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Graphene–nickel composites
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Graphene–nickel composites Da Kuang a,b , Liye Xu a , Lei Liu a,∗ , Wenbin Hu a , Yating Wu a a b
State Key Laboratory of Metal Matrix Composites, Shanghai Jiaotong University, Shanghai, 200240, People’s Republic of China Dept. of Mechanical and Manufacturing Engineering, University of Calgary, Calgary, AB, Canada T2N 1N4
a r t i c l e
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Article history: Received 12 August 2012 Received in revised form 16 February 2013 Accepted 17 February 2013 Available online xxx Keywords: Graphene/Ni composites Thermal properties Microstructure Electrodeposition Nanomechanical properties
a b s t r a c t Graphene/nickel composites were prepared by electrodeposition in a nickel sulfamate solution with graphene oxide (GO) sheets in suspension. Raman spectra demonstrated that the GO sheets had been reduced during the electrodeposition process and the graphene content was 0.12 wt%. X-ray diffraction patterns showed the preferred orientation of nickel growth changing from (2 0 0) to (1 1 1) in the composites. Transmission and scanning electron microscopy images were used to help explain how the introduction of graphene substrates leads to the change of preferred orientation. Measurements showed the thermal conductivity of the composites to be about 15% more than that of pure nickel electrodeposits. Significant improvement was also demonstrated in the hardness measured by nanoindentation. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Graphene, a single 2D sp2 -hybridized carbon sheet, has attracted considerable interests for both fundamental and applied researches since its discovery by Geim et al. [1,2]. The remarkable chemical and physical properties [3–8] of graphene lead it become potential applications in various fields, such as solar energy conversion [9], supercapacitors [10], field effect transistors [11], lithium secondary batteries [12] and fuel cells [13]. Graphene plays an important role in the area of polymer/graphene composites [14–16]. It has been concluded that graphene-based polymer composites exhibit superior mechanical properties and higher electrical conductivity than the neat polymer or conventional graphite-based composites [17]. In the area of metal composites, most researchers concern about using graphene as a support material for the dispersion of noble metal nanoparticles. These graphene-based metal materials can obtain catalytic, optoelectronic and magnetic properties and be potentially used in the fields of catalysis, chemical sensors and energy storage [18–20]. Graphene is also used as a substrate for the growth of metal oxides to render them electrically conductive and electrochemically active [21–24]. Ni-based composites have been widely adopted in automobile and aerospace industries because of their high specific strength, favorable corrosion resistance and toughness [25–27]. The Ni
∗ Corresponding author. Fax: +86 21 34202749. E-mail address: [email protected] (L. Liu).
matrix composites are usually reinforced with fibers or particles to improve their mechanical properties while keeping excellent thermal as well as electrical conductivities. Graphene incorporated into the Ni matrix are expected to exhibit even higher hardness and strength. However, to the best of our knowledge, there are few reports on the synthesis of graphene/metal composites where graphene works as enforcement. Due to the low density, poor dispersancy and complex interface reactions of graphene [7], it is difficult to produce metal-based graphene composites with traditional metallurgy method. With the development of electrochemical reduction of graphene oxide (GO) to graphene owing to its stable dispersion in water, green and fast nature [28–31], electrodeposition becomes an effective method to prepare graphene/metal composites. It is one of the most economically superior and technologically feasible techniques for fabricating metal-based composites [32–34]. The electrochemical synthesis of graphene is typically carried out through two steps, GO sheets being first assembled on the cathodes by solution deposition methods, and then being subjected to electrochemical reduction [35]. In this research, we firstly synthesized Ni-based graphene composites through electrodeposition. This one-step coelectrodeposition of graphene/Ni composites enables GO nanosheets and Ni2+ ions to occur reduction reactions simultaneously under cathodic conditions. The composites were characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), frequency infrared carbon and sulfur analyzer, energy dispersive X-ray spectrometer (EDS), Raman spectrum and atomic force microscopy (AFM). A laser flash method was employed to calculate the thermal conductivity of the composites. Furthermore, the
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Fig. 1. (a) Raman spectrum and (b) AFM image of GO nanosheets.
mechanical properties of the composite films were measured using the nanoindentation device.
Reagents were all of analytical reagent grades and deionized water was utilized to prepare the electrodeposition solution. GO nanosheets were purchased from Nanjing XFNANO Materials Tech Co., Ltd.
composites. The pH of the solution was adjusted to 4.0 by acid sulfamate. Electrodeposition solution was stirred by magnetic stirring, which could propel GO nanosheets and nickel ions onto the cathode surface, as well as prevent the nanosheets descending. The temperature was maintained at 55 ◦ C by an automatic heat control unit. During electrodeposition process, the current density was maintained at 5 A/dm2 . An electrolytic nickel plate bought from Canada Inco Ltd was used as the anode and a copper foil with 50 mm × 80 mm × 1 mm was employed as the cathode. In addition, pure Ni was electrodeposited under the same condition in comparison with the composites.
2.2. Preparation of graphene/Ni composites
2.3. Characterization
A nickel sulfamate solution, consisting of 350 g/L Ni(NH2 SO3 )2 ·4H2 O, 40 g/L H3 BO3 , 10 g/L NiCl2 ·6H2 O and 1 g/L GO nanosheets in suspension, were used to produce graphene/Ni
The Raman spectrum was obtained by a Bruker Senterra R200L Dispersive Raman Microscope at room temperature, with the 532 nm line of green Nd:YAG laser as an excitation source. AFM
2. Experimental 2.1. Materials
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Fig. 2. (a) SEM image and (b) Raman spectrum of the dissolved composites.
images were taken in tapping mode with the Multimode Nanoscope III from Veeco. Carbon content test of the composites was employed with a CS-206 frequency infrared carbon and sulfur analyzer from Shanghai Baoying Tech Co., Ltd, whose accuracy is 1.0 ppm. XRD (RINT-2000, Rigaku) was used to examine the composites with 35 kW, K␣ copper radiation of = 0.15 nm at the steps of 2 = 0.02◦ . SEM images were observed using a FEI SIRION 200 field-emission scanning electron microscope. The TEM images were acquired using a JEOL 2000F transmission electron microscope operated at 200 kV and the HRTEM measurements were carried out on the FEI Tecnai G2 F20 S-Twin microscope at 200 kV. The EDS analysis was performed with an EDAX Genesis detector. The thermal conductivity of this material was calculated by a laser flash technique, the same method described by Xie et al. [36] in the study of multiwalled carbon nanotube arrays. The testing error of this measurement is estimated within ±1.0% and the detailed estimation was presented in Ref. [36]. Nanomechanical properties of the composites were measured by the load-penetration depth curves obtained from nanoindentation test (Tribo Indenter) using 1 mN force indentation. The indentation tip used is the standard three-sided pyramidal Berkovich probe with an average radius of curvature of about 150 nm. We have tested three independently prepared films. The hardness and modulus of the composites were measured by the load-penetration depth curves obtained from the Hysitron data files according to the nanoJKR model and the experimental error did not exceed 0.01 GPa for nanohardness. 3. Results and discussion 3.1. The characterization of GO Raman spectrum is widely used to investigate the structure, crystallization and defects of carbon materials. And it is also a potential method for characterizing the number of graphene layers. Fig. 1a shows the Raman spectrum of GO sheets used in this experiment. It displays two prominent peaks at 1324 and 1600 cm−1 , which correspond to the characterization D and G bands of GO, respectively. The G band is usually assigned to the E2g phonon of C sp2 atoms, while the D band signifies the disorder in the carbon lattice. The second order 2D band originates from a two phonon double resonance Raman process [37]. Ferrari et al. [38] have demonstrated that the 2D band can be used as an efficient way to confirm the presence of single layer graphene. It can be found the 2D band in Fig. 1a is single and sharp. Comparing with the researches of graphene layers studied by Ni et al. [39] and Pimenta
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et al. [40], we can conclude that the GO sheets used in the experiment are single layers. AFM is one of the most important methods to characterize the single-layer crystals. And the AFM image in Fig. 1c also shows that the thickness of GO sheets is 1.2 nm and its size is about 1–2 m. 3.2. The characterization of graphene/Ni composites In this work, Ni2+ ions and GO nanosheets were electrochemically reduced to form the Ni-based graphene composites. Volume fraction is usually used to represent the enforcement content in composites. However, it is difficult to measure the volume fraction of graphene in the composites and there is no more information about the density of graphene except its surface density of 0.77 mg/m2 . In order to analyze the graphene content in the composites, frequency infrared carbon and sulfur analyzer was put to measure the mass fraction of graphene and it has been discovered that the graphene content is 0.12 wt%. Raman spectrum was used to characterize the reduced GO sheets in the composites. To verify the existence of graphene sheets in the composites and enhance the intensity of its Raman bands, we used 30 vol% HNO3 to corrode the composite surface 5 min. It can be found that typical curled graphene sheets situate on the corrosion surface from Fig. 2a. Raman spectrum of the corroded composites in Fig. 2b exhibits two characteristic peaks of graphene, corresponding to the D band at ∼1345 cm−1 and G band at ∼1595 cm−1 , and they can also be found in the spectrum of GO. Comparing with Fig. 1a, the intensities of two bands increase greatly. In addition, the intensity of the D and G bands is reversed. It has been demonstrated when GO is reduced to graphene, the intensity of the D and G peaks is reversed [41], which is caused by the increasing defect concentration appear in reduced GO sheets compared to that in GO. All of above prove that GO has been reduced to graphene in the process of electrochemical reduction, accompany with the coelectrodeposition of Ni in the composites. It has been discovered that chemical reduction of GO sheets in aqueous solutions could cause their irreversible agglomerate [42], in accordance with the disappearance of 2D band in Fig. 2b, which means the reduced graphene are multi-layers in the composites. We used XRD measurements to investigate the structure and phase of the synthesized composites. Fig. 3 shows the XRD patterns of the electrodeposited Ni coating and graphene/Ni composites. The diffraction peaks at 2 = 44◦ , 52◦ , 76◦ are assigned to (1 1 1), (2 0 0), (2 2 0) crystalline structures of Ni. These peaks are in good agreement with those of the reference patterns for the
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Ni (111)
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Ni (200) Ni (220) C (002)
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2θ (degree) Fig. 3. XRD patterns of (a) electrodeposited Ni coating, (b) graphene/Ni composites.
face-centered-cubic Ni. Besides, Fig. 3b shows a small diffraction peak appearing at 2 degrees of 26.32◦ , which corresponds to the lattice fringe of 0.34 nm and can be indexed into the (0 0 2) of reduced graphene sheets. On comparison with the two XRD patterns in Fig. 3, it can be found there is a change of preferred orientation of Ni growth in the composites after the electrochemical reduction of GO. The sharp diffraction peak of (1 1 1) with high
intensity in Fig. 3b demonstrates that the electrodeposited Ni in the composites has a tendency to grow along the (1 1 1) instead of (2 0 0). It can be speculated that Ni could grow on the graphene sheets after reduction during the electrodeposition process. Hence, these newly introduced graphene substrates probably lead to the change of preferred orientation of Ni growth from (2 0 0) to (1 1 1). More explicit explanations for this change of preferred orientation would be supported by the following microstructural analysis. The typical smooth surface morphology of electrodeposited Ni coating can be clearly observed in Fig. 4a. However, in Fig. 4b, the surface of the composites changes significantly with high roughness. It has been demonstrated that the reduced graphene layers interact with each other to generate an open pore structure, which provides an easy path for the insertion and extraction of electrolyte ions through the graphene surfaces [43]. It can be found some wrinkled sheets electrodeposited on the surface of the composites. These wrinkles are signs of homogeneous dispersion of reduced graphene sheets in the Ni matrix. The higher magnification SEM image displayed in Fig. 4c shows the more obvious curled sheets which are typically graphene sheets. EDS analysis from Fig. 4d also proves the existence of graphene in the composites. It should be noted that the presence of Ni composition in the EDS spectrum certifies that Ni2+ ions could be reduced on the conductive graphene surfaces. The composites were further investigated by TEM and HRTEM to obtain the structural and morphological information. Fig. 5a shows
Fig. 4. SEM images of (a) electrodeposited Ni coating, (b) graphene/Ni composites, (c) the higher magnification of the composites, (d) the EDS of the selected area in (c).
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Fig. 5. TEM images of the graphene/Ni composites: (a) plane view, (b) cross-sectional view, (c) the EDS spectrum of the selected area in (b). (d) HRTEM image of the composites, the arrow direction from left to right is the direction of coating growth.
the plane TEM image of the graphene sheets in the composites. It is obvious that the graphene sheets are curled and entangled together. It has been reported that scrolling and corrugation are part of the intrinsic nature of graphene sheets, which result from the fact that the 2D membrane structure becomes thermodynamically stable through bending [44]. And the cross-sectional TEM image of the composites is displayed in Fig. 5b. It can be found that some curled sheets insert in the composites and they are perpendicular with the direction of coating growth. Fig. 5c shows the EDS spectrum of the selected area in Fig. 5b, which demonstrates that the inserted sheets are graphene. Fig. 5d shows the HRTEM image of these inserted graphene sheets, we can further directly explain the change of preferred orientation of Ni in the composites. In Fig. 5d, the lattice fringe on the left side of the graphene sheets is 0.17 nm, a value that corresponds to (2 0 0) interplanar distance of face-centered-cubic Ni. And the fringe spacing on the right side is 0.2 nm, which corresponds to the interplanar distance of (1 1 1). The arrow direction from left to right in Fig. 5d is the direction of coating growth. Coupling with the results of XRD and SEM, it can be discovered that electrodeposited Ni grows along the preferred orientation of (2 0 0) under this process condition. However, the graphene sheets prevent the Ni growing along the original direction after they are reduced from GO. On these different graphene substrates, Ni prefers growing along the (1 1 1) instead of (2 0 0). Consequently, the introduction of graphene, which replaces part of the Ni matrixes, changes the preferred orientation of the electrodeposited Ni growing from (2 0 0) to (1 1 1).
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3.3. The thermal conductivity of graphene/Ni composites As a new kind of carbonaceous material, graphene possesses excellent intrinsic properties. It is expected that the electrodeposition of Ni with enforcement graphene sheets could produce a novel material with unusual properties useful in some potential applications. As a beginning research, we studied the thermal conductivity of the novel composites and laser flash method was employed to measure them. Fig. 6 presents the thermal conductivities of electrodeposited Ni coating and graphene/Ni composites as functions of the temperature from 20 ◦ C to 200 ◦ C. It can be observed that both of the thermal conductivities descend with the increase of temperature, which is in accordance with the properties of pure Ni and Ni-based composites. On comparison with the two lines in Fig. 6, it can be found that the composites have a higher thermal conductivity than the electrodeposited Ni. The thermal conductivity of the composites at 200 ◦ C (70.4 W/mK) is even higher than that of electrodeposited Ni at 20 ◦ C (68.4 W/mK). In conclusion, this material’s thermal conductivity has improved about 15% more than that of the pure Ni electrodeposits generally. Considering with the 0.12 wt% of graphene sheets in the composites, the effect of graphene in the coatings as a kind of reinforcement is obvious. 3.4. Nanomechanical properties of graphene/Ni composites Since hardness includes several properties such as resistance to deformation, friction and abrasion, measuring its value would be an
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λ (W/mK)
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80 79 78 77 76 75 74 73 72 71 70 69 68 67 66 65 64 63 62 61 60
better anchored in the embedding substrate. The increase of hardness value is concerned with the special structure and excellent properties of reinforced graphene sheets in the composites. The introduced graphene sheets disperse homogenously in the shallow surface and act like a net inside the substrate. When the indenter penetrates into the composite films, graphene sheets carry the load and impede the motion of dislocations. In contrast, the indentation tip could penetrate easily into a larger depth in the pure Ni electrodeposits.
Ni graphene/Ni
4. Conclusions
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T ( C) Fig. 6. The thermal conductivities of electrodeposited Ni coating and graphene/Ni composites.
Ni-based graphene composites were successfully produced through electrochemical reduction of GO sheets and Ni2+ ions with 0.12 wt% of graphene in this material. Raman spectrum and EDS analysis were used to demonstrate that graphene in the composites had been reduced from GO sheets during the electrodeposition process. Then we focused on the discovery that the electrodeposited Ni has a change of preferred orientation in the composites. XRD patterns and HRTEM image have been used to explain the preferred orientation of Ni changing from (2 0 0) to (1 1 1) is caused by the introduction of graphene substrates. Moreover, we preliminarily study the composites’ properties and measurements showed the thermal conductivity of Ni-based graphene composite films have improved 15% more than that of pure Ni electrodeposits in general. The hardness value of the composites was almost 4-fold higher than that of pure electrodeposited Ni. All of studies indicate that the reduced graphene sheets significantly influence the preferred orientation of Ni growth in the composite films and improve the properties of the composites.
important parameter in the study of composite properties. Nanoindentation can be used as a flawless and well-developed method for determining hardness and Young’s modulus values of composites. There is no difference in surface topography between the composites and pure electrodeposited Ni after polishing. So we randomly tested two groups of data and the hardness value for each coating was given as the average value from 20 measurements. Depth of indentation was controlled to be less than 30% of composite thickness for the purpose of minimizing the influences due to the substrate [45]. Fig. 7 shows the load-penetration depth curves for the graphene/Ni composites and pure Ni electrodeposits. By monitoring the curves, it can be observed an excellent improvement in nanomechanical properties of the graphene/Ni composites. On average, the composites have a hardness of 6.85 GPa and an elastic modulus of 252.76 GPa. Comparing to a hardness of 1.81 GPa and an elastic modulus of 166.70 GPa exhibited by the pure Ni electrodeposits, the increase in the hardness of composites is almost 4-fold. From the etched surfaces of composites, it can be clearly observed that the interfacial bonding between the graphene sheets and the Ni substrate is very compact which makes graphene sheets
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Fig. 7. Load–penetration depth curves of pure Ni electrodeposits and graphene/Ni composites.
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Acknowledgments This work was supported by the National Natural Science Foundation of China (no. 51073093), National Natural Science Foundation of China (No. 51204105) and Shanghai Natural Science Foundation (No. 11ZR1418000). The authors are grateful to Instrumental Analysis Center of Shanghai Jiaotong University for Raman spectra as well as AFM measurements and thank Shanghai Institute of Ceramics, Chinese Academy of Sciences for the thermal conductivity measurements.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Graphene–nickel composites Da Kuang a,b , Liye Xu a , Lei Liu a,∗ , Wenbin Hu a , Yating Wu a a b
State Key Laboratory of Metal Matrix Composites, Shanghai Jiaotong University, Shanghai, 200240, People’s Republic of China Dept. of Mechanical and Manufacturing Engineering, University of Calgary, Calgary, AB, Canada T2N 1N4
a r t i c l e
i n f o
Article history: Received 12 August 2012 Received in revised form 16 February 2013 Accepted 17 February 2013 Available online xxx Keywords: Graphene/Ni composites Thermal properties Microstructure Electrodeposition Nanomechanical properties
a b s t r a c t Graphene/nickel composites were prepared by electrodeposition in a nickel sulfamate solution with graphene oxide (GO) sheets in suspension. Raman spectra demonstrated that the GO sheets had been reduced during the electrodeposition process and the graphene content was 0.12 wt%. X-ray diffraction patterns showed the preferred orientation of nickel growth changing from (2 0 0) to (1 1 1) in the composites. Transmission and scanning electron microscopy images were used to help explain how the introduction of graphene substrates leads to the change of preferred orientation. Measurements showed the thermal conductivity of the composites to be about 15% more than that of pure nickel electrodeposits. Significant improvement was also demonstrated in the hardness measured by nanoindentation. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Graphene, a single 2D sp2 -hybridized carbon sheet, has attracted considerable interests for both fundamental and applied researches since its discovery by Geim et al. [1,2]. The remarkable chemical and physical properties [3–8] of graphene lead it become potential applications in various fields, such as solar energy conversion [9], supercapacitors [10], field effect transistors [11], lithium secondary batteries [12] and fuel cells [13]. Graphene plays an important role in the area of polymer/graphene composites [14–16]. It has been concluded that graphene-based polymer composites exhibit superior mechanical properties and higher electrical conductivity than the neat polymer or conventional graphite-based composites [17]. In the area of metal composites, most researchers concern about using graphene as a support material for the dispersion of noble metal nanoparticles. These graphene-based metal materials can obtain catalytic, optoelectronic and magnetic properties and be potentially used in the fields of catalysis, chemical sensors and energy storage [18–20]. Graphene is also used as a substrate for the growth of metal oxides to render them electrically conductive and electrochemically active [21–24]. Ni-based composites have been widely adopted in automobile and aerospace industries because of their high specific strength, favorable corrosion resistance and toughness [25–27]. The Ni
∗ Corresponding author. Fax: +86 21 34202749. E-mail address: [email protected] (L. Liu).
matrix composites are usually reinforced with fibers or particles to improve their mechanical properties while keeping excellent thermal as well as electrical conductivities. Graphene incorporated into the Ni matrix are expected to exhibit even higher hardness and strength. However, to the best of our knowledge, there are few reports on the synthesis of graphene/metal composites where graphene works as enforcement. Due to the low density, poor dispersancy and complex interface reactions of graphene [7], it is difficult to produce metal-based graphene composites with traditional metallurgy method. With the development of electrochemical reduction of graphene oxide (GO) to graphene owing to its stable dispersion in water, green and fast nature [28–31], electrodeposition becomes an effective method to prepare graphene/metal composites. It is one of the most economically superior and technologically feasible techniques for fabricating metal-based composites [32–34]. The electrochemical synthesis of graphene is typically carried out through two steps, GO sheets being first assembled on the cathodes by solution deposition methods, and then being subjected to electrochemical reduction [35]. In this research, we firstly synthesized Ni-based graphene composites through electrodeposition. This one-step coelectrodeposition of graphene/Ni composites enables GO nanosheets and Ni2+ ions to occur reduction reactions simultaneously under cathodic conditions. The composites were characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), frequency infrared carbon and sulfur analyzer, energy dispersive X-ray spectrometer (EDS), Raman spectrum and atomic force microscopy (AFM). A laser flash method was employed to calculate the thermal conductivity of the composites. Furthermore, the
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Fig. 1. (a) Raman spectrum and (b) AFM image of GO nanosheets.
mechanical properties of the composite films were measured using the nanoindentation device.
Reagents were all of analytical reagent grades and deionized water was utilized to prepare the electrodeposition solution. GO nanosheets were purchased from Nanjing XFNANO Materials Tech Co., Ltd.
composites. The pH of the solution was adjusted to 4.0 by acid sulfamate. Electrodeposition solution was stirred by magnetic stirring, which could propel GO nanosheets and nickel ions onto the cathode surface, as well as prevent the nanosheets descending. The temperature was maintained at 55 ◦ C by an automatic heat control unit. During electrodeposition process, the current density was maintained at 5 A/dm2 . An electrolytic nickel plate bought from Canada Inco Ltd was used as the anode and a copper foil with 50 mm × 80 mm × 1 mm was employed as the cathode. In addition, pure Ni was electrodeposited under the same condition in comparison with the composites.
2.2. Preparation of graphene/Ni composites
2.3. Characterization
A nickel sulfamate solution, consisting of 350 g/L Ni(NH2 SO3 )2 ·4H2 O, 40 g/L H3 BO3 , 10 g/L NiCl2 ·6H2 O and 1 g/L GO nanosheets in suspension, were used to produce graphene/Ni
The Raman spectrum was obtained by a Bruker Senterra R200L Dispersive Raman Microscope at room temperature, with the 532 nm line of green Nd:YAG laser as an excitation source. AFM
2. Experimental 2.1. Materials
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Fig. 2. (a) SEM image and (b) Raman spectrum of the dissolved composites.
images were taken in tapping mode with the Multimode Nanoscope III from Veeco. Carbon content test of the composites was employed with a CS-206 frequency infrared carbon and sulfur analyzer from Shanghai Baoying Tech Co., Ltd, whose accuracy is 1.0 ppm. XRD (RINT-2000, Rigaku) was used to examine the composites with 35 kW, K␣ copper radiation of = 0.15 nm at the steps of 2 = 0.02◦ . SEM images were observed using a FEI SIRION 200 field-emission scanning electron microscope. The TEM images were acquired using a JEOL 2000F transmission electron microscope operated at 200 kV and the HRTEM measurements were carried out on the FEI Tecnai G2 F20 S-Twin microscope at 200 kV. The EDS analysis was performed with an EDAX Genesis detector. The thermal conductivity of this material was calculated by a laser flash technique, the same method described by Xie et al. [36] in the study of multiwalled carbon nanotube arrays. The testing error of this measurement is estimated within ±1.0% and the detailed estimation was presented in Ref. [36]. Nanomechanical properties of the composites were measured by the load-penetration depth curves obtained from nanoindentation test (Tribo Indenter) using 1 mN force indentation. The indentation tip used is the standard three-sided pyramidal Berkovich probe with an average radius of curvature of about 150 nm. We have tested three independently prepared films. The hardness and modulus of the composites were measured by the load-penetration depth curves obtained from the Hysitron data files according to the nanoJKR model and the experimental error did not exceed 0.01 GPa for nanohardness. 3. Results and discussion 3.1. The characterization of GO Raman spectrum is widely used to investigate the structure, crystallization and defects of carbon materials. And it is also a potential method for characterizing the number of graphene layers. Fig. 1a shows the Raman spectrum of GO sheets used in this experiment. It displays two prominent peaks at 1324 and 1600 cm−1 , which correspond to the characterization D and G bands of GO, respectively. The G band is usually assigned to the E2g phonon of C sp2 atoms, while the D band signifies the disorder in the carbon lattice. The second order 2D band originates from a two phonon double resonance Raman process [37]. Ferrari et al. [38] have demonstrated that the 2D band can be used as an efficient way to confirm the presence of single layer graphene. It can be found the 2D band in Fig. 1a is single and sharp. Comparing with the researches of graphene layers studied by Ni et al. [39] and Pimenta
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et al. [40], we can conclude that the GO sheets used in the experiment are single layers. AFM is one of the most important methods to characterize the single-layer crystals. And the AFM image in Fig. 1c also shows that the thickness of GO sheets is 1.2 nm and its size is about 1–2 m. 3.2. The characterization of graphene/Ni composites In this work, Ni2+ ions and GO nanosheets were electrochemically reduced to form the Ni-based graphene composites. Volume fraction is usually used to represent the enforcement content in composites. However, it is difficult to measure the volume fraction of graphene in the composites and there is no more information about the density of graphene except its surface density of 0.77 mg/m2 . In order to analyze the graphene content in the composites, frequency infrared carbon and sulfur analyzer was put to measure the mass fraction of graphene and it has been discovered that the graphene content is 0.12 wt%. Raman spectrum was used to characterize the reduced GO sheets in the composites. To verify the existence of graphene sheets in the composites and enhance the intensity of its Raman bands, we used 30 vol% HNO3 to corrode the composite surface 5 min. It can be found that typical curled graphene sheets situate on the corrosion surface from Fig. 2a. Raman spectrum of the corroded composites in Fig. 2b exhibits two characteristic peaks of graphene, corresponding to the D band at ∼1345 cm−1 and G band at ∼1595 cm−1 , and they can also be found in the spectrum of GO. Comparing with Fig. 1a, the intensities of two bands increase greatly. In addition, the intensity of the D and G bands is reversed. It has been demonstrated when GO is reduced to graphene, the intensity of the D and G peaks is reversed [41], which is caused by the increasing defect concentration appear in reduced GO sheets compared to that in GO. All of above prove that GO has been reduced to graphene in the process of electrochemical reduction, accompany with the coelectrodeposition of Ni in the composites. It has been discovered that chemical reduction of GO sheets in aqueous solutions could cause their irreversible agglomerate [42], in accordance with the disappearance of 2D band in Fig. 2b, which means the reduced graphene are multi-layers in the composites. We used XRD measurements to investigate the structure and phase of the synthesized composites. Fig. 3 shows the XRD patterns of the electrodeposited Ni coating and graphene/Ni composites. The diffraction peaks at 2 = 44◦ , 52◦ , 76◦ are assigned to (1 1 1), (2 0 0), (2 2 0) crystalline structures of Ni. These peaks are in good agreement with those of the reference patterns for the
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Ni (111)
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2θ (degree) Fig. 3. XRD patterns of (a) electrodeposited Ni coating, (b) graphene/Ni composites.
face-centered-cubic Ni. Besides, Fig. 3b shows a small diffraction peak appearing at 2 degrees of 26.32◦ , which corresponds to the lattice fringe of 0.34 nm and can be indexed into the (0 0 2) of reduced graphene sheets. On comparison with the two XRD patterns in Fig. 3, it can be found there is a change of preferred orientation of Ni growth in the composites after the electrochemical reduction of GO. The sharp diffraction peak of (1 1 1) with high
intensity in Fig. 3b demonstrates that the electrodeposited Ni in the composites has a tendency to grow along the (1 1 1) instead of (2 0 0). It can be speculated that Ni could grow on the graphene sheets after reduction during the electrodeposition process. Hence, these newly introduced graphene substrates probably lead to the change of preferred orientation of Ni growth from (2 0 0) to (1 1 1). More explicit explanations for this change of preferred orientation would be supported by the following microstructural analysis. The typical smooth surface morphology of electrodeposited Ni coating can be clearly observed in Fig. 4a. However, in Fig. 4b, the surface of the composites changes significantly with high roughness. It has been demonstrated that the reduced graphene layers interact with each other to generate an open pore structure, which provides an easy path for the insertion and extraction of electrolyte ions through the graphene surfaces [43]. It can be found some wrinkled sheets electrodeposited on the surface of the composites. These wrinkles are signs of homogeneous dispersion of reduced graphene sheets in the Ni matrix. The higher magnification SEM image displayed in Fig. 4c shows the more obvious curled sheets which are typically graphene sheets. EDS analysis from Fig. 4d also proves the existence of graphene in the composites. It should be noted that the presence of Ni composition in the EDS spectrum certifies that Ni2+ ions could be reduced on the conductive graphene surfaces. The composites were further investigated by TEM and HRTEM to obtain the structural and morphological information. Fig. 5a shows
Fig. 4. SEM images of (a) electrodeposited Ni coating, (b) graphene/Ni composites, (c) the higher magnification of the composites, (d) the EDS of the selected area in (c).
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Fig. 5. TEM images of the graphene/Ni composites: (a) plane view, (b) cross-sectional view, (c) the EDS spectrum of the selected area in (b). (d) HRTEM image of the composites, the arrow direction from left to right is the direction of coating growth.
the plane TEM image of the graphene sheets in the composites. It is obvious that the graphene sheets are curled and entangled together. It has been reported that scrolling and corrugation are part of the intrinsic nature of graphene sheets, which result from the fact that the 2D membrane structure becomes thermodynamically stable through bending [44]. And the cross-sectional TEM image of the composites is displayed in Fig. 5b. It can be found that some curled sheets insert in the composites and they are perpendicular with the direction of coating growth. Fig. 5c shows the EDS spectrum of the selected area in Fig. 5b, which demonstrates that the inserted sheets are graphene. Fig. 5d shows the HRTEM image of these inserted graphene sheets, we can further directly explain the change of preferred orientation of Ni in the composites. In Fig. 5d, the lattice fringe on the left side of the graphene sheets is 0.17 nm, a value that corresponds to (2 0 0) interplanar distance of face-centered-cubic Ni. And the fringe spacing on the right side is 0.2 nm, which corresponds to the interplanar distance of (1 1 1). The arrow direction from left to right in Fig. 5d is the direction of coating growth. Coupling with the results of XRD and SEM, it can be discovered that electrodeposited Ni grows along the preferred orientation of (2 0 0) under this process condition. However, the graphene sheets prevent the Ni growing along the original direction after they are reduced from GO. On these different graphene substrates, Ni prefers growing along the (1 1 1) instead of (2 0 0). Consequently, the introduction of graphene, which replaces part of the Ni matrixes, changes the preferred orientation of the electrodeposited Ni growing from (2 0 0) to (1 1 1).
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3.3. The thermal conductivity of graphene/Ni composites As a new kind of carbonaceous material, graphene possesses excellent intrinsic properties. It is expected that the electrodeposition of Ni with enforcement graphene sheets could produce a novel material with unusual properties useful in some potential applications. As a beginning research, we studied the thermal conductivity of the novel composites and laser flash method was employed to measure them. Fig. 6 presents the thermal conductivities of electrodeposited Ni coating and graphene/Ni composites as functions of the temperature from 20 ◦ C to 200 ◦ C. It can be observed that both of the thermal conductivities descend with the increase of temperature, which is in accordance with the properties of pure Ni and Ni-based composites. On comparison with the two lines in Fig. 6, it can be found that the composites have a higher thermal conductivity than the electrodeposited Ni. The thermal conductivity of the composites at 200 ◦ C (70.4 W/mK) is even higher than that of electrodeposited Ni at 20 ◦ C (68.4 W/mK). In conclusion, this material’s thermal conductivity has improved about 15% more than that of the pure Ni electrodeposits generally. Considering with the 0.12 wt% of graphene sheets in the composites, the effect of graphene in the coatings as a kind of reinforcement is obvious. 3.4. Nanomechanical properties of graphene/Ni composites Since hardness includes several properties such as resistance to deformation, friction and abrasion, measuring its value would be an
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better anchored in the embedding substrate. The increase of hardness value is concerned with the special structure and excellent properties of reinforced graphene sheets in the composites. The introduced graphene sheets disperse homogenously in the shallow surface and act like a net inside the substrate. When the indenter penetrates into the composite films, graphene sheets carry the load and impede the motion of dislocations. In contrast, the indentation tip could penetrate easily into a larger depth in the pure Ni electrodeposits.
Ni graphene/Ni
4. Conclusions
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T ( C) Fig. 6. The thermal conductivities of electrodeposited Ni coating and graphene/Ni composites.
Ni-based graphene composites were successfully produced through electrochemical reduction of GO sheets and Ni2+ ions with 0.12 wt% of graphene in this material. Raman spectrum and EDS analysis were used to demonstrate that graphene in the composites had been reduced from GO sheets during the electrodeposition process. Then we focused on the discovery that the electrodeposited Ni has a change of preferred orientation in the composites. XRD patterns and HRTEM image have been used to explain the preferred orientation of Ni changing from (2 0 0) to (1 1 1) is caused by the introduction of graphene substrates. Moreover, we preliminarily study the composites’ properties and measurements showed the thermal conductivity of Ni-based graphene composite films have improved 15% more than that of pure Ni electrodeposits in general. The hardness value of the composites was almost 4-fold higher than that of pure electrodeposited Ni. All of studies indicate that the reduced graphene sheets significantly influence the preferred orientation of Ni growth in the composite films and improve the properties of the composites.
important parameter in the study of composite properties. Nanoindentation can be used as a flawless and well-developed method for determining hardness and Young’s modulus values of composites. There is no difference in surface topography between the composites and pure electrodeposited Ni after polishing. So we randomly tested two groups of data and the hardness value for each coating was given as the average value from 20 measurements. Depth of indentation was controlled to be less than 30% of composite thickness for the purpose of minimizing the influences due to the substrate [45]. Fig. 7 shows the load-penetration depth curves for the graphene/Ni composites and pure Ni electrodeposits. By monitoring the curves, it can be observed an excellent improvement in nanomechanical properties of the graphene/Ni composites. On average, the composites have a hardness of 6.85 GPa and an elastic modulus of 252.76 GPa. Comparing to a hardness of 1.81 GPa and an elastic modulus of 166.70 GPa exhibited by the pure Ni electrodeposits, the increase in the hardness of composites is almost 4-fold. From the etched surfaces of composites, it can be clearly observed that the interfacial bonding between the graphene sheets and the Ni substrate is very compact which makes graphene sheets
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Fig. 7. Load–penetration depth curves of pure Ni electrodeposits and graphene/Ni composites.
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Acknowledgments This work was supported by the National Natural Science Foundation of China (no. 51073093), National Natural Science Foundation of China (No. 51204105) and Shanghai Natural Science Foundation (No. 11ZR1418000). The authors are grateful to Instrumental Analysis Center of Shanghai Jiaotong University for Raman spectra as well as AFM measurements and thank Shanghai Institute of Ceramics, Chinese Academy of Sciences for the thermal conductivity measurements.
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