Research On Acetone Selective Detection And Gas Sensing Mechanism Of Nano-structured Metastable Phase WO₃ With Tunable Crystal Facets

Sensitive, selective, and working stability of gas sensors, which are applied on environmental monitoring and breath diagnosis analyzers, are strongly dependent on the control and design of gas sensing materials. Tungsten oxide（WO₃） is a wide-band n-type semiconducting material with excellent gas sensing properties. However, due to the polymorphism of WO₃, the gas sensing properties and sensing mechanism of metastable WO₃ phase are still lacking clear understanding. In this project, metastable WO₃ nano-materials with exposed of different crystal facets were prepared via hydrothermal method. Their ppm level acetone sensitive and selective sensing properties were investigated. On the basis of “volume depletion” model, acetone sensing model of metastable WO₃ nano-materials were proposed. All of this researches have provided a theoretical and experimental fundamental evidence for WO₃ nano-materials to be applied on the breath diagnosis analyzers.Hexagonal WO₃ nanorods fabricated by hydrothermal method with NH₄⁺ ions as directing agents exhibited sensitive and selective response to acetone, due to their optimal growth along [110] axis and exposure of active（002） facets. Triclinic WO₃ nanosheets with exposed（002） facets were prepared by hydrothermal method with citric acid as directing agent and the following heat treatment, which could detect acetone selectively. The active（002） facets of hexagonal and triclinic WO₃ nano-materials contained unsaturated coordinated oxygen atoms with asymmetric arrangement on the surface, which brought an extent of local polarization. Acetone molecules had large dipole moment, which could be adsorbed and coupled by polar（002） facets selectively. Hierarchical WO₃ microspheres assembled by WO₃ nanorods with exposed（002） facets were fabricated successfully, whose specific surface area was 62 m²/g, response to 1 ppm acetone was 3.53, response and recovery time were 9 and 14 s, respectively, indicating a promising canditate for applying as breath diagnosis analyzers. The acetone selective sensing mechanism was attributed to their porosity, exposure of（002） facets, and high level of structural defects.Based on these experiment results, the relationship between electrical resistance of semiconductors under “volume depletion” and surface adsorption behaviors was investigated. It was observed that the main oxygen adsorbing species of metastable triclinic and hexagonal WO₃ were O^-. For non-polar gas molecules H2, the surface adsorbing O^- of triclinic and hexagonal WO₃ could be reduced by H2, while the weak polar gas molecules CO could react with the surface by reducting not only the adsorbing O^- but also the lattice oxygen OL, in which the lattice oxygen OL was the dominated reduced species. The surface reduction behavior of h-WO₃ by CO were little deteriorated by humidity, and the response of triclinic WO₃ to CO even increased with humidness. On the basis of simple gas molecules surface reduction behavior, the surface redox model for acetone was established finally. Acetone molecules had large polar moment, so that they could react with triclinic and hexagonal WO₃ by reducing both their lattice oxygens and surface adsorbing oxygens, in which lattice oxygens reduction played a dominant role so that they could resist the variation of humidity. It could be inferred that gas sensors based on triclinic and hexagonal WO₃ nano-materials exhibited acetone sensing performance in high humidity during breath diagnosis as excellent as in the normal room atmosphere.