Fully Flexible MXene-based Gas Sensor on Paper for Highly Sensitive Room-Temperature Nitrogen Dioxide Detection

Flexible chemiresistive gas sensors have attracted growing interest due to their capability in real-time and rapid detection of gas. However, the performance of gas sensors has long been hindered by the poor charge transfer ability between the conventional metal electrode and gas sensing semiconductors. Herein, for the first time, a fully flexible paper-based gas sensor integrated with the Ti₃C₂T_x-MXene nonmetallic electrode and the Ti₃C₂T_x/WS₂ gas sensing film was designed to form Ohmic contact and Schottky heterojunction in a single gas sensing channel. Ti₃C₂T_x/WS₂ has outstanding physical and chemical properties for both Ti₃C₂T_x and WS₂ nanoflakes, showing high conductivity, effective charge transfer, and abundant active sites for gas sensing. The response of the gas sensor to NO₂ (1 ppm) at room temperature is 15.2%, which is about 3.2 and 76.0 times as high as that of the Au interdigital electrode integrated with the Ti₃C₂T_x/WS₂ sensor (4.8%) and the MXene electrode integrated with the Ti₃C₂T_x sensor (0.2%), respectively. Besides, this design performed at a limit of detection with 11.0 ppb NO₂ gas and displayed excellent stability under high humidities. Based on first-principles density functional theory calculation results, the improvement of the gas sensing performance can be mainly attributed to the heterojunction regulation effect, work function matching, and suppressing metal-induced gap states. This work provides a new approach for the design of flexible gas sensors on paper with MXene-based conductive electrodes and gas sensing materials.