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Date of Award
Stephanie L. Brock
SYNTHETIC LEVERS ENABLING CONTROL OF PHASE, SIZE AND MORPHOLOGY IN TRANSITION METAL PHOSPHIDE NANOPARTICLES (FE, NI)
Advisor:Dr. Stephanie L. Brock
Degree: Doctor of Philosophy
This dissertation study focuses on (1) development of a synthetic strategy to control phase in nanoscale iron phosphides; (2) extension of the developed phase control strategy to the nanoscale nickel phosphide system with simultaneous control on size and morphology and (3) illustration of the enhanced reactivity of nanoscale oxide systems.
A synthetic strategy to control phase in nanoscale iron phosphides was developed to prepare phase-pure samples of Fe2P and FeP. The metal nanoparticle conversion strategy first reported by Schaak and coworkers was selected as a starting point to carry out a detailed study on the effect of various synthetic levers on the phase of the final product. Spherical Fe nanoparticles were prepared by the decomposition of Fe(CO)5 and were subsequently treated with tri-octylphosphine (TOP) at temperatures in the range 350-385 °C to convert the Fe nanoparticles into iron phosphide nanoparticles. Optimized conditions for Fe2P and FeP were arrived at by evaluating temperature, heating time, order of addition of reagents and quantity of `Fe' and `P' precursors. The intrinsic magnetic properties of the nanoscale phosphides were determined by magnetic susceptibility measurements and attest to the purity of the samples.
The phase control strategy was successfully extended to the nickel phosphide system, resulting in the preparation of phase-pure samples of Ni12P5 and Ni2P. In addition, a handle on the size and morphology of both Ni12P5 and Ni2P was achieved by evaluating them as a function of precursor ratios and quantity of oleylamine. Thus, the ability to selectively prepare either Ni12P5 or Ni2P in a range of sizes from a few nanometers to 10's of nanometers, and as either hollow or dense spheres, was achieved. In addition, transformations of metal-rich phosphides (Ni12P5) to more P-rich phosphides (Ni2P) were carried out with retention of morphology (hollow and dense) of the starting product, indicating that topotactic transformations are possible in these systems.
The enhanced reactivity of oxide nanoparticle systems in their transformation to phosphides and sulfides was demonstrated by reactions with TOP and elemental sulfur, respectively. Oxide nanoparticles (NiO, Fe3O4, CoO and Mn3O4) were prepared by the oxidation of salts in a high boiling solvent/surfactant system under a slow and steady air flow. The completely oxidized nanoparticles (< nm) were treated with TOP at elevated temperatures (≥350 °C) under inert conditions to convert them to phosphide phases. Successful transformation was achieved for NiO, Fe3O4 and CoO nanoparticles, generating phase-pure Ni2P, FeP and CoP, respectively. Intriguingly, the method does not work for Mn3O4 nanoparticles, or to bulk oxides (size ≥ 50 nm). Transformation of all oxide nanoparticles (including Mn3O4) into their corresponding sulfide phases is also demonstrated.
Muthuswamy, Elayaraja, "Synthetic Levers Enabling Control Of Phase, Size And Morphology In Transition Metal Phosphide Nanoparticles (fe, Ni)" (2010). Wayne State University Dissertations. Paper 178.