Separation of photo-active nano-crystalline anatase from titanate nanotubes

Photo-active colloidal anatase was prepared from sodium titanate nanotubes by refluxing in 0.3 M HCl. The refluxing was carried out in cycles, replacing the acid each time. After the second reflux cycle a suspension of colloidal anatase was formed above the residual solids. After three reflux cycles all of the nanotubes were converted to a residual mixture of anatase, rutile and brookite. More colloidal anatase could be isolated from this mixture through a series of water rinses. The anatase suspension was found to be made up of particles with an average diameter of 40 nm as well as a very fine 10-15 nm diameter material. This latter dimension is in line with the crystallite size determined from the anatase isolated from the suspension. At pH 1 the anatase suspension was found to be significantly more photo-active in bleaching methyl orange than P25 at exposure times up to 1.5 h. The photo-activity after 1.5 h was found to be 29% higher than the best catalyst prepared by calcining the same titanate nanotube starting material. The increased activities can probably be attributed to the increased surface area, decreased crystallite size and decreased sodium content of the anatase suspension.