Relationship Between Electric Properties Variation Of Polyaniline Film In Reduction Process And Its Gas-sensing Characteristic

Studying the electric properties variation of polyaniline film during the redox process is a basic task at the field of the intrinsic conducting polymer. However, although many theoretical models have been proposed by scientists to explain it satisfactorily, and obtained some quanlitativly conclusions, the quantitative description about it is indiligent due to the complexity of this process.In this dissertation, the double potential step method was used to study the electric properties of polyaniline (PAn) film in acidic aqueous solutions. The resistance and capacitance of bulk film changed with prolonging of the reduction time and/or shifting of reduction potential in the negative direction. And also a shoulder appeared in the cathodic chronoamperometric curve indicate the two-stage reduction process of PAn. These experimental results have been satisfactorily explained by using an improved heterogeneous model, which takes the merits of both the percolation theory and the electrochemistry stimulated conformational relaxation model. This new model overcomes some difficulties that the previous two models may meet, and especially can quantitative explain the change of electric properties of PAn film in the reduction process, and rationally could describe the forming and development of phase separation in PANI film during this stage.Effects of dopants on the correlation between the polyaniline (PAn) film resistance (R) and the reduction charge (Q) injected to the PAn film was investigated in dry acetonitrile solutions during electrochemical reduction by using the proposed double potential step method. The R-Q correlation behaves as S-type curves, leading to the determination of the critical reduction charge (Q_c). The latter represents the reduction charge required for the formation of a continuous partially-reduced phase in the PAn film. It was observed that the PAn film doped with sodium dodecylbenzene sulphate (SDBS) yielded a smaller Q_c than that doped with perchlorate, when the PAn films were electrochemically reduced under given conditions. The resistance of the pre-doped PAn film will increase significantly when the film is injected with reduction charge more than Q_c. Hence, a smaller Q_c means that the film can respond to very light reduction (or dedoping), being indicative of better sensing ability toward alkaline and reducing gases. This was confirmed by the increased sensitivity of the PAn/SDBS sensor toward 100 ppm NH3 vapor, compared with the PAn/ClO₄^- sensor.Next, we have adopted a dynamic-potential technique to modifying the structure of polyaniline chains. And a peculiar cyclic voltammetric behavior of polyaniline (PANI) film was observed in acetonitrile (ACN) solution. When the potential was scanned in a narrow potential window corresponding to the first conventional redox pair for the conversion between leucoemeraldine and emeraldine in ACN solution, splitting of the anodic peak for the oxidation from leucoemeraldine to emeraldine was observed in the cyclic voltammograms. When the low limit of the potential window was shifted to a potential negative enough, the cyclic voltammograms of PANI changed significantly, and the electrical properties of PANI film also changed due to the treatment. These peculiar behaviors were tentatively suggested to be associated with the deep reduction, accompanied by the structural changes of PANI chains due to the de-doping. It is further found that when the PANI film was treated by pre-cycling in potential windows with low limits more negative than -0.8 V, its sensitivity was significantly increased toward ammonia vapor, being little affected by the humidity. This led to a successful fabrication of a concept ammonia sensor by using the PANI film pretreated by the deep reduction. And the mechanism related to the above phenomena has been studied primarily through a series of experiments.