Here come metal-oxide nanotubes

Researchers at the Georgia Institute of Technology are studying a remarkable class of tubular nanomaterials based on metal oxides with silicon and germanium added in.

Metal-oxide nanotubes have properties quite different from those of carbon nanotubes, which have been studied since they were discovered in the 1990s. “For example, the materials we’re working with are much more hydrophilic than carbon and can load nearly 50% percent of their weight with water,” says Sankar Nair, assistant professor in Georgia Tech’s School of Chemical and Biomolecular Engineering. “There is a whole range of behavior in oxide nanotubes that we cannot explore with carbon-based materials.”

The researchers hope to develop guidelines for controlling diameters with subnanometer precision and nanotube lengths with precision of a few nanometers. So far, they have had encouraging results with a method of making aluminosilicogermanate (AlSiGeO) nanotubes.

“We have shown there is a clearly quantifiable, molecularlevel structural and thermodynamic basis for tuning the diameter of these nanotubes,” says Nair. “We’re interested in developing the science to the point we can control curvature, length, and internal structure through inexpensive water - based chemistry.”

Using chemical reactions carried out in water at less than 100°C, Nair’s team varied the germanium and silicon content during nanotube synthesis that showed a clear link between composition and diameter. Simultaneously, calculations showed a strong correlation between composition, diameter, and internal energy of the nanotube. Simulations show that varying germanium and silicon content causes sheets of aluminum hydroxide to form nanotubes with diameters ranging from 1.5 to 4.8 nm and lengths of less than 100 nm.

The ultimate goal is to control the dimensions of nanotubes — and potentially other useful nanostructures — with different chemical-process conditions across a broad range of metal-oxide materials. “Almost all metals form oxides and many of them form layered sheetlike oxides, so if we can coax them into nanotubes with dimensions comparable to single-walled carbon nanotubes, we will have a range of useful properties,” says Nair.

Controlling dimensions of nanostructures is critical because properties such as electronic band-gap largely depend on dimensions. Dimension control has proven difficult in carbon nanotubes, leading to research in extracting only nanotubes of specific dimensions from a mixture of different-sized tubes.