ELECTROCHEMICAL QUARTZ-CRYSTAL MICROBALANCE STUDIES OF ELECTRON ADDITION AT NANOCRYSTALLINE TIN OXIDE WATER AND ZINC OXIDE/WATER INTERFACES- EVIDENCE FOR BAND-EDGE-DETERMINING PROTON UPTAKE/

Electrochemical quartz crystal microbalance (EQCM) measurements provid e compelling evidence for charge-compensating cation uptake by nanocry stalline SnO2 and ZnO electrodes during electron addition. Comparative light water/heavy water measurements establish that the adsorbed or i ntercalated ions are protons or deuterons. Additional studies as a fun ction of pH implicate water, rather than hydronium ions, as the proton source. The new results, when combined with previous results for tita nium dioxide in nonaqueous electrolytes, suggest that charge-compensat ing cation intercalation is a general mode of reactivity for metal oxi de semiconductors. Finally, the new observations raise significant fun damental questions concerning (1) chemical control of band energetics, (2) possible band-edge-unpinning phenomena, and (3) relationships bet ween band edge energies and driving forces for isolated electron trans fer reactions.