Photoelectrochemical Water Splitting using GaN Nanowires with Reverse-Mesa Structures as Photoanode Material

We report improved photoelectrochemical water splitting (PEC-WS) using GaN nanowires (NWs) with reverse-mesa structures (RMNWs) formed on Si(111) as a photoanode material. The GaN-RMNW photoanode exhibited a current density of 2.62 mA/cm² and an applied photon-to-current efficiency of 1.65% at 0.6 V versus a reversible hydrogen electrode. These values are considerably higher than those (1.16 mA/cm² and 1.24%) of the photoanode based on GaN NWs with uniform hexagonal-pillar structures. The improved PEC-WS using the GaN-RMNW photoanode is attributed to the increase in the number of carriers participating in the PEC-WS reaction. The increase in the effective carriers is primarily due to the high crystallinity of the GaN RMNWs and the increase in the absorption rate of the incident light by the reverse-mesa structures. In addition, the energy-band structure between the GaN RMNWs and Si(111) promotes the separation of photogenerated carriers. Consequently, it reduces carrier recombination inside the photoanode, thereby enabling a high-performance PEC-WS.