Structural investigation of anatase prepared at different calcination temperature of sol-gel process

TiO2, also know as titania, is a naturally occurring mineral compound, with melting temperature of approx. 1830°C. In nature it occurs in numerous polymorphic forms, mainly in the form of minerals such as: tetragonal rutile and anatase, and rhomboidal brookite, ilmenite, leukoxene (ilmenite weathering product), perovskite and sphene, also found in numerous iron ores. Rutile is the most stable form of titanium dioxide, generally present in igneous and metamorphic rocks. Anatase is a metastable form, created in lower temperatures than rutile.. Anatase is marked with higher specific surface area, porosity and a higher number of surface hydroxyl groups as compared to rutile. Under high temperatures reaching 800–900ºC (depending upon preparation method) and pressure, anatase is irreversibly transformed into rutile [1, 2].
In our work we present results of Raman studies of TiO2 coatings obtained in sol-gel method. In this method, an organometallic compound of a Ti is used as precursor. In the case of TiO2, the tetraisopropoxide [3-5] or titanium tetra-butoxide [6-8] are the most common. The first stage of this method is the hydrolysis of the organometallic precursor. Next, to the received hydroxide an appropriate amount of acid is added to obtain a colloidal solution (sol). The next step is the formation of a gel as a result of evaporation of the sol. Finally, in calcination process of the gel the final TiO2 product is achieved. In this work effect of calcination temperature on the structure of obtained coatings will be presented. Raman spectroscopy is a useful method of identification, allowing distinguishing the anatase phase from the rutile phase. In the rutile spectrum there are three main bands of positions: 605, 439, and 226 cm-1, and weaker bands of positions 570, 404 cm-1 [9]. In the anatase spectrum there are four main bands of clearly distinct positions 636, 515, 394, and very intense 143 cm-1, related to Eg vibrational mode of anatase primitive cell [10, 11]. We clearly observe a shift of Eg position as function of calcination temperature.