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Article

Facile Synthesis of Ni3S2/ZnIn2S4 Photocatalysts for Benzyl Alcohol Splitting: A Pathway to Sustainable Hydrogen and Benzaldehyde

by
Haibo Wang
1,
Chen Zhou
2,* and
Shengyang Yang
1,*
1
Department of Chemistry and Chemical Engineering, Yangzhou University, 180 Siwangting Road, Yangzhou 225002, China
2
Department of Physical Sciences, University of Central Missouri, Warrensburg, MI 64093, USA
*
Authors to whom correspondence should be addressed.
Catalysts 2025, 15(9), 830; https://doi.org/10.3390/catal15090830
Submission received: 5 August 2025 / Revised: 25 August 2025 / Accepted: 26 August 2025 / Published: 1 September 2025
(This article belongs to the Special Issue Heterogeneous Photocatalysis for Cyclic Compound Synthesis or Functionalization, a Promising Approach for Discovery Chemistry)
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Abstract

The escalating concerns about global warming and energy shortages have presented an urgent need for efficient and environmentally sustainable hydrogen production methods. This work presents an efficient Ni3S2/ZnIn2S4 (ZIS) composite photocatalyst synthesized via a hydrothermal process, demonstrating enhanced performance for hydrogen evolution and benzyl alcohol oxidation under visible-light irradiation. Specifically, the optimized 3.2% Ni3S2/ZIS composite achieves hydrogen and benzaldehyde production rates of 4.342 mmol g–1 h–1 and 4.213 mmol g–1 h–1, respectively, 1.79 and 1.76 times greater than those of pristine ZIS. The system exhibits excellent selectivity, producing benzaldehyde as the sole by-product, and maintains stability over multiple reaction cycles. Mechanistic studies reveal that Ni3S2 facilitates charge separation and accelerates reaction dynamics by providing conductive channels and enhancing catalytic activity at the ZIS interface. These findings highlight the potential of Ni3S2/ZIS composites as cost-effective, scalable, and noble-metal-free photocatalysts for hydrogen production and green chemical synthesis, offering a promising pathway toward energy sustainability.

1. Introduction

Hydrogen has become increasingly crucial in modern energy systems as an ideal energy carrier due to its cleanliness and efficiency, making it an essential player in reducing fossil fuel dependence and mitigating environmental pollution to support sustainable energy transition [1,2,3,4,5]. In this context, photoelectrocatalytic hydrogen production has gained significant attention, especially since Fujishima and Honda discovered the water splitting through titanium dioxide electrodes under ultraviolet light in 1972 [6,7]. This breakthrough in photocatalytic technology opened a new era for green hydrogen production, leveraging renewable energy sources (e.g., solar energy) and offering cost advantages [8,9,10,11]. However, photocatalytic water splitting faces intrinsic challenges, including efficiency limitation due to high energy barriers, slow reaction rates, and the production of oxygen as a by-product with limited utility [12,13,14,15,16]. While hole sacrificial agents are often used to enhance charge separation and catalytic activity, most are toxic, increase costs, and waste the oxidation capacity of photogenerated holes [17,18,19,20,21].
As an alternative strategy, replacing sacrificial agents with high-value-added organic conversions in photocatalytic hydrogen production systems appears to have significant potential to simultaneously achieve the production of fine chemicals using green energy [22,23]. To this end, benzyl alcohol (BA) has emerged as a leading candidate because of two key advantages [24]. First of all, the oxidation of BA requires less energy to enable higher hydrogen yields even at lower light intensities [25,26]. Secondly, benzaldehyde, a by-product formed during the photocatalytic reaction of BA, is a valuable industrial raw material used for pharmaceuticals, pesticides, and fine chemicals [27]. This dual benefit provides strong motivation to develop efficient, reusable photocatalytic systems for hydrogen production and BA oxidation, offering a pathway that integrates high efficiency, economic viability, and environmental sustainability that can be established [28,29,30].
Given that photocatalytic redox reactions are known to occur on semiconductor surfaces via photogenerated holes and electrons [31,32,33,34], the formation of a heterojunction between two or more materials with compatible band alignments is an effective scheme to enhance photocatalytic efficiency by reducing the e−/h+ recombination rate and improving e−/h+ pair separation between the materials [35,36,37,38]. Towards this goal, ZnIn2S4 (ZIS), a ternary metal chalcogenide with unique photoelectrocatalytic properties and high stability, has been widely studied [39,40]; its layered structure allows modification and coupling with other semiconductor catalysts, facilitating the development of photocatalytic systems with enhanced hydrogen production performance [41]. For example, Li et al. constructed a Z-scheme heterostructure of sulfur vacancies ZIS and MoSe2 with high hydrogen evolution rate through photocatalytic water splitting [42]; Guan et al. developed a 2D/2D CuInS2/ZnIn2S4 heterojunction photocatalyst to enhance solar hydrogen production through improved charge separation [43]; Yang et al. developed an adaptive S vacancy (Vs) and atomic Cu-doped ZnIn2S4 nanosheet photocatalyst that enhances charge separation and creates gradient channels for hydrogen migration [44]. Nevertheless, ZIS-based hybrid photocatalysts still face challenges in achieving stable overall water-splitting reactions, and their explorations in photocatalytic BA splitting remain very limited [45,46].
In the past few years, earth-abundant and low-cost nickel-based sulfides, such as NiS, NiS2, and Ni3S2, have emerged as rising candidates for water splitting, primarily due to the relatively high electronegativity (2.5) of sulfur when compared to most other metals [47]. Among these, the Ni3S2 electrocatalyst has received significant attention for its high conductivity and unique structural properties [48]. While Ni3S2 possesses limited exposed active sites and unbalanced hydrogen evolution reaction/oxygen evolution reaction performance, it is considered a promising co-catalyst for enhancing heterogeneous interfaces and increasing the density of active sites [49]. Here, we demonstrate a facile method for synthesizing Ni3S2/ZIS composite photocatalysts using a hydrothermal approach. The integration of Ni3S2 into ZIS not only significantly improves the separation of photogenerated charge carriers but also decreases the hydrogen reduction overpotential, which leads to enhanced visible-light photocatalytic activity for BA dissociation to produce hydrogen and benzaldehyde. The 3.2% Ni3S2/ZIS composite catalyst was used for simultaneous photocatalytic hydrogen production and BA oxidation to benzaldehyde under light with activities of 4.342 and 4.213 mmol g–1 h–1, respectively, which is 1.79 times and 1.76 times higher than that of pure ZIS. No other by-products were detected in the reaction process of this photocatalytic system except for the generation of benzaldehyde. Our results demonstrate that the Ni3S2/ZIS composite offers a novel route for efficient and nonprecious hybrid photocatalysts to the sustainable production of hydrogen and benzaldehyde through BA splitting.

2. Results and Discussion

The schematic in Figure 1 illustrates the synthesis of the Ni3S2/ZIS heterostructure, a highly efficient photocatalyst designed to selectively convert benzyl alcohol to hydrogen and benzaldehyde. The synthesis process involves a hydrothermal method to integrate Ni3S2 with ZIS. Initially, ZIS nanosheets are synthesized through a solvothermal process, followed by the in situ growth of Ni3S2 nanoparticles on the ZIS surface under controlled hydrothermal conditions. This approach ensures precise control over the interface and morphology of the heterostructure, which is critical for optimizing charge separation and enhancing catalytic performance. As a result, the Ni3S2/ZIS photocatalyst demonstrates exceptional efficiency in converting benzyl alcohol to hydrogen and benzaldehyde, underscoring its potential for applications in green chemical synthesis and renewable energy.
The X-ray diffraction (XRD) patterns of Ni3S2, ZIS, and Ni3S2-modified ZIS (Ni3S2/ZIS) composites with varying Ni3S2 contents (1.9%, 3.2%, 4.8%, and 6.4%) are shown in Figure 2a and Figure S1. The five main diffraction peaks of Ni3S2 were observed at 30.2°, 35.7°, 45.7°, 48.8°, and 53.5°, corresponding to the (110), (003), (202), (113), and (122) facets of Ni3S2 (JCPDS No. 44-1418), respectively. For ZIS, distinct diffraction peaks at 21.6°, 27.7°, 47.2°, 52.4°, and 55.6° can be ascribed to the (006), (102), (110), (116), and (022) planes, respectively, corresponding to the hexagonal phase of ZIS (JCPDS No. 65-2023) [50]. After doping with a small amount of Ni3S2, it is shown that the crystal structure of ZIS does not change significantly after doping with a small amount of Ni3S2 (Figure 2b). No prominent diffraction peaks related to Ni3S2 were detected in the Ni3S2/ZIS sample, which may be due to the low content of Ni3S2, so the diffraction peak of Ni3S2 at low doping may be masked by the main peak of ZIS, resulting in a signal that is too weak to be detected. In addition, the pore size distribution curves and nitrogen adsorption–desorption isotherms for ZIS and 3.2% Ni3S2/ZIS composites are investigated. As shown in Figure 2c, the pore size distributions of the samples range from 2 to 40 nm, consistent with those of pristine ZIS. While the 3.2% Ni3S2/ZIS photocatalyst also exhibits pristine ZIS-like type IV isotherms [51], the introduction of Ni3S2 records an over 20% increase in specific surface area, which is crucial for improving the photocatalytic efficiency by providing more adsorption reaction sites.
SEM and HRTEM analyses are performed to further examine the morphology of the prepared catalysts. While pure ZIS exhibits a distinctive flower-like structure composed of densely packed nanosheets (Figure 3a), the Ni3S2-modified ZIS composite retains the nanosheet architecture. Still, it displays rougher surfaces, confirming the successful deposition of Ni3S2 on ZIS (Figure 3b). HRTEM imaging of the 3.2% Ni3S2/ZIS composite (Figure 3c) reveals a well-defined lamellar structure with tight interfacial contact between ZIS and Ni3S2, consistent with the SEM observations. Elemental mapping images (Figure 3e–h) show the homogenous distribution of Zn, In, S, and Ni elements across the composite, with no evident phase separation. This uniform dispersion of Ni3S2 on the ZIS surface suggests effective integration, critical for enhancing photocatalytic performance.
To investigate the chemical states of the elements and the interaction between ZIS and Ni3S2, X-ray photoelectron spectroscopy (XPS) is performed (Figure 4 and Figure S2). For pure ZIS, the Zn 2p3/2 and Zn 2p1/2 peaks are located at 1021.28 eV and 1045.03 eV, respectively. Upon modification with Ni3S2, these peaks shift slightly to lower binding energies at 1020.98 eV and 1044.43 eV (Figure 4a), indicating an enhanced electron density on the ZIS surface due to Ni3S2 incorporation and improved separation of photogenerated electron–hole pairs. For In 3d peaks (Figure 4b), the In 3d5/2 and In 3d3/2 peaks of pristine ZIS appear at 444.63 eV and 452.08 eV, respectively, but shift to 444.93 eV and 452.48 eV in the 3.2% Ni3S2/ZIS composite, highlighting surface charge redistribution due to interaction with Ni3S2. Moreover, while the S 2p3/2 and S 2p1/2 peaks of ZIS are observed at 161.23 eV and 162.43 eV, respectively (Figure 4c), the S 2p3/2 peak in the Ni3S2-modified samples shifts slightly to 161.48 eV, indicating electronic coupling between sulfur atoms and Ni3S2 [52]. Figure 4d shows the Ni 2p XPS spectra of the 3.2% Ni3S2/ZIS composite material and pure Ni3S2. By comparison, it can be seen that the binding energies of Ni 2p3/2 and Ni 2p1/2 in the composite material decreased from 855.33 eV and 872.84 eV to 854.89 eV and 872.4 eV, respectively, and the positions of the satellite peaks also changed, indicating that the chemical environment and valence state of Ni have been adjusted. The reduction in binding energy indicates that ZIS, as an electron acceptor, accepts electrons from Ni3S2, reducing the oxidation state of Ni from the Ni2+ part in pure Ni3S2 to possibly close to Ni2+ or between Ni1+ and Ni2+. This interfacial electron transfer improves the redox properties and electron separation efficiency of the material, thereby enhancing the performance of the composite material in photocatalytic benzyl alcohol hydrogen production and benzaldehyde synthesis [53]. Overall, XPS results demonstrate that Ni3S2 effectively modulates the surface electronic structure of ZIS, enhances interfacial electron transfer, and inhibits photogenerated electron–hole recombination to allow for potentially improved photocatalytic performance.
The optical properties of the photocatalysts are evaluated by UV-vis absorption spectroscopy (Figure S3). Compared with pure ZIS, the 3.2% Ni3S2/ZIS photocatalyst illustrates an obvious enhancement in absorption intensity, demonstrating improved optical response and a broader spectral range through the incorporation of Ni3S2. Additionally, the band gap energy values are calculated using the Kubelka–Munk function [54]. The calculated band gap values for ZIS and 3.2% Ni3S2/ZIS are 2.35 eV and 2.15 eV [55], respectively (Figure 5a). The narrower band gap of 3.2% Ni3S2/ZIS further validates its improved visible light absorption, advantageous for photocatalytic applications. Furthermore, we determined the conduction band positions of ZIS and 3.2% Ni3S2/ZIS using Mott–Schottky diagrams. The measured flat-band potentials of ZIS and 3.2% Ni3S2/ZIS were found to be −1.69 V and −0.81 V (relative to Ag/AgCl), respectively (Figure 5b). By adjusting the flat-band potentials by 0.88 V, the conduction band positions relative to the NHE were calculated. As illustrated in Figure 4c, the introduction of Ni3S2 resulted in a more positive valence band (VB) position in the 3.2% Ni3S2/ZnIn2S4 composite compared to pristine ZIS. This more positive VB potential significantly facilitates the oxidation of benzyl alcohol, as the photogenerated holes are now stronger oxidants. The efficient consumption of these holes in the oxidation half-reaction suppresses the recombination of electron–hole pairs. Consequently, more electrons are available in the conduction band for the reduction of H+, leading to the observed enhancement in H2 production. Photoluminescence spectra (PL) can illustrate the role of photogenerated carrier separation on the composites, and the samples are analyzed at an excitation wavelength of 300 nm. As revealed in Figure 5d, all samples display emission peaks near 610 nm, with pure ZIS exhibiting the highest emission intensity, indicative of strong charge carrier recombination. Conversely, the reduced PL intensity in 3.2% Ni3S2/ZIS indicates effective suppression of electron–hole recombination due to Ni3S2 loading. This improvement could enable more photogenerated electrons to participate in the photocatalytic reaction and thereby enhance the overall photocatalytic performance.
To further investigate the photocatalytic mechanism of these composite photocatalysts, photoelectrochemical measurements, including electrochemical impedance spectroscopy (EIS) and transient photocurrent response, are conducted. The EIS results (Figure 6a) reveal semicircular Nyquist plots for all samples. Generally speaking, a smaller semicircular radius corresponds to a lower charge transfer impedance, which facilitates carrier mobility. From this perspective, the 3.2% Ni3S2/ZIS photocatalyst exhibits a notably reduced impedance compared to pure ZIS, indicating enhanced electron–hole transfer and separation, a key factor in promoting photocatalytic hydrogen evolution. Transient photocurrent responses of FTO electrodes coated with different catalysts under 300 W xenon lamp irradiation (Figure 6b) show that all samples produce photocurrent signals. The higher photocurrent density recorded from the 3.2% Ni3S2/ZIS photocatalyst over that of pure ZIS reflects improved light absorption and enhanced generation and separation of charge carriers, highlighting the efficacy of Ni3S2 loading in boosting photocatalytic performance.
The photocatalytic performance of the prepared Ni3S2/ZIS composites was evaluated by measuring hydrogen production rates under visible light irradiation. As shown in Figure 7a,b, pure ZIS exhibited a hydrogen evolution rate of 2.422 mmol g–1 h–1. Control experiments using only Ni3S2 showed no hydrogen evolution, confirming that Ni3S2 alone lacks intrinsic photocatalytic activity for hydrogen generation. Interestingly, the incorporation of Ni3S2 into ZIS significantly enhanced photocatalytic performance, as evidenced by the volcano-like trend observed in Figure 7b,c. The hydrogen evolution rate increased with Ni3S2 loading, peaking at 4.342 mmol g–1 h–1 at an optimal Ni3S2 content of 3.2% due to the decreased recombination of photogenerated electron–hole pairs. A similar trend was observed in the BA conversion rate, which reached a maximum of 4.213 mmol g–1 h–1 at the same Ni3S2 loading. These results represent improvements of 1.79 fold and 1.76 fold, respectively, compared to pristine ZIS, highlighting the synergistic role of Ni3S2 in boosting photocatalytic activity. However, beyond the optimal loading of 3.2% Ni3S2, the hydrogen evolution rate declined, likely due to excessive Ni3S2 coverage on the ZIS surface, which blocked active sites and impeded light absorption. Product analysis confirmed that benzaldehyde was the sole reaction product, with no detectable by-products, demonstrating an exceptional selectivity of the system. The strong correlation between hydrogen production and BA conversion rates indicates that the dehydrogenation of BA to benzaldehyde proceeds with high efficiency, achieving a 69% conversion rate after 24 h using the 3.2% Ni3S2/ZIS composite (Table S1). To contextualize our findings within the current literature, we have compiled the performance of recently reported photocatalytic systems for this reaction (Table S2). This comparison clearly demonstrates that our optimized Ni3S2/ZnIn2S4 photocatalyst shows a highly competitive, and in most cases superior, performance. Furthermore, stability and reusability tests demonstrated the practical viability of the photocatalyst, as shown in Figure S4. Over four consecutive reaction cycles spanning four days, the 3.2% Ni3S2/ZIS composite retained its hydrogen production efficiency with minimal loss, demonstrating its excellent stability and reusability. Figure S5 depicts the XRD spectrum of the composite catalyst 3.2% Ni3S2/ZIS before and after the cyclic experiment. Notably, there are no discernible distinctions between the two spectra. These findings, combined with the high performance, selectivity, and durability of the composite, statistically proved the potential of the 3.2% Ni3S2/ZIS composite as a sustainable and efficient photocatalyst for hydrogen production and green chemical synthesis under visible light.
To elucidate the mechanism of photocatalytic BA decomposition, controlled experiments with quenchers are conducted to identify the active species in the photocatalytic system. As summarized in Table 1, the introduction of the hole scavenger triethanolamine (TEOA) significantly reduces benzaldehyde production from 16.906 mmol g–1 to 7.936 mmol g–1, while the hydrogen production rate remains nearly unchanged. This suggests that photogenerated holes are primarily involved in the oxidation of BA in the photocatalytic process. On the contrary, the addition of the electron scavenger AgNO3 (0.1 mmol) results in a complete cessation of hydrogen production, demonstrating that photogenerated electrons are essential for the reduction of H+ to H2 in the system. These findings confirm that the photocatalytic reaction involves distinct roles for photogenerated holes in BA oxidation and photogenerated electrons in hydrogen evolution, emphasizing their synergistic contribution to the overall photocatalytic mechanism [56].
To further confirm the generation of free radicals during photocatalysis, electron spin resonance (EPR) spectroscopy is conducted using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a radical trapping agent [57]. As shown in Figure 8a,b, no radical signals are observed for either photocatalyst under dark conditions. However, under visible light irradiation, six characteristic signals corresponding to carbon-centered radicals, specifically α-hydroxybenzyl radicals, are detected for both ZIS and 3.2% Ni3S2/ZIS photocatalysts, suggesting that visible light activates the C-H bond in BA to induce radical formation [58]. Importantly, the DMPO-Cα radical signal intensity is significantly higher for the 3.2% Ni3S2/ZIS composite than for ZIS alone, which unravels the critical role of the Ni3S2 co-catalyst in accelerating C-H activation and dehydrogenation, thereby improving the performance of the composite photocatalytic system.
Based on the above findings, we propose a rational photocatalytic mechanism for the Ni3S2/ZIS composite, as illustrated in Figure 9. Firstly, the photocatalyst absorbs incident light energy to generate photogenerated electron–hole pairs, through which the efficient separation of charge carriers prevents their recombination. Following that, the photogenerated holes interact with BA molecules adsorbed on the catalyst surface, oxidizing them to benzaldehyde while releasing hydrogen ions. Simultaneously, the photogenerated electrons participate in the reduction of protons to produce hydrogen [59]. The incorporation of Ni3S2 significantly enhances charge separation efficiency and facilitates electron migration through conductive channels while confining photogenerated holes to the catalyst surface. This improved charge carrier dynamics underpins the superior photocatalytic activity of Ni3S2/ZIS over the pristine ZIS.

3. Experiment

3.1. Materials

Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, CP), nickel nitrate hexahydrate (Ni(NO3)2·6H2O, AR), L-cysteine (C3H7NO2S, BR), benzyl alcohol (C7H8O, 99.0%), acetonitrile (C2H3N, 99.8%), triethanolamine (C6H15NO3, AR), silver nitrate (AgNO3, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Indium nitrate tetrahydrate (In(NO3)2·4H2O, 99.9%), thioacetamide (C2H5NS, 99%), sodium citrate dihydrate (C6H5Na3O7·2H2O, 99.0%) were supported by Macklin Chemical Reagent Co., Ltd. (Shanghai, China). Ultrapure water, with an electrical resistivity of 18.2 MΩ·cm, was employed as the experimental solvent.

3.2. Synthesis of ZIS Photocatalyst

For the synthesis of ZIS, 3.5 mmol of zinc nitrate hexahydrate, 7 mmol of indium nitrate tetrahydrate, and 28 mmol of L-cysteine were dissolved in 40 mL of deionized water. The solution was sonicated and magnetically stirred for 30 min at room temperature until it became clear. The resulting solution was then transferred to a 100 mL Teflon-lined stainless-steel autoclave and heated at 180 °C for 3 h. After cooling the autoclave to room temperature, the product was collected by centrifugation, washed several times with deionized water and ethanol, and dried in an oven at 60 °C for 12 h.

3.3. Synthesis of the Heterogeneous Ni3S2/ZIS

To prepare the Ni3S2/ZIS composite, 0.24 mmol of nickel nitrate hexahydrate and 500 mg of sodium citrate dihydrate were first dissolved in 35 mL of deionized water and stirred for 30 min. Subsequently, 0.6 g of the synthesized ZIS was added to the solution and dispersed by stirring for 2 h. Afterward, 5 mL of an aqueous solution containing 0.32 mmol of TAA was gradually added, and the mixture was stirred for another hour. The resulting solution was transferred to a 100 mL Teflon-lined stainless-steel autoclave and heated at 160 °C for 6 h. After cooling to room temperature, the product was collected by centrifugation, washed multiple times with deionized water and ethanol, and dried at 60 °C for 12 h. To obtain different ratios of Ni3S2 in ZIS, the mass of Ni3S2 was kept constant, and the ratios were adjusted by varying the mass of ZIS.

3.4. Characterization

Scanning electron microscopy (SEM) analyses were performed using a Zeiss Supra 55 (Zeiss, Oberkochen, Germany) high-resolution field emission scanning electron microscope at 5.0 kV. Powder X-ray diffraction (XRD) patterns were obtained using Cu-Kα radiation on a Bruker D8 X-ray diffractometer (Bruker AXS, Karlsruhe, Germany). Morphological and elemental profiles were analyzed using a high-resolution transmission electron microscope (HRTEM, Tecnai G2 F30 S-TWIN). Fluorescence spectra were measured using an F-7000 fluorescence spectrophotometer (Hitachi, Japan) for photoluminescence (PL) spectroscopy. X-ray photoelectron spectroscopy (XPS) was performed using an ESCALAB 250 Xi electron spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). Nitrogen adsorption–desorption measurements were carried out using an Autosorb IQ3 instrument (Quantachrome, Boynton Beach, FL, USA) to analyze the Brunner–Emmett–Taylor (BET) surface area and pore size of the samples. The absorbance of the samples was measured using a UV-visible near-infrared absorption spectrophotometer (Cary 5000, Varian, Palo Alto, CA, USA), and barium sulfate was used as a blank. The formation of free radicals during the photocatalytic reaction was analyzed using EPR (Bruker A300-10/12, Billerica, MA, USA).

3.5. Photoelectrochemical Testing

Electrochemical tests were performed in a three-electrode system. 5 mg of the material was dissolved with 25 μL of Nafion solution in ethanol and uniformly coated on a conductive glass substrate as a working electrode with an effective area of 1 cm2. A platinum sheet was used as the counter electrode, the Ag/AgCl electrode was used as the reference electrode, and the electrolyte was a 0.1 M sodium sulfate solution. Electrochemical impedance spectroscopy (EIS) measurements were performed on a Gamry Reference 3000 workstation (Gamry Instruments, Warminster, PA, USA) with an applied voltage of 0.2 V and a frequency range of 105 Hz to 0.05 Hz. Photocurrent measurements were also performed using a Gamry Reference 3000 workstation under 300 W xenon lamp illumination (China Education Au-light, Beijing, China).

3.6. Photocatalytic Hydrogen Production Test

The photocatalytic reaction system employed a 100 mL quartz reactor with a 300 W xenon arc lamp as the light source. In the experiment, 20 mg of the prepared catalyst was uniformly dispersed in 40 mL of benzyl alcohol solution by ultrasonic treatment. To ensure an oxygen-free environment in the system, nitrogen was used as a protective atmosphere, and the reactor is evacuated to remove air by a vacuum pump for 15 min. Subsequently, the mixture was then stirred and irradiated from the top using a xenon arc lamp at a light intensity of 250 mW cm−2. The generated hydrogen was analyzed hourly during the reaction using a hydrogen chromatograph equipped with a thermal conductivity detector (GC-7920, Beijing CEJ Tech. Co., Ltd., Beijing, China, 5A sieve column, and N2 as carrier). To compare the performance of different catalysts, catalytic hydrogen production experiments with other catalysts were carried out under the same experimental conditions.
In addition, to assess the conversion and selectivity of benzyl alcohol oxidation for the production of benzaldehyde, the experiments were performed by adding 20 mg of photocatalyst, 20 μL of benzyl alcohol, and 50 mL of acetonitrile to the reaction system and subjected to light irradiation for 24 h (see Table S1 for details). At the end of the reaction, the supernatant was separated by centrifugation and analyzed using hydrogen chromatography (GC, Agilent 7890A-7697A, Tokyo, Japan). The conversion of benzyl alcohol and selectivity of benzaldehyde are calculated according to Equations (1) and (2), respectively.
Conversion = (C0 − C1)/C0 × 100%
Selectivity = C2/(C0 − C1) × 100%
where C0 represents the initial concentration of benzyl alcohol, C1 represents the remaining concentration of benzyl alcohol after the reaction, and C2 represents the concentration of benzaldehyde after the reaction.

4. Conclusions

In summary, this study presents the synthesis and application of a Ni3S2/ZIS composite photocatalyst with remarkable photocatalytic performance for simultaneous hydrogen production and BA oxidation under visible-light irradiation. The optimized 3.2% Ni3S2/ZIS composite demonstrated significantly enhanced photocatalytic activity compared to pristine ZIS, achieving hydrogen and benzaldehyde production rates of 4.342 mmol g–1 h–1 and 4.213 mmol g–1 h–1, respectively. These results represent an improvement of approximately 1.8-fold, attributed to the synergistic effects of Ni3S2, which enhance charge separation, suppress electron–hole recombination, and improve interfacial charge transfer. Mechanistic studies confirmed that Ni3S2 plays a crucial role in modulating the surface electronic structure and facilitating efficient photocatalytic redox reactions. Additionally, the system exhibited exceptional selectivity for benzaldehyde as the sole by-product and retained its stability across multiple reaction cycles, demonstrating its practicality for sustainable hydrogen production and green chemical synthesis. This work highlights the potential of Ni3S2/ZIS composites as cost-effective and scalable photocatalysts, paving the way for the development of noble-metal-free systems in renewable energy applications and fine chemical production.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15090830/s1, Figure S1: XRD patterns of x-Ni3S2/ZIS (x is the weight percentage of Ni3S2 to ZIS); Table S1: Photocatalytic conversion of benzyl alcohol and selectivity of benzaldehyde. References [60,61,62,63,64,65,66] are cited in Supplementary Materials.

Author Contributions

H.W.: Investigation, Methodology, Writing—original draft. C.Z.: Supervision, Writing—reviewing and editing, Funding acquisition. S.Y.: Conceptualization, Supervision, Writing - reviewing & editing, Funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This work is financially supported by the National Natural Science Foundation of China (21801219), the “Qing-Lan” Project of Jiangsu Province, the top-notch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP), and the start-up fund from Yangzhou University.

Data Availability Statement

Data will be made available on request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic flow chart of hydrothermal synthesis of Ni3S2/ZIS.
Figure 1. Schematic flow chart of hydrothermal synthesis of Ni3S2/ZIS.
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Figure 2. (a) XRD patterns of Ni3S2, ZIS, and 3.2% Ni3S2/ZIS. (b) Nitrogen adsorption–desorption isotherms for ZIS and 3.2% Ni3S2/ZIS. (c) ZIS and 3.2% Ni3S2/ZIS pore size distribution.
Figure 2. (a) XRD patterns of Ni3S2, ZIS, and 3.2% Ni3S2/ZIS. (b) Nitrogen adsorption–desorption isotherms for ZIS and 3.2% Ni3S2/ZIS. (c) ZIS and 3.2% Ni3S2/ZIS pore size distribution.
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Figure 3. (a,b) SEM images of (a) ZIS and (b) 3.2% Ni3S2/ZIS. (c) High-resolution TEM image of 3.2% Ni3S2/ZIS. (d) HAADF-STEM image of 3.2% Ni3S2/ZIS. (e–h) STEM-EDS mapping of 3.2% Ni3S2/ZIS.
Figure 3. (a,b) SEM images of (a) ZIS and (b) 3.2% Ni3S2/ZIS. (c) High-resolution TEM image of 3.2% Ni3S2/ZIS. (d) HAADF-STEM image of 3.2% Ni3S2/ZIS. (e–h) STEM-EDS mapping of 3.2% Ni3S2/ZIS.
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Figure 4. ZIS and 3.2% Ni3S2/ZIS high-resolution XPS spectra of (a) Zn 2p, (b) In 3d, (c) S 2p (d) Ni3S2 and 3.2% Ni3S2/ZIS high-resolution XPS spectra of Ni 2p.
Figure 4. ZIS and 3.2% Ni3S2/ZIS high-resolution XPS spectra of (a) Zn 2p, (b) In 3d, (c) S 2p (d) Ni3S2 and 3.2% Ni3S2/ZIS high-resolution XPS spectra of Ni 2p.
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Figure 5. (a) The bandgaps of ZIS and 3.2% Ni3S2/ZIS were determined from the UV-vis spectra using the Kubelka–Munk function. (b) Mott–Schottky plots of ZIS and 3.2% Ni3S2/ZIS. (c) The flat and conduction bands of ZIS and 3.2% Ni3S2/ZIS. (d) Steady-state photoluminescence (PL) spectra of ZIS and 3.2% Ni3S2/ZIS.
Figure 5. (a) The bandgaps of ZIS and 3.2% Ni3S2/ZIS were determined from the UV-vis spectra using the Kubelka–Munk function. (b) Mott–Schottky plots of ZIS and 3.2% Ni3S2/ZIS. (c) The flat and conduction bands of ZIS and 3.2% Ni3S2/ZIS. (d) Steady-state photoluminescence (PL) spectra of ZIS and 3.2% Ni3S2/ZIS.
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Figure 6. (a) Nyquist plot of electrochemical impedance spectra of ZIS and 3.2% Ni3S2/ZIS. (b) Transient photocurrent response of ZIS and 3.2% Ni3S2/ZIS.
Figure 6. (a) Nyquist plot of electrochemical impedance spectra of ZIS and 3.2% Ni3S2/ZIS. (b) Transient photocurrent response of ZIS and 3.2% Ni3S2/ZIS.
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Figure 7. (a) Trend of H2 evolution with time. (b) Ni3S2, ZIS, and x Ni3S2/ZIS H2 evolution rates (x is the weight percentage of Ni3S2 to ZIS). (c) Ni3S2, ZIS, and x Ni3S2/ZIS BAD evolution rates.
Figure 7. (a) Trend of H2 evolution with time. (b) Ni3S2, ZIS, and x Ni3S2/ZIS H2 evolution rates (x is the weight percentage of Ni3S2 to ZIS). (c) Ni3S2, ZIS, and x Ni3S2/ZIS BAD evolution rates.
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Figure 8. Spin-trapped EPR spectra with and without visible light irradiation. (a) ZIS, (b) 3.2% Ni3S2/ZIS.
Figure 8. Spin-trapped EPR spectra with and without visible light irradiation. (a) ZIS, (b) 3.2% Ni3S2/ZIS.
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Figure 9. (a) Oxidation and reduction reactions of benzyl alcohol during photocatalysis. (b) Diagram illustrating the photocatalytic synthesis of benzaldehyde and hydrogen generation using Ni3S2/ZIS as the photocatalyst.
Figure 9. (a) Oxidation and reduction reactions of benzyl alcohol during photocatalysis. (b) Diagram illustrating the photocatalytic synthesis of benzaldehyde and hydrogen generation using Ni3S2/ZIS as the photocatalyst.
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Table 1. Quenching experiments for photocatalytic benzyl alcohol oxidation and hydrogen evolution.
Table 1. Quenching experiments for photocatalytic benzyl alcohol oxidation and hydrogen evolution.
EntryQuenchersQuenching GroupH2 (mmol g–1)BAD (mmol g–1)
1nonenone17.36816.852
2TEOAholes16.9067.936
3AgNO3electrons016.503
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Wang, H.; Zhou, C.; Yang, S. Facile Synthesis of Ni3S2/ZnIn2S4 Photocatalysts for Benzyl Alcohol Splitting: A Pathway to Sustainable Hydrogen and Benzaldehyde. Catalysts 2025, 15, 830. https://doi.org/10.3390/catal15090830

AMA Style

Wang H, Zhou C, Yang S. Facile Synthesis of Ni3S2/ZnIn2S4 Photocatalysts for Benzyl Alcohol Splitting: A Pathway to Sustainable Hydrogen and Benzaldehyde. Catalysts. 2025; 15(9):830. https://doi.org/10.3390/catal15090830

Chicago/Turabian Style

Wang, Haibo, Chen Zhou, and Shengyang Yang. 2025. "Facile Synthesis of Ni3S2/ZnIn2S4 Photocatalysts for Benzyl Alcohol Splitting: A Pathway to Sustainable Hydrogen and Benzaldehyde" Catalysts 15, no. 9: 830. https://doi.org/10.3390/catal15090830

APA Style

Wang, H., Zhou, C., & Yang, S. (2025). Facile Synthesis of Ni3S2/ZnIn2S4 Photocatalysts for Benzyl Alcohol Splitting: A Pathway to Sustainable Hydrogen and Benzaldehyde. Catalysts, 15(9), 830. https://doi.org/10.3390/catal15090830

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