Preparation and Characterization of Titania-silica Composite Particles by Pechini Sol-gel Method

Two Pechini sol-gel processes were used to prepare titania-silica composite particles. The dynamic oxidation behavior of the TiO2-SiO2 powders has been characterized by thermogravimetry-differential scanning calorimetry (TG-DTG-DSC). The crystal phase and microstructure of the composite particles were investigated by X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM). The effects of Si:Ti molar ratio and sol-gel process on the TiO2-SiO2 powders were studied. The preparation of the polymeric precursors can influence the morphology of obtained TiO2-SiO2 composite particles. The spherical TiO2-SiO2 composite particles which are 20 nm~400 nm in diameter appear in gel-1 system. However, the TiO2-SiO2 powders obtained by gel-2 system are irregular in shape and 2~15 μm in diameter which show a loose porous structure consisted of very fine granules.


Introduction
Titania-silica composite particles exhibit enhanced ultraviolet photocatalytic activity when compared with titania particles.It is traditionally used for heterogeneous catalysis [1,2], electrochemical sensors [3], high-k dielectricmaterials [4], paints and so on.Titania-silica composite particles with various morphologies have been developed, such as porous particles, spheres and fibers [5,6].The microstructures, sizes and shapes have great influence on the properties of titania-silica composites, such as catalytic activity, thermal properties, photoactivity and chemical durability [7,8].The Pechini process [9][10][11] has the advantage of molecular level mixing of the components and low calcination temperature.And this method is usually applied to synthesize the nano or sub-micro powders with a more uniform size distribution.However, the influence of Pechini processing on the morphology of composite particles is unclear.
In the present work, Pechini sol-gel method was applied to prepare SiO 2 particles, TiO 2 particles and titania-silica composite particles.And a comparative study was carried out on the two Pechini sol-gel processes for the synthesis of titania-silica particles.We believe that the morphology of titania-silica composite particles can change with the preparation of polymeric intermediates.This study has general implications in the morphology controlled preparation of composite particles via Pechini sol-gel method.
Tetrabutyl titanate (TBOT, 98%, A.R.) solution was prepared by dissolving 0.02mol TBOT and 0.028mol citric acid in 18ml ammonia.22ml ethanol and 18ml distilled water were added to the TBOT solution, and 3.4g PEG-6000 was dissolved into the solution to obtain sol-2.The subsequent processes were as the same as sol-1, and then the TiO 2 powders were prepared.
Route I: Sol-1 was heated up to 85 for 3h to form intermediate-1.Sol-2 was then added into intermediate-1.The mixture was aged for 3h with stirring to finish the synthesis of gel-1.The gel was dried at 140 for 6h in an oven.Finally, the dried gel-1 was calcined in air at different temperatures ranging from 400 to 800 with a heating rate of 5 /min.The Si:Ti molar ratios of products A1, A2 and A3 were 5:1, 10:1 and 14:1 respectively.
Route II: Sol-2 was heated up to 85 for 3h to form intermediate-2.Sol-1 was then added into intermediate-2.The mixture was aged for 3h with stirring to finish the synthesis of gel-2.TiO 2 -SiO 2 powders were prepared as the same thermal process as route I. Differently the dried gel-2 was calcined in air at 600 .The Ti:Si molar ratio of products (B1) was 10:1.

2.2Characterization
XRD patterns of the calcined powders were carried out on an X-ray diffractometer (D/max-2200PC, Japan) using CuKĮ radiation ( =1.5406Å) in an effort to investigate the phase transformations.Thermal analysis was conducted with thermal analyzer (STA409PC, Germany) using a heating rate of 10 /min .FE-SEM (JSM-6700F, Japan) was utilized to examine the morphology of the obtained particles.

Results and Discussion
TG-DTG-DSC curves of gel-1 are shown in Fig. 1.The first exothermal peak centered at 368 in DSC curve can be related to the formation of anatase TiO 2 phase and the deformation of polyethylene glycol.The exothermal peak at 495 can be attributed to the combustion of polyethylene glycol derivatives and citric acid.And the exothermic peak at 760 is due to the transformation from anatase TiO 2 to rutile TiO 2 .
XRD patterns of TiO 2 -SiO 2 particles prepared by gel-1 are shown in Fig. 2. All patterns contain a halo at about 23°.This halo indicates that the synthesized composites contain an amorphous structure which is due to amorphous silica.The observed peaks in XRD pattern for the powders calcined at 400 and 600 are due to anatase TiO 2 phase (JCPDS 21-1272).At 800 , characteristic peaks of anatase TiO 2 and rutile TiO 2 (JCPDS 01-1292) appear.The anatase TiO 2 (101) peak is used to calculate crystallite size of the synthesized powders.It is found to be 15.1 and 11.4nm respectively for the powders calcined at 600 and 800 .The crystal size decreases with the increment of calcination temperature.This result may be due to the transformation from anatase TiO 2 to rutile TiO 2 .
As shown in FE-SEM images of SiO 2 powders and TiO 2 particles calcined at 600 (Fig. 3), the SiO 2 powders are well dispersed and maintain spherical shape with average particle diameter of about 20nm.The TiO 2 particles have irregular in shape.The blocks are about 5~20 μm in diameter.And the big blocks consist of nano-meter spherical particles with about 20nm in diameter which is close to crystal size.Fig. 4 shows FE-SEM images of TiO 2 -SiO 2 powders prepared by gel-1 calcined at 600 .When the Si:Ti molar ratio is 5:1, the obtained powders contain spherical particles and irregular particles.With the increment of Si:Ti molar ratio, the size of particles does not change, but TiO 2 -SiO 2 particles become nearly perfect spheres.The large particles of obtained TiO 2 -SiO 2 powders are about 200~400 nm in diameter.Some particles which are smaller in diameter (approximately 20~50 nm) are around the large particles.Both the particle sizes of large and small particles estimated from FE-SEM are larger than that calculated from the XRD data.The reason for the above result is that the particles of gel-1 system are agglomerated powders which contain amorphous SiO 2 and anatase TiO 2 .
Fig. 5 shows FE-SEM images of TiO 2 -SiO 2 powders (B1) prepared by gel-2 calcined at 600 .Compared B1 with A2 (see Fig. 4(b)), magnitude difference is found both in shape and particle size.Spheres can not be obtained by gel-2, and the TiO 2 -SiO 2 powders prepared by gel-2 are irregular in shape and 2~15 μm in diameter.As shown in Fig. 5(b), the smooth surface of B1 is solid plate, but the inside part shows a loose porous structure consisted of very fine granules (about 20 nm).
Compared Fig. 4 and Fig. 5 with Fig. 3, the morphology of TiO 2 -SiO 2 powders is similar with that of the TiO 2 or SiO 2 particles the sol of which is heated up first.However, owing to the addition of sol-2, the TiO 2 -SiO 2 powders prepared by gel-1 have smoother surface and bigger particle size.Although the low-magnification FE-SEM images indicate that the TiO 2 -SiO 2 powders prepared by gel-2 have similar morphology with that of TiO 2 powders, the high-magnification FE-SEM images of TiO 2 -SiO 2 powders present a loose porous structure which is caused by the addition of sol-1.In the Pechini sol-gel process, polymer resin forms via the chelations and polyesterifications.The viscosity of sol increases.So the sol which is heated up first acts as template and plays a decisive role in the morphology of TiO 2 -SiO 2 powders.In gel-1 system, because the SiO 2 template prevents the aggregation of TiO 2 particles, the micrometer-sized blocks can not be obtained.In gel-2

Conclusions
In this paper, the SiO 2 particles, TiO 2 particles and TiO 2 -SiO 2 composite particles could be synthesized by Pechini sol-gel processes.The obtained SiO 2 particles are spherical particles with average particle diameter of about 20nm.The obtained TiO 2 particles are big blocks consist of nano-meter spherical particles.The preparation of the gel can influence the morphology of the obtained TiO 2 -SiO 2 composite particles.In the gel-1 system, the gel starts to crystallize at 400 with the anatase TiO 2 phase.The rutile TiO 2 phase appears in the composite powders calcined at 800 .The TiO 2 -SiO 2 particles synthesized at 600 in gel-1 system are spherical particles which contain the larger spherical particles (about 200~400 nm) and smaller particles (approximately 20~50 nm).However, the spheres do not appear in the gel-2 system, and the prepared TiO 2 -SiO 2 powders are irregular in shape and 2~15 μm in diameter.Although the morphology of TiO 2 -SiO 2 powders is similar with that of the TiO 2 or SiO 2 particles, the surface or inner structures are different owing to the addition of another sol.

Acknowledgement
This work was supported by the National Natural Science Foundation of China (Grant No. 51302161, 51402179), the Education Foundation of Shaanxi University of Science and Technology (Grant No. BJ13-03).
TiO 2 particles exist as template and SiO 2 particles are well dispersed nano-spheres, the TiO 2 -SiO 2 powders have similar surface morphology with TiO 2 particles, but have loose porous structure in inside part.