Modulating the Refractive Index of a Three-Dimensional Alumina Network for Dynamic and Colorful Radiative Cooling

Abstract

An ideal radiative cooler requires high emissivity in the atmospheric transparency window and low solar energy absorption. Currently, most radiative coolers typically reflect sunlight in white, limiting their aesthetics for various applications. Achieving vibrantly colored radiative coolers poses a considerable barrier, since colors mostly are generated by absorbing visible light, conflicting with the demand of low solar absorption. In this work, we propose a refractive index modulation scheme based on a three-dimensional anodic alumina network (3D-AAN), which achieves nonabsorbing and color-switchable radiative cooling according to dynamic photonic bandgap principle. By using pulse anodizing and selective etching methods, we construct transverse channels inside conventional anodic alumina films, connecting hexagonal vertical channels to form a large-scale 3D-AAN with a colorful appearance. Outdoor daytime cooling experiments demonstrated that colored 3D-AAN can be 2.6 °C lower than the ambient temperature on average and can achieve an average temperature drop of 7.0 °C on the silicon substrate under sunlight. Meanwhile, a tunable color and cooling power can be achieved by reversibly wetting 3D-AAN with fluids. This scheme provides an attractive option for colorful radiative cooling demand in buildings and vehicles and inspires further development in photonic design for dynamic radiative cooling.