Self-assembled growth of surface-fluorinated TiO2 nanocrystal films with superior dual-band electrochromic and energy storage performance

Surface-fluorinated TiO2 nanocrystal (NC) films have been obtained on FTO substrates by the one-pot solvothermal reaction. It is found that the fluorine anions play a critical role in generating oxygen vacancies, and triggering the self-assembly of the NCs to form porous structures as well as modulating the NC sizes. Meanwhile, pristine TiO2 films show simultaneous electrochromic regulation behavior in the near-infrared and visible regions under electrochemical bias, which reveals that the surface-fluorinated TiO2 films demonstrate selective and independent modulation in the two regions with high optical modulation (77.5% at 700 nm and 62.5% at 1300 nm); unprecedentedly fast electrochromic response (coloring time of 12.8 s at 700 nm and 10.2 s at 1300 nm; bleaching time of 4.3 s at 700 nm and 4.8 s at 1300 nm); and superior cycling stability, respectively. The films also exhibit three typical energy efficient working modes and promising energy storage characteristics. In addition, the electrochromic reaction kinetics is scrutinized, and it is investigated that the effect of oxygen vacancies is of importance to realize NIR selective modulation. The present work may prove to provide an alternative approach for designing high performance TiO2-based single-component dual-band energy efficient electrochromic devices.