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.
High conductivity and transparency and sheet-like two-dimensional morphology of MXenes make them attractive for use as functional transparent thin films. In addition, because of the dense surface functional groups and negative surface charge of the MXene sheet, cationic species can be easily intercalated into MXene interlayers to largely enhance the film properties and device performance. In this paper, for the first time, we demonstrate a spontaneous self-assembly method to efficiently intercalate metal ions into MXene transparent thin films with cation-dependent properties. Unlike in previous methods that intercalate ions after film assembly, monovalent and divalent metal ions are easily intercalated during the self-assembly process within a very short period of time. The optoelectronic properties are dependent on the intercalated cation where uniformly assembled ion-intercalated Ti3C2T x MXene thin films exhibited on average a high optical transmittance of ∼90% at a wavelength of 550 nm. The ion-intercalated MXene films were utilized as gas sensors to detect ammonia gas. Interestingly, metal-ion-intercalated films showed a much higher signal-to-noise ratio upon exposure to ammonia gas compared to that of films assembled without metal ions, demonstrating the positive influence of metal-ion intercalation on enhancing the gas-sensing performance.
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