Interfacial Engineering of Graphene Nanosheets at MgO Particles for Thermal Conductivity Enhancement of Polymer Composites

An important task in facilitating the development of thermally conducting graphene/polymer nanocomposites is to suppress the intrinsically strong intersheet π-π stacking of graphene, and thereby to improve the exfoliation and dispersion of graphene in the matrix. Here, a pre-programmed intercalation approach to realize the in situ growth of graphene nanosheets at the inorganic template is demonstrated. Specifically, microsized MgO granules with controlled geometrical size were synthesized using a precipitation method, allowing the simultaneous realization of high surface activity. In the presence of a carbon and nitrogen source, the MgO granules were ready to induce the formation of graphene nanosheets (G@MgO), which allowed for the creation of tenacious linkages between graphene and template. More importantly, the incorporation of G@MgO into polymer composites largely pushed up the thermal conductivity, climbing from 0.39 W/m∙K for pristine polyethylene to 8.64 W/m∙K for polyethylene/G@MgO (60/40). This was accompanied by the simultaneous promotion of mechanical properties (tensile strength of around 30 MPa until 40 wt % addition of G@MgO), in contrast to the noteworthy decline of tensile strength for MgO-filled composites with over 20 wt.% fillers.


Introduction
In the areas of thermal management, heat dissipation and heat exchanging materials, intrinsic low thermal conductivity (TC) remains the main challenge for the application of polymer composites, regardless of low weight, desirable processing feasibility, good corrosion resistance, and low cost [1,2]. Incorporation of thermally conducting fillers, such as aluminum nitride [3], boron nitride [4], carbon nanotubes [5] and graphene nanosheets [6], represents the most common and affordable approach to TC promotion of polymers. Featuring extremely high TC values in the range of 3080-5150 W/m·K [7], graphene has garnered intense interest in the scientific and industrial communities. The graphene-enabled TC improvement, property improvement and multifunction are mainly related to the large surface area and high surface activity, which allow the creation of thermally conductive networks with high structural integrity and excellent reinforcing efficiency [8][9][10].
The surface chemistry of graphene is of importance for the stacking order and dispersion morphology in polymer composites [11], which profoundly affect the physicochemical properties of graphene/polymer composites, including the mechanical performance and TC, which are both linked to application-specific configurations [12]. Taking into account the large surface area and interaction

Materials and Methods
Sodium carbonate (Na 2 CO 3 ), magnesium chloride (MgCl 2 ), HCl, distilled water and other chemicals were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), and were used as received.
Using a precipitation method, aqueous Na 2 CO 3 solution (0.11 mol/mL) was gently added to aqueous MgCl 2 (0.1 mol/mL), which was accompanied by mechanical mixing. The mixture was sent to hydrothermal reaction in a reactor at 200 • C and 0.6 MPa within 30 min, yielding the formation of MgCO 3 granules with highly ordered crystal plates [18]. The precipitated MgCO 3 particles were filtered, followed by drying at 110 • C for 6 h. The MgCO 3 granules were calcined to MgO granules at 500 • C for 4 h under nitrogen atmosphere.
Using the MgO granules as the template, porous nanocarbons were grown at MgO by the chemical vapor deposition (CVD) method. The MgO granules were heated to 600 • C in a tube furnace at a rate of 20 • C/min and annealed at 600 • C for 0.5 h. The growth of graphene was triggered with the introduction of H 2 (100 sccm) into the gas bubbler containing acetonitrile, and the CVD reaction was held for 0.5 h to obtain the sufficient formation of graphene layers at the MgO template (G@MgO). After that, nitrogen was used to purge away any remaining H 2 , during the cooling procedure.
Using a HAAKE mill, the G@MgO was melt compounded with high-density polyethylene (PN049, Saudi Basic Industries Corporation, Tianjin, China) at 200 • C with a filler content ranging from 5 wt % to 40 wt %. The composites were compression molded into thin films with a thickness of 2 mm.
An SE-4800 scanning electron microscopy (SEM) (Hitachi, Tokyo, Japan), operating at an accelerated voltage of 5 keV, was employed to trace the morphological features of the MgCO 3 precursors, the MgO template, the G@MgO particles, and the mesoporous graphene without MgO template, as well as the fracture morphology of polyethylene composites. To observe the mesoporous graphene, the G@MgO particles were immersed in 1 M HCl solution for 24 h to etch the interior MgO template, followed by vacuum drying at 120 • C for 8 h. All the samples were sputter-coated with a 3.5 nm thick gold layer prior to the SEM observations.
Transmission electron microscopy (TEM) was used to image the morphology of graphene induced by the MgO template. Mesoporous graphene was obtained using the same method for SEM observation. Droplets of G@MgO and mesoporous graphene suspensions in ethanol were deposited onto a lacey carbon film 400 mesh copper TEM grid (Ted Pella, Inc., Redding, CA, USA) and allowed to dry in ambient conditions prior to TEM imaging (Hitachi HT7700, 80 keV). N 2 adsorption-desorption isotherms were measured on a Micromeritics ASAP 2020 at 77 K, using vacuum dried samples (150 • C/6 h). The isotherms were analyzed using the two-parameter Brunauer-Emmett-Teller (BET) model for specific surface area, the Dubinin-Radushkevitch model for micropore volume, the Horvath-Kawazoe method for micropore size distribution, and non-local density functional theory (NLDFT) isotherm fitting for meso-macropore size distribution.
Raman spectroscopy was performed on a Thermo Nicolet Almega XR Dispersive Raman microscope, comprising a 0.9 numerical aperture microscope objective, a ×50 lens and 10 exposures of 5 s. The laser wavelength is 532 nm (24 mW).
Following the standard GB/T3399-1982, the guarded hot plate method was used to evaluate the thermal conductivity (TC) of the composites on a DRH-V (Xiangyi Instrument, Xiangtan, China) at 25 • C.
Following the ASTM standard D638, tensile testing was performed on an Instron universal test instrument (Model 5944, Instron Instruments, Norwood, MA, USA) with a load cell of 500 N at 23 • C and relative humidity of 50%. The crosshead speed was set at 50 mm/min and the gauge length was 20 mm.
The composites were machined to thin films (thickness of~20 µm) and were imaged on an Olympus BX43 microscope equipped with a digital camera.

Results and Discussion
The morphological features of MgCO 3 precursors prepared by precipitation were directly examined by SEM observations (Figure 1a,b). It is clearly shown that the microsized MgCO 3 granules were orderly assembled by wafer-like nanosheets, unlike the general cubic structure with epitaxial growth along one crystalline direction [19]. The size of MgCO 3 granules was well controlled in the range of 5-10 µm, providing desirable platforms for the growth of graphene with microsized planar sheets [20]. Upon high-temperature decomposition, the MgCO 3 precursors were transformed to MgO templates with slightly decreased size, accompanied by the degradation of sheet thickness (Figure 1c,d). Moreover, a large amount of mesopores were created during the release of CO 2 , affording the enhancement of surface activity. With respect to the geometrical size and surface activity of the hard templates, the growth of graphene could be well controlled by the CVD method.
Using the activated MgO granules as the template, the graphene sheets were synthesized by the CVD method (Figure 2a). Our hypothesis on the synthetic route to graphene at the MgO template (G@MgO) was examined by SEM and TEM observations (Figure 2b-f). At the surfaces of MgO particles, graphene entities were closely attached to the template without the trace of individual carbon sheets (Figure 2b,c). High-resolution TEM images of G@MgO shows the existence of numerous few-layer graphene sheets grown from the hexagonal template units (Figure 2d-f). It is important to note that the basic hexagonal crystalline sheets constituting the MgO templates were closely wrapped by the graphene sheets with dense mesopores [21]. The CVD-grown graphene was characterized by a sheet-like morphology resembling the initial morphology of MgO template, essentially arising from Nanomaterials 2019, 9, 798 4 of 9 the formation of ordered carbons triggered by the crystalline entities of activated MgO particles. The role of the template was also verified by the observation of mesopores for the graphene sheets, in line with the structural features of MgO [22]. It is important to note that the basic hexagonal crystalline sheets constituting the MgO templates were closely wrapped by the graphene sheets with dense mesopores [21]. The CVD-grown graphene was characterized by a sheet-like morphology resembling the initial morphology of MgO template, essentially arising from the formation of ordered carbons triggered by the crystalline entities of activated MgO particles. The role of the template was also verified by the observation of mesopores for the graphene sheets, in line with the structural features of MgO [22].   It is important to note that the basic hexagonal crystalline sheets constituting the MgO templates were closely wrapped by the graphene sheets with dense mesopores [21]. The CVD-grown graphene was characterized by a sheet-like morphology resembling the initial morphology of MgO template, essentially arising from the formation of ordered carbons triggered by the crystalline entities of activated MgO particles. The role of the template was also verified by the observation of mesopores for the graphene sheets, in line with the structural features of MgO [22].  The structural features of graphene entities were further examined by etching the host MgO particles, as shown in Figure 3. The graphene sheets were characterized by interconnected pores with a pore size centered in the range of 50-100 nm (Figure 3a-c). Figure 3d-f reveals the high-resolution observation of morphological features for the typical graphene entities, which were composed of few-layer mesopores with high transparence. The morphological observations suggest that the proceeding base growth was probably triggered by the crystalline MgO substrate, leading to the Nanomaterials 2019, 9, 798 5 of 9 creation of nanosized graphitic cages encapsulating the underlying MgO. Wardle et al. reported the formation of turbostratic carbon nanotubes and nanofibers at the nanosized titania, yielding the presentation of a quantitative lift-off model in which several layers of carbon nanosheets lift off from the high-curvature corners of the template [23]. Here, we provide another example supporting the hypothesis in the case of the metal-oxide catalysts.
The structural features of graphene entities were further examined by etching the host MgO particles, as shown in Figure 3. The graphene sheets were characterized by interconnected pores with a pore size centered in the range of 50−100 nm (Figure 3a−c). Figure 3d−f reveals the high-resolution observation of morphological features for the typical graphene entities, which were composed of few-layer mesopores with high transparence. The morphological observations suggest that the proceeding base growth was probably triggered by the crystalline MgO substrate, leading to the creation of nanosized graphitic cages encapsulating the underlying MgO. Wardle et al. reported the formation of turbostratic carbon nanotubes and nanofibers at the nanosized titania, yielding the presentation of a quantitative lift-off model in which several layers of carbon nanosheets lift off from the high-curvature corners of the template [23]. Here, we provide another example supporting the hypothesis in the case of the metal-oxide catalysts. Measurements of surface area and porosity of the microporous graphene were conducted using N2 adsorption, as illustrated in Figure 4. The N2 adsorption-desorption isotherms (Type IV) allow us to hypothesize that the CVD grown graphene was primarily composed of mesopores (Figure 4a), giving rise to the significantly increased adsorption volume that corresponds to the filling of mesopores (in the P/P0 range of 0.2−0.8) [24,25]. It was accompanied by the ultimate rise (P/P0 > 0.8) that pointed to the capillary condensation in the finite volume of the mesopores. The specific surface area (SSA) of graphene was evaluated to be 175 m 2 /g, revealing the predominance of mesopores during the CVD growth at the MgO template. This was in line with the pore volume of 0.342 cm 3 /g, essentially arising from the existence of mesopores with probable collapsing of neighboring pores [26]. Measurements of surface area and porosity of the microporous graphene were conducted using N 2 adsorption, as illustrated in Figure 4. The N 2 adsorption-desorption isotherms (Type IV) allow us to hypothesize that the CVD grown graphene was primarily composed of mesopores (Figure 4a), giving rise to the significantly increased adsorption volume that corresponds to the filling of mesopores (in the P/P 0 range of 0.2-0.8) [24,25]. It was accompanied by the ultimate rise (P/P 0 > 0.8) that pointed to the capillary condensation in the finite volume of the mesopores. The specific surface area (SSA) of graphene was evaluated to be 175 m 2 /g, revealing the predominance of mesopores during the CVD growth at the MgO template. This was in line with the pore volume of 0.342 cm 3 /g, essentially arising from the existence of mesopores with probable collapsing of neighboring pores [26].
Raman spectroscopy affords an assessment of the ordering and graphitization degree for the carbons in the CVD-grown graphene ( Figure 5). The maximum was centered at 1563 cm −1 , assigned to the G band of graphite [27]. This indicates the existence of sp 2 carbons, which trigger the stretching motions of E 2g in-plane bonds. Unlike the generally observed peak at around 1580 cm −1 , the downshift of G band was probably due to the existence of numerous mesopores (Figure 3f), which was analogous to the case of carbon nano-onions featuring intensive curvature effect [28]. It was accompanied by the observation of D band located at 1330 cm −1 , pointing to the existence of structural defects of graphene. The 2D band (excited by a double-resonant Raman process) located at 2677 cm −1 was attributed to the double resonance effects of the D band, indicating the existence of few-layer graphene presented in G@MgO [20,29]. The presence of ordered and graphitized carbons attached to the MgO particles, instead of normal graphene oxide, probably contribute to the promotion of interfacial adhesion and TC [30].
To examine the unique functionality of the hierarchical structure design, the G@MgO particles were melt-compounded with polyethylene, while the MgO-filled composite counterparts were fabricated. The presence of graphene layers is expected to bridge the polymer matrix and the host MgO entities, thus conferring the enhancements of TC and mechanical properties. The plots of TC values in Figure 6a reveal the limited increase of TC for polyethylene composites with the addition of MgO, increasing from 0.39 W/m·K for pristine polyethylene to 1.08 W/m·K. It is in line with the available literature reporting the use of metal and metal oxide to improve the TC of polymer composites, generally yielding the limited improvements due to the distinct thermal resistance at the interfaces. In contrast to the poor performance of MgO-filled polyethylene, marvelous increase of TC was observed for polyethylene/G@MgO composites. With the presence of 20 wt % G@MgO, the TC of polyethylene composite was increased to 5.46 W/m·K, falling into the category of high TC. Upon the increase of filler loadings, the TC was further promoted to 8.64 W/m·K for polyethylene/G@MgO (60/40).  Raman spectroscopy affords an assessment of the ordering and graphitization degree for the carbons in the CVD-grown graphene ( Figure 5). The maximum was centered at 1563 cm −1 , assigned to the G band of graphite [27]. This indicates the existence of sp 2 carbons, which trigger the stretching motions of E2g in-plane bonds. Unlike the generally observed peak at around 1580 cm −1 , the downshift of G band was probably due to the existence of numerous mesopores (Figure 3f), which was analogous to the case of carbon nano-onions featuring intensive curvature effect [28]. It was accompanied by the observation of D band located at 1330 cm −1 , pointing to the existence of structural defects of graphene. The 2D band (excited by a double-resonant Raman process) located at 2677 cm −1 was attributed to the double resonance effects of the D band, indicating the existence of few-layer graphene presented in G@MgO [20,29]. The presence of ordered and graphitized carbons attached to the MgO particles, instead of normal graphene oxide, probably contribute to the promotion of interfacial adhesion and TC [30].   Raman spectroscopy affords an assessment of the ordering and graphitization degree for the carbons in the CVD-grown graphene ( Figure 5). The maximum was centered at 1563 cm −1 , assigned to the G band of graphite [27]. This indicates the existence of sp 2 carbons, which trigger the stretching motions of E2g in-plane bonds. Unlike the generally observed peak at around 1580 cm −1 , the downshift of G band was probably due to the existence of numerous mesopores (Figure 3f), which was analogous to the case of carbon nano-onions featuring intensive curvature effect [28]. It was accompanied by the observation of D band located at 1330 cm −1 , pointing to the existence of structural defects of graphene. The 2D band (excited by a double-resonant Raman process) located at 2677 cm −1 was attributed to the double resonance effects of the D band, indicating the existence of few-layer graphene presented in G@MgO [20,29]. The presence of ordered and graphitized carbons attached to the MgO particles, instead of normal graphene oxide, probably contribute to the promotion of interfacial adhesion and TC [30]. To examine the unique functionality of the hierarchical structure design, the G@MgO particles were melt-compounded with polyethylene, while the MgO-filled composite counterparts were fabricated. The presence of graphene layers is expected to bridge the polymer matrix and the host MgO entities, thus conferring the enhancements of TC and mechanical properties. The plots of TC values in Figure 6a reveal the limited increase of TC for polyethylene composites with the addition of MgO, increasing from 0.39 W/m•K for pristine polyethylene to 1.08 W/m•K. It is in line with the available literature reporting the use of metal and metal oxide to improve the TC of polymer composites, generally yielding the limited improvements due to the distinct thermal resistance at the  (Figure 6c,d). This was mainly ascribed to the enhanced interfacial adhesion between polyethylene matrix and G@MgO, as manifested by the observation of tenacious ligaments wrapping the G@MgO particles (inset SEM image in Figure 6d). This was in contrast to the smooth surfaces of MgO particles observed for the polyethylene/MgO (70/30), indicating the poor interfacial properties that were frequently reported for the traditional inorganic particles (inset SEM image in Figure 6c). The unusual combination of high TC and mechanical robustness confers great promise in the heat sink materials (e.g., heat-conducting plates for electronics). The design principles of the hierarchical heat-conducting fillers are significant for engendering functional particles with improved surface properties.
MPa for PE/MgO (80/20) from the initial value of 21.2 MPa for pristine polyethylene, the value fell to 15.6 MPa with the existence of 40 wt % probably due to the agglomeration of incorporated particles. Upon addition of G@MgO, the polyethylene composites witnessed large promotion of tensile strength to around 30 MPa without discernable decrease at high loadings of over 30 wt %, giving a combination of high TC and strength for the case of polyethylene/G@MgO (60/40). The optical images of the composites show the dispersion morphology for polyethylene composites, revealing the improved dispersion in polyethylene/G@MgO (70/30) compared to the counterpart (Figure 6c,d). This was mainly ascribed to the enhanced interfacial adhesion between polyethylene matrix and G@MgO, as manifested by the observation of tenacious ligaments wrapping the G@MgO particles (inset SEM image in Figure 6d). This was in contrast to the smooth surfaces of MgO particles observed for the polyethylene/MgO (70/30), indicating the poor interfacial properties that were frequently reported for the traditional inorganic particles (inset SEM image in Figure 6c). The unusual combination of high TC and mechanical robustness confers great promise in the heat sink materials (e.g., heat-conducting plates for electronics). The design principles of the hierarchical heat-conducting fillers are significant for engendering functional particles with improved surface properties.

Conclusions
Using a precipitation method, microsized MgO granules with a controlled geometrical size were synthesized, allowing the simultaneous realization of high porosity and surface activity. By incorporation of carbon and nitrogen source, the MgO templates were ready to induce the formation of few-layer graphene nanosheets, which allowed the creation of tenacious linkages between graphene and template. The structural features allowed the suppression of intrinsically strong intersheet π-π stacking of graphene, as well as the desired exfoliation and dispersion of graphene in the polyethylene matrix. More importantly, the incorporation of G@MgO into polymer composites largely pushed up the thermal conductivity, climbing from 0.39 W/m·K for pristine polyethylene to 8.64 W/m·K for polyethylene/G@MgO (60/40). Meanwhile, the simultaneous promotion of mechanical properties was observed after the addition of G@MgO, reaching a tensile strength of around 30 MPa until the content of 40 wt %, in contrast to the noteworthy decline of tensile strength for MgO-filled composites with over 20 wt % fillers. The concept of pre-programmed intercalation signifies a facile method to control the morphology and regioselectivity of graphene, laying down the essential prerequisites for the fabrication of high-performance polymer composites with a combination of high thermal conductivity and mechanical properties.