Synthesis and photocatalytic properties of fibrous titania prepared from protonic layered tetratitanate precursor in supercritical alcohols

Fibrous titania samples consisting of monoclinic TiO2, anatase, and rutile were synthesized by heat treatment of H2Ti4O9. nH(2)O, which was prepared b y the ion-exchange reaction of potassium tetratitanate in HCl, in different supercritical media. The phase transformation temperature and microstructu re of the products changed significantly depending on the heating environme nt. The critical temperature at which anatase appeared in liquid media was much lower than that at which it appeared in air, indicating that the phase transformation of the monoclinic TiO2 to anatase proceeded by the dissolut ion-precipitation mechanism. Fibrous titania consisting of nanosize crystal s of TiO2 possessing high crystallinity could be obtained by treatment usin g supercritical ethanol or methanol as the reaction medium. The photocataly tic activities for hydrogen evolution from an aqueous methanol solution and for phenol photodegradation were determined for various titania samples. A morphous TiO2, which possessed large amounts of crystal defects, showed no photocatalytic activity for either reaction. The titania that crystallized in supercritical ethanol or methanol consisted of a mixture of monoclinic T iO2 and anatase and showed excellent hydrogen evolution activity, and the h ydrogen evolution activity of fibrous titania changed with heat treatment m edia in the following sequence: ethanol > methanol > water > 8-propanol > n -butanol > n,-hexane > P-25 > air. On the other hand, the phenol degradatio n activity of titania decreased with decreasing specific surface area as fo llows: P-25 > water > ethanol > methanol > n-butanol > air.