Ionization of Water in High Electric Fields at and near a Metal (Electrode) Surface
ELECTROCHEMICAL UW SURFACE SCIENCE
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 Role of Electric Field in Electrochemistry
•High interfacial electric field (~1 V/Å) Interfacial Field
n– C– + •Field controlled by electrode – + – potential or chemical potential of + solution species Je – – + – + •What is role of field in C+ – electrochemical reactions? – +
Sub strate(IHP) Diffuse Layer Bulk Electrolyt •How to control field Interface independently? Use of Field Emitter Tips
• Characteristic field for bond breaking –All bonds similar strength: 1-5 eV 1-3 V/Å –All bonds similar length: 1-2 Å
•Field emitter tips – Concentrate field at tip
– 100-1000 Å tip radius; ß ≈ 5 FEM rt Tip Vt –1-5 kV => 1-5 V/Å
Vt Ft = βrt Field-Free Adsorbed Water
•Low temperature (< 170 K) Ice
–Variables: tw, T, Tads
•What can we study? –Ionization in ice layers –Dielectric properties
Dipole moment tw Dielectric constant –Crystalline vs. amorphous ice –Interaction of field and temp. Field Adsorbed/Condensed Water
•High temperatures (> 170 K) (H O) ·H+ H2O 2 n –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 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 •Mass spectroscopy with single ion resolution (+) –Time of flight (field pulse 1 V/Å 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 Ion –Thermo-cycling (TC) rate
cycle temp. at given field field UHV Chamber Lens Assembly Entrance Front Center Back Configuration Tip Assembly Diaphragm Electrode Electrode Electrode Coolant Down Tube
Tip Translation Apparatus
LD Mass Spectrometer 20 - 56 mm Variable Counter Electrode-Lens Distance
Gas Handling Lens Focus Drift Tube
Wien Filter Analytical Equipment Alternate Wien Filter •Rotatable Tip Assembly Configuration •FIM/FEM Imaging (no Drift Tube) •Wien Filter Turbomolecular Pump •Pulsed Potential Time of Flight •Quadrupole Mass Spectrometer Neon on Pt Field Ion Microscopy 107 K 1x10-4 Torr ~3.75 V/Å •Spatial Resolution of Ion Emission •Field Clean Pt Surface to Prevent Possible Contamination
MULTI-CHANNEL PHOSPHOR PLATES SCREEN
TIP Potential V Energy of Image Gas HV Electron In METAL (Pt) Applied FERMI LATTICE STEP LEVEL I IMAGE GAS (Ne) Field Near φ X ION (Ne+) Tip Surface
Adapted from Tsong,1990. δδ Magnetic m+ m Wien Filter Ion m δ Field (B) m-δm Characterization
•Continuous Mode Ion Mass to Charge Resolution Electric •Easily Separate Distinct Ion Signals without Disturbing Field (E) Formation Conditions
Ion E x B Drift Tube Ion Detector Click here to play Mass video of separation of Tip Lens Separator m + ∆x masses 19 (H3O ) Vt m + 0 and 37 (H2O)2H VCE VL
L L1 2eφ E L2 m eE = eνB E = B 0 ∆x = 0 + LL 1− 0 0 0 0 0 2φ 2 1 m m0 0
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 SEPARATION Masses 19 and 37 Field Distribution in Water
H2O •Modeled water layer on tip 3 ε 80 •Ice like (low dielectric) water near surface Field • Liquid like water far from surface •Field concentration at interface r t Distance •Thin layer behavior (tw < rt) t w Field at water/vacuum interf. Vacuum limit
•Thick layer behavior (tw > rt) Field at metal/water interf. rt Vt Electrochem. Limit +
Scovell et al., Chem. Phys. Lett. 294 (1998) 255. Ramped Field Desorption of Ice
x = 3.0 ± 0.3 •Field-free adsorbed water 148 K
• x = tw/rt = 3 (thick layer) 138 K •Increase field linearly in time •Measure ionization/desorption
of all ions 128 K
•Increasing temp. decreases field
needed for ionization Units) Ion Current (Arb. • Changes in peak shape with 118 K temperature Flash
103 K
00.40.81.2 1.6 Pinkerton, et al., Langmuir, 15 (1999) 851. -1 Fapp / V Å Onset Ionization Fields
0.6 •Phase transitions in ice layers H2O / Pt E x 0 •Amorphous ice (solid line) E E E •Crystalline ice (dashed line) E E
) E E •Ionization at only -1 0.4
E EE E E 0.2 – 0.5 V/Å ! E E E E 0.2 Onset Field, F (VÅ
Temp. Dipole Permittivity Amorph. < 130 K 3.9 D 10 0.0 100 120 140 160 180 200 220 Cryst. > 130 K 5.1 D 2.5 T / K
Scovell, et al., Surface Sci., 457 (2000) 365. Thermal Activation Barrier of Ionization
… HO–H OH2 •Gomer charge-exchange model E H3O
xc •Hydrogen motion along 0 xO–H V(x ) ∆ hydrogen bond (red curve) c Eres Q F Ha + PiF H ·(H 2O)m
Field dependent barrier
Thermal activation barrier Ionization in Thin and Thick Films
Thick Layers – Ionization at metal surface – Ions must diffuse through ice layer before detection –less sensitive to ice layer structure
Pt - H20 Interface Ion Formation
Pt Field Thin Layers Emitter Tip –Easy detection of ions –Sensitive to ice layer structure
H20 -Vacuum Interface Ion Formation
Water Layer Ramped Field Desorption of Ice Imaged in Total Ion Mode
r r r w =1.7 w =5 w =7 rt rt rt
108 K 108 K 133 K
Click here to play Click here to play Click here to play RFD video of thin, RFD video of thick, RFD video of thick, amorphous water amorphous water crystalline water layer adsorbed at layer adsorbed at layer adsorbed at 108 K 108 K 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 PROCEDURE RESULTS •Tip at optimum pot. for desired cluster •Ion cluster emission thermally deactivated -6 •Background H2O pressure: 5 x 10 Torr for given cluster size. •Temperature cycled linearly with time, •Termination of cluster n emission remaining above 165 K to avoid coincides with beginning of cluster (n – 1) condensation decay. •Mass selected ion signal measured and •Deactivation energies comparable to proton imaged with MCPs; for mass 73 (n = 4), solvation energies for the nth solvating etc. water molecule.
10 Minutes 300K 250K 200K 167K
250 240 230 220 210 H+(H O) 2 3 H+(H O) 200 ∆E = 0.85 ± 0.03 2 4 ∆
Log of Ion Signal E = 0.76 ± 0.02 + 190 H (H2O)5 Log Ion Signal 180 ∆
Tip Temperature, K E = 0.55 ± 0.02 170 160 3.0 3.5 4.0 4.5 5.0 5.5 6.0 150 Time 1000/T (K-1) Thermal Lit Exp a ∆ b ∆ c ∆ d Fapp Esolv Esolv Eemit Deactivation of V/Å eV eV eV H+ n/a n/a - 12.55 Ion Cluster + H (H2O) 1.00 -7.22 - 5.77 + Emission 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)n H (H2O)5 0.29 -0.50 -0.55 4.07 + H (H2O)6 0.27 -0.48 - 4.02 H+(H O) 0.26 -0.46 - 4.00 Ion Emission 2 7 * (Field Dependent) H2O
+ + H (H O)n H (H O)n 2 Desorption of nth 2 –1
Solvating H2O a (Thermally Activated) Optimal observed applied field for cluster formation. b H O H+ solvation energy associated with n-1 to n transition. [Meot-Ner] 2 c Experimentally observed solvation energies. Solvation by d ∆ + Calculated emission energy based on Esolv+lvap H (H2O)n+1 ∆ Additional H2O (e.g. Eemit(n=3) = 4.83eV - 0.82eV + 0.44eV = 4.45eV) (Positive ∆E) Direct Imaging of Ion Clusters
PROCEDURE •Ion signal directly imaged by FIM (c) p -6 • w = 5x10 Torr n>4 n=4 n=3 • Tip at optimum potential for desired cluster • 10 sec averaged pixel intensity n=2 n=1 H O+ 180K 2
230K n=2 n=1 Ne Water Ion Cluster Formation Proposed Mechanism
Dissociation Hump Emission Formation
Decreased Local Radius of Curvature
++++ +++ ++++ Pt Pt Pt Separating Ion Dissociation and Ion Emission RFD 2 1.00 + H2O Ramped Field Desorption 1 –Amorphous ice deposition 0.75 + H (H2O)n –Field ramp passes through 1 emission fields for all clusters n 0.50 RFD 1 ≥ 2 before dissociation 2 –When ramp reaches dissociation Applied Field (V/Å) Field Applied 3 field, clusters n ≥ 2 are emitted 0.25 4 - 6 Dissociation simultaneously. FIELD FREE CONDENSATION FIELD FREE 0.00 100 150 200 250 Ramped Field Desorption 2 Temperature, K –Field adsorbed layer –Field ramp activates dissociation Field dissociation (~ 0.7 eV) before emission –Strong temperature dependence –Cluster n emission observed, –Field-free dissociation at ~ 220K (Extrap) each in turn. Field ion emission (~ 4+ eV) –Not significantly temperature dependent Field Induced Ion Dissociation
Dissociation ~ 1.1Å Region H H O H H O + Field H
+ ~ 0.7eV
+ •Shifting proton along the H-bond requires ~0.6 V/Å + •Field in dissociation region is much lower than applied field or field at water + vacuum interface. Field Induced Ion Emission
Dissociation •Emission of ion cluster from Region water covered surface requires
∆E / eV H+(H O) + emit 2 n Field 5.77 1 4.832 2 + 4.453 3 4.124 4 4.075 5 + •Ion cluster can protrude near surface and experience enhanced + local field •Protrusion results in extended + local dissociation region 0.6
0.4
Summary of Results0.2 (I)
T / K / T
•Moderate fields (0.2-0.5 V/Å) promote 0.0 0 2 4 6 8 0 220 200 180 160 140 120 100 water ionization
•Ionization occurs at electrode surface or Onset FieldF in water adlayer o /VÅ -1
E E Pt / O 2 H E 0 x E E E E E Temp. Dipole Permittivity E E E E E Amorph. < 130 K 3.9 D 10 •Ionization and dielectric properties of E E E E water accessible Cryst. > 130 K 5.1 D 2.5 … HO–H OH2 E H3O
xc •Thermal activation barrier for 0 xO–H V(x ) ∆ c Eres Q ionization: 0.7 eV F Ha + PiF H ·(H 2O)m Summary II
+ •Water ion clusters H (H2O)n from field adsorbed water detected for n up to 7 for temps. of 170-300 K
•Low fields favor large clusters (energetically favorable)
300K 250K 200K 167K
•High fields favor small clusters H+(H O) 2 3 H+(H O) ∆E = 0.85 ± 0.03 2 4 (kinetically favorable) ∆ Log of Ion Signal E = 0.76 ± 0.02 + H (H2O)5 ∆E = 0.55 ± 0.02
3.0 3.5 4.0 4.5 5.0 5.5 6.0
1000/T (K-1) Summary III
•Ion emission of large clusters + * H (H2O)n deactivated at high temperatures due to desorption of solvating molecules * H2O
+ H+(H O) H (H2O)n 2 n–1
•Ion emission locally enhanced, possibly due to hump formation. Looking Ahead: E-Field Induced Ionization
•Ionic strength sensitive to electric field –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 greatly different than in bulk electrolyte •Enhance reactions that depend on H+ or OH– concentrations
•Hydrated ions may impede Greater ionic transport of species to/from surface strength in region of high field E-Field Effects in Electrocatalysis
•Increased ionic strength near surface can increase OH– Pt concentration Ru •Increased OH– can help remove CO poison from fuel cell anodes –C–O •Catalyst design, i.e. addition of O–C–O other elements, should be thought in terms of influence on local electric fields Future Experiments
• Temperature / flux dependence •Ion energy deficit of water –Quantify ionization rates ionization –Examine surface diffusion vs. –Measure appearance potential ionization – ∆H of ionization –Adlayer thickness –Influence of surface bonding –Ionic diffusion through ice layer – Combine with activation energy results
•Field ionization of methanol •Negative field ionization –Pure methanol –Probe OH– formation, –Methanol/water mixtures energetics, and hydration – Consider methanol –Work function measurements adsorption/ionization for DMFC UniversityUniversity of WashingtonWashington
ELECTROCHEMICAL UW SURFACE SCIENCE
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) World’s Largest Field Emitter Tip