Porous ZnO Polygonal Nanoflakes: Synthesis, Use in High-Sensitivity NO₂ Gas Sensor, and Proposed Mechanism of Gas Sensing

Unique porous ZnO polygonal nanoflakes were synthesized by the microwave hydrothermal method. The structural properties of the products were investigated by using X-ray diffraction, scanning electron microscopy, transmission electron microscopy (TEM), and high-resolution TEM techniques. In situ diffuse reflectance infrared Fourier transform spectroscopy technique was employed to investigate the mechanism of NO₂ sensing. Free nitrate ions, nitrate ions, and nitrite anions were the main adsorbed species. N₂O was formed via NO^– and N₂O₂^– that were stemmed from NO. Comparative tests for gas sensing between gas sensors based on the as-prepared porous ZnO nanoflakes and purchased ZnO nanoparticles clearly showed that the former exhibited more excellent NO₂ sensing performances. Photoluminescence and X-ray photoelectron spectroscopy spectra further proved that the intensities of donors (oxygen vacancy (V_O) and/or zinc interstitial (Zn_i)) and surface oxygen species (O₂^– and O₂), which were involved in the mechanism of gas sensing led to the different gas-sensing properties.