Fig. 7. Amperometric results of polyaniline exposed to varying ammonia concentrations from 2.5 to 20 ppm with n = 3 and correlation coefficient of 0.9944.
3.7. Discussion
The concept of change in conductivity is widely used in applying metal oxide as a gas sensor. It is thought that the gas chemisorbs on the surface of the metal oxide followed by a charge transfer reaction wherein the adsorbed gas undergoes a reduction or oxidation on the metal oxide surface [15]. The emeraldine oxidation state of polyaniline switches reversibly between the insulating and conducting forms. The polyemeraldine state is thought to be
equiproportionate in amine and imine bond. The protonated imine gives rise to the bipolaron structure. The polarons are thought to be the charge carriers. The extent of bipolaron–polaron conversion is dependent on the pH of the polymer or the dopant. The structure of emeraldine PANI is represented by Fig. 8(a), where the value of ‘y’ in the structure is ∽0.5 and ‘X’ is the chloride ion from HCl. This structure conducts inherently due to the presence of conjugation.
An excess of doping may lead the structure to extensive conjugation, also known as the pernigraniline state represented by Fig. 8(b) which is rather unstable.
Fig. 6. (a) Contact resistance (R1), charge transfer resistance predominant at high frequencies (R2) and charge transfer resistance predominant at low frequencies (R3) as a function of ammonia concentration. Each data point reported is an average of at least 3 readings to ensure repeatability. The experimental data was well within the error limit of ±1.0%, the radii of the circles over each point adequately covering the errors. The squares of the correlation coefficients are presented for each fit in Table 1. (b) Capacitance predominant at high frequency (C1, pF) and low frequency (C1, µF) as a function of ammonia concentration. Each data point reported is an average of at least 3 readings to ensure repeatability. The experimental data was well within the error limit of ±1.0%, the radii of the circles over each point adequately covering the errors. The squares of the correlation coefficients are presented for each fit in Table 1.