Engineering of Cu-Based Quaternary Sulfide Nanomaterials for Photocatalytic Applications

Zu, Bingqian; Chen, Song; Jin, Qiren; Wu, Liang

doi:10.70322/prp.2024.10009

Photocatalysis: Research and Potential

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Engineering of Cu-Based Quaternary Sulfide Nanomaterials for Photocatalytic Applications

Bingqian Zu Song Chen Qiren Jin Liang Wu *

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Key Laboratory of Functional Molecular Solids, Ministry of Education, School of Chemistry and Materials Science, Anhui Normal University, Wuhu 241002, China

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Photocatalysis: Research and Potential 2024, 1 (4), 10009; https://doi.org/10.70322/prp.2024.10009

Received: 14 June 2024 Accepted: 15 October 2024 Published: 22 October 2024

ABSTRACT: Semiconductor nanomaterials have been widely used as light-responsive photocatalysts for solar-to-fuel conversion over the past decade. However, the wide band gap of the most reported photocatalysts stems from light absorption in the visible region and results in low solar conversion efficiency. Therefore, it is necessary to develop new semiconductor nanomaterials with high absorption coefficients over the visible region as photocatalysts. The most promising candidates include Cu-based quaternary sulfide nanomaterials (CQSNs), such as Cu-II-III-S (e.g., CuZnInS, CuZnGaS), Cu-II-IV-S (e.g., Cu₂ZnSnS₄, Cu₂ZnGeS₄), and I-III-IV-S (e.g., CuInSnS₄, Cu₃GaSnS₅). This review provides a comprehensive overview of the recent progress in developing CQSNs for various photocatalytic applications. Firstly, we present an overview of the synthesis of CQSNs with precise control over crystal phase, composition, size, and shape using solution-based methods. Then, the enhancement of photocatalytic performance was presented via the engineering of CQSNs, which included surface engineering, elemental doping, cocatalyst loading, vacancy engineering, and interface engineering. Subsequently, we further introduce the photocatalytic applications in the fields of photocatalytic and photoelectrochemical hydrogen conversion, CO₂ reduction, organic synthesis, and pollutant degradation. Lastly, this review ends with views on the current challenges and opportunities of CQSNs in future studies.

Keywords: Cu-based quaternary sulfide nanomaterials; Photocatalytic hydrogen conversion; Photoelectrochemical hydrogen production; Photocatalytic CO₂ reduction; Photocatalytic organic synthesis; Photocatalytic pollutant removal

1. Introduction

The majority of the utilized energy is accompanied by CO₂ emissions, resulting in well-established greenhouse effects and numerous significant environmental concerns [1]. The global CO₂ emission must be limited to approximately 500 billion tonnes in order to effectively control the rise in global temperature to 1.5 °C [2]. In September 2020, China explicitly proposed the targets of achieving a “carbon peak” by 2030 and a “carbon neutrality” by 2060. To advance the implementation of the “dual carbon strategy” and mitigate carbon emissions, it primarily encompasses the following three aspects: (I) enhancing energy conservation on the demand side and optimizing energy utilization efficiency, (II) adjusting the energy mix and vigorously promoting low-carbon energy sources on the supply side, (III) concurrently advancing carbon sink development and carbon capture. The annual global CO₂ emissions amount to approximately 40 billion tonnes, with around 85% originating from the utilization of fossil fuels [3]. Therefore, there is an urgent need to explore clean, renewable, and carbon-neutral energy resources as substitutes for fossil fuels. Solar energy, the most abundant and environmentally friendly power source, holds great promise as a viable alternative that can play an indispensable role in all three aspects mentioned above. It will provide substantial support towards achieving the “double carbon” goal. Solar-driven photocatalysis technology holds significant potential for addressing environmental issues and energy shortages [4,5,6,7,8,9]. For example, converting solar energy to hydrogen provides an efficient approach to green hydrogen production. At the same time, photocatalytic CO₂ reduction serves as a valuable method for carbon capture and the generation of high-value-added products [10,11,12,13]. The photocatalytic reaction can be divided into three steps: (I) The photocatalysts absorb sunlight to generate carriers (excited electrons and holes); (II) The photogenerated carriers migrate to the surface of the photocatalysts; (III) The photogenerated carriers participate in the redox reaction [14,15]. The efficiency of photocatalytic conversion can be optimized by enhancing three key thermodynamic and dynamic parameters: light absorption efficiency, charge separation efficiency, and redox reaction efficiency [16,17]. The excitation of electrons to the conduction band (CB) of semiconductor photocatalysts occurs under solar light irradiation while simultaneously leaving holes in the valence band (VB) [18,19,20]. Since the groundbreaking discovery by Prof. Akira Fujishima and Prof. Kenichi Honda in 1972 [21], which reported the successful photocatalytic hydrogen production over a TiO₂ photocatalyst, there has been an increasing focus on the development of semiconductor photocatalysts, leading to remarkable achievements [22,23,24,25,26]. The rapid development of nanotechnology has strongly promoted the design and synthesis of nanostructured semiconductor photocatalysts. Compared to bulk semiconductor materials, nanostructured semiconductor photocatalysts offer distinct advantages due to their unique physical and chemical properties, which arise from quantum effects and a greatly increased surface area [27,28,29]. A series of semiconductors have been explored as photocatalysts, such as metal nitrides, metal oxides, metal oxysulfides, metal sulfides, and polymers [30,31,32,33,34]. However, most of the reported semiconductor photocatalysts exhibit limited sunlight absorption capability, thereby significantly constraining their potential for photocatalytic applications. The development of a nanostructured photocatalyst with a high absorption coefficient across the entire visible range is essential for achieving efficient photocatalytic reactions. Cu-based quaternary sulfide (CQS) compounds can be regarded as derivatives of ZnS/CdS by substituting Zn/Cd with environmentally friendly elements. These compounds exhibit exceptional properties in terms of visible light absorption and possess suitable energy band structures for efficient sunlight absorption, making them promising candidates for potential photocatalysts [35,36,37,38]. In addition, the energy band structure of CQSs can be easily regulated to optimize their visible light absorption capacity [39,40]. In recent years, the synthesis and photocatalytic applications of Cu-based quaternary sulfide nanomaterials (CQSNs) have garnered significant attention. In this review, we provide a summary of recent research on CQSNs—particularly those that belong to the Cu-II-III-S (e.g., CuZnInS, CuZnGaS), Cu-II-IV-S (e.g., Cu₂ZnSnS₄, Cu₂ZnGeS₄), and I-III-IV-S (e.g., CuInSnS₄, Cu₃GaSnS5) semiconductor families—as promising visible-light-driven photocatalysts (Figure 1). We initially present a concise overview of the solution synthesis method for fabricating CQSNs with varying compositions, shapes, sizes, and crystal phases, which have significantly impacted the photocatalytic performances of semiconductor materials. Then, the photocatalytic property of CQSNs can be considerably improved through strategies that modify the semiconductor band structure, facilitate efficient charge separation, and improve the efficiency of redox reactions. These strategies include surface engineering, elemental doping, cocatalyst loading, vacancy and interface engineering. We then present various photocatalytic applications of these CQSNs, focusing on solar-to-hydrogen conversion, CO₂ reduction, photoelectrochemical hydrogen production, organic synthesis, and pollutant removal. Finally, we also discuss the challenges and opportunities for future studies involving CQSNs.

Figure 1. Schematic illustration of the control synthesis, engineering, and photocatalytic applications of Cu-based quaternary sulfide nanomaterials.

2. Controlled Synthesis of Cu-Based Quaternary Sulfide Nanomaterials

Bulk CQS semiconductors can easily be obtained throuhigh-temperature solid-state methods [41,42]. In contrast, CQSNs are typically synthesized at relatively lower temperatures using solution-based approaches, such as colloidal chemical synthesis and hydrothermal/solvothermal methods [43,44]. The significant advancements in solution-based synthesis of nanomaterials have facilitated precise control over composition, shape, size, and crystal phase—all crucial factors influencing the photocatalytic activity of CQSNs. This section provides a summary of studies on the controlled synthesis of CQSNs. 2.1. Composition-Tunable Synthesis The band structure of CQSNs can be easily regulated to enhance their photocatalytic properties by adjusting the element composition [45,46,47]. The optical and electronic properties of copper-based quaternary chalcogenides, such as band structure, photogenerated charge carrier separation, and transfer, are significantly influenced by the ratio of Zn [48,49,50]. Tang and coauthors have developed a colloidal method for synthesizing one-dimensional (1D) Cu-Zn-Ga-S (CZGS) nanorods with varying Zn contents and investigated their photocatalytic hydrogen production properties [51]. As shown in Figure 2a, the synthesized CZGS nanorods have a wurtzite (WZ) crystal structure, which remains unaffected by the Ga:Zn feeding molar ratio. The incorporation of Zn facilitated the growth of quaternary Cu−Ga−Zn−S nanorods along the [0001] direction. It promoted the formation of elongated nanostructures while preserving their 1D-shaped morphology and wurtzite crystal phase. As a result, the presence of Zn cations increases the length of the obtained nanorods (Figure 2b–f). Figure 2g shows a high-resolution transmission electron microscopy (HRTEM) image of CZGS nanorods with a Ga:Zn feeding molar ratio of 7:1. There are two series of vertical lattice fringes with spacings of 3.09 and 3.29 Å, corresponding to the (0002) and (10-10) planes of WZ CZGS. The alteration of optical properties in CZGS nanorods induced by composition was further characterized via absorption spectroscopy. The absorption band edge exhibits a noticeable blue shift (Figure 2h), while the bandgap increases (Figure 2i) with an increasing Ga:Zn feeding molar ratio, respectively. The photocatalytic hydrogen production performances of CZGS nanorods can be effectively regulated by adjusting the Ga:Zn feeding molar ratio. The CZGS nanorods with a Ga:Zn feeding molar ratio of 7:1 exhibit the best photocatalytic performances with a hydrogen evolution rate of 2.101 mmol h⁻¹ g⁻¹ (Figure 2j).

Figure 2. (a–f) X-ray diffraction (XRD) patterns and TEM images of CZGS NRs synthesized with Ga:Zn feeding molar ratios of 10:0, 19:1, 8:2, 7:3, and 7:1. (g) HRTEM image of CZGS NRs synthesized with Ga:Zn molar ratios of 7:1. (h,i) Diffuse reflectance spectroscopy spectra and the corresponding Tauc plots of CZGS nanorods. (j) Photocatalytic hydrogen production rates of the synthesized CZGS nanorods [51].

The CQS compounds can be regarded as alloys composed of binary sulfides and ternary sulfides, allowing for the adjustment of their composition by manipulating the ratio between these two types of sulfides [52,53,54]. For example, Cu-Zn-In-S (CZIS)—a solid solution that is created by alloying with binary ZnS (ZS) and ternary CuInS₂ (CIS)—shows a ZS:CIS molar ratio-dependent absorption and photocatalytic performance. The bandgap of ZS is too wide for efficient visible-light utilization, whereas CIS has a suitable conduction band minimum to facilitate the reduction of H₂O to H₂. The alloying of ZS and CIS can eliminate these drawbacks and optimize the photocatalytic properties of CZIS nanocrystals. A series of alloyed CZIS nanostructures have been developed for photocatalytic applications, such as zero-dimensional (0D) zinc-blende (ZB) CZIS quantum dots, 1D WZ CZIS nanorods, two-dimensional (2D) WZ CZIS nanobelts and three-dimensional (3D) ZB nanostructures [55,56,57,58]. The photocatalytic hydrogen production performance of all CZIS nanostructures depends on the molar ratio of ZS:CIS. 2.2. Shape-Controlled Synthesis Refining the shape of nanostructures can enhance their functional capabilities, making them more suitable for advanced applications [59,60,61]. The synthesis of CQSNs with tailored morphology is crucial for their photocatalytic applications, as different shapes confer diverse physical and chemical properties [62]. Taking Cu₂ZnSnS₄ (CZTS) for an instant, WZ CZTS with different shapes can be achieved by selecting suitable synthesis methods. Cabot and coauthors reported a scalable colloidal method for the high-yield production of 0D CZTS nanocrystals [63]. As shown in Figure 3a, the obtained monodisperse CZTS nanocrystals exhibit a quasi-spherical shape with minimal size dispersion (below 10%). This can be attributed to the efficient mixing and heat transfer within the reaction solution and precise control of heating ramp stability achieved through intensive argon bubbling. The HRTEM image and the fast Fourier transform (FFT) pattern show that the CZTS nanocrystals have a WZ phase (Figure 3b). Ryan and coauthors have successfully prepared 1D WZ stoichiometric CZTS nanorods using 1-dodecanethiol (1-DDT) and tert-dodecyl mercaptan (t-DDT) as sulfur sources [64]. The obtained CZTS nanorods have an average diameter of 11 ± 0.5 nm and a length of 35 ± 3 nm (Figure 3c). The HRTEM image of an individual nanorod exhibited a lattice fringe with an interplanar spacing of 0.32 nm, corresponding to the (0002) plane of WZ CZTS (Figure 3d), clarifying the WZ phase of the obtained nanorods. 2D nanostructures (e.g., nanoplates and nanosheets) exhibit great potential as light-responsive photocatalysts due to their extensive surface area and exposed special facets [65,66,67]. Bao and coauthors synthesized WZ CZTS nanosheets using a one-pot thermal decomposition method using 1-DDT as both the reaction solvent and sulfur source [68]. The obtained thin CZTS nanosheets exhibited predominantly quasi-triangular and hexagonal morphologies, with an average size ranging from 300 to 400 nm (Figure 3e). The HRTEM image revealed lattice fringes exhibiting an interplanar spacing of 0.33 nm, corresponding to the (100) family planes of WZ CZTS (Figure 3f). The selected area electron diffraction (SAED) pattern (inset of Figure 3f), combined with the HRTEM image, indicates that the WZ CZTS nanosheets expose the high-energy (002) plane and exhibit a well-crystallized, single-crystalline nature. 3D nanostructures have also been synthesized to improve the photocatalytic activity of semiconductor photocatalysts, as these nanoscale assemblies can serve as active sites for photocatalytic reactions [69,70,71]. Xu and coauthors used a top-down synthetic strategy to prepare 3D WZ CZTS nanoboxes [72]. The CZTS nanoboxes were produced via reacting Cu₂O nanocubes with Zn²⁺ ions and tin metal chalcogenide complexes at pH = 7 (Figure 3g). The interplanar spacings of 0.33 nm shown in Figure 3f correspond to the (010) and (100) planes of WZ CZTS, illustrating the WZ structure of the synthesized nanoboxes.

Figure 3. TEM and HRTEM images of WZ CZTS (a,b) nanocrystals [63], (c,d) nanorods [64], (e,f) nanosheets [68], and (g,h) nanoboxes [72]. The insert in (a) is the graph of size distribution. The inserts in (b) are the HRTEM image of the selected nanocrystals and the corresponding FFT pattern. The inserts in (c,f) are the SAED patterns.

2.3. Size-Controlled Synthesis The size of a semiconductor photocatalyst is a key factor in modifying its photocatalytic performances as it directly affects the surface area, which determines the number of available surface-active reaction sites for photocatalytic [73,74,75]. Otherwise, size can be used to regulate the dynamics of electron-hole recombination. In general, reducing the size of a semiconductor photocatalyst enhances its photocatalytic performance by decreasing bulk charge recombination and increasing the availability of surface-active sites [76,77]. When nanomaterials are too small, the strong quantum confinement widens the band gap, thereby limiting their absorption range to visible light [78,79]. Moreover, surface carrier recombination becomes significant and can overshadow the benefits of a large surface area. Therefore, it is necessary to synthesize photocatalysts in various sizes to determine the optimum size for achieving maximum photocatalytic properties. The manipulation of reaction conditions such as solvent and ligand quantities, reaction time, and temperature allows for facile preparation of nanocrystals with different sizes. Xie and coauthors have successfully synthesized CZIS nanocrystals, achieving size control through temperature-dependent reaction [80]. The size of the CZIS nanocrystals increased from 3.0 to 7.0 nm as the reaction temperature was raised from 180 to 240 °C. Torimoto and coauthors successfully achieved the controlled synthesis of CZTS nanocrystals by manipulating the amount of oleylamine (OLA) and adjusting the reaction temperature [81]. As shown in Figure 4a–d, CZTS nanoparticles with an average diameter of ca. 2.8 and ca. 3.5 nm are produced at 150 °C by adding 0.13 and 2.0 mmol of OLA, respectively. Figure 4e–h shows that the CZTS nanocrystals with an average diameter of ca. 2.9 and 5.2 nm are obtained at 120 and 180 °C by adding 0.5 mmol of OLA, respectively. The obtained CZTS nanoparticles displayed size-dependent PEC properties.

Figure 4. TEM images and size distribution graphs of the CZTS nanoparticles synthesized by the addition of (a,b) 0.13 mmol of OLA at 150 °C, (c,d) 2.0 mmol of OLA at 150 °C, (e,f) 0.5 mmol of OLA at 120 °C, (g,h) 0.5 mmol of OLA at 280 °C [81].

2.4. Phase-Controlled Synthesis The crystal structure of semiconductor materials is an additional crucial factor that can impact their photocatalytic performance [82,83,84]. For example, the photocatalytic activity of anatase phase TiO2 nanocrystals is higher than that of the rutile phase [85]. CQSs are known to exist in two types of crystal phases: WZ and ZB [86]. ZB types derived from cubic ZB ZnS are classified into two structures: the cation-disordered cubic structure and the cation-ordered tetragonal structure (e.g., kesterite) [87]. Meanwhile, there are cation-disordered hexagonal structures and cation-ordered orthorhombic structures (e.g., WZ-kersterite, WZ-stannite) in WZ-type Cu-based quaternary sulfides [88]. The ZB phase is thermodynamically more stable than its WZ counterpart in the majority of situations and exhibits greater ease of synthesis. In order to synthesize phase-pure products, it is necessary to carefully select reaction conditions that favor the crystalline of a specific crystal phase over another. It has been established that the choice of ligands/solvents plays a crucial role in determining the crystal phase in colloidal methods. Previous studies have shown that long-chain alkanethiols (e.g., 1-DDT), which not only act as an additional sulfur source but also function as surfactants, are advantageous for promoting the formation of the metastable WZ Cu-based multinary sulfide nanomaterials, such as CIS, Cu₂SnS₃, Cu₂CdGeS₄, and CZTS [89,90,91,92]. Lam and coauthors have successfully synthesized kersterite and WZ CZTS nanocrystals using element S (Figure 5a) and 1-DDT (Figure 5d) as sulfur sources, respectively [93]. The synthesized kesterite CZTS nanocrystals have an irregular morphology with an average size of 19.8 ± 6.5 nm (Figure 5b). The lattice fringes (Figure 5c) were found to be 0.27 and 0.33 nm, corresponding to the (020) planes and (11-1) planes for kesterite CZTS. The obtained WZ CZTS nanocrystals possess a nearly spherical shape with an average size of 12.9 ± 1.2 nm (Figure 5e). The HRTEM image (Figure 5f) displays lattice fringes with an interplanar spacing of 0.33 nm, corresponding to the (100) planes for WZ CZTS. In addition, phase-selective synthesis can also be accomplished by manipulating other experimental conditions, such as reaction temperature, templets, and precursors [94,95,96].

Figure 5. (a) Photo of OLA-S solution. (b,c) TEM and HRTEM images of CZTS nanoparticles synthesized using sulfur. (d) Photo of OLA-S solution. (e,f) TEM and HRTEM images of CZTS nanoparticles synthesized using 1-DDT [93].

3. Strategies for Photocatalytic Activity Improvement

The photocatalytic performance of pristine CQSNs is often limited by their poor light absorption capability, rapid recombination rate of electron-hole pairs, and sluggish reaction kinetics. As a new type of photocatalysts, the studies and applications of CQSN photocatalysts are comparatively limited compared to ZnS/CdS photocatalysts. At the same time, their photocatalytic activity is relatively low and requires further enhancement [24,28,72,76]. Therefore, various optimization strategies have been employed to enhance their photocatalytic performances. These modulation strategies are categorized into surface engineering, elemental doping, co-catalyst loading, defect management, and interface engineering. 3.1. Surface Engineering Surface engineering, such as the regulation of surface crystal facets, facilitates the creation of uniform and abundant atomic-level sites for the photocatalytic reaction [97,98,99]. Different crystal facets in semiconductor photocatalysts typically exhibit varying levels of photocatalytic activity [100,101,102]. The manipulation of semiconductors to expose their most active facet represents an efficient approach to enhancing their photocatalytic performance [103,104,105]. In 2D materials, the exposure of more surface atoms enhances the adsorption of target molecules and provides additional reaction sites, which promotes catalytic reactions and results in reduced material thickness [106,107]. Furthermore, the shorter charge transfer distance significantly decreases the recombination probability of the electron-hole pairs, thereby enabling highly efficient carrier utilization. Therefore, engineering semiconductor photocatalysts to obtain ultrathin 2D structures that expose the most active crystal facets can effectively enhance the photocatalytic properties. We have reported the surface engineering of single crystalline CZIS and CZGS nanobelts for achieving efficient solar-to-hydrogen production [57]. We first explored the relevant Gibbs free energy (ΔG_H) for hydrogen evolution reaction of (0001), (1010), and (1011) facets of WZ CZGS (Figure 6a) via density functional theory (DFT) calculation. As shown in Figure 6b, the ΔG_H of (0001) is the smallest, indicating that the (0001) facet had the smallest binding strength to atomic hydrogen. Following the Bell–Evans–Polanyi principle, the (0001) is the most active facet for photocatalytic hydrogen generation. Then, we designed a simple colloidal method assisted by OLA and DDT to synthesize a 2D single crystalline WZ CZIS with exposed (0001) facets and 2D single crystalline WZ CZIS. The obtained CZIS possesses a typical WZ structure (Figure 6c). As shown in Figure 6d, the obtained 2D CZIS presents a nanobelt morphology. The HRTEM images (Figure 6e,f) and SAED patterns (Figure g,h) of the randomly selected areas in Figure 6c indicate that the synthesized CZIS nanobelts possess a WZ structure, the single crystalline nature and a surface crystal facet of (0001). Moreover, the obtained CZIS nanobelts exhibited an excellent photocatalytic performance with a hydrogen evolution rate of 3.35 mmol h⁻¹ g⁻¹, which is higher than that of the nanoparticles and nanorods which exposed lesser (0001) facet (Figure 6i). Importantly, the CZIS and CZGS nanobelts display excellent photocatalytic stability, as evidenced by the absence of any decrease in the rate of photocatalytic hydrogen production after six cycles (Figure 6j).

Figure 6. (a) Crystal model of WZ CZIS. (b) Reaction Gibbs energy diagram for H2 evolution on different crystal facets of WZ CZIS. (c) XRD patterns of the obtained CZIS nanobelts. (d–h) HAADF-STEM image, HRTEM images, and SAED patterns of CZIS nanobelts. (i) Comparison of the photocatalytic hydrogen production properties of CZIS nanoparticles, nanorods, and nanobelts. (j) Recycle hydrogen generation property of CZIS nanobelt photocatalyst [57].

3.2. Elemental Doping Regulating the electronic energy band structure is a fundamental approach for optimizing the photocatalytic performances of semiconductor photocatalysts. Incorporating heteroatoms into semiconductor materials is an effective strategy to modulate their physical and chemical properties, such as extending light absorption range, adjusting bandgap structure, providing active sites, improving electric conductivity, and facilitating the H adsorption and desorption process. This offers ample opportunities for fine-tuning their photocatalytic capabilities [20,108,109]. Till now, many metal atoms (e.g., Cu, Mo, Pt, Ni, Fe, etc.) have been utilized to modify the semiconductor materials to enhance their photocatalytic activity [24,110,111,112,113]. Metal heteroatoms can serve as trap sites to reduce carrier recombination and also facilitate the transfer of carriers to surface redox reaction sites, thereby enhancing photocatalytic performance. Zhan and coauthors incorporated Cu into ZnIn₂S₄ to optimize its photocatalytic properties and provided atomic-level insights into the role of Cu atoms in modifying ZnIn₂S₄ [114]. The doping of Cu atoms introduces electronic acceptor states in close proximity to VB maximum, facilitating efficient carrier transport and increasing charge density, resulting in improved performance in photocatalytic hydrogen evolution. However, excessive doping leads to a significant upshift of VB maximum and distortion in atomic structure, leading to accelerated carrier recombination rates and, consequentially, a drastic reduction in photoactivity. The previous reports indicate that non-metal dopants, such as C, O, and P elements, can easily substitute the exposed S atoms on the surface of sulfides to enhance their photocatalytic properties [115,116,117]. For example, Chen and coauthors prepared C-doped SnS₂ nanosheets as a photocatalyst for efficient solar fuel production [118]. The C-doped SnS₂ nanosheets show higher selectivity and activity for photocatalytic CO₂ conversion under visible light. We also have utilized the P atom to manipulate the carrier dynamics of single crystalline quaternary sulfide nanobelts [119]. The P-doped CZIS nanobelts were synthesized via annealing CZIS nanobelts with sodium hypophosphite monohydrate under an argon atmosphere (Figure 7a). As shown in Figure 2b, the P atom has been uniformly entered into the CZIS nanobelts. Importantly, the P-doped CZIS nanobelts inherited the crystal structure and exposed facet, which is beneficial for photocatalytic hydrogen generation (Figure 7c). As shown in Figure 7b, the P atom has been successfully injected into the CZIS nanobelts and evenly distributed throughout the nanoribbon. Importantly, P doping does not change the crystal structure and morphology of the CZIS nanobelts (Figure 7d). Meanwhile P doping can extend the light absorption range and upshift the VB of CZIS nanobelts (Figure 7d,e). Time-resolved photoluminescence (PL) decay spectra and optical pump THz probe spectroscopy (OPTPS) demonstrate that P doping efficiently suppresses the recombination of photogenerated carrier and enhances the rates of carrier transfer, thereby improving the photocatalytic performances (Figure 7f,g). As a result, the P-doped CZIS nanobelts exhibit a 3.5-fold higher visible-light photocatalytic hydrogen production rate than that of pristine CZIS nanobelts (Figure 7h). Additionally, the CZIS-P nanobelts exhibit remarkable stability in photocatalytic hydrogen production, as evidenced by the negligible decrease in hydrogen production rates observed after 6 consecutive cycles totaling 24 h of operation.

Figure 7. (a) The schematic diagram depicts the fabrication of CZIS-P nanobelts via an annealing process. (b) EDS element mapping of CZIS-P nanobelts. (c–g) XRD patterns, UV−vis absorption spectra, electric band structures, PL decay spectra, and time-resolved THz photoconductivities of P-doped and pristine CZIS nanobelts. (h) Photocatalytic hydrogen generation rates over the pristine CZIS nanobelts, P-doped CZIS nanobelts synthesized at 300 °C for different times, Pt decorated CZIS nanobelts (CZIS/Pt), and CZIS nanobelts annealed at 300 °C without sodium hypophosphite monohydrate (CZIS-A) [119].

3.3. Cocatalyst Loading Loading cocatalysts onto the semiconductor photocatalysts is one of the effective methods for improving the catalytic properties [120,121,122]. The incorporation of cocatalysts confers numerous advantages during the photocatalytic process [123,124]. (I) The cocatalysts can act as trap sites to capture the photogenerated carriers, thereby promoting charge separation. (II) The cocatalysts contribute to enhanced absorption of the target molecules. (III) The cocatalysts have the ability to regulate the surface reaction dynamics and modulate the activation energy for specific target molecules. (IV) By altering molecular coordination modes, the cocatalysts exert control over product selectivity. (V) Incorporation of cocatalysts in certain systems can effectively suppress photocorrosion and improve the stability of the photocatalysts. So far, a wide range of cocatalysts have been developed and used to improve the photocatalytic properties of semiconductors, including noble metal nanoparticles, transition metal oxides and sulfides, metal phosphide, and graphene [125,126,127,128,129,130]. The utilization of noble metal nanoparticles as cocatalysts is widely employed to enhance the photocatalytic performances of CQSNs [131,132,133]. For example, Han and the co-author decorated Pt nanoparticles on the tips of CZIS nanorods to enhance visible-light-driven photocatalytic hydrogen production [134]. Cabot and coauthors utilized Pt and Au nanoparticles as cocatalysts, strategically deposited onto WZ CZTS to enhance its photocatalytic capabilities (Figure 8a) [135]. Quasi-spherical WZ CZTS are synthesized, and served as seeds to synthesize CZTS-Pt and CZTS-Au heterostructure nanoparticles (Figure 8b,c). The addition of Pt/Au does not alter the size of CZTS nanocrystals, while preferential nucleation of Pt/Au nanocrystals occurs at the surface of CZTS nanoparticles. CZTS-Pt and CZTS-Au heterostructures showed approximately a 6-fold and 5-fold higher photodegradation rate of Rhodamine B (RHB) compared to that of pure CZTS nanocrystals, respectively (Figure 8d). Figure 8e that the loading of Pt and Au nanoparticles enhanced the photocatalytic hydrogen production rate of CZTS nanocrystals. The highest hydrogen evolution rate of CZTS-Pt with the Pt load at around 1% is 1.02 mmol g⁻¹·h⁻¹, which is 8 times higher than that of pristine CZTS (0.13 mmol g⁻¹·h⁻¹) (Figure 8e,f). In addition, metal sulfides can also be used as cocatalysts to enhance the photocatalytic performances of CQSNs [136]. For example, Niu and coauthors optimized the photocatalytic performance of CZTS nanoparticles by introducing ca.1 mol% of MoS₂ [137]. The MoS₂/CZTS heteronanostructures show about 7.8 times higher photocatalytic hydrogen evolution rates than CZTS nanoparticles.

Figure 8. (a) Schematic illustration of the photocatalytic hydrogen generation over CZTS-Au and CZTS-Pt heterostructured nanoparticles. (b,c) Bright-field and dark-field TEM images of CZTS-Au and CZTS-Pt heteronanocrystals, respectively. (d) Photodegradation performances of RHB. (e) Comparison of photocatalytic hydrogen production performances of CZTS, CZTS-Au, and CZTS-Pt heteronanocrystals. (f) Photocatalytic hydrogen production performances of CZTS-Pt heteronanocrystals with varying Pt loads [135].

3.4. Vacancy Engineering As a common point defect, atomic vacancies significantly enhance the photocatalytic performance of semiconductor catalysts by modulating their photoelectrochemical and physicochemical properties. These include extending the light absorption range, serving as active reaction sites on the surface that can trap photogenerated carriers and promote adsorption and desorption of intermediates, and adjusting the energy band structures [138,139,140,141]. Since Yanagida and coauthors first reported the sulfur vacancy engineering of ZnS to enhance its photocatalytic performance in 1998, increasing attention has been drawn to vacancy engineering in photocatalysis [142]. Recently, with the growing interest in multinary metal sulfide photocatalysts, research on vacancy engineering of CQSNs is also gaining momentum [9,55,143,144,145]. The wide range of cations and the crystal structure tolerance exhibited by CQSNs facilitate the formation of cation vacancies within these materials [50,146,147,148]. The modulation of photoelectric properties and electronic structure in semiconductors is predominantly influenced by cation vacancies rather than anion vacancies [149,150]. The literature contains numerous reports on the utilization of cation vacancy defect engineering in Cu-based quaternary chalcogenides for enhancing solar cell properties, reducing thermal conductivity, and modifying emission properties [151,152,153,154]. Recently, Wang and coauthors reported on the defect engineering of colloidal Zn-doped CuInS₂ (ZCIS) QDs, utilizing Cu vacancy and Cu²⁺ defect states to achieve photocatalytic CO₂ reduction. They also investigated the impact of defect−defect interactions on photogenerated carrier dynamics and photocatalytic processes [155]. The Cu-deficient and stoichiometric QDs exhibit three typical defect state types: type-A stoichiometric QDs containing antisite CuIn″-InCu^•• pairs, type-B Cu-deficient ZCIS QDs with Cu vacancies (V_Cu’), and type-C Cu-deficient ZCIS QDs with both Cu vacancies (V_Cu’) and Cu²⁺ defect states (Cu_Cu^•) (Figure 9a). They first used theoretical simulations to predict the CO₂ absorption capacity of the three ideal Cu defect sites (Figure 9b), revealing the affinity between Cu²⁺ defect sites close cooper vacancies and CO₂ molecules is the highest. Thus, preparing type-C Cu-deficient QDs for photocatalytic CO₂ reduction is desirable. Then, type-A stoichiometric colloidal QDs were prepared and denoted as 8/12ZCIS, and the Cu-deficient QDs, including 4/12ZCIS, 2/12ZCIS, and 1/12ZCIS, were produced by changing the atomic Cu/In ratios. The 2/12ZCIS sample has the highest EPR signal intensity tested, indicating the most abundant vacancy defects in this QD (Figure 9c). The Cu-deficient 2/12ZCIS sample exhibited the highest photocatalytic activity for the reduction of CO₂ to CO with a rate of 0.4–0.5 mmol h⁻¹ g⁻¹ (Figure 9d–f). However, the presence of Cu vacancy defect may lead to unexpected charge recombination and consequently diminish the photocatalytic performances. For example, Ning and coauthors used Zn to effectively mitigate surface defects and unexpected recombination of carriers of CIS QDs. Consequently, the obtained CZIS QDs exhibit excellent photocatalytic hydrogen production performances in both the visible and near-infrared regions [40].

Figure 9. (a) Schematic illustration of the different defect state types in Cu-deficient and stoichiometric ZCIS QDs. (b) The hypothetical CO2 adsorption allocation on type-A stoichiometric, type-B Cu-deficient, and type-C Cu-deficient ZCIS QDs, respectively. (c) EPR spectra of the synthesized ZCIS QDs. (d–f) Photocatalytic CO2 reduction performances of ZCIS QDs [155].

3.5. Interface Engineering Due to the rapid recombination of photogenerated carriers and low electron utilization, the photocatalytic performance of CQSN photocatalyst is insufficient for practical applications. To overcome these drawbacks of unmodified single semiconductor photocatalysts, fabricating heterojunction interfaces by coupling two kinds of different semiconductors is an effective and widely used strategy to extend the light absorption range and promote the separation and transfer of photogenerated carriers, thereby improving their photocatalytic performances [156,157,158,159]. Generally, based on the band alignments and carrier transfer pathways, type II heterojunctions are 、considered optimal for efficient separation and transfer of photogenerated carriers [160,161,162]. The construction of efficient heterojunction photocatalysts relies on two crucial factors: electronic band structure matching and geometric configuration matching [163,164]. Based on these principles, diverse heterostructured photocatalysts have been designed and developed to realize efficient solar-to-chemical energy conversions [157,165,166,167]. In recent years, CQSN-based heterostructured photocatalysts have been synthesized for energy and environmental applications [70,168,169]. For example, Wu and coauthors have synthesized CZTS-CdS heterostructured nanoparticles for photocatalytic hydrogen evolution [170]. When CdS comes into contact with CZTS nanocrystals, a p-n heterojunction with a built-in electric field is formed, facilitating the transfer of electrons from CZTS to CdS (Figure 10a). They fabricated CZTS-CdS heterostructured nanocrystals through a two-step method. After 3 cycles of CdS grew, the average diameter of CZTS-3CdS nanoparticles increased to 12 nm, and their morphology changed from quasi-sphere to quasi-square (Figure 10b). The HRTEM reveals distinct lattice fringes at the periphery of CZTS-3CdS heterostructured nanocrystal, exhibiting an interplanar spacing of 0.33 nm corresponding to (111) plane of cubic CdS (Figure 10c). The two peaks located at 604 and 299 cm⁻¹ in the Raman spectrum of CZTS-3CdS heterostructured nanocrystals proved the existence of CdS (Figure 10d). The photocatalytic performances of CZTS-xCdS heterostructured nanocrystals were found to be enhanced with increasing CdS thickness. Among the heterostructures, CZTS-3CdS heterostructured nanocrystals possess the highest photocatalytic hydrogen production rate of 937.6 μmol g⁻¹ h⁻¹ (Figure 10e), which is 15-fold higher than the original WZ CZTS (64.5 μmol g⁻¹ h⁻¹) and 31 times as high as CdS (30.2 μmol g⁻¹ h⁻¹), respectively. However, if the thickness of CdS exceeded a certain limit (more than 3 cycles), it could potentially restrict the utilization of visible light by the CZTS core and impede electron transfer, resulting in a reduction in the rate of photocatalytic H₂ evolution. Furthermore, the CZTS-3CdS heterostructured nanocrystals exhibit exceptional stability (Figure 10f). Zhou and co-author decorated CdS nanorods with CZTS nanocrystals to prepare CZTS-CdS 0D/1D heterostructures [171]. The hydrogen evolution rate of the obtained 0D/1D heterostructures was 165 times higher than that of individual CZTS nanoparticles and 48 times higher than that of individual CdS nanorods, respectively.

Figure 10. (a) Schematic illustration of band structure alignment and photogenerated carrier separation in CZTS-CdS heterojunction. (b,c) TEM and HRTEM images of CZTS-CdS heteronanocrystals. (d) The Raman spectrum of CZTS-CdS heteronanocrystals. (e,f) Photocatalytic performances of the synthesized CZTS-CdS heteronanocrystals [170].

However, the defects and interface stresses resulting from changes in composition and lattice mismatch at the heterojunction restrict the separation and migration of photogenerated carriers, posing limitations on photocatalytic reaction. On the other hand, constructing a homojunction interface using chemically identical but structurally different materials can avoid these troubles associated with compositional change and lattice mismatch at the heterointerfaces, facilitating efficient photogenerated carrier separation and efficient solar-to-hydrogen conversion [172,173,174,175]. There are two typical crystal phases of chalcogenide semiconductors: WZ and ZB structures. The atoms in the ZB structure are arranged in an ABCABC manner in the [111] direction, and the atoms in the WZ structure are stacked in an ABABAB manner in the [0001] direction. Due to the small energy difference between WZ and ZB and the tiny mismatch of the (111)_ZB and (0001)_WZ facets, WZ and ZB can be linked together along the [111]_ZB and [0001]_WZ directions to form a polytype. Alex Zunger and coauthors calculated the energy difference between WZ and ZB structures in II-VI semiconductors and predicted the existence of polymorphic structures in II-VI compounds in 1992 [176]. Subsequently, more and more scientists are paying attention to polytypic nanocrystals. In recent years, diverse colloidal polytypic Cu-based multinary chalcogenide nanocrystals have been designed and successfully synthesized [177,178,179,180,181]. We have developed a simple colloidal method to prepare a polytypic CQSNs library (Figure 11a) [182]. The homojunction number in polytypic CZTS nanocrystals can be precisely regulated by adjusting the amount of 1-DDT. Bullet-shaped single-homojunction polytypic (SHP) CZTS nanocrystals featuring a WZ section and a KS cusp were synthesized using 1 mL of 1-DDT.Rugby-shaped double-homojunction polytypic (DHP) CZTS nanocrystals with one WZ part and two opposite KS cusps are produced via using 0.5 mL of 1-DDT. The bandgaps of SHP and DHP CZTS nanocrystals are 1.51 and 1.47 eV, which are suitable for visible-light absorption (Figure 11b). The photocatalytic hydrogen evolution rate of the SHP and DHP CZTS nanocrystals are 2.8-fold and 3.9-fold than that of the KS CZTS nanocrystals, respectively (Figure 11c). In addition, the polytypic CZTS nanocrystals exhibited high stability against photocorrosion (Figure 11d). The DFT calculations illustrated that the homojunction formed with KS and WZ exhibited a type II band alignment, leading to the accumulation of photogenerated electrons in KS and holes in WZ. This facilitates charge separation across the homojunction, enhancing photocatalytic properties (Figure 11e–g).

Figure 11. (a) Schematic illustration for synthesizing SHP and DHP CZTS nanocrystals. (b–d) Absorption and photocatalytic hydrogen production properties of phase-pure and polytypic CZTS nanocrystals. (e,f) The calculation of DOS and bandgap alignments of WZ and KS phases in the polytypic structures. (g) The simulated charge distributions in the homojunction [182].

4. Photocatalytic Applications of Cu-Based Quaternary Nanomaterials

Due to their appropriate band gap, CQSN photocatalysts have shown great potential in various photocatalytic applications. The fundamental principle of a photocatalytic reaction is very simple. The photocatalysts absorb light, exciting electrons to the CB while leaving holes in the VB. Subsequently, the photogenerated electrons in the CB transfer to the surface of the photocatalyst and take part in reduction reactions such as hydrogen production and CO₂ reduction. Simultaneously, the remaining holes in VB participate in oxidizing reactions such as oxygen evolution and oxidation of organics [183,184,185]. In this section, we provide a summary of the photocatalytic applications of CQSNs, including photocatalytic water splitting to produce hydrogen, PEC hydrogen production, CO₂ reduction, organic synthesis, and pollutant removal. 4.1. Photocatalytic Hydrogen Production Due to its multiple usage modes, high specific energy, and clean reaction products, hydrogen has emerged as a promising candidate for future energy sustainability. Solar-driven hydrogen evolution via semiconductor photocatalysts is regarded as a green and sustainable hydrogen production method, garnering extensive research attention [186,187,188]. To date, a variety of semiconductor photocatalysts have been designed and utilized for solar-to-hydrogen conversion [19,124,189]. Among these photocatalysts, CQSNs are favored for photocatalytic hydrogen production due to their excellent stability, non-toxicity, and broad absorption range in the visible light spectrum [35,37,190,191]. Since the groundbreaking work on photocatalytic hydrogen evolution over CZIS solid solution under visible-light irradiation was reported by Kudo and coauthors in 2005 [41], significant efforts have been dedicated to this research field. CQSNs play a critical role in solar-to-hydrogen conversion; however, their efficiency is constrained by inadequate separation and utilization of photogenerated carriers. Shi and coauthors designed a ternary photocatalyst comprising CIZS QDs, MoS₂, and carbon dots (CDs) for efficient solar-driven photocatalytic hydrogen production [192]. CZIS QDs with a size distribution of about 4.5 ± 0.5 nm and CDs with a size distribution of about 2.5 ± 0.5 nm are synthesized first and then attached to the surface of MoS₂ nanosheets to form a ternary CZIS/NoS₂/CDs 0D/2D heterostructured composite (Figure 12a–d). Moreover, the EDX-elemental mapping images of CIZS/MoS₂/CDs exhibited the uniform distribution of Cu, In, Zn, S, Mo, C, N, and O elements, clarifying that the CIZS QDs and CDs were successfully attached and well dispersed on MoS₂ nanosheets (Figure 12e). The impact of MoS₂ and CDs on the photocatalytic activity of CZIS QDs was investigated by varying their loading amounts to determine the optimal load. The rate of photocatalytic hydrogen production increased with increasing loading amount of MoS₂, reaching a maximum value at 1.663 mmol h⁻¹ g⁻¹ (CZIS-MoS₂-5%), which was 2.98 times higher than that of the pristine CZIS (Figure 12f). Then, the effect of CDs was further investigated in detail, varying their loading amounts on CIZS/MoS₂-5%. The optimized CIZS/MoS₂/CDs photocatalysts exhibited a hydrogen evolution rate of 3.706 mmol h⁻¹ g⁻¹, which was 6.65 times and 148.24 times higher than that of the original CIZS and MoS₂, respectively (Figure 12g,h).

Figure 12. (a,b) TEM images of CZIS QDs and CDs, respectively. (c–e) HRTEM, HAADF-STEM, and EDX-elemental mapping images of CIZS/MoS2/CDs. (f–h) Photocatalytic hydrogen production performances of CZIS/MoS2 and CIZS/MoS2/CDs [172].

Except for the 0D CQSNs, successful synthesis and utilization of 1D, 2D, and 3D CQSNs have been achieved for photocatalytic hydrogen production. For example, 1D WZ CZIS nanorods were synthesized via a one-pot non-injection method and subsequently functionalized with Pt and Pd₄S cocatalysts to improve photocatalytic performances [134]. The introduction of Pt and Pd4S resulted in a remarkable improvement in the photocatalytic properties of CZIS by approximately 3.5-fold and 3-fold, respectively. Wang and coauthors prepared 2D graphene-like ultrathin CZTS nanosheets for highly stable photocatalytic hydrogen production [193]. The ultrathin CZTS nanosheets with a 2–3 nm thickness exhibit a high hydrogen evolution rate of 1.5 mmol h⁻¹ g⁻¹. Chen and coauthors synthesized 3D CZIS hollow sub-microspheres for efficient visible-light-driven photocatalytic hydrogen evolution [58]. The highest hydrogen evolution rate reaches 3.79 mmol h⁻¹ g⁻¹ for RuS_x decorated CZIS hollow sub-microspheres. 4.2. Photoelectrochemical Hydrogen Production Photoelectrochemical hydrogen evolution is an efficient method for producing hydrogen [194,195,196]. PEC water splitting, using commonly employed photovoltaic materials like Si, TiO₂, and CdTe, has achieved significant advancements [197,198,199]. Meanwhile the high energy fabrication process of Si, the narrow light absorption range, and the toxicity of Cd in CdTe hindered their large-scale applications in PEC water splitting. Recently, Cu-based quaternary chalcogenides have emerged as promising photocathode materials for PEC water splitting, owing to their excellent photovoltaic performance in thin-film solar cells [45,200,201,202,203]. In recent years, a variety of CQSNs have been utilized in the fabrication of photoelectrodes for PEC hydrogen production [204,205,206]. For example, Silvula and coauthors used CZTS colloidal inks to prepare thin-film photocathodes, which exhibited excellent PEC performances [207]. Kudo and co-author fabricated a PEC cell with a CoO_x-modified BiVO₄ photoanode and a Ru-loaded (CuGa)_0.5ZnS₂ photocathode for water splitting without applying an external bias, which exhibited excellent performances [208]. Wide-bandgap metal oxides (e.g., TiO₂ and ZnO) are widely utilized as anode materials for efficient PEC hydrogen production [209,210]. However, metal oxides limited visible light absorption hinders their solar-to-hydrogen conversion. Sensitizing metal oxides with quantum dots, particularly CQSN CDs, is one of the most effective strategies to expand the light absorption range and thereby enhance PEC performance [211,212,213,214]. Kim and coauthors developed a green chemistry method wherein vegetable oil is used as a non-toxic solvent to produce monodisperse and size-tunable CZTS nanocrystals. Then, a 5 nm thin atomic Zn(O,S) layer is covered onto previously prepared TiO₂ nanorod arrays (TNR) to prepare Zn(O,S)/TNR electrode. Lastly, the obtained CZTS nanocrystals are deposited onto Zn(O,S)/TNR electrode to fabricate a ternary CZTS NCs/Zn(O,S)/TNR photoelectrode for PEC water splitting [215]. As shown in Figure 13a,b, the TNRs are fully encapsulated by Zn(O,S) and CZTS nanocrystals. The formation of sequence type-II heterojunctions between CZTS, Zn(O,S), and TNR promotes efficient photogenerated carrier separation, thereby enhancing PEC performances (Figure 13c).

Figure 13. (a,b) TEM and HRTEM of the CZTS NCs/Zn(O,S)/TNR structure. (c) Illustration of the charge transfer mechanism. (d–h) Photocurrent densities-voltage (J-V) curves under dark and illumination, STH efficiency-voltage curves, chopped on/off cycles, incident IPCE as a function of wavelength, and EIS spectra of TNR, LE CZTS/TNR, and LE CZTS/Zn(O,S)/TNR. (i) Photocurrent-time (I-t) curves of LE CZTS/Zn(O,S)/TNRs [215].

The ligand exchange (LE) CZTS/TNRs, LE CZTS/Zn(O,S)/TNRs and TNRs are used as photoelectrodes for PEC measurements. All the photoelectrodes exhibit a negligible dark current (Figure 13d), a reproducible and rapid photocurrent response and the light on/off (Figure 13f). The LE CZTS/Zn(O,S)/TNR photoelectrode shows an extraordinary photocurrent density of 15.05 mA cm⁻² at 1.23 V (vs. the NHE), which is much higher than that of TNR photoelectrode (1.39 mA cm⁻²) and LE CZTS/TNR photoelectrode (7.13 mA cm⁻²) at 1.23 V (vs. the NHE) (Figure 13d). The LE CZTS/Zn(O,S)/TNR photoelectrode shows the highest photo conversion efficiency of 6.94% at 0.64 V (vs. the NHE), which is about 11.4 and 2 times that of TNRs (0.61% at 0.043 V) and LE CZTS/TNRs (3.40% at 0.62 V), respectively (Figure 13e). The LE CZTS/Zn(O,S)/TNR photoelectrode shows a greater IPCE in the visible region with the highest IPCE of 69.42% at 510 nm (Figure 13g). The electrochemical impedance spectroscopy (EIS) studies demonstrated that the LE CZTS/Zn(O,S)/TNR photoelectrode exhibited the lowest R_CT value (Figure 13h), suggesting the more effective separation of photogenerated carriers and the lowest charge transfer resistance, and thus exhibiting the best PEC performances. Additionally, the photocurrent of LE CZTS/Zn(O,S)/TNR photoelectrode showed a marginal decrease and maintained approximately 67% of its original value after 60 min (Figure 13i). 4.3. Photocatalytic CO₂ Reduction CO₂—a greenhouse gas—has caused serious environmental issues. Reducing CO₂ to hydrocarbon fuels (e.g., methane and formate) is a potential strategy to alleviate this trouble. Inspired by photosynthesis, solar-energy-driven photocatalytic reduction of CO₂ to hydrocarbon fuels emerges as a promising approach that simultaneously reduces greenhouse gas emissions and addresses global energy demands. Extensive research has been conducted on various photocatalysts for this purpose, including metal nitrides, polymers, metal oxides and metal sulfides [7,216,217,218,219]. Among them, metal sulfides exhibit excellent performances during the photocatalytic CO₂ reduction because the sulfur has light-effective mass carriers, as well as the S 3p orbital occupies the less positive VB, resulting in an extended photorespense range and higher carrier concentration for metal sulfides [220,221,222]. CQNSs have been widely used for photocatalytic CO₂ reduction [70,223,224,225,226,227]. Recently, Chang and coauthors employed a gel-assisted method to synthesize CIZS for visible-light photocatalytic CO₂ reduction [228]. When Ru was utilized as the cocatalyst, the CIZS nanocrystals exhibited commendable reducibility in CO₂ photocatalysis, resulting in a methane, hydrogen, and formate product mixture. Zhang and coauthors found that CuInSnS₄ (CITS) octahedral nanocrystal photocatalyst with exposed (111) plane possesses selective capability in reducing CO₂ to methane [229]. They successfully prepared CITS nanocrystals using a simple one-step hydrothermal method. The resulting CITS features a cubic spinel crystal structure and exhibits an octahedral morphology with a size of approximately 30 nm (Figure 14a). Furthermore, the HRTEM image (Figure 14b) and SAED pattern (Figure 14c) illustrate the obtained CITS nanocrystal has high-quality exposed (111) crystal facets. As shown in Figure 14d, the absorption band edges of In₂S₃, SnS₂, Cu₂S, and CITS nanocrystals are 641.6 nm, 552.7 nm, 747.0 nm, and 787.5 nm, which corresponds to the band gap of 1.93 eV, 2.24 eV, 1.66 eV, and 1.57 eV, respectively. The electronic band energies (Figure 14e) are calculated and illustrate that both CITS and single metal sulfide nanocrystals have the ability to reduce CO₂ to CO and CH₄ under solar irradiation. Under visible-light irradiation, CITS nanocrystals exhibited excellent photocatalytic performances and photocatalyzed the reduction of CO₂ into the main products of CH₄ (Figure 14f). The main product is CO on the corresponding individual In₂S₃, SnS₂, and Cu₂S (Figure 14f). Also, the CITS nanocrystals exhibited a good photocatalytic reduction of CO₂ performance with negligible activity decrement after three-cycle tests of a total of 27 h (Figure 14g). The photocatalytic activity obviously decreases with the wavelength of the incident light increasing (Figure 14h). Meanwhile, the GC-MS spectra prove that only CH₄ is produced in the CITS photocatalyzed CO₂ reduction. The non-metal sulfur atom in the CITS photocatalysts serves as the adsorption center, specifically targeting the carbon atom of CO₂ rather than general metal or defect sites. Consequently, this selective adsorption on the non-metal sulfur atom of CITS facilitates the stabilization of intermediates and thus boosts the reduction of CO₂ to CH₄.

Figure 14. (a–c) TEM image, HRTEM image, and SAED pattern of CITS nanocrystals. (d,e) Absorption properties and the optical band gap energy of In2S3, SnS2, Cu2S and CITS nanocrystals. (f) Comparison of the photocatalytic CO2 reduction properties over In2S3, SnS2, Cu2S and CITS nanocrystals. (g,h) Photocatalytic CO2 reduction stability and photocatalytic performance of CITS nanocrystals under monochromatic light irradiation. (i) GC-MS spectra [229].

4.4. Photocatalytic Organic Synthesis Photocatalytic organic synthesis offers an attractive approach for solar-to-chemical energy conversion and is an alternative to traditional high-energy chemical synthesis. Many classic reactions, including hydrogenation, alcohol oxidation, aerobic coupling, and epoxidation, have been carried out through photocatalytic reactions [230,231,232,233,234]. Due to their special light absorption properties, Cu-based quaternary sulfides are widely utilized for photocatalytic organic synthesis, such as PEC ammonia synthesis from NO_x reduction and methylcyclohexane synthesis from toluene [235,236]. CQSNs have been prepared for photocatalytic organic synthesis with the development of nanotechnology. For example, Pandey and coauthors reported the utilization of CuAlS₂/ZnS (CAZS) QDs for photocatalytic synthesis of organics from aqueous bicarbonate ions [237]. Biocompatible CAZS QDs were prepared for photocatalytic organics synthesis. The resulting CAZS QDs have an elongated morphology with a mean aspect ratio of 1:2.2 (Figure 15a). The distinct phase contrast within each QD indicates the presence of separate regions of CuAlS₂ and ZnS(Figure 15b). The HRTEM and the corresponding Fourier transform pattern also imply that CAZS QDs are core/shell materials (Figure 15c,d). The resulting QDs were used as a photocatalyst to reduce bicarbonate ions under visible light, producing both oxidizing and reducing products (O₂ and formate) over time(Figure 15f–i). The CAZS QDs exhibited an unheard-of visible light-driven photocatalytic activity for reducing carbon dioxide in the aqueous sodium bicarbonate without cocatalyst or sacrificial reagent. Additionally, devices utilizing these QDs exhibit exceptional energy conversion efficiencies, reaching up to 20.2 ± 0.2%.

Figure 15. (a–d) TEM image, STEM image, HRTEM image, and SAED pattern of CuAlZnS nanocrystals. (e) Schematic illustration of the visible light photosynthesis of organic from aqueous bicarbonate ions. (f) Production of O2 and formate with time by photoreduction of NaHCO3. (g–i) 1H NMR of photoreduction product, a reaction mixture, and the products formed by illuminating a light-harvesting device [237].

4.5. Photocatalytic Pollutant Removal Contemporarily, the issue of environmental pollutants has become increasingly severe due to industrialization. Solar-driven photocatalytic pollutant removal is considered an economical and effective approach to deal with this environment trouble [112,238,239]. Under visible light irradiation, semiconductor photocatalysts absorb light to produce excitons (electrons and holes), and then distinct reactive oxygen species are generated, such as •OH, O₂^•−, singlet oxygen molecules (¹O₂), etc. Photogenerated holes and O₂^•− are the key active species over the semiconductor photocatalysts. Metal oxide and sulfide semiconductors are widely employed as photocatalysts for photocatalytic reduction of pollutants [76,240]. In recent years, CQSNs have gained significant attention from researchers as a potential solution for addressing pollution issues [241,242,243,244]. CZTS nanocrystals are widely used in the photocatalytic degradation of organic pollutants and in industrial wastewater treatment [245,246]. Acharya and coauthors used the quaternary CuZnFeS (CZFS) nanocrystals as catalysts for degrading multiple refractory organic pollutants [247]. A remarkably fast and facile synthesis route produced colloidal CZFS nanocrystals in just 30 s. The obtained mostly faceted spherical CZFS NCs have an average size of 14 ± 3 nm (Figure 16a,b). Then, the organic ligands on the NCs are exchanged with hydrazine before photocatalytic applications. The internal energy levels of hydrazine-capped CZFS (H-CZFS) extracted using scanning tunneling spectroscopy and ultraviolet photoelectron spectroscopy show that the H-CZIS nanocrystals have suitable redox potential of oxidative species (Figure 16c–e). The H-CZIS nanocrystals exhibit excellent photocatalytic properties for the degradation of multiple refractory organic pollutants (Figure 16f). Moreover, a small amount of H-CZIS nanocrystal can speed up the conversion rate of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) at a much faster rate (Figure 16g,h).

Figure 16. (a,b) TEM and HRTEM images of CZFS nanocrystals. (c) Energy bandgap alignment of CZFS nanocrystals. (d) Standard redox potentials (E0) of different oxidative species. (e) A schematic of the catalytic pathways involved in degrading organic dyes and 4-NP by H-CZFS NCs. (f) Photocatalytic degradation of methylene blue solution by H-CZFS nanocrystals. (g,h) Catalytic performance of CZFS nanocrystals for reduction of 4-NP to 4-AP in the presence of NaBH. The insert in g is the time-dependent absorption spectra [247].

5. Conclusions and Outlook

CQSNs demonstrate remarkable photocatalytic performance, attributed to their suitable bandgap for visible light absorption, unique electronic states, tunable atomic structure, and exceptional optoelectronic properties. In conclusion, we provide a summary of recent progress in the design and development of CQSNs for photocatalytic applications. We initially present the solution-based synthesis of CQSNs with precise composition, shape, size and crystal phase control. Several strategies have been implemented to further enhance their photocatalytic performance, including surface engineering, elemental doping, cocatalyst loading, vacancy engineering, and interface engineering. Finally, various photocatalytic applications of CQSNs have been explored, including hydrogen production through both photocatalytic and PEC methods, CO₂ reduction, organic synthesis, and pollutant removal. Despite the enhanced photocatalytic performance of CQSN photocatalysts, they still have certain limitations compared to other types of photocatalysts. Therefore, there is potential for further advancements in the development of CQSN photocatalysts, along with a series of remaining challenges. Firstly, the PCE of current CQSNs is insufficient for feasible practical applications. It is expected to design and synthesize other favorable CQSNs with novel electronic properties or structural features. Also, it is desirable to design new modification approaches by borrowing from the optimizing strategies in other catalytic fields, such as strain engineering, polarization, and high-entropy structure. Besides, utilization of the structure-function relationship of photocatalysts in material design enables the highly efficient preparation of photocatalysts and facilitates the strategic integration of various advantageous factors in photocatalysis. Subsequently, the optimization and integration of synthetic technologies hold promise for fabricating novel types of CQSN photocatalysts. The conventional synthetic methods, including colloidal methods and hydrothermal and solvothermal approaches, still possess inherent limitations in fabricating nanocrystal structures. For example, the incorporation of additives can introduce novel structural characteristics that impact photocatalytic processes. Therefore, further innovative synthetic approaches are required to enhance the efficiency of CQSN photocatalysts. Lastly, there is still a lack of investigations on the photocatalytic mechanism of CQSNs. It is necessary to study the mechanisms of how the CQSN photocatalysts work in different photocatalytic reactions in detail via in situ characterizations, such as Fourier transform infrared spectroscopy, Electron paramagnetic resonance spectroscopy, X-ray photoelectron spectroscopy, X-ray absorption fine structure, Raman spectroscopy, etc. The determination of the charge transfer mechanism in various semiconductor heterojunctions is essential for advancing research on photocatalytic reactions. There are few reports about the research on the photogenerated carrier dynamics of CQSNs. The energy band matching, novel Z-scheme, and p-n heterojunctions all fall under the category of traditional type-II heterojunctions; however, they exhibit distinct charge transfer mechanisms. It is highly useful to explore the photogenerated carrier separation and migration of CQSN photocatalysts via transient absorption spectroscopy, transient emission spectroscopy, optical pump THz probe spectroscopy and theoretical calculations. A thorough study of photogenerated carrier dynamics in CQSN photocatalysts can aid in the rational design of catalysts to achieve improved photocatalytic performance.

Author Contributions

B.Z., S.C., Q.J. and L.W. conceived the idea. L.W. wrote the paper. B.Z., S.C. and Q.J. revised the paper. All authors assisted during the manuscript preparation.

Ethics Statement

Not applicable.

Informed Consent Statement

Funding

This research was funded by the National Natural Science Foundation of China (Grants 22101271, 22471005), the National Key Research and Development Program of China (Grants 2021YFA0715703).

Declaration of Competing Interest

The authors declare no competing interests.

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Photocatalysis: Research and Potential