Scalable Approach to Construct Self-Assembled Graphene-Based

Oct 24, 2018 - Large-area bulk oxidized cellulose nanocrystal (OCNC)/graphene nanocomposites with highly oriented structures were produced through a ...
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Scalable Approach to Construct Self-Assembled Graphene-Based Films with An Ordered Structure for Thermal Management Hongxia Zeng,† Jingyi Wu,† Yupu Ma,‡ Yunsheng Ye,*,† Jingwei Liu,† Xiongwei Li,† Yong Wang,† Yonggui Liao,† Xiaobing Luo,‡ Xiaolin Xie,*,† and Yiu-Wing Mai†,§

ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 11/10/18. For personal use only.

†

School of Chemistry and Chemical Engineering and ‡School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, Hubei, P. R. China § Centre for Advanced Materials Technology (CAMT), School of Aerospace, Mechanical and Mechatronic Engineering J07, The University of Sydney, Sydney, New South Wales 2006, Australia S Supporting Information *

ABSTRACT: Large-area bulk oxidized cellulose nanocrystal (OCNC)/graphene nanocomposites with highly oriented structures were produced through a straightforward, costeffective large-scale evaporation-induced self-assembly process followed by thermal curing. Well-aligned nano-sized graphene layers were evident and separated by the OCNC planar layers, which facilitate highly interconnected and continuous thermal transport parallel to the alignment. Hence, the laminated graphene-based nanocomposites possess an excellent in-plane thermal conductivity of 25.66 W/m K and a thermal conductivity enhancement (η) of 7235% with only a 4.1 vol % graphene loading. This value is the highest recorded among all laminated composite films with 99.5%) were obtained from Nanjing XFNANO Materials Tech Co., Ltd. Microcrystalline cellulose, diglycidyl ether of bisphenol-A-based E-51 epoxy (DGEBA), and sodium hypochlorite solution (NaClO, active chlorine ≥5.2%) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. 2,2,6,6-Tetramethylpiperidine-1-oxyl (98%) was obtained from Sigma-Aldrich Co., Ltd. Trimethylolpropane triglycidyl ether (TGPAP, 98%) was supported by Adamas-Beta Co., Ltd. Sodium chlorite (NaClO2, 80%) and N,N-dimethyl formamide (DMF, +99.9%) were purchased from Aladdin Industrial Corporation. Graphene oxide (GO), reduced GO (rGO), and CNC were synthesized according to previously published reports.26 B

DOI: 10.1021/acsami.8b13808 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 2. (a) AFM, (b) SEM, and (c) TEM images of the rGO/OCNC hybrid; (d) TEM image of the GP/OCNC hybrid; cross-sectional morphology SEM images of (e) rGO/OCNCVAF, (f) rGO/OCNCEISA, (g) rGO/OCNC/AAEISA, and (h) GP/OCNC/AAEISA films after thermal treatment; (bottom) a schematic representation of the VAF and EISA fabrication of rGO/OCNC nanocomposites from rGO/OCNC hybrid solution. Insets in (e−h) represent fast Fourier transform (FFT) frequency domain images (the ratio of the scattering vector lengths between the major and minor axis was define as ΦH/V). Technical details, which can be found in the Supporting Information, are briefly summarized here. A total of 100 mL of 2 mg/mL weight ratio GO/OCNC aqueous solution was transferred to a Teflon-lined autoclave and heated at 180 °C for 6 h. After cooling, the resulting rGO/OCNC hybrid aqueous solution was transferred to DMF giving a final concentration of 20 mg/mL. To prepare the GP/ OCNC hybrid DMF solution, 250 mg of GP was dispersed in aqueous OCNC (5 mg/mL) with 1 h sonication followed by solvent exchange to DMF solution with a concentration of 20 mg/mL. The following procedure was used to obtain the EISA films: (a) a mixture of 15 mL rGO/OCNC (or GP/OCNC) in DMF solution (20 mg/ mL) and 75−1200 mg of AA (20−80 wt %) was stirred continuously for 1 h at room temperature; and (b) the mixture was then dropcasted on a Teflon mold and heated at 60 °C for 4 h, 120 °C for 2 h, and 150 °C for 2 h. 2.2. Characterization. The morphology of the hybrids was studied using Shimadzu SPM-9700 AFM and a scanning electron microscope (SEM) (Hitachi S-4700). (Transmission electron microscope) (TEM) was performed on a JEOL JEM-1200CX-II electron microscope at 15 kV. The thermal properties of EISA films incorporated with AA were measured using differential scanning calorimetry (DSC) (DuPont TA Instrument Q2000) from −80 to 260 °C. The TCs of the films were tested with a LFA 467 NanoFlash apparatus (NETZSCH) calculated by K = αρCP (Figure S11), where α, CP, and ρ are thermal diffusivity, specific heat capacity, and density, respectively, of the EISA film. CP was measured by DSC (TA DSC Q2000) with the sapphire method. The water contact angles ϕ were measured and captured on an optical contact-angle measurement device (JC2000C1, Dataphysics Instruments Shanghai Zhongchen Digital Technic Apparatus Co., Ltd). Tensile tests of EISA film (width × length × thickness = 8 mm × 30 mm × ∼90 μm) were conducted

on a CMT4104 universal testing machine (SANS, Shenzhen, China) with 100 N load cell at 25 °C with a cross-head rate of 2 mm/min. A commercial laser diode (LD) (L450P1600MM, Thorlabs) driven by a current controller (LDC220C, Thorlabs) was used to obtain a focused laser spot by a pair of adjustable collimated and focused lenses. The output optical power of the LD was calculated by multiplying the input electrical power Pel (=∼400 mA) by the wallplug efficiency ηwp. The heating power was considered equal to the output power assuming all the optical energy was absorbed by the samples.

3. RESULTS AND DISCUSSION Avoiding the inherent difficulties of retaining a good dispersion of chemically inert graphene nanosheets with increasing solution concentration during solvent evaporation is a prerequisite to successfully fabricating graphene/OCNC nanocomposites by EISA. We first prepared highly water-dispersible thermal conductive rGO sheets and graphene nanoplates through a simple and green approach outlined in Figure 1, according to the method detailed in our previous study (see Experimental Section). Figure 2a−c shows (AFM), SEM and TEM images of rGO/OCNC hybrids, respectively. It can be seen that the compact spindle-like OCNCs are evenly decorated on exfoliated rGO sheets leading to an integrated network of hybrid materials with a sandwich-like structure. The AFM profile of the graphene sheet covered with OCNCs shows a 99.5% determined by elemental analysis), and hence stronger H-bonding interactions are formed at the rGO/ OCNC interface compared to the GP/OCNC interface. However, both hybrid samples stabilized by OCNCs are still readily dispersed in water and polar solvents by the solvent exchange process [Figure 1c], especially for the rGO/OCNC hybrid with a sandwich-like structure, which possesses a high rGO concentration (>2.5 mg mL−1). For fabrication of the aligned nanocomposites, the phase behavior of rGO/OCNC hybrid dispersion was first tested by drop-casting the mixture on a glass plate, allowing the resulting films to be examined by polarized optical microscopy (POM). For easy observation, the rGO concentrations (rGO/OCNC = 1:60−1:20) used in the mixture were lower than those used for the fabrication of the composites (rGO/OCNC = 1:10). As evaporation proceeded, bright regions belonging to the OCNC liquid-crystal phase (large-scale assembly) were to be seen at a ratio of rGO/OCNC = 1:60such patterns remained fixed throughout the remainder of the film drying process (see Figure S1), indicating that the self-assembly of OCNC can be triggered at the start of the evaporation process. This observed liquid crystal transition agrees well with those previously observed for pure CNC suspensions, which can be triggered at CNC concentrations of ∼3.5 wt % in the drying process.31 However, because of the low optical transmittance of graphene-based materials, increasing rGO concentrations in the mixture leads to a rather indistinct OCNC liquid crystal phase in the resulting rGO/OCNC films [see Figure S1b−f]; however, the bright regions corresponding to the OCNC liquid crystal phase can still be observed during drying. This suggests that the self-assembly of OCNCs is sensitive to changes of OCNC concentrations during the drying process, which with OCNCs as a host medium induces self-coassembly in nanomaterials. Self-assembled, aligned nanocomposites, or papers have been previously fabricated by using the VAF method, this approach was also used to prepare rGO/OCNC nanocomposites with oriented structures in this study. Filtrating the diluted rGO/OCNC DMF suspension (1 mg/mL) under vacuum, leads to the formation of well-aligned rGO/OCNC papers [denoted as rGO/OCNCVAF (Table S1)] by accumulating individual graphene sheets from bottom to top. In Figure 2e, it can be seen that graphene layers are alternately stacked, showing a highly aligned layered structure. This is consistent with our expectation that highly dispersed graphene sheets, stabilized by OCNCs, will tightly stack together and align. Subsequently, a rGO/OCNC mixture with a higher content of 20 mg/mL was directly drop-cast on a Teflon mold to fabricate a rGO/OCNC film (denoted as rGO/OCNCEISA) by a solvent evaporation process. A long-range aligned layered arrangement corresponding to the rGO/OCNC oriented parallel and stacked perpendicular to the planar surface was found in the final rGO/OCNCEISA film cross-section, confirming that the graphene layers were densely packed [Figure 2f]. We propose two schematics (bottom, Figure 2) to indicate self-alignment of the rGO/OCNC mixture, resulting from VAF and EISA processes. For VAF, the filtration through the PTFE membrane is quickly hindered by the deposition of the D

DOI: 10.1021/acsami.8b13808 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 3. (a) Stress−strain curves of rGO/OCNC/AAEISA films with different AA contents (inset shows a schematic depicting the formation of hybrid hydrogen and covalent bonding network in resultant films. The black sheets represent to the rGO and yellow needle-like sticks represent OCNC. Hydrogen and covalent bonding are highlighted in red dashed and blue lines, respectively). (b) KII and η of rGO/OCNC/AAEISA and GP/ OCNC/AAEISA films (TC of stainless steel and bronze are 17.6 and 25.7 W/m K, respectively). (c,d) Comparisons of η/vol % and KII values among laminated nanocomposite films/papers obtained via several different techniques with red stars representing results in the present study. Detail data are listed in Table S4 in the Supporting Information.

showing highly aligned structures in Figures 2h and S7, indicating that such methods would be applicable to other 2D nanomaterials. By using EISA followed by a thermally induced curing process to fabricate graphene-based nanocomposites with a highly oriented structure, we can obtain samples adjusted to a variety of shapes and sizes for particular applications including large-area thin films (10 cm wide, 130 μm thick) or bulk solids (2 cm wide, 4 mm thick), dimensions that are rarely achieved by standard methods (Figure S8). These well-aligned films obtained via VAF or EISA without AA are damaged in aqueous solutions, such as water; base and acid solutions (Figure S9), and frequently disintegrate with stirring. However, both rGO/ OCNC/AAEISA and GP/OCNC/AAEISA films incorporated with 20 wt % AA are extremely stable in water, base, and acid solutions, even after being soaked for more than 1 month statically, or under intense stirring. Moreover, water contact angle tests showed that the surface is more hydrophobic (ϕ = 78°−87°) when the films incorporate AA when compared to the rGO/OCNC film (ϕ = 41°) (Figure S10). The stress− strain curves of these EISA films [Figure 3a] also show that the well-aligned EISA film with 20 wt % AA has a high mechanical strength (116.5 MPa) and a high stiffness (i.e., Young’s modulus, E) of 3.56 GPa, the values that are much higher than those reported for other TMMs.34 This is mainly attributed to both the alignment of the graphene sheets and the hybrid hydrogen and covalent bonding network formed between stacked graphene sheets maximizing strength along the parallel axis. When pull forces are parallel to the orientation of rGO, OCNC and rGO could cooperatively take the tensile load. Moreover, because of the ultrahigh mechanical strength of

Figures S5 and 2g show the cross-sectional morphology of the rGO/OCNC/AAEISA films with different AA contents before and after thermal treatment. When the rGO/OCNC film is incorporated with 20 wt % AA, the aligned structure remains almost unchanged after thermal treatment [Figure S5a,e]; however, they are replaced with partially oriented structures when the AA content increases from 60 to 80 wt % [Figure S5c,d]. Before and after thermal treatment, the morphologies of the rGO/OCNC/AAEISA films were similar. To determine the orientation of the graphene nano-sheets within these films, cross-sectional TEM images of rGO/ OCNC/AAEISA films [see Figure S6a−d] incorporated with 20−80 wt % AA were obtained. Visible graphene stacks (30 W/m K). Table S5, films (code 1 and code 7) show the best KII values so far reported for GP- (178 W/m K) and BN-based composites (145.7 W/m K obtained by the steady state method) with layered architectures prepared by filtrating GP/PEI and BN/NFC mixture solution to form VAF papers, respectively. Interestingly, larger AI values are found in the composite films prepared by VAF compared to other techniques, such as hot pressing, magnetic alignment, electrospinning, and solvent highest TC on record among all laminated composite films with 100 with only a slight increase in the filler content, indicating that a highly ordered in-plane arrangement of the thermally conductive fillers leads to materials with ultra-high KII values. Figure 3b shows the KII values and the TCE [defined as η = (KII − Km)/ Km, where Km is the thermal conductivity of the matrix] as a function of the volume fraction for the rGO- and GP-based EISA films. The result shows that the ultra-high KII of 20.2 and 25.7 W/m K values can be obtained with only 4.1 vol % (7.3 wt %) rGO and GP loading, respectively, which is close to bronze (26.2 W/m K) and much higher than stainless steel (16.7 W/m K). Moreover, there is an extraordinary increase in η for both EISA films, which reaches the maximum value of 7234% for the EISA film made with only a 4.1 vol % GP loading. Such a value is ∼42 and ∼30 folds greater than those composites incorporated with 50 vol % conventional fillers such as alumina or silver and 7 vol % CNT, respectively.10,35 To examine the efficiency of the thermal conductive fillers in our EISA films, the TCE per 1 vol % filler (η/vol %) was calculated and the results compared with other reported laminated composite films/papers (see Figure 3c and Table S5). The data show that the η/vol % of our rGO- and GPbased EISA films are ∼1400 and ∼1800%, respectively, which are much higher than most laminated composite films/papers with graphene or BN. To put our work in perspective with respect to published studies of KII of laminated composite films/papers classified by their processing techniques, such as F

DOI: 10.1021/acsami.8b13808 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. Schematic of the thermal test setup, finite-element analysis of surface temperature variations with time and demonstration of thermal transport properties of EISA films with randomly dispersed graphene sheets and highly oriented graphene sheets.

TBR (RB) as 6.3 × 10−7 and 3.3 × 10−6 Km2/W, respectively. Although GP has a higher intrinsic thermal conductivity than rGO, the much stronger H-bonding interactions formed between rGO and the OCNC matrix lead to a lower TBR. We used the EMT to extract the theoretical KII (KII‑th) values of rGO- and GP-based EISA films versus θ calculated based on these obtained RB values (Figure 4a). It was found that only a small enhancement in KII‑th (η < 90%) is obtained when the filler loading is increased from 1.0 to 4.1 vol %. However, an ultra-high KII‑th (>11 W/m K) and large η increases of >400 and 650%, respectively, can be attained in the in-plane direction at θ < 25° and 10° with the same filler loading. Hence, for a composite with oriented nano-fillers (i.e., θ < 25°), better heat transfer is expected owing to the formation of continuous thermal conductive paths, thereby achieving a higher KII value (Figure 4b). For our laminated rGO- and GPbased EISA films, average θ values were taken at 10° and 13° from detailed measurements of θ in both the TEM and SEM images [Figures 4c and 2h] with >30 graphene-stacked layers: the results were calculated by the EMT model and designated as orange and green solid circles, respectively, in Figure 4a. It is observed that the EMT predicted KII‑th for both EISA films falls below the experimental KII (KII‑ex) at the same θ, implying that the thermal resistance is not just determined by the filler− matrix interfacial thermal resistance, which is the basic assumption for the EMT model. Comparing the influence of the filler−matrix and filler−filler thermal resistances, we found that heat transfer via the filler−filler interfaces plays a more important role in thermal conduction. In fact, filler−filler overlaps were seen in the TEM images of rGO/OCNC/AAEISA films. Thus, to account for the filler-overlaps of rGO or GP layers in both EISA films, we recalculated the contact resistance (RC) by: RC = (K0LVβC)−1, where K0 is the thermal conductivity of the graphene network, L is the length of graphene, VC is the critical volume fraction of graphene, and β is a critical exponent.37,38 The best fit of this model for our rGO- and GP-based EISA films gives RC values of 7.1 × 10−9 and 2.8 × 10−9 Km2/W, respectively. The relatively intact or complete surface structure of the GP is believed to have lowered the RC of the EISA film and decreased energy dissipation through filler−filler interfaces. We note that the

obtained RC value is more than 2 orders of magnitude lower than RB, which indicates that the filler−filler thermal resistance plays a critical role in determining KII in our EISA films. This result helps explain why our EISA films possess higher TCs compared to the EMT predictions. To understand the significantly enhanced KII in our EISA films, heat transfer mechanisms in the nanofillers, with various orientations are illustrated in Figure 4d. The random distribution of graphene sheets presents tortuous thermal transfer paths in the composites with large phonons scattered through the filler− matrix interfaces. Although alignment of these graphene sheets along the thermal transport direction to create continuous thermal transfer paths will dramatically improve this interface problem, the mean inter-filler distance is still high in laminated composites at low filler loadings. Hence, heat diffuses slowly and discordantly through these interfaces accompanied by extra phonon scattering. In our case, the graphene sheets align themselves along the OCNC planar domains into a long-range ordered structure to build a continuous heat transfer path and reduce the inter-filler distance. Thus, the heat transfer rate is easily increased by these “freeway-like” heat transfer paths to achieve ultra-high TCs at low graphene loading. To demonstrate the thermal management application of the EISA nanocomposite films, the heat dissipation properties were examined by heating the sample using a LD source with time (1−100 s). Their surface temperature variations with time during the process were also simulated by a finite-element method (FEM), based on the transient heat conduction equation: ρCP∂T/∂t = λ(∂2T/∂x2 + ∂2T/∂y2 + ∂2T/∂z2) + Q, where T and t are temperature and time, respectively. Other input parameters included: ρ (kg/m3), CP, (J/kg K), K (W/m K), and Q (W/m3), which are density, specific capacity, thermal conductivity, and heat power density, respectively (Table S4). All boundary conditions used in the FEM study were the same as those in the experiments. Figure 5 shows that the samples of the rGO/OCNC/AAEISA films with 1.0 and 4.1 vol % graphene were vertically placed on the homemade heat sink. As shown in simulated thermal images, during exposure to a focused laser spot (0.2 W), the laminated rGO/OCNC/ AAEISA film (with 4.1 vol % rGO) shows a more homogeneous thermal distribution with time compared to the film with (1.0 G

DOI: 10.1021/acsami.8b13808 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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vol %) randomly dispersed graphene. It was observed that after 100 s, there was a hole in the rGO/OCNC/AAEISA (1.0 vol %) graphene sample center caused by laser heating, but the laminated (4.1 vol %) graphene film remained stable even after a few minutes because of laminated EISA film’s ultra-high KII, conferring a much better thermal dissipation capability, benefiting heat dissipation along the graphene alignment direction (Figure 5). By contrast, in the former EISA film with 1.0 vol % randomly distributed graphene nanosheets, insufficient heat dissipation in the in-plane direction leads to localized heat accumulation in the sample’s center. The simulated results are consistent with our experimental observations. That is, the maximum temperature at the center of the laminated EISA film with 4.1 vol % graphene (107 °C) is much lower than that with 1 vol % graphene (215 °C). These notable heat dissipation properties offer a potential route to creating better performing TMMs.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.Y.). *E-mail: [email protected] (X.X.). ORCID

Yunsheng Ye: 0000-0002-2351-1845 Yong Wang: 0000-0003-4704-5436 Yonggui Liao: 0000-0003-2943-1501 Xiaobing Luo: 0000-0002-6423-9868 Xiaolin Xie: 0000-0001-5097-7416 Author Contributions

The manuscript was written through contributions of all authors. All the authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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4. CONCLUSIONS In summary, we have synthesized the first large-area bulk laminated graphene-based nanocomposites with high thermal stability, good mechanical properties, and ultra-high thermal conductivity (>20 W/m K) with low filler contents (4.1 vol %). The experimental design proposed here utilizes 1D OCNCs as a host matrix to coerce the guest 2D graphene sheets self-coassembled into a long-range ordered structure through a cost-effective and straightforward large-scale EISA followed by a thermally induced curing process. This method eliminates the need to use current aligned structure TMM fabrication techniques which are not amenable to scaling for bulk production. The uniquely oriented structure constructed by alternating nano-sized filler-rich and OCNC planar layers promotes fast and uniform heat transfer through “freeway-like” heat transfer paths, yielding ultra-high KII values that cannot be achieved in previously reported laminated composite films at low filler loadings. We believe our new approach can be applied to other large-area bulk nanomaterial systems for different applications.

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Research Article

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51673076 and 51433002). We acknowledge the analytical and testing assistance provided by the Analytical and Testing Center of Huazhong University of Science and Technology.

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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.8b13808. Technical details; statement of samples with different AA mass contents; thermal properties of OCNC and rGO/OCNC films; dynamic curing and kinetic analysis for EISA films; TCs of composite films with different AA contents; comparison of TCs in laminated nanocomposite films/papers; POM images; digital photos of EISA films before thermal treatment; thermal properties and kinetic analysis of EISA films; morphologies of rGO/OCNC/AAEISA films; cross-sectional morphology SEM images of BNNS/OCNC/AAEISA and MoS2/OCNC/AAEISA; large-area bulk thermal conductors; stability of EISA composites films; contact angle of a water drop on samples; thermal conductivity test model; schematic diagram showing the in-plane KII and through-thickness K⊥ TCs of samples; crosssectional morphology SEM image of GP/OCNC/AAEISA 80 wt % AA, and cross-sectional morphology TEM image of rGO/OCNC/AAEISA 80 wt % AA (PDF) H

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DOI: 10.1021/acsami.8b13808 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX