Low-temperature Preparation Of Well-aligned Titania Nanorods And Their Photocatalytic Activities

Materials with various nanostructures have attracted increasing interest due to their special properties. As an important semiconducting oxide, titanium dioxide (TiO₂) has been widely studied as photocatalysts, solar cells, gas sensors, etc. T1O2 with distinct nanostructures, such as nanotube, nanorod, nanowire and mesoporous film possess unique properties and have been extensively investigated in different fields.In this thesis, TiO₂ films on Ti sheets and TiO₂ powders in the remaining solution have been obtained through directly reacting Ti with 30 wt% H₂O₂ at 80℃ for 72 h. Field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) and Raman spectroscopic analysis demonstrate that the obtained TiO₂ film possesses a dual structure: well-aligned nanorods on the top and a condensed inner layer. The inner layer with a thickness of ca. 2μrn is aggregates of anatase nanoparticles with sizes of 10-30 nm; whereas the top layer, ca. 1μrn in thickness, is aggregation of tiny nanorods which are 20-30 nm in diameter and 150 nm in length. These nanorods are mixture of anatase and rutile, as revealed by XRD, Raman and high-resolution transmission electron microscopy (HR-TEM) examinations. The dual-structured film was formed through mainly a dissolution precipitation mechanism. The BET specific surface area of the nanorods powders, measured through a N₂ absorption method, is approximately 300 m²/g, which is much higher than that of commercial Degussa P-25 TiO₂ nanoparticles.Photocatalytic activities (PA) of TiO₂ films prepared through various methods are estimated by photodegradation of 0.01 mmol/L Rhodamine B (RB) aqueous solution. The TiO₂ film with well-aligned nanorods top layer was found to possess higher PA than that of P-25 coating with identical weight and that of sol-gel derived titania film. The prepared titania film was able to photodegrade RB thoroughly to CO2 and H2O, as identified by gas-chromatography/mass-spectrometry (GC-MS). The excellent PA of the prepared TiO₂ film was ascribed mainly to the high specific surface area arising from the unique nanostructures. However, after used for several runs, the PA of this film decreased rapidly due to the loss of surface hydroxyl radicals beneficial to PA.Effects of a subsequent heat treatment at various temperatures on the PA of the TiO₂ films are also studied. Heat treatment decreased the PA of the TiO₂ films but favored its durability. The best PA was obtained when the TiO₂ film was kept at 450℃ for 1 h. The specific area of the film and the beneficial Ti-OH groups decreased after the subsequent heat treatment which in turn resulted in a decreased PA; however, the crystallinity of the titania improved and hence improved the PA.