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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

Nanosheet-assembled tungsten oxide microspheres have been synthesized using rapid sonochemistry followed by thermal treatment. Transient observation of controllable synthesis reveals that the morphological evolution of the product is highly dependent on the ultrasonication time. An assembly mechanism based on oriented attachment and reconstruction is proposed for the sonochemical formation of the nanosheet-assembled microspheres. The obtained samples possess intrinsic nonstoichiometry and a hierarchically porous nano/microstructure, which is beneficial for their utilization in sensing materials and for fast diffusion of gas molecules. The maximum response of the tungsten oxide hierarchical microspheres is 3 times higher than that of commercial nanoparticles for NO 2 gas. The gas adsorption-desorption kinetics during the sensing process were mathematically simulated by a derivative method. The first-principles calculation reveals that the NO 2 molecule is most likely adsorbed at the terminal O 1c site of tungsten oxide, leading to the introduction of new surface states, which are responsible for the intrinsic NO 2 -sensing properties.

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