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. King 1 1 Department of Chemistry, University of Cambridge, Cambridge CB2 1EW, UK 2 Instituto de Ciencia de Materiales (CSIC), Cantoblanco, E Madrid, Spain 3 Instituto 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] Water Adsorption on TiO 2 (110) surface 1 ML

Structure of H 2 O adsorption on TiO 2 Surface M.A. Henderson, Surf. Sci. 335, 151 (1996), by TPD, HREELS nearly perfect surface at 135 K O HH O HH O HH (dipole moment) O H undergoes structural distortions hydrogen bonding O H H Ti 4+ first layer second layer multilayer dipole repulsion ice structure (different from ice structure) lack of interaction O HH Ti 4+ defect site O H H 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 molecules were observed at low coverage. T. Mitsui, M. K. Rose, E. Fomin, D. F. Ogletree, M. Salmeron, Science, 297, 1850 (2002) Water + 40 K

The Random Walk of Water Molecule on 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: D 1 =2.3x10 -3 A 2 s -1 Dimer: D 2 =50.0A 2 s -1 Trimer: D 3 =1.02A 2 s -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. A solution of Schrödinger equation for the potential The wave function of a rigid rotator in the potential J. E. Muller, J. Harris, Phys., Rev. Lett., 53, 2493 (1984). H 2 O + Al(100)

Dependence on tilt angle, z 0 =3.9 br Dependence on z 0, tilt angle 0 or 90 deg.

Binding Energy Lowering due to Charge Donation and s-p Promotion For the on-top site adsorption. -like 3a 1 and -like 1b 1 When tilt angle is largest,is smallest. is largest, is smallest. When tilt angle And they give an equilibrium geometry with the H-O-H plane tilted 60 deg from the surface normal.

H 2 O + 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, Surface Sci. 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 H 2 O 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 1b 1 MO. Where is the most favorable binding sites: atop, bridge, or threefold site? What is the orientation of adsorption water 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, H 2 O lies nearly parallel to the surface: The tilt angle between the molecular dipole plane and the surface is, on the average 10 Deg, with a minimum value of 6 Deg on Ru, and a maximum value of 15 Deg on Cu. Vertical displacement of the atop site metal atom Lateral displacement of O from the precise atop site H-O-H bond angle H-O-H plane Tilt angle to the surface Next most stable 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 azimuthal preference for water, with different orientation within ~ 0.02eV of each other. So H 2 O monomer will be randomly distributed about surface normal. Free Azimuthal Rotation ! Adsorption Energy is mainly due to tilt angle ! Water molecule orientation

Partial density of states (PDOS) projected onto the p orbitals of O Upright H 2 O favor interaction with the 3a 1 orbital, Flat H 2 O favor interaction with the 1b 1 orbital, Initially, the 1b 1 is closer to the Fermi level, so orientations that 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 (D 2 O) adsorption on T = 20 K Monomer Small cluster molecules Tetramer Formation of bilayer structure -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) D 2 O on Ni (110), T =180 K H - bonded OD stretch region Dangling OD stretch bonds Absorption Peak Area IR-adsorption increases very quickly. Then, especially, when into the linear region. It enters And the intensity is believed to decreases proportional with. And The D-O-D plane must lie close to the surface. B. W. Callen, K. Griffiths, P. R. Norton, Phys. Rev. Lett. 66, 1634 (1991)

PPE Intensity (CPS) Original Annealed Surface 0.1 L 0.23 L 0.34 L 0.45 L x10 4 Normalized Plot 0.7 L Annealed Surface Water Adsorption (less than 1 ML) Hot Electron Final Energy (eV) TiO 2 Experiment T = 90 K

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) Hot Electron Final Energy (eV) Normalized Plot T = 90 K

H 2 O on e - irradiated H 2 O on annealed Simulation of workfunction change by H 2 O adsorption  =   : dielectric constant of free space e : electron charge  : dipole moment  : polarizability N : molecular density  : structure parameter (  9) k: sticky factor  = 0.48 D Fitting Curves H 2 O in Gas phase : D (CRC Handbook)

Future Research High Resolution ESDIAD Experiment of water adsorption on metal/oxide surfaces, especially at large angle. Similar Theoretical Approach of water adsorption on Oxide Surfaces, for example TiO 2, 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.