Comparison of NO2 and NH3 gas adsorption on semiconductor polyaniline thin films
J. Elizalde-Torresa, b, H. Huc, *, and J.M. Sanigera
a Centro de Ciencias Aplicadas y Desarrollo Tecnológico, Universidad Nacional Autónoma de México, Ciudad Universitaria, Apartado Postal 70-186, México D.F., 04510, México.
b PhD student at the Facultad de Ciencias Químicas e Ingeniería, Universidad Autónoma del Estado de Morelos, Cuernavaca, Morelos, México.
c Centro de Investigación en Energía, Universidad Nacional Autónoma de México, Av. Xochicalco, S/N, Temixco, Morelos, 62580, México. * Corresponding author, e-mail: hzh@cie.unam.mx.
]]>Recibido el 17 de diciembre de 2004.
Aceptado el 10 de agosto de 2005.
Abstract
Semiconductor polyaniline (PANI) thin films have proven to be an optically sensitive element for detecting gaseous NH3 and NO2 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 NO2 saturated PANI thin films give an increasing blue shift of the polaron band (1.5 eV) as a function of the gas concentration. The NH3 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 NO2 oxidizes the semiconductor PANI just as this can be oxidized by an electrochemical potential in an aqueous acid solution, whereas NH3 deprotonates the polymer without taking out any electrons from it. The presence of NH+4 and of NH-2 species in the gas saturated PANI samples is shown by the analysis of their Fourier Transform Infrared (FT-IR) spectra.
Keywords: Polyaniline thin films; NO2 and NH3 gas detection; UV-VIS and FT-IR spectra.
Resumen
Se han comprobado que las películas delgadas de polianilina (PANI) son elementos ópticamente sensibles para detectar a los gases de NO2 y NH3 a temperatura ambiente. Aunque el resultado de la adsorción de ambos gases resulta en el cambio de color de las películas, de verde a azul, el mecanismo de interacción de cada uno con el polímero es distinto. Los espectros de ultravioleta (UV)- visible (VIS) de las películas delgadas de PANI saturadas con NO2 muestran un desplazamiento de la banda polaronica (1.5 eV) hacia el azul como función de la concentración del gas nocivo. En cambio las muestras de PANI saturadas con el gas amoniaco indican un corrimiento de la misma banda hacia el rojo y un incremento ligero de la intensidad relativa de la banda de 2.0 eV. Esto sugiere que NO2 oxida al semiconductor PANI de la misma forma que un potencial electroquímico en una solución ácida acuosa, mientras que NH3 desprotona al polímero sin quitarle ningún electrón. La presencia de las especies adsorbidas de NH+4 y NH-2 en las muestras de PANI saturadas con los dos gases se puede observar en sus espectros de infrarrojos por Transformada de Fourier (FT-IR).
]]> Descriptores: Películas delgadas de polianilina; detección de gases de NO2 y NH3; espectros de UV-VIS y FT-IR.
PACS: 71.20.Rv; 73.61.Ph; 78.40.Me; 78.66.Qn; 82.35.Cd
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Acknowledgements
The authors wish to thank Maricela Gutierrez for recording FT-IR spectra and Jose G. Bañuelos for PANI film thickness measurements. Financial support from CONACyT and from PAPIIT-UNAM (Project No.IN114902) of Mexico is gratefully acknowledged.
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