Comparison of NO$_{2}$ and NH$_{3}$ gas adsorption on semiconductor polyaniline thin films

Abstract

Semiconductor polyaniline (PANI) thin films have proven to be an optically sensitive element for detecting gaseous NH$_{3}$ and NO$_{2}$ at room temperature. Although the absorption of either of them on PANI films results in the color change of the films, from green to blue, the mechanism of the interaction of each gas with the polymer is different. Ultra-violet (UV)-Visible (VIS) spectra of the NO$_{2}$ saturated PANI thin films give an increasing blue shift of the polaron band (1.5 eV) as a function of the gas concentration. The NH$_{3}$ gas saturated PANI samples, however, show a small red shift of the same absorption band and a slight increase of the relative intensity of the 2.0 eV band. It is suggested that NO$_{2}$ oxidizes the semiconductor PANI just as this can be oxidized by an electrochemical potential in an aqueous acid solution, whereas NH$_{3}$ deprotonates the polymer without taking out any electrons from it. The presence of NH$_{4}^{+}$ and of NO$_{2}^{-}$ species in the gas saturated PANI samples is shown by the analysis of their Fourier Transform Infrared (FT-IR) spectra.