Of Nano Titanium Dioxide Crystal Modulation And Its Photocatalytic Properties

With the rapid develpment of industry,environmental problems are drawing more and more attentions.Extensive research such as analytical,biochemical,and physicochemical methods have been developed to address the pollution issue.Among varieties of techniques,the use of wide-band-gap semiconductor titania(TiO₂) as photocatalyst has proven attractive since this material is of high efficiency,low cost and environmentally sustainable.By TiO₂-based heterogeneous photocatalysis,total or partial decomposition of phenol,nitrobenzene,4-chlorophenol etc.has already been achieved in laboratory.Factors such as particle size,surface area,morphology and porosity etc.have been proved to affect the sample's absorptive ability as well as the recombination pathways of excited electron-hole pairs.One of those factors which arouse our interest is the so-called 'mixed-phase effect'.It is found that if anatase and rutile appear simultaneously in a certain sample,they will have a synergetic effect and be more photoactive than each single phase.Here,we will make studies on this unique phenomenon,and try to prepare TiO₂ crystallites under certain mild conditions.Nanocrystalline aerogel TiO₂ photocatalysts prepared by a facile epoxide sol-gel synthesis followed by subsequent supercritical fluid drying and thermal processing were studied in regard to their performance in the photocatalytic degradation of phenol.Xerogel TiO₂ catalysts obtained by direct drying were also examined.A variety of techniques including N₂ adsorption,X-ray powder diffraction(XRD), Diffuse reflectance infrared Fourier transform spectroscopy(DRIFTS),Raman spectroscopy,Transmission electron micrographs(TEM),and Diffuse reflection(DR) UV-vis spectra are employed to characterize the microstructural and surface properties of the as-obtained aerogel and xerogel TiO₂ samples.It is shown that the aerogel samples show particular advantage in controlling the microstructural properties in terms of crystallite size,surface area,crystallinity as well as anatase and rutile phase composition.The aerogel sample obtained by calcinations even exhibits a better photocatalytic activity than the commercial photocatalyst Degussa P-25. Further studies revealed a close relationship between the sample's unique phase transformation and its high photoactivity.Using TiCl₄ and Ti(OBu)₄ as the precursors and Pluronic P123 as the template, mesoporous titanium dioxide(TiO₂) was successfully prepared by the nonhydrolytic evaporation-induced self-assembly(EISA) method with a three-dimensional mesostructure obtained.XRD patterns show that the pore wall is composed of both anatase and rutile phases.TEM images reveal a well-ordered open-pore mesostructure. Further investigations on the effect of the "acid-base pair" find that by changing the molar ratio of TiCl₄/Ti(OBu)₄,both the pore texture and phase composition can be well-controlled.Finally,by using titanium sulfate(Ti(SO₄)₂) and methenamine(C₆H₁₂N₄) as reagents,we are able to obtain TiO₂ with pure anatase phase at room temperature. Photocatalytic test shows that such samples are good photocatalysts.Thermal analyses confirm that anatase does not transform to rutile phase even after calcination of 750℃,revealing the sample's high thermal stability.