Humidity sensors based on the composite of multi-walled carbon nanotubes and crosslinked polyelectrolyte with good sensitivity and capability of detecting low humidity

Multi-walled carbon nanotubes (MWNTs) were dispersed in the matrix of quaternized and crosslinked poly(4-vinylpyridine) (QC-P4VP) to prepare a composite via solution blending and heat treatment. The composite was characterized by FT-IR spectroscopy and scanning electron microscopy. Thin film humidity sensors based on the composite were fabricated and their electrical responses to relative humidity (RH) were investigated at room temperature. The composite sensor showed much higher response magnitude than the sensor based on MWNTs (impedance increase of ∼4700% from 7 to 90%RH). Meanwhile, its impedance at low humidity was significantly decreased with respect to that of QC-P4VP based sensor. Furthermore, it could detect very low humidity with good sensitivity (impedance change of 16%/%RH over 1-30%RH), and demonstrate its capability of realizing full-range measurement of humidity. The effect of content of MWNTs, poly(4-vinylpyridine) (P4VP) and the additive, molecular weight of P4VP, temperature and testing frequency on the humidity sensing properties of the composite sensors has been examined. A sensing mechanism was proposed by considering the MWNT conductive network in the polyelectrolyte matrix.