ARTICLE pubs.acs.org/JPCC
Magnetic Properties of Hematite Nanotubes Elaborated by Electrospinning Process Cynthia Eid,†,‡ Dominique Luneau,‡ Vincent Salles,‡ Roy Asmar,† Yves Monteil,‡ Antonio Khoury,† and Arnaud Brioude*,‡ †
Laboratoire de Physique Appliquee (LPA) associe a l’ecole doctorale des Sciences et Technologies, Departement de Physique, Universite Libanaise, Faculte des Sciences II, 90656 Jdeidet El Metn, Lebanon ‡ Laboratoire des Multimateriaux et Interfaces (UMR CNRS 5615), Universite Lyon1, Universite de Lyon, 43 Bd du 11 Novembre 1918, Villeurbanne Cedex, France ABSTRACT: Hematite nanotubes are successfully prepared using an electrospinning technique and studied by superconducting quantum interference device magnetometry. At room temperature, the hysteresis properties observed are strongly controlled by the size of the nanofibers. The coercivity force increases with the average diameter while the remnant magnetization decreases. The moment versus temperature data for samples with different nanotubes’ average diameter show two magnetic transitions assigned to Morin transitions. The origin of this phenomenon is traced to the presence of very thin nanotubes responsible for an additional Morin transition at low temperature. As a consequence, and based on previous works, a phenomenological description of the influence of hematite nanostructures size on the Morin temperature is proposed for the first time.
1. INTRODUCTION The interest in one-dimensional ferromagnetic nanotubes with controllable wall thickness continues to be an important aspect of magnetic nanotechnology.1 Compared to wires, tubes present a supplementary degree of freedom as their wall thickness can be varied in addition to the length and diameter. Changes in thickness are expected to strongly affect the mechanism of magnetization reversal and thereby the overall magnetic response. As a consequence, the enhancement of magnetic properties would make nanotubes suitable for many applications, particularly high density data storage.1,2 Among different anisotropic nanomaterials, the magnetic tubular structure presents an important advantage as its distinctive inner and outer surfaces can be functionalized differently.3 To date, various techniques have been used to produce nanotube structures, such as electrochemical deposition,4 hydrogen reduction in porous templates,5 or solvothermal processes.6 However, those methods could present common restrictions such as low aspect ratio, limited amount of materials produced, presence of impurities, and aggregation in the final product.5,7,8 In contrast to this, the electrospinning technique offers the possibility to easily generate nanotubes with controlled geometry and tunable magnetic properties. Among iron oxide forms, hematite (R-Fe2O3) is of great scientific and technological importance. It is an antiferromagnetic material exhibiting a weak ferromagnetism (WF) above the Morin temperature, TM (∼260 K), due to spin canting. In this case, the spins are lying in the basal plane (rhombohedral (111)). r 2011 American Chemical Society
The oxide transforms to an antiferromagnetic (AF) phase with two antiparallel magnetic sublattices lying along the c axis below the Morin transition temperature.9 On a nanometric scale, the particles behave in a different magnetic way. A decrease in the value of TM or even a suppression caused by the particle shape, size, and crystallinity has been reported.1013 The present paper details the tunable magnetic properties of hematite nanotubes obtained by electrospinning technique. Our focus on hematite nanotubes results from their unique position among the magnetic semiconducting materials in terms of multiple functionalities and low toxicity.
2. EXPERIMENTAL SECTION The R-Fe2O3 nanotubes were prepared by annealing in air PVP/FeAc2 nanofibers composed of poly(vinylpyrrolidone) (PVP) and iron acetate (FeAc2) produced by electrospinning. The detailed process has already been published in a previous work.14 By carefully varying the FeAc2/PVP wt ratio, tubular structures have been obtained with tunable wall thickness and average diameter. The morphology of the samples was analyzed by scanning electron microscopy SEM (Hitachi S800), and their structural properties were studied by transmission electron microscopy with a TOPCON 002B working at 200 kV. Magnetization data were taken through a Quantum Design SQUID Received: April 12, 2011 Revised: July 26, 2011 Published: July 27, 2011 17643
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Figure 1. SEM image of the PVP/FeAc2 nanofibers prepared with a wt ratio (FeAc2/PVP) of 2.63 and annealed under air atmosphere.
Table 1. Samples Prepared for Different Iron Acetate/PVP Weight Ratios FeAc2/PVP
average
wall thickness
sample
wt ratio
diameter (nm)
(nm)
B1
2.63
200
70
B2
1.75
160
37
B3
0.87
115
25
B4
0.43
95
13
Figure 2. TEM image of R-Fe2O3 prepared with a wt ratio (FeAc2/ PVP) of 0.87.
magnetometer. The magnetization versus temperature (MT) curves for an applied field of 25 Oe in a temperature range 5350 K and the magnetization hysteresis loops at 300 K in a magnetic field range up to 50 kOe were obtained.
3. RESULTS Figure 1 shows SEM pictures of a sample prepared with a wt ratio (FeAc2/PVP) of 2.63, which was annealed under flowing air up to 550 °C. It can be seen that the nanofibers morphology is maintained after thermal treatment. As for the as-spun fibers, the final material is composed of two fiber populations. Measured on 100 randomly chosen nanofibers, the smaller diameters are around 60 nm whereas the average diameter of greater ones is around 200 nm. For the other samples, the diameter of smaller nanofibers remains approximately the same while we measured an average diameter of 160, 115, and 95 nm14 (Table 1) for the greater ones. Such a formation of a “spider net” has already been reported and explained by the presence of anions and cations into the polymer solution which can be brought by salts.15 In the present case, the iron acetate can play itself this role of salt into the polymer solution (Fe2+, 2CH3COO) and induce this spider net emergence between the biggest spun fibers. A conventional TEM study has been performed in order to investigate more carefully the morphology of these nanofibers. A hollow structurelike nanotube is observed in Figure 2. The wall thickness measured was about 25 nm. For the other samples, a wall thickness of 13, 37, and 70 nm was calculated (Table 1). The magnetic hysteresis measurements of as-synthesized RFe2O3 nanotubes samples with different average diameters were carried out at 300 K in an applied magnetic field sweeping from 50 to 50 kOe. In Figure 3, the magnetic hysteresis loops indicate the presence of a ferromagnetic order for the four
Figure 3. Magnetic hysteresis loops at 300 K of the as-prepared RFe2O3 samples: B1, B2, B3 and B4 with an average diameter of 200, 160, 115, and 95 nm respectively. The inset is a magnified view of the curves at low magnetic fields.
samples at room temperature. It can be seen that no saturation of the magnetization as a function of the field is observed up to the maximum applied magnetic field, which may point to the disordered surface spin of the nanofibers. Considering the porous structure normally observed on samples prepared by the electrospinning process, the disordered surface spin may be attributed to the adsorption of adatoms or clusters on the surfaces of the nanotubes.16 The variation of the remnant magnetization and coercivity force as a function of the nanotubes’ average diameter is presented in Figure 4. The hysteresis properties observed are strongly controlled by the size of the nanofibers. The coercivity force increases with the average diameter while the remnant magnetization decreases. M. Graeser et al.17 have modified the direction of the applied magnetic field in order to show its effect on the remanence and the coercivity of iron fibers prepared by electrospinning. An increase in the coercivity and a decrease in the remanence were observed when 17644
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Figure 4. Variation of the remnant magnetization (square) and coercivity force (circle) as a function of the size of the nanotubes’ average diameter.
Figure 5. Sample B1 (average diameter ∼ 200 nm) moment versus temperature for cooling and subsequent warming run in a 25 Oe applied field.
Table 2. Values of Remnant Magnetization (Mr) and Coercivity (Hc) of the As-Synthesized Samples Compared to the Literature
samples Mr (emu/g) Hc (Oe)
B1
B2
B3
B4
0.2872 0.3622 0.4500 0.4512 628.18 328.20 286.30 256.71
spherical
porous
nanoparticles
nanorods
[79]
[79]
0.0087 20.31
0.0197 65.10
the applied field was perpendicular to the fiber axis. These differences in the magnetic properties are strongly related to the shape anisotropy. In our study, the magnetic properties of hematite nanotubes were measured without any specific orientation toward the magnetic field. This magnetic behavior can be related to the morphology of the nanostructures (size and shape) and is strongly affected also by the interparticle interactions. The magnetic interactions can be due to dipolar coupling and exchange coupling among nanoparticles surface atoms and play a fundamental role in the physics of these systems.18 The detailed values of remnant magnetization (Mr) and coercivity (Hc) of the as-synthesized R-Fe2O3 samples plotted in Figure 4 are summarized in Table 2. It is easy to see that the remnant magnetization and the coercivity force of the nanotubes are much larger than those obtained in a previous study for spherical nanoparticles and porous nanorods.13 These results confirm that the magnetization of ferromagnetic materials is very sensitive to the morphologies and the structures of as-synthesized samples. In fact, the higher remnant magnetization and coercivity of the nanotubes may be attributed to the shape anisotropy which prevents them from magnetizing in directions other than along their easy magnetic axes. The temperature dependences of the magnetization of samples B1 and B4 exhibited from the warming and cooling curves measured from 340 to 10 K in a 25 Oe magnetic field are shown in Figure 5 and Figure 6. For both samples, two magnetic transitions are clearly visible. Considering that the samples were pure, the possibility that one of the transitions is due to impurities can be precluded.14 As a consequence, we have assigned them as
Figure 6. Sample B4 (average diameter ∼ 95 nm) moment versus temperature for cooling and subsequent warming run in a 25 Oe applied field.
two specific Morin transitions. The magnetic transition observed at low temperature can be related to nanotubes with small average diameter, while the second magnetic transition observed at high temperature can be attributed to the nanotubes with a greater average diameter. For the sample B1, TM values of 127.5 and 251 K were measured (Figure 5). They are equal to the average of the heating and cooling values.19 Concerning sample B4, two transitions TM located at 125 and 237.5 K were also measured (Figure 6). Overall, the two samples of R-Fe2O3 nanotubes display similar magnetic behavior with a slight shift of the TM values. It has been reported that TM tends to decrease with particle size20,6 which can explain the reduced values of TM for the sample B4 and for small nanofibers present in each sample. In order to better understand the influence of size on the magnetism of R-Fe2O3 nanotubes, it is necessary to summarize 17645
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’ REFERENCES
Figure 7. Morin temperature versus hematite nanostructures. The data of previous work has been reported in this figure with error bars known. Experimental data from the work of Liu et al.6 (b), Goya et al.23 (O), Gee et al.24 (Δ), Xu et al.21 (2), Zysler et al.10 (9), Vasquez-Mansilla € et al.11 ([), and Ozdemir et al.22 (f). Experimental data from our study (g) extracted from Figure 6.
all the data extracted from previous works and to compare them with our results. This study has been realized and is reported in Figure 7. It is clear that the Morin temperature seems to evolve exponentially with the temperature. This behavior can be described by the following relation determined by a numerical fit based on all referenced experimental data and our results: d B TM ¼ TM 522:4 exp 19:6 with TBM as the Morin temperature for the bulk material and d the characteristic size of magnetic particles. This relation can be very useful to evaluate the average size of pure hematite nanocrystals for sizes greater than 15 nm. For smaller sizes inferior at 20 nm, this relation is no more relevant since the Morin transition has been reported to be absent. This size dependence of the Morin transition temperature can be explained by a size dependence of the magnetic anisotropy constants.
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4. CONCLUSION The magnetic properties of hematite nanotubes prepared by electrospinning technique have been studied by SQUID magnetometer from 10 to 340 K. All the samples prepared with different average diameters in the range of 95240 nm exhibit a ferromagnetic behavior at room temperature. Under a low external field of 25 Oe, two magnetic transitions have been observed. This is attributed to the Morin transition of two fiber populations with different diameters. We have shown the direct link between the average diameters of nanotubes and their magnetic properties. Based on previous works, a phenomenological description of the influence of hematite nanostructures size on the Morin temperature has been deduced. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. 17646
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