Highlighting Charge Transfer Phenomena in Photo-catalytic Compounds Using Optical and X-ray Techniques

Abstract

The best way to obtain significant results in science is to combine sophisticated experimental techniques and innovative samples. The aim of this thesis is to report an operative method to be used for the characterization of charge transfer phenomena in photo-active samples. This kind of analysis is extremely important for the doped TiO2 samples analyzed. These doped systems are promising photo-catalysts for hydrogen production from water splitting and pollutants degradation. Doping is an efficient method to decrease the energy needed to trigger the excitation of the photo-catalyst, otherwise possible only in the UV range. Identifying the role of dopants when the sample is excited is a mandatory step to understand the effects of dopants on the photo-catalytic properties of the systems. Both X-ray and optical techniques were used to characterize the samples and to follow those charge transfer phenomena induced by a visible excitation. Exploiting the unique characteristics of X Ray Absorption Fine Structure (XAFS) it was possible to determine the local structure of dopants inside the TiO2 matrix. With a differential illumination High Resolution Fluorescence Detected (HERFD) XAFS experiment an atomistic description of the light-induced charge transfer phenomena in V-doped TiO2 systems was obtained. The same physical process was also indirectly highlighted using optical ultrafast transient absorption spectroscopy. With this technique the inter-band transitions triggered by a visible light excitation were followed in real time. The combination of the two kinds of techniques made possible a complete description of the role of V dopants that, injecting charge carriers, lower the energy needed to trigger the photo-generation and surface trapping of charge carriers fundamental for the photo-catalytic processes.