Electron transfer at sensitized TiO2 electrodes

Electron transfer from the excited state of tetra-iodo, tetra-chloro fluorescein (rose bengal) to the conduction band of TiO2 has been studied through photoelectrochemical techniques. The measured transfer rate was correlated with information from absorbance and adsorption measurements of this dye molecule on the (001) surface of the single crystals used as electrodes. The quantum efficiency for the photoinjection of electrons was determined to be 4.0×10−3 independent of the pH of the electrolyte and the dye surface concentration. With these data, an argument is given supporting the existence of excitation transfer among the dye molecules on the surface. A temperature study of the electron transfer efficiency yielded an activation enthalpy of 3.2±0.3 kcal/mole with a pre-exponential factor of 1.0. A heat of adsorption of 7.6±0.5 kcal/mole for this dye on TiO2 single crystals was derived from analysis of adsorption isotherms measured at 21.5 and 36.5°C. In addition to the photo-oxidation process, a photoreduction was also observed upon cathodic polarization of the semiconductor. Action spectra revealed participation of solution dye molecules in this reaction; the reduction rate is dependent upon oxygen concentration in solution. It was concluded that the oxidized dye in solution accepts an electron from the TiO2 to generate this current. This photoreduction was found with triphenylmethane and thiazine dyes as well as with other fluorescein derivatives.