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Article Cite This: Langmuir 2018, 34, 15854−15863

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Rational Construction of Ti3C2Tx/Co-MOF-Derived Laminated Co/ TiO2‑C Hybrids for Enhanced Electromagnetic Wave Absorption Qiang Liao,†,‡,§ Man He,*,†,§ Yuming Zhou,*,†,§ Shuangxi Nie,‡ Yongjuan Wang,†,§ Beibei Wang,†,§ Xiaoming Yang,† Xiaohai Bu,† and Ruili Wang†,§ †

School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control, College of Light Industry and Food Engineering, Guangxi University, Nanning 530004, China § Jiangsu Optoelectronic Functional Materials and Engineering Laboratory, Nanjing 211189, China Langmuir 2018.34:15854-15863. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/21/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: Lightweight and compatible metal−organic framework (MOF)-derived carbon-based composites are widely used in electromagnetic (EM) absorption. Their combination with laminated TiO2-C (derived from Ti3C2Tx) is expected to further strengthen the EM attenuation ability. Herein, novel laminated Co/TiO2-C hybrids were derived from Ti3C2Tx/CoMOF using heat treatment. Compared with pristine MOF-derived carbon-based composites, the EM absorption ability of Co/ TiO2-C was improved by multiple reflections between multilayered microstructures and the improved polarization loss (due to the heterogeneous interfaces, residual defects, and dipole polarization) and the strengthened conductivity loss caused by the carbon layers. Specifically, for the Co/TiO2-C hybrids at thicknesses of 3.0 and 2.0 mm, the optimal reflection loss (RL) was −41.1 dB at 9.0 GHz and −31.0 dB at 13.9 GHz, with effective bandwidths (RL ≤ −10 dB) of 3.04 and 4.04 GHz, respectively. This study will underline the preparation of carbon-based absorbing materials starting from MXene/MOF hybrids.

1. INTRODUCTION The extensive use of wireless communications in military and civil fields has led to a potential threat of electromagnetic (EM) wave pollution.1,2 High-performance microwave absorbents may be an effective solution to the problems caused by EM pollution.3−5 Generally, promising EM wave absorbents should possess a strong absorption ability and satisfy the requirements of practical applications, such as being thin and lightweight with a broad frequency.6,7 So far, efficient EM absorbents include carbon-based materials,8,9 magnetic materials,10 and conduction polymers and their hybrids11−13 but are still limited by a high density, narrow absorbing bandwidth, low dispersibility, and risk of corrosion. Hence, viable methods are needed to handle these limitations. Metal−organic frameworks (MOFs), a category of organic− inorganic hybrids, have become increasingly attractive for a variety of potential applications.14−17 Owing to their unique structures, MOFs are considered to be ideal precursors for preparing nanostructured metal/C via facile pyrolysis and © 2018 American Chemical Society

demonstrate intriguing EM absorbing properties. For instance, Lv et al. successfully fabricated porous Co/C nanocomposites from Co-MOF through pyrolysis.18 Liu et al. prepared porous Ni/C composites from Ni-based MOF.19 However, MOFderived carbon is usually amorphous, and its dielectric loss should be enhanced so as to further improve the EM absorbing ability.20−24 Moreover, the EM attenuation performance can be further enhanced by combining magnetic loss and dielectric loss. One type of microwave absorbent, 2D transition-metal carbide/nitride materials labeled as MXenes,25,26 have high microwave absorption properties due to the their 2D multilayered microstructures, large specific surface area, abundant functional groups, metallicity, and good chemomechanical properties. However, the low dielectric loss of pristine MXene reduces the absorption ability. Noticeably, the TiO2 phase Received: September 24, 2018 Revised: November 14, 2018 Published: December 3, 2018 15854

DOI: 10.1021/acs.langmuir.8b03238 Langmuir 2018, 34, 15854−15863

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Figure 1. Schematic illustration of the synthesis of laminated Co/TiO2-C composites.

Figure 2. (a−c) XRD patterns of Ti3AlC2, Ti3C2Tx, Co-MOF, Ti3C2Tx/Co-MOF, TiO2-C, and Co/TiO2-C. (d) Raman spectra of TiO2-C and Co/ TiO2-C.

layer with TiO2 after MXene pyrolysis forms heterogeneous interfaces, which are beneficial to energy dissipation. Herein, given the weak dielectric loss of pure MOF-derived carbon-based composites for EM absorption, TiO2 with strong dielectric loss can be reasonably used to modulate the complex permittivity of MOF-derived carbon absorbents.29−34 Furthermore, the heterogeneous laminated microstructures of TiO2modified MOF-derived carbon-based composites (derived from Ti3C2Tx) further strengthen EM attenuation owing to the multiple reflections and prolonged EM transmission route. More importantly, the effective dual loss can be maintained by anchoring Co and TiO2 nanoparticles on a carbon layer, respectively, thus enhancing the EM attenuation ability.

derived from Ti3C2Tx by rapid annealing, which improves the dielectric loss and impedance match, can enhance the EM absorption ability. Han et al. made 2D laminated disordered C/ TiO2 nanoparticles from Ti3C2Tx through heat treatment.27 The reflection loss (RL) was minimized to −36 dB at 15.5 GHz for a thickness of 1.6 mm. Li et al. prepared Ti2CTx and its derivatives with different heterogeneous interfaces via simple heat treatment.28 RL was minimized to −50.3 dB at 7.1 GHz for the matching thickness of 2.1 mm. Moreover, as a class of EM absorbing materials with unique laminated structures and chemical stability, MXenes derivatives (TiO2/C) are potential supporting materials that may adjust dielectric loss MXenes in multicomponent systems. Additionally, the laminated carbon 15855


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Figure 3. SEM images of (a) Ti3C2Tx, (b) Co-MOF, (c) Ti3C2Tx/Co-MOF, and (d and e) the Co/TiO2-C hybrid derived from laminated Ti3C2Tx/ Co-MOF at different viewpoints.

Figure 4. (a and b) TEM and HRTEM images of Co/TiO2-C. 2.2. Synthesis of Laminated Ti3C2Tx. Laminated Ti3C2Tx particles were prepared by liquid etching in a 40 wt % aqueous HF solution to extract Al from Ti3AlC2, according to our previous study.6 2.3. Synthesis of Laminated Ti3C2Tx /Co-MOF Hybrids. Laminated Ti3C2Tx/Co-MOF hybrids were prepared using a facile liquid-phase deposition method. First, Ti3C2Tx (0.12 g) and Co(NO3)2·6H2O (0.291 g) were dissolved in 25 mL of methanol and ultrasonically treated at room temperature for 30 min. The resulting solution was added to 25 mL of a methanol solution containing 0.33 g of C4H6N2 under stirring for 5 min. After placement at 25 °C for 24 h, the precipitate was collected, centrifuged, washed with methanol consecutively, and vacuum-dried at 60 °C for 12 h. 2.4. Preparation of Laminated Co/TiO2-C Hybrids. The Ti3C2Tx/Co-MOF powder that was placed in a tube furnace was heated at a rate of 5 °C·min−1 to 800 °C and kept for 2 h in a flow of N2 to form laminated Co/TiO2-C hybrids. Ti3C2Tx powder was also held at 800 °C in N2, which was referred to as TiO2-C for property comparison. 2.5. Sample Characterization. The sample crystal structures were characterized on a Smart Lab X-ray diffractometer (XRD, Rigaku) with

On the basis of this, we successfully prepared laminated Co/ TiO2-C hybrids from Ti3C2Tx/Co-MOF through heat treatment. First, Co-MOF was anchored in situ onto the laminated Ti3C2Tx through simple liquid-phase deposition at room temperature. Then the Ti3C2Tx/Co-MOF was thermally decomposed in an inert atmosphere to form Co/TiO2-C hybrids. The EM absorbing capability and loss mechanism of Co/TiO2-C were comprehensively investigated. Co/TiO2-C significantly outperformed the magnetic metal/C composites (derived from MOF) from a previous report in the microwave absorption field.

2. EXPERIMENTAL SECTION 2.1. Materials. Ti3AlC2 powder (∼200 mesh, 98% purity, Forsman Technology (Beijing) Co., Ltd.), C4H6N2 (98%), HF (40 wt %), Co(NO3)2·6H2O (99%), and CH3OH (99%) were all used without further purification. 15856


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Figure 5. (a) Survey, (b) Co 2p, and (c) Ti 2p XPS spectra and (d) hysteresis loops of TiO2-C and Co/TiO2-C.

Figure 6. (a) N2 adsorption−desorption isotherms and (b) pore size distributions of TiO2-C and Co/TiO2-C. Cu Ka radiation at 2θ = 20−80°. The morphology and structures of the samples vacuum-coated with gold were observed under an FEI Inspect F50 scanning electron microscope (SEM). Samples coupled with copper grids were detected under a transmission electron microscope (TEM) and a JEM-2100 high-resolution TEM instrument. Magnetic hysteresis loops were detected with a model 7407 vibrating sample magnetometer (VSM, Lake Shore). Other instruments included a Thermo Fisher Raman microscope (laser wavelength 514 nm, spot size 150 μm) at 200−2000 cm−1 and a Thermo Scientifc Escalab 250 Xi Xray photoelectron spectrometer (XPS) at spot size of 650 μm and a pass energy of 30.0 eV. EM parameters of the hybrids were measured with an Agilent PNA N5224A vector network analyzer from 2 to 18 GHz. The test samples were prepared in paraffin wax and pressed into toroidal rings (outside diameter, 7.00 mm; inner diameter, 3.04 mm). Reflection loss (RL) was computed using the transmission line theory.

Table 1. Specific Surface Areas and Total Pore Volumes of TiO2-C and Co/TiO2-C sample

SBET (m2 g−1)

SLangmuir (m2 g−1)

average pore diameter (nm)

TiO2-C Co/TiO2-C

23.6 129.1

32.0 216.9

8.0 5.4

that Co2+ could be adsorbed completely on the Ti3C2Tx surfaces through electrostatic interaction. Then surface-adsorbed Co2+ was coordinated with 2-methylimidazole molecules to form Ti3C2Tx/Co-MOF hybrids. Finally, the Ti3C2Tx/Co-MOF precursor was annealed at high temperature under N2 to form laminated Co/TiO2-C hybrids. The XRD patterns for the crystallographic structures of Ti3AlC2, Ti3C2Tx, MOF, Ti3C2Tx/Co-MOF, Co/TiO2-C, and TiO2-C (Figure 2). Clearly, Ti3C2Tx does not show a peak at 39° in response to the (104) plane of Ti3AlC2 (Figure 2a), indicating that the Al layers were successfully removed after etching.35−37 Importantly, because of the exfoliation, the peak of the (002) plane shifted to a lower angle and was broadened compared to

3. RESULTS AND DISCUSSION The formation of the Co/TiO2-C hybrids is described in Figure 1. First, Ti3C2Tx was produced by selectively etching Al layers in Ti3AlC2 crystals with HF to form the laminated structures and then was infused in a methanol solution and sonicated for 1 h so 15857


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Figure 7. Complex permittivity of two samples: (a) real part, (b) imaginary part, and (c) dielectric loss tangent with the complex permeability of three samples: (d) real part, (e) imaginary part, and (f) magnetic loss tangent.

Figure 8. Cole−Cole curves of (a) TiO2-C and (b) Co/TiO2-C.

that of the original Ti3AlC2 powder.38 The XRD pattern of Ti3C2Tx/Co-MOF also shows peaks for Co-MOF (Figure 2b). For Co/TiO2-C, the main peaks (Figure 2c) are consistent with those of TiO2 and metal Co (JCPDS nos. 21-1276 and 15-0806, respectively). TiO2-C and Co/TiO2-C show a broad peak within 20−30°, which is probably the (002) peak of graphitic carbon.39 The Raman spectra show bands at 1340 and 1580 cm−1 after pyrolysis (Figure 2d), which are attributed to the characteristic D and G bands, respectively. A smaller ID/IG indicates a higher graphitization degree of carbon materials.40 The ID/IG ratios of TiO2-C and Co/TiO2-C are 1.34 and 1.04, respectively, indicating that the carbon in Co/TiO2-C is more graphitized as a result of metal Co catalysis for carbon after the hightemperature, long-time calcination.41 The microstructures of Ti3C2Tx, Co-MOF, Ti3C2Tx/CoMOF, and Co/TiO2-C were investigated by SEM (Figure 3). The laminated Ti3C2Tx prepared by the etching of bulk Ti3AlC2 shows a typical morphology (Figure 3a). The SEM image of CoMOF shows a well-defined rhombic dodecahedral morphology (Figure 3b), which is a typical shape of ZIF-67. Co-MOF could be adsorbed on the Ti3C2Tx sheet surface via crystallization in situ (Figure 3c). Ti3C2Tx/Co-MOF hybrids are also laminated,

but with rougher surfaces due to the Co-MOF precipitation. After Ti3C2Tx/Co-MOF was carbonized in N2, the Co-MOF mainly preserved the dodecahedral shape with considerable shrinkage (Figure 3d,e). Co/TiO2-C is sandwich-shaped, which contributes to microwave absorption.42 The shapes and structures of Co/TiO2-C were further observed by TEM and HRTEM. TEM shows the Co and TiO2 nanoparticles of Co/TiO2-C deposited evenly on a single C layer (Figure 4a). The lattice spacing analyzed by HRTEM verifies the presence of Co, TiO2, and C. Lattice spacings of 0.24 and 0.20 nm in Figure 4b correspond to TiO2(101)43 and Co(111), respectively.44 Moreover, the Co and TiO2 nanoparticles were also surrounded by coated graphitic layers. XPS spectra show the presence of elements Ti, Co, C, N, and O (Figure 5a). The chemical valence of element Co is shown in Figure 5b. The primary peaks of Co 2p at 779.9 (Co 2p3/2) and 795.3 eV (Co 2p1/2) correspond to metal Co, indicating that Co2+ in MOF was reduced to Co0 after hightemperature carbonization in an inert gas.44 Moreover, the two major peaks of Ti 2p at 459.2 and 464.9 eV (Figure 5c) are ascribed to Ti−O (2p3/2) and Ti−O (2p1/2), respectively. The above outcome is consistent with the XRD patterns and confirms the complete 15858


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Figure 9. Reflection loss of (top) TiO2-C and (bottom) Co/TiO2-C in the frequency range of 2−18 GHz.

phase transition from Ti3C2Tx MXenes to TiO2 and carbon phases.28 The magnetization hysteresis curves measured at room temperature (Figure 5d) show the magnetization value (Ms) of Co/TiO2-C and TiO2-C (39.8 vs 0.1 emu·g−1), which is mainly attributable to the existence ferromagnetic Co and agrees with the XRD patterns. With the BET method, the samples show a typical type-IV isotherm (Figure 6), which may be caused by the mixture of microporous and mesoporous structures. The SBET values of TiO2-C and Co/TiO2-C are 23.6 and 129.1 m2/g, respectively, and their average pore diameters are 8.0 and 5.4 nm, respectively (Table 1). In addition, the upward trend in SBET is ascribed to the pyrolysis of Ti3C2Tx/Co-MOF at higher temperature.

4. EM ABSORPTION PROPERTIES To evaluate the EM absorption properties of samples, we investigated the complex permittivity (εr = ε′ − jε″) and complex permeability (μr = μ′ − jμ″) from 2−18 GHz frequencies. In general, ε′ and ε″ (or μ′ and μ″) represent the storage and loss of electrical energy (or magnetic energy) in the material, respectively. tan δε = ε″/ε′ and tan δμ= μ″/μ′ are solutions for detecting the power loss relative to the power storage in a material.45,46 The frequency reliance of the electromagnetic indices of paraffin composites filled with 45 wt % TiO2-C or Co/TiO2-C was tested. The real permittivity ε′ decreased from 8.37 to 6.57 for TiO2-C and from 9.31 to 7.19 for Co/TiO2-C (Figure 7a,b). The ε″ value slightly increased over the whole tested frequency range, which suggests an enlargement of the energy loss ability. tan δε = ε″/ε′, a commonly used indicator of dielectric loss ability, changes similarly to ε″.47 In addition, the permittivity of the samples with the different mass ratios (35 and 55 wt %) of Co/TiO2-C hybrids in paraffin was investigated (details in Figure S1)

Figure 10. Dependence of the 1/4λ matching thickness on the RL peak frequency for Co/TiO2-C.

Dielectric loss is correlated with conductive loss and polarization loss. In this study, the conductive loss relies on electrical conductivity, which can be determined from the degree of graphitization. Raman spectra (Figure 2d) prove that Co can contribute to graphitizing pyrolysis products and thereby enhance the electrical conductivity. On the basis of the free electron theory, both electrical conductivity and complex permittivity will be promoted consequently.48 In the microwave 15859


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Figure 11. Schematic illustration of EM absorption mechanisms for Co/TiO2-C.

Table 2. Comparison of EM Wave Absorption of Typical Carbon-Based Materials sample

matrix

filler loading (wt %)

minimum RL (dB)

layer thickness (mm)

effective bandwidth (GHz)

ref

Co/C-500 Co@NPC@TiO2 TiO2/C Co/C-800 ZnO/NPC@Co/NPC Ni@C nanorod Co/TiO2-C Co/TiO2-C

paraffin paraffin paraffin paraffin paraffin paraffin paraffin paraffin

40 50 55 30 50 40 45 45

−35.3 −51.7 −50.3 −32.4 −28.8 −26.3 −41.1 −30.8

4.0 1.65 2.1 2.0 1.9 2.3 3.0 2.0

5.8 4.2 4.7 3.8 4.2 ∼4.0 3.04 4.6

18 20 28 51 21 52 this work this work

ment can be attributed to (1) the defect polarization of the carbon layer and (2) the multiple interfacial polarizations in Co/ TiO2-C (Figure 11). The absorption ability of each absorbent was evaluated using the reflection loss (RL), which was computed using εr and μr on the basis of the transmission line theory7,49

frequency range, polarization loss may be induced by the interface and dipole polarization effects. Multiple interface polarization would occur in the interfaces of Co nanoparticles/ crystalline carbon, TiO2/crystalline carbon, or carbon framework/wax. In this case, the dipole polarization can generally result from the dipole redirection and the EM interaction. Figure 7d,e illustrates that the real permeability (μ′) and imaginary permeability (μ″) both slightly fluctuate with the rising frequency. tan δμ = μ″/μ′ was also calculated and plotted as a function of frequency (Figure 7f). Clearly, tan δμ changes similarly as the dispersion of μ″ for all of these composites. μ″ and tan δμ both increase after Co filled in the spaces in the Ti3C2Tx sheets. On the basis of the Debye relaxation theory, ε′ and ε″ obey the following equation46,47 (ε ́ − (εs + ε∞)/2)2 + (ε″)2 = ((εs − ε∞)/2)2

Zin = Z0(μr /εr)1/2 tanh[j(2πfd /c)(με )1/2 ] r r

(2)

Zin − 1 Zin + 1

(3)

RL(dB) = 20lg

where Zin is the input impedance of the absorbent, d is its thickness, Z0 is the impedance of free space, f is the frequency of microwaves, and c is the velocity of electromagnetic waves in free space. Generally, RLs of EM materials should be lower than −10 dB (90% microwave absorption). Figure 9 displays the RLs of the samples with an optimal mass loading of 45 wt % at varying thickness and a frequency of 2−18 GHz. The TiO2-C composites, without the addition of Co nanoparticles, show a fair EM absorption ability. The minimum RL is −36.6 dB with the effective bandwidth (RL ≤ −10 dB) of 4.56 GHz for a thickness of 2 mm. Obviously, laminated Co/ TiO2-C hybrids have excellent MA properties, with the

(1)

where εs and ε∞ are the static and relative parts of the permittivity in the high-frequency limit, respectively. Thus, based on eq 1, the ε′ − ε″ curve would be a single Cole−Cole semicircle that corresponds to the multirelaxation process. Figure 8 show many Cole−Cole semicircles, indicating that the occurrence of multirelaxations improved the dielectric performances of TiO2-C and Co/TiO2-C composites. Such improve15860


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maximum RL being −41.1 dB at 9.00 GHz with an effective bandwidth of 3.04 GHz (from 7.24 to 10.28 GHz) at a thickness of 3.0 mm (Figure 8b). Furthermore, the maximum RL was −31.0 dB at 14.12 GHz with an effective bandwidth of 4.04 GHz for a thickness of 2.0 mm. Interestingly, the RL peak of samples shifts to lower frequency with the increase in layer thickness, which can be explained by the quarter-wavelength attenuation5,50 tm =

nλ nc = (n = 1, 3, 5, ...) 4 4fm |μr ||εr|

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b03238. Reflection loss of Co/TiO2-C/paraffin composites with filler loadings (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.H.). *E-mail: [email protected] (Y.Z.).

(4)

where f m is the frequency at peak RL, tm is the sample thickness, λ is the EM wavelength, and n = 1, 3, 5... Figure 10 shows the simulated curves of f m and the matching thickness of Co/TiO2C, which well fit the quarter-wavelength matching conditions. The quarter-wave principal effectively guides the thickness design of EM absorbents. Figure 11 illustrates a possible mechanism based on 2D carbon layers anchored by Co and planar TiO2 nanoparticles. First, the addition of Co and TiO2 nanoparticles to the carbon matrix creates more interfaces and generates abundant interfacial polarization, which much intensify the dielectric loss. Second, when EM waves enter the medium, the laminated structure causes multiple reflections between multilayered microstructures and thus leads to effective EM attenuation. Third, the defect polarization relaxation and electron polarization caused by the residual defects and groups in carbon layers contribute to EM attenuation. Fourth, more conductive paths are provided by the isolated carbon layers and the related TiO2 and Co and thereby intensify the conduction loss. The interface between C and a TiO2 (or Co) particle, which is linked to the surface of the carbon layer, can be considered to be a resistor− capacitor circuit model. When the charge carriers move to the carbon layer or jump across the nonuniform interfaces, the capacitor-like structures may attenuate the power of the incident EM wave. Finally, the Co nanoparticles buried in the amorphous carbon structure produce a magnetic loss.53−59 The microwaveabsorbing abilities of some C-based materials are illustrated in Table 2. The optimized Co/TiO2-C absorbent in this work comprehensively outperforms some common C-based materials in terms of EM wave absorption. Therefore, the Co/TiO2-C hybrids can be used as EM absorbing materials.

ORCID

Qiang Liao: 0000-0003-1416-1422 Man He: 0000-0003-4535-8068 Shuangxi Nie: 0000-0001-9695-3022 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the National Nature Science Foundation of China (51673040), the Natural Science Foundation of Jiangsu Province (BK20171357, BK20180366), the Prospective Joint Research Project of Jiangsu (BY201607601), the Opening Project of Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control (GD201802-5), Fundamental Research Funds for Central Universities (2242018k30008, 3207048418), the Scientific Innovation Research Foundation of College Graduate in Jiangsu (KYLX16_0266), a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (1107047002), and the Fund Project for Transformation of Scientific and Technological Achievements of Jiangsu (BA2016105). The authors especially thank Prof. He Jianping and his team for their kind help with the VNA test at Nanjing University of Aeronautics and Astronautics.

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REFERENCES

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5. CONCLUSIONS A facile method was proposed for the controlled fabrication of Co/TiO2-C starting from Ti3C2Tx/Co-MOF as a precursor. The laminated structure involving Co, TiO2 nanocrystals, and carbon was formed with the original two-dimensional shape and strengthened the EM absorbing capability. The EM wave attenuation was enhanced by the multiloss mechanism including multiple reflections and magnetic and dielectric loss. The reflection loss (RL) was maximized to−41.1 dB at 9.0 GHz. Moreover, the effective bandwidth (RL ≤ −10 dB) was 4.04 Hz under an absorbent thickness of 2.0 mm. Thus, the Co/TiO2-C hybrids with the novel structure may serve as a new guide for the plan and fabrication of MA materials and could be explored as an ideal candidate for microwave absorption materials. 15861


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