Chemical Engineering Journal 243 (2014) 436–447

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Chemical Engineering Journal

journal homepage: www.elsevier .com/locate /ce j

Studies on structural and magnetic properties of pristine andnickel-filled carbon nanotubes synthesized using LaNi5 alloyparticles as a catalyst

1385-8947/$ - see front matter � 2014 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cej.2013.12.087

⇑ Corresponding author. Tel.: +91 44 22358653; fax: +91 44 2220060.E-mail address: pandurangan_a@yahoo.com (P. Arumugam).

Anthuvan Rajesh John, Pandurangan Arumugam ⇑Department of Chemistry, Institute of Catalysis and Petroleum Technology, Anna University, Chennai 600025, Tamilnadu, India

h i g h l i g h t s

� Pristine CNTs and Ni-filled CNTs were synthesized over LaNi5 alloy catalyst by CVD.� The structural and magnetic properties of pristine CNTs and Ni-filled CNTs compared.� Catalyst oxidation step plays a key role in the formation of Ni-filled CNTs.� The surface segregation mechanism was suggested for the growth of Ni-filled CNTs.� The Ni-filled CNTs exhibit superior ferromagnetic behavior.

a r t i c l e i n f o

Article history:Received 20 August 2013Received in revised form 19 December 2013Accepted 28 December 2013Available online 8 January 2014

Keywords:Carbon nanotubesNi-filled CNTsLaNi5 alloy catalystChemical vapor deposition

a b s t r a c t

Pristine carbon nanotubes (CNTs) and nickel-filled CNTs (Ni-filled CNTs) were synthesized by chemicalvapor deposition using lanthanum nickel alloy (LaNi5) particles as a both catalyst and source for theNi-filling. Pristine CNTs and Ni-filled CNTs were formed without and with catalyst oxidation of LaNi5

alloy, respectively. Transmission electron microscopy analysis clearly illustrated that the formation ofNi-filling during the growth of CNTs was due to the catalyst oxidation. X-ray diffraction and selected areaelectron diffraction analysis demonstrated that the encapsulated Ni exists as a single crystal face centeredcubic structure. Compared with the pristine CNTs, the first order and second order Raman spectra ofNi-filled CNTs showed a disorder induced D band, an increased ID/IG ratio, and a decreased I0G/IG ratio.The magnetic properties of the Ni-filled CNTs were studied by vibrating sample magnetometer at roomtemperature. Coercivity value of Ni-filled CNTs reached the maximum at 445.87 Oe, which is significantlylarger than that of the bulk Ni at room temperature. The Ni-filling process through the catalyst oxidationis explained using the ‘‘surface segregation’’ mechanism. In addition, we have suggested the possiblegrowth model involved in their formation.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Extensive research on carbon nanotubes (CNTs) has beencarried out after the first successful synthesis by Iijima’s researchgroup in 1991 [1]. Owing to their unique mechanical, electrical,thermal and field emission properties [2] CNTs have been hailedas for their tremendous potential applications in electronics,energy, sensors, field emitters, biology, hydrogen storage andcomposite materials [3]. Due to their hollow nature, another possi-ble approach proposed is to use CNTs as a shielding agent for theforeign material encapsulated within. The encapsulation of CNTswith magnetic metals (Fe, Co and Ni) is of interest due to their dis-tinct electronic and magnetic properties, as well their potential

applications in electromagnetic and microwave absorption [4], bio-medicines [5], magnetic data storage devices [6] and sensors formagnetic force microscopy [7]. In addition, encapsulated magneticmaterials have been prevented by graphitic carbon layers from theoxidation reactions by providing long-term stability.

Regarding the synthesis of pristine CNTs and metal filled-CNTs, agreat deal of effort has been devoted and a lot of techniques havebeen developed, including laser ablation [8], arc-discharge [9,10],thermal (catalytic) chemical vapor deposition (CVD) [11–13], capil-lary infiltration [14], chemical methods [15] and electrochemicaldeposition [16]. Among these, catalyst assisted growth by CVD hasproved to be more effective method than others when aiming forat cost-effective mass production. Generally, transition metals (Fe,Co, Ni), supported catalyst, metallocenes and alloy catalysts werefound to be very effective for the production of both pristine CNTsand metal-filled CNTs [11–13,17–21]. However, several research

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groups have utilized and reported the growth of pristine CNTsand metal-filled CNTs over metallocenes and alloy catalysts[11–13,22–24]. The main reason for this is that they can act as a cat-alyst as well as source for the carbon and metal-fillings. Besides,understanding the growth mechanism is important for premeditatedsynthesis and it should be potential for various applications [25,26].

In our previous work, phenyltrimethylgermane (C6H5Ge(CH3)3)precursor has been developed to produce semiconducting germa-nium nanowires encapsulated within multi-walled CNTs using asimple one-step CVD synthesis method [27]. In this work we con-tinue to investigate the encapsulation of ferromagnetic metal insidethe CNTs. Lanthanum nickel alloy (LaNi5) was demonstrated to bean efficient catalyst for the synthesis of CNTs, carbon nanofibersand boron nitride nanotubes [11,28,29]. In addition, the surfacechemistry of LaNi5 alloy demonstrated that the oxidation of LaNi5

alloy forms lanthanum oxide (La2O3) surface and Ni precipitate,which is suitable for Ni-filling during the growth of CNTs [30].Therefore, the utilization of LaNi5 alloy as a both catalyst and sourcefor the Ni-filling could be reasonable and efficient way to produceNi-filled CNTs. Herein, we report an alloy catalytic route to synthe-size both pristine and Ni-filled CNTs by CVD using LaNi5 alloy as acatalyst and source for the Ni-filling. A simple oxidative dissocia-tion step was inserted for the filling of Ni during the catalyticgrowth of CNTs. Scanning electron microscopy (SEM), transmissionelectron microscopy (TEM), energy dispersive X-ray analysis(EDAX) and confocal Raman microscopy (CRM) were used to char-acterize the morphology, composition and structure of the pristineCNTs and Ni-filled CNTs. Thermal stability and the amount of filledNi in the CNTs were determined by thermogravimetric analysis(TGA) in air atmosphere. The magnetic properties of Ni-filled CNTswere investigated using a vibrating sample magnetometer (VSM).

2. Experimental section

2.1. Synthesis of pristine CNTs and Ni-filled CNTs

The pristine CNTs and Ni-filled CNTs were synthesized using aconventional CVD using LaNi5 alloy particles (99.5%, metal basis,Alfa Aesar) as a both catalyst and source for the Ni-filling. The syn-thesis method was similar to that described elsewhere [11] exceptfor catalyst oxidation at 400 �C for 20 min in air atmosphere. Aquartz boat loaded with 100 mg LaNi5 alloy catalyst was placed atthe centre of a tubular furnace. Pure nitrogen gas (N2) was intro-duced into the quartz tube at a flow rate of 200 ml/min for10 min then the flow rate was decreased to 50 ml/min; the temper-ature of the furnace was heated from room temperature to 400 �C ata rate of 5 �C/min with a flow of N2 at rate of 50 ml/min. When thetemperature reached 400 �C, N2 flow was switched to air at a flowrate of 100 ml/min for 20 min to oxidize the catalyst. After oxida-tion, the furnace temperature was raised to 550 �C, the oxidized cat-alyst was reduced with a flow of hydrogen (H2) gas at the rate of100 ml/min for 20 min. When the center of the furnace temperaturereached 700 �C, a gas mixture of acetylene (C2H2) at a flow rate of60 ml/min and H2 at a flow rate of 30 ml/min was introduced intothe reactor. After 20 min of growth, the sample was cooled to roomtemperature under N2 ambient. The pristine CNTs were synthesizedby the similar route without the catalyst oxidation step at 400 �C.The as-synthesized samples were treated with 1:3 sulfuric acid(H2SO4) and nitric acid (HNO3) mixture to eliminate metallic impu-rities and heated in air at 400 �C for removal of the residual carbon.

1 For interpretation of color in Fig. 2, the reader is referred to the web version ofthis article.

2.2. Instrumentations

The crystalline phase of the commercially purchased LaNi5

alloy catalyst, pre-treated catalyst (air oxidation at 400 �C

followed by H2 reduction at 550 �C) and purified samples (pristineCNTs and Ni-filled CNTs) were obtained from PANalytical X’ Pertusing Ni-filtered Cu Ka radiation and a liquid N2 cooled germa-nium solid-state detector. The average particle size of the LaNi5

alloy catalyst, H2 treated catalyst and pre-treated catalyst wasdetermined by laser scattering particle size distribution analyzer(Zetasizer Ver. 6.20). The morphology of the LaNi5 alloy catalyst,H2 reduced catalyst and pretreated catalyst prior to growth wascharacterized by SEM. SEM equipped with EDAX (Oxford Instru-ments) was carried out using an ESEM Quanta 200, FEI scanningmicroscope operated at 20 kV to study the morphology and com-position of the purified pristine CNTs and Ni-filled CNTs samples.TEM and high-resolution TEM (HRTEM) (JEOL 3010) operated at200 kV was used to study the microstructure analysis of the pris-tine CNTs and Ni-filled CNTs. The samples for HRTEM analysiswas prepared by placing droplets of a suspension of the samplesin acetone on a polymer micro grid supported on a copper grid.The Raman spectra for the purified samples was carried out usinga WiTec GmbH, alpha scanning near-field optical microscope withCRM 200 confocal Raman microscope having a 514 nm laser asthe excitation source. TGA was carried out on a Universal V4.5ATA instruments Q500. 1–2 mg of purified samples were loadedinto platinum pans and heated from room temperature to900 �C in flowing air with a 60 ml min�1 flow rate to the sampleand a 40 ml min�1 flow rate of N2 to the balance. Magnetizationmeasurements were performed on commercially purchased LaNi5

alloy catalyst, H2 treated catalyst, pre-treated catalyst, pristineCNTs and Ni-filled CNTs by using a VSM (Lakeshore 7410) atroom temperature.

3. Results and discussion

The average particle size and morphology of the LaNi5 alloycatalyst, H2 treated catalyst and pre-treated catalyst was stud-ied using a particle size analyzer and SEM, respectively. Theaverage particle size histograms are presented in Fig. 1a–c. Itcan be seen from that the average particles size of the LaNi5

alloy catalyst, H2 treated catalyst and pre-treated catalyst fellin the range of 2–6 lm, 50–500 nm and 100–200 nm, respec-tively. These histograms confirm that the LaNi5 alloy particleswere reduced to smaller particles after H2 treatment and pre-treatment.

The typical SEM image in Fig. 2a shows the morphology ofcommercially purchased LaNi5 alloy catalyst, which consists ofa broad range of crystallite sizes. The morphology of the H2 trea-ted catalyst and pre-treated catalyst (Fig. 2b and c) are distinctlydifferent from that of the untreated catalyst. In general H2 gastreatment is used for reducing metal catalyst [11]. Comparedto untreated catalyst, the catalyst particles distribution after H2

reduction shows the formation of refined, non-uniform andirregular shaped smaller particles (Fig. 2b). Controversially, themorphology of the pre-treated catalyst indicates clean andsmooth homogeneous catalyst surface distribution (Fig. 2c). Adetailed morphology of pre-treated LaNi5 alloy catalyst was car-ried out by TEM analysis. TEM image presented in Fig. 2d indi-cates the formation of La2O3 surface (light color) and surfacedepletion of Ni cluster (dark color)1. The formed Ni-cluster is akey for the synthesis of Ni-filled CNTs. The more discussion aboutthe change in phase, formation of La2O3 surface and surfacedepletion of Ni cluster as a result of H2 reduction and pre-treat-ment are discussed in the following XRD and growth mechanismsections.

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Fig. 1. (a)–(c) particle size histogram of LaNi5 alloy catalyst, H2 treated catalyst and pre-treated catalyst, respectively.

Fig. 2. SEM images of (a) LaNi5 alloy catalyst, (b) H2 treated catalyst, (c) pre-treated catalyst and (d) TEM image of pre-treated catalyst.

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3.1. Scanning electron microscopy equipped with energy dispersiveX-ray analysis

The morphology and composition of purified samples (pristineCNTs and Ni-filled CNTs) were initially examined by SEM andEDAX, respectively. The SEM images of the product synthesizedwithout catalyst oxidation step are shown in Fig. 3a and b, whichconsists high yield of carbon filaments with lengths up to severalmicrometers and their diameters nominally constant. The SEMimages presented in Fig. 3c and d clearly indicate the characteristicmorphology changes in the synthesized sample were due to cata-lyst oxidation at 400 �C. Compared to CNTs synthesized withoutcatalyst oxidation, the diameters of the CNTs synthesized withcatalyst oxidation were not uniform and showed a broad diameterdistribution. We believed that, the broad diameter distribution isdue to metallic Ni inside CNTs, which forms encapsulated Ni

Fig. 3. SEM images of (a), (b) pristine CNTs; (c), (d) Ni-filled C

nanowire in the hollow tubes. EDAX spectra of CNTs obtainedwithout and with catalyst oxidation are shown in Fig. 3e and f,respectively, which confirm that the nanotubes synthesized with-out catalyst oxidation were mainly composed of C, while the syn-thesized nanotubes with catalyst oxidation were composed of Cand Ni. Moreover, the EDAX results indicate that the presence ofoxygen signal in both spectra. After synthesis we performed acidtreatment on the both type of as-synthesized CNTs to remove cat-alyst particles. It is well known that the acid purification enhancesthe number of defects in CNTs via facile oxidation and introducesurface oxygen-containing groups [31]. Therefore, we concludedthat the surface oxidation of CNTs is responsible for the appearanceof oxygen signal in the EDAX spectra. In addition, it is worth notingthat the oxygen signal intensity in pristine CNTs is higher than thatof the Ni-filled CNTs in our case demonstrates that the pristineCNTs were more oxidizable than Ni-filled CNTs.

NTs. EDAX spectra of (e) pristine CNTs (f) Ni-filled CNTs.

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3.2. Transmission/high resolution transmission electron microscopyanalysis

Furthermore, the detailed morphology and structure of the pris-tine CNTs and Ni-filled CNTs was carried out using TEM, HRTEM,selected area diffraction pattern (SAED) and Fast Fourier transform(FFT) techniques. Fig. 4 shows the TEM and HRTEM images of pris-tine CNTs obtained without catalyst oxidation of LaNi5 alloy. Lowand high magnification TEM images shown in Fig. 4a and b clearlyreveal that the nanotubes were uniformly hollow in the centrewith metal particle embedded in the tips of the CNTs (marked witharrow). The average outer and inner diameters of the CNTs ranged

Fig. 4. (a) and (b) TEM images of pristine CNTs, (c) typical fragment of unfilled CNT, (d) Hunfilled CNT, respectively.

from 30 to 35 nm and 10–15 nm, respectively. Typical HRTEMimages of the CNTs presented in Fig. 4c and d confirms that thenanotubes were multi-walled with well graphitized and the inter-layer distance between two adjacent graphene sheets is 0.34 nm,consistent with the d value of (002) plane of graphite (JCPDSNo.: 75-1621). The SAED pattern of pristine CNTs in Fig. 4e showsstrong (002) ring and other three diffused rings (101), (100) and(004) of the graphite. Fig. 4f is the corresponding FFT of the HRTEMimage also reflecting the (002) plane of graphite.

Ni-filled CNT along the length of the tube obtained after the oxi-dation treatment of LaNi5 alloy catalyst at 400 �C is shown in Fig. 5.Fig. 5a shows a typical fragment of individual Ni-filled CNTs. The

RTEM image of graphitic layers, (e) and (f) SAED pattern and FFT obtained from this

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average diameter of this Ni-filled CNT is ranges from 50 to 60 nm,and the encapsulated Ni is structurally uniform with 20–25 nm indiameters and up to 700–800 nm in length (aspect ratios L/D up to>100). Fig. 5b is an enlarged TEM image taken from the squared re-gion in Fig. 5a. The atomic resolved HRTEM image of Ni core,Fig. 5c, exhibits lattice fringes and the d-spacing of two latticefringes 0.2 nm corresponding to the (111) of face centered cubic(FCC) Ni (JCPDS No.: 03-1051). Fig. 5d shows the graphitic sheetsof the Ni-filled CNT, which indicates the multi-walled nature withthe interlayer spacing between graphitic layers is equal to 0.34 nm,consistent with d-spacing obtained from the above pristine CNTsresult (as shown in Fig. 4d). The SAED pattern taken from this

Fig. 5. (a) and (b) TEM images of the typical fragment of Ni-filled CNT (c), (d) atomic resoFourier transform obtained from this filled CNT, respectively.

Ni-filled CNT (Fig. 5e) shows two types of diffractions, the diffrac-tion rings corresponding to the graphene layers of CNT and the dif-fraction spots (marked by the rectangles) correspond to themetallic Ni encapsulated in the nanotube. Fig. 5f displays a FFT spotdiagram from a representative TEM image which was indexed asthe (002) planes of graphite and (111) plane of FCC Ni.

3.3. X-ray diffraction analysis

The crystalline phase identification of LaNi5 alloy catalyst, pre-treated catalyst, as-synthesized and purified samples (pristineCNTs and Ni-filled CNTs) were performed using the XRD technique.

lved HRTEM images of Ni core and graphitic layers, (e) and (f) SAED pattern and Fast

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Fig. 6 shows the XRD patterns of commercially purchased, air oxi-dized and H2 reduced LaNi5 alloy catalyst. The XRD pattern of LaNi5

alloy catalyst with a CaCu5-type structure (JCPDS No.: 42-1191) isshown in Fig. 6a. XRD pattern of the oxidized catalyst at 400 �C for20 min in air atmosphere shown in Fig. 6b corresponds to theplanes of (111), (200), (220) and (311) of the cubic nickel oxide(NiO-JCPDS No.: 65-2901) and plane (101) of the hexagonalLa2O3 (JCPDS No.: 05-0602), respectively. It is seen that the peaksassociated with the LaNi5 phase have vanished; indicating thatthe CaCu5-type structure of LaNi5 disappeared and it changed intooxides of La and Ni during the oxidation treatment. H2 treatmentusually used to reduce the metal oxides. It can be understood fromFig. 6c that the cubic NiO phase disappeared and the planes of(111), (200) and (220) of the metallic Ni phase became strongwith weak diffraction peak of La2O3 located at 29.3� due to theplane of (101). It is noteworthy that the La2O3 remained as suchafter the H2 reduction step. This indicates that the powerful chem-ical affinity of La with oxygen is comparatively high and cannot bereduced to elemental La after H2 treatment. Our catalyst pre-treat-ment and the formation of La2O3 are similar to the surface chemis-try of LaNi5 alloy suggested by Wallace et al. [30].

Fig. 7a shows the XRD patterns of the as-synthesized andpurified pristine CNTs. The XRD pattern of as-synthesized pristineCNTs shows the broad diffraction peak of graphite plane (002)together with minor peaks of intermetallic compound of La andNi located at Bragg angles of 29.23� (111 La), 44.69� (111 Ni),52.11� (200 Ni), 76.72� (220 Ni), corresponding to cubic La(JCPDS No.: 89-2919) and cubic Ni (JCPDS No.: 03-1051) phases,respectively. It proved that the intermetallic compound of La andNi appeared during the synthesis. This is consistent with theprevious investigation on pristine CNTs synthesis using the sameLaNi5 alloy catalyst as reported by Zhang et al. [11]. The XRDpattern of acid purified pristine CNTs shows that the strongestpeak appeared in 2h = 26.1� and it corresponds to (002) plane withminor planes of (101), (100) and (004) were related to graphite(JCPDS No.: 75-1621), respectively. The fine black spherical catalystparticles in the purified pristine CNTs shown in TEM image Fig. 4aare not identified in the XRD pattern, which is due to the existenceof small amount of catalyst and consistent with the previousreports on acid purified CNTs [11].

The XRD patterns of the as-synthesized and acid purifiedNi-filled CNTs are shown in Fig. 7b. The XRD pattern of the

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Fig. 6. X-ray diffraction patterns: (a) commercially purchased LaNi5 alloy catalyst,(b) air oxidized catalyst and (c) pre-treated catalyst (air oxidized and H2 reducedcatalyst).

as-synthesized sample obtained after the catalyst pre-treatmentshows the broad diffraction peak of graphite and the sharp diffrac-tion peaks of cubic Ni with the remaining diffraction peaks of La2O3

(JCPDS No.: 05-0602) which are indexed to the plane of (101),(102), (110) and (112). The as-synthesized Ni-filled CNTs alsoconfirm the formation of La2O3 after the H2 reduction step. Moredetails about the formation of La2O3 phase under such high tem-perature synthesis process are discussed in the following growthmechanism section. The XRD pattern of the acid purified Ni-filledCNTs sample shows that the main peak at 26.35� is assigned to aplane of (002) of graphite, together with other sharp diffractionpeaks located at Bragg angles 44.7� (111), 52.1� (200) and 76.7�(220), corresponding to planes of cubic Ni phase (JCPDS No.:03-1051). It reveals that the presence of Ni showed single crystalFCC structure. It confirms that all the unfilled metallic Ni andLa2O3 were removed after purification and therefore, the occur-rence of cubic Ni peaks must be from encapsulated Ni in the CNTs.

3.4. Growth mechanism

Catalytic CVD is a general technique to produce CNTs using var-ious metal catalysts via the ‘‘dissolution and precipitation’’ growthmechanism [25]. According to this mechanism, the carbon precur-sors were dissociated into carbon atoms by pyrolysis and thenabsorbed on the surface of the catalyst. The adsorbed atoms were

Fig. 8. Schematic illustration of (a) LaNi5 alloy catalyst pre-treatment and (b)growth of Ni-filled CNTs.

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diffused over the catalyst through surface or bulk and then dis-solved on the catalyst. Finally, carbon atoms were precipitated asgraphitic layers on the opposite surface of the catalyst. In ourexperiments we used LaNi5 alloy both as a catalyst and as a sourcefor the Ni-fillings. A typical growth system involves a catalyst pre-treatment step and a growth step in which C2H2 gas is decomposedto form the tubular structure. According to the XRD results andTEM observation, the growth mechanism of Ni-filled CNTs can bereasonably proposed as follows: First, the LaNi5 alloy was exposedto air and converted into oxides of La and Ni. Second, a H2 reduc-tion step yielded Ni active sites from the oxidized catalyst, whileLa2O3 remained as such. This was systematically examined byXRD analysis discussed above (Fig. 6), which confirmed the pres-ence of La2O3 after the H2 reduction step. In keeping with Wallaceet al., the formation of La2O3 surfaces could take place in LaNi5 al-loy by the reaction of alloy with gaseous oxygen or by diffusion ofdissolved oxygen [30,32]. In the present case, the formation of sur-face oxide was due to temperature-induced air oxidation of LaNi5

alloy catalyst.To develop a better understanding of the growth process, the

‘‘surface segregation’’ mechanism was taken to examine thegrowth of Ni-filled CNTs [30,32]. The catalyst pre-treatment andthe formation of La2O3 surface are the most important factors to fillthe nanotube cavity with metallic Ni. Fig. 2d clearly illustrates thatthe formation of La2O3 surface and surface segregation of Ni frompre-treated LaNi5 alloy catalyst. As the surface oxide is formed,metallic Ni precipitated as cluster and inlays into the surface ofLa2O3, followed by the interaction of C2H2 and H2 gas, these Niaggregates change into fine Ni nanoparticles, which is responsiblefor the formation of Ni-fillings during the growth of CNTs. Based onthe above discussion the possible schematic illustration for thegrowth of Ni-filled CNTs is depicted in Fig. 8, which shows the for-mation of La2O3 surface, segregation of Ni precipitate (Fig. 8a) andthe growth of Ni-filled CNTs (Fig. 8b). The more details about theformation of La2O3 surface, segregation of Ni clusters and thegrowth of Ni-filled CNTs were discussed in our previous work [33].

The growth of nanotubes without catalyst oxidation treatmentcan result in free of metallic Ni encapsulation in CNTs. Accordingto the XRD results, the intermetallic compound of La and Ni origi-nates the growth of the pristine CNTs. The uniform hollow tubewith catalytic particle at the tips seen in Fig. 4a is consistent withthe previous report on CNTs synthesis using the same LaNi5 alloyparticles as catalyst [11].

3.5. Confocal Raman microscopy analysis

Raman spectroscopy was used to evaluate the degree of struc-tural ordering present in the purified samples. The First orderand second order Raman spectra of pristine CNTs and Ni-filledCNTs are compared in Fig. 9. The spectra show two main peaksaround 1360 cm�1 (D-band) and around 1580 cm�1 (G-band) bothof which correspond to the first order Raman peaks. The additionalpeak located around 2700 cm�1 (G0-band) was traditionally calledas second order Raman peak. For pristine CNTs (Fig. 9a) the disor-der induced D-band was at 1345 cm�1 and the calculated ID/IG ratiowas 0.72, indicating the well graphitic ordering in the pristineCNTs. In contrast, the higher intensity of the D-band peak as wellas the increase in the ID/IG ratio (0.95) appeared for Ni-filled CNTs,as shown in Fig. 9b. There are many factors, which can affect theintensity of the D-band, such as doping [34], layer numbers [35]and impurities [36]. Compared with the pristine CNTs, the D-bandintensity of Ni-filled CNTs is higher, which indicates that the Ni-filling (impurity) introduces more defects in the CNTs. A differencebetween the Raman spectra of pristine CNTs and Ni-filled CNTswas the decrease in the intensity of the G0-band. In our case, theNi filling in CNTs strongly affected the G’-band intensity, which

was in good agreement with the XRD patterns, this showed thatthe pristine CNTs were more graphitized than Ni-filled CNTs. Com-pared to pristine CNTs, the I0G/IG ratio of Ni-filled CNTs decreasedfrom 0.75 to 0.57.

3.6. Thermogravimetric analysis

TGA was used to determine the amount of filled Ni in the CNTsand the thermal stability of pristine CNTs and Ni-filled CNTs in airatmosphere. For pristine CNTs, the oxidation occurred between500 and 700 �C and the dominant weight loss due to combustionof graphitic layers of nanotubes (Fig. 10a). The TG curve exhibitedone apparent mass loss event, which shows the absence of amor-phous carbon and the high purity of the sample. Hence the samplewas almost composed of graphitic carbon and the purity of thesample was found to be 98.46%. The differential thermogravimetric(DTG) profile showed weight loss maxima at �665 �C.

The TG curve and corresponding DTG profile for the Ni-filledCNTs are shown in Fig. 10b. The oxidation occurred between 500and 650 �C and shows only one mass loss event indicate that ab-sence of amorphous carbon and no Ni nanoparticles present onthe outer surface of the CNTs [37]. Compared to pristine CNTs,the DTG profile of Ni-filled CNTs show that the shift in the massloss maxima from 665 to 615 �C suggests that the Ni-filling intothe CNTs increase the thermal oxidation of the nanotubes and de-crease the decomposition temperature [38,39]. The approximate Ni

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0.7

(b)

Temperature (ºC)

Fig. 10. Thermogravimetric analysis: (a) pristine CNTs and (b) Ni-filed CNTs.

444 A.R. John, P. Arumugam / Chemical Engineering Journal 243 (2014) 436–447

content in the sample was calculated from the final weight of theresidual material. Final residual weight of 22.6% obtained fromthe Ni-filled CNTs was due to the fully oxidized nickel.

3.7. Magnetic properties

The magnetic properties of LaNi5 alloy catalyst, H2 treated cat-alyst, pre-treated catalyst, pristine CNTs and Ni-filled CNTs weremeasured using a VSM. Fig. 11a–e shows plots of the magnetiza-tion versus the applied magnetic field (M–H) at room temperaturefor the LaNi5 alloy catalyst, H2 reduced catalyst, pristine CNTs, pre-treated catalyst and Ni-filled CNTs. It is well known that LaNi5 alloyis a Pauli paramagnet [40]. The M–H curve shown in Fig. 11a con-firms that the catalyst is paramagnetic and has low coercivity (Hci)value of 57.053 ± 16.478 Oe. Fig. 11b shows a thin open hysteresisloop and Hci of 139.88 ± 8.19 Oe for H2 treated LaNi5 alloy catalyst.It can be seen that the paramagnetic LaNi5 catalyst is changed toferromagnetic after H2 treatment. This result is quite expected be-cause previous hydrogenation studies indicate that LaNi5 particlesdisintegrate into a fine powder (smaller particles) and conse-quently make ferromagnetic [41]. A further possible explanationfor the observed ferromagnetism of H2 treated LaNi5 alloy catalystis due to precipitation of small amount of Ni clusters via surfacesegregation effect [41].

We examined the effect of acid treatment (purification method)on the magnetic properties of pristine CNTs. A significant ferro-magnetism was observed with Hci value of 140.40 ± 5.45 Oe

(Fig. 11c). In contrast, pure CNTs are known to be a diamagnetic[39]. However, the previous literature report on acid purified CNTsand chemically modified CNTs (structurally defected CNTs) showsferromagnetic properties instead of diamagnetic properties [42].The unavoidable existence of traces of catalyst particles (Fig. 4a)as well as introduction of defects due to oxygen functionalities inthe acid purified CNTs is reasonably proposed for the ferromag-netic behavior of our pristine CNTs.

Fig. 11d shows the M–H curve of pre-treated catalyst (catalystoxidation at 400 �C followed by H2 treatment at 550 �C). Again, athin open hysteresis loop obtained here indicates that the para-magnetic catalyst is transformed to the ferromagnetic state. Thisis possibly explain on the basis of surface chemistry of LaNi5 alloydemonstrates that the catalyst pre-treatment causes formation ofLa2O3 surface and surface segregation of Ni precipitate [30,32].Eventually, segregated Ni clusters (see Fig. 2d) show surface mag-netism with significant coercivity value of 149.49 ± 9.57 Oe [41].The M–H hysteresis loop of Ni-filled CNTs is presented inFig. 11e, which shows Hci, saturation magnetization (Ms), rema-nent magnetization (Mr) and Mr/Ms values of 445.87 ± 1.96 Oe,5.9108 ± 0.0027 emu/g, 1.9063 ± 0.0071 emu/g and 0.3225 emu/g, respectively. A typical ferromagnetic behavior observed withHci of 445.87 Oe exhibits increased coercivity by comparison tothe bulk Ni value (0.7 Oe) and with the previously reported coer-civity from Ni nanowires and Ni-filled CNTs [43–50]. The magneticcoercivity values for LaNi5 alloy, H2 treated catalyst, pre-treatedLaNi5 (oxidation at 400 �C followed by H2 reduction at 550 �C),pristine CNTs and Ni-filled CNTs are given with error values in Ta-

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(a) Hci: 57.053 Oe

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Hci: 445.87 Oe

Fig. 11. Hysteresis loops measured at room temperature for (a) LaNi5 alloy catalyst, (b) H2 treated catalyst, (c) pristine CNTs, (d) pre-treated catalyst and (e) Ni-filled CNTs.

A.R. John, P. Arumugam / Chemical Engineering Journal 243 (2014) 436–447 445

ble 1. In addition, the magnetic coercivity values for bulk nickel,previously reported Ni nanostructures and Ni-encapsulated CNTsare compared in Table 2. From Table 2, it can be seen that the mag-netic coercivity value of Ni-filled CNTs synthesized in our work issignificantly higher than that of Ni nanostructures and Ni-filledCNTs coercivity values. It is worth noting that the coercivity of the20–25 nm diameter Ni-nanowire encapsulated CNTs obtained byour alloy catalytic CVD method exceeds that of the coercivity val-ues of Ni nanoparticles (105 Oe, 580 ± 20 nm), Ni nanochains (101Oe, 360 ± 20 nm) and Ni nanowires (72–378 Oe, 30–200 nm) syn-thesized by microwave assisted method and combined techniqueof anodic anodization and direct current electrodeposition[47,48]. Moreover, it is even much higher than that of the Ni-filled

CNTs obtained by CVD, arc discharge and second order templatemethods with different encapsulated Ni diameters of 60 nm (94Oe), 185 nm (Hci|| � 184, Hci\ � 134), 9–12 nm (40 Oe) and20–50 nm (199 Oe) [43–46]. The high coercivity value obtainedhere is explained in terms of reduced size (confinement effect)and shape anisotropy (aspect ratio >100) of the Ni-nanowireencapsulated in the CNTs [51–53]. Moreover, the value of Mr/Msis 0.3225 indicates that the Ni-filled CNTs have strong ferromag-netic pseudo-single domain at room temperature. Hence, weconclude that the strong ferromagnetic properties of the Ni-filledCNTs obtained by our method make them suitable material for var-ious potential applications such as magnetic recording media, opti-cal devices, fuel cells, sensors and medicine.

Table 1Magnetic coercivity data for LaNi5 alloy catalyst, H2 treated catalyst, pre-treated catalyst (oxidation at 400 �C followed by H2 reduction at 550 �C), pristine CNTs and Ni-filled CNTs.

Materials Coercivity value (Hci) Coercivity value (+Hci) Coercivity value (�Hci) Error value

LaNi5 alloy 57.053 Oe 40.575 73.532 ±16.478H2 reduced LaNi5 alloy 139.88 Oe 131.69 148.07 ±8.19Pristine CNTs 140.40 Oe 134.96 145.85 ±5.45Pre-treated LaNi5 alloy 149.49 Oe 139.92 159.06 ±9.57Ni-filled CNTs 445.87 Oe 443.91 447.82 ±1.96

Table 2Comparison of magnetic coercivity values for various materials.

Materials Synthesis method Diameter of Ni (nm) Coercivity value (Oe) Temperature Refs.

Bulk nickel – – 0.7 R.T [43]Ni nanoparticles Microwave assisted route 580 ± 20 105 300 K [48]Ni nanochains 360 ± 20 101 300 K [48]Ni nanowires 200 145 300 K [48]Ni nanowires Combined AAO and electrodeposition 30 378 5 K [47]

50 273 5 K [47]200 72 5 K [47]

Ni-filled CNTs CVD 60 94 5 K [43]Array of Ni-filled CNTs Second order template 185 Hci|| �184 R.T [44]

Hci\ �134Ni-filled CNTs CVD 9–12 40 2 K [45]Carbon coated Ni nanocapsules Arc discharge 20–50 199 R.T [46]Ni-filled CNTs CVD 20–25 445.87 R.T This work

446 A.R. John, P. Arumugam / Chemical Engineering Journal 243 (2014) 436–447

4. Conclusion

We have successfully synthesized pristine CNTs and Ni-filledCNTs by CVD using LaNi5 alloy as a catalyst and source for theNi-filling. Ni-filled CNTs have been obtained through oxidative dis-sociation of LaNi5 alloy catalyst at 400 �C followed by H2 reductionprocesses. By using SEM, TEM, EDAX, XRD and Raman spectroscopywe have differentiated the significant changes in the nanotubemorphology, composition and structure of the pristine CNTs andNi-filled CNTs. Metallic Ni filled inside CNTs exists as a single crys-tal FCC structure, whereas, the Ni core is structurally uniform with20–25 nm in diameters and length in 700–800 nm. The growth ofNi-filled CNTs has been discussed on the basis of ‘‘surface segrega-tion’’ mechanism. We have observed the significant ferromagne-tism for pristine CNTs (140.40 Oe) and strong ferromagneticbehavior for Ni-filled CNTs (445.87 Oe), making them promisingmaterial for various potential applications, especially in magneticdata storage devices.

Acknowledgements

One of the authors J. Anthuvan Rajesh is thankful to UGC forproviding the Research Fellowship in Sciences for meritorious stu-dents. Instrumentation facility provided under FIST–DST and DRS–UGC to Department of Chemistry, Anna University is sincerelyacknowledged. The authors wish to thank Professor T. Pradeep ofthe DST unit of Nanoscience Indian Institute of Technology Madrasfor the HRTEM analysis and Dr. Dinesh Deva research scientist ofthe DST unit of Nanoscience Indian Institute of Technology Kanpurfor the CRM analysis.

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