Ionization of Water in High Electric Fieldsat and near a Metal (Electrode) Surface

Valentin Medvedev

Chris Rothfuss

Eric M. Stuve

University of Washington

Gordon Research Conference on Electrochemistry

Ventura, California

14-19 January 2001

Sponsored by the Office of Naval Research

UW ELECTROCHEMICALSURFACE SCIENCE

Role of Electric Field in Electrochemistry

• High interfacial electric field(~1 V/Å)

• Field controlled by electrodepotential or chemical potential ofsolution species

• What is role of field inelectrochemical reactions?

• How to control fieldindependently?

+

–+

++– –

–

–

– +–

+

n–

Je

(IHP)

C–

C+

Sub strate Bulk Elect rolytDiffuse Layer

In te rface

In te rfacial Field

• Characteristic field for bond breaking– All bonds similar strength: 1-5 eV– All bonds similar length: 1-2 Å

• Field emitter tips– Concentrate field at tip– 100-1000 Å tip radius; ß ≈ 5– 1-5 kV => 1-5 V/Å

Use of Field Emitter Tips

Ft = Vt

βrt

1-3 V/Š�����

VtrtFEM

Tip

Field-Free Adsorbed Water

• Low temperature (< 170 K)– Variables: tw, T, Tads

• What can we study?– Ionization in ice layers– Dielectric properties

Dipole momentDielectric constant

– Crystalline vs. amorphous ice– Interaction of field and temp.

tw

Ice

Field Adsorbed/Condensed Water

• High temperatures (> 170 K)– Variables: pw, T, F

• What can we study?– Monolayer & multilayer water– Surface vs. bulk ionization– Cluster formation: (H2O)n·H+

– Interaction of field and temp.– Room temp. studies possible

H2O (H2O)n·H+

How Are Ions Detected?

• Use principles of field ionization microscopy (FIM)• Tip usually in positive bias (to avoid e– emission)• Positive ions detected by microchannel plates

– Ion images– Ionization rates

(+)1 V/Å

• Mass spectroscopy with singleion resolution– Time of flight (field pulse

initiated)– ExB (Wien) filter

(continuous signal)

Experimental Techniques

• Imaging methods for spatio-temporal correlation– Field ionization (static and video)– Mass resolved ion images– Field emission micr. (FEM)

• Quantitative methods– Ramped field desorption (RFD)

similar to thermal desorp. (TDS)– Stepped field desorption (SFD)

similar to isothermal kinetics– Thermo-cycling (TC)

cycle temp. at given field field

Ionrate

Gas Handling

Turbomolecular Pump

Mass Spectrometer

WienFilter

Drift Tube

LensFocus

Tip TranslationApparatus

Alternate Wien FilterConfiguration (no Drift Tube)

CoolantDown Tube

UHV ChamberConfiguration

Analytical Equipment• Rotatable Tip Assembly• FIM/FEM Imaging• Wien Filter• Pulsed Potential Time of Flight• Quadrupole Mass Spectrometer

20 - 56 mm Variable CounterElectrode-Lens Distance

LD

EntranceDiaphragm

FrontElectrode

CenterElectrode

BackElectrode

Lens Assembly

Tip Assembly

• Spatial Resolution of Ion Emission• Field Clean Pt Surface to Prevent

Possible Contamination

Field Ion Microscopy Neon on Pt107 K

1x10-4 Torr~3.75 V/Å

METAL (Pt)LATTICE STEPIMAGE GAS (Ne)ION (Ne+)

Adapted from Tsong,1990.

TIP

HV

MULTI-CHANNELPLATES

PHOSPHORSCREEN

PotentialEnergy of

Image GasElectron In

AppliedField Near

Tip Surface

Iφ X

V

FERMILEVEL

ElectricElectricField (E)Field (E)

MagneticMagneticField (B)Field (B)

Lens: G.F. Rempter, J. Appl. Phys. 57 (1985) 2385.

E x B Mass Separator: M. Kato and K. Tsuno, Nucl. Instr. Methods A298 (1990) 296.

Wien Filter IonCharacterization

L1L

m

m0

Lens

Drift TubeE x BMass

Separator

Ion Detector

Tip

∆xVt

VCE VL

IonIon

m+m+δδmmm-m-δδmm

mm

• Continuous Mode Ion Massto Charge Resolution

• Easily Separate Distinct IonSignals without DisturbingFormation Conditions

00 BeeE ν=0

000

2

m

eBE

φ= ∆x =

E02φ0

L2

2+ LL1

1−m0m

WIEN SEPARATION Masses 19 and 37

Click here to playvideo of separation of

masses 19 (H3O+)and 37 (H2O)2H+

Field Distribution in Water

• Modeled water layer on tip• Ice like (low dielectric) water near

surface• Liquid like water far from surface• Field concentration at interface• Thin layer behavior (tw < rt)

Field at water/vacuum interf.Vacuum limit

• Thick layer behavior (tw > rt)Field at metal/water interf.Electrochem. Limit

rt tw

Field

Distance

3 80ε

H2O

+Vt

rt

Scovell et al., Chem. Phys. Lett. 294 (1998) 255.

Ramped Field Desorption of Ice

• Field-free adsorbed water• x = tw/rt = 3 (thick layer)• Increase field linearly in time• Measure ionization/desorption

of all ions

• Increasing temp. decreases fieldneeded for ionization

• Changes in peak shape withtemperature

0 0.4 0.8 1.2 1.6

Ion

Curre

nt (A

rb. U

nits

)

Fapp / V Å-1

148 K

138 K

128 K

118 K

103 K

x = 3.0 ± 0.3

Flash

Pinkerton, et al., Langmuir, 15 (1999) 851.

Onset Ionization Fields

• Phase transitions in ice layers• Amorphous ice (solid line)• Crystalline ice (dashed line)• Ionization at only

0.2 – 0.5 V/Å !

Temp. Dipole Permittivity

Amorph. < 130 K 3.9 D 10

Cryst. > 130 K 5.1 D 2.5

EEE

EE E

E

EEE

EEE E

EE

E

0.0

0.2

0.4

0.6

100 120 140 160 180 200 220T / K

H2O / Ptx 0

Ons

et F

ield

, F (V

Å-1

)

Scovell, et al., Surface Sci., 457 (2000) 365.

Thermal Activation Barrier of Ionization

• Gomer charge-exchange model

• Hydrogen motion alonghydrogen bond (red curve)

Field dependent barrier

Thermal activation barrier

0V(xc)

xc

∆Eres

H3O

H+·(H 2O)mF

PiF

HO–H … OH2

xO–H

E

QHa

Ionization in Thin and Thick FilmsThick Layers

– Ionization at metal surface

– Ions must diffuse through ice layerbefore detection

– less sensitive to ice layer structure

Pt FieldEmitter Tip

Water Layer

Pt - H20 InterfaceIon Formation

H20 -Vacuum InterfaceIon Formation

Thin Layers

– Easy detection of ions

– Sensitive to ice layer structure

108 K

rt

rw =1.7 rt

rw =5 rt

rw =7

108 K 133 K

Ramped Field Desorption of IceImaged in Total Ion Mode

Click here to playRFD video of thin,amorphous waterlayer adsorbed at

108 K

Click here to playRFD video of thick,amorphous waterlayer adsorbed at

108 K

Click here to playRFD video of thick,

crystalline waterlayer adsorbed at

133 K

Ion Cluster Formation

• Field condensed water layer on Pt• Ion clusters mass resolved with Wien

filter• Clusters of (H2O) n·H+ with n = 1-8• Trade-off between:

– Ionization potential(favors high n); lower ∆Hrxn

– Kinetics (favors low n);entropic effects

• Cluster field independent of temp.

Thermo-Cycling: Ion Cluster Formation

RESULTS•Ion cluster emission thermally deactivatedfor given cluster size.

•Termination of cluster n emissioncoincides with beginning of cluster (n – 1)decay.

•Deactivation energies comparable to protonsolvation energies for the nth solvatingwater molecule.

PROCEDURE•Tip at optimum pot. for desired cluster•Background H2O pressure: 5 x 10-6 Torr•Temperature cycled linearly with time,remaining above 165 K to avoidcondensation

•Mass selected ion signal measured andimaged with MCPs; for mass 73 (n = 4),etc.

3.0 3.5 4.0 4.5 5.0 5.5 6.0

1000/T (K-1)

Lo

g o

f Io

n S

ign

al

∆E = 0.85 ± 0.03 ∆E = 0.76 ± 0.02

H+(H2O)4H+(H2O)3

∆E = 0.55 ± 0.02

H+(H2O)5

300K 250K 200K 167K

Time

Lo

g Io

n S

ign

al

150

160

170

180

190

200

210

220

230

240

250

Tip

Tem

per

atu

re, K

10 Minutes

H+(H2O)n H+(H2O)n–1

H+(H2O)n+1

H+(H2O)n*

H2O

Ion Emission(Field Dependent)

Desorption of nth

Solvating H2O(Thermally Activated)

Solvation byAdditional H2O(Positive ∆E)

ThermalDeactivation ofIon ClusterEmission

aOptimal observed applied field for cluster formation.

bH+ solvation energy associated with n-1 to n transition. [Meot-Ner]

cExperimentally observed solvation energies.

dCalculated emission energy based on ∆Esolv+lvap

(e.g. ∆Eemit(n=3) = 4.83eV - 0.82eV + 0.44eV = 4.45eV)

Lit ExpFapp

a ∆Esolv b ∆Esolv

c ∆Eemitd

V/Å eV eV eV

H+ n/a n/a - 12.55H+(H2O) 1.00 -7.22 - 5.77

H+(H2O)2 0.64 -1.38 - 4.83

H+(H2O)3 0.45 -0.82 -0.85 4.45

H+(H2O)4 0.34 -0.76 -0.76 4.12

H+(H2O)5 0.29 -0.50 -0.55 4.07

H+(H2O)6 0.27 -0.48 - 4.02

H+(H2O)7 0.26 -0.46 - 4.00

H2O*

(c)

180K

230K

Direct Imaging of Ion Clusters

PROCEDURE• Ion signal

directly imagedby FIM

• pw = 5x10-6 Torr• Tip at optimum

potential fordesired cluster

• 10 sec averagedpixel intensity

n>4 n=4 n=3

n=2 n=1 H2O+

n=2 n=1 Ne

Dissociation HumpFormation

Emission

Pt Pt Pt+ + + + + + + + + + +

Water Ion Cluster FormationProposed Mechanism

DecreasedLocal Radiusof Curvature

0.00

0.25

0.50

0.75

1.00

100 150 200 250

Temperature, K

Ap

plie

d F

ield

(V

/Å)

H2O+

H+(H2O)n1

234 - 6

Dissociation

RFD 1

RFD 2

FIEL

D F

REE

CO

ND

ENSA

TIO

N

Field dissociation (~ 0.7 eV)– Strong temperature dependence– Field-free dissociation at ~ 220K (Extrap)

Field ion emission (~ 4+ eV)– Not significantly temperature dependent

Ramped Field Desorption 1– Amorphous ice deposition– Field ramp passes through

emission fields for all clusters n≥ 2 before dissociation

– When ramp reaches dissociationfield, clusters n ≥ 2 are emittedsimultaneously.

Ramped Field Desorption 2– Field adsorbed layer– Field ramp activates dissociation

before emission– Cluster n emission observed,

each in turn.

Separating Ion Dissociation and Ion Emission

+

+

+

+

+

Field

DissociationRegion

•Shifting proton along the H-bond requires~0.6 V/Å

•Field in dissociation region is muchlower than applied field or field at watervacuum interface.

Field Induced Ion Dissociation

OH

H OH

H

~ 1.1Å

H

~ 0.7eV

+

+

+

+

+

Field

DissociationRegion

•Emission of ion cluster fromwater covered surface requires

∆Eemit / eV H+(H2O)n5.77 14.832 24.453 34.124 44.075 5

•Ion cluster can protrude nearsurface and experience enhancedlocal field

•Protrusion results in extendedlocal dissociation region

Field Induced Ion Emission

Summary of Results (I)• Moderate fields (0.2-0.5 V/Å) promote

water ionization

• Ionization occurs at electrode surface orin water adlayer

• Ionization and dielectric properties ofwater accessible

• Thermal activation barrier forionization: 0.7 eV

E

EE

EE E

EE

EEE

EE

EE

E

E

0.00.2

0.40.6

100 120 140 160 180 200 220

Onset F

ield Fo / V

Å-1

T / K

H2O / Pt

x 0

Temp. Dipole Permittivity

Amorph. < 130 K 3.9 D 10

Cryst. > 130 K 5.1 D 2.5

0V(xc)

xc

∆Eres

H3O

H+·(H 2O)mF

PiF

HO–H … OH2

xO–H

E

QHa

Summary II

• Water ion clusters H+(H2O)n fromfield adsorbed water detected for n upto 7 for temps. of 170-300 K

• Low fields favor large clusters(energetically favorable)

• High fields favor small clusters(kinetically favorable)

3.0 3.5 4.0 4.5 5.0 5.5 6.0

1000/T (K-1)

Lo

g o

f Io

n S

ign

al

∆E = 0.85 ± 0.03 ∆E = 0.76 ± 0.02

H+(H2O)4H+(H2O)3

∆E = 0.55 ± 0.02

H+(H2O)5

300K 250K 200K 167K

Summary III

• Ion emission of large clustersdeactivated at high temperatures due todesorption of solvating molecules

• Ion emission locally enhanced, possiblydue to hump formation.

H+(H2O)nH+(H2O)n–1

H+(H2O)n*

H2O*

Looking Ahead: E-Field Induced Ionization

• Ionic strength sensitive to electricfield– Small fields (0.2-0.5 V/Å)

required for ionization– Fields obtainable at several

hundred mV from PZC– Ionic strength near electrode

surface could be greatlydifferent than in bulkelectrolyte

• Enhance reactions that depend onH+ or OH– concentrations

• Hydrated ions may impedetransport of species to/from surface

Greater ionicstrength in regionof high field

E-Field Effects in Electrocatalysis

• Increased ionic strength nearsurface can increase OH–

concentration• Increased OH– can help remove

CO poison from fuel cell anodes• Catalyst design, i.e. addition of

other elements, should be thoughtin terms of influence on localelectric fields

O–C–O

Pt

Ru

–C–O

Future Experiments

• Temperature / flux dependence– Quantify ionization rates– Examine surface diffusion vs.

ionization– Adlayer thickness– Ionic diffusion through ice layer

• Field ionization of methanol– Pure methanol– Methanol/water mixtures– Consider methanol

adsorption/ionization for DMFC

• Ion energy deficit of waterionization– Measure appearance potential– ∆H of ionization– Influence of surface bonding– Combine with activation energy

results

• Negative field ionization– Probe OH– formation,

energetics, and hydration– Work function measurements

UniversityUniversityofof

WashingtonWashington

Top (l-r)Nallakkan (Arvind) Arvindan,

Chris Rothfuss, Eric Stuve

Bottom (l-r)Tom Madden,

Valentin Medvedev,Seng-Woon (David) Lim

Not shown: Laura Roen

Former members:Tim Pinkerton (Intel)Dawn Scovell (Intel)

UW ELECTROCHEMICALSURFACE SCIENCE

World’s LargestField Emitter Tip