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

1. Surfaces of Metal Oxides
Ulrike Diebold
Institute of Applied Physics, TU Wien
Vienna, Austria

2. DFT Collaborations: Annabella Selloni (Princeton U.), Peter Blaha (TU Wien)

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
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
Credits: Forschungszentrum Jülich
Kizuka et al.,
Phys. Rev. B 55, R7398 (1
Main Tool:
Scanning Tunneling Microscopy (STM)*

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. &
CREDIT:
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*Flat, large single crystals. Ultrahigh Vacuum (10-11 mbar), low temperature (6K - 400 K)

8. &
8
1. Adsorption of Oxygen on TiO2
2. Single metal atoms on Fe3O4

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
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. &
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
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
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. 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
TiO2-x +
… see groups of Henderson, Iwasawa, Bowker, Yates, Thornton, Besenbacher,
Wendt, Dohnalek, Kummel, Selloni, Hammer, …
O2O2
Wendt et al. Science 2007

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
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
(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
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. 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. 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. 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. 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. Uli Aschauer, Annabella Selloni:
24
‘bridging peroxo’:
O2
2- replaces lattice O:
Interaction of O2 with (subsurface) O vacancy (VO) in TiO2:

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. 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

28. 2nd Topic - Fe3O4(001):
28
M. Haruta, CatalysisToday 36 (1997)153
• How active are the
smallest clusters?
• Single atoms?
• Sintering
• Reaction mechanism

29. Magnetite Fe3O4 (001)
Bulk: Inverse spinel structure, AB2O4
Tetrahedral Fe(A)3+
Octahedral Fe(B)2.5+
O2- (FCC sublattice)
(001)
(010)
(100)
29

30. 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

31. Bulk Termination:
31
R. Bliem, et al. Science 346 (2014) 1212
DFT+U, Ueff = 3.8
The Reconstructed Fe3O4(001) Surface

32. Maghemite (γ-Fe2O3) defect
32
The Reconstructed Fe3O4(001) Surface
R. Bliem, et al. Science 346 (2014) 1212

33. 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

34. 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

35. 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)

36. 36
Single, Stable Ad-atoms on Fe3O4(001):

37. Pd/Fe3O4(001)

38. 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

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)

40. 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

41. 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

42. 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

43. 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

44. 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

45. • 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

46. • One Pd spontaneously becomes mobile, starts
to diffuse around…
Fe3O4(001)
CO
46
Pd Carbonyl is formed through adsorption of CO

47. • 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).

48. CO  Pd/Fe3O4(001)
Pd clusters
270 pm
Dose CO, 100 Langmuir (1.33x10-4 mbar.sec)
OH

49. 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

50. Now…since CO causes sintering, we
can tune to an observable rate with
the CO pressure and watch…

52. WATCH
HERE

53. 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

54. Remember the red crosses?
x
x x
x
x
x
x
x x
x
x
x x
x
x
x

55. 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

56. 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

58. 58
Single, Stable Ad-atoms on Fe3O4(001):

59. 59
Single, Stable Ad-atoms on Fe3O4(001):

60. M. Haruta, CatalysisToday 36 (1997)153
• How active are the
smallest clusters?
• Single atoms?
• Sintering
• Reaction mechanism

61. 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

62. 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

63. 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

64. 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

65. 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

66. 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

67. 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