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Future Projects onMI Instrument
May 1, 2006
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Ul
timate Goal
While experiments done on ourUHV/LTSTM provide great insight into chemical
systems, the operating conditions are notpractical 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.), whichallows for easy application to industrialprocessing conditions.
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In situ ST
M We are unable to achieve
atomic resolution (except forHOPG) on the MI instrumentdue to the ease with which the
metal surface can becomecontaminated in air(hydrocarbons and water).
Sonnenfield and Hansma in1986 were the first to use STMto study a surface immersed ina liquid.1
In 1990, Magnussen et al.achieved atomic resolution ona metal surface.1
Figure from Ref. 2
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D
evel
opment of In Situ ST
M Depended on three advances1:
The development of the STM by Binnig and Rohrer
The development of surface preparation methods inambient conditions.
The development of methods and materials to coatthe STM tip and to couple the STM with a
biopotentiostat.This technique provides information on surface
processes such as phase transitions in adlayers on amolecular and atomic level.
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Comparing UHV and In Situ
Images of Au (herringbones)
Image of Au(111) under 0.1 MHClO4 solution1
Image of Au(111) underUHV
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Comparing UHV and In Situ
Images of Au (atomic res.)
Flame-annealed Au(111) underclean mesitylene3
Image of Au(111) underUHV
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Comparing Ambient and In Situ
Images ofHOPG
Image ofHO
PG underphenyloctane2Image ofHOPG in air
File: 3-9-06HOPG009
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Comparing Ambient and In Situ
Images of Molecules on Au
L-cyseteine molecules on Au(111)under perchlorate solution4
C10, C12 SAM on Au(111) in air
File: 3-15-06AuMicaSAMVap028
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El
ectroch
emistry in ST
M Schematic of a sample molecule
coadsorbed with referencemolecules on a substrate asprobed by an STM tip.
RE and CE represent thereference and counter electrodes,respectively.
Vsub and Vbias are the substratepotential (with respect to thereference electrode) and the tip-substrate bias voltage,respectively, which are controlledindependently by a bipotentiostat.5
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El
ectroch
emistry in ST
M Because the charge transfer event central to
electrochemical reactivity occurs within a fewatomic diameters of the electrode surface, thedetailed arrangement of atoms and molecules atthis interface strongly controls the correspondingelectrochemical activity1.
Cycling the potential causes significant changes
in th
e surface topograph
y, from ch
angingh
owmolecules adsorb to the surface to causingreconstructions of the metal atoms themselves.
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Insul
atingT
ips Because the faradaic background from a bare
metal wire immersed in solution can approach
several
mill
iamps of current wh
il
e tunnel
ingcurrents 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 insul
ator so th
at th
e tunnel
ingcurrent will not be overcome by theelectrochemical background.1
A variety of materials may be used to coat thetip, specifically wax and nail polish.
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T
ipE
tch
ing Extremely sharp tips withlow 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
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T
ipE
tch
ing As the etching proceeds,
the neck-like regionbecomes thinner and
thinner, and eventuallythe lower portion dropsoff.
This causes an abruptdecrease in the current.
A very sh
arp tip with
asmall protrusion at theend can be made byswitching off the circuit asthe current abruptlydrops.6
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W
ax Insul
ation ofT
ips Most common method uses Apiezon-brand wax The sharp etched tips are mounted vertically on
a manipul
ator. A copper plate is heated and used to melt thewax.
A rectangular slit in the plate provides atemperature gradient for the melted wax.
The tip is brought from underneath the slit bymeans of the manipulator.6
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W
ax Insul
ation ofT
ips 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 ofthe melt.
The tip is moved sideways out of the slitso as to leave the very end of the tipunperturbed.6
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Procedure forWax Insulation of
Tips
From Ref. 6
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Images ofW
axC
oatedT
ips
SE
M image ofEC
ST
M tips, insul
ated with
doubl
e (a
) andsingle (b) pulling methods7
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N
ail
Pol
ish
Insul
ation ofT
ips Multiple articles cited using nail polish to
coat their tips, however the exact coating
procedure could not be found.
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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 themetal.
Th
e el
ectroch
emical
environment offers anopportunity to systematically vary theelectronic state of a surface, through theapplication of potential and the influence of
adsorbed species in solution.1
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Adsorption
Adsorption induces changes in the workfunction modifications of the surface dipolarlayer
particularly if significant charge transfer occursbetween the adsorbate and surface
measurements of yield critical information
on the degree of charge reorganization uponadsorption
= adsorbate covered - clean
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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 revealschanges in the shape of the step edges on the surface,showing that the extra material required in thecompressed structure is taken from and returns to thestep edges.1
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Images of Au Reconstructions
TypicalAu(111) 23 X 3 reconstruction pattern.The image was obtained for Au under purewater at 0 mV.8
Typical image of Au(111) after thetransformation. The image was obtained for Auunder water after the surface potential was
raised to 400 mV.8
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Sul
fate on Au (111) Sulfate is known to form a (3 x 7)R19.1 structure on
Au(111) The coadsorption ofH3O+ 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 thesurface. The distance between them (2.47 ) is of thesame order of magnitude as the distance between Auatoms (2.88 ), so their geometrical arrangementmatches that of the Au (111) surface.9
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Sul
fate on Au (111) The reason for the presence of non-
uniform anion-anion distances is the
formation ofH-bridge bonds between theoxygen atoms of the oxoanions and thecoadsorbed H3O+ ions.9
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Images of Sul
fate on Au(111) In situ STM image (10x10 nm2) of a
Au(111) electrode in 0.1 M H2SO4showing both the (3 x 7)R19.1sulfate structure, (upper and lower
parts) and the (1x1) substrate (middlepart).
The potential was switched from 0.80to 0.65 V and then back to 0.80 V atthe points marked by the arrows.
The triangles and circles drawn on themiddle part of the image represent thepositions of the sulfate and hydroniumions, respectively.9
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Images of Sul
fate on Au(111) (B) Model of the(3 x
7)R19.1 sulfatestructure 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 bondswith the oxygen atoms ofsurrounding sulfate ions.9
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Intro.T
oC
ycl
icV
ol
tammetry The voltage is swept
between two values
at a fixed rate, whenthe voltage reachesV2 the scan isreversed and the
voltage is swept backto V1.11
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Intro.T
oC
ycl
icV
ol
tammetry In the forward sweep, as the voltage is swept
further to the right (to more reductive values) acurrent begins to flow and eventually reaches apeak before dropping. To rationalize thisbehavior we need to consider the influence ofvoltage on the equilibrium established at theelectrode surface. If we consider electrochemicalreduction, the rate of electron transfer is fast incomparison to the voltage sweep rate.11 (i.e.Fe3+ Fe2+)
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Intro.T
oC
ycl
icV
ol
tammetry When the scan is
reversed we simply moveback through the
equilibrium positionsgradually convertingelectrolysis product backto reactant.(Fe2+ Fe3+)The current flow is now
from the solution speciesback to the electrode andso occurs in the oppositesense to the forwardsweep.11
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Cyclic voltammogram of Au(111) in
0.1 M H2SO4 The peak at 0.55 V is
attributed to the lifting ofthe (23 x 3)
reconstruction th
at takesplace in the lowerpotential region.
The two sharp peaksaround 1.0 V are due tothe formation of an
ordered sulfate structureat more positivepotentials.10
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UnderpotentialDeposition
The electrodeposition of a metal on a foreignmetal at potentials less negative than theequilibrium potential of the deposition reaction.
Such a process is energetically unfavorable andit can occur only because of a strong interactionbetween the two metals, with their interactionenergy changing the overall energetics tofavorable. Consequently, only one (very seldomtwo) monolayer can be deposited this way, andthis is a very convenient way to produce well-controlled monolayer deposits.12
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UnderpotentialDeposition
Upd monolayers are formed by the deposition oflow work function metals onto high work functionmetals.
The monolayer originates from a relativelystrong adatom-substrate bond formed using lessenergy than required for adatom-adatom bondsformed during bulk deposition.
One of the most intriguing aspects of upd is theanion dependence, which derives fromcoadsoprtion of the anion and the adatom.1
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UnderpotentialDeposition ofCu 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
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Images ofUnderpotential
Deposition ofCu on Au (111) At positive potentials
(+300 mV), the bare
Au(111) surface isseen.1
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Images ofUnderpotential
Deposition ofCu on Au (111) Ordered adlayer with
(3 x 3)R30
structure, ascribed tocoadsorbed sulfate.
Formed between 200and 100 mV.1
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Images ofUnderpotential
Deposition ofCu on Au (111) FullCu monolayer in
registry (1x1) with
Au(111).1 At 5 mV
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UnderpotentialDeposition ofCu 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 theconc. of the anion.
On otherlow Miller index faces of Au, Cu doesnot exhibit the pronounced dependence on thetype and conc. of anion.1
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Conclusions
In situ STM allows for atomic resolution under ambientconditions.
Electrochemical STM can be used to understand theelectrochemical double layer and to correlate detailedstructure of the electrode surface with the double-layerstructure and ultimately with electrochemical response.
Studies of the upd processes reveal a rich structural andreactive chemistry, the detailed nature of which isdependent on potential, available anions, substrateorientation, and substrate identity.1
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References
1) Gewirth, A. A.; Niece, B. K. Chem. Rev. 1997, 97, 1129-1162.2) De Feyter, S.; Gesquiere, A.; Abdel-Mottaleb, M. M.; Grim, P. C.; De Schryver, F. C.; Meiners,
C.; Sieffert, M.; Valiyaveettil, S.; Mullen, K. Acc. Chem. Res. 2000, 33, 520-531.3) Han, W.; Li, S.; Lindsay, S. M.; Gust, D.; Moore, T. A.; Moore, A. L. Langmuir. 1996, 12, 5742-
5744.4) Dakkouri, A. S.; Kolb, D. M.; Edelstein-Shima, R.; Mandler, D. Langmuir. 1996, 12, 2849-2852.5) Tao, N. J. Phys. Rev. Lett. 1996, 76, 4066-4069.6) Nagahara, L. A.; Thundat, T.; Lindsay, S. M. Rev. Sci. Instrum. 1989, 60, 3128-3130.7) Kazinczi, R.; Szocs, E.; Kalman, E.; Nagy, P. Appl. Phys. A. 1998, 66, S535-S538.8) Tao, N.J.; Lindsay, S. M. J. Appl. Phys. 1991, 70, 5141-5143.9) Cuesta, A.; Kleinert, M.; Kolb, D. M. Phys. Chem. Chem. Phys. 2000, 2, 5684-5690.10) Climent, V.; Coles, B. A.; Compton, R. G. J. Phys. Chem. B 2001, 105, 10669-10673.
11) http://www.cartage.org.lb/en/themes/sciences/Chemistry/Electrochemis/Electrochemical/CyclicVoltammetry/CyclicVoltammetry.htm
12) http://www.corrosion-doctors.org/Dictionary/Dictionary-U.htm