Humidity Sensing Properties of Halogenated Graphene: A Comparison of Fluorinated Graphene and Chlorinated Graphene

This work presents a comparison of humidity sensing properties of fluorinated graphene (FG) and chlorinated graphene (ClG), using experimental data and atomic-level ab-initio simulations. The fabrication of the humidity sensor included drop-casting FG and ClG suspensions on silver (Ag)based interdigitated electrodes (IDEs) to form the sensing layer. The sensitivity of FG and ClG to humidity variations was investigated by measurement of relative resistance change ($\Delta R/R_{b}$) of the fabricated humidity sensors when the relative humidity (RH) was changed from 20% to 80%, in steps of 10%, at a constant temperature of 24° C. For RH transition from 20% to 80%, the $\Delta R/R_{b}$ of the FG-based and the ClG-based humidity sensors were measured as 13.3% and 10.8%, respectively, resulting in a sensitivity of 0.22%/%RH and 0.18%/%RH, respectively. Density functional theory (DFT) calculations showed adsorption energy (Eads) of -0.50 eV and -0.43 eV for the physisorption of water molecules on the FG and ClG, respectively, demonstrating the higher sensitivity of the FG to humidity. The density of states (DOS) calculations showed that the water-adsorbed FG has a larger DOS near the Fermi level when compared to water-adsorbed ClG, which can be attributed to the stronger interaction and more effective charge transfer between the FG and the water molecule.