Morphological Control of Metal Oxide for Semiconductor-Based Gas Sensor ^†

Morphological control of metal oxide (MO) is important for enhancement of sensing properties such as sensor response and response-recovery characteristics. Our research group has been developed MO based gas sensors fabricated by WO₃ and SnO₂ nanocrystals synthesized by hydrothermal method for high sensor response to NO₂ or H₂ [1,2]. On the other hand, shuttle-shape SnO₂ showed the sensor response to NO₂ and H₂S at room temperature.

The film sensor with cuboid-shape monoclinic WO₃ nanocrystal (Figure 1a) showed sensor response (Rg/Ra) of 10²–10⁴ to 0.05–1 ppm NO₂ at 200 °C (Figure 1c). In contrast, the sensor with hexagonal-shape hexagonal WO₃ nanocrystal (Figure 1b) showed sensor response lower one order of magnitude than that with cuboid-shape monoclinic WO₃ in above same detection condition (Figure 1d). This difference was related to surface states. XPS spectra of O1s showed that the content of OH^- was larger for hexagonal-shape hexagonal WO₃ (O²⁻/OH⁻/H₂O (in %) = 60.0/38.1/1.9, in Figure 1e) than for cuboid-shape monoclinic WO₃ (80.4/4.9/14.7, in Figure 1f). The results suggested that the high content of oxygen adsorbate (O²⁻) on the surface of WO₃ could be contributed to higher sensor response.