Electrokinetic behavior of spherical colloidal particles of cadmium sulfide

Materials Chemistryand Physics44 (1996) 51-58

ELSEVIER

Electrokinetic behavior of spherical colloidal particles of cadmium sulfide M.C. Guindo, L. Zurita, J.D.G. Durkn, A.V. Delgado* Departamento

de Fisica

Aplicada,

Facultad

de Ciencias,

Universidad

de Granada,

18071

Granada,

Spain

Received8 February 1995;accepted4 August 1995

Abstract The electrophoretic behavior of monodisperse, spherical cadmium sulfide particles is analyzed in this work with the aim of contributing new data on the electrical nature of the surface of this material. The electrophoretic mobility, pe, of CdS particles is studied as a function of both pH and concentration of several electrolytes. As in many metal sulfides, the surface characteristics are very sensitive to its degree of oxidation, as demonstrated by the slight increase in isoelectric point (iep) of samples obtained with longer synthesis times. The iep of CdS is found to be between pH 1 and 1.5, this value being unaltered by the addition of NaCl at different concentrations. The effect of the lattice ions (Cd” and S2-, or rather HS-) on the mobility is very significant: HS- anions adsorb on the particles, increasing the negative values of pe, whereas the behavior in the presence of Cd2+ salts suggests surface precipitation of Cd( OH), . The cations Ag+, Cu2 + , Mn2 + and La3+ can be considered as activating species for the cadmium sulfide/solution interface: their hydrolysis products adsorb onto the CdS particles and provoke up to three pH values of inversion of the sign of p,. The effect is most important for Cu2 +, Ag+ and La3 + cations, for which ,uereversalscan be achieved for initial concentrations as low as low4 and 5 x 10s5 M, respectively. If MnCl, solutions are used, their concentration must be > 10m3M to observe the same phenomenon. Keywords:

Cadmium sulfide;Electrophoreticmobility

1. Introduction Cadmium sulfide is a material thoroughly studied from the scientific and applied points of view due to its semiconducting properties, which make it useful in, the manufacture of solid-state solar cell windows by deposition of thin films on different substrates. Other deposition methods are carried out from chemical baths; hence the interest in properly characterizing the cadmium sulfide/solution interface [ 1,2]. Such characterization can be carried out on a more solid basis if the dispersed solid material is homogeneous in size, shape, chemical composition, etc. Hence we have carried out a study of the CdS/electrolyte solution interface using synthetic spherical, homogeneous colloidal particles [3]. Such a system can be considered an inorganic colloidal model; in fact, the shape and homogeneity of the particles allow a precise * Correspondingauthor. 0254-0584/96/$15.000 1996ElsevierScienceS.A. All rights reserved

calculation of such quantities as electrokinetic potential, interaction energy between particles or between particles and substrates, and so on. In this work we describe a study of the electrical properties of the CdS/electrolyte solution interface in different conditions, using electrophoresis of suspensions as the experimental method. The analysis of the electrophoretic mobility of the particles as a function of pH in the presence of several electrolytes shows substantially different behaviors, characteristic of indifferent, determinant or activating ions, for the interfacial potential. Previous work [2,4] has demonstrated the determinant effect on CdS surface potential of such ions as H+, OH-, Cd2 +, HS- as a consequence of their specific adsorption on Brsnsted (H+, OH-) or Lewis (Cd2+, S’-) acid sites. Concerning the activating effect of hydrolysable metal cations, it has been widely studied for other metal sulfides, mainly sphalerite [5-71; however, such a study has not been carried out with CdS, in particular for strongly activating ions like Ag+,

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+ NlNOJlOmM x N.N03Ill&, I I I -3 1 2 3 4 /

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Fig. 1. Electrophoretic mobility of CdS spheres as a function of pH for the concentrations of sodium chloride indicated.

Mn*+ or La3+, considered in this work. It will be shown that the mechanisms of interaction of these cations with the cadmium sulfide surface follows the ‘hydroxylation-precipitation’ model described for oxides and sulfides [5,8,9].

2. Experimental

Cadmium sulfide particles were prepared following the method described by Matijevic and Wilhelmy [3]: Cd(N03)* was reacted with thioacetamide (TA) at 26 “C in the presence of HN03 (pH < 1) during 14 h. At the end of this time, small CdS seedsare present in the medium; the final particles were grown on these seedsby adding TA (12.5 cm3 of 0.05 M TA solution per litre of seed suspension)and allowing the reaction to proceed during 20 min (sample A) or 100min (sample B). TEM micrographs showed that the particles were spherical and considerably monodisperse, with average diameters of 780 i: 70 nm and 1.3 + 0.1 pm for samples A and B, respectively. The chemical purity of the particles was ascertainedby EDX microanalysis; no atoms (heavier than Na) other than cadmium and sulfur were detected in the samples,within the experimental error of the instruments (Link Analytical QX2000), which amounts to x 0.5% (depending on the intensity of the electron beam).

It must be mentioned that, when working with metal sulfides, the possible surface oxidation of the particles must be kept in mind. For this reason, several authors [2,6,10] have insisted on working, in as much as possible, under inert atmosphere. Our CdS spheres were prepared in the presenceof (oxidizing) nitric acid solutions; hence our only precaution was to flush the suspension with pure nitrogen before their storage, since surface oxidation, if any, should have occurred during the synthesisprocessitself, All chemicals were analytical grade from Merck or Panreac. They were used as received, without any purification. Only TA used in the synthesiswas recrystallized from spectroscopic quality benzene. For the preparation of the suspensions,water was twice distilled, deionized and fitered through 0.2 /lrn membranes (Mill&Q Reagent Water System, Millipore). The suspensions obtained directly from the synthesis were cleaned by repeated centrifugation/redispersion cycles until the conductivity of the supernatant was close to that of pure water (l-2 ,uScm-‘). Such purified suspensions (pH 5-5.5) were kept in the dark in refrigerated polyethylene bottles. Electrophoretic mobilities were measured with a Malvern Zeta-Sizer 2c at 25 “C. The measurements were carried out 24 h after the preparation of the suspensions(volume fraction of solids ~0~05g 1-l). A relative error of < 5% can be ascribed to mobility data presented in this work. The pH of the suspensionswas adjusted, by addition of NaOH and either HCl or

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Fig. 3. Electrophoretic mobility of CdS (sample A) as a function of pH for different concentrations of (NH&S and Cd(NO,),.

-1.5

Log C W) Fig. 4. pe plotted as a function of (NH&S tions for pH 4 and 8.

and Cd(NO&

concentra-

HNO,, both at the moment of suspension preparation and immediately before measurements were carried out.

3. Results and discussion 3.1. Efect of pH on the electrophoretic CdS spheres

mobility

of

Let us first analyse how the electrophoretic mobility y, of the particles changes with pH in the presence of electrolytes that, like NaCl and NaNO,, can in principle be considered as indifferent for the CdS/solution interface. Fig. 1 shows the pe-pH trends for samples A and B in 10m3 and lo-’ M NaCl solutions. Note that sample A (20 min seed growth) has an isoelectric point (iep) close to pH = 1, regardless of the concentration of sodium chloride. This shows that NaCl is, as expected, an indifferent electrolyte: neither Na+ nor Cl- ions seem to adsorb at the interface. It is also worthwhile to observe that pe is approximately constant for this sample in the pH interval from 4 to 8. Concerning the iep value of CdS particles, values close to those found in this work have been reported by other authors [2]; however, values as high as 3.6 [3] or even 7.5 [4] have also been reported. The latter data were obtained in NaC104 and KNO, solutions, respectively; since both Cl07 and NO; anions are oxidizing, they could convert surface S2- ions into So or SO:- (such oxidations are thermodynamically favored; see data in

Ref. [2]). The iep must shift to higher pH values if sufficient surface oxidation has occurred, as shown in Refs. [ 1 1 - 131 for pyrite, galena and sphalerite. Similar arguments can be given for the existence of the mobility plateau between pH 4 and 8. According to Williams and Labib [ 141 and Moignard et al. [lo], the following oxidation-reduction reactions are likely to occur: CdS --f Cd2 + So + 2e (oxidation) 402 + 2H+ + 2e + H,O or

(reduction)

2H,O + 2e --f H2 + 20H- I As a consequence of such reactions, Cd’+ ions will be released from the solid, and elementary sulfur will be formed on the surface. The negative mobility plateau would then be related to the fact that ,ue of sulfur is negative and practically independent of pH between pH 4 and pH 10 [ 151. Furthermore, Pugh [ 131 has shown, by XPS data, that surface oxidation of a similar sulfide, ZnS, leads to So formation. Fig. 1 also includes pLe-pH data for sample B (100 min growth). The iep is slightly higher ( x 1.5) than for sample A; this fact could again be a consequence of a larger oxidation of these particles because of their longer reaction times in the presence of HNO,. It can also be observed that p, increases in absolute value for

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-5 1 2

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Log c Do

Fig. 5. Electrophoretic mobility of CdS as a function of pH for three AgNO, concentrations. Ionic strength lo-* M NaNO,.

Fig. 6. Electrophoretic mobility of CdS spheres as a function of total AgNOs concentration at pH 4 and 9.

the whole pH range, no pH interval of constant mobility being observed for sample B, unlike sample A. It can be suggested that more highly oxidated species, such as S,O;- [lo] are present on the CdS surface in this case. Hence most of the results referred to below were obtained with the less oxidized sample, A. A very interesting result can be seen in Fig. 2, corresponding to sample A in NaNO, solutions. Sodium nitrate does not seemto behave as an indifferent electrolyte: the iep shifts to higher pH values the larger the concentration of NaNO,, above the iep found with NaCl in both cases;the oxidizing nature of NO; is manifest in a more oxidized CdS surface with higher iep.

fact, it must be mentioned that, since S2- is readily hydrolyzed in solution [16], HS- and HIS cannot be ignored. Given that any hydrolysis reaction involves H+ and OH-, Sz- (or HS-) and Cd’+ would also be, in an indirect way, potential determining ions, as H+ and OH- (Figs. 1 and 2). In fact, it is well known that changes in electrochemical potential, d/l, of Cd2+ and S*- are related to dpn+ and d/con- [ 161,and hence the determining effect of Cd2+ and S2- (or HS-) on the zeta potential of the particles can be considered as a consequenceof the effect of these ions on the electrochemical potential of H+ and OH-, which would be true potential determining ions for the interface. It is hence interesting to analyze the behavior of the mobility of CdS in solutions containing Cd2+, S2ions, and their hydrolysis products. The results, corresponding to sample A, are shown in Figs. 3 and 4. Fig. 3 is a plot of the variation of [lc of CdS sphereswith pH for different concentrations of (NH4)2S and Cd(NO& (data corresponding to lo-’ M NaCl are included for comparison), for constant 10W2M ionic strength maintained with NaCl. This figure shows that in the presence of WH.M> I~el increasesconsiderably with pH. This behavior can be explained by taking into account that, as demonstrated by Ste-Marie et al, [17], HS- ions are always more abundant than S2- ions between pH 2 and 14, although the concentration of HS- increases very abruptly for pH > 4; their adsorption on the CdS

3.2. Lattice ions and electrophoretic mobility

According to Park and Huang [4], three types of facescan be found in the hexagonal wurtzite-type structure of CdS: non-polar surfaces containing equal surface densitiesof both Cd*+ and S2- ions (( 1010)crystal planes), and polar surfaces(0001) with only one type of atom in contact with the liquid phase. Specific adsorption of H+ and OH- ions can occur in the former planes (they would be Brranstedacid sites),whereasS*or Cd2+ can adsorb on polar faces (Lewis acid sites). From this point of view, it can be said that all these ions can be considered potential-determining ones. In

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particles would be responsible for the observed increase in I,u~/ (compare with the variation of ,u~ with pH in the presence of NaCl). The effect of pH on ~1, when Cd(NO& is in solution can also be observed in Fig. 3: at acid pH’s and [Cd(NO,),] > 10m3 M pu,is positive, instead of negative as it was in the absence of cadmium ions (Figs. 1 and 2); this fact suggests adsorption of Cd’+ (and perhaps also Cd(OH) + when we approach neutral pH values) at the inner part of the double layer. When the pH is neutral or basic, a sharp increase in II, is observed, the maximum mobility being obtained for pH = 8 (when the concentration of Cd(NO& is 10v2 M) and z.9 (for [Cd(NO,),] = 10W3M). The classical argument given by James and Healy [9] and Ralston and Healy [5] can be applied to the CdS/solution interface: the sharp increase in ,ue would be due to surface precipitation of Cd(OH), on the CdS particles; the latter would behave similarly to cadmium hydroxide for higher pH values (the higher the Cd(NO& concentration in solution, the more complete the surface coverage of the particles by Cd(OH),). Hence, after the mobility maximum, the subsequent change of sign of ,u~ (not observed for the pH range studied) must correspond to the point of zero charge (pzc) of Cd(I1) hydroxide. The effect on ,u, of the concentrations of (NH4)2S and Cd(NO,), (constant ionic strength 10e2 M NaCl) is plotted in Fig. 4. According to data in Fig. 1, p, should be approximately - 1 pm s-’ V-’ cm-’ between

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Log c (Ml Fig. 8. Same as Fig. 6, for Cu(NO,),

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pH 4 and 8 in the absence of S2- or Cd’+ ions. Note how the data in Fig. 4 suggest that S2- (or rather HS-, as explained above) ions do indeed interact strongly with the CdS surface: low5 M (NH,),S suffices for increasing the negative mobility by one (pH 4) or three (pH 8) units. Concerning the effect of Cd(N03)2 concentration on p,, Fig. 4 shows that, again, Cd2+ ions, and their hydrolysis species, must adsorb to a significant degree on the CdS particles: both at pH 4 and pH 8 an inversion of the sign of the electrophoretic mobility of CdS is observed for moderate cadmium nitrate concentrations. The surface coverage of the particles by Cd(H) hydroxide may account for the larger effect (almost seven mobility units increase) found at pH 8. 3.3. Efsects of Ag, Cu, Mn and La salts

Transition metal cations have been demonstrated to have a significant activating effect on the sulfide/solution interface, widely described for zinc sulfide in the presence of Cd’+, Pb2+ and Cu2+ [5,6,9,10]. In a previous work [7] we have described a similar effect of Ag+, Mn2+ and La3+ in the ZnS/solution interface; in this section we describe the results obtained with cadmium sulfide suspensions. Fig. 5 shows the variations of the electrophoretic mobility of CdS spheres with pH, for different concentrations of AgNO,, at a constant ionic strength (lo-* M

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Fig. 9. Electrophoretic mobility of CdS spheres as a function of pH for the concentrations of manganese(I1) chloride indicated. Ionic strength lo-‘M NaC1.

Fig. 10. Electrophoretic mobility of CdS spheres as a function of total MnCI, concentration at pH 4 and 9.

NaNO,). As observed, for the lowest AgN03 concentrations (0.1 and 0.5 mM) the mobility is zero for three different pH values (CR1 , CR2, CR,), in a similar way to the results reported by James and Healy [9] for the activation of Si02. It can be proposed that CR, corresponds to the iep of Ag,S; a surface coverage of CdS by silver will provoke the formation of surface Ag,S, since pK,(Ag,S) = 49.6, whereas pK,(CdS) = 26.1 [18]. The reaction would be:

CdS particles must be responsible for the existence of the CR, reversal pH. Finally, CR, is close to the point of zero charge of silver oxide; the latter completely covers the particles, and their electrokinetic behavior approaches that of AgzO. Ralston and Healy [5] have described the set of chemical reactions occurring under similar conditions with ZnS as solid and Cu(I1) as activating electrolyte. The effect of AgNO, concentration on /cc is depicted in Fig. 6 (constant ionic strength lo-’ M). The adsorption of Ag+ (the predominating species at pH 4; see Ref. [7]) is readily detected at pH 4: (A{,( decreases monotonically for [Ag+ltotnl > 10m4 M even though the ionic strength is constant, as a consequence of surface charge neutralization by silver ions. Fig. 6 shows, for pH 9, the expected inversion of the sign of the zeta potential of CdS because of the increasing surface coverage by silver oxide, as described above. Let us consider the effect of the addition of CU(NO~)~. Fig. 7 shows the /cc-pH curves obtained in the presence of low4 and low3 M copper nitrate solutions.at constant ionic strength (lo-’ M NaN03). The behavior observed in the acid pH range resembles that found with Ag+ (Fig. 5): CdS particles are covered with a black CL-IS precipitate (pK, = 35.1 [IS]), thermodynamically favored according to a reaction similar to that written above for Ag +. For neutral and basic pH values, reversals of the zeta potential are observed at

2Ag+(aq) + CdS(s) *(Cd,

Ag)S(s) + Cd2+(aq)

corresponding to a surface substitution of Cd2+ ions by Ag+ ions, thermodynamically favored by the fact that p&(Ag,V > K(CdS). When the total Ag concentration is 1 mM (Fig. 5), the lowest pH of zero mobility is ~4, i.e., much higher than the iep of CdS (Fig. 1). We suggest that this pH shift is due to specific adsorption of Ag+ on Ag,S-covered particles: a higher pH is needed to compensate for this extra positive charge absent when [Ag+] is lower. For the three concentrations studied, when the pH is neutral or basic, a typical surface activation process occurs: a sharp increase and change of sign in ,u, and a subsequent maximum. This behavior must be a consequence of Ag+ hydrolysis; simple chemical calculations [7] show that, for’that pH range, the concentration of Ag(OH) (as) is significant, as well as the precipitation of Ag,O (s): the surface precipitation of silver oxide on

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points CR2 and CR, (Fig. 7), due to surface precipitation of Cu(OH),. A similar phenomenon has been described by Pugh and Tjus [6] for ZnS activated with Cu2+. The effect of Cu(NO,), concentration on pL, is depicted in Fig. 8 for pH 4 and 7 ( IOU2 M ionic strength). In the former case, a monotonic decrease in the electrophoretic mobility of the particles is observed due to adsorption of Cu2+ cations on CuScovered cadmium sulfide particles. When the pH is 7, a sharp change in the sign of ,u~ is found for [Cu(NO,),] % 5 x 10m4 M; the surface precipitation of Cu(OH), seems to be confirmed by these data. The trend of variation of pe with pH, at constant ionic strength ( 10m2 M NaCl), for different concentrations of MnCl, is shown in Fig. 9. The reasoning applied to the data in Figs. 5 and 7, concerning surface formation of silver or copper sulfide, respectively, cannot be applied to Mn2+ ions, since MnS is more soluble than CdS. In the acid pH range the electrokinetic behavior does not essentially differ from that observed in the absence of MnCl, (Fig. 1); this suggests that no specific interactions between Mn2+ and CdS occur in such conditions, as expected from the fact that the loss of the hydration layer of Mn cations is thermodynamically unfavorable [8]. Only

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when the pH is neutral or basic do species like Mn( OH) + (as) or Mn( OH), (aq) appear in solution [7]; these can adsorb more easily (they are less strongly hydrated than Mn2+ ions). Their presence on the particle surface must be responsible for the existence of CR2, whereas CR, would be, as before, the pzc of MnO. The extent of surface coverage by MnO is a function of concentration; Fig. 10 is a plot of the y, versus [MnCl,] added at pH 4 and 9, in the presence of low2 M NaCl. Note that at pH 4, the effect of changes in the concentration of the Mn salt is almost negligible, since the mobility is very slightly reduced (less than 0.5 unit) in the concentration range lo-‘10m2 M. When the pH is 9, the mobility changes by almost five units, and the sign of p, is reversed when the initial MnCl, concentration is x 10m3 M. For higher concentrations, ,D~ continues to increase by the increases in the extent of surface covered by MnO. The effect of La(NO,), runs parallel to that of MnCl, (Fig. ll), so that the same mechanism of hydroxylation-precipitation could be involved to explain the changes of mobility with pH. The main significant differences are: (i) the reversal of the sign of pe may take place for lower concentrations in the case of La(NO,),; (ii) the values of 1.~~1for the highest La(NO,), concentration (1 mM) are significantly lower than for 0.1 or 0.05 mM (for acid and neutral pH). Both points might be explained by a very favorable adsorption of lanthanum complex ions, like La( OH)2+ [7], that would be more easily adsorbed due to their larger sizes and lower degree of hydration, as compared to Mn complexes.

Acknowledgement This work was financed by Fundaci6n ces, Spain.

Ram6n Are-

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[9] R.O. James andT.W. Healy, J. ColloidInterface Sci., 40( 1972) 53. [lo] D.J. Moignard, D.R. Dixon and T.W. Healy, Proc. Australas. Inst. Miner. Metall., 263 (1977) 31. [ 1l] D. Fornasiero, V. Eijt and J. Ralston, Colloids and Surfaces, 62 (1992) 63. [12] D. Fornasiero, L. Fengsheng, J. Ralston and R.S.C. Smart, J. Colloid Interface Sci., 164 (1994) 345. [13] R.J. Pugh, in E. Forssberg (ed.), 16th Int. Mineral Processing Congr., Amsterdam, 1988, p. 751. [14] R. Williams and M.E. Labib, J. Colloid Interface Ski., 206

(1985) 251. [ 151 E. Chibowski and J. Waksmundzki, J. Colloid InterJace Sci., 64 (1978) 380. [16] P.L. de Bruyn and G.E, Agar, in D.W. Fuerstenau (ed.), Froth Flotation, America1 Institute of Mining, Metallurgical and Petroleum Engineers, New York, 1962, p. 91. [17] J. Ste-Marie, A.E. Torma and A.O. Giibeli, Can. J. Chern., 42 (1964) 662. [18] J.N. Butler, Ionic E@libriwn: A Mathematical Approach, Addison-Wesley, London, 1964, p, 311.