Unraveling the Effect of Oxygen Vacancy on WO₃ Surface for Efficient NO₂ Detection at Low Temperature

Abstract

Oxygen vacancies (V_O) in metal oxide semiconductors play an important role in improving gas-sensing performance of chemiresistive gas sensors. Nonetheless, there is still a lack of clear understanding of the inherent mechanism of the influence of oxygen vacancies on gas sensing due to generally focusing on the concentration of V_O. Herein, oxygen vacancies were rationally modulated in WO₃ nanoflower structures via an annealing process, resulting in a transformation of V_O from neutral (V_O⁰) to a doubly ionized (V_O²⁺) state. Density functional theory (DFT) calculations indicate that V_O²⁺ is significantly more efficient than V_O⁰ for NO₂ detection in competition with atmospheric O₂. Benefiting from a high concentration of V_O²⁺, the WO₃-450 (WO₃ annealed at 450 °C) sensor exhibits excellent sensing performance with an ultrahigh sensitivity (3674.1 to 5 ppm NO₂), superior selectivity, and long-term stability (one month). Furthermore, the sensor with the wide range of concentration detection not only can detect NO₂ gas with parts per million (ppm) but also can detect NO₂ with parts per billion (ppb) level concentration, with a high sensibility reaching 2.8 to 25 ppb NO₂ and over 100 to 100 ppb NO₂. This study elucidates the oxygen vacancy mediated sensing mechanism toward NO₂ and provides an effective strategy for the rational design of gas sensors with high sensing performance.