In Operando X‑ray Absorption Fine Structure on Local Structure and Electronic State of Tungsten Oxide Nanoparticles with Different Crystal Structures under Electrochromism

The electrochromism (EC) of tungsten oxide (WO3–x ) is known to depend on its crystal structure. However, detailed information on the correlation between color, electronic, and local structural change in WO3–x with different crystal structures was missing. In this study, nanoparticles which were either monoclinic W18O49 or hexagonal (NH4)0.75W3O9 phases were used to make films (m-Film or h-Film) for the investigation and comparison of their EC behavior based on in operando X-ray absorption fine structure (XAFS). Based on color analysis, the h-Film was found to have a higher reversibility in color change than the m-Film. XAFS analysis of the W L3-edge showed periodic changes of the t2g and eg energy levels under repeated coloring/bleaching cycles, which was caused by the ligand field change upon Li+ intercalation/deintercalation. An irreversible increase in the eg level, decrease of the area intensity, and increase of W–O bond distance in the first shell after repeated cycles were observed in the m-Film but not in the h-Film, which suggested that distortions were accumulated in the local structure of the m-Film resulting in color deterioration. In the case of the h-Film, the reason for the color deterioration was explained by the trapping of Li+ in the optically inactive trigonal cavities. These results clearly indicate that different crystal structures have different color deterioration mechanisms.