Highly sensitive NO2 sensor based on ZnO nanostructured thin film prepared by SILAR technique

[Display omitted] •SILAR made ZnO thin films exhibited high sensor response (249) towards 20 ppm NO2 with fast response and recovery.•These films were found to be sensitive to as low as 500 ppb of NO2.•Improved gas sensing characteristics are attributed to presence of defects (oxygen vacancies & zinc interstitials.•Presence of these defects was verified by XPS, PL, Raman spectroscopy & work function measurements).•XPS studies confirm interaction of incoming gas with both adsorbed as well as lattice oxygen. Present study reports highly sensitive, selective and rapid sub-ppm level detection of nitrogen di-oxide (NO2) using pristine zinc oxide (ZnO) thin films prepared by a simple and efficient technique - Successive Ion Layer Adsorption and Reaction (SILAR). Synthesized ZnO thin films were characterized for their physical and chemical properties. Gas sensing studies revealed that these films are highly sensitive to NO2 with a sensor response of 249 for 20 ppm at an optimum temperature of 200 °C, with a lowest detection limit of 500 ppb. These sensors are found to be highly selective to NO2 with fast response and recovery times (11 and 44 s for 20 ppm). XPS studies of these films have been carried out after NO2 exposure in order to understand the sensing mechanism. Presence of nitride (N - 395.9 eV) species in N-1s spectra, decrease in binding energy position (∼ 1 eV) of Zn-2p peaks along with reduction of the concentration of total oxygen species on the surface of the film (from 73 % to 54 %), indicates that NO2 does not only interact with adsorbed oxygen but also with lattice oxygen. Optical studies (Raman and Photoluminescence) as well as observance of a higher work function value ∼ 5.24 eV in pure ZnO samples established the existence of defects in these ZnO thin films, which forms the basis for their superior sensor response and faster response kinetics.