A resistive-type humidity sensor based on polypyrrole and ZnO nanoparticles: hybrid polymers vis-a-vis nanocomposites

Abstract

Fabrication of resistive-type humidity sensors based on either hybrid polymers of polypyrrole(PPy)–ZnO nanoparticles or their nanocomposites is reported. Hybrid polymers with different ZnO content (14, 19, 32 and 46%) were synthesized via oxidative polymerization of free pyrrole monomer with pyrrole moieties covalently bonded on the surface of ZnO nanoparticles. Moreover, nanocomposites consisting of the above mentioned ZnO content were prepared by efficient mixing of PPy homopolymer with ZnO nanoparticles. The products were characterized via FTIR, TGA, XRD and SEM techniques. The surfaces of the sensing films were characterized with AFM showing uniform and continuous films. Sensors with the highest sensitivity were obtained using hybrid polymers with higher ZnO content and lower particle sizes. Compared to ZnO–PPy nanocomposite sensors, the hybrid polymer sensors showed higher sensitivity and more linear response (R² = 0.9879–0.9921) over a wide range of relative humidity (11–75%) at room temperature. It was concluded that the covalent bonding of PPy chains with ZnO nanoparticles in a hybrid polymer results in a p–n junction which, in contrast to the one in nanocomposites has a chemical nature. The p–n junction with such a nature enhances the charge transfer between PPy and ZnO nanoparticles in hybrid polymers compared to nanocomposite samples, and hence, imparts higher sensitivity to hybrid polymer based sensors. Investigation of the response–recovery behavior of sensors in 180 seconds of response period indicated a high repeatability and recovery time of 60 seconds without a noticeable hysteresis.