ARTICLE IN PRESS

Journal of Crystal Growth 270 (2004) 156–161

*Corresp

83595535.

E-mail

tangsl@nju

0022-0248/

doi:10.101

Simplified synthesis of single-crystalline magnetic CoFe2O4

nanorods by a surfactant-assisted hydrothermal process

G.B. Ji, S.L. Tang*, S.K. Ren, F.M. Zhang, B.X. Gu, Y.W. Du

National Laboratory of Solid State Microstructure and Department of Physics, Nanjing University, Nanjing 210093,

People’s Republic of China

Received 28 February 2004; accepted 4 June 2004

Available online 28 July 2004

Communicated by M. Kawasaki

Abstract

A simple hydrothermal route, using cetyltrimethylammonium bromide (CTAB) as the surfactant, has been proposed

for synthesizing single-crystalline spinal cobalt ferrite (CoFe2O4) nanorods. Transmission electron microscope (TEM)

and scanning electron microscopy (SEM) images indicated that the final product consists of nanorods with a diameter

of about 25 nm and length up to about 120 nm.Furthermore, high-resolution TEM (HRTEM) revealed a clear lattice

structure of the [1 0 0] planes in the nanorods. The magnetic properties of the CoFe2O4 nanorods were characterized

using vibrating sample magnetometer (VSM). The possible formation mechanism for the CTAB-assisted hydrothermal

synthesis of CoFe2O4 nanorods has been discussed.

r 2004 Elsevier B.V. All rights reserved.

PACS: 75.50.Ss; 81.10.Dn; 81.16.Be; 81.20.Ka

Keywords: A2. Hydrothermal crystal growth; B1. Nanomaterials; B2. Magnetic materials

1. Introduction

Cobalt ferrite (CoFe2O4), with a partiallyinverse spinel structure, is one of the mostimportant and most abundant magnetic materials.As a conventional magnetic material, with a Curietemperature (TC) around 793K, CoFe2O4 is well

onding author. Tel.: +86-25-3593817; fax: +86-25-

addresses: gbji@ufp.nju.edu.cn (G.B. Ji),

.edu.cn (S.L. Tang).

$ - see front matter r 2004 Elsevier B.V. All rights reserve

6/j.jcrysgro.2004.06.025

known to have large magnetic anisotropy, moder-ate saturation magnetization, remarkable chemicalstability and a mechanical hardness, which make ita good candidate for the recording media [1,2].CoFe2O4 ultrafine powders [3–5] and films [6–8]have attracted considerable attention for theirwide range of technological applications such astransformer cores, recording heads, antenna rods,memory, ferrofluids, biomedical application andsensors, etc. [9–11].Over recent years, the chemical solution routes

were successively emerging as effective, convenient,

d.

ARTICLE IN PRESS

G.B. Ji et al. / Journal of Crystal Growth 270 (2004) 156–161 157

less energy demanding and less materials consum-ing synthetic techniques for material preparation[12–13]. However, the hydrothermal route is oneof the most used ones, owing to its economics andthe high degree of compositional control [14]. Inaddition, the hydrothermal synthesis requiresneither extremely high processing temperaturenor sophisticated processing. For example, ferritescan be prepared via the hydrothermal method at atemperature of about 150�C, whereas the solid-state method requires a temperature of 800�C [15].Hydrothermal synthesis of several ferrites has beenreported. However, there is little report on thesynthesis of single-crystalline CoFe2O4 nanorods.We present here a simple hydrothermal routewithout a preformed template for the preparationof CoFe2O4 nanorods.

15 20 25 30 35 40 45 50 55 60 65 70

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

(440)(333)

(422)(400)

(311)

(220)

(111)

Inte

nsit

y (

a.u

.)

2θ(θ( degree)

a

b

Fig. 1. X-ray diffraction patterns of the prepared CoFe2O4

particles and nanorods via hydrothermal at 130�C treated for

15 h without CTAB (a) and with CTAB (b).

2. Experimental procedure

The purities of all the reagents used are ofanalytical grade without further purification. In atypical procedure for the preparation of CoFe2O4

nanorods, 1 g of cationic surfactant cetyltrimethy-lammonium bromide (CTAB) was dissolved in35ml deionized water to form a transparentsolution. Then ferric chloride hexahydrate(FeCl3.6H2O) of 1 g was added to the solution.After 10min stirring, stoichiometric amount ofCoCl2 was introduced into the mixed solutionunder vigorous stirring. Deionized water wasadded to make the solution for a total volume of40ml, and pH of the solution was adjusted to 11.0.Before being transferred to a Teflon-lined auto-clave of 50.0ml capacity, the solution mixture waspretreated under an ultrasonic water bath for 30–40min.Hydrothermal synthesis was carried out at130�C for 15 h in an electric oven without shakingor stirring. Afterwards, the autoclave was allowedto cool to room temperature gradually. The blackprecipitate collected was washed with distilledwater three times in an ultrasonic bath to removeany possible impurities. The solid was then heatedat 80�C and dried under vacuum for 2 h.The crystallographic information of the as-

prepared samples was analyzed by power X-raydiffraction (XRD) with Cu Ka radiation

(l=1.5406 (A) at a scanning rate of 1�min�1 andthe detective range from 15� to 70�. Morphologiesand structures of the samples were investigatedusing a JSM-6700F scanning electron microscopy(SEM) and a HITACHI-2000 transmission elec-tron microscopy (TEM). The X-ray photoelectronspectroscopy (XPS) data were obtained by aphotoelectron spectrometer with Mg Ka source,a concentric hemispherical analyzer operating in afixed analyzer transmission mode and a multi-channel detector. The magnetic properties weremeasured using vibrating sample magnetometer(VSM, Lakeshore, Model 7300 series).

3. Results and discussion

All the samples were obtained under hydro-thermal conditions at 130�C. Fig. 1a shows theXRD pattern of the samples obtained after hydro-thermal treatment in the absence of CTAB. Noimpurity phase such as FeCl3 and CoCl2 was detectedfrom this pattern. The broadened nature of thesediffraction peaks indicates that the grain sizes of thesamples are on nanometer scale. The XRD patternsof the samples obtained in the presence of CTAB aregiven in Fig. 1b. All XRD peaks can be indexed tothe spinal structure with lattice constant ofa=8.39 (A, which exhibits that the prepared CoFe2O4

nanorods are single phase. In addition, all peaks have

ARTICLE IN PRESS

Fig. 2. TEM images of the samples obtained in the absence of CTAB (a), in the presence of CTAB (b), magnified section of a bundle of

the CoFe2O4 nanorods and the corresponding diffraction pattern of these nanorods (inset) (c) and HRTEM image of an individual

nanorod (spacing=0.48 nm) (d).

Fig. 3. SEM images of the synthesized nanorods taken at different magnifications.

G.B. Ji et al. / Journal of Crystal Growth 270 (2004) 156–161158

much higher intensities than those measured for thesample synthesized without the addition of CTAB,indicating higher crystallinity.Fig. 2 presents the representative TEM images

of the samples prepared using the hydrothermalprocess. Fig. 2a is for the sample obtained in theabsence of CTAB, CoFe2O4 particles with anirregular-shape revealed. The typical morphologyof the samples obtained in the presence of CTABafter 15 h of hydrothermal treatment is shown inFig. 2b, which clearly reveal the effect of CTAB onthe nucleation and growth process of CoFe2O4.Interestingly, the morphology changed drastically

when the surfactant was applied.It has also beendemonstrated that a large number of CoFe2O4

nanorods were formed, accompanied by thedisappearance of the particles. The mean size isabout 120 nm in length and 25 nm in diameter. Theresults clearly show that the surfactant CTAB isresponsible for the formation of nanorods in largequantities. The electron diffraction (ED) pattern(shown in the inset of Fig. 2c) obtained along atypical individual nanorod confirms the singlecrystalline structure of the CoFe2O4 nanorods,and can be indexed as the [1 1 1], [3 1 1], and [2 2 0]zone axis of CoFe2O4. High-resolution TEM

ARTICLE IN PRESS

735 730 725 720 715 710 705 7007000

7500

8000

8500

9000

9500

Fe 2p3/2Fe 2p

1/2

Inte

nsi

ty [

CP

S]

Binding Energy (eV)

536 534 532 530 528 526 524

2000

3000

4000

5000

6000

7000O 1s

Inte

nsity

[CP

S]

Binding Energy (eV)

810 800 790 780 77010000

11000

12000

13000

14000

15000

16000

(a)

(b)

(c)

Co 2p3/2

Co 2p1/2

Inte

nsity

[CP

S]

Binding Energy (eV)

Fig. 4. XPS spectra of Co 2p (a), Fe 2p (b) and O 1 s (c) core

levels for the prepared cobalt ferrite nanorods.

G.B. Ji et al. / Journal of Crystal Growth 270 (2004) 156–161 159

(HRTEM) image shown in Fig. 2d furthersupports the single-crystalline nature of thesenanorods. It is clear that the nanorod is structu-rally uniform with a lattice spacing of about0.48 nm, which corresponds to the (1 1 1) latticeplane. Combined with the ED results it isconfirmed that in the presence of the surfactantCTAB the CoFe2O4 nanorods grow along [1 0 0],one of the easy magnetic axes of CoFe2O4. TheSEM images of the as-prepared samples shown inFig. 3 have revealed the presence of a largequantity of nanorods with typical length rangingfrom 100 to 130 nm and diameter of about 25 nm.The overall chemical composition for the as-

prepared samples was determined by X-rayphotoelectron spectroscopy (XPS). In the XPSanalysis, the samples were exposed to the mono-chromic X-radiation and the properties of theinner-shell electrons were probed. Fig. 4(a–c)display the XPS spectra of the prepared samples.The XPS analysis indicates that the sample iscomposed of Fe, Co and oxygen, with correspond-ing binding energies of 780.35 eV (Co 2p3/2),796.04 eV (Co 2p1/2), 710.55 eV (Fe 2p3/2),724.29 eV (Fe 2p1/2), 529.75 eV (O 1 s), respec-tively, and the value states were proved to beCo2+, Fe3+ and O2�. The analysis of the Co 2p,Fe 2p and O 1 s peaks gave Co:Fe and Fe:Oatomic ratios closely to 1:2 and 1:2, respectively, asexpected for the stoichiometric composition ofcobalt ferrite. XPS measurement results outlinedabove again support the result of XRD, showsthat the cobalt ferrite has been synthesized.These observations suggest that the surfactant

CTAB play a key role in controlling the nucleationand growth of the CoFe2O4 nanorods. For thereaction system in the presence of CTAB, thesurface tension of solution is reduced due tothe existence of surfactant, which lower the energyneeded for the formation of a new phase. CTAB isan ionic compound, which ionizes completely inwater [16]. The resultant CTA+ is a positivelycharged tetrahedron with a long hydrophobic tail,while the growth unit for CoFe2O4 crystal isconsidered to be Co–Fe–OH�. Therefore, ion-pairs between CTA+ and Co–Fe–OH� could beformed due to electrostatic interaction, the Co–Fe–OH� particles are negatively charged, and so

CTA+ ions were adsorbed on the particle surfaceto form a film. The film is assembled and floatable.When the surfactant molecules leave, Co–Fe–OH�

will be carried away in the form of ion pairs, in the

ARTICLE IN PRESS

-10000 -8000 -6000 -4000 -2000 0 2000 4000 6000 8000 10000-0.10

-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

M (

emu

/mg

)

H (Oe)

ab

Fig. 5. Typical magnetic hysteresis loops measured at room

temperature for the prepared particles (a) and nanorods (b),

respectively.

G.B. Ji et al. / Journal of Crystal Growth 270 (2004) 156–161160

crystallization process, surfactant molecules mayserve as a growth controller, as well as anagglomeration inhibitor, by forming a coveringfilm on the newly formed CoFe2O4 crystal. Asreported previously [17], the adsorption of growthunits on crystal surfaces strongly affects thegrowth rate and orientation of the crystals. Themolecules in the film tend to be perpendicular tothe absorbed surface, and the growth units wouldtend to face-land onto the growing interface. Sincethis kind of landing and dehydration will result inCo–O–Fe bonds, which make this landing modepredominant in competition with other ones suchas vertex- and edge-landing. CoFe2O4 crystalshould grow preferentially and the dehydrationsteps are repeated in the following procedures.Fig. 5 shows the typical hysteresis loops for the

as-prepared particles and nanorods measured atroom temperature, respectively. It shows that theCoFe2O4 particles and CoFe2O4 nanorods havemagnetization at 10 kOe of 83 and 66 m g�1,respectively. It is of interest to note that themagnetization of CoFe2O4 nanoparticles is largerthan that of the nanorods. The similar result wasalso obtained for Fe3O4 nanowires [18] and wasexplained in terms of the pinning of antiparallelmagnetic domains due to the spatial confinementeffect and the high shape anisotropy of thenanorods, preventing them from magnetizing indirections other than along their axes. The reason

of low magnetization is not yet clear. Furtherstudy is in progress to clarify the possible origin.

4. Conclusions

In summary, a simple synthesis route, theCTAB-assisted hydrothermal method, has beenused to fabricate single-crystalline phase CoFe2O4

nanorods. The prepared CoFe2O4 nanorodshave diameters of 25 nm and lengths up to120 nm.ED and HRTEM results indicate that thesingle-crystalline nanorods grow along [1 0 0].The magnetization of the CoFe2O4 nanorods islower than that of the CoFe2O4 nanoparticles,attributed to the spatial confinement effects andhigh shape anisotropy. Furthermore, the forma-tion mechanism of the CoFe2O4 nanorods hasbeen explained as the adsorption of CTA+, whichaffected the growth rate and orientation ofcrystals. It is expected that this method couldalso be a promising technique to fabricate one-dimensional nanostructures of other magneticmaterials.

Acknowledgements

This work was supported by the Project No.50171033 of National Natural Science Foundationof China and a national key project of funda-mental research (973, No. G 1999064508)

References

[1] E.S. Murdock, et al., IEEE Trans. Magn. 28 (1992)

3078.

[2] S.N. Okuno, et al., Appl. Phys. 71 (1992) 5926.

[3] V. Blaskov, V. Petkov, V. Rusanov, L.M. Martinez,

B. Martinez, J.S. Munoz, M. Mikhov, J. Magn. Magn.

Mater. 162 (1996) 331.

[4] V. Pillai, D.O. Shah, J. Magn. Magn. Mater. 163 (1996)

243.

[5] T. Meaz, S. Morup, J. Magn. Magn. Mater. 149 (1995)

14.

[6] J.W.D. Martens, W.L. Peeters, H.M. Van Noort,

M. Errnan, J. Phys. Chem. Solids 46 (1985) 411.

[7] K. Suzuki, T. Namikawa, T. Yamazaki, Jpn. J. Appl.

Phys. 27 (1988) 361.

ARTICLE IN PRESS

G.B. Ji et al. / Journal of Crystal Growth 270 (2004) 156–161 161

[8] S.N. Okuno, S. Hashimoto, K. Inomata, J. Appl. Phys. 71

(1992) 5926.

[9] F. Bodker, S. Morup, S. Linderoth, Phys. Rev. Lett. 72

(1994) 282.

[10] M. Abe, T. Itoh, Y. Tamaura, Thin Solid Films 216 (1992)

155.

[11] C.V. Gopal Reddy, S.V. Manorama, V.J. Rao, J. Mater.

Sci. Lett. 19 (2000) 775.

[12] F.F. Lange, Science 273 (1996) 903.

[13] W.S. Sheldrich, M. Wachhold, Angew. Chem. Int. Edit. 36

(1997) 206.

[14] M. Rozman, M. Drofenik, J. Am. Chem. Soc. 78 (1995)

2449.

[15] M.C. D’arrigo, C. Leonellic, G.C. Pellacani, J. Am.

Ceram. Soc. 81 (1998) 3041.

[16] L. Yan, Y.D. Li, Z.X. Deng, J. Zhuang, X. Sun, Int.

J. Inorg. Mater. 3 (2001) 633.

[17] T. Yoshida, M. Tochimoto, D. Schlettwein, D. Wohrle,

T. Sugiura, H. Minoura, Chem. Mater. 11 (1999) 2657.

[18] J. Wang, Q.W. Chen, C. Zeng, B.Y. Hou, Adv. Mater. 16

(2004) 137.