Underpotential surface reduction of mesoporous CeO2 nanoparticle films

The formation of variable-thickness CeO2 nanoparticle mesoporous films from a colloidal nanoparticle solution (approximately 1-3-nm-diameter CeO2) is demonstrated using a layer-by-layer deposition process with small organic binder molecules such as cyclohexanehexacarboxylate and phytate. Film growth is characterised by scanning and transmission electron microscopies, X-ray scattering and quartz crystal microbalance techniques. The surface electrochemistry of CeO2 films before and after calcination at 500 degrees C in air is investigated. A well-defined Ce(IV/III) redox process confined to the oxide surface is observed. Beyond a threshold potential, a new phosphate phase, presumably CePO4, is formed during electrochemical reduction of CeO2 in aqueous phosphate buffer solution. The voltammetric signal is sensitive to (1) thermal pre-treatment, (2) film thickness, (3) phosphate concentration and (4) pH. The reversible 'underpotential reduction' of CeO2 is demonstrated at potentials positive of the threshold. A transition occurs from the reversible 'underpotential region' in which no phosphate phase is formed to the irreversible 'overpotential region' in which the formation of the cerium(III) phosphate phase is observed. The experimental results are rationalised based on surface reactivity and nucleation effects.