Physicochemical regulations of nanoconfined two-dimensional spacing toward highly-selective NH₃ sensing

Nanoscale confinement environments often affect the transport mechanisms of nanofluids. Understanding the dynamic behavior of molecules in two-dimensional (2D) confined channels is of great importance in the areas of sensing, catalysis and energy storage. As a popular candidate for a new type of gas sensing material, MXenes have the problem of nonselectivity towards polar gases with slow responses, which severely limits their applications. Here, we report a study on regulating the confinement effect of 2D channels between MXene layers through annealing treatment and ion (Na⁺) intercalation for high-performance ammonia (NH₃) sensing. Firstly, the annealing treatment accurately modulates the size of the 2D channels to effectively block the entry of large-size gas molecules and improve the selectivity for NH₃. Ab initio molecular dynamics (AIMD) also confirms that the modulated channel size has a special "nano-pumping effect", which can accelerate the dynamic behavior of NH₃ molecules in the 2D confined space. Moreover, the intercalation of Na⁺ ions increases the adsorption capacity of NH₃. Therefore, the "nano-pumping effect" and theintercalation of Na⁺ ions effectively enhance the response speed and sensitivity of MXene to NH₃, respectively. The experimental results show that the modified Ti₃C₂ exhibits high sensitivity (0.17), rapid response (181 s), excellent selectivity and stability towards NH₃.