Carbon Nano-Onions-Based Matrix Nanocomposite as Sensing Film for Resistive Humidity Sensor

This paper reports several preliminary investigations concerning the relative humidity (RH) detection response of a chemiresistive sensor that uses a novel sensing film based on a matrix nanocomposite comprising pristine carbon nano-onions and polyvinylpyrrolidone polymer at 1/1 w/w. The sensing device, including a polyimide substrate and gold electrodes, is obtained by depositing by drop casting the sensing layer on the sensing structure. The sensing layer’s morphology and composition are analyzed by Scanning Electron Microscopy, Atomic Force Microscopy, Fourier Transform Infrared Spectroscopy, X-Ray diffraction, and Raman spectroscopy. The experimental measurements show that the resistance of the tested nanocomposite slightly increases with RH for RH less than 50% and has a sharp increase with RH for larger RH. Several types of possible RH sensing mechanisms are identified and discussed. The decrease of the hole concentration in the CNOs in interaction with water molecules, which act as electron donors, and the rapid swelling of the hydrophilic polyvinylpyrrolidone polymer at high RH levels are the RH sensing mechanisms that best explain the measured RH detection behavior. The hard–soft acid–base principle also supports the experimental data. The hysteresis characteristic of the sensor is improved after the first operating cycle.