Raman spectra for characterization of defective CVD multi-walled carbon nanotubes

Raman spectra for characterizationof defective CVD multi-walled carbon nanotubes

Vladimir L. Kuznetsov*,1,2,5, Sofya N. Bokova-Sirosh3,6, Sergey I. Moseenkov1, Arcady V. Ishchenko1,2,Dmitry V. Krasnikov1,2, Mariya A. Kazakova1,2, Anatoly I. Romanenko4, Evgeniy N. Tkachev4,and Elena D. Obraztsova3,6

1 Boreskov Institute of Catalysis SB RAS, Lavrentieva ave. 5, Novosibirsk 630090, Russia2 Novosibirsk State University, Pirogova str. 2, Novosibirsk 630090, Russia3 A. M. Prokhorov General Physics Institute, RAS, 38 Vavilova str., Moscow 119991, Russia4 Nikolaev Institute of Inorganic Chemistry, SB RAS, Lavrentieva ave. 3, Novosibirsk 630090, Russia5 National Tomsk State University, Lenina ave. 36, Tomsk 634050, Russia6 National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Kashirskoye shosse 31, Moscow 115409, Russia

Received 22 April 2014, revised 14 October 2014, accepted 16 October 2014Published online 20 November 2014

Keywords bimetallic catalysts, graphene flakes, multi-walled carbon nanotubes, Raman spectroscopy

* Corresponding author: e-mail [email protected], Phone: þ7 383 326 9510, Fax: þ7 383 330 8056

Curved graphene fragments can be considered as building blocksof carbon nanotubes. Thus, the size of graphene fragments maybe considered as one of the most important characteristics oftheir structural disorder. In this paper, we have performed acomparative Raman study of CVD multi-walled carbon nano-tubes (MWCNTs) and graphene flakes deposited on MWCNTs.Raman data have been considered in combination with HRTEMcharacterization of nanotubes. The basic attention has been paidto the behavior of D (disorder-induced), G (tangential mode), and2D (two-phonon scattering) bands in Raman spectra in order touse them for MWCNT characterization. A ratio of intensitiesof 2D and D bands (I2D/ID) demonstrates almost a lineardependence on the mean diameter of MWCNTs produced withtwo different types of catalysts (see abstract figure). It shouldbe mentioned that each type of catalyst provides the lineardependence with its own specific slope. The graphene fragmentshave been proposed to form amosaic structure of nanotubewalls.I2D/ID ratio depends on the amount of graphene flakes depositedon nanotube surface via ethylene decomposition.

A dependence of intensity ratio I2D/ID on the diameter ofnanotubes produced with different types of catalysts.

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1 Introduction Nanocarbons (namely, graphene, car-bon nanotubes, nanodiamond, onion-like carbons, full-erenes, etc.) are becoming promising materials innanotechnology. A great number of applications based onnanocarbons have been proposed [1–3]. A variety ofphysical methods are applicable for structural and electroniccharacterization of nanocarbons on both laboratory andmass-production scales (see ISO/TR 10929:2012(E) andreferences within).

A Raman spectroscopy has obvious advantages becauseit provides an express and nondestructive control ofstructural and electronic characteristics of carbon materials.Graphite, diamond, graphene, nanotubes, fullerenes in formof monocrystals, polycrystalline films, and powders showtheir own features in Raman spectra and, thus, can beidentified with this technique [4–12]. The graphene frag-ments can be considered as building blocks of sp2-carbonmaterials. The size of graphene fragments may be considered

Phys. Status Solidi B 251, No. 12, 2444–2450 (2014) / DOI 10.1002/pssb.201451195

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as one of the most important characteristics of structuraldisorder of these materials. The intensity ratio of D (disorder-induced) and G (tangential mode) bands of graphenefragments is often used as a measure of disorder in carbonmaterials. This ratio is empirically related with the size of in-plane graphene sheets according to Tuinstra & Koenig [13]:

LaðTKÞ ¼ 4:4IGID

� �; ð1Þ

where La is in-plane size in nm; 4.4 nm, empirical constantfor excitation wavelength 514.5 nm.

Later the integrated intensity ratio of D- and G-peakswas used for estimation of in-plane crystallite sizes La byCançado [9]:

LaðCÞ ¼ ð2:4� 10�10Þl4 AG

AD

� �; ð2Þ

where La is in-plane size in nm; l, excitation wavelength.A specific approach was developed for calculation of an

average distance between defects in irradiated defectivegraphenes [14].

The analysis of behavior of 2D (two-phonon scattering)band was mainly used for characterization of the number ofgraphene layers and their mutual orientation (stackingorder) [7, 8, 11]. At the same time authors of [15] havedescribed the relative areas of AD/AG, A

0D/AG, and A2D/AG

using a phenomenological model for single-, few-, andmany-layer graphene with increasing disorder produced viaArþ bombardment. It was found that the intensity of 2Dbands dramatically decreases with the decrease of distancebetween the radiation defects (LD).

This conclusion correlates with our observation of Ramanspectra of nanodiamond-derived series of sp2/sp3 compositesand onion-like carbon produced via a high-temperatureannealing [16]. Intensive 2D bands were observed only forthe samples annealed at temperature higher than 1900K whenthe prolonged graphene fragments were formed. An existenceof a critical size of defective (curved) graphene flakes (whichare responsible for the appearance of 2D bands) can beproposed on the base of data obtained.

Small graphene flakes are related to polyaromaticmolecules [17, 18] for which 2D bands are not characteristic.Thus, 2D band also can be used to characterize thedefectiveness of graphene-based materials. Previously, wehave observed almost a linear dependence of I2D/ID onthe average diameter of multi-walled carbon nanotubes(MWCNTs) produced with Fe–Co catalyst [19] in the rangeof 5–20 nm.

In the present paper, we have analyzed the peculiaritiesof Raman spectra for MWCNTs produced with Co–Mncatalyst and for lateral graphene-like flakes deposited onMWCNTs via an ethylene pyrolysis at 873–1023K. Thebasic attention has been paid to behavior of D (disorder-induced), G (tangential mode), and 2D (two-phononscattering) bands in the Raman spectra.

2 Experimental2.1 MWCNT preparation Two sets of MWCNTs

with different mean diameters have been produced bycatalytic decomposition of ethylene on Fe–Co catalysts (setA) and on Co–Mn/Al–Mg catalysts (set B) at 953K. Bothtypes of catalysts were synthesized via a polymerizedcomplex route, based on the Pechini-type reaction [20–24].A mean diameter of MWCNTs was controlled using thevariation of an active component concentration in catalyst.MWCNT diameter distribution and the structural changes ofgraphene flakes were monitored by HR TEM (JEM-2010).For each sample the diameter distribution were determinedusing 300–600 MWCNT images obtained by TEM atmagnification �50,000 and �400,000. The MWCNTs withmean diameters 5.5, 7.2, 8.7, 9.4, 10.2, and 20 nm have beenproduced for series A, and 6.2, 8.3, 9.1, and 16.8 nm forseries B, respectively. Figures 1 and 2 present examples ofTEM images and diameter distribution of two MWCNTsamples from sets A and B with similar mean diameter(d� 9 nm).

2.2 Deposition of graphene flakes on MWCNTsDeposition of defective graphene flakes on the MWCNTsurface was carried out using a non-catalytic ethylenepyrolysis at 873–1023K in a quartz tube reactor (50:50C2H4–Ar flow). TwoMWCNT samples from set A and set B

Figure 1 TEM images of MWCNT (set A, d¼ 9 nm).

Phys. Status Solidi B 251, No. 12 (2014) 2445

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with the mean diameters 9 and 15 nm, respectively, werechosen for the experiments. In order to remove catalysttraces, MWCNTs were refluxed with 50% HCl and thenwashed with a distilled water. The amount of depositedgraphene flakes was controlled by the sample weightincrease (Dm/m0 up to 160% of initial sample weight (m0))and by the gas chromatography of exhaust gas (detection ofH2 of and polyaromatic compounds).

The Raman spectra of MWCNT powders have beenmeasured with a Jobin Yvon S-3000 spectrometer inmicroscopic configuration with the spectral resolution of2 cm�1. The radiation of Ar–Kr laser with the wavelength514.5 nmwas used for spectra excitation. The Raman spectraof MWCNTs have been registered in three spectral regions:D (disorder-induced), G (graphene) and 2D (two-phononscattering) bands.

3 Results and discussion3.1 Influence of the catalysts nature on the

defectiveness of MWCNTs Figure 3 shows the Ramanspectra of MWCNTs with different mean diametersproduced with Co–Mn/MgAl2O4 catalysts (set B). Apresence of D bands with the frequency of 1350 cm�1 isrelated to a disorder-induced breaking of the Ramanselection rules for small graphene fragments, a presenceof G band (1580 cm�1) corresponds to tangential modes and2D band (2680 cm�1) corresponds to two-phonon scattering.

All spectra have been normalized to the D-modeintensity. The original spectra of MWCNT set A werepresented in [19]. For set A the ratio of intensities ID/IG isalmost invariable. The intensities of D and G bands arealmost equal in wide region of nanotube diameters (5–15 nm). At the same time, the spectrum of nanotubes with a20 nm-diameter shows a considerable difference of intensi-ties ID/IG. For set B there was also no systematic variation ofthe ratio ID/IG.

However, the ratio of intensities of 2D and D bands(I2D/ID) has demonstrated almost a linear dependence onthe mean nanotube diameter for both sets of MWCNTs(A and B, see Fig. 4). The increase of I2D/ID with the increaseof nanotube diameter means that the thicker tubes are lessdefective so far as the most defective graphene-basedmaterials are characterized the most intensive D band. Atthe same time, the comparison of I2D/ID ratio of sets A and Bleads to conclusion that nanotubes of set B are moredefective.

Figure 2 TEM images of MWCNTs (set B, d¼ 9 nm).

Figure 3 Raman spectra of MWCNTs (set B) with the differentmean diameters.

Figure 4 A dependence of intensity ratio I2D/ID on the diameter ofnanotubes produced with different types of catalysts, namely withFe2Co/Al2O3 (set A [19]) and with Co–Mn/MgAl2O4 (set B).

2446 V. L. Kuznetsov et al.: Raman spectra for characterization of defective CVD MWCNTs

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These data are also confirmed by the HRTEMmeasurements. Figures 1 and 2 demonstrate TEM imagesof MWCNT samples from sets A and B, respectively.One can see that tubes of set B are more defective. Thedifferences of number of graphene shells forming thenanotube walls are widely observed. It should be mentionedthat MWCNTs of set B were produced using Co–Mn–Mg–Al catalyst with a similar composition, which was patentedby Bayer [25] for a catalytic production of nanotubes with aspiral-like cross section.

The difference in slope of the I2D/ID dependences on thediameter of nanotubes for different type of catalysts can beexplained in terms of differences in kinetics and energyparameters of the main steps of MWCNT growth, namely(i) a decomposition of ethylene on an active metal particlewith the formation of carbon atoms and hydrogen evolution;(ii) a carbon atom diffusion (bulk and/or surface) to theinterface of an active metal particle with the growingnanotube; and (iii) a carbon atom insertion into a metal–carbon bond in the interface of metal particle and nanotube.Despite the similarities of reaction mechanism of MWCNTformation with a metal catalyst (Fe, Co, Ni, Mo, etc.) thedifferences in their specific energetic parameters aresignificant (an energy of M–C bonds, a carbon diffusioncoefficient D, etc.). As a result, each type of catalystcomponent provides the formation of specific carbondeposits with the characteristic size of graphene flakes. Asa consequence, MWCNTs supplied by different producersusing a variety of catalysts are characterized with significantvariation of structure and quality [26].

The Raman data allow us to estimate a typical size ofgraphene fragment using the value of ID/IG ratio [9, 13].For MWCNTs (set A) La(TK) (according to the empiricalequation of [13]) varies from 4.6 up to 9.3 nm while theapproach, used in [9], gives the variation of graphene flakesize in the range La(C)¼ 12–30 nm.

Thus, according to our results, the average size ofgraphene fragments for two types of MWCNTs producedwith different catalysts increases with MWCNT diameter.This observation is in agreement with our previous anindependent electophysical study of MWCNTs [27]. Thedecrease of graphene fragment size in MWCNTs of smallerdiameter estimated by Raman technique correlates withthe decrease of electron coherence length (LT). LT can beestimated using our previously published data on themagnetic field dependences of the conductivity (s(B)) ofMWCNTs with different mean diameter [27]. The proce-dure of measurement of s(B) by four-point probe techniquefor powder of MWNTs, which was pressed in a glasscylinder was described in [27]. We have fitted the data ofexperimental magnetic field s(B) dependences of theconductivity of MWCNTs (produced using Fe–Co cata-lysts [22]) by the Eqs. (3) and (4) with two parameters g(T),LT [28]:

DsintðBÞ / gðTÞw22eL2TBp�hc

� �; ð3Þ

where

w2 ¼Z 1

0

tdt

sh2t1� xt

shðxtÞ� �

¼ 0:3x2; x � 1;

ln x; x � 1:

(ð4Þ

Dsint(B), a contribution to conductivity from an electron–electron interaction, B, a value of magnetic field, g(T), aconstant of e–e interaction, LT¼ (�hD/kBT)1/2, e, an electroncharge, D, a diffusion coefficient of current carriers, c, avelocity of light (Eq. (5) and (5a) in [28]). Thus, we havegot LT, which varies from 17.5 up to 24 nm for MWCNTswith the different mean diameter ranging from 9 to 24 nm.Therefore, while the nanotube diameter decreases theelectron coherence length LT decreases and the tubesbecome more defective from the viewpoint of electrontransport. This observation correlates with the fact thatMWCNTs of smaller diameter (less than 10 nm) demon-strate temperature dependences of the conductivity s(T)behavior characteristic for systems with variable rangehopping conductivity (VRHC), while s(T) of nanotubeswith bigger diameters (more than 10 nm) demonstrates thebehavior typical for quantum corrections to conductivity forinteraction electrons in two-dimensional conductors witha local disorder [27]. These phenomena correspond to ahigher defectiveness of MWCNTs with smaller diameter.Thus, Raman, s(B) and s(T) measurement data correspondto a higher defectiveness of MWCNT with smallerdiameters and require a specific consideration.

At the same time the conclusions based on Raman dataand measurements of s(T) and s(B) contradict withobservations of the elastic modulus increase of MWCNTswith the decrease of their diameters [29]. This observationcan be interpreted as a decrease of MWCNT defectivenesswith an external diameter decrease. This contradiction can beeliminated in case of suggestion that graphene flakes inMWCNT walls form the mosaic graphene sheets with thedifferent orientation of single graphene fragments and grainboundaries containing covalent C–C bonds. Authors of [29]indeed demonstrated that grain boundaries in graphenecan achieve an ultrahigh strength. So, if one adopts thesuggestion that nanotube walls consist of bonded mosaicgraphene fragments, the contradiction concerning the factthat the more defective MWCNTs with the smaller diameterdemonstrate higher elastic modules can be omitted.

It should be mentioned that authors [30] demonstratethat graphene fragment of MWCNTs are also joint intostacks. So, when one estimates the stability (strength) ofnanotubes, the effect of the variation of block sizes can alsooccur. The systems consisting of large blocks are regularlyless stable (have a lower fracture strength) than thoseconsisting of the smaller blocks.

3.2 Graphene flakes deposited onMWCNTs Thedata on kinetics of ethylene decomposition on MWCNTsand the structure of deposited flakes have been discussedelsewhere [31]. Figure 5 demonstrates TEM images of



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MWCNTs (set A, d¼ 9 nm) before and after treatment inC2H4–Ar flow at 1000K. Contrast lines correspond tographene layers and small fragments with the distancebetween the lines equal 0.34–0.35 nm (d002¼ 0.3354 nm forhexagonal graphite). Figure 5B shows double structure ofMWCNTs treated in C2H4: the inner layers have an orderlylayer structure attributed to pristine nanotube, while the outerlayers separated by white dotted line have less orderly layerstructure consisting of short curved graphene fragmentsresembling the flakes of size 2–5 nm. These fragmentscannot be eliminated by soxhlet extraction with dimethyl-formamide that allows an assumption of the formationof irregular C–C bonds between them. Thus, MWCNTtreatment in ethylene results in formation of graphene-likeflakes on the nanotube surface.

The Raman spectra of MWCNTs after deposition ofgraphene flakes are presented in Figs. 6 and 7. Data on theamount of lateral carbon (Dm/m0) deposited on MWCNTs(set A) at 940 and 1000K are presented in Fig. 7. One canmention that the values of IG/ID and I2D/ID vary only duringan initial period of carbon deposition and after the period of

60–80min they keep almost constant. It can be explained interms of formation rather thick layers of defective grapheneflakes shielding the internal MWCNT walls.

The Raman data demonstrate two controversial effects.In the one hand, we found that during the initial period ofMWCNT treatment in C2H4 the ratio IG/ID increases whichimplies that a relative content of amorphous carbon speciesdecreases and a part of graphitic carbon increases. In theother hand, the ratio I2D/ID decreases (see Fig. 8). This can beattributed to the decrease of an average length of graphiticfragments, which can be notably smaller for deposited flakesthan for nanotube planes. The observed increase of the ratioIG/ID which conflicts with the decrease of graphene-likeflake size can be explained by the hydrogenation of theedges of graphene flakes in the manner of polyaromaticcompounds [18] or by the deviation from the equationsproposed by Tuinstra & Koenig [13] and Cançado [9], whichcan occur for small and curved graphene flakes and by apossible presence of sp3 carbon atoms. Moreover, after

Figure 5 HR TEM image of MWCNTs (set A, d¼ 9 nm) before(A) and after treatment in C2H4–Ar flow at 734 8C for 120min (B).The white dotted lines show the interfaces between the CVD-grownnanotube surface and deposited defective graphene flakes.

Figure 6 The Raman spectra of MWCNTs of set A (a meandiameter of 10 nm) after the treatment in C2H4–Ar flow at 1000K(60min) and 940K (120min).

Figure 7 The Raman spectra ofMWCNTs of set B (mean diameter15 nm) after the treatment in C2H4–Ar flow at 1000K (60min).



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deposition of graphene flakes on the nanotube surface thesystem becomes rather heterogeneous, and the dependenciesbeing characteristic for homogeneous system should notwork for this system.

4 Conclusions In this paper, we showed a highsensitivity of Raman spectroscopy for the defectivenessestimation of MWCNTs and of size of graphene flakesdeposited on the nanotube surface. This analysis required aconsideration of behavior of D (disorder-induced), G(tangential mode), and 2D (two-phonon scattering) bands.The estimation of in-plane graphene fragment size (La) ofMWCNTs via IG/ID ratio correlates with the electroncoherent length (LT) calculated on the base of magneto-conductivity data for these samples. I2D/ID ratio demon-strates almost linear dependences on the diameter ofMWCNTs produced with two different types of catalysts(Fe2Co/Al2O3 and Co–Mn/Mg–Al). It should be mentionedthat each type of catalyst provides the linear dependencewith its own specific slope. The data obtained are interpretedin terms of the size variation of graphene flakes considered asbuilding blocks for MWCNTs. It was proposed that thegraphene flakes form a mosaic structure of nanotube walls.It should be added, that high temperature annealing ofMWCNTs at 2470–3073K results in structural defectelimination (i.e. graphene biulding block growth) anddramatic increase of I2D/ID ratio (up to 10 times) [19].

Acknowledgements This study was partially supported bythe Russian Foundation for Basic Research (Project Nos. 14-03-31355 and 13-02-01354), Russian President Scholarship (SP-1540.2012.3), Program No. 24 of the Basic Research of PresidiumRAS, and Grant of Ministry of Science and EducationRFMEFI60714X0046.

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Raman spectra for characterization of defective CVD multi-walled carbon nanotubes