The production of solar fuels such as dihydrogen or methanol requires electrons to reduce protons or CO2, respectively. These electrons can be liberated by the photocatalytic oxidation of water. The last few decades have led to a large increase in fast and stable catalysts for the water-oxidation reaction, but their application in photocatalytic systems is limited. During my PhD I studied the mechanisms of photocatalytic electron transfer and water oxidation at the surface of liposomes. Liposomes were chosen as self-assembled scaffolds to perform photocatalysis: they can bring together the photosensitizer and the water-oxidation catalyst, which is critical for the efficiency of any photocatalytic reaction. The addition of liposomes to a photocatalytic system is not innocent. Photocatalytic electron transfer reactions between an electronically excited photosensitizer and a charged electron acceptor occur much less efficiently when the photosensitizer is bound to a charged membrane of the same sign, compared to homogeneous solution. In a photocatalytic system containing an electron acceptor, a photosensitizer, and a water-oxidation catalyst, the rate-limiting step in homogeneous solution was shown to be the electron transfer between the photosensitizer and the water-oxidation catalyst. By bringing these molecules close from each other on the surface of a liposome, the rate of electron transfer between these components is dramatically increased. However, the rate of electron transfer between the electron acceptor and the photosensitizer is decreased and this step becomes rate-limiting. Overall the rate of water oxidation was slightly lower on the surface of liposomes compared to homogeneous systems, but the photoreaction took place for a longer time. Quite unexpectedly, both in homogeneous solution and on the surface of liposomes, the water-oxidation catalyst was found to be quick enough, as electron transfer steps were found to limit the reaction rate, rather than catalytic turnover. From the results of my PhD, it appears that future research should not only be focused on improving water oxidation catalysts, but that more efforts should be put in increasing the efficiency of electron transfer reactions, and in the end, in better integrating the different components of a photocatalytic system.
