syntheses, properties and electrochemical activity of carbon microtubes modified with amino groups

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DOI: 10.1002/adfm.200701020
PAPER
Syntheses, Properties and Electrochemical Activity of CarbonMicrotubes Modified with Amino Groups**

By Xiaofeng Wang, Xiaoqing Liu, Linfei Lai, Siyue Li, Jian Weng,* Zhimin Zhou, Qiang Cui,Xin Chen, Minyuan Cao, and Qiqing Zhang*

Amine-modified carbon micro/sub-microtubes that encapsulate magnetite cores have been synthesized by decomposing a

ferrocene/hexabromobenzene mixture in the presence of ammonia under solvothermal conditions at 250 8C for 24 h in a

one-step process. The as-prepared carbon microstructures (NH2-ME-CMTs) were 90–2000 nm in diameter and from tens to

hundreds of micrometers in length that could be tuned in various solvents. The surface of the carbon microtubes can be modified

with amino groups by the synthetic process, as confirmed by infra-red (IR) spectroscopy and X-ray photoelectron spectroscopy

(XPS). Ammonia in the reaction system plays a key role in the formation of the microtube morphology and was the source of the

surface functionalization groups. Fluorescent fluorescein isothiocyanate (FITC) was selected as a model compound and

successively attached to the amino groups of the carbon microtubes. This result confirms the reactivity of the amino groups

on the surface of the carbon microtubes. The inner magnetite cores were removed after immersion in 1 M HCl solution at room

temperature over two months, and hollow carbon microtubes (NH2-H-CMTs) were obtained. The magnetite-encapsulated

carbon microtubes and the hollow carbon microtubes were coated on gold electrodes to prepare carbon microtube-modified

gold electrodes. The two electrodes have been used to investigate the oxidative properties of dopamine (DA) and ascorbic acid

(AA). Different from the magnetite-encapsulated microtube-modified electrode, the hollow microtube-modified electrode can

be utilized in the selective detection of DA in the presence of a large excess of AA. The electrochemical behaviour of DA and

AA on the hollow carbon microtubes modified with amino groups is similar to that on carbon nanotubes. This result suggests

that the one-step synthesis method will not change the electrochemical properties or break the backbone structure of the carbon

microtubes.

1. Introduction

Carbon nanotubes (CNTs) have been extensively investi-

gated for their synthesis, properties, and applications. Because

[*] Assoc. Prof. J. Weng, Prof. Q. Zhang, Dr. X. Wang, X. Liu, L. Lai, S. Li,Q. Cui, X. Chen, M. CaoResearch Center of Biomedical Engineering, Technology ResearchCenter of Biomedical Engineering of Xiamen CityThe Key Laboratory of Biomedical Engineering of Fujian ProvinceCollege of Materials, Xiamen UniversityXiamen 361005 (P.R. China)E-mail: [email protected]; [email protected]

Dr. X. Wang, Prof. Q. ZhangCollege of Chemistry and Chemical Engineering, Xiamen UniversityXiamen 361005 (P.R. China)

Prof. Q. Zhang, Z. ZhouInstitute of Biomedical Engineering, Chinese Academy of MedicalScience & Peking Union Medical College, The Key Laboratory ofBiomedical Material of TianjinTianjin 300192 (P.R. China)

[**] This work is supported by 973 Program (NO: 2006CB933300), Pro-gram for New Century Excellent Talents in Fujian Province (NO:Z03131), Scientific Research Foundation for the Returned OverseasChinese Scholars, the Ministry of Education (NO: K13003), NaturalScience Foundation of China (NO: 20701031) and Natural ScienceFoundation of Fujian Province of China (NO: C0710045).

Adv. Funct. Mater. 2008, 18, 1809–1823 � 2008 WILEY-VCH Verlag

of their high mechanical strength and modulus,[1] high

electrical conductivity,[2] and high thermal conductivity,[3]

CNTs are potentially useful in nanoelectro-mechanical systems

and structural/functional composites.[4] These materials have

been synthesized by employing various strategies. Many

approaches have been reported for the synthesis of CNTs,

such as arc-discharge,[5,6] gas-phase catalytic process,[7]

chemical vapor deposition (CVD),[8] hydrothermal,[9–13] and

solvothermal methods.[14–18]

The solvothermal synthesis of single-walled carbon nano-

tubes (SWNTs) has many advantages over other methods: it’s

simple and inexpensive. For example, CNTs can be prepared

under hydrothermal conditions at 700–800 8C from reactants

such as polyethylene,[9] amorphous carbon,[10] and ethylene

glycol[11] in the presence of a catalyst. The reaction tempera-

ture to obtain CNTs by a hydrothermal process can be as low as

160 8C.[13] By a solvothermal method, CNTs can be obtained at

various temperatures from different precursors and catalysts,

such as reduction of ethanol using metal oxides at 550 8C,[14]

reduction of hexachlorobenzene using Co/Ni catalyst at 350 8C,[15]

reduction of hexachlorobenzene using nickel chloride at

230 8C,[16] reduction of tetrachloroethylene using metallic

potassium at 200 8C,[17] and decomposition of ferrocene in

toluene at 650 8C.[4] The presence of a transition metal is

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X. Wang et al. /Amino-Modified Carbon Microtubes

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essential for the formation of nanotubes by pyrolysis carbon

precursors, and the diameter of the nanotubes is determined by

the size of the metal nanoparticles. Organometallic precursors

such as ferrocene act as a source of the metal as well as part of

the carbon precursors to produce CNTs.[19]

At the same time, some groups have also reported the

preparation of carbon microtubes (CMTs).[20–24] Graphite

tubes with outer diameters that range from 70 to 1300 nm were

grown hydrothermally in a C–H–O–Ni system.[11] Hu et al.

synthesized CMTs by heating a mixture of ZnS and activated

carbon powers at 1400 8C for 1.5 h.[20] CMTs with wide hollow

interiors might be suitable for large-diameter particle encap-

sulation such as in nano/microfluidic, drug-delivery, and

nanoelectronic applications.[20] However, compared with

CNTs, the synthetic method and application of CMTs have

not been fully investigated, and like CNTs, their applications in

certain fields (biomedical applications) are restricted because

of lack of solubility and tailored surface chemistry. The surface

functionalization of CMTs by a one-step process remains a

challenge.

Several strategies have been developed to tackle this

challenge, such as covalent and non-covalent approaches.

The covalent approach was attachment of functional groups to

the sidewall or defect sites of CNTs. The modification

procedures included refluxing with nitric acid[25] or treatment

by g-irradiation[26] to produce acid sites that were composed

mostly of carboxylic groups for covalent modification.

However, the intrinsic electrical and mechanical properties

of the CNTs could be affected by these harsh treatments

because of the severe disruption of p-networks on the CNT

walls. The non-covalent approach immobilized amphiphilic

molecules that contained aromatic moieties on the surface of

the SWNTs, which maintained their geometric structure. The

interactions involved van der Waals, electrostatic, and p–p

interactions between the graphite surface and the aromatic

component of amphiphilic molecule.[27] However, compared

with the covalent method, it usually suffered from some

disadvantages, such as instability, low loadings in solutions, and

the need for specific complex and synthetic reagents for the

functionalization.[28,29]

Higher reaction temperatures are advantageous in the

graphitization of CNTs, however, generally it is a disadvantage

to introduce functional groups through the synthesis process.

Strategies to functionalize carbon nanomaterials by their

synthetic process might overcome this problem. A relative

lower reaction temperature and suitable reaction precursors

are important to obtain functionalized carbon materials in a

one-step process. The linkage of organic or biological

molecules to functionalized groups of CNTs opens up the

possibility for their applications in medicinal and biological

filelds. However, the conventional method is complex and

tedious.[29,30] Amino groups have a high reactivity, a wealth of

chemistry, and can react with many chemicals such as organic

or biological molecules. The aim of this paper is to develop a

simple and effective method to prepare amine-modified CMTs

in one-step process.

www.afm-journal.de � 2008 WILEY-VCH Verlag GmbH

In this study, we prepared amine-functionalized CMTs

(NH2-ME-CMTs) by decomposing a ferrocene/hexabromo-

benzene mixture in the presence of ammonia under solvother-

mal conditions at 250 8C in a one-step process. The lengths of

the NH2-ME-CMTs are 0.01–0.1 mm. The most important

contribution of this work is to introduce a simple and effective

method by which the surface of the microtubes or nanotubes is

functionalized with amino groups by adding ammonia during

the one-step synthetic process.

Fluorescent fluorescein isothiocyanate (FITC) was selected

as a model compound to react with the amino groups on the

surface of the CMTs to confirm the reactivity of the amino

groups. Finally, we also investigated the oxidative properties of

DA and AA on NH2-ME-CMT and NH2-H-CMT electrodes

to confirm whether the introduced amino groups would affect

the electrochemical properties of the CMTs.

2. Results and Discussion

2.1. Preparation of ME-CMTs

The production of carbon nanomaterials is sensitive to

processing parameters such as reaction temperature,[31,32] the

concentration of carbon precursors,[33] the mixing ratio of

reactant agents,[34] and the reaction time.[17] The effect of these

parameters on the growth characteristics of CMTs have been

systematically investigated in our solvothemal system. Some

experiments were performed to selectively obtain CMTs with

high yield. The reaction molar ratio of ferrocene to

hexabromobenzene, reaction time, temperature, and solvents

used were varied to investigate their effect on both the

morphology and yield of the microtubes.

Ferrocene has long been studied as a catalyst reagent in

preparing CNTs by different methods. Iron particles produced

from ferrocene act as nuclei for the condensation of large

aromatic molecules and amorphous carbon. The hexabromo-

benzene and ferrocene were selected as the carbon shell and

magnetite core precursors. Variation of the molar ratio of

ferrocene to hexabromobenzene between 0.33 to 3 has a

negligible affect on the morphology of the as-prepared CMTs.

However, excess hexabromobenzene did not react completely

when the molar ratio was lower than 2. Therefore, all the

following experiments were performed at a molar ratio of

ferrocene/hexabromobenzene¼ 2 : 1 in the presence of 1 mL of

ammonia except where mentioned.

2.1.1. Effect of Reaction Temperature and Time

Figure 1 shows the scanning electron microscopy (SEM)

images of products prepared at 250 8C with a reaction time of

4 to 24 h. The products were carbon nanoparticles and an

amorphous carbon film at 250 8C for 4 h (Fig. 1a). Prolonging

the growth time to 8 h, the products were mostly sphere-like

nanoparticles and amorphous carbon sheets (Fig. 1b). There

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Figure 1. SEM images of products prepared in toluene/DMF (1:1) solvent ([ferrocene]¼ 1� 10�3M,

[hexabromobenzene]¼ 0.5� 10�3M, [ammonia]¼ 1mL) at 250 8C for: a) 4, b) 8, c) 12, d) 20, and 24 h (see

Fig. 2d).

were some short curved CMTs in low yield. When the time was

extended to 12 and 20 h, several long microtubes were

observed (Fig. 1c and d), and further prolonging the reaction

to 24 h produced well-crystallized CMTs (Fig. 2d). The yield of

CMTs increased with prolonged reaction time, but did not

increase significantly when the reaction time was 48 h (data not

shown), compared with that of 24 h. The effect of reaction time

on the formation of CMTs is similar to that of CNTs. Wang et

al. prepared CNTs through a benzene thermal-

reduction–catalysis route and found that the graphitization

of CNTs could be enhanced through prolonging the growth

process time from 9 to 27 h.[17]

The reaction temperature is also an important parameter

that may affect both the morphology and yield of the products.

Figure 2 shows the SEM images of the products prepared at

reaction temperatures from 180 to 250 8C. Below 210 8C, the

yield of CMTs is low and the products are a mixture of

microtubes and sheets (Fig. 2a and b). The yield of CMTs at

250 8C (Fig. 2d) is higher than that at 210 8C (Fig. 2c). At the

same time, the diameter and length of CMTs increased slightly

with the increase of reaction temperature from 180 to 250 8C.

Therefore, we can conclude that the optimal temperature for

the formation of CMTs is about 250 8C for 24 h in our

experiment. This result is reasonable because the hexabro-

mobenzene began to decompose at 250–300 8C.[35] Zhang et al.

reported similar results that carbon ‘test tubes’ and carbon

particles could be produced in glycol until the reaction

Adv. Funct. Mater. 2008, 18, 1809–1823 � 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

temperature reached over 200 8C,

and carbon test tubes were in

high yield when the temperature

was over 250 8C.[32]

2.1.2. Solvent Effect

Because of the good solubility

of hexabromobenzene and fer-

rocene in N,N-dimethylformamide

(DMF), ethanol, and toluene,

we selected the mono-solvent or

mixed solvent systems of DMF,

ethanol, and toluene to explore

both the morphology and yield

of the product in our experiment

system. Results show that the

reaction solvent had a great

effect on the morphology of

products, which could be tuned

from nanoparticles to micro-

tubes or sub-microtubes. The

results are summarized in Table 1.

Figures 3–5 show some typical

SEM images of products pre-

pared in different reaction sol-

vents at 250 8C for 24 h.

Figure 3 shows typical SEM

images of products prepared

with different volume ratios of toluene to ethanol. Aggregated

nanoparticles were formed with 100% toluene (5 : 0) (Table 1,

also see Fig. 4a) and with 20% ethanol (4 : 1) (Fig. 3a). With an

increase in the ethanol to 40% (3 : 2) (Fig. 3b), the main

products are straight sub-microscale tubes with a diameter of

about 200 nm. Upon a further increase in the amount of

ethanol to 60% (2 : 3) (Fig. 3c) and 80% (1 : 4) (Fig. 3d),

twist-like or coiled tubes and short branch-like microtubes can

be obtained. The morphology was tuned from micro/

nanoparticles to sub-microtubes with an increase in the volume

ratio of ethanol from 0 to 40% in the toluene/ethanol system.

However excess ethanol can produce a coiled tube and short

branch-like microtubes with diameters from 500 nm to 1mm.

With a further increase of ethanol to 100%, the main products

are nanoparticles and sheets (Table 1, also see Fig. 5a).

Figure 4 shows typical SEM images of products prepared in

different volume ratios of toluene to DMF. The products with

100% toluene (5 : 0) are nanoparticles in aggregate form

(Fig. 4a). With 20% DMF (4 : 1), the products are a mixture of

microtubes and some nanoparticles (Fig. 4b). Straight micro-

tubes in a high yield can be obtained with a ratio of 50% DMF

(1 : 1) (Fig. 2d). Upon increasing the ratio of DMF to 80%

(1 : 4) (Fig. 4c) and 100% (0 : 5) (Fig. 4d), the products are

mostly nano/microparticles. The amount of carbon nano/

microparticles decreases with an increasing amount of DMF

from 0 to 50%. DMF was advantageous for the formation of

straight microtubes when the ratio of DMF was 50%. With

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Figure 2. SEM images of products in toluene/DMF (1 : 1) solvent ([ferrocene]¼ 1� 10�3M, [hexabromoben-

zene]¼ 0.5� 10�3M, [ammonia]¼ 1mL) for 24 h at a) 180, b) 200, c) 210, and d) 250 8C.

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higher amounts of DMF, i.e., over 80%, carbon nanoparticles

were produced.

Figure 5 shows typical SEM images of products prepared in

different volume ratios of ethanol to DMF. The product

consists mainly of nanoparticles and sheets when ethanol is

Table 1. Effect of the reaction solvent on the morphology of products.

No. Reaction solvent (v:v)[a] Morphology of products

Toluene Ethanol DMF

1 4 1 0 nanoparticles

2 3 2 0 sub-micro CMTs

3 1 1 0 nanoparticles



6 4 0 1 CMTs, nanoparticles

7 1 0 1 CMTs

8 1 0 4 nanoparticles, sheets

9 0 4 1 CMTs

10 0 1 1 CMTs, sheets

11 0 1 4 CMTs




15 5 0 0 particles, sheets



[a]Each experiment was performed in the presence of 1mL of 30% ammonia at 250 8Cfor 24 h under solvothermal conditions.

www.afm-journal.de � 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

100% (5 : 0) (Fig. 5a). The pro-

ducts are curved microtubes with

20% DMF (4 : 1) (Fig. 5b), and

straight CMTs mixed with some

slightly curved CMTs with 50%

DMF (1 : 1) (Fig. 5c). The yield

of straight CMTs was improved

with an increase in the volume

ratio of DMF from 0 to 50% in

the ethanol/DMF system. The

products are composed of curved,

coiled tubes or belts, and amor-

phous carbon with 80% DMF

(1 : 4) (Fig. 5d). However with

100% DMF, the products are

mostly nano/microparticles and

sheets (Fig. 4d).

The optimal experiment para-

meters to prepare straight CMTs

in high yield are ferrocene/hex-

abromobenzene (2 : 1) with a

mixture of toluene/DMF (1 : 1)

in the presence of ammonia

(1 mL) at 250 8C for 24 h. The

outer diameter of the CMTs is

about 0.5–2.0mm and the length

is up to 100mm (Fig. 6a). Figure

6b is a magnified image of Figure

6a, which shows some bamboo-

like microtubes. A similar structure is also obtained by heating

a mixture of ferrocene and glycerol to prepare CMTs in an

autoclave at 600 8C for 16 h.[22]

Chen et al. produced magnetite nanorods from poly(ethy-

lene glycol) (PEG) and ferrous ammonium sulfate. They

concluded that the hydroxy groups in solution played an

important role in the formation of magnetite nanorods.

Hydroxy groups attacked and coordinated Fe by replacing

PEG, because the bond strength between the metal ion and

ligands follows the order of OH�>PEG.[35] Wan et al.

prepared Fe3O4 nanorods with ethylenediamine as a strong

coordinating agent. The formation of Fe3O4 nanorods may be a

result of the intermolecular interactions including hydrogen

bonding, van der Waals, and electrostatic interactions.[36] The

different solvents in the mixed solvent system could adsorb

onto the surface of Fe with different strengths on different

facets, which leads to the formation of nanorods. Previous

literature has shown that non-equilibrium growth is a possible

mechanism to lead to the formation of tubular or nanorod

structures. A similar mechanism is proposed for the CMTs

formed from non-equilibrium growth in the given toluene,

DMF, and ethanol complex solvent systems. The mixed solvent

in our system may play an important role in leading to the

non-equilibrium growth of magnetite nanorods. The different

morphology of our products obtained in various mixed solvent

systems may be a result of the different solvent ratio and the

different morphologies of magnetite during the initial growth

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Figure 3. SEM images of products prepared in toluene/ethanol as mixed solvents ([ferrocene]¼ 1� 10�3M,

[hexabromobenzene]¼ 0.5� 10�3M, [ammonia]¼ 1mL) at 250 8C for 24 h. The volume ratio of toluene and

ethanol are 5 : 0 (see Fig. 4a), a) 4 : 1, b) 3 : 2, c) 2 : 3, d) 1 : 4, and 0 : 5 (see Fig. 5a).

Figure 4. SEM images of products prepared in toluene/DMF as mixed solvents ([ferrocene]¼ 1� 10�3M,

[hexabromobenzene]¼ 0.5� 10�3M, [ammonia]¼ 1mL) at 250 8C for 24 h. The volume ratio of toluene and

DMF are 5 : 0 (a), 4 : 1 (b), 1 : 1 (see Fig. 2d), 1 : 4 (c), and 0 : 5 (d).

Adv. Funct. Mater. 2008, 18, 1809–1823 � 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

stage. However, the real mecha-

nism of the solvent effect on the

morphology of the product is

not very clear at this stage.

2.1.3. Ammonia Effect

Ammonia had a significant

effect on the morphology, yield,

and characteristics of the CMTs.

Therefore, it is necessary to

investigate the effect of the

amount of ammonia on the mor-

phology and yield of the CMTs

in detail. We selected the opti-

mal experiment parameters to

prepare straight CMTs: ferro-

cene/hexabromobenzene (2 : 1)

with mixture of toluene/DMF

(1 : 1) at 250 8C for 24 h. Figure 7

shows the morphology change

of the product with various

amounts of ammonia. Without

ammonia, the morphology of

the products was dominated by

nanoparticles (Fig. 7a), when

the amount of ammonia was

increased to 0.8 mL (Fig. 7c),

there was a substantial increase

in the amount of short CMTs,

and the straight CMTs even-

tually became the dominant

constituent at 1 mL (Fig. 6a).

It is worth noting that the

products present an ammonia-

dependent change in morphol-

ogy and yield. When the fraction

of ammonia is changed from 1.2

(Fig. 7d) to 1.8 mL (Fig. 7f), the

CMTs become short and curved

segments. Therefore the opti-

mal experimental parameters to

prepare straight CMTs in high

yield is ferrocene/hexabromo-

benzene (2 : 1) with a mixture

of toluene/DMF (1 : 1) in the

presence of ammonia (1 mL) at

250 8C for 24 h (Fig. 6a).

Han et al. found that NH3 was

essential to form CNTs in the

PECVD system. NH3 as a feed-

ing gas may provide an addi-

tional etching effect on the

growing carbon surface and

suppress carbon supplies, which

facilitates the formation of

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Figure 5. SEM images of products prepared in ethanol/DMF as mixed solvents ([ferrocene]¼ 1� 10�3M,

[hexabromobenzene]¼ 0.5� 10�3M, [ammonia]¼ 1mL) at 250 8C for 24 h. The volume ratio of ethanol and

DMF are 5 : 0 (a), 4 : 1 (b), 1 : 1 (c), 1 : 4 (d), and 0 : 5 (see Fig. 4d).

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CNTs.[37] In our experiment, with an increase in the amount of

ammonia from 0 to 1 mL, there is a significant change from

nanoparticles to CMTs. The proper fraction of ammonia was

advantageous for the formation of microtubes.[38] Ammonia in

our system may play two roles. First, alkaline ammonia

neutralizes the Br� produced from the debromination of

hexabromobenzene to accelerate the formation of a carbon

shell. Second, ammonia stabilizes the dangling bond produced

from the debromination of the hexabromobenzene as a group

of surface functionalization (the presence of the amino group

Figure 6. Typical SEM images of the straight CMTs in toluene/DMF (1 : 1) mixed solvent([ferrocene]¼ 1� 10�3

M, [hexabromobenzene]¼ 0.5� 10�3M) at 250 8C for 24 h (a). A magnified

image of the straight CMTs shows the bamboo-like structure (b).

www.afm-journal.de � 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

will be confirmed in the follow-

ing discussion). However, Juang

et al. found that NH3 in a high

concentration hampered the

CNT growth and decreased the

degree of straightness and ver-

tical alignment.[39] Similar

results were obtained in our

system. The as-prepared CMTs

were short with a rough wall

surface when the ammonia frac-

tion was increased from 1 to

1.8 mL.

2.2. Structure

Characterization

The ME-CMTs were treated

with dilute 1 M HCl solution to

remove the inner magnetite cores

to obtain hollow CMTs (H-CMTs).

The hollow structure of the

H-CMTs was characterized by

high-resolution transmission

electron microscopy (HRTEM).

Figure 8a shows the TEM

images of the H-CMTs after

etching with 1 M HCl solution

over two months at ambient temperature. The hollow-tube

structure can be clearly observed. Figure 8b shows a magnified

image of a single microtube. The (002), (100), and (101) crystal

planes of graphite can be indexed. The thickness of the carbon

shell is about 10 nm, which indicates that the acid etching

process would remove the magnetite cores. The electron

diffraction pattern shows concentric circles, which implies that

the H-CMTs were polycrystalline materials (Fig. 8c).

Figure 9 shows the X-ray powder diffraction (XRD)

patterns of the NH2-ME-CMTs and the H-CMTs. All the

peaks labelled by asterisks in the XRD

pattern of the ME-CMTs can be indexed

as cubic magnetite (JCPDS: 01-1111), and

the peaks of graphite are almost unob-

servable. The broad peak around

2u¼ 26.1 8can be indexed as the (002)

crystal plane of graphite carbon in the

XRD pattern of the H-CMTs (JCPDS:

01-0640). The peak at 2u¼ 42.8 8can be

indexed as the (100) crystal plane of

graphite. These results suggest that the

ME-CMTs were mainly formed by amor-

phous carbon and graphite carbon. Simi-

larly, Zhang et al. prepared sub-

microscale amorphous carbon tubes at

250 8C by the solvothermal method.[32]

Therefore, the low graphitic crystallinity

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Figure 7. Typical SEM images of the products prepared from different fractions of ammonia ([ferrocene]¼ 1� 10�3M, [hexabromobenzene]¼

0.5� 10�3M) at 250 8C for 24 h. The ammonia fraction was 0 (a), 0.4 (b), 0.8 (c), 1 (see Fig. 6a), 1.2 (d), 1.4 (e), and 1.8mL (f).

Figure 8. The hollow tubular structure of the H-CMTs obtained from ME-CMTs by HCl etching at room temperature for over 2 months (a). The HRTEMimages of a single CMT with a wall thickness of about 10 nm (b) and the electron diffraction pattern of the H-CMTs (c).

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Figure 9. a) XRD patterns of the ME-CMTs and H-CMTs. All the peaks labelled with asterisks can be indexed as magnetite in the XRD pattern of theME-CMTs. b) XRD patterns of ME-CMTs prepared with different fractions of ammonia from 0 to 1.8mL at 250 8C for 24 h ([ferrocene]¼ 1� 10�3

M,[hexabromobenzene]¼ 0.5� 10�3

M).

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of our products was mainly caused by the low temperature of

our reaction system. With a change in the ammonia fraction

from 0 to 1.8 mL, the corresponding XRD patterns of the

as-prepared ME-CMTs are shown in Figure 9b. All the

products have the same XRD pattern, which means that the

crystalline structure of the ME-CMTs remains unchanged

while the ammonia fraction is tuned from 0 to 1.8 mL.

Figure 10 shows typical Raman spectra of the ME-CMTs

prepared with ammonia (0, 1, and 1.8 mL). Two broad peaks

around 1360 and 1550 cm�1 can be observed. The peak around

1360 cm�1 is usually associated with the vibrations of dangling

carbon bonds at the edges of graphite defects and is labelled as

the D-band. The peak around 1550 cm�1 is assigned to the

Figure 10. Typical Raman spectra of the ME-CMTs in the presence ofammonia (0, 1, and 1.8mL). The broad peak around 1360 cm�1, as well asthe adsorption peak around 1550 cm�1, could be assigned to the D-bandand G-band, respectively.


G-band, which might be caused by the E2g mode of the

graphite carbon, and comes from the vibration of sp2-bonded

carbon atoms in a two-dimensional graphite plane.[34] The ID/

IG ratio has been used to correlate the structure of graphitic

and the amorphous component of the carbon materials.[38] The

ID/IG values were 1.02, 1.02, 1.07, 1.05, 0.92, 0.94, 1.10, 1.25, and

1.32 when the amount of ammonia was 0, 0.2, 0.4, 0.6, 0.8, 1, 1.6,

and 1.8 mL, respectively. These data indicate that the

graphitization was decreased with an increase of the ammonia

fraction. The graphitization was slightly improved with 0.8 and

1 mL of ammonia. Therefore, we can conclude that the proper

amount of NH3 would facilitate the formation of ME-CMTs,

excess ammonia, on the contrary, is disadvantageous for the

graphitization of CMTs. The results here are similar to the

results reported by Choi et al. that increasing the ammonia

concentration resulted in a complete morphology change from

nanoparticle to nanotube and the degradation of graphitic

sheet quality. The high concentration of ammonia resulted in

an increase in the intensity ratio of ID/IG in the Raman

spectrum and degradation of the graphitization.[40]

2.3. Functional Groups Characterization

The IR spectrum of the NH2-ME-CMTs is shown in

Figure 11. The peaks at 3430 and 3466 cm�1 can be assigned as

symmetric and antisymmetric stretching vibrations of the

primary amine (–NH2) group. The absorption bands at 1650

and 1386 cm�1 were attributed to the –NH2 bending vibration

and C–N stretching vibration, respectively.[40] The IR data

indicates the presence of amino groups in the NH2-ME-CMTs.

The IR result is consistent with amine group-modified CNTs by

post-treatment methods.[41,42] The IR spectrum of the

NH2-H-CMTs was similar to that of NH2-ME-CMTs (data

not shown).


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Figure 11. IR spectrum of the NH2-ME-CMTs that shows the double-peakof –NH2 groups at 3466.6 and 3430.1 cm�1, which indicated the presenceof amino groups. The absorptions at 1650.2 and 1386.1 cm�1 wereassingned to the –NH2 bending vibration and C–N bond stretchingvibration, respectively.

XPS is of interest in the present study because of the

inherently high sensitivity of this technique to the valence state

of the surface element. Therefore, XPS was selected to analyze

the surface groups of the NH2-ME-CMTs. It allows for

semi-quantitative analysis of functional groups, however, this

technique only analyzes the first 0.5–10 nm of the surface of the

sample. The thickness of the carbon shell in our products is

about 10 nm as confirmed by HRTEM (Fig. 8). Therefore, it is

reasonable to select XPS to analyze the bonding form of

nitrogen. The XPS data of the NH2-ME-CMTs are shown in

Figure 12. The asymmetric C1s peak is decomposed into two

peaks at 284.8 and 285.5 eV (Fig. 12a). The main peak at

284.8 eV for the NH2-ME-CMTs is a result of sp2 C–C bonding

in pristine highly oriented pyrolytic graphite (HOPG).[43,44]

The second peak centred at 285.5 eV can be ascribed to C–N

bonds. The amine functionalization is further confirmed in the

N1s spectrum (Fig. 12b). The peak at 399.7 eV is ascribed to the

amine on the surface of the NH2-ME-CMTs. In comparison to

the pyridine nitrogen at 398.4 eV,[45,46] quaternary nitrogens at

402.3 eV, and imines C––NH at 400.5 eV,[47] this exclusively

single peak is ascribed to primary amine C–NH2 bonding with

carbon.[43] The N content, which is defined as the N/C atomic

ratio, was estimated by the ratio of N and C 1s peak areas, with

their relative sensitive factor taken into account (Fig. 12c).

Upon increasing the fraction of ammonia from 0 to 0.6 mL, the

N/C atomic ratio (%) changes from 1 to 15%. The N/C atomic

ratio of the product with 1 mL of ammonia has a maximum

value, which represents the high surface functionalization of

NH2-ME-CMTs. However, the atomic ratio of N/C decreased

slightly while increasing the content of ammonia from 1 to

1.8 mL. This phenomenon could be interpreted as inhibition by

the excess ammonia (from 1 to 1.8 mL), which is consistent with

the results of Juang et al. that ammonia in high concentrations

hampered CNT growth and decreased the degree of straight-

Adv. Funct. Mater. 2008, 18, 1809–1823 � 2008 WILEY-VCH Verl

ness and vertical alignment.[39] Similar results were reported by

Choi et al.: an increase in the ammonia concentration resulted

in a complete morphology change and a degradation of the

graphitic sheet quality.[40] The as-prepared CMTs were short

with a rough wall surface (see Fig. 7) and had low graphi-

tization (see Fig. 10) when the ammonia fraction was increased

from 1 to 1.8 mL. The degradation of the graphitization and

decrease of the CMT growth may be the main reason that leads

to the decrease of nitrogen concentration.

IR and XPS data confirm that the surface of the CMTs is

functionalized with amino groups. The functionalization of the

carbon surface with amino groups was performed during the

synthetic process of the CMTs in one-step without any post

treatment to introduce functional groups. So this solvothermal

method might provide a new starting point in the modification

and application of CMTs.

Thermal gravimetric analysis (TGA) is often used to

investigate the distribution and species of the carbon phase

in carbon nanomaterials.[32] Therefore TGA was also used to

analyze the samples prepared with different fractions of

ammonia. The main weight loss point was at about 400 8C,

which corresponds to the temperature of amorphous carbon

burning. The residue weight ratio was about 80%, therefore,

the total content of carbon was about 20%. The XPS analysis

shows that the atomic ratio of nitrogen and carbon is 1 : 7. The

nitrogen weight ratio was about 2% in the NH2-ME-CMTs.

Considering the precision and sensitivity, TGA can not be

considered a useful technique for nitrogen content analysis.

In order to confirm the reactivity of the amino groups, a

fluorescent model compound, FITC, was selected to react with

the amino groups on the surface of the CMTs. FITC can

covalently attach to primary amine groups through a stable

thiourea linkage.[48,49] Figure 13 shows the confocal laser

scanning fluorescence microscope (CLSFM) images of

FITC-NH-H-CMTs and pristine CNTs (without amino

groups). In order to confirm that the green fluorescence of

FITC-NH-H-CMTs was a result of the chemical binding of

FITC and not from the physical adsorption of FITC onto the

NH2-H-CMTs themselves, we also investigated the fluorescent

characteristics of NH2-H-CMTs and CNTs. The results show

that NH2-H-CMTs and CNTs themselves are not fluorescent

under the conditions employed. FITC was covalently attached

to the outer wall of NH2-H-CMT, and the green fluorescence

could be clearly seen (Fig. 13a). In comparison with

FITC-NH-H-CMTs, the CNTs are not fluorescent after the

same treatment (Fig. 13b). This result confirms that the green

fluorescence of the FITC-NH-H-CMTs was a result of the

chemical binding of FITC and not physical adsorption, and also

further confirms the presence of primary amino groups on the

outer wall.

2.4. Formation Mechanism

Ferrocene is widely used to prepare CNTs by thermal

decomposition to generate iron nanoparticles as

ag GmbH & Co. KGaA, Weinheim www.afm-journal.de 1817

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Figure 12. XPS spectra of carbon (a) and nitrogen (b) on the surface of the NH2-ME-CMTs. c) The nitrogen/carbon atomic ratio (%) of the NH2-ME-CMTsproduced from different fractions of ammonia from 0 to 1.8mL.

1818

catalysts.[4,22,24] Usually the decomposition temperature of

ferrocene is higher than 500 8C.[40] In our solvothermal system,

mixed reaction solvents might decrease the decomposition

temperature which results in the change of morphology of the

products in various reaction solvents. The iron nanoparticles

might be encapsulated by carbon atoms quickly and form

carbon-coated nanoparticles.[19]

Ferrocene (Fe(Cp)2, Cp¼C5H5) is a sandwich organome-

tallic compound, the Fe–Cp bond is formed through the

d-electron of the metal and the p-electrons of the Cp groups,

and this bond is generally less stable than the bonds in the Cp

ring itself. The atomic iron produced from ferrocene could

agglomerate into iron clusters and act as a catalyst for the

pyrolysis of carbon precursors to produce CNTs.[19,22]

Hexabromobenzene has been debrominated by CaO.[35]

However, iron nanoparticles have been found to be highly


effective in the dechlorination and debromination of chlori-

nated or brominated aliphatics and aromatics.[51–53] Jiang et al.

prepared MWNTs at 350 8C through metallic potassium

reduction of hexachlorobenzene in the presence of Co/Ni.[15]

They proposed that the carbon clusters produced from

dechlorination would assemble into nanotubes and result in

the formation of MWNTs. Hexabromobenzene was the carbon

precursor in our system. The presence of Br� in the residual

solution after solvothermal reaction was confirmed by adding

dilute AgNO3/HNO3 solution. This result implies that

hexabromobenzene was decomposed in the reaction system.

It is similar to the dechlorination process,[15–17,50] hexabromo-

benzene would first be reduced by the iron clusters through

debromination and form carbon clusters. The carbon clusters

would nucleate on the surface of the metal particles and form

the walls of the CMTs.


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Figure 13. Typical fluorescent images of FITC-NH-H-CMTs (a) and CNTs (b). NH2-H-CMTs andCNTs (without amino groups) were treated with FITC under the same conditions, and then capturedby CLSFM.

Ammonia plays a key role in the system, and can tune the

morphology of the products from nanoparticles to CMTs.

Ammonia also acts as the source of surface functionalization.

First, alkaline amomonia neutralizes the Br� produced from

the debromination of hexabromobenzene to accelerate the

formation of the carbon shell. Second, the ammonia stabilizes

the dangling bond produced from debromination of hexabro-

mobenzene as a group of the surface functionalization. The

addition of ammonia to the upgrowth site of the carbon atom

dangling bonds is required for energy minimization and

stabilization of the CMTs.

According to previous literature, non-equilibrium growth is

a possible factor in the formation of tubular structures. Results

in Figure 1 also show that an increase in the reaction time

benefits the formation of NH2-ME-CMTs. The products are

Scheme 1. Schematic of the ME-CMTs and H-CMTs formation under solvothermal conditions.

mostly nanoparticles after 4 h

(Fig. 1a). With an increase in the

reaction time to 8 h, sub-

microscale particles can be

observed (Fig. 1b). The CMTs

can be produced by prolonging

the reaction time to 24 h

(Fig. 2d). These results indicate

that small catalyst clusters

would form large particles or

rod structures by prolonging the

reaction time.

On the basis of the experi-

mental results and previous

literature,[19,33,37] we propose

the formation mechanism of

NH2-ME-CMTs (Scheme 1):

first, ferrocene is decomposed

into Fe atoms, the Fe atoms then

Adv. Funct. Mater. 2008, 18, 1809–1823 � 2008 WILEY-VCH Verlag GmbH & Co. KGaA, We

form iron nanoparticles. Second, the iron

nanoparticles grow into microsca-

le-structures under solvothermal condi-

tions and in the presence of ammonia.

Finally, hexabromobenzene is decom-

posed on the surface of the in-situ

generated catalyst and forms carbon

shells. The surface dangling bonds of

the carbon clusters/shells are stabilized by

excessive NH3, which would generate a

surface functionalized with amino groups.

The exact formation mechanism of the

as-prepared NH2-ME-CMTs is still not

very clear at this stage. However, this

paper demonstrates a facile and cost-

effective protocol for functionalized

CMTs in a one-step process. The

as-developed one-step functionalization

opens up the possibility for applications in

medicinal and biological filelds.

2.5. Electrochemical Activity

The oxidation of dopamine (DA) and ascorbic acid (AA) on

the surface of as-prepared CMTs was investigated to confirm

whether the introduced amino groups would affect the

electrochemical properties of the CMTs. CNTs have been

widely investigated for applications of biosensors or electrode

modifications.[54,55] However, little research has been reported

on the electrochemical properties of CMTs. DA is well known

to play a significant role in the function of the central nervous,

renal, and hormonal systems.[56] The detection of DA in body

fluid is generally interfered with by the presence of ascorbic

acid (AA). Our previous work developed a method to

selectively detect DA in the presence of a high concentration

of AA on boron-doped diamond electrodes.[57] The detection

inheim www.afm-journal.de 1819

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1820

of DA in the presence of AA using CNT-modified electrodes

has also been reported.[58–61]

The oxidation properties of DA and AA at the

NH2-ME-CMT and NH2-H-CMT surface were investigated

by cyclic voltammetry (CV) and differential pulse voltammetry

(DPV). The oxidation properties of DA (0.1� 10�3M) and AA

(0.1� 10�3M) with NH2-ME-CMTs and NH2-H-CMTs are

shown in Figure 14a (at a scan rate 0.1 V s�1). The oxidation

potential of DA shifts from 265 mV with NH2-ME-CMTs to

210 mV with NH2-H-CMTs, as a result of the hollow cavities

and higher conductivity of the NH2-H-CMTs. The peak-to-peak

separations (DEp) of DA at the NH2-ME-CMT and

NH2-H-CMT electrodes are 65 and 40 mV, respectively. These

two values are lower than those of boron-doped diamond

(387 mV) and gold cluster-modified diamond (79 mV),[57] and

close to that of pristine CNTs (30 mV).[62] However, the DEp of

DA with NH2- H-CMTs is consistent with that of acid-treated

CNTs (41 mV).[61] Compared with the NH2-ME-CMTs, the

reversibility of the electrode reaction of DA is improved

significantly at the NH2-H-CMTs electrode and the peak

Figure 14. The cyclic voltammograms (a) and the differential plus voltammNH2-H-CMT electrodes, respectively. c) The differential plus voltammograms(from top to bottom), respectively, in the presence of AA (10�4

M) at the NH2-voltammogram peak currents for the determination of DA at the NH2-H-CMT e


current is also increased (Fig. 14a), which also indicates faster

electron transfer kinetics at the NH2-H-CMT electrode than

the NH2-ME-CMT electrode. The separation between the

voltammetric peaks of AA and DA is about 220 mV with

NH2-H-CMT. Oxidation peaks of AA and DA at the

NH2-ME-CMT electrode can not be separated. The peak

currents for oxidation of DA at the NH2-H-CMT electrode

increased linearly with the square root of the potential sweep

rate, which suggests that the electron transfer at the NH2-H-

CMTs electrode was diffusion controlled (Fig. 15), which is

consistent with DA oxidation on other carbon electro-

des.[57,61,62] The result confirms that the introduced amino

groups do not affect the electrochemical properties of CMTs.

DPV is a high sensitivity technique. Figure 14b shows the

differential pulse voltammograms obtained for DA and AA

oxidation at the NH2-H-CMT and NH2-ME-CMT electrodes.

The oxidation peak of DA at the NH2-H-CMT electrode is

150 mV, and the oxidation peak of AA at the NH2-H-CMT

electrode is 370 mV. The separation between the oxidation

peaks of DA and AA is about 220 mV at the NH2-H-CMT

ograms (b) for the oxidation of DA (0.1� 10�3M) at NH2-ME-CMT and

for the oxidation of DA with a concentration of 80, 10, 5.0, and 0.1� 10�6M

H-CMT electrode. d) The linear fit of the calibration plot of differential pluslectrode while changing the DA concentration in the presence of 10�4

M AA.


FULLPAPER


Figure 15. Plot of anodic peak current for the oxidation of DA (10�4M) at a

NH2-H-CMTs/Au electrode against the square root of the potentialsweep-rate in 0.1 M PBS (pH 7.4).

electrodes while it is about 70 mV at the NH2-ME-CMT

electrode. The separation between the oxidation peaks of DA

and AA at the NH2-H-CMT electrode is higher than those of

boron-doped diamond (60 mV), gold cluster-modified dia-

mond (150 mV),[57] acid-treated CNTs (200 mV),[63] poly(1,2-

phenylenediamine)-coated carbon fibers (200 mV),[64] gold

nanoparticle-modified glassy carbon electrodes (190 mV),[65]

and titanate nanotube-modified glassy carbon electrodes

(135 mV),[66] which are significantly superior to that observed

at other carbon electrodes. Furthermore, the peak current for

DA oxidation at the NH2-H-CMT electrode is larger than for

AA oxidation. These results indicate that the oxidation of DA

at the NH2-H-CMT electrode is easier and is more sensitive

than the oxidation of AA. Britto et al. considers that the hollow

cavities of CNTs is one of the reasons for the better

performance of nanotube electrodes in comparison with other

forms of carbon electrodes.[62] We propose that it might also

come from the higher conductivity of the NH2-H-CMTs in

comparison to that of the NH2-ME-CMTs.

DPV was also employed to investigate the effect of AA on

the response of DA at the NH2-H-CMT electrode. Figure 14c

shows the DPV results for NH2-H-CMTs to selectively

determine DA in the presence of 0.1� 10�3M AA. The

oxidation peak current of DA increases with the increase of

DA concentrations from (0.1 to 80)� 10�6M. Figure 14d shows

the calibration plot of DPV peak currents for DA oxidation in

the presence of 0.1� 10�3M AA (typical physiological con-

centration of AA), which is linear (I¼ 0.077þ 0.016C,

correlation coefficient of 0.995). The minimal detection limit

of DA in the presence of 0.1� 10�3M AA was 1.0� 10�6

M

(signal to noise ratio is 2) and the linear range of detection is

5.0� 10�6M to 0.1� 10�3

M, which is close to that of pristine

CNTs ((20–800)� 10�6M),[62] acid-treated CNTs ((0.5–10)�

10�6M),[63] titanate nanotube-modified glassy carbon electro-


des ((0.1–30)� 10�6M),[66] and higher than that of poly(1,2-

phenylenediamine)-coated carbon fiber ((0.05–10)� 10�6M),[64]

gold nanoparticle-modified glassy carbon electrodes ((0.075–

20)� 10�6M),[65] and gold cluster-modified diamond ((0.01–

10)� 10�6M).[57] The result further confirms that the intro-

duced amino groups do not affect the electrochemical

properties of the CMTs. However, further research is

necessary to investigate the structure of the NH2-ME-CMTs.

3. Conclusions

The decomposition of a ferrocene/hexabromobenzene

mixture in organic solvent in the presence of ammonia using

a solvothermal method has been performed to prepare CMTs

encapsulated with magnetite cores. The surface of the obtained

ME-CMTs is functionalized with amino groups through the

synthetic process in one step. FITC has been conjugated to

the NH2-H-CMTs by a thiourea bond, which confirms the

reactivity of the amino groups on the surface of the microtubes.

The inside magnetite cores are well protected by the carbon

shell. Etching by HCl solution to remove the inside cores gives

rise to hollow CMTs. The electrochemical properties of the

NH2-ME-CMTs and the NH2-H-CMTs have been investigated

for the oxidation of DA in the presence of AA. The results

suggest that the one-step synthesis method does not change the

electrochemical properties or break the backbone structure of

the CMTs. Our research might provide a low-cost and

convenient way to improve the application potential of CMTs

in the fields of chemistry, physics, materials, and biosensors.

4. Experimental

NH2-ME-CMTs Synthesis: In a typical experiment to generateNH2-ME-CMTs, 1 mmol of ferrocene and 0.5 mmol of hexabromo-benzene were dissolved in 20 mL of reaction solvent. After addition of1 mL of 30% ammonia, the transparent yellow solution was transferredinto a 25 mL Teflon tube and sealed in a steel autoclave. The autoclavewas maintained at the desired temperature for a specified time. Afterreaction, the autoclave was cooled to room temperature naturally. Theblack products were collected by centrifugation, washed with tolueneand ethanol, and then dried under vacuum at 40 8C overnight.

NH2-H-CMTs Synthesis: The as-prepared NH2-ME-CMTs wereimmersed in 50 mL of 1 M HCl solution at room temperature for over2 months. After the acid etching, the NH2-H-CMTs were collected bycentrifugation, washed with water and ethanol three times, and thendried under vacuum at 40 8C overnight.

FITC-NH-H-CMTs Synthesis: About 1 mg of NH2-H-CMTs and200mL (0.1 mg mL�1) of FITC were dissolved in DMF and stirredmagnetically for 12 h. The FITC-NH-H-CMT conjugate solution wasprotected from light during the reaction and stored to preventphoto-bleaching. The unbounded FITC was removed by centrifugationat 3 000 rpm and redispersed in 0.1 M NaHCO3/Na2CO3 buffersolution (pH¼ 9.4) three times, until a colorless solution was obtained.The CNTs without amino groups underwent the same treatment.Fluorescent characteristics of the FITC-NH-H-CMTs conjugate wereexamined with a confocal laser scanning microscope (MRC 1024) witha 505–550 nm band-pass emission filter. The excitation source was anargon laser (488 nm).


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1822

Electrochemical Experiments: Gold electrodes (f¼ 2.0 mm) werepolished and electrochemically cleaned in 0.1 M H2SO4. TheNH2-ME-CMTs and the NH2-H-CMTs were then coated on the goldelectrodes to obtain CMT-modified electrodes. DA and AA weredissolved in 0.1 M phosphate buffer solution (PBS, pH¼ 7.4). CV andDPV were used to investigate the electrochemical properties of theNH2-ME-CMTs and NH2-H-CMTs. The electrochemical measure-ments were performed on an electrochemical workstation (CHInstrument CHI 660C) using a 3.0 M KCl-Ag/AgCl reference electrodeand a platinum (Pt) wire as an auxiliary electrode.

Characterization: All samples were examined using a field emissionscanning electron microscope (FESEM, Leo 1530), a high-resolutiontransmission electron microscope (HRTEM, FEI TENAI F30) withelectron diffraction and an acceleration potential of 200 kV. An X-raypowder diffractometer (Philips PANalytical X’Pert) equipped with CuKa radiation (l¼ 1.542 A) over the 2u range of 10–90 8 was used tocharacterized the morphologies and structure of the NH2-ME-CMTsand the NH2-H-CMTs. Micro Raman spectroscope (DiLor SALABRAM) with an argon-ion laser at the excitation wavelength of514.5 nm, an infra-red spectroscope (IR, Nicolet AVATR 360), and anX-ray photoelectron spectroscope (XPS, PHI Quantum 2000) with anX-ray source of Mg Ka were used to study the surface functionalgroups of the NH2-ME-CMTs. Thermal gravimetric analysis (TGA)data were collected with a TA instruments Q500. NH2-ME-CMTs(1–5 mg) were loaded into platinum pans and heated to 900 8C at aspeed of 10 8C min�1with a 10 mL min�1 flow rate of N2 (99.998%).

Received: September 5, 2007Revised: February 25, 2008

Published online: June 6, 2008

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