Surface State Trapping and Mobility Revealed by Junction Electrochemistry of Nano-Cr(2)O(3)

Hydrous chromium oxide nanoparticles (similar to 15 nm diameter) are assembled from a colloidal solution onto tin-doped indium oxide (ITO) substrates by layer-by-layer electrostatic deposition with aqueous carboxymethyl-cellulose sodium salt binder. Calcination produces purely inorganic mesoporous films (average thickness increase per layer of 1 nm) of chromia Cr(2)O(3). When immersed in aqueous carbonate buffer at pH 10 and investigated by cyclic voltammetry, a chemically reversible oxidation is observed because of a conductive layer at the chromia surface (formed during initial potential cycling). This is attributed to a surface Cr(III/IV) process. At more positive potentials higher oxidation states are accessible before film dissolution. The effects of film thickness and pH on voltammetric responses are studied. X-Ray photoelectron spectroscopy (XPS) evidence for higher chromium oxidation states is obtained. ITO junction experiments are employed to reveal surface conduction by Cr(III/IV) and Cr(IV/V) 'mobile surface states' and an estimate is obtained for the apparent Cr(III/IV) charge surface diffusion coefficient D(app) = 10(-13) m(2) s(-1). The junction experiment distinguishes mobile surface redox sites from energetically distinct deeper-sitting 'trapped states'.