Humidity-Enabled Ionic Conductive Trace Carbon Dioxide Sensing of Nitrogen-Doped Ti₃C₂T_<i>x</i> MXene/Polyethyleneimine Composite Films Decorated with Reduced Graphene Oxide Nanosheets

Abstract

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 Ti₃C₂T_<i>x</i> (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.