Solution-mediated strategies for synthesizing metal oxides, borates and phosphides using nanocrystals as reactive precursors
Henkes, Amanda Erin
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Because of their high surface area (and hence, increased reactivity) nanocrystals can be used as reactive precursors in the low-temperature synthesis of solid state materials. When nanocrystals are used as reactants, the temperatures needed for diffusion between them can be significantly lower than for bulk-scale reactions?often at temperatures attainable using solution-based techniques. In the following work, two synthetic strategies are defined and developed for accessing metal oxides, borates and phosphides using nanocrystalline precursors and solution-mediated techniques. Broadly, the strategies involve either 1) the formation of a nano-sized precursor in solution which is post-annealed after isolation to form a target metal oxide or borate or 2) the solution-mediated diffusion of phosphorus into a nanocrystalline metal to form target metal phosphides. To form multi-metal oxides using the first strategy, metal oxide nanoparticle precursors are mixed in stoichiometric ratios in solution to form a nanocomposite. After isolation, the nanocomposite is annealed in air at 700-800 ?C to form target ternary metal oxides, including Y2Ti2O7, Eu2Ti2O7, NiTiO3, Zn2SnO4 and CuInO2. As a variation of this method, rare earth borate nanoparticle precursors can be formed in solution by the reaction of RE3+ with NaBH4. After isolation, annealing in air at 700-800 ?C crystallizes a range of REBO3 and Al3RE(BO3)4 powders. Using solution-based techniques, metal phosphides can be formed by the reaction of pre-formed metal nanocrystals with trioctylphosphine (TOP), which acts as a mild phosphorus-source, at 300-370 ?C. A range of transition metal phosphide nanocrystals are accessible using this strategy, including the polyphosphides PdP2, AgP2 and Au2P3. Furthermore, shape and size of the metal phosphide product can be influenced by the shape and size of the metal precursor, allowing for the templated-design of nanostructured metal phosphides. The utility of this technique is not limited to the nano-regime. Bulk-scale metal powders, wires, foils, thin films and nanostructured metals can be converted to metal phosphides using analogous reactions with hot TOP. The two-fold purpose of this work is to extend these solution-mediated nanocrystal-based synthetic strategies to new classes of materials, and to compliment the existing library of low-temperature methods for making solid state materials.