Highly Compressive Boron Nitride Nanotube Aerogels Reinforced with

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Mingmei Wang,†,‡,○ Tao Zhang,†,○ Dasha Mao,†,§ Yimin Yao,†,§ Xiangliang Zeng,†,‡ Linlin Ren,† Qiran Cai,∥ Srikanth Mateti,∥ Lu Hua Li,∥ Xiaoliang Zeng,*,† Guoping Du,*,‡ Rong Sun,*,† Ying Chen,*,∥ Jian-Bin Xu,⊥ and Ching-Ping Wong# †

Shenzhen Institute of Advanced Electronic Materials - Shenzhen Fundamental Research Institutions, Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, China ‡ School of Materials Science and Engineering, Nanchang University, Nanchang 330031, China § Shenzhen College of Advanced Technology, University of Chinese Academy of Sciences, Shenzhen 518055, China ∥ Institute for Frontier Materials, Deakin University, Waurn Ponds, Victoria 3216, Australia ⊥ Department of Electronics Engineering, The Chinese University of Hong Kong, Hong Kong 999077, China # School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States S Supporting Information *

ABSTRACT: Boron nitride nanotubes (BNNTs), structural analogues of carbon nanotubes, have attracted significant attention due to their superb thermal conductivity, wide bandgap, excellent hydrogen storage capacity, and thermal and chemical stability. Despite considerable progress in the preparation and surface functionalization of BNNTs, it remains a challenge to assemble one-dimensional BNNTs into three-dimensional (3D) architectures (such as aerogels) for practical applications. Here, we report a highly compressive BNNT aerogel reinforced with reduced graphene oxide (rGO) fabricated using a freeze-drying method. The reinforcement effect of rGO and 3D honeycomb-like framework offer the BNNTs/rGO aerogel with a high compression resilience. The BNNTs/rGO aerogels were then infiltrated with polyethylene glycol to prepare a kind of phase change materials. The prepared phase change material composites show zero leakage even at 100 °C and enhanced thermal conductivity, due to the 3D porous structure of the BNNTs/rGO aerogel. This work provides a simple method for the preparation of 3D BNNTs/rGO aerogels for many potential applications, such as high-performance polymer composites. KEYWORDS: boron nitride nanotubes, reduced graphene oxide, aerogel, mechanical elasticity, polymer composite

A

To give full play to nanomaterials’ advantageous performances, one promising route is to assemble them into macroscopic articles. For example, Xue et al. found that the construction of cellular BN architectures could enhance its mechanical tolerance and showed great potential in highperformance polymer composites and oil/water separation.10 However, it is still difficult to assemble the three-dimensional (3D) BNNTs, due to their strong hydrophobicity and the lack of surface functional groups. For the frequent functionalization

s structural analogues of carbon nanotubes, onedimensional (1D) boron nitride nanotubes (BNNTs) have many excellent properties such as high thermal conductivity (∼400 W/mk) but electrical insulation,1,2 prominent thermal stability (up to 900 °C in air),3,4 low dielectric constant,5 significant hydrogen storage capacity (0.85 wt %), and radiation absorption capacity.6 In addition, BNNTs possess good mechanical performances with an extremely high Young’s modulus (1.3TPa) and tensile strength (∼40 GPa).7 These superior properties enable BNNTs to be used in a wide variety of applications as structural, thermal, electronic, and optical materials. Because of the recent advances in highthroughput preparation and surface functionalization, the research on BNNTs is moving toward practical applications.8,9 © XXXX American Chemical Society

Received: April 26, 2019 Accepted: June 14, 2019 Published: June 15, 2019 A

DOI: 10.1021/acsnano.9b03225 ACS Nano XXXX, XXX, XXX−XXX

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Cite This: ACS Nano XXXX, XXX, XXX−XXX

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ACS Nano methods, such as treated with strong acid, hydrogen peroxide, and polyhedral oligosilsesquioxane,5,11,12 there exists some drawbacks limiting their practical applications, including low production, high energy consumption, damage to crystalline structure, and the use of poisonous solvents.13 To address these issues, Song et al.14 used a carbon aerogel as a template to prepare a BNNTs/BNNSs hybrid aerogel via chemical vapor deposition method. However, special templates and harsh experimental conditions (high temperature and pressure) were required. Thus, 3D assembly of BNNTs into aerogels via a facile and mild process will be more economical and operable for industrial applications. Unfortunately, it remains a challenge to assemble BNNTs into 3D aerogels, due to the lack of surface functional groups. In this work, taking full advantages of the ability of graphene oxide (GO) nanosheets to stably disperse nanomaterials in water and their self-assembly in the followed reduction process,15−17 we propose an effective strategy to prepare 3D BNNTs/rGO aerogels. The two-dimensional (2D) GO nanosheets act as both the reinforced material and the surfactant for stabilizing and assembling the 1D BNNTs based on affinity interactions between GO and BNNTs. The resultant BNNTs/rGO aerogel with 50 wt % BNNT content was proved to have good elasticity and fatigue resistance, which recover to its original volume even after 100 cycles of 50% compression and retain above 85% of the maximum stress. After the infiltration of polyethylene glycol (PEG), the obtained BNNTs/rGO/PEG phase-change composites showed good shape stability without obvious leakage even under 100 °C treatment, increased fusion enthalpy of 195.6 J/ g, and an enhanced thermal conductivity by 32.2% compared with that of PEG. We attribute the satisfactory properties to the 3D porous structure of the BNNTs/rGO aerogel. This work provides a facile method for the preparation of 3D BNNTs-based aerogels for many potential applications, such as high-performance polymer composites.

Figure 1. Interactions between BNNTs and GO in aqueous media. (a) Digital photographs of BNNTs and BNNTs/GO dispersions and (b) bottom views of BNNTs/GO suspension (the number represents the mass ratio of BNNTs in BNNTs/rGO). (c)TEM images of single BNNT (c1: TEM image, c2: HRTEM image, c3: SAED pattern). (d) AFM images of the BNNTs/GO hybrids and the red circles schematically indicate BNNTs. (e) UV−vis spectra of BNNTs/GO (with 0, 30, and 50 wt % BNNT fraction) aqueous dispersions. (f) Raman spectra of GO and BNNTs/GO aqueous dispersions.

size of the BNNTs remain intact after mild sonication and stirring. To investigate the interactions existing between BNNTs and GO nanosheets, we characterized the UV−vis absorbance spectra of the BNNT/rGO aqueous dispersions (Figure 1e). The main peak of GO (BNNTs = 0 wt %) spectrum is at 233 nm, representing the transition of π−π*.18 With the increasing mass ratio of BNNTs, the absorption peak of the BNNTs/rGO dispersion changes slightly from 233 to 228 nm. It is attributed to the electron transfer between the BNNTs and GO nanosheets originated from strong π−π stacking interactions, which is similar to previous reports.19 Besides, hydrophobic interaction and van der Waals force are two other vital binding forces between GO and BNNTs similar to previously reported systems of 2D GO and 1D nanomaterials.20 Raman spectra were recorded for GO, BNNTs, and BNNTs/rGO, respectively (Figure 1f). Typically, GO has two characteristic peaks representing D and G bands at 1370 and 1570 cm−1, respectively.21 For BNNTs, a characteristic peak at 1363 cm−1 is observed in the spectra, which originates from the E2g phonon vibration pattern in its crystal lattice, similar to the G peak in the graphite material.22 Strong D (∼1358 cm−1) and G (∼1590 cm−1) bands are evident for BNNTs/GO. The characteristic peak for BNNTs cannot be detected due to overlap with the strong D band for GO nanosheets.17 The interactions between BNNTs and GO are believed to play a positive part in the stable dispersion of BNNTs. In addition,

RESULTS AND DISCUSSION Dispersion of BNNTs Assisted by GO in Aqueous Solution. Figure 1a shows the digital optical photographs of the BNNTs and BNNTs/GO aqueous suspension. Almost all of the raw BNNTs in water have been deposited at the bottom of the vial in water, indicating that the BNNTs fail to stably disperse in water due to the lack of polar groups on their surfaces. Interestingly, the BNNTs/GO mixture maintains stable dispersion after 24 h. The maximum dispersion ratio of BNNTs reaches 50 wt % (Figure 1b). When the BNNT content is up to 60 wt %, some BNNTs are deposited at the bottom of the vial, indicating the important role of GO for good dispersion of BNNTs. TEM image shows that the bamboo-like BNNTs possess representative lengths in the range of 1−5 μm and a diameter of about 75 nm (Figure 1c1). The diffraction rings indexed to the (002), (100), (110), and (004) planes of h-BN, conforms to the relevant HRTEM image, indicating that the hexagonal structure of BNNTs has been well maintained through the dispersion process (Figure 1c2,c3) (PDF no. 45-0896). The GO nanosheets have a micrometer size with a thickness of around 1 nm (Figure S1, Supporting Information). For the BNNTs/rGO dispersion, the AFM image in Figure 1d exhibits that long and thick BNNTs are distributed separately and closely attached to the surface of GO nanosheets. It suggests that the GO has an efficient dispersion capacity for BNNTs and that the basic structure and B

DOI: 10.1021/acsnano.9b03225 ACS Nano XXXX, XXX, XXX−XXX

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ACS Nano previous studies have demonstrated that GO suspensions with high viscosity can be applied to hinder the sedimentation of the second components.23 The GO solution used in this work had a concentration of 4.0 mg/mL, and its relatively high viscosity may be also beneficial to the stable dispersion of BNNTs. Preparation and Properties of the BNNTs/rGO Aerogel. The BNNTs/rGO aerogels were prepared by first mixing BNNTs with GO dispersion, followed by reductioninduced gelation and freeze-drying, as shown in Figure 2a. The

Figure 3. Macroscopic and microscopic morphology of BNNTs/ rGO aerogels. (a) Digital image of an ultralight BNNTs/rGO aerogel with a density of 16.3 mg/cm3. (b) Typical honeycomblike cellular structures of BNNT/rGO aerogels. SEM images of microscopic morphology of hybrid aerogels with different BNNT contents: (c) 10 wt %, (d) 30 wt %, and (e, f) 50 wt %.

surface morphology of the hybrid aerogels with different BNNT contents was observed at a higher magnification (Figure 3c−e). Compared with the raw material BNNTs (Figure 1c,d), the BNNTs with mild sonication stirring and hydrothermal reduction exhibit no significant damage in length and structure. When the content of BNNTs is 10 wt %, the individual BNNTs are sparsely distributed on the surface of the rGO nanosheets. With the increased content (30 wt %), the BNNTs begin to loosely lap together with each other. After further increasing to 50 wt %, a large amount of BNNTs are interdigitated to form a network-like structure. As shown in Figure 3f, BNNTs are either distributed on the surface of GO sheets or wrapped in a rGO layer to form a 1D/2D hybrid structural unit on a microscopic scale and then connect with each other to obtain a robust macroscopic porous aerogel. The morphology of BNNTs/rGO hybrid aerogel was further confirmed by TEM. There are some long and dispersed BNNTs fixed on the wrinkled rGO nanosheets. The length of BNNT is in the range of 1−5 μm, with the diameter of about 75 nm (Figure S3, Supporting Information), which is consistent with the SEM images (Figure 3). In addition, TGA curves provide further evidence of the enhanced thermal stability of the BNNTs/rGO aerogel with a lower weight loss ratio, compared to the pure rGO aerogel (Figure S4, Supporting Information). To further investigate the reduction process and interactions between BNNTs and rGO nanosheets in the BNNTs/rGO aerogel, we determined the elemental composition and content of BNNTs/rGO by X-ray photoelectron spectroscopy (XPS) spectroscopy. Figure 4a shows the survey spectrum of three kinds of materials with the present elements of B, C, N, and O. The C/O ratio representing the degree of reduction shows a significant increase from GO to BNNTs/rGO (as in the case of GO to rGO, Table S1, Supporting Information), indicating an increased content of sp2-hydridized C situated at 284.7 eV and simultaneously a decreased fraction of oxygen-containing functional groups. The chemical environment of the individual element in the BNNTs-50/rGO aerogel was further analyzed, as presented in Figure 4b−f. The BNNTs-50/rGO aerogel is composed of boron (10.24%), nitrogen (16.36%), carbon (38.01%), and oxygen (13.15%), as presented in Figure 4b.

Figure 2. Fabrication procedure and reaction characteristics of BNNTs/rGO aerogels. (a) Schematic illustration of the preparation process of BNNTs/rGO aerogel. (b) Gelation time varies with the content of BNNTs in GO solution (the red circles show the inclination degree of samples after 2 h of reduction).

gelation time for BNNTs/rGO dispersion with 10, 30, and 50 wt % BNNT contents was about 2, 3, and 4 h, respectively (Figure 2b). This suggests that the BNNTs attached to GO nanosheets can modulate the rate of cross-linking between GO nanosheets during the assembling process. On the one hand, the coating of BNNTs with GO increases the average distance between the GO nanosheets, which hinders the self-assembly process. On the other hand, there exists physical or chemical interactions between the BNNTs and the GO nanosheets, which sacrifice a part of the active sites on their surface, leading to increased surface inertia of the GO nanosheets. The asprepared BNNTs/rGO aerogels with 10, 30, and 50 wt % BNNT contents are named as BNNTs-10/rGO, BNNTs-30/ rGO, and BNNTs-50/rGO, respectively. Figure 3a shows that the BNNTs-50/rGO aerogel stands freely on the petal of a flower, due to its high porosity and low density. The typical morphology and structure of the BNNTs/ rGO aerogels were characterized by SEM. As shown in Figure 3b, the rGO nanosheets are interconnected to form a honeycomb-like pore structure with a pore size of about 200 μm, which is consistent with the typical structure and size of pure rGO aerogel prepared (Figure S2, Supporting Information). It suggests that the addition of BNNTs has no significant effect on the final pore structure of the BNNTs/rGO aerogels, even though it will prolong the gelation time. The specific C

DOI: 10.1021/acsnano.9b03225 ACS Nano XXXX, XXX, XXX−XXX

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Figure 4. XPS survey spectra of the BNNTs/rGO aerogel. (a) XPS spectra of the BNNTs/rGO compared with raw GO and rGO. (b) Individual atomic distribution in BNNTs/rGO. (c−f) Narrow scan results of B 1s, N 1s, C 1s, and O 1s in BNNTs/rGO.

Figure 5. Mechanical properties of BNNTs/rGO aerogels. (a) Snapshots of BNNTs-50/rGO aerogel under compression and recovering process, (b) The σ−ε curves of BNNTs-50/rGO aerogel at different maximum strains. (c) The σ−ε curves of BNNTs-50/rGO aerogel at the maximum strain of 50% for 10 cycles. (d) The corresponding energy loss coefficient, Young’s modulus, and maximum stress for 10 different compression cycles. (e) The variation of height as a function of cycle number at the loading rate of 50% strain/min. (f) The compressive σ−ε curves of BNNTs/rGO aerogels with different BNNTs ratios at the maximum strain of 80%.

aerogels.26 Polar covalent bonding between B and C is possibly contributed to their small difference in electron negativity.25,27 N 1s spectrum is given by Figure 4d. It suggests that the main bonding is B−N bonds corresponding to a peak at 398.3 eV. Two peaks at binding energies of 399.8 and 401.5 eV indicate the presence of an amino groups on the GO nanosheets.28,29 Figure 4e shows main peaks of C 1s at binding energies of 284.7 eV (major), 286, and 287.7 eV corresponding to CC,

The main peak of the B 1s spectrum at 190.8 eV is assigned to the B 1s in h-BN (Figure 4c).24 This suggests that there mainly exists B−N bonds, as in h-BN where a B atom is surrounded by three N atoms. There was another peak at 191.9 eV corresponding to B−O bonds (sp3-hybridized N3B(OH)).25 A small shoulder peak located at about 189.8 eV was also observed, contributing to the broadening of the B 1s spectrum and indicating a possible B−C bonds in BNNTs/rGO hybrid D

DOI: 10.1021/acsnano.9b03225 ACS Nano XXXX, XXX, XXX−XXX

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ACS Nano C−OH, C−N groups, respectively.28 There also exists a shoulder peak at a lower binding energy indicating the presence of C−B bonds (in agreement with Figure 4c).29 The O 1s of BNNTs/rGO can be divided in to two different peaks (Figure 4f). A main peak located at 531.7 eV corresponds to the C−O bond, while another peak at 533.4 eV can be attributed to B−O bond.29 XPS spectra have proved successful reduction reactions occur during the formation of the BNNTs/rGO aerogels as well as possible chemical bonds that exist between BNNTs and GO nanosheets. The mechanical properties of the BNNTs/rGO aerogels were evaluated by analyzing their stress−strain (σ−ε) curves. Even when the content of BNNTs is 50 wt %, the as-prepared BNNTs-50/rGO aerogel can still completely recover to its original macro-configuration after the applied pressure is released (Figure 5a, Video S1). This property is attributed to the complete recovery of a honeycomb-like cell structure after large deformation.30 The compressive σ−ε curves of BNNTs50/rGO aerogel with strains up to 20%, 40%, 60%, and 75% are shown in Figure 5b. During the loading process, the cells gradually shrink, followed by conformal densification with three typical deformation states being observed.31 The states reflected from the σ−ε curves include Hookean region (ε < 8%) where the stress has a linear increasement with the strain, the plateau region (8%< ε < 43%) from which most of the absorbed energy dissipated, and the densification region (ε > 43%) where the stress increases rapidly due to the continuously decreasing pore volume. The corresponding Young’s modulus decreased from 8.13 to 3.65 kPa. During the unloading process, the strong cell walls timely stretch with the retreating stress (Figure 5b), and the aerogel fully springs back to its original volume when the stress is completely removed. It is found that the aerogels follow the cross-head closely during the unloading process even at a higher retreat speed (80%/min), similar to the property in previous researches.28,32 The mechanical stability of the BNNTs/rGO aerogel was evaluated by the cyclic compression tests at the maximum strain of 50% for 10 cycles, as shown in Figure 5c. During the first cycle, the curve shows a higher energy loss coefficient, Young’s modulus, and maximum stress than subsequent ones, corresponding to 0.73, 8.05, and 3.23 kPa, respectively. The hysteresis loop for the second cycle presents a shrinkage compared to the first one. However, the degradation of maximum stress located at strain 50% is only 6.3%, which is lower than that of the rGO aerogel reported previously.28 In the third compression cycle, the σ−ε curve patterns almost remain constant, energy loss coefficient, Young’s modulus, and maximum stress are, respectively, stable at 0.65, 4.6, and 2.8 kPa (Figure 5d). Figure 5e shows the cyclic compression testing of the BNNTs-50/rGO aerogel with strains up to 75% for 10 times, and the height of aerogel remains nearly the same as its original value, indicating the repeatable and reversible compressive deformability. Even after 100 compression cycles of strain at 50%, the aerogel can still retain above 85% of the maximum stress (Figure S5, Supporting Information) and recovers to its original volume. The effect of different BNNT contents in the BNNTs/rGO aerogels on the mechanical properties was also investigated. As presented in Figure 5f, there exists obvious differences between maximum stresses with the changed proportion of BNNTs in the BNNTs/rGO aerogels, which increases from 6.3 to 12.24 kPa. This may be attributed to the incorporation of BNNTs resulting in a cross-network structure on the surface of GO

nanosheets, along with increased thickness of the cell walls, which both enhance the resistance to cell wall bending and cell collapse, leading to a higher maximum stress. Thermal performances of BNNTs/rGO/PEG Composites. Infiltrating phase change materials within 3D porous aerogels has been demonstrated to be capable of simultaneously realizing direct thermal energy storage, high thermal conductivity, and leakage-proof capability.33−35 The BNNTs/ rGO aerogels with different BNNT contents (10, 30, and 50 wt %) were compounded with PEG by vacuum-assisted infiltration and named as BNNTs-10/rGO/PEG, BNNTs30/rGO/PEG, and BNNTs-50/rGO/PEG, respectively (Figure 6a). The microscopic morphology of the BNNTs/rGO/

Figure 6. Thermal properties of BNNTs/rGO/PEG composites. (a) Schematic diagram of the preparation of BNNTs/rGO/PEG composites. (b) DSC heating scan and cooling scan curves for pure PEG, rGO/PEG, BNNTs-10/rGO/PEG, BNNTs-30/rGO/PEG, and BNNTs-50/rGO/PEG. (c)The shape stability levels of the pure PEG, rGO/PEG, and the BNNTs/rGO/PEG composites at 100 °C for 10 min. (d) Thermal conductivities of pure PEG and BNNTs/rGO/PEG composites with different BNNT contents.

PEG composites was characterized by SEM. Due to the crystalline nature of pure PEG, a layered structure is presented in the composites, and the observed BNNT has almost no change in length and diameter (Figure S6, Supporting Information). The differential scanning calorimetry (DSC) measurements show that the BNNTs/rGO/PEG composites have almost the same fusion temperature (∼59 °C) and solidification temperature (∼38 °C) as pure PEG, which indicates that the introduction of porous BNNTs/rGO aerogel has no effect on the normal solid−liquid phase change of PEG. In addition, the calculated phase transition enthalpy of BNNTs-50/rGO/PEG composites can reach 195.6 J/g, higher than that of pure PEG of 188.5 J/g and rGO/PEG of 182.3 J/g (Table S2, Supporting Information). The small increase of phase change enthalpy should be attributed to the addition of BNNTs. The interactions between PEG molecules and the BNNTs walls impede the melting or crystallization process, thereby increasing the enthalpy, as demonstrated in a recent report in which the intermolecular attraction based on the Lennard-Jones potential is the main reason.33,36 In order to evaluate the shape stability of the BNNTs/rGO/PEG composite, the samples were placed on a piece of paper and heated up to 100 °C at a rate of 10 °C/min and held at this temperature for 10 min to ensure complete melting of the PEG, as shown in Figure 6b. After the heating procedure, the E

DOI: 10.1021/acsnano.9b03225 ACS Nano XXXX, XXX, XXX−XXX

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ACS Nano pure PEG is completely melted into a flowing liquid and spread out on the paper. The rGO/PEG can maintain its basic shape but exhibits a small amount of liquid leakage around its bottom. In contrast, the BNNTs/rGO/PEG composite remains its original shape, and almost no leakage of the loaded PEG is observed. The underlying rationale for good shape stability of BNNTs/rGO/PEG composites can be twofold: (1) The porous structure and ultrahigh specific surface area of the hybrid aerogel are advantageous for adsorbing the flow state PEG; and (2) the coating of BNNTs makes the surface of 3D framework hydrophobic and rough, producing a strong capillary force and preventing the melted PEG from leaking. Thermal conductivity is also relevant to the application in phase change materials, because it directly affects the absorption and conversion rates of thermal energy.37 The thermal conductivity of pure PEG was measured to be only 0.29 W/mK. With the addition of BNNTs/rGO aerogels, the thermal conductivity of composites is improved from 0.29 W/ mK to 0.43 W/mK (Figure 6d). The enhancement efficiency of thermal conductivity for 3D BNNTs/rGO aerogels in PEG is characterized by enhancement per 1 wt % loading (η), which is defined as

η=

K − Km 100Wf K m

Wf =

ma mc

facile handling, eco-friendly, and nondamaging to the structure of BNNTs, which can further be used in high-performance composites.

EXPERIMENTAL SECTION Materials. The BNNTs were prepared from boron powder via a ball milling and annealing method according to a previous report.38 GO dispersion was provided by Shanxi Institute of Coal Chemistry (Shanxi Province, China), which was fabricated from natural graphite.39 Ethylenediamine (EDA, purity >99.5%) was supplied by Aladdin Reagent Co., Ltd. Ultrapure water (18.25 MΩ) was fabricated in the laboratory using ULUPURE (UPHeI-20T). Absolute ethanol (EtOH, purity >99.7%) was obtained from Sinopharm Chemical Reagent Co., Ltd. (China). Polyethylene glycol (PEG, Mn = 4000) was purchased from Aladdin Reagent Co., Ltd. Preparation of the BNNTs/rGO Aerogels. The BNNTs/rGO aerogels were prepared via sol−gel self-assembly and freeze-drying processes. In a typical process, a certain amount of BNNTs (12.0 mg) was added to the GO solution (4.0 mg/mL, 3.0 mL) in a circular glass vial, followed by an ultrasonic dispersion at a relatively low power (150 W, 1 h) and a magnetic stirrer (1000 rpm, 1 h). After the addition of EDA as a chemical reducing agent (20.0 μL, GO/EDA = 150:1, v/v), the mixture was extensively stirred to obtain uniform dispersion (600 rpm, 5 min). Subsequently, the dispersion was heated at 95 °C for 12 h to reduce GO and induce self-assembly, and then the obtained hydrogel was dialyzed in an aqueous solution of ethanol (EtOH:H2O = 1:10, v/v) for 24 h. Lately, the BNNTs/rGO aerogel was fabricated via freeze-drying the frozen hydrogel for over 48 h (−50 °C, ∼30 pa). The hybrid aerogels with different BNNT contents at 10, 30, and 50 wt % were prepared. In addition, pure rGO aerogel was prepared for comparison. Preparation of BNNTs/rGO/PEG Composites. The composites were fabricated by infiltrating PEG into 3D BNNTs/rGO aerogels with the assistance of vacuum. The raw PEG powder was first heated at 90 °C for 15 min to obtain the fully melted PEG with a good fluidity. The as-prepared BNNTs/rGO aerogels were placed in an oven at 90 °C, and the molten PEG was introduced into the aerogels in a slow dropwise manner and incubated for 15 min. Then the samples were placed in a vacuum oven at 85 °C for 24 h to discharge bubbles inside and subsequently cooled at room temperature to obtain the BNNTs/rGO/PEG composites. Characterization. Transmission electron microscopy (TEM) micrographs were obtained using a FEI Tecnai G2 F20 S-TWIN transmission electron microscope (FEI Corporation, USA). Atomic force microscope (AFM) images were taken with a Dimension Icon instrument (Bruker, America) in ScanAsyst mode. Ultraviolet−visible (UV−vis) spectra were acquired with a spectrophotometer (Shimadzu, Japan). Raman spectroscopy analysis was obtained by using a Labram HR800 (Germany). The morphological features of the aerogels were characterized using a scanning electron microscope (SEM, FEI Nova NanoSEM 450, America). The density of BNNTs/ rGO aerogel is calculated by measuring its mass and volume. Thermogravimetric analysis (TGA) was conducted by using a TA Instrument (Q600) under nitrogen atmosphere. The chemical compositions of GO, BNNTs, rGO, and BNNTs/rGO samples were analyzed using X-ray photoelectron spectroscopy (XPS, RBD upgraded PHI-5000CESCA, PerkinElmer, USA). Compression stress−strain (σ−ε) curves of the BNNTs/rGO aerogels with different BNNT contents were measured by a dynamic mechanical analyzer (Q800, TA Instruments) at a ramp rate of 50%/min. The Young’s modulus (E) is numerically equal to the slope of the fitted curve with the first 5% of the σ−ε data. The energy loss coefficient (η) stands for the degree to which the aerogels dissipate mechanical energy and is calculated as:

(1)

(2)

where K and Km are the thermal conductivities of the composites and PEG matrix, respectively, and Wf is the loading of BNNTs/rGO in composites. The quality of aerogels and composites are represented by ma and mc, respectively. It is found that η increases with the increase of filler loading, which can be ascribed to the formation of a more effective thermal conduction network at a higher filler content (Figure S7, Supporting Information). The η of the BNNTs-50/rGO/PEG composite reaches 32.2% at an extremely low filler content (1.5 wt %). This indicates that the introduction of the 3D BNNTs/ rGO aerogel is effective in enhancing thermal conductivity of the PEG. The thermal conductivity skeleton consisting of BNNTs/rGO ensures that heat is transferred from the heat transfer path more effectively, which benefits energy storage and release of BNNTs/rGO/PEG composites.

CONCLUSION In summary, BNNTs can form a stable dispersion in aqueous medium with the assistance of GO via facile sonication and stirring treatment. The BNNTs/rGO aerogels with robust 3D honeycomb-like framework were prepared by freeze-drying methods. AFM, UV−vis, and XPS analyses demonstrated interactions of π−π stacking and chemical bonding between BNNTs and GO nanosheets. The as-prepared BNNTs/rGO aerogels have good elasticity and fatigue resistance, which recover to their original volume even after 100 cycles of 50% compression. After the infiltration of PEG, the obtained BNNTs/rGO/PEG phase-change composites showed good shape stability even under 100 °C treatment, increased fusion enthalpy of 195.6 J/g, and an enhanced thermal conductivity by 32.2% compared with that of PEG. We attribute the satisfactory properties to the 3D porous structure of the BNNTs/rGO aerogel. Compared with the other reported methods to prepare 3D BNNTs-based aerogels, this method is

η= F

ΔUi Ui

(3) DOI: 10.1021/acsnano.9b03225 ACS Nano XXXX, XXX, XXX−XXX

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ACS Nano ΔUi = Ui =

∮ σdε

∮0

σmax

σdε

51362020), National and Local Joint Engineering Laboratory of Advanced Electronic Packaging Materials (Shenzhen Development and Reform Committee 2017-934), Shenzhen International Cooperation Project (no. GJHZ20180420180909654), Shenzhen Key Laboratory (ZDSYS20140509174237196), SIAT CAS-CUHK Joint Laboratory of Materials and Devices for High Density Electronic Packaging, and Shenzhen Key Laboratory (ZDSYS20140509174237196).

(4) (5)

where ΔUi is the energy dissipated in the ith load-unload cycle, and Ui is the elastic energy stored in the tested sample, when it is compressed elastically to a maximum stress (σmax) in the ith cycle. The thermal properties of the BNNTs/rGO/PEG composites were analyzed with differential scanning calorimetry (DSC) using a TA Instrument (Q2000 analyzer) under nitrogen atmosphere. The stability of the composites was evaluated by heating the samples on a piece of A4 paper, with a heating plate (IKA, RT5 3690625, Germany). Thermal conductivities of pure PEG and BNNTs/rGO/PEG composites were determined by using a thermal constant analyzer with an isotropic standard module (Hot Disk, TPS 2500S, Sweden), and the type of Hot Disk sensor sandwiched between two halves of tested samples is 5465 (shown in Figure S8, Supporting Information). Optical pictures were taken by an iphone 7 camera.

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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/acsnano.9b03225. Video S1: The aerogel fully recovering after a large strain (AVI) AFM image and height profile of GO nanosheets; microscopic morphology of pure rGO aerogel; TEM images of the BNNTs/rGO hybrids; TGA results of BNNTs, rGO and BNNTs-30/rGO; stress−strain curves of BNNTs/rGO hybrid aerogels before and after cycled 100 times; microscopic morphology of BNNTs/rGO/PEG composites; enhancement efficiency of thermal conductivity for BNNTs/rGO/PEG composites; pictures of a Hot Disk device; elemental analysis of GO, rGO and BNNTs/rGO; phase change temperature and phase change enthalpy of pure PEG and rGO/PEG composites with different BNNT contents (PDF)

AUTHOR INFORMATION Corresponding Authors

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

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

ORCID

Lu Hua Li: 0000-0003-2435-5220 Xiaoliang Zeng: 0000-0002-1389-8484 Rong Sun: 0000-0001-9719-3563 Ying Chen: 0000-0002-7322-2224 Jian-Bin Xu: 0000-0003-0509-9508 Ching-Ping Wong: 0000-0003-3556-8053 Author Contributions ○

These authors contributed equally

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Key R & D Project from Minister of Science and Technology of China (2017YFB0406000), Leading Scientific Research Project of Chinese Academy of Sciences (QYZDY-SSW-JSC010), National Natural Science Foundation of China (21571095 and G

DOI: 10.1021/acsnano.9b03225 ACS Nano XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsnano.9b03225 ACS Nano XXXX, XXX, XXX−XXX