## 3 Results and Discussion
### Characteristics of the Graphene Materials
The percentage of pyridinic N species was slightly higher for the samples obtained from smaller sizes and quaternary nitrogen was higher for the samples obtained from bigger ones. The pyridinic nitrogen is the most basic among the different N species. Therefore, the basicity of carbon catalysts is related to the content of pyridinic groups [48; 49]. Figure 4: XPS spectra of the N 1s region for NrGO100-4, NrGO10-4, and NrGO325-4 samples. The C1s spectra was solved considering five components, which can be assigned to graphitic sp\({}^{2}\) carbon atoms (284.6-285.1 eV), the C-O bonds present in alcohol or ether groups (286.3-287.0 eV), C=O functional groups (287.5-288.1 eV), carboxyl or ester groups (289.3-290 eV), and a fifth wide shake-up satellite peak representing the \(\pi\)-\(\pi^{\star}\) transitions of aromatic rings (291.2-292.1 eV) [46; 50]. Concerning O1s, the curve was fitted considering four contributions corresponding to carbonyl groups (531.1-531.8 eV), epoxide and hydroxyl groups (532.3-533.3 eV), carboxylic groups (534.0-534.4 eV), and chemisorbed H\({}_{2}\)O or oxygen (535.5-536.1 eV) [43; 46; 50; 51; 52]. The spectra analysis revealed that surface oxygen content varies from 3.6-4.1 at % to 6.8-7.0 at % for N-doped and non-doped samples, respectively. The relatively small amount of oxygen with respect to that of carbon may be attributed to the process temperature. It is known that higher temperatures during the thermal treatment produce a decrease of the amount of oxygen functional groups [25]. The oxygen/carbon atomic ratios also confirm this, as can be seen in Table 2. The differences in these ratios for the various samples can be ascribed to an enhancement in oxidation degree from the original GO\({}_{\text{m}}\). For samples from raw graphite with smaller particle sizes, the oxidation degree of GO is higher. After the thermal treatment, the final content of oxygen is slightly higher for samples from graphite with a smaller particle size. The point of zero charge (PZC) was used to assess the surface chemistry of the graphene materials [9], because the surface charge of carbon materials is directed by the type and population of functional groups and the pH. The NrGO samples (Figure 5) show a PZC of 8.5-8.7, while rGO samples exhibit lower PZC values of 7.2-7.4. Therefore, the rGO samples have a practically neutral surface, while the NrGO surfaces are more basic. The incorporation of nitrogen atoms into the graphene structure means more electrons in comparison with carbon atoms and this fact favors the delocalization of p electrons in N-doped samples, leading to changes in the hydrophobicity of the surface. This excess of electrons produces a higher basic strength of NrGO surfaces [47]. Raman spectroscopy is a very useful tool to evaluate the degree of disorder in the structure of graphene [8]. Figure 6 shows Raman spectra for rGO\({}_{\text{m}}\)-3 and NrGO\({}_{\text{m}}\)-4 samples. As Raman spectra at discrete spots cannot provide an overall picture in the case of non-uniform defects distributions in the sample, spectral mapping was used to acquire 25 points over a \(50\times 50\)\(\upmu\)m\({}^{2}\) area. Two main peaks corresponding to vibrations with E2g symmetry in the graphitic lattice (G band) at 1580 cm\({}^{-1}\) and to graphite edges or structural defects (D bands) at 1345 cm\({}^{-1}\) were observed. Another two featured peaks have been reported in previous studies. A band D\({}^{\prime}\) peak appears at 1625 cm\({}^{-1}\) as a shoulder of the G band. It arises from alterations in the tension of sp\({}^{2}\) carbon atoms in the lattice caused by the arrangement of the electronic cloud [50]. A second peak assigned as 2D (historically called G\({}^{\prime}\)) is always present at 2700 cm\({}^{-1}\) in the spectra of graphene materials [40].