Page 83 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanomaterials Fabrication 69

200 nm

100 nm 100 nm
100 nm 200 nm
(a) (b) (c)

150 nm 150 nm
(d) (e)
Figure 3.28 (a) TEM image of truncated tetrahedral gold nanocrystals. (b) SEM images
of several partially developed gold tetrahedra. (d) SEM image of icosahedral gold nanopar-
ticles. (d and e) Gold nanocubes dispersed on a TEM grid and a silicon substrate [125].

have been obtained by the polyol process (Figure 3.28) [125]. Chloroauric
acid and polyvinyl pyrolidone (PVP) are introduced in boiling ethylene
glycol (EG). EG serves as both solvent and reducing agent, PVP stabilizes
the particles and also, in conjunction with the concentration of the gold
precursor, controls their shape. A low concentration of silver ions in the
medium orients the process toward the formation of gold nanocubes.
Another interesting seeding growth method has been used to produce
nanorods [126]. The basic principle for the shape-controlled synthesis
involves two steps: first, the preparation of spherical gold nanoparticles
of around 3,5 nm in diameter (seeds), by reduction at RT of HAuCl 4 by
NaBH 4 in the presence of citrate [citrate serves only as a capping or pro-
tective agent because it cannot, at room temperature, reduce Au(III)];
second, growth of the seeds in rod-like micellar environments acting as
templates [127]. The growth solution contains HAuCl 4 , CTAB (cetyl-
trimethylammonium bromide), and ascorbic acid to which the seed solution
is added. Ascorbic acid is a mild reducing agent, which cannot reduce the
gold salt in the presence of micelles without the presence of seeds. Gold
nanorods are so obtained with various aspect ratios (Figure 3.29). The
aspect ratio is controlled by varying the ratio of metal salt to seed, if some

Ag ions are present [128]. Silver ions are not reduced under these exper-
imental conditions, and their role in controlling the shape of gold nanorods