Ni₃(PO₄)₂ Cocatalyst-Supported β–Ga₂O₃/GaN Photoanodes for Highly Stable Solar Water Splitting

This paper investigated the chemically surface-modified gallium nitride (GaN) films used as photoanodes for photoelectrochemical (PEC) water splitting. Here, the porous GaN is prepared by a simple chemical etching process (referred to as β–Ga2O3/GaN–10 s, β–Ga2O3/GaN–30 s, and β–Ga2O3/GaN–1 min) and then annealed in an air ambient atmosphere at 600 °C to introduce the crystalline monoclinic β–Ga2O3 oxide into the GaN surface. X-ray photoelectron spectroscopy and focused ion beam-high-resolution-transmission electron microscopy studies confirmed the chemically robust Ga–O bonds as well as the formation of highly crystalline β–Ga2O3 with a thickness of 15 nm. Notably, the 15-nm-thick β–Ga2O3-integrated GaN photoelectrode provided a large photovoltage of ∼1.2 VRHE, resulting in extended band bending for efficient charge separation, leading to efficient PEC water oxidation. This phenomenon reduces the charge recombination in the surface state, protecting semiconductors from chemical corrosion of the pristine GaN film. As expected, the engineered β–Ga2O3/GaN–30 s photoanode displayed an excellent photocurrent density (Jph) of 1.2 mA·cm–2 at 1.23 V versus a reversible hydrogen electrode (VRHE) under a stable working condition, up to 25, 21, and 34% greater than those of the bare GaN, β–Ga2O3/GaN–10 s, and β–Ga2O3/GaN–1 min of 0.95, 0.99, and 0.86 mA/cm2, respectively, in 0.5 M NaOH alkaline solution. Moreover, nickel phosphate (Ni–Pi) electrocatalysts were supported on the β–Ga2O3/GaN–30 s photoanode film by a simple photoelectrodeposition method, revealing a Jph of 1.4 mA·cm–2 at 0 VRHE. The superior performance of the Ni–Pi/β–Ga2O3/GaN photoanode system clearly demonstrates the possibility of highly stable water splitting by the modulation of the reaction kinetic parameters, such as passivation and surface kinetics.