Journal of Magnetism and Magnetic Materials 252 (2002) 23–25

Size control of MnFe2O4 nanoparticles in electric doublelayered magnetic fluid synthesis

R. Aquinoa, F.A. Tourinhoa, R. Itrib, M.C.F.L. e Larac, J. Depeyrotc,*aComplex Fluids Group, Instituto de Qu!ımica, Universidade de Bras!ılia, Caixa Postal 04478, 70919-970 Bras!ılia (DF), Brazil

b Instituto de F!ısica da Universidade de S *ao Paulo, Caixa Postal 66 318, 05315-970 S *ao Paulo (SP), BrazilcComplex Fluids Group, Instituto de F!ısica, Universidade de Bras!ılia, Caixa Postal 04455, 70919-970 Bras!ılia (DF), Brazil

Abstract

We propose a method based on the pH of the synthesis to control the nanoparticle size during the ferrofluid

elaboration. The particle diameter is determined by means of X-ray diffraction experiments. The measured mean size

depends on the type of buffer used during the coprecipitation process. The results therefore confirm that the

nanoparticle size can be monitored by the hydroxide concentration and suggest to consider the induced interplay

between nucleation and crystal growth.

r 2002 Published by Elsevier Science B.V.

Keywords: Electric double layered magnetic fluid; Size control; X-ray diffraction

1. Introduction

In a very near future, a promising development for the

magnetic fluids (MF) applied technology will be their

use for biological purpose [1]. In such applications an

improved control of the magnetic nanoparticle size is

necessary. Although both synthesis and properties of

MF have been intensively studied over the last 35 years,

to our knowledge, no systematic synthesis methodology

has been proposed in order to elaborate MF with

controlled nanoparticle sizes. Nevertheless the size

variation of coprecipitated zinc ferrite particles has been

obtained by varying the pH of the aqueous solution of

FeCl3–ZnCl2 mixture in alkaline medium [2]. Moreover,

the changes induced by addition of several amounts of

citrate ions on the size of maghemite particles have been

investigated [3]. In this case, the complex formation

between citrate and iron ions leads to small magnetic

nanoparticles. Furthermore, oil in water micelles have

been used to make cobalt ferrite MF, where the particle

size has been controlled by the surfactant concentration

[4].

The aim of the present work is to control the

nanoparticle diameter during the MF synthesis. Then,

using the usual procedure [5], MnFe2O4 nanoparticles

syntheses were successively performed on the pH range

from 9 to 14. The best molar fraction in manganese has

been checked by measurements of the magnetic material

yield [6]. X-ray diffraction experiments have been made

in order to determine, both the crystalline structure and

the respective mean diameter. These results are discussed

based on equilibria of condensation processes derived

from hydrated metal ions that reveal the role played by

the hydroxide ions concentration in the size control of

ferrite magnetic nanoparticles.

2. Experimental

Sample synthesis: The EDL-MF synthesis is carried

out as in the usual procedure [6]. MnFe2O4 particles

were prepared using hydrothermal coprecipitating aqu-

eous solutions of MnCl2–FeCl3 mixture in alkaline

medium [5]. In order to strictly control the pH of the

synthesis medium, ammonium or methylamine buffers

solutions were used in the pH range from 9 to 13 and a

sodium hydroxide solution 2mol l�1 was employed at*Corresponding author. Fax: +55-61-307-23-63.

E-mail address: depeyrot@fis.unb.br (J. Depeyrot).

0304-8853/02/$ - see front matter r 2002 Published by Elsevier Science B.V.

PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 0 6 0 7 - 8

pH equal to 14. The measurements of the pH were done

with a pHmeter Metrohm 713.

Synthesis characterization: Our samples composition

has been checked by both chemical analysis and

measurements of the magnetic material yield: Fe (III)

titration was performed by dichromatometry and Mn

(II) was quantified by atomic absorption spectrometry.

The experimental procedure used in this work to

determine the best value in divalent metal molar fraction

is an adaptation of the method of continuous variation

or ‘‘Job’s method’’ and has been described elsewhere [6].

Structural characterization and particle size determina-

tion: The size determination and the structural char-

acterization were performed by X-ray diffraction using

the Cu-Ka radiation monochromatized by a graphite

monochromator. The measurements were made on a

powder obtained after evaporation of the liquid carrier

of the sample.

3. Results and discussion

The inset of Fig. 1 displays the variations of the

magnetic material yield as a function of the manganese

molar fraction. As it can be seen, our measurements

show a very good agreement with the theoretical curve

deduced from the chemical reaction of ferrite formation

[6]. The maximum yield corresponds to that provided by

the exact ferrite stoichiometry with a value of XMnþ2

equal to 0.33.

A typical powder diffraction spectrum is shown in

Fig. 1 and exhibits several lines corresponding to the

characteristic interplanar spacings 220, 311, 400, 422,

511 and 440 of the spinel structure. The cubic lattice cell

deduced from the peak position has an average value of

0.848 nm to be compared to the ASTM one equal to

0.849 nm [7].

In Fig. 2, we present typical powder diffractogram

obtained for manganese-based nanoparticles synthesized

in the pH range from 9 to 14. According to the Scherrer

equation [8,6] the nanoparticle size is related to the

width of the diffractogram peaks and calculated using

the strongest one. Then, in Fig. 3 the measured

nanoparticle diameters are plotted as a function of pH.

As it can be seen, in both figures, the nanoparticle size

increases markedly as pH increases in the system.

Let us therefore consider a possible process for the

ferrite nanoparticles formation. The inset of Fig. 3

propose a simplified schematic condensation mechanism

[9] leading to d-block metal oxide colloidal particles. As

the pH of the synthesis medium is increased, the aqua

ions of metal (M), after successive stages in the

deprotonation, undergo polymerization and condensa-

tion (simultaneous nucleation and crystal growth)

resulting in a precipitate of colloidal dimension. A

similar mechanism leading to ferrite particles seems very

reasonable. In this case, the increase in the nanoparticle

size with increasing pH could correspond to the inter-

play between nucleation and crystal growth. At high

hydroxide concentration (about 1mol l�1), the crystal

growth is more efficient (see the equilibria displacement

in the inset of Fig. 3). On the contrary, at small

hydroxide concentration (about 10�3mol l�1) and main-

taining the medium degree of relative supersaturation,

20 30 40 50 60 70

440

511

422

400

311

220

MnFe2O4

Inte

nsi

ty (a

.u.)

2 θ (deg)

0.0 0.5 1.00.0

0.5

1.0

∆m /

∆mm

áx

Molar Fraction XMn2+

Fig. 1. X-ray powder diffractrogram of MnFe2O4 nanoparti-

cles. The inset displays the normalized mass variation or

magnetic material yield as a function of the manganese molar

fraction.

20 30 40 50 60

pH = 14

pH = 11

pH = 9

DXR= 1.3 nmBuffer NH3

Buffer CH3NH2

+ NaOH

DXR= 7.0 nm

DXR= 3.6 nm

BufferCH3NH2

Buffer NH3

DXR=24.0 nm

DXR=18.8 nm

DXR=8.7 nm

NaOH

Buffer NH3

+ NaOH

2 θ (deg)

(Ix3)

Rel

ativ

e In

tens

ity

Fig. 2. X-ray diffraction spectra of samples synthesized at

several values of pH.

R. Aquino et al. / Journal of Magnetism and Magnetic Materials 252 (2002) 23–2524

the deprotonation of intermediate structures is less

efficient. Then the solution is in a state of metastable

equilibrium and this favors rapid nucleation to form a

larger number of small nanoparticles.

However, at pH=11, different sizes are also obtained

depending on the used buffer synthesis. This fact results

from complex formation of manganese ions with

ammonia or methylamine ligands. Since the manganese

(II) amine complex constant of the first step in the

complex formation is equal to 100.8 [10] and the

manganese (II) methylamine complexes are unstable

[11], this effect is more pronounced in the former case

(see both Figs. 2 and 3). In order to check this

assumption, we carried out new syntheses using these

same buffers, increasing the pH synthesis until 14 by

addition of small amounts of NaOH solution. As it can

be seen in both figures, the nanoparticles are smaller as

compared to particles synthesized in pure NaOH

medium, where no complex formation is expected.

According to our simplified mechanism, the interplay

between the formation of manganese (II) amine complex

and the formation of aqua ions (precursor in the ferrite

particle synthesis) therefore leads to smaller particles.

In conclusion, our results confirm that the nanopar-

ticle size can be monitored during the ferrofluid

synthesis by the hydroxide concentration.

Acknowledgements

We acknowledge the Brazilian agencies: Funda-c*ao de

Apoio a Pesquisa do Distrito Federal (FAP-DF),

Funda-c*ao de Apoio a Pesquisa do Estado de S*ao Paulo

(FAPESP), Coordena-c*ao de Aperfei-coamento de Pes-

soal de N!ıvel Superior (CAPES) and Conselho Nacional

de Desenvolvimento Cient!ıfico e Tecnol !ogico (CNPq).

References

[1] U. H.afeli, M. Zborowski, J. Magn. Magn. Mater. 225

(2001).

[2] T. Sato, K. Haneda, M. Seki, T. Iijima, J. Appl. Phys. A 50

(1990) 13.

[3] A. Bee, R. Massart, S. Neveu, J. Magn. Magn. Mater. 149

(1995) 6.

[4] N. Moumen, M.P. Pileni, J. Phys. Chem. 100 (1996) 1867.

[5] F.A. Tourinho, R. Franck, R. Massart, J. Mater. Sci. 25

(1990) 3249.

[6] M.H. Sousa, F.A. Tourinho, J. Depeyrot, G.J. da Silva,

M.C.F.L. Lara, J. Phys. Chem. B 105 (2001) 1168.

[7] ASTM No. 10-0319.

[8] C. Hammond, The Basics of Crystallography and Diffrac-

tion, Oxford University Press, Oxford, 1997, p. 148.

[9] D.F. Shriver, P.W. Atkins, C.H. Langford, Inorganic

Chemistry, Oxford University Press, Oxford, 1994, p. 199.

[10] L. Meites, Handbook of Analytical Chemistry, MacGraw-

Hill Book Company, New York, 1980, pp. 1–37.

[11] K. Osseo-Asare, Inst. Min. Metall. Trans. C 90 (1981) 152.

Fig. 3. Synthesis pH dependence on the nanoparticle size. The

inset shows a simplified schematic condensation mechanism of

d-block metal oxide colloidal particles.

R. Aquino et al. / Journal of Magnetism and Magnetic Materials 252 (2002) 23–25 25