Controlled synthesis of cobalt flowerlike architectures by a facile hydrothermal route

Article abstract of DOI:10.1002/ejic.200800137

Cobalt flowerlike architectures composed of hexagonal nanoplatelets have been synthesized by a simple hydrothermal reduction method. The architectures are fabricated by the reaction of CoCl₂ with NaOH at 140-180 °C in the presence of sodium dodecyl benzenesulfonate (SDBS), with NaH ₂PO₂·H₂O as reducing agent. The diameters of the flowers range from 8 to 10 μ m, and the average thickness of the hexagonal sheets is about 100 nm. Higher reaction temperatures and the proper concentration of sodium hydroxide (NaOH) are key requirements for the fabrication of the flowerlike architectures. A growth mechanism for these architectures is proposed on the basis of the characterization by X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The magnetic hysteresis loops at 5 K and 295 K of the cobalt flowerlike architectures show ferromagnetic characteristics with coercivities of 371 Oe and 197 Oe, respectively. Our work may shed light on the designed fabrication of complex 3D architectures of other materials. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200800137

FULL PAPER
DOI: 10.1002/ejic.200800137
Controlled Synthesis of Cobalt Flowerlike Architectures by a Facile
Hydrothermal Route
Ya-jing Zhang,*[a] Ying Zhang,[a] Zhen-hua Wang,[a] Da Li,[a] Tie-yu Cui,[a] Wei Liu,[a] and
Zhi-dong Zhang[a]
Keywords: Hydrothermal synthesis / Cobalt / Nanostructures / Magnetic properties
Cobalt flowerlike architectures composed of hexagonal nano-
platelets have been synthesized by a simple hydrothermal
reduction method. The architectures are fabricated by the re-
action of CoCl2 with NaOH at 140–180 °C in the presence of
sodium dodecyl benzenesulfonate (SDBS), with NaH2PO2·
H2O as reducing agent. The diameters of the flowers range
from 8 to 10 µm, and the average thickness of the hexagonal
sheets is about 100 nm. Higher reaction temperatures and
the proper concentration of sodium hydroxide (NaOH) are
key requirements for the fabrication of the flowerlike archi-
posed on the basis of the characterization by X-ray diffrac-
tion, scanning electron microscopy (SEM), and transmission
electron microscopy (TEM). The magnetic hysteresis loops
at 5 K and 295 K of the cobalt flowerlike architectures show
ferromagnetic characteristics with coercivities of 371 Oe and
197 Oe, respectively. Our work may shed light on the de-
signed fabrication of complex 3D architectures of other mate-
rials.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
tectures. A growth mechanism for these architectures is pro- Germany, 2008)
particle count, when dispersed by C-undecylcalixresorcinar-
Introduction
ene.[7] A lot of wet chemical processes for the fabrication of
hierarchical nanomaterials,[8–10]based on different driving
mechanisms including surface tension, capillary effects,
electric and magnetic forces, and hydrophobic interactions,
have been developed. Among all approaches, the hydrother-
mal process has been considered as one of the most promis-
ing routes because it is simple and inexpensive for large-
scale preparation. However, as these processes are compli-
cated and the structures of the products are usually unpre-
dictable, the controlled synthesis of hierarchical structured
nanomaterials is still an intensive challenge.
It is well known that assemblies of metallic and inorganic
nanobuilding blocks yield collective physical properties, de-
pending on their size, spacing, and high-order structure.[1,2]
Therefore, considerable attention has been paid to synthe-
size and investigate nanomaterials with hierarchical archi-
tectures, because of their novel properties and potential ap-
plication in optics, electronics, magnetism, and biology.[3]
Because their properties have a close relationship with their
morphologies, a wide variety of hierarchical architectures,
which are composed of low-dimensional nanostructured
building blocks, including 2D nanoplatelets, nanosheets
and nanodisks,[4] 1D nanobelts, nanowires, nanorods and
tubes, and 0D nanoparticles,[5] have been extensively investi-
gated. For example, two kinds of morphologies of 3D flow-
erlike β-NiS architectures, nanorods or nanoneedles, were
synthesized, and their morphologies can be modified conve-
niently by varying the reaction temperature;[6] 3D flowerlike
bismuth tungstate structures, assembled by nanosheets
composed of numerous square nanoplatelets, were obtained
by varying the amount of surfactant of poly(vinylpyr-
rolidone) (PVP);[4c] weak ferromagnetic cobalt nanopar-
ticles self-assembled into braceletlike rings with discrete
Cobalt nanomaterials have received increasing attention
due to their important applications in the fields of high-
density data storage, medical diagnosis, and cataly-
sis.[10a,11,12] Cobalt nanocrystals (below their critical size)
are superparamagnetic at room temperature due to their
small sizes, and thus they cannot be used in many fields. It
is known that the magnetic anisotropy can be increased by
increasing the shape anisotropy of magnetic materials. As
a result, great efforts have been focused on the controlled
synthesis of cobalt nanostructures with a high aspect ratio
and their multidimensional assemblies. 1D cobalt
nanowires/nanobelts/nanorods,[13–18] 2D cobalt nanoplate-
lets/nanodisks and superlattices have been prepared for en-
hancing the magnetic anisotropy.[19,20] For instance, cobalt
nanodisks and their assembly of ribbons were obtained by
rapid decomposition of carbonylcobalt in the presence of
trioctylphosphane and oleic acid.[19] Cobalt nanobelts and
nanowires were obtained by a surfactant- and complex-as-
[a] Shenyang National Laboratory for Materials Science, Institute
of Metal Research, and International Centre for Materials
Physics, Chinese Academy of Sciences,
Shenyang 110016, PR China
E-mail: yjzhang@imr.ac.cn
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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FULL PAPER
sisted hydrothermal reduction process, respectively.[14,15] In hexagonal nanosheets with an average thickness of 100 nm.
addition, some cobalt hierarchical architectures were fabri- These results indicate that the flowerlike cobalt hierarchical
cated to this end. Cobalt chainlike structures composed of structures can be synthesized by the present simple route of
cobalt nano/microspheres or hollow spheres are further ex- surfactant-assisted hydrothermal reduction. Interestingly,
amples.[21–23] However, few efforts have been focused on the flowerlike structure can still maintain its morphology
fabricating cobalt 3D hierarchical structures. By a hydro- even after sonication for 30 min, indicating that the struc-
thermal reduction approach, 3D cobalt microspheres com- ture is stable.
posed of nanoplatelets were synthesized with surfactant so-
dium dodecyl benzenesulfonate (SDBS) and capping rea-
gent sodium tartrate.[24] It is highly desirable to synthesize
3D cobalt hierarchical architectures.
Besides the technological application in high-density
storage and catalysis, the synthetic method for obtaining
cobalt 3D hierarchical structures is very interesting. Un-
doubtedly, simplifying the synthetic method as well as
simultaneously controlling the structures will contribute to
improving their magnetic properties, understanding the fab-
rication mechanism, and extending the potential applica-
tion of hierarchical magnetic nanomaterials. In this study,
3D flowerlike hierarchical architectures consisting of cobalt
hexagonal nanosheets were fabricated with SDBS by a sim-
ple hydrothermal reduction method. The effects of the reac-
tion parameters, such as the reaction temperature and the
amount of NaOH in the starting solution, on the morpho-
logies of the products were investigated. On the basis of the
evolution of the structure and the morphology with increas-
Figure 1. XRD patterns of the products collected at different stages
of the reaction at 180 °C: after (a) 6 h, (b) 16 h, and (c) 24 h.
ing reaction time, a growth mechanism of the 3D flowerlike
cobalt architectures is proposed. Moreover, the magnetic
measurements show that the flowers have ferromagnetic
characteristics with a coercivity of 197 Oe at 295 K, which
is greatly enhanced relative to their bulk counterpart.
Results and Discussion
Controlled Synthesis and Characterization of Cobalt
Flowerlike Architectures
The CoCl2 and NaOH solution with SDBS was prepared
in
a beaker. After the addition of reducing agent
NaH2PO2·H2O, the solution was stirred and autoclaved for
hydrothermal treatment. Typical XRD patterns of the prod-
ucts obtained after the hydrothermal treatment at 180 °C
for 6–24 h are shown in Figure 1. The XRD pattern of the
product obtained after the hydrothermal treatment at
180 °C for 24 h exhibits a predominant crystalline Co hex-
agonal phase, in consistent with the standard card JCPDS
No.5–727 (space group P63/mmc; a = 2.503 Å; c = 4.060 Å).
Figure 2. EDS pattern of the as-prepared cobalt flowerlike archi-
tectures at 180 °C after reaction for 24 h.
It was found that the morphologies of the products are
The energy-dispersive spectrum (EDS) shown in Figure 2 greatly affected by the reaction temperature. When the reac-
confirms that the product is pure cobalt (the element Au tion is performed at higher temperatures (ranging from
originates from the thin Au layer sputtering on the sample 140 °C to 180 °C), the products are flowerlike hierarchical
in the test). The morphology and size of the samples were architectures, which are completely composed of hexagonal
examined by scanning electron microscopy (SEM). Fig- nanosheets with a thickness of 100 nm, similar to those ob-
ure 3a shows the general morphologies of the product tained at 180 °C (Figure S1). However, it is interesting to
(180 °C, 24 h), revealing that it consists of relatively uni- note that when the reaction temperature is decreased to
form flowerlike architectures with diameters of 8–10 µm. 120 °C, three different kinds of morphologies coexist in the
Under the conditions reported, the products are all in this product, as shown by the arrows in Figure 4a. One is the
morphology. High-magnification SEM images in Figure 3b typical flowerlike architecture (similar to that shown in Fig-
shows that each flowerlike structure is composed of many ure 3), the second one is a spherical architecture, which is
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Eur. J. Inorg. Chem. 2008, 2733–2738

Cobalt Flowerlike Architectures by a Facile Hydrothermal Route
Figure 3. (a) Low-magnification and (b) high-magnification SEM
images of the cobalt flowerlike architectures obtained at 180 °C
after reaction for 24 h.
densely stacked by small hexagonal nanosheets (Figure 4b),
another is monodisperse hexagonal sheets, with a diameter
of 10 µm and a thickness of about 200 nm. When the reac-
tion temperature is decreased further to 100 °C, as shown
in Figure 4c, the product is composed of irregular thin
sheets in macroscale to nanoscale sizes, with widths of sev-
eral hundred nanometers to a few micrometers. It is worth
noting that most edges of the cobalt sheets have a tendency
of curling, which may be due to their thin nature. The corre-
sponding transmission electron microscopy (TEM) (Fig-
ure 4d) images and selected area electron diffraction
(SAED) pattern (the inset in Figure 4d), demonstrate the
sheet morphology of the products and indicate that the
sheets are single crystals with a (0001) lattice plane as the
basal plane. Due to the thin nature of and the magnetic
interaction between the cobalt sheets, they tend to over-
lap.[20] In general, the morphology of the product depends
strongly on the reaction temperature: lower temperature
tends to generate cobalt sheets, while high temperature fa-
cilitates flowerlike architectures. This result is in agreement
with other reports on the formation of CuO “dan-
delions”.[25] The results above suggest that the morphology
of the product can be easily modified by changing the reac-
tion temperature.
It was found that the surfactant SDBS is crucial for the
growth of cobalt flowerlike architectures. In the absence of
SDBS, the products are dominantly aggregations of spheri-
cal particles of various sizes (Figure S2). In order to investi-
gate the influence of the surfactant, other surfactants have
been examined. SDBS was substituted by sodium dodecyl
sulfate (SDS) and cetyltrimethylammonium bromide
(CTAB). No flowerlike architectures were observed in ex-
periments with these surfactants. Furthermore, even upon
varying the amount of the surfactants SDS or CTAB, the
products were still spherical aggregations with irregular
shapes (Figure S3). It is believed that the surfactant can
selectively adsorb on a certain crystal facet of the as-pre-
pared nanoparticles as well as prevent random aggregation
of the nanoparticles, resulting in the assembled struc-
tures.[26,27] During the formation of the flowerlike architec-
tures, SDBS possibly plays a role in both aspects. It is rea-
sonable that the anionic surfactant has a stronger interac-
tion with the cobalt ion relative to the cationic surfactant.
The results reveal that not only the type but also the molec-
Figure 4. (a) Low-magnification and (b) high-magnification SEM
images of the products prepared at 120 °C after reaction for 24 h,
(c) SEM image and (d) TEM image of the products at 100 °C after
reaction for 24 h (the inset in Figure 4d is the SAED of the cobalt
nanosheets).
of the flowerlike architectures. In contrast to SDS, the sur-
factant SDBS favors the formation of flowerlike architec-
tures, probably because of different interactions between the
surfactant molecule and the cobalt ion.
To investigate the role of NaOH in the reaction, con-
trolled experiments were carried out by keeping other pa-
rameters constant. When the concentration of NaOH was
increased to 2–5 , only irregular mesoscale sheets were ob-
served. As shown in Figure 5, the size of the sheets increase
with the concentration of NaOH. Xu et al. reported a sim-
ilar result in the synthesis of metal (cobalt, nickel, copper)
nanosheets in the presence of SDS.[20] It is understandable
that the reaction rate is accelerated by increasing the NaOH
concentration and the higher reaction rate favors the forma-
tion of larger sheet structure. This indicates that the NaOH
concentration is a critical parameter for controlling the
growth of flowerlike hierarchical architectures.
Figure 5. SEM images of the products prepared with different con-
centration of NaOH: (a) 2 , (b) 3 , (c) 5 .
Growth Mechanism for the Formation of Cobalt Flowerlike
Architectures
To understand the growth mechanism of the flowerlike
ular structure of surfactant is important for the formation cobalt hierarchical architectures, systematical time-depend-
Eur. J. Inorg. Chem. 2008, 2733–2738
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Y.-j. Zhang, Y. Zhang, Z.-h. Wang, D. Li, T.-y. Cui, W. Liu, Z.-d. Zhang
FULL PAPER
ent experiments illustrating the evolution of the structure et al.,[24] the cobalt complex is first formed with sodium
and the morphology were carried out by XRD and SEM at tartrate, it is then reduced, and cobalt nanoplates directly
various growth stages (6 h, 16 h, 24 h) of the hydrothermal form, possibly because SDBS kinetically controls the
process. When the reaction proceeds for 6 h, the XRD growth rates of the different faces of a cobalt nanocrystal,
pattern (Figure 1a) shows that the product is a mixture of and finally spherical structures are generated, the process
metallic Co and Co(OH)2 (JCPDS NO. 30–443). The prod- being driven by the interfacial tension and the hydrophilic
uct exhibits an irregular morphology composed of a lot of surface of the cobalt plates. However, in our experiments
tiny, close-stacked hexagonal sheets (Figure 6a). When the without the capping agent, the formation mechanism of co-
reaction time extends to 16 h, most of the product is met- balt sheets is absolutely different. Here, at the beginning of
allic Co (Figure 1b) with flowerlike morphology (Fig- the reaction, Co(OH)2 is formed, which is then reduced to
ure 6b). When the reaction time is prolonged to 24 h, the Co sheets, and the intrinsic lamellar structure of Co(OH)2
reaction is complete. The product is metallic cobalt with plays a crucial role in forming Co nanosheets. The pro-
flowerlike architectures consisting of many perfect hexago- cedure for assembling cobalt nanosheets into flowers is also
nal sheets (Figure 3). The reactions taking place during the similar to that for the generation of cobalt spheres.[24] The
hydrothermal process can be expressed as follows [Equation average size of the flowers is larger than that of the spheres,
(1), Equation (2), and Equation (3)]:
which may be ascribed to the higher reaction temperature
adopted here. In summary, the generation of flowerlike
architectures occurs in two main steps: the first step [from
Co(OH)2 sheets to Co sheets] is the morphology mainte-
nance process, and the second step is a typical Ostwald rip-
ening process (from cobalt sheets to cobalt flowers).
Co2+ + 2DBS– Ǟ Co(DBS)2
(1)
(2)
(3)
Co(DBS)2 + 2OH– Ǟ Co(OH)2 + 2DBS–
–
2–
Co(OH)2 + H2PO2 + OH– ǞCo + HPO3 + 2H2O
Figure 6. SEM images of the products recorded at different growth
stages: (a) 6 h, (b) 16 h.
Figure 7. Schematic illustration of a possible formation procedure
for the cobalt flowerlike architectures (HT: hydrothermal treat-
ment.).
On the basis of the results above, a growth mechanism is
proposed, as illustrated in Figure 7. Three consecutive
stages are included as follows: (1) Numerous cobalt hydrox-
ide colloids are formed in the basic solution of CoCl2 (Fig-
ure 7, step a). (2) The cobalt hydroxide crystals appear un-
der hydrothermal treatment. As a result of its intrinsic lam-
ellar structure, cobalt hydroxide is prone to forming plate-
lets.[28] Then, the Co(OH)2 is reduced, and thus metallic
cobalt nuclei that sustain the hexagonal sheet morphology
of Co(OH)2 are obtained (Figure 7, step b). (3) As the reac-
tion proceeds further, these Co hexagonal sheets diffuse and
aggregate together as a result of the strong magnetic inter-
action and the driving force of entropy.[27,29] Meanwhile,
the small aggregations may serve as seeds and grow by the
Ostwald ripening process. The flowerlike architectures are
thus generated (Figure 7, step c).[30] It should be noted that
the present mechanism for the generation of the flowerlike
structures is different from that reported in our earlier
work[23] and in another report,[24] in which cobalt hierarchi-
cal microspheres were reported to form by a typical
Table 1. A comparison of experimental parameters and results be-
tween the product in ref.[24] (sample 1) and the present product
(sample 2).
Sample Reaction system
Temperature Morphology
Size
1
CoCl2 + Na2-
C4H4O6
110 °C
cobalt
2.8 µm
NaH2PO2
SDBS
spheres
NaOH + H2O
2
CoCl2
NaH2PO2
SDBS
180 °C
cobalt
flowers
8–10 µm
NaOH + H2O
Magnetic Properties
The magnetic hysteresis loops of the cobalt flowerlike
Ostwald ripening process. A comparison of experimental architectures obtained after hydrothermal treatment at
parameters and results between the present study and that 180 °C for 24 h were measured at 5 K and 295 K. The hys-
of Hou et al.[24] is presented in Table 1. In the work of Hou teresis loops were recorded at each temperature after a mag-
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Cobalt Flowerlike Architectures by a Facile Hydrothermal Route
netic field of 20 kOe was applied. As shown in Figure 8,
the coercivities at 5 K and 295 K are 371 Oe and 197 Oe,
Experimental Section
All reagents used were commercial products of analytical pure
grade, and they were used without further purification. In a typical
procedure, CoCl2·6H2O (0.238 g, 1 mmol), dodecyl benzenesulfo-
nate (SDBS) (0.116 g, 0.5 mmol), and NaOH (1.6 g, 40 mmol) were
first dissolved in distilled water (40 mL), and the solution was con-
stantly stirred for 15 min at room temperature. After addition of
NaH2PO2·H2O (1.7 g, 16 mmol), the solution was constantly
stirred for another 15 min. Then the solution was transferred into
a 50 mL Teflon-lined stainless steel autoclave. The autoclave was
sealed and maintained at 180 °C for 6–24 h and then cooled to
room temperature naturally. The product was filtered off, washed
with distilled water and ethanol for several times, finally dried in a
vacuum oven at 60 °C for 4 h. Control experiments were carried
out by adjusting the reaction temperature (100–180 °C) and the
amount of NaOH (1–5 ), while keeping other reaction parameters
unchanged.
respectively. Relative to bulk cobalt (a few tens of Oe[16]
)
and cobalt hollow spheres (66 Oe, 300 K[21]), the coercivity
at room temperature is enhanced, which can probably be
attributed to the high shape anisotropy of the hexagonal
sheets on the flowerlike architectures. However, although
their morphologies are similar, the coercivity (371 Oe) of
the present product at 5 K is much lower than that of the
reported spheres (590 Oe, 5 K),[24] which might be because
of the larger size of the present product (Table 1) and the
higher reaction temperature. The saturation magnetization
at 295 K of the flowerlike structures is 150.4 emu/g, which
is slightly lower than that of its bulk counterpart (168 emu/
g).[14] The result is consistent with other previous reports,
which was ascribed to the reduction of the saturation mag-
netization because of the existence of impurities,[31] surface
antiferromagnetic oxidation,[31] and surface spin disor-
der.[32] In the present case, all of these factors may be re-
sponsible for the reduction of the saturation magnetization.
The phases of all products were identified with a Rigaku D/max
2500pc X-ray diffractometer (XRD) with Cu-Kα radiation (λ =
1.54156 Å) at a scan rate of 0.04 °s–1. The morphology was investi-
gated with a Shimadzu SSX-550 scanning electron microscope
equipped with an EDS system operated at an acceleration voltage
of 15.0 kV. Transmission electron microscopy (TEM) was carried
with a TECNAI 20 instrument having an emission voltage of
200 kV. Magnetic hysteresis loops were measured by using a
MPMS-7 superconducting quantum interference device (SQUID)
magnetometer at fields up to 20 kOe.
Supporting Information (see footnote on the first page of this arti-
cle): SEM images of the products at 140 °C and 160 °C; SEM im-
age of the product at 180 °C in the absence of SDBS; SEM images
of the products at 180 °C with SDS and CTAB.
Acknowledgments
This work has been supported by the National Natural Science
Foundation of China under Grant No. 50331030.
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Mesoscale formation of cobalt flowerlike hierarchical
architectures self-assembled by hexagonal nanosheets has
been fabricated in a controlled manner by a hydrothermal
process in the presence of SDBS. It has been found that the
surfactant SDBS, the reaction temperature, and the concen-
tration of NaOH are key parameters for the fabrication of
the complex 3D structures. The growth mechanism for the
formation of the flowerlike architectures has been eluci-
dated: the intermediate product Co(OH)2 plays a critical
role in the generation of sheet-shaped cobalt, and then the
cobalt nanoplates assemble into flowers. The products exhi-
bit ferromagnetic behavior. Such a simple and mild syn-
thetic approach can be extended to the controlled synthesis
of 3D hierarchical architectures of other materials.
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Received: March 27, 2008
Published Online: May 6, 2008
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