Zirconia (ZrO₂) is one of the most common ceramic materials. Zirconia is used in engineering and in dentistry (tooth crowns), 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 and oxygen sensors (e.g., lambda sensor for exhaust gas of cars).

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 ZrO₂ 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) ZrO₂: Their structure is essentially the same, and both possess a similar band gap.

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.

1. Pt₃Zr(0001): A substrate for growing well-ordered ultrathin zirconia films by oxidation
   M. Antlanger, W. Mayr-Schmölzer, J. Pavelec, F. Mittendorfer, J. Redinger, P. Varga, U. Diebold, and M. Schmid
   Phys. Rev. B 86, 035451 (2012) ⋅ full text
2. The growth of ultra-thin zirconia films on Pd₃Zr(0001)
   J. I. J. Choi, W. Mayr-Schmölzer, F. Mittendorfer, J. Redinger, U. Diebold, and M. Schmid
   J. Phys.: Condens. Matter 26, 225003 (2014) ⋅ full text
3. Water adsorption at zirconia: from the ZrO₂(111)/Pt₃Zr(0001) model system to powder samples
   P. Lackner, J. Hulva, E.-M. Köck, W. Mayr-Schmölzer, J. I. J. Choi, S. Penner, U. Diebold, F. Mittendorfer, J. Redinger, B. Klötzer, G. S. Parkinson, M. Schmid
   J. Mater. Chem. A 6, 17587 (2018) ⋅ full text

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. Our sputter-deposited ZrO₂ are also a perfect model system for understanding x-ray-photoelectron spectroscopy (XPS) of wide-bandgap insolators!

1. Surface structures of ZrO₂ films on Rh(111): From two layers to bulk termination
   P. Lackner, Z. Zou, S. Mayr, J.-I. J. Choi, U. Diebold, M. Schmid
   Surf. Sci. 679, 180 (2019)
2. Construction and evaluation of an ultrahigh-vacuum-compatible sputter deposition source
   P. Lackner, J. I. J. Choi, U. Diebold, M. Schmid
   Rev. Sci. Instrum. 88, 103904 (2017) ⋅ full text
3. Substoichiometric ultrathin zirconia films cause strong metal–support interaction
   P. Lackner, J. I. J. Choi, U. Diebold, M. Schmid
   J. Mater. Chem. A 7, 24837 (2019) ⋅ full text
4. Using photoelectron spectroscopy to observe oxygen spillover to zirconia
   P. Lackner, Z. Zou, S. Mayr, U. Diebold, M. Schmid
   Phys. Chem. Chem. Phys. 21, 17613 (2019) ⋅ full text
5. Few-monolayer yttria-doped zirconia films: Segregation and phase stabilization
   P. Lackner, A. J. Brandt, U. Diebold, M. Schmid
   J. Chem. Phys. 152, 064709 (2020)

The zirconia project was financed by the Austrian Science Fund as part of the SFB Functional Oxide Surfaces and Interfaces