Switching on efficient photocatalytic water oxidation reactions over CaNbO2N by Mg modifications under visible light illumination

In spite of a strong visible light absorbance as far as 600 nm, CaNbO 2 N generally exhibits poor photocatalytic activity under normal conditions because of various structural defects and poor charge mobility. In this work, we have synthesized a series of Mg-modified CaNbO 2 N, i.e. CaNb 1-x Mg x O 2+y N 1-y (0 ≤ x ≤ 0.2), and performed a detailed investigation on their crystal structures, optical absorption and other physicochemical properties. Our results show that there is a slight shrinkage of the unit cell and a blue-shift of absorption edges upon Mg incorporation into CaNbO 2 N. The nitrogen contents as well as defects levels can be effectively tuned by altering the content of Mg. More strikingly, photocatalytic oxygen productions are much improved after Mg modifications under visible light irradiation (λ ≥ 420 nm). An average oxygen production rate as much as ∼126.8 umol h −1 and an apparent quantum efficiency as high as ∼3.4 % at 420 ± 20 nm is achieved for CaNb 0.9 Mg 0.1 O 2+y N 1-y (x = 0.1). These improvements probably stem from a substantial decrease of Nb 4+ defects in CaNbO 2 N as well as slight positive shift of valence band maximum (VBM) after Mg modifications. Meanwhile, photoelectrochemical analysis suggests charge migration is somewhat enhanced in response to Mg modifications.