Granular Growth of Electrochemically Deposited Metals BYALEKSANDAR DRAGUTIN AND MILUTIN R. DESPI~ M. DRA~I~ D. MIRJANI~ Faculty of Technology and Metallurgy University of Beograd Beograd Yugoslavia Received 8th September 1977 Conditions of deposition of cadmium from cadmium perchlorate solutions on copper electrodes leading to the appearance of granular deposit (boulders) have been investigated. It was found that this type of deposit appears in a limited pH range only (pH 3 to 6) in which colloidal cadmium hydroxide is found to appear. The probability of appearance of granules is diKerent at different crystal planes of the copper substrate. The rate of growth of the granules has been determined microscopically. It was found that the size of the granules increases proportionally to the square root of time.This indicates that the rate of growth is controlled by the spherical diffusion of de- positing ions from the bulk of solution. Fundamental research in electrodeposition of metals in recent years has advanced to the extent that basic steps leading to the formation of the first few monolayers of the deposit are now fairly well understood and the theory has been developed to a semi-quantitative level. In the deposition of thicker metal layers however only the understanding of phenomena relating to difficulties in the transport of the reacting species (amplification of surface roughness dendritic growth and the levelling effect of some addtitives) seems to have reached a similar 1evel.l Except for some speculations concerning predominant orientation of crystal^,^-^ understanding of the development of texture of thicker deposits has made little progress since that reviewed in the classi- cal treatise of Fischer.6 In particular little seems to be known about conditions caus- ing the appearance of granular deposits of relatively high porosity and highly developed surface area which are obtained in some instance^.^ This type of growth has attracted our attention for some time n0~.~9~ In a study of the deposition of granular cadmium from solutions of simple cadmium salts9 it was shown that such a deposit dissolves anodically more readily than does a compact deposit not only because of the larger specific surface area but also because of a somewhat higher activity of the metal in such a state.Yet such deposit growth is clearly distinguishable from that leading to the formation of dendrites. The metal particles are of similar dimensions in all directions and in the extreme tend to become globular with outwardly poorly developed crystal structure.1° Hence it is obvious that the reasons for its appear- ance should not be sought along the lines of the theory of dendritic growth.llJ2 Globular growth of copper on graphite has been obtained upon the addition of an adsorbing and codepositing polyelectrolyte (polyvinylpyridine) to the so1ution.l' Hence the search for conditions leading to granular growth from simple salt solutions has been directed to the study of some solutions containing species which might exhibit similar action to that of the polyelectrolyte and in particular to the effect of the hydro- lysis of salts at increasing pH values.Also since granular growth has been observed as a rule on some parts of the electrode surface only while on others a fine-grained relatively smooth deposit is obtained the effect of the crystal orientation of the substrate on the appearance of the granulae has been studied. A. R. DESPIC D. M. DRA~I~ AND M. D. MIRJANIC EXPERIMENTAL Cadmium was deposited on copper electrodes from cadmium perchlorate solutions with perchloric acid and sodium perchlorate as pH adjusting electrolytes using the potentiostatic technique with simultaneous microscopic observation. ELECTRODES Copper electrodes were made from 99.95% copper wire 2 mm in diameter which was subjected to special thermo-mechanical treatment (repeated stretching and annealing) leading to an increase in grain size.When cut across and polished the cross sectional surfaces were found to consist of a very few grains and in some cases of a single grain only. Hence deposition on individual grains could be followed. The orientation of the grains was deter- mined by X-ray examination. Two orientations were found to predominate the (I 11) and (200) crystal planes appeared on the surface at inclinations to the axis of the wire ranging from 0.8 to 14". By mechanical polishing to a mirror finish followed by electropolishing layers of higher indices were removed and smooth surfaces of a given orientation exposed. In some instances anodic dissolution was extended to the appearance of dislocation pits.ELECTROLYTIC CELL A thin layer cell was used with an optical window through the counter electrode facing the electrode under Observation. This consisted of the wires described above embedded in Teflon. The counter electrode was a thin foil (80 pm) of pure cadmium (99.95%). Its surface was about 160 times larger than that of the working electrode so that it could serve also as a reference electrode. The gap between the two electrodes was 0.2 mm and the electrolyte was made to flow through the gap at a rate of 0.6 dm3 h-I in all the experiments. SOLUTIONS The solutions were prepared by dissolving cadmium oxide (" Zorka "-Sabac) in perchloric acid (" Merck "-p.a.)and triply distilled water to a concentration of 1.5mol dm-3.Stoichio-metric amounts of the oxide and the acid gave solutions of pH 6.40,with some oxide remaining undissolved. The solutions were therefore opaque with particles in the colloidal range (passing through fine grade filter paper). The pH was lowered by adding acid; solutions became clear below pH 5.20. However time effects and some hysteresis characteristic of colloidal systems were encountered in titration of the solution with the acid and retitration with alkalis. It appears that in the pH region between 5 and 6there is a maximum concentra- tion of Cd(OH)2 dissolved or in acolloidal state with colloidal particles below the turbidity limit. PROCEDURE Prior to the experiments the electrolyte was made to flow through the cell and the working electrode was potentiostatted with aWenking potentiostat to a potential 10 mV more positive than that of the cadmium reference electrode.The electrode was then subjected to a poten- tial step of a selected constant cathodic overpotential and the current was recorded on a Hewlett-Packard X-Y recorder for a period of up to 110 s. Simultaneously microscopic observations were made using a Reichert Zetopan Pol. microscope with a magnification of 400 (in some experiments 500x ). When granular growth appeared photographs were taken with an automatic exposure system at regular time intervals (5 s). At the end of the cathodic step the polarity was reversed and the electrode was n~aintained at the anodic overpotential 128 GRANULAR GROWTH OF ELECTROCHEMICALLY DEPOSITED METALS until the current dropped to zero.The electrode surface could then be seen to return to its original state. Experiments were carried out at room temperature (25-27 "C). RESULTS Out of a total of 7 electrodes subjected to the above treatment granular growth was observed on two electrodes only both having at least one larger grain with the plane (200) exposed to the electrolyte while the others had none. In a sequence of in-E 0 10 20 30 40 50 60 t/s FIG.1.-Time responses of current to steps of different cathodic overpotentials in the case of appear-ance of granular growth pH = 5.33. creasing cathodic overpotential a series of time responses obtained in the cases when granular growth was observed in the microscope is shown in fig.1. At any given time there is a rather sharp increase in current with overpotential (10-fold for 20-30 mV). The amount of deposited cadmium within the duration of the overpotential step could be obtained by integrating the time response curve. By anodic dissolution it was found that cathodic deposition occurred with roughly 100% faradaic efficiency. For a given overpotential the amount of deposited cadmium was found to depend on pH. As shown in fig. 2 this dependence exhibits a maximum at around pH 5. Cor-respondingly microscopic observations revealed a maximum incidence of granular growth in the same pH range. Below pH 4 granular growth was rare and below pH 3 this type of growth was never observed. At a given pH the number of granulae in the field of vision increases with increasing overpotential as shown in fig.3 although at the highest overpotential used some decrease is observed. The granulae are evidently becoming more even in size while the average size is increasing. PLATE 1.-Microphotographs of granular cadmium deposit on copper. (a)Example of a well grown hexagonal grain; (6) area of the electrode where granulae appear upon the application of cathodic potential step (arrows point to locations where nucleation occurs) ; (c)-(f)a sequence taken during growth at pH 4.98 and overpotential of 50 mV at 10 s intervals. [Toface page 129 A. R. DESPI6 D. M. DRAilc AND M. D. MIRJANIC PH FIG.2.-Total amount of electricity passed during 50 s as a function of pHfor different constant values of cathodic overpotential.0 t i 4 O /,pH 4'9s 5.92 I 30 40 50 __I 1' mV FIG.3.-Number of granulae counted in one and the same field of vision as a function of overpotential. A typical sequence of microphotographs recording the growth of granulae during a single overpotential pulse is shown in plate I. In some instances ideally hexagonal crystals grew as seen in picture (a) indicating that nuclei are formed with the hex- agonal plane lying flat on the substrate. Most granulae however were deformed exhibiting a tendency to spheroidize. The majority of the granulae appeared at some points where some imperfection (different from a pit-producing dislocation) occurs on the electrode surface (black dot).Some granulae however appeared at the edge or even at the bottom of triangular dislocation pits. Others developed at locations at which nothing but a flat crystal surface could be seen before the current pulse as shown in the plate (picture b). 130 GRANULAR GROWTH OF ELECTROCHEMICALLY DEPOSITED METALS When a series of cathodic deposition pulses was employed followed by total anodic dissolution of the deposit it was observed (fig. 4) that the majority of the granulae appeared repeatedly at the same locations. Some new locations however were also activated as the overpotential was increased while at some others growth was absent. A general finding was that once the granules start developing no new ones appear. u. 0 L aJ n E C Location FIG.4.-Number of pulses which produced granulae at a certain location. Type of location I-imperfection of uncertain origin ; D-dislocation pit ; shaded-flat surface. Series of micrographs of the type shown in plate I for different pH and different overpotential enabled a quantitative evaluation of growth. The rate of growth could be estimated by measuring the change in the linear dimension of the granulae with time. A typical example is shown in fig. 5. It is seen that in one and the same overpotential pulse all crystals do not grow at the same rate. Also fluctuations in the rate of growth are observed which are outside the limits of the error in the measure- ment. Nevertheless all the growth followed a square root time dependence as shown in 4 0 1 I I I I FIG.5.-Growth of a number of granulae with time pH = 5.95; 7 = 50 mV; A-the average size.L-the size of the largest measured granula; S-the size of the smalIest measured granula. A. R. DESPI~,D. M. DRA~I~ AND M. D. MIRJANI~ fig. 6 for a set of overpotential pulses. Even though the average size of a number of crystals is taken the growth is seen to be somewhat erratic at lower overpotentials while at 50 mV a very smooth linear dependence is found. The slopes of the t* dependences seem to be equal in all cases i.e. independent of either the pH or the overpotential as seen in fig. 7. The straight lines however do not extrapolate to the origin of coordinates. In some cases there is a positive intercept with ordinate indicating a faster growth at the very beginning.In others however the growth seems to exhibit a delay. 30 20 E \\ 0 10 0 / I b /I 1 I 1 I I 01234567 "'2 1 22 Frc 6 -Plot nf the si7e of the average ~ranirla against saiiare root of time for different overnntentials 8 LO 132 GRANULAR GROWTH OF ELECTROCHEMICALLY DEPOSITED METALS DISCUSSION Granular growth in electrodeposition of cadmium on copper is found to occur in a relatively narrow range of conditions (pH -5 -+ 1; overpotential 30-50 mV) and on certain electrodes only. This latter finding indicates that different crystal planes favour different types of nucleation. A possible representation can be seen on the model representation of a (1 1 1) and a (200)plane in fig.8. A two dimensional nucleus can be better situated on the (1 1 1) plane exh adatom of cadmium falling close to the ( 111 1 FIG.8.-Model of formation of a hexagonal nucleus of cadmium on two crystal planes of copper. recess between copper atoms in the plane. This implies that epitaxial growth in con- tinuation of the copper lattice at steps or kinks should also be more favourable on the (111) plane. On the (200) plane on the other hand two-dimensional nucleation produces less stable nuclei and hence a delay in two-dimensional growth occurs which makes room for three-dimensional nucleation. This idealized picture is of course complicated by the fact that at imperfections or dislocations at which most of the growth was found to occur the atom arrangements in the substrate could be significantly different from those shown in the model.Hence other structural elements in the surface must also have a role. This is sup- ported by the fact that granules also appear at some parts of the surface which accord- ing to the shape of the dislocation pits must be of the (1 11) orientation. As for the supply of cadmium adatoms for a continuous growth of the three- dimensional nuclei into granulae of considerable size (20-30 pm) one can calculate the necessary current by the following reasoning if we assume that granulae are approximately spherical a granulus of measured linear dimension a,consumes a vol-ume of metal rc V=-a3. 6 The change in volume with time dV/dt is related to the change in the molar amount of metal dN/dt and further to the current by the expression A.R. DESPI~,D. M. DRA~I~ AND M. D. MIRJANIC Hence by differentiating eqn (1) and substituting into eqn (2) one obtains Fpx da I=-a2-M dt' (3) The supply of atoms by the current given by eqn (3) can occur by discharge on the substrate or on the granules. The fact that the number of granulae within the field of vision does not increase once the granulae start developing supports the view that adatoms deposited at the substrate are transferred to the granulae by the mechanism of surface diffusion (the " clear field effect " observed by Markov Boynov and Toschev).I3 It is however difficult to conceive that this is the main supply route considering that the required amount of material increases as a2and that the surface diffusion path to the point of incorporation also increases with the size of the granulae.Hence it was assumed that most of the supply is met by discharge on the granulae themselves. In such a case the local current density on the granulae is . I (FpnIM)a2(da/dt) l=-=-S a2n (4) i.e. it is directly proportional to the slope of the a against t graph. Since the linear a against t dependences obtained in most experiments provide more precise values of the slope k one could further relate i with k in the case when the straight line passes close to the origin of coordinates from the following a = kt2 (5) and hence da k2 -=-dt 2a so that . Fpk2 1 1=-2M a (7) and I=-Fpnk2 2M a.It is seen that as long as the crystal growth follows the t* dependence the current density is inversely proportional to the diameter of the granule and the total current directly proportional to it. That is a characteristic of steady-state spherical diff~si0n.l~ This as well as the virtual independence of the rate of growth from overpotential indi- cates that once the granules start growing the rate of growth is diffusion controlled. In such a case14 . 2FDCO 1=-a12 . (9) Hence the slope k can be estimated theoretically from eqn (7)and (9) as k=(--). 8MDCO Taking D -loq5cm2s-l one obtains k -7 x cm s-* which is close to the slopes arising from fig. 7. 134 GRANULAR GROWTH OF ELECTROCHEMICALLY DEPOSITED METALS The fact that the i against t3 dependences often do not go through the origin can be due to two effects (a)At an early stage of growth the supply of material from that discharged on the substrate by the mechanism of surface diffusion can be significant and hence the growth be faster than if supplied by spherical diffusion only.This must lead to posi- tive intercepts with the ordinate and eqn (5) should read a2-a2(t=o,= 2kt. (5') (b)At low overpotentials nucleation of the granulae can be slow so that an induc- tion time ti is expected. In such a case the correct eqn (5) should be a = k(t -ti)+. (5") The observed behaviour is a consequence of the interplay of the two effects; the fact that experimental data fit the approximate eqn (5) fairly well indicates that these two disturbing effects (a) and (b)are together quite small.Also the fact that a delay in the development of the grains and a generally smaller grain size after a given period of time is observed at lower overpotentials indicates that the second effect then pre- vails. An indirect insight into the number of granulae over the entire electrode surface area n' can be obtained if it is assumed that at least at the advanced stages of growth all the current is used for their development and that the current spent on epitaxial growth is negligible. In such a case n' is found by dividing the total recorded current with that spent on developing an average granula. The latter was evaluated for each of the reviewed experiments using eqn (9) and on using the corresponding total current data presented in fig.9 were obtained. These confirm the finding that A 500 /pH 4.98 I 400 -c 300-3.46 200 -100-1 I 0 b the incidence of granular growth is largest around pH = 5 and that it increases with increasing overpotential. The sharp increase in the total current with overpotential at any given pH is actually due to the increased rate of nucleation since the rate of growth of individual nuclei is unaffected by overpotential under the conditions of diffusion control. A. R. DESPIt D. M. DRAiIC AND M. D. MIRJANIk Finally one notes that undisturbed growth under diffusion control should lead to the development of dendrites-l The fact that the granulae remain spherical throughout the growth period can be explained only if one assumes the existence of a dendrite suppressor in the electrolyte.Colloidal hydroxide might very well play such a role since it is likely to be adsorbed faster at micro-protrusions appearingon the crystal than on flat crystal planes and might prevent their further development with an overall effect of spheroidization. This as well as a possible effect of the adsorption of colloids on two-dimensional nucleation explains the fact that a maximum in the granular growth is found in the pH range where the formation of colloidal hydroxide occurs. The authors are indebted to NSF (USA) and to the Research Fund of SR Serbia whose grants have made this work possible. cf. A. R. Despid and K. J. Popov Transport-controlled Deposition and Dissolution of Metals in Modern Aspects of Electrochemistry ed.B. E. Conway and J. O’M. Bockris (Plenum Press New York 1st edn 1972) vol. 7 chap. 4 p. 199. K. M. Gorbunova 0. S. Popova A. A. Sityagina and Y. M. Polukarov Crystal Growth (Rept. Congr. Crystal Growth March 1 1956 Akad. Nauk S.S.S.R. Moscow 1957) p. 58. N. A. Pangarov and St. Rashkov Bull. Inst. Phys. Chem. 1960 1 79; Compt. Rend. Acad. Bulgare Sci. 1960 13 555. N. A. Pangarov and S. D. Vitkova Electrochim. Acta 1966 11 1733. A. K. N. Reddy J. Electroanalyt. Chem. 1963,6 141. H. Fischer Elektro lytische Abscheidung iind Elektrokristallisation von Metallen (Springer Verlag Berlin 1954). ’J. O’M. Bockris Z. Nagy and D. Draft J. Electrochem. SOC. 1973,120,30.J. N. JoviCeviC D. M. DraiiC and A. R. DespiC Electrochim. Acta 1977,22 589. J. N. JoviCeviC A. R. DespiC and D. M. DraLiC Electrochim. Acta 1977 22 577. lo A. R. Despid G. SaviC MagliC and M. Jadovid Paper presented at the 28th Meeting of ISE Varna Sept. 1977. J. L. Barton and J. O’M. Bockris Proc. Roy. SOC.A 1962,268,485. l2 J. W. Diggle A. R. DespiC and J. O’M. Bockris J. Electrochem. Soc. 1969 116 1503. l3 J. Markov A. Boynov and S. Toschev Electrochim. Acta 1973 18 377. l4 cf. P. Delahay New Instrumental Methods in Electrochemistry (Interscience London 1954) p. 61.