Humidity-Enabled Ionic Conductive Trace Carbon Dioxide Sensing of Nitrogen-Doped Ti3C2Tx MXene/Polyethyleneimine Composite Films Decorated with Reduced Graphene Oxide Nanosheets.

Continuous emission of carbon dioxide gas (CO2) poses a significant effect on ambient environment, crop production, and human health, necessitating further improvement of CO2 monitoring especially at low concentrations. To overcome the obstacles of elevated operation temperatures and faint response encountered by traditional CO2-sensitive materials such as metal oxides and perovskites, a nitrogen-doped MXene Ti3C2Tx (N-MXene)/polyethyleneimine (PEI) composite film decorated with reduced graphene oxide (rGO) nanosheets was initiatively leveraged in this work to detect 8-3000 ppm CO2 gas. Through subtle optimization in the aspects of componential constitutions, operation temperatures, PEI loading amounts, and relative humidity (RH), the ternary sensors with a PEI concentration of 0.01 mg/mL exhibited a reversible and superior performance over other counterparts under 62% RH at room temperature (20 °C). Apart from the inspiring detection limit of 8 ppm, favorable selectivity, repeatability, and long-term stability were demonstrated as well. During the humid CO2 sensing of the composites, few rGO nanosheets acted as an excellent conduction platform to transfer and collect charge carriers. Layered N-MXene offered more active sites for coadsorption of both CO2 and water, thereby facilitating the water-involving reactions. Rich amino groups of the PEI polymer were beneficial to bind CO2 molecules and thus induce appreciable density variation of charge carriers via proton-conduction behavior. This work initiatively offers an alternative ion-conduction strategy to detect ppm-level CO2 gas by harnessing rGO/N-MXene/PEI composites under a humid atmosphere at room temperature, simultaneously broadening the discrimination range of MXene-related gas sensing.