Electrochromic Films Composed of MnO[sub 2] Nanosheets with Controlled Optical Density and High Coloration Efficiency

Electrochemical and electrochromic properties of nanosheet electrodes prepared by layer-by-layer assembly on transparent conductive indium-tin oxide substrates were examined. Cyclic voltammograms (CVs) of multilayer film electrodes of nanosheets in nonaqueous -containing electrolyte showed well-defined redox peaks which are accounted for by the reduction/oxidation of of the nanosheets associated with the insertion/extraction of ions into/from the nanosheet galleries. The magnitude of the redox peaks was linearly enhanced as the number of layers increased. The nanosheets themselves were stable against the CV cycles, as revealed by in-plane X-ray diffraction, although the regular stacking structure of nanosheets was lost in the initial several cycles of potential sweeps. The redox processes induced changes in the UV-visible absorption spectra of nanosheet electrodes, exhibiting electrochromism. The films were bleached with the electrochemical reduction of and reverted to their original state by the reoxidation of of the nanosheets. The coloration efficiency was estimated to be at 385 nm. The absorbance changes were proportional to the number of layers of nanosheets, suggesting that appropriate changes in optical density can be easily achieved by controlling the number of deposition cycles.