Zinc oxide (ZnO) is a semiconductor with a wide band gap of 3.37 eV and is therefore often used in, for example, gas sensors, solar cells and photocatalysts. The size, as well as the morphology of the ZnO nanoparticles, determines both the physical and chemical properties for the application. Researchers have studied the low-dimensional nanoscale building blocks, such as 1D nanorods, 1D nanowires and 2D nanosheets. Recently it was found that mixing these materials with 3D hierarchical ZnO microstructures revealed special optical, electrical and catalytic properties.
In order to create 3D hierarchical ZnO structures, various techniques have been used, including chemical vapour deposition, sol-gel methods, electrochemical depositions and microwave-assisted techniques. The more conventional method of hydrothermal synthesis has often been criticised for its long reaction time (~20 h) and high temperatures (~200 °C).
Ping Li and co-workers from the Hebei Normal University in China have demonstrated in their publication in physica status solidi a (pssa) a simple one-step method to produce ZnO crystals at lower temperatures (85 °C) and shorter reaction times (2.5 h). The resulting hierarchical porous ZnO microstructures showed excellent absorption and degradation performance towards methylene blue, as they possess a large surface area and good optical quality.
The tuning of the morphology of the ZnO particles was performed by the researchers by regulating the citric acid concentration. By increasing the concentration of the anionic surfactant the morphology of the ZnO nanoparticles changed from hexagonal prisms to solid spheres via a porous flower structure, porous microspheres and a solid flower structure. The mechanisms behind the various structures are explained and depicted in detail by the authors in their publication.
The photoluminescence spectra of the different morphologies of the ZnO nanoparticles revealed that the ZnO catalyst with porous flower and porous microsphere structures exhibit the most efficient degradation efficiency for methylene blue under UV radiation. This can be explained by the larger surface area of the porous structures, which enables the enhanced absorption of methylene blue. Secondly, the oxygen vacancies present in the crystal structure can effectively trap electrons and hinder the recombination between electrons and holes, which improves photocatalytic activity.
This work contributes to the further understanding of the photocatalyic properties of metal oxide semiconductor materials and for the effective application of such materials in treatment of wastewater containing organic dyes.