Unraveling the Effect of Oxygen Vacancy on WO 3 Surface for Efficient NO 2 Detection at Low Temperature.

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 WO 3 nanoflower structures via an annealing process, resulting in a transformation of V O from neutral (V O 0 ) to a doubly ionized (V O 2+ ) state. Density functional theory (DFT) calculations indicate that V O 2+ is significantly more efficient than V O 0 for NO 2 detection in competition with atmospheric O 2 . Benefiting from a high concentration of V O 2+ , the WO 3 -450 (WO 3 annealed at 450 °C) sensor exhibits excellent sensing performance with an ultrahigh sensitivity (3674.1 to 5 ppm NO 2 ), superior selectivity, and long-term stability (one month). Furthermore, the sensor with the wide range of concentration detection not only can detect NO 2 gas with parts per million (ppm) but also can detect NO 2 with parts per billion (ppb) level concentration, with a high sensibility reaching 2.8 to 25 ppb NO 2 and over 100 to 100 ppb NO 2 . This study elucidates the oxygen vacancy mediated sensing mechanism toward NO 2 and provides an effective strategy for the rational design of gas sensors with high sensing performance.