V₂CT_x and Ti₃C₂T_x MXenes Nanosheets for Gas Sensing

In this paper, the performance and fundamental mechanisms on gas sensing of V2CTx and Ti3C2Tx MXenes were investigated. The MXenes nanosheets with the same chemical composition can show positive or negative response to the same gas due to the change of surface chemistry, which, in turn, is affected by the etching solution to make the samples. The residual Li or Na atoms on the surface of MXenes have a weak impact on the gas sensing performance; however, the ratio of Cl/F has a strong impact. V2CTx made in HF showed positive/zero response to the eight gases tested in this paper and had excellent sensing performance to methane. However, V2CTx made in LiF+HCl showed negative/zero response, while that made in NaF+HCl displayed positive response only to formaldehyde. Ti3C2Tx made in NaF+HCl showed a positive response to all detected gases while that made in ZnCl2 had better gas detection ability but showed a negative response to ammonia, triethylamine, and toluene. On the basis of the theoretical simulations, the structure-sensitive sensing behavior of MXenes as gas sensors is mainly ascribed to the structure-dependent electron transmission mechanism upon adsorbed gas molecule. This study is significant to the application of MXene sensors for designing specific gases.