Highly sensitive g-C3N4 nanosheets as a potential candidate for the effective detection of NO2 gas via langasite-based surface acoustic wave gas sensor

Abstract

In view of the fact, the poor stability and inadequate chemical properties of conductive polymers inhibit their use in gas sensor applications. Moreover, the elegant two-dimensional (2D) polymeric graphitic carbon nitride (g-C₃N₄) has attracted great attention for room-temperature (RT) gas sensing applications due to its intrinsic porous structure and lone pair electrons on nitrogen atoms. Herein, we report surface acoustic wave (SAW) NO₂ gas sensors using 2D g-C₃N₄ nanosheets deposited on langasite (LGS) SAW device. Furthermore, the thickness effect on gas sensing performances was manifested and it was noticed that 220 nm thickness of g-C₃N₄-coated LGS SAW sensors showed better sensing performances. The g-C₃N₄/LGS SAW sensor showed a significant negative differential frequency shift (Δf) of ∼3.1 kHz at room temperature (RT ∼ 27 °C) for 100 ppm of NO₂ gas with a quick response/recovery time (42/22 s). Furthermore, our experimental results indicated that the effect of temperature remarkably influences NO₂ sensing responses, which escalated from ∼3.1 kHz to ∼23.3 kHz in the temperature range of 27 °C to 200 °C for 100 ppm of NO₂. Moreover, the device showed immense sensitivity towards NO₂ (100 ppm) under various relative humidity (RH) conditions (20–80%) at RT. Besides, the g-C₃N₄/LGS SAW sensor showed low detection limit (∼158 ppb), excellent long-term stability, high sensitivity and selectivity to NO₂ gas, which could be ascribed to the enhancement in mass loading effect resulting from the absorption of NO₂ gas molecules. Finally, using resistive gas sensor, we deliberately discuss the underlying gas sensing mechanism associated with the 2D g-C₃N₄ nanosheet–deposited LGS SAW device.