Fast-speed, high-sensitivity polyimide humidity sensors with superhydrophilic carbon nanotube network electrodes

We present a highly fast capacitive humidity sensor based on superhydrophilic carbon nanotube network electrodes prepared by using spray deposition of multiwalled carbon nanotubes (MWCNT). Spray-deposited MWCNT films form a well-entangled and interconnected porous network with nano-scale open pores. These properties are highly desirable for fast-speed capacitive humidity sensors. In addition, the effect of plasma functionalization (pf) of the MWCNT surface on sensor response was evaluated. The pf-MWCNT electrode sensor exhibited a much faster response time (about 1.5 s) and a higher sensitivity (0.647 pF/%RH) to humidity, compared to untreated MWCNT and porous Cr electrodes. The large improvement in performance was explained by the percolated pore network, which is more accessible to water molecules; the superhydrophilic surface of the MWCNTs favorable to more adsorption of water vapor; and capillary condensation, which can amplify the capacitance response to relative humidity.