Superhydrophobic SiO2–Glass Bubbles Composite Coating for Stable and Highly Efficient Daytime Radiative Cooling

Abstract

Energy-free radiative cooling is a green and ideal solution to replace air conditioning by reflecting sunlight spontaneously and radiating excess heat through atmospheric transparency windows to outer space for passive cooling. However, most radiative cooling materials are susceptible to contamination by dust, rain, etc., which reduces the cooling capacity in outdoor environments. Herein, we report on a superhydrophobic daytime radiative cooling coating based on SiO2-coated glass bubble (SiO2-GB) powder that achieves strong sunlight reflectivity (96%) and high mid-infrared emissivity (98%), effectively producing an ambient temperature drop of 11.1 °C in direct outdoor sunlight. More importantly, the coating has good superhydrophobic properties with a water contact angle of 157°, which allows the coating to be self-cleaning to keep the coating free from contamination and effectively maintain good radiation cooling performance. In addition, the prepared coatings remain hydrophobic and keep good radiative cooling properties when exposed to different pH solutions and long-term exposure to UV irradiation, which has important implications for sustainable applications, and our work holds great promise for the energy efficiency of building materials and their long-term outdoor service.