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

TL;DR

This study uses spectroscopy to understand how protons and electrons change the structure and properties of hexagonal tungsten oxides during electrochromic processes.

Contribution

The paper reveals how proton insertion affects structural and electronic properties of HTOs, offering new insights into electrochromic mechanisms.

Findings

Proton insertion in Na-WO3 nanorods modifies lattice parameters and reduces local symmetry.

Protons prefer small trigonal windows, and their insertion is linked to water molecule dynamics in larger tunnels.

Electrochromic coloration leads to band gap narrowing and a new near-infrared absorption band due to charge injection.

Abstract

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.

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