Date on Master's Thesis/Doctoral Dissertation

12-2014

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemical Engineering

Degree Program

Chemical Engineering, PhD

Committee Chair

Sunkara, Mahendra K.

Committee Co-Chair (if applicable)

Amos, Delaina A.

Committee Member

Alphenaar, Bruce W.

Committee Member

Starr, Thomas L.

Committee Member

Sumanasekera, Gamini

Subject

Nanowires; Metallic oxides

Abstract

Recently, the use of materials with nanoscale morphologies has expanded, particularly with applications in energy conversion, catalysis, and energy storage. One drawback to single crystalline nanomaterials, especially 1D nanowires, is the difficulty in synthesizing these compounds, as the material chemistry becomes more complicated. Metal oxide nanowires can be synthesized easily and can be produced in relatively large quantities. However, more complex materials, such as metal chalcogenides, cannot typically be synthesized using facile methods, and currently very few techniques involve scalable methods. Phase transformation of metal oxides nanowires to form metal sulfides and selenides by gas-solid reactions is one viable route to scalable chalcogenide production. However, the transformation of 1D materials from oxides to other compositions using gas-solid reactions has not been well studied and the underlying mechanisms involved with phase transformation are minimally understood. A fundamental understanding of how gas phase reactants interact with nanomaterials is critical to not only making new materials on the nanoscale, but also allowing the engineering and optimization of nanomorphologies for functional applications.Iron sulfide and molybdenum sulfide nanowires are chosen as the two model materials to investigate crystal phase transformations in 1D systems. In reaction of Fe2O3 single crystal nanowires with H2S gas to form FeS, the formation of a hollow nanotube morphology is observed, caused by unequal diffusion of the cation and anion, resulting in the accumulation of voids. These findings suggest that the reactant (sulfur) does not diffuse into the nanostructure; rather, iron atoms diffuse outward and react on the surface. The resulting FeS compound also has interesting optical absorption properties that could be of use in solar applications. The conversion of MoO3 nanowires in an attempt to form MoS2 nanowires resulted in a MoS2/MoOx shell/core nanowire morphology, with strong diffusion limits in formation of the sulfide shell. In this case, the sulfur diffusion through the MoS2 shell limits the reaction, leading to core-shell morphology. The MoS2/MoOx shell/core architecture shows considerable activity for electrocatalysis of the hydrogen evolution reaction due to the high surface area architecture for edge plane sites. Chemical intercalation of the MoS2 shell shows a further change in the crystal morphology and an improvement in catalytic activity. Reducing agents (hydrazine) are also investigated in attempts to chemically modify the MoS2 surface, changing the surface charge carrier concentrations. This is the first report of the effect of hydrazine in any chalcogenide system and the hydrazine treated MoS2 nanowire architecture shows one of the best electrocatalytic performance to date. The chemical modifications of MoS2 1D structures suggest the electrocatalytic activity can be tuned and corresponding architectures could be synthesized through phase transformation by design. To further understand oxide to sulfide phase transformations in a generic sense for 1D systems, compounds with differing cationic diffusion rates and kinetics, i.e. tin oxide and zinc oxide single crystal nanowires, are reacted with H2S and the transformation effects studied. Zinc oxide forms a hollow, polycrystalline ZnS structure similar to FeS, but the hollowing and crystallinity is much less defined, highlighting the importance of epitaxial relationships during transformation reaction. The reacted tin oxide forms single crystal tin sulfide branches, while the original oxide core remains unaffected. This morphology is due to the tendency of tin cations to surface diffuse, rather than typical bulk diffusion. These experiments help to demonstrate that crystal phase transformation is more complicated than simply the diffusion rates of cations and anions. In addition, these results suggest some insight to nanomaterial degradation during use in reactive environments.

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