synthesis, structure and magnetic properties of iron-doped tungsten oxide nanorods

Physica B 392 2007 154–158Synthesis, structure and magnetic properties of iron-doped tungsten oxide nanorods P.Z.. The nanorods were mainly composed of tungsten, iron and their oxides..

Physica B 392 (2007) 154–158

Synthesis, structure and magnetic properties of

iron-doped tungsten oxide nanorods P.Z Sia,b,c, , C.J Choib, E Bru¨ckc, J.C.P Klaassec, D.Y Genga, Z.D Zhanga

a Shenyang National Laboratory for Materials Science and International Centre for Materials Physics, Institute of Metal Research,

Chinese Academy of Sciences, Shenyang 110016, China

b

Korea Institute of Machinery and Materials, 66 Sangnam-dong, Changwon 641-010, South Korea

c

Van der Waals Zeeman Institute, University of Amsterdam, Valckenierstr 65, NL-1018 XE Amsterdam, The Netherlands

Received 19 June 2006; received in revised form 6 November 2006; accepted 9 November 2006

Abstract

Iron-doped tungsten oxide nanorods of 20–30 nm in diameter and 60–2000 nm in length have been prepared by an arc discharge route using W as cathode and a mixture of Fe and NiO as anode, in which NiO serves as oxygen source The characteristics of the nanorods were investigated systematically by using X-ray diffraction, transmission electron microscopy, energy dispersive spectra, X-ray photoelectron spectroscopy, and superconducting quantum interference device magnetometer The nanorods were mainly composed of tungsten, iron and their oxides The iron-rich phase in the nanorods exhibits soft ferromagnetic behaviors with zero coercivity and zero remanence and a decreased Curie temperature of 1000 K Heat-treatment of the sample in air induces oxidation of elemental Fe, resulting

in the reduction of the magnetization

r2006 Elsevier B.V All rights reserved

PACS: 75.75.þa; 81.07.Wx

Keywords: Iron; Nanorods; Magnetic properties; Tungsten oxide

1 Introduction

Nanomaterials have been the subject of intense research

in recent years because of their unique properties in

comparison with the bulk counterparts and their existing

and/or potential applications in a wide variety of areas

such as information storage, electronics, sensors, structural

components, catalysis, etc Two-dimensional WO3 films

have been widely studied for their use in gas sensors [1]

One-dimensional WO3 nanorods, which can be prepared

by using a few different approaches, as partially described

below, are attracting increasingly attention recently

Nanorods of the mixtures of WO2and WO3were obtained

via amorphous tungsten oxide nanoparticles [2]

Electro-chemical etching followed by heating yielded WO3

nanorods on W substrates [3] Through the controlled

removal of surfactant from the pre-synthesized mesola-mellar at elevated temperature, WO3 nanowires were obtained[4] WO3 nanorods have also been generated by heating the tungsten filament using SiO2[5], B2O3 [6], air [7], and H2O as oxygen sources[8] In this work, we report

on the formation of Fe-doped tungsten oxide nanorods by arc discharge method, using NiO as oxygen sources The magnetic behaviors of atomic and bulk transition metals are intrinsically different Consequently, the magnetic properties of nanoparticles as a bridge in the atomic and bulk materials are very sensitive to size, composition, and local atomic environment, thus showing a wide variety of intriguing phenomena[9] In this work, the magnetic properties of the

WO3/Fe nanorods were investigated systematically

2 Experimental The WO3/Fe nanorods were prepared by using the traditional arc discharge method, which had been widely

www.elsevier.com/locate/physb

0921-4526/$ - see front matter r 2006 Elsevier B.V All rights reserved.

doi: 10.1016/j.physb.2006.11.011

Corresponding author Department of Physics, China Jiliang

Uni-versity, 310018, Hangzhou, China Tel.:+86 571 81302373.

E-mail address: pzsi@mail.com (P.Z Si).

employed to synthesize magnetic nanocapsules in our

previous work [10–12] The compacted mixture of 120 g

Fe and 14 g NiO powders was used as anode, while a W

needle was used as cathode The chamber was vacuumized

to be below 1 Pa and further filled with Ar to 14 000 Pa An

arc with a current of 200 A was struck between the anode

and the cathode Part of the as-prepared products was

annealed in air at 573 K for 10 h

Powder X-ray diffraction (XRD) was performed with Cu

Ka radiation (l ¼ 1:54178 ˚A) at room temperature to

identify the crystal structure of the products The products

were then dispersed in ethanol and deposited on copper

grids for transmission electron microscope (TEM) imaging

and energy dispersive X-ray spectroscopy (EDX)

Addi-tionally, the chemical bonding structure of the as-prepared

products was determined by X-ray photoelectron

spectro-scopy (XPS) employing a 1486.8 eV source Spectra of the

original sample surface and surface after argon–ion

bombardment for 150 s were recorded by XPS on a

compacted plate with diameter of 10 mm and thickness of

1 mm Magnetic hysteresis were measured by using a

superconducting quantum interference device

magnet-ometer (SQUID) in fields up to 5 T The hysteresis loops

were measured at selected temperatures Curie points were

determined by using a Faraday magnetometer from 330 to

1150 K in a magnetic field of 0.05 T The as-prepared

products for Faraday magnetometer measurements were

first loaded into a quartz tube and then sealed in 0.14 bar

argon atmosphere

3 Results and discussion

Fig 1shows the XRD patterns of the as-prepared and

the annealed samples In order to show up the weaker

peaks, the data in Fig 1 were plotted on a logarithmic

intensity scale Both spectra show WO3and W diffraction

peaks The additional weaker WO2 lines could also be indexed in the spectra for the as-prepared sample The results indicate that most of the WO2 and part of the W could be oxidized to WO3after air annealing at 573 K Iron and its oxide could be indexed in the XRD patterns of both samples However, the weak diffraction peaks for iron and its oxide were broadened significantly, indicating very tiny crystallites in size or a very small weight percentage The high saturation magnetization of the as-prepared sample and EDX analysis of the air-oxidized sample as discussed below indicate a considerable weight percentage of iron and its compounds in the samples

The morphologies of the as-prepared products are demonstrated in Figs 2a and b The products show obvious rod-like shape up to 2000 nm in length and 20–30 nm in diameter Shown in Fig 2b is a typical TEM image, in which the nanorods were covered by a thin film of approximately 3–4 nm in thickness, estimated by the contrast Nanoclusters adhering to the nanorods or protuberances could also be observed over the surface of the nanorods At higher magnification, as shown inFigs 2c and d, the nanorods exhibit clear fringes parallel to their long axis The lattice spacing of two parallel planes was 0.39 nm, which could be indexed best as (0 0 1) of WO3, according to JCPDS card No 20-1324.Figs 2c and d also show that the thin layer covering the nanorods is in amorphous The thickness of the amorphous coatings and the size of the nanoclusters adhering to the nanorods are much smaller, compared with the diameter and size of the well-crystallized WO3 matrix Therefore, there should be more elemental W than Fe in the sample

The XPS technique probes mostly the surface atoms of the sample.Fig 3represents the XPS spectra of the W 4f,

Fe 2p3/2, Fe 2p1/2, and Ni 2p photoelectrons in the as-prepared nanorods for original surface and surface after argon-ion etching for 150 s, respectively The original surface consists of WO3and non-stoichiometric tungsten oxide (WO3/W)[13] Additional W peaks could be detected

in the spectra for the etched surface of the sample The XPS results are in good agreement with that of the XRD analysis, which could be indexed to WO3, WO2and W In fact, a number of tungsten oxides, including WO3, WO2,

W3O8, W5O14, W17O47, W18O49, W20O58, and W40O118etc., could be formed, which could coexist or progressively change one into the other with changing temperature and oxygen partial pressure Usually, oxidation to metal is controlled by oxygen atomic diffusion process Therefore, the non-stoichiometric tungsten oxide could be formed in the nanoscale sample consisting of WO3and W There are two characteristic binding energies (707.4 and 711.4 eV) for the photoelectron line of Fe 2p3/2, as illustrated inFig 3 The binding energies of 707.4 and 711.4 eV are in good agreement with that of Fe and FeOx(Fe3O4or Fe2O3)[14], respectively However, the binding energy (724 eV) for the

Fe 2p1/2 peak agrees well with that of Fe2O3 instead of

Fe3O4[15] Therefore, we can conclude that Fe is present in the forms of elemental Fe and Fe O in the as-prepared

Fig 1 X-ray diffraction patterns of (a) the as-prepared powders and (b)

that after annealing at 573 K in air for 10 h.

sample As shown inFig 3, no Ni 2p peak was observed in

both the original and the etched surfaces of the sample

Since XPS technique is very sensitive to elements, the

absence of Ni 2p peak indicates the absence of elemental Ni

or its compounds in the products, in good agreement with

the XRD results above and the EDX results to be discussed

below Note that the photoelectron lines for elemental W

are much stronger than those for elemental Fe, indicating a

larger W content than Fe content in the sample

The electron-induced X-ray fluorescence (EDX) analysis

was employed to determine the composition of the

air-oxidized WO /Fe nanorods Since air oxidation could not

change the elemental ratio except ratio to oxygen, the EDX results for the air-oxidized sample can to some extent represent that of the as-prepared sample.Fig 4shows the TEM images and the EDX spectra for the air-annealed products It is obvious that the rod-like shape and morphology of the products were maintained after air-oxidation Only W, Fe, O, and Cu elements were detected,

as shown inFig 4 The presence of a Cu signal arises from the sample holder, thus the nanorods were composed of W,

Fe, and O The peak intensity for W is much stronger than that for Fe, indicating that the nanorods, at least within the selected area for recording EDX spectra, contain more W than Fe It should be noted that the Ni atoms expected from the anode were not detected by EDX

InFig 5we show the results of magnetic measurements for the as-prepared and the heat-treated samples Even though the XPS spectra proved the presence of Fe2O3 in the as-prepared sample, we cannot exclude the presence of

Fig 2 TEM images of the as-synthesized Fe-doped tungsten oxide

nanorods: (a) low magnification image shows the morphology of

the nanorods, (b) high magnification image shows a thin film covering

the nanorods and several nanoparticles adhering to the surface of the

nanorods, ðc; dÞ high resolution TEM images show amorphous film over

the surface of a nanorod and well-crystallized nanorods with 0.39 nm

interplanar distance corresponding to the (0 0 1) interplanar spacing

of WO 3

Fig 3 XPS spectra of the W 4f, Fe 2p 3/2 , Fe 2p 1/2 and Ni 2p photoelectrons in the as-prepared nanorods.

other iron oxides because of the limited difference between

Fe 2p binding energies in different iron oxides In order to

determine the magnetization contribution of different

magnetic phases, a basic knowledge for the magnetic

properties of bulk Fe and its oxides is crucial It is well

known that bulk iron (or Fe3O4) is a ferromagnet (or

ferrimagnet) with TC¼1043 K (or 850 K) and Ms¼

222 Am2=kg (or 84 Am2=kg), while FeO is

antiferromag-netic with TN¼198 K The Fe2O3 exists in amorphous

form or other four polymorphs (alpha, beta, gamma, and

epsilon) [16] Amorphous Fe2O3 is paramagnetic at

temperatures above TN¼80 K with a magnetic moment

of 2:5mB per atom of iron [17] The a-Fe2O3 phase is

antiferromagnetic (paramagnetic) at temperatures

To260 K (T4TN950 K) while a destabilization of their

perfect antiparallel arrangement and development of weak

ferromagnetism occurs between 260 and 950 K [16]

b-Fe2O3 exhibits paramagnetic behavior at temperatures

above 119 K[16] The thermal instability of ferrimagnetic

g-Fe2O3disables direct determination of its TC For well

developed g-Fe2O3 crystals (Ms¼74 Am2=kg) a direct

g-Fe2O3!a-Fe2O3 transformation occurs at

approxi-mately 673 K [16] For very small g-Fe2O3 particles, a

notably higher transformation temperature was observed

with e-Fe2O3 being an intermediate of the g-Fe2O3!

a-Fe2O3 structural transformation [18] The e-Fe2O3 is a

non-collinear ferrimagnet with TCnear 470 K [16,18]

In the plot of M vs H, both the samples reach saturation

in fields above 0.8 T The magnetization at 5 K of the

as-prepared sample in an applied field of 5 T is as large as

56 Am2=kg, arising from the magnetization of metallic Fe

and its oxides Assuming a magnetic moment of 2:2mB per

iron atom in the sample, we find the most conservative

estimation of Fe content in the sample is 25.5 wt%

Considering the effects that could reduce the total

magnetization, including the formation of iron oxides

and atomic disorder in small particles, the actual elemental

Fe content in the sample should be much higher than 25.5 wt% However, most analytical results mentioned above, including XRD, XPS, EDX, and TEM observa-tions, support a lower content of elemental Fe than that of

W in the sample We speculate that the large magnetization

of the sample might partially be due to a possible enhanced magnetic moment of Fe atom in the Fe nanoclusters In fact, an enhanced magnetic moment, 3mB per Fe atom at

120 K for clusters containing 25–130 Fe atoms, has been observed in small iron clusters [19] In comparison with iron, iron oxides have a much lower saturation magnetiza-tion, being just slightly larger than the saturation magnetization of the WO3/Fe nanorods Both the as-prepared and the heat-treated samples exhibit soft ferro-magnetic behavior as shown by hysteresis loops inFig 5 Even at 5 K, both the samples exhibit zero coercivity (less than 5 Oe) and zero remanence, which are quite different from those of well-crystallized Fe nanoparticles, in which enhanced coercivity and enhanced remanence magnetiza-tion in comparison with that of bulk Fe were observed[10] Shown in the left inset of Fig 5 is the plot of the magnetization at an applied field of 5 T vs T3=2 The magnetization for the heat-treated sample is approximately 48% of that of the as-prepared sample, owing mainly to the oxidation of the Fe clusters The as-prepared and heat-treated samples show a linear T3=2 dependence of M at temperatures above 50 and 90 K, respectively, following the well-known Bloch’s law[20,21] However, the curves at temperatures below these temperatures deviate significantly from Bloch’s law Magnetization deviation from Bloch’s law towards lower magnetization has been observed in amorphous iron at temperatures below 50 K[22] However, our samples exhibit magnetization deviations to larger magnetization as shown in the left inset of Fig 5 The sharp magnetization increase in the low temperature region

Fig 4 EDX spectrum shows the composition profiles of the Fe-doped

tungsten oxide nanorods after annealing in air The inset shows the

morphology of the annealed sample and the area for recording EDX

spectrum.

Fig 5 M vs H plot for the as-prepared (black) and heat-treated (gray) samples at several temperatures The left inset shows the magnetization vs the 3 power of temperature for both the samples The line is a fit to Bloch’s law Shown in right inset is the Curie point determination curve for the as-prepared sample obtained on a Faraday balance.

might indicate the presence of paramagnetic phases in the

sample, which usually has little (large) contribution to

magnetization in high (low) temperature regions The

air-oxidized sample and the as-prepared samples show

deviation at temperatures below 90 and 50 K, respectively

This indicates that the presence of iron oxides should be

one reason for these deviations because iron oxides usually

do not follow Bloch’s law

In the right inset of Fig 5 we show the magnetization

measurement for the as-prepared nanorods in an applied

field of 0.05 T The magnetization starts to decrease at

800 K with increasing temperature and vanishes at 1000 K

Among all iron oxides, Fe3O4 and g-Fe2O3 exhibit the

largest saturation magnetization of 84 and 74 Am2/kg,

respectively The slightly decreasing feature between 800

and 900 K is very likely due to the Curie point of Fe3O4

and the structural transformation of g-Fe2O34a-Fe2O3

with e-Fe2O3being an intermediate[18] A notably higher

transformation temperature for g-Fe2O34a-Fe2O3 than

673 K of bulk g-Fe2O3has been observed in nanoscale

g-Fe2O3 [18] The sharp magnetization decrease feature at

temperatures between 940 and 1000 K, which is slightly

lower than the TC (1043 K) of bulk Fe, is ascribed to the

Curie point for tiny Fe clusters Usually, amorphous iron

or very small iron clusters exhibit decreased Curie

temperature [19,22]

4 Conclusions

In summary, iron-doped tungsten oxide nanorods with

diameters ranging from 20 to 30 nm and lengths up to

60–2000 nm have been synthesized by an arc discharge

route using W as cathode and a mixture of Fe and NiO as

anode, in which NiO serves as an oxygen source The

nanorods were composed of W, Fe, and their oxides Most

of the nanorods were covered by an amorphous film with

3–4 nm in thickness, in which nanoclusters adhering to the

surface of the nanorods were frequently observed XPS

shows that the surface layers were mainly composed of

tungsten oxide, iron and its oxide Faraday balance

measurements show that the magnetization of the sample

vanishes at temperatures above 1000 K, indicating a

decreased Curie temperature for tiny Fe clusters comparing

with that of bulk Fe Heat-treatment of the sample in air

induces oxidation of elemental Fe, resulting in the

reduction of the magnetization Both the as-prepared and

the heat-treated samples show zero coercivity and zero

remanence

Acknowledgments The work was supported by the Center for Nanostruc-tured Materials Technology under ‘21st Century Frontier R&D Programs’ (Grant no 05K1501-00310), the National Natural Science Foundation of China (Grants nos

59725103, 50332020, and 50171070), and the scientific exchange program between China and The Netherlands

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