Atomic structures: Titanium coming out on top
High-resolution electron microscopy settles the problem of the atomic structure at the surface of titania, an important catalyst material
Precise knowledge of the arrangement of atoms at the surface of a crystal is crucial for catalytic materials, which are used to initiate chemical reactions through interactions at the surface. Settling existing controversy, Yuichi Ikuhara and colleagues from the University of Tokyo in collaboration with the Advanced Institute for Materials Research (AIMR) at Tohoku University1 have now determined the atomic arrangement at the surface of titania (TiO2), which is an important material for catalytic processes.
Crystalline materials are regular three-dimensional assemblies of atoms, and study of the bulk crystal structure is generally straightforward. The surface of a crystalline material, however, is notoriously difficult to study, as the disruption to the perfect crystal symmetry leads to compromises in the positions and chemical bonds of the surface atoms. “Determining the surface structure is very important for understanding the nature and the mechanism of the catalytic properties,” says Ikuhara.
Although there have been many proposals for the surface structure of titania, it has remained difficult to determine the positions of top surface atoms precisely by experimental observations. The researchers were able to derive the titania surface structure for the first time through a combination of high-voltage electron microscopy (HVEM) and scanning electron microscopy techniques (Fig. 1).
The surface structure was studied with atomic precision from two directions, and the three-dimensional image thus obtained was compared with two structures that have recently been proposed in theoretical studies. In one of the models, the outer surface is comprised predominantly of oxygen atoms, whereas in a more recent study, titanium was predicted to be located at various positions near the surface such that the surface composition is more balanced between oxygen and titanium. The experimental results clearly confirmed the latter prediction.
The findings suggest that titanium plays a more active role on the surface of titania than previously thought, which provides valuable feedback for developing a better understanding of the reaction kinetics on the surface of titania. “I believe that our findings will contribute significantly to our understanding of the surface kinetics,” comments Ikuhara.
In catalytic applications, the surface of titania is often functionalized with noble metals such as gold or platinum, which act as co-catalysts. As a next step it will be important to study the structure of the interfaces between these metals and titania. “This,” says Ikuhara, “will enable us to understand the origin and nature of their catalytic properties.”
Shibata, N., Goto, A., Choi, S.-Y., Mizoguchi, T., Findlay, S.D., Yamamoto, T. & Ikuhara, Y. Direct imaging of reconstructed atoms on TiO2 (110) surfaces. Science 322, 570–573 (2009). | article
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