j. electrochem. soc. 1989 fukunaka 3278 83

7/25/2019 J. Electrochem. Soc. 1989 Fukunaka 3278 83

1/6

3 2 7 8

J . E l e c t r o c h e m . S o c . ,

Vo l . 136 , No . 11 , Nov em be r 1989 9 The E l ec t rochem i ca l Soc ie ty , Inc .

pacity is acceptable, these films are capable of even higher

discharge currents.

Conclusion

From these studies it appears that high-quality amor-

phous molybdenum sulfide films can be prepared by

chemical vapor deposition. The density of films prepared

close to 200~ corr espon ds well to the den sity of bulk MoS3

prepared by classical techniques. As the temperature of

film preparation is increased, there is a rapid increase in

density, corre sponding to a loss of sulfur, as expected. For

both the bulk and thin film MoS3, it seems impossi ble to be

certain of the exact stoichiometry. However, the density

and FTIR results support that the materials are the same.

The films can be readily deposited on various substrates,

includin g simple a lumin um foil. The films are very flexible

and, ev en when deposited on A1 foil, the adhere nce is re-

markably good. Under our deposition conditions we ob-

tain pa cking densities of approxi matel y 60%. The films can

easily be prepare d even in our small research reactor with

surface areas of several hundre d square centimeters. The

growth rate has not been optimized, but the rates de-

scribed in this work (1000/~Jmin) are sufficient for most

applications. If indeed one of the main proble ms of the ap-

plication of MoS3 as a cathode material is the intrinsic re-

sistivity of the material, this problem has been diminish ed

but not eliminated by thin film preparation. The careful

control of temperature has allowed us to minimize the re-

sistivity of the films, and the mi nim um occurs when the

stoichiometry is close to MoS3. We are continuing our

work on thin films of MoS3 to obtai n resistivi ties substan-

tially lower than the l owest observed, of 90 k~-cm. It is our

feeling that the only possible met hod of lowering the

resistance is to heavily dope the material.

The thi n film work has allowed us to rapidly analyze sev-

eral materials electrochem ically and to observe the intrin-

sic limits of MoSs without binders or additives. The elec-

trochem ical behavi or of thin films prepared in the

tem per atu re range of 200~176 closel y parallels the re-

sults obta ined for bulk MoS3; however, superior long-term

cycling behavior was demonstrated. Two-thirds of the

original cycling capacity was maintained after 100 deep

discharge cycles at a rate of 0.2 mA/cm 2, with almo st all of

this loss occurring during the first 6 cycles. In addition, in-

creasing the current density by a factor of 20 caused only

moderate change in the voltage characteristics. The elec-

trochemical films show promising properties for use in

high surface area, high rate batteries, and for solid electro-

lyte, thin film batteries.

Acknowledgments

We would like to thank A. M. Williams for the molyb de-

num analysis, and J. J. Aubor n for many help ful discus-

sions. This work was supported in part by the Strategic

Defense Initiative Organization and ma naged by the Office

of Naval Research.

Manuscript submitt ed Nov. 28, 1988; revised manuscr ipt

received March 27, 1989.

P o l y t e c h n i c U n i v e r s i t y a s s i s t e d i n m e e t i n g t h e p u b l i c a -

t io n c o s t s o f t h i s a r t i c l e .

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Electrodepo sition of S i lver un de r Direct and Pulsed Cu rrent

Y. Fukunaka, T . Yam am oto , and Y . Kondo*

D e p a r t m e n t o f M e t a l l u r g y , K y o t o U n i v e r s it y , K y o to , J a p a n

ABSTRACT

Ele ctro depo siti on of Ag ions on a ve rtical p lane cath ode imm ers ed in s tagnan t 0.5M AgNO~ and 0.5M AgNO3-0.5M

HNO3 solutions was carried out under direct and pulsed current. The morph ologica l variations with electrolytic condi-

tions were examine d by measuring the average diameters of the electrodeposits using the intercept method. The ratio of

the particle diamete r obtained in a AgNO3-HNO3 solution under pulsed electrolysis to that und er direct current at the

same average curren t density was min imu m near 50-60 mA/cm2. It was further enh anced with a decrease in the duty cycle

when the pulse-on time and the charge density were maintaine d at 1 ms and 9 C/cm 2, respectively. The concentra tion pro-

files of Ag ions in AgNO3 solution were also measur ed by holographic interferometry. In these experime nts, the duplex

diffusion layer model prop osed by Ibl (18) was found to adequately describe the ph enom ena provide d that the pulse-on

time was sufficiently long. Based on the concentration profile measurem ents, it was deduce d that the appearanc e of the

min im um ratio of particle diamet er was closely related to the deplet ion of the surface concen tration of Ag + ions during the

pulse~on time, that is, to the c oncept of limiting current pulse.

It is well known that industrial silver electrorefining in

nitrate solution has certain disadvantages such as the need

for scrapers and a large distance between electrodes due to

dendrite formation. A numbe r of studies have been re-

porte d (1-4) on resear ch to find ways to over com e these dif-

ficulties.

*Electrochemical Society Active Member.

Many papers (5-11) have also been published on electro-

crystallization. Fro m the standpoint of technologic al im-

portanc e, th e stud ies on p olycryst alline subst rate (7, 10, 11)

are noteworthy. Fisher observed that nitric acid had an in-

hibit ing eff ect on the silver depos ition (7, 10). Vere ecke n

and Winand (11) studied the effect of electrolyte composi-

tion, temperature, current density, and duration on mor-

phology. They qualitatively summarize d the nature of the

)unless CC License in place (see abstract).ecsdl.org/site/terms_useaddress. Redistribution subject to ECS terms of use (see140.115.63.220wnloaded on 2015-07-28 to IP
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2/6

d. E lec t rochem. Soc . , Vol. 136, No. 11, November 1989 9 The E l e c t r o c h e m i c a l S o c i e t y, Inc. 3279

d e p o s i t a s a f u n c t i o n o f b o t h t h e r a t i o of c u r r e n t d e n s i t y t o

c o n c e n t r a t i o n o f t h e b u l k e l e c tr o l y te a n d t h e c o n c e n t r a -

t i o n o f th i o u r e a .

A n o t h e r w a y t o i m p r o v e t h e q u a l i t y o f d e p o s i t i s t o a p p l y

a p u l s e d e l e c t r o l y s i s t e c h n i q u e i n w h i c h a v e r y h i g h c u r -

r e n t d e n s i t y c a n b e i n s t a n t a n e o u s l y a p p l i e d a n d a v e r y

t h i n d i f f u s i o n l a y e r i s f o r m e d n e a r t h e c a t h o d e s u r f a c e .

T h e s e r e s u l t s s h o w t h a t t h e p h y s i c a l p r o p e r t i e s o f e l e c t ro -

d e p o s i t e d f i lm c a n b e d r a s t i c a ll y i m p r o v e d .

T h e e f f e c t s o f p u l s e d c u r r e n t o n t h e p h y s i c a l p r o p e r t i e s

o f e l e c t r o d e p o s i t s o f s il v e r w e r e r e p o r t e d b y D i n i ( 12 ) a n d

P a v l o v i c e t a k ( 13 ). T h e v a r i a t i o n s i n b o t h t h e m o r p h o l o g y

a n d c r y s t a l s t r u c t u r e o f s i lv e r e l e c t r o d e p o s i t e d i n a s i l v e r

a m m o n i a c o m p l e x b a t h a n d s i lv e r c y a n i d e b a t h a s a f u n c-

t i o n o f t h e p u l s e d c u r r e n t d e n s i t y a n d p u l s e - o n t i m e w e r e

e x a m i n e d b y F u k u m o t o

e t a l .

( 14 ). M o r e o v e r , t h e e f f e c t o f

t h e c a t h o d i c o v e r p o t e n t i a l o n t h e n u c l e a t i o n r a t e o f s i l v e r

i n A g N O 3 s o l u t i o n w a s s t u d i e d u s i n g t h e p o t e n t i o s t a t i c

d o u b l e p u l s e m e t h o d . T h e t w o - d i m e n s i o n a l n u c l e a t i o n

m o d e l w a s a p p l i e d t o t h i s p h e n o m e n a b y L a n

e t a l .

(15).

I n s p i t e o f m u c h w o r k o n t h e e l e c t r o d e p o s i t i o n o f s il v e r

a s m e n t i o n e d a b o v e , t h e g r o w t h m e c h a n i s m o f t h e p r ec i p i-

t a t e d p a r t i c l e o n t h e c a t h o d e s u r f a c e a t r e la t i v e l y h i g h

o v e r p o t e n t i a l i s n o t w e l l u n d e r s t o o d . T h i s i s p a r t l y b e -

c a u s e o f o u r p o o r k n o w l e d g e o n t h e i o n ic m a s s t r a n s f e r

r a t e o f A g i o n s . T h e p r e v i o u s p a p e r ( 16 ) d e a l t w i t h t h e

i o n i c m a s s t r a n s f e r o f A g i o n s a s s o c i a t e d w i t h n a t u r a l

c o n v e c t i o n a l o n g a v e r t i c a l s i l v e r c a t h o d e u n d e r d i r e c t c u r -

r e n t e l e c t r o ly s i s . I n t h e p r e s e n t p a p e r , t h e m o r p h o l o g i c a l

v a r i a t i o n s o f si l v er w i t h c u r r e n t d e n s i t y a n d c h a r g e d e n -

s i t y w i l l b e r e p o r t e d . T h e e f f e c t o f t h e p u l s e s c h e d u l e o n

m o r p h o l o g i c a l v a r i a t i o n s a n d i o n i c m a s s t r a n s f e r r a t e s w il l

b e e x a m i n e d b y c o m p a r i n g t h e r e s u lt s o b t a i n e d i n d ir e c t

c u r r e n t e l e c t ro l y s i s.

E x p e r i m e n t a l

T h e e l e c t r o l y t i c c e ll w a s m a d e o f a c r y l ic r e s i n p l a t e s . T h e

i n n e r d i m e n s i o n w a s 3 0 m m h i g h , 3 0 m m w i d e , a n d 1 0 m m

t h i c k . T w o e l e c t r o d e s 3 0 m m h i g h , 1 0 m m w i d e , a n d

0 .5 m m t h i c k w e r e p r e p a r e d f r o m a ro l l e d s i l v e r p l a te . A

p o l i s h e d c a t h o d e w a s e t c h e d i n 30 % H N O3 s o l u t i o n f o r a

f e w s e c o n d s . T h e e l e c t r o d e s w e r e c o a t e d w i t h P V C , e x c e p t

f o r t h e p a r t b e t w e e n 5 -1 5 m m f r o m t h e l o w e r e d g e o f t h e

e l e c t r o d e s s o t h e e f f e c t i v e s u r f a c e a r e a w a s 1 0 1 0 m m .

T h e e l e c t r o d e p o s i t i o n w a s c a r r ie d o u t u n d e r g a l v a n o s t a ti c

c o n d i t i o n s .

A c a t h o d e o f 1 m m w i d t h w a s e m p l o y e d w h e n t h e c o n -

c e n t r a t i o n p r o f i le s o f A g + i o n s f o r m e d n e a r t h e c a t h o d e

w e r e m e a s u r e d b y t h e h o l o g r a p h i c i n t e r f e ro m e t r y t e c h -

n i q u e . T h i s w a s n e c e s s a r y t o r e d u c e t h e o p t i c a l p a t h

l e n g t h u n d e r v e r y h i g h c u r r e n t d e n s i t y i n o r d e r to m e a s u r e

t h e r e f r a c t i v e i n d e x p r o f i l e v e r y c l o s e t o t h e c a t h o d e s u r -

f a ce . T h e e l e c t r o l y t i c c e l l f o r t h e s e m e a s u r e m e n t s w a s

m a d e o f P y r e x g l as s p l a te s o f 3 m m t h i c k n e s s w i t h i n n e r

d i m e n s i o n s o f 4 0 m m h i g h , 25 m m w i d e , a n d 1 m m t h i c k .

T h e e f f e c t i v e s u r f a c e a r e a o f t h e c a t h o d e i n s t a l l e d i n t h i s

e l e c t r o l y t i c c e l l w a s 1 1 0 m m .

T h e c o m p o s i t i o n s o f e l e c t r o l y t e s a r e s h o w n i n T a b l e I .

T h e e l e c t r o d e p o s i t i o n w i t h p u l s e d c u r r e n t w a s m a i n l y

c o n d u c t e d i n 0 . 5M Ag N O 3- 0 .5 M H N O 3 s o l u t i o n , b e c a u s e

u n i f o r m i t y o f c u r r e n t d e n s i t y d i s t r i b u t i o n w a s o b t a i n e d

e v e n a t t h e h i g h e r c u r r e n t d e n s i t y . H o w e v e r , t h e i o n i c

m a s s t r a n s f e r r a t e s w e r e s t u d i e d o n l y i n 0. 5M Ag N O 3 so l u -

t i o n , b e c a u s e o p t i c a l a n d p h y s i c a l p r o p e r t i e s o n A g N O3 -

H N O 3 s o l u t i o n a r e n o t k n o w n .

Table I. Physical prop erties of A gN03 solution

0.5M AgNO3

DI = 1.4 x 10 ~ (cm2/s)

a = 130.5 (cm3/m ol)

u = 0.965 x 10 -2 (cm2/s)

*t, = 0.482 (-)

0.5M AgNO3-0.5M HNO~

D~ = 1.7 x 10 5(cm2/s)(ll)

a(~-r - ~/a2~2) = 88.9 (cm3/mol)

v = 1 x 10 -2 (cm2/s)

t, = 0

-)

t h e g a l v a n o s t a t i c c o n d i t i o n s . S E M p i c t u r e s a b o v e

6 0 m A / c m 2 w e r e d i s c a r d e d b e c a u s e o f s i g n i f i c a n t d i f f er -

e n c e i n m o r p h o l o g y b e t w e e n t h e c e n t r a l a n d e d g e p a r t o f

t h e c a t h o d e c a u s e d b y n o n u n i f o r m i t y i n c u r r e n t d e n s i t y .

W i t h i n c r e a s in g c u r r e n t d e n s i t y u n d e r a c h a r g e d e n s i t y

o f 9 C / c m 2, t h e n u m b e r o f p r e c i p i t a t e d p a r t i c l e s o n t h e s il -

v e r c a t h o d e b e c a m e s i g n if i c an t l y l a r ge r a n d t h e d i a m e t e r

o f t h e p a r t i c l e s w e r e c o n s i d e r a b l y r e d u c e d . T h i s o c c u r r e d

b e c a u s e o f a n i n c r e a s e i n t h e a ct i v e s u r f a c e a r e a c a u s e d b y

a l a r g e r c a t h o d i c o v e r p o t e n t i a l . T h e p r e c i p i t a t e d p a r t i c l e s

o b s e r v e d a t t h e l o w e r c u r r e n t d e n s i t y s h o w t h e e x i s t e n c e

o f a l e v e l p r o b a b l y c o r r e s p o n d i n g t o a l o w e r i n d e x o f p l a n e

l i k e (1 11 ), a s P a n g a r o v ( 1 7) p o i n t e d o u t . F u r t h e r i n c r e a s e s

i n t h e c u r r e n t d e n s i t y i n d u c e t h e h i g h e r o v e r p o t e n ti a l . A t

t h e s a m e t i m e , t h e p r e c i p i t a t e d p a r t i c l e s s h o w l e s s o f a

l e v el a n d t h e ir s h a p e s b e c o m e s o m e w h a t r o u n d .

A s e l e c t r o d e p o s i t i o n p r o g r e s s e s a t a c u r r e n t d e n s i t y o f

3 0 m A / c m 2, i t is c l e a r l y s e e n t h a t t h e i n i t i al l y p r e c i p i t a t e d

p a r t i c l e s o n t h e c a t h o d e a r e g r a d u a l l y g r o w i n g . T h e n u m -

b e r o f p r e c i p i t a t e d p a r t i c l e s a r e s u b s t a n t i a l l y c o n s t a n t d u r -

i n g e l e c t r o d e p o s i t i o n , a s l o n g a s t h e c h a r g e d e n s i t y i s n o t

e x c e s s i v e . H o w e v e r , c o n t i n u a l e l e c t r o d e p o s i t i o n r e s u l t s i n

a d e n d r i t i c s t r u c t u r e a s s e e n i n t h e p i c t u r e .

T h e v a r i a t i o n i n t h e m o r p h o l o g y o f el e c t r o d e p o si t s o b -

t a i n e d i n A g N O ~ - H N O 3 s o l u t i o n i s s h o w n i n F i g . 2 . T h e e f -

f e c t s o f c u r r e n t d e n s i t y a n d c h a r g e d e n s i t y o n t h e m o r -

p h o l o g y a r e q u a l i t a t i v e l y s i m i l a r t o t h o s e i n A g N O 3

s o l u t i o n . H o w e v e r , t h e a d d i t i o n o f H N O 3 to t h e e l e c t r o l y t e

i n d u c e s a l a r g e r p o l ar i z a t i o n , as p o i n t e d o u t i n a p r e v i o u s

p a p e r ( 16 ) a n d r e s u l t s i n m o r e s p h e r o i d a l s h a p e d p a r t i c l e s

e s p e c i a l l y a t lo w e r c u r r e n t d e n s i t i e s .

T h e p a r t i c l e d i a m e t e r d e c r e a s e s w i t h i n c r e a s e i n c u r r e n t

d e n s i t y a n d t h e w h o l e a r e a of t h e c a t h o d e b e c o m e s

c o v e r e d w i t h p r e c i p i t a t e d p a r t i c l e s . A t a h i g h e r c u r r e n t

d e n s i t y o f 1 0 0 m A / c m 2, t h e e l e c t r o d e p o s i t f o r m s n e e d l e -

s h a p e d p a r t i cl e s , e v e n a t a c h a r g e d e n s i t y o f 9 C / c m 2. T h e

l i m i t i n g c u r r e n t d e n s i t y i n 0 . 5 M A g N O 3 - 0. 5 M H N O 3 s o l u -

R e s u lt s a n d D i s c u s s i o n

E l e c t r o d e p o s i t i o n o f s i l v e r u n d e r d i r e c t c u r r e n t . - - I t is

i m p o r t a n t t o u n d e r s t a n d t h e m o r p h o l o g i c a l v a r i a t io n o f

s i l v e r e l e c t r o d e p o s i t e d u n d e r d i r e c t c u r r e n t b e f o r e t h e ef -

f e c t o f p u l s e d e l e c t r o l y s i s i s d i s c u s s e d . F i g u r e 1 s h o w s t h e

m o r p h o l o g i c a l v a r i a t i o n o f t h e e l e c t r o d e p o s i t o b t a i n e d i n

0 .5 M A g N O 3 a q u e o u s s o l u t i o n a s a f u n c t i o n o f c u r r e n t d e n -

s i ty a n d c h a r g e d e n s i ty . A s m e n t i o n e d i n t h e p r e v i o u s

p a p e r ( 1 6), t h e o h m i c d r o p o f e l e c t r o l y t e w a s s i g n i f i c a n t

d u r i n g e l e c t r o d e p o s i t i o n o f s il v e r a n d i t w a s d i f f i cu l t to o b -

t a i n g o o d r e p r o d u c i b i l i t y i n m e a s u r e m e n t o f t h e p o l a ri z a -

t i o n c u r v e . T h e r e f o r e , e l e c t r o l y s i s w a s c a r r i e d o u t u n d e r

Fig. 1. Morphology of si lver electroplated und er various kinds of

electrolytic cond itions n O.5M Ag N03 solution.



3/6

3 2 8 0 J . E l e c t r o c h e m . S o c . , V o l . 1 3 6 , N o . 11 , N o v e m b e r 1 9 8 9 9 T h e E l e c t r o c h e m i c a l S o c i e t y , I nc .

d u c e s t h e p a r t i c l e s i z e b y 2 0 - 3 0 % . A c c o r d i n g t o t h e p o l a r -

i z a t io n c u r v e s ( 16 ), t h e o v e r p o te n t i a l in 0 .5 M Ag NO3 - 0 . 5M

HNO3 i s tw ic e a s l a r g e a s th a t in 0 . 5 M Ag NO3 s o lu t io n a t

t h e s a m e c u r r e n t d e n s i t y b e l o w 1 00 m A / c m 2. T h i s h i g h e r

o v e r p o t e n t i a l i n d u c e s a f in e r si z e o f p r e c i p i t a t e d p a r t i c le s .

N o n e t p r o d u c t i o n o f p r e c i p i t a t e d p a r t i c l e s d u r i n g e l e c -

t r o d e p o s i t i o n w a s n o t i c e d i n F ig . 1 a n d 2 . I f t h e n u m b e r o f

p r e c i p i t a t e d p a r t i c l e s o n t h e c a t h o d e i s s u b s t a n t i a l l y c o n -

s t a n t , t h e a v e r a g e d i a m e t e r o f t h e p r e c i p i t a t e d p a r t i c l e

s h o u l d b e p r o p o r t i o n a l t o t h e c u b e r o o t o f t h e c h a r g e d e n -

s it y . T h e d e p e n d e n c e o f t h e a v e r a g e p a r t i c le d i a m e t e r o n

t h e c h a r g e d e n s i t y i s s h o w n i n F i g . 4 . T h e a s s u m p t i o n o f n o

n e t p r o d u c t i o n o f p r e c i p i t a t e d p a r t i c l e s d u r i n g e ] e c t ro d e p -

o s i t i o n a p p e a r s t o b e v a l i d a s l o n g a s t h e c h a r g e d e n s i t y i s

l e s s t h a n 9 C / c m 2, s i n c e l i n e a r i t y b e t w e e n t h e a v e r a g e p a r -

t i c l e d i a m e t e r a n d t h e c u b e r o o t o f t h e d u r a t i o n o f e l e c t r o l -

y s i s ( c h a r g e d en s i t y ) i s o b s e r v e d i n b o t h e l e c t r o l y t e s u n d e r

t h e p r e s e n t e x p e r i m e n t a l c o n d i t i o n s .

Fig. 2. Morphology of silver electroplated under various kinds of

electrolytic conditions in 0.5M Ag NO3-0.SM HNO3 solution.

t i o n i s a b o u t 6 5 mA /c m 2 ( 16 ), a n d th e r e f o r e s i lv e r io n s a r e

p r e f e r e n t i a l l y t r a n s p o r t e d t o p a r t i c u l a r d e p o s i t s o n t h e

c a t h o d e u n d e r a v e r y h i g h c o n c e n t r a t i o n o v e r p o t e n ti a l .

I t is r e c o g n i z ed f r o m t h e s e t w o p h o t o g r a p h s t h a t t h e

s h a p e o f t h e e l e c t r o d e p o s i t e d p a r t i c l e s d o e s n o t s i g n i fi -

c a n t l y v a r y w i t h c o n v e n t i o n a l e l e c t r o l y t ic c o n d i t i o n s s u c h

a s e l e c t r o l y t e c o m p o s i t i o n , c u r r e n t d e n s i t y , a n d c h a r g e

d e n s it y . F u r t h e r m o r e , t h e v a r i a n c e i n t h e d i a m e t e r o f t h e

p r e c i p i t a t e d p a r t i c l e s i s n o t e x t r e m e l y l a r g e . T h u s , i t w a s

d e c i d e d t o m e a s u r e t h e a v e r a g e s i z e o f p a r t i c l e s u s i n g t h e

i n t e r c e p t m e t h o d .

T h e m e a s u r e d d i a m e t e r s a r e p l o t t e d a ga i n s t t h e c u r re n t

d e n s i t y i n F i g . 3. T h e y a r e s i g n i f i c a n t l y r e d u c e d w i t h i n -

c r e a s e i n c u r r e n t d e n s i t y b e l o w 3 0 m A / c m 2, w h i l e t h e v a r i -

a t i o n w i t h c u r r e n t d e n s i t y b e c o m e s s m a l l e r a b o v e

6 0 m A / c m ~, a s l o n g a s t h e c h a r g e d e n s i t y i s m a i n t a i n e d a t

9 C /c m 2. I t i s a l s o c l e a r ly s e e n th a t t h e a d d i t io n o f HNO~ r e -

40

30

-t

20

10

C/cm2

0 l I

0 30 60 90

120

i (mAIcm 2 )

Fig. 3. Effect of current density on the diameter of electrodeposited

particle.

E l e ct r o d e p o si t io n o f s i l v e r u n d e r p u l s e d c u r r e n t . - - F i g u r e

5 s h o w s t h e m o r p h o l o g y o b t a i n e d u n d e r p u l s e d c u r r e n t i n

0 .5 M A g N O 3 -0 . 5M H N Q s o l u t i o n . T h e c h a r g e d e n s i t y a n d

t h e p u l s e - o n t i m e w e r e m a i n t a i n e d a t 9 C / c m 2 a n d i m s , r e -

s p e c t iv e l y . A s m e n t i o n e d p r e v i o us l y , t h e d i a m e t e r o f e l e c-

t r o d e p o s i t i s d e c r e a s e d t o a b o u t 1 0 ~ m a t 60 m A / c m 2 w i t h

i n c r e a s e i n c u r r e n t d e n s i t y , a n d a n e e d l e - l i k e e l e c t r o d e p o s -

i t is f o r m e d a t 1 00 m A / c m 2 w h e n s i l v e r is e l e c t r o d e p o s i t e d

u n d e r d i r e c t c u r r e n t . H o w e v e r , p u l s e d e l e c t r o l y s i s h a s

c o n s i d e r a b l y i m p r o v e d t h e q u a l i t y o f e l e c t r o d e p o s i t i o n

e v e n a t a d u t y c y c l e o f 0.5 . T h e p a r t i c l e s i z e i s r e d u c e d

a b o v e a n a v e r a g e c u r r e n t d e n s i t y o f 30 m A / c m 2, a n d n o f o r -

m a t i o n o f n e e d l e - l i ke e l e c t r o d e p o s i t i s o b s e r v e d a t

10 0 m A / c m 2. F u r t h e r r e d u c t i o n i n d u t y c y c l e i n t r o d u c e s

m u c h f i n er p r e ci p i t a t e d p a r t i c l es u n i f o r m l y d i s t r i b u t e d o n

t h e c a t h o d e s u r f a c e f o r c a s e s a b o v e 3 0 m A / c m 2. H o w e v e r ,

a t a d u ty c y c le o f 0. 00 5 , s o m e l a r g e r p a r t i c l e s w i th 1 5 ~m

d i a m a r e p r e ci p i t a t e d u n d e r a n a v e r a g e c u r r e n t d e n s i t y o f

30-60 m A /c m 2.

T h e m o r p h o l o g y o f e l e c t r o d e p o s i t e d p a r t i cl e s b e c o m e s

v e r y c o m p l i c a t e d a t a l o w e r a v e r a g e c u r r e n t d e n s i t y o f

3 m A / c m 2. M o r e o v e r , a r e g u l a r w a v y p a t t e r n w h o s e w i d t h

i s a b o u t 1 0 0 t r m a p p e a r s o n t h e c a t h o d e s u r f a c e a t

r = 0 .0 05 . T h i s i s p r o b a b l y d u e t o e l e c t r o d e p o s i t i o n o f p r e -

c i p i t a t e s a l o n g a s c r a t c h m a r k o n t h e c a t h o d e . T h i s p h e -

n o m e n a w a s n o t r e p r o d u c i b l e a n d a s t r u c t u re r e s e m b l i n g a

b a m b o o g r a s s l e a f o f 7 0 ~ m d i a m a n d l e s s t h a n 1 ~ m i n

t h i c k n e s s w a s s o m e t i m e s o b s e r v e d . S i n c e t h e m o r p h o l o g y

a t 3 m A / c m 2 i s n o t r e p r o d u c i b l e , t h e s e r e s u l t s w i l l b e

o m i t t e d .

T h e r e l a t i o n s h i p b e t w e e n t h e a v e r a g e d i a m e t e r a n d t h e

p u l s e c u r r e n t d e n s i t y i s s h o w n i n F i g . 6 . T h e d a t a m e a s -

u r e d u n d e r d i r e c t c u r r e n t i s i n c l u d e d i n t h e s a m e f i g u r e t o

d e m o n s t r a t e t h a t t h e l o g a r i t h m o f t h e a v e r a g e d i a m e t e r d e -

c r e a s e s l i n e a r l y w i t h i n c r e a s e i n t h e l o g a r i t h m o f t h e d i r e c t

c u r r e n t d e n s i t y o v e r 1 0 m A / c m 2. W h e n p u l s e p l a t i n g i s c a r -

r i e d o u t a t a n a v e r a g e c u r r e n t d e n s i t y o f 3 m A / c m 2, n o s u b -

s t a n ti a l d e p e n d e n c e i n t h e a v e r a g e d i a m e t e r o n t h e p u l s e

50

o 0.5~ AgNO~

~0 0,SM AgNO3- 0,5M HNO3

~ 3o

2O

1

5 10 5

t113(s,3)

Fig. 4. Dependence of electrodeposited particle diameter on the du-

ration time of electropleting.



4/6

J . E l e c t r o c h e m . S o c . , V o l . 1 3 6 , N o . 11 , N o v e m b e r 1 9 8 9 9 T h e E l e c t r o c h e m i c a l S o c i e t y , I nc . 3281

, o o I t

0 .8

0.6

C~

o 4

0-2

0 30 60 90 120

iav (mA/crn2 )

Fig . 7 . Ef fec t o f average cu rrent dens i ty on dp /dDc 0 .5M Ag NO 3-

0 .5 M HN O3 , To , = 1 ms , q = 9 C/cm2) .

F ig . 5 . Morphology of s i lver p la ted by pu lsed e lec tro lys is in 0 .5 M

A g N O r 0 . S M H N O 3 s o lu t io n q = 9 C / c m 2) .

c u r r e n t d e n s i t y i s n o t i c e d . H o w e v e r , t h e d a t a o b t a i n e d a t

a n a v e r a g e c u r r e n t d e n s i t y o v e r 3 0 m A / c m 2 s h o w a c l e a r

s i z e r e d u c t i o n e f f e c t w i t h i n c r e a s e i n t h e p u l s e c u r r e n t

d e n s i t y , a l t h o u g h i t is l e s s s ig n i f i c a n t i n c o m p a r i s o n w i t h

t h e c a s e o f d i r e c t c u r r e n t .

T h u s , t h e r a t i o s o f t h e p a r t i c le d i a m e t e r o b t a i n e d u n d e r

p u l s e d c u r r e n t t o t ha t u n d e r d i r e c t c u r re n t a r e p l o t t e d

a g a i n s t t h e a v e r a g e c u r r e n t d e n s i t y i n o r d e r t o e m p h a s i z e

t h e e s s e n t i a l e f f e c t s c a u s e d b y p u l s e p l a t i n g . F i g u r e 7 d e m -

o n s t r a t e s t h a t a m i n i m u m r a t i o at t h e s a m e d u t y c y c l e is a t -

t a i n e d a t a r o u n d 5 0- 60 m A / c m 2. M o r e o v e r , i t s h o w s t h a t t h e

s m a l l e r t h e d u t y c y c l e , a m o r e s i g n i f i c a n t r e d u c t i o n o c c u r s

i n p a r t i c l e d i a m e t e r .

F r o m t h e s e r e s u l ts , i t i s w e l l u n d e r s t o o d t h a t t h e p r i m a r y

funct ion of the p ulse electrolysis is to raise the ca thod e

overpotential, which enhances nucleation which in turn

results in a smoo ther surface. With increase in the aver age

c u r r e n t d e n s i t y f r o m 6 0 t o 1 00 m A / c m 2, t h e c o n t r i b u t i o n o f

t h e c o n c e n t r a t i o n p o l a r i z a t i o n t o t h e g r o w t h o f t h e e l e c t r o -

d e p o s i t e d p a r t i c l e b e c o m e s p r e d o m i n a n t t o d i m i n i s h t h e

s i z e r e d u c t i o n e f f e c t i n d u c e d b y t h e l a r g e r c a t h o d i c o v e r -

p o t e n t i a l .

Ionic mass transfer rate associated with the eLectrodep

osition under pulsed current. In

o r d e r t o e x a m i n e t h e

i o n i c m a s s t r a n s f e r r a t e a s s o c i a t e d w i t h t h e e l e c t r o c r y s t a l -

l i z a t i o n, t h e c o n c e n t r a t i o n p r o f i l e o f A g + i o n s w a s m e a -

s u r e d b y h o l o g ra p h i c i n t e r f er o m e t r y . T h e m e a s u r e m e n t s

w e r e c o n d u c t e d o n l y i n 0. 5M A g N O 3 s o l u t i o n , s i n c e n o

d a t a i s a v a i l a b l e o n t h e r e l a t i o n s h i p b e t w e e n t h e r e f r a c t i v e

i n d e x a n d c o m p o s i t i o n s o f e l e c t r o l y t e ac i d i fi e d w ~t h

H N O 3 . F i g u r e 8 r e p r e s e n t s t h e i n t e r f e r o g r a m r e c o r d e d 4 5 s

a f t e r t h e s t a r t o f t h e e l e c t r o l y s i s u n d e r a n a v e r a g e c u r r e n t

d e n s i t y o f 60 m A / c m 2, w i t h a p u l s e p e r i o d o f 2s a n d a d u t y

c y c l e o f 0.5 . A v e r y s t e e p r e f r a c t i v e i n d e x g r a d i e n t i s s e e n

n e a r t h e v e r t i c a l p l a n e c a t h o d e . W h e n t h e p u l s e - o n t i m e i s

a s l o n g a s l s , i t is p o s s i b l e t o o b s e r v e o s c i l l a t i o n o f t h e i n -

t e r f e r o g r a m n e a r t h e c a t h o d e s u r f a c e w i t h p u l s e s c h e d u l e

w h e n u s i n g a 1 6 m m m o v i e c a m e r a . B e y o n d t h e o s c i l la t i n g

l a y e r , s u b s t a n t i a l l y s t e a d y c o n c e n t r a t i o n p r o f i l e s a r e

f o r m e d . T h i s i s q u a l i t a t i v e l y i d e n t i c a l t o t h e d u p l e x d i f f u -

s i o n l a y e r m o d e l ( F ig . 9) p r o p o s e d b y I b l (1 8). U n d e r s u c h

c o n d i t i o n s , t h e s u r f a c e c o n c e n t r a t i o n o f A g i o n s a t t h e

e n d o f t h e p u l s e - o n t i m e c o n v e r g e s t o a c e r t a i n v a l u e a n d a

q u a s i - s t e a d y s t a t e i s e s t a b l i s h e d .

T h e s u r f a c e c o n c e n t r a t i o n e s t a b l i s h e d u n d e r d i r e c t c u r -

r e n t c a n b e e x p r e s s e d b y

Sh x = 0.628 (Ra*x) lj~ [1]

a s l o n g as t h e c u r r e n t d e n s i t y d i s t r i b u t i o n i s u n i f o r m a l o n g

th e v e r t i c a l p l a n e c a th o d e ( 1 6 , 1 9 , 2 0 ) . F r o m th i s e q u a t io n ,

1.5

::L

v 1.0

$

' ' ~'(3~0~) O.5M Ag NO 3 - 0 5M HN O~

x~ "~o (~x , r)=(3,0,5) o(3 ,0.02 )

9 C l c m 2 TON= ms

\ ]

I

key

r'(--)

13 0.5

A 01

o 0.02 ~ ~

0.5 9 0,005

--X-- DC

' 4 ' i , i ' ;.

log ip

mAlcrn2)

Fig . 6 . Ef fec t o f the pu lsed current dens i ty on the overage d iameter

F ig . 8 . In te r fe rogram recorded a t 45s a f te r the s ta r t o f pu lsed p la t ing

in O .5M Ag NO 3 so lut ion io , = 60 m A/cm 2 , r = 0 .5 , T = ls ) .



5/6

3 2 8 2 J . E l e c t r o c h e m . S o c . , V o l . 1 3 6 , N o . 1 1 , N o v e m b e r 1 9 8 9 9 T h e E l e c t ro c h e m i c a l S o c i e ty , I nc .

Y

C

9 s .

i

- k . - . =

Co tho d e

C*

c ;

Fig. 9. Duplex diffusion layer model

to the surface concentration but to a value close to the

bound ary va lue b e tween t he osc i l la t ing layer and the v i r -

tually steady layer involved in the duplex diffusion layer

model, as dem ons tra ted in Fig. 9.

For pulse plat ing with a smaller duty cycle r , Ibl (18) pro-

posed the fo l lowing re la t ionship based on a s imple d i f fu-

s ion mode l

5p = ~/2DTon (1 - r) [3]

ipr

C'o= Co- ~( ~N- 8p )

[4]

C 7 - - ~ ( z F ) 2 ( C e ) 2 D [5]

4(ip) 2

the surface co ncent ratio n of Ag ions was calculated at an

average cur rent dens i ty wi th t he physica l proper t ie s l i s ted

in Table I. It is dep ict ed by a solid line in Fig. 10. Moreov er,

the mass t r ansfe r ra te unde r the l imi t ing cur rent dens i ty i s

given by (16, 21, 22)

Sh= = 0.499 (Ra=) 1/4 [2]

If there is no essenti a l change in th e effective surface area,

the ca lcula ted l imi t ing cur rent dens i ty for the present s i tu-

ation is 115 mA/ cm 2. The solid line rep res ent in g Eq. [1] is

ext rapola ted to th is l imi t ing cur rent dens i ty by the dot ted

l ine as the re i s no cor re la t ion equa t ion a t such h i gh cur rent

density.

The sur face concentra t ion dur ing pulse p la t ing was es t i -

mated . The ob se rved sur face concentra t ions a t the end of

the pulse-on t ime of ls are located on the solid l ine. The

sur face concentra t ion a t the end of the pulse -on t ime of l s

at a duty cycle of 0.5 can be appro xim ate d by the value pre-

d ic ted under a d i rec t cur rent dens i ty equiva lent to the

same average current density, as long as the dr iving force

of natural conve ctio n is small . However , the surface con-

cent ra t ions unde r a pulse -on t ime of 1 ms and a t smal le r

duty cycle are located above the l ine, especially a t higher

average current density. Under such a pulse schedule , i t is

cons ide red tha t the th ickness of the osc i lla t ing layer i s

abou t 2 I~m, whi ch is less than the pres ent re solutio n

power of at most 5-10 p.m based on diffraction theory (19).

I t i s the re fore conc lud ed tha t the measur ed concentra t ion

of Ag ion und er the pulse schedule of shor te r pulse -on

t ime and h igh er pulse cur rent dens i ty does not cor respond

O.S

0.4

I I I

key r Ton

ra 0.5 l s

9 0.5 1 m s

0.05 1m s

0 0.02 1 m s

- - -

~- ~. , %. % , - . ~

o ~ ,,, % - . \

t F t \

30 60 9 0 120

"6

E

m

0.3

0.2

(~1

J ay ( m A / c m 2 )

Fig. 10. Effect of average current density on surface concentration of

Ag + ions (0.5M AgNO3).

Once the diffusion layer th ickne ss 3N is given, the oscil la t-

ing diffusion layer thickness, 5p, and the boundary value

bet wee n these t wo dif fusion layers, C 'e , can be calculated.

Then, th e so-called transit io n t ime, ~, a t which the interfa-

cial conc entra tion drop s precisely to zero at the end of the

pulse- on t ime, is assessed.

Unde r the assu mpt io n tha t the d i f fus ion layer th ickness ,

~N, can be evaluated from the correlation at the average

cur rent dens i ty , the boundary va lues ,

C ~ ,

are calculated,

as show n in Table I I. We see the re i s cons is tency be twee n

the calculated C'e , and the observed concentration C~ TM

when the duty cyc le i s smal l and the pulse cur rent dens i ty

is not too high. Since the dup lex diffusion layer model is

applicable to estimate the boundary value C'~ in 0.5M

AgNOa solution when the ionic mass transfer ra te associ-

a ted wi th na tura l con vec t ion dur ing of f t ime is not s igni fi -

cant , th is techniqu e i s fur the r ext ended to t he case of 0 .5M

AgNO3-0.5M HNO3.

The sur face concentra t ion of Ag + ion es tabl i shed und er

direct curr ent is demo nst rat ed in Fig. 10. The l imiting cur-

rent den sit y for th is solu tion is 61.6 mA/ cm 2 from Eq. [2],

while 65 mA/cm e was obtai ned from the polar ization meas-

urement . Table I I I shows the boundar y va lue C ' , and the

transi t ion t ime ~ calculated for the pul se-on t ime o f 1 ms in

0.5M AgNO3-0.5M HNO3 solution. Th e p uls e-on t ime is very

Table II. Comparison between the observed surface concentration

and the calculated values (0.5M AgNO3)

ip r Lay ~N 5p Ce~ Ct e e l s

T (A/cm2) (- ) (A]cm2) (cm) (cm) (M) (M) (M)

ls 0.006 0.5 0.003 0.0217 -- 0.470 -- 0.475

ls 0.06 0.5 0.0 3 0.0137 -- 0.338 -- 0.344

ls 0.12 0.5 0.0 6 0.0119 -- 0.223 -- 0.229

1 ms 0.06 0.5 0. 03 0.0 137 0.0001 0.338 0.344 0.344

1ms 0.15 0.02 0. 00 3 0.0217 0.0002 0.475 0.475 0.475

1 ms 1.5 0.02 0.0 3 9.0 137 0.0002 0.355 0.346 0.344

1m s 1.2 0.05 0.0 6 0.0 119 0.0002 0.278 0.233 0.229

l ms 2.0 0.05 0.1 0.0117 0.0002 0.117 0.101 0.06a

aCl~ was take n from Fig. 1O. Exc ept for thi s value, C1~was calcu-

lated from Eq. [1].

Table III. Calculated boundary value C'e under pulsed plating in

0.SM AgNO3-0.SM HNOa

ip iav 8N : ~ C'e C] s

(A/c m 2)

(~) (A/cm2) (cm) ( ) (M) (M) (ms)

0.06 0.5 0. 03 0. 01 40 0.0001 0.24 6 0.243 2 x 103

0.12 0.5 0.0 6 0.0 122 0.0001 0.057 0.053 28.1

0.2 0.5 0.1 0.0110 0.0001 - - - - 0

0.03 0.1 0.003 0. 0223 0.0002 0.460 0.459 3 10a

0.3 0.1 0. 03 0. 01 40 0.0002 0.248 0.243 85

0.6 0.1 0.0 6 0.0 122 0.0002 0.061 0.053 1.3

1 0.1 0.1 0.0110 0.0002

0.15 0.02 0.003 0.0223 0.0002 0.460 0.459

1.2 103

1.5 0.02 0.0 3 0.0 140 0.0002 0.248 0.243 3.4

3 0.02 0.0 6 0.0 122 0.0002 0.061 0.053 0.05

5 0.02 0.1 0.0110 0.0002

6 0.005 0.03 0.0 140 0.0002 0.248 0.243

0.21

12 0.005 0 .0 6 0.01 22 0.0002 0.061 0.053 0.003

20 0.005 0.1 0.0110 0.0002 - - - - 0



6/6

J . E l e c t r o c h e m . S o c . ,

Vo l . 136 , No . 11 , No ve m be r 1989 9 The E l ec trochem ica l Soc ie ty , I nc . 328 3

short, so the difference between C'~ and the surface con-

centration predicted by Eq. [1] at an average current den-

sity is minor. However, the calculated transition time, ~, is

less than the pulse-on time of 1 ms for many cases.

Thus, the critical condition under which the transition

time is reduced to be less than the pulse-on time of 1 ms is

demon strat ed by a dotted line in Fig. 7. Above this dotted

line corresponding to the limiting current pulse character-

ized by T = ~, the pulse plati ng tec hni que can be ade-

quately applied to produce finer precipitates, while less

substantial i mproveme nt due to pulse plating is expected

below it. This dotted line crosses near the minimum point

of each cur ve at a particular duty cycle, except for

r = 0.005. Therefore, it will prov ide a good basis for the de-

termi nation of the opti mum scheduling of pulse plating.

Summary

Ag ~ ions were electrodeposi ted on a plane vertical silver

cathode installed in 0.5M AgNO3 and 0.5M AgNOa-0.5M

HNO3 solutions under direct and pulsed current. It was

found, through the experiments under direct current, that

Ag ions wer e prefe rentially electro deposited on the sur-

face of particles that had initially precipitated onto the

cathode and that the particle diameter was proportional to

the cube root of the charge density and decreased with in-

crease in curr ent density. The addition of HNO3 induced a

higher polarization and made a spheroidal shape of the

precipitated particle.

Pulse plating was conducted mainly in 0.5M AgNO3-

0.5M HNQ solution and the average diameter was meas-

ured. Comparing this data with data obtained under direct

current showed that the effect of pulsed current on the

ratio of the diamet ers obtained by both processes was

most significant near the average current density of

50-60 mA /cm 2. Moreover, this effect was enh anc ed with re-

duction of the d uty cycle, that is, with corr esponding in-

crease in pulse current density.

By measur ing t he surface con centration ofA g ions with

holographic interferometry, the qualitative soundness of

the duplex diffusion layer model proposed by Ibl was no*

ticed. Based on the duplex diffusion model, the transition

time at which the surface conce ntration of Ag ions is de-

pleted to zero at the end of the pulse-on time was evalu-

ated. This conc ept of limiting current pulse was reasona-

bly compared with the experimental observation on the

ratio of precipitated particle diameter under pulsed cur-

rent to that under direct current. The soundness of the

present way of estimating the limiting current pulse must

be further examined in many other metals and alloys in

order to provide a reasonable method to provide optimum

schedulin g on pulse plating.

Acknowledgment

Part of this work was perfo rmed un der financial aid

given to Y. F. by The Institute of Space and Astronaut ical

Science, to which the authors are grateful.

Manuscript submitt ed Nov. 28, 1988; revised manu script

received April 21, 1989.

h : y o t o U n i v e r s i t y a s s i s t e d i n m e e t i n g t h e p u b l i c a t i o n

c os t s o f t h i s a r t i c le .

C*

C S

D1

d

iav

ip

q

LIST OF SYMBOLS

concentration in the bulk solution (mol/cm3)

surface concentrat ion of Ag ions (mol/cm3)

dif fusi vit y of Ag ion (cm2/s)

particle diamete r (cm)

avera ge c urren t densi ty (A/cm 2)

pulsed current density (A/cm2)

amo unt of elec trici ty (C/cm 2)

Ra~ Rayleigh numb er = g a x 3 0 1 ( - )

vD1

gaiav(1 - * t , ) x 4

Ra*= modified Rayleigh num ber - (-)

z lFvD12

r duty cycle (-)

ia~(1 -

* h x

Sh= Sherwood number - (-)

z l F D l ~ l

Ton puls e-o n tim e (s)

t ti me (s)

*t, transfer numb er (-)

x height from the lower edge of cathode (cm)

densifi cation coeffici ent (cm3/mol)

~N diffusion layer thic kness (cm)

~ p thick ness of oscillating diffusion layer ( cm)

01 concentrat ion difference betwee n the cathode sur-

face and bulk elect rolyte (mol/cm3)

v kine mat ic visco sity (cm2/s)

transition time (s)

p* densi ty of bulk elec troly te (g/cm 3)

pS density of electrolyte at the cathode surface (g/cm3)

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j. electrochem. soc. 1989 fukunaka 3278 83

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