Hetero-nanostructured Films of Titanium and Manganese Oxide Nanosheets:  Photoinduced Charge Transfer and Electrochemical Properties

Multilayer heterostructured films of two different nanosheets, semiconducting Ti0.91O2 nanosheets and redoxable MnO2 nanosheets, were fabricated by the layer-by-layer sequential adsorption method using a polyelectrolyte as a linker. Successful deposition of these two nanosheets in various sequences was confirmed by monitoring UV−visible absorption spectra and X-ray diffraction data on the films. We examined how nanostructures of the heteroassembled films affect the behaviors of photoinduced electron transfer between the two types of nanosheets and electrochemical properties of composite film electrodes. The excitation of Ti0.91O2 nanosheets under light irradiation at 280 nm was found to cause a gradual decrease in intensity of the optical absorption peak at 372 nm, which can be ascribed to the reduction of Mn4+ to Mn3+ of MnO2 nanosheets as confirmed by XANES spectra. This behavior can be understood by a scheme that the excited electrons produced in Ti0.91O2 nanosheets are injected into MnO2 nanosheets. The injection rate depended on the nanostructures of the multilayer films. Electrochemical studies of composite multilayer films showed an additional oxidation peak in comparison with those for films solely composed of MnO2 nanosheets, which may be accounted for by the extraction of Li+ ions inserted into the interlayer gallery with a hetero-environment between Ti0.91O2 and MnO2 nanosheets.