Znd. Eng. Chem. Res. 1995,34, 2493-2498
2493
Mechanism of Formation of Composite CdS-ZnS Ultrafine Particles in Reverse Micelles Hiroshi Sato, Takayuki Hirai,* and Isao Komasawa Department of Chemical Engineering, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan
The mechanism of formation of composite cadmium sulfide and zinc sulfide ultrafine particles (Cdl-,Zn,S) by simultaneous precipitation of cadmium sulfide and zinc sulfide in sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/isooctane reverse micelles has been studied. The particle formation process was followed by the change in W-visible absorption spectra. The effects of the reactant concentration, the molar ratio of cadmium to zinc ions, and the water content on the particle formation process were investigated. The resultant particles were richer in CdS than the feed composition of the cadmium to zinc ions in the reaction solution. The particle composition was found to be controlled by the solubility of the CdS and ZnS ultrafine particles. The particle coagulation process was analyzed on the basis of a statistical distribution of particles among the reverse micelles. The coagulation rate constant was controlled by the composition and the size of the particles and by the size of the reverse micelles.
Introduction Nanometer-sized ultrafine particles are of interest with respect to their characteristics differing from the bulk materials. Nanometer-sizedultrafine semiconductor particles have band gaps greater than those of the bulk semiconductors owing to a quantum size effect (Brus, 1984). Composite particles of differing semiconductors have unique optical and electronic properties such as fluorescence activation (Hasselbarth et al., 1993) and quenching (Gopidas et al., 1990) or novel photocatalytic activities (Ueno et al., 1985;Youn et al., 1988). It is important that the ultrafine particles are protected from uncontrolled aggregation. The nanometersized water cores in reverse micelles have been used as the microreactors for the preparation of ultrafine particles (Osseo-Asare and Arriagada, 1990; Pileni, 1993). Composite ultrafine particles can be prepared in reverse micelles. CdSe-coated ZnS and the reverse composition composite particles have been prepared by Kortan et al. (1990). The two types of composite CdS and ZnS particles (core-shell structure and solid solution structure) have been prepared in reverse micelles, and their photocatalytic activity for water cleavage to hydrogen has been investigated (Hirai et al., 1994b). In order to prepare composite ultrafine particles having a controlled structure, a complete picture of the mechanism of particle formation is important. In the cases of metal boride particles (Ravet et al., 1987) and titanium dioxide particles (Hirai et al., 19931, the size of the formed particles was dominated by the statistical distribution of the reactants and particles among the reverse micelles. For the formation of metal sulfide ultrafine particles by fast reaction between metal and sulfide ions (Towey et al., 1990; Hirai et al., 1994a),the particle formation is found to be rapid and to be followed by a rapid coagulation process dominated by the intermicellar exchange process for the reverse micelles. The rate of coagulation decreases with increasing particle diameter and is dominated by the statistical distribution of the particles among the reverse micelles. A kinetic model for the reduced rate of particle coagulation process has been proposed (Hirai et al., 1994a). It is now necessary t o extend the previous work for the preparation of ultrafine particles composed of the 0888-588519512634-2493$09.00/0
single components, CdS and ZnS, t o that of the composite ultrafine particles. The mechanism of formation of the coprecipitated ultrafine particles of CdS and ZnS has therefore been studied in the present work. The effect of the solubility of the materials on the composition of the particles has also been studied. Special attention was paid to the solubility of the materials in the form of the ultrafine particles, since this solubility may be different from that of the bulk material. A kinetic model applicable to single component particles is also examined for its applicability to the present case of composite particles.
Experimental Section Sodium bis(2-ethylhexyl) sulfosuccinate (AOT), sodium sulfide, cadmium nitrate, and zinc nitrate were supplied by Wako Pure Chemical Industries, Ltd., and isooctane (2,2,44rimethylpentane)was supplied by Ishizu Seiyaku, Ltd. All reagents were used without further purification. Distilled water was filtered with a 0.45-pm membrane filter and any dissolved oxygen was purged by argon bubbling, prior to use. The reverse micellar solution was prepared by dissolving 0.1 M (M = mol/L) AOT in isooctane, followed by filtration using a 0.2-pm membrane filter. The water content (water to surfactant molar ratio, W O= [HzOHAOTI) was varied in the range 3-10. Aqueous solutions containing cadmium nitrate, zinc nitrate, or sodium sulfide were prepared daily. The reverse micellar solutions containing reactants were prepared by injecting the required amount of the aqueous solutions and were then used within a few minutes. The coprecipitated particles of CdS and ZnS (Cdl-,Zn,S) were prepared by rapid mixing of a reverse micellar solution (1.2 mL) containing both cadmium nitrate and zinc nitrate and an equal volume of a reverse micellar solution containing sodium sulfide. This was done at 25 "C, using the mixer part of a stoppedflow spectrophotometer. A stopped-flow spectrophotometer (Otsuka Electronics RA-401) equipped with a diode array detector (Otsuka Electronics RA-415) was used for the measurement of the W-visible absorption spectra. Two parameters Cf and xf respectively were defined in order t o represent the initial composition of each 0 1995 American Chemical Society
2494 Ind. Eng. Chem. Res., Vol. 34, No. 7, 1995 0.1 M AOT/ isooctane W, = 6
0.6s
h
F
1
W"
3 bulk
.' .--
.-,' I
300
350
400
Wavelength (nm)
Figure 1. Absorption spectra for single component and composite particles a t various feed ion ratios.
reactant in the reverse micellar solution, where
The concentrations of all materials are defined in terms of moles per liter of micellar solution. The concentration of the reverse micelles (C,) was calculated from the micellar size distribution. The values used for W O = 3, 6, and 10 reverse micellar solutions were 58.9 x 9.27 x and 4.31 x M, respectively (Hirai et al., 1994a). The size of AOT micelles has been reported t o be independent of the salt concentration (Aveyard et al., 1986) and also of the change in countercation of AOT (Dunn et al.,1990). In the present study, the size of the micelles was therefore considered to be independent of reactant concentration.
Results and Discussion Absorption Spectrum of Coprecipitated Particles. Semiconductormaterials absorb the part of the UV-visible spectrum having a greater energy than the band gap. The band gaps for bulk CdS and ZnS are 2.5 and 3.7 eV, respectively. The bulk band gap for the Cdl,Zn,S solid solution (&,bulk) changes parabolically from 2.5 to 3.7 eV with increasing values of x as expressed by eq 3 (Suslina et al., 1976).
Figure 1shows the UV-visible absorption spectra for Cdl-,Zn,S particles for various values of xf measured &r 0.06 and 0.6 s following the mixing of the solutions. Since Cd(NOd2, Zn(NOd2, and Na2S have very weak absorption characteristics, the observed absorption can be attributed to that of the formed particles and thus the magnitude of the absorption can be taken to indicate the quantity of formed particles. The absorption threshold for ZnS ultrafine particles (xf = 1)is observed a t a shorter wavelength than for CdS (xf = 01, since the band gap of ZnS is greater than that of CdS. The coprecipitated particles also exhibited only one threshold in the absorption spectrum, and wavelength of this blues h i h d with increasing xf as in the case of bulk materials and as shown by eq 3. Since core-shell structure composite semiconductor particles exhibit two thresholds in the absorption spectrum (Youn et al., 1988;Hirai et al., 1994b1, the observed spectra indicate the formation of solid solution particles of CdS and ZnS.
0 CdS
.
I
I
0.5 Xf
(-1
I
1
I
1 0 ZnS CdS
1,*1'
I
I
I
I
%a1
I
1
0.5
(4
ZnS
Figure 2. Band gap for Cdl-,Zn,S particles. (a) Band gap versus feed ion ratio; (b) band gap versus estimated zinc content in particles.
The absorption threshold wavelength red-shifted with increasing time from 0.06 to 0.6 s following the initial mixing. The increase in absorption magnitude however is not observed during this period, indicating that the conversion of ions to particles had already finished at time 0.06 s. The composition of the particles ( x ) can thus be assumed to remain constant with time following a time of 0.06 s after mixing and the red-shift can thus be attributed to particle coagulation, as was observed in the cases for CdS and ZnS single component particles (Hirai et al., 1994a). The band gap (E,) for a direct gap semiconductor can be determined by fitting the absorbance data to the following equation (Wang et al., 1987): O ~ = V A
( ~ -v E,)'"
(4)
where o is the molar absorption coefficient, A is a proportionality factor, and hv is the photon energy. The solid lines in Figure 2 show the band gap for Cdl-,Zn,S particles for various values of the feed composition (xf). The band gap for the bulk materials is also shown by means of the dotted line. The quantum size effect is clearly seen. The band gap increases parabolically with increasing xf as for bulk materials. With increasing feed ion concentration (Cf), however, the relationship became a little more linear. This result can be explained by the effect of feed concentration on particle composition. Composition of the Formed Particles. Cdl-,Zn,S particles prepared by the simultaneous precipitation of CdS and ZnS were richer in CdS as compared to the feed ratio of the Cd2+and Zn2+ions CYoun et al., 1988; Hirai et al., 1994b). This is due to a difference in the concentration of the ions unconverted to particles. The concentration of the unconverted ions in solution is controlled by the solubility product of the particle material, where [Cd2+1[S2-l/M2= Ksp,CdS [z~~+I[s~-YM' =K
~
(5)
~
, (6) ~
From these relationships, the ratio of the residual Cd2+and Zn2+ions in solution can be expressed by the following equation, [Zn2+l/[Cd2+1= Ksp,Zn@&,,CdS
(7)
Since the solubility product of bulk CdS is less than that for ZnS (Ksp,CdS,ba = 1.4 x io-29, &p,ZnS,bullr = 1 x io-23) (Youn et al., 19881, the concentration of the residual
~
~
Ind. Eng. Chem. Res., Vol. 34, No. 7,1995 2496 Table 1. Particle Composition Estimated from Solubility ( X C 3
CdS
XF
Cf (M) 1 10-4 2 x 10-4 4 x 10-4
0 0 0 0
0.25 0.09 0.18
0.22
0.5 0.41 0.46 0.48
0.75 0.74 0.74 0.75
1 1 1 1
Zn2+ion in the solution is greater than that of the Cd2+ ion, and hence, the formed particles become richer in CdS than the feed composition. With an increasing conversion of the metallic ions to particles, namely, with decreasing quantities of residual metallic ions in the solution, the composition of the formed particles approaches more closely that of the feed composition. As expected from eqs 5 and 6, an excess sulfide ion (Hirai et al., 1994b)or an increase in the concentrations of both metallic and sulfide ions can increase the conversion of the metallic ions to particles. One problem encountered is that the solubility of the particles in the size range of ultrafine particles increases with decreasing particle size (Nielsen, 1964). Thus, the actual solubility for ultrafine CdS and ZnS particles must be estimated from the change in the conversion of the ions induced by an excess sulfide or metal ion. The solubility product of ultrafine CdS particles was estimated as follows. A series of experiments were carried out in which the concentration of one of the Cd2+ and S2- ions was continuously increased from 1 x M, while the feed concentration of the other ion was M. The maintained constant a t a value of 1 x absorbance of the formed particles was increased by about 10% above that for the case of [Cd2+l= IS2-] = 1 x M. Hence, since about 10% of the supplied ions reside in the solution in this case, the solubility for the ultrafine CdS particles can be estimated at about 1 x M. Similarly the solubility of ultrafine ZnS particles can be estimated at 2 x M. Hence, the values for Ksp,cdsand Ksp,znsare estimated to be 1 x and 4 x 10-lo, respectively, for the particles employed in the present study. The composition of the formed particles was estimated from the above solubility values. When the concentration of all the supplied metallic ions is equal to that for the sulfide ion, the concentration of the residual ions in the solution is expressed by the condition that
+
[Cd2+l [Zn2+1= [S2-l
Table 2. Parameters for CdS, ZnS, and Their Mixed Particles (Solid Solution)
(8)
The concentrations of residual Cd2+, Zn2+, and S2ions were calculated as 4.5 x 1.8 x and 2.2 x M, respectively, based on eqs 5, 6, and 8 and the values of the solubility products. Table 1 shows the estimated composition for the formed particles ( x d ) based on the feed and the residual quantities of ions. The calculated value ( x , 3 approaches that of the feed (xf) with increasing feed concentration M. The band gap (Eg)is (Cf) up to a value of 4 x now plotted as a function of Xcal as compared to xf. The results shown in Figure 2b indicate that the Eg-Xc-1 relationship is almost independent of Cf.This shows that the xed value gives a good estimate of the particle composition. Size of Formed Particles. The size of the Cdl,Zn,S particles was estimated from the band gap in the same manner as that used for the CdS and ZnS single component particles (Hirai et al., 1994a). The relationship for band gap (&) versus particle diameter (d,) is
ZnS 3.7 Eg,bulk(eV) 2.5 0.25 0.19 mdmo 0.59 mho 0.8 5.2 €/to 5.7 a (nm) 0.4136 0.3820 c (nm) 0.6714 0.6260
Cdl-,Zn,S 2.5 + (3.7- 2.5 - 0.61)~ -'r 0.6Lz2 0.19+ (0.25- 0.19)Z 0.8 (0.59- 0.8h 5.7 (5.2 - 5.7h 0.4136 + (0.3820- 0.4136)Z 0.6714 + (0.6260- 0.6714)x
+ +
u ioo
2io-3 5
10-2
lo-1
Time (s)
Figure 3. Change in particle diameter during preparation of the particles.
expressed by the following equation (Brus, 19841,
where h is the Planck constant, e is the charge of electron, E is the dielectric constant of semiconductor, and me and mh are the effective masses of electron and hole, respectively. In order to calculate this, the electric parameters for Cdl-,Zn,S particles at various values of x are needed. The bulk band gap was calculated by means of eq 3. The other parameters, E , me, and mh were assumed to have a linear dependence on particle composition. The relationships used for the parameters in terms of x are shown in Table 2. Figure 3 shows the time course in the change of particle size for 4 x M Cdl-,Zn,S particles prepared a t different values of xcal. The particle size is seen t o increase continuously with time, and the ZnS particles are shown to be smaller than both CdS and Cdl-,Zn,S particles for the same reaction time. This indicates that the kinetics of coagulation is dependent on the particle composition. Coagulation Kinetics of Cdl-,Zn,S Particles. For the case of CdS and ZnS single component particles, the rate of coagulation in the initial stage was very rapid and was controlled by the rate for the intermicellar exchange process only as found by Towey et al. (1990). With particle diameters greater than about 2.4 nm, however, the rate of coagulation decreased with particle size and was dominated by particle material and by the statistical distribution of the particles among the micelles (Hirai et al., 1994a). Consequently, the effect of particle composition on the reduced rate of coagulation for Cdl-,Zn,S particles larger than 2.4 nm was investigated in the same manner as for the single component particles. Since in this case the rate of coagulation is slower than the rate of intermicellar exchange process, the particles will distribute among the micelles according t o an equilibrium distribution. Also, since coagulation is considered t o proceed in micelles containing more than two particles (Hirai et al., 1993, 1994a1, the
2496 Ind. Eng. Chem. Res., Vol. 34, No. 7, 1995 L
-1
0.48
0
0 0.75 0
1
VA
A
l
2.5
I
I
1
3
.
I
3.5
&lo4 lo2[
,;Y
4x10-4
2.5 I
'
'
I
'
'
'
* I
"
'
,
I
I
open key: xf=O (CdS) closed key: xf=0.5
hail closed: xf= 1 (ZnS) ,
,
,
3 d, (nm)
,
,
0 ,
1
3.5
Figure 5. Effect of reactant concentration on coagulation rate constant. c
I
composite particles with differing xcal values. The rate constants for the composite particles are shown t o decrease with an increase in particle diameter as occurred in the cases of CdS and ZnS single component 0 0.95 102- 0 1 particles (xcal = 0, 1). The rate constants are almost I ~ ~ I * ~ ~ I l ~ * * independent * l I of the composition in the range of xcal of 2.5 3 3.5 0-0.95, and this is in spite of the containment of ZnS. dp (nm) When the Cd content is reduced to 5% or lower, the behavior of the composite particles sharply approaches Figure 4. Effect of particle composition on coagulation rate constant. that of single component ZnS. One possible explanation of this result lies in the coagulation kinetics may be assumed to follow a pseudo change in the crystal structure of Cdl-,Zn,S. CdS and first-order kinetic relationship in respect of the concenZnS exist in two structural types, namely, wurtzite and tration of micelles containing more than two particles zinc blende (Sakaguchi et al., 1977). Wurtzite is the (Cmc), hence: low-temperature form for CdS, whereas zinc blende is that for ZnS. The structure of ultrafine particles -dCddt = k,,C,, (10) prepared in reverse micelles is reported to be wurtzite for CdS (Jain et al., 1992)and zinc blende for ZnS (Motte where k,, is the first-order rate constant and C, is the et al., 1992). The structure of the Cdl-,Zn,S will concentration of the formed particles. Assuming that therefore change from wurtzite t o that of zinc blende the particles distribute according to a Poisson distribuas the values of x increase (Sakaguchi et al., 1977). The tion (Atik and Thomas, 19811, the values of C,, is given region for x in which the transformation occurs depends by on the environment, but should be somewhere between the values of 0.95 and 1 for the present case. The assumption of a wurtzite structure at all values of x was used in the calculation of the number of the i=2 formed particles. This assumption however no longer applies the case of ZnS particles. When the value of 0.5409 nm is used for the lattice constant of zinc blende ZnS, however, the number of zinc or sulfur ions in a zinc blende ZnS corresponds to 357.3 for a value of d, where Npis the number of particles in the solution and = 3 nm and this is nearly equal to the value for that of wurtzite (357.4 for d, = 3 nm). Hence, the number of C, is the concentration of micelles in the micellar particles can be regarded to be independent of the form solution. of the crystal structure of the ZnS. The first-order rate constant, k m c , was thus calculated Factors for Particle Coagulation. The effects of based on eq 11from the number of particles expressed the feed ion concentration and the size of the micelles in a concentration unit (C,) and using a value of CmC. on the coagulation kinetics for Cdl-,Zn,S particles were The number of formed particles was calculated by dividing the total number of ions converted to particles studied. Figure 5 shows the effect of the feed ion concentration (Cf) on the coagulation rate constant (Kmc). by the number of metallic and sulfur ions in a particle. In the cases of CdS and ZnS single component particles, The former quantity can be estimated from the solubilthe value of k m c is almost independent of Cf, as reported ity as mentioned previously, and the latter quantity can previously (Hirai et al., 1994a). For xf = 0.5 coprecipibe obtained from the diameter and the lattice constants for the particle. Assuming that the structure of the tated particles, k,, is also independent of the feed formed Cdl-,Z&S particles is a wurtzite structure, the concentration, Cf. The value of xcal however decreases with decreasing Cfas shown in Table 1. Since It,, is lattice constants (a and c ) can be calculated from the independent of xCd in the range of 0-0.95 as shown in equations given in Table 2, since the lattice constants Figure 4, the value of k,, of the coprecipitated particles for wurtzite Cdl-,Zn,S have a linear dependence on x is thus independent of Cffor this range of Xcal values. (Skinner and Bethke, 1961). The effect of the size of the reverse micelles on the Figure 4 shows the relationships between the values coagulation kinetics was then investigated. In this, the of K, and d, for single component particles and for
Ind. Eng. Chem. Res., Vol. 34, No. 7, 1995 2497
r"'"''''''''''''l
1,
key A
&2J
0 (CdS)
0
2.5
open key: WO = 10 6, ,
, , c,b;;d: : y T k
(ZnSl half closed:WO = 3 3
3.5
,I 4
dp (nm)
Figure 6. Effect of water content of reverse micellar solution on coagulation rate constant.
size of micelles was changed by varying the water content of the micellar solution (WO). The results obtained are shown in Figure 6 . The value of k,, decreased with decreasing water content, i.e., decreasing size of micelles. The decrease may be caused by a restraint on the rate of coagulation by the micellar interface owing to an approach in the relative sizes of the micelles and particles, as also applied for single component particles (Hirai et al., 1994a). The effect of water content on k,, of ZnS particles is much smaller than that for CdS particles, confirming previous findings (Hirai et al., 1994a), and also smaller than Cdl-,Zn,S particles. The effect for ZnS particles, however, is slightly greater than the previous case obtained with Cf= 1 x M. Consequently, the coagulation model based on single component metal sulfide particles is found to be applicable also to the coprecipitated particles.
Conclusion The mechanism of formation of the coprecipitated ultrafine particles of cadmium sulfide and zinc sulfide in AOThsooctane reverse micelles by the reaction of Cd(NO&, Zn(NO&, and NazS was investigated. The particle formation process was followed by W-visible absorption spectra. The following results were obtained: 1. The absorption spectrum of the coprecipitated particles following a period of 0.06 s indicates a single band gap position that varies with the particle composition. This shows that in this period the mixing of CdS and ZnS was complete. A red-shift in the spectrum was then observed following this initial period and expressing an increase in the particle diameter, caused by particle coagulation. 2. The composition of the formed particles was controlled by the solubility of the particle materials. The solubility of CdS and ZnS for the present ultrafine particles was estimated. Since the solubility of the ZnS particles was greater than that of the CdS particles, the formed particles were richer in CdS than the feed composition. As the feed concentration increased, the particle composition approached that of the feed composition more closely. 3. The kinetics of coagulation was determined from the rate of decrease in the number of particles. The rate of coagulation was dominated by the distribution of the particles among the reverse micelles. The coagulation rate constants for Cdl,Zn,S particles decreased with increasing values of x in the range of x = 0.95-1, but were almost independent ofx in the range ofx = 0-0.95.
This effect may be due to a change in the crystal structure. When the water content of the reverse micellar solution was decreased, the rate constant of coagulation became less owing to the decrease in the size of the micelles.
Acknowledgment The authors gratefully acknowledge financial support from a Grant-in-Aid of Izumi Science and Technology Foundation. H.S.is supported by Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists.
Nomenclature a = lattice constant of wurtzite crystal for the a-axis, nm c = lattice constant of wurtzite crystal for the c-axis, nm Cf = [Cd2+] [Zn2+l= molar concentration for all metallic
+
ions in the feed solution, M C, = molar concentration of the reverse micelles, M (moll
L) C,, = molar concentration of the reverse micelles containing two or more particles, M (moVL) C, = molar concentration of the formed particles, M (moll
L) d, = average diameter of the particles, nm e = charge of electron, C Eg = band gap of semiconductor particles, eV = band gap of bulk semiconductor, eV = first-order rate constant for the coagulation process,
Eg,bulk
k,,
S-1
Ksp= solubility product mo = free electron mass, kg me = effective mass of electron, kg mh = effective mass of hole, kg .WO = [H2OY[AOT]= water to surfactant molar ratio (water content) x = molar fraction of zinc in all the metallic ions in the particles xcal= calculated molar fraction of zinc in all the metallic ions in the particles xf = molar fraction of zinc in all the metallic ions in the feed solution 6 = dielectric constant of semiconductor, C2 J-l m-l EO = dielectric constant of vacuum, C2 J-l m-l Y = frequency of light, Hz 0 = molar absorption coefficient, cm-l M-l [ ] = molar concentration of the species shown in the brackets, M (mol&)
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2498 Ind. Eng. Chem.Res., Vol. 34, No. 7,1995 Hlsselbarth, A.; Eychmiiller, A.; Eichberger, R.; Giersig, M.; Mews, A.; Weller, H. Chemistry and Photophysics of Mixed CdSiHgS colloids. J . Phys. Chem. 1993,97 (201,5333-5340. Hirai, T.; Sato, H.; Komasawa, I. Mechanism of Formation of Titanium Dioxide Ultrafine Particles in Reverse Micelles by Hydrolysis of Titanium tetrabutoxide. Znd. Eng. Chem. Res. 1993,32(121,3014-3019. Hirai, T.; Sato, H.; Komasawa, I. Mechanism of Formation of CdS and ZnS Ultrafine Particles in Reverse Micelles. Znd. Eng. Chem. Res. 1994a,33 (121,3262-3266. Hirai, T.; Shiojiri, S.; Komasawa, I. Preparation of Metal Sulfide Composite Ultrafine Particles in Reverse Micellar Systems and Their Photocatalytic Property. J . Chem. Eng. Jpn. 1994b,27 (51, 589-596. Jain, T. K.; Billoudet, F.; Motte, L.; Lisiecki, I.; Pileni, M. P. Photochemical Studies of Nanosized CdS Particles Synthesized in Micellar Media. Prog. Colloid Polym. Sci. 1992,89, 106109. Kortan, A. R.;Hull, R.; Opila, R. L.; Bawendi, M. G.; Steigerwald, M. L.; Carroll, P. J.; Brus, L. E. Nucleation and Growth of CdSe on ZnS Quantum Crystallite Seeds, and Vice Versa, in Inverse Micelle Media. J. Am. Chem. SOC. 1990,112 (41,1327-1332. Motte, L.;Petit, C.; Boulanger, L.; Lixon, P.; Pileni, M. P. Synthesis of Cadmium Sulfide in Situ in Cadmium Bis(ethy1-2-hexyl) Sulfosuccinate Reverse Micelle: Polydispersity and Photochemical Reaction. Langmuir 1992,8 (41,1049-1053. Nielsen, A.E.Kinetics ofprecipitation; Pergamon Press: London, 1964. Osseo-Asare, K.; Aniagada, F. J. Synthesis of Nanosize Particles in Reverse Microemulsions. Ceram. Trans. 1990,12,3-16. Pileni, M. P. Reverse Micelles as Microreactors. J . Phys. Chem. 1993,97(271,6961-6973. Ravet, I.; Nagy, J. B.; Derouane, E. G. On the Mechanism of Formation of Colloidal Monodisperse Metal Boride Particles from Reversed Micelles Composed of CTAB-1-Hexanol-Water. Stud. Surf Sci. Catal. 1987,31,505-516.
Sakaguchi, M.; Ohta, M.; Satoh, M.; Hirabayashi, T. The Phase Transformation during Crystallization of ZnS. J . Electrochem. SOC.1977,124(41,550-553. Skinner, B. J.; Bethke, P. M. The Relation between Unit-Cell Edges and Composition of Synthetic Wurtzites. Am. Mineral. 1961,46,1382-1398. Suslina, L. G.; Panasyuk, E. I.; Konnikov, S. G., Federov, D. L. Exciton Spectra and Band Structure of Zinc Cadmium Sulfide (Zn,Cdl-,S) Mixed Crystals (in Russian). Fiz. Tekh. Poluprouodn. 1976,lO (lo),1830-1838. Towey, T. F.; Khan-Lodhi, A.; Robinson, B. H. Kinetics and Mechanism of Formation of Quantum-sized Cadmium Sulphide Particles in Water-Aerosol-OT-Oil Microemulsions. J . Chem. Soc., Faraday Trans. 1990,86(22),3757-3762. Ueno, A,; Kakuta, N.; Park, K. H.; Finlayson, M. F.; Bard, A. J.; Campion, A.; Fox, M. A.; Webber, S. E.; White, J. M. SilicaSupported ZnS-CdS Mixed Catalysts for Photogeneration of Hydrogen. J . Phys. Chem. 1985,89(18),3828-3833. Wang, Y.;Suna, A.; Mahler, W.; Kasowski, R. PbS in Polymers. From Molecules to Bulk Solids. J . Chem. Phys. 1987,87 (12), 7315-7322. Youn, H A ; Baral, S.; Fendler, J. H. Dihexadecyl Phosphate, Vesicle-Stabilized and In Situ Generated Mixed CdS and ZnS Semiconductor Particles. Preparation and Utilization for Photosensitized Charge Separation and Hydrogen Generation. J. Phys. Chem. 1988,92(221,6320-6327.
Received for review October 7, 1994 Accepted April 17, 1995 *
IE940584B
* Abstract published in Advance ACS Abstracts, J u n e 1, 1995.
