The Introduction of Defects in Ti3C2Tx and Ti3C2Tx‐Assisted Reduction of Graphene Oxide for Highly Selective Detection of ppb‐Level NO2

At present, the main gas‐sensing mechanism of oxidized MXene (Ti3C2Tx) is commonly regarded as Schottky barrier modulation, but the influence of surface defects generated by oxidation is ignored and ambiguous. Herein, oxidized Ti3C2Tx crumpled spheres (MS) are obtained, accompanying numerous surface defects through thermal oxidation of MS synthesized by ultrasonic spray pyrolysis technology and gas‐sensing properties of oxidized MS with Ti3C2Tx/TiO2 crumpled spheres (MT‐10‐1) without new surface defects are compared. It is demonstrated that the significant improvement of the gas‐sensing properties of oxidized MS is due to the introduction of Ti atom defects rather than Ti3C2Tx/TiO2 heterojunction in‐situ generated by oxidation. First‐principles density functional theory calculations show that Ti atom vacancy can greatly improve the adsorption ability of Ti3C2Tx to gases (especially for NO2). Subsequently, with the facile oxidability, Ti3C2Tx is utilized as a reductant to assist the reduction of graphene oxide, and Ti3C2Tx/TiO2/rGO crumpled spheres are subtly designed and successfully synthesized for further enhancing the gas‐sensing performance. The MG‐2‐1 sensor achieves a low detection limit of NO2 (10 ppb), great NO2 selectivity, and high NO2 response. The clarification of the gas‐sensing mechanism of oxidized Ti3C2Tx and the utilization of oxidation of Ti3C2Tx provide a new idea for the application of MXenes. Utilizing ultrasonic spray pyrolysis technology, a Ti3C2Tx sphere is used to replace Ti3C2Tx film for mechanism analysis. It is confirmed that Ti atom defects caused by oxidation is the main reason for the improvement of gas‐sensing performance of oxidized Ti3C2Tx. Ti3C2Tx‐assisted reduction of graphene oxide to construct a Ti3C2Tx/TiO2/rGO heterostructure has achieved highly selective ppb‐level NO2 sensing.