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

E. G. Wang

Why ice surface is slippery?

E. G. Wang

E. G. Wang

Free Water ClustersGregory et al. Science 275, 814 (1997)

E. G. Wang

Water Adsorbate on Carbon Nanotubes

Maiti et al., PRL 87, 155502 (2001)

E. G. Wang

Water in confined system

Koga et al., Nature 412, 802 (2001)

E. G. Wang

Water on surfaceH2O/MgO H2O/Ru

E. G. Wang

Water on metal (Pt, Pd, Ru, Rh, Au) surfaces I:

Energetics and Kinetics

With Sheng Meng & Shiwu GaoPRL 2002, 2003; PRB 2004; CPL 2005

E. G. Wang

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

E. G. Wang

304 meVSmall

Clusters

433 meV

359 meV

520 meV

H-bond: 450 meV (adsorbed dimer) >>250 meV (free dimer)

E. G. Wang

The 1D water chains at a <110>/{100} step on the Pt (322) surface.

E. G. Wang

Water bilayer/Pt(111)

Morgenstern et al., PRL 77, 703 (1996)

E. G. Wang

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

E. G. Wang

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

E. G. Wang

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

E. G. Wang

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

E. G. Wang

The unit cell and charge density

E. G. Wang

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

E. G. Wang

Is this behavior applicable at microscopic level?

α

E. G. Wang

Experiments

Wetting order:Pt(111) >Ru(0001) >Cs/graphite >graphite > octane/Pt(111) > Au(111)

Surf. Sci. 367, L13; L19 (1996)

E. G. Wang

Gold and Platinum in Periodic Table

E. G. Wang

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.

E. G. Wang

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.

E. G. Wang

<>

==cHydrophilicHydrophobi

EEw

ads

HB

11

E. G. Wang

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

E. G. Wang

With Jianjun YangPRL 2004; PRB 2005, 2006

Water on silica surface: Tessellat ion ice

E. G. Wang

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.

E. G. Wang

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)

E. G. Wang

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

E. G. Wang

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)

E. G. Wang

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)

E. G. Wang

(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;

E. G. Wang

“We find strong evidence of ordering of the a-SiO2 surface and adsorbed H2O monolayer.”

E. G. Wang

* 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)

E. G. Wang

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;

E. G. Wang

Adsorbed water

Exp: 2γOH+δOH;8241(s)/8260(p), 8199(p), 8389(s)

Theo: γOH;406(degenerate modes), 428, 456

E. G. Wang

E. G. Wang

With Yong YangPRB 2006; PRE 2005; JPCM 2006

NaCl in w ater:

Dissolution and Nucleation

E. G. Wang

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

E. G. Wang

• 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

E. G. Wang

Cl-, Na+, Cl-, Na+…

Superscripts:

1~32 for Na+

33~64 for Cl-

Dissolution sequences:

E. G. Wang

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 !

E. G. Wang

Pathway

Site and orientation selection in the early stage of dissolution: corner sites, [111] direction.

_

molekcalEE bb /20~])111[]111[(−

−

E. G. Wang

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.

E. G. Wang

Hydration structures

Hydration structures of Na+, Cl- ions : Radial Distribution Functions (RDFs).

E. G. Wang

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

E. G. Wang

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

E. G. Wang

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

E. G. Wang

At the water-NaCl(001) interface, NaCl growth takes a 3D growth mode.

E. G. Wang

A positively-charged surface is found at early stage.

E. G. Wang

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

E. G. Wang

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

E. G. Wang

• 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

E. G. Wang

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

E. G. Wang

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 :

E. G. Wang

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

E. G. Wang

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

E. G. Wang

An order parameter for proton disorder at the surface of ice

Fully random ice Ih surface

COH=2

COH=2.67

E. G. Wang

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

E. G. Wang

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