IAP logo

The Many Faces of Titania

Titanium dioxide (TiO2, titania) has an incredible variety of applications, as a photocatalyst, in dye-sensitized solar cells, in electronic elements (memristors, varistors), as a white pigment, in optical coatings, and in sun lotion to protect your skin against harmful UV radiation. No wonder that it took the leading role among all research on oxide single crystals!

Also the list of publications on TiO2 by Surface Physics Group leader Ulrike Diebold is pretty long - more than 100 entries! So we can only mention very few of her (and our) results on this page.

  1. U. Diebold
    The surface science of titanium dioxide
    Surf. Sci. Rep. 48, 53-229 (2003)full text*

How simple molecules adsorb

How O2 adsorbs on TiO2(110) Any material interacts with its environment via its surface, and one of the first things to understand is how molecules bind to the material when they come from the gas phase (adsorption). On rutile TiO2(110), it is well known that O2 molecules adsorb at oxygen vacancies and dissociate there, healing the vacancy by filling it and leaving a single oxygen adatom at the surface. We could first detect the precursor to this process by STM: An O2 molecule in the previous vacancy appears very faint in STM images, but then explodes into two separate adatoms. Thereafter, one of them jumps back into the vacancy.

We have recently revisited the same process with non-contact AFM. We could confirm the STM result, but it turns out that the behavior of O2 on TiO2(110) is much richer than expected! Watch out for our new results (will be published in 2020)!

Whereas O2 dissociates in a rather benign manner, dissociation of the chlorine molecule is a more hefty process: Already a decade ago, we have found evidence that Cl2 adsorbed on rutile TiO2(110) literally explodes, and the the Cl atoms fly apart by 26 Å, a long distance on the atomic scale.

  1. P. Scheiber, A. Riss, M. Schmid, P. Varga, U. Diebold
    Observation and destruction of an elusive adsorbate with STM: O2/TiO2(110)
    Phys. Rev. Lett. 105 (2010) 216101full text
  2. U. Diebold, W. Hebenstreit, G. Leonardelli, M. Schmid, and P. Varga
    High transient mobility of chlorine on TiO2(110): evidence for “cannon-ball” trajectories of hot adsorbates
    Phys. Rev. Lett. 81 (1998) 405-408full text


Organic molecules

Catechol and hydroxyls on TiO2(110) Considering applications of TiO2 as a photocatalyst and in dye-sensitized solar cells, understanding adsorption of organic molecules on TiO2 is of paramount importance. One of the nicest results of these studies was finding out how catechol diffuses on a titania surface. This molecule does not simply hop from place to place, but always keeps one of it's “feet” on the ground, lifting the other “foot” by means of a hydrogen atom. Hydrogen allows the molecules to dance back and forward over the surface!

  1. S-C. Li, L.-N. Chu, X.-Q. Gong, U. Diebold
    Hydrogen bonding controls the dynamics of catechol adsorbed on a TiO2(110) surface
    Science 328 (2010) 882-884full text*


Anatase

TiO2 comes in three different crystalline forms, rutile, anatase and brookite. Rutile is the stable form for macroscopic crystals, and therefore most of the previous work on TiO2, including everything described above, was on rutile surfaces. In practical applications, nanometer-sized crystals are often anatase; we have therefore started to study its surfaces. In contrast to rutile, the anatase TiO2(101) surface usually has no stable oxygen vacancies on the surface: O vacancies are below, so it's all different! Nevertheless, O2 adsorbed at the surface can interact with an O vacancy, and then a O22- (peroxo) species may replace an oxygen atom in the surface. See our papers (listed below) for details.

  1. P. Scheiber, M. Fidler, O. Dulub, M. Schmid, U. Diebold, W. Hou, U. Aschauer, A. Selloni
    (Sub)surface mobility of oxygen vacancies at the TiO2 anatase (101) surface
    Phys. Rev. Lett. 109 (2012) 136103full text
  2. M. Setvín, U. Aschauer, P. Scheiber, Y.-F. Li, W. Hou, M. Schmid, A. Selloni, and U. Diebold
    Reaction of O2 with Subsurface Oxygen Vacancies on TiO2 Anatase (101)
    Science 341 (2013) 988-991Abstract with link to full text
  3. M. Setvin, J. Hulva, G. S. Parkinson, M. Schmid, and U. Diebold\\Electron transfer between anatase TiO2 and an O2 molecule directly observed by atomic force microscopy
    Proc. Natl. Acad. Sci. USA 114 (2017) E2556


Polarons

When rutile TiO2 is n-doped (e.g. by formation of oxygen vacancies), the excess electrons get localized at Ti atoms (which change the oxidation state from 4+ to 3+) and the lattice around each Ti3+ distorts, trapping the electron there (negative oxygens are less attracted by a 3+ than 4+ charge, positive Ti neighbors feel less repulsion). The electron and the lattice distortion can be described by a quasiparticle, the polaron. Polarons are very important to understand the physics and chemistry of TiO2. We could show that polarons have a decisive influnece on surface structure and the bonding of adsorbates on the surface. Anatase TiO2 is different, however!

  1. M. Setvin, C. Franchini, X. Hao, M. Schmid, A. Janotti, M. Kaltak, C. G. Van de Walle, G. Kresse, and U. Diebold
    Direct view at excess electrons in TiO2 rutile and anatase
    Phys. Rev. Lett. 113, 086402 (2014)full text.
  2. M. Reticcioli, M. Setvin, X. Hao, P. Flauger, G. Kresse, M. Schmid, U. Diebold, and C. Franchini
    Polaron-driven surface reconstructions
    Phys. Rev. X 7 (2017) 031053full text.
  3. M. Reticcioli, I. Sokolović, M. Schmid, U. Diebold, M. Setvin, and C. Franchini
    Interplay between adsorbates and polarons: CO on rutile TiO2(110)
    Phys. Rev. Lett. 122 (2019) 016805full text.


* Please note: access to full text (PDF files) of some articles is restricted due to copyright reasons.

For more papers on TiO2 by our group (still far from all of Ulrike Diebold's papers on this subject), see our Publications page.

surface/research/tio2.txt · Last modified: 2020-04-17 16:55 by Michael Schmid