Carbon nanotube films change color when subject to an applied voltage. (© 2011 Wiley-VCH) |
Smart
glass can change color or even go from opaque to transparent with just
the flick of a switch. Indium tin oxide is used as an electrical contact
in many of these ‘electrochromic’ devices because it is both
transparent to visible light and a good conductor of electricity. But
indium and tin are both becoming increasingly expensive as the global
supply diminishes.
Kazuhiro
Yanagi from the Tokyo Metropolitan University, working with colleagues
from across Japan, has now shown that carbon could be the perfect
replacement.
Graphene
sheets, consisting of a single atomic layer of carbon atoms in a
honeycomb framework, can be rolled into a tube just a nanometer or so in
diameter. These carbon nanotubes are highly conductive, mechanically
strong, electrochemically stable and can show bright colors depending on
how the sheet is rolled. Yanagi and his team have now shown that carbon
nanotubes are also electrochromic.
The
optical properties of carbon nanotubes can be altered by changing the
density of electrons in the tube. Visible color change is achieved by
applying a voltage of at least 2 V across tube when suspended in an
electrolyte solution. Previous research has suggested that the nanotubes
become photo-electrochemically unstable under these conditions. Yanagi
and his colleagues, however, were able to prepare samples with good
electrochemical stability using ionic liquids and density-gradient
purifications. This combination reduced possible unexpected
electrochemical reactions.
The
novel electrochromic device consisted of a thin film of carbon
nanotubes on a glass substrate. The team demonstrated the electrochromic
function of their device using three different samples with different
tube diameters. On application of a –3 V potential, 1.4 nm-diameter
nanotubes went from a blue-green in color to yellow, 1.0 nm tubes turned
from magenta to yellow-orange, and the initially yellow 0.84 nm sample
changed to light yellow (see image). In all cases, the color returned to
normal when the voltage was switched off. “Next, we would like to
control the optical absorption causing the yellow color so we can get a
highly transparent sheet of nanotubes, which could be important for
electrochromic display applications,” says Yanagi.
