General Model for Water Monomer Adsorption on Close-Packed Transition and Noble Metal Surfaces

A. Michaelides,1 V. A. Ranea,2,3 P. L. de Andres,2 and D. A. King1

1Department of Chemistry, University of Cambridge, Cambridge CB2 1EW, UK

2Instituto de Ciencia de Materiales (CSIC), Cantoblanco, E-28049 Madrid, Spain

3Instituto de Investigaciones Fisicoquimicas Teoricas y Aplicadas (CONICET, UNLP, CICPBA)

Sucursal 4, Casilla de Correo 16 (1900) La Plata, Argentina

Presented by Bin LI

April 16, 2004

[001] direction

[110]

[110]

Water Adsorption on TiO2 (110) surface

A9.5

A5.6

)(1020.5 214 cm1 ML

Structure of H2O adsorption on TiO2 Surface

M.A. Henderson, Surf. Sci. 335, 151 (1996), by TPD, HREELS

nearly perfect surface at 135 K

OHH

OHH

OHH

(dipole moment)OH

undergoes structural distortions

hydrogen bonding

OHH

Ti4+ Ti4+ Ti4+

first layer

second layer

multilayer

dipole repulsion

ice structure

(different from ice structure)

lack of interaction

OHH

Ti4+

defect site

O HH O H

H

1. Intermolecular Hydrogen Bond of Adsorption Water

2. Molecule – Substrate bonding Strength

3. Water Cluster Size (Monomer, Dimer, Trimer and so on …)

4. Binding Sites and Orientation of Water Molecular Dipole Plane

5. Diffusion Properties of Different Cluster Size

6. The Key factors that determine the wetting properties of materials

Interest Questions to Ask

Condition for Isolated Water Molecule (Monomer) Adsorption

Adsorption at Low Temperature and under Low Coverage.

(A) to (B): Two monomers join to form a dimer

(C): Dimer diffuses rapidly, STM tip producing a streak

(D): Dimer encounters a third monomer and forms a trimer

(E): Trimer approaching a pair of nearby monomers

(F): Pentamer formation by collision

At 40 K, mostly isolated water moleculeswere observed at low coverage.

T. Mitsui, M. K. Rose, E. Fomin, D. F. Ogletree, M. Salmeron, Science, 297, 1850 (2002)

Water + Pd(111) @ 40 K

The Random Walk of Water Molecule on Pd(111) @ 52.4 K

STM tip tracks a water Molecule, then gives a trajectory.

The Scanning Parameters:

150 pA, -100 mV, 18 nm x 18 nm

Mobilities of different clusters

Monomer: D1=2.3x10-3A2s-1

Dimer: D2=50.0A2s-1

Trimer: D3=1.02A2s-1

T. Mitsui, M. K. Rose, E. Fomin, D. F. Ogletree, M. Salmeron, Science, 297, 1850 (2002)

Stabilization of Water Clusters on Pd (111)

Small clusters encounter other molecules forming larger clusters.

Hexagonal clusters are stable and grow into the honeycomb Island structures.

The Scanning Parameters:

(A) to (C) 100 pA, 120 mV

(D) 100 pA, 80 mV

Image size: 9 nm x 9 nm

T. Mitsui, M. K. Rose, E. Fomin, D. F. Ogletree, M. Salmeron, Science, 297, 1850 (2002)

Molecular Orbital Energy Level Diagram of Gas-Phase Water

P. A. Thiel and T. E. Madey,S. Sci. Report, 7, 211 (1987)

Photoemission Spectrum of Gas-phase Water

D. W. Turner, C. Baker, A. D. Baker, and C. R. Brundle, Molecular Photoelectron Spectroscopy (Wiley-Interscience, London, 1970)

He-I Irradiation

Previous Study of Water Adsorption on Metal Surface

Water Molecule on a 9-atom cluster simulating the local environment of an Al(100) on-top site.

imlmvvlm ezz )()(),,( 00

A solution of Schrödingerequation for the potential

The wave function of a rigid rotator in the potential

)( 0zv

)( lm

J. E. Muller, J. Harris, Phys.,Rev. Lett., 53, 2493 (1984).

)60,()( 00 zEzV b

),9.3()( 0 zEV b

H2O + Al(100)

sin)](1[),(

)(cos)](1[),(

)(),(

00

000

,3

0 0

zaz

zzaz

rrrdz

wy

ctwz

z

Dependence on tilt angle, z0=3.9 br

Dependence on z0, tilt angle 0 or 90 deg.

Binding Energy Lowering due to Charge Donation and s-p Promotion

)/(||3][ 3 LpLVpL

)/(3||3]3[ 33 spsVps

For the on-top site adsorption.

-like 3a1 and -like 1b1

When tilt angle 0]3[ 1a

]3[ 1a]1[ 1b

]1[ 1bis largest, is smallest.

is largest, is smallest.

When tilt angle 90

And they give an equilibrium geometry with the H-O-H plane tilted 60 deg fromthe surface normal.

H2O + Ni(100)

Comprehensive Study of

(1) H-O-H bond angle

(2) Binding sites

H. Yang, J. L. Whitten, S. Sci., 223, 131 (1989).

(3) Geometries of Adsorption water on the surface

(4) Tilt angle of dipole plane

a) H. Yang, J. L. Whitten, J. Chem. Phys. 91, 126 (1989)

b) Hartree-Fock calculation by M. Dupuis, in P. A. Thiel and T. E. Madey, Surface Sci. Rept. 91, 126 (1989)

c) Koopmans’ theorem values

d) Self-consistent-field solution (SCF) and Configuration integration calculation (CI)

e) C. Nobl and C. Benndorf, SurfaceSci. 182, 499 (1987)

Hydroxyl group + Ni(100)

(1) –OH Bond (2) O-M Bond

(3) Binding sites ---- It is not atop site !

Here, the authors present the results of a density functional theory (DFT) study of H2O monomer adsorption on a variety of metal substrates. The total energy calculations within the DFT framework were performed with the CASTEP code [1]. Ultrasoft pseudopotentials were expanded within a plane wave basis set with a cutoff energy of 340 eV. Exchange and correlation effects were described by the Perdew - Wang generalized gradient approximation (GGA). [2] A p(2 x 2) unit cell was employed and a single water molecule was placed on one side of the slab. Monkhorst - Pack meshes within the surface brillouin zone was used. Water mixes with the surface mainly through its occupied 1b1MO.

Where is the most favorable bindingsites: atop, bridge, or threefold site?

What is the orientation of adsorptionwater ----- tilt angle of H-O-H plane?

Results in Current Paper

From this extensive set of DFT calculations for various metal surfaces, they find:

On every surface, the favored adsorption site for water is the atop site.

At this site, H2O lies nearly parallel to the surface: The tilt angle betweenthe molecular dipole plane and the surface is, on the average 10 Deg, witha minimum value of 6 Deg on Ru, and a maximum value of 15 Deg on Cu.

Vertical displacement ofthe atop site metal atom

Lateral displacement of Ofrom the precise atop site

H-O-Hbond angle

H-O-H planeTilt angle to the surface

Next moststable site

Water molecule doesn’t sit on the atop site up-right, the molecular dipole plane has a very large tilt angle from the surface normal!

How about the azimuthal angle? ----- There tends not to be a clear azimuthalpreference for water, with different orientation within ~ 0.02eV of each other. So H2O monomer will be randomly distributed about surface normal.

Free Azimuthal Rotation !

Adsorption Energy is mainly due to tilt angle !

Water moleculeorientation

Partial density of states (PDOS) projected onto the p orbitals of O

Upright H2O favor interaction with the 3a1 orbital, Flat H2O favor interaction with the 1b1 orbital, Initially, the 1b1 is closer to the Fermi level, so orientationsthat maximize this interaction will be preferred ----- Flat!

Besides the covalent interaction, the interaction between the water permanent dipole and its image beneath the surface also has to be considered.

Using Mulliken analysis, it shows that the perpendicular configuration is favored over the parallel configuration by 0.05 eV and 0.02 eV on Pt and Ag, respectively.

Although it is a competing interaction with the covalent interaction, it is small, so it is not decisive.

Image dipole moment favor upright orientation

[1] M. C. Payne, M. P. Teter, D. C. Allan, T. A. Arias and J. D. Joannopoulus, Rev. Mod. Phys. 64, 1045 (1992)

[2] J. P. Perdew, J. A. Chevary, S. H. Vosko, K. A. Jackson, M. R. Pederson, D. J. Sigh and C. Fiolhais, Phys. Rev. B 46, 6671 (1992)

Experiment Results Infrared reflection absorption spectroscopy (IRAS)

Water molecules (D2O) adsorption on Ru(0001) @ T = 20 K

Monomer

Small clustermolecules

Tetramer

Formationof bilayerstructure

-OD stretching modes

M. Nakamura, M. Ito, Chem. Phys. Lett. 325, 293 (2000)

Experiment Results Fourier-transform IR-reflection-absorption spectroscopy IR-radiation angle is 82 Deg (FTIR-RAS)

Chemisorbed c(2 x 2) D2O on Ni (110), T =180 K

ML5.0H - bonded OD stretch region

Dangling OD stretch bonds

Absorption Peak Area

ML0.15.0 IR-adsorption increases very quickly.

Then, especially, when

ML0.2into the linear region.

It enters

And the intensity is believed todecreases proportional with

cos . And The D-O-D planemust lie close to the surface.

B. W. Callen, K. Griffiths, P. R. Norton,Phys. Rev. Lett. 66, 1634 (1991)

6000

4000

2000

0

2PP

E I

nte

nsi

ty (

CP

S)

6.56.05.55.04.54.03.5

Original Annealed Surface

0.1 L

0.23 L

0.34 L

0.45 L

6.56.05.55.04.54.03.5

1.0

0.8

0.6

0.4

0.2

0.0

x104

Normalized Plot0.7 L

Annealed Surface Water Adsorption (less than 1 ML)

Hot Electron Final Energy (eV)

TiO2 Experiment

T = 90 K

5000

4000

3000

2000

1000

0

6.46.26.05.85.65.45.25.04.84.64.44.2

Original

0.1 L

0.23L

0.34 L

0.45 L 0.56 L

0.68 L

0.8 L

Electron Irradiation Surface Water Adsorption (less than 1 ML)

5000

4000

3000

2000

1000

0

6.46.26.05.85.65.45.25.04.84.64.44.2Hot Electron Final Energy (eV)

Normalized Plot

T = 90 K

-1.5

-1.0

-0.5

0.0

0.5

Wo

rkfu

nct

ion

ch

ang

e (e

V)

6543210Dosage (L)

H2O on e- irradiated

H2O on annealed

Simulation of workfunction change by H2O adsorption

+-

+-

+-

+-

+-

+-

+-

+-

+-

+-

+-

+-

+-

+-

+-

+-

))exp(1(49

1

))exp(1(

23

00

0

kxN

kxeN

=

: dielectric constant of free spacee : electron charge : dipole moment : polarizabilityN : molecular density: structure parameter ( 9)k: sticky factor

= 0.48 D

Fitting Curves

H2O in Gas phase : 1.854 D (CRC Handbook)

75)854.1/48.0(cos 1

Future Research

High Resolution ESDIAD Experiment of water adsorption on metal/oxidesurfaces, especially at large angle.

Similar Theoretical Approach of water adsorption on Oxide Surfaces, for example TiO2, especially, including the unoccupied LUMO states due to hybridization with substrates.

Simulation of other small molecules adsorption on different substrates, their possible surface geometric configurations and energy levels.