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Fabrication of flower-like Ni0.5Co0.5(OH)2@PANI and its enhanced microwave absorption performances
Materials Research Bulletin 98 (2018) 59–63
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Fabrication of flower-like Ni0.5Co0.5(OH)2@PANI and its enhanced microwave absorption performances
MARK
⁎
Yan Wang , Xinming Wu, Wenzhi Zhang, Chunyan Luo, Jinhua Li, Yujing Wang School of Materials and Chemical Engineering, Xi’an Technological University, Xi’an 710021, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Binary hydroxides Microstructure Polyaniline Microwave absorption properties
Ni0.5Co0.5(OH)2 with flower-like structure@PANI was designed and synthesized via a two-step route. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) results illustrated that the Ni0.5Co0.5(OH)2 had a 3D flower structure with a homogeneous distribution. The Ni0.5Co0.5(OH)2 was coated by polyaniline (PANI) and embedded in polyaniline matrix. The as-synthesized Ni0.5Co0.5(OH)2@PANI possessed a BET specific surface area of 58.82 m2 g−1, and its pore diameter was in the mesoporous range (3.93 nm). Investigations of EM wave absorption properties indicated that the maximum absorption intensity of Ni0.5Co0.5(OH)2@PANI was as high as −39.8 dB at 6.4 GHz and the absorption bandwidth exceeding −10 dB was 3.1 GHz (4.9–8 GHz) with a matching thickness of 2.5 mm. The excellent EM wave absorption properties of the material were ascribed to its high permittivity and improved impedance matching effect. Therefore, it is anticipated that the Ni0.5Co0.5(OH)2@PANI composite would serve as a promising microwave absorber.
1. Introduction In recent years, with the rapid development of electronic devices and communication equipment, electromagnetic (EM) pollution is becoming a threat to human health and also affecting the operation of electronic instruments [1–3]. Therefore, it is essential to develop highperformance microwave absorption materials to solve the issues caused by EM wave interference [4]. According to the EM wave absorption principle, the microwave absorbers are comprised of magnetic and dielectric materials, such as carbon materials [5–7], ferrite [8,9] and metal oxides [10,11]. Compared with carbon materials, metal oxides have the advantages of low cost and chemical stability. However, they have some drawbacks such as high density and low permittivity, so they are not suitable as high performance absorbers which are required to have low thickness, light weight, wide absorption bandwidth and strong absorption intensity [12]. Recently, metals oxides with hierarchical and porous structures have attracted much attention as microwave absorption and shielding materials because of their unique properties [13]. Furthermore, the impedance matching between permittivity and permeability by microstructure design is also crucial to microwave absorption performance. For instance, Wang et al. reported the microwave absorption performances of Ni crystals with branch-like and flower-like shapes. The branch-like sample showed better microwave absorption properties (maximum reflection loss of −17 dB at 6 GHz) compared with the flower-like structure, due to its high
⁎
permeability and favorable impedance matching [14]. The mesoporous NiCo2O4 with a surface area of 54.47 m2 g−1 displayed a maximum reflection loss of −35.76 dB at 14.86 GHz [15]. Lu reported MOF-derived porous Co/C nanocomposites with the maximum reflection loss of −35.3 dB, which was attributed to synergetic effect and multiple scattering [16]. Based on the previous research, it is evident that in addition to dielectric loss and magnetic loss, the impedance matching and synergistic effect based on the size and shape of materials also significantly influence the microwave absorption properties [17–19]. Transition metal oxides, such as Co3O4 and NiO, have been reported as EM wave absorption materials [20,21]. However, there is no report to date on nickel and cobalt binary hydroxides as microwave absorption materials. Because of their three-dimensional structures, nickel and cobalt binary hydroxides possess larger specific surface area, which is beneficial to EM wave absorption performances [22]. Polyaniline (PANI) is an important conducting polymer which has drawn much attention in the field of EM wave absorption materials due to its controllable conductivity, low cost, light weight and easy production [23]. In this paper, we successfully prepared three dimensional flower-like Ni0.5Co0.5(OH)2@PANI by a two-step method. The addition of PANI can not only decrease the weight of the EM absorbing material, but also enhance the interfacial polarization and dielectric properties, leading to a proper impedance matching. Therefore, the flower-like Ni0.5Co0.5(OH)2@ PANI can be expected to exhibit better EM wave
Corresponding author. E-mail address: [email protected] (Y. Wang).
http://dx.doi.org/10.1016/j.materresbull.2017.10.004 Received 12 August 2017; Received in revised form 25 September 2017; Accepted 3 October 2017 Available online 04 October 2017 0025-5408/ © 2017 Elsevier Ltd. All rights reserved.
Materials Research Bulletin 98 (2018) 59–63
Y. Wang et al.
absorption properties at 2–18 GHz. The microstructure, morphology and microwave absorption performance of Ni0.5Co0.5(OH)2@PANI were systematically investigated. 2. Experimental To prepare the flower-like Ni0.5Co0.5(OH)2@PANI sample, first 1.454 g Ni(NO3)2·6H2O, 1.455 g Co(NO3)2·6H2O and 2.1 g hexamethylenetetramine (HMT) were dissolved in 50 ml deionized water at 10 °C for 20 min. The mixture was transferred into a 100 ml Teflon-lined autoclave and maintained at 120 °C for 5 h. After cooling down to room temperature, the sample was washed with ethanol and water to remove residual impurities, and dried at 100 °C for 8 h. Subsequently, the dried product was annealed in nitrogen atmosphere at 400 °C for 1 h in a tube furnace to form Ni0.5Co0.5(OH)2. Then, 0.25 g of the as-prepared Ni0.5Co0.5(OH)2 was dispersed in 100 ml acidic aqueous solution (1 M) and ultrasonicated for 30 min. Subsequently, 0.1 ml aniline was added to this mixture and stirred for 30 min. A solution of (NH4)2S2O8 (0.25 g) in 30 ml distilled water was slowly added to the above mixture with constant stirring for 12 h at 0 °C. Finally, the product was washed and dried to obtain Ni0.5Co0.5(OH)2@PANI. The crystal phase was detected by X-ray diffraction (XRD, Bruker D8, Germany, with Cu-Kα radiation) and the structure was analyzed by Fourier transform infrared spectroscopy (FTIR, Thermo Scientific Co. USA). The FTIR analysis was performed on a KBr diluted sample, with the KBr to sample weight ratio of about 100:1. The morphology and microstructure were characterized by FESEM (Quanta 600FEG) and TEM (JTM-2100). The N2 adsorption-desorption curve was measured on a Quad-rasorb-SI instrument, and the specific surface area was determined by BET process. The electromagnetic parameters of toroidal sample (Ni0.5Co0.5(OH)2 to paraffin weight ratio is 3:7) were recorded using a vector network analyzer (HP8720ES) at 2–18 GHz. The actual power level of incident microwave radiation was −20 ∼ 0 dBm. The reflection loss (RL) of the sample was calculated as follows [24,25]:
R(dB)=20 lg
Zin =
Zin − 1 Zin + 1
μr 2πfd tanh(j μ r εr ) c εr
Fig. 2. FTIR spectra of Ni0.5Co0.5(OH)2@PANI.
and Co(OH)2 (46-0605) standard samples and no impurity peaks were observed [26]. For Ni0.5Co0.5(OH)2@PANI (Fig. 1b), all diffraction peaks belonging to Ni0.5Co0.5(OH)2 were much weaker due to the PANI coating. The PANI peak (around 25°) was not detected, which may be due to the weaker intensity of its diffraction peak compared with Ni0.5Co0.5(OH)2. Apart from Ni0.5Co0.5(OH)2, no other peaks were detected, revealing that the Ni0.5Co0.5(OH)2@PANI was successfully synthesized. Fig. 2 shows the FTIR spectrum of Ni0.5Co0.5(OH)2@PANI. The characteristic bands at 1576, 1498 and 1394 cm−1 are associated with the C]N and C]C stretching vibrations of quinonoid and benzene rings, respectively [5]. The peaks at 1306 and 1142 cm−1 are ascribed to the CeN stretching of the aromatic amine and quinonoid moieties of doped PANI. The band at 806 cm−1 is related to the CeH out-of-plane bending [10], while the peak at 619 cm−1 is ascribed to the vibration of the metal ions and oxygen. All of the above reveal the existence of Ni0.5Co0.5(OH)2 in the composite. The morphology and microstructure of the as-synthesized samples were investigated by FESEM and TEM analyses. From the results shown in Fig. 3(a–c), it is seen that Ni0.5Co0.5(OH)2 has a homogeneous 3D flower-like structure with a size of 2–3 μm, which consists of numerous vertically-standing nanosheets with thickness of 20–30 nm. These loose nanosheets connect to each other to form homogeneous flower-like architecture, which is beneficial to multiple absorption and reflection of electromagnetic waves. As can be seen in Fig. 3(d–e), the exposed surfaces of Ni0.5Co0.5(OH)2 are not smooth and are coated with a layer of PANI, thus indicating the formation of a core-shell 3D structure, namely Ni0.5Co0.5(OH)2@PANI. The TEM images in Fig. 4(a–b) also display that Ni0.5Co0.5(OH)2 possesses a hydrangea-like structure and the particle size is in accordance with the results of FESEM analysis. From Fig. 4c, it is clearly seen that the Ni0.5Co0.5(OH)2 are dispersed in polyaniline and the hierarchical nanosheets of Ni0.5Co0.5(OH)2 become rough. Based on the above analysis, it is evident that the 3D flower-like Ni0.5Co0.5(OH)2@PANI with core-shell structure was successfully fabricated. The surface area and pore size were determined by N2 adsorptiondesorption method. It is observed from Fig. 5a that the as-prepared Ni0.5Co0.5(OH)2@PANI displays a typical type IV curve with a hysteresis loop at relative pressure between 0.45 and 1, revealing the existence of porous structure. As seen in Fig. 5b, the distribution of pore size is mainly around 4 nm, which indicates that flower-like Ni0.5Co0.5(OH)2@PANI is mesoporous (2–50 nm). The BET surface area, average pore diameter and pore volume of Ni0.5Co0.5(OH)2@PANI are determined to be about 58.82 m2 g−1, 3.93 nm and 0.31 cm3 g−1, respectively. The large surface area and pore volume of Ni0.5Co0.5(OH)2@ PANI may be ascribed to the presence of nanopores between the vertically-standing nanosheets, which suggests that the large BET surface area and high pore volume are important parameters for electromagnetic wave absorption materials. To evaluate the microwave absorption performances of
(1)
(2)
Where, Zin is the input impedance of the absorber, f is the microwave frequency, d is the thickness of the absorber, and c is the velocity of electromagnetic wave in vacuum. 3. Results and discussion The XRD pattern of Ni0.5Co0.5(OH)2 is shown in Fig. 1a. All characteristic peaks were found to match well with the Ni(OH)2 (38-0715)
Fig. 1. XRD pattern of Ni0.5Co0.5(OH)2 (a) and Ni0.5Co0.5(OH)2@PANI (b).
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Fig. 3. FESEM images of Ni0.5Co0.5(OH)2 (a-c) and Ni0.5Co0.5(OH)2@PANI (d, e).
where εs and ε∞ are static permittivity and relative dielectric permittivity at high frequency limit, respectively. According to equation (3), the curve of ε′ versus ε″ is a single semicircle and represents one Debye relaxation process. As shown in Fig. 6e, there are at least five semicircles, which shows that Debye relaxation process plays a crucial role in the enhanced dielectric properties of Ni0.5Co0.5(OH)2@PANI [27]. It is reported that magnetic loss is also a key factor influencing the EM wave absorption properties [18]. Generally speaking, magnetic loss derives from domain wall resonance, eddy current effect and natural resonance. The domain wall resonance usually occurs over the 1–100 MHz range [28], and thus domain wall resonance can be excluded in magnetic loss. If eddy current effect is the main contribution to magnetic loss, the values of C0 (C0 = μ″(μ′)−2f−1) should be constant between 2 and 18 GHz [29]. However, as shown in Fig. 6f, the values of Co fluctuate over 2–18 GHz. Therefore, we can conclude that the magnetic loss results from natural resonance of Ni0.5Co0.5(OH)2@PANI. According to electromagnetic wave principle, in addition to permittivity and permeability, impedance matching characteristics (Z = Zin Z0 ) is also a vital factor for microwave absorption performance [17,30]. If Z is close to 1, the superior microwave absorption properties can be obtained. The calculated Z values for Ni0.5Co0.5(OH)2@ PANI for the thickness of 2.5 mm are shown in Fig. 6g. It can be found that the relevant Z of Ni0.5Co0.5(OH)2@PANI is close to 1 at 2–18 GHz, revealing that a good impedance matching results in superior microwave absorption performance.
Ni0.5Co0.5(OH)2 and Ni0.5Co0.5(OH)2@PANI, the reflection loss (RL) curves were shown in Fig. 6(a–b). As illustrated in Fig. 6a, the Ni0.5Co0.5(OH)2 exhibits a maximum RL value of −14.9 dB at 5.9 GHz with a thickness of 3 mm. After PANI coating on the Ni0.5Co0.5(OH)2, the maximum RL for Ni0.5Co0.5(OH)2@PANI is as high as −39.8 dB at 6.4 GHz and the absorption bandwidth (RL < −10 dB) is 3.1 GHz (from 4.9 to 8 GHz) for a matching thickness of 2.5 mm, as seen from Fig. 6b. In order to elucidate the possible absorption mechanism, EM parameters of Ni0.5Co0.5(OH)2@PANI were investigated. Fig. 6c shows the relative complex permittivity and the relative complex permeability of Ni0.5Co0.5(OH)2@PANI. It can be seen that the ε′ values of Ni0.5Co0.5(OH)2@PANI decrease from 6.39-2.28 and the ε″ values vary between 0.71 and 1.94. It can be observed that the μ′ and μ″ values exhibit several variations between 2 and 18 GHz. From Fig. 6d, we can clearly see that the dielectric loss (tan δε) values of Ni0.5Co0.5(OH)2@ PANI are larger than those of magnetic loss (tan δμ), which implies that the dielectric loss is crucial to microwave absorption performances of Ni0.5Co0.5(OH)2@PANI. In addition to the dielectric loss mechanism, the microwave absorption performance also involves the relaxation process of Ni0.5Co0.5(OH)2@PANI, which can be depicted by Cole-Cole semicircle. For the Debye dipole relaxation, the relative complex permittivity can be expressed as follows [24]:
(ε′ −
εs + ε∞ 2 ε − ε∞ 2 ) + (ε ″)2 = ( s ) 2 2
(3)
Fig. 4. TEM images of Ni0.5Co0.5(OH)2 (a, b) and Ni0.5Co0.5(OH)2@PANI (c).
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Fig. 5. N2 adsorption-desorption isotherms (a) and pore diameter distribution (b) of Ni0.5Co0.5(OH)2@ PANI.
Fig. 6. Reflection loss curves of Ni0.5Co0.5(OH)2 (a) and Ni0.5Co0.5(OH)2@ PANI (b), complex permittivity, complex permeability (c), tangent loss (d), cole-cole semicircle curve (e), C0-f curve (f) and impedance matching (g) of Ni0.5Co0.5(OH)2@ PANI.
among branches helps enhance the interfacial polarization. Moreover, the interfaces and charge transfer between Ni0.5Co0.5(OH)2 and PANI can cause charge accumulation and enhanced interfacial polarization, which is advantageous for EM wave attenuation. More importantly, the high specific surface area and the large pore volume of Ni0.5Co0.5(OH)2@PANI can lead to multiple scattering and reflection of
The superior microwave dissipation route of Ni0.5Co0.5(OH)2@PANI is shown in Fig. 7. It is likely that the following absorption mechanism is involved: first, the flower-like structure of Ni0.5Co0.5(OH)2 with its unique hierarchical morphology facilitates the formation of numerous conductive networks within the structure and results in microwave energy attenuation [13]. Secondly, the formation of multi-interfaces 62
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Y. Wang et al.
[8]
[9]
[10]
[11]
[12] Fig. 7. A schematic representation for the possible dissipation route of EM wave in Ni0.5Co0.5(OH)2@PANI composite.
[13]
[14]
EM waves [31,32].
[15]
4. Conclusion In summary, Ni0.5Co0.5(OH)2@PANI with a novel flower-like architecture was fabricated by a two-step route. The Ni0.5Co0.5(OH)2 was firstly synthesized by one-step hydrothermal method, followed by sintering in tube furnace, and then PANI was grown around the Ni0.5Co0.5(OH)2 via in-situ polymerization. Studies of microwave absorption properties indicated that the 3D flower-like architecture possessed high specific surface area, large permittivity and improved impedance matching effect. The maximum RL values of Ni0.5Co0.5(OH)2@ PANI were as high as −39.8 dB at 6.4 GHz and the absorption bandwidth exceeding −10 dB was almost 3.1 GHz with a thickness of 2.5 mm, as a result of the multiple scattering and reflection of EM waves within the unique pore structure. Therefore, it is believed that the 3D flower-like architecture of Ni0.5Co0.5(OH)2@PANI can pave the way for the design of high-performance microwave absorbers.
[16]
[17]
[18]
[19]
[20]
[21] [22]
Acknowledgements
[23]
The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant No 61701386, No 51303147, No 51502233 and No 21506167), Natural Science Basic Research Plan in Shaanxi Province of China (Grant No 2017JQ5060, No 2016JQ5011) and President’s Fund of Xi’an Technological University (project No. XAGDXJJ16002).
[24]
[25]
[26]
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Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Fabrication of flower-like Ni0.5Co0.5(OH)2@PANI and its enhanced microwave absorption performances
MARK
⁎
Yan Wang , Xinming Wu, Wenzhi Zhang, Chunyan Luo, Jinhua Li, Yujing Wang School of Materials and Chemical Engineering, Xi’an Technological University, Xi’an 710021, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Binary hydroxides Microstructure Polyaniline Microwave absorption properties
Ni0.5Co0.5(OH)2 with flower-like structure@PANI was designed and synthesized via a two-step route. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) results illustrated that the Ni0.5Co0.5(OH)2 had a 3D flower structure with a homogeneous distribution. The Ni0.5Co0.5(OH)2 was coated by polyaniline (PANI) and embedded in polyaniline matrix. The as-synthesized Ni0.5Co0.5(OH)2@PANI possessed a BET specific surface area of 58.82 m2 g−1, and its pore diameter was in the mesoporous range (3.93 nm). Investigations of EM wave absorption properties indicated that the maximum absorption intensity of Ni0.5Co0.5(OH)2@PANI was as high as −39.8 dB at 6.4 GHz and the absorption bandwidth exceeding −10 dB was 3.1 GHz (4.9–8 GHz) with a matching thickness of 2.5 mm. The excellent EM wave absorption properties of the material were ascribed to its high permittivity and improved impedance matching effect. Therefore, it is anticipated that the Ni0.5Co0.5(OH)2@PANI composite would serve as a promising microwave absorber.
1. Introduction In recent years, with the rapid development of electronic devices and communication equipment, electromagnetic (EM) pollution is becoming a threat to human health and also affecting the operation of electronic instruments [1–3]. Therefore, it is essential to develop highperformance microwave absorption materials to solve the issues caused by EM wave interference [4]. According to the EM wave absorption principle, the microwave absorbers are comprised of magnetic and dielectric materials, such as carbon materials [5–7], ferrite [8,9] and metal oxides [10,11]. Compared with carbon materials, metal oxides have the advantages of low cost and chemical stability. However, they have some drawbacks such as high density and low permittivity, so they are not suitable as high performance absorbers which are required to have low thickness, light weight, wide absorption bandwidth and strong absorption intensity [12]. Recently, metals oxides with hierarchical and porous structures have attracted much attention as microwave absorption and shielding materials because of their unique properties [13]. Furthermore, the impedance matching between permittivity and permeability by microstructure design is also crucial to microwave absorption performance. For instance, Wang et al. reported the microwave absorption performances of Ni crystals with branch-like and flower-like shapes. The branch-like sample showed better microwave absorption properties (maximum reflection loss of −17 dB at 6 GHz) compared with the flower-like structure, due to its high
⁎
permeability and favorable impedance matching [14]. The mesoporous NiCo2O4 with a surface area of 54.47 m2 g−1 displayed a maximum reflection loss of −35.76 dB at 14.86 GHz [15]. Lu reported MOF-derived porous Co/C nanocomposites with the maximum reflection loss of −35.3 dB, which was attributed to synergetic effect and multiple scattering [16]. Based on the previous research, it is evident that in addition to dielectric loss and magnetic loss, the impedance matching and synergistic effect based on the size and shape of materials also significantly influence the microwave absorption properties [17–19]. Transition metal oxides, such as Co3O4 and NiO, have been reported as EM wave absorption materials [20,21]. However, there is no report to date on nickel and cobalt binary hydroxides as microwave absorption materials. Because of their three-dimensional structures, nickel and cobalt binary hydroxides possess larger specific surface area, which is beneficial to EM wave absorption performances [22]. Polyaniline (PANI) is an important conducting polymer which has drawn much attention in the field of EM wave absorption materials due to its controllable conductivity, low cost, light weight and easy production [23]. In this paper, we successfully prepared three dimensional flower-like Ni0.5Co0.5(OH)2@PANI by a two-step method. The addition of PANI can not only decrease the weight of the EM absorbing material, but also enhance the interfacial polarization and dielectric properties, leading to a proper impedance matching. Therefore, the flower-like Ni0.5Co0.5(OH)2@ PANI can be expected to exhibit better EM wave
Corresponding author. E-mail address: [email protected] (Y. Wang).
http://dx.doi.org/10.1016/j.materresbull.2017.10.004 Received 12 August 2017; Received in revised form 25 September 2017; Accepted 3 October 2017 Available online 04 October 2017 0025-5408/ © 2017 Elsevier Ltd. All rights reserved.
Materials Research Bulletin 98 (2018) 59–63
Y. Wang et al.
absorption properties at 2–18 GHz. The microstructure, morphology and microwave absorption performance of Ni0.5Co0.5(OH)2@PANI were systematically investigated. 2. Experimental To prepare the flower-like Ni0.5Co0.5(OH)2@PANI sample, first 1.454 g Ni(NO3)2·6H2O, 1.455 g Co(NO3)2·6H2O and 2.1 g hexamethylenetetramine (HMT) were dissolved in 50 ml deionized water at 10 °C for 20 min. The mixture was transferred into a 100 ml Teflon-lined autoclave and maintained at 120 °C for 5 h. After cooling down to room temperature, the sample was washed with ethanol and water to remove residual impurities, and dried at 100 °C for 8 h. Subsequently, the dried product was annealed in nitrogen atmosphere at 400 °C for 1 h in a tube furnace to form Ni0.5Co0.5(OH)2. Then, 0.25 g of the as-prepared Ni0.5Co0.5(OH)2 was dispersed in 100 ml acidic aqueous solution (1 M) and ultrasonicated for 30 min. Subsequently, 0.1 ml aniline was added to this mixture and stirred for 30 min. A solution of (NH4)2S2O8 (0.25 g) in 30 ml distilled water was slowly added to the above mixture with constant stirring for 12 h at 0 °C. Finally, the product was washed and dried to obtain Ni0.5Co0.5(OH)2@PANI. The crystal phase was detected by X-ray diffraction (XRD, Bruker D8, Germany, with Cu-Kα radiation) and the structure was analyzed by Fourier transform infrared spectroscopy (FTIR, Thermo Scientific Co. USA). The FTIR analysis was performed on a KBr diluted sample, with the KBr to sample weight ratio of about 100:1. The morphology and microstructure were characterized by FESEM (Quanta 600FEG) and TEM (JTM-2100). The N2 adsorption-desorption curve was measured on a Quad-rasorb-SI instrument, and the specific surface area was determined by BET process. The electromagnetic parameters of toroidal sample (Ni0.5Co0.5(OH)2 to paraffin weight ratio is 3:7) were recorded using a vector network analyzer (HP8720ES) at 2–18 GHz. The actual power level of incident microwave radiation was −20 ∼ 0 dBm. The reflection loss (RL) of the sample was calculated as follows [24,25]:
R(dB)=20 lg
Zin =
Zin − 1 Zin + 1
μr 2πfd tanh(j μ r εr ) c εr
Fig. 2. FTIR spectra of Ni0.5Co0.5(OH)2@PANI.
and Co(OH)2 (46-0605) standard samples and no impurity peaks were observed [26]. For Ni0.5Co0.5(OH)2@PANI (Fig. 1b), all diffraction peaks belonging to Ni0.5Co0.5(OH)2 were much weaker due to the PANI coating. The PANI peak (around 25°) was not detected, which may be due to the weaker intensity of its diffraction peak compared with Ni0.5Co0.5(OH)2. Apart from Ni0.5Co0.5(OH)2, no other peaks were detected, revealing that the Ni0.5Co0.5(OH)2@PANI was successfully synthesized. Fig. 2 shows the FTIR spectrum of Ni0.5Co0.5(OH)2@PANI. The characteristic bands at 1576, 1498 and 1394 cm−1 are associated with the C]N and C]C stretching vibrations of quinonoid and benzene rings, respectively [5]. The peaks at 1306 and 1142 cm−1 are ascribed to the CeN stretching of the aromatic amine and quinonoid moieties of doped PANI. The band at 806 cm−1 is related to the CeH out-of-plane bending [10], while the peak at 619 cm−1 is ascribed to the vibration of the metal ions and oxygen. All of the above reveal the existence of Ni0.5Co0.5(OH)2 in the composite. The morphology and microstructure of the as-synthesized samples were investigated by FESEM and TEM analyses. From the results shown in Fig. 3(a–c), it is seen that Ni0.5Co0.5(OH)2 has a homogeneous 3D flower-like structure with a size of 2–3 μm, which consists of numerous vertically-standing nanosheets with thickness of 20–30 nm. These loose nanosheets connect to each other to form homogeneous flower-like architecture, which is beneficial to multiple absorption and reflection of electromagnetic waves. As can be seen in Fig. 3(d–e), the exposed surfaces of Ni0.5Co0.5(OH)2 are not smooth and are coated with a layer of PANI, thus indicating the formation of a core-shell 3D structure, namely Ni0.5Co0.5(OH)2@PANI. The TEM images in Fig. 4(a–b) also display that Ni0.5Co0.5(OH)2 possesses a hydrangea-like structure and the particle size is in accordance with the results of FESEM analysis. From Fig. 4c, it is clearly seen that the Ni0.5Co0.5(OH)2 are dispersed in polyaniline and the hierarchical nanosheets of Ni0.5Co0.5(OH)2 become rough. Based on the above analysis, it is evident that the 3D flower-like Ni0.5Co0.5(OH)2@PANI with core-shell structure was successfully fabricated. The surface area and pore size were determined by N2 adsorptiondesorption method. It is observed from Fig. 5a that the as-prepared Ni0.5Co0.5(OH)2@PANI displays a typical type IV curve with a hysteresis loop at relative pressure between 0.45 and 1, revealing the existence of porous structure. As seen in Fig. 5b, the distribution of pore size is mainly around 4 nm, which indicates that flower-like Ni0.5Co0.5(OH)2@PANI is mesoporous (2–50 nm). The BET surface area, average pore diameter and pore volume of Ni0.5Co0.5(OH)2@PANI are determined to be about 58.82 m2 g−1, 3.93 nm and 0.31 cm3 g−1, respectively. The large surface area and pore volume of Ni0.5Co0.5(OH)2@ PANI may be ascribed to the presence of nanopores between the vertically-standing nanosheets, which suggests that the large BET surface area and high pore volume are important parameters for electromagnetic wave absorption materials. To evaluate the microwave absorption performances of
(1)
(2)
Where, Zin is the input impedance of the absorber, f is the microwave frequency, d is the thickness of the absorber, and c is the velocity of electromagnetic wave in vacuum. 3. Results and discussion The XRD pattern of Ni0.5Co0.5(OH)2 is shown in Fig. 1a. All characteristic peaks were found to match well with the Ni(OH)2 (38-0715)
Fig. 1. XRD pattern of Ni0.5Co0.5(OH)2 (a) and Ni0.5Co0.5(OH)2@PANI (b).
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Fig. 3. FESEM images of Ni0.5Co0.5(OH)2 (a-c) and Ni0.5Co0.5(OH)2@PANI (d, e).
where εs and ε∞ are static permittivity and relative dielectric permittivity at high frequency limit, respectively. According to equation (3), the curve of ε′ versus ε″ is a single semicircle and represents one Debye relaxation process. As shown in Fig. 6e, there are at least five semicircles, which shows that Debye relaxation process plays a crucial role in the enhanced dielectric properties of Ni0.5Co0.5(OH)2@PANI [27]. It is reported that magnetic loss is also a key factor influencing the EM wave absorption properties [18]. Generally speaking, magnetic loss derives from domain wall resonance, eddy current effect and natural resonance. The domain wall resonance usually occurs over the 1–100 MHz range [28], and thus domain wall resonance can be excluded in magnetic loss. If eddy current effect is the main contribution to magnetic loss, the values of C0 (C0 = μ″(μ′)−2f−1) should be constant between 2 and 18 GHz [29]. However, as shown in Fig. 6f, the values of Co fluctuate over 2–18 GHz. Therefore, we can conclude that the magnetic loss results from natural resonance of Ni0.5Co0.5(OH)2@PANI. According to electromagnetic wave principle, in addition to permittivity and permeability, impedance matching characteristics (Z = Zin Z0 ) is also a vital factor for microwave absorption performance [17,30]. If Z is close to 1, the superior microwave absorption properties can be obtained. The calculated Z values for Ni0.5Co0.5(OH)2@ PANI for the thickness of 2.5 mm are shown in Fig. 6g. It can be found that the relevant Z of Ni0.5Co0.5(OH)2@PANI is close to 1 at 2–18 GHz, revealing that a good impedance matching results in superior microwave absorption performance.
Ni0.5Co0.5(OH)2 and Ni0.5Co0.5(OH)2@PANI, the reflection loss (RL) curves were shown in Fig. 6(a–b). As illustrated in Fig. 6a, the Ni0.5Co0.5(OH)2 exhibits a maximum RL value of −14.9 dB at 5.9 GHz with a thickness of 3 mm. After PANI coating on the Ni0.5Co0.5(OH)2, the maximum RL for Ni0.5Co0.5(OH)2@PANI is as high as −39.8 dB at 6.4 GHz and the absorption bandwidth (RL < −10 dB) is 3.1 GHz (from 4.9 to 8 GHz) for a matching thickness of 2.5 mm, as seen from Fig. 6b. In order to elucidate the possible absorption mechanism, EM parameters of Ni0.5Co0.5(OH)2@PANI were investigated. Fig. 6c shows the relative complex permittivity and the relative complex permeability of Ni0.5Co0.5(OH)2@PANI. It can be seen that the ε′ values of Ni0.5Co0.5(OH)2@PANI decrease from 6.39-2.28 and the ε″ values vary between 0.71 and 1.94. It can be observed that the μ′ and μ″ values exhibit several variations between 2 and 18 GHz. From Fig. 6d, we can clearly see that the dielectric loss (tan δε) values of Ni0.5Co0.5(OH)2@ PANI are larger than those of magnetic loss (tan δμ), which implies that the dielectric loss is crucial to microwave absorption performances of Ni0.5Co0.5(OH)2@PANI. In addition to the dielectric loss mechanism, the microwave absorption performance also involves the relaxation process of Ni0.5Co0.5(OH)2@PANI, which can be depicted by Cole-Cole semicircle. For the Debye dipole relaxation, the relative complex permittivity can be expressed as follows [24]:
(ε′ −
εs + ε∞ 2 ε − ε∞ 2 ) + (ε ″)2 = ( s ) 2 2
(3)
Fig. 4. TEM images of Ni0.5Co0.5(OH)2 (a, b) and Ni0.5Co0.5(OH)2@PANI (c).
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Fig. 5. N2 adsorption-desorption isotherms (a) and pore diameter distribution (b) of Ni0.5Co0.5(OH)2@ PANI.
Fig. 6. Reflection loss curves of Ni0.5Co0.5(OH)2 (a) and Ni0.5Co0.5(OH)2@ PANI (b), complex permittivity, complex permeability (c), tangent loss (d), cole-cole semicircle curve (e), C0-f curve (f) and impedance matching (g) of Ni0.5Co0.5(OH)2@ PANI.
among branches helps enhance the interfacial polarization. Moreover, the interfaces and charge transfer between Ni0.5Co0.5(OH)2 and PANI can cause charge accumulation and enhanced interfacial polarization, which is advantageous for EM wave attenuation. More importantly, the high specific surface area and the large pore volume of Ni0.5Co0.5(OH)2@PANI can lead to multiple scattering and reflection of
The superior microwave dissipation route of Ni0.5Co0.5(OH)2@PANI is shown in Fig. 7. It is likely that the following absorption mechanism is involved: first, the flower-like structure of Ni0.5Co0.5(OH)2 with its unique hierarchical morphology facilitates the formation of numerous conductive networks within the structure and results in microwave energy attenuation [13]. Secondly, the formation of multi-interfaces 62
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[8]
[9]
[10]
[11]
[12] Fig. 7. A schematic representation for the possible dissipation route of EM wave in Ni0.5Co0.5(OH)2@PANI composite.
[13]
[14]
EM waves [31,32].
[15]
4. Conclusion In summary, Ni0.5Co0.5(OH)2@PANI with a novel flower-like architecture was fabricated by a two-step route. The Ni0.5Co0.5(OH)2 was firstly synthesized by one-step hydrothermal method, followed by sintering in tube furnace, and then PANI was grown around the Ni0.5Co0.5(OH)2 via in-situ polymerization. Studies of microwave absorption properties indicated that the 3D flower-like architecture possessed high specific surface area, large permittivity and improved impedance matching effect. The maximum RL values of Ni0.5Co0.5(OH)2@ PANI were as high as −39.8 dB at 6.4 GHz and the absorption bandwidth exceeding −10 dB was almost 3.1 GHz with a thickness of 2.5 mm, as a result of the multiple scattering and reflection of EM waves within the unique pore structure. Therefore, it is believed that the 3D flower-like architecture of Ni0.5Co0.5(OH)2@PANI can pave the way for the design of high-performance microwave absorbers.
[16]
[17]
[18]
[19]
[20]
[21] [22]
Acknowledgements
[23]
The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant No 61701386, No 51303147, No 51502233 and No 21506167), Natural Science Basic Research Plan in Shaanxi Province of China (Grant No 2017JQ5060, No 2016JQ5011) and President’s Fund of Xi’an Technological University (project No. XAGDXJJ16002).
[24]
[25]
[26]
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