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Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 25369−25377

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Compressible Highly Stable 3D Porous MXene/GO Foam with a Tunable High-Performance Stealth Property in the Terahertz Band Wenle Ma,† Honghui Chen,† Shengyue Hou,† Zhiyu Huang,† Yi Huang,*,† Shitong Xu,‡ Fei Fan,‡ and Yongsheng Chen† †

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National Institute for Advanced Materials, Tianjin Key Laboratory of Metal and Molecule Based Material Chemistry, Key Laboratory of Functional Polymer Materials, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), School of Materials Science and Engineering, and ‡Institute of Modern Optics, Key Laboratory of Optical Information Science and Technology, Nankai University, Tianjin 300350, China S Supporting Information *

ABSTRACT: In this work, a three-dimensional (3D) porous MXene/GO foam (MGOF) was successfully synthesized and exhibited an excellent terahertz stealth property covering a whole measurement frequency of 0.2−2.0 THz. This is due to the ingenious assembly of two functional two-dimensional materials that have different advantages. The multiscale micronanostructure constructed with the 3D porous MGOF can effectively increase the terahertz scattering and refraction. Furthermore, MXene sheets with high conductivity can enhance the responsiveness to the terahertz wave. By adjusting the content of MXene in the MGOF, it exhibits a maximum reflection loss (RL) of 37 dB with a 100% qualified frequency bandwidth (RL > 10 dB), which is the most outstanding result in the available reference. In addition, the optimal average terahertz RL values of MGOF were up to 30.6 dB, which is 100% higher than the best data presented in previous work. Benefitting from an ultralow density, a high RL value, and a wide bandwidth, the maximum specific average terahertz absorption performance can reach 4.6 × 104 dB g−1 cm3, which is more than 4000 times that of other materials. In addition, the regulation of the terahertz absorption property through microstructure and morphology control is reported for the first time. KEYWORDS: graphene oxide, MXene, MGOF, ultrahigh performance terahertz absorber, 3D, tunable

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INTRODUCTION Stealth technology is a comprehensive technology that can reduce the detectability of targets and make them difficult to find, track, identify, and attack by reducing and controlling various characteristic signals of targets in a certain detection environment. In actual combat, stealth technology can effectively enhance the survivability and penetration ability and weapons battle effectiveness and its viability. At present, most research regarding stealth technology mainly focuses on the radar band.1,2 With increasing interest in high-frequency electronics, terahertz technology has received widespread attention in a wide range of fields including electro-optic apparatus, radar systems, wireless communication, imaging and spectroscopy devices, sensors for biological and chemical applications, and others.3−6 For instance, the distinct signature of explosives and narcotics in the terahertz spectra allows © 2019 American Chemical Society

terahertz spectroscopy to distinguish dangerous explosives and illegal drugs from their original compounds.4,7 The frequency of terahertz is 1−4 orders of magnitude higher than that of the existing microwave communication, allowing it to carry more information. Terahertz communication can easily solve the problems that cause current battlefield information transmission to be limited, such as the bandwidth, and meets the requirements of high data transmission rates.8,9 In the fields of national defense and military affairs, since the 1990s, terahertz radar has been highly valued and has achieved breakthrough progress. The earliest reports from the United States developed the groundbreaking airborne radar system based on the 0.225 Received: February 23, 2019 Accepted: June 25, 2019 Published: June 25, 2019 25369

DOI: 10.1021/acsami.9b03406 ACS Appl. Mater. Interfaces 2019, 11, 25369−25377

Research Article

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loading into 3D graphene skeletons have not been reported in the terahertz field. There are still great challenges in the design, preparation, and property studies of related materials. MXenes are an emerging family of 2D transition-metal carbides and nitrides with the general formula Mn+1XnTx.29,30 MXenes have drawn great interest because of their intriguing and diverse mechanical, structural, physical, and chemical properties. Above all, they exhibit super-high electrical conductivity (1500 S cm−1) with hydrophilic surfaces.31 Because of such unique properties, MXenes have attracted much attention from the scientific world for various applications including energy storage,32−34 selective ion sieving,35,36 smart material,37 electromagnetic interference shielding,38 and laser.39 According to current research, MXenes have excellent responses to electromagnetic waves. Jhon reported that Ti3C2 has giant optical absorption and extinction coefficients in the terahertz range through first-principles study.40 For example, Gogotsi et al. reported that a metallically conductive and hydrophilic 2D MXene composite film can exhibit a superior electromagnetic interference (EMI)-shielding performance (ranging from 8 to 12.5 GHz) compared to other synthetic materials.41 Yu et al. prepared an ultrathin, lightweight, and flexible MXene foam that showed greatly enhanced EMI-shielding effective (ranging from 8 to 12.5 GHz) compared to its unfoamed film counterpart because of highly efficient wave attenuation.42 Very recently, Xu et al. reported that lightweight Ti2CTx/poly(vinyl alcohol) composite foams exhibit electromagnetic wave shielding with absorption-dominated feature (ranging from 8.2 to 12.4 GHz).43 In addition, excellent terahertz shielding performance of MXene was proven by Gogotsi et al.38 However, little attention has been paid to explore the potential of MXenes in electromagnetic absorption application. Very recently, Yin et al. reported a self-assembled, core−shell graphene-bridged, hollow MXene sphere 3D foam that showed excellent specific EM absorption performance (ranging from 8 to 12.5 GHz).44 In contrast to radar stealth, the terahertz frequency range is very wide, and it is difficult to have both a high absorption intensity and ultrawide absorption frequency. Graphene oxide (GO) and MXene as 2D nanomaterials have similar structural characteristics, making it facile to match and fuse with each other to form strong interactions. Specifically, GO has good mechanical and temperature stability while MXene has better electromagnetic response properties. Designing a binary composite foam that combines the advantages of GO and MXene is an effective method to achieve high-performance terahertz stealth. In addition, the rational control of structure and morphology could further enhance the terahertz stealth property. In this work, we demonstrate the tunable and broad-band ultra-high-performance terahertz absorption property of ultralight compressible 3D porous MXene/GO foam (MGOF) for the first time. Designing of a binary composite 3D MGOF that combines the advantages of MXene and GO with highly electromagnetic response can optimize the performance and obtain a synergistic effect. Abundant micro-nanometer porous structures can cause multiple reflection and scattering of electromagnetic wave. Introducing MXene with a high terahertz response capacity can further improve the terahertz absorption performance. The MGOF exhibit a good terahertz absorption performance in the measured frequency range of 0.2−2.0 THz. The effective absorption bandwidth covers the whole tested frequency, and a maximum RL value of 37 dB at

THz frequency band, which verified the feasibility of the terahertz radar program.10 The rapid development of terahertz technology also generates the amounts of terahertz wave that may threaten human health, delicate electric devices, and even national security and information. Thus, high-performance terahertz stealth materials are in high demand. Strong absorption ability and ultrawide absorption bandwidth are two main factors for ideal terahertz stealth materials. Additionally, terahertz absorbers with tunable performance via compositional and structural adjustments are attractive because of their great flexibility in practical application. Metamaterials (MMs) comprising highly conductive and specific shaped sub-wavenumber structures of metals are the most widely studied materials for terahertz absorbers.11,12 They utilize intrinsic loss, with the aid of a complicated structure design and precise control of permeability and conductivity, to achieve absorption at a certain frequency. Unfortunately, unimode MMs have the common shortcomings of narrow absorption bandwidth and lower absorption intensity, which lead to a very poor average absorption intensity (AAI). An effective method to improve the absorption performance of MMs is to incorporate multiple resonant structures within one-unit cell. For example, dualband,13 triple band,14,15 and ultra-multi-layer16 terahertz absorbers with extremely complex structure and crafts have been reported in a relevant spectral range including microwave, terahertz, infrared, and optical frequencies. Nevertheless, the qualified frequency bandwidths of most MM-based terahertz absorbers only reach 50% of the measured frequency. It is difficult to the achieved effective regulation of absorption intensity and a wide absorption frequency range. In addition, property modulation of MMs means only shifting the frequency response to another frequency range, which limits the potential in practical application.17 Later, terahertz stealth materials with a 3D porous conductive network have attracted wide attention for broadband high-performance terahertz absorption applications.18 Specifically, because of the unique two-dimensional (2D) structural properties and the tunable dielectric properties, graphene nanosheets with large specific surface areas have desirable features for establishing 3D conductive networks,19−21 enabling the delivery of effective electromagnetic energy conversion and attenuation. In our previous reports, we preliminarily reported the good radar and terahertz stealth property of the 3D graphene foam.22 However, limited by a single structural design, defects associated with the 3D graphene foams, and the intrinsic electromagnetic properties of graphene, the stealth performance of unimode system remains must be improved.23,24 For the terahertz frequency band, the maximum reflection loss (RL) only reaches 19 dB. The qualified frequency bandwidth cannot cover 100% of the entire measured bandwidth (ranging from 0.1 to 1.2 THz). Moreover, high-temperature calcination process (≥1500 °C) is needed to improve and regulate the terahertz stealth property. Such a high temperature can damage the mechanical stability of the 3D graphene foam, which limited the prospect of practical applications. Our previous studies showed that introducing other low-dimensional nanomaterials such as zero-dimensional metal nanoparticles,25 one-dimensional carbon nanotube,26 and graphite,27,28 which are highly responsive to high-frequency electromagnetic wave, into the 3D graphene skeleton is an effective strategy to regulate the terahertz absorption performance. However, the studies of 2D material 25370

DOI: 10.1021/acsami.9b03406 ACS Appl. Mater. Interfaces 2019, 11, 25369−25377

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ACS Applied Materials & Interfaces

Figure 1. Schematic diagram of the MGOF preparation process.

Figure 2. AFM image of (a) GO flake and (d) Ti3C2Tx. Scale bar, 1 μm. Inset is the corresponding height profile along the crossed line. SEM images of (b) MGOF1:3 (c) MGOF1:5, (e) MGOF1:7, and (f) MGOF1:9. Scale bar, 2 μm.

2.0 THz was realized. In addition, the maximum AAI of the MGOF1:5 can reach 30.6 dB, which is the best available terahertz absorption material that has been reported. The design of 3D porous MGOF is the key factor in obtaining such an excellent terahertz absorption performance. The 3D skeleton constructed with GO nanosheets can reduce the surface reflection to a great extent and can serve as a channel that allows terahertz waves to shuttle back and forth. The highly conductive MXene sheets in the MGOF could form a long-range conductive network. When THz waves into the MGOF, an induced current could generate, which could convert the electromagnetic wave into thermal energy. At the same time, the terahertz absorption performance of the MGOF can be optimized by regulating and controlling its microstructure and morphology through mechanical compression because of its excellent compressible properties. Additionally, good mechanical stability further enhances its potential for practical applications. In general, through the rational design of a 3D porous structure and effective control of the proportion of materials, massive terahertz waves can be eliminated in the internal environment of the MGOF.

sheets and GO were synthesized according to previously published protocols.45 Ti3C2Tx MXene was synthesized through etching the Al layer of Ti3AlC2 in a mixture solution of hydrochloric acid and lithium fluoride followed by delamination through manual shaking, leading to a stable aqueous colloidal solution of MXene. The successful exfoliation of Ti3C2Tx MXene was verified by the shift of the (002) peak from 9.5° to 5.9° and the greatly weakened peak at 39° in the X-ray diffraction (XRD) (shown in the Supporting Information S2). Figure 2a and inset show the atomic force microscopy (AFM) of delaminated Ti3C2Tx is ultrathin monolayer flakes with hexagonal structure. The high-quality MXene sheets have an average lateral dimension of 1−4 μm and thicknesses of ∼1.5 nm.46 The GO nanosheets made by a modified Hummers’ method also show a predominantly singlelayer flake (Figure 2d and inset) with a lateral dimension range from 8 to 20 μm and a thicknesses of ∼1 nm.47 The porous structure of MGOF was characterized by scanning electron microscopy (SEM) as shown in Figure 2b,c,e,f. It is obvious that all the MGOF with different MXene contents shows a similar morphology, which is randomly lap by GO and MXene flakes and cellular-like structures with average pore sizes of approximately 10 μm. There are two possible reasons for this interesting phenomenon. First, the concentration of GO nanosheets for preparing MGOF is fixed, leading to a relatively stable GO skeleton. Second, the similar lamellar

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RESULTS AND DISCUSSION As illustrated in Figure 1, the MGOF was fabricated through the self-assembly of MXene and GO ultrathin nanosheets with different mass ratios of MXene/GO. Delaminated Ti3C2Tx 25371

DOI: 10.1021/acsami.9b03406 ACS Appl. Mater. Interfaces 2019, 11, 25369−25377

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Figure 3. (a) XRD spectra and (b) Raman spectra of MGOF and pure GO foam.

Figure 4. (a) Schematic diagram of the THz-TDS system. The RL curves of MGOF with different contents of MXene added to the GO foam with a thicknesses of (b) 1, (c) 2, (d) 3, and (e) 4 mm. (f) Average RL value for each thickness of different MGOF contents and qualified bandwidth of the sample with a mass ratio of MXene/GO = 1:5.

structural features of MXene and GO are advantageous to assemble with each other, which can form strong π−π interactions. A similar morphology of MGOF with different contents of MXene allows for a single-factor study of the effect of content on terahertz stealth without considering the morphological factors. To understand the impact of MXene on the crystallographic structure of MGOF, the XRD characterization of various

MGOF samples were performed. Figure S2 shows sharp peaks at 2θ = 5.9°, 18.9°, and 25.4°, which are characteristic of the (002), (006), and (008) crystal planes of the singe-layered Ti3C2Tx.48 This agrees well with AFM observations (Figure 2d). In addition, no impurity peaks were detected, indicating the high purity of the Ti3C2Tx phase. As shown in Figure 3a, there is only one broad peak in the XRD curves for each sample, but the XRD peaks of the MGOF are quite different 25372

DOI: 10.1021/acsami.9b03406 ACS Appl. Mater. Interfaces 2019, 11, 25369−25377

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Figure 5. RL curves for the MGOF1:5 under different compression ratios with thicknesses (a) 1, (b) 2, (c) 3, and (d) 4 mm. (e) Best average RL value for each thickness and qualified bandwidth of MGOF1:5 with and without compressive strains. (f) Average terahertz absorption performance of MGOF1:5 with thicknesses of 2−4 mm (marked as red stars) and reported data in the available literature (marked as black dot).

from those of pure GO foam. The degree of stacking order of MXene layers is so low that XRD cannot detect because of the random distribution in the 3D porous skeleton of the MGOF. For pure GO foam, the broad peak, approximately 23.2°, which corresponds to the (002) peak of GO, indicates weak long-range restacking of GO.49 The (002) diffraction peak shifts from 23.2° for pure GO foam to 25.3° for all MGOF samples. This corresponds to a decrease in interlayer spacing from 0.38 to 0.35 nm, indicating the strong interaction between GO and MXene layers. There are two possible reasons for this phenomenon. First, the MXene may be crosslinked to GO sheet, which increases the stacking density of GO sheets. In addition, we consider that the abundant oxygencontaining functional groups during self-assembly of GO will cause steric hindrance. The oxygen-containing functional group on MXene sheets is much less than the GO sheets, which will cause a smaller effect of steric hindrance. Second, MXene has a certain reduction effect, which reduces the surface oxygen-containing functional group of GO. This is beneficial to the transfer and transport of electrons between different dielectric layers, which would result in good terahertz absorption capacity.50

To further illustrate the structure of the MGOF, characterization by Raman spectra was implemented (shown in Figure 3b). Specifically, the mode at 201 (ω2) cm−1 is A1g symmetry out-of-plane vibrations of Ti, whereas the modes at 403(ω5), 508 (ω6), and 617 cm−1 (ω4) are Eg group vibrations, including in-plane (shear) modes of Ti, C, and surface functional group atoms.33,51 For the Raman shift range from 175 to 700 cm−1, the Raman spectra of MGOF depend on the content of MXene. With increasing the content of MXene, the Raman characteristic lines of MGOF associated with MXene become stronger. This confirms the successful synthesis of MXene/GO composite foams and that the content of MXene in the MGOF can be regulated by changing the added amount of MXene. Compared with the MXene film, the MGOF displays two evident peaks at approximately 1330 and 1589 cm−1, which are characteristics peaks of the D and G bands of graphitic carbon. Compared with pure GO foam, the D band of MGOF1:3 red shifts by 4 cm−1. This is probably caused by the interaction between MXene and GO flakes.33 This result agrees well with the XRD tests. Figure 4a shows the terahertz time domain spectroscopy (THz-TDS) system, which was applied to study the accurate THz absorption properties of the MGOF.9 The terahertz 25373

DOI: 10.1021/acsami.9b03406 ACS Appl. Mater. Interfaces 2019, 11, 25369−25377

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Figure 6. (a) Digital images of the compression rebound process of MGOF1:5. (b) Experimental THz time domain signals of the air reference and MGOF1:5 after 200 sustained compressions. (c) RL curves and (d) average RL value of MGOF1:5 with different thicknesses after 200 sustained compression.

parameter for terahertz absorbers. The higher the AAI value is, the better the broad-band terahertz absorbing ability of MGOF. Combined with the average RL value of the sample (as shown in Figure 4f), it is obvious that the optimal mass ratio of MGOF for a terahertz absorber is approached to 1:5. Because of a strong terahertz absorption property in the measured frequency band, MGOF1:5 with a thickness of 4 mm exhibits a very excellent AAI (30.6 dB), which exceeds all previously reported terahertz absorption materials. It is clear that the introduction of MXene into GO foam can dramatically improve the terahertz stealth performance. Compared to traditional MMs with fixed structures, the terahertz absorption property of MGOF can be further optimized through simple control of its structure and morphology. The terahertz absorption properties of a material rely not only on its chemical constituents but also on its physical structure including the absorber’s morphology and the interfacial microstructure. However, the control of the terahertz absorption performance of traditional MM materials is limited to its single structure. In this work, the MGOF has good compressibility, which can be used to adjust the compression ratio to examine the effect on terahertz absorption behavior in the measured frequency bands. As illustrated in Figure S6, increasing the compressive strain would cause the void space to shrink and the solid matrix to become denser, leading to a high-density graded hole structure of MGOF. This would increase the multiscale scattering and refraction of electromagnetic waves, thus enhancing the absorption capacity. Furthermore, the oriented and compressed pore structure is beneficial to the transverse scattering. Combining both structural features, the terahertz absorption properties can be tuned effectively by the morphology control. Figure 5a−d shows the RL curves of MGOF1:5 with various thicknesses under different compressive strains of 0, 20, 50, 70, and 85% in the range of 0.2−1.6 THz. It is indicated that the RL value of MGOF1:5 with a thickness of 1 mm, when

reflection simulation (Figure S1) was established to calculate the terahertz absorption capability using the electromagnetic properties achieved in the terahertz transmission experiment described above. In this way, the refractive index n, the extinction coefficient κ, and the dielectric constant ε of the samples are facile to be obtained, which can be used to obtain a deep understanding of the terahertz absorption mechanism (Figures S4 and 5). Figure 4b−e shows the RL curves of MGOF with different MXene contents and thicknesses in the range of 0.2−2.0 THz. It is obvious that the thickness and the MXene content are crucial to the terahertz stealth performance of MGOF. In our previous works, it was demonstrated that GO foam without any treatment exhibits a very inferior terahertz absorption ability (the average RL loss value is approximately 1 dB with a thickness of 4 mm) because of the severe disruption of in-plane conjugated structures.22 In this work (shown in Figure 4b−e), with the addition of MXene that has a high electromagnetic response to the GO foam, the absorptive property is greatly improved compared to the pure GO foam without any posttreatment. In addition, the absorptivity of MGOF with different thicknesses (1−4 mm) exhibits a gradient growth with an increase in the content of MXene nanosheets at a low load of MXene. However, when the mass ratio of MXene and GO nanosheets is higher than 1:5, the absorptivity of the sample will not increase. As shown in Figure S3, the overall conductivity of MGOF increases with the increasing MXene content, which causes stronger surface reflection. It is worth noting that the maximum RL value of MGOF1:5 with a thickness of 4 mm is 37 dB at 2.0 THz, which is much higher than the best data that have been reported in previous reports. It is apparent in Figure 4d,e that the qualified bandwidth of all the MGOF samples with a thickness of 3−4 mm covers the whole simulated frequency band ranging from 0.2 to 2.0 THz. Even at a lower thickness (2 mm), MGOF1:3, MGOF1:5, and MGOF1:9 still cover the entire qualified bandwidth. The AAI is another important 25374

DOI: 10.1021/acsami.9b03406 ACS Appl. Mater. Interfaces 2019, 11, 25369−25377

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mechanical compression of MGOF can further adjust the microstructural properties to yield enhanced absorption ability over the frequency range. Meanwhile, the MGOF has good compression stability, and the terahertz absorption performance remains stable even after 200 continuous compressions (50% compression ratio), which further enhances its potential in practical application ability. In general, MGOF with excellent terahertz absorption performance depends on controlling its composition and microstructure, which realizes the properties that current stealth materials do not have. This is beneficial to important military, information protection, and electronic applications.

applying compressive strains, all surpass that without any treatment. When compressed to 70% of its original thickness, MGOF demonstrates a substantial increase in the AAI from 8.7 to 19.5 dB, and the qualified frequency bandwidth changes remarkably from 20 to 100%. As shown in Figure 5e, MGOF1:5 with 50% compression exhibits the best absorption performance among all samples when the thickness of the sample is 2 mm. As the thickness increases to 3−4 mm, the sample with the optimum terahertz absorption performance is MGOF1:5 with 20% compression. Most interestingly, the terahertz absorption performance of MGOF1:5 without any treatment gradually approached that of the lower compression ratio with a thickness increasing from 1 to 4 mm. When the thickness gradually increases to 4 mm, the MGOF with different compression ratios can completely attenuate the terahertz wave into the material. The stronger the surface reflection, the less electromagnetic waves enter the material. Thus, the main factor determining the absorption performance of MGOF is the strength of surface reflection. As illustrated in Figure 5f, compared to the terahertz absorption materials reported in previous literature, the value of specific average terahertz absorption performance for the MGOF is approximately 4000 times higher. The durability of terahertz absorption materials is a key factor that influences their practical application. Next, the terahertz absorption performance of MGOF15 after repeated compressions was tested. The MGOF in this work demonstrates highly repeatable compression stability (as shown in Figure 6a), which can be compressed via 50% compressive strain more than 200 times. This is due to the cross-linking/condensation of oxygen-containing functional groups that are located on GO and MXene sheets, such as the COOH, OH, and epoxy groups, which are expected to covalently cross-link the sheets. The terahertz absorption performance after 200 sustained compression of the MGOF1:5 is shown in Figure 6b−d. The THz domain signals of MGOF1:5 (shown in Figure 6b) after 200 compressions did not show obvious changes. Similarly, the RL curve and the AAI of the MGOF1:5 for different thicknesses after various numbers of sustained compression almost unchanged (shown in Figure 6c,d). This illustrates that continuous compression has no effect on the absorption properties of MGOF1:5, which benefitted from the good compression tolerance of the MGOF. This is for the first time that this important property of compression stability of MGOF has been reported for terahertz stealth materials.

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EXPERIMENTAL SECTION

Synthesis of GO. GO was prepared by the oxidation of natural graphite using a modified Hummers’ method.52 In short, natural flake graphite was placed in a flask containing concentrated NaNO3 and H2SO4. The solution was mechanical-stirred in a low-temperature ice−water bath for several hours. After pre-oxidation, KMnO4 was gradually added into the solution mentioned above over the course of approximately 30 min and stirred continuously for 1 h. Finally, the product was transferred to a breaker and continued to react at 60 °C for 6 h in the oven. Then, the product was gradually added into deionized (DI) water. The volume of the DI water is more than twice the volume of the product. When the temperature reduced to 60 °C, H2O2 (30 wt % in aqueous solution) was added. After repeated centrifugation purification with DI water and redispersion into ethanol, we can get ethanol-dispersed GO solution. Synthesis of Delaminated Ti3C2Tx Colloidal Solution. The MILD method reported by Gogotsi and co-workers was used to get MXene sheets. First, an excess etching solution consist of 10 mL of HCl aqueous solution (9 M) and 1.0 g of LiF was used to etch the Al layer of the multilayered MXene.45 Ti3AlC2 (1.0 g) raw material (400 mesh, purchased from 11 Technology Co., Ltd.) was gradually added to the above solution in 5 min while magnetically stirred. After that, the mixture was continuously stirred for 24 h at 35 °C in a water bath. The acidic product was washed via centrifugation with DI water at 3500 rpm for 5 min per cycle until the pH of the supernatant reached approximately 6, accompanied by noticeable swelling of the sediment into a jelly-like material. Then, 20 mL of DI water was added to the as-prepared sediment. In addition, it was vigorously shaken by hand for approximately 10 min for exfoliation. Finally, the unexfoliation sheets were removed by centrifugation at 3500 rpm for 1 h. The supernatant that comprised abundant delaminated Ti3C2Tx sheets with a concentration of approximately 9 mg mL−1 was carefully collected. Synthesis of 3D MGOF. Typically, to make the MGOF, 1 mL of aqueous dispersions of Ti3C2Tx sheets with different concentrations were add into the ethanol-dispersed GO solution (35 mL, 0.8 mg/ mL), which was magnetically stirred 0.5 h to achieve homogeneity. After sonication, the MGOF was fabricated by a solvothermal method in a 50 mL Teflon-lined container at 180 °C for 12 h. After that, the as-prepared MGOF was carefully solvent-exchanged with water, followed by freeze-drying, and then MGOF was obtained. Finally, MGOF samples (average diameter greater than 1.5 cm) were cut into 2 mm thicknesses using a mechanical cutting device. For convenience, the MGOF with a mass ratio of MXene/GO = 1:3, 1:5, 1:7, 1:9 is labeled as MGOF1:3, MGOF1:5, MGOF1:7, and MGOF1:9, respectively. Measurement of Terahertz Absorption Performance. A homemade four-parabolic mirror THz-TDS system was used to get the time domain signals of MGOFs. The excitation source of this system is a Ti:sapphire laser with 800 nm wavelength and 75 fs pulse width. A low-temperature-grown GaAs photoconductive antenna with a 50 μm slit was used to generate THz pulses. The emitted terahertz wave transmitted through the first two gilded parabolic mirrors land and then focused on the center of samples with a spot size of 2.5 mm. Finally, the terahertz wave was detected by a (110) ZnTe crystal. The

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CONCLUSIONS In summary, a series of lightweight MGOF was successfully fabricated using a facile solvothermal process. The terahertz stealth performance of the MGOF is closely related to the content of MXene sheets and the MGOF skeleton microstructure. Compared to the traditional terahertz absorbers, MGOF1:5 with a 4 mm thickness exhibits an excellent terahertz stealth performance. The maximum RL value is up to 37 dB, and effective absorption bandwidth covers 100% of the whole measured frequency. Additionally, MGOF1:5 with a 4 mm thickness exhibits the best AAI (30.6 dB) compared to the reported typical terahertz stealth materials. The excellent terahertz absorption performance combined with a low density endows the MGOF a perfect specific average terahertz absorption efficiency, nearly 4000 times higher than those of the materials available in the literature. In addition, the 25375

DOI: 10.1021/acsami.9b03406 ACS Appl. Mater. Interfaces 2019, 11, 25369−25377

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ACS Applied Materials & Interfaces test is measured at room temperature and the air humidity was controlled below 5%. Characterizations. The delaminated 2D MXene and GO sheets were characterized using a ScanAsyst mode AFM (Bruker Instruments Dimension Icon). SEM images were achieved on a LEO 1530 VP field emission scanning electron microscope with a 5.0 kV accelerating voltage. The Raman spectra of MGOFs were determined using a Renishaw inVia Raman spectrometer with an excited laser with a frequency of 514.5 nm. Structural information about MGOFs was acquired via XRD carried out on a Rigaku D/Max-2500 diffractometer with Cu Kα radiation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b03406. Detailed experimental procedures, including measurement and simulation of calculation of terahertz transmission properties, XRD, electrical conductivities, electromagnetic parameters, electromagnetic parameters under compression, SEM under compression, and comparison of terahertz electromagnetic wave absorption performance (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yi Huang: 0000-0001-9343-207X Yongsheng Chen: 0000-0003-1448-8177 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the Ministry of Science and Technology of China (MoST, 2016YFA0200200), the National Natural Science Foundation of China (NSFC, 21875114, 51373078, and 51422304), NSF of Tianjin City (15JCYBJC17700), and the 111 project (B18030).

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REFERENCES

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Research Article

ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.9b03406 ACS Appl. Mater. Interfaces 2019, 11, 25369−25377