Bilayer-structured composite sensor based on polyaniline and polyelectrolyte for sensitive detection of low humidity

A bilayer-structured composite humidity sensor based on quaternized and crosslinked poly(4-vinylpyridine) (QC-P4VP) and polyaniline (PANI) was fabricated by depositing thin films of QC-P4VP and PANI onto interdigitated gold electrode in sequence. The composite was characterized by Fourier transform infrared spectroscopy, Ultraviolet–visible spectroscopy, scanning electron microscopy, and atomic force microscopy. The composite sensor could sensitively detect very low humidity (down to ∼1%RH) (impedance increase of ∼860% from 15% to 1%RH). Furthermore, the sensor exhibited relatively fast response (t90% of 24 s and 35 s for adsorption and desorption processes, respectively), small hysteresis (∼3%RH) and good repeatability. In addition, the composite sensor revealed impedance change close to 103 from 1% to 98%RH, suggesting its capability of detecting full-range humidity with high sensitivity. The effect of the concentration of poly(4-vinylpyridine) and PANI, deposition sequence of the sensitive layers on the humidity sensing characteristics of the composite has been examined. The humidity sensing mechanism of the composite sensor was proposed by considering the electrical properties of QC-P4VP and PANI at different humidity levels and the special bilayer structure.