Photoelectrochemical Properties of GaN for Solar Hydrogen Gas Generation Applications

This study aimed to investigate the photoelectrochemical properties of GaN for solar hydrogen gas applications. The thin film case is initially considered. In order to understand the effect of the polar crystallographic facets of GaN, photoelectrochemical measurements of free-standing GaN film was investigated. In 1 M HCl and 100 mW/cm2 Xe lamp illumination, the Ga-polar demonstrates a more negative onset potential compared to the N-polar surface. At more positive applied voltages, however, the N-polar shows higher photocurrent. Investigations indicate that these PEC performance profile may be explained by difference in the positions of the conduction and valence band edges of the polar surfaces. Using a simple and inexpensive photoelectrochemical and crystallographic etching process of a GaN thin film, the hydrogen generation efficiency was increased by 100%. Prior to etching, the thin film’s efficiency at the applied bias of 0.5 V versus counter electrode in 1.0 M HCl solution is 0.37%. After etching, the efficiency doubled to 0.75%. After five hours of continuous gas collection, the unetched GaN thin film yielded a stable photocurrent of 0.41 mA cm-2 which produced 0.10 mL of H2 gas. The etched sample, on the other hand, resulted to an improved stable photocurrent of 0.83 mA cm-2 and yielded a greater volume of 0.70 mL of H2 gas, with the presence of H2 confirmed through gas chromatography. The oxidation catalyst cobalt phosphate was also deposited as a thin film on GaN. The cobalt phosphate improved on the oxidation properties of GaN. Furthermore, the deposition of cobalt phosphate improved the photostability of the GaN surface. Finally, GaN nanowires were tested for their photoelectrochemical performance. Nanowires were successfully synthesized through Au-catalyzed thermal reconstruction method on GaN thin film. Photoelectrochemical measurements in 1 M HCl solution under 100 mW/cm2 of Xe light illumination of the nanowire system has shown markedly improved on the performance of the GaN nanowires compared to GaN nanowires synthesized by other methods (thermal chemical vapor deposition and metallo-organic chemical vapor deposition methods). High density nanowires fabricated from the three methods shows those that were synthesized through thermal reconstruction demonstrated photocurrent that was about one order higher than those synthesized through other techniques. The improved photoelectrochemical performance is attributed to the good interfacial contact between the nanowires and the GaN thin film substrate that allows good charge transfer during the photoelectrochemical process, and some improvements in the photoelectrochemical areas. One aspect of the nanowires growth that needed to be addressed, however, is the oxygen content of the sample. Further tests indicate the presence of oxides in the system, which may be core-shell structure with the GaN, or mixed GaN-GaOx system, a N-doped Ga2O3, or an O-doped GaN system