Low-temperature hydrothermal synthesis of WO ₃ nanorods and their sensing properties for NO ₂

Tungsten trioxide (WO ₃) nanorods with an aspect ratio of ∼50 have been successfully synthesized by hydrothermal reaction at a low temperature of 100 °C. The crystal structure, morphology evolution and thermal stability of the products are characterized in detail by XRD, FESEM, FTIR, and TG/DTA techniques. The diameter evolution and distribution of WO ₃ nanorods strongly depend on hydrothermal temperature and time. Hydrothermal conditions of 100 °C and 24 h ensure the formation of well-defined WO ₃ nanorods. The transition of the crystal structure from monoclinic WO ₃ to hexagonal WO ₃ occurs after calcination at 400 °C. The appropriate calcination conditions of the WO ₃ nanorods are defined to be 600 °C and 4 h for gas-sensing applications. Response measurements reveal that the WO ₃ sensor operating at 200 °C exhibits high sensitivity to ppm-level NO ₂ and small cross-sensing to CO and CH ₄, which makes this kind of sensor a competitive candidate for NO ₂-sensing applications. Moreover, impedance measurements indicate that a conductivity mechanism of the sensor is mainly dependent on the grain boundaries of WO ₃ nanorods. A possible adsorption and reaction model is proposed to illustrate the gas-sensing mechanism.