# Effect of Nitrogen Atoms in the CNT Structure on the Gas Sensing Properties of PANI/CNT Composite
## 3 Results
Nitrogen in each nanotube type is in 5 different states, which correspond with pyridinic (N1), pyrrol (N2) and graphitic (quaternary, N3) configurations as well as to oxidized (N4) and molecular nitrogen (N5) [25; 29]. The spectral fitting results indicate that the intensity ratio of the components corresponding to different states of nitrogen is hardly affected by the degree of doping. Figure 4 shows C1s NEXAFS spectra of the studied PANI/CNT composites. The absorption spectra for carbon reflect the transition from the C 1s core level to the unoccupied C 2s and 2p states of the conduction band, which have \(\pi^{\star}\) and \(\sigma^{\star}\) symmetry. It is known that C1s NEXAFS spectra of carbon nanotubes have two local maxima located at the photon energies \(\sim\)285.3-285.4 and \(\sim\)291.8 eV corresponding to the \(\pi^{\star}\) and \(\sigma^{\star}\) resonance, respectively (the main absorption structures of nanotubes are shown in Figure 4, vertical lines with resonance designations) [25; 29]. The most intense peak at the photon energy \(\sim\)285.3 eV corresponds to the \(\pi^{\star}\) states of carbon atoms in the composition of carbon-carbon bonds. The low-energy (\(\sim\)283.7 eV) maximum corresponds to the \(\pi^{\star}\) states of carbon atoms bonded to imine nitrogen (=N-) [30]. The maximum at the photon energy \(\sim\)286.8 eV is associated with the \(\pi^{\star}\) states of carbon atoms bonded to amine nitrogen (-NH-) [30]. In doing so, the maxima at the photon energies \(\sim\)288.6 and 290.0 eV conform to the \(\pi^{\star}\)(C=O) and \(\sigma^{\star}\)(C-O) states of carbon atoms in the carbon-oxygen groups [31]. In the far region of the spectrum, the maxima at the photon energies \(\sim\)294.6 and 302 eV reflect carbon states with the \(\sigma^{\star}\) symmetry. It is important to note that the position of the \(\sigma^{\star}\)(C=C) resonance in the NEXAFS spectrum of MWCNTs is \(\sim\)291.8 eV [25]. This fact allows us to conclude that the main signal in the C1s NEXAFS spectra of the composites is introduced by carbon, which is part of PANI. Thus, the spectra of the composites have the same set of local maxima and almost coincident relative intensities, which suggests close structural-chemical states of  
Figure 3: N1s PE spectra of N-CNT (**a**) and N\({}^{+}\)-CNT (**b**). Figure 2: C1s PE spectra of uCNT (**a**), N-CNT (**b**) and N\({}^{+}\)-CNT (**c**). their surface layers. This conclusion is also supported by the Fourier-transform infrared spectroscopy data (see Figure S1). The conductivity of nanotubes (as expected) increases at a higher content of nitrogen heteroatoms (Figure 5A), which increases the concentration of charge carriers in CNTs [32]. The conductivity of the PANI/N-CNT and PANI/N\({}^{+}\)-CNT composites is higher than that of the undoped nanotube composite. However, the high conductivity of N\({}^{+}\)-CNT does not provide an increase in the conductivity of the PANI/N\({}^{+}\)-CNT composite relative to PANI/N-CNT. This effect can be associated with a decrease in the contribution of nanotubes to the conductivity of composites due to a higher energy barrier between the polymer and nanotube. The barrier value can be estimated from the difference between the work functions (\(\Phi\), eV) of the polymer and the nanotube. It was determined by the scanning Kelvin probe microscopy (SKPM) technique described elsewhere [21; 33]. The work function (Figure 5B) for uCNT, N-CNT, and N\({}^{+}\)-CNT are 4.80 \(\pm\) 0.07, 4.65 \(\pm\) 0.08, and 4.56 \(\pm\) 0.12 eV, respectively. The work functions for all PANI composites are \(\Phi\) = 4.95 \(\pm\) 0.15 eV. Thus, the PANI/N\({}^{+}\)-CNT composite has the highest energy barrier. Another important factor in reducing the conductivity of PANI/N\({}^{+}\)-CNT from that of PANI/N-CNT may be the disruption of the highly ordered PANI layer due to serious damage to the N\({}^{+}\)-CNT graphene plane [34].