Ultra-rapid formation of ZnO hierarchical structures from dilution-induced supersaturated solutions

Abstract

A facile additive-free solution method has been developed to fabricate ZnO star-like assemblies, nanosheet-based microspheres and nanorod assemblies by rapid dilution of zinc-bearing alkaline solution with water at 60 °C. The shape of the nanoscale building blocks of the hierarchical structures depends on the supersaturations, which can be adjusted by varying the dilution ratios or the [OH⁻] to [Zn²⁺] ratios. The experimental results indicate that with the increase of supersaturation, the building blocks of the hierarchical structures evolve from nanorods to nanosheets and then to nanoparticles. The formation of ZnO nanorod assemblies involves the initial precipitation of the ε-Zn(OH)₂ intermediate followed by a relatively slow phase transformation process, while ZnO nanosheet-based microspheres and star-like assemblies are constructed within 30 seconds due to the rapid increase of supersaturation induced by dilution, involving the oriented attachment and the random aggregation mechanisms, respectively. Photoluminescence results reveal that the hierarchical structures are rich with singly and doubly ionized oxygen vacancies. The photocatalytic activities of the ZnO hierarchical structures were also evaluated by the degradation of rhodamine B under ultraviolet light. The photocatalytic performance depends on the content of oxygen vacancies as well as the specific surface area.