Surface modification of TiO2: Correlation between structure, charge separation and reduction properties

Surface complexation of colloidal titanium dioxide nanoparticles (40 Angstrom, anatase structure) by alpha-mercapto-substituted carboxylic acid results in the formation of a charge transfer complex with an optical absorption threshold at 520 nm. It was found by EPR spectroscopy that illumination of these surface-modified TiO2 particle, which leads to the charge transfer complex and/or band gap excitation, results in the efficient spatial separation of photoinduced charges. Photogenerated holes are transferred to the derivatives at 4.2 K, while photogenerated electrons are trapped on the TiO2 particle. Visible excitation (400 nm cut-off filter) results in localization of photogenerated holes on the carboxy group, while UV excitation results in localization on the most distant hydrocarbon group of the derivative. Both band gap and CT excitation of TiO2 lead to the reduction of Pb2+ ions to metallic lead (E degrees = -0.126 V vs. NHE), while only band gap excitation leads to the reduction of Cd2+ ions to its metallic form (E degrees = -0.405 V). The EPR signal for trapped electrons disappeared as a result of the reduction of Pb2+ or Cd2+ ions, and metallic lead was observed to precipitate.