Spectroscopic properties of hexagonal sodium tungstate nanorods

Sodium tungstate (Na-WO₃) nanorods with typical diameters of 10-200 nm and lengths of several microns were prepared via hydrothermal synthesis. X-ray diffraction showed that the material crystallized in a hexagonal phase (space group P6/mmm) with unit cell dimensions of a = 7.3166(8) Å and c = 3.8990(8) Å. The as-prepared Na-WO₃ nanorods showed a distinctive visible-light-driven photochromism related to a proton-electron double injection process. The involved local structural evolutions were monitored by Fourier transform infrared (FTIR) and Raman scattering spectroscopy. One diagnostic FTIR absorption at 585 cm^-1 and one Raman band at 813 cm^-1 were identified and assigned to the O-W-O stretching vibration. These two modes were strongly affected by the proton and electron insertion, showing promises for studying the chromogenic properties of hexagonal WO₃ materials.