# Effect of Nitrogen Atoms in the CNT Structure on the Gas Sensing Properties of PANI/CNT Composite
## 3 Results  
TEM images for the nanotubes under study are shown in Figure 1A,D,G. According to the fast Fourier transform data, the interlayer spacing for all nanotubes is approximately equal to 0.35 nm, which is typical for this synthesis method [25]. It is clearly seen that the integrity of the outer walls of nanotubes deteriorates when nitrogen is introduced into the structure of nanotubes synthesized from acetonitrile taken as the carbon source (Figure 1D,G). From the SEM images of the PANI/CNT composites (Figure 1B,E,H) it can be seen that the polymer layer completely covers the nanotubes. At the same time, there are significant differences in the polymer morphology on different types of nanotubes. The most uniform and thin polymer layer is formed on uCNT (Figure 1B,C). A thicker PANI layer is formed on the N-CNT (Figure 1E,F) and N\({}^{+}\)-CNT (Figure 1H,I) surfaces, locally reaching 200 nm. Polymer in the PANI/N-CNT composite is mostly globular, and the TEM images show that the composite is rich in pores (Figure 1F). The morphology of the PANI/N\({}^{+}\)-CNT composite is generally similar to that of PANI/N-CNT, and however sheet-like structures are observed on the surface. An increase in the thickness of the polymer layer in the PANI/N-CNT and PANI/N\({}^{+}\)-CNT composites can be associated with a larger number of defects in nanotubes, which contributes to the growth of the polymer outside the nanotube plane. The results of the quantitative elemental XPS show that the nitrogen content in N-CNT and N\({}^{+}\)-CNT is \(\sim\)2 and \(\sim\)4 at.%, respectively (Table 1). A further increase in the concentration of nitrogen atoms in CNTs makes no sense since this leads to a critical decrease in the length of nanotubes with a slight change in the electronic characteristics [26]. The presence of oxygen in the XPS spectra of all synthesized nanotubes is due to its adsorption from the atmosphere, as well as the presence of chemically bonded functional groups of various types, which are always present on the surface [27]. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**CNTs Type**} & \multicolumn{3}{c}{**Concentration, at. \%**} \\ \cline{2-4}  & **[C]** & **[N]** & **[O]** \\ \hline uCNT & 96.0 & - & 4.0 \\ N-CNT & 93.4 & 2.1 & 4.5 \\ N\({}^{+}\)-CNT & 92.6 & 4.0 & 3.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical analysis of the samples by XPS. For a detailed analysis of the chemical state of carbon, C1s PE spectra were decomposed into 5 components [25] (Figure 2). The C1 component (-284.5-284.6 eV) corresponds to the state of sp\({}^{2}\)-hybridized carbon atoms that form the framework of carbon nanotubes. The C2 component (-285.1 eV) is associated with structural defects in graphene layers, as well as with the state of carbon atoms located in the graphene network near carbon atoms chemically bonded to C\({}^{*}\)-C(O) and C\({}^{*}\)-C(N) heteroatoms. The C3 component (-287 eV) conforms to carbon atoms chemically bonded to oxygen or nitrogen atoms by a single covalent bond (C-O, C-N). The C4 component (-288 eV) is associated with carbon atoms chemically bonded to oxygen or nitrogen atoms by a double covalent bond (C=O, C=N). The high-energy C5 component of the spectrum corresponds to the state of carbon atoms in the composition of -COOH groups [25]. It can be seen from the C1s spectra presented in Figure 2 that the full width at half maximum (FWHM) line in the spectra of doped CNTs is higher as compared to that in the spectrum of undoped CNTs, increasing with a higher degree of doping. This is evidence for the growth in defectiveness of graphene layers due to the presence of nitrogen heteroatoms [28]. Figure 1: TEM images of uCNT (**A**), N-CNT (**D**), and N\({}^{*}\)-CNT (**G**); SEM and TEM images of PANI/uCNT (**B**,**C**), PANI/N-CNT (**E**,**F**) and PANI/N\({}^{*}\)-CNT (**H**,**J**) composites. Figure 3 shows nitrogen spectra for nanotubes with various degrees of doping.