Enhanced Gas Sensing Performance of M₂CT_x/MO (M = Ti, V, Nb, Mo) Nanocomposites

Gas sensors are in high demand across various fields, from disease diagnosis through breath analysis to ensuring environmental safety and enhancing food and agriculture processes. The continuous discovery of new materials drives advances in gas sensor technology. Notably, 2D materials have gained significant attention for gas sensing due to their remarkable electrical, optical, and mechanical properties. Among them, 2D MXenes have emerged as compelling candidates due to their high specific area and their rich surface functionalities with tunable electronic structure make them compelling for sensing applications. MXenes are generally represented by M<i>_n</i>₊₁X<i>_n</i>T<i>_x</i>, in which M represents an early transition metal, X denotes either carbon or nitrogen or carbonitrides or oxy-carbides, T<i>_x</i> signifies surface terminations (T_x = -OH, -O, -F, -Cl), and <i>n</i> can be 1-4. M₂CT_x MXenes, in particular, show promise for VOC/gas detection based on theoretical calculations. However, research has predominantly focused on Ti₃C₂T<i>_x</i> MXenes due to their established status, leaving limited exploration of other MXene variants as gas sensors. By altering the transition metal in MXenes, selectivity for specific gases can be achieved. Our research involves synthesizing MXenes from parent precursors (MAX and non-MAX phases) and conducting thorough property characterizations. We employ optimized synthesis methods to obtain single-/few-layered MXenes via selective etching and delamination. However, when comes to practical applications, pristine MXene-based gas sensors have low sensitivity, significant baseline drift, susceptibility to cross-interference, and a narrow band gap, which limits gas reaction and responsiveness. The self-stacking of MXene layers also impedes the diffusion of gas molecules and hinders surface-active sites, restricting the gas sensor response and limiting the detection of low-concentration gases [1]. To enhance gas sensing capabilities (response and stability), we fabricate M₂CT<i>_x</i>/MO (M = Ti, V, Mo, Nb) nanocomposites, introducing multiple in-situ Schottky barriers through surface oxidation to enhance VOC sensing capabilities. The MXene/MO nanocomposites were prepared by using the concept of in-situ conversion using MXene surfaces as reactants. MXene layers expose metallic atomic layers on their outer basal planes, which are highly oxophilic and prone to surface reactions like oxidation (e.g., Ti to TiO₂ in Ti₂CT<i>_x</i> or Ti₃C₂T<i>_x</i>). The MXene surface is transformed partially, to form MO material in these processes. This approach lays the foundation for understanding the surface-sensitive behavior of MXenes in gas sensing by investigating the impact of surface chemistry. It represents a critical step towards expanding the scope of MXene-based gas sensors beyond Ti₃C₂T_x and advancing the field of gas sensing technology.<br/><b>Keywords</b>: Ti₃C₂T_x; M₂X MXenes; surface chemistry; gas sensing; in-situ Schottky barriers.<br/><b>References:</b><br/>[1]. M Sai Bhargava Reddy, Saraswathi Kailasa, Bharat CG Marupalli, Kishor Kumar Sadasivuni, Shampa Aich*. "A Family of 2D MXenes: Synthesis, Properties, and Gas Sensing Applications". ACS Sensors 2022, 7, 8, 2132–2163.