Highly Anisotropic Thermal Conductivity of Layer-by-Layer Assembled

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Highly Anisotropic Thermal Conductivity of Layer-by-Layer Assembled Nanofibrillated Cellulose/Graphene Nanosheets Hybrid Films for Thermal Management Na Song,*,† Dejin Jiao,† Siqi Cui, Xingshuang Hou, Peng Ding,* and Liyi Shi Research Centre of Nanoscience and Nanotechnology, Shanghai University, 99 Shangda Road, Shanghai 200444, People’s Republic of China S Supporting Information *

ABSTRACT: An anisotropic thermally conductive film with tailorable microstructures and macroproperties is fabricated using a layer-by-layer (LbL) assembly of graphene oxide (GO) and nanofibrillated cellulose (NFC) on a flexible NFC substrate driven by hydrogen bonding interactions, followed by chemical reduction process. The resulting NFC/reduced graphene oxide (RGO) hybrid film reveals an orderly hierarchical structure in which the RGO nanosheets exhibit a high degree of orientation along the in-plane direction. The assembly cycles dramatically increase the in-plane thermal conductivity (λX) of the hybrid film to 12.6 W·m−1·K−1, while the cross-plane thermal conductivity (λZ) shows a lower value of 0.042 W·m−1·K−1 in the hybrid film with 40 assembly cycles. The thermal conductivity anisotropy reaches up to λX/λZ = 279, which is substantially larger than that of similar polymeric nanocomposites, indicating that the LbL assembly on a flexible NFC substrate is an efficient technique for the preparation of polymeric nanocomposites with improved heat conducting property. Moreover, the layered hybrid film composed of 1D NFC and 2D RGO exhibits synergetic mechnical properties with outstanding flexibility and a high tensile strength (107 MPa). The combination of anisotropic thermal conductivity and superior mechanical performance may facilitate the applications in thermal management. KEYWORDS: graphene nanosheets, nanofibrillated cellulose, layer-by-layer assembly, layered structure, thermal conductivity, anisotropic properties

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INTRODUCTION Flexible films with highly anisotropic thermal conductivity have elicited substantial attention in the thermal management of modern electronics, in which the in-plane thermal conductivity (λX) is substantially higher than the cross-plane thermal conductivity (λZ).1,2 The anisotropic thermally conductive films have the capacities to remove the heat from the hot spots along the in-plane direction while keeping the electronic components away from being heated by the neighboring hot spots. The graphite papers are typical film-like materials with anisotropic thermal conductivities.3,4 Despite the ultrahigh λX and λX/λZ, unprocessed graphite paper can be hardly used for thermal management due to its brittleness. Thus, there is a necessity to exploit new film materials with excellent mechanical properties as well as high λX/λZ that can be utilized as thermal management materials, including the removal of extra heat (call for high λX) and shielding from excess heat (call for low λZ). Due to the extremely high in-plane thermal conductivity (2000−5000 W·m−1·K−1 near room temperature), graphene has simulated substantial insight in fabricating the graphenebased efficient thermally management materials.5−9 Generally, to exploit the potential of graphene completely as a superior filler in anisotropic thermally conductive composite, one of the © 2017 American Chemical Society

critical technologies is to assemble the graphene nanosheets rationally along a certain direction.10,11 The assembly of graphene nanosheets into a layered structure can result in distinctly different thermal conductivities between the alignment direction and the perpendicular direction.10,12−14 There has been a range of approaches to fabricate the thermally conductive composites with hierarchical architectures, and the most notable ones are the solution-based methods such as vacuum filtration and solution casting15−17 or the melt-based methods such as polymer infiltration and hot pressing.10,18,19 Another high-potential route to achieve the highly ordered layered structures is layer-by-layer (LbL) assembly. The LbL assembly is realized by the sequential deposition of platelets and polymers, allowing a nanoscale-level uniformity of the nanocomposite and preserving the unique characteristic of each constituent.20−22 Moreover, in the search for innovative matrices and a powerful combination of complementary components beyond those in recent studies, some advanced nanocomposite matrices should be explored to replace commercial polymers in polymer-based nanocompoReceived: September 21, 2016 Accepted: January 3, 2017 Published: January 3, 2017 2924

DOI: 10.1021/acsami.6b11979 ACS Appl. Mater. Interfaces 2017, 9, 2924−2932

Research Article

ACS Applied Materials & Interfaces

Figure 1. LbL-assembled (NFC/RGO)n hybrid films. (a) Schematic drawing of the preparation of the hybrid films. Fabrication of the (NFC/RGO)n hybrid film architecture includes two steps: The first step is the sequential assembly of NFC and GO on a flexible NFC substrate. The second step is the reduction of GO. (b) TEM image of GO. (Insert) Photograph of GO dispersion. (c) TEM image of NFC. (Insert) Photograph of NFC suspension. (d) FTIR spectra of the (NFC/GO)n hybrid films at wavenumbers 3100−3500 cm−1 showing the vibration of −OH groups. assembly cycle, the flexible NFC substrate was first immersed into the GO dispersion (1 mg/mL) for 5 min, rinsed with DI water for 2 min, and then dried under an infrared oven with air flow. Subsequently, the film was dipped into the NFC dispersions (1 mg/mL) for 5 min, followed by similar rinsing and drying. This procedure was a single deposition cycle. The procedure could be repeated multiple times to obtain the (NFC/GO)n hybrid films with the desired number of bilayers. Afterward, the hybrid film was immersed into hydrazine solution and then heated to 90 °C to allow for reduction of GO in the hybrid films. The resulted films are named as (NFC/RGO)n, where n is the number of cycles (n = 0 represents the pure NFC film). GO was synthesized from natural graphite powder using a modified Hummer’s method.29 NFC was prepared according to the method reported by Saito.30 A detailed description of GO and NFC preparation can be found in the Supporting Information. Characterization. The reduction of GO in hybrid films was testified using X-ray diffraction (XRD, a D/MAX2200/PC, with Cu Kα radiation), Fourier transform infrared (FTIR, Avatar 370 FTIR spectrometer), and Raman spectra (using INVIA confocal micro Raman spectrometer with a 633 nm laser). The RGO content was determined through Thermogravimetric analysis (TGA, using a T.A. Instruments Q500 Thermo Gravimetric Analyzer, nitrogen atmosphere, heating rate of 10 °C·min−1). Samples were dried at 40 °C for 24 h under vacuum ahead of TGA analysis. UV−vis transmittance spectra were performed on a UV-2501PC spectrophotometer from Shimadzu. Zeta potentials were obtained using a Zetasizer 3000HS zeta potential analyzer (MALVERNLNSTVUMENTS) at ambient temperature. The morphology of the samples was found using transmission electron microscopy (TEM, 200CX 178 transmission electron microscope) and scanning electron microscopy (SEM, JSM6700F emission scanning electron microscope). The oxygen content on the RGO was measured using X-ray photoelectron spectroscopy (XPS, PHI-5000C ESCA system, PerkinElmer, hν =1253.6 eV). The thermal conductivities of the hybrid films were determined through a Netzsch LFA 447 Nanoflash at 25 °C. Each test was repeated six times, and the values with large errors were removed. The stress−strain curves were collected through dynamic thermomechanic analysis (DMA, T.A. Instruments Q800, with a film tension mode, rectangular samples of 3 × 20 mm with varying thickness were tested at a rate of 0.500 N·min−1 at room temperature).

sites. Herein, we suggest the LbL assembly of the thermally conductive 2D graphene nanosheet component with the 1D nanofibrillated cellulose (NFC), which possesses excellent mechanical properties (high strength and high stiffness), as well as biodegradability, transparency, lightweight, and facility for functionalization.23−25 In addition, the high concentration of the surface hydroxyl segment of NFC presents possibilities for effectively “gluing” the graphene nanosheets to assemble into macroscopic nanocomposites that have exciting properties of their nanometer-scale building blocks via hydrogen bonding.26,27 The focus of the present study is the strongly anisotropic thermal conductivity of NFC/RGO hybrid film, which is achieved by providing a well-aligned structure using the LbL assembly technique. Contrary to ordinary film-fabricating LbL approaches on rigid solid substrates, we adopt a flexible NFC film as the substrate for sequential deposition of NFC and graphene oxide (GO). In this method, the laminated hybrid films can be directly used after the reduction process as the flexible heat conductor without peeling off from the substrate. Therefore, the intrinsic drawback of LbL assembly, the requirement of a high number of deposition steps to fabricate a bulk film, can be avoided.28 The hierarchical structure in NFC/RGO hybrid film leads to substantially increased λX and, simultaneously, extremely low λZ. The anisotropy of thermal conductivity reaches a high value of λX/λZ = 279, which is much higher than previous research on polymer-based nanocomposites. Moreover, the mechanical flexibility and tensile strength of the hybrid film are also studied in detail.

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EXPERIMENTAL SECTION

Fabrication of the NFC/RGO Hybrid Films by Layer-by-Layer Assembly. In this study, we use the pure NFC films as the substrates. The NFC film was prepared by filtration of NFC suspensions on mixed cellulose ester membranes (47 mm in diameter, 0.45 μm pore size) and dried in vacuum at 40 °C. Afterward, the substrates were thoroughly rinsed with ethanol and DI water. In a typical LbL 2925


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RESULTS AND DISCUSSION A layer-by-layer assembly of NFC and GO nanosheets on an ultrathin NFC substrate and a chemical reduction procedure for conversion of GO to RGO was employed to prepare the hybrid films, as shown in Figure 1a. NFC and GO (the 1D and 2D morphology of the two components are shown in Figure 1b and 1c), two slightly negatively charged water-soluble components, were used in the LbL technique. Nevertheless, the NFC could associate with the GO nanosheets. The zeta potential studies (Figure S-1) proved this observation where the GO (ζ = −38.7 mV) and NFC (ζ = −6.5 mV) formed less stable GO/NFC hybrids (ζ = −3.0 at 5 wt % GO).12,31 Figure S-2 shows that the NFC nanofibers are absorbed on the surfaces of the GO nanosheets, further confirming the existence of strong interactions between NFC and GO nanosheets. The primary contributors to the interactions were considered to be the hydrogen bonds between plentiful −OH groups on NFC and −OH/−COOH groups on GO sheets. The intermolecular hydrogen bonds in composites can cause a downshift of the broad vibration peak position relating to the −OH group.32,33 In the present study the −OH peaks at approximately 3100−3500 cm−1, shown in Figure 1d, clearly illustrate a downshift in the NFC/GO hybrid films, confirming the hydrogen bonding between the NFC and the GO nanosheets. It is supposed that the intermolecular hydrogen bond between GO and NFC can induce the repeatable LbL assembly to fabricate the ordered inorganic/organic layer structure, which would have analogies with the brick-andmortar arrangement in natural nacre. In order to characterize and evaluate the reduction of GO in hybrid films, XRD, FTIR, and Raman techniques were carried out. Figure 2 shows the XRD patterns of GO, RGO, NFC

nanosheets. The new peak correlated to RGO nanosheets overlaps with the diffractions from the NFC around 2θ = 22.5°. In the FTIR spectra (Figure 3), typical peaks of the CO

Figure 3. FTIR spectra of GO, RGO, NFC substrate, and (NFC/ RGO)40 hybrid film.

group at 1726 cm−1 were observed from GO. This peak disappeared in the FTIR spectra of RGO and (NFC/RGO)40 hybrid film, suggesting the reduction of GO, and this is in agreement with the XRD data. Figure 4 reveals the Raman spectrum of GO, RGO, NFC substrate, and the NFC/RGO hybrid films with increasing deposition numbers. The Raman spectra of RGO and GO (Figure 4a) have two peaks: the D band at approximately 1330 cm−1 is associated with the vibration of sp3-hybridized carbons

Figure 2. XRD patterns of GO, RGO, NFC substrate, and (NFC/ RGO)40 hybrid film.

substrate, and the resulting hybrid film. A characteristic diffraction peak at 2θ = 10.2° can be observed in the XRD pattern of GO, which corresponds to a d spacing of 0.84 nm. After chemical reduction, the RGO exhibits a broad peak centered at 2θ = 23.5°, implying that the RGO nanosheets were loosely stacked in the obtained samples, and it is different from crystalline graphite.34,35 The NFC substrate illustrates two broad reflection peaks at approximately 2θ = 15.2° and 22.9°, which are typical of cellulose I crystalline structure.36,37 After LbL assembly and reduction, the typical peaks correlated to an interlayer spacing of GO completely disappear in (NFC/ RGO)40 hybrid film, indicating reduction of GO to graphene

Figure 4. Raman spectra of (a) GO and RGO. (b) LbL-assembled (NFC/RGO)n hybrid films on a flexible NFC substrate with increasing deposition cycles. 2926


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ACS Applied Materials & Interfaces in the edges or defects in the lattice, and the G band at approximately 1597 cm−1 is related to vibration of sp2hybridized carbons in graphene nanosheets.38 The D/G intensity ratio [I(D/G)] increased appreciably from 1.01 to 1.33 upon chemical reduction. This can be explained by the decrease in the size of sp2 domains when GO transformed to graphene.39,40 The NFC substrate exhibited two weak characteristic peaks around 1096 and 1380 cm−1, whereas hybrid films showed strong peaks at approximately 1335 (D band) and 1598 (G band) cm−1, confirming successful assembly of RGO nanosheets. More importantly, with the increasing of assembly cycles, the intensity of D bonds and G bonds of (NFC/RGO)n hybrid films gradually increased, which means gradual growth of repeating assemblies. The assembly process of NFC/RGO multilayer films was further monitored by UV−vis transmittance spectra. Figure 5

Figure 5. Transmittance spectra of (NFC/RGO)n hybrid film with different numbers of deposited cycles on a NFC substrate. (Insert) UV−vis transmittance of (NFC/RGO)n hybrid film at 550 nm.

Figure 6. (a) Photograph of the as-obtained (NFC/RGO)40 hybrid film. (b−d) SEM images of the surface of (NFC/RGO)40 hybrid film. (e−h) Cross-sectional SEM images of (NFC/RGO)40 film; among them, g presents the cross-section of thermally conductive coating layer and h is the NFC-substrate.

illustrates the deposition cycles dependence of the transmittance spectra of (NFC/RGO)n hybrid films. The transmittance spectra display an obvious trend of a decrease in the range of 400−800 nm. The optical transmittance of hybrid films at 550 nm decreased almost linearly from the NFC substrate’s initial value of 87−59% as the deposition cycles increased to 40. This linear relationship indicates a reproducible and uniform LbL assembly process. The thermally conductive films with a tunable optical transmission property are promising for broadening its applications in thermal management in electronics. Besides, it can be calculated from the TGA that the RGO content in (NFC/RGO)40 hybrid films is 1.0 wt % (for details, see Supporting Information). After depositing, a film of homogeneity and transparency was obtained, as shown in Figure 6a. Such an homogeneity and transparency is indicative of a well-defined structure within the films and the absence of detrimental graphene aggregation found earlier during reduction of GO before the vacuumassisted self-assembly.26 The SEM image of (NFC/RGO)40 hybrid film depicts a relatively smooth top surface (Figure 6b) and reveals that RGO nanosheets are well oriented along the plane direction to maximize the attractive energy. In addition, it is easy to find wrinkling at the RGO nanosheet edges (Figure 6c and 6d). The aligned structure favors the construction of consecutive 2D thermally conductive paths. The SEM images of the cross-section of the (NFC/RGO)40 hybrid film depict a sandwich construction within the hybrid film, in which the

interlayer is the NFC substrate and the coating layer consists of NFC and RGO (Figure 6e and 6f). In addition, the coating layer has an excellent extent of RGO nanosheets orientation that arranges into a well-defined layer structure, which is conceptually like that of nacre, as demonstrated in Figure 6g. This combination of the highly aligned structure of the active coating layer and the thermal insulation property of the interlayer is the cause of the ultralow cross-plane thermal conductivity. In order to understand the LBL assembly of NFC and GO better, we studied the morphology of one time deposition of NFC on an ultrathin GO film (Figure S-4). It is interesting to note that the morphology is apparently different from the (NFC/RGO)40 hybrid film. The reason for this is the different compositions of the outermost shell in the two different films. There are ample 1D nanofibrous structures of NFC present on the surface, indicating that the NFC nanofibers have assembled onto the film by hydrogen bonding in each deposition cycle. To support the claim on the orientation of RGO nanosheets, the fast Fourier transform (FFT) approach was adopted to quantitatively evaluate the degree of RGO nanosheets orientation.41 On the basis of a self-compiled program by Matlab, FFT is executed on each SEM picture with the same area (1 × 2 μm), where the image is transformed into a 2927


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The thermal conductivity anisotropy is believed to derive from the hierarchical structure consisting of NFC matrix and aligned RGO nanosheets. The LbL assembly led to an ordered laminated structure coating on the surface of flexible NFC substrate. As shown in Figure 6, the RGO nanosheets are oriented along the film plane. Large contacts are attained between the neighboring RGO nanosheets in the thermally conductive coating; the thermal contact resistance is thus minimized while heat conducts in the planar direction of the hybrid film.48 A recent study proved that the overlap area between flakes (correlating to the contact resistance) is a vital affecting factor for the in-plane thermal conductivity of graphene laminates through a combined atomistic-continuum hierarchical multiscale method.49 In other words, continuous heat conduction pathways with lower phonon scattering have been constructed in the planar direction in the deposition cycle, contributing to improving the thermal conductivity.45,50,51 As the deposition cycle increased, the increasing RGO content provided more 2D thermally conductive pathways, which can be regarded as paralleling low thermal resistances within the total resistance of NFC/RGO hybrid films according to the paralleling thermal resistance model proved by flake graphite/ polymer composites.52−54 The total thermal resistance along the in-plane direction in the bulk material reduced as deposition cycles increased, and therefore, the λX increased with the deposition cycle. However, for the case of λZ, the negligible increase with the deposition cycles should be attributed to two factors: (a) The NFC existed between the RGO nanosheets in the thermally conductive coating in nanoscale, and the λZ of the neat NFC is extremely low (λZ of NFC is 0.034 W·m−1·K−1. Detailed explanations for such a low thermal conductivity have been provided in the Supporting Information). The discontinuous thermally conductive pathways greatly limit the phonon transmission. Thus, the λZ of hybrid film depends not significantly on the RGO content. For example, even when the RGO content of hybrid film is up to 20 wt % the λZ is still only 0.059 W·m−1·K−1.26 (b) More importantly, the NFC substrate is located between the two thermally conductive coatings in the micrometer-scale sandwich structure. This heat-insulated substrate would further obstruct the heat flow when it transfers through thickness direction. In conclusion, the hierarchical structure is responsible for the different performance between the cross-plane and the in-plane components of thermal conductivity. Overall, the LbL assembly on the NFC substrate produces a hierarchical structure, which leads to distinct heat-transfer capabilities along the cross-plane and in-plane directions. The thermal conductivity anisotropy was in accordance with the observations in which the well-oriented fillers impart composites with an anisotropic thermal conductivity.55,56 It is meaningful to compare the scope of thermal conductivity anisotropy (λX/λZ) realized through the LbL assembly to previous data on these of polymeric composites. A number of design principles for the construction of appropriate structures can be derived, even if techniques toward understanding the thermal conductivity anisotropy in such composites are still a rarity. Table 1 sums up the anisotropy values reported in previous studies on polymeric composites. It can be seen from the data that the anisotropy of the LbL-assembled NFC/RGO hybrid films [approximately 279 for (NFC/RGO)40] is indeed at a higher level. Therefore, the hybrid films based on NFC and RGO prepared using LbL assembly on a flexible substrate could be particularly useful in applications requiring efficient

frequency domain. In FFT frequency domain images, the intensity of the pixel presents an angular dependence on patterns of spatial alignment. The pixels that display high intensity values will be arranged along the orientation of the highest degree of directional anisotropy. As shown in Figure S5, the FFT white spots of the (NFC/RGO)40 hybrid film present an elliptical distribution, illustrating a good orientation of the nanoplates. We define the ratio of the scattering vector lengths between the major and the minor axis as ΦH/V, which reflects the orientation degree of FFT spots, and ultimately corresponding to the orientation degree of RGO nanosheets in SEM images. Generally, the higher ΦH/V represents the higher degree of orientation of nanosheets. Compared with the layered-structured NFC/RGO hybrid film prepared using vacuum-assisted self-assembly,26 the thermally conductive coating in the (NFC/RGO)40 hybrid film exhibits a higher ΦH/V of 4.1, revealing a higher degree of orientation of RGO nanosheets achieved by LbL assembly, and this can be responsible for the superior λX and λX/λZ (discussed below). The thermal conductivity was determined using a laser flash system. The laser flash system is introduced in detail in the Supporting Information.42 Figure 7 and Table S-1 summarize

Figure 7. λX and λZ of LbL-assembled (NFC/RGO)n hybrid film with different numbers of deposited cycles on a NFC substrate.

the thermal conductivity testing results. A fundamentally different behavior can be observed between the cross-plane and the in-plane direction of the films, even though only up to 40 bilayers are assembled onto the flexible substrate. For the inplane direction, one can see that the λX increased monotonically as the assembly of RGO nanosheets progresses. The λX increases from 1.1 W·m−1·K−1 for NFC substrate to 12.6 W· m−1·K−1 (enhanced by 910%). Considering the content of RGO in (NFC/RGO)40 hybrid film is only 1.0 wt %, the hybrid films show high efficiency for thermal conductivity improving. Compared to former studies on polymer/graphene nanocomposites, similar levels of thermal conductivity often depended on higher filler loadings, mostly several and even dozens of times more than the present study.10,43−47 Directly on the other end, one notices that the cross-plane thermal conductivity, λZ, is small for all samples with different assembly cycles. The λZ increases from 0.034 W·m−1·K−1 for (NFC/ RGO)0 film to only 0.042 W·m−1·K−1 for (NFC/RGO)40 film. The in-plane components of the thermal conductivity are significantly superior to those along the cross-plane direction. These interesting features impart a strong anisotropy in the thermal conductivities of the NFC/RGO hybrid films. 2928


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ACS Applied Materials & Interfaces Table 1. Anisotropy of Thermally Conductive Polymer-Based Nanocomposites in Previous Literature ref 58 59 60 10 13 61 62 63 64 55 65 44 66 67 56 68 19 69 57 26 this work 1

matrixa epoxy PCL epoxy epoxy epoxy epoxy PVDF epoxy PI epoxy CPE PA PS CNF foam PI PVDF PA PVA NFC NFC

fillerb

anisotropy (λX/λZ)c f

hBN BN nanosheets BN nanosheets multilayer graphene CNTs graphite nanoplatelet graphene nanoflake RGO graphene carbon nanosheets SWCNTs GNR RGO GO/SEP/BA hBN CNT/graphene RGO hBN exfoliated graphite nanoplatelets RGO RGO RGO

2 2 2.5f 4 2−5f ∼5 6 8 9 5−10 11 12 13d ∼14 ∼14d 23 26 ∼30 139 150 279 675

filler contente

preparation method

20 wt % 20 wt % 10 vol % 11.8 wt % 10−20 vol % 11 wt % 25 vol % not given 12 wt % 33 vol % ∼50 vol % 0.5 wt % 3 wt % 10 wt % 50 vol % 80 wt % 5 wt % 50 vol % 100% 30 wt % 1.0 wt % 100%

magnetic alignment hot pressing magnetic alignment polymer infiltration polymer infiltration hot pressing hot pressing polymer infiltration polymer infiltration solution casting vacuum filtration hot pressing hot pressing freeze casting spin casting solution casting hot pressing solution casting vacuum filtration vacuum filtration LbL assembly solution casting and annealing

a In this column: PCL = poly(caprolactone), PVDF = poly(vinylidene fluoride), CPE = conjugated polyelectrolytes, PA = polyamide, PS = polystyrene, CNF = cellulose nanofiber. bIn this column: SWCNTs = single-walled carbon nanotubes, GNR = graphene nanoribbon, GO/SEP/BA = GO/sepiolite nanorods/boric acid. cThe maximum anisotropy values in references, which were calculated based on the reported λX and λZ. dThe thermal diffusivities (α) were only reported in the literature. However, the anisotropy can be calculated from λX/λZ = αX/αZ, as λ is proportional to α: λ = α·Cp·ρ. eThe filler content of the composite with the highest thermal conductivity anisotropy in the literature. fThe thermally conductive anisotropy refer to λZ/λX due to the vertical alignment of the fillers in these works.

this regard, the LbL assembly might be a powerful technique in fabricating the flexible heat spreader. The thermal conductivity improvement of the composite is often accompanied by an increase of the electrical conductivity. Furthermore, knowing the electrical conductivity can highly enrich the hybrid film for practical applications in thermal management. For this reason, there is a need to experimentally investigate the electrical conductive property. The electrical conductivity of (NFC/RGO)40 hybrid film was measured to be 1.37 × 10−5 S·m−1 using the standard four-probe technique. Compared with prior similar works,71,72 the electrical conductivity is comparatively low (maybe due to the fact that the value is still below the electrical conductive percolation threshold) and slightly above the antistatic criterion of 10−6 S· m−1.73 Thus, the electrical conductivity should not be a serious barrier for the film’s application in thermal management of electronic devices. The different performances in electrical and thermal conductivities are due to the difference in the mechanism between thermal and electrical conductivities.10 It is worth reminding that graphene papers consist of graphene−graphene linkages displaying high brittleness and low tensile strength.70,74 The graphene−graphene linkages intrinsically anchored into the NFC matrix, however, become mechanically flexible and strong. The λX and λZ of (NFC/ RGO)40 are negligibly changed before and after bending for 500 cycles, and they changed in a narrow range from ±1 to ±0.010 W·m−1·K−1. The λX/λZ of (NFC/RGO)40 hybrid film is also comparatively steady (Figure 8a). From Figure S-8 (Supporting Information) one can notice that the sample maintained its shape completely after the bending test. Though the wrinkles of the RGO nanosheets on top-face increased

directional thermal transportation. It is worth noting that the pure graphene films always demonstrate relatively high anisotropy, which can be explained by the porous structures,1,57 as the “air pockets” hinder the cross-plane heat transportation greatly and meanwhile not influencing the in-plane thermal conductivity strongly. Despite the significant increased λX of (NFC/RGO)40 hybrid film with a low mass fraction of RGO (λX = 12.6 W·m−1·K−1 with RGO content of only 1.0 wt %), it is still far from the inherent performance of pristine graphene. This should be a common problem in carbon nanomaterial-based polymer nanocomposites and mostly results from the discontinuous interfaces of matrix/filler and lacking valid heat conduction paths in nanocomposites compared with the perfect sp2hybridized pristine graphene.43 A more representative comparison is the λX of hybrid film with that of RGO. In a recent study, the RGO films prepared from a filtrated GO film through hightemperature thermal treatment exhibited excellent thermally conductive properties. Especially when the temperature reached 1200 °C, the RGO film achieved an unusual thermal conductivity of 1043.5 W·m−1·K−1.70 The reason for the noticeable gap between the RGO film and the NFC/RGO film is that the phonon thermal transport in hybrid film is still limited by the varieties of phonon transmission channels as a result of the different graphene content of composites not only by the intrinsic properties of RGO naosheets. However, in the LbL assembly system the λX of hybrid film can be tailored through the number of deposition cycles and still have a huge possibility to increase with the increment of deposition cycles and meanwhile maintain the good mechanical properties. In 2929


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using the LbL assembly technique. Driven by hydrogen bonding interactions GO and NFC were sequentially deposed on a flexible NFC substrate. Afterward, the fully exfoliated GO nanosheets in hybrid films were reduced using hydrazine. The resulted (NFC/RGO)40 hybrid film had a RGO content of 1.0 wt %, which have a hierarchical structure in which the RGO nanosheets exhibit a high degree of planar orientation, leading to considerable different heat conduction performance along the in-plane and cross-plane directions. The λX can easily achieve 12.6 W·m−1·K−1 after 40 assembly cycles, while the λZ can hardly exceed 0.050 W·m−1·K−1. The anisotropy λX/λZ is up to 279, which is almost the highest reported to date for polymer-based materials, indicating that the LbL assembly technique on a flexible NFC substrate is an efficient method to prepare the anisotropic thermally conductive composites. Additionally, this combination of 2D graphene and 1D NFC nanostructures in a layered structure promotes high mechanical flexibility along with excellent tensile strength. Therefore, this study has developed an efficient approach to prepare the anisotropic thermally conductive nanocomposites with outstanding mechanical properties, which are promising for applications in thermal management.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b11979. Experimental Section (PDF)

Figure 8. (a) λX, λZ, and λX/λZ of (NFC/RGO)40 hybrid film under different numbers of bending cycles. (b) Stress−strain curves of NFC substrate and (NFC/RGO)40 hybrid film.

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AUTHOR INFORMATION

Corresponding Authors

somewhat, the layered structure of the thermally conductive coating was preserved well. Then the FFT results demonstrate that the orientation degree of RGO nanosheets in hybrid films after the bending test reduced slightly (Figure S-5). This can be the cause of the slight decrease of the λX. Nonetheless, the λX and λX/λZ are still on a relatively high level. Figure 8b demonstrates the stress−strain curves of NFC substrate and (NFC/RGO)40 hybrid films. The (NFC/RGO)40 shows a high tensile strength of 107 MPa, slightly below that for NFC substrate (142 MPa), indicating that the (NFC/ RGO)40 hybrid films prepared from LbL assembly retained the high strength characteristics of NFC. The outstanding flexibility and tensile strength are in all likelihood ascribed to the potent combination of the intrinsic flexibilities of the two building blocks RGO and NFC, as well as the hydrogen bonding interactions between them. After chemical reduction most oxygen-containing groups have been removed; in spite of this, the RGO nanosheets still possess a certain extent of oxygen coverage. This issue was further confirmed using XPS measurement (for details, see Supporting Information 10), and the oxygen content is measured to be 14.1%. These functional groups on the RGO make the hydrogen bonding still exist between NFC and RGO.32,75 Because of the structure superiority, exceptional thermal conductivity, and sound mechanical properties, the NFC/RGO hybrid film can be a high-promising candidate as anisotropic thermal conductors in modern electronics.

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Na Song: 0000-0002-3343-3000 Author Contributions †

N.S. and D.J.: These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (no. 51303101). The authors thank Prof. Yuliang Chu and Prof. Weijun Yu for help with the SEM and TEM measurement. The authors acknowledge Prof. Zhiying Zhang from Tianjin Polytechnic University for discussion about the experimental results.

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REFERENCES

(1) Renteria, J. D.; Ramirez, S.; Malekpour, H.; Alonso, B.; Centeno, A.; Zurutuza, A.; Cocemasov, A. I.; Nika, D. L.; Balandin, A. A. Strongly Anisotropic Thermal Conductivity of Free-Standing Reduced Graphene Oxide Films Annealed at High Temperature. Adv. Funct. Mater. 2015, 25 (29), 4664−4672. (2) Prasher, R. Thermal Interface Materials: Historical Perspective, Status, and Future Directions. Proc. IEEE 2006, 94 (8), 1571−1586. (3) Klemens, P.; Pedraza, D. Thermal Conductivity of Graphite in the Basal Plane. Carbon 1994, 32 (4), 735−741. (4) Wei, X. H.; Liu, L.; Zhang, J. X.; Shi, J. L.; Guo, Q. G. Mechanical, Electrical, Thermal Performances and Structure Characteristics of Flexible Graphite Sheets. J. Mater. Sci. 2010, 45 (9), 2449−2455.

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CONCLUSION In summary, the uniform anisotropic thermally conductive hybrid film with high orientation was successfully fabricated 2930


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Highly Anisotropic Thermal Conductivity of Layer-by-Layer Assembled

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