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A facile aqueous phase synthesis of cobalt microspheres at room temperature
Colloids and Surfaces A: Physicochem. Eng. Aspects 336 (2009) 41–45
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
A facile aqueous phase synthesis of cobalt microspheres at room temperature Yunling Li a , Jingzhe Zhao b,∗ , Xiaodan Su a , Yanchao Zhu a , Yi Wang c , Lanqin Tang a , Zichen Wang a a
College of Chemistry, Jilin University, Changchun 130023, PR China Chemistry and Chemical Engineering, Hunan University, Yuelu Mountain, Changsha 410082, PR China c Department of Chemistry, Yanbian University, Yanji 133000, PR China b
a r t i c l e
i n f o
Article history: Received 31 August 2008 Received in revised form 7 November 2008 Accepted 9 November 2008 Available online 19 November 2008 Keywords: Cobalt Aqueous phase Microspheres Magnetic Glycerin
a b s t r a c t Cobalt microspheres constructed by the assembly of nanoplatelets have been synthesized by a wet chemical reductive procedure at room temperature with the help of glycerin and citric acid and without additional surfactants. The size of the microspheres is about 2–5 m and that of the nanoplatelets assembled the microspheres is tens of nanometers in thickness. In this synthetic system, cobalt acetate was employed as Co source, sodium hydroxide was used to manipulate the pH value of the reaction system, and hydrazine hydrate was used as a reducing agent. A series of experiments were performed with different amounts of glycerin, from 0.5 mL to 4 mL, the results reveal that the formation of cobalt microspheres is assisted by glycerin. The shape, structure, and magnetic properties of the final products were investigated by XRD, SEM and VSM. This kind of Co nanostructures shows a ferromagnetic behavior at room temperature with enhanced coercivity, and has potential uses in magnetic recording devices and other related nanodevices. A possible mechanism for the formation of microspheres is proposed. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Nanoscale magnetic materials, especially Fe, Co and Ni, have attracted intensive interest because of their physical properties and various applications in catalysis, high-density data storage, magnetic sensors, etc. [1–3]. The magnetic properties of nanomaterials have been considered to be highly dependent on their size, structure, shape, crystallinity, etc. Ferromagnetic cobalt has received considerable attention both in theoretical and experimental purposes. In recent years, cobalt nanocrystals with a wide range of morphologies such as monodisperse particles [4], nanowires [5,6], nanorods [7,8], nanodisc/nanoplatelets [9,10], nanorings [11] and two- and three-dimensional (2D and 3D) superlattices [12] have been successfully synthesized. Various synthetic methods, including thermal decomposition of cobalt carbonyl and organometallic precursors [13,14], template-mediated synthesis [15], solvothermal methods [16] and solution-phase metal salt reduction [17,18] have been proposed and demonstrated to prepare ferromagnetic cobalt. Recently, Zhu and co-workers synthesized dendritic cobalt nanocrystal in ethanol phase via a reduction synthetic route and they also substituted ethanol with other solvents (glycol and glycerin) to study the phase and morphology changes of the cobalt products. The viscosity and the coordinating ability of solvents were their fun-
damental factors influencing the formation of the dendrites. Ohta and co-workers assembled Co nanoplatelets into microspheres through a surfactant-assisted hydrothermal procedure, in which NaH2 PO2 ·H2 O was employed as reducing agent and the particle size of the spheres is about 2.8 m [19]. Guo et al. have synthesized chainlike nanostructures consisting of Co hollow mesospheres in a polymer solution [20]. Liu et al. reported a work on highly ordered snowflake-like Co microcrystals via a hydrothermal reduction route [21]. Citric acid as complexing reagent and glycerin/glycol as reducing agent or solvent in synthesizing Co nanoparticles have been frequently reported [18,20,22]. However, up to now, there are few reports on using little amount of them as assistants to control the morphologies of the samples. In our work, we use an aqueous reduction strategy to prepare Co microspheres at room temperature with citric acid and glycerin as assistants, no surfactants was introduced. The self-assembled Co microspheres with size of 2–5 m were successfully prepared by reducing Co(II) to Co(0) via a onepot aqueous process, in which hydrazine hydrate was the reducing agent, sodium hydroxide was used to achieve favorable pH values of the reaction system. The high yields, simple apparatus, and mild conditions would make this synthetic method a good prospective for metal material applications in future. 2. Experiments
∗ Corresponding author. Tel.: +86 731 8809278; fax: +86 731 8809278. E-mail address: [email protected] (J. Zhao). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.11.012
All chemical reagents in this work are of analytical grade purity and were used as staring materials without further purification.
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Y. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 336 (2009) 41–45
2.1. Preparation In a typical reaction, a homogeneous solution was first prepared by dissolving 2.49 g of cobalt acetate (Co(Ac)2 ·4H2 O), 0.1 g of citric acid (CA) and 0.5–4 mL of glycerin into 20 mL of distilled water. The solution was pink in color. Under vigorously stirring, a restrained volume of NaOH solution and 10 mL of 40% hydrated hydrazine (N2 H4 ·H2 O) were added into the above solution, respectively. The color of the solution changed from pink to dark blue at once. All of the manipulations were performed at room temperature. The molar ratio of Co(Ac)2 ·4H2 O to N2 H4 ·H2 O was 1:16. After 30-min reaction, black precipitates appeared. The precipitates were separated from the solution by placing a magnet under the container, and then washed several times with distilled water and absolute alcohol to remove any residual alkaline salt. The resulted Co products were obtained by drying wet precipitates in a vacuum system at room temperature. 2.2. Characterization The obtained samples were characterized by X-ray powder diffraction (XRD), scanning electronic microscopy (SEM) and vibrating sample magnetometer (VSM). The crystallization of Co microspheres was determined by a Shimadzu model XRD-6000 using Cu K␣ radiation. The morphology and particle size of the powders were characterized by a field-emission scanning electron microscope (FESEM, JEOL JSM-6700F). VSM measurements were performed for cobalt power in a VSM. 3. Results and discussion 3.1. Crystal structure The crystallization and chemical composition of the resulting products were examined by X-ray diffraction. Fig. 1 is a representative XRD pattern of the as-prepared Co sample synthesized at room temperature. As shown in Fig. 1, the diffraction peaks at 2 = 41.54◦ , 44.54◦ , 47.36◦ , 63.12◦ and 71.66◦ are well indexed to the reflection planes of (1 0 0), (0 0 2), (1 0 1), (1 0 2) and (1 1 0) of hexagonal cobalt (JCPDS 05-0727). Broadening of the peaks exhibits the nanocrystalline nature of the sample. In the formal PCPDF card of hcp Co, the diffraction peak of (1 0 1) is the strongest one, while the (0 0 2) peak is weaker. However, the relative intensity of the (0 0 2) peak for the sample prepared under current conditions increase significantly, which indicates that highly oriented growth of cobalt particles occurred. This agrees with the previous reports [5,23,24]. No characteristic peaks due to the impurities of cobalt oxide or hydroxides were detected. The XRD results indicate that Co products with high purity were obtained by our simple aqueous strategy.
Fig. 1. XRD patterns of as-prepared Co samples.
3.2. Morphology The morphology of the samples was characterized by SEM measurement, as shown in Fig. 2. Fig. 2a is the SEM image of the Co microspheres obtained at room temperature, and Fig. 2b is the magnified image taken from a selected section in Fig. 2a. The size of the microspheres is about 2–5 m. It is clearly seen that a microsphere is composed of ordered platelets with a thickness of tens of nanometers. And the state of the nanoplatelets is uprighting on the surfaces of the microspheres. To shed light on the formation of cobalt microspheres, a series of reactions were performed by changing reaction parameters, such as the amount of glycerin, reaction temperatures, concentration of reactants, feeding order and so on. The effects of glycerin in controlling the morphology of products are prominent in our experiments. Fig. 3 is the SEM images of the products obtained at room temperature with different amounts of glycerin. The blank sample without glycerin in Fig. 3a exhibits a tree-like morphology. Fig. 3b–d gives the morphologies of the products with increasing amount of glycerin from 2.5% to 10% in volume percent. It is clearly seen that the experiments in parallel conditions with less glycerin also produce branched particles as shown in Fig. 3b. When the addition of glycerin reached a certain amount, such as 5%, Co microspheres appeared with relatively uniform diameter of 2–5 m and better distribution, as shown in Fig. 3c. Further introduction of glycerin into the reaction system
Fig. 2. (a) SEM images of the Co microspheres obtained at room temperature and (b) a further magnification SEM image taken from a selected section in (a).
Y. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 336 (2009) 41–45
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Fig. 3. SEM images of the products obtained at room temperature with an increasing amount of glycerin: (a) 0%, (b) 2.5%, (c) 5% and (d) 10%.
led to the formation of larger spheres as shown in Fig. 3d, and the aggregation of the as-synthesized product was more serious. These results reveal that an appropriate amount of glycerin should be involved in the reactions to form Co microspheres in our strategy. As we believe, cobalt microspheres come into being through steps of complexation, reduction, nucleation and assembly in a multicomponent system. The reduction reaction in the presence of CA can be formulated as follows [25]: 2[Co(C6 H5 O7 )2 ]4− + N2 H4 + 4OH− → 2Co ↓ + N2 ↑ + 4H2 O + 4C6 H5 O7 2− Based on the results obtained so far, we propose that the glycerin assists the aggregation of nanoparticles to form microspheres. The whole process is illustrated in Fig. 4. In the beginning, citrate ions can coordinate with cobalt ions to form [Co(C6 H5 O7 )2 ]4− complexes in the solution, as observed previously [26]. And this step is very important, because the succeeding reduction reactions cannot go on without CA in our current system. So, CA is indispensable for the reactions. Subsequently, the glycerin was dispersed into our system to form minireactors. The reducing agent, N2 H4 ·H2 O, entered into the minireactor and converted [Co(C6 H5 O7 )2 ]4− to small Co nanopar-
ticles with help of OH− . According to the literatures [9,19,27], Co as well as its compounds is preferred to form platelets, same result was found in our experiments. These nanoplates have a tendency to aggregate, and at the same time, the miscells formed by glycerin limited the range of their growth, which led to the assembly of nanoplates to microspheres. So, in the situation of no or less glycerin, the products extended a branched morphology instead of microsphere. However, compared to the blank sample, the sample with less glycerin of 2.5% also tends to be spherical aggregates (Fig. 3b). This reveals that glycerin in the reaction system plays an important role in forming Co microspheres. And we presume that hydroxyls ( OH) in glycerin determine the effect of glycerin in the reductive reaction. In order to prove our presumption and confirm the effects of hydroxyl, we substituted glycol for glycerin in our experiments. Fig. 5 is the SEM images of the products obtained at room temperature in the presence of glycol. Fig. 5a gives a flower-like morphology of the sample, which was prepared with 5% glycol in volume percent. Fig. 5b corresponds to the sample with 25% glycol, microspheres in the size of 1–5 m can be seen clearly in the micrograph with better dispersion. These results reveal that Co microspheres can also be obtained with adequate amount of glycol. The action of glycol is similar to glycerin in our experiments to get Co micro-
Fig. 4. An illustration of the possible formation mechanism of the cobalt microspheres.
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Y. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 336 (2009) 41–45
Fig. 5. The SEM images of the products obtained at room temperature with the help of glycol: (a) 5% and (b) 25%.
spheres, the amount of glycol is required more comparing with glycerin because of their different hydroxyls in one molecule. Above all, we can conclude that polyol determine the formation of Co microspheres in our strategy. Otherwise, the reaction temperature also played an important role in our experiments on controlling the morphologies of the samples. As the temperature increased, the morphologies of the products changed from spheres at room temperature to branched assemblies at higher temperature of 353 K with similar amount of polyol in the samples of different temperatures (see supporting information Fig. S1). Furthermore, experiments under different reaction parameters, such as the concentration of NaOH and N2 H4 ·H2 O, feeding order, the ratio of the N2 H4 /Co2+ , reveal that they have little influences on the morphology and the particle size of the products. 3.3. Magnetic properties The magnetic properties of the as-synthesized cobalt microcrystals were measured at room temperature. Here we chose the microspheres under controlled amount of glycerin as examples to characterize the magnetic properties of cobalt microcrystals. The magnetic hysteresis loop (M–H loop) (Fig. 6a) of the as-synthesized cobalt microspheres measured at 300 K indicates the magnetic properties including saturation magnetization Ms and the coercivity Hc. From the M–H curves, the saturation magnetization value
Ms of the cobalt microspheres is 155.3 emu/g, and the coercivity value Hc for the samples is 259 Oe. In order to compare with the microspheres, magnetic properties of the cobalt microcrystal with tree-like morphology corresponding to the sample of Fig. 3a were also given in Fig. 6b. And the saturation magnetization (Ms) and coercivity (Hc) values of the tree-like particles are 105.2 emu/g and 268 Oe, respectively. Both the powdered Co microspheres (a) and tree-like particles (b) show a ferromagnetic behavior and their saturation magnetization are both reduced relative to the bulk cobalt (168 emu/g) [28]. But, compared to the tree-like particles, the saturation magnetization of the microspheres shows an increase. Many possible mechanisms on the influence of the saturation magnetization have been proposed, such as existence of impurities [29], surface antiferromagnetic oxidation [30], surface spin disorder [19,29–31], crystallinity, etc. The above two samples are similar in purity and crystallinty, which can be indicated by XRD patterns (see supporting information Fig. S2). So, such a difference on the saturation magnetization of microspheres and tree-like particles in our work would be attributed to a difference of the pinned surface magnetic moments in overall magnetization [16,31]. The decrease of the saturation magnetization of microspheres compared to bulk material might be partly due to a very small quantity of citrate or glycerin molecules absorbed on the Co microspheres and partly due to the surface oxidation of Co microcrystals. The coercivity value, Hc, is greatly enhanced as compared with the value of bulk Co (a few tens Oe) [5,23,24]. This enhancement might be due to the effect of surface anisotropy or smaller sizes of basic cells as nanoplatelets. However, the coercivity value of microspheres is much lower than those of the 2D and 3D superlattice of nanorods (740 Oe, 3200 Oe and 7200 Oe) with smaller sizes [8]. Because a higher coercivity is an important factor for high-density information storage, further experiments are in progress to improve the coercivity of these microspheres. 4. Conclusion In summary, we emphasize that cobalt microspheres were synthesized under controlled glycerin and citric acid via an aqueous solution reduction. Glycerin is introduced as a shape modifier to control the growth of the Co nanostructures. And the citric acid is a key reactant to accelerate the reducing reaction. The cobalt microspheres with size of 2–5 m show a ferromagnetic behavior, which is slightly better than that of the bulk and has potential uses in nanodevices. Acknowledgments
Fig. 6. Hysteresis loops of the as-synthesized Co microcrystals measured at 300 K: (a) microspheres assembled with nanoplatelets and (b) tree-like particles.
This work was supported by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education
Y. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 336 (2009) 41–45
Ministry (SRF for ROCS, SEM), the Natural Science Foundation of Jilin Province for Excellent Young Scholars (Grant No. 20040117). We also acknowledge the financial support from Scientific Research Foundation of Hunan University. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.colsurfa.2008.11.012. References [1] P. Dutta, B.C. Dunn, E.M. Eyring, N. Shah, G.P. Huffman, A. Manivannan, M.S. Seehra, Chem. Mater. 17 (2005) 5183–5186. [2] V.F. Puntes, K.M. Krishnan, A.P. Alivisatos, Appl. Phys. Lett. 78 (2001) 2187–2189. [3] D.J. Sellmyer, M. Yu, R.D. Kirby, Nanostruct. Mater. 12 (1999) 1021–1026. [4] Y. Bao, M. Beerman, A.B. Pakhomov, K.M. Krishnan, J. Phys. Chem. B 109 (2005) 7220–7222. [5] Q. Xie, Y.T. Qian, S.Y. Zhang, S.Q. Fu, W.C. Yu, Eur. J. Inorg. Chem. (2006) 2454–2459. [6] X.H. Huang, L. Li, X. Luo, X.G. Zhu, G.H. Li, J. Phys. Chem. 112 (2008) 1468–1472. [7] N. Shukla, E.B. Svedberg, J. Ell, A.J. Roy, Mater. Lett. 60 (2006) 1950–1955. [8] F. Dumestre, B. Chaudret, C. Amiens, M. Respaud, P. Fejes, P. Renaud, P. Zurcher, Angew. Chem. Int. Ed. 42 (2003) 5213–5216. [9] R. Xu, T. Xie, Y.G. Zhao, Y.D. Li, Cryst. Growth Des. 7 (2007) 1904–1911. [10] J.G. Li, J.J. Huang, Y. Qin, F. Ma, Mater. Sci. Eng. B 138 (2007) 199–204. [11] S.L. Tripp, R.E. Dunin-Borkowski, A. Wei, Angew. Chem. Int. Ed. 42 (2003) 5591–5593. [12] F. Wetz, K. Soulantica, M. Respaud, A. Falqui, B. Chaudret, Mater. Sci. Eng. C 27 (2007) 1162–1166.
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[13] C. Gvrler, M. Feyen, S. Behrens, N. Matoussevitch, A.M. Schmidt, Polymer 49 (2008) 2211–2216. [14] F. Dumestre, B. Chaudret, C. Amiens, M.C. Fromen, M.J. Casanove, P. Renaud, P. Zurcher, Angew. Chem. Int. Ed. 41 (2002) 4286–4289. [15] A.K. Srivastava, S. Madhavi, T.J. White, R.V. Ramanujan, J. Mater. Chem. 15 (2005) 4424–4428. [16] X. Wang, F.L. Yuan, P. Hu, L.J. Yu, L.Y. Bai, J. Phys. Chem. C 112 (2008) 8773– 8778. ˜ [17] V. Salgueirino-Maceira, M.A. Correa-Duarte, M. Farle, M.A. López-Quintela, K. Sieradzki, R. Diaz, Langmuir 22 (2006) 1455–1458. [18] Y.C. Zhu, H.G. Zheng, Q. Yang, A.L. Pan, Z.P. Yang, Y.T. Qian, J. Cryst. Growth 260 (2004) 427–434. [19] Y. Hou, H. Kondoh, T. Ohta, Chem. Mater. 17 (2005) 3994–3996. [20] L. Guo, S. Yang, C. Chen, Adv. Funct. Mater. 17 (2007) 425–430. [21] X. Liu, R. Yi, Y. Wang, G. Qiu, N. Zhang, X. Li, J. Phys. Chem. C 111 (2007) 163–167. [22] M.Y. Guan, J.H. Sun, C.L. Gao, X. Li, X. Zheng, ChemPhysChem 8 (2007) 2182–2184. [23] X.B. Cao, Y. Xie, L.Y. Li, Adv. Mater. 15 (2003) 1914–1918. [24] Q. Xie, Z. Dai, W.W. Huang, J.B. Liang, C.L. Liang, Y.T. Qian, Nanotechnology 16 (2005) 2958–2962. [25] P.Z. Lu, H.M. Xiao, W.D. Zhang, Y. Yang, S.Y. Fu, Cryst. Growth Des. 8 (2008) 1113–1118. [26] M. Matzapetakis, M. Dakanali, C.R. Raptopoulou, V. Tangoulis, A. Terzis, N. Moon, J. Giapintzakis, A. Salifoglou, J. Biol. Inorg. Chem. 4 (2000) 469– 474. [27] Y.L. Hou, H. Kondoh, M. Shimojo, T. Kogure, T. Ohta, J. Phys. Chem. B 109 (2005) 19094–19098. [28] G. Dumpich, T.P. Krome, B. Hausmans, J. Magn. Magn. Mater. 248 (2002) 241–247. [29] L. Zhang, A. Manthiram, Phys. Rev. B 54 (1996) 3462–3467. [30] D. Lin, A.C. Nunes, C.F. Majkrzak, A.E. Berkowitz, J. Magn. Magn. Mater. 145 (1995) 343–348. [31] J. Ye, Q.W. Chen, H.P. Qi, N. Tao, Cryst. Growth Des. 8 (2008) 2464–2468.
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
A facile aqueous phase synthesis of cobalt microspheres at room temperature Yunling Li a , Jingzhe Zhao b,∗ , Xiaodan Su a , Yanchao Zhu a , Yi Wang c , Lanqin Tang a , Zichen Wang a a
College of Chemistry, Jilin University, Changchun 130023, PR China Chemistry and Chemical Engineering, Hunan University, Yuelu Mountain, Changsha 410082, PR China c Department of Chemistry, Yanbian University, Yanji 133000, PR China b
a r t i c l e
i n f o
Article history: Received 31 August 2008 Received in revised form 7 November 2008 Accepted 9 November 2008 Available online 19 November 2008 Keywords: Cobalt Aqueous phase Microspheres Magnetic Glycerin
a b s t r a c t Cobalt microspheres constructed by the assembly of nanoplatelets have been synthesized by a wet chemical reductive procedure at room temperature with the help of glycerin and citric acid and without additional surfactants. The size of the microspheres is about 2–5 m and that of the nanoplatelets assembled the microspheres is tens of nanometers in thickness. In this synthetic system, cobalt acetate was employed as Co source, sodium hydroxide was used to manipulate the pH value of the reaction system, and hydrazine hydrate was used as a reducing agent. A series of experiments were performed with different amounts of glycerin, from 0.5 mL to 4 mL, the results reveal that the formation of cobalt microspheres is assisted by glycerin. The shape, structure, and magnetic properties of the final products were investigated by XRD, SEM and VSM. This kind of Co nanostructures shows a ferromagnetic behavior at room temperature with enhanced coercivity, and has potential uses in magnetic recording devices and other related nanodevices. A possible mechanism for the formation of microspheres is proposed. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Nanoscale magnetic materials, especially Fe, Co and Ni, have attracted intensive interest because of their physical properties and various applications in catalysis, high-density data storage, magnetic sensors, etc. [1–3]. The magnetic properties of nanomaterials have been considered to be highly dependent on their size, structure, shape, crystallinity, etc. Ferromagnetic cobalt has received considerable attention both in theoretical and experimental purposes. In recent years, cobalt nanocrystals with a wide range of morphologies such as monodisperse particles [4], nanowires [5,6], nanorods [7,8], nanodisc/nanoplatelets [9,10], nanorings [11] and two- and three-dimensional (2D and 3D) superlattices [12] have been successfully synthesized. Various synthetic methods, including thermal decomposition of cobalt carbonyl and organometallic precursors [13,14], template-mediated synthesis [15], solvothermal methods [16] and solution-phase metal salt reduction [17,18] have been proposed and demonstrated to prepare ferromagnetic cobalt. Recently, Zhu and co-workers synthesized dendritic cobalt nanocrystal in ethanol phase via a reduction synthetic route and they also substituted ethanol with other solvents (glycol and glycerin) to study the phase and morphology changes of the cobalt products. The viscosity and the coordinating ability of solvents were their fun-
damental factors influencing the formation of the dendrites. Ohta and co-workers assembled Co nanoplatelets into microspheres through a surfactant-assisted hydrothermal procedure, in which NaH2 PO2 ·H2 O was employed as reducing agent and the particle size of the spheres is about 2.8 m [19]. Guo et al. have synthesized chainlike nanostructures consisting of Co hollow mesospheres in a polymer solution [20]. Liu et al. reported a work on highly ordered snowflake-like Co microcrystals via a hydrothermal reduction route [21]. Citric acid as complexing reagent and glycerin/glycol as reducing agent or solvent in synthesizing Co nanoparticles have been frequently reported [18,20,22]. However, up to now, there are few reports on using little amount of them as assistants to control the morphologies of the samples. In our work, we use an aqueous reduction strategy to prepare Co microspheres at room temperature with citric acid and glycerin as assistants, no surfactants was introduced. The self-assembled Co microspheres with size of 2–5 m were successfully prepared by reducing Co(II) to Co(0) via a onepot aqueous process, in which hydrazine hydrate was the reducing agent, sodium hydroxide was used to achieve favorable pH values of the reaction system. The high yields, simple apparatus, and mild conditions would make this synthetic method a good prospective for metal material applications in future. 2. Experiments
∗ Corresponding author. Tel.: +86 731 8809278; fax: +86 731 8809278. E-mail address: [email protected] (J. Zhao). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.11.012
All chemical reagents in this work are of analytical grade purity and were used as staring materials without further purification.
42
Y. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 336 (2009) 41–45
2.1. Preparation In a typical reaction, a homogeneous solution was first prepared by dissolving 2.49 g of cobalt acetate (Co(Ac)2 ·4H2 O), 0.1 g of citric acid (CA) and 0.5–4 mL of glycerin into 20 mL of distilled water. The solution was pink in color. Under vigorously stirring, a restrained volume of NaOH solution and 10 mL of 40% hydrated hydrazine (N2 H4 ·H2 O) were added into the above solution, respectively. The color of the solution changed from pink to dark blue at once. All of the manipulations were performed at room temperature. The molar ratio of Co(Ac)2 ·4H2 O to N2 H4 ·H2 O was 1:16. After 30-min reaction, black precipitates appeared. The precipitates were separated from the solution by placing a magnet under the container, and then washed several times with distilled water and absolute alcohol to remove any residual alkaline salt. The resulted Co products were obtained by drying wet precipitates in a vacuum system at room temperature. 2.2. Characterization The obtained samples were characterized by X-ray powder diffraction (XRD), scanning electronic microscopy (SEM) and vibrating sample magnetometer (VSM). The crystallization of Co microspheres was determined by a Shimadzu model XRD-6000 using Cu K␣ radiation. The morphology and particle size of the powders were characterized by a field-emission scanning electron microscope (FESEM, JEOL JSM-6700F). VSM measurements were performed for cobalt power in a VSM. 3. Results and discussion 3.1. Crystal structure The crystallization and chemical composition of the resulting products were examined by X-ray diffraction. Fig. 1 is a representative XRD pattern of the as-prepared Co sample synthesized at room temperature. As shown in Fig. 1, the diffraction peaks at 2 = 41.54◦ , 44.54◦ , 47.36◦ , 63.12◦ and 71.66◦ are well indexed to the reflection planes of (1 0 0), (0 0 2), (1 0 1), (1 0 2) and (1 1 0) of hexagonal cobalt (JCPDS 05-0727). Broadening of the peaks exhibits the nanocrystalline nature of the sample. In the formal PCPDF card of hcp Co, the diffraction peak of (1 0 1) is the strongest one, while the (0 0 2) peak is weaker. However, the relative intensity of the (0 0 2) peak for the sample prepared under current conditions increase significantly, which indicates that highly oriented growth of cobalt particles occurred. This agrees with the previous reports [5,23,24]. No characteristic peaks due to the impurities of cobalt oxide or hydroxides were detected. The XRD results indicate that Co products with high purity were obtained by our simple aqueous strategy.
Fig. 1. XRD patterns of as-prepared Co samples.
3.2. Morphology The morphology of the samples was characterized by SEM measurement, as shown in Fig. 2. Fig. 2a is the SEM image of the Co microspheres obtained at room temperature, and Fig. 2b is the magnified image taken from a selected section in Fig. 2a. The size of the microspheres is about 2–5 m. It is clearly seen that a microsphere is composed of ordered platelets with a thickness of tens of nanometers. And the state of the nanoplatelets is uprighting on the surfaces of the microspheres. To shed light on the formation of cobalt microspheres, a series of reactions were performed by changing reaction parameters, such as the amount of glycerin, reaction temperatures, concentration of reactants, feeding order and so on. The effects of glycerin in controlling the morphology of products are prominent in our experiments. Fig. 3 is the SEM images of the products obtained at room temperature with different amounts of glycerin. The blank sample without glycerin in Fig. 3a exhibits a tree-like morphology. Fig. 3b–d gives the morphologies of the products with increasing amount of glycerin from 2.5% to 10% in volume percent. It is clearly seen that the experiments in parallel conditions with less glycerin also produce branched particles as shown in Fig. 3b. When the addition of glycerin reached a certain amount, such as 5%, Co microspheres appeared with relatively uniform diameter of 2–5 m and better distribution, as shown in Fig. 3c. Further introduction of glycerin into the reaction system
Fig. 2. (a) SEM images of the Co microspheres obtained at room temperature and (b) a further magnification SEM image taken from a selected section in (a).
Y. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 336 (2009) 41–45
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Fig. 3. SEM images of the products obtained at room temperature with an increasing amount of glycerin: (a) 0%, (b) 2.5%, (c) 5% and (d) 10%.
led to the formation of larger spheres as shown in Fig. 3d, and the aggregation of the as-synthesized product was more serious. These results reveal that an appropriate amount of glycerin should be involved in the reactions to form Co microspheres in our strategy. As we believe, cobalt microspheres come into being through steps of complexation, reduction, nucleation and assembly in a multicomponent system. The reduction reaction in the presence of CA can be formulated as follows [25]: 2[Co(C6 H5 O7 )2 ]4− + N2 H4 + 4OH− → 2Co ↓ + N2 ↑ + 4H2 O + 4C6 H5 O7 2− Based on the results obtained so far, we propose that the glycerin assists the aggregation of nanoparticles to form microspheres. The whole process is illustrated in Fig. 4. In the beginning, citrate ions can coordinate with cobalt ions to form [Co(C6 H5 O7 )2 ]4− complexes in the solution, as observed previously [26]. And this step is very important, because the succeeding reduction reactions cannot go on without CA in our current system. So, CA is indispensable for the reactions. Subsequently, the glycerin was dispersed into our system to form minireactors. The reducing agent, N2 H4 ·H2 O, entered into the minireactor and converted [Co(C6 H5 O7 )2 ]4− to small Co nanopar-
ticles with help of OH− . According to the literatures [9,19,27], Co as well as its compounds is preferred to form platelets, same result was found in our experiments. These nanoplates have a tendency to aggregate, and at the same time, the miscells formed by glycerin limited the range of their growth, which led to the assembly of nanoplates to microspheres. So, in the situation of no or less glycerin, the products extended a branched morphology instead of microsphere. However, compared to the blank sample, the sample with less glycerin of 2.5% also tends to be spherical aggregates (Fig. 3b). This reveals that glycerin in the reaction system plays an important role in forming Co microspheres. And we presume that hydroxyls ( OH) in glycerin determine the effect of glycerin in the reductive reaction. In order to prove our presumption and confirm the effects of hydroxyl, we substituted glycol for glycerin in our experiments. Fig. 5 is the SEM images of the products obtained at room temperature in the presence of glycol. Fig. 5a gives a flower-like morphology of the sample, which was prepared with 5% glycol in volume percent. Fig. 5b corresponds to the sample with 25% glycol, microspheres in the size of 1–5 m can be seen clearly in the micrograph with better dispersion. These results reveal that Co microspheres can also be obtained with adequate amount of glycol. The action of glycol is similar to glycerin in our experiments to get Co micro-
Fig. 4. An illustration of the possible formation mechanism of the cobalt microspheres.
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Fig. 5. The SEM images of the products obtained at room temperature with the help of glycol: (a) 5% and (b) 25%.
spheres, the amount of glycol is required more comparing with glycerin because of their different hydroxyls in one molecule. Above all, we can conclude that polyol determine the formation of Co microspheres in our strategy. Otherwise, the reaction temperature also played an important role in our experiments on controlling the morphologies of the samples. As the temperature increased, the morphologies of the products changed from spheres at room temperature to branched assemblies at higher temperature of 353 K with similar amount of polyol in the samples of different temperatures (see supporting information Fig. S1). Furthermore, experiments under different reaction parameters, such as the concentration of NaOH and N2 H4 ·H2 O, feeding order, the ratio of the N2 H4 /Co2+ , reveal that they have little influences on the morphology and the particle size of the products. 3.3. Magnetic properties The magnetic properties of the as-synthesized cobalt microcrystals were measured at room temperature. Here we chose the microspheres under controlled amount of glycerin as examples to characterize the magnetic properties of cobalt microcrystals. The magnetic hysteresis loop (M–H loop) (Fig. 6a) of the as-synthesized cobalt microspheres measured at 300 K indicates the magnetic properties including saturation magnetization Ms and the coercivity Hc. From the M–H curves, the saturation magnetization value
Ms of the cobalt microspheres is 155.3 emu/g, and the coercivity value Hc for the samples is 259 Oe. In order to compare with the microspheres, magnetic properties of the cobalt microcrystal with tree-like morphology corresponding to the sample of Fig. 3a were also given in Fig. 6b. And the saturation magnetization (Ms) and coercivity (Hc) values of the tree-like particles are 105.2 emu/g and 268 Oe, respectively. Both the powdered Co microspheres (a) and tree-like particles (b) show a ferromagnetic behavior and their saturation magnetization are both reduced relative to the bulk cobalt (168 emu/g) [28]. But, compared to the tree-like particles, the saturation magnetization of the microspheres shows an increase. Many possible mechanisms on the influence of the saturation magnetization have been proposed, such as existence of impurities [29], surface antiferromagnetic oxidation [30], surface spin disorder [19,29–31], crystallinity, etc. The above two samples are similar in purity and crystallinty, which can be indicated by XRD patterns (see supporting information Fig. S2). So, such a difference on the saturation magnetization of microspheres and tree-like particles in our work would be attributed to a difference of the pinned surface magnetic moments in overall magnetization [16,31]. The decrease of the saturation magnetization of microspheres compared to bulk material might be partly due to a very small quantity of citrate or glycerin molecules absorbed on the Co microspheres and partly due to the surface oxidation of Co microcrystals. The coercivity value, Hc, is greatly enhanced as compared with the value of bulk Co (a few tens Oe) [5,23,24]. This enhancement might be due to the effect of surface anisotropy or smaller sizes of basic cells as nanoplatelets. However, the coercivity value of microspheres is much lower than those of the 2D and 3D superlattice of nanorods (740 Oe, 3200 Oe and 7200 Oe) with smaller sizes [8]. Because a higher coercivity is an important factor for high-density information storage, further experiments are in progress to improve the coercivity of these microspheres. 4. Conclusion In summary, we emphasize that cobalt microspheres were synthesized under controlled glycerin and citric acid via an aqueous solution reduction. Glycerin is introduced as a shape modifier to control the growth of the Co nanostructures. And the citric acid is a key reactant to accelerate the reducing reaction. The cobalt microspheres with size of 2–5 m show a ferromagnetic behavior, which is slightly better than that of the bulk and has potential uses in nanodevices. Acknowledgments
Fig. 6. Hysteresis loops of the as-synthesized Co microcrystals measured at 300 K: (a) microspheres assembled with nanoplatelets and (b) tree-like particles.
This work was supported by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education
Y. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 336 (2009) 41–45
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