Transcript Slide 1
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 2Instituto of Chemistry, University of Cambridge, Cambridge CB2 1EW, UK 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 Water Adsorption on TiO2 (110) surface [110] 5 .9 A 6 .5 A [110] 2 1 ML 5.2010 (cm ) 14 Structure of H2O adsorption on TiO2 Surface ice structure multilayer undergoes structural distortions H H O H O H H O H second layer (different from ice structure) hydrogen bonding lack of interaction dipole repulsion H H H O Ti4+ H O Ti4+ H H H O Ti4+ defect site H O Ti4+ H O first layer (dipole moment) nearly perfect surface at 135 K M.A. Henderson, Surf. Sci. 335, 151 (1996), by TPD, HREELS Interest Questions to Ask 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 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 + 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 He-I Irradiation D. W. Turner, C. Baker, A. D. Baker, and C. R. Brundle, Molecular Photoelectron Spectroscopy (Wiley-Interscience, London, 1970) Previous Study of Water Adsorption on Metal Surface H2O + Al(100) vlm( z0 , , ) v ( z0 )lm ( )eim v ( z0 ) A solution of Schrödinger equation for the potential V ( z0 ) Eb ( z0 , 60 ) lm ( ) The wave function of a rigid rotator in the potential V ( ) Eb ( z0 3.9, ) Water Molecule on a 9-atom cluster simulating the local environment of an Al(100) on-top site. J. E. Muller, J. Harris, Phys., Rev. Lett., 53, 2493 (1984). ( , z0 ) d r r , z ( r ) 3 0 z ( , z0 ) w [1 a ( z0 )]cos ct ( z0 ) y ( , z0 ) w [1 a ( z0 )]sin 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 [ L] 3 p | V | L /( 3 p L ) [3s] 3 p | V | 3s /( 3 p 3s ) -like 3a 1 and -like 1b1 For the on-top site adsorption. 0 When tilt angle [3a1 ] is largest, When tilt angle [1b1 ] 90 is largest, [1b1 ] is smallest. [3a1 ] is smallest. And they give an equilibrium geometry with the H-O-H plane tilted 60 deg from the surface normal. Comprehensive Study of H2O + Ni(100) (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 (3) Binding sites ---- It is not atop site ! (2) O-M Bond Results in Current Paper 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 binding sites: atop, bridge, or threefold site? What is the orientation of adsorption water ----- tilt angle of H-O-H plane? 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 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 H2O 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 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 orientations that maximize this interaction will be preferred ----- Flat! Image dipole moment favor upright orientation 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. [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 Tetramer Formation of bilayer structure Small cluster molecules Monomer -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 0.5ML H - bonded OD stretch region Dangling OD stretch bonds 0.5 1.0ML IR-adsorption increases very quickly. Absorption Peak Area Then, especially, when 2.0ML It enters into the linear region. And the intensity is believed to decreases proportional with cos . 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) TiO2 Experiment 0.45 L Annealed Surface Water Adsorption (less than 1 ML) 2PPE Intensity (CPS) 6000 T = 90 K 0.34 L 4000 0.23 L Original Annealed Surface 0.1 L 2000 0 3.5 4.0 1.0 4.5 5.0 5.5 6.0 6.5 0.7 L Normalized Plot 0.8 x10 4 0.6 0.4 0.2 0.0 3.5 4.0 4.5 5.0 Hot Electron Final Energy (eV) 5.5 6.0 6.5 5000 T = 90 K 0.23L 0.45 L 0.56 L 0.34 L 4000 3000 Original Electron Irradiation Surface Water Adsorption (less than 1 ML) 0.1 L 0.68 L 2000 0.8 L 1000 0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5000 5.6 5.8 6.0 6.2 6.4 6.2 6.4 Normalized Plot 4000 3000 2000 1000 0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 Hot Electron Final Energy (eV) 5.8 6.0 Simulation of workfunction change by H2O adsorption + + + + + - - - - - Workfunction change (eV) +++++++++++ - - - - - - - - - - - D = N 0e (1 exp( kx)) 9 0 1 N 0 (1 exp( kx)) 4 3 0.5 0 : dielectric constant of free space e : electron charge : dipole moment : polarizability N : molecular density : structure parameter ( 9) k: sticky factor 2 Fitting Curves 0.0 = 0.48 D -0.5 H2O in Gas phase : 1.854 D (CRC Handbook) H2O on e- irradiated -1.0 H2O on annealed -1.5 0 1 2 3 Dosage (L) 4 5 6 cos1 (0.48/ 1.854) 75 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 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.