with its ecologically beneficial attributes. However, the rapid recombination of photogenerated carriers severely restricts the photocatalytic activity of ZnIn₂S₄ nanosheets, despite their potential for photocatalytic CO₂ reduction. Therefore, suppressing the recombination of photogenerated carriers by incorporating co-catalysts or forming heterojunctions is an excellent way to boost the photocatalytic activity of ZnIn₂S₄. Extreme conditions often expedite the transfer of photogenerated carriers, leading to the frequent use of the photothermal effect in photocatalytic CO₂ reduction. Thus, to boost the activity and selectivity of photocatalytic CO₂ reduction, photothermal effects must be included. Also, to date, there is limited research on the photocatalytic production of organic substances, the extraction of significant metal elements, and anti-cancer treatments. In this case ZnIn₂S₄-based S-scheme heterojunctions present significant opportunities in photocatalytic applications, warranting extensive investigation to optimize the performance of photocatalytic methodologies in further research.

Data availability

No primary research results, software, or code have been included and no new data were generated or analyzed as part of this review.

Author contributions

Muzamil Ahmad: writing the original draft, reviewing and data curation. Kaili Wu: conceptualization, supervision. Adeel Ahmed: review and editing. Adnan Muhammad: validation. Rafiq Muhammad: visualization. Hailin Cong, Bing Yu: project administration, funding acquisition.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (22074072, 22274083), the Shandong Provincial Natural Science Foundation (ZR2022LZY022, ZR2023LZY005), the Science and Technology Planning Project of South District of Qingdao City (2022-4-005-YY), the Exploration project of the State Key Laboratory of BioFibers and EcoTextiles of Qingdao University (TSKT202101), the National Key R&D Program of China (2024YFE0104100), and the Medical Plus Key Project of Qingdao University (YX2024201).

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Footnote

† These authors contributed equally to this work.

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