Electrochromic Properties of Metal Oxide Nanoparticles/Viologen Composite Film Electrodes

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.