(a) Sensor responses of the pure SnO2 NSAs, α-Fe2O3 NRs, and α-Fe2O3/SnO2 HNAs toward 5 ppm acetone as a function of operating temperature (280-380°C). (b) Sensor responses vs. acetone concentration (0.4-20 ppm) at 340°C. The corresponding transient response curves of (c) SnO2 NSAs and α-Fe2O3/SnO2 HNAs. Resistance curves of (d) SnO2 NSAs and (e) α-Fe2O3/SnO2 HNAs at 340°C. (f) Selectivity of the sensors to various gases (1 ppm) at 340°C.