Fabrication and measurement of gas sensor

The detail of the sensor fabrication and the testing process were described as follows⁴³. The as-prepared NiO or α-Fe₂O₃/NiO nanotubes were dispersed in deionized water to form paste and then coated them on the ceramic tube (length: 4 mm, external diameter: 1.2 mm, and internal diameter: 0.8 mm) with a pair of Au electrodes, which each electrode was connected with two Pt wires. Then the coated device was sintered at 400 °C for 2 h after the paste was become dry. After that a Ni-Cr alloy coil heater was inserted to the alumina tube. The operating temperature was controlled by adjusting the heating current flowed through the heater. The schematic of the fabricated gas sensor can be shown in Supplementary Information-Fig. S6. In the testing procedure of gas sensor, a certain amount of test gases were injected into the airtight chamber. The carrier gas is high purity air (21 vol.% O₂ and 79 vol.% N₂). The sensor resistances in different environmental atmospheres were measured. The gas response of the sensor (p-type, reducing gas) was defined as R_g/R_a, which R_a and R_g were the electrical resistance of the gas sensor exposed to pure air and in the presence of target gases, respectively. The response and recovery times were defined as the time taken by the resistance value changes to 90% in the case of adsorption and desorption, respectively.