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Preparation and properties of magnetic iron oxide nanotubes
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Particuology 6 (2008) 334–339
Preparation and properties of magnetic iron oxide nanotubes Baoliang Lv a,b , Yao Xu a,∗ , Dong Wu a , Yuhan Sun a a
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China b Graduate University of the Chinese Academy of Sciences, Beijing 100039, China Received 7 March 2008; accepted 4 April 2008
Abstract Magnetite (Fe3 O4 ) nanotubes were prepared by reducing synthesized hematite (␣-Fe2 O3 ) nanotubes in 5% H2 +95% Ar atmosphere, and then maghemite (␥-Fe2 O3 ) nanotubes were obtained by re-oxidizing the Fe3 O4 nanotubes. The nanotube structure was kept from collapsing or sintering throughout the high temperature reducing and re-oxidizing processes. The coercivities of the Fe3 O4 and ␥-Fe2 O3 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 scientifically interesting and technologically 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 protection (Wu, Qu, & Chen, 2005), sensors (Sun, Yuan, Liu, Han, & Zhang, 2005), drug delivery (Wu et al., 2007) and water splitting (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 applications. 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, catalysts, 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: [email protected] (Y. Xu).
(2004b), Wang, Wang, Li, Xu, and Zhou (2006), and Shen et al. (2004) used porous anodic aluminium oxide (AAO) as template to prepare Fe3 O4 and ␣-Fe2 O3 nanotube arrays; Sun et al. (2005) used carbon nanotubes as templates to fabricate ␣Fe2 O3 nanotubes; Liu et al. (2005) used MgO nanowires as templates to fabricate single-crystal Fe3 O4 nanotubes. However, templates not only introduced extraneous impurities but also increased production cost, not to say the many problems 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 ␣-Fe2 O3 nanotubes by a hydrothermal method without using template. In this work, we improved their work by first synthesizing ␣-Fe2 O3 nanotubes, followed by reducing ␣-Fe2 O3 and re-oxidizing the Fe3 O4 to ␥-Fe2 O3 nanotubes. The magnetic properties of the Fe3 O4 and ␥-Fe2 O3 nanotubes were investigated by using vibrating sample magnetometry (VSM). 2. Experimental All the reagents were A.R. grade and were used in preparation without further purification: ferric chloride (FeCl3 ·6H2 O, China Medicament Co.), sodium dihydrogen phosphate (NaH2 PO4 ·2H2 O, 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
B. Lv et al. / Particuology 6 (2008) 334–339
distilled water. Reduction gas was composed of 5 v% H2 (high purity) and 95 v% Ar (high purity). The preparation of ␣-Fe2 O3 nanotubes was an improved approach based on literature (Jia et al., 2005). In a typical synthesis, 40 mL of FeCl3 aqueous solution (46.2 mmol/L) and 40 mL of NaH2 PO4 aqueous solution (1.9 mmol/L) were first mixed and then dispersed uniformly by ultrasonic irradiation. The solution 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 ␣-Fe2 O3 nanotubes, named as S1. Fe3 O4 nanotubes were obtained by reducing S1 in a tubular oven at 500 ◦ C for 2.5 h in a 5% H2 +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 ␣-Fe2 O3 or FeO would be present in the reduction product. To prepare ␥-Fe2 O3 nanotubes, the as-prepared Fe3 O4 nanotubes 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␣ radiation (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 hysteresis loops were measured by vibrating sample magnetometry (VSM, Lakeshore 7407). 3. Results and discussion Fig. 1 shows the XRD patterns of samples S1 (a), S2 (b) and S3 (c). In Fig. 1(a), the initial synthesized product (sample S1)
Fig. 1. XRD patterns of samples S1 (a), S2 (b) and S3 (c).
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Fig. 2. Raman spectra of samples S2 (a) and S3 (b).
can be exclusively indexed to ␣-Fe2 O3 , according to standard data (JCPDS 33-0664). In Fig. 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 ␥-Fe2 O3 (JCPDS 39-1346) or Fe3 O4 (JCPDS 19-0629). However, it is well-known that clear identification of ␥-Fe2 O3 and Fe3 O4 based on ordinary 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 Fe3 O4 and ␥-Fe2 O3 , respectively, the purity of the samples cannot be simply identified by their appearance. To differentiate samples S2 and S3 clearly, further characterization 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 sample S2, shown in Fig. 2(a), exhibits two clear peaks at 665 and 540 cm−1 , which can be indexed to the A1g and T2g transitions of the Fe3 O4 phase (Shebanova & Lazor, 2003). In Fig. 2(b), the Raman spectrum of sample S3, the different characteristic bands of ␥-Fe2 O3 (700, 500 and 350 cm−1 ) can be observed (Varadwaj, Panigrahi, & Ghose, 2004). Consequently, it should be reasonable to think that sample S2 is Fe3 O4 and sample S3 is ␥-Fe2 O3 . Fig. 3 presents the SEM and TEM images of samples S1, S2 and S3. Fig. 3(a) and (b) show the morphologies of initial synthesized ␣-Fe2 O3 nanotubes (sample S1), in which the nanotubes can be seen clearly, with length of 160–300 nm,
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Fig. 3. SEM and TEM images of ␣-Fe2 O3 nanotubes in sample S1 (a and b), Fe3 O4 nanotubes in sample S2 (c and d) and ␥-Fe2 O3 nanotubes in sample S3 (e and f).
B. Lv et al. / Particuology 6 (2008) 334–339
and outer and inner diameters of 70–120 nm and 45–80 nm, respectively. Fig. 3(c) and (d) show the morphologies of Fe3 O4 nanotubes (sample S2), with their well retained nanotube structure. Fig. 3(e) and (f) show the nearly same morphologies of ␥-Fe2 O3 (sample S3) together with their well retained tube structure. 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 Fe3 O4 and ␥-Fe2 O3 nanotubes are conglomerated with each other, while ␣-Fe2 O3 nanotubes are well dispersed, apparently due to the mutual attraction of the magnetic Fe3 O4 and ␥-Fe2 O3 particles, though no obvious difference can be found from the morphologies of the three samples. Generally, nanostructures are often destroyed due to sintering or collapsing during treatment at high temperature. But Fig. 3 shows that the nanotube structure was well preserved after reduction and re-oxidization at high temperature. There might be two reasons for this. First, according to Jiao et al. (2006), conversion of ␣-Fe2 O3 to Fe3 O4 involves a change from a hexagonal closepacked oxide ion array (␣-Fe2 O3 ) to a cubic close-packed array (Fe3 O4 ). 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 destroying 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. Second, according to Jia et al. (2005), phosphate could be adsorbed on ␣-Fe2 O3 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 ␣Fe2 O3 nanotubes, with the result shown in Fig. 4(a). The binding energies obtained in the XPS analysis were corrected by referencing the C1s line to 284.5 eV. Seen from Fig. 4(a), the binding energy of P2p was found at 133.6 eV in the spectrum, which agreed with the reported value of PO4 3− (Wang et al., 2003). To further identify the existence of the phosphate layer, a highmagnification image of sample S1 was obtained on TEM, as shown in Fig. 4(b), indicating the presence of a 2.5-nm adsorption layer, thus confirming the presence of a phosphate layer on the surface of synthesized ␣-Fe2 O3 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 ␣-Fe2 O3 went on, the phosphate on the surface could not be reduced, and only the inner ␣-Fe2 O3 was reduced by hydrogen. Therefore, the nanotubes could be kept from sintering or collapsing. To confirm the stabilization of phosphate on nanotubes, pure iron phosphate (FePO4 ) sample was treated under the same reduction condition as that for ␣-Fe2 O3 reduction. The XRD pattern (not given here) of the reduction product showed that FePO4 was reduced to Fe2 PO5 . Oxidation of Fe3 O4 nanotubes to ␥-Fe2 O3 nanotubes involved a decrease in the number of Fe atoms per unit cell of 32 oxygen ions, from 24 in Fe3 O4 to 21(1/3) in ␥-Fe2 O3 . This reaction proceeded with outward migration of the Fe2+ cations towards the surface of the crystal
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Fig. 4. (a) XPS spectrum of ␣-Fe2 O3 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 ␥-Fe2 O3 , 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 structures, 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 nanotubes, 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 Fe3 O4 (a) and ␥-Fe2 O3 (b) nanotubes at room temperature. From Fig. 5(a), the M–H loop of Fe3 O4 nanotubes shows ferromagnetic behavior with a saturation magnetization (Ms) of 60.92 emu/g, a remanent magnetization (Mr) of 18.56 emu/g and a coercivity of 340.22 Oe at room temperature. Compared to bulk Fe3 O4 (Ms = 92 emu/g, coercivity 115–150 Oe) (Liu, Fu, & Xiao, 2006), the Ms was obviously lower and the coercivity was obviously higher. Fe3 O4 nanotubes also possess higher coercivity than other Fe3 O4 nanostructures 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 ␥-Fe2 O3 nanotubes shows ferromagnetic behavior with a Ms of 42.71 emu/g, a Mr of
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Fig. 5. M–H loops of Fe3 O4 nanotubes (a) and ␥-Fe2 O3 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 ␥-Fe2 O3 (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 Fe3 O4 nanotubes, ␥-Fe2 O3 nanotubes also have a higher coercivity than other reported ␥-Fe2 O3 nanostructures, such as nanofibres (78.11 Oe), nanoparticles (106 Oe), and some reported superparamagnetic ␥-Fe2 O3 particles (0 Oe) (Han et al., 2007; Jing, 2006; Zhang et al., 2008). It is noted that both these two magnetic iron oxide nanotubes have a higher coercivity than other nanostructures with the same phase and of similar size. Furthermore, the M–H loops of Fe3 O4 nanotubes and ␥-Fe2 O3 nanotubes indicate the similar magnetic domain type. On the basis of the criteria given by Dunlop (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. From Fig. 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 nanotubes, which has been confirmed previously by XPS, and the phosphate is not a magnetic material. From Fig. 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 structures 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 particles. 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 coordination 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 coercivity. 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 (din and 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 /dout value 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 Fe3 O4 nanotubes were prepared by reducing synthesized ␣Fe2 O3 nanotubes with a gas mixture of 5% H2 +95% Ar at 500 ◦ C for 2.5 h, and then ␥-Fe2 O3 nanotubes were obtained by reoxidizing the Fe3 O4 nanotubes with air at 300 ◦ C for 2 h. The nanotube structure was well retained without collapsing or sintering, for which, adsorbed phosphate and the type of crystal structure conversion should be the two most important reasons. Investigation of the magnetic properties of Fe3 O4 and ␥-Fe2 O3 nanotubes revealed that both the two magnetic iron oxide nanotubes 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. References Bødker, F., Hansen, M. F., Bender Koch, C., Lefmann, K., & Mørup, S. (2000). Magnetic properties of hematite nanoparticles. Physical Review B, 61(10), 6826–6838. Cesar, I., Kay, A., Gonzalez Martinez, J. A., & Grätzel, M. (2006). Translucent thin film Fe2 O3 photoanodes for efficient water splitting by sunlight: Nanostructure-directing effect of Si-doping. Journal of the American Chemical Society, 128(14), 4582–4583.
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Particuology 6 (2008) 334–339
Preparation and properties of magnetic iron oxide nanotubes Baoliang Lv a,b , Yao Xu a,∗ , Dong Wu a , Yuhan Sun a a
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China b Graduate University of the Chinese Academy of Sciences, Beijing 100039, China Received 7 March 2008; accepted 4 April 2008
Abstract Magnetite (Fe3 O4 ) nanotubes were prepared by reducing synthesized hematite (␣-Fe2 O3 ) nanotubes in 5% H2 +95% Ar atmosphere, and then maghemite (␥-Fe2 O3 ) nanotubes were obtained by re-oxidizing the Fe3 O4 nanotubes. The nanotube structure was kept from collapsing or sintering throughout the high temperature reducing and re-oxidizing processes. The coercivities of the Fe3 O4 and ␥-Fe2 O3 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 scientifically interesting and technologically 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 protection (Wu, Qu, & Chen, 2005), sensors (Sun, Yuan, Liu, Han, & Zhang, 2005), drug delivery (Wu et al., 2007) and water splitting (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 applications. 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, catalysts, 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: [email protected] (Y. Xu).
(2004b), Wang, Wang, Li, Xu, and Zhou (2006), and Shen et al. (2004) used porous anodic aluminium oxide (AAO) as template to prepare Fe3 O4 and ␣-Fe2 O3 nanotube arrays; Sun et al. (2005) used carbon nanotubes as templates to fabricate ␣Fe2 O3 nanotubes; Liu et al. (2005) used MgO nanowires as templates to fabricate single-crystal Fe3 O4 nanotubes. However, templates not only introduced extraneous impurities but also increased production cost, not to say the many problems 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 ␣-Fe2 O3 nanotubes by a hydrothermal method without using template. In this work, we improved their work by first synthesizing ␣-Fe2 O3 nanotubes, followed by reducing ␣-Fe2 O3 and re-oxidizing the Fe3 O4 to ␥-Fe2 O3 nanotubes. The magnetic properties of the Fe3 O4 and ␥-Fe2 O3 nanotubes were investigated by using vibrating sample magnetometry (VSM). 2. Experimental All the reagents were A.R. grade and were used in preparation without further purification: ferric chloride (FeCl3 ·6H2 O, China Medicament Co.), sodium dihydrogen phosphate (NaH2 PO4 ·2H2 O, 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
B. Lv et al. / Particuology 6 (2008) 334–339
distilled water. Reduction gas was composed of 5 v% H2 (high purity) and 95 v% Ar (high purity). The preparation of ␣-Fe2 O3 nanotubes was an improved approach based on literature (Jia et al., 2005). In a typical synthesis, 40 mL of FeCl3 aqueous solution (46.2 mmol/L) and 40 mL of NaH2 PO4 aqueous solution (1.9 mmol/L) were first mixed and then dispersed uniformly by ultrasonic irradiation. The solution 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 ␣-Fe2 O3 nanotubes, named as S1. Fe3 O4 nanotubes were obtained by reducing S1 in a tubular oven at 500 ◦ C for 2.5 h in a 5% H2 +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 ␣-Fe2 O3 or FeO would be present in the reduction product. To prepare ␥-Fe2 O3 nanotubes, the as-prepared Fe3 O4 nanotubes 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␣ radiation (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 hysteresis loops were measured by vibrating sample magnetometry (VSM, Lakeshore 7407). 3. Results and discussion Fig. 1 shows the XRD patterns of samples S1 (a), S2 (b) and S3 (c). In Fig. 1(a), the initial synthesized product (sample S1)
Fig. 1. XRD patterns of samples S1 (a), S2 (b) and S3 (c).
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Fig. 2. Raman spectra of samples S2 (a) and S3 (b).
can be exclusively indexed to ␣-Fe2 O3 , according to standard data (JCPDS 33-0664). In Fig. 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 ␥-Fe2 O3 (JCPDS 39-1346) or Fe3 O4 (JCPDS 19-0629). However, it is well-known that clear identification of ␥-Fe2 O3 and Fe3 O4 based on ordinary 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 Fe3 O4 and ␥-Fe2 O3 , respectively, the purity of the samples cannot be simply identified by their appearance. To differentiate samples S2 and S3 clearly, further characterization 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 sample S2, shown in Fig. 2(a), exhibits two clear peaks at 665 and 540 cm−1 , which can be indexed to the A1g and T2g transitions of the Fe3 O4 phase (Shebanova & Lazor, 2003). In Fig. 2(b), the Raman spectrum of sample S3, the different characteristic bands of ␥-Fe2 O3 (700, 500 and 350 cm−1 ) can be observed (Varadwaj, Panigrahi, & Ghose, 2004). Consequently, it should be reasonable to think that sample S2 is Fe3 O4 and sample S3 is ␥-Fe2 O3 . Fig. 3 presents the SEM and TEM images of samples S1, S2 and S3. Fig. 3(a) and (b) show the morphologies of initial synthesized ␣-Fe2 O3 nanotubes (sample S1), in which the nanotubes can be seen clearly, with length of 160–300 nm,
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Fig. 3. SEM and TEM images of ␣-Fe2 O3 nanotubes in sample S1 (a and b), Fe3 O4 nanotubes in sample S2 (c and d) and ␥-Fe2 O3 nanotubes in sample S3 (e and f).
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and outer and inner diameters of 70–120 nm and 45–80 nm, respectively. Fig. 3(c) and (d) show the morphologies of Fe3 O4 nanotubes (sample S2), with their well retained nanotube structure. Fig. 3(e) and (f) show the nearly same morphologies of ␥-Fe2 O3 (sample S3) together with their well retained tube structure. 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 Fe3 O4 and ␥-Fe2 O3 nanotubes are conglomerated with each other, while ␣-Fe2 O3 nanotubes are well dispersed, apparently due to the mutual attraction of the magnetic Fe3 O4 and ␥-Fe2 O3 particles, though no obvious difference can be found from the morphologies of the three samples. Generally, nanostructures are often destroyed due to sintering or collapsing during treatment at high temperature. But Fig. 3 shows that the nanotube structure was well preserved after reduction and re-oxidization at high temperature. There might be two reasons for this. First, according to Jiao et al. (2006), conversion of ␣-Fe2 O3 to Fe3 O4 involves a change from a hexagonal closepacked oxide ion array (␣-Fe2 O3 ) to a cubic close-packed array (Fe3 O4 ). 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 destroying 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. Second, according to Jia et al. (2005), phosphate could be adsorbed on ␣-Fe2 O3 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 ␣Fe2 O3 nanotubes, with the result shown in Fig. 4(a). The binding energies obtained in the XPS analysis were corrected by referencing the C1s line to 284.5 eV. Seen from Fig. 4(a), the binding energy of P2p was found at 133.6 eV in the spectrum, which agreed with the reported value of PO4 3− (Wang et al., 2003). To further identify the existence of the phosphate layer, a highmagnification image of sample S1 was obtained on TEM, as shown in Fig. 4(b), indicating the presence of a 2.5-nm adsorption layer, thus confirming the presence of a phosphate layer on the surface of synthesized ␣-Fe2 O3 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 ␣-Fe2 O3 went on, the phosphate on the surface could not be reduced, and only the inner ␣-Fe2 O3 was reduced by hydrogen. Therefore, the nanotubes could be kept from sintering or collapsing. To confirm the stabilization of phosphate on nanotubes, pure iron phosphate (FePO4 ) sample was treated under the same reduction condition as that for ␣-Fe2 O3 reduction. The XRD pattern (not given here) of the reduction product showed that FePO4 was reduced to Fe2 PO5 . Oxidation of Fe3 O4 nanotubes to ␥-Fe2 O3 nanotubes involved a decrease in the number of Fe atoms per unit cell of 32 oxygen ions, from 24 in Fe3 O4 to 21(1/3) in ␥-Fe2 O3 . This reaction proceeded with outward migration of the Fe2+ cations towards the surface of the crystal
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Fig. 4. (a) XPS spectrum of ␣-Fe2 O3 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 ␥-Fe2 O3 , 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 structures, 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 nanotubes, 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 Fe3 O4 (a) and ␥-Fe2 O3 (b) nanotubes at room temperature. From Fig. 5(a), the M–H loop of Fe3 O4 nanotubes shows ferromagnetic behavior with a saturation magnetization (Ms) of 60.92 emu/g, a remanent magnetization (Mr) of 18.56 emu/g and a coercivity of 340.22 Oe at room temperature. Compared to bulk Fe3 O4 (Ms = 92 emu/g, coercivity 115–150 Oe) (Liu, Fu, & Xiao, 2006), the Ms was obviously lower and the coercivity was obviously higher. Fe3 O4 nanotubes also possess higher coercivity than other Fe3 O4 nanostructures 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 ␥-Fe2 O3 nanotubes shows ferromagnetic behavior with a Ms of 42.71 emu/g, a Mr of
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Fig. 5. M–H loops of Fe3 O4 nanotubes (a) and ␥-Fe2 O3 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 ␥-Fe2 O3 (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 Fe3 O4 nanotubes, ␥-Fe2 O3 nanotubes also have a higher coercivity than other reported ␥-Fe2 O3 nanostructures, such as nanofibres (78.11 Oe), nanoparticles (106 Oe), and some reported superparamagnetic ␥-Fe2 O3 particles (0 Oe) (Han et al., 2007; Jing, 2006; Zhang et al., 2008). It is noted that both these two magnetic iron oxide nanotubes have a higher coercivity than other nanostructures with the same phase and of similar size. Furthermore, the M–H loops of Fe3 O4 nanotubes and ␥-Fe2 O3 nanotubes indicate the similar magnetic domain type. On the basis of the criteria given by Dunlop (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. From Fig. 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 nanotubes, which has been confirmed previously by XPS, and the phosphate is not a magnetic material. From Fig. 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 structures 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 particles. 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 coordination 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 coercivity. 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 (din and 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 /dout value 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 Fe3 O4 nanotubes were prepared by reducing synthesized ␣Fe2 O3 nanotubes with a gas mixture of 5% H2 +95% Ar at 500 ◦ C for 2.5 h, and then ␥-Fe2 O3 nanotubes were obtained by reoxidizing the Fe3 O4 nanotubes with air at 300 ◦ C for 2 h. The nanotube structure was well retained without collapsing or sintering, for which, adsorbed phosphate and the type of crystal structure conversion should be the two most important reasons. Investigation of the magnetic properties of Fe3 O4 and ␥-Fe2 O3 nanotubes revealed that both the two magnetic iron oxide nanotubes 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. References Bødker, F., Hansen, M. F., Bender Koch, C., Lefmann, K., & Mørup, S. (2000). Magnetic properties of hematite nanoparticles. Physical Review B, 61(10), 6826–6838. Cesar, I., Kay, A., Gonzalez Martinez, J. A., & Grätzel, M. (2006). Translucent thin film Fe2 O3 photoanodes for efficient water splitting by sunlight: Nanostructure-directing effect of Si-doping. Journal of the American Chemical Society, 128(14), 4582–4583.
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