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Articles Preparation of Ag/SiO2 Nanosize Composites by a Reverse Micelle and Sol-Gel Technique Tuo Li, Jooho Moon, Augusto A. Morrone, John J. Mecholsky, Daniel R. Talham, and James H. Adair* Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611-6400 Received July 17, 1997. In Final Form: March 9, 1999 Spherical nanosize Ag/SiO2 composite particles have been synthesized within reverse micelles via metal alkoxide hydrolysis and condensation. The size of the particles and the thickness of the coating can be controlled by manipulating the relative rates of the hydrolysis and condensation reactions of tetraethoxysilane (TEOS) within the microemulsion. Composite particles in the size range 20-35 nm are produced. As the molar ratio of water to surfactant is increased above 10, the size distribution broadens. Absorption spectra have been used to dynamically monitor the reaction and growth. The effects of other synthesis parameters, such as the molar ratio of water to TEOS and the amount of base catalyst, are discussed. Possible mechanisms for the formation of the nanocomposite particles are also discussed.
Introduction The motivation for recent research on colloidal metal particles has stemmed from their unique electro-optical and electrochemical properties. Many of the phenomena exhibited by semiconductor particles, for example, storage of excess electrons and holes, changes in electronic properties upon surface modification, photoelectron emission, and sensitization of photoreactions of other solutes, are also observed in metallic colloids. The first experimental study of the nonlinear optical properties of metal colloids less than a wavelength in diameter was reported by Ricard et al. in 1985.1 It was observed that opticalphase conjugation was resonantly enhanced near the surface-plasma resonance frequency. Since composite materials are macroscopically isotropic, their nonlinear response is mainly due to the third-order Kerr effect, which is well suited to the study of optical nonlinearities and is related to applications such as real-time holographic and bistable memory devices, optical correlators, phaseconjugator devices, and optical polarizers.2 Theoretical calculations of nonlinear optical behavior have been reported for model composites of nanospheres with a metallic core (e.g., Ag and Au) and a nonlinear shell (e.g., CdS) or with a nonlinear core and a metallic shell suspended in a nonlinear medium.3,4 Optical phase conjugation was shown to be enhanced from each nonlinear region because the optical field can be concentrated in both the interior and the exterior neighborhoods of the particle and magnified at the surface-mediated plasma resonance. However, so far no such composite model * Corresponding author. Present address: Materials Research Laboratory, Pennsylvania State University, University Park, PA 16802. (1) Ricard, D.; Roussignol, P.; Flytzanis, C. Opt. Lett. 1988, 10, 511. (2) Haus, J. W.; Kalyaniwalla, N. J. Appl. Phys. 1989, 65, 1420. (3) Neeves, A. E.; Birnboim, M. H. Opt. Lett. 1988, 13, 1087. (4) Haus, J. W.; Zhou, H. S.; Takami, S.; Hirasawa, M.; Honma, I.; Komiyama, H. J. Appl. Phys. 1993, 73, 1043.
particles have been available to verify these theoretical predictions. Surface coating or surface modification of nanometer semiconductor and metal particles offers a new challenge to synthesis. By changing the thickness of the shell and the particle radius, the overlap of the wave functions and the band gap can be changed.5 Chang et al. have previously shown that composite coatings on the nanoscale can be readily achieved with a variety of morphological features.6 Silica spheres with CdS cores of different morphology were prepared by Chang et al.6 The physical parameters of each material as well as the interface between the “heterostructures” will have a great influence on the optical properties.4,6,7 A wide variety of methods have been used to prepare such small clusters, including the use of hydrosols,8-11 irradiation,12 reduction and stabilization,13,14 and microemulsion and reverse micelle systems.6,15-20 This paper describes a method for the preparation of (5) Neeves, A. E.; Birnboim, M. H. J. Opt. Soc. Am. B 1989, 6, 787. (6) (a) Chang, S.; Liu, L.; Asher, S. A. J. Am. Chem. Soc. 1994, 116, 6739; (b) 1994, 116, 6745. (7) Mulvaney, P. Langmuir 1996, 12, 788. (8) Frens, G.; Overbeek, J. Th. G. Kolloid Z. Z. Polym. 1969, 233, 922. (9) Zsigmody, R. J. Phys. Chem. 1906, 56, 65. (10) Wilenzick, R. M.; Russell, D. C.; Morris, R. H.; Marshall, S. W. J. Chem. Phys. 1967, 47, 533. (11) Liz-Marza´n, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329. (12) Itakura, T.; Torigoe, K.; Esumi, K. Langmuir 1995, 11, 4129. (13) Liz-Marza´n, L. M.; Lado-Tourin˜o, I. Langmuir 1996, 12, 3585. (14) Huang, H. H.; Yan, F. Q.; Kek, Y. M.; Chew, C. H.; Xu, G. Q.; Ji, W.; Oh, P. S.; Tang, S. H. Langmuir 1997, 13, 172. (15) Markowitz, M. A.; Chow, G. M.; Singh, A. Langmuir 1994, 10, 4095. (16) Lisieki, I.; Bjo¨rling, M.; Motte, L.; Ninham, B.; Pileni, M. P. Langmuir 1995, 11, 2385. (17) Tanori, J.; Pileni, M. P. Langmuir 1997, 13, 639. (18) Adair, J. H.; Li, T.; Kido, T.; Havey, K.; Moon, J.; Mecholsky, J.; Morrone, A.; Talham, D. R.; Ludwig, M. H.; Wang, L. Mater. Sci. Eng. Rep. 1998, R23[4-5], 139-242. (19) Qi, L.; Ma, J.; Shen, J. J. Colloid Interface Sci. 1997, 186, 498. (20) Osseo-Asare, K.; Arrigada, F. Ceram. Trans. 1990, 12, 3.
10.1021/la970801o CCC: $18.00 © 1999 American Chemical Society Published on Web 05/18/1999
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Figure 2. Absorption spectra of Ag metal clusters formed in Igepal/cyclohexane/water reverse micelles at various water contents R. Optical path length is 1 cm, and Ag concentration is 10-3 M.
Figure 1. Flowchart for the synthesis of Ag/SiO2 nanocomposite particles.
nanocomposites composed of SiO2 with Ag nanometer size cores inside a microemulsion reaction matrix. The process is performed in conjunction with hydrolysis of organometallic precursors such as tetraethoxysilane (TEOS), followed by condensation in the water nanodroplets to form a coating on the nanosize metal within reverse micelle structures. Not only metal/silica nanocomposites, but also semiconductor/oxide and even semiconductor/insulator/ metal multiple-layer heterostructures can be prepared using this method.18 Materials and Methods Reverse micelles were prepared from both ionic and nonionic surfactants using procedures similar to those described by OsseoAsare and Arrigada.20 The anionic surfactant was sodium bis(2-ethylhexyl)sulfosuccinate (Aerosol OT or AOT, Aldrich Chemical Co.), and the nonionic surfactant was poly(oxyethylene) nonylphenyl ether (Igepal CO-520, Aldrich Chemical Co.). Both surfactants were used without further purification. AgNO3 (Aldrich Chemical Co.), tetraethoxysilane (TEOS, 99%), cyclohexane, isooctane, and NH4OH (29%) (all from Fisher Scientific) were used as received. Deionized water was used for all experiments. The general procedure used to prepare Ag/SiO2 nanocomposites is outlined in Figure 1. Typically, microemulsions of total volume 20 mL were prepared at ambient temperature in a 30 mL vial with rapid stirring, and they consisted of 4 mL of Igepal, 10 mL of cyclohexane, 1.64 mL of 10-2 M Ag(NO)3 solution, and deionized water. The size of the resulting particles was controlled by varying the ratio R ) [water]/[surfactant]. The microemulsion was mixed rapidly, and after 5 min of equilibration, one drop (∼0.05 mL) of hydrazine hydrate (9 M N2H4‚xH2O, Aldrich Chemical Co.) was added as a reducing agent. To expedite the condensation reaction, base in the form of NH4(OH) was injected into the stirred microemulsion at various ratios X ) [NH4(OH)]/[TEOS]. This ratio is specifically for the purpose of consistency, as the pH is measured independently in the organic solvent. After another 5-10 min equilibration period, TEOS stock solution consisting of 50% TEOS and 50% cyclohexane by weight was injected into the microemulsion at various molar ratios H ) [water]/[TEOS].
The structure, size, and morphology of the resulting composite particles were determined by transmission electron microscopy (TEM) (200CX, JEOL, Boston, MA), X-ray diffractometry (XRD) (APD 3720, Phillips Electronics, Mahwah, NJ), X-ray photon spectroscopy (XPS) (PHI 5100 ESCA, Perkin-Elmer, Norwalk, CT), and UV-visible absorption spectroscopy (Lambda 9 spectrophotometer, Perkin-Elmer, Norwalk, CT). For the TEM studies, samples were prepared by adding drops of freshly prepared cluster solution on a carbon film supported on a Cu grid. The mean diameter and standard deviation of the clusters were determined from TEM micrographs by averaging 100 particles. A series of absorption spectra were taken as a function of reaction time to monitor the hydrolysis and condensation processes by measuring changes in the optical intensity. UVvisible spectroscopy was also used to study the effect of size on the nanometer-sized metallic clusters.
Results and Discussion Influence of the Synthesis Conditions for the Ag Core Particles. Spherical Ag/SiO2 nanometer size composite particles with a narrow size distribution were obtained by reduction of Ag in reverse micelles followed by in-situ hydrolysis and condensation in the microemulsion. While the core particles are formed by homogeneous nucleation and growth, the shells are most likely formed through heterogeneous nucleation and growth. Because the two steps are different in mechanism, controlling the formation of the nanocomposites is very sensitive to modest processing changes. Most studies of the Ag nanosize particle formation process in reverse micelles have been based on a two-step model.21 The first step is rapid, complete reduction of the metal to the zero valence state. The second step is growth via reagent exchanges between micelles. The nucleation and growth of Ag particles is likely to be a diffusioncontrolled process through interaction between micelles, but it can be influenced by many other factors such as phase behavior and solubility, average occupancy of reacting species in the aqueous pool, and the dynamic behavior of the microemulsion.19 Figure 2 shows the absorption spectra of colloidal Ag particles obtained from the Igepal/cyclohexane/water reverse micelles at various water/surfactant ratios R. At low R, the surface-plasmon resonance peaks in the absorption spectra of the small Ag particles were broad (21) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1993, 97, 12974.
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and low in optical intensity. With decreasing R, the peak at 400 nm narrowed and increased in intensity with a slight red shift, which can be correlated with the size of the particles.3 The peak at 320 nm is attributed to the surfactant, as suggested by the photoluminescence spectrum of the pure microemulsion. The change in R influenced the size distribution of the resulting particles. As R increased above 10, the size of the metallic cluster remained unchanged, but the distribution broadened, probably due to movement outside the region of the psuedoternary phase diagram where the microemulsion is stable. A uniform size distribution for the Igepal system was obtained only for R less than 10, possibly due to the decreasing stability of the micellular phase. The monodispersity of the AOT system was maintained reasonably well. The absorption spectrum (not shown) narrowed and became more intense on increasing the concentration of Ag+ from 5 × 10-3 to 10-2 M, indicating that the size of the Ag particles increased. This change in the spectrum can be explained in terms of the classical mean free path effect and size-quantized theory.22 The ionic strength and the concentration have a great influence on the formation and stability of microemulsion systems.23 Compared with ionic surfactants, nonionic surfactants are relatively insensitive to changes in electrolyte concentration.24 On increasing the salt concentration from 10-3 to 10-2 M, the nonionic surfactant microemulsions studied here (Igepal/ cyclohexane/water) were stable over several days, whereas the anionic (AOT/isooctane/water) microemulsions were only stable for several hours. The Ag particles showed slight changes in the intensity of the absorption spectra with different solvents, due to the increasing size of the particles, indicating different particle growth mechanisms. It is probable that the intermicelle exchange process varies with solvent, resulting in slightly different size particles.25 Influence of Synthesis Method for the Ag/SiO2 Nanocomposite Particles. Formation of a thin layer of SiO2 on the Ag cluster was achieved by in-situ hydrolysis and condensation of metal alkoxide precursors of tetraethoxysilane (TEOS) in reverse micelles. To control the thickness and uniformity of the SiO2 layer, a variety of processing parameters such as the ratio of water to surfactant R, the ratio of water to metal alkoxide precursors H, and the amount of base added to enhance the hydrolysis and polycondensation (referred to as the ratio of TEOS to base X) have been paid special attention. The size and size distribution of the resulting nanometer-sized composites were quantitatively characterized by electron microscopy. Optical properties of the nanocomposite particles characterized by linear and nonlinear optical spectroscopy are the subject of current work and will not be reported at the present time. The TEM micrographs and histograms presented in Figures 3 and 4 show that the Ag/SiO2 nanosize composites are essentially monodisperse and spherical in shape. The size of the composite particles increases linearly with R (water/surfactant) and H (water/TEOS). Figure 5 shows the increasing thickness of the SiO2 coating on the nanosize core as the ratio H is increased while R, and hence the Ag core size, is kept relatively constant. To determine the median size and standard deviation, the log-normal probability of particle size was plotted against t values collected from the Standard Mathematical Tables for (22) Kreibig, U. J. Phys. F: Met. Phys. 1974, 4, 999. (23) Isrealachvili, J. N. Intermolecular and Surface Forces; Academic Press: London, 1992. (24) Shinoda, K.; Lindman, B. Langmuir 1987, 3, 135. (25) Shinoda, K. Prog. Colloid Polym. Sci. 1983, 68, 8.
Li et al.
Figure 3. TEM micrographs of Ag/SiO2 nanocomposites formed in the Igepal/cyclohexane/water system at R ) 4, X ) 1, and (a) H ) 100, (b) H ) 200, and (c) H ) 300.
normal distributions,26 and a linear regression was performed in the cumulative percentage range from 15% to 85%. In this way, the Ag/SiO2 nanocomposites were determined to have a median diameter from 12.5 to 35 nm as H varied from 100 to 300 at R ) 4, with a standard deviation of 0.25 nm. The electron diffraction pattern in Figure 5d indicates that the Ag core has a crystalline facecentered cubic (fcc) structure, with a lattice constant of 4.08 Å. Similar monolayer aggregates have been described (26) Scheaffer, R. L.; McClave, J. T. Probability and Statistics for Engineers; PWS-KENT: Boston, 1990.
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be due to intermicellar collisions which result in a distribution of micelle sizes with increasing R or due to the temperature sensitivity of the nonionic surfactant. To control the relative rates of hydrolysis and condensation reactions and improve the suspension stability, the ratio X of base to TEOS was varied. It was found that a stable suspension was obtained only for X ≈ 1. The results in Figure 6 present the effects on the resulting particles of changing the water content R when the amount of TEOS and the ratio of TEOS to catalyst X were kept constant. At low water content most of the water was bound to the hydrophilic head groups of amphiphilic molecules and the rate of hydrolysis of the TEOS was slow. Consequently, the thickness of the oxide layer increased slowly. As the R ratio increased, more free water inside the reverse micelle was available for the hydrolysis reaction of the TEOS, resulting in a thicker or a larger size of the SiO2 layer. Changes in the oxide layer thickness as a function of the R and H ratios have been plotted in Figure 7a. It was also found that at low water content the size distribution of the resulting particles remained fairly narrow and increased linearly with R. However, increasing the water content to R ) 10 gave an increased particle size with the size distributions of both the core metal and the oxide layer becoming more polydisperse, as shown in Figure 7b. The reverse micelles become unstable and more polydisperse as water content is increased.29 Similar results have been reported by Guizard et al.29 in the hydrolysis of Ti and Fe alkoxides in a nonionic surfactant (a polyoxyethylene alkylphenyl ether/decane/water reverse micellar system). This deviation could be due to the intermicellar collisions which result in a larger micellar size distribution with increasing R or due to the temperature sensitivity of the nonionic surfactant. Discussion of the Formation Mechanisms for the Nanocomposite Particles. Metal-organic derivatives within the microemulsion reaction matrix undergo a hydrolysis reaction and two possible condensation reactions, which can be represented as follows:20
M(OR)n + H2O f M(OR)n-1(OH) + R(OH)
(1)
M(OR)n + M(OR)n-1(OH) f M2O(OR)2n-2 + R(OH) (2) M(OR)n-1(OH) + M(OR)n-1(OH) f M2O(OR)2n-2 + H2O (3)
Figure 4. Particle size histograms for Ag/SiO2 nanocomposites synthesized at R ) 4, X ) 1, and (a) H ) 100, (b) H ) 200, and (c) H ) 300.
by Aksay27 for a suspension of colloidal gold particles and by Alexander and Iler28 for silica sols. In the latter case, it was suggested that the negative charge of the silica particles under alkaline conditions induces growth of the aggregate only around the edges of the monolayer of particles, where the ionic repulsion is minimal. On increasing the water content, the rate of hydrolysis is increased; however, for R greater than 10, the core particles have a wide size distribution. This deviation could (27) Aksay, I. L. In Ceramic Powder Science II, Vol. I, Part B; Messing, G. L., Fuller, E. R., Hausner, H., Eds.; American Ceramic Society: Westerville, OH 1988; p 633. (28) Alexander, G. B.; Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979.
As a first approximation, it may be assumed that the reverse micelle aggregates present in the solution are not affected by the addition of TEOS molecules or by subsequent reactions and in particular that the aggregation numbers of the micelles remain unchanged. The TEOS alkoxide molecules would then interact rapidly with the water molecules inside the reverse micelles, forming partially hydrolyzed species. These hydrolyzed species remain bound to the micelles due to their enhanced amphiphilic character brought about by the formation of silanol groups. All further reactions are restricted to the micelle region, and thus the overall mechanisms of oxide layer formation and growth involve both intra- and intermicelle events.20 It is likely that hydrolysis occurs within each reverse micelle, whereas condensation (particle growth) may occur also by intermicellar contacts. (29) Guizard, C.; Stitou, M.; Larbot, A.; Cot, L.; Bouviere, J. In Better Ceramics Through Chemistry; Materials Research Society Symposium Proceedings Volume 121; Materials Research Society: Pittsburgh, PA, 1988; p 317.
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Figure 5. TEM micrographs of Ag/SiO2 nanocomposites synthesized at R ) 6, X ) 0.75, and (a) H ) 100, (b) H ) 200, and (c) H ) 300; (d) diffraction pattern of the nanocomposites.
Therefore, the size of the composite particles depends on the relative rates of the hydrolysis and condensation reactions. Figure 8 shows the UV-visible absorption spectra obtained during hydrolysis of 1 × 10-4 M TEOS for various R values. The changes in the absorption bands were used to qualitatively monitor the relative rate of hydrolysis and condensation. The results are consistent with the hypothesis that the rate of the hydrolysis decreases as
the R ratio decreases. There are several factors which may affect the rate of hydrolysis in reverse micelles. With low water content (small R), most of the water molecules are bound to the polar head of the surfactant molecules; hence, the hydrolysis rate is slow. With increasing water content, more free water is available to participate in the hydrolysis reaction. Thus, the hydrolysis reaction rate is expected to increase for large R. The ratio H affects the rate of hydrolysis by increasing the rate of diffusion of
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Figure 6. TEM micrographs of Ag/SiO2 nanocomposites synthesized at X ) 1 and H ) 100 and at different water contents: (a) R ) 2; (b) R ) 4; (c) R ) 6; (d) R ) 10.
TEOS alkoxide molecules into the reverse micelles. It has been found that the rate of hydrolysis of the organometallic precursors in Ag-containing reverse micelles is much faster than that of CdS-containing reverse micelles, and this results in a more uniform distribution of the size and thickness of the shell.18 This implies that the nature of the core surface plays an important role in the interaction between the core material and the hydrolyzed silanol groups, influencing the surface modification and epitaxial growth. The rate of condensation through the intermicelle interaction can be controlled by altering the chain length of the bulk solvent. SiO2 particles without metallic cores have been found in the final products. This may be attributed to the distribution of ionic species in the reverse micelles or may be associated with the dynamic nature of the intermicelle interactions.30 The reverse micelles may not be fully occupied by metallic clusters of the core material in the first stage if the concentration of micelles is higher than
that of reactants. Empty droplets released during the intermicelle interaction may provide a microenvironment for TEOS precursors to nucleate and grow through hydrolysis and condensation. Thus, the formation of the pure SiO2 particles is presumably an inherent part the synthesis process. Conclusions Spherical nanocomposite Ag/SiO2 particles with uniform size distribution have been prepared using self-assembly molecules, in conjunction with the hydrolysis and condensation of organometallic precursors. These metallic nanometer size composite particles have excellent stability and reproducibility under optimum synthesis conditions. The average size of the metallic cluster was found to depend on the micelle size, the nature of the solvent, and (30) Fletcher, P. D. I.; Howe, A. M.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1987, 83, 985.
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Figure 8. Increase of the rate of hydrolysis as a function of water content R, quantitatively monitored through the changes of the slopes of absorption spectra taken from 5 min to 1.5 h after initiation of hydrolysis of TEOS (at constant X ) 1 and H ) 100 and different R: R ) 2; R ) 4; R ) 8).
process. By controlling the ratio of water to surfactant, the ratio of water to TEOS, and the ratio of TEOS to catalyst, the core particle size and the coating thickness can be adjusted. The process can also be modified to form a transparent gel instead of nanocomposite particles. The versatility of this method has potential for applications such as optical electronic devices. Part of ongoing work directed toward optical characterization of materials synthesized using the composite nanoparticles will be given in a future report. Figure 7. (a) Variations of the thicknesses of SiO2 layers (rL) as a function of H at R ) 4 and X ) 1. (b) Size of the Ag clusters with changing R ratio.
the concentration of reagent. UV-visible spectroscopy and TEM studies of particle formation indicate that the reaction process in the complex system containing reverse micelles and TEOS is governed by a diffusion-controlled
Acknowledgment. The Major Analytical Instrumentation Center (MAIC) at the University of Florida is thanked for assistance in the characterization of materials. The partial financial support of NASA (Grant No. NAG81244) for this work is also gratefully acknowledged. LA970801O
