C. Friesen, 4_16_09, A guest lecture in John Venables’ Surfaces Course Succinct, to-the-point...

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


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


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)


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


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


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…


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?


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)


A few practical matters…


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


3-electrode cells and potentiostats

http://en.wikipedia.org/wiki/Potentiostat

Feedback circuit

Working Electrode

Reference ElectrodeCounter Electrode


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


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


-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

+

+ +

+

+


Surface excess quantities


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


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).

C. Friesen, 4_16_09, A guest lecture in John Venables’ Surfaces Course Succinct, to-the-point...

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