Annealing induced structural evolution and electrochromic properties of nanostructured tungsten oxide films

• Surfactant-assisted WO3 films have been prepared by sol–gel method.
• Nanostructure of porous WO3 film is retained after crystallization.
• Kinetic properties of WO3 can be improved by nanostructure and crystallinity.

The effect of microstructure on the optical and electrochemical properties of nanostructured tungsten oxide films was evaluated as a function of annealing temperature. The films using block copolymer as the template were prepared from peroxotungstic acid (PTA) by spin-coating onto the substrate and post-annealed at 250–400 °C to form tungsten oxide films with nanostructure. The microstructure of the films was measured by X-ray diffraction and surface electron microscopy. The films annealed at temperatures below 300 °C are characterized by amorphous or nanocrystalline structures with a pore size of less than 10 nm. The evaluated annealing temperature caused a triclinic crystalline structure and microcracks. Cyclic voltammetry measurements were performed in a LiClO4-propylene carbonate electrolyte. The results showed that the ion inserted capacity were maximized for films annealed at 300 °C and decreased with the increasing of annealing temperature. The electrochromic properties of the nanostructured tungsten oxide films were evaluated simultaneously by potentiostat and UV–vis spectroscopy. The films annealed at 300 °C exhibit high transmission modulation (∆T ~ 40%) at λ = 633 nm and good kinetic properties. As a result, the correlation between the microstructure and kinetic properties was established, and the electrochromic properties have been demonstrated.