Hierarchical
Co
Nanoflowers:
Fabrication,
Magnetic Property and Catalytic Activity
Samarpita Senapati1, Suneel Kumar Srivastava1*, Shiv Brat Singh2
1
Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India
Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur-721302,
India
2
Keywords: Hierarchical, nanoflowers, magnetic property, catalytic activity, capping agents.
Anisotropic magnetic nanomaterials including magnetic metals, multilayer structure of various sizes,
crystal structures and morphology have attracted intensive research due to their interesting physical properties
and potential applications in the field of catalysis, high-density data storage, magnetic sensors etc.1 Of the three
ferromagnetic transition metals, more attention is focused on the size dependent growth of cobalt nanometal
crystallizing in hexagonal close packing (hcp), face centered cubic (fcc) and -phase structures and exhibiting
variable morphologies, which could alter its properties significantly. For example, the anisotropic hcp phase
with a high magnetic coercivity is used as permanent magnet, whereas, more symmetric and low coercivity fcc
phase is suitable for soft magnetic applications. On the other hand, the non zero-dimensional Co nanocrystals
possessing special shapes often show enhanced coercivity compared with the zero-dimensional spherical Co
nanoparticles, and this enhanced value is preferable for application in high density recording media as well as in
permanent magnet.2 In addition, hierarchical Co nanostructures are receiving considerable attention in catalysis
due to their dimensions and high surface areas.
A simple one pot synthesis method for the silver catalyzed growth of pure hexagonal close packed
cobalt by the reduction of cobalt salt using hydrazine hydrate in the presence of triethanolamine (TEA),
diethanolamine (DEA) and ethylene glycol (EG) as capping agents at 90 °C within 10 min has been reported.
Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the product
prepared in the presence of capping agents show the formation of the well defined porous flowery architecture
originating from the interlinked 2D wavy nanoflakes. When the same reaction is performed in the absence of
any capping agent, the agglomeration of the flakes of cobalt with irregular spherical morphology is observed.
The effect of reaction conditions on the size and morphology of the products and possible growth mechanism
have also been investigated. Based on the SEM images of the product at different reaction times, it is anticipated
that at the earlier stage of the reaction, hydrazine hydrate reduces cobalt hydroxide to form cobalt nuclei, where
the flake like structure of the former is retained and it coexists with cobalt. These cobalt nanoflakes, driven by
the interfacial tension and the hydrophilic cobalt surface, tend to aggregate and finally form the flowerlike
architecture of Co.2Vibration sample magnetometer (VSM) shows, that the products are ferromagnetic in nature
irrespective of the capping agents used and possess high value of coercivity, when prepared in the absence of
any capping agent. This enhanced coercivity of Co prepared in absence of any capping agent may be due to the
presence of Co nanoflakes.3 These cobalt microflowers have also been proved as an alternative to the other
available expensive catalysts (Au, Ag, Pt) in the room temperature production of p-aminophenol for its
applications in pharmaceutical, photographic and plastic industries.
Acknowledgment. Financial assistance from CSIR, New Delhi is acknowledged.
References
[1] Zhang, Y.-J.; Yao, Q.; Zhang, Y.; Cui, T.-Y.; Li, D.; Liu, W.; Lawrence W.; Zhang, Z.-D. Solvothermal
Synthesis of Magnetic Chains Self-Assembled by Flowerlike Cobalt Submicrospheres. Cryst. Growth Des. 8,
2008, 3206-3212.
[2] Senapati, S.; Srivastava, S. K.; Singh, S. B. Synthesis, Magnetic Properties and Catalytic Activity of
Hierarchical Cobalt Microflowers J. Nanosci. Nanotechnol. 12, 2012, 3048–3058.
[3] Zhu, Y.; Yang, Q.; Zheng, H.; Yu W.; Qian, Y. Flower-like cobalt nanocrystals by a complex precursor
reaction route. Mater. Chem. Phys. 91, 2005, 293-297.