Solvothermally grown WO3·H2O film and annealed WO3 film for wide-spectrum tunable electrochromic applications

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Tungsten trioxide (WO 3 ) is a classical electrochromic (EC) material with advantages of abundant reserves, high coloration efficiency and cyclic stability. However, WO 3 films are often accompanied by a narrow spectrum of modulation due to a single-color change from transparent to blue. In this work, we report a wide-spectrum tunable WO 3 ·H 2 O nanosheets EC film solvothermally grown on fluorine-doped tin oxide (FTO) glass. Interestingly, the crystalline WO 3 ·H 2 O nanosheets film is transformed into amorphous WO 3 after annealing at 250 °C for 1 h. The amorphous film can be transformed into crystalline WO 3 film by increasing the annealing temperature to 450 °C. After annealing at 250 °C, the WO 3 film exhibits an optical modulation of 75.8% in a broad solar spectrum range of 380–1400 nm and blocks 88.9% of solar irradiance. Fast switching responses of 4.9 s for coloration and 6.0 s for bleaching, and a coloration efficiency of 86.4 cm 2 C −1 are also achieved. Additionally, the WO 3 film annealed at 250 °C also demonstrates an excellent cyclic stability, where 99.6% of the initial optical modulation can be retained after 1500 cycles. This simple and mild solvothermal method used in this work provides a new idea for the preparation of wide-spectrum tunable WO 3 EC films.