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

Stress-induced ZnO· x H 2 O and TiO 2 · x H 2 O nanocrystals were spontaneously grown on ZnO and TiO 2 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 TiO 2 was in situ and ex situ studied. The formation of unstable near-amorphous belt-like orthorhombic ZnO·1.5H 2 O nanowires and partially crystalline column-like (pyramid top) monoclinic TiO 2 ·2.5H 2 O nanorods was initiated in a high relative humidity of 98%, whereas more stable polycrystalline faceted orthorhombic ZnO·H 2 O nanowires and bamboo leaf-shaped monoclinic TiO 2 ·0.4H 2 O nanoflakes were formed in 70% humidity. Upon receiving energy through in situ exposure to an electron beam or ex situ thermal annealing, the ZnO· x H 2 O and TiO 2 · x H 2 O transformed into hexagonal (wurtzite) ZnO and orthorhombic (brookite) TiO 2 , respectively. Exposure of non-polar TiO 2 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 [101&cmb.macr;0] orientation, via a self-assembly process that involved nanocrystallite agglomeration, subgrain tilting and boundary elimination. Stress-induced spontaneously ambient-grown orthorhombic ZnO· x H 2 O and monoclinic TiO 2 · x H 2 O nanostructures transform into single-crystalline wurtzite ZnO and polycrystalline brookite TiO 2 .