Photocatalytic Applications of ReS2-Based Heterostructures

ReS2-based heterostructures, which involve the coupling of a narrow band-gap semiconductor ReS2 with other wide band-gap semiconductors, have shown promising performance in energy conversion and environmental pollution protection in recent years. This review focuses on the preparation methods, encompassing hydrothermal, chemical vapor deposition, and exfoliation techniques, as well as achievements in correlated applications of ReS2-based heterostructures, including type-I, type-II heterostructures, and Z-scheme heterostructures for hydrogen evolution, reduction of CO2, and degradation of pollutants. We believe that this review provides an overview of the most recent advances to guide further research and development of ReS2-based heterostructures for photocatalysis.


Introduction
Currently, the need for clean and renewable energy sources in order to mitigate the increasing environmental pollution and global energy crisis has become more urgent. Photocatalytic technology has emerged as a promising avenue for utilizing solar energy due to its low cost, abundance, and eco-friendly nature [1]. In 1972, Fujishima and Honda demonstrated for the first time the ability to produce hydrogen by splitting water on a TiO 2 electrode [2]. Since then, TiO 2 has been extensively studied due to its strong redox abilities, eco-friendliness, and chemical stability, which have made it a promising material for photocatalysis over the past few decades [3][4][5][6]. Despite its advantages, TiO 2 suffers from a narrow spectral response range, a high carrier recombination rate, and a lack of catalytic active sites, thus limiting its photocatalytic hydrogen evolution (PHE) activity [7][8][9]. The emergence of two-dimensional (2D) photocatalysts, such as transition metal dichalcogenides (TMDs), has reinvigorated the progress in photocatalytic and photoelectrocatalytic research [10][11][12]. Due to their easy exfoliation into monolayers, TMDs provide numerous benefits such as a high surface area and decreased migration distance for photogenerated electron-hole pairs. This not only offers an increased number of reaction sites but also reduces the likelihood of electron-hole recombination, potentially improving photocatalytic performance [13]. Nevertheless, the electronic structure of TMDs is highly dependent on the number of layers [14]. Achieving precise control of the number of layers is critical to obtaining optimal catalytic performance, as exemplified by the fact that the photocatalytic hydrogen production of MoS 2 is substantially improved when it converts from a multi-layer to a single-layer configuration [15]. As such, controlling the number of layers remains a daunting task that limits the utility of TMDs in photocatalytic applications [16,17].

Synthetic Methods for ReS2 and Its Composite Material
Various synthesis techniques such as mechanical/chemical exfoliation, mal, and CVD methods have been developed to prepare ReS2-based photocat different morphologies including nanospheres, nanosheets, etc. [45,47], all of be used as core components in the construction of photocatalytic heterostructu

Exfoliation from the Bulk ReS2
The weak interlayer coupling of ReS2 permits the separation of bulk nanosheets through exfoliation, which is a simple, high-yield method for ReS2 p [29,48,49]. Ultrathin nanostructures can be obtained by employing a hybrid of m and liquid exfoliation. Bulk ReS2 produced using either commercial or hydroth thesis can be dispersed in organic solvents (such as ethanol or N,N-dimethylfo through ultrasound-assisted liquid-phase exfoliation at 20-25 °C using an ice lowed by centrifugation and filtration to yield the nanosheets [34,47,[50][51][52][53]. The The electronic band structure of bulk (orange solid curves) and monolayer (purple dashed curves) ReS 2 , as calculated by DFT, is predicted to be a direct bandgap semiconductor with a nearly identical bandgap value at the Γ point [29]. Reproduced with permission: Copyright 2014, Nature Publishing Group.
In 2018, Fu's group undertook an experimental study on the optoelectronic properties of ReS 2 and its performance in photocatalyzed hydrogen evolution [33]. The conduction and valence band edges of ReS 2 are more closely spaced to the water redox potentials than CdS, thus rendering ReS 2 more efficient for PHE. Moreover, the monolayer-like structure of the catalyst facilitates the formation of trions (a bound state of two electrons and one hole). These trions can then migrate to the surface and participate in two-electron reactions at the abundant active sites. Furthermore, the relatively intact basal planes of ReS 2 can promote electron transport to the edge-reactive sites. Such a two-electron catalytic reaction endows ReS 2 with a PHE rate of 13.023 mmol g −1 h −1 under visible light irradiation [34].
ReS 2 is a versatile material but relatively less-explored among TMDs. It has garnered significant attention from the scientific community due to its unique properties compared to other TMDs. Rahman et al. critically analyzed the synthesis, structure growth mechanisms, properties, and contemporary applications of ReS 2 in different areas [32]. Zhang et al. has highlighted the weak coupling characteristics of ReS 2 [35], and the intrinsic anisotropic properties are summarized by Cao et al. [36]. Other recent reviews have summarized advances in ReS 2 research with respect to electronics, optics, magnetism, and alkali-metal ion batteries [37][38][39][40]. Academic research regarding the photocatalytic properties of ReS 2 materials is currently limited.
It is well established that heterostructures have numerous advantages in enhancing photocatalyst functions [1,41,42]. Rational alignment of ReS 2 with other semiconductors into a tandem structure can increase the range of light absorption across the solar spectrum and optimize its photocatalytic performance by inhibiting charge carrier recombination as well as promoting charge separation [34,[43][44][45][46]. In this review, we provide mainly an overview of the recent progress on ReS 2 heterostructure, focusing on the synthesis methods and their photocatalytic applications. Firstly, the preparation methods are discussed, including hydrothermal reaction, chemical vapor deposition (CVD), and mechanical or chemical exfoliation. Furthermore, we focus mainly on photocatalytic applications based on ReS 2 -based heterostructure, such as photocatalytic hydrogen evolution, photocatalytic reduction of CO 2 , and pollutant degradation. Finally, the review is summarized, and the prospects are discussed from current views.

Synthetic Methods for ReS 2 and Its Composite Material
Various synthesis techniques such as mechanical/chemical exfoliation, hydrothermal, and CVD methods have been developed to prepare ReS 2 -based photocatalysts with different morphologies including nanospheres, nanosheets, etc. [45,47], all of which can be used as core components in the construction of photocatalytic heterostructures.

Exfoliation from the Bulk ReS 2
The weak interlayer coupling of ReS 2 permits the separation of bulk ReS 2 into nanosheets through exfoliation, which is a simple, high-yield method for ReS 2 preparation [29,48,49]. Ultrathin nanostructures can be obtained by employing a hybrid of mechanical and liquid exfoliation. Bulk ReS 2 produced using either commercial or hydrothermal synthesis can be dispersed in organic solvents (such as ethanol or N,N-dimethylformamide) through ultrasound-assisted liquid-phase exfoliation at 20-25 • C using an ice bath, followed by centrifugation and filtration to yield the nanosheets [34,47,[50][51][52][53]. The bulk ReS 2 can be exfoliated into a few layers (Figure 2a), and the original structure of ReS 2 is maintained. Figure 2b,c show the intact basal plane and edge of ReS 2 nanosheets, respectively [50]. Figure 2d,e display the ReS 2 nanosheets with a thickness of about 4.20 nm [54]. The undercoordinated sulfur sites at the edges of ReS 2 nanosheets serve as highly active centers for H 2 generation, while the intact basal plane facilitates efficient electron transport to these edge S active sites for H 2 evolution [34]. Chemical methods, such as the addition of diazonium salts, can also be employed to modify the surface structure of TMDs during the stripping process [55]. For example, the incorporation of hydroxyl functional groups such as carboxyl can enhance the hydrophilicity of ReS 2 and improve its dispersibility in the ReS 2 −C 6 H 5 COOH/TiO 2 physical mixture, thus preserving the photocatalytic active sites [56]. KGaA. (f) Schematic illustration of the solvothermal synthesis process of g-C3N4 nanotubes and 1T'-ReS2/g-C3N4 composites [57]. Reproduced with permission: Copyright 2022, ELSEVIER. (g) XRD patterns of 1T'-ReS2 and 2H-ReS2 after heat treatment [57]. (h) TEM images of 1T'-ReS2/g-C3N4 composite (12 wt%) [57]. Reproduced with permission: Copyright 2022, ELSEVIER. (i) Schematic diagram of the CVD-synthesized ReS2 and the two-electron photocatalytic reaction [33].

Hydrothermal Synthesis Reaction
The hydrothermal method is one of the simplest methods for the preparation of TMDs [16]. Specific precursors are used to carry out chemical reactions under high temperature and high pressure in a specially designed polytetrafluoroethylene-lined autoclave. For the synthesis of pristine ReS2, ammonium perrhenate (NH4ReO4), hydroxylamine hydrochloride (NH2OH•HCl), and thiourea (CS(NH2)2) are dissolved in deionized water. After stirring, they are transferred to a polytetrafluoroethylene autoclave for hydrothermal reaction by heating to 200-240 °C for 12-48 h [58][59][60]. After cooling, the ReS2 black powder is repeatedly washed with deionized water and ethanol, and the final product is collected after drying. The ReS2 prepared using this method typically displays the morphology of nanospheres, and the size of the ReS2 nanospheres can be adjusted by regulating the concentration of rhenium sources in the reaction [45].

Hydrothermal Synthesis Reaction
The hydrothermal method is one of the simplest methods for the preparation of TMDs [16]. Specific precursors are used to carry out chemical reactions under high temperature and high pressure in a specially designed polytetrafluoroethylene-lined autoclave. For the synthesis of pristine ReS 2 , ammonium perrhenate (NH 4 ReO 4 ), hydroxylamine hydrochloride (NH 2 OH·HCl), and thiourea (CS(NH 2 ) 2 ) are dissolved in deionized water. After stirring, they are transferred to a polytetrafluoroethylene autoclave for hydrothermal reaction by heating to 200-240 • C for 12-48 h [58][59][60]. After cooling, the ReS 2 black powder is repeatedly washed with deionized water and ethanol, and the final product is collected after drying. The ReS 2 prepared using this method typically displays the morphology of nanospheres, and the size of the ReS 2 nanospheres can be adjusted by regulating the concentration of rhenium sources in the reaction [45].
The enhanced photocatalytic activity of ReS 2 can be achieved by combining it with another semiconductor material to construct heterojunctions [61]. There are two ways to prepare ReS 2 -based heterojunctions via the hydrothermal method. The first is to incorporate the synthesized host semiconductor material into the precursor mixture for the preparation of ReS 2 , or vice versa, using a two-step hydrothermal method [46,62]. Wang et al. dispersed the generated TiO 2 nanofibers into the ReS 2 preparation solution for the hydrothermal reaction to anchor the ReS 2 nanosheets on the porous TiO 2 nanofibers to form TiO 2 @ReS 2 composites. The few-layer ReS 2 nanosheets with twisted 1T phase that were prepared via the hydrothermal method show large catalytic active sites, high electron mobility, and matching band structure [58]. The other is to fabricate a heterostructure with all the raw material in the one-pot hydrothermal method [63]. Chen et al. prepared a 1T'-ReS 2 /g-C 3 N 4 heterostructure via the one-pot solvothermal reaction (Figure 2f). The obtained 1T'-ReS 2 nanosheets can be converted to 2H-ReS 2 after subsequent heat treatment (Figure 2g). The thin 1T'-ReS 2 nanosheets with multi-layers grown on the g-C 3 N 4 nanotubes (green circle in Figure 2h) show characteristic 2D-layered structures of TMDs with the single-layer thick-ness~0.27 nm and the interlayer spacing~0.65 nm (blue circle in Figure 2h). Alternatively, ReS 2 prepared via the hydrothermal method can also be subsequently combined with other semiconductor materials through grinding or liquid-phase dispersion, thereby enabling the construction of heterojunctions [51,64].

Chemical Vapor Deposition Technique
ReS 2 nanosheets prepared through hydrothermal and exfoliation methods contain numerous impurities that can act as electron-and-hole recombination centers, thus reducing the photocatalytic activity of ReS 2 [65]. Compared to other methods, CVD-prepared TMDs have many advantages, including uniform appearance, controllable thickness, low defect rate, clean interface, and high synthesis efficiency [66,67]. On the one hand, the weak van der Waals interaction between TMDs makes it possible to stack them to form a variety of vertical heterostructures. On the other hand, chemical bonds can be formed at the interface to realize the interlocking of different 2D materials to form horizontal heterostructures with special interfacial properties. Vertical heterostructures can be prepared through methods such as mechanical exfoliation and transfer [68,69], direct CVD growth [70], electrochemical exfoliation [71], and liquid-phase exfoliation [47]. Horizontal heterostructures can generally be prepared only by the CVD approach [72][73][74].
The CVD preparation of ReS 2 heterostructures can also be divided into a "one-step method" and a "two-step method". The one-step method puts the precursor containing the elements of the synthesized heterostructure into the reaction system at one time. This method is based on the difference in the substrate adsorption energy or the control of growth conditions, such as growth temperature or carriers, to realize the growth of two materials one after another. For example, by utilizing the difference in adsorption energy between Re and W atoms on a Au substrate, a one-step synthesis of a vertical ReS 2 /WS 2 heterojunction on a Au/W-Re alloy foil can be achieved [75]. The two-step method means that the growth of heterostructures is carried out in two separated steps. These methods can realize the high-quality and large-area preparation of single-crystal ReS 2 , which has been widely used in micro-nano electronic devices and photoelectric detection fields [22,23,76]. A substantial amount of work has reported the CVD preparation and growth mechanism of ReS 2 heterostructure; readers can refer to the related papers [72][73][74][75]77,78] and reviews [79].
The majority of TMDs produced via CVD techniques exhibit a parallel growth relative to the underlying substrate, resulting in a flat film morphology. Under similar CVD growth conditions, ReS 2 nanosheets display a propensity to bend and adopt a vertically oriented configuration on the underlying substrate [80]. Specifically, the vertical growth of twodimensional ReS 2 is highly spontaneous and independent of the substrate [81]. The high packing density (∼10 8 cm −2 ) of the ReS 2 nanosheets promotes a substantial number of active Re atoms to be exposed at the sheet edges, providing abundant active sites for catalytic processes [33,70,80,[82][83][84]. The common precursors for the CVD synthesis of ReS 2 are ammonium rhenate (NH 4 ReO 4 ), Re powder [82], and rhenium-containing oxides (ReO 3 or Re 2 O 7 ), with sulfur powder used as the predominant sulfur source and heated in a different zone [33,85,86]. The ReS 2 prepared via the CVD method exists mostly in petal-like ReS 2 nanowalls arranged vertically without agglomeration [28,33,87]. Zhang et al. anchored the ReS 2 nanowalls vertically on the surface of the Si/SiO 2 substrate (Figure 2i), which can distribute ultra-uniformly with a dense structure over a wide area ( Figure 2j) [33]. The average length of the ReS 2 nanowalls can be up to 200 nm with domain sizes of about 10 layers. The interplanar distance of the ReS 2 nanowalls is about 0.62 nm ( Figure 2k), corresponding to the (001) crystal plane. The beneficial exposure of the (001) facet ensured that more active sites were accessible for hydrogen adsorption, consequently enhancing the catalytic activity. The growth of the ReS 2 single-crystal nanosheets does not need a polished substrate or a small lattice mismatch [87]. Cheng et al. grew ReS 2 tightly on the TiO 2 -coated b-Si substrate with the shape of nanopyramids via the CVD method. The obtained ReS 2 nanosheet was of high quality and exhibited a good crystal structure without defects [28]. Before the characterization of photocatalytic properties, ReS 2 generally needs to be scraped off from the substrate, and then via the solution reaction (such as hydrothermal or self-assembly) to prepare heterostructures.

Heterostructured ReS 2 Composites
The heterojunctions formed between the host semiconductors generate an internal electric field that facilitates the separation of electron-hole pairs and accelerates the migration of carriers, which is constructive for improving the photocatalytic efficiency [88][89][90]. ReS 2 -based heterostructures can fulfill the aforementioned requirements and demonstrate outstanding photocatalytic performance. According to the bandgaps and the electronic affinity of semiconductors, ReS 2 -based heterostructures can be divided into three different types: type-I (straddling gap), type-II (staggered gap), and Z-scheme system, the band alignment of which is shown in Figure 3. The band gap, the electron affinity, and the work function of the combined semiconductors can determine the dynamics of the electron and hole in the semiconductor heterojunctions. [33]. The average length of the ReS2 nanowalls can be up to 200 nm with domain about 10 layers. The interplanar distance of the ReS2 nanowalls is about 0.62 nm 2k), corresponding to the (001) crystal plane. The beneficial exposure of the (00 ensured that more active sites were accessible for hydrogen adsorption, conseque hancing the catalytic activity. The growth of the ReS2 single-crystal nanosheets d need a polished substrate or a small lattice mismatch [87]. Cheng et al. grew ReS on the TiO2-coated b-Si substrate with the shape of nanopyramids via the CVD The obtained ReS2 nanosheet was of high quality and exhibited a good crystal s without defects [28]. Before the characterization of photocatalytic properties, ReS ally needs to be scraped off from the substrate, and then via the solution reaction hydrothermal or self-assembly) to prepare heterostructures.

Heterostructured ReS2 Composites
The heterojunctions formed between the host semiconductors generate an electric field that facilitates the separation of electron-hole pairs and accelerates gration of carriers, which is constructive for improving the photocatalytic efficie 90]. ReS2-based heterostructures can fulfill the aforementioned requirements and strate outstanding photocatalytic performance. According to the bandgaps and t tronic affinity of semiconductors, ReS2-based heterostructures can be divided in different types: type-I (straddling gap), type-II (staggered gap), and Z-scheme sys band alignment of which is shown in Figure 3. The band gap, the electron affinity, work function of the combined semiconductors can determine the dynamics of t tron and hole in the semiconductor heterojunctions.

Type I Heterostructure
In a type-I heterostructure (shown in Figure 3a), the photo-excited electr holes in the wide-gap material transfer to the narrow-gap material (ReS2), as demo

Type I Heterostructure
In a type-I heterostructure (shown in Figure 3a), the photo-excited electrons and holes in the wide-gap material transfer to the narrow-gap material (ReS 2 ), as demonstrated by the arrows [46,50,51,53,54,[91][92][93]. The quantum confinement of the accumulated electrons and holes in the same semiconductor facilitates high carrier recombination rate and radiative recombination, which is incredibly detrimental to photocatalytic activity but desirable in light-emitting applications [91,94]. To inhibit recombination of photo-generated charge, a cocatalyst such as Pt that is deposited in situ onto the surface [54] or abundant sulfur vacancies [93] can act as an electron trapper to facilitate electron accumulation and transfer. Additionally, the abundant edge reaction sites in ultrathin ReS 2 nanosheets in the composites can also accumulate electrons, thus accelerating reaction kinetics of the hydrogen evolution [53].

Type II Heterostructure
In a type-II heterojunction semiconductor, both valence band (VB) and conduction band (CB) of semiconductor 1 are situated at higher energy levels than those of semiconductor 2 (Figure 3b). The alignment of the energy levels facilitates electrons transferring from semiconductor 1 to semiconductor 2, while the holes migrate in the opposite direction. If both semiconductors are in close contact, an effective charge separation will take place upon light illumination. This leads to a decrease in charge recombination and an increase in the lifetime of charge carriers, resulting in higher photocatalyst activity. As one of the typical systems, the VB and CB of MX 2 are both higher than that of ReS 2 . They can form a Type-II heterojunction structure in which the electrons transfer to ReS 2 with holes transferred to MX 2 [21,95,96]. Zhang et al. constructed a type-II heterojunction between ReS 2 and XS 2 (X = Mo, W), which can promote the electron transfer to further improve the photocatalytic performance of the counter electrode in dye-sensitized solar cells [96].
Traditional type-II heterojunctions (including n-n junctions and p-n junctions) have several obvious limitations. (1) From the thermodynamic point of view, the improvement of charge separation efficiency is at the expense of oxidation/reduction ability. Specifically, photo-generated electrons will accumulate on the CB of semiconductor 2 with a weaker reduction potential, and photo-generated holes will accumulate on the VB of semiconductor 1 with a lower oxidation potential. Following the type-II charge transfer mechanism, it will inevitably weaken the thermodynamic driving force of the photocatalytic reaction. (2) In terms of dynamics, the transition electrons of semiconductor 2 and the electrons transferred from semiconductor 1 will produce repulsive force. Similarly, the holes between them will also repel each other, which greatly hinders the continuous charge transfer in the type-II heterostructure. (3) Finally, the electrostatic attraction of electrons in semiconductor 2 and holes in semiconductor 1 also render the interfacial charge transfer of type-II composites difficult to achieve [61].

Z-Scheme Heterojunction
Thanks to inspiration from natural photosynthesis, in which two-step photoexcitation and electron transfer are connected in series to form a letter "Z", similar artificial bionic Z-type heterostructures have been studied and developed in the photocatalysis field [97,98]. The first generation of the liquid-phase Z-scheme system has shuttle oxidation/reduction medium pairs, which are played by electron acceptor/donors, respectively, while there is no physical contact between oxidized semiconductors and reduced semiconductors. However, the first-generation Z-scheme photocatalytic system is limited to use in liquidphase reactions, severely constraining its practical applications in gas-or solid-phase processes [99]. The second-generation all-solid-state Z-scheme heterojunction uses solid conductors (generally noble metal nanoparticles) instead of redox pairs to make it suitable for both liquid-solid and gas-solid reactions (Figure 3c). After the two semiconductors are excited at the same time, the electrons on the CB of the oxidized semiconductor will migrate to the solid conductor and then transfer to the VB of the reduced semiconductor. The length of this charge transfer path is significantly shortened compared to the initial liquid-phase Z-type heterostructure, which can greatly accelerate the charge transfer and separation efficiency. However, due to the difficulty in accurately controlling the heterojunction structure, the difficulty in directional electron transfer on the interface, and the competition between the light absorption of the mediator itself and the main catalytic components, these two heterostructures have not achieved the ideal photocatalytic effect [100].
The third-generation direct Z-type heterojunction is composed of only a reduced semiconductor and an oxidized semiconductor with a matching band structure and an ohmic contact at the interface (Figure 3d). This ohmic contact interface has certain defects and can be used as a recombination center for the electrons on the CB of the oxidized semiconductor and the holes on the VB of the reduced semiconductor. In addition, the impurity orbits contribute quasi-continuous energy levels and become charge transfer mediators. In addition to inheriting the advantages of the first two generations of Z-scheme systems, the direct Z-scheme without a solid medium greatly reduces the construction cost. Meanwhile, the direct Z-type heterostructure overcomes the light-shielding effect caused by redox pairs or noble metal particles [101]. Although the resistance of the direct Z-scheme at the solid-solid interface is generally considered to be higher than that of the all-solid-state Z-scheme, it is often possible to improve the conductivity of the interface by regulating or optimizing the chemical-binding force through molecular polarization to maximize the advantages of the Z-scheme heterojunction [56].

Photocatalytic Applications of Heterostructure ReS 2
The direct band gap of ReS 2 in both monolayer and bulk form is approximately 1.4 eV [29], allowing for the absorption of light across the entire visible spectrum and into the near-infrared region, making it an ideal material for visible light photocatalysis. The weak interlayer coupling of ReS 2 results in optoelectronic properties that are largely independent of the number of layers, and the unsaturated edge sites are particularly exposed [35]. These advantages endow ReS 2 with considerable potential as an efficient photocatalyst or cocatalyst. Considering the limitations of a single photocatalyst, coupling other semiconductors with ReS 2 into heterojunctions can not only improve the utilization of solar energy but also reduce electron-hole pair recombination, thereby improving redox capacity. ReS 2 -based heterostructure composites have provoked extensive research attention and exhibit excellent photocatalytic activity in the fields of hydrogen production, CO 2 reduction, and pollutant degradation.

Photocatalytic Hydrogen Production
Hydrogen, which is generated from photocatalytic water splitting, has been regarded as a potential source of energy for the development of a sustainable economy and society. Qualified photocatalysts should have a bandgap over 1.23 eV. The conduction band minimum (CBM) should be higher in energy than the H + /H 2 water reduction potential, while the valence band maximum (VBM) should be lower in energy than the OH − /O 2 water oxidation potential. The band-gap and band-edge positions of monolayer and multilayer ReS 2 are highly compatible with the energy levels required for water splitting [102]. Pioneer work by Zhang et al. reported that a multilayer structure ReS 2 with monolayer-like features can generate a tremendous number of trions consisting of two electrons and one hole. This can trigger a two-electron catalytic reaction, offering a PHE rate of 13 mmol g −1 h −1 under visible light, which is larger than that of most reported TMD composite photocatalysts [33]. Due to its distorted 1T phase structure, transition metal σ bond, and direct band gap, ReS 2 can serve as an electron reservoir with fast electron transport speed and active electronic states. Consequently, ReS 2 has been widely recognized as an excellent cocatalyst for constructing a heterostructure with classical photocatalysts such as TiO 2 [27,34,58,103,104], CdS [44,45,59,62], and g-C 3 N 4 [46,54,57,92]. It can not only markedly enhance the visible light absorption, charge transfer, and carrier separation efficiency but also offer abundant edge active sites. Additionally, when Re vacancy is produced in ReS 2 , some unsaturated S atoms around the defect sites can promote the adsorption of a H + proton, thus improving the HER performance more than perfect ReS 2 [34,53,60]. The performance for ReS 2 as a cocatalyst has been reported to be comparable to widely used noble-metal cocatalysts such as Pt [44,57]. Various photocatalysts have been summarized in Table 1 on the water splitting for the H 2 production. The morphology and structure of cocatalysts are important factors that can influence the photocatalytic performance. Cocatalysts typically exhibit morphologies such as nanoparticles, nanospheres, flakes, clusters, nanotubes, and quantum dots. Inspired by nature, Lin et al. designed novel sea-urchin-like ReS 2 nanosheet/TiO 2 nanoparticle heterojunctions through a simple one-pot hydrothermal method (Figure 4). Compared with the bulk ReS 2 cocatalyst composed of a large number of agglomerated nanoparticles or nanosheets, the sea-urchin-like ReS 2 cocatalyst shows a significantly accelerated charge transfer due to the unusual charge edge-collection effect. The photocatalytic H 2 evolution rate is 3.71 mmol h −1 g −1 (an apparent quantum efficiency (AQE) of 16.09%), which is 231.9 times that of P25 TiO 2 [104]. unusual charge edge-collection effect. The photocatalytic H2 evolution rate is 3.71 mmol h −1 g −1 (an apparent quantum efficiency (AQE) of 16.09%), which is 231.9 times that of P25 TiO2 [104]. TMDs with a metallic 1T phase and semi-metallic 1T' phase generally have high electrical conductivity [106,107]. However, most TMDs with the 1T-phase and 1T'-phase are thermodynamically unstable and can easily convert to the sTable 2H-phase [108,109]. Inspiringly, ReS2 has a thermodynamically sTable 1T'-phase crystal structure, which shows great promise for better catalytic performance [32,58]. As cocatalysts, the conductive 1T'-ReS2 nanosheets can trigger the host g-C3N4 nanotubes to generate more photoexcited charges, promote the carrier migration and separation, and supply abundant active sites for photocatalysis. Thus, the 1T'-ReS2/g-C3N4 composite containing 12 wt% 1T'-ReS2 exhibits an excellent photocatalytic H2 evolution rate of 2275 mmol h −1 g −1 [57]. Regulating phase structure and constructing an in-plane heterostructure of ReS2 can also provide more active sites and increase electron transfer [26]. Chen et al. reported that simply phosphorization treatment led to a shift in the Fermi level due to the extra electrons filling in the d orbital, and thus, ReS2 partially transformed from the T' to the T phase [62]. The developed 2% ReS2-CdS/P-0.2 photocatalyst with in-plane T-T' heterophase junctions enhanced the interfacial effect between CdS and ReS2, which can facilitate the electron transfer and exhibit an optimal hydrogen generation rate of 14.68 mmol g −1 h −1 .
Heterostructures with well-identified interfaces are crucial for enhancing photocatalytic performance. The intimate interfacial contact between the individual components via the covalent bond can greatly accelerate the separation of photogenerated electrons and holes. Taking the TiO2@ReS2 nanocomposites for example, the photoexcited electrons can rapidly transfer to ReS2 and diffuse to the active sites due to the chemical interaction of the Ti−O−Re bond [58]. The optimized TiO2@ReS2 nanocomposites with 30 wt% ReS2 exhibit superior photocatalytic H2 evolution activity with a rate of 1404 μmol h −1 g −1 . In another 2D/2D heterostructure, the cocatalyst ReS2 with abundant active sites is strongly anchored on the TiO2 by the chemical interaction of the Ti-S bond, and the established intimate interfacial contact facilitates efficient carrier transfer across the interface [103]. Recently, the strongly coupled interface between atomic-level regulated ReSe2 and various TMDs with a metallic 1T phase and semi-metallic 1T' phase generally have high electrical conductivity [106,107]. However, most TMDs with the 1T-phase and 1T'-phase are thermodynamically unstable and can easily convert to the sTable 2H-phase [108,109]. Inspiringly, ReS 2 has a thermodynamically sTable 1T'-phase crystal structure, which shows great promise for better catalytic performance [32,58]. As cocatalysts, the conductive 1T'-ReS 2 nanosheets can trigger the host g-C 3 N 4 nanotubes to generate more photoexcited charges, promote the carrier migration and separation, and supply abundant active sites for photocatalysis. Thus, the 1T'-ReS 2 /g-C 3 N 4 composite containing 12 wt% 1T'-ReS 2 exhibits an excellent photocatalytic H 2 evolution rate of 2275 mmol h −1 g −1 [57]. Regulating phase structure and constructing an in-plane heterostructure of ReS 2 can also provide more active sites and increase electron transfer [26]. Chen et al. reported that simply phosphorization treatment led to a shift in the Fermi level due to the extra electrons filling in the d orbital, and thus, ReS 2 partially transformed from the T' to the T phase [62]. The developed 2% ReS 2 -CdS/P-0.2 photocatalyst with in-plane T-T' heterophase junctions enhanced the interfacial effect between CdS and ReS 2 , which can facilitate the electron transfer and exhibit an optimal hydrogen generation rate of 14.68 mmol g −1 h −1 .
Heterostructures with well-identified interfaces are crucial for enhancing photocatalytic performance. The intimate interfacial contact between the individual components via the covalent bond can greatly accelerate the separation of photogenerated electrons and holes. Taking the TiO 2 @ReS 2 nanocomposites for example, the photoexcited electrons can rapidly transfer to ReS 2 and diffuse to the active sites due to the chemical interaction of the Ti−O−Re bond [58]. The optimized TiO 2 @ReS 2 nanocomposites with 30 wt% ReS 2 exhibit superior photocatalytic H 2 evolution activity with a rate of 1404 µmol h −1 g −1 . In another 2D/2D heterostructure, the cocatalyst ReS 2 with abundant active sites is strongly anchored on the TiO 2 by the chemical interaction of the Ti-S bond, and the established intimate interfacial contact facilitates efficient carrier transfer across the interface [103]. Recently, the strongly coupled interface between atomic-level regulated ReSe 2 and various semiconductor photocatalysts and the abundant Re/Se active sites have been demonstrated to boost the performance for photocatalytic H 2 evolution [110]. Chemical modification can adjust the optical and electronic properties of ReS 2 nanosheets and produce new catalytic properties by increasing the catalytic active sites exposed in solution, which is an important goal for the development of practical and efficient solar hydrogen production devices. Wu et al. used C 6 H 5 COOH on ReS 2 to form a molecular linkage between 2D ReS 2 and 0D two-phase TiO 2 through carboxylate ( Figure 5) [56]. This structure provides a Z-scheme molecular electron channel, while the non-functionalized ReS 2 provides an unstable and inefficient type-II path. The ReS 2 -C 6 H 5 COOH-TiO 2 composite photocatalyst can not only provide good wettability, connection, and charge transport for solar hydrogen production but also prevent ReS 2 from being oxidized by photogenerated holes. The composite photocatalyst exhibited high photocatalytic activity for solar hydrogen production (9.5 mmol h −1 g −1 ReS 2 nanosheets, 4750 times higher than bulk ReS 2 ) and high cycle stability over 20 h.

Photocatalytic CO 2 Reduction
Anthropogenic emissions of greenhouse gases, such as CO 2 , into the environment can lead to global warming. To reduce CO 2 production, various strategies have been developed. Using clean renewable energy to fix CO 2 can promote the carbon cycle and alleviate the global energy crisis. Therefore, photocatalysis has received extensive attention because it can convert carbon dioxide into renewable fuels. Various photocatalysts have been summarized in Table 2 for the CO 2 reduction.
The valence band edge of ReS 2 (Ev = 1.36 V vs. NHE) is more positive than the oxidation potential of H 2 O oxidation (0.82 V vs. NHE at pH = 7), while its conduction band edge (E C = −0.20 V vs. NHE) is much lower than the reduction potential of CO 2 (−0.52 V vs. NHE at pH = 7), indicating that ReS 2 on its own does not have the ability of photocatalytic reduction of CO 2 [111]. However, the introduction of defects on the ReS 2 surface can not only modulate its band structure but also activate the inert surface to realize efficient charge transfer [51]. For example, Zhang et al. synthesized a type I heterojunction which was composed of ReS 2 nanosheets and CdS nanoparticles via a self-assembly approach (Figure 6a-c). ReS 2 with S vacancies induced by visible-light illumination is beneficial to the chemical adsorption and activation of CO 2 , as well as the electron transfer between CO 2 and ReS 2 . The optimized ReS 2 /CdS heterostructure exhibits an enhanced photocatalytic CO 2 -to-CO conversion activity of 7.1 µmol g −1 accompanied by a prominent selectivity of 93.4% [51].
Owing to the excellent light absorption, large specific surface area, and abundant active sites of ReS 2 , it can serve as an ideal cocatalyst for photocatalytic CO 2 reduction [51,64,111]. Huang et al. constructed an indirect Z-scheme heterostructure Au-Pt/Cu 2 O/ReS 2 via chemical reduction and photodeposition (Figure 6d,e). The photocatalytic efficiency of the prepared heterojunction was significantly improved, and the CO yield was 30.11 µmol g −1 , which was higher than that of pure Cu 2 O quantum dots (22.87 µmol g −1 ). In addition, the selectivity of CH 4 and CO can be controlled by manipulating the mass ratio of Au/Pt (Figure 6f,g). Because of the high work function and excellent conductivity of Au and Pt, the electron transfer was accelerated between Cu 2 O and ReS 2 (Figure 6h) [111]. 5) [56]. This structure provides a Z-scheme molecular electron channel, while the nonfunctionalized ReS2 provides an unstable and inefficient type-II path. The ReS2-C6H5COOH-TiO2 composite photocatalyst can not only provide good wettability, connection, and charge transport for solar hydrogen production but also prevent ReS2 from being oxidized by photogenerated holes. The composite photocatalyst exhibited high photocatalytic activity for solar hydrogen production (9.5 mmol h −1 g −1 ReS2 nanosheets, 4750 times higher than bulk ReS2) and high cycle stability over 20 h.

Degradation of Organic Pollutants
The increase in energy consumption associated with the rapid development of modern society has led to a corresponding rise in environmental pollution, particularly water pollution, which poses a serious threat to human health and global ecological balance. To address this issue, advanced energy production and pollution control technologies should be developed. Among them, photocatalysts have demonstrated their efficacy in degrading organic pollutants. Various photocatalysts have been summarized in Table 3. Specifically, due to the nonbonding d electron of the Re atom, the 1T-ReS 2 phase exhibits a diamondchain structural distortion caused by the Re−Re bond interaction [112]. Therefore, the transitions of exciton in ReS 2 are highly polarization-dependent [113]. More electron-hole pairs and a longer carrier lifetime parallel to the diamond-chain direction give rise to a faster photodegradation rate constant up to 12 times greater than that perpendicular to the diamond-chain direction [114].

Degradation of Organic Pollutants
The increase in energy consumption associated with the rapid development of modern society has led to a corresponding rise in environmental pollution, particularly water pollution, which poses a serious threat to human health and global ecological balance. To address this issue, advanced energy production and pollution control technologies should be developed. Among them, photocatalysts have demonstrated their efficacy in degrading organic pollutants. Various photocatalysts have been summarized in Table 3. Specifically, due to the nonbonding d electron of the Re atom, the 1T-ReS2 phase exhibits a diamondchain structural distortion caused by the Re−Re bond interaction [112]. Therefore, the transitions of exciton in ReS2 are highly polarization-dependent [113]. More electron-hole pairs and a longer carrier lifetime parallel to the diamond-chain direction give rise to a faster photodegradation rate constant up to 12 times greater than that perpendicular to the diamond-chain direction [114].   The construction of a p-n heterojunction can induce a charge diffusion near the interface in nanocomposites until equilibrium is reached at the Fermi level between the two materials. For example, in the TiO 2 @ReS 2 nanocomposites, this effect will lift the conduction band position of ReS 2 higher than that of TiO 2 , leading to a better electron−hole separation efficiency. Under sunlight irradiation, the degradation activity of organic pigments can be significantly improved compared with pure TiO 2 nanoparticles [47]. Similar strategies take effect such that the ReS 2 /MIL-88B(Fe) heterojunction can extend the lifetime of photoexcited electron−hole pairs, absorb more visible light, and enhance persulfate activation, leading to a better catalytic capacity for ibuprofen degradation compared with the individual component [52]. Electrostatic attraction is significant for fabricating heterojunctions. For instance, the g-C 3 N 4 and defect-rich ReS 2 attain close contact, resulting in faster generation of the reactive oxygen species than pristine g-C 3 N 4 [116].
It is an efficient strategy to improve the photocatalytic performance by constructing a dynamic built-in electric field in a "self-driven" manner to continuously drive the separation of carriers [117]. Recently, a BaTiO 3 @ReS 2 piezo-assisted photocatalysis heterostructure has been successfully synthesized by Liu et al. [115] through the coupling of interfacial covalent bonds and piezotronics, showing high degradation efficiency for refractory pollutants. The introduction of distorted 1T-ReS 2 can enhance the light absorption of the hybrid nanostructure. Moreover, the formed interfacial Re-O covalent bond (Figure 7a,b) can serve as the channels for charge carriers' transfer. In addition, the internal polarization field in the Schottky interface can reduce the Schottky barrier of photogenerated charge transfer. Consequently, the BaTiO 3 @ReS 2 system obtained high O 2 activation activity and an excellent synergistic effect under ultrasonic and light conditions. The BaTiO 3 @ReS 2 exhibits ultrahigh pollutant degradation activity. The apparent rate constant of piezoelectricassisted photocatalysis is 0.133 min −1 , which is 16.6 and 2.44 times that of piezoelectric catalysis and photocatalysis, respectively. (Figure 7c,d) [115].

Photocatalytic Reduction of Metal Ions
Heavy metals in aquatic systems and drinking water sources are acutely toxic and carcinogenic to most organisms. Taking Cr (VI) as an example, photocatalytic reduction of Cr (VI) to low-toxic Cr (III) is generally considered to be a promising and practical option because this method is more effective and low-cost and does not produce any hazardous chemicals. Zhou et al. proposed a simple and feasible atomic hybridization strategy to accelerate the reaction kinetics process via constructing electronic pathways [86]. The reduced carbon quantum dots (rCQDs)/ReS2 heterostructure is synthesized via the CVD and hydrothermal method (Figure 8a,b). Compared with pure ReS2 nanosheets and

Photocatalytic Reduction of Metal Ions
Heavy metals in aquatic systems and drinking water sources are acutely toxic and carcinogenic to most organisms. Taking Cr (VI) as an example, photocatalytic reduction of Cr (VI) to low-toxic Cr (III) is generally considered to be a promising and practical option because this method is more effective and low-cost and does not produce any hazardous chemicals. Zhou et al. proposed a simple and feasible atomic hybridization strategy to accelerate the reaction kinetics process via constructing electronic pathways [86]. The reduced carbon quantum dots (rCQDs)/ReS 2 heterostructure is synthesized via the CVD and hydrothermal method (Figure 8a,b). Compared with pure ReS 2 nanosheets and CQDs/ReS 2 , the reduction reaction rate constants of the pseudo-first-order kinetic model were increased by about 13.1 and 4.3 times, respectively. (Figure 8c). C=O double bonds reduced and then anchored onto ReS 2 nanosheets can form an electronic channel via Re-5d and O-2p orbital hybridization (Figure 8d), in which a photoinduced carrier of the rCQDs can freely migrate to ReS 2 for hexavalent chromium reduction (Figure 8e) [86].

Photocatalytic Water Disinfection
The treatment cost of freshwater pollution is high, and the shortage of freshwater resources has become a more complex problem. Photocatalysis techniques have been regarded as one of the most promising strategies for environmental remediation. It is well

Photocatalytic Water Disinfection
The treatment cost of freshwater pollution is high, and the shortage of freshwater resources has become a more complex problem. Photocatalysis techniques have been regarded as one of the most promising strategies for environmental remediation. It is well known that when the photocatalyst absorbs light, electron−hole pairs are formed, which interact with water and dissolved oxygen to generate activated oxygen species (ROS), such as hydroxyl radicals, singlet oxygen, and superoxide [118]. These strong oxidants can destroy essential macromolecules in bacteria, resulting in bacterial disinfection [119]. The field of ReS 2 photocatalytic sterilization is still in the preliminary stages. Pioneer work by Ghoshal et al. proved that vertically oriented ReS 2 nanosheets show great potential for photocatalytic water disinfection [78]. However, due to the rapid recombination of the carriers, its inactivation efficiency still needs to be further enhanced.
It has been found that the construction of a 0D/2D heterostructure can greatly improve the sterilization efficiency of a photocatalyst because of the effective separation of charge carriers. Recently, Zhang et al. have reported that a Ag-modified ReS 2 heterostructure grown on a series of substrates can be used as a visible-light-driven photocatalyst for highly efficient water sterilization. Different substrates and the size of Ag nanoparticles are critical to the photocatalytic bactericidal performance (Figure 9a,b). The Ag nanoparticles on the surface of ReS 2 nanosheets can not only act as electron capture agents to promote the separation of photogenerated carriers but also inhibit the recombination of photogenerated carriers (Figure 9c). The main active substances in the photocatalysis process are h + , •OH, and •O 2− . All Escherichia coli can be inactivated within 30 min at a concentration of 1.72 mg L −1 by the reusable Ag/ReS 2 heterostructure under visible light irradiation (Figure 9d) [85].
cules 2023, 27, x FOR PEER REVIEW 17 of photocatalytic water disinfection [78]. However, due to the rapid recombination of t carriers, its inactivation efficiency still needs to be further enhanced. It has been found that the construction of a 0D/2D heterostructure can greatly im prove the sterilization efficiency of a photocatalyst because of the effective separation charge carriers. Recently, Zhang et al. have reported that a Ag-modified ReS2 heterostru ture grown on a series of substrates can be used as a visible-light-driven photocatalyst f highly efficient water sterilization. Different substrates and the size of Ag nanoparticl are critical to the photocatalytic bactericidal performance (Figure 9a,b). The Ag nanopa ticles on the surface of ReS2 nanosheets can not only act as electron capture agents to pr mote the separation of photogenerated carriers but also inhibit the recombination of ph togenerated carriers (Figure 9c

Summary and Outlook
Two-dimensional ReS 2 , a special new material in the family of TMDs, has great potential as a photocatalyst due to its favorable band structure, which is independent of the number of layers, as well as its high specific surface area and numerous active sites. This review summarizes the basic properties and heterostructure synthesis methods of ReS 2 . Several types of ReS 2 -based heterostructures and their recent progress in photocatalysis fields, such as photocatalytic hydrogen production, CO 2 reduction, and pollutant degradation, are then reviewed, demonstrating that ReS 2 is an efficient and stable photocatalyst. However, the research on ReS 2 is still in its early stages, and there is room for improvement of its photocatalytic performance. Based on synthetic chemical mechanisms, semiconductor physics of photocatalysis, and recent research results, the following aspects can be further explored for the rational design of efficient heterostructure photocatalysts:

1.
Developing new synthetic methodologies to achieve precise synthesis of ReS 2 and its composites. Mechanical mixing is usually used in the construction of heterojunctions for ReS 2 , but precise design for the interactions between the two or more materials has not yet been realized in the preparation of photocatalysts, resulting in random morphology and exposed crystal faces of the composites. With the growth mechanism of ReS 2 as a basis, rational control of the growth conditions is expected to precisely control the size and crystal face orientation. In addition, reducing the size of ReS 2 down to sub-10-nanometers is another intriguing direction due to the enhanced quantum confinement effect and derived novel photonic properties; however, structural controllability including particle diameter, edge structure, phase transition, etc. is highly important and is also well deserving of more research attention.

2.
Surface engineering strategies toward tailorable physicochemical properties of ReS 2 . Few studies on the design and modification of the surface structure of ReS 2 have been reported for photocatalytic research. Surface defects can become centers of the electron−hole complex, but they are not entirely inutile. For example, introducing Re vacancies in a reasonable way can enhance the adsorption ability of H + . In addition, organic molecules can form chemical bonds on the surface S vacancies, which can improve the interface wettability of ReS 2 on the one hand and fine-tune its energy band position on the other hand.

3.
Combining novel characterization methods with theoretical calculations. The surface and interface structure changes in photocatalysis should be thoroughly investigated from both microscopic and transient aspects. Real-time monitoring of intermediates and catalytic products is essential for understanding the photocatalytic mechanism and further optimizing the performance of photocatalysts. For instance, in situ Fourier Transform Infrared spectroscopy and online mass spectrometry can probe the source of hydrogen and the fate of the sacrifice reagents in photocatalytic hydrogen evolution [59]. Ultra-high spatial and temporal resolution technique, such as tipenhanced Raman spectroscopy, can greatly improve the signal-to-noise ratio and spatial resolution, allowing for the characterization of single molecules and even single chemical bonds. Moreover, the theoretical simulation of the model systems, particularly first-principles, external field simulations, and micro-reaction dynamics simulations, is essential for exploring the fundamental mechanisms of photocatalysis. The binding energy between various intermediates and catalysts can be obtained by calculating the adsorption energy and charge density difference, further reflecting the configuration of the reaction intermediates on the catalyst surface.
Overall, ReS 2 has the potential to excel as a photocatalytic material, and further investigation into its photocatalytic performance and mechanism could propel the development of photocatalysis and provide new insights into environmental and energy issues.