Improving visible light photocatalytic activity of NaNbO3: a DFT based investigation

Depleting sources of fossil fuels and their adverse environmental impact drive global interest to find efficient materials for hydrogen generation through solar water splitting. Although, NaNbO3 has several key features as an efficient photocatalyst, its large band gap restricts its photoactivity only in the UV-region of the solar spectrum. In this theoretical study, we investigate the effect of doping on the electronic structure of NaNbO3 aiming at improving its visible light photocatalytic activity. For this purpose, we employ hybrid density functional theory (DFT), which successfully reproduces the experimental band gap of NaNbO3. Doping with N has been found to reduce the effective band gap significantly by introducing localized acceptor states which however are known to promote the electron hole recombination rate. To overcome this, we propose codoping with W at the Nb lattice site. Interestingly this completely passivates those localized acceptor states. The band gap is found to be sufficiently reduced to enhance the visible light activity. The present strategy reduces the band gap in such a controlled way that (W, N)NaNbO3 satisfies the thermodynamic criteria to execute the overall decomposition of H2O, indicated in the relative position of its band edges with respect to water redox levels. Moreover, doping of N is found to be facilitated in the presence of W due to reduction in formation energy. Additionally, spontaneous formation of charge compensated valence defects is expected to be reduced in the presence of the (W, N) pair in comparison to individual dopant elements due to maintaining the total electrical charge neutrality.