Future Projects on MI Instrument

1
Future Projects on MI Instrument

• May 1, 2006

2
Ultimate Goal

• While experiments done on our UHV/LT STM provide
  great insight into chemical systems, the
  operating conditions are not practical for real
  world application.
• The advantages of the MI instrument is that it
  works in an ambient environment (i.e. room temp.
  and at 1 atm.), which allows for easy application
  to industrial processing conditions.

3
In situ STM

• We are unable to achieve atomic resolution
  (except for HOPG) on the MI instrument due to the
  ease with which the metal surface can become
  contaminated in air (hydrocarbons and water).
• Sonnenfield and Hansma in 1986 were the first to
  use STM to study a surface immersed in a liquid.1
• In 1990, Magnussen et al. achieved atomic
  resolution on a metal surface.1

Figure from Ref. 2
4
Development of In Situ STM

• Depended on three advances1
• The development of the STM by Binnig and Rohrer
• The development of surface preparation methods in
  ambient conditions.
• The development of methods and materials to coat
  the STM tip and to couple the STM with a
  biopotentiostat.
• This technique provides information on surface
  processes such as phase transitions in adlayers
  on a molecular and atomic level.

5
Comparing UHV and In Situ Images of Au
(herringbones)
Image of Au(111) under UHV
Image of Au(111) under 0.1 M HClO4 solution1
6
Comparing UHV and In Situ Images of Au (atomic
res.)
Image of Au(111) under UHV
Flame-annealed Au(111) under clean mesitylene3
7
Comparing Ambient and In Situ Images of HOPG
Image of HOPG in air File 3-9-06HOPG009
Image of HOPG under phenyloctane2
8
Comparing Ambient and In Situ Images of Molecules
on Au
C10, C12 SAM on Au(111) in air File
3-15-06AuMicaSAMVap028
L-cyseteine molecules on Au(111) under
perchlorate solution4
9
Electrochemistry in STM

• Schematic of a sample molecule coadsorbed with
  reference molecules on a substrate as probed by
  an STM tip.
• RE and CE represent the reference and counter
  electrodes, respectively.
• Vsub and Vbias are the substrate potential (with
  respect to the reference electrode) and the
  tip-substrate bias voltage, respectively, which
  are controlled independently by a bipotentiostat.5

10
Electrochemistry in STM

• Because the charge transfer event central to
  electrochemical reactivity occurs within a few
  atomic diameters of the electrode surface, the
  detailed arrangement of atoms and molecules at
  this interface strongly controls the
  corresponding electrochemical activity1.
• Cycling the potential causes significant changes
  in the surface topography, from changing how
  molecules adsorb to the surface to causing
  reconstructions of the metal atoms themselves.

11
Insulating Tips

• Because the faradaic background from a bare metal
  wire immersed in solution can approach several
  milliamps of current while tunneling currents are
  typically on the order of nanoamps, the STM tip
  must be insulated.
• The tip is insulated by coating all but the very
  end with an insulator so that the tunneling
  current will not be overcome by the
  electrochemical background.1
• A variety of materials may be used to coat the
  tip, specifically wax and nail polish.

12
Tip Etching

• Extremely sharp tips with low aspect ratios are
  prepared by chemically etching the tip in a 1 M
  basic solution (KOH).
• The etching current, which depends on the area of
  immersed wire and applied voltage is adjusted to
  an initial value.
• This process produces a neck shape near the
  air-solution interface.6

13
Tip Etching

• As the etching proceeds, the neck-like region
  becomes thinner and thinner, and eventually the
  lower portion drops off.
• This causes an abrupt decrease in the current.
• A very sharp tip with a small protrusion at the
  end can be made by switching off the circuit as
  the current abruptly drops.6

14
Wax Insulation of Tips

• Most common method uses Apiezon-brand wax
• The sharp etched tips are mounted vertically on a
  manipulator.
• A copper plate is heated and used to melt the
  wax.
• A rectangular slit in the plate provides a
  temperature gradient for the melted wax.
• The tip is brought from underneath the slit by
  means of the manipulator.6

15
Wax Insulation of Tips

• The tip is first moved slowly into the hot wax
  and allowed to attain a thermal equilibrium and
  uniform wetting.
• The tip is then raised through the wax and
  allowed to break the top surface region of the
  melt.
• The tip is moved sideways out of the slit so as
  to leave the very end of the tip unperturbed.6

16
Procedure for Wax Insulation of Tips
From Ref. 6
17
Images of Wax Coated Tips
SEM image of EC STM tips, insulated with double
(a) and single (b) pulling methods7
18
Nail Polish Insulation of Tips

• Multiple articles cited using nail polish to coat
  their tips, however the exact coating procedure
  could not be found.

19
Reconstructions

• Metal surfaces in UHV reconstruct in order to
  minimize their surface energy.
• The extent of reconstruction is strongly
  dependent on the work function of the metal.
• The electrochemical environment offers an
  opportunity to systematically vary the electronic
  state of a surface, through the application of
  potential and the influence of adsorbed species
  in solution.1

20
Adsorption

• Adsorption induces changes in the work function
• modifications of the surface dipolar layer
• particularly if significant charge transfer
  occurs between the adsorbate and surface
• measurements of ?F yield critical information on
  the degree of charge reorganization upon
  adsorption
• ?? ?adsorbate covered - ?clean

21
Au Reconstructions

• Reconstructions can be removed electrochemically
  by placing the electrode at sufficiently positive
  potential.
• The removal of reconstruction can be attributed
  to the adsorption of electrolyte anions at higher
  potentials.
• Cycling the potential to a region where the
  herringbone reconstruction is removed and then
  back reveals changes in the shape of the step
  edges on the surface, showing that the extra
  material required in the compressed structure is
  taken from and returns to the step edges.1

22
Images of Au Reconstructions
Typical Au(111) 23 X v3 reconstruction pattern.
The image was obtained for Au under pure water at
0 mV.8
Typical image of Au(111) after the
transformation. The image was obtained for Au
under water after the surface potential was
raised to 400 mV.8
23
Sulfate on Au (111)

• Sulfate is known to form a (v3 x v7)R19.1
  structure on Au(111)
• The coadsorption of H3O ions is necessary to
  stabilize the ordered oxoanion adlattices.
• Both species in H2SO4, sulfate (10) and
  bisulfate (90) have 3 free oxygen atoms to
  interact with the surface. The distance between
  them (2.47 Å) is of the same order of magnitude
  as the distance between Au atoms (2.88 Å), so
  their geometrical arrangement matches that of the
  Au (111) surface.9

24
Sulfate on Au (111)

• The reason for the presence of non-uniform
  anion-anion distances is the formation of
  H-bridge bonds between the oxygen atoms of the
  oxoanions and the coadsorbed H3O ions.9

25
Images of Sulfate on Au(111)

• In situ STM image (10x10 nm2) of a Au(111)
  electrode in 0.1 M H2SO4 showing both the (v3 x
  v7)R19.1 sulfate structure, (upper and lower
  parts) and the (1x1) substrate (middle part).
• The potential was switched from 0.80 to 0.65 V
  and then back to 0.80 V at the points marked by
  the arrows.
• The triangles and circles drawn on the middle
  part of the image represent the positions of the
  sulfate and hydronium ions, respectively.9

26
Images of Sulfate on Au(111)

• (B) Model of the(v3 x v7)R19.1 sulfate structure
  on Au(111) in 0.1 M H2SO4
• The H3O ions are placed on top of the Au atoms.
• Every H3O adsorbed can form 3 H-bridge bonds
  with the oxygen atoms of surrounding sulfate
  ions.9

27
Intro. To Cyclic Voltammetry

• The voltage is swept between two values at a
  fixed rate, when the voltage reaches V2 the scan
  is reversed and the voltage is swept back to V1.11

28
Intro. To Cyclic Voltammetry

• In the forward sweep, as the voltage is swept
  further to the right (to more reductive values) a
  current begins to flow and eventually reaches a
  peak before dropping. To rationalize this
  behavior we need to consider the influence of
  voltage on the equilibrium established at the
  electrode surface. If we consider electrochemical
  reduction, the rate of electron transfer is fast
  in comparison to the voltage sweep rate.11 (i.e.
  Fe3 ? Fe2)

29
Intro. To Cyclic Voltammetry

• When the scan is reversed we simply move back
  through the equilibrium positions gradually
  converting electrolysis product back to
  reactant.(Fe2 ? Fe3) The current flow is now
  from the solution species back to the electrode
  and so occurs in the opposite sense to the
  forward sweep.11

30
Cyclic voltammogram of Au(111) in 0.1 M H2SO4

• The peak at 0.55 V is attributed to the lifting
  of the (23 x v3) reconstruction that takes place
  in the lower potential region.
• The two sharp peaks around 1.0 V are due to the
  formation of an ordered sulfate structure at more
  positive potentials.10

31
Underpotential Deposition

• The electrodeposition of a metal on a foreign
  metal at potentials less negative than the
  equilibrium potential of the deposition reaction.
  Such a process is energetically unfavorable and
  it can occur only because of a strong interaction
  between the two metals, with their interaction
  energy changing the overall energetics to
  favorable. Consequently, only one (very seldom
  two) monolayer can be deposited this way, and
  this is a very convenient way to produce
  well-controlled monolayer deposits.12

32
Underpotential Deposition

• Upd monolayers are formed by the deposition of
  low work function metals onto high work function
  metals.
• The monolayer originates from a relatively strong
  adatom-substrate bond formed using less energy
  than required for adatom-adatom bonds formed
  during bulk deposition.
• One of the most intriguing aspects of upd is the
  anion dependence, which derives from coadsoprtion
  of the anion and the adatom.1

33
Underpotential Deposition of Cu on Au (111)

• One of the first examples of atomic resolution in
  the electrochemical environment was Cu monolayers
  on Au (111) in H2SO4.
• Three different structures are seen before bulk
  Cu deposition.1

34
Images of Underpotential Deposition of Cu on Au
(111)

• At positive potentials (300 mV), the bare
  Au(111) surface is seen.1

35
Images of Underpotential Deposition of Cu on Au
(111)

• Ordered adlayer with (v3 x v3)R30 structure,
  ascribed to coadsorbed sulfate.
• Formed between 200 and 100 mV.1

36
Images of Underpotential Deposition of Cu on Au
(111)

• Full Cu monolayer in registry (1x1) with
  Au(111).1
• At 5 mV

37
Underpotential Deposition of Cu on Au (111)

• Different solutions of anions give rise to
  different structures on the electrode surface.
• Cl- anions form both (2 x 2) and (5 x 5)
  incommensurate structures depending on the conc.
  of the anion.
• On other low Miller index faces of Au, Cu does
  not exhibit the pronounced dependence on the type
  and conc. of anion.1

38
Conclusions

• In situ STM allows for atomic resolution under
  ambient conditions.
• Electrochemical STM can be used to understand the
  electrochemical double layer and to correlate
  detailed structure of the electrode surface with
  the double-layer structure and ultimately with
  electrochemical response.
• Studies of the upd processes reveal a rich
  structural and reactive chemistry, the detailed
  nature of which is dependent on potential,
  available anions, substrate orientation, and
  substrate identity.1

39
References

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  1129-1162.
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• Han, W. Li, S. Lindsay, S. M. Gust, D. Moore,
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