Electrodeposition and Surface Treatment, 3 (1975) 369 - 377 © Elsevier Sequoia S.A., Lausanne -- Printed in Switzerland
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ELECTRODEPOSITION OF COPPER ON A COPPER SINGLE CRYSTAL (111) FACE IN THE PRESENCE OF BROMIDE IONS
s. NAGESWAR Department of Chemistry, Central College, Bangalore University, Bangalore 1 (India) (Received September 8, 1975; in final form October 27, 1975) Summary Observations were made of copper electrodeposits on the (111) plane of copper from highly purified solutions of copper sulphate containing known concentrations from 10-3 to 10 - l ° mol/1 of HBr. The pyramidal growth becomes truncated and transforms to a layer growth which gives rise to ridges and then to a polycrystalline deposit. At higher current densities, transition from one type of growth to another is gradual.
Introduction It is k n o w n t h a t the nature o f the metal, the crystallographic orientation and the bath composition affect the nature of electrodeposits on metals. The nature o f the metal surface influences the nature of thin deposits [1], and the properties of thicker deposits are mainly influenced by the bath conditions [2]. It is also known that the presence of some anions, especially halide ions, affects the nature o f the electrodeposits [3 - 5]. The present paper reports the effect o f bromide ions on the kinetic parameters and the morphology of copper deposits in the presence of various concentrations of bromide ions in the electrolytic bath. It was found that there was a transition from pyramidal to layer ridge and then to polycrystalline growth.
Experimental The experimental procedure has been described in detail elsewhere [6]. The (111) face of the copper single crystal used was prepared by a modified Bridgman method. The orientation of the electrode surface was controlled and corrected by a successive approximation method. The orientation of the final (111) face was determined by Laue back reflection. The (111) face of the single crystal was electropolished in a phosphoric acid bath [7]. A bath composition of 0.25M CuSO4 and 0.1M H2SO 4 was used, to which a known a m o u n t of HBr was added. The deposition of copper at a given current den-
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Fig. 1. Pyramidal2growth of copper deposited on Cu(111) from an acid copper sulphate bath at 2 mA/cm (x 625).
Fig. 2. Truncated pyramids when copper is deposited on Cu(111) from an acid copper sulphate bath in the presence of 10 -9 mol/1 of bromide ions at 2 mA/cm 2 (× 625). sity was carried o u t u p t o a thickness equivalent t o 10 C / c m e. T h e surface o f t h e c o p p e r d e p o s i t was e x a m i n e d b y phase c o n t r a s t m i c r o s c o p y .
Results A t 2 m A / c m 2 current density T h e characteristic triangular p y r a m i d a l g r o w t h was o b s e r v e d w h e n c o p p e r was d e p o s i t e d o n t h e ( 1 1 1 ) plane o f a single crystal o f c o p p e r f r o m a p u r e acid c o p p e r sulphate b a t h (Fig. 1). Macrosteps were o b s e r v e d at the base o f t h e triangular p y r a m i d s . When a 1 0 - l o m o l / l c o n c e n t r a t i o n o f bromide ions was a d d e d , the height o f t h e p y r a m i d s diminished and m o r e layers a p p e a r e d at t h e base. At a 10 - 9 mol/1 c o n c e n t r a t i o n o f b r o m i d e ions t h e r e was a change in m o r p h o l o g y : occasional large t r u n c a t e d p y r a m i d s w e r e observed. With a b r o m i d e c o n c e n t r a t i o n o f 1 0 - s mol/1 t h e d e p o s i t consisted o f irregular layers with some t r u n c a t e d p y r a m i d s (Fig. 2). At a 1 0 - 7 molfl c o n c e n t r a t i o n t h e t r u n c a t e d p y r a m i d s lost shape and pitting o c c u r r e d a r o u n d
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Fig. 3. P r e c i p i t a t i o n o f c u p r o u s b r o m i d e o n a p o l y c r y s t a l l i n e surface w h e n col~per is dep o s i t e d o n C u ( l l l ) f r o m a n acid c o p p e r s u l p h a t e b a t h in t h e p r e s e n c e o f 10 - 0 tool/1 o f b r o m i d e ions at 2 m A / c m 2 (× 625).
Fig. 4. L a y e r t y p e o f g r o w t h w i t h t r i a n g u l a r p y r a m i d s w h e n c o p p e r is d e p o s i t e d o n C u ( l l l ) f r o m a n acid c o p p e r s u l p h a t e b a t h in t h e p r e s e n c e o f 10 - 8 mol/1 o f b r o m i d e ions at 5 m A / c m 2 (× 625).
them. As the concentration was increased to 10-6 mol/1 the pitting around the pyramids increased and the distorted pyramids became smaller. On increase of the bromide content to 10-5 mol/1, pitting increased in the of the pyramids. At a 10-a mol/1 concentration of bromide ions the pyramids almost disappeared and only pits were observed. With a concentration of 10-s mol/1, precipitation of cuprous bromide occurred on a polycrystalline surface (Fig. 3). On increasing the concentration further, the black spots of cuprous bromide multiplied, the surface became rough and non-uniform and consisted mainly o f a polycrystalline type of deposit. A t 5 m A / c m 2 current density
There was no essential difference between the growth habit of the deposit obtained at current densities of 5 m A / c m 2 and 2 m A / c m 2. With a 10 - l ° mol/1 concentration o f bromide ions the height of the pyramids was reduced. With a concentration of 10-9 mol/1 large truncated triangular pyramids appeared. With a concentration o f 10- a mol/1, the truncated triangular
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Fig. 5. Etched pits with polycrystalline material when cop4Peris deposited on Cu(lll) from an acid copper sulphate bath in the presence of 10- mol/l of bromide ions at 5 mA/cm 2 (x 625).
Fig. 6. Truncated hexagonal pyramids when copper is deposited on Cu(111) from an acid copper sulphate bath at 10 mA/cm2 (× 625). pyramids almost disappeared and a layer t y p e of growth p r e d o m i n a t e d (Fig. 4). On a f u r t h e r increase o f bromide ions to 10- 7 mol/1 the triangular pyramids almost disappeared, giving rise t o occasional hexagonal pyramids in a layer-type growth. With a c o n c e n t r a t i o n of 10- 6 mol/1 o f b r o m i d e ions the n u m b e r o f t r u n c a t e d hexagonal pyramids increased, t o g e t h e r with pitting r o u n d tl~e pyramids. With a c o n c e n t r a t i o n o f 10-5 mol/1 t he pyramids became distorted and pitting increased. At 10 - 4 tool/1 a polycrystalline material with etched pits was observed (Fig. 5). Twinned triangular pyramids of cuprous b r o mid e f o r m e d with a c o n c e n t r a t i o n o f 1 0 - a mol/1 o f bromide ions. The deposit was non- uni f or m and a dirty grey in colour with a concentration o f 10- 2 mol/1. A t 10 m A / c m 2 c u r r e n t d e n s i t y A large n u m b e r o f t r u n c a t e d hexagonal pyramids were deposited from a pure acid co p p e r sulphate bath at a c ur r e nt density o f 10 m A / c m 2 (Fig. 6). With a c o n c e n t r a t i o n o f 10 - l ° tool/1 o f b r o m i d e ions in t he bat h t he deposit
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Fig. 7. Layers and pyramids disappear and pitting occurs when co_pTperis deposited on C u ( l l l ) from an acid copper sulphate bath in the presence of 10- mol/l of bromide ions at 10 mA/cm 2 (× 625).
Fig. 8. Hexagonal blocks and occasional triangular pyramids of copper deposited on Cu(111) from an acid copper sulphate bath at 15 mA/cm 2 (× 625). c o n s i s t e d m a i n l y o f h e x a g o n a l blocks. As t h e c o n c e n t r a t i o n was increased t o 1 0 - 9 mol/1, m o r e t r u n c a t e d h e x a g o n a l b l o c k s w e r e n o t i c e d , o v e r l a p p i n g o n e a n o t h e r . A t a c o n c e n t r a t i o n o f 1 0 - 8 mol/1 o f b r o m i d e ions the h e x a g o nal p y r a m i d s lost t h e i r s h a p e a n d t h e r e was p i t t i n g r o u n d t h e p y r a m i d s . At a c o n c e n t r a t i o n o f 1 0 - 7 mol/1 the p y r a m i d s d i s a p p e a r e d a n d a p i t t e d l a y e r b a c k g r o u n d was o b s e r v e d (Fig. 7) w h i c h c o n t i n u e d t o a c o n c e n t r a t i o n o f 1 0 - ~ mol/1 o f b r o m i d e ions in the e l e c t r o l y t i c b a t h . A t a 1 0 4 mol/1 c o n c e n t r a t i o n o f b r o m i d e ions, p i t t i n g o f t h e surface o c c u r r e d . A n o n - u n i f o r m p a t c h y p o l y c r y s t a l l i n e p i t t e d d e p o s i t was o b s e r v e d w h e n the b r o m i d e c o n t e n t was 1 0 2 mol/1. A t 1 5 m A / c m 2 current d e n s i t y In a p u r e s o l u t i o n at a c u r r e n t d e n s i t y o f 15 m A / c m 2, h e x a g o n a l b l o c k s w i t h o c c a s i o n a l t r i a n g u l a r p y r a m i d s in a l a y e r g r o w t h o c c u r r e d (Fig. 8). On a d d i n g b r o m i d e ions to a c o n c e n t r a t i o n o f 10 - 9 mol/1 t h e r e w a s little change, e x c e p t t h a t t h e n u m b e r o f h e x a g o n a l b l o c k s d e c r e a s e d . As t h e b r o m i d e ion c o n t e n t was increased t o 10 6 mol/1, the b l o c k s lost t h e i r s h a p e and pitting
374
Fig. 9. Pitting round the blocks when copper is deposited on Cu(111) from an acid copper sulphate bath in the presence of 10-6 moll1 of bromide ions at 15 mA/cm 2 (× 625).
Fig. 10. Pyramids with occasional hexagonal blocks of copper deposited on Cu(111) from an acid copper sulphate bath at 20 mA/cm 2 (× 625). occurred (Fig. 9). At a c o n c e n t r a t i o n o f 10-5 mol/1, only an et ched surface was observed. Small speck-like pyramids f o r m e d at a c o n c e n t r a t i o n of 10-4 mol/1 o f b r o mid e ions, and a polycrystalline t y p e of deposit f o r m e d at a c o n c e n t r a t i o n o f 10- 3 mol/1, t oget her with dark specks o f cuprous bromide. A n o n - u n i f o r m rough grey and pa t c hy polycrystalline deposit form ed at a 10-2 mol/1 c o n c e n t r a t i o n o f br om i de ions. A t 2 0 m A / c m 2 current d e n s i t y
A triangular pyramidal growth with occasional hexagonal blocks occurred when copper was deposited f r o m a pure acid copper sulphate bath (Fig. 10). There was no change in t he m o r p h o l o g y of the copper deposit when the bromide c o n t e n t was increased f r o m 1 0 - l o t o 1 0 - s mol/1. At a 10- 7 mol/1 c o n c e n t r a t i o n t he pyramids had almost lost their regular shapes and were truncated. Etching occurred r o u n d the pyramids, similar t o t hat at lower cu r r en t densities. At a 10-5 mol/1 c o n c e n t r a t i o n t he pyramids almost disappeared and more etch pits were observed. At a b r o m i d e c o n t e n t o f 10 - 4 mol/1, small pyramids covered the whole surface. At a 10-3 mol/1
375
I 20C
I
* Pure Solution o l(~°M HBr s 10-sM H Br o IO-TM HBr
16C IE
_~12c ~ sc 4( I
2
L
L
I
i
F
F
4 Thickness 6C/CmS~
Fig. 11. O v e r p o t e n t i a l
versus
I
1
I0
t h i c k n e s s at various c u r r e n t densities o n t h e ( 1 1 1 ) plane.
concentration, small dark specks of cuprous bromide in a background o f polycrystalline-type deposit were observed. On further increase of bromide ions to 1 0 - e mol/1, only a patchy non-uniform and dirty polycrystaUine deposit formed.
Overpotentials
On a (11 I) face The overpotential in a pure solution decreases with time and finally reaches a steady value at all current densities, as observed earlier (Fig. 11). The initial value o f the overpotential on the (111) plane is always higher than that on the (100) and (110) planes [8]. The overpotential gradually decreases with time and reaches a steady value in the presence of low concentrations o f bromide ions in the electrolytic bath. When there is a transition from pyramidal to layer-type growth, the overpotential increases and then attains a steady value (Fig. 12). This tendency is observed at all current densities. When the concentration of bromide ions is more than 10-6 mol/1 the steady-state value o f the overpotential is much higher than those obtained either in a pure solution or at a lower concentration of bromide ions. When steady-state overpotential is plotted against concentration of bromide ions, there is no regular change in overpotential value. The overpotential-log i relation, b o t h for initial and steady-state values, is linear in a pure solution, with a slope o f 120 + 5 mV. At a low concentration of bromide ions (10-a mol/1) the linear relationship still holds good.
376
•
300
2mA/Cm ~
o
5mA/Cm E
•
10mA/Crn 2
/
~
250
1
2OO
5O
o
I
tO-~"
10-e
I
J
10-m
10-4
i 0 -s
Log C
Fig. 12. Deposition of copper on the ( l l l ) plane in the presence of bromide ions at 5 m A / c m 2.
• b=131.0 o b=130,0
PURE SOLUTION i o =T'AmA/Cm z io =1"t mA/Cm
I i • L °
lO-SM HBr b = 1 3 1 - 0 mv b=130.3mv
i o = 1.4 rnA/Cm~l io = l"lmA/CmEJ
3OO
-~2oo
•
i° 1
I
101
/~ /
10~ 1 Current Density (mA/Cm 2)
r
101
102
Fig. 13. Overpotential versus current density on the (111) plane.
At a higher c o n c e n t r a t i o n t h a n ( 1 0 - s mol/1), t h e Tafel relationship n o longer holds g o o d (Fig. 13). (In pure s o l u t i o n f o r kinetic p a r a m e t e r s b = 1 3 1 . 0 and 130.0, i0 -- 1.4 and 1.1 m A / c m 2, respectively; with 10 - s mol/1 o f HBr, b = 1 3 1 . 0 and 130.3, i0 = 1.4 and 1.1 m A / c m 2, respectively.)
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Discussion With a low concentration of bromide ions at low current densities pyramidal growth transforms to macrosteps resulting in a layer growth. At critical concentrations, precipitation o f cuprous bromide occurs as triangular pyramids. A polycrystalline t y p e of deposit is obtained at higher concentrations. With a bromide ion concentration of 10- 7 mol/1, the highly polarisable bromide ions may be specifically absorbed on active sites on the copper (111) face and also on the apexes of pyramids thus hindering the vertical growth of the pyramids. Further growth may occur sideways as the steps cannot generate from the apexes of the pyramids. These layers are aligned in the [112] direction. As the concentration of bromide ions is increased the growth of layers in the [112] direction may be hindered and copper adions may diffuse in the [110] direction, giving rise to a broken ridge t y p e of growth. At critical concentrations of halide ions the copper adions nucleate randomly causing polycrystalline growth as all the active sites are blocked b y cuprous bromide. When layers form there is an increase of overpotential with time which attains a steady state on the (100) face. It m a y be concluded that at low concentrations of bromide ions deposition is facilitated w i t h o u t change in the mechanism. At higher concentrations there will be specific absorption and precipitation o f cuprous bromide leading to deviation in Tafel relationships, as studied b y Bockris and Enyo [9] in the case of polycrystalline copper. Acknowledgements The author is grateful to the late Professor T. H. V. Setty for suggesting the problem and for his guidance throughout the preparation of this work. The author also thanks Dr. M. Shadaksharaswamy for his interest and encouragement.
References 1 2 3 4 5 6 7 8
G. I. Finch, H. Wilman and L. Yang, Disc. Faraday Soc., 43(a) (1947) 144. T. H. V. Setty and H. Wilman, Trans. Faraday Soc., 51 (1955) 984. S. C. Barnes, Electrochim. Acta, 79 (1961) 169. D.W. Hardesty, J. Electrochem. Soc., 117 (1970) 169. T. H. V. Setty and S. Nageswar, Current Sci., 35 (1966) 235. S. Nageswar and T. H. V. Setty, Proc. Indian Acad. Sci. A, LXVIII (4) (1968) 178. P. Jacquet, Compt. Rend. Acad. Sci. Paris, (1933) 202. A. Damjanovic, T. H. V. Setty and J. O'M. Bockris, J. Electrochem. Soc., 113 (1966) 129. 9 J. O'M. Bockris and M. Enyo, Trans. Faraday Soc., 55 (1959) 1586.