Journal of Colloid and Interface Science 349 (2010) 519–526

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Journal of Colloid and Interface Science

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Self-assembly behavior of hematite nanoparticles with controllableanisotropic morphology

Lili Wang, Lian Gao *

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China

a r t i c l e i n f o

Article history:Received 25 March 2010Accepted 2 June 2010Available online 8 June 2010

Keywords:Hematite nanoparticlesAnisotropic morphologySelf-assemblyMagnetic dipolar interactionExchange couplingChain structure

0021-9797/$ - see front matter � 2010 Elsevier Inc. Adoi:10.1016/j.jcis.2010.06.003

* Corresponding author. Fax: +86 21 52413122.E-mail address: liangao@mail.sic.ac.cn (L. Gao).

a b s t r a c t

In this work, complex structures such as long chain, ‘‘semi-flexible” chain, ‘‘threefold junction” and net-work were formed by self-assembly of colloidal hematite nanoparticles. Morphology of these colloidalnanoparticles used as building blocks transformed from truncated rhombohedra, hexagon to pseudo-hexagon by altering reaction time and surfactants. By further observation using HRTEM, these nanopar-ticles were confirmed to grow along the c-axis. It was found that the molecular structures of surfactantsmake great influence on the transformation of bonding modes between carboxyl and iron atom on thesurface. Then crystal growth rate was changed. It led to two opposite growth trends along the c-axis.More interestingly, the chains formed by these colloidal nanoparticles were also assembled along thec-axis. Meanwhile, configuration diversity seemed related to the morphological anisotropy along the c-axis. It was believed that two main forces between the nanoparticles were responsible for the variousconfigurations, magnetic dipole–dipole and exchange-coupling interaction. The morphological anisot-ropy was considered to play a key role in the coordination of the two interactions which led to differentcomplex structures by self-assembly. Discussion was taken to explain the formation of these interestingconfigurations.

� 2010 Elsevier Inc. All rights reserved.

1. Introduction

In recent years, it is of considerable interest in controlling andunderstanding the arrangement of colloidal nanoparticles to formlinear chains, [1–3] rings [4,5] and higher structures [6,7] for itsinfluence on properties. Many researches have been made drivenby the motivation of both fundamental insight and practical impli-cation. It is believed that two main dominant factors are responsi-ble for these unique phenomena: drying-medicated assembly anddipolar-directed assembly. Ohara and Gelbart [8] have deeplyinvestigated the interplay between solvent evaporation and forma-tion of nanoparticle array. For magnetic materials, it is usuallydominated by the magnetic dipolar attraction to form an aniso-tropic chain-like structure. Butter et al. [9] have reported the firstdirect experimental observation of dipolar chains in iron ferrofl-uids by using cryogenic electron microscopy. They have controlledthe structure and properties of these aggregates by adjusting theparticle sizes and found that aggregates caused by magnetic dipo-lar moment favored a head-to-tail orientation.

In view of the wide application of magnetic fluids, great efforthas been made to investigate the self-assembly behavior ofmagnetic materials with or without assistance of an externalmagnetic field [10–13]. Compared with magnetite or maghemite,

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hematite is generally considered as a weakly magnetic material.Only a few reports have concerned self-assembly of hematite forits weak ferromagnetic property at room temperature [14,15].However, recently, hematite has been found to self-assemblecaused by exchange coupling which is orders of magnitude largerthan dipolar interaction [16]. It seems that exchange-couplinginteraction has great potential to dominate arrangement of hema-tite. In order to obtain controllable complex structure, it will behelpful to study coordination of exchange coupling and magneticdipolar interaction.

According to the work of Sun et al. [17,18] it can be concludedthat the self-assembly behavior has been in relation to the symme-try properties of organizing particles. Yamamuro and Sumiyama[19] have studied the self-assembly mechanism of nanocubes inthe light of energy and found that morphology played a decisiverole in the arrangement. So it is important to choose nanoparticleswith specific morphology as building blocks to explore the assem-bly mechanism. In the previous literatures, morphologies of nano-particles prepared for arrangement in certain conditions are mostlyspherical and other less-anisotropic shapes. Currently, Chen et al.[20] have reported the assembly of magnetite nanocubes intoflux-closure rings. Their experiments have confirmed that a criticalparticle size affects the formation of dipolar structure, but theyhave not further discussed the relation between anisotropic shapesand those structures. Lee and Liddell [15] have reported the syn-thesis of peanut-shaped colloids and employed them as building

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blocks to study the effect of hierarchical structure and colloidalproperty on their self-assembly behavior. It seems more difficultto understand how nanoparticles with anisotropic shapes formordered structures because there are usually more assembly pref-erences in their arrangement. As anisotropic nanoparticles tendto form complex structures, it is of great importance to study therelationship between their morphological anisotropy and assemblybehavior.

Herein, we used truncated rhombohedral, hexagonal and pseu-do-hexagonal nanoparticles as building blocks to make observationon their self-assembly behavior. And then we studied the influenceof morphological anisotropy on the interaction acting in thearrangement of nanoparticles.

2. Materials and methods

All chemical agents were purchased from Shanghai ChemicalReagent Co., Ltd. and used as received without further purification.The water used in this work was distilled and deionized.

2.1. Synthesis of truncated hematite nanorhombohedras

Synthesis of truncated hematite nanorhombohedras was car-ried out by a surfactant-mediated hydrothermal method accordingto the references, with some alteration [21]. In a typical synthesis,oleic acid (15 mL) and sodium oleate (1.68 g) were mixed in etha-nol (45 mL) with magnetic stirring at room temperature. Then ironchloride hexahydrate (FeCl3�6H2O) aqueous solution (10 mL, 0.2 M)was added to the mixture with vigorous stirring to form a dark-redhomogeneous solution. The final solution was transferred into aTeflon-lined stainless autoclave with a capacity of 90 mL. Thesealed autoclave was heated to 120 �C for 20 h and spontaneouslycooled to room temperature. Then the autoclave was opened anddark-red precipitates of hematite nanoparticles on the bottomwas obtained by removing upper flaxen mixture. The productwas dispersed in cyclohexane and precipitated by adding ethanolto remove the residual oleaginous mixture centrifugally. To under-stand the possible growth mechanism of the nanorhombohedra,controlled experiments were carried out.

2.2. Synthesis of hexagonal hematite nanorhombohedras

The experimental procedure of synthesizing hexagonal hema-tite nanoparticles was similar to that of nanorhombohedras, exceptthat the reaction time was shortened to 15 h.

2.3. Synthesis of pseudo-hexagonal hematite nanoparticles

The experimental procedure of synthesizing pseudo-hexagonalhematite nanoparticles was similar to that of nanorhombohedras,except that the sodium oleate was replaced by sodium stearate.

2.4. Preparation of samples for observation on self-assembly behavior

The product was dispersed in cyclohexane (4 mL) as initial solu-tion. The TEM samples were obtained by dropping hematite nano-particles dispersion onto the carbon-coated Cu-grids andevaporating for 90 s, and wicking away the excessive solvent witha filter paper. And the default solution was obtained by diluting theinitial solution 15 times.

2.5. Characterization

The crystalline structure of the samples was analyzed on aX-ray diffraction (XRD, D/max 2550 V, Rigaku, Tokyo, Japan) using

Cu Ka radiation (k = 1.5406 Å) and the angular instrument resolu-tion is 0.02� in 2h. The powder sample for XRD analysis wasprepared by drying the cyclohexane dispersion of hematite nano-particles at room temperature and the single-crystalline Si (1 0 0)wafer was used as a substrate. The size, morphology and structurewere probed by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selectedarea electron diffraction (SAED) on a field emission transmissionelectron microscope (JEM-2100F, JEOL, Tokyo, Japan; acceleratingvoltage: 200 kV). The organic groups absorbed on the surface ofnanoparticles were analyzed by Fourier transform infrared spec-troscopy (FTIR, Nicolet 7000-C with 4 cm�1 resolution). The ther-mal analysis was taken on a thermogravimetric-diffierentialscanning calorimetric analyzer (STA 449C, Netzsch) in air to inves-tigate the contents of oleic group absorbed on the surface of theproducts.

3. Results and discussion

3.1. Morphology of the as-prepared nanoparticles

The as-prepared nanoparticles with relatively narrow shape andsize distribution were shown in Fig. 1a. The typical morphology ofthese nanoparticles was truncated-rhombohedra shown in Fig. 1b.To further certify the shape of the truncated nanorhombohedra, aset of TEM tilting images were shown in Fig. 1c (the rotation axiswas noted by the white arrow). It indicated that the length alongthe c-axis became shorter with increase of the rotation angle,which was consistent with our hypothesis about the shape. A sta-tistical treatment of size (Fig. 1d) showed that the average lengthof these nanoparticles was 48 nm along the c-axis.

The XRD patterns illustrated in Fig. 1e revealed good crystallin-ity of as-synthesized nanoparticles matching with the hematitestructure corresponding to the literature data (JCPDS No. 33-0664, a = 5.0356 Å, c = 13.7489 Å).

Further observation on the morphology of as-prepared nanopar-ticles was presented by a high-resolution TEM image shown inFig. 1f. Two different lattice spacing were corresponding to {0 1 2}and {1 0 4} planes, respectively, and further assigned to ð�1012Þand ð10 �14Þ by the SAED. There was an angle of 57.6� betweenthe ð�1012Þ plane and long axis of the particle. It seemed that thelong axis of nanocrystal was parallel to the [0 0 0 6] direction,which was confirmed by the SAED shown in the inset of Fig. 1f.

To explore the growth mechanism of the nanorhombohedras,controlled experiments were carried out. The morphology of nano-particles obtained by shorter reaction time (15 h) was shown inFig. 2a. It could be seen that there were polyhedral with hexagonalappearance and smaller irregular nanoparticles as well. The typicalmorphology of these nanoparticles was shown in Fig. 2b. Com-pared with the morphology shown in Fig. 1a, it indicated that thosesmaller irregular nanoparticles disappeared along with morphol-ogy transformation from the hexagonal polyhedral to the nano-rhombohedras. It was coincident with the phenomenon causedby the Ostwald Ripening mechanism. Further observation of thepolyhedral was conducted through HRTEM (Fig. 2c). It was foundthat the lattice distances were 0.37 nm and 0.27 nm correspondingto {0 1 2} and {1 0 4} planes respectively, which was similar to thecrystal structure shown in Fig. 1f. According to the description ofCornell and Schwertmann [22], hematite consisted of the hexago-nal-close-packed arrays of oxygen ion stacked along with the[0 0 1] direction. Generally, the close-packed planes of crystaltended to grow slowly for high concentration of lattice point onthese faces. Meanwhile, the addition of surfactant would also alterthe growth rate as a consequence of different interactions betweensurfactant molecules and certain faces. Desorption of surfactant

Fig. 1. TEM observation, size distribution and XRD pattern of the nanoparticles synthesized by using sodium oleate. (a) Low-resolution TEM image of the 20-h sample withaddition of sodium oleate; (b) close observation on the as-synthesized nanoparticles; (c) a statistical result of these nanoparticles shown in (a); (d) XRD diffraction spectra; (e)a series of TEM tilting images of a typical nanoparticle with the same scale bar and the white arrow represents the rotation axis; (f) high-resolution TEM image of ananoparticle and the corresponding SAED pattern is shown in the bottom inset.

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molecules due to thermal disturbance was easier from specificplanes and resulted in preferential growth of low-index surfaces.Since the nanoparticles were elongated along [0 0 1] direction byprolonging reaction time, it could be concluded that the nano-rhombohedra was gained by preferential growth of these hexago-nal polyhedral along the [0 0 1] direction. While due to the excessof oleic acid, the corners of the as-synthesized nanorhombohedraswere found smooth along the long axis in our experiment.

The FTIR spectra of the as-synthesized nanorhombohedras(curve a) and hexagonal nanoparticles (curve b) were shown inFig. 3a. The separation of m (COO�) bands was about 235 cm�1 intwo curves. It indicated that the surfactant groups were chemi-sorbed onto the surface by coordination of oxygen atoms in car-boxylate and iron atoms on the surfaces of nanoparticlesasymmetrically in the both samples [23,24]. Noteworthily, therewas a slight peak near to 1500 cm�1 in curve b. By further

Fig. 2. TEM observation of the 15-h sample prepared by using sodium oleate. (a) Low-resolution TEM image; (b) close observation on the as-synthesized nanoparticles; (c)high-resolution TEM image of a nanoparitcle and the corresponding FFT image is shown in the bottom inset.

Fig. 3. FTIR spectra of the as-synthesized samples. (a) Curves a and b respectivelycorrespond to 20-h (curve a) and 15-h (curve b) samples obtained by using sodiumoleate and curve c corresponds to 20-h sample obtained by using sodium stearate.The partial enlargement of the curves from 4000 to 1000 cm�1 is shown in theinset; (b) curve d and e respectively corresponds to the 4-h samples obtained byusing sodium oleate (curve d) and stearate (curve e).

Fig. 4. TEM observation of the samples obtained by using sodium stearate. (a) Low-resolution TEM image of the 20-h sample; (b) high-resolution TEM image of atypical nanoparticle; (c) low-resolution TEM image of the 15-h sample; (d) closeobservation on the 15-h sample.

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shortening the reaction time to 4 h, there was dramatic change inthe FTIR spectrum (curve d) shown in Fig. 3b. The peak at1635 cm�1 was assigned to the asymmetrical vibration of carbox-ylate groups as well as the peak at 1520 cm�1. It indicated thattwo coordinate modes coexisted in the intermediate sample: uni-dentate and bidentate [23,24]. As the bidentate mode disappearedin curve a shown in Fig. 3a, it seemed that the coordinate modetransformed from bidentate to unidentate with the growth ofhematite nanocrystals. In this transformation process, one of thetwo bonds between iron and oxygen atom was broken and accord-ingly a carbonyl double bond was formed. Thus the bond energyinfluenced the bond breaking process, which was subject to themolecular structure of surfactant.

Based on this, we substituted the sodium stearate for the so-dium oleate, and then pseudo-hexagonal hematite nanoparticleswere obtained by heating for 20 h and their morphology wasshown in Fig. 4a. From the corresponding HRTEM image of a typicalnanoparticle shown in Fig. 4b, the lattice spacing was 0.46 nmwhich was consistent with the distances among (0 0 3) planes.And the FTIR spectrum of this sample (curve c) was also shownin Fig. 3a. Similarly, no obvious peak between 1625 and1390 cm�1 was found. When the reaction time was 4 h, therewas a peak at 1520 cm�1 (curve e in Fig. 3b) as well. It indicatedthere was also a bond transformation process. The controlledexperiment by shorter reaction time (15 h) was also taken. Mor-phology of the 15-h sample was shown in Fig. 4c and d. There wereno smaller nanoparticles in this sample and it seemed that hexag-onal nanoparticles got narrow along the [0 0 1] direction by pro-longing the reaction time. As growth of nanoparticles was greatlyinfluenced by the surfactant, difference between the growth trendsof two samples was attributed to the use of different surfactants,sodium stearate and oleate. It was due to the absence of carbon–carbon double bond that influenced the bond energy of iron and

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oxygen atom. Faster bond breaking led to faster crystal growth.According to previous literature [25] iron oleate was formed byreaction between hematite and oleic acid at high temperature. Itseemed consistent with our observation that morphology of thenanoparticles transformed from hexagon into pseudo-hexagon. Itwas believed that the same phenomenon occurred in the prepara-tion process by using sodium oleate. However, smaller particles inthat 15-h sample were more easily ‘‘dissolved” by oleic acid thanthose bigger hexagonal ones, according to the classical OstwaldRipening mechanism. The formation of iron oleate afforded the fur-ther growth of those bigger hexagonal nanocrystals. And thennanorhombohedras were obtained by prolonging the reactiontime. The schematic diagram was shown in Fig. 5.

Compared with the truncated rhombohedras, the morphologi-cal anisotropy of pseudo-hexagon was inhibited and less distin-guished along the c-axis orientation.

3.2. Self-assembly behavior of as-synthesized nanoparticles

By controlling the concentration of dispersion and the rate ofevaporation, the head-to-tail chains were formed through theself-assembly of as-synthesized nanorhombohedras (shown inFig. 6a–d). It was interesting to find that these chains were arrangedalong the long axis. Compared to the chain in Fig. 6a, it showed obvi-ous distortion for interference of a smaller nanoparticle in Fig. 6b(marked by white arrow). However, the chains bent spontaneouslywhen consisting of more nanorhombohedras (Fig. 6c and d). Furtherobservation on joint part of two adjacent particles was conducted byHRTEM. As shown in Fig. 6e, the lattice spacing of particle A was0.37 nm indexed to {0 1 2} planes. And the plane formed an angleof 32� with the orientation of the chain which was equal to the anglebetween (0 1 2) plane and [0 0 1] direction. The distance betweenadjacent lattice planes of particle B was 0.27 nm in accordance with{1 0 4} planes. The angle between {1 0 4} plane and the orientationwas 77� instead of 38� which was the angle between (0 1 4) and[0 0 1] direction. In Fig. 6f, a similar phenomenon was observed.The angle between {1 1 3} plane, whose lattice spacing was0.22 nm, and the orientation of chain was 48� rather than 28� (theangle between (1 1 3) and [0 0 1] direction). However, the angle be-tween two {1 1 3} planes was bisected along the orientation direc-tion indicated by the white arrow. As [0 0 1] direction was alwaysparallel to the angular bisector of {1 1 3} planes, it could be con-cluded that the long axis of some nanoparticles was not parallel tothe plane of paper.

For the shape of nanorhombohedras was corner-burnishedhexagon-bipyramid, their arrangement fashions were diverse.Some particles lay on the carbon film along the c-axis, while otherslay along the orientation with a certain angle diverging from the c-axis. The angular deviation observed in Fig. 6e and f could beattributed to diverseness of the arrangement fashion, which wasillustrated by the schematic diagram demonstrated in Fig. 6g. Itwas similar to that reported by Frandsen et al. [16] The diversenesswas due to the rough surface of carbon film on the Cu-grid. And itwas also attributed to that the sample was prepared by a solvent-evaporating method to some extent. Wherein the surfactantgroups absorbed on the surface of the nanorhombohedras playedan important role. A TG-DTA analysis (shown in Fig. 7) was taken

Fig. 5. Growth schematic diagram of the samples prepared by using sodium oleateand stearate respectively.

to determine the amount of surfactant group. It was found thatonly about 4.0 wt.% of surfactant groups absorbed on the surfaceof nanorhombohedras from the TG data, which was less than thatreferred in the literatures [26]. Compared with the oleic acid tailsinterdigitating at �2.2 nm, there was almost no interparticle spac-ing between two adjacent nanorhombohedras (shown in Fig. 6eand f). So in our case, it excluded a simple surfactant-medicatedself-assembly although the surfactant group on the surface actedon the assembly behavior of nanoparticles [27]. In other words, de-crease of surfactant group inhibited their effect on the self-assem-bly behavior in the evaporation process. As mentioned above, thenanorhombohedras was arranged to form a chain by head-to-tailalong the [0 0 1] axis. According to the previous work [16] orientedattachment caused by exchange excoupling of hematite nanoparti-cles have taken place along the [0 0 1] direction, which the sixpuckered Fe-layers in the hematite was perpendicular to. And thespins of adjacent Fe-layers were confined in the (0 0 1) plane andcoupled antiparallelly (shown in Fig. 6h schematically) [16,28–31]. So it seemed that exchange coupling rather than dipolar forcedominated the interaction between two adjacent nanoparticles inclose proximity, which was attributed to fewer oleic groups coat-ing on the interface in our experiment.

Nevertheless, the dipolar–dipolar interaction also influencedthe arrangement of the nanorhombohedras along the c-axis.According to the previous literatures, the dipolar interaction en-ergy between two magnetic particles could be written as [15,32].

Udipoleðr; hÞ ¼ k � 1� 3 cos2 hr3 ð1Þ

where k was a parameter determined by magnetic dipole momentper particle and permeability of free space, h was the angle of thechain (between the dipolar orientation and the line linking the par-ticle centers), r was the dipole–dipole separation. Since the dipolarorientation was perpendicular to the c-axis, the magnetic dipolarenergy reached the maximum when the nanoparticles were ar-ranged along the c-axis. Such chain configuration was not theoret-ically stable as predicted by Philipse and Maas [33] Then thechain tended to transform into a closed ring possessing lower en-ergy by overcoming a potential barrier. Yet no ring configurationwas found for the strong exchange-coupling interaction. Accordingto the work of Philipse et al. [9] the morphology of ferrofluids couldbe controlled by adjusting the particle size. Chen et al. [20] furtherconfirmed that it was determined by the particle size whether achain could degrade into a ring through dipolar–dipolar interaction.Compared with the chain A shown in Fig. 6c, the radian of the chainB shown in Fig. 6d was larger. It seemed that there were some smallparticles with length of 30–40 nm along the c-axis in chain A, whilethe particle size in chain B was distributed narrowly about 50 nm. Itindicated that the dipole–dipole interaction played a role inarrangement of the nanorhombohedras as well.

While there was no network configuration found in the sample,even if the concentration of colloidal nanoparticles dispersion wasaltered. The nanoparticles tended to aggregate closely with face-to-face attachment [12] to lower the overall energy and then thecorner-to-face attachment of truncated rhombohedral nanoparti-cles was unstable. Fig. 6b showed a typical case of such attachmentwhere a smaller particle distorted the chain obviously. It made thecomplex configuration such as network absent. It seemed that themorphological anisotropy had great influence on the coordinationof exchange coupling and dipolar interaction.

In contrast, the single chain formed by hexagonal nanoparticlesshown in Fig. 8a was straight for the formation of face-to-faceattachment dominated by exchange-coupling interaction and thebent configuration was also absent from the aligned chains (shownin Fig. 8b). As edges of the nanoparticles were approximately equal,

Fig. 6. TEM observation on self-assembly behavior of the nanorhombohedras. (a–d) Head-to-tail chains; (e) and (f) enlargement of the adjacent parts; (g) schematic diagramof the nanorhombohedras lying on the Cu-grid; (h) schematic diagram of exchange-coupling interaction between the adjacent particles.

Fig. 7. TG-DTA of the as-synthesized nanorhombohedras.

Fig. 8. TEM observation on self-assembly behavior of the hexagonal nanoparticles.(a) Single chain and (b) aligned chains assembled by the hexagonal nanoparticles.

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no complex configurations caused by different orientations werefound in this sample.

By the same experimental procedure, various configurationsassembled by pseudo-hexagonal nanoparticles were formed,which were extremely similar to the models provided in Safran’sreport [34]. A ‘‘semi-flexible” chain was shown in Fig. 9a. It con-sisted of two parts and there was a turn in the middle of the chain.Fig. 9b displayed a ‘‘threefold junction” formed by nanoparticles.Such configuration was predicted unstable unless the overallenergy was lower than that of head-to-tail unpaired structure.Besides these simple configurations, self-assembled networks

formed by pseudo-hexagonal nanoparticles were also found(shown in Fig. 9c and d). They could be considered consisting ofboth the simple structures aforementioned. To lower the overallenergy, ring-like structures were formed coordinately by severalchains instead of one chain, especially notable in the configurationshown in Fig. 9d). To make further understanding, a schematicdiagram was displayed in Fig. 9e. As mentioned, the pseudo-hexagonal nanoparticles grew along the [0 0 1] direction whichwas parallel to the short axis and the characters a and b respec-tively represented the short and long edges of each particle. Thedipolar distance was minimal between two adjacent particlesarranged along the c-axis, which was denoted by r1 in Fig. 9e.

Fig. 9. TEM observation on self-assembly behavior of the pseudo-hexagonal nanoparticles. (a) Semi-flexible chain; (b) threefold junction; (c and d) self-assembled networksformed by pseudo-hexagonal nanoparticles; (e) schematic diagram of the different interaction between adjacent particles; (f–h) Schematic diagram of the differentconfiguration shown in (a–c) formed by exchange coupling coordinated with dipolar interaction. The scale bar in (b) is 50 nm.

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And the distance denoted by r2 was second minimal to r1. Whentwo neighboring particles assembled by sharing the short-edge a,the distance r3 was maximal. As the dipolar orientation was per-pendicular to the c-axis, h was inversely proportional to the dis-tance between the particle centers when the particles aggregatedclosely. So according to the Eq. (1), dipolar interaction energy Udi-

pole was inversely proportional to the distance r. It meant thatthe energy was maximal when particles were arranged along thec-axis. While the other two arrangement fashions resulted in lowerenergy and it was minimal by sharing short-edge (the distance wasr3). Contrarily, such energy diversity aroused by dipolar interactionwas absent in those configuration formed by hexagonal nanoparti-cles for their low morphology anisotropy. The coordination ofexchange coupling and dipolar interaction led to the diverseconfiguration assembled by pseudo-hexagonal nanoparticles.Fig. 9f–h respectively demonstrated how the exchange couplingworked on the configurations shown in Fig. 9a–c schematically,from simple to complex, coordinating with magnetic dipolar inter-action especially at the joint parts of the chains. Compared withtruncated nanorhombohedras, the pseudo-hexagonal nanoparti-cles were successfully arranged to form high structures. It couldbe contributed to that the unstable corner-to-face attachmentwas replaced by face-to-face attachment at the joint of two chains.

It indicated the morphological anisotropy played a key role therein.Since the anisotropy along the c-axis was reduced in the pseudo-hexagon, the effect of exchange coupling was weakened and thatof magnetic dipole was strengthened contrarily. And then it in-creased the possibility of the coordination of exchange couplingand magnetic dipolar interaction, which led to the formation ofcomplex network structures.

4. Conclusions

Colloidal hematite nanoparticles with different anisotropicmorphologies, truncated rhombohedral, hexagonal and pseudo-hexagonal were synthesized. And their crystal structures weredeeply observed and it was confirmed that the growth directionwas parallel to [0 0 1] direction, along which the exchange cou-pling took place. Those nanoparticles were employed as buildingblocks for studying their self-assembly behavior. It was found thatthe arrangement fashion was subject to two main factors, magneticdipole–dipole and exchange-coupling interaction, with the lattermore dominant. Morphological anisotropy played a key role inthe coordination between the two forces. It resulted in the forma-tion of diverse configurations. The truncated nanorhombohedras

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were too anisotropic along the long axis to form stable networkconfiguration for the unstable corner-to-face attachment. Thehexagonal nanoparticles had low morphological anisotropy so thatthey tended to form aligned chains instead of complex structure.The pseudo-hexagonal nanoparticles were easy to aggregate byface-to-face attachment. Their morphology was appropriate toform self-assembled network structures for their unequal edgesand low anisotropy along c-axis that promoted the coordinationof exchange coupling and magnetic dipolar interaction. It mightprovide a new approach to organize the colloidal nanoparticles toform complex structures.

Acknowledgments

This work was supported by the National Key Project of Funda-mental Research (Grant no. 2005CB6236-05), the National NaturalScience Foundation of China (nos. 50572116, 50602049), andShanghai Nanotechnology Promotion Center (no. 0852nm01900).We are grateful to Ms. Meiling Ruan for her help with TEM. Specialthanks are given to Dr. Songwang Yang for his guidance on theresearch work.

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