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44886-AC10
New Structures of Old Elements: Low-Temperature Solution Routes to Metastable Polymorphs
Our ACS PRF Type AC grant set out to explore the formation of metastable elemental allotropes with target magnetic and catalytic properties. Toward that goal, we have uncovered several key results. First, we have learned how to routinely generate nanoparticles of nickel that crystallize in either the fcc or hcp structure types. We can selectively access either of these structures, and have learned empirically how the synthetic variables influence the structure. Moving beyond the elements, we have also discovered that a simple (and unintended) modification to the procedure for making hcp Ni nanoparticles generates crystalline nickel boride (Ni3B) – the first example of a crystalline metal boride generated directly in solution using standard low-temperature solution chemistry techniques. We have also spent a lot of time on the Sn and In systems. While we have not been able to generate alpha-Sn (stable below 13 degrees C) as hoped, we have learned how to routinely synthesize nanocrystals of beta-Sn. Importantly, we have learned how to control the morphology of these beta-Sn nanocrystals, providing easy access to nanocubes, nanorods, and nanospheres. These shape-controlled beta-Sn nanocrystals can then serve as precursors for transformation into a variety of shape-controlled intermetallic compounds. For the In system, we learned how to exploit the chemistry (pKa of solvent, solvent polarity, NaBH4 reactivity, etc.) to kinetically control the formation of shape-controlled In nanocrystals. Using these concepts, we have been able to reproducibly generate In nanocrystals with shapes that include spheres, triangles, octahedra, icosahedra, rods, wires, belts, and decahedra. These nanocrystals also have interesting superconducting and plasmonic properties, and will likely be able to serve as templates for conversion into other shape-controlled multi-element nanocrystals. We are currently expanding these efforts to the shape-controlled synthesis of other metal nanoparticles (Sn, In, Bi, Pb, Ge), as well as looking toward other methods for generating metastable elemental allotropes (as proposed). We are also beginning to explore the use of biological templates for the formation of metastable elemental allotropes, e.g. bcc and fcc Ru, fcc Bi, etc.
