Langmuir l995,lI, 2285-2292
2285
Preparation of Size-QuantizedCdS and ZnS Particles in Nanophase Reactors Provided by Binary Liquids Adsorbed at Layered Silicates Imre DBkany, Lasz16 Turi, and Etelka Tombacz Department of Colloid Chemistry, Attila Jbzsef University, Szeged H-6720, Aradi V.t.1, Hungary
Janos H. Fendler” Department of Chemistry, Syracuse University, Syracuse, New York 13244-4100 Received August 29, 1994. I n Final Form: December 8, 1994@ Liquid sorption measurements of hexadecylpyridiniummontmorillonite (HDPM)dispersionsin ethanol (1)-cyclohexane (2) and methanol (1)-cyclohexane (2)binary mixtures established the formation of a 0.5to 5-nm-thickalcohol (1)-richadsorption layer at the organoclay complex interfaces. This adsorptionlayer was used as a nanophase reactor for the in situ generation of size-quantized cadmium sulfide and zinc sulfide semiconductor particles from cadmium (or zinc) acetate and equivalent amounts of H2S. The volumes of the nanophase reactor in ethanol (1)-cyclohexane (2) at 1:2 = 0.05:99.95were determined, by adsorption excess isotherm and X-ray diffraction measurements, to be 0.692 cm3/(gof HDPM) and 0.746 cm3/(g of HDPM), respectively. Adsorption excess isotherm and X-ray diffraction measurements gave corresponding values of 0.170 cm3/(gof HDPM) and 0.231 cm3/(gof HDPM) in methanol (1)-cyclohexane (2) at 1:2 = 0.01:99.99. As expected, smaller sized semiconductor particles were generated in the smaller nanophase reactor, provided by the methanol-rich adsorption layer. Further reduction of semiconductor particles was accomplished by decreasing the concentration of their parent ions in the nanophase reactor. Incorporation of semiconductornanocrystallites into HDPM manifested themselves in increased viscosity of the suspension. Information on the fractal dimensions of the semiconductor-clay organocomplex suspensions and that on the radius of gyration of the nanoparticles has been determined by small-angle X-ray scattering measurements. The use of selectively adsorbed polar liquids at the solid binary polarapolar liquid mixture interfaces as versatile nanophase reactors is discussed.
Introduction Nanoparticles and nanoparticulate films are increasingly utilized as advanced materials.lY2 Their large surface-area-to-volume ratios are fruitfully exploited in developing novel, nanoparticle-based ~ a t a l y s t s Electron .~ and hole confinements in size-quantized semiconductor nanoparticles have led to unique optical, electrical, and electro-optical properties and, hence, to the construction of advanced devices and sensor^.^ Furthermore, a systematic change of particle diameters permitted fine tuning of the band-gaps, as well as of the oxidation and reduction potentials of a variety of semiconductor^.^^^ Nanoparticles and nanoparticulate films are customarily prepared in the solid state, usually in ultrahigh vacuum.’ More recently, alternative “wet”colloid chemical techniques have been developed for the generation of nanoparticles.8 Metallic, semiconducting, and magnetic particles have been in situ generated in a variety of organized media, as well as under monolayers and between Langmuir-Blodgett f i l m ~ . ~ - l ~ Abstract published in Advance A C S Abstracts, J u n e 1,1995. (1)Ozin, G. A. Adu. Mater. 1992,4,612. (2)Siegel,R.W. In Mechanical Properties and Deformation Behavior of Materials Having Ultra-Fine Microstructures; Nastasi, M., et al., Eds.; Published in The Netherlands, 1993;p 509. (3)Thomas, J. M. Angew. Chem., Int. Ed. Engl. 1994,33,913. (4)Kamat, P.V.; van Wijngaarden, M. L.; Hotchandani, S. Zsr. J . Chem. 1993,33,47. (5)Henglein, A. J. Phys. Chem. 1993,97,5457. (6)Kamat, P. V. Chem. Rev. 1993,93,267. (7)Siegel, R.W. Springer Series i n Materials Sciences; Fujita, F. E., Ed.; Springer-Verlag: Berlin, Heidelberg, 1994;Vol. 27,p 65. (8) Fendler, J. H. Membrane-Mimetic Approach to Advanced Materials; Advances in Polymer Science Series, Vol. 113;Springer-Verlag: Berlin, 1994. (9)Enea, 0.; Bard, A. J. J. Phys. Chem. 1986,90,301. (10)Yoneyama, H.; Haga, S.;Yamanaka, S. J.Phys. Chem. 1989,93, 4833. @
0743-7463/95/2411-2285$09.00/0
In a preliminary communication, we have reported the use of surfactant-modified montmorillonites, layered silicates (often referred to as clay o r g a n o c ~ m p l e x e s ~ ), ~ - ~ ~ as templates for CdS nanoparticle formation.25 Generalization of the colloid chemical principles which are involved in the in situ generation of subcolloidal particles in nanophase reactors, provided by binary liquids adsorbed a t solid surfaces, is the subject of the present report. The site of reaction between the semiconductor precursors (Cd2+and S2-, or HS-in the case of CdS) has been shown in the present work to be the 5.0- to 10-nm-thick adsorbed layer of organic liquid at the clay organocomplex interfaces. Manipulation of the nature and composition of the adsorption layer, composed ofbinary liquid mixtures (xls and x 2 9 , is expected to profoundly influence the (ll)Herron, N.; Wang, Y.; Eddy, M. M.; Stucky, G. D.; Cox, D. E.; Moller, K.; Bein, T. J. Am. Chem. SOC.1989,111, 530. (12)Jentys, A.; Grimes, R. W.; Gale, J . D.; Catlow, C. R. A. J.Phys. Chem. 1993,97,13535. (13)Tkachenko, 0. P.;Shpiro, E. S.; Wark, M.; Schulz-Ekloff, G.; Jaeger, N. I. J. Chem. SOC.,Faraday Trans. 1993,89,3987. (14)Wang, Y.; Herron, N. Res. Chem. Intermed. 1991,15, 17. (15)Willner, 1.;Eichen,Y.; Frank, A. J.J.Am. Chem. SOC.1989,111, 1884. (16)DQkany,I.; Nagy, L. G.; Schay, G. J. Colloid Interface Sci. 1978, 66,197. (17)DQklny, I.; Szdnt6, F.; Nagy, L. G. Prog. Colloid Polym. Sci. 1978,65,125. (18) DBkPny, I.; Szdntb, F.; Weiss, A,; Lagaly, G. Ber. Bunsenges. Phys. Chem. 1986,89,62. (19)DBkdny, I.; Szlnt6, F.; Weiss, A.; Lagaly, G. Ber. Bunsenges. Phys. Chem. 1986,90,422. (20)DBklny, I.; Szdnt6, F.; Weiss, A.; Lagaly, G. Ber. Bunsenges. Phys. Chem. 1986,90,427. (21) DBklny, I.; Nagy, L. G. Colloids Surf. 1991,58,151. (22)Pratum, T. K. J. Phys. Chem. 1992,96,4567. (23)Carminati, S.;Carniani, C.; Miano, F. Colloids Surf: 1990,48, 209. (24)Liu, X.;Thomas, J. K. J. Colloid Interface Sci. 1989,129,476. (25)Kotov, N. A.;Putyera, K.; Fendler, J. H.; Tomblcz, E.; DBkdny, I. Colloids and Surfaces 1993,71,317.
0 1995 American Chemical Society
Dkkany et al.
2286 Langmuir, Vol. 11, No. 6, 1995 formation and growth of semiconductor nanoparticles. The binary liquid mixture which is selected should have the following properties in order to allow efficient control of the semiconductor nanoparticle formation. First, one component of the liquid mixture (XI) should be preferentially adsorbed a t the clay organocomplex interface (indicated by the superscript s); hence its concentration in the bulk (XI)should be negligible @e.,xlS> xlb). Second, the other component of the liquid mixture ( X Z ) should not be effectively adsorbed at the clay organocomplexinterface (i.e.,xZsXI),then the volume of the adsorption layer can be expressed by Vs = nlsVm,l. Porous surfaces (zeolite and activated charcoal, for example) adsorb one of the components of the liquid mixture highly selectively (see Figure lb). In fact, according to the pore-fillingmodel, the adsorption capacity is simply taken to be the volume of the pores. Thus, knowing the surface area allows the assessment of the thickness of the absorption layer and, hence, the reaction volume of the “nanophase reactor” from eq 1. 2. Volume of the Adsorption Layer (the Nanophase Reactor) in Layered Silicates and Clay Organocomplexes. Volumes within interlamellar spaces are assessed by the interlamellar space filling (see Figure IC). A n important advantage of layered systems is that interlamellar distances can be accurately determined by X-ray diffraction measurements. The interlamellar free volume, Vint,is given by
Vint = (a x b)(d, - 6 )
(4)
where a is the length and b is the width of a silicate, (SiA.l)4010, unit cell (its area, a x b, is taken to be 0.232 nm2),6 is the thickness of the unit cell of the silicate layer (0.96 nm), and d~ is the basal distance, available from X-ray diffraction measurements (from Bragg reflections). Interlamellar distances in clay organocomplexeshave been shown to increase upon the introduction of a n appropriate organic liquid.18-20 This process is the swelling of the clay organocomplex particles. By use of different organic liquids, the interlamellar free volume (the volume of the nanophase reactor), Vnf,can be assessed by
V,, = Vint - V,
= 0.232(dL= 0.96) [0.205(0.127nC 0.28)
+
+ VpnlIij(5)
where Vint is the volume of the interlamellar space, Valk is the volume occupied by the surfactant alkyl chains (containing n, number of carbon atoms) in the interlamellar space, Vpol is the volume of the surfactant polar headgroups in the interlamellar space, and 5 is the surface charge density per unit cell of the clay. Previous experi-
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Langmuir, Vol. 11, No. 6, 1995 2287
ments have demonstrated good agreement between the values of Vd which were calculated by eq 5 and those of Vs which were assessed by eq For the present purpose, VsIV,,f.29 3. Assessment of Reactant Concentrations in the Adsorption Layer and Growth Control of Nanopartides. Selection of a binary liquid in an appropriate composition permits, as described above, the preferential adsorption of one component liquid at the clay organocomplex interface (Vs)or in the interlamellar free volume (Vd= Vint- Valk). Further, the thickness ofthe adsorption layer and, hence, the volume of the nanophase reactor, is also governed by the binary liquid which is selected. The reactants should be highly soluble in the preferentially adsorbed liquid and highly insoluble in the second liquid. Under these optimal conditions, eq 1becomes l.1s-209zg
V" = aBt= nlsVm,l+ '&lB~m,r indicating the presence of nrsmolar reaction partners in Vm,rvolume. Thus, eq 6 permits the calculation ofreactant concentrations in the adsorbed layer. Sizes of the semiconductor nanoparticles, in situ grown in the nanophase reactor, are determined by the volume of the adsorption layer and by the concentration of the precursors introduced. Thus, selection of a binary liquid with a nanometer-thick adsorption layer will lead to nanometersized semiconductor particles. Further reduction in the size of the incipient semiconductors is accomplished by decreasing the concentrations of their precursors below their solubility in the nanophase reactor.
Experimental Section Hexadecylpyridinium montmorillonite (HDPM)was prepared from sodium montmorillonite (Sudchemie AG, Germany) and hexadecylpyridinium chloride (Fluka AG, Switzerland), by the established methodology.17,30-32 Hydrogen sulfide was prepared, as needed, from FeS and HC1 in a Kipp apparatus, purified by washing with water, and dried over calcium carbonate. Appropriate amounts were withdrawn from the vessel by using a microliter syringe. Reagent-grade cadmium acetate and zinc acetate (p.a., Reanal, Hungary) were used as received. Ethanol, methanol, and cyclohexane (p.a., Reanal, Hungary) were dried over a 0.4-nm molecular sieve (Merck AG, Germany). Semiconductor nanoparticles were prepared by adding alcoholic solutions of cadmium (or zinc) acetate in amounts appropriate to those calculated for 0.5-1.0 g of the clay organocomplex particles. The composition of the desired binary liquids was then adjusted by adding appropriate amounts of cyclohexane (to give ethanol, xl0, = 0.05 and cylohexane, xzo, = 0.95; and methanol, xl0, = 0.01 and cyclohexane,xz0, = 0.99) to the clay organocomplex suspensions. These suspensions remained stable for at least 1month (as evidenced by absorption spectroscopicand electron microscopic measurements). Injection of H2S (in amounts equivalent to Cd2+or Zn2+)resulted in the formation of CdS (or ZnS). Adsorption excess isotherms n l d n ) = f(x1) on HDPM clay organocomplexeswere determined in methanol-cyclohexane and ethanol-cyclohexane m i x t ~ r e s . ~ OFine - ~ ~ powders (sieved by a 100-mesh sieve) of HDPM (typically 0.3-0.5 g) were allowed to equilibrate with a given 10 mL of liquid (25 "C, 48 h) and the compositions of the supernatants were determined by a Zeiss liquid interferometer. The excess isotherms were calculated by
(7) wherex1° andxl are the molar ratios of alcohoVcyclohexaneprior (29)DBkany, I.; Szbnt6, F.; Weiss, A. Colloids Surf: 1989,41, 107. (30)DBklny, I.; Szlnt6, F.; Nagy, L. G.; F6ti, G. J. Colloid Interface Sci. 1976,50,265. (31)DBkany, I.; Szant6, F.;Nagy, L. G. J . ColloidInterface Sci. 1986, 103,321. (32)DBklny, I.;Szanto, F.;Nagy, L. G. J . Colloid Interface Sci. 1986, 109, 376.
and subsequent to adsorption and no is the total amount of the liquid mixture in mmoVg adsorbent. The standard deviation of nldn)was 5 4 % depending on the equilibrium concentration of the liquid mixture. X-ray diffraction measurements were taken on a Phillips PW 1830 diffractometer (Cu Ka,A = 1.54 A). The basal distances, d~ values, were calculated from the peak positions by the Bragg equation, by using the PW 1877 automated powder diffraction program, with an accuracy of hO.1 A. Small-angle X-ray scattering (SAXS) measurements were performed in a capillary cell by using a compact Kratky Camera (Model KCEC/3 1129, Anton Paar Co.), with 80 and 100 pm entrance and detector slits, and 40 kV and 35 mA current. Samples, placed in a 1.00 mm diameter 2.0 cm long capillary, were introduced in the middle of the beam and aligned by monitoringCsC1fluorescence. The absolute intensity ofthe X-ray beam was determined by the moving slit method. ScatteredX-ray intensities (0 were routinely taken of the sample (CdS, incorporated in the nanophase reactor which was provided by the binary liquids adsorbed at layered silicates placed in the capillary either as a suspension or as dried powder) and the blank (identical to the sample except that it did not contain CdS) as a function of the scattering wave vector, h ( h = [4n/A] sin 8" and 1 = 1.54 A, 6 = half of the scattering angle). The data were treated in terms of the so-called Porod law33
log AI = -p(log h )
(8)
where AI is defined as
and the correction factors, A, were obtained as Asample = Nsampl$ No and fibla& = Nblank/No, where Nsample, Nblank, and No are the intensity counts of the sample, the blank, and the background (in the absence of the capillary in the beam) in the primer beam for a given time (2 min). In AI us h2values were used to assess information on the semiconductor particle sizes in the Gunier region34by
In AI = In AI, - 11$$h2
(10)
where AI, is the correctedintensity at 0 = 0" and& is the Gunier radius of the particles. Fractal dimensions, D, in the Porod region were calculated from the slopes (p) of the plots of eq 9 in the Porod region by 1 -pi 1 = D, since our system utilized a line focus, rather than a point source for the X-ray beam. Rheological measurements were taken by a Haake rotational viscosimeter, RV20-CV100 configuration, equipped with a ME15 head in the low shear range at 25.0 "C. The Bingham-yield stress, TB was obtained from the z = f ( D )flow curves, where z is the shear stress and D is the shear rate gradient. Absorption spectra of the CdS and ZnS nanoparticles in the HDPM clay dispersions in the liquid mixtures were taken on a UVIKON 930 spectrophotometerby using HDPM clay dispersions (in the absence of semiconductor nanoparticles) as blanks.
+
Results 1. Adsorption Excess Isotherms of Clay Organocomplexes in Binary Liquids. Volume of the Nanophase Reactor. The adsorption excess isotherms for HDPM in ethanol (1)-cyclohexane (2)liquid mixtures are shown in Figure 2a. From the linear portions of these plots, the composition of the adsorption layer, xlS = flxl), was calculated by the Schay-Nagy method.16-20 In the range of X I = 0.05-0.1, the ethanol concentration at the adsorption layer (xis) is seen to vary between 0.5 and 0.6 and, at x1 > 0.2, xslremains constant at 0.58 (Figure 2b). Similar excess isotherms and xlS = Axl) plots are presented in Figure 3 for HDPM in methanol (1)cyclohexane (2) liquid mixtures. There is a remarkable (33)Porod, G.Kolloid-2. 1962,125, 51. (34)Gunier, A.; Fournet, G. Small-Angle Scattering of X-rays; Wiley: New York, 1955.
Dkkany et al.
2288 Langmuir, Vol. 11, No. 6, 1995 4 ,
I
4 3bo 3
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I
0
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I
I
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I
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Xl Figure 2. Adsorption excess isotherm on HDPM in ethanol (1)-cyclohexane (2) liquid mixtures according to eq 3 (a) and a plot of the adsorption layer composition according to eq 2 (b). Table 1. CdS Particles in HDPM Nanophase Reactors in Ethanol (1)-Cyclohexane (2) Liquid Mixtures" CdSb n16,nz5 Vs, cm3/g of HDPMd dL, nm (dryY d ~nm , (in suspensionr Vnf, cm3/gof HDPMf TB, Pas bandgap, eVh RG,'nm D,d (in suspension)
0 3.6,2.7
0.692 2.06 4.21 0.746 0.15 2.151
0.02
0.04
0.06
1.o
0.004
0.04
0.4
1.99 4.14 0.724 0.20 2.70 8.0 2.118
2.10 3.85 0.637 0.215 2.52 9.0 2.071
2.08 3.98 0.677 0.375 2.46 10.4 1.951
*
a q0= 0.05. Formed frommillimoles of added cadmium acetate per grams of HDPM and equivalent amounts of H2S. Adsorption capacity, mmol/g, calculated by eq 3. Volume of the adsorption layer, calculated by eqs 1-3; Vs = h s Vml. e Basal spacings determined by X-ray diffraction measurements using the Bragg equation. f Volume ofthe nanophase reactor, calculated from X-ray diffraction measurements by eq 6.g Bingham yield value. * Assessed from absorption edge. & Radius of gyration from SAXS measurements (by the Gunier plot, eq 10).J Mass fractal dimension from SAXS measurements (by the Porod plot, eq 8).
difference between methanol- and ethanol-containing binary liquids. Thus, the adsorption layer is seen to consist essentially of methanol (xis = 0.8-0.90, Figure 3b), even though the composition of methanol in the methanolcyclohexane mixture is an order of magnitude smaller (notice the a - s c a l e in Figure 3 a ) than that of ethanol in the ethanol-cyclohexane mixture (Figure 2a). The adsorption excess isotherms permit the evaluation of the adsorption capacity (np,n&, the adsorption volume (Vs, by eq 1or 41, a n d , most importantly, the volume of the adsorbed layer composition at the clay organocomplex
Xl Figure 3. Adsorption excess isotherm on HDPM in methanol (1)-cyclohexane (2) liquid mixtures according to eq 3 (a) and a plot of the adsorption layer composition according to eq 2 (b). Table 2. CdS Particles in HDPM Nanophase Reactors in Methanol (1)-Cyclohexane (2) Liquid Mixturesa CdSb nls, nZE
Vs,cm3/g of HDPMd d ~nm , (dry)' dL, nm (in suspensiony Vnf, cm3/g of HDPMf tg, Pas
0 3.02 0.170 2.06 2.51 0.231 0.29
bandgap, eVh
RG,'nm D d (in suspension)
2.151
0.004
0.04
0.4
2.01 2.48 0.224 0.36 2.80 7.5 1.933
2.06 2.53 0.239 0.65 2.69 9.1 1.817
1.98 2.51 0.231 1.15 2.45 9.5 1.757
Formedfrommillimolesofaddedcadmium acetate per grams of HDPM and equivalent amounts of HzS. Adsorption capacity, mmoVg, calculated by eq 3. Volume of the adsorption layer, calculated by eqe 1-3; V8 = &is V,i. e Basal spacings determined by X-ray diffraction measurements using the Bragg equation. f Volume ofthe nanophase reactor, calculated fromX-ray diffraction measurements by eq 6.g Bingham yield value. Assessed from absorption edge. Radius of gyration from SAXS measurements (bythe Gunier plot, eq 10).J Mass fractal dimension from SAXS measurements (by the Porod plot, eq 8). rl0= 0.01.
interface ( ~ 1 9 Le., , the volume of the nanophase reactor. Calculated values of nls,nzs,and Vs for HDPM in ethanol (1)-cyclohexane (2) and in methanol (1)-cyclohexane (2)
are given in Tables 1-4. X-ray diffraction measurements provided an alternative way for assessing the volume of the nanophase reactor. The basal spacing of HDPM in ethanol (1)-cyclohexane (2) liquid mixtures, determined by X-ray diffraction measurements, is shown in Figure 4a. Similar data are plotted in Figure 4b for the methanol (1)-cyclohexane (2) liquid mixtures. In the ethanol (1)-cyclohexane (2) liquid mixture, d~ is largest in the X I = 0.02-0.4 region.
Size-Quantized CdS and ZnS Particles
Langmuir, Vol. 11, No. 6, 1995 2209
Table 3. ZnS Particles in HDPM Nanophase Reactors in Ethanol (1)-Cyclohexane (2) Liauid Mixturesa
4.6
ZnSb nls, nZs
Vs, cm3/gof HDPMd d L , nm (dry? d L , nm (in suspensionr Vnf,cmVg ofHDPMf tg, Pas bandgap, eVh RG,’ nm Dd (in suspension)
0 3.6, 2.7 0.692 2.06 4.21 0.746 0.15
1
4.4 4.2
0.004
0.04
2.01 4.02 0.689 0.19 2.93
2.15 3.95 0.667 0.23 2.76 9.2 2.143
2.151
0.4
2.13 3.99 0.679 0.46 2.29 9.7 1.817
4.0
@
3.6
*d
3.6
..
3.4 3.2
1
3.0
I
2.8
0.0
*
a x l 0 = 0.05. Formed from millimoles of added zinc acetate per grams of HDPM and equivalent amounts of H2S. Adsorption capacity, mmoVg, calculated by eq 3. Volume of the adsorption layer, calculated by eqs 1-3; VE = Cnis Vmi. e Basal spacings determined by X-ray diffraction measurements using the Bragg equation. f Volume of the nanophase reactor,calculated from X-ray diffraction measurements by eq 6. g Bingham yield value. Assessed from absorption edge. Radius of gyration from SAXS measurements (by the Gunier plot, eq 10).J Mass fractaldimension from SAXS measurements (by the Porod plot, eq 8).
1 0.2
0.4
0.6
0.8
1.0
XI,Ethanol I
Table 4. ZnS Particles in HDPM Nanophase Reactors in Methanol (1)-Cyclohexane (2) Liquid Mixturesa ZnSb nls, nZ8
Vs, cmVg of HDPMd CEL, nm (dry)e &, nm (in suspensionr Vd, cmVg of HDPMf SB, Pas bandgap, eVh R G ,nm ~ Dd (in suspension)
0 3.02
0.170 2.06 2.51 0.233 0.48 2.250
0.004
0.04
0.4
2.03 2.31 0.173 0.52 2.89 7.3 2.105
2.17 2.40 0.200 0.66 2.87 8.2 2.096
2.08 2.44 0.212 0.91 2.92 8.4 1.975
xl0 = 0.01. Formed from millimoles of added zinc acetate per grams of HDPM and equivalent amounts of H2S. Adsorption capacity, mmol/g, calculated by eqs 1 and 3. Volume of the adsorption layer, calculated by eqs 1-3; VE = Zn,s Vml.e Basal spacings determined by X-ray diffraction measurements using the Bragg equation. f Volume of the nanophase reactor, calculated from X-ray diffraction measurements by eq 5. g Bingham yield value. Assessed from absorption edge. Radius of gyration from SAXS measurements (by the Gunier plot, eq 10). Mass fractaldimension from SAXS measurements (by the Porod plot, eq 8). J
Increasing the ethanol concentration in the liquid mixture results in a steady decrease of the basal spacing (Figure 4a). Thus, a t the largest basal spacing (dL = 4.2-4.4 nm) and in the x1 = 0.02-0.05 composition range, the concentration of adsorbed alcohol isxlSx 0.6. The ethanol concentration is seen, therefore, to be higher in the adsorption layer a t the clay organocomplex surface than in the bulk. This will ensure the preferential solubilization of polar components (cadmium ions, for example) within the nanophase reactor. Solvent sorting in the adsorption layer of HDPM is even more pronounced in the methanol (1)-cyclohexane (2) liquid mixture. Methanol concentration in the bulk phase was arranged to be a n order of magnitude smaller (XI = 0.005-0.02, see Figure 4b) than that of ethanol ( X I = 0.05-0.4, see Figure 4a). At the same time, the methanol concentration in the nanophase reactor was xlSx 0.9, while the bulk phase was essentially cyclohexane, xzS = 0.98-0.995 (see Figure 3b). Volumes of the “nanophase reactors”, Vnf,provided by the swelling of HDPM at different basal spacings, were calculated by eq 5 using n, = 16 and = 0.32/Si4010 (see solid line in Figure 5). Variations in the hydrocarbon chain length of the surfactant (n,), the surface charge density (E), and the basal spacing (dL)control, as seen by eq 5 , are the factors which influenced the interlamellar space (V,f), Le., the volume of the nanophase reactor. The dotted lines (or lines i, ii, iii, etc.) in Figure 5 were
e
1.1
1.0 0.0 0.8 0.7 0.6
0.5 0.4 0.3 0.2
0.1 0.0
1
2
3
-
4
dL,
5
6
7
Figure 6. A plot of the volume of the nanophase reactor (Vnf) provided by the swelling(i.e.,intercalation of liquids)ofHDPM, against the basal spacing ( d ~ )The . solid line was calculated from eq 5 by taking n, = 16 and 5 = 0.32.Dotted lines were also calculated from eq 5 by using n, = 16 and 5 = 0.50 (i),n, = 16 and 6 = 0.75 (ii), and n, = 16 and 6 = 1.0 (iii).
calculated by using different parameters of n, and 5. Manipulation of eq 5 also permits a n independent assessment of Vd from changes in the basal spacing, dL, caused by swelling of the clay organocomplexes(see Tables 1-4 for obtained values). As can be seen, changes of the basal spacing from 2 to 6 nm permit variation of the volume of the nanophase reactor from 0.08 to 1.30 cm3/g. 2. Generation of CdS and ZnS within the Nanophase Reactor. Characterization by Absorption
DCkany et al.
2290 Langmuir, Vol. 11, No. 6, 1995
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D, sec'' Figure 7. Rheological flow curves of HDPM in an ethanol (1)-cyclohexane (2) liquid mixture, x l 0 = 0.05, prepared in the absence (i) and in the presence of CdS semiconductor nanoparticles, in situ generated from 0.004 mmol of cadmium acetatelg of HDPM and equivalent amounts of HzS (ii), 0.04 mmol of cadmium acetate/g of HDPM and equivalent amounts of H2S (iii),and 0.4 mmol of cadmium acetate/g of HDPM and equivalent amounts of H2S (iv).
Wavelength, nm
'
, 700
"
'
I 800
Wavelength, nm Figure 6. (a)Typical absorption spectra of CdS on HDPM in an ethanol (1)-cyclohexane (2) liquid mixture, x1O = 0.05, in situ prepared from 0.04 mmol of cadmium acetate/g of HDPM and equivalent amounts of HzS (i)and formed by adding solid crystalline CdS (0.04 mmol of zinc acetate/l g of HDPM) in an ethanol (1)-cyclohexane (2) liquid mixture, = 0.05 (ii).(b) Typical absorption spectra of ZnS on HDPM in an ethanol (1)cyclohexane (2) liquid mixture, x1O = 0.05, in situ prepared from 0.04 mmol of cadmium acetate/g of HDPM and equivalent amounts of HzS (i) and formed by adding solid crystalline ZnS (0.04 mmoVl g of HDPM) in an ethanol (1)-cyclohexane (2) liquid mixture, x1O = 0.05 (ii). Different baselines represent sample scatterings. Spectrophotometry and Transmission Electron Microscopy. The formation of cadmium sulfide nanoparticles was easily observable by the development of coloration upon the exposure of the clay organocomplex dispersions to hydrogen sulfide. The process was conveniently monitored by absorption spectrophotometry. Typical absorption spectra of CdS and ZnS nanoparticles, in situ formed in the clay organocomplex dispersions, are illustrated in Figure 6. The absorption edge of the spectra permitted the evaluation of the bandgap of a given preparation. 3. StructuralAlteration of the Layered Silicates by Semiconductor Particle Incorporation. a. Rheological Measurements. Formation of size-quantized semiconductor particles also affected the rheological properties of their clay organocomplex hosts. Plots of shear stress (z) versus shear rates ( D )for HDPM dispersions in ethanol-cyclohexane (x1O = 0.05) in the absence and in the presence of different amounts of CdS are shown in Figure 7. An increase in the concentration of the semiconductors is seen to increase the Bingham yieldvalue (zB), as is expected for stronger interparticle interactions (see Tables 1-4). Apparently, the formation of CdS and ZnS nanoparticles between the hydrophobic
-
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Figure 8. Wide-angleX-ray powder pattern for HDPM in the absence (i)and in the presence of CdS formed from 0.004 mmol of cadmium acetate/gof HDPM and equivalent amounts of H2S (ii),0.04 mmol of cadmium acetate/g of HDPM and equivalent amounts of H2S (iii),0.4 mmol of cadmium acetate/g of HDPM and equivalent amounts of HzS (iv),and 0.8 mmol of cadmium acetate/g of HDPM and equivalent amounts of H2S (v)after the evaporationof the ethanol (1)-cyclohexane (2)liquid mixture, xio = 0.05.
silicate layers results in the reorientation of the dispersed system by decreasing the number of parallel lamellas and increasing adhesion between the clay sheets, because the nanosized material makes bridges between the lamellas (see Figure 12) in the organocomplex suspensions. The ZB values are indicative of very strong structure building in the organic suspension. b. Wide-Angle X-ray Diffraction Measurements. Typical XRD plots of HDPM in the absence and in the presence of CdS particles are shown in Figure 8. CdS particle incorporation does not have a n appreciable effect on the X-ray diffraction of the clay organocomplexes. The value of d~ obtained for the dry clay organocomplex is practically unaltered by the presence of CdS (1.99-2.08 nm, Tables 1and 3). Incorporation of CdS does decrease, however, the X-ray diffraction intensities (Figure 9), indicating a decrease in the number of lamella oriented parallel to each other in the clay organocomplex. These results are in accord with the increased ZB values (vide supra). Similarly, the presence of ZnS did not affect the
Size-Quantized CdS and ZnS Particles
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Langmuir, Vol. 11, No. 6, 1995 2291
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log h,nm" Figure 9. (a) SAXS curves for HDPM in ethanol (1)cyclohexane (2) liquid mixtures, x1O = 0.05, in the presence of CdS, formed from 0.004 mmol of cadmium acetate/g of HDPM and equivalent amounts of HzS (i), 0.04 mmol of cadmium acetate/g of HDPM and equivalent amounts of HzS (ii),and 0.4 mmol of cadmium acetate/g of HDPM and equivalent amounts of HzS (iii). (b) SAXS curves for HDPM in methanol (1)cyclohexane (2) liquid mixtures, x1O = 0.01, in the presence of CdS, formed from 0.004 mmol of cadmium acetate/g of HDPM and equivalent amounts of HzS (i), 0.04 mmol of cadmium acetate/g of HDPM and equivalent amounts of HzS (ii),and 0.4 mmol of cadmium acetate/g of HDPM and equivalent amounts of H2S (iii).
d~ values of dry clay organocomplexes (dL = 2.06-2.13, Tables 2-4). c. Small-Angle X-ray Scattering Measurements. Structural changes accompanying CdS formation in the clay organocomplexes were also monitored by small-angle X-ray scattering measurements (SAXS). Porod plots of the logarithms of the relative scattering intensities (AI) us scattering wave vector (h, eqs 8 and 9) for HDPM suspensions in ethanol (1)-cyclohexane (2) and methanol (1)-cyclohexane (2)liquid mixtures in the absence and in the presence of CdS and ZnS nanoparticles are shown in Figures 9 and 10. Taking the slopes 0, in eq 8) of the relatively long linear portions permitted the assessment of the fractal dimensions, D, (see Tables 1-4). In the absence of semiconductor nanoparticles, D , values for HDPM dispersions in both ethanol (1)-cyclohexane (2) and methanol (1)-cyclohexane (2) liquid mixtures were found to be 2.151 and 2.250, respectively. CdS and ZnS nanoparticles, in situ generated in the HDPM suspensions, increased AZ and decreasedp (Figures 9 and 10 and Tables 1-4). These results support the structural reorganization of clay organocomplexes in the presence of semiconductor nanoparticles. Gunier plots, according to eq 10,allowed the assessment of the radius of gyration, RG,of CdS and ZnS nanoparticles in the clay organocomplexes (see Tables 1-4).
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log h,nm-' Figure 10. (a) SAXS curves for HDPM in ethanol (1)cyclohexane (2) liquid mixtures, x l 0 = 0.05, in the presence of ZnS, formed from 0.04 mmol of zinc acetate/g of HDPM and equivalent amounts of H2S (i) and 0.4 mmol of zinc acetate/g of HDPM and equivalent amounts of HzS (ii).b) SAXS curves for HDPM in methanol (1)-cyclohexane (2)liquid mixtures,x1O = 0.01, in the presence of ZnS, formed from 0.004 mmol of zinc acetatelg of HDPM and equivalent amounts of H2S (i), from 0.04 mmol of cadmium acetate/g of HDPM and equivalent amounts of H2S (ii),and from 0.4 mmol of cadmium acetate/g of HDPM and equivalent amounts of HzS (iii).
Discussion Size-quantized semiconductor particles have been in situ formed in polar organic liquids which were selectively adsorbed at hydrophobic layered silicates. This ultrathin adsorption layer has been shown to provide a versatile nanophase reactor whose volume could be theoretically estimated and experimentally determined. Partial (or complete) replacement of the inorganic cations in montmorillonite by hexadecylpyridinium ions permitted an "accordion-like" swelling of the clay layers in binary organic liquids. Importantly, this swelling could be conveniently and accurately quantified by X-ray diffraction measurements of the interlamellar or basal distance, dL. Thus, d~ for HDPM organoclay adsorbents has been shown to be variable from 2.2 to 4.4 nm by changing the mole fractions (i.e.,1:2 ratios) in the ethanol (1)-cyclohexane (2) or in the methanol (1)-cyclohexane (2) liquid mixture. The availability of experimentally determined dL values at a given composition of the binary liquid and surface density of the alkyl chains allows the calculation of the volume of the nanophase reactor, Vd (from eq 6). Thus, V,f = 0.746 cm3/(gof HDPM) has been calculated for the nanophase volume in the ethanol (1)cyclohexane (2) liquid mixture adsorbents a t x1 = 0.05 (Table 1). This V,fvalue is only slightly larger than that which was determined independently from adsorption isotherm measurements (Vs = 0.692 cm3/(g of HDPM), Table 1). Similarly, X-ray diffraction measurements yielded V, = 0.231 cm3/(gof HDPM) for the nanophase volume in the methanol (1)-cyclohexane (2)liquid mixture
2292 Langmuir, Vol. 11, No. 6, 1995
Dkkany et al.
(a) Figure 11. Schematic illustration of the nanophase reactors which were provided by binary liquids adsorbed at hydrophobic clay interfaces in the absence (a) and in the presence (b) of semiconductor nanoparticles.
a t XI = 0.01 (Table 2). This Vd value is, however, substantially larger than that which was determined independently from adsorption isotherm measurements (Vs= 0.170 cm3/(g of HDPM), Table 2). Changing the surface density of HDPM ( E ) appreciably altered the volume of the nanophase reactor a t a given basal spacing. This is illustrated by the calculated linear Vd us d ~plots , at different 5 values (Figure 5). The nature of the liquid mixture had the most profound effect on the volume of the nanophase reactor. This is most clearly seen on comparing the drastic differences between the behavior of ethanol and methanol in the alcohol (1)-cyclohexane (2) liquid mixtures (compare XI scales in Figures 2b and 3b at similar X I * values!). Overall, the type and amount of the surfactant which was used to exchange montmorillonite cations, as well as the type and composition of the liquid mixture, controlled the volume of the nanophase reactor. Restricting the volume of the nanophase reactor limited, of course, the size of the incipient semiconductor crystallites to the sizequantized regime. Absorption spectrophotometry provided direct evidence for the generation of semiconductor nanoparticles in the clay organocomplex dispersions. A shift of the absorption edges to energies which were higher than those assigned for bulk semiconductors has established size quantization.
Incorporation of size-quantized semiconductor particles profoundly influenced the structure of the clay organocomplexes. The number of parallel lamellas and adhesion between the clay sheets increased upon the in situ generation of CdS or ZnS nanocrystallites in the adsorption layer which was provided by the layered silicates. Most importantly, this structural reorganization could be conveniently characterized by rheological,XRD, and SAXS measurements. Although attention has been focused in the present work on nanophase reactors formed by the adsorption of alcohol-cyclohexane liquid mixtures onto clay organocomplexes,it is important to realize that the methodologies developed are general and can be extended to other appropriate surfaces and adsorbents. Similarly, the formation of semiconductor nanoparticles should only be considered to be a n example of the many potential reactions which could be f i t f u l l y performed in nanophase reactors.
Acknowledgment. Support of this research by grants from the National Science Foundation (U.S.A.),the U.S.Hungarian Joint Fund, and OTKA U5 (NFSR, Hungary) is gratefully acknowledged. LA940680H
