Origin of the Enhanced Visible Photocatalytic Activity in (N, C)-Codoped ZnS Studied from Density Functional Theory

Density functional theory calculations are used to investigate the origin of the experimentally observed changes in visible photoactivity of hexagonal wurtzite ZnS induced by (N, C) codoping. The accurate comparative analyses of geometric and electronic structures for the different doping models have been discussed. For the mono N- or C-doped ZnS systems, the substituted doping just induces a slight band gap narrowing (less than 0.2 eV), while the interstitial doping leads to more significant optical absorption red shift. However, the calculated energy indicates the formation of the interstitial doping is difficult with the lower impurity concentration. For the N+C-codoped systems, the electron transition from the impurity states in the gap to the conduction band will induce a large red shift, suggesting the visible-light absorption. However, the impurity states are partially occupied character, which may act as recombination center and reduce the photoinduced current density. The calculated results of 2N+C-codoped ZnS indicate the N-C-N timer structure was formed in the ZnS lattice, and the trimer doping induces a larger red shift of photoelectron transition and passivates the partially occupied states by the charge compensation effect in acceptor-donor-acceptor pair. Our work provides a solid basis for the rationalization of experimentally observed red shift of optical absorption in wurtzite ZnS as a consequence of (N, C) codoping and shows that 2N+C codoping will be a promising way for improving the visible-light activity of semiconductor photocatalysts