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

Abstract

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 crosssensing 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. † Electronic supplementary information (ESI) available: Gas-sensing test apparatus; TEM, HRTEM and SAED images of as-synthesized nanorods; and FESEM images of the WO 3 nanorods after calcination at different temperatures. See

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