Theoretical investigation of the effect of phosphate doping on the aggregation of Au atoms on an Al2O3 (0001) surface

Using density functional theory, the adsorption of Au atoms and Au dimers onto alpha-Al2O3 (0001) surfaces with various phosphate coverages was investigated. When the phosphate coverage is low (0% or 25%), Au dimers are more stable than isolated Au atoms on the surfaces, and therefore, aggregation of Au atoms into Au dimers is exothermic. In contrast, when the phosphate coverage is high (50% or 100%), isolated Au atoms are more stable than Au dimers on the surface, and Au atom aggregation is endothermic. This difference is caused by differences in the distance between adjacent phosphate groups; at certain distances, the phosphate groups are capable of bridging Au atoms. The Au-Au bond of a Au dimer at a phosphate-phosphate bridge site is destabilized by hole transfer from the Al2O3 surfaces to the bonding sigma orbital of the Au dimer. Isolated phosphate groups cannot cause the destabilization. These results provide a good explanation for the mechanism of the suppression of aggregation of Au particles caused by phosphate doping on an Al2O3 surface, and show the importance of charge transfer from a surface to Au atoms for suppressing Au sintering.