Sandwich-Structured ZnO/MXene Heterojunction for Sensitive and Stable Room-Temperature Ammonia Sensing

2D metal carbides/nitrides (MXenes) have attracted considerable interest in NH₃ sensing due to their high electrical conductivity and abundant terminal groups. However, the strong interlayer interactions between MXene nanosheets result in challenges related to recovery and rapid response decay in MXene-based sensors. Here, a one-step hydrothermal strategy is developed that anchors Zn atoms and grows ZnO polycrystals on the Ti vacancies of Ti₃C₂T_x layers, forming a sandwich-structured ZnO/Ti₃C₂T_x heterojunction. At room temperature, the NH₃ sensitivity of ZnO/Ti₃C₂T_x is a remarkable 45-fold higher than that of Ti₃C₂T_x, with a low detection limit of 138 ppb and a rapid recovery time of 39 s. Furthermore, the heterojunction exhibits exceptional long-term stability, maintaining a consistent response over 21 days. The results confirm that in situ intercalation of the ZnO polycrystals effectively solves the recovery problem in MXene substrates by completely exfoliating the Ti₃C₂T_x nanosheets. Meanwhile, the room-temperature sensing performance and recovery speed of the sandwich-structured ZnO/Ti₃C₂T_x is enhanced by rapid electron conduction. This straightforward and effective route for in situ exfoliation and intercalation of MXene layers promises the expanded use of 2D material heterojunctions in sensing applications.