Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

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7.5 CNT–AuNP Composites 275

capping/linking shells of the AuNPs induced an agglomeration of the AuNPs that

attached effectively to the CNTs surface. These composite materials were very

stable, with even sonication in hydrophobic solvents being unsuccessful in dis-

sociating the composite material. In time, this approach may ﬁ nd important

implications for the design of nanostructured and nanocomposite catalyst and

sensory materials. Since it is possible to control the size, shape, loading, and dis-

persion of the AuNPs on the surface of the conductive CNTs by using a combina-

tion of hydrophobic and hydrogen - bonding interactions, this system might also

become important for addressing some of the fundamental issues in fuel cell

catalysis applications.

Sainsbury and Fitzmaurice [72] reported a smart, noncovalent, self - assembly

coverage of dibenzylammonium - cation - modiﬁ ed MWNTs with crown - modiﬁ ed

(dibenzo[24]crown) AuNPs, where ammonium cations thread the crown ethers

present on the AuNPs surface (Figure 7.19 ). From the different experimental

conditions utilized, it was found that the MWNT - templated self - assembly of a solid

nanowire was observed only when uncomplexed cations were present on the

surface of the MWNTs and uncomplexed crown was present on the surface of the

AuNPs (Figure 7.19 a – c). This signiﬁ cant observation led to the perception that

the possibility of driving force for the templated nanowire assembly via charge

transfer from the crown - modiﬁ ed AuNPs to the cation - modiﬁ ed MWNT could

be excluded. Therefore, it was concluded that the templated self - assembly of

nanowires was driven by the formation of the surface - conﬁ ned to pseudorotaxanes

that resulted from the electron - poor cation threading the electron - rich crown. In

order to initiate the templated self - assembly of a gold nanowire in solution, a

dispersion of crown - modiﬁ ed AuNPs in chloroform was added to a freshly soni-

cated suspension of cation - modiﬁ ed MWNTs in chloroform. The stable dispersion

of dodecane - thiol - modiﬁ ed AuNPs was prepared using the well - known method

introduced by Brust and coworkers [73] . The nearly size - monodisperse fraction

was subsequently modiﬁ ed by the exchange of the adsorbed thiol for thiol -

incorporating crown molecules (dodecanethiol incorporating a crown moiety

in the terminal position) [74] . The MWNTs were modiﬁ ed by amide - coupling

reactions either between the carboxy - modiﬁ ed MWNTs and ethylenediamine,

or between the remaining amine of the coupled ethylenediamine and the

cation precursor [ N - (4 - carboxydibenzylamine) carbamate]. This approach may

offer the vision of diverse nanoscale components for which many applications can

be anticipated.

A facile approach has been developed to prepare a hybrid material of

dodecanethiol - protected AuNPs decorated on the surfaces of SWNTs and MWNTs

simply by reacting dispersions of both components in dichloromethane [75] . The

as - prepared AuNP – SWNT and AuNP – MWNT systems exhibited a strong elec-

tronic coupling, which may inﬂ uence important physico - chemical properties.

Although the nature of the electronically coupled state was not clearly understood,

it was anticipated that it might involve a charge - transfer character, in which the

AuNPs functioned as an electron acceptor, receiving additional electron density

from the SWNTs or MWNTs. The well - known absorption features of SWNTs led

276 7 Gold Nanoparticles and Carbon Nanotubes: Precursors for Novel Composite Materials

Figure 7.19 A cation - modiﬁ ed MWNT and the

noncovalent self - assembly of AuNPs with a

crown ether - modiﬁ ed monolayer, and TEM

images of (a) crown - modiﬁ ed AuNPs

adsorbed at the surface of a cation - modiﬁ ed

MWNT. The same AuNPs are not adsorbed at

the surface of (b) a cation - precursor - modiﬁ ed

MWNTs or (c) an unmodiﬁ ed MWNTs.

Adopted and modiﬁ ed according to Ref. [72] ;

TEM images reprinted with permission from

Ref. [72] .

to considering the dispersion of AuNP – SWNT instead of the less - explored and

less - understood MWNTs characteristics.

In contrast to normal CNTs, Li and coworkers [76] produced bamboo - like carbon

nanotube s ( BCNT s) that were decorated with a high - density and uniform assembly

of AuNPs on the surface, by using a simple and effective method. BCNTs pro-

duced by the pyrolysis of iron(II) phthalocyanine were sonicated in toluene for

homogeneous dispersion, and then mixed, by gentle shaking, with the toluene

solution of AuNPs protected by organic shell. The resultant composite materials,

AuNP – BCNTs, were separated by centrifugation and washed repeatedly with

toluene. Apart from the hydrophobic interaction between the alkyl chains of the

capping/linking molecules and the hydrophobic backbones of the nanotubes, the

speciﬁ c interaction between AuNPs and the nitrogen atoms present on the surface

7.5 CNT–AuNP Composites 277

Figure 7.20 Schematic illustration of the direct assembly of

AuNPs on BCNTs. (a) TEM image of BCNTs; (b, c) BCNT –

AuNP nanohybrids; (d) AuNPs supported on an undoped

CNTs. The inset in (a) shows a higher - magniﬁ cation image.

Reprinted with permission from Ref. [76] .

of the BCNTs is responsible for the high - density and uniform assembly of AuNPs

on BCNTs (Figure 7.20 ). It is important to note that, compared to the N - doped

BCNTs, the pristine undoped CNTs were not efﬁ cient for the direct, highly dense

and uniform immobilization of AuNPs, due mainly to an absence of speciﬁ c

AuNP – N interactions or other activation processes. As evidently seen from the

TEM images, the assembled AuNPs (average size 3.9 nm) were quite uniform, well

dispersed, and decorated or packed onto the walls and ends of BCNTs with high

density. The unique properties of the BCNTs, combined with this simple and

straightforward assembly approach, may facilitate the use of CNTs in a variety of

pivotal ﬁ elds, including biosensors and fuel cell catalysis.

π – π Stacking Interactions This is a type of noncovalent interaction, which is

mainly caused by the intermolecular overlapping of p - orbitals in π - conjugated

systems (e.g., organic compounds or polymers containing aromatic moieties). The

results of different studies [68, 77] have conﬁ rmed that organics or polymers with

phenyl groups could interact with CNTs through π – π stacking, which is a stronger

interaction than the van der Waals forces. Compared to various aromatic com-

pounds, pyrene derivatives are more susceptible to stack on the surface of the

CNTs via π – π stacking interactions [78 – 80] . In this way, the intrinsic electronic

properties of the CNTs are preserved, and the surface functional groups on the

nanotubes can easily be varied by changing the pyrene derivatives.

By using bifunctionalized pyrene derivatives such as 17 - (1 - pyrenyl) - 13 -

oxo - hepta - decanethiol ( PHT ) as an interlinker, AuNPs were decorated on the

278 7 Gold Nanoparticles and Carbon Nanotubes: Precursors for Novel Composite Materials

Figure 7.21 Illustration of the connection of AuNPs onto

MWNTs through pyrene derivatives as an interlinker, and

TEM image of MWNTs decorated with AuNPs.

PHT = 17 - (1 - pyrenyl) - 13 - oxo - heptadecanethiol. Adopted and

modiﬁ ed according to Ref. [78] ; TEM image reprinted with

permission from Ref. [78] .

surface of the didecylamine - modiﬁ ed MWNTs, where PHT bound to the surface

of MWNTs via a π – π stacking interaction between its pyrenyl unit at one end and

the sidewall of MWNTs, while the other terminal thiol group was interacted to the

surface of AuNPs (Figure 7.21 ). In this way, AuNPs were self - assembled onto the

surface of nanotubes via an organic linker. It should be noted that, while the ﬂ uo-

rescence of the resultant composite material was entirely quenched, the Raman

response of the CNTs was enhanced considerably. These important consequences

imply that there are charge - transfer interactions between the CNTs and AuNPs

through the organic interlinker.

7.5 CNT–AuNP Composites 279

Figure 7.22 Representation for the fabrication

processes of the AuNP – MWNT hybrids and

molecular structure of MEPTCDI. (a) TEM

image of AuNP – MWNT hybrids; (b) Emission

from the AuNP – MWNT hybrid with MEPTCDI

in water (left) and MEPTCDI in DMF (right)

under UV irradiation at 365 nm.

MEPTCDI = N , N ′ - bi(2 - mercaptoethyl) -

perylene - 3,4,9,10 - tetracarboxylic diimide.

Reprinted with permission from Ref. [81] .

Very recently, the in situ - solution method was proposed for the synthesis of

water - dispersable AuNP – MWNT hybrids with high density and well - distributed

AuNPs by using an optoelectronic - active compound of N , N ′ - bi(2 - mercaptoethyl) -

perylene - 3,4,9,10 - tetra - carboxylic diimide ( MEPTCDI ) as an interlinker and stabi-

lizer for the formation of the AuNP – MWNT hybrids [81] . Here, the MEPTCDI

with both phenyl and mercapto groups, played a dual role as an interlinker (Figure

7.22 ) between the MWNTs and AuNPs (noncovalently wrapping of the MWNTs

through π – π stacking) and as a stabilizer for controlling the nucleation and growth

of AuNPs on the surface of the MWNTs. This endowed the AuNPs anchored

on the surface of the MWNTs with a good stability and dispersability, as well as

ﬂ exibility of size - control (Figure 7.22 a).

AuNP – MWNT hybrids were prepared using an in situ procedure in such a way

that the MWNTs were dispersed in aqueous solution containing sodium dodecyl

benzen sulfonate ( SDBS ) by sonication and a dimethylformamide ( DMF ) solution

of MEPTCDI was added to the MWNTs suspension, followed by the addition of

an aqueous solution of HAuCl

, and stirred for 12 h at room temperature. The

nanohybrids formed were further puriﬁ ed by centrifugation and redispersed in

water for further characterization. The as - prepared AuNP – MWNT hybrid with

280 7 Gold Nanoparticles and Carbon Nanotubes: Precursors for Novel Composite Materials

MEPTCDI showed an interesting photoluminescence property under UV irradia-

tion at 365 nm (Figure 7.22 b). Based on several blank experiments and reported

information [82] , it was concluded that the strong luminescence of the hybrids did

not result from the atomic Au clusters or from defects of the CNTs or from the

organically functionalized CNT hybrids, but rather originated from the AuNP –

MWNT hybrids with MEPTCDI, which may be due to energy transfer occurring

between the MWNTs and AuNPs or between AuNPs and the MEPTCDI. This

property of AuNP – MWNT hybrids in water may expand the potential applications

as building blocks to a wider range such as light - emitting sources, biomedical

labels, or even tracking materials for drug delivery, and so on.

AuNPs of 2 – 4 nm diameter were densely decorated on the walls of MWNTs

using pyrenealkylamine derivatives such as 1 - pyrenemethylamine as the inter-

linker in aqueous solution [83] . While the pyrene chromophore is noncovalently

attached to the terrace of nanotubes through a π – π stacking interaction, the

alkylamine substituent of 1 - pyrene - methylamine connects to the AuNPs (Figure

7.23 ). UV - visible absorption and luminescence spectroscopy were employed to

monitor the formation of functionalized AuNPs and AuNP – CNT composites. Fol-

lowing surface modiﬁ cation of the AuNPs with 1 - pyrenemethylamine, the absorp-

tion value of the pyrene chromophore was greatly decreased and the surface

plasmon resonance ( SPR ) of AuNPs showed a red shift from 508 to 556 nm (Figure

7.23 a), owing to interparticle plasmon coupling. Further, the photoluminescence

property of 1 - pyrenemethylamine and its emission intensity were drastically

quenched after formation of the AuNP – CNT composites, most likely due to the

energy and/or electron transfer from the excited pyrene ﬂ uorophore to the AuNPs.

This facile strategy for the formation of a high - density assembly of AuNPs onto

the surface of CNTs (Figure 7.23 b) has been applied to other linking organic

molecules with similar structures, such as N - (1 - naphthyl)ethylenediamine and

phenethylamine, thus demonstrating the general applicability of this approach for

preparing AuNP – CNT composites. The resultant composites may become impor-

tant for applications in low - temperature CO oxidation and other catalytic reactions.

Further, the present strategy of depositing spherical AuNPs may be extended to

produce CNT – Au nanorod heterojunctions for electronic connection and molecu-

lar sensing applications.

Electrostatic Interactions Electrostatic interactions are noncovalent dipole –

dipole or induced dipole – dipole interactions that can be either stabilizing or desta-

bilizing. A simple and proﬁ cient method has been introduced for the selective

anchoring of AuNPs on the surface of nitrogen - doped MWNTs by electrostatic

adsorption [84] . Nitrogen - doped MWNTs produced using a pyrolysis method were

initially chemically modiﬁ ed by acid treatment (mixture of H

and HNO

)

and the resultant oxidized MWNTs subsequently treated with a cationic polyelec-

trolyte, namely poly - (diallyldimethylammoniumchloride) ( PDDA ). The polyelec-

trolyte was adsorbed onto the surface of the nanotubes by an electrostatic interaction

between the carboxyl groups on the chemically oxidized nanotube surface and

the polyelectrolyte chains. Negatively charged AuNPs of 10 nm were attached

7.5 CNT–AuNP Composites 281

Figure 7.23 AuNP assembly on CNTs

through the 1 - pyrenemethylamine interlinker.

(a) UV - visible absorption spectra of: (curve a)

free 1 - pyrenemethylamine in ethanol; (curve

b) aqueous AuNP solution; (curve c) a

mixture of 1 - pyrenemethylamine and AuNPs;

(curve d) solution containing AuNP − CNT

composites; (b) TEM image of the resulting

AuNP − CNT composites. Reprinted with

permission from Ref. [83] .

to the surface of the nanotubes via electrostatic interactions between the polyelec-

trolyte and AuNPs (Figure 7.24 ). In this way, novel composites with homogene-

ously distributed AuNPs on the surface of nitrogen - doped MWNTs were obtained.

Therefore, by selecting different types of polyelectrolytes the surfaces of the

CNTs can be modiﬁ ed to be either negatively or positively charged, such that

accordingly different types of other nanoparticle (e.g., semiconductor nanocrystals,

magnetic nanoparticles) can be selectively anchored to the nanotube surfaces.

This strategy of decorating nanotubes can be used to identify the location of func-

tional groups on the CNT surfaces, while the resultant composite materials may

be used in various domains such as catalytic, electronic, optical, and magnetic

applications.

Another example of the electrostatic approach is the formation of AuNP – MWNT

composites by the LBL self - assembly technique using polyelectrolytes [85] . To this

282 7 Gold Nanoparticles and Carbon Nanotubes: Precursors for Novel Composite Materials

Figure 7.24 (a) Schematic view of the process for anchoring

AuNPs onto CNTs; (b) TEM image of AuNP – CNT hybrid

structures. Reprinted with permission from Ref. [84] .

end, chemically functionalized MWNTs were wrapped with a layer of a positively

charged polyelectrolyte (PDDA), followed by a layer of a negatively charged poly-

electrolyte such as poly(sodium 4 - styrene sulfonate) ( PSS ). Consequently, posi-

tively charged AuNPs prepared by the phase - transfer method [86] were immobilized

on negatively charged MWNTs prepared by LBL self - assembly due to electrostatic

interactions (Figure 7.25 ). Positively charged AuNPs also interacted with chemi-

cally functionalized MWNTs alone (without polyelectrolyte coating) through elec-

trostatic interaction with the carboxylic acid groups present on MWNTs sidewalls.

Hence, it is expected that positively charged AuNPs could be used to detect nega-

tively charged defect sites on CNTs. However, the binding interaction between

MWNTs and AuNPs is much more powerful and effective for the LBL approach

than the direct approach.

The noncovalent attachment of silica - coated AuNP monolayers and multilayers

onto CNT templates was achieved by using the polymer - wrapping technique,

7.5 CNT–AuNP Composites 283

combined with the LBL assembly process [87] . First, a stable dispersion of indi-

vidual CNTs was obtained by dispersing the MWNTs in water by sonication in the

presence of PSS, which acts as the wrapping polymer. PSS has negatively charged

sulfonate groups, which serve as primers for the homogeneous adsorption of the

cationic polyelectrolyte PDDA. In the LBL approach, the interactions accountable

for the assembly are mostly electrostatic, and permit an exploitation of the surface

properties of silica to obtain close - packed monolayers and multilayers. As a result,

the MWNTs were entirely packed with dense monolayers of silica - coated AuNPs.

Such composite nanowires were optically labeled and might have key applications

as components of nanoelectronic circuits and waveguides. The repetition of the

LBL process allows a good control of the thickness of the nanocomposites by

increasing the number of layers deposited. Another strategy for functionalizing

SWNT or MWNT surfaces noncovalently with polymer multilayers and AuNPs

was also reported by Carrillo and coworkers [88] .

Jiang and Gao reported a versatile method for the selective attachment of AuNPs

onto the surface of the MWNTs [38] . By using cationic poly(ethyleneimine) ( PEI )

or anionic citric acid ( CA ) as the dispersant, the surface properties of the MWNTs

were modiﬁ ed to yield a basic or acidic surface. Then, by electrostatic interactions,

AuNPs of approximately 10 nm were successfully anchored onto the sidewalls of

the MWNTs (Figure 7.26 ). In the case of the CA - coated MWNTs, the CA forms

an adsorption layer around the outer walls of the CNTs and then reduces in situ

the HAuCl

to produce the AuNPs. The PEI treatment of the nanotubes and expo-

sure to the HAuCl

solution generated nitrogen - containing functional groups on

Figure 7.25 Schematic illustration of the LBL self - assembly

technique. PDDA = poly(diallyldimethylammoniumchloride);

PSS = poly(sodium 4 – styrene sulfonate). Adopted and

modiﬁ ed according to Ref. [85] .

284 7 Gold Nanoparticles and Carbon Nanotubes: Precursors for Novel Composite Materials

Figure 7.26 The process for attaching AuNPs to: (a) Citric

acid - coated CNTs; (b) Poly(ethyleneimine) (PEI) - coated CNTs.

The TEM images show the respective products. Adopted and

modiﬁ ed according to Ref. [38] ; TEM images reprinted with

permission from Ref. [38] .

the outer walls of the MWNTs. This strategy may be extended to attach other

nanoparticles (e.g., semiconductor, electrical and magnetic nanoparticles) onto the

terrace of the CNTs by choosing different types of suitable dispersant.

Another straightforward in situ reduction procedure for the fabrication of CNT -

supported AuNP composite nanomaterials was demonstrated by Hu and cowork-

ers [89] . Here, linear PEI had a dual role as a functionalizing agent for the MWNTs

as well as a reducing agent for the formation of AuNPs (Figure 7.27 ). The driving

force involved in the functionalization of PEI on the surface of CNTs is a combina-

tion of the electrostatic interaction between the oppositely charged CNTs and the

PEI and the physisorption process, which is analogous to the polymer - wrapping

process [87] . This synthetic method engages a mild heat - treatment process, which

persuades the in situ reduction of HAuCl

on the MWNTs sidewalls and does

not require the additional steps of oxidizing the MWNTs with mixed acids and