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Enhanced Electromagnetic Wave Interference by Nanoscale MixedDimensional C-MoS2 Magnetic van der Waals Heterostructures Erqi Yang, Xiaosi Qi, Guohui Zheng, Ren Xie, Zhongchen Bai, Yang Jiang, Shuijie Qin, and Wei Zhong ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01448 • Publication Date (Web): 05 Oct 2018 Downloaded from http://pubs.acs.org on October 8, 2018
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Abstract: It is generally believed that the matching of electromagnetic (EM) impedance or making the best of the interface polarization is an efficient way to achieve a good EM wave absorbing capability. Herein, the mixed-dimensional C-MoS2 based magnetic van der Waals heterostructures (vdWHs) were elaborately designed and successfully synthesized through the catalytic decomposition of C2H2 over Fe3O4-MoS2. By controlling the pyrolysis temperature, “3-0-2” type heterostructured C-Fe3O4-MoS2 ternary nanohybrid consisting of three-dimensional (3-D) carbon layers, zero dimensional (0-D) magnetite particles, two-dimensional (2-D) MoS2 nanosheets (NSs), and “1-0-2” type [email protected] vdWHs consisting of 1-D carbon nanotubes (CNTs), 0-D Fe and Fe3C nanoparticles, and 2-D MoS2 NSs were selectively synthesized. Compared to “0-2” type Fe3O4-MoS2, the obtained results demonstrated clearly that the introduction of carbon to form the mixed-dimensional C-MoS2 based magnetic vdWHs could greatly enhanced the EM wave absorption properties. Owing to the better optimization between magnetic loss and dielectric loss, more defect dipole and interface polarizations, the obtained “1-0-2” type [email protected] nanohybrid displayed enhanced microwave absorption property.
microwave absorption materials. Keywords: chemical vapor deposition, interface polarization, mixed-dimensional C-MoS2 based nanohybrids, carbon nanotubes, EM wave absorption capabilities
Introduction In the past decades, great efforts have been devoted to develop electromagnetic (EM) absorbers owing to the rapid development of wireless techniques, expansive problems of EM radiation and interference.1,2 High performance microwave absorption materials (MAMs) are required to possess a broad absorption frequency bandwidth, excellent EM attenuation capability, low density and good thermal stability features.3 Therefore, the applications of the traditional strong absorption MAMs such as ferrite, metals and their composites, 4,5 are greatly limited by their high density and large loading ratio.6 It is known that the EM loss characteristics can be well evaluated by the EM parameters ( µ and ε ) of MAMs. The optimization of EM parameters, namely EM impedance match, is important to improve microwave absorption performances (MAPs). And a good matching of EM impedance is able to achieve by modulating the categories and contents of nanohybrids.7,8 Accordingly, different categories of high-effective absorbents have been developed. For example, [email protected] nanotubes (CNTs) and [email protected] were proved to exhibit excellent MAPs.9,10 Very recently, graphene (G), reduced graphene oxide (RGO), and MXenes have been thought as a the representatives of novel MAMs because of their unique two-dimensional
properties,11,12 and similar results were also obtained over MoS2 nanosheets (NSs).13,14 However, due to the deficiency of their intrinsical characteristics (relatively low conductivity loss and high density), magnetic nanoparticles (NPs)-MoS2 NSs are very difficult to be used as the efficient MAMs. Moreover, the mixed-dimensional van der Waals heterostructures (vdWHs) based on 2-D layered nanohybrids have been extensively studied owing to their multiple interfaces, interlayer polarization and synergistic effects.15,16 Therefore, building the 3
mixed-dimensional 2-D materials based vdWHs can promote the attenuation of EM wave, which may be good candidates of high efficient MAMs.12 For example, 2-D G/zero-dimensional (0-D) metal oxide NPs, 2-D MoS2/one-dimensional (1-D) CNTs, and 2-D MoS2/three-dimensional (3-D) carbon layers were found to show excellent MAPs.17-19 However, the application of vdWHs in MAMs is subject to great restriction since the difficulty in their large-scale production at present. Based on the previously reported work,20,21 the optimization of permeability and permittivity and making full use of the interface polarization are primary strategies to achieve good MAPs. In order to utilize these two aspects effectively and develop high performance MAMs, in this paper, we elaborately designed and successfully synthesized the mixed-dimensional C-MoS2 based magnetic vdWHs by means of a combined hydrothermal process and chemical vapor deposition (CVD) method. By regulating the pyrolysis temperature, “3-0-2” type C-Fe3O4-MoS2 consisting of 3-D carbon layers, 0-D Fe3O4 NPs and 2-D MoS2 NSs, and “1-0-2” type [email protected] composing of 1-D CNTs, 0-D α-Fe and Fe3C NPs and 2-D MoS2 NSs could be selectively synthesized, respectively. The advantage of designing the nanohybrids can be mainly summed up five points: (i) the introduction of carbon can enhance the interface polarization and reduce the density of nanohybrids effectively; (ii) the EM parameters may be optimized through regulating the contents of Fe3O4/Fe NPs (magnetic loss substances) and C/CNTs/MoS2 (dielectric loss materials); (iii) CNTs and 2-D MoS2 with large-surface may enhance the interface polarization greatly, which can elevate their MAPs in theory; (iv) the construction of heteronanostructure may provide the synergetic effect, which can strengthen their MAPs greatly; and (v) the proposed strategy for the production of mixed dimensional magnetic C-MoS2 based vdWHs is simple and practical applied. 4
Experimental Synthesis of “0-2” Type Fe3O4-MoS2 Nanohybrid. Single layered MoS2 NSs were purchased from Xianfeng Nanomaterials Technology Co., Ltd (Nanjing, China). According to the work of Song et al., heterostructured “0-2” type Fe3O4-MoS2 nanohybrid was fabricated by a hydrothermal method.22 As illustrated in the I part of Figure S1 (Supporting Information), firstly, the transparent solution was obtained through ultrasonic mixing 30 mL ethanol with 30 mL deionized water, and MoS2 powder (50 mg) was dispersed in this mixture. 183.8 mg of FeCl3·6H2O and 70.3 g of FeCl2·4H2O were added in 10 mL deionized water under stirring. Subsequently, the configured Fe2+ and Fe3+ solution was mixed with the formed MoS2 liquid under stirred at 80 °C for 30 min. After that, 3 mL of NH4OH was dropwise added into the obtained solution. Finally, the formed solution was transferred to a autoclave (200 mL) and maintained at 120 °C for 120 minutes. And “0-2” type Fe3O4-MoS2 nanohybrid could be obtained after the washing and dry processes. Synthesis of Mixed-Dimensional C-MoS2 Based Magnetic vdWHs. As shown in the II part of Figure S1, similar to our previous work,23 the mixed-dimensional C-MoS2 based magnetic vdWHs were fabricated by means of the CVD method. Typically, 0.1 g of “0-2” type Fe3O4-MoS2 was used as the catalyst precursor, which was put into a tube furnace. And the furnace was heated to 400 °C in N2. Then, N2 was shut off and C2H2 was immediately introduced into the tube at atmospheric pressure. After C2H2 catalytically decomposed at 400 °C for 30 min and cooled to room temperature (RT) in N2, the black sample (denoted as S400) could be obtained. With the other experimental condition unchangeable, the decomposition of C2H2 was conducted at 500 °C. After cooling to RT in N2, much larger quantities of the as-prepared sample at this case (labeled as S500) could be obtained than S400. 5
Characterization and Measurement. The as-prepared Fe3O4-MoS2 and C-MoS2 based nanohybrids were characterized by X-ray powder diffractometer (XRD) (model D/Max-RA, Rigaku), transmission electron microscope (TEM) (model Tecnai-G20), field-emission scanning electron microscope (FE-SEM) (model ZEISS SUPRA-40), Raman spectroscopy (Jobin-Yvon Labram HR800 instrument), X-ray photoelectron spectroscopy (XPS) (PHI Quantera system), and Quantum Design MPMS SQUID magnetometer (Quantum Design MPMS-XL), respectively. For microwave measurement, 10 wt% and 30 wt% of Fe3O4-MoS2, C-Fe3O4-MoS2 and [email protected] were mixed with paraffin and compacted into toroidal shaped samples (Rout: 7.0 mm, Rin: 3.0 mm), respectively. The measurement of complex permittivity ( ε = ε ′ − jε ′′ ) and complex permeability ( µ = µ ′ − jµ ′′ ) of the as-prepared composites were carried out by a vector network analyzer (Agilent E8363B). Results and discussion Microstructures and Components. Figure 1 presents the typical morphologies of the catalyst precursor, S400 and S500. As displayed in Figures 1a and 1b, large number of Fe3O4 NPs (as indicated by the black arrow) distribute on the surface of MoS2 NSs (as indicated by the red arrow), and the MoS2 NSs can effectively suppress the aggregation of Fe3O4 NPs. Similar to the previously reported Fe3O4 NPs-MoS2 and MoS2-Ni NPs,22,24 it can be seen clearly that the “0-2” vdWHs between the 0-D Fe3O4 NPs and 2-D MoS2 NSs are well established. Figures 1c and 1d display the microstructure of S400. Typical TEM images of S400 show that different sizes of Fe3O4 NPs are deposited on the surface of MoS2 NSs. Compared to that of the catalyst precursor (as shown in Figure 1b), the closer TEM investigation (as shown in Figures 1d and S2b) reveals that 0-D Fe3O4 NPs are fully encapsulated and joined together by 6
3-D layered carbon (as marked by the yellow square in Figure 1d and the blue ellipse in Figure S2b). The high resolution TEM (HRTEM) investigation (Figures S2c and S2d) obviously reveals the lattice fringe of graphite and Fe3O4 (311) plane, and their lattice distance are ca. 0.34 and 0.25 nm, respectively. Similar to the recently reported “201” type nanohybrid obtained over MoS2-Ni NPs,24 the “3-0-2” type C-Fe3O4-MoS2 vdWHs consisting of 3-D carbon layers, 0-D Fe3O4 NPs and 2-D MoS2 NSs can also be produced in large scale. Moreover, On basis of the previous results,23 it is understandable that “3-0-2” type C-Fe3O4-MoS2 vdWHs can grow over Fe3O4 NPs-MoS2. Figures 1e and 1f show the microstructure of S500. As provided in Figure 1e, large scale of flexible 2-D MoS2 NSs and CNTs are found throughout the S500. And the TEM images in the Figures 1f and S3b shows that Fe NPs are encapsulated tightly in turn by Fe3C and CNTs. And the HRTEM investigation (Figure S3c) illustrates that the lattice distance is about 0.61 nm, which corresponds to the (002) plane lattice fringe of MoS2.19 The obtained morphology of S500 is similar to the recently reported heterostructured “201” type MoS2-Ni-CNTs and G-Co-CNTs.23,24 In light of the obtained results, unlike that of S400, one can find that the obtained S500 is “1-0-2” type [email protected] vdWHs. To confirm the phases of the as-prepared samples, their XRD patterns are presented. The obtained catalyst precursor (Figure 2a) exhibits nine evident diffraction peaks, in which the diffraction peaks centered at 14.4 and 39.5° are well-indexed to the hexagonal phase of MoS2 (JCPDS No.24-0513). And a weak and broad peak (centered at ca. 14.4°) corresponding to (002) plane of MoS2, which can be ascribed to the few-layer MoS2 NSs used in our experiment,25 can also be observed. And the other labeled peaks located at the ca.18.3, 30.1, 35.4, 37.1, 43.1, 57.0 and 62.5° match well with the face-center cubic structure of Fe3O4 (JCPDS No.74-0748). There are no other 7
peaks corresponding to ferric oxide such as Fe2O3 and FeO are found. The existence of MoS2 can also be further proved by the XPS (Figure S4). The binding energy peaks of S 2p 3/2, Mo 3d 5/2 and Mo 3d 3/2 can be seen clearly at 161.1, 228.3 and 231.3 eV, respectively. Additionally, the peak at ca. 164.5 eV indicates the existence of bridging disulfides S 2, which confirms further that the existence of MoS2. The 2 results of XRD, XPS and microstructure (Figures 1) confirm the used catalyst precursor is “0-2” type Fe3O4-MoS2 vdWHs. Figure 2b gives the XRD pattern of S400. One can find that the characteristic peaks of MoS2 and Fe3O4 coexist in the obtained XRD results. Additionally, a new broad peak centered at about 26.2° is obviously detected, which should be corresponded to the shell layer of carbon (JCPDS No.03-0401). Based on the aforementioned microstructure images and compared to that of the catalyst precursor, the obtained results demonstrate that the as-prepared S400 is “3-0-2” type C-Fe3O4-MoS2 vdWHs. The similar result was also reported before.26 Unlike that of S400, as displayed in Figure 2c, the aforementioned characteristic peaks of Fe3O4 cannot be seen over the as-prepared S500. And the relative XRD peak intensity ratio of carbon evidently become high, which is attributed to the increasing carbon content with the enhancement of pyrolysis temperature as mentioned in the experimental section. Moreover, as indicated by the symbols, some new obvious diffraction peaks located at ca. 37.8, 41.6, 43.4, 57.3 and 45.1°well corresponds to the standard XRD data of Fe3C (JCPDS No.06-0670) and Fe (JCPDS No.01-1267), respectively. Based on the obtained results of phase and morphology, it can be seen that the obtained S500 is heterostructured “1-0-2” type [email protected] quaternary nanohybrid. In order to further confirm the existence of carbon material in the as-prepared samples, their Raman spectra were 8
obtained. As presented in Figure 2d, the obtained S400 and S500 exhibit two strong peaks at 1345 (D band) and 1586 cm−1 (G band), respectively. It is well known that the defects and/or disorder of carbon materials result in the appearance of D band while the G band originates from crystallite graphitic structure.27 Generally, the decomposition temperature plays a great impact on the formation of mixed dimensional C-MoS2 based vdWHs. The similar temperature effect has also been reported before.28 Overall, the obtained results can be summarized as two points: (1) all the nanohybrids obtained by the catalytic decomposition of C2H2 at 400 and 500 °C over Fe3O4-MoS2 are the mixed dimensional magnetic vdWHs; (2) different categories of mixed dimensional C-MoS2 based vdWHs can be selectively synthesized by regulating the decomposition temperature. Therefore, a convenient and effective scheme is designed to produce the mixed dimensional magnetic C-MoS2 based vdWHs. Possible Formation Mechanism for the Mixed Dimensional C-MoS2 Based vdWHs. On the basis of the experimental aim and our acquired results, different categories of C-MoS2 based vdWHs are selectively produced by controlling the experimental temperature. According to the previously related reports,29,30 the interactions between C2H2 and Fe3O4 occur at high temperature, in which C2H2 induces the carbothermic reduction to synthesize Fe, iron oxide and/or Fe will generate crystalline graphite. Similar to that of [email protected][email protected],30 our obtained results suggest that the chemical reaction between C2H2 and Fe3O4 does not occur under this conditions ( ≤ 400 °C), which is the reason for the formation of “3-0-2” type C-Fe3O4-MoS2 vdWHs. However, the carbothermic reduction will be happen immediately when the temperature rises to 500 °C, and metal Fe will be generated without the residual Fe3O4 phase. And the formation of “1-0-2” type 9
[email protected] quaternary nanohybrid may be explained by the following reaction: 8C 2 H 2 + 2 Fe 3 O4 → 15C + Fe 3 C + 3 Fe + 8 H 2 O
In this reaction, the reduction reaction between C2H2 and Fe3O4 will occur, and the element of Fe will be easily carbonized to produce Fe3C. According to the pervious results,31,32 the metal carbide is very important to form [email protected]@shell structured CNT-based hybrids. Therefore, the formation of “1-0-2” type [email protected] quaternary nanohybrid can be attributed to the reduction reaction between C2H2 and Fe3O4-MoS2. In light of our obtained experimental outcomes and the growth models for
[email protected] vdWHs can be well interpreted by the mode of tip-growth, and the diagram of the growth process is displayed in Figure 3. The possible pathways to grow “1-0-2” type [email protected] vdWHs are in this manners: (1) the reduction reaction between Fe3O4 and C2H2 to form Fe and Fe3C NPs; (2) the decomposition of C2H2 on the surface of Fe and Fe3C NPs to produce carbon atoms; (3) the dissolution and diffusion of carbon atoms around the catalyst; (4) the growth of carbon film along the surface of catalyst and inducing the generation of [email protected] vdWHs. And the formation of heterostructured “3-0-2” type C-Fe3O4-MoS2 ternary nanohybrid is ascribed to the in situ low-temperature carbonization. Therefore, the designed plan is efficient and applicable for the synthesis of mixed dimensional vdWHs, which can fulfill the simultaneous construction of double attenuation mechanism, synergistic effect and interface engineering to solve EM wave interference issue. Properties of Mixed Dimensional C-MoS2 Based vdWHs. Figure 4 displays 10
the RT magnetic hysteresis (M-H) curves of S400 and S500. It can be seen that the Ms and Hc values of S400 and S500 are 44.7 emu/g and 107 Oe, 19.9 emu/g and 202 Oe. Moreover, the comparison results show that: (1) the obtained samples exhibit the typical ferromagnetic properties at RT, which is attributed to the existence of magnetic Fe3O4 and Fe NPs; (2) compared with the Ms value of Fe3O4-MoS2 reported elsewhere,22
Fe3O4-MoS2>S400>S500, which proves further that the introduction of carbon material during the CVD process. Based on the obtained experimental results, much more quantities of samples can be collected over the same amount of Fe3O4-MoS2 with a rise of temperature. That is to say, the content of magnetic NPs in the as prepared samples is become lower and lower with the increasing of temperature, which is consistent with their magnetic measurement. Moreover, after being placed in air for thirty days, the C-MoS2 based vdWHs exhibit no changes in phase and magnetic results, which confirms further that the magnetic NPs are well protected by carbon layers or CNTs. In general, although the introduction of carbon material decreases their Ms values, the as-prepared hybrids display good stabilities. Additionally, their complex permittivity should be enhanced evidently and tuned by the temperature, which is propitious to achieve a good EM impedance match and enhance the MAPs. Figure 5 displays the EM parameters of composites with the 30 wt% filler loading. As shown in Figures 5a and 5b, besides weak fluctuations, the ε ′ and ε ′′ values of the obtained mixed dimensional C-MoS2 based vdWHs decrease when the frequency increases, which can be interpreted by the Debye theory.34 And the obtained S500 displays evident fluctuating behaviors of ε ′ and ε ′′ , which is ascribed to a significant lag of displacement current at the ‘[email protected]@shell’ interfaces.10,35,36 In 11
addition, it can be noticed that the obtained S500 presents the negative permittivity values. Actually, the negative permittivity behaviors have been observed previously in the conductive filler (such as metal NPs, CNTs) when its content exceeds the percolation threshold.37,38 However, the origin for the negative values of permittivity is still not fully established. Combined with the previous result, we think that the transport of more electrons to form CNTs-MoS2 conductive network due to the mass production of CNTs. Large quantities of capacitor networks are changed to conductive paths, which can be considered as an inductor. Therefore, the obtained S500 displays the negative permittivity behaviors, which is similar to the metallic wires reported in previous study.38 By comparison, we can find that the ε values of the obtained samples are as follows: Fe3O4-MoS2
σ , where ε p′′ and σ is the polarisation loss and electric ε 0ω
conductivity, respectively.1 The dielectric loss mainly origins from conduction loss and/or polarisation loss. Therefore, the as-prepared S400 and S500 present high dielectric loss compared to Fe3O4-MoS2, which is in accord with the above formula. Similar to Gr/[email protected]/Fe3O4/PANI,40 the in-situ carbonization process is good for the generation of interfacial sites, which results in the enhanced dielectric performance of the as-prepared C-MoS2 based vdWHs. Figures 5c and 5d give the complex permeability. The µ values of the obtained Fe3O4-MoS2 and S400 are almost unchangeable in 2-18 GHz, while the obtained S500 presents multi-resonance 12
peaks, which is related to spin wave excitations and small size and surface effects. The detailed explanations are given in previous studies.41,42 Overall, it can be seen clearly that the difference in the µ values of the obtained samples is unobvious, which is ascribed to an uneven distribution of magnetic NPs, their low magnetizations and slow saturation processes.43 It is widely recognized that the magnetic loss mechanism mainly origins from three points in the measured frequency range: (i) eddy current loss, (ii) exchange resonance, (iii) natural resonance. If only the eddy current
-2 µ ′′(µ ′ ) f −1 = 2πµ 0 d 2σ should be a constant even if f is variable. Where µ 0 is -2 permeability of vacuum, f is frequency. The curve between µ ′′(µ ′ ) f -1 and f
(Figure S5) implies that the magnetic loss mainly results from the non-uniform exchange and natural resonance in the low-frequency region and eddy current loss in the high-frequency range44,45 In general, we can find that the EM parameters are greatly affected by carbon content, and the carbon content in our designed experiments can be controlled by the decomposition temperature and time, which is calculated by the equation: carbon content=
mtotal -mcatalyst mtotal
. Therefore, the carbon
content can be obtained through weighing the catalyst precursor and as-synthesized sample. The values of reflection loss (RL) can be obtained on basis of the transmission line theory, and the equations are as follows:46 Z in = Z 0
Where d, c, Z0, Zin is the thickness of absorber, velocity of light, impedance of air and input impedance of absorber. In the light of these formulas, the RL values of catalyst precursor, S400 and S500 are provided in Figure 6. The comparison results reveal that: (1) the optimal RL values for S400 and S500 are ca. -45.1 dB at 12.0 GHz and -58.5 dB at 8.8 GHz; (2) RL values lower than -20 dB (99% of EM wave energy is absorbed) for S400 and S500 can be achieved in the 11.0-14.4 and 8.6-18.0 GHz frequency range; (3) RL values less than -10 dB for S400 and S500 can be found in 10.2-18.0 and 8.2-18.0 GHz frequency range, respectively. By comparison, you can find that the as-prepared mixed dimensional C-MoS2 based vdWHs displays greatly improved MAPs compared to Fe3O4-MoS2. And the obtained S500 shows better MAPs than S400. Moreover, even with the low filling ratio of sample (10 wt%), the obtained results still prove that the as-prepared C-MoS2 based vdWHs exhibit enhanced MAPs (Figure S6) compared to the catalyst precursor. In addition, compared with those representatively related nanohybrids (as shown in Table S1), the as-prepared mixed dimensional C-MoS2 based vdWHs also exhibit the excellent MAPs, which indicates that the mixed dimensional vdWHs may also be used as the lightweight and efficient MAMs. As is known to all, the MAPs of MAMs are closely related to their structures, morphologies, degree of crystallinity, etc. Therefore, the obtained mixed dimensional C-MoS2 based nanohybrids do not show significantly improved performance in comparison with previously reported MoS2-based materials,13,14 which should be ascribed to the poor property of catalyst precursor. Generally, our obtained results prove well that the construction of the mixed-dimensional C-MoS2 based magnetic vdWHs is conducive to design high 14
performance nanohybrids. And we do think that much more categories and higher performance MAMs can be designed and synthesized based on the previously reported MoS2-based materials by the proposed manner. According to the recently reported results, zero reflection and geometrical effect were used to interpret the excellent MAPs of absorbers.3,47 About zero reflection, on basis of EM wave theory, the ε and µ should have equal values.4 However, the values of permittivity (Figure 5) for the as-prepared mixed dimensional C-MoS2 based nanohybrids are higher than their permeability. Therefore, their excellent MAPs are not expounded by this mechanism. As regards the second model, the following equation should be satisfied:48
d m = nc
(n = 1, 3 ,5 L )
where d m is the absorber thickness, f m is the peak frequency, µ and ε are the measured values at f m . If the matching thickness of the absorber satisfies this equation, which implies that a cancellation of the reflected waves from air-absorber interface and absorber-metal interface, the absorber will display a good MAPs owing to the phenomenon of quarter-wavelength interference. According to equation (4), the theoretical values for d m can be gained (labeled as d msim ). As depicted in Figure 7, the d msim values of the obtained S400 and S500 are in accord with their d mexp values (directly obtained in Figure 6). Therefore, similar to that of Fe3O4-Fe/G,21 the geometrical effect can well explain the excellent MAPs of the obtained mixed dimensional C-MoS2 based nanohybrids. It is well known that some characteristic physical parameters such as dielectric and magnetic loss tangents ( tan δ E and tan δ m ), and attenuation constant ( α ) play a 15
As given in Figures 8a and 8b, the as-prepared mixed dimensional C-MoS2 based vdWHs exhibit higher tan δ E values than their tan δ m values, which indicate that the dielectric loss is the main contribution to attenuate EM wave. In addition, compared to Fe3O4-MoS2, the as-prepared S400 and S500 have higher tan δ E values, suggesting their excellent EM wave absorption capabilities. And the obtained S500 displays better mutual compensation effect than Fe3O4-MoS2 and S400 through the comparison between Figure 8a and Figure 8b, which is beneficial to heighten the MAPs.50 According to the equation (5), the α values of the obtained sample can be achieved, which are provided in Figure 8c. The result reveal that the α values of mixed dimensional C-MoS2 based vdWHs are significantly enhanced compared to Fe3O4-MoS2. And the obtained S500 exhibits higher α values than those of S400, indicating its excellent microwave absorption ability. Generally, it can be seen evidently that the introduction of carbon material through the CVD process can significantly improve their dielectric loss abilities, α
values and mutual
compensation effect. Among these samples, the related physical parameters of as-prepared S500 are the best, indicating its enhanced MAPs. Additionally, it is generally believed that one of the efficient ways to improve MAPs of MAMs is to construct multiple interface structures, which can greatly enhance interface polarization. And the main purpose of the designed experiment is to enhance interface polarization through the introduction of carbon. In order to prove 16
this viewpoint, the plots of ε ′ versus ε ′′ (Figure S7) for the obtained samples are presented. On the basis of the Debye relaxation expression, ε ′ and ε ′′ satisfy the following equation:
ε + ε∞ ε ′ − s 2
ε − ε∞ 2 + (ε ′′ ) = s 2
ε +ε The above equation represents a single semicircle centered at s ∞ , 0 , which is 2 defined as the Cole-Cole semicircle and is also characteristic of the Debye relaxation process. As presented in Figures S7a-S7c, the plot of ε ′ versus ε ′′ for “0-2” type Fe3O4-MoS2 exhibits six relatively big semicircles, and the as-prepared “3-0-2” type C-Fe3O4-MoS2 and “1-0-2” type [email protected] have much more semicircles. The comparison result indicates that the introduction of carbon greatly enhance multirelaxations, which results from the existence of multiple interfacial polarizations in the mixed dimensional C-MoS2 based vdWHs compared to “0-2” type Fe3O4-MoS2.13 In order to corroborate the experimental results qualitatively, we also did some ab initio calculations using simple slab model. The calculations were carried out using Vienna ab initio simulation package (VASP) and Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional.51,52 The potential plots of Fe3O4-MoS2 and C-Fe3O4-MoS2 slab model in Figure S7d within which we show planar average electrostatic potential in the planes parallel to the interface. The macroscopic average technique introduced by A. Baldereschi and coworkers,53 is used to obtain the macroscopic potentials (the solid lines) and an immediate parameter of 8 Å is used to average out the oscillatory microscopic potential (the dashed lines) in certain extent. For the Fe3O4-MoS2 model (black lines), we can see a potential drop across MoS2/Fe3O4 interface, and thus conclude an induced dipole there. For the 17
C-Fe3O4-MoS2 model (blue lines), the potential drop across the MoS2/Fe3O4 interface is barely affected upon graphite deposition, owing to weak van de Waals interactions. Moreover, a potential drop and thus a dipole are also presented across the graphite/Fe3O4 interface with the dipole direction parallel with that across MoS2/Fe3O4 interface and enhancing the dipole field. Generally, the aforementioned results demonstrate that the introduction of carbon to form the mixed dimensional C-MoS2 based vdWHs greatly improve the MAPs. As depicted in Figure 9, firstly, the structures of the obtained nanohybrids are the key of excellent MAPs. The incident EM wave can be repeatedly reflected and scattered between numerous interfaces, which greatly improves the attenuation of the incident EM wave. Secondly, in this work, the formation of carbon layers and CNTs is the result of the catalytic decomposition of acetylene over Fe3O4-MoS2. And 2-D MoS2 NSs are annealed at high temperature, which destroys the inherent intrinsic between atoms. According to the previously related works,13,24 much more defect dipole polarizations were formed on the surface of amorphous carbon, CNTs and 2-D MoS2. And the interface polarization occurred between carbon layers, Fe3O4 and MoS2 or CNTs, Fe, Fe3C and MoS2. Therefore, the introduction of carbon provides much more defect dipole and interface polarizations, which enhances further their microwave absorption abilities. Thirdly, according to Cao’s work and the related papers,54,55 equivalent circuit mode is also applied in our work. C/CNTs and MoS2, Fe3O4 NPs, Fe and Fe3C mix are thought as resistance for hopping electrons (Rh) and resistance for migrating electrons (Rm), respectively. As shown as Figure 9, electrons will be greatly stimulated and directionally move, which ultimately results in the formation of induced microcurrent. According to Ohm’s law, the energy of EM wave will be converted into Joule heat, which is eventually released.56 18
Conclusion In order to improve the attenuation of EM wave, the mixed dimensional C-MoS2 based vdWHs were elaborately constructed and produced through the catalytic decomposition of C2H2 over “0-2” type Fe3O4-MoS2 for taking full advantage of dielectric and magnetic loss capabilities, their synergistic effect and the multiple interface structures. By controlling the pyrolysis temperature, heterostructured “3-0-2” type C-Fe3O4-MoS2 ternary and “1-0-2” type [email protected] quaternary vdWHs were selectively produced. The obtained results showed that the designed mixed dimensional C-MoS2 based vdWHs showed excellent MAPs, which were attributed to the geometrical effect. Owing to better optimization of dielectric and magnetic loss materials, and much more interface polarization, the as-prepared mixed-dimensional C-MoS2 based magnetic vdWHs exhibited significantly improved EM property and MAPs compared to the catalyst precursor. Therefore, mixed-dimensional C-MoS2 based magnetic vdWHs are essential to take full advantage of dielectric and magnetic loss capabilities, and control the relaxation behavior by regulating interfaces between 2-D MoS2 and other dimensional magnetic substances, which is a good alternative route to develop high performance MAMs. Supporting Information Available: Table S1, Detail comparison of EM wave absorption properties among the previously reported magnetic 2-D material based nanohybrids; Figure S1, Schematic illustration of the mixed-dimensional C-MoS2 based magnetic vdWHs production process; Figure S2, TEM images of S400; Figure S3, TEM images of S500; Figure S4, XPS spectra of S-400; Figure S5, Frequency dependent eddy-current loss curves of the samples; Figure S6, Three-dimensional RL profiles of composites with filler loadings of 10 wt %; Figure S7, Cole-Cole plots of Fe3O4-MoS2, S400, S500, and the microscopic potential and macroscopic-averaged 19
potential for Fe3O4-MoS2 and C-Fe3O4-MoS2 slab models. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]
Phone: +86-25-83621200. Fax: +86-25-83595535.
*E-mail: [email protected] Phone: +86-25-83621200. Fax: +86-25-83595535. ORCID Xiaosi Qi: 0000-0003-0987-1622 Wei Zhong: 0000-0003-2507-3479 Notes The authors declare no competing financial interest. Acknowledgments This work was supported by the Platform of Science and Technology and Talent Team Plan of Guizhou province (2017-5610 and 2017-5788), and the National Science Foundation of China (Grant Nos. 11604060). References (1) Qin, F. X.; Peng, H. X. Ferromagnetic Microwires Enabled Multifunctional Composite Materials. Prog. Mater. Sci. 2013, 58, 183-259. (2) Wang, Y. M.; Li, T. X.; Zhao, L. F.; Hu, Z. W.; Gu, Y. J. Research Progress on Nanostructured Radar Absorbing Materials. Energy Power Eng. 2011, 3, 580-584. (3) Liu, J. W.; Che, R. C.; Chen, H. J.; Zhang, F.; Xia, F.; Wu, Q. S.; Wang, M. Microwave Absorption Enhancement of Multifunctional Composite Microspheres with Spinel Fe3O4 Cores and Anatase TiO2 Shells. Small 2012, 8, 1214-1221. (4) Liu, X. G.; Jiang, J. J.; Geng, D. Y.; Li, B. Q.; Han, Z.; Liu, W.; Zhang, Z. D. Dual Nonlinear Dielectric Resonance and Strong Natural Resonance in Ni/ZnO Nanocapsules. Appl. Phys. Lett. 2009, 94, 053119. 20
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Figure Caption Figure 1. FE-SEM and TEM images of (a,b) Fe3O4-MoS2, (c,d) S400, and (e,f) S500, respectively. Figure 2. (a-c) XRD patterns, and (d) Raman spectra of the catalyst precursor, S400 and S500, respectively. Figure 3. Schematic diagram for the formation mechanism of “3-0-2” type C-Fe3O4-MoS2 and “1-0-2” type [email protected] vdWHs, respectively. Figure 4. Magnetic hysteresis loops for (a) S400, and (b) S500 at 300 K (inset is the enlarged part close to the origin). Figure 5. EM parameters of the obtained samples: (a,b) real and imaginary parts of permittivity, and (c,d) real and imaginary parts of permeability. Figure 6. Three-dimensional RL curves of (a) Fe3O4-MoS2, (b) S400, and (c) S500; (d) RL versus frequency of the as-prepared samples with the thicknesses of 4.9 mm. Figure 7. Comparison of dm for (a,c) S400, and (b,d) S500 under the quarter-wavelength matching model at fm. Figure 8. (a,b) Loss tangent, and (c) attenuation loss of the obtained samples. Figure 9. Schematic illustration of possible microwave absorption mechanism of (a) S400, and (b) S500.