Enhanced Water Oxidation on Ta3N5 Photocatalysts by Modification with Alkaline Metal Salts

Tantalum nitride (Ta3N5) is a promising nitride semiconductor photocatalyst for solar water splitting because it has band edge potentials capable of producing hydrogen and oxygen from water under visible light (lambda < 590 nm). However, the photocatalytic performance of Ta3N5 has been far below expectations because insufficient crystallization upon thermal nitridation of the oxide precursors enhances undesirable charge recombination limiting the quantum efficiency of the photocatalytic reaction. This problem was successfully rectified in this study by modifying the surface of the starting Ta2O5 with a small amount of alkaline metal (AM) salts. Compared with conventional Ta3N5, Ta3N5 nitrided from AM salt-modified Ta2O5 had better crystallinity and smaller particles with smoother surfaces and, most importantly, demonstrated a 6-fold improvement in photocatalytic activity for O-2 evolution under visible light. AM salt modification was compatible with the loading of an O-2 evolution cocatalyst, such as CoOx, yielding an apparent quantum efficiency of 5.2% at 500-600 nm. This indicates that the effects of AM modification were attributable to the changes in the crystallinity and the morphology of Ta3N5 rather than to catalytic effects. Detailed characterization of the Na2CO3-modified Ta3N5 suggested partial dissolution of Ta2O5 and nucleation of NaTaO3 in the early stages of nitridation, which gave rise to the characteristic particle morphologies and improved the crystallinity of the nitridation products. This study demonstrates that a facile pretreatment of a starting material can improve the physical and photocatalytic properties of photocatalysts drastically, enabling the development of advanced photocatalysts for solar water splitting.