IAP logo

Differences

This shows you the differences between two versions of the page.

surface:research:tio2 [2014-03-12 14:02]
Michael Schmid more on O2/anatase
surface:research:tio2 [2020-04-17 16:55] (current)
Michael Schmid
Line 3: Line 3:
Titanium dioxide (TiO<sub>2</sub>, titania) has an incredible variety of applications, as a photocatalyst, in [[wp>Dye-sensitized_solar_cell|dye-sensitized solar cells]], in electronic elements ([[wp>Memristor|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! Titanium dioxide (TiO<sub>2</sub>, titania) has an incredible variety of applications, as a photocatalyst, in [[wp>Dye-sensitized_solar_cell|dye-sensitized solar cells]], in electronic elements ([[wp>Memristor|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 TiO<sub>2</sub> by Surface Physics Group leader [[:surface:group:diebold:|Ulrike Diebold]] is pretty long - more than 6 dozen entries! So we can only mention very few of her results on this page.+Also the list of publications on TiO<sub>2</sub> by Surface Physics Group leader [[:surface:group:diebold:|Ulrike Diebold]] is pretty long - more than 100 entries! So we can only mention very few of her (and our) results on this page. 
 + 
 +  - U. Diebold\\ //The surface science of titanium dioxide//\\ [[http://dx.doi.org/10.1016/S0167-5729(02)00100-0|Surf. Sci. Rep. 48, 53-229 (2003)]] ⋅ {{http://www.iap.tuwien.ac.at/bin/surface_pdf/A2003F20.pdf|full text*}} 
===== How simple molecules adsorb ===== ===== How simple molecules adsorb =====
{{ :surface:research:tio2_110_o2-frames.jpg?120|How O2 adsorbs on TiO2(110)}} {{ :surface:research:tio2_110_o2-frames.jpg?120|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 TiO<sub>2</sub>(110), it is well known that O<sub>2</sub> molecules adsorb at oxygen vacancies and dissociate there, healing the vacancy by filling it and leaving a single oxygen adatom at the surface. It was only recently that we could first detect the precursor to this process by STM: An O<sub>2</sub> 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.+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 TiO<sub>2</sub>(110), it is well known that O<sub>2</sub> 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 O<sub>2</sub> 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 O<sub>2</sub> on TiO<sub>2</sub>(110) is much richer than expected! Watch out for our new results (will be published in 2020)!
Whereas O<sub>2</sub> 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 Cl<sub>2</sub> adsorbed on rutile TiO<sub>2</sub>(110) literally explodes, and the the Cl atoms fly apart by 26 Å, a long distance on the atomic scale. Whereas O<sub>2</sub> 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 Cl<sub>2</sub> adsorbed on rutile TiO<sub>2</sub>(110) literally explodes, and the the Cl atoms fly apart by 26 Å, a long distance on the atomic scale.
 +
 +  - P. Scheiber, A. Riss, M. Schmid, P. Varga, U. Diebold\\ //Observation and destruction of an elusive adsorbate with STM: O<sub>2</sub>/TiO<sub>2</sub>(110)//\\ [[http://link.aps.org/abstract/PRL/v105/p216101|Phys. Rev. Lett. 105 (2010) 216101]] ⋅ {{http://www.iap.tuwien.ac.at/bin/surface_pdf/A2010E07.pdf|full text}}
 +  - U. Diebold, W. Hebenstreit, G. Leonardelli, M. Schmid, and P. Varga\\ //High transient mobility of chlorine on TiO<sub>2</sub>(110): evidence for "cannon-ball" trajectories of hot adsorbates//\\ [[http://link.aps.org/abstract/PRL/v81/p405|Phys. Rev. Lett. 81 (1998) 405-408]] ⋅ {{http://www.iap.tuwien.ac.at/bin/surface_pdf/A1998E05.pdf|full text}}
<clear> <clear>
Line 16: Line 24:
{{ :surface:research:tio2_catechol.jpg?180|Catechol and hydroxyls on TiO2(110)}} {{ :surface:research:tio2_catechol.jpg?180|Catechol and hydroxyls on TiO2(110)}}
-Considering applications of TiO<sub>2</sub> as a photocatalyst and in dye-sensitized solar cells, understanding adsorption of organic molecules on TiO<sub>2</sub> is of paramount importance. One of the nicest results of these studies was finding out how [[wp>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!<clear>+Considering applications of TiO<sub>2</sub> as a photocatalyst and in dye-sensitized solar cells, understanding adsorption of organic molecules on TiO<sub>2</sub> is of paramount importance. One of the nicest results of these studies was finding out how [[wp>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! 
 + 
 +  - S-C. Li, L.-N. Chu, X.-Q. Gong, U. Diebold\\ //Hydrogen bonding controls the dynamics of catechol adsorbed on a TiO<sub>2</sub>(110) surface//\\ [[http://dx.doi.org/10.1126/science.1188328|Science 328 (2010) 882-884]] ⋅ {{http://www.iap.tuwien.ac.at/bin/surface_pdf/A2010E05.pdf|full text*}} 
 +<clear>
===== Anatase ===== ===== Anatase =====
Line 22: Line 33:
TiO<sub>2</sub> 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 TiO<sub>2</sub>, 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 TiO<sub>2</sub>(101) surface usually has no stable oxygen vacancies on the surface: O vacancies are below, so it's all different! Nevertheless, O<sub>2</sub> adsorbed at the surface can interact with an O vacancy, and then a O<sub>2</sub><sup>2-</sup> (peroxo) species may replace an oxygen atom in the surface.  See our papers (listed below) for details. TiO<sub>2</sub> 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 TiO<sub>2</sub>, 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 TiO<sub>2</sub>(101) surface usually has no stable oxygen vacancies on the surface: O vacancies are below, so it's all different! Nevertheless, O<sub>2</sub> adsorbed at the surface can interact with an O vacancy, and then a O<sub>2</sub><sup>2-</sup> (peroxo) species may replace an oxygen atom in the surface.  See our papers (listed below) for details.
- 
- 
- 
-===== References ===== 
- 
-  - U. Diebold\\ //The surface science of titanium dioxide//\\ [[http://dx.doi.org/10.1016/S0167-5729(02)00100-0|Surf. Sci. Rep. 48, 53-229 (2003)]] ⋅ {{http://www.iap.tuwien.ac.at/bin/surface_pdf/A2003F20.pdf|full text*}} 
-  - P. Scheiber, A. Riss, M. Schmid, P. Varga, U. Diebold\\ //Observation and destruction of an elusive adsorbate with STM: O<sub>2</sub>/TiO<sub>2</sub>(110)//\\ [[http://link.aps.org/abstract/PRL/v105/p216101|Phys. Rev. Lett. 105 (2010) 216101]] ⋅ {{http://www.iap.tuwien.ac.at/bin/surface_pdf/A2010E07.pdf|full text}} 
-  - U. Diebold, W. Hebenstreit, G. Leonardelli, M. Schmid, and P. Varga\\ //High transient mobility of chlorine on TiO<sub>2</sub>(110): evidence for "cannon-ball" trajectories of hot adsorbates//\\ [[http://link.aps.org/abstract/PRL/v81/p405|Phys. Rev. Lett. 81 (1998) 405-408]] ⋅ {{http://www.iap.tuwien.ac.at/bin/surface_pdf/A1998E05.pdf|full text}} 
-  - S-C. Li, L.-N. Chu, X.-Q. Gong, U. Diebold\\ //Hydrogen bonding controls the dynamics of catechol adsorbed on a TiO<sub>2</sub>(110) surface//\\ [[http://dx.doi.org/10.1126/science.1188328|Science 328 (2010) 882-884]] ⋅ {{http://www.iap.tuwien.ac.at/bin/surface_pdf/A2010E05.pdf|full text*}} 
  - P. Scheiber, M. Fidler, O. Dulub, M. Schmid, U. Diebold, W. Hou, U. Aschauer, A. Selloni\\ //(Sub)surface mobility of oxygen vacancies at the TiO<sub>2</sub> anatase (101) surface//\\ [[http://link.aps.org/abstract/PRL/v109/p136103|Phys. Rev. Lett. 109 (2012) 136103]] ⋅ {{http://www.iap.tuwien.ac.at/bin/surface_pdf/A2012E12.pdf|full text}}   - P. Scheiber, M. Fidler, O. Dulub, M. Schmid, U. Diebold, W. Hou, U. Aschauer, A. Selloni\\ //(Sub)surface mobility of oxygen vacancies at the TiO<sub>2</sub> anatase (101) surface//\\ [[http://link.aps.org/abstract/PRL/v109/p136103|Phys. Rev. Lett. 109 (2012) 136103]] ⋅ {{http://www.iap.tuwien.ac.at/bin/surface_pdf/A2012E12.pdf|full text}}
  - M. Setvín, U. Aschauer, P. Scheiber, Y.-F. Li, W. Hou, M. Schmid, A. Selloni, and U. Diebold\\ //Reaction of O<sub>2</sub> with Subsurface Oxygen Vacancies on TiO<sub>2</sub> Anatase (101)//\\ [[http://dx.doi.org/10.1126/science.1239879|Science 341 (2013) 988-991]] ⋅ [[http://www.iap.tuwien.ac.at/www-surface/abstracts/A2013e06.html|⋅]]<html><a href="http://www.iap.tuwien.ac.at/www-surface/abstracts/A2013e06.html" class="media mediafile mf_pdf" target="_blank">Abstract with link to full text</a></html>   - M. Setvín, U. Aschauer, P. Scheiber, Y.-F. Li, W. Hou, M. Schmid, A. Selloni, and U. Diebold\\ //Reaction of O<sub>2</sub> with Subsurface Oxygen Vacancies on TiO<sub>2</sub> Anatase (101)//\\ [[http://dx.doi.org/10.1126/science.1239879|Science 341 (2013) 988-991]] ⋅ [[http://www.iap.tuwien.ac.at/www-surface/abstracts/A2013e06.html|⋅]]<html><a href="http://www.iap.tuwien.ac.at/www-surface/abstracts/A2013e06.html" class="media mediafile mf_pdf" target="_blank">Abstract with link to full text</a></html>
 +  - M. Setvin, J. Hulva, G. S. Parkinson, M. Schmid, and U. Diebold\\//Electron transfer between anatase TiO<sub>2</sub> and an O<sub>2</sub> molecule directly observed by atomic force microscopy//\\ [[http://dx.doi.org/10.1073/pnas.1618723114|Proc. Natl. Acad. Sci. USA 114 (2017) E2556]]
 +<clear>
 +===== Polarons =====
 +
 +When rutile TiO<sub>2</sub> 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 Ti<sup>3+</sup> 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 TiO<sub>2</sub>. We could show that polarons have a decisive influnece on surface structure and the bonding of adsorbates on the surface. Anatase TiO<sub>2</sub> is different, however!
 +
 +  - 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 TiO<sub>2</sub> rutile and anatase//\\ [[http://link.aps.org/doi/10.1103/PhysRevLett.113.086402|Phys. Rev. Lett. 113, 086402 (2014)]] ⋅ {{http://www.iap.tuwien.ac.at/bin/surface_pdf/A2014E12.pdf|full text}}.
 +  - M. Reticcioli, M. Setvin, X. Hao, P. Flauger, G. Kresse, M. Schmid, U. Diebold, and C. Franchini\\ //Polaron-driven surface reconstructions//\\ [[https://link.aps.org/doi/10.1103/PhysRevX.7.031053|Phys. Rev. X 7 (2017) 031053]] ⋅ {{http://www.iap.tuwien.ac.at/bin/surface_pdf/A2017E08.pdf|full text}}.
 +  - M. Reticcioli, I. Sokolović, M. Schmid, U. Diebold, M. Setvin, and C. Franchini\\ //Interplay between adsorbates and polarons: CO on rutile TiO<sub>2</sub>(110)// \\ [[https://link.aps.org/doi/10.1103/PhysRevLett.122.016805|Phys. Rev. Lett. 122 (2019) 016805]] ⋅ {{http://www.iap.tuwien.ac.at/bin/surface_pdf/A2019E04.pdf|full text}}.
 +<clear>
<small>* Please note: access to full text (PDF files) of some articles is restricted due to copyright reasons.</small> <small>* Please note: access to full text (PDF files) of some articles is restricted due to copyright reasons.</small>
For more papers on TiO<sub>2</sub> by our group (still far from all of Ulrike Diebold's papers on this subject), see our [[http://www.iap.tuwien.ac.at/www/surface/publication_search?KEY1=TiO2|Publications page]]. For more papers on TiO<sub>2</sub> by our group (still far from all of Ulrike Diebold's papers on this subject), see our [[http://www.iap.tuwien.ac.at/www/surface/publication_search?KEY1=TiO2|Publications page]].
surface/research/tio2.txt · Last modified: 2020-04-17 16:55 by Michael Schmid