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One-pot synthesis of magnetite nanopowder and their magnetic properties
Powder Technology 197 (2010) 295–297
Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
One-pot synthesis of magnetite nanopowder and their magnetic properties S. Zhao, S. Asuha ⁎ Chemistry & Environment Science College, Inner Mongolia Normal University Key Laboratory of Physics and Chemistry of Function Materials, Inner Mongolia, 81 Zhaowudalu, Huhhot 010022, China
a r t i c l e
i n f o
Article history: Received 15 June 2009 Received in revised form 25 September 2009 Accepted 12 October 2009 Available online 17 October 2009 Keywords: Magnetite Magnetic materials Magnetic measurements Solvothermal method
a b s t r a c t Magnetite (Fe3O4) nanopowder can be prepared by one-pot solvothermal reaction of Fe-urea complex ([Fe (CON2H4)6](NO3)3) in ethanol. The result of X-ray powder diffraction (XRD) suggests the formation of Fe3O4 whose lattice constant is 8.385 Å. The formation of Fe3O4 is further confirmed from X-ray photoelectron spectroscopy (XPS) measurements. Transmission electron micrograph (TEM) observations show that Fe3O4 particles are nearly spherical in shape. The crystallite size of Fe3O4 can be controlled from 9.7 to 20.5 nm by varying the solvothermal reaction time. Room temperature magnetization hysteresis curves exhibit almost immeasurable values of coercivity and remanence, suggesting that the Fe3O4 nanopowder possesses superparamagnetic characteristics. The saturation magnetization (Ms) increases from 32.6 to 42.9 emu/g when the reaction time increases from 10 to 50 h. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In iron oxide crystalline forms, Fe3O4 is a ferrimagnetic material, and it has already been widely used in many applications such as magnetic recording [1], drug delivery [2], catalysis [3], ferro-fluids [4], and biotechnology [5–7]. For all of these applications, high quality Fe3O4 nanopowder with controlled size is required to satisfy new increasing demands. Especially in some cases (e.g., for medical applications), the Fe3O4 nanopowder are required to be superparamagnetic, which means that the particle size should be smaller than 30 nm. Therefore, the preparation of Fe3O4 nanopowder now attracts a lot of interest and various methods have been developed; each having their own advantages and disadvantages. Fe3O4 is generally prepared by a coprecipitation method using ferrous and ferric salts as starting materials. In this method, to obtain pure Fe3O4, the pH value of the solution has to be carefully controlled in both synthesis process and the subsequent purification process. In addition, the molar ratio of Fe(II) to Fe(III) ions also has to be fixed in stoichiometry; hence, to maintain it usually a protective gas (e.g., nitrogen gas) is necessary so that the oxidation of Fe (II) ions by air can be prevented. Another disadvantage of this conventional method is that the agglomeration of particles is easy to occur, and therefore the utilizations of various particle stabilizers (e.g., surfactants or polymer matrices) have been examined to obtain Fe3O4 nanopowder with a narrow particle size distribution or monodisperse [8]. Therefore, many alternative techniques have been developed to prepare Fe3O4 nanopowder, e.g., nonaqueous synthetic method [9,10], hydrothermal method [11,12], sonochemical approach [13], mechanochemical ⁎ Corresponding author. Chemistry & Environment Science College, Inner Mongolia Normal University, China. Tel.: + 86 471 43921214; fax: + 86 471 4392124. E-mail address: [email protected] (S. Asuha). 0032-5910/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2009.10.007
approach [14], etc. Among these methods, the nonaqueous synthetic process is a promising technique as the resulting nanopowder has rather narrow size distribution without particle agglomeration. Sun et al. have successfully prepared Fe3O4 nanopowder with narrow size distribution and controlled size by using this method [9]. The use of multireagents was typically required in this method. For some nonaqueous processes, iron pentacarbonyl (Fe(CO)5) is commonly used as a precursor. However, Fe (CO)5 is a very toxic and unstable chemical; hence, it may limit the usage of these methods for the mass production of Fe3O4 nanopowder. Therefore, an easy and environment-friendly synthetic method would be desirable for a large-scale production. In the present work, we show that the Fe3O4 nanopowder can be prepared by a simple one-pot solvothermal reaction process using [Fe (CON2H4)6](NO3)3 as a precursor. The main advantages of this method are (1) the preparation procedure of Fe3O4 nanopowder is simple, (2) the precursor, [Fe(CON2H4)6](NO3)3, a nontoxic material, can be easily synthesized from readily available Fe(NO3)3⋅9H2O and CON2H4, and (3) the solvent, C2H5OH, is an environment-friendly and most commonly used solvent. 2. Experimental 2.1. Synthesis The preparation of Fe3O4 nanopowder started from the synthesis of [Fe(CON2H4)6](NO3)3 as follows: Fe(NO3)3⋅9H2O and CON2H4 were mixed in a molar ratio of 1:6 in ethanol at room temperature, followed by intense stirring until the reactants were completely converted to a light green [Fe(CON2H4)6](NO3)3 powder. After removing the ethanol by filtering, the powder was dried in an oven at 45 °C. 1.0 g of [Fe(CON2H4)6](NO3)3 was added to 20.0 mL of ethanol and then the
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mixture was transferred into a 45-mL stainless autoclave with a Teflon liner. Then the autoclave was heated in an oven at 200 °C for a predetermined time. After cooling to room temperature, the precipitate was obtained by the centrifugation of the resulting suspension, and it was dried in air at room temperature. 2.2. Characterization X-ray diffraction patterns (XRD) were recorded on a Philips PW 1830 diffractometer using CuKα radiation. X-ray photoelectron spectroscopy (XPS) spectra were measured using an AXIS-Ultra instrument from Kratos Analytical with a monochromatic Al Kα radiation source; in this case, peak positions were calibrated using the C 1s peak position. For TEM studies, ca. 10 mg of Fe3O4 powder was dispersed in ca. 40 mL of hexane; then a droplet of the obtained suspension was dried on a carbon film and the measurements were carried out using a JEOL JEM-3000F transmission electron microscope. Magnetic measurements were performed using a Lake Shore 7407 vibrating-sample magnetometer (VSM) at room temperature. 3. Results and discussion Fig. 1 shows the XRD patterns of the samples prepared by the solvothermal reaction of [Fe(CON2H4)6](NO3)3 in ethanol at 200 °C for different times. The d-spacing values of the sample which were obtained with the reaction time of 30 h with the JCPDS files of Fe3O4 and maghemite (γ-Fe2O3) are listed in Table 1. The diffraction patterns and d-spacings match well with the JCPDS file of Fe3O4, indicating the formation of Fe3O4. In addition, the lattice constant was estimated to be 8.385 Å, which was in good agreement with that mentioned in the JCPDS file of standard Fe3O4. The reflection peaks became sharper and increased their intensities along with the increase of reaction time, showing the growth of Fe3O4 particles. The average crystallite sizes of Fe3O4 were calculated from the half-width of the diffraction lines by using the Scherrer's equation. For the samples prepared with reaction time of 10, 30 and 50 h, the average crystallite sizes of Fe3O4 were estimated to be 9.7, 13.8 and 20.5 nm, respectively. The result indicated that the crystallite size of Fe3O4 increased by the prolonging of reaction time. γ-Fe2O3 is also one of the iron oxides with a cubic spinel crystalline structure, but the XRD result obtained in the present work differs from its JCPDS file (e.g., No. 39-1346), as shown in Table 1. Therefore, we think that only pure Fe3O4 phase was formed in our samples without any phase transformation. Further evidence of the formation of Fe3O4 was obtained from Xray photoelectron spectroscopy. Fig. 2 shows XPS spectrum in the Fe
Fig. 1. XRD patterns of the samples prepared by the solvothermal reaction of [Fe(CON2H4)6] (NO3)3 in ethanol at 200 °C for (a) 10 h, (b) 30 h, and (c) 50 h.
Table 1 d-Spacings of the sample prepared by the solvothermal reaction at 200 °C for 30 h and those of Fe3O4 and γ-Fe2O3 from JCPDS files. d (Å)
Fe3O4a (Å)
γ-Fe2O3b (Å)
hkl
2.9630 2.5277 2.0931 1.7071 1.6143 1.4821
2.9670 2.5320 2.0993 1.7146 1.6158 1.4845
2.9530 2.5177 2.0866 1.7045 1.6073 1.4758
220 311 400 422 511 440
a b
JCPDS file No. 19-629. JCPDS file No. 39-1346.
2p region for the sample prepared by the solvothermal reaction of [Fe (CON2H4)6](NO3)3 in ethanol at 200 °C for 30 h. The peak positions of Fe 2p3/2 and Fe 2p1/2 were 710.6 and 724.2 eV, respectively. These values agree well with those for the Fe3O4 reported in literature [15,16]. Beside peak position, the appearance of satellite peak of Fe 2p3/2 is also one important feature that could be used for the discrimination of Fe3O4 and γ-Fe2O3. According to previous studies, the Fe 2p3/2 for Fe3O4 does not have a satellite peak [17,18]. As seen in Fig. 2, the satellite peak is absent. Therefore, the result of XPS measurements further confirms the formation of Fe3O4. According to our previous work, when [Fe(CON2H4)6](NO3)3 is heated at a high temperature above its melting point, it will decompose through a two-stage thermal decomposition process [19]. Fe(NO3)3 and CON2H4 are the products of the first stage thermal decomposition process. CON2H4 is a reducing agent, and it can reduce Fe3+ to Fe2+; on 2+ the other hand, NO− to 3 ion is an oxidizing agent, and it can oxidize Fe 3+ Fe . Therefore, the final decomposition product should depend on thermal decomposition conditions including heat treatment atmosphere and heat treatment state (open or closed). In the present case where [Fe (CON2H4)6](NO3)3 is heated in a closed autoclave, the released CON2H4 in the first stage thermal decomposition could not escape from the reactor by evaporation; hence, the coexistence of CON2H4 and NO− 3 ions results in the formation of Fe3O4 that contains Fe2+ and Fe3+. Based on such consideration, the formation reactions of Fe3O4 can be written as: ½FeðCON2 H4 Þ6 ðNO3 Þ3 →FeðNO3 Þ3 þ 6CON2 H4
ð1Þ
3FeðNO3 Þ3 þ CON2 H4 →Fe3 O4 þ CO2 þ 4NO þ 7NO2 þ O2 þ 2H2 O:
ð2Þ
In the case of our previous work where [Fe(CON2H4)6](NO3)3 was heated in an open state [19], however, the released CON2H4 in the first stage thermal decomposition was separated from Fe(NO3)3 by
Fig. 2. XPS spectrum of the samples prepared by the solvothermal reaction of [Fe(CON2H4)6] (NO3)3 in ethanol at 200 °C for 30 h.
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The hysteresis curves exhibited almost immeasurable values of coercivity and remanence, suggesting that the Fe3O4 nanopowder possess superparamagnetic characteristics at room temperature. The result is in agreement with the fact that the critical Fe3O4 particle size below which a superparamagnetic behavior will be observed is around 29 nm [20]. The saturation magnetization (Ms) increased from 32.6 to 42.9 emu/g when the reaction time increased from 10 to 50 h; i.e., the Ms increased with the increase of Fe3O4 particle size. At the same time, the values of Ms were all smaller than that of bulk Fe3O4 crystallite (i.e., 92 emu/g). This phenomenon has also been reported by other researchers [21,22], and it is most likely due to a surface spin canting effect according to the model proposed by Morales et al. [23]. 4. Conclusion Fig. 3. TEM micrograph of Fe3O4 nanopowder prepared by the solvothermal reaction of [Fe(CON2H4)6](NO3)3 in ethanol at 200 °C for 30 h.
In conclusion, this work demonstrates that the Fe3O4 nanopowder with superparamagnetic property can be formed by one-pot solvothermal reaction of [Fe(CON2H4)6](NO3)3 in ethanol. The crystallite size of Fe3O4 can be controlled from 9.7 to 20.5 nm by changing the reaction time. The proposed method is easy, nontoxic, and reproducible. Owing to these advantages, this method is quite promising for the mass production of Fe3O4 nanopowder. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 20861006) and Natural Science Foundation of Inner Mongolia (Grant No. MS0806). References
Fig. 4. Magnetization curves of the samples prepared by the solvothermal reaction of [Fe(CON2H4)6](NO3)3 in ethanol at 200 °C for (a) 10 h, (b) 30 h, and (c) 50 h.
evaporation; hence, the reduction of Fe3+ did not occur, resulting in the formation of γ-Fe2O3. Fig. 3 shows the TEM micrograph of Fe3O4 nanopowder prepared by the solvothermal reaction of [Fe(CON2H4)6](NO3)3 in ethanol at 200 °C for 30 h. The inset is the electron diffraction of a large zone. It is seen that Fe3O4 particles were nearly spherical in shape with an average size of around 10 nm. The average particle size is approximately in agreement with that estimated from the XRD data. The agglomeration of these particles on TEM grid was observed, probably due to the magnetic interaction of these particles. The presence of rings in the electron diffraction pattern suggests the crystalline nature of the particles and they can be indexed to Fe3O4. The magnetic properties of the Fe3O4 nanopowder were studied by magnetic measurements. Fig. 4 shows the room temperature magnetization hysteresis curves of the samples prepared by the solvothermal reaction of [Fe(CON2H4)6](NO3)3 in ethanol at 200 °C for different times.
[1] K. Yamaguchi, K. Matsumoto, T. Fujii, J. Appl. Phys. 67 (1990) 4493–4495. [2] V.G. Roullin, J.R. Deverre, L. Lemaire, F. Hindre, M.C.Y. Julienne, R. Vienet, J.P. Benoit, Eur. J. Pharm. Biopharm. 53 (2002) 293–299. [3] D.H. Zhang, G.D. Li, J.X. Li, J.S. Chen, Chem. Commun. (2008) 3414–3416. [4] R. Kaiser, G. Miskolcze, J. Appl. Phys. 41 (1970) 1064–1072. [5] L. Nixon, C.A. Koval, R.D. Noble, G.S. Slaff, Chem. Mater. 4 (1992) 117–121. [6] Z.M. Saiyed, M. Parasramka, S.D. Telang, C.N. Ramchand, Anal. Biochem. 363 (2007) 288–290. [7] K. Nishio, M. Ikeda, N. Gokon, S. Tsubouchi, H. Harimatsu, Y. Mochizuki, S. Sakamoto, A. Sandhu, M. Abe, H. Handa, J. Magn. Magn. Mater. 310 (2007) 2408–2410. [8] L.A. Harris, J.D. Goff, A.Y. Carmichael, J.S. Riffle, J.J. Harburn, T.G.S. Pierre, M. Sanders, Chem. Mater. 15 (2003) 1367–1377. [9] S. Sun, H. Zeng, J. Am. Chem. Soc. 124 (2002) 8204–8205. [10] W. Pei, H. Kumada, T. Natusme, H. Saito, S. Ishio, J. Magn. Magn. Mater. 310 (2007) 2375–2377. [11] J. Wang, J. Sun, Q. Sun, Q. Chen, Mater. Res. Bull. 38 (2003) 1113–1118. [12] N. Pinna, S. Grancharov, P. Beato, P. Bonville, M. Antonietti, M. Niederberger, Chem. Mater. 17 (2005) 3044–3049. [13] R. Vijayakumar, Y. Koltypin, I. Felner, A. Gedanken, Mater. Sci. Eng., A 286 (2000) 101–105. [14] C.R. Lin, Y.M. Chu, S.C. Wang, Mater. Lett. 60 (2006) 447–450. [15] M. Muhler, R. Schlögl, G. Ertl, J. Catal. 138 (1992) 413–444. [16] T. Yamashita, P. Hayes, Appl. Surf. Sci. 254 (2008) 2441–2449. [17] D.D. Hawn, B.M. DeKoven, Surf. Interface Anal. 10 (1987) 63–74. [18] C. Ruby, B. Humbert, J. Fusy, Surf. Interface Anal. 29 (2000) 377–380. [19] S. Zhao, H.Y. Wu, L. Song, O. Tegus, S. Asuha, J. Mater. Sci. 44 (2009) 926–930. [20] A. Angermann, J. Töpfer, J. Mater. Sci. 43 (2008) 5123–5130. [21] S. Si, A. Kotal, T.K. Mandal, S. Giri, H. Nakamura, T. Kohara, Chem. Mater. 16 (2004) 3489–3496. [22] S. Wan, J. Huang, H. Yan, K. Liu, J. Mater. Chem. 16 (2006) 298–303. [23] M.P. Morales, S. Veitemillas-Verdaguer, M.I. Montero, C.J. Serna, A. Roig, Ll. Casas, B. Martínez, F. Sandiumenge, Chem. Mater. 11 (1999) 3058–3064.
Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
One-pot synthesis of magnetite nanopowder and their magnetic properties S. Zhao, S. Asuha ⁎ Chemistry & Environment Science College, Inner Mongolia Normal University Key Laboratory of Physics and Chemistry of Function Materials, Inner Mongolia, 81 Zhaowudalu, Huhhot 010022, China
a r t i c l e
i n f o
Article history: Received 15 June 2009 Received in revised form 25 September 2009 Accepted 12 October 2009 Available online 17 October 2009 Keywords: Magnetite Magnetic materials Magnetic measurements Solvothermal method
a b s t r a c t Magnetite (Fe3O4) nanopowder can be prepared by one-pot solvothermal reaction of Fe-urea complex ([Fe (CON2H4)6](NO3)3) in ethanol. The result of X-ray powder diffraction (XRD) suggests the formation of Fe3O4 whose lattice constant is 8.385 Å. The formation of Fe3O4 is further confirmed from X-ray photoelectron spectroscopy (XPS) measurements. Transmission electron micrograph (TEM) observations show that Fe3O4 particles are nearly spherical in shape. The crystallite size of Fe3O4 can be controlled from 9.7 to 20.5 nm by varying the solvothermal reaction time. Room temperature magnetization hysteresis curves exhibit almost immeasurable values of coercivity and remanence, suggesting that the Fe3O4 nanopowder possesses superparamagnetic characteristics. The saturation magnetization (Ms) increases from 32.6 to 42.9 emu/g when the reaction time increases from 10 to 50 h. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In iron oxide crystalline forms, Fe3O4 is a ferrimagnetic material, and it has already been widely used in many applications such as magnetic recording [1], drug delivery [2], catalysis [3], ferro-fluids [4], and biotechnology [5–7]. For all of these applications, high quality Fe3O4 nanopowder with controlled size is required to satisfy new increasing demands. Especially in some cases (e.g., for medical applications), the Fe3O4 nanopowder are required to be superparamagnetic, which means that the particle size should be smaller than 30 nm. Therefore, the preparation of Fe3O4 nanopowder now attracts a lot of interest and various methods have been developed; each having their own advantages and disadvantages. Fe3O4 is generally prepared by a coprecipitation method using ferrous and ferric salts as starting materials. In this method, to obtain pure Fe3O4, the pH value of the solution has to be carefully controlled in both synthesis process and the subsequent purification process. In addition, the molar ratio of Fe(II) to Fe(III) ions also has to be fixed in stoichiometry; hence, to maintain it usually a protective gas (e.g., nitrogen gas) is necessary so that the oxidation of Fe (II) ions by air can be prevented. Another disadvantage of this conventional method is that the agglomeration of particles is easy to occur, and therefore the utilizations of various particle stabilizers (e.g., surfactants or polymer matrices) have been examined to obtain Fe3O4 nanopowder with a narrow particle size distribution or monodisperse [8]. Therefore, many alternative techniques have been developed to prepare Fe3O4 nanopowder, e.g., nonaqueous synthetic method [9,10], hydrothermal method [11,12], sonochemical approach [13], mechanochemical ⁎ Corresponding author. Chemistry & Environment Science College, Inner Mongolia Normal University, China. Tel.: + 86 471 43921214; fax: + 86 471 4392124. E-mail address: [email protected] (S. Asuha). 0032-5910/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2009.10.007
approach [14], etc. Among these methods, the nonaqueous synthetic process is a promising technique as the resulting nanopowder has rather narrow size distribution without particle agglomeration. Sun et al. have successfully prepared Fe3O4 nanopowder with narrow size distribution and controlled size by using this method [9]. The use of multireagents was typically required in this method. For some nonaqueous processes, iron pentacarbonyl (Fe(CO)5) is commonly used as a precursor. However, Fe (CO)5 is a very toxic and unstable chemical; hence, it may limit the usage of these methods for the mass production of Fe3O4 nanopowder. Therefore, an easy and environment-friendly synthetic method would be desirable for a large-scale production. In the present work, we show that the Fe3O4 nanopowder can be prepared by a simple one-pot solvothermal reaction process using [Fe (CON2H4)6](NO3)3 as a precursor. The main advantages of this method are (1) the preparation procedure of Fe3O4 nanopowder is simple, (2) the precursor, [Fe(CON2H4)6](NO3)3, a nontoxic material, can be easily synthesized from readily available Fe(NO3)3⋅9H2O and CON2H4, and (3) the solvent, C2H5OH, is an environment-friendly and most commonly used solvent. 2. Experimental 2.1. Synthesis The preparation of Fe3O4 nanopowder started from the synthesis of [Fe(CON2H4)6](NO3)3 as follows: Fe(NO3)3⋅9H2O and CON2H4 were mixed in a molar ratio of 1:6 in ethanol at room temperature, followed by intense stirring until the reactants were completely converted to a light green [Fe(CON2H4)6](NO3)3 powder. After removing the ethanol by filtering, the powder was dried in an oven at 45 °C. 1.0 g of [Fe(CON2H4)6](NO3)3 was added to 20.0 mL of ethanol and then the
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mixture was transferred into a 45-mL stainless autoclave with a Teflon liner. Then the autoclave was heated in an oven at 200 °C for a predetermined time. After cooling to room temperature, the precipitate was obtained by the centrifugation of the resulting suspension, and it was dried in air at room temperature. 2.2. Characterization X-ray diffraction patterns (XRD) were recorded on a Philips PW 1830 diffractometer using CuKα radiation. X-ray photoelectron spectroscopy (XPS) spectra were measured using an AXIS-Ultra instrument from Kratos Analytical with a monochromatic Al Kα radiation source; in this case, peak positions were calibrated using the C 1s peak position. For TEM studies, ca. 10 mg of Fe3O4 powder was dispersed in ca. 40 mL of hexane; then a droplet of the obtained suspension was dried on a carbon film and the measurements were carried out using a JEOL JEM-3000F transmission electron microscope. Magnetic measurements were performed using a Lake Shore 7407 vibrating-sample magnetometer (VSM) at room temperature. 3. Results and discussion Fig. 1 shows the XRD patterns of the samples prepared by the solvothermal reaction of [Fe(CON2H4)6](NO3)3 in ethanol at 200 °C for different times. The d-spacing values of the sample which were obtained with the reaction time of 30 h with the JCPDS files of Fe3O4 and maghemite (γ-Fe2O3) are listed in Table 1. The diffraction patterns and d-spacings match well with the JCPDS file of Fe3O4, indicating the formation of Fe3O4. In addition, the lattice constant was estimated to be 8.385 Å, which was in good agreement with that mentioned in the JCPDS file of standard Fe3O4. The reflection peaks became sharper and increased their intensities along with the increase of reaction time, showing the growth of Fe3O4 particles. The average crystallite sizes of Fe3O4 were calculated from the half-width of the diffraction lines by using the Scherrer's equation. For the samples prepared with reaction time of 10, 30 and 50 h, the average crystallite sizes of Fe3O4 were estimated to be 9.7, 13.8 and 20.5 nm, respectively. The result indicated that the crystallite size of Fe3O4 increased by the prolonging of reaction time. γ-Fe2O3 is also one of the iron oxides with a cubic spinel crystalline structure, but the XRD result obtained in the present work differs from its JCPDS file (e.g., No. 39-1346), as shown in Table 1. Therefore, we think that only pure Fe3O4 phase was formed in our samples without any phase transformation. Further evidence of the formation of Fe3O4 was obtained from Xray photoelectron spectroscopy. Fig. 2 shows XPS spectrum in the Fe
Fig. 1. XRD patterns of the samples prepared by the solvothermal reaction of [Fe(CON2H4)6] (NO3)3 in ethanol at 200 °C for (a) 10 h, (b) 30 h, and (c) 50 h.
Table 1 d-Spacings of the sample prepared by the solvothermal reaction at 200 °C for 30 h and those of Fe3O4 and γ-Fe2O3 from JCPDS files. d (Å)
Fe3O4a (Å)
γ-Fe2O3b (Å)
hkl
2.9630 2.5277 2.0931 1.7071 1.6143 1.4821
2.9670 2.5320 2.0993 1.7146 1.6158 1.4845
2.9530 2.5177 2.0866 1.7045 1.6073 1.4758
220 311 400 422 511 440
a b
JCPDS file No. 19-629. JCPDS file No. 39-1346.
2p region for the sample prepared by the solvothermal reaction of [Fe (CON2H4)6](NO3)3 in ethanol at 200 °C for 30 h. The peak positions of Fe 2p3/2 and Fe 2p1/2 were 710.6 and 724.2 eV, respectively. These values agree well with those for the Fe3O4 reported in literature [15,16]. Beside peak position, the appearance of satellite peak of Fe 2p3/2 is also one important feature that could be used for the discrimination of Fe3O4 and γ-Fe2O3. According to previous studies, the Fe 2p3/2 for Fe3O4 does not have a satellite peak [17,18]. As seen in Fig. 2, the satellite peak is absent. Therefore, the result of XPS measurements further confirms the formation of Fe3O4. According to our previous work, when [Fe(CON2H4)6](NO3)3 is heated at a high temperature above its melting point, it will decompose through a two-stage thermal decomposition process [19]. Fe(NO3)3 and CON2H4 are the products of the first stage thermal decomposition process. CON2H4 is a reducing agent, and it can reduce Fe3+ to Fe2+; on 2+ the other hand, NO− to 3 ion is an oxidizing agent, and it can oxidize Fe 3+ Fe . Therefore, the final decomposition product should depend on thermal decomposition conditions including heat treatment atmosphere and heat treatment state (open or closed). In the present case where [Fe (CON2H4)6](NO3)3 is heated in a closed autoclave, the released CON2H4 in the first stage thermal decomposition could not escape from the reactor by evaporation; hence, the coexistence of CON2H4 and NO− 3 ions results in the formation of Fe3O4 that contains Fe2+ and Fe3+. Based on such consideration, the formation reactions of Fe3O4 can be written as: ½FeðCON2 H4 Þ6 ðNO3 Þ3 →FeðNO3 Þ3 þ 6CON2 H4
ð1Þ
3FeðNO3 Þ3 þ CON2 H4 →Fe3 O4 þ CO2 þ 4NO þ 7NO2 þ O2 þ 2H2 O:
ð2Þ
In the case of our previous work where [Fe(CON2H4)6](NO3)3 was heated in an open state [19], however, the released CON2H4 in the first stage thermal decomposition was separated from Fe(NO3)3 by
Fig. 2. XPS spectrum of the samples prepared by the solvothermal reaction of [Fe(CON2H4)6] (NO3)3 in ethanol at 200 °C for 30 h.
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The hysteresis curves exhibited almost immeasurable values of coercivity and remanence, suggesting that the Fe3O4 nanopowder possess superparamagnetic characteristics at room temperature. The result is in agreement with the fact that the critical Fe3O4 particle size below which a superparamagnetic behavior will be observed is around 29 nm [20]. The saturation magnetization (Ms) increased from 32.6 to 42.9 emu/g when the reaction time increased from 10 to 50 h; i.e., the Ms increased with the increase of Fe3O4 particle size. At the same time, the values of Ms were all smaller than that of bulk Fe3O4 crystallite (i.e., 92 emu/g). This phenomenon has also been reported by other researchers [21,22], and it is most likely due to a surface spin canting effect according to the model proposed by Morales et al. [23]. 4. Conclusion Fig. 3. TEM micrograph of Fe3O4 nanopowder prepared by the solvothermal reaction of [Fe(CON2H4)6](NO3)3 in ethanol at 200 °C for 30 h.
In conclusion, this work demonstrates that the Fe3O4 nanopowder with superparamagnetic property can be formed by one-pot solvothermal reaction of [Fe(CON2H4)6](NO3)3 in ethanol. The crystallite size of Fe3O4 can be controlled from 9.7 to 20.5 nm by changing the reaction time. The proposed method is easy, nontoxic, and reproducible. Owing to these advantages, this method is quite promising for the mass production of Fe3O4 nanopowder. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 20861006) and Natural Science Foundation of Inner Mongolia (Grant No. MS0806). References
Fig. 4. Magnetization curves of the samples prepared by the solvothermal reaction of [Fe(CON2H4)6](NO3)3 in ethanol at 200 °C for (a) 10 h, (b) 30 h, and (c) 50 h.
evaporation; hence, the reduction of Fe3+ did not occur, resulting in the formation of γ-Fe2O3. Fig. 3 shows the TEM micrograph of Fe3O4 nanopowder prepared by the solvothermal reaction of [Fe(CON2H4)6](NO3)3 in ethanol at 200 °C for 30 h. The inset is the electron diffraction of a large zone. It is seen that Fe3O4 particles were nearly spherical in shape with an average size of around 10 nm. The average particle size is approximately in agreement with that estimated from the XRD data. The agglomeration of these particles on TEM grid was observed, probably due to the magnetic interaction of these particles. The presence of rings in the electron diffraction pattern suggests the crystalline nature of the particles and they can be indexed to Fe3O4. The magnetic properties of the Fe3O4 nanopowder were studied by magnetic measurements. Fig. 4 shows the room temperature magnetization hysteresis curves of the samples prepared by the solvothermal reaction of [Fe(CON2H4)6](NO3)3 in ethanol at 200 °C for different times.
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