Evolutionary Chlorination of Graphene: From Charge-Transfer Complex to Covalent Bonding and Nonbonding

Density functional theory (DFT) studies were performed to investigate the chlorination of graphene. Unlike hydrogenation and fluorination, where the adsorption of H and F is always by covalent C–H/C–F bonding, Cl atoms generate various states when single-sided graphene exposed. In the initial reaction stage, it forms Cl–graphene charge-transfer complex, where the C orbitals keep sp² hybridization and the graphene is p-type doped. Further chlorination may form two adsorption configurations: one is covalent bonding Cl pairs, where the structure of the C atom is close to sp³ hybridization. With the Cl coverage increases, this configuration may further cluster into hexagonal rings, and the resulting coverage is less than 25%. The other configuration is nonbonding. This configuration is energy preferable, while Cl atoms will form Cl₂ molecules and escaped. When both sides of the graphene are exposed, the most stable adsorption configuration is a homogeneous ordered pattern with a Cl coverage of 25% (C₄Cl) rather than collective clusters. The electronic properties of various chlorinated forms were also obtained; these showed that it is possible to tune the graphene bandgap by chlorination in a range of 0–1.3 eV.