Bismuth-Doped Crystalline/Amorphous WO₃ Composite Film: Superior Electrochromic Performance and Structure Evolution

Tungsten trioxide (WO₃) exhibits exceptional electrochromic properties, positioning it as a promising material for the fabrication of high-performance smart glass devices. Here, a two-step sol-gel approach was employed to synthesize crystalline/amorphous bismuth-doped WO₃ (Bi-WO₃) nanoparticle composite films. The rationally designed composite film demonstrates superior electrochromic performance, featuring a high optical modulation of 82.7%, rapid switching times of 2.1 s for coloring and 2.0 s for bleaching, a prominent coloration efficiency of 102.23 cm²/C at 630 nm, and remarkable cycling stability (retaining 84.5% of the initial optical modulation after 10,200 cycles). These improved properties are ascribed to the complementary advantages of the amorphous and crystalline Bi-WO₃ layers as well as their interface synergy. Furthermore, the study investigates the phase transformation and morphological evolution accompanying performance degradation during continuous electrochromic cycling. Observations reveal that the film surface transitions from small-scale nanoparticles to anisotropic nanosheets, while the crystalline structure evolves from an amorphous state to the orthorhombic WO₃·H₂O. Notably, the open framework structure and reversible proton insertion/extraction behavior of the amorphous Bi-WO₃ layer mitigate irreversible proton trapping, thereby maintaining long cycle stability. This work presents an effective strategy for constructing high-performance WO₃-based electrochromic composite materials by leveraging structural heterojunctions and interface engineering, providing insights into the design of next-generation smart optical devices.