Rethinking the Reactivity of Nanoscale Metal Oxides

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In this project funded by the Macromolecular, Supramolecular and Nanochemistry Program in the Division of Chemistry, Professor James Mayer of Yale University is developing new fundamental understanding of chemical reactions of a variety of metal oxide nanocrystals. Nanocrystals are tiny particles that contain thousands of atoms. While they are much larger than chemical molecules, which contain only a few atoms, they are still much smaller than solid materials that are large enough to see with the naked eye, containing trillions of atoms. Nanoscale materials are unique because they have sizes intermediate between molecules and solids, but sometimes have properties not easily predicted by averaging the two size extremes. Nanocrystals are also unusual in that a large fraction of the atoms are at or very near the surface, so rearrangement, exchange and addition of atoms in the particle with the atmosphere or a surrounding liquid is much easier for a nanocrystal than for a bulk solid in contact with the same gas or liquid, where the atoms are typically inside the solid and not accessible. Professor Mayer is studying particular metal oxide nanocrystals, including titanium dioxide (the pigment in white paint and a primary component of dye-sensitized solar cells), zinc oxide (a UV protectant in some sunscreens), and cerium oxide (a key part of automobile catalytic converters). The PI and his team are performing experiments that elucidate a new way of thinking about chemical processes at the surfaces of these materials. The results from these studies should have broad impact in many fields, since metal oxides are extremely common materials. The broad and fundamental approach provides an excellent training environment for graduate students. The Mayer lab is also sharing with middle and high school students the excitement of developing a new way of thinking about an important area of science, through open houses and hands-on summer programs. Oxidation and reduction reactions at semiconducting metal oxide/solution interfaces are pervasive, from catalysis to the environment. Work in the Mayer laboratory at Yale University, supported by the Macromolecular, Supramolecular and Nanochemistry (MSN) Program at NSF, is developing a new paradigm for such chemical processes at oxide surfaces. Mayer and co-workers are studying the kinetics and thermodynamics of stoichiometric transfers of electrons, protons, and hydrogen atoms to colloidal nanocrystals of ZnO, TiO2, CeO2 and other materials, suspended in organic solvents. Their studies show that current focus on transferring electrons needs to be broadened to include the cations that balance the electron charge. Thus the reaction chemistry is in many cases better described as inner-sphere processes, in which electron transfer is coupled to the making and breaking of chemical bonds. Reactions such as hydrogen atom transfers and multi-electron processes are being examined. Part of this new approach is a rethinking of the thermodynamics of interfacial reactions, building a new thermochemical scale for oxide materials based on their affinities for hydrogen, rather than using electronic band energies. The ubiquity of interfacial redox reactions makes this work of substantial potential broader impact, from energy conversions to corrosion. The broad approach is providing a challenging and stimulating interdisciplinary environment for the scientific growth of a diverse group of graduate students. The excitement of developing a new way of thinking is the basis of the Mayer lab's outreach to middle and high school students, both via open houses and hands-on summer programs.