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(a)
(b)
Figure 7.3
TEM micrographs of gold nanorods synthesized from (a) CTAB-stabilized
8 nm seeds and (b) CTAB-stabilized 16 nm seeds (scale bars
=
500 nm). [Adapted
from Cole and Murphy (2004) .]
The Brust-Schiffrin reaction has been successfully used to produce spheri-
cal gold and silver nanoparticles (Ahmadi et al., 1996; Bradley et al., 2000;
Petroski et al., 1998). Recently, the use of this method has been expanded to
the formation of gold and silver nanorods by a seed-mediated growth approach
(Jana et al., 2001a; Landskron and Ozin, 2004). In the fi rst step, spherical gold
and silver nanoparticles (diameter of
3.5 nm) were produced by hydride
reduction of hydrogen tetrachloroaurate or silver nitrate, respectively, in the
presence of sodium citrate (Cole and Murphy, 2004; Jana et al., 2001a,b). Then,
these spherical nanoparticles were transferred to a mixture of cetyltrimethyl-
ammonium bromide (CTAB), hydrogen tetrachloroaurate, or silver nitrate to
produce rod-shaped nanoparticles (Fig. 7.3; Cole and Murphy, 2004). The
CTAB, an ionic surfactant capable of forming LLC phases, acts as a directing
agent, thereby forming a bilayer on the nanorods. Its stronger binding to the
side edges than to the ends of the rods allows growth only in one direction.
Moreover, it was further found that using a mixture of CTAB, sodium hydrox-
ide, and ascorbic acid in the second step achieved longer and monodispersed
nanorods (Jana et al., 2001a; Sertova et al., 2006). The ascorbic acid, being a
weak reducing agent, was incapable of reducing the hydrogen tetrachloroau-
rate in the absence of the gold seeds, therefore minimizing additional nucle-
ation during particle growth (Jana et al., 2001a).
A similar approach, for example, utilizing an amphiphilic molecule as a
directing agent, has been used to form silicate nanorods (Ying et al., 1999).
The inorganic aluminosilicate mesoporous material (belonging to the M41S
family of molecular sieves) was synthesized by the addition of an aluminum
source to a mixture of a silica source (e.g., tetraethylorthosilicate), CTAB, a
base (e.g., sodium hydroxide), and water. Initially the mixture was aged at
∼
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