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Fabrication, magnetic and luminescent properties of CoFe2O4@SiO2@Y2O3:Dy3+ composites
Journal of Alloys and Compounds 589 (2014) 76–81
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Fabrication, magnetic and luminescent properties of CoFe2O4@SiO2@Y2O3:Dy3+ composites Xiaozhen Ren a, Lizhu Tong a, Xiaodong Chen a, Hong Ding b, Hua Yang a,⇑ a b
College of Chemistry, Jilin University, Changchun 130012, China State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China
a r t i c l e
i n f o
Article history: Received 28 September 2013 Received in revised form 2 November 2013 Accepted 15 November 2013 Available online 26 November 2013 Keywords: Core–shell structure Nanoparticles Ferrimagnetism Luminescence
a b s t r a c t Depositing the luminescent Y2O3:Dy3+ onto the surface of ferrimagnetic CoFe2O4@SiO2 composites obtains the difunctional CoFe2O4@SiO2@Y2O3:Dy3+ nanoparticles. In spite to keeping the yellow luminescence property (4F9/2 ? 6H13/2 in 572 nm), the nanoparticles display the strongest luminescent intensity at 600 °C of calcination temperature due to the influence of ferrimagnetic CoFe2O4 particles. The increasing of maturation magnetization (Ms) and the nearly unchanging of coercivity (Hc) in the nanoparticles with the increasing calcination temperature can be contribute to the increasing of CoFe2O4 particles size. By altering annealing temperature, the mass rate of Y2O3:Dy3+ over CoFe2O4@SiO2, and the doping concentration of Dy3+, the size, the magnetic, and luminescence properties of nanoparticles can be controlled. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent years, multifunctional nanocomposites have attracted increasing attention and have been applied in many important research filed, such as drug delivery [1–3], diagnostic analysis [4,5]. Moreover, the nanoparticles with magnetic and luminescence properties provide an excellent platform to integrate different functional properties into one single entity. As the magnetic material, bulk CoFe2O4 is ferrimagnetic and has an inverse spinel structure with Co2+ ions distributed on B sites and Fe3+ ions distributed on A and B sites. Cobalt ferrite is a hard magnetic material with very high cubic magnetocrystalline anisotropy (Keff = 1.8 107 erg/cm3), high coercivity (Hc = 5.4 kOe), and moderate saturation magnetization (80 emu/g) [6]. These properties make it a promising material for permanent magnets. As the spinel ferrites, CoFe2O4 also displays excellent properties in electronic devices [7,8]. When the magnetic CoFe2O4 is coated with SiO2, the magnetic property has been modified much [9]. Moreover, in the nanoparticles with magnetic and luminescence properties, the insert SiO2 layer can effectively separate the magnetic and photoluminescence section and decrease the quenching effect on luminescence layer resulting from the magnetic material. As the luminescent materials, the photobleaching and quenching properties of dye molecules [10] and toxicity of quantum dot (QDs) [11] has been seriously limited their applications. Compared with them, lanthanide-doped nanoparticles have begun to gain ⇑ Corresponding author. Tel.: +86 43185167712. E-mail address: [email protected] (H. Yang). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.11.102
attraction due to their excellent luminescence properties. Particularly, Y2O3 is an excellent luminescent matrix. When different activators (such as Eu, Dy, and Tb) are doping into Y2O3 matrix [12–15], Y2O3:RE (RE = Eu, Dy et. al) shows different color. So we intend to combine magnetic SiO2 coated CoFe2O4 with luminescent Y2O3:Dy3+ together. Here in this work, we developed a method to obtain the core–shell structured CoFe2O4@SiO2@Y2O3:Dy3+ with ferrimagnetic and luminescence properties. As shown in Scheme 1, the magnetic OA-CoFe2O4 particles were obtained through co-precipitated method. Second, the OA-capped CoFe2O4 particles were conversed to cetyltrimethylammonium bromide (CTAB) modified CoFe2O4 nanoparticles. Third, the surface modification and coating of the SiO2 layer was taken by the modified Stöber method. Finally, the Y2O3:Dy3+ was deposited onto the surface of CoFe2O4@SiO2 composites by a hydrothermal method and post annealing temperature. Recently, Atabaev et al. [14] investigated the optical properties of Dy3+-doped Y2O3 nanoparticles. The results showed that the emission intensity significantly increased with increasing calcination temperature due to the increase of crystallinity and good dispersion of doping Dy3+ materials inside the host Y2O3. But in the present CoFe2O4@SiO2@Y2O3:Dy3+ nanoparticles, the strongest emission intensity is obtained when the calcination temperature is 600 °C due to the influence of CoFe2O4 particles. Similar to Y2O3:Dy3+ particles, the obtained nanoparticles showed the strongest emission intensity when the doping concentration was 1% in mol equivalent.
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Scheme 1. Schematic illustration for the formation of the CoFe2O4@SiO2@Y2O3:Dy3+ nanoparticles. 2. Experimental section 2.1. Materials FeCl36H2O (99%), CoCl26H2O (99%), Oleic Acid (OA) were purchased from Tianjin Guangfu Science and Technology Development Co., Ltd. Y2O3 (99.99%), Dy2O3 (99.99%) were purchased Beijing Chemical Reagent Co. Tetraethyl orthosilicate (TEOS), cetyltrimethylammonium bromide (CTAB), ammonia aqueous (28 wt.%) were obtained from Shanghai Chem. Reagent Co. HNO3, urea, NaOH, ethanol were obtained from Beijing Chemical reagents Co. All materials were used without further purification. Deionized water was used in all experiments. 2.2. Synthesis of OA-coated magnetic CoFe2O4 nanoparticles OA-coated CoFe2O4 nanoparticles were prepared by the co-precipitation method. Briefly, 0.02 mol FeCl36H2O and 0.01 mol CoCl26H2O were added to 25 ml deionized water and stirred, performing clear aqueous solution. The above solution was dropped into an aqueous solution containing 80 ml NaOH (1.25 M) at 80 °C. A specified amount OA was added to the solution as a surfactant and coating material and the suspension was kept in the same temperature for another 1 h. Then the products were cooled to room temperature. To isolate the supernatant liquid, the above contents were centrifuged and washed twice with water and ethanol respectively. The precipitates were dried overnight at 100 °C. The obtained products were marked with SRM. To investigate the effect of annealing temperature, the obtained products were calcined at 600 °C and 800 °C for 2 h, which were marked with S600 and S800, respectively. 2.3. Preparation of CTAB-containing CoFe2O4@SiO2 microspheres Typically, 0.2 g OA-modified CoFe2O4 nanoparticles (SRM, S600, and S800) were treated with CTAB aqueous solution (20 ml, 0.15 mol/ml) under sonication for 30 min. Subsequently the mixture was heated to 60 °C under magnetic stirring for 30 min and then cooled to room temperature. Finally, the resultant CTABcapped CoFe2O4 nanoparticles were kept in an oven at 40 °C for further experiment. Then the CoFe2O4@SiO2 core–shell microspheres were prepared by the modified Stöber method. Briefly, 80 ml ethanol and 20 ml water were added to the above CTAB-CoFe2O4 solution. Ammonia aqueous (28 wt.%) was added to solution until the pH value reached to 9–10. Then the solution was treated with sonication for 30 min. A mixture of ethanol (9.5 ml) and TEOS (0.5 ml) was added dropwise to the solution within 30 min. After few hours, the products were collected and washed with water and ethanol for several times. Then the CTAB-containing CoFe2 O4@SiO2 microspheres were obtained after dried at 50 °C. As the mount of TEOS is constant, we did not mark it differently. 2.4. Synthesis of core–shell structured CoFe2O4@SiO2@Y2O3:Dy3+ nanoparticles In a typical procedure, a mount Y2O3 and Dy2O3 (Dy3+ 1 mol% doping to Y3+) were dissolved into dilute HNO3 with stirring. The superfluous HNO3 was driven off by heating. Then amount urea was dissolved in the solution under vigorous stirring to form a clear solution. Subsequently, the as-prepared CTAB-containing CoFe2O4@SiO2 microspheres were added to above solution under ultrasonication for 30 min. The obtained homogeneous solution was transferred to a Teflon-lined stainless-steel autoclave, which was sealed for heating at 200 °C for 6 h. Then the autoclave was cooled to room temperature. The products were collected by centrifugation, and washed with distilled water and ethanol several times. The final CoFe2O4@SiO2@Y2O3:Dy3+ nanoparticles were obtained through a calcination process at different temperature in air for 3 h with a heating rate of 2 °C min 1. When the SRM was as the magnetic core, the nanocomposites with different annealing temperature were marked with SRM-450, SRM-500, SRM-550, SRM-600, SRM-700, and SRM-800, respectively. When the S600 and S800 were as the magnetic core, the products precursor were only annealed at 600 °C and marked with S600-600, and S800-600. To investigate the effect of Dy3+ doping concentration on the luminescence property, different doping rate of Dy3+ were performed without changing other experiment parameters. 2.5. Characterization X-ray diffraction measurements (XRD) were performed to determine the phase of the as-prepared samples at room temperature using Cu Ka radiation (ka = 1.54059 Å) in the range of 15–70° at a scan rate of 6° min 1. Transmission
Fig. 1. XRD patterns of (A) SRM, S600, S800 and the corresponding CoFe2O4@SiO2 (a, b, and c), (B) SRM-600, S600-600, and S800-600, and (C) SRM-450, SRM-500, SRM-550, SRM-700, and SRM-800.
electron microscopy (TEM) was used to analyze the particle size distribution and microstructure. Magnetic properties of the products were characterized at room temperature with vibrating sample magnetometer (VSM) system (JDM-13). Photoluminescence (PL) excitation and emission spectra were recorded on Hitachi F-4500 spectrofluorometer equipped with a 150 W xenon lamp as the excitation source at room temperature.
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Fig. 2. The TEM images of (A) OA-CoFe2O4 (SRM), (B) CoFe2O4@SiO2 (SRM as the magnetic core), (C) CoFe2O4@SiO2@Y2O3:Dy3+ (SRM-600), (D) OA–CoFe2O4 annealed at 600 °C (S600), and (E) 800 °C (S800).
Fig. 3. The magnetic hysteresis loops of (A) SRM, S600, and S800 and (B) SRM-600, S600-600, and S800-600 samples.
3. Results and discussion 3.1. Phase, structure, and morphology Fig. 1A shows the XRD patterns of OA-CoFe2O4 samples and the corresponding SiO2-coated CoFe2O4 samples. For the SRM sample, all the diffraction peaks can be assigned to the standard single phase cubic CoFe2O4 structure (JCPDS card No. 22-1086). No impurity peaks are observed, indicating the OA-CoFe2O4 nanoparticles consist of pure phase. After the SRM sample was annealed at 600 and 800 °C, we obtained the S600, S800 samples. Compared with the XRD patterns of S600, S800, and SRM, the intensity of diffraction
Table 1 The magnetic characterization of the samples. Samples
SRM
S600
S800
SRM-600
S600-600
S800-600
Ms. (emu/g) Hc (Oe)
16.5 1800
46.5 1200
54 1105
2.5 1190
3.7 1182
5.2 1185
peaks became stronger and the width of diffraction peaks became narrower with the increase of annealing temperature, which can be contribute to the growth size of the magnetic particles with increasing annealing temperature [16]. The sizes of the three
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Fig. 4. The excitation (left) and emission (right) spectra of (A) Y2O3:Dy3+ (1 mol%), (B) CoFe2O4@SiO2@Y2O3:Dy3+ (SRM-600), and (C) CIE chromaticity diagram showing the emission colors of (a in C) Y2O3:Dy3+ and (b in C) CoFe2O4@SiO2@Y2O3:Dy3+ (SRM-600) (1 mol%). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. The strongest peaks histogram (emission) of SRM-600, S600-600, and S800-600 samples.
samples were determined by Scherrer formula using the strongest peak. The average sizes of the particles for the SRM, S600, and S800 samples were 15, 30 and 60 nm. Thus the size variation of the magnetic particles has been achieved by varying the annealing conditions. After coating SiO2 on the surface of CoFe2O4, the corresponding CoFe2O4@SiO2 samples were achieved. The broad band marked with the rectangle at 2h = 22° (Fig. 1A-a, b and c) can be indexed to the amorphous SiO2 shell. After depositing luminescent materials on the surface of CoFe2O4@SiO2 and annealing in air, the nanoparticles were obtained as shown in Fig. 1B and C. Besides
the characteristic diffraction of cubic CoFe2O4 marked with (), all the diffraction peaks in Fig. 1B can be assigned to the cubic Y2O3, indicating the successful crystallization of Y2O3:Eu3+ on the surface of SiO2 shell. The peaks of CoFe2O4 are weaker because it is localized in the inner of the core–shell nanoparticles. Fig. 1C described the XRD patterns of nanoparticles annealed at different temperature. The four primary peaks were shown in SRM-450 sample at 450 °C of annealing temperature, indicating beginning crystallization at 450 °C. With increasing the calcination temperature, the width of the diffraction peaks became narrower and the intensity became stronger, which dues to the well crystallization of Y2O3:Dy3+. In addition, the doping of Dy3+ (mol%) with respect of Y3+ or the mass rate of Y2O3:Dy3+ over CoFe2O4@SiO2 were performed to investigate the crystallization of the nanoparticles by keeping other operational parameters unchanged (not shown in here). The results indicate that both of them do not have any effect on the crystallization of the composites. Base on the XRD, 600 °C of annealing temperature, 1:0.12 of the mass rate of Y2O3:Dy3+ over CoFe2O4@SiO2 and 1 mol% of Dy3+ doping (SRM-600) were chosen to perform the following SEM and TEM of the CoFe2O4@SiO2@Y2O3:Dy3+ nanoparticles. Fig. 2 describes the TEM images of OA-CoFe2O4 (Fig. 2A, D and E), SiO2 coated composites (Fig. 2B) and further Y2O3:Dy3+ deposited nanoparticles (Fig. 2C). The size of the synthesized OA-CoFe2 O4 particles is small with the average diameter of 10–20 nm (Fig. 2A). But the size dispersion is slightly lager because of the
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co-precipitation method. After coating SiO2 on the surface of magnetic particles, the size of CoFe2O4@SiO2 products became larger and the thickness of SiO2 is about 20 nm (Fig. 2B). Fig. 2C clearly showed the morphology character of the final CoFe2O4@SiO2@Y2O3:Dy3+ nanoparticles. The gray border outside the black inner (arrows pointed) confirmed the succession deposition of luminescent Y2O3:Dy3+ by the hydrothermal method. The size of the final nanoparticles is 80–100 nm and the thickness of the Y2O3:Dy3+ is about 20–30 nm. The slight aggregation phenomenon may be due to the not good dispersion of OA-CoFe2O4 and CoFe2O4@SiO2. The obtained OA-CoFe2O4 particles were also annealed at different temperature and the corresponding TEM images had been shown in Fig. 2D and E. When the calcination temperature is 600 °C and 800 °C, the size is about 50 nm and 70 nm, respectively. Compared with Fig. 2A, it can be seen that the size of OA-CoFe2O4 particles increased because that annealing provided energy for the increase of the particles. Moreover, with increasing the calcination temperature from 600 °C to 800 °C, the increase of size can be contributed to more energy at 800 °C.
We devoted to probing the magnetic characterization of the samples and the results were shown in Fig. 3 and Table 1. All the samples showed ferrimagnetic behaves because the magnetic anisotropic energy between different magnetic easy axes and between the spin up and spin down direction within the same easy axis is higher than the thermal energy of the nanocrystals. For the SRM, the saturation magnetization (Ms) and coercivity (Hc) are 16.5 emu/g and 1800 Oe, respectively. For the S600 and S800, the Ms. is 46.5 and 54 emu/g, respectively, which is higher than the SRM. And the Hc is 1200 and 1105 Oe, respectively, which is lower than the SRM. The increase of Ms. and decrease of Hc can be due to the increase of average size of the nanoparticles that results from the coalescence of crystallites [17]. And the results are consistent with the XRD results. When SiO2 and Y2O3:Dy3+ were deposited onto the surface of magnetic particles, the Ms. decreased dramatically (Fig. 3B and Table 1). The reduced saturation magnetization of nanoparticles is generally believed to be due to the decreased mass of CoFe2O4 on a per-gram basis resulted from the coating of nonmagnetic SiO2 and Y2O3:Dy3+ layer and the presence of a magnetic dead or antiferromagnetic layer on the surface
Fig. 6. The strongest peak cures of the excitation (black line) and emission (red line) of Y2O3:Dy3+ and nanoparticles with different annealing temperature. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 7. The emission spectra of nanoparticles with (A) different Dy3+ doping and (C) different mass ration of Y2O3:Dy3+ over CoFe2O4@SiO2 and (B, D) the responding strongest peaks cures of emission.
X. Ren et al. / Journal of Alloys and Compounds 589 (2014) 76–81
induced by the nonmagnetic silica coating. The nanoparticles prepared here are not superparamagnetic. Nevertheless, they can be readily manipulated by an external magnetic field. Interestingly, the Hc of the nanoparticles almost unchanged, which may be contributed to the influence of the luminescent Y2O3:Dy3+. Fig. 4 displayed the excitation (left) and emission (right) spectra of Y2O3:Dy3+ (1 mol%), SRM-600 nanoparticles and the corresponding CIE chromaticity diagram. The excitation spectrum of Dy3+ in Y2O3 monitored with 572 nm consists of a weak band from 200 to 270 nm and some sharp lines in the longer wavelength region. The former is due to the absorption of host, and the latter belongs to the f–f transition within Dy3+ 4f9 configuration. Basically, these transitions lines and be assigned to the transition from the 6H15/2 ground-state to the different excited states, that is, 4D7/2, 6P3/2, and 6P7/2, respectively. Upon excitation into the 6H15/2 ? 6P7/2 transition at 348 nm, the Y2O3:Dy3+ sample shows a strong yellow luminescence. In the emission spectrum, the characteristic transition lines from the lowest excited 4F9/2 level of Dy3+ to 6H15/2 at 486 nm and 6H13/2 at 572 nm are shown, dominated by the Dy3+ 4 F9/2 ? 6H13/2 hypersensitive transition. For the nanoparticles (Fig. 4B), it keeps the excitation and emission characteristic peaks of Y2O3:Dy3+. Compared with the value of the vertical coordinates of Fig. 4A and B, we can find that the luminescent intensity of SRM600 nanoparticles dropped to about 3%. The decrease of the luminescent intensity is due to the serious quenching impact caused by the magnetic core and the detained illustration will be shown below. The CIE coordinates for the emission spectrum of Y2O3:Dy3+ (1 mol%) determined as x = 0.3425, y = 0.3699, located in the yellow region (point a in Fig. 4). For the nanoparticles, the CIE coordinates determined as x = 0.3119, y = 0.2999, shifted to the blue region (point b in Fig. 4), suggesting that magnetic material have influence on the luminescent part. To investigate the quenching effect of magnetic particles to the luminescent part, the strongest emission peaks of SRM-600, S600-600, and S800-600 samples are shown in Fig. 5. The emission intensity decreases with increasing the calcination temperature of CoFe2O4. As we illustrate above, the size of magnetic particles and the saturation magnetization became larger with increasing annealing temperature. As a result, the quenching effect was enlarged with the increase of the saturation magnetization. Fig. 6A depicts the strongest spectra of the samples prepared at different calcination temperatures with same Dy3+ doping level (1 mol%). It can be seen that the luminescent intensity of Y2O3:Dy3+ became stronger with increasing annealing temperature from 450 to 800 °C. The increase of calcination temperature can provides more energy for the crystallization of Y2O3:Dy3+ which permits a better activation for the doped Dy3+ ions. But for the nanoparticles, the strongest luminescence intensity appeared at 600 °C (SRM-600). By elevating the calcination temperature from 600 to 800 °C, the emission intensity experienced a slightly decrease. In the progress of synthesizing nanoparticles, the calcination temperature influences not only the crystallization of Y2O3:Dy3+, but also the crystallization of CoFe2O4, which has been confirmed in the above VSM data of Fig. 3. As a result, the serious quenching effect decreases the luminescence intensity of nanoparticles at higher annealing temperature. Therefore, 600 °C is the optimum calcination temperature to present the best luminescence performance for the nanoparticles. Fig. 7A and B display the emission spectra of the products with different doping levels with same annealing temperature (600 °C).
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The peaks intensity fluctuated when the Dy3+ doping level was tuned from 0.5 mol% to 4 mol%. The results suggest the optimum doping concentration is 1%. When the doping level was higher than 1 mol%, the PL emission intensity gradually decreases as the Dy3+ ion doping level is elevated. This phenomenon results from the concentration quenching which was caused by the cross-relaxation between neighboring Dy3+ ions. In addition, the relation of emission intensity and the mass rate of Y2O3:Dy3+ over CoFe2O4@SiO2 are investigated and the corresponding information is shown in Fig. 7C and D. It can be seen that the quenching effect enhanced with increasing the mass of CoFe2O4@SiO2, which can be due to the increase of the number of magnetic ions. 4. Conclusion In conclusion, by depositing luminescent Y2O3:Dy3+ onto the surface of ferrimagnetic CoFe2O4@SiO2, we obtained the core–shell structured CoFe2O4@SiO2@Y2O3:Dy3+ nanoparticles. The nanoparticles shows ferrimagnetic property and well yell luminescence property of 4F9/2 ? 6H13/2 in 572 nm. As the existence of ferrimegnetic core CoFe2O4 particles, the nanoparticles showed the strongest luminescent intensity. The thickness of shell and difunctional properties can be tuned by controlling the experimental parameters. When the annealing temperature, the doping concentration of Dy3+, and the mass rate of Y2O3:Dy3+ over CoFe2O4@SiO2 is 600 °C, 1 mol%, and 1:0.12, respectively, the size of the nanoparticles, the thickness of the shell, the Ms. is 80–100 nm, 20–30 nm, and 16.5 emu/g respectively. Acknowledgement This work is supported by the National Natural Science Foundation of China (NSFC). References [1] M. Liong, J. Lu, M. Kovochich, T. Xia, S.G. Ruehm, A.E. Nel, F. Tamanoi, J.I. Zink, ACS Nano 2 (2008) 889–896. [2] L. Li, C. Liu, L.Y. Zhang, T.T. Wang, H. Yu, C.G. Wang, Z.M. Su, Nanoscale 5 (2013) 2249–2253. [3] L.J. Zhu, D.L. Wang, X. Wei, X.Y. Zhu, J.Q. Li, C.L. Tu, Y. Su, J.L. Wu, B.S. Zhu, D.Y. Yan, J. Contr. Release 169 (2013) 228–238. [4] Y. Hu, L.J. Meng, L.Y. Niu, Q.H. Lu, Appl. Mater. Interfaces 5 (2013) 4586–4591. [5] Z. Fan, M. Shelton, A.K. Singh, D. Senapati, S.A. Khan, P.C. Ray, ACS Nano 6 (2012) 1065–1073. [6] J. Teillet, F. Bouree, R. Krishnan, J. Magn. Magn. Mater. 123 (1993) 93–96. [7] A.K. Giri, E.M. Kirkpatrick, P. Moongkhamklang, S.A. Majetich, Appl. Phys. Lett. 80 (2002) 2341–2343. [8] C.H. Kim, Y. Myung, Y.J. Cho, H.S. Kim, S.H. Park, J. Park, J. Phys. Chem. C 113 (2009) 7085–7090. [9] J. Wang, F. Zhao, W. Wu, S.H. Cao, G.M. Zhao, Phys. Lett. A 376 (2012) 547–549. [10] F. Grasset, F. Dorson, Y. Molard, S. Cordier, V. Demange, C. Perrin, et al., Chem. Commun. 39 (2008) 4729–4731. [11] E.Q. Contreras, M.J. Cho, H.G. Zhu, H.L. Puppala, G. Escalera, W.W. Zhong, V.L. Colvin, Environ. Sci. Technol. 47 (2013) 1148–1154. [12] X.L. Liu, P.X. Zhu, Y.F. Gao, R.H. Jin, J. Mater. Chem. C 1 (2013) 477–483. [13] M. Dwivedi, V. Tripathi, S.P. Singh, D.K. Gupta, Adv. Sci. Lett. 6 (2012) 213–216. [14] T.S. Atabaev, H.H.T. Vu, H.K. Kim, Y.H. Hwang, J. Korean Phys. Soc. 60 (2012) 244–248. [15] G.A. Sotiriou, M. Schneider, S.E. Pratsinis, J. Phys. Chem. C 116 (2012) 4493– 4499. [16] K. Maaz, A. Mumtaz, S.K. Hasanain, A. Ceylan, J. Magn. Magn. Mater. 308 (2007) 289–295. [17] T.P. Raming, A.J.A. Winnubst, C.M. van Kats, A.P. Philipse, J. Colloid Interf. Sci. 249 (2002) 346–350.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Fabrication, magnetic and luminescent properties of CoFe2O4@SiO2@Y2O3:Dy3+ composites Xiaozhen Ren a, Lizhu Tong a, Xiaodong Chen a, Hong Ding b, Hua Yang a,⇑ a b
College of Chemistry, Jilin University, Changchun 130012, China State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China
a r t i c l e
i n f o
Article history: Received 28 September 2013 Received in revised form 2 November 2013 Accepted 15 November 2013 Available online 26 November 2013 Keywords: Core–shell structure Nanoparticles Ferrimagnetism Luminescence
a b s t r a c t Depositing the luminescent Y2O3:Dy3+ onto the surface of ferrimagnetic CoFe2O4@SiO2 composites obtains the difunctional CoFe2O4@SiO2@Y2O3:Dy3+ nanoparticles. In spite to keeping the yellow luminescence property (4F9/2 ? 6H13/2 in 572 nm), the nanoparticles display the strongest luminescent intensity at 600 °C of calcination temperature due to the influence of ferrimagnetic CoFe2O4 particles. The increasing of maturation magnetization (Ms) and the nearly unchanging of coercivity (Hc) in the nanoparticles with the increasing calcination temperature can be contribute to the increasing of CoFe2O4 particles size. By altering annealing temperature, the mass rate of Y2O3:Dy3+ over CoFe2O4@SiO2, and the doping concentration of Dy3+, the size, the magnetic, and luminescence properties of nanoparticles can be controlled. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent years, multifunctional nanocomposites have attracted increasing attention and have been applied in many important research filed, such as drug delivery [1–3], diagnostic analysis [4,5]. Moreover, the nanoparticles with magnetic and luminescence properties provide an excellent platform to integrate different functional properties into one single entity. As the magnetic material, bulk CoFe2O4 is ferrimagnetic and has an inverse spinel structure with Co2+ ions distributed on B sites and Fe3+ ions distributed on A and B sites. Cobalt ferrite is a hard magnetic material with very high cubic magnetocrystalline anisotropy (Keff = 1.8 107 erg/cm3), high coercivity (Hc = 5.4 kOe), and moderate saturation magnetization (80 emu/g) [6]. These properties make it a promising material for permanent magnets. As the spinel ferrites, CoFe2O4 also displays excellent properties in electronic devices [7,8]. When the magnetic CoFe2O4 is coated with SiO2, the magnetic property has been modified much [9]. Moreover, in the nanoparticles with magnetic and luminescence properties, the insert SiO2 layer can effectively separate the magnetic and photoluminescence section and decrease the quenching effect on luminescence layer resulting from the magnetic material. As the luminescent materials, the photobleaching and quenching properties of dye molecules [10] and toxicity of quantum dot (QDs) [11] has been seriously limited their applications. Compared with them, lanthanide-doped nanoparticles have begun to gain ⇑ Corresponding author. Tel.: +86 43185167712. E-mail address: [email protected] (H. Yang). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.11.102
attraction due to their excellent luminescence properties. Particularly, Y2O3 is an excellent luminescent matrix. When different activators (such as Eu, Dy, and Tb) are doping into Y2O3 matrix [12–15], Y2O3:RE (RE = Eu, Dy et. al) shows different color. So we intend to combine magnetic SiO2 coated CoFe2O4 with luminescent Y2O3:Dy3+ together. Here in this work, we developed a method to obtain the core–shell structured CoFe2O4@SiO2@Y2O3:Dy3+ with ferrimagnetic and luminescence properties. As shown in Scheme 1, the magnetic OA-CoFe2O4 particles were obtained through co-precipitated method. Second, the OA-capped CoFe2O4 particles were conversed to cetyltrimethylammonium bromide (CTAB) modified CoFe2O4 nanoparticles. Third, the surface modification and coating of the SiO2 layer was taken by the modified Stöber method. Finally, the Y2O3:Dy3+ was deposited onto the surface of CoFe2O4@SiO2 composites by a hydrothermal method and post annealing temperature. Recently, Atabaev et al. [14] investigated the optical properties of Dy3+-doped Y2O3 nanoparticles. The results showed that the emission intensity significantly increased with increasing calcination temperature due to the increase of crystallinity and good dispersion of doping Dy3+ materials inside the host Y2O3. But in the present CoFe2O4@SiO2@Y2O3:Dy3+ nanoparticles, the strongest emission intensity is obtained when the calcination temperature is 600 °C due to the influence of CoFe2O4 particles. Similar to Y2O3:Dy3+ particles, the obtained nanoparticles showed the strongest emission intensity when the doping concentration was 1% in mol equivalent.
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Scheme 1. Schematic illustration for the formation of the CoFe2O4@SiO2@Y2O3:Dy3+ nanoparticles. 2. Experimental section 2.1. Materials FeCl36H2O (99%), CoCl26H2O (99%), Oleic Acid (OA) were purchased from Tianjin Guangfu Science and Technology Development Co., Ltd. Y2O3 (99.99%), Dy2O3 (99.99%) were purchased Beijing Chemical Reagent Co. Tetraethyl orthosilicate (TEOS), cetyltrimethylammonium bromide (CTAB), ammonia aqueous (28 wt.%) were obtained from Shanghai Chem. Reagent Co. HNO3, urea, NaOH, ethanol were obtained from Beijing Chemical reagents Co. All materials were used without further purification. Deionized water was used in all experiments. 2.2. Synthesis of OA-coated magnetic CoFe2O4 nanoparticles OA-coated CoFe2O4 nanoparticles were prepared by the co-precipitation method. Briefly, 0.02 mol FeCl36H2O and 0.01 mol CoCl26H2O were added to 25 ml deionized water and stirred, performing clear aqueous solution. The above solution was dropped into an aqueous solution containing 80 ml NaOH (1.25 M) at 80 °C. A specified amount OA was added to the solution as a surfactant and coating material and the suspension was kept in the same temperature for another 1 h. Then the products were cooled to room temperature. To isolate the supernatant liquid, the above contents were centrifuged and washed twice with water and ethanol respectively. The precipitates were dried overnight at 100 °C. The obtained products were marked with SRM. To investigate the effect of annealing temperature, the obtained products were calcined at 600 °C and 800 °C for 2 h, which were marked with S600 and S800, respectively. 2.3. Preparation of CTAB-containing CoFe2O4@SiO2 microspheres Typically, 0.2 g OA-modified CoFe2O4 nanoparticles (SRM, S600, and S800) were treated with CTAB aqueous solution (20 ml, 0.15 mol/ml) under sonication for 30 min. Subsequently the mixture was heated to 60 °C under magnetic stirring for 30 min and then cooled to room temperature. Finally, the resultant CTABcapped CoFe2O4 nanoparticles were kept in an oven at 40 °C for further experiment. Then the CoFe2O4@SiO2 core–shell microspheres were prepared by the modified Stöber method. Briefly, 80 ml ethanol and 20 ml water were added to the above CTAB-CoFe2O4 solution. Ammonia aqueous (28 wt.%) was added to solution until the pH value reached to 9–10. Then the solution was treated with sonication for 30 min. A mixture of ethanol (9.5 ml) and TEOS (0.5 ml) was added dropwise to the solution within 30 min. After few hours, the products were collected and washed with water and ethanol for several times. Then the CTAB-containing CoFe2 O4@SiO2 microspheres were obtained after dried at 50 °C. As the mount of TEOS is constant, we did not mark it differently. 2.4. Synthesis of core–shell structured CoFe2O4@SiO2@Y2O3:Dy3+ nanoparticles In a typical procedure, a mount Y2O3 and Dy2O3 (Dy3+ 1 mol% doping to Y3+) were dissolved into dilute HNO3 with stirring. The superfluous HNO3 was driven off by heating. Then amount urea was dissolved in the solution under vigorous stirring to form a clear solution. Subsequently, the as-prepared CTAB-containing CoFe2O4@SiO2 microspheres were added to above solution under ultrasonication for 30 min. The obtained homogeneous solution was transferred to a Teflon-lined stainless-steel autoclave, which was sealed for heating at 200 °C for 6 h. Then the autoclave was cooled to room temperature. The products were collected by centrifugation, and washed with distilled water and ethanol several times. The final CoFe2O4@SiO2@Y2O3:Dy3+ nanoparticles were obtained through a calcination process at different temperature in air for 3 h with a heating rate of 2 °C min 1. When the SRM was as the magnetic core, the nanocomposites with different annealing temperature were marked with SRM-450, SRM-500, SRM-550, SRM-600, SRM-700, and SRM-800, respectively. When the S600 and S800 were as the magnetic core, the products precursor were only annealed at 600 °C and marked with S600-600, and S800-600. To investigate the effect of Dy3+ doping concentration on the luminescence property, different doping rate of Dy3+ were performed without changing other experiment parameters. 2.5. Characterization X-ray diffraction measurements (XRD) were performed to determine the phase of the as-prepared samples at room temperature using Cu Ka radiation (ka = 1.54059 Å) in the range of 15–70° at a scan rate of 6° min 1. Transmission
Fig. 1. XRD patterns of (A) SRM, S600, S800 and the corresponding CoFe2O4@SiO2 (a, b, and c), (B) SRM-600, S600-600, and S800-600, and (C) SRM-450, SRM-500, SRM-550, SRM-700, and SRM-800.
electron microscopy (TEM) was used to analyze the particle size distribution and microstructure. Magnetic properties of the products were characterized at room temperature with vibrating sample magnetometer (VSM) system (JDM-13). Photoluminescence (PL) excitation and emission spectra were recorded on Hitachi F-4500 spectrofluorometer equipped with a 150 W xenon lamp as the excitation source at room temperature.
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Fig. 2. The TEM images of (A) OA-CoFe2O4 (SRM), (B) CoFe2O4@SiO2 (SRM as the magnetic core), (C) CoFe2O4@SiO2@Y2O3:Dy3+ (SRM-600), (D) OA–CoFe2O4 annealed at 600 °C (S600), and (E) 800 °C (S800).
Fig. 3. The magnetic hysteresis loops of (A) SRM, S600, and S800 and (B) SRM-600, S600-600, and S800-600 samples.
3. Results and discussion 3.1. Phase, structure, and morphology Fig. 1A shows the XRD patterns of OA-CoFe2O4 samples and the corresponding SiO2-coated CoFe2O4 samples. For the SRM sample, all the diffraction peaks can be assigned to the standard single phase cubic CoFe2O4 structure (JCPDS card No. 22-1086). No impurity peaks are observed, indicating the OA-CoFe2O4 nanoparticles consist of pure phase. After the SRM sample was annealed at 600 and 800 °C, we obtained the S600, S800 samples. Compared with the XRD patterns of S600, S800, and SRM, the intensity of diffraction
Table 1 The magnetic characterization of the samples. Samples
SRM
S600
S800
SRM-600
S600-600
S800-600
Ms. (emu/g) Hc (Oe)
16.5 1800
46.5 1200
54 1105
2.5 1190
3.7 1182
5.2 1185
peaks became stronger and the width of diffraction peaks became narrower with the increase of annealing temperature, which can be contribute to the growth size of the magnetic particles with increasing annealing temperature [16]. The sizes of the three
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Fig. 4. The excitation (left) and emission (right) spectra of (A) Y2O3:Dy3+ (1 mol%), (B) CoFe2O4@SiO2@Y2O3:Dy3+ (SRM-600), and (C) CIE chromaticity diagram showing the emission colors of (a in C) Y2O3:Dy3+ and (b in C) CoFe2O4@SiO2@Y2O3:Dy3+ (SRM-600) (1 mol%). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. The strongest peaks histogram (emission) of SRM-600, S600-600, and S800-600 samples.
samples were determined by Scherrer formula using the strongest peak. The average sizes of the particles for the SRM, S600, and S800 samples were 15, 30 and 60 nm. Thus the size variation of the magnetic particles has been achieved by varying the annealing conditions. After coating SiO2 on the surface of CoFe2O4, the corresponding CoFe2O4@SiO2 samples were achieved. The broad band marked with the rectangle at 2h = 22° (Fig. 1A-a, b and c) can be indexed to the amorphous SiO2 shell. After depositing luminescent materials on the surface of CoFe2O4@SiO2 and annealing in air, the nanoparticles were obtained as shown in Fig. 1B and C. Besides
the characteristic diffraction of cubic CoFe2O4 marked with (), all the diffraction peaks in Fig. 1B can be assigned to the cubic Y2O3, indicating the successful crystallization of Y2O3:Eu3+ on the surface of SiO2 shell. The peaks of CoFe2O4 are weaker because it is localized in the inner of the core–shell nanoparticles. Fig. 1C described the XRD patterns of nanoparticles annealed at different temperature. The four primary peaks were shown in SRM-450 sample at 450 °C of annealing temperature, indicating beginning crystallization at 450 °C. With increasing the calcination temperature, the width of the diffraction peaks became narrower and the intensity became stronger, which dues to the well crystallization of Y2O3:Dy3+. In addition, the doping of Dy3+ (mol%) with respect of Y3+ or the mass rate of Y2O3:Dy3+ over CoFe2O4@SiO2 were performed to investigate the crystallization of the nanoparticles by keeping other operational parameters unchanged (not shown in here). The results indicate that both of them do not have any effect on the crystallization of the composites. Base on the XRD, 600 °C of annealing temperature, 1:0.12 of the mass rate of Y2O3:Dy3+ over CoFe2O4@SiO2 and 1 mol% of Dy3+ doping (SRM-600) were chosen to perform the following SEM and TEM of the CoFe2O4@SiO2@Y2O3:Dy3+ nanoparticles. Fig. 2 describes the TEM images of OA-CoFe2O4 (Fig. 2A, D and E), SiO2 coated composites (Fig. 2B) and further Y2O3:Dy3+ deposited nanoparticles (Fig. 2C). The size of the synthesized OA-CoFe2 O4 particles is small with the average diameter of 10–20 nm (Fig. 2A). But the size dispersion is slightly lager because of the
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co-precipitation method. After coating SiO2 on the surface of magnetic particles, the size of CoFe2O4@SiO2 products became larger and the thickness of SiO2 is about 20 nm (Fig. 2B). Fig. 2C clearly showed the morphology character of the final CoFe2O4@SiO2@Y2O3:Dy3+ nanoparticles. The gray border outside the black inner (arrows pointed) confirmed the succession deposition of luminescent Y2O3:Dy3+ by the hydrothermal method. The size of the final nanoparticles is 80–100 nm and the thickness of the Y2O3:Dy3+ is about 20–30 nm. The slight aggregation phenomenon may be due to the not good dispersion of OA-CoFe2O4 and CoFe2O4@SiO2. The obtained OA-CoFe2O4 particles were also annealed at different temperature and the corresponding TEM images had been shown in Fig. 2D and E. When the calcination temperature is 600 °C and 800 °C, the size is about 50 nm and 70 nm, respectively. Compared with Fig. 2A, it can be seen that the size of OA-CoFe2O4 particles increased because that annealing provided energy for the increase of the particles. Moreover, with increasing the calcination temperature from 600 °C to 800 °C, the increase of size can be contributed to more energy at 800 °C.
We devoted to probing the magnetic characterization of the samples and the results were shown in Fig. 3 and Table 1. All the samples showed ferrimagnetic behaves because the magnetic anisotropic energy between different magnetic easy axes and between the spin up and spin down direction within the same easy axis is higher than the thermal energy of the nanocrystals. For the SRM, the saturation magnetization (Ms) and coercivity (Hc) are 16.5 emu/g and 1800 Oe, respectively. For the S600 and S800, the Ms. is 46.5 and 54 emu/g, respectively, which is higher than the SRM. And the Hc is 1200 and 1105 Oe, respectively, which is lower than the SRM. The increase of Ms. and decrease of Hc can be due to the increase of average size of the nanoparticles that results from the coalescence of crystallites [17]. And the results are consistent with the XRD results. When SiO2 and Y2O3:Dy3+ were deposited onto the surface of magnetic particles, the Ms. decreased dramatically (Fig. 3B and Table 1). The reduced saturation magnetization of nanoparticles is generally believed to be due to the decreased mass of CoFe2O4 on a per-gram basis resulted from the coating of nonmagnetic SiO2 and Y2O3:Dy3+ layer and the presence of a magnetic dead or antiferromagnetic layer on the surface
Fig. 6. The strongest peak cures of the excitation (black line) and emission (red line) of Y2O3:Dy3+ and nanoparticles with different annealing temperature. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 7. The emission spectra of nanoparticles with (A) different Dy3+ doping and (C) different mass ration of Y2O3:Dy3+ over CoFe2O4@SiO2 and (B, D) the responding strongest peaks cures of emission.
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induced by the nonmagnetic silica coating. The nanoparticles prepared here are not superparamagnetic. Nevertheless, they can be readily manipulated by an external magnetic field. Interestingly, the Hc of the nanoparticles almost unchanged, which may be contributed to the influence of the luminescent Y2O3:Dy3+. Fig. 4 displayed the excitation (left) and emission (right) spectra of Y2O3:Dy3+ (1 mol%), SRM-600 nanoparticles and the corresponding CIE chromaticity diagram. The excitation spectrum of Dy3+ in Y2O3 monitored with 572 nm consists of a weak band from 200 to 270 nm and some sharp lines in the longer wavelength region. The former is due to the absorption of host, and the latter belongs to the f–f transition within Dy3+ 4f9 configuration. Basically, these transitions lines and be assigned to the transition from the 6H15/2 ground-state to the different excited states, that is, 4D7/2, 6P3/2, and 6P7/2, respectively. Upon excitation into the 6H15/2 ? 6P7/2 transition at 348 nm, the Y2O3:Dy3+ sample shows a strong yellow luminescence. In the emission spectrum, the characteristic transition lines from the lowest excited 4F9/2 level of Dy3+ to 6H15/2 at 486 nm and 6H13/2 at 572 nm are shown, dominated by the Dy3+ 4 F9/2 ? 6H13/2 hypersensitive transition. For the nanoparticles (Fig. 4B), it keeps the excitation and emission characteristic peaks of Y2O3:Dy3+. Compared with the value of the vertical coordinates of Fig. 4A and B, we can find that the luminescent intensity of SRM600 nanoparticles dropped to about 3%. The decrease of the luminescent intensity is due to the serious quenching impact caused by the magnetic core and the detained illustration will be shown below. The CIE coordinates for the emission spectrum of Y2O3:Dy3+ (1 mol%) determined as x = 0.3425, y = 0.3699, located in the yellow region (point a in Fig. 4). For the nanoparticles, the CIE coordinates determined as x = 0.3119, y = 0.2999, shifted to the blue region (point b in Fig. 4), suggesting that magnetic material have influence on the luminescent part. To investigate the quenching effect of magnetic particles to the luminescent part, the strongest emission peaks of SRM-600, S600-600, and S800-600 samples are shown in Fig. 5. The emission intensity decreases with increasing the calcination temperature of CoFe2O4. As we illustrate above, the size of magnetic particles and the saturation magnetization became larger with increasing annealing temperature. As a result, the quenching effect was enlarged with the increase of the saturation magnetization. Fig. 6A depicts the strongest spectra of the samples prepared at different calcination temperatures with same Dy3+ doping level (1 mol%). It can be seen that the luminescent intensity of Y2O3:Dy3+ became stronger with increasing annealing temperature from 450 to 800 °C. The increase of calcination temperature can provides more energy for the crystallization of Y2O3:Dy3+ which permits a better activation for the doped Dy3+ ions. But for the nanoparticles, the strongest luminescence intensity appeared at 600 °C (SRM-600). By elevating the calcination temperature from 600 to 800 °C, the emission intensity experienced a slightly decrease. In the progress of synthesizing nanoparticles, the calcination temperature influences not only the crystallization of Y2O3:Dy3+, but also the crystallization of CoFe2O4, which has been confirmed in the above VSM data of Fig. 3. As a result, the serious quenching effect decreases the luminescence intensity of nanoparticles at higher annealing temperature. Therefore, 600 °C is the optimum calcination temperature to present the best luminescence performance for the nanoparticles. Fig. 7A and B display the emission spectra of the products with different doping levels with same annealing temperature (600 °C).
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The peaks intensity fluctuated when the Dy3+ doping level was tuned from 0.5 mol% to 4 mol%. The results suggest the optimum doping concentration is 1%. When the doping level was higher than 1 mol%, the PL emission intensity gradually decreases as the Dy3+ ion doping level is elevated. This phenomenon results from the concentration quenching which was caused by the cross-relaxation between neighboring Dy3+ ions. In addition, the relation of emission intensity and the mass rate of Y2O3:Dy3+ over CoFe2O4@SiO2 are investigated and the corresponding information is shown in Fig. 7C and D. It can be seen that the quenching effect enhanced with increasing the mass of CoFe2O4@SiO2, which can be due to the increase of the number of magnetic ions. 4. Conclusion In conclusion, by depositing luminescent Y2O3:Dy3+ onto the surface of ferrimagnetic CoFe2O4@SiO2, we obtained the core–shell structured CoFe2O4@SiO2@Y2O3:Dy3+ nanoparticles. The nanoparticles shows ferrimagnetic property and well yell luminescence property of 4F9/2 ? 6H13/2 in 572 nm. As the existence of ferrimegnetic core CoFe2O4 particles, the nanoparticles showed the strongest luminescent intensity. The thickness of shell and difunctional properties can be tuned by controlling the experimental parameters. When the annealing temperature, the doping concentration of Dy3+, and the mass rate of Y2O3:Dy3+ over CoFe2O4@SiO2 is 600 °C, 1 mol%, and 1:0.12, respectively, the size of the nanoparticles, the thickness of the shell, the Ms. is 80–100 nm, 20–30 nm, and 16.5 emu/g respectively. Acknowledgement This work is supported by the National Natural Science Foundation of China (NSFC). References [1] M. Liong, J. Lu, M. Kovochich, T. Xia, S.G. Ruehm, A.E. Nel, F. Tamanoi, J.I. Zink, ACS Nano 2 (2008) 889–896. [2] L. Li, C. Liu, L.Y. Zhang, T.T. Wang, H. Yu, C.G. Wang, Z.M. Su, Nanoscale 5 (2013) 2249–2253. [3] L.J. Zhu, D.L. Wang, X. Wei, X.Y. Zhu, J.Q. Li, C.L. Tu, Y. Su, J.L. Wu, B.S. Zhu, D.Y. Yan, J. Contr. Release 169 (2013) 228–238. [4] Y. Hu, L.J. Meng, L.Y. Niu, Q.H. Lu, Appl. Mater. Interfaces 5 (2013) 4586–4591. [5] Z. Fan, M. Shelton, A.K. Singh, D. Senapati, S.A. Khan, P.C. Ray, ACS Nano 6 (2012) 1065–1073. [6] J. Teillet, F. Bouree, R. Krishnan, J. Magn. Magn. Mater. 123 (1993) 93–96. [7] A.K. Giri, E.M. Kirkpatrick, P. Moongkhamklang, S.A. Majetich, Appl. Phys. Lett. 80 (2002) 2341–2343. [8] C.H. Kim, Y. Myung, Y.J. Cho, H.S. Kim, S.H. Park, J. Park, J. Phys. Chem. C 113 (2009) 7085–7090. [9] J. Wang, F. Zhao, W. Wu, S.H. Cao, G.M. Zhao, Phys. Lett. A 376 (2012) 547–549. [10] F. Grasset, F. Dorson, Y. Molard, S. Cordier, V. Demange, C. Perrin, et al., Chem. Commun. 39 (2008) 4729–4731. [11] E.Q. Contreras, M.J. Cho, H.G. Zhu, H.L. Puppala, G. Escalera, W.W. Zhong, V.L. Colvin, Environ. Sci. Technol. 47 (2013) 1148–1154. [12] X.L. Liu, P.X. Zhu, Y.F. Gao, R.H. Jin, J. Mater. Chem. C 1 (2013) 477–483. [13] M. Dwivedi, V. Tripathi, S.P. Singh, D.K. Gupta, Adv. Sci. Lett. 6 (2012) 213–216. [14] T.S. Atabaev, H.H.T. Vu, H.K. Kim, Y.H. Hwang, J. Korean Phys. Soc. 60 (2012) 244–248. [15] G.A. Sotiriou, M. Schneider, S.E. Pratsinis, J. Phys. Chem. C 116 (2012) 4493– 4499. [16] K. Maaz, A. Mumtaz, S.K. Hasanain, A. Ceylan, J. Magn. Magn. Mater. 308 (2007) 289–295. [17] T.P. Raming, A.J.A. Winnubst, C.M. van Kats, A.P. Philipse, J. Colloid Interf. Sci. 249 (2002) 346–350.