SCANNING-TUNNELING-MICROSCOPY OF CR(III) CHEMISORBED ON ALPHA-FE2O3 (001) SURFACES FROM AQUEOUS-SOLUTION - DIRECT OBSERVATION OF SURFACE MOBILITY AND CLUSTERING

We report scanning tunneling microscope (STM) observations of Cr(III) monomer and oligomer complexes chemisorbed to alpha-Fe2O3 (001) surfaces from aqueous solution. Monomers appear to prefer a particular surface site, one for each surface unit cell. This site is probably associated with an octahedral vacancy in the underlying oxide structure. Monomers were mobile; although ''hops'' from one site to a neighboring site were observed, most commonly adsorbates appeared upon or disappeared from particular surface sites. This probably represents adsorption-desorption coupled with outer-sphere surface diffusion. Activation energies for Cr(III) monomer motion estimated from the observed lifetimes of Cr complexes in surface sites are lower than those for ligand substitution in aqueous Cr(III) ions or its hydrolysis products. This may reflect an artifactual influence of the STM upon adsorbed Cr(III), but could also suggest that Cr(III) ligand exchange is not rate-limiting for Cr(III) adsorption-desorption or surface diffusion. A dimerization event was recorded, and larger surface clusters were observed. Clusters were immobile within the time-scale of our observations.