Improved charge carrier separation in barium tantalate composites investigated by laser flash photolysis

Abstract

Charge carrier dynamics in phase pure Ba₅Ta₄O₁₅ and in a Ba₅Ta₄O₁₅–Ba₃Ta₅O₁₅ composite have been studied by means of diffuse reflectance laser flash photolysis spectroscopy in the presence and absence of an electron donor, in order to reveal the reason for the improved photocatalytic performance of the latter. For the first time the transient absorption of trapped electrons with a maximum at around 650 nm and of trapped holes with a transient absorption maximum at around 310 nm is reported for tantalates. The decay kinetics of the photogenerated charge carriers could be fitted by second order reaction kinetics, and the direct recombination of the trapped electrons with the trapped holes was proven. In the absence of an electron donor, no difference in the decay behavior between the phase pure material and the composite material is found. In the presence of methanol, for the pure phase Ba₅Ta₄O₁₅ the recombination of the charge carriers could not be prevented and the trapped electrons also recombine with the ˙CH₂OH radical formed via the methanol oxidation by the trapped holes. However, in the composite, the electron can be stored in the system, the ˙CH₂OH radical injects an electron into the conduction band of the second component of the composite, i.e., Ba₃Ta₅O₁₅. Thus, the electrons are available for an extended period to induce reduction reactions.