44886-AC10
New Structures of Old Elements: Low-Temperature Solution Routes to Metastable Polymorphs
Our ACS PRF Type ACS grant set out to explore the formation of metastable elemental allotropes with target magnetic, electronic, and catalytic properties. Toward that goal, we have uncovered several key results. Earlier we reported that we learned how to routinely and selectively generate nanoparticles of nickel that crystallize in either the fcc or hcp structure types. We also reported the synthesis of B-Sn nanocrystals with controllable shapes and sizes. As the project continues to mature, we have expanded the proposed work into some exciting new areas that fit in with the elemental polymorph theme.
First, we have used the shape-controlled B-Sn nanocrystals as templates for conversion into Sn-based alloys and intermetallic compounds that are of interest for their catalytic, superconducting, magnetic, and electronic properties. In this case, the chemistry that we developed for controlling the shapes and sizes of B-Sn (spheres, rods, cubes) can be applied directly to the generation of shape-controlled intermetallic nanocrystals, which has remained a formidable challenge.
Our earlier work with Ni nanoparticles has led to an unexpected synthetic modification that generates crystalline Ni3B nanoparticles – the first example of a crystalline nickel boride generated directly in solution using standard low-temperature solution chemistry techniques. We have characterized the crystal structure and formation mechanism of these nanoparticles, and are beginning to explore the physical properties (including catalytic activity).
Most exciting has been the extension of the ideas included in the proposal to novel elemental systems that have not previously been explored from a chemical perspective. For example, we learned that shape-controlled indium (In) nanocrystals can be synthesized at room temperature in water and isopropanol using dilute sodium borohydride as a reducing agent for the In3+ metal salt precursor. This is a significant result, because prior syntheses of In nanocrystals required very harsh physical or chemical methods. This “green” technique is superior to previous methods using alkalide/electride reduction, Na reduction, laser ablation, dispersion of molten In in paraffin oil, etc., since it yields shape controlled nanocrystals (spheres, nanowires, octahedra, truncated octahedra, triangles, decahedra). In addition, the nanocrystals are plasmonically active with a UV-Vis absorption band centered around 400 nm, and they are superconducting with significantly higher critical fields than bulk In. This mild green chemistry approach to metal nanocrystal synthesis is currently being applied to the germanium system, which is also challenging to prepare as solution-synthesized nanocrystals.
Building on this, we have begun to synthesize nanocrystals of early transition metal systems, including Mn, W, and Mo, which have been challenging to prepare using traditional methods. We are also using phage display methods to help template metastable elemental polymorphs using biological recognition.
