Surface Technology, 15 (1982) 161 - 165 161

FUNDAMENTAL ASPECTS OF PULSATING CURRENT METAL ELECTRODEPOSITION IV: TAFEL EQUATION IN THE DEPOSITION OF METALS BY A PULSATING CURRENT

K. I. POPOV and M. D. MAKSIMOVIC

Faculty of Technology and Metallurgy, University of Beograd, 11000 Beograd, Karnegijeva 4 (Yugoslavia)

V. M. NAKIC and M. D. SPASOJEVIC

Institute of Electrochemistry, Institute of Chemistry, Technology and Metallurgy, University of Beograd, 11000 Beograd, Karnegijeva 4 (Yugoslavia)

(Received September 8, 1981)

Summary

A Tafel relationship was established for the overpotential (i.e. the ampli- tude overpotential reduced by the value of the Nernst potential in the pause) for the case of copper deposition by a pulsating current. This proved the pre- liminary assumption that at sufficiently high pulsating current frequencies the surface concentration of active species remains unchanged. A new method is given for the determination of kinetic parameters in steady state conditions, which is equivalent to the galvanostatic pulse technique.

1. Introduct ion

Over recent years in a few instances an application of pulsating current in the determination of electrochemical kinetic parameters has been indicated [ 1- 4]. Gurovich and Krivtsov [1 ], for example, identified the Tafel relation- ship between the pulse current and the maximum overpotential which appears as the response of a system in the domain of pure activation control of a process. However, for processes under mixed control this t reatment has not been carried out.

It was recently shown that in the process of metal deposition by pul- sating current the surface concentration of metal ions does not change during the pulse and the pause at sufficiently high frequencies [5]. This conclusion leads to an evaluation of the mathematical relation which describes the influence of the pulsating current on the surface roughness and the porosity

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1 6 2

of the metal deposit. Good agreement between theoretically expected and experimental results was taken as reliable evidence for the validity of the theory [5]. The aims of this work were to establish the direct relationship between the overpotential during the pulse of the current and the potential during the pause and the surface concentration of ions in the process of deposition as well as to extend Tafel plots to the mixed control region by the pulsating current regime at frequencies where the influence of the double- layer capacitance can be neglected (up to 100 Hz) [6].

2. Statement of the problem

It is known that in the domain of mixed control the relation

= r~01n {/C~° ) \io Cs.,

(1)

is used, where

i5 Cs = Co . . . . .

nFD (2)

For a pulsating current, eqn. (1) applies only for the pulse period. During the pause, in the non-current condition, at frequencies well below those of double-layer effects (so that they may be neglected [6] ) the mea- sured potential is equal to the Nernst concentration potential:

C o '

where

R T ¢ O - - -

nF

(3)

{4)

At sufficiently high frequencies when the concentration of depositing metal ions remains steady at the surface, the overpotential during the current pulse may be expressed as

771 = ~ o In Co ( 5 ) Vo

For copper deposition from acid solutions,

r~ o = 4~ 0 (6)

and eqn. (5) can be rewritten as

o r

nl °l = + 4~o \ C J

\ Cs ] \io /

163

(7)

(8)

where ~t denotes the transfer overpotential. Since the term ~0 In (Co/Cs) expresses the value of the potential during the pause this is the way to define a direct relation between the activ~ation overpotential and the current density in the pulsating current regime.

3. Experimental details

The electrolyte was 0.075 M CuSO4 in 0.5 M H2SO 4. The working electrode was a platinum wire previously plated with copper by a high frequency pulsating potential [7] from the same solution. The counterelec- trode and the reference electrode were made of electrolytic copper. The solution was prepared from reagent-grade chemicals and triply distilled water. Purified nitrogen was bubbled through the cell before measurements. Exper- iments were carried out at 30 °C. Standard electronic circuitry included a PAR potentiostat-galvanostat , a PAR universal programmer and a Nicolet oscilloscope. This system was chosen in order to compare our results with those of Mattsson and Bockris [8] obtained by galvanostatic single pulses. The measurements were done first by a steady state potentiostatic technique, then by galvanostatic single pulses and finally by a pulsating current with a frequency of 10 Hz and pause to pulse ratio of 4:1.

i t iA - - - - ~ t

Fig. 1. The shape of the pulsating current corresponding to the oscillograph presented in Fig. 2 (frequency 10 Hz; pause-to-pulse ratio, 4:1; current density amplitude, 2.5 mA cm-2; average current density, 0.5 mA cm-2).

4. Results and discussion

A typical potent ia l - t ime response to a pulsating current form of Fig. J is shown in Fig. 2. The 7?t-log i dependences obtained by (A) the steady

164

J

t Fig. 2. Overpotential -time dependence in pulsating current metal electrodeposit ion (cur- rent density amplitude, 2.5 mA cm-2; pulse duration, 20 ms; pause-to-pulse ratio, 4:1).

200f !

i I

lOO F !

50

O

O

A ~" B ~ " (:

0 [ f t i , / k " " J J I J I i 1 i 0,8, 2 4 12

-i, mA/cm 2 ] i I , J J

4O Fig. 3. ~ - log i relationship for copper electrodeposit ion: ) , steady state potentiostat ic technique; A, galvanostatic technique; ~, technique described in this work.

s ta te p o t e n t i o s t a t i c technique~ (B) ga lvanos ta t ic single pulses and (C) the new m e t h o d p r o p o s e d here de f ined by the eqn. (7) are shown in Fig. 3.

The resul ts o b t a i n e d by the m e t h o d descr ibed in this t e x t (2.37? o = 120 m V decade -1 ; io = 1.6 m A cm -2 ) and by the classical ga lvanos ta t ic m e t h o d (2.3V o = 120 m V decade - 1 ; io = 1.4 m A cm 2) are close to each o the r as well as to the resul ts o f Mat t s son and Bockr is [8] (2.3V o = 120 m V decade 1 ; io = 4.5 m A cm 2) p e r f o r m e d in the same solu t ion .

Final ly , it appears t ha t eqn. (8) was s t r ic t ly valid in pulsa t ing cu r ren t me ta l depos i t i on and, at the same t ime , a new m e t h o d for Tafe l re la t ionsh ip eva lua t ion in s t eady s ta te cond i t i ons was accompl i shed .

N o m e n c l a t u r e

C$ Co D F i iA io

surface concentration bulk concentration diffusion coefficient Faraday constant current density amplitude current density exchange current density

165

n number of electrons R gas constant T temperature ~7 overpotential 77 A amplitude of overpotential 2.3~70 slope of Tafel line

concentration potential

R e f e r e n c e s

1 R. I. Gurovich and A. K. Krivtsov, Elektrokhim., 7 (1971) 1435. 2 K. Viswanathan and H. Y. Cheh, J. Electrochem. Soc., 125 (1978) 1616. 3 C. C. Wan, H. Y. Cheh and H. B. Linford, J. Appl. Electrochem., 9 (1979) 29. 4 P. C. Andricacos and H. Y. Cheh, J. Electroanal. Chem., 121 (1981) 133. 5 K. I. Popov, M. D. Maksimovi~, B. M. Ocokolji~ and B. J. Lazarevi6, Surf. Technol., 11

(1980) 99. 6 M. D. Maksimovi~, S. K. Ze~evid and B. M. Ocokoljid, Bull. Soc. Chim. Beograd, 45

(1980) 289. 7 K. I. Popov, D. N. Ke~a, S. I. Vidojkovi6, B. J. Lazarevid and V. B. Milojkovid, J. Appl.

Electrochem., 6 (1976) 365. 8 B. E. Mattsson and J. O'M. Bockris, Trans. Faraday Soc., 55 (1959) 1586.