Solvothermal Synthesis of Magnetic Chains Self-Assembled by

Solvothermal Synthesis of Magnetic Chains Self-Assembled by Flowerlike Cobalt Submicrospheres Ya-Jing Zhang,*,† Qi Yao,† Ying Zhang,† Tie-Yu Cui,† Da Li,† Wei Liu,† Whitmore Lawrence,‡ and Zhi-Dong Zhang†

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 9 3206–3212

Shenyang National Laboratory for Materials Science, Institute of Metal Research, and International Centre for Materials Physics, Chinese Academy of Sciences, Shenyang 110016, PR China, and UniVersity SerVice Centre for Transmission Electron Microscopy, Vienna UniVersity of Technology, Wiedner Hauptstrasse 8-10, Austria ReceiVed October 23, 2007; ReVised Manuscript ReceiVed May 11, 2008

ABSTRACT: Cobalt chains with lengths of up to 4-20 µm, self-assembled by flowerlike cobalt submicrospheres, have been synthesized at 200 °C for 4 h by a solvothermal method with the surfactant poly(vinyl pyrrolidone) (PVP). The average diameter of individual flowerlike submicrospheres is 700-900 nm, which are composed of compact nanosheets with an average thickness of about 50 nm. The products were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDS). The effects of synthetic conditions, such as reaction temperature and the amount of reducing agent, on the morphology and size of the chains were investigated. The growth mechanism of the chains was proposed, based on the evolution of the structure and the morphology with increasing the reaction time. The magnetic hysteresis loops at 5 and 295 K of the chains show ferromagnetic characteristics with coercivities of 347 and 90 Oe, respectively. Our work may shed light on the design fabrication of one-dimensional chainlike structures self-assembled by complex three-dimensional architectures of materials.

1. Introduction It is believed that the properties of nanomaterials strongly depend on their size, shape and dimensionality.1 Considerable attention has been paid to synthesize nanomaterials with various architectures and to investigate their properties, because of their potential application in optics, electronics, magnetics, and biology.2 A wide variety of architectures have been synthesized by assemblies of primary building blocks such as nanoparticles, nanorods, nanosheets, and nanoplates.3–23 For example, sizecontrollable nanostructures were assembled by gold nanoparticles by a mediator-template,21 and flowerlike bismuth tungstate was assembled by nanosheets by the hydrothermal method.22 CoPt nanopolypods derived from the assembly of nanorods were synthesized by a simple and large-scale-applicable thermolytic reaction.23 Several chemical methods have been developed for the fabrication of complex nanostructures mentioned above, among which the chemical-solution-phase self-assembly method is simple and inexpensive for large-scale preparation. In the chemical-solution-phase self-assembly synthesis route, the surfactant or complex agent (sometimes both) is usually used to promote (or tune) the ordered alignment of primary nanobuilding blocks.3,10 However, since the self-assembly process by this method is considerably complex, the controlled-synthesis of complex nanomaterials (in particular with hierarchical structures) is still a challenge. Co is an important ferromagnetic metal, which can crystallize in three structures: the hexagonal close-packed (hcp), the facecentered cubic (fcc) and the epsilon phases. Generally, the hcp phase with a high coercivity is of interest in the field of permanent magnets, while the fcc phase with a low coercivity can be used as soft-magnetic materials.24 Co can be used as an excellent catalyst for intra- and intermolecular Pauson-Khand reactions.25 Great efforts have been focused on the synthesis * To whom correspondence should be addressed. E-mail: [email protected]. † Shenyang National Laboratory for Materials Science. ‡ Vienna University of Technology.

of various cobalt nanostructures in a controlled fashion. Simple Co nanostructures, including nanoparticles, nanoribbons, nanobelts and nanowires, have been obtained.26–38 For instance, hcpCo nanorods were obtained by a decomposition process in oleic acid and a long amine solution.34 Co nanodisks and their assembly of ribbons were obtained by rapid decomposition of carbonylcobalt in the presence of trioctylphosphane and oleic acid.35 Co polycrystalline wires were formed by the selfassembly of nanocrystalline Co by a solvothermal reduction process.36 Co single-crystalline nanowires37 and nanobelts38 assembles were prepared by a surfactant-assisted hydrothermal reduction process. Although synthesis of a lot of monodispersed magnetic building blocks has been achieved, it is still a challenge to control simultaneously their shape, surface structure, and anisotropy. However, few reports have been devoted to the synthesis of Co nanostructures with complex hierarchical structures. Recently, it was reported that Co nanoplatelets were self-assembled into monodispersed microspheres by a hydrothermal reduction in the presence of the surfactant SDBS and capping reagent sodium tartrate.39 Magnetic chains of hollow cobalt mesospheres were synthesized by a refluxing method in ethylene glycol solution with poly(vinyl pyrrolidone) (PVP).40,41 Magnetic chains of hierarchical cobalt microspheres (with a diameter of 5-10 µm) were synthesized via a complicated surfactant-assisted hydrothermal method in our earlier work.42 Since ordered Co nanostructures have potential applications in catalysts, magnetic recording, and drug delivery, due to their dimensions and high surface areas, the synthesis of Co hierarchical architectures is desirable. In particular, it is highly expected to synthesize controlled Co hierarchical structures and modulate the morphology by a simple method such as the variation of the reaction time, temperature, and so forth. In this paper, we demonstrate the formation of hierarchical Co chainlike architectures consisting of flowerlike submicrospheres. Each flowerlike submicrosheres consist of nanosheets with an average thickness of about 50 nm. It is found that the reaction temperature and the amount of reducing agent have

10.1021/cg7010452 CCC: $40.75  2008 American Chemical Society Published on Web 07/23/2008

Solvothermal Synthesis of Magnetic Chains

Crystal Growth & Design, Vol. 8, No. 9, 2008 3207

great effects on the morphology and size of the flowerlike Co spheres. On the basis of the evolution of the structure and the morphology with increasing the reaction time, a growth mechanism of the Co chains is proposed. Furthermore, the magnetic measurements show that the coercivities of the Co chains are 90 and 347 Oe at 295 and 5 K, respectively, which are higher than those of the bulk counterpart.

2. Experimental Section All reagents were commercial products with analytical grade without further purification. In a typical procedure, 0.238 g (1 mmol) of CoCl2 · 6H2O and 0.5 g of PVP K30 were first dissolved in 30 mL of ethylene glycol under magnetic stirring at room temperature. The solution was intensively stirred for 1 h, then 2.0 mL of hydrazine monohydrate N2H4 · H2O (85 vol % analytical reagent) was added dropwise to the solution at room temperature by vigorous stirring. The vigorous stirring lasted for 1 h after dropping so that the reaction can be completed. Subsequently, the solution was transferred into a 50 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 200 °C for 4 h and then cooled to room temperature naturally. The product was filtered off, washed several times with distilled water and absolute ethanol for several times to remove PVP, and finally dried in a vacuum oven at 60 °C for 4 h. Control experiments were carried out by adjusting the reaction temperature (120-200 °C) and/or the amount of N2H4 · H2O (0.5-3 mL), while other reaction parameters were unchanged. The phases were identified by means of X-ray diffraction (XRD) with a Rigaku D/max 2500pc X-ray diffractometer with Cu KR radiation (λ ) 1.54156 Å) at a scan rate of 0.04° s-1. The morphology and composition of the as-prepared products were characterized by a Supra 35 field-emission scanning electron microscopy (FESEM) operated at an acceleration voltage of 20.0 kV and equipped with an energy dispersive X-ray spectroscopy (EDS). Transmission electron microscopy (TEM) and high-resolution TEM observations were carried out on a JEOL 2010 with an emission voltage of 200 kV. Magnetic hysteresis loops were measured using a MPMS-7 superconducting quantum interference device (SQUID) magnetometer at magnetic fields up to 20 kOe.

3. Results and Discussion 3.1. Synthesis of Co Chains Self-Assembled by Flowerlike Spheres. A typical XRD pattern of the product obtained by the solvothermal treatment at 200 °C for 4 h is shown in Figure 1a. All XRD peaks can be indexed as Co with hexagonal structure, consistent with the standard card JCPDS card No. 05-0727 (space group P63/mmc; a ) 2.503 Å; c ) 4.060 Å). No obvious peaks resulting from impurities (such as CoO or Co3O4) are observed, indicating that the hexagonal cobalt phase is the main product. The EDS analysis confirms that the main phase is pure Co. Si in the EDS spectrum (Figure 1b) originates from the silicon wafer to support the sample, while Au originates from a thin Au layer sputtered on the sample for obtaining clear SEM images. The morphology and size of the samples were examined by SEM. A typical low-magnification SEM image is shown in Figure 2a. A lot of chains are completely composed of relatively uniform spheres with an average diameter of 700-900 nm. The chains, with many branches, range from 4 to 20 µm. In fact, it is hard for us to discern where the chains’ ends are, because they almost attach together. A high-magnification SEM image in Figure 2b displays the detailed structure of the spheres. The spheres look like flowers, which are composed of a number of nanosheets with an average thickness of about 50 nm, as estimated from the standing nanosheets in the SEM image. This indicates that the Co chains can be classified as hierarchical structures. It is noteworthy that the chains as well as the

Figure 1. (a) XRD and (b) EDS patterns of the as-prepared Co chains at 200 °C.

flowerlike spheres are considerably stable and they maintain the structure even under ultrasonication for 2 h. 3.2. Influence of the Reaction Temperature on the Morphology and Size of the Co Chains. It is found that the reaction temperature has a great effect on the morphology and size of the products. Our control experiments reveal that Co with different morphologies and sizes can be achieved by adjusting the reaction temperature. When the reaction is performed at 180 °C, the Co chains self-assembled by flowerlike spheres which are completely composed of nanosheets are obtained (Figure 3a), similar to those at 200 °C (Figure 2a). However, the average diameter of the spheres is 1.2 µm, which is slightly larger than that at 200 °C. In addition, the nanosheets at 180 °C are compacted more densely, compared with those obtained at 200 °C (Figure 2b). Figure 3b shows that the products obtained at 160 °C are chains consisting of spheres with an average size of 1.5 µm. It is interesting that two kinds of surfaces of the spheres are observed (Figure 3b), in which one is relatively smooth and the other is composed of nanosheets. The result is different from that at 180 or 200 °C. When the reaction temperature is decreased to 140 °C, the product is in chainlike structures, self-assembled by solid spheres. The spheres have an average diameter of 2.4 µm, as shown in Figure 3c. Further decreasing the reaction temperature to 120 °C does not produce any chainlike structures, rather it results in irregular structures formed by aggregation of some smooth blocks (Figure 3d). These results indicate that the reaction temperature plays a crucial role in determining the morphology and size of the

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Figure 2. (a) Low-magnification and (b) high-magnification FESEM images of the Co chains at 200 °C for 4 h.

Figure 3. FESEM images of the products prepared at different temperatures: (a) 180 °C, (b) 160 °C, (c) 140 °C, (d) 120 °C.

products. All the XRD patterns are indexed to pure hcp-Co for the products at different reaction temperatures 120-200 °C (see Figure S1, Supporting Information). 3.3. Influence of the Amount of Reducing Agent and Solvent on the Morphology and Size of the Co Chains. Apart from the reaction temperature, the amount of N2H4 · H2O is also crucial for the formation of the hierarchical Co chainlike structures. In the experiment, hydrazine monohydrate (N2H4 · H2O) served as the reducing agent. The process of the reaction probably involves the following chemical reactions:43,44

Co2+ + nEG f Co(EG)n2+ Co(EG)n2++

3N2H4 f [Co(N2H4)3]2+ + nEG

+ [Co(N2H4)3]2+ + N2H4 f Co + 4 NH3 + N2 + H4 + 2H

Figure 4a-f shows that in the control experiment, different morphologies of the products could be achieved accordingly by adjusting the amount of N2H4 · H2O. When the amount of N2H4 · H2O is 0.5 mL, Co chains network and surface structure of the spheres can be observed in Figure 4a,b. The Co chains are found to be 4-20 µm in length, consisting of spheres with an average diameter of 2.5 µm. The spheres closely contact with each other. From the high-magnification SEM image of the Co chains (Figure 4b), it can be seen that the spheres are composed of many compact nanosheets. When the amount of N2H4 · H2O is increased to 1 mL (Figure 4c,d) and 2 mL (Figure 2a,b), the

morphologies of the samples are similar to those with 0.5 mL of N2H4 · H2O. However, it is surprising to find that the average diameters of the spheres reduce to 1.1 µm and 800 nm, respectively. When the the amount of N2H4 · H2O is further increased to 3 mL, the Co chains self-assembled by solid spheres are obtained as the products (Figure 4e,f). The average diameter of spheres is found to be 750 nm, and the surfaces of the spheres are smooth. This indicates that the spheres are formed by aggregated particles rather than nanosheets. The result means that the amount of the reducing agent can greatly affect the size of the products. The results showed that the size of the spheres decreased by increasing the amount of N2H4 · H2O. However, excessive reducing agent would alter the morphology, which is caused by the change of the reaction rate. By increasing the amount of N2H4 · H2O, the reducing rate is accelerated, and more Co nuclei may form before the growth process, which leads to the formation of more spheres with a smaller diameter. (All the XRD patterns of the samples with different amounts of N2H4 · H2O are indexed to pure hcp-Co, see Figure S2, Supporting Information). In addition, the solvent is an important parameter for synthesis. It is known that the physical and chemical properties of the solvent can influence the solubility, reactivity, and diffusion behavior of the reagents, and even influence the growth of morphologies of product.45 Herein it is found that the ethylene glycol is important for the formation of hierarchical flowerlike



Figure 4. Low-magnification FESEM images of the products prepared with different amounts of N2H4 · H2O: (a) 0.5 mL, (c) 1 mL, (e) 3 mL. High-magnification FESEM images of the products prepared with different amounts of N2H4 · H2O: (b) 0.5 mL, (d) 1 mL, (f) 3 mL.

Figure 5. FESEM images of the products prepared at different growth stages: (a) 1 h, (b) 2 h. Inset: images of the individual nanosheets and the flowerlike spheres, respectively.

spheres. When propylene glycol is substituted for ethylene glycol, it can be seen that the product is peanut-like smooth Co aggregates (see Figure S3, Supporting Information). No obvious reaction was observed in the case of replacing ethylene glycol with methanol. Ethylene glycol has been used as a solvent in some chemical processes, and even simultaneously as a reducing agent for obtaining metals from their metal salt solutions.46–48 In the present case, no obvious change was found in the absence of reducing agent N2H4 · H2O, which indicates that ethylene glycol only serves as a solvent in the reaction. 3.4. Growth Mechanism. To investigate the growth mechanism of the Co chains, time-dependent experiments were carried out at 200 °C. The SEM images (Figure 5a,b and 2) of the samples obtained at various growth stages illustrate the

evolution of morphology of the Co chains. When the solvothermal reaction proceeds for 1 h, the dominant products are Co chains self-assembled by nonuniform spheres, and a number of dispersed or aggregated nanosheets coexist with Co chains, as shown in Figure 5a, while the inset displays the highmagnification SEM image of the nanosheets. For the sample with a reaction time of up to 2 h, though a small amount of nanosheets can be observed from the SEM image (Figure 5b), the major products are Co chains self-assembled by flowerlike spheres. In this case, the flowerlike spheres look very loose and seem to be underdeveloped, as seen in the inset of Figure 5b. When the reaction time is increased up to 4 h, the uniform Co chains constructed by flowerlike spheres with numerous wellstacked nanosheets are found in all the samples.


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Figure 6. (a) TEM image of the Co chains obtained at 200 °C for 1 h, (b) TEM image of a sheet, (c) the corresponding SAED pattern of the sheet, (d) HRTEM image of the sheet.

Figure 7. Schematic illustration of the formation process for the Co chains self-assembled by the flowerlike spheres.

TEM is used to further confirm the growth process of the Co chains. From the bright field TEM image for the sample obtained at 200 °C for 1 h, a lot of nanosheets and spheres can be seen. This is in agreement with the SEM result (Figure 5a). Figure 6b represents a high magnification TEM image of an individual nanosheet in Figure 6a. The corresponding select area electron diffraction (SAED) pattern (Figure 6c) indicates that the Co nanosheets are polycrystalline. The interplanar spacings are 0.2159, 0.2018, 0.1903, 0.1479, 0.1253, 0.1141 and 0.1056 nm, which can be determined to be the (100), (002), (101), (102), (110), (103) and (112) planes of the hexagonal Co, respectively. The SAED results are in good agreement with the XRD data.

High-resolution TEM (HRTEM) image (Figure 6d) was recorded on the corner of the nanosheet as shown in Figure 6b. From Figure 6d, the lattice interplanar spacings are measured to be 0.22 nm, corresponding to the (100) plane of hcp Co. This confirms the polycrystalline nature of the nanosheets. It is also found that the nanosheet shown in Figure 6b is not a single layered one, rather composed of some randomly attached smaller nanosheets. On the basis of the results above, a growth mechanism is proposed, as illustrated in Figure 7. At first, numerous tiny Co crystalline nuclei appear in the solution and then the crystal growth follows. With proceeding the reaction, these tiny nuclei


grow larger so that the particles with different sizes appear in the solution (Figure 7, step a). In comparison with the smaller particles, the larger ones have smaller surface free energies and consequently grew at the cost of the smaller ones, based on a typical Ostwald ripening process.49 In the subsequent process, the Co particles diffused and aggregated together to form tiny Co nanosheets, and the tiny sheets grew into larger nanosheets (as shown in Figure 6b) through random attaching and overlapping, as illustrated by step b in Figure 7. For the growth of cobalt in the solution process, two main growth mechanisms have been put forward. One theory suggested that the surfactant in the solution kinetically controlled the growth rates of different faces of a Co nanocrystal through interaction in the adsorption and desorption process;39 the other mechanism indicated that the Co sheetlike structure was inherited from the layered hydroxide precursor (Co(OH)2).50 In the present case, the selective adsorption of PVP on some faces of Co crystals might be responsible for the initial formation of tiny Co nanosheets. The larger Co nanosheets were not formed by oriented aggregation of the tiny ones, rather than by random aggregation. Then the larger nanosheets attached together and assembled into flowerlike Co spheres driven by interfacial tension and the hydrophilic surfaces of Co (Figure 7, step c).51 Finally, chainlike Co self-assembled by flowerlike spheres consisting of various individual nanosheets formed, as illustrated in Figure 7 (step d). For the formation of the chains, both the magnetic dipole-dipole interaction and effect of the PVP contribute to the assembly process,52,53 because the magnetic dipole-dipole interaction itself only aids these primary building blocks contact together rather than results in particular structure directly. It is worth mentioning that the assembled Co chains are highly branched rather than straight, which may be due to the stronger anisotropic magnetic forces.54 Guo et al. reported that a hollow Co mesosphere chainlike structure was assembled via an ethylene refluxing method (at the boiling point of EG, ca. 197 °C), using gas bubbles and PVP as a dynamic template.40 Compared with Guo et al.’s work, we obtained hierarchical Co chainlike structures, which is possibly due to adopting different reaction methods. However, the exact reason is unclear so far and should be investigated further. PVP has been successfully applied in the synthesis of a number of nanomaterials with different nanostructures.55–57 It is well-known that PVP can selectively absorb on a certain crystal facet of the as-prepared primary building blocks such as nanoparticles, nanosheets, nanoplates, nanorods, and so on. Therefore, PVP can prevent these primary building blocks from entropy-driven random aggregation58 and promote or tune selfassembly. Controlled experiments showed that only solid Co spheres were obtained with a tiny amount (0.1 g) or without PVP (see Figure S4, Supporting Information), which indicates that PVP is very important for the formation of Co chainlike hierarchical structures. In our experiments, PVP might play three main roles in the process: one is to prevent the aggregation of Co nanoparticles during the initial growth stage of Co nanosheets; the other is selective adsorption on the facets of Co crystals and kinetic control growth rates of these facets; and another is to promote the attachment of Co spheres. However, the exact role of PVP should be investigated in future. 3.5. Magnetic Properties of the Co Chains. Figure 8 shows the hysteresis loops of the Co chains at 5 and 295 K. It reveals that the magnetizations at 5 and 295 K are saturated at 20 kOe. The saturation magnetizations at 5 and 295 K of the Co chains are 148.3 and 139.4 emu/g, respectively. The saturation magnetization of the Co chains at room temperature is much


Figure 8. Magnetic hysteresis loops at 5 K (filled circles) and 295 K (open circles) of cobalt chains. The inset shows the low-field part of the hysteresis loops.

lower than that of its bulk counterpart (168 emu/g).37 It is believed that three main reasons can account for the reduction of the saturation magnetization: existence of impurities,59 antiferromagnetic oxides on surface51 and surface spin disorder.60 Among them, the existence of impurities and surface antiferromagnetic oxides may be the main reasons. PVP can coexist with the products, although the samples are washed several times before the magnetic measurements. Surface antiferromagnetic oxide has been proved by X-ray photoelectron spectroscopy (XPS) (see Figure S5, Supporting Information). However, it cannot be detected by XRD due to its small amount. The coercivities of the Co chains are 347 and 90 Oe at 5 and 295 K, respectively, as shown in the inset of Figure 7. The coercivity field at room temperature is greatly enhanced, compared to the bulk counterpart (10 Oe).37 The enhancement in the coercivity is attributed to the shape anisotropy originating from a relatively high value of the average aspect ratio of 12, which prevents the present Co chains from being magnetized along directions other than their easy magnetization axes.61 Nevertheless, it should be noted that the coercivity field of the 1D Co chain is relatively lower than other 1 D nanostructures such as Co nanowires (166.8 Oe, 300 K)37 and nanobelts (410.6 Oe, 300 K),38 because the aspect ratio of the Co chains is much lower than that of nanowires and nanobelts. It should be pointed out that the magnetic measurement in this case was carried out with a powder sample so that the individual chains were randomly oriented, which is in agreement with other reports.40,42,53

4. Conclusions In conclusion, 1D magnetic Co chains self-assembled by 3D flowerlike spheres have been prepared by means of a simple solvothermal process in the presence of surfactant of PVP. The 3D flowerlike Co spheres consisting of a number of 2D nanosheets as the primary building unit have been observed. In addition, the size of the 3D flowerlike spheres can be conveniently modulated by varying the reaction temperature and the amount of the reducing agent. The growth mechanism for the formation of self-assembled Co chains with hierarchical structure has been proposed. The chains exhibit ferromagnetic behavior. A significantly enhanced coercivity has been achieved for the Co chains compared to the bulk Co. Such a simple and high yield synthetic approach can be applied to controlled fabrication of complex hierarchical structure of other materials.


Acknowledgment. We gratefully acknowledge the financial support from the National Nature Science Foundation of China under Project Nos. 50331030 and 50703046. One of the authors ¨ AD for finance within the China(Q.Yao.) would like to thank O Austria cooperation program. Supporting Information Available: XRD patterns of products obtained at different reaction temperature and using different amount of reducing agents, SEM images of products with different amounts of PVP and other solvent. XPS spectrum of the Co chains. This information is available free of charge via the Internet at http:// pubs.acs.org.

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CG7010452

Solvothermal Synthesis of Magnetic Chains Self-Assembled by

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