Acceleration of secondary dark reactions of intermediates derived from adsorbed dyes on irradiated TiO2 powders

Time-resolved diffuse reflectance flash photolysis been used to characterize the kinetic behaviors of transients derived from several dyes (eosin Y, thionine, oxazine 725, and nile blue A) adsorbed on titanium dioxide powder (Degussa P-25) suspended in aqueous solutions of various pH. Secondary reaction rates on these powders were compared with those observed on analogous colloidal TiO2 suspensions. Faster secondary reaction rates were observed on the powders, where the reaction surface is pseudo-two dimensional. Neither first nor second order decays were observed; instead, the observed kinetics depended on size distribution of the powder. With N,N,N',N'-tetraphenylenediamine (TMPD), the decay rate of the singly oxidized cation radical TMPD+ was found to depend on water concentration, thus implicating both direct electron transfer (ET) and OH-radical mediated oxidation pathways and complicating the observed kinetics.