Morphology, Dispersion, and Stability of Cu Nanoclusters on Clean and Hydroxylated (α-Al2O3(0001) Substrates

Using high-resolution dynamic scanning force microscopy (SFM) operated in the noncontact mode, we investigate here the detailed morphology, dispersion, and thermal stability of nanometer-sized three-dimensional Cu clusters on an alpha-Al2O3(0001)) substrate. We systematically study the effect of surface hydroxylation by depositing metallic Cu either on an ultrahigh vacuum prepared Al-terminated alumina surface or on surfaces prehydroxylated/hydrated by exposure to varying doses of H2O. Three-dimensional growth of Cu nanoclusters dispersed evenly on the surface is observed independent of surface preparation. As the top facet of the nanoclusters appears hexagonal in SFM images, we conclude that they are regular Cu crystallites in the equilibrium form. For strongly hydroxylated surfaces, however, a reduced aspect ratio (height-to-width) of the nanoclusters indicates an increased wetting of the Cu. We propose that Cu bonding to the hydroxylated surface is enhanced by the formation of Cu-O-Al bonds. Surprisingly when heating the surface, Cu remains dispersed on the surface, but it is observed that the total coverage of Cu on the surface is rapidly reduced when the sample is heated to temperatures above 450 degrees C degrees for clean and 550 T for hydroxylated surfaces. Our results unexpectedly show that Cu is desorbed from the similar to 3 nm wide Cu nanoclusters at temperatures well-below the melting temperature of bulk Cu, and the sublimation is completed before the onset of sintering or ripening of the Cu nanoclusters under the conditions of our experiment. Our observations thus reveal significant changes in the cohesive energies of nanometer-sized Cu clusters, which is an effect that should be taken into account when modeling the stability of Cu crystallites, for example, with regard to sintering of Cu-based catalysts.