STRUCTURE OF MULTICOMPONENT (TITANIA ZIRCONIA) COLLOIDS/

The structure of titania/zirconia colloids has been investigated using small-angle neutron scattering (SANS). The colloids were produced by: (i) hydrolysis of a mixture of titanium and zirconium alkoxides and p eptization of the resulting hydrolyzate with nitric acid (homogeneous) and (ii) hydrolysis of a titanium alkoxide and peptization of the res ulting hydrolyzate with aqueous zirconium(IV) nitrate solution (hetero geneous). The final titania/zirconia and metal oxide/nitrate mole rati os were 16.0 and 10.0, respectively. The sols were concentrated by eva poration, dried under controlled conditions and redispersed in D2O/H2O mixtures. FT-Raman spectra of the sols, and XRD powder patterns from the gels, showed that the material was crystalline anatase and amorpho us zirconia. Both TEM. and XRD line broadening indicate that the cryst allite size of the dried titania gels is similar to 8 nm. The results of SANS contrast variation experiments are described. The minimum-cont rast points for the homogeneous and heterogeneous colloids, determined by two different methods, gave similar results which differed signifi cantly from the expected value, due to the sorption of nitrate counter ions and hydroxyl species on the surface of the colloids. In both syst ems, the scattering at minimum contrast was consistent with a network of unidimensional zirconia, with a typical diameter of similar to 1.5 nm. At full contrast, the homogeneous colloids have fractal dimensions (d(f) of 1.6) similar to those from static light scattering,that is,c onsistent with an open, extended, chainlike aggregate structure. Howev er, the heterogeneous colloids have higher fractal dimensions (d(f) of 2.2-2.4), due to zirconia crystallites packing the interstices betwee n the titania crystallites, that is, consistent with a loosely packed, spherical structure. The apparent fractal dimension of the heterogene ous colloids in H2O (d(f) of 2.4) is higher than that observed in D2O (d(f) of 2.2), whereas no such effect is observed for the homogeneous colloids. These results infer that, in the homogeneous colloids, the z irconia is segregated within the matrix of the titania crystallites (o n the similar to 1 nm scale), whereas, in the heterogeneous colloids, the zirconia is segregated on the surface of the titania crystallites (on the similar to 10 nm scale).