Charge Carrier Dynamics at TiO2 Particles: Reactivity of Free and Trapped Holes

Details of the mechanism of the photocatalytic oxidation of the model compounds dichloroacetate, DCA-, and thiocyanate, SCN-, have been investigated employing time-resolved laser flash photolysis. Nanosized colloidal titanium dioxide (TiO2, anatase) particles with a mean diameter of 24 A were used as photocatalysts in optically transparent aqueous suspensions. Detailed spectroscopic investigations of the processes occurring upon band gap irradiation in these colloidal aqueous TiO2 suspensions in the absence of any hole scavengers showed that while electrons are trapped instantaneously, i.e., within the duration of the laser flash (20 ns), at least two different types of traps have to be considered for the remaining holes. Deeply trapped holes, h+tr, are rather long-lived and unreactive, i.e., they are transferred neither to DCA- nor to SCN- ions. Shallowly trapped holes, h+tr*, on the other hand, are in a thermally activated equilibrium with free holes which exhibit a very high oxidation potential. The ov...