N–p-Conductor Transition of Gas Sensing Behaviors in Mo₂CT_x MXene

It is highly important to implement various semiconducting, such as n- or p-type, or conducting types of sensing behaviors to maximize the selectivity of gas sensors. To achieve this, researchers so far have utilized the n–p (or p–n) two-phase transition using doping techniques, where the addition of an extra transition phase provides the potential to greatly increase the sensing performance. Here, we report for the first time on an n–p-conductor three-phase transition of gas sensing behavior using Mo₂CT_x MXene, where the presence of organic intercalants and film thickness play a critical role. We found that 5-nm-thick Mo₂CT_x films with a tetramethylammonium hydroxide (TMAOH) intercalant displayed a p-type gas sensing response, while the films without the intercalant displayed a clear n-type response. Additionally, Mo₂CT_x films with thicknesses over 700 nm exhibited a conductor-type response, unlike the thinner films. It is expected that the three-phase transition was possible due to the unique and simultaneous presence of the intrinsic metallic conductivity and the high-density of surface functional groups of the MXene. We demonstrate that the gas response of Mo₂CT_x films containing tetramethylammonium (TMA) ions toward volatile organic compounds (VOCs), NH₃, and NO₂ is ∼30 times higher than that of deintercalated films, further showing the influence of intercalants on sensing performance. Also, DFT calculations show that the adsorption energy of NH₃ and NO₂ on Mo₂CT_x shifts from −0.973, −1.838 eV to −1.305, −2.750 eV, respectively, after TMA adsorption, demonstrating the influence of TMA in enhancing sensing performance.