Using Time-Resolved Microwave Conductivity to understand the photocatalytic mechanisms in TiO2 materials

COLBEAU-JUSTIN C.; SORGUES S.; REMITA H.; MENDIVE C.; KUNST M.

Conferencia; 6th European Meeting on Solar Chemistry and Photocatalysis: Environmental Applications (SPEA6); 2010

Research Centre for Nanosurface Engineering

The photocatalytical activity of TiO2 compounds is related to the creation and the evolution of charge-carriers in the photocatalyst. Thus, the knowledge of the relation existing between charge-carrier lifetimes and material structural parameters can help to understand the mechanisms leading to the photoactivity. To follow the charge-carrier dynamics in TiO2, the variation of the sample conductivity after illumination must be determined. Time Resolved Microwave Conductivity (TRMC) is a contactless method, based on the measurement of the change of the microwave power reflected by a sample induced by laser pulsed illumination. The relative change of the reflected microwave power is caused by a variation of the sample conductivity induced by the laser. For small perturbations of conductivity, proportionality between the reflected microwave power and the sample conductivity has been established. The TRMC signal obtained by this technique is called (microwave) photoconductivity, it allows to follow directly, on the 10−9–10−3 s time scale, the decay of the number of electrons and of the holes after the laser pulse by recombination or trapping of the charge-carriers. In the specific case of TiO2, the electrons mobility is much larger than the hole’s one. This implies that the microwave absorbance (the TRMC signal) is due to free mobile electrons in the conduction band and in shallow traps. So the kinetics of holes is observed only indirectly, by their influence on electron kinetics. In this work, various samples of TiO2 powders obtained by different synthesis methods have been tested by TRMC. TiO2 nanoparticles have been elaborated by sol-gel route followed by supercritical drying in CO2 and isopropanol and by hydrothermal synthesis. Surface modified TiO2 has been obtained by wet impregnation with Pt salts and Pt clusters synthesized by radiolytic reduction. Commercial TiO2 nanoparticles provided by Sachtleben, Evonik and Crystal Global companies have also been studied. The photocatalytic activity of all TiO2 samples has been studied by photodegradation of phenol in water. In each case, the relation between elaboration, structure (crystalline structure and microstructure), charge-carrier dynamics (electronic properties) and photocatalytic activity has been investigated. A strong influence of structural parameters on photoconductivity is observed, and a relation between photoconductivity and photoactivity may be evidenced. A complete study has been realized over a series of Cristal Global TiO2 powders prepared at various sintering temperatures: PC10, PC50, PC100 and PC500. Those results show that PC10 outperforms PC500 for phenol photodegradation despite having 30-fold less surface area. Agrios et al. interpret this result pretending that the decrease in the electron–hole recombination rate outweighs the decrease in surface area resulting from the sintering temperature. This explanation has been fully confirmed by the TRMC experiments. It is observed that PC10 presents the highest number of charge-carriers with the longest lifetimes explaining its high photoactivity. The influence on the TRMC signal of phenol and oxalic acid adsorbed at the TiO2 surface has also been studied. The results thus allow drawing some explanations to the mechanisms leading to the photocatalytic activity.