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ZnO nanoparticles and their electromagnetic properties
Materials Letters 73 (2012) 143–146
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis of hexagonal CoFe2O4/ZnO nanoparticles and their electromagnetic properties Ji Zheng a,⁎, Xinzhao Song a, Xuejia Liu b, Wei Chen a, Yan Li a, Jing Guo a a b
Department of Materials Science and Engineering, Tianjin University, Tianjin 300072, China Beijing Institute of Aerial Materials, China Aviation Industry Corporation, Beijing 100095, China
a r t i c l e
i n f o
Article history: Received 29 October 2011 Accepted 6 January 2012 Available online 13 January 2012 Keywords: Nanocrystalline materials Nanoparticles Hexagonal CoFe2O4/ZnO Electromagnetic property
a b s t r a c t We present a facile method to synthetize CoFe2O4/ZnO nanoparticles with hexagonal shape through high temperature hydrolysis of chelated zinc diethylene glycol alkoxide complexes in alkaline Diethylene Glycol solution in the presence of CoFe2O4 nanoparticles. X-ray diffraction data show that the obtained nanoparticles are composed of CoFe2O4 and ZnO. The hexagonal particle morphology of the resulting nanoparticles was analyzed using transmission electron microscopy, indicating that the shape of the nanoparticles is hexagonal with a mean diameter below 100 nm. Electromagnetic parameters in GHz frequency range was measured by a network analyzer, which reveals that magnetic and dielectric properties are combined due to both the interfacial polarization process and multiresonance in the as-prepared hexagonal CoFe2O4/ZnO nanoparticles. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Synthesis of magnetic core/shell nanoparticles has long been of scientific and technological interest due to their unique chemical and physical properties and their potential applications in magnetic resonance imaging [1–3], tissue imaging [4], smart drug delivery [5,6], information storage [7], and electromagnetic compatibility [8]. Important progress has been made in chemical or physical rout synthesis of magnetic core/shell nanoparticles of metals and oxides [9]. Several approaches including precipitation [10], microemulsion [11], sol–gel processes [12], hydrothermal rout [13] and arc discharge [14] have been investigated and utilized for the synthesis of core/ shell nanostructure. These heterostructured core/shell systems could exhibit multiple functions and unique magnetic or electrical properties different from those of cores or shells. The performance of these materials will be dependent on the chemical and physical characteristics of the nanoparticles and their surfaces, which mainly depend on the method of synthesis chosen for their preparation [15]. ZnO is an important semiconductor with a wide band-gap of 3.37 eV at room temperature, which has been used in the fabrication of many composites, such as Au–ZnO core-shell composites [16], Ni/ ZnO nanocapsules [17], ZnO/SiO2 nanocomposites [18], etc. In addition, many doping attempts into ZnO to form diluted magnetic semiconductor or materials with Curie temperature higher than room temperature have been carried out. CoFe2O4 is a material with hard magnetic property, high coercivity, moderate saturation magnetization, and high
⁎ Corresponding author. Tel.: + 86 13821217929; fax: + 86 2227404724. E-mail address: [email protected] (J. Zheng). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.01.035
cubic magnetocrystalline anisotropy [19,20]. Until now, many coreshell structure composite nanoparticles showing magnetic properties have been reported, such as TiO2/BaFe12O19 [21], TiO2/Fe3O4 [22], however, core-shell structure nanocomposites with properties of magnetism based on CoFe2O4 and ZnO has rarely been reported. Based on those factors, the synthesis of CoFe2O4/ZnO nanoparticles and their electromagnetic properties were examined. Here we present a facile synthesis of novel core/shell structured CoFe2O4/ZnO hexagonal composite nanoparticles by a two-step solution process. The as-prepared nanoparticles exhibit magnetic loss in GHz frequency range due to the magnetic cores, while the dielectric loss might be provided by zinc oxides and the interfacial polarization between the magnetic cores and the zinc oxides.
2. Experimental 2.1. Synthesis of CoFe2O4 nanoparticles Nearly monodisperse CoFe2O4 core nanocrystals were prepared by a modified low-temperature liquid/liquid interface reaction process. Firstly, in a typical synthesis of the CoFe2O4 cores, 5.76 g FeCl3·6H2O and 2.35 g CoCl2·6H2O were dissolved in a mixture solvent composed of 30 ml ethanol, 29 ml oleic acid, 30 ml distilled water and 20 ml xylene. 3.4 g NaOH was dissolved in 20 ml distilled water, and the alkaline solution was added to the former mixture solution under vigorous magnetic stirring. Then, the mixture solution was heated to 80 °C under continuously magnetic stirring for 1 h. When the reaction was completed, the upper organic layer containing the iron/cobalt–oleate complex was washed three times with 50 ml distilled
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another 1 h. The mixture was slowly turning black indicating the formation of the CoFe2O4 nanocrystals. The product was then precipitated with ethanol, centrifuged (5000 rpm, 15 min) to remove the solvent, washed by ethanol three times, and redispersed into diethylene glycol (DEG), resulting in CoFe2O4 colloidal solution. 2.2. Preparation of hexagonal CoFe2O4/ZnO nanoparticles
Fig. 1. X-ray diffraction patterns of (a) CoFe2O4/ZnO hexagonal nanoparticles, (b) CoFe2O4 cores.
ZnCl2/DEG solution was obtained by dissolving ZnCl2 (1.5 g) in 20 ml DEG in the presence of Polyvinyl Pyrrolidone K-30 (0.02 g), and then mixed with the alkaline DEG solution which was obtained by dissolving NaOH (0.9 g) in 20 ml DEG. The CoFe2O4 colloidal solution (100 ml solution with 1 g CoFe2O4) was loaded into a 250 ml three-neck round-bottom flask coupled with a condenser and a syringe with 20 ml mixed alkaline solution. After being degassed and purged with argon 3 times, the CoFe2O4 colloidal solution was slowly heated to 210 °C under moderate stirring and then the previous mixed solution was quickly injected into flask. The system was kept at this temperature for 60 min. The heat source was then removed and the solution was allowed to cool down to room temperature. Subsequently, the black CoFe2O4/ZnO precipitate was separated by magnet from the product mixture and washed by ethanol several times. The black precipitate was dried at 60 °C resulting in CoFe2O4/ ZnO composite powders. 2.3. Characterization
water in a separatory funnel. After that, ethanol was removed, resulting in solution composed of xylene and iron/cobalt–oleate complex. In the second step, 5.0 g NaOH was dissolved in 150 ml distilled water and was heated to 90 °C. The hot alkaline solution became brown soon after the iron/cobalt–oleate complex solution was added. The mixture was vigorously stirring and kept at 90 °C for
The crystal structure of CoFe2O4/ZnO sample was investigated by a Rigaku D/max 2500v/pc X-ray diffractometer with Cu Kα radiation (λ = 0.15418 nm). TEM images were obtained through a Tecnai G2 F20 Transmission Electron Microscope (TEM). Samples for TEM analysis were prepared by spreading a drop of as-prepared products
Fig. 2. Representative TEM images of (a)/(b) CoFe2O4 cores and (c)/(d) CoFe2O4/ZnO hexagonal nanoparticles.
J. Zheng et al. / Materials Letters 73 (2012) 143–146
dilute dispersion on amorphous carbon-coated copper grids and then dried in air. The specimen for measurement of electromagnetic parameters was prepared by uniformly mixing 50 wt.% CoFe2O4/ZnO powder with paraffin and made into a toroidal shape (φout:7.00 mm and φin:3.04 mm), with a height of 2.00 mm. The relative permittivity and permeability values of the specimen were measured between 2 and 18 GHz using a network analyzer Agilent HP-8722ES. 3. Results and discussion The XRD patterns of CoFe2O4 and CoFe2O4/ZnO nanoparticles are shown in Fig. 1. Fig. 1(b) shows the XRD patterns of pure CoFe2O4 cores obtained by the liquid interface reaction process. Peaks observed in Fig. 1(b) were well indexed to the spinel cobalt ferrite (JCPDS file No. 22-1086). Fig. 1(a) shows the XRD pattern of the as-prepared CoFe2O4/ZnO sample. Compared with Fig. 1(b), four extra peaks were observed in Fig. 1(a). The position of these peaks match well with (100), (002), (102), and (112) planes of the data for bulk ZnO (JCPDS file No. 36-1451). No peaks corresponding to the impurities are detected, indicating that pure CoFe2O4/ZnO heterostructure were formed during the synthesis process. The results calculated by Scherrer formula demonstrate that the average particle diameters of CoFe2O4 and ZnO powders are 10 nm and 16 nm respectively. Fig. 2(a) and (b) shows the representative TEM images of asprepared CoFe2O4 cores. It is clear that the liquid/liquid interface reaction process leads to nearly monodisperse CoFe2O4 nanoparticles with very narrow size distribution. During the process of hightemperature hydrolysis of chelated zinc diethylene glycol alkoxide complexes in the CoFe2O4 colloidal solution, the CoFe2O4 nanoparticles act as heterogeneous nucleation site, which result in the formation of CoFe2O4/ZnO heterostructure. Fig. 2(c) and (d) shows the TEM images of CoFe2O4/ZnO heterostructure nanoparticles. It can be seen that the shape of the obtained nanoparticles is hexagonal with a mean diameter below 100 nm. The average particle sizes of CoFe2O4 and ZnO obtained from TEM are consistent with those calculated from the XRD peak broadening. It is interesting that the formation of ZnO under the above synthesis conditions has resulted in CoFe2O4/ZnO compound particles with hexagonal shape [23]. When the temperature increases up to 210 °C, the thermal decomposition starts from Zn(OH)2 to ZnO on the surface of CoFe2O4 nanoparticles in the alkaline DEG solution. The as-existed CoFe2O4 nanoparticles act as crystal nucleus during the thermal decomposition. With time, grain–boundary diffusion takes place between CoFe2O4 and ZnO. The grain–boundary diffusion directly leads to the emergence of lattice defects in CoFe2O4 and ZnO, these lattice defects change the lattice constant [24]. Therefore, a diffusion layer is formed between CoFe2O4 and ZnO, the CoFe2O4/ZnO hexagonal nanoparticles are formed. Based on the characterized properties and crystal structure of the as-prepared products, the synthesis mechanism of the CoFe2O4–ZnO nanoparticles was hypothesized and needs to further study. The frequency dependency on the real part(ε′)and imaginary part (ε′′) of the complex permittivity(ε)for paraffin-CoFe2O4/ZnO nanocomposite sample is shown in Fig. 3(a). The values of ε′ declined from 5.24 to 4.49 in the 2–18 GHz range, while ε′′ shows an increase from 0.46 to 0.65 in the whole frequency range. As shown in Fig. 3(a), there are frequency-intervals on the permittivity curve which presents resonant characteristics; three peaks can also be observed near the resonant frequencies on the ε′′ curve. These phenomena are the typical characteristics of nonlinear resonant behaviors [17]. The resonant frequencies of ε in the current frequency range are about 3.8, 9.2 and 13.8 GHz, respectively. The dielectric loss, a ratio of ε′′ to ε′, is plotted in Fig. 3(b). The dielectric loss also exhibits three peaks with peak values of 0.12, 0.13 and 0.16 at about 3.8, 9.2 and 13.8 GHz, respectively. In addition, the fluctuations of
145
the real and imaginary permittivities in the range of 2–18 GHz are ascribed to displacement current lag at the “core/shell” interface, since the interfacial polarization process and the associated relaxation process give rise to a loss mechanism in CoFe2O4/ZnO heterogeneous system, which is similar to in Fe(C) and Ni(ZnO) nanocapsules [17,25]. Fig. 3(c) shows the real (μ′) and imaginary part (μ′′) values of relative complex permeability for paraffin-CoFe2O4/ZnO composite samples as a function of frequency in the 2–18 GHz range. It reveals that the value of real part (μ′) is in the range of 0.94–1.04. Meanwhile, the image part (μ′′) is less than 0.1 over 2–18 GHz and exhibits broad multiresonance peaks at 2–16 GHz with a maximum value of 0.05 at 13.8 GHz, which reveals that natural resonance occurs in the CoFe2O4/ZnO nanoparticles. Furthermore, the multiresonance peaks shown in Fig. 3(c) can also be attributed to the small size of the particles, the surface effect, and spin-wave excitations
Fig. 3. (a) Frequency dependence of real (ε′) and imaginary (ε′′) parts of relative complex permittivity for the paraffin-CoFe2O4/ZnO. (b) Frequency dependence of dielectric loss (ε′′ /ε′) for the paraffin-CoFe2O4/ZnO. Frequency dependence of real (μ′) and imaginary (μ′′) parts of relative complex permeability for the paraffin-CoFe2O4/ZnO.
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defined as “exchange interaction” resonance [3], where the resonance frequency is dependent on the radii of the particles. The resonance frequencies are shifted towards higher frequencies (14 GHz) as the constraint of this symmetry is removed when the particle size become extremely small. 4. Conclusions In summary, we have developed a two-step solution-phase method to synthesize hexagonal CoFe2O4/ZnO heterostructure by pyrohydrolysis of zinc precursors in DEG in the present of CoFe2O4 nanoparticles. The XRD results and TEM images indicated that the reaction offered an effective way to fabricate hexagonal CoFe2O4/ZnO heterostructure. Electromagnetic measurements showed that the heterostructure exhibited interfacial polarization effect and multiresonance behavior at room temperature, indicating these novel hexagonal CoFe2O4/ZnO heterostructure may have significant potential for the application in the fields of microwave devices, spintronic devices as well as electromagnetic compatibility applications. References [1] Lai Chih-Wei, Wang Yu-Hsiu, Lai Cheng-Hsuan, Yang Meng-Ju, Chen Chun-Yen, Chou Pi-Tai, et al. Small 2008;4:218–24. [2] Xuan Shouhu, Hao Lingyun, Jiang Wanquan, Gong Xinglong, Hu Yuan, Chen Zuyao. Nanotechnology 2007;18:035602 (6 pp). [3] Cannas Carla, Musinu Anna, Ardu Andrea, Orru Federica, Peddis Davide. Chem Mater 2010;22:3353–61.
[4] Seo WS, Lee JH, Sun XM, Suzuki Y, Mann D, Liu Z, et al. Nat Mater 2006;5:971–6. [5] Chan Juliana M, Zhang Liangfang, Yuet Kai P, Liao Grace, Rhee June-Wha, Langer Robert, et al. Biomaterials 2009;30:1627–34. [6] Gong Xiuqing, Peng Suili, Wen Weijia, Sheng Ping, Li Weihua. Adv Funct Mater 2008;18:1–6. [7] Sun SH, Murray CB, Weller D, Folks L, Moser A. Science 2000;287:1989–92. [8] Liu XG, Geng DY, Meng H, Shang PJ, Zhang ZD. Appl Phys Lett 2008;92:173117. [9] Lee Chung-Che, Chen Dong-Hwang. Appl Phys Lett 2007;90:193102. [10] Cao Jing, Wuyou Fu, Yang Haibin, Qingjiang Yu, Zhang Yanyan, Wang Shuangming. Mater Sci Eng B 2010;175:56–9. [11] Lee DC, Mikulec FV, Pelaez JM, Koo B, Korgel BA. J Phys Chem B 2006;110: 11160–6. [12] Barnakov YA, Yu MH, Rosenzweig Z. Langmuir 2005;21:7524–7. [13] Wu XF, Song HY, Yoon JM, Yu YT, Chen YFn. Langmuir 2009;25:6438–47. [14] Dong XL, Zhang ZD, Jin SR, Kim BK. J Appl Phys 1999;86:6701–6. [15] Roca AG, Morales MP, O'Grady K, Serna CJ. Nanotechnology 2006;17:2783–8. [16] Sun Lanlan, Wei Gang, Song Yonghai, Liu Zhiguo, Wang Li, Li Zhuang. Mater Lett 2006;60:1291–5. [17] Liu XG, Jiang JJ, Geng DY, Li BQ, Han Z, Liu W, et al. Appl Phys Lett 2009;94: 053119. [18] Cao Mao-Sheng, Shi Xiao-Ling, Fang Xiao-Yong, Jin Hai-Bo, Hou Zhi-Ling, Zhou Wei. Appl Phys Lett 2007;91:203110. [19] Huang X-H, Chen Z-H. Scr Mater 2006;54:169–73. [20] Yan Xingbin, Chen Jiangtao, Xue Qunji, Miele Philippe. Microporous Mesoporous Mater 2010;135:137–42. [21] Fu WY, Yang HB, Li MH, et al. Mater Lett 2006;60:2723. [22] Sun XY, Wang LJ, Wang J, et al. Spectrosc Spectra Anal 2007;27:777. [23] Caruntu Daniela, Caruntu Gabriel, Chen Yuxi, O'Connor Charles J, Goloverda Galina, Kolesnichenko Vladimir L. Chem Mater 2004;16:5527–34. [24] Tang B, Deng H, Shui ZW, et al. Acta Phys Sin 2007;56:5176. [25] Zhang XF, Dong XL, Huang H, Lv B, Lei JP, Choi CJ. J Phys D: Appl Phys 2007;40: 5383–7.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis of hexagonal CoFe2O4/ZnO nanoparticles and their electromagnetic properties Ji Zheng a,⁎, Xinzhao Song a, Xuejia Liu b, Wei Chen a, Yan Li a, Jing Guo a a b
Department of Materials Science and Engineering, Tianjin University, Tianjin 300072, China Beijing Institute of Aerial Materials, China Aviation Industry Corporation, Beijing 100095, China
a r t i c l e
i n f o
Article history: Received 29 October 2011 Accepted 6 January 2012 Available online 13 January 2012 Keywords: Nanocrystalline materials Nanoparticles Hexagonal CoFe2O4/ZnO Electromagnetic property
a b s t r a c t We present a facile method to synthetize CoFe2O4/ZnO nanoparticles with hexagonal shape through high temperature hydrolysis of chelated zinc diethylene glycol alkoxide complexes in alkaline Diethylene Glycol solution in the presence of CoFe2O4 nanoparticles. X-ray diffraction data show that the obtained nanoparticles are composed of CoFe2O4 and ZnO. The hexagonal particle morphology of the resulting nanoparticles was analyzed using transmission electron microscopy, indicating that the shape of the nanoparticles is hexagonal with a mean diameter below 100 nm. Electromagnetic parameters in GHz frequency range was measured by a network analyzer, which reveals that magnetic and dielectric properties are combined due to both the interfacial polarization process and multiresonance in the as-prepared hexagonal CoFe2O4/ZnO nanoparticles. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Synthesis of magnetic core/shell nanoparticles has long been of scientific and technological interest due to their unique chemical and physical properties and their potential applications in magnetic resonance imaging [1–3], tissue imaging [4], smart drug delivery [5,6], information storage [7], and electromagnetic compatibility [8]. Important progress has been made in chemical or physical rout synthesis of magnetic core/shell nanoparticles of metals and oxides [9]. Several approaches including precipitation [10], microemulsion [11], sol–gel processes [12], hydrothermal rout [13] and arc discharge [14] have been investigated and utilized for the synthesis of core/ shell nanostructure. These heterostructured core/shell systems could exhibit multiple functions and unique magnetic or electrical properties different from those of cores or shells. The performance of these materials will be dependent on the chemical and physical characteristics of the nanoparticles and their surfaces, which mainly depend on the method of synthesis chosen for their preparation [15]. ZnO is an important semiconductor with a wide band-gap of 3.37 eV at room temperature, which has been used in the fabrication of many composites, such as Au–ZnO core-shell composites [16], Ni/ ZnO nanocapsules [17], ZnO/SiO2 nanocomposites [18], etc. In addition, many doping attempts into ZnO to form diluted magnetic semiconductor or materials with Curie temperature higher than room temperature have been carried out. CoFe2O4 is a material with hard magnetic property, high coercivity, moderate saturation magnetization, and high
⁎ Corresponding author. Tel.: + 86 13821217929; fax: + 86 2227404724. E-mail address: [email protected] (J. Zheng). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.01.035
cubic magnetocrystalline anisotropy [19,20]. Until now, many coreshell structure composite nanoparticles showing magnetic properties have been reported, such as TiO2/BaFe12O19 [21], TiO2/Fe3O4 [22], however, core-shell structure nanocomposites with properties of magnetism based on CoFe2O4 and ZnO has rarely been reported. Based on those factors, the synthesis of CoFe2O4/ZnO nanoparticles and their electromagnetic properties were examined. Here we present a facile synthesis of novel core/shell structured CoFe2O4/ZnO hexagonal composite nanoparticles by a two-step solution process. The as-prepared nanoparticles exhibit magnetic loss in GHz frequency range due to the magnetic cores, while the dielectric loss might be provided by zinc oxides and the interfacial polarization between the magnetic cores and the zinc oxides.
2. Experimental 2.1. Synthesis of CoFe2O4 nanoparticles Nearly monodisperse CoFe2O4 core nanocrystals were prepared by a modified low-temperature liquid/liquid interface reaction process. Firstly, in a typical synthesis of the CoFe2O4 cores, 5.76 g FeCl3·6H2O and 2.35 g CoCl2·6H2O were dissolved in a mixture solvent composed of 30 ml ethanol, 29 ml oleic acid, 30 ml distilled water and 20 ml xylene. 3.4 g NaOH was dissolved in 20 ml distilled water, and the alkaline solution was added to the former mixture solution under vigorous magnetic stirring. Then, the mixture solution was heated to 80 °C under continuously magnetic stirring for 1 h. When the reaction was completed, the upper organic layer containing the iron/cobalt–oleate complex was washed three times with 50 ml distilled
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another 1 h. The mixture was slowly turning black indicating the formation of the CoFe2O4 nanocrystals. The product was then precipitated with ethanol, centrifuged (5000 rpm, 15 min) to remove the solvent, washed by ethanol three times, and redispersed into diethylene glycol (DEG), resulting in CoFe2O4 colloidal solution. 2.2. Preparation of hexagonal CoFe2O4/ZnO nanoparticles
Fig. 1. X-ray diffraction patterns of (a) CoFe2O4/ZnO hexagonal nanoparticles, (b) CoFe2O4 cores.
ZnCl2/DEG solution was obtained by dissolving ZnCl2 (1.5 g) in 20 ml DEG in the presence of Polyvinyl Pyrrolidone K-30 (0.02 g), and then mixed with the alkaline DEG solution which was obtained by dissolving NaOH (0.9 g) in 20 ml DEG. The CoFe2O4 colloidal solution (100 ml solution with 1 g CoFe2O4) was loaded into a 250 ml three-neck round-bottom flask coupled with a condenser and a syringe with 20 ml mixed alkaline solution. After being degassed and purged with argon 3 times, the CoFe2O4 colloidal solution was slowly heated to 210 °C under moderate stirring and then the previous mixed solution was quickly injected into flask. The system was kept at this temperature for 60 min. The heat source was then removed and the solution was allowed to cool down to room temperature. Subsequently, the black CoFe2O4/ZnO precipitate was separated by magnet from the product mixture and washed by ethanol several times. The black precipitate was dried at 60 °C resulting in CoFe2O4/ ZnO composite powders. 2.3. Characterization
water in a separatory funnel. After that, ethanol was removed, resulting in solution composed of xylene and iron/cobalt–oleate complex. In the second step, 5.0 g NaOH was dissolved in 150 ml distilled water and was heated to 90 °C. The hot alkaline solution became brown soon after the iron/cobalt–oleate complex solution was added. The mixture was vigorously stirring and kept at 90 °C for
The crystal structure of CoFe2O4/ZnO sample was investigated by a Rigaku D/max 2500v/pc X-ray diffractometer with Cu Kα radiation (λ = 0.15418 nm). TEM images were obtained through a Tecnai G2 F20 Transmission Electron Microscope (TEM). Samples for TEM analysis were prepared by spreading a drop of as-prepared products
Fig. 2. Representative TEM images of (a)/(b) CoFe2O4 cores and (c)/(d) CoFe2O4/ZnO hexagonal nanoparticles.
J. Zheng et al. / Materials Letters 73 (2012) 143–146
dilute dispersion on amorphous carbon-coated copper grids and then dried in air. The specimen for measurement of electromagnetic parameters was prepared by uniformly mixing 50 wt.% CoFe2O4/ZnO powder with paraffin and made into a toroidal shape (φout:7.00 mm and φin:3.04 mm), with a height of 2.00 mm. The relative permittivity and permeability values of the specimen were measured between 2 and 18 GHz using a network analyzer Agilent HP-8722ES. 3. Results and discussion The XRD patterns of CoFe2O4 and CoFe2O4/ZnO nanoparticles are shown in Fig. 1. Fig. 1(b) shows the XRD patterns of pure CoFe2O4 cores obtained by the liquid interface reaction process. Peaks observed in Fig. 1(b) were well indexed to the spinel cobalt ferrite (JCPDS file No. 22-1086). Fig. 1(a) shows the XRD pattern of the as-prepared CoFe2O4/ZnO sample. Compared with Fig. 1(b), four extra peaks were observed in Fig. 1(a). The position of these peaks match well with (100), (002), (102), and (112) planes of the data for bulk ZnO (JCPDS file No. 36-1451). No peaks corresponding to the impurities are detected, indicating that pure CoFe2O4/ZnO heterostructure were formed during the synthesis process. The results calculated by Scherrer formula demonstrate that the average particle diameters of CoFe2O4 and ZnO powders are 10 nm and 16 nm respectively. Fig. 2(a) and (b) shows the representative TEM images of asprepared CoFe2O4 cores. It is clear that the liquid/liquid interface reaction process leads to nearly monodisperse CoFe2O4 nanoparticles with very narrow size distribution. During the process of hightemperature hydrolysis of chelated zinc diethylene glycol alkoxide complexes in the CoFe2O4 colloidal solution, the CoFe2O4 nanoparticles act as heterogeneous nucleation site, which result in the formation of CoFe2O4/ZnO heterostructure. Fig. 2(c) and (d) shows the TEM images of CoFe2O4/ZnO heterostructure nanoparticles. It can be seen that the shape of the obtained nanoparticles is hexagonal with a mean diameter below 100 nm. The average particle sizes of CoFe2O4 and ZnO obtained from TEM are consistent with those calculated from the XRD peak broadening. It is interesting that the formation of ZnO under the above synthesis conditions has resulted in CoFe2O4/ZnO compound particles with hexagonal shape [23]. When the temperature increases up to 210 °C, the thermal decomposition starts from Zn(OH)2 to ZnO on the surface of CoFe2O4 nanoparticles in the alkaline DEG solution. The as-existed CoFe2O4 nanoparticles act as crystal nucleus during the thermal decomposition. With time, grain–boundary diffusion takes place between CoFe2O4 and ZnO. The grain–boundary diffusion directly leads to the emergence of lattice defects in CoFe2O4 and ZnO, these lattice defects change the lattice constant [24]. Therefore, a diffusion layer is formed between CoFe2O4 and ZnO, the CoFe2O4/ZnO hexagonal nanoparticles are formed. Based on the characterized properties and crystal structure of the as-prepared products, the synthesis mechanism of the CoFe2O4–ZnO nanoparticles was hypothesized and needs to further study. The frequency dependency on the real part(ε′)and imaginary part (ε′′) of the complex permittivity(ε)for paraffin-CoFe2O4/ZnO nanocomposite sample is shown in Fig. 3(a). The values of ε′ declined from 5.24 to 4.49 in the 2–18 GHz range, while ε′′ shows an increase from 0.46 to 0.65 in the whole frequency range. As shown in Fig. 3(a), there are frequency-intervals on the permittivity curve which presents resonant characteristics; three peaks can also be observed near the resonant frequencies on the ε′′ curve. These phenomena are the typical characteristics of nonlinear resonant behaviors [17]. The resonant frequencies of ε in the current frequency range are about 3.8, 9.2 and 13.8 GHz, respectively. The dielectric loss, a ratio of ε′′ to ε′, is plotted in Fig. 3(b). The dielectric loss also exhibits three peaks with peak values of 0.12, 0.13 and 0.16 at about 3.8, 9.2 and 13.8 GHz, respectively. In addition, the fluctuations of
145
the real and imaginary permittivities in the range of 2–18 GHz are ascribed to displacement current lag at the “core/shell” interface, since the interfacial polarization process and the associated relaxation process give rise to a loss mechanism in CoFe2O4/ZnO heterogeneous system, which is similar to in Fe(C) and Ni(ZnO) nanocapsules [17,25]. Fig. 3(c) shows the real (μ′) and imaginary part (μ′′) values of relative complex permeability for paraffin-CoFe2O4/ZnO composite samples as a function of frequency in the 2–18 GHz range. It reveals that the value of real part (μ′) is in the range of 0.94–1.04. Meanwhile, the image part (μ′′) is less than 0.1 over 2–18 GHz and exhibits broad multiresonance peaks at 2–16 GHz with a maximum value of 0.05 at 13.8 GHz, which reveals that natural resonance occurs in the CoFe2O4/ZnO nanoparticles. Furthermore, the multiresonance peaks shown in Fig. 3(c) can also be attributed to the small size of the particles, the surface effect, and spin-wave excitations
Fig. 3. (a) Frequency dependence of real (ε′) and imaginary (ε′′) parts of relative complex permittivity for the paraffin-CoFe2O4/ZnO. (b) Frequency dependence of dielectric loss (ε′′ /ε′) for the paraffin-CoFe2O4/ZnO. Frequency dependence of real (μ′) and imaginary (μ′′) parts of relative complex permeability for the paraffin-CoFe2O4/ZnO.
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defined as “exchange interaction” resonance [3], where the resonance frequency is dependent on the radii of the particles. The resonance frequencies are shifted towards higher frequencies (14 GHz) as the constraint of this symmetry is removed when the particle size become extremely small. 4. Conclusions In summary, we have developed a two-step solution-phase method to synthesize hexagonal CoFe2O4/ZnO heterostructure by pyrohydrolysis of zinc precursors in DEG in the present of CoFe2O4 nanoparticles. The XRD results and TEM images indicated that the reaction offered an effective way to fabricate hexagonal CoFe2O4/ZnO heterostructure. Electromagnetic measurements showed that the heterostructure exhibited interfacial polarization effect and multiresonance behavior at room temperature, indicating these novel hexagonal CoFe2O4/ZnO heterostructure may have significant potential for the application in the fields of microwave devices, spintronic devices as well as electromagnetic compatibility applications. References [1] Lai Chih-Wei, Wang Yu-Hsiu, Lai Cheng-Hsuan, Yang Meng-Ju, Chen Chun-Yen, Chou Pi-Tai, et al. Small 2008;4:218–24. [2] Xuan Shouhu, Hao Lingyun, Jiang Wanquan, Gong Xinglong, Hu Yuan, Chen Zuyao. Nanotechnology 2007;18:035602 (6 pp). [3] Cannas Carla, Musinu Anna, Ardu Andrea, Orru Federica, Peddis Davide. Chem Mater 2010;22:3353–61.
[4] Seo WS, Lee JH, Sun XM, Suzuki Y, Mann D, Liu Z, et al. Nat Mater 2006;5:971–6. [5] Chan Juliana M, Zhang Liangfang, Yuet Kai P, Liao Grace, Rhee June-Wha, Langer Robert, et al. Biomaterials 2009;30:1627–34. [6] Gong Xiuqing, Peng Suili, Wen Weijia, Sheng Ping, Li Weihua. Adv Funct Mater 2008;18:1–6. [7] Sun SH, Murray CB, Weller D, Folks L, Moser A. Science 2000;287:1989–92. [8] Liu XG, Geng DY, Meng H, Shang PJ, Zhang ZD. Appl Phys Lett 2008;92:173117. [9] Lee Chung-Che, Chen Dong-Hwang. Appl Phys Lett 2007;90:193102. [10] Cao Jing, Wuyou Fu, Yang Haibin, Qingjiang Yu, Zhang Yanyan, Wang Shuangming. Mater Sci Eng B 2010;175:56–9. [11] Lee DC, Mikulec FV, Pelaez JM, Koo B, Korgel BA. J Phys Chem B 2006;110: 11160–6. [12] Barnakov YA, Yu MH, Rosenzweig Z. Langmuir 2005;21:7524–7. [13] Wu XF, Song HY, Yoon JM, Yu YT, Chen YFn. Langmuir 2009;25:6438–47. [14] Dong XL, Zhang ZD, Jin SR, Kim BK. J Appl Phys 1999;86:6701–6. [15] Roca AG, Morales MP, O'Grady K, Serna CJ. Nanotechnology 2006;17:2783–8. [16] Sun Lanlan, Wei Gang, Song Yonghai, Liu Zhiguo, Wang Li, Li Zhuang. Mater Lett 2006;60:1291–5. [17] Liu XG, Jiang JJ, Geng DY, Li BQ, Han Z, Liu W, et al. Appl Phys Lett 2009;94: 053119. [18] Cao Mao-Sheng, Shi Xiao-Ling, Fang Xiao-Yong, Jin Hai-Bo, Hou Zhi-Ling, Zhou Wei. Appl Phys Lett 2007;91:203110. [19] Huang X-H, Chen Z-H. Scr Mater 2006;54:169–73. [20] Yan Xingbin, Chen Jiangtao, Xue Qunji, Miele Philippe. Microporous Mesoporous Mater 2010;135:137–42. [21] Fu WY, Yang HB, Li MH, et al. Mater Lett 2006;60:2723. [22] Sun XY, Wang LJ, Wang J, et al. Spectrosc Spectra Anal 2007;27:777. [23] Caruntu Daniela, Caruntu Gabriel, Chen Yuxi, O'Connor Charles J, Goloverda Galina, Kolesnichenko Vladimir L. Chem Mater 2004;16:5527–34. [24] Tang B, Deng H, Shui ZW, et al. Acta Phys Sin 2007;56:5176. [25] Zhang XF, Dong XL, Huang H, Lv B, Lei JP, Choi CJ. J Phys D: Appl Phys 2007;40: 5383–7.