Ellipsometric study of the polymeric surface films formed on platinum electrodes by the electrooxidation of phenolic compounds

Surface Science @North-Holland

96 (1980) 461-475 Publishing Company

ELLIPSOMETRIC

STUDY OF THE POLYMERIC

FORMED ON PLATINUM ELECTRODES

SURFACE FILMS

BY THE ELECTROOXIDATION

OF PHENOLIC COMPOUNDS

M. BABAI and S. GOTTESFELD Department Received

of Chemistry,

20 August

University of Tel-Aviv, Ranat Aviv, Israel

1979

Ellipsometric measurements were used to follow in situ the growth of polymeric films formed on platinum electrodes by the eletrooxidation of phenolic compounds from aqueous solutions. The optical results provide information on the physical properties of the films and on the kinetics of growth. The dependence of film thickness and properties on applied potential is explained by competitive processes of growth and dissolution. Growth is shown to be controlled by field-assisted electron transport through the film. A simple interpretation is advanced for the selective inhibition of halidelion electrooxidation by the polymeric surface films.

1. Introduction The optical investigation of surface films formed on electrode surfaces by the anodic oxidation of phenolic compounds was initiated originally in an attempt to gain a better understanding of the electrochemical phenomenon of “selective inhibition”. It has been demonstrated that the oxidation of iodide ions at a Pt electrode in acid solutions could be drastically inhibited in the presence of phenol and some of its derivatives, while the electrooxidation of water was at the same time only very slightly affected [1,2]. While it was quite obvious that this behaviour is due to the presence of a surface film, it was not fully understood how it formed, how thick it was and, particularly, which were the properties of the film required to inhibit selectively the oxidation of halide ions. The purpose of this study was to investigate optically in-situ the nature of the surface layers formed on a Pt electrode in acid solutions, first in the presence of 1naphthol [3] and then in the presence of phenol. The first compound chosen was 1-naphthol [3] because its oxidation to form a film on the Pt surface occurs at relatively low anodic potentials, so that a potential region exists in which film growth and properties can be investigated without any complication due to simultaneous oxide growth. On the other hand, the higher solubility of phenol in aqueous solutions allows us to obtain a faster growth of much thicker films associated with a different rate law. It was found that the thickness and properties of the surface films depended strongly on the growth 461

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M Babai, S. Gottesfeld /Polymeric

surface films

potential: The variation of the potential allowed to follow optically a wide range of films in these systems, ranging from chemisorbed sub-monolayers to thicknesses of several microns. The optical measurements allowed us to evaluate the growth-rate laws of the films, to follow gradual changes in their composition with applied voltage, and to learn about their tendency to dissolve. Since the faradaic efficiency for the formation of these films is in most cases very low, it would be impossible to obtain this type of information from electrochemical measurements alone.

2. Experimental 2.1. Instrumentation 2.1.1. Optical A Gaertner L-l 19X Ellipsometer was used, with a 1.50 W tungstenhalogen light source powered by a highly stabilized Kepco type KS 36-10M power supply. Measurements of the ellipsometric parameters A and J/ during the slow growth of the polymeric film were taken by the conventional nulling method. The fast variations of A with V occurring in sinusoidal potential modulation experiments (conducted in the region of non-oxidative adsorption) were measured as relative changes in the intensity of the exit beam with the analyzer and polarizer set at precalculated off-null readings [4]. An Ithaca lock-in amplifier (Model 391) was used to extract the component of the signal in phase with the modulating voltage. Differential reflectometric measurements were taken in the potential region of non-oxidative adsorption employing the same optical system, with the polarizer and analyzer both set at 0” and 90” for parallel and perpendicular polarization, respectively [5]. Regular reflectometric readings were found to be unreliable in the region of thick film formation because of the finite variations of light absorption in the solution due to slow formation of a colored soluble product. Measurements were performed in an all-Teflon electrochemical-optical cell. The windows were oriented for an angle of incidence of 69”. 2.1.2. Computation The complex refractive index Ar= nf - ikf of the polymeric film as well as its thickness at the ith stage of growth were calculated from values of Ai and ~$i measured at about 20 stages during film growth. A single fti model was employed, assuming a uniform complex refractive index for this film. Thus n + 2 parameters (nf, kr, Lr, .... L,) were adjusted according to 2n experimental readings ($, , .... J/,, Ar, .. .. A,). Adjustment of the parameters was achieved with the MINUIT library program [6] of a CDC 6600 computer. 2.1.3. Electrochemical Potentials were controlled

with a PAR potentiostat

(Model 173). Sinusoidal volt-

M. Babai, S. Gottesfeld /Polymeric

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463

age modulations were imposed by a twochannel HP Model 3300A function generator. An Elsint waveform generator Model CHF-1 was employed for the linear potential scan. A Yokagawa X-Y recorder (Model 3078) was used to record transients. 2.1.4. Electrodes The platinum working electrode (99.99%) was in the form of a cylinder, pressuremounted in a teflon holder, exposing a single surface (surface are 0.32 cm’) to the solution. The electrode was polished with alumina powder (0.3 pm particle size) to a mirror finish. The counter electrode was a platinum wire mounted in a glass tube. A sintered glass disc was used to separate the counter electrode compartment from the working electrode. A dynamic hydrogen electrode operated at 4 mA/cm2 in 0.5 M H2S04 served as the reference electrode. All potentials reported were measured against this reference, which was found to be -312 mV versus SCE in 0.5 M H&04. 2.1.5. Chemicals Solutions of sulfuric acid were prepared from Suprapur (Merck) reagent diluted by triple distilled water. Phenol and l-naphthol @ideal-De Haen) were purified by sublimation at a reduced pressure and a fresh stock solution was prepared before each experiment.

3. Results and discussion 3.1. The chemisorbed

layer of I-naphthol

- an optical criterion for charge-transfer

An optical investigation of the adsorption of 1-naphthol limited to the level of one monolayer could be conducted by holding the Pt electrode immersed in the acidic naphthol solution at potentials lower than 0.65 V versus DHE. In this potential region the rate of the electrooxidative process in which thick polymeric films are formed is negligible, but strong non-oxidative adsorption of 1-naphthol does take place, as could be verified from the strong lowering of the hydrogen coverage on Pt [3]. The apparent optical properties of the adsorbed layer were evaluated from the recorded changes in the ellipsometric and reflectometric parameters caused by the addition of l-naphthol up to a concentration of 1 mM, while the potential was held constant at 0.4 V DHE. Measurements were made at three wavelengths in the visible region. A uniform isotropic film with a real part of the refractive index between that of water (1.33) and that of naphthol (1.62) was assumed in the computer analysis. The results of the analysis are given in table 1. While the thickness and the real part of tierm obtained are of the order expected for a mixed water-natphthol monomolecular layer, the non-zero value of k indicates some interaction between the adsorbed naphthol molecules and the metal substrate. (Naphthol itself does not absorb in the visible.) Some further insight into the nature

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surface films

Table 1 Vahws of nf and kf measured for the adsorbed layer of I-naphthol at 0.4 V versus DHE ___--.___. Refractive Wavelength Extinction Thickness, index, nf coefficient, CA) z‘ (‘Q kf ._ 4047 5460 6330

1.44 * 0.03 1.45 f 0.03 1.46 f 0.04

0.08 * 0.02 0.06 f 0.02 0.08 f 0.03

6*2 6*2 6t2

of this interaction was obtained from measurements of the differential optical parameters @A/6 V)O, (1/Rp)(6Rp/6 v) 0, and (l/R,) (6RJ6 V)e at frequencies suffciently high to ensure constant coverage, so that only the effects of field modulation on the properties of the adsorbed layer were recorded [3,7]. The pronounced peculiar behaviour of @A/6 V)e due to the chemisorbed naphthol layer is shown in fig. 1: As naphthol is added and the coverage increases, as evident from the lowering of the capacitance, a change in the sign of @A/& u)e from negative to positive takes place. Simultaneously, a sign change in (l~R~)(~R~~~~~ from positive to negative and stronger positive readings of (l/R,)@R& V), were also recorded [3]. Very similar changes in the differential optical parameters were measured before for Pt in the presence of adsorbed halide ions [7]. They were inter-

6Or

Naphthol

Concentration

(M) x104

Fig. 1. The dependences of the differential optical parameter @A/S V)e and the different&I capacitance on the concentration of 1-naphthol, measured for a Pt electrode at 0.5 V versus DHE. Modulation frequency = 20 Hz, modulation amplitude = 40 mV for the monitoring of &A/s Y and 2 mV for the monitoring of C; ;\ = 4047 A.

M. Babai, S. Go ttesfeld

465

/ Polymeric surface films

[7f as due to ~eld~nh~~ed charge transfer from the electron-rich adsorbed anion to the metal substrate, which modifies the adsorbed layer to form a more compact film with a higher extinction coefficeint at more anodic potentials. The recording of the same behaviour in the presence of adsorbed naphthol indicates a similar increased interaction, most probably between the 71electron system of the adsorbed molecules and the Pt substrate. It seems that this behaviour of the differential optical parameters is typical for cases of chemisorption associated with significant charge transfer from the adsorbed species to the metal substrate. Further investigation seems warranted to clarify if this behaviour could be used as a general criterion for appreciable charge transfer in adsorption. preted

3.2.2. Formation o~~o~yrne~c films by the e~ectro~xi~~ti~~ of I-naphthol At potentials higher than 0.65 V, 1-naphthol is electrooxidized at the Pt surface in the acid solution employed, forming a thick polymeric layer. The growth of the film is maintained only in the presence of the applied anodic potential and is completely halted if the circuit is opened, indicating that the polymerization is an electrochemical process and not an electrochemically-induced chemical process. (The same is true for the electrochemical polymerization of phenol, to be described below .) 35

I

I

1

1

I (5;o)

I

I

I

I

(660)

30 ~%o/-x

*-

25 lz ‘p20-

Y/ A331

s!

I

15-

/ x

IO -: 7 * (62) “!

0

I

I

I

I

I

I

I

I

I

I

2

3

4

5

6

7

8

9

10

6’#’ (degf

Fig. 2. Comparison between measured values of A and j, (X) and the computer-generated solution based on a uniform film growth model for a polymeric layer grown in a 1 mM l-naphthol, 0.5 M HzS04 solution at 0.75 V versus DHE. The measured firnet= = 2.07 - 4.3Oi; the solution for n^ftlm = 1.71 - 0.12i (A = 6330 A).

M. Babai, S. Gottesfeld /Polymeric

466

surface films

I-

540 0a :: u ,’ .u f E .T !A.

460

380

300

I 65

1

I

I

1.05 Potential

f I 85

1.45 (V

vs

DHE)

Fig. 3. Film thickness reached after a fixed time as a function I-naphthol, 0.5 M HzS04): (0) 16 h; (0) 32 h.

of applied

potential

(Pt/l

mM

Films were grown at constant potential and their optical properties and thicknesses were evaluated by assuming uniform layer growth (cf. experimental section). The fit between the experimental points and the computer generated solution can be judged according to fig. 2. The quality of this fit, as found in all the growth experiments conducted, confirmed the validity of a uniform film growth model for these polymeric layers. In fig. 3 the thickness of the polymeric film obtained after 16 and 32 h of growth is shown as a function of the potential of growth. A maximum is observed in the dependence of the growth rate on anodic applied potential at about 1 V versus DHE. Beyond this potential the film seems to undergo partial oxidative dissolution. (A slight coloration of the solution indicated the formation of soluble products.) At potentials anodic to 1 V changes in the composition of the film could be detected in the measured optical properties, as shown in fig. 4: The lower extinction coefficients measured at higher anodic potentials seem to reflect

M. Babai, S. Gottesfeld /Polymeric

Potential

surface films

467

(V.vs.DHE)

Fig. 4. The

dependence of the real and imaginary components films grown from the acidic I-naphthol solution (h = 6330 A).

of no on applied potential for

an early stage in film dissolution in which partial bre~down of the long range order jn the polymer takes place. The result is a film with practically the same density, as judged from the insignificant variation of nf (fig. 4), but with a smaller absorption in the visible region, apparently caused by the partial breakdown of larger conjugated aromatic units. Calculated values of nf and kf for film growth at 0.85 V are shown in table 2 for three different wavelengths. A si~i~c~t absorption throu~out the visible is apparent with a maximum around 5000 8. Since 1-naphthol itself does not absorb in the visible, the non-zero krmust be due to the formation of larger conjugated units during polymerization. The value of nf seems to imply a fairly compact layer: Refractive indices quoted in the literature for bulk homopolymers are typically 0.05-0.10 units Iarger than those of the liquid monomer [8], and that seems to be true also in this case. However. some water

Table 2 Values of nf and kf for the thick film formed by electrooxidation sured at three wavelengths Wavelength (A)

Refractive index, nf

Extinction coefficient, kf

4050 5460 6330

1.72 1.69 1.71

0.068 0.158 0.116

of l-naphthol

at 0.75 V, mea-

M. Babai, S. Gottesfeld / Polymeric surface films

468 0.10 I-

I

I

I

I

I

I

1.0

1.2

I 1.4

I

0.06

0,02

- 0.10

- 0.14

I ()

0.2

I

0.4

I

I

I

0.6

0.8

Potential

(V.vs.

I

DHE)

Fig. 5. Cyclic voltammograms recorded on: (a) (solid curve) bare Pt surface; (b) (dashed surface covered with a 500 A thick polymeric film grown at 1.20 V versus DHE.

curve)

in the film could be confirmed from the persistence of the typical cyclic voltammogram for the Pt/H,O interface even in the presence of a thick polymeric film, as shown in fig. 5. This result means that the film has to be somewhat porous, SO as to allow penetration of water down to the metal-film interface even in the presence of quite thick polymeric films. The porosity of the fnm is an important requirement for the attainment of selective inhibition at the filmed electrode, as discussed below. content

M. Babai, S. Gottesfeld /Polymeric

I

0

I

IO

I

I

30

I

I

50

t”’

surface films

I 7.0

469

I

I

I

9.0

II 0

(hour”2)

Fig. 6. The thickness of a film grown at 0.75 V versus DHE from acidic 1 mM 1-naphthol tion as a function of t’ /2, showing the parabolic rate law for its growth. Measurements taken at three wavelengths: (0) 4047 A; (+) 5460 A; (0) 6330 A.

At potentials

lower than

1 V DHE (i.e. where

the rate of film dissolution

soluwere

is negli-

film could be evaluated and is shown in fig. 6. Independent measurements at three different wavelengths gave the same results, as can be seen from fig. 6. A parabolic growth law is in many cases an indication for diffusion through the growing film (a concentration gradient) being the rate-limiting step. This does not seem to be the case here, since the average measured parabolic rates, dL/d(t1’2), increased with applied potential in the relevant region (0.7-l .O V versus DHE), showing that the rate-limiting process is field-assisted. The mechanism that seems to explain successfully the parabolic rate of growth in this case involves charge-transfer through the film to the reacting 1-naphthol molecule, where the migration of an electron (or hole) through the film under the assistance of a relatively small applied voltage is the rate determining gible),

a simple

parabolic

rate law for the growth

of the polymeric

470

M. Babai, S. Gottesfeld /Polymeric

step. The reate equation dL/dt = A exp[(-AG$

surface f%ns

in this case is [9] : + zaFE)/R?]

-A

exp [(-AG,# - zaFE)/RT]

,

(1)

where E is the electric field in the film, z is the charge on the moving particle (z = 1 for an electron), a is half the distance between adjacent sites in the electron hopping trajectory, AG,# is the standard free energy of activation for a single jump from one site to the next, and A is a frequency factor. Under small fields such that 2aEF
(2)

and assuming that E = V/L where V is the voltage across the film and L is the film thickness, one obtains a linearized form of (1): dLldt = (2AzaFV/RTL)

exp(-AGo#lRT) ,

(3)

resulting in L* = (4AzaFV/RT)

t exp(-AG,#RT)

,

(4)

or, L =K(V) t'/'

.

(5)

The low field approximation is justified at the low anodic potentials applied. For example, at an applied potential of 0.75 V only about 100 mV fall across the layer, while the rest is required to drive the interfacial reaction, the rate of which becomes significant only above 0.65 V. For a voltage of 0.10 V across the film and assuming a = 5 8, eq. (2) is fulfilled for film thicknesses above 60 8. 3.3. Formation of polymeric films bJ1 the electrooxidation of phenol Polymeric films were grown by the electrooxidation of phenol from solutions 0.1 M in phenol and 0.5 M in H,S04. As long as the anodic growth potential was kept sufficiently low, the optical properties of the film and the rate law for the film growth were found to be similar to those observed during the electrooxidation of 1-naphthol. Fig. 7 shows the linear dependence of film thickness on t”* as measured at the lowest potential employed for the growth of the phenol surface polymer. The refractive index evaluated in this case was 1.70 - 0.06i. The main difference between the surface films formed in phenol and in naphthol solutions was found under higher applied potentials. While prolonged electrooxidation from the 1 mM naphthol solutions at potentials well inside the oxygen evolution region (0.e.r.) resulted in a very thin residual fdm (-50 a), thick layers of the order of 1 pm could be grown from the 0.1 M solutions of phenol under the same conditions. The reason for this apparent difference is the limited concentration of 1-naphthol, as dictated by the low solubility of naphthol in water, resulting in a low rate of mass transport from the unstirred solution to the electrode surface. Indeed, similar very thin residual layers were obtained inside the o.e.r. in phenol solutions

M. Babai, S. Gottesfeld /Polymeric surface films

471

Fig. 7. The thickness of a film, grown at 1125 mV versus DHE from a 0.5 M H2S04 + 0.1 M phenol solution, as a function of tllZ.

as well when the concentration of phenol was lowered to the level of 1 mM. The higher rate of monomer transport to the surface is apparently essential to overcompensate the destructive effect of Oa evolution through the film, as well as the effects of other dissolution processes which are enhanced at these higher anodic voltages. The thick films formed from the concentrated phenol solutions simultaneously with significant evolution of O2 were of a different nature: Their growth followed a linear rate law, as shown in fig. 8. It can be seen that layers as thick as 1 pm could be grown in less than an hour. These thick films still followed a uniform growth pattern, as demonstrated in fig. 9. Each optical cycle in this figure corresponds to a thickness increase of ca. 2300 A. The optical properties again showed a drop in kf at higher growth potentials, as demonstrated in fig. 10. These low values of kf allowed the ellipsometric monitoring of film growth through several optical cycles (fig. ,9). The linear growth law for these films (i.e. a rate of growth which does not depend on ftim thickness) indicates that these are most probably thick porous layers which form on top of a very thin “barrier layer” adjacent to the metal surface through which electrons have to be transferred. The presence of a thin barrier layer at the base of the thick film is suggested by the dependence of growth rate on

r

412 8000

7000

6000 LG z 5000 s p 4000 l5 3000 i; 2000

1000

t

(HOURS

Fig. 8. Film thickness versus time in the linear growth tials. Solution: 0.5 M HzS04 + 0.1 M phenol. I

I

I

I

I

1

region,

measured

I

I

I

at three applied

poten-

I

-10 0 i3

W

IO

0

3 20

ccl

30

40

1

I-

I

-40

-20

0

20

40 60 6A (DEG)

80

100

Fig. 9. Comparison between measured values of A and J, and the computer-generated solution based on the uniform growth model for a polymeric layer grown at 2.24 V versus DHE from a 0.5 M Has04 + 0.1 M phenol solution. The measured tiM = 1.89 - 4.17i; the solution for ti*film = 1.73 ~ 0.014i; A = 5460 A. o, o, A, 0, and A designate points in the A-$ plane recorded during the lst, 2nd, 3rd, 4th and 5th optical cycles, respectively.

M. Babai, S. Gottesfeld /Polymeric

surface films

413

I

I 1.8

I 1.9

I 2.1

I 2.0

GROWTH POTENTIAL

I 2.2

Fig. 11. Dependence of log(growth o.e.r. from phenol solutions.

rate)

POTENTIAL

on growth

I

2.4

(V vs DHE)

Fig. 10. The dependence of kF on applied potential for films grown o.e.r. region from a 0.5 M HzS04 + 0.1 M phenol solution.

GROWTH

I

2.3

potentiostatically

inside the

(VvsDHE)

potential,

for thick

films grown

inside the

M. Babai, S. Gottesfeld /Polymeric surface films

474

applied voltage in the region of thick layer formation, which was found to fit a semi-logarithmic plot, as shown in fig. 11. This behaviour can be described in terms of an abnormally high Tafel slope for the electrochemical growth process: av/a log(growth rate) = 200 mV. Such high Tafel slopes are typical for processes at filmed electrodes [lo], and their interpretation involves the assumption that comparable activation barriers exist within the film for the transport of electrons and at the interface for heterogenous electron transfer. If Vr is the total metal-solution potential difference and V is the fraction which operates across the barrier film, then a v,ja

log

i = 2.3RT/flzF(

i - a v/a vr)

(see ref. [IO]), which yields a slope of 200 mV/decade of current density when aI’/aI’, = 0.4, (assuming p = i). Thus, the combined results of a linear growth law under potentiostatic conditions (fig. 8) and the abnormally high Tafel slope (fig. 11) seem to suggest that the mechanism of thick polymeric film formation inside the o.e.r. involves transport of electrons through a thin barrier layer of constant thickness and transport of the reactant molecules and intermediate products through the porous growing outer part of the film. Further buildup of the outer layer seems to occur at the film-solution interface, while the rate-determining step throughout the growth of the film is apparently the transport of electrons from the metal across the barrier layer to the reactant phenol molecules. 3.4. The phenomenon

of selective inhibition

It was found that the selective inhibition of halide ion oxidation in the presence of phenol and its derivatives appears to be brought about by polymeric layers with a thickness ranging typically between 100-l 0 000 A. These layers are porous, the porosity being probably somewhat higher for the thick films formed in more concentrated solutions at high anodic potentials simultaneously with O2 evolution. The chemical composition of these layers was not found to be critical for the obtaining of selective inhibition: Even some porous hydrated nylon films on carbon electrodes were found to exhibit a similar behaviour [ 111. The key seems, therefore, to be simply the presence of a porous hydrated surface film. One very simple effect that such a film may produce is a drastic lowering in the rate of mass transport to the metal electrode surface. This will affect more dramatically the rate of the electrooxidation of a solute, such as the halide ion, while a smaller effect will be observed on the electrooxidation rate of the solvent. A simple calculation may elucidate this point: On a bare Pt surface the limiting current for iodide oxidation from a 0.1 M I- solution is about 100 mA cm-*. If we assume that the porous layer has an effective diffusion coefficient of lo-” cm2 s-r, which is a typical value for porous and hydrous oxide films [ 121, the limiting current for I- oxidation from the 0.1 M solution under control of mass transport through a 100 A thick film will

M. Babai, S. Gottesfeld / Polymeric surface films

415

drop to in,ritm = F’DritmCbdrJL

= 0.1 mA cme2 .

On the other hand, the calculated limiting rate for the diffusion of water through the same film, using the same thickness and diffusion coefficient, will be much higher: -55 mA cmd2 corresponding to Cburk = 55 M. This will result in an apparent selectivity of 550 : 1 for the oxidation of water if both electrochemical oxidations will be limited by transport through the film. In fact the selectivity may be even higher since the transport of the smaller Hz0 molecules through a partially hydrated phase may be faster than that of the halide ion, resulting in a relatively higher effective diffusion coefficient for water in the film.

4. Conclusion Ellipsometric measurements were found in this work to be effective in characterizing electrochemically formed polymeric surface films as well as in evaluating their growth laws. It is felt that the use of ellipsometry as a source of information during polymeric film growth on electrode surfaces, for purposes such as metal protection or the production of modified electrode surfaces [ 131, is promising.

References [I] (21 [3] [4] [5] [6] [7] [8]

T. Bejerano, Ch. Forgacs and E.Gileadi, J. Electroanal. Chem. 27 (1970) 69. T. Bejerano and E. Gileadi, J. Electroanal. Chem. 38 (1972) 137. M. Babai, S. Gottesfeld and E. Gileadi, Israel J. Chem. 18 (1979) 110. S. Gottesfeld and B. Reichman, Surface Sci. 49 (1974) 377. S. Gottesfeld, M. Babai and B. Reichman, Surface Sci. 57 (1976) 251. F. James and M. Roos, Computer Phys. Commun. 10 (1975) 393. S. Gottesfeld and B. Reichman, J. Electroanal. Chem. 67 (1976) 169. L. Bohn, in: Polymer Handbook, 2nd ed., Eds. J. Brandrup and E.H. Immergut (Wiley, New York, 1975) p. 111-241. [9] M.A. Genshaw, in: Electrosorption, Ed. E. Gileadi (Plenum, New York, 1967) p. 76. [lo] See, for example, A. Vijh, Electrochemistry of Metals and Semiconductors (Dekker, New York, 1973) p. 170. [ 111 S. Gottesfeld, unpublished results. [12] See, for example, M. Green, W.C. Smith and J.A. Weiner,Thin Solid Films 38 (1976) 89. [ 131 L.L. Miller and M.R. Van De MarkJ. Am. Chem. Sot. 100 (1978) 639.