Abstract
In this article, we demonstrate a new, facile and green antisolvent process to prepare nanostructure ZnO. The deep eutectic solvent (DES) is a new class green solvent that is negligible vapor pressure, high polar and biocompatible. UCC is one of DES that shows high solubility of many metal oxides that can be used as solvent in this approach. The antisolvent should be miscible with the DES UCC and non-solvent for the solute. By controlling the nucleation and growth, various morphologies of ZnO can be made by this method. At low ZnO concentration in UCC, ZnO twin-cone and rods can be prepared with morphology and size controls by injection rate and the ethanol content in antisolvent. The ethanol in the antisolvent can reduce the growth rate of ZnO and also lower the product dimensions. If the injection rate of ZnO-containing DES was reduced, the supply of Zn source became limited and the crystal growth became 1D dominated and grows on the preferred direction. It turns the product structure from twin-cones to nanorods. Mesocrystal ZnO is a mesoporous material fabricated from nanocrystals (NCs) with ordered orientation superstructure. Thus the material possesses high surface area and good crystallinity as single crystal. We use a biological buffer Tris as an oriented agent in antisolvent. ZnO NCs were made in antisolvent then Tris can increase the concentration of hydroxyl group in antisolvent. The hydroxyl group can attach on the oxygen vacancies on the polar O-terminated surface then increased the polarity of the ZnO NCs and then be attached together with the same orientation to form a mesocrystal ZnO. The increasing amount of Tris can make a larger size, better crystallinity and smaller surface area mesocrystal ZnO. The crystallinity is the more important factor rather than surface area. Thus a mesocrystal ZnO prepared in high Tris concentration shows better photocatalytic reactivity. At high ZnO dissolved concentration and low injection rate, the ZnO-ZCH nanosheet can be made by this antisolvent approach. After the annealing, single crystal mesoporous ZnO nanosheet can be parpared. The sheet is thin with a thickness of 10 nm and many pores with size of 5-60 nm can be found on the nanosheets. The ZnO nanosheet is single crystal and high surface area (~90 m2/g) with the exposed plane of (11-20). It can be used as photocatalyst for degradation of methylene blue (MB) in aqueous solution. The single crystal mesocrystal ZnO nanosheet shows performance as good as TiO2 P25.