© Michael Schmid/IAP

Ultra-thin zirconia on Pt3Zr (with simplified structure model)

Ultra-thin zirconia on Pt3Zr (with simplified structure model)

Having a perfect insulator for electrons (but an ionic conductor) is a blessing for applications, but a curse if we want to study its surface by scanning tunneling microscopy (STM) or many other techniques. The way out is studying very thin zirconia films on metals; such films are still sufficiently conductive for STM. We have developed a technique to create well-ordered ultrathin zirconia films by oxidation of alloys containing Zr, Pt₃Zr or Pd₃Zr. These zirconia films are very thin indeed, they consist only of two oxygen layers and one Zr layer in between! Nevertheless, they are very similar to the “real” material, thick (bulk) zirconia: Their structure is similar, and both possess a similar band gap, opens an external URL in a new window.

STM shows us the Zr atoms, perfectly well ordered on this surface. Density functional theory calculations (done in the Computational Materials Science group at our institute) tell us how the film binds to the substrate and much more. The structure model is a simplification, however; the stoichiometry is not exactly ZrO₂.

© Michael Schmid/IAP

Two crystal structures of ZrO2 and corresponding STM images

Two crystal structures of ZrO2 and corresponding STM images

While the ultra-thin zirconia films obtained by oxidation of Zr alloys share many properties with bulk ZrO₂, it is much nicer to study thicker ZrO₂ films (but still thin enough for STM). As mentioned above, we need a suitable model system for this, to ensure at least some conductivity. We have constructed a sputter deposition source, which allows us to produce ZrO₂ films with much better quality than any other method known so far. With these films, we could explain a previously astonishing phenomenon, the observation of the so-called strong-metal support interaction (SMSI) effect for ZrO₂-supported metal catalysts. We could show that ultrathin ZrO₂ films can be oxygen deficient, although ZrO₂ is normally non-reducible, and this is the reason for the SMSI effect. We could also clearly demonstrate the relation between the reduction state and the crystal structure (see the image). Our sputter-deposited ZrO₂ films are also a perfect model system for understanding x-ray photoelectron spectroscopy (XPS) of wide-bandgap insulators: Usually reduction of an oxide shifts the XPS binding energies to lower values, but for reduced zirconia the opposite is true!