Molybdenum-modified ZnIn₂S₄ with a bifunctional cocatalyst for efficient photocatalytic overall water splitting under simulated sunlight

Abstract

Photocatalytic overall water splitting (POWS) is a promising technique for sustainable hydrogen production that can potentially tackle the current energy and environmental challenges. As a visible-light-active photocatalyst, ZnIn₂S₄ is an ideal candidate for POWS; however, it exhibits low efficiency due to poor charge separation and improper surface reactions. Here, ZnIn₂S₄ was modified by doping Mo and depositing a bifunctional cocatalyst for efficient POWS under sunlight. Charge separation and photodeposition of Pt and CrO_x were accelerated by Mo introduction to promote surface POWS reactions and suppress reverse reactions. Under simulated sunlight irradiation, the modified ZnIn₂S₄ achieved POWS with an apparent quantum efficiency (AQE) of 0.17% at 420 ± 20 nm and a solar-to-hydrogen (STH) conversion efficiency of 0.016%. Theoretical calculations indicated that Mo dopants introduced spin-polarized states at both the conduction band bottom and the valence band top, which are spin-forbidden for charge recombination. This study served as a useful guideline for the design and development of efficient photocatalysts for sunlight-driven POWS.

---

1. Introduction

Developing hydrogen energy, particularly through sustainable routes, is essential to tackle the challenges of increasing energy needs and environmental degradation.¹ This is typically exemplified by photocatalytic overall water splitting (POWS) using solar energy where hydrogen and oxygen can be produced at the surface of a particulate photocatalyst. Previous studies on POWS have predominantly focused on wide-bandgap semiconductors whose light absorption is limited to UV-light.^2–6 The solar-to-hydrogen (STH) conversion efficiency thereby has intrinsic limitations (<5%) due to their inadequate utilization of solar energy. Developing narrow-bandgap semiconductors with visible light absorption is essential to achieve efficient POWS using solar energy.⁷

In this context, transition metal sulfides have garnered significant attention due to their simple structure and ability to absorb visible light photons.^8–11 Among them, ZnIn₂S₄ is widely studied because of its promising photoelectric properties and catalytic performance.^12–16 However, the rapid recombination of photogenerated electrons and holes in ZnIn₂S₄ poses a great challenge for POWS.^17,18 Moreover, the surface of ZnIn₂S₄ is usually deficient in active reaction sites for redox reactions and is subject to photocorrosion due to the vulnerability of the S-terminated surface during water splitting, which is another factor that inhibits POWS reactions.^19–21 Besides, the POWS activity hinges on the proper construction of both reduction and oxidation sites, which would otherwise induce reverse reactions for the product gases H₂ and O₂.^22,23 Rational cocatalyst deposition is indispensable for POWS not only for the rapid collection of electrons and holes but also for the inhibition of the reverse reaction as well as photocorrosion.^24–28

In this work, Mo has been uniformly doped into ZnIn₂S₄, i.e. ZnIn₂S₄:Mo, using a simple one-step hydrothermal method. Mo doping significantly enhances the capture efficiency of photogenerated holes, thereby facilitating charge separation. Meanwhile, Pt and CrO_x cocatalysts with a core–shell structure have been deposited onto ZnIn₂S₄:Mo, which substantially increases the surface charge transfer efficiency and inhibits reverse reactions. Under optimal conditions, the photocatalyst exhibits H₂- and O₂-evolution rates of 122.8 μmol g⁻¹ h⁻¹ and 65.9 μmol g⁻¹ h⁻¹ under simulated sunlight, respectively.

2. Experimental section

2.1. Chemicals

Zinc acetate dihydrate (C₄H₆O₄Zn·2H₂O, Aladdin, A.R.), indium(III) chloride tetrahydrate (InCl₃·4H₂O, Macklin, A.R.), thioacetamide (CH₃CSNH₂, Aladdin, A.R.), citric acid (C₆H₈O₇, Aladdin, A.R., ≥99.5%(T)), sodium molybdate dihydrate (Na₂MoO₄·2H₂O, Aladdin; A.R., 99.0%), chromium(III)nitrate nonahydrate (Cr(NO₃)₃·9H₂O, Aladdin, A.R., 99.0%), and chloroplatinic acid hexahydrate (H₂PtCl₆·6H₂O, Aladdin, A.R., Pt ≥ 37.5%) were used. All chemicals were used as received without further purification, and deionized water was employed throughout the entire experimental procedure.

2.2 Sample preparation

2.2.1. Synthesis of ZnIn₂S₄. ZnIn₂S₄ was prepared through hydrothermal reactions according to the literature.²⁹ Typically, 2 mmol of zinc acetate dihydrate, 4 mmol of indium(III) chloride tetrahydrate, 10 mmol of thioacetamide, and 1 g of citric acid were dissolved in 60 mL of deionized water with stirring for 30 min. The solution was transferred into a Teflon-lined autoclave (100 mL) and maintained at 180 °C for 24 h. After cooling to room temperature naturally, the obtained product was collected by centrifugation, washed with deionized water and absolute ethanol, and then dried at 80 °C for further characterization.

2.2.2. Synthesis of Mo doped ZnIn₂S₄ (ZnIn₂S₄:Mo). To prepare Mo-doped ZnIn₂S₄, sodium molybdate dihydrate was added to the reaction solution for hydrothermal synthesis at molar contents of 1%, 3%, and 5%. The reaction conditions were kept the same as those for the synthesis of ZnIn₂S₄. The products were denoted as ZnIn₂S₄:Mo(x) (x = 1, 3 and 5), where x represents the molar percent of Mo doped into ZnIn₂S₄.

2.2.3. Depositing cocatalysts onto ZnIn₂S₄:Mo (Pt/CrO_x@ZnIn₂S₄:Mo). The deposition of Pt and CrO_x onto ZnIn₂S₄:Mo was carried out via photo-reduction of H₂PtCl₆ and photooxidation of Cr(NO₃)₃ simultaneously over ZnIn₂S₄:Mo. Taking ZnIn₂S₄:Mo(3) as an example, 100 mg of ZnIn₂S₄:Mo was suspended in an aqueous solution containing appropriate amounts of H₂PtCl₆ and Cr(NO₃)₃. The resultant suspensions were irradiated with a 300 W Xe lamp for 1 h. The resultant products were filtered, washed with deionized water and absolute ethanol and dried in a vacuum oven at 60 °C. The resulting catalyst exhibited a deep yellow color and was designated as Pt/CrO_x@ZnIn₂S₄:Mo(3) (Scheme 1).

Scheme 1 The schematic representation of ZnIn₂S₄:Mo(3) nanospheres.

2.3. Catalyst characterization

The microstructure of the sample was examined by field emission scanning electron microscopy (FESEM, Hitachi SU8010, Japan) and transmission electron microscopy (TEM, JEOL JEM-F200, Japan). X-ray diffraction (XRD) patterns were recorded on an X-ray diffraction instrument (Bruker D8 Advanced, Germany) operating at 60 kV and 50 mA with Cu-Kα radiation. The surface state was investigated utilizing the X-ray photoelectron spectroscopy (XPS) technique with the Thermo Fisher Scientific ESCALAB 250Xi model. The nitrogen adsorption and desorption isotherms at 77 K were obtained using a nitrogen adsorption analyser (Micromeritics ASAP 2460, USA). Prior to measurement, all samples were outgassed under a vacuum at 120 °C for a minimum of 5 hours. UV-vis diffuse reflection spectra (DRS) were obtained using a UV-vis spectrophotometer (Shimadzu UV-3600 plus, Japan) equipped with an integrating sphere assembly. The photoluminescence (PL) spectra were recorded at room temperature with a fluorescence spectrophotometer (Edinburgh FLS1000) using 375 nm as the excitation wavelength. Decay curves were obtained using an FLS980 fluorescence spectrophotometer (Edinburgh Instruments, UK) under the excitation wavelength at 325 nm. The average lifetime (Ave. τ) was calculated according to τ = (A₁·τ₁² + A₂·τ₂²)/(A₁·τ₁ + A₂·τ₂) (τ_i is the lifetime; A_i is the relative intensity). A Bruker EMXplus-6/1 (Germany) was used for the characterization of electron paramagnetic resonance (EPR) with a 300 W Xenon lamp at room temperature. The steady-state surface photovoltage (SPV) spectra were collected using a CEL-SPS1000 spectroscopic analysis system containing a 500 W Xenon lamp (CEL-S500).

2.4. Electrochemical measurements

The experiment was conducted using a CHI760E electrochemical workstation. The experimental setup consisted of a three-electrode cell, with a calomel electrode as the reference electrode, platinum foil as the counter electrode, and a conductive glass electrode coated with a photocatalyst as the working electrode. The electrolyte used was a 0.5 mol L⁻¹ Na₂SO₄ solution. For the photoanode preparation, 24 mg of the photocatalyst, 3 mg of acetylene black, and 3 mg of polyvinylidene difluoride were mixed uniformly. Afterwards, an appropriate amount of N-methyl pyrrolidone was added through droplet-wise addition. Finally, the resultant suspensions were deposited onto conductive glass and dried at room temperature. Transient photocurrent response measurements were conducted using a 300 W xenon lamp coupled with an AM1.5G filter as the light source. Electrochemical impedance spectroscopy analysis was performed in the frequency range of 1 to 10⁶ Hz. Mott–Schottky measurements were carried out at a frequency of 2000 Hz.

2.5. Computational details

The theoretical calculations of the band structures for ZnIn₂S₄ and ZnIn₂S₄:Mo(3) were performed using density functional theory (DFT) with a commercial Vienna ab initio simulation package. The calculations were performed using the generalized gradient approximation (GGA), Perdew–Burke–Ernzerhof (PBE) function, and projector augmented-wave pseudopotential. A 2 × 2 × 2 cubic unit cell (a = b = c = 8.26 Å, α = β = γ = 90°) was used as the structural model. The structures were fully relaxed following the criteria that the forces on each atom are less than 0.02 eV Å⁻¹. An energy cutoff of 400 eV and a total energy of less than 10⁻⁵ eV were used for static electric potential calculations and geometry optimization, respectively. A Monkhorst–Pack k-points mesh of 5 × 5 × 5 was sampled for all structures.

2.6. Photocatalytic overall water splitting measurement

Photocatalytic tests were performed on a commercial photocatalytic analysis system (Labsolar-6A, Beijing Perfectlight) at ambient temperature. Typically, 50 mg of photocatalysts was dispersed in 100 mL of deionized water (pH = 7). Air in the reaction equipment and solution was vacuumed first. Subsequently, the reactor was exposed to a 300 W Xenon lamp source with an AM1.5G filter. Gas evolution was monitored by on-line gas chromatography (GC9790II, FULI). The stability test was performed under the same conditions over consecutive testing cycles (5 h). The reaction equipment and solution were vacuumed before each testing cycle. The apparent quantum efficiency (AQE) was evaluated using the same system under illumination with different bandpass filters. AQE was calculated according to the following equation:

To obtain the STH efficiency, 100 mg of photocatalyst and a 300 W xenon lamp with an AM1.5G filter (100 mW cm⁻²) were used.

3. Results and discussion

3.1. Microstructure characterization

The ZnIn₂S₄ and ZnIn₂S₄:Mo(x) samples were synthesized via a one-step hydrothermal method and were characterized by various analytical techniques. Taking ZnIn₂S₄:Mo(3) as an example, both pristine ZnIn₂S₄ (Fig. 1a and b) and ZnIn₂S₄:Mo(3) (Fig. 1c and d) exhibit hierarchical flower-like microstructures comprising nanosheets, being consistent with ZnIn₂S₄ samples synthesized via a hydrothermal method.³⁰ Mo doping, therefore, can preserve the flower-like microstructures of ZnIn₂S₄. To further elucidate the microstructural characteristics, the samples were inspected under HRTEM conditions. The ultra-thin two-dimensional nanosheets were similar before and after Mo doping, as depicted in the figure of pristine ZnIn₂S₄ (Fig. 1e) and ZnIn₂S₄:Mo(3) (Fig. 1g). In the magnified images, a lattice spacing of 0.32 nm is observed, corresponding to the (102) crystal plane of hexagonal ZnIn₂S₄. The elemental mapping of ZnIn₂S₄:Mo(3) (Fig. 1i) reveals a uniform dispersion of Zn, In, S, and Mo, providing evidence of the homogeneous doping of Mo in ZnIn₂S₄. The molar ratio of Zn/In/S in the ZnIn₂S₄:Mo(3) nanosheets is determined to be 1 : 1.66 : 4.00, as detailed in Table S1.† This ratio, in comparison with that of the pristine ZnIn₂S₄, indicates a significant reduction in In content, suggesting that Mo atoms likely substitute a fraction of the In atoms within the ZnIn₂S₄ lattice.

Fig. 1 Scanning electron microscopy images of (a and b) ZnIn₂S₄ and (c and d) ZnIn₂S₄:Mo(3); transmission electron microscopy and high-resolution transmission electron microscopy images of ZnIn₂S₄ (e and f) and ZnIn₂S₄:Mo(3) (g and h); (i–m) elemental mapping images of ZnIn₂S₄:Mo(3).

3.2. Structural and surface analysis

The crystal structures of pristine ZnIn₂S₄ and ZnIn₂S₄:Mo(x) (where x = 1, 3, 5 atom%) were characterized via X-ray diffraction (XRD). As depicted in Fig. 2a and Fig. S1,† pristine ZnIn₂S₄ and ZnIn₂S₄:Mo(x) showed XRD patterns that matched well with the hexagonal ZnIn₂S₄ standard pattern (JCPDS:00-065-2023). The characteristic reflections at 21.6°, 27.7°, 47.5°, 52.4°, and 55.6° correspond to the (006), (102), (110), (116), and (022) crystal planes of hexagonal ZnIn₂S₄, respectively. The light absorption capacity of Mo-modified samples was then assessed utilizing UV-vis absorption spectroscopy in Fig. 2b. The spectra indicate that the light absorption capacity of ZnIn₂S₄ has been significantly enhanced through Mo doping, as indicated by its increased absorption across the UV, visible, and near-infrared regions.

Fig. 2 (a) X-ray diffraction patterns; (b) UV-vis absorption spectra; (c) XPS spectra of In 3d and (d) S 2p of ZnIn₂S₄ and ZnIn₂S₄:Mo(3).

The elemental states and chemical composition of ZnIn₂S₄ before and after Mo doping were investigated by XPS. All core-level binding energies were adjusted with reference to the adventitious C 1s peaks at 284.7 eV. It is noteworthy that the peaks at 443.9 eV and 451.5 eV of ZnIn₂S₄:Mo(3) correspond to the In 3d_5/2 and In 3d_3/2 states, exhibiting a slight shift towards higher binding energy compared to ZnIn₂S₄ (Fig. 2c). This is probably due to a higher electronegativity of Mo compared to In that tends to attract electrons. Similar results were also observed in the S 2p state, where the peak positions underwent a blue shift of ∼0.2 eV (Fig. 2d). The increase can be attributable to the high electronegativity of Mo that results in the electron cloud shift from S towards Mo. The peaks of ZnIn₂S₄ and ZnIn₂S₄:Mo(3) at 1020.9 eV and 1043.9 eV, respectively, correspond to the Zn 2p_3/2 and Zn 2p_1/2 states (Fig. S2a†). The peaks observed at 228.1 eV and 231.4 eV in ZnIn₂S₄:Mo(3) correspond respectively to the Mo 3d_5/2 and Mo 3d_3/2 states, being indicative of Mo⁴⁺ (Fig. S2b†).³¹ Furthermore, Fig. S3a and b† present the N₂ adsorption–desorption isotherms that reveal hysteresis loops for both pristine ZnIn₂S₄ and ZnIn₂S₄:Mo(3) nanosheets, indicating mesoporous structures (type-IV isotherms). Using the Brunauer–Emmett–Teller (BET) method, we calculated their specific surface area and pore volume. The pristine ZnIn₂S₄ nanosheets have a BET specific surface area of 107.2 m² g⁻¹ and a pore volume of 0.22 cm³ g⁻¹. In contrast, the ZnIn₂S₄:Mo(3) nanosheets have a specific surface area of 117.8 m² g⁻¹ and a pore volume of 0.23 cm³ g⁻¹. These results suggest that Mo doping has only trivial impacts on the surface area and pore volume of ZnIn₂S₄.

3.3. Photocatalytic performance characterization

The photocatalytic performance of these samples was assessed based on their activity for overall water splitting under simulated AM1.5G conditions. No H₂ and O₂ evolution was detected when using naked ZnIn₂S₄ and ZnIn₂S₄:Mo(3), implying that the surface of ZnIn₂S₄ is unsuitable for POWS reactions. To facilitate water redox reactions, Pt and CrO_x cocatalysts have been deposited onto these samples, which are critical to achieve POWS activity.³² The successful deposition of these cocatalysts is confirmed by XPS and TEM analysis (Fig. 4a and b). The Pt 4f spectra are characterized by four overlapping peaks, being attributable to the Pt⁰/Pt²⁺ valence state. The surface of ZnIn₂S₄ is typically terminated with S anions, which can strongly interact with the metallic Pt cocatalysts by forming Pt–S bonds according to the literature.³³ Similarly, the Cr 3d spectra exhibit two peaks with binding energies of 586.2 eV and 576.1 eV, corresponding to the 2p_3/2 and 2p_1/2 orbitals of Cr³⁺ from CrO_x.³⁴ The cocatalysts were further inspected by using TEM, HRTEM, and elemental mapping (Fig. 5). The Pt cocatalyst was characterized as small granules with a diameter of approximately 5 nm, while CrO_x appears as an amorphous layer covering the Pt particles. The metallic Pt can be verified by its typical crystal plane spacing of 0.23 nm, corresponding to the Pt (111) crystal plane.³⁵ Elemental mapping analysis confirms that the deposition of Pt and CrO_x is uniform on the surface of ZnIn₂S₄:Mo(3) (Fig. S5c†).

Fig. 3 (a) Comparison of photocatalytic H₂ and O₂ evolution rates for ZnIn₂S₄, Pt/CrO_x@ZnIn₂S₄, and Pt/CrO_x@ZnIn₂S₄:Mo(3); (b) cycling tests of the POWS activity of Pt/CrO_x@ZnIn₂S₄:Mo(3); (c) comparison of photocatalytic H₂ and O₂ evolution rates for Pt/CrO_x@ZnIn₂S₄:Mo(x) (x = 0, 1, 2, 3); (d) UV-vis light absorption and corresponding wavelength-dependent AQE of Pt/CrO_x@ZnIn₂S₄:Mo(3).

Fig. 4 (a and b) XPS spectra of Pt 4f and Cr 2p for Pt@CrO_x/ZnIn₂S₄:Mo(3); (c) comparison of photocatalytic H₂ and O₂ evolution rates for Pt@ZnIn₂S₄:Mo, CrO_x@ZnIn₂S₄:Mo(3) and Pt/CrO_x@ZnIn₂S₄:Mo(3) nanosheets; (d) reverse reaction of Pt/CrO_x@ZnIn₂S₄:Mo(3) and ZnIn₂S₄:Mo(3).

Fig. 5 (a and b) Transmission electron microscopy and elemental mapping images of Pt/CrO_x@ZnIn₂S₄:Mo(3); (c) Transmission electron microscopy and elemental mapping images of Pt/CrO_x@ZnIn₂S₄:Mo(3).

By depositing the Pt and CrO_x cocatalysts, both pristine ZnIn₂S₄ and ZnIn₂S₄:Mo(3) exhibit POWS activity under simulated sunlight irradiation. The H₂ to O₂ ratio approaches approximately 2 to 1, suggesting successful overall water splitting for these samples.

As illustrated in Fig. 3a, compared with pristine Pt/CrO_x@ZnIn₂S₄, a significant enhancement in H₂ and O₂ evolution rates is observed for Pt/CrO_x@ZnIn₂S₄:Mo(3), highlighting the importance of Mo doping. The doping content was also optimized to further enhance the POWS activity. As shown in Fig. S4,† doping 3% Mo delivers the highest performance, with H₂ and O₂ evolution rates of 122.8 μmol g⁻¹ h⁻¹ and 65.9 μmol g⁻¹ h⁻¹, respectively. Further increasing the Mo content may lead to the formation of an impurity phase like MoS₂, which serves to block light penetration and impairs the POWS activity (Fig. S5†). ZnIn₂S₄ doped with 3% Mo was then used for further stability testing. Fig. 3b demonstrates that Pt/CrO_x@ZnIn₂S₄:Mo(3) can stably produce H₂ and O₂ for four consecutive cycles, indicating good stability for POWS reactions. The importance of depositing both Pt and CrO_x can be realized by several control experiments. First, the Pt content was varied from 0% to 3%. The activity of Pt/CrO_x@ZnIn₂S₄:Mo(3) clearly shows an optimal point at 2% (Fig. 3c), suggesting that the presence of Pt modulates the photocatalytic activity. The role of CrO_x can be elucidated through the control experiments with Pt and CrO_x alone. As shown in Fig. 4c, depositing Pt alone on ZnIn₂S₄:Mo(3) substantially inhibits O₂ evolution. This is probably due to the fact that Pt is effective in triggering reverse reactions between H₂ and O₂ or the oxygen reduction reaction (ORR). This is confirmed by the decrease in H₂ and O₂ levels in the presence of Pt loaded ZnIn₂S₄:Mo(3) (Pt@ZnIn₂S₄:Mo(3), Fig. 4d). This decrease is not observable when Pt is coated with CrO_x. Therefore, CrO_x has major contributions to the inhibition of reverse reactions. In addition, depositing CrO_x alone (CrO_x@ZnIn₂S₄:Mo(3)) shows a clear degradation of POWS activity, although the evolution of both H₂ and O₂ with the correct ratio is observed. This result indicates that Pt is more effective in promoting POWS reactions than CrO_x. Combining these results, it is feasible to rationalize the roles of Pt and CrO_x during POWS reactions. Upon the generation of photo-generated charges in ZnIn₂S₄:Mo(3) by sunlight, Pt efficiently collects charges for surface redox reactions. The product gases H₂ and O₂ are isolated from Pt due to the presence of CrO_x which effectively inhibits reverse reactions and/or the ORR. Therefore, there is a collaborative effect between Pt and CrO_x for improving the POWS activity.³⁶ The POWS activity was further evaluated by collecting the action spectra, as shown in Fig. 3d. It is clear that the photocatalytic activity is photon-driven as the apparent quantum efficiency (AQE) of Pt/CrO_x@ZnIn₂S₄:Mo(3) matches well with the light absorption spectra. The AQE at 420 ± 20 nm reaches 0.17% and the STH is determined to be 0.016%. To the best of our knowledge, these results are competitive with those of ZnIn₂S₄-based photocatalysts reported for POWS reactions (Table S2†).

3.4. Photoelectrochemical analysis

To clarify the improved POWS activity after Mo doping, we performed photoelectrochemical measurements on ZnIn₂S₄, ZnIn₂S₄:Mo(3) and Pt/CrO_x@ZnIn₂S₄:Mo(3). Electrochemical impedance spectroscopy (EIS) spectra are presented in Fig. 6a. The EIS results demonstrate that compared with pristine ZnIn₂S₄, ZnIn₂S₄:Mo (3) exhibits a lower charge transfer resistance (R_ct), while a further reduced R_ct is observed after cocatalyst deposition, indicating that Mo doping and cocatalyst deposition synergistically optimize the water redox reaction conditions. Furthermore, under chopped light irradiation, ZnIn₂S₄:Mo(3) exhibits a photocurrent density significantly higher than that of ZnIn₂S₄ yet slightly lower than that of Pt/CrO_x@ZnIn₂S₄, which aligns well with the photocatalytic performance. These results validate that Mo doping and cocatalyst deposition synergistically accelerate charge separation in ZnIn₂S₄ (Fig. 6b). This deduction was further confirmed by surface photovoltage (SPV) testing that shows a significant increase in surface photovoltage in ZnIn₂S₄:Mo(3) and Pt/CrO_x@ZnIn₂S₄:Mo(3) compared to ZnIn₂S₄ (Fig. 6c). All these results jointly confirm that the synergistic interaction between Mo doping and the cocatalyst contributes to enhanced charge separation, with the separated charges subsequently migrating to the catalyst surface to drive water redox reactions.

Fig. 6 (a) Electrochemical impedance spectroscopy spectra of ZnIn₂S₄, ZnIn₂S₄:Mo(3) and Pt/CrO_x@ZnIn₂S₄:Mo(3); (b) photocurrent measurements of ZnIn₂S₄, ZnIn₂S₄:Mo(3) and Pt/CrO_x@ZnIn₂S₄:Mo(3); (c) surface photovoltage of ZnIn₂S₄, ZnIn₂S₄:Mo(3) and Pt/CrO_x@ZnIn₂S₄:Mo(3); (d) photoluminescence spectra of ZnIn₂S₄, ZnIn₂S₄:Mo(3) and Pt/CrO_x@ZnIn₂S₄:Mo(3); (e) time-resolved fluorescence spectra of ZnIn₂S₄, ZnIn₂S₄:Mo(3) and Pt/CrO_x@ZnIn₂S₄:Mo(3); amplitude-weighted average lifetime is shown in the inset.

Photoluminescence (PL) is commonly employed to assess the ability of separating and recombining electrons and holes. Fig. 6d shows the photoluminescence spectra of pristine ZnIn₂S₄, ZnIn₂S₄:Mo(3), and Pt/CrO_x@ZnIn₂S₄:Mo(3). An emission peak attributable to the band gap transition of ZnIn₂S₄ nanosheets is evident at 500 nm. The fluorescence intensity of the ZnIn₂S₄:Mo(3) nanosheets is significantly reduced compared to that of pristine ZnIn₂S₄. This observation confirms the enhanced separation efficiency of photogenerated electrons and holes by Mo doping. Moreover, Pt/CrO_x@ZnIn₂S₄ shows further reduced PL intensity relative to ZnIn₂S₄:Mo(3), which highlights that cocatalyst deposition improves photocatalytic activity through additional charge separation promotion. The separation dynamics of photoinduced carriers were studied using time-resolved transient photoluminescence (TRPL) decay spectra as shown in Fig. 6e. A two-exponential model was used to fit the TRPL profiles (Table S3†). The prolonged average lifetime (τ) of ZnIn₂S₄:Mo(3) compared to pristine ZnIn₂S₄ supports more effective electron–hole pair separation in the doped material, while the cocatalyst deposition further enhances charge separation efficiency through optimized carrier migration pathways.

3.5. Theoretical calculations

The impact of Mo doping on the electronic structures of ZnIn₂S₄ was investigated using density functional theory (DFT). As depicted in Fig. 7, the band structures, density of states (DOS), and projected density of states (PDOS) of pristine ZnIn₂S₄ and ZnIn₂S₄:Mo(3) were calculated based on DFT. Both ZnIn₂S₄ and ZnIn₂S₄:Mo(3) are inherently semiconductors, with calculated bandgaps of 1.136 and 0.671 eV. The band gap values derived from the generalized gradient approximation (GGA) method are known to be underestimated when compared to experimental measurements.^37,38 However, the calculated results can be used for qualitative analysis.³⁹ It can be seen from the band structure that Mo doping slightly reduces the bandgap of ZnIn₂S₄. The bandgap reduction arises from the fact that Mo doping introduces spin-polarized states both at the bottom of the conduction band and at the top of the valence band. These states are mainly contributed by the Mo 4d orbitals that are also strongly hybridized with In 5s and S 3p orbitals according to the PDOS. The behaviour of photo-generated charges, in either the conduction or the valence band, is modulated by these Mo-dominated states that are probably the origin of improved charge separation. For instance, the photo-generated electrons and holes could have opposite spins, making their recombination spin-forbidden according to the selection rules, resulting in ameliorated charge separation.

Fig. 7 Band structures, density of states (DOS) and projected density of states (PDOS) for (a) ZnIn₂S₄:Mo(3) and (b) ZnIn₂S₄. The Fermi level is marked by the dotted lines.

3.6. Band structure analysis

For better understanding the POWS process, the band edge positions of ZnIn₂S₄ and ZnIn₂S₄:Mo(3) were determined by Mott–Schottky (MS) analysis in conjunction with X-ray photoelectron spectroscopy (XPS) valence band scans. The flat-band potential (V_fb) was calculated based on the Mott–Schottky plot (Fig. 8a) using the following formula:
where C and A are the capacitance at the interface and electrode area, respectively, V is the bias applied, k_B is Boltzmann's constant, N_D is the donor concentration, T is the absolute temperature, ε is the dielectric constant, and e is the electric charge. It can be seen from the MS plot that both samples exhibit positive slopes, indicative of n-type semiconductivity. By examining the intercepts of the MS curves with the energy axis, the flat band potentials of ZnIn₂S₄ and ZnIn₂S₄:Mo(3) are determined to be 0.50 and 0.36 V vs. RHE, respectively.^40,41 As illustrated in Fig. 8b, the onsets of the valence band of ZnIn₂S₄ and ZnIn₂S₄:Mo(3) are found to be 1.51 eV and 1.36 eV, respectively. Utilizing the Kubelka–Munk method, the bandgap (E_g) energies were derived from the UV-Vis diffuse reflectance spectra, as depicted in Fig. 8c. The calculated bandgap for ZnIn₂S₄:Mo(3) is determined to be approximately 2.40 eV, which is lower than that of ZnIn₂S₄ (2.50 eV), being consistent with theoretical calculations. Combining these results, the band edge positions of these two compounds can be deduced, as illustrated in Fig. 8d. ZnIn₂S₄:Mo(3) exhibits a slightly reduced band gap and negatively shifted band edge positions compared to ZnIn₂S₄ (Fig. 8).

Fig. 8 (a) Mott–Schottky plots of ZnIn₂S₄ and ZnIn₂S₄:Mo(3); (b) XPS VB spectra of ZnIn₂S₄ and ZnIn₂S₄:Mo(3); (c) Tauc plot of ZnIn₂S₄ and ZnIn₂S₄:Mo(3); (d) band structures of ZnIn₂S₄ and ZnIn₂S₄:Mo(3).

3.7. Photocatalytic mechanism analysis

Based on the above findings and analysis, the mechanism of POWS of Pt/CrO_x@ZnIn₂S₄:Mo(3) under simulated sunlight is illustrated in Scheme 2. Mo doping can effectively modulate the electronic structure of ZnIn₂S₄, facilitating charge separation. This improved charge separation is responsible for the high photocatalytic activity. Meanwhile, photodeposition of co-catalysts (Pt and CrO_x) plays a vital role during POWS reactions. Pt is mainly responsible for the collection of photo-generated charges and promotes water redox reactions. CrO_x serves as a functional layer that prevents H₂ and O₂ reverse reactions and the ORR. The synergistic effects between Mo doping, Pt and CrO_x facilitate a smooth POWS reaction at the surface of ZnIn₂S₄, with stoichiometric H₂- and O₂- evolution, under simulated sunlight.

Scheme 2 Mechanism for POWS in the Pt/CrO_x@ZnIn₂S₄:Mo(3) system under simulated sunlight irradiation.

4. Conclusions

In this work, Mo is doped into ZnIn₂S₄ using a simple one-step hydrothermal method, followed by photo-deposition of Pt and CrO_x cocatalysts for POWS reactions. The Mo doping introduces spin-polarized states at both the conduction band bottom and the valence band top, which are spin-forbidden for charge recombination and beneficial for charge separation. Depositing Pt facilitates charge collection and surface redox reactions. CrO_x serves as a functional layer covering Pt, which is critical for inhibiting water-splitting reverse reactions and the ORR. Under optimal conditions, the photocatalytic systems achieve average H₂- and O₂- evolution rates of 122.8 μmol g⁻¹ h⁻¹ and 65.9 μmol g⁻¹ h⁻¹, respectively, under simulated sunlight irradiation. An AQE of 0.17% at 420 ± 20 nm and an STH of 0.016% have been obtained. This study proposes a novel strategy for POWS through the synergistic effect of element doping and co-catalyst deposition, offering a promising guideline for the development of photocatalysts for efficient solar-driven water-splitting reactions.

Author contributions

Yutong Chen: investigation, conceptualization, data curation, methodology, formal analysis, and writing – original draft. Rong Wu: supervision and writing – review & editing. Guoan Lin: theoretical calculations. Jianyong Yue: investigation, methodology, and validation. Dezheng Kong: investigation, methodology, and validation. Chen Zhang: investigation, methodology, and validation. Shunhang Wei: supervision and writing – review & editing. Xiaoxiang Xu: supervision and writing – review & editing. All authors have given approval to the final version of the manuscript.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22066024, 52302291 and 52172225), the Special Project for Key Research and Development Program of Xinjiang Autonomous Region (2022B01033); the Key Projects of the Natural Science Program in Xinjiang Autonomous Region (2023D01D03).

References

1. S. Chen, T. Takata and K. Domen, Particulate photocatalysts for overall water splitting, Nat. Rev. Mater., 2017, 2, 17050.
2. W. Lei, Y. Wang, H. Wang, N. Suzuki, C. Terashima and A. Fujishima, Gelation-induced controlled synthesis of TiO₂ with tunable phase transition for efficient photocatalytic hydrogen evolution, Inorg. Chem. Front., 2024, 11, 2178–2186.
3. C. Li, H. Che, P. Huo, Y. Yan, C. Liu and H. Dong, Confinement of ultrasmall CoFe₂O₄ nanoparticles in hierarchical ZnIn₂S₄ microspheres with enhanced interfacial charge separation for photocatalytic H₂ evolution, J. Colloid Interface Sci., 2021, 581, 764–773.
4. X.-J. Lu, I. Ullah, J.-H. Li, S. Chen, C.-Z. Yuan and A.-W. Xu, A bimetallic CoZn metal–organic-framework derived CoZnS@NSC Co-catalyst loaded on g-C₃N₄ for significantly augmented photocatalytic H₂ evolution, Inorg. Chem. Front., 2024, 11, 3435–3445.
5. F. Xing, J. Li, C. Wang, S. Jin, H. Jin and J. Li, Efficient photocatalytic hydrogen evolution of g-C₃N₄/Vs-SnS₂/CdS through a sulfur vacancy-rich SnS₂ induced charge storage effect, Inorg. Chem. Front., 2024, 11, 2884–2893.
6. S. Zhang, X. Liu, C. Liu, S. Luo, L. Wang, T. Cai, Y. Zeng, J. Yuan, W. Dong, Y. Pei and Y. Liu, MoS₂ Quantum Dot Growth Induced by S Vacancies in a ZnIn₂S₄ Monolayer: Atomic-Level Heterostructure for Photocatalytic Hydrogen Production, ACS Nano, 2018, 12, 751–758.
7. X. Liu, L. Yan, W. Li, K. Chen, F. Wang, J. Xiao, T. Hisatomi, T. Takata and K. Domen, Enhancing the Photocatalytic Activity of CaTaO₂N for Overall Water Splitting through Surface Nitride Ion Enrichment, ACS Catal., 2024, 14, 10561–10567.
8. P. Kulkarni, S. K. Nataraj, R. G. Balakrishna, D. H. Nagaraju and M. V. Reddy, Nanostructured binary and ternary metal sulfides: synthesis methods and their application in energy conversion and storage devices, J. Mater. Chem. A, 2017, 5, 22040–22094.
9. Y. Li, H. Dong, L. Li, L. Tang, R. Tian, R. Li, J. Chen, Q. Xie, Z. Jin, J. Xiao, S. Xiao and G. Zeng, Recent advances in waste water treatment through transition metal sulfides-based advanced oxidation processes, Water Res., 2021, 192, 116850.
10. R. Kalia, B. M. Pirzada, R. K. Kunchala and B. S. Naidu, Noble metal free efficient photocatalytic hydrogen generation by TaON/CdS semiconductor nanocomposites under natural sunlight, Int. J. Hydrogen Energy, 2023, 48, 16246–16258.
11. R. Kalia, P. Yadav, A. Verma, J. Yadav and B. S. Naidu, Natural sunlight induced photocatalytic hydrogen evolution by SrTaO₂N/CdS nanocomposites, J. Alloys Compd., 2025, 1015, 178790.
12. S. Shen, J. Chen, X. Wang, L. Zhao and L. Guo, Microwave-assisted hydrothermal synthesis of transition-metal doped ZnIn₂S₄ and its photocatalytic activity for hydrogen evolution under visible light, J. Power Sources, 2011, 196, 10112–10119.
13. R. Yang, L. Mei, Y. Fan, Q. Zhang, R. Zhu, R. Amal, Z. Yin and Z. Zeng, ZnIn₂S₄-Based Photocatalysts for Energy and Environmental Applications, Small Methods, 2021, 5, 2100887.
14. T. Zhang, T. Wang, F. Meng, M. Yang and S. Kawi, Recent advances in ZnIn₂S₄-based materials towards photocatalytic purification, solar fuel production and organic transformations, J. Mater. Chem. C, 2022, 10, 5400–5424.
15. X. Zheng, Y. Song, Y. Liu, Y. Yang, D. Wu, Y. Yang, S. Feng, J. Li, W. Liu, Y. Shen and X. Tian, ZnIn₂S₄-based photocatalysts for photocatalytic hydrogen evolution via water splitting, Coord. Chem. Rev., 2023, 475, 214898.
16. P. Yadav and B. S. Naidu, in Green Hydrogen Economy for Environmental Sustainability. Volume 1: Fundamentals and Feedstocks, American Chemical Society, 2024, ch. 6, vol. 1473, pp. 119–146.
17. W. Guan, L. Zhang, P. Wang, Y. Wang, H. Wang, X. Dong, M. Meng, L. Sui, Z. Gan, L. Dong and L. Yu, Highly Efficient Photocatalytic Hydrogen Evolution over Mo-Doped ZnIn₂S₄ with Sulfur Vacancies, Nanomaterials, 2022, 12, 3980.
18. H. Su, C. Rao, L. Zhou, Y. Pang, H. Lou, D. Yang and X. Qiu, Mo-Doped/Ni-supported ZnIn₂S₄-wrapped NiMoO₄ S-scheme heterojunction photocatalytic reforming of lignin into hydrogen, Green Chem., 2022, 24, 2027–2035.
19. F. Huang, Z. Li, Y. Xu, A. Yan, T. Zhang, Q. Wang, S. Li, S. Lu, W. Zhao, Y. Gao and J. Zhang, Excellent anti-photocorrosion and hydrogen evolution activity of ZnIn₂S₄-based photocatalysts: In situ design of photogenerated charge dynamics, Chem. Eng. J., 2023, 473, 145430.
20. H. Su, H. Lou, Z. Zhao, L. Zhou, Y. Pang, H. Xie, C. Rao, D. Yang and X. Qiu, In situ Mo doped ZnIn₂S₄ wrapped MoO₃ S-scheme heterojunction via Mo-S bonds to enhance photocatalytic HER, Chem. Eng. J., 2022, 430, 132770.
21. F. Xing, Q. Liu and C. Huang, Mo-Doped ZnIn₂S₄ Flower-Like Hollow Microspheres for Improved Visible Light-Driven Hydrogen Evolution, Sol. RRL, 2020, 4, 1900483.
22. A. S. Alotabi, T. D. Small, Y. Yin, D. J. Osborn, S. Ozaki, Y. Kataoka, Y. Negishi, K. Domen, G. F. Metha and G. G. Andersson, Reduction and Diffusion of Cr-Oxide Layers into P25, BaLa₄Ti₄O₁₅, and Al:SrTiO₃ Particles upon High-Temperature Annealing, ACS Appl. Mater. Interfaces, 2023, 15, 14990–15003.
23. R. Li, T. Takata, B. Zhang, C. Feng, Q. Wu, C. Cui, Z. Zhang, K. Domen and Y. Li, Criteria for Efficient Photocatalytic Water Splitting Revealed by Studying Carrier Dynamics in a Model Al-doped SrTiO₃ Photocatalyst, Angew. Chem., Int. Ed., 2023, 62, e202313537.
24. K. Chen, J. Xiao, J. J. M. Vequizo, T. Hisatomi, Y. Ma, M. Nakabayashi, T. Takata, A. Yamakata, N. Shibata and K. Domen, Overall Water Splitting by a SrTaO₂N-Based Photocatalyst Decorated with an Ir-Promoted Ru-Based Cocatalyst, J. Am. Chem. Soc., 2023, 145, 3839–3843.
25. L. Lin, P. Kaewdee, V. Nandal, R. Shoji, H. Matsuzaki, K. Seki, M. Nakabayashi, N. Shibata, X. Tao, X. Liang, Y. Ma, T. Hisatomi, T. Takata and K. Domen, Flux-Assisted Synthesis of Y₂Ti₂O₅S₂ for Photocatalytic Hydrogen and Oxygen Evolution Reactions, Angew. Chem., Int. Ed., 2023, 62, e202310607.
26. Y. Qi, B. Zhang, G. Zhang, Z. Zheng, T. Xie, S. Chen, G. Ma, C. Li, K. Domen and F. Zhang, Efficient overall water splitting of a suspended photocatalyst boosted by metal-support interaction, Joule, 2024, 8, 193–203.
27. J. Xiao, M. Nakabayashi, T. Hisatomi, J. J. M. Vequizo, W. Li, K. Chen, X. Tao, A. Yamakata, N. Shibata, T. Takata, Y. Inoue and K. Domen, Sub-50 nm perovskite-type tantalum-based oxynitride single crystals with enhanced photoactivity for water splitting, Nat. Commun., 2023, 14, 8030.
28. W. Li, W. Duan, G. Liao, F. Gao, Y. Wang, R. Cui, J. Zhao and C. Wang, 0.68% of solar-to-hydrogen efficiency and high photostability of organic-inorganic membrane catalyst, Nat. Commun., 2024, 15, 6763.
29. H. Jing, G. Xu, B. Yao, J. Ren, Y. Wang, Z. Fang, Q. Liang, R. Wu and S. Wei, Sulfur Vacancy-Enriched Rhombohedral ZnIn₂S₄ Nanosheets for Highly Efficient Photocatalytic Overall Water Splitting under Visible Light Irradiation, ACS Appl. Energy Mater., 2022, 5, 10187–10195.
30. H. Jing, J. Ren, J. Yue, S. Liu, Q. Liang, R. Wu, Y. Wang, Z. Fang, H. Li and S. Wei, ZnIn₂S₄ with oxygen atom doping and surface sulfur vacancies for overall water splitting under visible light irradiation, Catal. Sci. Technol., 2023, 13, 226–232.
31. Y. Liu, Y. Li, F. Peng, Y. Lin, S. Yang, S. Zhang, H. Wang, Y. Cao and H. Yu, 2H- and 1T- mixed phase few-layer MoS₂ as a superior to Pt co-catalyst coated on TiO₂ nanorod arrays for photocatalytic hydrogen evolution, Appl. Catal., B, 2019, 241, 236–245.
32. T. Ma, W. Li, J. Li, W. Duan, F. Gao, G. Liao, J. Li and C. Wang, Multisite Cocatalysis: Single atomic Pt²⁺/Pt⁰ active sites synergistically improve the simulated sunlight driven H₂O-to-H₂ conversion performance of Sb2S3 nanorods, J. Colloid Interface Sci., 2024, 658, 476–486.
33. R. Wang, L. Shi, E. Y. Konysheva and X. Xu, Selective Photocatalysis of Benzyl Alcohol Valorization by Cocatalyst Engineering Over Zn₂In₂S₅ Nanosheets, Adv. Funct. Mater., 2024, 2418074.
34. T. Higashi, K. Seki, Y. Sasaki, Y. Pihosh, V. Nandal, M. Nakabayashi, N. Shibata and K. Domen, Mechanistic Insights into Enhanced Hydrogen Evolution of CrO/Rh Nanoparticles for Photocatalytic Water Splitting, Chem. – Eur. J., 2023, 29, e202204058.
35. Q. Li, C. Cui, H. Meng and J. Yu, Visible-Light Photocatalytic Hydrogen Production Activity of ZnIn₂S₄ Microspheres Using Carbon Quantum Dots and Platinum as Dual Co-catalysts, Chem. – Asian J., 2014, 9, 1766–1770.
36. T. Takata, J. Jiang, Y. Sakata, M. Nakabayashi, N. Shibata, V. Nandal, K. Seki, T. Hisatomi and K. Domen, Photocatalytic water splitting with a quantum efficiency of almost unity, Nature, 2020, 581, 411–414.
37. B. Neupane, H. Tang, N. K. Nepal, S. Adhikari and A. Ruzsinszky, Opening band gaps of low-dimensional materials at the meta-GGA level of density functional approximations, Phys. Rev. Mater., 2021, 5, 063803.
38. Y.-N. Zhang, M. Zhang, P. Zhang and W.-B. Hu, Investigation of electronic structure and optoelectronic properties of Si-doped β-Ga₂O₃ using GGA + U method based on first-principle, Acta Phys. Sin., 2024, 73, 017102-017101–017102-017110.
39. H. Xiao, J. Tahir-Kheli and W. A. Goddard III, Accurate Band Gaps for Semiconductors from Density Functional Theory, J. Phys. Chem. Lett., 2011, 2, 212–217.
40. G. Lin and X. Xu, Ba-Modified LaTiO₂N as an Efficient Visible Light Active Photocatalyst for Water Oxidation, ACS Sustainable Chem. Eng., 2020, 8, 9641–9649.
41. Y. Wang, S. Jin, G. Pan, Z. Li, L. Chen, G. Liu and X. Xu, Zr doped mesoporous LaTaON₂ for efficient photocatalytic water splitting, J. Mater. Chem. A, 2019, 7, 5702–5711.

---

Footnote

† Electronic supplementary information (ESI) available: XRD, XPS, BET, TEM-EDX, photocatalytic water splitting performance, element content, comparisons of photocatalytic performance and fitting data of TRPL. See DOI: https://doi.org/10.1039/d5dt00316d

---