polymers Article
Core/Shell Structure of Ni/NiO Encapsulated in Carbon Nanosphere Coated with Few- and Multi-Layered Graphene: Synthesis, Mechanism and Application Ferial Ghaemi 1, *, Luqman Chuah Abdullah 1,2 and Paridah Tahir 1 1 2
*
Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia (UPM), 43400 Serdang, Malaysia;
[email protected] (L.C.A.);
[email protected] (P.T.) Department of Chemical and Environmental Engineering, Universiti Putra Malaysia (UPM), 43400 Serdang, Malaysia Correspondence:
[email protected]; Tel.: +60-3-8946-8427
Academic Editor: Mohamed Khayet Received: 7 September 2016; Accepted: 14 October 2016; Published: 9 November 2016
Abstract: This paper focuses on the synthesis and mechanism of carbon nanospheres (CNS) coated with few- and multi-layered graphene (FLG, MLG). The graphitic carbon encapsulates the core/shell structure of the Ni/NiO nanoparticles via the chemical vapor deposition (CVD) method. The application of the resulting CNS and hybrids of CNS-FLG and CNS-MLG as reinforcement nanofillers in a polypropylene (PP) matrix were studied from the aspects of mechanical and thermal characteristics. In this research, to synthesize carbon nanostructures, nickel nitrate hexahydrate (Ni(NO3 )2 ·6H2 O) and acetylene (C2 H2 ) were used as the catalyst source and carbon source, respectively. Besides, the morphology, structure and graphitization of the resulting carbon nanostructures were investigated. On the other hand, the mechanisms of CNS growth and the synthesis of graphene sheets on the CNS surface were studied. Finally, the mechanical and thermal properties of the CNS/PP, CNS-FLG/PP, and CNS-MLG/PP composites were analyzed by applying tensile test and thermogravimetric analysis (TGA), respectively. Keywords: carbon nanosphere; graphene; core/shell structure; chemical vapor deposition; polypropylene
1. Introduction The discovery of new carbon forms has triggered a great interest in the carbon material science community [1,2]. Among these nanomaterials, carbon nanospheres (CNS) and graphene (G) have drawn great attention due to their potential applications in different fields of science such as lithium-ion batteries, supercapacitors, polymer composites, medical sciences, etc., which result from their excellent properties such as superior chemical stability, thermal insulation, low density, as well as high compressive strength [3–8]. Among the methods employed to synthesize CNS and G, such as the arc plasma technique [9,10], hydrothermal reaction [11,12], spray pyrolysis [13,14] and chemical vapor deposition (CVD) [15–20], CVD as a simpler, lower-cost and easier-to-implement method shows some advantages including uniform surface coating by nanomaterials and homogenous growth of material with high purity and large yield [21,22]. In the CVD technique, certain catalysts are used for nanomaterial fabrication including Fe, Ni and Co for growing CNS, and Ni, Co and Cu for growing graphene [17,18]. Growing graphene as a two-dimensional structure on other nanomaterials including carbon nanospheres, carbon nanofibers and carbon nanotubes makes hybrid nanomaterials with remarkable
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properties, such as thermal and mechanical properties, which have the potential to be used in many fields of science and engineering [23–26]. On the other hand, based on the number of graphene layers, graphene sheets have different types including single-layered (one layer), double-layered (two layers), few-layered (three to five layers) and multi-layered (six to 10 layers), each of which have different properties [27–29]. So, in the CVD technique, using certain parameters such as temperature, different shapes of carbon nanomaterials including CNS and G are obtained. Also, by altering different reaction times, different layers of graphene are achieved. To date and to the best of the authors’ knowledge, there is no study reporting the synthesis of carbon nanosphere-few layer graphene (CNS-FLG) and carbon nanosphere-multi layer graphene (CNS-MLG) hybrids using the CVD method and studying its growth mechanisms. In some existing research, authors have mixed CNS and G sheets to achieve CNS-G, but in this research, the CVD technique was employed to synthesize CNS, followed by few-layered and multi-layered graphene on its surface to produce CNS-FLG and CNS-MLG. Besides, the mechanisms to synthesize few- and multi-layered graphene on the grown CNS were studied. Finally, the nanomaterials were used as nanofillers in a polymer matrix to produce a nanocomposite with improved mechanical properties and thermal stability [30,31]. Furthermore, the resulting nanomaterial hybrids were used as reinforcement nanofillers in a polypropylene matrix to improve the polymer’s properties. In order to characterize the produced nanomaterial by its surface morphology, structure, components, number of graphene layers, size and graphitization, scanning electron microscopy (SEM), energy dispersive X-ray (EDX), transmission electron microscopy (TEM) and Raman spectroscopy were used. 2. Materials and Methods The CVD reactor (Downwell Resources Sdn. Bhd, Puchong, Malaysia) was used to synthesize carbon nanomaterials including carbon nanosphere, and few layered and multi layered graphene. In order to obtain different morphologies and sizes of the nanomaterials, CVD parameters such as reaction temperature and time should be changed. Therefore in this case, the temperature was set to range from 950 to 1050 ◦ C to synthesize CNS and G, respectively. Besides, the number of graphene layers was changed from few layers to multi layers by changing the time from 30 to 50 min. Initially, quartz substrate (the boat of the CVD reactor) was immersed in Ni(NO3 )2 ·6H2 O solution for 2 h. Then it was dried at 100 ◦ C under airflow for 1 h. After that, the boat was placed in the middle of the CVD reactor and the system was set. Then, the furnace was turned on to increase the temperature and at this time, N2 gas was released into the CVD chamber to remove O2 inside the reactor. When the temperature reached about 300 ◦ C, the surface of the boat was calcinated and the nitrate compound was removed and the Ni/NiO particles remained. When the temperature reached 950 ◦ C, the CNS fabrication started for 30 min by the use of acetylene (carbon source), with a flow rate of 50 standard cubic centimeters per minute (sccm) under flowing H2 /N2 (flow rates of 50, 100 sccm) at atmospheric pressure (1 bar). After that, at the end of reaction time, the acetylene flow was stopped and the temperature was increased to 1050 ◦ C under flowing C2 H2 /H2 /N2 (flow rates of 50, 50, 100 sccm) to synthesize graphene sheets. To synthesize few layered and multi layered graphene, different time durations (30 to 50 min) were used. In order to analyze the structural characteristics and graphitization of the resultant CNS, CNS-FLG, CNS-MLG, Raman spectroscopy (alpha 300 R, WITec, Ulm, Germany) was used. Raman spectrometer with a green Argon laser excitation source having a wavelength of 532 nm to observe the spectrum of the unstressed specimen. Besides, the morphology and components of the product were inspected using scanning electron microscope (SEM) (FEI Company, Hillsboro, OR, USA), energy dispersive X-ray (EDX) (Oxford INCA EDX 300, Singapore)and transmission electron microscope (TEM) (HITACHI Limited, Tokyo, Japan). The last part of the experimental work was to prepare the polymer composite. In this section, Thermo Haake Poly Drive R600/610 (LabX, Midland, ON, Canada) as mixer was used to melt and
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of the experimental work was to prepare the polymer composite. In this section, 3 of 12 Thermo Haake Poly Drive R600/610 (LabX, Midland, ON, Canada) as mixer was used to melt and blend the polypropylene (PP) at 180 °C with a 55 rpm rotor speed for 5 min. After that, the blend the polypropylene (PP) at 180 ◦ C with a 55 rpm rotor speed for 5 min. After that, the nanofillers nanofillers (5 wt %) were added into the mixer and was blended again for 15 min [32]. Then, the (5 wt %) were added into the mixer and was blended again for 15 min [32]. Then, the mixture was mixture was put inside a mold of 15 cm × 15 cm area and 1 mm thickness and melted again at 180 °C put inside a mold−2of 15 cm × 15 cm area and 1 mm thickness and melted again at 180 ◦ C under under 150 kg cm pressure by the use of a HSINCHU hot press machine (Tradekey, HsinChu, 150 kg·cm−2 pressure by the use of a HSINCHU hot press machine (Tradekey, HsinChu, Taiwan) and Taiwan) and then cooled. Finally, based on the guideline in ASTM D638 standard, the specimen and then cooled. Finally, based on the guideline in ASTM D638 standard, the specimen and cut into dog cut into dog bone shape [33]. bone shape [33]. In order to perform mechanical tests, an Instron Universal Testing Machine (Instron, Canton, In order to perform mechanical tests, an Instron Universal Testing Machine (Instron, Canton, MA, MA, USA) was used with a crosshead speed of 5 mm/min to determine and measure the strength USA) was used with a crosshead speed of 5 mm/min to determine and measure the strength modulus modulus and tensile stress of the PP, CNS/PP, CNS-FLG/PP and CNS-MLG/PP composites [34]. and tensile stress of the PP, CNS/PP, CNS-FLG/PP and CNS-MLG/PP composites [34]. Furthermore, Furthermore, to determine the thermal resistance of the produced nanocomposites, thermal to determine the thermal resistance of the produced nanocomposites, thermal gravimetric analysis gravimetric analysis (TGA) was employed using a Mettle Stare SW 9.10 thermal gravimetric (TGA) was employed using aColumbus, Mettle StareOH, SW USA) 9.10 thermal gravimetric analyzer (Polaris from Parkway, analyzer (Polaris Parkway, [35]. Firstly, to remove humidity the ◦C Columbus, OH, USA) [35]. Firstly, to remove humidity from the composite, it was heated to 200 composite, it was heated to 200 °C for few minutes. After that, a heating program was applied from forto few minutes. that, arate heating program was applied from 25 to 1000 ◦ C with a heating rate of 25 1000 °C withAfter a heating of about 10 °C/min under nitrogen flow. ◦ about 10 C/min under nitrogen flow. 3. Results and Discussion 3. Results and Discussion 3.1. 3.1. Nanomaterial Nanomaterial Characterization Characterization The agglomeration of ofproduced producedCNS CNScoated coatedwith withthe thegrown grown fewand multi-layered G formed The agglomeration fewand multi-layered G formed by by CVD was demonstrated by electron microscopy (SEM and TEM) images. Figure 1a–c show the CVD was demonstrated by electron microscopy (SEM and TEM) images. Figure 1a–c show the SEM SEM images the CNS, CNS-FLG and CNS-MLG, and Figure 2a–d illustrate the TEM images the images of theofCNS, CNS-FLG and CNS-MLG, and Figure 2a–d illustrate the TEM images of theof CNS, CNS, CNS-coated with graphene sheets, FLG and MLG, respectively. CNS-coated with graphene sheets, FLG and MLG, respectively.
Figure 1. SEM images of (a) CNS (carbon nanospheres); (b) CNS-FLG (Carbon nanosphere-few layer graphene) and (c) CNS-MLG (Carbon nanosphere-multi layer graphene). graphene).
The synthesizedCNS CNShad had a spherical shape, which it a porous structure 1a). The synthesized a spherical shape, which mademade it a porous structure (Figure (Figure 1a). Besides, Besides, the synthesis of graphene sheets onsurface the CNS surface in a rough on the the synthesis of graphene sheets on the CNS resulted in aresulted rough structure on structure the CNS surface. Therefore, by growing few-layered graphene on the CNS surface, a thin layer of graphene sheet
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CNS surface. Therefore, by growing few-layered graphene on the CNS surface, a thin layer of graphene sheet covered surface of1b), thewhile CNS the (Figure 1b),of while the growth of multi-layered covered the surface of thethe CNS (Figure growth multi-layered graphene led to thick graphene led CNS to thick layers the surface, as well as graphene sheets occupying the space layers on the surface, as on well as CNS graphene sheets occupying the space between CNS (Figures 1c between CNS (Figures 1c and 2b). This claim was proven by the width of the graphene sheet in and 2b). This claim was proven by the width of the graphene sheet in Figure 2c,d, whereby the Figure 2c,d,number whereby width and number graphene layers of thewere multi-layered graphene were width and ofthe graphene layers of the of multi-layered graphene greater than those of the greater thangraphene. those of So, thethe few-layered graphene. the diameter and sizewere of approximately the produced few-layered diameter and size of theSo, produced nanomaterials nanomaterials were for approximately diameter for nm CNSand (Figure 2a), × and 300nm nmfor× the 400FLG nm 200 nm in diameter CNS (Figure200 2a),nm andin300 nm × 400 1000 nm 2000 and nm ×2c) 2000 for sheet the FLG sheet2d), (Figure 2c) and MLG sheet (Figure 2d), respectively. sheet1000 (Figure andnm MLG (Figure respectively.
Figure Figure 2. 2. TEM TEM images images of of (a) (a) CNS; CNS; (b) (b) CNS CNS coated coated with with MLG; MLG; (c) (c) FLG FLG and and (d) (d) MLG. MLG.
Raman spectroscopy is a powerful yet relatively simple method to characterize the thickness Raman spectroscopy is a powerful yet relatively simple method to characterize the thickness and and crystalline quality of graphene layers [19]. Figure 3 depicts the Raman spectra for characterizing crystalline quality of graphene layers [19]. Figure 3 depicts the Raman spectra for characterizing the different nanomaterials including CNS, FLG-CNS and MLG-CNS with embedded Ni/NiO the different nanomaterials including CNS, FLG-CNS and MLG-CNS with embedded Ni/NiO nanoparticles as the catalyst. The spectra for Ni (II) particles which appeared at 317 and 555 cm−−11 nanoparticles as the catalyst. The spectra for Ni (II) particles which appeared at 317 and 555 cm corresponded to the Eg and A1g modes, respectively [36], while the peaks at 1350, 1580 and 2650 cm−−11 corresponded to the Eg and A1g modes, respectively [36], while the peaks at 1350, 1580 and 2650 cm corresponded to the D peak (breathing modes of the sp22 atom due to defective structure), G peak (E2g corresponded to the D peak (breathing modes of the sp atom due to defective structure), G peak (E2g stretching mode of the graphitic crystalline structure) and 2D peak (Raman signature of graphitic sp22 stretching mode of the graphitic crystalline structure) and 2D peak (Raman signature of graphitic sp materials) of the carbonic nanomaterials, respectively [37]. materials) of the carbonic nanomaterials, respectively [37].
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Figure 3. Raman spectra and(c)(c)Ni/CNS-MLG. Ni/CNS-MLG. Figure 3. Raman spectraofof(a) (a)Ni/CNS; Ni/CNS; (b) (b) Ni/CNS-FLG, Ni/CNS-FLG, and
order estimatethe thedegree degreeof ofgraphitization, graphitization, the ofof graphene In In order toto estimate the sp sp22cluster clustersize sizeand andnumber number graphene layers of the carbonic structures, the intensity ratio of the D peak and G peak (I D /I G ) and the intensity layers of the carbonic structures, the intensity ratio of the D peak and G peak (ID /IG ) and the intensity ratio peak and peak (I2D were calculated calculated [19,38]. /I/I G ratio of of thethe 2D2D peak and GG peak (I2D /I/IGG)) were [19,38]. ItIt isis observed observedthat that when whenIDID G decreased, the graphitizationincreased. increased. As As well, well, by by increasing increasing the /IG ratio, the number of the decreased, the graphitization the II2D 2D /IG ratio, the number of the graphene layers decreased. graphene layers decreased. The Raman spectra obtained for CNS and CNS coated with few-layered and multi-layered G The Raman spectra obtained for CNS and CNS coated with few-layered and multi-layered G sheets are revealed in Figure 3a–c, respectively. Based on the Raman spectra, the ID/IG ratio was sheets are revealed in Figure 3a–c, respectively. Based on the Raman spectra, the ID /IG ratio was about about 0.87 and the I2D/IG ratio was 0.52 for CNS, the ID/IG ratio was about 0.63 and the I2D/IG ratio was 0.87 and the I2D /IG ratio was 0.52 for CNS, the ID /IG ratio was about 0.63 and the I2D /IG ratio was 0.94 for CNS-FLG, and the ID/IG ratio was about 0.72 and the I2D/IG ratio was 0.81 for CNS-MLG. 0.94 for CNS-FLG, and the ID /IG ratio was about 0.72 and the I2D /IG ratio was 0.81 for CNS-MLG. Therefore, it can be concluded that the few growing layers of graphene sheets on the carbon Therefore, it can be concluded that the few growing layers of graphene sheets on the carbon nanosphere nanosphere surface led to the increment of the graphitization of the hybrid. surfaceThe led SEM/EDX to the increment ofthe thecarbon graphitization of the image of nanospheres at hybrid. 950 °C under C2H2/H2/N2 flow is revealed in ◦ C under C H /H /N flow is revealed The SEM/EDX imagethat of the carbon nanospheres 950 linked 2 other. 2 2 EDX 2 spectroscopy Figure 4a, which shows these carbon nanospheresatwere to each in indicates Figure 4a,that which shows that these carbon nanospheres were linked to each other. EDX the carbon nanosphere was composed of C, Ni and O atoms with aboutspectroscopy a 69:25:6 indicates that the nanosphere of C,Besides, Ni and the O atoms with about a 69:25:6 atomic atomic ratio andcarbon the C element was was mostcomposed predominant. TEM image in Figure 4b shows ratio the C element was most predominant. Besides, TEM image 4b shows that thatand carbon nanospheres with a diameter of about 200 nmthe surrounded theinNiFigure and NiO particles carbon with a diameter about 200images nm surrounded Ni and NiO particles (blackthe part). (blacknanospheres part). Moreover, elementalofmapping based onthe carbon nanospheres proved distribution of the Ni and C atoms 4c,d). Thesenanospheres results indicate that the Ni distribution elements existed inNi Moreover, elemental mapping images(Figure based on carbon proved of the the of(Figure nanoparticles with about 25% Ni particle content. and C form atoms 4c,d). These results indicate that Ni elements existed in the form of nanoparticles On the hand, content. based on the SEM/EDX images in Figure 5a, carbon nanospheres at with about 25%other Ni particle 1050 H2/N2based flow on andthe without C2H2images flow were composed of C,nanospheres Ni and O with an◦ C On°C theunder other hand, SEM/EDX in Figure 5a, carbon at 1050 atomic ratio of approximately 66:30:4. This result states that the amount of Ni particles under H2 /N2 flow and without C2 H2 flow were composed of C, Ni and O with an atomic ratio of increased while the This O atoms with carbonincreased nanospheres °C. approximately 66:30:4. resultdecreased states thatin thecomparison amount of Ni particles whileat the950 O atoms Hence, in bycomparison increasingwith the carbon temperature to 1050 °C ◦under H2/N flow, more salt was to decreased nanospheres at 950 C. Hence, by2 increasing theNi temperature converted to NiO and more NiO was converted into Ni particles. EDX spectroscopy and ◦ 1050 C under H2 /N2 flow, more Ni salt was converted to NiO and more NiO was converted into Ni elemental imaging in Figure 5b,c prove this claim. So, the percentage of Ni nanoparticles in particles. EDX spectroscopy and elemental imaging in Figure 5b,c prove this claim. So, the percentage of Ni nanoparticles in the carbon nanospheres was 30%, and it increased by about 5% in comparison
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the carbon nanospheres was 30%, and it increased by about 5% in comparison with the the carbon nanospheres was 30%, and it increased by about 5% in comparison with the previous carbon nanospheres, which had 25% Ni content, and which revealed the presence of previous carbon nanospheres, which had 25% Ni content, and which revealed the presence of Ni particles around and on top of the carbon nanosphere surface. particlescarbon aroundnanospheres, and on top of which the carbon surface. with theNi previous had nanosphere 25% Ni content, and which revealed the presence of After that, by inserting the C2H2 flow into the CVD reactor, the graphene sheets were After that, byon inserting the carbon C2H2 flow into the CVD reactor, the graphene sheets were Ni particles around and top of the nanosphere surface. grown on the Ni particles placed around and on top of the CNS surface. grown on the Ni particles placed around and on top of the CNS surface.
(a) (a)
(b) (b)
(c) (c)
(d) (d)
Figure 4. SEM/EDX image of Ni/C nanospheres (a); TEM image of Ni/C nanospheres (b); elemental
Figure 4. SEM/EDX image of Ni/C nanospheres (a);TEM TEMimage image Ni/C nanospheres (b); elemental Figure 4. SEM/EDX image of Ni/C nanospheres (a); of of Ni/C nanospheres (b); elemental mapping images of C at 950 °C (c) and Ni at 950 °C (d). ◦ ◦ mapping images of C at 950 °C (c) and Ni at 950 °C (d). mapping images of C at 950 C (c) and Ni at 950 C (d).
(a) (a)
(b) (b)
(c) (c)
Figure 5. SEM/EDX image of Ni/C nanospheres (a); elemental mapping images of C at 1050 °C (b) Figure 5. SEM/EDX image of Ni/C nanospheres (a); elemental mapping images of C at 1050 °C (b)
and Ni at 1050 °C (c). of Ni/C nanospheres (a); elemental mapping images of C at 1050 ◦ C (b) Figure 5. SEM/EDX image and Ni at◦ 1050 °C (c). and Ni at 1050 C (c).
3.2. Mechanism 3.2. Mechanism According to the results for characterization, the schematics for the formation of the different After that, by inserting the Cfor CVD reactor, for thethe graphene grown on 2 Hcharacterization, 2 flow into the the According to the results schematics formationsheets of the were different carbon nanomaterials including CNS, CNS-FLG and CNS-MLG are depicted in Figure 6. As a matter the Ni particles placed around and CNS, on top of the CNS surface. are depicted in Figure 6. As a matter carbon nanomaterials including CNS-FLG and CNS-MLG of fact, the defect structure of graphene has a negative effect on the mechanical and thermal of fact, the defect structure of graphene has a negative effect on the mechanical and thermal properties of the composite; hence, the large amount of the crystal part of graphene leads to 3.2. Mechanism properties of the composite; hence, the large amount of the crystal part of graphene leads to significantly improved mechanical and thermal properties. Based on the Raman results, the defect significantly improved mechanical and thermal properties. Based on the Raman results, the defect
According to the results for characterization, the schematics for the formation of the different carbon nanomaterials including CNS, CNS-FLG and CNS-MLG are depicted in Figure 6. As a matter of fact, the defect structure of graphene has a negative effect on the mechanical and thermal properties of the composite; hence, the large amount of the crystal part of graphene leads to significantly improved mechanical and thermal properties. Based on the Raman results, the defect structures are present in a lower amount in comparison with graphitization (compare the height of the D peak and the G peak). Figure 6a shows the substrate coated with Ni(NO3 )2 ·6H2 O particles. When the temperature is increased to about 300 ◦ C and the H2 /N2 gases are inserted into the CVD reactor, the decomposing process of Ni(NO3 )2 ·6H2 O particles starts as follows [39].
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in a lower amount in comparison with graphitization (compare the height 7 of of 12 the D peak and the G peak). Figure 6a shows the substrate coated with Ni(NO3)2·6H2O particles. When the temperature is Water increased toseparation: about 300 °C and the H2/N2 gases are inserted into the CVD reactor, the decomposing Ni(NOstarts = Ni(NO 3 )2 ·6Has 2 Ofollows 3 )2 ·2H2 O + 4H2 O process of Ni(NO3)2·6H2O particles [39]. Decomposition: Water separation: Ni(NO3)2·6H2O = Ni(NO3)2·2H2O + 4H2O Ni(NO3 )2 ·2H2 O = Ni(NO3 )(OH)1.5 ·O0.25 ·H2 O+ 0.25H2 O + NO2 Decomposition: ·H O = 0.5Ni2 O+3 NO + HNO 3 )(OH) 1.51.5·O 3 + 1.25H2 O Ni(NO3)2·2H2O =Ni(NO Ni(NO 3)(OH) ·O0.25 0.25·H22O+ 0.25H2O 2 Ni(NOdecomposition 3)(OH)1.5·O0.25·Hto 2ONiO: = 0.5Ni2O3 + HNO3 + 1.25H2O Oxide
Oxide decomposition to NiO: 3Ni2O3 = 2Ni3O4 + 0.5O2 Ni3O4 = 3NiO + 0.5O2
3Ni2 O3 = 2Ni3 O4 + 0.5O2 Ni3 O4 = 3NiO + 0.5O2
Reduction with with H H22/N Reduction /N2 2totoNi: Ni: Ni3O4 = 3NiO + 0.5O2 Ni3 O4 = 3NiO + 0.5O2 3NiO + 3/2H2 = 3Ni + 3/2H2O 3NiO + 3/2H = 3Ni + 3/2H O 2 2 After that, at a temperature of 950 °C, acetylene gas was released into the CVD reactor and the After that, at a temperature of 950 ◦ C, acetylene gas was released into the CVD reactor and the H–C–C–H bonds in the C2H2 molecules started to break up to form C atoms (Figure 6b). During the H–C–C–H bonds in the C2 H2 molecules started to break up to form C atoms (Figure 6b). During the reaction time, C atoms collected together to form a nanosphere of the deposited sp2 structure on reaction time, C atoms collected together to form a nanosphere of the deposited sp2 structure on active Ni atoms, and then surrounded the Ni/NiO nanoparticles (Figure 6c). Finally, CNS with a active Ni atoms, and then surrounded the Ni/NiO nanoparticles (Figure 6c). Finally, CNS with network structure were generated, in which Ni/NiO nanoparticles were embedded in the carbon a network structure were generated, in which Ni/NiO nanoparticles were embedded in the carbon nanosphere matrix. After that, the acetylene flow was stopped. Then, to grow the graphene sheets on nanosphere matrix. After that, the acetylene flow was stopped. Then, to grow the graphene sheets the CNS surface, the temperature was increased to about 1050 °C. While the reactor was reaching on the CNS surface, the temperature was increased to about 1050 ◦ C. While the reactor was reaching this temperature, more Ni particles were converted into Ni/NiO nanoparticles and deposited on the this temperature, more Ni particles were converted into Ni/NiO nanoparticles and deposited on the produced carbon nanosphere surface. Finally, when the temperature reached 1050 °C, acetylene gas produced carbon nanosphere surface. Finally, when the temperature reached 1050 ◦ C, acetylene gas filled the chamber and the graphene sheets started to grow on the CNS surface for 30 min and 50 min filled the chamber and the graphene sheets started to grow on the CNS surface for 30 min and 50 min to form few-layered graphene and multi-layered graphene, respectively (Figure 6d,e) [18]. to form few-layered graphene and multi-layered graphene, respectively (Figure 6d,e) [18].
Figure illustration of process: (a) NO and formation of Ni/NiO, synthesis Figure 6.6.Schematic Schematic illustration of process: (a)3 elimination NO3 elimination and formation of (b) Ni/NiO, (b) of carbon nanosphere, (c) zoon in of carbon nanosphere encapsulated Ni/NiO particles, (d) few-layer synthesis of carbon nanosphere, (c) zoon in of carbon nanosphere encapsulated Ni/NiO particles, (d) graphene (e) Multi-layer graphene productions on CNS. on CNS. few-layer and graphene and (e) Multi-layer graphene productions
nanofiller and polymer matrix. Moreover, the influence of using multi-layered graphene on the CNS surface as a nanofiller on the interaction with the polymer matrix is shown in Figure 7c. Indeed, the multi-layered graphene covered the agglomerate of CNS (refer Figure 1c) and also occupied the spaces between CNS. Therefore, when this nanofiller mixed with polypropylene, the porosity of 8,the Polymers 2016, 381 nanocomposite significantly decreased in comparison with the other mentioned 8 of 12 nanocomposites. Therefore, the presence of graphene sheets as nanofillers and as an interlocking factor with the polymer matrix is important, but not as much as the number of graphene layers. 3.3. Composite Moreover,Characterization Raman spectroscopy was employed to further analyze the interaction between the different nanofillers polymer matrix, which should beincluding reflected in the change in peak shiftand or SEM images ofand polypropylene (PP) composites CNS/PP, CNS-FLG/PP peak width [23].are Based on the in Figure the peaks related toofnanocomposites were CNS-MLG/PP depicted in results Figure 7. In these8a–d, images, the dispersion nanofillers in the PP sharper than those of pure polymer because of the resonance and absorbance effects of the matrix and the interfacial adhesion between the nanofillers and polymer matrix were observed. nanofillers. InFigure other words, peaks of the Raman for the nanocomposite were According to 7a, the the synthesized CNS had a spectra good interfacial interaction with PP;significant however, because ofporous the high intensity of the nanofillers with that, the polymer matrix, whileonthe pure it led to a polymer matrix structure. Besides the graphene sheets thepeak CNSfor surface polymer did not appear. Subsequently, the existence of nanofillers in the polymer matrix led to an enhanced the interaction with PP. In fact, the presence of the graphene on the CNS makes a rough increment in the intensity of the G and 2D peaks in the polymer composite because of the enhanced surface in comparison with the smooth surface of neat CNS, which leads an increased surface area. interaction theofpolymer and thewith filler. Furthermore,with the presence grapheneofsheets on Besides, thebetween interaction graphene-CNS polypropylene the porousofstructure CNS and CNS improved the interaction with the polymer matrix, and also it was finally understood that the hard structure of graphene leads to a significantly enhanced interaction between the polypropylene CNS-FLG had a stronger adhesion to the polymer matrix in comparison to CNS-MLG. and nanofillers.
Figure composites: (a) (a) CNS/PP; CNS/PP; (b) Figure 7. 7. SEM SEM pictures pictures of of different different composites: (b) CNS-FLG/PP CNS-FLG/PPand and(c) (c)CNS-MLG/PP. CNS-MLG/PP.
Therefore, regarding Figure 7b, the rough surface of the few-layered graphene sheets on the CNS surface led to a greater surface area, which proves the high interfacial adhesion between the nanofiller and polymer matrix. Moreover, the influence of using multi-layered graphene on the CNS surface as a nanofiller on the interaction with the polymer matrix is shown in Figure 7c. Indeed, the multi-layered graphene covered the agglomerate of CNS (refer Figure 1c) and also occupied the spaces between CNS. Therefore, when this nanofiller mixed with polypropylene, the porosity of the nanocomposite significantly decreased in comparison with the other mentioned nanocomposites. Therefore, the presence of graphene sheets as nanofillers and as an interlocking factor with the polymer matrix is important, but not as much as the number of graphene layers. Moreover, Raman spectroscopy was employed to further analyze the interaction between the different nanofillers and polymer matrix, which should be reflected in the change in peak shift or peak width [23]. Based on the results in Figure 8a–d, the peaks related to nanocomposites were sharper than those of pure polymer because of the resonance and absorbance effects of the nanofillers. In other words, the peaks of the Raman spectra for the nanocomposite were significant because of the high intensity of the nanofillers with the polymer matrix, while the peak for pure polymer did not appear. Subsequently, the existence of nanofillers in the polymer matrix led to an increment in the intensity of the G and 2D peaks in the polymer composite because of the enhanced interaction between the polymer and the filler. Furthermore, the presence of graphene sheets on CNS improved the interaction with the polymer matrix, and also it was finally understood that CNS-FLG had a stronger adhesion to the polymer matrix in comparison to CNS-MLG.
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Figure spectroscopy of (a) PP;PP; (b) (b) CNS/PP; (c) CNS-FLG/PP and (d) CNS-MLG/PP. Figure8.8.Raman Raman spectroscopy of (a) CNS/PP; (c) CNS-FLG/PP and (d) CNS-MLG/PP.
Based on the results achieved from the mechanical test, it was found that the nanofillers BasedFigure on the resultsspectroscopy achieved from the(b) mechanical it was found the nanofillers improved 8. Raman of (a) PP; CNS/PP; (c)test, CNS-FLG/PP and (d)that CNS-MLG/PP. improved the mechanical properties (see Table 1 and Figure 9). Besides, the study on the stiffness of the mechanical properties (see Table 1 and Figure 9). Besides, the study on the stiffness of the the compositeon fabricated from the different nanofillers, including CNS,found CNS-FLG andnanofillers CNS-MLG, the results achieved from the mechanical it CNS-FLG was thatCNS-MLG, the compositeBased fabricated from the different nanofillers, includingtest, CNS, and revealed revealed thatthe CNS coated with graphene a significant improvement thestiffness tensile of test improved mechanical properties (see layers Table 1led andtoFigure 9). Besides, the study oninthe that CNS coated with graphene layers led to a significant improvement in the tensile test results in results in comparison with from neat CNS becausenanofillers, of the highincluding interfacial adhesion. Also, the strength of the composite fabricated the different CNS, CNS-FLG and CNS-MLG, comparison with neat CNS because of the high interfacialwas adhesion. the that strength of CNS-FLG/PP CNS-FLG/PP nanocomposites greaterAlso, than CNS/PP revealed thatand CNS CNS-MLG/PP coated with graphene layers led to a significant improvement inofthethe tensile test and CNS-MLG/PP nanocomposites was greater than that of the CNS/PP nanocomposite due results in comparison with neat CNS because oflayers the high interfacial adhesion. Also, of to nanocomposite due to the presence of graphene on the CNS surface, which ledthe to strength a high stress thetransfer presence of graphene layers on the CNS which led to than a high stress between CNS-FLG/PP and CNS-MLG/PP nanocomposites was greater that of transfer thewhich CNS/PP between the CNS and polymer. On thesurface, other hand, the few-layered graphene, had a thehigher CNS and polymer. On the other hand, the few-layered graphene, which had a higher nanocomposite due to the presence of graphene layers on the CNS surface, which led to a high stress surface area in comparison with multi-layered grapheme, had a stronger interaction withsurface PP, transfer between the CNS and polymer. Onhaving the other the tensile few-layered graphene, which hadNote ato the area in comparison with multi-layered grapheme, had a stronger interaction with PP, leading leading to the CNS-FLG/PP nanocomposite thehand, highest strength and stress [18]. surface area in comparison multi-layered grapheme, stronger interaction with PP, CNS-FLG/PP nanocomposite havingwith the highest tensile strength anda stress [18]. Note the tensile thathigher the tensile strength of a material is the maximum amount of had tensile stress that it canthat take before leading to the CNS-FLG/PP nanocomposite having the highest tensile strength and stress [18]. Note strength of aand material is the maximum that it can take before breaking, and the breaking, the Young’s modulus isamount the ratioofoftensile stress stress upon strain. thatmodulus the tensileisstrength of of a material is thestrain. maximum amount of tensile stress that it can take before Young’s the ratio stress upon breaking, Table and the Young’s modulus is the the different ratio of stress upon strain. 1. Mechanical results of nanocomposites (5% nanofiller, 95% PP).
Table 1. Mechanical results of the different nanocomposites (5% nanofiller, 95% PP). Table Sample 1. Mechanical results of the different nanocomposites nanofiller, 95% PP). Tensile stress (MPa) Young’s(5% modulus (MPa)
PP Sample Sample CNS/PP PPPP CNS-FLG/PP CNS/PP CNS/PP CNS-MLG/PP CNS-FLG/PP
CNS-FLG/PP CNS-MLG/PP CNS-MLG/PP
15.1 ± 0.3
Tensile Tensile stress (MPa) 25.6 ± 0.4 15.1 ± ± 0.3 15.1 0.3 73.8 ±±0.6 25.6 25.6 ±0.4 0.4 40.1 ±±0.7 73.8 ±0.6 0.6 40.1 ± 0.7 ± 0.7
480.2 ± 11.7
Young ’s modulus Young’s modulus(MPa) (MPa) 830.2 ± 31.2 480.2 ± 11.7 480.2 ± 11.7 1,243.6 ±31.2 57.1 830.2 830.2± ± 31.2 1,097.5±±57.1 48.2 1,243.6 1,243.6 ± 57.1 1,097.5± ± 48.2 1,097.5 48.2
Figure 9. Tensile stress–strain graph of PP, CNS/PP, CNS-FLG/PP and CNS-MLG/PP. Figure 9. Tensile stress–strain graph of PP, CNS/PP, CNS-FLG/PP and CNS-MLG/PP. Figure 9. Tensile stress–strain graph of PP, CNS/PP, CNS-FLG/PP and CNS-MLG/PP.
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According to the thermogravimetric analysis (TGA) results, it was found that the existence of According to the thermogravimetric analysisthermal (TGA) resistance results, it of was thatnanocomposite the existence of nanofillers in the polymer matrix led to improved thefound polymer nanofillers in the polymer matrix led to improved thermal resistance of the polymer nanocomposite due to the high heat absorption capacity of the carbon nanomaterials (see Figure 10). Once the due to the high heat absorption capacityof of the carbon nanomaterials Figure 10). Oncetothea specimen specimen absorbs a certain amount heat, thermal degradation(see happens, leading single absorbs a certain amount of heat, thermal degradation happens, leading to a single degradation step. degradation step. Then, the structure of the polymer composite decomposed. Based on Figure 10, the Then, the structure of the polymer composite decomposed. Based on Figure 10, the neat PP was broken neat PP was broken down at about 330 °C and degraded completely at 430 °C without any char left. ◦ C withoutcomposites down aboutloss 330of◦ Cthe and degraded completelyand at 430 any char left. The weight lossand of the The at weight CNS/PP, CNS-MLG/PP CNS-FLG/PP started at 450, 500 ◦ C, and completed 530 °C, CNS-MLG/PP and completed at 550, 580 and 590 composites °C, respectively. Based on the had CNS/PP, and CNS-FLG/PP started at 450, 500 results, and 530CNS-FLG/PP ◦ C, respectively. than CNS/PP CNS-MLG/PP composites had because of the high thermal at greater 550, 580thermal and 590resistance Basedand on the results, CNS-FLG/PP greater thermal resistance resistance ofand the CNS-MLG/PP few-layered graphene in comparison Therefore, by few-layered growing than CNS/PP composites because of to theother high nanofillers. thermal resistance of the few-layered graphene on CNS, nanofillers. the thermalTherefore, resistance by of growing the nanocomposite significantly graphene in comparison to other few-layeredwas graphene on CNS, improved. the thermal resistance of the nanocomposite was significantly improved.
Figure TGAgraphs graphsofofthe thevarious variouspolymer polymer composites composites (1: PP; CNS-MLG/PP andand 4: 4: Figure 10.10.TGA PP; 2: 2:CNS/PP; CNS/PP;3:3: CNS-MLG/PP ◦ CNS-FLG/PP); Horizontal axis: Temperature (°C); Vertical axis: Loss weight percentage (wt %). CNS-FLG/PP); Horizontal axis: Temperature ( C); Vertical axis: Loss weight percentage (wt %).
Conclusions 4. 4.Conclusions The most important contribution of this research is the synthesis of the few- and multi-layered The most important contribution of this research is the synthesis of the few- and multi-layered graphene on the CNS, as well as the synthesis of the CNS encapsulating the Ni/NiO particles using graphene on the CNS, as well as the synthesis of the CNS encapsulating the Ni/NiO particles using the chemical vapor deposition technique. The reaction commenced by depositing carbon atoms on the chemical vapor deposition technique. The reaction commenced by depositing carbon atoms the Ni particle surface. Also, CNS was grown at 950 °C for 30 min and FLG and MLG were on the Ni particle surface. Also, CNS was grown at 950 ◦ C for 30 min and FLG and MLG were synthesized on CNS surfaces at 1050 ◦°C for 30 and 50 min, respectively. Moreover, we studied the synthesized on CNS surfaces at 1050 C for 30 and 50 min, respectively. Moreover, we studied the mechanism for hybrid growth and developed a model for growing a carbon nanosphere–graphene mechanism for growth and a model for growing a carbon hybrid, which hybrid encapsulated the Ni developed nanoparticles. Regarding the results, firstly,nanosphere–graphene CNS was grown on hybrid, which encapsulated the Ni nanoparticles. Regarding the results, firstly, was grown on Ni/NiO nanoparticles at 950 °C. Thereafter, by increasing the temperature underCNS H2/N 2 gas flow, ◦ Ni/NiO nanoparticles at 950 C. Thereafter, byCNS, increasing under H2 /N2 sheets gas flow, more Ni/NiO evaporated around the produced which the led temperature to the synthesis of graphene more Ni/NiO evaporated around the produced CNS, which led to the synthesis of graphene sheets on the CNS surface. Finally, the resultant nanomaterials including CNS, CNS-FLG and CNS-MLG on thewere CNSused surface. Finally, the resultantinnanomaterials including CNS, CNS-FLG and CNS-MLG were as reinforcing nanofillers the polymer matrix to improve the mechanical and thermal used as reinforcing in thematrix. polymer matrix totoimprove the mechanical properties properties of the nanofillers polypropylene According the obtained outputs, and it is thermal concluded that of CNS-FLG the polypropylene According to the obtained outputs,of it the is concluded that CNS-FLG had more amatrix. significant role in improving the properties polymer composite such ashad tensile strength, strain thermal resistance. more a significant role inand improving the properties of the polymer composite such as tensile strength, strain and thermal resistance. Acknowledgments: The authors would like to thank the University Putra Malaysia (UPM) for their financial support.
Acknowledgments: The authors would like to thank the University Putra Malaysia (UPM) for their financial support. Author Contributions: The experiments and results were derived by Ferial Ghaemi and the preparation and Author Contributions: The experiments and results were derived by Ferial Ghaemi and the preparation and correction thefinal finaldraft draftwas wasdone done by by Paridah Paridah Tahir. were proposed by by correction ofofthe Tahir. Some Somehints hintsofofthe thecharacterizations characterizations were proposed Luqman Chuah Abdullah. Luqman Chuah Abdullah. Conflicts Interest:The Theauthors authorsdeclare declareno no conflict conflict of interest. Conflicts ofof Interest: interest.
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