Multi-scale structural engineering enables thermochromic organic-inorganic hydrogels for robust smart windows.

Thermochromic smart windows have been widely investigated for regulating building energy exchange. However, most reported smart windows suffer from challenges such as low transparency, poor solar modulation ability, a narrow adjustable range of phase change temperature and poor long-term stability, which greatly limit their energy-saving performance. Herein, we report a scalable and recyclable thermochromic organic-inorganic hydrogel through multi-scale structural engineering that involves self-densification at the microscale, thermal-responsive clusters at the nanoscale and amorphous minerals at the molecular scale. The thermochromic hydrogel can rapidly switch between transparent and opaque states through the reversible phase separation of organic-inorganic nanoclusters. The resulting organic-inorganic hydrogel exhibits the highest combination of transparency (99%) and solar modulation capability (86.1%) among reported thermochromic materials, coupled with a broadly tunable transition temperature, long-term stability and recyclability. Additionally, the favorable energy absorption and adhesion properties of the organic-inorganic hydrogel layer endow the assembled smart windows with significantly enhanced impact resistance. The low-cost, easily fabricated, and mass-producible organic-inorganic hydrogel, combining superior thermal regulation, impact resistance and recyclability, offers a sustainable and innovative solution for thermochromic smart windows.