Inorganic multilayer films of manganese oxide nanosheets and aluminum polyoxocations: Fabrication, structure, and electrochemical behavior

Inorganic multilayer films of exfoliated MnO2 nanosheets and aluminum Keggin ions have been fabricated onto various substrates via sequential adsorption. Ultraviolet-visible absorption spectra in the multilayer buildup process indicated that nearly equal amounts of nanosheets could be assembled for each deposition cycle. X-ray photoelectron spectroscopic quantitative analysis and structural consideration based on charge balance indicate that aluminum Keggin ions should be sparsely distributed in the nanosheet galleries, occupying only 13% of the gallery area. This low density of Keggin ions can explain a Bragg peak observed with a relatively small periodicity of 0.73 nm, suggesting that a major gallery area accommodates H2O molecules. This unique structure is different from ordinary multilayer structures, in which two components alternate, contributing to the film thickness with their own sizes. The multilayer film showed a very stable cyclic voltammogram involving reversible redox reactions of Mn-III/Mn-IV, providing new potential electrodes for electrochemical applications. The observed electron transfer suggests that a predominant amount of MnO2 nanosheets in multilayer films was electrochemically active.