A researcher from the Tokyo Metropolitan University, Kazuhiro Yanagi with his colleagues from Japan has demonstrated that carbon nanotubes are electrochromic.
It is possible to roll graphene sheets into tubes with diameter measuring 1nm or so. These nanotubes are electrochemically stable, mechanically robust, and exhibit superior conductivity. They can display bright colors based on how rolling of the sheet is done.
Carbon nanotube films change color when subject to an applied voltage.
The carbon nanotubes’ optical properties can be changed by varying the electron density within the tube. When a voltage not less than 2 V is applied along the tube, which is immersed in an electrolytic solution, prominent color change is viewed. Past results show that nanotubes exhibit photo-electrochemical instability during these situations. Yanagi together with his colleagues prepared samples that are electrochemically stable utilizing density-gradient purifications and ionic liquids. This mixing reduced the occurrence of sudden electrochemical reactions.
The new electrochromic device comprises a thin carbon nanotube film placed on a glass substrate. The research team utilized three samples with varying tube diameters in order to demonstrate the device’s electrochromic function. On applying -3 V, samples measuring 1.0 nm changed from magenta to yellow-orange. On the other hand 1.4 nm nanotubes turned from blue-green to yellow, while the 0.84 nm nanotubes changed from yellow to light yellow. The three samples turned to normal color on switching off of the voltage.
Yanagi stated that the research team is planning to manipulate the optical absorption producing the yellow color in order to obtain a very fine transparent nanotube sheet. These transparent sheets of nanotubes can be used for electrochromic display applications, he added.