Specular reflectivity change due to the faradaic adsorption of some metal cations on Au electrode in HClO4

Electroanalytical Chemistry and Interfacial Electrochemistry, 41 (1973) 31-39 © Elsevier Sequoia S.A., Lausanne

31

Printed in The Netherlands

SPECULAR REFLECTIVITY CHANGE DUE TO THE FARADAIC ADSORPTION OF SOME METAL CATIONS ON Au ELECTRODE IN HC104

TSUTOMU TAKAMURA and YUICHI SATO

Research and Development Center, Tokyo Shibaura Electric Co., Ltd., 1 Komukai Toshiba-cho, Kawasaki 210 (Japan) KIYOKO TAKAMURA

Tokyo Colle9e of Pharmacy, Uenosakurayi, Taito-ku, Tokyo 110 (Japan) (Received 7th March 1972; in revised form 26th .~une 1972)

INTRODUCTION

Specular reflectivity of electrode surfaces has been shown to be very sensitive to the electrochemical surface states of the electrode 1-5. Adsorption of anions causes the reflectivity of Au to decrease and the decrease has been explained in terms of the decrease in negative charge density on the surface of the electrode 3. The marked change in reflectivity permits a quantitative analysis of the adsorption process to be carried out. Quantitative analysis of the time dependence of the reflectivity indicated that the anion adsorption at constant potential was controlled by diffusion when the concentration of adsorptive entities was very low 3. The reflectivity was also shown to be sensitive to the deposition of a metal film formed on Au by Faradaic adsorption 3. Since Haissinsky found that there is a so-called under-voltage in the process of formation of a surface monolayer 6, many studies have been reported on the under-voltage of film formation 7-x2. Deposition of Pb on Au was studied in detail by Schmidt and Gygax 11. When the potential is swept toward cathodic in a cyclic voltammetry in acidic solution containing a small amount of Pb 2+, a cathodic peak due to the deposition of a Pb layer can be observed on Au at a potential far positive of the standard redox pontential. Similar behaviour 2' 12 has been found for the adsorption of Cd. The deposition of these metals on Au might be expected to change the reflectivity in the visible because the light absorption of these metals is very different from that of gold. The light absorption of copper, however, is similar to gold in the short wavelengths 13, and accordingly, the reflectivity change due to copper deposition might be expected not to be less significant compared to the above metals. Such change in reflectivity caused by metal film formation has never been systematically studied. In spite of various studies, some physical properties of the metal mono- or submonolayer have remained unexplained. Since specular reflectance is sensitive to electrochemical surface states, it is worthwhile to study the reflectance properties of the metal film. The purpose of the present study is to see how the reflectivity changes during the formation of monolayer films of Pb, Cd and Cu, and to understand the nature of such reflectivity change.

32

T. TAKAMURA, Y. SATO, K. TAKAMURA

EXPERIMENTAL

Gold plate of 99.99% purity and 0.5 mm thickness was cut into two rectangular plates of areas 45 x 15 mm and 35 x 15 mm. The electrodes were polished with alumina polishing powder, progressively changing the grain size; at first large grain and finally 0.3 #m grain was used. After polishing and cleaning, the two plates were tightly fixed in a Teflon holder to keep them parallel with a gap of 9.0 mm. The two plates were connected together electrically with a gold wire and used as a working electrode. The part of the Teflon holder carrying the electrode was immersed in the electrolyte solution and the electrolyses were carried out. The electrode potential was referred to an Ag/AgC1 electrode placed in a compartment separated by a flitted glass plate. The base electrolyte of 1 M HC104 was prepared by dissolving Wako Pure Chemical perchloric acid for the use of lead determination in doubly distilled water. Solutions containing adsorbable metal ions were prepared by dissolving a known amount of the corresponding oxide in the base electrolyte. The concentration of the cation was adjusted by pipetting the stock solution into the electrolysis medium. Electrolysis and optical systems are the same as described in a previous paper a, but the number of reflections was kept to five, which is small enough to prevent erroneous results due to light scattering 5 etc. The output signal of the photomultiplier was fed to an X-Y recorder through the amplification system described below. Rectangular wave of 1 kHz was amplified to 60 V and fed to the first diode of an MS9SY photomultiplier (Toshiba Electric Co.). About 700 V d.c. was applied uniformly to the other electrodes. Superimposed a.c. signal on the output of the photomultiplier was amplified with a NF Co. model LI-572 A lock-in amplifier. By this procedure we could obtain a very stable signal and prevent noise due to mechanical vibration which may result from the use of a mechanical chopper. RESULTS AND DISCUSSION

Adsorption of lead Potential sweep voltammograms (i-E) and the corresponding reflectivitypotential (R/Ro-E) curves were recorded simultaneously on the Au electrode in M HC10 4 both in the absence and in the presence of Pb 2÷. Typical curves for the base solution are presented in Fig. 1, where no definite change is seen in either curve in the double layer region. The presence of Pb 2÷, however, gives rise to remarkable changes in this region as shown in Fig. 2. A peak is seen on both the anodic and cathodic i-E curves at around 0.07 V and is attributed to the reaction 1o-12: PbZ++2e ~ Pbaa

(1)

The cathodic peak corresponds to the formation of Pb.a and the anodic peak to the dissolution of Pb.d. The R/Ro-E curve shows a distinct change at exactly the same potential. A notable increase in R/Ro at 450 nm was attributed to the formation of Pbad. R/R o decreased at the potential of the anodic peak where Pbad dissolves.

33

REFLECTIVITY OF ADSORBED METALS ON Au

0.4

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E/V vs. A g / A g C I Fig. 1. Potential sweep ( ...... ) current-potential, ( ) reflectivity potential curves of Au in M HC104 at 25°C. Potential sweep rate, 37 mV s 1; R/Ro obtained at 450 rim with parallel polarization.

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E/V vs. Ag/AgCI Fig. 2. Potential sweep ( ...... ) current potential, ( - - ) reflectivity-potential curves of Au in M HC104 in presence of 1 x 10 -4 M Pb 2+. Potential sweep rate, 37 mV s - l ; R/R o obtained at 450 nm with parallel polarization.

T h e p o s i t i o n s of the a n o d i c a n d c a t h o d i c p e a k s o n the i-E c u r v e are very similar, i n d i c a t i n g t h a t r e a c t i o n (1) is fast. S c h m i d t a n d G y g a x p o i n t e d o u t the f o r m a t i o n of a P b - A u a l l o y w h e n the e l e c t r o d e is k e p t at a sufficient n e g a t i v e p o t e n t i a l for a p r o l o n g e d p e r i o d 11. T o a v o i d c o m p l i c a t i o n due to a l l o y f o r m a t i o n , the m e a s u r e m e n t s were c a r r i e d o u t in as s h o r t a t i m e as possible.

34

T. TAKAMURA, Y. SATO, K. TAKAMURA

The a m o u n t of charge due to the f o r m a t i o n of Pb.d was e s t i m a t e d from the i-E curve to be 230 # C c m - 2 i n d i c a t i n g t h a t the a m o u n t of Pbad is of the o r d e r o f a s u b - m o n o l a y e r since the generally a c c e p t e d value of the h y d r o g e n m o n o l a y e r is 210 # C c m - 2 at a P t surface 14. I n Fig. 2, a hysteresis is clearly seen on the R / R o - E curve for the metal d e p o s i t i o n . This seems to c o n t r a d i c t the p r e v i o u s suggestion that the r e a c t i o n rate of the f o r m a t i o n a n d d i s s o l u t i o n of Pb,d is very fast. To treat the

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Fig. 3. Plot of reflectivity change against square root of time. Reflectivity change observed at -0.16 V where Pbaa was formed on Au in M HC104 containing 10 4 M Pb 2+.

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E/V vs. Ag/AgCI Fig. 4. Potential sweep ( ...... ) current-potential, ( ) refiectivity-potentialcurves of Au in M HCIO 4 in the presence of 3× 10 - 3 M Pb z+. Potential sweep rate, 37 mV s - l ; R/R o obtained at 450 nm with parallel polarization.

REFLECTIVITYOF ADSORBED METALSON Au

35

hysteresis in some detail, two R/R o E curves obtained under the same condition but with different potential sweep rates were compared. The hysteresis was more significant on the curve obtained with the faster sweep rate, indicating that the hysteresis was controlled by diffusion. The adsorption rate was measured at constant potential. The potential was first kept at + 1.25 V for 5 min to oxidize and remove the impurity from the surface, then set at +0.5 V to reduce the oxide layer for 30 s with agitation by argon bubbling and finally stepped to - 0 . 1 6 V to allow the spontaneous adsorption of Pb. The R/Ro-time curve recorded showed that over 15 s was required to attain adsorption equilibrium. When the R/R o is replotted against the square root of the time, a linear relation is obtained for short times (Fig. 3). These results suggest that the adsorption rate of lead from a solution of low concentration 3 is controlled by the supply rate of Pb 2 + on to the gold surface. When the concentration of Pb 2 + is increased, another peak-like spike was seen on the i-E curve and the corresponding increase in R/R o also observed. These trends are seen in Fig. 4. Since the position of the spike is more positive than the reversible potential of the PbZ+/Pb ° couple ( E ° = - 0 . 3 4 8 V vs. Ag/AgC1), the spikes are attributed to the formation and the oxidation of another type of adsorbed Pb (Pb~a01)). The shape of the peak differs from that of a diffusion controlled wave 15 17 . The negative foot of the peak observed is much lower than the diffusion controlled wave at the same concentration 17. The reflectivity increase due to the adsorption of Pdaa~m is clearly seen on the reflectivity curve as a second increase which is shown in Fig. 4. The Figure shows no hysteresis on R/Ro-E curves, indicating that the adsorption and dissolution of Pbadm and Pdadlm is very fast.

Adsorption of cadmium In the case of Cd clear peaks were not well marked as in the case of Pb, but cathodic and anodic waves (dashed line) were observed as shown in Fig. 5. As the position of these waves is clearly more positive than the standard potential of Cd 2 +/Cd ° (E ° = - 0 . 6 2 V vs. Ag/AgC1) 18, they are attributable to the formation and dissolution of Cdae. The corresponding R/Ro-E curve shows an increase due to the formation of Cd,a. Although the current-potential curve does not show sharp peaks it suggests the existence of at least two types of adsorbed species. The separation of these species, however, requires more detailed experiment. The reaction rate of formation and desorption of Cd, d is thought to be slower than that of Pb, because a hysteresis still exists on the R/Ro-E curve even at 1.5 x 10 -3 M Cd z+ (curv e B in Fig. 5) which is not observed in the case of Pb z + at similar concentrations and because the cathodic peak due to the deposition of Cdae on the i-E curve is not clear. Adsorption of copper Figure 6 shows i-E and R/Ro-E curves obtained in the presence of 3 x 10-3 M Cu 2 +. Since the reversible potential of the Cu 2 +/Cu ° couple 19 is about 0.039 V (E°=0.115 V vs, Ag/AgC1), the anodic peak at 0.36 V on the anodic branch of the i-E curve is attributed to the reverse of reaction (2): Cu 2 + +

2e ~

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36

T. TAKAMURA, Y. SATO, K. T A K A M U R A

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Fig. 5. Potential sweep ( ...... ) current-potential, ( ) reflectivity-potential curves of Au in M HCIO 4 in presence of (A) 2 x 10 -4, (B) 1.5 x 10- 3 M Cd z +. Potential sweep rate, 104 mV s - 1; R/Ro obtained at 500 nm with parallel polarization.

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37

REFLECTIVITY OF ADSORBED METALS ON Au

As the potential of the cathodic wave is more positive than the reversible potential of CuZ+/Cu °, the main reaction of Cu 2+ at about 0.3 V is considered to be reaction (2). The change in reflectivity at 0.4 V (Fig. 6) is ascribed to the formation and disappearance of adsorbed Cu film. The enhancement of the reflectivity due to the adsorption of Cu is not so pronounced as those of Pb,d and Cdad. This may be explained on the basis of the small difference in the reflectivities of Au and Cu at the wavelengths 13 in question.

Reflectivity change due to metallic film formation As shown in preceding sections, reflectivity was markedly changed by metallic film formation. The change was however, found to be wavelength dependent, i.e., in the shorter wavelength region, film formation caused an increase in R/Ro but at the longer wavelengths, the change was reversed for both Pb and Cu, and no change was observed for Cd. Typical examples are shown in Fig. 7. The wavelength at which the change is reversed is about 550 nm where the reflectivity of gold increases significantly. The wavelength dependence of the reflectivity of each metal is com-

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38

T. TAKAMURA, Y. SATO, K. TAKAMURA

pared in Fig. 8, which shows that the reflectivity of gold is lower than that of Pb, Cd and Cu at wavelengths of around 450 nm 13. In the longer wavelength region, however, gold is the brightest among the four metals. 100 8o

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The simplest and clearest explanation of the reflectivity change due to metal film formation is based on the difference in the reflectivity of bulk metals. Reflectivity is a surface property and accordingly, when, for example, the surface of Au is covered by bulk metallic Pb, the measured reflectivity is that of Pb and not Au. If the reflectivity of bulk Pb is higher than that of Au at the wavelength measured, the reflectivity after covering by Pb would be higher than before. Assuming that such consideration for the bulk phase can be applied to a covering by an adsorption layer of atomic dimension, the present experimental results can be explained at least qualitatively. For example, at 450 nm, the reflectivity of bulk Pb is higher than that of Au (Fig. 8), while this is reversed at 700 nm. Therefore, when Au is covered by Pb, an increased reflectivity is expected at 450 nm, but a decreased reflectivity at 700 nm. This agrees with results shown in Fig. 7. The question however arises as to whether it is right to assume that the results of Fig. 8 (the properties of the bulk phase) are the same as those of a thin film of atomic dimensions. The question breaks down into two parts. The first is whether the adsorbed metallic atom behaves as a bulk metal or not and the second has regard to the applicability of optical methods to such a thin layer of atomic dimensions. Since one accepts the ellipsometric method as a tool for the determination of thickness z°-22, there should be no doubt regarding the second part of the problem. It seems that no satisfactory answer has yet been proposed for the first problem. Isolated metallic atoms have no bulk properties, but when an atom is attached to a bulk metal surface the atom is expected to have some metallic properties provided that each electronic orbital overlaps to some extent. There would be a definite charge density of conduction electrons also on the adsorbed atom, i.e., the atom would behave as a metal. This qualitative argument leads to the conclusion that the properties of adsorbed atoms are similar to those of a bulk phase. The proportionality between the reflectivity change and the amount of adsorbed metal has been ascertained a3.

REFLECTIVITY OF ADSORBED METALS ON Au

39

SUMMARY

Current-potential (i-E) and reflectivity-potential (R/Ro E) curves have been recorded for the Au electrode in M HC10 4 in the presence and in the absence of small amounts of metal cations such as Pb 2 +, Cd 2 + and Cu 2 +. All the metals investigated were adsorbed on Au to form atomic submonolayers at potentials more positive than the reversible potential of the bulk phase. The formation of Pbad or Cdaa gave rise to a marked increase in R/R o at 450 nm. This returned to the original value when the adsorbed layer dissolved. The rate of adsorption was measured for Pb at constant potential. The plot of R/R o against the square root of time gave a straight line, indicating that the rate is controlled by diffusion. At high concentration of Pb 2+ (3x 10 -3 M) the formation of another type of Pb adsorption was also observed on i-E and R/Ro-E curves, but the reaction became suppressed before monolayer saturation. Deposition of Cu occurred likewise at a more positive value than the reversible potential, but the reflectivity change was not so pronounced either at 450 or 700 nm. At 450 nm, it increased while at 700 nm it decreased. The wavelength dependence of the change in R/Ro was also examined. Pb, Cd, and Cu increased the reflectivity at shorter wavelengths but only Pb and Cu decreased it at wavelengths longer than 600 nm; Cd did not change the reflectivity. The reflectivity change was explained on the basis of the difference in the wavelength dependence of the reflectivity of bulk metals. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

J. D. E. Mclntyre, 135th ECS Meeting Abs. No. 232 (1969). T. Takamura, K. Takamura, W. Nippe and E. Yeager, J. Electrochem. Soc., 117 (1970) 626. T. Takamura, K. Takamura and E. Yeager, J. Electroanal. Chem., 29 (1971) 279. D. A. Koch and D. E. Scaife, J. Electrochem. Soc., 113 (1966) 302. B. D. Cahan, J. Horkans and E. Yeager, J. Electrochem. Soc., 118 (1971) 1322. M. Hainssinsky, Experientia, 8 (1952) 125. J. T. Byrne and L. B. Rogers, J. Electrochem. Soc., 98 (1951) 457. B. J. Bowles, Electrochim. Acta, 10 (1965) 731. E. Schmidt and H. R. Gygax, Helv. Chim. Acta, 48 (1965) 1178, 1584. E. Schmidt and H. R. Gygax, J. Electroanal. Chem., 12 (1966) 300; 33 (1971) 121. E. Schmidt and H. R. Gygax, J. Electroanal. Chem., 14 (1967) 126. F. Mikuni and T. Takamura, Denki Kaoaku, 37 (1969) 852; 38 (1970) 118; 39 (1971) 237, 579. landolt-B6rnstein, 6. Aufl., II Band, 8 Teil, Springer-Verlag, Berlin, 1962, p. 1-1. S. Gilman, J. Phys. Chem., 67 (1963) 78. T. Berzins and P. Delahay, d. Amer. Chem. Soc., 75 (1953) 555. M. M. Nicholson, J. Amer. Chem. Soc,, 79 (1957) 7. R. H. Wopschall and I. Shain, Anal. Chem., 39 (1967) 1514. W. M. Latimer, The Oxidation States of the Elements and their Potentials in Aqueous Solutions, Prentice-Hall Inc., New York, 2nd. ed., 1952, p. 168'.~ W. M. Latimer, The Oxidation States of the Elements and their Potentials in Aqueous Solutions, Prentice-Hall Inc., New York, 2nd. ed., 1952, p. 183. J. Kruger, Corros., 22 (1966) 88. J. O'M. Bockris, A. K. N. Reddy and B. Rao, J. Electrochem. Soc., 113 (1966) 1133. M. A. Genshaw and J. O'M. Bockris, J. Phys. Chem., 74 (1970) 4226. U. Moriyama and T. Takamura, Denki Kagaku, 40 (1972) 300.