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.
More solar energy hits the earth surface in an hour than is consumed globally in one year. Yet, despite the fact that solar energy is the largest energy resource on earth, its use remains limited primarily due to the challenge of developing highly efficient, photo-active materials for sunlight capture and storage. The overarching goal of this research was to improve the photocatalytic performance of novel nanoarchitectures of TiO2-based composite materials for energy production or energy efficiency applications. Our approach was to investigate how synthetic conditions create unique structural features that in turn improve the chemical reactivity of the materials. We interrogated the relationship between synthesis, surface features, and reactivity for three types of TiO2 nanomaterials. We assessed chemical reactivity by testing the materials’ ability to produce solar fuels (photocatalytically reduce CO2 to high energy products such as methane) or to improve energy efficiency (photocatalytically oxidize acetaldehyde, a indoor air pollutant, the buildup of which hinders air recycling in buildings and airplane cabins). Using physical vapor depostion (PVD; dc reactive magnetron sputtering) we fabricated titania thin films that displayed strong visible light absorbance by introducing niobium (Nb) into the anatase and rutile crystallites. These materials, however, failed to show improved chemical reactivity. Although Nb-substitution extended the range of light that activated the catalyst, it also created recombination centers that hindered charge transfer at the catalyst surface. In a rigorous use of X-ray diffraction (pole figure goniometry), we identified an optimum set of synthesis parameters that controls the important surface features of sputtered titania thin films. We demonstrated that the high photo-activity of these materials was associated with preferred orientation of the crystal facets in the films (Fig 1 & 2). We investigated two ways to make titania nanotubes (TiNT). High aspect ratios (length to diameter) impart large surface areas to TiNT. Using anodic oxidation synthesis we controlled the pore size and tube length of TiNT arrays to maximize their reactivie surface area. Post-synthesis anneal temperature was the most important factor for controlling TiNT crystal phase composition and was a major influence on reactivity. At temperatures higher than 500 °C, the nanotube structure begins to collapse due to phase transition from anatase to rutile, but reactivity with CO2 is enhanced. We attribute this to the creation of novel active sites located at the solid-solid interfaces of anatase and rutile crystals. We also demonstrated that calcination of hydrothermally synthesized TiNT powders allowed us to tune material structure and reactivity. Post-treatment temperatures above 500 °C collapsed the nanotubes to create titania nanorods (TiNR). TiNR synthesized in this way bear a strained and high energy crystal orientation or alignment that likely promotes enhanced chemical reactivity. Finally, a structural feature of TiO2 nanomaterials that exerts a major influence on reactivity with CO2 is the undercoordination of surface titanium. In order to probe its role on photocatalytic reaction, we must develop ways to synthesize materials that contain high and quantifiable densities of this structure. Using solgel synthesis on silica supports and by selecting the titanium precursor, we have developed a library of structures ranging from isolated undercoordinated titania sites to TiO2 clusters of varying size (Fig. 3). This continuum of materials allows us to control the amount and the effect of the neighboring structures so that we can systematically elucidate how this structure influences the mechanism of CO2 photochemical reaction. We have also developed a chemical method with which to quantify the undercoordinated sites and thereby, provide rigorous comparison of reactivity between materials. The intellectual merit of this research is associated with deeper understanding of how to engineer efficient photo-active materials for energy applications. This work is predicated on evidence revealing that certain structural defects formed at the solid-solid interface of TiO2 composite materials are likely responsible for the high activity of these photocatalysts. Yet, it is not possible to identify the sites and their role in the CO2 reaction mechanism without the ability to control material synthesis and to characterize the structures rigorously. In this research we have done both. The broader impact of this work is that it makes an important contribution to the fundamental and practical goals of assembling the ensemble of surface sites tailored to the photocatalytic conversion of CO2 and reducing the heterogeneity of surface sites that undermines reaction efficiency. The results of this research inform the rational design of a new generation of materials that one day will allow us to convert CO2 into fuels using sunlight. The research is strongly interdisciplinary reaching across materials, chemical and environmental engineering to facilitate advanced teaching and learning. This research produced four published manuscripts, with another in review and provided funding for one post-doc, two Ph.D. students, two MS students, five undergraduates, three of whom are now in top Ph.D. programs in either Chemistry or Chemical Engineering and three high school students.
