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

Abstract

Continuous\nemission of carbon dioxide gas (CO₂) poses\na significant effect on ambient environment, crop production, and\nhuman health, necessitating further improvement of CO₂ monitoring\nespecially at low concentrations. To overcome the obstacles of elevated\noperation temperatures and faint response encountered by traditional\nCO₂-sensitive materials such as metal oxides and perovskites,\na nitrogen-doped MXene Ti₃C₂T_<i>x</i> (N-MXene)/polyethyleneimine (PEI) composite film decorated\nwith reduced graphene oxide (rGO) nanosheets was initiatively leveraged\nin this work to detect 8–3000 ppm CO₂ gas. Through\nsubtle optimization in the aspects of componential constitutions,\noperation temperatures, PEI loading amounts, and relative humidity\n(RH), the ternary sensors with a PEI concentration of 0.01 mg/mL exhibited\na reversible and superior performance over other counterparts under\n62% RH at room temperature (20 °C). Apart from the inspiring\ndetection limit of 8 ppm, favorable selectivity, repeatability, and\nlong-term stability were demonstrated as well. During the humid CO₂ sensing of the composites, few rGO nanosheets acted as an\nexcellent conduction platform to transfer and collect charge carriers.\nLayered N-MXene offered more active sites for coadsorption of both\nCO₂ and water, thereby facilitating the water-involving\nreactions. Rich amino groups of the PEI polymer were beneficial to\nbind CO₂ molecules and thus induce appreciable density\nvariation of charge carriers via proton-conduction behavior. This\nwork initiatively offers an alternative ion-conduction strategy to\ndetect ppm-level CO₂ gas by harnessing rGO/N-MXene/PEI\ncomposites under a humid atmosphere at room temperature, simultaneously\nbroadening the discrimination range of MXene-related gas sensing.