# Ammonia Gas Sensing Characteristic of P3 HT-rGO-MWCNT Composite Films
## 3 Results and Discussion
### Ammonia Gas Sensing of P3 HT-Based Composite Films  
According to the gas sensing principle, when a gas sensor contacts a certain gas (such as NH\({}_{3}\), NO\({}_{x}\), H\({}_{2}\), etc. ), its resistance changes. Depending on the nature of the material used (semiconductor oxides and/or composite film) and the measured gas, the resistance of the gas sensor will be decreased or increased. In this study, P3 HT is considered as a p-type organic semiconductor [39], so when the sensor's surface comes into contact with NH\({}_{3}\) gas, the resistance of the PGC sensor will be increased due to the decrease in the number of holes in P3 HT. When NH\({}_{3}\) molecules are adsorbed on the sensor's surface, electrons tend to recombine with holes, leading to a decrease in the primary carriers (holes), followed by increase in P3 HT resistance. For the faster re-adsorption of NH\({}_{3}\), the PGC sensors are heated at about 150 \({}^{\circ}\)C. The molecules of ammonia gas quickly left from the sensor surface, taking with them electrons from the above-coupled charge carriers. Therefore, holes will be regenerated in the P3 HT polymer, resulting in the decrease of the sensor resistance. In the study of the sensing properties of a resistance sensor, a performance parameter usually used is the sensitivity (\(\eta\)) that shows the difference (in percentage) between the initial resistance (\(R_{0}\)) and the resistance (\(R\)) of the sensor exposed in the monitored gas. Thus, \(\eta\) is expressed by the following equation:  
\[\eta=\frac{R-R_{0}}{R_{0}}\times 100(\%) \tag{2}\]  
Figure 4: (a) UV-Vis absorptance spectra and (b) plots of (\(\alpha\hbar\
u\))\({}^{2}\) vs. \(\hbar\
u\) of drop-casting P3 HT and PGC-60 composite films. From experimental measurements of the response time of the sensor at a NH\({}_{3}\) gas concentration of 70 ppm, the adsorption/desorption time (t) dependence of the sensitivity of the PGC sensors is plotted in Figure 5, which consists of three \(\eta\)-t curves corresponding to PGC-40, PGC-60, and PGC-80 devices. Figure 5 demonstrates the adsorption and desorption processes of the PGC-based sensors. This figure shows that in the first 50 s, Ar gaseous flow eliminated the contamination agents from the PGC surface; consequently the surface resistance of the sensor was not stable. After the cleaning of the sensor surface during 30 s, the introduced NH\({}_{3}\) was adsorbed onto the sensor surface, resulting in the increase in resistance. In the subsequent cycles, the NH\({}_{3}\) desorption/adsorption process led to decrease and increase of the resistance of sensors, respectively, with results similar to those reported in [40]. The best sensitivity of the device was found to be about 5.0%; desorption ability, especially, was 95% in the 60 s for the PGC-60 sensor where rGO content in the composite film is 60 wt.% (Figure 5b). In case the amount of rGO is lower in the PGC-40 sample, the accumulated P3 HT chains become larger. On the opposite side, the larger amount of rGO in the PGC-80 device leads to the overlap of the rGO sheet on the P3 HT chain. In both cases, their gas sensitivity and desorption capability of composite film decrease significantly (Figure 5a,c). Thus, the optimum content of rGO in the composite NH\({}_{3}\) gas sensor was found to be 60 wt%, corresponding to the weight ratio of P3 HT:rGO:MWCNT equal to 20:60:20. A comparison of the sensor sensitivity caused by different active materials for NH\({}_{3}\) detection and the data obtained in this work are shown in Table 1.  
Figure 5: **(a-c)** Sensitivity vs. rGO content in the PGC composite sensors, and **(d)** NH\({}_{3}\) gas sensitivity of pristine P3 HT. Figure 5 demonstrates that, compared to the use of the pristine P3 HT polymer (Figure 5d, sensor sensitivity was significantly improved when using the rGO-MWCNTs-P3 HT composite. [PERSON] et al. also obtained similar results for sensors based on polypyrrole (Py)-rGO blends [44].