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

Подождите немного. Документ загружается.

7.5 CNT–AuNP Composites 265

Figure 7.10 Scanning electron microscopy images of

synthesized CNTs that are grafted with gold nanosphere and

nanocube systems. Reprinted with permission from Ref. [56] .

response, in line with the experimentally observed absorption peaks of such

systems in the optical range.

Tello and coworkers introduced a new method, known as solvated metal atom

dispersion ( SMAD ), in combination with CVD, to prepare MWNT and AuNP

hybrid materials, and subsequently investigated their thermal stabilities [57] . In

the SMAD protocol, a colloid with small and highly reactive gold clusters was

prepared by co - evaporation of the gold metal and acetone. The colloid was subse-

quently condensed into a frozen matrix in a liquid nitrogen atmosphere, and the

matrix then allowed to warm to room temperature. The as - prepared gold clusters

were reacted with previously incorporated MWNTs in the reactor to yield MWNT –

AuNP hybrid materials which were thermally stable up to 400 ° C. Moreover, no

appreciable changes in particle size were detected as the AuNPs were covered with

amorphous carbon after annealing at 200 ° C (Figure 7.11 ).

Interestingly, whilst the average AuNP size was increased approximately from

4 to 20 nm and the nanoparticles were detached from the MWNTs, no damage

was induced on the MWNTs when the hybrid material was annealed beyond 600 ° C

(Figure 7.11 c). Surprisingly, further heating (annealing up to 800 ° C) induced a

severe transformation of the MWNTs (perhaps due to a catalytic activity of the

AuNPs) into cylindrical solid carbon nanorods (Figure 7.11 d). It would appear that

this method provides a new approach to the synthesis of both carbon nanorods

and MWNT – AuNP composites that are useful in a variety of key applications.

A new and important approach was developed recently to form naked sub - 10 - nm

AuNPs on individual 2 nm bare CNTs [58] . These assemblies were produced on

the terrace of a porous anodic alumina ( PAA ) template, on which the CNTs (single -

or double - walled) were grown by using plasma - enhanced chemical vapor deposi-

tion ( PECVD ). The AuNPs were obtained via an indirect evaporation method using

a membrane mask, consisting of a suspended, patterned silicon nitride ﬁ lm; the

AuNPs then diffused along the PAA surface into the regions containing CNTs

(Figure 7.12 a). Three distinct regions were observed on the PAA surface: (i) a

semi - continuous gold ﬁ lm composed of multiple grains; (ii) a transition region,

266 7 Gold Nanoparticles and Carbon Nanotubes: Precursors for Novel Composite Materials

Figure 7.11 Transmission electron

microscopy images for the as - prepared

AuNP - CNT hybrids (a), and annealed up to

200 ° C (b), 600 ° C (c), and 800 ° C; (d) HR - TEM

images of AuNPs anchored on the CNT - walls

(e, f); for the as - prepared AuNP – CNT hybrids

(e), and annealed up to 200 ° C (f). A carbon

layer is encapsulating the AuNPs after the

thermal process. Lattice fringes on gold

particles are consistent with the (111) fcc

orientation (0.235 nm); (g) A schematic

representation of the process. Reprinted with

permission from Ref. [57] .

Figure 7.12 (a) Diagram of a cross - section of

the experimental set - up; (b) Schematic

representation of the three regions with

different Au particle distribution on the

porous anodic alumina (PAA) ﬁ lm after Au

evaporation through a nitride membrane

mask; (c) Enlarged view of region sparse

AuNPs, showing one CNT emerging from a

pore and growing along the PAA surface. The

AuNPs diffuse into this region and attach to

the CNT; (d) Field - emission scanning electron

microscopy images of an area from region

sparse AuNPS, showing two crossed CNTs

decorated with AuNPs; (e) A sub - 5 - nm AuNP

(indicated by a white arrow) is on a CNT and

above the pore in the PAA ﬁ lm. Scale

bar = 100 nm. Reprinted with permission from

Ref. [58] .

7.5 CNT–AuNP Composites 267

in which a sparse coverage of small AuNPs was observed; and (iii) a region where

negligible gold was observed. The schematic illustration of the three regions with

different gold particle distributions on the PAA ﬁ lm after the evaporation of gold

through a nitride membrane mask is shown in Figure 7.12 b. The presence of the

various regions is attributed to the migration of gold on the surface; as the gold

atoms migrate they also aggregate, forming grains (at large surface coverage) and

small nanoparticles (at lower surface coverage) [59] . As shown schematically in

Figure 7.12 c, within region (b) the AuNPs were observed on the PAA surface and

on the CNTs. The ﬁ eld - emission scanning electron microscopy ( FESEM ) image

of an area within region (b) shows the presence of a number of well - deﬁ ned par-

ticles with diameters ∼ 5 nm along the CNTs, most likely due to AuNPs (Figure

7.12 d). In addition, somewhat diffuse bright regions with the dimensions of 20 –

40 nm nearer to CNTs were also observed, though these may be due to local charg-

ing effects arising during the FESEM imaging. The AuNPs attached strongly to

the CNTs, as substantiated by the observations of nanoparticles that were sus-

pended over pores or that moved along with the CNTs (Figure 7.12 e). The strong

mechanical binding between the AuNPs and the CNTs revealed a comparatively

close contact between the two objects, and also showed that this binding energy

was larger than that between the cluster and the alumina surface. This behavior

is signiﬁ cant for understanding a strongly coupled electronic system. In contrast

to most other general methods for the direct evaporation of gold onto CNTs, defect

sites on the CNTs are not necessary in this method of creating preformed AuNPs.

This approach may provide a new strategy for functionalizing CNTs for chemical

or biological sensing and also for fundamental studies of nanoscale contacts to

CNTs. Thus, drawbacks such as the complexity in directly functionalizing CNTs,

and the inability to obtain individual SWNTs by using common bulk synthesis

methods are avoided, which conﬁ ne the applicability of CNTs as primary elements

in sensors.

7.5.3

Interaction Between Modiﬁ ed AuNPs and CNT s

A signiﬁ cant feature of nanoscience and nanotechnology concerns the progress of

experimental protocols for the preparation of nanoparticles of diverse chemical

compositions, sizes, shapes, and controlled dispersity with a facile approach and

no environmental risk [60] . There are many established methods for the synthesis

of AuNPs, including conventional chemical reduction, heat - treatment, microwave

irradiation, sonochemical, photolytical, seeding growth approaches, and self -

reduction using surfactants [4, 61] . It is well known that the reaction medium,

reducing agent, and capping or protecting agent are the three key factors for the

synthesis and stabilization of metal nanoparticles in general, and for AuNPs in

particular. By selecting these factors appropriately, it would be possible to modify

the nanoparticles according to their convenient purposes. Therefore, it should

also be possible to connect the AuNPs to the surface of the CNTs through either

covalent linking or supramolecular (noncovalent) interaction. Hence, the AuNPs

268 7 Gold Nanoparticles and Carbon Nanotubes: Precursors for Novel Composite Materials

prepared are modiﬁ ed with suitable functional groups so as to link the AuNPs

to the CNT surface. These connections can be attained by using the functional

groups of the modiﬁ ed AuNPs to covalently link with functional groups present

on the surface of the CNTs. An alternative method would be simply to adhere the

linker onto the CNT surface via supramolecular interactions such as hydrophobic

interactions, hydrogen bond linkage, π – π interactions, or electrostatic attraction.

7.5.3.1 Covalent Linkage

As CNTs are chemically inert, the triggering of their surface is a vital prerequisite

for linking the metal nanoclusters to them. Chemical functionalization is the

most common and widely used way to introduce the linkers, as well as to improve

dispersibility of CNTs, which is also signiﬁ cant for the proﬁ cient and uniform

deposition of nanoparticles. The functionalization of SWNTs by a chemical method

(covalent linkage) to enhance its dispersibility was introduced by Chen and

coworkers [62] .

Azamian and coworkers [63] employed carboxylate chemistry to covalently

link the AuNPs to defect sites in controllably oxidized SWNT termini and/or

sidewalls. The carboxylic acid groups created on a SWNT were converted to amides

by reaction with carbodiimide reagents [ N , N ′ - dicyclohexyl - carbodiimide ( DCC ) or

1 - ethyl - 3 - (3 - dimethyl - amino - propyl) - carbodiimide ( EDC )] and 2 - aminoethanethiol.

Generally, carbodiimide catalyzes the formation of amide bonds between carboxy-

lic acids or phosphates and amines by activating carboxyl or phosphate to form an

O - urea derivative. The introduced thiol functionality was consequently linked with

well - dispersed gold colloids (Figure 7.13 ). The attachment of AuNPs to SWNTs

sidewalls was corroborated by imaging the pristine SWNTs with atomic force

microscopy ( AFM ) before and after exposure to the coupling reagent and colloidal

AuNPs. This technique may be extended to test, with ease, the functionalization

of SWNTs with a range of groups.

A simple, direct, solvent - free approach was reported for decorating AuNPs

onto the surface of MWNTs functionalized with aliphatic dithiols such as 1,4 -

butanedithiol, 1,6 - hexanedithiol, 1,8 - octanedithiol, and 2 - aminoethanethiol [64] .

While AuNPs ( ∼ 1.7 nm) with a narrow particle size distribution were produced on

the 1,6 - hexanedithiol - functionalized MWNTs, the average size of AuNPs was

5.5 nm, obtained on MWNTs derivatized with aminothiol. This difference in

the AuNPs size was apparently due to a coalescence phenomenon of AuNPs in

aminothiol - functionalized MWNT samples; this could be the result of a nonuni-

form capping of the aminothiol over the AuNPs surface, avoiding the passivated

AuNPs. The main drawback of this approach is that attempts to attach AuNPs to

the derivatized MWNTs using water as a solvent medium were not particularly

successful, giving rise to a considerable agglomeration of gold over the nanotube

bundles, obviously due to the poor CNT dispersibility in water. Instead, the

MWNTs nicely decorated with well - dispersed AuNPs can be prepared only when

water is substituted by 2 - propanol. It is anticipated that this method may be useful

for attaching CNTs to gold tips for AFM and scanning tunneling microscopy

7.5 CNT–AuNP Composites 269

Figure 7.13 The chemistry used to connect the AuNPs to

oxidized SWNTs. AFM images of SWNTs before (a) and after

(b) exposure to the coupling reagents and colloidal gold

particles. DCC = N , N ′ - dicyclohexyl - carbodiimide. Adopted and

modiﬁ ed according to Ref. [63] ; AFM images reprinted with

permission from Ref. [63] .

( STM ), and also potentially for monitoring the adsorption and concentration of

trace metal ions.

An efﬁ cient method was developed to functionalize MWNTs with thiol groups,

after which AuNPs were anchored onto them to fabricate new composite materi-

als. Two different simple procedures were followed to produce MWNT – AuNP

composite materials, as summarized in the Figure 7.14 . The thiol - functionalized

MWNTs were stirred with already prepared AuNPs in toluene at room

temperature, and in another way AuNPs were produced in the presence of thiol -

functionalized MWNTs ( in situ formation of MWNT – AuNP composite) [65] . The

TEM images showed self - assembly of the AuNPs on the MWNTs, where the

AuNPs (Figure 7.14 a) of 25 nm were dispersed on the MWNTs. The shape and

size of the AuNPs obtained using the in situ method were different (Figure 7.14 b)

from those obtained with the former method. This might be explained by the

existence of thiol groups surrounded by AuCl

−

anions, which were reduced to

AuNPs in the presence of reducing agents. These AuNPs represent the cores for

further growth, as the thiol groups did not cover the whole particles.

Coleman and coworkers used the Bingel reaction to functionalize the SWNTs

with a cyclopropane group [66] . For this, the cyclopropane group was tagged using

270 7 Gold Nanoparticles and Carbon Nanotubes: Precursors for Novel Composite Materials

Figure 7.14 Experimental procedure for the fabrication of

AuNP – MWNT composites and TEM images of AuNP – MWNT

(a) and in situ AuNP – MWNT (b). TOAB = Tetra -

octylammonium bromide. Adopted and modiﬁ ed according

to Ref. [65] ; TEM images reprinted with permission from

Ref. [65] .

preformed ∼ 5 nm gold colloids by exploiting the gold sulfur binding interaction,

as shown in the Figure 7.15 . AuNPs were observed both on the sidewalls and at

the ends of the SWNTs. The Bingel reaction is an example of a [2+1] cycloaddition

reaction, and is a popular method in fullerene chemistry.

Recently, a novel DNA biosensor was fabricated for the detection of DNA

hybridization based on the layer - by - layer ( LBL ) self - assembly of MWNTs and

AuNPs via covalent linkage, which exhibited an excellent speciﬁ city and chemical

stability under the DNA - hybridization conditions [67] . The LBL assembly repre-

sents one of the simplest ways of producing fundamentally and practically interest-

ing multilayer ﬁ lms with unique mechanical properties; it also provides a precise

control over ﬁ lm composition and thickness, which in turn opens up many new

opportunities for achieving the ideal model surface, the properties of which are

controllable. For this, the cysteamine molecules simply act as a glue to connect

activated MWNTs and AuNPs into a 3 - D hybrid network on the Au electrode, after

which NH

- ssDNA ( ssDNA ; single - stranded DNA ) was immobilized onto multi-

7.5 CNT–AuNP Composites 271

Figure 7.15 Schematic illustration of the chemistry used to

connect AuNPs to SWNTs and AFM image of as - formed

AuNP – SWNT composites by using the Bingel reaction.

DBU = 1,8 - diazabicyclo[5.4.0]undec - 7 - ene. Adopted and

modiﬁ ed according to Ref. [66] ; AFM image reprinted with

permission from Ref. [66] .

layer ﬁ lms via the amino link at the 5 ′ end. Owing to the electron - transfer ability

of the CNTs and the catalytic activities of the AuNPs, the sensitivity of DNA bio-

sensors was improved and this DNA biosensor showed an excellent reproducibility

and stability under DNA hybridization conditions.

7.5.3.2 Supramolecular Interaction Between AuNPs and CNT s

Another strategy for preparing CNT – AuNP composites is that of the supramolecu-

lar or noncovalent functionalization of CNTs, and the subsequent attachment

of AuNPs based on noncovalent interaction such as hydrophobic – hydrophobic

interaction, weak hydrogen bond linkage, π – π stacking interaction, or electrostatic

attraction.

Hydrophobic Interactions and Hydrogen Bonding Hydrophobic interaction is the

attractive force between molecules owing to the close positioning of the nonhy-

drophilic portions of the two or more molecules. Hydrogen bonding is a type

of supramolecular interaction or a type of weak attractive (dipole – dipole) interac-

tion between an electronegative atom and a hydrogen atom bonded to another

electronegative atom such as nitrogen, oxygen, or ﬂ uorine. It can occur between

272 7 Gold Nanoparticles and Carbon Nanotubes: Precursors for Novel Composite Materials

Figure 7.16 Schematic illustrating the attachment of

octanethiol (OT) - capped Au nanoclusters to acetone - activated

MWNTs. Reprinted with permission from Ref. [69] .

molecules, or within parts of a single molecule. A hydrogen bond tends to be

weaker than a covalent or ionic bond, but stronger than van der Waals forces. One

method of noncovalent functionalization of CNTs involves their association with

amphiphilic molecules through hydrophobic interaction in aqueous medium [68] .

Hydrophobic interactions between the ligands forming the monolayer that

passivate the metal surface have been used to deposit the metal nanoparticles

onto the surface of the CNTs. A novel strategy was reported to link monolayer -

protected gold nanoclusters of 1 – 3 nm diameter to the sidewalls of nonoxidized

CNTs through hydrophobic interactions between acetone - activated CNTs and

octanethiol - protected gold nanoclusters [69] . The anchorage was provided by the

interdigitation of alkyl chains of self - assembled molecular layers protecting the

gold nanoclusters and molecular moieties adsorbed on the surface of CNTs (Figure

7.16 ). These molecularly interlinked hybrid nanoblocks may be signiﬁ cant for

exploring and manufacturing a rich variety of molecular nanostructures for poten-

tial device applications.

In addition to experimental studies, a theoretical model has been developed to

provide a detailed account of the interactions (charge transfer, van der Waals,

osmotic, elastic, nonelastic, and covalent) between tetraoctylammonium bromide -

stabilized AuNPs and alkyl - and alkylthiol - modiﬁ ed MWNTs, so as to estimate the

coverage of AuNPs at the surface of the MWNTs under different experimental

conditions [70] . A quantitative description of the interactions between AuNPs and

MWNTs was made by comparing between the predictions of the theoretical model

and the experimental results. From such a comparison, it was concluded that as

the length of the alkyl chains at the surface of the MWNTs increased, coverage of

7.5 CNT–AuNP Composites 273

Figure 7.17 Scheme showing the templated assembly of

tetraoctylammonium bromide - stabilized AuNPs at the surface

of unmodiﬁ ed, alkyl - modiﬁ ed, and alkylthiol - modiﬁ ed MWNTs.

Reprinted with permission from Ref. [70] .

the AuNPs decreased (i.e., noncovalent adsorption). In contrast, for alkylthiol -

modiﬁ ed MWNTs, coverage of the AuNPs at their surface remained constant,

irrespective of the length of the alkyl - thiol chain (covalent adsorption) (Figure

7.17 ). The theoretical model assumption was in good agreement with the experi-

mental ﬁ ndings, which proved the validity of the predictive model. A signiﬁ cant

insight is that, under certain conditions, the coverage of AuNPs is very sensitive

to the nature of the MWNT surface modiﬁ cation and the environment, pointing

274 7 Gold Nanoparticles and Carbon Nanotubes: Precursors for Novel Composite Materials

Figure 7.18 Schematic representation of the molecularly

mediated assembly of monolayer - capped AuNPs on CNTs,

and the characteristic TEM image of the AuNP – CNT

composite. Adopted and modiﬁ ed according to Ref. [71] ; TEM

image reprinted with permission from Ref. [71] .

the way to a rational design of functional CNT - based nanoscale devices with

potentially widespread applications.

A molecularly mediated assembly of alkanethiolate - capped AuNPs onto the

surface of MWNTs through a combination of hydrophobic interactions between

the alkyl chains of the capping/linking molecules and the hydrophobic backbones

of a nanotube and the hydrogen - bonding interaction between carboxylic groups of

the capping/linking molecules and the functional groups on the surface of the

nanotube, has been described [71] (Figure 7.18 ).

Here, the simplicity and effectiveness for assembling alkanethiolate - capped

AuNPs of 2 – 5 nm core size onto CNTs with controllable coverage and spatially

isolated character were achieved by adopting such a combination of interfacial

chemistry – that is, a combination of hydrophobic and hydrogen - bonding interac-

tions between the organic shells and the CNT surfaces. The beneﬁ t of this pathway

is that it does not require any difﬁ cult and tedious surface modiﬁ cation of

CNTs. Rather, the assembly, packing density and distribution of the AuNPs onto

the surface of the CNTs depended on the relative concentrations of the AuNPs,

CNTs, and capping or linking agents. A thermal treatment to remove any organic