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Method Article

Synthesis and Performance Evaluations of ZnCoS/ZnCdS with Twin Crystal Structure for Multifunctional Redox Photocatalysis in Energy Applications

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DOI:

10.3791/68828

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July 25th, 2025

 ,  ,  , 
1School of Energy and Chemical Engineering, Xiamen University Malaysia, 2Center of Excellence for NaNo Energy & Catalysis Technology (CONNECT), Xiamen University Malaysia, 3State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, 4Gulei Innovation Institute, Xiamen University, 5Shenzhen Research Institute of Xiamen University, 6Department of Chemical and Biological Engineering, College of Engineering, Korea University

In This Article

Summary

A synthesis technique to prepare the visible-light-responsive ZnCoS/ZnCdS with twin crystal structure photocatalyst and the photoredox reaction testing technique of its photocatalytic activity and selectivity in H2 evolution and benzaldehyde production is presented.

Abstract

This protocol presents both the synthesis of the visible-light-responsive ZnCoS/ZnCdS with twin crystal structure photocatalyst and the photoredox reaction testing of its photocatalytic activity and selectivity in H2 evolution and benzaldehyde production. This ZnCoS/ZnCdS heterostructure integrates the mixed-phase junction advantage of ZnCdS semiconductor and the cocatalytic function of ZnCoS acting as an electron reservoir to facilitate the surface reaction. Notably, the existence of twin crystal structure within a ZnCdS semiconductor purportedly presents a nano-scaled twin superlattice configuration with an alternative arrangement of long-range-ordered twinned planes, which gives rise to a unique wurtzite/zinc blende (WZ/ZB) interphase junction. This interphase junction possesses an interfacial electrostatic field, resulting in excellent photoexcited charge carrier separation and transport to the specimen surface, hence facilitating redox reactions. We report a facile hydrothermal approach to prepare the visible-light-responsive ZnCoS/ZnCdS with twin crystal structure, where the ZnCoS plays a crucial role in capturing photoinduced electrons on the catalyst surface. UV-Vis diffuse reflectance spectroscopy and N2 physisorption measurement were conducted to determine the photoabsorption ability and surface area of the sample, respectively. A detailed setup of solar-driven H2 production coupled with benzyl alcohol oxidation and an online gas chromatography was demonstrated. The ZnCoS/ZnCdS exhibited an enormous enhancement in both H2 and benzaldehyde formation. The correlation between ZnCoS co-catalyst and WZ/ZB phase junctions of ZnCdS towards the photocatalytic behavior was systematically assessed. This casts a novel idea to optimize the design of dual-functional photocatalysts for multifarious energy applications.

Introduction

Artificial photosynthesis, which harnesses solar energy for chemical fuel production, has emerged as a promising strategy within renewable energy systems, offering a green and sustainable pathway toward a low-carbon future1,2. Despite significant academic and industrial interest in solar-driven hydrogen (H2) evolution via water splitting for green hydrogen production, its practical implementation remains constrained by key challenges, including high overpotentials, rapid recombination of photogenerated charge carriers, and sluggish kinetics of the oxidative half-reaction3. In this context, the photoredox dual reaction involving hydrogen evolution and benzyl alcohol (BA) oxidation has recently gained attention as a promising approach for the effective utilization of photoinduced charge carriers4,5,6. In this system, protons are reduced to generate clean H2 fuel, while BA is selectively oxidized to produce the value-added product benzaldehyde (BAD)7,8. Consequently, the development of an efficient bifunctional heterogeneous photocatalyst capable of simultaneously facilitating the consumption of photogenerated electrons and holes is critical for realizing a high-performance cooperative photoredox coupling system9,10,11,12.

Metal sulfide semiconductors, particularly ternary zinc cadmium sulfide (ZnCdS) solid solutions, have been extensively studied for photocatalytic applications due to their tunable band gap and band edge positions, strong visible-light absorption, and high resistance to photocorrosion13,14. Notably, ZnCdS composites featuring a twin crystal structure composed of two distinct crystalline phases, zinc blende (ZB) and wurtzite (WZ), have recently garnered significant attention for their potential to enhance photocatalytic performance. The WZ/ZB interphase junction in ZnCdS composites possesses an interfacial electrostatic field that significantly enhances the separation and transport of photogenerated charge carriers to the catalyst surface, thereby promoting redox reactions15. Nevertheless, ZnCdS composites with WZ/ZB interphase junctions still encounter the challenge of rapid recombination of photoexcited electron-hole pairs, which detrimentally impacts their overall photocatalytic performance16. Among various enhancement strategies, the incorporation of co-catalysts has emerged as a promising and straightforward approach to improve the photocatalytic performance of ZnCdS-based semiconductors by facilitating more efficient separation and transfer of photoinduced charge carriers17,18,19. Recently, ZnCoS material has been extensively studied in supercapacitors20, electrocatalysis21, and batteries22 owing to its excellent structural stability and electrical conductivity. On this basis, ZnCoS/ZnCdS photocatalyst with optimal optoelectronic characteristics has been successfully developed by leveraging the distinct advantages of efficient photoinduced electron-hole pairs separation, catering to the needs of sustainable energy and environmental applications.

In this work, we present a comprehensive protocol for the synthesis and application of the ZnCoS/ZnCdS photocatalyst, aimed at guiding new researchers in the field of photoredox dual catalysis. Particularly, 20 mg of ZnCoS/ZnCdS photocatalyst is dissolved into 60 mL benzyl alcohol aqueous solution (1.8 mL of benzyl alcohol) and subsequently exposed to Xenon light source with light intensity of 84.6 mW cm-2. Additionally, this study offers a detailed demonstration of product analysis associated with photoredox dual reactions. Further information is available in our recently published article23.

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Protocol

The reagents and the equipment used in this study are listed in the Table of Materials.

1. Preparation of ZnCoS/ZnCdS photocatalyst

  1. Synthesis of ZnCoS co-catalyst
    1. Take a 100 mL beaker to contain 40 mL of ethylene glycol solution. Pour 0.22 g of Zn(CH3COO)2·2H2O, 0.5 g of Co(CH3COO)2·4H2O and 0.3 g of CH3CSNH2 into the solution.
    2. Subject the resultant solution to an ultrasonic treatment for 0.5 h, followed by a stirring process for 4 h at ambient temperature (25 °C) to ensure complete dissolution of the metal precursors.
    3. Transfer the mixed solution into a 100 mL synthetic polymer-lined stainless-steel autoclave and subsequently put it into an oven for heating at 180 °C for 12 h for the hydrothermal process.
    4. Collect the dark gray precipitate by using the centrifugation technique (~8800 x g for 10 min at 20 °C) to wash the sample three times each with deionized water and ethanol.
    5. Transfer the dark gray sample into an oven for the drying process at 60 °C overnight. The dark gray ZnCoS solid powder is finally obtained.
  2. Synthesis of ZnCdS with twin crystal structure
    1. Take a 100 mL beaker to contain 40 mL of deionized water. Pour 0.22 g of Zn(CH3COO)2·2H2O, 0.27 g of Cd(CH3COO)2·2H2O, 0.16 g of Na2S·xH2O and 0.15 g of CH3CSNH2 into the solution.
    2. Subject the resultant solution to an ultrasonic treatment for 0.5 h, followed by a stirring process for 3 h at ambient temperature to ensure complete dissolution of the metal precursors.
    3. Add the 0.2 M NaOH aqueous solution drop-wise into the solution to adjust the mixture to around pH 7.0.
    4. Transfer the mixed solution into a 100 mL synthetic polymer-lined stainless-steel autoclave and subsequently put it into an oven for heating at 180 °C for 24 h for the hydrothermal process.
    5. Collect the yellowish precipitate by using the centrifugation technique (~8800 x g for 10 min at 20 °C) to wash the sample three times each with deionized water and ethanol.
    6. Transfer the yellowish precipitate into an oven for drying at 60 °C overnight. The yellowish ZnCdS solid powder is finally obtained.
  3. Synthesis of ZnCoS/ZnCdS photocatalyst
    1. Dissolve 4 mg of ZnCoS and 0.196 g of ZnCdS into 40 mL of deionized water and subject the solution to an ultrasonication process lasting for 0.5 h, followed by a stirring process for 4 h at ambient temperature.
    2. Collect the yellowish precipitate by using the centrifugation technique (~8800 x g for 10 min at 20 °C) to wash the sample three times each with deionized water and ethanol.
    3. Transfer the yellowish precipitate into an oven for the drying process at 60 °C overnight. The yellowish ZnCoS/ZnCdS solid powder is finally obtained.

2. Photoredox dual reaction of benzyl alcohol oxidation and H2 production measurement

  1. Photoredox dual reaction setup
    1. Add 20 mg of as-synthesized photocatalyst and 60 mL of benzyl alcohol aqueous solution (1.8 mL of benzyl alcohol) into a 100 mL beaker.
    2. Subject the beaker to ultrasonication treatment for 0.5 h.
    3. Transfer the solution and put a magnetic stirrer into a three-necked top-irradiation reactor cell. The solution is under a slow stirring process throughout the whole reaction process.
    4. Connect a moisture trap at the downstream of the reactor cell. Then connect to the gas sampling loop inlet of the gas chromatography (GC). Connect the gas sampling loop outlet to the inlet of the reactor cell, thus forming a closed gas circulation system.
    5. Purge 50 mL/min of N2 gas throughout the reactor for 0.5 h to entirely remove the air inside the reactor after the reactor is sealed with a glass window. The reaction is conducted in a closed gas circulation system.
    6. Switch on a peristaltic pump and set the flow rate at 20 mL/min to circulate the N2 stream inside the closed gas circulation system.
    7. Switch on the Xenon lamp at 15 V and ensure the light penetrates through the glass window to reach the solution. The light intensity is about 84.6 mW/cm2, and the distance between the light source and the reactor glass window is around 15 cm.
  2. Photoredox dual reaction product analysis by online gas chromatography (GC) and high-performance liquid chromatography (HPLC)
    1. Employ a GC equipped with a combination of molecular sieve 5A and porous polymer adsorbent columns for gas products analysis during photoredox dual reaction.
    2. Use a thermal conductivity detector (TCD) to quantify H2 concentration.
    3. During photoredox dual reaction, maintain the flow rate at 20 mL min-1 to deliver gases into the GC.
    4. Determine the H2 content from the TCD signal.
    5. After the reaction is completed, use a 0.22 µm nylon syringe filter to filter 1 mL of suspension. Then, dilute the suspension with deionized water with a suspension: deionized water ratio of 1:9.
    6. Employ an HPLC equipped with a photodiode array detector and a high-performance 100 Å column. A mobile phase of water/acetonitrile (60:40) and a detection wavelength of 254 nm are employed to determine the composition of the analyte.

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Results

Morphology studies are conducted using high-resolution transmission electron microscopy (HRTEM) images, with the employment of a high-resolution microscope installed with a 200 kV field emission analytical electron microscope. High-resolution transmission electron microscopy (HRTEM) images are shown in Figure 1 for the twin crystal structure characterization of ZnCdS and ZnCoS/ZnCdS. UV-Vis diffuse reflectance spectra (DRS) are recorded with a wavelength rang...

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Discussion

Figure 1A-C displays HRTEM images of pristine ZnCdS and ZnCoS/ZnCdS photocatalysts. The pristine ZnCdS exhibits a granular nanostructure composed of highly crystalline nanoparticles. As shown in Figure 1A, lattice spacings of 0.32 nm and 0.36 nm are observed, corresponding to the ZB (111) and WZ (100) crystal planes of ZnCdS, respectively24. Prominent ZB/WZ interphase boundaries featuring mirror-symmetric...

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Acknowledgements

The authors would like to acknowledge the financial support provided by the Ministry of Higher Education (MOHE) Malaysia under the Fundamental Research Grant Scheme (FRGS) (Ref no: FRGS/1/2024/TK08/XMU/02/1). This work was supported by the PETRONAS-Academia Collaboration Dialogue (PACD 2023) grant, provided by PETRONAS Research Sdn. Bhd. (PRSB). The authors would like to thank the Ministry of Science, Technology and Innovation (MOSTI) Malaysia under the Strategic Research Fund (SRF) (S.22015). The authors gratefully acknowledge Agilent Technologies Malaysia Sdn Bhd for their contribution through chromatography. The authors would also like to acknowledge the financial support provided by the National Natural Science Foundation of China (Ref no: 22202168) and the Guangdong Basic and Applied Basic Research Foundation (Ref no: 2021A1515111019). We would also like to acknowledge the financial support from the State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen University (Ref no: 2023X11). This work is also funded by Xiamen University Malaysia Investigatorship Grant (Grant no: IENG/0038), Xiamen University Malaysia Research Fund (ICOE/0001 and XMUMRF/2021-C8/IENG/0041), and Hengyuan International Sdn. Bhd. (Grant no: EENG/0003).

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Materials

List of materials used in this article
NameCompanyCatalog NumberComments
Benzyl alcoholSigma-Aldrich402834ACS reagent grade, ≥ 99.0%
Cadmium acetate dihydrate (Cd(CH3COO)2·2H2O)Sigma-Aldrich289159Reagent grade, 98%
Cobalt(II) acetate tetrahydrate (Co(CH3COO)2·4H2O)Sigma-Aldrich403024ACS reagent, ≥ 98.0%
Ethylene glycolSigma-Aldrich102466ReagentPlus, ≥ 99%
Gas chromatographyAgilent8890 GC SystemCustomized, consists of flame ionization detector (FID), thermal conductivity detector (TCD), methanizer, two MolSieve 5A 60/80 mesh columns and three HayeSep Q 80/100 mesh columns
High-performance liquid chromatographyAgilent1260 Infinity II LC SystemConsists of photodiode array detector (PAD) and Kinetex 2.6 μm Phenyl-Hexyl 100 Å column
High-speed centrifuge Thermo Fisher Scientific75009760Sorvall X1R Pro
Magnetic stirrer IKA3622000RET basic
Mass flow controllerAlicatMC series
NitrogenAir Liquide99.9995% purity
OvenMemmertMMT-UN55
Sodium sulfide hydrate (Na2S·xH2O)Sigma-Aldrich13468≥60%, scales
Thioacetamide (CH3CSNH2)Sigma-Aldrich163678ACS reagent grade, ≥ 99.0%, solid
Three-necked top-irradiation glass reactor PTL Glass Aluminium Customized
Ultrasonicator bathCole-ParmerCP/P-08895-15
Xenon lampBeijing China Education Au-lightCEL-PF300-T6
Zinc acetate dihydrate (Zn(CH3COO)2·2H2O)Sigma-Aldrich383058ACS reagent, ≥ 98.0%

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Tags

Hydrogen EvolutionBenzaldehyde ProductionVisible Light PhotocatalystHeterojunction PhotocatalystHydrothermal SynthesisUV Vis SpectroscopyGas Chromatography
Synthesis and Performance Evaluations of ZnCoS/ZnCdS with Twin Crystal Structure for Multifunctional Redox Photocatalysis in Energy Applications
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