Artificial Photosynthesis: Molecular Approaches for Photocatalytic Hydrogen Production

The humankind today needs to face an epochal transition from a fossil fuel to a renewable source-based economy. Renewable sources are our chance to build a clean world with unlimited and widespread energy. Nowadays renewable energies could be properly harvested to produce electricity, while the development of a future clean fuel is less advanced. Since our energetic consumption is made essentially of fuels we need to build devices to transform renewable energy, such as solar radiation, into chemical energy of bonds. A promising future fuel is hydrogen since its carbon footprint is zero and it can be obtained from an abundant source such as water. Nature, through the photosynthesis, could inspire us to build our feed in the form of fuels. In this research project DSSC (dye-sensitized solar cells) have been modified to produce chemical energy instead of electricity. Attention has been focused on hydrogen production semi-reaction, thus the use of a sacrificial electron donor has been adopted. Such system is composed of TiO2 nanoparticles covered by a reduction catalyst and a metal-free organic sensitizer to harvest the visible spectrum of solar radiation. The aim of this research has been the development of molecular approaches to provide efficient light harvesting systems and reduction catalysts. Molecular design allowed a fine tuning of materials properties and a deep understanding of structure/performances relationships. The first part of the project has focused on designing push-pull structures to harvest visible light portion of solar spectrum. Fine molecular tuning of metal-free dyes afforded enhanced performances depending on the kind of modification. We modified a known phenothiazine dye in the donor, spacer and acceptor units in order to derive structure/performances relationships. Enhanced light harvesting properties and photo-stability have been afforded through π-spacer modification with various mono- and polycyclic simple and fused thiophene derivatives, while decoration of the donor core with glycolic or sugar chains gave better hydrophilicity and surface wettability. Lastly hydroxamic acids have been introduced as alternative anchoring groups to give stronger ester bonds on TiO2 surface and prevent hydrolysis in aqueous media. The second part of the research has concerned the study of cobaloximes as alternative noble metal free reduction catalysts. Starting from a mini-library of cobaloximes bearing various equatorial bridges, axial ligands, and starting oxidation numbers, molecular structure/efficiency studies have been done, while UV/Vis spectroscopy has been used to investigate the nature of the eventual Co(I) species transiently formed. For cobaloximes a Co(I) species is hypothesized but not confirmed in photocatalytic experiments and optimization of efficiency and stability of new catalysts need a deep understanding of the catalytic cycle in order to intervene in the critical intermediates.