Single-atom Pd confined in Ti ₃C ₂ nanosheet for boosted room-temperature trace NO ₂ monitoring

Two-dimensional MXenes have great potential for gas sensing applications due to their distinct electronic structure and unique surface properties. However, low sensitivity and poor selectivity to target gas at room temperature are major shortcomings of MXene materials. In this study, PdCl₂ was used to decorate Pd single atoms (Pd_SA) on Ti₃C₂ nanosheet (Ti₃C₂/Pd_SA). The Pd²⁺ was directly reduced (<i>in-situ</i>) into a Pd_SA due to abundant Ti vacancies and the inherent reducing ability of Ti₃C₂ MXene. The Ti₃C₂/Pd_SA sensor exhibited the highest response of 0.289 to 1 ppm NO₂, which is 28.9 and 7.8 times higher over pristine Ti₃C₂ and Pd nanoparticles (Pd_nano)-decorated on Ti₃C₂ (Ti₃C₂/Pd_nano), respectively. Simultaneously, the Ti₃C₂/Pd_SA sensor possessed an ultralow detection limit (10 ppb) and excellent selectivity towards NO₂. The enhanced gas sensing mechanism of Ti₃C₂/Pd_SA was investigated in detail through the activation energy calculation, <i>in-situ</i> diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and density functional theory (DFT) studies. The rich active sites, chemical sensitization effect, and NO₂ adsorption enhancement resulting from the Pd single atoms in Ti₃C₂/Pd_SA significantly boosted sensing performance. This work can provide new insights and guidelines for fabricating highly effective NO₂ sensors operated at room temperature.