First-principles investigations on the visible light photocatalytic activity of NaNbO3 by N and F doping Yong-Qiang Xu, Shao-Yi Wu, Li-Na Wu, Li-Juan Zhang and Si-Ying Zhong Eur. Phys. J. B, 92 4 (2019) 68 Published online: 01 April 2019 DOI: 10.1140/epjb/e2019-90467-5