# Effect of Nitrogen Atoms in the CNT Structure on the Gas Sensing Properties of PANI/CNT Composite
## 3 Results
Despite the fact that this layer is located only near nanotubes, it can be one of the main types of conduction channels in the composite (along with CNTs). Figure 4: C1s NEXAFS spectra of PANI/uCNT, PANI/N-CNT and PANI/N\({}^{+}\)-CNT. Figure 5: Conductivity (**A**) and work functions (**B**) of uCNT, N-CNT, N\({}^{+}\)-CNT and PANI/uCNT, PANI/N-CNT, PANI/N\({}^{+}\)-CNT composites. It should be noted that the conductivity measured on individual nanotubes is three orders of magnitude higher than the conductivity measured on pressed CNT pellets [35]. Such a decrease in conductivity can be explained by the contact resistance between individual nanotubes in a pellet. Polyaniline is a plastic electrically conductive material, and when pressed, good contact is formed between the individual components of the composite. Therefore, the close absolute values of the specific conductivity of nanotubes and composites should not be misleading. Summarizing, we can say that, during the CCVD synthesis of nanotubes with different nitrogen content, the defectiveness of the outer walls of CNTs increases. At the same time, the work function decreases from 4.8 eV for PANI/uCNT to 4.65 and 4.56 eV for N-CNT and N+-CNT, respectively. Since the conditions of PANI/CNT synthesis are identical, the work function is the same in all composites and amounts to 4.95 eV. Thus, the problem of an increased potential barrier at the PANI-CNT boundary has been successfully solved. However, the morphology of polymer grown on uCNT, N-CNT, and N+-CNT is fundamentally different, which can make changes in the measurement of gas-sensing properties of composites. A schematic diagram of measurement of gas sensitivity of composites and an example of the studied nanostructure are shown in Figure 6A. The change in resistance is recorded at the time step 1 s. Figure 6B shows the distribution of resistive response and resistance (R\({}_{0}\)) in air for five different PANI/N-CNT nanostructures. For all samples under study, the resistance lies in the range from several tens to several hundreds of k\(\Omega\). The response value is different for each nanostructure at the same ammonia concentration. However, the slope of \(S\) relative to the ammonia concentration (tg(\(S\))) is close within one composite. Figure 6C gives an example of the gas test results for typical PANI/CNT, PANI/N-CNT and PANI/N+-CNT nanostructures (three measurements per each NH\({}_{3}\) concentration). Response \(S\), calculated as a relative change in resistance, is positive for all composites and increases with increasing ammonia dose. The slope of \(S\) relative to the ammonia concentration (sensitivitytg(\(S\))) equals 10.5 \(\pm\) 0.2, 15.2 \(\pm\) 0.5, and 2.2 \(\pm\) 0.1 for PANI/CNT, PANI/N-CNT, and PANI/N+-CNT, respectively. This points to the possibility of using PANI/CNT nanostructures as an active layer of gas sensors for the quantitative determination of ammonia concentration. In addition, all composites show fast response time (15-25 s) and recovery time (100-150 s) regardless of the ammonia concentration (Figure S2). It should be noted that the sensory characteristics of PANI/CNTs of composites strongly depend on the structures on which the tests are carried out. For example, the response time can vary from units of seconds on nanofibers and sparse grids [36] up to hundreds of seconds on as-prepared films [37]. Sensitivity can also vary from fractions of units per ppm to several tens per ppm [38]. Thus, the sensor characteristics obtained by us show good values comparable to those obtained on nanofibers and sparse grids. Figure 6: Schematic of the experimental platform (**A**); response of various PANI/N-CNT nanostructures (**B**); response of typical PANI/uCNT, PANI/N-CNT and PANI/N+-CNT nanostructures (**C**). It is well known that PANI in the doped form has a great and reversible response to NH\({}_{3}\) gas. Ammonia reacts with the H\({}^{+}\) dopant proton, absorbing available carriers [39; 40].