A facile one-step hydrothermal synthesis of NiO/ZnO heterojunction microflowers for the enhanced formaldehyde sensing properties

NiO/ZnO heterojunction microflowers were successfully synthesized by using a facile one-step hydrothermal process followed by calcination. The structural features were mainly characterized by X-ray diffractometer (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). It is found that the obtained products showed flower-like structures assembled by a number of ZnO porous nanosheets, and which were successfully retained after introducing NiO. The formaldehyde gas sensing properties were systematically investigated between the pure and NiO/ZnO microflowers. NiO/ZnO microflower sensor exhibited the enhanced response and lower operating temperature compared to pure one. Especially, the 3 mol% NiO/ZnO microflower sensor exhibited the highest responses compared with others at a relatively lower operating temperature of 200 °C. In addition, fast response and recovery, good reproducibility and stability, and good selectivity to formaldehyde were also obtained, indicating the formation of NiO/ZnO heterojunction in favor of improving the sensing properties. Meanwhile, the sensing enhancement mechanism of the NiO/ZnO microflowers was also discussed, which is related to the formation of hetrojunction at interface and the high catalytic activity of NiO.