Metal Oxide Nanosheets Synthesized by Bottom-up Approach: Applications to Catalyst and Separation Membranes

Two-dimensional (2D) transition metal oxide nanosheets have useful characteristics such as high surface areas, and defined and controllable surface acidity, with potential for applications in solid acid catalysts and photocatalysts. The most common synthesis method to obtain 2D metal oxide nanosheets is the "exfoliation method" from the corresponding bulk layered oxide by the top-down approach. However, fragmentation and re-aggregation of nanosheets are some of the problems. In addition, this method requires calcination processes for the synthesis of layered compounds at high temperature and prolonged exfoliation treatment of the 2D crystal layer. In this study, layered titanate nanosheets and single layer niobate nanosheets were formed by the bottom-up approach utilizing hydrolysis and condensation of metal alkoxides in the presence of surfactants. Formation processes of the metal oxide nanosheets and effects of surface modification by the surfactants on their properties were studied. In addition, these nanosheets were evaluated for photocatalytic reactions such as photodegradation of organic dyes and hydrogen evolution from water decomposition. Composites of niobate nanosheets with other types of 2D nanosheets (graphene oxide and MoS2) achieved effective photocatalytic activities. Furthermore, stacked nanosheet membranes can be formed by assembling nanosheets. The nanosheet membranes have stable structures in water and high separation performance against aniomic dyes and salts, suggesting high potential as separation membranes.