The Ammonia Sensitivity Based On The Thin Films Of Conductive Polymers And Their Corresponding Metal Oxide Composites

The detection of the NH₃ is an important task in many technological fields such as industrial processes, clinical diagnosis and environmental minitoring. Many gas sensors based on metal-oxide thin films and/or thick films with dopants and cataluzers have been developed, but they need elevated working temperauture in the range from 250 to 500 oC in order to activate the adsorption and desorption phenomena of the test gas. On the contrary, the organic films and in particular the conductive polymers such as the polypyrrole have great advantages in comparison to metal-oxide devices for their higher sensitivitiy towards toxic gases, for the lower detectable limit in the range of few tens of ppm, and for their possibility to operate at or near room temperature.Heterogeneous conducting polymer nanocomposites, especially organic–inorganic nanocomposites, have drawn the attention of scientists over the last years due to a wide range of potential application of these composites. Inorganic nanoparticles stand for a class of novel materials having promising applicationin broad fields, and conducting polymer/inorganic nanocomposites are particularly challenging due to the combined nature of flexibilities and improved process ability of polymers, and attractive modulus, transparency, surface hardness and heat resistance properties of inorganic components. Based on the above background, we prepared gas sensors by in-situ polymerization in the presence of mental oxide on the sensor electrode to study the gas-sensitive of the composite materials.First, we prepared polypyrrole thin film with different thickness on sensor electrode by control polymerization time. The roughness of films was reduced and the polymer films gradually became smooth with polymerization time increased. Thickness dependence of polypyrrole films on the sensitive nature of ammonia was studied systematicly. The results showed that the thickness of polymer films affected the gas sensor sensitivity and response time directly. The thicker the polypyrrole thin films, the slower diffusion velocity of ammonia in polymer internal and the longer response time were. According to the simple model established on the Langmuir theory and developed by Hwang and Lin, we derived the relationship among the rate change of resistance, gas concentrations, film thickness and adsorption equilibrium constants. There was a proportional relationship between reciprocal of resistance changes rate of polypyrrole sensor and the reciprocal of gas concentration in our formula. The resistance change rate can be measured in accordance with the concentration of ammonia in experiment determined one linear equation which can be used for ammonia gas detection. Furthermore, from the relationship that the resistance change rate of the films were inverseflame proportional with the thickness of film, directly proportional with adsorption equilibrium and resistance change of polymer sensor after exposed to ammonia, but t related to sensor electrode spacing,.In order to improve the sensitivity of polypyrrole sensor to ammonia and shorten their response time, we have added Fe₃O₄ into polypyrrole to prepare the PPy/Fe₃O₄ composite film. The results of the ammonia gas test showed that the addition of Fe₃O₄ nanoparticles enhanced thermal stability of polypyrrole. It can be attributed to the presence of coordinate bond between the 3-d space orbit of Fe atoms in Fe₃O₄ and the lone pair electrons of N atom in PPy. Second, after introducing the Fe₃O₄ nanoparticles, the polymer sensor sensitivity of ammonia was decreased. The insulator for Fe₃O₄ decreased the ration of conductive polypyrrole in the composite and resulted in the resistance increscent of thin-film. In the sensitive gas sensor testing, the increasing of the initial resistance of film would lead to the sensitivity films resistance change rate decreased. Besides, we found that the addition of Fe₃O₄ nanoparticles into polypyrrole structure shortened the response time. It can be attributed to that the Fe₃O₄ nanoparticles in the polypyrrole increased the surface area and polymer porosity which accelerated the diffusion of ammonia in the film, thereby reducing the response time of the film.To further enhance the sensitivity of polypyrrole thin film sensor, we studied the effect of TiO₂ on the sensitivity of the polypyrrole film. The results showed that the polymer sensor sensitivity increased substantially, and the response time of gas sensors decreased as the TiO₂ particles were added. We considered the p-n junction between n-type TiO₂ and p-type polypyrrole was formed through depletion region. The width of depletion in the p-n junction increased when the composite film exposed to ammonia resulting the enhancement of the changes in resistance. At the same time, TiO₂ particles increased the polypyrrole/TiO₂ composite film porosity. Polypyrrole/TiO₂ composite porous structure reduced the response time of polymer sensors because of the acceleration of the diffusion rate of ammonia gas into polymer. In addition, polypyrrole/TiO₂ composite films showed detection limit at 2 ppm.We also investigated the sensitivity of the polyaniline/Fe₂O₃ composite film. Here, we prepared polyaniline/Fe₂O₃ composite dispersion by in-situ polymerization using dodecylbenzene sulfonate acid (DBSA) as both dopant and surfactant, followed by preparing composite thin film sensor by spin-coating the composite dispersion on gas electrode. We found that the sensitivity and response time of polyaniline has improved when the Fe₂O₃ was introduced. Compared with polypyrrole/oxide composite sensors, we found that polyaniline composite films had better sensitivity to NH₃. There was an acid-base reaction between the DBSA and ammonia when the base gas contacted with the doped polyaniline, which resulted in the reduction of doping degree of polyaniline.