Two-Dimensional Island Growth of ZnO on Monolayer WS₂

The composite photocatalysts exhibit enhanced photocatalytic efficiency due to the synergistic interaction between their constituent components. As such, the design and development of innovative composite photocatalysts represent a crucial step toward achieving efficient hydrogen production utilizing sunlight and water through photocatalysis. Here, we investigate the growth behavior and characterize nanoscale composite photocatalysts synthesized by depositing ultrathin ZnO on monolayer WS₂. To accomplish this, we first synthesized monolayer WS₂ on Al₂O₃(0001) substrate using the chemical vapor deposition (CVD) method. Subsequently, we utilized the monolayer WS₂ as a substrate to deposit ultrathin ZnO using physical vapor deposition (PVD) of ZnO powder.  Atomic force microscopy (AFM) measurements revealed that ZnO nucleates preferentially at the reactive edge sites and defects of WS₂ flakes. Continued deposition demonstrated ZnO spreads laterally over the WS₂ surface in a two-dimensional (2D) growth mode, with no significant vertical accumulation.   Additionally, the surface morphology measurements show that as-grown ZnO islands exhibit an uneven morphology, which can be attributed to the low ZnO deposition temperature of 90 °C. The Raman spectra of the film are consistent with those reported for ultrathin ZnO films in the literature.  The 2D island growth behavior of ultrathin ZnO on monolayer WS₂ is particularly fascinating, considering that monolayer WS₂ is generally chemically inert and tends to resist wetting, which usually promotes three-dimensional growth of oxides.