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Conceived and designed the experiments: CW XJH. Performed the experiments: CW. Analyzed the data: CW. Contributed reagents/materials/analysis tools: CW XJH. Wrote the paper: SRH WH LFT.
Chain-like Co flower is synthesized by simply modulating the reaction conditions via a facile liquid-phase reduction method. The morphology evolution process and transformation mechanism from particle to flower and finally to chain-like flower have been systematically investigated.  is the preferred growth orientation due to the existence of easy magnetic axis. The microwave loss mechanism can be attributed to the synergistic effect of magnetic loss and dielectric loss, while magnetic loss is the main loss mechanism. In addition, the special microstructure of chain-like Co flower may further enhance microwave attenuation. The architectural design of functional material morphology is critical for improving its property toward future application.
The microwave absorbing material has received extensive attention due to their prospective application in electromagnetic shielding coatings, microwave darkrooms, and self-concealing weapons in industry, commerce, and military affairs –. The conventional electromagnetic loss mechanisms are magnetic loss and dielectric loss, so most of microwave absorbers are composed of magnetic and dielectric materials. However, as a kind of representative magnetic material, only a few literatures have discussed the microwave absorption property of Co due to its poor microwave absorption ability. Kato et al. has validated cobalt particles could only exhibit strong microwave absorption during very narrow frequency band , while Cao et al. found hollow chain-like cobalt displayed absorption ability no stronger than −11 dB . Therefore, it is valuable to deeply explore the microwave absorption mechanism of Co and find the route to improve its absorption.
In our previous research , we reported the synthesis of differently shaped Ni and found the microwave absorbing ability could be apparently influenced by material microstructure. In addition, controllable preparation of hierarchical building components with specific structures and novel properties is now attracting much attention of scientists not just for its role in deeply comprehending the self-assembly mechanism but also for its prospective applications as functional materials –. As a sequence, here we exhibit the facile synthesis of chain-like Co flower via modulating experiment conditions and investigate the relationship between morphology and electromagnetic property.
CoCl2·6H2O, PVP K30, N2H4·H2O, and ethylene glycol (EG) were bought from Guangfu Chemical Co. Ltd. (Tianjin, China) and NaOH was bought from Dalu Chemical Co. Ltd. (Tianjin, China), which were analytical grade and used as received.
Differently shaped Co was synthesized via modulating the content of CoCl2·6H2O and NaOH. In a typical experiment, 0.01–0.02 g CoCl2·6H2O and 0.5 g PVP were added into 22.5 mL EG at 85°C in a three-necked flask. After vigorous stirring for 10 min, 0.2–0.8 g NaOH was added and 10 min later, 8 mL N2H4·H2O was injected, then the reaction was maintained until the solution became clear and some black particles emerged to make sure the reaction was completed. The product was collected by centrifugation at 3000 rpm for 10 min and washed with deionized water six times and with ethanol three times to eliminate residual PVP. Finally the product was dried in a vacuum oven at 60°C for 12 h.
The morphologies of the samples were characterized by scanning electron microscopy (SEM, FEI SIRION). The crystallite structure were determined using an XRD-6000 X-ray diffractometer (Shimadzu) with a Cu Kα radiation source (λ=1.5405 Å, 40.0 kV, 30.0 mA). The magnetic properties were measured by a vibrating sample magnetometer (VSM, Lake Shore 7307). A HP-5783E vector network analyzer was applied to characterize relative complex permeability and permittivity during 2–18 GHz for the calculation of reflection loss (microwave absorption). A sample containing 60 wt% of Co was pressed into a ring with an outer diameter of 7 mm, an inner diameter of 3 mm, and a thickness of 2 mm for electromagnetic measurement, in which wax was used as binder.
Typical SEM images of obtained product with different magnifications are presented in Fig. 1. 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 Figs. 1A and 1B. If 0.4 g NaOH is added, stings’ number and length increase, resulting in the formation of Co flower rudiment as seen by Figs. 1C and 1D. 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 Figs. 1E and 1F. Surprisingly and interestingly, Figs. 1D and 1F 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 Figs. 1G and 1H.
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.
Fig. 2 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 and NH3 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.
As shown in Fig. 3, 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 .
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 Fig. 4 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 .
Microwave absorption ability can be preliminarily judged by dielectric loss factor (tanδe=ε′′/ε′) and magnetic loss factor (tanδm=μ′′/μ′), which are shown in Fig. 5. The larger tanδm than tanδe 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.
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 –:
where Z0 is the impedance of free space, and Zin is the input characteristic impedance, which can be expressed as:
where c 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 Fig. 6.
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 Fig. 6 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/4 . 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 Fig. 5 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 and tanδm 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.
This work was funded by the Natural Science Foundation of China no. 51202228 (www.nsfc.gov.cn). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.