Fabrication and Characterization of Multilayer Ultrathin Films of Exfoliated MnO₂ Nanosheets and Polycations

Abstract

Multilayer ultrathin films composed of exfoliated nanosheets of manganese oxide and poly(diallyldimethylammonium) (PDDA) ions have been fabricated onto various substrates via electrostatic self-assembly technique. Dense monolayer coverage of the substrate surface by the nanosheets was confirmed by atomic force microscopy. UV−vis absorption spectroscopy provided evidence for subsequent growth of multilayer film, exhibiting progressive enhancement of optical absorption due to manganese oxide nanosheets. Evolution of a Bragg peak in the layer-by-layer deposition process also supported the growth of a nanostructured film of PDDA/MnO2 nanosheets with a repeating periodicity of ca. 0.9 nm. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy revealed the inorganic−organic hybrid nature of the obtained multilayer assemblies. Calcination of the films at temperatures above 500 °C removed the polycations, yielding an ultrathin film of manganese oxide Mn2O3. Cyclic voltammogram measurements revealed one reduction peak and two oxidation peaks for a monolayer film, which are attributable to the electrochemical conversion between MnIII and MnIV in manganese oxide nanosheets.