All-Inorganic Polynuclear Assemblies for Artificial Photosynthesis

An artificial photosynthetic system for the conversion of carbon dioxide and water to liquid transportation fuel requires the arrangement and coupling of light absorption, charge separation and catalytic units in a nanostructured membrane. The function of the membrane is to provide a physical barrier for separating evolving fuel and oxygen molecules, thereby minimizing back reaction. With the goal of combining the advantages of molecular components with the robustness of inorganic matter, we have developed well defined inorganic polynuclear units for visible light water oxidation and carbon dioxide reduction in inert nanoporous silica scaffolds. The chromophore is an all-inorganic oxo-bridged binuclear unit such as ZrOMn(II) or TiOCr(III) covalently anchored on the silica nanopore surface. These chromophores, which have unusually long excited state lifetimes of microseconds under ambient conditions, act as photon-driven electron pumps. When coupled to oxygen evolving catalysts such as iridium oxide nanoclusters inside the nanopores, efficient oxidation of water is achieved under visible light in neutral aqueous solution. Structural characterization of the units is based on vibrational, EPR, and X-ray spectroscopic analysis, while transient Fourier-transform-infrared and time resolved optical spectroscopy provide dynamic and mechanistic insights into the functioning of the photocatalysts. The molecular nature of the chromophore allows for the precise tuning of the redox properties of the donor and acceptor center by selecting appropriate metals and oxidation states. Such control is the key for optimizing thermodynamic efficiency and directional charge flow within the artificial photosynthetic system. Focusing on Earth abundant materials for reasons of scalability, we have developed cobalt oxide and manganese oxide nanostructured catalysts on mesoporous silica supports that evolve oxygen from water at high rates. The clusters have sufficiently high catalytic activity for developing integrated systems that are able to keep up with the photon flux at high solar intensity. Coupling of the photocatalytic units across nanoscale silica walls with embedded rectifying molecular wires for assuring directional charge flow and separation of evolving oxygen from the reductive catalysis is in progress.  

This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Chemical, Geological and Biosciences of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.