MXene Ti₃C₂T_<i>x</i>-Derived Nitrogen-Functionalized Heterophase TiO₂ Homojunctions for Room-Temperature Trace Ammonia Gas Sensing

Abstract

In this work, MXene Ti₃C₂T_<i>x</i>-derived nitrogen-functionalized heterophase TiO₂ homojunctions (N-MXene) were prepared via the urea-involved solvothermal treatment with varying reaction time as the sensing layer to detect trace NH₃ gas at room temperature (20 °C). Compared with no signal for the pristine MXene counterpart, the 18 h-treated sensors (N-MXene-18) achieved a detection limit of 200 ppb with an inspiring response that was 7.3% better than the existing MXene-involved reports thus far. Also, decent repeatability, stability, and selectivity were demonstrated. It is noteworthy that the N-MXene-18 sensors delivered a stronger response, more sufficient recovery, and quicker response/recovery speeds under a humid environment than those under dry conditions, proving the significance of humidity. Furthermore, to suppress the effect of the fluctuation of humidity on NH₃ sensing during the tests, a commercial waterproof polytetrafluoroethylene (PTFE) membrane was anchored onto the sensing layer, eventually bringing about humidity-independent features. Both nitrogen doping and TiO₂ homojunctions constituted by mixed anatase and rutile phases were primarily responsible for the performance improvement with respect to pristine MXene. This work showcases the enormous potential of N-MXene materials in trace NH₃ detection and offers an alternative strategy to realize both heteroatom doping and partial oxidation of MXene that is applicable in future optoelectronic devices.