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Combined in situ SPM and EIS studies of Pb UPD on Ag(111) and Ag(100)
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Electrochimica Acta, Vol. 43, Nos 19±20, pp. 2863±2873, 1998 # 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain S0013-4686(98)00027-9 0013±4686/98 $19.00 + 0.00
Combined in situ SPM and EIS studies of Pb UPD on Ag(111) and Ag(100) J. Sackmann, A. Bunk, R. T. PoÈtzschke, G. Staikov* and W. J. Lorenz Institute of Physical Chemistry and Electrochemistry, University of Karlsruhe, D-76128 Karlsruhe, Germany (Received in Newcastle 21 January 1998) AbstractÐExperimental results obtained by combined in situ scanning probe microscopy (SPM) and electrochemical impedance spectroscopy (EIS) studies of underpotential deposition (UPD) of Pb on Ag(111) and Ag(100) are correlated and interpreted in terms of the kinetics of local electrode processes. Exchange current densities at dierent underpotentials are related to dierent low-dimensional (1D and 2D) Pb phases directly observed by in situ SPM. First order phase transitions between expanded (gas-like) and condensed (liquid- or solid-like) 2D Pb phases as well as surface alloy formation processes are taken into account for the interpretation of impedance data. Relatively simple conditions are obtained on Ag(111) substrates where the formation of a condensed Pb monolayer starting exclusively at monatomic steps is observed by in situ SPM at low supersaturations with respect to the condensed 2D Pb phase. The analysis of SPM and EIS data indicates that the lateral growth of the condensed Pb monolayer preferentially occurs via a direct transfer of Pb2+ from the electrolyte to the step edges. # 1998 Published by Elsevier Science Ltd. All rights reserved Key words: UPD, EIS, SPM, low-dimensional metal phases, exchange current densities.
INTRODUCTION The kinetics of electrode processes involving phase formation phenomena are usually studied using conventional electrochemical methods such as transient techniques in the time domain and electrochemical impedance spectroscopy (EIS) in the frequency domain. These methods provide integral information on the overall electrode kinetics, but lack information on the local mechanism of electrode processes as well as the local structure and morphology of electrochemically formed phases. The introduction of scanning probe microscopy (SPM) techniques such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM) to electrochemical environments opened a way for in situ surface studies of electrode processes on an atomic level. These studies provide new important information necessary to develop appropriate physical models for a correct interpretation of experimental electrochemical data. *Author to whom correspondence should be addressed.
Correlations between in situ SPM data and cyclic voltammetric measurements were already performed by many authors in various systems with substrate surface reconstruction, adsorption and desorption of anions and underpotential deposition (UPD) and dissolution of metals [1±3]. These correlations mainly supply information on the thermodynamic and structural aspects of interfacial properties and reactions. A powerful electrochemical method for investigations of the kinetic aspects of electrode processes is EIS [4, 5]. During the last two decades EIS was intensively applied to investigate the kinetics and mechanism of electrocrystallization of metals, i.e. overpotential deposition (OPD) of metals on native substrates [1]. Dierent simpli®ed theoretical models for a quantitative transfer function analysis were developed and discussed by various authors [5±9]. These models were based on more or less hypothetical assumptions of the atomic structure of the electrode surface and the local mechanism of electrodeposition reactions. In most cases, however, crystal imperfections and surface inhomogeneities strongly in¯uence the local mechanism of the elec-
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trocrystallization process and the resulting surface morphology and therefore signi®cantly complicate the interpretation of experimental results [1]. A better understanding of the role of surface inhomogeneities in the kinetics and mechanism of electrochemical phase formation processes on an atomic level can be achieved by a combination of EIS measurements and in situ SPM. Recent in situ SPM studies showed that UPD of metals on single crystal substrates is an appropriate process for such combined studies. In contrast to the large number of EIS studies on OPD of metals on native substrates EIS investigations of UPD of metals are reported up to now only in two papers [10, 11]. These ®rst EIS investigations indicated an in¯uence of surface inhomogeneities but were interpreted in terms of a continuous adsorption process excluding ®rst order phase transitions leading to the formation of condensed metal UPD phases. However, in situ SPM studies in many UPD systems have given direct evidence for such ®rst order phase transitions which have to be taken into account for the interpretation of experimental data. The aim of this paper is to discuss the kinetic aspects and mechanism of underpotential metal deposition and dissolution correlating experimental results obtained by in situ STM and EIS in the model systems Ag(111)/Pb2+ and Ag(100)/Pb2+.
THEORETICAL CONSIDERATIONS UPD of a metal (Me) on a foreign substrate (S) is caused by a strong Me±S interaction and occurs at electrode potentials, E, more positive than the Nernst equilibrium potential, E3D Me , of the 3D Me bulk phase given by E > E3D Me E o3D Me
RT a z ln Me a3D Me zF
1
where E o3D Me denotes the standard potential, aMez the activity of Mez+ in the electrolyte and a3D Me the activity of the 3D Me phase, which is a constant equal to unity for a pure 3D Me bulk phase The potential dierence (a3D Me 1). DE E ÿ E3D Me > 0 represents the underpotential which is directly related to the undersaturation with respect to the 3D Me bulk phase (Dm ÿzFDE<0). Me UPD is an electrosorption process with a charge-coverage stoichiometry given by the socalled electrosorption valency [1]: 1 @q 1 @m 2 g F @G E F @E G where q is the relative ionic charge density, G the relative surface excess concentration of the adsorbed Me adatoms (Meads), and m the chemical potential of Mez+ in the electrolyte. In absence of cosorption and/or competitive sorption phenomena,
the electrosorption valency is equal to the ionic charge of Mez+ (g = z) and the Me UPD process can be described by a quasi-Nernst equation [1]: E E o3D Me
RT aMez RT ln ln f G E3D Me ÿ zF zF f G 3
where the function f(G) is related to the adsorption isotherm and depends on the adatom±adatom (Meads±Meads) and adatom±substrate (Meads±S) interactions and the crystallographic structure and heterogeneity of the substrate surface. Substrate surface inhomogeneities are characterized by signi®cant dierences in the adatom binding energy and can induce formation of various lowdimensional 1D and 2D Me phases with dierent energetics and structures in well separated underpotential ranges [1, 12, 13]. For instance a preferential decoration of 1D surface inhomogeneities such as monatomic steps by Meads chains can be considered as 1D Me phase formation [Fig. 1(b)]. The formation of 2D Me phases occurs on atomically smooth surface terraces [Fig. 1(c)]. Low-dimensional Me phases can be either expanded (gas-like) or condensed (liquid-like or solid-like).The formation of expanded low-dimensional Me phases is characterized by a continuous change of the equilibrium Me surface coverage G with the actual electrode potential E. Condensed low-dimensional Me phases are characterized by equilibrium potentials described by Nernst-type equations similar to equation (1) and are usually formed by a ®rst order phase transition involving nucleation and growth phenomena [1]. A ®rst order phase transition is re¯ected in a discontinuity of the G(E) adsorption isotherm at the corresponding equilibrium potential. It is well known [1] that the equilibrium potential E3D Me of a 3D Me bulk crystal phase de®ned by equation (1) is determined by the exchange frequency of 3D kink atoms [Fig. 1(a)]. Similarly, the equilibrium potentials E1D Me and E2D Me of condensed in®nitely large 1D and 2D Me phases formed at monatomic steps and atomically ¯at terraces of a foreign substrate are determined by the corresponding exchange frequencies of 1D and 2D kink atoms (Fig. 1(b) and (c)]. The concentration of kink sites niD kink (kinks cmÿ2) does not aect the equilibrium potential but signi®cantly in¯uences the partial exchange current density of kink atoms given by [1, 13]: io, iD kink zeo o, iD kink EiD Me niD kink zeo o, iD kink EiD Me
LS diD kink
where o o,iD kink EiD Me is the exchange frequency of iD kink atoms (i = 1,2,3) at the corresponding equilibrium potential EiD Me , LS (cm cmÿ2) denotes the density of monatomic steps, i.e. the total step
Combined in situ SPM and EIS studies of PbUDP
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Fig. 1. Schematic representation of iD kink atoms (i = 1,2,3) determining the equilibria of the corresponding condensed iD Me phases: (a) 3D Me phase, (b) 2D Me phase, and (c) 1D Me phase
length per unit surface area, and diD kink is the mean distance between iD kinks. As seen from equation (4) the partial exchange current density io, iD kink of iD kink atoms depends on the density of monatomic steps. Assuming that the lateral growth of a condensed monolayer is only controlled by a direct charge transfer (dt) of Mez+ to 2D kink sites the corresponding lateral growth rate is related to io, iD kink and for low supersaturations zF E ÿ E2D Me with respect to the condensed 2D Me phase is given by v2D, dt
io, 2D kink zF E ÿ E2D Me q2D Me LS RT
5
where q2D Me (As cmÿ2) denotes the charge amount per unit area for the formation of a 2D Me monolayer.
The overall exchange current density, io, S=Mez , in the UPD range depends on the underpotential DE and is given by the sum of the partial exchange current densities, io,j , related to dierent low-dimensional expanded or condensed Me phases contributing to the corresponding equilibrium: X io,j DE io,j DE 6 j
If the equilibrium involves an expanded Me phase with a low adatom concentration and a condensed iD Me phase with a relatively high concentration of iD kink atoms (high step density), the contribution of the partial exchange current density of the expanded Me phase can be neglected and the overall exchange current density will only be determined by the corresponding partial exchange current density of iD kink atoms:
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7
Generally, dierent steps must be considered in the kinetics of Me UPD: bulk diusion of Mez+ in the electrolyte, charge transfer of Mez+, surface diffusion of Meads, and ®rst or higher order phase transitions. In a ®rst model approach, the substrate surface was considered as quasi-homogeneous and the kinetics of the electrosorption UPD process were assumed to be controlled by Mez+ charge transfer and/or semiin®nite-linear bulk diusion of Mez+ excluding surface diusion of Me adatoms and phase transitions [1]. Later, a more sophisticated kinetic model including monatomic steps as 1D surface inhomogeneities was developed [1, 10]. In this model bulk diusion of Mez+ was neglected, while three dierent ¯uxes were considered: charge transfer of Mez+ at monatomic steps and on atomically ¯at terraces as well as a superimposed surface diusion leading to a leveling of Me adatom gradients. All these models, however, did not consider the changes in the condition of the electrode caused by the formation of dierent low-dimensional Me phases at dierent underpotentials as discussed above. Assuming g = z (no cosorption and/or competitive sorption phenomena), the transfer function of the overall interfacial impedance of an electrosorption UPD process controlled by Mez+ charge transfer and semiin®nite-linear bulk diusion of Mez+ can generally be described by: ÿ1 1 Z s sCdl 8 ÿ ÿ1 Rct sCads ZT s where s is the Laplace variable, Cdl ÿ @ q=@ E G is the double layer capacitance, Rct RT=zF 1=io, S=Mez the charge transfer resistance, Cads ÿzF @ G=@ E m the adsorption capacitance (pseudo-capacitance), and ZT s the mass transport impedance. In this approach, the charge transfer resistance is related to the overall exchange current density, io, S=Mez , containing the contributions of dierent low-dimensional Me phases formed at dierent underpotentials. Therefore, an exact interpretation of impedance data requires an additional direct information on low-dimensional Me phases which can be obtained by a combination of EIS measurements with in situ SPM. EXPERIMENTAL In situ SPM and EIS studies were performed at T = 295 K in the systems Ag(111)/Pb2+, ClOÿ 4, H+ (cPb2 = H+ and Ag(100)/Pb2+, ClOÿ 4, 5 10ÿ6 mol cmÿ3, pH = 2). Previous electrosorption valency measurements in these systems showed g = z = 2, indicating that cosorption and competitive sorption phenomena can be excluded [1]. The
electrolyte solutions containing 5 mM Pb(ClO4)2, 0.5 M NaClO4, and 10 mM HClO4 were prepared from suprapure chemicals and fourfold quartz-distilled water and deaerated by oxygen free nitrogen prior and during each experiment. Pt and Pb were used as counter and reference electrodes, respectively. Ag(111) and Ag(100) surfaces were prepared by mechanical and subsequent chemical polishing as previously described [14]. In situ STM characterization shows that chemically polished substrate surfaces preferentially consist of atomically smooth terraces separated by monatomic steps. An average step density in the range 1.5 106 cmÿ1
Combined in situ SPM and EIS studies of PbUDP strongly compressed due to the signi®cant crystallographic Pb±Ag mis®t and the strong Pbads±Ag attraction. On Ag(111) the condensed hcp Pb monolayer is incommensurate, isotropically compressed and rotated with respect to the substrate [1, 14] whereas on Ag(100) the condensed hcp Pb monolayer is anisotropically compressed and in higher order registry with the substrate [1, 18]. In situ STM images in Fig. 2 show morphological changes during the UPD of Pb on a stepped Ag(111) substrate [Fig. 2(b)]. After the initial step decoration corresponding to peak A1 of the cyclic voltammogram [Fig. 2(a)] an appearance of a growth front of the condensed Pb monolayer starting at monatomic steps is clearly visible in the in
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situ STM image obtained during the potential scan in peak A2 [Fig. 2(c)]. The formation and dissolution of the corresponding condensed 2D Pb phase occurs via a ®rst order phase transition which is re¯ected in the appearance of a hysteresis between the deposition (A2) and dissolution (D2) peaks in the cyclic voltammogram [Fig. 2(a)]. The equilibrium potential of the condensed 2D Pb phase is located between the peaks A2 and D2 (DE = 150 mV) and is given by E2D Pb E3D Pb 150 mV
9
The formation and dissolution of the condensed Pb monolayer occur at E E2D Pb , respectively. As demonstrated by Siegenthaler and Ammann [16, 17] the corresponding propagation
Fig. 2. Morphology changes during Pb UPD on a stepped Ag(111) substrate in the system Ag(111)/Pb2+. (a): Cyclic voltammogram at vdE/dtv = 1 mV s-1; (b): In situ STM image of the bare substrate; (c): In situ STM image showing the change in the surface morphology during the potential scan in the range of peak A2. The intersected lines indicate the location of step edges on the bare substrate. STM scan size: 70 nm 70 nm
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rate can be measured by a technique involving a potential step polarization and a simultaneous in situ STM imaging in a linescan mode. In the system studied values ranging between 5 and 10 nm sÿ1 were measured for the lateral growth rate of the condensed Pb monolayer at potentials E2D Pb ÿ 10 mVRE350 mV) and is associated with a formation of pits with monatomic depth [19]. In situ STM images in Fig. 3 show changes of surface morphology during underpotential deposition and dissolution of Pb on a stepped Ag(100) substrate. The initial step decoration at high underpotentials [Fig. 3(b)] is followed by a slow lateral growth of a condensed Pb monolayer starting at monatomic steps [Fig. 3(c)]. This growth starts in peak A1 of the cyclic voltammogram but continues in peak A2 where also a formation of 2D islands on atomically smooth terraces is observed [Fig. 3(d)]. These islands are mainly formed on top of the ®rst condensed Pb monolayer and do not grow or dissolve in the underpotential interval 0 mV < DE < 100 mV. The dissolution of the 2D Pb islands occurs in the peak D2 [Fig. 3(e)]. The dissolution of the ®rst condensed Pb monolayer also starts in peak D2 but continues with a very low dissolution rate in peak D1. The dissolution process leads to pronounced changes of the shape of monatomic steps [Fig. 3(f)]. These morphological changes and the observed limited 2D growth and dissolution are most probably related to a surface alloy formation rapidly occurring at monatomic steps. Extended (long-time) polarization do not in¯uence the step morphology in this system. Typical impedance spectra obtained during the underpotential deposition and dissolution of Pb on Ag(111) and Ag(100) substrates are shown in Figs 4 and 5, respectively. Impedance spectra measured in both large conventional and small STM/AFM electrochemical cells did not show any signi®cant dierences in the applied frequency range. Impedance
data are corrected for the electrolyte resistance and represent only the interface impedance. At high underpotentials DE>250 mV the interface impedance is characterized by a capacitive behavior which is representative for an ideally polarizable electrode [Fig. 4(b)]. In the UPD range a semicircle appears in the interface impedance spectra at high frequencies which is obviously related to the Pb2+ charge transfer reaction. Experimental impedance spectra were ®tted by the transfer function of equation (8) using for ZT(s) a Warburg-impedance p ZT s sW = s with pa Warburg coecient sW RT = z2 F 2 cMez D as a ®rst approximation. The optimum ®t data are summarized in Tables 1 and 2. The ®t shows that bulk diusion of Pb2+ has to be considered at low frequencies. However, diusion coecients estimated from the Warburg coecients in Table 1 and Table 2 are three to four orders of magnitude too small which may be attributed to a local Pb2+ charge transfer only at surface inhomogeneities leading to a bulk diusion towards a partially active electrode surface [8, 9, 11]. Thus, an analysis of the low frequency impedance data would require a separation of the transport impedance, ZT(s), into a Warburgimpedance, ZW(s), and a lateral transport contribution, sZs(s) [8, 9]. The coecient s describes the ratio of blocked to active areas of the electrode surface. In the systems under investigation kinks can be considered as active sites for the charge transfer. s values between 100 and 1000 are derived from the experimentally obtained step density, resulting in an increase of the diusion coecients by two to three orders of magnitude. In addition, a concentration gradient within the diusion layer leads also to an increase of the diusion coecient. A quantitative analysis of the low frequency parts of impedance spectra, however, requires a development of an appropriate physical mass transport model including surface inhomogeneities and non-linear diusion. Charge transfer resistances Rct and corresponding overall exchange current densities io, Ag=Pb2 obtained from the high frequency parts of impedance spectra are related to the partial exchange current densities of low-dimensional Pb phases contributing to the equilibrium at the given potential. In the system Ag(111)/Pb2+ impedance spectra and overall exchange current densities obtained at an underpotential DE = 150 mV corresponding to the equilibrium potential of the condensed 2D Pb phase E2D Pb (cf. equation (9)) depend strongly on the polarization routine (cathodic or anodic sweep) used for adjusting this potential. This behavior re¯ects the occurring ®rst order phase transition. If E2D Pb is adjusted by a cathodic potential sweep (c.s.) a formation of the condensed 2D Pb phase cannot occur and only an 1D Pb phase (decoration of monatomic steps) and an extremely diluted expanded (gas-like) 2D Pb phase are formed.
Fig. 3. Morphology changes during Pb UPD on a stepped Ag(100) substrate. (a): Cyclic voltammogram at vdE/ dtv = 1 mV s-1; (b): Step decoration at DE = 200 mV. The dotted lines indicate the location of the step edges on the bare substrate; (c): Formation of the hcp Pb monolayer at DE = 120 mV; (d) Formation of 2D Pb islands at 0 mV < DE < 100 mV; (e) Dissolution of the 2D Pb islands and the hcp Pb monolayer during an anodic potential sweep around peak D2; (f) Substrate surface after stripping the underpotential deposit at DE = 250 mV. STM scan size: 200 nm 200 nm 2869
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Fig. 4. Interface impedance behavior in the system Ag(111)/Pb2+: Cyclic voltammogram (a), impedance spectra at DE = 300 mV (b), at_DE = 150 mV after a cathodic potential sweep (c), at DE = 20 mV (d), and at DE = 150 mV after an anodic potential sweep (e). The inserts in the impedance spectra schematically represent the condition of the substrate surface
Combined in situ SPM and EIS studies of PbUDP
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Fig. 5. Interface impedance behavior in the system Ag(100)/Pb2+: Cyclic voltammogram (a), impedance spectra after a cathodic potential sweep at DE = 160 mV (b), and at DE = 125 mV (c), at DE = 20 mV (d), and after an anodic potential sweep at DE = 125 mV (e) and DE = 160 mV (f). The inserts in the impedance spectra schematically represent the condition of the substrate surface
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Table 1. Fit parameters describing the EIS data in the UPD system Ag(111)/Pb2+ according to equation (8) DE (mV) 20 125 150 c.s. 150 a.s. 170
Cdl (mF cmÿ2)
Cads (mF cmÿ2)
sW (O cm2 sÿ1/2)
Rct (O cm2)
io, Ag=Pb2 (mA cmÿ2)
1922 2222 3322 3222 3822
2502 50 3502 50 8002 50 5002 50 8502 50
5502 10 5002 10 3502 10 6502 10 5502 10
3.2 20.1 3.7 20.1 8.1 20.1 4.3 20.1 8.7 20.1
4 20.15 3.5 20.1 1.6 20.05 3 20.1 1.5 20.05
Table 2. Fit parameters describing the EIS data in the UPD system Ag(111)/Pb2+ according to equation (8) DE (mV) 20 80 125 125 160 160
c.s. a.s. a.s. a.s.
Cdl (mF cmÿ2)
Cads (mF cmÿ2)
sW (O cm2 sÿ1/2)
Rct (O cm2)
io, Ag=Pb2 (mA cmÿ2)
2022 2122 2622 2722 3022 3122
2502 50 3502 50 3202 50 30002 50 22002 50 22002 50
2002 10 2502 10 502 10 602 10 702 10 1002 10
2.5 20.1 2.5 20.1 3.8 20.1 8.2 20.1 6.8 20.1 14 20.1
5.2 20.2 5.2 20.2 3.4 20.1 1.6 20.05 1.9 20.05 0.9 20.01
Neglecting the contribution of the expanded gaslike 2D Pb phase the overall exchange current density in Table 1 corresponding to this polarization routine and this potential can be attributed only to the partial exchange current density of the 1D Pb phase. If, however, E2D Pb is adjusted after an anodic potential sweep (a.s.) the surface is covered by the condensed Pb monolayer and the overall exchange current density only represents the partial exchange current density of 2D kink atoms of the condensed 2D Pb phase: io, Ag=Pb2 E2D Pb io, 2D kink E2D Pb . As seen from Table 1 the partial exchange current density of 2D kink atoms is about two times larger than the partial exchange current density of the 1D Pb phase. Relatively large ac perturbations at the equilibrium potential E2D Pb lead to a signi®cant in¯uence of the ac amplitude on the low frequency behavior of the interface impedance (Fig. 6). This in¯uence is obviously connected with the formation and dissolution of the condensed Pb monolayer during the impedance measurements. In the applied frequency range amplitude independent impedance spectra were only observed at this potential while applying ac amplitudes smaller than 1 mV. The value of io, 2D kink determined by the impedance measurements can be used for an estimation of the propagation rate of the condensed 2D monolayer on Ag(111) from equation (5). With io, 2D kink =3 mA cmÿ2, q2D Me=0.3 mC cmÿ2, LS 2 106 cmÿ1, and E ÿ E2D Me= ÿ 2 mV a propagation rate of v2D dt17 nm sÿ1 is obtained which stands is good agreement with the values attained by in situ STM in the same potential range. These results indicate that the lateral growth of the condensed Pb monolayer on Ag(111) prefer-
entially occurs via a direct transfer of Pb2+ from the electrolyte to the growing step edges. In the system Ag(100)/Pb2+ impedance spectra measured at high underpotentials (120 mV < DE < 220 mV) corresponding to the peaks A1/ D1 strongly depend on the polarization routine (cathodic or anodic sweep) used for adjusting the potential (Fig. 5(b), (c), (e), and (f)]. This is re¯ected in dierent overall exchange current densities obtained at a given underpotential adjusted by dierent polarization routines (Table 2). An interpretation of this behavior, however, is very dicult due to the simultaneous formation of dierent
Fig. 6. In¯uence of the amplitude of the ac perturbation on the interface impedance spectra in the system Ag(111)/ Pb2+ at DE = 150 mV; ac amplitude: 1 mV (.); 5 mV (T); 10 mV (W)
Combined in situ SPM and EIS studies of PbUDP low-dimensional Pb phases and aging phenomena observed by in situ STM in this UPD range (Fig. 3). At low underpotentials (0 mV < DE < 100 mV) the impedance spectra do not depend on the underpotential and the polarization routine used for adjusting [Fig. 5(d)]. The higher overall exchange current density obtained in this UPD range can be related to the increased step density due to the 2D Pb islands formed in peak A2. This indicates that at these underpotentials the charge transfer process is probably preferentially located at the step edges of the condensed Pb monolayer. CONCLUSIONS The results demonstrate the great advantages of a simultaneous in situ SPM and EIS study of phase formation processes taking place during underpotential deposition and dissolution of metals. The EIS data and in situ SPM results obtained in the system Ag(111)/Pb2+ are related to the formation and dissolution of dierent low-dimensional (1D and 2D) Pb phases at dierent underpotentials. The analysis of experimental data indicate that the propagation of the condensed Pb monolayer formed via a ®rst order phase transition preferentially occurs by a direct transfer of Pb2+ to the step edges. The interpretation of impedance data in the system Ag(100)/Pb2+ is more complicated. In this system the UPD process is very complex and includes a number of phase formation processes accompanied by aging phenomena. Thus, a correlation between in situ SPM and EIS results is very dicult and requires additional studies. ACKNOWLEDGEMENTS The authors gratefully acknowledge the ®nancial support of this work by ``Deutsche Forschungsgemeinschaft'' (DFG), ``Arbeitsgemeinschaft Industrieller Forschungs-vereinigungen'' (AIF) and ``Bundesministerium fuÈr Wirtschaft'' (BMWi). The authors also thank U. Schmidt and S. Vinzelberg, for successful scienti®c cooperation.
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REFERENCES 1. E. Budevski, G. Staikov and W. J. Lorenz, Electrochemical Phase Formation and GrowthÐAn Introduction to the Initial Stages of Metal Deposition. VCH, Weinheim, 1996. 2. J. Lipkowski and P. N. Ross (ed.), Structure of Electri®ed Interfaces. VCH, Weinheim, 1993. 3. H. Siegenthaler and A. Gewirth, Nanoscale Probes of the Solid/Liquid Interface. NATO ASI Series E: Applied Sciences, Vol. 288, Kluwer Academic Publishers, Dordrecht, 1995. 4. C. Gabrielli, Use and Applications of Electrochemical Impedance Techniques. Technical Report, Issue A, Schlumberger Technologies, Instruments Division, Hampshire, 1990. 5. R. D. Armstrong, M. F. Bell and A. A. Metcalfe, in ElectrochemistryÐSpecialist Periodical Reports, The Chemical Society, ed. H. R. Thirsk. Alden Press, Oxford, 1978. 6. C. Cachet, I. Epelboin, M. Keddam and R. Wiart, J. Electroanal. Chem. 100, 745 (1979). 7. R. Wiart, Electrochim. Acta 35, 1587 (1990). 8. J. Hitzig, J. Titz, K. JuÈttner, W. J. Lorenz and E. Schmidt, Electrochim. Acta 29, 287 (1984). 9. E. Schmidt, J. Hitzig, J. Titz, K. JuÈttner and W.J. Lorenz, Electrochim. Acta 31, 1041 (1986). 10. K. Engelsmann, W. J. Lorenz and E. Schmidt, J. Electroanal. Chem. 114, 11 (1980). 11. M. Klimmeck and K. JuÈttner, Electrochim. Acta 27, 83 (1982). 12. W. J. Lorenz and G. Staikov, in Electrochemically Deposited Thin Films III, ed. M. Paunovic and D. A. Scherson, PV 96-19, p. 171. The Electrochemical Society Proceedings Series, Pennington, NJ, 1997. 13. G. Staikov, W. J. Lorenz and E. Budevski, in Frontiers of Electrochemistry, Vol. V, Imaging of Surfaces and Interfaces, ed. J. Lipkowski and P. N. Ross. VCH Publishers, New York, in preparation. 14. W. Obretenov, U. Schmidt, W. J. Lorenz, G. Staikov, E. Budevski, D. Carnal, U. MuÈller, H. Siegenthaler and E. Schmidt, J. Electrochem. Soc. 140, 692 (1993). 15. D. Carnal, P. I. Oden, U. MuÈller, H. Siegenthaler and E. Schmidt, Electrochim. Acta 40, 1223 (1995). 16. H. Siegenthaler and E. Ammann, 189th Meeting of The Electrochemical Society. Abstract No. 1072, Los Angeles, 1996. 17. E. Ammann, Master thesis, University of Bern, 1995. 18. U. Schmidt, S. Vinzelberg and G. Staikov, Surf. Sci. 348, 261 (1996). 19. W. J. Lorenz, G. Staikov and H. Siegenthaler, in Proceedings of the International Symposium on Pits and Pores. ed. P. Schmucki, D. J. Lockwood, H. Isaacs and A. Bsiesy (eds.), PV.97-7, p. 493. The Electrochemical Society, Pennington NJ, 1997.
Electrochimica Acta, Vol. 43, Nos 19±20, pp. 2863±2873, 1998 # 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain S0013-4686(98)00027-9 0013±4686/98 $19.00 + 0.00
Combined in situ SPM and EIS studies of Pb UPD on Ag(111) and Ag(100) J. Sackmann, A. Bunk, R. T. PoÈtzschke, G. Staikov* and W. J. Lorenz Institute of Physical Chemistry and Electrochemistry, University of Karlsruhe, D-76128 Karlsruhe, Germany (Received in Newcastle 21 January 1998) AbstractÐExperimental results obtained by combined in situ scanning probe microscopy (SPM) and electrochemical impedance spectroscopy (EIS) studies of underpotential deposition (UPD) of Pb on Ag(111) and Ag(100) are correlated and interpreted in terms of the kinetics of local electrode processes. Exchange current densities at dierent underpotentials are related to dierent low-dimensional (1D and 2D) Pb phases directly observed by in situ SPM. First order phase transitions between expanded (gas-like) and condensed (liquid- or solid-like) 2D Pb phases as well as surface alloy formation processes are taken into account for the interpretation of impedance data. Relatively simple conditions are obtained on Ag(111) substrates where the formation of a condensed Pb monolayer starting exclusively at monatomic steps is observed by in situ SPM at low supersaturations with respect to the condensed 2D Pb phase. The analysis of SPM and EIS data indicates that the lateral growth of the condensed Pb monolayer preferentially occurs via a direct transfer of Pb2+ from the electrolyte to the step edges. # 1998 Published by Elsevier Science Ltd. All rights reserved Key words: UPD, EIS, SPM, low-dimensional metal phases, exchange current densities.
INTRODUCTION The kinetics of electrode processes involving phase formation phenomena are usually studied using conventional electrochemical methods such as transient techniques in the time domain and electrochemical impedance spectroscopy (EIS) in the frequency domain. These methods provide integral information on the overall electrode kinetics, but lack information on the local mechanism of electrode processes as well as the local structure and morphology of electrochemically formed phases. The introduction of scanning probe microscopy (SPM) techniques such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM) to electrochemical environments opened a way for in situ surface studies of electrode processes on an atomic level. These studies provide new important information necessary to develop appropriate physical models for a correct interpretation of experimental electrochemical data. *Author to whom correspondence should be addressed.
Correlations between in situ SPM data and cyclic voltammetric measurements were already performed by many authors in various systems with substrate surface reconstruction, adsorption and desorption of anions and underpotential deposition (UPD) and dissolution of metals [1±3]. These correlations mainly supply information on the thermodynamic and structural aspects of interfacial properties and reactions. A powerful electrochemical method for investigations of the kinetic aspects of electrode processes is EIS [4, 5]. During the last two decades EIS was intensively applied to investigate the kinetics and mechanism of electrocrystallization of metals, i.e. overpotential deposition (OPD) of metals on native substrates [1]. Dierent simpli®ed theoretical models for a quantitative transfer function analysis were developed and discussed by various authors [5±9]. These models were based on more or less hypothetical assumptions of the atomic structure of the electrode surface and the local mechanism of electrodeposition reactions. In most cases, however, crystal imperfections and surface inhomogeneities strongly in¯uence the local mechanism of the elec-
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trocrystallization process and the resulting surface morphology and therefore signi®cantly complicate the interpretation of experimental results [1]. A better understanding of the role of surface inhomogeneities in the kinetics and mechanism of electrochemical phase formation processes on an atomic level can be achieved by a combination of EIS measurements and in situ SPM. Recent in situ SPM studies showed that UPD of metals on single crystal substrates is an appropriate process for such combined studies. In contrast to the large number of EIS studies on OPD of metals on native substrates EIS investigations of UPD of metals are reported up to now only in two papers [10, 11]. These ®rst EIS investigations indicated an in¯uence of surface inhomogeneities but were interpreted in terms of a continuous adsorption process excluding ®rst order phase transitions leading to the formation of condensed metal UPD phases. However, in situ SPM studies in many UPD systems have given direct evidence for such ®rst order phase transitions which have to be taken into account for the interpretation of experimental data. The aim of this paper is to discuss the kinetic aspects and mechanism of underpotential metal deposition and dissolution correlating experimental results obtained by in situ STM and EIS in the model systems Ag(111)/Pb2+ and Ag(100)/Pb2+.
THEORETICAL CONSIDERATIONS UPD of a metal (Me) on a foreign substrate (S) is caused by a strong Me±S interaction and occurs at electrode potentials, E, more positive than the Nernst equilibrium potential, E3D Me , of the 3D Me bulk phase given by E > E3D Me E o3D Me
RT a z ln Me a3D Me zF
1
where E o3D Me denotes the standard potential, aMez the activity of Mez+ in the electrolyte and a3D Me the activity of the 3D Me phase, which is a constant equal to unity for a pure 3D Me bulk phase The potential dierence (a3D Me 1). DE E ÿ E3D Me > 0 represents the underpotential which is directly related to the undersaturation with respect to the 3D Me bulk phase (Dm ÿzFDE<0). Me UPD is an electrosorption process with a charge-coverage stoichiometry given by the socalled electrosorption valency [1]: 1 @q 1 @m 2 g F @G E F @E G where q is the relative ionic charge density, G the relative surface excess concentration of the adsorbed Me adatoms (Meads), and m the chemical potential of Mez+ in the electrolyte. In absence of cosorption and/or competitive sorption phenomena,
the electrosorption valency is equal to the ionic charge of Mez+ (g = z) and the Me UPD process can be described by a quasi-Nernst equation [1]: E E o3D Me
RT aMez RT ln ln f G E3D Me ÿ zF zF f G 3
where the function f(G) is related to the adsorption isotherm and depends on the adatom±adatom (Meads±Meads) and adatom±substrate (Meads±S) interactions and the crystallographic structure and heterogeneity of the substrate surface. Substrate surface inhomogeneities are characterized by signi®cant dierences in the adatom binding energy and can induce formation of various lowdimensional 1D and 2D Me phases with dierent energetics and structures in well separated underpotential ranges [1, 12, 13]. For instance a preferential decoration of 1D surface inhomogeneities such as monatomic steps by Meads chains can be considered as 1D Me phase formation [Fig. 1(b)]. The formation of 2D Me phases occurs on atomically smooth surface terraces [Fig. 1(c)]. Low-dimensional Me phases can be either expanded (gas-like) or condensed (liquid-like or solid-like).The formation of expanded low-dimensional Me phases is characterized by a continuous change of the equilibrium Me surface coverage G with the actual electrode potential E. Condensed low-dimensional Me phases are characterized by equilibrium potentials described by Nernst-type equations similar to equation (1) and are usually formed by a ®rst order phase transition involving nucleation and growth phenomena [1]. A ®rst order phase transition is re¯ected in a discontinuity of the G(E) adsorption isotherm at the corresponding equilibrium potential. It is well known [1] that the equilibrium potential E3D Me of a 3D Me bulk crystal phase de®ned by equation (1) is determined by the exchange frequency of 3D kink atoms [Fig. 1(a)]. Similarly, the equilibrium potentials E1D Me and E2D Me of condensed in®nitely large 1D and 2D Me phases formed at monatomic steps and atomically ¯at terraces of a foreign substrate are determined by the corresponding exchange frequencies of 1D and 2D kink atoms (Fig. 1(b) and (c)]. The concentration of kink sites niD kink (kinks cmÿ2) does not aect the equilibrium potential but signi®cantly in¯uences the partial exchange current density of kink atoms given by [1, 13]: io, iD kink zeo o, iD kink EiD Me niD kink zeo o, iD kink EiD Me
LS diD kink
where o o,iD kink EiD Me is the exchange frequency of iD kink atoms (i = 1,2,3) at the corresponding equilibrium potential EiD Me , LS (cm cmÿ2) denotes the density of monatomic steps, i.e. the total step
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Fig. 1. Schematic representation of iD kink atoms (i = 1,2,3) determining the equilibria of the corresponding condensed iD Me phases: (a) 3D Me phase, (b) 2D Me phase, and (c) 1D Me phase
length per unit surface area, and diD kink is the mean distance between iD kinks. As seen from equation (4) the partial exchange current density io, iD kink of iD kink atoms depends on the density of monatomic steps. Assuming that the lateral growth of a condensed monolayer is only controlled by a direct charge transfer (dt) of Mez+ to 2D kink sites the corresponding lateral growth rate is related to io, iD kink and for low supersaturations zF E ÿ E2D Me with respect to the condensed 2D Me phase is given by v2D, dt
io, 2D kink zF E ÿ E2D Me q2D Me LS RT
5
where q2D Me (As cmÿ2) denotes the charge amount per unit area for the formation of a 2D Me monolayer.
The overall exchange current density, io, S=Mez , in the UPD range depends on the underpotential DE and is given by the sum of the partial exchange current densities, io,j , related to dierent low-dimensional expanded or condensed Me phases contributing to the corresponding equilibrium: X io,j DE io,j DE 6 j
If the equilibrium involves an expanded Me phase with a low adatom concentration and a condensed iD Me phase with a relatively high concentration of iD kink atoms (high step density), the contribution of the partial exchange current density of the expanded Me phase can be neglected and the overall exchange current density will only be determined by the corresponding partial exchange current density of iD kink atoms:
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7
Generally, dierent steps must be considered in the kinetics of Me UPD: bulk diusion of Mez+ in the electrolyte, charge transfer of Mez+, surface diffusion of Meads, and ®rst or higher order phase transitions. In a ®rst model approach, the substrate surface was considered as quasi-homogeneous and the kinetics of the electrosorption UPD process were assumed to be controlled by Mez+ charge transfer and/or semiin®nite-linear bulk diusion of Mez+ excluding surface diusion of Me adatoms and phase transitions [1]. Later, a more sophisticated kinetic model including monatomic steps as 1D surface inhomogeneities was developed [1, 10]. In this model bulk diusion of Mez+ was neglected, while three dierent ¯uxes were considered: charge transfer of Mez+ at monatomic steps and on atomically ¯at terraces as well as a superimposed surface diusion leading to a leveling of Me adatom gradients. All these models, however, did not consider the changes in the condition of the electrode caused by the formation of dierent low-dimensional Me phases at dierent underpotentials as discussed above. Assuming g = z (no cosorption and/or competitive sorption phenomena), the transfer function of the overall interfacial impedance of an electrosorption UPD process controlled by Mez+ charge transfer and semiin®nite-linear bulk diusion of Mez+ can generally be described by: ÿ1 1 Z s sCdl 8 ÿ ÿ1 Rct sCads ZT s where s is the Laplace variable, Cdl ÿ @ q=@ E G is the double layer capacitance, Rct RT=zF 1=io, S=Mez the charge transfer resistance, Cads ÿzF @ G=@ E m the adsorption capacitance (pseudo-capacitance), and ZT s the mass transport impedance. In this approach, the charge transfer resistance is related to the overall exchange current density, io, S=Mez , containing the contributions of dierent low-dimensional Me phases formed at dierent underpotentials. Therefore, an exact interpretation of impedance data requires an additional direct information on low-dimensional Me phases which can be obtained by a combination of EIS measurements with in situ SPM. EXPERIMENTAL In situ SPM and EIS studies were performed at T = 295 K in the systems Ag(111)/Pb2+, ClOÿ 4, H+ (cPb2 = H+ and Ag(100)/Pb2+, ClOÿ 4, 5 10ÿ6 mol cmÿ3, pH = 2). Previous electrosorption valency measurements in these systems showed g = z = 2, indicating that cosorption and competitive sorption phenomena can be excluded [1]. The
electrolyte solutions containing 5 mM Pb(ClO4)2, 0.5 M NaClO4, and 10 mM HClO4 were prepared from suprapure chemicals and fourfold quartz-distilled water and deaerated by oxygen free nitrogen prior and during each experiment. Pt and Pb were used as counter and reference electrodes, respectively. Ag(111) and Ag(100) surfaces were prepared by mechanical and subsequent chemical polishing as previously described [14]. In situ STM characterization shows that chemically polished substrate surfaces preferentially consist of atomically smooth terraces separated by monatomic steps. An average step density in the range 1.5 106 cmÿ1
Combined in situ SPM and EIS studies of PbUDP strongly compressed due to the signi®cant crystallographic Pb±Ag mis®t and the strong Pbads±Ag attraction. On Ag(111) the condensed hcp Pb monolayer is incommensurate, isotropically compressed and rotated with respect to the substrate [1, 14] whereas on Ag(100) the condensed hcp Pb monolayer is anisotropically compressed and in higher order registry with the substrate [1, 18]. In situ STM images in Fig. 2 show morphological changes during the UPD of Pb on a stepped Ag(111) substrate [Fig. 2(b)]. After the initial step decoration corresponding to peak A1 of the cyclic voltammogram [Fig. 2(a)] an appearance of a growth front of the condensed Pb monolayer starting at monatomic steps is clearly visible in the in
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situ STM image obtained during the potential scan in peak A2 [Fig. 2(c)]. The formation and dissolution of the corresponding condensed 2D Pb phase occurs via a ®rst order phase transition which is re¯ected in the appearance of a hysteresis between the deposition (A2) and dissolution (D2) peaks in the cyclic voltammogram [Fig. 2(a)]. The equilibrium potential of the condensed 2D Pb phase is located between the peaks A2 and D2 (DE = 150 mV) and is given by E2D Pb E3D Pb 150 mV
9
The formation and dissolution of the condensed Pb monolayer occur at E
Fig. 2. Morphology changes during Pb UPD on a stepped Ag(111) substrate in the system Ag(111)/Pb2+. (a): Cyclic voltammogram at vdE/dtv = 1 mV s-1; (b): In situ STM image of the bare substrate; (c): In situ STM image showing the change in the surface morphology during the potential scan in the range of peak A2. The intersected lines indicate the location of step edges on the bare substrate. STM scan size: 70 nm 70 nm
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rate can be measured by a technique involving a potential step polarization and a simultaneous in situ STM imaging in a linescan mode. In the system studied values ranging between 5 and 10 nm sÿ1 were measured for the lateral growth rate of the condensed Pb monolayer at potentials E2D Pb ÿ 10 mVRE
data are corrected for the electrolyte resistance and represent only the interface impedance. At high underpotentials DE>250 mV the interface impedance is characterized by a capacitive behavior which is representative for an ideally polarizable electrode [Fig. 4(b)]. In the UPD range a semicircle appears in the interface impedance spectra at high frequencies which is obviously related to the Pb2+ charge transfer reaction. Experimental impedance spectra were ®tted by the transfer function of equation (8) using for ZT(s) a Warburg-impedance p ZT s sW = s with pa Warburg coecient sW RT = z2 F 2 cMez D as a ®rst approximation. The optimum ®t data are summarized in Tables 1 and 2. The ®t shows that bulk diusion of Pb2+ has to be considered at low frequencies. However, diusion coecients estimated from the Warburg coecients in Table 1 and Table 2 are three to four orders of magnitude too small which may be attributed to a local Pb2+ charge transfer only at surface inhomogeneities leading to a bulk diusion towards a partially active electrode surface [8, 9, 11]. Thus, an analysis of the low frequency impedance data would require a separation of the transport impedance, ZT(s), into a Warburgimpedance, ZW(s), and a lateral transport contribution, sZs(s) [8, 9]. The coecient s describes the ratio of blocked to active areas of the electrode surface. In the systems under investigation kinks can be considered as active sites for the charge transfer. s values between 100 and 1000 are derived from the experimentally obtained step density, resulting in an increase of the diusion coecients by two to three orders of magnitude. In addition, a concentration gradient within the diusion layer leads also to an increase of the diusion coecient. A quantitative analysis of the low frequency parts of impedance spectra, however, requires a development of an appropriate physical mass transport model including surface inhomogeneities and non-linear diusion. Charge transfer resistances Rct and corresponding overall exchange current densities io, Ag=Pb2 obtained from the high frequency parts of impedance spectra are related to the partial exchange current densities of low-dimensional Pb phases contributing to the equilibrium at the given potential. In the system Ag(111)/Pb2+ impedance spectra and overall exchange current densities obtained at an underpotential DE = 150 mV corresponding to the equilibrium potential of the condensed 2D Pb phase E2D Pb (cf. equation (9)) depend strongly on the polarization routine (cathodic or anodic sweep) used for adjusting this potential. This behavior re¯ects the occurring ®rst order phase transition. If E2D Pb is adjusted by a cathodic potential sweep (c.s.) a formation of the condensed 2D Pb phase cannot occur and only an 1D Pb phase (decoration of monatomic steps) and an extremely diluted expanded (gas-like) 2D Pb phase are formed.
Fig. 3. Morphology changes during Pb UPD on a stepped Ag(100) substrate. (a): Cyclic voltammogram at vdE/ dtv = 1 mV s-1; (b): Step decoration at DE = 200 mV. The dotted lines indicate the location of the step edges on the bare substrate; (c): Formation of the hcp Pb monolayer at DE = 120 mV; (d) Formation of 2D Pb islands at 0 mV < DE < 100 mV; (e) Dissolution of the 2D Pb islands and the hcp Pb monolayer during an anodic potential sweep around peak D2; (f) Substrate surface after stripping the underpotential deposit at DE = 250 mV. STM scan size: 200 nm 200 nm 2869
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Fig. 4. Interface impedance behavior in the system Ag(111)/Pb2+: Cyclic voltammogram (a), impedance spectra at DE = 300 mV (b), at_DE = 150 mV after a cathodic potential sweep (c), at DE = 20 mV (d), and at DE = 150 mV after an anodic potential sweep (e). The inserts in the impedance spectra schematically represent the condition of the substrate surface
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Fig. 5. Interface impedance behavior in the system Ag(100)/Pb2+: Cyclic voltammogram (a), impedance spectra after a cathodic potential sweep at DE = 160 mV (b), and at DE = 125 mV (c), at DE = 20 mV (d), and after an anodic potential sweep at DE = 125 mV (e) and DE = 160 mV (f). The inserts in the impedance spectra schematically represent the condition of the substrate surface
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Table 1. Fit parameters describing the EIS data in the UPD system Ag(111)/Pb2+ according to equation (8) DE (mV) 20 125 150 c.s. 150 a.s. 170
Cdl (mF cmÿ2)
Cads (mF cmÿ2)
sW (O cm2 sÿ1/2)
Rct (O cm2)
io, Ag=Pb2 (mA cmÿ2)
1922 2222 3322 3222 3822
2502 50 3502 50 8002 50 5002 50 8502 50
5502 10 5002 10 3502 10 6502 10 5502 10
3.2 20.1 3.7 20.1 8.1 20.1 4.3 20.1 8.7 20.1
4 20.15 3.5 20.1 1.6 20.05 3 20.1 1.5 20.05
Table 2. Fit parameters describing the EIS data in the UPD system Ag(111)/Pb2+ according to equation (8) DE (mV) 20 80 125 125 160 160
c.s. a.s. a.s. a.s.
Cdl (mF cmÿ2)
Cads (mF cmÿ2)
sW (O cm2 sÿ1/2)
Rct (O cm2)
io, Ag=Pb2 (mA cmÿ2)
2022 2122 2622 2722 3022 3122
2502 50 3502 50 3202 50 30002 50 22002 50 22002 50
2002 10 2502 10 502 10 602 10 702 10 1002 10
2.5 20.1 2.5 20.1 3.8 20.1 8.2 20.1 6.8 20.1 14 20.1
5.2 20.2 5.2 20.2 3.4 20.1 1.6 20.05 1.9 20.05 0.9 20.01
Neglecting the contribution of the expanded gaslike 2D Pb phase the overall exchange current density in Table 1 corresponding to this polarization routine and this potential can be attributed only to the partial exchange current density of the 1D Pb phase. If, however, E2D Pb is adjusted after an anodic potential sweep (a.s.) the surface is covered by the condensed Pb monolayer and the overall exchange current density only represents the partial exchange current density of 2D kink atoms of the condensed 2D Pb phase: io, Ag=Pb2 E2D Pb io, 2D kink E2D Pb . As seen from Table 1 the partial exchange current density of 2D kink atoms is about two times larger than the partial exchange current density of the 1D Pb phase. Relatively large ac perturbations at the equilibrium potential E2D Pb lead to a signi®cant in¯uence of the ac amplitude on the low frequency behavior of the interface impedance (Fig. 6). This in¯uence is obviously connected with the formation and dissolution of the condensed Pb monolayer during the impedance measurements. In the applied frequency range amplitude independent impedance spectra were only observed at this potential while applying ac amplitudes smaller than 1 mV. The value of io, 2D kink determined by the impedance measurements can be used for an estimation of the propagation rate of the condensed 2D monolayer on Ag(111) from equation (5). With io, 2D kink =3 mA cmÿ2, q2D Me=0.3 mC cmÿ2, LS 2 106 cmÿ1, and E ÿ E2D Me= ÿ 2 mV a propagation rate of v2D dt17 nm sÿ1 is obtained which stands is good agreement with the values attained by in situ STM in the same potential range. These results indicate that the lateral growth of the condensed Pb monolayer on Ag(111) prefer-
entially occurs via a direct transfer of Pb2+ from the electrolyte to the growing step edges. In the system Ag(100)/Pb2+ impedance spectra measured at high underpotentials (120 mV < DE < 220 mV) corresponding to the peaks A1/ D1 strongly depend on the polarization routine (cathodic or anodic sweep) used for adjusting the potential (Fig. 5(b), (c), (e), and (f)]. This is re¯ected in dierent overall exchange current densities obtained at a given underpotential adjusted by dierent polarization routines (Table 2). An interpretation of this behavior, however, is very dicult due to the simultaneous formation of dierent
Fig. 6. In¯uence of the amplitude of the ac perturbation on the interface impedance spectra in the system Ag(111)/ Pb2+ at DE = 150 mV; ac amplitude: 1 mV (.); 5 mV (T); 10 mV (W)
Combined in situ SPM and EIS studies of PbUDP low-dimensional Pb phases and aging phenomena observed by in situ STM in this UPD range (Fig. 3). At low underpotentials (0 mV < DE < 100 mV) the impedance spectra do not depend on the underpotential and the polarization routine used for adjusting [Fig. 5(d)]. The higher overall exchange current density obtained in this UPD range can be related to the increased step density due to the 2D Pb islands formed in peak A2. This indicates that at these underpotentials the charge transfer process is probably preferentially located at the step edges of the condensed Pb monolayer. CONCLUSIONS The results demonstrate the great advantages of a simultaneous in situ SPM and EIS study of phase formation processes taking place during underpotential deposition and dissolution of metals. The EIS data and in situ SPM results obtained in the system Ag(111)/Pb2+ are related to the formation and dissolution of dierent low-dimensional (1D and 2D) Pb phases at dierent underpotentials. The analysis of experimental data indicate that the propagation of the condensed Pb monolayer formed via a ®rst order phase transition preferentially occurs by a direct transfer of Pb2+ to the step edges. The interpretation of impedance data in the system Ag(100)/Pb2+ is more complicated. In this system the UPD process is very complex and includes a number of phase formation processes accompanied by aging phenomena. Thus, a correlation between in situ SPM and EIS results is very dicult and requires additional studies. ACKNOWLEDGEMENTS The authors gratefully acknowledge the ®nancial support of this work by ``Deutsche Forschungsgemeinschaft'' (DFG), ``Arbeitsgemeinschaft Industrieller Forschungs-vereinigungen'' (AIF) and ``Bundesministerium fuÈr Wirtschaft'' (BMWi). The authors also thank U. Schmidt and S. Vinzelberg, for successful scienti®c cooperation.
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