Energetics of the Nanocrystalline Titanium Dioxide/Aqueous Solution Interface: Approximate Conduction Band Edge Variations

A reflectance method has been used to assess conduction band edge energies (Ecb) 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 Ecb 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 Ecb. Uptake occurs over the entire EQCM-accessible pH range (H0 ) -5 to pH ) +11). The combined findings are inconsistent with Ecb control solely via surface protonation and deprotonation reactions, whose pKa’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 Ecb above H) +23 and below H0 ) -8 is ascribed to an eventual decoupling of proton intercalation from electron addition.