Lightweight, three-dimensional carbon [email protected]2 sponge with enhanced microwave absorption performance

Accepted Manuscript Lightweight, three-dimensional carbon Nanotube@TiO2 sponge with enhanced microwave absorption performance Zichao Mo, Rongliang Yang, Dongwei Lu, Leilei Yang, Qingmei Hu, Hongbian Li, Hai Zhu, Zikang Tang, Xuchun Gui PII:

S0008-6223(18)31198-9

DOI:

https://doi.org/10.1016/j.carbon.2018.12.064

Reference:

CARBON 13767

To appear in:

Carbon

Received Date: 19 October 2018 Revised Date:

4 December 2018

Accepted Date: 18 December 2018

Please cite this article as: Z. Mo, R. Yang, D. Lu, L. Yang, Q. Hu, H. Li, H. Zhu, Z. Tang, X. Gui, Lightweight, three-dimensional carbon Nanotube@TiO2 sponge with enhanced microwave absorption performance, Carbon (2019), doi: https://doi.org/10.1016/j.carbon.2018.12.064. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical Abstract:

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Lightweight, Three-dimensional Carbon Nanotube@TiO2 Sponge with Enhanced Microwave Absorption Performance

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Zichao Mo1, Rongliang Yang1, Dongwei Lu1, Leilei Yang1, Qingmei Hu1, Hongbian Li2, Hai Zhu3, Zikang Tang4, Xuchun Gui1, * 1

State Key Laboratory of Optoelectronic Materials and Technologies, School of Electronics

CAS Center for Excellence in Nanoscience, National Center for Nanoscience and

Technology, Beijing 100190, China. 3

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and Information Technology, Sun Yat-sen University, Guangzhou, 510275, China

State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics, Sun

Yat-Sen University, Guangzhou 510275, China

Institute of Applied Physics and Materials Engineering, University of Macau, Avenida da

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Universidade, Taipa, Macau, China

* Corresponding authors: Tel: +86 20 3994 3411. E-mail address: [email protected] (X. Gui)

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Abstract Porous, spongy absorbers are considered to be the most promising candidates for lightweight and highly efficient microwave absorption material. However, conventional sponges or foams

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reported previously are difficult to meet the high performance requirements. Here, a lightweight (42 mg/cm3), porous CNT@TiO2 sponge with core-shell structure CNT/TiO2 was successfully prepared via a simple combination of hydrolysis and heat treatment method. In

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the sponge, the thickness and crystal structure of the TiO2 shell layer, and the mass fraction of

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TiO2 and CNTs can be precisely controlled by the growth processes, resulting in adjustable dielectric loss properties of the samples. With the optimized component proportion, it was found that a minimum reflection loss value of -31.8 dB was observed at 10.35 GHz, and reflection loss exceeds -10 dB is up to 2.76 GHz for the absorber with the thickness of 2 mm.

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Moreover, the effective microwave absorption (< 10 dB) can be achieved in the range of 3.43 to 12.0 GHz by adjusting the thickness of the absorber. These results indicate that the CNT@TiO2 sponges can serve as attractive candidate materials for lightweight microwave

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absorption materials.

Key words: CNTs, TiO2, core-shell, dielectric loss, microwave absorption

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ACCEPTED MANUSCRIPT 1. Introduction Microwave pollution has become one of the threats to human in recent years. Therefore, the development of microwave absorption materials with high performance to reduce the

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pollution of microwaves is an urgent requirement [1-6]. Over the past few years, various of materials that are high-efficiency in microwave absorption have been developed [7-11]. Especially, sponge-like absorbers with porous structure have attracted much attention, due to

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its low density and easy to process [12]. In general, electromagnetic dissipation capability

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and impedance matching characteristics are the key factors in designing a high performance absorption material [13,14]. Carbon-based materials with high dielectric loss, including carbon nanotubes (CNTs) [15-18], carbon nanocoils [19] and graphene nanosheets [20], have been proved as effective microwave absorption materials. In particular, CNTs have been

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considered as promising candidates for high-performance microwave absorbers due to the advantages of low density, high aspect ratio, stable chemical property and low cost [21,22]. For instance, Salimbeygi et al. [23] fabricated Poly (vinyl alcohol) (PVA)/multi walled CNTs

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nanocomposite as the absorbers and it showed the minimum reflection loss of -28.0 dB at 8.6

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GHz with a thickness of 1 mm. Cheng et al. [24] developed a highly conducting CNT/polyaniline hybrid absorber by plasma pretreating and in situ polymerization, and an excellent reflection loss value of -41.37 dB was achieved with absorber thickness of 2.0 mm. The microwave attenuation properties of CNTs could be further improved by decorating CNTs with other high dielectric loss or magnetic loss materials, such as metals or metallic oxides [25-27]. For examples, Wen et al. [28] have fabricated multi-walled CNT powders compounded with magnetic metal (Fe/Co/Ni) nanoparticles. It shows enhanced absorption 3

ACCEPTED MANUSCRIPT properties benefiting from matching of complex permittivity and permeability. Zhao et al. [29] synthesized Fe3O4 modified CNT composites, and delivered a minimum reflection loss of -35.8 dB at 8.56 GHz with an effective absorber bandwidth (reflection loss <-10 dB) more

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than 2.32 GHz. Among all the alternative materials, titanium dioxide (TiO2) has attracted much attention for microwave absorption application owing to its strong dielectric loss characteristic [30]. Moreover, TiO2 exhibits a relative lower real part of complex permittivity

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than that of CNTs [31]. Therefore, it is feasible to adjust dielectric properties of the absorbers

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composed of CNTs and TiO2 by adjusting the proportion of two components. Recently, Tiwari et al. [32] synthesized PPy/TiO2(np)/CNT nanocomposites by chemical oxidative polymerization. The minimum reflection loss of this composite is up to -51.11 dB at the thickness of 3 mm. Zhao et al. [33] prepared a nest structural TiO2/CNTs/ poly(3-methyl

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thiophene) composite, which exhibit an optimal microwave absorption property with minimum reflection loss of -21.56 dB at 11.04 GHz. However, these materials are mainly powder-like, which is difficult to process and has high density. In addition, CNTs tend to

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aggregate during the coating of polymer shell, which also restrict the further improvement of

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their microwave absorption performance. Therefore, the development of light-weight CNT-based microwave absorption materials with uniform coating layer is highly required. Here, we developed a lightweight CNT@TiO2 sponge for microwave absorption with a simple hydrolysis and heat treatment method. The highly porous and flexible CNT sponges were directly used as a template and a TiO2 layer was uniformly coated on the surface of the CNTs. The crystal structure and morphology of CNTs/TiO2 sponge can be well controlled by adjusting the fabrication conditions. Furthermore, the CNT@TiO2 sponge exhibits excellent 4

ACCEPTED MANUSCRIPT performance in microwave absorption, and a minimum reflection loss value of -31.8 dB is achieved with the absorbers thickness of 2 mm.

2.1 Synthesis of CNTs sponges and CNT@TiO2 sponges

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2. Experimental

CNTs sponges were synthesized via a chemical vapor deposition (CVD) method using

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ferrocene/dichlorobenzene solution as carbon source, as reported in our early work [21, 34,

temperature was set as 860

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35]. Typically, the ferrocene concentration in the precursor was 200 mg/mL and the reaction . The flow rates of the carrying gas of Ar and H2 were set as

2,000 sccm and 300 sccm, respectively. The as-grown CNT sponge was firstly treated with UV/O3 for 2 hours to increase its hydrophilicity. Then, the obtained sponge was immersed

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into an tetrabutyl titanate (Ti(OBu)4)/ethanol solution with a concentration of 0.1-0.5 g/mL. The Ti(OBu)4/ethanol solution could rapidly infiltrate into the pores of the CNT sponge. Then the filled sponge was transferred into water and sonicated for 40 min, where the TiO2

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precursor hydrolyzed to form an amorphous TiO2 (a-TiO2) layer onto the surface of each

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CNT in the sponge. After being freeze-dried for 12 hours, a CNT@a-TiO2 sponge was obtained, in which the CNT was uniformly coated with TiO2 layer and formed a core-shell structure. For optimizing the microwave absorption properties, amorphous TiO2 should be transformed into crystalline TiO2 [36]. CNT@a-TiO2 sponge was annealed in Ar for 2 hours at temperatures from 200

to 800

. And CNT@TiO2 sponges with different crystalline

phases of TiO2 were obtained. Finally, in order to adjust the dielectric properties of the samples, CNT@TiO2 sponge was calcined at 360- 440 5

in air for 2-6 hours to remove part

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2.2 Characterizations

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The morphologies and microstructures of the samples were characterized by scanning electron microscopy (SEM, Hitachi, S-4800) and transmission electron microscopy (TEM, JEM-210HR). X-ray diffraction analysis and Raman spectra were collected by X-ray Powder

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Diffractometer and Raman spectroscopy system (HORIBA, LabRAM HR), respectively. The

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content of each component in the sample was measured by thermogravimetric analysis (TGA, NETZSCH, TG209 F1) from room temperature to 800

at a scanning rate of 10

/min in air.

For complex permittivity and permeability measurement, composites were prepared by mixing CNT@TiO2 sponge (30 wt. %) with paraffin (70 wt. %). The composite was pressed

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into toroidal shape with external diameter of 7.00 mm, inner diameter of 3.04 mm and thickness of 2.0 mm. In a typical coaxial method, the complex permittivity and permeability of the composites were measured in the range of 2.0-18.0 GHz via a vector network analyzer

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(VNA, HP-8722ES).

3. Results and discussion

The fabrication process of the CNT@TiO2 sponge is schematically illustrated in Figure 1a. The CNT sponge block with high porosity was firstly immersed into Ti(OBu)4/ethanol precursor solution for the sufficient absorption of Ti(OBu)4. The sample was then transferred into water for the hydrolysis of the precursor into amorphous TiO2 (a-TiO2), and each of the CNT in the sponge was uniformly coated with a layer of the a-TiO2. The obtained 6

ACCEPTED MANUSCRIPT CNT@a-TiO2 sponge was annealed in Ar at different temperatures and amorphous TiO2 was transformed into crystalline TiO2 (CNT@TiO2), which maintains the spongy structure and flexibility of the original CNT sponge (Figure 1b). Here, the sample after annealing was

500

and 800

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defined as CNT@TiO2(500) and CNT@TiO2(800), corresponding to the samples annealing at , respectively. The sample was further calcined in air to remove part of the

CNTs in the CNT@TiO2 sponge. The obtained CNT@TiO2 sponge is freestanding and has

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high porosity and low density (42 mg/cm3). With the increase of calcination time and

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temperature, the CNT@TiO2 sponge could shrink a little (<8%) and turned into greyish yellow gradually, indicating the decrease of CNTs in the sample. When the sample was oxidized at 440

longer than 3 hours, CNTs could be completely removed, and a porous

TiO2 block with the same shape of original CNT sponge was obtained. The sample after

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oxidation was defined as CNT@TiO2(500)-400(2h) for the samples oxidized at 400

for 2

hours. And other samples follow the same defined rules. Free standing property of the

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size sample.

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synthesized CNT@TiO2 sponge indicated great feasibility for the scalable synthesis of larger

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ACCEPTED MANUSCRIPT Figure 1 Fabrication and photos of the CNT@TiO2 sponge. (a)Schematic illustration of the fabrication processes of the samples. (b)Photos of the freestanding CNT sponge, CNT@a-TiO2 sponge, CNT@TiO2 sponge, and CNT@TiO2 sponge with most of the CNT

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been removed (CNT@TiO2(800)-400(6h)).

The representative SEM and TEM images of the CNT sponge, CNT@a-TiO2 sponge,

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CNT@TiO2 sponges and oxidized CNT@TiO2 sponges are shown in Figure 2. The as-grown

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free-standing CNT sponge consists of CNTs with diameter of 20-40 nm and length up to several micrometers, which self-assemble into a porous three-dimensional network (Figure 2a). Through the coating process, each CNT was homogeneously coated with a smooth TiO2 layer (Figure 2b). From the cracks (marked by arrows) in the sample, the core-shell structure

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of CNT/a-TiO2 can be clearly seen. The surface of the shell is smooth, indicating that the wrapped TiO2 is amorphous. Moreover, the TiO2 layer efficiently welded contact points of CNTs, which further increased the mechanical strength of the CNT/a-TiO2 sponge. As shown

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in Figure S1, after wrapping with TiO2 layer, the compression stress of the CNT sponge has

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increased from 0.12 MPa to 2.24 MPa at the strain of 75%. The thickness of TiO2 shell is about 10 nm through the TEM image (Figure 2e and 2f) and it can be well controlled via changing the precursor concentration. As shown in Figure S2, with the concentration of precursor increasing from 0.1 to 0.5 g/mL, the thickness TiO2 shell gradually increases from 10 nm to 30 nm. After annealing treatment, the smooth amorphous TiO2 shells crystallized into nanoparticles while the CNT cores remained unchanged (Figure 2c). The crystallization of the TiO2 shells was also confirmed by the transformation of the amorphous layer into 8

ACCEPTED MANUSCRIPT nanoparticles in TEM images (Figure 2g) and the sharp lattice fringes in the high resolution TEM (Figure 2h). As exhibited elemental mappings of the sample (Figure S3), the O and Ti elements are uniform distribution on shells, and the C element, corresponding to the core of

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CNTs, concentrated distribution in the central zone of nanotubes, which further demonstrate the core-shell structure of the sample. After removal of part CNTs in the calcination process, the residual CNT in CNT@TiO2 composites significantly decreased (Figure 2d, the sample is

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CNT@TiO2(800)-400(4h)), which will be further discussed in the following sections. No

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distinct change between oxidized and non-oxidized sample. The well-preserved nanotube morphology verifies the structural stability of the CNT@TiO2, which is consistent with the image in Figure 1b. The oxidation degree of the CNT@TiO2 can be adjusted precisely by varying oxidized temperature and treatment time (Figure S4). As the sample oxidized at 440

observed (Figure S4c).

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for 2 hours, the nanotube structure is slightly damaged while some shorter nanotubes can be

Figure 3a summarizes the XRD patterns of the samples treated at different conditions.

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There are two distinct diffraction peaks at 25.8° and 44.8° in the CNTs sponge, corresponding

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to the (002) and (100) planes of graphite, respectively. For the CNT@a-TiO2, no clear diffraction peak could be observed, indicating the amorphous structure of the coated TiO2 layer. When the annealing temperature is below 300 ℃, the XRD patterns of CNT@TiO2 show no significant change. With the increasing of the annealing temperature, the XRD patterns of CNT@TiO2 exhibit obvious and regular change, as shown in Figure S5. As the temperature increases to 500 ℃, a series of clearer and sharper diffraction peaks appeared, indicating higher crystallization of the samples. The diffraction peaks appeared at 25.3°, 9

ACCEPTED MANUSCRIPT 37.8°, 48.0°, 53.9°, 55.0°, 62.7° and 74.9° are well matched with (101), (004), (200), (105), (211), (204) and (215) crystal planes of anatase TiO2 (PDF#21-1272), respectively. As the annealing temperature further raises up to 800 ℃, some new diffraction peaks at 27.4 °, 36.0 °

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and 41.2 ° can be observed, which can be attributed to the (110), (101) and (111) crystal planes of rutile TiO2 (PDF#21-1276), respectively. These changes indicate that part of anatase TiO2 has been transformed into rutile phase TiO2. The complex diffraction peaks in

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CNT@TiO2(800) indicated that the TiO2 shells in the sample are composed of two different

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crystalline structures. With the representative phase TiO2, the CNT@TiO2 (500) and

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CNT@TiO2 (800) samples are selected for further study.

Figure 2 Morphology of the CNT@ TiO2 sponge. (a) SEM images of CNT sponge. (b) SEM images of CNT@a-TiO2 sponge, revealing a core-shell CNT/ TiO2 structure. (c) SEM images of CNT@TiO2 (800) sponge. (d) SEM images of CNT@TiO2(800)-400 (4h) sponge. (e) and (f) TEM images of CNT@a-TiO2 sponge. (g) and (h) TEM images of CNT/TiO2(800) 10

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The mass percentage of CNT@TiO2 sponges is determined by TGA in air, as shown in

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Figure 3b and 3c. The mass fraction of TiO2 in the sponge can be controlled in the range of 70-90 wt.% by the oxidation temperature (Figure 3b). At the same oxidation temperature, the content of TiO2 in the sponge can be further precisely controlled by the oxidation time

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(Figure 3c). For example, the unoxidized CNT@TiO2(800) sponge has a residual mass of

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70.2 wt.%, which resulted from the TiO2 shell and a small amount of catalyst oxide. However, the samples with residual mass of 72.7 wt.% and 91.3 wt.% are obtained by annealing the sample at 360

and 440

for 2 hours, respectively. The residual mass of 27.1 wt.% of the

CNT sponge resulted from the catalyst for growth. In addition, higher concentration of the

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precursor solution also can result in thicker TiO2 coating shell and thus higher residual weight (Figure S6a). TiO2 shell layer with different crystalline phases exhibit little difference on the

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thermal stability of the samples (Figure S6b).

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91.3 %

CNT@TiO2 (500)

A

Weight (%)

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A R R

A

70.2 % 60 CNT CNT@TiO2(800)

40 C

CNT@TiO2(800)-360(2h) CNT@TiO2(800)-400(2h)

C

20

30

40

50

60

70

0

80

27.1 %

CNT@TiO2(800)-440(2h)

20

200

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R

79.5 % 72.7 %

80

CNT@TiO2 (800)

A

Intensity(a.u.)

(b) 100

CNT CNT@a-TiO2

400

600

800

Temperature(℃ )

2θ(°)

(c) 100

(d)

95

CNT@TiO2(800)

CNT@TiO2(800)-360(2h)

94.0 %

85 CNT@TiO2(800)-400 (2h)

80

79.1 %

CNT@TiO2(800)-400 (4h) CNT@TiO2(800)-400 (6h)

75 0

200

400

600

Temperature(℃ )

CNT@TiO2(800)-440(2h)

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Intensity (a.u.)

88.1 %

D

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Weight(%)

CNT@TiO2(800)-400(2h)

90

800

500

1000

G

1500

2000

-1

Raman Shift (cm )

Figure 3 Structural characterization of the CNT@TiO2 sponge. (a) XRD patterns of different samples. (b) TGA curves of CNT sponges and different CNT@TiO2 sponge obtained by

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various oxidation temperatures. (c) TGA curves of CNT@TiO2 sponge obtained by various oxidation times. (d) Raman spectrum of CNT@TiO2 sponge obtained by various oxidation

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temperatures.

The changing of weight percent of CNTs and TiO2 in the samples was also reflected in

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the Raman spectrum of the samples (Figure 3d). In the Raman spectrum, the two peaks located at 1329 cm-1 (D band) and 1597 cm-1 (G band) are originated from CNTs. The peaks appear located at 153, 396, 513 and 635 cm-1 for the CNT@TiO2 samples can be indexed to the anatase TiO2 [37,38]. With the increase of oxidizing temperature, the strength of characteristic peak of CNTs gradually weakened, while those of TiO2 enhanced, which indicated that the content of CNTs decreased in the samples. Similar tendency can also be

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while the content of TiO2 keeps stable during the oxidation process. In consideration of the unique core-shell nanotube structure and the controllable component content, CNT@TiO2 sponges have been optimized to be applied as microwave absorber. The as-prepared

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CNT@TiO2(800)-400(4h) sponge was homogeneous mixed with 70 wt.% paraffin, which

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was used for electromagnetic parameters measurement. The measured electromagnetic parameters versus frequency are shown in Figure 4(a). It is well known that the real parts of complex permittivity (ε′) and complex permeability (µ′) are used for measuring the storage capability of electric and magnetic energy, while the imaginary parts (ε″ and µ″) stand for the

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dissipation capability of electric and magnetic energy. The µ′ and µ″ values of the sample are about 1.00-1.05 and 0.05-0.25 in the range of measurement frequency (2-18 GHz) with small fluctuations, indicating the weak magnetism and corresponding low magnetic loss of the

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composite. The weak magnetism derived from the Fe nanoparticles wrapped in CNTs, which

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originated from the ferrocene catalyst. For the permittivity, it can be observed that the values of ε′ decline slowly from 14.8 to 11.1, accompanied by the increasing of ε″ values from 3.4 to 5.7, suggesting the dielectric loss is the main microwave dissipation mechanism for the absorber and the dissipation capability becomes stronger in higher frequency zone. This should be ascribed to the dipole polarizations of dielectric TiO2 nanoparticles and also the interfacial polarization from the interface between CNTs and TiO2. To reveal the microwave absorption efficiency of the as-prepared samples, the reflection 13

ACCEPTED MANUSCRIPT loss (RL) values are calculated using the measured relative complex permittivity (εr) and complex permeability (µr) according to the transmission line theory, which can be summarized as the following equations: Zin = Z0(µr/εr)1/2tanh[j(2πfd/c)(µrεr)1/2]

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RL = 20 log|(Zin – Z0)/(Zin + Z0)|

(1) (2)

In these equations, Zin represents the input impedance of the absorber, Z0 is the impedance of

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free space, f is the frequency of microwaves, d is the thickness of the absorber layer, and c is

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the velocity of light. The dependence of RL values on absorber thickness is investigated, as shown in Figure 4(b). As the absorber thickness is 2 mm, the minimum RL value reach -31.8 dB at 10.35 GHz and RL values lower than -10 dB are obtained in the frequency range of 9.24-12.0 GHz. As the absorber thickness increases, the absorption peaks shift towards low

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frequency regions, while the peak values show small variations. Moreover, the quarter-wavelength matching model can be adopted to understand the dependence of reflection loss peak upon absorber thickness, in which the relationship between peak

|

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( = 1,2, . . )

(3)

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=

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frequency (f) and absorber thickness (d) can be expressed as the following equation:

According to the quarter-wavelength matching model, the absorption peaks will shift to a lower frequency region with an increase in thickness and the experimental results match well with calculated values, as it shows in the Figure 4c.

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ε′

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8 0.4 6

ε″

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0.2

µ″

2 2

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10

12

14

16

Permeability

Permittivity

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-25 -30 -35

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4

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Frequency(GHz)

(c)10

0 -5

-20 CNT@TiO2 (800)-400(2h) CNT@TiO2 (800)-400(3h) CNT@TiO2 (800)-400(4h) CNT@TiO2 (800)-400(6h)

-35 2

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Frequency (GHz)

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Calculated curve Experimental results

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Frequency (GHz)

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Reflection loss (dB)

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Reflection loss (dB)

(a)16

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10

12

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Frequency(GHz)

Figure 4 Microwave absorption performance of CNT@TiO2 sponges. (a) Complex permittivity and permeability of CNT@TiO2(800)-400(4h). (b) Reflection loss of

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CNT@TiO2(800)-400(4h) with different thickness. (c) Comparison of experimental data and calculated curve by quarter-wavelength matching model. (d) Reflection loss of CNT@TiO2

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sponge with different oxidation time.

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To further reveal the influences of CNT and TiO2 content on the microwave absorption properties of the samples, the electromagnetic parameters of the CNT@TiO2 sponges with different oxidized time, prepared at different preparation stages and by different concentrations of precursor solution were measured at room temperature, and the results are summarized in Figure S8-S10, respectively. The µ′ and µ″ values of all samples changed little in the range of the whole measurement frequency, suggesting all the samples are the same

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(CNT@TiO2(800)-400(2h)) decreases gradually from 101.8 to 30.7 with frequency, while ε″ value declines from 63.5 to 41.5, indicating the conduction loss of CNTs plays a major role in dielectric loss for the sample. It could lead to a strong dissipation against microwaves.

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However, high conductivity of CNTs would cause a high reflectivity against electromagnetic

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wave of the material, which is detrimental to absorption property. As the oxidation time increase to 6 hours, ε′ and ε″ values of the sample have been declined to about 3.9 and 0.1, respectively. It demonstrates that the dissipation mechanism has changed during the oxidation process. The conduction loss caused by CNT is the dominant factor before removing the CNT

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skeleton, then the loss mechanism turns into the dipole polarizations originating from the dielectric TiO2 [39-40] and the interfacial polarization originating from different components of the material. These suggest that it is feasible to tune the dielectric loss properties via

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simply changing the oxidation time. RL curves of these samples are calculated based on

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electromagnetic parameters at a thickness of 2 mm, as shown in Figure 4d. Peaks of RL curves shift to a higher frequency and the minimum RL values increase with the increasing of oxidation time. The sample with 4 hours’ oxidation shows an optimal performance, indicating the favorable ratio of components in the CNT@TiO2 sponges. The influence of TiO2 crystalline phase on the microwave absorption also has been investigated. The electromagnetism parameters of CNT@TiO2(500)-400(4h) with single anatase phase TiO2 shell are shown in Figure 5a. The ε′ and ε″ values are about 7.7-7.2 and 0.5-1.1, indicating the 16

ACCEPTED MANUSCRIPT poor dielectric loss performance compared with the samples processed at 800

. This further

indicated that the dielectric loss performances of the samples can be optimized via adjusting the ratio of CNTs and TiO2, attributed to the different characteristics of the two materials. The

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RL values of the sample with different thickness are investigated and shown in Figure 5b. Significantly, samples with multiple crystal structures of anatase and rutile TiO2 perform much better than those with single anatase structure. Compared to previous CNT-based

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absorbers, our CNT@TiO2 sponges show high minimum reflection loss at the same thickness

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(Table S1). The excellent microwave absorption of the CNT@TiO2 sponges can be attributed to the following reasons. Firstly, the introduction of TiO2 with the crystalline grain composed core-shell structure results in more interfacial polarization processes between CNTs-TiO2. Secondly, the porosity and hollow nanotube structures resulted from the oxidation can

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interrupt the propagation of microwaves and also consume the wave energy through multi-reflection. Thirdly, the complex TiO2 of anatase and rutile will lead to more relaxation processes in the composites and interfacial polarization between two phases.

6

0.9

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4 2

0.3

ε″

µ″

0

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-12 -15

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-2

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(b) 0 Reflection loss (dB)

ε′

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µ′ Permeability

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-18 12

14

16

18

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Frequency(GHz)

4

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Frequency (GHz)

Figure 5 Microwave absorption performance of CNT@TiO2(500)-400(4h). (a) Complex permittivity and permeability, and (b) Reflection loss of the sample.

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Conclusions

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In summary, a CNT@TiO2 sponge with core-shell nanotube structure was successfully prepared via a simple combining of hydrolysis with heat treatment method. The thickness and crystal structure of the TiO2 shell layer can be modified via changing the precursor

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concentration and heat treatment temperature. The CNTs core can be partially removed

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through further oxidation process and the ratio of CNTs to TiO2 in the sponge can be precisely tuned by changing the oxidation temperatures and time. The microwave absorption performances of the sponges have been optimized. CNT@TiO2(800)-400(4h) shows the best performances. As the absorber thickness is 2 mm, a minimum RL value of -31.8 dB is

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achieved at 10.35 GHz, while the absorption bandwidth with reflection loss below -10 dB is achieved in the frequency range from 3.43 to 12.0 GHz with a thickness of 2-5 mm. These results suggest the as-prepared core-shell CNT@TiO2 sponges can be used as lightweight and

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high efficiency microwave absorption materials.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (Grant No. 51772335), Guangdong Natural Science Foundation (Grant No. 2016A030313346), the Fundamental Research Funds for the Central Universities.

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