Shape, Morphology, and Phase Transitions during Co Oxide Growth on Au(111)

The surface structure and morphology of cobalt oxide thin films grown on Au(111) were studied using scanning tunneling microscopy (STM) and ultraviolet and X-ray photoelectron spectroscopies. Initial growth in O2 led to 2-D CoO clusters exhibiting a superstructure in STM images characteristic of the lattice mismatch between the rocksalt (111) surface and the substrate. The superstructure governed the CoO cluster shape; as a result, the shapes of clusters containing up to thousands of Co atoms were modulated by the most efficient packing of spheres. Continued growth led to a transition to 3-D, fully oxidized, spinel phase Co3O4 clusters. These Co3O4 clusters were embedded with characteristic “Y” shaped grain boundaries that result from high reactivity toward oxygen at specific edges of the CoO clusters, and the doubling of the periodicity when CoO is oxidized to Co3O4. Exposure of the Co3O4 films to O atoms induced oxidation toward Co2O3, which roughened the surface. Reducing these overoxidized films produced CoO in a 3-D morphology. The results demonstrate how the shape and morphology of Co oxide nanostructures are intertwined with the support, the phase, and the history of the structures.