Moving beyond
carbon nanotubes, researchers are developing insights
into a remarkable class of tubular nanomaterials that
can be produced in water with a high degree of control
over their diameter and length. Based on metal oxides
in combination with silicon and germanium, such
single-walled inorganic nanotubes could be useful in a
range of nanotechnology applications that require
precise control over nanotube dimensions.
At the Georgia
Institute of Technology, researchers are studying the
formation of these metal oxide nanotubes to understand
the key factors that drive the emergence of nanotubes
with specific diameters and lengths from a “soup” of
precursor chemicals dissolved in water. Their goal is
to develop general guidelines for controlling nanotube
diameter with sub-nanometer precision and nanotube
length with precision of a few nanometers.
So far, the researchers have obtained encouraging
results with a model system that produces
aluminosilicogermanate (AlSiGeO) nanotubes. The
research, which was presented August 23rd at the 234th
National Meeting of the American Chemical Society,
could open the door for developing a more general set
of chemical “rules” for dimensional control of
nanotubes that could lead to a range of new
applications for inorganic nanotubes and other
nanometer-scale materials.
The research has been sponsored by the American
Chemical Society Petroleum Research Fund.
“We have shown that there is a clearly quantifiable
molecular-level structural and thermodynamic basis for
tuning the diameter of these nanotubes,” said Sankar
Nair, an assistant professor in Georgia Tech’s School
of Chemical and Biomolecular Engineering. “We’re
interested in developing the science of these
materials to the point that we can manipulate their
curvature, length and internal structure in a
sophisticated way through inexpensive water-based
chemistry under mild conditions.”
Using chemical reactions carried out in water at less
than 100 degrees Celsius, Nair’s research team – which
included graduate students Suchitra Konduri and Sanjoy
Mukherjee – varied the germanium and silicon content
during the nanotube synthesis and then quantitatively
characterized the resulting nanotubes with a variety
of analytical techniques to show a clear link between
the nanotube composition and diameter.
Simultaneously, the group’s molecular dynamics
calculations showed a strong correlation between the
composition, diameter and internal energy of the
material.
“There appear to
be energy minima that favor or stabilize certain
nanotube diameters because they have the lowest
energy, and those stable diameters change with the
composition of the material,” said Nair. “This shows
that the nanotube dimensions are not just a fortuitous
coincidence of the many synthesis parameters, but that
there is an underlying thermodynamic basis arising
from the subtle balance of interatomic forces within
the material.”
Specifically, the molecular dynamics simulations –
which are corroborated by the experiments – show that
the variation of germanium and silicon content causes
sheets of aluminum hydroxide to form nanotubes with
diameters ranging from 1.5 to 4.8 nanometers and
lengths of less than 100 nanometers. If that turns out
to be a general principle applicable to other metal
oxides, it could be used to dramatically expand the
catalog of nanotube structures available.
Once the researchers fully understand the factors
affecting the formation of nanotubes from
aluminosilicogermanate materials, they hope to apply
similar principles to other metal oxides. The ultimate
goal will be an ability to predictably vary the
dimensions of nanotubes – and potentially other useful
nanostructures – employing different chemical process
conditions across a broader range of metal oxide
materials.
“One can get a large range of useful properties with
metal oxide materials,” Nair noted. “Almost all metals
form oxides and many of them form layered sheet-like
oxides, so if one can coax them into nanotube form
with dimensions comparable to single-walled carbon
nanotubes, the range of useful properties would be
great.”
Controlling the dimensions of nanostructures is
critical because properties such as electronic
band-gap depend strongly upon the dimensions.
Dimension control has proven to be difficult in carbon
nanotube fabrication processes, leading to an entire
area of research focused on purifying nanotubes of
specific dimensions from an initial mixture of
different sizes.
“If we are able to produce single-walled nanotubes of
specific and controllable diameter with inexpensive
water-based chemistry, devices based on them would
perform in a consistent and predictable manner,” Nair
explained. “If we could synthesize the same nanotube
structure with predictably different diameters and
lengths, we could tune the properties like the
band-gap across a wide range. We could even get a
limited toolbox of materials to do many different
things.”
Though the chemical reactions that produce the metal
oxide nanotubes are complicated, they occur over a
period of days at low temperatures and can be carried
out with simple laboratory apparatus. That facilitates
control over processing conditions and allows the
researchers to track many different aspects of the
reaction with a variety of characterization tools.
“There is a lot of complex chemistry that can be done
in the aqueous phase, which motivated us to understand
the processes by which metal ions dissolved in water
organize themselves together with oxygen into specific
nanotubular arrangements, perhaps aided by water and
other species present in the solution,” Nair added.
The metal oxide nanotubes have properties very
different from those of carbon nanotubes, which have
been studied heavily since they were discovered in
the 1990s. “For example, the materials that we are
working with are much more hydrophilic than carbon and
can load nearly 50 percent of their weight with
water,” Nair explained. “There is a whole range of
behavior in oxide nanotubes that we cannot explore
with carbon-based materials.”
Other recent results of the group’s research were
published May 5 in the Journal of the American
Chemical Society, and have also been reported in
the journals Physical Review B and
Chemistry of Materials
Source:- Georgia Institute of Technology
Published on the 31st August 2007
