Indian Journal of Chem istry Vol. 44A, May 2005, pp. 899-9 12

Atomistic aspects of electrochemical alloy formation: A review of nucleation and growth of nano-clusters and thin films

Alexander Milchev'" * & Georgi Staikov"

a Rosti slav Kai schev In stitute o f Phys ical C hemi stry, Bulgarian Academy of Sc iences, 111 3 Sofia, Bulgaria

Email: amil chev@ ipchp.ipc.bas.bg

b Institute of Thin Films and Inte rfaces (ISG 3), and c ni-Centre of Nanoelectronic Systems fo :·lnf()rr.:~:i : ~ il Technology,

Research Centre Jlili ch, D-52425 llilich, Germany Email: g.staikov@ fz-jue lich.de

Received 25 NOI'el1lber 2004

Atomi sti c aspects of e lectroche mical alloy formati on a re rev iewed and di scussed on the basis of theoretica l considera­ti ons and experimental results obtained in se lected mode l systems. The role of interac ti ons between co-deposited metal at­oms, as well as metal -substrate inte ractio ns during the initial stages of e lectroche mica l co-deposition has been di scussed. Nuc leati on kinetics in binary a lloy systems is con sidered theoreti ca lly, applying the atomi sti c approach. Experimenta l data on the nucleation ra te, the s ize and the compos iti on o f Ag- Hg a ll oy nucle i electrodepos ited on a platinum e lec trode a re pre­sented and di scussed in deta il. The fo rmation of surface a lloys is discussed in terms of mode ls includ ing place exchange between meta l adatoms and substrate atoms, so lid state d iffus ion and nucleation and growth phenomena. T he correlation between the depositi on conditions and the composition and properti es of alloy coatings obta ined in the advanced stages of e lec trochem ical all oy formation is demonstrated on the basis of experimental data fo r the g rowth o f Ag-Au a lloy.

Ii>C Code: Int. CI 7 B82B ; C25B: C25D3/56

Introduction The processes of electrochemical alloy formation

have been a subject of ex tensive investigations since a long time l

-5

. In recent years, the increasing require­ments of modern nanotechnologies have given ri se to a renewed interest in electrochemical alloy formati on, which offers attractive possibilities for preparation of coatings and structured material s with unconventional physical and chemical properties. The development of appropriate techniques, however, suggests a detailed underu .anding of the complex alloy formation phe­nomena on an atomic level. Two processes leading to form ation of metal alloy coatings in electrochemical systems can be generally di stinguished: (i) electro­chemical co-deposition, and (ii) surface alloying.

The formation of a binary alloy A XB y by electro-

chemical co-deposition from an electrolyte containing

ions A ZA + and B lB + of both components A and B can be expressed by Eq. 1.

(1)

The deposition conditions depend mainly on the nature of the co-depositing metals and their ions in the electrolyte and are determined by the corresponding

equilibrium potential EAB of the bulk alloy with an equilibrium composition3

. 6-8 :

E - EO kT I aA _ EO kT I a 13 - +-- n- - +-- n-AI3 A • 13 • ZA e aA ZB e a B

... (2)

Here E~ and Eg are the standard electrode poten­

tials of pure bulk metal s A and B, aA and aB are the

activities of the metal ions A ZA + and B l B + in the

electrolyte, and a~ and a~ represent the activiti es of

the components A and B in the equilibrium alloy. As can be seen from Eq. (2), the equilibrium potential EAB of the alloy is always more positive than the cor­responding equilibrium potentials EA and EB of the pure bulk metals.

In the case of electrochemical surface alloying a noble metal, A is used as a substrate and the electro-

lyte contains ions (B l B +) of a less noble metal B. The

formation of a binary surface alloy A xB y can be de-

scribed by the reaction

.. . (3)

900 INDIAN J CHEM, SEC A, MAY 2005

This process is induced by a strong A-B interaction and usually starts in the underpotential deposition (UPO) range of the less noble component B (i.e., at electrode potentials E more positive than the Nernst equilibrium potential EB of the pure bulk metal B)9,IO.

Recently , many authors have reported experimental results illustrating the important role of UPD phe­nomena during the electrochemical alloy formation by co-deposition in multi-component aqueous and mol­ten salt electrolytes" -18

• It has been shown that in many cases the co-deposition begins with an overpo­tential deposition (OPO) of the more noble metal, which plays the role of a substrate inducing UPO of the less noble component at more negative potentia ls. Under these conditions, the alloy formation occurs by UPO of the less noble component, taking place si­multaneously with the charge transfer and/or diffusion controlled OPD of the nobler one. UPD processes and surface alloying have to be considered also in the ini­tial stages of electrochemical co-deposition in multi­component systems exhibiting strong interaction be­tween the foreign substrate and co-depositing metals9

.

A common aspect of preparation of thin alloy films and/or modification of structures with small dimen­sions by electrochemical alloy formation is required to understand the relationship between the early stages of this process and the resulting structure and properties of alloy coatings.

In thi s article, we discuss the atomistic aspects and the mechanism of both the nucleation of small alloy clusters and the surface alloying, which are of major importance for the understanding of the initial and advanced stages of electrochemical alloy formation.

Nucleation of alloy clusters during electrochemical co-deposition

Nucleation work

If the working electrode is polarized to a potential E more negative than the equilibrium potential , EAB. of the bulk equilibrium alloy (Eq. 2), two- or three­dimensional nuclei of the new alloy phase can be formed on the electrode surface. The thermodynamic

work LlG of nucleus formation is an important quan­tity in the nucleation theory and in what follows we

derive a general expression for the work LlG(nAB ) to

form an nAB-atomic cluster on a foreign substrate

from a binary electrolyte of Ions A ZA + and (refs 6-8).

In the very initial stage, before the nuc leus forma­

tion, the Gibbs free energy, G 1, of the whole system

is given by Eq. (4):

.. . (4)

where NA and NB are the numbers of ions A and B in

the electrolyte solution, flA and flB are their electro-

chemical potentials and G, (E) takes into considera­

tion the contribution of the working electrode to the total energy ba lance. The last quantity depends on the electrode potential E.

In the final stage, when a I1AB-atomic nucleus is

formed, the Gibbs free energy, G2 , of the system

changes to:

Here Gs (E,I1.AB) is the energy contribution of the

foreign substrate as modified by the presence of the I1c.IIB-atomic alloy nucleus consisting of nA , A and 118 , B atoms. Thus the difference,

82 - (;1 = Ll8(I1 AB ) = G (E,I1AB) - nA fl A - nB flB

(6)

gives the thermodynamic work t!G(nAl1 ) of nucleus

formation. The quantity G (E, nA8) in Eq. (6) is given

by

- - -G (E,I1AB) = G, (E,I1AB ) - G, (E)

and coincides with the Gibbs free energy of the 11118-

atomic nucleus formed on the foreign substrate.

In order to introduce the thermodynamic supersatu­ration in Eq. (6), it is necessary to rewri te it as Eq. (7):

- -LlG(I1 AB ) = cP(I1 AB ) - (IlA LlflA + n8 LlflB) ... (7)

where

.. . (8)

is the free energy excess due to the creation of new

interfaces when the nucleus is born and fl : and fl ;

are the electrochemical potentials of the A and B at­

oms in the bulk allol-8. The quantities t1.fiA and Llfl8

MILCHEV & STAIKOV: ATOMISTIC ASPECTS OF ELECTROCHEMICAL ALLOY FORMATION 90 I

in Eq . (7) define two thermodynamic supersatura­tions:

... (9)

... (10)

that can be varied either by varying the electrode po­tential E at constant activities a A and aB of metal ions in the electrolyte or by varying the activities aA and aB

at a constant potential E of the working electrode. Correspondingly, the difference, EAB - E, defines the cathodic overpotential T) , which is considered positive in this study.

Equation (7) is a general formula, which gives us the nucleation work of any I1Awatomic cluster of the new alloy phase. This quantity has a maximal value

tJ.C(nAB) = tJ.C (nC.AB) for the cluster consisting of

Ilc,AB = n c,A + n c B atoms that is called a critical nucleus. In the case of low supersaturations, the critical nu­cleus consists of a large number of atoms and its properties can be described in terms of the classical nucleation theory developed in the works of a number of authors (see refs 19-26 and the literature cited thereiTi). For high supersaturations and very active substrates , which is usually the case of electrochemi­cal formation of a new phase on a foreign substrate the size of the nucleus is small and the macroscopic concepts like "surface" and "specific free surface en­ergy" used in the classical theory lose their physical significance. In this case, the process of nucleus for­mation must be described by means of atomistic con­siderations and the reader could find the basic ther­modynamic and kinetic principles of the atomistic model of electrochemical nucleation in refs 27-30. In what follows we consider the kinetic theory of nu­cleus formation of alloy clusters providing informa­tion also on some experimental data obtained in bi­nary electrolyte solutions.

Nucleation rate

The theoretical expression for the stationary nu­cleation rate, lSI, in a binary electrochemical system of

ions A Z,,'" and B ZB + reads:

... (11)

where 20 is the number of active sites on the substrate, A. is a non-dimensional factor accounting for the dif-

ference between the equilibrium and the stationary number of critical nuclei and W+

II, is the frequency of

attachment of single atoms to the critical nucleus con­sisting of n c.A B = I1 c.A + !le, n atoms. Equation (II) can be presented in two different formss revealing the de­pendence of lSI on the electrode potential E as:

_ [(nC' AZA +1lc, BZB + (X;z; )eC-E) ] ISI.AB - Kj(nc.AB) exp

kT

.., (12)

and on the activities aA and an of metal ions in the electrolyte solution as:

... (1 3)

In Eq. (12), (Xi is the cathodic charge transfer coef­ficient, e is the elementary electric charge, k is the Boltzman constant, T is the absolute temperature, Kj(nc.AB) is a frequency factor and i=A or i=8 depend­ing on whether the critical nucleus transforms into a stable cluster by attaching one A or one 8 atom. re-

spectively. In Eq. (13), Kj* (n c.A B) is a frequency factor

that does not depend on the activities of the metal ions and i=A , )=1 and 1=0 when the critical nucleus be­

comes stable by attaching one A atom whereas i=8, )=0 and 1= I when the critical nucleus becomes stab le by attaching one 8 atom.

As seen, Eq . (12) allows us to obtain information on the sum, n c.AZA + n c,/lZB and, if ZA = Z/3, also on the total size n c,AB = n e,A+ n e, B of the critical nucleus by studying experimentally the potential dependence of the stationary nucleation rate l.w(E) at constant activi­ties of the metal ions . At the same time, Eq. (13) pro­vides the possibility of obtaining information on !le,A

and on n e, B, separately, and therefore allows us to re­veal the composition of the critical nucleus by study­ing experimentally the dependence of the stationary nucleation rate on the activities of the metal ions at a constant electrode potential E. Such experimental studies havc already been performed in the system PtlHg/+, Ag+ (See ref. 8).

In the following we comment briefly upon some of the obtained results.

Size and composition of critical nuclei In order to obtain information on the nucleation

rate of mercury-silver alloy clusters on a platinum single crystal electrode, "number of nuclei - time" , N(t), relationships were registered experimentally at different constant electrode potentials E (Fig. I) in an

902 INDIAN J C1-IEM. SEC A, MA Y 2005

tIs

Fig. I- Number of nuclei versus time relationships obtained in O. IM Hg:(N03h + 0.05M AgNO, + 2M KNO) + 0.35M HNO, at T = 308 K. The electrode potential E measured versus MRE is given by fi gures in volts (after Milchev A el al. in ref. 8).

electrolyte so lution contai ning O.IM Hg2(N03) 2 + 0.05M AgN03 + 2M KN03 + 0.35M HN03 (ref. 8).

The nuclei are formed by means of a double pulse potentiostatic method described in details in refs 30-32. The reference e lectrode (MRE) was bulk mercury dipped in a solution ofO. 1M Hg2(N0 3) 2 + 2M KN03 + 0.35M HN03 with a stable potential of 0.055V meas­ured with reference to a Hg/HgS0 4' 0.5M H2S04

electrode. The stationary nucleation rate /'\"AB = dN(t)/dt was determined from the slopes of the linear portio ns of the N(t) curves and its potenti al depend­ence was studied . The In/".AB vs -E re latio nship was found to be linear, in accordance with the atomi stic theory of electrochemical nucleat ion27

•3o

, indicating that the size of the critical nucleus remains constant in sufficiently wide supersaturation intervals. The sum,

I1 c.AZA + Itc. BZB (A= Hg i+ , B= Ag +), is determined ac­

cording to :

kT r dIn I".AlJ ] 11 \z +It / 7 f = - - -a,'z,"

c .; II d~ I e d e-E) L (IA .lI8

... (14)

and with Z , = Z h = I and a -a - OS the AI.' Hg

2 A .... ' - H.d+ - ,

size I1c.Hg-Ag of the mercury-silver critical nucleus re­

sults in 8 atoms8. (The valence of the Hg~+ ion is

considered equal to I because each ion's discharge gives two mercury atoms).

The concentration dependence of the stationary nu­c leation rate determined by registering "the number of nuclei - time" , N(t), relationships at a constant elec­trode po tential E = -0,070V , a constant concentration

of mercury ions C Hgi+ = 0.1 M and four concentrations

of si lver ions, cA

' = 0.03, 0.05 , 0.07 and 0,1 OM g

showed a linear In1slAB vs. In C _ relationship. Bear-Ag

ing in mind that for d ilute solutions the ac tivities of the metal ions coincide with the cOITesponding con­centrations, the number n eA l.' of silver atoms in the critical alloy nucl eus was determined fro m the slope (dlnlsl.ABldln c

Ag+) of the straight line according to (see

Eq. 13):

_ (d In I ". AB ) II c 8 - -I

. dln clJ c £ A'

... ( IS )

The result showed IlAg = 3 atoms if 1=0 andllAg = 2 atoms if 1=1. The more detailed investigations of the concentration dependence of the stationary nucleation rate reported by Milchev ef al.8 clearly show that with increasing concentratio n of si lver ions in the work ing e lectrolyte, the critical nuc leus is gradually enriched of si I ver. In this sectio n. the basic thermodynamic and kinetic features of the nucleat ion theory of OPO electrochemical alloy formation is desc ribed and an ex perimental method proposed for determinati on of the composition of critical alloy clusters.

Surface alloy formation phenomena in UPD systems

UPO of a meta l B on a foreign substrate A, occurs if the A-B bonding is stronger than the B-B bonding and typica lly leads to the fo rmation of expanded and/or condensed adlayers (20 phases) of I3 on A9

.IO

.

In some UPO systems, however, the strong A-B bonding induces place exchange processes between surface atoms of A and the adatoms of B leading to surface alloy formation. A pronounced alloy fo rma­tion can be expected in bimetallic A-B systems ex­hibiting a large bulk mi scibility of the components. For the simple case of isomorphic metals A and B, and first nearest-neighbor interactions, the so-called mean field approach y ields the following condition for the formation of an ideal bulk solid solution of B in A33

. . . (16)

Here If! A-B' If! A-A and If! B-B represent the energies

of the A-B , A-A and B-B first nearest-neighbor

MILCHEV & STAIKOV: ATOMISTIC ASPECTS OF ELECTROCHEMICAL ALLOY FORMATION 903

bonds, respectively. Theoretical considerations based on the same model show that surface alloying can occur also in immiscible bimetallic systems, i.e., in A­B systems where the condition in Eq. (16) is not ful­filled34

. This possibility is illustrated in Fig. 2, which shows as an example the replacement of an atom from a surface telTace of the metal substrate A by an ad­atom of the depositing metal B. The process involves creation of an atomic vacancy in the substrate terrace by transferring an atom A in a kink site position and a subsequent filling of the created vacancy by an ad­atom of B. The energy balance leads to the following condition for the occurrence of such place exchange process34

1 IjI A· B > 21j1 A · A ... (17)

This condition shows clearly that surface alloys can be formed even in systems with a mixed bond energy

IjI A. B' which is lower than that required for a bulk

alloy ing (ef condition in Eq. 16). In such cases, the surface alloying is usually limited to the topmost sur­face layer of the substrate. Although the above simple

OA

w m xm -o B '-f:rY);"A..).....A)\.,A,A..A')

(a)

energetic considerations neglect kinetic effects , they predict qualitatively many surface alloy formation phenomena observed experimentally in electrochemi­cal systems9

. In this section , we discuss the mecha­ni sm and kinetics of such phenomena on the bas is of experimental results obtained in selected UPD sys­tems.

The system Ag(111)/Pb2+

This system is characterized by a negligible Ag-Pb bulk miscibility at room temperature and is one of the most extensively studied UPD systems in the last fe w years9

.10

. This interest is particularly due to the fact that the UPD processes occur in well-defined and di s­tinguished potential ranges. Detailed electrochemi cal and ill situ STM investigations in aqueous perchloric acid solutions have given direct evidence for a step­wise formation of low-di mensional ( 10, 20) Pb phases35

.38

. These results have also been confirmed recently by in situ AFM studies of this UPD systemJ9

.

The in situ linescan AFM image and the correspond­ing cycl ic voltammogram (CY) shown in Fig. 3 illus­trate the stepwise formation and dissolution of the Pb UPD overlayer on a stepped Ag( Ill) substrate. The

-(b) (e)

Fig. 2- Schematic representati on of the repl acement of an atom from a surface terrace o f the metal substrate A by an adatom o f the depositing metal B.

(b) (a) (e) 0.2 __ ' Ag

N.c

~I 0-:a 0-

!I vi 0.02 - --- r > >

2DPb

UJ

.r 0.2 --. . ------ Ag

· 3 0 x --. I--i

;.10', Acm-2 20 nm

Fi g. 3- (a) III situ linescan AFM image howing the fo rmati on and di sso lution of the Pb UPD overlayer in the system Ag( III )/5 mM Pb(CI O-l)z + 0 .5 M NaClO-l + 5 mM II C IO-l at T = 298 K; (b) Cycli c vo itamillogralll (ldEldtl = 10 mV/s) corresponding to the AFM image in (a), and (c) Selected single line sec ti ons from the image in (a) (a fte r Stai kov G in re f. 39).

904 [NDIAN J Cf-IEM , SEC A, MA Y 2005

deposition/di ssolution processes occur in three dis­tinct potential interval s related to the voltammetric peak pairs A11D 1, A2/D2 and AJ/D3. The peak AI is associated with a decoration c; the bottom part of monatomic step edges. This 10 Pb phase ex tends to about 2 nm and acts as a precursor for the formation of a condensed close-packed Pb monolayer, which starts in the peak A2 and differs significantly at differ­ent surface domains such as stepped terraces and monato mic islands and pits . In peak A2. the condensed Pb monolayer does not comple te ly cover the terraces leav ing about 2 nm wide uncovered substrate stripe at the terrace peripheries, which becomes covered only at mOf= cathod ic poten tials corresponding to peak A3. Detailed ill situ STM studies at potentials located between peaks A2 and A) show that the uncompleted close-packed Pb monolayer is unstable in the time scale of minutes and transforms after extended po lari­zation to a Ag-Pb surface alloy with a Ag( lll)­

(13 x 13) R300Pb structurelJ.IO.J5.36. The transforma-

tion starts at the step edges of the condensed Pb mono layer and the uncovered substrate stripes at the terrace peripheries and propagates inwards on the ter­races. It is suggested that the surface alloy ing is lim­ited only to the topmost layer of the substrate and in­volves an exchange of every third Ag atom by a Pb atom accompanied by di ssolution o f the excess Pb adatoms in the e lectrolyte solution. An interes ting

feature can be recognized in the AFM image shown in Fig . 3b. As seen, so me of the monatomic step edges do not retract after th e potential cycle. to their original positions (el the single line sections at the bottom and at the top in Fig. 3c), which indicates that some trans­formations take place at step edges even during the rel atively short period (36 s) of the deposi­tion/dissolution cycle. This behavior can be ass igned to a lateral exchange of Ag atoms of step edges by Pb atoms, which is limited only to several atomic rows and occurs in the initi a l stages of the Pb overlayer formation. Such limited lateral intermixing could al so explain the appearance of uncovered substrate stripes at the terrace peripheri es observed afte r fast deposi ­tion in peak A2.

Figure 4 shows the surface morpho logy change~ observed during the anodic dissolution (de-alloying) of the Pb-Ag surface alloy formed by ex tended po·· lari zation at a potential located between peaks A2 and A3 (el Fig. 3)37 . The extended po lariza ti n leads to an almost complete disappearance of ('he vo ltammetric peak, D2, and the appearance of a new and rather broad peak, Do, in the stripping curve (e.f Fig. 4c) . As seen in the STM images in Figs 4a and 4b, the de·· alloying process in peak Do results in the appearance of a large number of pits with monatomic depth in the vicinity of step edges. These pits form obviously by clustering of vacancies created duri ng removal of Pb

~:~I ___ (C_) ~I ~~_D_2 _D1~:-~~~I~~~ __ D_O ~_~===r1 __

o 0.1 0.2 0.3 0.4 0.5

E I V VS . Pb/Pb2+

Fig. 4--Anodic dissolution o f a Pb-Ag surface alloy formed in the system Ag( III )/5 mM Pb(C I04) 2 + 0 .5 M NaCI04 + 5 mM HCI04 at T = 298 K. [(a) /11 silu STM image of the substrate surface obtained after po larization time of Ip = 10 min. at an electrode potenti al of E := 0.l3 V vs . Pb/Pb2

+ located between CV peaks A2 and A3 (ef Fig. 5b); (b) In situ STM image of the urface o btained after dissolution at E = 0.5 vs. Pb/Pb2

+ for 100 s; and (c) Anodic stripping curve (ldE/d/l = 10 mV/s) corresponding '[0 (a) and (b) (after Lorenz W J el al. In ref. 37)].

MILCHEY & STAIKOY: ATOMISTIC ASPECTS OF ELECTROCHEMICAL ALLOY FORMATION 905

from the surface alloy and disappear quickly due to the relatively high mobility of Ag surface atoms at

d· . I 4041 these ano IC potentia s . . The important role of the substrate step density in

electrochemical surface alloying has been demon­strated previously by voltammetric measurements performed under the same polarization conditions on stepped real and quasi-perfect (electrochemically grown) Ag( Ill) substrates42

.43 . These studies show clearly that the rate of surface alloy formation in­creases significantly with increasing step density in <Yood a<Yreement with the in situ STM and AFM re-o 0

suits discussed above. Recently , Widmer and Siegenthaler4

-l also investigated the influence of cr anions on the mechanism and kinetics of surface al­loying and de-alloying in the system Ag( III )/Pb2

+ by means of cyclic voltammetry and ill situ STM. They found that the presence of cr ions inhibits signifi­cantly the rate of surface alloy formation processes but enhances the Ag mobility at step edges of Pb-free Ag(lll) substrates after de-alloying.

The above results demonstrate clearly that even in UPO systems with negligible metal-substrate bulk miscibility, the UPO metal overlayers can represent non-equilibrium systems, which undergo slow trans­formations leading to formation of surface-confined alloys. More detailed information obtained in various systems on an atomic scale is necessary , however, in order to understand better the local dynamics of in­volved place exchange processes and to develop real­istic kinetic models .

The system Ag(1l1)/Cd2+

This system, along with the Au/Cd2+ system, be­

longs to the UPO systems showing a pronounced sur­face alloy formation45-47. In the last few years, both systems have attracted renewed interest not only as model UPO systems for investigation of surface alloy formation processes4S056, but also as systems suitable for electrodeposition of ultrathin films of compounds such as CdS and CdSe by the so-called electrochemi­cal atomic layer epitaxy (ECALE)57.58

Figure Sa shows a typical cyclic voltammogram for Cd UPO in the system Ag( L 11)/5 mM CdS04 + 0.5 M Na2S04 + 5 mM H2S04• Recent in situ STM studies performed in this system provided new important in­formation on the atomic structure and the local mechanism of formation of the Cd adlayers acting as precursors of the Ag-Cd surface alloy formation 56. Experimental results show that the UPD process starts in the potential range of CY peaks A 1 and A2 with the

formation of an expanded adlayer exhibiting a characteristic "linear" superlattice structure

AgO 11)-(.J3 x M) R23.4° . The latter is suggested

to be stabilized by co-adsorbed sulfate anions. In the UPO range corresponding to peak A3, the expanded adlayer transforms into a condensed close packed Cd monolayer via first order phase transition invol vi ng 20 nucleation and growth. The formation of a second Cd monolayer and significant Ag-Cd alloy formation are observed in the UPO range corresponding to peak A4 (0 mV< E (vs. Cd/Cd2+) < 50 mY) . The surface alloy formation process is reflected in the anodic stripping curves. As an example, Fig. sb shows typi­cal stripping curves obtai~ed after extended polariz

2: ­

tion at an electrode potential E = 10m Y vs. Cd/Cd . The observed changes of the number and position of stripping peaks with increasing polarization time fp

indicate the occurrence of the surface alloy formation.

100~--------------------------------,

(a) 50

E (.)

« 0200 CD

o ...... . ...:

(b)

10

5

:0 Ll_~'-tl._===~"-=::I...-'''''-_-::'":-__ (_ C)-:: ~tr=30S

240 S

t, ~ o 0.1 0.2 0.3 0.4 0.5

E f V vs. CdfCcr+

Fig. 5-(a) Cyclic vo ltammogram for Cd UPD in the system Ag( Ill )/ 5 mM CdS04 + 0.5 M Na2S04 + 5 mM H2SO.j. (ldEMrl = 10 mY/s, T= 298 K ); (b) Anodic stripping curves obtained after extended polarization at E = 10 mY vs. Cd/Cd2

+ (polarization time Ip: 30 and 240 s: IdE/drl = 10 mY/s); and (c) Anodic stripping curves obtained after extended polarization at E = 70 mY vs. Cd/Cd2

+ (po lari zat ion time Ip: 30 and 240 s: IdE/e/rl = 10 mY/s) (after Garcia S G el ai. in ref. 56).

906 INDIAN J CHEM, SEC A, MA Y 2005

1.0 (a) .£>

(b)

10mV 1.0

~~ 0.8 ~ ":' ,,0

E '" 0" 'E 0.8 ~ 0.6 <.) 0"

0 a" ,0

" 0 ~ 0 .. .. ~0.4 ;

70mV <l0.6

0.2

o 0 0.4

200 400 600 800 0 10 20 30

tp's t ;t2/ S 1f2

Fig. 6-(a) Dependence of the stripping charge density t,q on the polari zati on time 'I' in the system Ag( 111 )/Cd2+ at elec trode potential s 10

mY anJ 70 mY vs. Cd/Cd"+, and (b) t,q - 1 ~2 plot of experimental data in (a) for E = 10 mY vs. Cd/Cdl + (after Garcia S G e/ al. in ref. 56).

Similar changes of the anodic stripping curves (Fig. 5c) are also observed after long time polarization at an electrode potential E = 70 mY vs. Cd/Cd2

+ corre­sponding to CY peak A3. In particular, the position of peak D3 (compare Figures Sa and 5c) shifts to more positive potentials, which indicates that surface al­loying takes place even at this potential. The cOIl'e­sponding stripping charge densities 6.q are plotted as a function of the polarization time, (p, in Fig. 6a. As seen, for polarization potential E = 10m Y vs . Cd/Cd2

+, 6.q increases s ignificantly with t~ due to sur­face alloying. At E = 70 mY vs. Cd/Cd +, however, 6.q does not depend on tp and is limited to about 0.41 mC/cm2

, which is close to the value required for the deposition of a close packed Cd monolayer. This sug­gests that place exchange processes between Cd at­oms and Ag surface atoms may take place under these conditions leading to a transformation of the con­densed Cd monolayer to a thin Ag-Cd surface alloy layer.

The 6.q - t~12 plot of experimental data for E = 10

mY vs. Cd/Cd2+ shows two Ii near sections at short

and long polarization times, respectively (Fig. 6b). This behavior is very similar to that reported recently for the systems Au(lOO)/Cd2

+ and Au(lOO)/Cd2+ by

Yidu and Hara50.5I, who explained experimental re­

sults on the basis of a model for linear solid state dif­fusion shown schematically in Fig. 7. Tn this model, the growing Ag-Cd surface alloy layer is considered as consisting of two regions with different diffusion properties . The region -I < x < 0 represents a thin sur­face alloy film ("skin") formed at short times and characterized by a diffusion coefficient D" which is larger than the diffusion coefficient D2 of the rest

Q c~ Cd 2+

.!:!. ~ en .-~

'0 '" <>:

'-? 0 Cl ttl

<>: .. : - / 0 x __

X = - I - ~ C = const = Co

Fig . 7- Schematic representation o f the diffus io n model proposed in ref. 50 and applied for the analysi s of ex perimental data il) Fig.6b.

region x > O. The Cd surface concentration, Co, at the Ag-electrode/electrolyte interface (x := -I) is consid­ered to be constant. In the present case, a surface con­centration of Co = 0.0735 molicr} corresponding to a close packed Cd monolayer is assumed. Following the procedure described by Vidu and Hara50

, from the data in Fig. 6b, one obtains diffusion coefficient val­ues of DI = 1.5 X 10.16 cm2/s and D2 = 3.5 X 10,19 cm2/s (ref. 56), which are in good agreement with those reported for the system Ag(lOO)/Cd2

+ (ref. 51 ). Most probably, the initial formation of the "skin" layer occurs by a relatively fast place exchange be­tween Cd atoms and Ag surface atoms and the diffu­sion coefficient DI can be formally expressed

as DI = d 2verr' where d is the atomic diameter of Cd

and Veff represents the effective place exchange fre­quency . With DI = 1.5 X 10,16 cm2/s and d = 2.89 X

10'8 cm, this equation yields a value of 1.8 x 10,1 S' I

for the effective place exchange frequency VetT.

MILCHEY & STAIKOY : ATOM ISTIC ASPECTS OF ELECTROCHEMICAL ALLOY FORMATION 907

Generally, O2 should be considered as a mutual dif­fusion coefficient, which is related to the tracer diffu-

sion coefficients O~d(Ag) and O :g(Cd) of Cd and Ag in

an Ag- and Cd-matrix, respectively47. Literature data59

show that at room temperature O ,:g(Cd) is more than 12

orders of magnitude higher than O~d(Ag). The experi­

mentally observed value of O2 = 3.5 x 10-19 cm2/s is very close to the value for the tracer diffusion coeffi­

cient of Ag in a Cd-matrix (D:g(CdJ = 5.8 x I 0-19 cm2/s)

(ref. 59). This result is in agreement with previous . h' 47 d h measurements 1\1 t IS system an suggests t at at

long polarization times the alloy formation occurs preferentially by a movement of Ag atoms through the vacancy-rich surface alloy layer and a simultane­ous Cd deposition at the Ag-Cd/interface.

The above considerations presume that the surface alloy formation processes do not involve first order phase transitions. Such phenomena, however, are ob­served in some cases and will be discussed in the next section.

The system AlfLi+

This system is characterized by a relatively large solid solubility of Li in Al at room temperature and forms along with the solid sol ution of Li in Al (a phase), and also an intermetallic compound LiAI ({3 phase) via a first order phase transition6o

.6 1

. The electrochemical Li-AI alloy formation has been inten­sively studied in various AIILi+ systems with molten salt electrolytes and organic electrolyte solutions62

-68

.

This is mainly due to the fact that {3-LiAl exhibits a

450

300

N 150 'E (J

~ - 0 '" 0 ~ . ..;

-1 50

-300

-450

a 0.5 1.0 1.5 2.0 E I V vs. lilli·

N

E (J

~

'" 0 ~ . ..;

very fast lithium solid state diffusion and is consid­ered as electrode material for rechargeable lithium batteries. The AIILi+ system is not only of practical but also of fundamental interest because the formation of {3-LiAI alloy involves typical electrocrystallization phenomena, which control the kinetics of the initial stages of alloy formation . These phenomena will be discussed on the basis of experimental results for electrochemical (3-LiAl alloy formation on polycrys­talline Al substrates in a 1M LiCI04 propylene car­bonate (PC) solution at room temperature66

.67.

Figure 8a shows a typical cyclic voltammogram obtained in the system AIILi+,ClO-l-(PC) in the UPD range of Li, i.e. , at electrode potentials E more posi­tive than the equ ilibrium potential Eli of the pure bulk lithium phase (E > ELi). The hysteresis of the cathodic current observed by the change of the potential scan­ning direction reflects the formation of the {3-LiAI alloy phase, which dissol ves at electrode potentials more anodic than 0.34 V (vs. Li/Li+). This value is very close to the value reported in the literature for the apparent equilibrium potential (EtJ-LiAI ;::0 0.335 V vs. Li/Li+) of {3-LiAI electrodes in I M LiCIO-l PC­solution63-65

.

Figure 8b shows a typical current transient ob­tained by stepping the potential in the potential range of {3-LiAI alloy formation. The transient is character­ized by a steep initial current decay followed by a rise of the current to an almost constant steady state value. The initial current decay corresponds to the formation of the a-phase (so lid solution of Li in AI) whereas the rising part of the transient is associated with nuclea-

(b)

100

50

~ 0 a 100 200 300 400

tis

Fig. 8-Electrochemical behavior of the system All 1M LiCI04-propylene carbonate (PC) so lution at T = 298 K. [(a) CY obtained in the UPO range of Li; (b) Typical current transient for the formation of the {)-LiAI alloy obtained at an electrode potential of E = 160 mY vs. Li /Li+ (after Geronov Y el al. in ref. 66).\.

908 INDIAN J CHEM, SEC A, MAY 2005

tion and growth processes during the initial stages of electrochemical formation of the j3-LiAl alloy.

A peculiarity of the AI/Li+ system is that the poly­crystalline aluminum working electrode plays the role of both a foreign substrate for the lithium ions dis­charge and a parent phase providing aluminum atoms for the nucleation and growth of the new j3-LiAl phase. Assuming that in the very initial stages the process is controlled by the transfer of Li+ ions from the electrolyte to the aluminum substrate, in the fol­lowing we describe nucleation and growth kinetics in terms of the models used in the case of conventional metal electrodeposition on foreign substrates9.69-71.

Transient measurements performed in the potential range 0.220 V ~ E (vs. Li/Li+) ~O.l 00 V show that the initial rising parts of the current transients fit well with the linear i_t3 relation67 (Fig. 9).

This corresponds to progressive nucleation with a constant nucleation rate and charge transfer growth mechanism of 3D j3-LiAl clusters when the CULTent­time relationship at short times is given b/,69-71.

I i 1= Kp [ 3 (18)

with

K" = g V 3 I st.f3_LiAI (19)

Here, the factor g depends on the geometry and the

composition of the growing clusters, while I st.f3-LiAI is

the stationary nucleation rate and V is a function of the electrode potential gi ven by the Butler-Volmer relation 19,30.

0.6 0.8 1.0 1.2

0.16 V

N 50 'E ()

019 V 4:

'" 0

'7 25

o~ __ ~ __ ~ __ ~ ____ ~ __ ~ ____ ~~ o 6 8 10 12

t 3 .10-5 /s 3

Fig, 9- The i_I) plots of the initial ri sing parts of experimc:n ta l cu rrent transients obtained at different electrode potentials E (vs. Li/Lt) ill the system AI/1M LiCI04(PC) at T = 298 K. The arrows indica te the abscissa of the corresponding data set (after Geronov Y el 01. ill ref. 67).

In the present case, the nucleation and growth take place at electrode potentials within the interval 0.220 V ~ E (VS. Li/Li+) ~ 0.100 V, which are much lower than the apparent equilibrium potential of the j3-LiAI phase (EfJLiAI ;::; 0.3 35 V vs. Li/Li+) . Therefore, the anodic term of the Butler-Volmer relation can be ne­glected and the function VeE) can be expressed by

V = ko exp [ ex e:;E)] ... (20)

where ko is a kinetic constant related to the exchange current density of the bulk alloy.

The potential dependence of the stationary nuclea­tion rate 1 ".f3-LiAI can be described by Eg. (12) rewrit-

ten as:

." (12')

where A=Li, B=AI , ZA = ZLi = I and !1j1/J is the super­

saturation referred to the aluminum atoms. The last

quantity is given by 6.i1 B = i18 - i1 ; where i18 and

i1 ; are the electrochemical potentials of the alumi­

num atoms in the bulk aluminum substrate and in the bulk j3-LiAl alloy , respectively .

Thus, from Eqs (1 9), (20) and (l2') for the potential dependence of Kp. one obtains

-18 r-------

-20

-22

'::co. -24 .5

-26

-28

0.15 0. 1

E I V vs. Li/Li+

Fig. IO-The In K" - E plot of the experimenta l dnta from the transient measurements in the system AIIlM Li C IO.1( PC) at T = 298 K (after Geronov Y el 0 / , in ref. 67).

MILCHEY & STAIKOY : ATOMISTIC ASPECTS OF ELECTROCHEM ICAL ALLOY FORMATION 909

[

(nC.li + 4a)e( - E) 1 Kp -exp

kT ... (2 1)

Figure 10 represents the slopes Kp of experimental i_f3 relationships as InKp-E plots, which shows two distinct linear regions in agreement with Eq. (21) and the predictions of the atomistic theory of electro­chemical nucleation27

-30

. From the slopes of the two straight lines for the number of lithium atoms in the critical alloy nucleus one obtains ilc.Li= 0 atoms in the potential interval, 0.100-0.160V and Ilc.Li= 1 atom in the potential interval, 0.160-0.220V. In terms of the atomistic theor/7

-3o

, n c.IF 0 implies that the single Li atom deposited on the substrate represents a stable "cluster" and the nucleatiol1 site itself can be r.onsid­ered as a critical nucleus. Assuming a sto ichiometric composition of the ,B-LiAI clusters, for the total num­ber of atoms in the critical nucleus in the potentJaI interval, 0.1 00-0.160V, one obtains a value of 17c.fJ- liAI

= 2. This is , however, on ly an approximate estimate because the structure and composition of critical nu­clei hardly coincide with that of the bulk alloy phase.

1n the next section we discuss briefly the correla­tion between the deposition conditions and the COfll­

position of thin alloy fi lms, which is of great theoreti­cal and practical importance.

Electrochemical deposition of thin alloy films The properties of thll1 alloy films obtained by eler:­

trochemical co-deposition depend on a number of kI­netic and thermodynamic factors. An important task of most of the experimental studies is to develop rou­tines for deposition of alloy films with a defined chemical compos ition. Gtnerally, a controlled co­deposition process C3f1 be achieved if both metals

1.2 r---------------,

09

0.6

·0.3

·0.6

(a)

.t.# ....... ~' J ~_···~ __ ~ __ L_ __ L! __

· 1.0 -0.5 0.5

E I Vvs. SHE

1.0

have very close equi librium potentials and si milar electrodeposition kinetics 1-5.8.12. The si mplest case is when the polarization curve for the alloy deposition can be considered as a sum of the partial polarization curves of co-depositing metals. In real systems. how­ever, the situation is often more complicated because of possible interactions between partial depositi on reactions l

-5

. Thus, detailed investIgations are usually needed in order to find out appropriate conditions fo r depositio!l of alloys with defined composition. In many cases, complexing agents are introduced in the electrolyte solution in order to bring the equilibrium potentials of both metals closer to each other. A typi­cal example is the binary system Ag-Au. The standard equi libriu m potentials of Ag and Au in electrolyte solutions containing simple Ag+ and Au+ ions differ by about 0.9 V. In alkaline cyanide medium, however, the equilibrium potentials of Ag and Au shift signifi­cantly in the negative direction due to the formation of the cyanide complexes [Ag(CN)2r and [Au(CN)2r. Moreover, the standard equilibrium potential of the couple Ag/[Ag(CN)2r is a bout 0.3 V more posit ive than thar of the AU/[Au(CNh) couple, i.e. , Ag be­haves as 3. more noble metal ill this medium. The bi­nary Ag-Au system fe ms a homogeneous solid solu­tior. in the entire conpositional range and therefore 8ffel's pod possibilities fo,: de",elopment of appropri­ate :··)utines fo r c 'ntrc 1]ecl ~)ect!-odeposition of Ag-Au alloys wl(h defined cO'."1pcsi,:ion .

In rer;ent years, the eler trodepcs: tion of Ag, Au and Ag .. Au al~oys from a:kali '1e cyanide electrolytes has :lttr~'.cted considerable j I1tf" ~S t d '_ e to the possibility of apphation of these materIals : ~ different micro- and nano-electronic devices. T'Je kinetics and mechanism of' deposition processes have been extensively studied

1.2 r-- -

0.9 (b)

·0.3

·0.6

·1.0 ·0.5 0.5 1.0

E I V V~ SHE

Fig . II -CYs ror deposition and di ssolution of Ag, Au <lnd Ag-Au a l ;oy~ .n 'h·. systems. [(a) GC![Ag(CNhr (pH=14, cfAg(C )2r= 5 mM) and _GC/[Au(C!'Ihr (pH= 14 , c[Au(CN)2r = 5 !'1M); '\:ld (b) GC:[Ag( :-::Nhr + [Au(CN X (PH= .I.4 , ~[Ag(CNhr = 5 mM, c[Au(CNh) = 5 rnAif\ Scan rate IdE/drl = 10 mV/s; T= 298 K (af~er Maro : ! ::~ J(.~ ' ~l. in ref. 78»).

910 INDIAN] CHEM, SEC A, MA Y 2005

on various foreign substrates (n-Si, glassy carbon (GC), Cu) applying electrochemical and surface ana-I . I h d 73-79 ytlca met 0 s .

As an example, Fig. 11 demonstrates typical CYs for electrodeposition of Ag, Au and Ag-Au in the systems: GC/[Ag(CNhL GC/[Au(CN)2r and GC/[Ag(CNhr + [Au(CNhr (ref 78). The CYs show initial deposition peaks followed by diffusion-limited current, indicating mass transport controlled processes at high cathodic potentials. As seen in Fig. 11 a, the diffusion-limited CUITent densities observed on the reverse scans in both one-component systems are practically equal (0.2 mA/cm2

), as expected from the equal concentrations of [Ag(CNhr and [Au(CNhr and the same diffusion coefficients in the electrolyte solutions. The CY obtained in a two-component elec­trolyte containing the same equal amounts of Ag and Au (ef Fig. 11 b) does not exhibit on the reverse scan a pronounced diffusion-limited current but indicates an approximately two-folds higher value for the diffu­sion-limited cUlTent density as compared to the one­component systems.

The electrodeposition of Ag, Au and Ag-Au alloys on H-terminated n-Si substrates shows in principle the same behavior as that on GC substrates76

-78

. In con­trast to GC, however, an electrochemical dissolution of the deposits on n-Si is impossible due to the for­mation of high Schottky barriers at the deposit/n-Si interfaces73

-78

. The composition of the Ag-Au alloys obtained under different deposition conditions was determined using EDX and XPS. Figure 12 shows the composition of Ag-Au alloy deposits obtained at dif­ferent electrode potentials on n-Si(l J I) from an elec­trolyte solution contall1l11g equal amounts of r Ag(CNh r and [Au(CNhr (ref. 76). As can be seen, for relatively positive electrode potentials (£ > -0.95 Y vs. SHE), corresponding to a deposition of both components under combined charge transfer and dif­fusion limitations, the Au amount in the alloy deposits increases with decreasing deposition potential. How­ever, at electrode potentials more negati ve than -1.0 Y (vs. SHE), corresponding to the diffusion-limited depos ition regime of both components, the Au amount in the alloy becomes potential independent and reaches a constant value of about 51 at. %. Ex­perimental results show that under these conditions the Au amount increases with increasing relative amount of Au in the mixed electrolyte solution. Re­cently Ji el al. 79 reported detailed studies of Ag-Au alloy formation in the system Cu/[Ag(CNhr +

100

~ ...: III - 75 >-E .. . !: c 50 0

~ c Q) 0 c 25 0 0 ::> <{

oL-__ ~L-__ -J ____ ~ ____ -L ____ ~

-0 .8 -o .g -1 .0 -1. 1 -1. 2

Deposition potential I V vs. SHE

Fig. 12-Concentrati on of Au in Ag-Au alloy deposi ts obtained by electrodeposition at different potential s in the system n-Si( III )/[Ag(CNhr + [Au(CN)2r (pH= 14, c[Ag(CNhr = 5 mM, c[Au(CNhr = 5 !TIM). Deposition time 20 s; T = 298 K (after Marquez K el al. in re f. 76).

[Au(CNhr (PH=13). They showed that under diffu­sion-limited depos ition regime, the Au concentration in the deposited Ag-Au alloy depends linearly on the Au concentration in the electrolyte, the slope of the linear relationship being close to unity. This indicates that under such electrodeposition conditions, the sys­tem behaves as an ideal binary system allowing preparation of Ag-Au alloys in the entire composi­tional range, changing only the concentrations of sil­ver and go ld ions in the electrolyte solution. These researchers have a lso demonstrated that Ag,Aulj AgyAul _y multilayers and multi segment nanowires can be deposited by sequential electrodeposition at differ­ent electrode potentials79

.

Conclusions The present article reviews theoreti al and experi­

mental results related both to the initial and to the ad­vanced stage of electrochemical alloy formation. General expressions are deri ved for the thermody­namic work of formation of binary alloy nucl ei and for their stationary nucleation rate in the case of OPD electrochemical alloy formation. An experimental method to study the size and the composition of criti­cal alloy nuclei obtained at constant supersaturation is proposed and is applied to the case of "mercury­silver" clusters electrodeposited on a platinum single crystal electrode. The method can be used to compare the composition of critical alloy nuclei with that of stable nano-clusters grown under various experimen­tal conditions.

Surface alloy formation processes observed in various UPD systems are discussed in terms of 1l10d-

MILCHEV & STAIKOV: ATOM ISTI C ASPECTS OF ELECTROCHEMICAL ALLOY FORMATION 911

els including place exchange between metal adatoms and substrate atoms, solid state diffusion and nuclea­tion and growth. It is shown that surface-confined alloys can be formed under long-time polarization conditions even in UPD systems with negligible metal-substrate bulk miscibility .

The possibilities for preparation of thin alloy films and multi layers with well defined composition by electrochemical co-deposition are demonstrated on the bas is of experimental results obtained in the bi­nary Ag-Au system.

Acknowledgement Thi s study was completed with the support of the

Bulgarian Mini stry of Education and Science under Contract X-91 2-2000 and the Research Centre JUlich (Germany).

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