Synthesis of Sulfur Vacancy-Bearing In₂S₃/CuInS₂ Microflower Heterojunctions via a Template-Assisted Strategy and Cation-Exchange Reaction for Photocatalytic CO₂ Reduction

Abstract

The synthesis of the accurate composition and morphological/structural design of multielement semiconductor materials is considered an effective strategy for obtaining high-performance hybrid photocatalysts. Herein, sulfur vacancy (Vs)-bearing In₂S₃/CuInS₂ microflower heterojunctions (denoted Vs-In₂S₃/CuInS₂) were formed in situ using In₂S₃ microsphere template-directed synthesis and a metal ion exchange-mediated growth strategy. Photocatalysts with flower-like microspheres can be obtained using hydrothermally synthesized In₂S₃ microspheres as a template, followed by Ostwald ripening growth during the metal cation exchange of Cu⁺ and In³⁺. The optimal heterostructured Vs-In₂S₃/CuInS₂ microflowers exhibited CO and CH₄ evolution rates of 80.3 and 11.8 μmol g⁻¹ h⁻¹, respectively, under visible-light irradiation; these values are approximately 4 and 6.8 times higher than those reported for pristine In₂S₃, respectively. The enhanced photocatalytic performance of the Vs-In₂S₃/CuInS₂ catalysts could be attributed to the synergistic effects of the following factors: (i) the constructed heterojunctions accelerate charge-carrier separation; (ii) the flower-like microspheres exhibit highly uniform morphologies and compositions, which enhance electron transport and light harvesting; and (iii) the vs. may trap excited electrons and, thus, inhibit charge-carrier recombination. This study not only confirms the feasibility of the design of heterostructures on demand, but also presents a simple and efficient strategy to engineer metal sulfide photocatalysts with enhanced photocatalytic performance.

1. Introduction

The photocatalytic reduction of CO₂ into carbonaceous fuels has attracted much attention because it offers a feasible strategy for reducing the consumption of fossil fuels and preventing the emission of greenhouse gases [1,2,3,4]. Various types of semiconductor-based photocatalysts, such as TiO₂, BiVO₄, In₂O₃, and WO₃, have been employed to transform CO₂ into high-value fuels, such as CO, CH₃OH, HCOOH, and CH₄ [5,6,7]. However, the performance of these photocatalysts is limited by their insufficient active sites, low band gap energies, and high charge-carrier recombination rate. Hence, the elaborate design of advanced photocatalysts with outstanding performance and excellent stability is of great importance.

Metal sulfides are fascinating photocatalytic materials because they possess excellent photosensitivity, low redox potential, appropriate band edges, moderate band potential, high stability, good charge-carrier mobility, etc. In₂S₃ and CuInS₂, in particular, have emerged as promising catalysts owing to their visible-light response and appropriate band gaps (In₂S₃: 2–2.3 eV; CuInS₂: 1.57 eV) for CO₂ conversion [8,9,10,11]. For example, In₂S₃-based photocatalysts, such as In₂S₃/In₂O₃/rGO [7], Bi₂S₃@In₂S₃ [12], and In₂S₃/C₃N₄ [13], can transform CO₂ into CO, CH₄, or C₂H₄. CuInS₂-based photocatalysts, such as Ti₃C₂ MXene@TiO₂/CuInS₂ [14], ZnIn₂S₄/CuInS₂ [15], and CuInS₂/ZnS [16], also exhibit photocatalytic activity for the reduction of CO₂ to CO and H₂. Heterojunctions have recently been demonstrated to maximize the performance of In₂S₃/CuInS₂ photocatalysts and increase their ability for CO₂ reduction [17,18]. However, the synthesis of the accurate composition and morphological/structural design of multielement CuInS₂ semiconductor materials on the surface of In₂S₃ is challenging because of the difficulty of balancing the various reactivities of multiple reactants. Several studies have pointed out that the accurate composition and morphological/structural design of semiconductors are effective strategies for obtaining photocatalysts with outstanding performance and excellent stability [19,20]. Thus, further explorations of suitable strategies for designing special morphological In₂S₃/CuInS₂ photocatalysts are necessary.

Photocatalysts with three-dimensional flower-like structures are intriguing materials that have recently shown great potential for photocatalytic CO₂ reduction because of their intrinsic properties, which include a large surface area enriched with exposed active sites, which promotes CO₂ capture and conversion catalysis, a lessened perpendicular charge-transfer distance, which prevents electron (e⁻)–hole (h⁺) recombination, and internal reflection/dispersion effects, which reinforce light utilization [21,22]. However, the synthesis of flower-like micro/nanocrystals is challenging because it requires the refined tuning of nucleation, growth of particles, and surface binding dynamics for selective crystal face exposure. Hence, common strategies to prepare semiconducting flower-like micro/nanocrystals with controllable compositions and structures are highly desirable for achieving outstanding photocatalysts for carbonaceous fuel production.

Cation-exchange reactions, in which the bonding cations of the host sublattice are replaced by external mobile cations, present a promising method for fabricating hollow photocatalysts based on the Kirkendall effect [23,24]. For instance, using the Kirkendall effect driven by cation exchange between Cu⁺ and In³⁺, Li et al. synthesized controllable compositions and morphology/structures of hollow CuInS₂ nanododecahedrons [25]. This type of cation exchange not only allows for multiple cations to be wholly or partly replaced with each other, but also achieves powerful synthetic control of the composition of the desired materials. Furthermore, semiconductor structures preserve the anisotropic shapes of the as-prepared templates. Wang et al. prepared various hollow multinary metal sulfides, including binary compounds (CdS, ZnS, Ag₂S, PbS, SnS), ternary compounds (CuInS₂, Zn_xCd_1−xS), and quaternary compounds (single-atom Pt-anchored Zn_xCd_1−xS, Zn_xCd_1−xS-Pt₁), using Cu_2−xS nanocubes as the initial template. In this case, Cu⁺ was replaced by cations, the Cu_2−xS nanocube framework was conserved, and precise control of distinct elementary compositions was achieved by adjusting the ratio and type of the metal cation source, all of which significantly improved the photocatalytic performance of the resulting catalysts [26]. It has been argued that this is a simple but efficient approach to attain precise compositions and morphological/structural of multicomponent semiconductor materials.

In this work, sulfur vacancy (Vs)-bearing In₂S₃/CuInS₂ with microflower heterojunctions (denoted Vs-In₂S₃/CuInS₂) were formed in situ using an In₂S₃ microsphere template-directed synthesis and metal ion exchange reaction. Hybrid photocatalysts with flower-like microspheres were obtained using hydrothermally synthesized In₂S₃ microspheres as a template, followed by Ostwald ripening growth during the metal cation exchange of Cu⁺ and In³⁺. The optimal heterostructured Vs-In₂S₃/CuInS₂ microflowers exhibited excellent activity, with CO and CH₄ evolution rates of 80.3 and 11.8 μmol g⁻¹ h⁻¹, respectively, under visible-light irradiation; these values are roughly 4 and 6.8 times higher than those reported for pristine In₂S₃, respectively. The enhanced photocatalytic performance of the Vs-In₂S₃/CuInS₂ catalysts can be attributed to the synergistic effects of the following factors: (i) the constructed heterojunctions accelerate charge-carrier separation; (ii) the flower-like microspheres exhibit highly uniform morphologies and compositions, which enhance electron transport and light harvesting; and (iii) the vs. may trap excited electrons and, thus, inhibit charge-carrier recombination.

2. Results and Discussion

Figure 1a shows a schematic of the synthetic process for the Vs-In₂S₃/CuInS₂ composites by using an In₂S₃ microsphere template-assisted and cation exchange-mediated growth strategy. Typically, In₂S₃ template materials with controllable morphology are first synthesized by a facile low-temperature solvothermal method. The mixed solution, including an appropriate amount of the preprepared In₂S₃, CuCl₂·2H₂O, and thioacetamide (TAA), is then heated. The SEM image in Figure 1b shows that the as-prepared In₂S₃ has a microspherical structure with a rough and uniform surface. The spherical architectures are composed of small nanoparticles, and the average diameter of all the spheres is in the range of 1–2 μm. Figure 1c,d show SEM images of Vs-In₂S₃/CuInS₂, while Figure 1e shows its TEM image. The images reveal that the as-derived Vs-In₂S₃/CuInS₂ maintains the spherical shape of the In₂S₃ microsphere template and that all of the spheres have a uniform morphology with an average diameter of 1–2 μm. However, the surfaces of the microspheres exhibit numerous flakes (thickness, <15 nm) aligned with each other. The unique morphology of Vs-In₂S₃/CuInS₂ may be derived from Ostwald ripening growth during the metal cation exchange of Cu⁺ and In³⁺. The HRTEM image shown in Figure 1f reveals lattice spacings of 0.32 and 0.29 nm, which correspond to the (112) crystal plane of CuInS₂ and the (400) crystal plane of In₂S₃, respectively; these findings imply that CuInS₂ is orthotopically grown on the surface of the In₂S₃ microspheres, leading to the successful preparation of Vs-In₂S₃/CuInS₂. The selected area electron diffraction (SAED; Figure 1f, inset) pattern of the Vs-In₂S₃/CuInS₂ composite shows that the flower-like microspheres are polycrystalline, indicating that CuInS₂ is generated along with Ostwald ripening growth spatial orientation during the cation-exchange process; this finding is consistent with the TEM results. The elemental composition of the composites, which includes Cu, In, and S, is confirmed by the elemental mappings shown in Figure 1g.

The XRD patterns shown in Figure 2a reveal that all the diffraction peaks of In₂S₃ can be well indexed to the standard card (JCPDS No. 25-0390), and no other phase is observed (Figure 2a). Furthermore, most of the diffraction peaks in the XRD pattern of Vs-In₂S₃/CuInS₂ can be indexed to the composite phase of In₂S₃ and CuInS₂ (In₂S₃: JCPDS No. 25-0390; CuInS₂: JCPDS No. 27-0159).

XPS was used to investigate the elemental composition and valence environment of the samples. The survey XPS profile of Vs-In₂S₃/CuInS₂ reveals the existence of Cu, In, and S (Figure S1). The two peaks in the Cu 2p spectrum (Figure 2b) with binding energies of 932.6 and 952.3 eV are assigned to Cu 2p_3/2 and Cu 2p_1/2 of Cu⁺, respectively. These peaks coincide with the literature description of the valence state of Cu in CuInS₂, and verify the successful growth of CuInS₂ on In₂S₃. The peak in the Cu LMM (Auger electron) spectrum (Figure 2c) at 917.6 eV reveals that the oxidation number of Cu is +1. The peaks in the XPS profile of In 3d (Figure 2d) at 444.8 and 452.4 eV are ascribed to the core lines of the In–S bond. The binding energy of In 3d in In₂S₃ is 0.10 eV lower than that in Vs-In₂S₃/CuInS₂ owing to the existence of CuInS₂. The S 2p spectra of In₂S₃ and Vs-In₂S₃/CuInS₂ can be deconvoluted into two peaks with binding energies of 162.8 and 161.7 eV, which are consistent with S²⁻ in the crystal lattice. The binding energy of S 2p in In₂S₃ is 0.10 eV lower than that in Vs-In₂S₃/CuInS₂, probably because of the existence of vs. or CuInS₂ (Figure 2e). The presence of vs. was further confirmed by EPR spectroscopy. Several vs. are formed on the In₂S₃/CuInS₂ surface because it is S-deficient owing to the sharing of S atoms (Figure 2f). Numerous studies have shown that vs. defects can not only regulate the electronic structure/state of a photocatalyst but also serve as active sites to adsorb and activate CO₂ molecules, thus leading to improved photoactivity [27,28].

Photocatalytic CO₂ conversion was performed in the presence of water vapor under simulated solar irradiation (Figure 3). CO as the major product and a small amount of CH₄ as the minor product were detected over In₂S₃ and Vs-In₂S₃/CuInS₂. In₂S₃ could produce 20 μmol g⁻¹ CO and 1.73 μmol g⁻¹ CH₄ after 1 h (Figure 3a), and these yields gradually increase with increasing irradiation time. Compared with that of In₂S₃, the photocatalytic activity of Vs-In₂S₃/CuInS₂ is significantly improved, and the yields of CO and CH₄ over the catalyst gradually increase as the amount of CuCl₂·2H₂O increases from 0.25 to 0.75 g (Figure 3b). Maximum yields of 80.3 μmol g⁻¹ CO and 11.8 μmol g⁻¹ CH₄ are achieved over 0.75-Vs-In₂S₃/CuInS₂ within 1 h; these yields are 4 and 6.8 times greater than those produced over In₂S₃, respectively. However, the use of 1.0 g of CuCl₂·2H₂O leads to a reduction in activity, possibly because excess vs. defects may accelerate the recombination of photogenerated carriers (Figure 3c). The enhanced photocatalytic performance of the Vs-In₂S₃/CuInS₂ composite could be attributed to its flower-like microsphere structure, which features a large surface area that could promote the exposure of active sites and increase charge separation owing to the short charge-transport distance from the bulk to the surface. Moreover, the existence of vs. may trap excited electrons, thereby inhibiting charge-carrier recombination.

Considering that the photocatalytic behavior of semiconductors is closely related to their optical properties, UV-visible absorption spectroscopy was conducted on the samples (Figure 4a). Vs-In₂S₃/CuInS₂ shows significant absorption over nearly the entire visible region up to 700 nm. Its corresponding band gap energy is estimated to be 2.02 eV (Figure 4a, inset), which is lower than that of pure In₂S₃ (2.2 eV) owing to the low band gap energy of CuInS₂. The calculated band gap energy of CuInS₂ is 1.2 eV (Figure 4b, right), which approximately matches the literature value of 1.57 eV [11]. Theoretical calculations indicate that the band gap energy of In₂S₃ is approximately 2.13 eV, which closely matches the experimental result (Figure 4b, left). These results illustrate the reliability of the experimental data. Additionally, the absorption intensity of Vs-In₂S₃/CuInS₂ is improved compared with that of In₂S₃ because of its flower-like microsphere structure. The flat band potentials (E_FB) of the samples were acquired from the tangent of their Mott–Schottky curves at a frequency of 1000 Hz (Figure 4c). The E_FB values of In₂S₃ and CuInS₂ are −0.81 and −1.12 eV, respectively. In general, the E_FB is approximately 0.2 eV more positive than the conduction band edge (E_CB) of n-type semiconductors. Thus, the E_CB values of In₂S₃ and CuInS₂ can be estimated to be −1.01 and −1.32 eV, respectively. According to the equation E_VB = E_CB + E_g, the minimum positions of the corresponding conduction bands (CBs) of In₂S₃ and CuInS₂ may occur at 1.19 and 0.25 eV, respectively. The energy band structures of In₂S₃ and CuInS₂ are illustrated in Figure 4d; the well-aligned bands shown in the figure meet the photocatalytic CO₂ reduction potential. Under light irradiation, the electrons are excited from the VB to the CBs of In₂S₃ and CuInS₂. Owing to the formation of heterojunctions, the excited electrons of CuInS₂ transfer to the CBs of In₂S₃. The photoinduced electrons then react with CO₂ to form CO and CH₄. Simultaneously, the photoinduced holes could transfer from the valence band of the In₂S₃ to that of CuInS₂. NaSO₃, as the sacrificial agent, reacts with holes to further inhibit the recombination of photoinduced electrons and holes [29]. Moreover, previous work has reported that when vacancies are generated, a defect energy level will form in the forbidden band and the higher the sulfur vacancy concentration, the closer the defect level is to the CB position [27]. The EPR test (Figure 2d) verified that Vs exist in the In₂S₃/CuInS₂ sample; therefore, a defect energy level forms below the CBs of CuInS₂ (Figure 4d), which could easily trap the photogenerated charge carriers.

Figure 5a shows the transient photocurrent measurements of the as-prepared In₂S₃ and Vs-In₂S₃/CuInS₂ photoelectrodes under intermittent light irradiation; here, the light was switched on and off at 250 s intervals. The transient photocurrent for Vs-In₂S₃/CuInS₂ is higher than that for In₂S₃, indicating a higher charge-separation efficiency of the photogenerated carriers when using the Vs-In₂S₃/CuInS₂ composite. This result implies that the formation of a flower-like microsphere structure and heterojunctions enhance the separation efficiency of photoinduced charges, leading to improved photocatalytic performance. The Nyquist plots shown in Figure 5b reveal that the extent of photocurrent enhancement exhibited by Vs-In₂S₃/CuInS₂ could be correlated with the reduction in the charge-transfer resistance of the material. Fitting the experimental data to an equivalent resistance/capacitance circuit model (Figure 4a, inset) reveals that compared with In₂S₃, Vs-In₂S₃/CuInS₂ presents lower charge-transfer resistance, indicating easier charge transfer on the catalyst surface owing to the existence of a flower-like microsphere structure, Vs, and heterojunctions in the composite. The room-temperature steady-state PL of the developed catalyst was further investigated to understand its photogenerated charge-transfer mechanism. The intensity of the room-temperature steady-state PL peak of Vs-In₂S₃/CuInS₂ is weaker than that of In₂S₃ (Figure 5c), indicating that the recombination efficiency of the photogenerated electrons and holes is low. Based on the above analyses, the enhanced photocatalytic performance of the Vs-In₂S₃/CuInS₂ catalysts could be attributed to the synergistic effect of the following factors: (i) the formation of a flower-like microsphere structure with high morphological and compositional uniformity, which enhances light harvesting and electron transport; (ii) the acceleration of charge-carrier separation by the constructed heterojunctions; and (iii) the introduction of Vs, which may trap excited electrons and inhibit charge-carrier recombination.

To reveal the underlying reasons for the improved CO₂ photoreduction performances over the Vs-In₂S₃/CuInS₂ heterostructure, a possible mechanism of selective photocatalytic CO₂ reduction is proposed. Prior reports [30,31] indicate that the selectivity of methane during CO₂ reduction is strongly related to the reaction path of CO* to CHO*, which determines whether the CO* intermediates desorb to CO (g) or continue protonation to CHO* to yield CH₄. Xie et al. [31] report that the reaction energy from CO* to CHO* of the In₂S₃ slab is much higher than that from CO* to CO (g), consistent with the low activity towards CH₄ during CO₂ photoreduction in Figure 3a. Compared with that of In₂S₃, the photocatalytic activity of Vs-In₂S₃/CuInS₂ is significantly improved, its products are still CO and CH₄, and the yields of CO always exceed those of CH₄ (Figure 3b). These results suggest that the reaction energy from CO* to CO (g) and CO* to CHO* of the Vs-In₂S₃/CuInS₂ heterostructure slab was distinctly lower than In₂S₃, which could be attributed to the optimized carrier dynamics on the hybrid photocatalyst slab, including the generation and separation of the carrier. Furthermore, vs. may act as adsorption sites for CO₂ [28], conducive to the further hydrogenation of the CO intermediate into CH₄. To understand the deep behavior of the photocatalytic reduction of CO₂, this aspect needs to be further studied in the future.

In short, this study offers a significant contribution to the literature because we not only confirmed the feasibility of the design of heterostructures on demand, but also demonstrated the benefits of such heterostructures for improving the photochemical properties of In₂S₃-based photocatalysts. However, satisfactory photocatalytic efficiency in applications such as solar energy conversion, environmental remediation, chemosensors, etc., has not been achieved until now. Thus, intensive investigations should be carried out in the future, in order to provide scientific references for maximizing the photocatalytic efficiencies of sulfide semiconductors.

3. Experimental Section

3.1. Materials

Indium chloride tetrahydrate (InCl₃·4H₂O, AR) and copper chloride dihydrate (CuCl₂·2H₂O, AR) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China. Thioacetamide (TAA, AR) was provided by Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. All reagents used during the experiment were as provided without further purification.

3.2. Synthesis of In₂S₃ and Vs-In₂S₃/CuInS₂

The In₂S₃ crystals were prepared using a facile one-pot solvothermal method. Briefly, 37 mg of InCl₃⋅4H₂O and 15 mg of TAA were dissolved in 40 mL of ethanediol under ultrasonication. The mixture was then transferred into a hydrothermal autoclave (50 mL in capacity) and maintained at 150 °C for 90 min. The In₂S₃ microspheres were obtained by multiple centrifugations and thoroughly washed with water and ethanol, using a centrifugal speed of 4000 r/min and maintaining time of 1 min. Vs-In₂S₃/CuInS₂ flower-like microspheres were subsequently synthesized using an In₂S₃ template-assisted and cation-exchange strategy. Typically, an appropriate amount of In₂S₃ was added to a 40 mL ethanediol solution containing CuCl₂·2H₂O and TAA under continuous stirring. The solution was then transferred to a sealed Teflon-lined autoclave and heated to 150 °C for 90 min. After the reaction was completed, the products were collected by centrifugation, washed several times with deionized water, and dried at 60 °C in a vacuum. The mass of CuCl₂·2H₂O was changed to 0.25, 0.5, 0.75, and 1.0 g, and the same procedures were performed to obtain Vs-In₂S₃/CuInS₂ with different Cu masses; these products were denoted as 0.25-Vs-In₂S₃/CuInS₂, 0.50-Vs-In₂S₃/CuInS₂, 0.75-Vs-In₂S₃/CuInS₂, and 0.10-Vs-In₂S₃/CuInS₂, respectively.

3.3. Characterizations

X-ray diffraction (XRD, Rigaku D/MAX-Ultima III, Tokyo, Japan) was used to investigate the purity information and crystallographic phase of the as-prepared powder samples. The XRD pattern was recorded by using Cu-ka radiation (λ = 0.154178 nm) at 40 kV and 40 mA with a scan rate of 10° min⁻¹. The morphology of nanomaterials was observed by field-emission scanning electron microscopy (FESEM, FEI NOVA NANOSEM 230). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were captured using JEM 200CX TEM apparatus. Chemical states were investigated by X-ray photoelectron spectroscopy (XPS; K-Alpha, Thermo Fisher Scientific, Waltham, MA, USA); here, the spectra obtained were standardized according to the binding energy of the adventitious C 1s peak at 284.8 eV. A UV-vis spectrophotometer (UV-3600iPlus, Shimadzu, San Jose, CA, USA) was used to record the UV-visible diffuse reflectance of the samples. The sulfur vacancy signals were examined using spin-trapping electron paramagnetic resonance (EPR, EXM-10/12, Bruker, Bremen, Germany) measurements. The photoluminescence (PL) decay profiles of the samples were analyzed using single-particle confocal fluorescence spectroscopy measurements (PicoHarp300, Berlin, Germany).

3.4. Photoelectrochemical Measurements

Photoelectrochemical measurements were performed using electrochemical analysis (CHI-630D, Shanghai Chenhua, Shanghai, China) in a standard three-electrode system. The electrolyte was 0.5 M NaSO₄ aqueous solution. The as-prepared In₂S₃ or Vs-In₂S₃/CuInS₂ sample was used as the working electrode. A saturated Ag/AgCl electrode and a Pt foil were used as the reference and counter electrodes, respectively. The preparation process of In₂S₃ and Vs-In₂S₃/CuInS₂ bulk samples was as follows. In₂S₃ or Vs-In₂S₃/CuInS₂ catalyst ink was first prepared by dispersing 10 mg of the as-prepared catalyst in 1 mL of ethanol under sonication. Then, 50 μL of the ink was evenly spread onto a piece of pretreated fluorine-doped tin oxide (FTO) glass within a 1 cm² area and dried at room temperature (25 °C). Thus, the catalysts were attached to the FTO surface. The transient photocurrent test curves exhibited ohmic characteristics, confirming the formation of an ohmic back contact between the sample and FTO. The working area of the electrode was 1 × 1 cm², and the scan rate was 5 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) was performed using a PAR2273 workstation (Princeton Applied Research, USA) under AM 1.5G illumination. The Mott–Schottky curves of the samples were measured in 0.5 M Na₂SO₄ aqueous solution using an electrochemical workstation (CHI-630D, Shanghai Chenhua, Shanghai, China).

3.5. Photocatalytic Activity Measurements

Photocatalytic reduction of CO₂ was performed on a photoreaction system. A 4–5 mg powder catalyst was dissolved under ultrasound for 10 min in the prepared solution, including 2 mL of deionized H₂O and 1 mL of NaSO₃ in a 460 mL reactor with a quartz glass cover. After the air in the reaction system was cleared away, high-purity CO₂ was introduced into the system until the pressure reached 70 kPa and circulated for 60 min to achieve uniform distribution of CO₂ gas. The temperature of photocatalytic CO₂ reduction was set at 10 °C by cooling the water circulation system, which can promote the adsorption of CO₂. A 300 W xenon arc lamp was used as the light source for the photocatalytic reaction. During irradiation, approximately 1 mL of gas was collected from the reaction cell at specific time intervals for CO and CH₄ concentration analysis using a gas chromatograph (GC-2014C, Shimadzu Corp., Kyoto, Japan).

3.6. Density Functional Theory Calculation Details

The calculations are performed within the framework of density functional theory (DFT) using a basis set consisting of plane waves. The electron–ion interactions are described by ultrasoft pseudopotentials and electron exchange, and correlation energies are calculated with the Perdew–Burke–Ernzerhof (PBE) formulation of the generalized gradient approximation (GGA). The geometric structure is optimized with the Broyden–Fletcher–Goldfarb–Shanno (BFGS) method, the forces on each ion converge to less than 0.01 eV/Å, and the stress is less than 0.02 GPa. All the atoms of the structures are fully relaxed to their equilibrium positions with an energy convergence of 5 × 10⁻⁶ eV, the atomic displacement is less than 5 × 10⁻⁴ Å, and the self-consistent field (SCF) tolerance is 5 × 10⁻⁷ eV. The kinetic energy cutoff (In₂S₃: 350 eV; CuInS₂: 440 eV) of the plane wave basis is used throughout, and the Brillouin zone is sampled with special k-points of a 3 × 3 × 1 grid for In₂S₃ and a 5 × 5 × 2 grid for CuInS₂ based on Monkhorst–Pack.

4. Conclusions

In this study, Vs-In₂S₃/CuInS₂ microflower heterojunctions were formed in situ using an In₂S₃ microsphere template-directed synthesis and metal ion exchange-mediated growth strategy. Hybrid photocatalysts with a flower-like microsphere structure were obtained using hydrothermally synthesized In₂S₃ microspheres as a template, followed by Ostwald ripening growth during the metal cation exchange of Cu⁺ and In³⁺. The as-fabricated Vs-In₂S₃/CuInS₂ heterojunctions exhibited significantly improved photocatalytic activity for CO and CH₄ evolution compared with pure In₂S₃ owing to the synergistic effects of enhancements in charge-carrier separation and visible-light absorption capacity. We believe that the findings could serve as guidance for future researchers aiming to engineer other metal sulfide-based photocatalysts with enhanced photocatalytic performance via a simple and efficient strategy.