(337g) Photocatalytic Water Splitting Using Titania Nanoparticles Functionalized with High-Valent Oxomanganese Complexes

The development of efficient photocatalytic systems for solar-energy production of chemical-fuels such as hydrogen remains an open problem of great academic and technological interest, despite significant breakthroughs in studies of solar energy conversion based on dye-sensitized semiconductors (i.e., Gratzel cells) and ultraviolet water photocatalysis (i.e., the Honda-Fujishima effect). In particular, water cleavage by visible light remains a highly prized goal of photoelectrochemical research. We describe a collaborative investigation of the fundamental aspects of hydrogen production by photocatalytic water-splitting based on oxomanganese-functionalized TiO₂ nanoparticles. Among the very few active non-biological catalysts, these robust and inexpensive high-valent oxomanganese complexes are known to be effective catalysts for water-oxidation in homogeneous solution, but have not previously been coupled to a photo-driven oxidant. By efficiently coupling these catalyst's oxygen-evolution at a photoanode to an energy source (e.g., solar light) and an electron sink (e.g., a counter-electrode where electrons are collected through hydrogen evolution), this scheme represents a significant departure from previous efforts in heterogeneous photocatalysis, and offers the opportunity to address structure/function relations critical to achieving the necessary efficiency breakthroughs that will make photoelectrochemical water oxidation an economically viable solar fuel resource. The overall efficiency of the photocatalytic cell for contrathermodynamic fuel-forming reactions depends on six operational steps: (1) photoexcitation of surface complexes; (2) interfacial electron transfer and surface charge separation (i.e., electron injection); (3) surface complex interconversion; (4) catalytic water-splitting at the photoanode; (5) electron transport through the mesoporous TiO₂ film; and (6) reduction at the counter-electrode. Optimizing the efficiency of the overall photocatalytic process thus requires design and understanding of all six operational steps. Synthesis of oxomanganese surface complexes is achieved by a novel stepwise assembly technique, confirmed by spectroscopic characterization and computational studies. Ab initio-DFT molecular dynamics simulations are combined with quantum dynamics propagation of transient electronic excitations in order to characterize the energetics and photoactivation mechanism at the detailed molecular level. Dynamics of electron injection into and transport within the TiO₂ nanoparticles are experimentally investigated by time-resolved terahertz spectroscopy.