Rethinking the Reactivity of Nanoscale Metal Oxides

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Metal-oxide materials are widespread, from rocks in the environment, to components of fuel cells, to ingredients in sunscreens. These materials are increasingly used as very small particles (nanoparticles - particles that are a billionth of a meter in diameter). The behavior and use of nanoparticles often change after adding or removing just a few atoms. In this research project, Professor Mayer's group at Yale University is studying how and why hydrogen (H) and oxygen (O) atoms move onto and out of metal oxide nanoparticles. Hydrogen and oxygen are ubiquitous, as they are in water (H2O), in air (O2) and in hydrocarbon fuels like gasoline. Understanding how metal oxides use or provide H and O could lead to new ideas and perhaps the development of new technologies. The development of young scientists and the engagement of the broader community are intertwined goals of this project. Public lectures are planned and a short classroom program is being developed. Professor Mayer and his team also prepare week-long workshops and month-long internships are being offered. These activities emphasize hands-on and discovery-based activities to engage a diverse group of high school students, especially those from groups currently under-represented in STEM fields. Redox (oxidation/reduction) reactions of metal oxide nanocrystals (MOx NCs) are typically described as outer-sphere electron transfer processes. However, there is increasing evidence that many, if not most, of these interfacial reactions are actually inner-sphere processes, involving the making and breaking of O-H and M-O chemical bonds. Reactions involving O-H bonds of oxide materials can be described as proton-coupled electron transfer or hydrogen atom transfer. Proton-coupled electron transfer to a MOx NC involves protonation of an oxide to a hydroxide and addition of an electron to a band state or trap state. The inner-sphere redox reactivity of a variety of MOx NCs is being studied, including the n-type semiconductors TiO2 and CeO2-x, p-type NiO and metallic RuO2. Oxygen atom transfer reactions are also being explored. These reactions are not yet well established for NC/solution interfaces. The experiments take a molecular approach, utilizing tools from both solution chemistry and materials research. For example, the nanocrystals are reacted with molecular substrates known to undergo hydrogen atom transfer and oxygen atom transfer. The hydrogen atom transfer reactions are providing thermochemical information, which is assembled into a novel hydrogen-atom affinity scale for nanocrystals. The kinetics of the reactions are correlated with the thermochemical driving forces to examine rate vs. driving force scaling relationships. Parallel studies of hydrogen atom transfer and oxygen atom transfer reactions show the connections between these basic reaction types. These fundamental studies of individual reaction steps build a potentially transformative new paradigm for nanomaterial reactivity, rooted in thermochemistry and kinetics.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.