Preparation, Characterization And Comparison Study On Different TiO₂-SiO₂ Nanocomposites

Titania-silica nanocomposite(Ti-Si oxides)recently has been attracted the attention of many application fields such as catalysis,photocatalysis,solar cell for it possesses a superior photocatalytic activity,a photo-induced hydrophilicity and a photoelectric conversion property.However,the active structures or accurate active sites producing catalytic and photocatalytic activities are not completely understood.Therefore,it is necessary to investigate the structure of Ti-Si oxides and its formation mechanism,comprehensively and detailedly,and these investigations will be significant for further controlling the structure and properties of Ti-Si oxides,better comprehending its photocatalytic and catalytic mechanism,expanding the application fields of Ti-Si oxides.In the present paper,for the first time,three different sol-gel processes have been designed to prepare Ti-Si oxides with three structures(silica-supported titania,intimately mixed silica-titania and silica-coated titania,named by composite structure of sol-gel),each in the form of nanocomposite powders containing 10-40 mol%SiO₂ and annealed at 773-1273K,and then SEM, HRTEM,EDS,XRD and XPS have been employed to systematically and comparatively study the similarities and dissimilarities in microstructure and surface structure of these Ti-Si oxides under different SiO₂ content and annealing temperature.It is found in the experimental range that the thermal diffusion and redistribution of Ti/Si are main factors inducing the structure changes of Ti-Si oxides,and these structure changes can be found from the following aspects:(1)With increasing annealing temperature,Si enrichs on the surface of Ti-Si oxides and Ti accumulats in the deep sites of Ti-Si oxides,tending to form an new shell(O/Si rich area)-core(Ti rich area)structure for all Ti-Si oxides derived by the three sol-gel processes.This induces the surface structure(down to 4-5nm)of Ti-Si oxides powder is a double layer with O-rich first and SiO_x /TiO_y(x,y＜2)second.The excess O in first layer is probably contributed by chem-adsorbed H₂O.The Ti/Si-suboxide in second layer could be induced by the thermal diffusions of Ti⁴⁺and Si⁴⁺,which perhaps relate to the strong interaction between Ti⁴⁺and Si⁴⁺.(2)Both grain growth and anatase-rutile transformation(ART)of ncTiO₂ are proved to be significantly inhibited by SiO₂ addtion for all Ti-Si oxides,and the inhibiting ability depands not only on the SiO₂ content,annealing temperature,but also on the composite structure.Among the three kinds of composite structure,intimately mixed Ti-Si oxide shows the best inhibiting ability.(3)A significant Si-atoms and O-vacancies could exist in the ncTiO₂ anatase in Ti-Si oxides,and both substitutional and interstitial Si cation centers could occur simultaneously in the lattice of ncTiO₂ anatase and contribute to the large lattice strains therein.At same annealing temperature,an increase in SiO₂ content could induce large lattice strain,lattice distortion and more lattice defects in ncTiO₂ anatase;with same SiO₂ content,a rise in annealing temperature could increase the tetroganality(c₀/a₀)of ncTiO₂ anatase.The unit-cell volume of ncTiO₂ anatase in Ti-Si oxides is generally smaller than that in pure TiO₂,and the variation tends of unit-cell volume of ncTiO₂ anatase with annealing temperature is different with different composite structure.(4)The mechanism for SiO₂ inhibiting the grain growth of ncTiO₂ anatase: the grain size,size distribution and dispersion of ncTiO₂ anatase in Ti-Si oxides possibly depend on the variation of the large lattice strain therein.The restrained grain growth of TiO₂ could result from the shrinkage of unit-cell volume of ncTiO₂ anatase and the formation of Ti-O-Si linkages,which might be induced by the substitution of Si for Ti in crystal lattice of TiO₂.Small variation in lattice strain in nc TiO₂ anatase with increasing annealing temperature could lead to small grain size.(5)The possible mechanism for ART of ncTiO₂:with increasing annealing temperature,maybe there is a definite value(0.1360nm³)for unit-cell volume of ncTiO₂ anatase in Ti-Si oxides that induces its ART.