Abstract: Sunflower straw was used as a biological template, a novel graded porous structure of nickel oxide doped zinc oxidenanocomposite (Bio NiO/ZnO) was synthesised.  The composition and structure of the composite were characterised by using X-ray diffraction (XRD),  scanning electron microscopy (SEM),  transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The experimental results show that the synthesised Bio NiO/ZnO preserves the original graded porous structure of the sunflower straw material,  which consists of a large number of hexagonal crystalline fibrillated ZnO-type structures of uniform size and shape stacked with spherical particles of nickel oxide in a cubic crystalline aragonal phase structure.  Compared with the undoped ZnO material,   a gas sensor made of Bio NiO/ZnO composite  material has enhanced gas sensitivity to formaldehyde.  The response of the gas sensor to formaldehyde of 10^-4   is 42 at an operating temperature of 240 ℃， the response time and recovery time are 7 s and 6 s respectively. The lower limit of detection is 1.48×10^-4, which is  related to the increased concentration of oxygen vacancies in the complex and the existence  of p-n type heterojunctions between the two phases.

Key words: biological template,  , nickel oxide/zinc oxide,  , heterojunction,  , formaldehyde,  , gas sensing