A scanning electron microscope image of the side of a stack of nanosheets. The inset is an optical microscope image of a single exfoliated nanosheet, to show it is optically transparent.

Cornell
materials scientists have developed an inexpensive, environmentally
friendly way of synthesizing oxide crystal sheets, just nanometers
thick, which have useful properties for electronics and alternative
energy applications.

The
work, led by Richard Robinson, assistant professor of materials science
and engineering, is featured on the cover of the April 7 Journal of
Materials Chemistry (Vol. 22, No. 13).

The
millimeter-length, 20-nm-thick sodium-cobalt oxide crystals were
derived through a novel method that combined a traditional sol-gel
synthesis with an electric field-induced kinetic de-mixing step. It was
this second step that led to the breakthrough of a bottom-up synthesis
method through which tens of thousands of nanosheets self-assemble into a
pellet.

The
material has fascinating properties, Robinson said, including high
thermoelectric power, high electrical conductivity, superconductivity
and potential as a cathode material in sodium ion batteries.

Usually
oxide materials, like a ceramic coffee mug, aren’t electrically
conductive; they’re insulating, Robinson said. Since the material is a
conductive oxide, it can be used in thermoelectric devices to convert
waste heat into power. Now that the researchers have made nanosheets,
they expect the material’s thermoelectric efficiency to improve,
enabling the creation of more efficient alternative energy
thermoelectric devices.

The
nanosheets also show the ability to bend, sometimes up to 180 degrees,
Robinson added. This is unusual for ceramics, which are normally
brittle.

The
material is based on common, abundant elements (sodium, cobalt and
oxygen), without toxic elements, such as tellurium, that are normally
used in thermoelectric devices.

The
paper’s co-authors are graduate students Mahmut Aksit and David Toledo.
The work was supported by the National Science Foundation and the U.S.
Department of Energy, through the Energy Materials Center at Cornell
(EMC2).

Scalable nanomanufacturing of millimetre-length 2D NaxCoO2 nanosheets

Source: Cornell University