## 3 Results
The last component at 291 eV corresponds to the \(\pi\)-\(\pi^{*}\) shake-up satellite, which arises due to the transition of an electron from the occupied state of the valence band to the unoccupied state of the conduction band during the C1s photoionization process [14]. This peak is observed in aromatic systems due to the presence of \(\pi\)-conjugated delocalized electrons and is often used to assess the degree of graphitization of a carbon material: the more intense this peak, the more sp\({}^{2}\) bonds there are in the material [47]. Taking into account the fitting results (a low value of the FWHM parameter C1s line equal to 0.77 eV, a high-intensity C=C component, and the presence of a \(\pi\)-\(\pi^{*}\) shake-up satellite), we can conclude that the original MWCNTs have a high degree of graphitization. This result is consistent with TEM and XRD data. It can be seen that in the C1s spectrum of irradiated N-MWCNTs (Figure 8, top panel), the main PE line is shifted toward higher binding energies by approximately \(\sim\)0.2 eV relative to its position in the spectrum of MWCNTs (284.7 eV), as well as being broadened from 0.77 to 1.21 eV. This result is mainly explained by an increase in the spectrum component at 285.2 eV, which is associated with the ion-induced formation of point and extended structural defects, including the incorporation of nitrogen atoms in various configurations into the graphene walls in the near-surface region of MWCNTs. In this case, the C1s binding energy of carbon atoms linked to nitrogen atoms (C-N) according to literature data lies between 285.5 and 285.9 eV, which overlaps with the range of C1s binding energies for carbon bonds in C-C (285.1-285.6 eV) [14; 57; 58; 59]. Thus, the component at 285.2 eV is a combination of C1s PE signals from C-C and C-N bonds. At the same time, it is clear that in the C1s PE spectrum of N-MWCNTs, the sufficiently intense sp\({}^{2}\) component (C=C) and a \(\pi\)-\(\pi^{*}\) shake-up satellite is observed. Considering that the exit depth of the C1s photoelectrons when using Al K\(\alpha\) radiation (\(l\
u\) = 1487 eV) is up to 2 nm, we can assume that all damages caused by ion irradiation and observed in the C1s PE spectrum occur to a large extent in the near-surface region (up to 6-7 graphene layers) of MWCNTs. In addition, it should be noted that during ion irradiation, the number of oxygen-containing functional groups (OCFGs) on the surface of MWCNTs increases noticeably (Table 1), since the intensity of their C1s PE lines becomes greater. This phenomenon is associated with the attachment of additional OCFGs at sites of structural defects formed after exposure of the irradiated samples to the atmosphere (environment) [48]. The evaluation of the chemical state of nitrogen was performed using N1s PE spectra (depicted in Figure 9a). As can be seen, in the spectrum of the pristine MWCNTs, nitrogen is completely absent, while in the spectrum of irradiated N-MWCNTs, due to fitting, three components are observed with maxima positioned at binding energies of approximately \(\sim\)401.2 eV (N-Q), 399.8 eV (N-pyrr), and 398.7 eV (N-pyrr). These binding energies correspond to 1s electron energies of the nitrogen atom in graphitic/quaternary, pyrrolic, and pyridinic forms [14] (as depicted in Figure 9b). It is noteworthy that the N-pyrr component has the highest intensity, and its relative contribution to the total intensity of the N1s PE line is 63%, wherein the relative proportions of N-pyr and N-Q are approximately the same and amount to 18.5%. Summing up the results of the analysis of the N1s PE spectrum of N-MWCNTs, it is important to note that as a result of irradiation with N\({}^{+}\) ions, all nitrogen atoms are incorporated into the structure of the nanotube walls and chemically bonds with carbon in three configurations. In this case, the fraction of catalytically active nitrogen (nitrogen in pyridinic and pyrrolic configurations) accounts for about 1 at.% of the total nitrogen concentration Ntot (see Table 1).