Surface Studies of Semiconducting Metal Oxides
or:
Oxides, STM, and DFT: A Happy Marriage
Ulrike Diebold Department of Physics, Tulane University, New Orleans, U.S.A.
Surfaces of Metal Oxides:
• Most metals are oxidized in the ambient. • Metal oxides exhibit an extremely wide variation in their chemical and physical properties (e.g., superconductors to best insulators) • Metal oxides are important for many applications. (And surfaces are critical in most of them.) • Surfaces of metal oxides are challenging (different phases, prepration /chemical potential dependent surfaces…) • Defects are important! – Local probes (STM) + DFT + spectroscopies
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1 TiO2 (mostly)
(some SnO2)
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Titanium - Oxygen Phase Diagram
4
2
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 type 2: 3-layer symmetrical type 3: two ! = 0, repeat unit -layer repeat unit dipole moment µ =0 ! = 0, ! ! 0, dipole moment µ =0 dipole moment µ ! 0
+ - + - - + - + - 2+ - + + - + - - + - + - - - + - + - 2+ + + - + - - - + - + - + very stable. stable. generally very unstable.
ZnO (0001): Rocksalt (100): CaF2 (111):
c
a
7 a = 3.25 Å c
TiO2 Rutile (110)
Ti O
[110]
[001]
8 [110]
4 Interest in TiO2
U. Diebold. “The Surface Science of Titanium Dioxide”, Surf. Sci. Rep. 48 (2003) 53 - 229 9
Interest in TiO2 120
Publications on TiO (110) 2
100 /year
80 300 Citations to U. Diebold, Surf. Sci. Rep. 2003
250 60 /year
200
40 150 Number of Publications Number 100
20 of Citations Number 50
0 0 1975 1980 1985 1990 1995 2000 2005 2010 2002 2003 2004 2005 2006 2007 2008 Year Year U. Diebold. “The Surface Science of Titanium Dioxide”, Surf. Sci. Rep. 48 (2003) 53 - 229 10
5 Optical Properties Biocompatibility (pigment in paints, (dental and bone implants) cosmetics, optical coatings, photonic material)
TiO2
Photo-active material (self-cleaning coatings, solar 1 µm cells, production of H ) Electronic properties 2 (gas sensing )
B ? Chemical properties (heterogeneous A catalysis, oxidation magnetic properties reactions at low temperatures) 11
Message # 1
• Metal oxides (particularly TiO2) are worth your while • For binary oxides: cut one Me -> O per O -> me bond.
12
6 Exception to Message # 1 (bond cutting):
• SnO2.
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SnO2 has the rutile (101) or (011) structure
(110) 1mm
(110)
Prof. R. Helbig (Erlangen)
M. Batzill, K. Katsiev, and U. D. , Surf. Sci. , 529/3 (2003) 295 M. Batzill, K. Katsiev, J.M. Burst, U. D., A. M. Chaka, and B. Delley, Phys. Rev. B, 72 (2005) 165414 14
7 Two surface terminations for SnO2(101):
O-terminated surface: Sn-terminated surface: 2+ O2- O2- Sn Sn4+
4+ 2- The surface maintains a Sn O2 Sn2+O2- stoichiometry at the stoichiometry surface.
Sn (5s25p2) has two stable oxidation states (+2,+4) SnO and SnO are stable 2 15
Tin terminated surface: Oxygen terminated surface: sputtered and annealed in UHV 10 mbar O2 at RT reversible
60 eV 106 eV
He+ ISS He+ ISS
16 M. Batzill, A.M. Chaka, U. Diebold, Europhysics Lett. 65 (1) (2004) 61
8 …much of the surface chemistry of metal oxide is defect-driven… V. Henrich, P.A. Cox, “The Surface Science of Metal Oxides” Cambridge University Press 1994
Surface Science: ‘The ‘Real World’: Oxygen Vacancies Step Edges Hydroxyls Impurities Impurities Step edges Hydroxyls Oxygen Vacancies Shear plances …. … 17
Relaxations? -- Charlton et al, PRL 78 (1997) 495 Ramamoorthy und Vanderbilt, Phys. Rev.Rutile B 49 (1994)16721 (110) Harrison et al. Faraday Discuss. 114 (1999) 305-312 - LEED study (Thornton et al.) -> DFTOxygen is correct Vacancy (Point Defect)
Ti O
[110]
[001]
18 [110]
9 Experimental: Oxygen Vacancies: UV Photoelectron Spectroscopy Changed Electronic Structure of the Valence Band Region 1 Band Gap State h" = 45 eV Clean Surface,
x annealed in UHV
u
Clearly Ti 3d- derived l 0.8
f
o
t e-
d
e h"
z 0.6
i
l
a
m
r
o
n 0.4
s
/
s
t
n
u 0.2 o Defect State
c
0
Rutile TiO2(110) 10 8 6 4 2 0 point defect Binding energy (eV) O(2) Ti(5) O(3)
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Scanning Tunneling Microscopy
• Annealing in ultrahigh vacuum creates missing 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 20
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2.5 ML H2O/TiO2(110) 163 TPD, m/e = 18 Adsorption of Water: Multilayers
175 Experiment: Molecular adsorption, c/s) 1 6 2nd layer H O (H-bonded to Obridging) dissociation only at defekts 2 Brinkley et al, Surf. Sci. 395 (1998) 292; Henderson, Langmuir 12 (1996) 5093; Hugenschmid et al. Surf. Sci. 302 (1994) 329 1st layer water (molecular at terraces) 265
m/e=18 QMS signal (x10 .5 Theory: mixed molecular/dissociative dissociated adsorption Adsorpti at O vacancies Lindan, Harrison, Gillan, Kresse, Noguera, on at Vogtenhuber, Vittadini, Pacchioni, Casarin, step 490 Fahmi & Minot, Ferris, Bredow, Jug, edges?332 Stefanovich & Truong, Langel, Nørskov,...
0
100 200 300 400 500 600 Temperature (K)
Henderson, Surf. Sci. 400 (1998) 203 21
Bikonda et al. Nature Materials 5 (2006) 176
Oxygen vacancies on TiO2: Water has a unity sticking coefficient [1] and dissociates at O vacancies => easily hydroxylated
Rutile TiO (110) 2 point defect O(2) Ti(5) O(3)
[1] T. Engel et al, Surf. Sci. 395 (1998) 292 22
11 Photoemission of Water Adsorption on TiO2(011)-2x1 at 110 K:
Difference Spectra h" = 26 eV h" = 47 eV
Band gap state increases and shifts with initial water adsorption
4 2 0 23 Di Valentin et al, JACS127 (2005) 9895; Beck et al, Surf. Sci. Lett. 591 (2005) L267
Electronic structure of O vacancies and OH groups
Hybrid B3LYP functionals describe localized gap state (C. Di Valentin, A. Selloni, G. Pacchioni, PRL 2006)
OH group Ganduglia-Pirovano, et al. Surf. Sci Rep. 2007
Oxygen vacancy
Di Valentin et.al. PRL 2006 submitted 24
12 Message # 2 Defects (O vacancies) are important, yet challenging
• Theorists: Mistrust the experimentalists • Experimentalists: Mistrust the theorists.
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Exception to Message # 2 (reduced surfaces are reactive)
• SnO2.
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13 Ultraviolet Photoemission Spectroscopy SnO2 Sn4+ h! = 34 eV
80
60
4[a.u.] Counts [a.u.] Counts 0 Sn-5s Temperature 2.6 eV 557373 K derived 667373 K surface 777373 K 20 state 2+ 887373 K Sn 997373 K
0 8 6 4 2 0 BBindinginding eenergynergy [[eV]eV]
27 M. Batzill, et al. Phys. Rev. B 72 (2005) 165414
h! = 34 eV Reduced SnO2 surface: 80
60 Is quite inert! Sn-5s derived surface state
4[a.u.] Counts 0 - Water physisorbs 2.6 eV 573 K M. Batzill, et al., Surf. Sci. 600 (2006) L29; J. 673 K Phys. C., 18 (2006) L129-L134 20 773 K 873 K 973 K -Benzene interacts very 0 8 6 4 2 0 weakly Binding energy [eV] M. Batzill et al., Applied Physics Letters, 85 Sn2+ (2004) 5766
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14 Water adsorption on oxidized and reduced SnO2(101) surfaces:
Undercoordinated Oxygen Fully coordinated Oxygen may act as Brønsted-base Sn2+ cations with sites=> dissociation of water chemically inert Sn- 5s lone pair.
DFT calculations by W. Bergermayer and I. Tanaka, University of Kyoto, Japan
E = -0.366 eV Eads = -0.989 eV Eads = -1.518 eV ads 29 M. Batzill, et al., Surf. Sci. 600 (2006) L29; J. Phys. C., 18 (2006) L129-L134
TiO2-Color Scheme
samples annealed in a furnace(Ar with 20 -3 ppm O2 ~4x10 Torr)
M. Li, et al., Journal of Physical Chemistry B 104 (20) (2000)4944 30
15 Defects in TiO2-x Rutile (110)
Tiinterstitial Ovac
[110]
[001]
31 [110]
Reduced Crystals - (1x2) Reconstruction
(1x2)strands
(1x1) with point defects
Added Ti2O3 rows - model for (1x2) reconstruction H. Onishi and Y. Iwasawa, Surf. Sci. 313, L783-L789 (1994).
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Message # 3
• Theorists: Think before you use ordered O vacancies to model reconstructions.
• Experimentalists: Don’t trust models with ordered vacancies.
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Re-oxidation of reduced TiO2 crystals:
18 O2 18 Ti O2
Tiinterstitial
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18 Pan, Maschhoff, Diebold, Madey, JVSTA 10 (1992) 2470 - 2476 37
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19 Reoxidation of reduced TiO2(110) single crystals: 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 at elevated temperatures can react with subsurface defects
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TiO2 -based Photocatalysis: TiO2: Applications and Promise
CFM Continental Fan titaniumart.com
• removal of organic pollutants, purifying of water or air
• self-cleaning/desinfecting coatings (bacteria, viruses, cancer cells)
• solar cells
•photocatalytic splitting of water, production of hydrogen
43 Sustainable Technologies International
TiO2-based Photocatalyst
Ohno et al. New J. Chem. 26 (2002) 1167
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22 How about anatase?
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TiO2 Anatase
(001)
(101) (011)
(101) (011)
Natural Mineral Sample from Calculated Wulff Shape Hangarsvidda, Norway M. Lazzeri, A. Vittadini and A. Selloni, Phys. Rev. B, 65 (2002) 119901/1, ibid. Phys. Rev. B, 63 (2001) 155409/1
U. Diebold et al. Catalysis Today (2003) 46
23 STM of TiO2 Anatase (101)
no evidence for point defects!
47 W. Hebenstreit, N. Ruzycki, G.S. Herman, and U.D., PRB Rapid Comm. 64 (24) (2000) R16334
H2O/Anatase(101): H2O/Rutile(110):
160 K 2.5 ML H2O/TiO2(110) D O Exposure 163 2 TPD, m/e = 18 2 14 Multilayers (molecules/cm, x 10 )
c/s) 175 0 (bkgd) 6 1.7 1 3.4 bridging . units) 2nd layer H2O (H-bonded to O ) 5.1 ca. 1 H2O/Ti(5)
arb 6.8 190 K 8.5 1st layer water 10.2 (molecular at 11.9 terraces) 13.6 15.3 265 .5m/e=18 QMS signal (x10 250 K
m/e = 20 Intensity ( dissociated Adsorpt at O ion at vacancies490 step 332 edges? 0
100 200 300 400 500 100 200 300 400 500 600 Temperature (K) Temperature (K) Henderson, Surf. Sci. 400 (1998) 203 G.S. Herman, et al. J. Phys. Chem.B 107 (2003) 2788; 48 A. Selloni, et al, Surf Sci 402-404 (1998), 219
24 STM of H2O/TiO2 Anatase:
coming soon!
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Lessons - Surfaces of Metal Oxides
(mainly TiO2):
TiO2 and other oxides: non-polarity of surfaces is good guidance for predicting surface structure. Consider chemical potential (gas phase)
Defects are important: Oxygen vacancies (challenging, theoretically and experimentally) Hydroxyls Bulk - surface interplay Step edges at oxides - interesting area of future research
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