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1

Surface Studies of Semiconducting MetalOxides

Ulrike DieboldDepartment of Physics, Tulane University, New Orleans, U.S.A.

diebold@tulane.edu

or:

Oxides, STM, and DFT:A Happy Marriage

2

Surfaces of Metal Oxides:

• Most metals are oxidized in the ambient.• Metal oxides exhibit an extremely wide

variation in their chemical and physicalproperties (e.g., superconductors to best insulators)

• Metal oxides are important for manyapplications. (And surfaces are critical in most ofthem.)

• Surfaces of metal oxides are challenging(different phases, prepration /chemical potentialdependent surfaces…)

• Defects are important!– Local probes (STM) + DFT + spectroscopies

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3

TiO2 (mostly)

(some SnO2)

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Titanium - Oxygen Phase Diagram

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7

Stability of Polar Surfaces Electrostatic Considerations:

(P.W. Tasker, J. Phys. C 12 (1979) 4977 More recent and improved: Noguera, Jackowski)

type 1: One-layer repeat unit

! = 0,

dipole moment µ =0

+ - + -

+ - + -

+ - + -

+ - + -

+ - + -

+ - + -

+ - + -

type 2: 3-layer symmetrical

repeat unit

! = 0,

dipole moment µ =0

-

2+

--

2+

-

-

+

-+

-

+

type 3: two

-layer repeat unit

! ! 0,

dipole moment µ ! 0

very stable. stable.

generally very unstable.

Rocksalt (100):CaF2 (111):

a

c

a = 3.25 Å

c

ZnO (0001):

8

[110]

[110]

[001]

TiO2 Rutile (110)

OTi

5

9

Interest in TiO2

U. Diebold. “The Surface Science of Titanium Dioxide”, Surf. Sci. Rep. 48 (2003) 53 - 229

10U. Diebold. “The Surface Science of Titanium Dioxide”, Surf. Sci. Rep. 48 (2003) 53 - 229

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40

60

80

100

120

1975 1980 1985 1990 1995 2000 2005 2010

Publications on TiO2(110)

Num

ber o

f P

ublications

Year

/year

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50

100

150

200

250

300

2002 2003 2004 2005 2006 2007 2008

Citations to U. Diebold, Surf. Sci. Rep. 2003

Num

ber

of

Citations

Year

/year

Interest in TiO2

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TiO2

Optical Properties(pigment in paints,cosmetics, opticalcoatings,photonic material)

Biocompatibility(dental and bone implants)

A

BChemical properties(heterogeneouscatalysis, oxidationreactions at lowtemperatures)

Photo-active material(self-cleaning coatings, solarcells,production of H2)

magnetic properties

?

Electronic properties(gas sensing )

1 µm

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Message # 1

• Metal oxides (particularly TiO2) areworth your while

• For binary oxides: cut one Me -> Oper O -> me bond.

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13

Exception to Message # 1(bond cutting):

• SnO2.

14

1mm

(110)

Prof. R. Helbig (Erlangen)

SnO2 has the rutilestructure

(101) or (011)

(110)

M. Batzill, K. Katsiev, and U. D. , Surf. Sci. , 529/3 (2003) 295M. Batzill, K. Katsiev, J.M. Burst, U. D., A. M. Chaka, and B. Delley,Phys. Rev. B, 72 (2005) 165414

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15

Two surface terminations for SnO2(101):

O2-

Sn4+

The surface maintains a Sn4+O22-

stoichiometry

O-terminated surface:

Sn (5s25p2) has two stable oxidation states (+2,+4)

SnO2 and SnO are stable

Sn-terminated surface:

O2- Sn2+

Sn2+O2- stoichiometry at thesurface.

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Oxygen terminated surface:10 mbar O2 at RT

Tin terminated surface:sputtered and annealed in UHV

60 eV106 eV

reversible

He+ ISSHe+ ISS

M. Batzill, A.M. Chaka, U. Diebold, Europhysics Lett. 65 (1) (2004) 61

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…much of the surface chemistry of metal oxide isdefect-driven…

V. Henrich, P.A. Cox, “The Surface Science of Metal Oxides” CambridgeUniversity Press 1994

Surface Science:

Oxygen VacanciesHydroxyls

Impurities

Step edges

Shear plances

…

‘The ‘Real World’:

Step Edges

ImpuritiesHydroxyls Oxygen Vacancies

….

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[110]

[110]

[001]

Rutile (110)

Oxygen Vacancy (Point Defect)

OTi

Relaxations? -- Charlton et al, PRL 78 (1997) 495Ramamoorthy und Vanderbilt, Phys. Rev. B 49 (1994)16721Harrison et al. Faraday Discuss. 114 (1999) 305-312- LEED study (Thornton et al.) -> DFT is correct

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Rutile TiO2(110)point defect

Ti(5)O(3)

O(2)

Oxygen Vacancies: Changed Electronic StructureBand Gap StateClearly Ti 3d- derived

UV Photoelectron Spectroscopy of the Valence Band Region

e-

h"

Experimental:

0

0.2

0.4

0.6

0.8

1

0246810

count s

/s nor m

alize

d to flux

Binding energy (eV)

Clean Surface,annealed in UHV

Defect State

h" = 45 eV

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Scanning Tunneling Microscopy

• Annealing in ultrahigh vacuum createsmissing oxygen atoms

• Point defects very reactive

• 5 - 10% Ovac, then reconstructions

U.D., J.F. Anderson, K.-O. Ng, D. Vanderbilt, PRL 77 (1996) 1322

• The image contrast in empty-states

STM is reversed to the

morphological topography

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Adsorption of Water:

Experiment:Molecular adsorption,dissociation only at defektsBrinkley et al, Surf. Sci. 395 (1998) 292;Henderson, Langmuir 12 (1996) 5093;Hugenschmid et al. Surf. Sci. 302 (1994)329

600500400300200100

Temperature (K)

1

.5

0

2.5 ML H2O/TiO2(110)TPD, m/e = 18

490

265

163

332

175

m/e

=1

8 Q

MS

sig

na

l (x

10

6c/s

)

Multilayers

2nd layer H2O (H-bonded to Obridging)

1st layer water(molecular at

terraces)

dissociatedat O vacanciesAdsorpti

on at

stepedges?

Henderson, Surf. Sci. 400 (1998) 203

Theory:mixed molecular/dissociativeadsorptionLindan, Harrison, Gillan, Kresse, Noguera,Vogtenhuber, Vittadini, Pacchioni, Casarin,Fahmi & Minot, Ferris, Bredow, Jug,Stefanovich & Truong, Langel, Nørskov,...

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Rutile TiO2(110) point defect

Ti(5) O(3)

O(2)

Bikonda et al. Nature Materials 5 (2006) 176

Oxygen vacancies on TiO2:Water has a unity stickingcoefficient [1] anddissociates at O vacancies=> easily hydroxylated

[1] T. Engel et al, Surf. Sci.395 (1998) 292

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Photoemission of Water Adsorption on TiO2(011)-2x1 at 110 K:

h" = 26 eVDifference Spectra

h" = 47 eV

2 04

Band gap state increases andshifts with initial wateradsorption

Di Valentin et al, JACS127 (2005) 9895; Beck et al, Surf. Sci. Lett. 591 (2005) L267

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Electronic structure of O vacancies and OH groups

Hybrid B3LYP functionals describelocalized gap state (C. Di Valentin, A. Selloni,

G. Pacchioni, PRL 2006)

Di Valentin et.al. PRL 2006 submitted

Oxygen vacancy

OH group

Ganduglia-Pirovano, et al.Surf. Sci Rep. 2007

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Message # 2Defects (O vacancies) areimportant, yet challenging

• Theorists: Mistrust theexperimentalists

• Experimentalists: Mistrust thetheorists.

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Exception to Message # 2(reduced surfaces are reactive)

• SnO2.

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8 6 4 2 00

20

40

60

80

2.6 eV

h! = 34 eV

Counts

[a.u

.]

Binding energy [eV]

573 K 673 K 773 K 873 K 973 K

8 6 4 2 00

20

40

60

80

2.6 eV

h! = 34 eV

Counts

[a.u

.]

Binding energy [eV]

573 K 673 K 773 K 873 K 973 K

8 6 4 2 00

20

40

60

80

2.6 eV

h! = 34 eV

Counts

[a.u

.]

Binding energy [eV]

573 K 673 K 773 K 873 K 973 K

8 6 4 2 00

20

40

60

80

2.6 eV

h! = 34 eV

Counts

[a.u

.]

Binding energy [eV]

573 K 673 K 773 K 873 K 973 K

Temperature

Ultraviolet Photoemission Spectroscopy

M. Batzill, et al. Phys. Rev. B 72 (2005) 165414

Sn-5s

derived

surface

state

8 6 4 2 00

20

40

60

80

2.6 eV

h! = 34 eV

Counts

[a.u

.]

Binding energy [eV]

573 K 673 K 773 K 873 K 973 K

Sn4+

Sn2+

SnO2

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8 6 4 2 00

20

40

60

80

2.6 eV

h! = 34 eV

Counts

[a.u

.]

Binding energy [eV]

573 K 673 K 773 K 873 K 973 K

Sn-5s derived

surface state

Sn2+

Reduced SnO2 surface:

Is quite inert!

- Water physisorbsM. Batzill, et al., Surf. Sci. 600 (2006) L29; J.Phys. C., 18 (2006) L129-L134

-Benzene interacts veryweaklyM. Batzill et al., Applied Physics Letters, 85(2004) 5766

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29Eads = -1.518 eV

Undercoordinated Oxygen

may act as Brønsted-base

sites=> dissociation of water

Water adsorption on oxidized and reduced SnO2(101) surfaces:

Fully coordinated Oxygen

Sn2+ cations with

chemically inert Sn-

5s lone pair.

DFT calculations by W. Bergermayer and I. Tanaka,

University of Kyoto, Japan

Eads = -0.989 eV Eads = -0.366 eV

M. Batzill, et al., Surf. Sci. 600 (2006) L29; J. Phys. C., 18 (2006) L129-L134

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TiO2-Color Scheme

M. Li, et al., Journal of Physical Chemistry B 104 (20) (2000)4944

samples annealed in afurnace(Ar with 20ppm O2 ~4x10-3 Torr)

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[110]

[110]

[001]

Defects in TiO2-x Rutile (110)

Tiinterstitial

Ovac

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Reduced Crystals - (1x2) Reconstruction

(1x1) with point defects

(1x2)strands

Added Ti2O3 rows - model for (1x2)reconstructionH. Onishi and Y. Iwasawa, Surf. Sci. 313,L783-L789 (1994).

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Message # 3

• Theorists: Think before you useordered O vacancies to modelreconstructions.

• Experimentalists: Don’t trust modelswith ordered vacancies.

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Ti18O2

Tiinterstitial

18O2

Re-oxidation of reduced TiO2 crystals:

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37Pan, Maschhoff, Diebold, Madey, JVSTA 10 (1992) 2470 - 2476

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Reoxidation of reduced TiO2(110) singlecrystals: cycles (1x1) -> (1x2) structure

R.A. Bennett et al, Phys. Rev. Lett. 82 (1999) 3831

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Kinetics of re-oxidation:LEEM (see Gary Kellog et al.)

Consequence:Adsorption of reactive species atelevated temperatures can reactwith subsurface defects

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TiO2:

CFM Continental Fan

• removal of organic pollutants, purifying of water or air

titaniumart.com

• self-cleaning/desinfecting coatings (bacteria, viruses, cancer cells)

Sustainable Technologies International

• solar cells

•photocatalytic splitting ofwater, production of hydrogen

TiO2 -based Photocatalysis: Applications and Promise

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Ohno et al. New J. Chem. 26 (2002) 1167

TiO2-based Photocatalyst

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How about anatase?

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TiO2 Anatase

(011)

(011)(101)

(101)

(001)

Calculated Wulff ShapeM. Lazzeri, A. Vittadini and A. Selloni, Phys. Rev. B,65 (2002) 119901/1, ibid. Phys. Rev. B, 63 (2001)155409/1

Natural Mineral Sample fromHangarsvidda, Norway

U. Diebold et al. Catalysis Today (2003)

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STM of TiO2 Anatase (101)

no evidence for point defects!

W. Hebenstreit, N. Ruzycki, G.S. Herman, and U.D., PRB Rapid Comm. 64 (24) (2000) R16334

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H2O/Rutile(110):

600500400300200100

Temperature (K)

1

.5

0

2.5 ML H2O/TiO2(110)TPD, m/e = 18

490

265

163

332

175

m/e

=1

8 Q

MS

sig

na

l (x

10

6c/s

)

Multilayers

2nd layer H2O (H-bonded to Obridging)

1st layer water(molecular at

terraces)

dissociatedat O

vacancies

Adsorpt

ion atstep

edges?

Henderson, Surf. Sci. 400 (1998) 203

m/e

= 2

0 I

nte

nsity (

arb

. u

nits)

500400300200100

Temperature (K)

D2O Exposure

(molecules/cm2, x 10

14)

0 (bkgd)

1.73.45.16.88.5

10.211.913.615.3

250 K

190 K

160 K

H2O/Anatase(101):

G.S. Herman, et al. J. Phys. Chem.B 107 (2003) 2788;A. Selloni, et al, Surf Sci 402-404 (1998), 219

ca. 1 H2O/Ti(5)

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STM of H2O/TiO2 Anatase:

coming soon!

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Lessons - Surfaces of Metal Oxides

(mainly TiO2):

Defects are important:

Oxygen vacancies (challenging, theoretically andexperimentally)

Hydroxyls

Bulk - surface interplay

Step edges at oxides - interesting area of future research

TiO2 and other oxides: non-polarity of surfaces is good

guidance for predicting surface structure. Consider chemical

potential (gas phase)