First-principles investigations on the visible light photocatalytic activity of NaNbO3 by N and F doping

To enhance the photocatalytic activity of NaNbO3 (NNO) for hydrogen production through water splitting, monodoping with N or F atoms and codoping with (N, F) atom pairs are applied to modulate the electronic structures of NNO, which are calculated with hybrid functional HSE06 in VASP code. F doping into NNO crystals yields little band gap reduction of the system. Although N doping can significantly reduce band gap and enhance the visible light response of the system, presence of the localized unoccupied states above Fermi level in forbidden zone may promote the electron-hole recombination and possibly result in a poor photoconversion efficiency. A favorable improvement is achieved with N and F codoping, which yields not only an uncontaminated forbidden band but also the significantly narrowed band gap suitable for visible light absorption. Moreover, the band edge positions of various codoped systems are also suitable for the overall water splitting. The calculated formation energy indicates that N doping becomes more feasible in the presence of F due to charge balance. So, the (N, F) pairs codoping can be justified for improving the visible light driven photoactivity of NNO. The effects of concentration and separation of dopants on the photocatalytic performances are also investigated.