Gold Composites

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Self-Assembly of Mesoporous Nanoscale Silica/Gold Composites Robert I. Nooney,† Dhanasekaran Thirunavukkarasu,‡ Yimei Chen,§ Robert Josephs,§ and Agnes E. Ostafin*,† Department of Chemical Engineering, University of Notre Dame, Notre Dame, Indiana 46556, Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556, and Department of Molecular and Cellular Biology, University of Chicago, Chicago, Illinois 60637 Received March 26, 2003. In Final Form: June 24, 2003 The effect of cetyltrimethylammonium bromide (CTAB) template concentration, solvent conditions, and silica source on the structure and self-assembly of mesoporous nanoscale silica/gold composites was investigated. Nanocomposites were analyzed using X-ray diffraction, transmission electron microscopy, and liquid nitrogen adsorption. The number of gold particles per silica shell, referred to as the cluster number, and the ratio of gold/silica to pure silica particles, referred to as the occupancy, varied considerably with the concentration of CTAB template. In addition, the shape of the particles and the direction of mesopore growth were found to depend highly on the solvent composition and the silica source used. The materials produced were thermally stable above 540 °C, had pore volumes greater than 0.53 cm g-1 at standard temperature and pressure, and had surface areas in excess of 690 m2 g-1. The overall particle sizes ranged in diameter from 93 to 520 nm, and the pore center-to-pore center distances of the mesochannels of calcined samples varied from 3.06 to 5.25 nm. From an analysis of the silica mesopore geometry and the position of the gold nanoparticle, a three-stage mechanism for the self-assembly process of the nanocomposites is proposed.

Since the invention of MCM41,1 there has been much interest in the use of mesoporous silica-supported metal or semiconductor nanoparticles of gold,2 silver,3 titanium oxide,4 rhodium oxide,5 and cadmium(II) selenide6 for catalysis, chemical sensing, nanoscale capacitors, and semiconductor devices. Recently, we showed that mesoporous nanoscale silica/gold composite (MNSGC) particles containing either a single gold nanoparticle or nanoparticle clusters could be synthesized using a liquid-phase seeded growth method.7 In the first step, each nanoparticle of gold was coated with either a vitreophilic ligand or a thin layer of silica using a method developed by Liz-Marzan et al.8 The second step used a modification of the Sto¨ber et al. method,9 where reactants were mixed in controlled stoichiometric amounts to synthesize nanoscale mesoporous silica coatings.10-12 In contrast to aerosol-assisted * Author to whom correspondence should be addressed. Fax: (219)-6318366. E-mail: [email protected]. † Department of Chemical Engineering, University of Notre Dame. ‡ Radiation Laboratory, University of Notre Dame. § University of Chicago. (1) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (2) Mukherjee, P.; Patra, C. R.; Kumar, R.; Sastry, M. PhysChemComm. 2001, 4, 5. (3) Cai, W.; Zhong, H.; Zhang, L. J. Appl. Phys. 1998, 83, 1705. (4) Aronson, B. J.; Blanford, C. F.; Stein, A. Chem. Mater. 1997, 9, 2842. (5) Mulukutla, R. S.; Asakura, K.; Kogure, T.; Namba, S.; Iwasawa, Y. Phys. Chem. Chem. Phys. 1999, 1, 2027. (6) Parala, H.; Winkler, H.; Kolbe, M.; Wohlfart, A.; Fischer, R. A.; Schmechel, R.; von Seggern, H. Adv. Mater. 2000, 12, 1050. (7) Nooney, R. I.; Dhanasekaran, T.; Chen, Y.; Josephs, R.; Ostafin, A. E. Adv. Mater. 2002, 14, 529. (8) Liz-Marzan, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329. (9) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62. (10) Bu¨chel, C.; Gru¨n, M.; Unger, K. K.; Matsumoto, A.; Tsutsumi, K. Supramol. Sci. 1998, 5, 253.

self-assembly,13 the liquid-phase seeded growth method discussed herein uses pre-existing gold nanoparticles as nucleation sites for the propagation of a mesoporous shell, and, therefore, not only can the nanoparticle diameter be defined independently of the silica mesopore geometry but also the occupancy of the metal nanoparticle within the mesoporous silicate matrix can be extremely high. In aerosol-assisted self-assembly, the maximum size of the nanoparticle must be less than the typical mesopore diameter, normally around 3 nm in diameter, and the dispersion of particles throughout the material may vary. The ability to trap nanoparticles of arbitrary size in a mesoporous matrix opens the door to a number of quantum dot-based sensing applications.8 An important question is whether the nature of the nanoparticle influences the resultant morphology of the mesoporous silicate shell, or whether the silicate morphology is controlled only by the reactant molar ratios and solvent conditions, as is the case for bulk mesoporous silicate materials. The mesopore phase, which ranges from lamella to hexagonal to cubic symmetry,14 and the thickness of the silica wall15 depend on the cetyltrimethylammonium bromide (CTAB)/silica ratio under controlled solvent conditions. Changing the base catalysts,12 charge on the surfactant headgroup,10,11 silica source,16 and solubility16 also effect the mesopore phase and overall particle shape. Mesoporous materials in which (11) Gru¨n, M.; Unger, K. K.; Matsumoto, A.; Tsutsumi, K. Microporous Mesoporous Mater. 1999, 27, 207. (12) Cai, Q.; Luo, Z.; Pang, W.; Fan, Y.; Chen, X.; Cui, F. Chem. Mater. 2001, 13, 258. (13) Lu, Y.; Fan, H.; Stump, A.; Ward, T. L.; Rieker, T.; Brinker, C. J. Nature 1999, 393, 223. (14) Firouzi, A.; Kumar, D.; Bull, L. M.; Besier, T.; Seiger, P.; Huo, Q.; Walker, S. A.; Zasadzinkski, J. A.; Glinka, C.; Nicol, J.; Margolese, D.; Stucky, G. D.; Chmelka, B. F. Science 1995, 267, 1138. (15) Galarneau, A.; Desplantier-Giscard, D.; Di Renzo, F.; Fajula, F. Catal. Today 2001, 68, 191. (16) Anderson, M. T.; Martin, J. E.; Odinek, J. G.; Newcomer, P. P. Chem. Mater. 1998, 10, 311.

10.1021/la034522e CCC: $25.00 © 2003 American Chemical Society Published on Web 07/25/2003

Silica/Gold Composites

form defines the functionality are potentially useful in a variety of applications ranging from the self-assembly of electromechanical machines, chromatography, surface polishing, biomolecular separations, medical implants, and drug delivery.17,18 In previous work, we showed that the nanoparticle size could also be controlled by varying the initial silicate and surfactant concentrations over a range of diameters from 65 to 740 nm.19 In the work presented here, we use a combinatorial approach to investigate the effects of changing the CTAB concentration, solubility, and reactivity of the silica source on the synthesis of a MNSGC. The experiments were performed under both pure water (heterogeneous) and water/alcohol cosolvent (homogeneous) conditions using tetramethyl orthosilicate, TMOS, tetraethyl orthosilicate, TEOS, or tetrapropyl orthosilicate, TPOS.11 The occupancy and clustering of gold within the silica was found to be highly dependent on the CTAB/silica ratio, even though the concentration of the surfactant, CTAB, was shown to be 1 order of magnitude below the critical micelle concentration (cmc).20 For the first time, the reasons for a particular mesopore geometry under a given set of conditions are explored using the gold seed of the MNSGC particles as a tag to monitor the mesopore formation process. A mechanism for the self-assembly of the MNSGC is proposed that consists of three main stages: silica oligomerization via conventional silicate chemistry, formation of silica/CTAB primary particles, and, finally, aggregation of the primary particles, similar to that of pure silica nanoparticles.14,19 Experimental Section Materials. TEOS (99 wt %), TMOS (99 wt %), TPOS (99 wt %), n-dodecylamine (99.8 wt %), ammonia (2.0 M NH3 in 2-propanol), aminopropyltrimethoxysilane (APS, 98 wt %), and mercaptopropyltrimethoxysilane (MPTS, 98 wt %) were purchased from Sigma Aldrich; methanol (99.9 wt %), sodium citrate (99 wt %), ammonium hydroxide (29 wt % NH3 in water), sodium hydroxide (98%), and sodium silicate (∼27 wt % SiO2, ∼14 wt % NaOH in water) were purchased from Fisher Scientific; hydrogentetrachloroauric acid (99.99 wt %) was purchased from Alfa Aesar; CTAB (99.8 wt %) was purchased from Calbiochem; and ethanol was purchased from AAPER Alcohol and Chemical Company. All materials were used without further purification. Water was deionized to 18.2 MΩ cm-1 using an E-pure Barnstead model D4641 instrument, and Snakeskin dialysis tubing was purchased from PIERCE (10 000 molecular weight cut-off, MWCO). Synthesis of Gold Nanoparticles. Colloidal gold particles with a mean diameter of 15 ( 0.7 nm were prepared following the method of Turkevich and co-workers, in which a boiling solution of 1 wt % hydrogentetrachloroauric acid (HAuCl4‚3H2O) in deionized water is reduced with sodium citrate.21 The concentration of 15-nm gold was estimated to be 2.9 × 1012 particles/mL. Particles with a mean diameter of 60 nm and a 20% size polydispersity were prepared using a seeded growth method developed by Brown and co-workers.22 The concentration of 60-nm gold was approximately 3.6 × 1010 particles/mL. The conductivity of each gold sol was reduced from greater than 800 µS to less than 20 µS by dialyzing against deionized water for 24 h at room temperature prior to use in the synthesis of the nanocomposites. (17) Yang, H.; Coombs, N.; Ozin, G. A. Nature 1997, 386, 692. (18) Yang, H.; Ozin, G. A.; Kresge, C. T. Adv. Mater. 1998, 10, 883. (19) Nooney, R. I.; Dhanasekaran, T.; Chen, Y.; Josephs, R.; Ostafin, A. E. Chem. Mater. 2002, 14, 4721. (20) Anderson, M. T.; Martin, J. E.; Odinek, J. G.; Newcomer, P. P. Chem. Mater. 1998, 10, 1490. (21) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 55. (22) Brown, K. R.; Walter, D. G.; Natan, M. J. Chem. Mater. 2000, 3, 306.

Langmuir, Vol. 19, No. 18, 2003 7629 Table 1. Molar Ratios of Reactants and Respective Particle Sizes for the MNSGC Synthesized Using TMOS with Increasing Amounts of CTABa expt

CH3OH

H2O

A B C D E F G

886 886 886 2123 4730 886

2362 2362 2362 5655 12 597 2362 4674

NaOH TMOS CTAB 0.161 0.161 0.161 0.143 0.320 0.161 0.507

1 1 1 1 1 1 1

0.126 0.27 0.54 0.54 0.67 1.25 0.29

particle sizeb (nm) 170 100c 93 120 180 80 97 (L); 75 (W)

a Experiment G is without ethanol. b L and W refer to the average length and width of the nanoparticles. c Size of sample containing only one gold nanoparticle.

Protection of Gold Nanoparticles. All gold seeds were stabilized against flocculation in the presence of a template with a layer of silicate prior to the synthesis of the mesopore coating. Fifteen-nanometer-diameter citrate-stabilized gold nanoprticles were protected with a thin layer of silica using the method suggested by Liz-Marzan and co-workers.8 In the first step, 2.5 mL of APS solution (1.0 mM in ethanol) was added to 500 mL of gold colloid (APS is a bifunctional ligand where the amine group binds to the gold surface and the silica group points into the solvent phase, thus making the surface vitreophilic).8 Assuming all of the added APS was bound to the gold nanoparticle, approximately 60% of the particle’s surface area would be covered. Next, 50 mL of sodium silicate (0.27 wt % NaSiO2 in deionized water) was added, followed by 4 days of stirring. Sixty-nanometer ammonium hydroxide-stabilized gold could be used in the concentration range of 6.2-7.0 mM without additional treatment. For template concentrations above 7 mM, the gold colloid was protected with a thin layer of silica following the procedure described previously, except MPTS was used as the vitreophilic ligand instead of APS. In this case, 512 µL of MPTS (0.0005 M in ethanol) was added to 317 mL of the 65-nm gold colloid solution. Assuming all of the added MPTS was bound to the gold nanoparticle, approximately 70% of the particle’s surface area would be covered. Synthesis of the Nanoscale Mesoporous Silica Shell. The effect of the CTAB concentration and source of silica was studied by dividing the experiments into five groups based on their respective silica solubilities. Silica is deemed highly soluble (homogeneous) if the silica dissolution time (SDT) is faster than the rate of silica polymerization or the silica gelation time (SGT). This is achieved under water/alcohol cosolvent conditions where the dielectric constant is sufficiently low.11,20 Heterogeneous conditions, where the SDT is slower than the SGT, are achieved in the pure water solvent. The five experimental groups were (1) homogeneous (SDT < SGT) with TMOS in a methanol/water cosolvent; (2) heterogeneous (SDT > SGT) with TMOS and a pure water solvent; (3) homogeneous with TEOS in a water/ethanol cosolvent; (4) heterogeneous with TEOS in a pure water solvent; and (5) heterogeneous with TPOS in a pure water solvent. Experiments with TPOS under homogeneous conditions yielded no nanocomposite product under the reactant concentrations used in this study, and the implications are discussed in the following. Fifteen-nanometer-diameter gold colloid was used in experiments 1-4, whereas 65-nm was used in experiment 5 for convenience because within the range of CTAB concentrations used, 65-nm gold required no protection with a microporous silica layer. In all cases, samples were recovered by pressure filtration using 10 000 MWCO filter paper (Amicon) and the mesopore template was removed by calcination in air at 540 °C for 6 h. Procedure 1: Homogeneous Synthesis with TMOS in a Methanol/Water Cosolvent. The molar ratios of reactants using this procedure are shown in Table 1, experiments A-F. For experiment A, 13.7 mL of 15-nm gold colloid was mixed with 12.3 mL of deionized water. To this solution, 0.0282 g of CTAB previously dissolved in 17.3 mL of methanol was added. The pH of the colloid was then raised to approximately 10 with the addition of 50 µL of a 1 N NaOH solution. Finally, 90 µL of TMOS was added, and the solution was stirred at room temperature.

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Table 2. Molar Ratios of Reactants and Respective Particle Sizes for the MNSGC Synthesized Using TEOS under Homogeneous and Heterogeneous Reaction Conditions expt

C2H5OH

H2O

NaOH

TEOS

CTAB

particle sizea (nm)

H I

611

2360 2379

0.161 0.161

1 1

0.54 0.269

280 160 (L); 95 (W)

a L and W refer to the average length and width of the nanoparticles.

Table 3. Molar Ratios of Reactants and Respective Particle Sizes for the MNSGC Synthesized Using TPOS under Heterogeneous Reaction Conditions expt

H 2O

NH3

TPOS

CTAB

particle sizea (nm)

J

2000

30.24

1

0.123

520 (L); 390 (W)b 600 (L); 330 (W)c

a L and W refer to the average length and width of the nanoparticles. b Top particles in Figure 11. cBottom particles in Figure 11.

After approximately 10 min, the suspension became slightly turbid, and after 2 h, the suspension was filtered. A dark red precipitate was recovered, rinsed with water followed by methanol, and then air-dried. Procedure 2: Heterogeneous Synthesis with TMOS in a Pure Water Solvent. The molar ratios of the reactants used in this procedure are shown in Table 1, experiment G. In the first step, 16.1 mL of gold colloid was mixed with 0.0645 g of CTAB previously dissolved in 34.9 mL of deionized water. The pH of the suspension was then raised to approximately 10 with the addition of 250 µL of a 1 N NaOH solution. The subsequent addition of TMOS, observations, and the treatment of the precipitate matched those of procedure 1. Procedure 3: Homogeneous Synthesis with TEOS in a Water/Ethanol Cosolvent. The molar ratios of the reactants used in this procedure are shown in Table 2, experiment H. In the first step, 13.7 mL of 15-nm gold colloid was mixed with 12.3 mL of deionized water. To this solution was added 0.121 g of CTAB, previously dissolved in 17.3 mL of ethanol. The pH of the suspension was then raised to approximately 10 with the addition of 50 µL of a 1 N NaOH solution. Finally, 137 µL of TEOS was added, and the solution was stirred at room temperature. The subsequent observations and treatment of the precipitate also matched those of procedure 1. Procedure 4: Heterogeneous Synthesis with TEOS in a Pure Water Solvent. The molar ratios of the reactants used in this procedure are shown in Table 2, experiment I. In the first step, 13.7 mL of 15-nm gold colloid was mixed with 0.06 g of CTAB previously dissolved in 12.6 mL of deionized water. The pH of the suspension was then raised to approximately 10 with the addition of 50 µL of a 1 N NaOH solution. The addition of TEOS, observations, and the treatment of the precipitate matched those of procedure 3. Procedure 5: Heterogeneous Synthesis with TPOS in a Pure Water Solvent. The molar ratios of the reactants used in this procedure are shown in Table 3, experiment J. In the first step, 16.4 g of ammonium hydroxide (27 wt %, NH3 in water) was added to 329 mL of a gold colloid solution. Next, 0.425 g of CTAB was added with rapid stirring, and after 5 min, 2.73 mL of TPOS was added. After approximately 10 min, the suspension became slightly turbid, and after 2 h, the solution was filtered to recover a pale red precipitate.

Characterization Techniques X-ray Diffraction. Powder samples, each weighing 100 mg, were evenly dispersed onto clean glass slides. Patterns were collected using a Scintag XDS 2000 diffractometer with a diffractometer beam monochromator and a Cu KR radiation source. Scattering patterns were collected from 1.5 to 10° with a scan time of 5.0 s/0.01° step. Transmission Electron Microscopy (TEM). Micrographs were obtained using either a Philips CM120

electron microscope operating at 125 kV with a 35-µm objective aperture (Electron Microscopy Facility, University of Chicago) or a Hitachi H-600 operated at 80 kV with a 35-µm objective (Department of Biology, University of Notre Dame). Micrographs were recorded at a magnification of 125 000 or higher using Kodak SO163 film that was developed in D19 for 5 min. Specimens for sectioning were embedded in LR White resin and cured prior to sectioning 70-nm-thick samples. Both sectioned and assynthesized samples were picked up on a carbon-coated copper grid (Ted Pella). The cluster number and gold seed occupancy were determined by counting an average of 30 particles. Liquid Nitrogen Isotherms. Adsorption isotherms were obtained using a Quantachrome Autosorb-1 (Department of Chemical Engineering, University of Notre Dame). Powder samples, each weighing 20 mg, were outgassed to less than 5 mTorr at 200 °C for 3 h. The adsorption of liquid nitrogen was recorded using equilibration times of approximately 10 min per point. Surface area calculations were made using the BrunauerEmmett-Teller equation fitted to the first 10 points of each isotherm. Pore size distributions were calculated using Schmidt et al.’s modification of the Kelvin equation to account for mono- and multilayer adsorption.23 Results and Discussion 1. Homogeneous Synthesis with TMOS in a Methanol/Water Cosolvent. Synthesis using TMOS in a methanol/water cosolvent yielded spherical MNSGCs, with a range of particle diameters from 80 to 170 nm. The samples A-F formed colloidal suspensions that remained stable under quiescent conditions for several days without precipitation. Samples could be filtered and the particles resuspended in deionized water by mild sonication. For samples A-C, the concentration of CTAB was increased from 1.6 to 3.4 to 6.9 mM while holding all other reactant concentrations constant under the same conditions. These concentrations are well below 27 mM, the cmc (cmc1) of CTAB in water and 40 wt % methanol.20 It is unlikely that any pure surfactant micelles are present prior to silica/ CTAB self-assembly. Anderson and co-workers observed no significant changes in the cmc1 of CTAB with changes in the ionic strength of the solution, and, thus, the addition of TMOS with subsequent silica hydrolysis is not likely to affect the cmc1 of CTAB.20 This result has significant implications on the reaction mechanism of the MNSGCs, indicating that the formation of a mesoporous shell cannot proceed via a preliminary step involving the precipitation of a purely organic liquid crystal supporting structure prior to silica condensation. Firouzi and co-workers suggested that an important step in the formation of bulk mesoporous silica is the formation of CTAB/silica primary particles (see Figure 1).14 Screening of the electrostatic repulsion between adjacent silica oligomers and a low CTAB dissociation constant would make self-assembly possible at concentrations well below the cmc1 of pure CTAB, therefore meeting the requirements for synthesis of silica gold nanocomposites in this study.24,25 The X-ray diffraction patterns and corresponding unit cell parameters obtained for samples A-F are shown in Figure 2 and Table 4, respectively. The diffraction peaks (23) Schmidt, R.; Hansen, E. W.; Stocker, M.; Akporiaye, D.; Ellestad, O. H. J. Am. Chem. Soc. 1995, 117, 4049. (24) Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schu¨th, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176. (25) Huo, Q.; Feng, J.; Schuth, F.; Stucky, G. D. Chem. Mater. 1997, 9, 14.


Langmuir, Vol. 19, No. 18, 2003 7631

Figure 1. Cartoon illustrating the expected structure of selfassembling primary particles at high and low CTAB/silica molar ratios.

Figure 3. TEM micrograph of a MNSGC particle of sample A, prepared under homogeneous conditions with TMOS and charged template CTAB. The inset is an expanded section where the light gray ridges are condensed silica walls and the black dots are mesopores that have aligned between them.

Figure 2. Powder X-ray diffraction patterns of MNSGC particles of samples A-F (homogeneous solvent) and sample G (heterogeneous solvent), as-synthesized and calcined, with TMOS and charged template CTAB. Table 4. X-ray Diffraction Data for All Samples as-synthesized expt

int.a

2θ

db (nm)

A B C D E F G H I J

2676 6428 5290 3744 2818 3549 5251 2620 2817 3777

2.74 2.32 2.37 2.42 2.37 2.53 1.91 2.68 2.95 2.52

3.22 3.80 3.72 3.65 3.72 3.49 4.62 3.29 2.99 3.50

calcined åc (nm)

int.a

2θ

db (nm)

åc (nm)

3.72 4.39 4.30 4.21 4.30 4.03 5.34 3.80 3.46 4.04

5133 5523 4732 4803 3012 2814 6562 2584 4850 20 141

2.95 2.43 2.50 2.66 2.65 2.85 1.94 3.02 3.33 2.53

2.99 3.63 3.53 3.32 3.33 3.10 4.55 2.92 2.65 3.49

3.45 4.19 4.08 3.83 3.85 3.58 5.25 3.38 3.06 4.03

a

Intensity. b Spacing for the (100) Bragg reflection plane. c Unit cell spacing.

in all the patterns could be indexed to the (100) diffraction plane of a hexagonal unit cell. The broadness of the leading peak at the 2θ values indicated in Table 4 suggests that the mesopores in these materials are not well-aligned, typical of samples prepared under homogeneous conditions.19 Upon calcination, the first peak in each sample shifted to higher 2θ values and either maintained or increased in intensity, indicating that all the samples remained stable. For as-synthesized sample A, the pore center-to-pore center distance was 3.7 ( 0.4 nm, considerably less than that observed for samples B and C at 4.4 ( 0.3 nm and 4.3 ( 0.3 nm, respectively. The more closely spaced pores in sample A can be explained by the lower molar ratio of CTAB/TMOS in the starting mixture of sample A at 0.126, compared to those of samples B and C at 0.27 and 0.54, respectively. A lower CTAB/TMOS ratio would be expected to lead to the formation of smaller CTAB/silica primary particles with fewer bound CTAB molecules (see Figure 1). Primary particles with CTAB headgroups spaced far

Figure 4. Effects of solvent conditions on the aspect ratio of self-assembling CTAB/silica rod micelles and, subsequently, the direction of particle growth.

apart would pack with a higher degree of headgroup curvature to maximize the hydrophobic tail dispersion interactions, leading to smaller pore diameters and more closely packed pores.26 In Figure 3 is shown a TEM micrograph of a single as-synthesized particle of sample A. The gold particle is at the core and is surrounded by disordered mesopores extending throughout the entire structure. We know that each gold particle is at the core of the nanocomposite and not on the surface because the gold is always observed at the center of the particle in the TEM micrograph. If the gold particle were on the surface, we would expect to find it in a variety of positions depending on the rotation of the nanocomposite relative to the direction of the electron beam. The phrase “disordered mesopores” means that the material contains pores that are clearly visible using TEM but are poorly aligned in the particle.19 The origin of this effect is related to the presence of the alcohol cosolvent, which lowers the solvent dielectric constant leading to shorter rod micelle lengths and an increasing frequency of defects in the long-range stacking of the micelles (see (26) Fontell, K.; Khan, A.; Lindstro¨m, B.; Maciejewska, D.; PuangNgern, S. Colloid Polym. Sci. 1991, 269, 727.

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Figure 5. TEM micrograph of as-synthesized MNSGC particles of samples A-F, prepared under homogeneous conditions with TMOS and charged template CTAB.

Figure 4).27 The high number of defects and subsequent nondirectional self-assembly of the rod micelles are also responsible for the spherical shape of the silica shell. The inset in Figure 3 shows an expanded section of the particle where the light gray ridges are condensed silica walls and the black dots are mesopores that have aligned between them. The average pore center-to-pore center distance is 3.5 ( 0.4 nm and agrees well with that from X-ray diffraction. A lower magnification micrograph of several particles of sample A is shown in Figure 5. All particles contain at least one gold particle (100% occupancy), separated by a thin layer of silicate, with an average cluster size of 5 nanoparticles per silica shell. For samples B and C, where the concentrations of CTAB were increased, the average cluster sizes dropped to 1.2 and 1, respectively (see Figure 5). The cluster size reflects the flocculation of silica-coated gold seeds at the start of the experiment prior to silica mesophase growth. No significant gold seed clustering was observed using TEM prior to the addition of TMOS and CTAB for any of the experiments described. Gold clustering can be detected by a small red shift in the plasmon absorption peak of the colloidal gold.8 The small cluster size observed even with a relatively high concentration of the CTAB template suggests that there is a competition between CTAB and either the silicacoated gold particles or oligomeric silica in solution. Typically, when an amphiphilic surfactant is added to silica colloid, the surfactant coats the surface of each particle and then particles flocculate via van der Waals interactions between carbon chains.28 Because CTAB is (27) Auvray, A.; Petipas, C.; Anthore, R.; Rico, I.; Lattes, A. J. Phys. Chem. 1989, 93, 7458.

also a salt and should destabilize the electric double layer of the gold/silica seed, it would be expected that higher concentrations of CTAB should lead to larger cluster sizes. Instead, it appears that at low CTAB/silica ratios, smaller primary particles, with fewer associated surfactant tails, are formed. These have a weaker dispersion potential and lower chemical activity toward self-assembly, and as a result, the CTAB-coated gold seeds are free to flocculate. For sample A, the reaction of CTAB/silica/gold seeds with other seeds occurred with greater frequency than that with other silica/CTAB primary particles, resulting in the high cluster number of 5. Higher CTAB/TMOS ratios, as were exhibited by samples B and C, formed greater numbers of larger CTAB/silica primary particles, with a correspondingly greater dispersion potential and reactivity (see Figure 1). These would assemble more quickly than the silica/gold seeds could flocculate and, hence, lead to lower cluster numbers. The occupancies for samples B and C also dropped to 70 and 41%, respectively, indicating that the probability of self-nucleation for larger primary particles also increased. For sensing and quantum-dot applications, the control of the occupancy and cluster numbers lies within the range of CTAB concentrations used in samples A and B. At the same time, for samples A-C, the particle size also dropped from approximately 173 to 100 to 93 nm with an increase in the CTAB concentration. For sample B, over 80% of the mesoporous silica/gold particles are fused together and, overall, the connectivities for samples B and C are higher than that for sample A. This result can also be attributed to the enhanced reactivity of larger (28) Iller, R. K. The Chemistry of Silica; Wiley: New York, 1979.


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Figure 6. Liquid nitrogen adsorption isotherm of MNSGC particles of sample C, calcined at 540 °C for 6 h. The inset shows the pore radius distribution determined using the modified Kelvin equation.21

and more numerous CTAB/silica primary particles or possibly a final coating of particles with residual CTAB and subsequent silica particle flocculation.19 A liquid nitrogen isotherm of sample C was taken, and the result is shown in Figure 6. The isotherm is type IV in IUPAC classification.29 No hysteresis was observed in the desorption branch of the first step, relating to capillary condensation typical of unidimensional, mesoporous-type materials.23 A total mesopore volume of 0.53 cm3 g-1 was recorded at 0.75 P/P0 and is slightly lower than 0.64 cm3 g-1 obtained for nonseeded growth nanoparticles.19 The second step starting at 0.9 P/P0 was assigned to adsorption between nanoparticles, giving a total sample capacity of 1.12 cm3 g-1. The inset of Figure 6 shows a pore radius distribution graph determined using the Kelvin equation with an average mesopore radius of 1.16 nm.23 Upon subtracting the pore diameter from the unit cell length, we obtained a very large pore wall thickness of 1.76 nm. However, it is thought that the Kelvin equation underestimates the pore diameter, and this could be partly responsible for the large pore wall thickness.30 An alternative is to use the method of Galarneau and coworkers, which combines X-ray diffraction with the total adsorption capacity and, therefore, avoids the problems associated with mesopore wall adsorption.31 Using this method, we have obtained a pore radius of 1.57 nm and a pore wall thickness of 1.10 nm. This pore wall thickness matches closely to conventional mesoporous MCM41 powder at approximately 1 nm.1 The MNSGC has a surface area of 690 m2 g-1 and is slightly lower than that obtained for nonseeded growth nanoparticles at 917 m2 g-1.19 For sample D, the silica concentration was reduced to 5.35 mM from the 12.8 mM used for samples A-C. At the same time, the CTAB/TMOS ratio was held at 0.54, as in sample C, corresponding to a CTAB concentration of 2.88 mM. These conditions yielded a silica-coated gold seed occupancy of 95% and a cluster number of 1 (see Figure 5). These nanoparticles were highly monodispersed in size with an average diameter of 120 ( 5 nm. For sample E, the ratio of CTAB/TMOS was further increased to 0.67, while the overall concentrations were further reduced by a factor of 2.2 (Table 1). The occupancy remained high at 98%, but the cluster number increased slightly to 1.7 (see Figure 5). The intensity of the X-ray diffraction peak of sample E was lower than that for samples A-D, indicating that the high CTAB ratio was (29) Sing, K. S. W. Pure Appl. Chem. 1985, 57, 603. (30) Maddox, M. W.; Gubbins, K. E. Int. J. Thermophys. 1994, 15, 1115. (31) Galarneau, A.; Desplantier, D.; Dutartre, R.; Di Renzo, F. Microporous Mesoporous Mater. 1997, 27, 297.

Figure 7. TEM micrograph of as-synthesized MNSGC particles of sample G, prepared under heterogeneous conditions with TMOS and charged template CTAB.

now affecting the mesopore self-assembly and stability. Galarneau and co-workers also studied the effects of changing the CTAB/TMOS ratio on the stability of bulk mesoporous silica and found that upon increasing the ratio the thickness of the silica wall thinned and the stability of the mesoporous silica was reduced.32 In sample E, primary particles with relatively small silica oligomers and large CTAB clusters were likely present, leading to the instability in the mesopore assembly. For the final experiment in this set, sample F, the silica concentration was set at 12.8 mM, the same as in samples A-C, while the CTAB/TMOS ratio was increased to 1.24, corresponding to a CTAB concentration of 15.9 mM. A cluster number of 1 was obtained just like in experiment C (see Figure 5). For sample F, three different particle sizes were obtained: gold seeded growth particles at about 80 nm in diameter and non-gold seeded particles with bimodal size distributions of 69 and 29 nm. The occurrence of a large number of non-gold seeded particles indicates a large degree of self-nucleation. 2. Heterogeneous Synthesis with TMOS in a Pure Water Solvent. Heterogeneous MNSGC synthesis with TMOS in a water solvent, sample G, yielded elliptical particles with an average length of 97 nm and width of 75 nm. The X-ray diffraction pattern and corresponding data for sample G are shown in Figure 2 and Table 4, respectively. The first peak was shifted to a low angle and appeared as a shoulder on the background radiation. The peak was indexed to the (100) diffraction plane of a hexagonal unit cell, with an as-synthesized unit cell length of 5.34 nm, and was considerably larger than the values recorded for samples A-F prepared under homogeneous conditions. This is expected, because there are no alcohol groups to interfere with the packing of the CTAB headgroups, allowing larger micelles to form.26 Anderson and co-workers also observed a significant increase in the pore size of mesoporous silica powders under heterogeneous conditions.16 Upon calcination of this sample, the (100) diffraction peak shifted to 4.55 nm. Figure 7 shows a TEM micrograph of the as-synthesized sample G. The gold seed occupancy is about 70%, and the cluster number is 1.7. The CTAB concentration at 3.45 mM is higher than the cmc1 limit for CTAB in pure water at 2.1 mM, and, therefore, spherical micelles existed prior (32) Galarneau, A.; Desplantier-Giscard, D.; Di Renzo, F.; Fajula, F. Catal. Today 2001, 68, 191.

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Figure 8. Powder X-ray diffraction patterns of MNSGC particles of samples H (homogeneous solvent) and I (heterogeneous solvent), as-synthesized and calcined, with TEOS and charged template CTAB.

to self-assembly.20 However, the concentration is 2 orders of magnitude below the cmc2 value at 0.3 M for the formation of rod-shaped micelles and 3 orders below the concentration for the self-assembly of CTAB into a hexagonal liquid crystal at 1.1 M.24,33 It is clear that silica oligomerization continues to play an important role in the formation and self-assembly of silica/CTAB primary structures. However, unlike under homogeneous conditions, we cannot exclude the contribution of small purely CTAB micellar structures. The occupancy and cluster number for sample G is comparable to that of sample B under homogeneous conditions at a similar high CTAB/TMOS ratio, showing again that the activity of CTAB/silica primary particles toward self-assembly became competitive with gold seed flocculation at higher CTAB/silica ratios. The difference of 0.5 in the cluster numbers is likely due to the lower solubility of CTAB in water. Under these conditions, it is likely that more CTAB would bind at the gold seed surface and lead to more clustering. For sample G in Figure 7 the position of gold in the majority of particles was located close to the edge. This reproducible positioning of the gold particle is suggestive of directional growth of the mesoporous shell. Under heterogeneous conditions, the aspect ratio of rodlike micelles of CTAB is almost infinite in length prior to the hexagonal liquid-phase transition16,27 and is probably the reason for the directional growth of these particles. 3. Homogeneous Synthesis with TEOS in a Water/ Ethanol Cosolvent. Homogeneous synthesis with TEOS in an ethanol/water cosolvent, sample H, yielded spherical MNSGCs with an average particle diameter of 280 nm. The X-ray diffraction pattern and corresponding data for sample H are shown in Figure 8 and Table 4, respectively. The pattern is indexed to the (100) diffraction plane of a hexagonal unit cell with a unit cell length of 3.80 nm. The TEM micrograph of the as-synthesized sample H is shown in Figure 9. The molar ratio of CTAB/TEOS of 0.54 and the concentration of CTAB at 7.0 mM matched very closely with those of experiment C for homogeneous synthesis using TMOS, with a CTAB/TMOS ratio of 0.54 and a CTAB concentration of 6.9 mM. However, the occupancy of gold per silica shell for synthesis using TEOS at 100% is much greater than that for synthesis with TMOS at 41%. This difference may be related to the rate of hydrolysis that is 1-2 orders faster for TMOS over TEOS.16 A slower rate of hydrolysis would lead to a lower initial concentration of monomeric silica and, in turn, a lower concentration of silica oligomers.34 With less silica (33) Choudhary, S.; Yadav, R.; Maitra, A. N.; Jain, P. C. Colloids Surf., A 1994, 82, 49. (34) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic: New York, 1990.

Figure 9. TEM micrographs of as-synthesized MNSGC particles of samples H (homogeneous solvent) and I (heterogeneous solvent), with TEOS and charged template CTAB. The inset in the micrograph of I is expanded from the circled section to highlight the ordered mesopore structure.

oligomers in solution at the start of the experiment, the probability of collision and assembly at the gold seed increases, leading to the higher occupancy. For synthesis using TEOS and TMOS, an average cluster number of 1 gold nanoparticle per silica shell was observed. As was already stated, this result does not fit with the expected behavior of silica particles at a high CTAB/silica ratio. It may be that larger silica/CTAB primary particles form with a greater number of associated surfactant tails and, subsequently, a lower solubility. As a result, even at a lower concentration, the primary particles would bind and self-assemble at the gold seed surface and reduce the probability of flocculation. The CTAB concentration used was 7.0 mM, and given the very low dielectric constant of ethanol at 24.3, this concentration is well below cmc1. As with studies using TMOS, there can be no precipitation of a purely surfactant phase prior to silica condensation, and, therefore, an important step is the formation of silica/ CTAB primary particles. 4. Heterogeneous Synthesis with TEOS in a Pure Water Solvent. Heterogeneous synthesis with TEOS in a water solvent, sample I, yielded MNSGCs with an average particle length of 160 nm and width of 95 nm. The X-ray diffraction pattern and corresponding data for sample I are shown in Figure 8 and Table 4, respectively. A narrow peak and a low intensity broad peak were observed and indexed to a hexagonal unit cell, with a unit cell length of 3.46 nm. The narrowness of the first peak indicated highly uniform pore center-to-pore center distances and a long-range alignment of mesochannels,


consistent with the results of the other heterogeneous synthesis conditions discussed earlier. Unlike the previously described syntheses employing TMOS as the silicate source, where switching from homogeneous to heterogeneous conditions increased the pore center-to-pore center distance by about 1.0 nm, switching to heterogeneous synthesis conditions using TEOS as the silicate source yielded particles with the pore center-to-pore center distances smaller by about 0.36 nm. This result also contrasts with TEOS nonseeded growth methods, where increases in the pore center-topore center distances of 0.6 nm were observed upon the switch from homogeneous to heterogeneous conditions.19 The unexpected pore shrinkage observed here is likely due to a combination of effects from the gold seed and hydrolysis of the TEOS molecules. First, the gold seed would remove the requirement of a nucleation step speeding up the overall self-assembly process. Next, the rate of hydrolysis of TEOS is slower than that of TMOS, and, therefore, self-assembly may proceed at the gold seed without the complete hydrolysis of alkoxide groups on the silica species. Finally, the release of ethanol molecules from alkoxide hydrolysis reactions in silica/CTAB primary particles would disrupt micelle formation and lead to a reduction in the pore diameter.26 The TEM micrograph of the as-synthesized sample I is shown in Figure 9. Unlike the other samples, the MNSGCs were faceted with an overall hexagonal symmetry. The inset shows an expanded section of a single particle where the mesochannels are highlighted using parallel black lines. The pore center-to-pore center distance at 3.2 nm is slightly shorter than the 3.46 nm recorded from X-ray diffraction, and the difference is likely due to TEM artifact. The mesochannels align parallel to the longest side of the crystal, and it is most probable that directional growth occurred along this direction. Similar to synthesis under homogeneous conditions, the seed occupancy for synthesis using TEOS under heterogeneous conditions at 100% is greater than that for synthesis with TMOS at 70%. Again, the difference is likely due to the slower rate of hydrolysis of TEOS as compared to that of TMOS. The cluster number is low at 1.2 and, again, fits with the hypothesis that mesopore self-assembly is favored over seed flocculation at high CTAB/TMOS ratios. For the majority of the MNSGC, the gold particles are located close to the particle edge, in agreement with those synthesized using TMOS. 5. Heterogeneous Synthesis with TPOS in a Pure Water Solvent. No particle formation was observed under homogeneous conditions with TPOS. This is likely due to slow and incomplete hydrolysis of the alkoxide groups of TPOS in the low-dielectric-constant medium.20 Synthesis under heterogeneous conditions with TPOS, sample J, yielded large particles with highly faceted morphologies and ordered mesopores. The phrase “ordered mesopores”, in contrast to the disordered mesopores described previously, refers to a material that contains pores that align over a long range to form a pattern observable with TEM.19 The concentration of the 65-nm gold particles at 3.6 × 1010 particles/mL is approximately 2 orders of magnitude lower than that used in the previous studies at 2.9 × 1012 particles/mL. The low concentration reduced the probability of flocculation between gold seed particles, leading to an average cluster number of 1. However, the selfnucleation of silica/CTAB primary particles increased and led to a low gold seed occupancy of 1%. The driving force for self-nucleation under heterogeneous conditions is the high dielectric constant of water at 78, in which silica/ CTAB primary particles would form with larger dipole moments and, as a consequence, a greater activity toward

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Figure 10. Powder X-ray diffraction patterns of MNSGC particles of sample J, as-synthesized and calcined, prepared under heterogeneous conditions with TPOS and charged template CTAB.

self-assembly. The CTAB concentration at 3.4 mM is greater than the cmc1 for CTAB in pure water at 2.1 mM, and, therefore, small purely CTAB micellar structures may be important.20 The X-ray diffraction patterns and corresponding data for sample J are shown in Figure 10 and Table 4, respectively. Four peaks are clearly visible for both the as-synthesized and the calcined samples and were indexed to a hexagonal unit cell. Upon calcination, the first peak, indexed to the (100) Bragg reflection, remained at almost a constant position but increased in intensity almost 5 times. However, the sample did not improve in long-range crystallinity because the fourth peak, indexed to the (210) Bragg reflection, broadened significantly upon calcination. For this sample, the powder became very fine upon calcination, allowing a better distribution of powder crystallites on the sample plate in the diffractometer, contributing to the improved intensity reading. The TEM micrograph showing the mesochannel structure of a MNGSC of sample J is shown in the bottom micrograph of Figure 11. The mesopores are clearly ordered with a pore center-to-pore center distance of 3.6 nm, slightly smaller than the 4.04 nm recorded from X-ray diffraction, but the small difference is likely a TEM artifact due to tilting of the sample in the electron beam. The gold particle appeared at the edge of the majority of the particles, with mesopores extending along the length of the particle in the direction of self-assembly, as is shown in the middle micrograph of Figure 11. The top micrograph shows a MNSGC with an overall nanoparticle shape like that of the gold seed. However, more work is needed to show that this mimicking characteristic is reproducible. 6. Mechanism of Self-Assembly. The results of the work presented previously were used to construct a mechanism for self-assembly of a MNSGC. We propose that it proceeds via three main stages: (1) the hydrolysis of alkoxysilanes and subsequent condensation reactions to form silica oligomers; (2) the formation of silica/CTAB primary particles; and (3) mesopore growth via either aggregation of primary particles or deposition of monomeric silica and CTAB molecules. The mechanism proposed herein builds on the work of others for the synthesis of bulk mesoporous silicate materials and agrees fundamentally with our previous mechanism proposed for the nonseeded growth of mesoporous silica particles. However, the more extensive approach used in this study has enabled us to develop the model further.19 A scheme of the selfassembly mechanism is presented in Figure 12. The first stage in the MNSGC assembly follows basic silica chemistry, where the rate-limiting step is the hydrolysis of alkoxysilanes, whose rate decreases from TMOS to TEOS to TPOS.20 Monomeric silica species, in

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Figure 12. Scheme of the proposed mechanism for the formation of MNSGC particles via three main stages: (1) the hydrolysis of monomeric silicon esters and subsequent condensation reactions to form silica oligomers; (2) the formation of silica/CTAB primary particles; and (3) mesopore growth via either aggregation of primary particles or deposition of monomeric silica and CTAB molecules.

Figure 11. TEM micrographs (top two) that contain assynthesized MNSGC particles of sample J, prepared under heterogeneous conditions with TPOS and charged template CTAB. The bottom micrograph is an expanded section of a particle to highlight the ordered mesopore structure.

the form of Si(OR)4-nOn-n, where R is the organic ester, would begin to form silica oligomers almost immediately upon addition to water. Frasch and co-workers have also suggested that silica oligomers formed prior to mesoporous surfactant/silica precipitation, and any existing CTAB micelles simply act as reservoirs, supplying surfactant ions to the preformed silicate polymers.35 Although the first stage should be thought of not as a self-assembly process but as a precursor step because no mesopore assembly has occurred, it does affect the gold seed occupancy. The slower rates of hydrolysis for the case of TEOS and TPOS lowered the initial concentration of active primary particles at the start of the experiment and reduced the likelihood of primary-particle self-nucleation. The second stage involves the formation of silica/CTAB primary particles that probably interact via very strong

multidentate linkages between the ammonium headgroup and the siloxide ions.14 This step is the key step in the self-assembly of MNSGC. The interactions of a selfassembling silicate system have been described by Huo and co-workers as a four-part free energy term that includes (1) interactions within the silica wall; (2) forces between carbon chains in the surfactant phase; (3) interactions at the interface of the silica and organic phases; and (4) changes in the chemical potential of the solvent.24 This same point of view should be taken for MNSGC synthesis, with the additional complication that these contributions should be further separated into parts, one for primary-particle formation and one for subsequent primary-particle aggregation. The structure of the primary particles in the synthesis of the MNSGC may be equivalent to that of the precursor species in the synthesis of bulk mesoporous silica. Firouzi and co-workers using NMR under specific conditions observed small precursor species consisting of a double four-ring silica oligomer attached to one or two CTAB molecules that were able to assemble reversibly into bulk mesoporous silica.14 Evidence for the presence of larger primary particles has been given by Cai and co-workers, who proposed a mechanism for mesoporous silicate formation involving large silicate/surfactant rod micelles that pack together in an ordered fashion.12 In MNSGC (35) Frasch, J.; Lebeau, B.; Soulard, M.; Patarin, J.; Zana, R. Langmuir 2000, 16, 9049.


formation, high CTAB/TMOS ratios led to both lower silicacoated gold seed occupancies and lower cluster numbers and is consistent with the formation of larger CTAB/silica primary particles with a greater dispersion potential and higher reactivity. The third stage, aggregation of primary particles, may proceed via one or more of three different routes: (1) directional aggregation, where primary particles agglomerate together in an ordered fashion;36 (2) nondirectional aggregation, where primary particles aggregate in a disordered fashion;36 or (3) continued deposition of monomeric silica and surfactant ions onto existing primary particles. Sample J synthesized under heterogeneous conditions with TPOS with highly faceted morphologies and ordered mesopores likely proceeded via route one. Sample A containing the disordered mesopores prepared under homogeneous conditions and a low CTAB/TMOS ratio likely proceeded via route 2. It is conceivable that mesopore self-assembly can proceed via route 3, the addition of monomeric silica and CTAB surfactant. In fact, this is the mechanism behind the La Mer method for the synthesis of monodispersed microporous silica particles in which silica nucleation sites are coated with successive additions of monomeric silica at very low concentrations (2000 ppm for sample A), leading to considerable silica oligomerization prior to self-assembly, and, therefore, synthesis most probably proceeds via assembly of primary particles in route 1 or 2. Primary-particle assembly is also the accepted mechanism for the synthesis of monodispersed microporous silica particles via the more common Sto¨ber method.9,38 (36) Ocana, M.; Rodriguez-Clemente, R.; Serna, C. J. Adv. Mater. 1995, 7, 212. (37) La Mer, V. K.; Dinegar, R. H. J. Am. Ceram. Soc. 1950, 72, 4847. (38) Van Blaaderen, A.; Van Geest, J.; Vrij, A. J. Colloid Interface Sci. 1992, 154, 481.

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Conclusion The effects of the CTAB concentration, silica source, and solvent conditions on the liquid-phase seeded growth of the MNSGC were investigated. In general, increasing the CTAB/TMOS ratio reduced the flocculation of gold seed particles and also increased the probability of nonseeded self-assembly. A silica concentration of 5.35 mM and a CTAB/TMOS ratio of 0.54 yielded a MNSGC with a gold occupancy of 95% and a cluster number of 1. Changing the solvent conditions from heterogeneous to homogeneous using either TMOS or TEOS yielded particles with either spherical or faceted morphologies, respectively. Finally, a mechanism for self-assembly was proposed that proceeds via three main stages with the key step being the formation of silica/CTAB primary particles. MNSGC materials have potential applications in drug delivery, quantum-dot technology, and the design of electromechanical devices. In the future, it may be possible to substitute silica with oligomers of different materials and test if these can produce novel primary particles with CTAB and, subsequently, new nanostructured materials. Acknowledgment. We thank Professor Arvind Varma and Professor Paul J. McGinn from the Department of Chemical Engineering, University of Notre Dame, for use of the Quantachrome Autosorb-1 and the Scintag XDS 2000 diffractometer, respectively. The research described here was partially supported by the U.S. Department of Energy Office of Basic Energy Sciences. This is document no. NDRL-4420 of the Notre Dame Radiation Laboratory. Note Added after ASAP Posting. This article was released ASAP on 7/25/2003 with minor errors in the captions of Figures 10 and 11. The correct version was posted on 8/8/2003. LA034522E

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