Raman spectroscopy of silver plating from a cyanide electrolyte

207

.I. Electroanal. Chem., 325 (1992) 207-217 Elsevier Sequoia S.A., Lausanne

JEC 01896

Raman spectroscopy of silver plating from a cyanide electrolyte G. Lacconi

l, B. Reents and W. Plieth

Free University of Berlin, Institute of Physical Chemistry, Takustr. 3, W-1000 Berlin 33 (Germany) (Received 17 July 1991; in revised form 22 October 1991)

Abstract

The electrodeposition of silver from a cyanide electrolyte is followed by surface-enhanced in-situ Raman spectroscopy (SERSJ using the SERS signals of the CN vibrations in.the 2000-2150 cm-’ range. The SER spectra are generated by electrode-position processes without anodic activation. The intensity of the SERS line reflects the microstructure of the silver surface.

INTRODUCTION

The deposition of silver is one of the oldest plating processes. The chemical reduction of silver was described by Liebig in 1856. Electrochemical plating was mentioned even earlier, in 1840 in a British patent. Plating is mostly done from a cyanide bath [1,2]. The importance of silver for decorative purposes as well as for functional coatings has maintained interest in the plating process. Surface-enhanced Raman spectroscopy (SERS) was introduced in 1974 131.This method of investigation was primarily restricted to silver metal because of its unusual optical and chemical properties. It was suggested in 1981 that the micro-roughness of the silver surface might be a major reason for the unusual Raman enhancement on this metal [4]. In particular, Ag, and somewhat larger clusters were suggested as origins of the Raman signal. Later on, this was partly confirmed by Ray and Furtak [5]. This enhancement mechanism suggests that Raman spectroscopy can be used to study the nucleation process of silver and, possibly, other metals such as copper and gold which also show an enhancement effect.

Permanent address: Dpto de Fis&oquimica, Fat. Cs. Quimicas. Univ. Nat. de Gkdoba, Sue. 16, C.C. 61, 5016 Cordoba, Argentina. l

0022-0728/92/$05.00

0 1992 - Elsevier Sequoia S.A. All rights resewed

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In this paper we report experiments with electroplated silver films which demonstrate the sensitivity of the Raman signal for the silver nucleation process on a SERS inert platinum surface. The dependence of the Raman intensity and the size morphology of the silver surface were investigated in detail. The method was used to follow the kinetics of the nucleation and growth processes. EXPERIMENTAL

The experiments were performed in an electrochemical cell containing an inert platinum electrode which could be positioned very near an entrance window of optical quality, a counter-electrode, also made of platinum, and a saturated calomel reference electrode. The working electrode was polished with diamond paste down to 0.25 brn. A standard silver cyanide plating bath was used in two concentrations: electrolyte I, 0.4 M KNO, + 4.8 x 10m3 M AgNO, + 2.8 X lo-* M KCN; electrolyte II, 0.5 M KNO, + 1.1 x 10e3 M AgNO, + 4.5 X lop3 M KCN. The temperature was kept at approx. 23°C and the electrolyte was usually deaerated by bubbling nitrogen through the solution. The electrochemical control was performed using a galvanostat/potentiostat (HEKA Electronics) which was under direct computer control using an IEEE interface. Results were recorded using an X-Y recorder and could also be stored digitally in the computer. The Raman equipment consisted of a Spex 1406 monochromator which was used in a spectrographic mode. The Raman spectra were detected using an optical multichannel analyser (Spectroscopy Instruments) which was also computer controlled. The detection device was a 512-diode array. For demonstration purposes, a simple cyclic voltammetric experiment could be run sweeping the potential from zero to a negative value (e.g. - 1.0 VI and then switching back to a positive sweep. Most of the experimental measurements were performed in the following manner. The electrode was first kept at 0 V where no deposition took place. Then the potential was switched to the cathodic deposition region where it was kept for

E/V 0

time Fig. 1. Pots ltial pulse programme used for SERS activation of the electrode by silver metal deposition on platinur

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a variable length of time allowing the growth of a complete silver layer on the platinum surface. Afterwards, the potential was switched back to less negative potentials in the deposition region or to a stationary value where the current was zero or to the region where anodic dissolution of the silver layer had already started. In a double-pulse experiment a very short nucleation pulse to high negative potentials was applied prior to the deposition potential. The various potential-time diagrams are shown in Fig. 1. As a result of this electrode treatment, a microcrystalline silver layer was produced on the inert platinum electrode. The microcrystalline structure of the surface produced a Raman signal of the cyanide stretch vibration which either increased or decreased in intensity depending on the development of the morphology of the Ag deposit on the platinum surface as well as on the development of the surface coverage with CN- ions. Such an activation procedure was first reported in ref. 5. RESULTS

Raman measurements

Figure 2(a) shows the development of the Raman signal during a potentiodynamic cycle from 0 V to - 1.0 V and back to 0 V; Fig. 2(b) shows a cut at 2111 cm-‘. When the reversible Ag/Ag(CN); potential is reached, deposition starts and a Raman signal appears at 2111 cm - ‘. The Raman signal grows continuously and then becomes approximately constant. When the scan direction is reversed at - 1.0 V in the anodic scan, the intensity decreases for a short period of time and then starts to increase again during the dissolution. This behaviour is even more pronounced in a potential pulse experiment (Fig. 3). In the first pulse, continuous growth of a silver layer is initiated and the Raman signal increases until an approximate saturation value is reached (Fig. 3(b)). When the potential is switched back to the dissolution region, the change in potential is followed by an immediate decrease in the Raman intensity followed by a further increase to intensities higher than the saturation value. With the disappearance of the silver layer from the platinum surface the Raman signal also disappears. The behaviour of the Raman signal during the deposition process depends mainly on the experimental conditions. For example, as shown in Fig. 4, at lower concentrations of silver ions a continuous increase in the Raman signal during the deposition process is observed (at the same potential as in Fig. 3). Chronoamperometry

to determine the nucleation number

The kinetics of nucleation and growth of silver nuclei on the platinum surface was also followed by chronoamperometry. The experimental data for j(t), corresponding to the potential pulse programme in Fig. 3, are shown in Fig. 5 and were interpreted on the basis of the theoretical approach of Scharifker and coworkers

210

10 --lam

-400

cl’, x)0

,

,

(

300

t/s

,

500

,

,

700

,

,

9m

,’

Fig. 2. Development of the SER signal of the CN stretch vibration during a potential cycle 0 V -+ - 1.0 V + 0 V versus SCE (scan rate, 2 mV/s-‘; electrolyte I): (a) three-dimensional representation; (b) cut at 2111 cm-‘.

[7,8] (see also ref. 9) considering hemispherical nuclei distributed randomly on the electrode surface and growing under diffusion control. The surface microstructure after 15 s of deposition can be seen in the scanning electron micrograph shown in Fig. 6. Figure 7 shows the dimensionless plot j/j: versus t/t,, where j, and f, are the corresponding maximum values indicated in Fig. 5. From this plot, it is evident that the nucleation of silver on platinum in cyanide solutions follows closely the response predicted for progressive nucleation. The deviation at t > t, is caused by hemispherical diffusion while the theoretical plot in this region assumes partially linear diffusion. Coulometry The growth of the silver deposit and the dissolution of the silver layer in the potential step experiment of Fig. 3 were also followed by coulometry. The

211

20'38

2ili

wovenumber

21'91

/cm-’

t/s Fig. 3. Development of the SER signal of the CN stretch vibration after a potential pulse into the Ag deposition potential region ( - 0.8 V/SCE) and the time dependence after switching the potential back to the region of dissolution (- 0.3 V/SCE) (electrolyte I): (a) three-dimensional representation: (b) cut at 2111 cm-‘.

current-time diagram is shown in Fig. 8. The charge-time silver deposit was determined from these measurements.

dependence

of the

DISCUSSION

According to the theory of Raman enhancement by small metal particles, the Raman intensity is determined by the size and number of particles (nuclei) and by the surface coverage by CN- ions. The results observed with the Raman spectrometer reflect the crystalline structure of the silver layer on the metal surface; the adsorption of CN- ions is approximately constant. The SERS signal for adsorbed cyanide complex ions

18 2iii 2i91 wavenumber/cm-' Fig. 4. Development

of the SER signal with time in the dilute electrolyte

II.

(2100-2150 cm-‘) appears at the beginning of the deposition process and changes during crystal growth. With increasing growth of the nuclei, they start to overlap as described by the Avrami theory [lo]. The growth rate dN/dt =AN, as well as the size of the nuclei can be calculated using the model of Scharifker and coworkers [7,8]. The diffusion coefficient in the case of progressive nucleation can be calculated from (ref. 6, eqn. (23)) D =j&,,/O.26

( Fc,)~

(1)

-237 -397 i -557 3 -717 .-887 lm -

0

Fig. 5. Current-time - 800 mV/SCE.

t

;

lb

plot for the nucleation

iI period

of Ag deposition

on platinum;

potential

pulse 0 to

213

Fig. 6. Scanning electron micrograph of silver deposited on platinum after 15 s at E = -0.8 V/SCE.

where F is the Faraday constant and c,, is the bulk concentration. The equation used to calculate the growth rate dN/dt =AN,, of the nuclei is (ref. 6, eqn. (21)) AN, = 4.67/t;rk’D

(2)

with 4 k’=3

87Tc,M i~

P

1’2 1

(3)

where A4 is the molar mass and p is the density of the deposit. Values of D = 4.5 X lo-’ cme2 s-t and AN, = 9.7 X lo4 cme2 s-l can be calculated from the chronoamperogram (Figs. 5 and 7) CM= 107.87 g mol-‘, pAg = 10.5 g cmP3). When the kinetic factor AN,, is known, it is possible to calculate the maximum value of the number of nuclei (saturation number NJ (ref. 7, eqn. (28)): N, = 1.5 x lo5 cme2

(4)

214

I

2:o

6:O

4:o t/tm

Fig. 7. Comparison of the reduced current-time dependence of Fig. 5 (j/j,)* versus r/t,: experimental ( n ) and theoretical calculations assuming instantaneous nucleation t- - - - -1) or progressive nucleation ( -1. The experimental value in the critical region before the maximum follows the model of progressive nucleation.

(a)

I

I I

125-

(b)

075-

0

I coo

ml

I

I 1200

t/s Fig. 8. Current-time and charge-time plots for the growth of silver films on platinum: potential pulse program, 0 + - 800 --f - 300 mV/SCE; electrolyte I.

215

Fig. 9. Development of the number of nuclei with time. A saturation value N, is reached.

At this time, a stationary situation is attained between the formation of new nuclei, the growth of the nuclei and the breakdown of the growth process. The development of the number of nuclei with time was calculated according to eqn. (32) of ref. 7 and is shown in Fig. 8. The charge is given as a function of time in Fig. 9. Using the value of Q at the current maximum t, in Fig. 5, the average charge q of the crystal is Q(&J N,

=,-=

-3.1 x 10-a C

(5)

This equation can be used if N = N, at time c,. The average number of atoms per crystal is approximately It = 2 x 10”

(6)

This gives a mean radius of 1.2 X 10e4 cm (hemispherical nuclei). This value corresponds to the size of the crystals shown in Fig. 6 (this micrograph was obtained after 15 s, somewhat later than the time f,,,). The Raman intensity corresponding to this surface morphology is of the order of 100 counts s-l. Scharifker and coworkers [7,8] did not consider the transition from spherical diffusion to linear diffusion, which explains the discrepancy between the theoretical (j/j,,,)’ versus t/t, calculations and the experimental plots in Fig. 7. Using the Cottrell equation [ll] for the Z(t) versus t - ‘I* plot in the region dominated by linear diffusion, a value of D = 3.8 x 10m5 cm* s-l was calculated for the diffusion coefficient, which differs only slightly from the value found using the Scharifker

216

model. When the more general equation proposed by Hills et al. [12] for progressive nucleation Z(t) =

4FA&“(

Dc0)3’2M1’2 t3,2 3P i/2

is used for the linear part of the I(r) versus t312 plot, a value AN, = 8.6 x lo4 cm2 s-l was found, which is very near the value obtained using the Scharifker model. These calculations indicate that the Scharifker approach is adequate for our case. A stationary Raman signal is achieved when a compact silver layer is formed after 300-400 s. Following refs. 7 and 8, the average size of these microcrystallites is of similar size to that calculated using eqn. (6). We assume that the constant value of the Raman signal is reached when the layer from which scattered photons can escape into the electrolyte becomes less than the thickness of the deposit. In some cases, with dilute electrolytes at the same potential, such a stationary situation is not reached (Fig. 4). When the potential is stepped back to the dissolution region we observe an immediate change in the Raman intensity due to the influence of potential. This effect is much smaller than the larger changes which might be related to the average size and the number of silver microcrystallites in the silver layer. With the onset of dissolution of the silver the size and the number of nuclei is reduced. Nevertheless the Raman signal continues to increase and reaches a maximum before disappearing owing to the complete dissolution of the silver layer (Fig. 3). CONCLUSIONS

The experiments demonstrated show that it is possible to use SERS as a tool for in-situ investigation of the electrocrystallization of silver and other metals such as copper and gold. The Raman signal can be followed to study the development of the morphology of the surface. The SERS intensity is related to the surface structure (size, geometry and number of microcrystals) and indirectly to the real surface area. Experiments with other bath components such as organic additives have been described in the literature [13]. Thus the molecular structure of additives can also be monitored in situ during the electrocrystallization process. Of course, this method is in competition with other in-situ methods to study these processes. Although in-situ X-ray diffraction suffers from very low sensitivity, scanning tunnelling microscopy can be used to give additional information. We believe that a combination of these methods will reveal further details of the molecular picture of the electrocrystallization process. ACKNOWLEDGEMENTS

This work was supported by the Deutsche Forschungsgemeinschaft within the Research Programme “Kinetik der Keimbildung und des Keimwachstums”. We

217

also thank the Fonds der Chemischen Industrie for supporting our research. One of us (GL) thanks the Alexander von Humboldt Foundation for a research grant. REFERENCES 1 F.A. Lowenheim, Electroplating, McGraw-Hill, New York, 1978. 2 J.I. Duffy (Ed.), Electroplating Technology, Recent Developments, Noyes Data Corporation, New Jersey, 1981. 3 M. Fleischmann, P.J. Hendra and A.J. McQuillan, Chem. Phys. Lett., 26 (1974) 123. 4 W.J. Plieth, J. Phys. Chem., 86 (1982) 3166. 5 D. Roy and T.E. Furtak, Phys. Rev. B, 36 (1986) 5111. 6 W. Plieth, B. Roy and H. Bruckner, Ber. Bunsenges. Phys. Chem., 86 (1982) 273. 7 B.R. Scharifker and G. Hills, Electrochim. Acta, 28 (1983) 879. 8 B.R. Scharifker and J. Mostany, J. Electroanal. Chem., 177 (1984) 13. 9 E. Bosco and S.K. Rangarajan, J. Electroanal. Chem., 134 (1982) 213. 10 M. Avrami, J. Chem. Phys., 7 (1939) 1103; 8 (1940) 212; 9 (1941) 177. 11 K.J. Vetter, Elektrochemische Kinetik, Springer-Verlag, Berlin, 1961. 12 G.J. Hills, D.J. Schiffrin and J. Thompson, Electrochim. Acta, 19 (1974) 6547. 13 M. Fleischmann, G. Sundholm and Z.Q. Tian, Electrochim. Acta, 31 (1986) 900.