Enhanced Polarization from Hollow Cube-like ZnSnO3 wrapped by

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Functional Nanostructured Materials (including low-D carbon)

Enhanced Polarization from Hollow Cube-like ZnSnO3 wrapped by MWCNTs: As a Lightweight and High-performance Microwave Absorber Lei Wang, Xiao Li, Qingqing Li, Yunhao Zhao, and Renchao Che ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05414 • Publication Date (Web): 12 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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ACS Applied Materials & Interfaces

Enhanced Polarization from Hollow Cube-Like ZnSnO3 Wrapped by MWCNTs: As a Lightweight and High-Performance Microwave Absorber Lei Wang, Xiao Li, Qingqing Li, Yunhao Zhao and Renchao Che* Laboratory of Advanced Materials, Department of Materials Science and Collaborative Innovation Center of Chemistry for Energy Materials (iChem), Fudan University, Shanghai 200438, P. R. China * E-mail: [email protected]

ABSTRACT: Polarization and conduction loss play fundamentally important roles in the non-magnetic microwave absorption process. In this paper, uniform and monodisperse hollow ZnSnO3 cube wrapped by multi-wall carbon nanotubes (ZSO@CNTs) were successfully synthesized via a facile hydrothermal treatment. A reasonable mechanism related to Ostwald ripening was proposed to design the varied ZSO@CNTs for the special hollow conductive network. Scanning electron microscopy images clearly indicate that reaction temperature is the key factor for the composite structure, which has a significantly effect on its electromagnetic properties. Electron holography proves the inhomogeneous distribution of charge density in the ZSO@CNTs system, leading to the occurrence of interface polarization. Complex permittivity properties of ZSO@CNTs composites under different reaction temperatures were investigated to optimize the morphology that can distinctly enhance microwave absorption performance. The maximum reflection loss of ZSO@CNTs130°C composite can reach −52.1 dB at 13.5 GHz and the absorption bandwidths range from 11.9 GHz to 15.8 GHz with a thickness as thin as 1.6 mm. Adjusting the simulation thicknesses from 1 to 5 mm, the efficient absorption bandwidth (RL< -10 dB) of ZSO@CNTs composite could reach 14.16 GHz (88.8% of 2-18 GHz). The excellent microwave absorption performance may attribute to the synergistic effects of polarization, conduction loss and special hollow cages structure. It is proposed that the

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specially controlled structure could provide an effective path to achieve highperformance microwave absorber. KEYWORDS: ZnSnO3, microwave absorption, MWCNTs, hollow structure, polarization, electron holography.

1 Introduction Microwave absorption (MA) materials have been attracting considerable attention because of their broad prospects for the defense and civilian fields, such as electronic signal shielding, electromagnetic contamination, and stealth camouflage applications. 1-4

Generally, MA materials are basically classified into two types: magnetic loss (Fe,

Co, Ni, Fe3O4)

5-8

and dielectric loss (carbon, semiconductor and conductive

polymers).9-12 For the non-magnetic system, polarization and dielectric loss play important roles in the process of dissipating microwave energy. 13, 14 Facing the electromagnetic signal with various wavelength, single component of the absorber, especially for the carbon material, cannot meet the wide-band frequency response request. Thus, various strategies have been explored to decorate multi-walled carbon nanotube (MWCNTs) to improve their MA properties by enhancing their polarization dipoles and electronic conductive transportation. Cao and co-workers introduced ZnO nanocrystals on the surface of Multi-wall carbon nanotubes to adjust their interface polarization and permittivity. The high concentration of the MWCNTs easily leads to form a conductive network, which is favor to the microwave absorption.15 Tong and co-workers synthesized Ni microspheres coated with oriented multiwall carbon nanotubes, reporting that such radial nanostructures can generate special orientation and interface polarizations and enhance dielectric losses.16 Through tuning complex permittivity and conductivity, Robert and co-workers fabricated an ultralight multi-walled carbon nanotube (MWCNTs)/graphene foams via regulating MWCNT loading and thermal reduction temperature for the lightweight microwave absorption materials.17 Yin et al synthesized graphene-wrapped ZnO absorber and discussed the electrical conductivity and polarization between graphene sheets and ZnO nanoparticles.18 Briefly, the complex of carbon nanotubes can provide effective methods for obtaining efficient microwave absorbers. Besides, the optimized morphology occupies essential factor in tuning electromagnetic parameter and ameliorating MA capacity. From this perspective, a


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large number of materials with special structures are studied, such as the highly graphitized carbon, spheres, nanocoils, nanotubes, and two-dimensional graphene oxide.19-22 Wei and co-workers reported a self-assembled helical hollow polymer superstructure with enhanced MA properties.23 Xing and co-workers synthesized porous flower-like NiO@graphene composites to clarify the impact from 3D porous structure.19 Che and co-workers have successfully prepared a series of Co20Ni80 hierarchical structures with different surface morphologies, including flower-, urchin-, ball-, and chain-like shapes.24 Definitely, dielectric materials with designed morphologies become an effective path to improve microwave absorption capability, which still a great challenge for actual MA absorption. As an important multifunctional material, zinc stannate (ZnSnO3) have been extensively explored and applied in various applications, including piezoelectric, gas sensors, Li-ion batteries, electronics materials and nanogenerators.25-28 ZnSnO3 belongs to an intrinsically dielectric crystal, while the dominant polarization and the associated Debye relaxation could contribute to the microwave absorption. Inaguma and coworkers investigated the dielectric properties of a polar LiNbO3-type ZnSnO3, which was synthesized under high-pressure and discussed the relationship between its structure and complex permittivity in low frequency range.29 Due to the specials perovskite octahedral structure and ion position, more polarization occurred along the c-axis than other direction under electromagnetic imparting. Defect-dipoles generated by the oxygen vacancies (c-axis) also could improve the conductivity. According to the Debye theory, permittivity imaginary parts (εʺ) represents dielectric loss ability resulted from the remarkable polarization and conductivity (σ). In other words, high polarization and matching conductivity can enhance the electromagnetic attenuation ability. The design of composites constructed by MWCNTs and ZnSnO3 might be an effective way to achieve a lightweight and highly efficient MA material, in which MWCNTs has adjustable conductivity, high surface area, unique dielectric properties, onedimensional tubular character and plenty of interfaces. In this paper, we designed a facile and efficient strategy for MWCNTs wrapped hollow cube-like ZnSnO3 particle via a hydrothermal treatment. Tuning structure and electromagnetic parameters of ZSO@CNTs composites under different reaction temperatures were investigated to pursuit the ideal morphology that could enhance MA performance. As a result, all the ZSO@CNTs exhibits an enhanced MA capacity



compared to that of pure ZnSnO3 powder. The morphology caused by reaction temperature has a significantly influence in the complex permittivity. The excellent MA absorption performance was ascribed to the polarization loss and the conductive loss. While, the synergistic effect, special hollow structure, and reflection in the MWCNTs wrapped cube-like ZnSnO3 system also benefit to microwave absorption. Consequently, the synthesized ZSO@CNTs composite is a potential candidate for a highly efficient MA material. 2 EXPERIMENTAL SECTION 2.1 Materials. MWCNTs were provided by Chengdu Organic Chemicals Co., Ltd. (China). Raw MWCNTs were refluxed at 120 °C for 12h in the HNO3 solution. All the chemical reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. Tin (chloride pentahydrate) SnCl4·5H2O, Zinc nitrate hexahydrate Zn(NO3)2·6H2O and ammonia solution NH3·H2O were of analytical grade and used without further purification. Deionized water obtained from a milli-Q system was used throughout. 2.2 Synthesis of ZSO@CNTs composites The ZSO@CNTs composites were prepared by a hydro-thermal process described as follows. Acid-treated MWCNTs 40 mg were dispersed in 60 mL distilled water with ultrasonic 2 h, then directly added with 1 mmol SnCl4·5H2O and 1 mmol Zn(NO3)2·6H2O. After 30 min of stirring treatment, ammonia solution (25 wt%) was added drop-wise into the reaction mixture to adjust pH until to 10. The mixed solution was transferred into a Teflon-lined stainless steel autoclave, and maintained at 100 °C, 130 °C and 160 °C for 15 h, respectively. Finally, the ZSO@CNTs composite was collected by centrifugation, washed with DI water and ethanol, dried, and signed as ZSO@CNTs-100 °C, ZSO@CNTs-130 °C, ZSO@CNTs-160 °C, respectively. 2.3 Characterization The chemical composition of the product was characterized using powder X-ray diffraction (XRD) (XRD, Bruker, D8-Advance X-ray diffractometer, Germany) with Ni-filtered Cu Ka radiation, l = 1.5406 Å, 40 kV, 40 mA. The morphology and size of the ZSO@CNTs samples were examined by a field-emission scanning electron microscope (FESEM, S-4800) and a field-emission transmission electron microscope


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(TEM, JEOL, JEM-2100F, 200 kV). The X-ray Photoelectron Spectroscopy (XPS) measurements were recorded on KRATOS Axis Ultra Dld equipped with a monochromatic X-ray source (Al Kα, hv = 1486.6 eV). Before the spectra acquisition the samples were pelletized and outgassed for 1 h at 50 °C, and the pressure for the analysis chamber is 10−9 mbar. Raman spectra were recorded at room temperature in ambient conditions on a Renishow Invia spectrometer equipped with a Leica DMLM confocal microscope and a CCD detector with a 514 nm laser excitation. A HP8510C vector network analyzer measured electromagnetic parameters over the 2–18 GHz range. The measured samples were prepared by uniformly mixing the absorbents with paraffin matrix according to a same mass fraction of 50% and compacted into a columnar ring of 7.00 mm outer diameter and 3.04 mm inner diameter. 3 RESULTS AND DISCUSSION The synthesized process of ZSO@CNTs is described in Fig.1. First, suspension solution containing well dispersed acid-treated MWCNTs was added with SnCl4·5H2O, Zn(NO3)2·6H2O. Then ammonia is added to the solution to react with the Sn4+ ions to form SnO32-, while ammonia also react with the Zn2+ to form Zn(OH)2 and the precipitation will continue to complex with NH4+ and transform to [Zn(NH3)4]2+, which will release Zinc ions. Finally, the Zn2+ react with SnO32- to synthesize ZnSnO3 nanoparticle as a cube nuclei with small size (Equation:1-3). Because of the key role of hydrogen bonding, the ZnSnO3 nanocrystals were adhered to the surfaces of MWCNTs. Thus, during the self-assembled Oswald ripening process, the large cube-like ZnSnO3 with ～400 nm size are successfully obtained.30-32 Zn2+ + 𝑁𝐻3 ∙ 𝐻2 𝑂 → Zn(𝑂𝐻)2 ⋯ [𝑍n(𝑁𝐻4 )]2+ → Zn2+

(1)

Sn4+ + 𝑁𝐻3 ∙ 𝐻2 𝑂 → SnO2− 3

(2)

Zn2+ + SnO2− 3 → ZnSnO3

(3)

The structures and phase information were conﬁrmed by X-ray diffraction (XRD, Fig. 2). All of the high intensity diffraction peaks can be well attributed to ZnSnO3. The typical peaks at 2θ = 19.62°, 22.72°, 32.41°, 40.04°, 46.63°, 52.42° and 57.71° are assigned to the (111), (200), (220), (222), (400), (420) and (422) planes of the perovskite ZnSnO3 (JCPDS: 11−0274), respectively, and the sharp peaks confirm the



large size and high purity of ZnSnO3 crystals. Weak diffraction peak at 26.5° from the graphitized MWCNTs can also be observed from the inset. Hence, the special composite of ZnSnO3 wrapped by CNTs were fabricated. Fig. 3 shows the Raman spectrum of ZSO@CNTs composites. The intensity ratio of D-band to G-band (ID/IG) is usually employed to evaluate the order graphitization degree of carbon atoms. The D-band is a breathing mode of A1g symmetry involving phonons near the K zone boundary, which cannot be detected in perfect graphite and becomes active in the presence of disorder or finite-size crystals of graphite. Corresponding to the E2g mode, the G-band was caused by the stretching vibrations of the sp2 bond, which can be produced by all sp2 sites.33As shown in Fig.3, two distinguishable peaks come from MWCNTs assigned to D- and G-bands at about 1320 cm-1 and 1570 cm-1. The intensity value ID/IG increases from 1.12 to 1.19 with the increased reaction temperature, revealing the increased degree of defects in the ZSO@CNTs composites. It may be attributed to the interfacial interaction between ZnSnO3 and MWCNTs, which may enhance the microwave absorption performance. The morphology and structure of the ZSO@CNTs samples were characterized by SEM and TEM. The ZnSnO3 particles possess a uniform cubic shape with an average size about 400 nm and exhibits a smooth surface (Fig. 4a, b). Wrapped by the MWCNTs, the cubic ZnSnO3 particles can be considered as a linker to bind individual MWCNTs to form a spatial network. As the reaction temperature was increased to 130 °C, the morphology of ZnSnO3 changed significantly. Cracks and voids began to appear on the surface, while the internal solid interior turned into a partially hollow nanocage (Fig, 4c, d). Due to the severe thermal environment derived from the high reaction temperature of 160 °C, the structural stability deteriorates and the block begins to collapse, leading to a large number of small units are separated from the original cubes (Fig.4e, f). The elemental maps of Zn, Sn, and O (Fig.S2 Support Information) demonstrate a remarkably homogeneous distribution. EDS quantitative analysis indicates that the atomic ratio of Zn, Sn and O is 1:0.74:2.24, suggesting certain degree of oxygen vacancy in ZnSnO3 crystal (Fig.S1). The involvement of MWCNTs help our ZnSnO3 nanocrystals becomes more dispersed, which can facilitates the block-free transportation of microwave.


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Uniform hollow ZnSnO3 cubes were synthesized via a facile and hydrothermal method with a controlled temperature. The well-resolved crystal lattice fringes with dspacing values of 0.276, 0.390 and 0.452 nm can be assigned to the (220), (200) and (111) planes of the ZnSnO3, respectively.34,35 Lattice fringes also can be clearly distinguished with a spacing of 0.34 nm, which corresponds to MWCNTs.36 Although the MWCNTs exist on the surface of the cube (Fig. 5a, b, TEM), the ZnSnO3 particles still can maintain its special cubic shape. Therefore, plenty of interfaces between the ZnSnO3 and MWCNTs can be formed in this framework (Fig.5c, d, e, f). X-ray photoelectron spectroscopy (XPS) is an effective way to detect the element composition and chemical valence state (Fig. 6). Fig. 6a shows the wide span spectra of ZSO@CNTs-130°C composites, indicating the existence of C, O, Sn and Zn elements. In Fig. 6c, the spectrum of C1s shows four sub-peaks at 284.6 eV, 285.6 eV, 286.7 eV and 288.9 eV, corresponding to C–C or C=C from the carbon skeleton, C–O, C=O, and O–C=O groups of acid MWCNTs, respectively.37,38 Fig. 6d exhibits the spectrum of Zn in ZSO@CNTs-130°C composites. The peaks at 1025 eV and 1045 eV are attributed to Zn 2p3/2 and Zn 2p1/2. Herein, the splitting energy between Zn 2p3/2 and Zn 2p1/2 is measured to be 23 eV, suggesting that the Zn ions are present in the materials. The Sn peaks loaded at 487.5 eV and 495.9eV (Fig. 6 e) can be assigned to Sn 3d5/2 and Sn 3d3/2 with an energy gap of 8.4 eV. The spin–orbit splitting can be described as the existence of Sn ions in the ZnSnO3 lattice.39 In terms of chemical valence state of O element, the O 1s peaks at 531.0 eV, 531.6 eV and 532.8 eV can be referred to Zn–O or O–Sn–O group respectively (Fig. 6f). Both XPS and XRD results definitely confirms our ZnSnO3 crystals belong to perovskite phase (Fig. 6b), which might contribute dielectric polarization for the incident microwave. Generally, the electromagnetic wave absorption properties are highly depended on the complex permittivity and complex permeability. The real parts of complex permittivity (εʹ) and complex permeability (μʹ) indicate the storage capability of electric and magnetic energy, while the imaginary parts (εʺ, μʺ) represent the loss capability of electric and magnetic energy.40-42 To analyze the impacts of chemical composition and microstructure on the microwave absorption property, complex permittivity and complex permeability of ZSO@CNTs were measured by a vector network analyzer (Fig. 7).



The real permittivity (εʹ) values of three ZSO@CNTs materials decreased sharply from 26.7 to 14.4, 17.5 to 12.1 and 12.3 to 8.2 in the 2–12 GHz range, respectively (Fig. 7a). However, a noticeable valley peak occurred at ~9 GHz and the decreased tendency with the increased frequency follow the general rule of a typical dielectric system. The higher permittivity real parts may dominate more energy storage and polarization ability. Similarly, the imaginary part (εʺ) of ZSO@CNTs composites exhibits relatively smooth downward trend as the frequency less than 8 GHz. Interestingly, the εʺ values exhibit obvious Debye dielectric relaxation peaks in the 8~9 GHz and 14-16 GHz (Fig. 7b). Relaxation peaks occurred at 8-9 GHz may be induced by the defect dipole polarization onside the surface of MWCNTs and the partially collapsed ZnSnO3 cube. Some other peaks could be contributed from the abundant number of interfacial polarization in ZSO@CNTs composites.

43,44

When ZnSnO3 particles maturate to a

certain extent, the ZSO@CNTs-100°C composites maintains a complete structure without being destroyed. However, the relatively low dielectric constant indicates the poor electromagnetic storage and loss capacity of the material, especially the lower imaginary part value (~3) exposes to the weak conductivity and polarization. Increasing reaction temperature, there have a significantly change happened at ZSO@CNTs130°C. Cracks and voids began to appear on the surface and internal solid interior turned into a partially hollow shape. As Debye theory says, electromagnetic wave attenuation ability is greatly improved from the increased imaginary part (εʺ) and optimized dielectric loss tangent value (Fig. S3). Due to the severe thermal environment, the structural stability deteriorates and the block begins to collapse, leading to a large number of small units are separated from the original cubes and exposing more MWCNTs surfaces to the free space. Undesired high dielectric parameters will lead to skin effect, resulting electromagnetic waves reflected at ZSO@CNTs-160°C surfaces. Real part (μʹ) and imaginary part (μʺ) of permeability at the 2–18 GHz of the ZSO@CNTs samples with elevated temperature range were shown in Fig. 7c, d. Both μʹ in the range of 0.95 to 1.15 and the μʺ values exhibit a slightly decreasing trend. The similar low values of μʹ and μʺ display limited magnetic storage and loss. As the absence of magnetic components, ZSO@CNTs failed to generate strong magnetic loss ability, resulting that its complex permeability are close to 1 and 0, respectively.45,46 Meanwhile, it can be easily observed from the loss tangent that the tan δe values are much higher than that of tan δm, further verifying that ZSO@CNTs is a dielectric loss type microwave absorbing material (Fig.S3).


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The electromagnetic wave absorption properties of absorbers were evaluated with reflection loss (RL), which is calculated using relative complex permittivity and permeability according to transmit line theory. RL is defined in the following equations:47,48 2𝜋𝑓𝑑

Z = |𝑍𝑖𝑛 /𝑍0 | = √|𝜇𝑟 /𝜀𝑟 |𝑡𝑎𝑛ℎ [𝑗 (

𝑐

) √𝜇𝑟 𝜀𝑟 ]

RL = 20log|(𝑍𝑖𝑛 − 𝑍0 )/(𝑍𝑖𝑛 + 𝑍0 )|

(4) (5)

where Zin is the normalized input impedance of absorber, Z0 is the impedance of free space, εr is the complex permittivity, μr is the complex permeability, f is the frequency, c is the light velocity, and d is the thickness of the absorber, respectively. The reflection loss values of the ZSO@CNTs materials were shown in the Fig. 8. It can be seen that all the ZSO@CNTs materials display an excellent microwave absorption performance, the minimal RL value of ZSO@CNTs-100°C reach -17.4 dB at 8.2 GHz with only 2 mm thickness (Fig. 8a). When the hollow structure occurred inside the ZnSnO3 cube, the ZSO@CNTs-130°C reveals an excellent MA property in terms of minimum reflection loss, thinnest thickness and widest absorption bandwidth. When the thickness is only 1.6 mm, the RLmin can reach 52.1 dB at 13.5 GHz and the efficient absorption bandwidths (≤ 10 dB) covered 4 GHz from 11. 8 GHz to 15.8 GHz (Fig. 8b). Moreover, adjusting the simulation thicknesses from 1 to 5 mm, the efficient absorption bandwidth of ZSO@CNTs-130°C composite could reach 14.16 GHz from 3.5 to 18 GHz (Fig.S4). However, with the temperature increasing, the ZSO@CNTs160°C composites exhibited a clear morphology change with plenty of broken cubelike structure. The RLmin can reach -38.6 dB at 7.6 GHz with a 3.5 mm thickness (Fig. 8c). Compared with all ZSO@CNTs materials, we found that ZSO@CNTs-130°C composites could maintain excellent microwave absorption capacity at a thin thickness, which is cater to the requirements for modern MA materials (Fig. 9b). Some typical MWCNTs-based materials display their corresponding electromagnetic loss properties with varied thickness shown in Fig.9. The maximum reflection loss value of ZSO@CNTs-130 °C composite can reach −52.1 dB at 13.5 GHz and the absorption bandwidths cover from 11.9 GHz to 15.8 GHz with a thickness 1.6 mm (Fig.9b). Adjusting the simulation thicknesses from 1 to 5 mm, the efficient absorption bandwidth (RL< -10 dB) of ZSO@CNTs composite could reach 14.16 GHz. Compared



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with other MWCNTs-based absorbers (Fig. 9a and c), the ZSO@CNTs-130°C synthesized in this work show superior advantages owing to lower thickness, strong absorption properties and tuning absorption frequency, which distinctly cater to the requirement of low thickness and high efficient microwave absorber. Besides, because of the material distribution is different in paraffin, there is a theoretical addition range for the optimum loss capacity of absorbent. For example, the absorber of magnetic material will have a high additive amount and the excellent microwave absorption ability will be carried out at high frequency and high thickness. For the non-magnetic composites, the strongest absorption summit appears in the middle and low frequency band at low addition mass. The microwave absorption mechanism of the hollow cube-like ZnSnO3 wrapped by MWCNTs was proposed based on microstructure analysis (Fig. 10). According to the Debye theory, εʺ is proportional to conductivity (equation 4,5 ) and represents dielectric loss ability resulted from the remarkable polarization and conductivity in the ZSO@CNTs composites (Fig. 10, 11). The εʺ is mainly enhanced by the σ of the ZSO@CNTs 3D network, interfaces polarizations and electron conduction. 𝜀 −𝜀

𝑠 ∞ 𝜀 ′ = 𝜀∞ + 1+𝜔 2𝜏2

𝜀 −𝜀

(6) 𝜎

𝑠 ∞ 𝜔𝜏 + 𝜔𝜀 𝜀 " = 1+𝜔 2 𝜏2

0

(7)

The excellent MA properties of ZSO@CNTs-130°C can be attributed to following reasons. Firstly, an abundant of interface can be clearly observed from the ZSO@CNTs-130°C which was caused by the internal structure collapse during Ostwald ripening and the associated special hollow volume (Fig. 11a, e, i; TEM holograms). Such structures could enhance interfacial polarization relaxation. To understand the relationship between dielectric polarization and MA properties of the ZSO@CNTs composites, electron holography analysis was carried out to quantitatively map the charge density distribution (Fig. 11b, f, j). Obviously, there is both negative and positive distributed around the interfacial region area of ZnSnO3-ZnSnO3 and ZnSnO3-MWCNTs (Fig. 11c, g, k). Positive charge gathers on one side of the interface while the negative charge accumulates on the opposite side (Fig. 11d, h, l), leading to an intensive interfacial polarization to dissipate the electromagnetic. Under the applied high-frequency electromagnetic field, the symmetry center of positive and negative


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charge will deviate somehow from the original balance point and therefore contribute dipole polarization. Meanwhile, the unique hollow structure with plenty of interfaces and defects (Fig. 10) can provide enough active sites for repeated scattering. Hence, the hollow structure and interfaces of ZSO@CNTs-130 ℃ play vital roles in the MA performance. Secondly, as electromagnetic wave permeates into the composites, strong ballistic transportation might happen in the electron system of MWCNTs. The imaginary part of permittivity could be enhanced by the effect that the electron migrate from inter to outer shells or hop to the ZnSnO3 crystal. Because εʺ is proportional to conductivity, the higher value of σ means better electron movement, in which electromagnetic energy were quickly converted to thermal energy. Thirdly, the specific shape of ZSO@CNTs could build a 3D conductive network and increase the conductive pathways between ZnSnO3 and MWCNTs. The unique hollow interior structure not only expose more surface to contact with the air, which is conducive to match the impedance, but also benefit to the accumulation of spatial polarization charge. In this hetero-structure, bulk ZnSnO3 cube can be considered as a scaffold to contract MWCNTs on its surface to form a solid network. The conductive network constructed by MWCNTs contributes to conductive loss, which enhances the microwave attenuation. Meanwhile, the synergistic effects between ZnSnO3 and MWCNTs may improve the impendence matching and generate resonance relaxation, which facilitates the microwave to propagate through the MA materials rather than reflect at the boundary between the absorber and the free space. 4 CONCLUSIONS Here, a simple, facile and one-step strategy is established to synthesize the cube-like ZnSnO3 wrapped by multi-wall carbon nanotubes (ZSO@CNTs). The special hollow structure of the composites can be controlled only by changing the temperature. By tuning and optimizing the complex permittivity, the MA performance of ZSO@CNTs composites are greatly improved. The minimum reflection loss of ZSO@CNTs composite with the hollow structure can reach −52.1 dB at 13.5 GHz with a film thickness of only 1.6 mm. It is the interfacial polarization and conductivity losses that play the key roles in the process of microwave energy dissipation in the ZSO@CNTs system. Electron holography proved the inhomogeneous distribution of charge density around the interfaces of ZSO@CNTs, which has a significantly influence on the polarization and dielectric loss ability. The synergy effect between micro-nano ZnSnO3



and MWCNTs not only strengthen the polarization and conductivity but also enhance the impedance matching. Moreover, the unique 3D structure with hollow shape and large surface areas （98 m2·g-1, Fig.S5.）could ameliorate the MA performance, provide more multiple reflections and boost the microwave energy attenuation. The excellent MA performance was ascribed to the polarization loss, conduction loss, and special hollow cages structure. In addition, the above-mentioned discussion may give a novel approach to design new microwave absorbers based on regulated the polarization and conductivity. Acknowledgements This work was supported by the National Natural Science Foundation of China (11727807, 51725101, 51672050), Science and Technology Commission of Shanghai Municipality (2016YFE0105700).

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Figures

Fig.1 schematic illustration of the synthetic process of ZSO@CNTs composites

Fig.2 XRD pattern of the ZSO@CNTs composites


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G-

D

Intensity (a.u.)

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


1319

ID/IG=1.19 1326

ID/IG=1.15 ID/IG=1.12 1322

1000

1200

G+ 1563

ZSO/CNTs-160C

1575

ZSO/CNTs-130C

1566

1400

ZSO/CNTs-100C

1600

1800

Raman shift (cm-1)

Fig. 3 Raman spectrum of ZSO@CNTs composites


2000


Fig.4 SEM images of (a, b) ZSO@CNTs-100, (c, d) ZSO@CNTs-130,and (e, f) ZSO@CNTs-160 composites


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Fig.5 TEM images (a-b) and HRTEM images (c-f) of ZSO@CNTs -130°C sample



Fig. 6 XPS spectra of ZSO@CNTs-130°C composites: wide span (a), crystal structure (b) C 1s spectrum (c), Zn 2p spectrum (d), Sn 3dspectrum (e), and O 1s spectrum (f)


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Fig. 7 The electromagnetic parameters of ZSO@CNTs composites: (a) the real parts of complex permittivity (εʹ), (b) the imaginary parts of complex permittivity (εʺ), (c)the real parts of complex permeability (μʹ) and (d) the imaginary parts of complex permeability (μʺ)

Fig. 8 The reflection loss values of (a) ZSO@CNTs-100°C, (b) ZSO@CNTs-130°C, (c) ZSO@CNTs-160°C composites and (d) the RL values at same thickness 1.6 mm.



Fig. 9 The microwave absorption data of related MWCNTs-based materials (a), the reflection loss histogram of ZSO@CNTs system (b) and table of microwave absorption performance of relative absorber (c)

Fig. 10 The microwave absorption mechanism in the ZSO@CNTs system


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Fig. 11 The TEM images (a, e, f), holograms (b, f, j), charge density maps (c, g, k), and the profile of charge density (d, h, l) in the region of the white arrow for the ZSO@CNTs-130°C composites



Table of content 163x167mm (300 x 300 DPI)


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Enhanced Polarization from Hollow Cube-like ZnSnO3 wrapped by

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