Sulfur vacancy-engineered FeS_2-x /graphene heterostructures for ultrasensitive ppb-level NO₂ sensing

A transfer-free strategy is reported to fabricate FeS_2-x/graphene heterostructures by direct chemical vapor deposition (CVD) growth of graphene on sapphire followed by sulfurization of a pre-deposited Fe film. We uncover a thickness-dependent phase evolution mechanism, in which graphene acts as a structural template that promotes the preferential formation of sulfur-vacancy-rich FeS_2-x at an optimal Fe precursor thickness of 10 nm. Experimental characterization and mechanistic analysis reveal that these engineered sulfur vacancies-related donor states contribute to n-type doping, improving the apparent optical absorption behavior and increasing the carrier concentration of FeS_2-x. Furthermore, the sulfur-vacancy-rich interface acts as active charge-transfer centers, amplifying adsorption-induced modulation and facilitating charge transfer upon gas adsorption. As a result, the optimized sensor exhibits competitive room-temperature sensing performance toward NO₂, featuring a high response of 10.27% to 100 ppb NO₂, a remarkable calculated detection limit of 1.08 ppb, and rapid response/recovery kinetics. In addition, excellent stability, reproducibility, selectivity, and favorable humidity tolerance are achieved. This work elucidates the synergistic interplay between vacancy engineering and heterointerface modulation in 2D sensing systems and provides a scalable pathway toward advanced high-performance chemiresistive sensors.