Fast and recoverable NO₂ detection achieved by assembling ZnO on Ti₃C₂T_<i>x</i> MXene nanosheets under UV illumination at room temperature

Abstract

Recently, Ti₃C₂T_<i>x</i> MXenes have begun to receive attention in the field of gas sensors owing to their characteristics of high conductivity and abundant surface functional groups. However, Ti₃C₂T_<i>x</i>-based gas sensors still suffer from the drawbacks of low sensitivity and sluggish response/recovery speed towards target gases, limiting their development in further applications. In this work, Ti₃C₂T_<i>x</i>-ZnO nanosheet hybrids were fabricated through a simple sonication method. The Ti₃C₂T_<i>x</i>-ZnO nanosheet hybrids exhibited a short recovery time (10 s) under UV (ultraviolet) illumination, a short response time (22 s), a high sensitivity (367.63% to 20 ppm NO₂) and selectivity. Furthermore, the Ti₃C₂T_<i>x</i>-ZnO sensor has prominent anti-humidity properties, as well as superior reproducibility in multiple tests. The abundant active sites in the Ti₃C₂T_<i>x</i>-ZnO nanosheet hybrids, including surface groups (-F, -OH, -O) of Ti₃C₂T_<i>x</i> and oxygen vacancies of ZnO, the formation of Schottky barriers between Ti₃C₂T_<i>x</i> and ZnO nanosheets and the rich photogenerated charge carriers of ZnO under UV illumination, together result in excellent gas-sensing performance. Density functional theory calculations have been further employed to explore the sensing performance of Ti₃C₂T_<i>x</i> and ZnO nanosheets, showing strong interactions existing between the NO₂ and ZnO nanosheets. The main adsorption sites for NO₂ were present on the ZnO nanosheets, while the Ti₃C₂T_<i>x</i> played the role of the conductive path to accelerate the transformation of charge carriers. Our work can provide an effective way for improving the gas-sensing performances of Ti₃C₂T_<i>x</i>-based gas sensors.