Formation of double-cone-shaped ZnO mesocrystals by addition of ethylene glycol to ZnO dissolved choline chloride-urea deep eutectic solvents and observation of their manners of growth

ZnO mesocrystals have been explored for various physical and chemical applications. In spite of effort by a number of researchers, it is still difficult to grow specific crystal shapes in an environmentally-friendly, simple, and large-scale fabrication pathway. In this study, we report on a facile fabrication of ellipsoidal and double-cone-shaped ZnO mesocrystals elongated along the c-axis by an ionothermal method in choline chloride-urea deep eutectic solvents (CU-DESs) with addition of ethylene glycol (EG). It was found that as high as 16.7 mass% of ZnO could dissolve in CU-DESs at a high temperature by forming zinc cyanate complexes. Addition of EG to the above concentrated ZnO dissolved CU-DESs caused precipitation of ZnO particles which were confirmed as mesocrystals with transmission electron microscope images and selected area electron diffraction patterns. The crystal growth process and shape evolution in the CU-DESs were investigated and discussed. Photoluminescence study with excitation by ultraviolet light showed that the ZnO mesocrystals showed strong ultraviolet emission and relatively weak visible emissions, suggesting their high crystal quality.

Automated summary

In this study, ZnO mesocrystals were successfully synthesized in a novel solvent system. The experiments demonstrated their high crystal quality and photoluminescence performance, showing strong ultraviolet emission and weak visible emissions.