The Reading GroupThe Reading Group

Georg HeldGeorg Held

Water-Metal InterfaceWater-Metal Interface Chiral Surface SystemsChiral Surface Systems

The Reading MONET Team

Tugce

Andrey

Chemical interaction takes place at a length scale < 1nm→ chemical composition→ molecular orientation

Surface relaxations / reconstructions: < 0.1 nm→ electronic structure

Internal structure, crystallinity of ‘nano-objects’: < 0.1 nm→ electronic, magnetic properties

There is Plenty of Room at the Bottom of the Nano-Scale!

Low Energy Electron Diffraction (LEED)

Nobel prize 1937 for C.J. Davisson: Proof that electrons behave like waves (together with Germer).

Electron energy 30-300 eV (wavelength around 1 Å).

Electrons penetrate about 10 Å into the surface.

Elastically scattered electrons are detected at the fluorescent screen.

Low Energy Electron Diffraction

LEED pattern

Information about the long-range structure.

LEED-IV analysis

Information about the local surface geometry.

H2O

D2O

LEED-IV Analysis

CLEED Program Package

Water – Ru InterfaceWater – Ru Interface Heterogeneous catalysis:Heterogeneous catalysis:

reactant, product, intermediate.reactant, product, intermediate. Electrochemistry:Electrochemistry:

Fuel Cells.Fuel Cells.

Water-water hydrogen Water-water hydrogen bonding competes with water-bonding competes with water-metal bond.metal bond.

Small energy difference Small energy difference between intact and partially between intact and partially dissociated water.dissociated water.

Water molecules in ice

Every water molecule is involved in 4 hydrogen bonds.

Hexagonal bilayer structure.

Ice Ih

Water on Ru{0001}:Feibelman’s Model

Problem: No geometry found by DFT

with intact coplanar water molecules as found by LEED. (Held & Menzel Surf. Sci. 316 (1995))

Ice-like bilayer would not wet Ru{0001} surface.

Ice clusters are more stable. Solution: Partially

dissociated bilayer. Overlayer consists of

H2O, OH, and H. Positions of O and Ru atoms

agree with those from LEED. All hydrogen bonds parallel

to surface. Dissociation barrier ~0.5eV.

Bilayer modelDoering & MadeySurf. Sci. 123 (1982)

Partially dissociated bilayer

Feibelman, Science 295 (2002).Michaelides et al. JACS 125 (2003).

X-ray Photoelectron Spectroscopy

Core levels: element specific binding

energies (BE). chemical shifts in BE

depending on chemical environment (molecular species, adsorption site).

Surface sensitive (electron energy < 1000 eV)

Quantitative for high Ekin. Synchrotron XPS:

Photon energy tunablefor high cross section.

XPS: Ekin = hv – BE – Φ

XPS XPS

H2O on Ru{0001}:Temperature Programmed XPS

535 534 533 532 531 530 529 528Binding Energy [eV]

250 K

170 K

130 K

1 ML H2O adsorbed at 110 K

Heating rate 0.1K/s(1.5K / spectrum)

Sharp transition from low T phase to partially dissociated bilayer around 150K.

H2O OH

O

170K

130K

OH

H2O

H2O

Beam damage

528529530531532533534535536

Binding Energy (eV)

528529530531532533534535536

Binding Energy (eV)

0.14e/mol

0.05e/mol

Beam damage

Experiments at MAX-lab (Lund), beamline 311:

Relatively large X-ray spot on surface (0.3 x 2 mm2).

Photon flux ~ 1.2 x 1013 ph s-1 cm-2 electron flux ~ 1.5 x 1012 e s-1 cm-2.

Shortest spectra correspond to0.01 e per molecule.

Low T phase very beam sensitive:spectrum changes after irradiation with ~0.1 e / molecule

Partially diss. bilayer less sensitive: no changes up to several e / molecule.

Andersson et al. PRL 93 (2004)Faradzhev et al. CPL 415 (2005)

528529530531532533534535536

Binding Energy (eV)

528529530531532533534535536

Binding Energy (eV)

0.14e/mol

0.05e/mol

Beam damage

Experiments at MAX-lab (Sweden), beamline 311:

Relatively large X-ray spot on surface (0.3 x 2 mm2).

Photon flux ~ 1.2 x 1013 ph s-1 cm-2 electron flux ~ 1.5 x 1012 e s-1 cm-2.

Shortest spectra correspond to 0.01 e per molecule.

Low T phase very beam sensitive:spectrum changes after irradiation with ~0.1 e / molecule

Partially diss. bilayer less sensitive: no changes up to several e / molecule.

Andersson et al. PRL 93 (2004)Faradzhev et al. CPL 415 (2005)

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0.22

0.24

0.26

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4

electrons/molecule

OH

pe

ak (

ML

)

D2O H2O

0.45ML H2O bilayer 0.67ML H2O bilayer

155K

100K

H2O on Ru{0001}: Thermodynamic Considerations

Two configurations of water

Metastable intact water layer• Adsorption energy similar to sublimation energy of ice.• Clusters or 2D-layer.

Partially dissociated layer• Most stable configuration.

•Barriers for desorption and dissociation are similar.

Surface composition determined by kinetics rather than equilibrium thermodynamics.

Michaelides et al. JACS 125 (2003)Meng et al. CPL 402 (2005)

H2O

DFT: Desorption

H2O+OH

Intact H2O

En

erg

y (e

V/m

ol) ~0.5

H2O

~0.3

0.53

Dissociation

Break O-H bond

Break H2O-surface bondand hydrogen bonds

H2O coadsorbed with oxygen

H2O adsorption on O-precovered surface: H2O + Oad 2 OHad (disproportionation)

Clay et al. CPL 388 (2004)

H2O + Oad HOH—Oad (H-bonding)Doering & Madey Surf. Sci. 123 (1982)

Oad Oad

OH H

Run Oad

OH H

Run Oad

HO H

H2O coadsorbed with oxygen: TP-XPS

0.5 ML O (> 0.25ML): High BE O1s peak 180-220K No OH formation

0.1 ML O (< 0.20ML): Oat peak converts to OH H2O + OH disappear at

200K

OatOat

H2O

H2O

OH

200K

220K

180K

Low O coverage High O coverage

Gladys, GH, et al. Chem. Phys. Lett. 414 (2005) 311

H2O coadsorbed with oxygen

0.5 ML O (> 0.25ML): High BE O1s peak 180-220K Stronger bond than H2O-Ru. Non-recombinative desorption

0.1 ML O (< 0.20ML): Oat + H2O 2OH below

140K Recombinative desorption:

2OH Oat + H2O at 200K

Gladys, GH, et al. Chem. Phys. Lett. 414 (2005) 311

528529530531532533534535Binding Energy (eV)

230K

140K

185K

0.1ML O

OH

O

H2O

528529530531532533534535536

Binding Energy (eV)

140K

195K

210K

O

OHH2O

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

140 160 180 200 220 240Temperature (K)

Co

vera

ge

(ML

) H2O

H2O

+OH

OH

Oat

H2O coadsorbed with oxygen

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

140 160 180 200 220 240Temperature (K)

Con

cent

ratio

n (M

L)

H2O(1)

H2O(2)

Oat

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

140 160 180 200 220 240Temperature (K)

Con

cent

ratio

n (M

L)

H2O(1)

H2O(2)

Oat

0.1ML O 0.5ML O

Plot XPS intensity (~ coverage) of each adsorbate species vs temperature.

H2O+OH

d/d

T(

ML

)

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

Temperature (K)

140 160 180 200 220 240

H2O(2)

H2O(1)

d/d

T(

ML

)

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

Temperature (K)

140 160 180 200 220 240

H2O(2)

H2O(1)

0

0.02

0.04

0.06

0.08

0.1

0.12

140 160 180 200 220 240Temperature (K)

d/d

T (

ML

)

H2O coadsorbed with oxygen

Differentiate = Desorption Rate: Approx. TPD spectra. Good agreement with published TPD spectra (Doering & Madey Surf. Sci. 123, 1982) Non-recombinative and recombinative desorption at similar temperatures.

220K

180K

(Doering & Madey Surf. Sci. 123, 1982)

0.1ML O 0.5ML OH2O+OH

H2O coadsorbed with 0.5ML oxygen: NEXAFS

Big difference in angle dependence of NEXAFS spectra:

Large differences between normal (N.I.) and grazing incidence (70º) spectra for PDB (H2O + OH).Hydrogen bonds parallel to surface.

N.I. and 70º spectra more similar for H2O on 0.5ML O.Hydrogen bonds tilted.

- 0 . 2

0

0 . 2

0 . 4

0 . 6

0 . 8

1

1 . 2

5 2 5 5 3 0 5 3 5 5 4 0 5 4 5 5 5 0

P h o t o n E n e r g y [ e V ]

- 0 . 2

0 . 2

0 . 6

1

1 . 4

1 . 8

2 . 2

2 . 6

3

5 2 5 5 3 0 5 3 5 5 4 0 5 4 5 5 5 0

P h o t o n E n e r g y [ e V ]

H 2 O + 0 . 5 M L Oa n n . t o 1 8 5 K

H 2 O + O HN . I .

7 0 º

N . I .

7 0 ºO

O OH H

O

OH H

O

O OH H

O

OH H

Is Ru the exception or the rule?Is Ru the exception or the rule?

Ru (4d) hcp lattice, a(0001) = 2.71Å

Pd (4d)fcc lattice, a(111) = 2.75Å

Ir (5d) fcc lattice, a(111) = 2.71Å

Ru Rh Pd

Os Ir Pthcp

fcchcp

fcc2.71Å

fcc2.69Å 2.75Å

2.77Å2.71Åfcc

2.74Å

Water adsorption on hexagonal surfaces Water adsorption on hexagonal surfaces of Pt group metals with similar lattice constants,of Pt group metals with similar lattice constants,

using the same method (XPS)using the same method (XPS)

H2O + O on Pd{111}

O coverage up to 0.25 ML (p(2x2)-O overlayer).

100K: No reaction between H2O and O.

160K: Reaction between H2O and O: mixed H2O+OH layer(O coverages up to 0.25 ML)

p(√3 x√3) LEED pattern. [H2O] : [OH] not stoichiometric.

Desorption between 175-180K. 0.00

0.10

0.20

0.30

0.40

0.50

0.60

526527528529530531532533534535536537538

Binding Energy (eV)

Pd_Background

OH

H2O

H2O ads. at 160 K

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

526527528529530531532533534535536537538

Binding Energy (eV)

Pd_Background

Oad

H2O

H2O ads. at 100 K

H2O OH

O

Pd 3p3/2

H2O on Pd{111} surface oxide

Higher O coverage (~ 0.67ML O): p(√6 x√6) surface oxide.

No dissociation of H2O (170K). No stabilisation: desorption ~ 180K.

67.5 eV

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

527528529530531532533534535536

STM: Lundgren et al. PRL 88 (2002) 246103

O1s

0.000.050.100.150.200.250.300.350.400.450.50

527528529530531532533534535536537

Binding Energy (eV)

H2O

Oat

OH

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

527528529530531532533534535536537

Binding Energy (eV)

H2O(Monolayer)

H2O(Multilayer)

H2O + O on Ir{111}

O coverage up to 0.25 ML (p(2x2)-O overlayer).

100K: No reaction between H2O and O.

170K: O-induced partial dissociation of H2O:mixed O + OH + H2O layer.

Amount of atomic O unchanged.

[OH] : [H2O] = 0.4ML : 0.5ML

H2O ads. at 170 K

H2O ads. at 100 K

H2O OHO

Reactivity of Water on Reactivity of Water on O-covered Ru{0001}O-covered Ru{0001}

Low O coverage (< 0.25 ML):Low O coverage (< 0.25 ML): Mixed (HMixed (H22O + OH) layerO + OH) layer Temperatures around 140K.Temperatures around 140K. Ru, Pd, Pt.Ru, Pd, Pt. Ir{111}: atomic O not part of the reaction.Ir{111}: atomic O not part of the reaction.

High O coverage (> 0.25 ML):High O coverage (> 0.25 ML): No dissociation of HNo dissociation of H22O.O. Stabilisation of HStabilisation of H22O through hydrogen bonds.O through hydrogen bonds. Pd{111}: no stabilisation on oxidised surface.Pd{111}: no stabilisation on oxidised surface.

Desorption temperatures similar for Desorption temperatures similar for dissociative and intact adsorption.dissociative and intact adsorption.

Acknowledgement

Cambridge: Mick Gladys, Ali El Zein

Lund: Jesper Andersen, Anders Mikkelsen.

Sandia Albuquerque: Peter Feibelman

Andrey’s ProjectAndrey’s Project

Modified Metal / Oxide SurfacesModified Metal / Oxide Surfaces Effect of oxygen on water dissociationEffect of oxygen on water dissociation

Growth of ‘thick’ ice layers:Growth of ‘thick’ ice layers: Adsorption on iceAdsorption on ice Mesoscopic structure (nanostructures, porous Mesoscopic structure (nanostructures, porous

ice).ice).

Metal interface with aqueous solutionsMetal interface with aqueous solutions Alcohols (fuel cells)Alcohols (fuel cells) Fatty acids, Amino acids Fatty acids, Amino acids

(biological systems).(biological systems).

Experimental Methods:Experimental Methods: Low-energy Electron DiffractionLow-energy Electron Diffraction Photoelectron SpectroscopyPhotoelectron Spectroscopy NEXAFSNEXAFS

Chiral Systems

Molecular Recognition at SurfacesMolecular Recognition at Surfaces Enantioselectivity / Enantiospecificity Enantioselectivity / Enantiospecificity

requires multiple adsorbate-surface requires multiple adsorbate-surface bonds/interaction.bonds/interaction.

Lock and key effects.Lock and key effects. Depends on geometry of the Depends on geometry of the

adsorption complexadsorption complex

Chiral Adsorbates / ReactantsChiral Adsorbates / Reactants Amino acids (Alanine)Amino acids (Alanine)

Chiral SubstratesChiral Substrates Non-symmetric surface planes: {531}Non-symmetric surface planes: {531}

Geometry of the adsorption complex:LEED, STM

NEXAFSXPS, RAIRS

DFT

Intrinsically Chiral Surfaces: fcc{531} No mirror plane. High Miller indices, h k l 0.

Templates for enantio-selective adsorption or heterogeneous catalysis.(e.g. G. Attard J. Phys. Chem. B 103, 1381)

{531} has smallest unit cell of all chiral fcc surfaces.Highest density of low (6-fold) coordinated kink atoms.

R(D) surface if {111}–{100}–{110} facets clockwise.(McFadden, Gellman, et al. Langmuir 12, 2483). 111

110

100

R (D)

111111110

100

R (D)

Cu{531}R and Cu{531}S

Cu{531}R Cu{531}S

Intrinsically Chiral Surfaces: fcc{531} No mirror plane. High Miller indices, h k l 0.

Templates for enantio-selective adsorption or heterogeneous catalysis.(e.g. G. Attard J. Phys. Chem. B 103, 1381)

{531} has smallest unit cell of all chiral fcc surfaces.Highest density of low (6-fold) coordinated kink atoms.

R(D) surface if {111}–{100}–{110} facets clockwise.(McFadden, Gellman, et al. Langmuir 12, 2483). 111

110

100

R (D)

111111110

100

R (D)

(STM: Driver et al. in preparation)

Pt{531} – thermal instability

Pt{531}

200 Å

Almost no energy cost involved in the creation of adatom-vacancy pairs, but gain in entropy.

Kinked surfaces are unstable.(Power et al. Langmuir 18, 3737)

Zero spot intensities over large energy ranges (~100 eV). (Different from close packed surfaces.)

Can only be explained by high degree of surface roughness (interference between atoms from different layers).

Pt{531} – LEED IV curves

Puisto et al. Phys. Rev. Lett. 95 (2005) 036102.Puisto et al. J. Phys. Chem. B 109 (2005) 22456.

ExperimentTheory flat surfaceTheory rough surface

Surface Structure of Pt{531}

Alternating contraction and expansion of inter-layer spacings.

Large gap between 4th and 5th layer

Lateral shifts of surface atoms between 0.06 and 0.10 Å .

top view

bulk : 0.66 Å

0.44 Å (-)0.70 Å (+)0.50 Å (-)0.94 Å (+)

(0.53 Å) (0.54 Å)

(0.73 Å)

(0.78 Å)

0.56 Å (-)

(0.66 Å)

side view

Puisto et al. J. Phys. Chem. B 109 (2005) 22456.

(DFT)

Alanine on chiral Cu{531} surfaces

Cu{531} more stable than Pt{531}. Two ways of matching alaninate

‘footprint’: {110} and {311} facets. Asymmetric C not involved in bonding. Enantioselective adsorption?

LEED: R/S-Alanine on Cu{531}R/S

Alanine adsorbed at 300K, annealed to 390K.

Sharp (1x4) LEED pattern (good long-range order) for S/{531}R and R/{531}S.

Diffuse superstructure but still (1x4) for R/{531}R and S/{531}S

{531}R

{531}S

23 eV

S-Alanine

S-Alanine R-Alanine

R-Alanine

{531}S

{531}R

XPS: R/S-Alanine on Cu{531}R

C 1s and N 1s spectra identical (intensity and peak positions/shape) for both enantiomers.

Identical peak position for O 1s but intensity difference of about 15%.

Low kinetic energy: different photoelectron diffraction effects due to different local geometries.

C 1s, N 1s and O 1s peak positions identical (within 0.1 eV) to alaninate on Cu{110}:

Adsorbed as alaninate Surbstrate bond through two O and N.

hv = 630 eV

Williams et al. Surf Sci. 368 (1996) 303 (RAIRS)Barlow et al. Surf. Sci. 590 (2005) 243 (RAIRS, XPS, STM)Rankin & Scholl Surf. Sci. 548 (2004) 301 (DFT)Jones et al. Surf. Sci. 600 (2006) 1924 (NEXAFS, XPS, DFT)

1s *

CO forbidden

allowed

Near Edge X-ray Absorption Fine Structure

XPS: Ekin = hv – BE – ΦNEXAFS: hv = BEocc - BEunocc

NEXAFS Excitation into unoccupied

molecular orbitals near the Fermi level.

Needs tunable light source(Synchrotron).

Cross section depends on: Polarisation of X-rays. Symmetry of orbitals. Molecular Orientation.

XPS

NEXAFS

(3x2) Alanine on Cu{110}

Molecular orientation from NEXAFSusing dipole selection rules:

O-C-O in-plane tilt angle. E.g -resonance for Alanine on

Cu{110} disappears almost completely when E parallel to [1-10] (close packed rows).

Intensity of -resonance ~ cos2( = angle between E and normal of O-C-O)

E

Jones et al. Surf. Sci. 600 (2006) 1924

O1s(C1s) *

not allowed if

E parallel to O-C-O triangle.

0.0

0.2

0.4

0.6

0.8

1.0

0 30 60 90Angle of E

Inte

ns

ity

[a

rb. u

nit

s]

S-Alanine

R-Alanine

π-Resonance

In-plane NEXAFS of R/S-Alanine on Cu{531}R

0º

Ē Rotate E within the surface plane

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

280 285 290 295 300 305 310

Photon Energy [eV]

Inte

nsi

ty [a

rb. u

nits

]

R-Alanine

90º

0º

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

280 285 290 295 300 305 310Photon Energy [eV]

Inte

nsity

[arb

. un

its]

S-Alanine

90º

0º

90º

Large difference between R and S-alanine (ca. factor 2). -intensity does not go to zero.

ñ1

ñ2

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

-60 -30 0 30 60 90 120 150

Angle of E

Inte

ns

ity

[a

rb. u

nit

s]

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

-60 -30 0 30 60 90 120 150

Angle of E

Inte

ns

ity

[a

rb. u

nit

s]

Alanine on Cu{531}R: single or multiple adsorption sites?

Single adsorption site

ñ

Multiple adsorption sites

I = Iocos2()

I = I1cos2() + I2cos2()

Large oscillations - intensity goes to zero

Small oscillations -intensity does not go to zero

R/S-Alanine on Cu{531}: Fit to Data Fit results compatible with adsorption

on {311} (1~ 25º) and {110} (2~ - 45º) facets:

S-Alanine: 1= 23º, 2= - 57º, equal amounts. distorted molecules on {110} facetts.

Ambiguous result for R-Alanine: 1= 5º, 2= - 55º, equal amounts; 1= 29º, 2= - 39º, I311 : I110 = 0.5.

All kink-sites can be involved in adsorbate bond in p(1x4) superstructure.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

-60 -30 0 30 60 90 120 150

Angle of E

Inte

ns

ity

[a

rb. u

nit

s]

S-Alanine

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

-60 -30 0 30 60 90 120 150

Angle of E

Inte

ns

ity

[a

rb. u

nit

s]

R-Alaninemodel 1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

-60 -30 0 30 60 90 120 150

Angle of E In

ten

sit

y [

arb

. un

its

]

R-Alaninemodel 2

R/S-Alanine on Cu{531}: Local geometries

{110}

{311}

S-Alaninate: distorted molecules on {110} facetts.Θ110 = Θ311

R-Alaninate (model 1): distorted molecules on {110} and {311} facets;Θ110 = Θ311

Hydrogen bonds between molecules(O-N, O-O ~ 2.5 Å).

Distortion of molecules induced by interaction with metal atoms (?).

Surplus of molecules on {110} facets would be compatible with diffuse LEED pattern.

R-Alaninate (model 2): distortion on {110} facets relaxed;Θ110 > Θ311

Enantiospecific Adsorption ofEnantiospecific Adsorption ofAlanine (Alaninate) on Cu{531}Alanine (Alaninate) on Cu{531}

Different degrees of long-range order (LEED):Different degrees of long-range order (LEED): better order for S-Ala/Cu{531}better order for S-Ala/Cu{531}R R (sharper LEED spots).(sharper LEED spots).

Two adsorption sites occupied by R and S-Ala Two adsorption sites occupied by R and S-Ala (NEXAFS):(NEXAFS):triangular footprints on {311} and {110} facets.triangular footprints on {311} and {110} facets.

Possibly Possibly differentdifferentoccupation numbersoccupation numbers of of{311} and {110} sites:{311} and {110} sites:R-Ala R-Ala ΘΘ110110 > > ΘΘ311311

S-Ala S-Ala ΘΘ110110 = = ΘΘ311311

Different molecularDifferent moleculardistortions:distortions: induced by intermolecular induced by intermolecular hydrogen bonding and/or hydrogen bonding and/or interaction with substrate.interaction with substrate.

Acknowledgement

Mick J. GladysAmy V. Stevens,

Nicola ScottJaspreet S. Ottal

Glenn JonesUniversity of Cambridge

David Batchelor , Berlin

Amy V. Stevens

Mick J. Gladys

Nicola Scott

Tugce’s ProjectTugce’s Project

Lock and key effects / Lock and key effects / EnantioselectivityEnantioselectivityon Catalyst Surfaceson Catalyst Surfaces

Enantioselective Enantioselective heterogeneous Catalysis.heterogeneous Catalysis.

Chiral Adsorbates / Chiral Adsorbates / ReactantsReactants Amino acidsAmino acids

Chiral SubstratesChiral Substrates Non-symmetric surface planesNon-symmetric surface planes Enantioselective Enantioselective

adsorption/reactionsadsorption/reactions