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Abstract: MoO3 is an interesting oxide prototype because its catalytic activity is sensitive to the presence and nature of defects. In this work, we demonstrate that we can control the number of defects in single-layer MoO3 nanostructures grown on Au(111) by a thermal reduction treatment. X-ray photoelectron spectroscopy demonstrates the formation of Mo5+ species and oxygen vacancies during annealing at 650K. The percentage of Mo5+ increases with the duration of annealing, until a stable composition containing 50% Mo6+ and 50% Mo5+ is obtained. Surprisingly, the formation of lower oxidation states such as Mo4+ was not observed. The reduced MoO x islands remain one layer high, based on scanning tunneling microscope (STM) images. The two-dimensional nature of the reduced oxide nanocrystals may be due to a large barrier for structural reorganization and, thus, may account for the absence of Mo oxidation states lower than +5. Based on scanning tunneling microscopy images and density functional calculations, we propose that the formation of Mo5+ ions during annealing is not associated with formation of oxygen point defects, but can be attributed to the formation of extended one-dimensional shear defects. These reduced structures are useful for studying the dependence of reactivity on defect type, and present exciting possibilities for chemical sensors and other applications. [Copyright &y& Elsevier]

Abstract: Ultrathin oxide layers can exhibit special behavior by enabling the coupling of structural distortions and charge transfer beyond that allowed in the bulk. In this work, we show from first-principles calculations that ultrathin layers of titania, a prototypical oxide, are active in stabilizing adsorption of O2 on Au overlayers. The adsorbed O2 molecules induce charge redistribution in Au that penetrates to the Au–titania interface, which responds through structural distortions that lower the total energy of the system. We suggest that this effect may be of more general nature and useful in catalysis. [Copyright &y& Elsevier]

Abstract: We report experimental and theoretical investigations of monolayer MoO3 nanocrystals grown on Au(111), in contrast to the bilayered structure of the bulk oxide crystal. The Au surface acts as the other half of the bilayer by satisfying local bonding requirements through charge redistribution at the interface. Epitaxy with the Au lattice is achieved through the ability of the Mo–O bonds to rotate about one another. The oxide layer becomes semimetallic as it strains to enhance bonding with the Au substrate. This flexibility of the oxide lattice suggests the possibility of tuning electronic properties of transition metal oxides via interface interactions. The effects of electronic structure on the surface chemistry of oxides are in turn illustrated by H adsorption energetics on MoO3. [Copyright &y& Elsevier]