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A surface resistance study of lead underpotential deposition on epitaxial silver thin film electrodes
Journal of Electroanalytical Chemistry 554 /555 (2003) 325 /331 www.elsevier.com/locate/jelechem
A surface resistance study of lead underpotential deposition on epitaxial silver thin film electrodes C. Hanewinkel, D. Schumacher, A. Otto * Lehrstuhl fu ¨ r Oberfla ¨ chenwissenschaft (IPkM), Heinrich-Heine-Universita ¨ t Du ¨ sseldorf, Universita ¨ tsstraße 1, 40225 Du ¨ sseldorf, Germany Received 23 November 2002; received in revised form 24 February 2003; accepted 26 March 2003
Abstract The method of dc surface resistance is used as a tool to measure trace amounts of Pb2 in an aqueous ClO4 electrolyte. Long term stability is obtained with epitaxial Ag(111) films grown on Si(111). Various potential step and linear scan experiments were applied, yielding nearly the same resistance dependence on the surface coverage. The limit of detection is below 10 6 M of Pb2 . # 2003 Elsevier B.V. All rights reserved. Keywords: Thin silver film resistance; Lead underpotential deposition; Epitaxial silver thin film electrode
1. Introduction Here we study the resistance change of thin (111) oriented silver films by upd of lead as an analytical tool for the detection of trace concentrations of Pb2, following previous publications [1,2]. Mechanistic studies, e.g. the structure of the adsorbed Pb layer, the possible formation of a surface alloy, etc. are not within the scope of this article. However, in some cases we will quote former [3] and recent [4] mechanistic studies on the applicability of the surface resistance method in electrochemical surface science. Surface resistance is a phenomenon, which has been known for a long time [5 /8]. The resistivity of a thin metal film increases with the decrease in thickness if the thickness becomes comparable to the mean free path of the conducting electrons. For silver at 300 K the mean free path is about 50 nm [9]. Adsorption processes under vacuum have a significant influence on the surface resistance of a thin metal film which is caused by the scattering of conduction electrons with the atoms or molecules adsorbed at the thin film surface. This method is especially useful for submonolayer deposits. Exposing a smooth 23 nm thick silver film in vacuum to 5 L (L / Langmuir is the commonly used unit in surface science * Corresponding author. E-mail address: [email protected] (A. Otto). 0022-0728/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0022-0728(03)00257-2
for exposure, corresponding to 10 6 mm Hg s /1.33 / 104 N s m 2) of ethylene changes the thin film resistance by about 1.3% [10]. 6 L carbon monoxide on an epitaxial 42 nm thick Cu(111) thin film induces a change of 20% [11]. For a thin metal electrode film only contact adsorption (specific adsorption) has an important influence on the resistivity [12]. At low adsorbate coverages u the effective cross section S for diffuse scattering against an adsorbed species on a smooth surface of free electron metal can be defined as: S
16 ne2 d @r 3 mnF @na
j
na 00
where n is the number of conduction electrons per unit volume, na the number of adsorbates per unit area, d the thickness of the metal film, nF the Fermi velocity, m the electron mass. The last factor in the equation above is the increase of film resistivity r with increasing adsorbate concentration na in the range of linear increase [13]. Persson derived a relation between the change in resistivity of a thin metal film due to adsorption of adparticles and the adsorbate-induced resonance level in the Newns/Anderson model [14]. Thus resistance measurements should also provide information on chargetransfer processes. Under an electrochemical environment, the measurement of the electrode resistance can be performed as a
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potential-controlled technique. These potential controlled resistance measurements are possible only due to the very small influence of adsorbed water on the thin film resistance. Using atomic scale friction experiments, Krim et al. [15] deduced a value of S :/0.0006 nm 2 for a water molecule in a complete monolayer of adsorbed water on silver. This value of the cross-section S for diffuse scattering by adsorbed water can be neglected compared to those for other adsorbates. Under an electrochemical environment an appropriate setup provides that the electrochemical current does not influence the measurement of the surface resistance [16]. The surface resistance in the electrochemical environment was used to examine the adsorption of different halides on metal substrates by Hansen and coworker [17,18], and also for molecular anions, e.g. thiocyanate SCN , by Winkes et al. [16]. The deposition and desorption of a submonolayer of metal onto a foreign metal electrode occur in many cases at a potential which is positive with respect to the potential of the bulk metal deposition (underpotential deposition, upd). This effect has been known for a long time and intensively investigated during the past 25 years [19 /21]. Much work on resistance and upd for various metals on silver and gold has been done by Hansen [17] and Rath [22] as well as by Tucceri and coworkers [23 /25] and by Lilie [4]. In this work the highest concentration of Pb2 in aqueous electrolytes of 0.05 M KClO4 is 5 /105 M and the accumulation times of Pb-deposition never exceed the limit of the initial multilayer deposition. Usually the chosen potentials for the surface accumulation of Pb are positive of the bulk deposition of Pb. Therefore we speak generally of upd of lead, though this phenomenon is usually studied at higher concentrations, for instance at 10 3 to 102 M [3], at 5 /103 M [26] or at 5 /102 M [4].
2. Experimental The silver thin film electrodes were grown epitaxially on single crystal silicon substrates under ultra high vacuum conditions as described in detail in previous publications [1,16,27]. The silicon substrates with a (111) orientation had a lateral dimension of 15 /15 mm2 and a thickness of 400 mm. The electrical co-conduction in the silicon substrate is negligible due to the high resistivity /4000 V cm. The thickness of the prepared silver thin films is about 26 nm, they show a (111) orientation and the resistivity achieved is usually /2/ 108 V m. The resistance was obtained by passing a constant current IM through the thin film electrode and measuring the voltage with a four-point method in order to detect small resistance changes. A special film geometry,
in which the potentiostat working electrode contact is in the middle of the stripe shaped thin film electrode suppresses almost completely the influence of the electrochemical current IEC on the resistance measurement [16]. In this paper the stated thin film resistance is always the sheet resistance. A three-electrode quartz glass cell with a cell volume of 60 cm3 was used. A Pt wire with a large area was employed as the counter electrode. The potentials were measured against a saturated calomel electrode, which was separated from the actual cell by a Luggin capillary. The supporting electrolyte in each experiment was a 0.05 M aqueous solution of KClO4. The content of lead cations varied from 1 /107 up to 5/105 M, where Pb(ClO4)2 was the salt. All solutions were made from p.A. chemicals and bi-distilled water. Before the experiments were carried out, the electrolytes were intensively deoxygenated with a nitrogen gas bubbler. During the experiments the bubbling was continued and the electrolyte was stirred additionally. The measurements were performed at room temperature.
3. Results and discussion 3.1. Perchlorate adsorption In contrast to adsorption experiments under vacuum conditions, one has to consider, at electrode surfaces, the influence of the majority anions of the electrolyte, when traces of minority species, here lead anions, are to be examined by dc resistance. Therefore it is essential to use a suitable background electrolyte, whose anions show only a slight influence on the thin film resistance. The adsorption of less than a monolayer of chloride, bromide or iodide on a silver electrode initiates a thin film resistance change of some percent [16,23], but perchlorate, which is known as a weakly contact adsorbing species, should induce only a small resistance change on adsorption. We used linear scan and potential step experiments for the study of metal upd, so we also applied these techniques to an aqueous solution of perchlorate. First, a linear scan of the resistance is shown in Fig. 1. This resistance change versus potential curve shows a hysteresis but is closed and reproducible. A minimum in the resistance is visible close to the potential of zero charge (/0.64 VSCE) [28]. The increase of the resistance in the positive scan (maximum increase at /410 mV) and the decrease (a maximum at /520 mV) is attributed to the adsorption and desorption of the perchlorate anion. The measured resistance change is smaller than values obtained upon adsorption of chloride, bromide or iodide [16]. The contribution of the potassium cations to the resistance can be disgarded (as has already been observed in Ref.
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Fig. 1. Sheet resistance R ? vs. potential ESCE curve from a Ag(111) thin film of 26 nm thickness in 0.05 M KClO4 aqueous electrolyte. Linear sweep rate of 50 mV s 1.
[29]). The surface resistance method is susceptible only to contact adsorbed (specifically adsorbed) species [12] in agreement with theoretical estimations by Persson [14]. K cations have a large solvation energy and therefore are separated from the surface by at least one or two layers of water molecules. The decrease in resistance when perchlorate desorbs from the electrode surface and hence increases the amount of adsorbed water, respectively, could also be observed in potential step experiments. In Fig. 2 a silver thin film in KClO4 was held at a potential of /200 mV. The thin film resistance was measured 5 times s 1. At t/90 s the potential was changed from /200 to /550 mV for 30 s. The initial resistance of the thin film was 0.861 V. It reached a minimum of 0.855 V after the potential was changed to /550 mV. After returning to /200 mV the initial resistance of 0.861 V was regained. Applying further identical potential pulses to the thin film led to the same resistance versus time curve. This decrease in resistance of about 0.7% is attributed to the desorption of perchlorate from the silver surface. The results above are surprising, in view of the large hysteresis and the long time transients, which were already observed for Br on Ag(100) [12], where even
Fig. 2. Resistance R ? (thick points, left hand scale) of a Ag(111) thin film of 26 nm thickness in 0.05 M KClO4 vs. time t (taken 5 times s 1) due to a potential pulse (dashed line, right scale).
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a cyclic voltammogram at 2 mV s 1 delivered a considerable hysteresis. These phenomena have been carefully studied by Lilie [4] under ultraclean conditions (MilliQ-water, R /18.2 MV cm, all salts ‘ultrapure’ (Alfa-Aesar), all vessels and instruments coming into contact with the chemicals and the electrolytes boiled immediately before use in several steps in deionized water, milliQ-water with perchloric acid, pure milliQwater. Four-point-contacts and working electrode contact to the silver film samples outside the electrolytical cell, Ag(100) films grown epitaxially on ‘epipolished’ MgO-crystals ‘ready for epitaxy’(CrysTec), annealed in UHV at 1000 8C for 5 h, Au (111) films on borosilicate float glass fire-tempered on a CERAN plate in a hydrogen flame (purity 5.0) and cooled in a N2 stream.). In many simple adsorbate systems, the surface resistance up to the limiting time of 10 s after a potential step followed, within a very small error margin, the integrated electrode current. However, the charge and resistance transients were different for the reversed potential step. This must necessarily mean that there will be a hysteresis in the surface resistance during a cyclic voltammogram, as for instance in Fig. 1. Secondly, there are long (/10 s) time transients as in Fig. 2, whose assignment to impurities is most unlikely. As yet it is undecided, whether one may attain the theoretical ideal of a thermodynamical equilibrium at all, independent of the prehistory of a sample (here the same electrode potential reached from the positive or negative side). Interesting as such questions are, they are out of the scope of the analytical work in this article. 3.2. Upd of lead at low concentrations Obviously at the Pb2 concentration at and below 105 M all processes are controlled by bulk diffusion of Pb2 to the silver surface and by the long time formation of the adsorbate layer structure and maybe alloy formation. Again, these phenomena are not the topic of this paper. Rather, the following sections will demonstrate that the accumulated charge q by the complete reduction of Pb2 determines the surface resistance, irrespective of the potential /time program chosen. The programs involved in this work were different potential step and linear scan experiments, with data sampled at different arbitrarily chosen times and potentials. Not surprisingly, these programs give different reduction charges for the different programs, as collected in Fig. 7. For the convenience of the reader, we have formally converted the charge q into a coverage, which can be easily be converted back into q . This formal coverage scale demonstrates only that a coverage above one monolayer is never reached, because in this case the resistance would react differently. The important point of this work is that one may choose freely just one potential /time program and
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obtain a calibration curve like that in Fig. 7 to measure unknown aqueous concentrations of Pb2 down to 106 M. 3.3. Upd of lead */potential step experiments Applying a suitable potential step to the system of an aqueous solution of weakly adsorbing anion, perchlorate, with a small varying amount of the Pb2 cation should lead to the formation of a submonolayer of lead. The surface resistance of a silver thin film electrode should increase by deposition of lead. In the following sections we will focus on this dependence. We had to take care that the properties of the thin film electrode itself were not influenced by the adsorption and desorption of lead, especially during several adsorption and desorption cycles. In negative potential step experiments the lead layer is built up with time. In Fig. 3 the thin film resistance increases (though perchlorate desorbs) during the lead growth induced by the potential step to /1.0 VSCE in 1 /106 M Pb2. (Though the potential ESCE //1.0 V would induce bulk growth of metallic lead, the Pb2 concentration is too low to achieve more than a monolayer of Pb during the accumulation times in Fig. 3, see also the discussions below.) Holding the silver electrode at a potential of / 200 mV resulted in a lead free surface. A potential step to /1000 mV enables the formation of a lead layer and hence the increase of the thin film resistance. The initial thin film resistance of 0.699 V stayed the same as that noted in Fig. 3. There was no measurable deviation in resistance from the initial film resistance after several deposition and dissolution cycles. This measurement shows that several deposition and desolution cycles do not alter the Ag(111) thin film (roughening the surface by place exchanges between silver and
Fig. 3. Resistance R ? of a Ag(111) thin film of 26 nm thickness in 0.05 M KClO4; 1/10 6 M Pb2 vs. time t due to potential steps of ESCE indicated at the bottom. The minima of the resistance are marked.
lead and the eventual formation of a surface alloy [26] would increase the resistance after the pulse). This permits the following quantitative investigations on lead adsorption at different Pb2 concentrations by potential step experiments. The potential was held at /250 mV, a potential at which the electrode is not covered with lead. At t/0 the potential was stepped to /450 mV for 60 s so that perchlorate desorbed (probably completely) and lead adsorbed. The thin film resistance was measured twice per second. Despite the small influence of perchlorate on the resistance as shown in the section an perchlorate adsorption (Figs. 1 and 2) we had to subtract the blank potential step experiment. Especially at very low lead concentration, below 5 /106 M, it is essential to pay attention to these resistance changes from perchlorate desorption. ln Fig. 4 this potential step experiment is shown for different concentations of Pb2. The resistance change due to deposition of lead is given from the start of the applied potential steps. The electrochemical current is used as a tool to determine the lead coverages, assuming full discharging of the lead cations [30]. Simultaneously to the measurement shown in Fig. 4 the electrochemical current response to the potential step was recorded. Since the desorption of perchlorate contributes to the current we subtracted the signal from the blank measurement. The integrated electrochemical current due to the deposition of lead on the electrode surface is shown in Fig. 5 as surface charge density q transferred for different concentrations. From Figs. 4 and 5, one obtains in Fig. 7 the surface resistance change and the transferred surface charge converted to surface coverage of lead accumulated 10 s after the potential step to /450 mVSCE. Additionally values at 1 /10 7 M Pb2 are displayed. Here the surface concentration of lead is formally calculated by dividing the transferred charge by 2eN , where N is the density of silver atoms at the Ag(111) surface and e the
Fig. 4. Resistance change DR ? of a Ag(111) thin film of 26 nm thickness vs. time t due to a potential step at t/0 s from /250 to / 450 mV in 0.05 M KClO4, sampled twice s 1. The solution contained 1/10 6 M (1), 5/10 6 M (2), 1/10 5 M (3), 5/10 5 M (4) Pb2 .
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Fig. 5. Transferred surface charge density q vs. time t due to a potential step at t /0 s from /250 to /450 mV in 0.05 M KClO4, sampled twice s 1. The solution contained 1/10 6 M (1), 5/10 6 M (2), 1/10 5 M (3), 5/10 5 M (4) Pb2 .
elementary electron charge. The surface coverages with lead are shown as at the right hand axis in Fig. 7. The concentration dependence of the thin film resistance as well as of the accumulated charge after 10 s at an electrode potential of /450 mVSCE is evident. The deposition process is faster at higher Pb2 concentrations; this will be discussed further below.
3.4. Upd of lead */linear scan experiments In this experiment a thin film electrode was introduced in an aqueous solution with 1/10 6 M lead cations. In Fig. 6, the potential is varied linearly between /1360 and /130 mV with a sweep rate of 50 mV s 1. The resistance versus potential curve is closed. Due to this closed shape of the resistance versus potential curve it is obvious, that during several adsorption and desorption cycles the thin film electrode does not change its characteristics. The resistance is changed from 0.775 V at /300 mV to 0.818 V at the most negative potential of /1360 mV in the negative sweep. The sharp changes are attributed to the upd and the corresponding stripping of lead on the Ag(111) thin film. The measured potentials of the resistance responses are in agreement
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with the observed electrochemical current responses [31]. The resistance change at very positive potentials by the perchlorate adsorption discussed in the previous section is also observed in Fig. 6, though distorted by the hysteresis. Compared to the resistance change induced by adsorption of lead this change is very small. The ? is taken between the overall resistance change DR max minimum at /0.31 V in the negative scan and the maximum at /0.46 V in the positive scan. Such linear scan experiments were carried out in solutions of different cation concentrations (1 /106, 5 /106, 1 /05 and 5/105 M) to obtain the overall ? resistance change DR max as a function of lead concentration. The potential was cycled between /200 and /1000 mV in electrolytes and simultaneously the electrochemical current response was recorded. At a lead concentration of 1 /106 M the deposition and dissolution peaks in the voltammogram became visible. By integrating the current peaks due to the lead dissolution one obtains the maximum formal lead coverage as a function of the lead concentration in the solution. In Fig. 7, both the overall resistance change ? DR max (as defined above) and the maximum coverage are plotted versus lead concentration in the solution. It is evident that both the DRmax and coverage values obtained from the linear scan experiments exceed the corresponding values from the step experiments. There may be two reasons for this: I)
the values of the step experiments were sampled after 10 s, from the linear scan experiments after about 40 s of accumulation. II) The most negative potential of the linear scan experiments exceeds the Nernst potential of bulk Pb deposition (between /496 mVSCE at a Pb2 concentration of 5/105 mol l 1 and about /577 mVSCE at a Pb2 concentration of 10 7 mol l1) whereas the potential of /450 mVSCE falls only into the Pb-upd range at all concentrations used in this work. This may lead to faster adsorption at more negative potentials in the bulk potential range. (Nevertheless this does not lead to a significant signal at a Pb2 concentration of 10 7 mol l1.) These results are proof that resistance measurements at thin films are a suitable tool to study trace adsorption. 3.5. Upd of lead */comparison of the different methods
Fig. 6. Resistance change DR ? of a Ag(111) thin film of 26 nm thickness in 0.05 M KClO4 with 1/10 6 M Pb2 versus potential ESCE during linear sweeps at 50 mV s 1.
In this section we would like to show that the resistance change does depend only on the transferred charge q caused by lead adsorption, but does not depend on the deposition conditions in the present concentration, potential and coverage ranges. Potential step experiments as well as linear scan experiments both at very different cation concentrations and potentials
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? , respectively, and formal lead coverage, given in monolayers (ML), as a function of the lead concentration Fig. 7. Resistance change DR ? or DR max ? from the linear scan experiments to /1000 clead in the solution. Circles denote potential steps to /450 mV (Figs. 4 and 5), rectangles denote DR max ? (left hand scale), full symbols: formal coverage (right hand scale). mV (as in Fig. 6) Open symbols: DR ? or DR max
can lead to equal coverage on different time scales. ? Nevertheless, the resistance change DR ? or DR max depends only on q . In Fig. 8 the experimental values obtained with the three different methods, (/) time evolution after a potential step, (m) 10 s after potential steps in different Pb2 concentrations and (I) from linear scan experiments in different Pb2 concentrations are displayed. Within an error bar of about 9/0.005 V the curves agree, also for the measurments at concentra-
? , respectively, vs. transferred Fig. 8. Resistance change DR ? or DR max charge q of a 26 nm thick Ag(111) thin film derived from: (/) a potential step from /250 to /450 mV with 5/10 5 M Pb2 , this work; (m) a potential step from /250 to /450 mV with Pb2 contents from 1/10 7 to 5/10 6 M, values taken 10 s after the step, this work; (I) linear scan experiments with lead contents from 1/10 6 to 5/10 5 M, this work; (w) normalized (see text) resistance change DR ? vs. transferred charge q of a 30-nm thick Ag(100) thin film in 5/10 2 M Pb(ClO4)2/5/10 1 M NaClO4/ 5/10 3 M HClO4; jdE /dt j/10 mV s 1, from [4].
tions not displayed in Fig. 8. There is no indication of irreversible lead deposition. Fig. 8 shows the comparison between the results of this work and the analogous results in the upd range at a Pb2 concentration which is at least 1000 times greater than that used in this work, as obtained by Lilie for an Ag(100) oriented film of 30 nm thickness during a negative scan in an aqueous electrolyte of 5 /102 M Pb2 with a scan rate of 10 mV s 1, between a potential of 300 to nearly 0 mV above the potential of bulk deposition of Pb [4]. Lilie’s curve is in qualitatively good agreement with the analogous resistance data of Pb underpotential deposition on a gold film by Ganon and Clavilier [3]. However, the weak structures in the DR (q) plot of Ganon and Clavilier were not observed for upd Pb on Ag(100) by Lilie in spite of modern means of analogue /digital conversion with the possibility of arithmetic operations. Because the surface resistance scales like d2, the experimental DR ? values of the Ag(100) have been multiplied by (30 nm/26 nm)2 /1.33 for the comparison in Fig. 8. The error of this quotient for Dd :/9/1 nm is /0.20 to /0.15. Given this error, there is a relatively good correspondence of the results in this work and the results of Lilie at low q , but there is a great difference at higher q . This may be caused by the different faces of silver used and maybe by different surface roughness. Another reason to consider is the difference between instantaneous (in Lilie’s case) and delayed (in this work 10 /40 s) measurement of the resistance due to lead deposition. One might expect a kind of slow Oswald ripening, the smaller number of larger upd-patches of Pb causing less resistance than a larger number of small upd
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patches. This question remains open, because an instantaneous measurement is not possible at the low concentrations used in this work.
4. Summary Traces of Pb2 in a perchlorate electrolyte at concentrations of 10 6 M and higher can be quantified by surface resistance, because the measured and reproducible resistance change of thin epitaxial silver layers of (111) orientation does not depend on the potential /time history, but only on the upd submonolayer coverage. Different kinds of calibrations, like the time development of the resistance after a potential step into the potential range of Pb deposition or linear negative potential scans are possible.
Acknowledgements We acknowledge the support of this work by the Ministerium fu¨r Wissenschaft und Forschung of Nordrhein-Westfalen. We thank Philipp Lilie for allowing us to use one of his results prior to publication.
References [1] C. Hanewinkel, H. Winkes, D. Schumacher, A. Otto, Electrochim. Acta 42 (1997) 3345. [2] H. Emons, O. Glu¨ck, B. Hu¨llenkremer, M.J. Scho¨ning, Electroanalysis 13 (2001) 677. [3] J.P. Ganon, J. Clavilier, Surf. Sci. 147 (1984) 583. [4] P. Lilie, Dissertation, Universita¨t Du¨sseldorf, 2002.
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[5] K. Fuchs, Proc. Camb. Phil. Soc 34 (1938) 100. [6] E.H. Sondheimer, Adv. Phys. 1 (1952) 1. [7] P. Wissmann, Thin Metal Films and Gas Chemisorption, Elsevier, Amsterdam, 1987. [8] D. Schumacher, Springer Tracts Mod. Phys. 128 (1992) 1. [9] F.W. Reynolds, G.R. Stilwell, Phys. Rev. 88 (1952) 418. [10] H. Grabhorn, A. Otto, D. Schumacher, Surf. Sci. 264 (1992) 327. [11] M. Hein, P. Dumas, A. Otto, G.P. Williams, Surf. Sci. 419 (1999) 308. [12] C. Hanewinkel, A. Otto, T. Wandlowski, Surf. Sci. 429 (1999) 255. [13] B.N.J. Persson, D. Schumacher, A. Otto, Chem. Phys. Lett. 178 (1991) 204. [14] B.N.J. Persson, Phys. Rev. B 44 (1991) 3277. [15] J. Krim, E.T. Watts, J. Digel, J. Vac. Sci. Technol. A 8 (1990) 3417. [16] H. Winkes, D. Schumacher, A. Otto, Surf. Sci. 400 (1998) 44. [17] W.N. Hansen, Surf. Sci. 101 (1980) 109. [18] D.L. Rath, W.N. Hansen, Surf. Sci. 136 (1984) 195. [19] D.M. Kolb, M. Przasnyski, H. Gerischer, J. Electroanal. Chem. 54 (1974) 25. [20] E. Budevski, G. Staikov, W.J. Lorenz, Electrochemical Phase Formation and Growth, Verlag Chemie, Weinheim, 1996. [21] E. Herrero, L.J. Buller, H.D. Abrun˜a, Chem. Rev. 101 (2001) 1897. [22] D.L. Rath, J. Electroanal. Chem. 159 (1983) 521. [23] F.M. Romeo, R.I. Tucceri, D. Posadas, Surf. Sci. 203 (1988) 186. [24] R.I. Tucceri, D. Posadas, J. Electroanal. Chem. 283 (1990) 159. [25] R.I. Tucceri, D. Posadas, J. Electroanal. Chem. 270 (1989) 415. [26] J. Sackmann, A. Bunk, R.T. Po¨tschke, Electrochim. Acta 43 (1998) 2863. [27] H. Winkes, Dissertation, Heinrich-Heine-Universita¨t, 1996. [28] G. Valette, A. Hamelin, Electroanal. Chem. 45 (1973) 301. [29] D. Diesing, H. Winkes, A. Otto, Phys. Stat. Sol. (A) 159 (1997) 243. [30] W.J. Lorenz, L.M. Gassa, U. Schmidt, W. Obretanov, G. Staikov, V. Bostanov, E. Budevski, Electrochim. Acta 37 (1992) 2173. [31] S. Bharathi, V. Yegnaraman, G. Prabhakara Rao, Electrochim. Acta 36 (1991) 1291.
A surface resistance study of lead underpotential deposition on epitaxial silver thin film electrodes C. Hanewinkel, D. Schumacher, A. Otto * Lehrstuhl fu ¨ r Oberfla ¨ chenwissenschaft (IPkM), Heinrich-Heine-Universita ¨ t Du ¨ sseldorf, Universita ¨ tsstraße 1, 40225 Du ¨ sseldorf, Germany Received 23 November 2002; received in revised form 24 February 2003; accepted 26 March 2003
Abstract The method of dc surface resistance is used as a tool to measure trace amounts of Pb2 in an aqueous ClO4 electrolyte. Long term stability is obtained with epitaxial Ag(111) films grown on Si(111). Various potential step and linear scan experiments were applied, yielding nearly the same resistance dependence on the surface coverage. The limit of detection is below 10 6 M of Pb2 . # 2003 Elsevier B.V. All rights reserved. Keywords: Thin silver film resistance; Lead underpotential deposition; Epitaxial silver thin film electrode
1. Introduction Here we study the resistance change of thin (111) oriented silver films by upd of lead as an analytical tool for the detection of trace concentrations of Pb2, following previous publications [1,2]. Mechanistic studies, e.g. the structure of the adsorbed Pb layer, the possible formation of a surface alloy, etc. are not within the scope of this article. However, in some cases we will quote former [3] and recent [4] mechanistic studies on the applicability of the surface resistance method in electrochemical surface science. Surface resistance is a phenomenon, which has been known for a long time [5 /8]. The resistivity of a thin metal film increases with the decrease in thickness if the thickness becomes comparable to the mean free path of the conducting electrons. For silver at 300 K the mean free path is about 50 nm [9]. Adsorption processes under vacuum have a significant influence on the surface resistance of a thin metal film which is caused by the scattering of conduction electrons with the atoms or molecules adsorbed at the thin film surface. This method is especially useful for submonolayer deposits. Exposing a smooth 23 nm thick silver film in vacuum to 5 L (L / Langmuir is the commonly used unit in surface science * Corresponding author. E-mail address: [email protected] (A. Otto). 0022-0728/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0022-0728(03)00257-2
for exposure, corresponding to 10 6 mm Hg s /1.33 / 104 N s m 2) of ethylene changes the thin film resistance by about 1.3% [10]. 6 L carbon monoxide on an epitaxial 42 nm thick Cu(111) thin film induces a change of 20% [11]. For a thin metal electrode film only contact adsorption (specific adsorption) has an important influence on the resistivity [12]. At low adsorbate coverages u the effective cross section S for diffuse scattering against an adsorbed species on a smooth surface of free electron metal can be defined as: S
16 ne2 d @r 3 mnF @na
j
na 00
where n is the number of conduction electrons per unit volume, na the number of adsorbates per unit area, d the thickness of the metal film, nF the Fermi velocity, m the electron mass. The last factor in the equation above is the increase of film resistivity r with increasing adsorbate concentration na in the range of linear increase [13]. Persson derived a relation between the change in resistivity of a thin metal film due to adsorption of adparticles and the adsorbate-induced resonance level in the Newns/Anderson model [14]. Thus resistance measurements should also provide information on chargetransfer processes. Under an electrochemical environment, the measurement of the electrode resistance can be performed as a
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potential-controlled technique. These potential controlled resistance measurements are possible only due to the very small influence of adsorbed water on the thin film resistance. Using atomic scale friction experiments, Krim et al. [15] deduced a value of S :/0.0006 nm 2 for a water molecule in a complete monolayer of adsorbed water on silver. This value of the cross-section S for diffuse scattering by adsorbed water can be neglected compared to those for other adsorbates. Under an electrochemical environment an appropriate setup provides that the electrochemical current does not influence the measurement of the surface resistance [16]. The surface resistance in the electrochemical environment was used to examine the adsorption of different halides on metal substrates by Hansen and coworker [17,18], and also for molecular anions, e.g. thiocyanate SCN , by Winkes et al. [16]. The deposition and desorption of a submonolayer of metal onto a foreign metal electrode occur in many cases at a potential which is positive with respect to the potential of the bulk metal deposition (underpotential deposition, upd). This effect has been known for a long time and intensively investigated during the past 25 years [19 /21]. Much work on resistance and upd for various metals on silver and gold has been done by Hansen [17] and Rath [22] as well as by Tucceri and coworkers [23 /25] and by Lilie [4]. In this work the highest concentration of Pb2 in aqueous electrolytes of 0.05 M KClO4 is 5 /105 M and the accumulation times of Pb-deposition never exceed the limit of the initial multilayer deposition. Usually the chosen potentials for the surface accumulation of Pb are positive of the bulk deposition of Pb. Therefore we speak generally of upd of lead, though this phenomenon is usually studied at higher concentrations, for instance at 10 3 to 102 M [3], at 5 /103 M [26] or at 5 /102 M [4].
2. Experimental The silver thin film electrodes were grown epitaxially on single crystal silicon substrates under ultra high vacuum conditions as described in detail in previous publications [1,16,27]. The silicon substrates with a (111) orientation had a lateral dimension of 15 /15 mm2 and a thickness of 400 mm. The electrical co-conduction in the silicon substrate is negligible due to the high resistivity /4000 V cm. The thickness of the prepared silver thin films is about 26 nm, they show a (111) orientation and the resistivity achieved is usually /2/ 108 V m. The resistance was obtained by passing a constant current IM through the thin film electrode and measuring the voltage with a four-point method in order to detect small resistance changes. A special film geometry,
in which the potentiostat working electrode contact is in the middle of the stripe shaped thin film electrode suppresses almost completely the influence of the electrochemical current IEC on the resistance measurement [16]. In this paper the stated thin film resistance is always the sheet resistance. A three-electrode quartz glass cell with a cell volume of 60 cm3 was used. A Pt wire with a large area was employed as the counter electrode. The potentials were measured against a saturated calomel electrode, which was separated from the actual cell by a Luggin capillary. The supporting electrolyte in each experiment was a 0.05 M aqueous solution of KClO4. The content of lead cations varied from 1 /107 up to 5/105 M, where Pb(ClO4)2 was the salt. All solutions were made from p.A. chemicals and bi-distilled water. Before the experiments were carried out, the electrolytes were intensively deoxygenated with a nitrogen gas bubbler. During the experiments the bubbling was continued and the electrolyte was stirred additionally. The measurements were performed at room temperature.
3. Results and discussion 3.1. Perchlorate adsorption In contrast to adsorption experiments under vacuum conditions, one has to consider, at electrode surfaces, the influence of the majority anions of the electrolyte, when traces of minority species, here lead anions, are to be examined by dc resistance. Therefore it is essential to use a suitable background electrolyte, whose anions show only a slight influence on the thin film resistance. The adsorption of less than a monolayer of chloride, bromide or iodide on a silver electrode initiates a thin film resistance change of some percent [16,23], but perchlorate, which is known as a weakly contact adsorbing species, should induce only a small resistance change on adsorption. We used linear scan and potential step experiments for the study of metal upd, so we also applied these techniques to an aqueous solution of perchlorate. First, a linear scan of the resistance is shown in Fig. 1. This resistance change versus potential curve shows a hysteresis but is closed and reproducible. A minimum in the resistance is visible close to the potential of zero charge (/0.64 VSCE) [28]. The increase of the resistance in the positive scan (maximum increase at /410 mV) and the decrease (a maximum at /520 mV) is attributed to the adsorption and desorption of the perchlorate anion. The measured resistance change is smaller than values obtained upon adsorption of chloride, bromide or iodide [16]. The contribution of the potassium cations to the resistance can be disgarded (as has already been observed in Ref.
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Fig. 1. Sheet resistance R ? vs. potential ESCE curve from a Ag(111) thin film of 26 nm thickness in 0.05 M KClO4 aqueous electrolyte. Linear sweep rate of 50 mV s 1.
[29]). The surface resistance method is susceptible only to contact adsorbed (specifically adsorbed) species [12] in agreement with theoretical estimations by Persson [14]. K cations have a large solvation energy and therefore are separated from the surface by at least one or two layers of water molecules. The decrease in resistance when perchlorate desorbs from the electrode surface and hence increases the amount of adsorbed water, respectively, could also be observed in potential step experiments. In Fig. 2 a silver thin film in KClO4 was held at a potential of /200 mV. The thin film resistance was measured 5 times s 1. At t/90 s the potential was changed from /200 to /550 mV for 30 s. The initial resistance of the thin film was 0.861 V. It reached a minimum of 0.855 V after the potential was changed to /550 mV. After returning to /200 mV the initial resistance of 0.861 V was regained. Applying further identical potential pulses to the thin film led to the same resistance versus time curve. This decrease in resistance of about 0.7% is attributed to the desorption of perchlorate from the silver surface. The results above are surprising, in view of the large hysteresis and the long time transients, which were already observed for Br on Ag(100) [12], where even
Fig. 2. Resistance R ? (thick points, left hand scale) of a Ag(111) thin film of 26 nm thickness in 0.05 M KClO4 vs. time t (taken 5 times s 1) due to a potential pulse (dashed line, right scale).
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a cyclic voltammogram at 2 mV s 1 delivered a considerable hysteresis. These phenomena have been carefully studied by Lilie [4] under ultraclean conditions (MilliQ-water, R /18.2 MV cm, all salts ‘ultrapure’ (Alfa-Aesar), all vessels and instruments coming into contact with the chemicals and the electrolytes boiled immediately before use in several steps in deionized water, milliQ-water with perchloric acid, pure milliQwater. Four-point-contacts and working electrode contact to the silver film samples outside the electrolytical cell, Ag(100) films grown epitaxially on ‘epipolished’ MgO-crystals ‘ready for epitaxy’(CrysTec), annealed in UHV at 1000 8C for 5 h, Au (111) films on borosilicate float glass fire-tempered on a CERAN plate in a hydrogen flame (purity 5.0) and cooled in a N2 stream.). In many simple adsorbate systems, the surface resistance up to the limiting time of 10 s after a potential step followed, within a very small error margin, the integrated electrode current. However, the charge and resistance transients were different for the reversed potential step. This must necessarily mean that there will be a hysteresis in the surface resistance during a cyclic voltammogram, as for instance in Fig. 1. Secondly, there are long (/10 s) time transients as in Fig. 2, whose assignment to impurities is most unlikely. As yet it is undecided, whether one may attain the theoretical ideal of a thermodynamical equilibrium at all, independent of the prehistory of a sample (here the same electrode potential reached from the positive or negative side). Interesting as such questions are, they are out of the scope of the analytical work in this article. 3.2. Upd of lead at low concentrations Obviously at the Pb2 concentration at and below 105 M all processes are controlled by bulk diffusion of Pb2 to the silver surface and by the long time formation of the adsorbate layer structure and maybe alloy formation. Again, these phenomena are not the topic of this paper. Rather, the following sections will demonstrate that the accumulated charge q by the complete reduction of Pb2 determines the surface resistance, irrespective of the potential /time program chosen. The programs involved in this work were different potential step and linear scan experiments, with data sampled at different arbitrarily chosen times and potentials. Not surprisingly, these programs give different reduction charges for the different programs, as collected in Fig. 7. For the convenience of the reader, we have formally converted the charge q into a coverage, which can be easily be converted back into q . This formal coverage scale demonstrates only that a coverage above one monolayer is never reached, because in this case the resistance would react differently. The important point of this work is that one may choose freely just one potential /time program and
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obtain a calibration curve like that in Fig. 7 to measure unknown aqueous concentrations of Pb2 down to 106 M. 3.3. Upd of lead */potential step experiments Applying a suitable potential step to the system of an aqueous solution of weakly adsorbing anion, perchlorate, with a small varying amount of the Pb2 cation should lead to the formation of a submonolayer of lead. The surface resistance of a silver thin film electrode should increase by deposition of lead. In the following sections we will focus on this dependence. We had to take care that the properties of the thin film electrode itself were not influenced by the adsorption and desorption of lead, especially during several adsorption and desorption cycles. In negative potential step experiments the lead layer is built up with time. In Fig. 3 the thin film resistance increases (though perchlorate desorbs) during the lead growth induced by the potential step to /1.0 VSCE in 1 /106 M Pb2. (Though the potential ESCE //1.0 V would induce bulk growth of metallic lead, the Pb2 concentration is too low to achieve more than a monolayer of Pb during the accumulation times in Fig. 3, see also the discussions below.) Holding the silver electrode at a potential of / 200 mV resulted in a lead free surface. A potential step to /1000 mV enables the formation of a lead layer and hence the increase of the thin film resistance. The initial thin film resistance of 0.699 V stayed the same as that noted in Fig. 3. There was no measurable deviation in resistance from the initial film resistance after several deposition and dissolution cycles. This measurement shows that several deposition and desolution cycles do not alter the Ag(111) thin film (roughening the surface by place exchanges between silver and
Fig. 3. Resistance R ? of a Ag(111) thin film of 26 nm thickness in 0.05 M KClO4; 1/10 6 M Pb2 vs. time t due to potential steps of ESCE indicated at the bottom. The minima of the resistance are marked.
lead and the eventual formation of a surface alloy [26] would increase the resistance after the pulse). This permits the following quantitative investigations on lead adsorption at different Pb2 concentrations by potential step experiments. The potential was held at /250 mV, a potential at which the electrode is not covered with lead. At t/0 the potential was stepped to /450 mV for 60 s so that perchlorate desorbed (probably completely) and lead adsorbed. The thin film resistance was measured twice per second. Despite the small influence of perchlorate on the resistance as shown in the section an perchlorate adsorption (Figs. 1 and 2) we had to subtract the blank potential step experiment. Especially at very low lead concentration, below 5 /106 M, it is essential to pay attention to these resistance changes from perchlorate desorption. ln Fig. 4 this potential step experiment is shown for different concentations of Pb2. The resistance change due to deposition of lead is given from the start of the applied potential steps. The electrochemical current is used as a tool to determine the lead coverages, assuming full discharging of the lead cations [30]. Simultaneously to the measurement shown in Fig. 4 the electrochemical current response to the potential step was recorded. Since the desorption of perchlorate contributes to the current we subtracted the signal from the blank measurement. The integrated electrochemical current due to the deposition of lead on the electrode surface is shown in Fig. 5 as surface charge density q transferred for different concentrations. From Figs. 4 and 5, one obtains in Fig. 7 the surface resistance change and the transferred surface charge converted to surface coverage of lead accumulated 10 s after the potential step to /450 mVSCE. Additionally values at 1 /10 7 M Pb2 are displayed. Here the surface concentration of lead is formally calculated by dividing the transferred charge by 2eN , where N is the density of silver atoms at the Ag(111) surface and e the
Fig. 4. Resistance change DR ? of a Ag(111) thin film of 26 nm thickness vs. time t due to a potential step at t/0 s from /250 to / 450 mV in 0.05 M KClO4, sampled twice s 1. The solution contained 1/10 6 M (1), 5/10 6 M (2), 1/10 5 M (3), 5/10 5 M (4) Pb2 .
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Fig. 5. Transferred surface charge density q vs. time t due to a potential step at t /0 s from /250 to /450 mV in 0.05 M KClO4, sampled twice s 1. The solution contained 1/10 6 M (1), 5/10 6 M (2), 1/10 5 M (3), 5/10 5 M (4) Pb2 .
elementary electron charge. The surface coverages with lead are shown as at the right hand axis in Fig. 7. The concentration dependence of the thin film resistance as well as of the accumulated charge after 10 s at an electrode potential of /450 mVSCE is evident. The deposition process is faster at higher Pb2 concentrations; this will be discussed further below.
3.4. Upd of lead */linear scan experiments In this experiment a thin film electrode was introduced in an aqueous solution with 1/10 6 M lead cations. In Fig. 6, the potential is varied linearly between /1360 and /130 mV with a sweep rate of 50 mV s 1. The resistance versus potential curve is closed. Due to this closed shape of the resistance versus potential curve it is obvious, that during several adsorption and desorption cycles the thin film electrode does not change its characteristics. The resistance is changed from 0.775 V at /300 mV to 0.818 V at the most negative potential of /1360 mV in the negative sweep. The sharp changes are attributed to the upd and the corresponding stripping of lead on the Ag(111) thin film. The measured potentials of the resistance responses are in agreement
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with the observed electrochemical current responses [31]. The resistance change at very positive potentials by the perchlorate adsorption discussed in the previous section is also observed in Fig. 6, though distorted by the hysteresis. Compared to the resistance change induced by adsorption of lead this change is very small. The ? is taken between the overall resistance change DR max minimum at /0.31 V in the negative scan and the maximum at /0.46 V in the positive scan. Such linear scan experiments were carried out in solutions of different cation concentrations (1 /106, 5 /106, 1 /05 and 5/105 M) to obtain the overall ? resistance change DR max as a function of lead concentration. The potential was cycled between /200 and /1000 mV in electrolytes and simultaneously the electrochemical current response was recorded. At a lead concentration of 1 /106 M the deposition and dissolution peaks in the voltammogram became visible. By integrating the current peaks due to the lead dissolution one obtains the maximum formal lead coverage as a function of the lead concentration in the solution. In Fig. 7, both the overall resistance change ? DR max (as defined above) and the maximum coverage are plotted versus lead concentration in the solution. It is evident that both the DRmax and coverage values obtained from the linear scan experiments exceed the corresponding values from the step experiments. There may be two reasons for this: I)
the values of the step experiments were sampled after 10 s, from the linear scan experiments after about 40 s of accumulation. II) The most negative potential of the linear scan experiments exceeds the Nernst potential of bulk Pb deposition (between /496 mVSCE at a Pb2 concentration of 5/105 mol l 1 and about /577 mVSCE at a Pb2 concentration of 10 7 mol l1) whereas the potential of /450 mVSCE falls only into the Pb-upd range at all concentrations used in this work. This may lead to faster adsorption at more negative potentials in the bulk potential range. (Nevertheless this does not lead to a significant signal at a Pb2 concentration of 10 7 mol l1.) These results are proof that resistance measurements at thin films are a suitable tool to study trace adsorption. 3.5. Upd of lead */comparison of the different methods
Fig. 6. Resistance change DR ? of a Ag(111) thin film of 26 nm thickness in 0.05 M KClO4 with 1/10 6 M Pb2 versus potential ESCE during linear sweeps at 50 mV s 1.
In this section we would like to show that the resistance change does depend only on the transferred charge q caused by lead adsorption, but does not depend on the deposition conditions in the present concentration, potential and coverage ranges. Potential step experiments as well as linear scan experiments both at very different cation concentrations and potentials
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? , respectively, and formal lead coverage, given in monolayers (ML), as a function of the lead concentration Fig. 7. Resistance change DR ? or DR max ? from the linear scan experiments to /1000 clead in the solution. Circles denote potential steps to /450 mV (Figs. 4 and 5), rectangles denote DR max ? (left hand scale), full symbols: formal coverage (right hand scale). mV (as in Fig. 6) Open symbols: DR ? or DR max
can lead to equal coverage on different time scales. ? Nevertheless, the resistance change DR ? or DR max depends only on q . In Fig. 8 the experimental values obtained with the three different methods, (/) time evolution after a potential step, (m) 10 s after potential steps in different Pb2 concentrations and (I) from linear scan experiments in different Pb2 concentrations are displayed. Within an error bar of about 9/0.005 V the curves agree, also for the measurments at concentra-
? , respectively, vs. transferred Fig. 8. Resistance change DR ? or DR max charge q of a 26 nm thick Ag(111) thin film derived from: (/) a potential step from /250 to /450 mV with 5/10 5 M Pb2 , this work; (m) a potential step from /250 to /450 mV with Pb2 contents from 1/10 7 to 5/10 6 M, values taken 10 s after the step, this work; (I) linear scan experiments with lead contents from 1/10 6 to 5/10 5 M, this work; (w) normalized (see text) resistance change DR ? vs. transferred charge q of a 30-nm thick Ag(100) thin film in 5/10 2 M Pb(ClO4)2/5/10 1 M NaClO4/ 5/10 3 M HClO4; jdE /dt j/10 mV s 1, from [4].
tions not displayed in Fig. 8. There is no indication of irreversible lead deposition. Fig. 8 shows the comparison between the results of this work and the analogous results in the upd range at a Pb2 concentration which is at least 1000 times greater than that used in this work, as obtained by Lilie for an Ag(100) oriented film of 30 nm thickness during a negative scan in an aqueous electrolyte of 5 /102 M Pb2 with a scan rate of 10 mV s 1, between a potential of 300 to nearly 0 mV above the potential of bulk deposition of Pb [4]. Lilie’s curve is in qualitatively good agreement with the analogous resistance data of Pb underpotential deposition on a gold film by Ganon and Clavilier [3]. However, the weak structures in the DR (q) plot of Ganon and Clavilier were not observed for upd Pb on Ag(100) by Lilie in spite of modern means of analogue /digital conversion with the possibility of arithmetic operations. Because the surface resistance scales like d2, the experimental DR ? values of the Ag(100) have been multiplied by (30 nm/26 nm)2 /1.33 for the comparison in Fig. 8. The error of this quotient for Dd :/9/1 nm is /0.20 to /0.15. Given this error, there is a relatively good correspondence of the results in this work and the results of Lilie at low q , but there is a great difference at higher q . This may be caused by the different faces of silver used and maybe by different surface roughness. Another reason to consider is the difference between instantaneous (in Lilie’s case) and delayed (in this work 10 /40 s) measurement of the resistance due to lead deposition. One might expect a kind of slow Oswald ripening, the smaller number of larger upd-patches of Pb causing less resistance than a larger number of small upd
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patches. This question remains open, because an instantaneous measurement is not possible at the low concentrations used in this work.
4. Summary Traces of Pb2 in a perchlorate electrolyte at concentrations of 10 6 M and higher can be quantified by surface resistance, because the measured and reproducible resistance change of thin epitaxial silver layers of (111) orientation does not depend on the potential /time history, but only on the upd submonolayer coverage. Different kinds of calibrations, like the time development of the resistance after a potential step into the potential range of Pb deposition or linear negative potential scans are possible.
Acknowledgements We acknowledge the support of this work by the Ministerium fu¨r Wissenschaft und Forschung of Nordrhein-Westfalen. We thank Philipp Lilie for allowing us to use one of his results prior to publication.
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