preparation and properties of magnetic iron oxide nanotubes

Particuology 6 2008 334–339Preparation and properties of magnetic iron oxide nanotubes Baoliang Lva,b, Yao Xua,∗, Dong Wua, Yuhan Suna aState Key Laboratory of Coal Conversion, Institute

Particuology 6 (2008) 334–339

Preparation and properties of magnetic iron oxide nanotubes

Baoliang Lva,b, Yao Xua,∗, Dong Wua, Yuhan Suna

aState Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China

bGraduate University of the Chinese Academy of Sciences, Beijing 100039, China

Received 7 March 2008; accepted 4 April 2008

Abstract

Magnetite (Fe3O4) nanotubes were prepared by reducing synthesized hematite (␣-Fe2O3) nanotubes in 5% H2+95% Ar atmosphere, and then maghemite (␥-Fe2O3) nanotubes were obtained by re-oxidizing the Fe3O4nanotubes The nanotube structure was kept from collapsing or sintering throughout the high temperature reducing and re-oxidizing processes The coercivities of the Fe3O4 and␥-Fe2O3 nanotubes synthesized were found to be 340.22 Oe and 342.23 Oe, respectively, both higher than other nanostructures with the same phase and of similar size Both adsorbed phosphate and the nanotube structure are considered responsible for this high coercivity

© 2008 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences Published by Elsevier B.V

All rights reserved

Keywords: Nanostructures; Iron oxides; Nanotubes; Magnetic properties

1 Introduction

Magnetic materials with special nanostructures are

scientif-ically interesting and technologscientif-ically important in research for

future applications (Sui, Skomski, Sorge, & Sellmyer, 2004a)

Iron oxides as an important class of magnetic materials have

been widely used in catalysis (Zhang et al., 2005), magnetic

devices (Zeng, Li, Liu, Wang, & Sun, 2002), environment

pro-tection (Wu, Qu, & Chen, 2005), sensors (Sun, Yuan, Liu, Han,

& Zhang, 2005), drug delivery (Wu et al., 2007) and water

split-ting (Cesar, Kay, Gonzalez Martinez, & Grätzel, 2006) Up to

now, iron oxides with nanostructures have attracted a great deal

of attention because of their promising properties and

appli-cations Many iron oxide particles with zero-, one-, two- and

three-dimensional (0D, 1D, 2D and 3D) nanostructures have

been synthesized Ferromagnetic nanotubes were considered

as candidates for recording head, biomagnetic sensors,

cata-lysts, etc., because of their expected vortex magnetization state

and floatability in liquid as a result of their hollow structure

(Goldstein, Gelb, & Yager, 2001;Haberzettl, 2002;Khizroev,

Kryder, Litvinov, & Thomson, 2002) Iron oxide nanotubes

have been synthesized mostly via the so-called template-directed

growth method For example,Sui, Skomski, Sorge, and Sellmyer

∗Corresponding author Tel.: +86 351 4049859; fax: +86 351 4041153.

E-mail address:xuyao@sxicc.ac.cn (Y Xu).

(2004b),Wang, Wang, Li, Xu, and Zhou (2006), and Shen et

al (2004)used porous anodic aluminium oxide (AAO) as tem-plate to prepare Fe3O4and␣-Fe2O3 nanotube arrays;Sun et

al (2005)used carbon nanotubes as templates to fabricate

␣-Fe2O3 nanotubes; Liu et al (2005) used MgO nanowires as templates to fabricate single-crystal Fe3O4 nanotubes How-ever, templates not only introduced extraneous impurities but also increased production cost, not to say the many prob-lems to prepare these materials at large scale Therefore, it

is of practical significance to develop a template-free and somewhat easier method to synthesize magnetic iron oxides nanotubes

Recently,Jia et al (2005)synthesized␣-Fe2O3nanotubes by

a hydrothermal method without using template In this work, we improved their work by first synthesizing␣-Fe2O3nanotubes, followed by reducing␣-Fe2O3 and re-oxidizing the Fe3O4 to

␥-Fe2O3nanotubes The magnetic properties of the Fe3O4and

␥-Fe2O3nanotubes were investigated by using vibrating sample magnetometry (VSM)

2 Experimental

All the reagents were A.R grade and were used in prepara-tion without further purificaprepara-tion: ferric chloride (FeCl3·6H2O, China Medicament Co.), sodium dihydrogen phosphate (NaH2PO4·2H2O, Tianjin Chemical Reagent Co.), double-1674-2001/$ – see inside back cover © 2008 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences Published by Elsevier B.V.All rights reserved doi: 10.1016/j.partic.2008.04.006

distilled water Reduction gas was composed of 5 v% H2(high

purity) and 95 v% Ar (high purity)

The preparation of ␣-Fe2O3 nanotubes was an improved

approach based on literature (Jia et al., 2005) In a typical

synthe-sis, 40 mL of FeCl3aqueous solution (46.2 mmol/L) and 40 mL

of NaH2PO4aqueous solution (1.9 mmol/L) were first mixed

and then dispersed uniformly by ultrasonic irradiation The

solu-tion was then sealed in a 100-mL Teflon-lined stainless steel

autoclave and hydrothermally treated for 36 h at 240◦C At last,

a red precipitate was obtained at the bottom of the autoclave and

was separated by centrifugation The precipitate was washed

three times with distilled water, and then dried at 60◦C in air.

The resulting powder was␣-Fe2O3 nanotubes, named as S1

Fe3O4 nanotubes were obtained by reducing S1 in a tubular

oven at 500◦C for 2.5 h in a 5% H

2+95% Ar atmosphere, and the resulting black powder was named as S2 In this process,

the temperature and reduction time were very important, or else

␣-Fe2O3or FeO would be present in the reduction product To

prepare␥-Fe2O3nanotubes, the as-prepared Fe3O4nanotubes

were oxidized by air at 300◦C for 2 h, to produce a red powder,

named as S3

X-ray diffraction (XRD) measurement was performed on a

D8 Advance Bruker AXS diffractometer using Cu K␣

radia-tion (λ=1.5406 Å) Raman spectra were recorded using a Horiba

Labram HR800 spectrometer equipped with a Spectra Physics

514 nm argon ion laser The morphologies of the samples were

observed by scanning electron micrograph (SEM, LEO 1530VP)

and transmission electron micrograph (TEM, Hitachi H-600)

X-ray photoelectron spectroscopy (XPS) measurements were

performed on a PHI 5300× multi-technique system with Mg K␣

X-ray source (PerkinElmer Physical Electronics) Magnetic

hys-teresis loops were measured by vibrating sample magnetometry

(VSM, Lakeshore 7407)

3 Results and discussion

Fig 1shows the XRD patterns of samples S1 (a), S2 (b) and

S3 (c) InFig 1(a), the initial synthesized product (sample S1)

Fig 1 XRD patterns of samples S1 (a), S2 (b) and S3 (c).

Fig 2 Raman spectra of samples S2 (a) and S3 (b).

can be exclusively indexed to␣-Fe2O3, according to standard data (JCPDS 33-0664) InFig 1(b) and (c), the reflection peaks

of XRD patterns of S2 and S3, can be well assigned to a spinel structure with the characteristic reflections of␥-Fe2O3(JCPDS 39-1346) or Fe3O4(JCPDS 19-0629) However, it is well-known that clear identification of ␥-Fe2O3and Fe3O4based on ordi-nary XRD pattern is an arduous task due to their same spinel structure and their similar lattice parameters (Xiong, Ye, Gu, & Chen, 2007) Although the color of S2 was black and S3, red, corresponding to Fe3O4and␥-Fe2O3, respectively, the purity

of the samples cannot be simply identified by their appearance

To differentiate samples S2 and S3 clearly, further characteriza-tion is needed for more convincing evidence, for which Raman spectrum was resorted to (Daou et al., 2006; Pinna et al., 2005; Xiong et al., 2007) A representative Raman spectrum of sam-ple S2, shown inFig 2(a), exhibits two clear peaks at 665 and

540 cm−1, which can be indexed to the A1g and T2g transitions

of the Fe3O4 phase (Shebanova & Lazor, 2003) In Fig 2(b), the Raman spectrum of sample S3, the different characteristic bands of ␥-Fe2O3 (700, 500 and 350 cm−1) can be observed (Varadwaj, Panigrahi, & Ghose, 2004) Consequently, it should

be reasonable to think that sample S2 is Fe3O4and sample S3

is␥-Fe2O3 Fig 3presents the SEM and TEM images of samples S1, S2 and S3 Fig 3(a) and (b) show the morphologies of ini-tial synthesized␣-Fe2O3nanotubes (sample S1), in which the nanotubes can be seen clearly, with length of 160–300 nm,

Fig 3 SEM and TEM images of ␣-Fe O nanotubes in sample S1 (a and b), Fe O nanotubes in sample S2 (c and d) and ␥-Fe O nanotubes in sample S3 (e and f).

and outer and inner diameters of 70–120 nm and 45–80 nm,

respectively.Fig 3(c) and (d) show the morphologies of Fe3O4

nanotubes (sample S2), with their well retained nanotube

struc-ture.Fig 3(e) and (f) show the nearly same morphologies of

␥-Fe2O3(sample S3) together with their well retained tube

struc-ture.Fig 3(d) and (e) show that there was no obvious change

in the length and diameter of the nanotubes Comparison of the

TEM images of the three samples indicates that the Fe3O4and

␥-Fe2O3 nanotubes are conglomerated with each other, while

␣-Fe2O3 nanotubes are well dispersed, apparently due to the

mutual attraction of the magnetic Fe3O4and␥-Fe2O3particles,

though no obvious difference can be found from the

morpholo-gies of the three samples

Generally, nanostructures are often destroyed due to sintering

or collapsing during treatment at high temperature ButFig 3

shows that the nanotube structure was well preserved after

reduc-tion and re-oxidizareduc-tion at high temperature There might be two

reasons for this First, according toJiao et al (2006), conversion

of␣-Fe2O3to Fe3O4involves a change from a hexagonal

close-packed oxide ion array (␣-Fe2O3) to a cubic close-packed array

(Fe3O4) This conversion is not merely topotactic, but involves

a sheave of oxide ion planes from AB to ABC stacking, and this

significant structural change can occur without much

destroy-ing the tube structure The thin walls of the nanotubes endowed

the solids with a structural flexibility that made such solid/solid

transformation smooth while preserving the tube structure

Sec-ond, according toJia et al (2005), phosphate could be adsorbed

on ␣-Fe2O3 by reacting with the singly coordinated surface

hydroxy groups to form a monodentate or bidentate inner-sphere

complex Here, the amount of adsorbed phosphate was so small

that it could not be detected by XRD To confirm the existence

of phosphate on the surface of the nanotubes, XPS analysis was

carried out on the surface element composition of the initial

␣-Fe2O3nanotubes, with the result shown inFig 4(a) The binding

energies obtained in the XPS analysis were corrected by

refer-encing the C1s line to 284.5 eV Seen fromFig 4(a), the binding

energy of P2p was found at 133.6 eV in the spectrum, which

agreed with the reported value of PO4 −(Wang et al., 2003).

To further identify the existence of the phosphate layer, a

high-magnification image of sample S1 was obtained on TEM, as

shown inFig 4(b), indicating the presence of a 2.5-nm

adsorp-tion layer, thus confirming the presence of a phosphate layer on

the surface of synthesized ␣-Fe2O3 nanotubes The adsorbed

phosphate would be very stable in the reduction process, and act

as a framework or a protection shell for the nanotubes When

the reduction of␣-Fe2O3went on, the phosphate on the surface

could not be reduced, and only the inner␣-Fe2O3was reduced by

hydrogen Therefore, the nanotubes could be kept from sintering

or collapsing To confirm the stabilization of phosphate on

nan-otubes, pure iron phosphate (FePO4) sample was treated under

the same reduction condition as that for␣-Fe2O3reduction The

XRD pattern (not given here) of the reduction product showed

that FePO4 was reduced to Fe2PO5 Oxidation of Fe3O4

nan-otubes to␥-Fe2O3nanotubes involved a decrease in the number

of Fe atoms per unit cell of 32 oxygen ions, from 24 in Fe3O4

to 21(1/3) in␥-Fe2O3 This reaction proceeded with outward

migration of the Fe2+cations towards the surface of the crystal

Fig 4 (a) XPS spectrum of ␣-Fe 2 O 3 nanotubes and (b) high magnification TEM image of a nanotube in sample S1.

together with the creation of cation vacancies and the addition

of oxygen atoms At the surface the Fe2+cations were oxidized through interacting with adsorbed oxygen to form of␥-Fe2O3, too The whole process involved a topotactic reaction in which the original crystal morphology was preserved throughout the process (Cornell & Schwertmann, 2003)

Magnetic nanoparticles, especially those with special struc-tures, often exhibit unusual magnetic behaviors different from that of bulk solids, owing to finite size effects and microstructure (Bødker, Hansen, Bender Koch, Lefmann, & Mørup, 2000) To investigate the magnetic properties of the as-synthesized nan-otubes, magnetic hysteresis (M–H) loop measurements were carried out in an applied magnetic field at room temperature, with the field sweeping from −18 to 18 kOe Fig 5 shows the M–H loops of Fe3O4 (a) and ␥-Fe2O3 (b) nanotubes at room temperature From Fig 5(a), the M–H loop of Fe3O4 nanotubes shows ferromagnetic behavior with a saturation mag-netization (Ms) of 60.92 emu/g, a remanent magmag-netization (Mr)

of 18.56 emu/g and a coercivity of 340.22 Oe at room tem-perature Compared to bulk Fe3O4 (Ms = 92 emu/g, coercivity 115–150 Oe) (Liu, Fu, & Xiao, 2006), the Ms was obviously lower and the coercivity was obviously higher Fe3O4nanotubes also possess higher coercivity than other Fe3O4 nanostruc-tures of similar size, such as octahedral nanoparticles (141 Oe), nanocubes (62 Oe) and hollow spheres (40 Oe) (Daou et al., 2006;Huang & Tang, 2005;Xiong et al., 2007; Yu et al., 2006) From Fig 5(b), the M–H loop of ␥-Fe2O3 nanotubes shows ferromagnetic behavior with a Ms of 42.71 emu/g, a Mr of

Fig 5 M–H loops of Fe 3 O 4 nanotubes (a) and ␥-Fe 2 O 3 nanotubes (b) The

inset diagrams are their corresponding expanded low-field curves.

13.56 emu/g and a coercivity of 342.23 Oe at room temperature

Compared to bulk␥-Fe2O3(Ms = 76 emu/g, coercivity 300 Oe)

(Zhang, Tang, and Hu, 2008), the Ms is obviously lower and

the coercivity is somewhat higher Similar to Fe3O4nanotubes,

␥-Fe2O3 nanotubes also have a higher coercivity than other

reported␥-Fe2O3nanostructures, such as nanofibres (78.11 Oe),

nanoparticles (106 Oe), and some reported superparamagnetic

␥-Fe2O3particles (0 Oe) (Han et al., 2007; Jing, 2006; Zhang et

al., 2008) It is noted that both these two magnetic iron oxide

nan-otubes have a higher coercivity than other nanostructures with

the same phase and of similar size Furthermore, the M–H loops

of Fe3O4nanotubes and␥-Fe2O3nanotubes indicate the similar

magnetic domain type On the basis of the criteria given by

Dun-lop (Cornell & Schwertmann, 2003), the Mr/Ms value should

be larger than 0.5 for single domain (SD) particles, between 0.1

and 0.5 for pseudosingle-domain (PSD) particles and lower than

0.1 for multidomain (MD) particles FromFig 5, both the two

samples possess PSD-type magnetic domains, and their Mr/Ms

values are 0.30 and 0.32, respectively

There might be two reasons for the high coercivity First is

the influence of adsorbed phosphate at the surface of these

nan-otubes, which has been confirmed previously by XPS, and the

phosphate is not a magnetic material FromFig 5, both the two

samples have a Mr/Ms value between 0.1 and 0.5, indicating that

they may possess the magnetic properties of SD and MD

struc-tures simultaneously If the synthesized products possess more

properties of MD structures, the magnetic domain walls would exist inside the particles For MD materials, the movement of magnetic domain walls is the main reason for coercivity It is well known that there exist surface domain walls for MD par-ticles Here, the surface domain walls should be present at the interface between iron oxide and the adsorbed phosphate The phosphate as an uninterrupted adsorbed layer can easily block the movement of the surface domain walls and result in domain wall pinning, which contributes to the high coercivity Even if the synthesized products possess more properties of SD particles, the coercivity would also increase For SD magnetic material, magnetic domain wall does not exist, and spin flip conversion

is mainly responsible for the coercivity In this case, the coor-dination bonds between adsorbed phosphate ions and iron ions would form spin pinning and block spin flip conversion, directly resulting in the increase of coercivity of the samples Second, the nanotube structure may be another reason for the high coerciv-ity.Torres-Heredia, López-Urías, and Mu˜noz-Sandoval (2005) simulated the micromagnetic property of iron nanorings, and

they found large coercive fields for din/dout> 0.5 (dinand dout are the inner and outer diameters of the rings, respectively) and

t = 160–200 nm (t is the thickness of the rings or length of the

tubes) nanorings due to the absence of the vortex states and the presence of out-plane and in-plane spin configurations In our

samples, the average din/doutvalue of nanotubes is about 0.7, and the length of many nanotubes is about 200 nm, which snugly fall into the thickness range of nanorings mentioned in the literature (Torres-Heredia et al., 2005) So the nanotubes can be thought

as nanorings with immensely large thickness, and this structure can contribute to the high coercivity

4 Conclusions

Fe3O4nanotubes were prepared by reducing synthesized

␣-Fe2O3nanotubes with a gas mixture of 5% H2+95% Ar at 500◦C for 2.5 h, and then ␥-Fe2O3 nanotubes were obtained by re-oxidizing the Fe3O4nanotubes with air at 300◦C for 2 h The nanotube structure was well retained without collapsing or sin-tering, for which, adsorbed phosphate and the type of crystal structure conversion should be the two most important reasons Investigation of the magnetic properties of Fe3O4and␥-Fe2O3 nanotubes revealed that both the two magnetic iron oxide nan-otubes possess higher coercivity than other nanostructures with same phase and of similar size The adsorbed phosphate and the tube structure should be responsible for the high coercivity Research on applications of these two magnetic nanotubes is in progress

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