Cu-Modified Zn₆In₂S₉ Photocatalyst for Hydrogen Production Under Visible-Light Irradiation ^†

Abstract

Copper-doped indium zinc sulfides were synthesized by heating and stirring a mixture of zinc chloride, indium chloride tetrahydrate, thioacetamide, and copper chloride at 180 °C for 18 h. Among these, Zn_5.7Cu_0.3In₂S₉ exhibited a hydrogen-producing activity of 1660 μmol/g·h, which was approximately five times higher than that of pristine indium zinc sulfide. Therefore, the catalyst was characterized to investigate the effect of Cu addition. PL results revealed that the incorporation of Cu reduced the fluorescence intensity, indicating suppressed recombination of photogenerated electron–hole pairs. DRS showed that the Cu addition enhanced optical absorption in the visible-light region and narrowed the band gap. These findings suggest that the incorporation of copper into indium zinc sulfide improves its photocatalytic activity.

1. Introduction

In recent years, the increasing consumption of fossil fuels, such as petroleum, coal, and natural gas, driven by intensified economic activities in developed countries, has exacerbated environmental issues and accelerated the depletion of energy resources [1,2]. Consequently, hydrogen has garnered significant attention as a clean and sustainable next-generation energy carrier. Therefore, the development of environmentally friendly hydrogen production technologies is urgently needed. In particular, photocatalytic hydrogen evolution utilizing renewable energy sources, such as solar irradiation, is regarded as a promising and sustainable alternative that can reduce the reliance on fossil fuels [3,4].

The development of efficient photocatalysts is of paramount importance to achieve this goal. Among the various materials investigated, zinc indium sulfide has emerged as a promising candidate owing to its appropriate bandgap, high responsiveness to visible light, and non-toxicity [5,6]. However, the intrinsic photocatalytic performance of zinc indium sulfide remains poor. Thus, further modification strategies such as co-catalyst loading or elemental doping are required to enhance their activity [7,8]. Among these strategies, doping is considered particularly effective because it can introduce impurity levels and narrow the bandgap, thereby facilitating charge carrier separation and transport. Elements, such as Cu, Ni, and N, have been widely employed for this purpose. Cu doping has attracted considerable research interest across various material systems, including metal sulfides, metal oxides, carbon-nitrogen compounds, and organic materials [8].

In this study, Cu-doped indium zinc sulfide photocatalysts were synthesized via a solvothermal method, and their photocatalytic hydrogen evolution performance was systematically evaluated. Among the prepared samples, the Zn₅.₇Cu₀.₃In₂S₉ catalyst exhibited the highest hydrogen production rate, achieving 1660 μmol/g·h, which is approximately five times higher than that of the undoped zinc indium sulfide (320 μmol/g·h). This significant enhancement in photocatalytic activity was attributed to the incorporation of copper, which effectively broadened the visible light absorption range, suppressed electron–hole recombination, and reduced the charge transfer resistance. These findings highlight the potential of Cu-doped indium zinc sulfide as a promising material for the development of next-generation photocatalysts for hydrogen production.

2. Methods

2.1. Chemicals

All reagents used in this study were of reagent grade and used without further purification. Indium (III) chloride tetrahydrate (InCl₃·4H₂O), thioacetamide (TAA), hexachloroplatinic acid (H₂PtCl₆), sodium sulfide pentahydrate (Na₂S·5H₂O), and sodium sulfite (Na₂SO₃) were purchased from the Fujifilm Wako Pure Chemical Corporation. Zinc chloride (ZnCl₂) and copper(I) chloride (CuCl) were obtained from Nacalai Tesque. Aqua pura were acquired using an ultrapure water system (Advantec, MFS Inc., (Dublin, OH, USA)).

2.2. Synthesis of Zinc Indium Sulfide Photocatalysts

In this study, indium zinc sulfide was prepared using a solvothermal method. Indium chloride tetrahydrate and zinc chloride were added to 80 mL water and purged with nitrogen. Thioacetamide and copper chloride were then added, and the mixture was heated and stirred at 180 °C for 18 h. The product was then washed with water and ethanol and vacuum dried.

Table 1. Amounts of precursors.

	ZnCl2 (mmol)	InCl3 · 4 H2O (mmol)	TAA (mmol)	CuCl (mmol)
Zn6In2S9	2.700	0.900	4.050	0.000
Zn5.9Cu0.1In2S9	2.655	0.900	4.050	0.045
Zn5.8Cu0.2In2S9	2.610	0.900	4.050	0.090
Zn5.7Cu0.3In2S9	2.565	0.900	4.050	0.135
Zn5.6Cu0.4In2S9	2.520	0.900	4.050	0.180
Zn5.4Cu0.6In2S9	2.430	0.900	4.050	0.270

lists the amounts of zinc indium sulfide precursors.

2.3. Characterization

X-ray diffraction (XRD) patterns of the pretreated photocatalysts were recorded using an Ultima IV powder X-ray diffractometer (Rigaku Co, Tokyo, Japan) with a Cu Kα radiation source. The specific surface area and pore size distribution were determined from nitrogen adsorption–desorption isotherms using a BELSORP-mini II surface area analyzer (BEL Japan, Inc., Osaka, Japan). The surface morphologies of the photocatalysts were observed using scanning electron microscopy (SEM; S-4300, Hitachi, Tokyo, Japan). The chemical states of the samples were analyzed by X-ray photoelectron spectroscopy (XPS) using an Al Kα radiation source, with measurements performed on a PHI Quantera SXM (ULVAC-PHI, Kanagawa, Japan) instrument. The binding energies were calibrated against the C 1 s peak at 284.8 eV as a reference. UV–visible (UV–vis) diffuse reflectance spectra were collected using a V-750 spectrophotometer equipped with an ISV-922 integrating sphere (Jasco, Tokyo, Japan). Photoluminescence (PL) spectra were obtained using an FP-8500 fluorescence spectrometer (Jasco, Tokyo, Japan). Time-resolved photoluminescence (TRPL) spectra of the samples were recorded using a Quantaurus-Tau (C11367-21, HAMAMATSU Photonics, Shizuoka, Japan).

2.4. Photocatalytic Hydrogen Evolution Activity Tests

Photocatalytic experiments were conducted by dispersing 40 mg of the prepared photocatalyst in 40 mL of an aqueous solution containing 20 mL of 0.50 M Na₂SO₃/0.70 M Na₂S and 1.2 mL of hexachloroplatinic acid (H₂PtCl₆). In all hydrogen production experiments, platinum was loaded onto the samples at 1.2 mg (3 wt%) using the photodeposition method. The suspension was purged with nitrogen gas for 30 min to remove dissolved oxygen prior to visible light irradiation (λ ≥ 420 nm) for 6 h. The photocatalytic activity was evaluated based on the hydrogen evolution rate, which was calculated from the amount of hydrogen generated after 3 and 6 h of irradiation. The apparent quantum yield (AQY) of ZIS was determined using the following equation:

$$AQY \left(\%\right)= {\displaystyle \frac{number of reacted electrons}{number of incident photons}} \times 100\%= {\displaystyle \frac{number of evolved H_2 molecules \times 2}{number of incident photons}} \times 100\%$$

(1)

2.5. Electrochemical Characterization

Electrochemical analyses were conducted using electrochemical impedance spectroscopy (EIS), Mott–Schottky measurements, and transient photocurrent response measurements. A conventional three-electrode configuration was employed, consisting of a platinum wire counter electrode, an Ag/AgCl (saturated KCl) reference electrode, and a working electrode immersed in a 0.2 M Na₂SO₄ aqueous electrolyte.

The working electrode was prepared as follows: (1) a photocatalyst sample (5.0 mg) was dispersed in a mixed solution of 90 μL 2-propanol and 10 μL Nafion solution, and (2) the suspension was ultrasonicated and subsequently drop-cast onto a fluorine-doped tin oxide (FTO) glass substrate. The potential of the Ag/AgCl electrode at 25 °C was +0.196 V vs. the normal hydrogen electrode (NHE).

3. Results and Discussion

3.1. Structural and Morphological Analyses

The crystallinity of Zn₆In₂S₉ and Cu-doped Zn₆In₂S₉ samples was investigated by X-ray diffraction (XRD). As shown in Figure 1, four characteristic diffraction peaks corresponding to indium zinc sulfide were observed for all catalysts, which can be indexed to the (102) and (104) planes at approximately 27.9°, the (112) plane at around 47.5°, and the (202) plane at approximately 56.5° [9,10]. The consistent presence of these diffractions in all samples indicates that Cu incorporation does not significantly alter the primary crystal structure of Zn₆In₂S₉.

Scanning electron microscopy (SEM) was performed to examine the surface morphology of the photocatalysts, as shown in Figure 2. The results revealed that the incorporation of copper had no discernible impact on the surface morphology of indium zinc sulfide. Elemental mapping images (Figure S1) confirm the presence of Zn, In, S, and Cu in the composite. These images also reveal that Zn, In, S, and Cu are uniformly distributed throughout the material, indicating the successful preparation of Cu-loaded indium zinc sulfide.

As shown in Figure S2, the XPS spectra of Zn₆In₂S₉ and Zn_5.7Cu_0.3In₂S₉ were analyzed to investigate the chemical environments of the constituent elements. The spectra confirm the presence of Zn, In, and S in Zn₆In₂S₉, and Zn, In, S, and Cu in Zn_5.7Cu_0.3In₂S₉. The Zn 2p₁/₂ and Zn 2p₃/₂ peaks are observed at 1045.56 and 1022.56 eV for Zn₆In₂S₉, and at 1045.4 and 1022.39 eV for Zn_5.7Cu_0.3In₂S₉, which are characteristic of Zn²⁺ (Figure S2b). As shown in Figure S2c, the In 3d₃/₂ and In 3d₅/₂ peaks are located at 452.9 and 445.3 eV for Zn₆In₂S₉, and at 452.74 and 445.17 eV for Zn_5.7Cu_0.3In₂S₉, respectively, confirming the presence of In³⁺. In Figure S2d, the S 2p₁/₂ and S 2p₃/₂ peaks appear at binding energies of 162.4 and 163.6 eV for Zn₆In₂S₉, and 162.1 and 163.3 eV for Zn_5.7Cu_0.3In₂S₉, respectively, indicating the presence of S²⁻. Furthermore, the Cu 2p₃/₂ and Cu 2p₁/₂ peaks appear at 932.65 and 952.65 eV, respectively, demonstrating that Cu is present in the monovalent state (Figure S2e).

Nitrogen adsorption–desorption measurements were performed to evaluate the specific surface areas and pore size distributions of the samples, and the corresponding isotherms are presented in Figure S3. As depicted in Figure S3a, all samples exhibited type IV isotherms accompanied by H3-type hysteresis loops, indicating mesoporous structures comprising slit-like pores [11]. As depicted in Figure S3b, the pore size distribution profiles demonstrate a dominant peak at approximately 3 nm for all photocatalysts. Importantly, the addition of Cu did not result in an increase in the specific surface area. These observations suggest that the enhancement of photocatalytic activity is not attributable to changes in the surface area.

3.2. Optical and Electrochemical Properties

UV–visible diffuse reflectance spectroscopy (UV–vis DRS) was conducted to evaluate the optical absorption properties of all zinc indium sulfide photocatalysts (Figure 3) [12]. The results revealed that pristine zinc indium sulfide exhibited the shortest absorption edge, indicating a limited light absorption in the visible region. Upon Cu doping, a significant red-shift of the absorption edge was observed, with enhanced absorption extending beyond 500 nm. This expansion of the absorption region facilitates the generation of additional photogenerated electron–hole pairs, thereby enhancing the photocatalytic activity.

The optical band gaps of the samples were estimated using the Tauc plots. A progressive reduction in the bandgap energy was observed with increasing Cu content. Specifically, the band gap energy decreased from 2.90 eV for pristine Zn₆In₂S₉ to 2.40 eV for Zn_5.7Cu_0.3In₂S₉, which exhibited the highest photocatalytic activity among the samples.

Photoluminescence (PL) spectra were recorded for all the zinc indium sulfide samples to evaluate the recombination behavior of the photogenerated charge carriers (Figure S4). Compared to pristine zinc indium sulfide, the Cu-doped zinc indium sulfide samples exhibited markedly reduced PL emission intensities, indicating a more efficient separation of the photoinduced electron–hole pairs. Furthermore, the PL intensity decreased progressively with increasing Cu content, suggesting further enhancement of the charge carrier separation efficiency. TRPL measurements were performed for Zn₆In₂S₉, Zn_5.7Cu_0.3In₂S₉, and Pt/Zn_5.7Cu_0.3In₂S₉ (Figure S5). Platinum was supported using the photodeposition method. The fluorescence lifetimes of Zn₆In₂S₉, Zn_5.7Cu_0.3In₂S₉, and Pt/Zn_5.7Cu_0.3In₂S₉ were determined to be 2.7 ns, 1.4 ns, and 0.71 ns, respectively. Furthermore, Pt/Zn_5.7Cu_0.3In₂S₉ exhibits a shorter fluorescence lifetime compared to Zn_5.7Cu_0.3In₂S₉, indicating that the Pt single atoms provide an additional pathway for electron transfer [13].

An electrochemical impedance spectroscopy (EIS) Nyquist plot is shown in Figure 4a. Compared with pristine zinc indium sulfide, the Zn_5.7Cu_0.3In₂S₉ sample exhibited a significantly smaller semicircle diameter, indicating improved charge carrier mobility and reduced electron transfer resistance [14]. Additionally, Zn_5.7Cu_0.3In₂S₉ demonstrated the highest photocurrent density (Figure 4b), suggesting efficient charge transfer. The Mott–Schottky plot for Zn_5.7Cu_0.3In₂S₉, shown in Figure 4c, exhibits a positive slope, confirming that it is an n-type semiconductor [15,16]. The flat band potential (E_FB) of Zn_5.7Cu_0.3In₂S₉ was determined to be −1.45 V (vs. Ag/AgCl), and E_FB (vs. NHE) = E_FB (vs. Ag/AgCl) + 0.196 eV, the corresponding value was −1.25 V (vs. NHE). In n-type semiconductors, the conduction band (CB) is typically located approximately 0.1 eV below the flat band potential (EFB). Therefore, for Zn_5.7Cu_0.3In₂S₉, the CB is situated at −1.35 V. The valence band (VB) was calculated using the equation E_BG = E_VB − E_CB. As a result, the VB was located 1.05 V.

3.3. Photocatalytic Activity and Mechanism of the Photocatalysts

Hydrogen production from the photocatalyst was evaluated using a Na₂SO₃/Na₂S mixture as a sacrificial reagent and platinum (Pt) as a co-catalyst with varying copper loading (Figure 5a). The results demonstrate that Zn_5.7Cu_0.3In₂S₉ exhibits the highest performance, achieving a hydrogen production rate of 1660 μmol/g·h, which is approximately five times higher than that of pristine zinc indium sulfide.

The stability of Zn_5.7Cu_0.3In₂S₉, which exhibited the highest activity, was assessed using a photocatalytic cycling test (Figure 5b). The results showed that the photocatalyst maintained a high reactivity after 30 h of continuous irradiation.

Figure 5c shows the wavelength-dependent apparent quantum yield (AQY) of Zn_5.7Cu_0.3In₂S₉. The action spectra were measured at specific wavelengths (420, 450, 500, 550, 600, 650, and 700 nm). These spectra exhibit a trend similar to that of the DRS absorption spectra, suggesting that hydrogen production is driven by the band excitation of the photocatalyst. A comparison of hydrogen production rate with those reported in previous studies is shown in Table S1 [11,17,18,19,20].

Finally, the energy band structure of Zn_5.7Cu_0.3In₂S₉, derived from the Tauc plot (Figure 3b) and Mott–Schottky plot (Figure 4c), along with the proposed reaction mechanism, are illustrated in Figure 6. Upon irradiation, electrons in the valence band were excited to the conduction band, generating photoinduced electrons and holes. The photoexcited holes efficiently oxidize sulfide (S²⁻) and sulfite (SO₃²⁻) ions, resulting in proton generation. Concurrently, the photoexcited electrons are transferred to sulfide and sulfite ions, effectively suppressing electron–hole recombination and enhancing charge separation. Additionally, the photoexcited electrons in the conduction band are transferred to platinum (Pt), where they reduce protons (H⁺) to generate hydrogen gas.

4. Conclusions

In this study, the solvothermal method was employed to reduce the Zn ratio in Zn₆In₂S₉ and to dope it with Cu, resulting in a hydrogen evolution rate of 1660 μmol/g·h. This enhancement is attributed to the reduction in the bandgap energy and the increased visible-light absorption capacity due to Cu doping. In addition, Zn_5.7Cu_0.3In₂S₉ exhibited excellent stability. The discovery of this zinc indium sulfide photocatalyst offers valuable insights into the design of future photocatalysts for hydrogen production.