thermal oxide synthesis and characterization of fe3o4 nanorods and fe2o3 nanowires

The influence of temperature on the morphology cubic structure and the average diameter of 0.5－0.8 µm grow after reaction at 200－500℃ for 1 h in the thermal oxide process, nanorods, na

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Thermal oxide synthesis and characterization of Fe 3 O 4

nanorods and Fe 2 O 3 nanowires

JIAO Hua1,2† & YANG HeQing1

1 Key Laboratory of Macromolecular Science of Shaanxi Province, School of Chemistry and Materials Science, Shaanxi Normal

University, Xi’an 710062, China;

2 Department of Chemistry and Chemical Engineering, Weinan Teacher’s University, Weinan 714000, China

mi-croscopy and transmission electron mimi-croscopy The influence of temperature on the morphology

cubic structure and the average diameter of 0.5－0.8 µm grow after reaction at 200－500℃ for 1 h in the

thermal oxide process, nanorods, nanowires, C 2 H 2 O 4 , iron sheet

1 Introduction

Fe3O4 is an important magnetite material having cubic

inverse spinel type structure, which has been widely

used as magnetic fluid and magnetic recording materials,

due to its unique electrical and magnetic properties[1,2]

Nanoscale Fe3O4 has been applied in magnetic ink[3],

electronics and bio-sensitive materials[4,5], high density

magnetic recording media and biomedical fields[6－ 9],

because of its good compatibility with organism and its

electrical and magnetic characteristics of its size and

morphology Therefore, the preparation of Fe3O4

nanos-tructures and its properties research are extremely active

in recent years

At present, different kinds of Fe3O4 nanostructures

have been successfully synthesized via various physical

and chemical methods For example, the monodisperse

Fe3O4 nanoparticles were prepared by solvothermal and

high temperature organic liquid reflux method[10－ 13], on

the basis of which three-dimensional superlattice has

been assembled[14] Recently, Yu et al prepared the

structure of octahedron by reflux method[15] The Fe3O4 nanorods, nanowires,branch-likenanowires,nanochains, octahedral structure, nanoflakes, peanut-like Fe3O4, nanotubes and nanopyramid arrays were prepared by hydrothermal method[16－ 23], electroprecipitation meth-

od[24], ultrasound irradiation[25], PLD-assisted VLS[26,27], and microwave plasma chemical vapor deposition tech-nique (MWCVD)[28], respectively

Recently, some researchers focused on the investiga-tion of the synthesis of Fe3O4 nanorods Wan et al.[29] obtained Fe3O4 nanorods with an average diameter of 25

nm, length of 200 nm via hydrothermal reaction of FeSO4·7H2O and FeCl3 at 120℃ for 20 h Kumar et

al.[25] prepared the Fe3O4 nanorods with acetic ferrous and the stabilizer of cyclodextrin under Ar atmosphere

Received May 5 2008; accepted November 18, 2008 doi: 10.1007/s11426-009-0092-1

† Corresponding author (email: jiaohua0106@yahoo.com.cn) Supported by the Fund of Weinan Teacher’s University (Grant No 08YKZ008), the National Natural Science Foundation of China (Grant No 20573072) and the Doc-toral Fund of Ministry of Education of China (Grant No 20060718010)

of 0.15 MPa These synthesized methods of nanorods

usually require organic solvents and complex operation

In the present investigation, iron sheet as source and

dripping acid solution on the surface of iron sheet were

adopted to prepare Fe3O4 nanorods The Fe3O4 nanorods

with the rectangular cross-section and approximate

0.5－0.8 μm length were obtained by oxidizing at low

temperature for 1 h Subsequently, Fe2O3 nanowires in

the range of 100－300 nm were obtained at 600℃

2 Experimental

2.1 Experimental materials

All the chemical reagents in our experiments are

ana-lytical grade and they are used without further

purifica-tion Iron (Fe, 99.6%) was obtained from Shaanxi Huaou

Industry Ltd, and oxalic acid (C2H2O4·2H2O, analytical

grade) was purchased from Xi’an reagent factory

In a typical experiment, a sheet of iron with size of 1×1

cm was polished by the sand paper and dealt with

alco-hol in ultrasonic for 15 min Then, it was placed in

quartz boat and a drop of oxalic acid (0.75 mol·L−1)

so-lution was taken onto the iron surface After that, the

quartz boat was placed in the oven and maintained at

200, 300, 400, 500 and 600℃ for 1 h with heating rate

of 10℃·min−1 before being naturally cooled to room

temperature There was a red and black thin film on the

surface layer of the iron sheet

2.3 Characterization of products

X-ray powder diffraction (XRD) patterns of the products

were obtained on a Japan Rigaku D/Max-ⅢC

diffrac-tometer at a voltage of 60 kV and a current of 80 mA

with Cu Kα radiation (λ=1.5406 Å), employing a

scan-ning rate of 8° min−1 in the 2θ ranging from 10° to 70°

Scanning electron microscopy (SEM) images were

ex-plored on a Holand model FEI Quanta 200 microscope

Transmission electron microscopy (TEM) images were

taken on a JEOL JEM-3010 transmission electron

mi-croscope at an accelerating voltage of 200 kV

3 Results and discussion

3.1 SEM analysis

Figure 1(a)－(f) showed the SEM images of samples

synthesized by reactions of C2H2O4 with Fe at 300℃ for 1 h and Figure 2(a)－(d) showed the SEM images of the samples synthesized at 200, 400, 500 and 600℃ for

1 h, respectively It can be seen clearly that samples were nanorods with rectangular cross-section and the size between 0.5－1.0 μm at 200℃, as seen in Figure 2(a) When the reaction temperature was increased to 300℃, the different magnification of the front SEM im-ages were shown in Figure 1(a), (c)－(f) and the side SEM image was shown in Figure 1(b) As seen in Figure 1(c) and (e), the shape of nanorods samples were the appearance of rectangular cross-section and with the length range of 0.5－0.8 μm A number of rods were split along the same axis The results showed that the smaller rods and lines were split by the relatively coarse rods, as seen in Figure 1(d) Figure 1(f) was the cross-section image of single nanorod with the length of 0.8 μm and width of 0.6 μm under the high multiple When the reaction temperature was increased to 400℃, the samples were a small amount of nanolines

Figure 1 SEM images of samples synthesized by reactions of C2 H 2 O 4

with Fe at 300℃ for 1 h

expect for the nanorods of rectangular cross-section, as

shown in Figure 2(b) When the temperature was 500℃,

the morphology of the sample was changed continuously

from nanorods with the size range of 0.3－0.5 µm to

nanowires with the size range of 100－300 nm in Figure

2(c) When the reaction temperature was increased to

600℃, the morphology of the sample was nanowires

with the size range of 100－300 nm, as shown in Figure

2(d) From the analysis of the reaction kinetic, a part of

lower energy molecules became activated as the

tem-perature increased Later, the increasing chance of the

effective collisions made the reaction rate (ν) increase

As the temperature was increased, the decomposition

rate of oxalic acid (νd) increased with the gas-liquid

in-terface of the oxalic acid solution contacting with air At

the same time, the reaction rate (νr) was also increased

with the liquid-solid interface of the oxalic acid solution

contacting with Fe When the temperature was up to

600℃, νd>νr, the sample morphology was mainly

de-cided by the reaction of water vapor and Fe[30]

Figure 2 SEM images of samples synthesized by reactions of C2 H 2 O 4

with Fe at different temperatures for 1 h (a) 200℃; (b) 400℃; (c) 500℃;

(d) 600℃

3.2 XRD analysis

Figure 3(a)－(e) showed the XRD patterns of the

sam-ples prepared from 200－600℃ for 1 h The samsam-ples

were obtained at 200℃ with the two diffraction peaks

corresponding to the cubic structure of Fe (110) (200)

crystal plane (JCPDS No 06-0696), as shown in Figure

3(a) It indicated that the crystal sample was not com-plete at 200℃ When the temperature was increased to 300－500℃, the diffraction peak corresponding to the cubic phase of Fe3O4 (111), (220), (311), (222), (331), (511) crystal plane (JCPDS No 65-3107) and hexagonal phase of Fe2O3 (012), (104), (311), (113), (024), (116), (214) crystal plane (JCPDS No 33-0664) became wide and weak, which indicated that the product was multi- crystalline structure Further, the Fe3O4 diffraction peaks disappeared gradually When the temperature was up to 600℃, the Fe3O4 diffraction peaks were not obvious, which indicated that Fe2O3 could be obtained in higher temperatures

Figure 3 XRD images of samples synthesized by reactions of C2 H 2 O 4

with Fe at different temperatures for 1 h (a) 200℃; (b) 300℃; (c) 400℃; (d) 500℃; (e) 600℃

3.3 TEM analysis

In order to determine the detailed crystalline structure, TEM measurements were employed to investigate the samples prepared at 300℃ for 1 h A typical TEM image

of the single Fe3O4 nanorod was shown in Figure 4(a) The size of the nanorod with the length of 2.4 µm and the width of 0.5 µm was in good agreement with the above SEM image shown in Figure 1 Figure 4(b) is the top TEM image of Figure 4(a) It can be seen from the Figure 4(b) that the samples of nanorods were fibri-form-like structure self-assembly A selected area elec-tron diffraction (SAED) pattern was presented in Figure 4(c) according to the rectangular frame of Figure 4(b),

Figure 4 TEM images of samples synthesized by reactions of C2 H 2 O 4

with Fe at 300℃ for 1 h (a)－(c)TEM; (d) SAED

indicating the cubic phase of Fe3O4 (311), (400), (220),

(511) crystal plane diffraction Meanwhile, the energy

dispersive spectrometer (EDS) was used to analysis the

chemical composition of the sample, and the results can

be seen in Figure 4(d) It can be clearly identified that

the nanorods were composed of Fe and O elements, and

the ratio of the number of atoms Fe and O was about

3︰4 The TEM, SAED and EDS analyses revealed that

Fe3O4 nanorods were of polycrystalline cubic phase

structure

3.4 The influence of oxalic acid on the morphology

of samples

In order to study the role of oxalic acid, the morphology

of the products from water reacting with Fe sheet at

300℃ and 600℃ for 1 h in the air were investigated,

respectively, as shown in Figure 5 From Figure 5(a), it

can be observed that, when the reaction temperature was

300℃, the surface of the Fe sheet had not shaped

regu-larity morphology, only sporadic small particles When

the reaction temperature was up to 600℃, the surface of

Fe sheet was nanowires with the size of 100－300 nm in

Figure 5(b) It was obvious that the addition of oxalic

acid was benefit for the formation of nanorods in the

temperature range of 200－500℃

Figure 6 showed the XRD patterns of the samples

above It can be seen from the patterns, the two

diffrac-tion peaks were corresponding to the cubic phase Fe

(110) (200) crystal plane (JCPDS No 06-0696) under

Figure 5 SEM images of samples synthesized by reactions of H2 O with

Fe at 300℃ (a) and 600℃(b) for 1 h in air

Figure 6 XRD images of samples synthesized by reactions of H2 O with

Fe at 300℃ (a) and 600 ℃(b) for 1 h in air

300℃ When the temperature was up to 600℃, the sam-ples of the diffraction peaks corresponding to the hex-agonal phase of the Fe2O3 (JCPDS No 33-0664) indi-cated that dropping water on the iron surface did not react at 300℃, and the pure Fe2O3 products were ob-tained at 600℃ The results of SEM and XRD indicated that Fe3O4 nanorods on the surface of Fe sheet were complexation reaction of oxalic acid and iron at a rela-tively low temperature

In order to determine the detailed crystalline structure, TEM measurements were employed to investigate the samples prepared at 600℃ for 1 h in air, on the surface

of which water dripped, as shown in Figure 7 A typical TEM image of single Fe2O3 nanowire was shown in Figure 7(a) The size of the nanowire was in good agreement with the above SEM image shown in Figure 5(b) with the length of about 100 nm Figure 7(b) is the high resolution TEM image of Figure 7(a) It can be seen from the image that the crystal plane spacing was 0.37 nm, corresponding to the distance of hexagonal

Figure 7 TEM images of samples synthesized by reactions of H2 O with

Fe at 600℃ for 1 h (a),(d)TEM; (b) HRTEM; (c) SAED

phase of Fe2O3 (012) crystal plane A SAED pattern was

presented in Figure 7(c) according to Figure 7(a),

indi-cating the hexagonal phase of Fe2O3 [0001] zone axis

diffraction The growth of nanowires was from the

rough to the fine in Figure 7(d) The TEM and HRTEM

analyses revealed that Fe2O3 nanowires were of single

crystalline hexagonal phase structure

3.5 Mechanism

Based on the above results, the reaction process was: the

oxalic acid solutions contacted with air and formed an

interface of the gas-liquid phase after oxalic acid

drip-ping on iron surface, which occurred as reaction of

ox-alic acid decomposition (νd) Meanwhile, the reaction of

the oxalic acid solution and Fe happened on the

liq-uid-solid interface (νr) When the Fe sheet with a drop of

acid was placed in the oven, before reaching the

de-composition temperature of 190℃, νd<νr, the oxalic acid

occurred complexation reaction with iron and obtained

ferrous oxalate, as seen eq (1); latter, the unstable

rous oxalate decomposition became FeO (eq (2));

fer-rous oxide was oxidized to Fe3O4 by oxygen in the air

(eq (3)); the νd>νr was increasing with the temperature

increasing at the same time When the temperature was

up to 600℃, νd>νr, Fe2O3 nanowires were the result of

Fe reacted with water vapor (eq (4))

The chemical reactions can be expressed as:

Firstly, at a relatively low temperature, Fe3O4

nano-rods were obtained in situ with oxalic acid solution

dripped on The ferrous oxalate was obtained via heat treatment in the air (Figure 8(b)) FeO was obtained from the unstable ferrous oxalate decomposition Whereafter, FeO was oxidized to Fe3O4 grains by oxy-gen in the air (Figure 8(c)) The nanorods grew from saturation Fe3O4 grains as the reaction going on (Figure 8(d)) When the reaction temperature was up to 600℃, the product was only the nanowires due to the high reac-tion temperature Actually, the reacreac-tion happened be-tween the water vapor and iron, and the growth process was depicted in Figure 9 Fe2O3·nH2O grains were gained in the air under high temperature (Figure 9(a) and (b)) Fe2O3·nH2O grain began to decompose and became

Fe2O3 nanocrystals as the temperature increased (Figure 9(c)) The nanowires grew from saturation Fe2O3 grains

as the reaction going on (Figure 9(d))

Figure 8 Schematic diagram of the growth process of Fe3 O 4 nanorods

Figure 9 Schematic diagram of the growth process of Fe2 O 3 nanowires

4 Conclusions

In summary, we successfully prepared Fe3O4 nanorods and Fe2O3 nanowires via a simple thermal oxide process

We investigated the influence of reaction temperature on the samples morphology A possible mechanism was

also proposed to account for the growth of these samples

These Fe3O4 nanorods and Fe2O3 nanowires have

poten-tial applications in future magnetic materials, sensor materials and new type of catalysts

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