A multifunctional thermal management paper based on functionalized graphene oxide nanosheets decorated with nanodiamond

Journal Pre-proof A multifunctional thermal management paper based on functionalized graphene oxide nanosheets decorated with nanodiamond Bingfei Nan, Kun Wu, Zhencai Qu, Luqi Xiao, Changan Xu, Jun Shi, Mangeng Lu PII:

S0008-6223(20)30063-4

DOI:

https://doi.org/10.1016/j.carbon.2020.01.056

Reference:

CARBON 14991

To appear in:

Carbon

Received Date: 25 November 2019 Revised Date:

13 January 2020

Accepted Date: 18 January 2020

Please cite this article as: B. Nan, K. Wu, Z. Qu, L. Xiao, C. Xu, J. Shi, M. Lu, A multifunctional thermal management paper based on functionalized graphene oxide nanosheets decorated with nanodiamond, Carbon (2020), doi: https://doi.org/10.1016/j.carbon.2020.01.056. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Credit Author Statement

Title A multifunctional thermal management paper based on functionalized graphene oxide nanosheets decorated with nanodiamond

Authors Bingfei Nan, Kun Wu, Zhencai Qu, Luqi Xiao, Changan Xu, Jun Shi, Mangeng Lu

Statement Bingfei Nan: Conceptualization, Methodology, Investigation, Data curation, WritingOriginal Draft, Review & Editing, Kun Wu: Funding acquisition, Resources, Writing-Review, Supervision Zhencai Qu: Visualization, Software Luqi Xiao: Investigation Changan Xu: Validation Jun Shi: Methodology Mangeng Lu: Resources

Graphical Abstract

A multifunctional thermal management paper based on functionalized graphene oxide nanosheets decorated with nanodiamond Bingfei Nan

a, b, c

, Kun Wu

a, c,

*, Zhencai Qu

a, b, c

, Luqi Xiao

a, b, c

, Changan Xu

b, c

,

Jun Shi a, b, Mangeng Lu a, c a

Guangzhou Institute of Chemistry, Chinese Academy of Sciences, Guangzhou

510650, China b

CAS Engineering Laboratory for Special Fine Chemicals, Chinese Academy of

Sciences, Guangzhou 510650, China c

University of Chinese Academy of Sciences, Beijing 100049, China

ABSTRACT In this study, we developed a multifunctional thermal management nanocomposite paper (NPG) consisting of cationic poly (diallyldimethylammonium chloride) (PDDA)-functionalized graphene oxide (PG) and nanodiamond (ND). Due to the functionalized reduction of graphene oxide (GO) by PDDA as well as electrostatic interactions between the positively charged PG and negatively charged ND, a three-dimensional (3D) hybrid NPG paper constructed by two-dimensional (2D) PG layers and zero-dimensional (0D) ND particles was successfully prepared via a vacuum-assisted self-assembly strategy. The resulting NPG papers were characterized by various techniques including Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) pattern and X-ray photoelectron spectroscopy (XPS). In this analogous 3D "panel-bead" structure, adjacent PG nanosheets and ND particles are bridged through electrostatic interactions, which strengthens the interface connection and reduces phonon scatterings, resulting in an effective phonon transport pathway. Thus, a superior in-plane thermal conductivity of 16.653W m-1K-1 was obtained at the mass ratio of GO:ND =3:1 (NPG3), which is about 80 times higher than that of traditional pure polymers. In addition, the peak heat release rate (PHRR) of NPG3 paper was only 99.16 W g-1 at 439.2 oC while that of GO was 438.4 W g-1 at 210.9 oC, * Corresponding author, E-mail address: [email protected] (Kun Wu).

indicating that NPG paper possesses excellent flame retardancy. More interestingly, although the NPG3 paper has excellent insulation (electrical resistivity reaches up to 2.647×1011 Ω cm), the ultrafast flame alarm response was found within about 1s after the paper exposed to the flame. This design idea provides an alternative approach for preparing multifunctional thermal management nanocomposites in the future. Keywords: Nanodiamond; Poly(diallyldimethylammonium chloride); Graphene oxide; Thermal conductivity

1. Introduction Along with the rapid development of smart electronic technologies and portable devices, especially with the advent of the 5th-Generation (5G) era, heat dissipation has become an essential issue in integrated and miniaturized electronic industries [1, 2]. Poor thermal spread often adversely affects the lifetime and reliability of the electronic equipment [3]. To improve thermal transfer efficiency, developing high-quality and versatile thermal management materials (TMMs) is of critical importance [4]. Kargar’s group, for instance, prepared a dual-functional graphene composites with excellent thermal conductivity (8W m−1K−1) and electromagnetic interference shielding (45dB, f=8.2-2.4GHz), which can effectively dissipate heat and shield electromagnetic waves at the same time [5]. Jiang et al. [6] reported a self-healable thermal interface nanocomposite containing boron nitride nanosheets (BNNSs) and polyacrylic acid (PAA) hydrogel, and the thermal conductivity reaches up to 3.5W m-1 K-1 at appropriate fraction of BNNSs and water fraction. An alternating multilayered poly (vinyl alcohol)/transition metal carbide (PVA/MXene) film with excellent thermal conductivity, electromagnetic interference shielding and flame retardancy performance was developed by Jin et al. in their recent work [7]. Apparently, multifunctional TMMs can not only engage extensive research interests but bring tremendous technological and economic advantages [5]. Over the past decade, carbon materials, especially graphene oxide (GO), as a precursor of graphene, have been widely prepared from natural graphite by Hummers’ method and have drawn great concern [8]. Different from the poor aqueous dispersion

of graphene, GO is easy to disperse in water and can be made into macroscopic films due to the presence of many oxygen-containing functional groups (e.g., hydroxyl, carbonyl and epoxy groups) on its surface [9, 10]. However, previous literature has shown that the thermal conductivity of GO is not desirable because of its surface defects, edge disorder or surface functional groups. Thus, various reduction methods are used to reduce graphene oxide (RGO) and improve its thermal and electrical properties [11], and graphene-based films were prepared by reducing GO with chemical reducing agents or high-temperature treatment [12]. Unfortunately, these methods often bring environmental problems and high energy consumption. For example, Song et al. [13] fabricated RGO/nanocellulose films using in-situ reduction of GO with highly toxic hydrazine hydrate. Meng et al. [14] developed a highly thermal conductive graphene-based composite film consisting of cellulose nanocrystals and GO under extremely high temperature (1500 ℃). Zhang and collaborators found that cation poly(diallyldimethylammonium chloride) (PDDA) can be used as an environmentally friendly reductants and stabilizers of GO aqueous solution [15]. Inspired by this, PDDA was used to functionalize GO to impart positive charges on the surface of nanosheets, and a possible self-assembly system was formed at the same time [16]. To the best of our knowledge, this is the first time that PDDA-functionalization GO has been used for thermal management and flame retardancy research. Nanodiamond (ND), as another carbon allotrope, has also attracted researchers’ considerable attention in the fields of biomedicine, biosensors, energy storage and tribology applications, etc. [17] due to its remarkable biocompatibility, hardness, excellent thermal stability and chemical inertness, and so forth [18]. Despite its high thermal conductivity (~2000Wm-1K-1), ND is less of concern in the thermal management. This is mainly because 1D and 2D fillers such as nanowires, nanotubes and nanosheets, can form more continuous thermally conductive network in the polymer matrix due to their high aspect ratio [19]. More recently, a large number of studies have revealed that 3D interconnected thermally conductive networks are more effective for the transfer of phonons and can maximize the thermal conductivity of

TMMs [20]. The rGO wrapped copper spheres (Cu@rGO) hybrid structure was used to construct 3D epoxy composites. A high thermal conductivity of 7W m-1 K-1 was obtained when the Cu@rGO content was 80wt% due to the formation of 3D heat transfer architecture of Cu microspheres-graphene nanosheets [21]. Fang and coworkers found that 0D Al2O3 nanoparticles could be used as linking bridges to connect 2D BN sheets for building 3D thermal conductive networks, and these hybrid fillers exhibits obviously synergistic effect on the thermal conductivity of epoxy [22]. Based on this viewpoint, we put forward a green and facile method to fabricate a flexible

3D

nanocomposite

paper

by combining

the

advantages

of

the

PDDA-functionalization GO nanosheets and ND, which may be more competitive in achieving multifunctional TMMs. Herein, ND particles were introduced into graphene nanosheets to form a "panel-bead" structure to heighten the thermal management ability. First, positively charged reduced graphene oxide nanosheets were prepared by incorporation of cationic PDDA. Subsequently, the nanocomposite paper was fabricated via electrostatic interactions and vacuum-assisted filtration. It’s worth mentioning that the entire film preparation process is facile and green, and no toxic solvent was used. The properties of nanocomposites were characterized by a series of characterization techniques such as FTIR, XRD and XPS spectra. Additionally, the thermal conductivity, flame retardancy and ultrafast fire alarm response of nanocomposite papers were investigated. 2. Experimental Section 2.1. Materials Natural graphite flakes (99.95%, 325 mesh) and poly(diallyldimethylammonium chloride) (PDDA, 35%; Mw<100000) were purchased from Aladdin Co., Ltd. (Shanghai, China). Concentrated sulfuric acid (H2SO4, 98%), hydrogen peroxide (H2O2, 30%), sodium nitrate ((NaNO3) and potassium permanganate (KMnO4) were supplied by Guangzhou Chemical Reagent Factory, China. Nanodiamond (ND, purity>97%, average particle size of 5-10 nm) prepared by detonation method was

obtained from Nanjing XFNANO Material Tech. Co. Ltd., China. Deionized water (DI water) were used throughout the experiments. 2.2. Preparation of PDDA-functionalized graphene oxide (PG) GO was prepared from natural graphite flakes by a modified Hummers' method (for detailed preparation process, see experiment section in the Supporting Information) [8, 23]. Preparation of PDDA-functionalized graphene oxide was described as follows [15]. First, 60ml of 0.25mg/mL homogenous GO dispersion was obtained by uniformly mixing and sonicating GO powders and DI water. After that, 750 µL of PDDA solution was added to the above dispersion, and refluxed for 5 hours at 90 ℃. Finally, excess PDDA was washed and removed thoroughly by filtration and centrifugation (10,000 rpm). The resulting product was denoted as PG and saved for characterization. 2.3. Preparation of the NPG nanocomposite paper NPG nanocomposite papers were fabricated by using vacuum-assisted self-assembly technique. Brief, a certain amount of ND powders was dispersed into DI water and was sonicated for 1 hour to form uniform dispersion (0.25 mg/ml). Subsequently, the as-prepared PG solution was mixed with ND dispersion under bath sonication for another 1 hour and accompanied with vigorous stirring for 3 hours to form homogeneous suspension. Finally, the mixture was transferred into a mixed cellulose ester filter (0.22 µm in pore size, 50 mm diameter) and was filtered via vacuum-assisted filtration method. The film was then dried in a vacuum oven at 60 ℃ overnight. Thus, a series of resulting thin papers were obtained and named as NPG. The samples with different mass fractions of GO to ND (Table S1) were prepared as above procedure. GO and PG papers were also prepared by the similar method for comparation. The thickness of thin papers was controlled by adjusting the amount of PG and ND suspensions. 2.4. Characterization The zeta potential values of GO, ND, and PG aqueous solution were tested at room temperature on a Malvern Zetasizer (Zetasizer Nano). Morphological changes of nanocomposite papers were performed on a scanning electron microscope (SEM,

S-3400N (Hitachi, Japan). Energy-dispersive X-ray spectrum (EDS) was tested at the same condition. Atom force microscopy (AFM) was carried out on a Dimension Ion PT (Bruker, Germany). Transmission electron microscope (TEM) was measured by using a Tecnai G20 transmission electron microscope (FEI company, American). The functional groups of GO, ND and PG were characterized using Fourier infrared spectra (FTIR, Bruker, Germany). The X-ray diffraction patterns (XRD) were conducted on a Bruker D8 ADVANCE with a Cu target (λ=0.154 nm), and the scan rate was 5o/min (5-90o). X-ray photoelectron spectroscopy (XPS AXIS Ultra DLD) was performed using an ESCALAB 250Xi XPS spectrometer. The mechanical property tests of nanocomposite papers (5×20 mm strips) were evaluated using a universal tensile machine (Dongguan Kejian, China), and each sample was tested five times. Thermal conductivity measurements of nanocomposite papers were conducted using the laser flash apparatus (NETZSCH LFA 467) at 25 ℃, and the thermal diffusivity α (mm2 s−1) was directly obtained. Specifically, the thermal conductivity K (W m-1K-1) of samples was calculated according to the following equation: K (T)= α (T) Cp (T) ρ (T)

(1)

where Cp is the specific heat capacity (J g-1 K-1), obtained by differential scanning calorimetry (METTLER DSC1). ρ (g cm-3) is the density of samples, which was calculated using the formula: ρ=m/v. Infrared thermal camera (FLIRONE PRO) was used to illustrate the heat dissipation performance of nanocomposite papers. The heat release rate of samples was obtained using a microscale combustion calorimeter (MCC-2, GOVMARK) at the heating rate of 1 oC/s under the N2 (80%) and O2 (20%). Thermal stabilities of nanocomposites were analyzed using a thermal gravimetric analysis (TGA, TG 209F3 Netzsch, Germany) at a heating rate of 10 ℃/min from 30-700 ℃ under nitrogen atmosphere. The measurements of volume electrical resistivity were carried out on an Agilent 4339B high resistance meter. 3. Results and discussion 3.1. Fabrication of NPG nanocomposite papers

Scheme 1. Schematic illustration of the fabrication procedure of NPG nanocomposite papers via vacuum-assisted self-assembly. As clearly displayed in the schematic procedure of Scheme 1, the NPG nanocomposite paper with different mass ratio of GO:ND was fabricated by an environmentally friendly vacuum-assisted self-assembly route under the electrostatic absorption (Table S1). Surface charge variations obtained by the zeta potential measurement are usually used to confirm the electrostatic attraction. As shown in Figure 1a, the zeta potential of pure GO aqueous solution is -12.4 mV, which mainly owes to the ionization of numerous oxygen-containing functional groups (e.g., carboxyl and phenolic hydroxyl groups) on the GO nanosheets surface [21, 24, 25]. To complete self-assembly process, the surface charge of GO nanosheets was changed by cationic PDDA. After GO was functionalized with PDDA solution at 90 ℃ for 5 h, the color of solution changed from yellow-brown to black (the PG vial in Figure1b), meaning that GO has been reduced [19]. The UV-vis spectrum and Raman were also certified the successful reduction of GO (Figure S1). The thickness variation of nanosheets was measured by the AFM. Pristine GO has fairly thin thickness, which is only about 0.96 nm and corresponds to the GO thickness of monoatomic layer (Figure1c). The average thickness of PG nanosheets reaches up to 1.58 nm (Figure1d), indicating that

PDDA coating layer formed on the surface of GO via π–π interactions [15, 26]. The zeta potential value of PG remarkably increased to +15.6 mV due to the presence of N+ centers of cationic PDDA macromolecules [27]. The zeta potential value of ND (-20.4 mV) was also evaluated. Thus, negatively charged ND particles were spontaneously adsorbed onto the surface of positively charged platforms of PG nanosheets under the electrostatic drive, resulting in the formation of hybrid nanostructure. The surface micro-morphologies of GO, PG and NPG nanosheets were further observed by TEM images. GO nanosheets possess flat flakes structure, while the surface of PG has apparent wrinkles and becomes darker due to the presence of PDDA coating (Figure 1e, f) [28]. Figure 1g presents the TEM image of the NPG, and ND particles with particle sizes of 5-10 nm are dispersed on the surface of graphene nanosheets.

Figure 1. Zeta potential of GO, PG and ND particles dispersed in DI water (a). The dispersion states of GO, PG and NPG3 in water with the same content (0.25 mg ml-1) (b). Typical AFM images of GO (c) and PG (d). Typical TEM images of GO (e), PG (f), NPG (g). 3.2. Characterizations of nanocomposite papers FTIR spectra were used to detect changes in functional groups of GO, PG, ND and NPG. As shown in Figure 2a, GO has a broad absorption peak at about 3415 cm-1, which is attributed to the hydroxyl stretching vibrations (νO-H). The peaks at 1731 and 1621 cm-1 correspond to the carbonyl stretching vibrations (νC=O) and bending vibrations from hydroxyl groups (δO-H) in water molecules, respectively. Epoxy

groups stretching vibrations (νC-O-C) were found at approximately 1049 cm-1. After GO was functionalized by PDDA, the typical absorption peaks like νC=O and νC-O-C were almost disappeared, indicating that GO has been reduced. The FTIR spectrum of pristine ND shows a typical broad peak at around 3426 cm-1, which attributes to the νO-H or/and absorbed H2O on the ND surface. Other characteristic peaks of ND include νC=O (1753 cm-1) and δO-H (1629 cm-1). For NPG, the main peaks of ND and C-N groups (1463 cm-1) derived from PDDA were found. XRD patterns of GO, PG, ND and NPG were shown in Figure 2b. Pristine GO has a strong diffusion peak at 2θ=9.7o (d-spacing=0.91 nm), which is corresponding to the (002). The (002) plane disappeared and a new broad peak at 2θ=23.5o (d-spacing=0.38 nm) was observed in the PG, indicating the successful reduction of GO [29]. The crystal structure of ND exhibits three diffraction peaks at about 21.2o, 43.8o and 75.3o, which are associated with the sp2 carbon of the amorphous graphite, (111) and (220) crystal faces, respectively. NPG has a broad peak at 20.9 o. The characteristic peaks of ND at 43.8 o can be observed in the NPG whereas the peak at 75.3 o was almost disappeared.

Figure 2. FTIR spectra (a) and XRD patterns (b) of GO, PG, ND and NPG. XPS spectra were carried out to further confirm the successful preparation of NPG films. As shown in Figure 3a, the pristine GO has two peaks of C1s and O1s. The functionalization of GO by PDDA results in an increase in C/O atomic ratio, with N1s and Cl 2p peaks appearing at about 401.9 and 196.8 eV, respectively. For C1s of GO, PG and NPG (Figure 3b, c), the four peaks and their corresponding binding

energy were assigned to C-C (sp2-carbon, at ~284.6 eV), C-O (hydroxy carbon, at ~286.0 eV), C=O (epoxy carbon, at ~286.6 eV), and O-C=O (carboxyl carbon, at ~288.1 eV) [30]. The oxygen content decreases from the 34.22 % of GO to 20.65% of PG due to the effective reduction of GO [26]. PG exhibits C-N bond at around 285.7 eV arising from PDDA, and C-N bond (~285.2 eV) was also found in the NPG because of the presence of PDDA and ND. Figure 3e shows the peak of N1s of PG and NPG, which illustrates the presence of strong interactions between PDDA chains and

graphene

nanosheets

(Figure

3e)

[26].

Figure 3. XPS survey spectra of GO, PG and NPG (a). C1s high resolution XPS spectra of GO (b), PG (c), NPG (d). N1s high resolution XPS spectra of PG and NPG (e). 3.3. Morphologies, structures and mechanical properties of nanocomposite papers The micro-morphologies of as-prepared papers were observed by the cross-section SEM and displayed in Figure 4. The GO paper exhibits stacked multilayers architecture after the vacuum-assisted filtration, and interlayer pores were found between the GO nanosheets (Figure 4a). The fracture morphology of the PG paper presents more denser layered arrangements owing to the presence of cationic polymers, which is similar to the “brick-and-mortar” microstructure of nacre (Figure

4b). During the electrostatic interactions and vacuum-assisted self-assembly process, ND particles were inserted and uniformly dispersed between graphene layers (Figure 4c-f), and a hierarchical and ordered "panel-bead"-like microstructure was observed. This ND-filled hierarchical structure facilitates the transfer of phonons and heat flow along the parallel direction. In addition, EDS elemental mapping collected from the fracture surface of NPG3 paper was used to display the distribution of element components (Figure 4g). Obviously, uniform element distribution of C, O, N and Cl in the NPG paper suggests that homogenous distribution of ingredients.

Figure 4. Cross-section SEM images of pristine GO (a), PG (b), NPG10 (c), NPG6 (d), NPG3 (e-f). EDS elemental mapping consists of carbon (C), oxygen (O), nitrogen

(N) and chlorine (Cl) collected from the corresponding cross-section red box area of NPG3 (g) (the white scale bar of all elemental mapping images is 2µm). Macroscopic morphologies of the PG and NPG nanocomposite papers with different mass ratio were shown in Figure 5a. To display good mechanical flexibility of nanocomposite papers, the NPG3 paper was bending or was completely curled up forming a cylindrical shape without any fracture (Figure 5a1, a2). Furthermore, the mechanical performance of nanocomposite papers is a significant indicator for TMMs. The tensile tests were carried out to evaluate the mechanical strength of nanocomposite papers (Figure 5b). Compared with PG paper, the tensile stress and strain failure of NPG composites are decreased with increasing ND content. For NPG3 paper, the tensile stress is 11.32 MPa, which could meet practical applications for the mechanical strength of TMMs. Young’s modulus and toughness were also displayed in Figure 5c. Specifically, the introduction of ND particles significantly increases the Young's modulus while reducing toughness of papers. A Young's modulus of 22.49 GPa and a toughness of 4.71 MJ m-3 for NPG3 paper were obtained. These flexibility and relatively mechanical strength features of NPG papers facilitate potential applications for TMMs.

Figure 5. Digital photographs of PG paper and NPG papers with different mass ratio

(a), and NPG3 paper exhibits flexibility (a1, a2). Tensile stress-strain curves (b) and Young's modulus/toughness (c) of tensile tests for PG paper and NPG papers with different mass ratio. 3.4. Thermal conductivity (K) and application for thermal management of LED The thermal diffusion capability of heat flow in time is of great importance for TMMs. A laser flash apparatus (LFA, see in Figure S2) was widely used to evaluate thermally conductive capability of nanocomposites. As shown in Figure 6a and b, the in-plane and cross-plane thermal conductivity of GO, PG and NPG nanocomposite papers were measured by LFA method at 25 °C. The in-plane thermal conductivity of pristine GO paper was only 3.108 W m-1K-1, which was consistent with reported values in the literature [31]. For PG paper, the in-plane thermal conductivity value reaches up to 5.467 W m-1K-1 duo to the reduction of GO. As expected, the in-plane thermal conductivity of NPG films was sharply improved with the increasing of ND loadings. The NPG3 paper exhibits excellent in-plane thermal conductivity of 16.653 W m-1K-1, which is approximately 80 times higher than that of traditional pure polymers. This indicates that the embedding of ND into the 2D nanosheet layers has a significant thermal conductivity improvement in the horizontal direction. Moreover, compared with previously reported literature on thermally conductive materials (Table 1), the NPG paper has a remarkable superiority in the in-plane heat conductive direction. The cross-plane thermal conductivity of papers was also investigated to display the thermal conductivity changes in the vertical direction. However, a change trend different from the in-plane direction was found, and the cross-plane thermal conductivity for PG and NPG papers with various ND mass ratio were 0.203, 0.288, 0.48 and 0.26 W m-1K-1, respectively. Obviously, the increasing ND concentration is not always advantageous for cross-plane thermal conductivity of nanocomposite papers. The reduced cross-plane thermal conductivity is ascribed to the weak Van der Waals interactions between the graphene-ND interfaces [32, 33]. The anisotropy index (AI), a parameter used to account for the anisotropy difference between the in-plane and cross-plane thermal conductivity directions, can be obtained by the formula AI=Kin-plane/Kcross-plane [34]. As shown in Figure 6c, the maximum AI value

was 64 for the NPG3 paper. In order to quantify the contribution of the ND on the in-plane thermal conductivity of nanocomposite papers, the thermal conductivity enhancement per 1wt% ND loading (η) was evaluated according to the following formula: η % =

×

× 100%

(2)

where KNPG and KPG are thermal conductivity values of NPG and PG papers, respectively. WND is the mass fraction of ND in the nanocomposites. For the in-plane thermal conductivity, the η values are 844.4 (NPG10), 1111.4 (NPG6), 818.4%(NPG3), respectively. The incorporation of ND exhibits excellent in-plane thermal conductivity enhancement. In this work, the thermal conductivity results demonstrate that dramatic improvement of in-plane thermal conductivity of NPG nanocomposite papers compared with the PG paper. A simple thermal transfer model of "panel-bead" structure was proposed to better understand the thermal conductive change. On one hand, ND as low dimensional nanoparticles, is difficult to form effective heat conduction pathways in the matrix at low loading. On the other hand, the 2D GO has limited thermal conductivity. Therefore, in our design, the incorporation of ND particles plays an important role of bridge in adjacent PDDA-functionalized graphene oxide layers, which can effectively fill the spaces and edges of the 2D nanosheets and further lead to accelerated phonon transmissions. In this case, thermal conductive networks of nanocomposite papers changes from 2D nanosheets to 3D interconnected graphene-nanodiamond architecture ("panel-bead"-like structure) and the thermal conductive performance exhibits obvious increase (Figure 6d) [35]. From the perspective of the interface thermal resistance (ITR) [36, 37], the interspace-filling and edge-bridging between nanosheets and nanoparticles can effectively reduce ITR and promote the phonon transportation under electrostatic interactions. The heat flow will preferably transfer along the 3D thermally conductive channels (the red arrows in Figure 6d). As a result, a significant improvement in thermal conductivity for NPG3 nanocomposites was achieved.

Figure 6. In-plane (a) and cross-plane (b) thermal conductivity of GO, PG and NPG films. The AI values of PG and NPG films with various ND mass ratio (c). The schematic diagram of thermal conductive mechanism (d). Table 1. Comparison of thermal conductivity of our nanocomposite papers with previously reported literature. Testing method

Ref. year

0.42

LFA LFA LFA LFA

[31]2016 [38] 2015 [31] 2016 [39]2017

3.45

-

LFA

[40] 2017

13.42 10.3 16.653

0.69 0.38 0.26

LFA LFA LFA

[41]2017 [42]2018 This work

Sample

Fraction

Thermal conductivity (W m-1K-1) In-plane Through-plane

GO BNNs BNNS-GO h-BN/f-G/P2E HA h-BN– RGO/epoxy GPF a GO-BNMPs b NPG3

30 wt%h-BN 1wt%f-G

3.75 4.0 29.8 4.20

26.04 vol% GO:dopamine= 2.4:1 GO:BNMPs =1:1 ND:GO =1:3

Note: a graphene/polydopamine; b boron nitride micro-platelets Here, we displayed the practical thermal management application of the NPG

paper for a light-emitting-diode (LED, 24W, current: 280 mA±5%) light strip. The details were shown in Figure S3 and Figure 7a. The PG paper and NPG3 paper were respectively adhered to the surface of the LED device, and the temperature change of central spot (Tcs) of the LED source with time was captured and recorded by an infrared thermal camera. The infrared thermal images of the entire heating and cooling process were shown in Figure 7b and c. The color of the infrared thermal images gradually brightens during the heating process, indicating the surface temperature increases with the heating time. For the PG paper and NPG3 paper, the Tcs is 81.1 and 97.7 oC at 25s, respectively. It is obvious that Tcs of the NPG3 paper increased significantly faster than that of PG paper (△T=18 oC at 27s). In addition, from the temperature-time curves of the PG paper and NPG3 paper during the heating and cooling process (Figure 7d), one can see that NPG3 has faster heating rate. After 27s, the power supply of the LED device was cut off. The NPG paper was cooled rapidly from 101 to 41.3 oC within 5 s during the cool process, while PG was only cooled from 83.1 to 41.3 oC at the same time. The decrease of Tcs of the NPG3 paper is much faster than PG paper. These temperature variation differentials illustrate that NPG3 paper has faster heating and cooling rate compared with the PG paper. Therefore, the NPG nanocomposite paper has great potential for heat dissipation in electronic devices.

Figure 7. Thermal management application of the NPG nanocomposites. Schematic

diagram of the infrared thermal imaging design (a). Infrared thermal images of the surface temperature variations of PG and NPG3 nanocomposite papers during the heating (b) and cooling process (c). Temperature-time curves of the PG paper and NPG3 paper (d). 3.5. Flame retardancy and rapid fire alarm response application In the past years, extensive research has been conducted on graphene-based materials with excellent flame retardancy [43-45]. Unfortunately, for TMMs, their fire resistance has not been paid much attention. In fact, good fire retardancy is also critical for TMMs in real life. To display the flame resistance of nanocomposite papers, the combusting process of all samples was recorded by an ignition device, as shown in Figure 8. Pristine GO paper completely burned off within 30s, and only some residue left (Figure 8a). The rich oxygen-containing groups on the surface of GO is an important factor for its easy flammability [45]. In contrast, the PG and NPG3 papers kept initial shape and exhibited nonflammable behaviors (Figure 8b and c). In addition, the flame behaviors of the NPG10 and NPG6 papers were displayed in Figure S4.

Figure 8. Optical photos of combustion process of GO (a), PG (b) and NPG3 (c)

papers. The thermal stability measurements of the GO, PG and NPG3 were displayed in TGA curves of Figure 9 (a). All nanocomposites begin to lose weight below 100 oC due to the evaporation of absorbed water, especially NPG3 shows the maximum weightlessness at this stage. The weight loss occurred from 150 to about 300 oC, which was assigned to the thermal degradation behaviors of oxygen-containing groups such as hydroxyl, carboxyl and epoxy [46, 47]. For PG, the thermal stability obviously improved because of the reduction of GO [48]. It’s found that PG and NPG3 composites have two weight losses at approximately 300 ℃ and 430 ℃. The residual chars of PG and NPG3 are 43.08% and 55.21% at 700 ℃, respectively, and the result shows that the incorporation of ND further improves the thermal stability of nanocomposites. Microscale Combustion Calorimetry (MCC) is considered to be one of the most effective measurements to evaluate the combustion of materials. Here, the flame retardancy properties of GO, PG and NPG3 papers were tested by using MCC. Detailed MCC data and heat release rates (HRR) curves versus temperatures were shown in Table S2 and Figure 9. Pristine GO paper has a sharp HRR curve in the range from approximative 190 ℃ to 230 ℃, and the peak heat release rates (PHRR) reaches up to 438.4 W g-1 at 210.9 ℃, which fully illustrates that pristine GO paper has a strong heat release process in this temperature range. On the contrary, the PG and NPG papers exhibit decreased HRR curves. In detail, the PG paper shows PHRR value of 110.2 W g-1 at 440.9 ℃, while NPG3 paper has a lower PHRR (99.16 W g-1) at 439.2 ℃. This phenomenon is also authenticated by the TGA results, indicating that the incorporation of ND particles can be used to improve the thermal stability of PG. Compared with GO paper and PG paper, the PHRR value of NPG3 is reduced by 77 % and 10 %, respectively. In addition, the heat release capacity (HRC) is an essential parameter to evaluate fire resistance of materials. For pristine GO paper, the HRC value is up to 490 J g-1 K-1. When GO was functionalized with PDDA, this value was reduced to 122 J g-1 K-1. After the incorporation of ND, the NPG3 paper has a lower

HRC value of 111 J g-1 K-1, which is decreased to 77 % of GO and 9 % of PG. These data reveal that NPG3 nanocomposite paper has less fire hazard.

Figure 9. TGA curves of GO, PG and NPG3 nanocomposites (a). Heat release rates (HRR) curves of pristine GO, PG and NPG3 nanocomposite films (b). To deeply analyze flame-retardant mechanism, the morphologies and properties of char residues of nanocomposites were analyzed through various techniques such as SEM, FTIR and Raman spectroscopy. The SEM morphologies of both PG (Figure 10a) and NPG3 (Figure 10b, c) show continuous and compact residual char surfaces while micropores is found on the surface of residual PG surface. These images show that wrinkles and rugged graphene nanosheets (or hybrid of graphene-nanodiamond) are tightly interconnected, and thus release of mass and heat will be effectively suppressed during the whole combustion [49]. Specifically, some white deposits were found on the SEM images of char residues surface, which would be ascribed to the inorganic residual matter or soot deposition after a long time of combustion and carbonization [50, 51]. The FTIR spectra of char residues of PG and NPG3 nanocomposites are shown in Fig. 10d. Some organic groups (O-H bond at 3200 cm-1, C=O band at 1700 cm-1) can’t be found due to the combustion process. The graphitization degree of the char residues has an extremely important effect on improving flame retardancy of composites [52]. As shown in Fig. 10e and f, two main peaks of Raman spectra at approximately 1365 and 1586 cm−1 are assigned to the D and G bands of all residual nanocomposites, respectively. The area ratio of D to G bands (ID/IG) is an essential parameter for evaluating the graphitization degree of

the char residue, and the relatively lower ID/IG value means the higher graphitization degree of char layers [53]. The ID/IG value ranges from 1.80 to 1.69, indicating that residual char of NPG3 has higher graphitization degree. The residual chars with higher graphitization degree not only suggest higher thermal stability, but also display better protection role for the further damage of composite papers [54], corresponding to the aforementioned results of TGA and SEM images. Hence, the incorporation of ND particles can further increase the graphitization capability of nanocomposites, thereby forming a denser and protective barrier char layer on the surface of the NPG3 paper, inhibiting the output of the heat and mass transfer, and exhibiting greatly flame retardancy.

Figure 10. SEM images of char residues surface of PG (a) and NPG3 (b, c) after combustion. Char residues of FTIR spectra (d) and Raman spectra (e, f) of PG and NPG3 after combustion. As is well known, rapid and effective early flame alarm is essential for people to evacuate at the beginning of the potential fire disasters. Here, the nanocomposite paper was attached to a self-designed electrical circuit (voltage: 220V) containing an alarm buzzer and volt-ammeter, and the schematic diagram was shown in Figure 11a. As shown in Figure 11c, d and Figure S5a, b, the buzzer didn’t flash when the power (220V) was turned on because the nanocomposite paper has high electrical resistivity. The volume electrical resistivities of nanocomposite papers were measured by using

Agilent 4339B high resistance meter. As shown in Figure 11b, the volume resistivities of the PG paper and NPG3 paper are 1.283×1010 and 2.647×1011 Ω cm, respectively. Obviously, the introduction of ND increases the electrical resistivity by an order of magnitude due to the excellent insulation of ND. These values are significantly higher than the critical state of electrical insulation (109 Ω cm). Interestingly, as shown in Figure 11c and d, Figure S5, Movie S1 and Movie S2, the buzzer flashed and ultrafast flame warning was triggered within ~1s after the paper encountered a flame attack. More importantly, the flash alarm was continuous for a long time even removing the fire. The decreased intensity ratio of the ID/IG of Raman spectra after combustion signifies thermal reduction of PG and NPG3 in flames (Figure S6) [55]. Therefore, the occurrence of graphitization and thermal reduction of PG and NPG3 has established effective conductive networks, and so it shows rapid fire warning response after the flame attack [55]. As far as we know, this is highly sensitive early flame response detection and flame alarm sensor compared with previous reports (Table 2). These results illustrate that the nanocomposite paper has rapid fire response and can be used as a promising fire alarm sensor for fire prevention and early fire warning detection

applications.

Figure 11. Rapid fire alarm response design of nanocomposite papers. The electrical circuit constructed by an alarm buzzer and volt-ammeter (a). Volume electrical resistivities of the PG and NPG3 papers (b). Fire warning process of PG paper (c) and

NPG3 paper (d). Table 2. Comparison of response time of different fire alarm sensor after encountering flame.

a

Sample

Response time (s)

Ref. year

GO/silicone coatings FGO/CNTs coated WPP a MF@GOWR b silane-GO papers NPG3 paper

2-3 5 ∼2 ∼1.6 ~1

[55] 2018 [56] 2018 [43] 2019 [57] 2019 This work

wood pulp paper (WPP);

b

graphene oxide wide-ribbon (GOWR) and

melamine-formaldehyde sponges (MF). 4. Conclusions In summary, a kind of multifunctional carbon-based nanocomposite paper with functionally reduced GO and ND was successfully prepared via the electrostatic driving force and vacuum-assisted self-assembly method. The incorporation of ND particles was distributed to the interlayer and edge of graphene nanosheets, which led to form a hierarchical 3D interconnected "panel-bead" architecture of the NPG paper. A high in-plane thermal conductivity of 16.653 W m-1K-1 was obtained while the cross-plane thermal conductivity value was only 0.26 W m-1K-1. Thus, the highly thermally conductive anisotropy degree (AI=64) of the NPG3 nanocomposite was discovered. Furthermore, this nanocomposite paper shows excellent flame retardancy and ultrafast fire alarm response ability. The peak heat release rate (PHRR) of NPG3 paper (99.16 W g-1 at 439.2 oC) was reduced by 77% compared with that of GO (438.4 W g-1 at 210.9 oC). In addition, the ultrafast flame alarm response was found within about 1s after the paper encountered flame due to the occurrence of graphitization and thermal reduction of nanocomposites. We anticipate that would present a feasible strategy for preparing multifunctional TMMs with excellent thermally conductive and electrical insulating properties, remarkable flame retardancy and ultrafast early fire alarm response capabilities.

Author information Corresponding Author *E-mail: [email protected] ORCID Kun Wu: 0000-0001-6494-8851 Notes The authors declare no competing financial interest.

Acknowledgments This work was financially supported by the Key Technologies Research and Development Program, China (2017YFD0601003) and Guangzhou Science and Technology Program Key Projects (201904010244). In addition, Bingfei Nan would like to thank Miss Cheng for her care and encouragement over the past years. Will you marry me?

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Highlights

1. A series of 3D hybrid NPG paper constructed by positively charged PDDA-functionalized GO and negatively charged ND particles under electrostatic interactions were successfully prepared via a vacuum-assisted self-assembly strategy. 2. The NPG nanocomposite paper has superior in-plane thermal conductivity of 16.653W m-1K-1 at the mass ratio of GO:ND =3:1, which is about 80 times higher than that of conventional polymers. 3. In addition, the NPG nanocomposite paper shows excellent flame retardancy, electrical insulation and ultrafast flame alarm response capability. 4. This

nanocomposite

paper

provides

an

advantageous

multifunctional development of thermal management materials.

option

for

the

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: