Metal oxides-based systems for efficient visible light conversion

Nowadays, the research in the field of renewable energy sources is fundamental, to stem the climate crisis and to overcome the reducing availability of fossil fuels. Thanks to the high magnitude of solar energy a lot of research is focused on efficient methods to convert it into other energy forms (e.g. electric or chemical). One of the most promising methods to convert solar into chemical energy is visible light photocatalysis. The main aim of this thesis is the investigations of systems at the nanoscale that are promising candidates for visible light photocatalysis, and in particular of thin films of cuprous oxide and of cerium oxide combined with plasmonic nanoparticles (NPs). In the latter, cerium oxide has been coupled with NPs because the band gap of the bare oxide is too wide for the absorption of visible radiation, but previous works demonstrated that the formation of heterojunctions by coupling plasmonic nanostructures with semiconductors can greatly enhance the activity of photocatalysts by plasmonic energy transfer from the metal nanostructure to the semiconductor. The first part of the thesis will describe the growth and characterization of these systems, aimed to extract information on their optical properties, with a specific focus on the ultrafast dynamics and temporal evolution of excited states. For this purpose, systems composed by Ag, Au and Cu NPs surrounded by CeO2 have been investigated by means of time-resolved and static absorbance and emission analysis. First, systems composed by Ag NPs with CeO2 have been studied with time-resolved photoemission spectroscopy and free electron laser time-resolved X-ray absorption spectroscopy. Secondly, the ultrafast dynamics of excited states induced by ultra-violet and visible light excitation has been explored in Au NPs combined with cerium oxide, aimed at understanding the excitation pathways, using femtosecond transient absorption spectroscopy. Finally, the last part of the thesis is focused on Cu NPs, also embedded in CeO2 films, or surrounded by oxides, in particular on Cu2O, that, thanks to its band gap in the visible region, is a promising candidate for solar light catalysis. Cu NPs have been investigated in terms of their morphology, optical properties, and stability in air conditions, and a procedure for growing metallic core-Cu2O shell has been developed and investigated. Finally, Cu2O crystals and films of different thickness have been grown and analyzed by means of low energy electron diffraction, scanning tunneling microscopy and photoluminescence spectroscopy in a wide temperature range to obtain information on the behavior of excitons