Improving KNbO3 photocatalytic activity under visible light

Abstract

An increasing number of photocatalytic applications of KNbO₃ in different fields motivated us to find an efficient strategy to reduce its band gap so that it can utilize the solar spectrum. Using density functional theory (DFT) with a hybrid functional proposed by Heyd, Scuseria, and Ernzerhof, the experimental band gap (3.24 eV) of KNbO₃ was successfully reproduced (3.23 eV). In the present study, we systematically investigated the effect of doping with N and W on the geometry and electronic structure of KNbO₃. Because of the closeness of the ionic radius small changes in the parent crystal structure occurs. However, the electronic structure showed major changes in both the cases. N introduces impurity states adjacent to the top of the valence band and the bottom of the conduction band (CB), thus reducing the band gap significantly. Doping with W results in an n-type semiconductor, and introduces occupied states adjacent to the CB. Although both the dopant elements can improve the visible light absorption, it may accelerate electron–hole recombination. Therefore, individually they may not be able to improve the photocatalytic activity of KNbO₃. Interestingly, a highly favourable band structure was produced with a reduced band gap when both N and W are simultaneously doped into the crystal structure of KNbO₃. The calculated formation energy indicates that the doping of N becomes more feasible in the presence of W. This may be due to the formation of a charge compensated system, which also reduces the vacancy defect formation. More importantly, the band edge shifting in the presence of both N and W occurs in such a controlled fashion that KNbO₃ still remains suitable for overall water splitting. Therefore, one can justify the choice of the (N, W) pair for improving the visible light driven photoactivity of KNbO₃.