Electrochromic
substances supported on metal oxide nanoparticle
layers have found application in electrodes for electronic paper and
smart windows. In addition to applications of technological importance,
the nanospaces created by the nanoparticles are scientifically important
as they provide specific electrochemical reaction fields. Herein,
the electrochromic properties of the composite film electrodes consisting
of viologen and a metal oxide (SnO2, Al-doped ZnO, and
ZnO) on an indium tin oxide (ITO)-coated glass plate were investigated.
All the electrodes exhibited electrochromism ranging from purple to
white (or yellowish-white). However, the coloring uniformity, coloring
concentration, and response times of coloring/decoloring depended
on the nature of the nanoparticles, the reason for which is discussed
on the basis of the difference in the mixing state of the nanoparticle–viologen
system and the difference in the hydrophilicity of the nanospaces.
The SnO2–viologen/ITO electrode demonstrated the
most promising electrochromic performance. The ZnO–viologen/ITO
electrode exhibited specific electrochromism in that the supported
amount of viologen was controlled by pre-exposure to white light,
thereby allowing image patterns and gradation images to display electrochromic
behavior. Finally, an indium-free electrode, SnO2–viologen/FTO
(fluorine-doped tin oxide-coated glass plate), was prepared. Reflecting
the higher chemical affinity of SnO2 for FTO, the electrode
demonstrated the highest electrochromic performance.