Facile Synthesis and Hierarchical Assembly of Flowerlike NiO

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Facile synthesis and hierarchical assembly of flower-like NiO structures with enhanced dielectric and microwave absorption properties Peijiang Liu, Vincent Ming Hong Ng, Zhengjun Yao, Jintang Zhou, Yiming Lei, Zhihong Yang, Hualiang Lv, and Ling Bing Kong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 01 May 2017 Downloaded from http://pubs.acs.org on May 2, 2017

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ACS Applied Materials & Interfaces

Facile synthesis and hierarchical assembly of flower-like NiO structures with enhanced dielectric and microwave absorption properties Peijiang Liua, b, c, Vincent Ming Hong Ngc, Zhengjun Yaoa, b, *, Jintang Zhoua, b, *, Yiming Leia, b, Zhihong Yanga, Hualiang Lva, Ling Bing Kongc, * a

College of Materials and Technology, Nanjing University of Aeronautics and

Astronautics, Nanjing 211100, People’s Republic of China b

Jiangsu Laboratory of Advanced Materials and Application Technology, Jiangsu

211100, People’s Republic of China c

School of Materials Science and Engineering, Nanyang Technological University, 50

Nanyang Avenue, Singapore 639798 *Correspondence to: Zhengjun Yao (E-mail: [email protected]) Jintang Zhou (E-mail: [email protected]) Ling Bing Kong (Email: [email protected])

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Abstract In this work, two novel flower-like NiO hierarchical structures, rose-flower (S1) and silk-flower (S2), were synthesized by using a facial hydrothermal method, coupled with subsequent post-annealing process. Structures, morphologies, magnetic and electromagnetic properties of two NiO structures have been systematically investigated. SEM and TEM results suggested that S1 had a hierarchical rose-flower architecture with diameters in the range of 4-7 µm, while S2 exhibited a porous silk-flower architecture with diameters of 0.7-1.0 µm. Electromagnetic performances indicated that the NiO hierarchical structures played a crucial role in determining their dielectric behavior and impedance matching characteristic, which further influenced the microwave attenuation property of absorbers based on them. Due to its hierarchical and porous architectures, S2 had higher microwave absorption performances than S1. The maximum RL value for sample S2 can reach –65.1 dB at 13.9 GHz, while an efficient bandwidth of 3 GHz was obtained. In addition, the mechanism of the improved microwave absorption were discussed in detail. It is expected that our NiO hierarchical structures synthesized in this work could be used as a reference to design novel microwave absorption materials. Keywords: Nickel oxide, Hierarchical structure, Dielectric property, Interfacial polarization, Microwave absorption, 1. Introduction In recent years, radiated electromagnetic waves have become a severe environmental problem, not merely affecting the running of electronic equipment, but


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also influencing humans’ health and military weapons’ application.1-4 In this case, it is necessary to explore high performance microwave absorption materials working at frequencies of 2-18 GHz. To satisfy the requirements of actual applications, an ideal microwave absorption material should have several advantages, such as low density, strong absorbing capability and broad absorbing bandwidth, which is still a challenge 5

. Up to now, various absorbing materials, such as pure metal powders, ferrites,

carbon derivatives, nanosized particles, conductive polymers, frequency selective surfaces, metallic oxides and chiral materials, have been explored.6-11 Among these absorber materials, metallic oxides possess relative better complex permittivity and complex permeability properties that can be controlled more readily, which lead to microwave absorbers with improved impedance matching characteristic and thus broad absorbing bandwidth.12 For examples, Fe3O4 nanocrystals prepared by a normal hydrothermal way showed a minimum loss value of -21.2 dB at 8.16 GHz.13 CoO nanobelts had a good impedance matching characteristic and thus exhibited promising microwave absorption behavior, as well as a reflection loss being -12.3 dB with a thickness of 3 mm.14 As one of the most important antiferromagnetic metallic oxides, nickel oxide (NiO) with NaCl-type structure has been extensively studied as catalysts, battery electrodes, gas sensors, solar cells, semiconductors, and magnetic materials.15-17 Due to its high hole mobility, moderate dielectric loss, environmentally friendly, low cost, natural abundance, large surface area, low conductivity, and unique magnetic property, NiO is also a potential candidate for microwave absorption at gigahertz



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frequencies.18-20 Liu and his colleagues prepared microporous Ni/NiO nanocomposite by using a chemical dealloying method. The maximum reflection loss reached -49.1 dB, while the absorbing frequency bandwidth was as broad as 5.8 GHz.21 Yang et al. investigated electromagnetic absorption behavior of NiO nanorings on SiC.11 The nanocomposites not only displayed a reflection loss value being -46.9 dB at 673 K, but also exhibited a broader absorbing bandwidths covering the whole X-band at 673 and 773 K. However, it should be noticed that these NiO-based nanomaterials mentioned above usually suffer from complicated assembled process and have to be filled into the microwave absorbing materials with a high mass to reach a large reflection loss and a wide effective bandwidth. It simultaneously increases the production cost and lowers the overall mass efficiency of the absorber materials, which limit their practical applications. Therefore, until now, exploration of simple absorbers with effective process and light weight has been of scientific importance. Generally, physical and chemical characteristics for nanomaterials are closely related to the structures and morphologies. In this respect, various NiO nanomaterials with multitudinous structures and morphologies have been reported in the open literature, such as nanoplates, polyhedrons, nanorings and nanoparticles.22-24 Among these various structures currently available, 3D flower-like structures have attracted considerable attentions, because of their large specific surface area, massive interior contact sites and large number of grain boundaries,19,

25

which all contribute to

polarization that lead to enhanced microwave absorbing properties. In addition, achieving a high permeability at high frequency areas is difficult for most absorbers


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since the eddy current effect can induce magnetic field, which neutralize the exterior field, leading to poor absorption performance.26-28 However, recent studies showed that the eddy current can be effectively mitigated by using plate-like architectures, owing to the shift of resonance to the high frequency range.29 Therefore, it is expected that hierarchical flower structured NiO assembled from massive flake-like units should have promising microwave absorption performance. More importantly, the density of flower-like NiO is much lower than those of other types of solid structures. In this case, weight loading can be effectively decreased. As a result, development of strategies to construct and assemble 3D microsized and nanosized structures is of utmost importance.22-23, 30 To the best of our information, the effect of structure and morphology of 3D flower-like NiO on its dielectric and microwave absorption performance have not been reported, although its composites have been studied in many fields, such as gas sensor, battery and photocatalysts.31-33 In present work, aiming at elucidating the dielectric properties and clarifying the mechanism of the microwave absorption property of hierarchical flower-like structures, two hierarchical NiO flowers consisting of numerous nanosheets were experimentally synthesized by using a facial hydrothermal method with nickel salts as the starting materials, coupled with post-annealing. Structural and morphology of the two types of particles, with different nanosheet thicknesses, were characterized. Furthermore, the microwave absorption properties of the nanomaterials were also evaluated based on permeability and permittivity. Due to their large surface to volume ratio and hierarchical structures, the NiO nanoflowers exhibited strong absorbing



performance, which could be could be considered as a new generation of absorption materials. 2. Experimental section 2.1 Materials All Alcohols were provided by Sinopharm Chemical Reagent, China. Precursor nickel salts (NiCl2·6H2O, Ni(NO3)2·6H2O), sodium acetate (NaAc), urea and ammonium fluoride (NH4F) were commercially purchased from Aladdin Reagent, China. DI water used in the synthesis of NiO was produced in our lab. 2.2 Synthesis of rose-flower NiO To synthesis rose-flower NiO, 0.89 g Ni(NO3)2·6H2O, 0.22 g NH4F and 0.9 g urea were mixed with 25 mL DI water, with aid of ultrasonication for 0.5 h. After that, the green transparent solution was transferred to a Teflon-lined reactor. The hydrothermal reaction was conducted at 120 oC for 6 h. After cooling down to room temperature, the resulting green slurry was filtered and thoroughly washed with a mixture solution of alcohol and water for three times, followed by drying at 60 oC. Rose-flower NiO powders were received after the products were annealed at 350 oC for 120 min. 2.3 Synthesis of silk-flower NiO To synthesize silk-flower NiO, 1.08 g NiCl2·6H2O and 3.80 g NaAc were dispersed in 28 mL ethylene glycol at temperatures of 50-60 oC in water bath. The mixture was ultrasonicated for a few minutes. After that, a mixture of 40 mL of D.I. water and 0.6 g of polyethylene glycol was added until all solids were dissolved. The


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green transparent mixture was added into a Teflon-lined reactor and then kept at 190 o

C for 8 h. The dark-green products were obtained through filtration, washed for

several times to remove excess polyethylene glycol and ethylene glycol, and then dried in air at 60 oC. Finally, after annealed at 350 oC for 2 h in vacuum, black silk-flower NiO structures were thus obtained. For convenience, the two samples, rose-flower and silk-flower NiO, are denoted as S1 and S2, respectively. 2.4 Characterization Fourier transform infrared spectroscopy (FT-IR, Bruker Vertor 33) was recorded over 4000-4500 cm-1. Before test, all samples were pressed into wafers with purified KBr. Crystalline structures of the NiO nanoparticles were checked by using X-ray powder diffraction (XRD ) with a Bruker-D8 DISCOVER X-ray diffractometer (US, λ = 0.15406 nm). Static magnetic properties of NiO structures were measured by using a vibrating sample magnetometer (VSM, Lakeshore, Model 7400 series). Structures and sizes of the NiO structures were examined by using field emission scanning electron microscope (FE-SEM, Hitachi S-4800N), assisted with an energy dispersive X-ray spectroscopy (EDX) and transmission electron microscopy (TEM, JEM-2010, JEOL). Electromagnetic properties were measured by using an Agilent PNA N5224A vector network analyzer. Before test, composites were prepared by homogeneously adding the NiO powders into softening wax at a concentration of 30 wt%. The samples used for measurement were shaped into a toroid with 2.00 mm in thickness, 3.04 mm in inner diameter and 7.00 mm in outer diameter. 3. Result and discussion



Fig. 1 shows FT-IR spectra of the flower-like NiO samples and their precursors. As a shown in Fig. 1a, the rose-flower NiO precursor has a large band at about 3400 cm-1, which is ascribed to O–H stretching vibration from the absorbed water and free hydroxyl groups,34 whereas the bands located at 2186 cm-1 and 1506 cm-1 originate from the C=N vibration of OCN– anions and C=O vibration of the carbonate ions, respectively.35,36 These ions are the hydrolysis byproduct of urea at high temperature, demonstrating that the byproducts are present in the rose-flower NiO precursor. In addition, the intense peak at 1384 cm-1 is ascribed to the vibration of NO3– ions.37 Meanwhile, the band at 400 cm-1 can be corresponding to the metallic lattice vibration of Ni–O

38

. Compared with the precursor, the band of NO3– in the nitrate ions and

byproduct of the urea almost disappear in the annealed sample (S1), as shown in Fig. 1b, which indicates the high purity oxide has been obtained. As for silk-flower NiO precursor (Fig. 1c), two bands referred to absorbed water molecules and the metallic oxide are also observed in the spectra. The bands located at about 2852 cm-1 and 1087 cm-1 come from the CH2 stretching vibration and C–OH stretching vibration, respectively, which are both derived from the ethylene glycol unit.33 Two big bands at 1571 cm-1 and 1413 cm-1 are also detected, which are ascribed to C=O absorption in acetate 39. In addition, the bands at about 879 cm-1 and 667 cm-1 are due to the C–O stretching and bending vibrations, further proving the presence of acetate ions in the silk-flower NiO precursor.40 After being annealed at high temperature, the acetate groups disappeared in the composition of S2 (Fig. 1d), and S2 retain the peak at 400 cm-1, confirming the successful preparation of NiO.


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Based on the FT-IR results and experimental observations, possible formation mechanisms of the two samples have been proposed, as schematically illustrated in Figure 2. For S1, we suggest that the formation of the rose-like structures consisted of two major steps: the formation of the solid core, and subsequent interlocking and growth of particles on the surface of the core into two-dimensional nanosheets (as seen in Fig. 2a). At the initial stage of the reaction, Ni2+ ions coming from Ni(NO3)2·6H2O was high and then reacted with F– anions to form [NiFx](x–2)– anions. While heating, urea dissolved in aqueous solution and decomposed to generate OH– and CO32– ions. During this period of time, many Ni2(OH)2CO3 nuclei were formed due to the occupation of the empty orbitals of [NiFx](x–2)– anions by reaction pairs of both OH– and CO32– ions

32

. Subsequently, with increasing reaction time, the nuclei

interlocked and then grew larger to form two-dimensional nanosheets, while the nanosheets were developed into interlocked networks at high temperatures.41-42 To reduce the surface energy, the nanosheets were gradually assembled to rose-like 3D structures.42-43 In the hydrothermal reaction process, we believe that the existence of plenty of NH3 might have played a significant role

44

. Because excessive NH3 was

produced from the hydrolysis of NH4F and thermal decomposition of urea, the Ni2(OH)2CO3 nuclei could be coordinated with abundant NH3. Subsequently, the nuclei were continuously grown up, so as to develop interconnected networks during the hydrothermal reaction, resulting in the formation of the two dimensional flakes of S1. In the case of S2, a plausible growth mechanism has been proposed in Fig. 2b.



Previous work has confirm that there exists strong coordination bond between metallic ions and ethylene glycol. For example, Lou and Zhu found that ethylene glycol could be regarded as the ligand, which would combine with nickel ions to construct a wire-like coordination compound. When these wires grew enough long, the as-formed coordination compounds would self-assembled into spherical structures.45 In this work, the Ni2+ ions firstly diffused in ethylene glycol and then were attached onto ethylene glycol molecules to generate glycolates (Ni-EG) by the strong coordination interaction. During the procedure, NaAc served as stabilizer and dispersant to prevent particle agglomeration and promote the generation of homogeneous Ni-EG particles at relative low concentration.45-46 After that, the polymerization process of nickel glycolates occurred, which is an important part that has been discussed in many other nanostructured metallic oxides.47 According to the classical nucleation theory, the homogeneous nucleation would occur, when the degree of supersaturation reached a high level.33,

48

With the increase of the

supersaturation, the nucleation rate increased. Thus, large amounts of nickel glycolates nanosheets formed continuously via the reaction of Ni2+ and EG ion. Afterwards, in order to minimize the surface energy, these nanosheets spontaneously interconnected to form clusters to reduce the exposed areas. 33 After nickel glycolates subsequently grew into a big size, the hierarchical and porous silk-flower structures were formed. XRD patterns of the NiO structures are exhibited in Fig. 3. Five major diffraction peaks are observed, related to the (111), (200), (220), (311), and (222)


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crystallographic planes, in agreement with the cubic NiO pattern (JCPDS No. 71-1179). At the same time, no impurity peaks are present, evidently proving that the products are highly purified. The two NiO samples, rose-flower NiO and silk-flower NiO, have Scherrer’s average crystal sizes of 11.7 and 10.9 nm, respectively. Hysteresis loop curves of the NiO samples are illustrated in Fig. S1. Saturation magnetization (Ms) values of S1 and S2 are 10.3 emu/g and 9.3 emu/g, respectively, while the coercivity (Hc) values are 211 Oe and 105 Oe, respectively. Therefore, both samples are ferromagnetism materials 49. It is believed that the magnetism of the NiO samples is ascribed to the synergistic effects of the broken bonds and lattice distortion 50

. Representative SEM images of the two NiO structures are displayed in Fig. 4. In

Fig. 4a, S1 has a hierarchical flower-like structure, and each monodisperse flower exhibits a relatively narrow size distribution of 4-7 µm, as shown in Fig. S2. By magnifying single NiO flower (Fig. 4b), each flower structure have pretty smooth surfaces and integrated edges. Moreover, it seems that the nanosheets were wrapped around an invisible axis to form the flower-like structure. Average thickness of the petals is about 25 nm. In comparison, the S2 sample has 3D hierarchical silk-flower architectures containing large number of nanosheets, with diameters of 0.7-1.0 µm (Fig. 4c and S2). Magnified image (Fig. 4d) reveals that the nanosheets consist of randomly dispersed silk build blocks with a uniform thickness of about 11 nm. At the same time, the extremely thin nanosheets with similar arc-shaped morphologies are densely packed, thus forming the multilayered porous structure. Comparing the two



flower-like samples, it should be noted that the monodisperse flower’s diameter of S2 sample is much smaller than that of S1, and the silk-flower NiO structures have much more curled nanosheets, pores, and intervals, implying that the silk-flower structure has advantage of keeping the large surface area in the even region, which could largely improve microwave absorbing capabilities. High purities of the two NiO samples are further proved by EDX spectra (Fig. 4e and f). The SEM images of two NiO precursors are present in Fig. S3. The result confirm that the precursors almost have the same morphology compared with the two flower-like NiO. TEM was employed to further analyze the structures of the two NiO samples (Fig. 5a and b). S1 shows a 3D rose-flower structure, composed of numerous 2D well-defined ultrathin sheets (Fig. 5a), which is in good accordance with the SEM results. For S2, it clearly shows the presence of silk-constructed intact sphere and the sphere is composed of radially oriented flake-like nanosheets. Due to the nanometer size and sufficient amount of the sheets, abundant conical pores are generated due to the interconnection of the nanosheets (Fig. 5b). The corresponding high resolution TEM (HRTEM) images of two flower-like NiO are exhibited in Fig. 5c and d. The interplanar spacing of 0.245 nm is related to the cubic NiO (111) plane for S1; while the spacing of 0.204 nm corresponds to the (200) lattice plane for S2. Selected area electron diffraction (SAED) and distinct white lattice fringes demonstrate that the two NiO structures consist of highly crystalline nanocrystals (Fig. 5c and d). To evaluate microwave absorption capabilities of the two NiO samples, reflection loss (RL) was calculated. Based on the measured complex permittivity and


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permeability, RL values usually are given by the following equation:51-52 RL (dB) = 20log(Zin − 1)/(Zin + 1)

(1)

where Zin is the normalized input impedance (377 Ω), which is calculated as follow: Zin =

[

µr /εr tanh j(2πfd/c) εr µr

]

(2)

where d is the thickness of the absorb layers, c is the light velocity in free space, f is the frequency of electromagnetic wave, and εr and µr are the complex permittivity and permeability of absorber, respectively. RL of –10 dB corresponds to 90% electromagnetic wave attenuation. In other words, a proper microwave absorber should have RL values to be less than –10 dB. Fig. 6 shows the calculated RL curves of S1 and S2 with different thicknesses over 2-18 GHz. In this frequency region, S2 exhibits significantly higher microwave absorption capability than S1. As displayed in Fig. 6a, S1 has a very poor microwave absorption ability and the maximum RL value is only –5.2 dB at an optimal sample thickness of 7.5 mm. It is obvious that most RL peaks of S1 are corresponding to the magnetic resonant peaks as shown in inset of Fig. 6a. Accordingly, the poor microwave absorption performance of S1 is ascribed to the fact that absorption behavior mostly comes from the weak magnetic losses. Comparatively, as shown in Fig. 6b, S2 has a maximum RL value of –55.6 dB at 14.6 GHz with a thickness of 7.0 mm. Furthermore, the effective bandwidth corresponding to the RL value below –10 dB is as wide as 2.7 GHz (from 13.3 GHz to 16.0 GHz). Moreover, along with the change of thickness at around 6.0-8.0 mm, S2 still has desirable microwave absorption performance, with maximum RL values of –34.5 dB, –30.9 dB, –41.4 dB and –27.5 dB, at thicknesses of 6.0 mm, 6.5 mm, 7.5 mm and 8.0



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mm, respectively. It should be point out that the optimal RL peaks are shifted toward lower frequencies as the absorber thickness is increased from 6.0 mm to 8.0 mm. These peak shifting should be expressed by the 1/4 wavelength cancellation equation 53

:

tm = nλ / 4 = nc / 4 fm(εr µr )

1/2

(3)

where fm and tm are the frequency of complex permeability and permittivity and the matching thickness, respectively, and c is the light velocity. Obviously, the peak frequency range is inversely proportional to the thickness of absorbers. To present a visual effect of microwave absorption performance, the reflection loss data of S1 and S2 have been converted into 3D maps and contour maps, as shown in Fig. 7a-d. Clearly, S2 exhibit a broader absorption area, which is located over 3.1-4.5 GHz and 9.8-18 GHz, at corresponding absorber thicknesses of 7.7-10 mm and 5.7-10 mm, respectively. The maximum RL value can even reach –65.1 dB at 13.9 GHz with the thickness of 7.9 mm, while the effective bandwidth is as wide as 3 GHz (from 12.4 GHz to 15.4 GHz). However, S1 has no apparent effective microwave absorption area and all RL values for S1 are less than –5.0 dB, confirming its poor absorption performance (Fig. 7a and b). The large differences in microwave absorption properties between the two NiO samples can be reasonably assigned to their difference in the hierarchical structures. In Fig. 7d, the two red lines are simulated results of thickness (tm) vs. peak frequency (fm) for S2, which are derived from the quarter-wavelength equation according to the electromagnetic parameters.Clearly, both lines just go through the effective absorption


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area (the RL value below –10 dB). Moreover, when n=3, all the microwave absorption peaks are exactly located on the λ/4 red line, proving that the relationship between peak frequency of microwave absorption and matching thickness conforms to the quarter-wavelength matching conditions. As we all know, microwave absorption performance of absorbing materials is closely associated with their magnetic loss, dielectric loss and impedance matching behaviors. At the first stage, the impedance matching ratio directly determine the microwave absorption efficiency. It is easy to understand that the incident electromagnetic wave should effectively propagate into the absorbers as many as possible, so that electromagnetic waves could be changed into heat energy or dissipated by interference and less electromagnetic wave will be reflected at surface of absorber. Most desirably, the impedance of the absorbers is equal to that of free space, leading to a zero reflection of electromagnetic waves.52, 54 Impedance matching ratios of the two flower-like NiO structures are shown in Fig. 8a. Both samples exhibit comparatively stable impedance matching ratio values, with only small fluctuations. More importantly, all values exceed 0.4 over 2-18 GHz, implying that only a small fraction of the wave is reflected at the interface. Compared with the two samples, S1 has a little bigger impedance matching ratio value than S2, which may be due to the unbalanced electromagnetic parameters caused by their unique structures. These impedance matching ratios are higher than other similar absorbers, such as Fe3O4/activated-carbon 55, Fe3O4/MWCNTs,55 Fe3O4/graphene 55, carbon@Fe@Fe3O4 56

, and MnO2@Fe-GNS 57.



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On the basis of transmission line theory, desirable absorbing performance is also related to electromagnetic attenuation capability, another crucial factor that can be expressed by attenuation constant α:58-60 α =

2 πf

c

×

(µ ′′ε ′′ −

µ ′ε ′ ) +

(µ ′′ε ′′ −

µ ′ε ′ ) + (µ ′ε ′′ + µ ′′ε ′) 2

2

(4)

In this case, the attenuation constant α refers to an integrated attenuation capability. Although great dielectric loss and magnetic loss may result in a large α value, a large α does not mean a great dielectric loss or magnetic loss capability. It is clearly demonstrated in Fig. 8b that S2 has a much superior electromagnetic attenuation capability than S1. The attenuation constant α value of S2 is increased from 17.3 to 80.5 with the increase of frequency, whereas α value of S1 is only increased from 4.0 to 18.4. Although S1 show a larger impedance matching ratio, S2 has a moderate impedance matching characteristic but a much larger attenuation constant α. As a result, S2 has a much better microwave absorption performance than S1. In order to verify the above explanations, relative complex permittivity and relative complex permeability of the two NiO structures have been carefully evaluated. It is well known that the real parts of complex permittivity (ε′) and permeability (µ′) stand for the storage ability of electromagnetic energy, while imaginary parts (ε″ and µ″) indicate the inner dissipation.61 Relative complex permittivity of the two NiO samples over 2-18 GHz are shown in Figure 9a and b, respectively. As shown in Fig. 9a, ε′ values are 2.5 and 5.5, respectively, both of which are moderate in terms of impedance matching. For S1, both ε′ and ε″ values are almost a constant in the measurement frequency region, except for a certain degree of fluctuations. S1 has


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relatively constant ε′ and ε″ of ~2.7 and ~0.1, respectively, suggesting that it has a relatively low dielectric loss. In contrast, S2 has obviously larger ε′ and ε″ values, implying that it has both high storage capability and high dielectric loss. S2 has a similar variation trend in ε′ and ε″ as a function of frequency. ε′ value is decreased from 5.6 to 4.1, while ε″ value is decreased from 1.5 to 0.8, as frequency is increased from 2 to 18 GHz. The frequency dispersion behavior of S2 is similar to those observed in other metallic oxide absorbers.61-62 However, such phenomenon is only observed in S2. Therefore, it is reasonable to suggest that the difference in dielectric behavior of the two types of NiO samples is related to their difference in morphology and structural features. In our opinion, the hierarchical silk-flower NiO structure offers more interfaces and junctions, as compared with the hierarchical rose-flower NiO structure. Therefore, in the composite based on S2, more migrating charges are induced. Such charges present at the interfaces and junctions would be responsible for interfacial polarization (Maxwell-Wagner effect) as well as related relaxation. As a consequence, the dielectric properties of S2 are superior in terms of microwave absorption. This frequency dispersion behavior has also appeared in dielectric loss curve of S2 (Fig. 9e), where the dielectric loss values decrease from 0.27 to 0.18 over the measurement frequency range. According to Debye theory, the ε″ values are related to both polarization and electrical conductivity σ of absorbers 63. As NiO is a p-type semiconductor 64, the cause for ε″ peaks in S2 should be mainly ascribed to the polarizations and its associated relaxations. The polarizations include interfacial polarization due to the hierarchical porous NiO architecture, along with the dipole



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polarizations owing to external defects on/in NiO.65 Real and imaginary parts (µ′ and µ″) of relative complex permeability of the two samples are shown in Fig. 9c and d. The two NiO samples possess similar µ′ values, with fluctuations between 0.99 and 1.15. They also have a similar variation trend as a function of frequency, demonstrating their similar storage capability of magnetic energy. Due to the relatively high real parts of the permeability curves, large impedance matching properties will be obtained over whole frequency range. The imaginary part (µ″) is usually used to explain the magnetic loss. In Fig. 9d, the µ″ curves of two NiO structures show nearly the same variation trends in the whole measurement frequency range, and six obvious resonance peaks exist in the corresponding range, which corresponds to multiple magnetic resonance (MMR) 66. It is necessary to mention that a weak resonance peak appears at 16-18 GHz for S2, whereas S1 has not exhibited such resonance peak, as shown in inset of Fig. 9d. The explanation for the appearance of this resonance peak could not be exactly understood at present. Possible mechanisms are explored as follows: As an important antiferromagnetic metallic oxide, NiO structures should exhibit a similar magnetic properties. However, according to a recent study, absorbers with flake-like architectures can effectively restrain the eddy current effect

66

. As everyone knows,

eddy current effect could hinder the electromagnetic wave penetrating into the absorber and play a negative role on magnetic loss performance, which usually happens at high frequency range. Therefore, due to the flat and large flake architectures of S1, eddy current effect would be suppressed to some extent in the


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corresponding frequency range. As a result, no obvious peak appears. The frequency dependence of dissipation factors displayed by the dielectric loss tangent (tanδε = ε″/ε′) and magnetic loss tangent (tanδµ = µ″/µ′) for the two flower-like NiO structures are exhibited in Fig. 9e and f. Generally, the loss tangents are usually used to evaluate the corresponding dielectric loss and magnetic loss abilities. In Fig. 9e, S2 shows a quite higher dielectric loss ability over the whole frequency range compared to S1, which indicates that the hierarchical silk-like NiO structure may generate more interfacial polarization at interfaces and junctions, resulting in high ε″ values and enhanced dielectric loss. As we all know, interfacial polarization and its related relaxation contribute to the absorbing capability, which is related to the Cole-Cole semicircle.38 According to Debye dipolar relaxation, the complex permittivity (εr) can be calculated as follow:67 εr = ε∞ +

εs − ε∞ = ε ′− j ε″ 1 + j 2 π fτ

(5)

where τ and f are the polarization relaxation time and frequency, respectively, and εs and ε∞ are the stationary dielectric constant and optical dielectric constant at the high-frequency limit, respectively. Thus, from above equation (5), it is further calculated that: ε ′= ε ∞ +

ε″=

εs − ε∞ 1 + (2π f

(6)

)2 τ 2

2πfτ (εs − ε∞ )

(7)

1 + (2πf ) τ 2 2

As inferred from equations (6) and (7), the relationship between ε′ and ε″ is as follow:

(ε′- ε∞ )2 + (ε″)2 = (εs - ε∞)2

(8)

On the basis of the mathematical equation (8), it is concluded that the plot ε′ vs ε″ may



be a single semicircle, which is usually called the Cole-Cole semicircle, while each semicircle is assigned to a Debye relaxation process.68 ε′ vs ε″ curves for the two flower-like NiO structures are presented in Fig. 10. It can be clearly observed that S1 shows a completely disordered curve. If the scale of the relative complex permittivity is further expanded to reach a normal size, the curve of S1 seems like a complex dot (as seen in inset of Fig. 10a), demonstrating that there is no obvious dielectric relaxation process in the S1 structure. For S2, five distinguishable Cole-Cole semicircles are observed, implying that there are probable multiple dielectric relaxation processes, which is ascribed to the hierarchical and porous structures of S2. Generally, under the alternating electromagnetic field, the relaxation processes in a dielectric medium are caused by the delay of induced charges. In present work, due to the hierarchical and porous structure of S2, large amounts of NiO-NiO and NiO-paraffin interfaces have been introduced, which can result in the accumulation of bound charges at the interfaces and cause the interfacial polarization or Maxwell-Wagner effect

69

. Additionally, the hierarchical structures can provide

numerous surfaces with unsaturated coordination bonds and dangling atoms, resulting in dipoles and dipole polarization. These polarization effects should be responsible for the increase of the dielectric loss. Fig. 9f shows tanδµ values of the two flower-like NiO structures. It’s clear that no obvious difference can be found between the two curves of magnetic loss except that S2 shows a slight resonance peak at 15-18 GHz, which is inherited from imaginary part of permeability. It is widely believe that the magnetic loss mainly


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derive from domain wall resonance, eddy current effect, hysteresis loss, exchange resonance, dimensional resonance and natural resonance.70-72 For magnetic NiO materials, the hysteresis loss can be negligible in weak filed

38

. Meanwhile, the

magnetic loss could not originate from the domain wall resonance, which generally appears at megahertz frequency region. In addition, the actual sample thickness is much smaller than the minimum thickness at which the dimension resonance could happen. Therefore, the exchange resonance, eddy current effect and natural resonance are three important elements that could be responsible for magnetic loss. Especially, the eddy current loss is associated with the particle diameter (d) and the conductivity (σ) of the materials, which can be expressed as µ″ ≈ 2πµ0(µ′)2σd2f/3, where µ0 is the permeability of vacuum

73

. Based on this equation, when the absorber has the eddy

current effect, the values of µ″(µ′) –2f–1 will be a constant. From Fig. S4, the values of µ″(µ′)–2f–1 are nearly constant over 8-18 GHz. Furthermore, the magnetic loss is very small. Thus, the magnetic loss of NiO is mainly originated from eddy current effect. In addition, when the microwave inters the helix, it will produce magnetic polarization along the axis and further causes an electric polarization partly because the electromagnetic waves flow in the direction parallel to the axis.74 The efficient polarization produces planar and/or circularly polarized waves, which are reflected or dispersed, so as to be attenuated. It is worth noting that S2 only shows relatively low magnetic loss ability, suggesting that the contribution to microwave absorption properties mainly depends on dielectric loss. In general, the microwave absorption properties of absorbers arise from the



impedance matching characteristics, dielectric loss, magnetic loss and layer thickness, which are correlated with their size, shape, structure and nature. In this work, the improved microwave absorption performance of sample S2 can be explained as follows. Firstly, due to the complicated hierarchical and porous structures of S2, a plenty of NiO-NiO and NiO-paraffin interfaces and junctions are formed in the silk-like NiO particles, which could lead to the bound charges aggregating at the interfaces and junctions, and cause the interfacial polarization as well as related relaxation (Fig. 11). Such process is favorable for microwave absorption. Furthermore, under a sufficient altering EM field, electrons at the interfaces may gain enough energy to freely move along the interfacial planes, and the assembled movement of collective electrons could also enhance the microwave absorption ability 20. Secondly, we suggest that there is a large part of the microwaves attenuated by a multiple scattering effect for S2. As shown in Fig. 11, it is not difficult to find that every single structure of S2 has more nanosheets, folds and voids compared to that of S1. Thanks to these nanosheets, folds and voids, large amounts of specific surface areas will generated. When the electromagnetic waves are projected into this structure, the specific surface areas will provide massive active sites to induce multiple reflection and scattering of electromagnetic waves.28, 75 After constantly reflecting and scattering, the microwave waves can be effectively consumed by NiO. Thirdly, because of the different impedance matching characteristics, the porous structures may interrupt the spreading routes of waves and change the path of the waves into disorder. There are more opportunities for two microwaves to run across (see Fig. 11). During the process,


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some microwaves may be attenuated by the opposite electromagnetic waves with similar vibrational frequency

75

. Fourthly, the clustered defects on NiO surfaces

caused by the existence of oxygen vacancies would break the balance of the charge distribution and give rise to the dipole polarization and related relaxations

76

. Under

altering electromagnetic filed, the induced dipole polarization can increase energy dissipation, further enhancing microwave absorption capability of NiO particles. 4. Conclusion In summary, two flower-like NiO hierarchical structures were experimentally synthesized using a facial hydrothermal method with the post-annealing treatment. Structure, morphology and magnetic and electromagnetic performances of the two structures were investigated in detail. Sample S1 showed a hierarchical rose-flower architecture with an average diameter of 4-7 µm, while S2 exhibited a porous silk-flower architecture with a diameter of about 0.7-1.0 µm. The structural distinctions between S1 and S2 lead to a total difference in dielectric and microwave absorption properties. Specifically, compared to S1 structure, the S2 structure exhibited significantly enhanced dielectric property and microwave attenuation performance with the maximum RL value of –65.1 dB at 13.9 GHz and effective bandwidth of about 3 GHz (from 12.4 GHz to 15.4 GHz). The improved microwave attenuation performance was mainly attributed to hierarchical and porous architectures of S2, which may induce more interfacial polarizations, multiple scattering and energy dissipation. It can be concluded that the structures synthesized in present work may provide theoretical guidance for designing novel microwave



absorption materials. Acknowledgements This work was supported by the National Natural Science Foundation of China (51672129), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and AcRF Tier 1 (477) of MOE, Singapore. Supporting Information Magnetic hysteresis loops, eddy current curves and particle size distributions of rose-flower NiO and silk-flower NiO, SEM images of the precursors of the rose-flower NiO and silk-flower NiO. References 1.

Shahzad, F.; Alhabeb, M.; Hatter, C. B.; Anasori, B.; Hong, S. M.; Koo, C. M.; Gogotsi, Y.,

Electromagnetic Interference Shielding with 2D Transition Metal Carbides (MXenes). Science 2016, 353, 1137-1140. 2.

Bai, S.; Shen, X.; Zhong, X.; Liu, Y.; Zhu, G.; Xu, X.; Chen, K., One-pot Solvothermal

Preparation of Magnetic Reduced Graphene Oxide-Ferrite Hybrids for Organic Dye Removal. Carbon 2012, 50, 2337-2346. 3.

Cao, M.-S.; Yang, J.; Song, W.-L.; Zhang, D.-Q.; Wen, B.; Jin, H.-B.; Hou, Z.-L.; Yuan, J.,

Ferroferric Oxide/Multiwalled Carbon Nanotube vs Polyaniline/Ferroferric Oxide/Multiwalled Carbon Nanotube Multiheterostructures for Highly Effective Microwave Absorption. Acs Appl. Mater. Interfaces 2012, 4, 6949-6956. 4.

Liu, P.; Yao, Z.; Zhou, J., Fabrication and Microwave Absorption of Reduced Graphene

oxide/Ni0.4Zn0.4Co0.2Fe2O4 Nanocomposites. Ceram. Int. 2016, 42, 9241-9249. 5.

Liu, J.; Cao, M.-S.; Luo, Q.; Shi, H.-L.; Wang, W.-Z.; Yuan, J., Electromagnetic Property and

Tunable Microwave Absorption of 3D Nets from Nickel Chains at Elevated Temperature. Acs Appl. Mater. Interfaces 2016, 8, 22615-22622. 6.

Liu, P.; Yao, Z.; Zhou, J., Controllable Synthesis and Enhanced Microwave Absorption Properties

of Silane-modified Ni0.4Zn0.4Co0.2Fe2O4 Nanocomposites Covered with Reduced Graphene Oxide. RSC Adv. 2015, 5, 93739-93748. 7.

Che, R. C.; Peng, L. M.; Duan, X. F.; Chen, Q.; Liang, X. L., Microwave Absorption

Enhancement and Complex Permittivity and Permeability of Fe Encapsulated within Carbon Nanotubes. Adv. Mater. 2004, 16, 401-405. 8.

Wen, B.; Cao, M.; Lu, M.; Cao, W.; Shi, H.; Liu, J.; Wang, X.; Jin, H.; Fang, X.; Wang, W.; Yuan,

J., Reduced Graphene Oxides: Light-Weight and High-Efficiency Electromagnetic Interference Shielding at Elevated Temperatures. Adv. Mater. 2014, 26, 3484-3489.


Page 24 of 43

Page 25 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


9.

Cao, W.-Q.; Wang, X.-X.; Yuan, J.; Wang, W.-Z.; Cao, M.-S., Temperature Dependent Microwave

Absorption of Ultrathin Graphene Composites. J. Mater. Chem. C 2015, 3, 10017-10022. 10. Zhang, X.-J.; Wang, G.-S.; Cao, W.-Q.; Wei, Y.-Z.; Liang, J.-F.; Guo, L.; Cao, M.-S., Enhanced Microwave Absorption Property of Reduced Graphene Oxide (RGO)-MnFe2O4 Nanocomposites and Polyvinylidene Fluoride. Acs Appl. Mater. Interfaces 2014, 6, 7471-7478. 11. Yang, H.-J.; Cao, W.-Q.; Zhang, D.-Q.; Su, T.-J.; Shi, H.-L.; Wang, W.-Z.; Yuan, J.; Cao, M.-S., NiO Hierarchical Nanorings on SiC: Enhancing Relaxation to Tune Microwave Absorption at Elevated Temperature. Acs Appl. Mater. Interfaces 2015, 7, 7073-7077. 12. Batlle, X.; Labarta, A., Finite-size Effects in Fine Particles: Magnetic and Transport Properties. J. Phys. D: Appl. Phys. 2002, 35, R15-R42. 13. Ni, S.; Lin, S.; Pan, Q.; Yang, F.; Huang, K.; He, D., Hydrothermal Synthesis and Microwave Absorption Properties of Fe3O4 Nanocrystals. J. Phys. D: Appl. Phys 2009, 42, 055004. 14. Sun, G.; Zhang, X.; Cao, M.; Wei, B.; Hu, C., Facile Synthesis, Characterization, and Microwave Absorbability of CoO Nanobelts and Submicrometer Spheres. J. Phys. Chem. C 2009, 113, 6948-6954. 15. Skoufa, Z.; Heracleous, E.; Lemonidou, A. A., On Ethane ODH Mechanism and Nature of Active Sites over NiO-based Catalysts via Isotopic Labeling and Methanol Sorption Studies. J. Catal. 2015, 322, 118-129. 16. Yu, T.; Cheng, X.; Zhang, X.; Sui, L.; Xu, Y.; Gao, S.; Zhao, H.; Huo, L., Highly Sensitive H2S Detection Sensors at Low Temperature Based on Hierarchically Structured NiO Porous Nanowall Arrays. J. Mater. Chem. A 2015, 3, 11991-11999. 17. Fu, J.; Zhao, C.; Zhang, J.; Peng, Y.; Xie, E., Enhanced Gas Sensing Performance of Electrospun Pt-Functionalized NiO Nanotubes with Chemical and Electronic Sensitization. Acs Appl. Mater. Interfaces 2013, 5, 7410-7416. 18. Xie, Y.; Wu, Z.; Wu, Q.; Liu, M.; Piao, L., Effect of Different Base Structures on The Performance of The Hierarchical TiO2 Photocatalysts. Catal. Today 2014, 225, 74-79. 19. Zhou, W.; Liu, H.; Boughton, R. I.; Du, G.; Lin, J.; Wang, J.; Liu, D., One-dimensional single-crystalline Ti-O Based Nanostructures: Properties, Synthesis, Modifications and Applications. J. Mater. Chem. 2010, 20, 5993-6008. 20. Yang, H.; Cao, M.; Li, Y.; Shi, H.; Hou, Z.; Fang, X.; Jin, H.; Wang, W.; Yuan, J., Enhanced Dielectric Properties and Excellent Microwave Absorption of SiC Powders Driven with NiO Nanorings. Adv. Opt. Mater. 2014, 2, 214-219. 21. Liu, T.; Pang, Y.; Xie, X.; Qi, W.; Wu, Y.; Kobayashi, S.; Zheng, J.; Li, X., Synthesis of Microporous Ni/NiO Nanoparticles with Enhanced Microwave Absorption Properties. J. Alloy. Compd. 2016, 667, 287-296. 22. Liu, B.; Yang, H.; Wei, A.; Zhao, H.; Ning, L.; Zhang, C.; Liu, S., Superior Photocatalytic Activities of NiO Octahedrons with Loaded AgCl Particles and Charge Separation between Polar NiO {1 1 1} Surfaces. Appl. Catal., B 2015, 172–173, 165-173. 23. San, X.; Wang, G.; Liang, B.; Ma, J.; Meng, D.; Shen, Y., Flower-like NiO Hierarchical Microspheres Self-assembled with Nanosheets: Surfactant-free Solvothermal Synthesis and Their Gas Sensing Properties. J. Alloy. Compd. 2015, 636, 357-362. 24. Wang, J.; Zeng, W.; Wang, Z., Assembly of 2D Nanosheets into 3D Flower-like NiO: Synthesis and The Influence of Petal Thickness on Gas-Sensing Properties. Ceram. Int. 2016, 42, 4567-4573. 25. Gund, G. S.; Dubal, D. P.; Shinde, S. S.; Lokhande, C. D., Architectured Morphologies of Chemically Prepared NiO/MWCNTs Nanohybrid Thin Films for High Performance Supercapacitors.



Acs Appl. Mater. Interfaces 2014, 6, 3176-3188. 26. Zhang, X. K.; Ekiert, T.; Unruh, K. M.; Xiao, J. Q.; Golt, M.; Wu, R. X., High Frequency Properties of Polymer Composites Consisting of Aligned Fe Flakes. J. Appl. Phys. 2006, 99, 08M914. 27. Deng, L. J.; Zhou, P. H.; Xie, J. L.; Zhang, L., Characterization and Microwave Resonance in Nanocrystalline FeCoNi Flake Composite. J. Appl. Phys. 2007, 101, 103916. 28. Lv, H.; Liang, X.; Ji, G.; Zhang, H.; Du, Y., Porous Three-Dimensional Flower-like Co/CoO and Its Excellent Electromagnetic Absorption Properties. Acs Appl. Mater. Interfaces 2015, 7, 9776-9783. 29. Wang, T.; Han, R.; Tan, G.; Wei, J.; Qiao, L.; Li, F., Reflection Loss Mechanism of Single Layer Absorber for Flake-shaped Carbonyl-iron Particle Composite. J. Appl. Phys. 2012, 112, 104903. 30. Lin, L.; Liu, T.; Miao, B.; Zeng, W., Synthesis of NiO Nanostructures from 1D to 3D and Researches of Their Gas-sensing Properties. Mater. Res. Bull. 2013, 48, 449-454. 31. Zhu, G.; Xi, C.; Xu, H.; Zheng, D.; Liu, Y.; Xu, X.; Shen, X., Hierarchical NiO Hollow Microspheres Assembled from Nanosheet-stacked Nanoparticles and Their Application in A Gas Sensor. RSC Adv. 2012, 2, 4236-4241. 32. Chen, Z.; Xiao, A.; Chen, Y.; Zuo, C.; Zhou, S.; Li, L., Highly Porous Nickel Oxide Thin Films Prepared by A Hydrothermal Synthesis Method for Electrochromic Application. J. Phys. Chem. Solids 2013, 74, 1522-1526. 33. Song, X.; Gao, L., Facile Synthesis and Hierarchical Assembly of Hollow Nickel Oxide Architectures Bearing Enhanced Photocatalytic Properties. J. Phys. Chem. C 2008, 112, 15299-15305. 34. Liu, P.; Yao, Z.; Li, L.; Zhou, J., In situ Synthesis and Mechanical, Thermal Properties of Polyimide Nanocomposite Film by Addition of Functionalized Graphene Oxide. Polym. Compos. 2016, 37, 907-914. 35. Mavis, B.; Akinc, M., Cyanate Intercalation in Nickel Hydroxide. Chem. Mater. 2006, 18, 5317-5325. 36. Xu, L.; Ding, Y.-S.; Chen, C.-H.; Zhao, L.; Rimkus, C.; Joesten, R.; Suib, S. L., 3D Flowerlike Alpha-nickel Hydroxide with Enhanced Electrochemical Activity Synthesized by Microwave-assisted Hydrothermal Method. Chem. Mater. 2008, 20, 308-316. 37. Du, H.; Jiao, L.; Cao, K.; Wang, Y.; Yuan, H., Polyol-Mediated Synthesis of Mesoporous alpha-Ni(OH)(2) with Enhanced Supercapacitance. Acs Appl. Mater. Interfaces 2013, 5, 6643-6648. 38. Liu, P.; Yao, Z.; Zhou, J., Preparation of Reduced Graphene Oxide/Ni0.4Zn0.4Co0.2Fe2O4 Nanocomposites and Their Excellent Microwave Absorption Properties. Ceram. Int. 2015, 41, 13409-13416. 39. Zhen, W.; Ma, J.; Lu, G., Small-sized Ni(1 1 1) Particles in Metal-organic Frameworks with low Over-potential for Visible Photocatalytic Hydrogen Generation. Appl. Catal., B 2016, 190, 12-25. 40. Li, J.; Yan, R.; Xiao, B.; Liang, D. T.; Lee, D. H., Preparation of Nano-NiO Particles and Evaluation of Their Catalytic Activity in Pyrolyzing Biomass Components. Energy Fuels 2008, 22, 16-23. 41. Nwanya, A. C.; Offiah, S. U.; Amaechi, I. C.; Agbo, S.; Ezugwu, S. C.; Sone, B. T.; Osuji, R. U.; Maaza, M.; Ezema, F. I., Electrochromic and Electrochemical Supercapacitive Properties of Room Temperature PVP capped Ni(OH)2/NiO Thin Films. Electrochim. Acta 2015, 171, 128-141. 42. Miao, R.; Zeng, W., Hydrothermal Synthesis of Flake-Flower NiO Architectures: Structure, Growth and Gas-Sensing Properties. Mater. Lett. 2016, 171, 200-203. 43. Du, H.; Jiao, L.; Cao, K.; Wang, Y.; Yuan, H., Polyol-Mediated Synthesis of Mesoporous α-Ni(OH)2 with Enhanced Supercapacitance. Acs Appl. Mater. Interfaces 2013, 5, 6643-6648.


Page 26 of 43

Page 27 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


44. Bai, L.; Yuan, F.; Hu, P.; Yan, S.; Wang, X.; Li, S., A Facile Route to Sea Urchin-like NiO Architectures. Mater. Lett. 2007, 61, 1698-1700. 45. Zhu, T.; Chen, J. S.; Lou, X. W., Highly Efficient Removal of Organic Dyes from Waste Water Using Hierarchical NiO Spheres with High Surface Area. J. Phys. Chem. C 2012, 116, 6873-6878. 46. Deng, H.; Li, X. L.; Peng, Q.; Wang, X.; Chen, J. P.; Li, Y. D., Monodisperse Magnetic Single-Crystal Ferrite Microspheres. Angew. Chem., Int. Ed. 2005, 44, 2782-2785. 47. Murakami, Y.; Matsumoto, T.; Yahikozawa, K.; Takasu, Y., Monte-carlo Simulation of The Sol-Gel Process for The Hydrolysis and Polycondensation of Tin Tetra-N-butoxide. Catal. Today 1995, 23, 383-389. 48. Auer, S.; Frenkel, D., Prediction of Absolute Crystal-Nucleation Rate in Hard-Sphere Colloids. Nature 2001, 409, 1020-1023. 49. Sutradhar, S.; Das, S.; Chakrabarti, P. K., Magnetic and Enhanced Microwave Absorption Properties of Nanoparticles of Li0.32Zn0.26Cu0.1Fe2.32O4 Encapsulated in Carbon Nanotubes. Mater. Lett. 2013, 95, 145-148. 50. Bi, H.; Li, S. D.; Zhang, Y. C.; Du, Y. W., Ferromagnetic-like Behavior of Ultrafine NiO Nanocrystallites. J. Magn. Magn. Mater. 2004, 277, 363-367. 51. Singh, K.; Ohlan, A.; Pham, V. H.; R, B.; Varshney, S.; Jang, J.; Hur, S. H.; Choi, W. M.; Kumar, M.; Dhawan, S. K.; Kong, B. S.; Chung, J. S., Nanostructured Graphene/Fe(3)O(4) Incorporated Polyaniline as A High Performance Shield Against Electromagnetic Pollution. Nanoscale 2013, 5, 2411-2420. 52. Liu, P.; Li, L.; Yao, Z.; Zhou, J.; Du, M.; Yao, T., Synthesis and Excellent Microwave Absorption Property of Polyaniline Nanorods Coated Li0.435Zn0.195Fe2.37O4 Nanocomposites. J. Mater. Sci.: Mater. Electron. 2016, 27, 7776-7787. 53. Yong, Y.; Yang, Y.; Wen, X.; Jun, D., Microwave Electromagnetic and Absorption Properties of Magnetite Hollow Nanostructures. J. Appl. Phys. 2014, 115, 17A521. 54. Li, Y.; Cao, M., Enhanced Electromagnetic Properties and Microwave Attenuation of BiFeO3-BaFe7(MnTi)2.5O19 Driven by Multi-relaxation and Strong Ferromagnetic Resonance. Mater. Des. 2016, 110, 99-104. 55. Wang, X.; Huang, X.; Chen, Z.; Liao, X.; Liu, C.; Shi, B., Ferromagnetic Hierarchical Carbon Nanofiber Bundles Derived from Natural Collagen Fibers: Truly Lightweight and High-performance Microwave Absorption Materials. J. Mater. Chem. C 2015, 3, 10146-10153. 56. Lv, H.; Ji, G.; Liu, W.; Zhang, H.; Du, Y., Achieving Hierarchical Hollow Carbon@Fe@Fe3O4 Nanospheres with Superior Microwave Absorption Properties and Lightweight Features. J. Mater. Chem. C 2015, 3, 10232-10241. 57. Lv, H.; Ji, G.; Liang, X.; Zhang, H.; Du, Y., A Novel Rod-like MnO2@Fe Loading on Graphene Giving Excellent Electromagnetic Absorption Properties. J. Mater. Chem. C 2015, 3, 5056-5064. 58. Cao, M.; Qin, R.; Qiu, C.; Zhu, J., Matching Design and Mismatching Analysis towards Radar Absorbing Coatings Based on Conducting Plate. Mater. Des. 2003, 24, 391-396. 59. Liu, P.; Yao, Z.; Zhou, J.; Yang, Z.; Kong, L. B., Small Magnetic Co-doped NiZn Ferrite/Graphene Nanocomposites and Their Dual-Region Microwave Absorption Performance. J. Mater. Chem. C 2016, 4, 9738-9749. 60. Zhao, B.; Shao, G.; Fan, B.; Zhao, W.; Xie, Y.; Zhang, R., Facile Preparation and Enhanced Microwave Absorption Properties of Core-Shell Composite Spheres Composited of Ni Cores and TiO2 Shells. Phys. Chem. Chem. Phys. 2015, 17, 8802-8810.



61. Du, Y.; Liu, W.; Qiang, R.; Wang, Y.; Han, X.; Ma, J.; Xu, P., Shell Thickness-Dependent Microwave Absorption of Core-Shell Fe3O4@C Composites. Acs Appl. Mater. Interfaces 2014, 6, 12997-13006. 62. Wu, H. J.; Wu, G. L.; Ren, Y. Y.; Yang, L.; Wang, L. D.; Li, X. H., Co2+/Co3+ Ratio Dependence of Electromagnetic Wave Absorption in Hierarchical NiCo2O4-CoNiO2 Hybrids. J. Mater. Chem. C 2015, 3, 7677-7690. 63. Mingming, L.; Xixi, W.; Wenqiang, C.; Jie, Y.; Maosheng, C., Carbon Nanotube-CdS Core–Shell Nanowires with Tunable and High-efficiency Microwave Absorption at Elevated Temperature. Nanotechnology 2016, 27, 065702. 64. Ge, C.; Hou, Z.; He, B.; Zeng, F.; Cao, J.; Liu, Y.; Kuang, Y., Three-dimensional Flower-like Nickel Oxide Supported on Graphene Sheets as Electrode Material for Supercapacitors. J. Sol-gel Sci. Techn. 2012, 63, 146-152. 65. Li, Y.; Fang, X.; Cao, M., Thermal Frequency Shift and Tunable Microwave Absorption in BiFeO3 Family. Sci. Rep. 2016, 6, 24837-24842. 66. Wang, H.; Dai, Y. Y.; Geng, D. Y.; Ma, S.; Li, D.; An, J.; He, J.; Liu, W.; Zhang, Z. D., CoxNi100-x Nanoparticles Encapsulated by Curved Graphite Layers: Controlled in Situ Metal-Catalytic Preparation and Broadband Microwave Absorption. Nanoscale 2015, 7, 17312-17319. 67. Frenkel, J.; Dorfman, J., Spontaneous and Induced Magnetisation in Ferromagnetic Bodies. Nature 1930, 126, 274-275. 68. Cao, M.-S.; Wang, X.-X.; Cao, W.-Q.; Yuan, J., Ultrathin Graphene: Electrical Properties and Highly Efficient Electromagnetic Interference Shielding. J. Mater. Chem. C 2015, 3, 6589-6599. 69. Liu, T.; Xie, X.; Pang, Y.; Kobayashi, S., Co/C Nanoparticles with Low Graphitization Degree: A High Performance Microwave-Absorbing Material. J. Mater. Chem. C 2016, 4, 1727-1735. 70. Lu, B.; Dong, X. L.; Huang, H.; Zhang, X. F.; Zhu, X. G.; Lei, J. P.; Sun, J. P., Microwave Absorption Properties of The Core/Shell-Type Iron and Nickel Nanoparticles. J. Magn. Magn. Mater. 2008, 320, 1106-1111. 71. He, J.-Z.; Wang, X.-X.; Zhang, Y.-L.; Cao, M.-S., Small Magnetic Nanoparticles Decorating Reduced Graphene Oxides to Tune The Electromagnetic Attenuation Capacity. J. Mater. Chem. C 2016, 4, 7130-7140. 72. Chen, Y.-H.; Huang, Z.-H.; Lu, M.-M.; Cao, W.-Q.; Yuan, J.; Zhang, D.-Q.; Cao, M.-S., 3D Fe3O4 Nanocrystals Decorating Carbon Nanotubes to Tune Electromagnetic Properties and Enhance Microwave Absorption Capacity. J. Mater. Chem. A 2015, 3, 12621-12625. 73. Lu, M.-M.; Cao, M.-S.; Chen, Y.-H.; Cao, W.-Q.; Liu, J.; Shi, H.-L.; Zhang, D.-Q.; Wang, W.-Z.; Yuan, J., Multiscale Assembly of Grape-Like Ferroferric Oxide and Carbon Nanotubes: A Smart Absorber Prototype Varying Temperature to Tune Intensities. Acs Appl. Mater. Interfaces 2015, 7, 19408-19415. 74. Wang, G.; Gao, Z.; Tang, S.; Chen, C.; Duan, F.; Zhao, S.; Lin, S.; Feng, Y.; Zhou, L.; Qin, Y., Microwave Absorption Properties of Carbon Nanocoils Coated with Highly Controlled Magnetic materials by Atomic Layer Deposition. ACS nano 2012, 6, 11009-11017. 75. Lv, H.; Zhang, H.; Ji, G.; Xu, Z. J., Interface Strategy To Achieve Tunable High Frequency Attenuation. Acs Appl. Mater. Interfaces 2016, 8, 6529-38. 76. Jeon, J.; Yu, B. D.; Hyun, S., Adsorption Properties of Transition Metal atoms on Strongly Correlated NiO(001) Surfaces with Surface Oxygen Vacancies. Curr. Appl. Phys. 2015, 15, 679-682.


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Figure Captions Figure 1. FT-IR spectra of (a) rose-flower NiO precursor, (b) rose-flower NiO, (c) silk-flower NiO precursor, and (d) silk-flower NiO. Figure 2. A possible schematic illustration of the formation of (a) rose-flower NiO and (b) silk-flower NiO. Figure 3. XRD patterns of two flower-like NiO samples. Figure 4. SEM images of (a and b) rose-flower NiO and (c and d) silk-flower NiO structures; EDS spectra of (e) rose-flower NiO and (f) silk-flower NiO structures. Figure 5. TEM images of (a) rose-flower NiO and (b) silk-flower NiO structures; HRTEM images of (c) rose-flower NiO and (d) silk-flower NiO samples (the insets are SAED patterns). Figure 6. Reflection losses of (a) S1 and (b) S2 structures at different absorber thicknesses. Figure 7. Three-dimensional representation of the reflection loss values for (a) S1 and (c) S2 structures, and two-dimensional representation of the reflection loss for (b) S1 and (d) S2. Figure 8. (a) Impedance matching ratio, and (b) attenuation constant α of two flower-like NiO structures. Figure 9. Frequency dependence on (a and b) relative complex permittivity curves, (c and d) relative complex permeability curves, (e) dielectric loss tangent, and (f) magnetic loss tangent of two NiO samples. Figure 10. Typical Cole-Cole semicircles of (a) S1 and (b) S2.


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Figure 11. Schematic illustration of microwave absorption mechanisms for silk-flower NiO.



In this study, two novel flower-like NiO hierarchical structures, rose-flower (S1) and silk-flower (S2), were synthesized, which exhibited much different dielectric properties and microwave absorption behaviors due to their difference in structures.


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Figure 1. FT-IR spectra of (a) rose-flower NiO precursor, (b) rose-flower NiO, (c) silk-flower NiO precursor, and (d) silk-flower NiO. 57x33mm (300 x 300 DPI)



Figure 2. A possible schematic illustration of the formation of (a) rose-flower NiO and (b) silk-flower NiO. 52x27mm (300 x 300 DPI)


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Figure 3. XRD patterns of two flower-like NiO samples. 78x61mm (300 x 300 DPI)



Figure 4. SEM images of (a and b) rose-flower NiO and (c and d) silk-flower NiO structures; EDS spectra of (e) rose-flower NiO and (f) silk-flower NiO structures. 110x121mm (300 x 300 DPI)


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Figure 5. TEM images of (a) rose-flower NiO and (b) silk-flower NiO structures; HRTEM images of (c) roseflower NiO and (d) silk-flower NiO samples (the insets are SAED patterns). 99x99mm (300 x 300 DPI)



Figure 6. Reflection losses of (a) S1 and (b) S2 structures at different absorber thicknesses. 57x21mm (300 x 300 DPI)


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Figure 7. Three-dimensional representation of the reflection loss values for (a) S1 and (c) S2 structures, and two-dimensional representation of the reflection loss for (b) S1 and (d) S2. 122x100mm (300 x 300 DPI)



Figure 8. (a) Impedance matching ratio, and (b) attenuation constant α of two flower-like NiO structures. 156x245mm (300 x 300 DPI)


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Figure 9. Frequency dependence on (a and b) relative complex permittivities, (c and d) relative complex permeabilities, (e) dielectric loss tangent, and (f) magnetic loss tangent of two NiO samples. 168x190mm (300 x 300 DPI)



Figure 10. Typical Cole-Cole semicircles of (a) S1 and (b) S2. 75x28mm (300 x 300 DPI)


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Figure 11. Schematic illustration of microwave absorption mechanisms for silk-flower NiO. 80x43mm (300 x 300 DPI)


Facile Synthesis and Hierarchical Assembly of Flowerlike NiO

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