Nanostructures templated on biological scaffolds for light harvesting, energy transfer, charge transfer, and redox reactions

Abstract

Solar energy provides an unparalleled promise to generate enormous amounts of clean energy. As the solar industry grows rapidly with a focus on power generation, new, but equally important challenges are emerging, including how to store and transfer the generated solar energy. Light-driven water splitting to generate hydrogen has received increasing attention as a means of storing solar energy. However, in order to evolve hydrogen with no energy input beyond sunlight, it is important to develop a stable and efficient catalytic system for water oxidation, which is the more challenging half-reaction of photocatalytic water splitting. Over several billion years, cyanobacteria and plants have evolved highly organized photosynthetic systems for the efficient oxidation of water. Water oxidation by mimicking photosynthesis has been pursued since the early 1970s; however, the approaches have been primarily limited to the extraction and reconstitution of the existing natural pigments, photosystems, and photosynthetic organisms, which suffer from instability. Metal oxide catalysts, often coupled with pigments, are similar to the reaction centers in natural photosystems and have been shown to photochemically oxidize water. Unfortunately, various approaches involving molecular design of ligands, surface modification, and immobilization still show low catalytic efficiencies unless they are used under relatively harsh conditions (i.e., in highly alkaline or acidic solutions under ultraviolet radiation). The current work aims to demonstrate the impact of nano-scale assembly of organic and inorganic molecules on energy and charge transfers, and related redox reactions. Genetically modified M13 viruses are explored as biological scaffolds to guide the formation of metal oxide catalysts-pigments hybrid nanostructures that enable efficient transports of both energy and electrons for photochemical water oxidation. This dissertation deals with three aspects of the virus-templated nanostructures - photonic, photochemical, and electrochemical properties. First, organic pigments are arranged into a one-dimensional light-harvesting antenna on the M13 virus. Chemical grafting of zinc porphyrins to the M13 virus induces spectroscopic changes, including fluorescence quenching, the extensive band broadening and small red-shift of their absorption spectrum, and the shortened lifetime of the excited states. Based on these optical signatures a hypothetical model is suggested to explain the energy transfer occurring in the supramolecular porphyrin structures templated on the virus. Second, through further genetic engineering of M13 viruses, iridium oxide hydrosol clusters (catalysts) are co-assembled with zinc porphyrins. When illuminated with visible light, this system evolves about 100 oxygen molecules per surface iridium molecule per minute in a prolonged manner. In addition, porous polymer microgels are used as an immobilization matrix to improve the structural durability of the assembled nanostructures and enable the recycling of the materials. The system also maintains a substantial level of its catalytic performance after repeated uses, producing about 1,200 oxygen molecules per molecule of catalyst during 4 cycles. These results suggest that the multiscale assembly of functional components, which can improve energy transfer and structural stability, should be a promising route for significant improvement of photocatalytic water oxidation. Lastly, electrochemical properties of the virus-templated iridium oxide nanowires are examined as an electrochromic film on a transparent conductive electrode. The prepared nanowire film has a highly open porous morphology that facilitates ion transport, and the redox responses of the nanowires are limited by the electron mobility of the nanowire film. These results demonstrate that a bio-templating approach provides a versatile platform for designing complex nanostructures that can facilitate the transport of electrochemical molecules in a broad range of photoelectrochemical devices.