Research Article www.acsami.org
Yolk−Shell Ni@SnO2 Composites with a Designable Interspace To Improve the Electromagnetic Wave Absorption Properties Biao Zhao,*,†,‡ Xiaoqin Guo,†,‡ Wanyu Zhao,§ Jiushuai Deng,∥ Gang Shao,§ Bingbing Fan,§ Zhongyi Bai,†,‡ and Rui Zhang*,†,§ †
Provincial Key Laboratory of Aviation Materials and Application Technology, Zhengzhou University of Aeronautics, Zhengzhou, Henan 450046, China ‡ School of Mechatronics Engineering, Zhengzhou University of Aeronautics, Zhengzhou, Henan 450046, China § School of Materials Science and Engineering, Zhengzhou University, Zhengzhou, Henan 450001, China ∥ State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Faculty of Land Resource Engineering, Kunming University of Science and Technology, Kunming, Yunnan 650093, China S Supporting Information *
ABSTRACT: In this study, yolk−shell Ni@SnO2 composites with a designable interspace were successfully prepared by the simple acid etching hydrothermal method. The Ni@void@SnO2 composites were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. The results indicate that interspaces exist between the Ni cores and SnO2 shells. Moreover, the void can be adjusted by controlling the hydrothermal reaction time. The unique yolk−shell Ni@void@SnO 2 composites show outstanding electromagnetic wave absorption properties. A minimum reflection loss (RLmin) of −50.2 dB was obtained at 17.4 GHz with absorber thickness of 1.5 mm. In addition, considering the absorber thickness, minimal reflection loss, and effective bandwidth, a novel method to judge the effective microwave absorption properties is proposed. On the basis of this method, the best microwave absorption properties were obtained with a 1.7 mm thick absorber layer (RLmin= −29.7 dB, bandwidth of 4.8 GHz). The outstanding electromagnetic wave absorption properties stem from the unique yolk−shell structure. These yolk−shell structures can tune the dielectric properties of the Ni@ air@SnO2 composite to achieve good impedance matching. Moreover, the designable interspace can induce interfacial polarization, multiple reflections, and microwave plasma. KEYWORDS: yolk−shell structure, Ni@void@SnO2, microwave absorption, electromagnetic parameter, interfacial polarization, multiple reflections
1. INTRODUCTION Nowadays, electromagnetic (EM) radiation pollution has become a serious threat, not only influencing the operation of electronic equipment, but also affecting the human health and raising problems regarding military application areas. Researchers have focused on the design and preparation of highly effective microwave-absorbing materials to address serious electromagnetic issue.1−5 The microwave absorption performances of the absorbent are largely related with the complex permeability and permittivity, as well as their synergetic match. These electromagnetic parameters can be effectively tuned by constructing magnetic and/or dielectric components. In particular, dielectric loss can be remarkably improved by suitable integration of specific microstructures6−10 and/or heterogeneous interfaces.11−15 Core−shell and yolk−shell structured materials are expected to be promising electromagnetic absorbers because of their multiple reflections and scattering dissipation. There are lots of © 2016 American Chemical Society
reports about microwave absorption of core−shell structured composites, such as Ni@polyaniline,16 Ni/polypyrrole,17 CuO/ Cu2O-coated Ni,18 Ni@TiO2,19 Ni/ZnO,20,21 and Ni@ carbon22 composites. In these core−shell composites, the magnetic Ni metals are expected as cores, which bring magnetic loss, and the dielectric or conductive materials are supposed to be shells, which can cause dielectric loss. The synergetic effects between magnetic loss and dielectric loss as well as interfacial polarization are favorable for enhancement of microwave absorption capabilities. A yolk−shell structure with a void is a special type of core−shell structure,23−27 and it has the features of low density, a designable interspace, a large surface area, and a functional interface. Compared with core−shell structures, the specific yolk−shell structures used as microwave absorbers have Received: August 30, 2016 Accepted: October 4, 2016 Published: October 4, 2016 28917
DOI: 10.1021/acsami.6b10886 ACS Appl. Mater. Interfaces 2016, 8, 28917−28925
Research Article
ACS Applied Materials & Interfaces
products were centrifugally separated and washed with ethanol and distilled water several times. Characterization. The morphology of the as-prepared products was performed in field emission scanning electron microscopy (FESEM, JEOL JSM-7001F) and transmission electron microscopy (TEM, JEOL JEM-2010). In detail, all of the samples were pasted on conductive tape and sputtered with gold before being analyzed by scanning electron microscopy. The samples dispersed at an appropriate concentration were placed on a copper grid before being analyzed by transmission electron microscopy. The crystal phases of the samples were determined by X-ray diffraction (XRD, Rigaku Ultima IV) using Cu Kα radiation (λ = 0.15418 nm). X-ray photoelectron spectroscopy (XPS) was carried out on a XSAM 800 spectrometer (Kratos Co., U.K.) with Al Kα excitation radiation (1486.6 eV). Fourier transform infrared (FTIR) spectroscopy was performed with KBr powder-pressed pellets on a Nicolet iS10 FTIR spectrometer. A vector network analyzer (Agilent, N5244A) was used to determine the permeability and permittivity with the transmission method in the frequency range 1−18 GHz for simulation of reflection loss. The tested samples were fabricated by uniformly mixing a certain amount of functional fillers with paraffin wax. The mixture was moved into a toroidal mold with an inner diameter of 3.0 mm and an outer diameter of 7.0 mm. The amount of yolk−shell Ni@void@SnO2 in the mixture was 50 wt %. In the coaxial wire measurement, the complex permeability and permittivity were calculated from the experimental scattering parameters S11 (or S22) and S12 (or S21) by Nicolson−Ross theoretical calculations.36
much superior properties, such as a light-weight structure, tunable dielectric properties, multiple reflections, and scattering, which satisfy the current requirements of absorbers (light weight, thin, high absorption, and wideband).28−30 The approaches for fabrication of yolk−shell structures are mostly the galvanic replacement reaction,31 the Kirkendall effect,32 and Ostwald ripening effects.33,34 It has been reported that yolk− shell structured Fe3O4@TiO2,25 Fe3O4@ZrO2,26 Fe3O4@ SnO2,27 and Fe3O4@copper silicate24 present improved electromagnetic wave absorption capabilities in comparison with pristine Fe3O4 microspheres. However, investigation of these yolk−shell structured materials has mostly concentrated on enhancing reflection loss without analysis of the mechanism of interspace and absorption adjustment. Therefore, an effective strategy to fabricate void−shell structures and investigate the void function is required. In this study, yolk−shell Ni@void@SnO2 composites with designable interspaces were successfully prepared by the simple acid etching method. Compared with Ni@TiO219 and Ni@ carbon22 composites materials, the yolk−shell Ni@void@SnO2 composites present some advantages. On one hand, the void between Ni and SnO2 can induce multiple reflection and scattering under alternated electromagnetic field. On the other hand, the designable void can tailor complex permittivity to obtain proper impedance match, which makes microwaves penetrate into absorbing materials as much as possible. Moreover, the void was introduced into the core−shell structure, which can meet the current absorber requirement of a light weight characteristic. The Ni@void@SnO 2 composites show excellent electromagnetic wave absorption. Moreover, we propose a new way to reasonably judge the efficiency of microwave absorption. The void can also cause microwave plasma under an alternating electromagnetic field, which is helpful for absorbing microwave energy.
3. RESULTS AND DISCUSSION The overall crystal structure and phase purity of the as-obtained yolk−shell structured Ni@void@SnO 2 composites were determined by XRD, as shown in Figure 1. The diffraction
2. EXPERIMENTAL SECTION Materials. All of the analytical reagents were commercially available and used without purification. Anhydrous ethanol, hydrochloric acid, and SnCl2·2H2O were purchased from Guangfu Chemical Reagent Technologies Co., Ltd. (Tianjin, China). NiCl2·6H2O, trisodium citrate dehydrate, sodium acetate, and glycerol were purchased from Xilong Chemical Reagent Co., Ltd. (Guangdong, China). NaH2PO2·H2O was obtained from Kemiou Chemical Reagent Co. Ltd. (Tianjin, China). Preparation of Submicrometer Ni Particles. The monodispersed submicrometer Ni particles were synthesized on the basis of our previous studies.3,35 In brief, sodium acetate (3.0 g), NiCl2·6H2O (1.2 g), and trisodium citrate dihydrate (0.2 g) were dissolved in the mixture of glycerol (30 mL) and distilled water (10 mL) at room temperature. The homogeneous suspension was then transferred into a Teflon-lined stainless steel autoclave. Sodium hydroxide (1.6 g) and NaH2PO2·H2O (3.2 g) dissolved in 20 mL distilled water was slowly brought into the autoclave. After reacting at 140 °C for 15 h, the solution was cooled to room temperature. The resultant product deposited on the bottom of autoclave was then rinsed with distilled water and absolute ethanol, and finally dried under vacuum at 60 °C for 12 h for further characterization. Preparation of Yolk−Shell Ni@void@SnO2 Composites. To obtain yolk−shell Ni@void@SnO2 composites, the as-received submicrometer Ni particles (0.5 g) were dispersed in a mixture of 40 mL of distilled water and 20 mL of anhydrous ethanol. SnCl2·2H2O (1 mmol) and hydrochloric acid (4 mL, 1.0 M) were then successively introduced to the above mixture with stirring for 0.5 h. The obtained solution was then transferred to a Teflon-lined autoclave, and the hydrothermal reaction was carried out at 200 °C for 15 h. The
Figure 1. XRD curve of as-obtained yolk−shell structured Ni@void@ SnO2 composites.
peaks at 2θ = 26.7°, 33.9°, 38.0°, and 52.0° match well with the (110), (101), (200), and (211) reflections of the standard XRD pattern of tetragonal rutile SnO2 (JCPDS no. 41-1445), whereas the peaks at 2θ = 44.7°, 52.0°, and 76.6° can be attributed to the (111), (200), and (220) planes of Ni (JCPDS no. 04-0850). The XRD results reveal that the yolk−shell composites are composed of crystalline SnO2 and Ni. No diffraction peaks from other materials are observed, indicating the high purity of the products. Moreover, the broad diffraction peaks of SnO2 indicate that the SnO2 crystals are small. The morphologies and microstructures of the yolk−shell Ni@void@SnO2 products were clarified by FESEM observations. The inset in Figure 2a exhibits a FESEM image of the submicrometer Ni particles. It shows that the Ni particles are irregular walnut-like spheres with a relatively narrow size distribution and a mean diameter of 500 nm. A panoramic FESEM image of the as-prepared Ni@void@SnO2 products is shown in Figure 2a. Compared with pure Ni walnut-like particles, the Ni/SnO2 composites have smoother surfaces and 28918
DOI: 10.1021/acsami.6b10886 ACS Appl. Mater. Interfaces 2016, 8, 28917−28925
Research Article
ACS Applied Materials & Interfaces
Figure 3. (a, b) Different magnification TEM images of yolk−shell Ni@void@SnO2 composites, (c) HRTEM image, and (d) SAED image taken from rectangle area in part b.
0.33 and 0.27 nm could be assigned to the (110) and (101) planes of the rutile SnO2 phase, respectively. The SAED pattern recorded from the rectangle area in Figure 3b also confirms that the layer shell has high crystallinity and can be assigned to the rutile SnO2 crystal. From the above analysis, we can conclude that the yolk−shell composite is composed of a Ni core, an outer SnO2 shell, and an interspace between the core and the shell. It should be mentioned that this unique yolk−shell structure with an interspace is helpful for improvement of electromagnetic wave attenuation, which will be discussed in the following paragraph. XPS is supposed to be a sensitive and powerful instrument to investigate the surface chemical compositions of materials. Figure 4a shows the Sn 3d XPS spectrum of the yolk−shell Ni@void@SnO2 composite. The two main peaks at 495.1 and 486.7 eV can be assigned to Sn 3d3/2 and Sn 2p5/2, respectively. These are in good accordnce with the mentioned values for SnO2.37 The inset in Figure 4a gives the XPS data for the valence of Ni. The peaks located at 855.3 and 861.4 eV are close to the binding energies of Ni 2p3/2 and Ni 2p1/2, which indicates zerovalent Ni.38,39 The FT-IR pattern of the yolk− shell Ni@void@SnO2 composite is shown in Figure 4b. The peak around 610 cm−1 corresponds to the stretching vibration of the Sn−O bond, which is in agreement with previous studies.40,41 These results further verify that the yolk−shell composite consists of Ni and SnO2. To reveal the growth process of the yolk−shell structured Ni@void@SnO2 composites and the possible growth mechanism, the morphology evolution of the yolk−shell Ni/SnO2 composite with reaction time was investigated at 200 °C while the other reaction conditions were kept constant. The corresponding results are shown in Figure 5. With a hydrothermal time of 3 h, the product is entirely composed of core−shell Ni@SnO2 composites with Ni cores and rough shells of SnO2 nanoparticles (Figure 5a). In this stage, hydrochloric acid begins to erode the surfaces of the submicrometer Ni particles and provides sites for subsequent SnO2 nucleation. When the hydrothermal process is increased to 7 h, core−shell Ni@SnO2 composite microspheres with relatively smooth SnO2 shells are obtained (Figure 5b). Interestingly, from the structure of some broken core−shell structured composite microspheres, there is a clear interspace
Figure 2. (a) Low-magnification and (b) high-magnification FESEM images of as-prepared yolk−shell Ni@void@SnO2 composites. Inset is the FESEM photograph of submicron Ni particles.
a similar size, which indicates that the Ni particles are consumed and surrounded by other products. In combination with the XRD results, it can be deduced that core−shell Ni@ SnO2 composites with Ni cores and SnO2 shells can be obtained by this method. In the high-magnification FESEM photograph of the core−shell composites (Figure 2b), the interspace between the Ni core and the SnO2 shell can be obviously observed in some broken core−shell composites. Therefore, according to the FESEM and XRD results, we can conclude that yolk−shell Ni@void@SnO2 composites were successfully synthesized using this simple, low-cost, and benign method. The structure was further examined by TEM. The TEM images in Figure 3a,b show that the shape of the product is similar to that observed by FESEM. The TEM images clearly show that the composite microspheres hold a unique yolk− shell structure, with a dark particle encapsulated in a separate gray shell and a clear interstitial void between the core and the shell (Figure 3a). From the high-magnification TEM image (Figure 3b), the diameter of the dark Ni core is estimated to be ∼300 nm, which is smaller than that of raw Ni walnut-like particles. The decrease of the dark Ni core is attributed to the corrosion reaction between hydrochloric acid and magnetic Ni. To further confirm the constituents of the shell layer, a HRTEM image and selected-area electron diffraction (SAED) pattern obtained from rectangle areas of the shell layer are shown in Figure 3c,d, respectively. As shown in Figure 3c, the HRTEM image of the rectangle area in Figure 3b confirms that the layer shell has high crystallinity. The interplanar spacings of 28919
DOI: 10.1021/acsami.6b10886 ACS Appl. Mater. Interfaces 2016, 8, 28917−28925
Research Article
ACS Applied Materials & Interfaces
acid. In this process, the reaction can be expressed by the equations 2H+ + Ni → Ni2+ + H2↑, 2Sn2+ + 2H2O + O2 → 2SnO2 + 4H+. At first, the surfaces of Ni walnut spheres were eroded by hydrochloric acid, which is beneficial for subsequent nucleation of SnO2 nanoparticles. Moreover, the SnO2 was generated through oxidation of the Sn 2+ by the acid of dissolving oxygen and existed stably under the acid condition, which was reported in a previous paper.43 The electromagnetic absorption abilities of the absorbing materials are mainly determined by the relative complex permeability (μr = μ′ − jμ″) and permittivity (εr = ε′ − jε″), in which the real parts (ε′ and μ′) stand for the storage ability of the electric and magnetic energy, and the imaginary parts (ε″ and μ″) are associated with the attenuation abilities of electric and magnetic energy, respectively.19,44,45 Figure 6a exhibits the
Figure 4. (a) XPS spectra of Sn 3d and (b) FTIR spectra of the asprepared yolk−shell Ni@void@SnO2. Inset is the XPS spectra of Ni 2p of yolk−shell Ni@void@SnO2.
Figure 6. (a) Relative complex permittivity (εr = ε′ − jε″) of yolk− shell Ni@void@SnO2 composites and (b) the Cole−Cole plots of the complex permittivity in the dielectric phenomenon.
frequency dependence of the relative complex permittivity for the yolk−shell Ni@void@SnO2 composites. The ε′ values present a decreasing tendency in the measured frequency of 1− 18 GHz. Furthermore, the ε′ values are in the range 7.7−8.1 with little fluctuation. It is noteworthy that the ε″ values show an increasing tendency and strong fluctuation compared with the ε′ values. Importantly, multiple resonance peaks are found in the ε″ values. The resonance peaks are maybe correlated with the interfaces between Ni and air, SnO2 and air, and Ni and SnO2, which are because of the displacement current lag caused by the interfaces.46 On the basis of the above analysis, the combined losses of the dipole polarizations and interfacial polarizations are suggested to account for the dielectric loss.47−49 The dipole polarizations probably originate from defects in the Ni and SnO2 materials, while the interfacial polarizations come from the existence of plentiful interfaces among Ni, air, and SnO2. Smart design of the interspace between Ni and SnO2 leads to a lot of interfaces, which induce interfacial polarization as well as relaxation that can lead to dielectric loss. In general, the relaxation process, which is expressed by a Cole−Cole semicircle,50,51 can account for the permittivity
Figure 5. FESEM photographs of the Ni/SnO2 composites synthesized under different hydrothermal times: (a) 3, (b) 7, (c) 11, and (d) 15 h.
between the Ni cores and the SnO2 shells, which indicates that yolk−shell Ni@void@shell composites formed. In this stage, metal Ni is further corroded by hydrochloric acid, and relatively smooth SnO2 shells form by dissolution−recrystallization because of the decrease of the surface energy of the SnO2 nanoparticles. Further increasing the reaction time to 11 and 15 h results in high-yield yolk−shell Ni@void@shell composite microspheres, as shown in the overview FESEM images (Figure 5c,d). From the above results, we can conclude that unique yolk−shell Ni@void@shell composites can be obtained with an appropriate hydrothermal reaction time. Moreover, the size of the interspace between the core and shell can also be tuned by adjusting the corrosion reaction42 between Ni and hydrochloric 28920
DOI: 10.1021/acsami.6b10886 ACS Appl. Mater. Interfaces 2016, 8, 28917−28925
Research Article
ACS Applied Materials & Interfaces behavior of electromagnetic wave-absorbing materials. On the basis of Debye dipolar relaxation,52−54 the relative complex permittivity (εr) could be described by the following equation: εr = ε′ − jε″ = ε∞ +
εs − ε∞ 1 + j 2πfτ
(1)
Here, εs, τ, and ε∞ are the static permittivity, polarization relaxation time, and relative dielectric permittivity at the highfrequency limit, respectively. From eq 1, it can be deduced that ε′ = ε∞ +
ε″ =
εs − ε∞ 1 + (2πf )2 τ 2
(2)
2πfτ(εs − ε∞) 1 + (2πf )2 τ 2
(3)
On the basis of eqs 2 and 3, the relationship between ε′ and ε″ is ⎛ εs + ε∞ ⎞2 ⎛ εs − ε∞ ⎞2 2 ⎜ε′‐ ⎟ + (ε″ ) = ⎜ ⎟ ⎝ ⎝ 2 ⎠ 2 ⎠
Figure 7. (a) Relative complex permeability (μr = μ′ − jμ″) and (b) eddy current curve of yolk−shell Ni@void@SnO2 composite.
(4)
If we plot ε′ against ε″, it may be a single semicircle, which is denoted as a Cole−Cole semicircle, and each semicircle is associated with one Debye relaxation process. A plot of ε″ against ε′ for yolk−shell Ni@void@SnO2 is shown in Figure 6b, in which several Cole−Cole semicircles are observed. This indicates that the mechanisms of permittivity dispersion can be described by the Debye dielectric relaxation model (Cole−Cole model). The existence of interfaces in this heterogeneous composite would cause interfacial polarization.55 However, the Cole−Cole semicircles are distorted, which indicates that, besides the Debye relaxation, other mechanisms also exist in the yolk−shell Ni@air@SnO2 composites, such as Maxwell− Wagner relaxation and electron polarization.56 The former behavior happens in heterogeneous media due to the accumulation of charges at the interfaces. In yolk−shell structured composites, the existence of interfaces leads to interfacial polarization under an alternating electromagnetic field (Maxwell−Wagner effect). Figure 7a shows the relative complex permeability of the yolk−shell Ni@void@SnO2 composite. The μ′ values are relatively constant with a range 1.10−1.14, which indicates maintenance of steady μ′ values and impedance matching properties at high frequency. Interestingly, as with the μ″ values, three obvious resonance peaks are observed in the measured frequency range 1−18 GHz. Conventionally, the resonance can be assigned to natural resonance at low frequency,57,58 whereas other resonances would originate from nonuniform exchange resonance modes at high frequency.59,60 The magnetic loss can be explained by hysteresis loss, domain-wall resonance, the eddy current effect, natural resonance, and exchange resonance.61 The magnetic hysteresis coming from irreversible magnetization is only generated in a strong magnetic field. The domain wall resonance originating from multidomain materials is only existed in the low-frequency range (
