preparation of ultrafine fe3o4particles by precipitation in the presence of pva at high ph

JOURNAL OF COLLOID AND INTERFACE SCIENCE 177, 490–494 (1996)ARTICLE NO. 0062

Preparation of Ultrafine Fe3O4 Particles by Precipitationin the Presence of PVA at High pH

JIWON LEE, TETSUHIKO ISOBE, AND MAMORU SENNA

Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223, Japan

Received February 7, 1995; accepted June 9, 1995

the present work, we tried to precipitate magnetite in theUltrafine magnetite particles of average diameter 4–7 nm were presence of PVA at the moment of precipitation, in contrast

prepared by precipitation in a poly(vinylalcohol) (PVA) aqueous to the subsequent addition of PVA after precipitation, in ansolution. Crystallinity of the particles decreased with increasing attempt to modify the surface of freshly prepared particlesPVA concentration, while the morphology and particle sizes re-

with the coexisting hydrophilic polymer, PVA. We exam-mained almost unchanged. The dispersion of the magnetite parti-ined particularly carefully the effects of the PVA concentra-cles prepared at 1 wt% PVA was particularly stable. The saturationtion on the particle size, crystallinity, and magnetic proper-magnetization of the particles was above 50 emu/g, in spite ofties of magnetite particles.their small size and low crystallinity compared with conventionally

prepared fine magnetite particles. q 1996 Academic Press, Inc.

Key Words: ultrafine magnetite particles; PVA; superparamag- MATERIALS AND METHODSnetism; saturation magnetization.

Preparation of Magnetite

A mixed solution of ferrous and ferric ions in the molarratio 1:2 was prepared by dissolving 0.15 mol FeCl2r6H2OINTRODUCTION and 0.3 mol FeCl3r3H2O in a 1 dm3 aqueous solution ofpoly(vinylalcohol) (PVA) at 507C. PVA with a saponification

Magnetite is used in numerous industrial processes, e.g., value of 87% and on average molecular weight of 22,000 wasfor printing ink (1) and for recording media (2) . Wider used to obtain aqueous solutions at concentrations up to 20applications to magnetic fluids, drugs and biomedical de- grdm03 . Magnetite was precipitated by adding 30 cm3 of thevices are now being developed (3–5). Conventionally, mag- above-mentioned mixed solution to 100 cm3 aqueous solutionnetite is prepared by adding a base to an aqueous mixture of 0.37 molrdm03 NaOH (pH 13.8) at 807C while stirring atof ferrous and ferric chloride at a 1:2 molar ratio (6, 7) . 400 rpm with an impeller. The mixture was subsequently agedSince particles are attracted magnetically, in addition to the at 807C for 30 min and cooled to room temperature. After theusual flocculation due to van der Waals force, surface modi- end of magnetite precipitation, pH decreased to 5.2.fication is often indispensable. Various surfactants, e.g., so- Precipitates and supernatant fluid were separated by cen-dium oleic acid, dodecylamine, and sodium carboxymethyl- trifugation. The precipitates were washed with deionizedcellulose, are usually used to enhance dispersibility in an water under ultrasonication for 10 min and then separatedaqueous medium (8–10). Poly(vinylalcohol) (PVA) serves by centrifugation at 10,000 rpm for 30 min. Precipitates wereas a protective agent to stabilize colloidal dispersions of dried after washing three times under reduced pressure atmagnetite (11, 12). 507C for 24 h for the purpose of characterization.

For better dispersion, magnetite particles are often modi-fied after precipitation (9) . For the purpose of preparing a Characterizationwell-dispersed magnetic fluid, Papell’s grinding method iswidely used, where powdered ferrites are ball-milled in a In order to observe morphology and particle size, more

than 100 particles were observed under a transmission elec-carrier fluid containing surfactants (13, 14). This methodis recognized to be extremely versatile (15), but is time tron microscope (TEM) (JEM-2000FX, JEOL). The

average size of particles in a supernatant fluid was also deter-consuming and results in a broad distribution of particlesizes, typically between 2 and 50 nm. mined by a dynamic light scattering method (DLS) (LPA-

3000 Photal, Otsuka Electronics) . Before DLS measure-Precipitation of inorganic particles in a cross-linking poly-mer matrix or network of gel often prevents coagulation of ment, we tried to eliminate flocculated particles by centrifu-

gation at 10,000 rpm for 1 h.particles, giving rise to monodisperse particles (16–18). In

4900021-9797/96 $12.00Copyright q 1996 by Academic Press, Inc.All rights of reproduction in any form reserved.

/ m4420$3896 12-28-95 08:11:17 coida AP: Colloid

491PRECIPITATION OF ULTRAFINE Fe3O4

FIG. 1. Transmission electron micrographs of the Fe3O4 particles obtained from aqueous solution (a) without PVA and (b) with 1 wt% PVA, and(c) after being dispersed in 1 wt% PVA aqueous solution.

The stability of magnetite colloidal dispersions was also Table 1 shows the change in particle size with PVA concen-evaluated from the absorbance of the visible light (670 nm) tration, determined by two different methods. The values forin the supernatant. Crystallographic state was determined Fe3O4–P were 4.2 { 1.5 nm by TEM and 4.3 { 0.1 nm byby X-ray diffractometry (XRD). Relative crystallinity was DLS, agreeing fairly well with each other. For Fe3O4–NP,determined from the integral intensity of the X-ray diffrac- however, the difference between the two values were verytion peak (311) by using well-crystallized silicon particles large, 9.5 { 0.5 nm by TEM and 26.5 { 1 nm by DLS. Theas a standard reference. The lattice distortion of each sample increase of the size determined by DLS is due to very fastwas estimated by a conventional Hall method (19). flocculation after centrifugation. When remeasurement of the

Adsorption of PVA on the surface of Fe3O4 powders was same sample was carried out after 1 h, the particle sizeexamined by infrared spectroscopy, FT-IR (FTS-65, BIO- substantially increased to about 300–400 nm. This is muchRAD). The amount of PVA attached to the magnetite parti- larger than the initial average size of 26.5 { 1 nm, so itcles was analyzed by thermogravimetry under linear heating is evident that Fe3O4–NP flocculates rapidly without thein air at 10 Krmin01 up to 7007C. Saturation magnetization presence of PVA.was determined at room temperature by vibration sample Figure 2 shows visible light absorption spectra for a sus-magnetometry (VSM-3S-15, Rinken Denshi) with an exter- pension of magnetite obtained from the upper part of thenal magnetic field of 10 Arm01 . fluid after centrifugation at 10,000 rpm for 1 h. When the

concentration of PVA aqueous solution was 1 wt%, remark-RESULTS AND DISCUSSIONable absorption was observed at 670 nm. This indicates the

Morphology and Particle Size presence of magnetite particles well dispersed in the upperpart of the fluid, even after thorough centrifugation.Figures 1a and 1b show the morphology of the samples,

Fe3O4–NP (without PVA) and Fe3O4–P (with 1 wt% PVA). When the Fe3O4–NP was dispersed subsequently in a 1

TABLE 1Properties of Magnetite Particles Obtained from PVA Aqueous Solution at pH 13.8

PVA concentration(wt%) 0 0.35 0.7 1 1.35 1.7 2

Relative crystallinity 1 0.67 0.62 0.61 0.55 0.44 0.24Size (nm) 26.5 { 1a 6.6 { 0.1 4.7 { 0.4 4.2 { 0.1 4.4 { 0.1 3.8 { 0.5 4.3 { 0.5

9.5 { 0.5b 4.2 { 1.5 4.3 { 1.0Saturation magnetizationss (emu/g) 53.8 56.1 55.8 55.1 55.6 49.5 46.6

a From dynamic light scattering.b From TEM photographs.


492 LEE, ISOBE, AND SENNA

FIG. 2. Absorbance of visible light (670 nm) in the supernatant aftercentrifugation (104 rpm, 1 h).

wt% PVA aqueous solution, the average particle size ofFe3O4–NP was much larger, as shown in Fig. 1c, than thatof Fe3O4–P (Fig. 1b), although the same amount of PVAwas added to the dispersing medium. This clearly demon-strates the advantage of the coexistence of PVA moleculesat the moment of precipitation for protecting the newly bornminute particles from rapid flocculation. Similar protectionby the coexisting polymeric species is known in the field of FIG. 4. IR spectra measured by a transmission method: (a) Fe3O4–Pbiomineralization (11). (PVA 2 wt%); (b) Fe3O4–P (PVA 1 wt%); (c) Fe3O4–P (PVA 0 wt%).

Coverage and Adsorption by PVAAfter storage for 3 months in a desiccator at room temper-

ature, black Fe3O4–NP powder changed into reddish brown.Figure 3 shows that the amount of total weight loss afterthe PVA-coated Fe3O4 is heated to 7007C increased with The surface of the magnetite was oxidized to a-Fe2O3, as

confirmed by X-ray diffractometry. In contrast, no colorincreasing PVA concentration. The weight loss after heatingis attributed predominantly to the amount of PVA adsorbed change was observed in Fe3O4–P powder. This indicates

that the PVA layer on the magnetite particles prevents directon the Fe3O4. The possible weight gain due to partial oxida-tion of Fe3O4 to a-Fe2O3 is at most ca. 5 wt%. contact with air. It is known that oxidation of the particle

occur easily due to an increase on the specific surface areawith decreasing particle size. Fe3O4-P powder was resistantto oxidation in air even after drying, although the similarphenomenon was reported that magnetite is stable againstoxidation only in the solvent (20, 21).

Figure 4 shows IR spectra of the precipitated Fe3O4 parti-cles. As shown in Figs. 4a and 4b, three absorption bandswere observed at 1380, 1330, and 1083 cm01 , the formertwo bands being assigned to the C–H deformation vibrationof PVA and the last to the C–O stretching vibration ofPVA. The intensity of the band from the Fe3O4 at 600 cm01

decreased relative to the bands of PVA with increasing PVAconcentration. These results indicate that PVA irreversiblyadsorbed on the surface of Fe3O4 particles even after carefulwashing.

FT–IR spectra (Fig. 5) measured by a diffuse reflectionmethod give further information from the surface of parti-FIG. 3. The amount of total weight loss as a function of initial PVA

concentration after heating PVA coated Fe3O4 up to 7007C. cles. As shown in Fig. 5a, the band of Fe3O4 disappeared at


493PRECIPITATION OF ULTRAFINE Fe3O4

FIG. 7. Change in relative crystallinity with respect to PVA concentra-tion.

spinel structure in all the samples. These peaks did not shiftbut became broader with increasing PVA concentration. TheHall method (19) revealed the lattice distortion of each sam-ple to be negligible. The broadening of XRD peaks is there-fore predominantly attributed to the decrease in the crys-

FIG. 5. IR spectra measured by a diffuse reflection method: (a) Fe3O4– tallite size, which is directly related to the decrease in theP (PVA 2 wt%); (b) Fe3O4–P (PVA 1 wt%); (c) Fe3O4–P (PVA 0 wt%). particle size. The relative crystallinity of magnetite, deter-

mined from the relative XRD intensity, decreased with in-2 wt% PVA. This indicates that the surface of particles is creasing PVA concentration, as shown in Fig. 7.entirely covered by PVA. Chemical interaction between The absolute values of saturation magnetization, ss , de-Fe3O4 and PVA was not significant, since little chemical creased with decreasing particle size and were lower thanshift of the IR band due to PVA was observed. that of bulk magnetite particles (sbulk Å 92 emu/g) (6, 22),

as shown in Table 1. The ss decreased when the relativeCrystallographic and Magnetic Propertiescrystallinity becomes smaller than 0.6, as shown in Fig. 8.

Figure 6 shows the change in the X-ray diffraction patterns A decrease in ss is reported to occur when the particle sizewith PVA concentration. The XRD peaks correspond to the of magnetite decreases below 30 or 20 nm due to superpara-

FIG. 8. Relationship between saturation magnetization and relativeFIG. 6. X-ray diffraction profiles of Fe3O4–P (PVA1 and 2 wt%) andFe3O4–NP. crystallinity.


494 LEE, ISOBE, AND SENNA

ACKNOWLEDGMENTS

The authors are grateful to Mr. H. Kurokawa of Toda Industrial Co.,Ltd., for valuable discussions and the measurement of magnetic properties.We thank the staff of Kawaguchi Laboratory of Keio University for support-ing the particle size measurement.

REFERENCES

1. Atarashi, T., Imai, T., and Shimoiizaka, J., J. Magn. Magn. Mater.85, 3 (1990).

2. Yamaguchi, K., Matsumoto, K., and Fujii, T., J. Appl. Phys. 67, 4493(1990).

3. De Cuyper, M., and Joniau, M., Prog. Colloid Polym. Sci. 82, 353(1990).

4. Raj, K., and Moskowitz, R., J. Magn. Magn. Mater. 85, 233 (1990).5. Yoshimoto, T., Mihama, T., Takahashi, K., Saito, Y., Tamura, Y.,

and Inada, Y., Biochem. Biophys. Res. Commun. 145, 908 (1987).6. Sato, T., Iijima, T., Seki, M., and Inagaki, N., J. Magn. Magn. Mater.

65, 252 (1987).FIG. 9. The relation between primary particle sizes determined from7. Massart, R., IEEE Trans. Magn. MAG-17, 1247 (1981).DLS and coercive force.8. Scamehorn, J. F., Schechter, R. S., and Wade, W. H., J. Colloid

Interface Sci. 85, 463 (1982).9. Wooding, A., Kilner, M., and Lambrick, D. B., J. Colloid Interface

magnetism (16, 23). The critical particle size of ferromagne- Sci. 144, 236 (1991).tism of magnetite is ca. 25 nm, based on theoretical calcula- 10. Pillai, V., Kumar, P., and Shah, D. O., J. Magn. Magn. Mater. 116,tion from the equation KVÉ 25 kT , where k , T , K , and V are L299 (1992).

11. Burdon, J. W., and Calvert, P., in ‘‘Materials Synthesis Based onthe Boltzmann constant, the absolute temperature, anisotropyBiological Processes’’ (M. Alper, P. Calvert, R. Frankel, P. Riekeconstants, and the particle volume, respectively (24, 25).and D. Tirrell, Eds.) , p. 203. Symposium Proceedings of Materials

The coercive force, Hc , drops sharply and becomes almost Research Society, Pennsylvania, 1991.zero when the particle size decreases below 4 nm (see Fig. 12. Iler, R. K., J. Colloid Interface Sci. 51, 388 (1975).9). Since the particle size of the magnetite prepared in the 13. Kaiser, R., and Miskolczy, G., J. Appl. Phys. 44, 1064 (1970).

14. Berkowitz, A. E., Lahut, J. A., and VanBuren, C. E., IEEE Trans.present study was much less than 25 nm, these samples areMagn. MAG-16, 184 (1980).expected to be superparamagnetic. However, the superpara-

15. Wooding, A., Kilner, M., and Lambrick, D. B., IEEE Trans. Magn.magnetism becomes appreciable only partly. The suppres- MAG-24, 1650 (1988).sion of the superparamagnetism might be attributed, in part, 16. Sugimoto, T., Adv. Colloid Interface Sci. 28, 65 (1987).to the hindrance of thermal agitation at the surface due to 17. Sobon, C. A., and Bowen, H. K., J. Mater. Sci. L6, 901 (1987).

18. Harada, S., Yamanashi, T., and Ugaji, M., IEEE Trans. Magn. MAG-polymer adsorption.8, 468 (1972).

19. Hall, W. H., Proc. Phys. Soc. London Sect. A 62, 741 (1949).20. Yokoi, H., Yagishita, K., and Nakaishi, Y., Bull. Chem. Soc. Jpn. 63,CONCLUSION

746 (1990).21. Sato, T., Nakatsuka, K., Toita, K., and Shimoizaka, J., Funtai oyobi

Stable magnetite colloidal dispersions with average parti- Funmatsu Yakin 14, 71 (1967).cle size about 4 nm were prepared by precipitation in a PVA 22. Han, D. H., Wang, J. P., and Luo, H. L., J. Magn. Magn. Mater. 136,

176 (1994).aqueous solution. Each particle was covered by a PVA layer.23. Haneda, K., Kojima, H., and Morrish, A., Funtai oyobi FunmatsuThe value of saturation magnetization, ss (ca.54emu/g),

Yakin 30, 1 (1983).was smaller than that of bulk magnetite (92emu/g) but still24. Chikazumi, S., Taketomi, S., Ukita, M., Mizukami, M., Miyajima, H.,

comparatively high, in spite of the particle size being smaller Setogawa, M., and Kurihara, Y., J. Magn. Magn. Mater. 65, 245than the theoretically expected critical size for superpara- (1987).

25. Meikiejohn, W. H., Rev. Mod. Phys. 25, 302 (1953).magnetism.


preparation of ultrafine fe3o4particles by precipitation in the presence of pva at high ph

Documents