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

Подождите немного. Документ загружается.

7.5 CNT–AuNP Composites 255

of composite material is important not only for fundamental and academic

studies of the interactions between the matrix and the metallic nanoparticles, but

also for various applications such as catalysts as well as electronic, optical, and

sensor devices.

7.5

CNT – AuNP Composites

A variety of resourceful techniques has been reported for the production of

CNT – AuNP composites (in general, the deposition of AuNPs onto the surface of

the CNT substrate), each offering different degrees of particle size control

and distribution along the CNTs. By changing the size and concentration of the

AuNPs deposited/incorporated, the electronic properties of the CNTs can in turn

be controlled. Composites of CNTs with AuNPs may be created via different path-

ways. One approach is to grow and/or incorporate the AuNPs into the hollow

cavities of the CNTs, while a second approach is to grow and/or deposit naked

AuNPs directly onto the terrace of the CNTs. In yet another process, the AuNPs

may be prepared and modiﬁ ed with suitable functional groups that can be con-

nected to the CNT surface via covalent bonding through organic moieties; alter-

natively, the modiﬁ ed AuNPs may simply be linked to the surface of the CNTs via

supramolecular interactions.

7.5.1

Filling of CNT s with AuNPs

In order to synthesize CNTs with predetermined characteristics, it is essential to

identify and control the mechanisms that direct CNT growth. The major challenge

here is to identify effective ways in which to ﬁ ll the metal nanoparticles (notably

AuNPs) into the hollow CNT cavities, without affecting the latter ’ s individual

characteristic properties.

A simple procedure for producing a composite is to ﬁ ll the MWNTs with AuNPs,

simply by mixing an aqueous citric acid solution containing NH

- treated MWNTs

and aqueous auric chloride solution [38] . Heat treatment in NH

causes most of

the nanotubes to be open, such that functional basic groups are created on their

inner walls. Mixing and ultrasonication will then help the citric acid to combine

strongly with the basic groups via electrostatic attraction, thus facilitating the in

situ reduction and subsequent attachment of AuNPs (1 – 2 nm) inside the nano-

tubes (Figure 7.2 ). These hybrid materials may in time become important for

investigating and creating a rich variety of electrical and sensor devices.

In another “ wet chemistry ” technique, a two - step procedure was used to produce

a composite material by ﬁ lling gold metal into the cavities of the CNTs. Here, the

nanotubes were opened by oxidation with HNO

[39] and then stirred overnight

with a concentrated aqueous solution of AuCl

. The resultant CNTs, when sepa-

rated from excess concentrated solution, were calcined in a furnace under argon

256 7 Gold Nanoparticles and Carbon Nanotubes: Precursors for Novel Composite Materials

at 600 ° C, whereby the AuCl

was decomposed to produce elemental gold [40] . Most

of the gold entrapped within the hollow CNTs was seen to be crystalline in nature,

spherical in shape, and to range in size from 1 to 5 nm diameter. A HR - TEM image

of a CNT encapsulated with a gold crystal, together with the analytical energy -

dispersive spectrum ( EDS ) of the gold particle, are shown in Figure 7.3 . More

importantly, by using this method it was possible to produce a relatively high

percentage ( ∼ 70%) of opened nanotubes to be ﬁ lled with metallic gold [33, 39] .

7.5.2

Deposition of AuNPs Directly on the CNT Surface

When depositing AuNPs onto the terrace of the CNTs, it is possible to use gold

salts as the precursors for the AuNPs; these salts are produced by a variety of

reduction processes, using either reducing agents and/or external energies such

as heat (thermal), photochemical, and light, in the presence of the CNTs. The

interactions between the AuNPs and CNTs are mostly based on van der Waals

forces which, in some cases, appear to be sufﬁ ciently strong so as to ensure

signiﬁ cant adhesion.

Figure 7.2 Schematic illustration for the attachment of AuNPs

to NH

- treated CNTs and the TEM image of AuNP - ﬁ lled

CNTs. Adopted and modiﬁ ed according to Ref. [38] ; TEM

image reprinted with permission from Ref. [38] .

7.5 CNT–AuNP Composites 257

Figure 7.3 (a) High - resolution transmission electron

microscopy (HR - TEM) image of a carbon nanotube ﬁ lled with

a spherical Au crystal. The solid arrow shown in the HR - TEM

image indicates where there has been intercalation into the

gaps where carbon layers are missing; (b) ED spectrum of the

Au particle. Reprinted with permission from Ref. [40] .

An effective method was introduced by Xue and coworkers for depositing

AuNPs onto the walls of CNTs [41] . For this, AuNPs (average size 8 nm)

were grown on the surface of the CNTs by the thermal decomposition (400 ° C) of

gold salts under a hydrogen atmosphere. This synthetic strategy has also been

shown to be a generalized process that can easily be extended to the synthesis of

different types of metal nanoparticle (Pt, Ag, Pd) onto the CNT surface. It appears

258 7 Gold Nanoparticles and Carbon Nanotubes: Precursors for Novel Composite Materials

Figure 7.4 (a, b) Low - and (c) high - magniﬁ cation bright - ﬁ eld

TEM images showing the decoration of MWNTs with AuNPs

of 3 – 10 nm diameter. Reprinted with permission from

Ref. [42] .

that the CNTs play a vital role here, not only as a template for tuning the metal

nanoparticles size but also acting as a support material. This ﬁ nding was sup-

ported by the fact that larger particles were observed when metal salts were

reduced in the presence of graphite or amorphous carbon. Xue and coworkers

suggested that these composite materials might be used as efﬁ cient catalysts for

certain environmentally advantageous reactions, as well as certain applications in

electronic devices.

Raghuveer and coworkers established a novel “ eco - friendly ” strategy of utilizing

microwave irradiation for the rapid introduction of carboxyl, carbonyl, hydroxyl,

and allyl terminal groups onto the surface of MWNTs, without using any aggres-

sive oxidants such as HNO

and/or ultrasonication [42] . Here, the functional

groups served as the preferred nucleation points for reducing gold ions from solu-

tion by a microwave - assisted reduction reaction. MWNTs were dispersed in water

and added to an aqueous solution containing HAuCl

and ethylene glycol. After

microwave irradiation, the surfaces of the MWNTs were seen to be decorated by

uniformly dispersed AuNPs (Figure 7.4 ) that ranged in size from 3 to 10 nm

(average ∼ 6 nm). The MWNTs were derivatized with nanoparticles synthesized by

an in situ gold - ion reduction during functionalization, all in a single - step process.

The notable point here was that the overall tubular structure of the MWNTs

remained intact, as was evident from TEM images. This was in great contrast to

7.5 CNT–AuNP Composites 259

the rupture and tube breakage observed during functionalization by aggressive

sonication and acid treatment [43] .

A different strategy for the formation of AuNPs on the surface of SWNTs, based

on the spontaneous reduction of metal ions in solution without the use of a reduc-

ing agent, was reported by Choi and coworkers [44] . This approach differed from

a typical electroless deposition, which requires either a reducing agent or a catalyst,

as a result of direct redox reactions between the ions and nanotubes. Electroless

deposition methods rely on a chemical (as opposed to an electrochemical) reduc-

tion process, whereby a chemical species with a redox potential suitably lower than

that of the metal species being reduced provides the driving force for the reaction

[45] . A spontaneous decoration of AuNPs (average size 7 nm) on the sidewalls of

the SWNTs was observed following their immersion in HAuCl

(Au

) solution

for 3 min (Figure 7.5 a). The Au

ion reduction and SWNT oxidation during elec-

troless metal deposition was investigated by measuring the electrical conductance

of SWNTs immersed in solutions (Figure 7.5 b). When the SWNTs act as electron

donors, hole insertion into SWNTs would be expected to cause an increase in the

electrical conductance [46] to the already p - type nanotubes due to O

doping under

ambient conditions [47] . Spontaneous metal deposition onto the SWNTs by an

electroless process allows for a facile, efﬁ cient, and selective immobilization of

metal species on nanotubes, which may be useful for sensor and catalysis applica-

tions. However, this selective electroless metal deposition on SWNTs was shown

to be effective only for Au or Pt; other metal ions such as Ag

, Ni

, and Cu

, could

not be reduced in the same way, perhaps due to their lower redox potentials.

Figure 7.5 (a) Atomic force microscopy image of AuNPs

formed on an individual SWNT; (b) Monitoring the change in

current across a SWNT during exposure to a 5 mM HAuCl

solution. The period before the exposure corresponds to the

nanotube in a mixture of ethanol and water (1 : 1). Inset:

Schematic for the experimental set - up. Spacing between

points = 1 s. Reprinted with permission from Ref. [44] .

260 7 Gold Nanoparticles and Carbon Nanotubes: Precursors for Novel Composite Materials

Figure 7.6 (a) Schematic illustration of the metal nanoparticle

deposition on CNTs via the substrate - enhanced electroless

deposition process; (b, c) Scanning electron microscopy

images of (b) MWNTs and (c) SWNTs supported by a copper

foil after immersion in an aqueous solution of HAuCl

(3.8 mM). Reprinted with permission from Ref. [48] .

Qu and Dai introduced for the ﬁ rst time another facile, but versatile and effec-

tive, method for the electroless deposition of AuNPs on both SWNTs and MWNTs

in the absence of any additional reducing agent [48] . Upon immersion of the

copper - supported MWNTs or SWNTs into an aqueous solution of HAuCl

(3.8 mM), the AuNPs were deposited spontaneously onto the terrace of the nano-

tubes. The advantage of this technique over the previous method is that a large

variety of metal nanoparticles (even for metals with a lower redox potential than

that of the CNTs, such as Cu and Ag) could be reduced and decorated onto the

surface of both MWNTs and SWNTs. The general scheme of the reaction process,

together with scanning electron microscopy ( SEM ) images of AuNPs decorated

onto MWNTs and SWNTs, are shown in Figure 7.6 .

The deposition of metal nanoparticles is achieved via the redox reaction of a

galvanic cell, in which the nanotube acts as a cathode for the metal nanoparticle

deposition (M

, e.g., Au

) from the reduction of metal ions (M

, e.g., Au

) in

solution, while the metal substrate (e.g., Cu) serves as an anode where metal atoms

(Sub

) are oxidized into corresponding ions (Sub

) followed by dissolution [49]

(Figure 7.6 ). This process, which is known as substrate - enhanced electroless depo-

sition ( SEED ), allows the electroless deposition of many metal nanoparticles onto

conducting CNTs, and indicates a great potential for the functionalization of CNTs

with various metal nanoparticles.

Geckeler and coworkers have established an unprecedented approach to prepare

SWNT – AuNP hybrids in homogeneous phase by using the reaction of gold salts

7.5 CNT–AuNP Composites 261

and SWNTs in the presence of a surfactant in aqueous solution, without the addi-

tion of a reducing agent [50] . The AuNPs decorated on the sidewalls of the SWNTs

were uniform in size and well dispersed (Figure 7.7 ). Statistical calculations indi-

cated the average AuNP size to be 2.94 ± 0.75 nm, with 7.5 × 1 0

− 17

g of gold being

coated on an individual nanotube that in turn contained approximately 300 AuNPs.

Interestingly, the size of the AuNPs decorated on the surface of the SWNTs could

be tailored by altering the concentration of the gold salt solution.

The chemistry of the hybrid material was described by using a frontier - orbital

picture. As the relative position of the Fermi level of nanotubes with respect to the

mixed metal ion/nanotube highest occupied molecular orbital ( HOMO ) and

lowest unoccupied molecular orbital ( LUMO ) is suitable for charge transfer, both

semiconducting and metallic CNTs may establish attractive interactions with the

metal ions, either by four - electron interactions involving two occupied orbitals, or

by zero - electron interactions involving two empty orbitals (Figure 7.7 d). It was

noted that the HOMO level of

AuCl

−

is partly occupied with electrons. This

method is easy to scale - up, such that a uniform size of AuNPs may be decorated

on the walls of the SWNTs, and size of the AuNPs can also be controlled. More

importantly, as the method yields water - soluble composite materials, a much

greater variety of applications can be envisaged.

A simple method has been developed recently to prepare hybrid materials from

SWNTs and AuNPs, including Pt and Rh nanoparticles. For this, nanoparticles

were deposited on the surface of the SWNTs by the mild reduction of metal salts

using poly(ethylene glycol) - 200 as the reducing agent [51] . The free surface of

the nanoparticles attached to the SWNTs was then coated with organic aliphatic

molecules such as oleylamine, which enhanced the dispersion of the resulting

hybrid material in organic solvents. This method avoids chemical functionaliza-

tion of the sidewalls and open ends of the SWNTs, and the ﬁ nal hybrid material

may be used for the application in the catalysis of organic reactions.

Recently, a simple UV irradiation method was developed to grow uncoated

naked AuNPs on carboxy - modiﬁ ed MWNTs by performing UV irradiation on

mixed solution containing oxidized MWNTs, HAuCl

, and acetone (acting as a

photosensitive agent) at room temperature (Figure 7.8 ) [52] . The size of AuNPs

deposited on the terrace of the MWNTs was found to depend heavily on the diam-

eter of MWNTs and the solution pH. The size of the AuNPs was indirectly pro-

portional to the diameter of MWNTs and the solution pH. As a high catalytic

activity (especially for AuNPs) mainly depends on the steps, edges, and corner

sites of surface, and also on the electrical interactions between AuNPs and sup-

porting materials, the resultant composites may prove to be advantageous in cata-

lytic reactions.

Although a variety of ingenious strategies to decorate AuNPs onto CNTs is avail-

able, the electrodeposition method has its own advantages. Electrochemistry rep-

resents a potent technique for the deposition of diverse metals and/or the surface

modiﬁ cation of CNTs, being both rapid and facile, and thus allowing the chemist

and materials scientist to control with ease the nucleation and growth of the metal

nanoparticles [45, 53, 54] . It is very feasible to control the size and distribution of

262 7 Gold Nanoparticles and Carbon Nanotubes: Precursors for Novel Composite Materials

Figure 7.7 (a) Transmission electron

microscopy (TEM) image, and (b, c) HR - TEM

images of the CNT/AuNPs hybrid;

(d) Frontier - orbital picture representation

of four - electron interactions (plot a), and

zero - electron interactions (plot b) between

AuCl

−

and a metallic CNT. The

AuCl

−

frontier - orbitals are designated as HOMO and

LUMO, and the nanotube orbitals are

represented by the density of states plots.

= Fermi level. ( Reprinted with permission

from Ref. [50] ); (e) Schematic showing

decoration of the surface of CNTs with

AuNPs. (Kyungjae Lee is gratefully

acknowledged for his support in the

preparation of this image).

7.5 CNT–AuNP Composites 263

Figure 7.8 Scheme showing the growth of AuNPs on the

surface of MWNTs initiated by UV irradiation, and a TEM

image of AuNPs attached to the surface of MWNTs at pH 12.

Scale bar = 100 nm. Adopted and modiﬁ ed according to Ref.

[52] ; TEM image reprinted with permission from Ref. [52] .

metal nanoparticles simply by varying the deposition potential, time, and sub-

strate. However, the other imaginative methods of depositing nanoparticles onto

the CNTs surface involve some tricky, tedious, and time - consuming treatments

that allow the impurities in the bath solutions to be included either into the nano-

particles or onto the terrace of the CNTs themselves, thus affecting the optical or

catalytic properties of the nanoparticles [54] . Electrochemically deposited nanopar-

ticles – particularly of noble metals such as Au, Pt, or Pd – are often of very high

purity, are formed rapidly, and have good adhesion to the CNT substrate [53, 54] .

Quinn and coworkers [53] reported a general electrodeposition method for

depositing noble metals such as Au, Pt, and Pd onto the surface of the SWNTs by

immersing the latter in a solution of the respective metal salts, namely HAuCl

PtCl

, and (NH

)

PdCl

. Whilst the size of the nanoparticles was tuned by the

concentration of the metal precursor salt and the electrochemical deposition

parameters, the coverage of the nanoparticles on the surface of the SWNTs

was controlled by the nucleation potential. The resultant composite material was

264 7 Gold Nanoparticles and Carbon Nanotubes: Precursors for Novel Composite Materials

Figure 7.9 General schematic illustration of the SWNT as an

electrode used for the electrodeposition of metal

nanoparticles, and AFM images of the predeposition (a) and

postdeposition (b) of Au. The deposition time was 20 s. Scale

bar = 300 nm. Reprinted with permission from Ref. [53] .

considered as a metallic wire, as the surface of the SWNTs was thoroughly deco-

rated by the metal nanoparticles (Figure 7.9 ). It was noted that, under the experi-

mental conditions, both the sidewalls and ends of the SWNTs were coated equally

by metal nanoparticles, even though it is believed that the CNTs sidewalls are less

reactive than the tube ends. The SWNT serves a dual function in such a way that

initially acts as the electrodeposition template, and subsequently as a wire to elec-

trically connect the deposited Au, Pt, and Pd nanoparticles.

The cubic and spherical nanoparticles of gold, with a fairly narrow size distribu-

tion, deposited on the surface of the CNTs were produced by immersing copper

foil - supported CNTs into an aqueous solution of HAuCl

with and without CuCl

at room temperature under different reaction conditions (e.g., different concentra-

tion and reaction time) [55, 56] . Both, the shape and size of the AuNPs were found

to depend heavily on the gold salt concentration and reaction time, providing

considerable room for regulating the morphological features of the resultant nano-

particles. The size of the AuNPs deposited on the surface of CNTs is approximately

60 – 100 nm, although their size can be tailored by controlling the reaction condi-

tions, especially the deposition time [56] . Gold nanospheres and nanocubes were

multisite deposited along the CNT length, with individual nanotubes even thread-

ing through the nanoparticles (Figure 7.10 ). The facile and versatile technique for

the shape - and size - controlled syntheses of AuNPs for the site - selective modiﬁ ca-

tion of CNTs is very attractive for producing various multicomponent nanoparticle

– nanotube hybrid structures that might be useful in a wide range of potential

applications, including fuel cells, catalytic, sensing, and optoelectronic systems.

Further, in order to investigate and explain the optical response of the CNT – AuNP

hybrid from a theoretical standpoint, a 3 - D electrodynamic model was built using

the ﬁ nite - difference time - domain [56] . These studies proposed an anisotropic