Development of a Fluorinated Graphene-Based Resistive Humidity Sensor

This work presents the development of novel fluorinated graphene (FG)-based resistive humidity sensor. The humidity sensor was fabricated by drop-casting FG suspension, as the humidity sensing material, on silver (Ag)-based interdigitated electrodes (IDEs). The silver-based IDEs were screen printed on a flexible polyimide substrate. The FG suspension was synthesized by uniform dispersion of FG in isopropyl alcohol (IPA), using the ultra-sonication process. The resistive response of the fabricated humidity sensors towards varying relative humidity (RH) levels was investigated, when the RH was varied from 20% to 80%, in steps of 10%, and at a temperature of 24 °C. A relative resistance change of 13.3% was observed when the RH was changed from 20% to 80%, with a sensitivity of 0.22%/%RH for the FG-based humidity sensor. Response time and recovery time of 82 s and 125 s, respectively, was obtained for the fabricated sensor. In addition, the effect of varying operating temperatures on the response of the fabricated humidity sensors was investigated. The average temperature coefficient of resistance of sensors was obtained as approximately −0.3%/°C. A linear relation between the temperature and the relative resistance change of sensors was observed. Further, first-principles study, employing density functional theory calculations, was performed to investigate interactions between the fluorine atom and graphene substrate, as well as humidity sensing behavior of the FG. DFT calculations showed that hydrogen atoms of the water molecule move towards the fluorine atom of the FG during the relaxation process, confirming the hydrogen bonding between FG and water molecules. The E ads of −0.50 eV was calculated for the adsorption of water molecule on the FG, demonstrating the strong humidity sensing property of the FG. The results demonstrate that FG, a highly stable derivative of graphene, is a potential material for humidity sensing applications.