Photo-thermochromic W18O49/hydrogel hybrid smart windows for graded and dual-band sunlight control

Thermochromic hydrogel-based smart windows (SWs) have received wide attention due to their high solar energy modulation efficiency by switching transparent and opaque state in the visible region. However, SWs are opaque to visible light rather than near-infrared (NIR) light at high temperatures, and the very limited NIR modulation greatly limits their energy-saving effect and practicality. Herein, we propose a reversible photo-thermochromic hybrid smart window (SW) consisting of photochromic W18O49 and thermochromic polyacrylamide-poly (N-isopropylacrylamide) (PAM-PNIPAM) hydrogel to address above issues. It enables graded and dual-band sunlight regulation. In the initial state, the SW is transparent to visible and NIR light with luminous transmittance (Tlum) of 79.49 %. As solar irradiation increases, it gradually blocks near-infrared light and maintains Tlum due to the ultraviolet (UV)-triggered photochromic effect of W18O49. The increase in indoor temperature is delayed. When the solar irradiation reaches the maximum, the increase in ambient temperature and the photothermal effect of colored W18O49 trigger the thermochromism of the hydrogel, thus the full-band sunlight is blocked and the sunlight transmittance is 0 %. The SW can reduce indoor temperature by 8.5 °C. This work sheds light on saving energy by fully utilizing solar energy through muti-mode switching in hybrid SW.