Tetramethylammonium-Intercalated M₂CT_x (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 Ti 3 C 2 T x 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 (T x ), and gas-MXene interactions in M 2 CT x (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 Mo 2 CT x, as evidenced by a reversal in response trends. Among the tested MXenes, Ti 2 CT x, V 2 CT x, and Nb 2 CT x demonstrated excellent selectivity for nitrogen dioxide (NO 2 ), with Ti 2 CT x achieving an outstanding response of ∼ 660% to 50 ppm and a detection limit below 1 ppm. In contrast, Mo 2 CT x 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.