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 [dbnd]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 [dbnd]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 (N–COOH or N–OH) can reduce the bandgap of CNNR significantly, and the bandgap further reduces sharply at high concentration of N–COOH or N–OH, while attaching [dbnd]O does not change its bandgap much regardless the position of the groups expect replacing H atom at the right edge. Therefore, attaching N–OH at the middle, replacing H atoms by [dbnd]O at the right edge and attaching N–COOH or N–OH 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.