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Surfaces of Metal Oxides.

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Plenary lecture of the XIV SBPMat Meeting, given by Prof. Ulrike Diebold (UT Wien, Austria) on September 29, 2015, in Rio de Janeiro (Brazil).

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Surfaces of Metal Oxides.

  1. 1. Surfaces of Metal Oxides Ulrike Diebold Institute of Applied Physics, TU Wien Vienna, Austria
  2. 2. DFT Collaborations: Annabella Selloni (Princeton U.), Peter Blaha (TU Wien)
  3. 3. Surfaces of Metal Oxides: Ulrike Diebold Institute of Applied Physics, TU Wien Vienna, Austria Understanding fundamental mechanisms and processes at the atomic scale
  4. 4. & 4 TiO2 U. Diebold, “The Surface Science of Titanium Dioxide”, Surf. Sci. Rep. 48 (2003) 53 [~4000 cites] Oxide materials we have studied Fe3O4, In2O3, SrTiO3, Sr3Ru2O7, SnO2, ZnO, ZrO2 Heterogeneous Catalysis (support & active catalyst) Gas sensing Photocatalyst Dye-sensitized solar cells Li-ion batteries Memristor Optical Properties Biocompatibility
  5. 5. & 5 Credits: Forschungszentrum Jülich Kizuka et al., Phys. Rev. B 55, R7398 (1 Main Tool: Scanning Tunneling Microscopy (STM)*
  6. 6. & in combination with Density Functional Theory 6 Credits: Forschungszentrum Jülich Dulub et al., Science 2007 *Flat, large single crystals. Ultrahigh Vacuum (10-11 mbar), low temperature (6K - 400 K) Main Tool: Scanning Tunneling Microscopy (STM)*
  7. 7. & CREDIT: VERGRÖSSERUNG Hinter dieser Scheibeliegt das HerzdesRastertunelmikrosko Hier tastet dieNadel rasterförmigdieProbeab. Dieresultierende Vergrößerun 50000000-fach. OKULAR Kleinesbinokulares Mikroskop– zur Positionsüberprüfung desObjektsundder tastendenNadel. OSKOP, FA.C.REICHERT 20 12 RASTERTUNNELMIKROSKOP,TECHN.UNIVERSITÄT WIEN brauchen zum n. Dafür spüren . ope he Blutgruppen-Entdecker einander kombinierte, Gegenstandoptisch meter-Abstände auflösen. spiegel als Lichtlieferant. Dieser 1Meter hohe Stahkolosswürde heute dasHerz desNobelpreisträgers Landsteiner höher schlagenlassen:Inder Stahlkammer ertastet eine elektrische Nadel das Objekt, erkennt Atome anhanddesElektronenstromsundbildet sie als Schwarz-Weiß-Bildab.Auflösung?500 Milionstel Milimeter. Wichtig:-200° C Kammertemperatur. Dasfriert die beweglichen Atome einundmacht sie fotogen. VERGRÖSSERUNG Luxusanno1890:Drei optionaleObjektivlinsendirekt über demzuuntersuchenden Gegenstand.InKombination mit der Okular-Linseliefert diesesMikrsoskopein270-fach vergrößertesBild. GEHÄUSE Robuster Edelstahl. Schließlichherrscht in der Kammer auch Vakuum– um Verunreinigungenaus der Luft zuvermeiden. KÜHLUNG Über dieseAnschlüsse wirddieKammer auf -200°Cabgekühlt - mittelsflüssigem Sticksoff. BELEUCHTUNG Der Konkavspiegel dient der Lichbündelungvonuntenauf denGegenstand.Licht damals? Tageslicht oder Öllampe. Sosieht ein Kristall (Indiumoxid) durchdas Okular desMikroskops vonC. Reichert aus. Sosieht das Kristall auf einem RTM-Fotoaus:Die Atome sindhier alsgrau Kügelchenmühelos erkennbar. VERGLEI Sokleinist d Lichtmikros nebendemR *Flat, large single crystals. Ultrahigh Vacuum (10-11 mbar), low temperature (6K - 400 K)
  8. 8. & 8 1. Adsorption of Oxygen on TiO2 2. Single metal atoms on Fe3O4
  9. 9. & 9 1. Adsorption of Oxygen on TiO2 - O2 does not stick to fully oxidized surface. - Needs extra electrons (from oxygen vacancies, dopants, hydroxyls, etc.) - Can assume many different configurations Oads, (O2 -)ads , (O2 2-)ads , (O2)O - Role of electric field
  10. 10. & 10 Heterogeneous Catalysis (support & active catalyst) Gas sensing Biocompatibility Photocatalyst M. Haruta, CatalysisToday 36 (1997)153 Adsorption of Oxygen on TiO2 M. Batzill and UD, Progr. Surf. Sci. 2005
  11. 11. & TiO2-based Photocatalysis: The Basic Mechanism Linsebigler, Lu, Yates, Chem. Rev. 95 (1995) 735 Ohno et al. New J. Chem. 26 (2002) 1167 Photocatalyst: mixture of TiO2 rutile and anatase • Rutile and anatase have different photocatalytic activity. • Under typical photocatalytic conditions, more O2 is adsorbed on anatase than rutile. • How does O2 adsorb on TiO2 rutile and anatase?
  12. 12. 12 Rutile vs. Anatase: Wulff Shape (DFT Results) M. Lazzeri, A. Vittadini and A. Selloni, Phys. Rev. B, 65 (2002) 119901/1, ibid. Phys. Rev. B, 63 (2001) 155409/1 Ramamoorthy and Vanderbilt Phys. Rev. B 49, 16721 (1994) • Rutile and anatase have different photocatalytic activity. • Under typical photocatalytic conditions, more O2 is adsorbed on anatase than rutile. • How does O2 adsorb on TiO2 rutile and anatase?
  13. 13. 13 TiO2- Color Scheme (Rutile Samples) M. Li, et al., Journal of Physical Chemistry B 104 (20) (2000)4944 Samples heated in furnace to different temperatures (Ar with 20 ppm O2 ~4x10-3 Torr)
  14. 14. Diebold, Li, and Schmid Annual Rev. Phys. Chem 2010 Ti5c O2c VO OH [110] [001] O3c TiO2 Rutile (110): Model & Scanning Tunneling Microscopy S.-C. Li, et al., JACS 130, 9080 (2008) 14 Empty-states STM image Vsample=+0.8 V, const. height, T=78 K
  15. 15. 15 TiO2-x + … see groups of Henderson, Iwasawa, Bowker, Yates, Thornton, Besenbacher, Wendt, Dohnalek, Kummel, Selloni, Hammer, … O2O2 Wendt et al. Science 2007
  16. 16. 16 Ph. Scheiber et al., Phys. Rev. Lett., 105 (2010) 216101 Vsample=+1.8 V, I = 0.03 nA, Tsample = 17 K
  17. 17. 17 Natural Mineral Sample from Hangarsvidda, Norway U.D. et al., Catalysis Today, 85 (2003) 93-100. Vendor of cut and polished anatase(101) samples (minerals): http://www.surfacenet.de TiO2 anatase (Wulff shape)
  18. 18. 18 (c) STM of cleaved* Anatase (101) Anisotropic step edges (Gong et al, Nature Mater. 2006) [10 ]1 [010] 300 Å x 300 Å, Vs=+1.3V, It=1.9 nA *Dulub and Diebold, J. Phys. Cond. Mat. 2010 Adsorption of water (He et al, Nature Mater. 2009) No surface oxygen vacancies (He et al, PRL 2009)
  19. 19. 19 DFT Calculations*: He et al. PRL 2009 Eact for diffusion (Nudged elastic band, 19 configurations) V1 -> V4: 0.74 eV V4 -> V1: 0.95 eV ‘Stiff surface’ (Vacancies move & stay below) *GGA-PBE, 216 atom slab, O=0, Formation energies w/respect to O2 in the gas phase. 5.40 eV 4.15 eV4.73 eV 3.69 eV 3.65 eV Cheng & Selloni, PRB 2009, J. Chem. Phys. 2009
  20. 20. Diffusion of Ovac’s into TiO2 anatase (STM images @78K) After electron irradiation @ 100 K STM @ 295 K Ph. Scheiber et al PRL 109 (2012) 136103 0 0.2 0.4 0.6 0.8 1 0 100 200 300 400 500 600 V O density(normalized) Annealing Temperature (K)
  21. 21. STM tip: Pulling O vacancies back to the surface +5.2 V, 0.7 nA +1.0 V, 0.1 nA (78 K) +1.2 V, 0.1 nA +0.9 V, 0.12 nA (6 K) 1111 1014 1115 2140 Setvin et al. Science, 341 (2013) 988
  22. 22. STM - tip: induced VO migration + VO -5.2V - -- - -- - - -- E Charges positively because of tip- induced band bending – reversal of energetics. Ballistic electrons – overcoming kinetic barriers 22D. B. Strukov et al., Nature 453, 80 (2008) J. O. Lee et al., Nat. Nanotechnology 8, 36 (2013) Memristor M. Setvin et al. Science, 341 (2013) 988 Phys. Rev. B. 91 (2015) 195403
  23. 23. Uli Aschauer, Annabella Selloni: 23 *Car-Parrinello, timestep of 5 au, fictitious electron mass 500 au First-Principles Molecular Dynamics at T = 200 K* ‘bridging peroxo’: O2 2- replaces lattice O Interaction of O2 with (subsurface) O vacancy (VO) in TiO2: Movie available: Supplement to DOI: 10.1126/science.1239879
  24. 24. Uli Aschauer, Annabella Selloni: 24 ‘bridging peroxo’: O2 2- replaces lattice O: Interaction of O2 with (subsurface) O vacancy (VO) in TiO2:
  25. 25. Uli Aschauer, Annabella Selloni: 25 ‘bridging peroxo’: O2 2- replaces lattice O: Interaction of O2 with (subsurface) O vacancy (VO) in TiO2: Experiment: VO + O2 → O2 2- bridging Vo+ O2 2- → O2 2- bridging Movie available: Supplement to DOI: 10.1126/science.1239879
  26. 26. Summary: How O2 adsorbs on TiO2 26 Rutile (110): Anatase (101): O2 + (VO)surface→ Oadatom (via fragile (O2)bridging intermediate) O2 2- +(VO)bulk→ O2 2- bridging Ph. Scheiber et al , Phys. Rev. Lett. 105 (2010) 216101 Phys. Rev. Lett. 109 (2012) 136103 M. Setvin et al. Science, 341 (2013) 988 Angew. Chem. Int. Ed. 53 (2014) 4714
  27. 27. 2nd Topic - Fe3O4(001): 28 M. Haruta, CatalysisToday 36 (1997)153 • How active are the smallest clusters? • Single atoms? • Sintering • Reaction mechanism
  28. 28. Magnetite Fe3O4 (001) Bulk: Inverse spinel structure, AB2O4 Tetrahedral Fe(A)3+ Octahedral Fe(B)2.5+ O2- (FCC sublattice) (001) (010) (100) 29
  29. 29. The Reconstructed Fe3O4(001) Surface In STM we “see” wavy rows of Fe(B) atoms Vsample = +1 V, Itunnel = 0.35 nA STM image of the clean surface Ar+ sputter + anneal 600 °C 1 nm 30 Vapor-deposited metal adsorbs as isolated atoms here
  30. 30. Bulk Termination: 31 R. Bliem, et al. Science 346 (2014) 1212 DFT+U, Ueff = 3.8 The Reconstructed Fe3O4(001) Surface
  31. 31. Maghemite (γ-Fe2O3) defect 32 The Reconstructed Fe3O4(001) Surface R. Bliem, et al. Science 346 (2014) 1212
  32. 32. Subsurface Cation Vacancy Structure 33DFT+U, Ueff = 3.8 The Reconstructed Fe3O4(001) Surface LEED-IV: SE > 11 000 eV, Rpendry = 0.125 R. Bliem, et al. Science 346 (2014) 1212
  33. 33. Adatom Adsorption • DFT+U: Au adsorbs strongly at one specific site 34 Subsurface Cation Vacancy Structure + Au Au(I), Eads = 2.03 eV (46 kcal/mol), DFT+U, Ueff = 3.8 R. Bliem, et al. Science 346 (2014) 1212
  34. 34. Au/Fe3O4(001) 35(30×30 nm2) Vsample = +1 V, Itunnel = 0.38 nA Isolated Au adatoms stable at room temperature nearest neighbour 8.4 Å no clusters STM image of 0.12 ML Au at room temperature (UHV) Z. Novotny, G. Argentero, Z. Wang, M. Schmid, U. Diebold, G. S. Parkinson Phys. Rev. Lett. 108, 216103 (2012)
  35. 35. 36 Single, Stable Ad-atoms on Fe3O4(001):
  36. 36. Pd/Fe3O4(001)
  37. 37. Pd/Fe3O4(001) 50 x 50 nm2; V=1.2 V ; I=0.3 nA; 0.2 ML Pd in adatoms, 0.2ML in clusters Adatoms Clusters Fuzzy Clusters (mobile) G.S.Parkinson et al. Nature Materials 12 (2013) 724
  38. 38. Cluster Coarsening • The growth of active catalyst material • A major cause of catalyst deactivation • Is often thermally driven • Can also be induced by gas molecules Michael Bowker, Nature Materials 1, 205 - 206 (2002)
  39. 39. Cluster Coarsening • The growth of active catalyst material • A major cause of catalyst deactivation • Is often thermally driven • Can also be induced by gas molecules Michael Bowker, Nature Materials 1, 205 - 206 (2002) Next: Follow aggregation of single atoms, step-by-step
  40. 40. STM Movie (50×50 nm2, +1 V, 0.2 nA) 78 frames, 5.33 hours 0.2 ML Pd/Fe3O4(001) sample in ultrahigh vacuum (p < 10-11 mbar) Room Temperature
  41. 41. Close Up… One Pd adatom becomes mobile Jumps from Pd to Pd across the surface (6.5×8.5 nm2, +1 V, 0.2 nA) G.S.Parkinson et al. Nature Materials 12 (2013) 724
  42. 42. PdFeoct O 2.02 Å DFT+U (Ueff= 3.8 eV for the Fe-3d states) Pd atom binds in a Fetet bulk continuation site, 2.2 eV binding energy Pd is similar to undercoordinated Fetet (2+) Magnetic moment 0.47 μB Fetet Calculations by Rukan Kosak and Peter Blaha, TU Wien Pd/Fe3O4(001) - Theroy G.S.Parkinson et al. Nature Materials 12 (2013) 724
  43. 43. CO-Pd/Fe3O4(001) – DFT+U Pd C O Pd-O bond broken 2.05 Å 1.86 Å 1.16 Å CO lifts Pd from surface Energy gain 1.6 eV Calculations by Rukan Kosak and Peter Blaha, TU Wien G.S.Parkinson et al. Nature Materials 12 (2013) 724
  44. 44. • One Pd spontaneously becomes mobile, starts to diffuse around… Fe3O4(001) Pd Adsorption landscape experienced by Pd atom. Pd trapped in deep well at “N” sites. 45
  45. 45. • One Pd spontaneously becomes mobile, starts to diffuse around… Fe3O4(001) CO 46 Pd Carbonyl is formed through adsorption of CO
  46. 46. • One Pd spontaneously becomes mobile, starts to diffuse around… 47 NOTE: This suggests the (Pd carbonyl) – Pd bond is not strong enough for cluster nucleation. “skyhook effect” S. Horch, et al. Nature 398, 134–136 (1999).
  47. 47. CO  Pd/Fe3O4(001) Pd clusters 270 pm Dose CO, 100 Langmuir (1.33x10-4 mbar.sec) OH
  48. 48. CO  Pd/Fe3O4(001) Dose CO, 100 Langmuir (1.33x10-4 mbar.sec) 270 pm x x x x x x x x x x x x x x x x x x x No more Pd adatoms. Large clusters and 270 pm features CO induces sintering OH Pd clusters
  49. 49. Now…since CO causes sintering, we can tune to an observable rate with the CO pressure and watch…
  50. 50. WATCH HERE
  51. 51. Selected Frames… CO induces mobility in the system Multiple Pd-CO needed to nucleate cluster Homogeneous nucleation Clusters grow through diffusion and coalescence “Large cluster” contains 19 Pd atoms (14×14 nm2, +1 V, 0.2 nA) G.S.Parkinson et al. Nature Materials 12 (2013) 724
  52. 52. Remember the red crosses? x x x x x x x x x x x x x x x x
  53. 53. 270 pm species (10×10 nm2, +1 V, 0.2 nA) The Role of Hydroxyls G.S.Parkinson et al. Nature Materials 12 (2013) 724
  54. 54. Summary Pd carbonyls are the mobile species PdCO temporarily trapped at stable Pd adatoms Homogenous cluster nucleation Growth through cluster diffusion and coalescence OH groups stabilize Pd adatoms against CO induced mobility G.S.Parkinson et al. Nature Materials 12 (2013) 724
  55. 55. 58 Single, Stable Ad-atoms on Fe3O4(001):
  56. 56. 59 Single, Stable Ad-atoms on Fe3O4(001):
  57. 57. M. Haruta, CatalysisToday 36 (1997)153 • How active are the smallest clusters? • Single atoms? • Sintering • Reaction mechanism
  58. 58. O O + CO CO - CO2 Oxygen vacancy 1) CO adsorption on Pt 2) Abstraction of O-lattice + 1/2 O2 Reparation of the surface 61 Mars-van Krevelen Mechanism
  59. 59. O O + CO CO - CO2 Oxygen vacancy 1) CO adsorption on Pt 2) Abstraction of O-lattice + 1/2 O2 PROX reaction* in excess H2 *Preferential Oxidation of CO in H2 steam 62 Reparation of the surface Mars-van Krevelen Mechanism Next: Follow each step individually For Pt supported on Fe3O4
  60. 60. Pt on Fe3O4(001), after heating to 550 K 1x10-7 mbar CO, 20 min • Holes in the surface • Clusters are located at edges • Not all clusters have holes R. Bliem, et al., Angew. Chem. Intl. Ed 54 (2015) DOI:10.1002/ ange.201507368
  61. 61. Pt on Fe3O4(001), after heating to 550 K 1x10-7 mbar CO, 20 min O Fe O Fe O Fe O Fe O Fe O Fe O C • Holes in the surface • Clusters are located at edges • Not all clusters have holes O Fe O Fe O Fe O Fe O Fe O Fe O C Fe O Fe O Fe O Fe O Fe O Fe CO2 R. Bliem, et al., Angew. Chem. Intl. Ed 54 (2015) DOI:10.1002/ ange.201507368
  62. 62. Pt on Fe3O4(001), after heating to 550 K 1x10-7 mbar O2, 10 min • Hillocks around each cluster Growth of Fe3O4: • O2 adsorption, dissociation, and spillover • Reaction with excess Fe from the bulk R. Bliem, et al., Angew. Chem. Intl. Ed 54 (2015) DOI:10.1002/ ange.201507368
  63. 63. Pt on Fe3O4(001), after heating to 550 K 1x10-7 mbar O2, 10 min Fe O O - O - O - Fe • Hillocks around each cluster Growth of Fe3O4: • O2 adsorption, dissociation, and spillover • Reaction with excess Fe from the bulk R. Bliem, et al., Angew. Chem. Intl. Ed 54 (2015) DOI:10.1002/ ange.201507368
  64. 64. O Fe O Fe O Fe O Fe Fe O Fe CO2 O O - O Fe O Fe O Fe Fe +CO +O2 etching re-growth p CO = 1x10-7 mbar, T = 550 K R. Bliem, et al., Angew. Chem. Intl. Ed 54 (2015) DOI:10.1002/ ange.201507368

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