Au nanoparticles grown on mildly sputtered Highly Ordered Pyrolytic Graphite (HOPG) surfaces were studied using Scanning Tunneling Microscopy (STM) and X-ray Photoelectron Spectroscopy (XPS). The results were compared with those of Ag nanoparticles on the same substrate. By varying the defect densities of HOPG and the Au coverages, one can create Au nanoparticles in various sizes. At high Au coverages, the structures of the Au films significantly deviate from the ideal truncated octahedral form: the existence of many steps between different Au atomic layers can be observed, most likely due to a high activation barrier of the diffusion of Au atoms across the step edges. This implies that the particle growth at room temperature is strongly limited by kinetic factors. Hexagonal shapes of Au structures could be identified, indicating preferential growth of Au nanostructures along the (111) direction normal to the surface. In the case of Au, XPS studies reveal a weaker core level shift with decreasing particle size compared to the 3d level in similarly sized Ag particles. Also taking into account the Auger analysis of the Ag particles, the core level shifts of the metal nanoparticles on HOPG can be understood in terms of the metal/substrate charge transfer. Ag is (partially) positively charged, whereas Au negatively charged on HOPG. It is demonstrated that XPS can be a useful tool to study metal-support interactions, which plays an important role for heterogeneous catalysis, for example.

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A change of behavior: Deposition of atomic oxygen on Au nanoparticles larger and smaller than about 0.7 nm (as determined by STM, see image) in height results in the formation of oxygen species that exhibit very different oxidation behaviors toward CO at room temperature. X‐ray photoelectron spectroscopy has been used to explain these differences.

Even–odd effects: Oxidation patterns of the mass‐selected Aun (n=2–10) clusters deposited on silica surfaces are reported. An additional Au atom can drastically change the oxidation pattern of the Au clusters. Au5 and Au7 (see picture) are suggested to be highly stable upon oxygen exposure. Formation of AuO bonds is not observed.

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