# Effect of Nitrogen Atoms in the CNT Structure on the Gas Sensing Properties of PANI/CNT Composite
## 4 Conclusions  
The CCVD method was used to prepare CNTs with different content of nitrogen heteroatoms (0, 2, and 4 at.%). This made it possible to obtain nanotubes with the work functions 4.80 \(\pm\) 0.07, 4.65 \(\pm\) 0.08, and 4.56 \(\pm\) 0.12 eV for uCNT, N-CNT, and N\({}^{+}\)-CNT, respectively. With an increase in the nitrogen concentration in nanotubes, an increase is observed in their conductivity (uCNT--0.62 \(\pm\) 0.02 S/cm, N-CNT--1.76 \(\pm\) 0.16 S/cm, and N\({}^{+}\)-CNT--2.17 \(\pm\) 0.06 S/cm). The structural and chemical analysis of the obtained nanotubes showed that the chemical state of nitrogen in both doped CNTs is practically the same. The analysis of the chemical state of carbon showed that an increase in the concentration of nitrogen atoms in nanotubes causes a decrease in the content of sp\({}^{2}\)-hybridized carbon atoms (uCNT--88.2%, N-CNT--82.6%, and N\({}^{+}\)-CNT--73.6%). This indicates a higher degree of defectiveness of the outer CNT layers during doping, which is also confirmed by morphological studies of CNTs using TEM. It was found that PANI in all PANI/CNT composites has identical structural and chemical states, which is confirmed by NEXAFS and FTIR methods. The work function value is also the same for all composites (4.95 \(\pm\) 0.15 eV). Thus, the value of the potential barrier at the PANI-CNT interface was increased from 0.15 eV for PANI/uCNT to 0.3 and 0.39 eV for PANI/N-CNT and PANI/N\({}^{+}\)-CNT, respectively. At the same time, the dependence of the conductivity of both composites and nanotubes on the nitrogen concentration in nanotubes is different. Thus, \(\sigma\)(PANI/N-CNT) > \(\sigma\)(PANI/N\({}^{+}\)-CNT), while the conductivity of nanotubes increases at a higher concentration of nitrogen in them. This is indicative of a decrease in the contribution of N\({}^{+}\)-CNTs to the conductivity of the respective composite. Figure 7: Schematic of changes if the SCR of PANI/uCNT, PANI/N-CNT and PANI/N\({}^{+}\)-CNT composites during ammonia adsorption. The polymer morphology critically depends on the degree of imperfection of the nanotube surface. A uniform and thin polymer layer is formed on uCNT. On both nitrogen-doped CNTs, a polymer layer is of much greater thickness and has a relief morphology: globular on N-CNT and globular with sheet-like structures on N\({}^{+}\)-CNT. The analysis of the response of composites to ammonia was carried out on PANI/CNTs nanostructures. All samples showed similar response (\(\sim\)20 s) and recovery (\(\sim\)120 s) times. It was shown that the response value S of different nanostructures (they may contain different amounts of CNTs) of the same composite can vary in a wide range. However, the dependence of the response on the ammonia concentration (2-10 ppm) is linear, and the slope of this dependence (sensitivity, tg(S)) is the same for all composites. This indicates the possibility of using PANI/CNT nanostructures as an active layer of gas sensors for the quantitative determination of ammonia concentration. Thus, the sensitivity is 10.5 \(\pm\) 0.2, 15.3 \(\pm\) 0.5 and 2.2 \(\pm\) 0.1 ppm\({}^{-1}\) for PANI/uCNT, PANI/N-CNT and PANI/N\({}^{+}\)-CNT composites, respectively. An increase in the sensitivity of PANI/N-CNT as compared to PANI/uCNT is associated with a growth in the specific surface area of the sensitive layer. A significant decrease in the sensitivity to ammonia was found in PANI/N\({}^{+}\)-CNT, which is associated with a reduction in the effective surface area of polymer due to the presence of sheet-like structures on the surface, which are not involved in the conduction due to the full occupancy of the SCR. Thus, the morphological features of the composites had a dramatic effect on the gas sensing properties, which must be taken into account when developing new materials for sensitive elements of sensors. The length of this work does not allow us to separate the contribution of the morphology from the contribution of the potential barrier at the PANI-CNT interface to the sensing properties of nanostructures.