Ultrathin WO 3 ·H 2 O nanosheets derived from the Aurivillius phase for high-performance dual-band electrochromic smart windows

The performance of electrochromic materials is determined by the ion and electron transport. The ion diffusion within active materials generally represents a fundamental limitation for improving electrochromic performance. To enable efficient ion diffusion and ion insertion/extraction, reducing the size and thickness of the active materials is highly desirable. Herein, we report a facile top-down approach for efficiently producing ultrathin WO₃·H₂O nanosheets via ion etching of Aurivillius phase Bi₂WO₆ and liquid-phase exfoliation. Thanks to the ultrathin nanosheet with a thickness of ∼4.5 nm, the WO₃·H₂O film achieves excellent dual-band modulation of 90% at 633 nm and 90.5% at 1200 nm, fast switching within 10 s, and remarkable cycling stability of 5000 cycles. In situ measurements disclose that reversible valence change and negligible structural strain of ultrathin nanosheets enable stable electrochromism of WO₃·H₂O. Furthermore, self-powered and large-scale smart windows are demonstrated using the WO₃·H₂O film and Al foil, realizing a remarkable temperature modulation of 12 °C in a house model. The simulation results reveal that the smart window can reduce the energy consumption of buildings by up to 20.7% (168.8 MJ m^-2), demonstrating its excellent thermal regulation and energy-saving capabilities. This ultrathin nanosheet fabrication approach breaks through the inherent inaccessibility of ions within active materials, shedding new light on the exploration of next-generation high-performance electrochromic materials and devices.