Zirconia – In Your Mouth and Your Car?
Zirconia (ZrO2) is one of the most common ceramic materials. Zirconia is used in engineering and in dentistry (tooth implants), and it has a unique property that makes it ideal for many applications: At high temperatures, doped zirconia is a good conductor for oxygen ions, but it remains an insulator for electrons. Thus, any electric current through zirconia means oxygen transport, and oxygen transport is accompanied by a an electric current and voltage. Therefore, zirconia is used for solid oxide fuel cells, opens an external URL in a new window and oxygen sensors (e.g., lambda sensor, opens an external URL in a new window for exhaust gas of cars).
Ultrathin Zirconia Films
© Michael Schmid/IAP
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.
M. Antlanger, W. Mayr-Schmölzer, J. Pavelec, F. Mittendorfer, J. Redinger, P. Varga, U. Diebold, M. Schmid
Pt3Zr(0001): A substrate for growing well-ordered ultrathin zirconia films by oxidation
Physical Review B 86, 035451 (2012); doi: 10.1103/PhysRevB.86.035451
J. I. J. Choi, W. Mayr-Schmölzer, F. Mittendorfer, J. Redinger, U. Diebold, M. Schmid
The growth of ultra-thin zirconia films on Pd3Zr(0001)
Journal of Physics: Condensed Matter 26, 225003 (2014); doi: 10.1088/0953-8984/26/22/225003
Sputter-Deposited Zirconia Films
© Michael Schmid/IAP
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!
P. Lackner, Z. Zou, S. Mayr, J.-I. J. Choi, U. Diebold, M. Schmid
Surface structures of ZrO2 films on Rh(111): From two layers to bulk termination
Surface Science 679, 180 (2019); doi: 10.1016/j.susc.2018.09.004
P. Lackner, J. I. J. Choi, U. Diebold, M. Schmid
Substoichiometric ultrathin zirconia films cause strong metal–support interaction
Journal of Materials Chemistry A 7, 24837 (2019); doi: 10.1039/C9TA08438J
P. Lackner, Z. Zou, S. Mayr, U. Diebold, M. Schmid
Using photoelectron spectroscopy to observe oxygen spillover to zirconia
Physical Chemistry Chemical Physics 21, 17613 (2019); doi: 10.1039/C9CP03322J
Ao.Univ.Prof. Dipl.-Ing. Dr.techn. Michael Schmid
Research Unit of Surface Physics