Self-assembly of [10(1)over-bar0] grown ZnO nanowhiskers with exposed reactive (0001) facets on hollow spheres and their enhanced gas sensitivity

In this work, we demonstrate a simple gas-phase process to synthesize ZnO with exposed reactive {0001} facets without the use of any capping agent. The method is based on controlling the ZnO growth direction, causing it to switch from the typical [0001] to [10 (1) over bar0], resulting in [10 (1) over bar0] grown nanowhiskers with exposed reactive {0001} facets. Moreover, these nanowhiskers self-assemble on ZnO hollow spheres to form a three-dimensional hierarchical nanostructure, which stabilizes these nanowhiskers against agglomeration. Such a designed ZnO nanostructure endows the ZnO sensing material with both small crystal sizes and exposed reactive {0001} facets, which leads to greatly enhanced gas sensitivities compared to typical [0001] grown ZnO nanowires. Such reactive-facet-enhanced gas sensitivity is also shown by our calculation of the chemisorption energy of oxygen, based on density functional theory.