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Abstract:
The influence of an oxygen atmosphere on the growth of Au nanoparticles on Mo-doped CaO films has been investigated by means of low temperature scanning tunneling microscopy. Whereas at ideal vacuum conditions, only 2D Au islands develop on the oxide surface, the fraction of 3D deposits increases with increasing O2 partial pressure until only 3D particles are formed in 10-6 mbar of oxygen. This morphology crossover is attributed to changes in the interfacial charge flow between Mo donors in the CaO lattice and different ad-species on the oxide surface. At vacuum conditions, most of the donor charges get trapped by the incoming Au atoms which therefore experience a stronger bonding to the oxide surface and follow a 2D growth behavior. In an O2 ambience, on the other hand, a substantial fraction of the excess electrons occupies states in adsorbed oxygen species, while the Au atoms remain neutral and return to a 3D growth regime that is typical for non-doped oxide films. Our experiments demonstrate how charge competition between different adsorbates governs the physical and chemical properties of doped oxides, as widely used in heterogeneous catalysis.
