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
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 can 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.�
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