Materials Chemistry and Physics 113 (2009) 675–679

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Materials Chemistry and Physics

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Shape-controlled synthesis and self-assembly ofhexagonal cobalt ultrathin nanoflakes

Xi Yang, Qian-wang Chen ∗, Ju-zhou ZhangHefei National Laboratory for Physical Sciences at Microscale, and Department of Materials Science & Engineering,University of Science and Technology of China, Hefei 230026, China

a r t i c l e i n f o

Article history:Received 22 May 2008Received in revised form 1 August 2008Accepted 9 August 2008

Keywords:

a b s t r a c t

Hexagonal cobalt microspheres composed of ultrathin nanoflakes were obtained via a hydrothermalreduction route by using the metal complex cobalt bis (4-pyridine carboxylate) tetrahydrate as a pre-cursor of cobalt. The diameter of these spheres is about 2 �m. The thickness of the cobalt nanoflakes isabout 10 nm along the easy magnetization axis [0 0 1], which is approximate to the critical size (12 nm)for single-domain behavior in Co. The hysteresis loop of the sample shows an interesting soft magnetic

Magnetic materialsNanostructuresChemical synthesisMagnetic properties

behavior with low coercivity (24.5 Oe). It is noticed that the coercivity is much lower than that of thehcp cobalt nanoflakes reported previously. The peculiar magnetic properties may result from both theanisotropy and the self-assembly manner of the nanoflakes.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Anisotropic hexagonal Co nanocrystals have been paid muchattention for their high saturation magnetization and magneticcoercivity [1–2]. It is found that magnetic properties of nanomate-rials depend significantly on the size and morphology of materials.For instance, the spherical cobalt nanoparticles with diameterssmaller than the critical size (12 nm) of single magnetic domain areusually superparamagnetic at room temperature [3], which makesthese particles impractical for many applications [4]. There are twofeasible ways to increase the magnetic anisotropy to overcome theproblem. One is modifying the shape of the nanoparticles [5–9], andthe other is assembling nanocrystals into multidimensional mor-phologies [10–13]. Single-crystalline hexagonal close-packed (hcp)cobalt nanoflakes with thickness of ∼8 nm show a ferromagneticbehavior and an obvious magnetic anisotropy, though the thick-ness of the nanoflakes is smaller than that of the single magneticdomain. The coercivity values of these nanoflakes are unequal witheach other when the external magnetic field is applied parallel(H||) and perpendicular (H⊥) to the flakes. The H|| and H⊥ valuesare 218 Oe and 176 Oe at 300 K, respectively. And the H|| value is772 Oe at 3 K. The predominantly exposed planes of these flakesare (0 0 1) [9]. The coercivity (Hc) value of the hexagonal cobaltspheres consisting of nanoplatelets is 590 Oe at 5 K. And the thick-

∗ Corresponding author. Tel.: +86 551 3607292; fax: +86 551 3607292.E-mail address: cqw@ustc.edu.cn (Q.-w. Chen).

ness of these nanoplatelets which growth orientation is [0 0 1] isabout 20 nm [10]. In addition, the flowery assembly of hcp-cobaltflakelets shows the Hc value of 308 Oe at room temperature, whilethe Hc value is 946 Oe at a low temperature of 2 K. The flakeletsare about 50–100 nm thick and their top/bottom faces are (0 0 1)[11]. Based on the Hc values at low temperatures, the coercivityof the assembly is reduced when the [0 0 1] thickness decreases.The [0 0 1] direction is the easy magnetization axis of hexagonalcobalt phase [14]. It is proposed that the thickness along the easymagnetization axis is an important factor influencing the mag-netic properties of these nanostructures. This paper aims to preparehexagonal cobalt ultrathin nanoflakes and understand the effectof self-assembly of hexagonal cobalt nanoflakes on the magneticproperties. The metal organic complex cobalt bis (4-pyridine car-boxylate) tetrahydrate (CoL2(H2O)4) was selected as a precursor toprepare hcp cobalt ultrathin nanoflakes. The CoL2(H2O)4 moleculehas a special spatial structure [15] which may control the thicknessof the nanoflakes.

2. Experimental

All chemicals were of analytical grade and used without further purification.In the first step, the metal organic complex CoL2(H2O)4 was synthesized by ahydrothermal method according to the literature [15]. In a typical synthesis of micro-spheres, 0.03 g of CoL2(H2O)4 was dispersed in 20 mL 85 wt% hydrazine hydrate(N2H4·H2O). A stable orange solution was formed after vigorous magnetic stirringfor 30 min. Then 10 mL NaOH aqueous solution (10 mol L−1) was added dropwisewith constant stirring and the solution turned blue. The final mixture was trans-ferred into a 60 mL Teflon-lined stainless steel autoclave, which was closed tightand maintained at 160 ◦C for 36 h. After the autoclave cooled to room temperature,the black powder settled at the bottom of the autoclave and was collected by a mag-

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676 X. Yang et al. / Materials Chemistry and Physics 113 (2009) 675–679

Fig. 1. (a) XRD pattern of as-prepared sample; (b) SAED pattern of an isolated nanoflake.

net. Then the solid product was rinsed with distilled water and absolute ethanolseveral times respectively and finally dried in air at room temperature.

3. Characterization

The X-ray diffraction data of as-obtained products were col-lected by a Rigaku (Japan) D/max-�A X-ray diffractometer withgraphite monochromatized Cu K� radiation (� = 1.5418 Å). TheSEM images were taken by a field emission scanning electronmicroscope (FESEM, JEOL JSM-6700F). The selected area electrondiffraction (SAED) pattern was obtained with a Hitachi H-800electron microscope and high-resolution transmission electron

microscope (HRTEM) images were recorded with a JEOL-2010transmission electron microscope. A superconducting quantuminterference device (SQUID) magnetometer (Quantum DesignMPMS XL-7) was used to measure the magnetic properties of as-prepared samples.

4. Results and discussion

The chemical composition and the phase of as-obtained prod-uct were determined by X-ray diffraction. The XRD pattern of arepresentative sample is presented in Fig. 1a. It can be identifiedas hexagonal cobalt, in which no other phases can be detected

Fig. 2. SEM images at low magnification (a) and high magnification (b) and TEM images at low resolution (c) and high resolution (d) of the microspheres composed ofnanoflakes.

X. Yang et al. / Materials Chemistry and Physics 113 (2009) 675–679 677

Fig. 3. SEM images of the samples obtained at various stages of reaction process: (a) 20 min; (b) 2 h; (c) 6 h; (d) 48 h.

Fig. 4. SEM images of the samples obtained in mediums with different NaOH concentrations: (a) 10 mol L−1; (b) 5 mol L−1; (c) 1 mol L−1; (d) 0.5 mol L−1.

678 X. Yang et al. / Materials Chemistry and Physics 113 (2009) 675–679

and all the diffraction peaks are well consistent with the standardcard (JCPDS 89-4308, P63/mmc, a = 2.505 Å, c = 4.089 Å). Further-more, the relative intensity of the (0 0 2) peaks compared with thatof the neighboring (1 0 1) peaks increased, which indicates highlyoriented growth of Co particles occurred. This agrees with the previ-ous reports [8–11]. A SAED pattern (Fig. 1b) of an isolated nanoflake(the inset of Fig. 1b) further reveals that the cleavage plane of thenanoflake is (0 0 1) of hcp cobalt, which conforms to the character-istic of the XRD pattern of Fig. 1a.

Field emission scanning electron microscope was employed toinvestigate the morphologies and structures of as-obtained sam-ples. From Fig. 2a, the product fabricated at 160 ◦C for 36 h iscomposed of flocky microspheres with about 2 �m in diameter. Thehigher magnification image of an individual microsphere (Fig. 2b)demonstrates that the sphere is comprised of a large number ofnanoflakes. The cobalt flakelets with irregular shape are ∼10 nmthick. The HRTEM image (Fig. 2d), which was taken from the edgeof the microsphere in Fig. 2c, shows a hexagonal symmetry struc-ture with the lattice spacing of 0.21 nm, which can be indexed tothe {1 0 0} planes of hexagonal Co.

To understand the growth process of the microspheres, theshape evolution of the products at various stages of reaction processwas investigated by FESEM. SEM images of the samples preparedat 160 ◦C for 20 min, 2 h, 6 h and 48 h are shown in Fig. 3. Seenfrom Fig. 3a, the product grown for 20 min comprises spherical andflowery particles, which is similar to the sample prepared for 1 h(Supporting Information S1). A higher magnification view of a smallpart of a flower-like crystal is shown in the inset of Fig. 3a and it indi-cates that they are composed of nanoflakelets. After 2 h, the flowerycrystals tend to be spherical and more microspheres consisting ofnanoflakes are formed (Fig. 3b). As seen from Fig. 3c, the flowerycrystals disappear after 6 h. It is observed that the sizes of thesemicrospheres are not uniform and the surface of microsphere israther coarse. After 12 h, the sample (Supporting Information S2) issimilar to the microspheres grown for 6 h and the diameter of thesespheres is in the range of 2–5 �m. However, the size of the micro-spheres heated for 48 h (shown in Fig. 3d) was uniform and thediameter is about 2 �m, though a few smaller nanospheres couldbe observed.

On the basis of the above studies, a possible growth mechanismof the microspheres was proposed. CoL2(H2O)4 is a stable metalcomplex, which cannot be dissolved in water but can be dissolved inhydrazine hydrate. After the aqueous solution of NaOH was added,the Co2+ ions reacted with OH−, which can be judged from the colorchange of the solution. The cobalt (II) complexes were reduced byhydrazine in the initial stage, and then the cobalt nuclei were gen-

erated. In the growth process of Co nanoflakes, the present of thegroups of isonicotinic acid may play a role as an organic surfactant.The groups of isonicotinic acid are adsorbed selectively by somespecial crystallographic facets of the Co crystals and this varies thesurface energy of these planes. Then the varieties yield anisotropiesin the kinetic limit and induce the growth of the cobalt flakelets[5–7]. On the other hand, the CoL2(H2O)4 molecules containing car-boxyl groups tend to agglomerate into spherical clusters due to thesteric effect and hydrophilicity of carboxylate [16–17]. As a result,microscale spherical clusters would form, in which flakelets weregenerated simultaneously. It is supported by the phenomenon thatthe flowery crystals were formed on the early stage of reaction. Andthese clusters were different in size, which led to obvious differencein crystal size. With the whole reaction carried through, they self-assembled into microspheres to lower the energy of the system dueto the high surface energy of the nanoflakes and the strong mag-netic interaction. And the sizes of the spheres tend to be smallerand uniform because of the high pressure with the prolonging ofreaction time (the estimated pressure was about 640 atm at 160 ◦Cin the autoclave).

In order to control the shape of the products, we varied the reac-tion conditions and found out that the concentration of a NaOHsolution and the complex precursor in the system are importantfactors. SEM images of these samples obtained at various NaOHconcentrations are shown in Fig. 4. It can be observed that the finalproducts are more irregular by decreasing the amount of NaOHin the initial solution. This phenomenon indicates that the highconcentration of NaOH is advantageous to the formation of themicrospheres. The reason may be that the reducibility of hydrazineis stronger in high concentration of NaOH than that in a lowconcentration solution. In addition, Fig. 5b reveals that a higherconcentration of precursor CoL2(H2O)4 (0.18 g) produces thorn-likemicrospheres. The surface is more irregular and the size is moreuneven compared with the product fabricated under the lower con-centrations (Fig. 5a). This means that low concentrations of thecomplex precursor tend to produce microspheres.

The hysteresis loop of as-synthesized sample was obtained at300 K (Fig. 6a), which demonstrates that the sample exhibits softmagnetic properties at room temperature. The coercivity (Hc) andsquareness ratio (Mr/Ms) are 24.5 Oe and 0.009, respectively. The Hcis much smaller compared with that of bulk cobalt (Hc = 1500 Oe[18]). Particularly, it is noticed that the Hc of our sample is alsosmaller than that of the hcp cobalt nanoflakes reported previously[9–11,18]. There are several possible reasons responsible for thisphenomenon, such as the surface-oxidation of the flakelets [1].Moreover, magnetic properties of metallic nanomaterials signifi-

Fig. 5. SEM images of the samples prepared with varied concentration of CoL2(H2O)4: (a) 0.03 g CoL2(H2O)4; (b) 0.18 g CoL2(H2O)4.

X. Yang et al. / Materials Chemistry and Physics 113 (2009) 675–679 679

Fig. 6. (a) Magnetic hysteresis loop of the sample measured at room temperature; (b) magnetization distribution of cobalt flakes (redrawn from the figure in ref. [1]).

cantly depend on the size and morphology. It is proposed that thepeculiar magnetic property of our microspheres may be a resultof anisotropy of the cobalt nanoflakes and self-assembly morphol-ogy of the microspheres. The normal direction of the cobalt flakelet[0 0 1] is the easy magnetization axis of the flake, which makesthe nanoflakes demonstrate a remarkable magnetic anisotropy [9].The magnetic anisotropy and shape anisotropy cause the nanoflake∼10 nm thick to exhibit a vortex-like magnetization distribution(shown in Fig. 6b) [1,19–21]. The magnetic moment directionschange gradually without an external magnetic field. With an exter-nal magnetic field applied, these moment directions turn parallelto the external magnetic field. This magnetic structure is an impor-tant reason for the significant decrease of the coercivity of ourproduct. Moreover, the assembly manner of the Co nanoflakesis isotropic, while the nanoflake itself is anisotropic [22]. Thismay be the reason why the coercivity of our products is lowerthan that of the isolated hcp cobalt nanoflakes in ∼8 nm thick[9].

5. Conclusions

Cobalt microspheres were prepared through the self-assemblyof nanoflakelets formed by reduction of a metal complex precur-sor CoL2(H2O)4 with hydrazine at 160 ◦C for 36 h. The Co flakeletis ultrathin with the thickness of about 10 nm, whose normaldirection [0 0 1] is parallel to the easy magnetization axis. Thegroups of isonicotinic acid of CoL2(H2O)4 molecule and the agglom-erate morphology of the molecules in solution play importantroles in the formation of microspheres. Low concentration ofCoL2(H2O)4 and high concentration of NaOH in the system arebeneficial to the fabrication of the microspheres. The hystere-sis loop shows that the assembly exhibits peculiar soft magneticproperties at room temperature, which may result from both theanisotropy and the self-assembly manner of the nanoflakes. Theself-assembly manner is isotropic, while the ultrathin nanoflakeitself is anisotropic.

Acknowledgement

This work was supported by the Natural Science Foundation ofChina.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version, at doi:10.1016/j.matchemphys.2008.08.023.

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