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

Abstract

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.

A Family of 2D-MXenes: Synthesis, Properties, and Gas Sensing Applications

“…196 Furthermore, Choi et al demonstrated the n−p-conductor transition of gas sensing behaviors in Mo 2 C 2 T x MXene, in which the organic intercalants (such as TMA + ) and film thickness significantly influenced the sensing performance. 101 The TMA + intercalation on Mo 2 C 2 T x induced a transition from n-type sensing behavior to p-type, while thicker films displayed a conductor-type gas sensing behavior. This transition indicates that the presence of TMA (and likely other intercalants) improves accessibility to gas molecules, in addition to tuning the electronic properties of MXene sensors.…”

“…99 Besides Ti-based MXenes, Mo-MXenes are more stable 100 and have a smaller number of surface termination (T x ), as Mo atoms provide fewer electrons to surface functional groups. 101 V-MXenes have multiple oxidation states but showed the lowest chemical stability in their delaminated form in the colloidal state. 102 However, recent efforts showed progress in the chemical stability of V 2 CT x through the ion exchange process coupled with flocculation.…”

“…30,40 The intercalated cations which were used for the delamination process come as a chemical residual intercalant from the etchants, and intercalating agents during synthesis can endow semiconductor-like properties on multilayer MXene films despite individual single layers being intrinsically metallic. 101 Therefore, it is expected that these species can also influence the chemiresistive sensing properties of MXenes.…”

“…However, the sensing setup and the sensing performance were observed in an artificial background of N 2 instead of air. 101,182 Moreover, along with the detection of VOCs, the control of humidity in atmospheric air is essential for many processes and applications. Particularly, Ti 198 The fabricated porous silicon/ Mo 2 CT x sensor working at 30 °C and 50 ppm of CO 2 demonstrated fast response and recovery times of 30 and 45 s, respectively.…”

“…It is clear from the analysis that MXene-based gas sensors have far superior sensitivity to NH 3 when compared to others. 101,179,180,185,187,188,201 This is due to the presence of surface functionalities, which act as specific adsorption sites for NH 3 due to higher adsorption energies and bond length. Furthermore, these sensors have shown a significant sensing response to various VOCs, 181,182,191 and some showed excellent RH sensing capabilities.…”