A Flexible Ammonia Gas Sensor Based on a Grafted Polyaniline Grown on a Polydopamine-Coated Polymer Film (2020)

Ammonia gas is generated by various industrial and agricultural activities and is considered an alternative energy to replace petroleum. However, if such a toxic gas becomes widespread as an energy source, leakage detection will be essential. Therefore, the development of a small and low-cost sensor that can easily measure the concentration of ammonia gas will be increasingly needed. In order to develop a new ammonia gas sensor that meets various other requirements such as low power consumption and good flexibility, while also improving basic performance, it is essential to develop novel gas-sensing materials. We report a new method that can uniformly and conveniently prepare a grafted conductive polyaniline (PANI) film on a flexible polyethylene terephthalate (PET) film, which can realize the development of a sensor that is flexible and does not require a heater. A flexible ammonia gas sensor was prepared by graft polymerizing PANI on PET film. In order to carry out graft polymerization, reaction sites are needed on the PET surface. For that purpose, polydopamine (PD) was coated on the PET film first. Subsequently, the PD surface was modified with p-phenylenediamine (PPD) to further increase the amino groups for reaction sites and then reacted with the aniline monomer. As the oxidative polymerization progressed, a polyaniline grafted film was grown on the film. After the growth of the PANI grafted film, the uniform formation of a granular morphology with a diameter of around 50 nm was observed on the PET film surface. After the graft polymerization, the initial resistance value of the PET film decreased and the film became responsive to ammonia gas. The sensor characteristics of the PANI grafted film prepared under the optimized conditions increased the response value more than tenfold compared with the sensor composed of deposited film. The response was sufficiently large to 250 ppm of ammonia gas, and it can be concluded that ammonia gas detection below 25 ppm is sufficiently possible. In addition, the response time was shortened, and the recovery rate reached 90% or more even at 50 °C. These good results were not observed when graft polymerization was carried out without PPD treatment. These results indicate that PPD treatment increased the concentration of amino groups on the PD surface, increased the surface density of the PANI grafted film, and formed a brush-like morphology, resulting in easier diffusion of gas into the graft chains. The flexible sensor is supposed to be attached to a curved surface for use. Therefore, it is required that the gas can be detected even when the sensor is bent, and the gas response characteristics must be the same as in the case where the sensor is not bent. When we actually held the grafted film sensor by hand and bent it, we confirmed that there was no problem with mechanical flexibility. The results of the measurement of the sensor characteristics are shown in the figure without bending and with bending inward at an angle of -60°. The vertical axis represents the value when the response value in the unbent state is 100%. Even when bent at -60°, the sensor responded to ammonia gas and the response time was significantly shortened. However, the response value decreased to a little more than half of the value without bending. Therefore, in terms of practicality for use as a flexible sensor, further improvement is necessary to achieve the goal. In addition, we also examined the change in sensor characteristics from before to after the test in which the sensor was bent and stretched 500 times repeatedly. Figure 1