C A R B O N 4 6 ( 2 0 0 8 ) 8 8 4 – 8 8 9

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The synthesis of hybrid nanostructures of gold nanoparticlesand carbon nanotubes and their transformation to solidcarbon nanorods

Alejandra Telloa,b, Galo Cardenasb, Patricio Haberlea, Rodrigo A. Seguraa,*

aDepartamento de Fısica, Universidad Tecnica Federico Santa Marıa, Avenida Espana 1680, 2390123 Valparaıso, ChilebFacultad de Ciencias Quımicas, Universidad de Concepcion, Chile

A R T I C L E I N F O

Article history:

Received 3 January 2008

Accepted 15 February 2008

Available online 4 March 2008

0008-6223/$ - see front matter � 2008 Elsevidoi:10.1016/j.carbon.2008.02.024

* Corresponding author: Fax: +56 32 2797656.E-mail address: rodrigo.segura@usm.cl (R

A B S T R A C T

Hybrid nanostructures composed of gold nanoparticles and multiwall carbon nanotubes

(AuNPs@MWCNT) were prepared by the so called solvated metal atom dispersion method

(SMAD), combined with chemical vapor deposition (CVD). In the SMAD procedure, bulk gold

and an organic solvent are co-evaporated and later condensed into a frozen matrix at liquid

nitrogen temperature. After warming up this matrix to room temperature, a colloid with

very small and highly reactive gold clusters is obtained. These clusters react in the same

vessel with MWCNTs, previously synthesized by CVD, by anchoring themselves to the side-

walls of the tubes. The resulting hybrids are very stable at temperatures up to 400 �C since

the AuNPs get coated in this process by a thin layer of amorphous carbon. At temperatures

beyond 600 �C the hybrid structures separate into CNTs and aggregated clusters of AuNPs.

Further heating induces a severe transformation of the CNTs, due mainly to the catalytic

activity of the AuNPs. The long MWCNTs are transformed into sub-micrometric solid car-

bon nanorods, comparable in diameter to the final size of the sintered AuNPs. The descrip-

tion of the synthesis products have been based on transmission electron microscopy and

thermogravimetric analysis.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

CNTs are extremely resistant to mechanical tension, are

highly flexible and can exhibit semiconducting, metallic or

even superconducting properties. In short, they are the ideal

building blocks for molecular nanoelectronics and at the

same time very strong materials, which have already been

incorporated into light reinforced composites. On the other

hand, noble metal nanostructures, particularly gold nanopar-

ticles (AuNPs), are of widespread interest mainly due to their

remarkably high catalytic activity and their special electronic

and optical properties. The hybrid structures, formed by com-

bining these two kinds of materials, can improve and even ex-

tend the possible uses for these new composites. Their

er Ltd. All rights reserved.A. Segura).

technological perspectives in fields such as catalysis, gas

and electrochemical sensing and solar energy conversion

have attracted the interest of many researchers in the last

few years.

Several groups have successfully synthesized hybrids of

AuNPs on the surface of CNTs. They have mostly used cova-

lent linkage through bifunctional molecules [1–3], while oth-

ers have prepared hybrids only taking advantage of the

intermolecular interaction between the ligand molecules,

usually long carbonated molecular chains bound to the AuNP

surface, and the CNTs side walls [4–6]. These new structures

could be employed in optoelectronic devices as well as in bio-

medicine, allowing the possibility to carry and selectively de-

liver into the body different therapeutic drugs [7], or by using

.

C A R B O N 4 6 ( 2 0 0 8 ) 8 8 4 – 8 8 9 885

them in combination with electromagnetic irradiation to pro-

voke localized heating which could eventually destroy sur-

rounding cancer cells [8]. Other metals have also been used

to synthesize hybrids with CNTs. For example, AgNPs have

been electro-crystallized onto functional MWCNTs surfaces

[9]. The resulting nanohybrids show a high catalytic activity

to the electro-oxidation of hydrazine. Magnetic iron [10], co-

balt [11] and nickel [12] NPs have also been linked to CNTs

to form hybrids structures. The use of these hybrids in mag-

netic storage as well as in nuclear magnetic resonance, as

contrast agents for imaging and diagnosis, has been consid-

ered [8]. Other metals such as Pd [13], Pt [14], Rh [15], and

Ru [16] have also been incorporated into CNTs mainly with

the purpose of using them as catalysts or gas sensors.

One important aspect which must be considered in the

incorporation of this type of hybrid materials as integral part

of devices is their stability, under thermal treatments and/or

in a harsh chemical environment. The hybrid nanostructures

must behave as a unit and preserve their structural integrity

and functionality in the whole range of temperature needed

for their synthesis or operation. In other words we must know

what will happen if the temperature of a certain device based

on hybrid structures is raised beyond certain critical limits.

In this contribution we present a detailed study of the ther-

mal stability of hybrid structures formed by AuNPs and

MWCNTs. The AuNPs@MWCNTs hybrids were prepared by

the so called solvated metal atom dispersion method (SMAD)

[17,18]. The use of this method to synthesize these Me-

tal&CNTs hybrids, has never been reported before, neither

have we found reports related to their thermal stability. The

material evolves from a CNT supported AuNPs dispersion to

the formation of the hybrid structure, which at higher tem-

peratures forms a new material constituted mainly by solid

carbon rods of nanometer dimensions. There are only very

few reports related to the synthesis of carbon nanorods

(CNRs). Liu et al. [19] have observed rod-like carbon nano-

structures as a byproduct of arc discharge process to produce

CNTs. In this case the CNRs have graphitic planes aligned par-

allel to the rod axes. In a different study, CNRs with Y-junc-

tions structures were produced by the copyrolysis of C6H6

and C5H6 at 600 �C, under the cocatalysis effects of Fe and

Mg [20]. In this work the direction normal to the graphitic

planes is perpendicular to the CNR axis. More recently, El

Hamaoui et al. [21] have reported the solid-state pyrolysis of

polyphenylenes with dicobalt octacarbonyl. Under certain

synthesis conditions graphitic CNRs could be obtained in-

stead of CNTs. In contrast with these previous reports on

CNRs, in this contribution we present a simple procedure to

produce the rods starting from of CNTs annealed in the pres-

ence catalytic AuNPs.

2. Experimental section

2.1. Nanotubes growth and purification

MWCNTs were synthesized by chemical vapor deposition

(CVD), through the decomposition of acetylene at 800 �C over

a Pd/c-Al2O3 catalyst [22]. The raw MWCNTs were purified by

standard air oxidation, alkali and acid treatments [23,24] to

eliminate amorphous carbon, alumina and the catalytic parti-

cles. The CNTs obtained by this procedure have a mean diam-

eter of 27 nm.

2.2. Synthesis, purification and thermal treatment ofnanohybrids

AuNPs@MWCNTs hybrids were prepared by the solvated

metal atom dispersion method (SMAD) [17,18]. In brief, this

procedure consists in the resistive evaporation of bulk metal

(gold) into an organic solvent (acetone) atmosphere, which

is subsequently incorporated by condensation, into a frozen

matrix at liquid nitrogen temperature. After warming up this

matrix to room temperature, a colloid with very small and

highly reactive gold clusters is obtained. These gold clusters

react, in the same vessel, with the CNTs, which have been

previously incorporated into the reactor. The chemically

reactive AuNPs bind to the CNTs side walls to form the hybrid

nanostructures. After synthesis, the as-prepared hybrids were

purified by microfiltration with a nitrocellulose membrane

(pore 3 lm), washed and dried. Following purification the hy-

brids were annealed for 1h at different temperatures (200, 400,

600 and 800 �C) in an argon atmosphere.

2.3. Characterization techniques

Conventional and high resolution transmission electron

microscopy (TEM) measurements were performed on dis-

persed samples. For these studies, a drop of the dispersed

sample was left to dry out on a commercial carbon coated

Cu TEM grid. Bright field micrographs were taken in a Jeol

1200EX operating at 120 kV, with a point resolution of �4 A.

For the HRTEM measurements a FEI Tecnai G2 F20 S-Twin

microscope, operated at 200 kV and with a point resolution

of �2.4 A, was employed. Gatan CCD cameras were used to

acquire the images. TEM images were processed and analyzed

with DigitalMicrograph 3.9.0 (Gatan Inc) and The Gimp 2.4.0

software packages. A statistical study of TEM images was car-

ried out in order to quantify the effects of the annealing treat-

ment over the particle size. This study consists in the size

measurement (diameter) of about 150 particles for each sam-

ple. The counts were then plotted as frequency histograms.

The mean particle sizes and the standard deviations were

also calculated.

Thermogravimetric analyses were performed on the

MWCNTs, AuNPs and AuNPs@MWCNT hybrid samples. To

obtain the thermograms, a small amount of the products

(0.5–1.0 mg) was heated from room temperature up to

1000 �C, with a ramp rate of 20 �C/min in a nitrogen flow.

The sample mass was monitored as a function of temperature

and registered in a thermobalance from TA Instruments,

model SDT 2960.

3. Results and discussion

Gold nanoparticles were successfully incorporated in the out-

er walls of carbon nanotubes. Fig. 1 shows the TEM micro-

graphs of (a) the as-prepared AuNPs@MWCNT nanohybrids;

(b) after a one hour anneal up to 200 �C, and (c) after being

Fig. 2 – HRTEM micrographs of AuNPs anchored on the

CNTs-walls. (a) and (b) are for the as prepared AuCNT

hybrids; (c) and (d) are for the Au-CNT hybrids annealed up

to 200 �C. A carbon layer is encapsulating the AuNPs after

the thermal process. Lattice fringes on gold particles are

consistent with (111) fcc orientation (0.235 nm) and (e) is a

schematic representation the model of this process.

(a)

(b)

(c)

As prepared

Annealed to 200 ºC

Annealed to 400 ºC

0 2 4 6 8 100

10

20

30

40d = 3.5 nm

σ = 0.8 nm

Freq

uenc

y (%

)

Particle Size (nm)

Freq

uenc

y (%

)

Particle Size (nm) 0 2 4 6 8 10

0

10

20

30

40d = 3.8 nm

σ = 1.4 nm

0 2 4 6 8 100

10

20

30

40d = 3.9 nm

σ = 1.0 nm

Freq

uenc

y (%

)

Particle Size (nm)

Fig. 1 – TEM micrographs and gold particle size distributions

for the as prepared AuCNT hybrids (a) and annealed up to

200 �C (b) and 400 �C (c). The TEM analyses of the

micrographs did not show any appreciable changes in the

particle sizes for the different treatments. The

corresponding mean particle size ð�dÞ and the standard

deviation (r) are also indicated.

886 C A R B O N 4 6 ( 2 0 0 8 ) 8 8 4 – 8 8 9

annealed to 400 �C. The TEM images are accompanied by the

corresponding frequency size distribution plots. From this

information, it is possible to determine the AuNPs improved

their adhesion to the CNTs sidewalls after the thermal treat-

ment. From the statistical analysis, we found in both cases

the AuNPs diameters and shape (almost spherical) did not

show a substantial variation. For the as-prepared sample,

Fig. 1a, the mean diameter was close to 3.5 nm. After the ther-

mal treatments up to 200 �C (Fig. 1b) and 400 �C (Fig. 1c), the

sizes of the AuNPs increase slightly from 3.5 nm (in the as-

prepared sample) to 3.8 nm and 3.9 nm, respectively.

Although these enlargements are within the error margins

of the measurements, we have noted the distributions are a

bit wider after the thermal procedures were completed. These

results are rather unexpected for a NPs distribution loosely at-

tached to the CNTs. The usual behavior of AuNPs subjected to

these temperatures would be to grow or sinter after a thermal

annealing [25,26], forming large clusters of material by aggre-

gation of the NPs. Our results indicate the AuNPs are effec-

tively anchored on the CNTs walls, even in the as-prepared

samples.

To gain a better understanding of the above mentioned

phenomena we performed HRTEM of the synthesis products.

(a)

(b)

Annealed to 600 ºC

Annealed to 800 ºC

0 10 20 30 40 500

10

20

30

40d = 19.2 nm

σ = 4.2 nm

0

10

20

30

40d = 21.4 nm

σ = 5.1 nm

0 10 20 30 40 50

Freq

uenc

y (%

)

Particle Size (nm)

Freq

uenc

y (%

)

Particle Size (nm)

Fig. 3 – TEM micrographs and AuNPs size distribution in the

AuNPs@MWCNT hybrids after being annealed up to: (a)

600 �C and (b) 800 �C. After each thermal treatment the

hybrids nanostructures suffer a dramatic transformation.

The AuNPs grow up to 20 nm in diameter. After annealing to

800 �C the CNTs evolve to form solid carbon nanorods.

0 200 400 600 8000

5

10

15

20

25

Aver

age

Parti

cle

Size

(nm

)

Annealing Temperature (ºC)

Fig. 4 – AuNPs average diameter on the AuNPs@MWCNTs

hybrids as function of annealing temperature. The error

bars represents the standard deviation for the

corresponding distribution. Thermal annealing at

temperatures higher than 400 �C induces sintering of the

AuNPs.

C A R B O N 4 6 ( 2 0 0 8 ) 8 8 4 – 8 8 9 887

Fig. 2 shows a series of detailed micrographs from the inter-

face between the AuNPs and CNTs, (a) and (b) for the as-pre-

pared sample and (c) and (d) for the one annealed up to

200 �C. The lattice fringes associated to the AuNPs have an

interplanar spacing of 0.24 nm, consistent with the (111)

planes of fcc Au [JCPDS File No. 4-0784]. From these high res-

olution images we can clearly determine the thermal treat-

ment induces the formation of a thin layer of amorphous

carbon over the AuNPs, even at temperatures as low as

200 �C. A reasonable explanation for the formation of this

layer, at such low temperature, is the catalytic behavior of

the AuNPs surfaces. The carbon overlayer reduces the parti-

cles mobility and precludes the AuNPs sintering, thus avoid-

ing the formation of larger Au clusters. The AuNPs are now

embedded in the CNTs. Fig. 2e shows a schematic diagram

representing the formation of this layer. The most probable

explanation for this phenomenon is that C-atoms migrate

from the neighboring carbon segments of CNTs to the free

AuNP surface to form the amorphous layer. This seems to

be a thermally activated process with a characteristic temper-

ature in a range between room temperature and 200 �C.

Even though the micrographs in Fig. 2a and b seem to indi-

cate the AuNPs, in the as-prepared sample, have penetrated

into the CNTs as they bind to the side walls, this effect could

be due to an artifact. The AuNPs could be displaced either be-

low or above the CNTs midplane, thus giving the impression,

in the image, as if they were incorporated into the tubes. Nev-

ertheless, it could very well be that the actual binding of the

AuNPs to the wall is due to a ‘‘lego’’ type physical attachment

instead of a chemical bonding with the CNTs walls.

One could take advantage of this C-coating process as a

way of protect the particles against reactive environments,

which could possibly oxidize or even dissolve them. This pro-

tective coating could even be used to improve the NPs’ struc-

tural stability and also as a way to modify and even tune the

optical response of the system.

Additional thermal treatments, at temperatures of 600 �Cand beyond, induce the dissociation of the hybrid nanostruc-

ture. Fig. 3 shows two TEM micrographs, together with the

corresponding AuNPs size distributions, after annealing the

AuNPs@MWCNT hybrids to 600 �C (Fig. 3a) and 800 �C(Fig. 3b). It is possible to note the nanostructures have under-

gone severe transformations when compared with the hy-

brids previously annealed up to 400 �C. In the case of the

sample annealed up to 600 �C, the AuNPs get separated from

the CNTs. They grow up to an average diameter of 19 nm. It is

now apparent from the result of this procedure, that the car-

bon layer formed at low temperatures is not stable beyond

600 �C. On the other hand the CNTs have not suffered mayor

transformations, retaining their original form. When the

sample is annealed to 800 �C the AuNPs grow even larger, sta-

bilizing at a slightly larger average size (21 nm). At this tem-

perature the CNTs are transformed into solid carbon bars or

rods with diameters in the range: 15–50 nm and lengths vary-

ing in the range: 0.2–0.5 lm.

The effect of annealing on the AuNPs size, while being part

of the hybrids, can be summarized by the graph shown in

Fig. 4. The NPs average size shows a sharp transition between

400 �C and 600 �C, which coincides with the hybrid nanostruc-

tures dissociation temperature.

Fig. 5 – (a) HRTEM micrograph of a carbon nanorod obtained

after annealing the AuNPs@MWCNT hybrids up to 800 �C.

The enlargement of the rod (b) and (c) show they have a

highly ordered structure, with an interlayer spacing similar

to that of graphite (0.349 nm) [28]. In contrast, (d) shows a

TEM micrograph of pure MWCNTs annealed up to 900 �C,

without the presence of AuNPs. No significant changes were

observed in the pure CNTs sample and no rods were formed.

a

b

0 200 400 600 800 100050

60

70

80

90

100

Wei

ght (

%)

Temperature (ºC)

AuNPs AuNP@MWCNT CNTs purified

0 200 400 600 800 1000

-0.12

-0.08

-0.04

0.00

AuNPs AuNP@MWCNT CNTs purified

dM/d

T (º

C-1)

Temperature (ºC)

Fig. 6 – Thermogravimetric analysis of CNTs (continuous/

blue line), AuNPs (dotted/black line) and AuNPs@MWCNT

hybrid (dash/red line). (a) Thermograms (TG) and (b)

differential thermograms (DTG). (For interpretation of the

references to colour in this figure legend, the reader is

referred to the web version of this article.)

888 C A R B O N 4 6 ( 2 0 0 8 ) 8 8 4 – 8 8 9

Fig. 5a–c show HRTEM micrographs of the AuN-

Ps@MWCNTs hybrids after being annealed up to 800 �C. The

sidewalls and the caps of the rods do display some structural

disorder; nevertheless the prevailing structure of the solid

rods is that of a highly ordered crystalline structure with axial

symmetry. The characteristic interplanar spacing of the con-

centric cylindrical structure is close to 0.35 nm. This value is

very close to the interlayer spacing of graphite (0.335 nm)

[27]. In order to verify explicitly if the AuNPs played a funda-

mental role in the formation of these CNRs, we have annealed

a sample of CNTs, from the same batch used to synthesize

the hybrids, up to a temperature of 900 �C for 1 h in the same

atmosphere. Fig. 5d shows a TEM micrograph of the purified &

annealed CNTs. It is indeed evident form the image; the CNTs

did not suffer any transformations. They retain the tubular

shapes and they explicitly display an empty core. This last re-

sult indicates the AuNPs do play a catalytic role in the forma-

tion of the CNRs.

To complement our TEM results we have performed TGA

on CNTs, AuNPs and nanohybrid samples. Fig. 6a and b show

the TG signal and the differential TG (DTG) curves. The pure

CNTs display no significant weight loss up to 500 �C and the

total weight loss up to 1000 �C is close to 10% of its original va-

lue. The DTG curve for CNTs does not show any decomposi-

tion peak, probably only non-graphitic carbon is being lost

during the thermal treatment. For both the AuNPs and the hy-

brid samples there is a DTG peak close to 100 �C. This weight

loss is probably due to the desorption of water and other sol-

vents used during synthesis and purification. The TG curve

for the hybrid samples shows a wide range of weight loss be-

tween 300 �C and 500 �C with a DTG loss peak close to 400 �C.

The total weight loss for the hybrids sample up to 900 �C is

again not higher than 10%. This behavior contrasts with that

exhibited by pure AuNPs, which show an important weight

loss between 200 and 500 �C. For the AuNPs one of the charac-

teristic decomposition peaks is centered at 270 �C while the

other appears as a shoulder close to 400 �C. The total weight

loss for the AuNPs sample up to 900 �C is close to 30% the ori-

ginal weight. The strong DTG peak of the AuNPs centered at

270 �C comes from the decomposition of acetone fragments

anchored on the NPs surfaces. This temperature is rather

similar than reported for decomposition of acetone fragments

from other metals [28,29]. This particular peak is absent in the

nanohybrids since in this case, the AuNPs are already sur-

rounded by a carbon protecting layer. In conclusion, the

TGA results are consistent with the formation of a hybrid

structure.

4. Summary

AuNPs@MWCNTs hybrid nanostructures were successfully

prepared by the combination of SMAD and CVD techniques.

Gold nanoparticles were incorporated in the outer walls of

C A R B O N 4 6 ( 2 0 0 8 ) 8 8 4 – 8 8 9 889

carbon nanotubes. After a thermal treatment, no appreciable

changes in particle size (AuNPs) were detected between the

as-prepared hybrid (3.5 nm) and samples annealed up to

400 �C (3.9 nm). The analysis of HRTEM micrographs indicates

that thermal annealing to temperatures as low as 200 �C, in-

duces the formation of a thin layer of amorphous carbon,

encapsulating the anchored gold nanoparticles. This C-layer

avoids subsequent sintering of the AuNP, making the hybrid

structures very stable up to temperatures close to 400 �C.

For temperatures beyond 600 �C the AuNPs detach from the

tubes and undergo a sintering process. An important particle

size increase occurs (from 4 nm to almost 20 nm in average),

but no damage is induced on the CNTs. A completely different

situation is observed if the samples are annealed up to 800 �C.

In this case the CNTs undergo a transformation into cylindri-

cal solid nanorods. It is likely this process occurs due to the

presence of the AuNPs in the hybrids. TGA from the hybrids

shows no important weight loss up to 400 �C, which is a fairly

different behavior to that displayed by the pure AuNPs. The

almost featureless TGA signal from the CNTs and the differ-

ence in the weight loss between the pure AuNPs and the hy-

brids indicate the existence of a strong link between them in

the hybrid structure.

Acknowledgements

Partial funding for this work has been provided by the follow-

ing PBCT Grants: PSD031 and ACT027, Chile.

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