Typical SEM images of obtained product with different magnifications are presented in . When only 0.2 g NaOH is added, spheric Co particles with a few short stings growing on the surface are obtained as shown in . If 0.4 g NaOH is added, stings’ number and length increase, resulting in the formation of Co flower rudiment as seen by . In addition, some stings present a symmetrical structure as hexagon with adjacent ones displaying an angle of 60°. Once the NaOH is increased to 0.8 g, well-crystallized Co flower with the maximum and longest stings is obtained as shown in . Surprisingly and interestingly, demonstrate that the bottoms are flat regardless the morphology of flower. If CoCl2·H2O is increased from 0.01 g to 0.02 g and keep NaOH still at 0.8 g, chain-like Co flower is obtained as exhibited in .
Figure 1 SEM images of the Co prepared at varying experiment conditions: (A, B: Co particles) 0.2 g, (C, D: rudiment of Co flower) 0.4 g, (E, F: Co flower) 0.8 g NaOH is added at the CoCl2·H2O content of 0.01 g; (G, H: chain-like cobalt flower) 0.8 g NaOH (more ...)
PVP is a conventional surfactant that can be used to activate certain crystal plane, which may lead to newly reduced Co atoms adsorbing on the plane of existed Co particles and finally the stings are formed. In addition, N2H4·H2O only exhibits strong reduction ability under alkaline condition, that’s why the growth of stings is slow under low NaOH content. While increasing NaOH addition may raise the number of reduced Co atoms and promote the rise of stings’ number and length and finally induce the Co flower rudiment growth. Once NaOH reaches 0.8 g, newly reduced Co atoms will be much more and enough to accelerate stings’ growth and the well organized Co flower is shaped.
Under the preparation conditions of well grown Co flower, chain-like Co flower can be obtained without any external assistant magnetic field if CoCl2·6H2O addition is increased from 0.01 g to 0.02 g. This may be due to each Co flower generates its own magnetostatic field and the increase of CoCl2·6H2O will result in large number of Co flowers, which enhance the magnetostatic field in the reaction system. Hence, these flowers are arranged linearly along magnetic force line to form chain-like Co flower. The following discussions are based on the measure data of these chain-like Co flower.
shows an XRD pattern of obtained chain-like Co flower (upper) and the pattern from data bank entry (lower). The characteristic peaks arise at 2θ
41.61°, 44.42°, 47.29°, 75.89° can be indexed to (100), (002), (101), (110) planes, respectively, of hexagonal-phase cobalt (JCPDS: 05-0727). No characteristic peaks arising from impurities of cobalt oxides or hydroxides are detected, which may be due to the floating reduction product N2
can exclude air to inhibit oxidation of newly reduced Co. It is found that the peaks relative intensities corresponding to (002)/(100) and (002)/(101) planes are significantly higher than that of standard values, indicating the preferred growth orientation is along  direction, which is very likely to the reported dendritic cobalt 
. In fact,  direction is the easy magnetic axis of Co 
, while magnetostatic energy will be the lowest and reaction system can maintain steady if Co atoms are stacked along easy magnetic axis. Therefore, the growth rate of  direction is faster than others.
XRD pattern of chain-like Co flower (upper) and standard diffraction pattern of Co (JCPDS Card No. 05-0727) (lower).
As shown in , the saturation magnetization (Ms
), remanent magnetization (Mr
), coercivity (Hc
) of chain-like Co flower are 181.38 emu/g, 2.56 emu/g, 33.74 Oe, respectively. It should be noted that the Ms
of chain-like Co flower is a little higher than that of bulk Co 
, which may arise from its special chain-like microstructure or tiny experimental error. In addition, the high Ms
can induce high initial permeability which is beneficial for microwave absorption 
Magnetic hysteresis loop at room temperature for chain-like Co flower.
The real parts of relative complex permittivity and permeability symbolize storage ability to electromagnetic energy, while the imaginary parts represent loss ability. The low permittivity imaginary part (ε
′′) as shown in demonstrates dielectric loss is weak. It is worthy to be noted that ε
′′ is a little below zero under some bands while there is not a convincing interpretation for the negative value, Deng et al. explain the phenomenon as the magnetic energy being radiated out 
, Chiu et al. point out it is meaningless and might be due to noise 
, while Chen et al. point out it does not arise from test error and needs further study 
. The real part of relative complex permeability (μ
′) decreases as the increase of frequency displays slight frequency-scattering effect. The imaginary part of permeability (μ
′′) weak peak around 6 GHz exhibits the existence of natural resonance just as reported dendritic Co 
The relative complex permittivity (ε′ and ε′′) and permeability (μ′ and μ′′) of chain-like Co flower.
Microwave absorption ability can be preliminarily judged by dielectric loss factor (tanδe
) and magnetic loss factor (tanδm
), which are shown in . The larger tanδm
demonstrates that main loss mechanism is magnetic loss rather than dielectric loss. There are three dielectric loss factor peaks around 4, 10, and 14.5 GHz which arise from relative high ε′′
and low ε′
at above frequencies. The tanδm
rapidly increases in 2–6 GHz and undulately increases during 6–16.4 GHz, then dramatically decreases. Due to magnetic loss is main loss mechanism and tanδm
is large around 16.4 GHz, the microwave absorption peak may emerge at 16.4 GHz.
The dielectric loss factor (tanδe) and magnetic loss factor (tanδm) of chain-like Co flower.
According to the measured data of permittivity and permeability, reflection loss (RL) usually can be calculated by following equation according to transmission line theory and assuming the absorber is attached on high reflective metal plates 
is the impedance of free space, and Zin
is the input characteristic impedance, which can be expressed as:
is the velocity of light and d
is the thickness of an absorber. The reflection loss (microwave absorption) ability of chain-like Co flower is shown in .
Absorbing ability of chain-like Co flower at 2 mm thickness.
Conventional measure frequency band of microwave absorbing materials is 2–18 GHz. It is reported that Co nanochains can reach reflection loss peak about −11 dB at 17.2 GHz at the thickness of 2.5 mm, and Co nanoparticles can only reach the maximum reflection loss −5 dB in 2–5 GHz at the thickness of 3.16 mm 
. While chain-like Co flower at a thinner thickness of 2 mm as shown in exhibits stronger reflection loss (−11.5 dB at 14.8 GHz) than the above reported Co materials, which may be illustrated from its special microstructure. There are many sharp petals that can be tuned with the incident microwave for the point discharge effect just as the action mechanism of lightning rod to lightning, and electromagnetic energy will be induced into dissipative current. Then, the chain-like structure is beneficial for transmission of dissipative current and finally leads to energy attenuation by transforming to heat.
It should be noted that chain-like Co flower reaches the loss peak at 14.8 GHz rather than the maximum tanδm
frequency of 16.4 GHz. Loss peak frequency is determined by loss mechanism or absorber thickness that can be characterized as times of λm
. Although Co is a kind of magnetic loss microwave absorbing material, free electronic will be polarized due to its electronic conductive essentiality and dielectric loss can be induced under external alternated electromagnetic field. Therefore, dielectric loss is also one of the loss mechanisms even though it is not strong. The tanδe
at 16.4 GHz in demonstrates dielectric loss is weak, so microwave absorption can not be the strongest even though tanδm
is the largest at this frequency. However, there is a tanδe
peak around 14.8 GHz and tanδm
is also very large at that frequency, so chain-like Co flower reaches the strongest microwave absorption. After 14.8 GHz, the variation tendencies of tanδe
are just opposite and result in microwave absorption becoming weaker and weaker in total. The above analysis adequately demonstrates the loss nature is coming from synergistic effect of magnetic loss and dielectric loss, while magnetic loss is the main loss mechanism.
In summary, we successfully realize the evolution progress from Co particle to chain-like Co flower via modulating reaction conditions. The increasing number of newly reduced Co atoms can accelerate petals’ growth, and the flowers will be arranged linearly under magnetostatic force to form chain-like structure. Both magnetic loss and dielectric loss are beneficial for microwave absorption though the former is the main loss mechanism. In addition, the special sharp petal microstructure can be tuned with incident microwave and induces electromagnetic energy into dissipative current, while chain-like microstructure is beneficial for dissipative current transmission and finally leads to energy attenuation, which enhances the microwave absorption. Our research demonstrates that the architectural microstructure design of functional material may be a promising route to improve property towards to future application.