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.

Introduction

Carbon-based materials have been playing significant roles in defining the research and applications of nanoscience and nanotechnology.[1,2] Their fabrication and functionalization have facilitated massive interdisciplinary research involving chemistry, physics, and materials.[3,4] Oxidation is one of the most basic routes to manipulating the carbon-based materials by functionalizing the chemically inert, sp2 hybridized carbon structure.[5,6] To realize the desired functionalization of carbon-based materials, controlled oxidation reactions are critical to a low density of lattice defects during carbon evolution.[7,8] With more than 160 years of synthesis history, the fabrication of graphite oxide (GO) is the most typical oxidation reaction of carbon-based materials.[9,10] To synthesize GO with controlled chemical composition and properties, various processes and oxidizing agents have been attempted for the oxidation of graphite.[11−13] Nevertheless, the details of the chemical structure of GO and the oxidation chemistry of sp2 carbon sites have not been fully understood,[14,15] which essentially hinders the success of usage of the surface chemistry of GO and the correct acquaintance of the fundamental oxidation perspectives of carbon-based materials.

To reveal the fine chemical structure of GO and the oxidation chemistry of sp2 carbon sites, generations of chemists have tried to propose a suitable structural model.[16−21] The most recognized Lerf–Klinowski (LK) structural model concludes that GO is a highly functionalized graphite with randomly distributed oxidized domains containing epoxy and hydroxyl groups on the basal planes and carboxyl and hydroxyl groups at the edges.[20] The atomic-level resolution transmission electron microscopy images confirmed that the oxidized domains form a continuous network and the graphitic domains form isolated islands.[22,23] The solid-state nuclear magnetic resonance (SSNMR) characterization also shows that the major functional groups situating on the basal plane of GO are C–OH and epoxide. Carbonyl is spatially separated from the majority sp2, C–OH and epoxide carbons, indicating that carbonyl groups mainly distribute at the edge.[24,25] Nevertheless, because of the complexity of the oxidizing system and the chemical instability of GO, the published works still cannot completely resolve the mystery of the oxidation process and the GO chemistry.

There are several unrevealed questions for the oxidation process of graphite: (i) What is the intrinsic formation and conversion path of the oxygen-containing functional groups on GO? Several groups proposed that epoxy is the primary functional group, and the local strain induced by the oxygen-containing functional groups results in the cutting of graphene sheets.[26−28] After exposure to water, acid catalyzes the hydrolysis of epoxide to form hydroxyl groups.[29,30] Chen et al. proposed that epoxy and hydroxyl groups can be formed by directly attacking graphite sheets with •O• and HO• radicals.[31] (ii) How can the oxygen-containing functional groups be stabilized in the strong oxidizing medium? The oxidized domains decorated with reactive functional groups such as epoxy and hydroxyl groups should be unstable in the strong oxidizing system. While during the oxidation process, the GO composition and the structure do not change significantly after reaching a threshold oxidation degree,[32,33] implying the chemical stability of functional groups in the oxidizing medium. (iii) Why does GO consist of randomly distributed oxidized domains and graphitic domains? Manipulation of the stoichiometry of GO tailors its optoelectronic and physicochemical properties.[7,8] The formation mechanism and size control of the graphitic and oxidized domains should be of great importance for the synthesis of GO with tunable properties.

In this work, we have carefully prepared unhydrolyzed oxidized graphite to avoid the interference of water and other impurities, and the chemistry of oxidized graphite has been scrutinized by the most relevant characterization techniques. We also have considered the chemical reactivity of interim oxidized products and the energy variation during the graphite oxidation in order to understand the chemistry stability of oxygen-containing functional groups in the strong oxidizing medium and the self-regulating feature of the oxidation process. We aim to determine the main oxidizing agent species and an intrinsic formation and conversion path of oxygen-containing functional groups, which have to be consistent with the experimental observations, characterization results, and theoretical calculations.

Results and Discussion

Identification of the Oxidizing Agents

For the modified Hummers’ method, the identity of the specific oxidizing species that attacks graphite seems to be still in controversy. Thus far, the proposed oxidizing species in the H2SO4–KMnO4 medium include Mn2O7, MnO3+, MnO4–, O3, and some radicals such as •O•. Lerf et al. suggested that the oxidizing species is a nucleophile, which should be MnO4–.[20] Dreyer et al. proposed that the active oxidizing species is M2O7, which is sensitive to high temperature and organic compounds.[15] Dimiev et al. mentioned that in H2SO4, the Mn(VII) most likely exists in the form of a MnO3+ cation, which closely associates with HSO4– and SO42– ions in the forms of MnO3HSO4 or (MnO3)2SO4.[34] Chen et al. proposed that epoxy and hydroxyl groups can be formed by directly attacking graphite sheets with •O• and HO• radicals.[31]

However, the main oxidizing species should not be a nucleophile or an electrically neutral species. The oxidation of graphite is a self-regulating process, as the GO composition and the structure do not change significantly with the addition of an excess oxidizing agent after reaching a certain threshold oxidation degree.[32,33] Sulfuric acid with pKa = −10 is considered strong and can easily donate proton. According to the Brønsted–Lowry acid–base theory, in the concentrated sulfuric acid, part of the oxygen-containing functional groups such as epoxy and hydroxyl groups will become conjugate acids with Keq ≫ 1 (see detailed calculation in Supporting Information, Discussion S1), and the corresponding domain will carry positive charges by the induction effect. If the oxidizing species is a nucleophile MnO4–, it will violently react with the as-formed conjugate acids and the positively charged domain. In such case, the oxygen-containing functional groups should not be stable in the oxidizing medium, which is in conflict with the experimental results. Research evidenced that GO consists of aromatic domains with sp2-hybridized carbon atoms and nonaromatic oxidized domains with an sp3-hybridized carbon structure. If the oxidizing agents do not carry charges, such as Mn2O7 and •O•, the oxidation reaction sites will be random, and the formed oxygen-containing functional groups can be further oxidized by the oxidizing agent. In this case, the oxidation degree of GO will increase with the reaction time and does not have a threshold oxidation degree. Therefore, the main active oxidizing species must carry a positive charge, and only MnO3+ meets this principle.

The substrate for the electrophilic addition reaction of the oxidizing agent MnO3+ is the sulfuric acid-graphite intercalation compound (H2SO4-GIC). H2SO4-GIC, also known as graphite sulfate, is formed by the intercalation of HSO4– and H2SO4 and has the characteristic diffraction peak at 22.5° (7.98 Å) (Figure a).[35−37] The ideal structure of H2SO4-GIC was revealed as (C24+(HSO4–)(H2SO4)2).[38,39] Hence, it seems impossible for the electrophile MnO3+ to intercalate into the interlayer spaces of the positively charged carbon skeleton of H2SO4-GIC and oxidize it. Actually, H2SO4-GIC is not just an ionic compound formed by the intercalation of HSO4– and H2SO4 between the layers of the graphite matrix; it also contains a small amount of oxygen-containing functional groups formed on the basal plane of graphite sheets.[39,40] As depicted by the thermogravimetric analysis (TGA) curve of the solvent-washed H2SO4-GIC sample (Figure b), besides the weight loss of HSO4– and H2SO4 at around 450 °C, the weight loss between 150 and 170 °C is because of the decomposition of the labile oxygen-containing functional groups such as hydroxyl and epoxide groups.[35,41] The high-resolution C 1s spectrum of the H2SO4-GIC sample also shows a small peak at 286.5 eV coming from the C–O bond and another small peak at 288.8 eV coming from the C=O bond (Figure c). The low degree oxidation of H2SO4-GIC can be attributed to the intercalation of an electrically neutral oxidizing agent into the graphite layers. We name this early oxidation of H2SO4-GIC that occurs in the intercalation step as intercalating-oxidation to distinguish it from the diffusive-oxidation that occurs during the diffusion process, where MnO3+ is functioning as the main oxidizing species. The intercalating-oxidation is the origin of the diffusive-oxidation of H2SO4-GIC. It damages the aromatic carbon skeleton and transfers the charge of H2SO4-GIC by the formation of oxygen-containing functional groups and thus enables the diffusion of the electrophile MnO3+ into the interlayer of H2SO4-GIC through electron-rich domains. The 13C SSNMR spectrum of H2SO4-GIC also suggests the existence of intercalating oxidation. As shown in Figure d, besides the resonance signal of sp2 conjugation carbon and C=O bond appearing at 127.0 and 165.5 ppm, respectively,[42] a resonance signal at 135.0 ppm can be attributed to the formation of an isolated sp2 carbon pair.[43] The intercalating-oxidation process of the electrically neutral oxidizing agent and subsequent diffusive-oxidation process of the electrophile also can be illustrated by another oxidation experiment (see detailed experiment procedure and results analysis in Supporting Information, Discussion S2 and Figure S1). Once the electrophile MnO3+ starts to attack the carbon skeleton, formed oxygen-containing functional groups will stop the electrically neutral oxidizing agent from diffusing into the interlayer of H2SO4-GIC with the steric hindrance effect, so the system enters the diffusive-oxidation stage and exhibits the self-regulating feature of graphite oxidation.

Figure 1

(a) XRD patterns of graphite and unhydrolyzed H2SO4-GIC. (b) TGA curve, (c) high-resolution C 1s scan, and (d) solid-sate 13C magic-angle spinning NMR spectrum of unhydrolyzed H2SO4-GIC.

Formation and Conversion Path of Oxygen-Containing Functional Groups

The diffusion of the oxidizing agent MnO3+ into the interlayer galleries of H2SO4-GIC and subsequent electrophilic attack on the carbon skeleton result in the formation of oxygen-containing functional groups. These functional groups and their conversion path in strong acidic oxidizing medium need to be experimentally examined in order to reveal the oxidation chemistry of graphite. In the modified Hummers’ method, the oxidized graphite is usually washed by deionized water to remove the remaining oxidizing agent. However, the hydrolyzed sample does not contain the genuine chemical composition of oxidized graphite because it reacts with water constantly.[29,32] Dimiev et al. introduced a new purification protocol to prepare pristine GO, while the H2O2 aqueous solution was inevitably added to remove Mn7+ and MnO2.[32] We thus developed a unique method to prepare the fully unhydrolyzed oxidized graphite (Experimental and Computational Methods section), and the prepared samples are free of impurities of MnSO4 and K2SO4 (Figure S2). The unhydrolyzed oxidized graphite samples were characterized by 13C SSNMR, TGA, and X-ray photoelectron spectroscopy (XPS) to reveal the genuine chemical composition of oxidized graphite.

Figure a shows the 13C SSNMR spectra of the solvent-washed oxidized graphite. For the partly oxidized graphite sample (A2, 24 min), in addition to the signal of the aromatic carbon skeleton and the C=O bond, new resonance signals arise at 56 and 85 ppm, assignable to the epoxide and monosulfate, respectively.[24,32] In addition, a new resonance signal appears at 97 ppm, which may be caused by the carbons of five- and six-membered-ring lactols.[44] With the oxidation degree of graphite further increasing, the relative peak intensity of epoxide (56 ppm) shows a notable increasing accompanied by the decrease of monosulfate (85 ppm) and C=O bond (165.5 ppm). For the fully oxidized graphite sample (A8, 96 min), the resonance signal of sp2 conjugation carbon at 135.0 ppm is weakened because of the formation of oxygen-containing functional groups. Correspondingly, the intensity of the epoxide signal at 61 ppm reaches a maximum intensity.

Figure 2

(a) Solid-sate 13C magic-angle spinning NMR spectra and (b) TGA curves of unhydrolyzed oxidized graphite. Atomic fractions of (c) composition elements derived from the XPS atomic content in Figure S2 and (d) oxygen-containing groups and C–C/C=C bonds corresponding to various forms of C atoms deconvolved from C 1s spectra in Figure S4.

The TGA analysis of unhydrolyzed oxidized graphite also provides useful information on the composition of samples. As shown in Figure b, all the oxidized graphite samples show two major weight losses. The first major weight loss happens between 150 and 170 °C because of the decomposition of the labile oxygen-containing functional groups such as hydroxyl and epoxide groups.[41] The rapid exothermic process in this temperature range corresponds to the substantial emission of CO2/CO from the thermal decomposition of epoxy functional groups (Figure S3). The second major weight loss between 250 and 280 °C can be attributed to the decomposition of monosulfate, as reported by Eigler et al.[40,45] The weight loss in both temperature windows increases with the oxidation degree, indicating that formation of hydroxyl and epoxide groups as well as monosulfate is a continuous process.

The atomic fraction of composition elements obtained from XPS was investigated to reflect the composition evolution during the oxidation process (Figure c). With the increase of the oxidation degree, the atomic fraction of C atoms declines, while those of O and S increase, indicating that the formation of monosulfate is accompanied with the whole oxidation process. Combining them with the quantitative atomic fraction of oxygen-containing groups and C–C/C=C bonds (Figure d), it can be calculated that the content of monosulfate accounts for 23.4% of the C–O bonds for the fully oxidized graphite. In addition, the content of the C=O bond remains almost stable through the whole oxidation process (Figure d), which indicates that the carbonyl and carboxyl groups mainly forms during the intercalating-oxidation stage of H2SO4-GIC.

The above characterization results show that the diffusive-oxidation originates from the electrophilic attack of MnO3+ on H2SO4-GIC in the medium of HSO4– and H2SO4, and the formed oxygen-containing functional groups are mainly hydroxyl and epoxide groups, as well as monosulfate.[32,45] Based on the experimental results, we proposed an intrinsic formation and conversion path of oxygen-containing functional groups during the graphite oxidation process as follows: after the Mn atom at the positive charge center acts as an electrophile to attack the nonaromatic graphite domain, epoxide groups are formed; in the medium of concentrated sulfuric acid, the as-formed epoxy groups are protonated to oxonium ions, namely [C2OH]+, forming conjugate acid-base pair with HSO4–; the as-formed oxonium ions transopen to form monosulfate and hydroxyl groups at ortho-positions because the α-carbon atom, which bonds to OH+, has the lowest electron density (Figure S5) and is susceptible to the nucleophilic addition of HSO4–. Figure schematically illustrates the intrinsic formation and conversion path of oxygen-containing functional groups with three interim oxidized products (epoxy-GO, oxonium-GO, and sulfate-GO).

Figure 3

Schematic illustration of the intrinsic formation and conversion path of oxygen-containing functional groups on oxidized graphite with three interim oxidized products (epoxy-GO, oxonium-GO, and sulfate-GO).

Stability of Oxygen-Containing Functional Groups in Strong Oxidizing Medium

In addition to the experimental evidence, we also attempted to interpret the proposed intrinsic formation and conversion path with theoretical support and reveal the reasons behind the unique features of the oxidation process of graphite. We first researched the chemical reactivity of each entity with the frontier molecular orbital (FMO) theory, which helped us to understand the chemistry stability of oxygen-containing functional groups in strong oxidizing medium. Table lists the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) spatial distribution and energy of MnO3+, H2SO4-GIC, and oxidized microdomains containing different functional groups. In a redox reaction, the HOMO reflects the electron losing potential of a reducing agent, while the LUMO reflects the electron capturing potential of an oxidizing agent.[46] The H2SO4-GIC shows delocalized electrons across the conjugated π bonds of an aromatic structure and is capable of electron transfer. The LUMO energy of MnO3+ is −14.37 eV, indicating its high oxidizing strength. The LUMO spatial distribution of MnO3+ overlaps with the HOMO spatial distribution of H2SO4-GIC through σ bonding, which allows π electrons to transfer from the HOMO of H2SO4-GIC to the LUMO of the oxidizing agent MnO3+, forming a C–O(MnO2)–C bond on the basal plane of H2SO4-GIC through the electrophilic addition reaction. The subsequent cleavage of the Mn–O bond results in the formation of epoxide on the basal plane of H2SO4-GIC and the solid byproduct MnO2. After the formation of epoxy group, the HOMO energy becomes −4.70 eV (Table , epoxy-GO), slightly lower than that of H2SO4-GIC (−4.93 eV). The formation of the epoxy group leads to the activation of six-membered rings in the form of sp2 to sp3 conversion.

Table 1

HOMO and LUMO Spatial Distribution and Energy of the Oxidizing Agent and the Simulated Interim Products for the Oxidation of Graphite

The as-formed epoxy group can be protonated to an oxonium ion [C2OH]+ in the medium of concentrated sulfuric acid, forming a conjugate acid-base pair with HSO4–. The electron cloud gathers around the positively charged oxonium ion because of the inductive effect, which causes the adjacent aromatic carbons to carry positive charges. The positive charges on the oxonium ion and aromatic carbons can repel the oxidizing agent MnO3+ and thus hinder their further oxidation. The HOMO energy of oxonium-GO increases to −7.47 eV (Table , oxonium-GO), reflecting a low potential of losing electrons, and the oxidized microdomain thus gets passivated.

However, as depicted in the LUMO spatial distribution of oxonium-GO, the α-carbon carries the positive charge, which is responsible for the instability of [C2OH]+ in the medium of sulfuric acid and the cleavage of a part of C–O bonds by nucleophilic attack to form monosulfate and trans-hydroxyl groups. The formation of monosulfate and trans-hydroxyl groups from the nucleophilic attack on [C2OH]+ results in the change of HOMO energy of the oxidized microdomain to −4.68 eV (Table , sulfate-GO), so the oxidized microdomain gets activated because of the cleavage of the oxonium ion, and the carbon skeleton can be further oxidized by the oxidizing agent MnO3+. The further oxidation reaction preferentially occurs at the activated sites with an anti-Hückel structure through para-position addition reaction.

As shown in the HOMO spatial distribution of sulfate-GO, the monosulfate −OSO3H lacks a molecular orbital, which matches with the energy level of an oxidizing agent MnO3+, so it is chemically inert. Furthermore, the steric hindrance effect from the tetrahedral structure of monosulfate hinders the collision of its C–O bond with the oxidizing agent MnO3+, which accounts for the stability of nearby oxygen-containing functional groups in the strong oxidizing medium. In addition, similar with the formation process of the oxonium ion [C2OH]+, the trans-hydroxyl can be protonated to form [OH2]+ in the medium of sulfuric acid, avoiding further oxidation by MnO3+.

Self-Regulating Feature of Graphite Oxidation

We also studied the proposed formation and conversion path from the point of view of energy, which helped to explain the self-regulating feature of graphite oxidation. The oxidization of graphite by the modified Hummers’ method is a spontaneous process. To investigate the energy variation of the system, the Gibbs free energy (ΔGθ) of each oxidation process (the formation of epoxy-GO, oxonium-GO, and sulfate-GO) was calculated by using the calculated standard Gibbs free energy of formation (ΔGθ) of each reactant and an interim oxidized product (see detailed calculation process in Supporting Information, Calculation data S1). The complex process of functionalization is rationally divided into two possible routes, and the corresponding Gibbs free energy (Figures and S6) was calculated based on the simulated model of 19 six-membered carbon rings. For the epoxidation of H2SO4-GIC, the reactants are graphite supercell and MnO3+, and the oxidized graphite can be divided into the infinitely extended aromatic domains and nonaromatic oxidized microdomains, which carries a part of the positive charge from MnO3+. On Route 1, the positive charge can transfer from the oxidized microdomains to the aromatic domains through conjugated π bonds, and the corresponding free energy is −10.87 eV. In other words, the carbon skeleton with an intact conjugation structure tends to donate one electron to the oxidized microdomains. The oxidized microdomains will advance toward the aromatic domains until the carried charge of aromatic domains reaches the redox potential of HSO4– (E° = 2.01 V for HSO4–/HSO4•). For the protonation process of an epoxide to an oxonium ion, the calculated free energy varies from −10.87 to −12.32 eV, releasing 1.45 eV energy. During the conversion of oxonium ion to monosulfate and trans-hydroxyl groups, the ring-opening reaction of the oxonium ion in the medium of sulfuric acid is an exothermic process, and the released energy is 3.17 eV.

Figure 4

Gibbs free energy of the simulated interim products with two routes of the oxidation process obtained by DFT calculations.

However, the oxidized microdomains will not infinitely advance toward the aromatic domains because the carried positive charge numbers of the aromatic domains can significantly affect its charge acceptance. For the 19-ring aromatic domain, which carries one positive charge, the conversion of it to the ground-state consumes 3.52 eV free energy. The conversion of aromatic domains with two positive charges to the ground-state consumes 14.56 eV free energy, which is larger than the energy 10.87 eV released from the epoxidation process of graphite, so the two positive charges is the threshold for the 19-ring aromatic domain. In this case, the epoxidation on Route 1 gets self-terminated and switches to Route 2. Therefore, the oxidation of the aromatic domain is self-regulated by its carried positive charge. On Route 2, the positive charge cannot transfer from the oxidized microdomains to the aromatic domains, and the corresponding Gibbs free energy is −5.21 eV. Because of the charge repulsion, the epoxy groups formed on Route 2 will not be protonated to oxonium ions but can be converted to monosulfate and hydroxyl groups. The ring-opening of epoxide propels the removal of the positive charge accumulated on the oxidized domain so that the oxidation will continue till the content of the monosulfate reaches its maximum, that is to say, all the epoxy groups are under the steric shielding of monosulfate.

The oxidation of aromatic domains is self-terminated after the carried charge of aromatic domains reaches the redox potential of HSO4–. As a result, the area of the aromatic islands is dependent on the concentration of HSO4–, and it decreases with the increase of HSO4– content. This is why a high oxidation degree of GO can generally be obtained using modified Hummers’ method, which applies auxiliary agents such as H3PO4, HNO3, H2O, and so forth and thus provides sufficient conjugate base of HSO4–.[11−13,30]

Conclusions

The oxidation of graphite by the modified Hummers’ method has been investigated through a combination of theoretical calculations and practical experiments. The major results are as follows:

The oxidizing agent MnO3+ is identified by its electrophilic nature. The intercalating-oxidation process damages the aromatic carbon skeleton of H2SO4-GIC and provides the origin for the diffusion and electrophilic addition of MnO3+.

The SSNMR, TGA, and XPS analyses of unhydrolyzed oxidized graphite indicate that the formed oxygen-containing functional groups are mainly hydroxyl and epoxide groups, as well as monosulfate. The intrinsic formation and conversion path of oxygen-containing functional groups is proposed with three corresponding interim oxidized products (epoxy-GO, oxonium-GO, and sulfate-GO).

The intrinsic formation and conversion path is interpreted with the FMO theory. The simulation results also show that the C–O bond of GO can be protected by the electrostatic repulsion between the oxonium ion and the oxidizing agent, as well as the steric hindrance of tetrahedral monosulfate, resulting in the chemistry stability of the oxidized domains.

The density functional theory (DFT) method is used to calculate the Gibbs free energy of oxidized microdomains, illustrating that the oxidized domains can capture electrons from the surrounding graphitic domains and result in the electron deficiency of the aromatic domains, elucidating the self-regulating feature of the aromatic islands.

Experimental and Computational Methods

Materials

Natural flake graphite with a uniform size of about 150 μm was purchased from Qingdao Meizhen Co., Ltd. Potassium permanganate (KMnO4, 99.5%, AR), sulfuric acid (H2SO4, 98%, AR), and barium chloride (BaCl2, 99.5%, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. Potassium persulfate (K2S2O8, 99.0%, AR), acetic acid (CH3COOH, 99.5%, AR), ethyl acetate (C4H8O2, 99.0%, AR), periodic acid (H5IO6, 99.0%, AR), and hydrogen peroxide aqueous solution (H2O2, 30.0 wt %, AR) were purchased from Aladdin Chemical Reagents Co., Ltd. All chemicals were used as received without further purification.

Preparation of Oxidized Graphite

A modified Hummers’ method was used to oxidize graphite.[11,47] First, 50 mL of concentrated H2SO4 was added into a 100 mL conical flask at 20 °C, and 3.0 g KMnO4 was dispersed in the acid with vigorous stirring for 10 min. Then, the KMnO4–H2SO4 mixture was heated to 50 °C in a water bath, and 0.5 g of graphite was carefully added into it as the formed oxidizing agent species is highly reactive and known to detonate at high temperatures or in contact with organic compounds.[15] The addition time of graphite was controlled at about 3 min, and vigorous stirring was applied to form a homogeneous mixture. To reveal the chemical and structural evolution during the graphite oxidization, the whole oxidation process was monitored at 9 points with 12 min intervals, namely, 0, 12, 24, 36, 48, 60, 72, 84, and 96 min. The 0 min is the starting point of oxidization, at which the graphite was just added into the KMnO4–H2SO4 mixture, while the 96 min is the endpoint of oxidization, where the graphite domain completely disappeared. The nine sampling points were named A0, A1, A2, A3, A4, A5, A6, A7, and A8, respectively. For each sampling point, the reaction mixture was poured into 150 mL of concentrated H2SO4 and then aliquoted into six 50 mL plastic centrifuge tubes, followed by repeated centrifugal washing with concentrated H2SO4 at 6000 rpm until the supernatant became clear and colorless, indicating a complete removal of the oxidizing agent (MnO3+). The centrifuge pellet of A0 was examined with X-ray diffraction (XRD).

To characterize the chemical composition of the unhydrolyzed oxidized graphite, each collected pellet (A0–A8) was first dispersed in K2S2O8–H2SO4 mixture (200 mL, 50 mg/mL) with stirring for 10 min to chemically reduce solid MnO2, followed by 5 min centrifugation at 6000 rpm. Although the standard electrode potential of K2S2O8 (+1.96 V for S2O82–/SO42–) is higher than that of MnO2 (+1.69 V for MnO4–/MnO2), in the medium of concentrated H2SO4, K2S2O8 has to decompose to form H2O2, which then reduces the solid MnO2 (see a detailed explanation in Supporting Information, Discussion S3). Second, the pellet was dispersed in concentrated H2SO4 and centrifuged for another 5 min at 6000 rpm to remove K2S2O8 so as to avoid its contact with the washing solvent in the subsequent step. Third, the pellet was redispersed in 200 mL of anhydrous acetic acid and centrifuged at 6000 rpm for 5 min repeatedly until the complete removal of H2SO4, K2SO4, and MnSO4 (no precipitate formed in supernatant in the presence of BaCl2 aqueous solution and no color change on pellet in the presence of H5IO6 aqueous solution). For an efficient and thorough drying, the pellet was further redispersed in anhydrous ethyl acetate and centrifuged (6000 rpm, 5 min) for two times to displace the anhydrous acetic acid, as the boiling point and saturation vapor pressure of ethyl acetate are lower than that of acetic acid. Finally, the pellet was transferred to a vacuum freeze-dryer and maintained at −40 °C until it completely dried. The unhydrolyzed oxidized graphite powder samples without any impurity ions were thus prepared and used for SSNMR, XPS, and TGA–differential scanning calorimetry (DSC) analyses.

Characterization

The crystalline phase and structure of oxidized graphite were characterized by XRD using a D8-Advance, Bruker AXS diffractometer (Cu Kα radiation, λ = 1.5418 Å) in the continuous scan mode over 5–60° (2θ) with a scan rate of 10° (2θ)/min, operating at 40 kV and 40 mA. Direct 13C pulse SSNMR (13C SSNMR) spectra were acquired on a Bruker Advance 400D spectrometer (50.3 MHz 13C, 200.1 MHz 1H). In particular, the spectra shown in Figures d and 2a were acquired with the 4 mm rotor spinning at 12.0 kHz, 90° 13C pulse, 39.9 ms FID acquisition time, and 3-relaxation delay for all samples and processed with 50 Hz (1 ppm) of line broadening. Relatively long relaxation delays were used to ensure meaningful relative signal intensities. TGA and DSC were performed on a NETZSCH STA 449F5 thermogravimetric analyzer. The samples (5 mg) were heated in a N2 atmosphere from room temperature to 800 °C at 5 °C·min–1. XPS analysis was carried on a PHI-5000 Versaprobe X-ray photoelectron spectrometer.

Computation

The spin density distribution, molecular orbital spatial distribution, and energy variation of the reactants and the simulated interim oxidized products were calculated at the B3LYP/6-31G(d) level with the Gaussian 09 program package.[48] The structure of the graphene supercell with n = 19 (n is the number of benzene rings in the graphene) was used in calculations. The edge of graphene was terminated with hydrogen atoms. Because the oxygen-containing functional groups are randomly distributed on the carbon skeleton (LK structural model), we selected a pair of adjacent carbon atoms on the plane of the carbon skeleton to generate an oxygen-containing group (epoxy group), which remained at the fixed position in the subsequent calculations for the protonation of epoxy and ring-opening of the oxonium ion. The atomic coordinates of calculated reactants and the simulated interim oxidized products are provided in Supporting Information (Calculation data S2).