Highly Electrically Conductive Three-Dimensional Ti3C2Tx MXene

Oct 19, 2018 - Herein, we demonstrate an efficient approach for constructing highly ... The porous and highly conductive architecture (up to 1085 S mâ...
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Highly Electrically Conductive Three-Dimensional Ti3C2Tx MXene/Reduced Graphene Oxide Hybrid Aerogels with Excellent Electromagnetic Interference Shielding Performances Sai Zhao, Hao-Bin Zhang, Jia-Qi Luo, Qi-Wei Wang, Bin Xu, Song Hong, and Zhong-Zhen Yu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b05739 • Publication Date (Web): 19 Oct 2018 Downloaded from http://pubs.acs.org on October 19, 2018

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Highly Electrically Conductive Three-Dimensional Ti3C2Tx MXene/Reduced Graphene Oxide Hybrid Aerogels with Excellent Electromagnetic Interference Shielding Performances Sai Zhao,† Hao-Bin Zhang,*,† Jia-Qi Luo,† Qi-Wei Wang,† Bin Xu,† Song Hong,‡ and ZhongZhen Yu*,† †

State Key Laboratory of Organic-Inorganic Composites, College of Materials Science and

Engineering, Beijing University of Chemical Technology, Beijing 100029, China ‡

Analysis and Test Centre, Beijing University of Chemical Technology, Beijing 100029, China

Corresponding Author: E-mail: [email protected] (H.-B. Zhang); [email protected] (Z.-Z. Yu)

ABSTRACT: Two-dimensional transition metal carbides/carbonitrides (MXenes) networks with both superb electrical conductivity and hydrophilicity are promising for fabricating multifunctional nanomaterials and nanocomposites. However, construction of three-dimensional (3D) and lightweight MXene macroscopic assemblies with excellent electrical conductivity and mechanical 1 ACS Paragon Plus Environment

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performances has not been realized due to the weak gelation capability of MXene sheets. Herein, we demonstrate an efficient approach for constructing highly conductive 3D Ti3C2Tx porous architectures by graphene oxide-assisted hydrothermal assembly followed by directional-freezing and freeze-drying. The resultant hybrid aerogels exhibit aligned cellular microstructure, in which the graphene sheets serve as the inner skeleton while the compactly attached Ti3C2Tx sheets present as shells of the cell walls. The porous and highly conductive architecture (up to 1085 S m-1) is highly efficient in endowing epoxy nanocomposite with highly electrical conductivity of 695.9 S m-1 and outstanding electromagnetic interference (EMI) shielding effectiveness of more than 50 dB in the X-band at a low Ti3C2Tx content of 0.74 vol%, which are the best results for polymer nanocomposites with similar loadings of MXene so far. The successful assembly methodology of 3D and porous architectures of Ti3C2Tx would greatly widen the practical applications of MXenes in the fields of EMI shielding, supercapacitors and sensors.

KEYWORDS: Ti3C2Tx MXene aerogel; electromagnetic interference shielding; reduced graphene oxide; electrical conductivity; polymer nanocomposites

Recently, two-dimensional (2D) transition metal carbides/carbonitrides (MXenes) with versatile surface chemistry, high aspect ratio, excellent mechanical and electrical properties have been intensively exploited for various potential applications, such as supercapacitors,1-3 batteries,4-8 catalysts,9 sensors,10, 11 molecular sieving,12 and electromagnetic interference (EMI) shielding.13, 14 Different from the inert and hydrophobic surface of graphene, the high hydrophilicity of MXenes 2 ACS Paragon Plus Environment

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provides a great opportunity to modify MXene surfaces while retaining their excellent electrical conductivity. For example, Shahzad et al. reported a flexible Ti3C2Tx film with remarkable EMI shielding effectiveness (SE) of 92 dB at a small thickness of 45 μm.13 Liu et al. developed a hydrazine-induced foaming process to convert a hydrophilic Ti3C2Tx film to a lightweight, flexible and hydrophobic porous film with an excellent EMI SE of more than 70 dB.14 Despite these impressive progresses, however, it is still a great challenge to fabricate highly conductive polymer nanocomposites at low loadings of Ti3C2Tx sheets due to their unsatisfactory dispersion in polymers and contact resistance between the Ti3C2Tx sheets.15,

16

To achieve a satisfactory electrical

conductivity, high loading of Ti3C2Tx is usually adopted to ensure the formation of threedimensional (3D) conducting networks. For instance, a polyacrylamide nanocomposite has an electrical conductivity of 3.3 S m-1 with 75 wt% of Ti3C2Tx,17 and a polyvinyl alcohol film with 40 wt% of Ti3C2Tx exhibits a conductivity of 0.04 S m-1.3 Fundamentally, the electrical properties of polymer nanocomposites depend mainly on the intrinsic conductivity, dispersion and distribution of MXenes, and the key is to form a high-quality conductive network in a polymer matrix. By electrostatic assembling of negative Ti3C2Tx sheets on positive polystyrene microspheres followed by compression molding, Sun et al. fabricated highly conductive Ti3C2Tx@polystyrene nanocomposites with a low percolation threshold of 0.26 vol%.18 In addition, preconstruction of a conducting network of nanofillers before compounding with polymers is also an effective strategy to endow polymers with excellent electrical and EMI shielding performances.19-23 However, assembling of MXene sheets into a 3D porous architecture 3 ACS Paragon Plus Environment

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remains difficult because of their weak gelation capability.24 To address this issue, a few significant attempts have been made. Bian et al.25 assembled Ti3C2Tx-based architectures by tuning the hydrophilic-hydrophobic balance of Ti3C2Tx at water/oil interfaces. Gao et al. fabricated Ti3C2Tx/reduced graphene oxide (RGO) aerogel as piezo-resistive sensor11 and supercapacitor26 by an ice-template freezing technique followed by thermal annealing or chemical reduction with hydroiodic acid/acetic acid solution. However, the construction of 3D MXene porous architectures with satisfactory EMI shielding performances and mechanical properties has rarely been reported.27, 28 Herein, we develop a graphene oxide (GO)-assisted hydrothermal assembly followed by directional-freezing and subsequent freeze-drying to assemble Ti3C2Tx sheets into highly conductive, porous and 3D architectures. Combined with the well-retained intrinsic structure of Ti3C2Tx, the Ti3C2Tx MXene/RGO hybrid aerogel (MGA) with aligned core-shell structure provides its epoxy nanocomposite with an outstanding electrical conductivity of 695.9 S m-1 and EMI shielding performance of >50 dB at a low Ti3C2Tx content of 0.74 vol%. The assembled microstructures of Ti3C2Tx and RGO sheets in the hybrid aerogel are identified and the assembly mechanisms are proposed. RESULTS AND DISCUSSION Construction of Ti3C2Tx MXene/RGO Hybrid Aerogels Scheme 1 illustrates the fabrication process of a MGA by GO-assisted hydrothermal assembly followed by directional-freezing treatment and subsequent freeze-drying. To assemble Ti3C2Tx 4 ACS Paragon Plus Environment

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sheets into a 3D porous architecture, GO sheets with strong gelation capability are utilized as a gelation agent.29, 30 Stable aqueous Ti3C2Tx/GO suspensions are readily formed due to their similar hydrophilicity and negative charges. In view of the easy oxidization of MXene sheets in water at high temperatures,24 mild hydrothermal conditions, such as low temperatures, are selected to avoid severe oxidation and structural degradation of Ti3C2Tx. During the hydrothermal assembling process, GO is reduced by both ascorbic acid and hydrothermal reaction and the resultant RGO sheets are interconnected with each other to form a 3D framework, while the Ti3C2Tx sheets are assembled onto the surfaces of the RGO framework to get a MXene/RGO hybrid hydrogel (Figure S1). It is observed that Ti3C2Tx sheets are totally assembled into 3D architectures when their initial content in the suspension does not exceed 80 wt% (Figure S2). After dialyzing in 10 vol% of ethanol solution, the Ti3C2Tx MXene/RGO hybrid hydrogel is put on a copper disk that is immersed in liquid nitrogen for directional-freezing treatment, during which numerous vertically grown ice cylinders tend to expel the RGO and Ti3C2Tx sheets and compel them assemble between the numerous ice cylinders. Finally, a lattice-like cellular MGA is formed by freeze-drying at -60 C to remove the ice crystals by sublimation. Ti3C2Tx is synthesized by selectively etching Ti3AlC2 precursor followed by exfoliation process. As shown in the X-ray diffraction (XRD) patterns, the clay-like Ti3C2Tx presents an enlarged interlayer spacing evidenced by the smaller 2θ degree of (002) peak in relative to that of Ti3AlC2, because of the removal of Al layers and the intercalation of lithium ions into the intra-gallery (Figure 1a, S3a).31 The remained weak peaks related to (002), (004) and (104) faces of Ti3AlC2 5 ACS Paragon Plus Environment

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indicate that there is incompletely etched compenent,18, 32 which can be eliminated in subsequent exfoliation process. Thus, the successful exfoliation of Ti3C2Tx sheets is confirmed by the enlarged interlayer spacing of 1.3 nm (002) and the ultrathin and transparent features in the images of atomic force microscopy (AFM), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) (Figure 1b, c, S3b).31, 33 The selected area electron diffraction (SAED) pattern with a typical hexagonal symmetry reveals the polycrystal feature of Ti3C2Tx sheets (Figure S3c).2, 34

The strong gelation feature, excellent mechanical flexibility and large size of GO (Figure S4) make it well suitable for assisting Ti3C2Tx sheets to form 3D architectures.35 The similar hydrophilicity and negative charges of Ti3C2Tx and GO components make their aqueous suspension stable, evidenced by the typical Tyndall effect (Figure 1d), which is a prerequisite for constructing 3D hybrid frameworks. Figure 2 shows side-view SEM images of MGAs. In comparison, graphene aerogel (GA) was also prepared by the same method. It is seen that the MGAs show a uniform, and overall aligned cellular structure with unidirectional assembly of graphene and Ti3C2Tx sheets (Figure 2a-c, Figure S5). Similar regular cellular structure is observed for MGAs with different initial ratios of Ti3C2Tx/GO components. With increasing the Ti3C2Tx content, the average channel width between the parallel cells becomes smaller, from ~16 μm for GA to ~10 μm for MGA-2 and ~8 μm for MGA-4. Although the exact reasons for the variation of channel width are unclear, it is reasonable to speculate that the nucleation density of ice crystals is crucial in determining the cellular structures, which is influenced by various parameters including the concentration, viscosity, 6 ACS Paragon Plus Environment

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and cooling rate of the suspension.36, 37 The smaller nuclei density of ice crystals for GO suspension produces wider channels in GA,38, 39 while the higher ice nuclei density results in the narrower channels of MGAs. Similar phenomena were also found in the hybrid aerogels with graphene and other 2D materials of g-C3N4 or MoS2.11, 29, 30 The structural differences are expected to vary the electrical and thermal performances of MGAs and their polymer nanocomposites. Some sheets are found to periodically bridge and intersect with the long-range aligned cell walls, forming lattice-like networks for efficient load transfer and electron transport (Figure 2c). Note that these entangled sheets are punctured by the continuously grown ice crystals and in turn the rigid sheets would hinder the total exclusion of the cell walls among the ice crystals during the directional-freezing process. This can be attributed to the addition of ethanol during the dialysis process that modulates the growth of ice crystals.40, 41 The MGA-4 exhibits a similar top-view microstructure with the reported 3D architecture prepared by directional freezing (Figure S6).42 The porous and aligned structure allow the MGA-4 to stand on a flower (Figure 2d) and a MGA-4 weight 100 mg to support a load of 500 g without fracture when the load was applied along the direction of directional freezing, indicating the good strength of the skeleton (Figure 2d, Figure S7). The presence of both Ti3C2Tx and RGO in MGA is further confirmed by comparing the XRD patterns (Figure 3a). The retained (002) peak of MGA-4 at 6.4o indicates the well-preserved structure of Ti3C2Tx sheets after the hydrothermal treatment and the enlarged interlayer spacing results from the intercalated RGO sheets between Ti3C2Tx sheets. The presence of RGO is reflected 7 ACS Paragon Plus Environment

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by the broad and weak peak at ~24.6, which is ascribed to the reduced GO by ascorbic acid.43 Raman spectrum of MGA-4 combines the main characteristic peaks from Ti3C2Tx and GA despite their different peak intensities, verifying the combination of both Ti3C2Tx and RGO components (Figure S8). The peaks from 150 to 750 cm-1, including the out of plane vibrations of Ti (198 cm1)

and C (716 cm-1) atoms, and the in-plane modes of Ti (283 cm-1), C (365 cm-1) and surface

termination (624 cm-1), are assigned to the signals of Ti3C2Tx, which are the evidence for wellpreserved intrinsic structure of Ti3C2Tx. The characteristic peaks of D (1328 cm-1) and G (1594 cm-1) are correlated to graphitic structures from the RGO sheets in MGA-4, and the peak shifts are due to the strong interaction between the Ti3C2Tx and RGO, consistent with the results of Ti3C2Tx /RGO films.44 X-ray photoelectron spectroscopy (XPS) of MGA-4 reveals the main constituents of Ti, C, O and F elements (Figure 3b). MGA-4 shows stronger C 1s and O 1s signals than those of neat Ti3C2Tx due to the addition of the partially reduced GO (C/O = 4.2) at the mild hydrothermal assembly conditions. Also, some amounts of oxygen-containing groups assigned to the introduced oxygen terminations on Ti3C2Tx during the etching and exfoliation processes and the formed TiO2 by the weak oxidation of Ti3C2Tx can be detected (Figure 3c, d). Notably, it can be observed from Figure 3d that the intensity of the peak assigned to TiO2 slightly increased, which indicates the weak oxidation of Ti3C2Tx during the assembly process. 24, 45-47 High resolution C 1s spectrum of MGA-4 further indicates the presences of Ti-C-O (282.3 eV), Ti-C-O (283.2 eV), C-C (284.8 eV), C-O (285.9 eV) and C=O (C-F) (287 eV) bonds (Figure 3e, f). The typical peak at 289.2 eV, 8 ACS Paragon Plus Environment

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corresponding to O-C=O group of GA (Figure S9), reflects incomplete reduction of GO in MGA.44, 48

All these results indicate that the 3D porous architecture is well constructed by RGO and Ti3C2Tx

components, and the well-retained intrinsic structure of Ti3C2Tx sheets endows MGAs with high electrical conductivities. Assembly Mechanisms of Ti3C2Tx MXene/RGO Aerogels Although the presence of both Ti3C2Tx and RGO in MGA is well confirmed, it is still difficult to identify their spatial locations in the Ti3C2Tx MXene/RGO porous architecture. Thus, the microstructure of the MGA is examined with TEM, scanning transmission electron microscopy (STEM) and elemental mapping to ascertain the assembly mechanisms. As shown in Figure S10a, GA exhibits a transparent texture with weak diffraction rings like RGO in SAED patterns;49 whereas, MGA-4 contains the characteristics of both RGO and Ti3C2Tx, and the small particles observed on the large transparent sheets may be the generated tiny nanosheets during the etching and exfoliation processes as reported previously (Figure S10b).50 In addition, the well distinguished edges of Ti3C2Tx sheets are identified in Figure S10c. Note that the undetectable characteristics of TiO2 is another evidence for the good retaining of Ti3C2Tx sheets even after the assembly process. To explore how GO induces the assembly of Ti3C2Tx sheets, the energy-dispersive X-ray spectroscopy (EDS) elemental mapping is employed to determine the distribution of Ti3C2Tx in MGA. The statistic results demonstrate the constituents of MGA-4 with Ti (19.6%), C (39.8%), F (10.4%) and O (30.2%) species (Figure S11). More specifically, the Ti and C species distribute uniformly except for the strut area, where the other parts are covered by the O element that mainly 9 ACS Paragon Plus Environment

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originates from the oxygen functionalities of RGO. Moreover, the microstructure of MGA is characterized with high angle annular dark field STEM and elemental mapping images (Figure 4). The high sensitivity of STEM to the type and number of atoms highlights the Ti species with high image contrast, where the closely stacked Ti3C2Tx sheets are illustrated as a bright continuous hollow-like framework (Figure 4a, b). However, the inner parts encapsulated by the outer Ti3C2Tx layers are not really hollow but carbon-rich area, as identified by the overall elemental distribution of the MGA-4 (Figure 4c, d). The Ti and C distributions indicated by the EDS mapping prove that the core part of the framework is mainly dominated by the carbon element from the RGO sheets. Therefore, it is reasonable to expect that the cell walls of MGA possess a highly conductive coreshell structure with RGO sheets as the main core skeleton and the compactly attached Ti3C2Tx sheets as the shell. Based on the porous architecture, an assembly mechanism of MGA is proposed as follows: the strong self-gelation capability of GO sheets makes them to form a 3D framework driven by the enhanced interactions with the reduction process. The MXene sheets are then attracted to and adhered on the outside surface of the RGO network driven by the strong polar interactions, and ultimately giving birth to a high-quality, continuous and core-shell Ti3C2Tx MXene/RGO hybrid architecture. Electrical and EMI Shielding Performances of Epoxy/MGA Nanocomposites The optimized core-shell framework of MGA affords far superior electrical conductivity to GA, highlighting the crucial contribution of Ti3C2Tx to the functionality of MGA (Figure 5a and Table S1). Under the mild hydrothermal conditions, GA exhibits a relatively low electrical conductivity 10 ACS Paragon Plus Environment

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of 5.4 S m-1 at 36 mg cm-3 due to the poor reduction of GO, indicating that the poorly reduced GO has little effect on conductivity of MGA. However, in striking contrast, MGA-4 has much higher conductivity without any further reduction treatment, such as 516, 837, and 1085 S m-1 for the hybrid aerogels with densities of 24, 30, and 44 mg cm-3, respectively. These results emphasize the significant contribution of well-preserved Ti3C2Tx to the outstanding conductivities of MGAs. In addition, these outstanding conductivities are significantly higher than those of reported porous carbon materials, such as 200 S m-1 for carbon-based aerogel (50 mg cm-3),51 and 20 S m-1 for carbon nanotube foam (60 mg cm-3).52 It should be mentioned that the high electrical conductivities of carbon materials are usually achieved by chemical vapor deposition method or high-temperature graphitization. Fortunately, our highly conductive MGA is readily fabricated at mild conditions. Since the important contribution of Ti3C2Tx to the MGA, the influence of Ti3C2Tx contents on electrical conductivity of the hybrid aerogels is explored by adjusting Ti3C2Tx/GO ratio while maintaining a similar bulk density of 29-32 mg cm-3. As shown in Figure 5b and Table S2, the electrical conductivity increases rapidly with the Ti3C2Tx content due to the formation of more conductive networks. With only 15 wt% of Ti3C2Tx, the conductivity increases from 5.4 S m-1 for GA to 49.3 S m-1 for MGA-1, and it further reaches 647 and 837 S m-1 when the Ti3C2Tx contents are 65 and 80 wt%, respectively. The excellent conductivity of MGA is also illustrated by its effective connection of a circuit to power LED bulbs with increasing the brightness due to the better conductivity (Figure 5b). As well established, developing a preformed 3D porous conducting network is an effective way 11 ACS Paragon Plus Environment

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to fabricate high-performance polymer nanocomposites with excellent electrical properties at low filler loadings.19 To prepare solid polymer nanocomposites with high EMI shielding properties and reasonable mechanical properties, the highly conductive MGA is compounded with epoxy monomer by a vacuum-assisted impregnation and curing processes to prepare epoxy-based nanocomposites. The Fourier-transform infrared spectrometer (FT-IR) results (Figure S12) indicate that the epoxy/MGA-4 nanocomposite combines the main characteristic peaks of both the epoxy matrix and MGA-4, and no new peaks are observed, indicating the well-preserved chemical composition and intrinsic structure of Ti3C2Tx in the epoxy matrix that are important for providing satisfactory electrical and EMI shielding performances of the epoxy/MGA nanocomposites. Figure 5c shows the electrical conductivities of MGA and its epoxy nanocomposites. The Ti3C2Tx contents can be varied by adjusting the mass ratio of Ti3C2Tx/GO in the MGA (Table S3). As expected, larger Ti3C2Tx contents lead to higher conductivities for epoxy/MGA nanocomposites due to the more compact and effective conducting networks, consistent with the tendency of the MGAs. It is also seen that the conductivity of the nanocomposite is indeed slightly lower than that of corresponding MGA, which is due to the possible damage of the integrity and interconnections of the conducting networks from barrier effect of the insulating epoxy matrix between the MXene nanosheets the during the impregnation process. Even so, the epoxy/MGA nanocomposites exhibit significantly higher conductivities than those of reported polymer nanocomposites filled with MXenes and other carbon nanomaterials. With only 0.15 vol% of Ti3C2Tx, a conductivity of 23.3 S m-1 is obtained for the nanocomposite and it increases further to 296.5 (0.45 vol%) and 695.9 S 12 ACS Paragon Plus Environment

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m-1 (0.74 vol%), higher than 0.04 S m-1 for PVA/Ti3C2Tx nanocomposite (40 wt%),3 and 0.033 S m-1 for PAM/Ti3C2Tx nanocomposite (6 wt%).17 Even our recently reported PS/Ti3C2Tx nanocomposites, prepared by electrostatic assembly approach with PS microspheres as the template, shows a conductivity of 50 S m-1 at 0.72 vol% of Ti3C2Tx.18 All these results suggest the high efficiency of the preformed 3D conducting network of Ti3C2Tx in the epoxy matrix for electron transport, rendering the epoxy/MGA nanocomposites great promises for various potential applications. Because of the outstanding capability of highly conductive materials in attenuating microwave, EMI shielding performances of the epoxy/MGA nanocomposites are measured (Figure 5d and Table S3). In accordance with the electrical properties, the EMI shielding performance presents a similar continuous enhancement with increasing the Ti3C2Tx content. Compared to neat epoxy (~ 3 dB) and its GA nanocomposite (18.2 dB), the epoxy/MGA nanocomposite with only 0.15 vol % of Ti3C2Tx shows a high EMI shielding effectiveness of 27.3 dB over the whole X-band range, surpassing the standard shielding requirement for commercial applications.19,

53, 54

The overall

shielding performance is dramatically elevated to 40 dB at 0.45 vol% and further to 50 dB over Xband with a maximum value of 56.4 dB with only 0.74 vol% of Ti3C2Tx, superior to those of the reported polymer nanocomposites filled with graphene and other nanofillers. The advantages of preforming the highly conductive 3D MGA network are further elucidated by the much higher EMI shielding performance of the epoxy/MGA-4 nanocomposite than that of the epoxy nanocomposite filled with the same loadings of randomly dispersed graphene and Ti3C2Tx (Figure S13a). It is also 13 ACS Paragon Plus Environment

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noted that the epoxy/MGA-4 nanocomposite shows comparable EMI shielding performance to that of its precursor MGA-4 (Figure S13b). Moreover, the EMI shielding performances are measured from the top-view and side-view incident directions to explore the effect of the aerogel structure on ultimate EMI shielding performances. Interestingly, similar EMI shielding performances over the explored frequency range are obtained when the electromagnetic wave incident from different directions (Figure S13c). This may relate to the ordered lattice-like structure capable of efficient electron transmission and microwave attenuation from either top-view or side-view directions. Besides the electrical conductivity and incident direction, the specimen thickness that the electromagnetic wave needs to pass through is another vital factor that determines the EMI shielding effectiveness.55, 56 Figure 5e shows the influence of specimen thickness on EMI shielding performance of the epoxy/MGA-4 nanocomposite (0.74 vol%). Even the 1-mm thick nanocomposite displays an EMI SE of 24.8 dB, giving a shielding performance of > 20 dB over the whole frequencies explored which satisfies the minimum standard required for the commercial applications of EMI shielding materials. The main advantage of our epoxy/MGA-4 nanocomposite lies in the achievement of satisfactory EMI shielding performances over the whole X band with low filler loading (0.74 vol%) and small thickness as compared to the reported results of other polymer nanocomposites.57 By simply increasing the thickness to 1.5 and 2 mm, the mean values are rapidly improved to 39.0 and 52.7 dB, respectively, providing a great flexibility to tune the EMI shielding properties. To analyze the EMI shielding mechanisms, the contributions from reflection (SER) and 14 ACS Paragon Plus Environment

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absorption (SEA) to the total attenuation of electromagnetic wave (SETotal) are compared (Figure S13d).58,59 It is seen that SEA is significantly higher than that of SER regardless of the Ti3C2Tx contents, implying an absorption-dominated EMI shielding for the epoxy/MGA nanocomposites.60 For example, the values of SETotal, SEA and SER of the epoxy nanocomposite (0.74 vol%) at 12.4 GHz are 56.4, 50.7 and 5.7 dB, respectively. As previously reported,61,

62

the EMI shielding

mechanisms of polymer nanocomposites are more complicated than those of monolithic materials because of the influences of intrinsic conductivity, dispersion, distribution and contents of conductive fillers. Different from the bulk solid conductive materials,13,

14

the porous 3D

MXene/RGO network in epoxy nanocomposites is expected to facilitate penetration of incident wave into the inner porous structure by providing improved impedance matching. Then, the incident wave can be effectively dissipated and attenuated on the large surface area and interfaces by multiple scattering and interfacial electric polarization. Consequently, the contribution of reflection to the overall shielding performance decreases while the absorption contribution instead increases.22 The SER values increase slightly with the increment in conductivity (Figure S13d), similar to the reported results of polymer nanocomposites.19,

22, 62, 63

In addition, the surface

terminations, structural defects and the heterogeneous interfaces between the Ti3C2Tx and RGO provide versatile dipole polarizations to improve the electromagnetic wave attenuation.18, 64, 65 To further highlight the merits of the preformed highly conductive architecture in improving EMI shielding performances of the epoxy nanocomposites, the results of SE divided by sample thickness (SE/d) are summarized in Figure 5f and Table S4 to eliminate the effect of specimen 15 ACS Paragon Plus Environment

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thickness.54 Traditionally, the preformed 3D networks consisting with carbon nanotubes (CNTs) or graphene are promising for EMI shielding applications. For example, 20 dB mm-1 for the PDMS nanocomposite with 0.36 vol% graphene,19 and 20 dB mm-1 for the epoxy nanocomposite with 1.34 vol% CNTs sponge.55 By contrast, it is worth noting that our epoxy/MGA-4 nanocomposite shows a comparable or even superior shielding performance such as 28.2 dB mm-1 with 0.74 vol% of Ti3C2Tx. To explore the reasons for the excellent electrical and EMI shielding properties, the microstructures of the epoxy/MGA-4 nanocomposite are examined with SEM and TEM. Due to the aligned structure of MGA, the fracture surface of the nanocomposite exhibits distinct morphologies from different views (Figure 6a, b and Figure S14). No obvious cracks or voids are observed on the fracture surface from the top view, and no clear sheet-like interfaces can be seen even in the magnified image, indicating the strong interfacial interactions and good compatibility between the hydrophilic Ti3C2Tx sheets and the polar epoxy matrix (Figure 6c, d). This is also evidenced by the higher glass-transition temperatures (Tg) of the epoxy/MGA nanocomposites as compared to that of neat epoxy due to the strong constraint of the filler network on the movement of the epoxy molecular chains (Figure S15). The excellent filler-matrix compatibility derives from the surface terminations of Ti3C2Tx and the polar epoxide groups of epoxy matrix.33, 66 The fracture surfaces of the MGA-4 and its epoxy nanocomposite show highly aligned cell walls of MGA-4, indicating the aligned Ti3C2Tx network is well retained in the nanocomposite (Figure 6a, b). The integrity of the 3D framework is directly observed (Figure 6c, d), which is responsible 16 ACS Paragon Plus Environment

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for the efficient transport of electrons and thus the outstanding electrical and EMI shielding properties. Figures 6c and d present an interconnected graphene framework encapsulated by numerous Ti3C2Tx sheets. Additionally, the presence of the characteristic interlayer spacing of 1.34 nm for Ti3C2Tx sheets also reflects the effective protection of Ti3C2Tx from oxidation. Notably, almost all the Ti3C2Tx sheets are assembled into the porous architecture and no individual sheets randomly dispersed in the matrix are observed, which explains the superior electrical and EMI shielding properties obtained at such ultralow loadings.

CONCLUSION We fabricate highly conductive 3D Ti3C2Tx MXene architecture by GO-assisted hydrothermal assembly followed by unidirectional-freezing and subsequent freeze-drying. The hybrid aerogel shows uniform, ordered lattice-like cellular structure, in which the cell walls are formed by RGO sheets and the compactly attached Ti3C2Tx sheets serving as the inner skeleton and the shell. The favorable ordered nanostructure and well-retained intrinsic structure of Ti3C2Tx afford an outstanding electrical conductivity of 1085 S m-1, which is much higher than that of graphene aerogel (5.4 S m-1) obtained at the same protocol without further reduction process. The highly conductive network endows its epoxy nanocomposite with excellent electrical and EMI shielding performances at very low loadings. Only 0.15 vol% of Ti3C2Tx provides its epoxy nanocomposite a high electrical conductivity of 23.3 S m-1 and EMI shielding effectiveness of 27.3 dB. The conductivity and EMI SE are respectively increased to 695.9 S m-1 and 50 dB at a low loading of 17 ACS Paragon Plus Environment

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0.74 vol%, which are the best results for polymer/Ti3C2Tx nanocomposites. The well-preserved 3D conducting network is responsible for the remarkable electrical and EMI shielding performances obtained at the ultralow loadings. Thus, the successful assembly of 3D Ti3C2Tx MXene architectures would greatly widen the applications of MXene nanomaterials in various potential areas.

METHODS Synthesis of Ti3C2Tx: Ti3C2Tx was obtained by selectively etching Al species from MAX phase (Ti3AlC2). In a typical process, 1 g of LiF (Aladdin Industrial) was dissolved in 20 mL of 9M HCl in a Teflon vessel, and then 1g Ti3AlC2 powders (< 38 μm) was added slowly under stirring. The mixture reacted for 24 h at 35 C to obtain a clay-like Ti3C2Tx suspension, which was repeatedly washed using deionized water and centrifuged at 3500 rpm for 5 min until the pH reaches 6. A homogeneous supernatant of Ti3C2Tx sheets was prepared by ultrasonicating the clay-like Ti3C2Tx suspension under Ar flow for 1 h and centrifuging at 3500 rpm for 1 h. Finally, the Ti3C2Tx suspension was further concentrated by centrifuging at 8000 rpm for 40 min and the obtained Ti3C2Tx paste was stored in a fridge for further uses. Fabrication of Ti3C2Tx MXene/RGO Hybrid Aerogels: GO was prepared by oxidizing natural graphite flakes according to a modified Hummers method.67 Here, a typical MGA with 80 wt% Ti3C2Tx was synthesized as following: Ti3C2Tx suspension (2.4 mL, 10 mg mL-1) and GO dispersion (0.6 mL, 10 mg mL-1) were mixed in a 15 mL cylindrical glass vessel by ultrasonication. 18 ACS Paragon Plus Environment

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Then 6 mg ascorbic acid (Beijing Aoboxing Bio-tech) (ascorbic acid/GO = 1:1, w/w) was added into the mixture under mechanical stirring for 30 min, then the vessel was put into a Teflon vessel for hydrothermal treatment at 65 C for 3 h followed by 70 C for 1 h. After cooled to room temperature, the resultant hydrogel was dialyzed in a 10 vol% ethanol solvent for 6 h. Then it was subjected to directional-freezing process, in which the cylinder hydrogel was put on a copper plate which was pre-cooled in liquid nitrogen. Freeze-drying at -60 C (with pressure < 10 Pa) was used to obtain the hybrid aerogel. Note that ethanol was chosen as the anti-freezing agent to tune the crystallization of the ice crystals during the directional-freezing process. To explore the effect of Ti3C2Tx on the performances of MGAs, the Ti3C2Tx content was tuned by varying the mass ratio of Ti3C2Tx suspension with a constant total concentration of Ti3C2Tx and GO in the suspension (10 mg mL-1). The ultimate MGA specimens with 15, 50, 65 and 80 wt% of Ti3C2Tx were denoted as MGA-1, MGA-2, MGA-3 and MGA-4, respectively. For comparison, in the absence of Ti3C2Tx component, neat graphene aerogel was fabricated by the same procedure under similar conditions. Fabrication of epoxy/MGA nanocomposites: To prepare dense polymer nanocomposites with high EMI shielding performances and satisfactory mechanical properties, epoxy was selected as the polymer matrix to provide necessary strength and stabilize the preformed porous 3D conductive MGA network. The epoxy monomers (Jiafa Chemicals), curing agent (methyl hexahydrophthalic anhydride, Adamas Reagent), reactive diluent (Ethylene Glycol Diglycidyl Ether, Adamas Reagent) and accelerant (2,4,6-tris(dimethylaminomethyl) phenol, Macklin) were homogeneously mixed firstly, and then the resultant mixture was filled into the porous MGA with a vacuum-assisted 19 ACS Paragon Plus Environment

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impregnation technique. Finally, epoxy/MGA nanocomposites were obtained by curing reaction at 80 C for 4 h followed by 120 C for 2 h. The epoxy nanocomposites were machined into specific shapes for electrical and EMI shielding performances measurements. For comparison, epoxy nanocomposites filled with the same contents of randomly dispersed graphene and MXene sheets were also prepared by simple mixing route and using the abovementioned curing process. The volume fractions of Ti3C2Tx and graphene were calculated on the basis of the densities of the constituents using a reported method: 68 Take Ti3C2Tx as an example: vol.%(Ti3C2Tx) = wt.%(Ti3C2Tx)×A/M

A = mA/VA

(1) (2)

where mA, VA and A are the mass, volume, and apparent density of a MGA sample, which can be easily determined for the round plate sample with a diameter of 13 mm and a thickness of 2 mm. wt.%(Ti3C2Tx) is the weight content of Ti3C2Tx in MGA, which is calculated on the basis of the initial mass ratio of Ti3C2Tx/GO and that all the constituents were assumed to be assembled into the MGA. The true density of Ti3C2Tx (M) and that of RGO are 3.2 and 2.26 g cm-3, respectively.6, 69, 70

The volume fraction of RGO, and the total volume fractions of Ti3C2Tx and graphene were

obtained using the similar method. Characterization: Zeta potentials of Ti3C2Tx and GO suspensions were measured with a Malvern Nano-ZS Zetasizer. X-ray diffraction patterns were recorded with a Rigaku D/Max 2500 X-ray diffractometer (XRD). Scanning electron microscope (SEM, Hitachi S 4700) and transmission electron microscopes (TEM, JEOL JEM-ARM200F, Tecnai G2 F20 S-TWIN, and 20 ACS Paragon Plus Environment

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Hitachi HT 7700) were used to characterize the morphology, microstructure and elemental mapping of Ti3C2Tx, GO, MGA-4 and their epoxy nanocomposites. The components of Ti3C2Tx, GA, and MGA were characterized with a ThermoFisher Scientific ESCALAB 250 XPS with an Al K radiation. The samples for the electrical conductivity and EMI shielding performance measurements were round plates with a diameter of 13 mm and a thickness of 2 mm. The electrical conductivity above 10-4 S m-1 is determined with a 4-probeTech RST-8 resistivity meter (China). Fixed current of 100 mA was applied by metal electrodes and the voltage drop was tested over the distance of 4 mm along the aerogel. Keysight N5247A PNA series vector network analyzer was employed to analyze the EMI shielding performances of epoxy nanocomposites in the frequency range of 8.2-12.4 GHz on the basis of a coaxial flange method. Glass transition temperatures of samples were measured by a Mettler DSC1 differential scanning calorimeter (DSC) with a heating rate of 10 C min-1 from 20 to 350 C. Epoxy and its nanocomposite were also characterized with a NICOLET-iS50 Fourier-transform infrared spectrometer (USA). ACKNOWLEDGEMENTS Financial support from the National Natural Science Foundation of China (51673015, 51373011, 51533001), the Fundamental Research Funds for the Central Universities (BHYC1707B), and the National Key Research and Development Program of China (2016YFC0801302) is gratefully acknowledged. ASSOCIATED CONTENT Supporting Information 21 ACS Paragon Plus Environment

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Photograph of hydrothermal process and MGA bearing a load; SEM, TEM and AFM images of GO; SEM and TEM images of Ti3C2Tx; TEM images and SAED pattern of GA; SEM images, TEM image, EDS elemental mapping and Raman spectra of MGA-4, XPS spectra of GA; EMI SE performance, SEM images and DSC curves of epoxy/MGA-4 composites; FT-IR spectra of MGA4, epoxy and epoxy/MGA-4 composites; EMI shielding performances of MGA-4 and epoxy nanocomposite containing randomly dispersed Ti3C2Tx and graphene; EMI shielding performances of epoxy/MGA-4 composite. The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. The authors declare no competing financial interest. AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected] (H.-B. Zhang); [email protected] (Z.-Z. Yu)

ORCID Hao-Bin Zhang: 0000-0003-1156-0495 Zhong-Zhen Yu: 0000-0001-8357-3362

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(49) Krishnamoorthy, K.; Veerapandian, M.; Yun, K.; Kim, S. J. The Chemical and Structural Analysis of Graphene Oxide with Different Degrees of Oxidation. Carbon 2013, 53, 38-49. (50) Li, H.; Hou, Y.; Wang, F.; Lohe, M. R.; Zhuang, X.; Niu, L.; Feng, X. Flexible All-SolidState Supercapacitors with High Volumetric Capacitances Boosted by Solution Processable MXene and Electrochemically Exfoliated Graphene. Adv. Energy Mater. 2017, 7, 1601847. (51) Gao, H.-L.; Zhu, Y.-B.; Mao, L.-B.; Wang, F.-C.; Luo, X.-S.; Liu, Y.-Y.; Lu, Y.; Pan, Z.; Ge, J.; Shen, W.; Zheng, Y.-R.; Xu, L.; Wang, L.-J.; Xu, W.-H.; Wu, H.-A.; Yu, S.-H. SuperElastic and Fatigue Resistant Carbon Material with Lamellar Multi-Arch Microstructure. Nat. Commun. 2016, 7, 12920. (52) Worsley, M. A.; Kucheyev, S. O.; Jr., J. H. S.; Hamza, A. V.; Baumann, T. F. Mechanically Robust and Electrically Conductive Carbon Nanotube Foams. Appl. Phys. Lett. 2009, 94, 073115. (53) Ameli, A.; Jung, P. U.; Park, C. B. Electrical Properties and Electromagnetic Interference Shielding Effectiveness of Polypropylene/Carbon Fiber Composite Foams. Carbon 2013, 60, 379391. (54) Zeng, Z.; Jin, H.; Chen, M.; Li, W.; Zhou, L.; Xue, X.; Zhang, Z. Microstructure Design of Lightweight, Flexible, and High Electromagnetic Shielding Porous Multiwalled Carbon Nanotube/Polymer Composites. Small 2017, 13, 1701388. (55) Chen, Y.; Zhang, H.-B.; Yang, Y.; Wang, M.; Cao, A.; Yu, Z.-Z. High-Performance Epoxy Nanocomposites

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Electromagnetic Interference Shielding. Adv. Funct. Mater. 2016, 26, 447-455. 30 ACS Paragon Plus Environment

Sponge

for

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(56) Li, X.-H.; Li, X.; Liao, K.-N.; Min, P.; Liu, T.; Dasari, A.; Yu, Z.-Z. Thermally Annealed Anisotropic Graphene Aerogels and Their Electrically Conductive Epoxy Composites with Excellent Electromagnetic Interference Shielding Efficiencies. ACS Appl. Mater. Interfaces 2016, 8, 33230-33239. (57) Chen, Y.; Wang, Y.; Zhang, H.-B.; Li, X.; Gui, C. X.; Yu, Z.-Z. Enhanced Electromagnetic Interference Shielding Efficiency of Polystyrene/Graphene Composites with Magnetic Fe3O4 Nanoparticles. Carbon 2015, 82, 67-76. (58) Saini, P.; Choudhary, V.; Singh, B. P.; Mathur, R. B.; Dhawan, S. K. Enhanced Microwave Absorption Behavior of Polyaniline-CNT/Polystyrene Blend in 12.4-18.0 GHz Range. Synth. Met. 2011, 161, 1522-1526. (59) Arjmand, M.; Apperley, T.; Okoniewski, M.; Sundararaj, U. Comparative Study of Electromagnetic Interference Shielding Properties of Injection Molded versus Compression Molded Multi-Walled Carbon Nanotube/Polystyrene Composites. Carbon 2012, 50, 5126-5134. (60) Cao, M.-S.; Wang, X.-X.; Cao, W.-Q.; Yuan, J. Ultrathin Graphene: Electrical Properties and Highly Efficient Electromagnetic Interference Shielding. J. Mater. Chem. C 2015, 3, 65896599. (61) Chung, D. D. L. Electromagnetic Interference Shielding Effectiveness of Carbon Materials. Carbon 2001, 39, 279-285. (62) Al-Saleh, M. H.; Sundararaj, U. Electromagnetic Interference Shielding Mechanisms of CNT/Polymer Composites. Carbon 2009, 47, 1738-1746. 31 ACS Paragon Plus Environment

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Superelastic and Resistant to Fatigue. Nat. Nanotechnol. 2012, 7, 562-566.

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Figures:

Scheme 1. Schematic illustrating the fabrication process of a Ti3C2Tx MXene/RGO hybrid aerogel by GO-assisted hydrothermal assembly, directional-freezing, and freeze-drying.

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Figure 1. (a) XRD patterns of Ti3AlC2, clay-like Ti3C2Tx, and Ti3C2Tx. (b) AFM image and (c) TEM image of Ti3C2Tx sheets. (d) Zeta potentials of Ti3C2Tx, GO, and Ti3C2Tx MXene/GO suspensions, with insets show the Tyndall effect of Ti3C2Tx, GO and their mixture suspensions.

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Figure 2. (a-c) Side-view SEM images of (a) GA, (b) MGA-2 and (c) MGA-4. (d) Digital images of MGA-4 (100 mg) standing on a flower (left) and supporting a weight of 500 g (right).

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Figure 3. (a) XRD patterns of Ti3C2Tx, GA and MGA-4; (b) XPS survey spectra of Ti3C2Tx, GA and MGA-4; Ti 2p spectra of (c) Ti3C2Tx and (d) MGA-4; and C 1s spectra of (e) Ti3C2Tx and (f) MGA-4.

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Figure 4. (a, b) STEM images of epoxy/MGA-4 nanocomposite. (c, d) STEM image and the corresponding elemental mapping images of Ti, C and O in the same area.

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Figure 5. (a) Comparison of electrical conductivities of GA and MGA-4 with different bulk densities. (b) Influence of Ti3C2Tx content on conductivity of MGA. (c) Electrical conductivities and (d) EMI shielding performances of epoxy/MGA nanocomposites. (e) Effect of sample thickness on the EMI SE of epoxy/MGA-4 nanocomposite (0.74 vol%). (f) Comparison of shielding performances of epoxy/MGA nanocomposites (red star) with that of shielding materials ever reported: SE/d as a function of filler volumetric content. The number inside the plot are the reference numbers listed in Table S4.

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Figure 6. (a, b) Side-view SEM images of MGA-4 and epoxy/MGA-4 nanocomposite; (c, d) TEM images of the epoxy/MGA-4 nanocomposite under different magnifications.

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Table of Contents Three-dimensional and lightweight MXene porous assemblies with excellent electrical conductivity are constructed by a graphene oxide-assisted hydrothermal process, directionalfreezing and freeze-drying. The resultant MXene/RGO hybrid aerogel shows an outstanding electrical conductivity, and its epoxy nanocomposite exhibits a remarkable electromagnetic interference shielding performance of >50 dB over the X-band.

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