Physics & Astronomy ETDs

Darcy Kruse

Publication Date

7-2-2011

Abstract

Hydrogen fuel, derived from clean solar energy, easily stored, and free of greenhouse gas emissions, is one part of a multipronged approach to address future global energy shortages. The purpose of this thesis is to investigate metal core/semiconductor shell nanoparticles that can be illuminated with solar light to produce hydrogen fuel by splitting (reducing and oxidizing) water. Metal nanoparticles corrode in water, so a chemically stable shell needs to be placed over metal nanoparticle cores. Nanoscale semiconductors, which are chemically stable in water, have several advantages over bulk semiconductors. First, their bandgaps can be manipulated depending on their size and shape, which is not possible with bulk semicondutors. Secondly, whereas hot' electrons excited above the bottom edge of the conduction band in bulk semiconductors relax within picoseconds, nanoscale semiconductors have been shown to exhibit slow electron, cooling which allows time for excited electrons to interact with and split water, producing hydrogen. Upon illumination with visible light, electrons on the surface of metal nanoparticles exhibit high energy localized surface plasmon resonance (LSPR), increasing the local electric field 10^14-10^15 times. These hot electrons can tunnel to the shell surface to reduce water and produce H2. Wide-bandgap semiconductor shells absorb ultraviolet light, and the holes produced upon excitation serve to oxidize water. In this way, metal core/semiconductor shell nanoparticles, along with visible and ultraviolet radiation, can be used to split water and produce hydrogen. In this work, various nanomaterials were synthesized, including silver core nanoparticles, mixtures of silver and titanium dioxide nanoparticles, Ag/TiO2 core-shell nanoparticles with spherical, onion-like structures, and Ag/TiO2 core-shell nanowires with bristled surfaces. Absorption measurements were made to determine the silver cores' LSPR energies. Differential pulse voltammetry experiments were performed to determine both semiconductor bandgaps as well as conduction and valence band edges relative to the normal hydrogen electrode (NHE). In order for hydrogen to be produced, the reduction (-0.41 eV vs. NHE) and oxidation (0.82 eV vs. NHE) potentials of water must lie inside the conduction and valence band edges of the semiconductor shell. With differential pulse voltammetry experiments, synthesized nanomaterials were all shown to fulfill that requirement. The optical and electrochemical experiments performed show that the synthesized silver core/metal shell nanomaterials are viable candidates for hydrogen production upon solar illumination.

Degree Name

Physics

Level of Degree

Masters

Department Name

Physics & Astronomy

First Committee Member (Chair)

Pihlstrom, Ylva

Second Committee Member

Koch, Steve

Language

English

Keywords

Water--Electrolysis, Nanoparticles.

Document Type

Thesis

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