Conversion Reaction Synthesis: A Versatile Synthesis Pathway to Nanoporous Metals
A scalable, room-temperature synthesis process named conversion reaction synthesis was developed that can spontaneously produce nanoporous metal structures from simple ionic precursor compounds. This process involves reacting an organolithium reducing agent (specifically n-butyllithium) with a metal halide to produce a metal/lithium halide nanocomposite comprised of nanocrystalline phases of the constituent compounds. The lithium halide is then dissolved with a polar solvent, leaving behind a continuous, three-dimensional network of metal filaments that form a pure metal nanoporous structure. This approach is highly versatile and can produce nanoporous metals made from both more-noble metals (Au, Ag, Cu) and less-noble transition metals (Co, Fe, Ni) with pore sizes ranging from 2 to 50 nm with specific surface areas from 1.0 m2/g to 160 m2/g. These nanoporous microstructures can be designed to have a specific pore size by engineering the precursor or by annealing the nanocomposite phase prior to purification. Annealing the metal-lithium halide nanocomposites allowed for controlled growth of the metal phase, the rate of which depends on diffusivity of the metal. Conversion reaction synthesis can also form hybrid nanoporous structures between different metals and between metals and non-metals from co-precipitated nanocomposite precursors. Numerous nanoporous morphologies prepared with conversion reaction synthesis were analyzed primarily using a combination of electron microscopy, nitrogen adsorption analysis and x-ray diffraction to investigate the nanoporous metals’ formation mechanism and to demonstrate the wide capabilities of this method. Conversion reaction synthesis expands our access to additional compositions and microstructures of nanoporous metals.