Formation and characterization of gray Ta2O5 and its enhanced photocatalytic hydrogen generation activity

White and gray Ta2O5 powders were fabricated by sol-gel and then annealed at 850 degrees C in air and vacuum, respectively. Analyses by Raman spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, electron paramagnetic resonance spectroscopy, UV-visible reflectance spectroscopy, photoluminescence spectroscopy, and density functional theory calculations indicated that oxygen vacancies are formed on the surface of gray Ta2O5, resulting in a disordered shell and more absorption of visible light. Its surface concentration ratio of oxygen to tantalum was determined to be 2.31. It generated 48% higher photocatalytic hydrogen than that of the white one, and cycling test indicated that it remained stable during 12 h irradiation. It also exhibited higher incident photon-to-current conversion efficiency than that of white Ta2O5 in the wavelength range of 300-600 nm. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.