Photochemistry of nanostructured materials for energy applications

A number of major disciplines have separately developed as distinct fields of energy research utilizing nanostructure materials: i. Heterogeneous photocatalysis; ii. Photoelectrochemistry-including electrochemical photovoltaic cells; iii. Photochemistry in zeolites and intercalated materials; iv. Photochemistry of thin films and membranes-including self assembled structures; and v. Supramolecular photochemistry. Photophysical properties of small particles, in the angstrom to nanosized regime-depending on specific material, resulting in band gap broadening as compared to bulk properties, and corresponding phenomena with organic dyes as a function of aggregate size having relevance to energy related applications are discussed, as are dielectric confinement effects controlling the geometric distribution of light absorption within a particle, aggregate or adsorbed molecular deposit. Synergism among fields has emerged, as for example with transition metal oxide photocatalysts and photoelectrodes, combined with supramolecular spectral sensitizing transition metal ligand complexes used to harvest light and vectorially transfer photo-generated electrons and holes along selected energetic pathways. Two systems have already demonstrated potential for significantly reducing reliance on fossil fuels and concomitant environmental stress. These are: i. Pollution remediation with wide band gap semiconducting particulate and nanoporous photocatalysts; and ii. Electrochemical photovoltaic cells utilizing nanoporous semiconducting electrodes fabricated by lightly sintering nanosized TiO(2) particulates, followed by spectral sensitization with tri-nuclear ruthenium ligand dyes. Heterojunction contacts between inorganic photoconducting particulates, termed photocatalytic diodes, and three phase systems, termed photocatalytic transistors, have been demonstrated to increase photocatalytic conversion efficiency in catalytic processes and to increase light sensitivity of analogous silver halide photographic systems. Research being carried out in laboratories throughout the world, aimed at improving the efficiency and understanding of the multi-disciplinary processes involved are described. Suggested areas of investigation for achievement of short (similar to 5 years) and long term (5-20 years) goals are reviewed.