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Size Effects on the Microchemistry and Plasticity of Sto1 ber Silica Particles: A Study Using EFTEM, FESEM, and AFM-SEPM Microscopies Carlos Alberto Rodrigues Costa,† Carlos Alberto Paula Leite,† Elizabeth Fa´tima de Souza,‡ and Fernando Galembeck*,† Instituto de Quı´mica, Universidade Estadual de Campinas, UNICAMP, PO Box 6154, CEP 13083-970, Campinas SP, Brazil and Instituto de Cieˆ ncias Biolo´ gicas e Quı´mica, Pontifı´cia Universidade Cato´ lica de Campinas, PUC-Campinas, PO Box 1111, CEP 13020-904, Campinas SP, Brazil Received March 16, 2000. In Final Form: October 19, 2000 Two different samples of monodisperse Sto¨ber silica particles were examined using three different microscopies: energy-filtered analytical transmission electron microscopy (EFTEM), high-resolution fieldemission scanning electron microscopy (FESEM), and scanning probe microscopy, in the noncontact atomic force (AFM) and scanning electric potential microscopy (SEPM) modes. Upon drying the silica dispersions, the larger (ca. 141 nm) particles were only partially deformed by capillary adhesion, whereas the smaller particles (ca. 36 nm) were strongly deformed and closely packed into dense films of a low porosity, which is evidence of their larger plasticity, or superplasticity. Electric potential distribution maps obtained by SEPM showed a significant interparticle as well as intraparticle contrast, especially in the case of the smaller particles. Examination by electron backscattering also revealed a larger contrast among the smaller particles, thus evidencing a nonuniformity of chemical composition. The results are interpreted considering the changes in the synthetic medium and other aspects of the particle growth mechanism, and they point toward the possibility of exploiting the plasticity of the nanosized silica particles in the making of silica monoliths.
Introduction Size effects on the properties of nanosized colloidal particles have been receiving great attention, due to their interesting properties of quantum confinement1 and superplasticity.2 On the other hand, the access to monodisperse, well-defined particles is facilitated by many achievements during the past decades concerning the preparation of uniform colloid dispersions.3,4 It is currently possible to obtain many particle samples, prepared following a uniform procedure and covering a broad range of particle sizes. These particles are expected to have a uniform chemical composition, but this is seldom demonstrated experimentally. On the other hand, there is circumstancial evidence for nonuniform, size-dependent particle behavior, which is not easily understood. For instance, a recent attempt to prepare amorphous and crystalline monolayers of silica particles with diameters of 100, 200, 300, 400, 500, and 1000 nm met with a greater success when the 300 nm particles were used, as compared to the smaller or larger particles. This may be due to chemical differences among the particles of the various sizes or to some other factor, which has not yet been elucidated.5 * Corresponding author. † Universidade Estadual de Campinas. Tel.: +55 51 788 3080. Fax: +55 51 788 3023. E-mail:
[email protected]. ‡ Pontifı´cia Universidade Cato ´ lica de Campinas. Tel.: +55 51 729 8313. Fax: +55 51 729 8517. E-mail:
[email protected]. (1) Henglein, A. Chem. Rev. 1989, 89, 1861. (2) Galembeck, F.; Lima, E. C. O.; Masson, N. C.; Monteiro, V. A. R.; Souza, E. F. In Fine Particles Science and Technology: From Micro to Nanoparticles; Pelizzetti, E., Ed.; Kluwer: Dordrecht, 1996; p 267. (3) Matijevic, E. Chem. Mater. 1993, 5, 426. (4) Goodwin, J. W.; Ottewill, R. H.; Pelton, R,; Vianello, G.; Yates, D. E. Brit. Polym. J. 1978, 10, 173. (5) Dimitrov, A. S.; Tiwa, T.; Nagayama, K. Langmuir 1999, 15, 5257.
An elegant method of quasi-monodisperse spherical silica particle preparation was reported by Sto¨ber et al.6 many years ago, based on the hydrolysis of tetraethoxysilane (TEOS) in alcoholic media under catalysis by ammonia. Silanol groups are first formed by TEOS hydrolysis, and siloxane bridges are then formed by a silanol condensation reaction. Spherical particles are obtained when enough ammonia is present in the initial reaction mixture. The final particle size depends mainly on the initial water and ammonia concentrations, and the particles thus obtained cover the whole colloidal range. Sto¨ber silica particles have been used as model colloids in a large number7-9 of experimental investigations. In this laboratory, we have recently observed significant chemical heterogeneity within another important group of model colloids, the monodisperse polymer latex particles. This chemical heterogeneity affects particle aggregation, macrocrystallization, and film formation.10,11 This information is not yet available for other model colloidal particles; for this reason, we decided to do a detailed examination of Sto¨ber silica particles using different microscopy techniques. Experimental Section Preparation of the Silica Particles. Silica particles were prepared by the method of Sto¨ber et al.6 Reagent-grade ethanol and ammonium hydroxide were used. Tetraethyl orthosilicate (6) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62. (7) Van Helden, A. K.; Vrij, A. J. Colloid Interface Sci. 1980, 78, 312. (8) Kirkland, J. J. J. Chromatogr. 1979, 185, 273. (9) Kops-Werkhoven, M. M.; Fijnaut, H. M. J. Chem. Phys. 1981, 74, 1618. (10) Cardoso, A. H.; Leite, C. A. P.; Galembeck, F. J. Braz. Chem. Soc. 1999, 10, 497. (11) Cardoso, A. H.; Leite, C. A. P.; Galembeck, F. Langmuir 1999, 15, 4447.
10.1021/la000408+ CCC: $20.00 © 2001 American Chemical Society Published on Web 12/13/2000
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(TEOS) was supplied by Aldrich. Glassware was cleaned with 10% aqueous hydrochloric acid and rinsed with distilled water and absolute ethanol. The volumes of the components used were (i) 4 mL TEOS, 4 mL ammonium hydroxide (sat.) and 50 mL ethanol, and (ii) 4 mL TEOS, 2 mL ammonium hydroxide (sat.) and 50 mL ethanol. Particle formation was detected by an increasing opalescence of the mixture, starting between 1 and 5 min after the addition of TEOS, and reaction completion was evidenced by the stability of sample turbidity. Following the synthesis, ca. 50 mL of each as-prepared silica dispersion was stored in 100-mL glass bottles. Photon Correlation Spectroscopy (PCS). Effective diameter (by PCS) was measured in a ZetaPlus instrument (Brookhaven Instruments) with Bi-MAS software and a solid-state laser (15 mW, l ) 670 nm) as the radiation source. Samples were contained within 3-mL acrylic cuvettes. Energy-Filtered Analytical Transmission Electron Microscopy (EFTEM). A Carl Zeiss CEM 902 transmission electron microscope equipped with a Castaing-Henry energy filter spectrometer within the column was used. For individual silica particle examination, one drop of the silica dispersion (1% solids content) was deposited on carbon-coated parlodion films supported in 400-mesh copper grids (Ted Pella). High-Resolution Scanning Electron Microscopy (FESEM). Scanning electron microscopy was performed in a JEOL JSM 6340F instrument. Silica dispersion (1% w/w, 2 drops) samples were placed on top of mica sheets, previously mounted on a brass sample holder with double-face adhesive tape. The dry silica films were carbon-coated in a Bal-Tec MD 020 instrument. Atomic Force (AFM) and Scanning Electric Potential (SEPM) Microscopies. AFM microscopy was used to obtain topographic information on the films formed by drying silica dispersions, using a Topometrix Discoverer instrument. The sample was prepared by dropping 0.05 mL of 1% w/w silica dispersions on top of a mica sheet and allowing it to dry at room temperature. Topography changes were sensed by monitoring detector signal amplitude, at a 300 × 300 pixel resolution and 1.81 µm/s scan rate. The tips used were made of silicon coated with platinum and had a pyramidal form with a spherical 20-nm nominal radius tip, which was verified by imaging the tip in the scanning electron microscope. SEPM in this instrument uses the standard noncontact AFM setup, but with the following modifications: the Pt-coated conducting tip was fed with an AC signal, 10 kHz below the frequency of the normal AFM oscillator, which matches the natural frequency of mechanical oscillation of the cantilever-tip system (40-70 kHz). During a measurement, the mechanical oscillation of the tip was tracked by the four-quadrant photodetector and analyzed by two feedback loops. The first loop was used in the conventional way to control the distance between tip and sample surface, while scanning the sample at constant oscillation amplitude. The second loop was used to minimize the electric field between tip and sample: a second lock-in amplifier measured the AC frequency oscillation while scanning and added a DC bias to the tip to recover the undisturbed AC oscillation. This technique differs from that used by Terris,12 who measured the phase displacement of the AC voltage; in the Topometrix set up, we cancelled the phase displacement by DC biasing. The image was built using the DC voltage fed to the tip, at every pixel, thus detecting electric potential gradients throughout the scanned area. This technique is reminiscent of the oscillating electrode technique13 for monolayer study: both use an oscillating electrode separated from the sample by an air gap. The major difference between both techniques is the detection technique used, since SEPM uses a phase detection of the perturbations on the applied AC voltage. Image processing was performed in an IBM PC microcomputer using the Image-Pro Plus 4.0 (Media Cybernetics) or the Topometrix image analysis program. (12) Terris, B. D.; Stern, J. E.; Rugar, D.; Mamin, H. J. J. Vac. Sci. Technol., A 1990, 8, 374. (13) Shaw, D. J. Introduction to Colloid and Surface Chemistry; Butterworth: London, 1996.
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Results Particle effective diameters determined by PCS were 141.5 ( 2.5 and 36.5 ( 1.0 nm, and the larger particles were obtained in the presence of a higher ammonia concentration, as expected.6 EFTEM Images. Transmission electron micrographs are shown in Figure 1: bright-field images as well as elemental distribution maps are given for both particle samples. The larger particles are roughly spherical, and some interstitial material is seen as dark-gray areas (or “necks”) amidst the particles in the bright-field picture. The silicon and oxygen maps for both the larger and smaller particles show a major difference: the borders between neighboring particles are clearly seen in the Si map, but they are almost undistinguishable in the O map. This is observed in a more quantitative way in the densitometric line-scans presented in Figure 2. The scanned line is shown in the bright-field image in Figure 1, and measurements were made on both the Si and O maps. In these curves, the elevations correspond to the brighter domains (this means, the Si- or O- rich regions), and the valleys are the Si- and O-depleted interparticle regions. There are three main observations drawn from these line-scans: the sharp ups and downs in these curves correspond to large composition fluctuations, which are probably due to pores and to local changes in the Si/O elemental ratio. Moreover, the particle to the left appears broader in the O map than in the Si map, since the corresponding half-height widths in the line-scans extend for, respectively, 33 and 26 pixels. The smaller particle shapes (see the bottom bright-field image in Figure 1) are less regular than in the case of the larger particles. From the observations, we can draw three conclusions: (i) The larger particles have a well-defined core-andshell nature, and the outer shells appearing brighter in the O map than in the Si map have a higher O/Si content than the particle bulk. The shells are thus made out of silica with a higher degree of hydration than the bulk. (ii) The interstitial material between the particles is evidence for the presence of a nonparticulate, nondializable solute with a larger O/Si content than the particles bulk, probably a polysilicic acid. (iii) The smaller particles have a lower interfacial tension in the medium from which they were dried, which justifies both their smaller sizes and irregular shapes. FESEM Images. Backscattered electron image contrast depends largely on chemical composition fluctuations across the sample, as well as on local specimen surface inclination, crystallography, and internal magnetic fields. Elastic scattering efficiency increases with the atomic number of the target atom, and sample areas with a larger average atomic number scatter more strongly than areas with a lower average atomic number.14 The beam electrons actually penetrate to a significant sample depth before reversing their course and returning to the surface to escape as a backscattered electron. Electrons that travel along such trajectories may clearly be influenced by subsurface features of the specimen structure (e.g., inclusions of different composition, voids, etc.), and they carry information on that structure upon escaping. Therefore, the BEI signal is sensitive to bulk and surface composition at conventional SEM beam energies (>10 keV), and the brighter areas in the pictures are those richer in the heavier elements in a noncrystalline sample. Compared to the backscattered electrons, the (14) Goldstein, J. I.; Newbury, D. E.; Echlin, P.; Joy, D. C.; Romig, A. D., Jr.; Lyman, C. E.; Fiori, C.; Lifshin, E. Scanning Electron Microscopy and X-ray Microanalysis; Plenum: New York, 1990.
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Figure 1. Brightfield (bf) transmission electron micrographs and silicon and oxygen elemental maps of the larger (ca. 141 nm, top) and smaller (ca. 36 nm, bottom) silica particles. In the maps, the brighter pixels correspond to Si- or O-rich domains.
Figure 2. Line-scan profiles of pixel intensity across oxygen (upper curve) and silicon (lower curve) elemental images of the larger (ca. 141 nm) silica particles. Full white corresponds to a 255 Gy level, and full black corresponds to a zero gray level. Line-scans were measured across the arrow drawn in Figure 1.
secondary-electron coefficient is relatively insensitive to the local average atomic number.14 Field-emission scanning electron micrographs of the silica particles obtained both in the SEI and BEI modes are presented in Figure 3. As opposed to the TEM pictures, these samples were prepared with higher particle loadings, which is allowed in SEM but not in TEM due to the experimental peculiarities of both techniques (TEM detects unscattered electrons, or electrons inelastically scattered at low angles; SEM detects electrons scattered at large angles). Comparing the two SEI pictures, we observe that the larger particles are closely packed, but
the individual particles are still distinguished in the aggregates. On the other hand, the smaller particles are coalesced, forming a dense film with few pores that allows the identification of only a few individual particles. The BEI pictures give three important results: (i) In the larger particle sample, the particle borders are clearly seen, and the particles appear smaller and more clearly separated than in the SEI picture. Since a major factor for BEI imaging contrast is the local average atomic number, we conclude that the particle cores are richer in Si (the heavier element) than the particle shells. This conclusion is in exact agreement with the results from TEM elemental mapping, confirming that the shells are made out of a more hydrous silica than the cores. (ii) There are bright areas in the background of the BEI micrograph of the larger particles in which particulate material is not detected. This is independent evidence for the existence of nonparticulate, nondializable solute, thus confirming the TEM results. (iii) The BEI picture of the smaller particles shows small brighter dots, even in domains appearing rather smooth in the SEI picture. Considering that brighter dots are silicon-rich (or oxygen poor) areas in these images, we conclude that the silica film is made out of domains with varying degrees of hydration. AFM and SEPM Images. Noncontact AFM and SEPM images are in Figure 4. These samples were also prepared at high particle loading, and the following observations can be made from these images: (i) The larger particles are strongly deformed in the AFM picture. Particles are deformed in every direction; consequently, their deformation does not follow a preferential direction that could be assigned to a scanning artifact. However, we note that this picture was taken
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Figure 3. Secondary electrons (SEI) and backscattered electrons (BEI) scanning electron micrographs of the larger (ca. 141 nm, top) and smaller (ca. 36 nm, bottom) silica particles.
Discussion
and plasticity. According to Iller,15 in most sols that consist of discrete spherical particles of amorphous silica, the interior of the particles is made out of anhydrous SiO2 and the surface holds silanol groups that are not lost when the silica is dried to remove free water. The present results show that silica particles have indeed a core-and-shell nature, but, from the comparison of the SEI and BEI pictures, we observe that the thickness of the shell layer extends over many nanometers. An independent verification of these results by analytical electron microscopy using plasmon mapping, together with elemental imaging obtained in the transmission electron microscope is in another publication of this laboratory.16 Consequently, silanol groups are found not only in a silica particle surface monolayer, but rather throughout a thick particle shell. The smaller particles are quite heterogeneous, as detected by every technique used in this work, specially SEPM. The shapes of these particles depart strongly from spherical, even when they are well apart. This result is interesting, because it points toward a low particlesolvent interfacial tension, which is in turn consistent with their persistence as very small particles, in a liquid medium. A larger interfacial tension would drive particles more strongly toward aggregation, thus preventing their existence as nanoparticles. Moreover, the small particles are very plastic, since they easily form dense films, thus undergoing extensive distortions. The larger particles also undergo distortion, but they retain their identities without merging into a coalesced film even though they are poorly connected by a thin film, visible in the bright-field picture. The core-and shell nature of the larger particles and the chemical heterogeneity of the smaller particles may
The examination of two Sto¨ber silicas in different size ranges, by different microscopy techniques, reveals that these particles are quite different in their chemical nature
(15) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979. (16) Leite, C. A. P.; Souza, E. F.; Galembeck, F. J. Braz. Chem. Soc., in press.
from a border region, from the area covered by the dispersion droplet used for sample preparation. Images taken from the central area of the droplet (not shown) display particles aligned in the same direction. (ii) The film obtained by drying the smaller particles is very smooth and nonporous, and its low roughness corresponds to that of a mirrorlike reflective surface (less than 1/20 visible light wavelength). Individual particles are hardly distinguishable, as opposed to the largerparticle sample. (iii) The SEPM picture of the larger particles shows that particle cores are electrically negative, as compared to particle shells. There is also significant contrast within the particles, evidenced by bright tiny dots and lines and showing that particle bulk is electrically inhomogeneous. The smaller-particle SEPM image shows black and gray dots in a bright matrix. Dot sizes are highly variable, but most extend for a few tens of nanometers, which is the same order of magnitude of the PCS particle sizes. We can estimate very large electric potential gradients throughout the film, in the range of 40 kV/cm. A further conclusion can be obtained considering the small bright dots in the BEI picture together with the black dots in the SEPM picture, both for the smaller particles sample: the domains containing less hydrous silica are more negative than the domains made with more hydrous silica. This agrees with the observation of darker cores in the SEPM image for the larger particles. A comparison of the results of particle examination by these three techniques is in Table 1.
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Figure 4. Noncontact mode AFM (left) and SEPM (right) images of the larger (ca. 141 nm, top) and smaller (ca. 36 nm, bottom) silica particles. Table 1. Comparison of the Results of Particle Examination by EFTEM, FESEM, AFM, and SEPM larger particles
smaller particles
EFTEM, brightfield
Coarsely spherical, smooth surfaces, thin “necks” joining clusters of particles.
Uniform sizes, irregular shapes, marked “neck” formation.
EFTEM, Si and O maps
Core-and-shell particles: the outer shells have a higher O/Si content than the particle bulk.
Particle borders are seen in the Si map, but they are almost indistinguishable in the O map.
FESEM, secondary electrons
Contacting but discrete particles, few coalescing particles.
Highly coalesced particles forming a film with some pores. Few individual particles are discerned.
FESEM, backscattered electrons
Contrast between bulk and borders (particles appear smaller than in the SEI mode), broader interparticle voids.
Significant contrast with bright domains smaller than the particles, and within domains with large numbers of coalesced particles. Also, nonparticulate material.
AFM, noncontact
Particles appear strongly distorted, without a preferential direction for distortion.
Formation of a continuous, dense packed film with low roughness and without morphological features identifiable as the original particles.
SEPM, electric potential
Particle cores are electrically negative, relative to particle shells. Significant contrast within particles.
Black (negative) and gray dots in a bright (positive) matrix. Variable dot sizes, but most extend for a few tens of nanometers, in the same range as the particle diameters.
be understood considering the particle synthesis procedure and the intervening reactions.17,18 Harris and co-workers19 proposed that the first hydrolysis of an alkoxide group is the rate-limiting step in the formation of small nuclei, and particle growth after the formation of the stable seed particles occurs mainly by addition of monomer to the surface, not by aggregation of the small nuclei particles as claimed by Bogush and
Zukoski.20 Bailey and Mecartney21 postulated that the hydrolyzed monomer reacts to form microgel polymers that collapse upon reaching a certain size and cross-linking density. The denser seed particles grow by addition of hydrolyzed monomer or polymer addition to their surface. Boukari and co-workers22 have proposed that the polymeric particles are better described as mass fractals, in which the nucleating backbones or seeds are used to build
(17) Yamane, M.; Inoue, S.; Yasumori, J. J. Non-Cryst. Solids 1984, 63, 45. (18) Wood, D. L.; Rabinovich, E. M.; Johnson, D. W., Jr.; MacChesney, J. B.: Vogel, E. M. J. Am. Ceram. Soc. 1983, 66, 693. (19) Harris, M. T.; Brunson, R. R.; Byers, C. H. J. Non-Cryst. Solids 1990, 121, 397.
(20) Bogush, G. H.; Zukoski, C. F. In Ultrastructure Processing of Advanced Ceramic; Mackenzie, J. D., Ed.; Wiley: New York, 1988; p 477. (21) Bailey, J. K.; Mecartney, M. L. Colloids Surf., A 1992, 63, 151. (22) Boukari, H.; Lin, J. S.; Harris, M. T. Chem. Mater. 1997, 9, 2376.
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the compact and stable particles observed later in the growth. The contribution of the present work may be summed up as follows: we found that small particles are highly heterogeneous and plastic, whereas the larger particles are more uniform and less easily deformable. Both types of particles have internal domains, characterized by different degrees of hydration: in the larger particles, the more hydrated domains concentrate in a thick surface layer, but these are randomly scattered throughout the smaller particles. Nonparticulate material was detected in the finished dispersions using two independent techniques, which adds a new element to the understanding of these silica dispersions. The present results are consistent with the formation of different kinds of siloxane chains in solution followed by their heteroaggregation, forming small gel particles of
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nonuniform chemical composition. However, many chains persist as polymeric solutes. The particles are strongly deformable due to their hydrated domains, and the more hydrophilic (more branched) chains accumulate at the outer particle shells, driven by the minimization of interfacial tension. This proposal parallels the mechanism put forward by El-Aasser23 for core-and-shell latex formation, and it helps to explain the detectable but limited plasticity of the larger particles, as well as their higher uniformity. Acknowledgment. The authors thank FAPESP, Pronex/Finep/MCT, and CNPq. LA000408+ (23) Dimonie, V. L.; El-Aasser, M. S.; Vanderhoff, J. W. Polym. Mater. Sci. Eng. 1988, 58, 821.
