Hexagonal Tungsten Oxide Based Electrochromic Devices: Spectroscopic Evidence for the Li Ion Occupancy of Four-Coordinated Square Windows

• Published in: Chemistry of materials 2009-04, Vol.21 (7), p.1381-1389
• Main Authors: Balaji, Subramanian, Djaoued, Yahia, Albert, André-Sébastien, Ferguson, Richard Z, Brüning, Ralf
• Format: Article
• Language: English
• Subjects: Characterization of Materials; Coatings, Thin films, and Monolayers; Layered Materials (including Intercalation Compounds and Layer-by-Layer Assembly)
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/cm8034455

Macroporous hexagonal WO3 (h-WO3) films were obtained at 400 °C from a sol containing tungstic acid with organically modified silane as a template. Asymmetric electrochromic devices based on the macroporous h-WO3 layer were constructed. XRD and micro-Raman studies of the intercalation/deintercalation of lithium into the h-WO3 layer of the device as a function of the applied voltages were performed. In h-WO3, Li+ can be intercalated into three potential sites: trigonal cavity (TC), hexagonal window (HW), and four-coordinated square window (SW). XRD measurements show systematic changes in the lattice parameter, which was associated with the amount of Li intercalated into the h-WO3 layer. Correspondingly, Raman spectroscopy shows that at 1.0 V Li+ completely fill TC and partially fill HW sites. For potentials ≥1.5 V, Li+ are inserted into the SW, as evidenced from the vanishing of the ν(O−W−O) Raman modes. The reversible characteristics of the device from optical measurements and Raman spectra demonstrated that the coloration process in the electrochromic device is mainly due to the Li+ that occupy HW and SW sites of the h-WO3. Optical measurements performed as a function of applied potentials, show excellent contrasts between colored and bleached states and qualifies the macroporous h-WO3-based device for smart window applications.