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 TiO ₂, 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 ZrO ₂ shifts negatively with increasing pH. Comparisons of surface-based ligand-to-metal charge-transfer band energies point to an E_cb 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 lie low enough in energy to permit efficient injection from photoexcited dyes under certain conditions.