Phase and structure development of spontaneously ambient-grown ZnO·xH2O and TiO2·xH2O nanostructures towards oxide single crystals

Abstract

Stress-induced ZnO·xH₂O and TiO₂·xH₂O nanocrystals were spontaneously grown on ZnO and TiO₂ films in an ambient atmosphere based on a bond breaking–hydrolysis–reconstruction mechanism, without the use of any other precursors. The development of their phase and structure towards ZnO and TiO₂ was in situ and ex situ studied. The formation of unstable near-amorphous belt-like orthorhombic ZnO·1.5H₂O nanowires and partially crystalline column-like (pyramid top) monoclinic TiO₂·2.5H₂O nanorods was initiated in a high relative humidity of 98%, whereas more stable polycrystalline faceted orthorhombic ZnO·H₂O nanowires and bamboo leaf-shaped monoclinic TiO₂·0.4H₂O nanoflakes were formed in 70% humidity. Upon receiving energy through in situ exposure to an electron beam or ex situ thermal annealing, the ZnO·xH₂O and TiO₂·xH₂O transformed into hexagonal (wurtzite) ZnO and orthorhombic (brookite) TiO₂, respectively. Exposure of non-polar TiO₂ to an electron beam or annealing generated a polycrystalline structure. Exposure of polar ZnO to a low-energy electron beam caused the formation of aligned subgrains, while high-energy annealing yielded a single-crystalline structure, both with a longitudinal [100] orientation, via a self-assembly process that involved nanocrystallite agglomeration, subgrain tilting and boundary elimination.