Tuning the Oxidation Degree of Graphite toward Highly Thermally

Article Cite This: Chem. Mater. 2018, 30, 7473−7483

pubs.acs.org/cm

Tuning the Oxidation Degree of Graphite toward Highly Thermally Conductive Graphite/Epoxy Composites Wen Sun,†,‡,§ Lida Wang,† Zhengqing Yang,† Tianzhen Zhu,† Tingting Wu,†,∥ Chuang Dong,‡ and Guichang Liu*,† †

School of Chemical Engineering, ‡Key Lab for Materials Modification by Laser, Ion and Electron Beams of Education Ministry, and State Key Lab of Fine Chemicals, Carbon Research Laboratory, Centre for Nano Materials and Science, Dalian University of Technology, 2 Linggong Road, Dalian 116024, People’s Republic of China § Material Corrosion and Protection Key Laboratory of Sichuan Province, Sichuan University of Science & Engineering, Zigong 643099, People’s Republic of China Chem. Mater. 2018.30:7473-7483. Downloaded from pubs.acs.org by UNIV PARIS-SUD on 11/13/18. For personal use only.

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ABSTRACT: Efficient removal of heat is one of the most challenging issues in the development of modern electronic devices. However, traditional thermal interface materials (TIMs) may exhibit limited thermal conductivity when loaded with thermally conductive fillers at high contents due to irrational filler/polymer structures. In this work, graphite oxide (GrO) with different oxidation degrees was prepared by a controlled Hummers method and used as fillers for the enhancement of thermal conductivity of epoxy. Our experiment revealed that the thermal conductivity of the obtained composites greatly depends on the oxidation degree of GrO. Furthermore, GrO with a mild oxidation degree exhibits edgefunctionalization characteristics and possesses a strong ability to enhance the thermal conductivity of epoxy because edge-functionalized GrO is able to provide a preferential conducting pathway for heat transfer in the epoxy matrix. This work provides new insight into the preparation of high-performance TIMs for microelectronic packaging.

1. INTRODUCTION The continuous downscaling of modern devices with high power densities have caused escalating hot-spot temperature, making efficient heat dissipation a crucial issue for the high performance and reliability of numerous systems including computers, light-emitting diodes, communication devices, energy storage elements, automotive and aerospace products, etc.1−3 In order to address the heat dissipation problem, developing high-performance TIMs has aroused worldwide attention because TIMs with high thermal conductivity can facilitate heat transfer across interfaces by reducing the thermal resistance between heat sources and heat sinks.4−6 To date , it is generally believed that polymeric nanocomposites (PNCs) would be the most promising TIMs candidates.7,8 However, given that most polymers have a low thermal conductivity of around 0.2 W/(m·K), preparing highly thermal-conductive PNCs needs to embed thermal-conductive fillers into polymer matrix.9,10 At present, choosing highly thermal-conductive fillers and increasing the volume fraction of fillers are two of the most conventional methods to achieve high-performance PNCs. However, it is reported that PNCs filled with a high-volume fraction of fillers (>50%) may have a poor integrity, degraded mechanical property, bad barrier performance, and poor processing properties.11−13 In addition, © 2018 American Chemical Society

utilizing ultrahighly thermal-conductive nanomaterials (for example, carbon nanotubes, graphene, AlN, etc.) as fillers in PNCs have not led to wide practical applications due to their prohibitive cost or difficulty in scalable production.7,14,15 Therefore, developing methods that can achieve PNCs with a thermal conductivity as high as possible by reinforcing conventional polymer with cheap thermal-conductive particles at a low volume fraction has a fatal research significance. The structure of PNCs decides their nature. Highly thermalconductive PNCs can be prepared by fine tuning the structure of the polymer matrix (including molecular structure, chain length, chain stiffness, arrangement, crystallinity, intra/ interchain interactions),16−23 filler (including material composition, shape, size, morphology, dispersion, volume fraction, orientation, continuous three-dimensional network),24−31 and filler/polymer interface (e.g., interaction between fillers and polymer matrix).32−34 To date, there are numerous quantitative rules about how the structure of the polymer matrix or filler affects the thermal conductivity of PNCs; however, there are relatively few papers that report the influence of the Received: May 8, 2018 Revised: October 15, 2018 Published: October 16, 2018 7473

DOI: 10.1021/acs.chemmater.8b01902 Chem. Mater. 2018, 30, 7473−7483

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Chemistry of Materials

Figure 1. Optical microphotographs of (a) pristine graphite, (b) GrO0, (c) GrO0.25, (d) GrO0.5, (e) GrO0.75, (f) GrO1.0, (g) GrO1.25, (h) GrO1.5, and (i) GrO1.75.

structure of the filler/polymer interface on the PNCs thermal conductivity. In addition, it has now been widely accepted that increasing the interactions between fillers and polymer matrix can help to improve the thermal conductivity of PNCs; however, such an understanding is proposed to be rather ambiguous. For example, it has been revealed that the increase of filler/polymer interactions may not always lead to improved thermal conductivity because it also may lead to the formation of defects, which impede the acoustic phonon transport in the fillers and can further reduce thermal conductivity. Therefore, it can be inferred that the filler/polymer interaction has an optimum degree. Furthermore, it should be noted that the structure of the filler/polymer interface on the thermal conductivity of PNCs needs more quantitative and systematic investigations when taking into consideration filler/polymer contact area, polymer crystallization, interaction type (e.g., chemical bond, hydrogen bond, π−π stacking, and van der Waals force), interaction density, interaction location, and interaction strength in filler/polymer interface as component parts of the interface structure.35−43 Although more and more computational studies have revealed that the filler/polymer interface structures greatly affect the thermal conductivity of PNCs, to our best knowledge, there are still very few experimental papers on how to prepare PNCs with high thermal conductivity through precisely tuning the structure of fillers/polymer interfaces. In addition, pathways for heat transfer in the polymer matrix are another critical issue for the thermal conductivity improvement of PNCs.7 It has been demonstrated that PNCs with a 3D graphene network for heat transfer exhibit a thermal conductivity of 2.13 W/(m·K) at a graphene loading of 0.92 vol %.3 Therefore, it is inferred that the combination of fine-designed filler/polymer interface structures and optimized pathways for heat transfer in polymer matrix would give rise to high-performance TIMs.

Herein, graphite/epoxy composites were prepared by tuning the oxidation degree and the microstructure of graphite. Organosilicon functionalization of fillers was used in this work because it is a cheap and easy method that helps to increase the interaction between fillers and polymer matrix. Specifically, the preparation of graphite/epoxy composites with featured covalent interactions between natural graphite (far below the thresholds) and epoxy resin matrix was achieved by carefully regulating the oxidation and silicon modification of the graphite. Our experimental results revealed that the thermal conductivity of graphite/epoxy composites greatly depends on the oxidation degree of graphite. Additionally, edge-functionalized graphite prepared by mildly oxidizing graphite provides a greater thermal conductivity enhancement when embedded into epoxy as compared to raw graphite and overoxidized graphite. These results would offer new inspirations to the design and fabrication of highly thermal-conductive PNCs.

2. RESULTS AND DISCUSSION Graphite oxide (GrO) fillers with eight different degrees of oxidation were synthesized via a modified Hummers method by adjusting the mass ratio of KMnO4/graphite used in the oxidation reaction. The as-prepared GrO fillers were denoted as GrO0, GrO0.25, GrO0.5, GrO0.75, GrO1.0, GrO1.25, GrO1.5, and GrO1.75, where the subscript number denotes the KMnO4/ graphite ratio. It should be noted that the oxidation treatment is not independently associated with the functional groups on the GrO surface but also affects the size and thickness of GrO, defects within the GrO layers, etc.,44−46 which would further influence the thermal conductivity of the resultant GrO/epoxy composites. Therefore, analyzing the variations of microstructure and oxygen-containing groups of GrO after controlled oxidation treatment is necessary for preparing PNCs with high thermal conductivity. 7474


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Chemistry of Materials A metallographic microscope was used to roughly observe the microstructure of graphite. Figure 1 shows the optical microphotographs of the GrO samples. It is revealed that the microstructure of GrO is closely associated with its oxidation degree. As shown in Figure 1a−c, the pristine graphite, GrO0, and GrO0.25 samples are very similar in appearance, which indicates that H2SO4 or H2SO4 with a low concentration of KMnO4 has a relatively weak ability to change the microstructure of graphite. Increasing the utilization of KMnO4, the edge of graphite become blue and purple, which is believed to be associated with the diffusion and oxidation of KMnO4 at the edge of GrO0.5 (Figure 1d). Further increasing the KMnO4 concentration, the whole GrO0.75 flake become purple, blue, and green, which is supposed to be related to the diffusion and oxidation of KMnO4 within graphite interlayer galleries. In addition, it can be clearly observed that large-size graphite flakes are exfoliated into small-size GrO flakes (Figure S1). Some exfoliated GrO flakes are yellow, indicating that they are strongly oxidized. When the concentration of KMnO4 is higher enough, the graphite flakes gradually become yellow from edge to center (Figure 1f−h). In particular, for GrO1.25−1.75, GrO could even be exfoliated into few-layer graphene oxide nanosheets (GO) (Figure 1i). X-ray diffraction (XRD) was employed for further studying the microstructure of the GrO samples. As shown in Figure 2, the XRD pattern of pristine graphite shows a diffraction peak at 2θ = 26.5°, which corresponds to an interlayer spacing of about 0.34 nm. The XRD patterns of all of the GrO samples clearly show that the intensity of all of the peaks of raw graphite start to decrease with the increase of oxidation levels and finally disappears at high oxidation levels. Simultaneously, a new peak gradually appears and finally disappears around 2θ = 13.0°. The XRD pattern reveals that significant changes in the crystallinity of GrO samples occurred during oxidation processes, especially at the edge of graphite according to previous papers.44 Comparing with pristine graphite, peak broadening can be seen around 2θ = 26.5° in the XRD pattern of GrO0, GrO0.25, GrO0.5, GrO0.75, GrO1.0, and GrO1.25, which indicates that lattice distortion occurs in the AB stacking order of the graphite lattice due to mild oxidation.47 Increasing the quantity of KMnO4, the new broad peak appearing at around 2θ = 13.0° shows an increasing intensity and slowly shifts to a low angles, which indicates that graphite domains are gradually turning into oxidized graphite and interlayer spacing in oxidized graphite is becoming lager with the increasing oxidation levels. When using a KMnO4/graphite ratio of 0.25−1.25, the interlayer spacing in GrO was calculated to be about 6.81, 6.89, 6.99, 7.13, and 7.37 Å, respectively. However, for GrO1.5 and GrO1.75, obviously amorphous feature can be observed in their XRD patterns because the formation of massive oxygenated groups in strong oxidation conditions signifies the increase in interlayer spacing and exfoliation of GrO into few-layer graphene oxide. The XRD patterns indicate that the existence of a heterogeneous nature of the GrO samples consists of both sp2 domains from graphite and the sp3 domains from oxidized graphite. After washing with distilled water until the wash was neutral and centrifuging at 10 000 rpm for 60 min, the apparent volume (Vp) of GrO was also measured (inset of Figure 2). It is revealed that with the increase of oxidation degree the Vp of mildly oxidized GrO is almost the same (prepared with a KMnO4/graphite ratio of 0−0.5), whereas the Vp of strongly oxidized increases sharply to ∼40 mL (prepared with a

Figure 2. XRD patterns of pristine graphite and the GrO samples. (Inset) Apparent volume and photograph of the as-prepared GrO after repeatedly washing with distilled water until the last wash was neutral (centrifuged at 10 000 rpm for 60 min).

KMnO4/graphite ratio of 0.75−1.75). Therefore, it can be inferred that the increase of interlayer spacing in mildly oxidized GrO is not significant and mainly occurs at the graphite edge while that in strongly oxidized GrO is rather obvious and occurs at both the edge and the domain of graphite. The photographs of all of the samples without washing are presented in Figure 3a. With the increase of KMnO4 quantity, a gradual change in color from black into yellowish brown was observed because the samples had a higher level of oxygenated functional groups when graphite was exposed to a stronger oxidation conduction. Fourier transform infrared (FT-IR) spectroscopy was employed to study the types of functional groups formed on the surface of GrO at different degrees of

Figure 3. (a) Photographs of the graphite oxides, and (b) FT-IR spectra of pristine graphite and graphite oxides prepared by adjusting the mass ratio of KMnO4/graphite. 7475


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Figure 4. C 1s spectra of (a) pristine graphite, (b) GrO0, (c) GrO0.25, (d) GrO0.5, (e) GrO0.75, (f) GrO1.0, (g) GrO1.25, (h) GrO1.5, and (i) GrO1.75.

graphite gradually decreases and the peaks of C−C (sp3 carbon bonds) and the oxygenated functional group become more and more obvious. The oxygenated functional groups are composed of hydroxyl (C−OH), epoxyl (C−O−C), carbonyl (CO), and carboxyl (−COOH) groups, which are formed due to oxidation of graphite. The XPS spectra further reveal that the sp2/sp3 ratio decreases with the increases of oxidation level. The sp2/sp3 ratio measured from the C 1s XPS spectra of GrO0, GrO0.25, GrO0.5, GrO0.75, GrO1.0, GrO1.25, GrO1.5, and GrO1.75 is found to be 22.66, 17.54, 5.03, 3.03, 2.05, 0.92, 0.75, and 0.65, respectively. Furthermore, as shown in Figure 5, the deconvolution results of the C 1s peaks from XPS spectra shows that with the increases of oxidation levels, the relative percentage of carbon atoms in C−O−C bonds obviously increases, that in C−OH bonds tends to increase slowly or even decrease, while that in CO and −COOH bonds keeps fluctuating in a low content range. The reason why C−OH and −COOH bonds increase slightly is that, on further oxidation, the C−OH and −COOH groups can lead to the formation of the C−O−C groups. In addition, it has been reported that, on further oxidation, the C−OH and −COOH groups are converted to C−O−C groups, which would further result in the increasing of interlayer spacing in GrO and the exfoliation of GrO in to small-size flakes, as proposed by the results of XRD (Figure 2) and optical microphotographs (Figure 1). Dimiev and Tour demonstrated that the oxidation of graphite goes through three stages.44 The first stage is the

oxidation. The FT-IR spectra of pristine graphite and all GrO samples are presented in Figure 3b. The FT-IR spectra revealed the formation of oxygenated functional groups in graphite. The FT-IR spectra of pristine graphite, GrO0, and GrO0.25 have the same characteristic peaks, of which the bands at 3428 and 1380 cm−1 corresponding to O−H stretching and bending vibrations from hydroxyl groups (water molecular and carboxyl and hydroxyl groups), the weak peak at 1721 cm−1 originating from CO stretching vibration of carboxyl groups, and the strong peak at 1632 cm−1 can be ascribed to the presence of CC stretching in graphitic domains. The FT-IR spectra reveal that there are oxygen groups in raw graphite. With further increases in oxidation level, the FT-IR spectra reveal the presence of C−O−C stretching vibration at 1244 cm−1 and the C−OH stretching vibration peak at 1077 cm−1, indicating the occurrence of graphite oxidation. X-ray photoelectron spectroscopy (XPS) was also used to analyze the components of functional groups in the asprepared GrO. The C 1s XPS spectra of pristine graphite and the GrO samples are shown in Figure 4. In the C 1s XPS spectra of pristine graphite, a strong peak corresponding to C− C/CC bonds at 284.7 eV and several feature peaks of oxygen-containing groups can be observed, indicating the presence of a few oxygenated functional groups (∼5 atom %) in raw graphite. With the increase of oxidation degree, the intensity of the CC peak (sp2 carbon bonds) and π−π* (πelectrons delocalized at the aromatic network in graphite) in 7476


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diffuse into the graphite interlayers and oxidize graphite more easily. It should be noted that the diffusion and oxidation of oxidizing agents would stop when KMnO4 is consumed. Therefore, if KMnO4 mass is not enough to oxidize graphite completely, pristine GrO with an oxidized outside and an unoxidized inside could be prepared. In addition, due to the formation of large amounts of oxygen-containing groups, small-size pristine GrO debris could be exfoliated from largesize pristine GrO under agitation, resulting in the lateral size and thickness of GrO tending to decrease with the increase of oxidation degree (Figures S2−S4). After adding H2O and raising the temperature to 90 °C, several processes occur simultaneously, including the exfoliation of GrO, additional modification of oxygen groups caused by reaction with water, further oxidation of GrO due to reaction with MnO4−, etc.44−51 Finally, during washing, shear force caused by centrifugation, stirring, and shaking can exfoliate the GrO layers while maintaining their lateral size.52 As shown in Figure 6, when KMnO4 is not enough for complete oxidation of graphite, it is possible to obtain GrO nanosheets with an edge functionalized by oxygen-containing groups, which is called edge-functionalized GrO (EGrO) in this work. To verify the presence of EGrO, the GrO samples were characterized by Raman spectroscopy. It has been demonstrated that sp3 carbon atoms of defects and disorders in graphite or GrO induce a D band in the Raman spectra and the in-plane E2g vibrational modes of sp2 carbon atoms induce a G band, Figure 7; therefore, the intensity ratio between the D band and the G band (D/G ratio) is an indication of the disorder degree in graphene layer of GrO. It has been revealed that GrO has a low ID/IG ratio (0.065) at the basal plane and a slightly higher ID/IG ratio (1.012) at the edge, which is completely different from that of pristine graphite (0.077 at the basal plane and 0.104 at the edge) and overoxidized graphite (1.018 at the basal plane and 1.003 at the edge) (Figure 2a−c). The Raman spectra indicate that graphite is edge selectively oxidized under suitable mild oxidation conditions, forming EGrO.

Figure 5. Percentage of carbon atoms in different bonding structures obtained from deconvoluted C 1s spectra.

conversion of graphite into H2SO4−graphite intercalation compounds (H2SO4−GICs). The second stage is the oxidizing agent-diffusion-controlled oxidation of H2SO4−GICs, which forms “pristine graphite oxide” (PGO). The third stage is the conversion of PGO into graphene oxide by the interaction between PGO and water. Kang and Park et al. proposed that the oxidation of graphite after the addition of water is only an edge-selective process.45,46 In addition, through comprehensively analyzing the results of the optical microscope, XRD, FT-IR, and XPS, the oxidation mechanism of graphite through a modified Hummers method is proposed and illustrated in Figure 6. First, H2SO4/HSO4− intercalates into graphite interlayers, forming H2SO4−GICs with augmented interlayer spacing in a short time at 0 °C. Then diffusion of oxidizing agents into the spaced graphite interlayers and oxidization of graphite occurs successively. Both the intercalation of H2SO4/ HSO4− and the diffusion and graphite oxidation of oxidizing agents occur preferentially at the edge and defects of graphite. When the temperature rises to 35 °C, the oxidizing agents can

Figure 6. Schematics of the mild oxidation mechanism of graphite into GrO and GO. 7477


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Figure 7. Raman spectra of (a) graphite, (b) GrO0.5, and (c) GrO1.25, and 2D mapping images (×100) for (d) graphite, (e) mildly oxidized GrO, and (f) strongly oxidized GrO.

Figure 8. Schematic illustration of preparing the GrO/epoxy composites.

In order to establish covalent interactions between filler and polymer in GrO/epoxy composites, the obtained GrO samples are modified with (3-aminopropyl) triethoxysilane (APTES). The schematic diagram for the preparation of GrO/epoxy composites is presented in Figure 8. XPS was also employed to survey the elemental composition of pristine graphite, GrO and APTES-functionalized GrO (GrO−NH2). Figure 9 shows the XPS spectra. Compared with pristine graphite and GrO, three characteristic peaks (Si 2p, Si 2s, and N 1s) originating from APTES appear in the XPS spectrum of GrO−NH2, providing additional evidence of successful grafting of APTES on GrO. Additionally, C 1s XPS spectra of GrO−NH2 further reveal that the functionalization of GrO by APTES may result

in the reduction of GrO because it is shown that the relative intensity of C−O−C bonds which appears around 286.4 eV decreases obviously after functionalization (Figure S5). Therefore, functionalizing GrO with APTES holds the promise of restoring the thermal conductivity of GrO and reducing Kapitza resistance, both of which are beneficial to high thermal conductivity. Figure 10 presents the thermal conductivity of different composites prepared by embedding 5 wt % GrO in epoxy at room temperature. To evaluate influences of the oxidation degree of GrO, epoxy composites containing 5 wt % pristine graphite are also prepared. It can be seen in Figure 10 that pristine graphite improves the thermal conductivity of the 7478


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conductive property. However, as presented in Figure 10, chemical reduction of GrO−NH2 is not a cost-effective way to fabricate GrO/epoxy composites with a good thermal conductivity because the chemical reduction is not able to completely remove the structure defects in the graphene lattice and the redundant oxygen-containing groups in GrO, which are caused by over oxidation and are bad for in-plane heat transfer.53 Besides the KMnO4/graphite ratio, the oxidation time also affects the oxidation degree of GrO significantly. Figure S6 presents the XRD patterns of GrO prepared through oxidizing raw graphite with a KMnO4/graphite ratio of 0.5 for different times. Compared to raw graphite, the intensity of all characteristic peaks of the as-prepared GrO samples decreases and a new peak appears around 2θ = 13.0°, which indicates that the lattice distortion of graphite occurs and the interlayer spacing in graphite increases due to mild oxidation. With the increasing of oxidation time, the positions of characteristic peaks are basically the same, while the intensities decrease slightly, which demonstrates that the microstructure of GrO changes a little. The XPS spectra of the as-prepared GrO samples are shown in Figure S7. It is revealed that the intensity of the CC and π−π* peaks gradually tends to decrease as the oxidation time increases; however, the intensity of the peaks corresponding to the C−C, C−OH, and C−O−C groups increases with oxidation. Further analysis on C 1s XPS spectra of the GrO reveals that, during 40 min oxidation, the percentage of C−C, C−OH, and C−O−C bonds increases, the percentage of CC and π−π* bonds decreases, and the percentage of CO and −COOH bonds increases slightly; during 40−60 min oxidation, the percentage of each bond remains unchanged due to the exhaustion of KMnO4 (Figure S8). After being functionalized with APTES, XPS spectra reveal that the content of Si atom in GrO−NH2 gradually increases to a stable value, which exhibits a similar varying tendency with the percentage of C−OH in GrO due to the reaction between −OC2H5 of APTES and −C−OH of GrO (Figure S9). Optical photographs of the GrO show that oxidation mainly occurs at the edge of graphite during 30 min oxidation; the core oxidation and exfoliation of graphite can be observed obviously during 40−60 min oxidation (Figures S10 and S11). The thermal conductivity of GrO/epoxy composites prepared with loading 5 wt % GrO filler, which is prepared by oxidizing graphite for different time, is shown in Figure 11. It can be seen that the thermal conductivity of GrO/epoxy composites is closely related to the oxidation time of GrO. For different KMnO4/graphite ratios, all of the thermal conductivity increases first and then decreases with the prolongation of oxidation time. In addition, it has been revealed that with the increase of KMnO4/graphite ratio, the optimum oxidation time is shortened (30, 20, and 15 min) while the maximum thermal conductivity decreases (0.210, 0.189, and 0.175 W/(m·K)). Graphite oxidation is also associated with the dispersibility and size of GrO−NH2, which may influence the thermal conductivity of the composites. The zeta potential of GrO− NH2 was measured to analyze its dispersibility (Zetasizer Nano ZS, UK). It can been seen that the zeta potential varies with the oxidation degree of GrO (Figure S12). However, for almost all of the GrO−NH2 samples, the zeta potential is smaller than ±40 mV, which indicates a relatively poor dispersibility. Furthermore, the GrO−NH2 dispersion in epoxy

Figure 9. XPS spectra of pristine graphite, GrO, and GrO−NH2.

Figure 10. Thermal conductivity of GrO/epoxy composites prepared with different ratios of KMnO4/graphite.

epoxy composites to 0.168 W/(m·K), as compared to 0.157 W/(m·K) of neat epoxy. Obviously, the GrO fillers with mild oxidation degree are able to provide greater thermal conductivity enhancement when embedded into epoxy as compared to pristine graphite and overoxidized GrO fillers. In addition, for mildly oxidized GrO, the thermal conductivity of graphite/epoxy composite increases as the oxidation degree of GrO increases, and the thermal conductivity of the composite reaches a value of 0.210 W/(m·K) at the oxidation degree of KMnO4/graphite = 0.5. However, the thermal conductivity of the composite gradually decreases with further increasing the oxidation degree of GrO. In particular, GrO1.5 and GrO1.75 decrease the thermal conductivity of the epoxy composites to 0.156 and 0.154 W/(m·K), respectively, when compared with that of neat epoxy. Therefore, it can be concluded that the contribution of GrO filler to the thermal conductivity is closely related to the oxidation degree or, essentially, the microstructure and the content and distribution of oxygencontaining groups. In order to demonstrate that overoxidation is unfavorable for the enhancement of thermal conductivity of GrO/epoxy composites, we reduced APTES-functionalized GrO1.25, GrO1.5, and GrO1.75 in N2H4·H2O/NH3 aqueous solution. Measurements on the thermal conductivity of reduced GrO/epoxy composites reveal that recovering the CC bonds in overoxidized GrO improves its thermally 7479


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epoxy matrix, which indicates the graphite fillers are not well connected and there are gaps between adjacent fillers (Figure S12). Thus, filler thermal conductivity, Kapitza resistance, and thermal contacting resistance, rather than GrO size, are the major factors that prevent high thermal conductivity.55,56 However, on one hand, it is proposed that reducing thermal contacting resistance needs a high content of fillers which is higher than thermal percolation threshold; on the other hand, reducing Kapitza resistance by excessive convent functionalization fails to improve the thermal conductivity of graphite/ epoxy composites. Therefore, we believe that it is reasonable to elaborate the enhancement mechanism of thermal conductivity based on the graphite oxidation behavior, which simultaneously influences the filler thermal conductivity of GrO−NH2 and the Kapitza resistance between GrO−NH2 and epoxy. On the basis of the proposed oxidation mechanism of graphite prepared by the modified Hummers method, we propose one possible explanation for the notable enhancement of thermal conductivity of the GrO/epoxy composites endowed by mildly oxidized GrO fillers (Figure 12). It has been revealed by the optical photographs and Raman analyses that GrO with mild oxidation is mainly composited of EGrO. The most notable feature of EGrO is that EGrO has a defectfree graphite/graphene structure in nonedge area and significant amounts of active functional groups in edge area. Therefore, the nonedge area of EGrO has a very high thermal conductivity. Furthermore, strong interaction between polymer chains and EGrO can be provided through the covalent grafting of APTES onto the edge of EGrO to reduce the interfacial thermal resistance and weaken the effect of edgelocalized phonons and boundary scattering at the edge of GrO on reducing thermal conductivity.57,58 For the EGrO/epoxy composite, energy transfers from heat sources to heat sinks mainly through three parts, including polymer chains, interfacial junction between filler and polymer, and in-plane crystallites of graphite, as shown in Figure 12. It should be noted that the thermal conduction of inner/inter polymer

Figure 11. Thermal conductivity of GrO/epoxy composites prepared with different oxidation times and KMnO4/graphite ratios.

matrix was observed using a transmission optical microscope. The results show that the GrO−NH2 dispersion in epoxy matrix almost does not change with increasing the oxidation of graphite and obvious GrO−NH2 agglomeration is absent (Figures S13 and S14). Therefore, the dispersibility of GrO− NH2 is not a determining factor affecting the thermal conductivity of graphite/epoxy composites in this work. The size of GrO−NH2 was also statistically analyzed using dynamic light scattering (DLS, Mastersizer 2000, UK). It is revealed that the lateral size of GrO or GrO−NH2 tends to decrease with the increase of oxidation degree (Figures S2, S3, and S15). Chatterjee et al. and Wu et al. have revealed that phonon transport through a conducting pathway is better for graphite materials with a bigger particle size.54,55 However, this work demonstrated that the thermal conductivity of graphite/epoxy composites may increase with the decrease of GrO size. Furthermore, for almost all of the samples there is no 3Dpercolated network for thermal conductivity forms in the

Figure 12. Heat transfer from the high-temperature zone to the low-temperature zone in (a) EGrO/polymer composites, (b) GO/polymer composites, and (c) neat polymer materials. 7480


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by controlling the reaction time or the mass ratio of KMnO4/graphite. In a typical preparation, graphite powders (0.684 g) were added into a NaNO3 (0.342 g)/concentrated H2SO4 (29.4 mL) solution in an ice bath under magnetic stirring. Then KMnO4 (0−1.197 g) was slowly added to the solution. The mixture was stirred in the ice bath for 0−1 h and for another 0−1 h at 35 °C in a water bath. Next, about 34.2 g of ice was added into the reaction system. The obtained mixture was stirred in a 90 °C water bath for 0−1 h. A 68.4 mL amount of DI was continuously added, followed by adding 4.0 mL of H2O2 solution to terminate the reaction. After cooling to room temperature, the mixture was filtrated and washed with a sufficient amount of DI to completely remove the excessive acid and inorganic salts. Finally, the GrO was dried in a vacuum oven at 60 °C overnight. Preparation of GrO−NH2. In a typical procedure, the obtained GrO was ultrasonically dispersed in 200 mL of ethanol for 1 h (250 W, Shanghai Kudos Ultrasonic instrument Co., Ltd.). Then, 1 mL of APTES was added dropwise into the dispersion under magnetic stirring. The dispersion was maintained in a 70 °C oil bath for 5 h. After cooling to room temperature, the mixture was filtrated and washed with ethanol to completely remove the excessive APTES. Finally, the GrO−NH2 was dried in a vacuum oven at 60 °C overnight. Preparation of Composites. In a typical procedure, the obtained GrO−NH2 was ultrasonically dispersed in 100 mL of acetone for 30 min with continuous stirring. Then 10.4 g of epoxy resin was added to the dispersion. The mixture was then heated in an air-dry oven at 80 °C for 12 h to remove the acetone. After cooling to room temperature, 2.6 g of 593 curing agent and 0.1 g of defoaming agents were uniformly mixed into the mixture, followed by degassing in a vacuum oven at room temperature. Subsequently, the mixture was slowly poured into two glass molds (composite size φ 3.8 cm × 0.6 cm). The samples were cured at room temperature overnight. Extra epoxy adhered on the composites surface was removed by polishing with 1000 # SiC paper (composite size φ 3.8 cm × 0.5 cm). For comparison, graphite/epoxy composites were also prepared by simply dispersing graphite powders in the epoxy matrix. Characterization. The Fourier-transform infrared (FT-IR) spectrum was recorded on Nicolet 6700 infrared spectrometry using the KBr pellet method. The X-ray diffraction (XRD) pattern was collected by an X-ray powder diffractometer (Bruker, D8 ADVANCE). X-ray photoelectron spectroscopy (XPS) was performed on Thermo escalab 250Xi using focused monochromatized Al Kα radiation. Raman spectrum was obtained using a laser Raman confocal microscope (Thermo Fisher, DXR Microscope) with a 532 nm laser source. Thermal conductivity was measured based on transient plane source method (Hesheng, HS-DR-5) according to standard ISO 22007-2:2008. A sensor with a radius of 7.5 mm acts as both a heat source and a temperature recorder. For measurement, the sensor was sandwiched between two pieces of the as-prepared composites. The conductivity measurements were carried out by applying a power of 0.25 W for 160 s. Optical microscopy analysis was carried out on a metallographic microscope (Olympus, BX51M).

chains coiled randomly in bulk polymers is very low (0.1−1 W/(m·K))59,60 whereas that of EGrO is 2−5 orders of magnitude higher than that of polymers.61 Therefore, for PNCs-based TIMs with a certain thickness, EGrO is able to maximize the length of the heat conduction path within the inplane crystallites of graphite and reduce the length of the heat conduction path in polymer chains, which significantly improves the thermal conductivity of polymeric materials. For strongly oxidized graphite, the thermal conductivity of graphite is much lower than pristine graphite because extended π-conjugated structure is destroyed during oxidization (GO has a thermal conductivity as low as 0.14−2.87 W/(m·K)).53 Hence, embedding overoxidized GrO into polymer matrix cannot effectively accelerates heat transfer in polymers; on the contrary, low thermally conductive GrO is even able to reduce the thermal conductivity of PNCs.

3. CONCLUSIONS GrO with different degrees of oxidation was synthesized using a modified Hummers method through adjusting the ratio of KMnO4/graphite and the oxidation time. The formation features of various oxygen-containing groups at different degrees of oxidation and their influences on the ability of GrO in improving the thermal conductivity of epoxy were investigated. The morphological studies using optical microscopy revealed that the oxidation of graphite occurred from outside to inside, which may be accompanied by the exfoliation of GrO at higher oxidation degrees. The XRD analyses demonstrated that the graphitic nature of the material was destroyed gradually with increasing oxidation degree. FT-IR and XPS studies showed more oxygen-containing groups formed in the graphite lattice during oxidation, especially epoxyl groups. In addition, a detailed study on the C 1s XPS spectra showed that the sp3 carbons increased remarkably with the increase of oxidation degree. Raman spectra further demonstrated that EGrO formed at mild oxidation degree. The oxygenated functional groups in GrO samples significantly altered the structures and thermally conductive properties of GrO. In particular, EGrO is able to enhance the thermal conductivity of epoxy remarkably because the covalent bonds in EGrO/epoxy interface reduces interfacial thermal resistance and the reserved sp2 carbon in EGrO offers an optimized pathways for heat transfer in polymer matrix. The new finding of this work throws light on the preparation of highly thermalconductive graphene-based TIMs for microelectronic device applications by precisely tuning the functionalization of graphene.

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4. EXPERIMENTAL SECTION

ASSOCIATED CONTENT

S Supporting Information *

Materials. Natural graphite powders (425 mesh, 98.9%) and concentrated H2SO4 (98%) were provided by Sinopharm Chemical Reagent Co., Ltd. Deionized (DI) water was produced from a Barnstead Smart2 Pure Water Purification system (Thermo Scientific). All other chemicals, including HCl (36.5%), NaNO3 (A.R.), KMnO4 (A.R.), H2O2 (30%), ethanol (A.R.), and acetone (A.R.), were obtained from Tianjin Kermel Chemical Reagent Co., Ltd. and used without further purification. The epoxy used was bisphenol A (E44, Nantong Xingchen Synthetic Material Co, Ltd.) with an adduct of diethylenetriamine and epoxypropane butyl ether (593, Guangzhou Yezeng Chemical Co., Ltd.) as the curing agent. Defoaming agents were provided by Changzhou Runxiang Chemical Co., Ltd. Oxidation of Graphite. GrO was prepared by a modified Hummers method. GrO with different oxidation degrees was obtained

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b01902.

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Optical microphotographs, XRD patterns, XPS spectra of GrO prepared under different oxidation times; AFM images, DLS results, and zeta potential of GrO or GrO− NH2 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 7481


Article

Chemistry of Materials ORCID

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Tianzhen Zhu: 0000-0002-1403-053X Guichang Liu: 0000-0002-1839-3020 Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the General Financial Grant from the China Postdoctoral Science Foundation (No. 2017M610177), the National Natural Science Foundation of China (Nos. 21703026, 21403030, 51671047), the Opening Project of Material Corrosion and Protection Key Laboratory of Sichuan Province (No. 2017CL16), Fundamental Research Funds for the Central Universities (No. DUT16RC(3)106), and National Key R & D Program of China (No. 2016YFB0601100).

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Tuning the Oxidation Degree of Graphite toward Highly Thermally

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