ECS Paper

1
ECS Paper 116Correlating the Kelvin Probe Work
Function with In-Solution Electrochemical
Potentials

• F. J. Martin1, D. C. Hansen2, P. M. Natishan3
• 1GEO-Centers, Inc. Ft. Washington, MD
• 2Princeton Applied Research, Oak Ridge, TN
• 3U. S. Naval Research Laboratory, Washington, DC
• Tuesday, May 11, 2004

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Research Objectives

• Stratmann (and others) have shown direct
  correlation between Ecorr and work function
  (Volta potential) for metals in solution
• We want to answer some practical application
  questions
• Progression of WF with Ecorr is it a function of
  potential or simply bulk metal composition?
• Protective passive films does WF follow Ecorr
  for a passive metal surface?
• Scan rate effects how soon after changing
  potential will WF manifest the change?

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Work function

• Defined as The potential that an electron at
  the Fermi level must overcome to reach the level
  of zero kinetic energy away from the surface to
  infinity.

SR Bare and GA Somajai, Surface Chemistry in
Encylopedia of Phys. Sci. and Techn. 18401-403
(2002).
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Kelvin Probe Principle and Operation
?sample
? Work Function
?probe
E Fermi Level
Vc
Metal Probe
EProbe
EProbe
ESample
Metal Sample
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Kelvin Probe Principle and Operation
Vb
Iac 0 when Vb -Vc
Iac
Lock-in Amplifier
30 ?m
Iac
EProbe
ESample
Metal Sample
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In-Situ Work Function andEcorr Measurements
Vb
Iac
Reference Electrode
Ecorr
Iac
EProbe
ESample
Thin layer solution
Metal Sample
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Correlation of ? to Ecorr(Simultaneous
measurement of wf and Ecorr)
Source S. Yee, R.A. Oriani and M. Stratmann,
JECS 138 (1991)
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WF related to Volta, surface, and Galvani
Potentials

• When 2 metals having different work functions
  are electrically connected,
• electrons will distribute themselves such that
  an equilibrium of charge will be
• established
• This redistribution of electrons establishes a
  contact potential, ?, and any contact
• potential difference (CPD) between the work
  functions of two metals in contact at
• thermal equilibrium is defined as
• The work function of a solid material at a
  solid/liquid interface can be divided into
• two components
• - the contact potential (also defined as the
  Volta potential), ?
• - the surface potential, ?
• which when added together is defined as the
  Galvani potential
• ?surface ? ?

?? ?probe - ?metal surface
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Potential Distribution at Metal/Oxide/Solution
Interface
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In-Situ Work Function andPolarization
Measurements
Vb
Iac
Reference Electrode
Iac
Pstat E, i
EProbe
ESample
Thin layer solution
Metal Sample
Counter Electrode
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Cyclic Polarization of Alloy 625

• Alloy 625 in 0.6M NaCl, area7.42 cm² (screened
  by anti-crevice tape)
• Cyclic Polarization Scans 1V vs AgCl
• Scan rates range from 1mV/s to 1V/s
• Thin layer solution, approx 300-600µm
• Operating in conditions where passive film is
  stable, and pitting does not occur

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SKP Response to Cyclic Polarization 1mV/s
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SKP Response to Cyclic Polarization 1V/s
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SKP Response to Cyclic Polarization

• SKP responded to externally applied potentials,
  matching response very closely for slower scan
  rates (1mV/s) and with significant loss for
  higher scan rates.
• SKP appeared to follow changes in electrochemical
  potential, and NOT changes in current density
• Problem how to know if signal attenuation is due
  to interfacial work function phenomena at passive
  film, or SKP amplifier electronics?

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SKP Electronics Verification Experiment
Vb
Iac
Iac
EProbe
ESample
1 kOhm Resistor
Zinc Sample
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SKP Electronics Verification 1V/s
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SKP Electronics Verification 300mV/s
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SKP Electronics Verification 100mV/s
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SKP Peak Comparison SummaryAlloy 625/In-Line
Resistor
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Conclusions

• SKP Work Function followed changes in
  (potentiostatically-controlled) electrochemical
  potential for a passive alloy specimen immersed
  in a thin later electrolyte
• SKP WF showed some signal loss for higher CV scan
  rates (in excess of 30mV/s)
• SKP amplifier losses were responsible for part of
  higher frequency signal loss in CVs
• Some portion of SKP signal loss may also be due
  to presence of passive film/water interface