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An investigation of structural phase transformation of monosize γ-Fe2O3 nanoparticles fabricated by arc discharge method
Materials Letters 89 (2012) 140–142
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
An investigation of structural phase transformation of monosize g-Fe2O3 nanoparticles fabricated by arc discharge method Mansoor Farbod n, Akbar Movahed, Iraj Kazeminezhad Physics Department, Shahid Chamran University, Ahvaz, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 August 2012 Accepted 19 August 2012 Available online 27 August 2012
Iron oxide (g-Fe2O3) nanoparticles were produced via DC arc plasma method. To find the optimum production conditions, arc electrical currents of 75, 100 and 125 A at pressures of 1, 2 and 3 atm of oxygen were used. The samples were examined by using XRD, SEM and EDX. The SEM images showed that the mean size of fabricated nanoparticles is less than 100 nm and dependent on the production conditions. Monosize nanoparticles with size of 55–60 nm were produced under 1 atm pressure and different arc currents. The XRD data confirmed that the produced light brown powders are maghemite (g-Fe2O3) phase of iron oxide. Post-annealing of the nanoparticles from 200 to 800 1C showed that a structural phase transition from maghemite to hematite with rhombohedral lattice structure occurs at 700 1C. The observed magnetic properties were characteristic of a- and g-Fe2O3. & 2012 Elsevier B.V. All rights reserved.
Keywords: Crystal structure Nanoparticles Phase transformation
1. Introduction Iron oxide nanoparticles have received much attention due to their broad applications, including information storage disks, ferrofluids, pigments and medical applications such as magnetic resonance imaging (MRI), targeted drug delivery and cancer diagnoses [1–3]. Up to now, several methods of iron oxide nanoparticles synthesis such as mechanochemical [4], sol–gel [5], wire electrical explosion [6], electrochemical [7] and arc discharge [8] have been reported. The arc discharge method is a technique which has the potential to produce as prepared nanoparticles by optimizing different parameters such as the arc discharge current, arc chamber geometry and volume, chamber pressure, etc. Among different iron oxides phases, the magnetite and maghemite are the two phases that have attracted much attention due to their practical applications. Maghemite is the ferrimagnetic cubic form of Fe(III) oxide and differs from the inverse spinel structure of magnetite through vacancies in the cation sublattice. Here we report the fabrication of maghemite nanoparticles using the arc method and the hematite nanoparticles by heating the maghemite nanoparticles. The characterizations of samples were performed by measuring SEM and EDX using a Leo 1455VP scanning electron microscope and XRD using a PW-1840 Philips diffractometer at room temperature utilizing ˚ The room temperaCu Ka radiation wavelength of l ¼1.5418 A. ture magnetization, hysteresis and coercivity of samples were
n
Corresponding author. Tel./fax: þ 986113331040. E-mail address: [email protected] (M. Farbod).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.08.091
determined using a commercial vibrating magnetometer (VSM) under a cycling magnetic field of 710 kOe. 2. Experimental 2.1. Nanoparticles preparations The apparatus was a home-made [9] cylindrical and multi-port stainless steel chamber, with an approximate volume of 7000 cm3, designed and built for this purpose. In order to produce iron oxide nanoparticles and find the optimum fabrication conditions, the experiments were performed at 1, 2 and 3 atm pressures of pure oxygen. Also the electrical currents of 75, 100 and 125 A were tested. A pair of iron rods was employed as cathode and anode electrodes. The cathode rod had 10 cm length and 8 mm diameter and the length of anode was chosen to be 7 cm due to geometrical limitations. When arc discharge is formed, an intensive evaporation of iron rods into the chamber environment occurs. The atoms, ions and clusters of iron, then react with environmental oxygen immediately and form many nuclei particles. The collision of nuclei particles with each other causes growth and formation of the iron oxide nanoparticles. Due to the consumption of the anode during the arc formation, the cathode was fixed and the anode was moved toward the cathode by a DC motor. In this way it was possible to control the distance between the electrodes. When the anode reaches to the cathode at a certain distance of about 1 mm, by passing a high current through the electrodes, an arc plasma is formed and a layer of iron oxide nanoparticle powders is deposited on the inner walls of the chamber. The produced light brown color powders then were collected and used for characterization.
M. Farbod et al. / Materials Letters 89 (2012) 140–142
3. Results and discussions 3.1. SEM and EDX analysis In order to observe the size and morphology of the prepared nanoparticles, SEM measurements were performed. Fig. 1 shows the SEM images of samples which were prepared at 1 atm of oxygen under arc current of 75, 100 and 125 A. It was observed that the particles have mostly spherical shape. Table 1 has listed the mean particle size of the prepared nanoparticles at different conditions. Although the average particle size is different for different conditions, for all conditions the nanoparticles are formed and the average particle sizes are less than or about 100 nm. At 1 atm pressure, the nanoparticle’s sizes are all in the range of 55–60 nm and it seems the size of nanoparticles does not depend on the current significantly. At 2 atm pressure, the size of nanoparticles decreases by increasing the arc current but at 3 atm. pressure, the size of nanoparticles increases by increasing the arc current. So the optimal condition to produce nearly monosize iron oxide nanoparticles is the atmospheric pressure with arbitrary currents between 75 and 125 A. EDX analysis was used to identify the elemental composition of the samples. The EDX analysis was performed on different sites of each sample. The results showed that the samples consist of iron and oxygen and no sign of impurity was detected. 3.2. XRD analysis and heating effects on the phase transformation of nanoparticles The powder x-ray diffraction (XRD) measurement was performed to identify the phase composition of nanoparticles. Iron oxides have many phases such as magnetite (Fe3O4) and maghemite (g-Fe2O3) with cubic crystal structure and hematite (a-Fe2O3) with rhombohedral crystal structure. Though magnetite
141
and maghemite have black and light brown colors respectively [6] due to the same crystal structure and close lattice parameters (a ¼8.396 A˚ for magnetite and a¼8.351 A˚ for maghemite) they are very similar in their x-ray diffraction patterns [10]. Fig. 2 (left-hand) shows the x-ray diffraction (XRD) patterns of iron oxide nanoparticles produced at pressure of 1 atm and currents of 75, 100 and 125 A. As can be seen all the peaks belong to the maghemite (JCPDS 39–1346) or magnetite (JCPDS 88-0866) but the light brown color of the produced iron oxide nanoparticles confirmed that they are definitely g-Fe2O3. Of course a small trace of hematite is observed at the angle 33.41. Sidhu has shown that there is a phase transformation from maghemite to hematite when the maghemite is heated up at 650 1C [11]. In order to find whether such a transformation occurs to maghemite in the nanoparticles form, and at what temperature the nanoparticles of maghemite were studied systematically by heating them at temperatures of 200, 300, 400, 500, 600, 700 and 800 1C for 3 h. The phase transformation was then examined by XRD measurements. Fig. 2 (right-hand) shows the XRD patterns of the heated samples at different temperatures. As can be observed the phase transition starts at 600 1C and is almost completed at 700 1C. The main peak of hematite at 33.151 is quite large and the peak at 301 which belongs to maghemite disappeared. Also another four peaks of hematite at 24.151, 40.861, 49.461 and 54.071 appeared and the single peak of maghemite at 62.981 is converted to the two distinct peaks at 62.431 and 641. So, all the peaks can be indexed to the pure rhombohedral phase of hematite according to JCPDS card number of 87-1165. Heating at 800 1C makes the peaks sharper indicating better crystallization of nanoparticles. We believe that by the arc discharge method it is possible to produce different nanoparticles and control the size and morphology of the particles by optimizing the arc parameters [12]. This method is simple, cheap and reproducible and provides an effective and rapid large scale production of nanoparticles.
Fig. 1. SEM image of iron oxide nanoparticles, formed at 1 atm of oxygen and arc current of a) 75 A, b) 100 A, and c) 125 A.
Table 1 Mean particle size of the prepared nanoparticles at different conditions. Pressure (atm)
1
1
1
2
2
2
3
3
3
Electrical current (A) Mean particle size (nm)
75 58
100 57
125 60
75 102
100 91
125 69
75 67
100 86
125 91
Fig. 2. XRD patterns of iron oxide nanoparticles produced under different arc currents at pressure of 1 atm (left) and maghemite nanoparticles annealed at different temperatures (right).
142
M. Farbod et al. / Materials Letters 89 (2012) 140–142
Fig. 3. Hysteresis loops of a- and g-Fe2O3 at room temperature. The inset shows the low field part of the loop.
3.3. Magnetic measurements
Although at all used conditions, the size of nanoparticles was less than 100 nm, the results showed that the atmospheric oxygen pressure and arc currents of 75–125 A are the optimum conditions which yielded 55–60 nm nanoparticles in size. The effect of annealing on the prepared nanoparticles showed that a phase transformation from maghemite to hematite nanoparticles starts at 600 1C and is almost complete at 700 1C. The magnetic parameters showed that the phases are pure and no mixing of them occurs.
Fig. 3 shows the typical hysteresis loops of a-Fe2O3 and g-Fe2O3 samples. The magnetic parameters of the samples are
Acknowledgments
Table 2 The magnetic parameters of the samples. Phases
Mr (emu/g)
Ms (emu/g)
Hc(Oe)
g-Fe2O3 a-Fe2O3
9.5 1.1
65 7.8
112 100
listed in Table 2. As expected the saturation magnetization (Ms) of g-Fe2O3 sample is larger than that of a-Fe2O3. A relatively large slope of the g-Fe2O3 magnetization curve indicates a high level of magnetic susceptibility for this sample. Also the shapes of both hysteresis loops are symmetric indicating that the samples are not a mixture of soft and hard magnetic phases. As one can observe from Fig. 1, the samples are in the form of nano-grains containing both a very developed free surface and extremely high specific area and grain boundaries. These areas are ideal for growing other iron oxide phases [13] which may not be seen by XRD. Nevertheless, the magnetic measurements, particularly the ferromagnetic behavior of the samples show that the samples obtained from arc and after annealing are of single phase.
4. Conclusion
g-Fe2O3 nanoparticles were produced by the arc discharge method, which can be optimized for industrial purposes. The observations and analyses of SEM images clarified that the nanoparticles have spherical shape but the sizes of the g-Fe2O3 nanoparticles more or less depend on processing conditions.
The authors acknowledge Shahid-Chamran University of Ahvaz for financial support of this work. References [1] Bate G. J. Magn Magn Mater 1991;100:413–24. ¨ [2] Murbe J, Rechtenbach A, Topfer J. Mater Chem Phys 2008;110:426–33. [3] Ichiyanagi Y, Moritake M, Taira S, Setou M. J. Magn Magn Mater 2007;310:2877–9. [4] Iwasaki T, Kosaka K, Mizutani N, Watano S, Yanagida T, Tanaka H, et al. Mater Lett 2008;62:4155–7. [5] Xu J, Yang H, Fu W, Du K, Sui Y, Chen J, et al. J Magn Magn Mater 2007;309:307–11. [6] Wang Q, Yang H, Shi J, Zou G. Mater Res Bull 2001;36:503–9. [7] Cabrera L, Gutierrez S, Menendez N, Morales MP, Herrasti P. Electrochim Acta 2008;53:3436–41. [8] Balasubramaniam C, Khollam YB, Banerjee I, Bakare PP, Date SK, Das AK, et al. Mater Lett 2004;58:3958–62. [9] Farbod M, Hasani Matin MM. Curr Nanosci 2011;7:794–6. [10] Cornell RM, Schwertmann U. The Iron Oxide: Structure and Properties. Wiley-VCH Verlag GmbH; 2003. [11] Sidhu PS. Clay Clay Miner 1988;36:31–8. [12] Shoushtari MZ, Parhoodeh S, Farbod M. J. Phys: Conf Ser 2008;100:052017–20. [13] Straumal B, Baretzky B, Mazilkin A, Protasova S, Myatiev A, Straumal P. J. Eur Ceram Soc 2010;29:1963–70.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
An investigation of structural phase transformation of monosize g-Fe2O3 nanoparticles fabricated by arc discharge method Mansoor Farbod n, Akbar Movahed, Iraj Kazeminezhad Physics Department, Shahid Chamran University, Ahvaz, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 August 2012 Accepted 19 August 2012 Available online 27 August 2012
Iron oxide (g-Fe2O3) nanoparticles were produced via DC arc plasma method. To find the optimum production conditions, arc electrical currents of 75, 100 and 125 A at pressures of 1, 2 and 3 atm of oxygen were used. The samples were examined by using XRD, SEM and EDX. The SEM images showed that the mean size of fabricated nanoparticles is less than 100 nm and dependent on the production conditions. Monosize nanoparticles with size of 55–60 nm were produced under 1 atm pressure and different arc currents. The XRD data confirmed that the produced light brown powders are maghemite (g-Fe2O3) phase of iron oxide. Post-annealing of the nanoparticles from 200 to 800 1C showed that a structural phase transition from maghemite to hematite with rhombohedral lattice structure occurs at 700 1C. The observed magnetic properties were characteristic of a- and g-Fe2O3. & 2012 Elsevier B.V. All rights reserved.
Keywords: Crystal structure Nanoparticles Phase transformation
1. Introduction Iron oxide nanoparticles have received much attention due to their broad applications, including information storage disks, ferrofluids, pigments and medical applications such as magnetic resonance imaging (MRI), targeted drug delivery and cancer diagnoses [1–3]. Up to now, several methods of iron oxide nanoparticles synthesis such as mechanochemical [4], sol–gel [5], wire electrical explosion [6], electrochemical [7] and arc discharge [8] have been reported. The arc discharge method is a technique which has the potential to produce as prepared nanoparticles by optimizing different parameters such as the arc discharge current, arc chamber geometry and volume, chamber pressure, etc. Among different iron oxides phases, the magnetite and maghemite are the two phases that have attracted much attention due to their practical applications. Maghemite is the ferrimagnetic cubic form of Fe(III) oxide and differs from the inverse spinel structure of magnetite through vacancies in the cation sublattice. Here we report the fabrication of maghemite nanoparticles using the arc method and the hematite nanoparticles by heating the maghemite nanoparticles. The characterizations of samples were performed by measuring SEM and EDX using a Leo 1455VP scanning electron microscope and XRD using a PW-1840 Philips diffractometer at room temperature utilizing ˚ The room temperaCu Ka radiation wavelength of l ¼1.5418 A. ture magnetization, hysteresis and coercivity of samples were
n
Corresponding author. Tel./fax: þ 986113331040. E-mail address: [email protected] (M. Farbod).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.08.091
determined using a commercial vibrating magnetometer (VSM) under a cycling magnetic field of 710 kOe. 2. Experimental 2.1. Nanoparticles preparations The apparatus was a home-made [9] cylindrical and multi-port stainless steel chamber, with an approximate volume of 7000 cm3, designed and built for this purpose. In order to produce iron oxide nanoparticles and find the optimum fabrication conditions, the experiments were performed at 1, 2 and 3 atm pressures of pure oxygen. Also the electrical currents of 75, 100 and 125 A were tested. A pair of iron rods was employed as cathode and anode electrodes. The cathode rod had 10 cm length and 8 mm diameter and the length of anode was chosen to be 7 cm due to geometrical limitations. When arc discharge is formed, an intensive evaporation of iron rods into the chamber environment occurs. The atoms, ions and clusters of iron, then react with environmental oxygen immediately and form many nuclei particles. The collision of nuclei particles with each other causes growth and formation of the iron oxide nanoparticles. Due to the consumption of the anode during the arc formation, the cathode was fixed and the anode was moved toward the cathode by a DC motor. In this way it was possible to control the distance between the electrodes. When the anode reaches to the cathode at a certain distance of about 1 mm, by passing a high current through the electrodes, an arc plasma is formed and a layer of iron oxide nanoparticle powders is deposited on the inner walls of the chamber. The produced light brown color powders then were collected and used for characterization.
M. Farbod et al. / Materials Letters 89 (2012) 140–142
3. Results and discussions 3.1. SEM and EDX analysis In order to observe the size and morphology of the prepared nanoparticles, SEM measurements were performed. Fig. 1 shows the SEM images of samples which were prepared at 1 atm of oxygen under arc current of 75, 100 and 125 A. It was observed that the particles have mostly spherical shape. Table 1 has listed the mean particle size of the prepared nanoparticles at different conditions. Although the average particle size is different for different conditions, for all conditions the nanoparticles are formed and the average particle sizes are less than or about 100 nm. At 1 atm pressure, the nanoparticle’s sizes are all in the range of 55–60 nm and it seems the size of nanoparticles does not depend on the current significantly. At 2 atm pressure, the size of nanoparticles decreases by increasing the arc current but at 3 atm. pressure, the size of nanoparticles increases by increasing the arc current. So the optimal condition to produce nearly monosize iron oxide nanoparticles is the atmospheric pressure with arbitrary currents between 75 and 125 A. EDX analysis was used to identify the elemental composition of the samples. The EDX analysis was performed on different sites of each sample. The results showed that the samples consist of iron and oxygen and no sign of impurity was detected. 3.2. XRD analysis and heating effects on the phase transformation of nanoparticles The powder x-ray diffraction (XRD) measurement was performed to identify the phase composition of nanoparticles. Iron oxides have many phases such as magnetite (Fe3O4) and maghemite (g-Fe2O3) with cubic crystal structure and hematite (a-Fe2O3) with rhombohedral crystal structure. Though magnetite
141
and maghemite have black and light brown colors respectively [6] due to the same crystal structure and close lattice parameters (a ¼8.396 A˚ for magnetite and a¼8.351 A˚ for maghemite) they are very similar in their x-ray diffraction patterns [10]. Fig. 2 (left-hand) shows the x-ray diffraction (XRD) patterns of iron oxide nanoparticles produced at pressure of 1 atm and currents of 75, 100 and 125 A. As can be seen all the peaks belong to the maghemite (JCPDS 39–1346) or magnetite (JCPDS 88-0866) but the light brown color of the produced iron oxide nanoparticles confirmed that they are definitely g-Fe2O3. Of course a small trace of hematite is observed at the angle 33.41. Sidhu has shown that there is a phase transformation from maghemite to hematite when the maghemite is heated up at 650 1C [11]. In order to find whether such a transformation occurs to maghemite in the nanoparticles form, and at what temperature the nanoparticles of maghemite were studied systematically by heating them at temperatures of 200, 300, 400, 500, 600, 700 and 800 1C for 3 h. The phase transformation was then examined by XRD measurements. Fig. 2 (right-hand) shows the XRD patterns of the heated samples at different temperatures. As can be observed the phase transition starts at 600 1C and is almost completed at 700 1C. The main peak of hematite at 33.151 is quite large and the peak at 301 which belongs to maghemite disappeared. Also another four peaks of hematite at 24.151, 40.861, 49.461 and 54.071 appeared and the single peak of maghemite at 62.981 is converted to the two distinct peaks at 62.431 and 641. So, all the peaks can be indexed to the pure rhombohedral phase of hematite according to JCPDS card number of 87-1165. Heating at 800 1C makes the peaks sharper indicating better crystallization of nanoparticles. We believe that by the arc discharge method it is possible to produce different nanoparticles and control the size and morphology of the particles by optimizing the arc parameters [12]. This method is simple, cheap and reproducible and provides an effective and rapid large scale production of nanoparticles.
Fig. 1. SEM image of iron oxide nanoparticles, formed at 1 atm of oxygen and arc current of a) 75 A, b) 100 A, and c) 125 A.
Table 1 Mean particle size of the prepared nanoparticles at different conditions. Pressure (atm)
1
1
1
2
2
2
3
3
3
Electrical current (A) Mean particle size (nm)
75 58
100 57
125 60
75 102
100 91
125 69
75 67
100 86
125 91
Fig. 2. XRD patterns of iron oxide nanoparticles produced under different arc currents at pressure of 1 atm (left) and maghemite nanoparticles annealed at different temperatures (right).
142
M. Farbod et al. / Materials Letters 89 (2012) 140–142
Fig. 3. Hysteresis loops of a- and g-Fe2O3 at room temperature. The inset shows the low field part of the loop.
3.3. Magnetic measurements
Although at all used conditions, the size of nanoparticles was less than 100 nm, the results showed that the atmospheric oxygen pressure and arc currents of 75–125 A are the optimum conditions which yielded 55–60 nm nanoparticles in size. The effect of annealing on the prepared nanoparticles showed that a phase transformation from maghemite to hematite nanoparticles starts at 600 1C and is almost complete at 700 1C. The magnetic parameters showed that the phases are pure and no mixing of them occurs.
Fig. 3 shows the typical hysteresis loops of a-Fe2O3 and g-Fe2O3 samples. The magnetic parameters of the samples are
Acknowledgments
Table 2 The magnetic parameters of the samples. Phases
Mr (emu/g)
Ms (emu/g)
Hc(Oe)
g-Fe2O3 a-Fe2O3
9.5 1.1
65 7.8
112 100
listed in Table 2. As expected the saturation magnetization (Ms) of g-Fe2O3 sample is larger than that of a-Fe2O3. A relatively large slope of the g-Fe2O3 magnetization curve indicates a high level of magnetic susceptibility for this sample. Also the shapes of both hysteresis loops are symmetric indicating that the samples are not a mixture of soft and hard magnetic phases. As one can observe from Fig. 1, the samples are in the form of nano-grains containing both a very developed free surface and extremely high specific area and grain boundaries. These areas are ideal for growing other iron oxide phases [13] which may not be seen by XRD. Nevertheless, the magnetic measurements, particularly the ferromagnetic behavior of the samples show that the samples obtained from arc and after annealing are of single phase.
4. Conclusion
g-Fe2O3 nanoparticles were produced by the arc discharge method, which can be optimized for industrial purposes. The observations and analyses of SEM images clarified that the nanoparticles have spherical shape but the sizes of the g-Fe2O3 nanoparticles more or less depend on processing conditions.
The authors acknowledge Shahid-Chamran University of Ahvaz for financial support of this work. References [1] Bate G. J. Magn Magn Mater 1991;100:413–24. ¨ [2] Murbe J, Rechtenbach A, Topfer J. Mater Chem Phys 2008;110:426–33. [3] Ichiyanagi Y, Moritake M, Taira S, Setou M. J. Magn Magn Mater 2007;310:2877–9. [4] Iwasaki T, Kosaka K, Mizutani N, Watano S, Yanagida T, Tanaka H, et al. Mater Lett 2008;62:4155–7. [5] Xu J, Yang H, Fu W, Du K, Sui Y, Chen J, et al. J Magn Magn Mater 2007;309:307–11. [6] Wang Q, Yang H, Shi J, Zou G. Mater Res Bull 2001;36:503–9. [7] Cabrera L, Gutierrez S, Menendez N, Morales MP, Herrasti P. Electrochim Acta 2008;53:3436–41. [8] Balasubramaniam C, Khollam YB, Banerjee I, Bakare PP, Date SK, Das AK, et al. Mater Lett 2004;58:3958–62. [9] Farbod M, Hasani Matin MM. Curr Nanosci 2011;7:794–6. [10] Cornell RM, Schwertmann U. The Iron Oxide: Structure and Properties. Wiley-VCH Verlag GmbH; 2003. [11] Sidhu PS. Clay Clay Miner 1988;36:31–8. [12] Shoushtari MZ, Parhoodeh S, Farbod M. J. Phys: Conf Ser 2008;100:052017–20. [13] Straumal B, Baretzky B, Mazilkin A, Protasova S, Myatiev A, Straumal P. J. Eur Ceram Soc 2010;29:1963–70.