Potential barrier modulation in ZnO nanostructures and their CO sensing properties

Colloidal ZnO nanostructures were synthesized by using a high-temperature solution-phase hydrolysis approach. ZnO nanoparticles (NPs) with ∼25 nm particle size were synthesized in ethylene glycol (EG), whereas ZnO nanospheres with an average diameter of ∼500 nm (composed of 10-15 nm ZnO NPs) were formed in polyethylene glycol (PEG). The effect of solvents on the morphology of ZnO nanocrystal clusters was investigated. UV-visible absorption band of both samples was blue-shifted as compared to bulk ZnO. ZnO NPs showed strong UV emission and weak green emission, whereas ZnO nanospheres displayed strong green emission, along with UV emission, in the photoluminescence (PL) spectrum. As prepared ZnO nanostructures were applied for CO sensing, where ZnO NPs exhibited higher response than ZnO nanospheres, although the grain size of ZnO NPs was almost double that of ZnO nanospheres. A relationship between sensing layer microstructure and inter-electrode gap was established. It was demonstrated that the arrangement of grains in the sensing layer between the electrodes plays an important role in gas sensing.