*3.2. Characterizations*

The phase and purity of the unloaded SnO2 and CeO2-doped SnO2 samples was investigated by powder X-ray diffraction (XRD, Bruker-AXS D8, Bruker, Madison, WI, USA) with Cu Kα radiation at 40 kV and 150 mA in a scanning range of 20–80◦ (2θ) and in the continuous mode with step size of 0.02◦ (2θ) by scanning speed of 10◦/min. The morphology and nanostructure were studied by field-emission scanning electron microscopy (FESEM, Quanta™ FEG 250) (FEI, Eindhoven, The Netherlands). Chemical composition analysis was tested by energy dispersive spectroscopy (EDS, INCA ENERGY 250) (FEI, Eindhoven, The Netherlands) integrated into the FESEM system.

#### *3.3. Gas Sensor Fabrication and Analysis*

The sensor is prepared similarly to the method we reported previously [46,47]. In brief, the sample was mixed with distilled water to form a uniform paste and coated onto a ceramic substrate (13.4 mm × 7 mm) with an Ag-Pd electrode to obtain a resistive sensor. The structure was shown in Figure 7. In order to improve the stability and repeatability of the sensor, the gas sensitivity test was performed after aging for 12 h at 60 ◦C. The gas sensitivity test was conducted on the intelligent gas sensing analysis system of CGS-4TPS (Beijing Elite Tech Co., Ltd., Beijing, China) under laboratory conditions (30 RH%, 30 ◦C). The sensitivity of the sensor was defined as S = Ra/Rg, where Ra and Rg represent the resistance of the sensor in air and target gas, respectively.

**Figure 7.** Structure diagram of gas sensor.
