Hierarchical Structured SnOx/g-C3N4 nanocomposites with n-n heterojunctions for NO2 detection at low working temperature

• Constructed novel SnO x /g-C 3 N 4 n–n heterojunctions via a hydrothermal route for low-temperature NO 2 detection. • Interfacial charge transfer and built-in electric field enhance electron depletion and sensing response. • Hierarchical meso-/macroporous structure accelerates gas diffusion, enabling high sensitivity and fast response. Nitrogen dioxide (NO 2 ) is a harmful air pollutant that requires sensitive, low-temperature detection. Here, n–n heterojunction gas sensors based on SnO x /g-C 3 N 4 composites were synthesized via a hydrothermal method and thermal treatment. The optimized GNSn-10% sensor exhibited a superior NO 2 response (R g /R a = 54.8 at 48 ppm) at only 80 °C, over four times that of pristine SnO x (12.2). XRD, XPS, and UV–Vis DRS analyses confirmed strong interfacial coupling, with a reduced band gap and elevated chemisorbed oxygen ratio (from 44.41% to 63.21%), enabling enhanced electron withdrawal and surface reactivity. DFT calculations revealed an adsorption energy of −1.31 eV for NO 2 on the composite, higher than pristine materials, indicating strong charge transfer and interfacial bonding. Moreover, hierarchical meso-/macroporous structures improved gas diffusion, shortening response/recovery times (63/142 s) and lowering the detection limit (69.8 ppb). The synergistic effects of heterojunction formation, enriched oxygen species, and optimized porosity underpin excellent NO 2 sensing performance.