Analysis of Electronic and Photocatalytic Properties of Semiconductor Powders through Wavelength-Dependent Quasi-Fermi Level and Reactivity Measurements

Wavelength-dependent measurements of the quasi-Fermi level of electrons by the Suspension photovoltage method allows locating intra-bandgap states involved in interfacial electron transfer. Employing in these experiments reducing agents with current amplification properties may lead to false results due to the strongly reducing properties of oxidation intermediates. The observation that upon UV excitation holes oxidize water or an other donor instead of relaxing to nearby intra-bandgap states suggests an only weak electronic Coupling between these and valence band states. This interpretation is corroborated by the wavelength-dependent reactivity of the two nitrogen-modified anatase photocatalysts TiO2-N and TiO2-N,C prepared from NH3 and urea, respectively. Although both materials have a very similar energy level scheme, the former is inactive in visible light mineralization of formic acid but the latter is very active. In TiO2-N,C the photogenerated hole can be stabilized through delocalization within an extended aromatic heterocycle component rendering electron-hole recombination less favorable. This hole stabilization is not possible for TiO2-N which contains nonaromatic small amidic or oxidic nitrogen species. As consequence thereof, electron-hole recombination becomes more efficient than interfacial electron exchange and formic acid degradation does not occur.