Colloids and Surfaces, 41 (1989) 15-23 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

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Surface Properties of Cuprous Sulphide in Aqueous Solutions

PAWED NOWAK and ANDRZEJ POMIANOWSKI

Polish Academy of Sciences, Institute of Catalysis and Surface Chemistry, ul. Niezapominajek,

30239 Krakdw (Poland)

(Received 22 August 1988; accepted 14 November 1988)

ABSTRACT

The interfacial impedance of non-stoichiometric cuprous sulphide electrodes in solutions of indifferent electrolytes was measured in order to study the electrical double layer (e.d.1.) at the sulphide-aqueous solution interface. A significaint dispersion of impedance components was ob- served, which was ascribed to the influence of roughness and inhomogeneity of the surface. The measured capacitive part of the impedance is influenced both by the solution side and by the solid- body side of the e.d.1.

INTRODUCTION

Copper sulphides are the world’s main source of copper and the copper sul- phide minerals are processed in industry in very large quantities in the form of aqueous suspensions. It is well known that electrical properties of the inter- face influence significantly the behaviour of the suspension. Surprisingly, in- formation concerning the surface electrical properties of copper sulphides in the literature is rather spare. Oestreicher and McGlashan [l] have investi- gated the surface electrical properties of copper sulphides, studying electroki- netic phenomena, and have reached the conclusion that the properties of the surface of sulphide are similar to the oxide; but that was obviously due to the fact that the surface of sulphide was significantly oxidized. The electrical con- ductivity of non-stoichiometric cuprous sulphides is high, and the concentra- tion of free charge carriers is approximately equal to the number of copper atoms missing (in comparison to the ideal stoichiometry Cu,S ). The electro- chemical properties of copper sulphides have been investigated very inten- sively but in most works copper sulphides were investigated from the point of view of hydrometallurgy - no information concerning the electrical double layer (e.d.1. ) structure at the surface of the sulphide may be drawn from these works. There are only a few works [ 2-41 in which cyclic voltammetry in solutions of indifferent electrolytes was used to investigate the behaviour of the surface,

0166-6622/89/$03.50 0 1989 Elsevier Science Publishers B.V.

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and there are no works (except our preliminary investigations [ 5,6] ) in which impedance methods were used.

Therefore, measurements of the interfacial impedance of the electrodes made of non-stoichiometric cuprous sulphide have been performed in order to inves- tigate the properties of the e.d.1. at the surface of the sulphide in contact with aqueous solution.

EXPERIMENTAL

Reagents, materials and apparatus

Non-stoichiometric cuprous sulphide of compsition Cu1.$3, similar in com- position to many naturally occurring cuprous sulphides, was synthesized from analytical-grade copper and sulphur. The sulphide obtained was cast in a large lump and cut on a diamond saw to convenient dimensions. The composition of the sulphide was checked by chemical analysis, X-ray diffraction analysis showed that it was composed of djurleite and digenite [ 731. Inspection of the surface of the samples by optical microscopy did not show any cracks or pores. All electrochemical experiments were performed using a typical electrochem- ical setup in a three-electrode configuration, with the saturated calomel elec- trode (SCE) as reference electrode. All potentials in this work are quoted ver- sus that electrode. The interfacial impedance was measured using a computerized ESM 700 system (produced by Center for Design of Scientific Apparatus of the Academy of Sciences of G.D.R., Berlin) or by a home-made system. A more detailed description of the measuring system used may be found in our previous paper [ 61, In both cases the measured interfacial impedance (2) was expressed in terms of series connection of pure resistive (ReZ) and pure capacitive (ImZ) parts. From the capacitive part the series capacitance C, was calculated by the formula C,= l/ (2xfImZ), where f is the frequency of the measuring signal. Of course, C, is not equal to either the interfacial capac- itance or to the high frequency limit of the interfacial capacitance. The mea- surements were performed in the frequency range 5 kHz-5 mHz in solutions of NaF, KNO, or Na2B,0, of different concentrations. All reagents used were of analytical grade and were additionally recrystallized from doubly distilled water, which was also used for preparation of the solutions. All experiments were performed at the temperature of 298 K. Before measurements the solu- tions were bubbled with argon containing < 1 ppm of oxygen. The behaviour of cuprous sulphide electrodes was compared to the behaviour of copper elec- trodes - polycrystalline copper electrodes were made by melting analytical- grade copper into the form of a rod, which was next cut on a diamond saw.

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Ex~~rirnent~~ procedure

The crucial problem in studying the interfacial properties of the solid body is the preparation of the surface prior to measurements. In the case of semi- conductors, etching of the surface in proper reagents is usually applied - in the case of metal electrodes, polishing and electropolishing are used. These methods may not be used in the studies of soft material such as copper sul- phides. It has been stated that intensive polishing, as well as etching of the surface with typical etching agents, changes significantly the properties of the surface in comparison with the surface obtained by cleavage of a piece of the sulphide in the solution. In our opinion, only the surface obtained by cleavage reflects perfectly the properties of the surface of a mineral particle in the min- eral suspension. However, cleavage always produces a very irregular surface and no quantitative measurements may be made for such a surface. It was found that the surface obtained by gentle polishing on rather coarse emery paper (up to 800), followed by gentle polishing on filter paper and left for N 5 min in an argon atmosphere, differs only very little from the surface obtained by cleavage - such preparation of the surface was used during this work. How- ever, the surface prepared in such a way is rather rough, which causes signifi- cant problems in interpretation of the results.

For mounting the electrodes, the so-called dipping technique [9] was used to minimize the problems of contamination of the solution and possible leak- age of the solution into the electrode mounting system. After introduction of the electrode to the measuring cell, it was kept for 5 min in an argon atmo- sphere and brought into contact with the solution. This was done either under potentiostatic control or at free potential - in the latter case the potential was measured for some time after having established the contact. For electrodes with the surface created in the solution, the rod of the sulphide with the side walls insulated with paraffin wax was introduced in the solution and broken. The changes of potential during breaking were noted using a fast measuring system with a digital memory.

RESULTS AND DISCUSSION

In our previous paper [6] we established that the interface of non-stoichio- metric cuprous sulphide-aqueous solution of indifferent electrolyte may be de- scribed by the equivalent electrical circuit, consisting of solution resistance R,,

high frequency differential interfacial capcitance C,, faradaic reaction resis- tance R,, diffusional (Warburg) impedance Wand the so-called constant phase element CPE (Fig. 1). The latter element arises due to the roughness and inhomogeneity of the surface, and is a complicated combination of solution resistance and interfacial capcitance. The behaviour of rough electrodes may be described using the so-called fractal theory [lO,ll], which explains per-

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J

CPE

Fig. 1. Electrical equivalent circuit for the cuprous sulphide electrode in solutions of indifferent electrolyte. R,, solution resistance; C,, high frequency limit of electrode capacitance; R,, faradaic reaction resistance; W, diffusional impedance; CPE, constant phase element.

fectly, in our opinion, the behaviour of rough surfaces. Unfortunately, its mathematical formalism does not make it possible to calculate the most inter- esting quantity (from the point of view of surface studies), the low frequency limit of the interfacial capacitance (thermodynamic capacitance). The only indication from the fractal theory is that the measured interfacial capacitance should be closer to the true interfacial capacitance for a higher concentration of electrolyte and a lower frequency of the sinusoidal signal.

The influence of surface oxides on measured impedance

The surface created at the free access of air is always covered to some extent by surface oxides. When such a surface is brought into contact with the solu- tion, the measured series impedance is low and irreproducible. The surface oxides may be reduced at the appropriate potential [ 12 ] and, after having reached that potential, reproducible and stable results may be obtained (Fig. 2) providing that the potential range of the experiment from Fig. 2 is not ex- ceeded. The behaviour of the electrode depends also on the pH of the solution. For pH values lower than - 5 and higher than - 10, the accessible range of the potential is very narrow. Outside the potential range shown in Fig. 2, rather large currents of anodic or cathodic decomposition begin to flow and no imped- ance measurements are possible. Once the potential of the reduction of surface oxides is reached, the measurements in the range of potentials below the po- tential of the surface oxide reduction also give stable results. This behaviour is completely different from the behaviour of metallic copper. When the copper electrode is polarized in slightly alkaline solution in the anodic direction, the surface oxides are formed in an electrochemical process, which is a well-estab- lished fact (see, for example, the work of Deutscher and Woods [ 131). These surface oxides block the surface and suppress the measured interfacial capac- itance. Such behaviour of copper is not an exception but is rather typical for metals. However, for cuprous sulphide no formation of surface oxides in an electrochemical process was observed in neutral and moderately alkaline so- lutions. Instead, the formation of soluble copper species was observed, which

80.

70.

60.

so-

L I

-600 -400 -200 0

E/mVvs SCE

Fig. 2. Changes of the measured series capacitance of the cuprous sulphide electrode with the surface initially covered with surface oxide during consecutive measurements of interfacial imped- ance. 0.1 mol dm-” NaF.

was proved in the cyclic voltammetry experiments on rotating disc electrodes. Similar behaviour was reported for natural cuprous sulphide [ 141.

The influence of measurement conditions on measured capacitance

As stated in the previous paper [ 61, there is no frequency range in which the dispersion of impedance parameters on frequency caused by the roughness of the surface ceases. Even for very low frequencies (0.01 Hz for example), for which the faradaic currents (caused by the tails of the anodic and cathodic decomposition reactions) are large in comparison with the a.c. current, the dispersion is still significiant. Therefore, on the basis of the known equivalent electrical circuit, the frequency range lo-20 Hz was chosen to present the re- sults of the measurements. In Fig. 3, the dependence of the measured series capacitance on potential for two concentrations of sodium nitrate are shown. Note that there was no correction for the ohmic potential drop in the solution, so, due to some faradaic current flowing in the range of extreme values of po- tential, the flattening of the curve may be expected for the lower concentration

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400 400 -200 d

EmvvsSCf /

Fig. 3. Dependence of the measured series capacitance of the cuprous sulphide electrode on poten- tial for 5 mol dm-” (upper curve) and 5.lo-” mol dm-” (lower curve) NaNO,, solutions.

of electrolyte. Similar curves, both in shape and in the measured values, were obtained for other electrolytes investigated (NaF and Na,B,07). For the mer- cury electrode, which is the one most comprehensively studied, the measured series capacitance does not depend significantly on electrolyte concentration, except at the potential close to the potential of zero charge. Therefore, one may conclude that the observed strong dependence of measured series capacitance ensues from the dependence of the constant phase element on concentration (at the frequency used the dispersion of impedance components on frequency is very significant), not from the dependece of the interfacial capacitance on concentration. At the potential of about -450 mV versus SCE, a depression on the curve for a 0.005 mol dm-” solution may be seen. Assuming that this minimum occurs at the potential of zero charge for the sulphide, the so-called Parsons-Zobel plot [ 151 was drawn. Indeed, a straight line was obtained, but only for rather low frequencies - for frequencies higher than h 20 Hz the plot was always curved. For NaF solutions, the extrapolated interfacial capacitance was 110 ,LLF cm-’ and the roughness factor calculated from the tangent of the line was 11. This leads to an interfacial capacitance of 9 ,uF cmp2. Note that

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the procedure of extrapolation in the coordinates used in construction of the Parsons-Zobel plots, in reality, is the extrapolation to infinitely large concen- tration and should also cancel the influence of the dispersion of the impedance components versus frequency (for an infinitely large conductivity of the so- lution, the impedance of the CPE should be infinitely low). Therefore, the true interfacial capacitance should be obtained after extrapolation. The value of 9 ,DF cm-’ obtained is slightly low but still in the range of values expected for polycrystalline metal electrodes [ 151. This is not surprising, because the den- sity of free charge carriers in the sulphide used is very large (of the order of 1021 cm-“), so one cannot expect a very significant influence of the space charge layer capacitance on the total interfacial capacitance. It is well known that the compact layer capacitance is very sensitive to the presence of adsorbed species. Therefore, measurements of the interfacial impedance of the sulphide elec- trode in 5 mol dmp3 NaNO, at a frequency of 10 Hz (to minimize the influence of CPE impedance) were performed with an without the addition of 10-l mol dmW3 of n-butyl alcohol. Only a slight decrease of the measured series capaci- tance C, (of N 20%) was observed for the sulphide electrode. (In the same solution, the measured series capacitance for a polycrystalline copper electrode dropped to 50% of its initial value. ) The value of 11 obtained for the roughness factor seems to be too large; however, from the theory of fractals [ 161 it follows that for surfaces which may be described in terms of that theory one may ex- pect very high values of the roughness factor, even for surfaces which are ap- parently flat.

The surface newly formed in the solution should attain, in the first moment after creation, the potential of zero charge. In Fig. 4 an experiment is presented

Fig. 4. Changes of potential of the cuprous sulphide electrode with newly formed surface during

the first second after cleaving. 0.1 mol drnm3 NaF.

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in which the rod made from the sulphide was broken in the solution and its potential was registered on a fast digital recorder. It is seen that the value of the potential after cleavage, extrapolated to zero time, coincides roughly with the potential of the minimum observed for a solution of the same electrolyte. (For all electrolytes used, the minimum on the curve for the lowest concentra- tion appears always at approximately the same potential, about - 450 mV ver- sus SCE.)

CONCLUSIONS

For metal electrodes made from monocrystals, the real area of the electrode is usually equal to the geometric surface area. For polycrystalline metal elec- trodes, the roughness factor may be diminished by polishing or electropolish- ing of the surface. For polycrystalline mineral electrodes, one may always ex- pect significant roughness, which obscures interpretation of the results of impedance measurements. Unfortunately, the theory of the rough electrodes is not so well developed to permit calculation of the interfacial capacitance from impedance data. It was proposed in the present work that extrapolation of the same coordinates as in the Parsons-Zobel plots should eliminate the influence of surface roughness too. This assumption requires further verifica- tion. The observed weak sensitivity of the measured capacitance to adsorption of surfactant and changes in concentration of the solution (note that the ob- served dependence of the measured series capacitance on concentration was ascribed to the influence of surface roughness, not to the changes in double layer capacitance) may be explained by the different electronic structure of the interface. In the case of a metal, both the activity of the surface to adsorp- tion and the sensitivity of double layer capacitance to different factors are caused by the fact that the electronic gas, present in the metal, sticks out of the surface and interacts with the molecules in the compact part of the e.d.1. In the case of non-stoichiometric cuprous sulphide, which is a degenerate p- type semiconductor, there is no such interaction, so the interface is less sen- sitive to factors influencing the structure of the compact layer. The same ex- planation may be given in the case of observed differences in the formation of surface oxides. The results suggest (especially the significant curvature of the measured capacitance-potential curve in concentrated electrolyte solutions) that there is a non-negligible contribution from the solid-body side to the in- terfacial capacitance. This is probably not due to the space charge layer capac- itance but rather to the capacitance of the surface states.

ACKNOWLEDGEMENT

This work was supported by Research Program CPBP 02.07.

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REFERENCES

1 C.A. Oestreicher and D.W. McGlashan, Min. Eng., 23 (1971) 75. 2 A. Kowal and A. Pomianowski, J. Electroanal, Chem., 46 (1973) 411. 3 S. Chander and D.W. Fuerstenau, J. Electroanal. Chem., 56 (1974) 217. 4 C.S. O’Dell, R.K. Dooley, G.W. Walker and P.E. Richardson, in P.E. Richardson, S. Srini-

vasan and R. Woods (Eds), Proc. Int. Symp. on Electrochemistry in Mineral and Metal Processing, Cincinnanti, 1984, The Electrochemical Society, Pennington, NJ, 1984, p. 81.

5 P. Nowak and A. Pomianowski, Anal Chem. Symp. Ser., 22 (1985) 591. 6 P. Nowak, Bull. Pol. Acad. Sci. Ser. Sci. Chim., 35 (1987) 451. 7 R.W. Potter II and H.T. Evans Jr., J. Res. U.S. Geol. Surv., 4 (1976) 20. 8 ASTM 23962. 9 D. Dickertman, F.D. Koppitz and J.W. Schultze, Electrochim. Acta, 21 (1976) 967.

10 L. Nyikos and T. Pajkossy, Electrochim. Acta, 30 (1985) 1533. 11 T. Pajkossy and L. Nyikos, J. Electrochem. Sot., 131 (1984) 2061. 12 T. Czeppe and P. Nowak, Mater. Sci. Forum., 25/26 (1988) 525. 13 R.L. Deutscher and R. Woods, J. Appl. Electrochem., 16 (1986) 413. 14 G.W. Walker, J.V. Stout III and P.E. Richardson, Int. J. Miner. Process., 12 (1984) 55. 15 M.A. Vorotyntsev, in J. O’M. Bockris, B.E. Conway and R.E. White, Mod. Aspects Electro-

them., 17 (1986) 131. 16 B.B. Mandelbrot, The Fractal Geometry of Nature, Freeman, San Francisco, 1982.