Engineering the optoelectronic properties of ZnS (1100) surface using selected 3d transition metal dopants for enhanced Photoelectrochemical water Splitting: A DFT study

ZnS (1100) surface has emerged as a promising photocatalyst for water split- ting due to its rapid generation of electron-hole pairs upon photoexcitation and high hydrogen evolution efficiency. However, its widespread use has been limited by its response to UV spectrum and rapid recombination of charge carriers. Studies have shown that doping ZnS with transition metals can modify its band gap edge and alter its optical properties. Despite this, there are few comprehensive studies that have systematically explore the potential of doped ZnS (1100) surface for Photo-electrochemical (PEC) applications. In this work, the effects of selected transition metal (TM) dopants (Mn, Cu, Co and Fe) on the optoelectronic properties of ZnS (1100) surface using density functional theory approach has been explored. The results showed that the stability of TM dopants in ZnS (1100) surface is dependent on the d character of the TM dopant as well as their concentration and doping site. Further, it was noted that Zn-rich synthesis conditions were favorable for introduction of TM dopants compared to S-rich conditions. Notably, among the dopants studied, Cu exhibited the highest stability, whereas Co, Mn and Fe displayed decreasing levels of stability. Mn and Fe (for dopant concentration between 1–6%) induces reduction of band-gap energy by 15–60% and 19–51%, respectively, while Cu and Co dopants of similar concentrations induced a more dramatic reduction of band gap energy between 37–78% and 26–75%, respectively. Additionally, band-edge alignment analysis showed that ZnS (1100) surface doped with 4% Cu and 2% Co falls below the redox potential of water (H⁺/H₂). Therefore, Cu and Co are anticipated to induce significant blue shift and offer improved PEC activity.