Electrochimica Acta 47 (2001) 899– 904 www.elsevier.com/locate/electacta

Potential scan rate dependence of underpotential and bulk depositions of lead on screen-printed silver electrodes Jyh-Myng Zen *, Chih-Chio Yang, Annamalai Senthil Kumar Department of Chemistry, National Chung-Hsing Uni6ersity, Taichung 402, Taiwan, ROC Received 9 May 2001; received in revised form 8 August 2001

Abstract This is the first report on studies of the underpotential deposition of Pb (upd-Pb) on a disposable screen-printed silver electrode (AgSPE, non-single crystalline surface). The dependence of upd-Pb and bulk Pb deposition (bulk-Pb) on potential scan rate (w) was demonstrated for the AgSPE in pH 3 KNO3/HNO3 solutions. At w =20 mV s − 1, the extent of formation of reversible upd-Pb was  15 times less than that of irreversible bulk-Pb. However, for w\100 mV s − 1, only the upd-Pb was observed. Scanning electron microscopic (SEM) analysis speculated 3D-nucleation (hcp) of bulk-Pb on the AgSPE at low scan rate (w=1 mV s − 1). On the other hand, at a faster scan rate (w = 300 mV s − 1), no marked crystallites were obtained. By interpretation with the CV results, the later case was proved to be formation and complete stripping of the upd-Pb on the AgSPE and this property was further utilized for analytical application. The nearly reversible behavior of the upd-Pb process at the disposable AgSPE is a judicious choice for the effective stripping analysis of Pb by square-wave voltammetry. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Underpotential deposition; Lead; Silver; Screen-printed electrode

1. Introduction Stripping analysis using mercury film electrodes is considered the most effective approach for Pb determination [1–3]. However, because of the toxicity of Hg, future regulations and occupational health considerations may severely restrict the use of Hg as an electrode material. This problem should be considered in developing screen-printed disposable sensors due to the inherent problem of Hg contamination in the context of disposability. If a screen-printed electrode (SPE) could be employed without Hg film, this would provide an excellent, and environmentally friendly approach to Pb determinations. We report here information about the behavior of an electrodeposited Pb layer on a disposable screen-printed silver electrode (AgSPE). Our prim aim is to determine suitable electrochemical conditions for the formation of upd-Pb on the AgSPE. The ulti-

* Corresponding author. Fax: + 886-4-2286-2547. E-mail address: [email protected] (J.-M. Zen).

mate goal is to take advantage of the disposable SPE technique to construct an efficient stripping procedure for Pb determination. The underpotential deposition (upd) of Pb is an important electrocrystallization phenomena associated with the deposition of a metal (Me) monolayer onto a foreign substrate (S) at a potential more positive than the redox potential of the bulk metal, i.e. E− E°Me \0. It often occurs at a single crystal electrode surface like Au(111), Pt(111), Ag(111), and Ag(100) [4–14]. The main problem in upd formation with non-single crystal electrode surface is the relatively faster deposition rate of bulk-Pb, complicated by simultaneous deposition on the underlying electrode [15 –19]. Separation of bulk-Pb from upd-Pb is a very challenging and most desirable goal for upd applications. Interestingly, the formation of upd-Pb on the AgSPE was found to depend on the potential scan rate (w) in this study and, at particular scan rates, the formation of bulk-Pb was totally arrested. Here, we disclose the conditions for predominance of upd-Pb formation of usefulness for electroanalytical assays.

0013-4686/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 1 ) 0 0 7 9 0 - 3

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J.-M. Zen et al. / Electrochimica Acta 47 (2001) 899–904

2. Experimental Standard solutions of AAS grade Pb2 + (1000 mg l ) from E-Merck and Pb(NO3)2 from Aldrich were used as received. All the other compounds (ACS-certified reagent grade) were used without any further purification. A supporting electrolyte of pH 3 KNO3/ HNO3 (I=0.1 M) was used in all electrochemical experiments. Aqueous solutions were prepared with doubly distilled deionized water. Electrochemical studies were carried out with a BAS CV-50W electrochemical analyzer. The three-electrode system consists of either an SPE or the AgSPE working electrode, an Ag/AgCl reference (Model RE-5, BAS), and a platinum auxiliary electrode. The SEM measurements were performed using a Topcon ABT-150S instrument. A semi-automatic screen-printer was used to prepare the disposable AgSPEs as per our earlier report [20]. Briefly, a stencil with structure of five continuous electrodes was used to print the conducting silver ink on a flexible polypropylene film (50 ×70 mm) and was cured under UV irradiation at 3.25 mW cm − 2 for 1 h. After drying, an insulator layer was spread over the AgSPE leaving the working geometric area (A) of 0.196 cm2 with a conductive track dimension of 5×3 mm. The amount of Ag loaded on SPE was calculated as  1.3 mg cm − 2 corresponding to a silver film thickness of 638 mm. The resistance (Rf) was determined using a twopoint probe digital multimeter and the average Rf for the five strips is 0.4790.05 V cm − 1. Surface charge −1

Fig. 1. CV responses of AgSPE with (solid line) and without (dotted line) 96.5 mM Pb2 + in pH 3 KNO3/HNO3 solution at various scan rates of (a) 1, (b) 10, and (c) 300 mV s − 1.

(Q) for the adsorbed upd-Pb and bulk-Pb films on AgSPE was determined by simple integration of respective CV peaks a built-in BAS program. The equation of Q= nFAYs was used for the calculation of surface coverage (Ys) of the films. The AgSPE was first equilibrated in pH 3 KNO3/ HNO3 electrolyte for about 1 min before electrochemical experiments. It was then pretreated by continuous scan in the window of − 0.7 to − 0.1 V at w=50 mV s − 1 until a stable background current was obtained. All cyclic voltammetric (CV) experiments were done in the window − 0.1 to −0.7 V at various scan rates.

3. Results and discussion The CV responses of Pb2 + (96.5 mM) on the AgSPE under various scan rates are shown in Fig. 1. At w=1 mV s − 1, a couple of redox peaks at around − 300 mV (A1/C1) and − 450 mV (A2/C2) were noticed on the AgSPE. The feeble cathodic deposition peak at −500 mV (C2) with irreversible characteristic (ipa/ipc 1) is a typical example of bulk-Pb deposition and is in good agreement with the observation on solid electrodes [20–22]. The very small Efwhm value (= 25 mV) of the bulk-Pb peak implies the adsorptive behavior of the bulk-Pb at the AgSPE. The surface charge (Qc) is calculated to be 1.01 mC cm − 2 corresponding to a Ys = 52.4× 10 − 10 mol cm − 2 specifying the multilayer formation of the bulk-Pb crystallites at this relatively slower scan rate. The much smaller peak at − 300 mV with reversible characteristic (ipa/ipc : 1) reveals a minor upd-Pb formation on the AgSPE. The measured Ys for this peak is 3.27× 10 − 10 mol cm − 2 corresponding to small monolayer coverage of the upd-Pb film. Both the bulk-Pb and upd-Pb processes occur under this slow scan rate of 1 mV s − 1. However, the reversibility between upd-Pb and bulk-Pb is different; it may be due to the influence of site–site surface interaction parameters [23]. When the scan rate increased to 10 mV s − 1, the upd-Pb peak was found to increase by five times at the expense of the bulk-Pb peak (Fig. 1B). A complete diminishing of the bulk-Pb was noticed when the scan rate was further increased to 300 mV s − 1 (Fig. 1C). The variation of bulk-Pb and upd-Pb with scan-rate indicated different formation rates of these two processes. Fig. 2 shows the results obtained from potential segment analysis at scan rates of 2 mV s − 1 (A) and 300 mV s − 1 (B) for 0.1 mM of Pb2 + . At the scan rate of 2 mV s − 1, a weak faradaic process was noticed in the window of − 100 to −300 mV. Whereas, a specific redox peak at about −300 mV started to appear once the potential scanned beyond − 350 mV. The cyclic voltammogram showed two peaks at around − 500 mV and −300 mV corresponding to bulk-Pb and upd-Pb,

J.-M. Zen et al. / Electrochimica Acta 47 (2001) 899–904

Fig. 2. CV response of 0.1 mM Pb2 + on the AgSPE in pH 3 KNO3/HNO3 solution at different potential windows of (i) − 100 to −300 mV, (ii) −100 to −350 mV, (iii) − 100 to − 450 mV, and (iv) − 100 to − 700 mV at 2 mV s − 1 (A) and 300 mV s − 1 (B).

Fig. 3. (A) The ipa vs. w plots for (a) upd-Pb and (b) bulk-Pb processes. (B) The log(ipa) vs. log(w) plot for upd-Pb. [Pb2 + ]= 96.5 mM.

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respectively, when the potential scanned further to −700 mV. The same phenomenon is also observed at the scan rate of 300 mV s − 1 except with a near absence of bulk-Pb. These results confirm that it is not necessary to form bulk-Pb before the formation of upd-Pb on the AgSPE. However, the baseline-corrected anodic currents for the upd-Pb in the narrower potential window (curves iii) are lowered by about 5% (w=2 mV s − 1) and 25% (w= 300 mV s − 1) relative to those obtained from the wide potential window (curves iv). This implies that part of the bulk-Pb layer might be chemically transformed to the upd-Pb layer (particularly at faster scan rates). The electrochemical responses of the above processes are summarized in Fig. 3. As can be seen, the quick drop of ipa for bulk-Pb starts to level off at w= 20 mV s − 1; while it is almost linear up to w= 100 mV s − 1 for upd-Pb. A log(ipa) versus log(w) plot yielded a slope of 0.99 up to w= 100 mV s − 1 and after that the slope is close to 0.5 for upd-Pb. This observation shows the alternation in the nature of the stripping process at  100 mV s − 1. In other words, the rate-controlling process may convert from adsorption (B100 mV s − 1) to diffusion (\ 100 mV s − 1) under these conditions. Based on the above results, we offer three tentative conclusions: (i) the existence of scan rate dependence and different kinetics for the bulk-Pb and upd-Pb processes, (ii) the total absence of the bulk-Pb process at w\ 20 mV s − 1 and (iii) a switch from adsorption- to diffusion-controlled stripping behavior at w100 mV s − 1 for the upd-Pb process. Examination of the coulometric data can further clarify the above tentative conclusions. Fig. 4 shows the dependence of Q values on w for the upd-Pb and bulk-Pb processes. As can be seen, the bulk-Pb charge sharply decreased with the increase of w and after 20 mV s − 1 no bulk-Pb charge could be detected. This in

Fig. 4. A plot of surface charge (Q) against w on the AgSPE. [Pb2 + ]= 96.5 mM.

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Fig. 5. A plot of ipa vs. log[Pb2 + ] for upd-Pb and bulk-Pb at (A) 2 mV s − 1 and (B) 300 mV s − 1 in pH 3 KNO3/HNO3 solution.

fact supports conclusions (i) and (ii) and indicates limitation of the bulk-Pb process at faster scan rates on the AgSPE. Meanwhile, the measured upd-Pb charges showed a complex dependence on w (insert Fig. 4). The measured Qc sharply increased up to 20 mV s − 1, followed by a decreasing trend up to 100 mV s − 1 and finally a relatively constant charge. A similar trend in Qc was also observed for Qa except that the results were more irregular. It is expected that bulk-Pb on the AgSPE has some positive interaction with upd-Pb, leading to the increase in charge below 20 mV s − 1. The same reasoning can be applied to the decreasing trend above w\ 20 mV s − 1 due to the absence of bulk-Pb process. At the time scale of lower scan rate, the probability to observe adsorption is higher. Thus, the decreasing charge from 20 to 100 mV s − 1 is due to the desorption-controlled stripping of upd-Pb. On the other hand, when w \100 mV s − 1, a constant charge value (  10 mC) is observed, corresponding to a slope  0.5 due to diffusion-controlled stripping process. These criteria support conclusion (iii). Note that the chemical transformation ( 25%) of bulk-Pb “upd-Pb at fast scan rate and the positive interaction of the bulk-Pb with upd-Pb at slow scan rate should take into consideration. At present, the behavior of the upd-Pb process on non-single crystal AgSPE will require additional experiments for fundamental understanding. Information on limitation of bulk-Pd deposition and the behavior of upd-Pd acquired from these studies can be helpful for analytical purposes.

The effect of the concentration of Pb2 + on these two processes was studied at the scan rates of 2 and 300 mV s − 1, as shown in Fig. 5. At w= 2 mV s − 1, a predominant bulk-Pb stripping peak at −450 mV was observed for which ipa was two orders of magnitude higher than that of upd-Pb. The ipa of bulk-Pb sharply increased when [Pb2 + ]\ 0.2 mM (i.e. log[Pb2 + ]= − 0.3 to +0.3 in Fig. 5a). However, when [Pb2 + ]B 0.2 mM (i.e. [Pb2 + ]= − 1.3 to −0.3 in Fig. 5a), the ipa of bulk-Pb was observed only at w= 2 mV s − 1. This is probably because, at lower concentration of Pb2 + , upd-Pb is a competitive reaction on the AgSPE and thus leads to the decline in the bulk-Pb current signal. At faster scan rate (300 mV s − 1), up to 0.2 mM Pb2 + , only upd-Pb current signals at − 312 mV were observed as the reasons mentioned earlier. The upd-Pb process considerably decreased at the expense of the bulk-Pb formation when [Pb2 + ]\ 0.2 mM. The concentration limitation is actually not a concern since the AgSPE, without chemical modification, is employed as a disposable sensor for measurement of trace levels of Pb2 + . It was reported earlier that both the upd-Pb and bulk-Pb layers are in hexagonal close pack (hcp) arrangements [7,11,13]. A well-ordered 2D Pb adsorbedlayer occurs in the underpotential range, while 3D metal deposits as well as further epitaxial crystallite growth is obtained at the formal potential on Ag single crystal electrodes [7,11,13]. Surface morphological studies with different Pb deposition conditions confirm some of the expectation. Fig. 6 shows SEM pictures of Pb on the AgSPE at different scan rates. After ten successive linear scans at w=300 mV s − 1 (i.e. upd-Pb as indicated in Fig. 1C), no noticeable crystallites were found on the surface (Fig. 6B). Indeed, it is very difficult to discuss the upd layer by the SEM images obtained by a low magnification. Nevertheless, there is still a possibility that the absence of any crystallite behavior in SEM may be due to instability of 2D crystallites under the sample preparation conditions. This may also be due to relatively weaker surface–site interactions [23] of the upd-Pb layer with underlying metal. However, many islands of 3D nucleated crystallites were formed after ten successive scans at w=2 mV s − 1 (i.e. bulk-Pb) (Fig. 6C). Magnification of small crystallites showed a majority are hexagonal structures on the AgSPE (Fig. 6D), perhaps due to the strong adsorption and positive attraction between the surface sites. Based on the surface interaction model [24], the interaction parameter (r) is estimated from the surface activity relationship for the interacting sites with the following equation: ipa = nFQw/RT(4−2rYs). A positive ‘‘r’’ value of 4.68× 1011 cm2 mol − 1 was obtained for the bulk-Pb at w =2 mV s − 1, which clearly indicates the attractive interaction between the active sites on the electrode surface. These results distinguish the two nucleation processes (upd-Pb and bulk-Pb).

J.-M. Zen et al. / Electrochimica Acta 47 (2001) 899–904

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Fig. 6. SEM images of (A) AgSPE, (B) upd-Pb/AgSPE, (C) bulk-Pb/AgSPE, and (D) magnification of one of the crystals in (C). The SEM films are prepared by scanning in the window of − 0.1 to −0.7 V by linear scan voltammetry (n =10) using [Pb2 + ]= 0.1 mM (i.e. 20 ppm) at w = 300 mV s − 1 (upd-Pb) and 1 mV s − 1 (bulk-Pb) in pH 3 KNO3/HNO3 solution.

Overall, the weaker surface interaction with effective formation of upd-Pb at w \100 mV s − 1 may afford a favorable route to the development of rapid Pb-trace

metal analyses based on electroanalytical techniques with a renewable underlying surface. The nearly reversible behavior of upd-Pb is a suitable choice for the

Fig. 7. SWV responses for the increasing concentration of Pb2 + at the AgSPE in pH 3 KNO3/HNO3. Inset figure shows the typical calibration plot from the SWV responses. SWV parameters are: fHz = 50 Hz, Eamp =50 mV, and Estep =4 mV.

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effective stripping analysis of Pb by square-wave voltammetry (SWV), since the sensitivity of this technique is directly proportional to the reversibility of the system [25]. Fig. 7 shows the preliminary quantitative SWV analysis for the Pb2 + on the AgSPE. Note that, in this potential window, there are no complications from bulk-Pb. This is a desirable feature of the present system for sensitive electroanalytical Pb assays using disposable AgSPEs.

4. Conclusion The potential scan rate strongly influences the updPb and bulk-Pb formation on the AgSPE. Potential segment analysis identified that upd-Pb at low concentrations (B0.2 mM) is nearly independent of bulk-Pb formation. At low concentration of Pb2 + the irreversible 3D-nucleation (hcp) bulk-Pb is only favorable at wB 20 mV s − 1, whereas, at w \100 mV s − 1, the bulk-Pb is completely suppressed and 2D-nucleation (hcp) of the reversible upd-Pb is dominant. SEM studies indicated 3D-nucleation and hcp structure of the bulk-Pb at w=2 mV s − 1, while the absence of any crystallites at faster scan deposition indicated the formation and complete stripping (i.e. reversible) of the upd-Pb on the AgSPE. Most important of all, the reversible behavior of upd-Pb with comparatively weak surface interaction is suitable for adsorptive stripping analysis of Pb using the AgSPE. Further work is in progress with emphasis on solid-state characterization using XRD, AFM and development of a high precision analytical assay for Pb.

Acknowledgements The authors gratefully acknowledge financial support from the National Science Council of the Republic of China.

References [1] D.A. Roston, E.E. Brooks, W.R. Heineman, Anal. Chem. 51 (1979) 1728. [2] E. Ya Neiman, L.G. Petrova, V.I. Ignatov, G.M. Dolgopolova, Anal. Chim. Acta 113 (1980) 277. [3] J.-M. Zen, H.-H. Chung, A. Senthil Kumar, Anal. Chim. Acta 421 (2000) 189 and references therein. [4] E. Herrero, L.J. Buller, H.D. Abrunˇ a, Chem. Rev. 101 (2001) 1897 and references therein. [5] W. Obretenov, N. Dimitrov, A. Popov, J. Cryst. Grow. 167 (1996) 253. [6] X. Xing, I.T. Bae, D.A. Scherson, Electrochim. Acta 40 (1995) 29. [7] A. Popov, N. Dimitrov, T. Vitanov, D. Kashciev, E. Budevski, Electrochim. Acta 40 (1995) 1495. [8] K.J. Stevenson, D.W. Hatchett, H.S. White, Langmuir 12 (1996) 494. [9] S. Takahashi, K. Hasebe, A. Aramata, Electrochem. Comm. 1 (1999) 301. [10] C. Nishihara, K. Iwata, T. Tai, M. Yuasa, I. Sekine, Electrochem. Comm. 1 (1999) 104. [11] W.J. Lorenz, H.D. Hermann, N. Wuthrich, F. Hilbert, J. Electrochem. Soc. 121 (1974) 1167. [12] W. Obretenov, U. Schmidt, W.J. Lorenz, G. Staikov, E. Budevski, D. Carnal, U. Muller, H. Seigenthaler, E. Schmidt, J. Electrochem. Soc. 140 (1993) 692. [13] R.R. Adzic, J. Wang, C.M. Vitus, B.M. Ocko, Surf. Sci. 293 (1993) L876. [14] U. Schmidt, S. Vinzelberg, G. Stailkov, Surf. Sci. 348 (1996) 261. [15] I. Shain, S.P. Perone, Anal. Chem. 33 (1961) 325. [16] T. Ishiyama, T. Tanaka, Anal. Chem. 68 (1996) 3789. [17] M. Brand, I. Eshkenazi, E.K. Eisner, Anal. Chem. 69 (1997) 4660. [18] T. Miwa, Y. Nishimura, A. Mizuike, Anal. Chim. Acta 140 (1982) 59. [19] H. Shen, J.E. Mark, C.J. Seliskar, H.B. Mark Jr., W.R. Heineman, J. Solid State Electrochem. 1 (1997) 241. [20] J.-M. Zen, H.-H. Chung, A. Senthil Kumar, Analyst 125 (2000) 1633. [21] J.-M. Zen, H.-H. Chung, G. Ilangovan, A. Senthil Kumar, Analyst 125 (2000) 1139. [22] A.J. Bard, R. Parsons, J. Jordan, Standard Potential in Aqueous Solutions, IUPAC, Marcel Dekker, New York, 1985, p. 221. [23] A. Senthil Kumar, K. Chandrasekara Pillai, J. Solid State Electrochem. 4 (2000) 408. [24] A.P. Brown, F.C. Anson, Anal. Chem. 49 (1977) 1589. [25] J.G. Osteryoung, R.A. Osteryoung, Anal. Chem. 57 (1985) 101A.

Potential scan rate dependence of underpotential and ...

was demonstrated for the AgSPE in pH 3 KNO3/HNO3 solutions. .... line) 96.5 mM Pb2+ in pH 3 KNO3/HNO3 solution at various scan .... 101 (2001) 1897.

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