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Fe3O4 hybrid microspheres
Materials Letters 65 (2011) 264–267
Contents lists available at ScienceDirect
Materials Letters 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 / m a t l e t
One-step synthesis of Fe-phthalocyanine/Fe3O4 hybrid microspheres Fanbin Meng, Rui Zhao, Yingqing Zhan, Yajie Lei, Jiachun Zhong, Xiaobo Liu ⁎ Research Branch of Functional Polymer Composites, Institute of Microelectronic and Solid State Electronic, University of Electronic Science and Technology of China, Chengdu 610054, PR China
a r t i c l e
i n f o
Article history: Received 11 July 2010 Accepted 27 September 2010 Available online 31 October 2010 Keywords: Nanomaterials Magnetic materials Bis-phthalonitrile oligomer Magnetite Monodispersed hybrid microspheres Electromagnetic properties
a b s t r a c t Fe-phthalocyanine/Fe3O4 hybrid microspheres were synthesized from bis-phthalonitrile and FeCl3·6H2O through a simple and effective solvent-thermal route. The hybrids were monodispersed solid microspheres with diameter of ~ 400 nm. The ferromagnetic signature emerged with the saturated magnetization of ~ 55.7 emu g−1, and the coercive force of ~ 93.7 Oe at 300 k. The addition of bis-phthalonitrile oligomer brought Fe3O4 nanoparticles novel dielectric property: a new dielectric loss peak appeared at ~ 8 GHz. Considering the microwave magnetic loss properties, two microwave magnetic loss peaks were presented at ~ 1.5 GHz and ~10 GHz, the former peak was attributed to the natural properties of the Fe3O4, and the latter originated from the interface effects between the bis-phthalonitrile oligomer and Fe3O4. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Magnetite (Fe3O4) as one of the most important transition magnetic metal oxides has received increasing attention due to its extensive applications. It has been considered as an ideal material for recording media, magnetic heads [1], and a drug-delivery carrier for antitumor therapy [2]. We have previously succeeded in the preparation of hierarchically nanostructured Fe3O4 microspheres which exhibit high magnetic saturation and novel electrometric properties [3]. But the conventional inorganic Fe3O4 particles have the problems of poor processability and easy oxidation. These limit some of the above-mentioned applications. The incorporation of polymers and Fe3O4 particles into the hybrid material can address the above problems [4]. Furthermore, magnetic nanoparticle/organic hybrids can integrate unique properties of the entrapped particles with the existing qualities of the existing qualities of the organic substance and even show novel properties due to synergistic effects derived from the interaction between them [5–7]. The phthalocyanine polymer, as a kind of high temperature materials, has a variety of potential uses for adhesive, electronic and structural applications [8–10]. Furthermore, the most remarkable feature which makes these molecules play an exceptional role in the area of materials science is their versatility [11]. The two hydrogen atoms of the central cavity can be replaced by more than 70 central metals and a variety of substituents can be incorporated, namely the metal phthalocyanine molecules (denoted as MPc) which display
⁎ Corresponding author. Tel./fax: + 86 28 83207326. E-mail address: [email protected] (X. Liu). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.09.075
interesting and useful optical, electronic, catalytic, and biological properties [11,12]. Planar MPc molecules invariably crystallize in the solid state as weakly coupled one-dimensional stacks, forming “herringbone structures”, and therefore, their various physical properties, such as conductivity, magnetism, and optical response, are strongly anisotropic. Based on the versatile reactivity of phthalocyanine with metal and outstanding physical properties of MPc, introducing the phthalocyanine to Fe3O4 nanoparticles is believed to produce a new kind of hybrid material whose electromagnetic properties are different from not only from the single polymer but also from the Fe3O4. In this study, we extended our work to the synthesis of Fephthalocyanine/Fe3O4 hybrid microspheres through a solvent-thermal route. The nano-scale hybrid microspheres showed good dispersity, ferromagnetism and novel electromagnetic properties. 2. Experimental The hybrid microspheres were prepared by one-step process through a solvent-thermal route. The procedure is as follows according to the reference [3] with minor modification: FeCl3·6H2O (6.75 g) was dissolved in EG (200 ml) at room temperature, followed by the addition of PEG 2000 (5.0 g) and bis-phthalonitrile (1.0 g) to form a brown solution with the help of an ultrasonic bath. The NaAc (18.0 g) was slowly added into the solution with vigorous stirring for 30 min and then sealed in a Teflon-lined stainless-steel autoclave. The autoclave was heated to and maintained at 200 °C for 15 h, and allowed to cool to room temperature. The products were filtered and washed several times with ethanol and distilled water, dried at 60 °C over night.
F. Meng et al. / Materials Letters 65 (2011) 264–267
265
Fig. 1. SEM (a), TEM (b), TEM at higher magnification images (c) and EDX image of monodispersed Fe-phthalocyanine/Fe3O4 hybrid microspheres.
The synthesized products were characterized by Fourier transform infrared spectrophotometer (FTIR) (Shimadzu, 8000S) by a KBr pallet, X-ray diffraction (XRD) (Rigaku RINT2400 with Cu Kα radiation), scanning electron microscopy (SEM) (JSM, 6490LV) and transmission electron microscopy (TEM) (Hitachi, H-600). TGA analysis was carried out under N2 atmosphere at a heating rate of 10 °C/min using TA Q50 series analyzer system. Magnetic study was performed by a vibrating sample magnetometer (VSM, Riken Denshi, BHV-525). The samples used for electromagnetic measurements were prepared by homogeneously mixing the hybrid microspheres with wax in a mass ratio of 3:1. The electromagnetic properties were measured using a vector network analyzer (Agilent 8720ET) at 0.5–18 GHz. 3. Results and discussion From the FTIR spectrum, a new absorption band at 837 cm−1 is clearly observed, which is attributed to the typical formation of iron phthalocyanine [13], indicating the interaction between Fe3O4 and the bis-phthalonitrile oligomer in the final hybrid materials. The absorption bands at 1006 cm−1 and 2231 cm−1 correspond to phthalocyanine cycle and cyano groups, respectively. The absorption bands 580 cm−1,
1630 cm−1, 3405 cm−1 are attributed to Fe3O4 [14]. These results indicate that the formation of Fe-phthalocyanine/Fe3O4 hybrid material. Fig. 1(a) shows a representative SEM image of the products. It can be found that the as-prepared hybrids consisted of a large quantity of spheres with a diameter of ~ 400 nm. TEM (Fig. 1(b)) exhibits spherical morphology with good dispersity and a rough appearance, which was consistent with the result of SEM. A TEM image at higher magnification indicates that the obtained hybrid particles are a loose structure (Fig. 2(b)), which should be a result of the coalescence of small particles to grow big particles. The EDX spectrum of the hybrid microspheres (as shown in Fig. 1(d)) reveals the presence of four elements (C, Fe, O, and N), suggesting the presence of bisphthalonitrile oligomer based on Fe-phthalocyanine. Fig. 2 shows the XRD patterns of (a) hierarchical Fe3O4 microspheres (b) Fe-phthalocyanine/Fe3O4 hybrid microspheres. From Fig. 2(b), the main diffraction peaks of hybrid microspheres at (111), (220), (311), (400), (422), (511) and (440) are all consistent with those of hierarchical Fe3O4 microspheres (Fig. 2a). The peak intensity decreases and the full width of the peak increases (Fig. 2a), which indicate the low crystallinity and small crystallite size. It can be predominantly attributed to the existence of the bis-phthalonitrile
Fig. 2. The XRD patterns of (a) hierarchical Fe3O4 microspheres (b) Fe-phthalocyanine/Fe3O4 hybrid microspheres.
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F. Meng et al. / Materials Letters 65 (2011) 264–267
Fig. 3. Magnetization curve of Fe-phthalocyanine/Fe3O4 hybrid microspheres at 300 k.
oligomer which affects the crystal size and crystallinity. About 3.682 wt.% of the organic phthalocyanine was incorporated in the resulting hybrids from TGA. Fe-phthalocyanine/Fe3O4 hybrids have good thermal stability, and the decomposition temperature of the hybrids is about 350 °C. Fig. 3 shows the magnetization hysteresis loop of Fe-phthalocyanine/Fe3O4 at 300 K, which demonstrates its ferromagnetic properties with saturation magnetization (Ms) and coercivity (Hc) values of ~ 55.7 emu g−1 and ~93.7 Oe, respectively. The Ms value obtained here is much lower than that of the pure Fe3O4, which should be attributed to the lower crystallization of magnetite in the hybrid microspheres. Additionally, the magnetic properties allow them to be easily manipulated by an external magnetic field, which is important for their applications in the biological field. Fig. 4 shows the complex permittivity and complex permeability of the prepared hybrid microspheres/wax in the range of 0.5–18 GHz.
From Fig. 4(a), the real part ε′ decreases with an increase of frequency except for a variation observed at ~ 6 GHz which is attributed to the enhancement of charge polarization between adjacent spheres [15]. The imaginary part (ε″) of complex permittivity increases and a resonance peak appears at ~8 GHz which is due to atomic and electronic polarizations [16]. When compared with the hierarchical Fe3O4, an obvious peak of dielectric loss is detected at ~ 8 GHz (shown in Fig. 4c). It can be indicated that the presence of bis-phthalonitrile oligomer brought Fe3O4 nanoparticles novel dielectric property. For the complex permeability shown in Fig. 4b, with the frequency increased to 8 GHz, the real part μ′ decreases monotonically; while imaginary part μ″ exhibits a peak with a maximum value of 1.1 at 1.5 GHz which results in a broad magnetic loss peak from 0.5 to 8 GHz, due to domain wall resonance which is supposed to occur at a lower frequency [17]. However, compared with hierarchical Fe3O4, as shown in Fig. 4d, at the frequency ~ 10 GHz, another magnetic loss peak is clearly observed. We hypothesize that the magnetic loss peak at high frequency is attributed to the interface effects between the bis-phthalonitrile oligomer and Fe3O4. More detailed investigations on electromagnetic properties are still in progress. 4. Conclusion In summary, the nano-scale Fe-phthalocyanine/Fe3O4 hybrid microspheres were synthesized through a simple and effective solventthermal route. The hybrid microspheres showed good dispersibility, ferromagnetism and novel electromagnetic properties. In future study, we will explore the correlations between the synthesis parameters and the physical properties of hybrid microspheres in order to control the characteristics for specific applications. It is believed that the hybrid microspheres have broad applications in the biomedical fields and microwave absorption materials.
Fig. 4. (a) Complex permittivity (b) complex permeability (c) dielectric loss and (d) magnetic loss of hybrid microspheres in the range of 0.5–18 GHz.
F. Meng et al. / Materials Letters 65 (2011) 264–267
References [1] O'Handley RC. Modern Magnetic Materials: Principles and Applications. New York: Wiley; 2000. [2] Alexiou C, Schmid RJ, Jurgons R, Kremer M, Wanner G, Bergemann C, et al. Biophys Lett 2006;35:446–50. [3] Zhao R, Jia K, Wei JJ, Pu JX, Liu XB. Mater Lett 2010;64:457–9. [4] Zhitomirsky I, Niewczas M, Petric A. Mater Lett 2003;57:1045–50. [5] Deng YH, Yang WL, Wang CC, Fu SK. Adv Mater 2003;15:1729–32. [6] Deng Y, Wang L, Yang W, Fu S, Elaīssari A. J Magn Magn Mater 2003;25:69–78. [7] Deng Y, Qi D, Deng C, Zhang X, Zhao D. J Am Chem Soc 2008;130:28–9. [8] Dominguez DD, Jones HN, Keller TM. Polym Compos 2004;25:554–61.
[9] [10] [11] [12] [13] [14] [15] [16] [17]
267
Walton TR, Griffith JR, Reardon J. J Appl Polym Sci 1985;30:2921–39. Sastri SB, Armistead JP, Keller TM. Polym Compos 1996;17:816–22. Torre de la G, Torres T. Claessens C.G. Chem Commun 2007;20:2000–15. McKeon NB. Phthalocyanine Materials. Cambridge: Cambridge University Press; 1998. Ercolani C, Gardini M, Monacelli F, Pennese G, Rossi G. Inorg Chem 1983;22: 2584–9. Kryszewski M, Jeszka JK. Synth Met 1998;94:99–104. Zhang BS, Lu G, Feng Y, Xiong J, Lu HX. J Magn Magn Mater 2006;299:205–10. Verma A, Saxena AK, Dube DC. J Magn Magn Mater 2003;263:228–34. Ni SB, Lin SM, Pan QT, Yang F, Huang K, He DY. J Phys D 2009;42:055004–8.
Contents lists available at ScienceDirect
Materials Letters 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 / m a t l e t
One-step synthesis of Fe-phthalocyanine/Fe3O4 hybrid microspheres Fanbin Meng, Rui Zhao, Yingqing Zhan, Yajie Lei, Jiachun Zhong, Xiaobo Liu ⁎ Research Branch of Functional Polymer Composites, Institute of Microelectronic and Solid State Electronic, University of Electronic Science and Technology of China, Chengdu 610054, PR China
a r t i c l e
i n f o
Article history: Received 11 July 2010 Accepted 27 September 2010 Available online 31 October 2010 Keywords: Nanomaterials Magnetic materials Bis-phthalonitrile oligomer Magnetite Monodispersed hybrid microspheres Electromagnetic properties
a b s t r a c t Fe-phthalocyanine/Fe3O4 hybrid microspheres were synthesized from bis-phthalonitrile and FeCl3·6H2O through a simple and effective solvent-thermal route. The hybrids were monodispersed solid microspheres with diameter of ~ 400 nm. The ferromagnetic signature emerged with the saturated magnetization of ~ 55.7 emu g−1, and the coercive force of ~ 93.7 Oe at 300 k. The addition of bis-phthalonitrile oligomer brought Fe3O4 nanoparticles novel dielectric property: a new dielectric loss peak appeared at ~ 8 GHz. Considering the microwave magnetic loss properties, two microwave magnetic loss peaks were presented at ~ 1.5 GHz and ~10 GHz, the former peak was attributed to the natural properties of the Fe3O4, and the latter originated from the interface effects between the bis-phthalonitrile oligomer and Fe3O4. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Magnetite (Fe3O4) as one of the most important transition magnetic metal oxides has received increasing attention due to its extensive applications. It has been considered as an ideal material for recording media, magnetic heads [1], and a drug-delivery carrier for antitumor therapy [2]. We have previously succeeded in the preparation of hierarchically nanostructured Fe3O4 microspheres which exhibit high magnetic saturation and novel electrometric properties [3]. But the conventional inorganic Fe3O4 particles have the problems of poor processability and easy oxidation. These limit some of the above-mentioned applications. The incorporation of polymers and Fe3O4 particles into the hybrid material can address the above problems [4]. Furthermore, magnetic nanoparticle/organic hybrids can integrate unique properties of the entrapped particles with the existing qualities of the existing qualities of the organic substance and even show novel properties due to synergistic effects derived from the interaction between them [5–7]. The phthalocyanine polymer, as a kind of high temperature materials, has a variety of potential uses for adhesive, electronic and structural applications [8–10]. Furthermore, the most remarkable feature which makes these molecules play an exceptional role in the area of materials science is their versatility [11]. The two hydrogen atoms of the central cavity can be replaced by more than 70 central metals and a variety of substituents can be incorporated, namely the metal phthalocyanine molecules (denoted as MPc) which display
⁎ Corresponding author. Tel./fax: + 86 28 83207326. E-mail address: [email protected] (X. Liu). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.09.075
interesting and useful optical, electronic, catalytic, and biological properties [11,12]. Planar MPc molecules invariably crystallize in the solid state as weakly coupled one-dimensional stacks, forming “herringbone structures”, and therefore, their various physical properties, such as conductivity, magnetism, and optical response, are strongly anisotropic. Based on the versatile reactivity of phthalocyanine with metal and outstanding physical properties of MPc, introducing the phthalocyanine to Fe3O4 nanoparticles is believed to produce a new kind of hybrid material whose electromagnetic properties are different from not only from the single polymer but also from the Fe3O4. In this study, we extended our work to the synthesis of Fephthalocyanine/Fe3O4 hybrid microspheres through a solvent-thermal route. The nano-scale hybrid microspheres showed good dispersity, ferromagnetism and novel electromagnetic properties. 2. Experimental The hybrid microspheres were prepared by one-step process through a solvent-thermal route. The procedure is as follows according to the reference [3] with minor modification: FeCl3·6H2O (6.75 g) was dissolved in EG (200 ml) at room temperature, followed by the addition of PEG 2000 (5.0 g) and bis-phthalonitrile (1.0 g) to form a brown solution with the help of an ultrasonic bath. The NaAc (18.0 g) was slowly added into the solution with vigorous stirring for 30 min and then sealed in a Teflon-lined stainless-steel autoclave. The autoclave was heated to and maintained at 200 °C for 15 h, and allowed to cool to room temperature. The products were filtered and washed several times with ethanol and distilled water, dried at 60 °C over night.
F. Meng et al. / Materials Letters 65 (2011) 264–267
265
Fig. 1. SEM (a), TEM (b), TEM at higher magnification images (c) and EDX image of monodispersed Fe-phthalocyanine/Fe3O4 hybrid microspheres.
The synthesized products were characterized by Fourier transform infrared spectrophotometer (FTIR) (Shimadzu, 8000S) by a KBr pallet, X-ray diffraction (XRD) (Rigaku RINT2400 with Cu Kα radiation), scanning electron microscopy (SEM) (JSM, 6490LV) and transmission electron microscopy (TEM) (Hitachi, H-600). TGA analysis was carried out under N2 atmosphere at a heating rate of 10 °C/min using TA Q50 series analyzer system. Magnetic study was performed by a vibrating sample magnetometer (VSM, Riken Denshi, BHV-525). The samples used for electromagnetic measurements were prepared by homogeneously mixing the hybrid microspheres with wax in a mass ratio of 3:1. The electromagnetic properties were measured using a vector network analyzer (Agilent 8720ET) at 0.5–18 GHz. 3. Results and discussion From the FTIR spectrum, a new absorption band at 837 cm−1 is clearly observed, which is attributed to the typical formation of iron phthalocyanine [13], indicating the interaction between Fe3O4 and the bis-phthalonitrile oligomer in the final hybrid materials. The absorption bands at 1006 cm−1 and 2231 cm−1 correspond to phthalocyanine cycle and cyano groups, respectively. The absorption bands 580 cm−1,
1630 cm−1, 3405 cm−1 are attributed to Fe3O4 [14]. These results indicate that the formation of Fe-phthalocyanine/Fe3O4 hybrid material. Fig. 1(a) shows a representative SEM image of the products. It can be found that the as-prepared hybrids consisted of a large quantity of spheres with a diameter of ~ 400 nm. TEM (Fig. 1(b)) exhibits spherical morphology with good dispersity and a rough appearance, which was consistent with the result of SEM. A TEM image at higher magnification indicates that the obtained hybrid particles are a loose structure (Fig. 2(b)), which should be a result of the coalescence of small particles to grow big particles. The EDX spectrum of the hybrid microspheres (as shown in Fig. 1(d)) reveals the presence of four elements (C, Fe, O, and N), suggesting the presence of bisphthalonitrile oligomer based on Fe-phthalocyanine. Fig. 2 shows the XRD patterns of (a) hierarchical Fe3O4 microspheres (b) Fe-phthalocyanine/Fe3O4 hybrid microspheres. From Fig. 2(b), the main diffraction peaks of hybrid microspheres at (111), (220), (311), (400), (422), (511) and (440) are all consistent with those of hierarchical Fe3O4 microspheres (Fig. 2a). The peak intensity decreases and the full width of the peak increases (Fig. 2a), which indicate the low crystallinity and small crystallite size. It can be predominantly attributed to the existence of the bis-phthalonitrile
Fig. 2. The XRD patterns of (a) hierarchical Fe3O4 microspheres (b) Fe-phthalocyanine/Fe3O4 hybrid microspheres.
266
F. Meng et al. / Materials Letters 65 (2011) 264–267
Fig. 3. Magnetization curve of Fe-phthalocyanine/Fe3O4 hybrid microspheres at 300 k.
oligomer which affects the crystal size and crystallinity. About 3.682 wt.% of the organic phthalocyanine was incorporated in the resulting hybrids from TGA. Fe-phthalocyanine/Fe3O4 hybrids have good thermal stability, and the decomposition temperature of the hybrids is about 350 °C. Fig. 3 shows the magnetization hysteresis loop of Fe-phthalocyanine/Fe3O4 at 300 K, which demonstrates its ferromagnetic properties with saturation magnetization (Ms) and coercivity (Hc) values of ~ 55.7 emu g−1 and ~93.7 Oe, respectively. The Ms value obtained here is much lower than that of the pure Fe3O4, which should be attributed to the lower crystallization of magnetite in the hybrid microspheres. Additionally, the magnetic properties allow them to be easily manipulated by an external magnetic field, which is important for their applications in the biological field. Fig. 4 shows the complex permittivity and complex permeability of the prepared hybrid microspheres/wax in the range of 0.5–18 GHz.
From Fig. 4(a), the real part ε′ decreases with an increase of frequency except for a variation observed at ~ 6 GHz which is attributed to the enhancement of charge polarization between adjacent spheres [15]. The imaginary part (ε″) of complex permittivity increases and a resonance peak appears at ~8 GHz which is due to atomic and electronic polarizations [16]. When compared with the hierarchical Fe3O4, an obvious peak of dielectric loss is detected at ~ 8 GHz (shown in Fig. 4c). It can be indicated that the presence of bis-phthalonitrile oligomer brought Fe3O4 nanoparticles novel dielectric property. For the complex permeability shown in Fig. 4b, with the frequency increased to 8 GHz, the real part μ′ decreases monotonically; while imaginary part μ″ exhibits a peak with a maximum value of 1.1 at 1.5 GHz which results in a broad magnetic loss peak from 0.5 to 8 GHz, due to domain wall resonance which is supposed to occur at a lower frequency [17]. However, compared with hierarchical Fe3O4, as shown in Fig. 4d, at the frequency ~ 10 GHz, another magnetic loss peak is clearly observed. We hypothesize that the magnetic loss peak at high frequency is attributed to the interface effects between the bis-phthalonitrile oligomer and Fe3O4. More detailed investigations on electromagnetic properties are still in progress. 4. Conclusion In summary, the nano-scale Fe-phthalocyanine/Fe3O4 hybrid microspheres were synthesized through a simple and effective solventthermal route. The hybrid microspheres showed good dispersibility, ferromagnetism and novel electromagnetic properties. In future study, we will explore the correlations between the synthesis parameters and the physical properties of hybrid microspheres in order to control the characteristics for specific applications. It is believed that the hybrid microspheres have broad applications in the biomedical fields and microwave absorption materials.
Fig. 4. (a) Complex permittivity (b) complex permeability (c) dielectric loss and (d) magnetic loss of hybrid microspheres in the range of 0.5–18 GHz.
F. Meng et al. / Materials Letters 65 (2011) 264–267
References [1] O'Handley RC. Modern Magnetic Materials: Principles and Applications. New York: Wiley; 2000. [2] Alexiou C, Schmid RJ, Jurgons R, Kremer M, Wanner G, Bergemann C, et al. Biophys Lett 2006;35:446–50. [3] Zhao R, Jia K, Wei JJ, Pu JX, Liu XB. Mater Lett 2010;64:457–9. [4] Zhitomirsky I, Niewczas M, Petric A. Mater Lett 2003;57:1045–50. [5] Deng YH, Yang WL, Wang CC, Fu SK. Adv Mater 2003;15:1729–32. [6] Deng Y, Wang L, Yang W, Fu S, Elaīssari A. J Magn Magn Mater 2003;25:69–78. [7] Deng Y, Qi D, Deng C, Zhang X, Zhao D. J Am Chem Soc 2008;130:28–9. [8] Dominguez DD, Jones HN, Keller TM. Polym Compos 2004;25:554–61.
[9] [10] [11] [12] [13] [14] [15] [16] [17]
267
Walton TR, Griffith JR, Reardon J. J Appl Polym Sci 1985;30:2921–39. Sastri SB, Armistead JP, Keller TM. Polym Compos 1996;17:816–22. Torre de la G, Torres T. Claessens C.G. Chem Commun 2007;20:2000–15. McKeon NB. Phthalocyanine Materials. Cambridge: Cambridge University Press; 1998. Ercolani C, Gardini M, Monacelli F, Pennese G, Rossi G. Inorg Chem 1983;22: 2584–9. Kryszewski M, Jeszka JK. Synth Met 1998;94:99–104. Zhang BS, Lu G, Feng Y, Xiong J, Lu HX. J Magn Magn Mater 2006;299:205–10. Verma A, Saxena AK, Dube DC. J Magn Magn Mater 2003;263:228–34. Ni SB, Lin SM, Pan QT, Yang F, Huang K, He DY. J Phys D 2009;42:055004–8.