A novel exposed facet dependent topological transformation mechanism from hydrozincite microspheres into hierarchical mesoporous ZnO structures

Distinct from normal hierarchical mesoporous ZnO structures exposed with (2 −1 −1 0) facets derived from polycrystal hydrozincite precursors exposed with (0 1 0) facets, single-crystal hydrozincite precursors with exposed (0 0 1) facets were prepared by a hydrothermal method assisted with sodium citrate and then topologically transformed into hierarchical mesoporous ZnO microspheres with exposed (0 0 0 1) facets by calcination. To reveal the transformation process, a novel exposed facet dependent topological transformation mechanism was proposed as follows. Firstly, CO bonds and OH bonds in Zn5(CO3)2(OH)6 were broken with H2O and CO2 gases releasing; secondly, the remaining Zn atoms and O atoms rearranged their positions, leading to a transformation from a unit cell of monoclinic hydrozincite into eight unit cells of wurtzite ZnO based on their relationship of crystal lattice parameter. As a result, the (0 1 0) and (0 0 1) facets of hydrozincite can be transferred into the (2 −1 −1 0) and (0 0 0 1) facets of wurtzite ZnO, respectively. In this sense, the exposed facets of hydrozincite precursors determined the exposed facets of ZnO micro/nano structures. Therefore, the transformation mechanism can be also used to synthesize other metal oxide structures with special exposed facets.