Enhanced Water Oxidation on Ta_3N_5 Photocatalysts by Modification with Alkaline Metal Salts

摘要

Tantalum nitride (Ta_3N_5) 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 (λ < 590 nm). However, the photocatalytic performance of Ta_3N_5 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 Ta_2O_5 with a small amount of alkaline metal (AM) salts. Compared with conventional Ta_3N_5, Ta_3N_s nitrided from AM salt-modified Ta_2O_5 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 CoO_x, 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 Ta_3N_5 rather than to catalytic effects. Detailed characterization of the Na_2CO_3-modified Ta_3N_5 suggested partial dissolution of Ta_2O_5 and nucleation of NaTaO_3 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.