Sulfur doping effects on the electronic and geometric structures of graphitic carbon nitride photocatalyst: insights from first principles

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.