Tetramethylammonium-Intercalated M₂CT_<i>x</i> (M = Ti, V, Nb, Mo) MXene Nanosheet-Based Gas Sensors

Gas sensors have become indispensable tools across various fields, with advanced materials playing a crucial role in enhancing their reliability and efficiency. Among two-dimensional nanomaterials, MXenes stand out due to their high surface area, tunable surface chemistry, and unique electronic properties. While Ti3C2Tx has dominated MXene-based gas sensing research, other MXenes remain relatively underexplored despite their strong theoretical potential. This study bridges this gap by investigating the roles of transition metal (M)-sites, intercalants, surface terminal groups (Tx), and gas-MXene interactions in M2CTx (M = Ti, V, Nb, Mo) systems. Employing an optimized synthesis approach, comprising selective etching of both MAX and non-MAX phase precursors followed by delamination using tetramethylammonium hydroxide (TMAOH), we successfully produced multi- and few-layered MXenes with enhanced gas sensing performance. TMA+ intercalation expands interlayer spacing, amplifying swelling-driven responses in metallic MXenes (Ti/V/Nb), while inducing pseudosemiconducting behavior in Mo2CTx, as evidenced by a reversal in response trends. Among the tested MXenes, Ti2CTx, V2CTx, and Nb2CTx demonstrated excellent selectivity for nitrogen dioxide (NO2), with Ti2CTx achieving an outstanding response of ∼ 660% to 50 ppm and a detection limit below 1 ppm. In contrast, Mo2CTx showed superior selectivity for carbon monoxide (CO: −3.2% for 50 ppm). These findings underscore the critical influence of M-site selection, intercalation, and surface chemistry on MXene-gas interactions, providing a roadmap for designing tailored sensors for applications such as air quality monitoring or hazardous leak detection.