The effects of sonification and TiO₂ deposition on the micro-characteristics of the thermally treated SiO₂/TiO₂ spherical core-shell particles for photo-catalysis of methyl orange

SiO₂/TiO₂ core-shell particles were prepared to decompose methyl orange (MO) in a laboratory scale study prior to its use in the treatment of waste containing azo dyes that might potentially pose adverse environmental consequences and health hazard to human. The preparation of silica cores produced perfectly spherical and smooth surfaces, but when TiO₂ shell/film was deposited using titanium butoxide (TBOT) precursor at concentration >0.24 mol L^-1, the resulting shell was rougher and loose, giving rise to Ostwald ripening that caused the core-shell particles to agglomerate. The use of sonification improved the dispersion at the expense of thin shell. When multi-step shell coating was applied to increase the shell thickness, loose titania shells were observed despite increasing surface coverage at higher number of coating step. The isoelectric point of the core-shell particles was equivalent to that of pure titania (4.5) due to the complete encapsulation of silica cores by titania. The silica core crystals exhibited highly amorphous structures, whereas titania covered silica cores demonstrated pure anatase phase that grew in size upon heat treatment at 900 °C. Rutile phase started to develop at 5 coatings, suggesting that anatase crystallite size was attainable for as long as the critical crystalline size smaller than ca. 30 nm was sustained or equivalently, 3 coatings were applied. Complete decomposition of MO was achieved in less than 370 min using core-shell particles prepared from triple coating.