Iron oxide-based nanostructured films for water splitting

Meeting the escalating energy demands of the 21st century requires sustainable solutions, urging a shift from finite fossil fuels to scalable and renewable sources. Solar energy, a promising contender, demands efficient storage methods due to its intermittent nature. To address this issue, ongoing research is focused on converting sunlight into chemical energy, specifically hydrogen production through water splitting — the reduction of water to H2 and its oxidation to O2. The study presented in this thesis falls within the research scope of photoelectrochemical water splitting, where the reaction is assisted by the application of an external potential and illumination from a light source simulating sunlight. It particularly focuses on the solid-state photoanode where the oxidation semi-reaction occurs, utilizing a hematite-based electrode. For this purpose, hematite (or iron oxide III) attracted interest almost four decades ago thanks to its favourable attributes, such as a suitable bandgap, stability, abundance, cost-effectiveness, and very low toxicity. Nevertheless, its efficiency is hampered by poor optoelectronic features, necessitating research to improve conductivity and charge dynamics, such as controlling the physical composition and morphology in a way that is not yet fully understood. The thesis explores the preparation and characterization of hematite nanostructured films, achieved through the physical vapour deposition of iron on FTO (Fluorin-doped Tin oxide) substrates, followed by thermal treatment in an oxidizing atmosphere. This less common synthesis method provides full control of the formation of Fe oxide, together with the possibility of inducing the formation of iron-oxide nanowhiskers for an increased anode-water interface. Various characterization techniques are employed to conduct the study. Surface sample morphology is studied using Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM), while the composition and crystalline structure are investigated through Raman spectroscopy and Grazing Incidence X-Ray Diffraction (GIXRD). The analysis of optical properties is conducted through absorbance measurements, and finally, the study regarding the ability for water splitting is carried out via photoelectrochemical (PEC) measurements, including current-tovoltage, impedance under dark and illuminated conditions (EIS), and Controlled Intensity Modulated Photocurrent Spectroscopy (CIMPS) technique. Employing this range of experimental techniques reveals that low Fe thickness deposited and higher annealing temperatures contribute to augmented photocurrent values, attributed to the improvement of charge transport, enhanced crystalline order, and potential Sn diffusion from the substrate. Additionally, the research delves into the impedance response of the nanostructured films, unveiling promising correlations with photocurrent performance. The employed equivalent circuit, delineating charge transfer processes with characteristic times, provides insights into the physical processes at the interface between the photoanode and the electrolyte solution. The findings underscore the substantial impact of synthesis parameters on photocurrent performance and suggest a route for the fabrication of Fe-based nanostructured photoanodes with improved efficiency.