Facile Synthesis of Metallic Co Hierarchical Nanostructured

Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China, Graduate School of Chinese Academy of Sciences, Beij...
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J. Phys. Chem. C 2008, 112, 10073–10078

10073

Facile Synthesis of Metallic Co Hierarchical Nanostructured Microspheres by a Simple Solvothermal Process Lu-Ping Zhu,†,‡ Wei-Dong Zhang,†,‡ Hong-Mei Xiao,† Yang Yang,†,‡ and Shao-Yun Fu*,† Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China, Graduate School of Chinese Academy of Sciences, Beijing 100039, China ReceiVed: March 2, 2008; ReVised Manuscript ReceiVed: April 9, 2008

The fabrication of hierarchical and complex micro/nanostructures using nanoparticles such as nanorods, nanoribbons, nanoplatelets, etc. as building blocks at different levels has become a hot topic in recent material research fields. In this study, facile synthesis of metallic cobalt hierarchical nanostructured microspheres was reported by a simple solvothermal process. The as-obtained products were well characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Brunauer-Emmett-Teller analysis, and vibrating sample magnetometery. It was shown that the Co hierarchical nanostructured microspheres with a diameter of several micrometers are assembled from nanoplatelets with a thickness of about 20 nm and a width of 0.5-1.5 µm. A rational mechanism of formation was proposed on the basis of a range of contrasting experiments. The as-synthesized material exhibits ferromagnetic characteristics and a high surface area, showing potential applications for catalysts and other related micro- and nanodevices. Introduction Magnetic nanomaterials have aroused extensive attention because of the technological applications such as ferrofluids, advanced magnetic materials, catalysts, optical and mechanic devices, high-density magnetic recording media, and medical diagnostics.1–6 Several methods are available for the production of magnetic nanomaterials.7 In most cases, the magnetic nanomaterials are superparamagnetic at room temperature because of their small dimensions. Thus, they are not suitable for some important applications, such as magnetic recording, etc., in which anisotropic magnetic nanomaterials such as nanorods, nanowires, or supernanostructures are desirable. Anisotropic magnetic nanoparticles or superstructures are expected to exhibit interesting magnetic properties such as enhanced coercivity. As an important ferromagnetic material, cobalt particles have attracted much attention and have been the focus of intense research because of its structure-dependent magnetic and electronic properties.8 It has been reported that the anisotropic high magnetic coercivity of hexagonal-close-packed (hcp) Co is the preferred structure for permanent magnet applications, as compared with the more symmetric low coercivity facecenterd cubic (fcc) phase used for soft magnetic applications.9 Different one-dimension (1D) Co nanostructrures, such as nanorods, nanowires, and nanofibers, have been successfully synthesized via various methods, including thermal decomposition of organometallic precursor,10 γ-irradiation methods,11 templated-mediated synthesis,12 electrospinning techniques,13 magnetic-field-induced processes,14 and hydro/solvothermal methods.15 An alternative way to improve the magnetic anisotropy is to assemble nanocrystals into multidimensional morphologies.10a,16 The fabrication of hierarchical and complex micro/nanostructures that are assembled from nanoparticles, nanorods, nanoribbons, nanoplatelets, nanodisks, and so forth, * Corresponding author e-mail: [email protected]. † Technical Institute of Physics and Chemistry. ‡ Graduate School of Chinese Academy of Sciences.

as building blocks at different levels has become a hot topic in recent material research fields.17–20 This process should be helpful to a deeper understanding of the oriented-attachment mechanism, offering opportunities in searching for exciting new properties of materials and in fabricating functional micro/ nanodevices, etc.21 So far, various kinds of metal oxide, sulfide, and hydrate with controlled hierarchical/complex morphologies have been successfully synthesized.22–24 However, it is still a great challenge to develop simple and reliable synthetic methods for facile synthesis of hierarchical nanostructured magnetic nanoarchitectures. In our previous works, we reported the synthesis of chainlike ferromagnetic CoNi alloy particles and Co dendritic superstructures using wet-chemical methods.25 Herein, we report the facile synthesis of Co hierarchical nanostructured microspheres by an ethylene glycol (EG)-mediated self-assembly process. The typical synthesis is based on simple solvothermal reduction of a Co hydrazine complex in the presence of cetyltrimethylamonium bromide (CTAB). The chemical reduction can be formulated as shown in eqs 1 and 2.

Co2++3N2H4 f Co(N2H4)32+ Co(N2H4)32+ + N2H4 f CoV + 4NH3v + 2N2 v + H2 v + 2H+

(1) (2)

Magnetic measurements demonstrate that as-synthesized hexagonal phase Co microspheres with a hierarchical nanostructure show ferromagnetic behaviors. Experimental Section All reagents were analytical grade and used without further purification. In a typical experiment, 0.48 g of CoCl2 · 6H2O and 0.50 g of CTAB were dissolved in 30 mL of ethylene glycol (EG) by intensive stirring for 2 h, and a homogeneous transparent mauve solution was obtained. A solution of hydrazine monohydrate (40% v/v) was added dropwise to the wellstirred mixture at room temperature by simultaneous, vigorous

10.1021/jp8019182 CCC: $40.75  2008 American Chemical Society Published on Web 06/14/2008

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Figure 1. (a) XRD pattern of the Co hierarchical nanostructured microspheres synthesized at 200°for 4 h; (b) SEM image of the as-synthesized Co hierarchical nanostructured microspheres. Inset: the energy dispersive X-ray (EDX) analysis; (c) SEM image of one typical Co hierarchical nanostructured microsphere.

agitation. The mixture was stirred vigorously for 30 min and then sealed in a Teflon lined stainless-steel autoclave (50 mL capacity). The autoclave was heated to and maintained at 200 °C for 4 h and then allowed to cool to room temperature. The black fluffy solid products were washed several times with ethanol and dried at 60 °C for 6 h. The phase purity of the products was examined by X-ray powder diffraction (XRD) using a Rigaku D/max 2500 diffractometer at a voltage of 40 kV and a current of 200 mA with Cu KR radiation (λ ) 1.5406 Å), employing a scanning rate 0.02 °/s in the 2θ ranging from 30 to 80°. Scanning electron microscopy (SEM) images and energy dispersive X-ray (EDX) analysis were obtained using a HITACHI S-4300 microscope (Japan). Transmission electron microscopy (TEM) images and the corresponding selected electron diffraction (SAED) pattern were taken on a Hitachi-600 transmission electron microscope at an accelerating voltage of 200 kV. High-resolution transmission electron microscopy (HRTEM) images were carried out for the as-prepared Co sample using a JEOL JEM-2010 transmission electron microscope at an accelerating voltage of 200 kV. The size distribution of the Co sample was measured using a scale on the magnified SEM micrographs. The nitrogen adsorption and desorption isotherms were obtained at 77 K after heating the sample at 100 °C for 2 h. The surface area was measured using a Micromeritics (NOVA 4200e) analyzer. Magnetic measurements for the samples were carried out at room temperature using a vibrating sample magnetometer (VSM, Lakeshore 7307, USA) with a maximum magnetic field of 10 kOe. Thermogravimetric (TGA) and differential scanning calorimetric (DSC) analyses were carried out on a NETZSCH STA-409 PC thermal analyzer with a heating rate of 10 °C · min-1 in a flowing oxygen atmosphere.

Results and Discussion The phase structure and purity of the as-synthesized sample were examined by powder X-ray diffraction (XRD). The XRD pattern of the as-obtained product is shown in Figure 1a. Five obvious diffraction peaks can be indexed to the (100), (002), (101), (102), and (110) crystal faces of the hcp phase Co, which is consistent with the standard values reported in the Joint Committee on Powder Diffraction Standards (JCPDS) 05-0727. No characteristic peaks due to the impurities of cobalt oxides or hydroxides were detected, which indicates that the as-prepared products obtained by the present synthetic route consist of a pure hcp phase. Figure 1b shows the SEM image of a typical sample composed of hierarchical nanostructured microspheres with a diameter of approximately 4 µm, showing a relatively narrow size distribution (see the Supporting Information, Figure SI-1). The energy dispersive X-ray (EDX) analysis (Figure 1b inset) of the magnetic Co microspheres indicated that the sample is essentially pure Co. To confirm this result, the product was characterized using thermogravimetric analysis (TGA). As shown in Supporting Information Figure SI-2, the as-synthesized cobalt product began to be oxidized at around 200 °C, receiving a weight gain. This indicates that the as-synthesized product was slightly oxidized, which is basically consistent with the EDS result. Moreover, Figure SI-2 shows that at around 550 °C the product was fully oxidized and the final weight gain was around 40%, which was almost consistent to the theoretical weight gain (40.7%) for the perfect conversion of pure Co to Co2O3, indicating that the product consists of pure cobalt. The detailed morphology of the hierarchical nanostructured microspheres is shown in Figure 1c, which reveals that the entire structure of the spherical architecture is built from several dozen nanoplatelets with smooth surfaces. These nanoplatelets, with a thickness

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Figure 2. (a) TEM image of the as-synthesized Co hierarchical nanostructured microspheres; (b) HRTEM image taken from the age of the hexagonal phase Co hierarchical nanostructured microspheres and the corresponding selected-area electron diffraction (SAED) pattern (inset in lower leftt corner).

Figure 3. Nitrogen adsorption-desorption isotherm for the product obtained in the presence of CTAB with an aging time of 4 h.

of about 20 nm and a width of 0.5-1.5 µm, are connected to each other to form spherical hierarchical structures. The size and microstructure of the products were further examined with transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). As shown in Figure 2a, the samples dispersed on the TEM grids show hierarchical nanostructured microspheres with a diameter of 4 µm, which is consistent with those of the samples observed by SEM. The corresponding HRTEM image (Figure 2b) shows that the fringe spacing is about 0.22 nm, which can be indexed to the (100) plane of hcp Co. The selected-area electron diffraction (SAED) pattern shown in the lower left corner of Figure 2b gives clear concentric rings, showing that the as-obtained product is polycrystalline and can be indexed respectively to the (100), (002), (101), (102), and (110) planes of the hexagonal Co, in complete agreement with the inference from the above XRD data. Nitrogen adsorption-desorption measurements were conducted to characterize the Brunauer-Emmett-Teller (BET) surface area and hierarchical nanostructures. The recorded adsorption and desorption isotherms for the Co hierarchical nanostructured microspheres show a significant hysteresis (Figure 3), exhibiting a characteristic behavior of plate materials such as clays.26 The BET surface area for the as-obtained samples calculated from the linear part of the BET plot is about 38 m2/g, which is smaller than the calculated value (70.8 m2/g) for an ideal Co nanosheet with a thickness of 20 nm.27 This is attributed to the fact that the sample has hierarchical structure

with diverging nanoplatelets. And this result, combined with the TEM images to be shown below, suggests that the Co spherical hierarchical architecture should have a core-shell structure. To understand the formation process of the Co spherical hierarchical architectures, we carried out time-dependent experiments during which samples were collected at different time intervals. As shown in Figure 4a, at the early stage, the sample was composed of ca. 2 µm microspheres. The sample collected 1 h later (Figure 4b) showed ca. 2.5 µm microspheres with fine hierarchical nanoarchitectures as well as ca. 2 µm microspheres. As the reaction proceeded (Figure 4c), the amount of microspheres with fine hierarchical nanoarchitectures increased at the expense of the microspheres. At the same time, the size of the spherical structures gradually grew, and the morphology became microspheres with fine hierarchical nanoarchitectures. The corresponding TEM images (see the Supporting Information, Figure SI-3) of the Co samples obtained at 200 °C for 2 h showed that a lot of nanoplatelets are derived from the core of the Co microspheres, which is in agreement with the morphological evolving progress shown by the above SEM micrographs. Eventually, the sample was composed entirely of the hierarchical nanostructured microspheres assembled with nanoplatelets, which are about 20 nm in thickness and 0.5-1.5 µm in width, as shown in Figures 4d and 1c. From this point, the size and morphology of the product remained the same even at longer reaction times. The whole evolution process is illustrated in Figure 4e. In this formation process, time was the most important controlling factor. By quenching at different stages, we could easily obtain four kinds of samples: (1) microspheres; (2) microspheres and microspheres with fine hierarchical nanoarchitectures; (3) microspheres with fine hierarchical nanoarchitectures; and (4) a spherical hierarchical nanoarchitectures with a high yield. Such a process is consistent with previous reports of a so-called two-stage growth process, which involves a fast nucleation of primary particles followed by a slow aggregation and crystallization of primary particles.28–31 In our experiment, NH2NH2 was first coordinated with CoCl2 to produce a complex that was then reduced to become the nuclei and quickly grew into the primary particles at high temperature. In the following secondary growth stage, the primary particles aggregated into microspheres that became the core of the hierarchical nanostructured microspheres. The microspheres continued to grow by combining with the remaining primary particles in the presence of CTAB, finally forming the spherical hierarchical architectures.

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Figure 4. SEM images of the Co particles collected at different intervals after the appearance of the precipitate: (a) 0.5, (b) 1, (c) 2, and (d) 4 h, and (e) schematic illustration of the morphological evolution process of the Co hierarchical nanostructured microspheres.

Figure 5. XRD patterns of the as-prepared samples at various growth stages of the solvothermal process: (a) 0.5, (b) 1, and (c) 2 h.

All the XRD patterns are indexed to pure hcp-Co of the samples at different time intervals (see Figure 5). Notably, when the reaction time was prolonged, all the peaks belonging to the Co hexagonal phase were strengthened, suggesting that the longer reaction time does favor the crystallization of the Co phase.5 In Figure 1a, the Co sample was synthesized under the reaction time of 4 h, and the (102) crystal face can be clearly seen even though its intensity is low. In Figure 5, the relative intensity becomes too low to be seen since the reaction time is

shorter than for Figure 1a. The surface of the platelets assembled into the hierarchical nanostructured microspheres was very smooth, probably due to Ostwald ripening.28 The compellent mechanism for the formation of the final morphology by interaction between primary particles remains a mystery to materials chemists.30 Several factors, including crystal-face attraction, van der Waals forces, hydrophobic interactions, and hydrogen bonds may have effects on the self-assembly. The role of CTAB was found to be very critical for the formation feature of magnetic Co hierarchical nanostructured microspheres. In a control experiment, when no CTAB was added under the same reaction conditions, only solid Co microspheres with a diameter of 4 µm were obtained without spherical hierarchical architectures (see Figure 6a). As a surfactant, CTAB is widely used as a stabilizer to prevent nanoparticles from aggregation in chemical preparations of metals, alloys, and metal oxides nanoparticles,25a,32 similar to other surfactants used in the hydrothermal system to control the growth of nanostructures, which can be confirmed by its dramatic effect on the morphologies of the product. Here, it could be believed that the surfactant CTAB might play roles in at least two aspects. On the one hand, CTAB is a cationic surfactant, which could form a shell surrounding the particles to prevent Co nanoparticles from being aggregated into larger particles to a certain extent25a,32 and ensured the occurrence of steps D and E in the mechanism proposed above. In the absence of CTAB, the Co nanoparticles, once formed, would rapidly aggregate to large solid Co microspheres. On the other hand,

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Figure 6. (a) SEM image of Co spherical particles obtained without CTAB. (b-c) SEM and TEM images of the product obtained using anhydrous hydrazine as reductant.

the surfactant can be thought to act as a template. A few studies reported that the growth of nanostructures was related to selective absorption of organic surfactants onto particular crystallographic facets of a growing crystal.15a This selective absorption controls the growth rates of various crystallographic faces and promotes or induces them for ordered self-assembly in the kinetic limit.33 On the basis of the proposed mechanism for the growth of nanostructures under the confinement of capping reagent,34 we herein propose that one possible function of CTAB is to kinetically control the growth rates of the different crystalline planes through the selective interaction in the adsorption and desorption processes, similar to the function of cetyltrimethylamonium chloride (CTAC), sodium dodecyl benzenesulfonate (SDBS), poly(vinyl pyrrolidone)(PVP), and tetrabutylamonium bromide (TBAB) in the formation of metal or metal oxide hierarchical nanostructures.25a,27,35 In addition, the concentration of hydrazine hydrate also plays an important role in controlling the morphology of the product. When anhydrous hydrazine was chosen as reductant, there were no spherical Co hierarchical architectures but only chainlike structures of solid cobalt submicroparticles. A SEM image of these (Figure 6b) shows that the chains are too long to determine where they start or end. In addition, most of the particles are 500-800 nm in diameter (Figure 6c). The reducing strength of anhydrous hydrazine is higher than that of hydrazine hydrate (N2H4 · H2O, 40% v/v); therefore, the reduction reaction of anhydrous hydrazine occurs more acutely, and the reaction rate is much higher. Co nanoparticles, thus, grow faster in anhydrous hydrazine and form larger particles. As such, step D proposed in the formation mechanism of spherical hierarchical architectures does not happen. Finally, under the effects of the CTAB and magnetic dipole-dipole attractions, the Co submicrospheres are assembled into chainlike structures.15c As is well-known, the physical and chemical properties of nanoscale materials strongly depend on the size, size distribution,

Figure 7. Hysteresis loop for the Co hierarchical nanostructured microspheres at room temperature.

defect structures, and dimensions. The magnetic properties were investigated by the magnetization dependence of applied fields at 300 K, as shown in Figure 7. The hysteresis loop reveals ferromagnetic behavior with the saturation magnetization (Ms) and coercivity (Hc) values of 134.98 emu/g and 95.45 Oe, respectively. The saturation magnetization value of the sample was found to be lower than that of the bulk (168 emu/g).36 However, compared to the coercivity value of bulk Co (a few tens of oersteds),37 Co hierarchical nanostructured microspheres exhibit an enhanced value, which may be contributed to their spherical hierarchical architectures and the small thickness of the platelets. Conclusions In summary, facile synthesis of Co hierarchical nanostructured microspheres has been reported by a simple one-pot solvother-

10078 J. Phys. Chem. C, Vol. 112, No. 27, 2008 mal process. The spheres with an aveage diameter of about 4 µm are composed of nanoplatelets of about 20 nm in thickness and 0.5-1.5 µm in width. The results indicated that the reaction time, surfactant CTAB, and concentration of hydrazine hydrate play critical roles in the formation of spherical Co hierarchical architectures. The magnetic measurement indicated that the asobtained product had a ferromagnetic behavior and an enhanced coercivity value relative to the bulk Co. Because of the high surface area and ferromagnetism, the as-obtained products offer potential applications in water treatment, catalysts, and other related micro/nanoscale devices. Further studies will show that the present synthetic route is not limited only to synthesis of Co hierarchical nanostructured microspheres but can also be extended to the synthesis of other transition metals or their alloys. Acknowledgment. This work was financially supported by the Beijing Municipal Natural Science Foundation (No. 2082023), the National Natural Science Foundation of China (Nos. 50573090 and 10672161) and the Overseas Outstanding Scholar Foundation of Chinese Academy of Sciences (Nos. 2005-1-3 and 2005-2-1). Supporting Information Available: The size distribution of the Co sample, TGA and DSC curves, and TEM and HRTEM images of the as-prepared sample. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Caruso, F.; Spasova, M.; Susha, A.; Giersig, M.; Caruso, R. A. Chem Mater. 2001, 13, 109. (2) Hyeon, T.; Lee, S. S.; Park, J.; Chung, Y.; Na, H. B. J. Am. Chem. Soc. 2001, 123, 12798. (3) Xiong, Y.; Xie, Y.; Chen, S.; Li, Z. Chem.—Eur. J. 2003, 9, 4991. (4) Woo, K.; Lee, H. J.; Ahn, J.; Park, Y. S. AdV. Mater. 2003, 15, 1761. (5) Teng, X.; Yang, H. J. Mater. Chem. 2004, 14, 774. (6) Yu, A.; Mizuno, M.; Sasaki, Y.; Kondo, H. Appl. Phys. Lett. 2002, 81, 3768. (7) Dumestre, F.; Chaudret, B.; Amiens, C.; Fromen, M.; Casanove, J.; Renaud, P.; Zurcher, P. Angew. Chem., Int. Ed. 2002, 41, 4286. (8) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (9) Dinepa, D. P.; Bawendi, M. G. Angew. Chem., Int. Ed. 1999, 38, 1788. (10) (a) Dumestre, F.; Chaudret, B.; Amiens, C.; Respaud, M.; Fejes, P.; Renaud, P.; Zurcher, P. Angew. Chem., Int. Ed. 2003, 42, 5213. (b) Vivekch, S. R. C.; Gundiah, G.; Govindaraj, A.; Rao, C. N. R. AdV. Mater. 2004, 16, 1842. (11) Wang, F.; Gu, H.; Zhang, Z. Mater. Res. Bull. 2003, 38, 347. (12) Knez, M.; Bittner, A. M.; Boes, F.; Wege, C.; Jeske, H.; Mai, E.; Kern, K. Nano Lett. 2003, 3, 1079. (13) Wu, H.; Zhang, R.; Liu, X.; Lin, D.; Pan, W. Chem. Mater. 2007, 19, 3506. (14) (a) Niu, H. L.; Chen, Q. W.; Zhu, H. F.; Lin, Y. S.; Zhang, X. J. Mater. Chem. 2003, 13, 1803. (b) Athanassiou, E. K.; Grossmann, P.; Grass, R. N.; Stark, W. J. Nanotechnology 2007, 18, 165606.

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