Physica B 392 (2007) 154–158 Synthesis, structure and magnetic properties of iron-doped tungsten oxide nanorods P.Z. Si a,b,c,Ã , C.J. Choi b , E. Bru ¨ ck c , J.C.P. Klaasse c , D.Y. Geng a , Z.D. Zhang a 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. r 2006 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 an d their existing and/or potential applications in a wide variety of areas such as information storage, electronics, sensors, structural components, catalysis, etc. Two-dimensional WO 3 films have been widely studied for their use in gas sensors [1]. One-dimensional WO 3 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 WO 2 and WO 3 were obtaine d via amorphous tungsten oxide nanoparticles [2]. Electro- chemical etching followed by heating yielded WO 3 nanorods on W substrates [3]. Through the controlled removal of surfactant from the pre-synthesized mesola- mellar at elevated temperature, WO 3 nanowires were obtained [4].WO 3 nanorods have also been generated by heating the tungsten filament using SiO 2 [5],B 2 O 3 [6], air [7], and H 2 O 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 i ntrinsically d ifferent. Consequently, t he 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 o f t he WO 3 /Fe nanorods were investigated systematically. 2. Experimenta l The WO 3 /Fe nanorods were prepared by using the traditional arc discharge method, which had been widely ARTICLE IN PRESS 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 Ã 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 K a 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. 1 shows 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 WO 3 and W diffraction peaks. The additional weaker WO 2 lines could also be indexed in the spectra for the as-prepared sample. The results indicate that most of the WO 2 and part of the W could be oxidized to WO 3 after air annealing at 573 K. Iron and its oxide could be indexed in the XRD patterns of both samples. However, the weak diff raction 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 in Figs. 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 WO 3 , 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 WO 3 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. 3 represents the XPS spectra of the W 4f, Fe 2p 3/2 ,Fe2p 1/2 , 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 WO 3 and non-stoichiometric tungsten oxide (WO 3 /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 index ed to WO 3 ,WO 2 and W. In fact, a number of tungsten oxides, including WO 3 ,WO 2 , W 3 O 8 ,W 5 O 14 ,W 17 O 47 ,W 18 O 49 ,W 20 O 58 ,andW 40 O 118 etc., 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 WO 3 and W. There are two characteristic binding energies (707.4 and 711.4 eV) for the photoelectron line of Fe 2p 3/2 , as illustrated in Fig. 3. The binding energies of 707.4 and 711.4 eV are in good agreement with that of Fe and FeO x (Fe 3 O 4 or Fe 2 O 3 ) [14], respectively. However, the binding energy (724 eV) for the Fe 2p 1/2 peak agrees well with that of Fe 2 O 3 instead of Fe 3 O 4 [15]. Therefore, we can conclude that Fe is present in the forms of elemental Fe an d Fe 2 O 3 in the as-prepared ARTICLE IN PRESS 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. P.Z. Si et al. / Physica B 392 (2007) 154–158 155 sample. As shown in Fig. 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 indica tes 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 3 /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. 4 shows 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 in Fig. 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. In Fig. 5 we show the results of magnetic measur ements for the as-prepared and the heat-treated samples. Even though the XPS spectra proved the presence of Fe 2 O 3 in the as-prepared sample, we cannot exclude the presence of ARTICLE IN PRESS 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 ,Fe2p 1/2 and Ni 2p photoelectrons in the as-prepared nanorods. P.Z. Si et al. / Physica B 392 (2007) 154–158156 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 Fe 3 O 4 ) is a ferromagnet (or ferrimagnet) with T C ¼ 1043 K (or 850 K) and M s ¼ 222 Am 2 =kg (or 84 Am 2 =kg), while FeO is antiferromag- netic with T N ¼ 198 K. The Fe 2 O 3 exists in amorphous form or other four polymorphs (alpha, beta, gamma, and epsilon) [16]. Amorphous Fe 2 O 3 is paramagnetic at temperatures above T N ¼ 80 K with a magnetic moment of 2:5m B per atom of iron [17]. The a-Fe 2 O 3 phase is antiferromagnetic (paramagnetic) at temperatures To260 K (T4T N 950 K) while a destabilization of their perfect antiparallel arrangement and development of weak ferromagnetism occurs between 260 and 950 K [16]. b- Fe 2 O 3 exhibits paramagnetic behavior at temperatures above 119 K [16]. The thermal instability of ferrimagnetic g-Fe 2 O 3 disables direct determination of its T C . For well developed g-Fe 2 O 3 crystals (M s ¼ 74 Am 2 =kg) a direct g-Fe 2 O 3 ! a-Fe 2 O 3 transformation occurs at approxi- mately 673 K [16]. For very small g -Fe 2 O 3 particles, a notably higher transformation temperature was observed with e-Fe 2 O 3 being an intermediate of the g-Fe 2 O 3 ! a-Fe 2 O 3 structural transformation [18]. The e-Fe 2 O 3 is a non-collinear ferr imagnet with T C near 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 Am 2 =kg, arising from the magnetization of metallic Fe and its oxides. Assuming a magnetic moment of 2 :2m B 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 atomi c 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, 3m B 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 WO 3 /Fe nanorods. Both the as- prepared and the heat-treated samples exhibit soft ferro- magnetic behavior as shown by hysteresis loops in Fig. 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 T 3=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 T 3=2 dependence of M at temperatures above 50 and 90 K, respectively, following the well-known Bloch’s law [20,21]. How ever, 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 ARTICLE IN PRESS 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 2 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. P.Z. Si et al. / Physica B 392 (2007) 154–158 157 might indicate the presence of paramagnetic phases in the sample, which usually has little (large) contrib ution 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, Fe 3 O 4 and g-Fe 2 O 3 exhibit the largest saturation magnetization of 84 and 74 Am 2 /kg, respectively. The slightly decreasing feature between 800 and 900 K is very likely due to the Curie point of Fe 3 O 4 and the structural transformation of g -Fe 2 O 3 4a-Fe 2 O 3 with e-Fe 2 O 3 being an intermediate [18]. A notably higher transformation temperature for g-Fe 2 O 3 4a-Fe 2 O 3 than 673 K of bulk g-Fe 2 O 3 has been observed in nanoscale g- Fe 2 O 3 [18]. The sharp magnetization decrease feature at temperatures between 940 and 1000 K, which is slightly lower than the T C (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. Conclusi ons In summary, iron-doped tun gsten 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 mainl y 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 eleme ntal 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 Techn ology under ‘21st Century Frontier R&D Programs’ (Grant no. 05K1501-00310), the National Natural Science Foundation of China (Gr ants nos. 59725103, 50332020, and 50171070), and the scientific exchange program between China and The Netherlands. References [1] A. Hoel, L.F. Reyes, P. Heszler, V. Lantto, C.G. Granqvist, Curr. Appl. Phys. 4 (2004) 547. [2] Y. Koltypin, S.I. Nikitenko, A. Gedanken, J. Mater. Chem. 12 (2002) 1107. [3] G. Gu, B. Zheng, W.Q. Han, S. Roth, J. 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Physica B 392 (2007) 154–158 Synthesis, structure and magnetic properties of iron-doped tungsten oxide nanorods P.Z. Si a,b,c,Ã , C.J. Choi b ,. 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