Effects of halogen termination (F, Cl, and Br) and surface symmetry on the adsorption performance of Ti3C2 MXenes for H2S, SO2, and NO2 gases

The real-time detection of toxic and hazardous gases under ambient conditions is crucial for industrial safety and human health. However, traditional gas sensors suffer from low sensitivity, poor repeatability, high working temperature, and poor selectivity. MXenes are one of the frontier functional two-dimensional materials with potential in gas sensing. In this work, we systematically investigate the adsorption of three hazardous gases (H₂S, SO₂, and NO₂) on halogen-functionalized Ti₃C₂ MXenes using first-principles calculations, including both symmetric (Ti₃C₂F₂, Ti₃C₂Cl₂, and Ti₃C₂Br₂) and asymmetric (Ti₃C₂FCl, Ti₃C₂FBr, and Ti₃C₂ClBr) ones. It is found that Ti₃C₂FCl–F and Ti₃C₂FBr–F are suitable for high-sensitive and fast-recovery chemiresistor NO₂ sensors, while Ti₃C₂FBr–Br can be used for H₂S and SO₂ sensors. It is worth mentioning that Ti₃C₂F₂, Ti₃C₂FCl–Cl, and Ti₃C₂BrCl–Cl acquire magneticity after NO₂ adsorption, which makes them potentially useful in spintronic or magnetic gas sensors. Besides, the adsorption mechanism is explained. The adsorption behaviour is more relevant to the terminal atoms on the surface, compared with the material itself; the intrinsic electric fields caused by the electrostatic potential difference between the two ends of asymmetric Janus structures can influence the charge transfer behaviour of gas molecules, which is promising for surface engineering of MXenes to meet specific demands.