Effects of calcination on the microstructures and photocatalytic properties of nanosized titanium dioxide powders prepared by vapor hydrolysis

Ultrafine titanium dioxide powders are produced in an aerosol reactor using vapor hydrolysis of titanium tetraisopropoxide (TTIP) at 260 degrees C and higher temperatures (600 degrees, 700 degrees, 800 degrees, and 900 degrees C). The effect of calcination on the microstructure characteristics and the photoactivity is studied. The powders are characterized using Brunauer-Emmett-Teller (BET) surface area, X-ray diffraction (XRD), and transmission electron microscopy (TEM) analyses. The photocatalytic activity of the powders is also studied using degradation of phenol in water as a test reaction. The powder produced at 260 degrees C is calcined at 500 degrees to 900 degrees C while those produced at higher temperatures are calcined at 600 degrees C for 3 h, Raw powder produced at 260 degrees C is amorphous but becomes crystalline after calcination. As the calcination temperature increases, the surface area decreases but the rutile-to-anatase ratio and the anatase and rutile crystallite sizes increase. The photoactivity increases as calcination temperature increases to 900 degrees C, when the powder becomes densified and the surface area drops significantly because of sintering, Powders produced at higher temperatures are predominantly anatase and are generally more photoactive, Calcination of the powders at 600 degrees C for 3 h results in little loss of surface areas and enhances the photoactivity, Among the factors examined, large surface area and good dispersion of the powders in the reaction mixture are favorable to photoactivity, Conversely, prolonged calcination at high temperatures is detrimental to photoactivity, However, surface area, crystallite size, anatase-to-rutile ratio, and dispersity of the powders alone cannot account for the observed trend of photoactivity, The role of crystallinity needs to be investigated.