Energetics of the Nanocrystalline Titanium Dioxide/Aqueous Solution Interface: Approximate Conduction Band Edge Variations between H0 = −10 and H- = +26

Abstract

A reflectance method has been used to assess conduction band edge energies ( E cb ) for nanocrystalline TiO 2 (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 H 0 = −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 (H 0 = −5 to pH = +11). The combined findings are inconsistent with E cb control solely via surface protonation and deprotonation reactions, whose p K 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 H 0 = −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 H 0 = −8 is ascribed to an eventual decoupling of proton intercalation from electron addition.