Future Projects on MI Instrument

May 1, 2006

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.

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

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.

Comparing UHV and In Situ Images of Au (herringbones)

Image of Au(111) under 0.1 M HClO4 solution1

Image of Au(111) under UHV

Comparing UHV and In Situ Images of Au (atomic res.)

Flame-annealed Au(111) under clean mesitylene3

Image of Au(111) under UHV

Comparing Ambient and In Situ Images of HOPG

Image of HOPG under phenyloctane2

Image of HOPG in air

File: 3-9-06HOPG009

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

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

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.

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.

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

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

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

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

Procedure for Wax Insulation of Tips

From Ref. 6

Images of Wax Coated Tips

SEM image of EC STM tips, insulated with ‘double’ (a) and‘single’ (b) pulling methods7

Nail Polish Insulation of Tips

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

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

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 ΔΦ yield critical information

on the degree of charge reorganization upon adsorption

ΔФ = Фadsorbate covered - Фclean

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

Images of Au Reconstructions

Typical Au(111) 23 X √3 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

Sulfate on Au (111)

Sulfate is known to form a (√3 x √7)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

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

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 (√3 x √7)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

Images of Sulfate on Au(111)

(B) Model of the(√3 x √7)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

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

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+)

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

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 √3) 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

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

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

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

Images of Underpotential Deposition of Cu on Au (111)

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

Images of Underpotential Deposition of Cu on Au (111)

Ordered adlayer with (√3 x √3)R30° structure, ascribed to coadsorbed sulfate.

Formed between 200 and 100 mV.1

Images of Underpotential Deposition of Cu on Au (111)

Full Cu monolayer in registry (1x1) with Au(111).1

At 5 mV

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

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

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/Cycli

cVoltammetry/CyclicVoltammetry.htm12) http://www.corrosion-doctors.org/Dictionary/Dictionary-U.htm