Solar water splitting for hydrogen production: development of photocatalysts based on earth abundant and biocompatible materials (TiO2 and Fe2O3)

Fossil fuels have been critical to the development of modern society, but concerns over pollution, environmental degradation and climate change demand humans transition to renewable sources of energy. Solar energy is, among renewables, by far the largest exploitable resource, providing more energy in 1 hour to the earth than all of the energy consumed by humans in an entire year. The principal problem related to solar energy use is its intermittency. Collecting and storing solar energy in chemical bonds (solar fuel), as nature accomplishes through photosynthesis, is possible through photo-electrochemical water splitting, a clean and sustainable way for hydrogen production. The materials used as photo-electrodes in a photo-electrochemical cell must fulfil a variety of thermodynamic and kinetic requirements to ensure good efficiency and durability. Since there is no material in nature satisfying all these requirements, tailoring the optical, electrical, and morphological properties of the existing materials to construct photo-electrodes with the desired performance is a big task for materials scientists. In this thesis, we study TiO2 based photo-catalysts and Fe2O3 based water oxidation catalysts. TiO2 thin films were deposited by radio frequency magnetron sputtering technique and their optical, electrical and morphological properties were changed to enhance the visible light absorption and/or limit the recombination rate of charge carriers. More specifically, the effect of compensated (V and N) and non compensated (Cu and N) n-p codoping of TiO2 was studied. The role of coupling TiO2 thin films with indium tin oxide films in single and multilayer structures, compact and porous morphologies was underlined. The effect of hydrogen doping in passivating dangling bonds in TiO2 was demonstrated. Fe2O3 nanoparticles assembled coatings were synthesized by pulsed laser deposition and studied for the functionalization of electrodes and absorbers surfaces as water oxidation catalysts. The response of the optical and electrochemical properties of the coating to the tuning of film morphology was studied, ranging from a low-transmittance compact layer to a porous nanoparticle-assembled coating, which resulted to be highly transparent. Materials properties were characterized by various techniques such as Raman spectroscopy, x-ray diffraction, UV-vis spectroscopy, x-ray photoelectron spectroscopy, energy dispersive x-ray spectroscopy, and scanning electron microscopy. Electrochemical and photo-electrochemical properties of the samples were studied by testing them as electrodes in a photoelectrochemical cell. Both materials were chosen because they are widespread, non-hazardous, biocompatible and scalable. This enables the large-scale application of photo-electrochemical water splitting and the full exploitation of the green potential of this technology.