Dynamics of semiconductor-to-dye electron transfer for anthracene dyes bound to different sized TiO2 particles

The photosensitization of different sized TiO2 nanoparticles by anthracenecarboxylic acid dye molecules has been studied using transient absorption spectroscopy. These experiments primarily yield information about the semiconductor-to-dye electron transfer reaction. Our measurements show that for TiO2 particles in the 4 to 40 nm size range, the rate of this reaction does not depend on the particles' size. The results were analyzed using a model where the electrons are assumed to be evenly distributed over the surface of the particle. To reproduce our experimental results with this analysis, the scaling parameter that describes the distance dependence of the semiconductor-to-dye electron transfer reaction must be > 180 nm. We do not consider this to be physically reasonable. An alternative explanation is that the injected electrons are in localized trap sites that are spatially close to the dye radical cation, i.e., they are not randomly distributed over the particle surface. The observation of single exponential decays in the transient absorption data implies that these trap sites have a narrow energy distribution.