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

Macroporous hexagonal WO3 (h-WO3) films were obtained at 400 degrees 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 v(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.