Sub 5 nm magnetite nanoparticles: Synthesis, microstructure, and magnetic properties

Materials Letters 61 (2007) 3124 – 3129 www.elsevier.com/locate/matlet

Sub 5 nm magnetite nanoparticles: Synthesis, microstructure, and magnetic properties Jun-Hua Wu a , Seung Pil Ko b , Hong-Ling Liu c , Sangsig Kim d , Jae-Seon Ju e , Young Keun Kim b,⁎ a

Research Institute of Engineering and Technology, Korea University, Seoul 136-713, Republic of Korea Department of Materials Science and Engineering, Korea University, Seoul 136-713, Republic of Korea c Institute for Nano Science, Korea University, Seoul 136-713, Republic of Korea d Department of Electrical Engineering, Seoul 136-713, Republic of Korea Cooperative Center for Research Facilities, Sungkyunkwan University, Suwon 440-746, Republic of Korea

b

e

Received 3 July 2006; accepted 2 November 2006 Available online 27 November 2006

Abstract Magnetite particles of 2–4 nm were synthesized by an economic, biocompatible chemical coprecipitation route, with their size tuned by the reaction temperature. The microstructure and morphology of the nanoparticles were characterized by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM), whereas the magnetic properties were investigated by vibrating sample magnetometry (VSM) and superconducting quantum interference device (SQUID). It is found that the nanoparticles demonstrate well-defined superparamagnetic behavior as prepared and after annealing. Distinct lattices were observed which manifest the high crystallinity of such ultrasmall particles and the finite-size effect was revealed by analyzing the corresponding microstructure and magnetism. © 2006 Published by Elsevier B.V. PACS: 61.46; 81.05.Y; 75.50.V; 75.60 Keywords: Magnetite; Nanoparticle; Coprecipitation; Superparamagnetism

1. Introduction Magnetic nanoparticles offer exciting opportunities in fundamental study and technological applications, such as in high density data storage [1], biomedical applications [2,3], color imaging [4], bioprocessing [5], magnetic refrigeration [6], catalysts [7], ferrofluids [8], among many others. The appeals of those nanoparticles arise from their reduced dimension with unique properties different from their bulk counterparts. The large surface-to-volume ratio leads to unusual coupling and interaction of the surface atoms sitting a modified local environment. The role of the surface atoms becomes more important as the contributions from surface and volume change substantially with the decreasing size. Unlike bulk ferromagnetic materials which easily form multiple crystal and magnetic domains, magnetic nanoparticles can take single crystal and ⁎ Corresponding author. Tel.: +82 2 3290 3281; fax: +82 2 928 3584. E-mail address: [email protected] (Y.K. Kim). 0167-577X/$ - see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.matlet.2006.11.032

magnetic domains as a result of the dimension shrinkage. The applications, however, are still subject to many limitations, such as nanoparticle size, size monodispersity, biocompatibility, magnetization and stability. Practically, an efficient, economic and scalable synthesis of ultrasmall magnetic particles is highly desirable for potential applications and fundamental research. Magnetite (Fe3O4) nanoparticles occupy a singular position in the field of magnetic materials, owing to its special physiochemical properties. For instance, it exhibits many interesting phenomena such as charge ordering, mixed valence, and metal– insulator transition known as the Verwey transition [9]. For its exceptional biocompatibility, magnetite including other forms of iron oxide nanoparticles has been highlighted for biomedical applications [10,11], with ultrasmall superparamagnetic iron oxides mostly interesting for rapid elimination of nanoparticles and perfusion imaging [12,13]. Fe3O4 nanoparticles can be obtained by several methods, for example, polyol process [14], precipitation route [15], sonochemical synthesis [16], and microemulsion technique [17]. In this work, we synthesize ultrasmall

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size effects revealed in the structural analysis and magnetic measurements. It is expected that a product from such a clean process could find applications that require the implementation of ultrasmall magnetic particles with fine magnetic performance. 2. Experimental

Fig. 1. XRD patterns for the magnetite nanoparticles obtained at 60 °C: (a) before and (b) after annealing. See text for more explanation.

magnetite particles of 2–4 nm by an economic, biocompatible coprecipitation route at different reaction temperatures and report herein the characterization of the morphology, microstructure and magnetic properties of those superparamagnetic nanoparticles. The ultrasmall magnetic particles show high crystallinity and well-behaved nanomagnetism, with the finite-

Typically, 30 ml of FeCl3 and FeCl2 aqueous solutions with a concentration ratio of 2:1 was added dropwise into 200 ml of an alkali solution under vigorous stirring for 40 min at a controlled temperature (20 °C, 40 °C, 60 °C or 80 °C) to tune the size of the resultant nanoparticles. Subsequently, the solution was decanted and centrifuged to obtain dried samples in an oven. Annealing of the synthesized particles was conducted at 300 °C for 90 min in vacuum or nitrogen. The samples could be well dispersed in hexane. The microstructure and grain size of the nanoparticles was acquired by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) with selected-area electron diffraction (SAED). The magnetic property of the nanoparticles was analyzed by vibrating sample magnetometry (VSM) and superconducting quantum interference device (SQUID) at a magnetic field of 100 Oe for both field-cooling (FC) and zero-field-cooling (ZFC).

Fig. 2. TEM morphology of magnetite nanoparticles obtained at 20 °C (a), 40 °C (b), 60 °C (c) and 80 °C (d), respectively.

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3. Results and discussion Ultrasmall magnetite particles were synthesized by the economic, non-toxic coprecipitation method at different reaction temperatures. The crystal structure of the synthesized magnetic nanoparticles was first analyzed by XRD and HRTEM/SAED. As given in Fig. 1(a) for the case of the sample prepared at 60 °C, the XRD pattern shows a single-phased cubic inverse spinel structure. In comparison to the situation of larger particle sizes [18–21], the broadening of the peaks in the present investigation is primarily attributed to the finite-size of the ultrasmall particle sizes, which is particularly true for the ones before annealing (Fig. 1(a)). We note that the existence of a defective structure in the nanosized magnetite is plausible [21]. The finding is further substantiated by SAED patterns in which diffusive rings prove the finite dimension of the nanoparticles. According to the Scherrer's equation [22] in the application to the (311) peak, the grain size of the nanoparticles as prepared is estimated to be ∼ 5 nm (Fig. 1(a)), close to the HRTEM evaluation of ∼4 nm (refer to Figs. 2 and 3). After Fig. 4. Individual dispersion of magnetite nanoparticles obtained at 20 °C, assisted with the addition of oleic acid.

Fig. 3. HRTEM microimages of magnetite nanoparticles obtained at 20 °C (a) and 60 °C (b). The distinctive lattices represent respective (311) and (220) projections of the nanoparticles with high crystallinity.

annealing, the original peaks turn up much sharper and some weak peaks become stronger (see Fig. 1(b)). Moreover, the appearance of the new peaks labeled with asterisk may be associated with the possible forbidden Fe3O4 diffractions and inception of γ- and α-Fe2O3 phases, owing to the defective structure and oxidation from annealing [21,23]. It is evident that the width of the diffraction peaks reduces significantly, indicating an increase in the crystallinity and grain size of the annealed nanoparticles. Revealed by transmission electron microscopy (TEM) imaging, the nanoparticles are near spherical in shape, with an average grain size of 2–4 nm and a narrow grain size distribution. Fig. 2 is the morphologies of the samples prepared at 20 °C, 40 °C, 60 °C and 80 °C, respectively. It is apparent that the reaction temperature tailors the grain size of the nanoparticles, from 2–3 nm at 20 °C to ∼ 4 nm at 60 °C. An appraisal of the images in Fig. 2 shows that the samples synthesized at 20 °C (Fig. 2(a)) and 80 °C (Fig. 2(d)) appear similar and the ones at 40 °C (Fig. 2(b)) and 60 °C (Fig. 2(c)) seem alike. We consider the outcome as a consequence of the competition between nanocrystallite nucleation and growth commanded by the reaction temperature. The nuclei or the ratios of the nucleation to growth rates decrease from 20 °C to 40– 60 °C, and then increase again at 80 °C. Hence, the grain size changes alongside the coprecipitating temperature. As reflected in the XRD patterns of Fig. 1, the nanoparticles obtained from the coprecipitation process possess high crystallinity, which is sustained by the HRTEM observation in the way of distinct lattices from the nanoparticles. As shown in Fig. 3, the (311) and (220) projections of the lattices are visible for the specimens prepared at 20 °C and 60 °C, respectively, implying different faceting of the nanoparticles as a function of the precipitating temperature. It is straightforward that the physicochemistry and pharmacokinetics of such nanoparticles is further affected significantly as the lateral dimension of such single nanoparticles is only 3–5 unit cells, owing to their large surface area to volume ratio. It is worthy to mention that the lattice imaging is observable in large-sized magnetite nanoparticles as a result of high crystallinity due to increasing dimension [18]. Dispersion of nanoparticles is an important issue in the application of nanoparticles. In our case, the particle dispersion may be improved by adding a few droplets of oleic acid in the solution of the magnetite nanoparticles in hexane. Compared to the specimens directly cast from the samples in hexane (Fig. 2), oleic acid facilitates the physical

J.-H. Wu et al. / Materials Letters 61 (2007) 3124–3129

separation of the nanoparticles by surface coating and preventing aggregation of the nanoparticles during the evaporation of the solvent molecules (Fig. 4) [19]. Subsequently, the magnetic properties of the magnetite nanoparticles were investigated by VSM and a SQUID magnetometer. As presented in Fig. 5, the finite-size effect is demonstrated (as addressed below) and the results show that the nanoparticles were superparamagnetic at room temperature before and after vacuum annealing. Panels (a) and (b) of Fig. 5 are hysteresis loops for the samples obtained at 20 °C and at 60 °C, respectively. Overall, the two samples give almost the same order of magnitude of magnetization at the field of 4 T, 19.2 and 24.1 emu/g for the 20 °C sample before and after annealing, in comparison to 16.8 and 20.2 emu/g for the 60 °C sample before and after annealing. As prepared, the 20 °C sample is somehow harder to saturate than the 60 °C one; the phenomenon is reversed after annealing where the former demonstrates a higher susceptibility. Fig. 5(c)–(f) presents

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both FC and ZFC M–T curves obtained from the SQUID measurements conducted under a magnetic field of 100 Oe. The curves follow a typical superparamagnetic behavior of magnetic nanoparticles [18]. Before annealing, the samples have freezing temperatures of ∼60 K (Fig. 5(c)) for the sample prepared at 20 °C and ∼ 75 K (Fig. 5(d)) for the sample prepared at 60 °C, respectively, with slightly higher blocking temperatures. The outcome of the difference in the freezing temperatures is consistent with the change in the grain size, as a reduced grain size downshifts the temperature. Compared to the polyol process, the coprecipitation technique produces magnetite of higher freezing temperatures [18]. After annealing, the freezing temperatures shift correspondingly to ∼170 K (Fig. 5(e)) and ∼150 K (Fig. 5(f)), whereas the blocking temperatures increase to ∼300 K (Fig. 5(e)) and ∼ 250 K (Fig. 5(f)), respectively. The origin of the findings is recognized as a consequence of increase in the particles sizes due to annealing (see the detailed analysis below). Furthermore, it is noticeable that there are

Fig. 5. Magnetic measurements for the samples prepared at 20 °C (a, c, and e) and at 60 °C (b, d, and f). (a, b) M–H loops before and after annealing, (c, d) M–T loops before annealing, and (e, f) M–T loops after annealing.

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Fig. 6. The fitting of the experimental data to Eq. (4). (a) 20 °C samples as prepared, (b) 20 °C samples after annealing, (c) 60 °C samples as prepared, and (d) 60 °C samples after annealing. Legends: experimental data (circles) and fits (lines).

dips in the field-cooling magnetization at ∼ 60 K (Fig. 5(c)) and 75 K (Fig. 5(d)) for the as-prepared samples and ∼150 K (Fig. 5(f)) for the annealed one. The phenomenon could be a manifestation of spin glass behavior and/or the evidence of a Verwey transition [9], with the first two much smaller than the bulk value (125 K) which could be due to the finite-size of the nanoparticles. In contrast, such dips were not detected in the case of magnetite from polyol process [18]. To elucidate the relation between the particle size and magnetic properties discussed above, we address the issue of the superparamagnetic magnetization describable by the Langevin function on assemblies of magnetic nanoparticles with freely rotating moments [24,25], M 1 ¼ cothðaÞ− MS a

ð1Þ

where a¼

lH kB T

 M¼

MS1

1 cothða1 Þ− a1

 þ

MS2

  1 cothða2 Þ− a2

ð4Þ

where M1S, M2S, α1 and α2 are the fitting quantities corresponding to particle size 1 and 2. Fig. 6 shows the fits of the experimental data by

Table 1 The parameters derived from the fitting of Fig. 5(a) and (b) to Eq. (4)

ð2Þ

M/MS is the magnetization (M) normalized to the saturation magnetization (MS), H the applied magnetic field, T the temperature and kB the Boltzmann constant. For the Langevin-function fit to experimental data will produce the magnetic moment μ of the individual particles, hence it is possible to estimate the dimension of the nanoparticles. The diameter of a nanoparticle, d, is related to the corresponding magnetic moment μ by k l ¼ MS d 3 6

In our situation, using the bulk value of the saturation magnetization of MS = 6000 G at 20 °C for the magnetic moment of Fe3O4 in the nanoparticle [26], the size of the magnetic nanoparticles is estimable. The analysis shows that the data of Fig. 5(a) and (b) can be described well by a bimodal superposition of Eq. (1), that is, the nanoparticle systems here have two particle sizes in each sample. So, we use the equation below to fit the data of Fig. 5(a) and (b),

ð3Þ

As prepared

Annealed

R MS (emu/g) β (Oe) d (nm) R MS (emu/g) β (Oe) d (nm)

20 °C sample

60 °C sample

1

1

0.99998 22.5 (0.1) 15855.9 (320.8) 2.2 0.99837 14.2 (0.4) 6073.8 (551.3) 3.0

2 5.6 (0.2) 2753.3 (63.2) 3.9 11.7 (0.4) 300.5 (20.0) 8.2

0.99983 13.4 (0.1) 9418.3 (145.2) 2.6 0.99907 17.6 (0.7) 3115.1 (154.8) 3.7

2 6.6 (0.1) 881.6 (13.4) 5.7 5.1 (0.7) 496.6 (72.6) 6.9

J.-H. Wu et al. / Materials Letters 61 (2007) 3124–3129 Table 2 Comparison of the averaged particle size (d¯) to the freezing temperature (Tf)

As prepared

Annealed

As prepared

Annealed

by the Grant A050750 of the Korea Health 21 R&D Project, Ministry of Health & Welfare, and by Grant M1050000010505J0000-10510 from the National Research Laboratory Program of the Korea Science and Engineering Foundation.

2.5 60

5.3 170

3.6 70

4.5 150

References

20 °C sample d¯ (nm) Tf (K)

60 °C sample

Eq. (4) and Table 1 sums up the fitting results, where R is the correlation coefficient and b ¼ kBlT , in addition to the conventions mentioned above (errors in brackets). From Fig. 6 and Table 1, the fittings agree well with the experimental data and the nanoparticles are composed of two particle sizes with different fractions. Both particle sizes increase after annealing. To correlate with the freezing temperature, the averaged particle size is calculated in the way of M d d1 þ MS d d2 d¯ ¼ S 1 MS þ MS2 1

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2

ð5Þ

with d1 and d2 are the particle size 1 and 2. Table 2 compares the averaged particle size with the freezing temperature. Under the different experimental conditions, the averaged particle size obtained from the Brillouin equation, 2.5 and 5.3 nm for the 20 °C sample before and after annealing, compared to 3.6 and 4.5 nm for the 60 °C sample before and after annealing, is close to the ones estimated from the TEM and XRD analysis. It is obvious that the averaged particle size becomes bigger after annealing, with more increase of the 20 °C sample than the 60 °C sample. Moreover, the freezing temperature monotonically increases with the averaged particle size, indicative of the finite-size effect in determining the freezing temperature.

4. Conclusions Ultrasmall magnetite particles of 2–4 nm were successfully synthesized by an economic, non-toxic aqueous coprecipitation process, with their size tuned by the precipitating temperature. From the structural characterization and magnetic measurements, it is shown that the nanoparticles with such small dimension exhibit well-defined superparamagnetic behavior as prepared and after annealing. Distinct lattices are observed demonstrating the high crystallinity of such nanoparticles and the finite-size effect was revealed by analyzing the relevant microstructure and magnetism. It is expected that a product from the clean process could be directly employed in applications that require the implementation of ultrasmall-sized magnetic particles with outstanding magnetic performance. Acknowledgements This work was supported by the Korea Research Foundation Grants KRF-2005-210-D00023 and KRF-2004-005-D00057,

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