Date on Master's Thesis/Doctoral Dissertation

12-2015

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemical Engineering

Degree Program

Chemical Engineering, PhD

Committee Chair

Sunkara, Mahendra

Committee Member

Starr, Thomas

Committee Member

Fu, Xiao-an

Committee Member

Jasinski, Jacek

Committee Member

McNamara, Shamus

Author's Keywords

GaSbxN1-x; MOCVD; PEC water splitting; III-V alloys; Light absorber; Solar Fuels

Abstract

Solar energy conversion to fuels via photoelectrochemical water splitting is one of the most important technological directions toward meeting the global energy and environmental challenge. However, till date, there are no suitable semiconductor materials available that can absorb visible light, possess right kind of band edge energetics and are stable in aqueous environments. In this work, a new III-V alloy material Ga(Sbx)N1-x with dilute antimony concentration is proposed and developed for photoelectrochemical water splitting. Experimental studies were conducted first to synthesize the proposed alloy materials to understand structure-property relationships and compare them to those obtained using first principles computations. Finally, efforts were made to improve the quality of materials synthesized within the context of improving their photoactivity with water splitting reaction. In general, III-V nitrides have garnered immense interest as suitable materials for solar hydrogen generation due to their tunable band gaps with composition, high carrier mobilities and high absorption coefficients. Computations using first principles density functional theory (DFT+U) revealed that a small amount of Sb incorporation is sufficient to achieve a significant band gap reduction in GaN from 3.4eV to 2eV. Theoretical computations predicted that Ga(Sb)xN1-x alloys with 2 eV band gap straddle the electrochemical redox potentials. The synthesis of dilute GaSbxN1-x alloys is conducted using a custom-built metalorganic chemical vapor deposition reactor. Extensive characterization of the resulting films suggests that there is a large band gap bowing even with small amounts (few percent ~ 2-3%) of antimony incorporation into GaN. In addition, photoelectrochemical characterization confirmed the band edges straddling redox potentials. All the experimental data regarding band gap bowing, lattice expansion and band edge straddling matched very well with the theoretical predictions. Moreover, the alloys with Sb incorporation >7% exhibited indirect band gap transition as predicted by DFT + U calculations. The polycrystalline Ga(Sbx)N1-x thin films were shown to be capable of unassisted water splitting but with low efficiencies. So, two different approaches are investigated to improve the quality of resulting films: thick films with high texture and single crystal quality, Ga(Sbx)N1-x nanowires. The use of a pre-treatment step at 900° C, 40:1 ratio of antimony to gallium precursors and temperatures above 750 °C allowed for good quality crystal growth while allowing for incorporation of antimony. Photoactivity as high as 1 mA/cm2 was obtained. In addition, VLS approach has been demonstrated to obtain high crystalline quality films using copper as catalyst. Vapor-liquid-solid growth experiments using copper particles allowed for tip led growth of GaSbxN1-x nanowires at temperatures beyond 600° C. The antimony composition in the resulting nanowires increased with growth temperature up to 5 at% while improving the quality. Also the photoactivity obtained from nanowires has been increased by two orders of magnitude when compared with polycrystalline films. In summary, a new class of III-V nitride alloys using dilute antimonides is demonstrated to have suitable properties for solar fuels applications but can find other applications.

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