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Mass synthesis of nanocrystalline spinel ferrites by a polymer-pyrolysis route
Materials Science and Engineering C 27 (2007) 750 – 755 www.elsevier.com/locate/msec
Mass synthesis of nanocrystalline spinel ferrites by a polymer-pyrolysis route Xian-Ming Liu a , Guo Yang a,b , Shao-Yun Fu a,⁎ a
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China b Graduate School, Chinese Academy of Sciences, Beijing 100039, PR China Received 17 January 2006; received in revised form 22 July 2006; accepted 22 July 2006 Available online 1 September 2006
Abstract Nanocrystalline MeFe2O4 (Me = Mn, Ni and Zn) spinel ferrites have been synthesized by polymer-pyrolysis method. The pyrolysis behaviors of the polymeric precursors prepared via in situ polymerization of metal salts and acrylic acid are analyzed by use of simultaneous thermogravimetric and differential thermal analysis (TG-DTA). Then, the structural characteristics of the products are studied by powder X-ray diffraction (XRD), infrared spectroscopy (IR), transmission electron microscope (TEM) and electron diffraction (ED) pattern. The results revealed that the spinel ferrites have nano-sized morphology and good crystallinity even if calcined at moderate temperature like 500 °C for 3 h. The average sizes of nanocrystalline spinel ferrites range from 10 to 30 nm with narrow size distributions. Magnetic measurements at room temperature show that Mn, Ni and Zn ferrites with the small coercivity and remanence exhibit soft magnetic behaviors. The spinel ferrites (MnFe2O4 and NiFe2O4) obtained here show higher saturation magnetization than the corresponding spinel ferrites produced by other methods such as conventional ceramic and wet chemical route. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanocrystalline MeFe2O4; Polymerization; Pyrolysis; Saturation magnetization; Coercivity
1. Introduction Development of spinel ferrite nanoparticles has been intensively pursued because of their technological and fundamental scientific importance [1,2]. Ferrites have received great attention as a result of their magnetic and electronic properties [3–5]. Such magnetic nanoparticles are currently used in magnetic recording media [6], microwave devices [7], contrast enhancement in magnetic resonance imaging [8], magnetic carriers for drug targeting and catalysis [9–11]. The magnetic properties of spinel ferrites can be varied systematically by changing the identity of the divalent Me2+ cations (Me = Co, Mn, Ni, Zn, etc.) without changing the spinel crystal structure [4]. It is well known that the chemical, structural, and magnetic properties of spinel ferrite nanoparticles are strongly influenced by their composition and microstructures, which are sensitive to the preparation methodologies [12–15]. Ferrite nanoparticles have been prepared by mechanical-milling [16,17], coprecipita⁎ Corresponding author. Tel./fax: +86 10 62659040. E-mail address: [email protected] (S.-Y. Fu). 0928-4931/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2006.07.026
tion reaction [18–20], sonochemical reaction [21], microwave route [22], reverse and normal micelles [23–25], chemie douce approach [26], alkalide reduction [27], sol–gel method [28–30] and microemulsion method [31]. However, it is very difficult for those methods to synthesize nanocrystalline spinel ferrites with high yield and large scale. Polymer pyrolysis method has the advantages for preparation of ferrite nanoparticles: (a) it can be easily operated, (b) it can be readily scaled up in the form of a batch and (c) it can produce highly homogeneous nanocrystalline spinel ferrites with excellent magnetic performance. In our previous paper [32], this method was reported for synthesis of nanocrystalline CoFe2O4 spinels. This method is versatile so that various metals can be used. It involves the preparation of a polymer precursor that reflects the precise stoichiometry of the end product (MeFe2O4, where Me = Mn, Ni and Zn) and allows preparation of 10–30 nm nanoparticles with a narrow size distribution at a moderate temperature. Nanocrystalline spinel ferrites with discrete structures were prepared by pyrolysis of Me–Fe polyacrylate precursors prepared via in situ polymerization. The structural and magnetic characteristics of the spinel ferrites were studied and
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discussed. In addition, the saturation magnetization of the MeFe2O4 (Me= Mn, Ni and Zn) spinel ferrites prepared here was compared with that of the corresponding spinel ferrites produced by other methods such as conventional ceramic and wet chemical route. 2. Experimental The chemical reagents ferric nitrate (Fe(NO3)3·9H2O), nickel nitrate (Ni(NO3)2·6H2O), zinc nitrate (Zn(NO3)2·6H2O), manganese acetate (Mn(CH3COO)2·4H2O), ammonium persulfate ((NH4)2S2O8) and acrylic acid are of analytical grade and used without further purification. In a typical experiment, nanocrystalline spinel ferrites were prepared by a polymer-pyrolysis method using polyacrylate of Me (Ni, Zn or Mn) and Fe as precursor compounds. The simultaneously polymeric precursors were made by in situ polymerization of the mixed aqueous solution of acrylic acid in the presence of metal salt and Fe(NO3)3·9H2O, with (NH4)2S2O8 as the initiator. First, ferric nitrate and target metal salt were dissolved in 10 g of acrylic acid aqueous solution (acrylic acid:H2O = 70:30 wt.%) under stirring. The molar fraction of Me/Fe was fixed at 1:2. Afterwards, a small amount (0.5 g) of 5% (NH4)2S2O8 aqueous solution as the initiator was added to the mixed acrylic acid solution to promote the polymerization. Under heating at 70–90 °C for 2 h, the mixed solution was dried to form the well-distributed polyacrylate salt. The obtained polyacrylates were dried at 100 °C for 24 h, and calcined at 500 °C for 3 h in air, then the final products were obtained after being slowly cooled to room temperature. The decomposition processes of the polymer precursor during heat treatment were characterized by thermogravimetric and differential thermal analysis (TG/DTA) on a model WCT1A thermobalance (TA-5000 apparatus) at the temperature range of 30–900 °C with a heating rate of 10 °C/min in air. To reveal the crystalline structure of the spinel ferrite powders, XRD analysis was carried out on a Rigaku D/max2500 diffractometer at a voltage of 40 kV and a current of 200 mA with Cu-Kα radiation (λ = 1.5406 Å), employing a scanning rate 0.02° s− 1 in the 2θ ranging from 10 to 70°. TEM images and the electron diffraction (ED) patterns were recorded on a Hitachi-600 transmission electron microscope (TEM) at an accelerating voltage of 200 kV. The particle sizes were
Fig. 1. Schematic representation of polymeric chain for the co-polymeric precursor of Me–Fe polyacrylates. (Me = Mn, Ni and Zn).
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measured from their TEM images. The infrared (IR) spectra were recorded on a Bruker Equinox-55 spectrometer on samples palletized with KBr powder. Magnetic measurements were carried out at room temperature using a vibrating sample magnetometer (VSM, Lakeshore 7307) with a maximum magnetic field of 10 kOe. The actual stoichiometric compositions of the obtained powders were determined by a Shimadzu ICPS75000 inductively coupled plasma atomic emission spectrometer (ICP-AES). 3. Results and discussion As the first step of the ferrite synthesis, an aqueous solution of acrylic acid is mixed with the appropriate amount of Me and Fe salts. Ammonium persulfate ((NH4)2S2O8) is used as the initiator of polymerization reaction. This affords a composition which has the predetermined ratio of Me to Fe to be compatible with the final product (MeFe2O4) according to the following reactive formula:
In the reactive formula, x and y represent the polymerization degrees, which are presumably distributed randomly along the polymer chain. It is preferable to chose x ≪ y in order to avoid the spontaneous preparation of ill-defined products. To avoid compromising the purity and properties of spinel ferrite and related materials it would be desirable to prepare them from a single solid precursor which can be prepared in a pure state in which the Me2+ and Fe3+ cations are uniformly distributed on an atomic level. The co-polymeric precursor compounds are schematically represented in Fig. 1. As is shown, Me (II) and Fe (III) ions are bound by the strong ionic bonds between the metallic ions and carboxylate ions in a polymeric chain or between the polymeric chains. This uniform immobilization of metallic ions in the polymer chains favors the formation of uniformly distributed solid solution of the metallic oxides in the following pyrolysis process. To clarify the chemical reactions of the co-polymeric precursors occurring in the pyrolysis process, we measured TG and DTA curves of the co-polymeric Me–Fe precursors, as shown in Fig. 2. In the TG-DTA curves of Mn ferrite precursors (Fig. 2A), two exothermic peaks are observed to appear at 242–350 and 350–454 °C, respectively, corresponding to two steps of weight loss. The first step shows a weight loss of 39.9% and the latter has a weight loss of 43.4%. Fig. 2B shows the TG and DTA curves of Ni ferrite precursors. The main feature in the DTA curve is a big exothermic peak at 371 °C, corresponding to a weight loss reaction between 246 and 408 °C, while a small exothermic peak that appears at 278 °C. The total weight loss is 82.2%, possibly due to the decomposition of polyacrylates to form oxides. The TG and DTA curves of the co-polymeric Zn– Fe precursor are shown in Fig. 2C. The total weight loss is
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peak appears at 37.2° corresponding to the crystal plane (222) of spinel ferrite except ZnFe2O4 (Fig. 3C). All the ferrites showed a single-phase spinel ferrite with no impurities in the XRD patterns. The crystallite size can be evaluated from the major diffraction peak (311) using the well-known Scherrer's formula. The size of nanocrystalline NiFe2O4, MnFe2O4 and ZnFe2O4 were found to be about 21, 9 and 19 nm, respectively. In addition, the elemental analyses of these nanoparticles using ICP-AES showed that the molar ratios of Me2+/Fe3+ are close to 0.5. The detailed microstructures of the products were characterized by transmission electron microscopy (TEM). Fig. 4A–C showed the bright field TEM images of MnFe2O4, NiFe2O4 and ZnFe2O4 ferrite powders. It can be seen that the particles have nanometer-scale morphology and are dispersive apart from each
Fig. 2. TG-DTA curves of metal ferrite precursors: (A) MnFe2O4; (B) NiFe2O4; (C) ZnFe2O4.
82.3% and it occurred in two steps up to 446 °C. In the first step of the weight loss (30–316 °C), a 32.9% weight loss associated with an exothermic peak at 279 °C may be attributed to the decomposition of nitrate. In the second step (316–446 °C), the exothermic peaks at 387 °C may be related to the decomposition of Zn–Fe precursor. The observed differences in the decomposition temperatures for different metal ferrite precursors are due to the differences in the strength of the interaction between metallic cation and the polymeric chain. It is noted that the weight of different metal ferrite precursors is almost unchanged above the decomposition temperature, indicating that the final decomposed products are ferrites. Fig. 3 shows the XRD patterns of metal ferrite powders prepared by the polymer-pyrolysis route. The diffraction patterns and relative intensities of all diffraction peaks match well with those of JCPDS card 10-0325 for NiFe2O4 (Fig. 3A), 10-0319 for MnFe2O4 (Fig. 3B) and 22-1012 for ZnFe2O4 (Fig. 3C), respectively. The peaks appear at around 18.5, 30.2, 35.6, 43.0, 53.4, 57.1, and 62.5° which are well indexed to the crystal plane of spinel ferrite (111), (220), (311), (400), (422), (511), and (440), respectively. Further careful observation can find that a shoulder
Fig. 3. XRD patterns of spinel ferrites calcined at 500 °C: (A) NiFe2O4; (B) MnFe2O4; (C) ZnFe2O4.
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This method used in this work is to synthesize the co-polymeric precursor of the mixed metalic ions and then to pyrolyze the precursor into the required oxide structures. The polymerization reaction of acrylate has been well studied and discussed in detail in previous papers [33]. It is now well accepted that the reaction proceeds mainly through a free radical polymerization. The IR spectra of the precursor compound of copolymerized Me–Fe polyacrylates are shown in Fig. 6A. It is found that the C_O stretching vibration (∼ 1702 cm− 1) of α and β-unsaturated carboxylic acid, the C_C stretching vibration (∼ 1640 cm− 1) and the _C–H vibrations (∼ 1045 and ∼ 984 cm− 1) of acrylic acid, characteristic of the vinylidene group in the acrylic monomers, are absent, suggesting that the polymerization of the acrylates took place in the acrylates of Me and Fe. Furthermore, the spectra show the appearance of the symmetric (∼ 1618 cm− 1) and asymmetric (∼ 1451 cm− 1) stretching bands of carboxylate salts. Compared with the standard spectrum of acrylic acid, these two bands are positively shifted, implying an associating interaction between the metallic ions and carboxylate ions. In addition, the IR bands for H2O (∼ 3428 cm− 1) and NO3− vibration (∼ 1384 and ∼ 839 cm− 1)
Fig. 4. TEM images and ED patterns of ferrites: (A) MnFe2O4; (B) NiFe2O4; (C) ZnFe2O4.
other. And their average particle sizes for metal ferrites are consistent with the calculated results by XRD observations. The solid materials obtained showed a very narrow particle size distribution for different metal ferrites shown in Fig. 5. Moreover, the average size of MnFe2O4 is the smallest. The SAED pattern shown in the up-left of each picture, should be indexed to further confirm that the structure is a spinel-type.
Fig. 5. Histograms of the respective size distributions in each TEM image: (A) MnFe2O4; (B) NiFe2O4; (C) ZnFe2O4 nanoparticles.
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Fig. 8. Hysteresis loops for nanocrystalline spinel ferrites.
Fig. 6. IR spectra of the samples. (A) a: Zn ferrite precursors; b: Mn ferrite precursors; c: Ni ferrite precursors; (B) a′: ZnFe2O4; b′: MnFe2O4; c′: NiFe2O4.
are also observable due to the residual H2O and NO3−, while characteristic bands of other anions with small quantity are overlapped. Fig. 6B exhibits IR spectra of spinel ferrites obtained by pyrolysis of Me–Fe polyacrylates at 500 °C. It can be seen from Fig. 6B that the absorption bands around 588 and 441 cm− 1 are assigned to Fe–O stretching vibration mode, indicating that the pyrolysis products were inorganic metal
Fig. 7. IR spectra of the samples under the same synthetic conditions, (A) acrylic acid; (B) acrylic acid and metal nitrate without the persulfate catalyst; (C) Me–Fe polyacrylates obtained by the polymerization reaction using the persulfate as catalyst.
oxide species. These results are in good agreement with the analysis of XRD that the oxides consist of spinel ferrites. In order to convincingly demonstrate that the polyacrylates formed, comparing experiments without the persulfate catalyst was performed. It was found that the viscosity of solutions was increased, but the polymerization reaction didn't occur. IR spectra of the samples obtained under the same synthetic conditions were recorded on a Bruker Equinox-55 spectrometer, as shown in Fig. 7. It can be seen from Fig. 7 that Fig. 7B is similar to IR spectrum of acrylic acid except that IR bands for NO3− vibration (∼ 1384 and ∼ 839 cm− 1) can't be observable in Fig. 7A. IR spectra of the samples made with and without the persulfate catalyst were completely different, which is almost in agreement with the literature [33]. Thus, it can be inferred from IR spectra that the polyacrylates can be formed by the polymerization reaction using the persulfate as catalyst. An attractive feature of this method, compared to the ceramic method [16,17], is that it does not involve milling of the precursor which introduces defects and strains into the ferrite and hence affects magnetic properties of the materials. In addition, this pyrolysis method is simple and convenient, greatly feasible for preparation of nanocrystalline spinel ferrites. The complex oxide synthesized from the polymer-pyrolysis method shows a uniform morphology and narrow size distribution, and the preparation process is much simpler without coprecipitating, gelling, and grinding processes. However, it should be mentioned that the pretreatment process in the polymer-pyrolysis method can produce some oxides of nitrogen and CO2 due to the decomposition of the residual NO3− and polyacrylate salts in a similar way as the sol–gel method. But the waste gas could be treated during drying and pyrolysis process. Thus, the polymerpyrolysis method is still a commercially worthy route compared with other methods. The field dependence of the magnetization of spinel ferrites was measured using a vibrating sample magnetometer (VSM) at room temperature with an applied field − 10 kOe ≤ H ≤ 10 kOe, shown in Fig. 8. Clearly, the hysteresis varies with different samples. At low external field, the hysteresis loops of Mn, Ni and Zn ferrites exhibit very small coercivity and remanence. The saturation magnetization is evaluated by extrapolated value of 10 kOe. The saturation magnetization depends on the divalent metal ions and ranges from 48.6 emu/g for MnFe2O4 and
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50.6 emu/g for NiFe2O4 at the applied magnetic field of 10 kOe. However, it can also be seen from Fig. 8 that the magnetization of zinc ferrites increases almost linearly with the given field and shows a lack of saturation at a field as high as 10 kOe. The saturation magnetization of spinel ferrites (MnFe2O4 and NiFe2O4) produced by a polymer-pyrolysis route have much higher values than those of MeFe2O4 produced by conventional ceramic and wet chemical methods at 1100 °C for 10 h [16,18– 20]. This is because the polymer-pyrolysis route possesses several advantages as a precursor to form spinel ferrites that lead to enhanced saturation magnetization. Use of a molecular precursor with cations randomly distributed with no long order facilitates the synthesis of a homogeneous spinel phase [34]. The close structural relationship between the Me–Fe-PA precursors and its calcination products is also a key factor. Furthermore, the fact that the spinel is produced from a single solid precursor rather than a mixture means that the calcination process requires a much shorter time and lower temperature, leading to a lower chance of side reactions to occur. 4. Conclusions In summary, highly homogeneous nanocrystalline spinel ferrites have been produced by the pyrolysis of Me–Fe (Me = Mn, Ni and Zn) co-polymeric precursors in situ polymerized by reaction of the metal salt and acrylic acid. TG-DTA measurements showed that the pyrolysis process could occur at a relatively low temperature to form spinel ferrites. The results from the XRD, TEM and ED measurements revealed that the ferrite powders even calcined at 500 °C have good crystallinity with the spinel structure and an average particle size of 10–30 nm with a uniform size distribution. Moreover, the saturation magnetization of spinel ferrites derived from the Me–Fe PA precursors has been shown to be higher than that of the same materials produced by the conventional ceramic and wet chemical route. Furthermore, this method is easy to operate and can be used for industrial production of highly homogeneous metal oxide magnetic nanomaterials. In addition, the method can be particularly suitable for preparation of mixed metal oxides, such as MnxZn1−xFe2O4, NixMn1−xFe2O4 and CoxMnyZn1−x−yFe2O4. In the specific case of MeFe2O4 reported here, the preliminary magnetic properties are promising for the reason that the synthesis can be easily scaled up for industrial production. Acknowledgements We appreciate the financial support of the National High Technical Research and Development Program of China (No 2003AA305890).
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Mass synthesis of nanocrystalline spinel ferrites by a polymer-pyrolysis route Xian-Ming Liu a , Guo Yang a,b , Shao-Yun Fu a,⁎ a
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China b Graduate School, Chinese Academy of Sciences, Beijing 100039, PR China Received 17 January 2006; received in revised form 22 July 2006; accepted 22 July 2006 Available online 1 September 2006
Abstract Nanocrystalline MeFe2O4 (Me = Mn, Ni and Zn) spinel ferrites have been synthesized by polymer-pyrolysis method. The pyrolysis behaviors of the polymeric precursors prepared via in situ polymerization of metal salts and acrylic acid are analyzed by use of simultaneous thermogravimetric and differential thermal analysis (TG-DTA). Then, the structural characteristics of the products are studied by powder X-ray diffraction (XRD), infrared spectroscopy (IR), transmission electron microscope (TEM) and electron diffraction (ED) pattern. The results revealed that the spinel ferrites have nano-sized morphology and good crystallinity even if calcined at moderate temperature like 500 °C for 3 h. The average sizes of nanocrystalline spinel ferrites range from 10 to 30 nm with narrow size distributions. Magnetic measurements at room temperature show that Mn, Ni and Zn ferrites with the small coercivity and remanence exhibit soft magnetic behaviors. The spinel ferrites (MnFe2O4 and NiFe2O4) obtained here show higher saturation magnetization than the corresponding spinel ferrites produced by other methods such as conventional ceramic and wet chemical route. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanocrystalline MeFe2O4; Polymerization; Pyrolysis; Saturation magnetization; Coercivity
1. Introduction Development of spinel ferrite nanoparticles has been intensively pursued because of their technological and fundamental scientific importance [1,2]. Ferrites have received great attention as a result of their magnetic and electronic properties [3–5]. Such magnetic nanoparticles are currently used in magnetic recording media [6], microwave devices [7], contrast enhancement in magnetic resonance imaging [8], magnetic carriers for drug targeting and catalysis [9–11]. The magnetic properties of spinel ferrites can be varied systematically by changing the identity of the divalent Me2+ cations (Me = Co, Mn, Ni, Zn, etc.) without changing the spinel crystal structure [4]. It is well known that the chemical, structural, and magnetic properties of spinel ferrite nanoparticles are strongly influenced by their composition and microstructures, which are sensitive to the preparation methodologies [12–15]. Ferrite nanoparticles have been prepared by mechanical-milling [16,17], coprecipita⁎ Corresponding author. Tel./fax: +86 10 62659040. E-mail address: [email protected] (S.-Y. Fu). 0928-4931/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2006.07.026
tion reaction [18–20], sonochemical reaction [21], microwave route [22], reverse and normal micelles [23–25], chemie douce approach [26], alkalide reduction [27], sol–gel method [28–30] and microemulsion method [31]. However, it is very difficult for those methods to synthesize nanocrystalline spinel ferrites with high yield and large scale. Polymer pyrolysis method has the advantages for preparation of ferrite nanoparticles: (a) it can be easily operated, (b) it can be readily scaled up in the form of a batch and (c) it can produce highly homogeneous nanocrystalline spinel ferrites with excellent magnetic performance. In our previous paper [32], this method was reported for synthesis of nanocrystalline CoFe2O4 spinels. This method is versatile so that various metals can be used. It involves the preparation of a polymer precursor that reflects the precise stoichiometry of the end product (MeFe2O4, where Me = Mn, Ni and Zn) and allows preparation of 10–30 nm nanoparticles with a narrow size distribution at a moderate temperature. Nanocrystalline spinel ferrites with discrete structures were prepared by pyrolysis of Me–Fe polyacrylate precursors prepared via in situ polymerization. The structural and magnetic characteristics of the spinel ferrites were studied and
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discussed. In addition, the saturation magnetization of the MeFe2O4 (Me= Mn, Ni and Zn) spinel ferrites prepared here was compared with that of the corresponding spinel ferrites produced by other methods such as conventional ceramic and wet chemical route. 2. Experimental The chemical reagents ferric nitrate (Fe(NO3)3·9H2O), nickel nitrate (Ni(NO3)2·6H2O), zinc nitrate (Zn(NO3)2·6H2O), manganese acetate (Mn(CH3COO)2·4H2O), ammonium persulfate ((NH4)2S2O8) and acrylic acid are of analytical grade and used without further purification. In a typical experiment, nanocrystalline spinel ferrites were prepared by a polymer-pyrolysis method using polyacrylate of Me (Ni, Zn or Mn) and Fe as precursor compounds. The simultaneously polymeric precursors were made by in situ polymerization of the mixed aqueous solution of acrylic acid in the presence of metal salt and Fe(NO3)3·9H2O, with (NH4)2S2O8 as the initiator. First, ferric nitrate and target metal salt were dissolved in 10 g of acrylic acid aqueous solution (acrylic acid:H2O = 70:30 wt.%) under stirring. The molar fraction of Me/Fe was fixed at 1:2. Afterwards, a small amount (0.5 g) of 5% (NH4)2S2O8 aqueous solution as the initiator was added to the mixed acrylic acid solution to promote the polymerization. Under heating at 70–90 °C for 2 h, the mixed solution was dried to form the well-distributed polyacrylate salt. The obtained polyacrylates were dried at 100 °C for 24 h, and calcined at 500 °C for 3 h in air, then the final products were obtained after being slowly cooled to room temperature. The decomposition processes of the polymer precursor during heat treatment were characterized by thermogravimetric and differential thermal analysis (TG/DTA) on a model WCT1A thermobalance (TA-5000 apparatus) at the temperature range of 30–900 °C with a heating rate of 10 °C/min in air. To reveal the crystalline structure of the spinel ferrite powders, XRD analysis was carried out on a Rigaku D/max2500 diffractometer at a voltage of 40 kV and a current of 200 mA with Cu-Kα radiation (λ = 1.5406 Å), employing a scanning rate 0.02° s− 1 in the 2θ ranging from 10 to 70°. TEM images and the electron diffraction (ED) patterns were recorded on a Hitachi-600 transmission electron microscope (TEM) at an accelerating voltage of 200 kV. The particle sizes were
Fig. 1. Schematic representation of polymeric chain for the co-polymeric precursor of Me–Fe polyacrylates. (Me = Mn, Ni and Zn).
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measured from their TEM images. The infrared (IR) spectra were recorded on a Bruker Equinox-55 spectrometer on samples palletized with KBr powder. Magnetic measurements were carried out at room temperature using a vibrating sample magnetometer (VSM, Lakeshore 7307) with a maximum magnetic field of 10 kOe. The actual stoichiometric compositions of the obtained powders were determined by a Shimadzu ICPS75000 inductively coupled plasma atomic emission spectrometer (ICP-AES). 3. Results and discussion As the first step of the ferrite synthesis, an aqueous solution of acrylic acid is mixed with the appropriate amount of Me and Fe salts. Ammonium persulfate ((NH4)2S2O8) is used as the initiator of polymerization reaction. This affords a composition which has the predetermined ratio of Me to Fe to be compatible with the final product (MeFe2O4) according to the following reactive formula:
In the reactive formula, x and y represent the polymerization degrees, which are presumably distributed randomly along the polymer chain. It is preferable to chose x ≪ y in order to avoid the spontaneous preparation of ill-defined products. To avoid compromising the purity and properties of spinel ferrite and related materials it would be desirable to prepare them from a single solid precursor which can be prepared in a pure state in which the Me2+ and Fe3+ cations are uniformly distributed on an atomic level. The co-polymeric precursor compounds are schematically represented in Fig. 1. As is shown, Me (II) and Fe (III) ions are bound by the strong ionic bonds between the metallic ions and carboxylate ions in a polymeric chain or between the polymeric chains. This uniform immobilization of metallic ions in the polymer chains favors the formation of uniformly distributed solid solution of the metallic oxides in the following pyrolysis process. To clarify the chemical reactions of the co-polymeric precursors occurring in the pyrolysis process, we measured TG and DTA curves of the co-polymeric Me–Fe precursors, as shown in Fig. 2. In the TG-DTA curves of Mn ferrite precursors (Fig. 2A), two exothermic peaks are observed to appear at 242–350 and 350–454 °C, respectively, corresponding to two steps of weight loss. The first step shows a weight loss of 39.9% and the latter has a weight loss of 43.4%. Fig. 2B shows the TG and DTA curves of Ni ferrite precursors. The main feature in the DTA curve is a big exothermic peak at 371 °C, corresponding to a weight loss reaction between 246 and 408 °C, while a small exothermic peak that appears at 278 °C. The total weight loss is 82.2%, possibly due to the decomposition of polyacrylates to form oxides. The TG and DTA curves of the co-polymeric Zn– Fe precursor are shown in Fig. 2C. The total weight loss is
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peak appears at 37.2° corresponding to the crystal plane (222) of spinel ferrite except ZnFe2O4 (Fig. 3C). All the ferrites showed a single-phase spinel ferrite with no impurities in the XRD patterns. The crystallite size can be evaluated from the major diffraction peak (311) using the well-known Scherrer's formula. The size of nanocrystalline NiFe2O4, MnFe2O4 and ZnFe2O4 were found to be about 21, 9 and 19 nm, respectively. In addition, the elemental analyses of these nanoparticles using ICP-AES showed that the molar ratios of Me2+/Fe3+ are close to 0.5. The detailed microstructures of the products were characterized by transmission electron microscopy (TEM). Fig. 4A–C showed the bright field TEM images of MnFe2O4, NiFe2O4 and ZnFe2O4 ferrite powders. It can be seen that the particles have nanometer-scale morphology and are dispersive apart from each
Fig. 2. TG-DTA curves of metal ferrite precursors: (A) MnFe2O4; (B) NiFe2O4; (C) ZnFe2O4.
82.3% and it occurred in two steps up to 446 °C. In the first step of the weight loss (30–316 °C), a 32.9% weight loss associated with an exothermic peak at 279 °C may be attributed to the decomposition of nitrate. In the second step (316–446 °C), the exothermic peaks at 387 °C may be related to the decomposition of Zn–Fe precursor. The observed differences in the decomposition temperatures for different metal ferrite precursors are due to the differences in the strength of the interaction between metallic cation and the polymeric chain. It is noted that the weight of different metal ferrite precursors is almost unchanged above the decomposition temperature, indicating that the final decomposed products are ferrites. Fig. 3 shows the XRD patterns of metal ferrite powders prepared by the polymer-pyrolysis route. The diffraction patterns and relative intensities of all diffraction peaks match well with those of JCPDS card 10-0325 for NiFe2O4 (Fig. 3A), 10-0319 for MnFe2O4 (Fig. 3B) and 22-1012 for ZnFe2O4 (Fig. 3C), respectively. The peaks appear at around 18.5, 30.2, 35.6, 43.0, 53.4, 57.1, and 62.5° which are well indexed to the crystal plane of spinel ferrite (111), (220), (311), (400), (422), (511), and (440), respectively. Further careful observation can find that a shoulder
Fig. 3. XRD patterns of spinel ferrites calcined at 500 °C: (A) NiFe2O4; (B) MnFe2O4; (C) ZnFe2O4.
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This method used in this work is to synthesize the co-polymeric precursor of the mixed metalic ions and then to pyrolyze the precursor into the required oxide structures. The polymerization reaction of acrylate has been well studied and discussed in detail in previous papers [33]. It is now well accepted that the reaction proceeds mainly through a free radical polymerization. The IR spectra of the precursor compound of copolymerized Me–Fe polyacrylates are shown in Fig. 6A. It is found that the C_O stretching vibration (∼ 1702 cm− 1) of α and β-unsaturated carboxylic acid, the C_C stretching vibration (∼ 1640 cm− 1) and the _C–H vibrations (∼ 1045 and ∼ 984 cm− 1) of acrylic acid, characteristic of the vinylidene group in the acrylic monomers, are absent, suggesting that the polymerization of the acrylates took place in the acrylates of Me and Fe. Furthermore, the spectra show the appearance of the symmetric (∼ 1618 cm− 1) and asymmetric (∼ 1451 cm− 1) stretching bands of carboxylate salts. Compared with the standard spectrum of acrylic acid, these two bands are positively shifted, implying an associating interaction between the metallic ions and carboxylate ions. In addition, the IR bands for H2O (∼ 3428 cm− 1) and NO3− vibration (∼ 1384 and ∼ 839 cm− 1)
Fig. 4. TEM images and ED patterns of ferrites: (A) MnFe2O4; (B) NiFe2O4; (C) ZnFe2O4.
other. And their average particle sizes for metal ferrites are consistent with the calculated results by XRD observations. The solid materials obtained showed a very narrow particle size distribution for different metal ferrites shown in Fig. 5. Moreover, the average size of MnFe2O4 is the smallest. The SAED pattern shown in the up-left of each picture, should be indexed to further confirm that the structure is a spinel-type.
Fig. 5. Histograms of the respective size distributions in each TEM image: (A) MnFe2O4; (B) NiFe2O4; (C) ZnFe2O4 nanoparticles.
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Fig. 8. Hysteresis loops for nanocrystalline spinel ferrites.
Fig. 6. IR spectra of the samples. (A) a: Zn ferrite precursors; b: Mn ferrite precursors; c: Ni ferrite precursors; (B) a′: ZnFe2O4; b′: MnFe2O4; c′: NiFe2O4.
are also observable due to the residual H2O and NO3−, while characteristic bands of other anions with small quantity are overlapped. Fig. 6B exhibits IR spectra of spinel ferrites obtained by pyrolysis of Me–Fe polyacrylates at 500 °C. It can be seen from Fig. 6B that the absorption bands around 588 and 441 cm− 1 are assigned to Fe–O stretching vibration mode, indicating that the pyrolysis products were inorganic metal
Fig. 7. IR spectra of the samples under the same synthetic conditions, (A) acrylic acid; (B) acrylic acid and metal nitrate without the persulfate catalyst; (C) Me–Fe polyacrylates obtained by the polymerization reaction using the persulfate as catalyst.
oxide species. These results are in good agreement with the analysis of XRD that the oxides consist of spinel ferrites. In order to convincingly demonstrate that the polyacrylates formed, comparing experiments without the persulfate catalyst was performed. It was found that the viscosity of solutions was increased, but the polymerization reaction didn't occur. IR spectra of the samples obtained under the same synthetic conditions were recorded on a Bruker Equinox-55 spectrometer, as shown in Fig. 7. It can be seen from Fig. 7 that Fig. 7B is similar to IR spectrum of acrylic acid except that IR bands for NO3− vibration (∼ 1384 and ∼ 839 cm− 1) can't be observable in Fig. 7A. IR spectra of the samples made with and without the persulfate catalyst were completely different, which is almost in agreement with the literature [33]. Thus, it can be inferred from IR spectra that the polyacrylates can be formed by the polymerization reaction using the persulfate as catalyst. An attractive feature of this method, compared to the ceramic method [16,17], is that it does not involve milling of the precursor which introduces defects and strains into the ferrite and hence affects magnetic properties of the materials. In addition, this pyrolysis method is simple and convenient, greatly feasible for preparation of nanocrystalline spinel ferrites. The complex oxide synthesized from the polymer-pyrolysis method shows a uniform morphology and narrow size distribution, and the preparation process is much simpler without coprecipitating, gelling, and grinding processes. However, it should be mentioned that the pretreatment process in the polymer-pyrolysis method can produce some oxides of nitrogen and CO2 due to the decomposition of the residual NO3− and polyacrylate salts in a similar way as the sol–gel method. But the waste gas could be treated during drying and pyrolysis process. Thus, the polymerpyrolysis method is still a commercially worthy route compared with other methods. The field dependence of the magnetization of spinel ferrites was measured using a vibrating sample magnetometer (VSM) at room temperature with an applied field − 10 kOe ≤ H ≤ 10 kOe, shown in Fig. 8. Clearly, the hysteresis varies with different samples. At low external field, the hysteresis loops of Mn, Ni and Zn ferrites exhibit very small coercivity and remanence. The saturation magnetization is evaluated by extrapolated value of 10 kOe. The saturation magnetization depends on the divalent metal ions and ranges from 48.6 emu/g for MnFe2O4 and
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50.6 emu/g for NiFe2O4 at the applied magnetic field of 10 kOe. However, it can also be seen from Fig. 8 that the magnetization of zinc ferrites increases almost linearly with the given field and shows a lack of saturation at a field as high as 10 kOe. The saturation magnetization of spinel ferrites (MnFe2O4 and NiFe2O4) produced by a polymer-pyrolysis route have much higher values than those of MeFe2O4 produced by conventional ceramic and wet chemical methods at 1100 °C for 10 h [16,18– 20]. This is because the polymer-pyrolysis route possesses several advantages as a precursor to form spinel ferrites that lead to enhanced saturation magnetization. Use of a molecular precursor with cations randomly distributed with no long order facilitates the synthesis of a homogeneous spinel phase [34]. The close structural relationship between the Me–Fe-PA precursors and its calcination products is also a key factor. Furthermore, the fact that the spinel is produced from a single solid precursor rather than a mixture means that the calcination process requires a much shorter time and lower temperature, leading to a lower chance of side reactions to occur. 4. Conclusions In summary, highly homogeneous nanocrystalline spinel ferrites have been produced by the pyrolysis of Me–Fe (Me = Mn, Ni and Zn) co-polymeric precursors in situ polymerized by reaction of the metal salt and acrylic acid. TG-DTA measurements showed that the pyrolysis process could occur at a relatively low temperature to form spinel ferrites. The results from the XRD, TEM and ED measurements revealed that the ferrite powders even calcined at 500 °C have good crystallinity with the spinel structure and an average particle size of 10–30 nm with a uniform size distribution. Moreover, the saturation magnetization of spinel ferrites derived from the Me–Fe PA precursors has been shown to be higher than that of the same materials produced by the conventional ceramic and wet chemical route. Furthermore, this method is easy to operate and can be used for industrial production of highly homogeneous metal oxide magnetic nanomaterials. In addition, the method can be particularly suitable for preparation of mixed metal oxides, such as MnxZn1−xFe2O4, NixMn1−xFe2O4 and CoxMnyZn1−x−yFe2O4. In the specific case of MeFe2O4 reported here, the preliminary magnetic properties are promising for the reason that the synthesis can be easily scaled up for industrial production. Acknowledgements We appreciate the financial support of the National High Technical Research and Development Program of China (No 2003AA305890).
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