A novel route in the synthesis of magnetite nanoparticles

Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎

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A novel route in the synthesis of magnetite nanoparticles Mira Ristić a,n, Tatsuo Fujii b, Hideki Hashimoto b, Ivana Opačak a, Svetozar Musić a a b

Ruđer Bošković Institute, Bijenička 54, P.O. Box 180, HR-10002 Zagreb, Croatia Department of Applied Chemistry, Faculty of Engineering, Okayama University, Tsushima-naka 3-1-1, Okayama 700-8530, Japan

art ic l e i nf o

a b s t r a c t

Article history: Received 24 October 2012 Accepted 6 March 2013

A novel route in the synthesis of magnetite nanoparticles is reported. This synthesis is based on autoclaving of iron choline citrate dissolved in mixed water/ethanol or water solution and neutralized with NH4OH solution. The magnetite precipitates were characterized by XRD, Mössbauer, FT-IR, HRTEM and magnetometric measurements. All the precipitated magnetite particles were in the nanosize range as shown by HRTEM. They did not contain organic residues or any other iron (hydrous) oxide phase. & 2013 Elsevier B.V. All rights reserved.

Keywords: Magnetite XRD Mössbauer HRTEM Magnetometry

1. Introduction Magnetite (Fe3O4) is a naturally occuring magnetic material found in sediments and rocks. In nature, magnetite particles can be produced with the help of magnetotactic bacteria[1]. Magnetite has found applications in ferrofluids, magnetic inks, cosmetics, as black pigment, catalyst, sorbent or filler in polymers and rubber. In the last decade much attention has been paid to magnetite nanoparticles and the functionalization of their surfaces due to important biomedical applications [2,3]. For that reason, different ways of their synthesis have been sought. Magnetite nanoparticles 4 nm to 18 nm in diameter were prepared by the thermal decomposition of an iron-oleate precursor [4]. Gržeta et al. [5] investigated the thermal decomposition of iron choline citrate salt under different experimental conditions. Magnetite and hematite (α-Fe2O3) were obtained with the crystallite size of magnetite varying from 10(1) nm to 21(2) nm and of hematite from 25(2) nm and 39(3) nm. Mössbauer spectra showed substoichiometric magnetite Fe3−xO4. Several synthesis routes for the preparation of magnetite nanoparticles in aqueous media were reported [6–9]. The syntheses of magnetite nanoparticles starting from the Fe(III)-acetylacetonate precursor were also reported [10–12]. Gotić and Musić [13] reported the solvothermal synthesis of magnetite nanoparticles. The measured crystallite sizes of these nanoparticles varied between 11.1 nm and 22.6 nm and the stoichiometry was Fe2.89O4. Gotić et al. [14] also investigated the formation mechanism of magnetite nanoparticles in γ-irradiated microemulsions containing the Fe3 þ precursor. In the synthesis of magnetite by the precipitation method the key role is played by n

Corresponding author. Tel.: þ385 1 4680 107; fax: þ 385 1 4680 098. E-mail address: [email protected] (M. Ristić).

the nature of the starting Fe-containing compound. In the present work we report a novel synthesis route for the precipitation of magnetite nanoparticles. The proposed synthesis route is based on the autoclaving of iron choline citrate (C33H57Fe2N3O24) in a mixed water/ethanol solution or water with added NH4OH solution. 2. Experimental Iron choline citrate (C33H57Fe2N3O24) was supplied by SIGMA, triton X-100 (C34H62O11) by Merck, absolute ethanol (C2H5OH) by Carlo Erba and NH3∙aq (25%) by Kemika. Twice distilled water was prepared in own laboratory. The experimental conditions for sample preparation are given in Table 1. After 24 h of autoclaving the reactions were stopped by abrupt cold water cooling of the autoclave. Autoclaving was performed using the Parr Instruments teflon–lined non-stirred pressure vessel, model 4744, in a Yamoto DX 300 gravity oven with temperature uniformity 71.9 1C at 100 1C or 73 1C at 200 1C, as supplied by Cole–Parmer. The precipitates were separated from the liquid phase by centrifugation (Sorvall, model RC2-B), then washed two times with twice distilled water and once with absolute ethanol. The precipitates were dried at RT in a Cole–Parmer vacuum dryer coupled with a Pfeiffer vacuum pump. The samples were characterized with XRD (ItalStructures APD 2000), Mössbauer (WissEl GmbH), FT-IR (Perkin Elmer,model 2000), HRTEM, ED (Jeol JEM-2100 F) and magnetometric measurements (MPMS SQUID-VSM, Quantum Design).

3. Results and discussion Fig. 1 shows XRD patterns of samples S1 to S4, which can be assigned to the magnetite phase in accordance with the JCPDS

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Please cite this article as: Ristić M, et al. A novel route in the synthesis of magnetite nanoparticles. Mater Lett (2013), http://dx.doi.org/ 10.1016/j.matlet.2013.03.013i

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M. Ristić et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎

2

Table 1 Experimental conditions for the preparation of samples S1 to S4 and XRD analysis of precipitates. Sample

ICC/g

H2O/ml

C2H5OH/ml

S1 S2 S3 S4

0.79 0.79 0.79 0.79

15 20 35 15

20 10 20

T-X-100/g

25% NH3∙aq./ml

Temp./1C

Time/h

XRD analysis

0.4

5 10 5 5

160 160 160 160

24 24 24 24

magnetite magnetite magnetite magnetite

Key: ICC¼ iron choline citrate;T-X-100¼Triton X-100

311

S1 S1 440 220

511

400

111

422 220 511

S2 400

111

422

440

S3

220

511

111

S4 111

400

S2

Count rate / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66

422

440

220

S4

511

400 422

20

30

40

50

60

70

2θ / o (CuKα)

-10

Fig. 1. Characteristic XRD patterns of samples S1 to S4, recorded at 20 1C. Fig. 2.

Powder Diffraction File [15]. The broadening of diffraction lines is visible in all samples, indicative of very small crystallites. It should be noted that the phase identification of magnetite and maghemite (γ-Fe2O3) by XRD is not simple, because both phases have the same cubic structure and their lattice parameters are very close. Recently Kim et al. [16] proposed to determine the presence of magnetite and maghemite in their mixtures by deconvoluting the diffraction lines at (511) or (440). Magnetite is an inverse spinel in which Fe3 þ occupies the tetrahedral A-sites, whereas the octahedral B-sites are occupied by both Fe3 þ and Fe2 þ ions. The structural formula of magnetite can be written as (Fe3 þ )Td(Fe2 þ ,Fe3 þ )OhO4, whereas Td ¼tetrahedral and Oh ¼octahedral. Magnetite shows the Curie temperature at about 850 K and below that temperature the spin arrangement on

0

10

Velocity/ mm s-1 57

Fe Mössbauer spectra of samples S1, S2 and S4 recorded at RT.

the A- and B-sites is antiparallel. Since the magnitudes of the spins on these sites differ, magnetite shows a ferromagnetic behavior. Above the Vervey transition at about 119 K electron hopping takes place, (Fe2 þ Fe3 þ )B-(2Fe3 þ þ e−)B, between Fe2 þ and Fe3 þ ions at the octahedral B-sites. The charge transfer is rapid enough for the nucleus to sense the average charge “Fe2.5” at the B-sites. Therefore, the Mössbauer spectrum of magnetite at RT shows a superposition of two sextets. Mössbauer spectroscopy is especially useful in studying the magnetite nonstoichiometry [17]. The Mössbauer spectroscopic results are summarized in Figs. 2 and 3 and in Table 2. RT Mössbauer spectra of samples S1, S2 and S4 (Fig. 2) showed the superposition of two collapsing sextets which deviated in shape and line intensities typical of

Please cite this article as: Ristić M, et al. A novel route in the synthesis of magnetite nanoparticles. Mater Lett (2013), http://dx.doi.org/ 10.1016/j.matlet.2013.03.013i

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M. Ristić et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎ Table 2 Mössbauer parameters calculated for samples S1 to S4. Sample

Lines

δ (mm s−1)

Δ or Eq (mm s−1)

HMF (T)

Γ (mm s−1)

A (mm s−1)

T (K)

S1

M1 M2 M1 M2 Q M M1 M2 M1 M2

0.44 0.46 0.47 0.52

−0.02 −0.07 0.05 0.02 0.79

49.8 45.5 50.8 46.6

0.75 1,08 0.65 1.12

38.8 48.4 42.5 50.2 46.0

0.73 1.14

52 48 54 46 41.3 58.7 44.3 55.7 53 47

120 120 120 120 RT RT 120 120 120 120

S2 S3

S4

0.44 0.51

−0.01 −0.05

Key: Q¼central quadrupole doublet; M¼ sextet; δ ¼ isomer shift relative to α-Fe; Δ or Eq¼ quadrupole splitting; HMF¼ hyperfine magnetic field; Γ ¼line width; A¼ area under peaks; T ¼ temperature of measurement

RT

S3 120 K

Count rate / a.u.

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3

S3 120 K

S2 -10

0

10

Velocity / mm s-1 Fig. 3. 57Fe Mössbauer spectra of (a) sample S2 recorded at 120 K and (b,c) sample S3 recorded at 120 K and RT.

stoichiometric magnetite. At 120 K, the Mössbauer spectra of these samples were characterized by one sextet of very broad lines which could be fitted for two subspectra (sextets) with corresponding parameters given in Table 2. Fig. 3a shows the Mössbauer spectrum of sample S2 recorded at 120 K. Mössbauer spectra of sample S3 recorded at 120 K and RT are shown in Fig. 3b,c. The spectrum at RT represents the superposition of the central quadrupole doublet and the collapsing sextet(s). At 120 K, sample S3 showed one sextet of very broad lines, similar to the case of samples S1, S2 and S4. Disappearance of central quadrupole doublet at 120 K can be assigned to magnetite particles showing the superparamagnetism. It is important to note that the FT-IR spectra of samples S1 to S4 did not show IR bands that may be indicative of the presence of organic residues (contaminants) due to the starting precursor used. Moreover, the characteristic bending vibrations, δOH and γOH, which usually can detect the traces of goethite (α-FeOOH) are not noticed in the FT-IR spectra. Generally, in many reported syntheses of precipitated magnetite the formation of unwanted goethite and/or hematite is also possible. HRTEM images in Fig. 4 show the presence of magnetite nanoparticles in samples S1 to S4. The smallest nanoparticles with their maximum around 3–10 nm were observed in sample S3. This corresponds to the features of Mössbauer spectra at RT and 120 K, recorded for sample S3. Electron diffraction (ED) also showed that the precipitated nanoparticles in samples S1 to S4 consisted of the magnetite crystal phase. The magnetization hysteresis curves measured at 300 K showed different saturation values (Ms). Magnetization saturation increased in the order of 48.7 emu g−1 for sample S3, 51.9 emu g−1 for sample S1, 54.7 emu g−1 for sample S4 and 63.8 emu g−1 for sample S2. A similar effect for magnetite nanoparticles was also noticed by Gingasu et al. [18]. Goya et al. [19] recorded the magnetization hysteresis curves for magnetite particles of 5, 10, 50 and 150 nm and found an increase in saturation magnetization with increasing magnetite particle size. The substoichiometry of magnetite particles can also influence Ms values and this is more likely for very fine particles. Wang et al. [20] measured Ms ¼85.8 emu g−1 for well-crystallized magnetite nanoparticles (40 nm), whereas for bulk magnetite particles Ms ¼92 emu g−1 was reported [21]. Fig. 5a,b (left side) shows the magnetization hysteresis curves for sample S3, as recorded at 300 K and 10 K. Fig. 5c (left side) shows the magnetization of sample S3 as function of temperature in the field cooled (FC) and zero field cooled (ZFC) case. ZFC curve recorded for sample S3 increased up to∼95 K and from this temperature up to∼205 K there is a very broad maximum. ZFC and FC curves merge at∼300 K. The shape of ZFC and FC

Please cite this article as: Ristić M, et al. A novel route in the synthesis of magnetite nanoparticles. Mater Lett (2013), http://dx.doi.org/ 10.1016/j.matlet.2013.03.013i

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M. Ristić et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎

4

5 nm

5 nm

5 nm

5 nm

Fig. 4. HRTEM images of sample (a) S1, (b) S2, (c) S3 and (d) S4.

S3

15

FC

10

FC H = 200 Oe

10

5

6 0

100

200

T/K

T = 10 K

S1

300 16

60 40

ZFC

8

ZFC

H = 200 Oe

Magnetization / emu g-1

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FC

S3

H = 200 Oe

12

20

ZFC

0 -20

8

-40

S2 12

-60 40

FC

T = 300 K

S3

20

H = 200 Oe

9

ZFC

0

6

-20 -40

S4

3 -60

-40

-20

0

20

Magnetic field / kOe

40

60

0

100

200

300

T/K

Fig. 5. (Left side) Magnetization hysteresis curves of sample S3 recorded at (a) 300 K, (b) 10 K, and (c) magnetization as a function of temperature for sample S3 in the field cooled (FC) and zero field cooled (ZFC) case under 200 Oe. (Right side) FC and ZFC curves under 00 Oe, recorded for samples S1, S2 and S4.

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curves of sample S3 can be assigned to the distribution of nanoparticles (superparamagnetic) which is in line with Mössbauer results (Fig. 3b,c). ZFC and FC curves of samples S1 and S4 (Fig. 5 right side) showed similar behavior, whereas ZFC curve of sample S2 (Fig. 5 right side) showed a different feature which can be related with the results shown in the corresponding Mössbauer spectrum recorded at RT (Fig. 2). This Mössbauer spectrum showed high increase of the relative intensity of outher sextet thus indicating a strong oxidation of Fe2 þ ions in octahedral positions (inner sextet). Generally, the magnetometric behavior of magnetite particles is of a complex nature, because influences are simultaneously exerted by particle size and morphology, stoichiometry and magnetic coupling between the particles. References [1] Arakaki A, Nakazawa H, Nemoto M, Mori T, Matsunaga T. J R Soc Interface 2008;5:977–99. [2] Majewski P, Thierry B. Crit Rev Solid State Mater Sci 2007;32:203–15. [3] Kluchova K, Zboril R, Tucek J, Pecova M, Zajoncova L, Safarik I, et al. Biomaterials 2009;30:2855–63.

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