The influence of solution composition on the process of lead, cadmium and thallium UPD on (111) oriented silver single crystal surfaces has been investigated ...
229
J. Electroanal. Chem, 288 (1990) 229-243 Elsevier Sequoia S.A., Lausarme
The influence of solution composition on lead, cadmium and thallium underpotential deposition on ( 111) oriented silver single crystal surfaces V.D. JoviC Institute of Technical Sciences of the Serbian Academy of Science and Arts, 11001 Belgrade, P.O. Box 745 (Yugoslavia)
B.M. JoviC Chemical Power Sources Institute, 11080 Zemun Polje, Batajnicki put 23 (Yugoslavia)
A.R. Despid Faculty of Technology and Metallurgy,
University of Belgrade, 11000 Belgrade, Karnegijeva
4 (Yugoslavia)
(Received 11 December 1989; in revised form 26 February 1990)
ABSTRACT The influence of solution composition on the process of lead, cadmium and thallium UPD on (111) oriented silver single crystal surfaces has been investigated by using linear sweep voltammetry and differential capacity measurements. It is shown that the Y’-E curves can be used qualitatively as a criterion for the occurrence of partial charge transfer reactions in the double layer region and the commencement of the faradaic reaction of UPD. The C-E curves obtained in the presence and in the absence of depositing metal cations can give useful information about the nature of species present in the solution and adsorbed onto the substrate surface. It is also demonstrated that the presence of different complexes of cations investigated and the kinetics of their reduction can be responsible for a change in the UPD voltammograms.
INTRODUCTION
The theoretical background for the process of under-potential deposition (UPD) of metals on foreign substrates was given by Gerischer et al. [l] in 1974. More recently, this phenomenon has been studied extensively in a great variety of systems [2-51. These studies have shown that the UPD is connected with the work function difference between the depositing metal and the substrate and that UPD is possible only if this difference is positive. After this theory had been given, the main interest was directed towards the investigation of UPD onto single crystal substrates in 0022-0728/90/$03.50
0 1990 - Elsevier Sequoia S.A.
230
order to find some correlation between the substrate orientation and the characteristics of the monolayer deposition and dissolution processes [6-151. The results obtained were explained mainly by the formation of well ordered superlattice structures with defined equilibrium adsorption properties [6-91, or by the existence of a 2D nucleation and growth mechanism [lo-151. It has also been shown that the shape of the monolayer deposition and dissolution voltammograms depends on the solution composition and the change of the voltammograms has been ascribed to the influence of anion adsorption [12,16]. The aim of the present paper is to elucidate the influence of the solution composition on the process of UPD of different metals (Pb, Cd and Tl) on the (111) oriented silver single crystal surface. For that purpose, in addition to linear sweep voltammetry, the method of differential capacity measurements was used [17]. EXPERIMENTAL
All experiments were carried out under a purified nitrogen atmosphere at 25 f 1” C in a standard electrochemical cell. The procedure of single crystal surface preparation was the one already explained in great detail in previous papers [12,13]. The counter electrode was a large area platinum sheet. In the case of lead and cadmium UPD the reference electrodes were chemically polished wires of pure metal (Cd and Pb 99.999%, Johnson Matthey) in a solution of the corresponding cations, while for the UPD of thallium and the differential capacity measurements in the supporting electrolytes a saturated calomel electrode was used as the reference electrode. All solutions were made of analytical grade chemicals and triply distilled water. Linear sweep voltammetry was performed by using a universal programmer (PAR-M175) and a potentiostat (PAR-M173), while the differential capacity measurements were made by using a potentiostat (PAR-M273 with a built-in sweep generator) and a two-phase sensitive detector (Lock-in Amplifier PAR-M5028). The in-phase and out-of-phase components of the electrode admittance and the linear sweep voltammograms were recorded on a two-channel X-Y,-Y, recorder (Philips PM8033). RESULTS
Figures l-3 present the linear sweep voltammograms of lead, cadmium and thallium monolayer deposition and dissolution in different supporting electrolytes. The main characteristic of these voltammograms is that they are measured vs. the reversible potential of bulk deposition of these metals. It can be seen that the peaks of monolayer deposition and dissolution are the most sensitive to the solution composition in the case of lead UPD (Fig. l), while in the case of thallium UPD the voltammograms obtained in all solutions investigated are almost the same (Fig. 3). In Figs. 4-6, together with the linear sweep voltanmrograms of lead, cadmium and thallium UPD, the differential capacity measurements obtained in the support-
231
0.0
0.1
0.2 E /V(vs.
0.3
OL
Pb*+lPb,
Fig. 1. Linear sweep voltammograms of lead UPD at a sweep rate of 1 mV s-l. Solution composition: (1) 0.05 M Pb(ClO,), + 0.5 M NaC104 +0.005 M HClO,; (2) 0.05 M Pb(CH,COO), + 0.5 M NaCH,COO + 0.01 M HCH,COO; (3) 0.05 M Pb,(C$H,O,), +0.5 M Na,C,H,O,.
ing electrolytes are presented. The common feature of these diagrams is that in all cases the UPD commences at about the same value of differential capacity (about 60 I_IF cme2) and is not dependent on the anions present in the solution. Figures 7 and 8 present linear sweep voltammograms of the deposition and dissolution of the first structure during cadmium monolayer formation and the C-E and Y’-E curves obtained in the supporting electrolytes (2a and la) and in the presence of Cd2+ in the solution (1 and 2). It can be seen that the C-E curves obtained in the presence (2) and in the absence (2a) of cadmium ions are identical in the potential region where UPD does not take place only in the solution containing
232
2-
l-
?
5
0,
? '-7 xf 0
-1
-2
-3 I
I
0.2
1
I
OX
I
0.6
E/V(vs. Cd*+/Cd) Fig. 2. Linear sweep voltammograms of cadmium UPD at a sweep rate of 50 mV s-‘. Solution composition: (1) 0.1 M CdSO, +OS M Na$O, +O.Ol M H,SO,; (2) 0.1 M Cd(ClO,), +0.5 M NaClO, +0.005 M HCIO,; (3) 1 +O.Ol M Na&H,07.
sulphates. This is an indication that together with the Cd’+ ions some other species are present in the solution containing perchlorates. Also, a sharp increase of both C and Y’ is seen to occur when the UPD commences in both solutions. In Fig. 9 are shown C-E curves obtained in the supporting electrolytes containing perchlorate (la), acetate (2a) and citrate (3.a) ions and in the same electrolytes with the addition of 0.01 M lead ions (1, 2 and 3), but only in the UPD region. It can be seen that the C-E curves obtained in the presence (1) and in the absence (la) of lead cations are identical in the potential region before UPD takes place only in the case of perchlorate containing solution.
233 DISCUSSION
Differential capacity measurements The most common way of measuring the differential capacity is to measure only the out-of-phase component of the electrode admittance, neglecting the contribution of an in-phase component and assuming that the charge transfer resistance is very high. Hence, the electrode immersed in the solution of supporting electrolyte can be represented by an ohmic resistance connected in series with the double layer capacitance. In such a case the in-phase component of the impedance represents the ohmic resistance (R,) which cannot change with the potential, while the out-of-phase component is a pure capacitance (l/oCdl) and usually changes with the potential. If
0.0
I
1
0.2
0.1
E/V(vs.TI+/TI) Fig. 3. Linear sweep voltammograms of thallium UPD at sweep rate of 10 mV s-l. tion: (1) 0.01 M Tl,SO, +0.5 M Na,SO, +O.Ol M H,SO,; (2) 0.01 M TlCH,C00+0.5 +O.Ol M HCH,COO; (3) 0.01 M TlCH,COO+0.5 M Na&H,O,.
Solution composiM NaCH,COO
234
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.,
.:’
“...,,. za
,:.
;:
\ ,“.. ‘\
I
-0.6
I
1
-0.4
I
-0.2
0.0
E IV vs. SCE Fig. 4. Linear sweep voltammograms of lead UPD (10 mV s-l) and the C-E curves obtained in the supporting electrolytes (0 = 2 mV s-l; ac ampl. = 5 mV; freq. = 30 Hz). Solution, composition: (la) 0.5 M NaClO, +0.005 M HClO,; (2a) 0.5 M NaCH,COO+O.Ol M HCH,COO; (3a) 0.5 M Na,C,H,O,; (1) la+O.Ol M Pb(ClO,),; (2) 2a+0.01 M Pb(CH,COO),; (3) 3a+0.01 M Pb3(GH507)2.
some charge transfer reaction takes place in the double layer region (e.g. a partial charge transfer corresponding to the anion adsorption), the electrode impedance can be represented by an equivalent circuit in which the charge transfer resistance (R,), pseudo-capacitance (C,,) and the double layer capacitance (C,,) are connected in parallel, while the ohmic resistance ( RO) is cormected in series with them (neglecting the diffusional components in the concentrated electrolytes, > 0.1 M). In such a case neither the in-phase, nor the out-of-phase component of the cell admittance
235
I
1510-
$;
5-
s .7 O% -5 -lO-151
E/Vvs.SCE Fig. 5. Linear sweep voltammograms of cadmium UPD (50 mV s-l) and the C-E curves obtained in the (la) 0.5 supporting electrolytes (u = 2 mV s-l; ac ampl. = 5 mV; freq. = 30 Hz). Solution composition: M Na,SO, +O.Ol M H,SO,; (2a) 0.5 M NaClO, +0.005 M HClO,; (3a) la+O.Ol M Na,C,H,O,; (1) la+O.l M CdSO,; (2) 2a+O.l M Cd(ClO,),; (3) 3a+O.l M CdSO,.
represents pure resistance or pure capacitance; tions:
they are given by following equa-
(1) (2) Recently, Cooper and Harrison [18] have developed a new model for the ion distribution at the metal/electrolyte interface, based on charge redistribution in response to a change in potential across the interface. They have also presented a
,... . r\:
236
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.
3
E/Vvs.SCE Fig. 6. Linear sweep voltammograms of thallium UPD (10 mV supporting electrolytes (u = 2 mV s-‘; ac ampl. = 5 mV; freq. M Na,SO, + 0.01 M H,SO,; (2a) 0.5 M NaCH,COO f0.01 M la+O.Ol M Tl,SO,; (2) 2a+0.01 M TlCH,COO; (3) 3a+0.01
s-l) and the C-E curves obtained in the = 30 Hz). Solution composition: (la) 0.5 HCH,COO; (3a) 0.5 M Na,GH,O,; (1) M TlCH,COO.
suitable way for measuring the differential capacity in the presence of a faradaic reaction [19], which is based on correction of the potential to the true potential at each point by subtracting the ohmic drop according to the following equation: 1 Z($+R,
= &
Ct
+ jw(C,, + C,,)
As can be seen, the in-phase component contains only a resistance (l/R,,)and the out-of-phase component only a capacitance (C,, + C,,). It is very important to take this into account with low concentration electrolytes (K 0.001 M), but in electrolytes with a high concentration of ions (> 0.1 M) the ohmic resistance is very small and can probably be neglected. In such a case the in-phase component would represent a frequency independent charge transfer
237
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i
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cu 'E LY 2 UY s
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1
50
0
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-0.4
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QO
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E/V vs.SCE Fig. 7. (a) UPD (u = admittance (u = 2 mV M H2S04;
Linear sweep voltammogram of deposition and dissolution of the first structure in cadmium 50 mV s-l). (b) In-phase (la and 1) and out-of-phase (2a and 2) components of the electrode obtained in the presence (1 and 2) and in the absence (la and 2a) of Cd*+ in the solution s-l; ac ampl. = 5 mV, freq. = 30 Hz). Solution composition: (la and 2a) 0.5 M Na,SO, +O.Ol (1 and 2) la and 0.1 M CdSO,.
resistance cc,,
only (l/R,,)
+ c,,)Preliminary
and the out-of-phase
component
is a pure capacitance
experiments on polycrystalline silver electrodes in a solution of 0.5 M NaCH,COO have shown that the real component of the cell admittance changes with the frequency, indicating that the ohmic resistance, even if very small (= 3 &I cm2), cannot be neglected, most probably because the charge transfer resistance is also very small (the results of more detailed investigation will be presented soon). This means that the Y’-E and C-E curves do not represent a pure charge transfer
238
0.2
00
0.4
0.6
0.8
1Cl
(b)
100
T
s
LL
2 CD
0
50
I 0
I
I
I
I
-0.6 -0.4 -0.2 00 E/Vvs.SCE
I
0.2
OX
Fig. 8. (a) Linear sweep voltammogram of deposition and dissolution of the first structure in cadmium UPD (u = 50 mV s-l). (b) In-phase (la and 1) and out-of-phase (2a and 2) components of the electrode admittance obtained in the presence (1 and 2) and in the absence (la and 2a) of Cd2+ in the solution ( u = 2 mV s- ‘; ac ampl. = 5 mV; freq. = 30 Hz). Solution composition: (la and 2a) 0.5 M NaC104 + 0.01 M HClO,; (1 and 2) la+O.l M Cd(ClO,),.
and a pure capacitance, but can be used qualitatively as a criterion for the occurrence of partial charge transfer reactions in the double layer region and the commencement of the faradaic reaction of UPD. It is shown in Figs. 7 and 8 that the in-phase component (curves la) changes with the potential in both sulphate and perchlorate solutions, indicating that some partial charge transfer between the adsorbed anions and the substrate occurs during anion adsorption on the (111) oriented silver single crystal surface. This change is of the resistance
239
I
-0.6
I
- 0.4
I
- 0.2
I3.a
E/V vs. SCE Fig. 9. The C-E curves obtained in the absence (la, 2a and 3a) and in the presence (1, 2 and 3) of Pb2+ in the solution (u= 2 mV s-‘; ac ampl. = 5 mV; freq. = 30 Hz). Solution composition: (la) 0.5 M NaCIO, +O.OOSM HClO,; (2a) 0.5 M NaCH,COO+O.Ol M HCH,COO; (3a) 0.5 M Na,GH,O,; (1) la+O.Ol M Pb(ClO,),; (2) 2a+0.01 M Pb(CH&OO),; (3) 3a+0.01 M Pb,(CsH,O,),.
same kind as the change in the out-of-phase component (C). At the same time the curves possess the predicted shape for electrolytes containing high concentrations of ions [l&19]. It is interesting to note that the potential of the well pronounced maximum on the C-E curves is moved in the negative direction and the maximum value of the capacitance increases with increasing energy of anion adsorption (citrate > acetate > perchlorate). Keeping in mind that the maximum value of the capacitance reaches about go-120 PF cme2, it is to be expected that the C-E
240
partial charge transfer reaction occurs in the double layer region on the silver surface. The difference between the UPD reaction and the partial charge transfer reaction is clear from Figs. 7 and 8, where the faradaic current (Y ‘) rises sharply when the UPD process commences (curves 1). It should be emphasized here that the C-E curves of the UPD process cannot be treated as a pure capacitance, because there are two faradaic reactions taking place at the same time, the partial charge transfer reaction of anion adsorption and the faradaic reaction of UPD. The equivalent circuit of such an electrochemical reaction is very complex and the way of measuring C-E curves in the presence of a faradaic reaction proposed by Cooper and Harrison [18] cannot be applied. It should be noted also that the proposed technique [18] can be applied only in the case of electrochemical reactions represented by a simple equivalent circuit containing R,,
R,, , Cad and G, only. The influence of ion adsorption on the process of UPD It is well known that complexing of metal cations with different anions can provoke a change in the reversible potential of the bulk deposition of metals. To avoid the influence of complexing processes on the UPD of metals it is necessary to measure the reversible potential of the investigated metal in the solution investigated, or to use the same metal in a solution of the corresponding cations as reference electrode and compare voltammograms of UPD vs. the reversible potential of the metal investigated. The results of such an investigation are shown in Figs. 1-3. It can be seen that the UPD of lead (Fig. 1) is very sensitive to the solution composition. It commences at more positive potentials in the solution of perchlorate anions than in the solutions of acetate and citrate anions and the shape of the voltammograms is changed. In our first paper [12] (also in the paper of Vilche and Jtittner [16]) this was ascribed to the influence of anion adsorption. If this assumption is correct, the same influence should be expected for cadmium and thallium UPD on the same substrate and in the same solutions. As shown in Figs. 2 and 3, this is not the case. There is some difference between the voltammograms of cadmium UPD in sulphate and perchlorate solutions (curves 1 and 2 in Fig. 2), but not as pronounced as in the case of lead UPD. It is interesting that thallium UPD takes place at almost the same potential in all solutions investigated (Fig. 3). Additional measurements of the differential capacity of the supporting electrolytes used were made and the results are shown in Figs. 4-6 together with the voltammograms of lead, cadmium and thallium UPD. It is interesting to note that in all cases the UPD commences at almost the same value of the differential capacity (about 60 FF cm-*, marked in the figures with C,). This means that adsorption (desorption) of anions on the substrate cannot alone be responsible for the change in the UPD potential of these metals. It seems that a much better way to investigate the influence of ion adsorption is to measure the differential capacity of a (111) oriented silver single crystal electrode
241
in the absence and in the presence of depositing cations. It can be seen from Figs. 7 and 8 that the voltammograms of deposition and dissolution of the first structure (most probably a 20 x 26 structure) in the UPD of cadmium are different in different solutions (sulphates and perchlorates) and that the potential of deposition of that structure is moved in the negative direction in perchlorate containing electrolyte in comparison with the sulphate solution. Knowing that the adsorption energy of ClO; on Ag is lower than the adsorption energy of SO,‘-, such a change is unexpected. Taking into account the C-E curves shown in these figures, it is easy to conclude that in the perchlorate solution some other species (probable some hydroxy complex of Cd) is adsorbed in a very small amount (small peak) on the surface before the UPD commences, which is not the case for sulphate solutions. Comparing the C-E curves obtained in the presence and in the absence of lead ions (Fig. 9), it is clear that before the UPD commences these curves are identical only in the perchlorate containing solution (curves la and 1). The possible explanation for such a behaviour could be connected with the chemistry of lead in the solutions investigated. For the acetate containing solution it could be calculated [20] that there are three different complexes of lead with acetate anions: [Pb(CH,COO)]+ = lo%, [Pb(CH,COO),] = 35% and [Pb(CH,COO),]-= 55%. Also, for the citrate containing solution at pH = 4-7, two different lead complexes are present in the solution [20]: [PbH(C,H,O,)]and [PbH2(C,H,0,),14-. According to this, it seems that only in the perchlorate containing solution are all lead cations free (as Pb2’), while in the other solutions some of the existing complexes are adsorbed (desorbed) on the substrate surface before the UPD commences, which can provoke the change of the reversible potential of monolayer deposition in the negative direction. The mechanism and kinetics of reduction of complexes could also influence the process of UPD of metals. In our previous paper [13], we found that the potentiostatic j-t transients of lead monolayer deposition on the (111) oriented silver single crystal surface are fastest in perchlorate containing solution (Fig. 4 of ref. 13). The transients presented in that figure were obtained at high overpotentials and were not very sensitive to the solution composition. If one compares the j-t transients obtained at very low overpotentials (up to - 20 mV vs. Ef”), it can be seen that in perchlorate containing solution deposition of a monolayer is twenty times faster than in citrate containing solution for the same value of overpotential and that a full monolayer of lead is deposited in more than 100 s at an overpotential of - 2 mV vs. Ef” in the citrate containing solution [15]. Keeping in mind that cadmium and thallium do not form complexes in the solutions investigated [20] and that the influence of the solution composition is more pronounced in the case of lead UPD, it seems reasonable to ascribe this influence to the adsorption (desorption) of complexes and to possible limitations connected with their reduction mechanisms. It is obvious that more information about the stability constants of the complexes present in the solution and about the mechanism and kinetics of their reduction is
242 TABLE 1 Dependence of AE,!” and A{ on the anions present in the solution for lead UPD on a (111) oriented silver single crystal surface Anion
A.EF/mV
ClO,CH,COOC,H,O;-
147 133 123
(vs. E,)
AC I A{//.tF cm-’ AE 29 190 161
= 50mV (vs. Er)
AE = 10 mV (vs.
E,)
33 110 115
needed for a better understanding of the influence of solution composition on the process of metal UPD on foreign substrates. One of the theoretical treatments of UPD processes, given by Pangarov [21] and based on the “half lattice position”, predicts that the reversible potential of monolayer deposition (Ef”) should depend on the difference between the energy of anion adsorption on top of a monolayer (lr) and on a substrate (la), A[ = {i - &,. According to the theory, if the value of Al increases, the reversible potential of monolayer deposition should become more negative, i.e. AE,!! = Erm - E, (E, is the reversible potential of bulk deposition) should decrease. Voltammograms of lead UPD are very convenient for such an analysis because in all solutions investigated the monolayer is completed at about 70 mV vs. E, (AE = 70 mV vs. Pb’+/Pb). Assuming that after the monolayer formation no faradaic reaction takes place and that the value of the differential capacity can be used as a measure of the anion adsorption energy (at least arbitrarily), it is possible to calculate A5 = AC for different potentials [17] from the C-E curves shown in Fig. 9 (AS, 5, and 5, are marked in the figure for the C-E curves obtained in citrate solution). The results of such an analysis are given in Table 1. It can be seen that there is good agreement between AErm and A{ only at a very low value of AE (10 mv). ACKNOWLEDGEMENT
The authors are indebted to Prof. R. Parsons (Department of Chemistry, University of Southampton) for helpful discussions and to the Research Fund of SR Serbia for financial support of this work. REFERENCES 1 H. Gerischer, D.M. Kolb and M. Przasnyski, Surf. Sci., 43 (1974) 662. 2 D.M. Kolb in H. Gerischer and C.W. Tobias (Eds.), Advances in Electrochemistry and Electrochemical Engineering, Vol. 11, John Wiley, New York, 1978, p. 125. 3 W.J. Lorenz, H.D. Herrman, H. Wtthrich and F. Hilbert, J. Electrochem. Sot., 121 (1974) 1167. 4 K. Jiittner and W.J. Lorenz, Z. Phys. Chem. NF, 122 (1980) 163.
243 5 R.R. AdZif in H. Gerischer (Ed.), Advances in Electrochemistry and Electrochemical Engineering, Vol. 13, John Wiley, New York, 1984, p. 159. 6 K. Jiittner and W.J. Lorenz, EIectrochim. Acta, 21 (1976) 117; 23 (1978) 741. 7 M. Khmmeck and K. Jtittner, EIectrochim. Acta, 27 (1982) 83. 8 H. Bort, K. Jttttner and W.J. Lorenz, J. EIectroansI. Chem., 90 (1978) 413. 9 K. Takayanagy, D.M. Kolb, K. Kambe and G. LempfuhI, Surf. Sci., 100 (1980) 40. 10 A. Bewick and B. Thomas, J. Electroanal Chem., 65 (1975) 911; 70 (1976) 239; 84 (1977) 127; 85 (1977) 329. 11 A. Bewick, J. Jovicevic and B. Thomas, Faraday Symp. Chem. Sot., 12 (1978) 24, 165. 12 J.N. Jovicexic, V.D. Jovic and A.R Despic, Electrochim. Acta, 29 (1984) 1625. 13 V.D. Jovic, J.N. JovZevic and A.R. Despic, Electrochim. Acta, 30 (1985) 1455. 14 J.N. Jovi&-ic, PhD Thesis, University of Southampton, 1978. 15 V.D. Jovic, PhD Thesis, University of Belgrade, 1985. 16 J.R. Vilche and K.J. Jlittner, EIectrochim. Acta, 32 (1987) 1567. 17 B.M. Jovic, V.D. Jovic and A.R. Despic, 11th Yugoslav Symposium of Electrochemistry, Rovinj, 1989, Ext. Abstr., p. 99. 18 I.L. Cooper and J.A. Harrison, Electrochim. Acta, 29 (1984) 1147. 19 I.L. Cooper and J.A. Harrison, Electrochim. Acta, 29 (1984) 1165. 20 Gmehns Handbuch der Anorganische Chemie, 8. Aufl., Verlag Chemie, GmBH, Weinheim/Bergstr., 1970. 21 N. Pangarov, Electrochim. Acta, 28 (1983) 763.
