Synthesis of transition-metal polyphosphides

This dissertation describes the synthesis of several transition-metal polyphosphides including orthorhombic FeP_c2, cubic CoP₃, cubic NiP₂, monoclinic PdP₂ and monoclinic CuP₂. The investigation of these materials was initiated by the discovery of MP_x formation upon reacting a metal halide with yellow P₄ in superheated toluene. When these MP_x products were annealed at moderate temperatures (350-500 °C), crystalline phosphorus-rich phases were afforded. Upon further investigation, we found that these phases have been primarily synthesized from high-temperature elemental reactions and that low temperature routes to these phase-pure polyphosphides using at least one non-elemental source were non-existent in literature. The absence of low-temperature, "bottom up" routes to these materials encouraged us to investigate our initial findings further.

Metal-rich counterparts to the aforementioned polyphosphides have been reported on several occasions and have been successfully produced as nanoparticles via molecular, solvothermal reactions. These reports consistently used an excess of phosphorus in their reactions and still afforded metal-rich products, and often times produced materials with a combination of metal-rich and phosphorus-rich phases. These routes showed the inability to dial in the phosphorus content of the produced MP_x phases and did not use balanced stoichiometry to rationalize the chemistry, and rarely identified reaction byproducts. This prompted us to design a synthesis in which discrete amounts of phosphours reagent could be used to target specific phases and in which byproducts could be identified allowing us to propose stoichiometric reactions.

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 and balanced reactions could then be performed 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 MP_x 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, pellet products reminiscent of the reagent pellet could be afforded.

After identifying the PCl₃ byproduct, the stoichiometry used in solid-state reactions was translated to solvothermal reactions. In these reactions, amorphous MP_x 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 which 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, 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.