Fabrication and Magnetic Properties of Cobalt Microcrystals

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Fabrication and Magnetic Properties of Cobalt Microcrystals L. J. Zhao, L. F. Duan, Y. Q. Wang, and Q. Jiang* Key Laboratory of Automobile Materials (Jilin UniVersity), Ministry of Education and School of Materials Science and Engineering, Jilin UniVersity, Changchun 130022, China ReceiVed: NoVember 25, 2009; ReVised Manuscript ReceiVed: May 21, 2010

Marked morphological changes in Co microcrystals are achieved by varying the reaction conditions, such as dosages of precursors (D), reaction temperatures (T), and types of alkali during the solvothermal treatment. Discussion is focused on the fundamental understanding and experimental designs in the control of shape of cobalt particles in solvothermal systems. UV-vis spectra show that the adsorption peak for the Co microcrystals ranges from 216 to 222 nm, which corresponds to the morphologies of disk, truncated prism, chain, hierarchical structure, and sphere, respectively. The final morphologies are complex results of the time-dependent oriented attachment and growth processes. By combining experimental results gained from XRD and SEM characterization, the formation mechanisms of Co microcrystals in solvothermal systems are conjectured. 1. Introduction Control over crystallization is one of the most important techniques in modern materials science due to its potential to produce well-defined particles with unique shapes and structures from nanometer to micrometer size range. As nucleation and growth processes are highly sensitive, crystallization could be controlled by various additives in solvothermal systems. Several groups have investigated the effect of polymers,1 synthetic macromolecules,2 low molecular weight compounds,3 organic small molecules,4–7 and others on the crystallization of cobalt from solution under mild conditions. Hereinto, small organic molecules play increasingly important roles in the design of shape-controlled particles since they are used to inhibit the growth of a particular crystallographic direction by selective adsorption during the growth of a crystal. The nucleation can be modulated through controlling surface free energy, reaction temperature, and degree of supersaturation.8,9 The growth can be controlled by both thermodynamic and kinetic factors which are dictated by both the intrinsic structural properties of cobalt and reaction systems such as, solvent, capping agent, and reducing agent. The rate constants for growth are exponentially dependent upon surface energy of facets of the nuclei, and small energy differences on different facets of crystals can be exploited to yield major anisotropies in the kinetic limit. By investigation, we found that ethylene diamine (EDA) was such a contributing small molecule to control the nucleation and growth of Co morphologies. Therefore, special attention is paid to the effects of reaction conditions on the Co morphologies during the solvothermal process. In this contribution, we show a more comprehensive picture for the shape control via selective adsorption in the Co system. These results are of interest from not only the viewpoint of understanding the crystal growth, but also because of the interesting magnetic, electrical, optical, and catalytic properties of the resulting Co-based materials. Ferromagnetic Co crystals with well-defined superstructures have attracted considerable attention for their structure characteristics that endow them with a wide range of potential applications.10,11 Controlled synthesis of Co microcrystals is thus * To whom correspondence should be addressed. E-mail: [email protected]. Phone: +86-431-85095371. Fax: +86-431-85095876.

important to achieve practical applications with uncovering properties. Now, there exist numerous and diverse synthetic methodologies, surface analytical techniques, and materials fabrication methods which build structures with different compositions and shapes on the nanometer and micrometer length scales.12–16 For the compound possessing hcp structure, there are many reports about the polyhedral morphologies, for example, ZnO disks,17 β-NaYF4 prisms, and octadecadedra.18 However, it is rare to see a report about the polyhedral cobalt with hcp structure. Among metallic particles with different shapes, cobalt polyhedra are a class of particularly interesting structures, because they allow researchers to investigate the influence of particle size and shape on their physical and chemical properties. Note that a well-controlled shape and a well-defined reactive crystalline facet of catalyst particles play an essential role in determining catalytic properties.19 Besides, large-scale Co polyhedra can be prepared by the facile synthesis method. The dependence of morphology on the dosage of precursors (D), the reaction temperature (T), and the type of alkali has been rationally investigated. 2. Experimental Section All reagents were analytical grade and were used without further purification. In a typical experiment, 1 g of CoCl2 · 6H2O was dissolved in 30 mL of ethylene glycol (EG) by intensive stirring for 30 min, and a homogeneous transparent solution was obtained. Ethylene diamine (EDA) (3 mL) was added dropwise to the solution at room temperature with simultaneous vigorous agitation. The mixtures were stirred vigorously for 30 min and then sealed in a Teflon-lined stainless-steel autoclave (50 mL capacity). Other samples were prepared by changing the reaction conditions during the solvothermal treatment. Detailed experimental parameters and corresponding brief results are listed in Table 1 (from S1 to S9). The autoclave was maintained at 473 K for 24 h and then cooled to room temperature. The gray products were washed several times with ethanol and dried in a vacuum oven at 313 K for 6 h. The fabricated phases from the above solutions were identified by means of X-ray diffraction (XRD) with a Rigaku D/max 2500 pc X-ray diffractometer with Cu KR radiation (λ ) 1.54156 Å) at a scan rate of 0.04 deg/s. The morphologies and

10.1021/jp9112377  2010 American Chemical Society Published on Web 06/01/2010

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TABLE 1: Brief Summary of the Experimental Parameters and Corresponding Results, Where EDA, TEA, and LA Denote Ethylenediamine, Triethylamine, and L-Arginine CoCl2 · 6H2O (g) samples

temp (K)

S1 S2

1 1.5

473 473

S3 S4 S5 S6

2 1 1 1

473 493 513 533

S7 S8 S9

1 1 1

473 473 473

alkali

morphology

3 mL of EDA microdisks 3 mL of EDA chains with truncated prisms 3 mL of EDA chains 3 mL of EDA mixed shapes 3 mL of EDA octadecahedra 3 mL of EDA prism with a hierarchical structure 3 mL of TEA irregular spheres 0.4 g of LA twists 1 g of NaOH spheres

compositions of the as-prepared products were characterized by a JEOL JSM-6700F field-emission scanning electron microscopy (FESEM) and equipped with energy dispersive X-ray spectroscopy (EDS). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) observations were carried out on a JEOL 2100F with an emission voltage of 200 kV. The UV-visible absorption spectra were measured with a Hitachi U-4100 UV-visible spectrophotometer, using a 1 cm pathlength cell, and the samples were dispersed in acetone to form solutions. Magnetic measurements were carried out with a Quantum Design Superconducting Quantum Interference Device (SQUID) magnetometer (LakeShore 7307). 3. Results and Discussion XRD patterns of the as-prepared samples are shown in Figure 1. Four obvious diffraction peaks can be indexed to the (100), (002), (101), and (110) facets of the hcp Co phase, which are consistent with the standard values reported in the Joint Committee on Powder Diffraction Standards (JCPDS 05-0727). No characteristic peaks for Co oxides or hydroxides are detected. Compared with the representative S1, we can first conclude that synthetic conditions change the crystalline structure of Co microcrystals only a little. As far as the S9 is concerned, the diffraction peak of the (101) facet is the broadest among the samples. We suppose that the uneven crystallite sizes of S9 may cause the broadening of the (101) peak. In addition, the intensities of diffraction peaks vary directly with the tested amount of powder samples. Besides the phase purity of Co, we investigate the effect of D in the initial solution on the shapes of Co crystals. The morphologies and sizes of the as-synthesized S1, S2, and S3 as a function of D are shown in Figure 2. Figure 2a shows the large area and regular Co hexagonal microdisks, which have a edge length of 1.5-2.5 µm and a height of about 600 nm. The formation process of the microdisks is estimated as a nucleation-crystallization growth mechanism. Particles are formed by a conventional nucleation process at the initial stage, and are difficult to aggregate together in the following stage due to the obstruction of the amines in the reaction solution. However, the particles grow continually and crystallize into hexagonal disks based on the intrinsic crystal symmetry. Figure 2b presents the morphologies of chains consisting of truncated prisms where D increases to 1.5 g. The formation of chain-like structure may be induced due to the following several reasons. Because the increasing dosage of free Co ions is in favor of a relatively rapid growing rate of Co crystals, the Co

Figure 1. XRD patterns of S1, S3, S6, and S9.

particles diffused and aggregated to form chains comprising polyhedra. The reaction environment promotes the surface domains on neighboring particles to match up driven by magnetic dipole attraction.20–23 Subsequently, chemical reduction of Co ions continued and newly produced Co crystals would connect to the chains. The Co particles tended to join with each other to reduce both magnetic anisotropic energy and surface energy. Thereby, the growth mechanism of S2 may be a nucleation-aggregation-crystallization growth one. With remarkably enhanced D, the cobalt chains are prepared (S3), and are composed of polyhedra with lengths of several tens of micrometers (Figure 2c). On the one hand, a mass of increase in the Co atoms production rate shifts the balance between structural transition and growth to different extents, forming a range of products of different morphologies. On the other hand, the lack of a globally homogeneous reaction environment for each particle in the solution results in the formation of particles with different morphologies or fluctuation of particle structures in the final product. These results imply that the kinetic control can be a versatile and effective tool for tuning the morphology of crystals. Note that as D increases, the crystal sizes increase. In addition to D, the reaction temperature affects the crystals morphologies too. This is because the increasing T enhances the diffusion rate of metal atoms into the surfaces of the initially formed crystals, and then we obtain high-quality crystals. The effect of T on the morphologies of Co crystals is shown in Figure 3 where T is changed from 493 to 533 K. At T ) 493 K, the Co crystallites grow up and the morphologies of Co microcrystals change from uniform hexagonal disks to mixed shapes of prisms with truncated corners (Figure 3a). When T increases to 513 K, the octadecahedra are built, as shown in Figure 3b. The growth mechanisms of S4 and S5 should be similar to the nucleation-crystallization growth mechanism. At T ) 533 K, hierarchical microstructured prisms are formed, which are composed of ca. 100-nm-thick hexagonal disks with an edge length of about 1-2 µm, as shown in Figure 3c. The concave surfaces create reentrant grooves that increase coordination numbers of surface atoms, providing extra stability and promoting faster growth. The energy dispersive X-ray (EDX) analysis (Figure 3d) of S6 indicates the sample is essentially pure Co. The SAED pattern (Figure 3e) exhibits the single crystal nature of Co hierarchical microstructured prisms. Furthermore, the SAED spots can be steadily indexed to (101), (103), and (002) facets being in good agreement with XRD data. HRTEM analysis of random hierarchical microstructural prisms of S6 is shown in Figure 3f to understand the crystallographic orientation. The clear lattice fringes in Figure 3f correspond to (002)

Fabrication and Properties of Cobalt Microcrystals

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Figure 2. SEM images of (a) S1, (b) S2, and (c) S3.

Figure 3. SEM images of (a) S4, (b) S5, and (c) S6; (d) EDX pattern; (e) SAED and (f) HRTEM images of S6; and (g) schematic illustration for the formation of S6.

facets where the lattice space of 2.0 Å indicates that each disk grows along the [100] direction according to the equivalent crystallographic facet principle. The uniformity and continuity of the lattice fringes imply the high level of crystallinity of the hierarchical microstructural prisms. The parallel fringes confirm again that these microcrystals are composed of single crystals. To shed light on the possible growth processes of the Co hierarchical structured microcrystals, a schematic diagram was drawn in Figure 3g. While in these systems the bigger particles are grown from small primary particles through an oriented attachment mechanism, the adjacent particles share a common crystallographic orientation and dock these

particles at a planar interface. In the classical nucleation theory, the nucleation process can thermodynamically be described according to Gibbs free energy change of the considered system.24–26 The excess free energy, which contains two competing terms, i.e., the changes in surface and bulk free energies, reaches the maximum when clusters grow to the critical size. It is noteworthy that, for Co crystals with hexagonal structure, the surfaces are typically (001) for top/bottom facets and a family of six energetically equivalent (110) facets on basis of the known or similar models.27–31 The surface energy of (100) is smaller than that of (110) for hcp Co,32 thus it is easier to grow along the [110] direction.

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Figure 4. Schematic illustration for the morphology transformation of Co microcrystals.

Figure 5. SEM images of (a) S7, (b) S8, and (c) S9; and structural formulas of (d) ethylenediamine, (e) triethylamine, and (f) L-arginine.

When a certain amount of amines adsorb selectively on the Co (100) facets, disk-shaped particles were formed because of the remarkable restriction along the [100] direction. With increasing T during the solvothermal process, EDA was removed from the surfaces of (100) facets, which directly leads to the elevation of its surface energy. Hence, S6 may result from the oriented attachment growth of the primary disks through the (100) facets to drop the surface energy. Moreover, microstructural features, such as edge dislocations, porosity, and particle morphology, yield important clues to the aggregation-growth mechanism. Therefore, the observation of microstructural features (e.g., gap, “dimples”, and “creases”) retained in crystals implies that the oriented attachment growth is the dominant mechanism for the formation of the hierarchical microstructured prisms. The driving force for this spontaneous oriented attachment should be the elimination of the pairs of high-energy surfaces. The formation of hierarchical structure in Co crystals can lower the symmetry in crystal structures and changes the natural growth habits of Co particles, leading to various new morphologies. The evolution processes of the morphologies of Co crystals affected by both D and T are schematically shown in Figure 4. It is known that the preferential adsorption of molecules and ions in solution onto different crystal facets directs the growth of particles into various shapes by controlling the growth rates along different crystal axes.33 In our case, EDA may play a crucial role in directing the growth of Co microdisks due to the interaction between the -NH2 and the Co ions or the -NH2 selective adsorption on the (100) facets. SEM results show that the increases of both D and T bring on the disappearance of (100) facets. The morphological changes in a series of microdisks, truncated prisms, and long chains connected by (100) facets are due to a preferential removal of the capping agent of EDA from the (100) facets of the particles and then bonding. In addition, since the nucleation rate is proportional to D, a quick nucleation rate benefits from the presence of (111) facets. In summary, either a quick nucleation or a fast growth is in favor of the presence of (111) facets.

After studying the effects of D and T on the morphologies of Co crystals in the initial solution, the types of alkali were further investigated. Here, both the D and T are fixed, while the types of alkali are variable. Figure 5a presents irregular spheres in a diameter of 1-2 µm fabricated by triethylamine. Moreover, a twist structure is obtained by using L-arginine as the precipitator (Figure 5b). Figure 5c shows that NaOH could prepare regular spheres with smooth surface with a diameter of ca. 2 µm. The distinct structures of alkali may play a part in the formation of different morphologies of Co microcrystals. Herein, NaOH and L-arginine as solid powders are dissolved beforehand into a small quantity of water. Hence, they exist in the forms of OH- and +H3NC(R)H-COO- in the reaction system, respectively. EDA and TEA as liquids are directly dissolved into EG solvent in a form of functional groups. When metal salt is reduced in EG solvent, the alkali can play dual roles on decreasing the reduction potential and blocking particles from excessive growth. The complexation activity of TEA is much weaker than that of EDA, because of the large steric hindrance of TEA. Thus, Co2+ ions are difficult to complex with TEA. From the kinetic point of view, the TEA has little effect on the process of nucleation. Moreover, the inert triethyl groups of TEA may just block the growth of cobalt in all directions rather than adsorb selectively on the special crystal planes of cobalt. Hence, the irregularly spherical cobalt particles are obtained. To the best of our knowledge, this is the first synthesis of hcp Co with a twist structure. At room temperature, the COO- ions of L-arginine first connect with Co2+ ions. With the increasing solvothermal temperature, the ability for COO- ions to bind to certain crystal facets of Co may be different, which leads to the selective loss of COOions on certain crystal facets, and thus the presence of twist structure. For the synthesis of S9, the apparently spherical shape of the resulting particles indicates that several different families of slow-growing facets grow at a similar rate. In a moderate concentration of OH- ions, they play a key role in determining the morphology by adsorbing on every crystal facet of Co. In summary, the difference of complexation

Fabrication and Properties of Cobalt Microcrystals

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Figure 6. UV-vis spectra for (a) S9, (b) S6, (c) S3, (d) S2, and (e) S1.

activity between CoII and alkali causes the different free Co2+ concentration in the solution, which results in a diverse rate to generate Co atoms. Thereby, different types of alkali kinetically control the nucleation and growth rates and conceivably modulate the size and shape of the final product.34 Figure 6 is the UV-vis spectra of the samples. The slight red shift of the absorption peaks from about 216 nm for S1 to 222 nm for S9 indicates the shape transformation of Co crystals. Furthermore, the broad and rough absorption peaks imply inhomogeneous crystal sizes of the samples. Compared with the absorption peak of the representative S1, the largest difference of adsorption peaks is between S1 and S9. It is noteworthy that the absorption peak position shows a red-shift with the increase of edge length.35–37 Here, the edge length of S9 is infinite, and the edge lengths of S6 and S3 are longer than that of S2. S1 has the shortest edge length among the samples. In view of the UV-vis results, we speculate that the effects of reaction conditions on controlling the morphologies of Co microcrystals are types of alkali, T, and D. Thus, morphology-controlled synthesis may provide a powerful tool to create morphologies with unique signatures for optical labeling and sensing applications.38 To understand the shape-dependent physical properties of Co microcrystals, we make an investigation of their magnetic properties. Figure 7 presents the hysteresis loops of S1, S2, and S3 tested at 300 and 2 K by using SQUID. The corresponding magnetic parameters are listed in Table 2. The coercivity (Hc) and remnant magnetization (Mr) of the samples at 2 K are larger than those at 300 K due to the reduced influence of thermal fluctuation on the rotation of magnetic dipoles. In addition, the saturation magnetizations (Ms) of the Co microcrystals measured at 300 and 2 K are similar since the symmetry and crystal field of the surface metal ions resemble that of the core atoms. Ms

S2

S3

300 K

2K

300 K

2K

300 K

2K

154 2.0 38

155 5.3 125

163 2.9 49

162 5.3 108

160 4.3 77

159 8.5 176

values of S4, S5, and S6 at 300 K are 164, 167, and 172 emu/ g, respectively, while their Hc values are similar at around 30 Oe. For S7, S8, and S9, their Ms values are 156, 150, and 160 emu/g, and the corresponding Hc values are separately 33, 120, and 24 Oe. The above magnetic data obtained from hysteresis loops show that the morphologies play a role in the magnetic properties of Co microcrystals, especially the coercivity. We conclude that the one-dimensional (1D) Co microcrystals possess higher coercivity, relative to the zero- (0D) and two-dimensional (2D) Co microcrystals. The particles with ellipse morphology had the lowest shape anisotropy,39 so we speculated that the Co chains had a higher shape anisotropy energy. 0D and 2D Co microcrystals were closer to ellipse, so they had lower shape anisotropy energy. Besides, it also can be explained by considering the reduced influence of 1D architecture on the rotation of magnetic dipoles. As a result, an increased amount of energy (or higher magnetic field strength) was required to change the magnetization direction of these aligned dipoles. When subjected to an external magnetic field, those isolated 1D crystals could all be aligned in the same direction, while only one 0D or 2D crystal could be arranged along the direction of the external magnetic field;40 thus a relatively higher Hc was exhibited for S3 and S8. Comparing S3 with S8, the Hc value of S3 is lower than that of S8; perhaps an integral crystallite possesses lower anisotropy. Moreover, high solvothermal temperature would make for high crystallinity to some extent, so S6 shows the highest Ms value. 4. Conclusion Reaction conditions act on the morphologies of Co microcrystals during the solvothermal treatment while the growth mechanisms of Co microcrystals may be nucleationcrystallization growth, nucleation-aggregation-crystallization growth, and oriented attachment in the solvothermal systems. The effects of reaction conditions on controlling the morphologies of Co microcrystals are types of alkali, T, and D. Red-shift can be observed from the UV-vis absorption spectra because of the shape transformation of Co microcrystals. In addition, the final shapes decide the magnetic properties of Co microcrystals. Acknowledgment. We acknowledge support by the National Key Basic Research and Development Program (Grant No.

Figure 7. Hysteresis loops of S1 (a), S2 (b), and S3 (c) tested at 300 and 2 K. Inset: A magnified view of the hysteresis loops tested at 2 K.

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