Synthesis of one-dimensional hexagonal sodium tungsten oxide and its near-infrared shielding property

Sodium tungstate nanofibers have been successfully synthesized via a facile hydrothermal reaction route in the presence of sulfate. The as-prepared sodium tungstate nanofibers exhibited great homogenity with diameters of 200-300 nm and length of 15-20 μm. The results also suggested that atomic ratio of Na/W and aspect ratio of nanofibers can be easily adjusted by changing the concentration of Na₂SO₄ in the starting materials. After reduction under an atmosphere of H₂ (5 vol.%)/N₂, the sodium tungstate transformed to sodium tungsten bronze without loss of one-dimensional nanostructure. The sodium tungstate (M_xWO_3+x/2) showed no NIR shielding performance, however, the sodium tungsten bronze (M _xWO₃) with high content of sodium showed promising optical characteristics such as high transmittance for visible lights as well as high shielding performance for near-infrared lights, indicating its potential application as solar filter. It was observed that NIR shielding performance increased with increment of Na/W atomic ratio while the transparency in visible light region was almost the same.