SURFACE MODIFICATION OF TIO2 - CORRELATION BETWEEN STRUCTURE, CHARGE SEPARATION AND REDUCTION PROPERTIES

Surface complexation of colloidal titanium dioxide nanoparticles (40 A ngstrom, anatase structure) by alpha-mercapto-substituted carboxylic a cid results in the formation of a charge transfer complex with an opti cal absorption threshold at 520 nm. It was found by EPR spectroscopy t hat 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. Photogenerat ed holes are transferred to the derivatives at 4.2 K, while photogener ated electrons are trapped on the TiO2 particle. Visible excitation (4 00 nm cut-off filter) results in localization of photogenerated holes on the carboxy group, while UV excitation results in localization on t he 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 le ads 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.