(0001)-Oriented Single-Crystal-Like Porous ZnO on ITO Substrates via Quasi-Topotactic Transformation from (001)-Oriented Zinc Hydroxychloride Crystals

Novel ZnO nanostructures composed of ⟨0001⟩-oriented porous ZnO were prepared on indium tin oxide (ITO) substrates from ⟨001⟩-oriented zinc hydroxychloride (ZHC) crystals. The ZHC crystals were obtained by simple electrochemical deposition on layer-by-layer (LbL)-coated ITO in aqueous solution and transformed into porous ZnO via calcination in air. X-ray diffraction and scanning electron microscopy were used to characterize the ZHC crystals and porous ZnO. The well-faceted ZHC crystals obtained on the LbL-coated ITO had hexagonal-plate shapes with diameters of 3–7 μm and exhibited strongly ⟨001⟩-oriented simonkolleite crystal structures. The calcination of ZHC crystals at 450–550 °C for 1 h in air led to the formation of ⟨0001⟩-oriented porous ZnO comprising nanocrystals with diameters of ∼50 nm while maintaining the original hexagonal-plate shapes. Electron backscatter diffraction analyses and epitaxial growth of ZnO nanorod array revealed that the porous ZnO hexagonal plates exhibited features of single crystals. Finally, a plausible mechanism for the topotactic-like transformation of ZHC crystals to porous ZnO is discussed on the basis of similarities in their Zn–O frameworks.