Inorganic Multilayer Films of Manganese Oxide Nanosheets and Aluminum Polyoxocations: Fabrication, Structure, and Electrochemical Behavior

Inorganic multilayer films of exfoliated MnO 2 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 H 2 O 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 MnO 2 nanosheets in multilayer films was electrochemically active.