Physical Vapor Deposition of Ultrathin ZnO on Monolayer WS₂

Designing innovative composite photocatalysts is the first step in realizing the efficient production of hydrogen utilizing sunlight and water via the photocatalytic process. The composite photocatalysts offer enhanced photocatalytic efficiency due to the synergistic effects of their constituents. As such, we explore the growth behavior of nanoscale composite photocatalysts by synthesizing ultrathin ZnO layers on monolayer WS₂. To achieve this, we synthesized monolayer WS₂ using chemical vapor deposition (CVD) on Al₂O₃(0001) substrate and utilized this monolayer WS₂ to deposit ultrathin ZnO using physical vapor deposition (PVD) of ZnO powder. Atomic force microscopy (AFM) investigation reveals that the growth of ZnO initiates at the reactive edge sites and defects of WS₂ flakes and gradually expands laterally over the WS₂ surface. The continued ZnO deposition reveals that the growth follows a two-dimensional (2D) mode rather than vertical accumulation. The as-grown ZnO islands exhibit an uneven surface morphology, primarily due to the low deposition temperature of 90 °C. The 2D growth mode of ultrathin ZnO on monolayer WS₂ is particularly fascinating given the generally inert nature of monolayer WS₂, which typically resists wetting by oxide materials, favoring three-dimensional growth.