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 <i>via</i> 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. <i>In situ</i> 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.