Carbon Nano-Onions-Polyvinyl Alcohol Nanocomposite as Sensing Film for Resistive Monitoring of Relative Humidity
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This paper reports several preliminary investigations concerning the relative humidity (RH) detection response of a chemiresistive sensor that uses a novel sensing layer based on pristine carbon nano-onions (CNOs) and polyvinyl alcohol (PVA) at 1/1 and 2/1 w/w ratio. The sensing device, including a Si/SiO2 substrate and gold electrodes, is obtained by depositing the CNOs-PVA aqueous suspension on the sensing structure by drop casting. The composition and morphology of the sensing film are explored by mean of Scanning Electron Microscopy, Raman spectroscopy, Atomic Force Microscopy, and X-Ray diffraction. The room temperature RH detection performance of the manufactured sensor is examined by applying a continuous flow of the electric current between the interdigitated electrodes and measuring the voltage as the RH varies from 5% to 95%. For RH below 82% (sensing layer based on CNOs-PVA at 1/1 w/w ratio) or below 50.5% (sensing layer based on CNOs-PVA at 2/1 w/w ratio), the resistance varies linearly with RH, with a moderate slope. The newly developed sensor, using CNOs-PVA at a 1:1 ratio (w/w), responded as well as or better than the reference sensor, while the recorded recovery time was about 30 seconds, which is half the recovery time of the reference sensor. Additionally, the changes in resistance (ΔR/ΔRH) for different humidity levels showed that the CNOs-PVA layer at 1:1 was more sensitive at humidity levels above 80%. The main RH sensing mechanisms considered and discussed are the decrease of the hole concentration in the CNOs during the interaction with an electron-donor molecule, such as water, and the swelling of the hydrophilic PVA. The experimental RH detection data are analyzed and compared with the RH sensing results reported in previously published work of RH detectors employing sensing layers based on oxidized carbon nanohorns-polyvinylpirrolidone, oxidized carbon nanohorns-PVA and CNOs-polyvinylpyrrolidone (PVP).