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Silica Coating on Colloidal Maghemite Particles
Journal of Colloid and Interface Science 220, 357–361 (1999) Article ID jcis.1999.6517, available online at http://www.idealibrary.com on
Silica Coating on Colloidal Maghemite Particles Michaela Klotz,* Andre´ Ayral,* ,1 Christian Guizard,* Christine Me´nager,† and Vale´rie Cabuil† *Laboratoire des Mate´riaux et Proce´de´s Membranaires, UMR CNRS 5635, ENSCM, 8, rue de l’Ecole Normale, F34296 Montpellier cedex 5, France; and †Laboratoire Liquides Ioniques et Interfaces Charge´es, UMR CNRS 7612, Universite´ P. et M. Curie, 4, place Jussieu, 75252 Paris cedex, France Received May 3, 1999; accepted August 31, 1999
charge and the mean particle radius during the coating stage. The porosity of the silica coating is analyzed by nitrogen adsorption. The maghemite–silica interface is studied by Fourier transform infrared spectrometry.
Maghemite colloidal particles are coated with a silica layer using a silicon alkoxide as silica precursor. The coating process is studied by electrophoresis, quasi-elastic light scattering, nitrogen adsorption, and infrared spectrometry analyses. The conditions of complete coverage of the iron oxide particles by silica and the nature of the maghemite–silica interface are discussed. © 1999
EXPERIMENTAL
Academic Press
Key Words: maghemite; silica coating; zetametry; quasi-elastic light scattering; infrared spectrometry.
INTRODUCTION
Magnetic nanoparticles offer attractive properties for various industrial applications. Stable colloidal sols, for instance, are applied as ferrofluids in magnetic joints (1). Ferric oxide particles, as maghemite (g-Fe 2O 3) particles, are often used but aqueous dispersions of uncoated g-Fe 2O 3 particles are stable only for highly acidic or highly basic conditions. Surface modifications are required to increase the stability domain in aqueous media or to permit the dispersion in organic solutions (2). This study deals with the coating of maghemite particles with silica. Silica-coated particles should be easily dispersed in a wide pH range in aqueous solutions (3). On the other hand, methods of grafting organic groups on silica could be advantageously used to promote the dispersibility in organic media. Various routes for coating iron oxide particles with silica have been investigated: aggregation of small colloids (4), condensation of silica oligomers produced by solubilization of silica particles in a highly basic medium (5), hydrolysis of silicon alkoxides (6). The alkoxide route was chosen for this study. To evaluate the potentials associated with this method, it is first important to understand the mechanisms of formation of the silica coating and to characterize the maghemite–silica interface. Zetametry and quasi-elastic light scattering techniques are applied to determine the evolution of the surface 1
To whom correspondence should be addressed. Fax: 33 4 67 14 43 47. E-mail: [email protected].
a. Materials Seven samples with increasing amounts of tetramethoxysilane [Si(OCH 3) 4] (TMOS) are prepared. The chemical compositions of the various samples are given in Table 1. In Table 1 is also reported the corresponding theoretical thickness of the coating, t, assuming that all the added silicon alkoxide is condensed on the surface of the maghemite particles to produce dense amorphous silica (r 5 2.2 g cm 23). The colloidal maghemite particles are prepared using the synthesis method described by Massart (7). They are stabilized at pH 2 in a nitric aqueous solution. At this pH value, the particles exhibit a positive surface charge since the point of zero charge of g-Fe 2O 3 is equal to about 7.3 (2). The mean magnetic radius determined from magnetization curves is equal to 3.8 nm with a polydispersity of 0.35 (8). This mean magnetic radius is always smaller (;10%) than the radius measured by electron microscopy because the external part of the particle is poorly crystallized and does not contribute to the magnetic response (9). Methanol and TMOS are successively poured into the ferrofluid dispersion. The pH of the dispersion is then adjusted to 2.5 using a 1 M NH 3 aqueous solution to promote the condensation of the hydrolyzed alkoxide without destabilization of the dispersion. After 5 days of aging, the pH of the samples is increased to 4. Aliquots are taken and diluted in acidic solutions of the same pH for zetametry and quasi-elastic light scattering analyses. To study the particles coated at pH 4 by nitrogen adsorption and Fourier transform infrared spectroscopy, they are extracted from their dispersion medium containing free and growing silica clusters. The separation is based on a destabilization of the dispersion by quenching in liquid nitrogen. This method was previously developed for silica sols
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0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
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TABLE 1 Chemical Composition of the Samples and Theoretical Thickness of Dense Silica, t Sample I Volume of 1 vol% g-Fe 2O 3 dispersion (ml) Volume of methanol (ml) Volume of TMOS (ml) t (nm)
0.001 0.02
II
0.05 0.08
III
IV
V
VI
VII
0.1 0.15
15 0.3 0.25 0.35
0.5 0.65
0.75 0.92
1 1.15
(3). The resulting aggregates are separated from the supernatant solution using a magnetic field, washed several times with absolute ethanol, and finally dried at 150°C in an oven. For the unmodified ferrofluid and sample I, the destabilization by quenching is not observed. These two samples are thus first dried and then washed with ethanol before the final drying treatment at 150°C. b. Methods A Coulter Delsa 440 apparatus is used for the electrophoretic analyses. The temperature is fixed at 25°C. The applied electric field ranges from 14.5 to 21.0 V cm 21 and is applied for periods of 5 s followed by periods of 2.5 s without the electric field. The measurement of the electrophoretic mobility of the particles is based on a quasi-elastic light scattering method. The zeta potential is calculated from the electrophoretic mobility using the Smoluchowski equation (10). Quasi-elastic light scattering measurements are carried out using an apparatus composed of an Amtec SM 200 goniometer and a Sematech RTG correlator. The scattered light is measured at 90° from the incident beam. The temperature is regulated at 20°C. The cumulant method is used to determine the mean diffusion coefficient of the particles and the Stokes– Einstein equation is applied to calculate their mean hydrodynamic radius (11). The porous texture of the washed and dried samples is analyzed from the nitrogen adsorption– desorption isotherms at 77 K. The specific surface area (SSA) is determined using the BET method (12). The infrared spectra are recorded in the range 400 – 4000 cm 21 using a Nicolet Impact 400s Fourier transform infrared (FTIR) spectrometer (resolution, 2 cm 21). The dried powders are dispersed in a KBr matrix and compacted as thin pellets. To facilitate their comparison, the various spectra are normalized from the maximum of the absorption band located at 635– 640 cm 21 and associated with g-Fe 2O 3. Additional spectra are obtained after subtraction of the normalized spectrum of the unmodified ferrofluid.
FIG. 1. Effect of the added volume of TMOS (a) on the zeta potential of the particles and (b) on the mean hydrodynamic radius of the particles.
RESULTS AND DISCUSSION
a. Study of the Dispersions from Electrophoretic and Quasi-Elastic Light Scattering Measurements The zeta potential of the particles first decreases with the increase in added volume of TMOS; then it is nearly constant for the highest volumes of TMOS (Fig. 1a). At pH 2.5, the zeta potential remains positive. The positive charge of the unmodified ferrofluid is not fully reversed as expected in the case of homogeneous coating by a silica layer. The inversion of sign
FIG. 2. Nitrogen adsorption– desorption isotherms for unmodified maghemite and sample VII.
SILICA COATING ON COLLOIDAL MAGHEMITE
359
for the surface charge is observed at pH 4, except for samples I–IV which do not contain enough TMOS to reach complete silica coverage. The evolution of the mean hydrodynamic radius of the colloidal particles versus the volume of TMOS added to the dispersion is reported in Fig. 1b for two selected pH values: 2.5 and 4. Two kinds of dispersion must be distinguished here. The first group is composed of samples IV–VII dispersed at pH 4. These dispersions are not stable since coagulation (3) occurs 5 days after their preparation. All the other dispersions are stable. They exhibit the same mean hydrodynamic radius after an aging time of 9 months. These results are in full agreement with a criterion of stability usually valid for electrostatically stabilized aqueous dispersions at room temperature: the absolute value of the zeta potential must be larger than 25–30 mV (13). The jump observed in particle size between sample III and sample IV at pH 4 can thus be explained by the beginning of slow particle aggregation in samples IV–VII. For the other samples which are stable dispersions, the measured mean hydrodynamic radius corresponds to the radius of the individual solid nanoparticle plus the layer of ions and molecules adsorbed at the surface of the solid. It must, moreover, be noted that the diffusion coefficient of charged particles, as in this work, the uncoated maghemite particles, can be reduced by several percent if the double-layer thickness is comparable to the particle radius (14). On the other hand, for silica, pH values near its point of zero charge (pH ' 2) lead to low ionization of the surface and favor a multilayer adsorption of molecular water (15). The hydrodynamic radius is thus always clearly larger than the radius of the solid nanoparticle (16). Moreover it appears that the mean hydrodynamic radius increases both with the added volume of TMOS and with the pH of the dispersion. This last result is in agreement with the fact that the condensation is favored by an increase in pH. The increase in the mean hydrodynamic radius, however, is more than one order of magnitude larger than the theoretical thickness for a dense silica layer (Table 1). This result can be explained by the highly porous nature of the silica layer and significant hydration of the silica surface. The initial porosity of the silica layer is confirmed by the residual porosity evidenced by N 2 adsorption measurements on the dried samples. In Fig. 2 are shown the isotherm curves obtained for unmodified maghemite and sample VII. The important increase in the adsorbed volume at low relative pressure is related to the presence of microporosity in the silica layer. This additional microporosity induces an important variation increase in the SSA (Fig. 2). Moreover the lower stability of sample VII leads to more open aggregates
FIG. 3. FTIR spectra for samples I, II, and V: (a) original spectra and spectrum of unmodified maghemite in the range 400 to 1300 cm 21; (b) spectra obtained after subtraction of the maghemite spectrum in the range 400 to 1300 cm 21; (c) original spectra and spectrum of unmodified maghemite in the range 400 to 750 cm 21; (d) spectra obtained after subtraction of the maghemite spectrum in the range 400 to 750 cm 21.
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TABLE 2 Assignment of the IR Bands for Amorphous Silica Maximum absorbance (cm 21)
Assignment
460 800 980 1080 1220
Si–O–Si bending Si–O–Si stretching Si–OH stretching Si–O–Si asymmetric stretching Si–O–Si asymmetric stretching
Source. Ref. 19.
after drying and causes a strong increase in the pore size and the pore volume associated with interparticle mesoporosity (high relative pressure domain on the curves in Fig. 2). b. Study of the Dried Materials by FTIR Spectrometry We focus the analysis of the spectra on the spectral domain from 400 to 1300 cm 21 corresponding to the location of the absorption bands associated with the iron oxide and silicon oxide networks. The normalized spectra for unmodified maghemite and samples I, II, and V are illustrated in Fig. 3a. The spectra obtained after subtraction of the spectrum of unmodified maghemite are given on Fig. 3b. Figures 3c and 3d show magnifications of the previous spectra between 400 cm 21 and 750 cm 21. The spectrum of the unmodified maghemite shows two main maxima related to Fe–O vibrations, at 570 and 635 cm 21. This spectrum is consistent with that obtained by Preudhomme (17) in the case of a poorly crystallized and partially ordered maghemite. This result is also in agreement with the data established by Haneda and Morrish (18) who demonstrated that the decrease in size for the crystallized domains favors a random distribution of the vacancies in the spinel structure. The existence of an order for the vacancies in octahedral sites gives rise to a more defined spectrum with bands at 560, 587, 636, 690, and 724 cm 21 (17). The coating of the maghemite particles with silica induces the presence of new absorption bands at 800, 970, 1070, and 1190 cm 21 which can be related to the formation of the silica layer (Table 2). As expected the intensity of these bands increases with the added volume of TMOS. Subtraction of the maghemite spectrum enables visualization of a band at 460 cm 21 which is assigned to the bending of the siloxane bridges. The strong band located at 970 cm 21 is associated with the presence of a large amount of silanol groups (Si–OH) as expected for a poorly reticulated silica layer. The silica coating induces an increase in the intensity and a sharpening for the absorption bands located at 560, 590, 640, 700, and 730 cm 21. This phenomenon can be related to the ordering of the octahedral vacancies in the reverse spinel structure of g-Fe 2O 3. This result could be explained by an
ordering of the amorphous layer at the surface of the maghemite particles. Another explanation is that, during the drying treatment, the silica shell generates compression stresses on the maghemite core, which promote the structure ordering. These mechanical stresses are expected because the drying induces an important shrinkage of the porous silica layer (19). Moreover, the linear thermal expansion coefficient of amorphous silica is more than one order of magnitude smaller than that of maghemite. A wide and weak absorption band centered around 900 cm 21 can be observed in the spectra of samples containing small amounts of silicon alkoxide. It appears as a shoulder of the band at 970 cm 21 for the other samples. This band cannot be assigned to iron or silicon single oxides. On the basis of data obtained by Szostak et al. (20) who studied ferrisilicate zeolites, this band could be attributed to an asymmetric stretching vibration of the Fe–O–Si bond. Other authors observed this band at higher frequency: 1015 cm 21 (21). Jung (22) associated it with a vibration of the Fe–OH bond. In our case, this band is not present in the spectrum of the unmodified maghemite and does not decrease with improvement of maghemite crystallinity as could be expected in the case of a Fe–OH band. CONCLUSION
Measurements of zeta potential and hydrodynamic radius show that complete coverage of the maghemite particles with silica is obtained at pH 4 if enough tetramethoxysilane is added. The microporosity of the dried silica coating is evidenced by nitrogen adsorption analyses. FTIR spectrometry shows an increase in the crystallinity of the maghemite core of the coated particles and the presence of a weak and wide band at 900 cm 21 assigned to Fe–O–Si bonds at the maghemite– silica interface. REFERENCES 1. Massart, R., Bacri, J. C., and Perzynski, R., “Techniques de l’Inge´nieur,” Vol. D2180, pp. 1–10. Techniques de l’Inge´nieur, Paris, 1995. 2. Bacri, J. C., Perzynski, R., Salin, D., Cabuil, V., and Massart, R., J. Magn. Mater. 85, 27 (1990). 3. Iler, R. K., “The Chemistry of Silica.” Wiley–Interscience, New York, 1979. 4. Homola, A. M., Lorenz, M. R., Sussner, H., and Rice, S., J. Appl. Phys. 61, 3898 (1987). 5. Philipse, A. P., van Bruggen, M. P. B., and Pathmamanoharan, C., Langmuir 10, 92 (1994). 6. Ohmori, M., and Matijevic, E., J. Colloid Interface Sci. 150, 594 (1992). 7. Massart, R., IEEE Trans. Magn. Magn. Mag. 17, 1247 (1981). 8. Bacri, J. C., Perzynski, R., Salin, D., Cabuil, V., and Massart, R., J. Magn. Mater. 62, 36 (1986). 9. Berkovski, B. (Ed.), “Magnetic Fluids and Applications Handbook.” Begell House, New York, 1996. 10. Hunter, R. J., “Zeta Potential in Colloid Science: Principles and Applications.” Academic Press, London, 1981.
SILICA COATING ON COLLOIDAL MAGHEMITE 11. Ford, N. C., Jr., in “Dynamic Light Scattering, Applications of Photocorrelation Spectroscopy” (R. Pecora, Ed.), p. 7. Plenum, New York, 1985. 12. Lowell, S., and Shields, J. E., “Powder Surface Area and Porosity.” Chapman & Hall, London, 1984. 13. Moortgat, G., Silicates Ind. 5/6, 75 (1989). 14. Schumacher, G. A., and Van de Ven, T. G., Faraday Discuss. Chem. Soc. 83, 75 (1987). 15. Greenberg, S. A., Jarnutowski, R., and Chang, T. N., J. Colloid Sci. 20, 20 (1965). 16. Ayral, A., and Phalippou, J., J. Eur. Ceram. Soc. 6, 179 (1990).
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17. Preudhomme, J., Ann. Chim. 9, 31 (1974). 18. Haneda, K., and Morrish, A. H., Solid State Commun. 22, 779 (1977). 19. Brinker, C. J., and Scherer, G. W., “Sol–Gel Science, The Physics and Chemistry of Sol–Gel Processing.” Academic Press, San Diego, 1990. 20. Szostak, R., Nair, V., and Thomas, T. L., J. Chem. Soc. Faraday Trans. 1 83, 487 (1987). 21. Scarana, D., Zecchina, A., Bordiga, S., Geobaldo, F., Spoto, G., Petrini, G., Leofanti, G., Padovan, M., and Tozzola, G., J. Chem. Soc. Faraday Trans. 89, 4123 (1993). 22. Jung, C. W., Magn. Reson. Imaging 13, 675 (1995).
Silica Coating on Colloidal Maghemite Particles Michaela Klotz,* Andre´ Ayral,* ,1 Christian Guizard,* Christine Me´nager,† and Vale´rie Cabuil† *Laboratoire des Mate´riaux et Proce´de´s Membranaires, UMR CNRS 5635, ENSCM, 8, rue de l’Ecole Normale, F34296 Montpellier cedex 5, France; and †Laboratoire Liquides Ioniques et Interfaces Charge´es, UMR CNRS 7612, Universite´ P. et M. Curie, 4, place Jussieu, 75252 Paris cedex, France Received May 3, 1999; accepted August 31, 1999
charge and the mean particle radius during the coating stage. The porosity of the silica coating is analyzed by nitrogen adsorption. The maghemite–silica interface is studied by Fourier transform infrared spectrometry.
Maghemite colloidal particles are coated with a silica layer using a silicon alkoxide as silica precursor. The coating process is studied by electrophoresis, quasi-elastic light scattering, nitrogen adsorption, and infrared spectrometry analyses. The conditions of complete coverage of the iron oxide particles by silica and the nature of the maghemite–silica interface are discussed. © 1999
EXPERIMENTAL
Academic Press
Key Words: maghemite; silica coating; zetametry; quasi-elastic light scattering; infrared spectrometry.
INTRODUCTION
Magnetic nanoparticles offer attractive properties for various industrial applications. Stable colloidal sols, for instance, are applied as ferrofluids in magnetic joints (1). Ferric oxide particles, as maghemite (g-Fe 2O 3) particles, are often used but aqueous dispersions of uncoated g-Fe 2O 3 particles are stable only for highly acidic or highly basic conditions. Surface modifications are required to increase the stability domain in aqueous media or to permit the dispersion in organic solutions (2). This study deals with the coating of maghemite particles with silica. Silica-coated particles should be easily dispersed in a wide pH range in aqueous solutions (3). On the other hand, methods of grafting organic groups on silica could be advantageously used to promote the dispersibility in organic media. Various routes for coating iron oxide particles with silica have been investigated: aggregation of small colloids (4), condensation of silica oligomers produced by solubilization of silica particles in a highly basic medium (5), hydrolysis of silicon alkoxides (6). The alkoxide route was chosen for this study. To evaluate the potentials associated with this method, it is first important to understand the mechanisms of formation of the silica coating and to characterize the maghemite–silica interface. Zetametry and quasi-elastic light scattering techniques are applied to determine the evolution of the surface 1
To whom correspondence should be addressed. Fax: 33 4 67 14 43 47. E-mail: [email protected].
a. Materials Seven samples with increasing amounts of tetramethoxysilane [Si(OCH 3) 4] (TMOS) are prepared. The chemical compositions of the various samples are given in Table 1. In Table 1 is also reported the corresponding theoretical thickness of the coating, t, assuming that all the added silicon alkoxide is condensed on the surface of the maghemite particles to produce dense amorphous silica (r 5 2.2 g cm 23). The colloidal maghemite particles are prepared using the synthesis method described by Massart (7). They are stabilized at pH 2 in a nitric aqueous solution. At this pH value, the particles exhibit a positive surface charge since the point of zero charge of g-Fe 2O 3 is equal to about 7.3 (2). The mean magnetic radius determined from magnetization curves is equal to 3.8 nm with a polydispersity of 0.35 (8). This mean magnetic radius is always smaller (;10%) than the radius measured by electron microscopy because the external part of the particle is poorly crystallized and does not contribute to the magnetic response (9). Methanol and TMOS are successively poured into the ferrofluid dispersion. The pH of the dispersion is then adjusted to 2.5 using a 1 M NH 3 aqueous solution to promote the condensation of the hydrolyzed alkoxide without destabilization of the dispersion. After 5 days of aging, the pH of the samples is increased to 4. Aliquots are taken and diluted in acidic solutions of the same pH for zetametry and quasi-elastic light scattering analyses. To study the particles coated at pH 4 by nitrogen adsorption and Fourier transform infrared spectroscopy, they are extracted from their dispersion medium containing free and growing silica clusters. The separation is based on a destabilization of the dispersion by quenching in liquid nitrogen. This method was previously developed for silica sols
357
0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
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KLOTZ ET AL.
TABLE 1 Chemical Composition of the Samples and Theoretical Thickness of Dense Silica, t Sample I Volume of 1 vol% g-Fe 2O 3 dispersion (ml) Volume of methanol (ml) Volume of TMOS (ml) t (nm)
0.001 0.02
II
0.05 0.08
III
IV
V
VI
VII
0.1 0.15
15 0.3 0.25 0.35
0.5 0.65
0.75 0.92
1 1.15
(3). The resulting aggregates are separated from the supernatant solution using a magnetic field, washed several times with absolute ethanol, and finally dried at 150°C in an oven. For the unmodified ferrofluid and sample I, the destabilization by quenching is not observed. These two samples are thus first dried and then washed with ethanol before the final drying treatment at 150°C. b. Methods A Coulter Delsa 440 apparatus is used for the electrophoretic analyses. The temperature is fixed at 25°C. The applied electric field ranges from 14.5 to 21.0 V cm 21 and is applied for periods of 5 s followed by periods of 2.5 s without the electric field. The measurement of the electrophoretic mobility of the particles is based on a quasi-elastic light scattering method. The zeta potential is calculated from the electrophoretic mobility using the Smoluchowski equation (10). Quasi-elastic light scattering measurements are carried out using an apparatus composed of an Amtec SM 200 goniometer and a Sematech RTG correlator. The scattered light is measured at 90° from the incident beam. The temperature is regulated at 20°C. The cumulant method is used to determine the mean diffusion coefficient of the particles and the Stokes– Einstein equation is applied to calculate their mean hydrodynamic radius (11). The porous texture of the washed and dried samples is analyzed from the nitrogen adsorption– desorption isotherms at 77 K. The specific surface area (SSA) is determined using the BET method (12). The infrared spectra are recorded in the range 400 – 4000 cm 21 using a Nicolet Impact 400s Fourier transform infrared (FTIR) spectrometer (resolution, 2 cm 21). The dried powders are dispersed in a KBr matrix and compacted as thin pellets. To facilitate their comparison, the various spectra are normalized from the maximum of the absorption band located at 635– 640 cm 21 and associated with g-Fe 2O 3. Additional spectra are obtained after subtraction of the normalized spectrum of the unmodified ferrofluid.
FIG. 1. Effect of the added volume of TMOS (a) on the zeta potential of the particles and (b) on the mean hydrodynamic radius of the particles.
RESULTS AND DISCUSSION
a. Study of the Dispersions from Electrophoretic and Quasi-Elastic Light Scattering Measurements The zeta potential of the particles first decreases with the increase in added volume of TMOS; then it is nearly constant for the highest volumes of TMOS (Fig. 1a). At pH 2.5, the zeta potential remains positive. The positive charge of the unmodified ferrofluid is not fully reversed as expected in the case of homogeneous coating by a silica layer. The inversion of sign
FIG. 2. Nitrogen adsorption– desorption isotherms for unmodified maghemite and sample VII.
SILICA COATING ON COLLOIDAL MAGHEMITE
359
for the surface charge is observed at pH 4, except for samples I–IV which do not contain enough TMOS to reach complete silica coverage. The evolution of the mean hydrodynamic radius of the colloidal particles versus the volume of TMOS added to the dispersion is reported in Fig. 1b for two selected pH values: 2.5 and 4. Two kinds of dispersion must be distinguished here. The first group is composed of samples IV–VII dispersed at pH 4. These dispersions are not stable since coagulation (3) occurs 5 days after their preparation. All the other dispersions are stable. They exhibit the same mean hydrodynamic radius after an aging time of 9 months. These results are in full agreement with a criterion of stability usually valid for electrostatically stabilized aqueous dispersions at room temperature: the absolute value of the zeta potential must be larger than 25–30 mV (13). The jump observed in particle size between sample III and sample IV at pH 4 can thus be explained by the beginning of slow particle aggregation in samples IV–VII. For the other samples which are stable dispersions, the measured mean hydrodynamic radius corresponds to the radius of the individual solid nanoparticle plus the layer of ions and molecules adsorbed at the surface of the solid. It must, moreover, be noted that the diffusion coefficient of charged particles, as in this work, the uncoated maghemite particles, can be reduced by several percent if the double-layer thickness is comparable to the particle radius (14). On the other hand, for silica, pH values near its point of zero charge (pH ' 2) lead to low ionization of the surface and favor a multilayer adsorption of molecular water (15). The hydrodynamic radius is thus always clearly larger than the radius of the solid nanoparticle (16). Moreover it appears that the mean hydrodynamic radius increases both with the added volume of TMOS and with the pH of the dispersion. This last result is in agreement with the fact that the condensation is favored by an increase in pH. The increase in the mean hydrodynamic radius, however, is more than one order of magnitude larger than the theoretical thickness for a dense silica layer (Table 1). This result can be explained by the highly porous nature of the silica layer and significant hydration of the silica surface. The initial porosity of the silica layer is confirmed by the residual porosity evidenced by N 2 adsorption measurements on the dried samples. In Fig. 2 are shown the isotherm curves obtained for unmodified maghemite and sample VII. The important increase in the adsorbed volume at low relative pressure is related to the presence of microporosity in the silica layer. This additional microporosity induces an important variation increase in the SSA (Fig. 2). Moreover the lower stability of sample VII leads to more open aggregates
FIG. 3. FTIR spectra for samples I, II, and V: (a) original spectra and spectrum of unmodified maghemite in the range 400 to 1300 cm 21; (b) spectra obtained after subtraction of the maghemite spectrum in the range 400 to 1300 cm 21; (c) original spectra and spectrum of unmodified maghemite in the range 400 to 750 cm 21; (d) spectra obtained after subtraction of the maghemite spectrum in the range 400 to 750 cm 21.
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TABLE 2 Assignment of the IR Bands for Amorphous Silica Maximum absorbance (cm 21)
Assignment
460 800 980 1080 1220
Si–O–Si bending Si–O–Si stretching Si–OH stretching Si–O–Si asymmetric stretching Si–O–Si asymmetric stretching
Source. Ref. 19.
after drying and causes a strong increase in the pore size and the pore volume associated with interparticle mesoporosity (high relative pressure domain on the curves in Fig. 2). b. Study of the Dried Materials by FTIR Spectrometry We focus the analysis of the spectra on the spectral domain from 400 to 1300 cm 21 corresponding to the location of the absorption bands associated with the iron oxide and silicon oxide networks. The normalized spectra for unmodified maghemite and samples I, II, and V are illustrated in Fig. 3a. The spectra obtained after subtraction of the spectrum of unmodified maghemite are given on Fig. 3b. Figures 3c and 3d show magnifications of the previous spectra between 400 cm 21 and 750 cm 21. The spectrum of the unmodified maghemite shows two main maxima related to Fe–O vibrations, at 570 and 635 cm 21. This spectrum is consistent with that obtained by Preudhomme (17) in the case of a poorly crystallized and partially ordered maghemite. This result is also in agreement with the data established by Haneda and Morrish (18) who demonstrated that the decrease in size for the crystallized domains favors a random distribution of the vacancies in the spinel structure. The existence of an order for the vacancies in octahedral sites gives rise to a more defined spectrum with bands at 560, 587, 636, 690, and 724 cm 21 (17). The coating of the maghemite particles with silica induces the presence of new absorption bands at 800, 970, 1070, and 1190 cm 21 which can be related to the formation of the silica layer (Table 2). As expected the intensity of these bands increases with the added volume of TMOS. Subtraction of the maghemite spectrum enables visualization of a band at 460 cm 21 which is assigned to the bending of the siloxane bridges. The strong band located at 970 cm 21 is associated with the presence of a large amount of silanol groups (Si–OH) as expected for a poorly reticulated silica layer. The silica coating induces an increase in the intensity and a sharpening for the absorption bands located at 560, 590, 640, 700, and 730 cm 21. This phenomenon can be related to the ordering of the octahedral vacancies in the reverse spinel structure of g-Fe 2O 3. This result could be explained by an
ordering of the amorphous layer at the surface of the maghemite particles. Another explanation is that, during the drying treatment, the silica shell generates compression stresses on the maghemite core, which promote the structure ordering. These mechanical stresses are expected because the drying induces an important shrinkage of the porous silica layer (19). Moreover, the linear thermal expansion coefficient of amorphous silica is more than one order of magnitude smaller than that of maghemite. A wide and weak absorption band centered around 900 cm 21 can be observed in the spectra of samples containing small amounts of silicon alkoxide. It appears as a shoulder of the band at 970 cm 21 for the other samples. This band cannot be assigned to iron or silicon single oxides. On the basis of data obtained by Szostak et al. (20) who studied ferrisilicate zeolites, this band could be attributed to an asymmetric stretching vibration of the Fe–O–Si bond. Other authors observed this band at higher frequency: 1015 cm 21 (21). Jung (22) associated it with a vibration of the Fe–OH bond. In our case, this band is not present in the spectrum of the unmodified maghemite and does not decrease with improvement of maghemite crystallinity as could be expected in the case of a Fe–OH band. CONCLUSION
Measurements of zeta potential and hydrodynamic radius show that complete coverage of the maghemite particles with silica is obtained at pH 4 if enough tetramethoxysilane is added. The microporosity of the dried silica coating is evidenced by nitrogen adsorption analyses. FTIR spectrometry shows an increase in the crystallinity of the maghemite core of the coated particles and the presence of a weak and wide band at 900 cm 21 assigned to Fe–O–Si bonds at the maghemite– silica interface. REFERENCES 1. Massart, R., Bacri, J. C., and Perzynski, R., “Techniques de l’Inge´nieur,” Vol. D2180, pp. 1–10. Techniques de l’Inge´nieur, Paris, 1995. 2. Bacri, J. C., Perzynski, R., Salin, D., Cabuil, V., and Massart, R., J. Magn. Mater. 85, 27 (1990). 3. Iler, R. K., “The Chemistry of Silica.” Wiley–Interscience, New York, 1979. 4. Homola, A. M., Lorenz, M. R., Sussner, H., and Rice, S., J. Appl. Phys. 61, 3898 (1987). 5. Philipse, A. P., van Bruggen, M. P. B., and Pathmamanoharan, C., Langmuir 10, 92 (1994). 6. Ohmori, M., and Matijevic, E., J. Colloid Interface Sci. 150, 594 (1992). 7. Massart, R., IEEE Trans. Magn. Magn. Mag. 17, 1247 (1981). 8. Bacri, J. C., Perzynski, R., Salin, D., Cabuil, V., and Massart, R., J. Magn. Mater. 62, 36 (1986). 9. Berkovski, B. (Ed.), “Magnetic Fluids and Applications Handbook.” Begell House, New York, 1996. 10. Hunter, R. J., “Zeta Potential in Colloid Science: Principles and Applications.” Academic Press, London, 1981.
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