Effect of Sulfur Contents in NiZnS Composite Photocatalysts on Solar Water Splitting

Solar energy is attracting much attention as an eco-friendly source for future energy needs. Herein, NiZnS photocatalysts were synthesized with a hydrothermal method at various sulfur contents. The ZnS material is widely used as a photocatalyst because of its high stability, low toxicity, and excellent charge separation characteristics. Nickel is considered a co-component in the ZnS base to improve hydrogen evolution efficiency, because nickel sulfide has a narrow band gap. Field emission scanning electron microscopy analysis was used to observe particle size and shape. As the sulfur ratio increased, the particle size increased, and relatively uniform particle sizes were obtained at the 2:2 molar ratio of NiZn:S. X-ray diffractometer analysis showed the formation of ZnO crystals at low sulfur contents in the NiZnS photocatalysts. Among the various NiZnS compositions, the NiZn:S ratio of 2:2 resulted in the highest hydrogen production rate (1541.5 μmol/g/h) with stable reproducibility. UV-vis spectroscopy was used to analyze light absorbance, and the band gap changed with different sulfur contents due to the oxygen vacancies in ZnO, as identified by X-ray photoelectron spectroscope. High amounts of thiourea used to introduce the sulfur increased the particle sizes and blocked sunlight coming to NiZnS surfaces, thereby degrading photocatalytic performance. Therefore, changing the sulfur content when fabricating the NiZnS composite photocatalysts affected the crystalline structures and band characteristics of the materials, and it finally resulted in improved light absorption, charge separation, and the hydrogen production rate of the photocatalysts.