Electrochemical, spectral, and quartz crystal microgravimetric assessment of conduction band edge energies for nanocrystalline zirconium dioxide/solution interfaces

Electrochemical quartz crystal microgravimetry studies of porous nanocrystalline ZrO2 electrodes in acetonitrile containing 1 M LiClO4 show that surface electronic states can be accessed at potentials as far positive as 0 V versus Ag/AgCl, as evidenced by uptake of charge-compensating cations. A much higher density of surface states is encountered beginning at about -1.3 V. Based on previous work with TiO2. SnO2, and ZnO, this potential is tentatively identified with E-cb for ZrO2 and is about 0.5 V more negative than E-cb for TiO2. In water, cation uptake is replaced by efficient reduction of H3O+ or water to hydrogen, a finding that has interesting parallels in radiation chemistry. Identifying the onset potential for hydrogen evolution with either E-cb or a potential characteristic of a high density of trap states, the value obtained is about 0.3 V negative of E-cb for TiO2. Like the conduction band edge energy for titanium dioxide, the putative E-cb value for ZrO2 shifts negatively with increasing pH. Comparisons of surface-based ligand-to-metal charge-transfer band energies point to an Ecb value for colloidal ZrO2 in water that is about 0.4 V negative of the value for colloidal TiO2. Consistent with three recent literature reports, empty states should he low enough in energy to permit efficient injection from photoexcited dyes under certain conditions. (C) 2004 Elsevier B.V. All rights reserved.