Two-Dimensional Semiconducting Covalent Organic Nanosheets for Highly Sensitive and Stable NO2 Sensing under Humid Condition

Semiconductor metal oxide gas sensors do not provide selective and sensitive gas adsorption under humid conditions owing to the deactivation of active sites by environmental humidity, leading to inaccurate signals in practical applications. Therefore, it is essential to explore organic-based gas sensors that are highly stable against variations in humidity without loss of selectivity and sensitivity toward the target gas. Herein, we demonstrate that a gas sensor integrated with two-dimensional covalent organic nanosheets (CONs) can show highly sensitive and reliable NO2 sensing performance at high humidity levels. The CON-based sensor can selectively detect the target gas to the 20 ppb level, and is thermally stable up to 300 °C, which is much above the typical working temperature for NO2 sensing. More importantly, the relative humidity change from 0 to 87% neither affects the baseline resistance nor the gas response of the CON sensor; the initial gas response is almost constant in repeated NO2 adsorption/desorption tests. Such excellent sensing stability at high humidity levels is ascribed to the superhydrophobic nature of CONs originating from their strong covalent bonds and additional nitrogen–sulfur bonds in the networks as confirmed by water contact angle (135.4°) measurements and density functional theory calculations.