Accepted Manuscript Carbon nanospheres induced high negative permittivity in nanosilver-polydopamine metacomposites Hongbo Gu, Xiaojiang Xu, Mengyao Dong, Peitao Xie, Qian Shao, Runhua Fan, Chuntai Liu, Renbo Wei, Zhanhu Guo PII:
S0008-6223(19)30251-9
DOI:
https://doi.org/10.1016/j.carbon.2019.03.028
Reference:
CARBON 14033
To appear in:
Carbon
Received Date: 25 January 2019 Revised Date:
28 February 2019
Accepted Date: 11 March 2019
Please cite this article as: H. Gu, X. Xu, M. Dong, P. Xie, Q. Shao, R. Fan, C. Liu, R. Wei, Z. Guo, Carbon nanospheres induced high negative permittivity in nanosilver-polydopamine metacomposites, Carbon (2019), doi: https://doi.org/10.1016/j.carbon.2019.03.028. 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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Carbon Nanospheres Induced High Negative Permittivity in
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Nanosilver-Polydopamine Metacomposites
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Hongbo Gu,1,* Xiaojiang Xu,1 Mengyao Dong,2,3 Peitao Xie,4
Qian Shao,5 Runhua Fan,4 Chuntai Liu,3 Renbo Wei,6,* Zhanhu Guo2,* 1
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Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, Shanghai 200092, People’s Republic of China Integrated Composites Lab (ICL), Department of Chemical & Biomolecular Engineering University of Tennessee, Knoxville, TN, 37966, USA
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College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, People’s Republic of China 6
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College of Ocean Science and Engineering, Shanghai Maritime University, Shanghai 201306, People’s Republic of China
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Key Laboratory of Materials Processing and Mold (Zhengzhou University), Ministry of Education; National Engineering Research Center for Advanced Polymer Processing Technology, Zhengzhou University, Zhengzhou 450002, People’s Republic of China
Research Branch of Advanced Functional Materials, School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu, 611731, People’s Republic of China
*Corresponding author E-mail:
[email protected] [email protected]
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ABSTRACT In this work, a unique high negative permittivity is observed in the carbon nanosphere (CNS)
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supported nanosilver-polydopamine (PDA) metacomposites (called CNS-PDA/Ag). The CNS possesses a uniform spherical structure (average diameter of about 200 nm) with a bridge to link CNS together. The Ag nanoparticles are observed to be uniformly deposited on the surface of
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CNS-PDA core-shell structures with an average diameter of around 10-20 nm. Both impedance
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and optical band gap results confirm that the CNS-PDA/Ag metacomposites can form a conductive network pathway for further improving their electrical conductivity (about 1 and 2 magnitudes of higher than that of CNS, and CNS-PDA, respectively). With the support of CNS, the enhanced high negative permittivity observed in the CNS-PDA/Ag metacomposites (about
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-6.1 × 105) compared with CNS (around -1.2 × 105), CNS-PDA (around 2.5 × 104), and PDA/Ag (around 2.0) is ascribed to uniform decoration of PDA and Ag nanoparticles, induced electric dipole polarization, excellent electrical conductivity as well as anisotropy in dielectric and
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electrical properties of CNS, PDA, and Ag. The negative permittivity and positive reactance
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demonstrate an inductance character of the CNS-PDA/Ag metacomposites. This work opens up a new strategy for design and development of metacomposites.
Keywords: Carbon Nanospheres; Polydopamine; Nano-Silver; Metacomposites; Negative permittivity.
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1. Introduction Metamaterials, as a class of artificially designed materials with unique physical properties
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that are not achievable in ordinary materials, have been highly valued in the fields of perfect lens, wireless power transfer, invisible cloak and magnetic resonance imaging due to their negative permittivity and/or negative permeability at a certain frequency [1]. Actually, there is nothing
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special about the composition of metamaterials, but their newly designed structures including precise shape, geometry, size, orientation and arrangement [2]. With the size of microstructures
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smaller than the wavelength, these materials start to influence the electromagnetic waves by blocking, absorbing, enhancing or bending these waves [3-6].
Recent years, more and more efforts have been made to develop metacomposites, in
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which the negative permittivity and/or negative permeability can be tuned mainly through controlling their ingredients rather than the microstructures [7]. It is a hot topic to fabricate metacomposites with superb properties by combining two or more different ingredients. Many
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researchers indicated that carbon materials, such as carbon nanofibers (CNFs) [8], multi-walled carbon nanotubes (MWCNTs) [9, 10], graphene oxide (GO) [11], reduced graphene oxide (rGO)
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[12], and graphene nanosheets [13] are suitable for reaching stable and weak negative permittivity in the bulk materials because of their moderate charge carrier concentration [14, 15]. For example, Sun et al. [16] noticed that polydimethylsiloxane (PDMS)/MWCNTs nanocomposite films fabricated by an in-situ polymerization method possessed a negative permittivity with a dielectric resonance for the induced electric dipoles. Zhu et al. [17] studied the polyaniline (PANI) nanocomposites incorporating different loadings of carbon nanostructures 3
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like graphene, CNTs and CNFs. Negative permittivity was found in all the fabricated PANI nanocomposites and could be easily tailored by adjusting the loading, morphology and surface
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functionality of these carbon nanostructures. Moreover, Gu et al. [18] prepared the β -SiC/PANI metacomposites through a surface initiated polymerization (SIP) method and reported a switching frequency at which the permittivity changed from negative to positive. Luo et al. [19]
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investigated the microwave behavior of glass fiber/epoxy-based metacomposites containing
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ferromagnetic Fe77Si10B10C3 microwires with CNFs and indicated that these metacomposites were promising for microwave cloaking and sensing applications for aerospace [20-23]. Compared with metals, alloys, carbon and ceramics [24-31], sliver (Ag) nanoparticles have been widely exploited in the applications including biological sensors [32], flexible
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electronic devices [33] and solar cells [34] for their unique properties, such as high electrical conductivity and chemical stability under the normal environment[35, 36]. Meanwhile, it can also establish a continuous conductive network in the system to provide and improve the
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electrical conductivity of nanocomposites [37, 38]. Sun et al. [39] reported a Ag/yrrium iron
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garnet composite and revealed that the dielectric loss was mainly caused by the conduction and polarization. Matsuhisa et al. [40] achieved a high-performance stretchable and printable elastic conductor with an initial electrical conductivity of 6168 S cm-1 by the in-situ formation of Ag nanoparticles. However, metacomposites with negative permittivity using carbon nanosphere (CNS) with Ag nanoparticles as ingredients have not been reported yet.[41]
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In this work, unique carbon nanospheres supported nanosilver-polydopamine (CNS-PDA/Ag) metacomposites with enhanced high negative permittivity were designed and
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synthesized. In this metacomposite, the CNS was fabricated with a hydrothermal and high temperature annealing method. Then the PDA was introduced on the surface of CNS through the self-polymerization of dopamine for further deposition of Ag nanoparticles via the in-situ
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reduction of a silver nitrate in the ammonia solution. The chemical structure and thermal
(SEM),
energy-dispersive
X-ray
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properties of prepared CNS-PDA/Ag were analyzed through the scanning electron microscope (EDX),
transmission
electron
microscopy
(TEM),
Fourier-transform infrared spectroscopy-Attenuated total reflectance (FTIR-ATR), Raman, X-ray diffraction (XRD) and thermogravimetric analysis (TGA). The impedance was investigated by
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the AC resistance and reactance. The resistivity (ρ) was measured by a standard four-probe method. The optical band gap of metacomposites was calculated based on the Ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS) spectra. More importantly, the effects of CNS as
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a supporter on the negative permittivity performance of CNS-PDA/Ag were systematically
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characterized by the real permittivity (ε'), imaginary permittivity (ε'') and dielectric loss (tan δ).
2. Experimental 2.1 Materials
Silver nitrate (AgNO3, 99.95-100.05%) was obtained from Shanghai Reagent Plant.
D-xylose (C5H10O5, 98%), dopamine hydrochloride (C8H11NO2·HCl, 98%), tris (hydroxymethyl) methyl aminomethane (Tris, C4H11NO3, 98%), anhydrous ethanol (C2H5OH), hydrochloric acid 5
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(HCl, 36.0-38.0 wt%) and ammonia (NH3·H2O, 25-28 wt%) were purchased from Shanghai Macklin Biochemical Co., Ltd. The 0.01 mol L-1 Tris-HCl buffer solution and 2 wt% ammonia
further treatment.
2.2 Fabrication of CNS-PDA/Ag metacomposites
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solution were prepared for further usage. Other chemical reagents were used directly without any
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Fig. 1 depicts the preparation procedure of CNS-PDA/Ag metacomposites. Firstly, the
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CNS was fabricated via a hydrothermal method combined with high temperature annealing. The D-xylose (1.5 g) was dissolved in 60 mL deionized water and then transferred into a 100 mL poly(p-phenylene)-line stainless steel autoclave. After that, the autoclave was sealed and maintained in a regular oven at 200 oC for 24 h. Later on, as the solution was cooled down to
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room temperature naturally, the product was vacuum filtered, washed with deionized water for several times, and dried in a freeze-dryer for 12 h. This sample was recognized as CNS precursor (CNSP). Then the collected sample was annealed in a tube furnace at 800 oC for 2 h to obtain
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CNS under the nitrogen atmosphere.
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Secondly, in order to deposit Ag on the surface of CNS, the PDA was introduced to the outside of the CNS by an in-situ polymerization of dopamine hydrochloride. CNS (0.125 g) was ultrasonically dispersed within 230 mL Tris-HCl buffer solution in a 500 mL three-necked flask for 5 min. Dopamine hydrochloride (0.25 g) was dissolved in 20 mL Tris-HCl buffer solution and added dropwise into the above three-necked flask. The mixed solution was mechanically stirred at 25 oC with a stirring rate of 300 r min-1 for 3 h. The filtered product was placed in a 6
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freeze-dryer for 12 h. The resulted polydopamine surface-modified carbon nanospheres were indexed as CNS-PDA.
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Finally, the PDA on the surface of CNS was used as a reductant to reduce the AgNO3 for the synthesis of Ag nanoparticles. CNS-PDA (0.125 g) was added into a 250 mL erlenmeyer flask with 50 mL deionized water and sonicated for 15 min. Then the fresh-made silver ammonia
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solution (which was prepared by dripping 2 wt% ammonia solution into 24.5 mL AgNO3 solution (0.12 mol L-1) until the precipitate was completely dissolved) was put into the above
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solution dropwise and magnetically stirred (800 r min-1) in a thermal water bath at temperature of 25 oC for 30 min. After that, the reaction was continued at 80 oC for 1 h and vacuum filtered by washing with deionized water for several times until the solution was neutral, then placed in a
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freeze-dryer for 12 h. The ultimate solid nanocomposite was named as CNS-PDA/Ag. In comparison, both pure PDA and PDA/Ag were also manufactured without addition of CNS. 2.3 Characterizations
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SEM (Hitachi S-4800) and TEM (Tecnai G20) were used to observe the microstructures
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of materials. XRD was applied to characterize the crystalline structure of samples by a D8 Advance X-ray powder diffractometer within the diffraction angle ranges from 10 to 70 º at a scanning speed of 10 º min-1. FTIR-ATR spectra were recorded in the range of 400 - 4000 cm-1 at 64 scans per spectrum at 2 cm-1. Raman spectra were collected in the range of 1000 - 2000 cm-1 on a Renishaw inVia Reflex Raman spectrometer. TGA was conducted at a heating rate of 20 oC min-1 within a temperature range from 25 to 1000 oC with an air flow rate of 20 mL min-1. LCR 7
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meter (Tonghui TH2838H) was employed to investigate the frequency dependent permittivity and AC impedance within the frequency range from 20 Hz to 2 MHz at room temperature. The
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samples were prepared through mixing with paraffin according to the weight ratio of 1:2 to form a disc pellet with a diameter of 25 mm by applying a pressure of 50 MPa in a hydraulic presser. The same sample was also applied to directly measure the resistivity (ρ) by a standard four-probe
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method (KDY-1, Guangzhou Kunde Technology Co., Ltd.) and the optical band gap by using
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UV-vis DRS in a UV-VIS/NIR spectrophotometer (PC, Agilent Carry 5000).
3. Results and Discussion
3.1 Optimal Condition Determination for Fabrication of CNS
Generally, in the hydrothermal method, the morphologies, size, and structures of a material
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can be easily tuned by various reaction parameters including reaction time, concentration of reactants, filled volumes of autoclave, and temperature. In order to obtain an optimal condition for the preparation of CNS, the effects of the parameters including D-xylose concentration,
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rection time, and filled volume of stainless steel autoclave on the morphology of CNS were
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explored and the obtained SEM images of these samples are listed in Fig. S1. As for different D-xylose concentrations in 60 mL deionized water (1.0, 1.5 and 2.0 g/60 mL, respectively), Fig. S1a&b&c, the sample with 1.5 g in 60 mL deionized water (Fig. S1b) exhibits the best uniformity both in the dimension and shape. For the reaction time effect in the stainless steel autoclave at 200 oC with a D-xylose concentration of 1.5 g (60 mL) for 12, 24, and 48 h, accordingly, Fig. S1d&e&f, the sample with a reaction time of 24 h (Fig. S1e) has the identical 8
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morphlogy. For the reaction time of 12 h, Fig. S1d, the dimension of CNS is not even. When the reaction time prolongs to 48 h, Fig. S1f, even the sample possesses a uniform dimension around
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100 nm and round shape, they tend to agglomerate together, which is not beneficial for the adhesion of PDA and further deposition of Ag nanoparticles. Therefore, 24 h was chosen as the best reaction time for the preparation of CNS. In the effect of filled volume (like 60, 70, and 75
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mL, respectively) of autoclave at a D-xylose concentration of 1.5 g (60 mL) at 200 oC for 24 h,
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it’s clearly seen that after the increase of filled volume of autoclave up to 70 and 75 mL, Fig. S1h&I, the size of CNS is not consistent. Only the filled volume of autoclave with 60 mL can obtain a stable, even and regular CNS. Consequently, the optimal condition for the fabrication of CNS by using D-xylose as precursor is discovered to be 1.5 g of D-xylose in the 60 mL filled
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volume of autoclave for reaction of 24 h at 200 oC. Besides, it’s interestingly found that under this optimal condition, the as-prepared CNS exhibits an interconnected bridge-like structure, which may favor the electrical transport with the neighboring CNS. The subsequent experiment
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and analysis are made based on the CNS fabricated in the optimal condition. [42, 43]
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3.2 Structure Characterization
The SEM and TEM images of CNS, CNS-PDA and CNS-PDA/Ag are displayed in Fig. 2. The as-synthesized CNS is observed to have a uniform spherical structure with a bridge-like structure interlinked each other and a rough surface with an average diameter of approximate 200 nm, Fig. 2a&1c. This connection structure of CNS still exists after surface modification by PDA, Fig. 2d. Meantime, the as-prepared CNS-PDA/Ag metacomposites still maintain the bridge 9
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structure between the neighboring CNS, and the Ag nanoparticles with an average diameter of about 10-20 nm are uniformly distributed on the surface of CNS, Fig. 2b&1e. Fig. 2f
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demonstrates a high-resolution TEM (HRTEM) image of CNS-PDA/Ag. The calculated d-spacing value of 2.359 Å in the lattice fringe corresponds to the (1 1 1) crystallographic planes of Ag. In this work, the EDX elemental mapping in Fig. 3 shows the (a) zero loss image,
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elemental maps of (b) C, (c) N, (d) O, (e) Ag and (f) the summation of the C, N, O, Ag elements,
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which further illustrates the successful deposition of Ag nanoparticles on the surface of CNS-PDA. On the contrary, without the substrate of CNS, the large Ag particles are randomly distributed on the surface of PDA, giving a non-uniform structure, Fig. S2b, (Fig. S2a is the SEM image of pure PDA). This demonstrates the significance of CNS as a support.
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Fig. 4A displays the XRD patterns of CNS, pure PDA, CNS-PDA, pure Ag nanoparticles and CNS-PDA/Ag. In Fig. 4A-a, the XRD diffraction pattern of CNS shows two wide diffraction peaks at 2θ = 24 and 42 o, respectively. The XRD diffraction pattern of PDA displays a broad
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diffraction peak at 2θ = 23 o, which is much wider than that of CNS around 2θ = 24 o, Fig. 4A-b. In comparison, the XRD diffraction pattern of CNS-PDA shows two wide diffraction peaks at 2θ
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= 23 and 42 o, Fig. 4A-c. These results indicate that the CNS, PDA and CNS-PDA are amorphous.[44] In the XRD diffraction pattern of pure Ag nanoparticles, Fig. 4A-d, the diffraction peaks 2θ = 38.3, 44.5, and 64.6
o
correspond to the (1 1 1), (2 0 0), (2 2 0)
crystallographic plane for the crystal structure of silver. The XRD diffraction pattern of CNS-PDA/Ag metacomposites (Fig. 4A-e) is similar to that of Ag nanoparticles, except that the positions of these diffraction peaks obviously shift. This implies that the Ag nanoparticles are 10
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successfully deposited on the surface of CNS-PDA,[45] and the presence of the CNS-PDA does not cause the crystal structure change of Ag nanoparticles.
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Fig. 4B depicts the FTIR spectra of CNS, pure PDA, CNS-PDA and CNS-PDA/Ag. Fig. 4B-a shows the FTIR spectrum of CNS, in which the absorption peak appears only at 1036 cm-1, corresponding to the stretching vibration of C-O-C arising from the oxygen-containing
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functional groups remained on the surface of CNS during the preparation process. Fig. 4B-b
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shows the FTIR spectrum of PDA. The strong absorption peaks at 1289, 1505 and 3227 cm-1 belong to the C-N stretching vibration, C-H stretching vibration, -OH, and -NH hydrogen bond vibration, respectively [46-49]. Compared with the FTIR spectra of CNS and PDA, the absorption peak of CNS-PDA is almost the same as that of PDA, but the peak position is shifted.
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This indicates the successful coating of PDA on the surface of CNS, Fig. 4B-c. The FTIR spectrum of C-PDA/Ag metacomposite is given in Fig. 4B-d. The absorption peak is similar to that of CNS-PDA. The presence of peak shifts indicate that there is a certain interaction between
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PDA, Ag nanoparticles and CNS.[50]
Fig. 4C shows the Raman spectra of (a) CNSP, (b) CNS, (c) CNS-PDA and (d)
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CNS-PDA/Ag. It turns out that these four samples have the D peak at 1347 cm-1, which is due to the formed C-C by the sp3 orbital hybridization, and G peak at 1595 cm-1 from C=C stretching vibration. Normally, a stronger D peak means more oxygen-containing functional groups and more defects on the carbon material. And a stronger G peak implies a higher degree of graphitization of a material [51]. In this work, the G peak is obviously enhanced after carbonized at high temperature, Fig. 4C-b, illustrating that the graphitization of CNS is enhanced after 11
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annealing. In general, the ratio of the integrated intensity of D-band and G-band (ID/IG) representing the peak area integral of D peak divided by the peak area integral of G peak, can be
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used to describe the degree of graphitization of a carbon material and the degree of oxygen-containing functional groups on the surface of a carbon material [52]. The computed ID / IG of CNSP, CNS, CNS-PDA and CNS-PDA/Ag metacomposite are shown in Table S1. The
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CNSP is concluded to have more oxygen-containing functional groups and a lower degree of
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graphitization than the CNS. After modified by PDA, the surface of CNS is covered with a layer of PDA, and the ID/IG value is increased due to the change of CNS surface structure during the modification process, resulting in an increase in the surface oxygen-containing functional groups. The increased ID/IG value of the CNS-PDA/Ag metacomposite is due to the surface structure
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change of the CNS during the deposition of Ag nanoparticles.[27, 31, 53] Fig. 4D illustrates the TGA curves of (a) CNS, (b) CNS-PDA and (c) CNS-PDA/Ag in the air condition. The thermal degradation curves of these three materials mainly have one
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degradation stage. The mass loss from 25 to 150 oC is mainly due to the water evaporation from
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the material. The thermal oxidative degradation of CNS is from 550 to 820 oC, Fig. 4D-a. The final residue percentage is essentially negligible in the case of error tolerance. The thermal oxidation degradation of CNS-PDA is wider than that of CNS, Fig. 4D-b. The interval starts from about 300 and ends at around 840 oC. Under the error tolerance range, the residual mass percentage is basically zero. The degradation of PDA molecular chain causes a widening of the range. According to the degradation range of CNS (550-820 oC), we can infer that the degradation before 550 oC is due to the PDA layer which is coated on the surface of CNS. The 12
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thermal degradation process of CNS-PDA/Ag metacomposites is between 350 and 720 oC, combined with the thermal degradation curves of CNS and CNS-PDA, Fig. 4D-c. The weight
residue is about 66%. [24, 28, 54] 3.2 Impedance, Electrical Conductivity and Optical Band Gap
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percentage of Ag nanoparticles in the CNS-PDA/Ag metacomposites estimated from the weight
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Fig. 5A illustrates the AC resistance (Zꞌ) of (a) CNS, (b) CNS-PDA and (c) CNS-PDA/Ag in different frequency ranges at room temperature. It comes out that in the measured frequency
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range of 20-2×106 Hz, the Zꞌ of CNS and CNS-PDA/Ag is basically fixed at about 16 and 5 Ω, respectively. However, the Zꞌ of CNS-PDA decreases from 320 to 290 Ω as the frequency increases from 20 to 2×106 Hz arising from the nonlinear polarization behavior of dipole in the
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PDA polymer,[55] which is due to the rotation between two equivalent equilibrium positions of dipole and the spontaneous arrangement of dipoles in an equilibrium position [56-58]. Fig. 5B shows the frequency dependent reactance for (a) CNS, (b) CNS-PDA and (c) CNS-PDA/Ag. The
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CNS (Fig. 5B-a) and CNS-PDA/Ag (Fig. 5B-c) possess positive reactance values, suggesting
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that both of them perform an inductance characteristic. Normally, the inductance character in a material implies the formation of conductive networks [3, 26]. In the sample of CNS-PDA, Fig. 5B-b, the reactance Zꞌꞌ is positive below 2000 Hz and changes to negative with further increasing the frequency, which means that the sample transfers from inductor to resistor and to capacitor [59]. In order to further confirm the reactance results, the electrical conductivity of CNS, CNS-PDA and CNS-PDA/Ag were performed. Fig. 5C presents the electrical conductivity of CNS, CNS-PDA and CNS-PDA/Ag being about 0.62, 0.03 and 1.21 S cm-1, accordingly. 13
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Generally, the electrical conductivity of a material is relevant to its band gap. In order to calculate the optical band gap (Eg) of CNS, CNS-PDA and CNS-PDA/Ag, the UV-vis DRS
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spectra were obtained. The plot of (αhν2) vs. photon energy (hν) for CNS, CNS-PDA and CNS-PDA/Ag is respectively listed in Fig. 5D, E&F, in which α is the absorption coefficient (α = (1 - R%)2/(2 × R%)), R is reflectance obtained from UV-vis DRS spectra, ν is the frequency,
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hν =1240/λedge, and λedge is the UV-vis absorption edge [60]. The Eg can be evaluated using
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Equation (1):
αhν = Α(hν-Eg)n
(1)
where Α is the constant, n = 0.5 offers the best fit for the optical absorption data of a material. The Eg can be obtained by extrapolating the straight portion of the graph as indicated in the Fig.
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5D, E&F. The acquired Eg of CNS, CNS-PDA and CNS-PDA/Ag is about 3.6, 3.9 and 3.2 eV, respectively, which is increased after coating with PDA polymer and then decreased with
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depositing the Ag nanoparticles, consistent with the obtained electrical conductivity and
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reactance results [61].
3.3 Permittivity Analysis
Fig. 5G, H&I depicts the real permittivity (ε'), imaginary permittivity (ε") and dielectric
loss tangent (tan δ, where tan δ = ε"/ε') as a function of frequency for (a) CNS, (b) CNS-PDA, (c) CNS-PDA/Ag and (d) CNSP at room temperature. Fig. 5G provides the ε' of the aforementioned materials within the frequency range of 20-2×106 Hz and the magnified image offers the corresponding ε' in the frequency range of 2×103-2×106 Hz. It is clearly seen that the CNSP 14
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exhibits a positive permittivity with a value of around 2 over the measured frequency range, Fig. 5G-d, whereas CNS displays a significant negative permittivity especially at low frequency
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(around -1.2 × 105), Fig. 5G-a. Normally, the negative permittivity is related to the formed continuous conduction networks in the system and correlated to the electrical conductivity of the materials [62-64]. This indicates that after carbonization, CNS has a higher degree of
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graphitization and an increase in the electrical conductivity (0.62 S cm-1) than CNSP (which is
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beyond the detection limit of the KDY-1 instrument (10-4 S cm-1)). After the PDA modification, the surface structure of CNS has more oxygen-containing functional groups, Fig. 5G-b, which causes the decrease in the electrical conductivity of CNS (0.03 S cm-1) and further affects the dielectric properties of CNS-PDA (decrease to 2.5 × 104 at low frequency). As the Ag
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nanoparticles are deposited on the surface of CNS-PDA by the reduction of PDA, Fig. 5G-c, Ag nanoparticles can not only enhance the overall electrical conductivity of the CNS-PDA (increases up to 1.21 S cm-1), but also expand the structure of CNS-PDA, forming a new continuous
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conductive path. Therefore, the negative permittivity value of the CNS-PDA/Ag is increased a
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lot (about -6.1 × 105 at low frequency) compared with CNS (around -1.2× 105) as expected. Generally, in a material, the permittivity is negative and the reactance is positive, implying this material is inductive in nature [65]. This is consistent with the results obtained in the reactance analysis. It’s reported that the electromagnetic response of metacomposites is reflected in the frequency dependent permittivity [66, 67]. The appearance of negative permittivity in the CNS and CNS-PDA/Ag is due to the increase in the electrical conductivity after both carbonization and Ag nanoparticles deposition. For the CNS-PDA/Ag sample, the enhanced negative 15
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permittivity may be also attributed to the Maxwell-Wagner-Sillars effect [68], in which three types of materials including CNS, PDA and Ag exhibit anisotropy in the dielectric properties and
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electrical conductivity, further influencing the electrical characteristics of the final metacomposites of CNS-PDA/Ag under an electric field. In the case of applying an external electromagnetic field, the charge accumulation occurs inside the medium, leading to the
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enhancement of negative real permittivity. Conversely, the ε' of PDA/Ag expresses a positive
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permittivity with a value of about 2.0, analogous to that of CNSP, verifying the crucial role of existence of CNS for the establishment of an identical metacomposites based on the PDA and Ag nanoparticles. This result is consistent with SEM and TEM outcomes, in which the CNS could serve as a frame for the uniform growth of PDA with a subsequent even deposition of Ag
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nanoparticles. In Fig. 5H-d, similar to ε', the ε" of CNSP also displays a constant with a value of approximate 0. The ε" value of CNS, CNS-PDA, and CNS-PDA/Ag reveals a gradual downward trend with the variations of the frequency. This is resulted from the energy consumed in the
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polarization process with forming an electric dipole [69]. Fig. 5I illustrates the corresponding tan
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δ for CNS, CNS-PDA, and CNS-PDA/Ag, which refers to the dielectric spent electricity and heat loss in the alternating electric field [70]. There is a very interesting phenomenon that the CNS-PDA/Ag has a higher tan δ than CNS and CNS-PDA below 1000 Hz, but a lower tan δ above 1000 Hz, which may be contributed to the interface polarization [71, 72].
4. Conclusions
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To sum up, CNS-PDA/Ag metacomposites have been successfully prepared by a hydrothermal process of D-xylose and high temperature annealing of CNSP combined with the
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self-polymerization of dopamine and in-situ reduction of silver nitrate verified by XRD, FTIR, Raman spectra. The SEM and TEM results confirm that the Ag nanoparticles have been uniformly dispersed on the surface of CNS-PDA with an average diameter of about 10-20 nm.
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The TGA analysis manifests that the loading of Ag nanoparticles in the CNS-PDA/Ag
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metacomposites is about 66%. The deposition of Ag nanoparticles on the surface of CNS-PDA can efficiently form the conductive pathway for charge charier transport, further increasing the electrical conductivity of CNS-PDA/Ag metacomposites. This is also affirmed by optical band gap acquired from UV-vis DRS. The CNS acts as a supporter for the uniform cover of PDA and
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even growth of Ag nanoparticle, and the CNS plays a vital role for obtaining the negative permittivity in CNS-PDA/Ag metacomposites. Relative to pure CNS and the CNS-PDA, the high negative permittivity in CNS-PDA/Ag metacomposites is also ascribed to the anisotropy in the
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dielectric properties and electrical conductivity of CNS, PDA, and Ag nanoparticles. The
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negative permittivity and positive reactance suggest that the CNS-PDA/Ag metacomposites are inductance in feature. In comparison, without CNS, the large Ag particles are randomly distributed on the surface of PDA, giving a non-uniform structure and a positive permittivity around 2.0. Our CNS-PDA/Ag nanocomposites if combined with other functional polymers or nanoparticles [73-78] have potential applications in the field of wearable electronic devices, sensors, microwave absorbing materials and electromagnetic interference shielding [79-94].
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Acknowledgement The authors are grateful for the support and funding from the Foundation of National
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Natural Science Foundation of China (No. 51703165), Young Elite Scientist Sponsorship Program by CAST (YESS, No. 2016QNRC001). This work is supported by Shanghai Science and Technology Commission (14DZ2261100).
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Figures and Figure Captions
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Fig. 1 Preparation procedure of carbon nanosphere-polydopamine/silver (CNS-PDA/Ag) metacomposites.
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Fig. 2 SEM images of (a) CNS, (b) CNS-PDA/Ag, TEM images of (c) CNS, (d) CNS-PDA, (e) CNS-PDA/Ag and (f) HRTEM image of CNS-PDA/Ag.
Fig. 3 EDX elemental mapping of CNS-PDA/Ag, (a) zero-loss image, (b) C map, (c) N map, (d) O map, (e) Ag map, and (f) C + N + O + Ag map. 26
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Fig. 4 (A) XRD patterns of (a) CNS, (b) PDA, (c) CNS-PDA, (d) Nano-Ag, (e) CNS-PDA/Ag. (B) FT-IR spectra of (a) CNS, (b) PDA (c) CNS-PDA, (d) CNS-PDA/Ag. (C) Raman spectra of (a) CNSP, (b) CNS (c) CNS-PDA, (d) CNS-PDA/Ag. (D) The TGA curves of (a) CNS, (b) CNS-PDA and (c) CNS-PDA/Ag.
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Fig. 5 (A) Frequency dependent resistance for (a) CNS, (b) CNS-PDA, (c) CNS-PDA/Ag; (B) frequency dependent reactance for (a) CNS, (b) CNS-PDA, (c) CNS-PDA/Ag; (C) electrical conductivity for CNS, CNS-PDA, CNS-PDA/Ag; Calculation of band gaps for (D) CNS; (E) CNS-PDA; and (F) CNS-PDA/Ag, obtained from UV-vis DRS spectra by plotting (αhν)2 vs. hν; (G) Real permittivity (ε'); (H) imaginary permittivity (ε"); and (I) dielectric loss tangent (tan δ) as a function of frequency for (a) CNS, (b) CNS-PDA, (c) CNS-PDA/Ag and (d) CNSP.
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