Colloids and Surfaces

iiiLolDS SURFACES A

A: Physicochemical and Engineering Aspects 109 (1996) 121-127

Magnetic properties of ultrafine magnetite particles and their slurries prepared via in-situ precipitation

Jiwon Lee, Tetsuhiko Isobe, Mamoru Senna * Keio University, Dept. of Applied Chemistry, Faculty of Science and Technology, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama

223, Japan

Received 2 August 1995; accepted 13 September 1995

Abstract

Ultrafine magnetite particles of 4-10 nm were prepared in aqueous solutions of polyvinylalcohol (PVA) or PVA with partially exchanged carboxyl groups. The mode of agglomeration and dispersion of the particles were different depending on the properties of polymers in the solvent. The dispersion of magnetite particles prepared in PVA 1 wt.% aqueous solution at pH 13.8, was particularly stable. In contrast, those precipitated in an aqueous solution of PVA with 0.1 mol.% exchanged carboxyl groups agglomerated in the form of a chain-like cluster. The magnetic properties were sensitive to the state of agglomeration of the magnetite particles with adsorbed polymers. An increase of the coercive force after ball-milling was observed, due to the formation of the network structure.

Keywords: Agglomeration; Coercive force; Dispersion; Magnetite; Polymer

1. Introduction

A large number of composite materials of biolog- ical origin contain well-dispersed iron oxide par- ticles. The oxide is formed from soluble precursors within a soft-tissue matrix [ 11. This suggests the possibility of using a similar method to prepare dispersions of fine magnetite particles in a polymer matrix by reaction in a solution of a soluble iron source dissolved in a polymer-containing solvent WI.

Well-dispersed magnetite particles are usually produced via a surface modification after the for- mation of magnetite particles [4,5]. The difference between these synthetic materials and biological composites is that the inorganic particles are grown in the presence of polymers in the latter cases [6].

* Corresponding author. Tel: (045)563-1141 (EXT.3423); fax: (045)563-0446; e-mail: senna@applc.keio.ac.jp

0927-7757/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDZ 0927-7757(95)03479-X

We tried to precipitate magnetite in a polymer matrix to mimic in-situ growth of biological com- posites. The initial problem was to discover a suitable reaction system enabling precipitation of inorganics in the presence of the polymer matrix. This problem surmounted to some extent for the system of magnetite and PVA in our previous study [7].

In the present work, the magnetic properties of ultrafine magnetite particles, precipitated in the presence of polymer such as polyvinylalcohol (PVA) or PVA with partially exchanged carboxyl groups (PVA-C) were measured. The PVA-C is expected to bind through its carboxyl groups on the surface of magnetite particles.

The effects of the polymer properties and the pH during precipitation are examined, and the relationship between the state of dispersion and magnetic properties of the suspension is also discussed.

122 J. Lee et al. JColloids Surfaces A: Physicochem. Eng. Aspects 109 (1996) 121-127

2. Materials and methods

2.1. Preparation of magnetite particles and its slurries

Magnetite particles were precipitated in an aque- ous solution of a respective polymer, e.g., PVA and PVA-C, by adding an aqueous mixture of ferrous and ferric chloride to an alkaline aqueous solution. Details of the preparation technique are explained elsewhere [7]. PVA and PVA-C with a saponifica- tion value and polymerization degree of 87 mol.% and 500, and 77 mol.% and 550 respectively, were used to obtain aqueous solutions of the concen- tration up to 2 wt.%. The PVA-C was produced by converting 0.1 mol.% of OH groups of PVA into COOH. Throughout this paper, the samples Mag-A, Mag-OH and Mag-COOH indicate mag- netite precipitated in an aqueous solution of PVA or PVA-C, respectively.

The initial pH of the NaOH aqueous solution was adjusted to 13.0, 13.7 or 13.8. The pH of the magnetite suspension after the end of precipitation was 1.5, 4.0 or 0.6 respectively. The suspension of the ultrafine magnetite particles was prepared by adding the washed particles to a 3 wt.% PVA aqueous solution and then ball-milling with one hundred 16 mm glass balls for 48 h. The ball-milled sample was denoted Mag-OH-48h.

2.2. Characterization

The magnetite particles were observed under a transmission electron microscope (TEM) (JEM-2OOOFX, JEOL). The crystallographic state was determined by X-ray diffractometry (XRD). The adsorption of polymer onto the surface of magnetite particles was examined by Fourier trans- form infrared (FT-IR) spectroscopy, (FTS-65, BIO- RAD). Saturation magnetization was measured at room temperature by vibration sample magnetom- etry (BHV-35H Riken Denshi) with an external magnetic field of 10 kOe. The samples used were a powder and a suspension of magnetite. In the case of the suspension, the concentration of magne- tite particles with respect to the PVA aqueous solution was 0.05 g ml-‘. The net weight of Fe304 was obtained by subtracting the amount of PVA

attached to the magnetic particles, the latter being measured by thermogravimetry.

The analysis by X-ray photoelectron spectro- scopy (XPS) was carried out using Mg Ka as the exiting radiation. All powder samples were com- pressed at 150 MPa to give the form of a thin pellet. Samples were first studied in the air-exposed form. They were successively etched with argon ion and spectra were recorded at the end of each etching period to obtain an elemental depth profile.

3. Results and discussion

3.1. Morphology and dispersion of particles

The XRD peaks shown in Fig. 1 correspond to the spine1 structure in all the samples. The peaks did not shift but became broader by adding PVA or PVA-C. The broadening is mainly attributed to the decrease in the crystallite size [ 71. When the magnetite particles are precipitated in an aqueous solution of PVA, the sizes of the particles are much smaller than those precipitated for an aqueous solution [7]. The dispersibility of particles also increased significantly in PVA aqueous solution.

Fig. 2 shows the morphology of the samples (a) Mag-A, (b) Mag-OH and (c) Mag-COOH. In

Mag-COOH

, I 1

30 40 50 60

Fig. 1. X-ray diffraction profiles of Mag-A, Mag-OH and Mag-COOH.

J. Lee et al./Colloids Surfaces A: Physicochem. Eng. Aspects 109 (1996) 121-127 123

Fig. 2. Transmission electron micrographs of the samples precipitated at pH 13.8. (a) Mag-A, (b) Mag-OH and (c) Mag- COOH.

the sample Mag-OH, the adsorbed PVA layer on the surface of ultrafine magnetite particles was not observed on the electron micrograph, Fig. 2(a). The adsorption of PVA was confirmed by other observations such as infrared spectroscopy and thermogravimetry [7]. On the sample Mag- COOH, a polymer layer was observed on the particles, forming a cluster. They are similar to magnetotactic bacteria, which contain chains of magnetic iron oxide particles of about 10 nm diam- eter [8].

Fig. 3 shows an IR spectrum of the Mag-COOH. The adsorption band 1700-1750cm-’ was assigned to an ester group. Therefore, an esterifica- tion must have occurred between -COOH and -OH along the chain of the PVA-C [9]. Hence, the sample Mag-COOH was not dispersed, but owing to agglomeration, primary particles were linked by the polymer in the form of a chain.

3.2. Adsorption of polymer

Polymer adsorption on the surface of magnetite particles was investigated from the atomic percent of carbon. The carbon concentration was different among the Mag-A, Mag-OH and Mag-COOH samples precipitated at the same pH 13.8. Fig. 4 shows the relative amount fraction of Fe 2p, 0 1s and C 1s on the respective samples. Quantitative analysis revealed that atomic percent of carbon for

2ooa 1500

Wavenumber (cm.‘)

loo0

Fig. 3. IR spectra of the sample Mag-COOH.

124 J. Lee et aL/CoNoids Surfaces A: Physicochem. Eng. Aspects IO9 (1996) 121-127

0

(4 Sputtering time (WC)

1

(W

10 20 30 40 50 60

Sputtering time (set)

0 10 20 30 40 50 60

Cc) Sputtering time (set)

Fig. 4. Relative fraction of (a) Fe, (b) 0 and (c) C for Mag-A, Mag-OH and Mag-COOH.

Mag-OH and Mag-COOH was about twice as much as that in Mag-A on the surface. In the sample of Mag-A, the carbon almost disappeared after 10 s with argon ion etching, but this remained

in the other samples even after etching for 60 s. Hence, the excess carbon must be attributed to the adsorption of PVA and PVA-C on the surface.

3.3. Magnetic properties of particles and their slurries

The coercive force of ferromagnetic particles decreases rapidly when the particle size decreases below the superparamagnetic size as shown in Fig. 5, with saturation magnetization as a function of the particle size.

At room temperature, a coercive force of a few Oe was observed for ultrafine magnetite particles. Therefore, all the samples look superparamagnetic as far as coercive force is concerned. The saturation magnetization (0,) per net weight of Fe,O, (about 54 emu g-‘) is also smaller than that of bulk magnetite (92 emu g- ‘) but is still significant. The superparamagnetism is therefore appreciable only partly. Berkowitz et al. (1968) found a(293 K)= 74 emu g-i for the largest crystallite size but this decreased to 34 emu g-’ for particles in which the crystallite size was about 50 A. In addition, the saturation magnetization of ferrites reduced cS by more than 50% in the presence of a surfactant layer on particles [lo]. Generally, the ferrite par- ticles coated with surfactant show a smaller magne- tization compared to bulk values [ 11 J. The observed decrease is not significant for the ultrafine

3 4 5 6 7 8 9 10

Particles size(nm)

Fig. 5. Change in the saturation magnetization and coercive force with size of magnetite precipitated in PVA aqueous solution.

J. Lee et al.!Colloids Surfaces A: Physicochem. Eng. Aspects 109 (1996) 121-127 125

magnetite particles prepared in the present study. The reason for this is not clear at present, but the polymer layer adsorbed on the surface of ultrafine particle may have suppressed thermal agitation of the spins of near-surface Fe atoms, and hence, suppressed superparamagnetism. This might explain why the saturation magnetization of MAG-OH samples is larger than that of MAG-A sample (about 10 nm) in spite of a smaller particle size (below 5 nm).

3.4. Effect of ball-milling

The state of dispersion is observed in Fig. 6. The sample Mag-A-48h was not changed from Mag-A (Fig. 2(a)). However, the sample Mag-OH-48h, prepared in the same way as Mag-A-48h, formed a network structure between particles. When the concentration of PVA increased to about 10 wt.%, black beads were produced with an average diame- ter of about 5 mm. The chain of polymer adsorbed on the surface of particles may have served as a binder while ball-milling was taking place.

The magnetization behavior of the magnetite suspension after ball-milling was examined by VSM at room temperature (shown in Fig. 7). The coercive force of Mag-OH-48h was 15 Oe, being three times larger than that of Mag-A-48h (5.2 Oe). A difference in the coercive force was also observed between Mag-OH (5.0 Oe) and Mag- COOH ( 15.2 Oe). The increase in the coercive force was observed on Mag-OH-48h and Mag- COOH, which have a similar network structure. Magnetic properties such as the saturation magne- tization and the coercive force are very sensitive to the microstructure. The increase in the coercive force seems to be attributed to the microstructure of the aggregates. A similar increase in the coercive force by an agglomeration of particles or broad size distribution has already been reported [ 121.

4. Conclusion

The PVA adsorbed on ultrafine Fe,O, particles (Mag-OH) with an average size of about 4 nm were formed in a well-dispersed state, owing to the presence of a nonmagnetic surface layer which

(4

(b)

Fig. 6. Transmission electron micrographs of the samples prepared after ball-milling for 48 h. (a) Mag-A-48h and (b) Mag- OH-48h.

prevents interparticle interaction. In contrast, Fe,O, (Mag-COOH) precipitated in an acidic aqueous solution of PVA with partly exchanged carboxyl groups (PVA-C) formed soft agglomer- ates with a chain-like cluster.

The coercive force of Mag-OH (about 3.0 Oe) was one third of that of Mag-COOH (ca. 9.0 Oe). It was similar to the difference between Fe,O, precipitated in an aqueous solution (5.2 Oe) and Mag-OH (15 Oe) after ball-milling for 48 h. The samples Mag-COOH and ball-milled Mag-OH

J. Lee et al. JColloids Surfaces A: Physicochem. Eng. Aspects 109 (1996) 121-127

M(emufg)

Applied Field H, 1OKOe

300K

(a)

M(emlg)

1

I -3

H@.lOe) / I I

1 2 3 H(KOe)

(b)

Fig. 7. Magnetization curves of the samples shown in Fig. 7. (a) Mag-A-48h and (b) Mag-OH-48h

showed a similar network structure of particles. References An increase of coercive force was attributed to this network structure. Cl1

c21

Acknowledgments c31

The authors are grateful to Mr. H. Kurokawa of Toda Industrial Co., Ltd. for valuable discus- sions and the measurement of magnetic properties.

c41

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