A nanoporous GaN photoelectrode on patterned sapphire substrates for high-efficiency photoelectrochemical water splitting

Abstract The photoelectrode of highly ordered nanoporous GaN on patterned sapphire substrate (PSS) is exploited to address issues of optical absorption and photocarrier separation efficiency in solar water splitting. The introduced PSS reduces the threading dislocation (TD) density and defects of the GaN epilayer, resulting in the improved crystalline quality, which suppresses recombination of electron-hole pairs. A single-step top-down etching approach is developed to fabricate the nanoporous GaN using an anodic aluminum oxide (AAO) mask conveniently and economically. The highly ordered nanoporous morphologies of AAO membrane are well transferred into the GaN surface grown on PSS via inductively coupled plasma (ICP) dry etching. Surface nanostructuring significantly increases surface-volume ratio of GaN, more incident light is trapped and absorbed by nanoporous structure, and the unabsorbed light scattered by PSS upwards is re-absorbed in nanoporous GaN. The ultraviolet light absorptance and reflectance of nanoporous GaN grown on PSS were improved significantly, close to 89% and 9% respectively. The resulting improved absorption in the nanoporous GaN further enhances the generation of photocarriers. The increasing surface-volume ratio also contributes to increasing of photoelectrochemical (PEC) reaction area and photocarrier separation efficiency, decrease of photocarrier migration distance towards the GaN-electrolyte interface, more holes participate in the PEC reaction, leading to an improved PEC efficiency and photocurrent density by 470% times with respect to planar counterpart. This work will pave the way towards low-cost and mass production of nanoporous GaN photoelectrode for efficient solar water splitting.