Recording high resolution transmission electron images from two orthogonal directions provides the solution to a long-standing problem in surface science.
Titanium dioxide has numerous applications including heterogeneous and photocatalysis, gas sensors and coatings. Material properties are often governed by the reactivity of the surface, thus a clear understanding of the surface structure is essential. While the bulk structure of TiO2 is well known, the nature of TiO2surfaces is not clear. Creating a surface implies the abrupt discontinuation of bonds found in the bulk structure, with atoms at the surface forming different arrangements—undergoing a so-called reconstruction—and the resulting structure has been the subject of much debate.
Now Naoya Shibata and co-workers from the University of Tokyo1 have developed a solution to this long-standing problem by applying atomic-resolution transmission electron microscopy (TEM). “Analyzing the surface of titania—especially after reconstruction—using conventional surface analysis has been problematic,” says Shibata.
Earlier reports proposed two very similar arrangements of the oxygen and titanium atoms at the surface, both of which had been shown to be energetically favorable. The surfaces differed in the stoichiometry—namely, the density of reactive oxygen atoms present at the surface. “It was only by recording images from two directions that we were able to identify which model was correct,” says Shibata.
Knowledge of the surface structure (Fig. 1) could be exploited for creating more accurate computer models, which could predict the reactivity, and be useful for the design of new catalysts, for example.
“We would like to look at how different preparations and surface treatments affect the structure,” says Shibata. “TEM contrast is sensitive to atomic number, and the images can thus be used to distinguish between atoms of different elements, it is possible that we will therefore be able to determine the structures of more complex material surfaces”.
Ultimately, the research team hopes that further advances in TEM will allow them to investigate more than just static surfaces. “There are lots of obstacles,” admits Shibata, “but such studies may allow us to see dynamic surface changes during reactions and thus could reveal the detailed mechanism of surface reaction."
References
Shibata, N. et al. Direct imaging of reconstructed atoms on TiO2 (110) surfaces. Science 322, 570–573 (2008).
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Surface Science: Reconstruction resolution. NPG Asia Mater (2009). https://doi.org/10.1038/asiamat.2009.107
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DOI: https://doi.org/10.1038/asiamat.2009.107