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Preparation of ferromagnetic γ - Fe2O3 nanocrystallites by oxidative co-decomposition of PEG 6000 and ferrocene
Solid State Communications 141 (2007) 573–576 www.elsevier.com/locate/ssc
Preparation of ferromagnetic γ -Fe2O3 nanocrystallites by oxidative co-decomposition of PEG 6000 and ferrocene Bi Hong a,b,∗ , Chen Qianwang b , Sun Tao a a Department of Chemistry, Anhui University, Hefei 230039, PR China b Structure Research Laboratory and Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, PR China
Received 2 February 2006; accepted 10 December 2006 by C.H.R. Thomsen Available online 28 December 2006
Abstract Highly crystalline and ferromagnetic γ -Fe2 O3 nanocrystallites were prepared by controlled oxidative co-decomposition of PEG 6000 and ferrocene at a temperature of 450 ◦ C under air atmosphere. The morphology, crystalline structure and preliminary magnetic properties of the as-synthesized nanocrystallites have been characterized by using transmission electron microscope (TEM), X-ray powder diffraction (XRD) and vibrating sample magnetometer (VSM). The highly crystalline γ -Fe2 O3 nanocrystallites are in quasi-cubic shape with an average size of 30 nm and exhibit room-temperature ferromagnetism. The capping effect of PEG 6000 has also been investigated by thermogravimetry analysis (TGA) and Fourier transform infrared (FTIR) regarding controlling the size of the nanocrystallites and preventing the volatilization of ferrocene and thus raising the yield of the products. This simple method has a high yield of over 80% as well as low cost. c 2007 Elsevier Ltd. All rights reserved.
PACS: 75.50.-y; 81.07.-b; 81.16.Be Keywords: A. Magnetically ordered materials; A. Nanostructures; B. Chemical synthesis; B. Oxidative co-decomposition
1. Introduction Maghemite (γ -Fe2 O3 ) nanocrystallites are widely used in ferrofluids, magnetic refrigeration, information storage, biomedical applications etc [1–3]. Over the past few years, considerable research has been done on the synthesis of γ -Fe2 O3 nanocrystallites [1–12]. Although uniform γ -Fe2 O3 nanoparticles have been synthesized by using wet chemical methods such as co-deposition [8,9], microemulsion [10], and others [11], the crystallinity of these nanoparticles is comparatively poor [3], most of them are quite small (with a size of several nanometers) and behave superparamagnetically at room temperature. Moreover, using inorganic ferrous or ferric salts as starting precursors and sodium dodecyl sulfate as surfactant often leads to the resultant products being contaminated by remaining inorganic ions such as Na+ and Cl− [5,9]. ∗ Corresponding author at: Department of Chemistry, Anhui University, Hefei 230026, PR China. E-mail addresses: [email protected], [email protected] (B. Hong).
c 2007 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2006.12.011
Therefore, a solid-state synthetic method of fabricating highly crystalline and pure γ -Fe2 O3 nanoparticles has attracted much more attention in recent years. Taeghwan Hyeon et al. reported that highly crystalline and monodisperse γ -Fe2 O3 nanocrystallites can be achieved by direct oxidation of iron pentacarbonyl (Fe(CO)5 ) in the presence of oleic acid with trimethylamine oxide ((CH3 )3 NO) as an oxidant [3]. Furthermore, the particle size of the nanocrystallites could be tuned through changing the molar ratio of Fe(CO)5 and oleic acid. However, the exact mechanism of this synthetic procedure was not elucidated; oxidizing at 300 ◦ C with a mild oxidant ((CH3 )3 NO) may not completely rule out the existence of carbon element in the products arising from the co-carbonization of Fe(CO)5 and oleic acid. Radek Zboril et al. studied thermally induced decomposition of a pure iron complex (Prussian Blue), it was found that a mixture of β-Fe2 O3 and γ -Fe2 O3 nanopowders was obtained and their contents are strongly dependent on the oxidation–diffusion conditions [12].
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Actually, it is well known that carbon-encapsulated magnetic metal or metal carbides nanocrystallites can be easily obtained by in situ pyrolysis of organometallic and organic/polymeric precursors under an inert or reductive gas. This method provides relative size and carbon content control of the individual particles by varying the carbon/metal concentration in precursors and the pyrolysis temperature during the codecomposition process. Many research results in terms of preparation, growth mechanism and magnetic properties of the carbon-encapsulated magnetic metal nanocrystallites have been reported over the past ten years [13–16]. Recently, it was proposed [17] that the organic/polymeric precursor approach to nano-sized magnetic oxides is of great interest, because of the overall simplicity of the technique. Therefore, we believe that magnetic metal oxide nanocrystallites may be prepared in a simple way via oxidative co-decomposition of organometallic and polymeric precursors. In this communication, a commonly used organometallic precursor ferrocene was chosen as the iron source. Ferrocene is a highly volatile but relatively stable organometallic compound with excellent vaporizability, and its decomposition temperature in the gas phase has been reported to be higher than 400 ◦ C [18,19]. Therefore, poly (ethylene glycol) with an average molecular weight of 6000 was employed as a capping polymer due to its thermal decomposition temperature reported to be near 400 ◦ C [20]. Highly crystalline γ -Fe2 O3 nanocrystallites were achieved by controlled oxidative codecomposition of PEG 6000 and ferrocene at a temperature of 450 ◦ C under air atmosphere, and the nanocrystallites exhibit room-temperature ferromagnetism. 2. Experimental 1.0 g of purified ferrocene powder (Tianjin Guangfu Fine Chemicals Industry, China) was mixed uniformly with PEG 6000 (Anhui Wanwei High-tech Materials Industry Co. Ltd., China) hydrosol (3.8 g PEG 6000/15 mL H2 O) at a temperature of 80 ◦ C, then the mixture was placed in a 25 mL ceramic crucible and exposed in an air furnace at a temperature of 450 ◦ C for 3 h; 0.36 g of yellow powder was obtained. In order to investigate the capping effect of PEG 6000, 1.0 g of pure ferrocene powder was placed in another 25 mL ceramic crucible and the same thermal treatment performed simultaneously; finally, 0.04 g of yellow powder was obtained. X-ray powder diffraction analysis was conducted on a Rigaku DX-2000 X-ray diffractometer (XRD) using Cu Kα ˚ with 2θ ranging from 20 to 70◦ . radiation (λ = 1.5418 A) JEOL JEM-200CX transmission electron microscope (TEM) and selected-area electron diffraction (SAED) at an accelerating voltage of 200 kV were employed to examine the morphology of the nanostructures. Their preliminary magnetic properties were evaluated on a vibrating sample magnetometer (VSM) (Riken Denshi Co. Ltd.) under an applied field of up to 0.5 T. Thermogravimetric analysis (TGA) was performed on a Pyris-1 thermogravimetric analyzer (Perkin-Elmer Corp.) with a 10 mg sample under an air flow at a heating rate of 10 ◦ C/min.
Fig. 1. TGA curves of pure ferrocene and PEG 6000.
Fig. 2. IR spectra of (a) ferrocene, (b) PEG 6000, and (c) ferrocene mixed with PEG 6000.
3. Results and discussion The chemical stability and thermal decomposition temperature of the organometallic and polymeric precursors in air were investigated by TGA. Fig. 1 shows TGA curves of pure ferrocene and PEG 6000 with temperature increasing from 50 to 700 ◦ C, respectively. The TG curve of pure ferrocene presents a sharp weight loss of 97% in the temperature range 100–200 ◦ C due to its volatilizing at a temperature higher than 100 ◦ C and then running away with the air flow [18,19]. The TG curve of PEG 6000 indicates a one-step decomposition in air, starting at about 400 ◦ C and finishing at almost 450 ◦ C; this result is consistent with that obtained by Seongok Han et al. [20], because PEG and oxygen in air can easily react to form PEG peroxide in excess air, which produces low-molecular-weight oxygenated products by the random chain scission process. The effect of PEG 6000 in preparation is considered to be capping the organometallic precursor due to its good swelling ability to produce highly viscous hydrosol and the relatively high decomposition temperature in air. This consideration was confirmed by the following experimental results. FTIR spectra of ferrocene (a), PEG 6000 (b) and ferrocene uniformly mixed with PEG 6000 (c) (as shown in Fig. 2) illustrate clearly that no chemical bonds having been formed between the ferrocene
B. Hong et al. / Solid State Communications 141 (2007) 573–576
Fig. 3. TEM image of the product generation from pure ferrocene.
and PEG 6000 molecules, since the spectrum (c) is simply an addition of that of (a) and (b), which means that no complex of ferrocene and PEG 6000 is generated in the precursors, thus the chemical stability and decomposition temperature of ferrocene and PEG 6000 will not be influenced by each other below a temperature of 400 ◦ C. As the temperature increases from 400 to 450 ◦ C, ferrocene decomposed to form small Fe and Cn clusters [21], while PEG 6000 reacted with oxygen in air to form low-molecular-weight oxygenated products [20]. These small Fe clusters are very active and easily oxidized by oxygen in excess air to form iron(III) oxide, and the small Cn clusters that were produced were simultaneously oxidized to gaseous carbon oxide. Although the exact mechanism of oxidative co-decomposition is not clear to date, the capping effect of PEG 6000 has been investigated to be mainly controlling the size of the as-synthesized nanocrystallites and preventing the volatilization of ferrocene below a temperature of 400 ◦ C and thus raising the yield of the products. The yield of the product originating from pure ferrocene is only 9.3%, which is much lower than that of the product from oxidative co-decomposition of PEG 6000 and ferrocene of 83.7%. Figs. 3 and 4 show TEM images of the products originating from pure ferrocene and ferrocene/PEG 6000 precursors, respectively. It can be seen obviously that both samples exhibit quasi-cubic shape, but the particles originating from pure ferrocene are about 240 nm in size, which is much larger than those from oxidative codecomposition of PEG 6000 and ferrocene of about 30 nm.
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The right-hand photograph in Fig. 4 shows a selectedarea electron diffraction (SAED) pattern of the nanoparticle generated from oxidative co-decomposition of PEG 6000 and ferrocene. The diffraction rings indicate that the as-synthesized nanoparticles are polycrystalline, and the labeled indices (220), (311), (511), (440) in Fig. 4 agree well with the lattice facets of cubic maghemite (γ -Fe2 O3 ) (JCPDS no. 39–1346). Fig. 5 shows the powder X-ray diffraction (XRD) pattern of the oxidative co-decomposition product, which confirms the result of SAED. By using Scherrer’s equation, D = kλ/β cos θ , where λ is the X-ray wavelength, β is the half-peak width, θ is the Bragg angle in degrees, and k is the shape factor (often assigned a value of 0.89), the average size of the γ -Fe2 O3 nanocrystallites has been estimated to be 26 nm, which is a little smaller than the result from TEM observation. Although the SAED and XRD patterns of the oxidative codecomposition product display an exclusively highly crystalline γ -Fe2 O3 structure with no indication of the presence of any other phases, we cannot completely exclude the presence of amorphous carbon or Fe3 O4 phases in the product. Further study of identifications using M¨ossbauer spectra and X-ray photoelectron spectra are in progress. It is reasonable to believe that the amorphous carbon may be generated by the pyrolysis of excessive PEG 6000. Actually, if the mass ratio of PEG 6000 and ferrocene in precursors was raised or the temperature was maintained at 450 ◦ C for longer periods, more pyrolytic carbon was obtained, which leads to a decrease in the purity of the γ -Fe2 O3 nanocrystallites. Thus, the mass ratio in precursors and oxidation time must be carefully controlled in the preparation of pure γ -Fe2 O3 nanocrystallites via the oxidative co-decomposition of PEG 6000 and ferrocene. The preliminary magnetic property of the co-decomposition product has been characterized by VSM, as shown in Fig. 6. It can be seen clearly that the product exhibits roomtemperature ferromagnetism; the saturation magnetization (Ms ) and coercivity (Hc ) were determined to be 20.9 emu/g and 510.2 Oe, respectively. However, the saturation magnetization of the as-synthesized γ -Fe2 O3 nanocrystallites is significantly lower than that of bulk γ -Fe2 O3 (76 emu/g) [1], which can probably be attributed to the nanoscale dimension and the
Fig. 4. TEM image of the product generation from co-decomposition of PEG 6000 and ferrocene (left), and the SAED pattern of the nanocrystallites (right).
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4. Conclusions
Fig. 5. XRD pattern of the co-decomposition product.
Highly crystalline and ferromagnetic γ -Fe2 O3 nanocrystallites were prepared by controlled oxidative co-decomposition of PEG 6000 and ferrocene in air at a temperature of 450 ◦ C. The capping effect of PEG 6000 has been investigated to be mainly controlling the size and raising the yield of the as-synthesized γ -Fe2 O3 product. The mass ratio of PEG 6000 and ferrocene in the precursors and the oxidation time must be carefully controlled in the preparation of γ -Fe2 O3 nanocrystallites to ensure high purity of the product. Simplicity, low cost, controllability and high yields are the key features of this preparation method. It provides an effective method for the synthesis of highly crystalline and ferromagnetic γ -Fe2 O3 nanocrystallites on a large scale. Acknowledgement This work was supported by the Natural Science Foundation of China (20401001). References
Fig. 6. VSM M–H loop of the co-decomposition product at room temperature.
surface defects [1,12,22]. Detailed magnetic studies of the γ -Fe2 O3 nanocrystallites are currently underway. In recent years, ferrocene has been widely used in the production of carbon nanotubes [21,23], carbon-encapsulted iron [13,15,24] or iron oxide-filled [25,26] carbon nanotubes by the catalyzed pyrolysis of hydrocarbon. However, in these processes of ferrocene-catalyzed pyrolysis, ferrocene should be pre-sublimed under a vacuum, and sealed in the reactor (usually a quartz glass tube in the lab) by high-purity H2 (or Ar) so as to prevent it from burning, and then the reactor is heated to temperatures higher than 450 ◦ C by external heating or a pressure source [13–15]. These steps (where careful control is needed) are responsible for the high-cost products [15]. In our experiments, highly crystalline and ferromagnetic γ -Fe2 O3 nanocrystallites were easily obtained by controlled oxidative co-decomposition of PEG 6000 and ferrocene in air, which means simplicity as well as lower cost. In addition, the high yield (over 80%) of this synthetic process indicates that scaleup of production can be achieved relatively easily.
[1] Z.-H. Jing, S.-H. Wu, J. Solid State Chem. 177 (2004) 1213. [2] J. Chatterjee, Y. Haik, C.-J. Chen, J. Magn. Magn. Mater. 257 (2003) 113. [3] T. Hyeon, S.S. Lee, J. Park, Y. Chung, H.B. Na, J. Amer. Chem. Soc. 123 (2001) 12798. [4] M.P. Morales, M.A. Verges, J. Magn. Magn. Mater. 203 (1999) 146. [5] Y. Konoshi, T. Kawamura, S. Asai, Ind. Eng. Chem. Res. 32 (1993) 2888. [6] B.R.V. Narasimban, S. Prabhakar, Mater. Lett. 52 (2002) 295. [7] D.-Z. Wang, C.-B. Cao, S.-H. Xue, H.-S. Zhu, J. Cryst. Growth 277 (2005) 238. [8] K.A. Easom, K.J. Klabunde, C.M. Sorensen, Polyhedron 13 (1994) 1197. [9] Y.S. Kang, S. Risbud, J.F. Rabolt, P. Stroeve, Chem. Mater. 8 (1996) 2209. [10] N. Feltin, M.P. Pileni, Langmuir 13 (1997) 3927. [11] C. Pascal, J.L. Pascal, F. Favier, M.L.E. Moubtassim, C. Payen, Chem. Mater. 11 (1999) 141. [12] R. Zboril, L. Machala, M. Mashlan, V. Sharma, Cryst. Growth Des. 4 (2004) 1317. [13] E.P. Sajitha, V. Prasad, S.V. Subramanyam, S. Eto, K. Takai, T. Enoki, Carbon 42 (2004) 2815. [14] J.-P. Huo, H.-H. Song, X.-H. Chen, Carbon 42 (2004) 3177. [15] Y. Lu, Z.-P. Zhu, Z.-Y. Liu, Carbon 43 (2005) 369. [16] V.V. Baranauskas, M.A. Zalich, M. Saunders, T.G. St. Pierre, J.S. Riffle, Chem. Mater. 17 (2005) 5246. [17] J.S. Miller, M. Drillon, Nanostructured magnetic materials, in: C.J.O’ Connor, J.-K. Tang, J.H. Zhang (Eds.), Magnetism: Molecules to Materials III, Wiley-VCH Verlag GmbH, 2002, pp. 1–36 (Chapter 1). [18] G.J.M. Dormans, J. Cryst. Growth 108 (1991) 806. [19] M. Bernhauer, M. Braun, K.J. Huettinger, Carbon 32 (1994) 1073. [20] S. Han, C. Kim, D. Kwon, Polymer 38 (1997) 317. [21] H. Hou, A.K. Schaper, F. Weller, A. Greiner, Chem. Mater. 14 (2002) 3990. [22] C.J. Serna, F. BØdker, S. MØrup, M.P. Morales, F. Sandiumenge, S. Veintemillas-Verdaguer, Solid State Commun. 118 (2001) 437. [23] C.N.R. Rao, R. Sen, B.C. Satishkumar, A. Govindaraj, Chem. Commun. 15 (1998) 1525. [24] N. Sano, H. Akazawa, T. Kikuchi, T. Kanki, Carbon 41 (2003) 2159. [25] B.K. Pradhan, T. Toba, T. Kyotani, A. Tomita, Chem. Mater. 10 (1998) 2510. [26] M.C. Schnitzler, M.M. Oliveira, D. Ugarte, A.J.G. Zarbin, Chem. Phys. Lett. 381 (2003) 541.
Preparation of ferromagnetic γ -Fe2O3 nanocrystallites by oxidative co-decomposition of PEG 6000 and ferrocene Bi Hong a,b,∗ , Chen Qianwang b , Sun Tao a a Department of Chemistry, Anhui University, Hefei 230039, PR China b Structure Research Laboratory and Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, PR China
Received 2 February 2006; accepted 10 December 2006 by C.H.R. Thomsen Available online 28 December 2006
Abstract Highly crystalline and ferromagnetic γ -Fe2 O3 nanocrystallites were prepared by controlled oxidative co-decomposition of PEG 6000 and ferrocene at a temperature of 450 ◦ C under air atmosphere. The morphology, crystalline structure and preliminary magnetic properties of the as-synthesized nanocrystallites have been characterized by using transmission electron microscope (TEM), X-ray powder diffraction (XRD) and vibrating sample magnetometer (VSM). The highly crystalline γ -Fe2 O3 nanocrystallites are in quasi-cubic shape with an average size of 30 nm and exhibit room-temperature ferromagnetism. The capping effect of PEG 6000 has also been investigated by thermogravimetry analysis (TGA) and Fourier transform infrared (FTIR) regarding controlling the size of the nanocrystallites and preventing the volatilization of ferrocene and thus raising the yield of the products. This simple method has a high yield of over 80% as well as low cost. c 2007 Elsevier Ltd. All rights reserved.
PACS: 75.50.-y; 81.07.-b; 81.16.Be Keywords: A. Magnetically ordered materials; A. Nanostructures; B. Chemical synthesis; B. Oxidative co-decomposition
1. Introduction Maghemite (γ -Fe2 O3 ) nanocrystallites are widely used in ferrofluids, magnetic refrigeration, information storage, biomedical applications etc [1–3]. Over the past few years, considerable research has been done on the synthesis of γ -Fe2 O3 nanocrystallites [1–12]. Although uniform γ -Fe2 O3 nanoparticles have been synthesized by using wet chemical methods such as co-deposition [8,9], microemulsion [10], and others [11], the crystallinity of these nanoparticles is comparatively poor [3], most of them are quite small (with a size of several nanometers) and behave superparamagnetically at room temperature. Moreover, using inorganic ferrous or ferric salts as starting precursors and sodium dodecyl sulfate as surfactant often leads to the resultant products being contaminated by remaining inorganic ions such as Na+ and Cl− [5,9]. ∗ Corresponding author at: Department of Chemistry, Anhui University, Hefei 230026, PR China. E-mail addresses: [email protected], [email protected] (B. Hong).
c 2007 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2006.12.011
Therefore, a solid-state synthetic method of fabricating highly crystalline and pure γ -Fe2 O3 nanoparticles has attracted much more attention in recent years. Taeghwan Hyeon et al. reported that highly crystalline and monodisperse γ -Fe2 O3 nanocrystallites can be achieved by direct oxidation of iron pentacarbonyl (Fe(CO)5 ) in the presence of oleic acid with trimethylamine oxide ((CH3 )3 NO) as an oxidant [3]. Furthermore, the particle size of the nanocrystallites could be tuned through changing the molar ratio of Fe(CO)5 and oleic acid. However, the exact mechanism of this synthetic procedure was not elucidated; oxidizing at 300 ◦ C with a mild oxidant ((CH3 )3 NO) may not completely rule out the existence of carbon element in the products arising from the co-carbonization of Fe(CO)5 and oleic acid. Radek Zboril et al. studied thermally induced decomposition of a pure iron complex (Prussian Blue), it was found that a mixture of β-Fe2 O3 and γ -Fe2 O3 nanopowders was obtained and their contents are strongly dependent on the oxidation–diffusion conditions [12].
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Actually, it is well known that carbon-encapsulated magnetic metal or metal carbides nanocrystallites can be easily obtained by in situ pyrolysis of organometallic and organic/polymeric precursors under an inert or reductive gas. This method provides relative size and carbon content control of the individual particles by varying the carbon/metal concentration in precursors and the pyrolysis temperature during the codecomposition process. Many research results in terms of preparation, growth mechanism and magnetic properties of the carbon-encapsulated magnetic metal nanocrystallites have been reported over the past ten years [13–16]. Recently, it was proposed [17] that the organic/polymeric precursor approach to nano-sized magnetic oxides is of great interest, because of the overall simplicity of the technique. Therefore, we believe that magnetic metal oxide nanocrystallites may be prepared in a simple way via oxidative co-decomposition of organometallic and polymeric precursors. In this communication, a commonly used organometallic precursor ferrocene was chosen as the iron source. Ferrocene is a highly volatile but relatively stable organometallic compound with excellent vaporizability, and its decomposition temperature in the gas phase has been reported to be higher than 400 ◦ C [18,19]. Therefore, poly (ethylene glycol) with an average molecular weight of 6000 was employed as a capping polymer due to its thermal decomposition temperature reported to be near 400 ◦ C [20]. Highly crystalline γ -Fe2 O3 nanocrystallites were achieved by controlled oxidative codecomposition of PEG 6000 and ferrocene at a temperature of 450 ◦ C under air atmosphere, and the nanocrystallites exhibit room-temperature ferromagnetism. 2. Experimental 1.0 g of purified ferrocene powder (Tianjin Guangfu Fine Chemicals Industry, China) was mixed uniformly with PEG 6000 (Anhui Wanwei High-tech Materials Industry Co. Ltd., China) hydrosol (3.8 g PEG 6000/15 mL H2 O) at a temperature of 80 ◦ C, then the mixture was placed in a 25 mL ceramic crucible and exposed in an air furnace at a temperature of 450 ◦ C for 3 h; 0.36 g of yellow powder was obtained. In order to investigate the capping effect of PEG 6000, 1.0 g of pure ferrocene powder was placed in another 25 mL ceramic crucible and the same thermal treatment performed simultaneously; finally, 0.04 g of yellow powder was obtained. X-ray powder diffraction analysis was conducted on a Rigaku DX-2000 X-ray diffractometer (XRD) using Cu Kα ˚ with 2θ ranging from 20 to 70◦ . radiation (λ = 1.5418 A) JEOL JEM-200CX transmission electron microscope (TEM) and selected-area electron diffraction (SAED) at an accelerating voltage of 200 kV were employed to examine the morphology of the nanostructures. Their preliminary magnetic properties were evaluated on a vibrating sample magnetometer (VSM) (Riken Denshi Co. Ltd.) under an applied field of up to 0.5 T. Thermogravimetric analysis (TGA) was performed on a Pyris-1 thermogravimetric analyzer (Perkin-Elmer Corp.) with a 10 mg sample under an air flow at a heating rate of 10 ◦ C/min.
Fig. 1. TGA curves of pure ferrocene and PEG 6000.
Fig. 2. IR spectra of (a) ferrocene, (b) PEG 6000, and (c) ferrocene mixed with PEG 6000.
3. Results and discussion The chemical stability and thermal decomposition temperature of the organometallic and polymeric precursors in air were investigated by TGA. Fig. 1 shows TGA curves of pure ferrocene and PEG 6000 with temperature increasing from 50 to 700 ◦ C, respectively. The TG curve of pure ferrocene presents a sharp weight loss of 97% in the temperature range 100–200 ◦ C due to its volatilizing at a temperature higher than 100 ◦ C and then running away with the air flow [18,19]. The TG curve of PEG 6000 indicates a one-step decomposition in air, starting at about 400 ◦ C and finishing at almost 450 ◦ C; this result is consistent with that obtained by Seongok Han et al. [20], because PEG and oxygen in air can easily react to form PEG peroxide in excess air, which produces low-molecular-weight oxygenated products by the random chain scission process. The effect of PEG 6000 in preparation is considered to be capping the organometallic precursor due to its good swelling ability to produce highly viscous hydrosol and the relatively high decomposition temperature in air. This consideration was confirmed by the following experimental results. FTIR spectra of ferrocene (a), PEG 6000 (b) and ferrocene uniformly mixed with PEG 6000 (c) (as shown in Fig. 2) illustrate clearly that no chemical bonds having been formed between the ferrocene
B. Hong et al. / Solid State Communications 141 (2007) 573–576
Fig. 3. TEM image of the product generation from pure ferrocene.
and PEG 6000 molecules, since the spectrum (c) is simply an addition of that of (a) and (b), which means that no complex of ferrocene and PEG 6000 is generated in the precursors, thus the chemical stability and decomposition temperature of ferrocene and PEG 6000 will not be influenced by each other below a temperature of 400 ◦ C. As the temperature increases from 400 to 450 ◦ C, ferrocene decomposed to form small Fe and Cn clusters [21], while PEG 6000 reacted with oxygen in air to form low-molecular-weight oxygenated products [20]. These small Fe clusters are very active and easily oxidized by oxygen in excess air to form iron(III) oxide, and the small Cn clusters that were produced were simultaneously oxidized to gaseous carbon oxide. Although the exact mechanism of oxidative co-decomposition is not clear to date, the capping effect of PEG 6000 has been investigated to be mainly controlling the size of the as-synthesized nanocrystallites and preventing the volatilization of ferrocene below a temperature of 400 ◦ C and thus raising the yield of the products. The yield of the product originating from pure ferrocene is only 9.3%, which is much lower than that of the product from oxidative co-decomposition of PEG 6000 and ferrocene of 83.7%. Figs. 3 and 4 show TEM images of the products originating from pure ferrocene and ferrocene/PEG 6000 precursors, respectively. It can be seen obviously that both samples exhibit quasi-cubic shape, but the particles originating from pure ferrocene are about 240 nm in size, which is much larger than those from oxidative codecomposition of PEG 6000 and ferrocene of about 30 nm.
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The right-hand photograph in Fig. 4 shows a selectedarea electron diffraction (SAED) pattern of the nanoparticle generated from oxidative co-decomposition of PEG 6000 and ferrocene. The diffraction rings indicate that the as-synthesized nanoparticles are polycrystalline, and the labeled indices (220), (311), (511), (440) in Fig. 4 agree well with the lattice facets of cubic maghemite (γ -Fe2 O3 ) (JCPDS no. 39–1346). Fig. 5 shows the powder X-ray diffraction (XRD) pattern of the oxidative co-decomposition product, which confirms the result of SAED. By using Scherrer’s equation, D = kλ/β cos θ , where λ is the X-ray wavelength, β is the half-peak width, θ is the Bragg angle in degrees, and k is the shape factor (often assigned a value of 0.89), the average size of the γ -Fe2 O3 nanocrystallites has been estimated to be 26 nm, which is a little smaller than the result from TEM observation. Although the SAED and XRD patterns of the oxidative codecomposition product display an exclusively highly crystalline γ -Fe2 O3 structure with no indication of the presence of any other phases, we cannot completely exclude the presence of amorphous carbon or Fe3 O4 phases in the product. Further study of identifications using M¨ossbauer spectra and X-ray photoelectron spectra are in progress. It is reasonable to believe that the amorphous carbon may be generated by the pyrolysis of excessive PEG 6000. Actually, if the mass ratio of PEG 6000 and ferrocene in precursors was raised or the temperature was maintained at 450 ◦ C for longer periods, more pyrolytic carbon was obtained, which leads to a decrease in the purity of the γ -Fe2 O3 nanocrystallites. Thus, the mass ratio in precursors and oxidation time must be carefully controlled in the preparation of pure γ -Fe2 O3 nanocrystallites via the oxidative co-decomposition of PEG 6000 and ferrocene. The preliminary magnetic property of the co-decomposition product has been characterized by VSM, as shown in Fig. 6. It can be seen clearly that the product exhibits roomtemperature ferromagnetism; the saturation magnetization (Ms ) and coercivity (Hc ) were determined to be 20.9 emu/g and 510.2 Oe, respectively. However, the saturation magnetization of the as-synthesized γ -Fe2 O3 nanocrystallites is significantly lower than that of bulk γ -Fe2 O3 (76 emu/g) [1], which can probably be attributed to the nanoscale dimension and the
Fig. 4. TEM image of the product generation from co-decomposition of PEG 6000 and ferrocene (left), and the SAED pattern of the nanocrystallites (right).
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4. Conclusions
Fig. 5. XRD pattern of the co-decomposition product.
Highly crystalline and ferromagnetic γ -Fe2 O3 nanocrystallites were prepared by controlled oxidative co-decomposition of PEG 6000 and ferrocene in air at a temperature of 450 ◦ C. The capping effect of PEG 6000 has been investigated to be mainly controlling the size and raising the yield of the as-synthesized γ -Fe2 O3 product. The mass ratio of PEG 6000 and ferrocene in the precursors and the oxidation time must be carefully controlled in the preparation of γ -Fe2 O3 nanocrystallites to ensure high purity of the product. Simplicity, low cost, controllability and high yields are the key features of this preparation method. It provides an effective method for the synthesis of highly crystalline and ferromagnetic γ -Fe2 O3 nanocrystallites on a large scale. Acknowledgement This work was supported by the Natural Science Foundation of China (20401001). References
Fig. 6. VSM M–H loop of the co-decomposition product at room temperature.
surface defects [1,12,22]. Detailed magnetic studies of the γ -Fe2 O3 nanocrystallites are currently underway. In recent years, ferrocene has been widely used in the production of carbon nanotubes [21,23], carbon-encapsulted iron [13,15,24] or iron oxide-filled [25,26] carbon nanotubes by the catalyzed pyrolysis of hydrocarbon. However, in these processes of ferrocene-catalyzed pyrolysis, ferrocene should be pre-sublimed under a vacuum, and sealed in the reactor (usually a quartz glass tube in the lab) by high-purity H2 (or Ar) so as to prevent it from burning, and then the reactor is heated to temperatures higher than 450 ◦ C by external heating or a pressure source [13–15]. These steps (where careful control is needed) are responsible for the high-cost products [15]. In our experiments, highly crystalline and ferromagnetic γ -Fe2 O3 nanocrystallites were easily obtained by controlled oxidative co-decomposition of PEG 6000 and ferrocene in air, which means simplicity as well as lower cost. In addition, the high yield (over 80%) of this synthetic process indicates that scaleup of production can be achieved relatively easily.
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