Study of transition metal oxide nanostructures for photoelectrocatalytic energy conversion and degradation of pollutants in water

As energy demand and pollution levels continue to rise, increasing attention is paid to green energy conversion and water purification technologies. Among these, photoelectrocatalysis stands out for its potential to produce low-cost hydrogen and degrade pollutants, using as a power source solar light, an abundant renewable energy. The main challenge here is to find efficient, stable, low-cost and non-critical raw materials to be used as photoanodes, as the water oxidation process is the bottleneck in hydrogen production from water. Furthermore, a thorough understanding of the charge transfer processes especially occurring at the material interfaces is essential for optimizing photoanode systems. This project aims to elucidate how specific structural and morphological features of selected Transition Metal Oxide (TMO) photoanodes impact on the functional properties in photoelectrochemical (PEC) applications, i.e. water splitting and degradation of pollutants in water solution. The research also investigates the formation processes of high-aspect-ratio nanostructures resulting from the thermal oxidation of thin metallic films. Additionally, it contributes to developing a consistent model for interpreting intensity-modulated spectroscopic experiments on these systems. The fabrication of the photoanodes was based on the sputtering deposition of metallic films and thermal annealing. The structural and morphological analyses were performed by Grazing Incidence X-ray diffraction (GIXRD) and Scanning Electron Microscopy (SEM). Additional techniques, X-ray Photoelectron Spectroscopy (XPS) and X-ray Absorption Spectroscopy (XAS) were employed for the compositional and structural studies. Electron Paramagnetic Resonance (EPR) experiments were performed to detect specific defects. Electrical properties were evaluated through Impedance Spectroscopy (IS) at the solid state and in solution, while the charge dynamics were studied through Intensity-Modulated Photocurrent Spectroscopy (IMPS), and Intensity-Modulated photoVoltage Spectroscopy (IMVS). This work focuses especially on two systems designed to address the challenges mentioned above: (i) cobalt oxide nanostructures supported on silicon and (ii) Sn-doped iron oxide thin films prepared on conductive substrate. In particular, the mechanism of the formation of Co3O4 petal-like nanostructures from a Co metallic film and thermal oxidation is elucidated. This layer, when coupled with n-Si has shown to work efficiently as a photoanode for sensing and degradation of an organophosphate pesticide. The same photoanode is deeply investigated to understand how the electric field associated with the heterojunction affects the final photoelectrochemical performance in water splitting. In particular, it is shown how defects at the solid-solid interface,which negatively impact the PEC behaviour, can be passivated by annealing in vacuum. The second investigated system, i.e. hematite-based photoanodes was used as a case of study to develop a consistent model to simulate both the steady-state and transient response of the photocurrent and the photovoltage. Finally, an investigation of the correlation between structural order, composition, and final PEC performance of this Sn-doped hematite-based photoanode gives insights into the design of efficient solar-to-energy photoelectrodes.