Succinct, to-the-point title:
Connections between Dry & Wet
Interfaces: 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
0.15
S+I (N/m)
S+I (N/m)
2.46
2.8
0.6
0.10
S+I
Relaxed
Relaxed
S+I
2.44
2.7
S+I
Unrelaxed
Unrelaxed
S+I
dS Surface
dS Surface
2.42
2.6
dI Interface
dI Interface
0.4
0.05
2.5
2.40
0.2
0.00
2.4
2.38
0.0
-0.05
2.3
2.36
-0.15
-0.2
2.2
2.34
-0.4
2.1
2.32
10
0
21
32
34
45
56
d
(A)
d (A)
-0.10
2.0
7 8 8 9 2.30
9
67
Al Thickness
(ML)
Ag Thickness
(ML)
film-bulk (e-/a.u. ) x1000
3
0.0
12 ML Al Slab
0
(7 ML Al)
12 ML Ag slab
-2
(6 ML Ag)
(6 ML Al)
3
film-bulk (e-/a.u. ) x1000
0.5
-0.5
-1.0
-1.5
-4
(5 ML Al)
(4 ML Al) (5 ML Ag)
-6
(4 ML Ag)
(3 ML Al)
-8
(3 ML Ag)
(2 ML Al)
-10
(1 ML Ag)
-2.0
1
1
2
(2 ML Ag)
(1 ML Al)
2
3
3
4
4
5
6
5
Depth (ML)
Depth (ML)
7
6
C. Friesen, 4_16_09, A guest lecture in John Venables’ Surfaces Course
Sputter deposition
Sputtering occurs by:
~10s keV Ar+ ions
-Fields of order ~100 kV/m
-~99% of ejected atoms are
not ionized.
-Sputtered atoms have kinetic
Energies of order 10-100s eV
-Sputtered atoms have high “T”
~106K while evaporated metal
Atoms might be 0.1 eV or ~103K
http://en.wikipedia.org/wiki/File:Sputtering.gif
C. Friesen, 4_16_09, A guest lecture in John Venables’ Surfaces Course
The Electrolyte-Solid Interface
1-10 nm
The double-layer region is:
↓
↓
+
↓
↓
↓ ↓ ↓
↓
↓
↓ ↓ ↓
Which means fields of order 107-8 V/m
“The effect of this enormous field at the electrodeelectrolyte interface is, in a sense, the essence of
electrochemistry.” [1]
l
IHL
~1 volt is dropped across this region…
↓
↓
↓
+
↓
l
↓
↓
↓
l
+
↓
l
↓
↓
Where the truncation of the metal’s
Electronic structure is compensated for
in the electrolyte.
1-10 nm in thickness
+
↓
↓ ↓
↓
↓
↓
l
↓
Solvated
ions
↓
l
↓ ↓ ↓
Electrode
surface
OHL
[1] Bockris,
of Electrodics,
2000
C. Friesen, 4_16_09,
A guestFundamentals
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 supersaturation
EC: 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
Supersaturation: overpotential vs. partial pressure
Take the case of Cu.
Vapor/Solid:
300 kJ/mol heat of vaporization
Boiling point: 2843 K
Electrolyte/Electrode:
Valency = 2
Eo=340 mV vs. SHE
 pi 
  RT ln  
 p
i  io  RT ln ai
G  nF E
RT
EE 
ln a
nF
o
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
Temperature (K)
+
activity (pi/p & [M ]/[M])
0
2000 4000 6000 8000 10000
10000
100
1
0.01
1E-4
1E-6
1E-8
RT
EE 
ln a
nF 0.3
0.0 0.1 0.2
o
1E-10
-0.3 -0.2 -0.1
Electrochemical Potential (V in Volts)
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
equilibrium
E° (volts)
-3.03
Pourbaix Diagrams
-2.92
-2.87
-2.71
-2.37
-1.66
-0.76
-0.44
-0.13
0
+0.34
+0.80
+1.50
C. Friesen, 4_16_09, A guest lecture in John Venables’ Surfaces Course
3-electrode cells and potentiostats
Feedback circuit
Working Electrode
Counter Electrode
Reference Electrode
http://en.wikipedia.org/wiki/Potentiostat
C. Friesen, 4_16_09, A guest lecture in John Venables’ Surfaces Course
i (
-100
Cyclic voltammograms, etc…
Pt {111}
-200
a) 100
2
A/cm )
if ((N/m)
b)
0.00
-0.4
-100
-0.8
-200
-1.2
b)
0.0
f (N/m)
Ru/Pt {111}
Ru {0001}
-0.4
-0.8
-1.2
Pt {111}
Ru/Pt {111}
Ru {0001}
-0.6 -0.4 -0.2 0.0
E (V vs MSE)
0.2
Current: “+” is oxidation or “anodic” current
“-” is reduction or “cathodic” current
Potential: positive is synonymous with anodic
negative 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 reaction
occurs and the cathode is where the reduction reaction occurs
M/M+
H+/ ½H2
V
C. Friesen, 4_16_09, A guest lecture in John Venables’ Surfaces Course
A Comment on Exchange Current Density
+
+
+
+
+
C2
+
Oxidation (M to M )
+
Reduction (M to M)
Net Current
C1
C2 > C1
Current
io
-io
C1
C2
-100 -75 -50 -25 0 25 50 75 100
o
Potential (mV vs E )
Butler-Volmer Equation:


 exp  (1 ) nF ( E  E  )  exp  nF ( E  E  )  

0
 RT

 RT


i i
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
Laplace
Liquid
P
Solid
2f
P 
r
Lippman
Liquid
Solid

 q
V


   q  2( f   )
V


 V

C. Friesen, 4_16_09, A guest lecture in John Venables’ Surfaces Course
Electrocapillarity
Pt & Ru
100
0
2
a)
i (A/cm )
Mercury-Drop
-100
-200
Pt {111}
Ru/Pt {111}
Ru {0001}
b)
f (N/m)
0.0
-0.4
-0.8
-1.2
-0.6 -0.4 -0.2 0.0
E (V vs MSE)
0.2
D. C. Grahame, “Theory of Electrocapillarity”,
Chem.
Rev.
41,in 441
C. Friesen, 4_16_09,
A guest
lecture
John (1947).
Venables’ Surfaces Course