Monoclinic-Hexagonal Heterophase Junctions of WO₃ Nanostructures with Brønsted Acid Sites for Sensitive and Selective Detection of NH₃ Gas

WO3 nanostructures with monoclinic-hexagonal heterophase junctions (S-400) and the pure monoclinic phase of WO3 were designed and prepared by the solvothermal method, combined with calcination at different temperatures. The WO3 sensors exhibited p-type responses under low operating temperatures (≤100 °C) and n-type responses under high operating temperatures (≥200 °C) to the reducing gas NH3. Particularly, the S-400 sensor displayed outstanding sensitivity (133.2 to 100 ppm NH3) and superior selectivity under a low operating temperature of 70 °C, which may be attributed to its intrinsic structure of heterophase junctions and surface state effects. Surprisingly, it was found that for the p-type conductive WO3, the environmental humidity (ranging from 2 to 28%) positively affects its gas sensing performances to NH3. Thus, we proposed for the first time that the Brønsted acid sites formed on the WO3 surface under 70 °C greatly facilitate adsorption of the alkaline gas NH3, resulting in excellent sensitivity and selectivity.