Synthesis and characterization of metallic co with different hierarchical structures prepared by a simple solvothermal method

Article abstract of DOI:10.1002/ejic.200901105

Magnetic metallic cobalt with different hierarchical structures, such as dendritic particles, flowery particles, and flocky spheres, has been successfully prepared, by using a simple solvothermal process in which the relative amounts of H₂C₂O₄ and N ₂H₄-H₂O in the reaction system were rationally adjusted. The Co crystals with hierarchical structure had a hexagonal-close- packed (hep) structure. The dendritic Co crystal, comprises several 1-5 um long dendrites, and. the hierarchical assemblies radiate from the center. The flowerlike Co submicrocrystals are composed of petal-like crystals around 1.5 μm in diameter and 100-200 nm thick. Interestingly, an intergrade from single-crystal to polycrystalline structures for flowerlike Co submicrocrystals is observed. The flocky spheres, about 600 nm in diameter, consist of 5 nm thick nanosheets. In addition, a growth mechanism is proposed which is based on the evolution of the structure and morphology with increasing reaction time. Further magnetic measurements for the Co crystals with different hierarchical structures indicated that the morphology of the Co crystals had an obvious influence on their magnetic properties.

Full text of DOI:10.1002/ejic.200901105

FULL PAPER
DOI: 10.1002/ejic.200901105
Synthesis and Characterization of Metallic Co with Different Hierarchical
Structures Prepared by a Simple Solvothermal Method
Lianfeng Duan,[a] Shusheng Jia,[a] and Lijun Zhao*[a]
Keywords: Hierarchical structures / Crystal growth / Cobalt / Magnetic properties
Magnetic metallic cobalt with different hierarchical struc-
tures, such as dendritic particles, flowery particles, and
flocky spheres, has been successfully prepared by using a
simple solvothermal process in which the relative amounts of
H2C2O4 and N2H4·H2O in the reaction system were rationally
adjusted. The Co crystals with hierarchical structure had a
hexagonal-close-packed (hcp) structure. The dendritic Co
crystal comprises several 1–5 µm long dendrites, and the
hierarchical assemblies radiate from the center. The flower-
like Co submicrocrystals are composed of petal-like crystals
around 1.5 µm in diameter and 100–200 nm thick. Interest-
ingly, an intergrade from single-crystal to polycrystalline
structures for flowerlike Co submicrocrystals is observed.
The flocky spheres, about 600 nm in diameter, consist of 5 nm
thick nanosheets. In addition, a growth mechanism is pro-
posed which is based on the evolution of the structure and
morphology with increasing reaction time. Further magnetic
measurements for the Co crystals with different hierarchical
structures indicated that the morphology of the Co crystals
had an obvious influence on their magnetic properties.
dro-/solvothermal methods, have been developed to fabri-
cate cobalt.[17–25] As one of the most common liquid-phase
methods, the hydro-/solvothermal method is often used to
prepare high-purity materials and to control the product
morphology.[26] Anisotropic magnetic particles or super-
structures are expected to exhibit interesting magnetic prop-
erties such as enhanced coercivity. Hence, it has recently
become a hot topic to fabricate magnetic materials with
hierarchical structures. Here, we report a tunable, facile, and
surfactant-free solvothermal synthesis of Co microcrystals
possessing a variety of hierarchical structures. By con-
trolling the content of H2C2O4 in the reaction system, we
obtained Co products with different sizes and morpho-
logies. Likewise, the effects of N2H4·H2O were studied un-
der conditions of fixed alkalinity. Moreover, the magnetic
properties of the Co products obtained at different condi-
tions were investigated to explore the dependence of the
magnetic properties on the sizes and morphologies of the
particles.
Introduction
Magnetic materials have attracted considerable interest
due to their novel properties and many applications in mag-
netic recording media, sensors, and other devices.[1–4] Co-
balt compounds display a wealth of magnetic, electronic,
optical, microwave, and catalytic properties.[5–9] It is be-
lieved that the properties of micro-/nanomaterials strongly
depend on their size, shape, and dimensionality. Because the
multiple crystal structures [such as hexagonal-close-packed
(hcp), face-centered-cubic (fcc), and ε-phase][10] and mor-
phologies of cobalt have a significant influence on its prop-
erties, there has been an increasing number of reports on
the synthesis of cobalt with various shapes. Nanorods,
micro-/nanowires, nanofibers, nanobelts, nanotubes, nano-
rings, nanodisks/nanoplatelets,[11–15] two- and three-dimen-
sional superlattices, and chain-like structures[16] have been
successfully synthesized. In this contribution, not only do
we study the transformation of morphologies, but we also
focus attention on an intergrade from single-crystal to poly-
crystalline hierarchical structures synthesized by the solvo-
thermal method.
Results and Discussion
To date, various synthetic methods, including thermal de-
composition of organometallic precursors, electrodeposi-
tion technology, templated/mediated synthesis, electrospin-
ning techniques, magnetic-field-induced processes, and hy-
Figure 1a shows XRD patterns of as-prepared cobalt
crystals (S1, S2, and S3). All of them are well crystallized.
The diffraction peaks for the as-formed samples at 2θ =
41.68, 44.24, 47.44, and 75.89°, which match the reflection
planes of (100), (002), (101), and (110), respectively, could
be indexed to a pure hcp cobalt phase (JCPDS 05-0727).
Trace carbon peaks (JCPDS 26-1076) can also be observed,
[a] Key Laboratory of Automobile Materials (Jilin University),
Ministry of Education and School of Materials Science and
Engineering, Jilin University,
Changchun 130022, China
2–
Fax: +86-431-85095876
which are derived from C2O4 ions formed during the sol-
E-mail: lijunzhao@jlu.edu.cn
vothermal process. However, it is observed that the inten-
sities of these reflections do not agree with those in the
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.200901105.
Eur. J. Inorg. Chem. 2010, 1957–1962
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1957

L. Duan, S. Jia, L. Zhao
FULL PAPER
PDF card (JCPDS 05-0727). For instance, in the card, the hierarchical Co structures was gained by using TEM and
diffraction peak of (101) is the strongest, whereas the (002) high-resolution TEM (HRTEM). Figure 2c shows the TEM
peak is weaker. However, the relative intensity of the (002) image of the as-prepared S1 sample. Figure 2d is the corre-
peak for the samples prepared under solvothermal condi- sponding selected area electron diffraction (SAED) pattern,
tions increases significantly, which might result from the which illustrates the single-crystal nature of the cobalt den-
oriented aggregation of the crystallites along the c axis. To drite. Some elongated diffraction spots in the SAED can be
determine the content of Co, energy dispersive X-ray spec- seen because of the deformation of several branches in the
troscopy (EDS) was carried out on the S2 sample, and the metallic cobalt superstructures. Furthermore, the SAED
result is shown in Figure 1b, where strong Co peaks and spots can be consistently indexed to (101), (103), and (002)
weak C peaks are observed. By calculation, the Co content facets, in good agreement with the XRD data. The
in the S1, S2, and S3 samples should be 98.5%, 92.6%, and HRTEM image of the arrow tip of the dendrite (Figure 2c)
97.6%, respectively. Note that Si, also observed in the EDS is shown in Figure 2e. From this figure, the lattice in-
spectrum, is due to the silicon wafer supporting the sample. terplanar spacing is measured to be 0.20 nm, corresponding
to the (002) plane of hcp Co. The [001] direction, corre-
sponding to the (002) plane of hcp Co, as shown in Fig-
ure 2e, is parallel to the growth direction of the main
branch, as displayed by the arrow in Figure 2c, indicating
that this growth is along the [001] direction.
Figure 2. (a) Low-magnification SEM, (b) high-magnification
SEM, (c) TEM, (d) SAED, and (e) HRTEM images of S1.
Figure 1. (a) XRD patterns of the as-obtained products S1, S2, S3
(diamonds represent cobalt; pentagrams indicate carbon) and (b)
EDS pattern of S2.
To investigate the effect of N2H4·H2O on the mor-
Figure 2a is a typical low-magnification SEM image of phology, the as-prepared S2 sample was examined by SEM.
S1, from which we can observe that novel and large-scale Figure 3a shows the overall morphology of S2, which indi-
dendritic superstructures with an average diameter of ap- cates that the products mainly consist of flowerlike struc-
proximately 10 µm were fabricated. A higher-magnification tures. The high-magnification image of a single Co submic-
SEM image of an individual crystallite of S1 indicates that rocrystal with a size of about 1.5 µm demonstrates that the
the superstructures comprise several dendrites having hier- architecture is constituted by a few petals with sizes of 100–
archical assemblies that radiate from the center (Figure 2b). 200 nm radiating from the center (Figure 3b). The mor-
The final product consists of many dendritic architectures phology of the flowerlike Co is further determined by TEM
with lengths of 1–5 µm along the trunks, as can be seen (Figure 3c), and is consistent with that observed by SEM.
from the SEM images displayed in Figures 2a and b. Fur- The SAED pattern (Figure 3d) illustrates the imperfect sin-
ther insight into the morphology and microstructure of gle-crystal nature of the cobalt, where some separated and
1958
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Metallic Co with Different Hierarchical Structures
elongated diffraction spots are observed. An unusual
The effect of H2C2O4 on the morphology was also rese-
HRTEM image (Figure 3e) was recorded on the fringe of arched. The S3 sample is composed of flocky spheres about
the S2 sample. From Figure 3e, the lattice interplanar spac- 600 nm in diameter (Figure 4a). A high-magnification SEM
ings are measured to be 0.22 nm, corresponding to the (100) image (Figure 4b) displays the detailed structure of the
plane of hcp Co in the “A” area. In the “B” area, the lattice spheres. These comprise a number of nanosheets with an
interplanar spacings are measured to be 0.20 nm, corre- average thickness of about 5 nm, as estimated from the
sponding to the (002) plane of hcp Co. In order to under- standing nanosheets in the SEM image. Figure 4c shows a
stand the unusual HRTEM image, we have drawn an ex- TEM image of an individual flocky sphere. The corre-
planatory diagram in Figure 3f. The grain boundary sepa- sponding SAED pattern (Figure 4d) denotes that the Co
rating the crystals having different crystallographic orienta- nanosheets are polycrystalline. According to the interplanar
tions in polycrystalline materials is shown in Figure 3f. spacing of the hexagonal Co, the (100), (002), (101), (102),
Within the boundary region, which is just several atoms (110), (103), and (112) planes could be determined. The
wide, there is some atomic mismatch in a transition from SAED results are in good agreement with the XRD data.
the crystalline orientation of one grain to that of an adja- To depict the nature of the as-prepared S3 product,
cent one. This orientation mismatch is slight, in the order HRTEM was also performed, and the micrograph is shown
of a few degrees, so the term small- (or low-) angle grain in Figure 4e. The inner lattice fringes of S3 can be seen
boundary is used.[27] These boundaries can be described in clearly in Figure 4e . The interplanar spacing was measured
terms of dislocation arrays. One simple small-angle grain to be 0.22 nm, corresponding to the (100) planar spacing of
boundary is formed when edge dislocations are aligned in hcp Co. However, the inner lattice fringes of S3, shown in
the manner shown in Figure 3f. The angle of misorienta- Figure 4e, are in every orientation. In conclusion, the S3
tion, θ, is indicated in the formula θ ≈ tgθ = b/D ≈ 11°, sample is composed of a single-crystalline sub-micro spheri-
where b is the Burgers vector and D is the interval of adja- cal core and polycrystalline nanosheets. It was also found
cent defect.
that the nanosheet was not single-layered, rather composed
of randomly attached smaller sheets.
Figure 3. (a) Low-magnification SEM, (b) high-magnification
SEM, (c) TEM, (d) SAED, and (e) HRTEM images of S2; (f) dem-
onstration of HRTEM images of S2.
Figure 4. (a) Low-magnification SEM, (b) high-magnification
SEM, (c)TEM, (d) SAED, and (e) HRTEM images of S3.
This is different from the twin structure, which could
To shed light on the growth processes of the hierarchical
form as a result of the erroneous attachment of atoms to a Co structures, we studied the products (obtained at dif-
growing crystal in such a way that two crystals appear to ferent stages of the reaction under 200 °C solvothermal
be growing out of each other, such as in gold and silver.[28] treatment), as shown in Figure 5 and Figures S1–S3 (Sup-
Hence, flowerlike structures may be a transitional mor- porting Information). After 6 h of solvothermal treatment,
phology from single-crystal to polycrystalline.
the S1 samples obtained were composed of irregular par-
Eur. J. Inorg. Chem. 2010, 1957–1962
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L. Duan, S. Jia, L. Zhao
FULL PAPER
Figure 5. Schematic illustration of how the hierarchical structures of the Co crystals vary with solvothermal treatment time.
ticles approximately 5 µm in size. The complete conversion more, polyols are strong complexing and reducing agents at
of the flowerlike congeries to dendritic Co through the high temperatures under basic conditions and certainly play
growth of the microparticles can be achieved after 12 h of a role in the formation of the Co nanoparticles. The solid
solvothermal treatment. After 24 h of solvothermal treat- intermediate phase Co(CH3COO)2 is obtained from the re-
ment, large-scale dendritic Co microcrystals were obtained. action mixture at the beginning of the reduction.[31] At
S2 with accidented surfaces appeared after 6 h of solvother- stage II, with the continuous advancement of the reaction,
mal treatment. Note that the average diameter of the protu- assembled nanoparticles with a relatively large size might
berances was 20 nm. Increasing the reaction time to 12 h in serve as crystallization nuclei. The thermodynamically
the same reaction system led to samples containing spheri- active sites, such as surface defects and active crystal sur-
cal nano-protuberances with average diameters of 50 nm. faces, are generated on the first Co nuclei formed as a result
After 24 h of solvothermal reaction, flowery S2 samples of the solvation of the Co surface.[32] These active sites can
were entirely composed of petals of approximately 400 nm. then serve as heterogeneous nucleation sites. Once nuclei
For S3, after 12 h of solvothermal treatment, flakelets were are formed at these active sites, growth with preferential
present. The hierarchical nanosheets were clearer and more orientation can start. When the reactants are consumed be-
complete on increasing the treatment time to 24 h.
cause of particle growth, the larger ones have smaller sur-
On the basis of the above experimental observations, we face free energies and consequently grow at the cost of the
have easily obtained three kinds of hierarchical architec- smaller ones, following a typical Ostwald ripening pro-
tures. The formation process of the hierarchical structures cess.[33] It was reported that the morphology and size of the
can be seen clearly in Figure 5. The preparation of metal products depend on the competition between crystal nucle-
Co probably involves the following chemical reactions:
ation and crystal growth, and the pH value has an impact
on both of these factors.[34]
Co2+ + H2C2O4 Ǟ Co(COOH)2
(I)
Compared with that of S1 at stage III, the surface of S2
is smoother. As mentioned above, S1 is composed of irregu-
lar particles of approximately 5 µm, whereas the average
diameters of the protuberances on the surface of S2 are
about 20 nm. Hence, we speculate that the addition of
N2H4·H2O can influence the roughness of the surface of the
Co microcrystals to some extent. Complexation of
Co(COOH)2 + 2(CH2OH)2 + 8OH–Ǟ
Co(CH3COO)2 + 2CO2 + 8H2O
(II)
(III)
(IV)
(V)
Co(CH3COO)2 + 14OH– Ǟ CoC2O4ȇ + 2CO2 + 10H2O
CoC2O4 + (CH2OH)2 + 10OH– Ǟ Co + 4CO2 + 8H2O
2–
CoC2O4 + N2H4 + 4OH– Ǟ Co + N2 + C2O4 + 4H2O
The chemical reactions involved in the formation of S1 N2H4·H2O with Co2+ ions will decrease the concentration
and S3 are described by reactions (I), (II), (III), and (IV) of Co2+ ions available for the formation of CoC2O4. More
above, whereas S2 is obtained as described by reactions (I), nuclei can always be formed at a higher Co2+ concentration
(II), (III), and (V).
due to a higher crystal growth rate, so particles with smaller
Small particles are formed by a conventional nucleation sizes can be obtained at higher Co2+ concentrations. As a
process at stage I. These then diffuse and aggregate to de- result, more smaller building blocks can aggregate into
crease their high surface energies. Moreover, the Co- microspheres with smooth surfaces at higher Co2+ concen-
(COOH)2 molecules containing carboxyl groups tend to ag- trations, as observed for S2; on the contrary, in the forma-
glomerate into spherical clusters as a result of the steric tion of S1, fewer particles of a larger size form microspheres
effect and hydrophilicity of the carboxylate.[29,30] Further- with rough surfaces. Comparing S1 and S2 at stages IV and
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Metallic Co with Different Hierarchical Structures
V, we can see that dendritic S1 and flowerlike S2 are ues of coercivity (Hc) are 75, 105, and 22 Oe for S1, S2, and
formed. Here, the addition of N2H4·H2O may accelerate the S3, respectively. Experimental errors may be responsible for
growth of S1, because of its strong reducing ability. The the different Ms values of the samples. For S2, N2H4·H2O
rapid growth of S1 may make the 5 µm irregular particles is not added into the reaction; therefore, the growth of Co
grow into dendritic shapes, whereas the relatively slow particles is relatively slow, which facilitates complete
growth of S2 may only give an opportunity for the 20 nm crystallization. Hence, S2 has the highest Ms value. Further-
protuberances to grow.
more, a double dosage of H2C2O4 was used to fabricate S3,
At stage III, the surface of S3 is also smoother than that which would decrease the reducing rate of Co and make for
of S1. As mentioned above, the double dosage of H2C2O4 complete crystallization to some extent. S2has a higher Ms
may play a key role. At stages IV and V, the reduction po- value relative to S1. However, Co hierarchical structures
tential in solution for S1 is higher than that for S3, because generally exhibit enhanced values, which may be contrib-
a double amount of H2C2O4 is used for the preparation of uted to their hierarchical architectures. The boundary dislo-
S3 in the reaction system. Higher pH values facilitate the cation arrays of S2 may be the main reason for its highest
reduction process for the preparation of Co in ethylene Mr and Hc. However, the coercivity of S3 was much lower
glycol (EG). Hence, the low reduction potential in solution than those of S1 and S2 by the reduced influence of its
leads to the slower reduction rate, which causes the Co architecture on the rotation of magnetic dipoles. Generally,
spherical clusters to become the reactive core of the flake- an increased amount of energy (or higher magnetic field
lets. In this case, the interaction between inorganic materials strength) is required to change the direction of magnetiza-
and the abundant H2C2O4 was the main driving force for tion of these aligned dipoles, which suggests larger coerciv-
self-assembly.
ity for S1 and S2, as observed. The difference of Co con-
We conclude that the formation of Co hierarchical archi- tents between S1 and S2 is another reason for the lower Hc
tectures is controlled by the rates of nucleation and growth. value of S1. Furthermore, such a decrease could also be
H2C2O4 plays an important role in determining the nucle- ascribed to the special 3D assembled structure of S3, which
ation rate, while the growth speed is under the control of is an integral crystallite possessing low anisotropy.[35] Com-
N2H4·H2O. We successfully fabricated different Co hierar- pared with S1 and S2, S3 may have the lowest shape anisot-
chical architectures by adjusting the dosages of H2C2O4 and ropy. Because elliptic morphology has the lowest shape an-
N2H4·H2O. Furthermore, the reaction systems of S2 and S3 isotropy, the morphologies of S3 are closer to an ellipse.
show lower pH values relative to that of S1. Hence, the rate Hence, S3 has the lowest Mr and Hc.
of crystal nucleation was slower than that of crystal growth
in the reaction systems of S2 and S3, relative to that for S1.
Table 1. Magnetic parameters obtained from hysteresis loops.
The above explanation may be the reason why the particle Samples
S1
S2
S3
sizes of S1 are larger than those of S2 and S3.
Ms (emu/g)
Mr (emu/g)
Hc (Oe)
124
3.6
75
139
5.4
105
131
0.9
22
To investigate the dependence of the morphologies on
the magnetic properties of Co hierarchical architectures,
magnetization measurements were performed. Figure 6
shows the hysteresis loops of S1, S2, and S3 measured in
fields between Ϯ1 T at room temperature.
Conclusions
In summary, magnetic metallic Co samples with different
hierarchical architectures, such as dendritic particles, flow-
ery particles, and flocky nanospheres, have been success-
fully prepared by using a simple solvothermal process and
adjustment of the dosages of H2C2O4 and N2H4·H2O in the
reaction solution. Among the three hierarchical architec-
tures, we can observe the transformation from single-crystal
to polycrystalline structures for flowerlike Co submicrocry-
stals. Moreover, magnetic measurements indicate that the
magnetic properties of Co hierarchical architectures depend
on their morphologies.
Experimental Section
Figure 6. Magnetization curve measured at room temperature for
(a) S1, (b) S2, (c) S3.
All reagents were analytical grade commercial products used with-
out purification. Detailed experimental parameters and corre-
sponding brief results are listed in Table 2.
The magnetic parameters obtained from Figure 6 are
listed in Table 1. The values of saturation magnetization
(Ms) are 124, 139, and 131 emu/g; the values of remnant
In the first step, CoCl2·6H2O (0.2 g) was dissolved in EG (20 mL)
magnetization (Mr) are 3.6, 5.4, and 0.9 emu/g; and the val- with magnetic stirring at room temperature for 20 min. Quantified
Eur. J. Inorg. Chem. 2010, 1957–1962
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1961

L. Duan, S. Jia, L. Zhao
FULL PAPER
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N2H4·H2O (2.0 mL, 55 vol% analytical reagent) was added drop-
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Subsequently, the solution was transferred into a 30 mL Teflon-
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Supporting Information (see footnote on the first page of this arti-
cle): To shed light on the growth process of the hierarchical Co
structures, we studied the products (obtained after different periods
of solvothermal treatment at 200 °C). Figures S1, S2, and S3 are
low-magnification and high-magnification FESEM images of the
as-synthesized S1, S2, and S3 samples collected after solvothermal
reactions for 6 h and 12 h, respectively.
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We acknowledge support by the Basic Research Expenses for the
Special Funds of Jilin University.
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Received: November 15, 2009
Published Online: March 24, 2010
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