Visible-Light-Driven Photochromism of Hexagonal Sodium Tungsten Bronze Nanorods

Abstract

Single-crystalline sodium tungsten bronze (Na-WO3) nanorods with typical diameters of 10–200 nm and lengths of several micrometers were prepared via hydrothermal synthesis. The as-prepared Na-WO3 nanorods crystallized in a hexagonal structure (space group P6/mmm) with unit cell parameters a = 7.3166(8) Å and c = 3.8990(8) Å and elongated along the ⟨001⟩ direction. Chemical analyses indicated a stoichiometry of Na0.18WO3.09·0.5H2O, revealing the existence of tunnel Na+ ions and water molecules in the structure, as confirmed also by the vibrational spectroscopic study. The as-prepared Na-WO3 nanorods exhibited a direct-allowed electronic transition with band-gap energy of about 2.5 eV, which allows a visible-light-driven photochromism related to photogenerated carriers and a proton–electron double injection process. The proposed photochromism was discussed in detail by means of Fourier transform infrared spectroscopy. The involved local structural evolutions such as water decomposition and ion intercalation during the photochromic process were identified.