TiO2-based nanocomposites for solar fuel production: Engineering the solid-solid interface for specialized photocatalytic function

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CBET-0829146 Gray There is a large gap between our present use of solar energy and its enormous untapped potential. This is due to a variety of technical challenges, but primary among them is the need to develop highly efficient, photo-active materials for sunlight capture and in this case, conversion to chemical fuels. The aim of the proposed research is to synthesize using dc-reactive magnetron sputtering TiO2-based nanocomposite materials that harvest visible light to drive CO2 reduction, thereby producing energy rich fuels selectively and efficiently. We believe that the key to this goal is to understand the role played by the solid-solid interface of the nano-structured composites. Recent findings in our laboratory reveal a number of surprising insights as to why TiO2 composites tend to display higher photo-activity than pure phase materials and point to the critical role of the solid-solid interface as the location of catalytic "hot spots." Efforts to probe the role of the solid solid interface in photocatalysis are stymied by an inability to synthesize under sufficiently controlled conditions and in sufficient quantities the "interface," which would then allow structural characterization and functional interrogation. We hypothesize that the solid-solid interface of TiO2-based, nanostructured composite materials can be synthesized by magnetron sputtering to yield an optimum combination of optical, electronic and chemical properties to improve solar fuel generation by CO2 reduction.The focus of the proposed work is to interrogate and then, manipulate the critical features of the sputtered solid-solid interface that are fundamental to the high efficiency, visible light photoreduction of CO2 to energy rich fuels such as CH4 or CH3OH. The phase transition occurring at the solid-solid interface of sputtered TiO2 composites may induce changes in the coordination state of Ti4+. The tetrahedral coordinated Ti has been proposed as the catalytic active site in a variety of photoactive materials that catalyze the reduction of CO2. We believe that interfacial tetrahedrally coordinated Ti sites explain the high activity of mixed phase titania powders and have shown this to be the case in both Degussa P25 and sol-gel materials. In this proposed work, we answer the following questions: (1) Is tetrahedrally coordinated Ti formed at the phase interface of oxygen deficient (TiO2-x) anatase/rutile nanocomposites; (2) Is this localized site associated with CO2 photoreduction in our sputtered composite materials and how can we optimize its function under visible light activation; (3) Will sputtered cation substitution of V, Nb or Ta produce bulk electronic and optical modification of our material to yield a red shift in the absorption edge without altering the surface or interfacial catalytic properties of our composites to reduce CO2?Intellectual Merit: This research will explain the structural and functional basis of the photocatalytic activity in TiO2-based nanostructured composites synthesized by reactive magnetron sputtering. The proposed research redirects the investigation of TiO2, widely regarded as well studied materials, but among the best suited for energy and environmental use. We seek to engineer, interrogate and exploit critical features of mixed phase, non-stoichiometric nanocomposites. Our goal is to reproducibly create and identify interfacial defects sites that drive reductive chemistry. The results of our proposed work will produce a continuum of knowledge that links the relationships between synthesis, structure and function, all focused on photocatalytic CO2 reduction to generate energy rich fuels. With this understanding we will be better able to tune catalytic performance for efficient solar energy harvest and storage.Broader Impact: The broader significance of the proposed work is that this deeper understanding will allow us to design a new generation of material systems tailored to energy applications, especially those associated with solar energy harvest, conversion and storage. The proposed work is fundamental, but also has implications to a wide range of applied engineering areas, particularly those related to the development of sustainable, carbon neutral technologies and renewable resources. The proposed work promises to push the design of photoactive materials far beyond where the technology exists today. The research has a strong interdisciplinary nature reaching across the materials, chemical and environmental engineering fields to facilitate advanced teaching and learning among a cadre of faculty, undergraduate, graduate and postgraduate students and the larger community. Undergraduate research opportunities abound as this project connects to the avid interest among our student population in topics related to energy and the environment. This project also provides numerous vehicles for reaching non-science students, middle and high school science teachers and the general public. The PIs have established expertise and ongoing collaboration particularly well suited to the ambitious research program described within this proposal.