Fig. 4. (a) Schematic illustration of the α-Fe₂O₃ based interferometric vapor sensor. (b–d) Reflected spectrum of the α-Fe₂O₃ with anodization time of 30 min, 50 min and 80 min. (e) Reflective interferometric Fourier transform spectroscopy in air and ethanol of the α-Fe₂O₃ (anodization time: 50 min).