Covalently Functionalized MXenes for Highly Sensitive Humidity Sensors

Transition metal carbides and nitrides (MXenes) are an emerging class of 2D materials, which are attracting ever-growing attention due to their remarkable physicochemical properties. The presence of various surface functional groups on MXenes' surface, e.g., F, O, OH, Cl, opens the possibility to tune their properties through chemical functionalization approaches. However, only a few methods have been explored for the covalent functionalization of MXenes and include diazonium salt grafting and silylation reactions. Here, an unprecedented two-step functionalization of Ti₃ C₂ T_x MXenes is reported, where (3-aminopropyl)triethoxysilane is covalently tethered to Ti₃ C₂ T_x and serves as an anchoring unit for subsequent attachment of various organic bromides via the formation of CN bonds. Thin films of Ti₃ C₂ T_x functionalized with linear chains possessing increased hydrophilicity are employed for the fabrication of chemiresistive humidity sensors. The devices exhibit a broad operation range (0-100% relative humidity), high sensitivity (0.777 or 3.035), a fast response/recovery time (0.24/0.40 s ΔH^-1 , respectively), and high selectivity to water in the presence of saturated vapors of organic compounds. Importantly, our Ti₃ C₂ T_x -based sensors display the largest operating range and a sensitivity beyond the state of the art of MXenes-based humidity sensors. Such outstanding performance makes the sensors suitable for real-time monitoring applications.