Surfaces of Metal Oxides

Oxide Surfaces at the Atomic Scale

Surfaces of Metal OxidesUlrike DieboldInstitute of Applied Physics, TU WienVienna, Austria

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

Surfaces of Metal Oxides:Ulrike DieboldInstitute of Applied Physics, TU WienVienna, AustriaUnderstanding fundamental mechanisms and processes at the atomic scale

&4TiO2U. 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

4

&5

Credits: Forschungszentrum Jlich

Kizuka et al., Phys. Rev. B 55, R7398 (1997)Main Tool: Scanning Tunneling Microscopy (STM)*

&in combination with Density Functional Theory6

Credits: Forschungszentrum Jlich

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)*

&

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

&81. Adsorption of Oxygen on TiO22. Single metal atoms on Fe3O4

&91. Adsorption of Oxygen on TiO2O2 does not stick to fully oxidized surface. Needs extra electrons (from oxygen vacancies, dopants, hydroxyls, etc.)Can assume many different configurationsOads, (O2-)ads , (O22-)ads , (O2)O

- Role of electric field

&10

Heterogeneous Catalysis (support & active catalyst)

Gas sensingBiocompatibility

Photocatalyst

M. Haruta, CatalysisToday 36 (1997)153 Adsorption of Oxygen on TiO2

M. Batzill and UD, Progr. Surf. Sci. 2005

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&

TiO2-based Photocatalysis: The Basic Mechanism

Linsebigler, Lu, Yates, Chem. Rev. 95 (1995) 735

Ohno et al. New J. Chem. 26 (2002) 1167Photocatalyst: mixture of TiO2 rutile and anataseRutile 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?

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12Rutile 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 VanderbiltPhys. 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?

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13TiO2- Color Scheme(Rutile Samples)

M. Li, et al., Journal of Physical Chemistry B 104 (20) (2000)4944Samples heated in furnace to different temperatures (Ar with 20 ppm O2~4x10-3 Torr)

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Diebold, Li, and SchmidAnnual Rev. Phys. Chem 2010

Ti5c

O2cVO

OH

[110][001]O3c

TiO2 Rutile (110): Model & Scanning Tunneling MicroscopyS.-C. Li, et al., JACS 130, 9080 (2008)14

Empty-states STM image

Vsample=+0.8 V, const. height, T=78 K

15TiO2-x + see groups of Henderson, Iwasawa, Bowker, Yates, Thornton, Besenbacher, Wendt, Dohnalek, Kummel, Selloni, Hammer, O2

O2

Wendt et al. Science 2007

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16Ph. Scheiber et al., Phys. Rev. Lett., 105 (2010) 216101

Vsample=+1.8 V, I = 0.03 nA, Tsample = 17 K

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Natural Mineral Sample fromHangarsvidda, 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)

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18(c)STM of cleaved* Anatase (101)

Anisotropic step edges(Gong et al, Nature Mater. 2006)

300 x 300 , 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)

18U

19DFT Calculations*:He et al. PRL 2009Eact for diffusion(Nudged elastic band, 19 configurations)

V1 -> V4: 0.74 eVV4 -> V1: 0.95 eVStiff surface(Vacancies move & stay below)*GGA-PBE, 216 atom slab, O=0, Formation energies w/respect to O2 in the gas phase.5.40 eV4.15 eV

4.73 eV

3.69 eV

3.65 eV

Cheng & Selloni, PRB 2009, J. Chem. Phys. 2009

19E bridge rutile = 3.68 eVEsubsurface rutile >4.5 eV

Diffusion of Ovacs into TiO2 anatase (STM images @78K)After electron irradiation @ 100 K

STM @ 295 KPh. Scheiber et al PRL 109 (2012) 136103

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Unit cell: 3.78 x 10.2 A 2 O2c per unit cell. 1 ML eq 5 O2c/nm2 Initial density: 0.8 O2/nm2 equals 0.15 ML

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)

1111101411152140Setvin et al. Science, 341 (2013) 988

Dopants (1% Nb)

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STM - tip: induced VO migration

+VO -5.2V

-

-

-

-

-

-

-

-

-

-

E

Charges positively because of tip-induced band bending reversal of energetics. Ballistic electrons overcoming kinetic barriers22

D. 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) 988Phys. Rev. B. 91 (2015) 195403

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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: O22- replaces lattice OInteraction of O2 with (subsurface) O vacancy (VO) in TiO2:Movie available: Supplement to DOI: 10.1126/science.1239879

Uli Aschauer, Annabella Selloni:24bridging peroxo: O22- replaces lattice O:Interaction of O2 with (subsurface) O vacancy (VO) in TiO2:

Uli Aschauer, Annabella Selloni:25bridging peroxo: O22- replaces lattice O:Interaction of O2 with (subsurface) O vacancy (VO) in TiO2:

Experiment:VO + O2 O22- bridging

Vo+ O22- O22- bridgingMovie available: Supplement to DOI: 10.1126/science.1239879

Summary: How O2 adsorbs on TiO226Rutile (110):

Anatase (101):

O2 + (VO)surface Oadatom

(via fragile (O2)bridging intermediate)O22- +(VO)bulk O22- bridging

Ph. Scheiber et al ,Phys. Rev. Lett. 105 (2010) 216101Phys. Rev. Lett. 109 (2012) 136103 M. Setvin et al. Science, 341 (2013) 988Angew. Chem. Int. Ed. 53 (2014) 4714

2nd Topic - Fe3O4(001):28

M. Haruta, CatalysisToday 36 (1997)153

How active are the smallest clusters?Single atoms?SinteringReaction mechanism

Magnetite Fe3O4 (001)Bulk: Inverse spinel structure, AB2O4

Tetrahedral Fe(A)3+Octahedral Fe(B)2.5+O2- (FCC sublattice)

(001)(010)(100)29

The Reconstructed Fe3O4(001) Surface

In STM we see wavy rows of Fe(B) atomsVsample=+1 V, Itunnel=0.35 nA STM image of the clean surface Ar+ sputter + anneal 600 C

1 nm

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Vapor-deposited metal adsorbs as isolated atoms here

30

Bulk Termination:

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

Maghemite (-Fe2O3) defect32The Reconstructed Fe3O4(001) SurfaceR. Bliem, et al. Science 346 (2014) 1212

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Subsurface Cation Vacancy Structure33DFT+U, Ueff = 3.8The Reconstructed Fe3O4(001) SurfaceLEED-IV: SE > 11 000 eV, Rpendry=0.125R. Bliem, et al. Science 346 (2014) 1212

Adatom AdsorptionDFT+U: Au adsorbs strongly at one specific site34

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

Au/Fe3O4(001)35

(3030 nm2) Vsample = +1 V, Itunnel = 0.38 nA

Isolated Au adatoms stable at room temperaturenearest neighbour 8.4 no clustersSTM image of 0.12 ML Auat room temperature (UHV)Z. Novotny, G. Argentero, Z. Wang, M. Schmid, U. Diebold, G. S. ParkinsonPhys. Rev. Lett. 108, 216103 (2012)

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

Catalytically active metals, Sr because its really different and H36

Pd/Fe3O4(001)

Pd/Fe3O4(001)

50 x 50 nm2; V=1.2 V ; I=0.3 nA; 0.2 ML Pd in adatoms, 0.2ML in clustersAdatoms

Clusters

Fuzzy Clusters(mobile)G.S.Parkinson et al. Nature Materials 12 (2013) 724

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Cluster CoarseningThe growth of active catalyst materialA major cause of catalyst deactivationIs often thermally drivenCan also be induced by gas molecules

Michael Bowker, Nature Materials 1, 205 - 206 (2002)

Cluster CoarseningThe growth of active catalyst materialA major cause of catalyst deactivationIs often thermally drivenCan also be induced by gas molecules

Michael Bowker, Nature Materials 1, 205 - 206 (2002)

Next: Follow aggregation of single atoms, step-by-step

STM Movie(5050 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

CloseUpOne Pd adatom becomes mobileJumps from Pd to Pd across the surface(6.58.5 nm2, +1 V, 0.2 nA) G.S.Parkinson et al. Nature Materials 12 (2013) 724

PdFeoctO2.02 DFT+U (Ueff= 3.8 eV for the Fe-3d states)Pd atom binds in a Fetet bulk continuation site, 2.2 eV binding energyPd is similar to undercoordinated Fetet (2+)Magnetic moment 0.47 B FetetCalculations by Rukan Kosak and Peter Blaha, TU WienPd/Fe3O4(001) - TheroyG.S.Parkinson et al. Nature Materials 12 (2013) 724

The density functional theory (DFT) based simulations utilize the augmented plane wave + local orbitals (APW+lo) method as embodied in the WIEN2k code(Ref.S1). We employ the generalized gradient approximation of Perdew et al. (Ref.S2) and treat the correlated Fe-3d electrons with a Hubbard U correction (GGA+U) and the double counting correction in the fully localized limit (Ref.S3). Specifically we employ an effective U value of 3.8 eV for the Fe-3d states. In the APW+lo method space is divided into spheres around the atoms and an interstitial region and we use atomic sphere radii of 1.86, 1.6, 2.1, 0.9 and 1.2 bohr for Fe, O, Pd, C and O (of CO), respectively. The plane wave cutoff is set to 20 Ry and the 2D Brillouin zone is sampled with a 2x2x1 k-mesh. We use a Fermi broadening of 0.08 eV.The Feoct-terminated Fe3O4 (001) surface is modeled using a symmetric slab with 17 layers and a 2x2 supercell (248 Fe and O atoms). In agreement with previous calculations(Ref.32,29) we find charge ordering of the sub-surface Feoct atoms with a characteristic dimerization leading to the well-known (2x2)R45 reconstruction of the clean surface. The larger supercell used in the present calculations allows for a better separation of the Pd (CO) ad atoms.

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CO-Pd/Fe3O4(001) DFT+UPdCOPd-O bond broken2.05 1.86 1.16 CO lifts Pd from surfaceEnergy gain 1.6 eVCalculations by Rukan Kosak and Peter Blaha, TU WienG.S.Parkinson et al. Nature Materials 12 (2013) 724

One Pd spontaneously becomes mobile, starts to diffuse aroundFe3O4(001)PdAdsorption landscape experienced by Pd atom.Pd trapped in deep well at N sites.45

One Pd spontaneously becomes mobile, starts to diffuse aroundFe3O4(001)CO46Pd Carbonyl is formed through adsorption of CO

One Pd spontaneously becomes mobile, starts to diffuse around47NOTE: This suggests the (Pd carbonyl) Pd bond is not strong enough for cluster nucleation.

skyhook effectS. Horch, et al. Nature 398, 134136 (1999).

CO Pd/Fe3O4(001)

Pd clusters

270 pmDose CO, 100 Langmuir (1.33x10-4 mbar.sec)

OH

CO Pd/Fe3O4(001)

Dose CO, 100 Langmuir (1.33x10-4 mbar.sec)

270 pmxxxxxxxxxxxxxxxxxxxNo more Pd adatoms.Large clusters and 270 pm featuresCO induces sinteringOHPd clusters

Nowsince CO causes sintering, we can tune to an observable rate with the CO pressure and watch

WATCH HERE

Selected Frames

CO induces mobility in the systemMultiple Pd-CO needed to nucleate clusterHomogeneous nucleationClusters grow through diffusion and coalescenceLarge cluster contains 19 Pd atoms

(1414 nm2, +1 V, 0.2 nA) G.S.Parkinson et al. Nature Materials 12 (2013) 724

Remember the red crosses?

xxxxxxxxxxxxxxxx

270 pm species(1010 nm2, +1 V, 0.2 nA) The Role of HydroxylsG.S.Parkinson et al. Nature Materials 12 (2013) 724

SummaryPd 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

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

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O

O

+ COCO

- CO2Oxygen vacancyCO adsorption on PtAbstraction of O-lattice

+ 1/2 O2Reparation of the surface61Mars-van Krevelen Mechanism

61

O

O

+ COCO

- CO2Oxygen vacancyCO adsorption on PtAbstraction of O-lattice

+ 1/2 O2PROX reaction*in excess H2*Preferential Oxidation of CO in H2 steam62Reparation of the surfaceMars-van Krevelen Mechanism

Next: Follow each step individually For Pt supported on Fe3O4

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Pt on Fe3O4(001), after heating to 550 K

1x10-7 mbar CO, 20 min

Holes in the surfaceClusters are located at edgesNot all clusters have holesR. Bliem, et al., Angew. Chem. Intl. Ed 54 (2015) DOI:10.1002/ ange.201507368


1x10-7 mbar CO, 20 min

OFeOFeOFeOFeOFeOFeOC=-Holes in the surfaceClusters are located at edgesNot all clusters have holesOFeOFeOFe

OFeOFeOFeOC=---Fe

OFeOFeOFeOFeOFeCO2R. Bliem, et al., Angew. Chem. Intl. Ed 54 (2015) DOI:10.1002/ ange.201507368


1x10-7 mbar O2, 10 minHillocks around each clusterGrowth of Fe3O4:O2 adsorption, dissociation, and spilloverReaction with excess Fe from the bulk

R. Bliem, et al., Angew. Chem. Intl. Ed 54 (2015) DOI:10.1002/ ange.201507368

Pt on Fe3O4(001), after heating to 550 K1x10-7 mbar O2, 10 minFe

O-O-

O-

O-FeHillocks around each clusterGrowth of Fe3O4:O2 adsorption, dissociation, and spilloverReaction with excess Fe from the bulk

R. Bliem, et al., Angew. Chem. Intl. Ed 54 (2015) DOI:10.1002/ ange.201507368

OFeOFeOFeOFeFeOFeCO2

O-O-OFeOFeOFeFe

+CO

+O2etchingre-growth

p CO = 1x10-7 mbar, T = 550 KR. Bliem, et al., Angew. Chem. Intl. Ed 54 (2015) DOI:10.1002/ ange.201507368