Defects Tailoring and Operando investigations of Transition Metal Oxides for water-splitting catalysis

This thesis examines the synthesis, characterization, and performance evaluation of transition metal oxide (TMO) films as catalysts for water-splitting reactions. It focuses on monitoring the electronic and structural changes occurring under realistic working conditions (i.e., operando). In this work, two different families of compounds were investigated: Mo oxides and Fe oxides. Molybdenum oxide-based films were investigated as efficient HER electrocatalysts in acidic and alkaline environments. The sample structure was tailored by controlling the oxygen vacancy concentration and distribution or introducing secondary metallic dopants via post-deposition thermal treatment and interfacial metallic diffusion. A custom-made cell was realized to monitor the sample structure during thermal treatments (i.e., in-situ) with Raman spectroscopy. The structural reconfiguration and oxygen site occupancy are duly investigated with a multi-spectroscopical approach. Samples’ structural dynamics under operando conditions were studied with Raman and X-ray absorption spectroscopy (XAS). The catalytic mechanisms under both acid and alkaline media are discussed. Our results highlight the preponderant role of O1 vacancies for acid HER electrocatalysis and metallic Ni co-catalyst for alkaline HER. Epitaxially grown Maghemite ultra-thin films, doped with different Ni concentrations, were studied as OER photocatalysts in alkaline media. The effect of Ni insertion on the maghemite structure was investigated via XPS and XAS analysis. The dopant’s role in the catalyst’s electrochemical and photoelectrochemical performance for the OER was studied using several electrochemical techniques. To gain insight into the evolution of the electronic structure occurring during the catalytic activity, an EC-XAS cell was realized to carry out X-ray absorption spectroscopy (XAS) measurements under reaction conditions. A novel pump-and-probe setup was also realized to observe even extremely small electronic modulations upon visible light irradiation. The sample’s photocatalytic mechanism and its surface-limited nature are discussed. Our results show the optimization of the maghemite photoelectrochemical performances for a relatively low (0.045 Ni/Fe) Ni dopant concentration. The findings presented in this thesis contribute to deepening the understanding of defect engineering in transition metal oxides. They also offer insights into the electrocatalytic and photocatalytic mechanisms occurring in the investigated systems during water-splitting reactions. The developed experimental setups and methodology could be applied to a wide range of functional materials, contributing to fundamental knowledge and the development of more efficient catalytic materials.