The occlusion of methionine and cystine during copper electrodeposition

Electroanalytical Chemistry and lnterfacaol Electrochemistry, 54 (1974) 189-196

189

© Elsevier Sequoia S.A, Lausanne - Printed m The Netherlands

THE OCCLUSION O F METHIONINE AND CYSTINE D U R I N G COPPER ELECTRODEPOSITION

R. J. GALE* and C A. WINKLER

Department of Chemzstry, McGill Universtty, Montreal (Canada) (Recewed 26th October 1973, m revised form 28th January 1974)

INTRODUCTION

The occlusion of addition agents in electrodeposits has long been recognizedt, but the mechanism of the process is little understood. Relatively few studies have attempted to determine surface concentrations of organic adsorbates at electrodes in the presence of appreciable faradaic discharge processes, even though detailed consideration has been given to the effects that an adsorbed neutral substance might exert on the electrode kinetics of the primary charge-transfer reaction (cf. refs. 2, 3). In the present study, a radiotracer method was used to analyse for occluded methionine and cystine in copper deposited at various current densities from acid copper sulphate electrolytes. By comparing their occlusion rates with the kinetics of adsorption of these amino acids on copper under open circuit conditions 4, it was hoped to assess the effects of the faradaic process on their adsorption and subsequent occlusion. EXPERIMENTAL

The method and the conditions for studying the occlusion of additives on copper foil cathodes, during the passage of current, were similar in all essentials to those used to study their adsorption on open circuit 4. Thin copper foil cathodes (1.0 cm x 1.0 cm x 0.004 cm) were suspended in 15(~ml beakers open to the air and containing freshly prepared, unstirred 50 ml portions of the electrolytes to which the radioactive additives had been added. The cathode was located between two copper anodes of polycrystalline sheet, 1 cm x 10 cm, each at a distance of 2 cm from the cathode. Before their use the foils were cleaned, as previously described 4, and the anodes were treated similarly between each experiment. The electrolytes were prepared from the standard electrolyte (125 g 1- t CuSO4- 5 H20/100 g 1-1 H2SO4) to have a nominal radioactivity of 50 #Ci 1-1 with one of 1-methionine-14CH3, D,L-methionine-2-~4C, D,L-cystine-3-14C (all from Int. Chem. and Nucl. Corp.), D,L-cystine-1-14C (New England Nucl. Co.), or L-cystine-35S (Amersham Searle Corp.). The experiments were made at 25.0_+ 0.1°C. Radioactivity measurements were made both on solid samples, with a Baird * Present address" Department of Chemistry, Colorado State University, Fort Collins, Colo 80521, U.S.A

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R.J. GALE, C. A. WINKLER

tll

z

0 ,,3 0

I

I

20

.40

I 60

I 80

MIN

Fig. 1. Change in count rate from occluded L-cystlne-aSSwith quantity of copper deposited ((1~) 1.0 A dm-2, (ill) 4.0 A dm-2. Cystme 1.0 x 10-4 M in the standard electrolyte. Atomic t-proportional counter, and on liquid samples with a Beckman model LS150 liquid scintillation counter. To obtain meaningful count rates from the solid foils containing occluded radioactive additive, it was necessary first to establish the minimum amounts of copper electrodeposit that would provide constant absorption and back-scattering of the t-rays emitted by the radioisotopes. The data in Fig. 1 illustrate that this was achieved after the passage of 36 C c m - 2 for current densities of 10 and 40 mA cm-2. All subsequent occlusion experiments were made with the passage of this quantity of electricity. The counting data for the solid foils and for the solutions of foils agreed qualitatively and, since the liquid counting method may be standardized 4, it was possible to obtain quantitative results for the solid samples from a calibration curve. RESULTS A marked decrease in the z4C or ass radioisotopic content of deposits occurred as the current density for copper electrodeposition was increased in the presence of these additives, as illustrated by the data for cystine in Fig. 2. Such a decrease has often been observed, e.g. refs. 5-7. It has been proposed that cystine may be reduced electrolyticaUy and occluded in copper electrodeposits as CuS 8' 9. On the other hand, Lahousse and Heerman 1° considered that experimental proof was still lacking for the desulphuration of methionine and cystine on the surface of copper during its deposition. From the relative extents to which cystine molecules with different radioactive centres (D,L-cystine-l-14C, L-cystine-35S, and D,Lcystine-3-14C) were apparently occluded from standard electrolytes that contained cystine in bulk concentrations of 1.0 x 10 -4 M, it seems most likely that at least the -S-CH2--CH(COOH)NH~ portion of the molecule was trapped stoichiometrically. Data for D,L-methionine-2-1~C and L-methionine-14CH3 suggested, similarly, that this additive molecule was occluded into the deposits without decomposition. Nevertheless, the radioisotopic evidence in this paper is not entirely conclusive, since there is a possibility that all fragments of a decomposed additive molecule were occluded to equivalent extents.

OCCLUSION OF METHIONINE AND CYSTINE IN Cu

0

0

_m o E w

Ld

Z

//o

z

_o (.9 -J U

04

W

Z

L~ z l-l.o >U

o

U ×

o I

I

C U R R E N T DENSITY / m A

c m -2

I

I

l.n [ C U R R E N T DENSITY / re.a,c m "2]

Fig. 2. Relation between count rate of occluded cystine and current density, for deposition of copper from electrolyte containing cystlne 1.0 x 10 -4 M (O) D,L-cystme-l-14C, (1~) L-cystine-35S, ( , ~ . D , L cystine-3- ~4C. Fig. 3. Occlusion ofcystlne m electrodeposits formed at different current densities from cystme 1 0 x 10 -4 M. (O) D,L-cystine-l-14C, standard electrolyte, (1~) L-cystine-3SS, standard electrolyte; (O) D,Lcystine-l-14C, 0.5 M CuSO 4 (no H2SO4 added).

When the mean rates of occlusion were calculated using geometrical areas, on the assumption that no degradation of additive occurred, it was found that the relations between the occlusion rates and current density were markedly different for eystine and methionine. This difference is particularly of interest, since the coverages of these two additives by adsorption on copper on open circuit were found to be similar in the 10-400 s period 4. For the current density range investigated, the plot of In (cystine occluded per second) against In (current density) was approximately linear with a slope of 0.50 (Fig. 3). The amounts of cystine occluded at particular current densities were similar with and without sulphuric acid in the electrolyte. In contrast, Fig. 4 illustrates variations in the mean rates of occlusion of methionine from a bulk concentration 2.0 x 10 -4 M in the standard electrolyte. In the current density range, 2.5-30 mA c m - 2, the occlusion rates were about four times higher than the mean rate of adsorption on open circuit in an aerated electrolyte 4, 1.4 x 10 - 1 2 mol c m - 2 s - 1 . With further increase in the current density, the occlusion rates increased until a maximum was reached at about 50 mA cm-2. DISCUSSION

Generalfeatures of occlusion If the rate of adsorption of an additive species is small compared with the rate of copper deposition, the surface density of adsorbed species at any instant

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R.J. GALE, C. A. W I N K L E R

O

2C

5 E L,J

z

5 v

3

x

© ×

I

I

lO

30

I

CURRENT DENSITY/ mA cr'n"2

Fig. 4. Occlusion of methlonine m electrodeposlts formed at dlfferefit current densities, from methlonlne 2.0 x 10-4 M m standard electrolyte. ( x ) D,L-methiomne-2-x4C, ( O ) L-methlomne-14CHa.

during deposition will be very much less than that corresponding to its monolayer coverage. The decrease in radioisotopic contents of deposits formed with increasing current densities, often observed, may be attributed to a reduction in density of occluded radioisotopes since, for an approximate assessment, the species may be considered to be adsorbed at an essentially constant rate, while the rate of deposition increases with current density. Indeed, it seems likely that, except perhaps at very low current densities, the rates of occlusion during electrodeposition from solutions with low additive concentrations should be controlled by the maximum rates of adsorption of the additive. Volk and Fischer 11 have determined differential capacities at electrodes during nickel deposition with the additives 2-butyn-l,4-diol and 2propyn-l-ol. The strongest decrease of the differential capacity, thus the strongest increase of surface coverage of additive, occurred at the lowest current densities, in general agreement with the occlusion mechanism proposed above. Theoretical lattice growth rates for the current density range used in the present study, 2.5-60 mA cm- 2, are presented in Table 1. These were calculated on the TABLE 1 IDEAL ATOMIC G R O W T H O F L O W - I N D E X PLANES

Cathodtc current denstty

Copper atomic growth rate /layers per second

/mA cm- 2

2.5 10 60

{111}

{1/90}

{110}

4.4 17.7 106

5.1 20.4 122

7.2 28.8 173

O C C L U S I O N O F M E T H I O N I N E A N D C Y S T I N E IN Cu

193

basis of 10070 cathodic current efficiency* and for perfectly formed low-index lattice planes {111}, {100}, and {110}, for which the atomic densities are 4/3 ½ a2o, 2/a 2, and 2/2 ~ a 2 atoms per unit area, respectively (ao=3.615 A). Ideally, to compare adsorption rates with occlusion rates obtained in the presence of a concurrent electrocrystallization reaction, accurate adsorption data would be needed for the time scale of layer regeneration i.e. ,~5-250 ms for the current densities used in this study. In addition, the surface heterogeneities and rugosities of the solid electrodes should be closely matched for precise comparisons of the two processes.

Occlusion behaviour of methionine From radiotracer investigations of methionine adsorption and desorption rates on copper 4, it appeared that the major species adsorbed on open circuit was a Cu(I)-methionine complex. It has been proposed 13'14 that the mechanism for cupric ion reduction, appropriate to the conditions of this study, may be represented by the reaction scheme, Cu 2+ + e rd.s. Cu +

(1)

Cu + + e ~

(2)

Cu (0)

The formation of a Cu(I)-methionine complex at the electrode surface might then be directly related to the deposition current density. No such simole relation between occlusion rate and current density was observed in the experimental data of Fig. 4. The concentrations of the complex species available for adsorption will depend upon the relative magnitudes of the rates for reactions (1) and (2), together with Cu(I) + methionine --, Cu(I)" methionine Cu(I). methionine + e -~ Cu(0) + methionine

(3) (4)

A negligible level of the complex species might result if reactions (1) and (2) are rapid in comparison with reaction (3), or if reaction (4) occurs very much faster than reaction (3). It follows that a comparison of the adsorption rate on open circuit with that obtained under conditions of electrodeposition may not always be vahd, since the adsorption of different species could be involved in the two processes. The variation observed in the rate of occlusion of methionine with- increase of current density might be explained on the basis that maximum adsorption of organic compounds at an electrode, without accompanying faradaic processes (i.e. at ideally polarized electrodes), often occurs at potentials in the vicinity of the potential of zero charge (p.z.c.) of the particular electrode/electrolyte system 15. The maximum in the mean rate of occlusion of methionine, shown in Fig. 4, could reflect a dependence of its adsorption on the potential at the electrode during electrodeposition. The current density range of the peak is not broad and it occurred at ~ 50 mA cm-2, for which current density the experimentally determined overpotential was - 3 7 2 mV vs. Cu electrode in standard electrolyte (or ~ - 0 . 0 5 V vs. NHE). By comparison, the maximum adsorption of additives on copper without concurrent deposition, e.g. naphthalene 16, is known to occur in a * In this current density range, faradalc etticlencles are ~ 9870 or better, for copper deposition from acid copper sulphate electrolytes 12.

194

R.J. GALE, C. A. WINKLER

small potential range and the value of the p.z.c, for copper (H2SO 4 electrolyte) is usually placed aT' 18 in the potential range 0.0 to - 0 . 2 V vs. NHE.

Occlusion behaviour of cystine As the overpotential of the cathode is made increasingly negative during copper deposition in the presence of cystine (RSSR), a potential may eventually be reached at which cystine is reduced at the copper surface by a direct chargetransfer reaction, kl

R S S R + 2 H + +2e ~ 2RSH

(5)

k2

Although disparities may occur at solid electrodes between the actual potentials of the cystine/cysteine couple and those calculated from the Nernst equation as applied to reaction (5), cf. refs. 19 and 20, determinaton ofa cystine/cysteine standard potential from heats of combustion gave a value21+0.025 V vs. NHE. This value is in good agreement with the results from a recent kinetic technique which similarly does not rely upon solid electrodes. It involves, instead, analyses of the thioldisulphide exchange glutathione (GSH) and cystine or oxidized glutathione (GSSG) and cysteine. From knowledge of the standard redox potential of GSSG/GSH, a value of + 0.02 V vs. N H E was computed for the cystine/cysteine standard potential 22, i.e. this overpotential is reached 2a in these copper electrodeposition experiments at a current density between 35-40 mA cm-2. For the reversible reduction of cystine to cysteine, electrochemical rate theory gives, for the forward reaction, kl = A a exp ( - atlnF/R T) and for the reverse reaction k2 = A2 exp [(1 - a)tlnF/g T] From these relations,

K = kl/k 2 = const, e x p ( - t l n F / R T ) [RSH] 2 [RSSR] [H+] 2 In a dynamic situation activities have been replaced by concentrations, a is a transfer coefficient for reaction (5), and n is the number of electrons of the faradaic process. Correspondingly, in the presence of an additive with only minimal influence on the rate of deposition of copper, it may be assumed that the Tafel relation is obeyed

q=a-b

lni

where ~/ is the overpotential for the copper charge-transfer reaction (formally negative for cathodic reactions), a and b are constants, and i is the current density for copper deposition; i,..,iexpt, if iexpt is the actual current density. The empirical relation, shown in Fig. 3, seems to indicate that a linear free energy function exists between the rate "of occlusion of the organic compound and the rate of deposition

OCCLUSION OF METHIONINEAND CYSTINEIN Cu

195

of copper. If cystine occludes mostly in the form of cysteine and the rate of occlusion of the latter depends on [RSH] at the electrode surface, the quantities of occluded additive will vary in a linear manner with K ~ as the potential at the electrode is decreased (becomes more negative), for [RSSR] and [H÷],-~constant. The surface concentration of [RSSR] may remain approximately constant if the electron-transfer reaction is relatively slow (quasi-reversible or irreversible). Cysteine, formed by the reduction of cystine, may react with cupric ion or cuprous ion to form complexes that become occluded into the electrodeposits. Arguments similar to those discussed previously for the behaviour of methionine apply to the formation of cystine complexes and their reduction. However, it may be noted that Cu(II) chelation to the amino acid functional groups of these additives will be restricted by the pH conditions of the electrolytes24. The observed increase in occluded additive, (2.2-10.8) x 10-11 mol cm- 2 s- ~ at current densities 2.5-60 mA cm- 2, respectively, calculated as cysteine, confirms that the limiting rate of diffusion in support of occlusion was not achieved in these expriments. It should be emphasized, perhaps, that the quantity of material occluded need not represent the total cystine reduction current, since diffusional losses of products to the bulk electrolyte may occur, followed by re-oxidation of any cysteine to cystine by cupric ion or dissolved oxygen25. More complete analytical data would be required to lreat the reaction kinetics of the surface processes in a rigorous manner. No maximum is evident in the data for occlusion of cystine at potentials near the p.z.c. (H2SO4 electrolyte). The maximum adsorption of cysteine may take place at a potential beyond the range of those used in the present study or, alternatively, cysteine may chemisorb to the copper surface more strongly than does methionine, cf. ref. 10. ACKNOWLEDGMENT The authors would like to thank the National Research Council of Canada for financial assistance. SUMMARY A radioisotopic technique has been used to measure the extents to which the addition agents methionine and cystine are occluded in copper electrodeposits. Increase of the current density from 2.5 to 60 mA cm-2 resulted in a decrease in the volume density of occluded radioisotopes. This behaviour would be expected if the net adsorption rates of additives vary only slightly relative to the change in the growth rates of the metallic lattice. The mean occlusion rates of methionine exhibited a maximum at about 50 mA cm-2, which may be due to a potentialdependent adsorption process. An empirical variation in the mean occlusion rates for cystine is interpreted on the assumption that cystine is reduced by a slow charge-transfer process, and is occluded either as cysteine or as a copper-cysteine complex. REFERENCES 1 F Foerster,Z. Elektrochem., 5 (1899) 512

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2 A. Aramata and P Delahay, J. Phys. Chem., 68 (1964) 880. 3 J. Llpkowskl, J Electroanal. Chem, 39 (1972) 333 4 R J. Gale and C. A. Wmkler, J Electroanal. Chem., 49 (1974) 209 5 S M Kochergm and L. L. Khomna, J. Appl. Chem. USSR, 35 (1962) 877; 36 (1963) 642 6 E Raub and K. Miiller, Fundamentals of Metal Deposttton, Elsevier, Amsterdam, 1967, p 105. 7 S. Venkatachalam and C. A Wlnkler, J. Electrochem. Soc., 115 (1968) 591. 8 V. N. Lebedeva, Soy. Electrochem., 3 (1967) 1305. 9 G. R. Johnson and D. R. Turner, J Electrochem. Soc, 109(1962) 798, 918. 10 G. Lahousse and L. Heerman, Bull. Soc. Chim. Belg, 80 (1971) 125. 11 O. Volk and H. Fischer, Electrochim Acta, 5 (1961) 112. 12 H.-U Hemtze, Ph.D. Thesis, McGlll University, 1973. 13 E. Mattson and J. O'M Bockrls, Trans. Faraday Soc., 550 (1959) 1586. 14 A K P. Chu and A J. Sukava, J Electrochem. Soc, 116(1969) 1188 15 E Gdea& (Ed), Electrosorptlon, Plenum Press, New York, 1967. 16 J. O'M. Bockrls, M. Green and D A. J. Swmkels, J. Electrochem Soc., 111 (1964) 743. 17 D. Armstrong, N. A. Hampson and R. J. Latham, J. Electroanal. Chem., 23 (1969) 361. 18 R. O. Loutfy, Electrochim Acta, 18 (1973) 227 19 W. Leyko, Bull. Soc. Sc~ Lett. Lodz, Classe 111, Vol 111, 14 (1952) 1 20 D. B Cater and I. A. Silver in D. J. G. Ires and G. J. Janz (Eds), Reference Electrodes, Academic Press, New York, 1969, p. 489. 21 H. Borsook, E. L Elhs and H M. Huffman, J Btol. Chem., 117 (1937) 281. 22 P. C. Jocelyn, Eur. J. Btochem., 2 (1967) 327 23 R. J. Gale, Ph.D Thests, McGlll Umversity, 1972 24 C J Hawkins and D. D. Pernn, Inoro Chem., 2 (1963) 843. 25 J. M. Swan, Aust J Chem., 18 (1965) 411.