Atomic Plasma Grafting: Precise Control of Functional Groups on Ti₃C₂T_x MXene for Room Temperature Gas Sensors

Gas sensing properties of two-dimensional (2D) materials are derived from charge transfer between the analyte and surface functional groups. However, for sensing films consisting of 2D Ti₃C₂T_x MXene nanosheets, the precise control of surface functional groups for achieving optimal gas sensing performance and the associate mechanism are still far from well understood. Herein, we present a functional group engineering strategy based on plasma exposure for optimizing the gas sensing performance of Ti₃C₂T_x MXene. For performance assessment and sensing mechanism elucidation, we synthesize few-layered Ti₃C₂T_x MXene through liquid exfoliation and then graft functional groups via in situ plasma treatment. Functionalized Ti₃C₂T_x MXene with large amounts of −O functional groups shows NO₂ sensing properties that are unprecedented among MXene-based gas sensors. Density functional theory (DFT) calculations reveal that −O functional groups are associated with increased NO₂ adsorption energy, thereby enhancing charge transport. The −O functionalized Ti₃C₂T_x sensor shows a record-breaking response of 13.8% toward 10 ppm NO₂, good selectivity, and long-term stability at room temperature. The proposed technique is also capable of improving selectivity, a well-known challenge in chemoresistive gas sensing. This work paves the way to the possibility of using plasma grafting for precise functionalization of MXene surfaces toward practical realization of electronic devices. © 2023 American Chemical Society.