Spectroscopic Insights into the Electrochromism of Hexagonal Tungsten Oxides (HTOs)

Spectroscopic techniques, including X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy, were employed to unravel the microscopic details of proton insertion and extraction in hexagonal tungsten oxides (HTOs), thereby providing new insights into the underlying electrochromic mechanisms. Hexagonal sodium tungsten bronze (Na-WO₃) was selected as a representative HTO material and prepared hydrothermally. The resulting Na-WO₃ nanorods had typical diameters of 10-200 nm and lengths of several microns and crystallized in a hexagonal structure (space group P6/mmm, No. 191) with unit cell parameters a = 7.3166(8) Å and c = 3.8990(8) Å and elongated along the ⟨001⟩ direction. Proton insertion during the electrochromic (EC) coloration process induced significant changes in both long-range and local structural order, as evidenced by modified lattice parameters (a = 7.4192(6) Å, c = 7.5440(2) Å) and reduced local symmetry (space group P6₃/mcm, No. 193) in the EC colored Na-WO₃ nanorods. The inserted protons preferentially occupied small trigonal windows rather than larger trigonal cavities, indicating a selective intercalation behavior analogous to that observed in the large hexagonal tunnels. FTIR analysis revealed that proton insertion and extraction in the small trigonal tunnels were closely coupled with the dynamics of water molecules residing in the large hexagonal tunnels. These ionic processes were accompanied by electron transfer, resulting in substantial modifications to the electronic structure of the material. Band gap narrowing (from 2.5 to 1.75 eV) and the emergence of a new near-infrared absorption band (centered at 1155 nm) were observed in the EC colored Na-WO₃ nanorods, which were attributed to increased free electron densities arising from proton-electron double charge injection.