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Research
Research in the Schaak group is driven by synthesis – developing new synthetic methodologies that fill critical gaps in the current “toolbox” of techniques available in the solid-state chemistry and nanoscience communities, and applying these new synthetic tools to important and often applied problems that could benefit from our unique capabilities. In all of our endeavors, we integrate ideas and techniques from solid-state chemistry, solution (molecular) chemistry, and nanoscience, and this allows us to tackle important and often longstanding scientific problems that lie at the interface between chemistry, physics, and materials science. For example, by approaching problems in traditional high-temperature solid-state chemistry with ideas and techniques from solution chemistry and nanoscience, we have been able to avoid solid-solid diffusion as the rate-limiting step in bulk-scale solid-state reactions and nucleate intermetallic compounds and other solids at low temperatures (often with structures not accessible using traditional high-temperature routes). Likewise, inspired by the dramatic and often unexpected changes in physical properties that can occur when solid-state materials are dimensionally confined as nanocrystals, we have been able to synthesize nanocrystals that are exceptionally complex in terms of composition and structure. These approaches are helping to establish a toolbox of reactions for generating well-controlled nanomaterials of complex solids, often of phases that are inaccessible using traditional synthetic strategies. Three of our current projects are described below.
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Nanoparticle Toolkit for the Low-Temperature Synthesis of Solid-State Materials . The rate-limiting step in traditional solid-state reactions is solid-solid diffusion, which generally necessitates high reaction temperatures and usually leads to thermodynamically stable structures. We have been actively developing alternative low-temperature strategies that side-step the diffusion problem. In particular, we have focused on using nanoparticles as highly-reactive synthons for accessing binary and ternary intermetallic compounds of the late transition metals and post transition metals (“Metallurgy in a Beaker”). In addition to establishing the generality of these nanoparticle-directed synthetic methods, we have accessed new compounds, e.g. AuCuSn 2, which are not observed using traditional methods. A key point of this work is that these low-temperature solution approaches provide new variables for influencing the structure of the phases that nucleate, providing a new medium and temperature regime for solid-state synthesis. As such, one of our primary goals is to use these low-temperature techniques to generate new structures and new materials that cannot be made by other methods. Also, this work is generating solid-state “chemical reactions” that convert metal and intermetallic nanocrystals into derivative intermetallics, phosphides, oxides, and sulfides in a predictable manner, and also is helping to establish mechanistic guidelines for accessing new solids and complex multi-metal nanocrystals.
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Synthesis and Self-Assembly of Complex Nanostructures . We are interested in developing and generalizing strategies for accessing size- and shape-controlled nanocrystals of compositionally and structurally complex solids for applications in catalysis, plasmonics, and magnetism, as well as their assembly into hierarchical nanostructures. We have been quite successful using both template and non-template assembly methods to generate nanoparticle superlattices. For example, bi-disperse FePt nanocrystals can be made to self-assemble into AB 2 (AlB 2), AB 5 (CaCu 5), and AB 13 (NaZn 13) superlattice structures. Other recent efforts have focused on rigorous shape control of alloy and intermetallic nanocrystals, which remains largely unexplored yet is important for the previously-described applications. While significant worldwide effort has been focusing on controlling the shape and size of single-metal nanocrystals, it is not clear whether these same strategies will translate to multi-metal systems because of differences in electronegativity, redox potentials, reduction kinetics, and reactivity among different elements. We discovered that if size- or shape-controlled single-metal nanocrystals (for which synthetic methods are well established) are used as templates, diffusion- or redox-mediated conversion reactions can be used to form derivative intermetallics with retention of shape and size dispersity. This project merges our low-temperature solution techniques with methods that are appropriate for controlling the morphology of metal nanocrystals to produce high-quality intermetallic nanocrystals with significantly greater diversity of compositions, structures, and properties than have been achievable in the past.
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Nanoscale Intermetallic Catalysts . The ordered atomic structures of intermetallic compounds (particularly Pt-based systems) are promising targets for highly active and selective catalysis, yet relatively little work has been done in this area because of the lack of appropriate and general methods for accessing intermetallic nanocrystals, which our group has helped to pioneer. We have learned how to synthesize and carefully fine-tune the compositions and structures of intermetallic nanoparticles, including as SiO 2- and Al 2O 3-supported nanoparticles in the catalytically-relevant 3-10 nm size regime. These nanoparticles are tunable, supported, and non-stabilized (e.g. they contain no deliberately-added surface stabilizers), making them ideal catalytic systems. We are exploring these systems as CO oxidation catalysts, which are useful for hydrogen reforming applications, as well as electrocatalysts for fuel conversion reactions that are central to emerging fuel cell technologies. |