Synthesis and characterization of Cu–Ni/Gr nanocomposite

Bull. Mater. Sci. (2020) 43:66 © Indian Academy of Scienceshttps://doi.org/10.1007/s12034-019-2031-x

Synthesis and characterization of Cu–Ni/Gr nanocompositecoatings by electro-co-deposition method: effect of current density

AJAY D PINGALE∗, SACHIN U BELGAMWAR and JITENDRA S RATHOREDepartment of Mechanical Engineering, Birla Institute of Technology and Science, Pilani 333031, India∗Author for correspondence ([email protected])

MS received 21 March 2019; accepted 10 October 2019

Abstract. Cu–Ni alloys are widely used and implemented due to their remarkable mechanical and electrochemical proper-ties in most of the engineering applications. The reinforcement of graphene nanoplatelets (Gr) in Cu–Ni alloy can be utilizedto enhance the properties of Cu–Ni alloy. In the present work, Ni as an alloying element and graphene nanoplatelets asreinforcing element were co-deposited with Cu to prepare Cu–Ni/Gr composite coatings by electro-co-deposition method.The influence of various current densities on surface morphology, composition, microstructure, crystallite size, lattice strain,microhardness, coefficient of friction and corrosion resistance of the resulting composite coatings were investigated andwere presented in detail. Based on the experimental results, the coatings prepared at 6 A dm−2, exhibit a reduced grain sizewith enhanced mechanical properties and corrosion resistance.

Keywords. Composite coatings; current density; corrosion; electro-co-deposition; mechanical properties.

1. Introduction

Pure copper and its alloys are commonly used in a wide rangeof automobiles and have extensive metallurgical applicationsdue to their remarkable properties [1–4]. Cu–Ni, an alloyof copper, is mainly used in the applications of corrosiveenvironment, such as piping, marine hardware, boat hulls,condensers and heat exchangers in seawater system due to itscorrosion resistance properties [5,6]. However, the incorpora-tion of Al2O3, SiC and CNTs, enhanced the mechanical andelectrical properties of alloy [7–9].

Graphene, a 2D structure of sp2 hybridized carbon atoms,has been mainly synthesized by chemical vapour depo-sition [10], mechanical exfoliation [11] and reduction ofgraphene oxide [12]. Graphene nanoplatelets are composedof short stacks of isolated single layers of graphite (calledgraphene) [13]. Graphene nanoplatelets have been provedas a promising reinforcing element due to its extraordinaryproperties, such as huge surface area, high electrical conduc-tivity, excellent mechanical properties and superior thermalconductivity [14,15]. Thus, graphene nanoplatelets reinforcedmetal matrix composites have gained considerable attentionin the past few years, due to its high potential for providinghigh strength, superior corrosion resistance, high electricaland thermal conductivities and light-weight to meet the chal-lenges of extreme conditions in various engineering fields[16,17].

Various processes like chemical vapour deposition, plasmaspraying, thermal spraying, electrodeposition and physicalvapour deposition have been used to form a composite coat-ing on the different substrates [9]. The electrodeposition

process is prominent in copper plating [9,18]. The surfacemorphology, composition, microstructure, mechanical andelectrochemical properties of the plating directly rely on theprocess parameters like electrolyte pH, current density, elec-trolyte bath composition, etc. [19–21].

In the present work, Ni as an alloying element and graphenenanoplatelets as reinforcing element were co-deposited withCu to prepare Cu–Ni/Gr composite coatings by electro-co-deposition method. The influence of various current densitieson the characteristics of Cu–Ni/Gr composite coatings wasexamined.

2. Experimental

2.1 Materials

Copper sulphate pentahydrate (CuSO4·5H2O), nickel sul-phate hexahydrate (NiSO4 ·6H2O) with the purity of 99% andtrisodium citrate dihydrate (Na3C6H5O7·2H2O) were sup-plied by Merck Specialties Pvt. Ltd. Graphene nanoplatelets(with surface area ≈500 m2 g−1) used in this study were pur-chased from Alfa Aesar.

2.2 Electrodeposition process

In a citrate type bath, Cu–Ni/Gr composite coatings wereelectrodeposited at various current densities. All the stepsinvolved are shown in figure 1. A volume of 250 cm3 ofcitrate bath was composed of 21 g l−1 of copper sulphate pen-tahydrate as the copper source, 105 g l−1 of nickel sulphate

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66 Page 2 of 9 Bull. Mater. Sci. (2020) 43:66

Figure 1. Scheme of the electro-co-deposition of Cu–Ni/Gr composite coatings.

Table 1. Used bath composition and operating conditions.

Bath composition and operating conditions Quantity

Nickel sulphate hexahydrate 105 g l−1

Copper sulphate pentahydrate 21 g l−1

Trisodium citrate dihydrate 59 g l−1

pH 4Temperature 35 ± 1◦CSteering 300 rpm (using magnetic stirrer)Graphene nanoplatelets 100 mg l−1

Current density 2, 4, 6 and 8 A dm−2

Plating time 90 min

hexahydrate as the nickel source and 59 g l−1 of trisodiumcitrate dihydrate as a complex agent. For comparison, pureCu–Ni coating was also prepared. The citrate bath com-position and operating conditions are given in table 1. Allelectrolyte solutions were formed by reagent grade chemicalsand deionized water. The vacuum filtration was used to sepa-rate insoluble impurities from the electrolyte solution. Then,100 mg l−1 graphene nanoplatelets were added to the elec-trolyte solution. A magnetic stirrer was used for agitation ofthe electrolyte solution with 300 rpm for 15 min. After stirring,ultrasonic treatment was given for 1 h to disperse the graphenenanoplatelets uniformly in the solution. A stainless steel plateof 20 × 20 × 1.5 mm3 was used as a cathode (substrate) and

a nickel plate was used as an anode. Stainless steel substrateswere polished with silicon carbide paper, degreased in ace-tone, and activated in nitric acid before use.

2.3 Characterization

The surface morphologies and elemental composition of theCu–Ni/Gr composite coatings were examined by using scan-ning electron microscopy (SEM) (FEI-Apreo S) attachedwith energy dispersive spectroscopy (EDS). The energy usedfor analysis was 10 kV. The effect of the current densitieson the microstructure of prepared coatings was examinedby X-ray diffraction (XRD) (Rigaku Mini Flex-II model)

Bull. Mater. Sci. (2020) 43:66 Page 3 of 9 66

(Cu Kα radiation, λ = 0.15418 nm). The examination wasacquired from 10 to 80◦ 2θ for the Cu–Ni/Gr coatings.The XRD experiments were done with a scanning rate of0.05◦ s−1. The average crystallite size of composite coatingswas estimated using the Scherrer technique. Also, the latticestrain was determined from XRD data. The microhardnessmeasurements of the composite coatings were performed witha period of 10 s and a load of 20 g by microhardness tester(Innovatest Nexus 4303).

Wear tests of composite coatings were performed by recip-rocating tribometer coupled with Kistler dynamometer witha ball-on-disk arrangement. The steel ball of 3 mm diameterwas used for testing. The experiments were performed with astandard load of 4 N, 5 Hz frequency and amplitude of 10 mm.The coefficient of the friction was recorded continuously dur-ing the tests.

The corrosion resistance of prepared composite coatingswas examined by a technique called potentio-dynamic linearpolarization using CHI604E (Potentiostat/Galvanstate, USA)instrument. The experimental tests were performed at roomtemperature in 3.5 wt% aqueous solution of NaCl using aconventional three-electrode system.

3. Results and discussion

3.1 Morphological and compositional characterizations

The surface morphology of Cu–Ni/Gr composite coatingsis depicted in figure 2. Figure 2 demonstrates the effect ofvarious current densities on the surface morphology of elec-trodeposited coatings. At the low current densities, a smoothand compact morphology of the produced Cu–Ni/Gr coat-ings are clearly noticed as shown in figure 2a–d. A noticeablechange is observed in the surface morphology of Cu–Ni/Grcoatings with less compactness and rough characteristics dueto the increase of current density from 6 to 8 A dm−2 as shownin figure 2e–h. Prominently, at increased current densities, therate of deposition is high (like 6 and 8 A dm−2), which resultsin less compactness and rough morphology [22].

As seen in figure 3, as the current density is increased,the graphene content of the Cu–Ni/Gr composite coatingsis also increased gradually up to 6 A dm−2 and then starteddecreasing with further increase in current density. At lowcurrent densities, there are less metallic ions which wouldbe embedded with graphene nanoplatelets and deposited ona substrate. At high current densities, metallic ion showshigh deposition rate compared to graphene nanoplatelets [23].Hence, experimentally, it has been observed that the graphenenanoplatelets content in Cu–Ni/Gr composite coatings isgradually increased from 2 to 6 A dm−2 and then decreasedat 8 A dm−2. However, Cu–Ni/Gr composite coating elec-trodeposited at 6 A dm−2 has higher graphene nanoplateletscontent.

Figure 4a and b shows SEM cross-sections of Cu–Ni/Grcomposite coatings, which depict proper bonding between

the substrate and coating as no cracks are observed betweenthem. The thickness of Cu–Ni/Gr composite coatings elec-trodeposited at various current densities, is measured. Theeffect of various current densities on the thickness of Cu–Ni/Gr composite coating is shown in figure 4c. It is noticedthat the thickness increases with an increase in current density.

Figure 5 shows EDS mapping of the Cu–Ni/Gr coatings,electrodeposited at various current densities. The differentelements present in the coatings are represented by variouscolours. Figure 5c shows the presence of graphene plateletsin the composite. It is observed that the graphene nanoplateletsand the nickel elements are distributed uniformly in the cop-per matrix. The EDS results are listed in table 2, which showthat the Cu–Ni/Gr composite coatings electrodeposited at thecurrent densities of 6 and 8 A dm−2 have lower copper con-tent, in comparison with the 2 and 4 A dm−2. On the otherhand, by increasing the current density, the content of nickelin the electrodeposited coatings is increased. Cu2+ species aredischarged with mass transport control, and Ni2+ species aredischarged with activation control [24,25]. Therefore, withthe increase in the current density from 2 to 8 A dm−2, therelative content of nickel in the electrodeposited Cu–Ni/Grcoating increases. The enhancement in the characteristics ofthe electrodeposited coating relies on a homogeneous distri-bution of the graphene nanoplatelets in the coating [26]. Thehighest graphene nanoplatelets content and the homogeneousdistribution of the elements are at 6 A dm−2 coating deposi-tion is seen in figures 3 and 5c, respectively.

3.2 Microstructural characterization

Figure 6 shows the XRD spectrum of graphene nanoplatelets,Cu–Ni and Cu–Ni/Gr composite coatings electrodeposited atvarious current densities. On XRD spectrum of Cu–Ni, onlyone peak of Cu–Ni is observed instead of two separate peaksof Cu and Ni, which confirms the formation of Cu–Ni alloy inall the coatings [27,28]. The major peak (111) correspondingto Cu–Ni has been observed at 2θ = 43.6◦, which is betweenthe 2θ values of pure copper at 43.35°(American mineralo-gist crystal structure database (AMCSD 0012903)) and purenickel at 44.53◦ (AMCSD 0011153) [29]. The peaks in XRDpattern of Cu–Ni and Cu–Ni/Gr composite coatings are cor-responding to the plane (111), (200) and (220), which confirmthe fcc structure of the electrodeposited coatings. The carbonpeaks belong to graphene nanoplatelets (around 2θ = 26◦)are not observed in XRD spectrum of Cu–Ni/Gr compositecoatings electrodeposited at various current densities, whichis due to very low graphene nanoplatelets content [30]. How-ever, the peak intensity of the Cu–Ni/Gr composite coatingshas been weakened with the increase in current density due tothe lattice distortion of electrodeposited coatings by the incor-poration of graphene nanoplatelets. In figure 6, the width ofthe fcc reflections peaks are changed across the distinct sam-ples due to the presence of graphene nanoplatelets. Moreover,peak broadening indicates the small crystallite size of the coat-ings [7]. However, no impurity (such as Cu2O) is observed


Figure 2. Surface morphology of the Cu–Ni/Gr composite coatings electrodeposited at (a, b) 2 A dm−2, (c, d)4 A dm−2, (e, f) 6 A dm−2 and (g, h) 8 A dm−2.

in the XRD spectrum of Cu–Ni and Cu–Ni/Gr compositecoatings.

Also, there is a slight decrease in crystallite size of Cu–Ni/Gr composite coatings with the rise in current density dueto the increase in the deposition rate. The crystal growth issignificantly inhibited due to the higher deposition rate.

Variation in crystallite size of Cu–Ni and Cu–Ni/Grcomposite coatings electrodeposited at various current densi-ties is depicted in figure 7. According to the Scherrer equationand XRD pattern, the crystallite size for the (111) reflectionwas calculated [31]. Moreover, Cu–Ni/Gr composite coatingelectrodeposited at 6 A dm−2 has the least crystallite size of


Figure 3. Graphene nanoplatelets content vs. current density ofCu–Ni/Gr composite coatings.

about 8 nm. The embedded graphene nanoplatelets increaseactive nucleation sites and hence, inhibit the crystal growth.Hence, incorporation of graphene nanoplatelets serves as amechanism for grain refinement of electrodeposited compos-ite coatings [17]. Also, the coating has high graphene contentat the current density of 6 A dm−2, and the coating has theleast crystallite size.

Figure 8 shows the change in the lattice strain of Cu–Niand Cu–Ni/Gr composite coatings at various current densities.The lattice strain was calculated with the help of XRD resultsby using equation (1) [32]:

ε =(

β

4 × tan θ

)× 100, (1)

where β is the full width at half maximum (FWHM) and θ theBragg’s angle corresponding to fcc (111) peak. As shown infigure 8, the lattice strain increases up to 6 A dm−2 and then

Figure 4. SEM cross-sections of Cu–Ni/Gr composite coatings electrodeposited at (a) 4 A dm−2, (b) 8 A dm−2 and(c) the thickness of Cu–Ni/Gr composite coatings electrodeposited at various current densities.


Figure 5. EDS mapping of the Cu–Ni/Gr coatings electrodeposited at (a) 2 A dm−2, (b) 4 A dm−2, (c) 6 A dm−2 and (d) 8 A dm−2.

Table 2. EDS data of Cu–Ni and Cu–Ni/Gr compositecoatings electrodeposited at various current densities.

Description Cu (wt%) Ni (wt%) C (wt%)

Cu–Ni 67.86 32.14 —Cu–Ni/Gr (2 A dm−2) 65.92 28.76 5.32Cu–Ni/Gr (4 A dm−2) 64.68 29.34 5.98Cu–Ni/Gr (6 A dm−2) 63.84 29.55 6.61Cu–Ni/Gr (8 A dm−2) 66.18 29.84 3.98

decreases. Also, the smaller the crystallite size, the higherthe lattice strain [33]. The generation of defects, such asvoids, twins, vacancies and dislocations can increase the lat-tice strain [32].

3.3 Microhardness testing

Figure 9 illustrates the microhardness variation of Cu–Niand Cu–Ni/Gr composite coatings electrodeposited at vari-ous current densities. The measured hardness of Cu–Ni/Grcomposite coatings is higher than Cu–Ni composite coat-ing. The improved microhardness of Cu–Ni/Gr compositecoatings is mainly due to the grain size refinement and anincrease in the percentage of grain boundaries after incor-poration of graphene nanoplatelets, as well as extraordinarymechanical properties of graphene nanoplatelets [34]. Themovement of dislocation in the Cu–Ni/Gr composite coat-ing is hindered by graphene nanoplatelets, which tends toincrease the deformation resistance and lattice distortionenergy [35,36]. As shown in figure 9, the microhardness ofCu–Ni/Gr composite coating first increases up to 6 A dm−2,


Figure 6. XRD spectrum of graphene nanoplatelets, Cu–Ni andCu–Ni/Gr composite coatings electrodeposited at various currentdensities.

Figure 7. Change in the crystallite size of Cu–Ni and Cu–Ni/Grcomposite coatings electrodeposited at various current densities.

then decreases with an increase in current density. Theadsorption force of graphene nanoplatelets surrounded bypositive ion cloud (Cu2+ and Ni2+) in the bath is increasedwith an increase in current density within a certain range [8].Hence, the content of graphene nanoplatelets in the coat-ings is gradually increased up to 6 A dm−2 as observed infigure 3. However, with further increase in current densityto 8 A dm−2, independent metal ions (Cu2+ and Ni2+) arequickly attracted to the substrate, the content of incorporatedgraphene nanoplatelets decreases [37,38]. Thus, the micro-hardness of Cu–Ni/Gr composite coating decreases at currentdensity of 8 A dm−2.

Figure 8. Change in the lattice strain of Cu–Ni and Cu–Ni/Grcomposite coatings electrodeposited at various current densities.

Figure 9. Variation in microhardness of Cu–Ni and Cu–Ni/Grcomposite coatings electrodeposited at various current densities.

3.4 Tribological performance

The average coefficient of friction graph of Cu–Ni and Cu–Ni/Gr composite coatings electrodeposited at various currentdensities is given in figure 10. The semiconductor straingauges measured the frictional forces for 500 cycles. Fromthe ratio of tangent force to the normal force, the coefficient offriction was calculated. The measured average coefficient offriction for Cu–Ni/Gr composite coatings is lower than Cu–Nicoating. The Cu–Ni/Gr composite coating electrodepositedat 6 A dm−2 has a lower value of the average coefficientof friction compared to the remaining current densities. Thereduction of coefficient of friction is attributed to the role ofgraphene nanoplatelets in the Cu–Ni/Gr composite coatings[39,40]. The graphene nanoplatelets present in the coatingact as a lubricant and mating surface slides easily over each


Figure 10. The average coefficient of friction graph of Cu–Ni andCu–Ni/Gr composite coatings electrodeposited at various currentdensities.

Table 3. Calculated corrosion results of Cu–Ni and Cu–Ni/Grcomposite coatings electrodeposited at various current densities.

Description Ecorr (V) Icorr (μA)

Cu–Ni −0.432 96Cu–Ni/Gr (2 A dm−2) −0.426 52Cu–Ni/Gr (4 A dm−2) −0.424 63Cu–Ni/Gr (6 A dm−2) −0.411 31Cu–Ni/Gr (8 A dm−2) −0.428 87

other [41]. In addition, according to Archard’s equation, thewear rate of a material is inversely proportional to its hard-ness [42]. The Cu–Ni/Gr composite coating electrodepositedat 6 A dm−2 has higher microhardness; thus, it controls theplastic deformation of coating and helps to reduce the coef-ficient of friction. However, the increase in the coefficient offriction in the other electrodeposited coatings is due to thedecrease in graphene nanoplatelets content in the coatings.From figure 3, it is clearly seen that when current densityincreases up to 6 A dm−2, the graphene content in the coatingincreases gradually and hence, the average coefficient of fric-tion also decreases with increase in graphene content, whichcan be seen in figure 10.

3.5 Corrosion performance

The potentio-dynamic linear polarization graphs of Cu–Niand Cu–Ni/Gr composite coatings electrodeposited at variouscurrent densities are given in figure 11 and the correspond-ing calculation are shown in table 3. In a potential range of+0.4 to −0.9 V at the scan rate of 0.01 V s−1, the electrodekinetics of the electrodeposited coatings were examined. Thecorrosion resistance of the Cu–Ni/Gr composite coatings is

Figure 11. Potentio-dynamic linear polarization graph of Cu–Niand Cu–Ni/Gr composite coatings electrodeposited at various cur-rent densities.

affected by the current density, which is displayed in table 3and figure 11. The Cu–Ni/Gr composite coatings electrode-posited at 6 A dm−2 shows lower corrosion current and highercorrosion potential. The low rate of corrosion is the cause oflower corrosion current, and high rate of corrosion is the causeof lower corrosion potential. At 6 A dm−2 current density,the Cu–Ni/Gr composite coating gives lower anodic current,which shows that higher corrosion resistance of the preparedcoating.

There are many reasons for enhancement in the corrosionresistance of the nanoparticles-reinforced composite coatings[43–45]. Incorporated graphene nanoplatelets avoid the initia-tion and growth of corrosion by filling the gaps and microholesin the composite coatings [17]. Cu–Ni/Gr composite coatingdeposited at 6 A dm−2 has high graphene content; therefore,it shows higher corrosion resistance. Moreover, the compositecoating deposited at 8 A dm−2 has high surface roughness andless graphene, results in high surface area exposed to the cor-rosive environment and decrease in corrosion resistance [44].

4. Conclusion

In the present study, Cu–Ni/Gr composite coatings were suc-cessfully electrodeposited at various current densities (suchas 2, 4, 6 and 8 A dm−2). The influence of various currentdensities on the different characteristics of Cu–Ni/Gr com-posite coatings was examined, and the results were reportedas follows:

1. The crystalline structure was fcc for the electrodepositedCu–Ni and Cu–Ni/Gr composite coatings.

2. In the coating area of Cu–Ni/Gr composite, the graphenenanoplatelets were homogeneously distributed.


3. The amount of graphene nanoplatelets in Cu–Ni/Grcomposites coatings was increased up to 6 A dm−2 andthen decreased.

4. The incorporation of graphene nanoplatelets in Cu–Nicoating influenced the crystallite size. As the graphenenanoplatelets content was increased, the crystallite size ofthe coating decreased, which in turn increased the micro-hardness, corrosion resistance and decrease the coefficientof friction of the coating.

5. Cu–Ni/Gr composite coating reached the maximum valuesof desirable properties at 6 A dm−2 of current density.

Acknowledgements

We gratefully acknowledge Birla Institute of Technology andScience (BITS), Pilani Campus, Rajasthan, India, for thefinancial support to this research. We are thankful to Ms Nee-lakshi Sharma, Mr M Dinachandra Singh and Prof AnshumanDalvi of BITS, Pilani, for their help and support in X-raydiffraction. We express our sincere thanks to Dr Girish Kantfrom Mechanical Department, BITS, Pilani, for his support intribological testing. We are thankful to Dr Surojit Pande fromChemistry Department, BITS, Pilani, for his support in cor-rosion testing. We are thankful to Material Research Centre,MNIT, Jaipur, for the technical assistance during microhard-ness characterization.

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Synthesis and characterization of Cu–Ni/Gr nanocomposite