Morphology-controllable ZnO rings: Ionic liquid-assisted hydrothermal synthesis, growth mechanism and photoluminescence properties

Abstract

Morphology-controllable ZnO rings with high crystallinity were synthesized by a simple hydrothermal approach, using ionic liquid (through surface energy conditioning) as a stabilizing agent or template. The growth mechanism of these ring-like ZnO crystals was explored based on first-principles calculations and a series of controlled experiments where the concentration of the ionic liquid 1-propyl-3-methylimidazolium bromide ([C₃mim]Br) was tuned. With the increase of the concentration of [C₃mim]Br, the aspect ratio of ZnO product increases, and the morphology changes from ring to tube. FTIR and theoretical calculations indicate that [C₃mim]Br prefers to adsorb on the ZnO (100) facets, which lowers the surface energy so as to protect the lateral facets from growing too fast to vanish and result in the formation of disk-like structures. Furthermore, the relatively high density of the defects and dislocations on the exposed (000) facet at the center of ZnO disks results in a higher etching rate of (000) facet by H⁺, driven by the reduction of strain energy associated with dislocations. Therefore, the selective etching of ZnO disks leads to formation of ring-like structure for final products. With the increase of the concentration of [C₃mim]Br, the selective adsorption of [C₃mim]Br will also promote the growth along the [0001] direction to form a tube-like structure. In addition, the photoluminescence spectra reveal that the as-prepared ZnO rings exhibit blue emission related to their oxygen vacancy as well as various other barely avoidable impurities and defects.