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

Abstract

Macroporous hexagonal WO 3 (h-WO 3 ) 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-WO 3 layer were constructed. XRD and micro-Raman studies of the intercalation/deintercalation of lithium into the h-WO 3 layer of the device as a function of the applied voltages were performed. In h-WO 3, 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-WO 3 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-WO 3 . Optical measurements performed as a function of applied potentials, show excellent contrasts between colored and bleached states and qualifies the macroporous h-WO 3 -based device for smart window applications.