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Effects of synthetical conditions on octahedral magnetite nanoparticles
Materials Science and Engineering B 136 (2007) 101–105
Effects of synthetical conditions on octahedral magnetite nanoparticles Yu Wen-Guang, Zhang Tong-Lai ∗ , Qiao Xiao-Jing, Zhang Jian-Guo, Yang Li State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, China Received 13 March 2006; received in revised form 29 July 2006; accepted 14 August 2006
Abstract By controlling the growth kinetics of nanoparticles, octahedral magnetite nanoparticles were successfully synthesized employing an oxidation–precipitation method with FeSO4 ·7H2 O, NaNO3 and NaOH as starting materials. Magnetite nanoparticles with 40 nm or less can be synthesized under the following conditions: pH 9.5, F = 10–90 ml/min, T = 60–90 ◦ C, C = 0.2–1 mol/l, t = 4 h, R = 200–500 rpm. The saturation magnetizations of octahedral magnetite nanoparticles are in the range from 80 emu/g to 90 emu/g. The remanence magnetizations of the as-prepared samples are lower than that of the bulk magnetite and the coercive forces of the as-prepared sample are around 140 Oe. © 2006 Published by Elsevier B.V. Keywords: Crystal growth; Magnetic materials; Magnetite; Nanoparticles; Morphology and synthetical conditions
1. Introduction
2. Experimental
Synthesizing magnetite (Fe3 O4 ) nanoparticles has been of great interest in recent years because they can be applied technologically in many important fields such as pigment, activator, magnetic fluids, magnetic record materials, etc. [1–4]. Various methods of synthesizing magnetite nanoparticles, for example, co-precipitation [4,5], micro-emulsion [6], electrochemical synthesis [7], hydrothermal synthesis [8,9], reduction–precipitation [10], etc. have been successfully developed. However, as can be seen in the literatures, the oxidization–precipitation method [11] was rarely employed in the process of synthesizing magnetite nanoparticles. In fact, this method is easier to be put in practice than micro-emulsion, electrochemical synthesis and hydrothermal, etc., and has merits of accurately controlling the proportion of Fe2+ and Fe3+ in the sample and largely synthesizing magnetite nanoparticles without any surfactant. Here, we chose NaNO3 as oxidant and successfully synthesized octahedral magnetite nanoparticles. The synthesis of nanoparticles having well-defined sizes, special shape and good material properties is of great importance [12]. To the best of our knowledge, the octahedral magnetite nanoparticles have not been reported in the literatures up to now. The main affecting factors on particle size and magnetic properties of magnetite were studied by using a single-factor method.
Iron sulfate heptahydrate (FeSO4 ·7H2 O), sodium nitrate (NaNO3 ) and sodium hydroxide (NaOH) were of analytical grade and were used as received. Magnetite nanoparticles were synthesized as follows: a predetermined volume of NaOH stock solution was placed into a three-neck flask after having used N2 to lustrate oxygen in it, and the NaOH solution in the three-neck flask was heated at the same time. When the temperature of the NaOH solution went up to 60–100 ◦ C, a predetermined volume of FeSO4 stock solution was added slowly into the three-neck flask under stirring at a rate of 200–500 rpm; after running out of the predetermined volume of FeSO4 solution for 5 min, a calculated NaNO3 stock solution was added slowly into the flask. When the NaNO3 stock solution was exhausted, the reaction was kept for 2–8 h. The magnetite particles were separated from reaction system by a strong magnet and washed five times with de-ionized water and ethanol, then dried in vacuum oven for 48 h. Thus a sample of magnetite nanoparticles was obtained and characterized by multiform techniques. X-ray diffraction (XRD) pattern was taken using a Rigaku D/max-2500 X-ray diffractometer with monochromatic Cu K␣ radiation (λ = 0.15405 nm) for crystal structure of the particle. Employing Scherrer’s equation, particle size was determined by an average of five values calculated according as the five strongest diffraction peaks in XRD pattern. Magnetic properties of the sample were measured by vibrating sample magnetometer (VSM) employing a LDJ-9500 magnetometer at room temperature. Using X L 30 S-FEG equipment,
∗
Corresponding author. Tel.: +86 10 689 13818; fax: +86 10 689 13818. E-mail address: [email protected] (T.-L. Zhang).
0921-5107/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.mseb.2006.08.030
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scanning electron microscopy (SEM) was employed to observe the morphology of the particles. 3. Results and discussions 3.1. The effect of reaction time (t) The samples used to study the effect of reaction time (t) on particle size and magnetic properties were synthesized at pH 9.5, F = 50 ml/min, T = 85 ◦ C, C = 0.3 mol/l, R = 400 rpm. It can be seen from Fig. 1 that the size of magnetite nanoparticles decrease from 38 nm to 34 nm as t was prolonged from 2 h to 4 h. However when t was prolonged from 4 h to 8 h, the particle size increased. Particle size can be affected by many factors [13]. Not-reacted Fe(OH)3 in a magnetite sample can make the particle size larger than the particle size of a pure magnetite sample. XRD (Fig. 2) proved that there is some not-reacted Fe(OH)3 in the sample synthesized at 2 h, and therefore, the particle size of the sample synthesized at 2 h is larger than that of the sample synthesized at 4 h, and the particle size arrives at the smallest value when the Fe(OH)3 was completely reacted into magnetite at 4 h. When t was longer than 4 h, some small mag-
netite particles were reunited to form larger ones, and the more material is added to magnetite nanoparticle surfaces to cause secondary nucleation and secondary growth, so that the particle sizes linearly increase with t after 4 h. It was accepted that it is usually associated with the smaller size of magnetic particles if the saturation magnetization of the magnetite particles is lower than that (92 emu/g) of bulk magnetite [14]. The principal effect of small size on a magnetic particle is the breaking of a larger number of exchange bonds for surface atoms. When some exchange bonds are removed at the surface, there can be frustration and spin disorder [12], thus the saturation magnetization of the magnetic particle decrease. On the other hand, the shape anisotropy and wide particle size distribution of the synthesized magnetite nanoparticles are also responsible for the relatively low magnetization [15]. All the values of saturation magnetization of samples in Fig. 1 are close to but lower than the value of 92 emu/g, proving that the particle in the samples are in good crystalline state but in smaller size. The sample synthesized at 2 h has lower magnetic properties than those synthesized at 4 h or 6 h. It proved further that there was some Fe(OH)3 in the sample synthesized at 2 h. In can be concluded that t ought to be controlled at 4 h because the sample synthesized at 4 h has good magnetic properties and the smallest size. 3.2. The effect of reactant concentration of Fe2+ (C)
Fig. 1. The effect of reaction time.
The samples used to study the effect of concentration of Fe2+ (C) on particle size and magnetic properties were synthesized at pH 9.5, F = 50 ml/min, T = 90 ◦ C, t = 4 h, R = 400 rpm. The results were plotted in Fig. 3, which shows that the size of magnetite nanoparticles increases with the increase of C. The formation of magnetite nanoparticles follows the nucleation-growth mechanism, and the particle size principally depends on their nucleation rate and growth rate [16]. If the nucleation rate of a particle is faster than the growth rate of it, its size is usually smaller; contrarily, its size is larger. When C was low, the nucleation rate of magnetite nanoparticles was faster than their growth rate because their growth was limited by
Fig. 2. XRD patterns of samples synthesized at 2 h and 4 h, respectively.
Fig. 3. The effect of reactant concentrations (Fe2+ ).
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the low C; therefore, the particle size synthesized at a lower C was correspondingly smaller. As C increased, their growth rate became faster; as a result their size was larger. Fig. 3 shows, furthermore, that the influence of C on the particle size becomes little when C is higher than 0.6 mol/l, revealing that the effect of C on the particle size has a limit. It was found in experiment that employing low concentration FeSO4 solution usually followed patina as intermediate. The samples synthesized at 0.2 mol/l or 0.4 mol/l are such ones that followed patina as intermediate. Some patina in the sample likely causes the low magnetic properties of the samples synthesized at low C (Fig. 3). 3.3. The effect of reaction temperature (T) Under other conditions unaltered, the effect of reaction temperature (T) on particle size and magnetic properties was studied. The samples used to study the effect of T were synthesized at pH 9.5, F = 50 ml/min, t = 4 h, C = 0.3 mol/l, R = 400 rpm. It can be seen from Fig. 4 that the size of magnetite nanoparticles increases when T was elevated. This result is in agreement with the results of other methods such as co-precipitation [17,18]. As we knew, the higher the T is, the faster the movement of a particle is. The particle growth is expedited with the increase of T and the rate of addition of material to the existing magnetite nuclei increases to quicken the secondary nucleation and secondary growth so that the larger particle can be synthesized at higher temperature. In the same way, the effect of T on magnetic properties of the sample is the same as the effect of T on the particle size. Noticeably, the nanoparticles synthesized at 90 ◦ C show higher saturation magnetization and larger size. 3.4. The effect of the flow rate of FeSO4 solution (F) The samples used to study the effect of flow rate of FeSO4 solution (F) were synthesized at pH 9.5, T = 90 ◦ C, C = 0.3 mol/l, R = 400 rpm, but the different flow rate of flow of FeSO4 solution.
Fig. 4. The effect of reaction temperature.
Fig. 5. The effect of flow rate of FeSO4 solution.
Fig. 5 shows that the size of magnetite nanoparticles increases with the increase of F from 10 ml/min to 90 ml/min. When the FeSO4 stock solution was dropped into the NaOH solution at the low F, the dropped FeSO4 solution was rapidly decentralized by the agitating to form a great deal of Fe(OH)2 nucleus, and almost everyone was surrounded by the superfluous NaOH solution. Magnetite nucleus began to form while NaNO3 was added into the reaction system, but the growth of magnetite nucleus was limited by the surrounding NaOH solution, so that the growth rate of magnetite nanoparticles was lower than their nucleation rate, therefore, magnetite in smaller size was synthesized. As F went up from 10 ml/min to 90 ml/min, the growth rate of magnetite nucleus increased, therefore, the size of magnetite nanoparticles became larger. However, when F went up to 130 ml/min, the dropping of FeSO4 stock solution almost became pouring of it, the plentiful FeSO4 solution in the reaction system made a mass of Fe(OH)2 nucleus form immediately, mass transfer was hampered by the high concentration and the growth of magnetite particles was confined to form smaller magnetite nanoparticles. From Fig. 5, we can see that the change of magnetization with F is the same as that of particle size with F, but the magnetic
Fig. 6. XRD patterns of the samples synthesized at the different F.
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Fig. 7. The effect of the rate of agitation.
properties of the magnetite nanoparticles come down when F was 130 ml/l. No impurity were revealed in the XRD patterns (Fig. 6) of the sample synthesized at 130 ml/l, therefore, the decline of its magnetic properties was probably caused by the smaller size of magnetite nanoparticles in the sample. 3.5. The effect of the rate of agitation (R) Under these conditions pH 9.5, F = 50 ml/min, T = 90 ◦ C, C = 0.3 mol/l unchanged, the rate of agitation (R) was changed
Fig. 8. The X-ray diffraction pattern of the magnetite sample synthesized at pH 9.5, F = 30 ml/min, T = 90 ◦ C, C = 0.6 mol/l, t = 4 h, R = 300 rpm.
from 200 rpm to 500 rpm to study the effect of R on the particle size and magnetic properties. The result was shown in Fig. 7, which shows that the particle size decreases as R increases. The shear force from the agitation can make the particle size smaller. The higher the R is, the stronger the shear force is. It is reasonable that the particle size go down with the increase of R. Noticeably, magnetic properties of magnetite nanoparticles were hardly influenced by the rate of agitation in the researched range (Fig. 7).
Fig. 9. (a–c) SEM images of magnetite nanoparticles of the sample synthesized at pH 9.5, F = 30 ml/min, T = 90 ◦ C, C = 0.6 mol/l, t = 4 h, R = 300 rpm.
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seen clearly that the as-synthesized sample shows a high saturation magnetization (87.1 emu/g) and a strong coercive force (132.3 Oe), but a low remanence magnetization (11.8 emu/g). 5. Conclusions
Fig. 10. The room-temperature magnetic hysteresis curve of the magnetite nanoparticles of the sample synthesized at pH 9.5, F = 30 ml/min, T = 90 ◦ C, C = 0.6 mol/l, t = 5 h, R = 300 rpm. It has a saturation magnetization of 87.1 emu/g and a coercive field of 132.3 Oe.
4. Characterization of the sample XRD pattern of the typical sample synthesized is seen in Fig. 8. The characteristic peaks at 2θ angles correspond well to the standard card of magnetite (JCPDS: 19-0629 [5]), which proves that the sample can be identified as Fe3 O4 with the spinel structure. Although all the reflections of Fe3 O4 and ␥-Fe2 O3 are similar, the characteristic reflections of 2 2 1, 2 1 0 and 2 1 3 planes corresponding to ␥-Fe2 O3 are found absent in Fig. 8. Therefore, the sample produced by our method can be identified as Fe3 O4 instead of ␥-Fe2 O3 [9]. Furthermore, the d values of the sample are observed to be closer to the standard values of Fe3 O4 phase than the other oxide of iron. From all the above observations, the as-synthesized sample can be identified as Fe3 O4 phase. The size of magnetite nanoparticles in the sample was calculated with the Scherrer’s equation, and the average size of nanoparticles is 40 nm or less. The strong peaks in XRD reveal that the particles were crystallized well. In order to observe the morphology of magnetite nanoparticles in the typical sample synthesized by this method, SEM was employed and the images obtained are shown in Fig. 9. The images of particles in the sample reveal a pattern with a very plain morphology of magnetite nanoparticles, which consists of nearly homogeneous grains (Fig. 9a) with the size from 80 nm to 200 nm (Fig. 9b). The figures of the particles present regular octahedron with very smooth surface (Fig. 9c), which further indicates that the particles were well crystallized. The measurement of the magnetic properties of the typical sample at room temperature is presented in Fig. 10. It can be
This work has presented an efficient method of synthesizing octahedral magnetite nanoparticles, by which octahedral magnetite nanoparticles can be easily mass-produced and no surfactant was used in the entire course. The average size of octahedral magnetite nanoparticles is 40 nm or less. The saturation magnetization of octahedral magnetite nanoparticles is in the range from 80 emu/g to 90 emu/g. The elementary conditions for synthesizing octahedral magnetite nanoparticles with 40 nm or less are pH 9.5, F = 10–90 ml/min, T = 60–90 ◦ C, C = 0.2–1 mol/l, t = 4 h, R = 200–500 rpm. Acknowledgement This work was supported by the National Science Foundation under grant no. NSFC20471008. References [1] G.P. Song, J. Bo, R. Guo, Chin. J. Chem. 8 (2005) 997. [2] D. Zhang, Z. Liu, S. Han, C. Li, B. Lei, P.S. Michael, M.T. James, C. Zhou, Nano Lett. 11 (2004) 2151. [3] D. Thapa, V.R. Palkar, M.B. Kurup, S.K. Malik, Mater. Lett. 58 (2004) 2692. [4] Y. Zhu, Q. Wu, J. Nanopaticle Res. 1 (1999) 393. [5] Y. Konishi, T. Nomura, K. Mizoe, Hydrometallurgy 74 (2004) 57. [6] Z.L. Liu, X. Wang, K.L. Yao, G.H. Du, Q.H. Lu, Z.H. Ding, J. Tao, Q. Ning, X.P. Luo, D.Y. Tian, D. Xi, J. Mater. Sci. 39 (2004) 2633. [7] S. Franger, P. Berthet, J. Berthon, J. Solid State Electr. 8 (2004) 218. [8] M. Wu, Y. Xing, Y. Jia, H. Niu, N. Qi, J. Ye, Q. Chen, Chem. Phys. Lett. 401 (2005) 374. [9] Y.B. Khollam, S.R. Dhage, H.S. Potdar, S.B. Deshpande, P.P. Bakare, S.D. Kulkarni, S.K. Date, Mater. Lett. 56 (2002) 571. [10] S. Qu, H. Yang, D. Ren, S. Kan, G. Zou, D. Li, M. Li, J. Colliod Interface Sci. 215 (1999) 190. [11] T. Deepa, V.R. Palkar, M.B. Kurup, S.K. Malik, Mater. Lett. 58 (2004) 2692. [12] R.H. Kodama, J. Magn. Magn. Mater. 200 (1999) 359. [13] L. Vayssi`eres, C. Chanac´eac, E. Tronc, J.P. Jolivet, J. Colloid Interface Sci. 205 (1998) 205. [14] H.B. Wang, Z.L. Liu, O.H. Lu, Chin. J. Inorg. Chem. 20 (2004) 1280. [15] G. Benito, M.P. Morales, J. Requena, V. Raposo, M. Vazquez, J.S. Moya, J. Magn. Magn. Mater. 234 (2001) 65. [16] Y.B. Khollam, A.S. Deshpande, A.J. Patil, S.B. Deshpande, H.S. Potdar, S.K. Date, Mater. Chem. Phys. 71 (2001) 304. [17] Q.X. He, H. Yang, Q.Q. Chen, S.Q. Liu, K.L. Huang, J. Magn. Mater. Devices 34 (2003) 9. [18] R.H. Qin, W. Jiang, H.Y. Liu, F.S. Li, Mater. Rev. 17 (2003) 66.
Effects of synthetical conditions on octahedral magnetite nanoparticles Yu Wen-Guang, Zhang Tong-Lai ∗ , Qiao Xiao-Jing, Zhang Jian-Guo, Yang Li State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, China Received 13 March 2006; received in revised form 29 July 2006; accepted 14 August 2006
Abstract By controlling the growth kinetics of nanoparticles, octahedral magnetite nanoparticles were successfully synthesized employing an oxidation–precipitation method with FeSO4 ·7H2 O, NaNO3 and NaOH as starting materials. Magnetite nanoparticles with 40 nm or less can be synthesized under the following conditions: pH 9.5, F = 10–90 ml/min, T = 60–90 ◦ C, C = 0.2–1 mol/l, t = 4 h, R = 200–500 rpm. The saturation magnetizations of octahedral magnetite nanoparticles are in the range from 80 emu/g to 90 emu/g. The remanence magnetizations of the as-prepared samples are lower than that of the bulk magnetite and the coercive forces of the as-prepared sample are around 140 Oe. © 2006 Published by Elsevier B.V. Keywords: Crystal growth; Magnetic materials; Magnetite; Nanoparticles; Morphology and synthetical conditions
1. Introduction
2. Experimental
Synthesizing magnetite (Fe3 O4 ) nanoparticles has been of great interest in recent years because they can be applied technologically in many important fields such as pigment, activator, magnetic fluids, magnetic record materials, etc. [1–4]. Various methods of synthesizing magnetite nanoparticles, for example, co-precipitation [4,5], micro-emulsion [6], electrochemical synthesis [7], hydrothermal synthesis [8,9], reduction–precipitation [10], etc. have been successfully developed. However, as can be seen in the literatures, the oxidization–precipitation method [11] was rarely employed in the process of synthesizing magnetite nanoparticles. In fact, this method is easier to be put in practice than micro-emulsion, electrochemical synthesis and hydrothermal, etc., and has merits of accurately controlling the proportion of Fe2+ and Fe3+ in the sample and largely synthesizing magnetite nanoparticles without any surfactant. Here, we chose NaNO3 as oxidant and successfully synthesized octahedral magnetite nanoparticles. The synthesis of nanoparticles having well-defined sizes, special shape and good material properties is of great importance [12]. To the best of our knowledge, the octahedral magnetite nanoparticles have not been reported in the literatures up to now. The main affecting factors on particle size and magnetic properties of magnetite were studied by using a single-factor method.
Iron sulfate heptahydrate (FeSO4 ·7H2 O), sodium nitrate (NaNO3 ) and sodium hydroxide (NaOH) were of analytical grade and were used as received. Magnetite nanoparticles were synthesized as follows: a predetermined volume of NaOH stock solution was placed into a three-neck flask after having used N2 to lustrate oxygen in it, and the NaOH solution in the three-neck flask was heated at the same time. When the temperature of the NaOH solution went up to 60–100 ◦ C, a predetermined volume of FeSO4 stock solution was added slowly into the three-neck flask under stirring at a rate of 200–500 rpm; after running out of the predetermined volume of FeSO4 solution for 5 min, a calculated NaNO3 stock solution was added slowly into the flask. When the NaNO3 stock solution was exhausted, the reaction was kept for 2–8 h. The magnetite particles were separated from reaction system by a strong magnet and washed five times with de-ionized water and ethanol, then dried in vacuum oven for 48 h. Thus a sample of magnetite nanoparticles was obtained and characterized by multiform techniques. X-ray diffraction (XRD) pattern was taken using a Rigaku D/max-2500 X-ray diffractometer with monochromatic Cu K␣ radiation (λ = 0.15405 nm) for crystal structure of the particle. Employing Scherrer’s equation, particle size was determined by an average of five values calculated according as the five strongest diffraction peaks in XRD pattern. Magnetic properties of the sample were measured by vibrating sample magnetometer (VSM) employing a LDJ-9500 magnetometer at room temperature. Using X L 30 S-FEG equipment,
∗
Corresponding author. Tel.: +86 10 689 13818; fax: +86 10 689 13818. E-mail address: [email protected] (T.-L. Zhang).
0921-5107/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.mseb.2006.08.030
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scanning electron microscopy (SEM) was employed to observe the morphology of the particles. 3. Results and discussions 3.1. The effect of reaction time (t) The samples used to study the effect of reaction time (t) on particle size and magnetic properties were synthesized at pH 9.5, F = 50 ml/min, T = 85 ◦ C, C = 0.3 mol/l, R = 400 rpm. It can be seen from Fig. 1 that the size of magnetite nanoparticles decrease from 38 nm to 34 nm as t was prolonged from 2 h to 4 h. However when t was prolonged from 4 h to 8 h, the particle size increased. Particle size can be affected by many factors [13]. Not-reacted Fe(OH)3 in a magnetite sample can make the particle size larger than the particle size of a pure magnetite sample. XRD (Fig. 2) proved that there is some not-reacted Fe(OH)3 in the sample synthesized at 2 h, and therefore, the particle size of the sample synthesized at 2 h is larger than that of the sample synthesized at 4 h, and the particle size arrives at the smallest value when the Fe(OH)3 was completely reacted into magnetite at 4 h. When t was longer than 4 h, some small mag-
netite particles were reunited to form larger ones, and the more material is added to magnetite nanoparticle surfaces to cause secondary nucleation and secondary growth, so that the particle sizes linearly increase with t after 4 h. It was accepted that it is usually associated with the smaller size of magnetic particles if the saturation magnetization of the magnetite particles is lower than that (92 emu/g) of bulk magnetite [14]. The principal effect of small size on a magnetic particle is the breaking of a larger number of exchange bonds for surface atoms. When some exchange bonds are removed at the surface, there can be frustration and spin disorder [12], thus the saturation magnetization of the magnetic particle decrease. On the other hand, the shape anisotropy and wide particle size distribution of the synthesized magnetite nanoparticles are also responsible for the relatively low magnetization [15]. All the values of saturation magnetization of samples in Fig. 1 are close to but lower than the value of 92 emu/g, proving that the particle in the samples are in good crystalline state but in smaller size. The sample synthesized at 2 h has lower magnetic properties than those synthesized at 4 h or 6 h. It proved further that there was some Fe(OH)3 in the sample synthesized at 2 h. In can be concluded that t ought to be controlled at 4 h because the sample synthesized at 4 h has good magnetic properties and the smallest size. 3.2. The effect of reactant concentration of Fe2+ (C)
Fig. 1. The effect of reaction time.
The samples used to study the effect of concentration of Fe2+ (C) on particle size and magnetic properties were synthesized at pH 9.5, F = 50 ml/min, T = 90 ◦ C, t = 4 h, R = 400 rpm. The results were plotted in Fig. 3, which shows that the size of magnetite nanoparticles increases with the increase of C. The formation of magnetite nanoparticles follows the nucleation-growth mechanism, and the particle size principally depends on their nucleation rate and growth rate [16]. If the nucleation rate of a particle is faster than the growth rate of it, its size is usually smaller; contrarily, its size is larger. When C was low, the nucleation rate of magnetite nanoparticles was faster than their growth rate because their growth was limited by
Fig. 2. XRD patterns of samples synthesized at 2 h and 4 h, respectively.
Fig. 3. The effect of reactant concentrations (Fe2+ ).
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the low C; therefore, the particle size synthesized at a lower C was correspondingly smaller. As C increased, their growth rate became faster; as a result their size was larger. Fig. 3 shows, furthermore, that the influence of C on the particle size becomes little when C is higher than 0.6 mol/l, revealing that the effect of C on the particle size has a limit. It was found in experiment that employing low concentration FeSO4 solution usually followed patina as intermediate. The samples synthesized at 0.2 mol/l or 0.4 mol/l are such ones that followed patina as intermediate. Some patina in the sample likely causes the low magnetic properties of the samples synthesized at low C (Fig. 3). 3.3. The effect of reaction temperature (T) Under other conditions unaltered, the effect of reaction temperature (T) on particle size and magnetic properties was studied. The samples used to study the effect of T were synthesized at pH 9.5, F = 50 ml/min, t = 4 h, C = 0.3 mol/l, R = 400 rpm. It can be seen from Fig. 4 that the size of magnetite nanoparticles increases when T was elevated. This result is in agreement with the results of other methods such as co-precipitation [17,18]. As we knew, the higher the T is, the faster the movement of a particle is. The particle growth is expedited with the increase of T and the rate of addition of material to the existing magnetite nuclei increases to quicken the secondary nucleation and secondary growth so that the larger particle can be synthesized at higher temperature. In the same way, the effect of T on magnetic properties of the sample is the same as the effect of T on the particle size. Noticeably, the nanoparticles synthesized at 90 ◦ C show higher saturation magnetization and larger size. 3.4. The effect of the flow rate of FeSO4 solution (F) The samples used to study the effect of flow rate of FeSO4 solution (F) were synthesized at pH 9.5, T = 90 ◦ C, C = 0.3 mol/l, R = 400 rpm, but the different flow rate of flow of FeSO4 solution.
Fig. 4. The effect of reaction temperature.
Fig. 5. The effect of flow rate of FeSO4 solution.
Fig. 5 shows that the size of magnetite nanoparticles increases with the increase of F from 10 ml/min to 90 ml/min. When the FeSO4 stock solution was dropped into the NaOH solution at the low F, the dropped FeSO4 solution was rapidly decentralized by the agitating to form a great deal of Fe(OH)2 nucleus, and almost everyone was surrounded by the superfluous NaOH solution. Magnetite nucleus began to form while NaNO3 was added into the reaction system, but the growth of magnetite nucleus was limited by the surrounding NaOH solution, so that the growth rate of magnetite nanoparticles was lower than their nucleation rate, therefore, magnetite in smaller size was synthesized. As F went up from 10 ml/min to 90 ml/min, the growth rate of magnetite nucleus increased, therefore, the size of magnetite nanoparticles became larger. However, when F went up to 130 ml/min, the dropping of FeSO4 stock solution almost became pouring of it, the plentiful FeSO4 solution in the reaction system made a mass of Fe(OH)2 nucleus form immediately, mass transfer was hampered by the high concentration and the growth of magnetite particles was confined to form smaller magnetite nanoparticles. From Fig. 5, we can see that the change of magnetization with F is the same as that of particle size with F, but the magnetic
Fig. 6. XRD patterns of the samples synthesized at the different F.
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Fig. 7. The effect of the rate of agitation.
properties of the magnetite nanoparticles come down when F was 130 ml/l. No impurity were revealed in the XRD patterns (Fig. 6) of the sample synthesized at 130 ml/l, therefore, the decline of its magnetic properties was probably caused by the smaller size of magnetite nanoparticles in the sample. 3.5. The effect of the rate of agitation (R) Under these conditions pH 9.5, F = 50 ml/min, T = 90 ◦ C, C = 0.3 mol/l unchanged, the rate of agitation (R) was changed
Fig. 8. The X-ray diffraction pattern of the magnetite sample synthesized at pH 9.5, F = 30 ml/min, T = 90 ◦ C, C = 0.6 mol/l, t = 4 h, R = 300 rpm.
from 200 rpm to 500 rpm to study the effect of R on the particle size and magnetic properties. The result was shown in Fig. 7, which shows that the particle size decreases as R increases. The shear force from the agitation can make the particle size smaller. The higher the R is, the stronger the shear force is. It is reasonable that the particle size go down with the increase of R. Noticeably, magnetic properties of magnetite nanoparticles were hardly influenced by the rate of agitation in the researched range (Fig. 7).
Fig. 9. (a–c) SEM images of magnetite nanoparticles of the sample synthesized at pH 9.5, F = 30 ml/min, T = 90 ◦ C, C = 0.6 mol/l, t = 4 h, R = 300 rpm.
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seen clearly that the as-synthesized sample shows a high saturation magnetization (87.1 emu/g) and a strong coercive force (132.3 Oe), but a low remanence magnetization (11.8 emu/g). 5. Conclusions
Fig. 10. The room-temperature magnetic hysteresis curve of the magnetite nanoparticles of the sample synthesized at pH 9.5, F = 30 ml/min, T = 90 ◦ C, C = 0.6 mol/l, t = 5 h, R = 300 rpm. It has a saturation magnetization of 87.1 emu/g and a coercive field of 132.3 Oe.
4. Characterization of the sample XRD pattern of the typical sample synthesized is seen in Fig. 8. The characteristic peaks at 2θ angles correspond well to the standard card of magnetite (JCPDS: 19-0629 [5]), which proves that the sample can be identified as Fe3 O4 with the spinel structure. Although all the reflections of Fe3 O4 and ␥-Fe2 O3 are similar, the characteristic reflections of 2 2 1, 2 1 0 and 2 1 3 planes corresponding to ␥-Fe2 O3 are found absent in Fig. 8. Therefore, the sample produced by our method can be identified as Fe3 O4 instead of ␥-Fe2 O3 [9]. Furthermore, the d values of the sample are observed to be closer to the standard values of Fe3 O4 phase than the other oxide of iron. From all the above observations, the as-synthesized sample can be identified as Fe3 O4 phase. The size of magnetite nanoparticles in the sample was calculated with the Scherrer’s equation, and the average size of nanoparticles is 40 nm or less. The strong peaks in XRD reveal that the particles were crystallized well. In order to observe the morphology of magnetite nanoparticles in the typical sample synthesized by this method, SEM was employed and the images obtained are shown in Fig. 9. The images of particles in the sample reveal a pattern with a very plain morphology of magnetite nanoparticles, which consists of nearly homogeneous grains (Fig. 9a) with the size from 80 nm to 200 nm (Fig. 9b). The figures of the particles present regular octahedron with very smooth surface (Fig. 9c), which further indicates that the particles were well crystallized. The measurement of the magnetic properties of the typical sample at room temperature is presented in Fig. 10. It can be
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