Multifunctional Superelastic Foam-Like Boron Nitride Nanotubular

Dec 13, 2016 - Research Institute of Unconventional Petroleum and Renewable Energy, China University of Petroleum (East China), Qingdao 266580, P. R. ...
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Multifunctional Superelastic Foam-Like Boron Nitride Nanotubular Cellular-Network Architectures Yanming Xue,*,† Pengcheng Dai,*,⊥ Min Zhou,† Xi Wang,‡ Amir Pakdel,† Chao Zhang,† Qunhong Weng,† Toshiaki Takei,† Xiuwei Fu,† Zakhar I. Popov,# Pavel B. Sorokin,# Chengchun Tang,*,§,∥ Kiyoshi Shimamura,† Yoshio Bando,† and Dmitri Golberg*,† †

National Institute for Materials Science (NIMS) and International Center for Materials Nanoarchitectonics (MANA), Namiki 1, Tsukuba, Ibaraki 3050044, Japan ⊥ Research Institute of Unconventional Petroleum and Renewable Energy, China University of Petroleum (East China), Qingdao 266580, P. R. China ‡ Key Laboratory of Luminescence and Optical Information, Beijing Jiaotong University, Beijing 100044, P. R. China # National University of Science and Technology “MISiS”, Leninsky Prospect 4, Moscow 119049, Russian Federation § School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, P. R. China ∥ Hebei Key Laboratory of Boron Nitride Micro- and Nano-Materials, Tianjin 300130, P. R. China S Supporting Information *

ABSTRACT: Construction of cellular architectures has been expected to enhance materials’ mechanical tolerance and to stimulate and broaden their efficient utilizations in many potential fields. However, hitherto, there have been rather scarce developments in boron nitride (BN)-type cellular architectures because of well-known difficulties in the syntheses of BNbased structures. Herein, cellular-network multifunctional foams made of interconnective nanotubular hexagonal BN (h-BN) architectures are developed using carbothermal reduction-assisted in situ chemical vapor deposition conversion from N-doped tubular graphitic cellular foams. These ultralight, chemically inert, thermally stable, and robust-integrity (supporting about 25,000 times of their own weight) three-dimensional-BN foams exhibit a 98.5% porosity, remarkable shape recovery (even after cycling compressions with 90% deformations), excellent resistance to water intrusion, thermal diffusion stability, and high strength and stiffness. They remarkably reduce the coefficient of thermal expansion and dielectric constant of polymeric poly(methyl methacrylate) composites, greatly contribute to their thermal conductivity improvement, and effectively limit polymeric composite softening at elevated temperatures. The foams also demonstrate high-capacity adsorption-separation and removal ability for a wide range of oils and organic chemicals in oil/water systems and reliable recovery under their cycling usage as organic adsorbers. These created multifunctional foams should be valuable in many high-end practical applications. KEYWORDS: hexagonal boron nitride, cellular-network foams, superelastic behavior, polymeric composites, adsorption-separation purification fields;10 cellular graphene elastomers document exceptional ability toward ultrafast dynamic piezoresistive response within a broad frequency bandwidth;5 and 3D-architected Au microlattices with periodic porous structures and independently tunable surface compositions are efficiently used for positive

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ellular architectures constructed within a material body have enabled mechanically tolerant materials.1,2 Such architectures manufactured from any kind of desired solid constituents are expected to stimulate materials’ utilizations and broaden their application fields, e.g. energy damping,3 sensing,4,5 tissue engineering,6 environmental cleaning,7 and electrochemistry.8,9 For example, octet-truss hollow-tube Ni−P and Al2O3 microlattices, with a linear stiffness-density relationship (E ∝ ρ) and superelasticity, illustrate highly efficient applications in mechanics and energy © 2016 American Chemical Society

Received: September 30, 2016 Accepted: December 13, 2016 Published: December 13, 2016 558

DOI: 10.1021/acsnano.6b06601 ACS Nano 2017, 11, 558−568

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ACS Nano electrodes in Li−O2 batteries.9 These studies have triggered a continuous interest to the new-era lightweight cellular materials, which would result in more efficient technologies and promote a variety of smart applications. Currently, the efforts to construct and study cellular-like hexagonal boron nitride (h-BN) architectures have been intensified due to h-BN’s excellent electrical insulation,11,12 its exceptionally high thermal and chemical stabilities,12 and superb mechanical properties.12,13 For example, Yin and coworkers have fabricated ultralight hierarchical BN foams via a nickel-foam template-assisted chemical vapor deposition (CVD) and investigated their superelasticity (at a maximal strain of 70%) and ultralow permittivity (only 1.03−1.12-fold increase compared to air).14 Two years later, using a freezecasting technology, Zeng et al. prepared cellular-like BN nanosheet (BNNS) aerogels exhibiting superelasticity (maximal 75% strain) and ultralow dielectric constant (1.24) and loss (0.003).15 These pioneering works are encouraging with respect to overcoming well-known difficulties in the synthesis of h-BN-based materials. They also demonstrate bright application potentials of h-BN cellular materials, such as energy absorbing, elevated-temperature tunable capacitors, and force sensors. However, these studies have also risen some important issues. First, the approach14 of using 3D interconnected metal (such as Ni) foams with coarse microstructures (200−500 μm cell sizes) as templates to grow h-BN cannot guarantee sufficient bonding densities within a resultant cellular-network body. This leads to fragility during localized bending and rapid loss in load-carrying ability during deformations. Second, the freeze-casting processes15,16 result in heterogeneous bondings between cellular cells and polymers via hydrogen bonds and/or van der Waals interactions. These inhomogeneous interconnectivities are inherently weak and easily deteriorated when only a few percent of strain is applied. The cellular networks may also easily thermally collapse because of polymer pyrolysis. These factors bring to the front a timely warrant need for finding an approach to fabricate h-BN cellular structures which have better bonding densities and mechanical properties, reliable interconnectivities, and built-in integrity to satisfy the strict requirements for emerging applications. Herein, we develop covalently (partially ionic) bonded cellular lightweight foams made of interconnected tubular h-BN architectures, which reveal greatly enhanced stiffness (densitynormalized elastic modulus of ∼13.2 MPa cm3 g−1), excellent shape recovery after compressive deformations up to 90%, desired robust integrity and damage tolerance (they support a weight of ∼25,000 times their own without distortion), by a two-step CVD conversion method. A quasi-nanoscale tubular form of BN is used as cellular units instead of nanosheet15 morphologies and/or monolith-like bubble-assembled structures17 (obtained through integrative chemistry method) because of well-established and excellent properties of BN nanotubes (BNNTs), such as ∼1.3 TPa Young’s modulus,12,18 and bending strength of ∼260 MPa.12 Based on robust integrity within 3D tubular BN cellular-network foams (3D-BNFs), we were able to thoroughly investigate the shape recovery, the materials’ usage in polymeric composites, and hydrophobiclipophilic adsorption-separation cleaning. Our striking findings allow us to claim the creation of an excellent multifunctional foam-like material.

RESULTS AND DISCUSSION The 3D-BNFs were fabricated using a two-step templateassisted CVD method, as illustrated in Scheme 1. First, by using Scheme 1. Synthesis Schematic of 3D Tubular BN CellularNetwork Foams

SiO2 aerogels with cellular nano/microwire networks19,20 (Supporting Information (SI) Figure S1), template-directed N,N-dimethyformamide (DMF) CVD (SI Figure S2) at 1100 °C, and subsequent etching removal of silica templates, we successfully prepared 3D tubular N-doped graphitic foams (3DNGF). A homogeneous N doping (SI Figures S4a and S5) was synchronously achieved by this DMF CVD process when the primary graphitic layers have continuously been formed and uniformly segregated on the silica surfaces. After etching removal of the inner silica templates, the as-formed N-doped graphite (NG) structures completely replicated the structural outlines of 3D silica cellular-network aerogels, enabling a spatially tube-interconnected continuity and high porosity within the final 3D-NGF body retaining a macroscopically bulk shape (SI Figures S3 and S4c,d). Second, 3D-BNFs imitated the 3D-NGF-templates were prepared through a modified carbothermal-reduction CVD reaction (CR-CVD-R) (SI Figure S6). Homogeneous N dopant-introduced defects (SI Figure S4b,e) in 3D-NGFs play important on site restrictive roles during tubular-structure conversion and transformation from NG to BN phase: some B−N species have been preformed inside NG walls through CR-CVD-R when incipient B2O3 vapor diffuses into these N heteroatom sites, and then a continuous reaction and assembling of the preformed B−N species with other BN phases (those are generated from continuous CR-CVD-R along the tubular NG wall surfaces) take place. The material is spatially converted into BN nanotubular fibers while copying the prime forms of tubular NG. Morphologically, the final 3D-BNFs product is totally identical to the 3D-NGFs (SI Figures S3 and S7) in view of its microstructures and interconnective skeletons. However, while utilizing the cellular graphitic nanotubes (without N-doping) as a template, we found that the obtained tube walls consist of many more defective BN-nanosheet-assembled structures (SI 559

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Figure 1. (a) Photos of real products of 3D-NGF (black) and 3D-BNF (white). (b, c) SEM images of the 3D-BNF under different magnifications; inset of (c) shows an individual opened BN tube cell. (d) XRD patterns of 3D-BNF (red) and commercial bulk h-BN (black). Insets depict enlarged angle ranges of 24−28° and 40.4−44.4°. (e) Raman shifts of 3D-BNF (red) and commercial bulk h-BN (black). Inset illustrates the enlarged parts in the range of peak positions. (f) Electron energy loss spectra of 3D-BNF (red) and commercial bulk h-BN (black). Insets correspond to the enlarged regions of B−K and N−K edges.

further used to evaluate the crystalline ordering of the BN tubes: a slight shift from 1367 cm−1 natural for a standard highly crystallized bulk BN to 1368 cm−1 (inset of Figure 1e) results from the slightly enlarged (0002) spacing due to stacking within tubes; 22.8 cm−1 for fwhm of E2g mode indicates a highly crystallized material (as referred to the corresponding h-BN single crystals numbers of 1367 and 9.1 cm−1 fwhm).11 Distinct B and N core loss K-edges in electron energy loss spectra (EELS), seen at 191 and 401 eV, respectively, reveal sharp π* peaks and σ* bands similar to those for a bulk BN (Figure 1f and its insets), unveiling wellformed sp2 hybridization (similar to h-BN).25 No trace of any C K-edge (284 eV) confirms a complete CR-CVD-R conversion from 3D-NGF to 3D-BNF. Furthermore, full X-ray photoelectron spectra (XPS) and high-resolution B 1s (190.3 eV) and N 1s (398.0 eV) XPS signals (SI Figure S11) and B/N atomic ratio of ∼1/1, as determined by both EELS and XPS techniques, are indicative of perfect purity and high crystallinity of the resultant 3D-BNFs products. Spacially resolved elemental mapping is shown in Figure 2a− c. Individual B and N species (Figure 2b,c) are uniformly distributed along and across the tube walls and their junctions. An individual BN tube is depicted in Figure 2d; its walls are rather straight and well-ordered as well as for standard BNNTs.24,26 Several thin BN nanosheets and/or flakes are randomly distributed over the exteriors of tube walls (dotted lines marked in Figure 2d and SI Figure S12). These nanosheets are formed via nitridation of B-containing intermediates on original cell surfaces (herein, tubular NGs), as similarly reported in Wang’s work.27 Some thin layers may be a result of secondary reaction generated near tube walls, where a resultant CO (eq 1) further reduces the B2O3 vapor, followed by a reaction with N2 around tubes to produce new BN layers, along with the following reaction:

Figure S8), thus weakening the interconnected robustness of the 3D-BNFs. Approximately 5 at. % N-doping (SI Table S1) induces a change in conventional carbothermal reduction reaction21 (CCRR) toward BN formation. This goes along the following assumed reaction: 12B2O3(g) + 40C0.9N0.05(s) + 11N2(g) → 24BN(s) + 36CO(g)

(1)

Figure 1a shows photographs of a pristine 3D-NGF bulk (cut into square-like block) and the converted 3D-BNF product. The converted BN sample has no cracks and keeps original macroscopic volume and shape, while its body color turns from black (peculiar to N-doped C) to white (characteristic of BN). This BN foam has maintained a continuous homogeneous interconnection of cellular-networks constructed over tubular cells (Figure 1b,c), in the same manner as 3D-NGF does (SI Figure S7). A mean diameter of the primary tubular units is statistically averaged at ∼360 nm (inset of Figure 1b and SI Figure S9). Their interconnective nodes may be of two-, three-, four-, five-, and more than five-fold shapes. The trifurcate joints were the most common knots revealing more than 50% occurrence (SI Figure S10). X-ray diffraction (XRD) of a 3DBNF product compared with commercial bulk BN is illustrated in Figure 1d. This indicates high purity and perfect crystallinity of the material. Fine structure analyses of XRD patterns (inset of Figure 1d) illustrate that the (0002) (peaked at 2θ = 25.8°) interplanar spacing of 0.345 nm is slightly larger than that of 0.332 nm (2θ = 26.7°), peculiar to bulk BN. This enlargement usually appears in the tubular BN materials, such as BNNTs.22 Also, its (101̅0) diffraction reveals peaks at 41.7° and 42.6°, which are attributed to the h-BN and rhombohedral BN (r-BN) (101̅0) patterns, respectively.23 Such traces of r-BN arrangement have been frequently detected in highly crystallized BN nanotubular species.24 Raman shift (Figure 1e) and its full width at half-maximum (fwhm) of E2g vibration mode were 560

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The curves of compressive stress (σ) as a function of strain (ε) used to quantify superelasticity of a 3D-BNFs material are presented in Figure 3d−f. Interestingly, 13-cycles compressive σ−ε curves (with increasing ε from 5% to 65% in sequence with an interval of 5%) (Figure 3d) exhibit a continuous trend, i.e., each successive loading curve threads the maximum point of the preceding compressing-releasing cycle, demonstrating a perfect shape recovery effect. After increasing ε amplitude to 65%, further consecutive loading−unloading 10-cycles were carried out (Figure 3e), followed by 5-cycles with a compression ε up to 90% (Figure 3f). The loading processes reveal three typical deformation regimes, as evaluated by the tangent modulus in loading-unloading cycles (insets of Figure 3e,f, and SI Figure S14): the initial Hook-law regime at 1.5% < ε < 3.5%; a plateau regime for 3.5% < ε < 50%; and the final densification regime at ε > 50% with a rapidly rising σ. The Hook-law deformation region is attributed to decent bending of tubular cells in 3D-BNFs; the slowly rising long-range plateau results from overbending and postbuckling deformation of the interconnected branches and junction nodes; the severe volume reduction at ε > 50% greatly increases the frequency of intertouches and frictional contacts between tube walls due to localized compression at nodes (SI Figures S15, S27, and S28), causing a steep increase in σ. These features are similar to other superelastic materials, e.g., graphene foams,20 CNT aerogels,28 and metallic microlattices.1,29 Under unloading, ∼15% of ε recoveries lead to σ recovery ratios of ∼70% (cycles at 65% ε, Figure 3e) and ∼88% (case of ε at 90%, Figure 3f). However, the σ does not completely vanish until ε returns to ∼0%, after complete shape restoration. Inevitably, unrecoverable residual deformations (RDs) of 2.5% and 4.3% are introduced after 10cycles at 65% and 5-cycles of 90% strains, respectively, due to appearance of twists, bends, and fractured tubes during loadingunloading (SI Figures S15, S16c,e,f,h, S27, and S28). Hysteresis loops measured from σ−ε curves (Figure 3d−f) imply that energy dissipation occurs, probably because of cracks, and that adhesion and friction take place between tubular interconnects, similar to the compression-recovery cycles of most resilient cellular-network materials.20,28,29 The experimentally measured maximum compressive stresses (σM), elastic modulus (EM, given by slopes of σ−ε tangents, SI Figure S17), and energy loss coefficients (ΔU/U) (generally used to evaluate a superelastic behavior) are summarized in Figure 3g and SI Figure S18. The σM values for ε at 65% and 90% are 0.13 and 0.34 MPa in the prime cycle, confirming stiffening due to numerous frictional contacts between BN tubes under volume reduction. The first EM is 0.7 and 0.4 MPa for ε of 65% and 90%. Such σM and EM numbers are the highest values among the superelastic BN-based family members reported previously. These figures are ∼3 and ∼9 times larger than those for BNNS-based aerogels15 and ∼4800 and ∼5500 times higher than in the cases of 3D-BN foams derived from the Ni-foam CVD.14 Such remarkable superelasticity is attributed to the high-density bonding and strong tubeinterconnected skeletons inside 3D-BNFs. The σM and EM values tend to stabilize at ∼0.1 MPa and ∼0.4 MPa for ε - 65% cycles and ∼0.3 MPa and ∼0.2 MPa for cycles at ε - 90%. A density-normalized elastic modulus (E) is estimated to be ∼13.2 MPa cm3 g−1 after 10 cycles at ε - 65%, demonstrating a 5-fold improvement as compared with BNNS-based aerogels,15 and ∼300 times rise compared to 3D-BN foams (from Nifoam).14 The maximum compressive strain (εM = 90%) for a fully sustainable elasticity is superior to all previously reported

Figure 2. (a) Bright-field TEM image of tubular cells. (b, c) Boron and nitrogen elemental maps corresponding to the image in (a). (d) TEM image of an individual multiwalled BN nanotube; the segments marked with a red dotted line represent attached BN nanosheet structures. (e) HRTEM image of the well-structured BN nanotube wall. (f, g) Interplanar spacings of (101̅0) and (0002) planes of the tube corresponding to in-framed in pink and yellow wall domains in (e).

3CO(g) + B2O3(g) + N2(g) → 2BN(s) + 3CO2 (g) (2)

HRTEM image indicates dozens of BN (0002) layers within the tube walls (a wall thickness ranges from ∼10 to ∼20 nm) with an average interplanar spacing of 0.345 nm (Figure 2f). This number is consistent with the XRD-derived results (Figure 1d). Lattice distance along the tube axis is 0.216 nm (Figure 2g), which is assigned to the spacing of (101̅0) planes in h-BN (inset of Figure 1d).22 Hence, the tube axes are basically parallel to the [101̅0] orientation common for BNNTs.22,24,26 The 3D-BNFs possess robust interconnections leading to excellent shape recovery after compression (Figure 3). A lowdensity/lightweight nature of the 3D-BNFs is nicely demonstrated by supporting them on the spike awn of a plant and their adhesion to the bottleneck of a plastic bottle due to electrostatic forces (Figure 3a). The 3D-BNFs’ measured density of ∼30.4 mg/cm3 means about 1.45 wt % BN in the material, yielding a porosity of ∼98.5%. The measured specific surface area was 166 m2/g. The material revealed typical slitshaped pores (pore size distributions were centered at ∼3 nm and ∼15 nm; the total pore volume was 0.63 cc/g), as documented through Barrett−Joyner−Halenda (BJH) analysis of N2 adsorption-desorption isotherms at 77 K (SI Figure S13). Figure 3b displays a superelasticity of 3D-BNFs revealing a complete shape recovery without any traces of mechanical failure after a 65% compression strain (SI Movie S1). These results were additionally supported by the molecular dynamics (MD) simulations of compression-recovery cycles using a simplified foam model (see SI, Figures S27 and S28). Also, a block of 3D-BNF (∼16 mg) can easily support a metal ingot (∼400 g weight, thus ∼25,000 times of its own weight) without any visible shape distortions (Figure 3c), indicating the excellent foam robustness. 561

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Figure 3. (a) Photos of a 1.5 cm3 block of 3D-BNF placed on the spike awn of a plant and that adhered to the bottleneck of a plastic bottle due to electrostatic attraction. (b) Photos of the 3D-BNF compressed to 65% and recovered to its original shape: (i) original state, 0% strain; (ii) compressed state, deformed to 65%; and (iii) shape fully recovers. (c) 16 mg of 3D-BNF supporting a ∼ 400 g weight without visible distortion and damage. (d) Loading-unloading cycles and stress-strain curves in sequence of increasing the strain to 5% and then to 65% (with the strain intervals of 5%). During compression, every next loading curve intersects the preceding stress-strain curve at their maximum strain positions (labeled with different black circles) in sequence. Inset shows the enlarged stress-strain curves under compressive strains of 5%, 10%, and 15%. (e) Fatigue-resistance properties of 3D-BNF tested under 10-times cyclic compression at a strain of 65%. Inset depicts the tangent modulus in each compression cycle at a strain of 65%. (f) Fatigue resistance during compression with a 90% strain; the inset is the evolution of corresponding tangent modulus in every consecutive cycle. (g) Summary of ΔU/U, σM, and E during compressive cycles at 65% and 90% strains.

superelastic BN-based cellular-network materials (the latter did not exceed a 75% deformation)14,15 and other types of BNbased foams, such as sp2-bonded BNNSs aerogels (with a recovery strain of only ∼30%).30 Moreover, the superelastic properties measured by us are better than those for most of the reported carbon-/graphene-based foams (SI Table S3), such as carbon aerogels, multiarch lamellar carbon-based monoliths, graphene-complex cellular networks, and biomimetic graphenebased monoliths; and also they are comparable to some toplevel carbon-based foams (SI Table S3), for example, tubular graphene-connected cellular-network foams and graphenecoated CNTs aerogels. Such improved superelasticity is attributed to the efficient integration of numerous robust interconnective BNNTs. To further confirm the superb elasticity of our material, we additionally performed MD

theoretical simulations of the BN nanotube three- and fourterminal network compression-recovery cycles (SI Figures S27 and S28); double-walled nanotubes were taken for simplicity and clarity, while also considering the available computational resources. The ΔU/U values at ε of 65% and 90% are 60.5% and 68.7% in the first cycles, respectively, and tend to stabilize at ∼58% for ε at 65% after the seventh cycle and 65% for ε of 90% after the third cycle, meaning that larger ε leads to higher ΔU/U ratios and energy absorption properties. 3D-BNF-Poly(methyl methacrylate) (3D-BN-PMMA) composites were then fabricated via an acetone-assisted solution method (Experimental Section). Their properties vs blank PMMA are documented in Figure 4. 3D-BN-PMMA composites display a drastic decrease in the UV−vis transmission in the range of 350−850 nm with increasing composite 562

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Figure 4. Superb performance of 3D-BN/PMMA composites. (a) Photos: (i) a blank PMMA slice; and 3D-BN/PMMA composite slices having different thickness after cutting, (ii) 0.2 mm, (iii) 0.5 mm, and (iv) 1 mm. (b) Comparative optical transparency tests on blank PMMA and its 3D-BNF composite slices of various thicknesses. (c) Cross-sectional SEM images of the composite: (i) low-magnification image of cross-section and (ii) zoomed-in image of the framed section from the left-hand-side spot; arrows mark the interconnective tubular cellularnetwork knots and nodes visible within the cross-sectioned surface; black frames mark the open-ended tubes embedded into the composite; and yellow frames mark the empty bubbles. (d) Frequency-dependent DCs and the corresponding DLTs of a blank PMMA and the 3D-BNF polymeric composites (averaged values are shown). (e) Summary of measured TCs of blank PMMA and its composites. (f) Linear thermal expansion ratios of blank PMMA and composites determined via thermodilatometry analysis; different horizontal compression loading forces of 10 and 50 g acting on a sectional area of ∼6 mm 2 along with a heating rate of 5 °C/min were applied. (g) Photos of a blank PMMA and 3D-BNF PMMA composite samples before (i) and after (ii) thermodilatometry measurements under a force of 50 g and temperature of up to 120 °C.

tronic devices made up of multiply interconnected transistors.34,35 This fact indicates that the present 3D-BN-PMMA composites should find valuable application prospects in future integrated circuits (ICs) and packaging. The thermal conductivity (TC) was improved from 0.186 ± 0.016 W m−1 K−1 to 1.03 ± 0.072 W m−1 K−1 (thermal diffusivity (TD) was changed from blank PMMA (of ∼1.21 × 10−7 m2/s) to ∼6.42 × 10−7 m2/s for a composite) (Figure 4e and SI Figure S20), thus revealing a 5.5-fold gain. Pure 3DBNFs have ultrahigh TD values (∼1.2 × 10−5 m2/s) close to that of air (∼1.9 × 10−5 m2/s), showing excellent transmission capacity to temperature variations (SI Figure S20d). An ultralow BN filling fraction (BNFF) inside composites, which is only ∼2.7 wt % (∼1.5 vol %), should particularly be mentioned. Also it is noted that ∼3.5 vol % air filling is natural for the composites due to inherent hollow nature of BN connecting tubes and a number of bubbles formed during

sheet thickness because of increased superposition of continuous tubular 3D networks (Figure 4a,b). A continuous interconnection between tubes is maintained within the composites (Figure 4c (ii)). Such intact hollow-connection frames create many empty channels within the material, which spoil its dielectric ability. For example, the frequency-dependent dielectric constants (DCs) are reduced by 18%, as compared to blank PMMA. The obviously increased dielectric loss tangents (DLTs) in composites, as compared with pure PMMA, are predominantly attributed to the synergistic effects of interfacial polarization losses31 (Maxwell−Wagner−Sillars polarization)32,33 occurring between multiple interfaces within PMMA matrix, air voids, and BN phases, though all measured DLT values still exhibit the same order of magnitude (Figure 4d and SI Figure S19). Lowering DCs of insulators can efficiently slow down resistance-capacitance (RC) delay emerging in new-generation high-operation-speed microelec563

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Figure 5. (a) Photograph of a water droplet on the 3D-BNF surface; its contact angles of 124°, 131°, and 144° obtained at different pH values of 1, 7, and 14, respectively. (b) Photographs of hexane (dyed with a red agent) and its adsorption process by the 3D-BNF on the water surface. Scale bar, 2 cm. (c) Adsorption efficiency during 30 cycles. The inset shows the recovery process after rapid burning using a flame of liquidized gas spray nozzle; as a result, original white body color is entirely regained. (d) Schematic representation of the continuous collection of chloroform from under the water by introducing a suction force onto the 3D-BNF surface above the water surface, and photographs of the continuous collecting process: (i) 3D-BNF put into the water/chloroform system; (ii) starting collection; and (iii) finishing the chloroform collection without changing the amount of water. Scale bar, 2 cm. (e) Adsorption capacities of 3D-BNF for a wide range of oils and organic solvents. (f) Corrosion resistance of 3D-BNFs and their weight changes in the 1.5 M hydrochloric acid and 10 M sodium hydroxide solutions over 30 days. The inset illustrates that 3D-BNFs are still floating on the surfaces of two highly corrosive solutions after 30 days.

still a room for TC improvement using this strategy if high densification of a polymer filling into the 3D-BNFs may be properly addressed under processing, because the hollow channels and unfilled interspaces within composites present some heat reservoirs to obstruct heat flow paths and to restrict a heat transfer. Low coefficient of thermal expansion (CTE) is another important thermal parameter vital to microelectronic packaging. For example, differences in CTEs can lead to serious thermal/internal stresses between packaging composites and electronic components, which make a device unreliable. One effective solution to minimize mismatched CTEs is to reduce the CTEs of packaging composites. Herein, thermodilatometry measurements were used to investigate a coefficient of linear thermal expansion (CLTE) of PMMA and the designed 3DBNF polymeric composites (Figure 4f). A remarkable decrease

curing process (Figure 4c). TC enhancement efficiency, that is defined by TC improvement per 1 wt % or 1 vol % BN loading (BNFF-normalized TC improvement, SI Table S2), is 169 (weight-normalized) or 304 (volume-normalized). These values are superhigh, for example, they are 2-fold larger than those for 24 wt %-BNNTs-filled-PMMA composites36 and 6times larger compared to PMMA loaded with 18 wt % BNNSs.37 Such a striking efficiency is attributed to the continuously interconnected BN nanotubular cellular-networks enabling efficient thermally conductive pathways (heat transfer along the tube walls of ultrahigh thermal-conductivity BN (0002) planes) inside the composite. This means that the homogeneous conductive pathway interconnectivity presents the most effective approach to enhance polymeric TC, unlike traditional concepts relying on sole increasing of BNFFs or/and enhancing an interfacial thermal transfer.36 Actually, there is 564

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ACS Nano by ∼50% in LCTE (4.4 ± 0.64 × 10−5/°C) was observed in the composites (with only 2.7 wt % BNFF), as compared to PMMA (8.3 ± 0.16 × 10−5/°C) (inset of Figure 4f). Such a result is better than in all previous cases, e.g., CTE of epoxy decreased by ∼31% after filling with a 40 wt % AlN fraction;38 CTE of HDPE matrix decreased by ∼46% while introducing AlN/SiC fractions up to 30 wt %.39 This CLTE behavior results from a restriction of PMMA chain mobility through the embedment of low-expansion BN phase and its robust connective cellular-networks and well-developed π electronic surfaces of tubular h-BN. Besides, the 3D-BNFs can effectively restrain a temperature-driven softening failure of PMMA composites due to their interconnection integrity peculiar to the composites. As shown in Figure 4g, a blank polymer specimen has a serious deformation over softening temperature (ST) (corresponding to the steeply falling plots after ST in Figure 4f), however, its composite remains almost intact (nearly horizontal portions of thermodilatometry curves in Figure 4f) under the identical testing conditions. To sum up, the developed 3D-BNF-PMMA composites, combining low DC, DLT, and CLTE and high TC values and hightemperature robustness, would be excellent next-generation composites for ideal ICs and packaging. Finally, 3D-BNFs architectures were also tested as separation-purification agents in oils/water systems, owing to their inherent hydrophobic-lipophilic nature, as shown in Figure 5. The 3D-BNFs are superhydrophobic at different pH values of 1, 7, and 14, showing wetting angles of 124°, 144°, and 131°, respectively (Figure 5a). The foams nicely adsorb various hydrophobic oils from the oil-contaminated water. The oil is stored in their highly abundant porous spaces. For example, once we drop a block of 3D-BNF into the oil/water surface, the adsorption of red-dyed oil is immediately completed within only several seconds. The foam becomes red (Figure 5b and SI Movie S2). Due to its low density and superhydrophobicity, 3D-BNF still floats on the water surface after adsorbing, enabling easy and recyclable collection. Such fast kinetics of adsorption is attributed to the synergetic effects of high porosity, inborn lipophilicity and enhanced capillarity (as well as for previously reported BN porous nanosheets40 and sp2-bonded BN aerogels30) of h-BN cellular networks. The 3DBNFs’ high porosity provides a low-resistant pathways for oilmolecule adsorption and diffusion along the lipophilic tube surfaces. Their high-density interconnections result in highdensity capillarity filling within the tubular hollow channels. Due to oil-surface tension, the preadsorbed and -diffused molecules continuously pull the rest of the oil body toward pore spaces of 3D-BNFs, until the saturation takes place. As well as conventional BN and BNNTs, this 3D-BNF is chemically inert and does not oxidize even up to ∼1050 °C in air (SI Figure S21). Thus, oil-saturated 3D-BNF (with engine heavy oil) may repeatedly be recovered by fast flame burning (inset of Figure 5c and SI Movie S3) without shape distortion and adsorption efficiency deterioration (it remains ∼98.5% efficiency during 30 cycles) (Figure 5c). Moreover, the nearly invariable chemical composition, microstructure, and superhydrophobicity after regeneration (SI Figure S22) indicate an absolute reliability for 3D-BNFs as adsorbers. High-density oil species such as chloroform, which usually sink down deep into the water, can also be fully and quickly removed by 3D-BNFs (SI Figure S23 and SI Movie S4). This implies that there is an existence of intrusion pressure (IP) that can efficiently obstruct water to enter into a 3D-BN foam

interior. An estimated IP under water for 3D-BNFs is at least higher than 14.7 kPa (SI Figure S24), which is much better than such number for robust superhydrophobic graphdiynebased cellular foams (only 0.87 kPa).41 In view of this point, the 3D-BNFs can be used to remove the subfluvial chloroform under its insertion into water. The oil is absorbed at the bottom end, and a forceful pumping tube is placed on the upper end (∼1/5 volume above the water) (Figure 5d and SI Movie S5) in order to continuously collect the adsorbed chloroform. As a result, a high amount of oils (volume is much bigger than that of 3D-BNFs) can be fast separated and collected from water at a speed of ∼0.25 mL s−1 with an extracting pressure of ∼0.1 MPa over several cycles (SI Figure S25). Such adsorptionseparation mechanism is a synergetic result of the 3D-BNF adsorption-saturation performance, ultrahigh water-resistant capacity, and pumping-assisted continuous capture process (SI Figure S26). The robust integrities, while withstanding a powerful pumping force (∼0.1 MPa) and several cycles of recovery (including body color) under severe jet flame without cracking and distorting, are the best for any member of the BNbased foam family reported to date. For example, BNNSs/ BNNTs-hybrid aerogels were prone to serious deformation even after mild cyclohexane-burned recovery;42 severe body cracks and volume shrinkages occurred for sp2-bonded BNNSs aerogels when they had only experienced organic solvent adsorption and drying.30 A wide range of oils and solvents can be efficiently adsorbed (Figure 5e), revealing the ultrahigh adsorption and separation capacity, up to 16−46 times of own 3D-BNF weight, with highly pore-filling efficiency, up to ∼99%. Thus, the presently uncovered outstanding adsorption-separation capacity is superior to most of previously reported BNbased and carbon-based foam-like materials (SI Table S3). Figure 5f shows corrosion effects on the synthesized 3D-BNFs. Both foam shapes and foam superhydrophobicity are perfectly retained during 30 days afloat soaking, indicating outstanding anticorrosion behavior to strong acids and alkalis. This means that the present 3D-BNF would be particularly useful for environmental cleaning, especially in severely harsh environments.

CONCLUSIONS In summary, 3D architectural foams constructed from nanotubular BN cellular networks have been successfully fabricated via carbothermal reduction and in situ conversion from preformed 3D tubular N-doped graphitic foams. The 3D BN foams show a high-porosity (98.5%), sustainable compressive deformations, ultralight weight, superelastic behavior, superhydrophobic properties, superb water-anti-intrusion ability, high-thermal-diffusion/-stability, chemical inertness, robust integrity, ultrahigh strength and stiffness, and a perfect shape recovery (at a compressive strain of up to 90%). Such an impressive set of properties rivals the most optimal foam candidates known to date, such as graphene aerogels and foams, and surpasses all known hyperelastic 3D BN foams. The material remarkably reduces CLTE (by 50%) and DC (by 18%) of polymeric PMMA composites, exhibits an ultrahigh TC improvement efficiency, and effectively restrains polymer softening at elevated temperatures. The 3D-BN foams also possess high-capacity adsorption-separation and removal ability for diverse range of oils and organic chemicals in oil/water systems, thus documenting reliable recovery and recycling. We envisage that the presently designed and fabricated by us multifunctional 3D-BN foams would be highly valuable for 565

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ACS Nano

Thermo Plus EVO2 DSC-8231) using a sapphire as a reference. Thermal diffusivity and conductivity were analyzed using a temperature-wave analysis (ai-Phase Mobile 1u, ai-Phase Co., Ltd.) method based on ISO 22007-3:2008. Coefficient of linear thermal expansion (CLTE) was determined using a thermodilatometry (Bruker TD5000S) analysis (by heating rod-shaped specimens to 125 °C with a rate of 5 °C/min). Frequency-dependent dielectric constants (DCs) were determined by using an impedance analyzer (Keysight Agilent HP E4990A).

diverse and advanced applications, such as heat-dissipation packagings, adjustable dielectric devices, tunable capacitors, force sensors, catalyst carriers, and environmental cleaners.

EXPERIMENTAL SECTION Synthesis of 3D Tubular Cellular-Network BN Foams (3DBNFs). Preparation of 3D Silica Aerogel (3D-SiO2) Templates. As referred in refs 20, 43, and 44, a homogeneous sol of deionized water, 37% hydrochloric acid, P123, and TEOS with a mass ratio of 1:0.17:0.03:0.07 was prepared at 38 °C. This transparent sol was sealed into a polytetrafluoroethylene-lining autoclave and heated at 125 °C for 48 h. Wet 3D-SiO2 bulk aerogels were obtained, and these were dried naturally under chemical hood for 2 weeks. Removal of P123 surfactant was performed at 700 °C for 6 h in air. Preparation of 3D N-Doped Tubular Graphite (NG) Foams (3DNGFs). As illustrated in SI Figure S2, a 3D-SiO2 aerogel was used as a template to in situ growth of NG via a DMF CVD process. Such CVD was carried out at 1100 °C for 1 h. SiO2 inside the deposited NG coating layers was etched by a HF solution over 1 week. Final 3DNGF was obtained after several time washing and drying treatments. Fabrication of 3D-BNFs. As illustrated in SI Figure S6, the obtained 3D-NGF was directly used as an in situ template for 3D-BNF conversion through a carbothermal reduction CVD substitution reaction in the presence of B2O3 vapor and N2. This reaction was performed at ∼1700 °C for 4 h under a constant N2 flow of 50 mL/ min as protective and reactive atmosphere. Preparation of 3D-BNF-PMMA Composites. 3D-BNFs were soaked into an acetone-dilute PMMA solution (mass ratio of PMMA/ acetone was 1/100). Continuous adding of the PMMA-acetone solution during the natural evaporation of acetone was necessary. A precipitated solidification of PMMA (fully filled into the 3D-BNFs) was conducted along with the acetone evaporation. 3D-BNF-PMMA composites were fabricated once the acetone was completely evaporated (the whole procedure took about 2 months). Experiments for Adsorption-Separation Purifications. Weight-known 3D-BNFs were placed into an oil solvent (floating on water surface or underwater). After finishing adsorption, 3D-BNFs were taken out and weighed again. Adsorption capacities (Ca) were calculated using an equation: Ca = (Mt − M0)/M0 (where Mt and M0 are the weights of 3D-BNFs with and without adsorbed oils, respectively). Oil-saturated 3D-BNFs were rapidly burned under a flame of liquidized gas spray nozzle for recovery. Continuous adsorption-separation collection of underwater chloroform was performed by utilizing a drafting pressure of ∼0.1 MPa on the top exposed surface of cuboid 3D-BNF which soaked into chloroform through water (see SI Movie 5). Chloroform was continuously collected using forced pumping system from the top surface of 3DBNF when chloroform infiltration went up to the top surface. Fivecycle pumping cycles can be easily accomplished for 50 mL underwater chloroform without changing in 3D-BNF shape and weakening its efficiency. Characterizations. A XRD diffractometer (Rigaku Ultima III Cu Kα radiation), a Raman spectrometer (Horiba-Jovin Yvon T64000, 514.5 nm excitation laser), a scanning electron microscope (SEM, Hitachi S-4800), a high-resolution transmission electron microscope (HRTEM) equipped with the EELS system (JEM-3000F with EDS operated at 300 kV, and JEOL JEM-2100F, operated at 200 kV), a DTA-TG instrument (Rigaku Thermo Plus TG 8120), a XPS apparatus (PHI Quantera SXM), N2 adsorption-desorption isotherms and BET apparatus (Quantachrome Autosorb-IQ2 System), and a water contact angle measurement system (Drop-Master DM-701, KYOWA, Keyence VH-5000 optical instrument) were used to thoroughly characterize 3D-BNFs. UV−vis transmission was analyzed on a Hitachi U-4100 spectrometer. Mechanical loading-unloading tests were carried out using a Shimadzu EZ-S-100N machine (displacement rate of 0.5 mm/min, a sample was kept at the desired compressed strain conditions for 1 min to achieve a balance state before unloading). Heat capacities (Cp) of PMMA and its composites were measured by a differential scanning calorimetry analysis (DSC, Rigaku

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b06601. SEM image of SiO2 aerogel; DMF CVD process; photos of real products; characteristics of 3D-NGFs; elemental mapping of 3D-NGFs; carbothermal reduction CVD reaction for BN conversion; contrasting SEM images of 3D-NGFs and 3D-BNFs; SEM and TEM images of 3DBNFs by using different precursors with and without Ndoping; C, N, and O contents in 3D-NGFs under XPS analysis; diameter distribution of nanotube cells from SEM images; different types of junctions; XPS spectra of 3D-BNFs; TEM images of tubular cells in 3D-BNFs; BET and pore analyses of 3D-BNFs; evolution of tangent modulus of 3D-BNFs during loading-unloading processes; in situ SEM images of 3D-BNFs under compressive strain up to 90%; TEM images of tubular cells before and after compression; slop of Hook-law deformation region of a loading-carrying step; summary of ΔU/U, E, and σM in sequence of increasing compressive strain amplitudes from 5% to 65%; frequency-dependent dielectric constant and loss of individual 3D-BNFs-PMMA composite specimen; characterizations of blank PMMA and its 3D-BNF composites with respect to thermal conductivity; summary of thermal conductivity improvement of polymers in literature as compared to our work; oil adsorption capacity and superelasticity of 3D-BNFs as compared with BN-based foams and excellent carbonbased foams; TG-DTA of 3D-BNFs vs commercial bulk BN; FTIR, EELS, SEM, TEM, and water contact angle characterizations after recovery; fast oil adsorption process under water; an estimate for intrusion pressure to obstruct water; continuous adsorption-separation cycles; mechanism of the pumping-assisted continuous adsorption-separation process; MD simulations of 3DBNFs reversible compression (PDF) Movie 1: Shape recovery after compression (AVI) Movie 2: Ultra-fast oil adsorption on water surface (AVI) Movie 3: Fast recovery by flame burning in air (AVI) Movie 4: Ultra-fast oil adsorption under water (AVI) Movie 5: Continuous collection oil under water by pumping-assisted adsorption-removal process (AVI)

AUTHOR INFORMATION Corresponding Authors

*E-mail: *E-mail: *E-mail: *E-mail:

[email protected]. [email protected]. [email protected]. [email protected].

ORCID

Yanming Xue: 0000-0003-1061-229X 566

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Amir Pakdel: 0000-0001-5852-0808 Notes

The authors declare no competing financial interest.

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