MXene Ti3C2Tx-Derived Nitrogen-Functionalized Heterophase TiO2 Homojunctions for Room-Temperature Trace Ammonia Gas Sensing

In this work, MXene Ti3C2Tx-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.