Controllable Synthesis of Metal Nanoparticles for Electrocatalytic Activity Enhancement - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 231

isotropic shape. To achieve an anisotropic growth condition, the reaction
may be performed in a micelle structure. This micelle is formed when
amphiphilic molecules are dispersed in water. The hydrophilic groups are
exposed to water, leaving the hydrophobic tail regions buried in the micelle
center. The reversed case, i.e., reverse micelle, is amphiphilic molecules in a
nonpolar solvent, where the hydrophobic tails are in contact with solvent
with the hydrophilic end embedded in the center of the structure. De-
pending on the dimension of hydrophobic and hydrophilic units, aniso-
tropic environment can be created and reaction in such a structure at a
controlled temperature often leads to the formation of one-dimensional
structure. This synthetic approach has been highlighted in the synthesis of
Au nanorods, 15 where amphiphilic cetyltrimethylammonium bromide
(CTAB) molecules are used to form double-layer micelle structures in the
aqueous solution, and the Au precursors (usually HAuCl 4 ) are encapsulated
at the micelle center. Due to the slightly different bonding strength of CTAB
with Au crystal facets, preferential growth is introduced by forming the
elongated micelles. Another example is in the synthesis of FePt nanowires
(NWs) via decomposition of Fe(CO) 5 and reduction of Pt(acac) 2 in the mix-
ture of OAm and ODE. 24 The lengths of the resulting FePt NWs can be tuned
from 20 to 200 nm by changing the volume ratio of OAm to ODE
(Figure 9.5a-c). It is believed that OAm self-organizes into an elongated re-
verse micelle-like structure within which the FePt nuclei are formed. The
elongated nuclei result in the different OAm packing densities on different
surfaces, as indicated by (1), (2), and (3) in Figure 9.5d. In area (1), the

d n 9 r 4 n g | 5

.

Figure 9.5 (a)-(c) TEM images of Fe 55 Pt 45 NWs with a length of 200 nm (a), 50 nm

(b), and 20 nm (c). (d) Schematic illustration of the growth of FePt NWs
in reverse micelle.
Reprinted from ref. 24 with permission by Wiley-VCH.