Solar‐Tunable Reversible Photochromic Smart Window Based on BiOCl/CA Hydrogel

Abstract Traditional smart windows face challenges in concurrently achieving rapid optical modulation and thermal regulation. Addressing this, a photochromic smart window is developed by immobilizing BiOCl nanosheets within a calcium alginate (CA) hydrogel matrix, which exhibits adaptive solar modulation through synergistic photocorrosion‐driven oxygen vacancy（OV ） and metallic Bismuth（Bi ） generation. Under light irradiation, the BiOCl/CA composite demonstrates fast optical switching and enhanced photothermal efficiency, enabled by the co‐action of Bi's surface plasmon resonance (SPR) and OV‐induced bandgap narrowing. The window dynamically regulates solar transmission: it blocks 100% Ultraviolet Rays (UV) radiation while maintaining 50% visible light transmittance post‐coloration, effectively reducing UV damage and glare without compromising indoor visibility. Integrated with double‐glazing, the system achieves dual‐functional thermal management reducing indoor heat gain by 5.6 °C in summer and minimizing thermal loss in winter through CA‐mediated heat dissipation and insulation. Mechanistic studies combining experiments and density functional theory (DFT) reveal that light‐triggered OVs accumulation facilitates electron localization at Bi sites, weakening Bi─O bonds to drive photochromism, while hydroxyl groups in darkness mediate OVs repair via oxygen transfer, enabling self‐recovery. This work establishes a bioinspired strategy for energy‐efficient smart windows, merging human‐centric comfort with sustainable building design.