Structurally incorporated impurities have been shown to have systematic effects on the rate of the thermally driven transformations in titania nanoparticles. For example, the anatase-to-rutile transformation is slowed when anatase nanoparticles are doped with a cation of valence >+4, but favored when the valence < +4. Based on these observations, Y³⁺ dopants should promote the anatase-to-rutile transformation. However, prior studies showed that the transformation is actually inhibited by such impurities. So far these [1,2], observations have remained unexplained.

Recently, scientists from University of California Berkeley and Lawrence Berkeley National Laboratory, in collaboration with SSRL beam line scientists at BLs 10-2 and 11-2, used extended x-ray absorption fine structure (EXAFS) experiments on yttrium-doped titania nanoparticles to determine the local structural environment of Y³⁺ impurities.

Results indicate that yttrium impurities are mostly present as individual, oxygen-coordinated atoms at the titania surface (i.e., as YO₆ groups) and about 15% of the surface oxygen sites are bound to Y. Together with the observation of the structural modification and phase transformation retardation in complementary wide-angle x-ray scattering experiments for the study, the researchers found that the low concentrations of yttrium surface impurities on nano-anatase reduce surface energy and inhibit nanoparticle growth over a large temperature range. The findings demonstrate the effectiveness of surface bound impurities of stabilizing nanoparticle size and phase, an issue of great importance for retaining the materials properties of nanoscale catalysts that operate at high temperatures.

1. J. F. Banfield, B. L. Bischoff, and M.A. Anderson, Chem. Geo. 110, 211 (1993)
2. B. L. Bischoff, Thermal Stabilization of Anatase Membranes, Ph.D. dissertation, University of Wisconsin-Madison, 1992.

To learn more about this research see the full scientific highlight at:
http://www-ssrl.slac.stanford.edu/research/highlights_archive/ydopants.html

Bin Chen, Hengzhong Zhang, Benjamin Gilbert, Jillian F. Banfield, "Mechanism of Inhibition of Nanoparticle Growth and Phase Transformation by Surface Impurities", Phys. Rev. Lett. 98, 106103 (2007)