2.3. Sensor fabrication and test
The prepared ZnO micro/nano-material and distilled water were mixed to form a slurry, which was used to fabricate the gas sensor by coating the slurry onto a ceramic tube (length 4 mm, diameter 1 mm) to make a thin film structure. Two Au electrodes circled the both sides of tube and four Pt wires were stretched on both ends of the tube. A Ni–Cr alloy coil as a heater went through the ceramic tube to control the operating temperature. Target gas acetone was first diluted in the air and then a calculated amount of test gas was introduced by a microsyringe into the testing chamber on Chemical Gas
Sensor-8 (CGS-8, Ailite Technology Co., Ltd., Beijing, China) at a relative humidity of 20–30%. The sensor sensitivity signal S, was defined as the ratio Ra/Rg, where Ra and Rg were the electrical resistances
of the sensor in air and in test gas, respectively. The scheme of the sensor is shown in Fig. 1.