Synthesis of submicron-sized titania-coated silica particles with a sol-gel method and their application to colloidal photonic crystals

A previously proposed method for preparing monodispersed titania particles was extended to preparation of titania-coated silica spherical particles. The core silica particles with an average size of 264 nm were prepared with Stober method. The titania-coating was performed in ethanol/acetonitrile solvent in the presence of silica particles by a sol-gel method with the use of titanium tetraisopropoxide (TTIP) and methylamine (MA) catalyst. Average size of the silica-titania particles decreased from 457 to 292 nm with an increase in concentration of silica particles. Coefficient of variation for the particle size was less than 5%. Colloidal crystals could be fabricated with a dip-coating technique and a sedimentation process, respectively. Measurements of reflectance revealed peaks based on the Bragg diffraction. Those peaks red-shifted with an increase in titania shell thickness because of a high refractive index of titania. Annealing at high temperature transformed crystal structure of titania shell from amorphous to anatase (500 degrees C) and rutile (1000 degrees C), which led to red-shift of reflection peak because of an increase in refractive index of titania due to the crystallization.