Low-temperature hydrothermal synthesis of WO3 nanorods and their sensing properties for NO2

Tungsten trioxide (WO3) nanorods with an aspect ratio of similar to 50 have been successfully synthesized by hydrothermal reaction at a low temperature of 100 degrees 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 WO3 nanorods strongly depend on hydrothermal temperature and time. Hydrothermal conditions of 100 degrees C and 24 h ensure the formation of well-defined WO3 nanorods. The transition of the crystal structure from monoclinic WO3 to hexagonal WO3 occurs after calcination at 400 degrees C. The appropriate calcination conditions of the WO3 nanorods are defined to be 600 degrees C and 4 h for gas-sensing applications. Response measurements reveal that the WO3 sensor operating at 200 degrees C exhibits high sensitivity to ppm-level NO2 and small cross-sensing to CO and CH4, which makes this kind of sensor a competitive candidate for NO2-sensing applications. Moreover, impedance measurements indicate that a conductivity mechanism of the sensor is mainly dependent on the grain boundaries of WO3 nanorods. A possible adsorption and reaction model is proposed to illustrate the gas-sensing mechanism.