Date on Master's Thesis/Doctoral Dissertation

5-2016

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemical Engineering

Degree Program

Chemical Engineering, PhD

Committee Chair

Sunkara, Mahendra

Committee Co-Chair (if applicable)

Fu, Xiao-an

Committee Member

Fu, Xiao-an

Committee Member

Starr, Thomas

Committee Member

Jasinski, Jacek

Committee Member

McNamara, Shamus

Committee Member

Deutsch, Todd

Committee Member

Menon, Madhu

Abstract

Photoelectrochemical water splitting has been identified as a promising route for achieving sustainable energy future. However, semiconductor materials with the appropriate optical, electrical and electrochemical properties have yet to be discovered. In search of an appropriate semiconductor to fill this gap, GaSbP, a semiconductor never tested for PEC performance is proposed here and investigated. Density functional theory (DFT+U) techniques were utilized to predict band gap and band edge energetics for GaSbP alloys with low amount of antimony. The overall objective of this dissertation is to understand the suitability of GaSbxP1-x alloys for photoelectrochemical water splitting application. Specifically, the goals are to develop synthesis methods, grow GaSbxP1-x alloys, understand their optical and photoelectrochemical properties, and compare experimental values with theoretical predictions. DFT+U calculations suggested that with less than 1% Sb incorporated into GaP, an indirect to direct band gap transition should occur. Furthermore, predictions with band edge positions for GaSbxP1-x alloys with small amount of Sb composition suggest band edge straddling of the water splitting reaction. Preliminary experiments were performed using reactive vapor transport in a microwave plasma reactor. The experiments primarily resulted in growing GaSbxP1-x nanowires. Extensive characterization using electron microscopy and X-ray diffraction and photoluminescence spectroscopy corroborated the predictions using DFT+U calculations. Initial experimentation utilized a plasma transport scheme of Ga and Sb metals with di-tert-butyl-phosphine gas on the reactor to synthesis GaSbxP1-x nanowires. Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD) confirmed ternary alloying of these GaSbxP1-x nanowires. Direct band gaps were observed between 1.7 eV and 2.2eV with GaSbxP1-x compositions between 3% and 6%. However, the method used here is not suitable for growing single crystal films. In order to grow single crystal films on silicon substrates, a new reactor was designed and built for halide vapor phase epitaxy method. Experiments using Halide Vapor Phase Epitaxy (HVPE) reactor were conducted using silicon and sapphire substrates. Experiments using excessive Gallium yielded microwire morphologies. Further experiments with reduced Gallium precursor temperature allowed for growth of quality crystalline films on silicon substrate. The films grown at different temperatures exhibited different amounts of antimonide alloying. The resulting samples exhibited direct band gaps of 1.7 to 2.1 eV evidenced by UV-Vis diffuse reflectance spectroscopy.. Room temperature photoluminescence corroborated these findings. Photoelectrochemical studies of the HVPE grown samples show that they can be highly photoactive materials under the proper growth conditions. The best performing sample had saturated photovoltages of .75 eV and a photoactivity of 8 mA/cm2 under unbiased conditions and 4 suns illumination. This photocurrent saturated to 11 mA/cm2 at 1 V vs. Ag/AgCl external bias. In summary, the work presented here provides fundamental insight into growth and properties of GaSbP alloy samples with low amount of Sb incorporation. The experimental data corroborates predictions by DFT+U technique in terms of indirect to direct band gap transition, band gap as a function of Sb incorporation and band edge energetics for photoelectrochemical water splitting. This work also provides first of its kind use of halide vapor phase epitaxy technique for the growth of GaSbP alloys. Photoactivity data suggests that these materials are highly promising for photoelectrochemical devices.