New Molecules and Nano-materials for Artificial Photosynthesis

The Thesis project has been focused on innovative synthetic systems for artificial photosynthesis. This is a complex photocatalytic architecture that allows the conversion of solar light into chemical energy, enabling water splitting into hydrogen and oxygen under visible light irradiation. With the principal aim of orchestrating physical and chemical interfaces, a great research effort is currently dedicated at the optimization of the envisaged molecular components, including light-antennae, photosensitizers and multi-redox catalysts, as independent building blocks, together with their arrangement within nano-structured environments that define geometry, morphology and surface properties of the resulting photosynthetic system. In this Thesis, novel systems for photocatalytic water oxidation have been investigated, focusing on the design of catalyst-photosensitizers dyads by covalent (Chapter 2) or supramolecular strategies (Chapter 3). A final goal is the integration of these photosynthetic dyads on electroactive semiconductor surfaces, for the development of regenerative photoanodes (Chapter 4). The PhD work has been developed along three main research lines: 1) The synthesis and characterization of a novel covalent dyad based on a Co(II) catalyst and a Ru(II) photosensitizer moiety (E. Pizzolato et al. Phys. Chem. Chem. Phys. 2014, 16, 12000). Combined electrochemical and photophysical studies reveal that photoinduced, redox events involving the two metal centres occur within a short timescale of 15 ps, confirming efficient electronic interactions between the two units and functional water oxidation activity (Chapter 2). 2) The study of a novel supramolecular assembly, combining an organic metal-free bis-cationic perylene bisimide (PBI) photosensitizer with a totally inorganic anionic polyoxometalate (Ru4POM). This latter represents the state-of-the-art of molecular catalysts for water oxidation. This PBI self-assembles in water into 1-D structures and it provides one of the strongest photo-generated oxidant E(PBI*2+/1+) = 2.20 V vs NHE; its combination with Ru4POM is driven by electrostatic interactions and leads to the formation of a 2D porous hybrid architectures with a nano-lamellar sub-structure, alternating organic-inorganic molecular domains. This innovative supramolecular architecture shows: i) an ordered supramolecular structure; ii) fast photoinduced electron transfers (ET) in a 100 ps timescale (in the natural system, ET occur in the 40 µs-1.6 ms range); iii) oxygenic activity under visible light in neutral aqueous solution (E. Pizzolato et al. “Perylene bisimides-oxygenic/polyoxometalates photosynthetic assemblies”, manuscript in preparation) (Chapter 3). 3) The fabrication of composite photoanodes combining the photoactive PBI/Ru4POM nano-hybrid with nanocrystalline tungsten-oxide (nanoWO3) as the semiconductor acceptor layer. The photoelectrode demonstrates catalytic activity upon illumination with visible light (λ > 450 nm) in slightly acidic electrolyte (pH 3), with a maximum photocurrent density of 75 µA/cm2 at 1.20 V vs NHE and an Absorbed Photon to Current Efficiency (APCE) of 1.30%, superior to literature benchmark of 0.8% for PBI-sensitized photoelectrodes with IrO2 as oxygen evolving catalyst (Chapter 4). This result paves the way to further improvement concerning the decoration of the semiconductor surface to boost the photocatalytic performance and to improve the photoelectrode robustness.