Quantized Recombination Mechanism in TiO2 Photocatalysis

SCHNEIDER J.; MENDIVE C.; BAHNEMANN D.

Bangalore

Congreso; Temporally and Spatially Resolved Molecular Science: Faraday Discussion 177; 2015

Indian Institute of Science

In the present study, two techniques have been applied for the detection of charge carriers photogenerated in different TiO2 crystal phases and different particle sizes: Laser Flash Photolysis-Spectroscopy(LFPS) to monitor the reaction dynamics of trapped charge carriers and Time Resolved Microwave Conductivity (TRMC) for the tracking of mobile charge carriers, respectively. Trapped electrons could be detected just in the presence of a hole scavenger evincing that they are not easily trapped at the surface but remain mobile, as the TRMC signals show. In the absence of any hole scavenger all TiO2phases produce two transient absorption bands in the UV region (Lambda1) and in the NIR region (Lambda2), respectively, as detected upon flashing with a 249 nm laser in Air-atmosphere. Since the TRMC results have shown that the photogenerated electrons remain mobile during time scale of the measurements, the transient absorption spectrum is consequently assigned to the trapped holes as the only detectable trapped species.In the present work we suggest that both transient absorption bands arise from all type of holes present in the system, i.e. from shallow and deep trapped holes, as well as from localized and extended holes. We base our interpretation on the fact that bands, and no narrow lines, are detected. Moreover, the usually discussed shallow or deep trapped holes will be here considered as holes which may be trapped at different radial locations within the surface layer. Since the surface of the particles has a thickness ranging from 0.35 nm to several atomic layers, the trapped holes will be highly delocalized over this region. The observed decays of the transient absorption and of the microwave conductivity are assigned to the recombination of the trapped holes with the mobile bulk electrons. Assuming that the trapped holes are entities with quantized energy states, we suggest here that the recombination will proceed via two mechanisms: R1and R2, corresponding to Lambda1 and Lambda2, respectively. In any of these recombination mechanisms the mobile electrons need to have a mobility associated with the respective Lambda-value matching either Lambda1 or Lambda2