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.
A large number of defects and insufficient active sites are two important factors resulting in the poor photocatalytic activity of CaNbO2N. In this work, we successfully prepared one-dimensional (1D) single-crystal CaNbO2N (CNON-P) with highly crystallinity based on a chemical vapor deposition–re-evaporation strategy (Bi2O3 as the evaporation source). The strategy effectively suppressed the generation of defects, achieved spatial separation of the active facets, and increased active sites, promoting carrier separation and transfer. This strategy unexpectedly enhanced the hydrophilicity as well, facilitating intimate contact between the photocatalyst and water. Compared to the 1D CaNbO2N without using the chemical vapor deposition–re-evaporation strategy and the conventional CaNbO2N prepared by thermal ammonolysis of oxide particles, the photocatalytic performance of the CNON-P was significantly enhanced due to the coupling of these favorable factors. Its H2 and O2 production rates in the presence of sacrificial reagents reached 55.0 μmol h–1 and 11.7 μmol h–1 under visible-light (λ ≥ 420 nm) irradiation, respectively, with the H2 production rate being the highest reported for CaNbO2N so far. It is hoped that this work can provide a new pathway for defect control of different perovskite oxynitrides.
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