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 inve stigated employing time-resolved laser flash photolysis. Nanosized col loidal titanium dioxide (TiO2, anatase) particles with a mean diameter of 24 Angstrom were used as photocatalysts in optically transparent a queous suspensions. Detailed spectroscopic investigations of the proce sses occurring upon band gap irradiation in these colloidal aqueous Ti O2 suspensions in the absence of any hole scavengers showed that while electrons are trapped instantaneously, i.e., within the duration of t he 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 neith er to DCA- nor to SCN- ions. Shallowly trapped holes, h(tr)(+), on th e other hand, are in a thermally activated equilibrium with free holes which exhibit a very high oxidation potential. The overall yield of t rapped holes can be considerably increased when small platinum islands are present on the TiO2 surface which act as efficient electron scave ngers competing with the undesired e-/h(+) recombination. While molecu lar oxygen, 02, reacts in a relatively slow process with trapped elect rons (k(2) = 7.6 x 10(7) L mol(-1) s(-1)), the adsorption of the model compounds DCA- and SCN- on the TiO2 surface prior to the band gap exc itation appears to be a prerequisite for an efficient hole scavenging. In the case of DCA- the detailed kinetic analysis of the time-resolve d spectroscopic data reveals an extremely good correlation with indepe ndent adsorption measurements. Moreover, calculations using the Marcus electron transfer theory for adiabatic processes which result in a re orientation energy lambda = 0.64 eV suggest that also in the case of S CN- the hole transfer occurs in the adsorbed state. The competition of DCA- and SCN- for holes has also been analyzed in detail, revealing t he extremely complex nature of photocatalytic processes on tiny semico nductor particles.