SPECTROSCOPIE ET RÉFLECTANCEREFLECTANCE SPECTROSCOPY IN THE STUDY OF ELECTRODE SURFACES

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SPECTROSCOPIE ET

RÉFLECTANCEREFLECTANCE SPECTROSCOPY

IN THE STUDY OF ELECTRODE SURFACES

D. Kolb

To cite this version:

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SPEC TROSCOPIE ET

REFLECTANCE.

REFLECTANCE SPECTROSCOPY IN THE STUDY

OF ELECTRODE SURFACES

D. M. KOLB

Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 1 Berlin 33, W.-Germany

RBsumi5. - On dkcrit l'utilisation de la spectroscopie de rkflectance in situ pour l'ktude d'6lec- trodes nues ou recouvertes d'un absorbat. On montre d'abord que les proprietks du mktal nu plongk dans la solution peuvent 6tre facilement ktudiks par les techniques d'electrorkflectance.

Des mesures sur des klectrodes d'Ag monocristallines B diffkrents angles d'incidence et diffkrents potentiels donnent des informations sur l'interface metal-klectrolyte qui ne pouvaient &re obtenues avec des surfaces polycristallines. De plus, l'effet du potentiel de l'klectrode sur les propriktes optiques de surface est determine et discutk en detail. Par ailleurs, I'excitation des plasmons de surface sur des couches minces d'argent et d'or en fonction du potentiel d'klectrode peut 6tre ktudik par rkflexion totale attknu6e. Des rksultats preliminaires relatifs 2 i une influence des adsorbats sur l'excitation des plasmons de surface sont present&. Finalement, la formation et les proprietes de monocouches metalliques disposks s u . des klectrodes mktalliques ont kt8 ktudiks par rkflectance spectroscopique diffkrentielle. Des spectres de monocouches de cuivre sur du platine et de plomb sur de I'argent ont 6tB enregistrks dans 1'U. V. visible et pour diffkrents angles d'incidence. Diffkrentes mkthodes peuvent 6tre employkes pour calculer les constantes optiques B partir des spectres AR/R. La possibilitk d'une anisotropie dans les propriktb optiques du film est discutke bri6vement.

Abstract.

-

The use of in situ reflectance spectroscopy for the study of bare and adsorbate covered electrode surfaces is described. It is first shown that the electronic properties of bare metal surfaces immersed in an electrolyte can be conveniently investigated by electroreflectance techniques. Measurements on single crystal surfaces of Ag electrodes at various angles of incidence and bias potentials yield information about the metal/electrolyte interface which was previously not obtained withpolycrystalline surfaces. Furthermore, the effect of the electrode potential on the metal surface optical properties is determined and discussed in some detail. Secondly surface plasmon excitation on thin Ag and Au films as a function of electrode potential can be investigated by attenuated total reflection and preliminary results for the influence of adsorbates on the surface plasmon excitation are presented. Finally the formation and properties of metallic monolayers on metal electrode surfaces as studied by differential reflectance spectroscopy are considered. Spectra for monolayers of Cu on Pt and Pb on Ag electrodes were recorded in the UV-VIS range for various angles of incidence. Different methods can be employed to calculate the film optical constants from the AR/R-spectra and the resuIts are compared. A possible anisotropy in the film optical constants is briefly discussed.

1. Introduction.

-

In recent years a n increasingly growing number of investigators has recognized the power of spectroscopic methods for studies of the electrode-electrolyte interface. Besides ellipsometry, electroreflectance and differential reflectance spectroscopy in conjunction with classical electrochemical techniques have mostly been used and are now becom- ing valuable tools in characterizing electrode surfaces and in studying electrochemical processes. For example, electroreflectance measurements at metal- electrolyte interfaces have been employed to investigate changes in the surface properties of metal electrodes with potential [I-161, while differential reflectance spectroscopy and related modulation spectroscopy techniques have been applied for studying adsorption processes and film formation. Some typical examples of the latter are dye adsorption on metal and semi- conductor electrodes [17-231, anion adsorption on metals [24, 251, hydrogen adsorption on Pt [7, 261,

metal deposition on metals [27-341 and semiconductors [35-371, oxide formation on metal electrodes [27, 38-41], compound layer formation [42] and many more. The present state of the art is well documented in two review articles by McIntyre 16, 431.

Spectro-electrochemical methods such as reflectance spectroscopy in an electrochemical environment can yield information relevant to problems in electrochemistry as well as in surface physics. Optical methods provide information for the electrochemist, which is specific on an atomic scale, whereas the commonly used electrochemical techniques often yield only integral information. Thus, reaction products and adsorbates can be identified with more confidence and their electronic properties determined. In addition optical methods are non-destructive and they can be applied in situ over an extended wavelength range from the near I R to the near UV to monitor directly reactions at the electrode surface. On the other hand, the unique pro-

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perties of the electrode-electrolyte interface allow us to obtain information from spectroscopic measurements, which are often difficult to get from similar measurements a t the solid-vacuum interface. For example, high electric fields and large surface charges can easily be obtained at a metal surface by applying appropriate electrode potentials which makes this interface ideally suited for electroreflectance measurements. Further- more the equilibrium position of reversible reactions can be changed over a large range simply by changing the electrode potential. This means that the coverage of adsorbate can usually be varied repetitively by potential steps, allowing the use of the very sensitive modulation spectroscopy for detection. Adsorption processes can thus be investigated in an electrochemical cell at ther- modynamic equilibrium conditions, which are often impossible to achieve at the solid-vacuum interface. Therefore a thorough study of the solid-liquid interface is not only mandatory for a better understanding of fundamental problems in electrochemistry, but is also highly relevant for surface studies in general, despite the nature of the interface. Some of the more recent advances in reflectance spectroscopic studies at metal electrode-electrolyte interfaces will be discussed in the following.

2. Experimentat. - In reflectance spectroscopy studies on electrode surfaces minute changes in the reflectance, caused by adsorption processes or electrode reactions, have to be detected over an extended wavelength region. Usually relative reflectance changes ARJRIR are measured, thus avoiding the more difficult task of determining in situ absolute reflectivity values. Several experimental set-ups have been described in the literature 15-7, 18, 27, 30, 35, 42-45]. Measurements in the author's laboratory were mostly performed with a dual-beam rapid scanning spectrometer as has been described previously [45], which allows fast scanning and yet offers a sufficiently high sensitivity. A spectral range from about 800 nm to 200 nm was usually covered with wavelength scans, 200 to 300 nm wide. In each wavelength range a single scan was performed within about 23 ms with a repetition frequency of

36 cps. A total of 512 scans seemed to be the optimum

number to be sampled in a signal averager.

The electrochemical cell, which had a semicircular quartz window, was placed on a 9

-

2 cp goniometer table, with the sample photomultiplier directly attached to the 2 q-arm. Thus measurements at various angles of incidence (1 50

5

rp

,

5

850) could easily be achieved. All measurements were performed with linearly polarized light by using a Glan-Thompson prism. The electrode samples were either metal sheets, carefully polished to a mirror finish, metal films evaporated on glass or quartz substrates or, in the case of Ag and Au,

single crystal films of Ill-orientation evaporated on mica at elevated substrate temperatures ( w 270 OC). In the latter case the influence of surface roughness, which may play a crucial role in interpreting

optical spectra of adsorbed layers [44], is elimina- ted.

All potentials quoted in this work are given with respect to the saturated calomel electrode (SCE).

3. Results and discussion.

-

3.1 ELECTROREFLEC- TANCE STUDIES ON Ag (1 1 1)-ELECTRODES. - When an electric field is applied or modulated at a metal surface, a change in reflectance is observed. This electroreflectance (ER) effect is most conveniently studied a t the metal-electrolyte interface by modulating the potential across the Helmholtz layer in the so called double layer region, where no electrochemical reactions take place but only the metal surface is charged like a capacitor. Large surface charges and high electric field strengths can easily be achieved by appropriate changes in the electrode potential. Unlike the situation in semiconductors the static electric field is screened by the high charge density in the metal within the very first atomic layer at the surface, which makes this method extremely surface sensitive and therefore very attractive for studying the electronic properties of bare metal surfaces and their influence by the electrode potential. So far the metals Cu [3, 51, Ag [6, 15, 16,46, 471, Au [5, 7, 10, 111, Hg [14, 421, Pt [13] and Pb [8, 91 have been investigated, the measurements having usually been performed, with a few exceptions [46,47], on polycrystalline surfaces. In a recent work on Ag single crystals it has been demonstrated that much more information can be gained by using atomically smooth surfaces and studying the angle dependence of the ER effect [47]. Many experimental details were observed in these spectra, which were not seen in previously reported ER spectra of slightly rough polycrystalline Ag surfaces [6, 161, and which can be used as critical test for currently employed models explaining the origin of the ER in metals. In figure 1 the ER spectra of an Ag (1 11) electrode are shown for a potential step from

-

0.5 V (which is close to the potential of zero charge (pzc)) to _+ 0.0 V, taken at an angle of incidence

cp, = 450 with p- and s-polarized light. A few points

-2 0

2 0 3 0 L 0 5 0 6 0

Aw/eV

FIG. 1.

-

Electroreflectance spectra of Ag (111) in 0.5 M NaC104 (pH2). Potential step from - 0.5 V to 0.0 V. 91 = 45O.

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REFLECTANCE SPECTROSCOPY IN THE STUDY OF ELECTRODE SURFACES C5-169 are worthwhile noting. The spectral features around

3.5 eV due to surface plasmon excitation on rough surfaces are completely missing, as one would expect for a perfectly flat surface. This allows a more unambiguous assignment of the true ER effect of Ag in this wavelength region. Secondly the spectra in figure 1 as well as those taken at different angles of incidence q, [47] reveal, that the ER effect cannot be explained satisfactorily by the free-electron model alone [6], but the influence of bound electrons has also to be taken into account. According to the free-electron model proposed by McIntyre the metal surface complex dielectric constant, :2, is split into contributions from free and bound electrons [5, 61,

Since it is assumed that to a first approximation the bound electrons are not affected by the applied electric

A

field (therefore is

?2

,,

= 8, ,,), the change in the surface dielectric constant is solely determined by the change in the free electron concentration on the surface according to the Drude equation for a free electron metal (the indices 2 and 3 refer to the film and the bulk phases in the 3-phase model [48]) :

with

*

112

op = (4 n ~ e g l r n )

.

(2a) Thus the difference in surface and bulk dielectric constants is given by [6]

A A

62 - &3 = ($33

-

1) (ANIN)

,

(3) where ANIN, the normalized field-induced change in the free electron concentration with potential may be expressed as

(CDL : integral double layer capacity ; eo : electronic charge ; N : free electron concentration in the bulk metal ; d : penetration depth of the static electric field). AR/R spectra for Ag calculated on the basis of this model are smaller by about one order of magnitude and do not reproduce the observed spectral features other than those at 3.9 eV (main peak). This indicates, that the bound electrons are markedly affected by the applied electric field and the term, a:2,,/a~, can no longer be neglected. In reverse, this means that ER measurements will yield information on the band structure of metal surfaces, an important piece of information in surface physics. Assuming a field- induced shift in the electron energy states at the surface with respect to those in the unperturbed bulk (analo- guous to band bending in semiconductors), it becomes evident, that transitions from or to the Fermi level will be affected more strongly than interband transitions at critical points. The latter will only show up in the spec-

trum when the bending is different for the two states involved in the optical transition. Preliminary studies at Ag and Au electrodes confirm this picture [49]. E. g. the transition at the L point in Ag (L2 t L3) may be

unburried at high positive potentials (low EF) and show up at 3.7 eV [50].

Another interesting feature in the ER spectra of Ag (111) is the sharp dip at 3.84 eV for p-polarization. This dip, which is not seen in s-polarization at any q,

and in p-polarization at p,

<

30°, occurs exactly at the volume plasma excitation energy hop, where

&IAg = 0 1511. A possible explanation has been offered

by Kliewer [52]. For Ag in the energy region around

hop the existence of a longitudinal wave may have to be taken into account, since div G can no longer be set to zero in deriving the optics equations. Then it has been shown that around hop a field suppression effect may occur at the surface, making the surface suddenly insensitive to the probing light as ho approaches hop.

Hence ARIR drops to zero at this energy. Since in recent calculations for metal optics the existence of such an effect has been questioned [53], we have to await some more decisive experiments. In addition a

similar sharp dip, causing a sign change in AR/R, can be produced in classical model calcuIations using Fresnel equations, either in free electron ER calculations for rather large potential steps

( I

ANIN1

>

0.1)

or by assuming a film formation on Ag (see chap. 3.3). Up to now, however, it seems as if the observed dip is experimental evidence for the need of incorporating longitudinal waves in reflectance studies.

When the surface is roughened electrochemically, additional spectral features are observed in the ER spectra for both polarizations due to surface plasmon excitation at 3.5 eV, where

&A,

=

-

E,,,. This is show in figure 2, together with a static reflectance spectrum

FIG. 2.

-

Electroreflectance spectra of slightly rough Ag (111) in 0.5 M NaCl04 (pH2) for p (-) and s (-

-

-) polariza- tion. Potential step from

-

0.5 V to 0.0 V. qq = 4 5 O . The dotted lines represent the curves for the initially flat surface. The arrow indicates Amsp. The reflectancespectrum of a rough Ag electrode in 0.5 M NaC104 for p-polarized light at p l = 4 5 O is shown in

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of a rough Ag electrode as insert (the latter was taken from a much rougher surface for clarity sake), where surface plasmon excitation is seen as dip in the reflectance around 3.5 eV. It has been shown recently, that the surface plasmon excitation energy hm,, for an Ag electrode is potential dependent [47]. This result is reproduced in figure 3. It is seen, that in a wide poten-

RG. 3. -Surface plasmon excitation energy for a slightly rough Ag (111) electrode as a function of electrode potential in

0.5 M NaC104. AU = 0.4V. 91 = 45O. p (0) and s(U) polari- zation. pzc : potential of zero charge. (-- - -) calculated from

figure 4. After reference [47].

tial range anodic on the pzc the surface plasmon energy is shifted linearly with the electrode potential, the slope dAm,,/dU being

-

0.2 eV/V, whereas at potentials negative from the pzc no influence on Am,, is found. This shift, which implies that

can be explained on the basis of an electron density change at the surface with electrode potential 1471.

A

In figure 4 the E~,,,~,,, values (= 8;) were calculated according to eq. (3) as a function of photon energy for various ANIN values, using Johnson and Christy's data for bulk Ag

( 2

8;). It is clearly seen from this figure

3.0 3.5 4.0 4 5 5.0

h w/eV

FIG. 4.

-

Model calculation for the Ag surface dielectric cons- tants, taking only free electron effects into account. The bulk Ag data are from Johnson and Christy. Parameter is ANIN, the rela-

tive change in the free electron concentration at the surface.

how the potential dependence of Am,, arises from the condition for surface plasmon excitation :

The result from this calculation is also shown in figure 3 (dashed line), assuming an integral double layer capacity C,, of 15 y F cm-2 for this only slightly rpughened (111)-surface and d = 1

A.

The very good agreement in the slope must be considered as fortuitous in view of uncertainties in the values of C,, and d, and of neglecting the potential influence on the bound electrons. However, it can be concluded from this result that the surface plasmons at large k-values, which we are dealing with in the case of excitation via surface roughness, are obviously confined to a region at the surface of a few Angstroms thickness only.

It should be mentioned that a possible contribution to the shift in hm,, with potential may arise from the bound electrons. The transition, which mainly determines the shape of

?*,

around 4 eV is that to the Fermi level at the L point [50]. Lowering

EF

by a positive electrode potential would shift the onset of this transition also to lower energies. The net effect on ~k,,~,,~,~, would then be similar to that caused by a negative ANIN : the energies for the conditions E' = 0 (volume plasmon excitation) and E' =

-

(surface plasmon excitation) shift to lower values. However, from various considerations (the linearity over a wide potential range, the change in slope near the pzc, the close similarity of the results for Ag and Au as shown in chap. 3.2) we expect the change in ANIN to be the main reason for the shift in tlmsp, although the magnitude of the effect by bound electrons still has to be established.

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REFLECTANCE SPECTROSCOPY IN THI 2 STUDY OF ELECTRODE SURFACES C5-171

FIG. 5. - Electsoreflectance spectra of a rough Ag electrode at two different bias ~otentials. 0.5 M NaC104. Potential steps

from i 0.0 V to - 0.25 V (a) and from

-

0.75 V to

-

1.00 V (b).

9 1 = 45O. p-polarization.

with the free electron concentration, changed by the potential. Applying the same potential step at rather negative bias potentials (curve b), where the free electron concentration, should not be modulated by the potential, those spectral features between 2.5 and

3.8 eV are nearly completely suppressed. Similar results on the free electron contribution to ER have also been obtained for Au electrodes [49]. This finding allows in principle to study conveniently bound electron effects in ER, that is surface band structures, simply by an appropriate choice of the bias electrode potential, negative of the corresponding pzc.

It has been reported, that the measured ER spectrum of Cu deviates markedly from the spectral shape calculated by the free electron model [5], whereas the features of the ER spectra for Au, taken a t potentials slightly positive from the pzc, were well reproduced in detail by that model. The pronounced peak at 2.0 eV in the measured ER spectrum for Cu, which does not show up in the calculation, indicates a substantial contribution from field modulated interband transitions. We believe that this discrepancy can be explained by the above described effect. For Cu only measure- ment at potentials negative of the pzc are possible, since at potentials positive of the pzc Cu dissolution takes place. Hence one can only work under conditions where the free electron contribution to the ER effect is largely suppressed and the modulation of bound electrons determines the spectral shape.

3 . 2 SURFACE PLASMON EXCITATION BY ATR. - The study of surface plasmons in metal electrodes can yield additional information on the electrode surface electronic properties and their change with electrode potential, adsorption or film formation. Unfortunately for most metals surface plasmon excitation occurs in an energy range not accessable in reflectance studies at the metal/solution interface. Exceptions, however, are the metals Ag and Au, where the onset of interband transi-

tions around 4 and 2 eV modify the free electron dielectric constants such, that plasrnon excitation is possible in the VIS

-

near UV range. Surface plasmons with large k-values can be excited in Ag via surface roughness at hco,, = 3.54 eV, where &kg = - E,,,

and & l g

<

1. In Au, however, this effect is not observed,

because the interband transition sets in too early, causing 82, > 1 when &a, =

-

Furthermore, it has been found, that the conditions for surface plasmon excitation by roughness depend somewhat on the degree of roughness [47] rendering these studies more difficult to be reproduced in detail. We therefore employed the more sophisticated and sensitive method developed by Otto [54] and Kretschmann 1551, which allows surface plasmon excitation directly by the light wave in the small k-region (k

=

l o w 3

A-1)

via attenuated total reflection (ATR) [12, 561. The excitation occurs in thin f i h s of about 500

A

thickness, evaporated on the flat side of a glass or quartz hemicylinder. The light beam then enters the hemicylinder, which is immersed in an electrolyte, through the curved wall and is totally reflected at the flat side. This opticaI arrangement provides larger k-values inside the hemicylinder than outside and still facilitates total reflection. Since the component of the k-vector parallel to the surface is changed with the angle of incidence p,, the surface plasmon excitation energy ha,, also becomes a function of q, and can therefore be changed over a wide energy range (dispersion curve 1561). In figure 6

FIG. 6.

-

ATR spectrum of Ag (500 Pi) on a quartz hemicylin- der. 0.5 M NaC104. Uo = f 0.0 V. 91 = 77.50.p-polarization.

the ATR spectrum for Ag on a quartz hemicylinder is shown. The very pronounced dip at 1.9 eV is caused by direct optical surface plasmon excitation, while at

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2 050

2 OL5

2 OLO

-1 .O -0.5 0 0.5

FIG. 7. - Surface plasmon excitation energy, Amsp, for Ag and Au electrodes in 0.5 M NaC104 as a function of electrode potential Uo. = 60° (51°) for Ag (Au). The respective potentials of zero charge are marked by arrows. After reference

[561.

electrode potential on Ao,,. In figure 7 the position of that minimum which represents tzm,, for direct excita-

tion, is plotted as a function of the electrode potential U

for Ag and Au in the double layer region [56]. Again, a clear shift of Am,, with

U

is seen at potentials positive of the respective pzc's, the slopes being

-

0.02 and

-

0.01 eV/V for Ag and Au respectively, while no

change with potential is observed cathodic of the pzc's. Comparison of the slopes BAw,,/BU for Ag obtained by

external electroreflectance on rough surfaces and by direct optical excitation at small k shows that in the latter case the effect is smaller by one order of magnitude. This is easily understood if one considers that the plasma waves extend much further into the metal at small wave vectors. Thus any perturbation at the surface will have a correspondingly smaller influence on Am,,. The curves in figure 7 confirm the results

discussed in chap. 3.1, that the electron density (which determines Am,,) at the surface can be lowered consi-

derably by an applied electric field, but obviously cannot be increased significantly. However, since the surface excess charge can very well be increased in both directions by an electrode potential on either side of the pzc, we assumed that for positive potentials the electron density is decreased whereas for negative potentials the surface region is extended, in which the charge is accommodated [56].

When electrode reactions or adsorption processes are allowed to take place, much larger shifts in hmsp

are observed. In figure 8 the change in surface plasmon excitation energy is shown for Au when oxygen adsorption or lead deposition occurs. It becomes evident by comparison with the corresponding I- U-curve that the gross change in Aosp with potential reflects the coverage of the deposit. In general this effect could be used to determine important electrochemical quantities, such as pzc's in various electrolytes, or the electrosorption valency of adsorbates. A preliminary result for halide

I I I I

- 0 5 0 0 5 1 .O 1.5 USCE/ V

FIG. 8. - Shift in the surface plasmon excitation energy hmsp

for Au in 0.5 M NaC104 due to oxide formation and, after adding 2 x 10-4 M Pb(NO& to the solution, due to Pb monolayer

formation. = 51°.1RG 2.

adsorption on Ag is reproduced in figure 9. The changes in Ao,, with electrode potential for an Ag electrode in a chloride ion containing and in a blank solution clearly reflect the shift in pzc to more negative values in halide solutions. Although charge measurements indicate a substantial amount of Cl- being adsorbed at potentials up to +_ 0 V (just before AgCl is formed) [57, 581, only a slight increase in the slope from

-

0.021 to

-

0.026 eV/V is found for afim,,/BU. However, we

would expect ion adsorption to affect Am,, more drasti-

cally because of large changes in the free electron concentration by image forces.

FIG. 9. - Surface plasmon excitation energy, amsp, for Ag in 0.5 M NaC104 as a function of electrode potential. 91 = SO0. Quartz hemicylinder. (-0-0-) [CI-] = 0. (-0-0-)

[Cl-1 = 10-2 M.

The experimental result could tentatively be explained by assuming that C1- forms an almost covalent type of bonding with the surface, thus leaving the adsorbate in a nearly discharged state (electrosorption valency close to 1). Further experiments have to be done to find out, to what extent surface plasmon excitation can be used as diagnostic tool in electrochemistry and surface studies.

3.3 DIFFERENTIAL REFLECTANCE SPECTROSCOPY IN

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REFLECTANCE SPECTROSCOPY IN THE STUDY OF ELECTRODE SURFACES C5-173 perties of adsorbates and monolayer deposits are

conveniently studied by differential reflectance spectroscopy (DRS). Here the relative reflectance change

is measured, where R(0) and R(d) represent, respectively, the reflectances of the bare and the adsorbate covered surfaces [48]. In a simple 3-phase model with sharp boundaries ARIR can then be related to the dielectric functions of ambient, film and substrate phases, e l ,

22

and

g3

(complex quantities are denoted by a hat). For very thin films (d Q A) McIntyre and Aspnes have shown that to a very good approximation it is 1481

and

These expressions have proved to be very valuable for discussing the influence of the various parameters on AR/R [6]. They show that AR/R does not only depend on the film optical properties, described by g2, but also in a rather complex manner on the substrate dielectric function :3. E. g. AR/R often resembles the dielectric loss function of the substrate metal regardless of the film dielectric constants, especially when the substrate optical constants have a pronounced wavelength dependence. This makes it often impossible to determine absorption processes in the film just by visual inspection of the measured spectra. Furthermore it is believed that adsorbate induced electroreflectance (ER) may play a dominant role in those cases, where the substrate metal already exhibits a strong ER effect [33]. In a recent work we have studied the deposition of a Cu monolayer on Pt by DRS [45]. This system was chosen for a detailed study, since Cu is deposited in a potential range where neither oxygen adsorption on Pt nor hydrogen evolution interferes with the Cu mono- layer formation and because Pt has a vanishingly small ER effect. In this case we can assume that absorption processes in the monolayer or originating from it are already indicated clearly in the measured spectrum. In figure 10 the spectra due to the formation of submonolayer amounts of Cu on Pt are given in the photon energy range from 1.5 to 5.5 eV for various coverages. The (AR/R),, values were normalized for a m thickness and photon energy for better comparison of the different coverages and to eliminate the d/A-effect (see eq. (8)), which tends to suppress the long wavelength part of the spectrum. Surprisingly sharp structural features are seen in the spectra which indicate the

FIG. 10.

-

Spectra of the relative reflectance change ARIR per

photon energy and film thickness for various coverages of Cu on Pt. p l = 600. p-polarization. Parameter : coverage 8. After

reference 1451.

energetic positions of absorption processes in the monolayer. The main results from this work can be summarized as follows [45]. i) When ARlRe, the reflectance change per coverage, is plotted for one wavelength versus coverage 8, a clear step is found at

8 = 213 indicating a structural transition in the layer. This is supported by recent I-U measurements on single crystal Pt electrodes. ii) Inspection of the measured ARIR spectra at different angles of incidence, as well as evaluation of the film optical properties by inverting the linear approximation equations [38], reveal three absorption bands in the Cu monolayer at energies around 2.1 eV, 2.8 eV and 4.1 eV, the band at 2.8 eV being strongly angular dependent. The latter one was interpreted as charge transfer transition from an occupied adsorbate level to empty states near the Fermi level in the substrate. iii) An assignment of the electron

FIG. 11. - ARIR spectra for multilayer deposits of Cu on Pt. 1 N H2S04

+

2 x 10-4 M CuS04. p l = 60°. p-polarization. Parameter : averaged coverage. (- - -) model calculation for a 3 A thick Cu overlayer on Pt, assuming bulk Cu optical

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states involved in the optical transitions under conside- ration has tentatively been given. iv) The optical properties of one monolayer of Cu on Pt are found to be markedly different from those of bulk Cu.

In studying thicker films the transition of the monolayer optical constants to those of bulk Cu should be monitored. There is evidence that underpotential deposition of Cu (as well as of Ag) on Pt also influences the further growth, favoring the deposition of a rather uniform film. In figure 11 the measured (AR/R)II spectra for multilayer deposits of Cu on Pt are shown, together with a calculated spectrum for a 3

A

thick layer of bulk Cu on Pt (dashed line). The model calculation reveals that a sudden decrease in AR1.R near 2 eV should be observed when the film band structure is sufficiently bulk like to produce the interband transition at 2.0 eV. This is indeed the case for a Cu deposit with an average thickness of about 3 monolayers. Earlier findings for Ag on Pt are confirmed in that as little as 4-5 monolayers is necessary to obtain bulk metal optical constants in the VIS-near UV range [27], when the film structure is very homogeneous.

FIG. 12. - ARIR spectra for the deposition of a Pb monolayer (a) and a TI monolayer (b) on Ag. 91 = 45O.

Firstly, the absence of surface roughness would allow to extend that part of the spectrum up to about 3.7 eV,

which is not disturbed by contributions from ER of Ag. Secondly, an atomically flat substrate surface is the necessary prerequisite for detecting unambiguously any anisotropy effects in the film optical properties. The AR/.R spectrum for a complete monolayer of Pb on Ag (111) is shown in figure 13 for both polarizations.

FIG. 13.

-

AR/R spectra for the deposition of a Pb monolayer on Ag (111) in 0.5 M NaC104 $. 2 X 10-4 M Pb(N03)2.

91 = 45O.

The I-U curves for underpotential deposition of Pb on Ag (1 11) indicate 2-dimensional phase formation [59], that is, a close packed layer of Pb is formed within a rather narrow potential range. Therefore, one would expect this monolayer to exhibit a clear anisotropy in its optical behaviour. Two different ways were chosen to determine the film optical constants. The AR/R-spectra were measured in the range

for p-polarization at angles of incidence cp, from 150 In figure l2

ARIR

'pectra are shown for monolayer

up to 850, in 50 intervals. Then, the linear approxima- deposition of lead and thallium on polycrystalline Ag _tion_{equations derived}_{for uniaxial anisotropic film} electrodes in the underpotential region. From the

similarity of both spectra and by comparison with ER dielectric constants with components normal G2,") spectra on polycrystalline Ag (see e. g. fig. 5) it and tangential (c2,3 to the surface [60] were employed becomes evident that the main peaks around 4.0 eV to calculate AR/R values and to fit them to the measur- for s- and p-polarization are caused by substrate ed data by trial and error :

properties and that the structure around 3.5 eV has its origin in the surface roughness of Ag. Only for hw

<

3 eV the AR/R spectra are originating from film optical properties per se, and therefore can be used to evaluate the monolayer dielectric constants. It is worthwhile noting, that the sharp dip at 3.9 eV seen in both spectra for p-polarization is not likely to be connected with plasmon excitation, but is caused by the monolayer formation. Although only seen in p-polarization this effect can be explained on the basis of Fresnel equations and is reproduced in the corresponding model calculations using the linear approximation.

As a next step these measurements were performed on Ag (1 11) electrodes for two obvious reasons.

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REFLECTANCE SPECTROSCOPY I N THE STUDY OF ELECTRODE SURFACES C5-175 different minima may be found with optical constants

which all reproduce the measured AR/R values within experimental limits. In the second case the results depend on the tail fit as well as on the choice of the reference point for the phase change [62]. Different results are obtained for assuming the film to grow into the substrate (e. g. oxide formation) or on top of the substrate into the ambient phase (e. g. metal plating). Preliminary results are reproduced in figure 14. The data obtained by both methods indeed reveal a strong

FIG. 14. - Optical properties of a Pb monolayer on Ag (Ill), evaluated by different methods and assuming uniaxial a m anisotropy. (-1 multiangle method. (-

-

-) Kramers- Kronig-analysis. (.

. .

.) bulk Pb optical properties. t and n refer to the components of e tangential and normal to the

surface. d = 4 A.

anisotropy of the Pb monolayer dielectric function, the tangential components of E, resembling closely the spectral features of bulk Pb (dotted line). E;,, suggests in both cases a much weaker absorption normal to the surface (for clarity sake the data from KKA were omitted in figure 14). However, we feel that at present the tangential component is more trustworthy, since

A

E ~ , , is extremely sensitive to variations in AR1.R and hence to unavoidable errors. Much more experimental data have to be gathered for an unambiguous evaluation of monolayer optical properties.

4. Conclusion. - The use of reflectance spectroscopy for the investigation of solid-liquid interfaces seems to be a very promising approach, which yields a wealth of new, experimental information relevant to problems in electrochemistry as well as surface physics. However, the rather complex interconnection between the observable (reflectance change AR/.R) and the physical parameters of interest (e. g. energetic levels of electron states) makes it often necessary to evaluate the surface or film optical properties on the basis of model calculations. Here, the situation has hardly advanced during the last years. Even refinements of the classical macroscopic 3-phase-model applied to monolayers, such as e. g. the use of uniaxial anisotropic optical constants, are still a first order approach to the problem. A more sophisticated, microscopic model for the

interaction of light with monolayer adsorbates has not yet been developed in a form applicable to experimen- talists. Highly accurate measurements on well defined surfaces, e. g. of single crystal electrodes, may assist in deriving such a theory.

References

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47 (1973) 229.

141 MCINTYRE, J. D. E., Symp. Faraday Soc. 4 (1970) 50,55,61. [5] MCINTYRE, J. D. E., Surf. Sci. 37 (1973) 658.

161 MCINTYRE, J. D. E., in Advances in Electrochemistry and

Electrochemical Engineering, Vol. 9, ed. R. H. Muller (Wiley-Interscience, N. Y.) 1973, p. 61.

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SOC. 56 (1973) 122.

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[9] BEWICK, A. and ROBINSON, J., Surf: Sci. 55 (1976) 349. [lo] GARRIGOS, R., KOFMAN, R. and RICHARD, J., NUOVO

Cimento 13B (1973) 272 ; Solid State Commun. 14

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[I21 A B E L ~ , F., LOPEZ-RIOS, T. and TADJEDDINE, A., Solid State

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Elektrokhimiya 8 (1972) 982.

[14] LAZORENKO-MANEVICH, R. M. and BRIK, E. B., Elektro-

khimiya 11 (1975) 750 and 1498.

[15] BRIK, E. B. and LAZORENKO-MANEVICH, R. M., Opt.

Spektrosk. 39 (1975) 119.

[16] LAZORENKO-MANEVICH, R. M., BRIK, E. B. and KOLO- TYRKIN, Y. M., Electrochim. Acta 22 (1977) 151. [17] HANSEN, W. N., Symp. Faraday Soc. 4 (1970) 27. [18] PLIETH, W. I., Symp. Faraday SOC. 4 (1970) 137.

[19] MEMMING, R. and MBLLERS, F., Symp. Faraday Soc. 4 (1970) 145.

[20] MARK, H. B. and RANDALL, E. N., Symp. Faraday Soc. 4 (1970) 157.

1211 TAKAMLJRA, T., TAKAMURA, K. and WATANABE, F., Surf: Sci. 44 (1974) 93.

[22] PLIETH, W. J. and SCHAEFER, G., Surf. Sci. 37 (1973) 67.

[23] PLIETH, W. and BRUCKNER, H., 2. phys. Chem. NF98 (1975) 33.

[24] TAKAMLJRA, T., TAKAMURA, K. and YEAGER, E., J. Electro- anal. Chem. 29 (1971) 279.

[25] TAKAMURA, T. and TAKAMURA, K., J. Electroanal. Chem. 39

(1972) 478.

[26] BEWICK, A. and TUXFORD, A. M., J. Electroanal. Chem. 47 (1973) 255.

[27] MCINTYRE, J. D. E. and KOLB, D. M., Symp. Faraday Soc. 4 (1970) 99.

[28] TAKAMWRA, T., SATO, Y. and TAKAMURA, K., J. Electroanal.

Chem. 41 (1973) 31.

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chim. Acta 19 (1974) 933.

[32] HORKANS, J., CAHAN, B. D. and YEAGER, E., J. Electrochem. SOC. 122 (1975) 1585.

1331 KOLB, D. M., LEUTLOFF, D. and PRZASNYSKI, M., Surf. Sci. 47 (1975) 622.

[34] BEWICK, A. and THOMAS, B., J. Electroanal. Chem. 65 (1975) 911.

[35] KOLB, D. M. and GERISCHER, H., Electrochim. Acta 18 (1973) 987.

[36] KOLB, D. M., Ber. Bunsenges. phys. Chem. 77 (1973) 891. 1371 KOLB, D. M., Discuss. Faraday Soc. 56 (1973) 138. 1381 KOLB, D. M. and MCINTYRE, J. D. E., Surf: Sci. 28 (1971)

321.

1391 HORKANS, J., CAHAN, B. D. and YEAGER, E., Surf. Sci. 46 (1974) 1.

[40] NAEGELE, K. and PLIETH, W. J., Surf. Sci. 50 (1975) 64. [41] NAEGELE, K. and PLIETH, W. J., Surf. Sci. 61 (1976) 504. 1421 GOITESFELD, S. and CONWAY, B. E., J. C . S., Faraday 170

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[43] MCINTYRE, J. D. E., in Optical Properties of Solids-New

Developments, ed. B. 0. Seraphin (North-Holland, Amsterdam) 1976, p. 555.

1441 DIGNAM, M. J. and MOSKOVITS, M., J. C. S., f;araday ZI 69 (1973) 65.

[45] KOLB, D. M. and KOTZ, R., Surf: Sci. 64 (1977) 698.

(1975) 960.

[47] KOLB, D. M. and KOTZ, R., Surf. Sci. 64 (1977) 96. [48] MCINTYRE, J. D. E. and ASPNES, D. E., Surf: Sci. 24 (1971)

417.

1491 KOTZ, R. and KOLB, D. M., to be published.

[50] ROSEI, R., Cap, C. H. and WEAVER, 3. H., Phys. Rev. B 10 (1974) 484.

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DISCUSSION

1) Pr. E. YEAGER (Cleveland).

-

The control of where dO/dE is frequency or time dependent and goes impurities is a very severe problem in electrochemical to zero at sufficiently high frequencies (or short times). studies, particularly of noble metals at more cathodic It would be interesting to examine (aR/aE),/R as a potentials. Pre-electrolysis is often used but pre-electro- function of coverage for various UPD svstems.

_-

lysis with Pt electrodes usually leads to contamination _Dr._D._M._KOLB_{(Berlin). -}_{We have done such} with this How did you 'lean up the

however, we have chosen a slightly dige- used in your work and do you have evidence that the

rent approach for determining (IIR) (aR/aE),. We surfaces are free of impurities ?

introduced another electrode into the cell at which, Dr. D. M. KOLB (Berlin).

-

When taking the usual

precautions (triply distilled water, p. a. grade or supra- pur chemicals), we often find it sufficient to clean the electrode surfaces by cycling the potential in a carefully chosen range to avoid extensive roughening. A common test for the cleanliness of the electrode surface are cyclic current-potential curves. Furthermore, optical studies can be used to detect impurities at the surface. E. g. the potential dependence of the surface plasmon energy as determined in ATR experiments (see Fig. 7) sometimes shows a pronounced hysteresis which we attribute to impurities in solution.

2) Pr. E. YEAGER (Cleveland).

-

In studies of adsorbed species, the electroreflectance coefficient at constant coverage (dRldE), can be obtained by observ- ing the a. c. electromodulation coefficient at sufficiently high frequencies. Thus

after having deposited the desired amount of metal on the working electrode, all metal ions in solution were plated until the solution was nearly metal ion free. Then the electroreflectance spectrum of the partially or completely covered working electrode was recorded at a bias potential negative enough to maintain the coverage and to prevent oxidative desorption of the adsorbate. Preliminary results for TI on Au are shown elsewhere [I]. However, when measuring the ARIR spectra due to adsorbate formation, an adsorbate induced electroreflectance effect may occur which is inevitably combined with the term aR/aO and which cannot be easily separated from that part of ARIR caused by the adsorbate itself.

[I] D. M. KOLB, in Advances in Electrochemistry

and Electrochemical Engineering, ed. by H. Gerischer and C. W. Tobias, Vol. 11, in press.

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REFLECTANCE SPECTROSCOPY IN THE STUDY OF ELECTRODE SURFACES C5-177

rage situation i. e. at high modulation frequencies, I like to present again, by means of a slide, the results for the complex plane analysis of 6A/6 V in the case of Pt in H2S04 containing

--

M HBr. The particular point here is the different signs of (aA/aB), and (dA/aV)l'),. I also draw attention to the shape of the curve in the third quadrant, which proves the mixed control of diffusion and kinetics of adsorption, for Br- adsorption on Pt in this concentration range. I fully agree that (aA/aV)e may be an important parameter, like (aR/aV),, for elucidation of the nature of specific adsorption.

M. FROMENT. - Pour diminuer l'influence des

impuretbs au cours des exptriences d'ClectrorCflectance ne serait-il pas possible d'examiner des surfaces mttal- liques au cours meme de leur formation ? Par exemple, on peut effectuer 1'Clectrocristallisation de I'argent sur

des monocristaux du meme mCtal et obtenir des surfaces de tr&s haute perfection. On acckderait par la meme occasion aux phCnom&nes d'adsorption des anions qui sont tr&s importants dans les phCnom&nes d'Clectrocristallisation.

I.

K. SASS (to Froment). - In reply to Dr. Fro- ment's question :

We have studied the photoemission yield in a thin- layer cell consisting of two silver electrodes (Ag (1 11) films on mica). This arrangement allowed us to dissolve one electrode and to deposit the dissolved amount on the other electrode.

Neither on this fresh electrode nor on the one that was dissolved did we find changes of the photoelectric response, significantly larger than the experimental accuracy, with the exception of the vicinity of o,,