Titanium dioxide (TiO2, titania) has an incredible variety of applications, as a photocatalyst, in dye-sensitized solar cells, opens an external URL in a new window, in electronic elements (memristors, opens an external URL in a new window, 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.

How simple molecules adsorb

Dissociation of oxygen on TiO2 (schematic)

© Michael Schmid/IAP

Dissociation of oxygen on TiO2 (schematic)

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!

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.

  • I. Sokolović, M. Reticcioli, M. Čalkovský, M. Wagner, M. Schmid, C. Franchini, U. Diebold, M. Setvín
    Resolving the adsorption of molecular O2 on the rutile TiO2(110) surface by noncontact atomic force microscopy
    Proceedings of the National Academy of Sciences 117, 14827 (2020); doi: 10.1073/pnas.1922452117.

  • U. Diebold, W. Hebenstreit, G. Leonardelli, M. Schmid, P. Varga
    High transient mobility of chlorine on TiO2(110): Evidence for “cannon-ball” trajectories of hot adsorbates
    Physical Review Letters 81, 405 (1998); doi: 10.1103/PhysRevLett.81.405.

Organic Molecules

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, opens an external URL in a new window 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!

  • 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, 882 (2010); doi: 10.1126/science.1188328

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 for details:

  • M. Setvín, U. Aschauer, P. Scheiber, Y.-F. Li, W. Hou, M. Schmid, A. Selloni, U. Diebold
    Reaction of O2 with subsurface oxygen vacancies on TiO2 anatase (101)
    Science 341, 988 (2013); doi: 10.1126/science.1239879, Science 349, aac9659 (2015); doi: 10.1126/science.aac9659

  • M. Setvin, B. Daniel, U. Aschauer, W. Hou, Y.-F. Li, M. Schmid, A. Selloni, U. Diebold
    Identification of adsorbed molecules via STM tip manipulation: CO, H2O, and O2 on TiO2 anatase (101)
    Physical Chemistry Chemical Physics 16, 21524 (2014); doi: 10.1039/C4CP03212H

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 influence on surface structure and the bonding of adsorbates on the surface. Anatase TiO2 is different, however!

  • M. Setvin, C. Franchini, X. Hao, M. Schmid, A. Janotti, M. Kaltak, C. G. Van de Walle, G. Kresse, U. Diebold
    Direct view at excess electrons in TiO2 rutile and anatase
    Physical Review Letters 113, 086402 (2014); doi: 10.1103/PhysRevLett.113.086402

  • M. Reticcioli, M. Setvin, X. Hao, P. Flauger, G. Kresse, M. Schmid, U. Diebold, C. Franchini
    Polaron-driven surface reconstructions
    Physical Review X 7, 031053 (2017); doi: 10.1103/PhysRevX.7.031053

  • M. Reticcioli, I. Sokolović, M. Schmid, U. Diebold, M. Setvin, C. Franchini
    Interplay between adsorbates and polarons: CO on rutile TiO2(110)
    Physical Review Letters 122, 016805 (2019); doi: 10.1103/PhysRevLett.122.016805