with aqua regia. Relatively small changes in the pH of the solution, for example, in
the case of silver nanorods, leads to nanowires [26]. Gold, we find, is difficult to
make into nanowires, but is relatively easy to make into nanorods, for reasons that
are unclear. A few degrees difference in room temperature, as well as the timing of
the growth steps, also affects the final product’s size and shape. Changing relative
concentrations of seed, salt, and surfactant can eliminate nanorods entirely, but
produce nanospheres with well-controlled dimensions from 5–40 nm [30]. Also,
depending on reaction conditions, it is possible to have seeds promote the formation of more seeds instead of growth [31]!
There is an ever-increasing list of other methods to make inorganic nanorods
and nanowires. For example, porous alumina membranes with well-defined nanochannels can be used as hard templates in which to electrochemically deposit
metals; the metal forms nanorods dictated by the channel dimensions, and the
membrane must of course be attached to the electrode for this to work [5, 6, 19].
Metal nanowires can also be grown via electrodeposition along the step edges of
highly-ordered pyrolytic graphite from metal salt in solution, using a scanning
tunneling microscope tip to pulse the voltage in ‘‘activation’’, ‘‘nucleation’’ and
‘‘growth’’ steps [32].
Many methods of producing nonspherical, well-defined nanoparticles of various
shapes and sizes use a ‘‘soft template’’ (e.g., microemulsions, polymers, or surfactants) that directs nanoparticle growth, in the absence of preformed seed [24,
33–40]. Some of these preparations are performed at higher temperatures (up to
several hundred
C) in organic solvents, with organometallic precursors, while
others are simple reductions of metals, or arrested precipitation reactions, in water.
Metals, semiconductors, and metal oxides have been made by these soft solution
routes [24, 33–40]. Heterogeneous seeded methods, in which the seed is not the
same element or compound as the nanostructure grown from it, are also being
developed [41, 42], with great success in the case of semiconductor nanowires [41].
Intuitively, one might imagine that the mechanism of anisotropic growth of a
nanoparticle in the ‘‘soft template’’ methods would involve physical constraint on
the part of the template; for example, that the size and shape of rodlike micellar
templates would be mirrored in the size and shape of the resulting nanoparticles
made in those matrices. However, it is now more accepted that preferential adsorption of molecules and ions to different crystal faces of the growing nanoparticle leads to different nanoparticle shapes [1, 40, 43–46]. The underlying
mechanisms of anisotropic nanoparticle growth are of fundamental interest, but
are also of practical interest; for example, poisoning of platinum nanoparticle catalysts by adsorbed sulfur compounds is a problem of great commercial importance, and it has been suggested that the poisoning is due to adsorbate-induced
crystal facet changes of the Pt nanoparticle catalyst [47].
In the case of gold nanorods prepared in aqueous solution by the seed-mediated
growth method, we have performed high-resolution transmission electron microscopy (HRTEM) and electron diffraction experiments as a function of growth time
to crystallographically characterize the nanorods [45]. It is well-known that for the
face-centered cubic (fcc) structure of gold, the (111) face is the most stable, close9.2 Seed-Mediated Growth Approach to the Synthesis of Inorganic Nanorods and Nanowires 289
the case of silver nanorods, leads to nanowires [26]. Gold, we find, is difficult to
make into nanowires, but is relatively easy to make into nanorods, for reasons that
are unclear. A few degrees difference in room temperature, as well as the timing of
the growth steps, also affects the final product’s size and shape. Changing relative
concentrations of seed, salt, and surfactant can eliminate nanorods entirely, but
produce nanospheres with well-controlled dimensions from 5–40 nm [30]. Also,
depending on reaction conditions, it is possible to have seeds promote the formation of more seeds instead of growth [31]!
There is an ever-increasing list of other methods to make inorganic nanorods
and nanowires. For example, porous alumina membranes with well-defined nanochannels can be used as hard templates in which to electrochemically deposit
metals; the metal forms nanorods dictated by the channel dimensions, and the
membrane must of course be attached to the electrode for this to work [5, 6, 19].
Metal nanowires can also be grown via electrodeposition along the step edges of
highly-ordered pyrolytic graphite from metal salt in solution, using a scanning
tunneling microscope tip to pulse the voltage in ‘‘activation’’, ‘‘nucleation’’ and
‘‘growth’’ steps [32].
Many methods of producing nonspherical, well-defined nanoparticles of various
shapes and sizes use a ‘‘soft template’’ (e.g., microemulsions, polymers, or surfactants) that directs nanoparticle growth, in the absence of preformed seed [24,
33–40]. Some of these preparations are performed at higher temperatures (up to
several hundred
C) in organic solvents, with organometallic precursors, while
others are simple reductions of metals, or arrested precipitation reactions, in water.
Metals, semiconductors, and metal oxides have been made by these soft solution
routes [24, 33–40]. Heterogeneous seeded methods, in which the seed is not the
same element or compound as the nanostructure grown from it, are also being
developed [41, 42], with great success in the case of semiconductor nanowires [41].
Intuitively, one might imagine that the mechanism of anisotropic growth of a
nanoparticle in the ‘‘soft template’’ methods would involve physical constraint on
the part of the template; for example, that the size and shape of rodlike micellar
templates would be mirrored in the size and shape of the resulting nanoparticles
made in those matrices. However, it is now more accepted that preferential adsorption of molecules and ions to different crystal faces of the growing nanoparticle leads to different nanoparticle shapes [1, 40, 43–46]. The underlying
mechanisms of anisotropic nanoparticle growth are of fundamental interest, but
are also of practical interest; for example, poisoning of platinum nanoparticle catalysts by adsorbed sulfur compounds is a problem of great commercial importance, and it has been suggested that the poisoning is due to adsorbate-induced
crystal facet changes of the Pt nanoparticle catalyst [47].
In the case of gold nanorods prepared in aqueous solution by the seed-mediated
growth method, we have performed high-resolution transmission electron microscopy (HRTEM) and electron diffraction experiments as a function of growth time
to crystallographically characterize the nanorods [45]. It is well-known that for the
face-centered cubic (fcc) structure of gold, the (111) face is the most stable, close9.2 Seed-Mediated Growth Approach to the Synthesis of Inorganic Nanorods and Nanowires 289