| WO3 semiconductor gas sensors present interesting advantages in relation to other gas-sensor technology approaches because of their simple implementation, low cost, high sensitivity, short response/recovery time and good reliability for real-time control systems. In this study, WO3 nanocrystals were synthesized by facile hydrothermal method and further controlling of structure-directing agent, temperature, pH and heat treatment processing. Gas sensing properties of the as-synthesized WO3 nanocrystals were investigated and the effect of morphology, phase structure and exposed facet were studied. Furthermore, the mechanism of gas sensing selectivity and the method of improving gas selectivity were discussed in detail.The microstructure of WO3 nanocrystals has significant impacts on their gas response, response/recovery time, sensitivity and selectivity. Among the structures, WO3 nanosheet exhibited the highest gas response, sensitivity and the best selectivity in comparison with nanoparticle, nanorod and nanobulk for its polar structure, high specific surface area and good dispersibility. Thus, 2-dimensional WO3 nanocrystals will play an important role in the preparation of gas sensing devices due to their excellent gas sensing performances.The phase structure of WO3 nanocrystals has important influences on their gas sensing properties, especially gas selectivity. Triclinic WO3 nanosheets exhibited superior acetone selectivity and sensing performance to that of monoclinic and hexagonal WO3 nanosheets. The gas response of triclinic WO3 nanosheets is 9.6 to 20 ppm of acetone with a steady base-line at 230 °C. The response and recovery time are 15 and 16 s. However, the response to other tested gases are lower than 4. It is suggested that the acentric structure of triclinic WO3 which has a spontaneous electric dipole moment plays an important role on the selective detection of acetone. On the other hand, acetone has a much larger dipole moment than the other tested gases in this experiment. As a consequence, the interaction between triclinic WO3 and acetone molecules is much stronger than the other gases, resulting in the observed good selectivity to acetone detection. Triclinic WO3 nanosheet gas sensor exhibited excellent sensing properties to NO2 at relatively low operating temperature, and the response to 300 ppb NO2 was 18.8 at 100 °C. WO3 is known as an n-type semiconductor with main carrier are free electrons due to the oxygen vacancy, whereas NO2 is a strong oxidizer and each NO2 molecule has an unpaired electron. Therefore, upon adsorption of NO2, charge transfer is likely to occur from WO3 to adsorbed NO2 because of the strong electron-withdrawing power of NO2 molecules.Exposed facet and surface atomic structure could influence gas sensing properties of WO3 nanosheets. Surface structure study revealed that the triclinic WO3 nanosheets had more unsaturated coordinated O atoms than monoclinic and hexagonal WO3 nanosheets on their surface. So more gas molecules could adsorb and react directly at these chemical adsorption sites, which lead to the high sensitivity and good selectivity of triclinic WO3 nanosheets. |
PDF Full Text Request