NO2 sensing characteristics by α-Fe2O3 nanorod arrays with atomic layer deposited amorphous Al2O3 overlayer

Abstract

We have grown α-Fe2O3 nanorods by solution processing followed by the deposition of Al2O3 overlayer using atomic layer deposition. Al2O3 layer was deposited for two different thicknesses 4 nm and 8 nm and a post-deposition annealing at 550 °C for 2 h in air atmosphere was performed. Crystallinity analysis through x-ray diffraction (XRD) reveals that the α-Fe2O3 nanorods crystallized into rhombohedral structure, whereas the outer Al2O3 layers remained largely amorphous. Interestingly, the interface showed signs of AlFexOy formation as observed through high-resolution transmission electron microscopy images. Gas sensing characteristics were studied using NO2 with 10, 50, and 100 ppm concentrations at operating temperatures of 30 °C, 100 °C, 150 °C and 190 °C. The room temperature sensitivity values obtained in response to 10 ppm NO2 were 31%, which surpassed the previously reported values. A higher concentration of surface adsorbed oxygen on the Al2O3 overlayer, as revealed by the x-ray photoelectron spectroscopy (XPS) analysis, led to enhanced NO2 sensing at room temperature. A lower activation energy (0.29 eV) of barrier to charge transport for Al2O3 coated α-Fe2O3 nanorods compared to that of bare nanorods (0.45 eV), as calculated from the temperature dependent I-V measurements, supported observation of higher sensitivity at room temperature.