A Molecular Picture of Water Structure and Dynamics from Computer Simulation

Understanding the nature of O-H bonds is the key issue in the study of water and energy.

A molecular picture of water structure and dynamics from computer simulation

Enge Wang Institute of Physics (CAS)

Supported by National Natural Science Foundation of China Ministry of Science and Technology Chinese Academy of Sciences

E. G. Wang Outline

Water on metal surface - Energetics and Kinetics - Hydrophilic and hydrophobic behavior Water on silica surface - Tessellation 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 Clusters Gregory 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 surface

H2O/MgO H2O/Ru

E. G. Wang Water on metal (Pt, Pd, Ru, Rh, Au) surfaces I:

Energetics and Kinetics

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

E. G. Wang H2O/Pt(111)

Top Bridge Hollow

• 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

E. G. Wang 304 meV 359 meV Small Clusters

520 meV 433 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 H-up =291 meV;

* Two non-equivalent H bonds;

H-down

E. G. Wang Vibrational spectra

Translation and rotation

198 0.08 H-up bilayer 4 32 18 HOH bending

0.04 69 467 53 87 432 388 OH stretch 0.00

0.04 57 202 H-down bilayer 6 34 69 196 16

Intensity (arb. units) 0.02 91 424 384 438 0.00 0 100 200 300 400 500 Vibrational Energy (meV)

E. G. Wang Two types of Hydrogen Bonds

Strong H-bond

1.00

0.96 Free OH Weak H-bond 1.00

OH Bond Length Bond (Angstrom)OH

0.96 0 100 200 300 400 500 Time (fs)

E. G. Wang Nature of H-bond at surface Electron density differences

Free dimer Adsorbed dimer

Strong bond in Strong bond in H-up bilayer H-down bilayer

Weak bond in Weak bond in H-up bilayer H-down bilayer

E. G. Wang The unit cell and charge density

E. G. Wang Minimum energy path for H-up flipping to H-down

E. G. Wang RT3 vs RT39, RT37

RT3

RT39

600

400

RT3 200 RT39 RT37 RT37

Adsorption Energy (meV) 0 1 2 3 Water Coverage (bilayer)

E. G. Wang Water on Pt(111)

E. G. Wang Water monomer on different metal surfaces

E. G. Wang Water bilayer on metal surfaces

E. G. Wang Partial Dissociation on Ru(0001)

E. G. Wang Water on metal (Pt, Pd, Ru, Rh, Au) surfaces II:

Hydrophilic and hydrophobic behavior

With Sheng Meng & Shiwu Gao JCP 2003

E. G. Wang α

Is this behavior applicable at microscopic level?

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

E. G. Wang Vibrational Recognition

E. G. Wang 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 E > 1 Hydrophobic w = HB =  Eads < 1 Hydrophilic

E. G. Wang Hydrophilic vs. Hydrophobic

EHB in Ice: 315meV

Au:

Pt:

E. G. Wang Charge Densities Total charge Difference charge

Pt: d9s1 Au: d10s1

E. G. Wang Wetting order

H-up H-down d7s1 d8s1 d9s1 d9s1 d10s1 Wetting order: Ru > Rh > Pd > Pt > Au

E. G. Wang Water on silica surface: Tessellation ice

With Jianjun Yang PRL 2004; PRB 2005, 2006

E. G. Wang Typical hydroxyl groups The 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.

Single hydroxyl Geminal hydroxyls Vicinal hydroxyls

E. G. Wang (I) Monomer on β-cristobalite (100) surface

Definition: O…O < 3.3Å H-bond H-O…H > 140º

OH bond lengthened: 0.988 (0.973Å) HOH angle enlarged: 105.1 (104.9º)

Eads={[nE(H2O)+E(substrate)]-E(nH2O+substrate)}/n ∠ NHB Eads (meV/ H2O) dOH1 (Å) dOH2 (Å) HOH (º) A (bridge) 3 622 0.974 0.988 105.06 B (geminal) 2 508 0.973 0.992 106.03 C (top) 1 339 0.970 0.960 106.12

Free H2O — — 0.973 0.973 104.91 E. G. Wang (I) Dimer on β-cristobalite (100) surface

OO distance shortened: 2.53 (2.89 Å) H-bond strengthened

E ∠ ads NHB dOH1 dOH2 HOH dOO α β adsorbed 748 5 0.973 1.043 108.85 2.530 8.48 103.58 dimer 0.994 0.992 103.63 free dimer — — 0.973 0.984 104.79 2.895 2.79 126.00 0.973 0.973 105.08 E. G. Wang (I) Monolayer on β-cristobalite (100) surface

1ML: One hydroxyl Side view adsorbs one water molecule.

Results: . Forming a 2D H-bonded water network Top view . Half molecules is parallel and the rest is perpendicular to the surface

. Each H2O is saturated with 4 H- Side bonds. view

1hydroxyl＋3H2O

2D ice layer

E. G. Wang (I) 2D tessellation ice:

Each H2O is saturated with 4 H-bonds: 1hydroxyl＋3H2O; No free OH sticking out of surface Weak H-bond Strong 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 (I) Degenerated 2D ice configurations

This 2D ice structure can sit on different sites (left panels) with two possible orderings of H-bonds (right panels). ∆E <17meV E. G. Wang (I) Vibrational spectrum

(80K;0.5fs;3ps)

476

stronger H-bond more red-shifted of OH stretched vibration lower vibration energy

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 Cavity ring-down spectroscopy (CRDS)

2 * Ultrapure a-SiO2: 2X2 cm 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);

E. G. Wang (a) Vibration-combination

spectra of a-SiO2 surface hydroxyls. Peaks: 8119 and 8154 cm-1;

(b) Vibration-combination spectra of adsorbed wa t e r. Peaks: 8199(p), 8241(s), 8260(p) and 8389(s) cm-1;

A coverage of ~1 monolayer of water is estimated at 10% RH. 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 NaCl in w ater:

Dissolution and Nucleation

With Yong Yang PRB 2006; PRE 2005; JPCM 2006

E. G. Wang Dissolution

From ab initio calculations, at least six water molecules are needed to separate a NaCl pair.

Side Top view view

How about a nanocrystal ?

E. G. Wang Dissolution

• Classical MD performed by AMBER package with TIP3P model. • System investigated:

625H2O (liquid state) + 32NaCl. • NTP: ~350 K, ~1 bar. Size of unitcell : 27.86Å×27.88Å ×27.50Å

- + Cl Na H2O

E. G. Wang Dissolution sequences: Cl-, Na+, Cl-, Na+…

Superscripts: 1~32 for Na+ 33~64 for Cl-

E. G. Wang Role of water & dissolution pathway

25 29 53 24 64 Na -Cl Na -Cl 29 61 29 64 Na -Cl Na -Cl 20 29 63 Na32-Cl64 Na -Cl 15 ) (Angstrom) -

) (Angstrom) - -Cl

+ 10 -Cl + d(Na

d(Na 5

12 120 28 Cl36 Na 29 10 Cl64 10 Na

8 8

6 6

4 4 Coordination Number

2 Coordination Number 2

0 0 0 50 100 150 200 250 300 300 350 400 450 500 550 600 Time (ps) Time (ps) Breaking two of the three ionic bonds E. G. Wang simultaneously ! Pathway

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

−

(Eb[111]− Eb[111]) ~ 20kcal / mole

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 Nucleation 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

E. G. Wang Nucleation

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

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 !

1.4 O 2

/H 1.3 HB N 1.2

1.1

1.0 0 300 600 900 1200 Time (ps)

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)

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.

Different deposition rate ! E. G. Wang Water solid surface: unexpectedly cold

Proton Ordering

With D. Pan, L.M. Liu, G. Tribello, B. Slater, A. Michaelides PRL 2008; Faraday Discuss. 2009

E. G. Wang The Surface of Ice: One of Nature’s Best Catalysts

• What is ice like when it’s not slippery? …the influence of proton order on the surface energy

E. G. Wang Bernal-Fowler ice rules: Bulk Ice Ih: A Proton Disordered Solid

(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 XI Ice Ih “proton ordered” “proton disordered”

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]

[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). 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; [4] J. A. Hayward and J. R. Reimers, J. Chem. Phys. 106, 1518(1997).

E. G. Wang Computational Details:

* MC simulation Empirical 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 O) 600 2

620

640

660

680

700

720 Bulk Cohesive Energy (meV/H Energy Cohesive Bulk

E. G. Wang Surface energy :

For ice XI surface: ferroelectric & antiferroelectric proton structures; For ice Ih surface: > 20 proton structures.

Ice XI Ice Ih

Surface energies get converged quite well for ice XI.

E. G. Wang Surface Energy vs Bulk Cohesive Energy

320 O) 600 2 O)

2 300 620 280 640 260 660 240 680 200 700

Surface Energy Energy Surface (meV/H 200 720 Bulk Cohesive Energy (meV/H Energy Cohesive Bulk

Bulk variation with proton order ~ 5 meV/H2O Surface variation with proton order >100 meV/H2O

E. G. Wang Order Parameter

New order parameter on basal plane surfaces of ice Ih

c =3 i ci=4 Order parameter: COH : [2, 6)

The larger the order parameters, the more inhomogeneous the proton distribution.

COH ~ 3 COH = 2 E. G. Wang An order parameter for proton disorder at the surface of ice

COH=2.67

COH=2

Fully random ice Ih surface

E. G. Wang In a classical electrostatic model, we write the surface energy for various ice Ih surfaces as

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

2 It depends linearly on COH with a slope proportional to q . q is the “effective charge” on the H atoms of the dangling OH groups. If 2 dHH=4.42A and =16.92A , then the best fit for the charge q=0.21e based on our DFT PBE and BLYP results. Thank You ! 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 The order parameter COH is a very unique and sensitive factor to describe proton ordering on ice surface.

* 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 ! 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. Wang Sognel, 2007

ThankThank Youyou !! E. G. Wang