Scalable and waterborne titanium-dioxide-free thermochromic coatings for self-adaptive passive radiative cooling and heating

Titanium dioxide (TiO₂) is the most widely used white pigment for coatings, plastics, papers, and cosmetics, but its high price, high-pollution production process, and potential cancer risks have led to scientists and engineers attempting to find alternatives. However, little progress has been made thus far. Herein, we report the fabrication of TiO₂-free coatings by mixing common solvent-based or waterborne resins and hollow glass beads of specific sizes. The TiO₂-free white coatings exhibit much higher solar reflectance (∼0.96) and longwave infrared near-normal emittance (∼0.94) than many TiO₂-based coatings, thus realizing highly efficient subambient cooling of ∼7.8°C at night and ∼7.1°C under direct sunlight. Specifically, the scalable and self-adaptive subambient radiative cooling and above-ambient solar heating coatings (ΔT_{cooling-heating} = 9.5°C) are demonstrated when reversible thermochromic microcapsules are added, which significantly expands the scope of year-round energy-saving applications.