**5. Reactions on the surfaces of graphene and graphite**

The performances of different reactions on graphene and graphite such as catalysis, oxidation and reduction, as well as functionalization, are dependent on their surface properties, which are the primary areas of interest. N-doped carbon materials are promising catalysts that exhibit high electrocatalytic activity for the oxygen reduction reaction with high durability [37]. By using HOPG as a model, several types of N-doped HOPG model catalysts were created to investigate the active sites in the oxygen reduction reaction [38]. These N-doped HOPG models included clean-HOPG, Ar+ ion sputtered HOPG and HOPG modified with different types of N such as pyridinic and graphitic N on the surface. The introduction of different types of N on the HOPG surface was verified by a least-square curve fitting analysis of XPS N1s spectra. The pyridinic N and graphitic N component peaks were assigned at 398.5 and 401.1 eV, respectively. The active sites of the N-doped HOPG surface were then subjected to oxygen reduction reaction and followed by XPS characterization. It was found that

the concentration of pyridinic N decreased, while the concentration of pyridonic N increased. This result suggests that the C atom next to pyridinic N is the active site, which reacts with the OH species formed during the oxygen reduction reaction, accompanied by the transformation of the pyridinic N to the pyridonic N.

Surface functionalization of graphene and graphite is also an essential process to enable them to be used for various applications owing to the changes of the surface chemistry such as hydrophobicity and hydrophilicity. For example, hydrogenation reaction is a common way to introduce C-H bonds on the surfaces of graphene and graphite by converting sp2 carbons to sp3 carbons [39]. Birch reduction has been used to convert localized sp2 carbons to sp3 carbons at the edges of the graphite surface without creating additional defects in the graphite structure [40, 41]. **Figure 7a** shows a comparison of C1s spectra of graphite powder and hydrogenated graphite powder and a curve-fitting analysis of the two spectra. From the XPS measurements of graphite and hydrogenated graphite powders annealed at 500°C, the binding energy of the C1s main peak of the hydrogenated graphite powder shifted by 0.4 eV toward the high-binding-energy compared to that of the graphite powder [42]. In the C1s spectrum of the graphite powder, only a dominant component peak representing sp2 carbons and a small component peak representing sp3 carbons are observed. The decrease and increase in the intensity of the sp2 and sp3 carbon component peaks, respectively, observed in the C1s spectrum of the hydrogenated graphite powder confirm the conversion of sp2 to sp3 carbons. However, one key disadvantage of using XPS in the above analysis is that XPS cannot detect H and exactly confirm the increase in the hydrogen concentration on a hydrogenated graphite powder surface. By contrast, ToF-SIMS is extremely sensitive to hydrogen and can be used for this purpose instead. Both the ToF-SIMS negative ion spectra of graphite and hydrogenated graphite powders show Cx − and CxH− ion series; nevertheless, the major difference between these two spectra is the increase of normalized intensities of the H<sup>−</sup> and CxH− ions after hydrogenation (**Figure 7b**), strongly indicating a higher concentration of hydrogen on the hydrogenated graphite powder surface.

Functionalization of graphene and graphite usually involves oxidation of their surfaces by different processes [43]. Graphene and graphite oxides are compounds consisting of carbon, oxygen and hydrogen with different ratios, obtained by

#### **Figure 7.**

*(a) A comparison of the XPS C1s spectra of the HOPG, graphite powder and hydrogenated graphite powder, XPS C1s curve-fitting results for graphite powder and hydrogenated graphite powder after annealing at 500°C. The black ( ) and green ( ) lines represent the experimental and curve-fitted spectra, respectively. (b) ToF-SIMS negative ion spectra of graphite powder and hydrogenated graphite powder after annealing at 500°C [42].*

### *Surface Analysis of Graphene and Graphite DOI: http://dx.doi.org/10.5772/intechopen.108203*

treating graphite with strong oxidizers. One common method to produce graphene and graphite oxides is the modified Hummers' method (MHM) [44, 45]. The oxidation results in incorporation of hydroxyl and epoxy groups into the basal plane and carboxyl groups at the edges of graphene and graphite [46]. The existence of these functional groups, of which the chemical states and concentrations, can be determined by XPS. One of the key difficulties to differentiate between hydroxyl and epoxy groups is due to their similar C1s binding energies. To solve this problem, chemical derivatization XPS has been commonly used. Trifluoroacetic anhydride (TFAA) chemical derivatization has been applied to quantify the hydroxyl groups on different surfaces [47, 48]. TFAA mainly reacts with the hydroxyl groups, leading to the formation of CF3 and ester carbons with identical concentrations on the surface (**Figure 8a**). After the TFAA derivatization, the peak, which originally corresponds to epoxy and hydroxyl groups, becomes mainly associated with the epoxy groups. Following this ideal, graphite oxide powder was first prepared using the MHM. The surface of this resulting graphite oxide powder was analyzed by XPS and the results show that the surface mainly consists of C (74.4 at%) and O (25.2 at%) [49]. After the labeling of the hydroxyl groups with TFAA, a small decrease of the C-O concentration is observed as shown in **Figure 8b**, and only 1.4 at% of F was detected. According to the reaction mechanism, the reaction between one TFAA molecule and one hydroxyl group introduces three F atoms on surface, this means that the carbons representing the hydroxyl groups should be less than 0.5 at% and the C-O component peak in the C1s spectrum of graphite oxide powder is mostly due to epoxy groups on the surface.

The oxygen-containing functional groups on the surfaces of graphene and graphite oxides can be further modified with different molecules through covalent or noncovalent bonds. Theoretical calculations have shown that the epoxy groups on graphene and graphite oxide surfaces play an important role in the reactions with different gases, such as oxidizing SO2 to SO3 at room temperature [50]. Based on the XPS result showing that graphite oxide powder prepared using the MHM contains epoxy groups, the reaction between the graphite oxide powder prepared by MHM and SO2 was carried out [49]. The reaction between the graphite oxide powder and SO2 produced about 3.8 at% of S on its surface. A comparison of the spectra (**Figure 9a** and **b**) of

#### **Figure 8.**

*(a) Reaction mechanism between TFAA and the hydroxyl group on a surface. (b) A comparison of C1s spectra of graphite oxide powder before and after the reaction with TFAA at 25°C [49].*

**Figure 9.**

*XPS C1s normalized spectra of graphite oxide powder (a) before and (b) after the reaction with SO2 obtained at 25°C. The black ( ) and green ( ) lines represent the experimental and curve-fitted spectra, respectively. ToF-SIMS negative ion spectra of graphite oxide powder (c) before and (d) after the reaction with SO2 obtained at 25°C [49].*

graphite oxide powder before and after the reaction shows that the C-O peak decreases while peak representing the carboxyl group almost stays the same after the reaction, suggesting that the SO2 mainly reacts with the epoxy groups to produce the C-O-SO3H groups. ToF-SIMS spectra of graphite oxide powder before and after the reaction with SO2 were acquired. The Cx − and CxH− ion series representing the graphite peaks are the dominant components in the ToF-SIMS negative ion spectrum of graphite oxide powder (**Figure 9c**). After the reaction between the graphite oxide powder and SO2, the sulfate-related peaks, such as HSO4 − and SO4 − , become the dominant peaks in the spectrum (**Figure 9d**). The changes can also be seen from the increase of normalized ion intensities of HSO4 − , SO4 − and SO3 − ions after the reaction with SO2. In addition, the intensities of CH2SO4 − and C2H3SO4 − ions also increase, indicating that most of the SO2 had reacted at and attached to the oxidation sites on the surface of the graphite oxide powder. In summary, the epoxy groups on the surface of the graphite oxide powder are responsible for the oxidation of SO2 into bisulfate.
