(81b) DFT Studies On Adsorption, Diffusion, and Clustering of Cu and Au On the Non-Polar ZnO(10-10) Surface
Catalysts consisting of Cu or Au supported on ZnO surface have been widely used in a variety of hydrogenation reactions, especially in methanol synthesis. Recent experimental studies from our collaborators at Louisiana State University show that Cu grows as metallic 3D clusters preferring to nucleate at  step edges at low coverage, whereas Au initially grows as well-dispersed single-layer islands with nonmetallic character and no preferred nucleation sites below a certain critical coverage. To understand these experimental results, density functional theory (DFT) calculations were performed to investigate the single metal atom adsorption and diffusion, along with the morphology of small Cu and Au clusters on the non-polar ZnO(10-10) surface. DFT results show that Au and Cu bond with Zn and O atoms of the ZnO(10-10) surface, respectively, with the Cu atom binding around ~40% more strongly than the Au atom. Nudged elastic band calculations show Cu adatom diffuses rapidly along the [1-210] direction, while the diffusion along  is hindered by large barriers. For the Au adatom, no anisotropy is observed, with relatively smaller diffusion barriers than found for Cu. These DFT results suggest that the experimentally observed differences in dispersion of Au versus Cu clusters is due to differences in adatom diffusion, where Cu adatoms diffuse preferentially to the  step edge and nucleate while the Au adatoms diffuse and nucleate in a random manner. The nucleation process and the behavior of the metal atoms at the step edges are currently being explored. We also examined small clusters including the dimer, trimer, tetramer, and pentamer. Based on the structure of these small clusters, we generated larger clusters including 10 and 16 metal atoms in a single layer and a double layer configuration. The adsorption energy results for these large clusters verify a preference for Cu to grow in a stacking configuration, while the Au clusters prefer to spread out into 1D islands. These DFT results on the clusters again match well with experimental observations.