Phase formation and transformation of nanostructured titania in solution

Synthesis and phase transformation of nanosized titania has been paid great attention for decades. Nanoparticls of anatase and rutile with various sizes and morphologies were prepared herein in various acidic aqueous solutions via sol-hydrothermal reactions. Anatase forms first and then transforms to rutile in a certain medium. The transformation can be accelerated in acidic solution. However, the strong chemical coordination of some anions, such as CH3COO- (OAc-) and SO42- to titanium suppresses the phase transformation. Thus, phase-pure anatase was formed in either H2SO4 or HOAc aqueous solutions, and phase-pure rutile was formed in HCl solution, especially in concentrated HCl solutions. The phase transformation could also be facilitated by the presence of some minelizer or initial rutile particles. Besides the crystallographic structure, the size and shape of titania could be controlled in solution. Monodisperse anatase nanocrystals were prepared in solvothermal system, which shows high photocatalysis in the decomposition of organic compounds. As microemulsion approach can confine crystal growth and lead to the yield of monodisperse nanoparticles, microemulsion-mediated hydrothermal synthesis to prepare nanoparticles of titania was proposed here. The initial shape of microemulsions of amorphous titania show crucial effect on the eventual shape of nanocrystals of titania. The reverse micelles can confine the growth of nanocrystallites in cores, while the micelles can serve as templates to direct the generation of nanoporous materials. Mesoporous titania with anatase framework was constructed through alkylamine as surfactant in aqueous solution. Upon removal of surfactants, the porous materials exhibit reversible adsorption-desorption isotherms, indicating uniform pore sizes, which would find wide application in photocatalysis. Under hydrothermal condition, the mesoporous titania can be transformed into finely dispersed nanoparticles of anatase and rutile, respectively.