Photochemical water splitting is a promising avenue to sustainable, clean energy and fuel production. Gallium nitride (GaN) and its solid solutions are excellent photocatalytic materials; however, the efficiency of the process is low on pure GaN, and co-catalysts are required to increase the yields. We present the first time-domain theoretical study of the initial steps of photocatalytic water splitting on a GaN surface. Our state-of-the-art simulation technique, combining non-adiabatic molecular dynamics (NA-MD) and time-dependent density functional theory (TD-DFT), allows us to characterize the mechanisms and timescales of the evolution of the photogenerated positive charge (hole) and the subsequent proton transfer at the GaN/water interface. The calculations show that the hole loses its excess energy within 100 fs and localizes primarily on the nitrogen atoms of the GaN surface, initiating a sequence of proton transfer events from the surface N-H group to the nearby OH groups and bulk water molecules. Water splitting requires hole localization on oxygen rather than nitrogen, necessitating non-adiabatic transitions uphill in energy on pure GaN. Such transitions happen rarely, resulting in low yields of the photocatalytic water splitting observed experimentally. We conclude that efficient co-catalysts should favor localization of the photogenerated hole on oxygen-containing species at the semiconductor/water interface.

Non-adiabatic dynamics of photoholes during initial steps of the photocatalytic water splitting on GaN(10-10) surface: Charge localization governs splitting efficiency

246th National Meeting (2013)

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