Tailoring of Size and Porosity of TiO 2 ‐Particles

The importance of well-defined particles has been recognized in a number of applications, for example, in ceramics, catalysis, pigments and, more recently, as building blocks of photonic crystals. Titania particles are particularly interesting with regard to their high refractive index. As titania particles tend to agglomerate they must be stabilized either electrostatically or sterically [1]. In order to investigate the influence of electrostatic and steric stabilization on the formation mechanism, the size and the size distribution of titania particles in more detail, we varied the type of the stabilizing salt or polymer in the reaction solution. Monodisperse spherical TiO2 particles with diameters from 200 to 2500 nm were prepared by controlled hydrolysis of titianium tetraethoxide in ethanol. Ethanol was mixed with 0.1 M aqueous salt or polymer solution, and titanium tetraethoxide was added under stirring at ambient temperature. Depending on the concentration, precipitation occurred after a few seconds or minutes and homogeneous TiO2 beads were formed. After two hours, the reaction was finished and the spheres were filtered and washed with ethanol [2]. Scanning electron micrographs reveal the perfectly uniform spherical shape of the TiO2 particles (Fig. 1). The size and the size distribution were affected in a rather sensitive way by different salt or polymers which were added to the reaction Fig. 1 SEM picture of the titania particles solution. With alkali synthesized by addition of Lutensol ON 50. halides we observe that the particle size decreases with increasing ionic strength in the reaction solution. Beads with diameters of about 3000 nm were obtained with lithium chloride, whereas the use of cesium chloride yielded 200-nm particles. The electrophoresis results show that an increased positive zetapotential leads to a reduction in particles size. This is correlated with the ionic strength in the reaction medium. Bogush et al. established the growth of the particles is best described by an aggregation mechanism, rather than by the LaMer model. The aggregation mechanism implies that the colloidal particles are formed by aggregation of small particles with a size of 5 20 nm (primary particles). The AFM picture of our titania colloids demonstrates that the surface of the final particles is rough: The height variation amounts to about 5 nm. This result indicates that the final particles consist ofprimary particles with a diameter of about 10 nm. The influence of polymers on the size and size distribution of the colloidal particles was investigated next. Two different types of polymers, diblock-copolymers Lutensol (RO(CH2CH2O)xH) and triblock-copolymers Pluronic (PEOn-PPOm-PEOn), were used for ste-