Synthesis and Characterization of Structure Controlled ...

Mat. Res. Soc. Symp. Proc. Vol. 755 © 2003 Materials Research Society DD5.20.1

Synthesis and Characterization of Structure Controlled Nano-cobalt Particles

Shiqiang (Rob) Hui1, Mingzhong Wu1, Shihui Ge1, Dajing Yan1, Y.D. Zhang1 *, T.D. Xiao1,
M. J. Yacaman2, M. Miki-Yoshida3, W. A. Hines4, and J. I. Budnick4,

1Inframat Corporation, Farmington, CT 06032
2Department of Chemical Engineering, University of Texas, Austin, TX 78712
3Texas Materials Institute, University of Texas at Austin, Austin, TX 78712-2201

4Physics Department and IMS, University of Connecticut, Storrs, CT 06269

ABSTRACT

Nanostructured cobalt particles with and without a ceramic coating have been synthesized
using a wet chemical method. The structure and magnetic properties of synthesized powder
were characterized using x-ray diffraction (“XRD”), high-resolution transmission electron
microscopy (“HRTEM”), and a Quantum Design (SQUID) magnetometer. The cobalt
nanoparticles are of either face-centered cubic (“fcc”) and/or hexagonally close-packed (“hcp”)
crystalline structures. The average grain size is ~14 nm for cobalt (either fcc or hcp) with an
amorphous silica coating, and the average grain size is ~9 nm for hcp cobalt and 26 nm for fcc
cobalt without a silica coating. The effect of annealing temperature on grain size and magnetic
properties are addressed.

INTRODUCTION

Cobalt has been known with allotropic forms including fcc, hcp, epsilon (“ ”), and body-
centered cubic (“bcc”). Hull first reported the existence of fcc and hcp-cobalt after analyzing
different patterns of metallic powders prepared by several methods in 1921 [1]. The ε-cobalt, a
complex cubic primitive structure (P4332), was recently recognized by Dinega et al. through a
detailed structure analysis [2]. A non-equilibrium bcc structure, which does not naturally occur
in bulk materials, was also obtained using epitaxial growth [3]. The fcc structure is
thermodynamically preferred at higher temperatures and the hcp structure is favored at lower
temperatures. The ε-cobalt can be converted to common hcp and fcc-cobalt by annealing at
temperatures of 300oC and 500oC, respectively [4, 5]. However, the fcc structure appears to be
stable phase even below room temperature when the particle sizes are smaller than 20nm [6].
Temperatures of about 200oC are enough to trigger atom diffusion and phase transformations for
nanostructured cobalt crystals [7]. The fcc and hcp phases of cobalt are close-packed and nearly
isoenergetic crystal structures that differ only in the stacking sequence of atomic planes in the
[111] direction. Low activation energy for formation of stacking faults often results in the
formation of both phases in the same sample under high-temperature crystallization techniques,
such as melting-crystallization and evaporation-condensation. On the other hand, ε-cobalt is
most often found in nanostructured particles prepared at low temperatures by solution phase

* Author to whom correspondence should be addressed; email: [email protected], Tel: 860-
487-3838, Fax: 860-429-5911, Inframat Corp., Willington, CT 06279

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chemical synthesis technique, which is generally not thermodynamically controlled and thus can
allow the preparation of metastable phases [2, 4].

Cobalt nanocrystals display a wealth of size-dependent structural, magnetic, electronic, and
catalytic properties. There has been a considerable amount of research involving the preparation,
structure, and properties of magnetic cobalt nanoparticles in the past decade [8-17]. The high
magneto crystalline anisotropy of hcp Co has spurred intensive studies of Co-based
nanostructures for magnetic storage purposes [18]. Cobalt nanoparticles coated with insulators
have been prepared and studied for the applications in AC electrical and electronic devices [19-
21]. With the growing interest in building advanced materials using nanoscale building blocks,
there is a need to control the sizes, shapes, and structures. Different crystalline structures
provide some practical benefits. Symmetrically structured fcc-cobalt provides higher saturation
moment and lower magnetic anisotropy, which is suitable for linear applications such as
converter. On the other hand, asymmetric hcp-cobalt has been a basic element for high
temperature permanent magnets, and is also suitable for microwave applications such as in
circulator. To date, making magnetic cobalt nanocrystals has been difficult and required costly
size-selective precipitation methods. In practice, cobalt nanoparticles often possesses mixed
structures in which low energy stacking faults introduce a combination of fcc and hcp character,
making the synthesis of single structured cobalt a challenging task. Puntes et al achieved size
and shape-controlled cobalt nanorods as well as spherically shaped nanocrystals [22]. In this
work, we reported the successful preparation of structure-controlled cobalt nanoparticles with or
without coating.

EXPERIMENTAL

Cobalt nanoparticles with or without an amorphous SiO2 coating were synthesized by a wet
chemical approach [23]. The main procedures included: (1) preparing the starting precursors
containing cobalt and silicon, (2) atomizing the precursors to make colloidal solutions, (3)
annealing the solutions to form a pre-composite complex, and (4) low temperature calcination to
form cobalt or SiO2-coated cobalt nanoparticles. The cobalt concentration in the synthesized
nanoparticles was found to be dependent mainly on the precursor concentration and the
calcination temperature. In this paper, the nanoparticle samples were prepared by using the same
precursor but different calcination temperatures of 600, 700, 800 and 900 °C. The volume
fraction of cobalt in these samples was targeted at 50%. The structure of the Co nanoparticles
was determined by powder XRD analysis using Cu Kα1 radiation. Morphology and
microstructure was analyzed using HRTEM. The analysis was performed in a JEOL-2010F high
resolution field emission electron microscope, with a point to point resolution of 0.19 nm. The
microscope was also coupled with an EDS system to perform nanobeam analysis. Magnetic
properties were studied using a SQUID magnetometer.

RESULTS AND DISCUSSION

A series of cobalt samples have been prepared and their XRD patterns were shown in Fig. 1.
It is evident that the as-synthesized nanopowders correspond to either hcp or fcc structure of

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(a) #32602 100 (a) #52024
002
110 100
101 103 002
101
112 110
201 103
004
112
Intensity (b) #43022 Intensity 102 201
(c) #424021
(b) #121401 111
200
111 220 220
200 311 311

222 222

20 30 40 50 60 70 80 90 100 20 30 40 50 60 70 80 90 100

2θ 2θ

Fig. 1. X-ray diffraction patterns for as Fig. 2. X-ray diffraction patterns for as-
synthesized cobalt nanoparticles with synthesized nanocobalt particles coated with
different structures: (a) hcp, (b) mixed hcp silica (Co:SiO2 50:50 in vol): (a) hcp, (b) fcc.
and fcc, and (c) fcc.

cobalt depending on the controlled processing conditions, including starting materials, additives,
and heat-treatment temperatures [23]. The average grain size of the synthesized cobalt
nanoparticles was estimated by the Scherrer formula from the (101) and (111) diffraction peak
for hcp and fcc-cobalt, respectively [24]. A value of 9nm for hcp-cobalt and 26nm for fcc-cobalt
was obtained. It illustrates clearly from the XRD pattern that the (101) hcp-cobalt diffraction
peak was further broadened than other reflection peaks. This is most likely due to the formation
of stacking faults, leading to energetically growth along the b-axis, indicating that the
synthesized nanoparticles might be in the shape of rods rather than other shapes. A tempt to
directly observe the detailed microstructure by TEM was not successful. It is difficult to make
isolated nanoparticles of cobalt for TEM observation because of large attractive van der Waals
and magnetic forces between the particles.

Similarly, silica-coated cobalt (Co:SiO2 50:50 in vol) nanoparticles were prepared under
different conditions and the XRD patterns for the as-synthesized samples were shown in Fig. 2.
The as-synthesized samples can be signed as either hcp or fcc-cobalt with estimated being
approximately the same average grain size of 14nm from the XRD peak broadening analysis.
There were no peaks corresponding silica, indicating that the silica was most likely in its
amorphous state. HRTEM studies were carried out to study the nanostructure of these samples.
A typical nanoparticle image for sample 121401 and 52024 was shown in Fig. 3. (a) and (c),
respectively. In addition, an enlarged FFT filtered image of squared zone was shown in Fig. 3.
(b) for sample 121401, which shown atomic resolution images of planes (111) and (-111),
corresponding to the cubic phase of Co (PDF #15-0806). Fig. 3. (c) shows characteristic
images of a Co particle surrounded by SiO2. A core-shell structure is clearly evident in the
HRTEM image. The enlarged zone in Fig. 3. (d) shows FFT filtered image of the atomic
resolution of planes (101) and (002), corresponding to a hexagonal structure (PDF # 05-0727) for
sample 52024. The average particle size is about 18nm from the analysis of TEM particle
distribution for both samples, which is consistent with the calculation from XRD. The slight

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10 nm (b)
(c) (-111)
(111)

(d)

(002)

10 nm (101)

Fig. 3. HRTEM images for: (a) sample 121401 and (c) sample 52024. Enlarged zone shows
atomic resolution images of planes: (b) (111) and (-111) for sample 121401 and (d) (101) and
(002) for sample 52024.

difference between these two values is due to the methodology difference between XRD and
TEM.

The silica-coated cobalt nanoparticles with fcc structure were prepared at different
temperatures form 600 – 900 oC in order to study the property-temperature relationship. From
the (111) diffraction peak, the average grain size of the inner cobalt core in the synthesized
Co/SiO2 nanoparticles was estimated (see Fig. 4). The cobalt grain size increased slightly with
temperature when the calcination temperature was 800 °C or below. At above 800 oC, the cobalt
grain size increased significantly. This indicates that when the calcination was conducted at 800
°C or below, the SiO2 coating remained as an unbroken shell and prevented the coarsening of the

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cobalt nanoparticles. The SiO2 coating no longer acted as a barrier to effectively to prevent
abnormal cobalt grain growth at 900 °C.

Magnetic properties of the synthesized cobalt nanoparticles were also studied for these samples
prepared at different temperatures. Fig. 4 shows the coercivities measured at 10 K and 300 K as
a function of calcination temperature. The coercivity of the cobalt/silica nanocomposite material
decreases with increasing calcination temperature. Since the cobalt phase in the nanoparticles
calcined at lower temperature has a smaller effective size due to the presence of the inner Co-
oxide core, it can be inferred from Fig. 4 that the coercivity decreased with increasing particle
size. This result is consistent with the theoretical analysis given in reference [25], where R. C.
O’Handley argued that the coercivity went as H c ≈ a r 2 − b (r, particle size) for non- interacting
single-domain fine particles. The cobalt nanoparticles in our measured samples are free from
interaction, and their sizes are smaller than the single-domain critical size (76 nm) of fcc cobalt
particles [17].

Coercivity (Oe) 1500 Coercivity at 10 K 50 Particle size (nm)
1000 Coercivity at 300 K 40
Particle size (nm)

500 30

0 600 700 800 900 20
500 Calcination temperature (C) 1000

Fig. 4. Change of magnetic property and grain size as a function of temperature for silica-
coated nanoparticles of cobalt.

Fig. 4 also reveals that the coercivities measured at 10 K are notably higher than those
measured at 300 K for the nanoparticles calcined at 600, 700, and 800 °C. This is believed to be
due to the Co/CoO exchange coupling, which occurs at 10 K and disappears at 300 K. Besides,
although the nanoparticles calcined at low temperatures exhibited high coercivities up to 984.3
Oe, the nanoparticles calcined at 900 °C exhibited low coercivities of 126.7 Oe at 10 K and 63.3
Oe at 300 K. This indicates that, using the engineered wet chemical approach, the magnetic
softness of the nanoparticles can be readily controlled by the calcination temperature.

CONCLUSION
Cobalt nanoparticles with or without silica coating were synthesized using the engineered

wet chemical approach. Their structure was found to vary with preparation conditions and could
be controlled as either hcp or fcc structure. For silica-coated cobalt nanoparticles, their gain size

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increased slightly when the calcination temperature was lower than 800 °C. The silica shell as
observed by HRTEM hindered the grain growth of cobalt nanoparticles during the synthesis.

ACKNOWLEDGMENTS
This work was supported by DARPA through contract No. F7-6AM945-X05.

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