Improvement of Photocatalytic H₂-Generation under Visible Light Irradiation by Controlling the Band Gap of ZnIn₂S₄ with Cu and In

Abstract

The band gap controlled photocatalyst (Zn_0.74Cu_0.13In₂S_3.805) was prepared via a simple one-step solvothermal method. The effects of doping of Cu⁺ and excess In on the photocatalytic activity of ZnIn₂S₄ photocatalyst were investigated. In addition, optical properties, surface morphology and crystal structure were evaluated. The maximum H₂ evolution rate (2370 µmol h⁻¹ g⁻¹) was achieved with Zn_0.74Cu_0.13In₂S_3.805, which was about five times higher than that of untreated ZnIn₂S₄ under visible light (λ ≥ 420 nm). The band gap of Zn_0.74Cu_0.13In₂S_3.805 decreased to 1.98 eV by raising the maximum position of the valence band, compared to ZnIn₂S₄. Furthermore, the recombination of electron hole pairs was effectively reduced. This research contributes to the development of highly active photocatalysts under visible light.

1. Introduction

Hydrogen is one of the more demanded synthetic energy carriers. Until now, hydrogen can be produced chemically, thermochemically, biologically, biochemically, biophotochemically, etc. [1,2,3,4]. Among these techniques, hydrogen generation by water splitting using a photocatalyst has been expected as a clean and sustainable energy technology, because it can directly convert solar energy into chemical energy by using only water as a raw material [5,6,7]. It is important to develop highly efficient photocatalysts to replace the current hydrogen generation technology with the photocatalytic water splitting process. Sunlight contains ultraviolet light, visible light and infrared light, and it is ideal to use all the wavelengths when using sunlight. It is known that the wavelength ranges of light, in which the photocatalyst reacts, is dependent on the size of the band gap of the photocatalyst [8,9,10]. Therefore, photocatalytic activity can be exhibited even under longer wavelength light [11,12,13]. The addition of foreign elements, which is one of the ways to change photocatalytic ability, greatly affects the characteristics of the catalyst. Specific requirements to improve the activity of the photocatalytic material generally include efficient light absorption, effective separation of photogenerated charge carriers, and better efficiency of the interface for direct release of hydrogen and/or oxygen from water [14,15,16]. In efficient light absorption, the size of the band gap formed by the conduction band (CB) and the valence band (VB) of the semiconductor photocatalyst is the most important issue [17,18,19]. With regard to the effective separation of photogenerated charge carriers, many elements such as defects in the crystal structure and band structure are involved in a complex manner [20,21,22].

Sulfide photocatalysts are advantageous for visible light driven photocatalysts, because they have narrow band gaps and negative valence bands due to the electron orbit of S, compared to oxide based photocatalysts [23,24,25]. ZnIn₂S₄ has been tested to have a suitable band gap corresponding to the visible-light absorption region, high photocatalytic activity and considerable chemical stability for photocatalytic H₂ evolution [26,27,28]. Yao et al. described the fabrication of Z-scheme PtS–ZnIn₂S₄/WO₃-MnO₂ for overall photo-catalytic water splitting [29]. It was reported by Zhao et al. that the combined effects of octahedron NH₂-UiO-66 and flowerlike ZnIn₂S₄ microspheres for photocatalytic hydrogen evolution [30]. However, the absorption wavelength of pure ZnIn₂S₄ is limited to about 500 nm. Cu species such as Cu⁺ and Cu²⁺ affected the valence band of ZnIn₂S₄ and formed an advantageous band structure for photocatalysts [31,32,33]. In In-Zn-S compounds, the [Zn²⁺]/[In³⁺] ratio changes the structural and optoelectronic properties, and greatly affects the composition of In–Zn–S [34]. Thus far, there are few reports that ZnIn₂S₄ is co-doped with Cu⁺ and excess indium. In this study, we investigated photocatalytic activity, optical properties and surface morphology of ZnIn₂S₄ simultaneously doped with Cu⁺ and excess indium.

2. Results and Discussion

2.1. Structural Characterization

The XRD patterns of ZnIn₂S₄, Zn_0.87Cu_0.13In₂S_3.935, Zn_0.87In₂S_3.87 and Zn_0.74Cu_0.13In₂S_3.805 are shown in Figure 1. The XRD pattern of ZnIn₂S₄ could be indexed as the hexagonal structure (JCPDS No. 65-2023). The three diffraction peaks at around 20.8°, 27.5°, 47.2° and 56.4° could be assigned to the (006), (101), (110) and (202) planes, respectively [35,36]. The XRD patterns of other Zn_0.87Cu_0.13In₂S_3.935, Zn_0.87In₂S_3.87 and Zn_0.74Cu_0.13In₂S_3.805 photocatalysts also showed similar results. These XRD patterns were in agreement with those reported in previous studies [35], which revealed that Zn_0.74Cu_0.13In₂S_3.805 is a hexagonal structure and contains almost no impurities of ZnS. The reason that the Cu and In derived peaks were not observed could be due to the very low doping amount. In addition, with respect to the peak of the (006) plane, a slight shift toward the high angle was observed when doping Cu and increasing In. This means that the interplanar spacing was reduced by doping, suggesting that Cu and excess In may be incorporated into the crystal structure of ZnIn₂S₄ and exist as a solid solution.

The results of the X-ray photoelectron spectroscopy (XPS) measurement for further structural analysis of photocatalysts are shown in Figure 2. The elemental ratios determined from the XPS spectrum are shown in Table S2. XPS survey spectra of Zn_0.87In₂S_3.87, Zn_0.87Cu_0.13In₂S_3.935 and Zn_0.74Cu_0.13In₂S_3.805 matched the material ZnIn₂S₄. This indicates that the impurities were not contained regardless of the doping Cu⁺ and excess In. Furthermore, from the result of elemental ratio analysis of XPS, it could be confirmed that the elemental ratio of the prepared catalysts were substantially in agreement with the theoretical ratio. Typical narrow spectra of Zn_0.74Cu_0.13In₂S_3.805 are shown in Figure 2a. In the XPS spectrum of Zn 2p, the peak of Zn 2p_3/2 (1020.4 eV) was observed. This peak was derived from the ZnIn₂S₄ component in Zn_0.74Cu_0.13In₂S_3.805. In the XPS spectrum of In 3d from ZnIn₂S₄ and In₂S₃, binding energies of 443.7 (In 3d_5/2) and 451.0 eV (In 3d_3/2) were observed. The peaks of S 2p were at 161.3 eV (S 2p_3/2) and 162.6 eV (S 2p_1/2) [37,38]. In addition, since the peak position of Cu 2p_3/2 is observed only at 932 eV and Cu 2p_3/2 satellite peak derived from Cu²⁺ was not present at 942 eV, it can be seen that Cu was doped in a monovalent state [39,40]. Both results of XRD and XPS show that the basic structure of Zn_0.74Cu_0.13In₂S_3.805 is hexagonal ZnIn₂S₄, doped with Cu⁺ and excess In.

2.2. Morphological Analysis

In order to investigate the influence of Cu and excess In doping on the characteristic surface morphology, SEM images of the prepared photocatalysts were observed. The results are shown in Figure 3. In pure ZnIn₂S₄, a microsphere structure formed by the overlapping of nanosheets was observed [41]. When Cu and excess In was doped, the spherical structure was destroyed. The nanosheet structure also collapsed and aggregated. The shapes of pure ZnIn₂S₄ and Zn_0.74Cu_0.13In₂S_3.805 were very different. Maybe, Cu⁺ and excess In formed a solid solution with ZnIn₂S₄.

2.3. Optical Analysis

Figure 4 shows the absorption wavelength region of light using the UV-visible diffuse reflectance spectrum. Furthermore, a Tauc plot calculated from the UV-vis diffuse reflectance spectra (DRS) spectrum is shown in Figure S1, in order to obtain a band gap. As shown in the results of DRS and Tauc plot, the absorption edge of pure ZnIn₂S₄ was 480 nm, and the size of the band gap was 2.67 eV. When [Zn²⁺]/[In³⁺] mole ratio was changed (Zn_0.87In₂S_3.87), the absorption tendencies were almost similar to ZnIn₂S₄. However, when Cu⁺ doping (Zn_0.87Cu_0.13In₂S_3.935) and excess In doping (Zn_0.74Cu_0.13In₂S_3.805) were performed, the absorption edges were extended to about 700 nm. Consequently, the band gap of ZnIn₂S₄ similarly decreased in the case of the doping Cu and excess In. So as to analyze the band structure, valence band edge measurement for the prepared photocatalysts was performed by XPS. It is clear from the results in Figure S2 that the doping of Cu⁺ shifted the valence band edge to the negative side. It has been reported that the doping of In³⁺ forms a sub-band on the positive side of the conduction band of ZnIn₂S₄ [36]. Therefore, it is reasonable that the doping of Cu⁺ and In into the photocatalyst reduce the band gap energy of ZnIn₂S₄.

We investigated the photoluminescence spectra in order to investigate the electron–hole pair separation efficiency, and the results are shown in Figure 5. Luminescence in ultraviolet and visible regions was observed in the photoluminescence spectrum. In general, ultraviolet emission is associated with exciton transition and recombination from the conduction band level to the valence band, and visible light emission is mainly associated with intrinsic or extrinsic defects in the catalyst. The photoluminescence spectra of ZnIn₂S₄ was approximately close to Zn_0.87In₂S_3.87. On the other hand, the spectra from Zn_0.87Cu_0.13In₂S_3.935 and Zn_0.74Cu_0.13In₂S_3.805 were lowered by doping of Cu⁺ and excess In. This may have been due to the decrease of the recombination rate between the photogenerated holes and the electrons photogenerated in the valence band. The photogenerated electrons may have been trapped in the oxygen vacancies generated in the photocatalyst by doping.

2.4. Photocatalytic Activity

Hydrogen generation was conducted using the photocatalyst Zn_0.74Cu_0.13In₂S_3.805. ZnIn₂S₄, Zn_0.87In₂S_3.87 and Zn_0.87Cu_0.13In₂S_3.935 were used as comparative objects. All catalysts were loaded with 1wt% Pt as a cocatalyst. The results are shown in Figure 6. The photocatalyst ZnIn₂S₄ mono-doped with excess In showed slightly higher hydrogen generation activity than that of pure ZnIn₂S₄. On the other hand, ZnIn₂S₄ doped only with Cu⁺ greatly improved the photocatalytic activity. Furthermore, the photocatalyst doped with Cu⁺ and excess In showed the highest hydrogen generation activity. The maximum H₂ evolution rate was 2370 μmol h⁻¹ g⁻¹, which showed about five times better results than that of untreated ZnIn₂S₄. The reproducibility of the H₂ production (relative standard deviation (RSD), for hydrogen amounts) was better than RSD 4% for three repeated measurements.

The TEM images before and after the hydrogen generation of Zn_0.74Cu_0.13In₂S_3.805 photocatalyst are shown in Figure S3. Only after hydrogen generation, a 3–4 nm spot was observed on the sample surface. The spot deposition was Pt, which was reduced during the hydrogen production.

2.5. Proposed Hydrogenation Mechanism

The reaction mechanism is shown in Scheme 1. Irradiation of light having a wavelength corresponding to the band gap energy of the Zn_0.74Cu_0.13In₂S_3.805 photocatalyst excites electrons in the valence band to the conduction band to produce an electron–hole pair (Formula (1)). From the results of DRS and photoluminescence (PL), it could be considered that Cu⁺ and excess In doping enhanced charge separation by forming an impurity level on the negative side of the valence band of ZnIn₂S₄ and the positive side of the conduction band, narrowing the band gap. In Formula (2), a part of electrons excited in the conduction band is consumed for photodeposition of Pt. The deposited Pt reduces H⁺ by using electrons transferred from the photocatalyst to generate H₂ (Formula (3)). On the other hand, sulfite ions and sulfide ions consume the holes for promoting a hydrogen generation reaction. The presence of Na₂S is very important in enhancing the photocatalytic activity, as Na₂S stabilizes the surface of the metal sulfide by suppressing the formation of surface defects as a scavenger of holes. However, when the concentration of Na₂S is high, the pH becomes high. High pH values are thermodynamically disadvantageous in the reaction represented by Formula (4). As described in the Formulas (5)–(7), SO₃²⁻ and S²⁻ consume holes. According to the Formula (6), S₂²⁻ ions are generated and act as an optical filter. If S₂²⁻ is not consumed, it interferes with light absorption. As shown in Formula (8), the reaction between S₂²⁻ and SO₃²⁻ forms S₂O₃²⁻, which is colorless and can hardly affect to light absorption.

Photocatalyst + hν → e⁻ + h⁺

(1)

Pt²⁺ + 2e⁻_CB → Pt

(2)

2H⁺ + 2e⁻_CB → H₂

(3)

2H₂O + 2e⁻_CB → H₂ + 2OH⁻

(4)

SO₃²⁻ + H₂O + 2h⁺_VB → SO₄²⁻ + 2H⁺

(5)

2S²⁻ + 2h⁺_VB → S₂²⁻

(6)

SO₃²⁻ + S²⁻ + 2h⁺_VB → S₂O₃²⁻

(7)

S₂²⁻ + SO₃²⁻ → S₂O₃²⁻ + S²⁻

(8)

3. Materials and Methods

3.1. Preparation of Photocatalysts

All chemicals were analytical grade and used as received without further purification. Zn_0.74Cu_0.13In₂S_3.805 was prepared by a simple hydrothermal method. Cetyltrimethylammonium bromide (CTAB, 3.76 mmol) (Wako Pure Chemical Industries, Ltd., Osaka, Japan), stoichiometric mole of ZnSO₄·7H₂O (Nacalai Tesque, Inc., Kyoto, Japan), InCl₃·4H₂O and CuCl (I) and an excess of thioacetamide (TAA) (Wako Pure Chemical Industries, Ltd., Osaka, Japan) were dissolved in 50 mL of distilled water. At the same time, in order to keep Cu monovalent, nitrogen was purged into the solution for 10 min to remove dissolved oxygen. The mixed solution was then transferred into a 100 mL Teflon autoclave. The autoclave was sealed, kept at 160 °C for 1 h and cooled to room temperature naturally. After cooling, the product was dried in a vacuum at 40 °C for 4 h and was ground for 30 min. ZnIn₂S₄ (not doped), Zn_0.87Cu_0.13In₂S_3.935 (Cu⁺ doped) and Zn_0.87In₂S_3.87 (change of Zn²⁺/In³⁺) were also prepared by the same method as reference materials. The prepared photocatalysts are shown in F1.

3.2. Characterization of Samples

X-ray powder diffraction (XRD) measurements were performed using a Rigaku RINT Ultima-IV diffractometer. It was carried out by using Cu radiation at a scan rate of 0.04°/s in a scan range of 10°–80°. X-ray photoelectron spectroscopy (XPS) measurements were carried out with a PHI Quantera SXM photoelectron spectrometer using Al Kα radiation. To compensate for surface charge effects, binding energies were calibrated using the C1s peak at 284 eV as the reference. Scanning electron microscope (SEM) observations were performed using a Hitachi S-4000 SEM. The transmission electron microscopy (TEM) images were taken on JEOL product JEM1011. The UV–vis diffuse reflectance spectra (DRS) of the photocatalysts were recorded using a Shimadzu UV-2450 spectrophotometer equipped with an integral sphere assembly, using BaSO₄ as a reflectance standard. Photoluminescence (PL) spectra were obtained at an excitation wavelength of 350 nm by using a Shimadzu RF-5300PC spectrofluorophotometer.

3.3. Photocatalytic Hydrogen Generation

A Pyrex column vessel reactor (inner volume: 123 mL) was used for the photocatalytic production of hydrogen from aqueous sulfide solution. In all experiments, 40 mL of solution containing 40 mg of catalyst, 10 mL of 0.04 ppm H₂PtCl₆ solution and 0.25 M Na₂SO₃/0.35 M Na₂S mixed sacrificial agent was added into the reaction cell. The light source was a 4000–4500 µW/cm² Xe-lamp (300 W), with a cut-off filter (λ ≥ 420 nm). Nitrogen was purged into the system for 30 min before the reaction to remove oxygen. The concentrations of H₂ were measured with an online gas chromatograph (GC). Injection, column and detector in GC were 50 °C. A thermal conductivity detector (TCD) was used as detector. The hydrogen generation experimental conditions are shown in

Table 1. Hydrogen generation experimental conditions.

Photocatalyst	ZnIn2S4, Zn0.87In2S3.87, Zn0.87Cu0.13In2S3.935, Zn0.74Cu0.13In2S3.805
Cocatalyst	0.04 ppm H2PtCl6 10 mL (1.0 wt%)
Medium	0.25 M Na2SO3 / 0.35 M Na2S 40 mL
Reactor	Pyrex glass vessel (volume: 123 mL)
Temperature	Room temperature (25 °C)
Light source	Xenon lamp (λ ≥ 420 nm, 4500 µW/cm2)
Irradiation time	6 h
Analysis	Gas chromatography (TCD)

.

4. Conclusions

The Zn_0.74Cu_0.13In₂S_3.805 photocatalyst, in which ZnIn₂S₄ was doped with Cu⁺ and excess In, was prepared by a simple one-pot solvothermal method. From the SEM, XRD and XPS results, it is highly possible that Zn_0.74Cu_0.13In₂S_3.805 is a solid solution with a hexagonal ZnIn₂S₄ as a basic structure. Control of the band gap and suppression of electron–hole recombination were confirmed by doping ZnIn₂S₄ with Cu⁺ and excess In. In addition, an increase in the absorption wavelength range and improved catalytic activity were observed. The hydrogen generation rate by Zn_0.74Cu_0.13In₂S_3.805 was 2370 μmol g^-1 h^-1, which was almost five times larger compared with that obtained with ZnIn₂S₄. The present work provides a strategy for water splitting systems consisting of sulfide materials with narrow band gaps for efficient hydrogen production.