PICOSECOND FLASH SPECTROSCOPY OF TIO2 COLLOIDS WITH ADSORBED DYES

Spectra for electrons trapped in TiO2 have been reported. In this study, kinetic analysis of processes taking place when TiOO colloids are flashed in the presence of three dyes leads to assignment of the spectrum of a trapped hole. Within the duration of a 20-ps pulse at 350 nm, a transient is formed in TiO2 which decays with a second-order rate constant of 2.4 × 10-10 ne s-1, where ne is the number of electrons. The absorbance is probably attributable to electrons in the 'conduction band' (a term that must be used cautiously for these very amorphous systems), and the rate constant measures the rate of hole-electron recombination. Upon addition of a dye that may scavenge carriers, a new transient grows with a rate constant of 5 × 108 s-1. This feature, with an absortion maximum at 630 nm, is attributted to a trapped hole. The mechanism proposed for the results of this intense pulse experiment involves two photons and excitation of both dye and colloid. The evidence includes observation of spectra of reduced dyes and quantitative consistency not achieved with any other model.