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Properties of Cu(II) and Ni(II) Sulfides Prepared by Coprecipitation in Aqueous Solution D. Tsamouras,† E. Dalas,† S. Sakkopoulos,‡ and P. G. Koutsoukos*,§ Department of Chemistry, Department of Physics, and Department of Chemical Engineering, University of Patras, University Campus, GR 265 00 Patras, Greece, and Institute of Chemical Engineering and High-Temperature Chemical Processes, P.O. Box 1414, GR 265 00 Patras, Greece Received October 5, 1998. In Final Form: June 25, 1999 Among semiconductors, metal sulfides are very interesting materials that may be used for solar energy conversion arrangements. We prepared a series of CuxNi1-xS powders, where x ) 0.05, 0.20, 0.50, and 0.80, by spontaneous precipitation. Various stoichiometries of Cu(NO3)2, Ni(NO3)2, and (NH4)2S solutions at 25 °C and pH 2.50, at conditions in which the aqueous solutions were supersaturated with respect to CuS and NiS, yielded precipitates corresponding to intermediate mixed sulfides. The appearance of new peaks in the powder X-ray diffraction spectra suggested the formation of new phases. Measurements of the resistance of the bulk solids as a function of temperature for the preparations with x ) 0.05, 0.20, and 0.50 showed a decrease with increasing temperature. A hysteresis was found upon thermal cycling of the sulfide preparations, possibly due to changes in the solid state that affect the energy bands of the semiconductors. The Cu and Ni sulfide preparations with x ) 0.80 showed metallic behavior. All preparations were highly charged when suspended in aqueous media, and the electrical charge was found to depend strongly on the activity of copper ion in solutions equilibrated with the solid phase. Finally, current potential curves of electrodes prepared from the precipitated sulfides under constant illumination gave values for the open circuit potential as high as 0.868 V, with 0.119 mA for the photocurrent and 0.58 for the field factor. These values are encouraging for the potential application of these materials in cells for the conversion of solar into electric energy.
Introduction Mixed crystalline phases of metal sulfides that exhibit semiconductor behavior are an interesting class of compounds because, among others, they have a potential for use in energy conversion applications.1,2 Mixed-metal sulfide phases are both naturally encountered3-5 and prepared synthetically at controlled conditions in order to achieve desired particle size6 and/or surface charge.7 The simultaneous formation of two or more crystalline phases may result in the distortion of covalent bonds at the phase boundaries and/or in different placements of the building atoms in the lattice at these regions. This is particularly important for polycrystalline material imperfections (vacancies, interstitials, antisites). In these materials and at grain boundaries, the crystal point defects may introduce additional levels in the energy gap of the semiconductor, causing considerable changes in the electric and electronic characteristics of the semiconductor materials.8,9 The transition-metal sulfides have drawn * To whom correspondence should be addressed at the Department of Chemical Engineering, University of Patras. † Department of Chemistry, University of Patras. ‡ Department of Physics, University of Patras. § Department of Chemical Engineering, University of Patras. (1) Bard, A. J. J. Chem. Phys. 1982, 86, 172. (2) Jaegerman, W.; Tributsch, H. Prog. Surf. Sci. 1988, 29, 1. (3) Gullup, D. L.; Andersen, G. R.; Holligen, D. Trans. Geotherm. Resour. Counc. 1990, 14, 1583. (4) Pabencia, I.; Carranza, F.; Garica, M. J. Hydrometallurgy 1990, 23, 191. (5) Carranza, F.; Garcia, M. J.; Palencia, I.; Pereda, J. Hydrometallurgy 1990, 24, 67. (6) Bredol, M.; Merikhi, J. J. Mater. Sci. 1998, 33, 471. (7) Bebie, J.; Schoonen, M. A. A.; Fuhrmann, M.; Strongin, D. R. Geochim. Cosmochim. Acta 1998, 62, 633. (8) Takahashi, T.; Ebina, A. Appl. Surf. Sci. 1982, 11/12, 268. (9) Potz, W.; Ferry, D. Phys. Rev. B 1985, 31, 968.
considerable interest for their semiconducting properties because they show a high absorbance of luminescent radiation. Properties of the transition-metal sulfides depend very much on the method of preparation and also on the relative proportions of the metal and sulfide components.10 With the system of CuS and NiS, there is no information concerning the presence of mixed phases or the electrical and/or electronic properties of these polycrystalline materials. In the present work, we investigated systematically the synthesis and properties of polycrystalline Cu-Ni sulfides prepared in solutions that were supersaturated simultaneously with CuS and NiS at conditions in which formation of the precipitates occurred spontaneously. A series of polycrystalline powders with the stoichiometry CuxNi1-xS were prepared. The materials prepared were characterized, with respect to their composition, with chemical and spectroscopic analytical methods. The electronic properties of the semiconducting powder preparations were investigated by measurements of the electrical conductivity at various temperatures and the thermoelectric power. Finally, the morphology of the preparations and the particle-size distributions were analyzed in order to investigate the extent to which these factors influenced the electronic properties of the semiconductors. Experimental Section Stock solutions of nickel(II) chloride and copper(II) nitrate were prepared from crystalline reagents, NiCl2‚6H2O and Cu(NO3)2‚xH2O (Ferak, Zur Analyse), respectively, by dissolution in conductivity water followed by filtration through membrane filters (Millipore 0.22 µm). Stock sulfide solutions were prepared (10) Tsamouras, D.; Dalas, E.; Sakkopoulos, S.; Koutsoukos, P. G. Langmuir 1998, 14, 5298.
10.1021/la981384y CCC: $18.00 © 1999 American Chemical Society Published on Web 09/21/1999
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Figure 1. Variation of the specific conductivity during the precipitation of CuxNi1-xS by mixing Cu(NO3)2 and Ni(NO3)2 (Cu:Ni ) 0.2:0.8); 25 °C, pH 2.50. from a (NH4)2S standard solution (20% w/w Ferak, Pro Analyse). The standardization of nickel(II) and copper(II) stock solutions was done by atomic absorption spectrometry (AAS, Varian 1200). Preparation of all solids was done by spontaneous precipitation in aqueous solutions, by rapidly mixing equal volumes of nickel(II) chloride and copper(II) chloride in various proportions with ammonium sulfide prepared from the respective stock solutions. The precipitation took place in a 0.5 dm3 double-walled Pyrex glass reactor thermostatted at 25.0 ( 0.1 °C with circulating water. The pH of the working solution was rapidly adjusted to 2.50 by the addition of 5 N nitric acid solution. Under these conditions, the solutions were supersaturated with respect to both CuS and NiS, and the mixed sulfides precipitated immediately following establishment of the solution supersaturation. Finally, hydrazine sulfate was added to a final concentration that was equal to the final sulfide concentration in solution, both to ensure a reducing environment and to accelerate the sulfide formation. The precipitation process was monitored by measuring the specific conductance of the supersaturated solutions and was allowed to proceed until the specific conductance of the suspension remained constant. A typical plot of the variation of the solution specific conductivity as a function of time is shown in Figure 1. To ensure that the precipitation process had ended, the aqueous phase was analyzed for copper and nickel by atomic absorption spectrometry. Next, the suspensions were left to age for 24 h and were filtered though membrane filters (Millipore, 0.22 µm); the solids were dried at 50 °C overnight. An inert nitrogen atmosphere was maintained throughout. They were then placed in a desscator under an inert nitrogen atmosphere to avoid oxidation exposure to the air. The dried solids were characterized by powder X-ray diffraction (Philips PW 1830/40, CuKR radiation, Ni filter, 0.4 incoming slit) and scanning electron microscopy (JEOL, JSM 5200). Particle-size distributions were measured by withdrawing an aliquot from the final suspension and suspending it in the cell of a laser diffraction instrument (Spectrex ILI 1000) containing saturated Cu and Ni solutions. The electrical conductivity and the coefficient of thermal power of the precipitates were measured under nitrogen. Details are given elsewhere.10 Depending on the stoichiometry of the mixing solutions, the following CuII-NiII sulfides were prepared: precipitate
x
CuxNi(1-x)S
0.05, 0.20, 0.50, 0.80, 1.00
All preparations consisted of mixtures of NiS, CuS, and intermediate sulfides. Diffuse Reflectance Spectroscopy (DRS). DRS spectra were obtained with a UV-VIS spectrometer (Varian Cary 219) using MgO as reference and with automatic background subtraction. Electrophoretic Mobility (EM) Measurements. The electrophoretic mobility of the preparations was measured in suspensions of the sulfide powders in 0.05 M KClO4. The suspensions were kept in double-walled, glass reactors thermostatted and covered with a Perspex lid. A water-vapor
saturated nitrogen atmosphere was maintained by bubbling pure nitrogen through a water trap in the suspension, which was magnetically stirred. The pH was approximately 4.0, and it was not adjusted by acid or base addition. Dilute suspensions were introduced in a capillary (Rank Brothers MK II), at the ends of which a DC electrical field was applied by two Pt electrodes. The polarity was reversed frequently to avoid polarization. The velocity of at least twenty particles in each direction was measured. The suspensions, prior to their introduction to the measurement cell, were subjected to treatment for 10 min in an ultrasonic bath to avoid (or reverse) aggregation of the particles and were allowed to reach equilibrium for 20 h. Current-Voltage Measurements. Working electrodes were constructed from the precipitated sulfides in the shape of pellets formed by pressing a carefully weighed amount of the respective dried powder. Electrical contact was effected by attaching the copper wires to the specimens with silver paste. The pellets were coated either with a conducting polymer (polypyrrole) or with Au by sputtering. The electrodes were sealed with the appropriate thermal tubes and epoxy resins. The current-voltage curves were obtained under illumination. A tungsten-halogen lamp (400W, Philips EVD-7787) was used as a light source and was connected with a stabilized power supply. The light passed through a cylindrical water filter to cut off the infrared part of the radiation. The electrode was illuminated through a variablewidth aperture. A constant potential was applied to the working electrode through a potentiostat (Amel 2051), and the current was measured with a precision ammeter (Keithley 195 A). All measurements were done at a light intensity of 100 mW/cm2 and under an inert nitrogen atmosphere.
Results and Discussion (i) Thermodynamic Calculations. The thermodynamic driving force for the formation of a mineral phase in aqueous solutions depends on the ion activity product and the thermodynamic solubility product of the precipitating minerals.11 The formation of a sparingly soluble salt by precipitation from a supersaturated solution depends on the difference between the chemical potentials of the solute in the solution and at equilibrium. In the supersaturated solutions the chemical potential, µs, is
µs ) µ0s + kT ln Rs
(1)
At equilibrium, the chemical potential is
µ∞ ) µ0∞ + kT ln R∞
(2)
In eqs 1 and 2, k is Boltzmann’s constant, T the absolute temperature, µ0s and µ0∞ the standard chemical potentials of the precipitating salt and Rs and R∞ the activities of the salt in the supersaturated solution and at equilibrium, respectively. Assuming that µ0s ) µ0∞, the change in chemical potential for going from the supersaturated solution to equilibrium through the precipitation of the salt is
∆µ ) µ∞ - µs ) -kT ln
Rs R∞
(3)
or
∆G ) -RT ln
Rs R∞
(4)
The sulfides considered in the present work are of the type MS. Hence Rs ) [(RM2+)s(RS2-)s]1/2 and R∞ ) [(RM2+)∞(11) Garside, J.; So¨hnel ,O ¨ . Precipitation; Butterworth Heinemann: Oxford, 1991.
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Figure 2. Powder X-ray diffraction spectra of polycrystalline CuxNi1-xS precipitated in aqueous supersaturated solutions; 25 °C, pH 2.50: (a) x ) 0.05; (b) x ) 0.20; (c) x ) 0.50; (d) x ) 0.80. Table 1. Change in Gibbs Free Energy for the Formation of CuS and NiS at the Initial Conditions Used for the Preparation of CuxNi1-xS Powders from Supersaturated Solutions, Mixing Cu(NO3)2 and Ni(Cl)2 Solutions at pH 2.50 and 25 °C ∆G/kJ mol-1 x
CuS
NiS
0.00 0.05 0.20 0.50 0.80 1.00
-70.72 -72.48 -73.71 -74.44 -74.91
-44.12 -44.06 -43.85 -43.30 -42.24 -
(RS2-)∞]1/2. Equation 4 becomes
(RM2+)s(RS2-)s RT ln ∆G ) 2 (RM2+)∞(RS2-)∞
(5)
In eq 5, the numerator of the logarithmic term is the ion activity product in solution of the precipitating salt, and the denominator is the respective thermodynamic solubility product. The values of the change of the Gibbs free energy for going from the supersaturated solution to equilibrium for the solution compositions used and for the respective pure minerals are shown in Table 1. As may be seen, at all compositions used in the present work, the driving force was very favorable for the precipitation of both CuS and NiS. (ii) Physicochemical characterization. The powder X-ray diffraction spectra of the mixed sulfide precipitates are shown in Figure 2. In all spectra, the peaks [300], [021], [131], and [401], corresponding to millerite, (NiS)12 are clearly seen. In the precipitates with x ) 0.20 and x ) 0.50, the additional peaks observed corresponding to d ) 2.749, 1.946 and 1.666 Å were attributed to the formation of a new intermediate phase. It should be noted that no metal disulfides or oxides were found to form either directly (12) JCPDS card file No. 12-41.
Figure 3. Diffuse reflectance spectra of CuxNi1-xS polycrystalline powders prepared by coprecipitation; 25 °C, pH 2.50: (a) x ) 0.05; (b) x ) 0.20; (c) x ) 0.50.
in the precipitation process or due to possible oxidation during the drying step. For the preparation in which x ) 0.80, in which Cu(II) is in relative excess with respect to nickel, covellite (CuS) was also present.13 The diffuse reflectance spectra of the preparations in which x ) 0.05, 0.20, and 0.50, shown in Figure 3, show that the polycrystalline sulfide precipitates adsorb over a wide wavelength range of UV-visible light (200-800 nm), suggesting that these materials may be considered as candidates for use in solar energy conversion devices. The morphology of the sulfide preparations is presented in the scanning electron micrographs shown in Figure 4. As may be seen, there are differences in the morphology of the polycrystalline powders obtained. At low copper content (Figure 4a), a plate-like morphology is dominant, whereas an increase in copper content (Figures 4b and 4c) leads to powders with fine grains. At very high Cu content, however (Figure 4d), the formation of large particles covered with smaller crystallites is dominant. Measurements of the particle size showed that the size distributions were narrower (5-8 µm) for very low (x ) 0.05) and very high (x ) 0.80) copper contents in the precipitates, whereas broader distributions (5-11 µm) were found for x ) 0.20 and 0.50. The mean size of the particles of the various polycrystalline mixed sulfide precipitates as a function of copper content is shown in Table 2. It should be noted that in each preparation the powders consisted of NiS, CuS, and an intermediate sulfide phase with the respective metal content. (iii) Electrical Properties. Despite the increased interest in transition-metal sulfides, little is known concerning the electrical properties of mixed-metal sulfides prepared by coprecipitation of two or more metal sulfides. Important information may be obtained from measurements of the electrical resistivity of the materials as a function of temperature. According to their resistivity (bulk solids), the materials may be distinguished in semiconductors and/or metallic type materials. The polycrystalline sulfide preparations with x ) 0.05 and 0.20 showed a behavior typical for semiconductors containing dopants, (13) JCPDS card file No. 6-464.
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Figure 4. Scanning electron micrographs of CuxNi1-xS preparations precipitated in aqueous supersaturated solutions; 25 °C, pH 2.50: (a) x ) 0.05; (b) x ) 0.20; (c) x ) 0.50; (d) x ) 0.80. Table 2. Mean Particle Size of the CuxNi1-xS Powders Prepared by Spontaneous Precipitation, 25 °C and pH 2.50 solid stoichiometry
mean particle size (µm)
CuS Cu0.80Ni0.20S Cu0.50Ni0.50S Cu0.20Ni0.80S Cu0.05Ni0.95S
3.0 5.8 6.9 7.2 6.4
whereas for x ) 0.8 and 1.0, metallic character was shown. For the CuxNi1-xS preparations with x ) 0.05, 0.20, and 0.50, the corresponding plots of the variation of the specific resistance as a function of temperature is shown in Figures 5a, 5b, and 5c, respectively. It should be noted that XRD analysis before and after heating did not show any variation in the mineral phase composition of the solids tested. The complex form of these graphs, in comparison with the plots obtained for simple sulfides,14-16 is indicative of the fact that they correspond to semiconductors doped with impurities, which introduce additional energy levels in the semiconductor energy gap. These energy levels are activated in a well-defined temperature region.17 In graphs such as those presented in Figure 5, four regions may be distinguished: (14) Bither, T. A.; Bouchard, R. J.; Cloud, W. H.; Domotue, P. C.; Siemons, W. J. Inorg. Chem. 1968, 7, 2208. (15) Loseva, G.; Abramova, G. Phys. Status Solidi 1983, 80, K109. (16) Pakeva, S.; Germanova, K. J. Phys. D: Appl. Phys. 1985, 18, 1371. (17) Ioannides P., Ph.D. Thesis, University of Athens, Athens, 1969.
(1) The first represents a region of endogenous conductance, typical for high temperatures and characterized by a sharp drop of the specific resistance upon increasing the temperature above a limiting value. From the graph of the logarithm of the specific conductivity as a function of the inverse of the absolute temperature and through a linear fit of the experimental data according to eq 6
ln F ≈
E 2kT
(6)
where E is the activation energy of the current carriers and k is Boltzmann’s constant. E may be determined from the slope of the lnF versus 1/T line. (2) The region of exogenous conductance, or the doping region, corresponds to lower temperatures. In this region, the resistance is increasingly reduced with increasing temperature due to the excitation of carriers from the energy level of the dopants. The respective activation energy in this region may be calculated from the slope of the linear fit of the experimental measurements of lnF as a function of 1/T. (3) Between regions (i) and (ii), there is a limited region in which the specific resistance is approximately constant and its value depends only on the mobility of the carriers. (4)At low temperatures, there is often a region in which decreases in temperature cause a slow reduction of the specific resistance. This finding is encountered mostly in monomolecular materials such as a Ge semiconductor.
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Figure 6. Variation of the specific resistance of the Cu0.8Ni0.2S, prepared by coprecipitation at 25 °C and pH 2.50, with temperature.
Figure 5. Plot of the variation of the logarithm of the specific resistance with the inverse of the absolute temperature for polycrystalline CuxNi1-xS powders prepared by coprecipitation at 25.0 °C, pH 2.50: (a) x ) 0.05; (b) x ) 0.20; (c) x ) 0.50. Table 3. Activation Energy Values for CuxNi1-xS Polycrystalline Preparations Prepared by Coprecipatation at 25 °C and pH 2.50a E/10-3 eV x
endogenous region
exogenous region
0.05 0.20 0.50
27.7 16.5 30.6
4.8 2.7 3.5
a
Calculations were done from the linear fit of the logarithm of the measured specific resistance values as a function of the inverse of the absolute temperature.
This analysis was done for the CuxNi1-xS sulfides, for which the plots of the variation of the specific resistance as a function of the inverse of temperature are shown in Figure 5. The results from the linear regression of the experimental data are summarized in Table 3. As may be seen from the data shown in Table 2, the values for the excitation of carriers obtained from mixed sulfides are clearly lower than the respective values for the endogenous carriers. This fact suggests that the contribution of the mixed phases in the solid conductivity is more important than that of the unmixed phases. In general, increased values of the activation energy indicate decrease in the mobility of the carriers. The mixed sulfides with x ) 0.80, however, showed a markedly different behavior. In this preparation, as shown from the XRD spectrum, the presence of CuS was very strong, and the specific resistance and its variation with temperature showed behavior which was characterized by the dominance of CuS (Figure 6). An interesting feature shown by all preparations was the change of the electrical resistance at the limiting temperature of 370 K. Experimental measurements showed, however, that the overall shape of the lnF ) f(1/T) curves was not affected even though the values of the specific resistance changed. This effect is related mainly to the heat added to the system rather than with the aging process. The changes of the
Figure 7. Variation of the specific resistance of the Cu0.2Ni0.8S preparation with temperature.
specific resistance of the solid samples caused by heating the solid samples at the limiting temperature are shown by the behavior of the preparation Cu0.2Ni0.8S in Figure 7. As may be seen, starting from a value of F ) 37.1 mΩ cm (point 1) past the characteristic temperature of 37.5 K, reduction of the temperature resulted in a new curve which did not coincide with the original, thus showing hysteresis. At point 3 (the same temperature as that of point 1), the value of the specific resistance of the solid was significantly lower, 34.4 mΩ cm. All samples tested showed a similar hysteresis behavior. The final value of the specific resistance remained constant after 3 cycles. Similar changes of the electrical conductivity and of various other electric and magnetic properties of sulfides as a function of their thermal history have been reported in the literature.18-21 The observed changes in the values of the specific resistance may also be due to the rearrangement of the vacancies and /or of the defects of the crystalline lattice. The possible concomitant lattice distortion affects both the electrical and magnetic properties of the materials mainly because of carrier-scattering or charge-trapping effects.22 (18) Benoit, R. C. R. Acad. Sci. 1952, 234, 2174. (19) Neel, L. Rev. Mod. Phys. 1953, 25, 58. (20) Bertant, E. Bull. Soc. France Miner. Crist. 1956, 79, 276. (21) Pasquariello, D.; Kershaw, R.; Passaretti, J.; Dwight, K.; Wold, A. In Solid State Chemistry in Catalysis; Grasselli, R., Brazdil, J., Eds.; ACS Symposium Series 279; American Chemical Society: Washington, DC, 1985; p 247.
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Figure 8. Electrophoretic nobility of the CuxNi1-xS polycrystalline preparations as a function of pCu; 0.05 KClO4, pH 9.0, 25 °C.
(iv) Surface Charge Measurements. The electrical properties of the sulfide preparations depend on their surface charge, and very few studies have been published concerning this issue.7 It has been reported that protons may indirectly be acting as potential-determining ions (pdi), responsible for the development of surface charge on the metal sulfide, because they influence sulfide speciation. However, experimental measurements on NiS2, MnS2, and PbS show practically an independence of the measured ζ potential on the suspension pH.7 Because the pdi for crystalline materials are the lattice ions,23,24 we measured the electrophoretic mobilities of the mixed sulfide polycrystalline precipitates as a function of the Cu ion activity. The variation of the electrophoretic mobility of the CuxNi1-xS particles as a function of pCu is shown in Figure 8. As may be seen, a clear isoelectric point (iep is the pCu concentration at which the solid has zero charge (2)) was found at pCu ) 2.50 ( 0.20, with the exception of Cu0.5Ni0.5S, for which the iep was 3.4. All measurements were done in suspension, in which the ionic strength, adjusted with KClO4, was 0.05 M at pH 4.0. (v) Measurements of Current-Voltage Curves. Finally, following the detailed characterization of the CuxNi1-xS powders, they were tested with respect to their potential for use in photoelectrochemical cells. Earlier reports on the possibility of using nickel(II) sulfide containing molybdenum in devices used for the conversion of solar into chemical energy did not give encouraging results.25 For our preparations, we used two types of cells, for which the current-voltage curves at constant illumination were obtained:
Figure 9. Current-voltage curves obtained at constant illumination of the electrodes: (a) Cu0.2Ni0.8S|polypyrole|0.05 M Fe2+|Fe3+|Pt; (b) Cu0.2Ni0.8S|Au|0.01|M Ce3+/Ce4+|Pt.
Iph ) 0.119 mA, and FF ) 0.58. It is obvious that the performance of cell (2) is far better than that of cell (1). In cell (1), the sulfide was coated by a conducting polymer, polypyrrole, a p-type semiconductor. The polymer coating was developed by the direct polymerization of pyrrole at low pressure (0.1 mmHg) for 1 h.26 Gold coatings were obtained by sputtering under conditions necessary for the development of a 70 Å Au layer. The polypyrrole coatings were employed because it has been reported that they have a stabilizing effect on the semiconducting electrodes, especially in the presence of strong oxidants.27-29 It should be noted that both coatings yielded electrodes with better stability and resistance with time in comparison with the uncoated electrodes. Although Au coatings yielded electrodes that produced higher current densities in both cases, the current density was low, possibly due either to the formation of conductive layers or to the porosity of the electrode preparations. Conclusions
The respective current voltage curves are shown in Figure 9. From the current-voltage plot of Figure 9a, the values of Voc ) 0.320 V, Iph ) 0.08 mA, and FF ) 0.35 for the open circuit potential, the photocurrent density, and the field factor, respectively, were obtained. From Figure 9b, the respective values obtained for cell (2) were Voc ) 0.868 V,
In the present work, it was demonstrated that mixed Cu(II) and Ni(II) sulfides may be prepared by spontaneous precipitation from aqueous solutions that are supersaturated with respect to both NiS and CuS. Mixed sulfides were formed with variable stoichiometries, depending on the stoichiometric ratios of the respective soluble salts in the solutions. Spectroscopic analysis showed the formation of mixed and intermediate mineral phases corresponding to the formula CuxNi1-xS, with x ) 0.05, 0.20, 0.50, and 0.80. The sulfide powders prepared were strongly charged, their electric charge depending on the activities of the potential-determining ions, i.e., Cu2+ and S2-. The activation energy determined from measurements of the de-
(22) Yugo, S.; Kimura, T. Phys. Status Solidi A 1980, 59, 363. (23) Bijsterbosch, B. H.; Lyklema, J. Adv. Colloid Interface Sci. 1978, 9, 147. (24) Lyklema, J. In Fundamentals of Interface and Colloid Science; Academic Press: London, 1991; p 5.89. (25) Albertini, L.; Angel, A.; Gonzales, E. J. Appl. Electrochem. 1992, 22, 888.
(26) Mohammadi, A.; Lundstrom, I.; Salaneck, W. R.; Iganas, O. Synth. Met. 1987, 21, 169. (27) Lane, R.; Hubbard, A. J. Phys. Chem. 1978, 77, 1401. (28) Nigrey, R.; Heeger, A.; MacDiarmid, A. Mol. Cryst. Liq. Cryst. 1982, 83, 309. (29) Heinze, J.; Mortensen, J.; Mullen, K.; Schenk, L. J. Chem. Soc., Chem. Commun. 1987, 701.
[1] CuxNi1-xS | 0.05 M Fe2+/Fe3+| Pt [2] CuxNi1-xS | 0.01 M Ce3+/Ce4+| Pt
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pendence of the electrical resistance on temperature showed that the role of the mixed phases in determining the conductivity of the sample is very important. At high copper contents (x ) 0.80), metallic behavior of the powders was exhibited. Measurements of the current-potential curves using electrodes made from the precipitated
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sullfides coated by conducting polypyrrole and Au yielded encouraging results with respect to the stability and durability of the electrodes with time. LA981384Y
