Underpotential deposition of cadmium adatoms on Te and CdTe

Electrochimica Acta 52 (2006) 996–1002

Underpotential deposition of cadmium adatoms on Te and CdTe N.P. Osipovich ∗ , S.K. Poznyak Physico-Chemical Research Institute, Belarusian State University, Leningradskaya str. 14, 220050 Minsk, Belarus Received 20 February 2006; received in revised form 22 May 2006; accepted 16 June 2006 Available online 8 August 2006

Abstract Underpotential deposition (UPD) of Cd adatoms onto the surface of Te and CdTe films in Cd2+ -containing solutions has been studied using electrochemical methods and AFM. The electrochemical deposition of Cd adatoms on Te and CdTe begins at potentials 400 mV more positive than the reversible potential of Cd2+ /Cd couple and proceeds irreversibly. A strong chemical interaction of Cd adatoms with the surface Te atoms is the driving force of the UPD process. The deposition of Cd adatoms on the tellurium surface is accompanied by their stepwise interaction with tellurium to give CdTe nanophase. © 2006 Elsevier Ltd. All rights reserved. Keywords: Underpotential deposition; Cadmium; Electrodeposition; Cadmium telluride; Tellurium; Adatoms

1. Introduction The study of the processes of adatom electrodeposition on foreign substrates at potentials more positive than the equilibrium Nernst potential (underpotential deposition, UPD) provides an important information on the mechanism of some electrochemical processes, such as adsorption, charge transfer, nucleation and growth of a new phase, etc. [1–4]. Up to now most of the papers concerning UPD of adatoms dealt with “metallic electrode–foreign metal adatoms” systems and more than 50 systems of that kind were described [4,5]. But there is relatively little information on the UPD processes on the semiconductor surfaces. Electrodeposition of lead monolayers on the germanium surface [6], copper and silver monolayers on Hg1−x Znx Te [7], UPD of copper on diamond [8] were studied. We have found the Pb UPD on Se [9], Te [10], PbSe [11] and PbTe [12], the Bi UPD on Te [13]. Note that the UPD of metal adatoms on semiconductors is of importance in the electrochemical synthesis of binary AII BVI and AIII BV semiconductors. In particular, in ECALE (electrochemical atomic layer epitaxy) method, alternating UPD of metal and non-metal adatoms is used [14–22]. Also worthy of mention are the works on electrodeposition of CdTe and CdSe films [23–25], where the metal and the chalcogen are codeposited at the potentials more ∗

Corresponding author. E-mail address: [email protected] (N.P. Osipovich).

0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.06.038

positive than the equilibrium potential EMz+ /M0 . In those works the electrodeposition potential does not reach the values necessary to reduce chalcogen to chalcogen hydride, and, thus, the chalcogenide formation must include the step of metal UPD on semiconductor. Although the possibility of Cd UPD on CdSe and Te was reported [26,27], these processes have not been studied in detail. Here we present the results of a detailed investigation of the UPD of cadmium adatoms on tellurium and cadmium telluride. 2. Experimental 2.1. Electrodes Polycrystalline (according to the XRD analysis) Te and CdTe film electrodes were obtained by potentiostatic electrodeposition of these materials onto an Au substrate from solutions containing 0.002 M TeO2 + 0.1 M HNO3 (deposition potential: −0.2 V; T = 70 ◦ C), and 1 M CdSO4 + 0.001 M TeO2 + 0.05 M H2 SO4 (deposition potential: −0.3 V; T = 70 ◦ C), respectively. Before the deposition, the Au foil was polished using diamond paste, treated with hot concentrated HNO3 and H2 SO4 , then washed with bidistilled water and finally heated at 700 ◦ C for 20 min in the air. The prepared Te and CdTe films were about 0.5 ␮m thick; CdTe films had the p-type conductivity. Before the experiments the Te electrodes were cycled in 0.1 M HNO3 in the potential range from −0.5 to 0.38 V. To get the

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correct results, each of the voltammograms was measured on an individual electrode.

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3. Experimental results 3.1. Cadmium electrodeposition on tellurium

2.2. Apparatus and chemical reagents Electrochemical experiments were carried out in a standard three-electrode two-compartment cell with a platinum counter electrode and an Ag|AgCl|KCl(sat.) reference electrode. All potentials were determined with respect to this reference electrode and controlled by a conventional potentiostat with a programmer. Current densities reported in this work refer to geometric area of the electrodes. Spectral dependences of the photocurrent were obtained using a set-up equipped with a high intensity grating monochromator, a 1000 W xenon lamp and a slowly rotating light chopper (0.3 Hz). The photocurrent spectra were corrected for the spectral intensity distribution at the monochromator output. The polychromatic light from 100 W halogen lamp (j = 4.2 mW cm−2 ) equipped with an infrared filter was also used for the photoelectrochemical measurements. AFM images of the electrode surface were obtained with a Femto-Scan-001 microscope (Advanced Technologies Center, Moscow) operated in the constant force mode (1.5–5 nN) under ambient conditions. AFM tips were standard oxide-sharpened Si3 N4 cantilevers with spring constants of 0.06 and 0.12 N m−1 . The computer analysis of the AFM images was performed to estimate the true surface area (roughness factor) of Te films. Reagents of the highest purity and doubly distilled water were used in the electrolyte preparation. All solutions were deaerated by blowing purified argon through and over the solution. Analysis of the solutions for cadmium was carried out using an atomic absorption spectroscopy.

No Faradaic processes are observed on Te electrode in the background electrolyte (0.1 M HNO3 ) in a wide range of potentials (Fig. 1a, dotted line). Anodic Te oxidation begins at E > 0.4 V, while cathodic reduction starts at E < −0.5 V. With Cd2+ ions present in the solution, a cathodic current peak CTe is observed at potentials more positive than the equilibrium ECd2+ /Cd0 potential (−0.67 V against Ag|AgCl|KCl(sat.) ) for the negative potential scan. On the subsequent reverse scan, an anodic A1Te peak appears (Fig. 1a). When the cathodic limit of polarization is negatively shifted, the A1Te peak concurrently decreases and then disappears and another anodic peak A2Te appears. Study of the electrochemical processes on Te electrode in 0.01 M Cd(NO3 )2 + 0.1 M HNO3 solution revealed the following regularities: • Both of the anodic peaks and the cathodic one are positively shifted with increasing the Cd2+ concentration in solution. • The A1Te peak on the positive scan appears only after the cathodic CTe peak is recorded on the negative scan. • At a potential sweep rate of 20 mV s−1 the charge corresponding to the anodic A1Te peak is close to the cathodic charge passed through the electrode on the potential sweep down to −0.36 V (Fig. 1a, curve 1). But when the cathodic polarization limit is negatively shifted, the cathodic charge becomes greater than the anodic charge measured on the subsequent positive scan, which is accompanied by the appearance of the A2Te peak (Fig. 1a, curves 3 and 4). On increasing the

Fig. 1. (a) Voltammograms of Te electrode in 0.1 M HNO3 (dotted line) and in 0.01 M Cd(NO3 )2 + 0.1 M HNO3 solutions (solid lines). The potential was scanned from open circuit potential EOCP = 0.33 to −0.36 V (1), −0.385 V (2), −0.45 V (3), −0.61 V (4) and then to 0.35 V. Potential sweep rate: 20 mV s−1 . (b) Voltammograms of Te electrode in 0.01 M Cd(NO3 )2 + 0.1 M HNO3 solutions. The potential was scanned from open circuit potential EOCP = 0.33 to −0.35 V (1), −0.4 V (2), −0.46 V (3), −0.52 V (4) and then to 0.35 V. Potential sweep rate: 200 mV s−1 . The inset shows the dependence of the charges corresponding to the cathodic (solid dots) and anodic (open dots) peaks on the lower potential limit.

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Fig. 2. Anodic voltammograms of Te electrode in 0.01 M Cd(NO3 )2 + 0.1 M HNO3 solution recorded after its potentiostatic polarization at −0.45 V for 5, 10, 50 s, and 5 min. Potential sweep rate: 20 mV s−1 .

sweep rate up to 200 mV s−1 only one anodic A1Te peak is observed in the voltammograms (Fig. 1b). The charge corresponding to this peak increases initially with increasing the passed cathodic charge and then reaches the limiting value (Fig. 1b, inset). • If Cd is deposited on Te electrodes under potentiostatic conditions, the subsequent positive scan demonstrates A2Te peak growing and A1Te peak decreasing and then disappearing with deposition time (Fig. 2). In order to obtain additional information on the nature of the A1Te and A2Te peaks, we performed the chemical analysis of anodic dissolution products. To do this, the Te electrode after its negative polarization in Cd2+ -containing solution was withdrawn from the cell, rinsed and transferred into another cell with 0.1 M HNO3 solution. After recording the anodic peaks the solution in the cell was analyzed for cadmium. The obtained results showed that the charges corresponding to A1Te and A2Te peaks are in good agreement with the cadmium amount found in the solution, indicating that these peaks are associated only with cadmium stripping. Tellurium is a narrow-gap semiconductor (Eg = 0.4 eV) with a rather high electrical conductivity at room temperature. Hence, Te electrode behaves as a metallic one in solution. We could not notice any photoelectrochemical activity of Te electrode in background electrolyte in the potential range investigated. But after the cathodic CTe peak has been recorded in the Cd2+ containing solution, a cathodic photocurrent is observed on the illumination of Te electrode (Fig. 3). The long-wavelength edge of the photocurrent spectrum (the inset in Fig. 3) correlates well with the absorption edge of cadmium telluride, indicating that this compound is formed on the Te electrode during the Cd UPD. The CdTe formation is accompanied by a significant change in the surface morphology of Te electrode. As-prepared Te film consists of grains of about 100 nm in width (Fig. 4). The negative polarization of the electrode at −0.45 V in 0.01 M Cd(NO3 )2 + 0.1 M HNO3 solution for several minutes (the passed charge corresponds to the deposition of 12–15 monolayers of cadmium) leads to the appearance of smaller grains

Fig. 3. Voltammograms of Te electrode in 0.01 M Cd(NO3 )2 + 0.1 M HNO3 solution recorded at modulated polychromatic illumination. Inset shows the photocurrent spectrum of Te electrode with cadmium deposited on its surface at polarization to −0.61 V. The spectrum was recorded in 0.1 M HNO3 solution at −0.4 V.

with a mean diameter of 20 nm on the background of the initial surface structure (Fig. 4). 3.2. Electrodeposition of cadmium on cadmium telluride To elucidate the nature of the current peaks observed at Te electrode in Cd2+ -containing solutions, we also studied the electrochemical behavior of CdTe film electrodes in the same solutions. The surface of the CdTe electrodes, which were withdrawn from the deposition cell immediately after their preparation, washed and transferred into another cell filled with 0.01 M Cd(NO3 )2 + 0.1 M HNO3 solution under Ar atmosphere, have no time to be oxidized appreciably. If the potential of these electrodes (hereafter referred to as as-prepared electrodes) is shifted 1 positively from the open circuit potential (EOCP = −0.09 V), an 2 anodic peak ACdTe is observed at 0.15 ÷ 0.25 V, and at E > 0.4 V there is a drastic current rise due to the dissolution of whole CdTe film (Fig. 5a, dotted line). On negative potential scan, a cathodic CCdTe peak is observed at −0.4 V and anodic A1CdTe and A2CdTe peaks are recorded on the subsequent reverse scan (Fig. 5a). The charge corresponding to the anodic peaks in this case is greater than that for the cathodic peak by the value of the charge corresponding to the A2CdTe peak (Fig. 5a, dotted line) recorded at the initial anodic polarization. Furthermore, a small A1CdTe peak observed at E ≈ 0 V disappears on shifting the cathodic polarization limit into more negative potential region (Fig. 5a). If the as-prepared CdTe electrodes are kept in the deposition electrolyte for several minutes or in air for several tens of minutes (after having been removed from the deposition cell), their surface is oxidized strongly and the open circuit potential 2 becomes more positive (EOCP = −0.12 V). There is no A2CdTe peak on the anodic voltammograms of these electrodes (Fig. 5b, dotted line). On the negative potential scan, a cathodic CCdTe peak is observed and anodic A1CdTe and A2CdTe peaks are recorded

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Fig. 4. AFM images of the surface of as-prepared Te electrode (a) and the Te electrode with cadmium adatoms deposited potentiostatically at a potential of −0.45 V for 4 min (b).

on the subsequent positive scan (Fig. 5b). For these electrodes, the charges corresponding to the cathodic and anodic peaks are close, and the anodic A1CdTe peak is higher than the A2CdTe peak at initial stages of Cd electrodeposition. Moreover, in the case of strongly oxidized electrodes the A1CdTe peak is markedly higher as compared with that for the as-prepared electrodes (Fig. 5a and b).

4. Discussion 4.1. Electrochemical behavior of CdTe electrodes The electrochemical deposition of CdTe film in 1 M CdSO4 + 0.001 M TeO2 + 0.05 M H2 SO4 electrolyte proceeds in the regime of UPD of Cdad and overpotential deposition of Te [14]. The Cd adatoms are deposited on the surface Te atoms and the quantity of the deposited Cd is bounded by a monolayer since the UPD of Cd on Cd is impossible. Only after deposition of fresh Te atoms on the surface, the Cd UPD becomes again possible. Immediately after electrodeposition is stopped, both cadmium and tellurium atoms can be on the CdTe film surface. When we transfer quickly the electrode into the other cell with 0.01 M Cd(NO3 )2 + 0.1 M HNO3 solution and polarize it positively, cadmium is dissolved from the surface according to the reaction [28]: CdTe → Cd2+ + Te + 2¯e

Fig. 5. Voltammograms of as-prepared CdTe electrodes (a) and the CdTe electrodes with strongly oxidized surface (b). The potential was scanned from the open circuit potential (EOCP ) in the positive direction (dotted lines) and from the EOCP in the negative direction with different scan limits (−0.4 V (solid lines), −0.5 V (dashed lines) and −0.61 V (dashed-dotted lines)) and then back in the positive direction. Potential sweep rate: 20 mV s−1 .

(1)

giving the anodic A2CdTe peak (Fig. 5a, dotted line). Note that this process proceeds at more negative potentials than the direct electrooxidation of CdTe into Cd(II) and Te(IV). On negative polarization of as-prepared CdTe electrode, the UPD of Cd occurs on Te atoms which are on the telluride surface (cathodic CCdTe peak). On the subsequent positive scan, both the deposited Cd atoms and those, which were initially present on

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the surface of the as-prepared electrode, are dissolved, so that the charge corresponding to A2CdTe peak is greater than that for the cathodic CCdTe one (Fig. 5a). When as-prepared CdTe electrode is exposed to the deposition solution or to air for a long time, cadmium is lost from the surface and the latter is enriched with tellurium. It may be a result of its oxidation by oxygen or of displacement processes, for example: CdTe|2Cds + Te(IV) → CdTe|Tes + 2Cd(II)

(2)

where Cds and Tes are the surface cadmium and tellurium atoms participating in the displacement reaction. Such displacement processes were previously observed for cadmium adatomic layer on PbSe in Pb2+ -containing solution [29] and for copper adatomic layer on Au(1 1 1) in Pt(IV)- and Pd(II)-containing solutions [30]. The formation of a Te layer on the CdTe surface during its oxidation was shown in a number of works [28,31–34]. It was found that after immersion in an acidic solution (2 M H2 SO4 ) with or without Ce4+ ions the stoichiometric surface of CdTe is covered with a Te0 layer with a limiting thickness of about 1–2 nm [32–34]. On the photoelectrochemical oxidation in acidic medium (HClO4 , pH 0) CdTe is first oxidized into Cd2+ and elementary tellurium, the latter forming a passivation layer at the electrode surface [28]. According to the findings of Lincot and Vedel, the thickness of this layer does not exceed 1 nm. Taking into account the covalent radius of Te atom (0.137 nm), this corresponds to approximately 3–4 monolayers of tellurium. In addition, the enrichment of the CdTe surface with tellurium was found to proceed during the photoelectrochemical oxidation of CdTe electrodes [35,36]. It is reasonable that the peak of cadmium dissolution, A2CdTe , is absent on positive polarization of the electrode with strongly oxidized surface (Fig. 5b, dotted line), and cadmium is deposited all over the surface on negative polarization, so that the charge corresponding to CCdTe peak is greater than that for the asprepared electrode with slightly oxidized surface, a part of the latter being occupied by cadmium. The A1CdTe peak may be associated with the anodic stripping of Cd adatoms deposited onto Te-rich surface of the oxidized CdTe electrodes. This assumption is supported by a close similarity of the A1Te and A1CdTe peaks and by the fact that the A1CdTe peak is significantly higher for strongly oxidized CdTe electrodes in comparison with as-prepared ones. 4.2. The UPD of Cd adatoms on tellurium The results obtained suggest that the CTe , and A1Te peaks observed at Te electrodes are related to the UPD of cadmium adatoms on tellurium and their stripping, respectively. As to the peaks A2Te on Te and A2CdTe on CdTe, their similarity suggests that they are of the same nature, that is, both are related to the process of cadmium stripping from the CdTe surface. This fact as well as the appearance of the photoelectrochemical activity on Te with the deposited Cd adatoms and the change in the surface morphology evidence strongly that the UPD of Cdad on tellurium is accompanied by a progressive interaction of cadmium adatoms

and tellurium atoms to give CdTe phase on the surface. The evolution of the stripping voltammograms recorded after potentiostatic polarization of the Te electrode in the potential region of Cd UPD for different periods of time distinctly demonstrates the changes occurring on the Te surface. With increasing the Cdad deposition time, the peak (A1Te ) of Cdad dissolution from the Te surface decreases and disappears and the peak A2Te associated with the CdTe dissolution appears and then increases (Fig. 2). The fact that the cathodic charge passed during the Cd UPD becomes greater than that recorded on subsequent positive scan, is easy to be explained: only Cd atoms from the CdTe surface are dissolved, while the deeper ones are not oxidized in this potential region. When recording the voltammograms at a sufficiently high sweep rate (200 mV s−1 ), cadmium telluride has no time to be formed because of kinetic limitations. In this case the Cd UPD on Te is not yet complicated by the CdTe formation and only one anodic peak related to the stripping of Cd adatoms from the Te surface is observed in the voltammograms (Fig. 1b). The limiting value of the charge corresponding to the A1Te peak was used as the charge required for the UPD of Cdad monolayer on Te. It was determined on the Te electrode with a known roughness factor and was 400 ± 50 ␮C cm−2 of the real surface. This value is close to the calculated charge necessary to form a cadmium monolayer with hexagonal close packing. Thus, during the Cd UPD on Te the deposition of several monolayers of cadmium is possible owing to the interaction of Cd adatoms with tellurium resulting in the CdTe formation, and to the Cd diffusion into the Te bulk. Although this is not a classical UPD process, however, it should be noted that this process is similar to the process of surface alloying observed during the UPD of some metals onto the other metals. The study of the UPD of Pb on Au [37–39], Ag [39–46] and Tl on Ag [46,47] and Cd on Au [48,49] showed that the surface alloying is a rather common phenomenon in the UPD systems which are characterized by a strong adatom–substrate interaction. There is a large separation on the potential scale between the peaks of deposition (CTe ) and stripping (A1Te ) of cadmium adatoms on tellurium. According to [50], this suggests that the UPD of Cdad on Te is irreversible. The latter is also evidenced by the effect of the sweep rate on the potential of these peaks: increasing sweep rate results in a negative shift of the CTe peak and in a positive shift of the A1Te peak. An important characteristic of the UPD process is the underpotential shift, EUPD —the difference in peak potentials of adatomic monolayer adsorption–desorption and equilibrium EMz+ /M0 potential [4]. To estimate EUPD value for the UPD of Cdad on Te, we should take into account the process irreversibility. According to [50,51], for an electrochemical reaction Mz+ + ze → Mad with the rate-determining step being the charge transfer, the differences between the potential of the reversible cat ) process peak (Ep,r ) and those of the peaks of cathodic (Ep,irr an ) processes for an irreversible process depend and anodic (Ep,irr on the potential sweep rate, v, as follows: cat Ep,irr − Ep,r =

RT RT − (βzF )ln v0 (βzF )ln v

(3)

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Fig. 6. Voltammograms of Te electrode in 0.01 M Cd(NO3 )2 + 0.1 M HNO3 solutions recorded at different potential sweep rates (a) and the dependence of potentials corresponding to cathodic (1) and anodic (2) peaks on logarithm of the potential sweep rate (b). an Ep,irr − Ep,r = −

RT RT + ((1 − β)zF )ln v0 ((1 − β)zF )ln v

(4)

where v0 is the reversibility parameter, β is the symmetry factor for the surface process. From Eqs. (3) and (4) it follows that cat versus ln v and Ean versus ln v the slopes for the plots of Ep,irr p,irr are RT/βzF and RT/(1 − β)zF, respectively, and the intersection of the plots gives the values of ln v0 and Ep,r . The potential difference EMz+ /M0 − Ep,r gives the value of EUPD . The following aspects should be taken into account when determining the EUPD for the Cd–Te system. It is necessary to deposit a Cdad monolayer avoiding the CdTe formation because the latter process leads to a change of the surface nature. Furthermore, for definiteness sake the measurements should be performed at the same degree of surface coverage. These conditions can be met owing to the cathodic potential limit, the value of which should be changed with changing the potential sweep rate. The corresponding voltammograms and the dependence of potentials of the adsorption (CTe ) and desorption (A1Te ) peaks of Cdad monolayer on Te are shown in Fig. 6. For the UPD of Cdad on Te, the estimated EUPD value was about 0.47 V. The exact value is difficult to determine, because the potential of the anodic A1Te peak depends slightly on the potential of the cathodic polarization limit. In the works [52,53] from a thermodynamic point of view, the following expression was derived for the UPD shift: EUPD =

bind ze(EM –S

1 bind ) − EM –M

(5)

bind is the binding energy per atom of bulk metal M where EM –M bind and EM–S the binding energy of an atom M when adsorbed on the substrate S. Going to moles we obtain

EUPD =

1 zeNA (E bind M–S

− E bind M–M )

=

1 zF (E bind M–S

− E bind M–M ) (6)

If for the UPD of Cdad on Te we take f G298 CdTe (98.55 kJ mol−1 ) as a measure of strengthening of Cd–Te bond in comparison with Cd–Cd bond, we can obtain EUPD = 98,550/(2 × 96,500) ≈ 0.5 V. This calculated value is

in good agreement with the EUPD value estimated experimentally. 5. Conclusions The present study showed that the UPD of cadmium adatoms can be performed on Te electrode, with the UPD shift being about 0.5 V. This process is accompanied by a progressive interaction of cadmium adatoms and tellurium atoms to give CdTe nanophase, resulting in a noticeable photoelectrochemical activity of the Te electrode. A characteristic feature of the UPD of Cdad on tellurium surface is the irreversibility of the process, which may be related to a strong interaction of cadmium adatoms with surface tellurium atoms. The Cd UPD on CdTe electrodes occurs at the same potentials as on Te electrodes and Cd adatoms are deposited only on the surface Te atoms. However, the anodic stripping of the Cd adatoms from CdTe is observed at 200 mV more positive potentials in comparison with that from Te. The phenomenon of the UPD of cadmium adatomic layers on tellurium is of interest for understanding the mechanism of CdTe electrosynthesis and directs the possible ways of preparing semiconductor structures including monolayers or nanoparticles of cadmium telluride. Acknowledgments This work was supported by the NATO Collaborative Linkage Grant PST.CGL.979631 and the Belarusian state program “NANOTEKH”. References [1] R.R. Adzic, in: H. Gerischer, C.W. Tobias (Eds.), Advances in Electrochemistry and Electrochemical Engineering, vol. 13, Wiley, New York, 1978, p. 159. [2] D.M. Kolb, in: H. Gerischer, C.W. Tobias (Eds.), Advances in Electrochemistry and Electrochemical Engineering, vol. 11, Wiley, New York, 1978, p. 125. [3] K. J¨uttner, W.J. Lorenz, Z. Phys. Chem. 122 (1980) 163.

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N.P. Osipovich, S.K. Poznyak / Electrochimica Acta 52 (2006) 996–1002

[4] D.M. Kolb, M. Przasnysky, H. Gerischer, J. Electroanal. Chem. 54 (1974) 25. [5] E. Herrero, L. Buller, H.D. Abruna, Chem. Rev. 101 (2001) 1897. [6] A.B. Ershler, A.V. Kaisin, Kh.Z. Brainina, I.M. Levinson, Elektrokhimiya 22 (1986) 203. [7] C.N. van Huong, P. Lemasson, Phys. Rev. B 40 (1989) 3021. [8] F. Bouamrane, A. Tadjeddine, R. Tenne, J.E. Butler, R. Kalish, C. L´evyCl´ement, J. Phys. Chem. 102 (1998) 134. [9] E.A. Streltsov, S.K. Poznyak, N.P. Osipovich, J. Electroanal. Chem. 518 (2002) 103. [10] E.A. Streltsov, N.P. Osipovich, L.S. Ivashkevich, A.S. Lyakhov, Electrochim. Acta 44 (1998) 407. [11] E.A. Streltsov, N.P. Osipovich, L.S. Ivashkevich, A.S. Lyakhov, V.V. Sviridov, Electrochim. Acta 43 (1998) 869. [12] E.A. Streltsov, N.P. Osipovich, L.S. Ivashkevich, A.S. Lyakhov, A.N. Vesti, Belar. Ser. Khim. Nauk 3 (1997) 21 (in Russian). [13] N.P. Osipovich, E.A. Streltsov, A.S. Susha, Electrochem. Commun. 2 (2000) 822. [14] B.W. Gregory, J.L. Stickney, J. Electroanal. Chem. 300 (1991) 543. [15] I. Villegas, J.L. Sticney, J. Electrochem. Soc. 139 (1992) 686. [16] B.W. Gregory, D.W. Suggs, J.L. Sticney, J. Electrochem. Soc. 138 (1992) 1279. [17] F. Loglio, M. Innocenti, G. Pezzatini, M.L. Foresti, J. Electroanal. Chem. 562 (2004) 117. [18] R. Vaidyanathan, J.L. Stickney, U. Happek, Electrochim. Acta 49 (2004) 1321. [19] M. Innocenti, S. Cattarin, M. Cavallini, F. Loglio, M.L. Foresti, J. Electroanal. Chem. 532 (2002) 219. ¨ ul¨uer, U. ¨ Demir, J. Electroanal. Chem. 529 (2002) 34. [20] T. Ozn¨ [21] M. Innocenti, F. Forni, G. Pezzatini, R. Raiteri, F. Loglio, M.L. Foresti, J. Electroanal. Chem. 514 (2001) 75. ¨ Demir, C. Shannon, J. Phys. Chem. B. 102 [22] A. Gichuhi, B.E. Boone, U. (1998) 6499. [23] M.P.R. Panicker, M. Knaster, F.A. Kroger, J. Electrochem. Soc. 125 (1978) 566. [24] F.A. Kroger, J. Electrochem. Soc. 125 (1978) 2028. [25] Z. Loizos, N. Spyrellis, G. Maurin, Thin Solid Films 204 (1991) 139. [26] Kh.Z. Brainina, V.V. Nikiforov, Elektrokhimiya 25 (1989) 1237 (in Russian). [27] E.A. Streltsov, N.P. Osipovich, Dokl. Akad. Nauk Belar. 42 (1998) 58 (in Russian).

[28] D. Lincot, J. Vedel, J. Cryst. Growth 72 (1985) 426. [29] E.A. Streltsov, N.P. Osipovich, L.S. Ivashkevich, A.S. Lyakhov, Electrochim. Acta 44 (1999) 2645. [30] S.R. Brankovic, J.X. Wang, R.R. Adˇzi´c, Surf. Sci. 474 (2001) L173. [31] Y.-C. Lu, R.S. Feigelson, R.K. Route, J. Appl. Phys. 67 (1990) 2583. [32] C. Debiemme-Chouvy, F. Iranzo Mar´ın, U. Roll, M. Bujor, A. Etcheberry, Surf. Sci. 352–354 (1996) 495. [33] F.I. Mar´ın, J. Vigneron, D. Lincot, A. Etcheberry, C. Debiemme-Chouvy, J. Phys. Chem. 99 (1995) 15198. [34] A. Etcheberry, F.I. Mar´ın, E. Novakovic, R. Triboulet, C. DebiemmeChouvy, J. Cryst. Growth 184/185 (1998) 213. [35] H. Gerischer, W. Mindt, Electrochim. Acta 13 (1968) 1239. [36] P.P. Konorov, S.M. Repinskii, Elektrokhimiya 4 (1968) 226 (in Russian). [37] U. Schmidt, S. Vinzelberg, G. Staikov, Surf. Sci. 348 (1996) 261. [38] M.P. Green, K.J. Hanson, R. Carr, I. Lindau, J. Electrochem. Soc. 137 (1990) 3493. [39] M.P. Green, K.J. Hanson, Surf. Sci. 259 (1991) L743. [40] H. Siegenthaler, K. J¨uttner, Electrochim. Acta 24 (1979) 109. [41] T. Vitanov, A. Popov, G. Staikov, E. Budevski, W. Lorenz, E. Schmidt, Electrochim. Acta 31 (1986) 981. [42] A. Popov, N. Dimitrov, O. Velev, T. Vitanov, E. Budevski, Electrochim. Acta 34 (1989) 265. [43] A. Popov, N. Dimitrov, D. Kashchiev, T. Vitanov, E. Budevski, Electrochim. Acta 34 (1989) 269. [44] N. Dimitrov, A. Popov, T. Vitanov, E. Budevski, Electrochim. Acta 36 (1991) 2077. [45] N. Dimitrov, A. Popov, D. Kashchiev, T. Vitanov, E. Budevski, Electrochim. Acta 36 (1991) 1259. [46] D. Carnal, P.I. Oden, U. M¨uller, E. Schmidt, H. Siegenthaler, Electrochim. Acta 40 (1995) 1223. [47] H. Siegenthaler, K. J¨uttner, E. Schmidt, W.J. Lorentz, Electrochim. Acta 23 (1978) 1009. [48] J.W. Schultze, F.D. Koppitz, M.M. Lohrengel, Ber. Bunsen. Phys. Chem. 78 (1974) 693. [49] R. Vidu, S. Hara, J. Electroanal. Chem. 475 (1999) 171. [50] B.E. Conway, H. Angerstein-Kozlovska, US Nat. Bur. Stand. Spec. Publ. 455 (1976) 107. [51] H. Angerstein-Kozlowska, B.E. Conway, J. Electroanal. Chem. 95 (1979) 1. [52] C. S´anchez, E. Leiva, J. Electroanal. Chem. 458 (1998) 183. [53] W. Schmickler, Chem. Phys. 141 (1990) 95.