Photoinduced Water Oxidation at the Aqueous GaN (101̅0) Interface: Deprotonation Kinetics of the First Proton-Coupled Electron-Transfer Step

Photoelectrochemical water splitting plays a key role in a promising path to the carbon-neutral generation of solar fuels. Wurzite GaN and its alloys (e.g., GaN/ZnO and InGaN) are demonstrated photocatalysts for water oxidation, and they can drive the overall water splitting reaction when coupled with co-catalysts for proton reduction. The present work investigates the water oxidation mechanism on the prototypical GaN (101 ̅0) surface using a combined ab initio molecular dynamics and molecular cluster model approach taking into account the role of water dissociation and hydrogen bonding within the first solvation shell of the hydroxylated surface. The investigation of free-energy changes for the four proton-coupled electron-transfer (PCET) steps of the water oxidation mechanism shows that the first PCET step for the conversion of −Ga−OH to −Ga−O•− requires the highest energy input. The study further examines the sequential PCETs, with the proton transfer (PT) following the electron transfer (ET), and finds that photogenerated holes localize on surface −NH sites, and the calculated free-energy changes indicate that PCET through −NH sites is thermodynamically more favorable than −OH sites. However, proton transfer from −OH sites with subsequent localization of holes on oxygen atoms is kinetically favored owing to hydrogen bonding interactions at the GaN (101 ̅0)−water interface. The deprotonation of surface −OH sites is found to be the limiting factor for the generation of reactive oxyl radical ion intermediates and consequently for water oxidation.