Sythesis and Nanoengineering of Gallium Phosphide Nanostructures for Photoelectrochemical Solar Energy Conversion.

Abstract

Gallium phosphide (GaP) is a technically mature material widely used for LEDs with excellent optoelectroinic properties. It is also a good photocathode candidate for H2 and CO2 reduction in photoelectrochemical (PEC) cell. A crucial challenge lies at the center of GaP development in the PEC cell is to improve its efficiency. For thick GaP material its PEC efficiency is generally limited by the low carrier collection, while for thin GaP it is limited by the insufficient light adsorption. An effective strategy to overcome this problem is to use high aspect ratio nanostructures, for which the long axial direction allows sufficient light absorption while the short axial direction improves the carrier collection. This dissertation details synthetic methods to fabricate GaP nanostructures and the photoelectrochemical properties of GaP nanostructures. Uniform GaP nanowires averaging 150 nm in diameter and 20-30 μm in length were synthesized by direct sublimation chemical vapor deposition. The structural properties of GaP nanowires were characterized by a set of techniques, including scanning electron microscopy, transmission electron microscopy, x-ray diffraction, Raman spectroscopy and x-ray photon spectroscopy. The optical and electrical properties of GaP nanowires were further tailored by doping other elements through high temperature post-treatment. In particular, the effects of nitrogen alloying and zinc doping on the photoelectrochemical properties of GaP nanowires were comprehensively analyzed and discussed. The results demonstrate that photoelectrochemical carrier collection efficiency of GaP is remarkably improved by employing nanowire structures. By nitrogen alloying, the supra-bandgap conversion efficiency of GaP nanowires is further enhanced. Finally by zinc doping the electrical properties of GaP nanowires can be effectively controlled. This work has demonstrated GaP nanowires can be viable materials used for PEC system. As compared to other work in this area, this dissertation shows much better photoresponse than the achieved ones. It offers an efficient method to fabricate GaP nanowires as well as strategies to improve their PEC efficiency. Insights from our results and direction for future work are also discussed in this dissertation.