ZnS/ZnSe heterojunction photocatalyst for augmented hydrogen production: Experimental and theoretical insights

A facile chemical process for the synthesis of ZnS/ZnSe composites and theoretical and experimental insights into their sunlight-driven photocatalytic H₂ production via water-splitting reactions are demonstrated. ZnSe systems are varied by synthesizing at various temperatures such as 80, 150, and 180 °C for 12 h via hydrothermal process to tune their crystalline properties while maintaining ZnS as the host material, which is the main driving force to achieve increased photocatalytic H₂ production efficiency in this study. The prepared composite photocatalyst is found to have cubic ZnS and wurtzite ZnSe phases with good overall crystalline properties. The morphological investigation revealed that the composite consists of ZnS with a spherical structure coupled with irregular-structured ZnSe particles. The optimized ZnS/ZnSe (ZnSe prepared at 150 °C) photocatalyst showed the highest H₂ generation of around 84.8 mmol h⁻¹ g⁻¹_cat, with a UTH (i.e., UV–visible-to-H₂) conversion efficiency and turnover frequency of 20.4% and 0.097 Atom⁻¹ s⁻¹, respectively. This observed photocatalytic efficiency is presumed to be the formation of type-I heterojunction channelizing the effective transfer of hot photocarriers from ZnS to ZnSe for the rapid production of protons (H⁺) and their subsequent reduction to H₂ molecules. The achieved highest efficiency of the composite is around 56.4 and 4.2 folds higher than the pristine ZnSe and ZnS, respectively. In addition, the recycle experiments of the optimized catalyst showed consistent H₂ production for upto 5 cycles. Further, the developed composite systems are investigated via density functional theory and validated through various physio and electrochemical analyses to understand their structure-property relationships and photocatalytic mechanisms toward water splitting for H₂ production.