Studies of the charge carrier dynamics and photoelectrocatalytic properties of V-modified TiO2 thin films

This thesis aims to study the modifications induced by vanadium in titanium dioxide, an oxide semiconductor used for photoelectrocatalytic applications, such as water-splitting for hydrogen production and air and water remediation. The modification of this material is necessary to improve its conversion efficiency of light into chemical energy. In fact, its main limitation is the poor visible light absorption due to its wide energy gap (3.2 eV). In this work, V was introduced during the preparation of the sample, either in form of thin films or nanoparticles, using radio-frequency magnetron sputtering and inert gas phase condensation technique, respectively. A structural characterization by X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy showed that V does not induce critical changes in TiO2 matrix; X-ray near edge absorption spectroscopy was used to determine the local environment of V and Ti, revealing that V is substitutional. Femtosecond transient absorbance spectroscopy was adopted to provide the basis for the interpretation of the photoelctrocatalytic behavior of V-modified and unmodified TiO2, used as photoanodes in a photoelectrochemical cell. FTAS revealed that vanadium accelerates electron-hole recombination upon UV irradiation, explaining the lower conversion efficiency in the UV spectral range with respect to unmodified TiO2. In the visible range, for modified samples, FTAS revealed the presence of a transient signal due to free electrons and trapped holes after pumping at 530 nm. These results were supported by the new photoelectrocatalytic activity in the visible range, attributed to a V-induced introduction of intragap levels at ≈ 2.2 eV below the conduction band. Similar results were obtained for thin films and nanoparticle based samples. IPCE spectra showed that incorporation of vanadium in TiO2 extends water splitting in the visible range up to ≈ 530 nm, a significant improvement compared to unmodified TiO2 that is active only in the UV range.