Titanoniobate and niobate nanosheet photocatalysts: superior photoinduced hydrophilicity and enhanced thermal stability of unilamellar Nb3O8 nanosheet

Abstract

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 (TiNbO₅, Ti₂NbO₇, Ti₅NbO₁₄, and Nb₃O₈) 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 Nb₃O₈ 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 TiO₂. Moreover, the monolayer film of Nb₃O₈ nanosheet was found to have enhanced thermal stability and chemical resistance, particularly against diffusion of sodium ions at elevated temperature: Nb₃O₈ nanosheet film heated on sodium-rich glass (soda-lime glass) substrate maintained excellent hydrophilic conversion activity, whereas Ti_0.87O₂ nanosheet as well as anatase (TiO₂) based photocatalysts was virtually deactivated. These features are a great advantage of Nb₃O₈ nanosheet photocatalysts for developing the practical super-hydrophilic applications where post-annealing is indispensable.