Revisiting the Oxidation of Graphite: Reaction Mechanism, Chemical Stability, and Structure Self-Regulation

To fully understand the chemical structure of graphene oxide and the oxidation chemistry of sp2 carbon sites, we conducted a practical experiment and density functional theory combined study on the oxidation process of graphite. The nuclear magnetic resonance, thermogravimetric analysis, and X-ray photoelectron spectroscopy results of unhydrolyzed oxidized graphite indicate that the oxidation process involves the intercalating oxidation, where electrically neutral species is the oxidizing agent, and the diffusive-oxidation, where MnO3+ is the oxidizing agent. An intrinsic formation and conversion path of oxygen-containing functional groups is proposed based on the experimental results and further interpreted with the aid of frontier molecular orbital theory and density functional theory. Meanwhile, the two unique features of the oxidation process of graphite, the chemistry stability of oxygen-containing functional groups in the strong oxidizing medium, and the self-regulation of the oxidation process are theoretically reasoned.

In the modified Hummers method, the oxidation of graphite occurs in the medium of concentrated sulfuric acid. In our experiment, the concentration of sulfuric acid is 18.4 mol/L. Sulfuric acid is a strong acid with pK a =-10 and will easily donate one of its two hydrogens. After the oxygen-containing functional groups are formed on graphene surface, part of them such as epoxide group will react with excess sulfuric acid.
For example: R-O-R + H 2 SO 4 ⇋ R-OH-R + + HSO 4ˉ ( 1) For the forward reaction, H 2 SO 4 is functioning as an acid, and R-O-R is functioning as a base.
For the reverse reaction, R-OH-R + is functioning as an acid, and HSO 4ˉ is functioning as a base.
For the forward reaction, pK eq = pK a acid L -pK a acid R (2) = pK a (H 2 SO 4 ) -pK a (R-OH-R + ) = -10 -(-3.6) = -6.4 From pK eq = -log K eq = -6.4, we can get K eq = 10 6.4 ≫ 1. K eq ≫ 1 indicates that the equilibrium lies to the right and favors the weaker acid. S3 Discussion S2: verification of the intercalating-oxidation and subsequent diffusive-oxidation processes of graphite Sample characterization and analysis: the unhydrolyzed oxidized graphite sample was characterized with 13 C SSNMR, TGA-DSC, and XPS ( Figure S1). Figure S1a shows the 13 C SSNMR spectrum of oxidized product. Besides the resonance signal of C=O bond (168.8 ppm), a strong resonance signal at 135.8 ppm also appears, which can be attributed to the formation of isolated sp 2 carbon pair. The resonance signals of monosulfate and disubstituted tert-butyl sulfate are also observed at 85.8 and 75.4 ppm, respectively, indicating the relatively high oxidation degree of graphite oxide. Figure S1b shows the TGA-DSC curves of oxidized graphite, and the sample has two major weight losses. The first major weight loss happens between 150 and 170 °C due to the decomposition of the labile oxygen-containing functional groups such as hydroxyl and epoxide groups. The second major weight loss happens between 620 and 760 °C, and the 31.6% weight loss can be attributed to the loss of sulfur.
[32] Figures S1c and S1d show the XPS survey spectrum and high-resolution C 1s spectra of unhydrolyzed oxidized graphite, respectively. The atomic fractions of oxidized product are 39.18%, 46.49%, and 11.22% for C, O, and S atoms, respectively, indicating that the main oxygen-containing group is organosulfate. Meanwhile, the atomic fractions of C-C/C=C, C-O, and C=O are 61.9%, 29.1%, and 9.0%, respectively, indicating that the sample obtained with this method has a lower functional group content than that with the modified Hummers method.
For the modified Hummers method, 4 weight equivalent oxidizing agent KMnO 4 is needed for the oxidation of graphite. But in this procedure, only 0.5 weight equivalent KMnO 4 is used, which cannot realize the effective oxidation of graphite. 34 Meanwhile, although nitric acid acting as a Brønsted-Lowry base in the medium of concentrated sulfuric acid and forming H 2 NO 3 , which in turn decomposes into nitronium ion, treating graphite with a mixture of concentrated nitric and sulfuric acids can only generate products similar to H 2 SO 4 -GIC due to the repulsive interaction between S4 electrophile NO 2 + and positively charged H 2 SO 4 -GIC. The combination of the above two oxidizing agents can realize the complete oxidation of graphite, and the possible oxidation mechanism of this method is as follows: (i) the electrically neutral oxidizing agent forms in the medium of KMnO 4 -H 2 SO 4 , and then intercalates into the interlayer spaces of graphite, damages the aromatic carbon skeleton, and transfers the charge of H 2 SO 4 -GIC by forming oxygen-containing functional groups (C=O); (ii) electrophile NO 2 + forms in the medium of KNO 3 -H 2 SO 4 and then diffuses into the interlayer spaces of H 2 SO 4 -GIC through electron-rich domains, forming oxygen-containing groups by the oxidation of isolated C=C bonds; (iii) the reaction by-product (NO 2 ) is oxidized by the decomposition product of MnO 2 , forming electrophile NO 2 + .
The feasibility of the above procedure indicates that the oxidation reaction of graphite indeed includes the intercalating-oxidation process of an electrically neutral oxidizing agent and the subsequent diffusive-oxidation process of an electrophile.   Figure S6. The intrinsic formation and conversion path of oxygen-containing functional groups on oxidized graphite with two possible routes.
The reaction system exhibits the feature purple color of Mn(VII).
During the preparation of unhydrolyzed oxidized graphite, we dispersed oxidized graphite in