Thermal Stability of Ultrathin ZnO Layers Deposited on Monolayer WS₂

Nanoscale composite photocatalysts are emerging as promising materials for solar-driven hydrogen production via the photocatalytic process, since the synergistic interactions among their constituents enhance photocatalytic performance. In this work, we investigate the growth behavior and thermal stability of a nanoscale composite photocatalyst comprising ultrathin ZnO layers and monolayers of WS₂. To fabricate the composite, we synthesized monolayer WS₂ on Al₂O₃(0001) substrate by using the chemical vapor deposition (CVD) method and then used this as a substrate for depositing ultrathin ZnO using the physical vapor deposition (PVD) method. Atomic force microscopy (AFM) reveals that ZnO growth follows a two-dimensional growth mode, with as-grown ZnO islands displaying uneven surface morphology. Thermal stability studies indicate that annealing the ZnO film above 400 °C leads to marginally more uniform surface morphology. However, annealing at 550 °C leads to substantial degradation of the composite, which corresponds to the temperature at which monolayer WS₂ becomes thermally unstable. These findings demonstrate that the overall stability of the nanocomposite of ultrathin ZnO and monolayer WS₂ is determined by the intrinsic thermal instability of monolayer WS₂.