Fabrication Of High Photoreacative TiO₂

With the rapid development of economy,environmental pollution is becoming serious.As one of a typical photocatalyst,much attention was paid to anatase TiO₂due to its high photoreactivity.However,high photoreactive TiO₂ has the disadvantages of poor thermal stability and low water dispersity.It can easily transform into poor photoreactive rutile phase after calcination at temperature of higher than 600 ^oC.This hampers the practical application of anatase TiO₂photocatalyst in certain circumstances.This thesis aims to improve the thermal stability and water dispersity of anatase TiO₂,which are composed of the following two parts:Part Ⅰ:Fabrication of high thermally stable TiO₂ hollow microspheres（TiO₂-HMSs）.TiO₂-HMSs have attracted much attention due to the high photoreactivity,low density and good permeability.In this part,we tried to improve the thermal stability of anatase TiO₂-HMSs by surface fluorination,which can prevent the growth of the TiO₂ nanocrystals,increase the anatase-to-rutile phase transformation temperature.TiO₂-HMSs assembly from hollow nanoparticles was fabricated by hydrothermal reaction of the mixed solution of Ti（SO₄）₂-NH₄F-H₂O₂and followed by calcination.The photoreactivity of TiO₂-HMSs was evaluated by photocatalytic oxidation of acetone.It was found that（1）after calcination at 300°C,the photoreactivity of TiO₂-HMSs discreased from 1.39×10^-3（precursor）to 0.82×10^-3min^-1（T300 sample）due to the removal of surface adsorbed fluoride ions;（2）with increase in the temperature from 300 to 900°C,the photoreactivity of TiO₂-HMSs increased from 0.82×10^-3 to 2.09×10^-3 min^-1 because of the enhanced crystallization;and（3）Anatase-to-rutile phase transformation begins at 1100°C,which shows poor photoreactivity due to the formation of rutile phase.With the increase of temperature,the adsorption of fluoride ions gradually removed,which led to the formation of oxygen vacancies on the surface,preventing the formation of Ti-O-Ti chemical bonds between two adjacent titanium dioxide nanoparticles.Thus,the growth of TiO₂crystallization was inhibited and the thermal stability of anatase TiO₂ phase was therefore improved.Part Ⅱ:Water-dispersed TiO₂ nanocrystals with average crystalline sizes of less than 5 nm were prepared by sol-gel method.The suspensions almost keep unchanged even stored for 2 months,reflecting its excellent water dispersity and stability.The photoreactivity of the photocatalyst was evaluated by oxidation of acetone.The apparent reaction constant for photocatalytic oxidation of acetone was measured to be5.04×10^-3 min^-1 in the presence of prepared TiO₂ nanocrystals,which is 2.5 and 2.0times higher than that of commercially water-dispersed TiO₂ samples of CT1 and CT2,repectively.This reflects that our prepared water-dispersed TiO₂ nanocrystals are promising for commercial use.