packed face, and one might have expected that the surface of the nanorods would
consist mostly of this face (e.g., the long sides) and that the dimensions of the
nanorods would be limited to the rod-like micelle size of CTAB. But this is not
what we have observed [45].
The crystallographic structure and 3-D crystal morphology of individual gold
nanorods prepared by seed-mediated sequential growth in the presence of CTAB
were determined by selected area electron diffraction (SAED) in combination with
HRTEM. At zero degree tilt, not all the rods imaged on the TEM grid showed
Bragg diffraction. Of those that did, SAED gave two patterns in equal proportions.
Neither pattern could be indexed as a single zone, indicating that the gold nanorods were not single domain crystals. Instead, both patterns consisted of a superposition of two specific crystallographic zones of general form, h112i and h100i
and h110i and h111i, which were consistent with multiple twinning of a facecentered cubic structure. Both types of composite electron diffraction patterns can
be rationalized on the basis that the gold nanorods consist of an elongated variant
of a cyclic penta–tetrahedral twin crystal in which five {111} twin boundaries are
arranged radially to the direction of elongation. This type of twinning is common
in isotropic gold nanoparticles with decahedral (D 5h ) morphology because the interfacial angle between {111} planes (70.53
) is close to 2p/5 (72
). In the case of
the nanorods, the shape anisotropy originates from a specific elongation along the
common [110] five-fold axis to produce an idealized 3-D morphology based on a
pentagonally twinned prism with five {100} side faces and capped at both ends by
five {111} faces (Figure 9.4).
The absence of any preferred zone combination in the electron diffraction patterns suggests that the side faces of the nanorods are either not well-developed or
consist of two forms, e.g. {100} and {110} with approximately equivalent surface
area. HRTEM images of individual gold nanorods (Figure 9.5) show stripe patterns
characteristic of the superposition of two diffraction patterns, i.e. a twinned defect
structure, consistent with the SAED data.
In our experiments, growth of the isometric twinned ‘‘seeds’’ (diameter 4 nm) in
the presence of CTAB results in the initial transformation of ca. 4% of the seeds
into short nanorods, while the remaining crystals increase in size to around 17 nm
[45]. Once formed, the nanorods grow almost unidirectionally in length when immersed in a fresh reaction solution to produce cylindrical penta-twinned particles
with high aspect ratios and variable crystal lengths between 100 and 300 nm. Because the increase in width is minimal, the elongated crystals have a uniform
thickness that is determined by the width of the short nanorods formed in the
previous stage of the reaction sequence. At the same time, ca. 6% of the 17 nmsized isometric twins are transformed into a new population of short rod-shaped
nanoparticles, while the remaining crystals continue to grow isometrically. Subsequent transfer of the products into fresh reaction solutions reiterates the combination of isometric growth, nanorod elongation and nanorod formation to produce
a trimodal distribution in rod widths [45]. The distribution corresponds to three
types of nanorods with mean widths of 34, 40 and 58 nm and decreasing aspect
ratios with values between 17–20, 8–11, and 2–3, respectively. Each type can be
9 Synthesis, Assembly and Reactivity of Metallic Nanorods
290
consist mostly of this face (e.g., the long sides) and that the dimensions of the
nanorods would be limited to the rod-like micelle size of CTAB. But this is not
what we have observed [45].
The crystallographic structure and 3-D crystal morphology of individual gold
nanorods prepared by seed-mediated sequential growth in the presence of CTAB
were determined by selected area electron diffraction (SAED) in combination with
HRTEM. At zero degree tilt, not all the rods imaged on the TEM grid showed
Bragg diffraction. Of those that did, SAED gave two patterns in equal proportions.
Neither pattern could be indexed as a single zone, indicating that the gold nanorods were not single domain crystals. Instead, both patterns consisted of a superposition of two specific crystallographic zones of general form, h112i and h100i
and h110i and h111i, which were consistent with multiple twinning of a facecentered cubic structure. Both types of composite electron diffraction patterns can
be rationalized on the basis that the gold nanorods consist of an elongated variant
of a cyclic penta–tetrahedral twin crystal in which five {111} twin boundaries are
arranged radially to the direction of elongation. This type of twinning is common
in isotropic gold nanoparticles with decahedral (D 5h ) morphology because the interfacial angle between {111} planes (70.53
) is close to 2p/5 (72
). In the case of
the nanorods, the shape anisotropy originates from a specific elongation along the
common [110] five-fold axis to produce an idealized 3-D morphology based on a
pentagonally twinned prism with five {100} side faces and capped at both ends by
five {111} faces (Figure 9.4).
The absence of any preferred zone combination in the electron diffraction patterns suggests that the side faces of the nanorods are either not well-developed or
consist of two forms, e.g. {100} and {110} with approximately equivalent surface
area. HRTEM images of individual gold nanorods (Figure 9.5) show stripe patterns
characteristic of the superposition of two diffraction patterns, i.e. a twinned defect
structure, consistent with the SAED data.
In our experiments, growth of the isometric twinned ‘‘seeds’’ (diameter 4 nm) in
the presence of CTAB results in the initial transformation of ca. 4% of the seeds
into short nanorods, while the remaining crystals increase in size to around 17 nm
[45]. Once formed, the nanorods grow almost unidirectionally in length when immersed in a fresh reaction solution to produce cylindrical penta-twinned particles
with high aspect ratios and variable crystal lengths between 100 and 300 nm. Because the increase in width is minimal, the elongated crystals have a uniform
thickness that is determined by the width of the short nanorods formed in the
previous stage of the reaction sequence. At the same time, ca. 6% of the 17 nmsized isometric twins are transformed into a new population of short rod-shaped
nanoparticles, while the remaining crystals continue to grow isometrically. Subsequent transfer of the products into fresh reaction solutions reiterates the combination of isometric growth, nanorod elongation and nanorod formation to produce
a trimodal distribution in rod widths [45]. The distribution corresponds to three
types of nanorods with mean widths of 34, 40 and 58 nm and decreasing aspect
ratios with values between 17–20, 8–11, and 2–3, respectively. Each type can be
9 Synthesis, Assembly and Reactivity of Metallic Nanorods
290