We demonstrated that nanotubes synthesized from a NaOH treatment on TiO 2 with subsequent acid washing could proceed with repeatable crystalline-structure transformation through a simple acid-base washing step. By providing the unit cell parameters, we identified a divalent salt titanate (Na 2 Ti 2 O 5 ‚ H 2 O) with a layered structure as the structure formed after the NaOH treatment. With the increase in acidity during the post-treatment acid washing, the layered titaniate transformed into a nanotube through Na + -H + substitution, and eventually transformed into anatase TiO 2 . Crystalline-structure analysis has shown the feasibility of this titanate-titania transformation occuring through a simple structural rearrangement. A complete scheme for the formation and transformation of nanotubes induced by the NaOH treatment and the post-treatment washing was proposed.
Perovskite‐like NaTaO3 powders have potential applications in photoluminescence and photocatalysis. Sol–gel, hydrothermal and solid‐state methods were used to synthesize NaTaO3 powders of different crystalline structures, which were identified by Rietveld refinement simulation of X‐ray diffraction patterns and transmission electron microscopic diffraction. The refinement results show that the sol–gel specimen has a monoclinic phase with a Ta−O−Ta bond angle of 179° while the hydrothermal and solid‐state specimens have an orthorhombic phase with bond angles of 163° and 157°, respectively. By excitation with a 304 nm light source, these NaTaO3 specimens show photoluminescence emission at ca. 450 nm. The photoluminescence intensity of the specimens had an order solid state >hydrothermal >sol–gel, which is opposite to that of the Ta−O−Ta bond angle. On the other hand, the photocatalytic activity of the NaTaO3 specimens in water splitting showed the same order as that of the Ta−O−Ta bond angle. This paper directly evidenced that the Ta−O−Ta bond angle affects the separation rate of the photo‐induced charges, as well as that structure tuning of tantalates is achievable and crucial for applications in photoluminescence and photocatalysis.
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