Construction and characterization of tunable meso-/macroporous tungsten oxide-based transmissive electrochromic devices

Recent developments in the processing of porous transition metal oxide thin films have opened up new opportunities in the construction of electrochromic (EC) devices with enhanced properties. Tungsten oxide (WO₃) is one of the most promising materials for EC devices. In this work, we report on the synthesis of meso-/macroporous WO₃ thin films using tungstic acid as precursor and organically modified silane (ORMOSIL) as a templating agent. Calcination of the film at 500 °C resulted in nanocrystalline monoclinic tungsten oxide (m-WO₃) with crystallite sizes of ~16 nm. The meso-/macroporous structure is retained after calcination. An asymmetric EC device based on the meso-/macroporous m-WO₃ thin film was constructed. Thus constructed EC device was characterized by micro-Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction (XRD). Detailed micro-Raman and XRD studies of the intercalation and deintercalation of lithium (Li) in the meso-/macroporous m-WO₃ layer of the EC device as a function of the applied voltage were performed. The meso-/macroporous WO₃ layer, which was found to be monoclinic before Li intercalation, transforms to its higher symmetric phase of tetragonal and toward cubic phase when Li is intercalated into it. Upon complete deintercalation, these phase transitions were reversed and m-WO₃ is recovered. Optical transmission studies were performed in conjunction with Raman and XRD studies. This study shows that meso-/macroporous m-WO₃ layer-based EC device exhibit a high color contrast during the coloration and bleaching.