High-sensitivity NO2 gas sensors based on flower-like and tube-like ZnO nanomaterials

Hierarchical flower-like and 1D tube-like ZnO architectures were synthesized by a microemulsion-based solvothermal method. Technologies of XRD, SEM and TEM were used to characterize the morphological and structural properties of the products. The influence of the flower-like and tube-like morphologies on their NO₂ sensing properties was investigated. The experimental results showed that high-sensitivity NO₂ gas sensors were fabricated. The sensitivity of the tube-like ZnO gas sensor was much higher than that of the flower-like ZnO gas sensor and the tube-like ZnO gas sensor exhibited shorter response time. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) technique was employed to investigate the NO₂ sensing mechanisms. Free nitrate ions, nitrate and nitrite were the main adsorbed species during the adsorption, and NO also existed in the initial period of surface reoxidation. Furthermore, N₂O was formed via NO⁻ and N₂O₂⁻ stemmed from NO and increased upon rising temperature. Moreover, the PL spectra and the XPS spectra further proved that the intensity of donors (oxygen vacancy (V_O) and zinc interstitial (Zn_i)) and surface oxygen species (O₂⁻ and O₂) involved in the gas sensing mechanism leaded to the different sensitivities.