The structure of a side-heated type of sensor is schematically shown in Fig. 1(a). The construction of the WO3 nanocrystal sensor is described as follow [25,26]: WO3 nanocrystals were mixed with an adhesive in an agate mortar to form a paste. The paste was coated on an alumina tube with a pair of Au electrodes and four sticks of Pt wires, and then dried under infrared lamp for 10 min in air, followed by sintering at 500 ◦C for 2 h. Finally, a stick of Ni–Cr alloy coil was put through the tube as a heater, which provides operating temperature of the gas sensor. The gas sensors were aged at 300 ◦C for 10 days in order to improve its stability.