Electrochromic films composed of MnO2 nanosheets with controlled optical density and high coloration efficiency

Electrochemical and electrochromic properties of MnO2 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 MnO2 nanosheets in nonaqueous Li+-containing electrolyte showed well-defined redox peaks which are accounted for by the reduction/oxidation of Mn4+/Mn3+ of the nanosheets associated with the insertion/extraction of Li+ 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 MnO2 nanosheet electrodes, exhibiting electrochromism. The films were bleached with the electrochemical reduction of Mn4+ and reverted to their original state by the reoxidation of Mn3+ of the nanosheets. The coloration efficiency was estimated to be 36.7 cm(2) C-1 at 385 nm. The absorbance changes were proportional to the number of layers of MnO2 nanosheets, suggesting that appropriate changes in optical density can be easily achieved by controlling the number of deposition cycles.