Fahlman B.D. Materials Chemistry

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reaction will facilitate the displacement of Cu

with Ag

within the dendrimer

interior (Eq. 9).

Cu + 2 Ag

! Cu

2þ

+2Agð9Þ

In addition to simple metallic nanostructures, more complex intermetallic species

have also been synthesized through the introduction of mor e than one metal. For

instance, bimetallic nanoclusters may be generated via three routes within a den-

dritic host (Figure 6.43). In addition to already bein g proven for core–shell nanoclus-

ters, this route should also be amenable for the growth of trimetallic nanostructures

for interesting catalytic applications.

[129]

Other polymers may also serve as effective stabilizing agents for nanostruc-

tural growth. For instance, poly(vinylpyrrolidone) (PVP, III), poly(styrenesulfo-

nic acid) sodium salt (PSS, IV), and poly(2-ethyl-2-oxazoline) (PEO, V)were

recently used to generate a number of intermetallic nanoalloys via amild

metallurgy in a beaker approach developed by Schaak and coworkers.

[130]

Since individual metal nanoparticles are in intimate contact with high surface

reactivity and low melting points, the use of high-temperature annealing is not

required, unlike bulk-scale alloy synth eses. The PVP architecture has al so been

shown to facilitate the growth of Au@Ag co re–shell nanostructures, as well as

Ag nanowires and nanocubes.

[131]

Interestingly, one is able to ﬁne-tune the

LSPR frequency of core-shell nanoclusters by varying the shell thickness or

composition/size of the core.

[132]

Figure 6.42. Molecular structures of dendrimers modiﬁed with long-chain aliphatic cores. Unlike

traditional dendritic structures with smaller cores, as the generation size increases, there is an available

channel for external species to enter the dendrimer interior. Reproduced with permission from

Watkins, D. M.; Sayed-Sweet, Y.; Klimash, J. W.; Turro, N. J.; Tomalia, D. A. Langmuir 1997, 13,

508 6 Nanomaterials

1. Co-complexation Method

2. Sequential Method

3. Partial Displacement Method

NaBH

Alloy

Weak Reducing

Agent (i.e H

)

More Noble

Metal Salt

Gn-OH

Core/Shell

Figure 6.43. Schematic of the three methods used to generate bimetallic nanoclusters within a dendritic

host. Reproduced with permission from Scott, R. W. J.; Wilson, O. M.; Crooks, R. M. J. Phys. Chem. B

6.3. Nanoscale Building Blocks and Applications 509

For the synthesis of nanostructures within nonpolar solvents, one may use a

stabilizing agent that contains alkyl chains rather than —OH endgroups. One of

the ﬁrst capping agents to be used for noble metal colloidal growth was alkylthiols

(CH

—[CH

]

—SH).

[133]

In this system , the —SH end is bound to the surface

of the nanostructure, and the long organic tail is responsible for dispersion within

the organic solvent. Though these capping stabilizers worked well for colloidal

growth to prevent agglomeration, even allowing solvent removal/redispersion into

organic solvents, it was relatively difﬁcult to control the size dispersity of the

nanoparticles.

Accordingly, systems that contain a nanoreactor template have been used more

recently for controlled nanostructural growth. In fact, everyone who has washed

dishes or laundry already has some experience with these types of stabilizing agents,

known as micelles. These compounds contain both polar (—OH, cationic/anionic)

and nonpolar (aliphatic) ends. Soaps and surfactants work by surrounding the dirt

particle with the nonpolar ends, leaving the hydrophilic polar groups exposed to

the surrounding water molecules . This results in pulling the dirt particle from the

surface of the clothing ﬁber, forming an aqueou s suspension. In a similar fashion, an

oil-in-water microemulsion may be set up using common surfactants such as sodium

bis(2-ethylhexyl)sulfosuccinate (also referred to as Aerosol OT or AOT, VI), or

the nonionic surfactant Triton-X (VII) in the presence of the biphasic oil/water

mixture.

[134]

510 6 Nanomaterials

Since most precursors for solution-phase nanostructural growth are ionic metal

salts, a typical micelle would not be effective for since the precursor would not be

conﬁned to the interior of the microemulsion. Hence, reverse micelles (or inverse

micelles, Figure 6.44) are used to conﬁne the precursor ions to the aqueous interior,

which effectively serves as a nanoreactor for subsequent reduction, oxidation, etc.

en route to the ﬁnal nanostructure. Not surprisingly, either PAMAMOS den-

drimers (see Chapter 5) or dodecyl-terminated (hydrophobic) PAMAM dendrimers

(Figure 6.45) have been recently employed for this application.

[135]

It should be noted that dendrimer-entrained nanoclusters synthesized within

aqueous solutions may also be phase-transferred into an organic solvent by mixing

with alkylthiols dissolved in a nonpolar solvent.

[136]

This also results in monodis-

perse nanoclusters, with much less polydispersity than early colloidal syntheses that

employed thiol-based entraining agents. That is, the nanocluster size has already

been controlled via intradendrimer stabilization. In contrast, the use of alkylthiols

from the initial stages of growth is not as effective toward preventing agglomeration

during the nucleation step.

Inorganic-based entraining agents have also been used to stabilize nanoclusters.

Perhaps the earliest example is the use of polyoxometalates of general composition

n

or X

m

(where: X ¼ P or Si; M ¼ W, Mo, Nb or V) for the

synthesis of nearly-monodisperse Ir nanoclusters (e.g.,P

9

Figure 6.46).

[137]

In addition, one may also synthesize nanoparticulates by entrap-

ping the precursor within voids of a highly porous substrate. For instance, zeolites,

Figure 6.44. Comparison of a traditional micelle used to entrain organic oils/dirt using an anionic

surfactant, (a), and a reverse micelle used to stabilize aqueous nanoreactors within a nonpolar solvent, (b).

6.3. Nanoscale Building Blocks and Applications 511

layered solids, molecular sieves, gels and glasses have been used to sequester

reactive precursor species wherein hydrolysis, high temperature, or other chemical/

redox reactions are able to transform them into surface-immobilized nanostruc-

tures.

[138]

Of course, the encasing medium may then be removed by acid/base

dissolution, plasma/heat treatment, etc. if discrete nanostructures are desired.

Tri-n-octylphosphine oxide (TOPO, VIII) is another common inverse-micelle

stabilizing agent used to control the sizes of semiconductor nanocrystals such as

CdE (E ¼ S, Se, Te) quantum dots. The synthesis is performed at elevated tem-

peratures since TOPO is a low-melting (53



C), moisture-sensitive solid. A typical

procedure consists of heating/degassing TOPO at ca. 360



C under an inert atmo-

sphere, quickly injecting a tri-n-octylphosphine (TOP) suspension of Me

Cd and

chalcogen powder at a reduced temperature (ca. 180



C), and re-heating to 250–

260



[139]

The quick injection of precursors into the hot solvent generates homo-

geneous nucleation of CdE nanocrystals. Subsequent re-heating affords slow growth

HO CH HC OH

(CH

)

(CH

)

HN NH

G4-C

G5-PPI-C

(CH

)

(CH

)

HNNH

NH NH

Figure 6.45. Hydrophobic-functionalized PAMAM (G4–C

) and poly(propyleneimine) (PPI)

dendrimers, which serve as templates for Au nanocluster growth. Reproduced with permission from

American Chemical Society.

512 6 Nanomaterials

9−

8−

Ir(0) Metal

Core

W=O

W-O-W

Nb=O

Nb-O-Nb

Ir(0)

O62

9−

Stabilizing Matrix

Figure 6.46. Polyoxoanion-stabilized Ir

nanocluster formation. Shown is (a) Polyhedral representation

of the a-1,2,3-P

9

stabilizing anion, and (b) a space-ﬁlling representation of the [(1, 5 –

COD)Ir(P

)]

8

complex. In (a), the three Nb atoms are indicated by the striped octahedra in

positions 1–3. The WO

polyhedra occupy the 4–18 positions, and the internal PO

tetrahedral units are

illustrated in black. In (b), the black spheres represent M–O terminal groups, and the white spheres

represent M–O–M bridging groups. The Bu

and Na

counterions are omitted for clarity. Images (a)

and (b) reproduced with permission from Finke, R. G.; Lyon, D. K.; Nomiya, K.; Sur, S.; Mizuno, N.

Inorg. Chem. 1990, 29, 1784. The bottom image of the stabilized nanocluster is reproduced from Watzky,

6.3. Nanoscale Building Blocks and Applications 513

of nanocrystals via Ostwald ripening

[140]

– the growth of large particles at the

expense of smaller ones, due to the relatively higher surf ace free energy (less

stability) of small nanoclusters. This is important in order to achieve a very high

degree of monodispersity.

It is not always necessary for the precursor metal ions to be encapsulated withi n a

stabilizing polymer during chemical reduction. For instance, the reduced metal may

be entrained by polymerization precursors, such as post-reduction living radical

polymerization that takes place on the surface of gold nanoparticles (Figure 6.47).

This results in a dense “polymer brush”

[141]

that encapsulates the metallic nanopar-

ticle, effectively stabilizing the structure against agglomeration. Subsequent align-

ment and surface reactivity of the resultant nanostructures may be ﬁne-tuned by

varying the nature of the polymer coating.

Figure 6.47. Schematic of the formation of gold nanoparticles coated with free-radical polymerization

initiators that subsequently yield Au@polymer nanostructures through a surface-controlled living

polymerization process. Reproduced with permission from Ohno, K.; Koh, K.-M.; Tsujii, Y.; Fukuda,

514 6 Nanomaterials

Nanoshells

Increasingly complex stabilizing agents such as cross-linked micelles have also been

developed in recent years. These nanostructures consist of a polymeric core that is

cross-linked with surrounding polymer layer(s) (Figure 6.48). In contrast to solid

nanoparticles, a hollow nanoshell may be synthesized by sacriﬁcially removing the

core material by chemical or thermal decomposition. Such intriguing nanobuilding

blocks will likely be of extreme importance for next-generation medical treatment/

sensing, hydrogen storage, ion- exchange, and microelectronics applications.

Another strategy for nanoshell growth consists of applying a thin metallic coating

onto surface-functionalized silica or polystyrene templating spheres. Such gold

nanoshells were pioneered by Naomi Halas at Rice University, and have far-reaching

applications in photonics and biomedicine such as cancer diagnosis/treatment.

[142]

If desired, one may also subsequently remove the template by treating with hydro-

ﬂuoric acid (HF) or toluene for silica or polystyrene cores, respectively. The reverse

case of a polymer/ceramic coating onto removable metallic nanoparticles has also

been applied to yield nonmetallic nanoshells.

[143]

However, the removal of a rela-

tively large core (i.e., typicall y >200 nm) from a nanoscale coating generally results

in signiﬁca nt deformation of the resultant nanoshell.

Figure 6.48. Synthetic scheme for three-layer cross-linked micelles. Shown is the micellation of poly

[(ethylene oxide)-block-glycerol monomethacrylate-block-2-(diethylamino)ethyl methacrylate] (PEO-

GMA-DEA) triblock copolymers to the ﬁnal “onion-like” layered nanostructure. Reproduced with

permission from Liu, S.; Weaver, J. V. M.; Save, M.; Armes, S. P. Langmuir 2002, 18, 8350.

6.3. Nanoscale Building Blocks and Applications 515

The structural robustness of nanoshells has been recently improved through the

use of sacriﬁcial cores of smaller diameters. An example of this strategy is the

growth of silver nanoshells from nanostructural Co templates (Figure 6.49). It should

be noted that NaBH

reduction of ligand-capped Co

ions preferentially yields

B rather than metallic cobalt. However, a post-exposure of oxygen converts the

boride into metallic cobalt nanostructures (Eq. 10).

[144]

Once the Co core was

formed, silver ions were introduced into the system through addition of AgNO

Since Ag

red



¼ 0.799 V) is preferentially reduced relative to Co

red



–0.377 V), the exchange of Ag for Co occurs spontaneously as Ag

/Co

boundaries

are formed. This process continues as Ag

ions diffuse through the growing Ag

layer, until all of the metallic Co is consumed from the core.

4Co

B þ 3O

! 8Coþ 2B

ð10Þ

Rather than redox-governed nanoshell growth, diffusion-governed routes are also

possible. For example, the exposure of cobalt nanoparticles to oxygen, sulfur, or

selenium does not simply form a Co-chalcogenide coating, but rather hollow nano-

shells of the cobalt chalcogenide (Figure 6.50).

[145]

This interesting result is a

modern extension of an effect that has been studied since the late 1940s – the

Kirkendall Effect. This phenomenon describes the differential diffusion rates of

two species in direct contact with one another at elevated temperatures (e.g., Cu

and Zn in brass). During the growth of oxide or sulﬁde ﬁlms, pores are formed in the

solid due to a vacancy-exchange mechanism. That is, the outward move ment of

metal ions through the oxide layer is balanced with an inward migration of lattice

vacancies that settle near the metal/oxide boundary. Due to the large number of

defects and volume of bulk solids, the pores at the metal–oxide interface do not

coalesce into an ordered array. However, for a nanostructural system that is rela-

tively free of defects and exhibits a large surface/volume ratio, the pores readily

aggregate into a single hollow core.

Not surprisingly, 0-D nanostructural growth need not be limited to metallic

structures, but may also include other compounds such as metal oxides, sulﬁdes,

Ag nanoparticles

Co core Ag nanoshell

Figure 6.49. Schematic of Ag nanoshell formation from a nanostructural Co core. Due to favorable redox

couples between Ag and Co, a nanoshell of metallic silver forms at the expense of the inner Co core.

American Chemical Society.

516 6 Nanomaterials

etc. There are a copious number of applications for nanostructural oxide s such as

high-density magnetic storage, heterogeneous catalysis, gas sensors, electrolytes for

lithium batteries, and fuel cells. Sun and coworkers recently describ ed a polyol

[146]

route for the synthesis of hexane-suspended Fe

(magnetite) nanoclusters through

the reaction between Fe(acac)

, oleylamine, oleic acid, and 1,2-hexadecanediol at

ca. 200



C within phenyl ethe r.

[147]

This one-pot synthesis results in the thermal

replacement of the acetylacetonate ligands (recall Chapter 4 – CVD precursors)

from the iron center, followed by oxide formation from reaction with the alcohol.

The oleylamine and oleic acid condense in situ to form a long-chain inverse micelle

that is capable of suspending the nanoclusters in nonpolar media. For puriﬁcation,

one simply adds an excess of polar solvent (methanol or ethanol) to precipitate

the nanostructures. Following centrifugation, the nanocl usters are re-suspended in

hexane, with this precipitation/resuspension procedure repeated three to four times

to afford monodispersed, chemically pure Fe

nanoclusters. Another analogous

procedure uses oleic acid/trimethylamine N-oxide in trioctylamine (b.p. 365



C) to

decompose Fe(CO)

to yield monodispersed g-Fe

nanoclusters.

[148]

Interest-

ingly, shape control of nanoparticles may be afforded by altering the nature of the

solvent or injection timing of surfactants.

[149]

Another more recent strategy that we developed is the synthesis of metal oxide

nanoparticles from the simple reaction of dendritic-stabilized basic metalate salts

with CO

(Figure 6.51).

[150]

This strategy should work for any metalate salt that is

typically formed from the reaction of the metal hydroxide with a strong base (Eq. 11,

for sodium hafnate). Metal/nonmetal oxides may also be synthesized using a sol-gel

technique, analogous to that described in Chapter 4 for thin ﬁlm growth. Fo r

Figure 6.50. Hollow nanoshell formation via the Kirkendall Effect. Shown is the evolution of the hollow

nanoshell after reaction times of (left–right): 0 s, 10 s, 20 s, 1 min, 2 min, 30 min. An interesting feature is

the formation of “bridges” that connect the core to the sulﬁde shell, facilitating the fast outward migration

of Co. Reproduced with permission from Yin, Y.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai,

6.3. Nanoscale Building Blocks and Applications 517