Photosensitized electron injection in colloidal semiconductors

Abstract

Photosensitized electron injection in semiconductor particles was studied by using aqueous eosin (EO)/colloidal TiO/sub 2/ as the model system. Adsorption of EO onto the surface of TiO/sub 2/ occurs quantitatively at pH <6 and leads to red-shifted absorption and emission spectra. Electron injection in the conduction band of the TiO/sub 2/ particles takes place from the S/sub 1/ state of adsorbed eosin and occurs with a rate constant of 8.5 x 10/sup 8/ s/sup -1/ and a quantum yield of 38% (pH 3). The quantum yield decreases at high eosin occupancy of the particles due to concentration quenching. Back-electron transfer occurs via a rapid intraparticle reaction between EO/sup +/ e/sub CB//sup -/ pairs associated with the same TiO/sub 2/ host aggregate and via a slower process involving bulk diffusion. The rate constant for intraparticle recombination is 2 x 10/sup 5/ s/sup -1/, i.e., 4 x 10/sup 3/ times slower than that for electron injection. This enables slight-induced charge separation to be sustained on a colloidal TiO/sub 2/ particle for several microseconds, which is sufficient to trap the electron by a noble metal deposit. Implications for light energy conversion devices are discussed.