Fabrication and Computational Study of a Chemiresistive NO₂ Gas Sensor Based on the Carbon Dots-WO₃ Heterostructure...

For a long time, chemiresistive gas sensors based on metal oxide semiconductors (MOSs) suffer from higher operating temperatures, resulting in higher energy consumption and instability of the sensors. Generally, a MOS-based chemiresistive gas sensor being able to work at room temperature is considered to be outstanding already. Here, a highly sensitive NO₂ gas sensor based on the carbon dots-WO₃ heterostructure, which can work below room temperature at -6 °C, is fabricated. At 18, -1, and -6 °C, its detection limits are 200 ppb, 5 ppm, and 20 ppm, respectively, and the corresponding response values (R_a/R_g) are 1.11, 1.04, and 1.13, respectively. The sensor exhibits good selectivity, stability, and linearity between relative humidity and response values too. A peculiar response behavior was observed. Toward oxidizing gas NO₂, the resistance of the sensor based mainly on n-type WO₃ shows decrease behavior. Its peculiar response behavior and strong gas sensing ability at lower temperatures were elucidated theoretically using the results of first-principles calculations. The reduction of NO₂ into NO by surface oxygen vacancies of WO₃ and the following adsorption of NO on the surface of WO₃ lead to electron transfer from NO to WO₃, and the Fermi level shifts toward the conduction band, making the sensor exhibit the peculiar response behavior. The stronger adsorption capability of carbon dots toward NO₂ and a synergistic effect of carbon dots and WO₃ together make the sensor capable of working at lower temperatures and own higher sensitivity.