Effect of oxygen vacancy sites in exposed crystal facet on the gas sensing performance of ZnO nanomaterial

The development of a gas sensor for ppb-level detection has been required for the industrial and medical fields. Exposed facet control is an effective way to improve gas sensing performance without novel metal catalysts. Herein, we investigated the change in gas sensitivity of ZnO nanostructures depending on the exposed surface. ZnO-rod and ZnO-whisker were synthesized using zinc nitrate hexahydrate, hexamethylenetetramine, and polyethyleneimine (PEI) via a low-temperature aqueous solution process. As increasing the amount of PEI, the ZnO-rod with (0001) facet was converted to the ZnO-whisker with (101) facet by edge sharpening. Although both samples can detect acetone gas until 200 ppb, the ZnO-whisker exhibited superior sensing performance in all ranges. Especially, the gas response of ZnO-whisker was three times higher than that of ZnO-rod at 5 ppm. The result was related to the difference in oxygen vacancy site where generate pre-adsorbed oxygen species. In the analysis results of electron density distribution, two Zn atoms are associated with one oxygen vacancy site in (101) facet. On the other hand, one oxygen vacancy site was connected to only one Zn atom in the (0001) plane. Thus, electrons could be much easily provided to the oxygen vacancy site in (101) facet, and it causes the higher sensitivity of ZnO-whisker.