Date on Master's Thesis/Doctoral Dissertation

5-2017

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)

Starr, Thomas

Committee Member

Starr, Thomas

Committee Member

Fu, Xiao-An

Committee Member

McNamara, Shamus

Committee Member

Spurgeon, Joshua

Author's Keywords

water splitting; III-V; GaSbN; GaSbP; hydrogen production; renewable energy; CVD

Abstract

Direct conversion of solar energy to fuels by photoelectrochemical (PEC) water splitting has been identified as a promising route for sustainable power generation and storage. However, semiconductor materials with suitable opto-electronic properties have yet to be identified. In this dissertation, band gap engineering of III-V alloys, tuning the light sensitivity of GaP and GaN nitride through alloying Sb, is investigated for developing new semiconductor alloys with appropriate opto-electronic properties and their performance with overall water photoelectrolysis. In addition to III-V materials, copper oxide (Cu2O) electrode was investigated for p-type semiconductor material. The new III-V alloy anodes were studied in single-semiconductor PEC cells, as well as in z-scheme PEC cells involving p-Si substrates. Metal organic chemical vapor deposition, halide vapor phase epitaxy and hot filament chemical vapor deposition methods were employed for the synthesis of tunable III-V alloys and wide band gap semiconductors as photoanodes and photocathodes. Synthesis of p-Cu2O/WO3 core-shell nanowire arrays resulted in a five-fold improvement of the photocurrent density compared to TiO2-coated copper oxide nanowire arrays. The deposition of WO3 and CuWO4 reduced the CuO phase impurities in Cu2O leading to considerable enhancement of the photocathode activity in terms of charge separation and stability. Nearly epitaxial films of GaSbxN(1-x) were grown on lattice matching and miss-matching substrates, studying the effect of substrate temperature and metal precursor ratios in the incorporation of antimony. The photovoltages and the very high carrier concentration of nearly epitaxial GaSbxN(1-x) films suggests that un-intentional doping causes a high degree of degeneracy and a metallic character of the electrodes. This helps explain the lack of a depletion region in the epitaxial films of GaSbxN(1-x) due to continuous density of states between the valence and conduction bands.

Un-biased water splitting with 2% solar to hydrogen efficiency under 1.5 AM illumination is reported using GaSbxP1-x. The optoelectronic properties of GaP are modified to study the indirect-to-direct band gap cross-over point in this ternary III-V system. The photoanodes show a photovoltage of 750 mV, photocurrents of 7 mA cm-2 at 10 sun illumination, and corrosion resistance at pH 0. The growth of compact, free-standing films of GaSbxP1-x was acchieved with inexpensive metallic precursors. Experimental results indicate that only photon energies greater than 2.68 eV generate mobile and extractable charges. Here it is demonstrated that a low penetration depth and efficient carrier extraction to the indirect conduction band of the material can be acchieved above the energy level of the hydrogen reduction potential. This observation could potentially challenge the longstanding belief that direct band gap materials outperform indirect ones in regards to absorption. Single semiconductor water splitting can be accomplished by tuning the band structure of GaSbxP1-x through incorporation of antimony into the lattice of GaP. GaSbxP(1-x) was tested with p-type silicon as a wired dual PEC cell. Improved photocurrent with a z-scheme configuration results from enhanced absorption of low energy photons at the p-Si photocathode, and demonstrates the feasibility of using GaSbxP(1-x) in a tandem photoelectrochemical cell.

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