research article preparation and microwave absorbing...

Research ArticlePreparation and Microwave Absorbing Properties ofan Electroless Ni-Co Coating on Multiwall Carbon NanotubesUsing [Ag(NH3)2]+ as Activator

Qiao-ling Li Xiao-yong He Yue-qing Zhang and Xiao-feng Yang

Department of Chemistry School of Science North University of China Taiyuan 030051 China

Correspondence should be addressed to Qiao-ling Li qiaolingl163com

Received 16 December 2014 Accepted 17 April 2015

Academic Editor Taeseup Song

Copyright copy 2015 Qiao-ling Li et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Ni-Co-coated carbon nanotubes (CNTs) composites with different molar ratios of NiCo were synthesized using [Ag(NH3)2]+ as

activator and H2PO2

minus as reductant thereby replacing the conventional noble metal Pd salt activator and Sn2+ reductant Scanningelectron microscopy X-ray diffraction and X-ray energy dispersive spectrometry analyses demonstrated that the CNTs weredeposited with a dense uniformNi-Co coatingThe possiblemechanism of the electrolessmethodwas studied which indicates thatpure Ag0 acted as a nucleation site for subsequent Ni-Co-P deposition Network vector analyzer measurements indicated that thecomposite with only Ni coated had an absorbing value of minus126 dB and the composite with a NiCo ratio of four had the maximumwave absorption (minus156 dB) and the widest absorption bandwidth (800MHz RL lt minus10 dB) while the saturation magnetization(Ms) was 428 emusdotgminus1 and the coercive force (Hc) was 3133Oe

1 Introduction

Carbon nanotubes (CNTs) discovered by Iijima [1] haveattracted considerable attention due to their perfect struc-ture and outstanding mechanical electrical properties [2ndash4] However their magnetic properties are not ideal whichlimits their extensive application in electromagnetic energydissipation field especially in themicrowave absorption fieldConsequently it is necessary to improve their magnetic anddielectric properties with electroless magnetic metal plating[5 6] so as to make CNTs excellent microwave absorbingmaterials

Electroless plating is a surface modification method bywhich metallic ions were changed into metals under thecatalytic activation of the substrate surface and are depositedon the substrate surface without an external current [7]Considering the good conductivity chemical stability andunique one-dimensional structure of CNTs they were cho-sen as the substrate of electroless plating Besides CNTscomposites are light weight and high temperature resistantIn the case of a nonmetallic substrate such as CNTs thekey to electroless plating lies in activating the substrate and

coating a catalytic metal layer on the substrate surface [8]Palladium has been employed as an activator for many years[9] Using SnCl

2-PdCl

2as sensitization-activation solution

[10] the stannous ions adsorbed by the physical adsorptionof the substrate surface reduce the palladium ions to metallicpalladium seeds which can then act as catalytic nuclei forsubsequent metal electroless deposition [11] The palladiumactivation method is costly [12] and has low efficiency and arisk of contamination

In this paper considering that Ag particles have strongoxidability [13 14] and can also be used as catalysts of an elec-troless plating reaction [15 16] we developed a low-cost andlow-pollution palladium-free activation process acidulatedmultiwalled carbon nanotubes (MWCNTs) were activatedwith silver ammonia solution and then the adsorbed silverions were reduced to silver seeds by sodium hypophosphitesolution Silver seeds will catalyze succedent nickel-cobaltalloy deposition on the MWCNTs The key work lies in theinvestigation of the effect of the molar ratio of nickel-cobalton the magnetic properties and microwave absorbing prop-erties of composites Besides this the electroless mechanismwas studied to some extent

Hindawi Publishing CorporationJournal of NanomaterialsVolume 2015 Article ID 404698 7 pageshttpdxdoiorg1011552015404698

2 Journal of Nanomaterials

Table 1 Composition of nickel-cobalt plating solution

Compose Chemicals Content (gsdotLminus1)Main salt NiSO4sdot6H2O 45ndash182Main salt CoSO4sdot7H2O 82ndash194Reductant NaH2PO2 182Complexing agent Na3C6H5O7sdotH2O 413Buffering agent (NH4)2SO4 231Acidity pH 9

2 Materials and Methods

21 Pretreatment of MWCNTs MWCNTs (Nanoport CoLtd Shenzhen China) with 10ndash30 nm diameters 5ndash15120583mlength and a purity higher than 98 were roasted in a mufflefurnace at 500∘C for 2 h they were then stirred in a mixedacid solution of concentrated sulfuric and nitric acid (1 3 byvolume) at 80∘C for 2 h Afterward theMWCNTswere rinsedin distilled water several times until the pH value reachedneutral and were then dried at 80∘C for further use

22 Activation of PretreatedMWCNTs Silver ammonia solu-tion was first prepared by dropping dilute aqua ammoniainto 20 gL silver nitrate solution until the solution changedfrom turbidity to clarityThen the pretreatedMWCNTs wereadded to the silver ammonia solution sonicated for 05 h andstirred at 80∘C for 05 h after adding 25 gL sodiumhypophos-phite solution to the mixture just mentioned Finally theactivated solution was filtered and the residue was rinsed indistilled water until the pH value reached neutral and wasthen dried at 70∘C for 8 h for further use

23 Electroless Ni-Co Alloy Deposition onMWCNTs Figure 1shows the procedure used for the preparation of Ni-Co platedMWCNTs Table 1 shows the composition of the nickel-cobalt plating solution NiSO

4sdot6H2O CoSO

4sdot7H2O (119899Ni 119899Co

= 4 1 3 2 1 1 2 3) Na3C6H5O7sdotH2O and (NH

4)2SO4were

dissolved in distilled water of different volumes respectivelyorderly added to a certain quality of activated MWCNTs(the ratio of Ni-Co particles to MWCNTs was 9 1) andsonicated for 05 h sodium hypophosphite solution wasfinally added and ultrasonic treatment was continued for20minAfterward themixturewas stirred at amedium speedat 80∘C for 1 h During the entire reaction the pH value wasadjusted to 9 with aqua ammonia and sodium hypophosphitesolution being added until the solution no longer bubbledFinally the solution was filtered and the residue was rinsedin distilled water until the pH value reached neutral and wasthen dried for 8 h at 80∘C samples were marked as A B CandDTheNi-MWCNTs composite was prepared in a similarmanner and marked as E

24 Characterization The surface morphology and elemen-tal composition of the coating were determined by scanningelectron microscopy (SEM) and an X-ray energy dispersivespectrometer (EDS) The phase structure was analyzed by X-ray diffraction (XRD) The magnetic property was measured

Pristine MWCNTs

Acid-pretreated MWCNTs

[Ag(NH3)2]+ solution

NaH2PO2 solution

Electroless nickel-cobalt platingsolution

Nickel-cobalt-coated MWCNTs

Figure 1 Procedure for preparation of nickel-cobalt-coatedMWCNTs

at room temperature using a vibration sample magnetometer(VSM) with a maximum field of 10 kOe The absorbingproperty was detected by a Network Vector Analyzer

3 Results and Discussion

31 Morphology Analysis Figure 2 depicts the SEM micro-graphs of MWCNTs before and after electroless plating Thepristine MWCNTs are highly entangled (Figure 2(a)) theblack dots on the surface of the MWCNTs were expectedto be graphite metallic catalyst particles and amorphouscarbons After acid treatment the black dots were completelyremoved long MWCNTs were cut off the caps of MWCNTswere opened and etched off (Figure 2(b)) The surface ofthe activated MWCNTs appeared to be coated with manyuniform and white silver particles (Figure 2(c)) implyingthat the Ag(NH

3)

2

+ activator can also form well-distributedactive centers Figure 2(d) was only the Ni-coated compositewhich had a netty structure As observed from the images ofthe Ni-Co-coated MWCNTs with a different molar ratio ofnickel-cobalt apparentlyMWCNTswere completely coveredwith uniform and continuous alloy particles Ni-Co grew intosphericity along active centers and formed metal nanofiberswhich took carbon nanotubes as the core (Figures 2(e) and2(f)) In particular from Figure 2(g) we can see differentradial nickel and cobalt atoms clusters getting together andforming a dense coating Meanwhile with the increase ofCo content the size of the Ni-Co particles gets bigger from400 nm (Figure 2(e)) to 1 120583m (Figure 2(g))

32 Schematic Procedure of Preparation of Nickel-Cobalt-CoatedMWCNTs Figure 3 depicts the schematic procedureIn the process of electroless the MWCNTs act as a one-dimensional template for Ni-Co deposition At the sametime because of its outstanding conductivity it can alsoenhance the property of composites The whole nickel-cobalt

Journal of Nanomaterials 3

(a) (b)

(c) (d)

(e) (f)

(g)

Figure 2 SEM micrographs of MWCNTs before and after coated (a) pristine MWCNTs (b) acid-pretreated MWCNTs (c) activatedMWCNTs (d) Ni-MWCNTs (e) Ni

8Co2-MWCNTs (f) Ni

5Co5-MWCNTs (g) Ni

4Co6-MWCNTs



[Ag(NH3)2]+ activation

NaH2PO2 reduction

Ni-Co-coated MWCNTs

Co atom Ni atom

Figure 3 Schematic of electroless Ni-Co alloy-coated MWCNTs

plating process consisted of four steps After acid treatmentthe caps of MWCNTs were etched off and the MWCNTswere activated with more oxidized functional groups such ascarboxylic groups and hydroxyl groups which can improvethe dispersion water-solubility and activity of MWCNTs inthe electroless solution (step 1)

Ag(NH3)

2

+ ions were adsorbed on the MWCNTs sur-face with considerable OHminus ions forming an Ag(NH

3)2OH

complex by an electrostatic interaction (step 2) ImmersingMWCNTs into NaH

2PO2solution H

2PO2

minus will reduce theadsorbed Ag(NH

3)

2

+ ions into Ag particles as active centersbeing uniformly distributed on the MWCNTs surface [17]In alkaline condition the standard electrode potential ofHPO3

2minusH2PO2

minus couple is minus157V while Ag(NH3)

2

+Ag is+0373V The standard battery electromotive force of suchelectrode couples is positive value [Δ119864∘ = +0373V minus(minus157V) = 1943V] therefore the reaction is spontaneousand the equation is as follows (step 3)

2H2PO2minus+ 2 [Ag (NH3)2]

++ 2H2

= 2HPO32minus+ 4NH4

++ 2Ag +H2

(1)

Under the catalytic action of Ag complex Ni2+ and Co2ions in the plating solution are changed into Ni and Coparticles and deposited onto the surface by capturing elec-trons furnished by the reductant (H

2PO2

minus) by the followingreactions [18]

Ni2+ +mL119899minus + 4H2PO2minus+H2O

Ag997888997888rarr Ni + P + 3HPO3

2minus+ 4H+ + 3

2H2 +mL119899minus

(2)

4000

3000

2000

1000

0

20 30 40 50 60 70 80

Inte

nsity

2120579 (∘)

Ni4Co6

Ni6Co4

Ni8Co2

MWCNTs

Ni5Co5

120572-Co Ni (220)

PDF 4-0783)Ag (

Ni-Co-P (71-2336)

Figure 4 XRD patterns of different samples

(CoLx)2+ +H2PO2minus+ 3OHminus

Ag997888997888rarr Co +HPO3

2minus+ 2H2O + xL

(3)

L stands for a complexing agent and the overall reaction canbe written as follows

Ni2+ + Co2+ + 6H2PO2minus+ 2OHminus

= Ni + Co + 4H2PO3minus+H2 + 2P + 2H2O

(4)

The deposited Ni and Co atoms then can act as self-catalysts for obtaining a well-developed Ni-Co coating Withthe decrease in the Ni2+ and Co2+ ion concentration fewerbubbles are released implying that the reaction is graduallyslowing down In order to obtain a compact and continuouscoating we need to add an electroless solution and adjust thepH value constantly

33 X-Ray Diffraction Analysis and EDS Analysis Figure 4depicts the XRD patterns of nickel-cobalt-coated MWCNTscomposites without heat treatment It was found that themolar ratio of nickel-cobalt influenced the intensity of onlysome peaks A broad peak at about 2120579 = 45∘ was assignedto the Ni-Co-P amorphous diffraction peak which indicatedthat the Ni-Co-P plating was amorphous under the electro-less state The amorphous peak was the most obvious andthe sharpest when the molar ratio of nickel-cobalt was 1 1Diffraction peaks at 2120579 = 7722∘ 64∘ 382∘ were attributed toNi (220) 120572-Co and Ag (111) (PDF 4-0783) in each patternAn intense Ag diffraction peak demonstrated the formationof many active centers on the surface of theMWCNTs whichwere critical to the later electroless plating Additionally forthe Ni-Co-MWCNTs composites a diffraction peak assignedto the (002) plane of the MWCNTs at 261∘ has been lessobvious implying that theMWCNTs were completely coatedby the Ni-Co alloy

Figure 5 displays EDS spectra of the electrolessMWCNTs which mainly showed Ni and Co peaks Theelement of Au plating on the surface can strengthen the


150

100

50

00 1 2 3 4 5 6 7 8 9 10

C

ONi

NaAu

Mo BaBaCo

Ni

Ni

Cou

nts

Energy (keV)

Ni4Co6

Figure 5 EDS spectra of Ni-Co alloy-coated MWCNTs

reflectivity of the signal in the process of scanning Thecontents of Ni and Co were 412 and 1582wt whichdemonstrated that the main matter of coating was the Ni-Coalloy The intense O peak indicated that the surface of theMWCNTs was sufficiently oxidized the appearance of theweak C peak was attributed to a thin coating

34 Magnetic Property Analysis Figure 6 shows the hystere-sis line of the Ni-Co-coated MWCNTs composites Table 2lists the main magnetic properties of different samples Asobserved in the table Ni-Co-coated MWCNTs had moreexcellent magnetic properties than pristine MWCNTs andNi-Co alloy without MWCNTs With an increase of Co con-tent saturation magnetization (Ms) was gradually enhancedand coercive force (Hc) was typically improved Co is adensely hexagonal structure and has a great magnetic crystalanisotropic constant 119870 and a magnetostrictive coefficient120582

119878 which make the coercive force increase In addition the

atomic magnetic moment of Co is larger than that of Ni theincrease in Co content is helpful in improving the saturationmagnetization (Ms) [19]

35 Microwave Absorbing Analysis The Ni-Co-P-coatedMWCNTs composite is a kind of microwave absorbingmaterial which has both hysteresis losses and electric lossThe reflection loss can be defined by the loss tangent (tan 120575)which includes dielectric loss tangent and magnetic losstangent [20] Their relationship can be explained by thefollowing formulas

tan 120575120576120574=

120576

10158401015840

120574

120576

1015840

120574

tan 120575120583120574=

120583

10158401015840

120574

120583

1015840

120574

tan 120575 = tan 120575120576120574+ tan 120575120583

120574=

120576

10158401015840

120574

120576

120574

+

120583

10158401015840

120574

120583

1015840

120574

(5)

Here 120576120574and 120583

120574are the complex relative permittivity and

complex relative permeability respectively Electromagneticparameters (1205761015840

120574 120576101584010158401205741205831015840120574 and12058310158401015840

120574) are the real part and imaginary

part of relative permittivity and relative permeability respec-tively In this work both the loss tangent and reflection lossaremeasured by theNetworkVectorAnalyzerThe compositewas mixed with paraffin in themass ratio of 7 3 meanwhilethe absorbing layer thickness was 3mm As observed from

Table 2 The magnetic properties of different samples

Sample HcOe Msemusdotgminus1 Mremusdotgminus1

Ni-Co alloy without MWNTs 1109 321 027Pristine MWNTs 3221 056 006Ni8Co2 3133 428 056Ni6Co4 4856 899 081Ni5Co5 6674 1274 106Ni4Co6 8521 1589 115

15

10

5

0

minus5

minus10

minus15

minus20

minus25

minus6000 minus4000 minus2000 0 2000 4000 6000

Ms (

emu

g)

Hc (Oe)

Figure 6 Hysteresis line of the composites

Figures 7 and 8 the order of the dielectric loss tangentwasAgtC gt E gtD gt B while the magnetic loss tangent was B gtA gtDgt C gt E As depicted in Figure 9 the reflection loss of sampleA was the largest (minus156 db) next was that of the ordinalsamples E (minus126 db) C (minus98 db) B (minus96 db) and D (minus6 db)The frequency range of the attenuation peak was 3000MHzndash5500MHz the bandwidth (RL lt minus5 db) of sample E wasthe largest (18 GHz) with B (17 GHz) A (15 GHz) C(12 GHz) and D (08GHz) successively the bandwidth (RLlt minus10 db) of samples A and E was 800MHz and 400MHzrespectively Ni-Co-coated MWCNTs composites displayedstronger wideband microwave absorbing properties and thepossible reason was as follows the surface of the MWCNTsconsisted of coated metal nanoparticles which possessedmagnetism and microwave absorbing abilities of generalnanoparticles Coupling between the microwave field andinternal magnetic field of the nanoparticles caused magneticresonance absorption magnetic particles were magnetizedin an alternating magnetic field and produced a certainmagnetic flux density whose change produced magneticinduced electromotive force and eddy current thus resultingin an eddy current loss In addition the magnetic lag effectalso caused a part of electromagnetic energy loss

4 Conclusions

The surface of acid-treated MWCNTs was coated with adense and uniform Ni-Co plating by utilizing Ag(NH

3)

2

+ asan activator replacing a conventional costly and pollutionalPd (II) activator Results implied that coercive force (Hc)


18

16

14

12

10

8

6

4

2

0

minus20 1000 2000 3000 4000 5000 6000

Frequency (MHz)

tan 120575120576

Ni8Co2Ni6Co4Ni5Co5

Ni4Co6Ni

Figure 7 Dielectric loss tangent of different samples

18

20

16

14

12

10

8

6

4

2

0

minus2

0 1000 2000 3000 4000 5000 6000

Frequency (MHz)

Ni8Co2Ni6Co4Ni5Co5

Ni4Co6Ni

tan 120575120583

Figure 8 Magnetic loss tangent of different samples

and saturation magnetization (Ms) were gradually enhancedwith the increase of Co content the molar ratio of nickel-cobalt influenced the intensity of only some peaks TheNi8Co2composite possessed the best microwave absorption

(minus156 dB) in the lower frequency range (0ndash6000MHz) owingto its better dielectric loss tangent and magnetic loss tangentand the frequency bandwidth (RL lt minus10 db) was also thelargest (800MHz)The electroless mechanism was studied tosome extent indicating that pure Ag0 acted as a nucleationsite for subsequent Ni-Co-P deposition

2

0

minus2

minus4

minus6

minus8

minus10

minus12

minus14

minus16

RL (d

B)

0 1000 2000 3000 4000 5000 6000

Frequency (MHz)

Ni8Co2Ni6Co4Ni5Co5

Ni4Co6Ni

Figure 9 Reflection loss of different samples

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China (nos 20871108 and 51272239) YouthScience Foundation of Shanxi Province (no 2010021022-3)and the Scientific Research Foundation of the Higher Educa-tion Institutions of Shanxi Province China (no 2010115)

References

[1] S Iijima ldquoHelicalmicrotubules of graphitic carbonrdquoNature vol354 no 6348 pp 56ndash58 1991

[2] C G Hu W L Wang and W Zhu ldquoElectrochemical investi-gation on carbon nanotube film with different pretreatmentsrdquoActaMetallurgica Sinica (English Letters) vol 16 no 4 pp 295ndash301 2003

[3] J W Jiang and F Peng ldquoActuality and progress of applicationand research of Carbon Nanotubesrdquo Materials Science andEngineering Transaction vol 21 no 3 pp 464ndash468 2003

[4] H Hiura T W Ebbesen J Fujita K Tanigaki and T TakadaldquoRole of sp3 defect structures in graphite and carbon nan-otubesrdquo Nature vol 367 no 6459 pp 148ndash151 1994

[5] S J Tans A R M Verschueren and C Dekker ldquoRoom-tempe-rature transistor based on a single carbon nanotuberdquo Naturevol 393 no 6680 pp 49ndash52 1998

[6] C Niu E K Sichel R Hoch D Moy and H Tennent ldquoHighpower electrochemical capacitors based on carbon nanotubeelectrodesrdquoApplied Physics Letters vol 70 no 11 pp 1480ndash14821997


[7] Y Zhao Y-J Xue H Zheng and Y-X Duan ldquoElectroless plat-ing of CondashFe on the surface of MWCNTsrdquo New Carbon Mat-erials vol 25 no 1 pp 65ndash70 2010

[8] H Liu J Li and LWang ldquoElectroless nickel plating on APTHSmodified wood veneer for EMI shieldingrdquo Applied SurfaceScience vol 257 no 4 pp 1325ndash1330 2010

[9] P K Rohatgi ldquoCast aluminium alloy composites containingcopper-coated ground mica particlesrdquo Journal of MaterialsScience vol 16 no 6 pp 1599ndash1606 1981

[10] T J OrsquoKeefe ldquoMethod for stripping metals in solvent extrac-tionrdquo US Patent 5228903 1993

[11] W Sha X Wu and K G Keong Electroless Copper and Nickel-Phosphorus Plating Processing Characterisation and ModelingWoodhead Publishing Cambridge UK 2010

[12] J Li M J OrsquoKeefe Matthew and T J OrsquoKeefe Thomas ldquoActi-vation of non-metallic substrates for metal deposition usingorganic solutionsrdquo Surface and Coatings Technology vol 205no 10 pp 3134ndash3140 2011

[13] D Y Zemlyanov A Nagy and R Schlogl ldquoThe reaction of silverwithNOO

2rdquoApplied Surface Science vol 133 no 3 pp 171ndash183

1998[14] W L Dai Y Cao L P Ren et al ldquoAg-SiO

2-Al2O3composite as

highly active catalyst for the formation of formaldehyde fromthe partial oxidation of methanolrdquo Journal of Catalysis vol 228no 1 pp 80ndash91 2004

[15] S Shukla S Seal Z Rahaman and K Scammon ldquoElectrolesscopper coating of cenospheres using silver nitrate activatorrdquoMaterials Letters vol 57 no 1 pp 151ndash156 2002

[16] I Lee P T Hammond and M F Rubner ldquoSelective electrolessnickel plating of particle arrays on polyelectrolyte multilayersrdquoChemistry of Materials vol 15 no 24 pp 4583ndash4589 2003

[17] Z Aixiang XWeihao andX Jian ldquoElectrolessNi-Co-P coatingof cenospheres using [Ag(NH

3)2]+ activatorrdquoMaterials Letters

vol 59 no 4 pp 524ndash528 2005[18] X Jiang and W Shen The Fundamentals and Practice of

Electroless Plating National Defence Industry Press BeijingChina 2000

[19] D Cai J Gu Z Huang and F Kang ldquoCarbon nanotubes withnickel-cobalt alloy coatingrdquo Journal of Materials Science andEngineering vol 25 no 5 pp 713ndash714 2007

[20] TMMeaz SM Attia and AM Abo El Ata ldquoEffect of tetrava-lent titanium ions substitution on the dielectric properties ofCo-Zn ferritesrdquo Journal of Magnetism and Magnetic Materialsvol 257 no 2-3 pp 296ndash305 2003

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Table 1 Composition of nickel-cobalt plating solution

Compose Chemicals Content (gsdotLminus1)Main salt NiSO4sdot6H2O 45ndash182Main salt CoSO4sdot7H2O 82ndash194Reductant NaH2PO2 182Complexing agent Na3C6H5O7sdotH2O 413Buffering agent (NH4)2SO4 231Acidity pH 9

2 Materials and Methods

21 Pretreatment of MWCNTs MWCNTs (Nanoport CoLtd Shenzhen China) with 10ndash30 nm diameters 5ndash15120583mlength and a purity higher than 98 were roasted in a mufflefurnace at 500∘C for 2 h they were then stirred in a mixedacid solution of concentrated sulfuric and nitric acid (1 3 byvolume) at 80∘C for 2 h Afterward theMWCNTswere rinsedin distilled water several times until the pH value reachedneutral and were then dried at 80∘C for further use

22 Activation of PretreatedMWCNTs Silver ammonia solu-tion was first prepared by dropping dilute aqua ammoniainto 20 gL silver nitrate solution until the solution changedfrom turbidity to clarityThen the pretreatedMWCNTs wereadded to the silver ammonia solution sonicated for 05 h andstirred at 80∘C for 05 h after adding 25 gL sodiumhypophos-phite solution to the mixture just mentioned Finally theactivated solution was filtered and the residue was rinsed indistilled water until the pH value reached neutral and wasthen dried at 70∘C for 8 h for further use

23 Electroless Ni-Co Alloy Deposition onMWCNTs Figure 1shows the procedure used for the preparation of Ni-Co platedMWCNTs Table 1 shows the composition of the nickel-cobalt plating solution NiSO

4sdot6H2O CoSO

4sdot7H2O (119899Ni 119899Co

= 4 1 3 2 1 1 2 3) Na3C6H5O7sdotH2O and (NH

4)2SO4were

dissolved in distilled water of different volumes respectivelyorderly added to a certain quality of activated MWCNTs(the ratio of Ni-Co particles to MWCNTs was 9 1) andsonicated for 05 h sodium hypophosphite solution wasfinally added and ultrasonic treatment was continued for20minAfterward themixturewas stirred at amedium speedat 80∘C for 1 h During the entire reaction the pH value wasadjusted to 9 with aqua ammonia and sodium hypophosphitesolution being added until the solution no longer bubbledFinally the solution was filtered and the residue was rinsedin distilled water until the pH value reached neutral and wasthen dried for 8 h at 80∘C samples were marked as A B CandDTheNi-MWCNTs composite was prepared in a similarmanner and marked as E

24 Characterization The surface morphology and elemen-tal composition of the coating were determined by scanningelectron microscopy (SEM) and an X-ray energy dispersivespectrometer (EDS) The phase structure was analyzed by X-ray diffraction (XRD) The magnetic property was measured

Pristine MWCNTs


[Ag(NH3)2]+ solution

NaH2PO2 solution

Electroless nickel-cobalt platingsolution

Nickel-cobalt-coated MWCNTs

Figure 1 Procedure for preparation of nickel-cobalt-coatedMWCNTs

at room temperature using a vibration sample magnetometer(VSM) with a maximum field of 10 kOe The absorbingproperty was detected by a Network Vector Analyzer

3 Results and Discussion

31 Morphology Analysis Figure 2 depicts the SEM micro-graphs of MWCNTs before and after electroless plating Thepristine MWCNTs are highly entangled (Figure 2(a)) theblack dots on the surface of the MWCNTs were expectedto be graphite metallic catalyst particles and amorphouscarbons After acid treatment the black dots were completelyremoved long MWCNTs were cut off the caps of MWCNTswere opened and etched off (Figure 2(b)) The surface ofthe activated MWCNTs appeared to be coated with manyuniform and white silver particles (Figure 2(c)) implyingthat the Ag(NH

3)

2

+ activator can also form well-distributedactive centers Figure 2(d) was only the Ni-coated compositewhich had a netty structure As observed from the images ofthe Ni-Co-coated MWCNTs with a different molar ratio ofnickel-cobalt apparentlyMWCNTswere completely coveredwith uniform and continuous alloy particles Ni-Co grew intosphericity along active centers and formed metal nanofiberswhich took carbon nanotubes as the core (Figures 2(e) and2(f)) In particular from Figure 2(g) we can see differentradial nickel and cobalt atoms clusters getting together andforming a dense coating Meanwhile with the increase ofCo content the size of the Ni-Co particles gets bigger from400 nm (Figure 2(e)) to 1 120583m (Figure 2(g))

32 Schematic Procedure of Preparation of Nickel-Cobalt-CoatedMWCNTs Figure 3 depicts the schematic procedureIn the process of electroless the MWCNTs act as a one-dimensional template for Ni-Co deposition At the sametime because of its outstanding conductivity it can alsoenhance the property of composites The whole nickel-cobalt


(a) (b)

(c) (d)

(e) (f)

(g)


8Co2-MWCNTs (f) Ni

5Co5-MWCNTs (g) Ni

4Co6-MWCNTs




NaH2PO2 reduction

Ni-Co-coated MWCNTs

Co atom Ni atom



Ag(NH3)

2


3)2OH


2PO2solution H

2PO2


3)

2


2minusH2PO2


2



++ 2H2

= 2HPO32minus+ 4NH4

++ 2Ag +H2

(1)


2PO2



Ag997888997888rarr Ni + P + 3HPO3

2minus+ 4H+ + 3

2H2 +mL119899minus

(2)

4000

3000

2000

1000

0

20 30 40 50 60 70 80

Inte

nsity

2120579 (∘)

Ni4Co6

Ni6Co4

Ni8Co2

MWCNTs

Ni5Co5

120572-Co Ni (220)

PDF 4-0783)Ag (

Ni-Co-P (71-2336)




2minus+ 2H2O + xL

(3)




(4)





150

100

50

00 1 2 3 4 5 6 7 8 9 10

C

ONi

NaAu

Mo BaBaCo

Ni

Ni

Cou

nts

Energy (keV)

Ni4Co6







tan 120575120576120574=

120576

10158401015840

120574

120576

1015840

120574

tan 120575120583120574=

120583

10158401015840

120574

120583

1015840

120574

tan 120575 = tan 120575120576120574+ tan 120575120583

120574=

120576

10158401015840

120574

120576

120574

+

120583

10158401015840

120574

120583

1015840

120574

(5)

Here 120576120574and 120583



120574 120576101584010158401205741205831015840120574 and12058310158401015840






15

10

5

0

minus5

minus10

minus15

minus20

minus25


Ms (

emu

g)

Hc (Oe)



4 Conclusions


3)

2



18

16

14

12

10

8

6

4

2

0

minus20 1000 2000 3000 4000 5000 6000

Frequency (MHz)

tan 120575120576

Ni8Co2Ni6Co4Ni5Co5

Ni4Co6Ni


18

20

16

14

12

10

8

6

4

2

0

minus2

0 1000 2000 3000 4000 5000 6000

Frequency (MHz)

Ni8Co2Ni6Co4Ni5Co5

Ni4Co6Ni

tan 120575120583




2

0

minus2

minus4

minus6

minus8

minus10

minus12

minus14

minus16

RL (d

B)

0 1000 2000 3000 4000 5000 6000

Frequency (MHz)

Ni8Co2Ni6Co4Ni5Co5

Ni4Co6Ni




Acknowledgments


References

















2-Al2O3composite as

















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




(a) (b)

(c) (d)

(e) (f)

(g)


8Co2-MWCNTs (f) Ni

5Co5-MWCNTs (g) Ni

4Co6-MWCNTs




NaH2PO2 reduction

Ni-Co-coated MWCNTs

Co atom Ni atom



Ag(NH3)

2


3)2OH


2PO2solution H

2PO2


3)

2


2minusH2PO2


2



++ 2H2

= 2HPO32minus+ 4NH4

++ 2Ag +H2

(1)


2PO2



Ag997888997888rarr Ni + P + 3HPO3

2minus+ 4H+ + 3

2H2 +mL119899minus

(2)

4000

3000

2000

1000

0

20 30 40 50 60 70 80

Inte

nsity

2120579 (∘)

Ni4Co6

Ni6Co4

Ni8Co2

MWCNTs

Ni5Co5

120572-Co Ni (220)

PDF 4-0783)Ag (

Ni-Co-P (71-2336)




2minus+ 2H2O + xL

(3)




(4)





150

100

50

00 1 2 3 4 5 6 7 8 9 10

C

ONi

NaAu

Mo BaBaCo

Ni

Ni

Cou

nts

Energy (keV)

Ni4Co6







tan 120575120576120574=

120576

10158401015840

120574

120576

1015840

120574

tan 120575120583120574=

120583

10158401015840

120574

120583

1015840

120574

tan 120575 = tan 120575120576120574+ tan 120575120583

120574=

120576

10158401015840

120574

120576

120574

+

120583

10158401015840

120574

120583

1015840

120574

(5)

Here 120576120574and 120583



120574 120576101584010158401205741205831015840120574 and12058310158401015840






15

10

5

0

minus5

minus10

minus15

minus20

minus25


Ms (

emu

g)

Hc (Oe)



4 Conclusions


3)

2



18

16

14

12

10

8

6

4

2

0

minus20 1000 2000 3000 4000 5000 6000

Frequency (MHz)

tan 120575120576

Ni8Co2Ni6Co4Ni5Co5

Ni4Co6Ni


18

20

16

14

12

10

8

6

4

2

0

minus2

0 1000 2000 3000 4000 5000 6000

Frequency (MHz)

Ni8Co2Ni6Co4Ni5Co5

Ni4Co6Ni

tan 120575120583




2

0

minus2

minus4

minus6

minus8

minus10

minus12

minus14

minus16

RL (d

B)

0 1000 2000 3000 4000 5000 6000

Frequency (MHz)

Ni8Co2Ni6Co4Ni5Co5

Ni4Co6Ni




Acknowledgments


References

















2-Al2O3composite as

















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






NaH2PO2 reduction

Ni-Co-coated MWCNTs

Co atom Ni atom



Ag(NH3)

2


3)2OH


2PO2solution H

2PO2


3)

2


2minusH2PO2


2



++ 2H2

= 2HPO32minus+ 4NH4

++ 2Ag +H2

(1)


2PO2



Ag997888997888rarr Ni + P + 3HPO3

2minus+ 4H+ + 3

2H2 +mL119899minus

(2)

4000

3000

2000

1000

0

20 30 40 50 60 70 80

Inte

nsity

2120579 (∘)

Ni4Co6

Ni6Co4

Ni8Co2

MWCNTs

Ni5Co5

120572-Co Ni (220)

PDF 4-0783)Ag (

Ni-Co-P (71-2336)




2minus+ 2H2O + xL

(3)




(4)





150

100

50

00 1 2 3 4 5 6 7 8 9 10

C

ONi

NaAu

Mo BaBaCo

Ni

Ni

Cou

nts

Energy (keV)

Ni4Co6







tan 120575120576120574=

120576

10158401015840

120574

120576

1015840

120574

tan 120575120583120574=

120583

10158401015840

120574

120583

1015840

120574

tan 120575 = tan 120575120576120574+ tan 120575120583

120574=

120576

10158401015840

120574

120576

120574

+

120583

10158401015840

120574

120583

1015840

120574

(5)

Here 120576120574and 120583



120574 120576101584010158401205741205831015840120574 and12058310158401015840






15

10

5

0

minus5

minus10

minus15

minus20

minus25


Ms (

emu

g)

Hc (Oe)



4 Conclusions


3)

2



18

16

14

12

10

8

6

4

2

0

minus20 1000 2000 3000 4000 5000 6000

Frequency (MHz)

tan 120575120576

Ni8Co2Ni6Co4Ni5Co5

Ni4Co6Ni


18

20

16

14

12

10

8

6

4

2

0

minus2

0 1000 2000 3000 4000 5000 6000

Frequency (MHz)

Ni8Co2Ni6Co4Ni5Co5

Ni4Co6Ni

tan 120575120583




2

0

minus2

minus4

minus6

minus8

minus10

minus12

minus14

minus16

RL (d

B)

0 1000 2000 3000 4000 5000 6000

Frequency (MHz)

Ni8Co2Ni6Co4Ni5Co5

Ni4Co6Ni




Acknowledgments


References

















2-Al2O3composite as

















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




150

100

50

00 1 2 3 4 5 6 7 8 9 10

C

ONi

NaAu

Mo BaBaCo

Ni

Ni

Cou

nts

Energy (keV)

Ni4Co6







tan 120575120576120574=

120576

10158401015840

120574

120576

1015840

120574

tan 120575120583120574=

120583

10158401015840

120574

120583

1015840

120574

tan 120575 = tan 120575120576120574+ tan 120575120583

120574=

120576

10158401015840

120574

120576

120574

+

120583

10158401015840

120574

120583

1015840

120574

(5)

Here 120576120574and 120583



120574 120576101584010158401205741205831015840120574 and12058310158401015840






15

10

5

0

minus5

minus10

minus15

minus20

minus25


Ms (

emu

g)

Hc (Oe)



4 Conclusions


3)

2



18

16

14

12

10

8

6

4

2

0

minus20 1000 2000 3000 4000 5000 6000

Frequency (MHz)

tan 120575120576

Ni8Co2Ni6Co4Ni5Co5

Ni4Co6Ni


18

20

16

14

12

10

8

6

4

2

0

minus2

0 1000 2000 3000 4000 5000 6000

Frequency (MHz)

Ni8Co2Ni6Co4Ni5Co5

Ni4Co6Ni

tan 120575120583




2

0

minus2

minus4

minus6

minus8

minus10

minus12

minus14

minus16

RL (d

B)

0 1000 2000 3000 4000 5000 6000

Frequency (MHz)

Ni8Co2Ni6Co4Ni5Co5

Ni4Co6Ni




Acknowledgments


References

















2-Al2O3composite as

















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




18

16

14

12

10

8

6

4

2

0

minus20 1000 2000 3000 4000 5000 6000

Frequency (MHz)

tan 120575120576

Ni8Co2Ni6Co4Ni5Co5

Ni4Co6Ni


18

20

16

14

12

10

8

6

4

2

0

minus2

0 1000 2000 3000 4000 5000 6000

Frequency (MHz)

Ni8Co2Ni6Co4Ni5Co5

Ni4Co6Ni

tan 120575120583




2

0

minus2

minus4

minus6

minus8

minus10

minus12

minus14

minus16

RL (d

B)

0 1000 2000 3000 4000 5000 6000

Frequency (MHz)

Ni8Co2Ni6Co4Ni5Co5

Ni4Co6Ni




Acknowledgments


References

















2-Al2O3composite as

















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials













2-Al2O3composite as

















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials










CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



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