Facilely prepared composites of polyelectrolytes and graphene as the sensing materials for the detection of very low humidity

Two polyelectrolytes, i.e., cationic poly(diallydimethylammonium chloride) (PDDA) and anionic sodium poly(4-styrenesulfonate) (PSSNa), were separately mixed with graphene oxide (GO) and hydrazine in solution, and heat-treated to result in composites of PDDA/graphene (GN) and PSSNa/GN, respectively. The composites were characterized by scanning electron microscopy, transmittance electron microscopy and Raman spectroscopy. Impedance-type humidity sensors were constructed by depositing the composite films onto interdigitated gold electrodes via dip-coating. The electrical response of the composite humidity sensors toward relative humidity (RH) was investigated at room temperature. It was found that the composites with higher content of GN exhibited lower impedance at low humidity levels. Both composite sensors could detect very low humidity (down to 0.2%RH), and showed good response magnitude in the low humidity range (relative impedance change of 300% and 1000% from 0.2 to 30%RH for PSSNa/GN and PDDA/GN, respectively). Moreover, the composites exhibited slight hysteresis similar to that of the polyelectrolytes alone, while their response time for desorption was even reduced. The sensing mechanism of the composites was explored by complex impedance spectra analysis, and the good sensing behavior at low humidity was proposed to the special electrical property and unique two-dimensional nanosheet structure of GN.