Studies of electrode resistance in the electrochemical cell

J. Electroanal. Chem., 150 (1983) 521-534 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

521

S T U D I E S OF E L E C T R O D E R E S I S T A N C E IN T H E E L E C T R O C H E M I C A L CELL

D A V I D L. R A T H

Thomas J. Watson Research Center, International Business Machines Corporation, Yorktown Heights, N Y 10598 (U.S.A.) (Received 26th July 1982)

ABSTRACT With thin metal films used as electrodes, double-layer processes can be tracked and identified by the complimentary technique of electrode resistance monitoring. This is demonstrated with chloride adsorption and lead underpotential deposition on gold electrodes. An example of the latter system with the process of controlled electrode emersion is included where the continuously measured resistance dearly shows that the lead adlayer is removed completely intact with the electrode. A simple model for adionic effects on conduction electron scattering at metal surfaces is proposed and found to correlate well with the experiment. The merits of the electrode resistance measurement for in situ studies are discussed.

INTRODUCTION

The heart of electrochemistry is the region encompassing the electrified interface where the properties of both phases differ from their bulk value. For a solid metal electrode, the electronic properties are modified by the electrostatic fields of the double layer which extend into the bulk to distances on the order of a few atomic layers. When the electrode is in the form of a thin film, however, the condition of the surface strongly influences the transport properties of the metal as well. Thus, the measurement of the thin film resistance will reflect the microscopic structure of the double layer through the mutual interactions occurring there. The purpose of the present work is to clarify and update our knowledge on thin film resistance phenomenon and to exemplify how the application of this surface sensitive technique will aid in the unravelling of interfacial processes. It is hoped that the evidence accumulated here will stimulate further interest in this interdisciplinary method and will help bridge the gap between surface physics and electrochemistry where only unilateral communication seems to exist. In an effort to rectify this problem, we will attempt to describe the adsorption process of ionic species by modifying a theory proposed by Watanabe [ 1] where neutral adatom effects on metal resistivity are modeled.

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© 1983 Elsevier Sequoia S.A.

522 BACKGROUND The first observation that the resistivity of a thin continuous film is larger than its bulk value, led to a wealth of investigations intended to reveal the origin of this effect and had also resulted in a large number of conflicting interpretations [2]. Fortunately, many of these early controversies have been resolved, primarily because of improved vacuum technology, so that at present a fairly clear picture of the resistivity phenomenon can be drawn. In light of the near bulk resistivities found with epitaxially grown films, the generally accepted model of electron scattering is that it is specular at a smooth undamaged surface [3], with the large resistivities measured on polycrystalline samples now attributed to grain boundary scattering [4]. Externally imposed surface effects on the resistivity are of greater interest to the electrochemist. The simplest phenomenon, at least conceptually, is the electrostatic charging of the double-layer, termed the field effect, where the induced charges on the electrode reside entirely at the surface. For those metals which have been examined in the vacuum chamber and the electrochemical cell, the resultant changes in thin film resistivity have been normal and linear to the added charge [5-8], but having a magnitude larger than anticipated if only the number of conduction electrons was changed. Arguments to account for this discrepancy have been proposed where the surface scattering and the effective thickness of the thin film would also be changed with the number of charge carriers [9,10]. The resistivity of thin metal films with adsorption has been studied for many years in both the vacuum chamber and the electrochemical cell and remains the subject of active research. Studies of gaseous adsorption have empirically demonstrated that the resistivity response is caused by changes in the electron scattering at the surface and not by the possible mechanisms of surface demetalization or a change in the number of conduction electrons [ 11 ]. Additional evidence gained from the vacuum system indicates that not only is the resistivity sensitive to the number adsorbed species but also the adsorption state [12,13]. Use of the electrochemical cell for studies of adsorption effects on thin film resistivity offers experimental conditions far superior to those available in the vacuum chamber. In addition to an almost unlimited variety of adsorbing species to choose from, the electrode-electrolyte system allows the adsorption and desorption of ionic species to be precisely controlled and monitored through the electrode potential and charge. Significant progress towards the development of electrode resistance monitoring as a diagnostic tool of interfacial phenomena has been achieved. Shimizu [14-16] investigated the mechanisms of the adsorption states of hydrogen on platinum with galvanostatic methods. Bockris et al. [17] carefully considered the experimental conditions in their study on the effects of several acids on thin platinum films at elevated temperatures. Anderson and Hansen [6,7,18] have shown that the resistivity of gold electrodes is selective through its varying degree of sensitivity with different adsorbing species, and have correlated these results with optical measurements. Niki and Shirato [19] analyzed gold electrodes in the presence of organic compounds and found that the maximum changes in the resistivity occur

523 at the potential of zero charge. Dickinson and Sutton [20] analyzed the in situ dc method of resistance monitoring in their investigation of several anion and cation adsorption processes on platinum. Fujihira and Kuwana [8] correlated the platinum electrode charge with its conductance and found that hydrogen and copper adsorption increases the conductance while the adsorption of oxide species has an opposite effect. Petrii and co-workers demonstrated in a series of papers [21-24] the effects on the resistance measurement of platinum, rhodrium and palladium thin film electrodes for many types of adsorbing species. Ganon et al. [25] analyzed the oxide region of gold and through the resistance-charge variation, identified mechanisms governing the electrochemical adsorption of oxygen. Tucceri and Posadas [26,27] reconsidered the problem of the potential distribution along a thin metal film electrode under dc polarization and demonstrated a means to obtain the derivative of the voltammogram through the resistance measurement. Hansen [28] and coworkers [29,30] have demonstrated the unique role that the resistance measurement can play in the process of electrode emersion. In a very recent work, Rath and Hansen [31] analyzed many of the basic theoretical premises to describe the effects of adsorption on thin film resistance. In their work where specific adsorption of bromide and iodide on several gold films were studied, it was shown that the resistance is linear to the coverage of these species and that the ratio of the resistance sensitivities to these halides with a given electrode is invariant. In addition, techniques were developed in their study which enables the reduction of assignable parameters of theory, so that empirical evidence could be brought to bear. A common result that can be derived from these studies is that the resistance measurement is linear to the amount of coverage [7,8,23-25,28,31]. For the transition metals though, the resistance response seems to also depend on the specific interaction of the adlayer with the conduction d-bands of the metal, acting as traps for the nearly free s-electrons [32]. In this report, emphasis will be placed on the gold electrode which has a simpler Fermi surface and thus better fits the free-electron model on which theory is based. Furthermore, the large polarizable region of the gold electrode, allows a wide selection of adsorbing species for study, free of interfering electrode processes. THEORY The theoretical description of surface effects on thin film resistivity is based on the free-electron gas model. In this model the electrical resistivity of a monovalent bulk metal can be written as po = h k F / n oe21°

(1)

where k F is the Fermi wavevector, no is the volume density, and 1o is the mean-free-path (mfp) of the conduction electrons [33]. The mfp is the average distance a conduction electron will travel between collisions with lattice imperfections and phonons, of which the latter is dominant at room temperature [34]. w h e n the metal has a thickness comparable to the mfp the surfaces also impose a

524

constraint on the electron motion. Using the Boltzmann equation to describe a free electron gas distribution, Fuchs [35] and Sondheimer [36] presented the theory for this size effect on the electron transport properties where the electrons have a probability of being specularly reflected at the surface. Accounting for the different environments that the surfaces of a film evaporated onto a substrate have, the thin film resistivity is expressed by [37]

P : P° ( 1 - ~---xfol ( U - -pj-~l~xp(( U3)[1- exp( ~-2-- -x/u ~ X [2-po-pl

+ (Pu + P l - 2puP,) e x p ( - x / u ) ] du) -1

(2)

where x, the reduced thickness, is the ratio of the film thickness to the mfp, Pu ( P ~) is the specularity parameter of the upper (lower) surface of the film and u is the cosine of the incident angle of the conduction electrons to the surface. The specularity parameter is the probability of an electron being specularly reflected by the surface with values ranging from 0 for complete diffuse scattering, to 1 for complete specular scattering. Theoretical advances in modeling surface phenomena have shown that this parameter is intrinsically dependent on the electron incident angle [38], so that approximations of this equation where the specularity parameters are removed from the integral are not strictly valid. The initial adsorption of any foreign species on a clean thin film surface will increase the resistivity [11,39] provided that the metal has a simple Fermi energy surface. Within the framework of the presented theory, the adsorption process decreases the specularity of the electron-surface collisions by introducing discrete potential barriers located just at the surface. While theoretical treatments of the specularity parameters have been developed for a few select systems [12,38], to date, none are directly applicable to ionic adsorption on metal electrodes. Perhaps the closest facsimile of our problem is approached by Watanabe [1] where neutral adatoms on a metal are assumed to be similar in effect to the scattering of slow electrons by gaseous atoms. In his model, the potential of the adatom as seen by the incident electron has the form of a shielded Coulomb potential located at an infinite I I I I

ELECTRON DENSITY POSITIVE JELLIUMDENSITY METAL

]

X ~

xa

0

Fig. 1. Schematic representation of infinite surface barrier model with adsorbed ion potential barrier, after ref. 44.

525 barrier representing the metal surface (see Fig. 1). In a similar fashion, we will assume that the potential barrier of an adsorbed monoatomic ion may be represented by

V( x ) = - ( Zae 2/x) exp( - 2 x / x a)

(3)

where for our case, z a is the charge of the ion with x, as its crystal ionic radius which would approximate the adsorbate-adsorbent interaction. With the derivation supplied by Greene and O'Donnell [38] and subsequent modification by Watanabe [1], the dependence of the specularity parameter for the exposed (upper) surface on randomly placed adatoms can be expressed as p u = [ 1 - N a b A I ( U , Oa)] / [ 1 +NabA2(u , Oa)]

(4)

followed by

Al(U'°a)=,..

-KS.,8:{

1 1+8

Az(u'°~)=----~u

~1

2 ln/t+u81]

u,8

\,-uSJJ

2 + u26

+~ln(l+8)_

c = (4 + 48 + u282) 1/2

8 = o,/b

u c ~ ( l - u 2) b =

(5)

with

o a = ~(ZaXa/aokF) 2

(6)

where a 0 is the Bohr radius, and N, is the surface density of the adsorbed species with scattering cross section, % Since the theory used is based in part on the Born approximation, this equation for the specularity parameter will break down as the product of Na and oa approaches unity. Meanwhile, computations of this model have shown that the resistivity will be linear to the number of adsorbed species up to coverages approaching this limit. To describe thin film resistivity completely, we should also include the effects of surface roughness, and for polycrystalline samples, grain boundary scattering. We will instead adopt the method suggested by Rath and Hansen [31] where these additional complications can be avoided for determining the adatom cross section. In their work, an apparent scattering cross section of the adatom was defined by the relation o* = lim ( O2r/ONaOs¢-1 )

(7)

K --~ oO

where r isthe relative charge of the total film resistance. The experimental evaluation of this equation with electrochemical data requires that the electrosorption valency of the adion and the roughness factor of the film surface be known in order to determine the derivative with respect to the number of adsorbed species, OrlONa. The second derivative with respect to the inverse of the reduced thickness and the limiting operation is used to negate the innate physical properties of the metal film. Noting that the parameter r is equivalent to the relative change of the film

526

resistivity, the apparent scattering cross section as derived from the presented Fuchs-Sondheimer theory (eqn. 2) can be written as o*

=

-

(3/4) f01(u -

u3)(apu/aNa)du

(8)

which has no dependence on the lower (unexposed) surface scattering parameter, p]. Inclusion of surface roughness [40] and grain boundary scattering [41] as modeled in the literature, into the theory also reduces to this form. Thus the link between experiment and theory can be realized through the use of these equations since parameter assignments are not required. E X P E R I M E N T A L DESIGN A N D CONSIDERATIONS

The monitoring of thin film electrode resistance is a versatile technique which can be applied to many different electrochemical situations. Likewise, there are a correspondingly large number of experimental arrangements and electrode designs to implement this measurement. One such design that we have found to be particularly suitable for in situ investigations is schematically reproduced in Fig. 2. In this example, the thin film electrode is sealed to an open, rectangular window in the cell wall, so that the necessary electrical connections to the electrode are isolated from the electrochemistry. The shown rectangular geometry of the thin film allows the use of the four-probe resistance method where the measurement of the voltage drop resulting from a constant current applied along the length of the film, gives the film resistance, free of any contact resistance. An additional contact to the working area of the film may also be attached close so the position of the Luggin capillary tip of the reference electrode to minimize potentiostatic compensation of common path ohmic drops.

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POTENTIOSTAT

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Fig. 2. Thin film electrode configuration for monitoring electrode resistance by 4-probe method and schematic representation of monitoring and instrumentation.

527 The required instrumentation for the resistance measurement consists of a current supply and a monitor capable of tracking small changes in the voltage. The only stipulation being that its electronics must not interfere with that of the controlling electrochemical instruments. The simplest arrangement for the dc method is to have as the current supply, a battery placed in series with fixed resistors of value much larger than the film resistance, and a voltmeter with offset or a dc bridge as the monitor. For ac measurements, an isolation transformer powered by a frequency generator may replace the battery with the monitor being a lock-in amplifier or ac bridge. Monitoring the changes of the electrode resistance by the dc method is suitable for most electrochemical studies. However, the measurement itself requires a potential drop along the length of the film, so that the electrode no longer has an equipotential surface. This causes a smoothing effect on the voltammetric spectra which would be reflected by the measured resistance. Other difficulties may arise when the potentiostatic contact is asymmetrically positioned on the working electrode [20] and when the measurement is made in the presence of potential controlled faradaic processes [26]. These effects can be minimized by taking the resistance measurement in the limit of zero dc current but will result in a tradeoff of measurement sensitivity. Resistance measurement with ac currents has several advantages over the dc method, such as superior instrumentation giving an increased sensitivity with better stability and noise discrimination, and a net zero current through the film so that the dc limitations are avoided. However, the double-layer can no longer confine the ac current to the electrode, making this measurement sensitive to changes in the double-layer impedance when the solution is highly conducting. By proper choice of the electrolytic concentration, measurement errors resulting from the co-conduction can be made negligible. Confirmation that the measurement will faithfully represent the thin film resistance phenomenon is easily accomplished by simply repeating the measurement with either the small current dc method or with ac method at a different frequency because the double-layer impedance depends strongly on the measurement frequency [42]. DISCUSSION Many of the features of in situ resistance monitoring are exemplified by the chloride adsorption study on an epitaxial (111) gold film electrode as shown in Fig. 3. In the linear sweep voltammogram, the adsorption of chloride is seen as the broad current peaks at about 0.25 V. The large currents at the lower potential limit signal the onset of hydrogen evolution. Both of these phenomena are superimposed on the double-layer charging current, which in this case, makes up a sizable fraction of the total current in the adsorption region. Shown with this voltammogram is the resistance spectrum as measured by the ac method with precautions taken as outlined. Comparison of this curve to one obtained without chloride in the solution has shown that the sensitivity of the

528

0

o I

-

(c)

-Io

'on

-20 I •

~

] -0.4

I

I 0

UScE/V

I

I 0.4

I -0.4

0

0.4

UScE/V

Fig. 3. (a) Cyclic voltammogram, and (b) resistance and (c) derivative resistance vohammetric spectra for chloride adsorption with 150 n m (111) gold film at scan rate of 50 m V / s . Electrolyte is 50/~M NaC1 in 10 m M HCIO 4.

resistance to chloride adsorption is nearly an order of magnitude larger than its response to the background double-layer charging. Furthermore, with the linearity of resistance to specifically adsorbed ions [31] which has also been found in this case, this measurement tracks the covcrage of the adsorbed chloride with simple charging effects discriminated against. We also see here that the resistance is insensitive to the hydrogen evolution which gives definite proof that this process is a " t r u e " faradaic reaction in the sense that the interracial region essentially remains unchanged. Since the resistance is an integral measurement, the voltammetric current can be directly correlated to the derivative of the resistance. Displayed in Fig. 3c is the derivative-resistance spectrum obtained by differentiating the resistance measurement without any filtering of noise. The close resemblance of this curve to the voltammogram in the adsorption region lends additional credance for the concept of resistance linearity. Furthermore, with the enhanced sensitivity of the resistance measurement to adsorption, the peak potentials may be more accurately determined. Upon closer inspection of these curves, we notice that the derivative resistance spectrum is relatively smaller on the positive potential side of the adsorption peak. This effect, however, is not due to a saturation of the resistance response to higher coverage of chloride, but can be easily shown to result from changes in the nature of the adlayer cause by either an oxidation reaction or the coadsorption of an oxide

529 40

:J 2O

~'E

0

-20

-40

-04

0

04

08

12

USCE/V

Fig. 4. (a) Cyclic voltammogram and (b) resistance voltammetric spectrum for lead UPD with 200 nm gold film at scan rate of 10 mV/s. Electrolyte is 1 mM Pb(C104)2 in 10 mM HC104.

species. Thus, the-derivative technique enhances the deviations from the ideal adsorption system. A more interesting case is the underpotential deposition (UPD) of lead on evaporated gold where the formation of ordered structures in the adlayer causes an intricate resistance response [28]. Shown in Fig. 4 is the simultaneously measured voltammetric and resistance spectra for a potential range spanning the Nernst potential of lead and the oxide region of gold. In the more negative potential region, the voltammogram indicates that lead U P D is reversible with the exception of the small stripping peak at 40 mV which has no apparent counterpart in the deposition scan. The resistance curve, meanwhile, has a marked hysteresis with scan direction at lower coverages of lead while the traces nearly coincide as the monolayer is completed. In the oxide region the addition of lead to the solution causes only slight changes on the voltammogram while the resultant changes in the resistance spectrum are drastic. In the resistance curve presented, the pronounced dip in the positive scan at 0.8 V and the indentation at 0.7 V in the negative scan are not seen with a lead-free solution [25]. Further evidence for a lead residue remaining on the electrode is apparent from the behavior of the resistance measurement in the intermediate potential range. In this region the minima of the two resistance traces do not attain the same value. In addition, comparison with lead-free spectra shows us that the resistance increases much more rapidly here in the positive scan. The initial deposition of lead which occurs in the broad voltammetric peak at 100 mV, termed the monolayer peak, gives rise to a resistance response which is linear to the coverage as was found with halide adsorption [31]. As more lead is -

530 40

3o

~

~ 20

%° ~

°° 10

c~

~~

|

20

40

60

100 o-Q/ nm2

Fig. 5. Relation %* of eqn. (8) taken in the limit of zero coverage and modeled adion scattering cross section, % of eqn. (6). Data points for I-1 and Br-1 taken from ref. 31 with remaining points evaluated relative to I-1. Electrosorption valency for Br-1 is -0.81 [28,31] and for CI l taken as -0.8.

deposited, the relative sensitivity of the resistance decreases until at a potential lying in the valley of the voltammetric doublet peaks, the total resistance reaches a m a x i m u m and then decreases. In the region of the doublet peaks, the added lead m a y displace those adatoms already present to less preferential sites, so that the net result on the adlayer-electrode interaction may either increase or decrease, as reflected in the diametrically opposite effects seen in the resistance. Here it is interesting to note that the clarity of the resistance maxima has been seen to depend strongly on the scan rate [28] as does the relative position and magnitude of these voltammetric peaks [43]. With the formation of the close-packed layer the resistance decreases slightly until the Nernst potential is reached. At this point the bulk deposition of lead has little effect on this measurement. This suggests that the first layer remains the same with bulk lead deposited because the effective sampling distance of the conduction electrons lies within the first layer. In the stripping scan the resistance essentially retraces itself until reaching the more positive side of the doublet, at which point it does not fall off near as rapidly. This hysteresis is due to an irreversibly deposited lead at intercrystalline boundary sites which is stripped during the voltammetric peak at 40 mV. This conclusion is substantiated by measurements on the system with truncated potential limits and by supportive experiments using monocrystalline and single crystal (111) gold films. The sensitivity of the resistance to this irreversibly deposited lead is the same as was found for the monolayer lead, both in depositing and stripping. Even though no theoretical development to date can fully describe these effects on the resistance, we can compare the resistance response to the initial lead deposition with the sensitivities of this measurement to other adsorbing systems. This is because the lead at low coverages is mostly ionic in character. Using the approach developed by Ruth and Hansen [3 l] and their data, the apparent scattering cross section, o*, for several U P D species and the adsorbing halides on gold have been experimentally determined. Shown in Fig. 5 are the results of these measurements plotted against the modelled scattering cross sections, oa as outlined in the

531

50mY s-I

T°

I

EMERSE

i

IMMERSE

I -0.4

I

I 0

I

I 0.4

USCE/V

T

0.5%

\

i

200 i

sec_ j -i

time

Fig. 6. Resistance with electrode emersion at -0.4 V for lead UPD on 100 nm gold film as a function of time. Electrolyteis 1 mM Pb(C104)2 in 10 mM H C 1 0 4. Insert: in situ resistance spectrum at scan rate of 50 mV/s. theoretical section of this paper. The electrosorption valency for each of these ions was assumed to be equal to its solution valency except for bromide which has been determined to have a value of - 0 . 8 1 [28,31] and for chloride with its value taken as -0.8. Also included in this figure is the theoretical response curve calculated from equation 8, which relates the intrinsic electronic response to the observable changes in the measurement. The close fit of nearly all these points to the calculated curve is remarkable considering the simple model adapted to describe the adatom-electrode interaction. The relatively large departures seen with several of the U P D species can be explained within this model as being due to a changed effective distance of interaction, x a as has been done in gaseous adsorption studies [1,12]. In light of the more sophisticated models of the metal surface [44], further theoretical work directed at the electrode system should be rewarding. One last aspect of the resistance technique that deserves special mention is its unique capability to continuously monitor the electrode double-layer throughout the emersion process. In Fig. 6 an example of electrode emersion and immersion as monitored by the dc resistance method is shown as a function of time for lead U P D on gold. For comparison purposes, an in situ resistance-potential curve for this system is included as an insert. Prior to the emersion sequence, the electrode was potentiostatically cycled between + 0.5 V and the Nernst potential, until the resistance curve was reproducible. The selected potential for the emersion ( - 0.4 V) was

532 then held during the positive scan to assure equilibrium lead coverage. This is displayed in the resistance-time curve of the figure which also includes the last deposition scan. After a time to establish a base line for the resistance, the electrode was removed from the solution to the ambient while under potentiostatic control and then immersed at the same in situ potential. In this example the lead layer was removed completely intact as there are no perceptible changes in the measured resistance during the actual emersion time, which was less than 5 s. After the emersion the resistance-time curve attains a m a x i m u m and then decreases with a decay time on the order of minutes. By noting the shape of the in situ resistance spectrum and the relative position of the selected emersion potential (see arrow in insert), we can attribute this behavior as being due to an oxidation of the lead layer by the ambient which in essence follows the in situ stripping scan. Thus the lead which has the smallest interaction with the electrode is oxidized first while the more strongly bound lead is that much more stable. Upon immersion of the electrode at the same in situ potential, the resistance attains a higher value than it initially was which may be due to some lead oxide species remaining on the surface. Here it is interesting to note that in a similar study with U P D on silver with a nearly identical in situ resistance spectrum, the emersed adlayer was seen to be oxidized almost instantaneously by the ambient, and yet the lead was found to remain entirely in the removed double-layer [45]. CONCLUSION Electrode resistance monitoring has evolved into a powerful technique to probe electrode processes, yielding insight unavailable by classical electrochemical methods alone. Using the gold electrode with the specific adsorption of chloride and lead UPD, several attributes of the resistance method were demonstrated. The resistance of the single crystal (111) gold film, was found to be linear to the amount of chloride adsorbed with a sensitivity enhanced over that to double-layer charging. Thus, this technique can be used to conveniently track the adsorption-desorption process. With the demonstrated insensitivity of the ac resistance measurement to massive Faradaic reactions, this technique should be of particular value for studies, such as electrocatalysis where these reactions dominate electrochemical measurements. Also, from this study we note that the film thickness is no longer very restrictive. Extrapolating the displayed sensitivity in these curves, surface effects on gold films of about 1/~m in thickness should be resolvable with the resistance measurement. The U P D of lead on polycrystalline gold resulted in a more complex response on the resistance measurement. While the resistance was again found to be linear to the initially deposited lead, at higher coverages the intricate resistance response revealed that it is highly sensitive to the nature of the adlayer. Evidence for a residue lead species remaining on the gold electrode even in the oxide region was substantiated with the noted changes of the resistance spectrum. This demonstrated the ability of the resistance measurement to highlight certain electrode processes which are not observable by other methods.

533 U s i n g a c o m p a r i s o n t e c h n i q u e devised by R a t h and H a n s e n [31], the sensitivity of resistance to several different a d a t o m s was e x p e r i m e n t a l l y d e t e r m i n e d in a m a n n e r wh i ch allowed the testing of theory. T h ese results were f o u n d to well agree with a m o d e l i n t r o d u c e d there to describe ionic a d s o r p t i o n effects on m et al film resistivity. R e f i n e m e n t s of this theory m a y also offer b e t t e r insight into the m o r e c o m p l e x systems, such as those f o u n d in studies of a d s o r p t i o n on the transition m e t a l electrodes. T h e u n i q u e capability of the resistance m e a s u r e m e n t to m o n i t o r d o u b l e - l a y e r processes d u r i n g e le c t r o d e e m e r s i o n was d e m o n s t r a t e d with lead U P D on gold. In this e x a m p l e the d o u b l e layer was again sh o w n to be r e m o v e d intact with the electrode, even t h o u g h the o x i d a t i o n of the lead layer o c c u r r e d in the ambient. This feature of the resistance m e a s u r e m e n t thus p r o v i d e s an i m p o r t a n t link b e t w e e n the e l e c t r o c h e m i c a l cell where sample p r e p a r a t i o n can be precisely controlled, an d the v a c u u m c h a m b e r w h e r e n u m e r o u s structure sensitive techniques are available. ACKNOWLEDGEMENT T h e a u t h o r wishes to thank Professor H. Gerischer, F r i t z - H a b e r - I n s t i t u t , Berlin, W e s t G e r m a n y for s u p p o r t during the time m u c h of this w o r k was carried out. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

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