Detection of very low humidity using polyelectrolyte/graphene bilayer humidity sensors

An impedance-type humidity sensor was prepared by sequentially depositing the thin films of crosslinked and quaternized poly(4-vinylpyridine) (QC-P4VP) and reduced graphene oxide (RGO) onto interdigitated gold electrodes. The QC-P4VP/RGO bilayer sensor reveals much lower impedance than QC-P4VP based sensor in the environments with low relative humidity (RH) levels, and could detect ultra-low humidity (0.18%RH) with very high response magnitude (impedance increase of 500% between 2.1%RH and 0.18%RH) at room temperature. Moreover, the bilayer sensor shows small hysteresis (∼4.5%RH) and fast response (t90% of 21 s and 78 s for adsorption and desorption processes, respectively). The effect of the concentration of RGO precursor, the deposition order and thickness of the sensing films, temperature, and the structure of the electrodes on the humidity sensing behaviors of the bilayer-structured sensors has been thoroughly examined, and the humidity sensing mechanism was explored.