## 3 Results and Discussion
### Characteristics of the Graphene Materials
The average stacking number of graphene layers (N\({}_{\text{L}}\)) in the exfoliated samples was estimated by using the layer-to-layer distance (d\({}_{(002)}\)) and the size of the crystallite measured from the width of the diffraction peak of the (002) reflection, using the Scherrer equation [12]. The N\({}_{\text{L}}\) calculated show higher values than the ones expected according to the S\({}_{\text{BET}}\) obtained. This may be due to the fact that the XRD signal is strongly influenced by the thicker particles, because although these constitute a minor proportion in the total of the sample, they are the ones with the highest diffraction signal. The X-ray crystalline parameters are shown in Table 1.  
Some morphological evidences of the differences between rGO and NrGO were determined by TEM (Figure 2). The rGO TEM images exhibited the presence of winkled structures of graphene. The introduction of nitrogen in the graphitic structure--the NrGO samples--did not produce noticeable difference in the morphology of the graphene sheets. High resolution transmission electron microscopy (HRTEM) characterization further showed that rGO\({}_{\text{m}}\)-3 and NrGO\({}_{\text{m}}\)-4 samples consist of 5-12 graphene layers (Figure S3). The elemental analysis technique was used to determine the amount of nitrogen atoms in the graphene materials (see Table 1 for N wt %). For the rGO sample, nitrogen was not found. The  
Figure 3: XRD patterns of reduced graphene materials. NrGO\({}_{325}\)-4 sample presents the higher N content among the NrGO samples with 5.0 wt %, and a gradual decrease of the N content values was observed as the size of starting graphite increases. There are two explanations for this phenomenon. First, this tendency can be explained in terms of the higher degree of oxidation obtained in GO samples with smaller graphite particle sizes [38]. It is known that the oxygen functional groups in GO including carbonyl, carboxylic, lactone, and quinone groups are responsible for reacting with NH\({}_{3}\) to form C-N bonds, allowing the incorporation of N in the structure [34]. Thus, a smaller graphite particle size favors the formation of GO with a higher degree of oxidation, which could lead to higher N contents. Besides, the higher degree of exfoliation of small crystals also facilitates the contact of the ammonia with the oxygen groups in the basal plane of the sheets, leading to a better incorporation of N atoms. XPS is a powerful technique to identify the chemical states of the surface species. It was used to analyze the surface of the different graphene materials from the characteristic XPS peaks corresponding to C, N, and O regions (Figure S4 ESI shows general XPS spectra). The results obtained from the analysis of the C1s, O1s, and N1s individual high-resolution spectra are shown in Table 2. The assignment of the components of the N1s, O1s, and C1s region is not straightforward. The value of binding energy observed for the different functional groups of these elements varies in the literature. It may be due to the specific environments of the atoms and the redistribution of electrons after the ionization of the sample [30; 41; 42; 43]. Nitrogen peak deconvolution for the NrGO\({}_{\text{m}}\)-4 samples (Figure 4) indicated the presence of four elementary peaks: pyridinic nitrogen (399.5-398.5 eV), pyrrol nitrogen (400.8-399.8 eV), quaternary nitrogen (403.0-401.0 eV), and NOx groups (404.9-405.6 eV) [10; 44; 45; 46; 47; 48]. XPS analysis indicated that about 3.2-3.4 at % N was found in the surface of the graphene sheets after ammonia treatment. These values are lower than those corresponding to the bulk N% content obtained by elemental analysis. This difference could be attributed to inhomogeneous nitrogen doping of the graphenic materials, since it was inferior at the surface analyzed by XPS. For FGO samples, the nitrogen peak was undetected.