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IV. PREPARATION OF SINGLE-CRYSTAL-FACE ELECTRODES

IV. PREPARATION OF SINGLE-CRYSTAL-FACE ELECTRODES

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Double-Layer Properties at Metal Electrodes



29



during solidification from the seed. Methods developed from this

principle are:

1. The crystal-pulling method.37

2. The Bridgman method.

3. The zone-melting method.

4. The floating-zone method.

Hitherto, the last two methods have not been used by electrochemists. They have either melted the tip of a wire so as to obtain

a small sphere which in some cases is a single-crystal (platinum)

or used a modified Bridgman method.

For the Bridgman method the melt is contained in a crucible

(graphite for gold or silver); the freezing must commence at a

point—used as a seed—from which the solidification proceeds (a

conical tip of the crucible produces a "natural" but uncontrolled

selection of the seed). The solidification proceeds either by moving

the crucible or by regulating the temperature gradient, i.e., the rate

of movement of the freezing plane. This last possibility was used

for gold, silver, and copper, for example.

Vacuum

Quartz tube



D



~ - ^ \ _ Copper

/tube



o.

c

o



az



ZX5

^ _ _ Graphite



GZ

Water



Metal



Figure 13 One of the possible ways of growing a gold

(or silver) crystal in a crucible placed in the helix of an

induction oven



30



A. Hamelin



For gold or silver, the graphite crucible was placed in a quartz

tube; the quartz tube was positioned in the helix of an induction

oven; and the tube was evacuated (Fig. 13) and the temperature

gradient regulated. Mechanical vibrations must be avoided during

solidification (2-20 min for a cylinder 20 mm long). The experimenter has to know whether the sample is an individual crystal or not

when it is removed from the crucible. Its macrostructure (its general

gross structural distribution in the whole) has to be determined.

Therefore the ingot has to be macroetchedt; then grain boundaries,

if they exist, are visible to the naked eye.

The size of the crystal thus obtained may be 2-20 mm in

diameter and 5-200 mm in length.

(if) Growth from the Metal Vapor



The collector [polished glass, cleaved mica, (100) face of a

sodium chloride crystal, face of a metal, face of the metal] is

maintained at a temperature below the melting point of the metal

(in vacuum or in a neutral gas) so that crystal nuclei can be formed.

The area of the film can reach several square centimeters.

The metal is either evaporated or sputtered, so that either very

thin layers or thicker layers are obtained. In the following paragraphs, some examples are given.

Thin layers of gold, evaporated on polished glass (10-80 nm

thick) give monooriented polycrystalline electrodes; the film is made



of grains, the surfaces of which are (111) faces. The [111] axes of

the grains are nearly perpendicular to the substrate and the

azimuthal orientation is random.

Layers of gold (100-150 nm thick) sputtered on cleaved mica

give monooriented polycrystalline electrodes. The grains are a few



tenths of micrometers in area; they are (111) oriented, but the

azimuthal orientation is random. Since the whole electrode surface

is not made of one grain, there are grain boundaries. At present

the contribution of these to dl parameters does not seem important.

The behavior of these layers is close to that of massive conventional

single crystals which are (111) oriented.38

t Macroetching is done by dipping the ingot in an etching reagent (for gold a few

seconds in warm aqua regia, for silver a few seconds in tepid 50% nitric acid,

etc.) and rinsing.



Double-Layer Properties at Metal Electrodes



31



Heteroepitaxial layers of gold (100-150 nm thick) evaporated

on cleaved mica give nearly monooriented crystalline electrodes; with



big (lll)-oriented grains (a few micrometers to a few tenths of a

micron in area), nearly no azimuthal disorientations are observed

at the surface. Their electrochemical behavior is close to that of

massive conventional single crystals.38

Heteroepitaxial layers of gold sputtered on polished (100) faces

of sodium chloride crystals were made. The first deposited layers

of gold on the substrate have its topography—they are gold single

crystals which are (100) oriented. A supplementary deposit of gold

(about 2 jum thick) makes them "easy" to manipulate and to remove

from the NaCl crystal. The first layer built on the substrate was

used as an electrode surface for dl measurements; the results are

close to those of massive conventional single crystals which are

(100) oriented.39

Homoepitaxial layers of one metal on a substrate of the same

metal improve the quality of the surface.

The surfaces grown from the metal vapor do not require cutting

and polishing. However, isolation of the face of interest and electric

contact must be ensured.

(iii) Growth from Metal Electrolyte Solutions



The question of why electrochemists have so seldom used a

technique which is their own is of interest. First, only the faces of

the lowest surface energies can be obtained [the (111) and (100)

faces for the fee system]. Second, if an epitaxial growth is observable

on some metals it is not on others. Up to the present, dl measurements were done on faces grown from the metal electrolyte solutions

only in the case of silver.40"42 These silver faces were grown in glass

or Teflon capillaries on a substrate which was a massive silver

crystal grown from the melt (oriented, cut, and polished according

the desired face).

After electrolytic growth, only one crystal face fills the section

of the capillary. The surface of the face consists of terraces and

growth steps; by convenient cathodic treatment, one can change

the growth-step density and can thus partly modify the atomic

surface structure of the electrode although its co remains

unchanged.4042t The possibility to change this density at will is the

t Screw dislocation free faces are obtained.



32



A. Hamelin



main advantage of this type of electrode. It should be emphasized

that, by this method, the imperfections of the surface seem to be

minimized. Furthermore, the influence of these atomic irregularities

on dl measurements was studied by using silver (111) and (100)

faces with a definite growth-step density (2-20.104cm~1).

The working area of these electrodes is a few tenths of a square

millimeter.

2. Cutting the Crystal



Crystals grown from the melt are removed from the crucible and

have to be recognized as individual crystals [see Section IV.l(i)]

before being oriented by the X-ray back-reflection Laiie method

(see Section III.3). At the end of this procedure, the desired co is

perpendicular to the X-ray beam; therefore the crystal must be cut

perpendicularly to this beam.

The cutting can be done either with a saw (thread saw, with

carborundum, electrolytic saw, spark erosion), or by elimination

of part of the crystal by abrading (grinding on emery paper of fine

grade, for instance, or spark erosion). The position of the crystal

when cutting and the angle of cut have to be maintained with

precision. Either the crystal has to be left in the goniometer or some

precise guidemark has to be drawn on the crystal.t

Any face of determined co can be obtained by this method

and thus the variation of dl parameters can be investigated from a

general viewpoint; but a disturbed layer is made at the surface

which has to be removed by polishing (see Section IV.3).

In some cases the cylindrical crystal (of small diameter) can

be cleaved successfully in liquid nitrogen along the most densely

packed face. This is the case for the basal face of zinc44; then no

polishing is necessary.

t If the desired face, which is elliptical in shape, is parallel to the film and the major

axis of its ellipse horizontal, a guide mark (a thin line) can be drawn horizontally

on the crystal, i.e., along the major axis. In the holder (Teflon or another material)

a hole of the diameter and the cut angle of the crystal is driven. A line, as a

guidemark, is also drawn along the major axis of the elliptical hole (at the surface

of the holder). The lines drawn on the crystal and on the holder are brought into

coincidence. The bulging part of the crystal is then abraded flat. The crystal must

sometimes be fixed in the holder with a wax melting below 100°C. This technique

was used for gold crystals.43



Double-Layer Properties at Metal Electrodes



33



To introduce a personal comment, it is surprising that this part

of the work is considered as a menial task and every time, when

visiting a laboratory, I asked to see how the crystals were cut,

I felt that I was being indiscreet. However, any error introduced

at this stage will entail errors in the electrochemical results.

3. Polishing and Isolating the Face of Interest



After cutting along the desired crystal face, the surface is left with

a disturbed layer on top. The physical state of the electrode surface

must be that of an atomically flat face described above (see Section

III.4). Therefore the experimenter must try, by all means available,

to create a disturbed layer as thin as possible, or absent. Elimination

of this disturbed layer can be done either directly by electrochemical

polishing (e.g., possible for silver) or by mechanical polishing,

followed by electrochemical or chemical polishing.

(i) Mechanical Polishing



Any book of metallography provides good information for this

step of preparation of the surfaces.

The choice of the ingredients used for mechanical polishing

(felt or cloth, alumina powder or diamond paste, etc.) is dictated

by the hardness of the metal and its chemical properties. Soft metals

(such as gold) are more difficult to polish than hard metals because

the polishing material can possibly be "buried" into the metal and

consequently modify the chemical composition of the electrode

surface (see Section IV.6).

Mechanical polishing is generally first done with fine emery

paper. Care must be taken to work across the lines (scratches)

formed on the surface by continual random rotation of the sample.

Then alumina powders of different grades (on different felts) or

diamond pastes of different grades (on different cloths) are used

to remove, as well as possible, the disturbed layer.t A mirror finish

should be observed; for instance, no trace of the lines due to the

polishing should appear when observation is magnified 20 times.

t A lapping wheel is generally used; the sample can be held by hand but an automatic

apparatus is now available.



34



A. Hamelin



The elimination of the disturbed layer can be checked by X-ray

photographs.

However, electrochemistry takes place on the outermost layer

of atoms which was in contact with the products; therefore annealing, chemical or electrochemical polishing, and then further annealing are necessary, t

(11) Electrochemical (or Chemical) Polishing of the Face



Books have been published about electrochemical and

chemical polishing.46 Of course, the procedure depends on the

nature of the metal, the size of the electrode, and the skillfulness

of the experimenter.

Only the face of interest must be in contact with the polishing

bath; therefore the isolation of the face (see further: resin, O-ring,

and holder) is necessary. Creeping of polishing solution between

the crystal and the isolating material could round the edge of the

face and change the geometric working area; therefore creeping

must be avoided.

Electrochemical or chemical polishing, whatever is the rinsing

procedure, leaves traces of chemicals at the surface (cyanide in the

case of gold or silver, for instance). These polishing stages are

followed either by annealing or by one of the final surface preparations (see Section IV.4).

(MI) Isolating the Face of Interest



The problem is to leave only the face of interest in contact

with the solution and to do it in such a way that the geometric

working area can be known and to avoid contamination of the face

which could not be removed by a final preparation (see Section

IV.4). Of course, creeping between the holder and the walls modifies

the working geometric area of the electrode and then other co's

than the desired one are in contact with the solution.

The crystal was often positioned in a Teflon holder (which has

been thoroughly cleaned) and the face was limited with a RTV

t For silver, we succeeded in making good electrochemical dl measurements after

mechanical polishing just by annealing and cooling in argon and putting the crystal

in contact with the solution without contact with air.45



Double-Layer Properties at Metal Electrodes



35



(room-temperature vitrification) silicone resin (Fig. 14a). Polythene

dissolved in toluene is also used. In any case, it should be a

noncontaminating material. When using films, a viton O-ring can

be positioned on the face (Fig. 14b); this type of holder was also

used for faces of large geometric area. For crystals grown in capillaries no further isolation is necessary.



Kel-F



Kel-F



S



Wire for

electric

connection

Stainless

steel



.->f— Teflon

\~~- Non-contaminating



(a)



O-Ring



- Crystal

(b)



Wire



Figure 14. Three possibilities of

isolation of the face of interest are

by: (a) a noncontaminating resin

and holder; (b) an O-ring and holder;38 and (c) a hanging electrolyte

method.47 Only the face of interest

should be in contact with the solution.



36



A. Hamelin



Where the "hanging electrolyte" method is used,47 no isolating

material is necessary; just after annealing the crystal in a torch

(town gas and oxygen) and cooling it in a clean medium (water48

or argon45) and touching the surface of the solution, a noncontaminated interphase can be obtained without any interference of the

co's of the walls of the crystal (which have to be dry and smooth)

(Fig. 14c).

Once more, for each metal, solvent, and chosen technique, the

sequence of procedures will differ.

4. Final Surface Preparation



The final aim is to have an electrochemical interphase chemically

clean both on the solution side (water, chemicals, gas) and on the

metal side (no oxide, no sulfide, no traces of the polishing solutions),

and physically well-defined at the outermost layer, or layers, of atoms

being as they are in the bulk metal in a plane parallel to the surface.

We shall discuss two examples.

1. For silver faces, mounted as in Fig. 14a, before each series

of dl measurements a few seconds of electrochemical polishing are

necessary; therefore traces of cyanide are left at the surface which

are not eliminated by rinsing thoroughly with solvent (very pure

water). In the aqueous solvent, excursions, in a range of potentials

more negative than the double-layer region, produce a slight hydrogen evolution as a consequence, which carries the impurities existing

at the metal surface away into the volume of the solution, if the

latter is stirred. Then the solution has to be renewed. This procedure

can be applied several times until the C(E) curves are stable and

satisfactory. Of course, one must avoid by all means excessive

hydrogen evolution which could substantially disturb the first layers

of atoms of the electrode surface or work the resin loose.

It must be pointed out that oxides formed on silver cannot be

reduced electrochemically.

2. For gold faces, mounted as in Fig. 14a, no annealing and

cooling as described in Section IV.3(iii) is possible. In the aqueous

solvent a slight hydrogen evolution could be used (as for silver,

with the same disadvantages, although electrochemical polishing

was not done at this stage). But for gold, with no or slight adsorption,

a monolayer of oxide can be completed at the surface and removed



Double-Layer Properties at Metal Electrodes



37



electrochemically in the aqueous solvent; the formation and

removal of this oxide layer can be used as a cleaning procedure.

Do these formations and removals of oxide disturb the outermost

layer of gold atoms? Yes, if the variation of the applied potential

with time is fast (more than 100 mV s"1),49 but it does not seem to

be the case for slow sweep rate (lOmVs"1). For the (100) face,

comparison of C(E) curves obtained after numerous cycles of

formation and removal of oxide and obtained just after annealing

and cooling [see Section IV.3(iii)] are identical.50 If there is ionic

adsorption (bromide, iodide solutions), there is oxidation of the

anion before oxidation of gold and this final preparation has to be

done in another solution (perchloric acid, for instance) before dl

study of ionic adsorption.

For each metal and each solvent a final preparation of the face

has to be adopted. In some cases this final surface preparation is

at the same time a check of the quality of the electrochemical

interphase.

5. Ex situ Check of State of Electrode Surface



Ex situ checking of the electrode surface is done first by optical

microscopy, electron microscopy, and scanning microscopy. When

a UHV chamber is available in the laboratory then more sophisticated methods can be used, although the state of the surface in

vacuum is not necessarily what it is in contact with a solvent at the

electrochemical interphase.t However, the new powerful methods

used to provide information on the chemical state of surfaces,

namely, AES (Auger electron spectroscopy) and XPS (X-ray photoelectron spectroscopy) [also called ESCA (electron spectroscopy

for chemical analysis)], and on the physical state of the uppermost

region of the surface, namely, LEED (low-energy electron diffraction) and RHEED (reflection high-energy election diffraction), are

certainly of great help for the erectrochemist working on singlecrystal-face electrodes. The "gap" between observations in the UHV

chamber and observation at the electrochemical interphase due to

transfer is smaller every day, although it may never be completely

t After the present chapter was submitted, studies on emersed electrodes consisting

of single crystal faces were published; they give information on the dl.



38



A. Hamelin



closed. Therefore conclusions drawn from observations in UHV

can, to some extent, be valid for the electrochemical situation.

The reader can refer to References 5 and 51-53 for further

information.

6. In situ Check of State of Electrode Surface



In fact, all that follows in this chapter could be considered, in a

way, as an "in situ check" of the electrode surface, because, while

observing the desired electrochemical dl parameters, questions of

the chemical cleanliness of the electrode surface (and of the interphaset) and the physical state of the outermost layer of atoms at

the metal surface are always open.

In situ checks are absolutely necessary. Either they are provided by the experimental electrochemical results themselves or by

other methods such as optical techniques.

For instance, at the present, by optical measurements (by in

situ electroreflectance) it was shown that the symmetry properties

of the crystal faces were not perturbed by contact with aqueous

solutions. For copper, all faces studied present their azimuthal

anisotropy with respect to the plane of polarization of the incident

light, except for (100), (111), (211), and some neighboring faces of

(111) for which no azimuthal anisotropy is observed.54 For silver,

the twofold symmetry of the (110) face was observed: the electroreflectance spectra at normal incidence differ markedly when the

electric field vector of light is parallel or perpendicular to the surface

atomic "rails." This is not the case for the (111) and (100) faces55

which have higher symmetry. For gold, no azimuthal anisotropy

was observed for the (111) and (100) faces with respect to the plane

of polarization of the incident light, while the (110) and (311) faces

present an azimuthal anisotropy.56'57 It does not necessarily mean

that the outermost layer of surface atoms is 1 x 1, because a surface

reconstruction such as 1 x 2 for Au (110) would have the same

symmetry order as l x l . These are already studies of the metal

surface in the presence of the electrochemical dl.

t For further discussion of techniques required for obtaining ultraclean conditions,

see the chapter by H. A. Kozlowska in Comprehensive Treatise of Electrochemistry,

Ed. by E. Yeager, J. O'M. Bockris, and B. E. Conway, Plenum Press, New York,

1984, Vol. 9.



Double-Layer Properties at Metal Electrodes



39



Most electrochemistry laboratories have no optical setup and

therefore are limited to electrochemical observations. Furthermore,

the potentialities of i(E) curves and C(E) curves seem larger

every day; this means that under rigorously clean and well-defined

conditions, the wealth of information contained in these curves is

not yet fully exploited.

For any metal in a given solution, at a given temperature, the

range of potential E over which only a change (with potential

applied) of the electrostatic charge on the metal (and correspondingly in solution) is observable, is the "dl region."

In the dl region, comparison of the i(E) and C(E) curves can

be used as a first test [one should be able to deduce one from the

other, the current being only capacitive, (i = C dE/ dt)"\. The shape

of the minimum (corresponding to the capacity of the diffuse part

of the electrochemical dl) on the C{E) and i(E) curves (Fig. 15)

for the case of no adsorption in dilute solution, is another criterion;

it should not be too different than that observed for mercury, but

its position in the range of potential can be completely different.

The shape of the adsorption peak is another criterion (in the case

of adsorption); it should be sharp and reproducible. Of course, the

stability and reproducibility of the curves are important and should

be observed not only in the dl region but over all the range of

potential explored.

On the i(E) curves, the dl region is limited at negative potential

by a faradaic current corresponding to the reduction of the solvent

(or of the cation); the observation of the foot of the wave of this

current gives an indication of the cleanliness of the interphase.

Undoubtedly, a steep foot of the wave of current (Fig. 16) is

indicative of a cleaner (or less dirty) surface-than a curve that rises

slowly because the electrochemical reaction rate is slowed down

by some impurities (for instance, carbonaceous species) at the

surface. Slight hydrogen evolution will clean the surface; but care

must be taken to avoid strong hydrogen evolution which could

modify the outermost layer of metals atoms irreversibly.

On the i(E) curve the dl region is limited at positive potentials

by oxidation—either of the anion, the solvent, the metal, or of

impurities. For some metals, oxidation of the metal occurs at

potentials less positive than oxidation of the solvent. When the

oxygenated compound formed can be totally reduced at the



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