Counter-ion adsorption and homogeneity of TiO2/ZrO2 nanoparticles from SANS contrast variation

The local structure of titania/zirconia colloids has been investigated using small-angle neutron scattering (SANS). The colloids were produced by: hydrolysing a mixture of titanium and zirconium alkoxides, and peptising the resulting hydrolysate with nitric acid (homogeneous); hydrolysing a titanium alkoxide, and peptising the resulting hydrolysate with aqueous zirconium (IV) nitrate solution (heterogeneous). The final titania/zirconia and metal oxide/nitrate mole ratios were 16.0 and 10.0, respectively. The results of SANS contrast variation experiments are described. The minimum-contrast points for the homogeneous and heterogeneous colloids, determined using either the known analytic form of the scattering at q = 0 or the scattering invariant, gave similar results, which differed significantly from the expected value. These differences were attributed to the sorption of nitrate counter-ions and hydroxyl species on the surface of the colloids. In both cases, the scattering at minimum contrast was consistent with a fractal network of uni-dimensional zirconia, with a typical diameter of similar to 1.5 nm. The results infer that in the homogeneous colloids, the zirconia is segregated within the matrix of the titania crystallites (on similar to 1 nm scale), whereas in the heterogeneous colloids, the zirconia is segregated on the surface of the titania crystallites (on similar to 10 nm scale).