Flexible NO2 gas sensors based on sheet-like hierarchical ZnO1-x coatings deposited on polypropylene papers by suspension flame spraying

2D sheet-like ZnO coatings composed of nanoparticles were deposited on the flexible polypropylene substrates equipped with gold electrodes by in three steps. ZnO powders with sheet-like hierarchical structure was first synthesized by direct precipitation method. And then the as-synthesized powder was dispersed in anhydrous ethanol to form a suspension. At last, the suspension was injected into the acetylene-oxygen flame to deposit ZnO sensitive layers. XRD and FE-SEM characterization confirmed that the as-sprayed coatings were present a hexagonal phase and exhibited a porous hierarchical structure. The sensors using as-sprayed ZnO coatings as sensitive layers showed good response to sub-ppm level NO2 at room temperature under white light LED illumination.