Laser welding of aluminum battery tab to variable Al/Cu busbars in Li-ion battery joint

1.   Introduction

The demand for energy is increasing with the development of industrialization and information technology. Otherwise, the use of fossil fuels will emit a large amount of carbon dioxide and give rise to environmental pollution. Therefore, how to use new technology and materials to solve the increasingly tense energy crisis and severe environmental problems has become a hot spot in material and chemistry research fields. Solar energy is the most abundant and clean green energy.

Photocatalysis using solar energy is an appealing technology to address the energy crisis and environmental problems ^[1,2,3,4,5]. The photocatalytic reaction is generally divided into three steps. (Ⅰ) Photocatalysts absorb light to produce both excited holes and electrons. (Ⅱ) The holes and electrons transfer to the surface of the photocatalyst. (Ⅲ) The surface carriers participate in the redox reaction ^[6]. Specifically, semiconductor photocatalysts stir up the electrons in the valance band (VB) to the conduction band (CB) to produce hole-electron pairs ^[7,8,9]. Subsequently, the photogenerated carriers are separated and diffused from the bulk to the surface of the photocatalyst. Obviously, the light absorption efficiency, the charge separation and diffusion efficiency and the redox reaction efficiency are the three key parameters for determining the conversion efficiency of the photocatalytic reaction ^{[10,11,12,13]}. Therefore, the most important issue in photocatalysis is designing photocatalysts from the above three aspects to achieve high conversion efficiency.

Semiconductor nanomaterials have attracted extensive attentions due to the special properties stemming from the quantum effect ^[14,15,16]. As the size is less than the exciton Bohr radius, the electron density of the semiconductor nanomaterials will be discontinuous, and the carrier will have a spatial limiting effect and cause a change in the band structure ^[17,18,19]. On the other hand, the huge surface area of semiconductor nanomaterials will bring ignored surface effects. The tunable bandgaps and special surface effect of semiconductor nanomaterials will bring novel optical and electronic properties, rendering them as promising candidates for photocatalytic applications ^[20,21,22].

To date, a series of semiconductor nanomaterials have been developed as photocatalysts, such as metal oxides (TiO₂, ZnO, WO₃, etc.), metal chalcogenides (CdS, CdSe, etc.) and polymers ^{[23,24,25,26,27,28,29]}. Among the most widely investigated semiconductor photocatalysts, Cu-based multinary sulfide (CMS) nanomaterials have attracted wide interest in various photocatalytic applications due to their suitable and tunable bandgap, high absorption coefficient, environmental benign elemental components, unique electronic state and adjustable atomic structure ^{[30,31,32,33,34]}. CMSs are inorganic compounds consisting of at least two metal elements. The band structure of these CMSs can be conveniently adjusted through metal element alloying, then optimizing their photocatalytic performances ^{[35,36,37,38]}. The diversity of metal elements in CMSs will bring different crystal structures and allow precise control of the reactive atomic arrangement on the surface to construct specific active sites for diversified photocatalytic reactions ^{[39,40,41,42]}. At the same time, the differential charge distribution of diverse elements will result in microcosmic polarization and lead to a built-in electric field, which is beneficial for the effective separation of photogenerated holes and electrons ^[43,44]. Due to these specialties, CMS nanomaterials exhibit excellent performance in photocatalytic H₂ production, CO₂ reduction, organic synthesis, photoelectrochemical (PEC) H₂ production, and so on ^{[45,46,47,48,49]}.

In this review, we provide a comprehensive overview on recent research of CMS nanomaterials in photocatalytic applications. We focus on the most researched CMSs, namely Cu-based ternary and quaternary sulfide nanomaterials. We first introduce the bandgap alignments of Cu-based ternary sulfides (CuInS₂, CuIn₅S₈, Cu₃SnS₄, etc.) and Cu-based quaternary sulfides (CuZnInS, Cu₂ZnSnS₄, CuInGaS, etc.), which are the key factors for the photocatalytic activity of semiconductor photocatalysts. Subsequently, we provide a summary of recent progress in the photocatalytic applications of Cu-based ternary sulfides and Cu-based quaternary sulfide nanomaterials, focusing on photocatalytic H₂ production, CO₂ reduction, organic synthesis, degradation of pollutants and PEC H₂ production. Finally, we also summarize the current challenges and opportunities of Cu-based multinary sulfides in future studies.

2.   Cu-based multinary sulfide nanomaterials

To date, the developed CMS nanomaterials mainly focused on Cu-based ternary and quaternary sulfides (Figure 1). This section will introduce the bandgap alignments of Cu-based ternary (CuInS₂, CuIn₅S₈, Cu₃SnS₄, CuFeS₂, etc.) and quaternary sulfides (CuZnInS, Cu₂ZnSnS₄, CuZnGaS, CuInGaS, etc.) in detail.

Figure 1.  Schematic illustration of the photocatalytic applications of Cu-based ternary and quaternary sulfide nanomaterials.

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2.1.   Cu-based ternary sulfide nanomaterials

Cu-based ternary sulfides are ZnS-derived compounds produced by replacing Zn atoms with Cu cations and trivalent/tetravalent cations (e.g., In, Ga, Sn, Ge, Fe, etc.) ^[50,51]. Cu-Ⅲ-S compounds (Ⅲ = In, Ga) preferentially crystallize in a diamond-like structure inherited from ZnS. For example, the stoichiometric formula CuInS₂ (CIS) exhibits a stable chalcopyrite structure at room temperature and a metastable wurtzite structure at high temperature ^[52]. The tolerance of cation off-stoichiometry allows Cu-Ⅲ-S chalcopyrite compounds to introduce different cation ratios, producing nonstoichiometric ternary sulfides, such as CuIn₃S₅, CuIn₅S₈, and CuIn₁₁S₁₇ ^[53,54]. Moreover, the off-stoichiometry effect usually has great influences on the band structures, optical properties and crystal structures of Cu-Ⅲ-S compounds. The bandgaps of CuInS₂ and CuIn₅S₈ are 1.5 and 2.06 eV, respectively ^[55,56]. In addition, the VB and CB positions of CuInS₂ are also different from these of CuIn₅S₈ (Figure 2). The bandgap will change from 1.5 to 2.35 eV when the In cation is replaced by a Ga cation (Figure 2) ^[57].

Figure 2.  Band position of representative Cu-based ternary and quaternary sulfide semiconductors.

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The most reported Cu-Ⅳ-S compounds are stoichiometry Cu₂SnS₃ (CTS) and Cu₂GeS₃, which generally crystallize in zinc-blende, orthorhombic and wurtzite structures ^[58,59]. Similar to Cu-In-S compounds, tolerating cation off-stoichiometry usually leads to nonstoichiometric ternary sulfides, such as, Cu₃SnS₄, Cu₄SnS₄, and Cu₂Sn₃S₇ ^[60,61]. The different cation ratios will lead to different bandgap alignments of Cu-Sn-S compounds. For example, the bandgaps of Cu₂SnS₃ and Cu₃SnS₄ are 0.91 and 1.67 eV, respectively, and possess different VB and CB positions (Figure 2) ^[62,63]. Cu₂GeS₃ with a monoclinic structure has a bandgap of 1.49 eV (Figure 2) ^[34]. In addition to the above Cu-Ⅳ-S compounds, CuFeS₂ (CFS), obtained by replacing Zn atoms with Cu cations and Fe cations, is another well-studied ternary sulfide ^[64,65,66]. The special band structures and high visible-light absorption efficiency favor the utilization of Cu-based ternary sulfides in photocatalytic H₂ production, CO₂ reduction, organic synthesis and pollutant removal.

2.2.   Cu-based quaternary sulfide nanomaterials

Cu-based quaternary sulfides have attracted extensive research interests in photocatalytic applications as fascinating and promising nanomaterials. Many types of Cu-based quaternary sulfides have been designed and synthesized via regulation of metal types and ratios, such as Cu-Ⅱ-Ⅵ-S compounds obtained via successive replacement of the In atoms in CuInS₂ with Ⅱ and Ⅵ atoms obeying the octet rule ^[44], and Cu-Ⅱ-Ⅴ-S compounds obtained via partially replacing of the In atoms in CuInS₂ with Ⅱ atoms ^[35,67]. As far back as 1950, scientists designed quaternary chalcogenide via cation substitution ^[68]. A series of Cu-based quaternary sulfides have been synthesized for photocatalytic applications in recent decades, such as Cu₂ZnSnS₄ (CZTS), CuZnInS (CZIS), Cu₂CdSnS₄ (CCTS), Cu₂ZnGeS₄ (CZGS), and CuGaInS (CGIS) ^{[69,70,71,72,73]}. As the most researched quaternary chalcogenide, CZTS is an n-type semiconductor with a bandgap of 1.5 eV ^[31], which can absorb all visible light for photocatalytic applications. In addition, the other Cu-based quaternary sulfides also have suitable bandgaps for visible-light absorption, namely CuZn₄InS₆ with a bandgap of 2.23 eV, CZGS with a bandgap of 2.11 eV and CGIS with a bandgap of 1.82 eV (Figure 2) ^{[39,40,74,75]}. Due to their special band structures, high visible-light absorption efficiency and tunable surface atomic arrangements, Cu-based quaternary sulfides are widely used for photocatalytic H₂ production, organic synthesis, degradation of pollutants and PEC H₂ evolution.

3.   Photocatalytic applications of Cu-based multinary sulfide nanomaterials

3.1.   Photocatalytic H₂ production

Hydrogen energy is a kind of secondary energy with abundant sources and wide applications. However, almost no elementary hydrogen exists on Earth. Hydrogen can be produced by the decomposition of hydrogen-containing materials. Water is the most abundant hydrogen-containing compound on Earth, so water splitting is an efficient and sustainable method for hydrogen production. Photocatalytic hydrogen production is a green and sustainable hydrogen production program. In photocatalytic overall water splitting, water molecules are split into hydrogen and oxygen on the surface of photocatalysts under irradiation ^[8,79,80].

To date, various kinds of CMSs have been used for solar-to-hydrogen production. Kudo and coauthors first used CZIS powder for photocatalytic H₂ evolution from aqueous solutions containing K₂SO₃ and K₂S under visible-light irradiation ^[67]. With the development of nanotechnology, CMS nanomaterials have been designed and synthesized for solar-driven hydrogen production ^{[81,82,83,84]}. For example, Wu and coauthors reported the synthesis of CIS quantum dots (QDs) via an aqueous synthetic approach, which exhibited excellent photocatalytic hydrogen evolution performances ^[84]. Ning and coauthors successfully prepared water soluble CZIS QDs for photocatalytic hydrogen production (Figure 3). The CIZS QDs passivated by glutathione ligands are highly stable in aqueous conditions (Figure 3a) and show significant energy conversion efficiency in the visible and near-infrared regions with an external quantum efficiency (EQE) of ~1.5% at 650 nm (Figure 3b), which is the highest EQE reported in the near-infrared region ^[76]. In addition, many strategies have been used to improve the photocatalytic hydrogen production performances of CMS QDs. For instant, Pt and Au co-catalysts have been loaded on the surface of CZTS nanocrystals to significantly improve the photocatalytic H₂ evolution performances ^{[31,82,85,86]}. Heterojunctions and homojunctions are designed in CZTS to promote the separation of photogenerated charges and then optimize their photocatalytic performances ^{[43,87,88,89]}.

Figure 3.  (a, b) Transmission electron microscope (TEM) image and photocatalytic H₂ evolution properties of 0D CZIS nanocrystals (Reproduced from Ref. ^[76] with permission). (c, d) TEM image and photocatalytic H₂ evolution properties of 1D CZIS nanorods (Reproduced from Ref. ^[77] with permission). (e, f) TEM image and photocatalytic H₂ evolution properties of 2D CZIS nanobelts ^[40]. (g, h) Scanning electron microscope (SEM) image and photocatalytic H₂ evolution properties of 3D Cu₃SnS₄ flower-like microspheres (Reproduced from Ref. ^[78] with permission).

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One-dimensional (1D) nanostructures exhibit novel physicochemical properties brought about by changes in dimension, size and composition ^[90,91,92]. In particular, the continuity of charge transport brings out the unique advantages of 1D materials in photocatalytic and photoelectronic applications. Various types of 1D semiconductor nanomaterials have been developed for photocatalytic hydrogen production ^[26,93,94]. Han and coauthors have successfully synthesized wurtzite CZIS nanorods using a one-pot non-injection method for solar-to-hydrogen production (Figure 3c) ^[32,77]. Subsequently, Pt and Pd₄S nanoparticles were decorated on the tips of CZIS nanorods to improve their photocatalytic performances. The Pt and Pd₄S cocatalysts can accelerate charge separation and act as active sites for hydrogen production, resulting in remarkably improved photocatalytic performance (Figure 3d). Low-cost and noble-metal-free 1D CZTS nanofibers also exhibit excellent photocatalytic hydrogen production performance under visible-light irradiation ^[95].

Due to the solo ability of confinement of electrons in their layer, two-dimensional (2D) nanomaterials possess special optical and electronic properties. In addition, 2D semiconductor nanomaterials can reduce the recombination rate and shorten the migration distance of photogenerated carriers. The layered structure facilitates the light absorption process even at low flux density, and the photogenerated carriers are only required to transfer a very small distance. Moreover, the large surface area of 2D nanomaterials favors surface reactions. Thus, 2D semiconductor nanomaterials are widely used for solar-to-hydrogen production ^{[96,97,98,99]}. Wang and coauthor first reported the synthesis of graphene-like CZTS nanosheets using a facile template-free approach ^[100]. The synthesized CZTS nanosheets with a thickness of ca. 2–3 nm exhibited high activity and stability for solar-to-hydrogen production without loading cocatalysts. Note that the 2D structure can expose the highest active facet. We first used first-principle-density functional theory calculations to explore the reaction Gibbs energy (ΔG_H) of the (0001), (1010) and (1011) facets of wurtzite CZIS, finding that the (0001) facet has the smallest binding strength to atomic hydrogen. We then designed a simple colloidal method to synthesize single crystalline wurtzite CZIS nanobelts (NBs) exposing the (0001) facet assisted with oleylamine and 1-dodecanethiol (Figure 3e). The synthesized CZIS nanobelt photocatalysts show excellent photocatalytic performances under visible-light irradiation without cocatalyst (Figure 3f) ^[40]. Furthermore, we doped phosphorus into 2D single crystalline CZIS sulfide nanobelts to reduce the recombination rate and increase the electric conductivity of photogenerated carriers, enabling significantly improved photocatalytic H₂ evolution performance ^[39].

In addition, three-dimensional (3D) CMSs have also been developed as photocatalysts for solar-to-hydrogen production. Xie and coauthors have constructed monodisperse CIS hierarchical microarchitectures for photocatalytic hydrogen production ^[101]. The obtained monodisperse CIS hierarchical photocatalyst showed an average hydrogen production rate of 59.4 µmol h⁻¹ g⁻¹ under visible light irradiation. Qian and coauthors successfully synthesized 3D-hierarchical Cu₃SnS₄ flower-like microspheres via a solvothermal process (Figure 3g) ^[78]. The bandgap of the obtained Cu₃SnS₄ microspheres is 1.38 eV, which is suitable for UV and near-IR light absorption. The 3D Cu₃SnS₄ photocatalysts without cocatalyst show good photocatalytic activity with a H₂ evolution rate of 1.1 mmol h⁻¹ g⁻¹ (Figure 3h). 3D Cu-based quaternary sulfides also exhibit excellent photocatalytic hydrogen production performances, such as CZIS hollow microspheres and hollow sub-microspheres ^[102,103].

3.2.   Photoelectrochemical H₂ evolution

Photoelectrochemical water splitting provides a promising strategy to convert solar energy into hydrogen ^{[106,107,108]}. Previously, many common photovoltaic materials such as CdTe, TiO₂ and Si were successfully used to fabricate photoelectrodes for PEC solar water splitting and obtained significant results ^{[109,110,111]}. However, the toxicity of Cd in CdTe, the narrow light absorption range and the high energy fabrication process of Si limited their large-scale applications in PEC water splitting. Thus, scientists are searching for high-efficiency, low-cost and wide light absorption materials for PEC applications. Copper-based multinary chalcogenides such as CIS, Cu(InGa)Se₂ (CIGSe), and CZTS have been used as potential photocathode materials due to their excellent photovoltaic performances and suitable conduction band positions for water reduction ^{[48,112,113,114,115,116,117]}. For example, the CIGSe/CdS/ZnO photocathode exhibited excellent PEC performance with a current upping to 32.5 mA cm⁻² at 0 V_RHE ^[118]. Then, CMS nanomaterials were also used to fabricate photoelectrodes for PEC applications with the development of nanotechnology ^{[119,120,121]}.

Guo and coauthors fabricated CZTS photoelectrode using colloidal CZTS nanocrystals via electrophoretic deposition. The obtained CZTS photoelectrode exhibited composition-dependent PEC hydrogen production performances ^[122]. The (Cu₂Sn)_0.45Zn_1.65S₃ photocathode has the highest photocurrent with an incident photon to current conversion efficiency (IPCE) of 3.9% at 600 nm, which is higher than previously reported results of CZTS photocathodes ^[123]. Similarly, the CZIS photoelectrode fabricated by 2D CZIS nanoplates also showed a cathodic photocurrent ^[124]. Moon and coauthors synthesized chalcopyrite CIS nanorod arrays as photoelectrodes for PEC hydrogen production via anodic aluminum oxide template-assisted growth. The CIS nanorod arrays showed an obvious cathodic photocurrent response without co-catalyst loading ^[125]. Zhang and coauthors reported the synthesis of chalcopyrite CIS nanosheet-constructed nanowire arrays (NCNAs) (Figure 4a). The PEC properties of these NCNAs were investigated using a three-electrode system with a reference electrode of Ag/AgCl and counter electrode of a Pt mesh in a 1.0 M KCl aqueous solution. The CIS NCNA photocathode exhibited not only enhanced PEC performance, but also improved PEC stability (Figure 4b) ^[104]. Many strategies have been developed to improve the PEC performances of CMS photoelectrodes. For example, Sivula and coauthors used CdS, ZnSe and CdSe buffer layers to modify the surface of CZTS nanocrystal photocathodes to enhance the PEC performance ^[126]. Pradhan and coauthors integrated Au on CIS nanocrystals to improve the PEC performance of CIS nanoplates ^[127]. The exciton−plasmon coupling in the 0D−2D dot−disk Au-CIS heterostructures leads to a higher photocurrent with efficient production of H₂.

Figure 4.  (a) TEM, selected area electron diffraction (SAED) and high-resolution TEM (HRTEM) images of CIS nanosheet-constructed nanowires. (b) Linear sweep voltammetry (LSV) curves and intensity-time (I-t) curves (at 0 V versus reversible hydrogen electrode (RHE)) of the pristine Cu₂S nanowire arrays and CIS nanosheet-constructed nanowire arrays under simulated solar illumination (Reproduced from Ref. ^[104] with permission). (c) A schematic and SEM images of the ZnO/CZTS photoanode. (d) LSV, ABPE, chopped LSV and I-t (at 1.23 V versus RHE) curves of the three photoanodes of ZnO/CZTSSe, ZnO/CZTS and ZnO (Reproduced from Ref. ^[105] with permission).

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Among the widely used PEC materials, some wide-bandgap metal oxides (such as ZnO and TiO₂) are promising stable, efficient and inexpensive anode materials for efficient PEC hydrogen production ^[128,129]. However, the large bandgaps of metal oxides limit the absorption of visible light, thus reducing the solar-to-hydrogen conversion efficiency. Sensitization using QDs, especially CMS CDs, is one of the most promising strategies to expand the light absorption range of metal oxides, mainly due to the suitable bandgap of CMS QDs ^{[130,131,132,133]}. Teng and coauthors used colloidal CIS CDs as sensitizers for TiO₂ photoelectrodes ^[134]. The CIS-sensitized TiO₂ photoelectrodes exhibited a maximum conversion efficiency of 1.9% at +0.23 V bias. Wang and coauthors used CZIS CDs to sensitize TiO₂ photoelectrodes. The obtained CZIS/TiO₂ photoelectrodes showed composition-dependent performances with a maximum photocurrent density of 3.7 mA cm^{−2 [135]}. In addition, 2D CZIS nanosheet-sensitized TiO₂ nanorods can also boost their solar water splitting performance ^[136]. Xu and coauthors studied the PEC performance of CZTS-sensitized ZnO nanorod photoanodes in detail. CZTS/ZnO photoanodes are fabricated by spin-coating CZTS nanocrystals on ZnO arrays (Figure 4c). The ZnO/CZTSSe photoelectrode showed excellent PEC performances with a photocurrent density of 6.38 mA cm⁻² at 1.23 V_RHE and an optimal applied bias photon-to-current efficiency (ABPE) of 2.8% at a bias of 0.60 V (Figure 4d) ^[105].

3.3.   Photocatalytic CO₂ reduction

Excessive CO₂ emissions have led to greenhouse effects and several serious environmental problems. Converting CO₂ into hydrocarbon fuels is a promising approach to lower the CO₂ concentration in the atmosphere and can also reduce the dependence on fossil fuels. Inspired by the photosynthesis of plants, photocatalytic CO₂ reduction to hydrocarbon fuels using solar energy is an attractive method for CO₂ conversion and can mitigate greenhouse gas emissions. A series of photocatalysts have been explored for photocatalytic CO₂ reduction, such as metal oxides, metal nitrides, and metal sulfides ^{[137,138,139,140]}. Among them, metal sulfides are excellent catalysts for photocatalytic or PEC CO₂ reduction because the S 3p orbital occupies the less positive VB, which promote metal sulfides with a wider photoresponse range and higher carrier concentration, and sulfur has light-effective mass carriers ^[141,142]. CMS semiconductors with a narrow bandgap and excellent light absorption abilities have been widely studied for photocatalytic and PEC CO₂ reduction ^{[143,144,145,146]}.

Furthermore, the large specific surface area of CMS nanomaterials could bring a huge number of reactive sites, thus leading to excellent photocatalytic performance. For example, single-phase Cu₃SnS₄ with a bandgap of 1.76 eV is a robust photocatalyst that can selectively reduce CO₂ into CH₄ via a photocatalytic reaction ^[147]. The Cu₃SnS₄ photocatalyst exhibited a remarkable CH₄ production rate of 14 μmol g⁻¹ h⁻¹ with 80% selectivity without a cocatalyst or scavenger. To improve the activity and selectivity of the Cu₃SnS₄ photocatalyst, Yu and coauthors constructed S vacancies in Cu₃SnS₄ nanocrystals to introduce Cu(Ⅰ) and Sn(Ⅱ), which can effectively suppress the recombination of electrons and holes and improve their selectivity and reactivity ^[62]. The as-prepared S-vacancy Cu₃SnS₄ nanocrystals exhibited a CH₄ evolution rate of 22.65 μmol g⁻¹ h⁻¹ with a selectivity of 83.1%, without any cocatalyst or scavenger. Similarly, Xie and coauthors engineered S vacancies in Cu₅InS₈ single-unit-cells to improve the selectivity of photocatalytic CO₂ reduction (Figure 5a–d) ^[55]. The existence of S vacancies not only reduce the overall activation energy barrier (Figure 5a), but also transform the endoergic protonation process to an exoergic reaction step (Figure 5c–d), which can change the reaction pathway to improve the selectivity of CH₄. The obtained S-vacancy CuIn₅S₈ (Vs-CuIn₅S₈) single-unit-cell layers exhibited a CH₄ evolution rate of 8.7 μmol g⁻¹ h⁻¹ with a near 100% selectivity in the photocatalytic CO₂ reduction reaction (Figure 5b). Wang and coauthors constructed Cu_xInS₅-Cu_ySe S-scheme heterostructures to enhance the reactivity of photocatalytic CO₂ reduction ^[148]. The obtained S-scheme heterostructures showed a near 100% selectivity for photocatalytic CO₂ reduction to methanol, with a rate of 5.250 μmol g⁻¹ h⁻¹. In addition, Cu-based ternary and quaternary sulfide QDs are usually used as photosensitizers for photocatalytic CO₂ reduction assisted by molecular catalysts ^[46,149,150]. For example, Weiss and coauthors used CIS colloidal QDs as photosensitizers for the photocatalytic reduction of CO₂ to CO in pure water ^[46]. The photocatalysts exhibited excellent photoreduction performances with a turnover number of 72484−84101, a quantum yield of 0.96%–3.39%, and a CO selectivity of 99%.

Figure 5.  (a) Bandgap alignments of CuIn₅S₈ and Vs-CuIn₅S₈ single-unit-cell layers. (b) Photocatalytic CO₂ reduction performances of CuIn₅S₈ (blue bars) and Vs-CuIn₅S₈ (red bars) single-unit-cell layers. (c, d) Free energy diagrams of CO₂ photoreduction to CH₄ for the Vs-CuIn₅S₈ and pristine CuIn₅S₈ single-unit-cell layers (Reproduced from Ref. ^[55] with permission).

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3.4.   Photocatalytic organic synthesis

Photocatalytic organic synthesis is not only an attractive strategy enabling direct solar energy to chemical energy conversion, but it also provides a promising alternative to traditional high-energy chemical synthesis. Photocatalysts can absorb solar light to produce electrons and holes, thereby promoting chemical reactions on the surface of photocatalysts. A series of classic reactions, such as alcohol oxidation, hydrogenation, aerobic coupling and epoxidation have been achieved via photocatalytic reactions. Note that, the photocatalyst is essential for the photocatalytic reaction. CMSs are widely used for photocatalytic organic synthesis, such as PEC methylcyclohexane synthesis and PEC ammonia synthesis ^[47,152].

The development of nanotechnology promotes the preferable use of CMS nanomaterials for photocatalytic organic synthesis. For example, chalcopyrite CFS nanocrystals with a tetragonal structure show obvious localized surface plasmon resonance at 2.4 eV, which relaxes through nonradiative damping, thus generating hot holes or electrons and heat. In addition, due to their strong coordination preference of Fe–S units for organics and hydrogen atoms, CFS nanocrystals could be used as an attractive plasmonic photocatalyst for photocatalytic synthesis ^[151,153]. Zbořil and coauthors used CFS nanocrystals as plasmonic photocatalysts for the selective hydrogenation of nitroaromatics (Figure 6a–e) ^[151]. Under solar light irradiation, the CFS nanocrystals work through the synthetic action of photothermal effects and hot holes. The CFS nanocrystal catalysts exhibited a higher production rate than other top-rated photo and thermal catalysts. Importantly, these plasmonic catalysts can be reused many times. Sun and coauthors used S-doped biochar-supported CIS QD-sensitized Bi₂MoO₆ hierarchical flowers as photocatalysts for efficient photocatalytic biorefineries ^[154]. The obtained photocatalyst exhibited a xylonic acid yield of 86.59% under visible light irradiation. Chen and coauthors constructed a CIS modified polymeric carbon nitride S-scheme photocatalyst for H₂O₂ synthesis ^[155]. The optimized S-scheme photocatalyst showed a H₂O₂ yield of 1247.6 μmol L⁻¹ h⁻¹ with an apparent quantum yield (AQY) of 16.0% at 420 nm.

Figure 6.  (a) Photocatalytic reduction of nitrobenzene (NB) using CFS nanocrystals. (b) Photocatalytic reduction of NB for different reaction times. (c) Photocatalytic reduction of NB with different amounts of NB and catalysts. (d, e) Photocatalytic aniline yield at controlled temperature or light ^[151].

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3.5.   Photocatalytic pollutant removal

A pollutant-free environment is a fundamental need of human beings. A variety of pollutants effect living organisms. To solve these problems, the exploration of catalysts for the photocatalytic degradation of these pollutants using solar energy is necessary ^[156,157]. Semiconductor photocatalysis is an economical and effective strategy to remove pollutants. Metal oxide semiconductors are the most commonly used photocatalysts for photocatalytic pollutant removal ^[158,159]. Meanwhile, the greatest disadvantage of these photocatalysts is their wide bandgap which limits the absorption of visible light. CMS is a promising candidate for metal oxide due to its wide light absorption range ^{[49,160,161,162]}.

Under visible light irradiation, CMS photocatalysts absorb light to produce hole-electron pairs. Then, the electrons and holes migrate to the surface of the photocatalysts. Finally, the hole-electron pairs format different reactive oxygen species via a series of redox reactions, which are the dominant active species in the photooxidation of pollutants (Figure 7a). CTS flower-like architectures show obvious photocatalytic activity for photodegradation of methylene blue ^[163]. CZTS nanoparticles with a size of 2–5 nm was used for the photocatalytic degradation of industrial waste and organic pollutants in water ^[49]. The efficiency of CZTS nanoparticles was 75% for industrial waste and 98.4% for organic pollutants under sunlight (Figure 7b, c). Deka and coauthors also used surfactant-free hydrophilic CZTS nanoparticles as photocatalysts for the photoreduction of Cr(Ⅳ) into nontoxic Cr(Ⅲ) ^[164]. The high catalytic performance of the as-prepared CZTS NPs under visible light irradiation is attributed to the size, surface charge and electronic effect of the CZTS nanoparticles.

Figure 7.  (a) Photocatalytic pollutant removal mechanism of CZTS. (b, c) Absorption spectra for CZTS (organic pollutants) in sunlight and with lens ^[49].

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4.   Conclusions

Due to their suitable bandgap, adjustable atomic structure, unique electronic state, and outstanding optoelectronic properties, CMS nanomaterials are widely used in the field of photocatalysis and show exciting photocatalytic performances. This review provides a summary of recent progress on the photocatalytic applications of Cu-based ternary and quaternary sulfide nanomaterials, such as photocatalytic H₂ production, CO₂ reduction, organic synthesis, pollutant removal and PEC H₂ production.

Although remarkable progress has been made in the photocatalytic applications of Cu-based ternary and quaternary sulfide nanomaterials, there is a series of challenges that merit further investigation. First, the photocatalytic performances of current CMS nanomaterials are deficient for feasible scale applications. It is necessary to develop other CMS nanomaterials with novel structural features and electronic properties for photocatalytic applications. Additionally, engineering strategies should be developed to improve the photocatalytic performances of CMS nanomaterials, such as surface and defect engineering, element doping, cocatalyst loading, crystal structure engineering, and homojunction and heterojunction construction. Second, it is necessary to study the detailed photocatalytic mechanisms of how the CMS photocatalysts work using in situ methods, such as X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy, X-ray absorption fine structure (XAFS), Raman spectroscopy, and HRTEM. Deep exploration of the intrinsic mechanism of CMS photocatalysts will be beneficial for rational photocatalyst design to obtain higher photocatalytic activity. Third, the theoretical study of the CMS photocatalysis process is insufficient. Due to the surface atomic reconstruction and dynamic exciton behavior during the photocatalytic reaction, it is significant to use time-dependent density functional theory calculations to explore the excited-state chemistry. Furthermore, in situ characterizations assisted by calculations will provide deep insights into the relationship between atomic arrangements and photocatalytic properties. Fourth, the main products are CH₄ and CO for the photocatalytic CO₂ reduction of CMS nanomaterials. However, it is difficult to obtain high value-added C2+ products. Finally, hole-generated photocorrosion is a general problem for the metal sulfide photocatalysts. Further efforts should be made to develop other means of addressing the stability problems coming from photocorrosion.

Use of AI tools declaration

The author declares that no Artificial Intelligence (AI) tools were used in the creation of this article.

Acknowledgments

This work was supported by the National Key Research and Development Program of China (Grants 2021YFA0715700), the National Natural Science Foundation of China (Grants 22101271).

Conflict of interest

The author declares no conflict of interest.