Reversibility of photoelectrochromism at the TiO2/methylene blue interface

This paper describes the photoelectrochromic bleaching and recoloration of methylene blue in aqueous or methanolic slurry suspensions of TiO2. Using simple modification of a W-visible diode-array spectrometer setup, the spectral changes undergone by the dye in these solutions were monitored in situ during irradiation of the TiO2 particles and in the dark. A particular focus of this study was a systematic investigation of factors underpinning the chemical and electrochemical (kinetic) reversibility of the dye bleaching-recoloration sequence. Thus, choice of solution pH and hole capture agents is shown to be crucial to the suppression of parasitic chemical and photochemical reactions involving the dye. On the other hand, dye adsorption on the TiO2 surface, the solvent medium itself, and its O-2 content are all important factors in dictating the kinetic reversibility of the photoelectrochromic process. Finally, the practical implications of the findings are presented.