X-Ray diffraction (XRD) analysis between 20° and 80° in 2θ was conducted on a Scintag XDS-2000 X-ray diffractometer with Cu Kα radiation (λ=1.5418 Å). X-Ray photoelectron spectroscopy (XPS) data was obtained with a VG ESCA LAB MK II spectrometer with an Mg KR excitation (1253.6 eV). Energy-dispersive X-ray spectroscopy (EDX) was obtained on a JEM-ARM200F microscope. Scanning BW 373U86 microscopy (SEM) images were performed on a SHIMADZU SSX-550 (Japan) instrument.
2.4. Fabrication and measurement of gas sensor
The as-prepared sample was mixed with deionized water in weight ratio of 100:25 and ground in a mortar to form a paste. Then the paste was coated on a ceramic tube as a sensing film with a thickness of about 300 μm. A pair of Au electrodes was previously printed on the ceramic tube. After the ceramic tube was calcined at 300 °C for 2 h, a Ni–Cr heating wire was inserted in the tube as a heater for controlling the operating temperature. The schematic image of the sensor was shown in Fig.1.