Density functional theory study on the effects of oxygen groups on band gap tuning of graphitic carbon nitrides for possible photocatalytic applications

Graphitic carbon nitride (g-C3N4) has been considered to be a promising photocatalyst due to its photoresponse under visible light. It is known that different types of oxygen groups would be normally remained on g-C3N4 during synthesis, and g-C3N4 with oxygen groups was reported to have promising photocatalytic performance experimentally. To understand the mechanism of the enhanced photocatalytic performance of g-C3N4 with oxygen groups, density functional theory (DFT) calculations were carried out in this work to investigate the band structures of g-C3N4 with different types of oxygen groups (COOH, OH or O) systematically, thus predicting its capability of activation of electron-hole pair. In addition, in order to consider the position of oxygen groups on g-C3N4 and its corresponding effect on the band structure, graphitic carbon nitride nanoribbons (CNNR) is built. It is found that only OH and O groups can be stably attached at the center of CNNR, while all the three types of groups are stable at the edges. Additionally, COOH or OH group binding with N atoms (NCOOH or NOH) can reduce the bandgap of CNNR significantly, and the bandgap further reduces sharply at high concentration of NCOOH or NOH, while attaching O does not change its bandgap much regardless the position of the groups expect replacing H atom at the right edge. Therefore, attaching NOH at the middle, replacing H atoms by O at the right edge and attaching NCOOH or NOH at both sides are promising ways to reduce the band gap of CNNR and thus may improve the generation of electron-hole pair. Furthermore, the higher the concentration of the oxygen groups, the better the performance it has.