Revealing the Chemical Nature of Functional Groups on Graphene Oxide by Integrating Potentiometric Titration and Ab Initio Calculations

A new graphene oxide (GO) model with reasonable functionalgrouptypes and distribution modes was proposed by integrating potentiometrictitrations and ab initio calculations. Due to the complex synthesismechanism, the atomic structure of GO has been controversial for along time. Here, we use density functional theory calculations tomimic the oxidation process, and a series of GO fragments (GOFs) werededuced. A new pK (a) calculation method(RCDPKA) developed specifically in this work was further used to predictpK (a) values of the fragments. Then, weperformed potentiometric titrations on four different GO samples toconfirm the existence of these GOFs and determine the content of functionalgroups. Interestingly, different GO samples present the same pK (a) values in titration, and the results are consistentwith the predicted ones. Based on the evidence from titration andcalculation, prominent correlations between functional groups couldbe found. Groups at the edges are mainly double-interactive carboxyls(pK (a1) & AP; 3.4, pK (a2) & AP; 5.7) and double-adjacent phenolic hydroxyls(pK (a1) & AP; 8.8, pK (a2) & AP; 12.1), while groups on the plane are mainlycollocated epoxies and hydroxyls (pK (a1) & AP; 11.1, pK (a2) & AP; 13.8) onboth sides of the plane with a meta-positional hydrogen bond interaction.These findings were further validated by multiple characterizationsand GO modifications. These results not only stimulate a fundamentalunderstanding of the GO structure but also provide a quantitativeanalysis method for functional groups on GO.