Flower-like cobalt nanocrystals by a complex precursor reaction route

Materials Chemistry and Physics 91 (2005) 293–297

Flower-like cobalt nanocrystals by a complex precursor reaction route Yongchun Zhu, Qing Yang, Huagui Zheng, Weichao Yu, Yitai Qian∗ Structure Research Laboratory, Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China Received 29 June 2004; received in revised form 6 October 2004; accepted 21 November 2004

Abstract A complex reaction route was developed for the preparation of flower-like cobalt nanocrystals by using 2-hydroxy-4-(1-methylheptyl) benzophenone oxime (N530), a normally effective chelate extracting agent. X-ray powder diffraction pattern shows that the cobalt nanocrystals coexist of both hexagonal close-packed (hcp) and cubic close-packed (ccp) forms, with flower-like architecture characterized by a field emission scanning electronic microscopy. The magnetic coercivity Hc of the as-prepared cobalt crystals came up to 360 Oe at room temperature. The existence of the precursor N530–CoCl2 complex was determined by a Fourier transform infrared (FT-IR) spectrometer. Experiments suggested that the existence and the framework structure of the precursor complex have a considerable effect on the final architectures of cobalt nanocrystals. © 2004 Elsevier B.V. All rights reserved. Keywords: Magnetic materials; Nanostructures; Chemical synthesis; Magnetic properties

1. Introduction Transition metallic materials, especially Fe, Co and Ni, have been extensively studied due to their various properties and technological applications, such as catalysis, solar energy absorption, permanent magnets, magnetic fluids and magnetic recording media [1–6]. Of the three ferromagnetic transition metals of Fe, Co and Ni, oriented cobalt nanoclusters probably have the greatest potential application because ␣-Co (unlike iron and nickel) is uniaxial [7]. Co nanocrystals have been synthesized by several chemical methods, such as metal carbonyl pyrolysis [8–10], electrochemical deposition [11,12] and solution-phase metal salt reduction [13–15]. The redox procedure using hydrazine hydrate as reducing agent is a facile and inexpensive route, and the product produced displays less chemical contamination from the reaction system. For example, Gibson prepared cobalt nanoclusters by the reduction of Co2+ in hydrazine system initiated by a high-intensity ultrasound [16]. ∗

Corresponding author. Tel.: +86 5513601589; fax: +86 5513607402. E-mail address: [email protected] (Y. Qian).

0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.11.025

In recent years, one of the important goals of material scientists is to develop ways of tailoring the structure of materials with unusual and novel forms. The shape and the texture of metals are well known to determine the electronic, optical, catalytic and magnetic properties [11–13]. Recently, several methods have been attempted for the synthesis of metals with complex patterns and much has been made of polymers as the crystal morphology modifiers [17]. Up to now, considerable efforts have been put into the preparation of cobalt nanoparticles and nanowires [8–16], while the reports concerning sheet-like cobalt nanostructures are quite few. Here, we report a facile complex reaction route to flower-shaped cobalt structures composed of flake-like nanocrystals with precursor N530–CoCl2 complex reduced by hydrazine hydrate in ethanol at room temperature. 2-Hydroxy-4-(1-methylheptyl) benzophenone oxime (N530) is a normally effective chelate extracting agent and it has been widely used for the extraction of Cu, Fe, Co, Ni and some other metals from wastes [18]. These studies inspired us to utilize such an extracting agent to deduce complexes with metal ions, and thus tailor the growth of target nanomaterials with defined morphologies, subsequently control the corresponding properties.

294

Y. Zhu et al. / Materials Chemistry and Physics 91 (2005) 293–297

2. Experimental procedure All chemical reagents in this work were of analytical grade purity. N530 (containing 80 wt.% trans-N530) was developed by Institute of Shanghai Organic Chemistry of Chinese Academy of Science. In a typical procedure, CoCl2 ·6H2 O (2.0 g) and N530 (4.0 g) were dissolved into 40 ml ethanol. The mixture of hydrazine hydrate (10 ml 50%) and sodium hydroxide (5.0 g) was added dropwise into the mixture solution within 20 min under constant stirring at room temperature. The reaction solution was aged for 48 h. The resulting black solid powder was separated by placing a magnet under the container, and then washed several times with hydrazine hydrate (50%), distilled water and absolute alcohol. The black powders were collected for characterization. The obtained samples were characterized by X-ray powder diffraction (XRD) on a Japan Dmax-␥A X-ray diffractometer using graphite-monochromatized Cu K␣ radiation ˚ The overview morphologies of the prod(λ = 1.54178 A). ucts were observed by field emission scanning electronic microscopy (FE-SEM), performed on a JEOL-6700F scanning electron microanalyzer. Transmission electron microscopy (TEM), taken with a Hitachi H-800 microscope with an accelerating voltage of 200 kV, was used to observe the morphologies of as-prepared products. For the TEM studies, the samples were dispersed in ethanol in an ultrasonic bath and then were dropped onto Cu grids coated with lacey carbon films. FT-IR spectra were recorded on a Nicolet Model 759 Fourier Transmission Infrared spectrometer, at wave numbers 500–4000 cm−1 . The room-temperature magnetic characterization of the sample was carried out by a BHV-55 vibrating sample magnetometer.

3. Results and discussion 3.1. Phase and morphology of the products Fig. 1 is the XRD pattern of the as-prepared Co sample. The pattern shows that the sample is hexagonal close-packed (hcp) coexist with cubic close-packed (ccp) cobalt. Though the hcp structure is the most stable phase for bulk Co at room temperature, the coexistence of ccp and hcp phases in the same cobalt sample are relative stable phases, as reported by Sun and Murray [13]. The ccp and hcp phases of cobalt are close-packed structures that differ only in the stacking sequence of atomic planes in the cubic [1 1 1] direction. Low stacking faults energy could easily lead to the formation of both phases in the same sample [9,13]. This phase transition phenomena are often observed in other materials, such as CdSe [19] and SiC [20]. Fig. 2 depicts the FE-SEM images showing the cobalt crystals with flower-like architecture. These flower-like morphologies consist of crispate flakes observed from a highmagnification FE-SEM image (shown in Fig. 2b). The prod-

Fig. 1. XRD pattern of the product (F: ccp, H: hcp).

uct was further examined by TEM, as shown in Fig. 3. The TEM images demonstrate that the produced crystallites are thin nanostructured flakes, which are consistent with the morphologies observed by FE-SEM. The electron diffraction pattern (ED, inset in Fig. 3) from selected area reveals that the sample is also hcp coexisting with ccp phase. 3.2. Magnetic properties The hysteresis loop (Fig. 4) of the sample, measured at room temperature, shows a ferromagnetic behavior with saturation magnetization (Ms), remnant magnetization (Mr), and coercivity (Hc) values of ca. 90 emu/g, 33 emu/g and 360 Oe. Compared to the coercivity of bulk cobalt (a few tens of oersteds [11]), it exhibits an obviously enhanced value. It is also much higher than that of the previously produced spherical Co nanoparticles (Hc ≈ 150 Oe) [21]. The magnetic properties of materials have generally been believed to be highly dependent on the sample shape, crystallinity, magnetization direction, and so on. Our experimental results give a hint that the magnetic properties of our Co crystals can be explained considering shape anisotropy of the samples (the flakes exhibit higher shape anisotropy than spherical particles), as others have previously reported that the samples with high shape anisotropy often possess high coercivity [9,22,23]. 3.3. Complex precursor determination and reaction mechanism of Co In the present experiments, when CoCl2 ·6H2 O and N530 were dissolved into ethanol, an N530–CoCl2 complex was formed in solution. By a subsequent reaction with hydrazine, Co is synthesized through the reduction route from N530–CoCl2 complex at room temperature. This proposal can be supported by supplemental experiments with FT-IR measurements. Fig. 5 shows the FT-IR spectra of both pure N530 and N530–CoCl2 complex. In the spectra, it is found that the curve in Fig. 5b (from N530–CoCl2 ) is roughly close

Y. Zhu et al. / Materials Chemistry and Physics 91 (2005) 293–297

295

Fig. 2. FE-SEM images of the cobalt crystals with flower-like architecture: (a) low magnification SEM image and (b) a high-magnification FE-SEM image.

to that in Fig. 5a (pure N530) except that the two stretch absorption bands of O H and C N shift to lower wave number. In details, the frequency of the O H stretching vibration of ∼3383 cm−1 in pure N530 shifted to ∼3351 cm−1 in N530–CoCl2 , since oxygen atoms of the hydroxyls could partially donate the lone pair electrons to the vacant d-orbit of Co2+ ions. In such a case, the vibration constant of O H decreases, and accordingly the vibration of O H shifts to lower wave number. This lower wave number shifting vibration of associated hydroxyls has been observed previously in

PEG–NiCl2 due to the coordinative chemical bonding formed at the O H groups of PEG with Ni2+ ions [24]. Similarly, since nitrogen atoms of C N in N530 could donate pair electrons to the vacant d-orbit of Co2+ ions, the vibration constant decreases and the stretching vibration of C N moves to lower wave number. Thus, it is reasonable that the frequency of the C N stretching vibration of ∼1627 cm−1 in pure N530 (Fig. 5a) can shift to ∼1608 cm−1 in N530–CoCl2 (Fig. 5b). These results convince the formation of the complex of N530–CoCl2 in such a reaction system. With a reduction of hydrazine in basic solution, Co can be produced accordingly. The formation of Co can be formulated as follows [21]: CoCl2 + N530 → CoCl2 –N530(complex)

Fig. 3. TEM images of the product.

Fig. 4. Room-temperature hysteresis loop of the sample.

(1)

296

Y. Zhu et al. / Materials Chemistry and Physics 91 (2005) 293–297

Fig. 5. The FT-IR spectra of (a) pure N530 and (b) N530–CoCl2 complex.

CoCl2 –N530(complex) + mN2 H4 → Co(N2 H4 )m 2+ + N530 + 2Cl−

(2)

Co(N2 H4 )m 2+ + OH− → Co + N2 + (m − 1)N2 H4 + H2 O (3) 3.4. Formation mechanism of flower-like architectures The growth of Co flower-like crystals in the route is based on the above reactions. It is noted that the morphologies of the cobalt nanocrystals were intensively influenced by the precursor of N530–CoCl2 complex along its spatial conformation. According to previous reports [18], trans-N530 and CoCl2 may form a trans-N530–CoCl2 complex with a CoL2 (L = N530) form and the possible spatial conformation can be schematized as shown in Fig. 6. In such a complex, N530 and Co2+ ions form a planar square framework, which provides the special molecular environment favorable for the nucleation and growth of cobalt nanocrystals with anisotropic morphologies. Meanwhile, the strong crystalline intrinsic anisotropy of hexagonal cobalt itself also influences the crystal structure and the final shape in the present complex synthetic route [9]. Obviously, both the framework structure of N530–CoCl2 complex precursor and the cobalt crystalline nature direct the formation of cobalt crystals with flake-like

Fig. 6. (a) The possible spatial conformation of trans-N530–CoCl2 complex and (b) the spatial conformation of trans-N530.

structure. Determined by special hindrance, magnetic attraction of excessive crystallites, and other kinetic factors, the cobalt flakes can tend to curl and finally aggregated to 3D flower-like architectures. In the present synthetic process, some supplementary experiments with other polymers of polyethylene glycol (PEG, molecular weight = 400) and polyvinyl pyrrolidone (PVP) (K30, polymerization degree = 360) substituting for N530 were employed for comparison. In the case of PEG, it can combine with Co2+ ions and form PEG–CoCl2 complex due to the existence of lone pair electrons in oxygen atoms of its repeat units [24]. As for PVP, the oxygen atoms in carboxides and its nitrogen atoms in pyrrolidone rings coordinate the ions and lead the formation of a PVP–CoCl2 complex [25]. It is found that there are not any Co flake-like nanocrystals formed when PEG and PVP used, even though some similar polymer caped complexes [24,25] achieved in the process. TEM images demonstrate that only cobalt dendrites mixed with spherical nanoparticles in either PEG or PVP (as shown in Fig. 7), which are close to those we previously reported in alcohol synthetic systems [21]. In addition, reaction duration and aging period may also influence the morphologies of the cobalt nanocrystals. Fig. 8 is a TEM image of the products using N530 after the reaction occurred for 1 h, which show that a large amount of spherical nanoparticles (less than 10 nm in diameter) coexisted with flakes. As we know, longer aging time is favorable for a thermodynamic growth of cobalt with more stable shapes [26]. Commonly in hcp cobalt, the (0 0 2) surface (as the same to the (1 1 1) plane in ccp cobalt) typically has a higher surface energy than that of others. When N530 used, N530 can bind strongly to the energy surface of (0 0 2) plane with the as-above shown special structure and accordingly the surface energy of the (0 0 2) planes lowered. Thus, the lower growth (0 0 2) planes remained and the flake-like cobalt nanocrystals produced in such an anisotropic condition. As for PEG and PVP, they show more homogeneous

Fig. 7. A typical TEM image of the product synthesized using PVP, similar morphologies are investigated using PEG.

Y. Zhu et al. / Materials Chemistry and Physics 91 (2005) 293–297

297

to express our acknowledgement to Prof. Qianrong Li for his valuable discussion on Fourier transform infrared absorption (FT-IR) spectra.

References [1] [2] [3] [4] [5] [6] [7]

[8] [9] Fig. 8. TEM image of the product using N530 after the reaction occurred for 1 h.

situation due to their folded chain structures, and nanoparticles of dendrites produced are reasonable. These investigations suggest that mainly the structure of the complex plays an important role in the morphologies of cobalt nanocrytals.

4. Conclusion A complex reaction has been developed for the preparation of flower-like cobalt crystals with nanoscale flakes at room temperature. The experimental investigations along sample characterization demonstrate that the use of N530–CoCl2 complex precursor play an important role in the growth of the architectural flower-like cobalt crystals. Magnetic properties study exhibits that the flower-like cobalt crystals have an enhanced magnetic coercivity in comparison with the bulk cobalt crystals and the Co spherical nanoparticles. This demonstrates that the flower-like cobalt nanocrystals may be promising for use in magnetic recording devices.

Acknowledgements Financial support by the National Natural Science Foundation of China is gratefully acknowledged. We would like

[10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]

M.L. Wagner, L.D. Schmidt, J. Phys. Chem. 99 (1995) 805. S.W. Moore, Sol. Energy Mater. 12 (1985) 435. M. Ozaki, Mater. Res. Soc. Bull. 14 (1989) 35. K. Haneda, Can. J. Phys. 65 (1987) 1233. S.H. Lion, C.L. Chien, J. Appl. Phys. 63 (1988) 4240. K. Bridger, J. Watts, C.L. Chien, J. Appl. Phys. 63 (1988) 3233. Y. Maeda, M. Takahashi, Recent Advances in Magnetism and Magnetic Materials, World Scientific, Singapore, 1990, pp. 202– 213. V.F. Puntes, K.M. Kishnan, A.P. Alivisatos, Science 291 (2001) 2115. D.P. Dinega, M.G. Bawendi, Angew. Chem. Int. Ed. Engl. 38 (1999) 1788. J.R. Thomas, J. Appl. Phys. 37 (1966) 2914. H.Q. Cao, Z. Xu, H. Sang, D. Sheng, C.Y. Tie, Adv. Mater. 13 (2001) 121. T.M. Whitney, J.S. Jiang, P.C. Searson, C.L. Chien, Science 261 (1993) 316. S. Sun, C.B. Murray, J. Appl. Phys. 85 (1999) 4325. Y.P. Sun, H.W. Rollins, R. Guduru, Chem. Mater. 11 (1999) 7. G.N. Glavee, K.J. Klabunde, C.M. Sorensen, G.C. Hadjipanayis, Inorg. Chem. 32 (1993) 474. G.P. Gibson, K.J. Putzer, Science 267 (1995) 1338. T.S. Ahmadi, Z.L. Wang, T.C. Green, A. Henglein, M.A. ElSayed, Science 272 (1996) 1924. Y.D. Li, H. Wang, H.G. Zheng, C.W. Li, Y. Zhou, Chem. Res. Appl. 8 (1996) 173 (in Chinese). Q. Yang, K.B. Tang, C.R. Wang, Y.T. Qian, S.Y. Zhang, J. Phys. Chem. B 106 (2002) 9227. A.F. Well, Structure Inorganic Chemistry, fourth ed., Oxford University Press, 1975, p. 788. Y.C. Zhu, H.G. Zheng, Q. Yang, A.L. Pan, Z.P. Yang, Y.T. Qian, J. Cryst. Growth 260 (2004) 427. D.H. Qin, L. Cao, Q.Y. Sun, Y. Huang, H.L. Li, Chem. Phys. Lett. 358 (2002) 484. S.J. Park, S. Kim, S.Y. Lee, Z.G. Khim, K. Char, T. Hyeon, J. Am. Chem. Soc. 122 (2000) 8581. F. Xu, Y. Xie, X. Zhang, C.Z. Wu, W. Xi, J. Hong, X.B. Tian, New J. Chem. 27 (2003) 1331. Y.G. Sun, Y.N. Xia, Science 298 (2002) 2176. S.M. Lee, S.M. Cho, J. Cheon, Adv. Mater. 15 (2003) 441.