Fabrication and Computational Study of a Chemiresistive NO 2 Gas Sensor Based on the Carbon Dots-WO 3 Heterostructure for Operating below Room Temperature.

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 NO 2 gas sensor based on the carbon dots-WO 3 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 NO 2 , the resistance of the sensor based mainly on n-type WO 3 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 NO 2 into NO by surface oxygen vacancies of WO 3 and the following adsorption of NO on the surface of WO 3 lead to electron transfer from NO to WO 3 , 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 NO 2 and a synergistic effect of carbon dots and WO 3 together make the sensor capable of working at lower temperatures and own higher sensitivity.