Role of graphene quantum dots with discrete band gaps on SnO 2 nanodomes for NO 2 gas sensors with an ultralow detection limit.

NO 2 is a major air pollutant that should be monitored due to its harmful effects on the environment and human health. Semiconducting metal oxide-based gas sensors have been widely explored owing to their superior sensitivity towards NO 2 , but their high operating temperature (>200 °C) and low selectivity still limit their practical use in sensor devices. In this study, we decorated graphene quantum dots (GQDs) with discrete band gaps onto tin oxide nanodomes (GQD@SnO 2 nanodomes), enabling room temperature (RT) sensing towards 5 ppm NO 2 gas with a noticeable response (( R a / R g ) - 1 = 4.8), which cannot be matched using pristine SnO 2 nanodomes. In addition, the GQD@SnO 2 nanodome based gas sensor shows an extremely low detection limit of 1.1 ppb and high selectivity compared to other pollutant gases (H 2 S, CO, C 7 H 8 , NH 3 , and CH 3 COCH 3 ). The oxygen functional groups in GQDs specifically enhance NO 2 accessibility by increasing the adsorption energy. Strong electron transfer from SnO 2 to GQDs widens the electron depletion layer at SnO 2 , thereby improving the gas response over a broad temperature range (RT-150 °C). This result provides a basic perspective for utilizing zero-dimensional GQDs in high-performance gas sensors operating over a wide range of temperatures.