Synthesis of transition-metal polyphosphides.

摘要

This dissertation describes the synthesis of several transition-metal polyphosphides including orthorhombic FeP2, cubic CoP3, cubic NiP2, monoclinic PdP2 and monoclinic CuP 2. The investigation of these materials was initiated by the discovery of MPx formation upon reacting a metal halide with molecular yellow P4 in superheated toluene. When these MPx products were annealed at moderate temperatures (350--500 °C), crystalline phosphorus-rich phases were produced. We found that these phases have been previously synthesized from high-temperature elemental reactions and that low temperature routes to these phase-pure polyphosphides using at least one non-elemental source were not found in the literature. The absence of low-temperature, "bottom up" routes to these materials encouraged us to investigate our initial findings further, as such methods provide unique chemical and structural flexibility in materials synthesis.;Metal-rich counterparts to the aforementioned polyphosphides have been successfully produced via molecular solvothermal reactions. These reports consistently used an excess of phosphorus in their reactions but still afforded metal-rich products, and often produced materials with a combination of metal-rich and phosphorus-rich phases. These routes show the inability to dial in the phosphorus content of the produced MPx phases and so they were unable to use balanced stoichiometry to rationalize the chemistry, and rarely attempted to identify reaction byproducts. This prompted us to design a synthesis in which discrete amounts of a phosphorus reagent could be used to target specific compositions and phases and allow for byproduct identification.;In order to determine reaction byproducts, a metal halide and yellow P4 were reacted in together without solvent in sealed ampoules. The clear liquid byproduct was identified as PCl3 or PBr3 , indicating that stoichiometric and balanced reactions were possible. In these reactions, metal halides and phosphorus (red or yellow) were balanced such that the chloride was ideally removed has PCl3 and any remaining phosphorus was used to form targeted, phase-pure MPx phases. Using this stoichiometry in solid-state reactions, all of the aforementioned polyphosphide phases were synthesized as phase pure products at moderate temperatures (500--700 °C). By pelletizing the metal halide reagents in reactions with yellow P4 or by co-pelletizing the metal halide with red phosphorus, porous pellet products reminiscent of the reagent pellet could be afforded.;The reaction stoichiometries used in solid-state reactions were adapted to solvothermal reactions. In these reactions, amorphous MPx products were synthesized from metal halides and yellow P4 in various solvents (superheated toluene, 1-octadecene and hexadecane), and upon annealing (350--500 °C), targeted phase-pure CoP3, NiP2 and CuP2 were produced. Reactions substituting red phosphorus for yellow P4 in hexadecane reactions also yielded the same crystalline phases.;Solvothermal reactions were modified by the addition of Lewis base surfactants that led to lower reaction temperatures and in certain systems afforded nanoparticles (10--30 nm). In these reactions, a metal acetylacetonate or halide was complexed with a Lewis base, activating the metal halide or acetylacetonate bond and increasing their solubility and reactivity with yellow P4. The Lewis-base surfactants also acted to stunt particle growth. These systems afforded phase-pure NiP2 and CuP2 nanoparticles for the first time. In some cases, crystalline MP2 was produced directly from the solvothermal reaction.