Theoretical insights into origin of graphene oxide acidity and relating behavior of oxygen-containing groups in water

Understanding the acidic characteristics of graphene oxide (GO) and the relating deprotonation and proton transfer behaviors of oxygen-containing functional groups in the water environment is of great significance for the application of GO nanoscale materials. In this study, firstly, the density functional theory (DFT) method was applied to compute the explicit acidity constant (pK_a) of the carboxyl and hydroxyl groups on GOs. The good consistency of our results with experiments approves the applicability of the designed thermodynamic cycles for computing the pK_a of GOs. Secondly, the thermodynamics and kinetics of the deprotonation reactions of GOs in the presence of hydroxyl anions were investigated, quantitatively revealing that it is the carboxyl groups that mainly contribute to the surface charge acquisition of GO. Lastly, specific intramolecular proton transfer pathways were studied energetically, demonstrating that the proton is easily transferred between the adjacent hydroxyl group and the epoxy group on GO basal plane. This process could cause a change in the distribution of surface charge on GO surface, which will be very critical in understanding the surface properties of GOs.