N-Doped CsTaWO6 as a New Photocatalyst for Hydrogen Production from Water Splitting Under Solar Irradiation

Photocatalysts for efficient solar hydrogen production are highly sought after. Here a new type of nitrogen-doped tantalum tungstenate (CsTaWO6) material, which demonstrates excellent visible light absorption and improved photocatalytic activity, is demonstrated. X-ray diffraction (XRD) patterns reveal that the defect pyrochlore-type structure of CsTaWO6 remained intact upon nitrogen doping. UV-vis spectra indicate that nitrogen doping in the compound results in a red-shift of the absorption edge from 358 nm to 580 nm, thus offering significantly increased visible light absorption. X-ray photoelectron spectroscopy (XPS) further indicates that [Ta/W]-N bonds were formed by inducing nitrogen to replace a small amount of oxygen in the material, resulting in a compound of CsTaWO6-xNx. The explanation of the experimental results is supported by density functional theory calculations. The density of states (DOS) and the projected DOS after substitutional doping of nitrogen in CsTaWO6 indicated that N-doping reduces the bandgap significantly from 3.8 to 2.3 eV due to N 2p and O 2p orbital mixing. The role of the new N 2p states is also investigated by studying the production of the center dot OH radicals in the visible light region (>420 nm). In CsTaWO6-xNx, the N 2p orbitals are the main contributors to the top of the valence band, causing bandgap narrowing while the bottom of conduction band, due to Ta 4d orbitals, remains almost unchanged. Compared with its undoped counterpart, nitrogen-doped CsTaWO6-xNx exhibits a nearly 100% increase in solar hydrogen production efficiency.