Characterization of cobalt ferrite cofe2o4 nanoparticles synthesized by co-Precipitation and hydrothermal methods

L. T. Tam, N. H. Du, L. T. H. Nhung, D. T. N. Hang, P. T. Dung, N. M. Duc, T. T. P. Thu / Characterization 58 CHARACTERIZATION OF COBALT FERRITE CoFe2O4 NANOPARTICLES SYNTHESIZED BY CO-PRECIPITATION AND HYDROTHERMAL METHODS Le The Tam (1) , Nguyen Hoa Du (1) , Le Thi Hong Nhung (1) Duong Thi Ngoc Hang (1) , Phan Thi Dung (2) , Ngo Minh Duc (1) , Tran Thi Phuong Thu (1) 1 School of Natural Sciences Education, Vinh University 2 Nghe An College of Education Received on 10/5/2017, accepted for publication on 08/7/2017 Abstract. Crystalline nanoparticles CoFe2O4 with a spinel structure were prepared by co-precipitation and hydrothermal methods. The magnetic properties of calcined cobalt ferrite formed from nano crystalline powders by these methods have been compared to each other. The dependence of the particle size and crystalline structure of obtained nanoparticles on the synthesis conditions was examined and characterized using field emission scanning electron microscope (FESEM), X-ray diffraction analysis (XRD) and energy dispersive X-ray spectroscopy (EDX). The XRD analysis revealed a high degree of crystallinity and confirmed spinel structure. The FESEM images showed the presence of spherical ferrite particles with an average diameter about 20-25 nm. The results also showed that the formation of cobalt ferrite spinel structures is affected by synthesis methods. Both prepared techniques were effective for the production of spinel crystalline nanoparticles. Magnetic hysteresis loop data confirmed that the magnetic properties of nanoparticles depend on the structure and size of particles. The materials prepared by hydrothermal route and calcination at 600ºC have had higher magnetic saturation than the non-calcined and calcined co- precipitation method samples. 1. Introduction In recent years, nanocrystalline materials are becoming a subject of intense research because of their unique properties. Magnetic nanoparticles have been of interest for their typical physical and chemical properties as well as their potential applications in various fields such as information technology, environmental treatment, catalysis, biomedicine (extraction of biomolecules, targeted drug delivery, magnetic resonance imaging (MRI) contrast enhancement and thermal magnetic therapy) [1-3]. In particular, magnetic spinel ferrites (MxFe3−xO4, where M = Fe, Co, Ni, Mn, or Zn) are emerging as innovative nanostructures for many biological applications, where a superparamagnetic behavior, a high magnetization value, a diameter smaller than a critical value (typically around 10-20 nm), a narrow size distribution, and an appropriate surface coating are required. Among magnetic spinel ferrite nanoparticles (NPs), CoFe2O4 has received a lot of attention because of its unique magnetic properties, such as a large anisotropy energy, tunable coercivity, and high saturation magnetization, that make CoFe2O4 NPs become a good candidate for many applications such as in magnetic resonance image (MRI). There are some common ways to synthesize CoFe2O4 nanoparticles, including co- precipitation, sol-gel or hydrothermal methods [4-7]. Among these techniques, chemical co-precipitation has been reported to be the most economical one. In addition, hydrothermal method has been confirmed to be a high rate of production and simplicity. . Email: tamlt@vinhuni.edu.vn (L. T. Tam) Trường Đại học Vinh Tạp chí khoa học, Tập 46, Số 2A (2017), tr. 58-65 59 In this paper, we reported the effect of structural properties on magnetic properties of cobalt ferrite samples, prepared by hydrothermal and co-precipitation processes. 2. Chemicals, instruments and measurements CoFe2O4 NPs were prepared by hydrothermal and co-precipitation method using the analytically pure grade ferric chloride hexahydrate (FeCl3.6H2O), cobalt(II) chloride tetrahydrate (CoCl2.4H2O) and sodium hydroxide. All chemicals were purchased from Merck chemical company. Ultra-pure nitrogen gas (99.99%) was used to provide anaerobic condition in solution. Distilled deionized water was used to prepare all the solutions. All the synthesis experiments were conducted in the laboratory of Inorganic Chemistry at Vinh University. 2.1. Synthesis by co-precipitation method Superparamagnetic CoFe2O4 nanoparticles were prepared by co-precipitation of cobalt (II) and ferric chloride in nitrogen atmosphere. The chlorides of cobalt and iron were dissolved in deionized water at the determined molar ratio (Fe/Co = 2) under N2 with stirring at 400 rpm for 25 minutes. Aqueous solution of NaOH 3M was used as the precipitating agent. The obtained solution was added by dropwise into 15 ml sodium hydroxide (NaOH 3M) solution with rate of 3 ml/min, under vigorous stirring with a magnetic stirrer under N2 atmosphere, after which the color of the mixture turned to black and the pH value was higher than 12. Large pH values (above 12) were used because it controls the process of nucleation rate and reduces the particles sizes. The obtained solution was maintained at a fixed temperature for 9 hours under vigorous stirring with a magnetic stirrer under N2 atmosphere [4, 5]. This precursor (denoted A) was used for hydrothermal synthesis. For co-precipitation, the mixture was stirred strongly for 8 hours at 80ºC. The synthesized CoFe2O4 NPs were washed by decanting with assistant of magnet, using distilled deionized water until neutralization, and was dried at 80ºC for 8 hours (denoted as M1). A half of M1 was calcined at 600˚C for 2 hours and denoted as M2. 2.2. Synthesis by hydrothermal method The resulting suspension A was transferred into a teflon-lined stainless steel autoclave with a capacity of 50 ml, and treated at 180ºC for 15 hours. After the hydrothermal reaction time, the autoclave was taken out and cooled at room temperature naturally. The synthesized CoFe2O4 NPs were washed by decanting with assistant of a magnet, using distilled deionized water until neutralization. The obtained dark product was dried at 80ºC for 8 hours. This product was denoted as M3. A half of M3 powders were calcined at 600˚C for 2 hours and the obtained product was denoted as M4. 2.3. Measurements The crystal structures of the samples were characterized by XRD using diffractometer XD8 Advance Bruker with Cu-Kα radiation (λ=1.5406 Å) (Faculty of Chemistry, Vietnam National University, Hanoi). The morphology (size and shape) of the particle materials was obtained by field emission scanning electron microscopy FESEM (Hitachi S-4800) and hysteresis loops were measured at room temperature to the highest field of 11 kOe using a vibrating sample magnetometer (VSM) (Institute of Materials Science, Vietnam Academy of Science and Technology). L. T. Tam, N. H. Du, L. T. H. Nhung, D. T. N. Hang, P. T. Dung, N. M. Duc, T. T. P. Thu / Characterization 60 3. Results and discussion 3.1. X-ray Diffraction Analysis The XRD patterns of CoFe2O4 samples prepared by using hydrothermal and co- precipitation methods are shown in Figure 1. These patterns confirmed the formation of cubic spinel type lattice of CoFe2O4, which matches well with the XRD pattern of standard CoFe2O4 (ICDD card No: 22-1086), no other crystalline phase presented in the samples. Figure 1: XRD patterns of the CoFe2O4 NP samples prepared and treated with various conditions The crystallite size D of the samples was calculated from the data of peak at 2θ = 35.5º with Miller indices by (311), using Scherrer equation. (1) where D is the grain diameter, β is half intensity width of the relevant diffraction, λ is X- ray wavelength and θ is the diffraction angle. The lattice parameter was calculated according to the Eq. (2): a = dhkl(h 2 +k 2 +l 2 ) 1/2 (2) The crystallite size and lattice constants of the cobalt ferrite nanoparticles have been summarized in Table 1. Table 1: Characteristics of the CoFe2O4 NPs samples Sample Synthesis method Lattic parameter, a (Å) d311(Å) Particle size, (nm) D(XRD) DFESEM M1 (non-calcined Co-precipitation 8.381 2.527 27.06 25.5±1,6 M2 (calcined) 8.356 2.520 23.22 22.7±1.9 M3 (non-calcined Hydrothermal 8.381 2.525 21.32 21.4±0.6 M4 (calcined) 8.351 2.518 24.31 23.4±1.5 Trường Đại học Vinh Tạp chí khoa học, Tập 46, Số 2A (2017), tr. 58-65 61 3.2. Morphology characterizations Figures 2 and 3 (a, b) show the FESEM images of the CoFe2O4 NP samples. The average size of nano-particles prepared by hydrothermal and co-precipitation method is ~ 20 and 25 nm, respectively. This result is matching with the calculations from the XRD data (Table 1). The CoFe2O4 NPs prepared by hydrothermal method are uniform with a narrow size distribution. The size and size distribution have a dependence on nucleation and growth rates during the reaction, which may be controlled by concentration of reagents and reaction conditions. Smaller particles were obtained if the nucleation rate was higher than growth rate [8, 9]. Figure 2: FESEM of non-calcined samples: a) M1 and b) M3 Figure 3: FESEM of calcined at 600ºC samples: a) M2 and b) M4 The uniform spherical morphology of nanoparticles obtained in non-calcined and calcined at 600°C for 2 h as indicated in Figures 2 and 3, which display a spherical morphology with size of 27.06 nm (M1), 23.22 nm (M2), 21.32 nm (M3) and 24.31 (M4). It is interesting to note that the size distribution (Fig. 4) is more narrow for particles synthesized via hydrothermal than that via co-precipitation route. L. T. Tam, N. H. Du, L. T. H. Nhung, D. T. N. Hang, P. T. Dung, N. M. Duc, T. T. P. Thu / Characterization 62 Figure 4: Particle size distribution of the samples calcined at 600ºC for 2 h (M2 and M4) 3.3. EDX spectrum Figure 5 shows the EDX spectra of the CoFe2O4 NPs samples and confirms the ratio of the transition metal atoms in each material according to the nominal stoichiometry. Figure 5: The qualitative EDX analysis for CoFe2O4 powder of M3 The EDX analysis is considered a semi-quantitative analysis. A typical EDX spectrum obtained from the analyzed samples is presented in Figure 5 where the peaks corresponding to Co, Fe and O have been identified. The atomic ratio of Fe: Co for the entire calcined sample is ~ 2:1, according to the nominal stoichiometry of CoFe2O4 materials (Table 2). Table 2: Elemental composition of M3 from EDX data Element Weight% Atomic% O 30.53 60.98 Fe 45.13 25.82 Co 24.34 13.20 Total 100.00 100 M2 M4 Trường Đại học Vinh Tạp chí khoa học, Tập 46, Số 2A (2017), tr. 58-65 63 3.4. Magnetic Properties The magnetic properties of the CoFe2O4 nanoparticles were measured by using vibrating sample magnetometer (VSM) with maximum applied field of 11 kOe at room temperature. The results are shown in Table 3 and Figure 6. Table 3: Magnetic properties of cobalt ferrite samples prepared by different methods Sample Synthesis method Average particle size (nm) Ms (emu/g) at H=11 kOe Hc (Oe) M1 Co-precipitation 27.06 47.31 - M2 23.22 55.37 7.81 M3 Hydrothermal 21.32 53.48 14.50 M4 24.31 61.50 33.80 Figure 6: Hysteresis loop M(H) of nano-particle CoFe2O4 samples prepared by co-precipitation (M1, M2) and hydrothermal methods (M3, M4) Figure 7: The coercivity (Hc) of the samples prepared by co-precipitation and hydrothermal methods L. T. Tam, N. H. Du, L. T. H. Nhung, D. T. N. Hang, P. T. Dung, N. M. Duc, T. T. P. Thu / Characterization 64 The hysteresis loops of samples are depicted in Figure 6, and magnified the low field near the cordinate origin in Figure 7. The saturation magnetization (Ms) of the samples M1, M2, M3 and M4 are 47.31; 55.37; 53.48 and 61.50 emu/g, respectively. These values of saturation magnetization are much lower than the bulk value of 80 emu/g. The coercivity (Hc) of the samples M1 is much higher than the M2, M3, M4. The coercivities (Hc) are 7.81, 14.50 and 33.80 Oe for M2, M3 and M4 samples, respectively. The difference in Ms may be due to the difference in size of the nanoparticles. As for the CoFe2O4 synthesized by co-precipitation method, the decrease in Ms was ascribed to the existence of impure phase such as β-FeOOH as reported in [9]. The plots of magnetization (Ms) applied field are shown in Figure 6. It is demonstrated that the magnetic properties of samples prepared by co-precipitation were less than that of the samples obtained by hydrothermal method, in which the sample calcined was higher than non-calcined one. 4. Conclusion Spinel CoFe2O4 ferrite system was successfully prepared by co-precipitation and hydrothermal method with nano-size dimension. The XRD revealed that the prepared samples possess single phase cubic spinel structure. The nanocrystalline nature of the prepared samples was also confirmed from FESEM. The EDX confirmed the ratio of the transition metal atoms in each material according to the nominal stoichiometry. The saturation magnetizations of the sample at applied field 12 kOe were found to be 47.31, 55.37, 53.48 and 61.50 emu/g for M1, M2, M3 and M4, respectively. The coercivities (Hc) are 7.81, 14.50 and 33.80 Oe for the samples M2, M3 and M4, respectively. REFERENCES [1] X. Meng, H. C. Seton, T. Lu Le, I. A. Prior, N. T. Thanh, B. Song, Magnetic CoPt nanoparticles as MRI contrast agent for transplanted neural stem cells detection, Nanoscale 3, 2011, pp. 977-984. 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TÓM TẮT ĐẶC TÍNH HỆ HẠT NANO CoFe2O4 CHẾ TẠO BẰNG PHƢƠNG PHÁP THỦY NHIỆT VÀ ĐỒNG KẾT TỦA Trong nghiên cứu này, các hạt nano CoFe2O4 với cấu trúc spinel đƣợc tổng hợp bằng phƣơng pháp thủy nhiệt và phƣơng pháp đồng kết tủa với định hƣớng ứng dụng trong y sinh. Các đặc trƣng cấu trúc, kích thƣớc hạt và tính chất từ của vật liệu thu đƣợc bằng các phƣơng pháp đã đƣợc so sánh với nhau. Sự thay đổi về kích thƣớc tinh thể đƣợc khảo sát và đánh giá bằng phƣơng pháp nhiễu xạ tia X (XRD), hiển vi điện tử kiểu quét phát xạ trƣờng (FESEM) và phổ tán xạ năng lƣợng tia X (EDX). Phân tích kết quả XRD cho thấy tất cả các mẫu thu đƣợc đều đơn pha và có cấu trúc spinel. Kích thƣớc tinh thể trung bình thu đƣợc từ XRD tƣơng đƣơng với kích thƣớc hạt xác định từ các hình ảnh FESEM (20-25 nm). Kết quả cho thấy việc hình thành các hạt nano từ với cấu trúc spinel phụ thuộc vào phƣơng pháp tổng hợp. Kết quả đo từ kế mẫu rung (VSM) cho thấy tính chất từ phụ thuộc vào cấu trúc, kích thƣớc hạt và cách chế tạo vật liệu. Các mẫu chế tạo theo phƣơng pháp thủy nhiệt và đƣợc thiêu kết ở 6000C có giá trị từ độ bão hòa cao hơn so với các mẫu theo phƣơng pháp đồng kết tủa.