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Journal of Physics and Chemistry of Solids 69 (2008) 222–229

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The effect of chemical treatment on the crystallinity of multi-walledcarbon nanotubes

Kai Yanga,�, Haibo Hanb, Xifeng Pana, Na Chena, Mingyuan Gua

aState Key Laboratory of MMCs, Shanghai Jiaotong University, 800 Dongchuan Road, Shanghai 200240, ChinabInstrumental Analysis Center of Shanghai Jiaotong University, 800 Dongchuan Road, Shanghai 200240, China

Received 8 June 2007; received in revised form 17 August 2007; accepted 28 August 2007

Abstract

Multi-walled carbon nanotubes (MWCNTs) were chemically treated using nitric acid solution for different time. Quantitative analysis

of the crystallinity of the MWCNTs was performed by wide-angle X-ray diffraction (WAXD). The WAXD patterns were deconvoluted

into the crystalline diffraction peaks and the amorphous scattering peaks. The introduction of a correction factor for the integrated peak

intensity can enhance the computational accuracy of the crystallinity. With increasing the chemical treatment time, the crystallinity of

MWCNTs first increases, and then decreases. When the chemical treatment time is equal to 2 h, the crystallinity of MWCNTs reaches the

maximum of 85.9%. Moreover, the degree of order in the structures of chemically treated MWCNTs was further studied by

thermogravimetric analysis (TGA) and high-resolution transmission electron microscopy (HRTEM). It was found that the external walls

of chemically treated MWCNTs with high crystallinity consist of a series of perfectly continuous and straight graphite layers.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Multi-walled carbon nanotubes; Crystallinity; Correction factor; Chemical treatment; Wide-angle X-ray diffraction (WAXD)

1. Introduction

Carbon nanotubes (CNTs) have received widespreadattention since the landmark paper by Iijima [1], largelybecause of their unique structure and superior physicalproperties such as high Young’s modulus, electrical andthermal conductivities [2–4]. These outstanding physicalproperties are closely related to the special structure ofCNTs (carbon nanotubes consist of concentric cylinders ofgraphite layers). Presently, CNTs occur in several discreteforms (e.g. single-walled CNTs (SWCNTs), double-walledCNTs (DWCNTs) and multi-walled CNTs (MWCNTs))[5]. These CNTs possess different surface areas and aspectratios, which are important in their applications.

In recent years, CNT/polymer composites have beenseen as an extremely promising research topic in the field ofmaterial science. Subsequently, extensive researches havebeen focused on the use of CNTs as nano-fillers to promotethe performance of CNT/polymer system. Currently,

front matter r 2007 Elsevier Ltd. All rights reserved.

s.2007.08.069

ng author. Fax: +86 21 62822012.

ss: kyang_80@yahoo.com.cn (K. Yang).

thermally and electrically conductive polymer compositeshave attracted more attention because CNTs are theoreti-cally evaluated to have large thermal and electricalconductivities [6–9], for example, MWCNTs are reportedto have an electrical conductivity of approximately1.85� 103 S/cm [10] and a thermal conductivity of over3000W/mK [11]. So MWCNTs are suggested to beextensively used in polymer composites to improve theirthermal and electrical conductivities. Gojny et al. [12]researched the electrical and thermal conduction mechan-isms in different types of CNT/epoxy composites. Theyindicated that MWCNTs showed the highest potential forefficient enhancements of thermal and electrical conducti-vities of polymer composites, due to the relatively lowsurface area, high aspect ratio and better dispersion inepoxy matrix. Therefore, there have been more studiesconcentrated on the MWCNT/polymer composites.In order to acquire superior thermal and electrical

properties of MWCNT/polymer composites, it is indis-pensable to purify MWCNTs to get rid of diverseimpurities that not only impair the physical properties ofMWCNTs, but also cause poor dispersion of MWCNTs in

ARTICLE IN PRESSK. Yang et al. / Journal of Physics and Chemistry of Solids 69 (2008) 222–229 223

the polymeric matrix. Chemical treatment is always appliedin the purification process. Kim et al. [13] reported thatthey obtained well-purified MWCNTs through, respec-tively, using nitric acid solution and a mixture of oxydol(H2O2) and ammonia (NH4OH) to remove the impuritiespresent in MWCNTs. In the meanwhile, they also pointedout that strong chemical treatment conditions resulted inthe damage in the crystalline structures of MWCNTs. As isknown, the thermal and electrical transport properties ofCNTs are extremely sensitive to the degree of crystallinityand defects/contamination in them. Jin et al. [14] foundthat with increasing annealing temperature, both electricaland thermal conductivities of macroscopic bundles of longMWCNTs were prone to increase. They believed that theseresults were attributed to the improvement of the crystal-linity of MWCNTs through annealing. Likewise, Delpeux-Ouldriane et al. [15] indicated that annealing caused thetransition from the template CNTs to well-graphitizedMWCNTs and the latter possessed better crystallinity.According to these investigations, crystallinity is a signi-ficant factor to affect the thermal and electrical conductiv-ities of MWCNTs. Furthermore, thermal and electricalconductivities of MWCNT/polymer composites can bealso improved. The central issue is how to improve thecrystallinity and purify MWCNTs, so that their essentialproperties can be presented. It is proved that chemicaltreatment is beneficial to remove the impurities andimprove the dispersion of nanotubes [16–18]. However,whether or not the chemical treatment improves thecrystallinity of MWCNTs is not certain. Moreover,quantitative analysis of the crystallinity of chemicallytreated MWCNTs is also deficient.

There are some methods to calculate the crystallinity atpresent, such as differential scanning calorimetry (DSC),wide-angle X-ray diffraction (WAXD) and infrared spec-trum (IR). In these methods, WAXD is more extensivelyused to analyze the crystallinity because it possessesdefinite physical meaning and reliable values [19]. In thiswork, MWCNTs were purified using nitric acid solutionfor different treatment time. The research was chieflyfocused on the effect of chemical treatment on thecrystallinity of MWCNTs. Quantitative analysis of thecrystallinity was performed by WAXD.

2. Experimental

2.1. Chemical treatment of MWCNTs

MWCNTs were obtained from the Nanotech PortCompany, Shenzhen, China. These MWCNTs wereproduced by chemical vapor deposition (CVD). This kindof catalytic production method is simple and has a highproductivity. First, MWCNTs (the mass of every portionwas 1 g) were treated in a preheated nitric acid solutionwith a volume of 60ml (concentration (%) ¼ 63.01%) at90 1C for different time, 1, 2, 4, 6 and 10 h, respectively.And then, MWCNTs were washed with deionized water,

filtered until the pH value reached 7 and subsequently driedin a vacuum oven at 80 1C for 1 day. The chemically treatedMWCNTs were weighed again. The ratio of the decreasingmass to that of as-received MWCNTs was smaller than1%, which indicated that few impurities were removed.The numberings of MWCNT specimens are made by thehours of chemical treatment time with 0] representing theas-received MWCNTs.

2.2. WAXD characterization of MWCNTs

WAXD patterns were obtained using a Bruker-AXS D8advanced diffractometer with nickel-filtered Cu Ka radia-tion (l ¼ 0.15406 nm). The applied voltage and current ofthe X-ray tubes were 40 kV and 40mA, respectively. TheWAXD measurements were performed by step scan in the2y range from 101 to 701 at a scanning speed of 1 s/step(the step size is 0.021/step). The diffractograms wererespectively deconvoluted into crystalline diffraction peaksand amorphous scattering peaks using pseudo-Voigt peakfunction [20,21].

2.3. TGA characterization of MWCNTs

Thermogravimetric analysis (TGA) curves were obtai-ned with a Perkin-Elmer TGA7 instrument. The weightloss was recorded from 20 to 800 1C at a heating rate of20 1C/min. The mass of analyzed samples was 2.0mg. Thedegree of order in the structures of MWCNTs chemicallytreated at different time was further studied.

2.4. TEM/HRTEM characterization of MWCNTs

The morphologies of MWCNTs were observed throughtransmission electron microscopy (TEM; JEM 2100F,JEOL, Japan) with an accelerating voltage of 200 kV.High-resolution transmission electron microscopy (HRTEM)images were taken on JEM 2100F to characterize themicrostructures of MWCNTs.

3. Theoretical analysis

According to the two-phase model [22], MWCNTsconsist of remarkable crystalline and amorphous phasesand the influence of interfaces between crystalline andamorphous phases can be negligible. So the crystallinity isdefined as the ratio of mass of crystalline phase to that ofMWCNTs. It can be quantified by the following equation:

W c ¼Mc

Mc þMa� 100%, (1)

where Wc is the crystallinity of MWCNTs, Mc and Ma arethe masses of crystalline and amorphous phases, respectively.WAXD are used to investigate the crystallinity in terms

of the theoretical basis that overall coherent scatteringintensity in the whole reciprocal space is only related to thekinds and numbers of the atoms to attend scattering, but

ARTICLE IN PRESSK. Yang et al. / Journal of Physics and Chemistry of Solids 69 (2008) 222–229224

irrelated to their manners of aggregation. Since thescattering intensity is in direct proportion to the mass ofscatterer, the crystallinity obtained by WAXD (Wc,x) canbe revealed as follows:

W c;x ¼I c

I c þ Ia� 100%, (2)

10 20 30 40 50 60 700

20

40

60

80

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120

140

160

180

Lin

(co

un

ts)

Lin

(co

un

ts)

Lin

(co

un

ts)

2-Theta(°)

10 20 30 40 50 60 70

2-Theta(°)

10 20 30 40 50 60 70

2-Theta(°)

4 1 2 3

(002)

(100) (101)

0

50

100

150

200

250

4

1

2 3

0

50

100

150

200

250

4 1 2 3

(002)

(100) (101)

(002)

(100) (101)

Fig. 1. WAXD patterns for MWCNT specimens of (a) 0],(b) 1], (c) 2], (d) 4crystalline diffraction peaks marked with ‘1’, ‘2’, ‘3’ and amorphous scatterin

where Ic and Ia are the scattering intensities of crystallineand amorphous phases, respectively.Based on the discrepancy of electron densities in the crysta-

lline and amorphous regions (electron density in crystallineregion is greater than that in amorphous region), WAXDcan be used to analyze the crystallinity by calculating theintegral intensities of crystalline diffraction peaks and

10 20 30 40 50 60 70

2-Theta(°)

10 20 30 40 50 60 70

2-Theta(°)

10 20 30 40 50 60 70

2-Theta(°)

0

50

100

150

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Lin

(co

un

ts)

4 1 2 3

0

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Lin

(co

un

ts)

Lin

(co

un

ts)

4 1 2 3

0

20

40

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80

100

120

4 1 2 3

(100)(101)

(002)

(100) (101)(002)

(002)

(100) (101)

], (e) 6] and (f) 10]. These diffractograms are severally deconvoluted into

g peaks marked with ‘4’ using pseudo-Voigt peak function.

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Table 1

Calculation of the correction factors of MWCNTs

MWCNTs hkl 2y (deg) LP T f2 f2 LPT C(y)

0]-MWCNTs (0 0 2) 26.155 36.08 0.98 76.11 2691.13 1

(1 0 0) 43.202 12.15 0.92 52.19 583.38 4.61

(1 0 1) 44.564 11.31 0.92 36.07 375.32 7.17

A 25.243 39.16 0.98 105.72 4057.20 0.66

1]-MWCNTs (0 0 2) 26.194 36.08 0.98 76.11 2691.13 1

(1 0 0) 43.028 12.28 0.92 52.19 589.62 4.56

(1 0 1) 44.552 11.31 0.92 36.07 375.32 7.17

A 25.426 38.51 0.98 105.72 3989.85 0.67

2]-MWCNTs (0 0 2) 26.135 36.08 0.98 76.11 2691.13 1

(1 0 0) 42.821 12.41 0.92 52.19 595.86 4.52

(1 0 1) 44.345 11.43 0.92 36.07 379.30 7.09

A 25.079 39.82 0.98 105.72 4125.57 0.65

4]-MWCNTs (0 0 2) 26.250 36.08 0.98 76.11 2691.13 1

(1 0 0) 42.977 12.28 0.92 52.19 589.62 4.56

(1 0 1) 44.455 11.43 0.92 36.07 379.30 7.09

A 25.714 37.27 0.98 105.72 3861.38 0.70

6]-MWCNTs (0 0 2) 26.138 36.08 0.98 76.11 2691.13 1

(1 0 0) 42.819 12.41 0.92 52.19 595.86 4.52

(1 0 1) 44.454 11.43 0.92 36.07 379.30 7.09

A 25.534 37.88 0.98 105.72 3924.58 0.69

10]-MWCNTs (0 0 2) 26.152 36.08 0.98 76.11 2691.13 1

(1 0 0) 42.666 12.54 0.92 52.19 602.11 4.45

(1 0 1) 44.215 11.54 0.92 36.07 382.95 7.03

A 25.682 37.88 0.98 105.72 3924.58 0.69

K. Yang et al. / Journal of Physics and Chemistry of Solids 69 (2008) 222–229 225

amorphous scattering peaks (these peaks can be separatelygenerated from crystalline and amorphous phases and theirintegral intensities are the areas under the crystallinediffraction peaks and the amorphous scattering peaks) inreciprocal space, respectively. In order to enhance compu-tational accuracy of the crystallinity, it is indispensable tosubtract compton scattering and background scatteringfrom overall scattering intensity in the experiments andconduct the theoretical correction of the peak integralintensity. In this study, the WAXD patterns weredeconvoluted using the pseudo-Voigt peak function toobtain separated crystalline diffraction peaks and amor-phous scattering peaks, as efficiently took off the effects ofcompton scattering and background scattering. Further-more, the theoretic correction of integral intensities of thesepeaks was performed through the introduction of thecorrection factor (C(y)) [23]. Therefore, the crystallinity ofMWCNTs can be calculated by the following equation:

W c;x ¼

Pi

Ci;hklðyÞI i;hklðyÞP

i

Ci;hklðyÞI i;hklðyÞ þP

j

CjðyÞI jðyÞki

� 100%, (3)

where i and j are the numbers of crystalline diffractionpeaks and amorphous scattering peaks, respectively;Ci,hkl(y) and Ii,hkl(y) are the correction factor and integralintensity of the crystalline diffraction peak, respectively;Cj(y) and Ij(y) stand for the correction factor and integralintensity of the amorphous scattering peak; ki is thecorrection coefficient (when all crystalline diffraction peaksare considered to calculate the crystallinity, ki is equal to 1;in our experiments, ki ¼ 1). Ci,hkl(y) and Cj(y) can bejointly quantified by the following equation [24,25]:

CðyÞ ¼ f 2�

1þ cos2 2y

sin2 y cos ye�2Bðsin y=lÞ2

¼X

Nf 2i �

1þ cos2 2y

sin2 y cos ye�2Bðsin y=lÞ2 ð4Þ

where f is the total scattering factor of all carbon atoms in arepeating unit; N and fi are the number of carbon atoms ina repeating unit and atomic scattering factor, respectively(in our experiments, MWCNTs belong to Graphite-2Hstructure and N is equal to 4); y is the diffractionangle; (1+cos2 2y)/(sin2 y cos y) is the angle factor (LP);e�2Bðsin y=lÞ2 is the temperature factor (T). Regarding acertain Chkl(y) (the correction factor of this crystallinediffraction peak possessing the greatest diffraction inten-sity) as the standard, we can perform the normalization forthe values of C(y). Considering the two-phase model, thecrystallinity of MWCNTs can be calculated using Eqs. (3)and (4).

4. Results and discussion

4.1. WAXD analysis

WAXD patterns for as-received and chemically treatedMWCNTs are shown in Fig. 1. The WAXD patterns for

these MWCNT specimens exhibit three characteristicdiffraction peaks (0 0 2), (1 0 0) and (1 0 1). It can be seenthat the diffraction intensity of (0 0 2) is the greatest. Inorder to investigate the influence of chemical treatment onthe crystallinity, it is necessary to obtain the values of thecrystallinity of MWCNT specimens and regard the crystal-linity of as-received MWCNTs as a reference. So theWAXD diffractograms for the MWCNT specimensare deconvoluted into crystalline diffraction peaks andamorphous scattering peaks using pseudo-Voigt peakfunction. In Fig. 1(a)–(f), the crystalline diffraction peaks(0 0 2), (1 0 0) and (1 0 1) are, respectively, marked with ‘1’,‘2’ and ‘3’. The amorphous scattering peaks A are markedwith ‘4’. Taking the values of C002(y) as the standard(namely, C002(y) is equal to 1), the values of othercorrection factors can be correspondingly normalized.Calculation of the correction factors can be shown inTable 1. So the crystallinity of MWCNTs can be calculatedby Eq. (3).Fig. 2 describes the crystallinity of MWCNTs chemically

treated at different time. It is found that with the increaseof chemical treatment time, the crystallinity of MWCNTsfirst increases, and then decreases. When chemical treat-ment time is 2 h, the crystallinity of 2]-MWCNTs reachesthe maximum of 85.9%. The crystallinity of 0]-MWCNTs(as-received MWCNTs) with no chemical treatment is70.0%. As is known, chemical treatment is beneficial toremove impurities. It can promote the improvement of

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growing zones of MWCNTs and reduce the faults betweengraphitic carbon planes. So the enhancement of thecrystallinity of MWCNTs is attributed to the improvement

0 4 8 10

68

70

72

74

76

78

80

82

84

86

88

The c

rysta

llinity o

f M

WN

Ts(%

)

Chemical treatment time(h)

62

Fig. 2. The crystallinity of MWCNTs as a function of chemical treatment

time.

the TGA curve of 6#

the corresponding diffe

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

5004003002001000 600 700 800 900

0

20

40

60

80

100

the TGA curve of 0#

the corresponding differential curve

Temperature (°C)

4003002001000

Temperatu

We

igh

t (%

)

0

20

40

60

80

100

We

igh

t (%

)

1st

De

riva

tive

(%

/min

)

Fig. 3. TGA and DTG curves of MW

of their crystalline structures. However, longer chemicaltreatment time results in the decrease of the crystallinity ofMWCNTs. The cause is that nitric acid corrodes thegraphite layers and generates the damage to the crystallinestructures of MWCNTs. It can be also seen that thecrystallinity of 10]-MWCNTs is 70.9%, approximatelyamounting to that of 0]-MWCNTs. With regard to10]-MWCNTs, the crystallinity increases from 70.0% to85.9% in the incipient 2 h and drops down to 70.9% in thesubsequent 8 h. The escalation rate of the crysta-llinity of MWCNTs is greater than its rate of descent inchemical treatment process. Short chemical treatmenttime more efficiently enhances the crystallinity ofMWCNTs. Therefore, effectively controlling chemicaltreatment time is critical to obtain the MWCNTs withhigh crystallization.Additionally, MWCNTs with the length of 1–2 mm are

employed in our experiments. Compared to alignedMWCNTs or macroscopic bundles of long MWCNTs,the influence of the preferred orientation on the crystal-linity can be effectively eliminated in our study. So thevalues of the crystallinity in this work are more reliable.

rential curve

500 600 700 800 900

re (°C)

Temperature (°C)

0

20

40

60

80

100

We

igh

t (%

)

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riva

tive

(%

/min

)

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0.0

0.1

1st

De

riva

tive

(%

/min

)

the TGA curve of 2#

the corresponding differential curve

5004003002001000 600 700 800 900

CNTs (a) 0], (b) 2] and (c) 6].

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Fig. 4. (a) TEM image of as-received MWCNTs. Inset is SAED pattern of

as-received MWCNTs; (b) HRTEM image of close-up of the boxed region

in (a) (as-received MWCNTs). Black arrows indicate the impurities.

K. Yang et al. / Journal of Physics and Chemistry of Solids 69 (2008) 222–229 227

4.2. TGA analysis

The weight loss curves and the corresponding differentialcurves (DTG) for MWCNTs (0], 2] and 6]) are dis-played in Fig. 3(a)–(c). It can be seen that the onsetcombustion temperatures of 0]-MWCNTs, 2]-MWCNTsand 6]-MWCNTs are 616, 698 and 674 1C, respectively. Asis known, carbon materials with low degree of graphitiza-tion have good reacting activity at lower temperature,compared to those with high degree of graphitization.Better degree of graphitization indicates that there isgreater degree of order in the structures of carbonmaterials. Among the three specimens, the onset combus-tion temperature of 0]-MWCNTs is the lowest, so it hasgreater reacting activity and lower degree of order in thestructure, compared to the other two specimens. Bycontrast, 2]-MWCNTs has the largest onset combustiontemperature and the highest degree of order in itsstructure. Furthermore, the degree of order in the structureis closely related to the crystallinity of materials. Highcrystallinity denotes great degree of order in the structure.So this is consistent with the results obtained byWAXD analysis. Moreover, the residual mass factionsof 0]-MWCNTs, 2]-MWCNTs and 6]-MWCNTsare 13%, 30% and 15% at 800 1C, respectively. Theentire combustion of 2]-MWCNTs needs higher tempera-ture, as further proved that it possessed better degree ofgraphitization.

4.3. TEM/HRTEM analysis

The microstructures of as-received MWCNTs are shownin Fig. 4. The external and internal diameters of MWCNTsare 40 and 10 nm, respectively. The as-received MWCNTscontain few impurities (mainly including metallic catalystparticles), which have different shapes as seen in Fig. 4(a).The select area electron diffraction (SAED) pattern ofas-received MWCNTs (inset in Fig. 4(a)) shows concentricdiffraction rings, which indicate that MWCNTs arepolycrystal. The nanotube wall consists of a series ofgraphite layers, as is presented in Fig. 4(b). However, thegraphite layers do not stay continuous along the growthorientation in some regions, which results from pointdefects and the faults between graphite carbon planes. Theappearance of these defects has a negative effect on thecrystallinity of MWCNTs.

Fig. 5 displays the microstructures of MWCNTschemically treated at different times. It can be seen fromFig. 5(a) that 2]-MWCNTs is dispersed well owing tonegative charges formed at the external walls. Likewise,SAED pattern of 2]-MWCNTs (inset in Fig. 5(b)) revealsintact diffraction rings very clearly and brightly, whichindicates its good crystalline structure. Furthermore,from the HRTEM image (presented in Fig. 5(c)), theoverall straight graphite layers remain perfectly contin-uous. The HRTEM observation effectively confirms that2]-MWCNTs possesses high crystallinity. Since the loss of

mass does not exceed 1% of the mass of as-receivedMWCNTs, the chemical treatment may facilitate theimprovement of growing zones of MWCNTs and provideenergy for atom migration. Some amorphous phase in2]-MWCNTs transformed into the corresponding crystal-line phase, and so the crystallinity of MWCNTs increased.On the other hand, longer chemical treatment time willcause the damages to the structures of MWCNTs. It can beseen that some MWCNTs are truncated on account of thesevere corrosion of nitric acid in Fig. 5(d). With regard to10]-MWCNTs, it is found that the external walls of a fewMWCNTs are severely eroded by nitric acid and extremelygreat degree of disorder in the structures of MWCNTsexists, as is shown in Fig. 5(e) and (f). SAED pattern of

ARTICLE IN PRESS

Fig. 5. The microstructures of chemically treated MWCNTs. (a) TEM image of 2]-MWCNTs; (b) TEM image of 2]-MWCNTs. Inset is SAED pattern of

2]-MWCNTs; (c) HRTEM image of close-up of the boxed region in (b) (2]-MWCNTs); (d) TEM image of 10]-MWCNTs; (e) TEM image of 10]-MWCNTs. Inset is SAED pattern of 10]-MWCNTs; (f) HRTEM image of close-up of the boxed region in (e) (10]-MWCNTs). White arrows indicate the

damages to MWCNTs.

K. Yang et al. / Journal of Physics and Chemistry of Solids 69 (2008) 222–229228

ARTICLE IN PRESSK. Yang et al. / Journal of Physics and Chemistry of Solids 69 (2008) 222–229 229

10]-MWCNTs (inset in Fig. 5(e)) presents discontinuousdiffraction rings, which indicates low degree of order in itsstructures. From the HRTEM image (exhibited inFig. 5(f)), there are no continuous graphite layers consist-ing of the external walls of MWCNTs. A number offaults between graphite carbon planes appear. The severechemical treatment condition causes damage to the crystal-line phase of MWCNTs. These observations revealremarkable characteristic of amorphous phase and effi-ciently illustrate the decrease of the crystallinity ofMWCNTs.

5. Conclusions

MWCNTs are treated in nitric acid solution for differenttime. In order to better combine the purification ofMWCNTs with the improvement of crystallinity ofMWCNTs, it is very important to effectively controlchemical treatment time, so that their essential propertiescan be exhibited. Quantitative analysis of the crystallinityof MWCNTs was performed by WAXD. The introductionof the correction factor (C(y)) of the peak integral intensitycan enhance the computational accuracy of the crystal-linity. With the increasing chemical treatment time, thecrystallinity of MWCNTs first increases, and thendecreases. When chemical treatment time is 2 h, thecrystallinity of MWCNTs reaches the maximum of85.9%. Compared to the crystallinity of as-receivedMWCNTs (70.0%), chemical treatment is confirmed tobe very effective to enhance the crystallinity of MWCNTs.However, longer chemical treatment time would bedetrimental to the crystallinity. Moreover, the ascent rateof the crystallinity of MWCNTs is greater than its rate ofdescent in chemical treatment process.

The degree of order in the structures of MWCNTschemically treated at different times was further investi-gated by TGA. It is found that MWCNTs with greatercrystallinity have higher onset combustion temperature andbetter degree of graphitization. HRTEM image shows thatthe external walls of 2]-MWCNTs consist of very straightand perfectly continuous graphite layers, so it effectivelyconfirms that 2]-MWCNTs have good crystalline struc-tures. When chemical treatment time is long enough, theexternal walls of MWCNTs are corroded by nitric acid andthe appearance of an amount of faults between graphitecarbon planes results in the decrease of the crystallinityof MWCNTs.

Acknowledgment

The authors are grateful to Instrumental Analysis Centreof Shanghai Jiao Tong University for providing the XRDanalyses.

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