Density-functional theory study of the effects of atomic doping on the band edges of monoclinic WO$_{3}$

The effects of impurities in room temperature monoclinic WO$_{3}$ were studied using the local density approximation to density-functional theory. Our main focus is on nitrogen impurity in WO$_{3}$, where both substitutional and interstitial dopings were considered. We have also considered doping with transition-metal atoms and some co-doping approaches in WO$_{3}$. We find that, in general, band gap reduction was a common result due to the formation of impurity bands in the band gap. Also, the changes of band-edge positions, valence-band maxima and conduction-band minima, were found to depend on the electronic properties of the foreign atom and their concentration. Our results, therefore, provide guidance for making WO$_{3}$ a suitable candidate for photo-electrodes for hydrogen generation by water splitting.