Study of ZnO room temperature NO2 sensor under illumination prepared by auto-combustion

ZnO nanoparticles have been prepared by auto-combustion method. Morphological and structural properties of the prepared samples were investigated by SEM, XRD, Raman and XPS characterizations. The XRD diffractogram of the sample indicates that ZnO has a hexagonal wurtzite structure. The crystallites average size, calculated from the Williamson–Hall plot, was 69.3 nm and the estimated by the SEM image (76.86 nm). Raman investigation indicates different modes of atomic displacement which correspond to longitudinal/transversal optical components with different frequencies. These modes are due to the macroscopic electric fields associated with the basic phonon of hexagonal ZnO. The XPS spectra indicate the presence of Zn and O in the structure with a small number of interstitial Zn2+, oxygen vacancies (Vo), and a negligible amount of chemical bonds with carbon (=CO …), confirmed by FTIR spectroscopy. The UV absorbance and reflectance spectra show a high absorbance with gap energy of 3.17 eV, estimated by Tauc's model. The a.c. electrical spectroscopy can be described by the Jonscher universal power-law. The charge carriers move according to the correlated barrier hopping pattern over the dispersive region. At room temperature, the conductivity of ZnO is high (~ 8 × 10–6 S.m−1) making it promises for gas detection applications. The sensor was prepared by spraying the suspension of ZnO nanopowders on alumina substrates with pre-deposited gold interdigitated electrodes. The sensor responses of NO2, for concentrations of 0.5, 0.75 and 1 ppm, were investigated at room temperature under illumination with different wavelengths. The best response of the sensor was obtained for a concentration of 1 ppm NO2 excited by 400 nm (purple) and 380 nm (UV) wavelengths, which were 91 and 88 with response/recovery times equal to 4/6.7 min and 4.4/3.3 min. Higher responses at the lower wavelength are due to the higher excitation energy which tends to excite more electrons, at the material surface, subsequently participating in the detection mechanism with gas molecules