Synthesis of Metastable Wurtzite Cu–in-Doped ZnS Photocatalysts via Molten Salt Treatment

Herein, the crystallization process of doped ZnS photocatalysts using a molten salt treatment in air is explored. When ZnS-based photocatalysts with low crystallinity and a zincblende structure are heated in molten salt under conditions of a short holding time at high temperatures, the structure transitions from zincblende to the metastable wurtzite phase at 600 °C, which is significantly lower than the bulk transition temperature (1020 °C). Furthermore, with a longer holding time, ZnS reverted to the stable zincblende phase. This phenomenon is attributed to the unique crystal growth behavior of the nanoparticles in the molten salt environment. The evaluation of the synthesized photocatalysts reveals that ZnS with a wurtzite structure exhibits particularly high photocatalytic activity. Additionally, Cu and In co-doping induce visible-light responsiveness, achieving hydrogen evolution with a higher external quantum efficiency (EQE) than that of molten-salt-treated CdS photocatalysts synthesized in previous studies. The selective formation of nanoparticles with a metastable wurtzite structure or microparticles with a stable zincblende structure could be controlled by varying the holding time at 600 °C. Thus, Cu–In-doped ZnS photocatalysts may potentially be used as Cd-free alternative photocatalysts and semiconductor electrode materials.