Synergistic effect of protonation and oxygen vacancies in polyoxometalates for ppb-level NO2 detection at subzero temperatures

Accurate detection of nitrogen dioxide (NO₂) at low concentrations and sub-zero temperatures is essential for environmental monitoring and industrial safety, particularly in cold regions where conventional sensors often fail due to poor sensitivity and slow response. Here, we report a defect-engineered phosphotungstic acid-based sensor that enables ultrasensitive NO₂ detection with a limit of detection down to 0.17 ppb, rapid response/recovery times within 7/9 s, and robust operation even at −10 °C. This high-performance stems from a synergistic interaction between exposed Brønsted acid protons and thermally induced oxygen vacancies, which jointly facilitate NO₂ chemisorption and redox activation. In-situ infrared spectroscopy reveals a dynamic conversion of NO₂ to NO₂^- and NO₃^- on the sensor surface, corroborated by density functional theory (DFT) calculations showing enhanced adsorption energy and substantial charge transfer. Integrating this sensor into a smart NO₂-responsive ventilation system enables real-time air quality regulation, demonstrating its practical utility in intelligent environmental control. Our findings present a viable route for next-generation gas sensors operable in harsh, low-temperature environments.