Energetics of the nanocrystalline titanium dioxide/aqueous solution interface: Approximate conduction band edge variations between H₀ = -10 and H_- = +26

A reflectance method has been used to assess conduction band edge energies (E_cb) for nanocrystalline TiO₂(anatase) electrodes in contact with aqueous electrolytes. The measurements, which were made over a range of nearly 40 pH units, reveal a Nernstian dependence of E_cb upon pH over most of this range, i.e., a -64 mV shift per unit decrease in log(proton activity) between H₀ = -8 and H_- = +23. Electrochemical quartz crystal microbalance (EQCM) measurements have established that charge compensating proton uptake occurs at potentials negative of E_cb. Uptake occurs over the entire EQCM-accessible pH range (H₀ = -5 to pH = +11). The combined findings are inconsistent with E_cb control solely via surface protonation and deprotonation reactions, whose pK_a's occur in the vicinity of pH 4 and 10. They are consistent, however, with a mechanism whereby: (a) electrochemical generation of Ti(III) trap sites, in the log(proton activity) range from H₀ = -8 to H_- = +23, is accompanied quantitatively by proton intercalation, (b) conversion of the trap sites back to oxidation state IV is accompanied quantitatively by proton expulsion, and (c) the conduction band edge energy is controlled by the pH-dependent trap-based Ti(III/IV) couple. The pH independence found for E_cb above H_- = +23 and below H₀ = -8 is ascribed to an eventual decoupling of proton intercalation from electron addition.