Oxidation-Induced Dispersion of Gold on Ru(0001): A Scanning Tunneling Microscopy Study

With scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) we studied the redox properties of Au islands supported on Ru(0001) as a function of the island thickness. Both the size and the height of Au islands on Ru(0001) can be controlled by the density of the oxygen precoverage on ruthenium and the sample temperature during the deposition of gold. The oxidation of the Au islands at 300 K was accomplished by exposing atomic oxygen produced from a thermal gas cracker. Regardless of the lateral size of the three monolayer (ML) thick Au islands, the oxidation leads to a fragmentation into a number of small particles (3-5 nm) whose arrangement reflects the shape of the former intact Au islands. This oxygen-induced dispersion of Au on Ru(0001) is explained by a shoveling process. Quite in contrast, no fragmentation of the 4-5 ML thick Au islands into smaller entities is observed. Rather, the entire Au island transforms into one big particle. From Au 4f core level spectroscopy we provide evidence that the nanoparticles consist of Au oxide and metallic Au. The Au oxide/Au particles can be reduced by thermal annealing to 670 K under vacuum or by chemical reduction via CO exposure at 670 K, forming again extended Au islands. However, reduction of Au oxide/Au metal particles by CO exposure at room temperature retains the high dispersion of the prior formed nanoparticles.