Understanding charge transfer processes between graphene and functional materials is crucial from the perspectives of fundamental sciences and potential applications, including electronic devices, photonic devices, and sensors. In this study, we present the charge transfer behavior of graphene and amine-rich polyethyleneimine (PEI) upon CO2 exposure, which was significantly improved after introduction of hygroscopic polyethylene glycol (PEG) in humid air. By blending PEI and PEG, the number of protonated amine groups in PEI was remarkably increased in the presence of water molecules, leading to a strong electron doping effect on graphene. The presence of CO2 gas resulted in a large change in the resistance of PEI/PEG-co-functionalized graphene because of the dramatic reduction of said doping effect, reaching a maximum sensitivity of 32% at 5,000 ppm CO2 and an applied bias of 0.1 V in air with 60% relative humidity at room temperature. This charge transfer correlation will facilitate the development of portable graphene-based sensors for real-time gas detection and the extension of the applications of graphene-based electronic and photonic devices.
