SusChEM: Using theory-driven design to tailor novel nanocomposite oxides for solar fuel production

The nanoscale control of matter and energy remains a defining grand challenge for materials science and related applied disciplines. This research will design and investigate nanostructured oxide surfaces and oxide-oxide interfaces able to undergo controlled redox cycling involving reversible loss of framework oxygen and electron transfer to external reactants under the action of light and heat. Specifically, we will examine nanostructured composite oxides consisting of oxide nanoclusters (<5nm) supported on a second semiconductor oxide surface by a combined computational and experimental approach. These materials will be designed to support and control both photo-driven electron transfer and generation of particular types of reduced metal centers and oxygen vacancies. Materials design rules will be developed to enable particular types of chemical reactivity relevant to energy and sustainability. Because of the enormous complexity of these materials and the reactivity they support, work in this super seed will focus only on C-O and M-O bond forming and breaking behavior on these composite oxide surfaces. Ultimately, we then seek to understand and control the formation of transiently-reduced surface sites tailored to react with C-O containing molecules such as CO2, taking up 2 e- and breaking C-O bonds. Explicitly acknowledging the dynamic nature of the oxide surface and a focus on photo-driven electron transfer into C-O bonds distinguishes the work of this super seed from other groups studying photochemistry on related surfaces. Finally, our predictive design rules will go beyond existing knowledge of bulk materials and large nanoparticles to address effects of the very smallest nanoclusters and their chemical identity, and the support crystal face and chemical identity. This work has five sub-goals, which are elaborated in the planned research activities. Based on input from theory, which itself will be improved by subsequent feedback from experimental materials, we will synthesize supported oxide materials that allow us to understand and control: 1) …the light absorption of the composite material. Distinct from classical doped oxides or dye sensitized systems, supporting nanocluster oxides on a second crystalline support creates new states that narrow band gaps to red shift optical absorption spectra. 2) …the number and location of O vacancies and reduced metal centers arising from illumination into the bandgap, thermal activation, addition of reducing species, and their combination. 3) …the fate of electrons within the material – located at the nanocluster oxide, at the cluster-support interface, or in the support oxide. 4) …the ability to bind and activate (reduce) key molecules of interest for energy and sustainability with a particular focus on CO2. 5) …the ability to heal vacancies on the oxide material using O atoms from an external, ‘soft’ oxidant like CO2, completing the redox cycle and enabling revolutionary new processes for chemical energy storage.