cg060395j Fe3O4 八面体

Fabrication and Characterization of Fe3O4Octahedrons via an

EDTA-Assisted Route

DongEn Zhang,?,?XiaoJun Zhang,?XiaoMin Ni,?JiMei Song,?and HuaGui Zheng*,?

Department of Chemistry,Uni V ersity of Science and Technology of China,

Hefei230026,People’s Republic of China

Recei V ed June26,2006;Re V ised Manuscript Recei V ed June13,2007

ABSTRACT:Octahedral-like magnetite was fabricated using an ethylenediamine tetraacetic acid(EDTA)-assisted route under mild conditions.The as-prepared powders were characterized in detail by conventional techniques such as X-ray diffraction(XRD) and scanning electron microscopy(SEM).On the basis of these methods,the growth mechanism was discussed.EDTA plays important roles in the formation of the products.This method can be easily controlled and is expected to be applicable for the preparation of other metal oxides with special morphologies.Also,the magnetic properties of the samples were characterized on a vibrating sample magnetometer(VSM).The sample exhibits a high first discharge capacity of884mAh/g,which enables it to be used for a lithium-ion battery.

Introduction

Magnetic nanoparticles have been widely studied because of their fascinating properties and wide range of potential applica-tions in ferrofluids,information storage,and medicine.1Among magnetic particles,iron oxides(Fe2O3and Fe3O4)have been extensively investigated.Various methods have been reported in the literature for the preparation of ultrafine particles of Fe3O4, such as reduction of hematite by CO/CO22or H2,3coprecipi-tation of the solution of ferrous/ferric mixed salt,4microwave plasma synthesis,5microemulsion methods,6and ultrasound irradiation.7Among them,a hydrothermal process has been widely used to prepare single crystal and ceramic powders.In the past10years,it has been employed to generate novel oxides,8polyanion clusters,9and zeolite materials.10Homoge-neous precipitation using metal-ethylenediaminetetraacetic acid (EDTA)chelate precursors is an excellent method of preparing a powder with a controlled morphology at relatively low temperatures.Many researchers have investigated the synthesis of monodispersed particles of slightly soluble inorganic salts, such as barium sulfate,11,12metal oxides,13,14and phosphate salts,15-17by homogeneous precipitation,using the dissociation or decomposition of metal-EDTA chelates.In this paper,Fe3O4 octahedrons have been prepared by a one-step hydrothermal procedure from ferric chloride(FeCl3),diamine hydrate(H4N2?H2O),and sodium hydroxide(NaOH)using an EDTA-assisted route.

Experimental Section

All chemicals were analytical grade and used without purification.

A typical experiment was as follows:0.4g of EDTA was added to a 40mL aqueous solution of0.02M FeCl3under stirring.Then2.0mL of hydrated hydrazine(80%)and0.05g of NaOH was added.The whole mixture was stirred for another5min to obtain a homogeneous solution and it was subsequently transferred into a50mL autoclave,sealed, and maintained at160°C for12h.Afterward,the autoclave was allowed to cool to room temperature gradually.The black precipitate collected was washed with distilled water three times in an ultrasonic bath to remove any possible impurities.The solid was then heated at 80°C and dried under vacuum for4h.

X-ray powder diffraction(XRD)patterns were determined using a Philips X’Pert PRO SUPER X-ray diffractometer equipped with graphite-monochromatized Cu K R radiation(λ)1.5418?).Morphol-ogy of the sample was examined by scanning electron microscopy (SEM,X-650).The magnetic properties of the sample were measured on a BHV-55vibrating sample magnetometer at room temperature.

The positive electrodes were fabricated by pasting slurries of the as-prepared FeS2crystallites(85wt%),carbon black(Super P,10wt %),and polyvinylidene(PVDF,5wt%)dissolved in N-methylpyrro-lidinone(NMP)on Al foil strips by the“doctor blade”technique.Then the strips were dried at160°C for24h in an air oven,pressed under 20MPa pressure,and kept at120°C for12h in a vacuum.The electrolyte was1M LiPF6in a1:1mixture of ethylene carbonate(EC)/ diethyl carbonate(DEC);the separator was Celgard2500.The cells were assembled in the glovebox filled with highly pure argon gas.The cells were galvanostatically cycled in the3-0.5V range at a current density of0.1mA cm-2.

Results and Discussion

The reaction of hydrothermal synthesis of Fe3O4from Fe3+ and diamine hydrate(H4N2?H2O)can be expressed as follows: N2H4can serve as either an oxidant or a reducer.11

As an oxidant:

As a reducer:

whereΦ°is the criterion electromotance.N2H4,therefore,is able to reduce Fe3+into Fe2+in a highly alkaline medium.Then, Fe2+and Fe3+will form the Fe3O4.18Hydrothermal system provides an environment for Fe3O4to grow highly crystalline particles.

An XRD pattern(Figure1)of the prepared product can be clearly seen and indexed to the face-centered cubic spinel structure of pure Fe3O4with a lattice parameter of a)8.393?,which is very close to the reported value(JCPDS85-1436). No peaks of hematite,metal hydroxides,or other impurities were detected,suggesting the complete formation of Fe3O4.The strong and sharp peaks revealed that Fe3O4particles were well

*To whom correspondence should be addressed.Tel:+86-551-3606144. E-mail:zhenghuagui@https://www.sodocs.net/doc/f414827861.html,.

?University of Science and Technology of China.

?Present address:Department of Chemical Engineering,Huaihai Institute of Technology,Lianyungang222005,People’s Republic of China.N

2

H

4

+2H

2

O+2e-)2NH

3

+2OH-Φ°)0.1V

N

2

+4H

2

O+4e-)N

2

H

4

+4OH-Φ°)-1.15V

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&DESIGN

2007

VOL.7,NO.10

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10.1021/cg060395j CCC:$37.00?2007American Chemical Society

Published on Web10/03/2007

crystallized.The products obtained were further examined by X-ray photoelectron spectroscopy (XPS).The spectra of the products corresponding to the binding energies of Fe 2p and O1s are shown in Figure 1b,c.The figure shows that the binding energies relating to Fe 2p3/2,Fe 2p1/2,and O 1s are about 710,723,and 532eV,respectively.The data are consistent with the values reported for Fe 3O 4in the literature.19-21Therefore,XPS results also prove the composition of the products.

The morphology and size of the as-prepared Fe 3O 4were characterized with SEM shown in Figure 2.Figure 2a shows the overall morphology of Fe 3O 4,which indicates that the sample is composed of octahedrons.Few particles have been found during the observations.Figure 2b presents a higher-magnification SEM image of Fe 3O 4micro-octahedrons,reveal-ing that the as-obtained Fe 3O 4octahedrons have typical edges in the range of 1-1.2mm.These particles display a narrow size distribution as shown in SEM images.The statistical distribution of grains is illustrated in Figure 2c.It shows that most of the grains have sizes in the range of 0.8-1mm with a uniform shape.

Magnetic measurements of thus-prepared octahedral magne-tite were conducted,and the magnetization -hysteresis (M -H)loop was presented in Figure 3.The hysteresis loop of the products shows ferromagnetic behavior with saturation mag-

netization (M s ),remanent magnetization (M r ),and coercivity (H c )values of about 78,21emu/g,and 195Oe,respectively,and the values are different from those reported for Fe 3O 4nanowires,nanoparticles,and Fe 3O 4.22-25This may be due to the low shape anisotropy and multiple domains of Fe 3O 4octahedrons,which allowmagnetization in directions along their easy magnetic axes.The saturation magnetization (M s )reached 78emu/g,lower than that of bulk Fe 3O 4(90emu/g).26The magnetic properties of materials are influenced by many factors,such as size,structure,surface disorder,morphologies,etc..The lower saturation magnetization in this work may be caused by anisotropy,including crystal anisotropy and shape anisotropy,which play an important role in reducing the saturation magnetization because most materials contain some type of anisotropy.

The discharge curve of the sample is shown in Figure 4.The sample delivered a large discharge capacity of 884mAh/g at a current density of 0.1mA/cm 2in the voltage range 3.0-0.5V,corresponding to the reaction of 7.7Li +per Fe 3O 4.The discharge capacity includes two parts,(i)Fe(II)-Fe(0)and (ii)Fe(III)-Fe(0),which are consistent with the voltage plateaus at ca.1.0and 0.85V (Figure 4),respectively.The sample exhibiting a high first discharge capacity might be ascribed to the increased surface area of Fe 3O 4novel octahedrons and the smooth surfaces,which facilitate the deintercalation/intercalation of Li ions and allow Fe 3O 4octahedrons to be used for a lithium-ion battery.

In this EDTA-assisted route,the addition of EDTA is the key factor to the morphology of the products.Without the addition of EDTA,the final products are all particles in unordered shapes,even when other experimental conditions are kept the same.To improve the understanding of the effect of EDTA,we substituted EDTA with ammonia,diethylamine,and pyridine.Unfortunately,Fe 3O 4octahedrons could not be obtained.Experiments show that EDTA played a unique role in the preparation of Fe 3O 4octahedrons.The morphology of the crystal is determined by the relative growth rates of different crystal planes.Wang suggests 27that the shape of an fcc nanocrystal is mainly determined by the ratio of the growth rate in the <100>direction to that in the <111>direction,and octahedrons bounded by eight {111}planes will be formed when the ratio is relatively high.It should be noted that EDTA is a strong coordinating agent,which could form a very stable complex (K formation )1025)by coordinating with Fe 3+.This is confirmed by the fact that the solution color changed from yellow to carmine soon after the addition of EDTA.In the complex,EDTA acts as a hexadentate unit by wrapping itself around the metal ion with four oxygen atoms and two nitrogen atoms and forming a regular octahedral-like structure.The

Figure 1.(a)XRD pattern of the Fe 3O 4octahedron;(b,c)XPS spectra of as-obtained Fe 3O 4octahedron.

Figure 2.(a)A typical SEM image of octahedral Fe 3O 4;(b)typical high-magnification SEM image.(c)Edge size distribution histogram of the Fe 3O 4octahedrons.

2118Crystal Growth &Design,Vol.7,No.10,2007Zhang et

al.

formation of the complexes not only can kinetically control the reaction rates but also may provide the octahedral chemical environments for Fe3+ions.The theoretical growth habit of the Fe3O4crystal is the octahedron.Under the influence of the octahedral coordination environment provided by EDTA,the renascent Fe3O4nuclei are limited to an epitaxial microenvi-ronment and naturally prefer to follow the growth habit of Fe3O4 crystals growing into the octahedral.Moreover,the slow reaction rate is favorable for the faster growth rate along<100> compared to that along<111>due to the lowest energy of the {111}surfaces.So the obtained octahedral morphology is regular and uniform.At any rate,the growth habit of the Fe3O4 crystal needs to be probed further.

Conclusions

In conclusion,we have succeeded in synthesizing Fe3O4 octahedrons using an EDTA-assisted route under mild conditions for the first time.EDTA plays important roles in the formation of the products in the process.This method can be easily controlled and is expected to be applicable for the preparation of other metal oxides with special morphologies.The sample exhibits a high first discharge capacity of884mAh/g,which makes it useful for a lithium-ion battery. Acknowledgment.We are grateful to the electron micro-scope and X-ray diffraction facilities of University of Science &Technology of China for assistance with the XRD and SEM measurements.

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CG060395J

Figure3.Magnetization-hysteresis(M-H)loops of as-prepared

samples measured at room temperature.

Figure4.Discharge curve for the obtained magnetite sample at a

voltage window of3.0-0.5V.

Characterization of Fe3O4Octahedrons Crystal Growth&Design,Vol.7,No.10,20072119