XPS, SEM, EDX and EIS study of an electrochemically modified
electrode surface of natural chalcocite (Cu
2S)
P. Vela´squez
a, D. Leinen
a, J. Pascual
a, J.R. Ramos-Barrado
a,*, R. Cordova
b,
H. Go´mez
b, R. Schrebler
baLaboratorio de Materiales y Superficie,Unidad Asociada al CSIC,Departamento de Fı´sica Aplicada & Departamento de Ingenierı´a Ci6il,
Materiales y Fabricacio´n,Facultad de Ciencas,Uni6ersidad de Ma´laga E-29071Malaga,Spain
bInstituto de Quı´mica,Facultad de Ciencias Ba´sicas y Matema´ticas,Uni6ersidad Cato´lica de Valparaı´so Casilla 4059,Valparaı´so,Chile Received 20 January 2000; received in revised form 1 March 2001; accepted 13 May 2001
Abstract
An electrode surface of a natural chalcocite mineral obtained from the ‘Chuquicamata’ mine in Chile has been studied by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy including microanalysis (SEM/EDX). Different potentials were applied via CV to the electrode surface in a specially designed preparation chamber of inert (Ar gas) atmosphere. This chamber was coupled, in the case of XPS analysis, to the X-ray photoelectron spectrometer and, in the case of SEM/EDX analysis, to the electron microscope in order to study quasi-in-situ changes in the chemical composition and morphology at the electrode surface induced by the electrochemical treatment and to relate them to the EIS results obtained for the same applied potential. CV was performed in an aqueous 0.05 M borax electrolyte solution of pH 9.2 at 300 K (cycles: 0−850 mV (cathodic sense: reduction)+200 mV (anodic sense: oxidation)−400 mV, all vs. SCE). Along the cyclic voltammogram, which shows two anodic peaks, ten points of applied potential (for 1 or 400 s) have been studied by XPS. Partial oxidation from Cu(I) to Cu(II) is observed at +100 mV versus SCE applied for 400 s which is assigned to the formation of CuO, Cu(OH)2and possibly Cu3(SO4)(OH)4. For comparison, the fractured mineral surface was
also studied by XPS. The measurements of SEM and EDX did not show any relevant alterations, except for the potential of
+100 mV versus SCE in the positive-going potential, in which the formation of protrusions with a high concentration of oxygen (44%) was detected. The modifications detected by XPS, SEM and EDX reflect large changes in the electrochemical parameters obtained using EIS and are characteristic of a partially covered electrode; the charge-transfer resistance across the irregularly formed layer on the surface increases for the point +100 mV versus SCE in the positive-going potential and its capacitance is reduced. © 2001 Elsevier Science B.V. All rights reserved.
Keywords:Chalcocite; EIS; XPS; SEM/EDX
1. Introduction
Chalcocite, with a bulk composition of Cu2−xS, x ranging from 0 to 4×10−3, is a mixed conductor with
largely predominant electronic conduction (p-type semi-conductor) and ionic conduction due to copper(I) ion motion [1]. These minerals have a crystal structure based on antifluorite in which the Cu(I) partially occu-pies the tetrahedral interstitials in the fcc sulphur ma-trix allowing easy migration [2]. There have been many
electrochemical studies on copper sulphide minerals [3 – 7] most of them aimed at understanding their be-haviour in different stages of mineral processing, partic-ularly in the froth flotation steps, the latter involving complex interactions between collector reagents and sulphide surfaces [3,4,8,9]. These electrochemical pro-cesses, which are determined by the electrochemical interactions on dissolution at the electrode surface, are dependent on the composition and morphology of the mineral. During the flotation process, the surface mod-ification could be so great as to form a hydrated oxidised film of material with a heterogeneous composi-tion and morphology which depends on, among other factors, the applied potential.
* Corresponding author. Tel.: +34-952-131922; fax: + 34-952-132000.
E-mail address:[email protected] (J.R. Ramos-Barrado).
0022-0728/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 0 1 ) 0 0 5 3 3 - 2
The electrical interaction at the electrode surface with the electrolyte and its variations with different oxida-tion potentials can be ascertained by cyclic voltamme-try (CV)and electrochemical impedance spectroscopy (EIS). However, the relationship between the variations in electrochemical parameters and the surface chemical composition and morphology also assume the need for the determination of the variation of both characteris-tics for different oxidation potentials applied. This re-quires the use of analytical techniques such as X-ray photoelectron spectroscopy (XPS) and scanning elec-tron microscopy including microanalysis (SEM and EDX), which allow the study of changes in chemical composition and morphology of the surface as a func-tion of the applied potential. The use of these tech-niques [10] requires the construction of an inert atmosphere chamber, which serves to carry out the electrochemical processes and transfer the electrode to the interior of the XPS, or SEM/EDX chamber, thus avoiding contact with the atmosphere and consequent modification of the chemical composition of the elec-trode surface. SEM, EDX and XPS have all been used quasi-in-situ in previous studies to study the changes in oxidation state of different elements in the electrode surface of enargite throughout the voltammogram, us-ing a controlled inert atmosphere chamber communi-cating with the XPS or SEM/EDX chambers [11,12]. The aim of the present paper is the application of XPS, SEM and EDX to the study of the changes in morphology and chemical composition of the electrode surface of natural chalcocite during CV and to relate these results to the changes in the electrochemical parameters as determined from the EIS measurements for the same oxidation potential values applied throughout the voltammogram cycle. To this end the experiments with XPS, SEM, EDX, and EIS have been performed for the most significant potentials.
2. Experimental
2.1. Cyclic 6oltammetry and electrochemical impedance spectroscopy
The working electrode was natural chalcocite (Cu2S) obtained from the ‘Chuquicamata’ (Chile) mine; its crystal structure was determined by XRD and has shown to correspond to 20-0365 (Cu1.97S) or 12-0205
(Cu1.96S) copper sulphide according to the PDF data
base. The sample was placed on an epoxy resin. The area of electrode exposed to the electrolyte was 0.2 cm2.
The electrode was attached to the copper wire with In – Ga. The auxiliary electrode was made of graphite (Goodfellows) and the reference electrode was a satu-rated calomel electrode (SCE) from EG&G. Before each measurement, i.e. for each applied potential, a fresh electrode surface was prepared by wet abrading with 600-grade silicon carbide paper and then rinsing with deoxygenated deionised water. Finally, a fine pol-ish was achieved with an alumina suspension of first 0.3 and then 0.05 mm particle size. This procedure of electrode preparation was carried out in a glove box under an Ar atmosphere to prevent any oxidation process.
After each electrochemical treatment, each electrode surface was cleaned by rising with deoxygenated deionised water before XPS or SEM/EDX analysis. The electrolyte solution was prepared with disodium tetraborate decahydrate (borax) from Merck (EWG-Nr. 215-540-4) to 0.05 M in deoxygenated deionised and water, decarbonated by heat treatment, with a constant ionic strength of 0.2 at pH 9.2. Voltammetry measurements were performed with a Solartron EI 1286 controlled by computer by means of a specialised pro-gram. The Solartron EI 1286 was connected to the electrochemical cell via copper feedthroughs in the glove box with a diameter of 3 mm. Cycles were performed from 0 to −850 mV (negative-going poten-tial scan), then to +200 mV (positive-going potential scan) and back to −400 mV, all versus SCE at a scan velocity of 5 mV s−1. For the voltammogram (Fig. 1),
the points of study for surface modifications were − 100 (point 3), −300 (point 4), −500 (point 5), −700 (point 6), −700 (point 7), −500 (point 8), −100 (point 9), +100 mV (point 10), +100 (point 11), −150 (point 12), −300 mV (point 13). Once cycled to each of these potentials, the electrode potential was maintained for 1 s (instantaneous analysis: this was done only for XPS analysis to reveal the effect of time of applied potential as shown in Fig. 3) or at least 400 s or, if it was necessary, until a steady state current was observed. Once this steady state situation was reached XPS, SEM/EDX and EIS measurements were carried out. Impedance data were recorded with a Solartron 1255 frequency response analyser (FRA). For all Fig. 1. Cyclic voltammogram of a natural chalcocite electrode in
alkaline borax solution of pH 9.2 at room temperature recorded at 5 mV s−1. Arrows with numbers indicate points of XPS, SEM/EDX and EIS analysis.
impedance measurements, the Solartron FRA sine wave output was superimposed on an applied dc bias from the Solartron EI 1286. The FRA and the EI were both controlled by computer by means of a specialised pro-gram. Ac measurements were made for the frequency range from 10 mHz to 10 kHz for different dc applied potentials and an ac potential of 0.01 V rms. EIS measurement was carried out for each point of study (see above). All measurements were carried out at 300 K.
2.2. X-ray photoelectron spectroscopy
XPS measurements were carried out with a PHI 5700 spectrometer using Mg – Ka radiation (1253.6 eV) and Al – Ka radiation (1486.6 eV) as excitation sources. Multiregion spectra were recorded at 45° take-off-angle with a concentric hemispherical energy electron analy-ser operating in the constant pass energy mode at 29.35 eV, using a 720mm diameter analysis area. Under these conditions, the Au 4f7/2 line was recorded with 1.16 eV
FWHM at a binding energy of 84.0 eV. The spectrom-eter energy scale was calibrated using Cu 2p3/2, Ag 3d5/2
and Au 4f7/2 photoelectron lines at 932.7, 368.3 and
84.0 eV, respectively. Survey and multiregion spectra were recorded of the Cu 2p, S 2p, C 1s, O 1s, B 1s and Na 1s photoelectron peaks. The pressure in the analysis chamber was maintained lower than10−7Pa. A PHI
ACCESS ESCA-V6.0 F software package was used for data acquisition and analysis. The atomic concentra-tions were calculated from the photoelectron peak areas using Shirley background subtraction [13] and sensitiv-ity factors provided by the spectrometer manufacturer PHI (Physical Electronics, 6509 Flying Cloud Drive, Eden Prairie, MN 55344, USA). Recorded spectra were not shifted in binding energy. Cu 2p3/2 for Cu(I) was
found at 932.8 eV. After each electrochemical treatment (i.e. for each point of study) the electrode surface was cleaned from residual electrolyte solution by rinsing with double-distilled, deionised and decarbonated wa-ter, all in the glove box filled with Ar gas (99.999%) and coupled to the spectrometer. Then, Mg – Kaand Al – Ka survey spectra and multiregion spectra were recorded. In this way, the XPS analyses were carried out for the same points as the EIS experiments without exposure of the electrode surface to ambient atmosphere. For com-parison, a mineral surface, fractured in the same cham-ber (Ar atmosphere) was also studied.
2.3. Scanning electron microscopy and EDX
SEM micrographs and EDX spectra were obtained with a JEOL JSM 6400 scanning electron microscope equipped with a Link analytical system. The electron energy used was 20 keV. In this case, the glove box filled with Ar gas (99.999%) was coupled to the
micro-scope for analysis of each point of CV applied poten-tial. In this way, the SEM and EDX analysis were carried out for the same points as in EIS and XPS experiments. After each electrochemical treatment (point of study) and subsequent cleaning of the elec-trode surface from residual electrolyte solution by rins-ing with double-distilled, deionised and decarbonated water, all in the glove box filled with Ar gas (99.999%) and coupled to the SEM/EDX chamber, SEM mi-crophotographs and EDX spectra were recorded.
3. Experimental results and discussion
3.1. Cyclic 6oltammetry
Previous voltammetric experiments have been done to determine the main peak currents, which were associ-ated with redox processes at the electrode surface. Fig. 1 shows the voltammetry curve I/E in a buffer borate solution of pH 9.2. In the voltammogram, cathodic currents can be seen at −0.85 V versus SCE (peak A, charge of 5.5 mC cm−2), −300 mV versus SCE (peak
D, charge of 0.93 mC cm−2), and anodic currents at
−0.65 V versus SCE (peak B, charge of 3.1 mC cm−2),
+100 mV versus SCE (peak C, charge of 5.3 mC cm−2), both in the positive sweep direction
(posi-tive-going potential scan). The rapid current increase observed at the end of the negative scan corresponds to copper reduction according to the reaction [8]:
Cu2S+H2O+2e−2Cu+HS−+OH− (1)
The reverse of this reaction accounts for the process associated with anodic peak B, assuming that the HS−
required for this remains in the vicinity of the interface. This could be in the pores of the natural mineral, for instance. The anodic peak C has been related to copper sulphide electro-oxidation going through all possible non-stoichiometric sulphides with correlative formation of Cu(II).
Cu2S+2xOH−Cu2−xS+xCuO+xH2O+xe−
(2)
The cathodic peak D is very complex and corresponds to the reverse reaction of reaction (2) and, may be, to the reduction of the oxy – hydroxyl species of Cu(II) originating at the electrode surface:
CuO+H2OCu+2OH− (3)
3.2. X-ray photoelectron spectroscopy
The fractured chalcocite mineral surface shows a stoichiometry of Cu1.94S which is in good agreement with the bulk composition as determined by XRD to Cu1.97S (after PDF 20-0365). However, the polished
Table 1
XPS atomic concentrations andxvalues of the Cu/S atomic ratio obtained for the different chalcocite surfaces CuxS
Treatment Atomic concentration/%
DRL spectra
Cu S C O Na
1.94 41.0 21.1+2.0
1 Fracture surface 22.4 13.5 0
2 Polished surface 0.87 9.7 11.1 73.3 5.9 0
3 −0.1 V vs. SCE 1.38 20.9 15.2 40.0 21.0 2.9
1.47 25.3 17.2
−0.3 V vs. SCE 31.4
4 20.6 5.5
−0.5 V vs. SCE
5 1.38 22.5 16.3 29.1 22.6 9.5
1.13 12.0 10.6
6 −0.7 V vs. SCE 43.5 23.4 10.5
1.45 19.3 13.3
−0.7 V vs. SCE 35.6
7 21.2 10.6
−0.5 V vs. SCE
8 1.38 18.0 13.0 26.9 23.9 18.2
9 −0.1 V vs. SCE 1.58 29.4 18.6 32.3 16.2 3.5
1.48 13.5 9.1
+0.1 V vs. SCE 31.5
10 40.0 5.9
1.78 18.4 10.3
11 +0.1 V vs. SCE 35.4 33.4 2.5
1.65 27.8 16.8
−0.15 V vs. SCE 24.0
12 25.5 5.9
1.63 25.3 15.5 24.2
13 −0.3 V vs. SCE 28.2 6.8
The different chalcocite surfaces are the freshly fractured, the polished, and the surfaces treated at different electrochemical potentials. For spectra identification see also Fig. 1.
electrode surface of the natural mineral reveals a highly copper deficient composition (Cu0.9S) and an unusually
large amount of carbon. (Table 1). At the fractured mineral surface some SO42− species have been found
which disappear with polishing (Fig. 2, S 2p3/2 peak at
about 169 eV), and, if formed during the electrochemi-cal treatment, are dissolved and washed out by rinsing with water (Fig. 3). These species are due to the pres-ence of Cu3(SO4)(OH)4in the chalcocite mineral which is typical for minerals of the Chuquicamata mine [7]. The electrochemical treatment increases the amount of Cu at the polished electrode surface to CuxS with x:1.4. However, the value ofxis influenced greatly by the amount of residual surface contamination not effec-tively eliminated during rinsing with water just after the electrochemical treatment and before passing into the XPS analysis chamber (Table 1).
Fig. 2 shows the normalised Cu 2p and S 2p spectra recorded after each electrochemical treatment of ap-plied potential for 400 s. From these spectra it is evident that the only change in chemical state can be observed at an oxidation potential of +0.1 V versus SCE for Cu (spectra 10 and 11). Both Cu 2p spectra show, apart from the main Cu 2p3/2 peak at 932.8 eV,
a new contribution which can be seen as a shoulder on the high binding energy side of the peak. This new contribution is characteristic of Cu(II) indicating that the electrode surface has been oxidised partially. Hence, the corresponding current peak maximum in the voltammogram (Fig. 1) can be assigned clearly to an oxidation process of copper (Cu(I) to Cu(II)), i.e. the formation of species such as CuO, Cu(OH)2 and/or
even Cu3(SO4)(OH)4 . However, S 2p spectra remain
unchanged as well as for all other potentials of study (S 2p3/2at 162.0 eV) which is explained below (Fig. 3). As
can be seen by the subsequent spectra (spectra 12 ff.), the oxidation state of Cu(II) is unstable with reduction potentials and copper is found again in Cu(I).
Fig. 2. Normalised XPS Cu 2p and S 2p spectra (1 – 13) recorded after each electrochemical treatment of 400 s applied potential as indicated in Fig. 1 (to guide the eye a vertical scale numbering has been given).
Fig. 3. Normalised XPS Cu 2p and S 2p spectra at point 10, i.e.
+100 mV vs. SCE applied potential in the positive-going potential scan, showing the effect of time of applied potential and by rinsing the electrode with water. After having scanned to point 10 the potential was maintained at+100 mV vs. SCE for about 1 s (spectra 3 and 4) or for 400 s (spectra 1 and 2) and was rinsed with water (spectra 1 and 3) or not rinsed with water (spectra 2 and 4) before XPS analysis.
Fig. 4. Scanning electron microscopy view of the polished electrode surface.
Fig. 3 shows the effect of time with an applied potential and the effect by rinsing with water. Cu spectra demonstrate that it is not sufficient just to scan to +0.1 V versus SCE, but time is needed for the oxidation of Cu(I) to Cu(II), i.e. the oxidation process is dynamically slow which explains why the electrode surface even after 400 s of applied potential is only partially oxidised (the main contribution to the Cu 2p signal is still from Cu(I)). On the other hand, S 2p spectra show that SO42− species may always be formed
at this potential, but are eliminated during rinsing with water, which indicates that the oxidation and formation of SO42− species takes place only on the outermost
atomic layer of the electrode surface.
3.3. SEM and EDX analysis
The surface of the polished electrode shows a striated flat surface with small cracks and holes. (Fig. 4). Anal-ysis with EDX of the surface shows a composition, which takes into account that the range of error is similar to the stoichiometry of chalcocite, since Cu2.1S
is obtained. This is in agreement with XPS results for fractured chalcocite although it differs from the pol-ished electrode surface (Cu0.9S). This difference is due
to the differences in data sampling depth of each tech-nique for the photoelectrons (XPS) and the SEM elec-trons (approximately 20 keV). XPS provides information about very superficial layers (:10 A,), whereas EDX penetrates much more deeply and hence gives bulk composition. The voltammogram results ob-tained by SEM and EDX for different points show virtually no variation either in morphology or chemical composition except for point 10, +100 mV versus SCE in the positive-going potential scan, which corresponds to the highest anodic current. The micrographs for this point make it clear that small bright agglomerations in varying forms are present, being irregularly spread over the surface of the electrode. Furthermore, in the
micro-graphs obtained from electrons back scattering, the existence of protrusions and valleys can be observed which form a surface with gentle undulations. EDX did not detect the presence of oxygen on the surface but did show a high proportion of copper which assumes a stoichiometry enriched in copper. However, if the bright agglomerations are analysed, an elevated propor-tion of oxygen (43.5%) is detected and a decrease in copper (37%) and sulphur (19.2%), (Fig. 5). The holes have also been analysed showing oxygen always to be present, a typical result being 18% oxygen, although with a high presence of copper (48%) and some show-ing the presence of iron.
The presence of the bright agglomerations is depen-dant on the extent of rinsing with water of the electrode prior to passing into the SEM/EDX chamber, for which reason of its origin can be attributed to very superficial processes and formed from a mixture of different Cu(II) composites. In any case, the data ob-tained from XPS indicate that the oxidation of the electrode surface is only partial (Table 1) which coin-cides with antlerite, SO4Cu3(OH)4 [14].
3.4. Electrochemical impedance spectroscopy
In EIS measurements, values ofVdcwere applied to a
natural chalcocite mineral electrode from 0 to −850
Fig. 5. EDX spectra of the electrode surface for +100 mV vs. SCE in positive-going potential scan.
mV (negative-going potential scan), then to +200 mV (positive-going potential scan) and back to −400 mV all versus SCE at a scan velocity of 5 mV s−1. For
every EIS measurement, we started to apply the poten-tial from 0 V versus SCE; then cycling at a scan velocity of 5 mV s−1to the desired potential (point in Fig. 1) of
study where we stopped the scan and maintained the applied potential for 400 s or, if necessary, until a steady state current was observed, before carrying out the EIS measurements. Then a new fresh electrode surface was prepared by a wet abrading with 600-grade silicon carbide paper and fine polishing and then rinsing with deoxygenated deionised water and we repeated the process just to the next target potential. Data were fitted to equivalent electrochemical circuits by a non-linear method [15]. Fitting of the equivalent circuits was carried out applying criteria of simplicity and electro-chemical interpretation; the equivalent circuits repre-sent an approach to the description of the electrochemical processes into the electrodeelectrolyte interface and any models derived from them are only tentative.
Fig. 6 shows the Nyquist plot for the complex impedance for the natural chalcocite electrode in the borate buffer solution and Fig. 7 shows the Bode plot for angle phase. If we compare these plots, we can see that there is a slowly varying modification in the plots and as we raise the values of applied potential, the semicircle at high frequencies increases. For all applied potentials, we obtain the same type of equivalent cir-cuit, Fig. 8; this type of circuit corresponds to two faradic relaxation processes.Re is the resistance of the electrolyte and other ohmic resistance,Rslis the surface
layer resistance,Rctis the charge-transfer resistance and
Qdl corresponds to the double-layer capacitance; Qc is
associated with an irregular surface coating layer [16]; this circuit is often used to represent an imperfectly covered electrode [17 – 19]. The constant phase element is a distributed element defined asQ=1/Kpn, 05n5
1. In this expression, n is a dimensionless number,Kis a constant whose dimension is F sn−1cm−2 and p=j
with =2pf. Q is equivalent to the impedance of a capacitance C=K(2pf)n−1, that is, the CPE behaves
as a capacitance which varies with frequency. This modification to an ideal capacitance has already been explained by distribution effects [20], porosity [21], fractal geometry [22] or more recently by distribution of interfacial capacitances [23,24].
Nyquist plots derived from the results obtained are very similar (Fig. 6). Nevertheless, a more detailed analysis of these results by means of equivalent circuits reveals that there are important differences.
The resistanceRct presents a slow decrease along the
successive points of the voltammogram, except for
points 8 – 10, −500, −100 and +100 mV versus SCE in the positive-going potential scan, respectively, where a sharp rise is seen. Points 8 and 10 are situated immediately after the anodic peaks (Fig. 1) and the rise is accompanied by a highly pronounced decrease inQdl.
These changes are related to modification in chemical composition and morphology as has been deduced from XPS and SEM/EDX results. Hence the first an-odic peak, peak B, is associated with the oxidation of Cu(0) to Cu(I). Cu(0) was formed during the peak A (Eq. (1)). There is no evidence from XPS or EDX data for the formation of Cu(0) in the peak A, but in the processes of peak B, a very superficial layer of Cu2−xS (see Table 2) should form with lower conductivity than the substratum, although its permittivity would be very similar and with almost no change in Qc. For peak C, the higher anodic peak current, the changes in chemical composition in the electrode surface are more impor-tant than for the other anodic peaks. The oxidation of part of the Cu(I) from Cu2−xS to Cu(II) as CuO and
Cu(OH)2, and the presence of SO42−, Cu3(SO4)(OH)4in
peak C implies a greater modification of the electrode surface since the resistance of CuO is higher than that of the chalcocite and its permittivity is lower [3,18,19]. This reflects the rise in Rct and the highly pronounced
decrease in Qc.
The other characteristic resistance, the surface layer resistance, Rsl, decreases greatly at points 6 and 7.
These points are found in areas of rapid increase in the point 7 cathodic and anodic current, respectively, due to the reduction processes of Cu(I) from Cu2S to Cu(0)
with the cathodic current (peak A, Eq. (1)) and the inverse process of oxidation with the anodic current (peak B). Furthermore,Qdlrises sharply at these points
and behaves as a pure capacitor. The resistance of Rsl
rises sharply at points 8 and 9, situated after peak B. In this part of the voltammogram the current is virtually zero since up to peak C the oxidation of Cu(I) of the chalcocite to Cu(II) or the CuO or the Cu(OH)2 does
not occur. These results agree with those obtained from SEM, EDX, and XPS given that for these points, neither chemical nor morphological modifications are observed, and Cu2−xS for both points give very similar values for x (Table 2).
4. Conclusions
Oxidation of chalcocite in an alkaline medium results in a modified morphology and chemical composition of the surface. The joint application of analytical tech-niques (SEM, EDX, and XPS) and electrochemical techniques (CV and EIS) allows us to relate changes on the surface with changes in the most characteristic
Fig. 6. Nyquist plots for the complex impedance at points of applied potential 1 – 13 as indicated in Fig. 1. Plotted data sets correspond to measured values (×) and fitted values ().
electrochemical parameters as determined by EIS. The measurements taken from XPS, SEM, and EDX for different potentials along the voltammogram clearly show that the surface modification mainly occurs at the potential next to the highest anodic peak of the voltam-mogram. These changes, both in morphology and
chemical composition, give rise to a heterogeneous surface with protrusions of high oxygen content and with varying morphologies spread irregularly over the electrode surface. The transformations of the latter cause changes in the characteristic parameters of the equivalent circuit which best represents the EIS data,
this being characteristic of partially covered electrode surfaces; the charge-transfer resistance and the resis-tance of the holes in the surface show a heavy depen-dence on the potential considered in the voltammogram in which XPS, SEM, and EDX measurements have
been made. The resistance of the film rises sharply after the main anodic peak due to the formation of CuO, Cu(OH)2 and probably Cu3(SO4)(OH)4. On the other hand, the charge-transfer resistance rises for potentials between the two anodic peaks.
Fig. 7. Bode plots for the phase angle at points of applied potential 1 – 13 as indicated in Fig. 1. Plotted data sets correspond to measured values (×) and fitted values ().
Table 2
Values of element of the equivalent circuits (Fig. 8) for different applied potentials along the voltammogram
n Rct/kV Rsl/kV 104×Qdl/V−1sn
Points 10−4×Qc/V−1sn ndl
3.304 29.424
0.61 4.573
2.155 0.56
3
0.76
3.404 18.886 1.083
4 1.639 0.65
0.72 1.865
8.3232 1.911
1.769 0.64
5
1.329 0.235 289 1
6 4.590 0.56
1.854 0.076 909.9 1
7 4.866 0.55
0.62 8.711 2.869 6.733
8 2.755 0.85
17.405 3.519
0.61 6.748
4.927 0.72
9
0.69 5.719 9.803 7.709
10 0.595 0.79
0.73 2.506 – 3.489
11 0.448 0.36
1.214 26.422
0.033 0.58
0.70 3.158
12
0.017 33.078 3.199 0.61
13 0.347 0.93
Fig. 8. Equivalent circuit:Re, resistance of the electrolyte and other ohmic resistance; Rsl, surface layer resistance; Rct, charge transfer resistance;Qdl, double-layer capacitance;Qc, irregular surface coating layer.
[5] A.N. Buckley, R. Woods, J. Electroanal. Chem. 357 (1993) 387. [6] D.P. Tao, Y.Q. Li, P.E. Richardson, R.H. Yoon, Colloids Surf.
A 93 (1994) 229.
[7] A.N. Buckley, T.J. Parks, A.M. Vassallo, R. Woods, Int. J. Mineral Process. 51 (1997) R7.
[8] H. Go´mez, J. Vedel, R. Cordova, R. Schrebler, L. Basae´z, J. Electroanal. Chem. 388 (1995) 888.
[9] S. Sa´nchez, S. Cassaignon, J. Vedel, H. Go´mez, Electrochim. Acta 41 (1996) 1331.
[10] P. Vela´squez, J.R. Ramos-Barrado, R. Cordova, D. Leinen, Surf. Interface Anal. 30 (2000) 149.
[11] P. Vela´squez, D. Leinen, J. Pascual, J.R. Ramos-Barrado, R. Cordova, H. Go´mez, R. Schrebler, J. Electroanal. Chem. 494 (2000) 87.
[12] C. Klein, C.S. Hurbart, Manual of Mineralogy, 21st ed., Wiley, New York, 1998.
[13] D.A. Shirley, Phys. Rev. B 5 (1972) 4709. [14] J.J. Finney, T. Araki, Nature 197 (1963) 70. [15] B.A. Boukamp, Solid State Ionics 20 (1986) 31. [16] J.R. Macdonald, J. Appl. Phys. 58 (1985) 1971.
[17] D.C. Silverman, Electrochemical Impedance, ASTM, Philadel-phia, PA, 1993, p. 194-3455.
[18] J. Pang, A. Briceno, S. Chander, J. Electrochem. Soc. 137 (1990) 3447.
[19] G.B. Appetecchi, S. Scaccia, S. Passerini, J. Electrochem. Soc. 14 (2000) 4448.
[20] G.J. Brug, A.L.G. Van den Eden, M. Sluyters-Rehbach, H.J. Sluyters, J. Electroanal. Chem. 176 (1984) 275.
[21] L.M. Gossa, J.R. Vilche, M. Ebert, K. Su¨ttner, W.J. Lorenz, J. Appl. Electrochem. 20 (1990) 677.
[22] T. Pajkossy, L. Nikos, Electrochim. Acta 34 (1989) 171. [23] T. Pajkossy, Solid State Ionics 94 (1997) 123.
[24] Z. Kemer, T. Pajkossy, J. Electroanal. Chem. 448 (1998) 139. Acknowledgements
The financial support of the Junta de Andalucı´a (Spain) through research group FQM192 and FONDECYT (Chile) through contract LC No. 8000022 is gratefully acknowledged; moreover, the bursary holder, P. Vela´squez, wishes to thank AECI (Agencia Espan˜ola de Cooperacio´n Internacional).
References
[1] Th. Pauporte`, J. Vedel, Solid State Ionics 116 (1999) 311. [2] C. Conde Amiano, Electron Microsc. 1 (1980) 478. [3] R. Woods, G.A. Hope, Colloids Surf. A 146 (1999) 63. [4] R. Woods, R.H. Yoon, Lagmuir 13 (1997) 876.
