Porous orthorhombic tungsten oxide thin films: synthesis, characterization, and application in electrochromic and photochromic devices

Abstract

Porous orthorhombic tungsten oxide (o-WO₃) thin films, stabilized by nanocrystalline anatase TiO₂, are obtained by a sol–gel based two stage dip coating method and subsequent annealing at 600 °C. An Organically Modified Silicate (ORMOSIL) based templating strategy is adopted to achieve porosity. An asymmetric electrochromic device is constructed based on this porous o-WO₃ layer. The intercalation/deintercalation of lithium ions into/from the o-WO₃ layer of the device as a function of applied coloration/bleaching voltages have been studied. XRD measurements show systematic changes in the lattice parameters associated with structural phase transitions from o-WO₃ to tetragonal Li_xWO₃ (t-Li_xWO₃) and a tendency to form cubic Li_xWO₃ (c-Li_xWO₃). These phase transitions, induced by the Li ions, are reversible, and the specific phase obtained depends on the quantity of intercalated/deintercalated Li. Raman spectroscopy data show the formation of t-Li_xWO₃ for an applied potential of 1.0 V and the tendency of the system to transform to c-Li_xWO₃ for higher coloration potentials. Optical measurements show excellent contrasts between colored and bleached states. An alternate photochromic device was constructed by sensitizing the o-WO₃ layer with a ruthenium based dye. The nanocrystalline anatase TiO₂ in the o-WO₃ layer has led to an enhanced photochromic optical transmittance contrast of ∼51% in the near IR region. The combination of the photochromic and electrochromic properties of the synthesised o-WO₃ layer stabilized by nanocrystalline anatase TiO₂ opens up new vista for its application in energysaving smart windows.