Light-powered active colloids from monodisperse and highly tunable microspheres with a thin TiO₂ shell

The emerging field of active matter, and its subset active colloid, is in great need of good model systems consisting of moving entities that are uniform and highly tunable. In this article, we address this challenge by introducing core-shell SiO2-TiO2 microspheres, prepared by chemically coating a thin layer of TiO2 on an inert core, that are highly monodisperse in size (polydispersity 4.1%) and regular in shape (circularity 0.93). Compared with similar samples prepared by the classic sol-gel method, Janus TiO2-Pt active colloids prepared with core-shell TiO2 spheres move faster and boast a much clearer Janus interface. Moreover, a unique feature of these core-shell TiO2 microspheres is their great tunability in the colloid size, shell thickness, and even the type of the core particle. These advantages are highlighted in two examples, one demonstrating a TiO2-Pt active colloid with a magnetic core that enables magnetic manipulation, and the other demonstrating the collective expansion and contraction of a uniform cluster of core-shell TiO2 colloids under UV light illumination. We believe that TiO2 microspheres produced by this core-shell technique compare favorably with many other types of active colloids being employed as model systems, and thus open up many research possibilities.