We describe a novel method to synthesize colloidal particles with an eccentric core-shell structure. Titania-silica core-shell particles were synthesized by silica coating of porous titania particles under Sto¨ber (Sto¨ber et al. J. Colloid Interface Sci. 1968, 26, 62) conditions. We can control access of silica to the pores in the titania, allowing us to produce either core-shell or composite particles. Calcination of the core-shell particles gives unique eccentric core-shell structures, as a result of extensive shrinkage of the highly porous titania core with respect to the silica shell. However, when the titania particles are silica treated prior to drying they result in composite titania-silica spheres, where two materials are mixed uniformly. These spheres are interesting for catalysis, (switchable) photonic crystal applications, optical tweezing, and new titiana based materials. We demonstrate photocatalytic activity of the eccentric spheres where the silica layer acts as a size selective membrane
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