Magnetite – A Long Standing History

Magnetite (Fe3O4) is one of the Earths' oldest known, most naturally abundant and cost-effective materials. In the bulk it exhibits interesting phenomena such as ferrimagnetism, opens an external URL in a new window, half-metallicity, opens an external URL in a new window, and the Verwey transition, opens an external URL in a new window, in which almost all the properties change drastically on cooling through 123 K. Fe3O4 plays an important role in many natural and man made processes. For example, magnetite nanoparticles are present in the beaks of homing pigeons (thought to be related to the sensing of the Earths magnetic field). Fe3O4 particles can be dragged around the body using magnets to deliver drugs in a highly localized way. However, while the bulk properties are much studied, much less work has gone into studying the surfaces, which turn out to be critical to the function performed in the majority of applications.

Fundamental Surface Chemistry

structure of the magnetite (100) surface

© Michael Schmid/IAP

Our group has been interested in Fe3O4 surfaces since we published the first atomically resolved image of the Fe3O4(001) in 2000. In the recent years we have come back to this surface and begun to study the surface chemistry in detail. By systematically varying the preparation conditions, we found that several highly ordered metastable Fe terminated surfaces can be created. The ability to tailor the surface structure in this way presents an unrivalled opportunity to investigate the effect of cation concentration and co-ordination on the surface reactivity.

So far, our investigations have produced very interesting results. We have found that the surface structure differs from the known bulk structure, which plays a huge role in the adsorption of metal atoms and small molecules.

  • R. Bliem, E. McDermott, P. Ferstl, M. Setvin, O. Gamba, J. Pavelec, M. A. Schneider, M. Schmid, U. Diebold, P. Blaha, L. Hammer, G. S. Parkinson
    Subsurface cation vacancy stabilization of the magnetite (001) surface
    Science 346, 1215 (2014); doi: 10.1126/science.1260556

From Single Adatoms to Clusters in Catalysts

Single gold atoms at a magnetite surface (STM image)

© Michael Schmid/IAP

Metals such as gold, platinum, rhodium or palladium are often used as catalysts to speed up certain chemical reactions or increase the yield of a desired product. When the atoms of the catalyst ball together, most of them do not get into contact with the surrounding gas any more and the catalytic effect diminishes drastically, but these processes are insufficiently understood. While small clusters are highly efficient catalysts, it is still unknown what happens at the extreme, where isolated metal atoms reside on the surface — it was close to impossible to have single metal atoms on an oxide surface. The Fe3O4(001) surface allows us to study both, isolated metal adatoms that are stable up to surprisingly high temperature, as well as the processes leading to cluster formation and growth of clusters. The special properties of magnetite have spurred a new research field, single-atom catalysis!

  • Z. Novotný, G. Argentero, Z. Wang, M. Schmid, U. Diebold, G. S. Parkinson
    Ordered array of single adatoms with remarkable thermal stability: Au/Fe3O4(001)
    Physical Review Letters 108, 216103 (2012); doi: 10.1103/PhysRevLett.108.216103

  • R. Bliem, J. Pavelec, O. Gamba, E. McDermott, Z. Wang, S. Gerhold, M. Wagner, J. Osiecki, K. Schulte, M. Schmid, P. Blaha, U. Diebold, G. S. Parkinson
    Adsorption and incorporation of transition metals at the magnetite Fe3O4(001) surface
    Physical Review B 92, 075440 (2015); doi: 10.1103/PhysRevB.92.075440

  • G. S. Parkinson, Z. Novotny, G. Argentero, M. Schmid, J. Pavelec, R. Kosak, P. Blaha, U. Diebold
    Carbon monoxide-induced adatom sintering in a Pd–Fe3O4 model catalyst
    Nature Materials 12, 724 (2013); doi: 10.1038/nmat3667

  • R. Bliem, J. van der Hoeven, A. Zavodny, O. Gamba, J. Pavelec, P. E. de Jongh, M. Schmid, U. Diebold, G. S. Parkinson
    An atomic-scale view of CO and H2 oxidation on a Pt/Fe3O4 model catalyst
    Angewandte Chemie International Edition 54, 13999 (2015); doi: 10.1002/anie.201507368

Hematite

Hematite surfaces (schematic), STM image and structure model

© Florian Kraushofer & Michael Schmid

In contrast to magnetite hematite, Fe2O3, is a semiconductor with a band gap of approx. 2 eV. This would make it an ideal material for hydrogen production with solar energy, without the detour via electricity generation and electrolysis! Unfortunately, the efficiency of this process with Fe2O3 is very low, and substantial research efforts will be needed to make it work in practice. Here again, understanding the surface is decisive! Fe2O3 is also an important support material for catalysts.

Previously, mainly the Fe2O3(001) surface ("c cut") has been studied by various groups. This surface is not very stable, however, and many of the (001) structures are still unclear in spite of decades of work. The (012) surface ("r cut") is much more stable and in many cases it is also more important. Therefore, we have provided fundamental studies on this surface.

  • F. Kraushofer, Z. Jakub, M. Bichler, J. Hulva, P. Drmota, M. Weinold, M. Schmid, M. Setvin, U. Diebold, P. Blaha, G. S. Parkinson
    Atomic-scale structure of the hematite α-Fe2O3(11̅02) “r-cut” surface
    The Journal of Physical Chemistry C 122, 1657 (2018); doi: 10.1021/acs.jpcc.7b10515
  • G. Franceschi, F. Kraushofer, M. Meier, G. S. Parkinson, M. Schmid, U. Diebold, M. Riva
    A model system for photocatalysis: Ti-doped α-Fe2O3(11̅02) single-crystalline films
    Chemistry of Materials 32, 3753 (2020); doi: 10.1021/acs.chemmater.9b04908

  • F. Kraushofer, L. Haager, M. Eder, A. Rafsanjani-Abbasi, Z. Jakub, G. Franceschi, M. Riva, M. Meier, M. Schmid, U. Diebold, G. S. Parkinson
    Single Rh adatoms stabilized on α-Fe2O3(11̅02) by coadsorbed water
    ACS Energy Letters 7, 375 (2022); doi: 10.1021/acsenergylett.1c02405