Hasegawa, Daiji

Ogawa, Tomoyuki

Takahashi, Migaku

Yang, Haitao

Journal of Applied Physics

English

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Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

Synthesis and magnetic properties ofmonodisperse magnetite nanocubes著者 小川  智之journal orpublication titleJournal of Applied Physicsvolume 103number 7page range 07D526-1-07D526-3year 2008URL http://hdl.handle.net/10097/46585doi: 10.1063/1.2833820Synthesis and magnetic properties of monodisperse magnetite nanocubesHaitao Yang,1,a Tomoyuki Ogawa,2 Daiji Hasegawa,1 and Migaku Takahashi11New Industry Creation Hatchery Center, Tohoku University, Aoba-yama 10, Sendai 980-8579, Japan2Research Strategy and Support, Tohoku University, Aoba-yama 05, Sendai 980-8579, JapanPresented on 6 November 2007; received 11 September 2007; accepted 26 October 2007;published online 4 March 2008The monodisperse Fe3O4 nanocubes with controllable sizes from 6.5 to 30.0 nm have beensynthesized in one pot. The shape-induced texture of the self-assembled nanocube superlattices hasbeen deposited onto substrates by a convective assembly technique. The cubelike shapes affect thecrystalline orientation of individual particles within the self-assembled superlattices compared withthe nanoparticles with an isotropic spherical shape. The nanocubes with a size less than 25 nm showa typical superparamagnetic behavior at room temperature. Comparison of the hysteresis loop at 5 Kof cubes with different sizes reveals a size-dependent behavior of saturation magnetization andcoercivity. © 2008 American Institute of Physics. DOI: 10.1063/1.2833820INTRODUCTIONEnormous progress has been made in producing metaland semiconductor nanoparticles with various shapes such asrods, cubes, and prisms.1,2 Different shapes of the particlescan introduce electronic, optical, and magnetic propertiesthat are different from those observed in their sphericalcounterparts.3 Moreover, it is of great fundamental and prac-tical interest to prepare self-assembled arrays by using dif-ferent shapes as building blocks. Recently, various wetchemical approaches including hydrothermal synthesis andorganic-phase synthesis have widely been used to preparenonspherical nanoparticles since they provide great benefitsin control of size, component, and dispersion.4,5 However,there are several reports on the organic-phase synthesis ofmagnetic nonspherical nanoparticles, especially focused onFePt,6 Co,7 and MnFe2O4 Ref. 8 even though such mono-disperse materials are of great importance in a variety oftechnological applications ranging from biomedical areas tomagnetic data storage.Magnetite Fe3O4 is a common magnetic iron oxide andan important class of half-metallic materials.9 Recentorganic-phase synthesis studies on magnetite nanoparticleshave mainly focused on size control and phase transfer. Themonodisperse spherical magnetite nanoparticles with sizevaried from 4 to 20 nm have been synthesized by combiningorganic-phase reaction with seed-mediated growth.10In this work, we present the one-pot synthesis of mono-disperse Fe3O4 nanocubes with controllable size from6.0 to 30.0 nm by a modified polyol process in organicphase. A convective assembly by evaporation assisted with-drawal coating has been used to make superlattices of par-ticles from dilute toluene/hexane suspensions. X-ray diffrac-tion XRD of such a self-assembled texture of Fe3O4nanocubes on nonreflective Si substrate shows an intensified220 peak. The hysteresis loops of Fe3O4 nanocubes withdifferent sizes at 300 and 5 K have been investigated.EXPERIMENTALIn a typical synthesis of 15.0 nm Fe3O4 nanocubes, amixture of 0.5 mmol of Feacac3, 4 mmol of 1,2-hexadecandiol, 8 mmol of oleic acid OA, 2 mmol of oleylamine, and 10 ml of benzyl ether was heated at 110 °C for1 h with Ar flow. The temperature was then raised to 200 °Cand kept at that temperature for 30 min. Under Ar blanket-ing, the mixture was heated to a reflux temperature of about290 °C as a heating rate of 15 °C /min and kept refluxing for1 h. The solution mixture was cooled down to room tem-perature, and the nanocubes were precipitated out by addingethanol. The product was washed with ethanol and redis-solved in the mixture of toluene and hexane.Transmission electron microscopy TEM, JEOL-3010 at300 kV was employed to determine the morphology, size,and size distribution of nanoparticles. Samples were formedby depositing toluene/hexane dispersion of particles onamorphous-carbon-coated copper grids in a convective as-sembly apparatus.11 The sample was deposited on the nonre-flective Si substrate 55 mm2 using the convective assem-bly technique for scanning electron microscopy SEM, JEOLSM71010 and XRD Rigaku 2200 measurement. The Femass in particles was firstly measured by our reportedmethod12 and the Fe3O4 mass was then calculated via thestoichiometric ratio. Magnetic measurements were con-ducted using a Quantum Design MPMS-5 superconductingquantum interference device.RESULTS AND DISCUSSIONTo achieve a shape-controlled synthesis, we modifiedslightly the synthesis procedure in Ref. 10. The molar ratioof OA to Feacac3 was increased to 4, while the molar ratioof oleyl amine and 1,2-hexadecandiol to Feacac3 was re-duced. Such a modification based on that OA with a carboxy-lic group, –COOH, has a selective binding onto differingenergy crystal facets together with a nonpolar tail group forsteric hindering.13 However, oleyl amine with a –NH2 grouphas a weak and isotropic binding onto the surface of par-ticles. The modified approach enables the control of the re-activity of the carboxylic group and as a result the nucleationaAuthor to whom correspondence should be addressed. Telephone:81-22-795-7134. FAX: 81-22-263-9402. Electronic mail:htyang@ecei.tohoku.ac.jp.JOURNAL OF APPLIED PHYSICS 103, 07D526 20080021-8979/2008/1037/07D526/3/$23.00 © 2008 American Institute of Physics103, 07D526-1Downloaded 29 Mar 2010 to 130.34.135.83. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jspand growth dynamics of the particles. By careful adjustmentof the stabilizer molar amount as well as the reaction time wehave obtained the size-controlled monodisperse magnetitenanocubes.Figure 1 shows the TEM images of monodisperse Fe3O4nanocubes with different sizes from 6.5 to 30.0 nm. The IM-AGEJ 1.3.2J software was used to determine the average par-ticle size and the size distribution, analyzing at least 400particles. The particle size was determined as the average ofthe major and minor axes of particles. The 15.0 nm cubeswere synthesized with a heating rate of 15 °C /min and areflux time of 1 h. Through quickly raising the reflux tem-perature from 200 °C as a rate of 35 °C /min and a shortreflux time of 20 min, nanocubes with small sizes of lessthan 10 nm were obtained due to a relatively faster nucle-ation speed which favors the formation of more nuclei ini-tially and thus a smaller size. To achieve nanocubes with bigsizes, a slow nucleation speed adjusted by a heating rate of5 °C /min and a long reflux time such as 3 h for the synthe-sis of 30 nm nanocubes were necessary. Such one-pot syn-thesis of monodisperse nanocubes with controlled size is fac-ile only by adjusting the heating rate and the reflux timewithout a laborious multistep seed-mediated growth. With asize less than 15.0 nm, the nanocubes usually have a verynarrow size distribution of less than 7% and the aspect ratiosclose to 1. Moreover, the nanocubes with small sizes welldisperse in most of organic solvent such as hexane. However,with a size more than 25 nm, the size distribution increasesgreatly and the aspect ratios located between 1 and 1.5 to-gether with a poor disperse in hexane. Typically, the 30 nmnanocubes have a size distribution of about 20%. Such aphenomenon may be due to a pronounced Ostwald ripeninghappened in the synthesis of nanocubes with big sizes.Meanwhile, the strong magnetostatic interaction that existedin the nanocubes with big sizes lead to the aggregation ofnanocubes resulted in depositing from suspensions. The highresolution TEM image of single 15.0 nm nanocubes is shownin Fig. 1g. It shows that these nanocubes are single crys-talline with an interplanar spacing of 0.292 nm for the 220plane, which is consistent with the bulk values of magnetitewith inverse spinel structure.The convective assembly technique is illustrated in Fig.2a. The deposition was made directly onto an angled wallof the convective assembly apparatus. As the suspensionevaporated slowly, the total bath level fell and caused thecontact line to eventually sweep down the substrate andcover it with particles; the substrate, Si substrates, or TEMcopper grids were effectively “withdrawn” relative to thebath. The suspension was diluted to between 0.002 and0.05 vol %. Controlled evaporation of the carrier solventfrom toluene/hexane dispersion of the particles led to Fe3O4nanocube superlattices. The SEM images of 15.0 nm Fe3O4nanocubes as shown in Fig. 2b reveal that most ofnanocubes are the orientation ordering in the self-assemblysuperlattices though there are different defects such as cracksand holes. The 16.0 nm spherical nanoparticles synthesizedby the method in Ref. 10 were also formed nanoparticle su-perlattices by the convective assembly technique. Figure 2cshows the XRD patterns of superlattice assembly from the15.0 nm nanocubes and 16.0 nm spherical nanoparticles. Thedifferent shapes possessed by each group of the particles doaffect the crystalline orientation of individual particles withinthe self-assembled superlattices. XRD of the self-assemblednanocubes on nonreflective Si substrate shows the intensifiedFIG. 1. TEM images of Fe3O4 nanocubes with different sizes a 6.5 nm, b15.0 nm, and c 30.0 nm and the corresponding size distribution histogramsd–f. g HRTEM images of single 15.0 nm Fe3O4 nanocubes with aninterplanar spacing of 0.292 nm for the 220 plane.FIG. 2. a Diagram of the convective assembly process; b SEM image ofFe3O4 nanocube superlattices on the Si substrate; the up-right inset is theenlarged partial image; c XRD patterns of nanocube curve A and nano-particle curve B superlattices on nonreflective Si substrate.07D526-2 Yang et al. J. Appl. Phys. 103, 07D526 2008Downloaded 29 Mar 2010 to 130.34.135.83. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp220 and 440 peaks in curve A of Fig. 2c, which is mark-edly different from that of the self-assembled isotropicspherical particles with a strong 311 peak in curve B. Thefact that the XRD of the nanocube assembly shows a strong220 peak indicates that each of the cubelike particles in thesuperlattices has preferred crystal orientation with 110planes parallel to the substrate. These demonstrate clearlythat particle shape can indeed induce texture in a self-assembled particle superlattice.Samples for the magnetic measurements were preparedby sealing the diluted magnetite nanocube solution intoluene/hexane typically 0.1 vol % into a quartz tube 515 mm2 in the glovebox with Araldite and the diamag-netic background from the quartz tube was subtracted. Thehysteresis loops of the Fe3O4 nanocubes with different sizesat 300 and 5 K are shown in Fig. 3. The saturation magne-tizations Ms at 300 K of Fe3O4 nanocubes with sizes of6.5, 15.0, and 30.0 nm were 39.5, 80.5, and 83.0 emu /g,respectively. The Fe3O4 nanocubes with a size less than25.0 nm possess typical superparamagnetic features at roomtemperature. However, the 30.0 nm Fe3O4 nanocubes show aferromagnetic behavior with a small coercivity HC of100 Oe and a high Ms value close to that of the commercialmagnetite powder Aldrich catalog No. 31,006-9. For thenanocubes with small sizes, the loss of magnetization is pos-sibly due to the contribution from small-size effect or spincanting at the outer surfactant layer of the particles. Thefield-dependent magnetizations at 5 K are presented in Fig.3b. The insert displays partial hysteresis curves for Fe3O4nanocubes with three different sizes at an enlarged scale offield strength for clearer viewing. All of the Fe3O4 nanocubeswith three different sizes show a size-dependent ferromag-netic behavior at low temperature. As for the saturation mag-netization and coercivity, the trend for both of them is in-creased with increasing nanocube size. The Ms at 5 K ofFe3O4 nanocubes with sizes of 6.5, 15.0, and 30.0 nm were54.0, 84.0, and 88.0 emu /g, respectively, which are all alittle higher than the corresponding values at 300 K. The HCincreased from 190 Oe for 6.5 nm to 500 Oe for 15.0 nmand 790 Oe for 30.0 nm Fe3O4 nanocubes. The coercivity ofmagnetic nanocubes is surely related to the magnetic aniso-tropy, which originates from the spin-orbital coupling atcrystal lattices. According to the Stoner-Wohlfarth single do-main theory, the magnetocrystalline anisotropy energy EA ofsingle domain nanoparticles is approximated by EA=KV sin2 , where K is the magnetocrystalline anisotropyconstant, V is the volume of nanoparticles, and  is the anglebetween the easy axis of nanoparticles and the direction offield-induced magnetization. Both the magnetocrystalline an-isotropy constant K and the volume of particles control themagnetic anisotropy. The single ion model has been success-fully used in bulk magnetic materials to semiquantitativelyevaluate the magnitude of anisotropy constant K.14,15 Themagnitude of K is closely related to the strength of spin-orbital coupling. Since the ligand field is weak in spinel fer-rites, all cations assume high spin states. An Fe3+ cation with3d5 electron configuration usually has its orbital angular mo-mentum quenched in a weak ligand field. Therefore, the con-tribution to the magnetic anisotropy should only come fromFe2+ cations in Fe3O4. Considering the ligand field at octa-hedral lattice sites, the 3d6 electron configuration of the Fe2+cation gives a ground energy state of 5T2g under the Oh sym-metry. However, this cubic symmetry is often lowered to atrigonal field in spinel due to the structural distortion fromthe Jahn-Teller effect. As a result, the triplet degeneratedenergy state of T2g splits into two levels of E and B2 with B2as the ground state. The orbital angular momentum in anFe2+ cation with a B2 ground state is quenched and littlespin-orbital coupling should be expected. So the volume ofnanocubes is the dominant factor in the magnetic anisotropyof Fe3O4 nanocubes. As a result, the coercivity of Fe3O4nanocubes is clearly increased with increasing nanocubesize. In addition, the high coercivity of 30.0 nm Fe3O4nanocubes may also result from the shape anisotropy contri-bution since they have aspect ratios of more than 1.In summary, we have developed a facile one-pot synthe-sis method of monodisperse size-controlled Fe3O4 nanocubesin organic phase. Moreover, the shape-controlled synthesisand shape-induced texture of monodisperse Fe3O4 nanocubesreveal that chemical process can be used to regulate thegrowth of a particle and control the crystal orientation of aparticle in a superlattice assembly. A robust magnetic nano-particle superlattice array with an aligned magnetic easy axisand controlled magnetic interactions between the neighbor-ing particles may fabricate high-performance magnetic nan-odevices in the future.ACKNOWLEDGMENTSWe thank Professor Shouheng Sun for helpful discussionand suggestion. This work is supported by the New Energyand Industrial Technology Development OrganizationNEDO of Japan and the Japan Society for the Promotion ofScience JSPS.1X. Peng et al., Nature London 404, 59 2000.2T. S. Ahmadi et al., Science 272, 1924 1996.3Y. Y. Yu et al., J. Phys. Chem. B 101, 6661 1997.4J. Cheon et al., J. Am. Chem. Soc. 126, 1950 2004.5V. F. Puntes et al., Angew. Chem., Int. Ed. 45, 3414 2006.6O. Margeat et al., Phys. Rev. B 75, 134410 2007.7V. F. Puntes et al., Science 291, 2115 2001.8H. Zeng et al., J. Am. Chem. Soc. 126, 11458 2004.9S. Soeya et al., Appl. Phys. Lett. 80, 823 2002.10S. Sun and H. Zeng, J. Am. Chem. Soc. 124, 8204 2002.11J. A. Lee et al., Langmuir 22, 5217 2006.12H. T. Yang et al., J. Appl. Phys. 101, 09J112 2007.13P. D. Cozzoli et al., J. Am. Chem. Soc. 125, 14539 2003.14K. Yosida and M. Tachiki, Prog. Theor. Phys. 17, 331 1957.15W. P. Wolf, Phys. Rev. 108, 1152 1957.FIG. 3. Hysteresis loops at a 300 K and at b 5 K for Fe3O4 nanocubeswith different sizes of 6.5, 15.0, and 30.0 nm. The insert shows the enlargedpartial hysteresis curves for Fe3O4 nanocubes with different sizes.07D526-3 Yang et al. J. Appl. Phys. 103, 07D526 2008Downloaded 29 Mar 2010 to 130.34.135.83. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp

Synthesis and magnetic properties of monodisperse magnetite nanocubes

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Synthesis and magnetic properties of monodisperse magnetite nanocubes

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