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

Abstract

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 Å 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 overall yield of trapped holes can be considerably increased when small platinum islands are present on the TiO2 surface which act as efficient electron scavengers competing with the undesired e-/h+ recombination. While molecular oxygen, O2, reacts in a relatively slow process with trapped electrons (k 2 = 7.6 × 107 L mol-1 s-1), the adsorption of the model compounds DCA- and SCN- on the TiO2 surface prior to the band gap excitation appears to be a prerequisite for an efficient hole scavenging. In the case of DCA- the detailed kinetic analysis of the time-resolved spectroscopic data reveals an extremely good correlation with independent adsorption measurements. Moreover, calculations using the Marcus electron transfer theory for adiabatic processes which result in a reorientation energy λ = 0.64 eV suggest that also in the case of SCN- the hole transfer occurs in the adsorbed state. The competition of DCA- and SCN- for holes has also been analyzed in detail, revealing the extremely complex nature of photocatalytic processes on tiny semiconductor particles.

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