Date on Master's Thesis/Doctoral Dissertation

12-2011

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemical Engineering

Committee Chair

Sunkara, Mahendra K.

Author's Keywords

Nanowires; Water splitting; InGaN; Strain relaxing

Subject

Nanowires; Photoelectrochemistry; Water--Electrolysis

Abstract

Rising environmental concerns due to our rising population ad energy demand along with our excessive dependence on fossil fuels has created an urgent need to find clean, renewable and carbon free source of energy. Photoelectrochemical (PEC) water splitting is a clean and carbon free process where hydrogen is produced from water and sunlight using a semiconductor. To date, no material has been found that meets the stringent requirements of band gap, band edge positions and stability for spontaneous water splitting. It is however possible to use two materials to meet the criteria. In this regard, InGaN alloys with indium rich composition are interesting materials. However, very little is understood about the synthesis of thick (~200-300 nm), single crystal InGaN layers for PEC applications. Heteroepitaxial growth of InGaN films on planar substrates induces phase segregation due to stress. Here, we proposed to investigate the role of nanowires as strain relaxing substrates to mitigate phase segregation. GaN nanowires with controlled orientation and small diameters were synthesized on various substrates by controlling the temperature and material f1ux to control the nuclei formation. The mechanism to control the growth mode using equilibrium solubility was validated with the III-Sb system. InGaN layers with controlled composition were synthesized on the GaN nanowires in a custom built MOCVD reactor. The InGaN layers are single crystalline, without any phase segregation. It was observed that only nanowires with diameters < 30 nm led to the observation while nanowires with larger diameters (~100 nm) act as planar substrates resulting in polycrystalline growth. The heteroepitaxial growth was observed to evolve from initial InGaN islands coalescing into single crystalline shell on the GuN nanowires. Morphology of the InGaN shells was observed to depend on the orientation of the GaN nanowire substrates with c-GaN nanowires resulting in hexagonal shell while a-GaN nanowires had rectangular shell. We also investigated a novel material system GaSbN using theoretical techniques for its applicability toward PEC water splitting. The electronic structure of GaSbN system with dilute antimony was investigated using theoretical simulations. Results indicate that only very small antimony content (< 10%) is required to achieve the right band gap. Most importantly, the band edges of GaSbN alloy seem to straddle the water splitting potentials that makes it a potential direct water splitting material.

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