A MXene Decorated ZnO Nanorod Gas Sensor for Low Concentration NO₂ at Room Temperature via UV Irradiation

Zinc oxide (ZnO) is a well-known semiconductor material that has been extensively studied for gas sensing applications due to its low cost, chemical stability, and high sensitivity. However, one of its inherent drawbacks is the high operating temperature typically required for efficient gas sensing. Furthermore, it remains challenging to enhance the selectivity and sensitivity of ZnO-based gas sensors simultaneously at room temperature. To overcome these problems, ZnO nanorod structures have gained attention with large surface area, which can absorb more gases on the surface. In this work, ZnO nanorods were synthesized via a hydrothermal method and decorated with Ti₃C₂T_x MXene. Ti₃C₂T_x MXene is a 2D material known for its excellent electrical conductivity and large specific surface area. In addition, MXene has a higher signal-to-noise ratio than other 2D materials such as graphene.
MXene-decorated ZnO nanorod sensors were prepared with variation of MXene amount and the sensitivity of each samples were investigated. The crystal structure of ZnO/MXene gas sensor was analyzed using X-ray diffractometer (XRD) and morphology was measured by scanning electron microscopy (SEM), respectively. SEM images confirmed that MXene successfully decorated on ZnO nanorod.
The results revealed that MXene decorated ZnO nanorod gas sensor improved gas sensing properties compared to pristine ZnO nanorod gas sensors. The ZnO/MXene sensor exhibited a response of approximately 12 ppm to 2 ppm of NO₂ at room temperature. The response improved with increasing MXene content, reaching an optimal value when 8 mL of 1mg/mL MXene solution was used. However, further increases in MXene amount resulted in a decline in sensor performance. During the gas sensing process, we irradiated UV light to improve the sensor characteristics. UV irradiation further enhanced the sensing performance of the ZnO/MXene composite, particularly at room temperature. The UV light generates electron-hole pairs in the ZnO, which promotes increased surface adsorption of oxygen species. The UV-irradiated ZnO/MXene gas sensor react with NO_{<span style="font-size:10.8333px">2</span>}, leading to a more significant modulation in resistance. This allowed the sensor to detect NO₂ concentrations as low as 200 ppb, demonstrating a significant improvement in sensitivity compared to sensors without UV activation. In terms of selectivity, the ZnO/MXene sensor exhibited remarkable selectivity towards NO₂ when tested against various gases such as benzene (C₆H₆), toluene (C₇H₈), ammonia (NH₃), hydrogen sulfide (H₂S), and ethanol (C₂H₅OH). The response to 2 ppm of NO₂ gas was found to be approximately 12 times greater than that of 10 ppm for other gases. Due to the electron-withdrawing nature of NO₂ and the distinct surface properties of the ZnO/MXene composite, the sensor can exhibit a superior response to NO₂. The NO₂ gas sensing mechanism is primarily attributed to the strong electron affinity, which induces charge transfer at the ZnO/MXene interface, leading to measurable resistance changes.
In summary, the integration of MXene with ZnO nanorods, combined with UV light activation, has led to the development of a highly sensitive and selective NO₂ sensor capable of detecting concentrations as low as 200 ppb at room temperature. And the presence of MXene creates more active sites for gas interaction due to its large surface area and has abundant functional groups (-OH, -O, or -F), thus enhancing the overall sensitivity. These results suggest a promising pathway for the development of next-generation gas sensors for real-time environmental monitoring.