Facile Preparation\nof Raspberry-Like SiO<sub>2</sub>@Polyurea Microspheres with Tunable\nWettability and Their Application\nfor Oil–Water Separation

Raspberry-like microspheres have been widely used as\nsuperhydrophobic\nmaterials, photonic crystals, drug carriers, etc. Nevertheless, their\npreparation methods, usually consisting of multiple steps, are generally\ntime- and energy-consuming. Herein raspberry-like SiO<sub>2</sub>@polyurea\nmicrospheres (SiO<sub>2</sub>@PUM) are readily prepared via a one-step\nprecipitation polymerization of isophorone diisocyanate in a H<sub>2</sub>O/acetone mixture with the presence of SiO<sub>2</sub> particles.\nThe sphere size, surface roughness, and SiO<sub>2</sub> content of\nSiO<sub>2</sub>@PUM are easily adjustable by varying the experimental\nconditions. TEM and SEM observations reveal that the final SiO<sub>2</sub>@PUM exhibits a core–shell structure, with polyurea\n(PU) in the core and SiO<sub>2</sub> particles as the shell. In the\nprocess, the SiO<sub>2</sub> particles were initially located on the\nPUM surface as a monolayer. With the reaction proceeding, the monolayer\nof SiO<sub>2</sub> particles became thicker, forming a thicker layer\nof SiO<sub>2</sub> particles on PUM due to the accumulation of SiO<sub>2</sub> particles, leading to a multilayer structure of SiO<sub>2</sub> particles on the shell of SiO<sub>2</sub>@PUM. The formation mechanism\nof the raspberry-like SiO<sub>2</sub>@PUM was thoroughly discussed\nand ascribed to electrostatic attraction between the positively charged\nPU and negatively charged SiO<sub>2</sub> particles. Once dried, SiO<sub>2</sub>@PUM was superhydrophobic and turned hydrophilic if water-wetted.\nUsing a layer of SiO<sub>2</sub>@PUM, effective separation with good\nreusability for a variety of oil–water mixtures was achieved\nregardless of the oil density and types of oil–water emulsions.\nThis work presents a novel protocol for the preparation of raspberry-like\nmicrospheres with tunable wettability via a rapid and green process,\nand the resulting microspheres are highly effective for the separation\nof diverse types of oil–water mixtures.