NO2-induced optical absorbance changes in semiconductor polyaniline thin films

Chemically deposited semiconductor polyaniline (PANI) thin films have been exposed to detect nitrogen dioxide gas. Because of the high electron receptor behavior, NO2 molecules oxidize the conjugated polymer during the sensing process, leading to the modification of the electronic structure and consequently the change of the optical absorbance of PANI as a function of the toxic gas concentration. NO2-induced optical transmittance changes of the polymer films can be varied between 5 and 80% for a NO2 gas concentration of 3-50 ppm. By using a parametric method, the heat of adsorption (Q) as well as the activation energy of adsorption (EA) for NO2 molecules on PANI surface are obtained from the corresponding adsorption kinetic curves. It is found that Q is of the order of 0.50-0.58 eV, and EA of 0.28-0.50 eV, depending on the type of the PANI films and the NO2 gas concentration. We show that the same kinetic expression can also be obtained from a reaction-diffusion coupled problem reported in literature by assuming a couple of the relations between the thermodynamic variables and kinetic reaction rates of the two models. The desorption process of NO2 from PANI samples is kinetically irreversible in a dried air or N-2 during the experimental time, and slowly reversible if they have been kept in ambient air with a relative humidity greater than zero. (C) 2003 Elsevier B.V. All rights reserved.