Tungsten disulfide (WS 2 ) nanosheets (NSs) have become a promising room-temperature gas sensor candidate due to their inherent high surface-to-volume ratio, tunable electrical properties, and high on-state current density. For further practical applications of WS 2 -based gas sensors, it is still necessary to overcome the insensitive response and incomplete recovery at room temperature. In this work, we controllably synthesized high-performance ammonia (NH 3 ) gas sensor based on CuO decorated WS 2 NSs. The optimized p-p WS 2 /CuO heterojunctions improve the surface catalytic effect, thereby enhancing the gas-sensing performance. The pure WS 2 NSs-based gas sensors showed a low response and an incomplete recovery in the case of NH 3 sensing. After the functionalization of CuO nanoparticles, the WS 2 /CuO heterostructurebased gas sensor exhibits an improved response value of 40.5% to 5 ppm NH 3 and full recoverability without any external assistance. Density functional theory calculations illustrate that the adsorption of CuO for NH 3 is much superior to WS 2 . The p-p heterojunctions strategy demonstrated in this work has great potential in the design of sensitive materials for gas sensors, and provides useful guidance for enhancing the room-temperature sensitivity and recoverability.

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 Ti 3 C 2 T x -MXene nonmetallic electrode and the Ti 3 C 2 T x /WS 2 gas sensing film was designed to form Ohmic contact and Schottky heterojunction in a single gas sensing channel. Ti 3 C 2 T x /WS 2 has outstanding physical and chemical properties for both Ti 3 C 2 T x and WS 2 nanoflakes, showing high conductivity, effective charge transfer, and abundant active sites for gas sensing. The response of the gas sensor to NO 2 (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 Ti 3 C 2 T x /WS 2 sensor (4.8%) and the MXene electrode integrated with the Ti 3 C 2 T x sensor (0.2%), respectively. Besides, this design performed at a limit of detection with 11.0 ppb NO 2 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.

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