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C. Friesen, 4_16_09, A guest lecture in John Venables’ Surfaces Course
Succinct, to-the-point title:
Connections between Dry & WetInterfaces: An Intro to Electrochemistry
for Students Familiar with UHV…in 30 minutes or less.
C. Friesen
Please interrupt whenever necessary
C. Friesen, 4_16_09, A guest lecture in John Venables’ Surfaces Course
Explanation for NAN 546: April 09
This talk was given on 4/16/09 as a guest lecture to our NAN 546: Surfaces and Thin Films class
This material is copyright of the Author, Dr Cody Friesen. All queries for use other than private study should be directed to him personally.
Dr Venables' interest in this material is as a student of crystal growth. The phenomena of growth or evaporation in UHV, and growth or dissolution in solution are very similar, but the language is quite different, and so is some of the science, especially the importance of solvation in Electrochemistry. This talk explores some of these issues
C. Friesen, 4_16_09, A guest lecture in John Venables’ Surfaces Course
The Vacuum-Solid Interface
0 1 2 3 4 5 6 7 8 9
-0.4
-0.2
0.0
0.2
0.4
0.6
S+I
Relaxed
S+I Unrelaxed
dS Surface
dI Interface
S+
I (N
/m)
Al Thickness (ML)
2.0
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
d (A
)
1 2 3 4 5 6 7
-10
-8
-6
-4
-2
0
(1 ML Al)
(2 ML Al)
(3 ML Al)
(4 ML Al)
(5 ML Al)
(6 ML Al)
(7 ML Al)
12 ML Al Slab
film-
bulk (
e-/a
.u.3 )
x100
0
Depth (ML)
0 1 2 3 4 5 6 7 8 9
-0.15
-0.10
-0.05
0.00
0.05
0.10
0.15
S+I
Relaxed
S+I Unrelaxed
dS Surface
dI Interface
S+
I (N
/m)
Ag Thickness (ML)
2.30
2.32
2.34
2.36
2.38
2.40
2.42
2.44
2.46
d (A
)
1 2 3 4 5 6
-2.0
-1.5
-1.0
-0.5
0.0
0.5
(1 ML Ag)
(2 ML Ag)
(3 ML Ag)
(4 ML Ag)
(5 ML Ag)
(6 ML Ag)
12 ML Ag slab
fil
m-
bulk (
e-/a
.u.3 )
x10
00
Depth (ML)
C. Friesen, 4_16_09, A guest lecture in John Venables’ Surfaces Course
Sputter deposition
http://en.wikipedia.org/wiki/File:Sputtering.gif
Sputtering occurs by:
~10s keV Ar+ ions
-Fields of order ~100 kV/m
-~99% of ejected atoms are not ionized.
-Sputtered atoms have kineticEnergies of order 10-100s eV
-Sputtered atoms have high “T”~106K while evaporated metalAtoms might be 0.1 eV or ~103K
C. Friesen, 4_16_09, A guest lecture in John Venables’ Surfaces Course
The Electrolyte-Solid Interface
↓
↓↓
↓↓
↓
↓↓
↓↓
↓↓
↓
↓
↓ ↓↓+
↓
↓
↓ ↓↓+
↓
↓
↓ ↓↓+
↓
↓
↓ ↓↓+
↓
↓
↓ ↓↓+
↓
↓
↓ ↓↓+
↓
↓
↓
+
↓
↓
↓
+IHL OHL
l l l l l l
↓
Solvated ions
Electrode surface
1-10 nm
The double-layer region is:
Where the truncation of the metal’s Electronic structure is compensated forin the electrolyte.
1-10 nm in thickness
~1 volt is dropped across this region…
Which means fields of order 107-8 V/m
“The effect of this enormous field at the electrode-electrolyte interface is, in a sense, the essence of electrochemistry.” [1]
[1] Bockris, Fundamentals of Electrodics, 2000
C. Friesen, 4_16_09, A guest lecture in John Venables’ Surfaces Course
Sputtering vs. Electrochemical deposition……As in-the “Power of Solvation” (say it with an evangelists flair!)
-Sputtering results in ~100s eV atoms being generated-Electrochemical reactions usually involve ~1e*1V ~1eV
-Keep in mind that this could correspond to the same net result:-stripping of atoms from one surface and depositing them
on another
-PVD: Simple/easy to define interface : Complex equipment-EC: Complex/difficult to define interface : Simplest possible equipment
-PVD: Line-of-sight deposition, massive supersaturationEC: Surface-normal deposition, operating very close to equilibrium
Each has its own advantages and challenges…
C. Friesen, 4_16_09, A guest lecture in John Venables’ Surfaces Course
Take the case of Cu.
Vapor/Solid: 300 kJ/mol heat of vaporizationBoiling point: 2843 K
Electrolyte/Electrode:Valency = 2Eo=340 mV vs. SHE
Supersaturation: overpotential vs. partial pressure
ln ipRT
p
lnoi i iRT a
G nF E
lno RTE E a
nF
How do driving forces (T vs. V) compare in the two systems?
C. Friesen, 4_16_09, A guest lecture in John Venables’ Surfaces Course
Supersaturation: overpotential vs. partial pressure
lno RTE E a
nF
-0.3 -0.2 -0.1 0.0 0.1 0.2 0.31E-10
1E-8
1E-6
1E-4
0.01
1
100
10000
0 2000 4000 6000 8000 10000
activ
ity (
p i/p &
[M
+]/[
M])
Electrochemical Potential (V in Volts)
Temperature (K)
C. Friesen, 4_16_09, A guest lecture in John Venables’ Surfaces Course
A few practical matters…
C. Friesen, 4_16_09, A guest lecture in John Venables’ Surfaces Course
The Electrochemical Series & Electrochemical Phase Diagrams
+1.50
+0.80
+0.34
0
-0.13
-0.44
-0.76
-1.66
-2.37
-2.71
-2.87
-2.92
-3.03
E° (volts)equilibrium
+1.50
+0.80
+0.34
0
-0.13
-0.44
-0.76
-1.66
-2.37
-2.71
-2.87
-2.92
-3.03
E° (volts)equilibriumPourbaix Diagrams
C. Friesen, 4_16_09, A guest lecture in John Venables’ Surfaces Course
3-electrode cells and potentiostats
http://en.wikipedia.org/wiki/Potentiostat
Feedback circuit
Working Electrode
Reference ElectrodeCounter Electrode
C. Friesen, 4_16_09, A guest lecture in John Venables’ Surfaces Course
Cyclic voltammograms, etc…
-0.6 -0.4 -0.2 0.0 0.2
-1.2
-0.8
-0.4
0.0b)
f (N
/m)
E (V vs MSE)
a)
-200
-100
0
100
Pt {111} Ru/Pt {111} Ru {0001}
i (A
/cm
2 )
-0.6 -0.4 -0.2 0.0 0.2
-1.2
-0.8
-0.4
0.0b)
f (
N/m
)
E (V vs MSE)
a)
-200
-100
0
100
Pt {111} Ru/Pt {111} Ru {0001}
i (A
/cm
2 )
Current: “+” is oxidation or “anodic” current “-” is reduction or “cathodic” current
Potential: positive is synonymous with anodicnegative is synonymous with cathodic
C. Friesen, 4_16_09, A guest lecture in John Venables’ Surfaces Course
Naming electrodes
The colloquial use of “anode” and “cathode” can get confusing:-The anode is the *negative* electrode and the cathode is the *positive* electrode in a battery or fuel cell. -In an electrolyzer or other driven cell its just the opposite.
However, the formal definition is clear: the anode is where the oxidation reactionoccurs and the cathode is where the reduction reaction occurs
V
M/M+ H+/ ½H2
C. Friesen, 4_16_09, A guest lecture in John Venables’ Surfaces Course
-100 -75 -50 -25 0 25 50 75 100
io
C1
C2
C1
C2
-io
Cur
rent
Potential (mV vs Eo)
Oxidation (M to M+)
Reduction (M+ to M) Net Current
)(exp)()1(
exp0 EERT
nFEE
RT
nFii Butler-Volmer Equation:
A Comment on Exchange Current Density
C2 > C1
+
+ +
+
+
C. Friesen, 4_16_09, A guest lecture in John Venables’ Surfaces Course
Surface excess quantities
C. Friesen, 4_16_09, A guest lecture in John Venables’ Surfaces Course
Laplace pressure and Charge vs. the Lippmann Equation and Electrocapillarity
qV
2( )q f VV
P
Liquid
Solid 2 fP
r
Laplace
Lippman
Liquid
Solid
C. Friesen, 4_16_09, A guest lecture in John Venables’ Surfaces Course
Electrocapillarity
-0.6 -0.4 -0.2 0.0 0.2
-1.2
-0.8
-0.4
0.0b)
f (
N/m
)
E (V vs MSE)
a)
-200
-100
0
100
Pt {111} Ru/Pt {111} Ru {0001}
i (
A/c
m2 )
Mercury-Drop Pt & Ru
D. C. Grahame, “Theory of Electrocapillarity”, Chem. Rev. 41, 441 (1947).