Progress in functionalized WO3-based gas sensors for selective H2S and NH3: A review

With the deep integration of industries such as the Internet of Things (Iot), big data, and artificial intelligence, sensors are becoming more and more interconnected and intelligent, which also poses a higher challenge to the development of gas sensors with excellent gas sensitive performance. WO3-based gas sensors are widely used in the detection of H2S and NH3 gases because of their unique electrical characteristics. This paper reviews the progress of WO3-based chemoresistive gas sensors for the difficult-to-remove reducing gases H2S and NH3. First, the sensing mechanism of pure-phase WO3 as well as the enhancement of WO3 sensing through modification strategies, including the construction of heterojunctions, the electronic and chemical sensitization mechanisms of noble metal modifications, the protonation/deprotonation process of polyaniline (PANI), and photoexcitation, are discussed. In addition, the application of WO3 gas sensors using different nanostructures (such as quantum dots, nanoparticles, nanowires, nanotubes, nanofibers, and flowers) for the detection of H2S and NH3 gases is summarized in detail. Although pure-phase WO3 can effectively detect toxic and hazardous gases, it still has the disadvantages of low sensitivity, poor selectivity and high operating temperature. Metal oxide semiconductor (MOS) compounding, ion doping, noble metal modification, organic conductive polymer compounding, carbon-based material loading, transition metal-disulfide compounds (TMDCs) modification, and light can effectively improve the gas-sensitive properties. Finally, the challenges and prospects of WO3-based H2S and NH3 gas sensors are also thoroughly analyzed and discussed.