Thin Film and Monolayer Electrochemistry Robert J. Forster

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Thin Film and Monolayer Electrochemistry
Robert J. Forster Dublin City University Robert.F
orster_at_dcu.ie
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Density of Redox Acceptor States
Energy ?

Density of Metallic Electronic States
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Processes Within Films

• Heterogeneous electron transfer
• Electron diffusion or electron hopping in the
  film.
• Mediated interfacial electron transfer.
• Penetration of A into the film.
• Mass transfer of Q within the film.
• Movement of A through a pinhole or channel in the
  film.

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Redox Properties Finite Diffusion
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Finite Diffusion
   
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Os(bpy)2 PVPn L(PF6)x
Os(bpy)2 PVIn L(PF6)x
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STM Image 10 nm x 10 nm
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Experiment
Sweep rate dependence of the cyclic voltammograms
under finite diffusion conditions for an
Os(bpy)2(PVI)10 Cl film on a 3 mm glassy
carbon electrode. Sweep rates (top to bottom)
40, 20, 10 and 5 mV/s. Surface coverage ? is 1.8
x 10-8 mol cm-2. Supporting electrolyte is
aqueous 0.1 M p-toluene sulfonic acid.
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Outcomes

• Close to ideal electrochemical reversibility.
• Weak destabilising interactions.

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Homogeneous Charge Transport
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HOMOGENEOUS CHARGE TRANSPORT
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ip 2.69x105 n3/2 A DCT1/2C ?1/2
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Randles-Sevcik Analysis
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Ion Concentration Effects
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Randles-Sevcik Equation
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DCT vs. Electrolyte Concentration
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Distance Dependence
Separation when electron transfer occurs (10 Å)
Concentration of osmium centres
DCT Dphys 1/6 kex ?2 COs
Physical diffusion
Electron self-exchange dynamics
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Distance Dependence
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METALLOPOLYMER NANOCOMPOSITE
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SCANNING ELECTRON MICROSCOPY
Transmission electron micrographs of gold
nanoparticles in which the mole fraction of
Os(bpy)2 (PVP)10 ClCl is (A) 0.84
(magnification 1.25x106) and (B) 0.02
(magnification 8.0x105).
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DRY STATE CONDUCTIVITY
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ETCHED MICROELECTRODES
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CYCLIC VOLTAMMETRY
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CHRONOAMPEROMETRY
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FILM RESISTANCE
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CHARGE TRANSPORT - METALLOPOLYMER
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CHARGE TRANSPORT - NANOCOMPOSITE
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CHARGE TRANSPORT DIFFUSION COEFFICIENTS
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Conclusions

• Molecules can show close to ideal voltammetry
  within films.
• Electrochemistry can probe electron transfer
  rates.
• Electron transfer rates depend strongly on
  distance.
• Temperature dependence can reveal materials
  properties.

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Microelectrodes

• Defined as an electrode whose critical
  dimensions in the micrometre range.
• Typically 0.1 - 10 ?m.

• Main attributes are
• Small currents (nA - pA)
• Steady state responses (sigmoidal curves)
• Short response times (RC time constant)

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Steady State Response
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Microelectrodes vs. Macroelectrodes
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Microelectrodes Geometries
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Fabrication
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SEM image of a 1 ?m gold UME

• Note
• Elliptical shape
• Scratches on surface due to mechanical
  polishing procedure

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Mass Transport
1. How does shrinking the electrode radii affect
the diffusion process and how is this manifested
in the experimental response ?
2. How do diffusion fields evolve over time ?
To find out perform potential step experiment
from an initial value where no electrode reaction
occurs to one where electrolysis proceeds at a
diffusion controlled rate.
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Ficks law solved using Laplace transforms to give
the time evolution of the current a time
independent and time dependent term.
Two limiting regimes
Short Times
Long Times
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1. Short Times (Time dependent term)
Cottrell equation
Planar Diffusion field
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2. Long Times (Time independent term)
Spherical Diffusion field
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How to determine the times at which steady state
and transient behaviours will predominate.
Ratio of equations for short and long times
give dimensionless parameter
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• Steady state current for a disc electrode is
  given by
• iss 4nFDCr

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1. Electrode Response Times
The cell time constant is dictated by the ease
with which current can flow through solution and
the capacitance of the electrode.
In other words how to get the microelectrode to
the desired potential in the shortest possible
time.
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Why is the RC time constant important and why it
is desirable to minimise it ?
A. In order to extract meaningful data from
transient measurements it is important to
separate both the Faradaic and charging currents,
i.e., non-Faradaic currents.
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B. Meaningful electrochemical data (kinetics)
can only be extracted at timescales that are at
least 5 - 10 times longer than the RC cell
time constant.
C. The potential at the electrode interface does
not attain the desired potential until the
charging is complete.
D. The charging and Faradaic currents are
convolved at short times (High scan rates).
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How does shrinking the electrode effect the RC ?
1. Resistance
k conductivity of solution r radius of
electrode
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Note from previous equation that

• Resistance is inversely proportional to the
  electrode radius.

• R increases as the electrode radius decreases.

Therefore, microelectrodes do not reduce the
response time by reducing the resistance.
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2. Capacitance
When an electrode comes into contact with an
electrolytic solution a charge is generated on
the electrode surface (interface) which is
compensated by a layer of opposite charge coming
from the solution.
This charge is known as the electrical double
layer capacitance.
The magnitude of this capacitance is proportional
to the electrode area
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Therefore, the capacitance for microelectrodes is
dramatically reduced.
Example
Capacitance
1 mF
1 mm radius disc electrode 1 ?m radius disc
microelectrode
1 pF
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What is it and what effects does it have on
voltammetry?
1. Ohmic drop

• When Faradaic and charging currents flow
  through a solution, they generate a potential
  which acts to weaken the applied potential by an
  amount iR, where i is the total current and R is
  the resistance.

• The effect on voltammetry is distorted
  responses and a reduction in the variety of
  samples on which voltammetry can be applied.

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In order to understand ohmic drop you need to
know about resistance in solution and currents.
iR
R increases as electrodes radius decreases
6 fold reduction in current at micro
over macroelectrodes
Use of microelectrodes over macroelectrodes
enables measurements be performed in media
without interference from ohmic drop, e.g.,
organic solvents.
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Quinone monolayers
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Low Temperature Measurements
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Solids
Microelectrodes have been used to study
redox-state transitions of single crystals of
silicotungstic acid, H4SiW12O40.31H2O.
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Cross sectional view of 3 electrode
solid-state electrochemical cell
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H4SiW12O40.31H2O voltammogram
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Radical Reactions - NAD Model Systems
(A)
(B)
Cyclic voltammogram of (A) 25 mM solution of
1-Methyl-4-carbamidopyridinium perchlorate. The
sweep rate is 51.2 V/s. (B) 1-Methyl-3-carbamidopy
ridinium perchlorate, sweep rates, from top to
bottom, 1000, 500, 200, 100 and 50 V/s. The
working electrode is a 26 ?m Hg hemisphere. The
supporting electrolyte is 0.5 M TEAP in DMF.
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High Speed Electrochemistry
Resistance depends on conductivity and electrode
radius
Capacitance depends on the electrode area
CElectrode Co A
Thus, the time constant, RC, decreases as the
electrode radius decreases
RC a rs
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Reaction Rates
Dependence of logarithm of the reaction rate in
buffered aqueous solution for (a)
1-Methyl-3-carbamidopyridinium, (b)
1-Methyl-4-carbamidopyridinium and (c)
1-Methyl-3,4-dicarbamidopyridinium perchlorate on
the solution pH.