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

The photosensitization of different sized TiO2 nanoparticles by anthracenec arboxylic acid dye molecules has been studied using transient absorption sp ectroscopy. These experiments primarily yield information about the semicon ductor-to-dye electron transfer reaction. Our measurements show that for Ti O2 particles in the 4 to 40 nm size range, the rate of this reaction does n ot depend on the particles' size. The results were analyzed using a model w here the electrons are assumed to be evenly distributed over the surface of the particle. To reproduce our experimental results with this analysis, th e scaling parameter that describes the distance dependence of the semicondu ctor-to-dye electron transfer reaction must be > 180 nm. We do not consider this to be physically reasonable. An alternative explanation is that the i njected electrons are in localized trap sites that are spatially close to t he dye radical cation, i.e., they are not randomly distributed over the par ticle surface. The observation of single exponential decays in the transien t absorption data implies that these trap sites have a narrow energy distri bution.