  We present here results of our first principles studies of the sulfur doping
effects on the electronic and geometric structures of graphitic carbon nitride
(g-C3N4). Using the Ab initio thermodynamics approach combined with some
kinetic analysis, we reveal the favorable S-doping configurations By analyzing
the valence charge densities of the doped and un-doped systems, we find that
sulfur partially donates its px- and py- electrons to the system with some back
donation to the S pz-states. To obtain accurate description of the excited
electronic states, we calculate the electronic structure of the systems using
the GW method. The band gap width calculated for g-C3N4 is found to be equal to
2.7 eV that is in agreement with experiment. We find the S doping to cause a
significant narrowing the gap. Furthermore, the electronic states just above
the gap become occupied upon doping that makes the material a conductor.
Analysis of the projected local densities of states provides insight into the
mechanism underlying such dramatic changes in the electronic structure of
g-C3N4 upon the S doping. Based on our results, we propose a possible
explanation for the S doping effect on the photo-catalytic properties of g-C3N4
observed in the experiments.
