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Langmuir 1999, 15, 8018-8024
Physicochemical Characteristics of Mixed Copper-Cadmium Sulfides Prepared by Coprecipitation D. Tsamouras,† E. Dalas,† S. Sakkopoulos,‡ and P. G. Koutsoukos*,§ 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 March 2, 1999. In Final Form: July 8, 1999 A series of Cu-Cd sulfides were prepared by coprecipitation in aqueous supersaturated solutions at 25 °C, pH 2.50. The solids obtained were characterized by powder X-ray diffraction and their elemental composition was determined by chemical analysis. The results showed that in all cases mixed CuS-CdS precipitates were prepared, the ratio of each phase depending on the composition of the supersaturated solutions. A series of precipitates, of (CuS)x (CdS)1-x, were prepared with x ) 0.0, 0.20, 0.25, 0.30, 0.35, 0.50, and 1.0. All polycrystalline preparations with x ) 0.30 showed metallic behavior typical for CuS, while the mixed sulfides with x ) 0.20 and 0.25 showed semiconducting behavior, which however could not be described by any of the models reported in the literature. The band gap energies of 2.40 and 2.39 eV calculated for the two preparations showed that at this composition the material was dominated by the behavior of CdS. The activation energies estimated from the dependence of the solids conductivity on the temperature supported these results. Measurements of the current density at the preparations (x ) 0.20 and 0.25) -polypyrrole and Al junctions yielded low values. More promising behavior was shown by the mixed sulfide-polypyrrole junctions, which yielded relatively higher current densities and higher stability.
Introduction The potential applications of cadmium sulfide in photovoltaic devices has been the driving force for the increased attention of researchers over the past two decades.1 Cadmium sulfide as a bulk material is characterized by high stability and homogeneity. Moreover a relative advantage of CdS, related to the range of applications it may find, is the possibility of substitution of cadmium atoms of the respective lattice by other metal atoms. Substitutions of this type may be used to direct changes of the dielectric properties in the desired direction. A problem of the materials resulting from the substitution of cadmium for other metal atoms compatible with the CdS lattice, is the limited stability of the films prepared and the poor reproducibility of the composition and hence of the properties of these materials. In a previous work we presented an alternative method of preparing mixed metal sulfides by spontaneous precipitation in supersaturated solutions.2 This method has the advantages of being easily applied and the solids may be prepared reproducibly. The preparation of metal sulfides by coprecipitation, when more than one metal is present, may lead to the formation of either mixed sulfides of the respective metals or of intermediate sulfides in which one metal substitutes for the other. In the present work we investigated the properties of mixed sulfide solids formed by coprecipitation in solutions supersaturated simultaneously with cadmium and copper sulfides. The motivation for this investigation was the * Corresponding author. † Department of Chemistry. ‡ Department of Physics. § Department of Chemical Engineering and Institute of Chemical Engineering. (1) Heller, A. Acc. Chem. Res. 1981, 14, 5. (2) Tsamouras, D.; Dalas, E.; Sakkopoulos, S.; Koutsoukos, P. G. Langmuir 1998, 14, 5298.
preparation of cadmium sulfide polycrystalline powders with improved characteristics related to stability and dielectric properties, using a simple methodology in which the preparation parameters could easily be controlled. It is expected that the improvement of the dielectric properties of polycrystalline materials may enhance their applications spectrum.3-7 Experimental Section Stock solutions of copper(II) and cadmium(II) chloride were prepared from crystalline CuCl2•×H2O, and CdCl2×H2O (Ferrak Zur Analyse) by dissolution in water followed by filtration through membrane filters (Millipore 0.22 µm). Stock sulfide solutions were prepared from a (NH4)2S standard solution (20% w/w Ferrak, Zur Analyse). The standardization of the Cu(II) and Cd(II) stock solutions was done by atomic absorption spectrometry (AAS, Varian 1200). The preparation of all solids was done by spontaneous precipitation in aqueous solutions. Equal volumes of Cu(II) ammonium sulfide solutions (containing various amounts of Cd(II) chloride) prepared from the respective stock solutions were rapidly mixed through a Y shaped tube to ensure perfect mixing. The experimental conditions are summarized in Table 1. The precipitation took place in a 0.5 dm3 double-walled Pyrex glass reactor kept at 25.0 ( 0.1 °C by circulating water from a thermostat. The pH of the working solution was rapidly adjusted to 2.50 by the addition of 5 N nitric acid solution. At these conditions the solutions were supersaturated with respect to both CuS and CdS and the mixed sulfides precipitated immediately following the establishment of supersaturation. Finally, hydrazine sulfate was (3) Hayashi, T.; Nishikura, T.; Suzuki, T.; Ema, Y. J. Appl. Phys. 1988, 64, 3542. (4) Dalas, E.; Sakkopoulos, S.; Kallitsis, I.; Vitoratos, E.; Koutsoukos, P. Langmuir 1990, 6, 1356. (5) Fatas, E.; Herrasti, P.; Arjona, F.; Camarero, E. J. Electrochem. Soc. 1987, 134, 2799. (6) Sebastian, P.; Campus, J.; Nair, P. Thin Solid Films 1993, 277, 190. (7) Shi, K.; Dong, K.; Byung, A.; Ho, I. J. Mater. Sci.: Mater. Electron. 1993, 4, 178.
10.1021/la990245t CCC: $18.00 © 1999 American Chemical Society Published on Web 09/11/1999
Characterization of Semiconducting Heavy Metal Sulfides Table 1: Initial Experimental Conditions for the Preparation of Polycrystalline Mixed Sulfides by Coprecipitationa total CuCl2 (M)
total CdCl2 (M)
s 1.00 × 10-2 1.25 × 10-2 1.50 × 10-2 1.75 × 10-2 2.50 × 10-2 5 × 10-2
5.00 × 10-2 4.00 × 10-2 3.75 × 10-2 3.50 × 10-2 3.25 × 10-2 2.50 × 10-2 s
a
∆G/kJ mol-1 CuS CdS s 71.03 -71.34 -71.61 -71.85 -72.48 -74.91
-58.04 -57.66 -57.55 -57.44 -57.31 -56.85 s
precipitate CdS Cu0.2Cd0.8S Cu0.21Cd0.75S Cu0.30Cd0.70S Cu0.35Cd0.65S Cu0.5Cd0.5S CuS
pH 2.50, 25 °C, total hydrazine 0.1 M, total (NH4)2S 0.1M.
added to a final concentration equal to the final sulfide concentration in solution, to ensure both a reducing environment and to accelerate the sulfide formation. The precipitation process was monitored by measuring the specific conductance in the supersaturated solutions and it was allowed to proceed until the suspension specific conductance remained constant. It should noted that the conductance vs time profiles obtained were highly reproducible showing that the rates of formation of the solids were also reproducible ((5%). It was not easy however to compute rates because of the difficulties involved in constructing the appropriate calibration curves. The end of precipitation was also ensured by analyses of the aqueous phase for copper and cadmium by atomic absorption spectrometry. Next, the suspensions were left to age for 24 h, filtered through membrane filters (Millipore, 0.22 µm), and the solids were dried at 50 °C overnight. The dried solids were characterized by powder X-ray diffraction (Philips PW 1830/40, Cu KR radiation, Ni filter, 0.4 incoming slit) and scanning electron microscopy (JEOL, JSM 5200). Particle size distributions were measured by drawing aliquots from the final suspension and suspending them in the cell of a laser diffraction instrument (Spectrex ILI 1000) containing saturated Cu, Cd sulfide solutions. The electrical conductivity of the specimens and the coefficient of thermal power of the preparations were measured as detailed elsewhere.9 Depending on the stoichiometry of the mixing solutions the following mixed sulfides (CuS)x(CdS)1-x were prepared: mineral phases
x
(CuS)x(CdS)1-x
0.00, 0.20, 0.25, 0.30, 0.35, 0.50, 1.00
Diffuse Reflectance Spectroscopy (DRS). The 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 pH was approximately 4.0 and it was not adjusted by acid or base addition. Dilute suspension specimens were introduced in a capillary (Rank Brothers MK II) at the end 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 suspension, prior to introduction to the measurement cell, was subjected to treatment for 10 min in an ultrasonic bath to avoid (or reverse) aggregation of the particles, and it was 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 weighted amount of the respective dried powder. An electrical contact was effected by attaching the copper wires on the specimens with silver paste. The pellets were coated either with a conducting polymer (polypyrrole) or with Al 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 (400 W, Philips EVD-7787) was used as a light source, connected with a stabilized power supply. The light passed through a cylindrical
Langmuir, Vol. 15, No. 23, 1999 8019 Table 2: Equilibria in the Cd2+-Cu2+-H2S-H2O System equilibrium
log K (ionic strength 0.0)
HS-+H+
) H2S (aq) H2S(aq) ) H2S(g) Cd2++HS- ) CdHS Cd2++2HS- ) Cd(HS)20 Cd2++3HS- ) Cd(HS)3Cd2++4HS- ) Cd(HS)42CdS(s)+2H+ ) Cd2++H2S Cd2++OH- ) CdOH+ Cd2++2OH- ) Cd(OH)20 Cd2++3OH- ) Cd(OH)33Cd2++4OH- ) Cd(OH)422Cd2++OH- ) Cd2(OH)+3 4Cd2++4OH- ) Cd4(OH)4+4 Cd2++2OH- ) β-Cd(OH)2(s) Cd2++2OH- ) γ-Cd(OH)2(s) Cu2++OH- ) CuOH+ Cu2++2OH- ) Cu(OH)2) Cu2++3OH- ) Cu(OH)3Cu2++4OH- ) Cu(OH)422Cu2++OH- ) Cu2OH3+ 2Cu2++2OH- ) Cu2(OH)22+ 3Cu2++4OH- ) Cu3(OH)42+ Cu2++2OH- ) Cu(OH)2(s) Cu2++2OH- ) CuO(s) CuS(s)+2H+ ) Cu2++H2S a
7.02 -0.99 7.60a 14.60a 16.50a 18.90a -7.0 3.9 7.7 10.3 8.7 4.6 23.02 -14.35 -14.10 6.5 11.8 14.5 15.6 8.2b 17.4 35.2 -19.2 -20.35 -15.20
1.0 M. b 3M.
water filter to cut off the infrared part of radiation. The electrode was illuminated through a variable width aperture. A constant potential was applied to the working electrode through a potentiostat (Amel 2051) and the current was measured with an ammeter (Keithley 195 A). All measurements were done at a light intensity of 100 mW/cm2. It should be noted that the method of preparation described herein ensured the reproducibility of the properties of the materials prepared. Each of the preparations was repeated at least three times and the powders obtained exhibited the same properties within experimental error.
Results and Discussion Thermodynamic Aspects of the Preparations. The thermodynamic driving force for the formation of solid material from the supersaturated solutions formed upon mixing the metal and sulfide ion-containing solutions was calculated taking into consideration all species present in the solutions in which the solids were formed. The equilibria considered are presented in Table 2.8 The saturation ratio, Ω, for the formation of all possible solid phases Mv+Av- formed by the metals Mm+ and the anionic components Am- was computed from the equilibrium constants and the mass and charge balance equations for the components of the solids:
Ω)
(Mm+)v+(An-)vK0s
(1)
where K0s is the thermodynamic solubility constant for the formation of the respective solid phase. The results of the calculations and the experimental conditions employed for the preparation of the sulfides are summarized in Table 3. Characterization of the Nature of the Precipitates. In all preparations the characterization of the solids showed the presence of two mineral sulfides. As may be seen from the powder X-ray diffraction spectra presented (8) Martel, A. E.; Smith, R. M. Critically Selected Stability Constants of Metal Complexes Database v. 5.0, NIST, 1998.
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Figure 2. Diffuse reflectance spectra of the coprecipitade CuCd sulfides which showed semiconducting behavior. Figure 1. Powder X-ray diffraction spectra of the (CuS)x(CdS)1-x preparations (*) 102, 103, 110 CuS reflection (4) 100, 002, 101, 001 CdS reflection. Table 3: Mean Particle Size of (CuS)x(CdS)1-x Polycrystalline Mixed Sulfides precipitate
mean diameter (µm)
Cu0.5Cd0.5S Cu0.35Cd0.65S Cu0.30Cd0.70S Cu0.25Cd0.25S Cu0.20Cd0.80S
7.0 5.87 6.3 5.8 6.1
in Figure 1, the 102, 103, and 110 reflections of covellite9 and the 101, 100, and 002 reflections corresponding to greenokite10 are present in all preparations. A marked difference from the CuxZn1-xS2 and CuxNi1-xS11 systems in which the appearance of new peaks in the X-ray diffraction spectra was interpreted as an indication for the formation of new solid phases of intermediate composition with respect to the pure metal sulfides, is that in the CuS-CdS system the precipitates consist of mixed sulfides. This may be ascribed to the large differences between the mean atomic radius of Cu and Cd (128 and 151 pm, respectively) which contrary to the case of Zn and Ni (134 and 124 pm, respectively) do not favor lattice substitution of the sulfide precipitated phase at least within the time scale of the precipitation process used for the polycrystalline sulfide preparation. Physicochemical Properties of the Polycrystalline Preparations. The diffuse reflectance spectra (DRS) of the mixed sulfide preparations with x ) 0.20 and x ) 0.25 are shown in Figure 2. From the of intercept of the graph (9) JCPDS Card File No 6-464. (10) JCPDS Card File No 6-314. (11) Tsamouras, D.; Dalas, E.; Sakkopoulos, S.; Koutsoukos, P. G. Langmuir (In press).
of the variation of the values of function F(R∞) with the wavelength at the low-frequency region, the absorbance threshold for the preparations with x ) 0.20 and 0.25 equal to 2.40 and 2.39 eV was calculated. The function F(R∞) is given by eq 2:
F(R∞) )
(1 - R∞)2 K ) 2R∞ S
(2)
where R∞ is the ratio of light intensity of the light reflected from the sample to the intensity of the light reflected by the reference. K and S are the coefficients for absorbance and reflectance of the materials examined, respectively. These values are very close to the value 2.40 eV reported for the band gap energy of cadmium (II) sulfide.12 Further taking into consideration the findings of the powder X-ray diffraction analysis it may be suggested that in our preparations no intermediate phases were formed and that the polycrystalline powders obtained consisted of mixtures of cadmium and copper sulfides. Considering that in the XRD spectra of the semiconducting materials (x ) 0.20, 0.25) the intensity corresponding to the cadmium sulfide peaks was significant, it is reasonable to assume that these samples show in their DRS spectra absorbance corresponding to the energy gap, Eg, of CdS. It should be noted that the CuS prepared by precipitation in all cases has shown metallic behavior thus precluding the presence of an energy gap in this preparation. Moreover, the lack of any additional absorption bands suggested that there are no additional states in the energy gap which may absorb at the corresponding energy values. The morphological examination of the mixed sulfide preparations showed that they consisted of mixtures of (12) Bockris, J. O. M.; Khan, S. U. M. Surface Electrochemistry; Plenum Press: New York and London, 1993; Chapter 3.
Characterization of Semiconducting Heavy Metal Sulfides
Langmuir, Vol. 15, No. 23, 1999 8021
Figure 3. Scanning electron micrographs of the (CuS)x(CdS)1-x mixed sulfides preparations. (a) x ) 0.20; (b) x ) 0.30; (c) x ) 0.35, (d) x ) 0.50. Table 4: Characteristics of Cadmium Sulfide Crystalline Materials formula
name
crystallographic system
CdS CdS
Greenokite Hawleyite
hexagonal cubic
unit cell lattice parameters (Å)
larger and smaller particles, possibly reflecting the morphological differences between CuS which forms larger crystallites13 and CdS which upon precipitation forms small polycrystalline formations.14 As may be seen from the micrographs (Figure 3), the formation of the mixed phase precipitates did not yield any marked morphology differences over the entire range of the mixture composition. The results of the morphological examination of the samples by SEM were corroborated by the particle size distribution analysis of the polycrystalline powders presented in Table 3. In Table 3 the mean particle diameter of equivalent spheres is shown for the various (CuS)1-x(CdS)x preparations. Electrical Properties. Cadmium sulfide as a crystalline material of the hexagonal system is often found as Wurzite. Thermodynamically unstable phases have also been reported, greenokite being the most often encountered unstable phase. The properties of the cadmium sulfides are summarized in Table 4. The energy gap of CdS at 300 K is 2.42 eV while the electrical conductivity, (13) Andritsos, N.; Karabelas, A. J. Colloid Interface Sci. 1991, 145, 158. (14) Dalas, E.; Kallitsis, J.; Koutsoukos, P. G. Langmuir 1991, 7, 1822.
a ) b ) 4.140, c ) 6.719 a ) 5.818
stability (/°C)
conductivity
>525
