Access Type

Open Access Dissertation

Date of Award

January 2014

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Chemistry

First Advisor

Stephanie L. Brock

Abstract

This dissertation research is focused on synthesis, characterization and assembly of metal phosphide nanoparticles of relevance to catalysis, magnetism, and optoelectronic applications. These include (1) synthesis of discrete FexNi2-xP ternary nanocrystals of relevance to hydrodesulfurization (HDS) catalysis and magnetism, and (2) 3-dimensional nanostructures (gels and aerogels) of InP and Ni2P.

A three-step solution-phase arrested precipitation method was employed for bimetallic nanocrystal synthesis. The synthesis of nanocrystals involves preparation of amorphous NixPy amorphous particles, introduction of the Fe precursor to form amorphous Fe-Ni-P particles, and high temperature conversion of Fe-Ni-P particles into crystalline ternary phosphide nanocrystals. Ternary FexNi2-xP nanocrystals crystalize in the hexagonal Fe2P-type structure and the morphology of the nanocrystals showed a distinct compositional dependence. With the increase in the amount of Fe incorporation (x≥1.2), nanocrystals transition from a sphere to a rod morphology. Likewise, as the Fe content of the ternary phosphide phase increases, more polydisperse samples are produced. Magnetic measurements of FexNi2-xP (x=1.8, 1.4, and 1.2) phases were carried out to determine the blocking temperature (TB) and Curie temperature (TC). Both TB and TC showed composition-dependence and the highest TB (200 K) and TC (265 K) were observed when x=1.4. Preliminary dibenzothiophene (DBT) HDS activity of mesoporous silica encapsulated (to prevent sintering) FexNi2-xP (FexNi2-xP@mSiO2) was also investigated. FexNi2-xP@mSiO2 (x=0.03, 0.1, 0.2, and 0.3) showed composition dependent catalytic activity. The phase Fe0.1Ni1.9P1.1@mSiO2 showed similar catalytic activity to Ni2P@mSiO2, whereas all other compositions demonstrated lower DBT HDS activities.

The applicability of sol gel nanoparticle assembly routes, previously employed for metal chalcogenides, to phosphides is reported for the case of InP and Ni2P. With respect to InP gelation, two different sizes (3.5 and 6.0 nm) of InP nanoparticles were synthesized by solution-phase arrested precipitation, capped with thiolate ligands, and oxidized with H2O2 to induce gel formation. The gels were aged, solvent-exchanged, and then supercritically dried to obtain aerogels with both meso- (2-50 nm) and macropores (>50 nm); surface areas of ~200 m2/g can be achieved. InP aerogels showed higher band gap values relative to precursor nanoparticles, suggesting that during the process of assembling nanoparticles into 3D architectures, particle size reduction may have taken place. In contrast to metal chalcogenide gelation, InP gels did not form using tetranitromethane, a non-oxygen-transferring oxidant. The requirement of an oxygen-transferring oxidant, combined with X-ray photoelectron spectroscopy data showing oxidized phosphorus, suggests gelation is occurring due to condensation of phosphorus oxoanionic moieties generated at the interfaces. In addition to the oxidative sol gel method, for Ni2P gel formation a second method, metal-assisted gelation was developed. Gelation occurs by cross-linking of pendant carboxylate or thiolate functionalities on surface-bound thiolate ligands via metal ions to yield an interconnected particle network. Metal assisted gel networks were found to have surface areas and pore size distributions similar to those of gels formed by the chemical oxidation method. The method of gel network formation can be tuned by changing the surface ligand terminal functionalities and the nature (oxygen-transferring or non-oxygen transferring) of the oxidant. Both methods produce porous, high surface area materials with thermal stabilities above 400 °C.

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