Fast Hydrolysis in the Microdroplets of an Aqueous Solution of CuSO4 on the Surface of an Alkali Solution and the Formation of Ordered Arrays of Open Microspheres with Cu(OH)2 Nanocrystal Walls

Abstract It was shown for the first time that 1–10 µm microspheres with walls made of Cu(OH)2 nanocrystals and unique morphology can be formed on the surface of an alkali solution at room temperature without the use of surfactants. The microspheres are formed via fast hydrolysis of copper(II) cations when microdroplets of an aqueous solution of CuSO4 are sprayed onto an alkaline solution of Na2SO4. It was found that the microspheres formed in this way have a single hole in their walls with a size of fractions of a micrometer or a few micrometers and they are arranged on the surface of the alkaline solution with this hole facing the air. The microspheres can be transferred to various substrates using a vertical lift technique and are deposited as layers in which the holes are predominantly oriented away from the substrate. The walls of these microspheres are several hundred nanometers thick and are composed of sets of Cu(OH)2 nanocrystals shaped as nanorods with a diameter of 5–10 nm and a length up to 500 nm. When heated in air at 150°C, these nanocrystals lose water to give CuO single crystals without significant changes in their morphology. It was found that deposition of these microspheres as layers on various substrates such as silicon or titanium endows the surface with superhydrophilic properties.