Electrochromic behavior of WO3 nanotree films prepared by hydrothermal oxidation

Hexagonal structured WO3 films with tree-like morphology were synthesized on tungsten foils by a hydrothermal method. Each nanotree was composed of several (typically six) nanosheet-shaped “branches”. TEM examination revealed that the nanosheet was a single crystal and its long axis was oriented toward 〈0 0 1〉 direction. The WO3 nanotree films retain the hexagonal structure after being annealed up to 400 °C for 2 h, while they have a phase transition to monoclinic structure after being annealed at 500 °C for 2 h. Electrochromic (EC) performance of the films was examined in a propylene carbonate solution of 1 M LiClO4 using an electrochemical workstation and an UV–Vis spectrometer. Due to the large tunnels of hexagonal structure and highly porous surface morphology, a large modulation of optical reflectance of WO3 nanotree films up to 30% and coloration efficiency of 43.6 cm2 C−1 at 500 nm were achieved by annealing the WO3 nanotree films at 400 °C for 2 h.