Facet-Dependent Growth of Pd Particles on the Polar Surfaces of ZnO

Researching the dispersion as well as the stability of metals on the oxide surfaces is essentially important for understanding the interaction mechanisms at the metal/oxide interfaces and thus optimizing the activities of the heterogeneous catalysts. Here in this work, we have investigated the growth behavior of Pd particles on the polar surfaces of ZnO with scanning tunneling microscopy (STM). By systematically varying the coverages as well as the annealing temperatures, we found that at room temperature Pd tends to form three-dimensional (3D) round particles on the ZnO(000 \bar1 ) surface but grow into monolayer two-dimensional (2D) hexagonal islands on the ZnO(0001) surface. During annealing processes, the Pd particles on ZnO(000 \bar1 ) aggregate gently while maintaining the 3D morphology in the mild temperature range but then quickly transform into tall hexagonal islands at elevated temperatures. In contrast, the segregation of Pd islands on ZnO(0001) occurs gradually along with the temperature rising without changing the 2D morphology. These results directly reflect the different interaction strengths of Pd on the distinct ZnO polar surfaces. They also provide important input for understanding the facet-dependent catalytic mechanisms of the Pd/ZnO catalytic systems.