HighlyEfficient UV-Activated TiO₂/SnO₂ Surface Nano-matrixGas Sensor: Enhancing Stability for ppb-LevelNO_<i>x</i> Detection at Room Temperature

This study presents a new nanoporous TiO₂/SnO₂ heterojunction for NO_<i>x</i> gas detection by using a two-step sol–gel process. The unique TiO₂ and SnO₂ nanoheterojunction matrix right on the film surface enables the TiO₂ photocatalyst to absorb minimal UV power (3 μW/cm²) and effectively transfer electrons to the SnO₂ conduction band. The sensor detects NO and NO₂ gases down to 4 ppb (response of 0.6%) and 10 ppb (response of 1.3%) at 1 V at room temperature. It also exhibits a fast recovery time (100 ± 40 s at 500 ppb NO_<i>x</i>), an improved response over a wide relative humidity range (10–60%), and a long lifetime over 30 days. The ultralow UV power required can be easily harvested from sunlight, eliminating the need for UV LEDs. XPS and SEM analyses indicated that the unique nanoporous TiO₂/SnO₂ structure improves sensing performance, with oxygen vacancies playing a critical role in the NO_<i>x</i> gas sensing mechanism. This work demonstrated the highly efficient UV catalyst effect in sensors with the surface heterojunction matrix. The low-power ppb-level NO_<i>x</i> detection is suitable for environmental monitoring and respiratory disease detection.