Engineering the optical properties of SrZrO₃ crystals via Zn doping for superior passive daytime radiative cooling

As global energy consumption rises, developing efficient energy-saving materials is crucial for alleviating energy pressure. Passive daytime radiative cooling materials are promising for energy-free refrigeration. The wide band gap and the absorption of infrared photons through lattice vibrations of the SrZrO₃ crystal make it a potential passive radiative cooling material, but its solar reflectivity and atmospheric window emissivity can be further enhanced. For this purpose, a series of Zn-doped SrZrO₃ crystals were prepared by the sol–gel method combined with the solid-phase synthesis method. The effects of Zn doping on the phase structure, electronic structure, spectral radiative characteristics, and passive daytime radiative cooling performance of SrZrO₃ crystals were investigated by experiments and theoretical calculations. The results show that, on one hand, the changes in the grain morphology and electronic structure caused by Zn doping jointly improved the reflection performance of the crystals in the wavelength of 0.3–2.5 μm. On the other hand, the lattice distortion and decrease in the lattice symmetry caused by Zn doping lead to an increase in the photon emissivity for the 8–13 μm band. The solar reflectivity of the SrZr_0.75Zn_0.25O_2.75 crystal powder can reach 0.892, whereas the atmospheric window emissivity can reach 0.954, both of which are greater than that of SrZrO₃. The coating prepared with the SrZr_0.75Zn_0.25O_2.75 crystal powder as the radiative refrigerant can reach a maximum radiative cooling temperature of 15.3 °C under a solar irradiation of 654 W·m⁻², and the maximum net radiative power is 64.7 W·m⁻². These results indicate that SrZr_0.75Zn_0.25O_2.75 is an excellent passive daytime radiative cooling material.