Fabrication and characterization of multilayer ultrathin films of exfoliated MnO2 nanosheets and polycations

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 degreesC 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 Mn-III and Mn-IV in manganese oxide nanosheets.