a molecular picture of water structure and dynamics from … · 2009-04-08 · a molecular picture...
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E. G. Wang
A molecular picture of water structure and dynamics from computer simulation
Enge WangInstitute of Physics (CAS)
Supported byNational Natural Science Foundation of China
Ministry of Science and TechnologyChinese Academy of Sciences
Understanding the nature of O-H bonds is the key issue in the study of water and energy.
E. G. Wang
Outline
Water on metal surface - Energetics and Kinetics- Hydrophilic and hydrophobic behavior
Water on silica surface - Tessellat ion ice
NaCl in w ater - Dissolution and Nucleation
Water solid surface: unexpectedly cold- Proton ordering
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Why ice surface is slippery?
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Free Water ClustersGregory et al. Science 275, 814 (1997)
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Water Adsorbate on Carbon Nanotubes
Maiti et al., PRL 87, 155502 (2001)
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Water in confined system
Koga et al., Nature 412, 802 (2001)
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Water on surfaceH2O/MgO H2O/Ru
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Water on metal (Pt, Pd, Ru, Rh, Au) surfaces I:
Energetics and Kinetics
With Sheng Meng & Shiwu GaoPRL 2002, 2003; PRB 2004; CPL 2005
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H2O/Pt(111)
• Adsorption energy on top atom:~300 meV
• Flat on surface (13-14 °), freely rotates on the surface
• Rotational barrier:140~190meV• Charge transfer from O to Pt:0.02e
Top HollowBridge
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304 meVSmall
Clusters
433 meV
359 meV
520 meV
H-bond: 450 meV (adsorbed dimer) >>250 meV (free dimer)
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The 1D water chains at a <110>/{100} step on the Pt (322) surface.
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Water bilayer/Pt(111)
Morgenstern et al., PRL 77, 703 (1996)
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Adsorbed H-up and H-down bilayer with √3 × √3R30°(RT3) reconstruction
* H-up and H-down close in energy, 522 and 534 meV, whereas half-dissociated layer can be ruled out: Eads/molecule =291 meV;
* Two non-equivalent H bonds;
H-up
H-down
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Vibrational spectra
0 100 200 300 400 5000.00
0.02
0.04 H-down bilayer
438424
384
202196
91
6957
34166
Inte
nsity
(arb
. uni
ts)
Vibrational Energy (meV)
0.00
0.04
0.08
53
H-up bilayer
467432388
198
8769
32184
HOH bendingOH stretch
Translation and rotation
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0 100 200 300 400 500
0.96
1.00
Weak H-bond
OH B
ond
Leng
th (A
ngstr
om)
Time (fs)
0.96
1.00
Free OH
Strong H-bond
Two types of Hydrogen Bonds
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Nature of H-bond at surfaceElectron density differences
Adsorbed dimerFree dimer
Strong bond in H-up bilayer
Strong bond in H-down bilayer
Weak bond in H-down bilayer
Weak bond in H-up bilayer
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The unit cell and charge density
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Minimum energy path for H-up flipping to H-down
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RT3 vs RT39, RT37
RT3
RT39
RT371 2 3
0
200
400
600
Adso
rptio
n En
ergy
(meV
)
Water Coverage (bilayer)
RT3 RT39 RT37
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Water on Pt(111)
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Water monomer on different metal surfaces
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Water bilayer on metal surfaces
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Partial Dissociation on Ru(0001)
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Water on metal (Pt, Pd, Ru, Rh, Au) surfaces II:
Hydrophilic and hydrophobic behavior
With Sheng Meng & Shiwu GaoJCP 2003
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Is this behavior applicable at microscopic level?
α
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Experiments
Wetting order:Pt(111) >Ru(0001) >Cs/graphite >graphite > octane/Pt(111) > Au(111)
Surf. Sci. 367, L13; L19 (1996)
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Gold and Platinum in Periodic Table
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Adsoption Property of Various Water Candidates
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Vibrational Recognition
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Eads: the adsorption energy per molecule
Eads= (Emetal + n × EH2O - E(H2O)n/Metal)/ n
Here E (H2O)n/Metal is the total energy of the adsorption system, Emetal and EH2O are those for free a surface and a free molecule, respectively, and n is the number of water in the unitcell.
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EHB: the strength of H-bond
EHB= (Eads×n - Eads[monomer] × NM-H2O)/ NHB,for clusters and 1 BL;
or
(Eads[m BL]×2m - Eads[(m-1) BL] × 2(m-1))/ 4,for m BL, m> 1.
Here Eads[monomer] and NM-H2O are the adsorption energy of monomer and the number of molecule-surface bonds in the water structures; and Eads[m BL] is the adsorption energy for m bilayers.
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<>
==cHydrophilicHydrophobi
EEw
ads
HB
11
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Hydrophilic vs. HydrophobicEHB in Ice:315meV
Pt:
Au:
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Charge Densities
Pt: d9s1
Au: d10s1
Total charge Difference charge
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Wetting order
H-up H-down
Wetting order:Ru > Rh > Pd > Pt > Au
d7s1 d8s1 d9s1 d9s1 d10s1
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With Jianjun YangPRL 2004; PRB 2005, 2006
Water on silica surface: Tessellat ion ice
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Single hydroxyl Vicinal hydroxylsGeminal hydroxyls
Typical hydroxyl groupsThe presence of hydroxyl groups on silica is important as it impacts the reactivity and performance of the silica surfaces, which are so important both naturally and technologically.
Two typical hydroxyl groups are detected by experiments, the single (Si-OH) and geminal (Si-(OH)2), and some of them form hydrogen-bonding.
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NHB Eads (meV/ H2O) dOH1 (Å) dOH2 (Å) ∠HOH (º)A (bridge) 3 622 0.974 0.988 105.06B (geminal) 2 508 0.973 0.992 106.03
C (top) 1 339 0.970 0.960 106.12Free H2O — — 0.973 0.973 104.91
Eads={[nE(H2O)+E(substrate)]-E(nH2O+substrate)}/n
H-bondO…O < 3.3Å
H-O…H > 140º
Definition:
Monomer on β-cristobalite (100) surface
OH bond lengthened: 0.988 (0.973Å)
HOH angle enlarged: 105.1 (104.9º)
(I)
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Eads NHB dOH1 dOH2 ∠HOH dOO α β
adsorbed dimer
748 5 0.973 1.043 108.85 2.530 8.48 103.58
0.994 0.992 103.63free dimer — — 0.973 0.984 104.79 2.895 2.79 126.00
0.973 0.973 105.08
OO distance shortened: 2.53 (2.89 Å) H-bond strengthened
(I) Dimer on β-cristobalite (100) surface
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Monolayer on β-cristobalite (100) surface
Side view
Side view
Top view
1ML: One hydroxyladsorbs one watermolecule.
Results:
Forming a 2D H-bonded waternetwork
Half molecules is parallel and therest is perpendicular to the surface
Each H2O is saturated with 4 H-bonds.
1hydroxyl+3H2O
2D ice layer
(I)
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2D tessellation ice:(I)
Each H2O is saturated with 4 H-bonds: 1hydroxyl+3H2O;No free OH sticking out of surface
Strong H-bondWeak H-bond
The adsorption energy of the tessellation ice on β-cristobalite (100) is large,712 meV/H2O, almost the same as adhesive energy in bulk ice, 720 meV/H2O.It is stable up to room temperature (300K).
E. G. Wang
Degenerated 2D ice configurations
This 2D ice structure can sit on different sites (left panels) with two possible orderings of H-bonds (right panels).
meVE 17<∆
(I)
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(80K;0.5fs;3ps)
476
stronger H-bond
more red-shifted of OH stretched vibration
lower vibration energy
Vibrational spectrum(I)
The strong H bond inside the quadrangles: 406 and 428 meV modes;The weak H bond between the two neighboring quadrangles: 456 meV modes;The OH stretching: 347 and 378 meV modes;
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“We find strong evidence of ordering of the a-SiO2 surface and adsorbed H2O monolayer.”
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* Ultrapure a-SiO2: 2X2 cm2 and thickness of 0.5 cm;* Probed area: π√2(85 X 99) μm2;* Ambient temperature: 22 ˚C;* Using the idler of a seeded-tripled-Nd: YAG-pumped optical parametric oscillator operating at 30 Hz, laser pulses (~0.5 mJ/pulse, 6 ns, linewidth < 10 cm-1);
Cavity ring-down spectroscopy (CRDS)
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A coverage of ~1 monolayer of water is estimated at 10% RH.
(a) Vibration-combination spectra of a-SiO2 surface hydroxyls. Peaks: 8119 and 8154 cm-1;
(b) Vibration-combination spectra of adsorbed water.Peaks: 8199(p), 8241(s), 8260(p) and 8389(s) cm-1;
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Adsorbed water
Exp: 2γOH+δOH;8241(s)/8260(p), 8199(p), 8389(s)
Theo: γOH;406(degenerate modes), 428, 456
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With Yong YangPRB 2006; PRE 2005; JPCM 2006
NaCl in w ater:
Dissolution and Nucleation
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From ab initio calculations, at least six water molecules are needed to separate a NaCl pair.
Side view
Top view
How about a nanocrystal ?
Dissolution
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• Classical MD performed by AMBER package with TIP3P model.
• System investigated: 625H2O (liquid state) + 32NaCl.
• NTP: ~350 K, ~1 bar.
Cl- Na+ H2O
Size of unitcell : 27.86Å×27.88Å ×27.50Å
Dissolution
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Cl-, Na+, Cl-, Na+…
Superscripts:
1~32 for Na+
33~64 for Cl-
Dissolution sequences:
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Role of water & dissolution pathway
0 50 100 150 200 250 3000
2
4
6
8
10
12
d(
Na+ -C
l- ) (An
gstro
m)
Na24-Cl64
Na29-Cl64
Na32-Cl64
Coor
dina
tion
Num
ber
Time (ps)
Cl36
Cl64
0
5
10
15
20
25
300 350 400 450 500 550 6000
2
4
6
8
10
12
d(Na
+ -Cl- ) (
Angs
trom
)
Na29-Cl53
Na29-Cl61
Na29-Cl63
Coor
dina
tion
Num
ber
Time (ps)
Na28
Na29
Breaking two of the three ionic bonds simultaneously !
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Pathway
Site and orientation selection in the early stage of dissolution: corner sites, [111] direction.
_
molekcalEE bb /20~])111[]111[(−
−
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Why does Cl- dissolve prior to Na+ ?
* The difference of dissolution barrier ( Eb + Ehydration.) is very small. (Cl- slightly lower than Na+).
* Local density of water around the ions is the key factor.
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Hydration structures
Hydration structures of Na+, Cl- ions : Radial Distribution Functions (RDFs).
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A typical example: NaCl
Spontaneous nucleation of NaCl in supersaturated solution — irregular shape, Na+ serves as center of stability in early stage.
A more important case: Nucleation at solid-liquid interface
Nucleation
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Classic MD simulation in AMBER 6.0 package.
A five-layer NaCl (001) slab with 160 NaCl units.
At room temperature, in the supersaturated salt solution:
NNaCl : NH2O ~ 1 : 9.
The system was equilibrated at ~ 300 K for at least 300 ps with harmonic restraints applied on the Na+, Cl- solutes, before running.
NTP: 300 K, 1 atm. Na+ Cl- H2O
Nucleation
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Critical size
By statistical analysis, the critical size is found to consist of two atoms: one Na+ and one Cl-.
All the trajectories with different initial configurations and velocities were simulated for 1.2 ns
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At the water-NaCl(001) interface, NaCl growth takes a 3D growth mode.
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A positively-charged surface is found at early stage.
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1. Why 3D growth at interface ? (2D growth in vacuum)
2. Why do Na+ and Cl- show different deposition rate?
A relative stable water network occurs at the interface !
0 300 600 900 12001.0
1.1
1.2
1.3
1.4
1.5
1.6(c)
N HB /H
2O
Time (ps)
Interface Solution
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Water network results in a charged surface.
Na+(aq) H2O — Cl- (substrate) Easy (H-Cl weak bond)
Cl-(aq) H2O — Na+ (substrate) Hard (O-Na strong bond)
Different deposition rate !
Based on our ab initio calculation for water monomer on NaCl (001), we found the averaged resident time of the water molecules on the top sites of surface:
Na+ : about 8.95 ps; Cl- : about 4.12 ps.
E. G. Wang
With D. Pan, L.M. Liu, G. Tribello, B. Slater, A. MichaelidesPRL 2008; Faraday Discuss. 2009
Water solid surface: unexpectedly cold
Proton Ordering
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• What is ice like when it’s not slippery? …the influence of proton order on the surface energy
The Surface of Ice: One of Nature’s Best Catalysts
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Bulk Ice Ih: A Proton Disordered Solid
Bernal-Fowler ice rules:
(1) Each oxygen atom has two OH bonds.
(2) There is exactly one hydrogen atom between each two nearest neighbour oxygen atoms.
J. D. Bernal and R. H. Fowler, J. Chem. Phys. 1, 515 (1933).
E. G. Wang
Ice Ih“proton disordered”
Ice XI “proton ordered”
Ttrans = 72 K
(KT ~5meV)
S. J. Singer, et al., Phys. Rev. Lett. 94, 135701 (2005).
E. G. Wang
Computational Details:
* Density functional theory (DFT)CP2K/QUICKSTEP program [1]
Core Electrons: Goedecker-Teter-Hutter pseudo-potential [2];Valence Electrons: Gaussian functions with triple (TZV2P) -
and quadruple (QZV2P) – doubly polarized basis set; Generalized gradient approximation (GGA): PBE and BLYP
exchange-correlation functions;* Maximally localized Wannier functions [3]
Bulk: Hayward-Reiwers model [4], at least 96 waters;Surface: A slab with 8 - 48 waters per bilayer, up to 15 bilayers;Structures: All atoms are fully relaxed;Plane wave cutoff: 340 Ry;
[1] J. Vande Vondele, M. Krack, F. Mohamed, M. Parrinello, T. Chassaing and J. Hutter, Comp. Phys. Comm. 167, 103 (2005).[2] S. Goedecker, M. Teter, and J. Hutter, Phys. Rev. B 54, 1703 (1996).[3] N. Marzari and D. Vanderbilt, Phys. Rev. B 56, 12847 (1997); P.L. Silvestrelli, N. Marzari, D. Vanderbilt, and M. Parrinello, Solid State Commun. 107, 7 (1998).
[4] J. A. Hayward and J. R. Reimers, J. Chem. Phys. 106, 1518(1997).
E. G. Wang
Computational Details:
* MC simulationEmpirical potential with a six site, rigid body, potential modified to reproduce the DFT proton ordering energies of designated proton configurations (TIP6P). All simulations are run for 500,000 steps, and the mean and variation data are collected over final 15,000 accepted moves.
E. G. Wang
The Cohesive Energy of Ice Ih/XI
Bulk
Coh
esiv
e En
ergy
(meV
/H2O
)
720
700
680
660
640
620
600
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Ice XI Ice Ih
Surface energies get converged quite well for ice XI.
For ice XI surface: ferroelectric & antiferroelectric proton structures;For ice Ih surface: > 20 proton structures.
Surface energy :
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Surface Energy vs Bulk Cohesive Energy
Bulk
Coh
esiv
e En
ergy
(meV
/H2O
)
720
700
680
660
640
620
600
Surfa
ce E
nerg
y (m
eV/H
2O)
200
200
240
260
280
300
320
Bulk variation with proton order ~ 5 meV/H2OSurface variation with proton order >100 meV/H2O
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Order parameter:
COH ~ 3 COH = 2
ci=3 ci=4
The larger the order parameters, the more inhomogeneous the proton distribution.
COH : [2, 6)
Order Parameter
New order parameter on basal plane surfaces of ice Ih
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An order parameter for proton disorder at the surface of ice
Fully random ice Ih surface
COH=2
COH=2.67
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In a classical electrostatic model, we write the surface energy for various ice Ih surfaces as
Thank You !
where is the surface energy of surfaces with COH=2 and EHH a surface excess energy which COH>2 surfaces have due to the
additional repulsion between dangling OH groups brought about by their on average closer proximity to each other. We express the total repulsion between dangling EHH groups through a screened Coulomb interaction, which leads to
It depends linearly on COH with a slope proportional to q2. q is the “effective charge” on the H atoms of the dangling OH groups. If dHH=4.42A and =16.92A2, then the best fit for the charge q=0.21e based on our DFT PBE and BLYP results.
E. G. Wang
An order parameter for proton disorder at the surface of ice
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The surface of ice Ih is unexpectedly cold
• The energetics of proton order differs significantly at the surface compared to in the bulk
• Dangling OHs must maximise their separation – make ice surface more ordered
• Many proton configurational states will be inaccessible (KBT not sufficient to disorder the surface)
No order-disorder transition at any relevant temperature (i.e. below the onset of surface pre-melting)
E. G. Wang
* Under equilibrium the ice Ih surface will not become fully proton disordered at any relevant temperature - Unexpectedly cold;
* It is not yet possible to say with confidence if any one particular structure, for example of Fletcher’s striped phase, is the lowest energy structure;
* The present study is likely to have implication to the premelting of ice. We suggest that regions on the surface with high concentrations of dangling OH groups will melt first.
* It is plausible that other properties of the ice surface, such as adsorption and disorption probabilities for other molecules, will be sensitive to the degree of local order.
* It is no longer recommended to generate ice surface from bulk ice structure by Hayward and Reimers rules alone.
Thank You !
The order parameter COH is a very unique and sensitive factor to describe proton ordering on ice surface.
E. G. Wang
Acknowledgements
Previous Students: Collaborators:Sheng Meng (Harvard) Shiwu Gao (IOP/Chalmers)
Jianjun Yang (Saskatchewan) Lifang Xu, Qinglin Guo (IOP)
Yong Yang (Tohoku) Angelos Michaelides (UCL)Yinghui Yu (NIMS) Limin Liu, Ben Slater (UCL) Kefei Zheng (Parma) G. Tribello (UCL)
B. Slater(UCL)
M. Scheffler(Fritz-Haber-Institut-MPG)
Current Students:Ding Pan & Jie Ma (IOP)
E. G. WangThank You !Thank you !
Sognel, 2007