An ultra-high sensitive ethanol sensor through amending surface-functionalized groups by novel acidic synthesis methods

The surface structure and functional groups of MOS materials play key roles in sensor performance. Herein, we report the first attempt to use four organic acids (citric, ellagic, oxalic, and glycolic acids), instead of a traditional alkaline environment, to synthesize ZnO sensing materials. The results demonstrate that the morphology and structure of the ZnO material cab be modified by simply varying the type of organic acid, which can straightforwardly inhibit the growth of the crystal planes through acid etching. Among those selected organic acids for the synthesis, the synthesized ZnO-based citric acid has the best gas sensitivity to ethanol, with a state-of-the-art sensing response of 121.5 (R_a/R_g) to 100 ppm ethanol at a fairly low working temperature of 180 °C. All the sensors also show good selective ethanol detection compared to other gases, including methanol, carbon monoxide, carbon dioxide, hydrogen, methane, and propane. The improved sensor sensitivity to ethanol mainly originates from the surface defects increase in the organic-acids-synthesized sensing materials, which enhances adsorption and ionization of oxygen. Molecular dynamics (MD) simulations confirmed that the ethanol gas was preferentially absorbed on almost all (002), and (100) crystal planes of ZnO, explaining the selective sensing response to ethanol gas.