Hierarchical Three-Dimensional ZnO and Their Shape-Preserving Transformation into Hollow ZnAl₂O₄ Nanostructures

We present novel three-dimensional (3D) hierarchical ZnO and ZnAl 2 O 4 nanostructures produced by multitransformation reactions using Zn as the starting material. In two representative instances selected for illustrating this strategy, various Zn precursors like Zn nanowires and polyhedral Zn microcrystals were first prepared by physical vapor deposition without any catalyst. Subsequently, we successfully synthesized branched ZnO nanowires and hollow ZnO polyhedrons with vertically aligned ZnO nanowires by oxidizing Zn nanowires and polyhedral Zn microcrystals, respectively, under optimal conditions. The growth of both ZnO nanobranches and vertically aligned nanowires was suggested as a self-supplying and self-catalytic process following the vapor transport solid condensation mechanism. Furthermore, the obtained ZnO nanostructures were homogeneously covered by a shell of amorphous Al 2 O 3 using atomic layer deposition. Through the solid–solid interfacial reaction of ZnO and Al 2 O 3, the 3D nanostructures were transformed to hierarchical ZnAl 2 O 4 nanostructures conserving the shape of the corresponding ZnO nanostructures. Particularly, all the ZnAl 2 O 4 nanobranches or vertically aligned 1D structures exhibited a tubular structure, which was induced by the nanoscale Kirkendall effect. This work demonstrates a simple and efficient pathway to design hierarchical ZnO and complex hollow zinc spinel nanostructures, which are expected to have potential applications in nanotechnology.