Langmuir 1996, 12, 3709-3715
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Preparation and Characterization of CdS and ZnS Particles in Nanophase Reactors Provided by Binary Liquids Adsorbed at Colloidal Silica Particles I. De´ka´ny, L. Nagy, L. Tu´ri, and Z. Kira´ly Department of Colloid Chemistry, Attila Jo´ zsef University, Szeged H-6720, Aradi V.t.1, Hungary
N. A. Kotov and J. H. Fendler* Department of Chemistry, Syracuse University, Syracuse, New York 13244-4100 Received December 14, 1995X Determinations of adsorption excess isotherms of silica particle dispersions in ethanol-cyclohexane and in methanol-cyclohexane binary mixtures have established the formation of 0.5-5.0 nm thick alcohol rich adsorption layers at the silica particle interface. The adsorption layers on silica particles, formed in methanol-cyclohexane ) 0.01:99.99 (mol/mol) and ethanol-cyclohexane ) 0.05:99.95 (mol/mol), were used as nanophase reactors for the in situ generation of size-quantized CdS and ZnS particles. The semiconductor particles were generated by the addition of H2S to silica particle dispersions which contained different concentrations of Cd2+ or Zn2+. The smallest semiconductor particles were generated in dispersions which contained the least amount of precursors. Absorption spectrophotometry, small-angle X-ray scattering, rheological, and calorimetric measurements were used to characterize the dispersions. The presence of semiconductor particles increased the viscosities and decreased the immersion wetting enthalpies of the silica dispersions, indicating strong interparticle interactions.
Introduction The size dependent optical, electrical, electro-optical, and magnetic properties of dispersed nanoparticles have prompted the development of colloid chemical methodologies for their preparation and stabilization.1,2 The reproducible preparation of nanoparticles demands careful attention to the numerous experimental parameters which control the delicate balance between nucleation, particle growth, and agglomeration.3 Advantage is often taken of charged surfactants and/or polyelectrolytes to stabilize the nanoparticle dispersions. An alternative approach is to restrict the growth of the particles by confining them into cavities or pores of synthetic and naturally occurring materials. Porous vicor glass, organoclay complexes, zeolites, and nanoporous membranes have been employed as hosts for size-quantized semiconductor and metallic particles.1 More recently, size-quantized silver,4,5 gold,5 and cadmium sulfide6,7 particles have been prepared and stabilized on the surfaces and within the pores of colloidal silica particles.1,2 Judicious selection of the composition of the liquid used for dispersing the colloidal hosts provides an additional control for the growth and stabilization of nanoparticles. Thus, the dispersion of organoclay particles in ethanolcyclohexane and methanol-cyclohexane binary liquid mixtures has been shown to result in the formation of a 0.5-5.0 nm thick alcohol rich adsorption layer at the X
Abstract published in Advance ACS Abstracts, June 15, 1996.
(1) Fendler, J. H. Membrane-Mimetic Approach to Advanced Materials; Advances in Polymer Science Series, Vol. 113; Springer-Verlag: Berlin, 1994. (2) Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7, 607. (3) Myers, D. Surfaces, Interfaces, and Colloids. Principles and Applications; VCH: New York, 1991. (4) Lawless, D.; Kapoor, S.; Kennepohl, P.; Meisel, D.; Serpone, N. J. Phys. Chem. 1994, 98, 9619. (5) Arai, M.; Mitsui, M.; Ozaki, J.-I.; Nishiyama, Y. J. Colloid Interface Sci. 1994, 168, 473. (6) Chang, S. Y.; Liu, L.; Asher, S. A. J. Am. Chem. Soc. 1994, 116, 6739. (7) Chang, S. Y.; Liu, L.; Asher, S. A. J. Am. Chem. Soc. 1994, 116, 6745.
S0743-7463(95)01541-1 CCC: $12.00
surfaces of the clay particles.8,9 This interfacial adsorption layer has been shown to function as a nanophase reactor in which relatively monodisperse semiconductor particles could be grown in desirable size ranges.8,9 Relying on the established principles of adsorption equilibria has permitted the assessments of the volume of the nanophase reactor and of the concentrations of the reactants therein.10-16 In the present work we have generated CdS and ZnS nanoparticles in the nanophase reactors provided by alcohols, preferentially adsorbed from binary liquid mixtures, at the surfaces of colloidal silica particles. The general concept of this approach is illustrated in Figure 1. Conditions have been selected such that most of the alcohol is adsorbed at the silica particle interfaces (i.e., there is very little alcohol in the bulk solvent) and that the semiconductor particle precursors (Cd2+, Zn2+, SH-, S2-) partition favorably into the adsorption layer. The properties of the adsorption layer at the silica particle interfaces have been investigated by the determinations of adsorption excess isotherms. The effects of semiconductor nanoparticles on the structure of silica particles have been elucidated by rheological, calorimetric, and small-angle X-ray diffraction measurements. Brief theoretical descriptions of preferential liquid adsorption on solid surfaces and rheological and enthalpy measurements are provided in the subsequent sections of this paper. (8) De´ka´ny, I.; Turi, L.; Tomba´cz, E.; Fendler, J. H. Langmuir 1995, 11, 2285. (9) De´ka´ny, I.; Turi, L.; Tomba´cz, E.; Fendler, J. H. Mag. Ke´ m. Foly. 1995, 101, 296. (10) Kipling, J. J. Adsorption from Solution of Non-Electrolytes; Academic Press: London, 1965. (11) Everett, D. H. Trans. Faraday Soc. 1964, 61, 2478. (12) Everett, D. H. In Colloid Science; Everett, D. H., Ed.; Chemistry Society: London, 1979; Vol. 3, p 66. (13) De´ka´ny, I.; Sza´nto´, F.; Weiss, A. Colloids Surf. 1989, 41, 107. (14) De´ka´ny, I.; Sza´nto´, F.; Nagy, L. G. Prog. Colloid Polym. Sci. 1978, 65, 125. (15) De´ka´ny, I.; Nagy, L. G.; Schay, G. J. Colloid Interface Sci. 1978, 66, 197. (16) De´ka´ny, I.; Sza´nto´, F.; Nagy, L. G.; Fo´ti, A. J. Colloid Interface Sci. 1975, 50, 265.
© 1996 American Chemical Society
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De´ ka´ ny et al. in turn, the determination of the adsorption volume (Vs) and the composition of x1; i.e., the parameters which characterize the nanophase reactor. 2. Enthalpies of Immersion. In pure liquids the enthalpies of immersion at a solid-liquid interface are expressed by
∆wH ) H - n°H°m - hsm
(3)
where H is the total enthalpy of the system, H°m is the molar enthalpy of the liquid, n° is the molar number of the pure liquid, hs is the specific enthalpy of the solid, and m is the mass of the solid. The ∆wH values are characteristic of the solid-liquid interaction and, in the case of high energy (polar) surfaces, the interaction with polar liquids (water, alcohols, for example) is very high. Depolarization of the surface by making it more hydrophobic will result in decreased ∆wH values. 3. Rheological Properties of Dispersed Particles. The rheological properties of suspensions are determined by the particle-particle and particle-solvent interactions. The behavior of dilute Newtonian dispersions is given by Einstein’s viscosity equation.17 In this case, a plot of the shear stress, τ, against the shear rate, D, is described by τ ) ηD, where η is the viscosity of the dispersed system. For concentrated (pseudoplastic) systems where stronger particle-particle interactions occur, the shear stress is given by
τ ) τB + ηplD
Figure 1. Cartoon illustrating the formation of semiconductor particles, MeS, from their metal ion precursor, Men+, and H2S in the nanophase reactor which has a volume Vs, thickness ts, and composition x1s (or n1s), where x1s is the mole fraction of alcohol which had selectively adsorbed from the alcohol (x1) and cyclohexane (x2) liquid mixture.
Theoretical Section 1. Adsorption and Wetting on Solid Surfaces. An adsorption layer is formed on the surface of the dispersed particles due to solid-liquid interfacial interactions. The material content of this layer (adsorption capacity) can be determined if the adsorption excess isotherm is known. The composition of the adsorption layer (x1s) is different from those of the initial (x1°) and equilibrium (x1) values. In binary mixtures the preferential adsorption in any component in the interfacial layer (component x1, for example) can be expressed by the adsorption excess isotherm, by adapting the Ostwald-de Izaguirre equation:14
n1σ(n) ) n°(x1° - x1)/m ) n1s - (n1s + n2s)x1 ) n1sx2 - n1sx1 ) ns(x1s - x1) (1) where n1σ(n) is the adsorption excess amount per gram of adsorbent, n° is the number of moles of the liquid mixture in the dispersed system, n1s + n2s ) ns is the material content of the adsorption layer, and x1s ) n1s/ns is the composition of the adsorption layer. If preferential adsorption occurs on the solid particle surface, then x1s . x2s and x1s = 1. This means that in the adsorption layer practically only component 1 is present. However, we have only diluted mixtures of component 1 (i.e., x1 , l, x2 = 1, n1σ(n) = n1s). The volume of the adsorption layer (Vs) can be calculated by the adsorption space-filling model:15
Vs ) n1sVm,1 + n2sVm,2
(2)
where Vm,1 and Vm,2 are the molar volumes of the adsorbed components 1 and 2. If preferential adsorption of component 1 occurs, then Vs ) n1sVm,1. Knowing the adsorption excess isotherm, n1σ(n) ) f(x1), values of n1s and n2s can be calculated by the Schay-Nagy extrapolation method15 or by the Everett-Schay function.16 These data permit,
(4)
where ηpl is the plastic viscosity. The value of τB, the Bingham yield, can be determined from the rheological flow curve by linear extrapolation to D ) 0. The Bingham yield value is characteristic for a given interparticle interaction and of the structural properties of the colloidal dispersion.
Experimental Section 1. Materials. Two colloidal silica (SiO2) particles, hydrophilic A-200 and hydrophobic R-972, were obtained from Degussa (Degussa AG, Germany). The specific surface areas of the A-200 and R-972 silica particles were determined to be as ) 192 m2/g and as ) 121 m2/g, respectively, by BET measurements. Hydrogen sulfide was prepared, as needed, from FeS and HCl 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. Methanol, ethanol, benzene, and cyclohexane (p.a.; Reanal, Hungary) were dried over a 0.4 nm molecular sieve (Merck AG, Germany). 2. Methods. Adsorption excess isotherms on the silica particles were determined in methanol-cyclohexane and ethanol-cyclohexane mixtures.8 Silica particles (typically 0.5 g) were allowed to equilibrate with a given 10 mL of liquid (25 °C, 48 h), and the composition of the supernatant was determined by a Zeiss liquid interferometer. The excess isotherms and the volumes of the adsorption layers were calculated by eqs 1 and 2. The standard deviations of n1σ(n) and Vs were between 5 and 8%, depending on the equilibrium concentration of the liquid mixture. Semiconductor nanoparticles coating the silica particle surfaces 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 silica particles. The composition of the desired binary liquids was then adjusted by adding appropriate amounts of cyclohexane (to give ethanol, x1, ) 0.05 and cyclohexane, x2, ) 0.95; or to give methanol, x1, ) 0.01 and cyclohexane, x2, ) 0.99) to the silica particle suspensions. These suspensions remained stable for at least one month (as was evidenced by absorption spectroscopic and electron microscopic measurements). Injection of H2S, in amounts equivalent to [Cd2+] or [Zn2+], resulted in the formation of CdS or ZnS on the SiO2 particle surfaces. Dispersions of SiO2 semiconductor particles were stable and optically transparent. Furthermore, they could be filtered, washed by cyclohexane, dried, and stored as powders. Redispersion of the powdered particles in the appropriate binary liquids resulted in SiO2 semiconductor particles which were identical to (17) Tadros, Th. F. Dispersions of Solids in Liquids.
CdS and ZnS Particles in Nanophase Reactors
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those initially formed (as was evidenced by their absorption spectra and electron microscopic images). SiO2 semiconductor particles in their powdered form could be stored indefinitely. Some experiments were carried out in methanol-benzene mixtures. Small-angle X-ray scattering (SAXS) measurements were performed in a 1 mm solid-state capillary cell at 25.0 ( 0.1 °C by using a compact Kratky Camera (Model KCEC/3 1129, Anton Paar Co.) attached to a Philips PW 1830 diffractometer (Cu KR, λ ) 1.54 Å) with 80 and 100 µm entrance and detector slits and 40 kV and a 35 mA current.18 The absolute intensity of the X-ray beam was determined by the moving slit method. Scattered X-ray intensities (I) were routinely taken of the sample (SiO2 semiconductor particles either as powders or as dispersions in the appropriate binary liquid) and the blank (SiO2 powders or dispersions in the appropriate binary liquid in the absence of semiconductors) as a function of the scattering wave vector, h (h ) [4π/λ] sin θ° where λ ) 1.54 Å and θ ) half of the scattering angle). The data were treated in terms of the Porod law:19
log I ) -p log h
(5)
Knowing the slope (p) of this function, the surface fractal dimension, Ds, can be calculated (Ds ) p + 5). The ln ∆I vs h2 plots were used to assess information on the semiconductor particle sizes in the Gunier region20 by
ln ∆I ) ln ∆I0 - 1/3RG2h2
(6)
where ∆I0 is the corrected intensity at θ ) 0° and RG is the radius of gyration of the semiconductor particles, and ∆I is defined as
∆I ) Isample/Asample - Iblank/Ablank
(7)
and the correction factors, A, were obtained as Asample ) Nsample/ N0 and Ablank ) Nblank/N0, where Nsample, Nblank, and N0 are the intensity counts of the sample, the blank (SiO2 powders or dispersions in the appropriate binary liquid in the absence of semiconductors), and the background (in the absence of the capillary in the beam) in the primer beam for a given time (2 min). Rheological measurements were carried out in dilute suspensions (0.82 g of SiO2/100 cm3 of liquid) with a Haake rotational viscosimeter (RV20-CV100) equipped with a ME-15 head in the low shear range at 25.0 °C. The Bingham-yield stress, τB, was obtained from the τ ) f(D) flow curves, where τ is the shear stress and D is the shear rate gradient. Absorption and emission spectra of the SiO2 semiconductor particle dispersions were taken on a UVIKON 930 or HewlettPackard diode array spectrophotometer and on a SPEX Fluorolog spectrofluorometer, respectively. Transmission electron microscopy was carried out on an Opton microscope operating at 80 kV. Immersion wetting enthalpies in methanol (∆Hw values) were determined by means of a microcalorimeter (LKB 2107) at 25.0 ( 0.1 °C with a standard error of (8%.
Results and Discussion 1. Adsorption of Binary Alcohol (x1)-Cyclohexane (x2) Liquids at Silica Particle Surfaces. The adsorption excess isotherms for hydrophilic A-200 and hydrophobic R-972 SiO2 particles in methanol-cyclohexane and ethanol-cyclohexane are illustrated in Figure 2. The partial miscibility of methanol and cyclohexane necessarily limited the available range of solvent composition to a low mole fraction of methanol (x1 ) 0.1) in which the adsorption excess isotherms could be determined (Figure 2a), whereas in ethanol-cyclohexane mixtures, the entire mole fraction range could be investigated (Figure 2b). The composition of the adsorption layer, x1s ) f(x1), was assessed from the linear portions of the n1σ(n) vs x1 plots.15 (18) Stabiner, H.; Kratky, O. Macromol. Chem. 1978, 179, 1655. (19) Porod, G. 1951, 83, 1951. (20) Gunier, A.; Fournet, G. Small-Angle Scattering of X-rays; New York, 1955.
Figure 2. Adsorption excess isotherms on hydrophilic A-200 (b, O) and hydrophobic R-972 (9) SiO2 particles in methanolcyclohexane (a) and ethanol-cyclohexane (b).
As expected, hydrophilic A-200 SiO2 particles concentrate methanol to a greater extent than do hydrophobic R-972 SiO2 particles, as was evidenced by the plateau values of their n1σ(n) vs x1 plots (n1σ(n) for A-200 SiO2 ) 5.45 mmol/g and that for R-972 SiO2 ) 1.4 mmol/g) and by the greater adsorption capacity of the former (Vs ) 0.56 cm3/g) than the latter (Vs ) 0.22 cm3/g). The excess adsorption of ethanol is positive in the entire mole fraction region for the hydrophilic A-200 SiO2 particles, but there is a sign change for the hydrophobic R-972 SiO2 particles (Figure 2b). Apparently, cyclohexane is adsorbed preferentially onto the hydrophobic silica particle surfaces from the binary ethanol-cyclohexane liquid mixture at high cyclohexane mole ratios. The maximum amounts of ethanol which can be concentrated in the adsorption layer, provided by the hydrophilic A-200 and the hydrophobic R-972 SiO2 particles, are 4.32 and 2.82 mmol/g. These values correspond to the adsorption capacities of Vs ) 0.26 cm3/g and Vs ) 0.17 cm3/g. The concentrations of methanol and ethanol in the adsorption layer of the hydrophilic A-200 and hydrophobic R-972 SiO2 particles, x1s, against the compositions of the liquid mixtures are plotted in Figure 3. The separation factor, S, (S ) x1sx2/x2sx1) was calculated from the adsorption capacity of the liquid mixture, ns, (ns ) n1s + n2s and x1s ) n1s/ns) and is also plotted against the compositions of the liquid mixtures in Figure 3. The alcohols are seen to concentrate on the surfaces of both the hydrophilic A-200 and hydrophobic R-972 SiO2 particles, although their preferential adsorption is somewhat more pronounced on the hydrophilic A-200 SiO2 particles. These plots clearly illustrate the optimum compositions of liquid mixtures for providing the largest volumes of polar solvents in the adsorption layer, i.e., in the nanophase reaction. These
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Figure 4. Absorption spectra of CdS particles, generated in the nanophase reactor, provided by the hydrophobic R-972 SiO2 particles in methanol-cyclohexane (x1° ) 0.01) mixtures at 8.0 × 10-3, 8.0 × 10-2, and 8.0 × 10-1 M CdS concentrations. Table 1. CdS and ZnS Particles on Hydrophilic (A 200) SiO2 Silica Carrier Adsorbent in Different Liquid Mixtures Liquid Mixture: Methanol (1)-Cyclohexane (2); x1 ) 0.01 CdS (mmol/g of SiO2) 8 × 10-2
8 × 10-1
495 (2.50)
497 (2.49)
507 (2.45)
623
561
616
8× λg (nm) Eg (eV) d (nm) τB (mPa)
10-3
ZnS (mmol/g of SiO2) 8× λg (nm) Eg (eV) d (nm) τB (mPa)
10-3
311 3.99 4.24 148
8 × 10-2
4 × 10-1
315 3.94 4.59 131
320 3.87 5.17 142
Liquid Mixture: Ethanol (1)-Cyclohexane (2); x1 ) 0.05 CdS (mmol/g of SiO2) λg (nm) Eg (eV) d (nm) τB (mPa)
8 × 10-4
8 × 10-3
8 × 10-2
389 3.19 3.24 103
433 2.86 4.25 113
450 2.76 4.91 130
ZnS (mmol/g of SiO2) λg (nm) Eg (eV) d (nm) τB (mPa)
Figure 3. Plots of methanol mole fraction (a) in the adsorption layer (x1s) and separation factor (S) vs methanol mole fraction (b) in the methanol and cyclohexane solvent mixture (x1°) for the hydrophilic A-200 (•) and for the hydrophobic R-972 (9) SiO2 particles. Plots of ethanol mole fraction (c) in the adsorption layer (x1s) and separation factor (S) vs ethanol mole fraction (d) in the ethanol and cyclohexane solvent mixture (x1°) for the hydrophilic A-200 (O) and for the hydrophobic R-972 (9) SiO2 particles.
8 × 10-4
8 × 10-3
8 × 10-2
302 4.10 3.67 118
307 4.04 3.96 145
314 3.55 4.49 152
solvent compositions (x1 ) 0.01 for the methanolcyclohexane liquid pair and x1 ) 0.05 for the ethanolcyclohexane liquid pair) were then selected for the generation of CdS and ZnS nanoparticles on the surfaces of hydrophilic A-200 and hydrophobic R-972 SiO2 particles. 2. Generation of CdS and ZnS within the Nanophase Reactors Provided by SiO2 Particles. Formation of CdS and ZnS particles in the Cd2+- and Zn2+containing, alcohol-rich nanophase SiO2 reactors was readily observable by the development of color upon the introduction of H2S. The process was conveniently monitored by absorption spectrophotometry. Typical
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Table 2. CdS and ZnS Particles on Hydrophobic (R 972) SiO2 Silica Carrier Adsorbent in Different Liquid Mixtures Liquid Mixture: Methanol (1)-Cyclohexane (2); x1 ) 0.01 CdS (mmol/g of SiO2) 8× λg (nm) Eg (eV) d (nm) τB (mPa)
10-3
458 2.71 5.34 41.9
8 × 10-2
8 × 10-1
477 2.62 6.72 103.0
499 (2.48) (48.6)
ZnS (mmol/g of SiO2) 8× λg (nm) Eg (eV) d (nm) τB (mPa)
10-3
300 4.13 3.57 57.5
8 × 10-2
4 × 10-1
302 4.11 3.67 102.7
308 4.03 4.02 112.9
Liquid Mixture: Ethanol (1)-Cyclohexane (2); x1 ) 0.05 CdS (mmol/g of SiO2) λg (nm) Eg (eV) d (nm) τB (mPa)
8 × 10-3
8 × 10-2
8 × 10-1
438 2.83 4.14 17.7
458 2.71 5.34 14.4
514 (2.41) microsize 18.28
Figure 5. Plot of absorption edge of CdS particles vs methanol mole fraction, generated by the addition of stoichiometric H2S to a hydrophilic R-972 SiO2 dispersion in methanol-benzene which contained 0.4 mmol of Cd2+/g of silica particles.
ZnS (mmol/g of SiO2) λg (nm) Eg (eV) d (nm) τB (mPa)
8 × 10-3
8 × 10-2
4 × 10-1
295 4.20 3.35 168
302 4.11 3.67 226
304 4.08 3.78 208
absorption spectra are illustrated in Figure 4, and the absorption edges and optical band gaps of the CdS and ZnS particles are collected in Tables 1 and 2. Size quantization of the nanoparticles is clearly evident. The smallest CdS particles (3.2 nm mean diameter) are formed upon the introduction of H2S into the hydrophilic silica particles, dispersed in the ethanol-cyclohexane liquid, in the presence of the 8 × 10-3 M cadmium ions. The smallest ZnS particles (3.4 nm mean diameter) are formed upon the introduction of H2S into the hydrophobic silica particles, dispersed in the ethanol-cyclohexane liquid, in the presence of the 8 × 10-3 M cadmium ions. The effect of changing the composition of the binary liquid (methanol-benzene) on the absorption edge of the CdS particles generated is illustrated in Figure 5. A 1:1 molar mixture of methanol-benzene appears to be the optimal media for producing CdS particles with the shortest wavelength absorption edges. 3. Structural Alteration of the SiO2 Particles by Semiconductors. (a) Rheological Measurements. Formation of size-quantized CdS and ZnS particles was found to affect the rheological properties of the SiO2 dispersions. Plots of shear stress (τ) vs shear rates (D) of ethanolcyclohexane (x1 ) 0.05) and hydrophilic A-200 SiO2 particles in ethanol-cyclohexane (x1 ) 0.05) in the absence and in the presence of CdS and ZnS particles are shown in Figure 6, and the τB values are collected in Tables 1 and 2. An increase in the concentration of the semiconductor nanoparticles on the SiO2 surface increases the τB values, indicating, as expected, stronger interparticle interactions and structure building in the liquid suspension. It is tempting to speculate that the semiconductor nanoparticles on the SiO2 surface (Figure 1) bridge neighboring aggregates and form a cohesive three-dimensional network. (b) Calorimetric Investigations. The immersional wetting enthalpies, ∆Hw, of the hydrophilic A-200 SiO2
Figure 6. Plots of shear stress (τ) vs shear rates (D) for CdS (a) and ZnS (b) particles generated on hydrophilic A-200 SiO2 particles in ethanol (x1)-cyclohexane mixtures (x1° ) 0.05) (0.82 g of CdS or ZnS/100 mmol of liquid mixture).
particles were determined to be 41.5 J/g in methanol in the absence of semiconductors. Substantially lower, the ∆Hw values were observed for hydrophilic A-200 SiO2 particles which contained CdS or ZnS (Figure 7). The coating of the SiO2 surface by CdS or ZnS depolarizes the surface and thereby renders it more hydrophobic. Nucleation of the semiconductor nanoparticles is likely to occur at the most highly activated parts of the SiO2 surface.
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Figure 7. Plots of immersion enthalpies (∆Hw) vs the concentration of CdS (b) or ZnS (4), generated on hydrophilic A-200 SiO2 particles (expressed in terms of their precursor ion concentrations) in methanol. Figure 9. (a) Plots of the logarithm of the relative scattering intensities (log I) vs scattering wave vectors (h), according to the Porod equation (eq 5), for hydrophobic R-972 SiO2 particles containing 0.004 mmol (1), 0.04 mmol (2), and 0.4 mmol (3) of ZnS per gram of SiO2. (b) Data for ZnS nanoparticles only plotted as the natural logarithms of the relative scattering intensities (ln I) vs the square of the scattering wave vectors (h2), according to the Gunier equation (eq 6). SAXS measurements taken on dried powder samples. Table 3. SAXS Parameters of SiO2-CdS and SiO2-ZnS Nanoparticlesa CdS on R-972 SiO2 (mmol/gb)
I0,c cps RG,d nm Dse
ZnS on R-972 SiO2 (mmol/gb)
4.0 × 10-3
4.0 × 10-2
4.0 × 10-1
4.0 × 10-3
4.0 × 10-2
4.0 × 10-1
2.00
2298 5.56 2.05
3866 6.03 2.11
5218 4.68 2.00
5294 4.48 2.01
5431 5.27 2.02
a
Measurements were performed on powders of SiO2-CdS and SiO2-ZnS, prepared by exposing H2S to hydrophobic R-972 silica particles in methanol-cyclohexane ) 0.01:0.99. b Concentrations expressed in terms of Cd2+ or Zn2+ ions. c Intensities in terms of counts per seconds. d Radius of gyration of the semiconductor particles. e Surface fractal dimensions.
Figure 8. (a) Plots of the logarithms of the relative scattering intensities (log I) vs the scattering wave vectors (h), according to the Porod equation (eq 5), for hydrophobic R-972 SiO2 particles containing 0.004 mmol (1), 0.04 mmol (2), and 0.4 mmol (3) of CdS per gram of SiO2. (b) Data for CdS nanoparticles only plotted as the natural logarithms of the relative scattering intensities (ln I) vs the square of the scattering wave vectors (h2), according to the Gunier equation (eq 6). SAXS measurements taken on dried powder samples.
Subsequent growth of CdS (or ZnS) decreases the surface energy of the SiO2 particles, which manifests itself in a decreased wetting enthalpy in methanol. This result is in agreement with and supports the rheological data. The increase of τB values upon the growth of semiconductor nanoparticles on the SiO2 surface can be considered to originate the decreased wetting enthalpy as a result of increased interparticle interactions. (c) Small-angle X-ray Scattering Measurements. SAXS measurements also provided valuable insight into the structural changes which accompanied the formation of CdS and ZnS in the nanophase reactor provided by the SiO2 particles. Increasing concentrations of semiconduc-
tor particles led to increasing scattering intensities, as indicated by the log I vs log h plots and eq 5 (Figures 8a and 9a and Table 3). Treatment of the data in terms of the Guinier equation (eq 6) permitted the evaluation of the radius of gyration of the semiconductor particles, RG, from the Guinier plots (Figures 9a and 9b). The obtained RG values are collected in Table 3. It should be recalled, however, that eq 6 is valid only for monodispersed spherical particles. Considering the expected deviations from spherical symmetry and monodispersity, the agreement between RG values, calculated for CdS from SAXS data (Table 3), and the diameters, estimated from the absorption spectra of the corresponding CdS preparations (Table 2), is quite satisfactory. Fractal dimensions, Ds, obtained as the slopes of eq 6, are also given in Table 3. Ds values of 2.0 indicate the presence of relatively smooth particles, whereas Ds values of 3.0 are associated with rough particles. The obtained fractal dimensions correspond to relatively smooth silica particles, and apparently, the presence of semiconductors causes only a minimal change in the surface morphology.
CdS and ZnS Particles in Nanophase Reactors
Conclusion The ultrathin adsorption layer of methanol or ethanol, formed at colloidal silica particle surfaces dispersed in binary alcohol-apolar organic liquids, has been shown to function as a nanophase reactor for the in situ generation of semiconductor nanoparticles. Determination of adsorption excess isotherms has permitted the full characterization of the nanophase reactor in terms of its volume and composition. Apparently, the nucleation and growth of semiconductor nanoparticles decreases the surface energy of the host SiO2, which manifests itself in increased
Langmuir, Vol. 12, No. 15, 1996 3715
τB values and decreased enthalpies of wetting. The information obtained in the present work will permit additional and more refined controlled preparations of size-quantized semiconductor particles. Acknowledgment. Support of this research by grants from the National Science Foundation, the U.S.-Hungarian Joint Fund, and OTKA I/5 (NFSR, Hungary) is gratefully acknowledged. LA951541I
