Underpotential Deposition Topic Review and

1
Underpotential DepositionTopic Review and
Implementation Considerations Mohan
Karulkar February 2, 2004
2
Outline

• Underpotential Deposition (UPD) Topic Review
• (What is UPD?)
• Literature Review
• (What work has been done?)
• Project Review
• (Why is it important to our group?)
• Implementation
• (What are the implementation issues?)

3
Underpotential Deposition Characteristics

• Occurs at Potentials positive of the Nernst
  Potential 7
• Occurs with metal deposition onto a foreign metal
  substrate 24
• Depends strongly on
• potential
• electrolyte composition
• local coverage
• Deposits only up to one monolayer (ML)
• Proceeds slower than bulk deposition

Figure 1 CV for Tl Deposition onto Ag
Figure 1 D. M. Kolb, in Advances in
Electrochemistry and Electrochemical Engineering,
edited by H. Gerischer and C. W. Tobias, (Wiley,
New York, 1978), Vol. 11, p. 141
4
Different UPD Mechanisms

• Additive-Free 7
• More anodic potentials ordered structures
  (islands), up to 0.5 ML
• More cathodic potentials, phase transitions
  marked by onset of lateral, metal-like
  attractions, up to Full ML 1
• Additive-Assisted 1,-3,9-11,15,16,18,19,29-32,36,3
  7,39
• Three-region, two-step mechanism.
• Most anodic region hexagonal layer of additive
• Mid-range potentials replacement of some
  additive with metal, into hexagonal structure
• Most cathodic Formation of metal monolayer with
  additive on top
• Described in detail shortly

5
OPD/UPD Comparison
6
Halide-assisted UPD
3-Region, 2-Stage mechanism (Figure 2)

• Region III Substrate covered by halide
  hexagonal structure formed
• Region II Stage 1 of UPD begins at most anodic
  peak some halide is displaced, eventually
  forming Region II intermediate hexagonal
  structure with both Metal and halide present
• Region I Stage 2 begins at less-anodic peak
  halide further displaced by metal. Region I
  structure consists of pseudomorphic monolayer of
  Metal covered by ordered halide layer

Figure 2 CV of Cu UPD on Pt(111) 0.001M
Cu(ClO4)2 and 0.01M NaCl. Shows 3 regions of
UPD. Measured vs. SCE.
Figure 2 Tidswell, C. Lucas, N. Markovic, P.
Ross, Phys Rev B 51, 10205 (1995)
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Halide-assisted UPD Example
End of Region II
Region III
Region I
Start of Region II
Figure 3 Various Stages of halide-assisted UPD
Figure 3 N. Markovic, C. Lucas, H. Gasteiger, P.
Ross, Surface Science 372, 239 (1997)
8
Phenomena to Capture

• Geometry
• Region 3 (additive case) is hexagonal, region 2
  is hexagonal, region 1 is pseudomorphic, etc
• Necessary geometry affects what sites can
  experience adsorption

• Potential
• Growth regions separated by potential
• Coverage
• Deposit forms in each growth region only up to a
  certain local coverage

Fig 4 Examples of different geometric
interactions
Figure 4 M. Itoh, G. Bell, A. Avery, T. Jones,
B. Joyce, D. Vvedensky, Physical Review Letters
81, 633 (1998)
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UPD Literature Review
Review of Cu UPD Experimental Work

• Surfaces
• Pt(111) and (001) 2,10,15-17,29-32,36
• Au(111) and (001) 1,3,9,11,18,19,37,39
• Additives
• Halide, Sulphate, Perchlorate
• Results
• Same general 2-stage mechanism seen for all
  single-additive systems
• Differences in threshold concentrations, detailed
  structure of metal/additive intermediate,
  additive layer in Region I, etc

10
UPD Literature Review
Review of Cu UPD Simulation Work

• Pade approximants (Cu on Au(111)) 21, 22
• Interpolated between Langmuir isotherm and Ising
  model to derive coverage as function of potential
  or electrovalence
• Lattice Gas Model (LGM) (Cu on Au(111))
• Simulated two-component adsorption system to
  study lateral adsorbate-adsorbate interactions up
  to fourth-nearest neighbors.

11
UPD Literature Review
Review of Cu UPD Simulation Work

• Dynamic Monte Carlo Method (Cu on Au(111)) 13
• Combined with LGM to simulate thermally activated
  motion of particles adsorbing, desorbing, and
  diffusing
• Used LG Hamiltonian to get free energies for
  different geometric configurations
• Kinetic Monte Carlo Method (homoepitaxy,
  GaAs(001)) 25
• Included Ga deposition, hopping, As2 deposition
  and desorption
• Used to obtain energy barriers for different
  surface interactions

12
Project Review

• Experimental study and Simulation of Metallic
  Nanoclusters
• Potential gradient along resistive strip
• UPD-like conditions at deposition nose
• Detailed in 2003 NIRT Proposal

• Bohn group (Brian Coleman) experiments
• Experimental setup is a resistive strip
• Investigation of deposition nose

Fig 5 Deposition along linear potential gradient

13
Project Review

• Dima Lubomorskys Cell
• Mono-electrode
• Must calculate potential distribution
• Feng Xuis Cell
• Bi-potential
• Known potential distribution
• Both cells will exhibit regions of underpotential
  deposition

Fig 6Lubomorsky Cell design
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Implementation

• Need Rate Law to use for KMC algorithm
• Rate law must obey detailed balance
• Rate of going from state c to c in equilibrium
  is same as c going to c.
• Simplest rate law 13
• state2 and state1 are the energy of state 1 and 2
• u attempt frequency, D free-energy barrier
• Obtain energy of a state from Hamiltonian

15
Implementation

• Energy of a particular configuration given by
  Grand-canonical lattice-gas Hamiltonian12, 13

• m electrochemical potential
• ci concentration of adsorbate at site i (1 or
  0)
• Index n represents separation between sites
• represents sum over all pairs of neighbors
  of rank
• F 2-body interaction energy

16
Implementation

• Can combine Hamiltonian with simple rate law to
  obtain KMC-ready rate expression
• Can be extended to 2-component systems by using a
  two-component lattice gas Hamiltonian 12
• Hamiltonian can be extended to include more
  components

17
Implementation

• Recall important phenomena
• Potential
• Hamiltonian has a potential-dependent term, which
  carries over to the rate law. At more negative
  potentials, this overpowers body-interaction term

• Geometry
• Hamiltonian has term for 2-body interactions,
  which can be adjusted for certain orientations
• Coverage
• Body-interaction term overpowers potential term
  at less negative potentials, leading to lower
  coverage

Fig 7 Recall different geometric interactions

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Summary

• UPD
• Deposition at potentials more positive than
  Nernst Potential
• Important aspects Potential, Geometry, Coverage
• 3-stage, 2-step mechanism
• UPD Regions Separated by potential characterized
  by geometry and maximum coverage
• Literature Review
• Experiments Pt and Au, (111) and (001) Halide,
  Sulphate, Perchlorate additives
• Simulation Pade Approximants, Lattice Gas
  Models, Dynamic Monte Carlo, Kinetic Monte Carlo
• Projects
• Bi-potential and mono-electrode cells (Feng and
  Dima)
• Implementation
• Basic Arrhenius rate law
• Energy of states obtained from Lattice Gas
  Hamiltonian

19
References

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  Garza, N. Batina, Surface Science, 476, 139
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• C. Lucas, N. Markovic, P. Ross, Physical Review
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• D. Huckaby, J. Kowalski, J. Chem. Phys, 80, 2163
  (1984)
• D. Huckaby, L. Blum, Langmuir 11, 4583 (1995)
• D. Kolb, Z. Phys. Chem. N.F. 154, 179 (1987)
• D. M. Kolb, in Advances in Electrochemistry and
  Electrochemical Engineering, edited by H.
  Gerischer and C. W. Tobias, (Wiley, New York,
  1978), Vol. 11, p. 125
• D.M. Kolb in Scherling Lecture, (Sherling
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• I. Tidswell, C. Lucas, N. Markovic, P. Ross, Phys
  Rev B, 51, 10205 (1995)

20
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21
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