Titanoniobate and niobate nanosheet photocatalysts: superior photoinduced hydrophilicity and enhanced thermal stability of unilamellar Nb_3O_8 nanosheet

摘要

Unique photocatalysts based on novel two-dimensional (2D) oxide nanosheets have been synthesized and their photochemical activity has been examined. Monolayer films of titanoniobate and niobate (TiNbO_5, Ti_2NbO_7, Ti_5NbO_(14), and Nb_3O_8) nanosheets, synthesized by exfoliating layered oxide precursors through a soft-chemical procedure, were fabricated on quartz glass substrate via a sequential adsorption method. All the nanosheet films exhibited good photoinduced hydrophilicity, while their oxidation activity was very low. This behavior can be regarded as inherent features of nanosheet-type photocatalysts having molecularly thin 2D anisotropy. Such ultrathin flexible structures are advantageous for facilitating photo-driven surface wettability change. Especially, the hydrophilic conversion property of Nb_3O_8 nanosheet was highly efficient, showing activity that was at least one order of magnitude superior to that of the widely used photocatalyst film of polycrystalline anatase TiO_2. Moreover, the monolayer film of Nb_3O_8 nanosheet was found to have enhanced thermal stability and chemical resistance, particularly against diffusion of sodium ions at elevated temperature: Nb_3O_8 nanosheet film heated on sodium-rich glass (soda-lime glass) substrate maintained excellent hydrophilic conversion activity, whereas Ti_(0.87)O_2 nanosheet as well as anatase (TiO_2) based photocatalysts was virtually deactivated. These features are a great advantage of Nb_3O_8 nanosheet photocatalysts for developing the practical super-hydrophilic applications where post-annealing is indispensable.