Porous Orthorhombic Tungsten Oxide Thin Films Applied in Electrochromic Device and Photochromic Device

Tungsten oxide, also known as tungsten trioxide or tungstic anhydride, WO3, is a chemical compound containing oxygen and the transition metal tungsten. It is obtained as an intermediate in the recovery of tungsten from its minerals. The tungsten oxide nanoparticles can be applied in many areas. They can be used in colorant and analysis reagent of chinaware,  used in producing metal tungsten material, gas sensors,  fire-proofing fabrics, imaging; large-area displays, catalysts; ceramic pigments; humidity sensors, infrared switching devices; high-density memory devices and so on.

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℃. 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 shows 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 baseddye. 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 energy saving smart windows.

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