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

ABSTRACT

A new graphene oxide (GO) model with reasonable functional group types and distribution modes was proposed by integrating potentiometric titrations and ab initio calculations. Due to the complex synthesis mechanism, the atomic structure of GO has been controversial for a long time. Here, we use density functional theory calculations to mimic the oxidation process, and a series of GO fragments (GOFs) were deduced. A new pKa calculation method (RCDPKA) developed specifically in this work was further used to predict pKa values of the fragments. Then, we performed potentiometric titrations on four different GO samples to confirm the existence of these GOFs and determine the content of functional groups. Interestingly, different GO samples present the same pKa values in titration, and the results are consistent with the predicted ones. Based on the evidence from titration and calculation, prominent correlations between functional groups could be found. Groups at the edges are mainly double-interactive carboxyls (pKa1 ≈ 3.4, pKa2 ≈ 5.7) and double-adjacent phenolic hydroxyls (pKa1 ≈ 8.8, pKa2 ≈ 12.1), while groups on the plane are mainly collocated epoxies and hydroxyls (pKa1 ≈ 11.1, pKa2 ≈ 13.8) on both sides of the plane with a meta-positional hydrogen bond interaction. These findings were further validated by multiple characterizations and GO modifications. These results not only stimulate a fundamental understanding of the GO structure but also provide a quantitative analysis method for functional groups on GO.