Fabrication and performance of scalable bilayer colored daytime radiative cooling coating

Radiative cooling is a passive cooling strategy based on the principles of thermal radiation. Its core mechanism lies in leveraging the substantial temperature difference between the Earth’s surface and outer space (3 K) to dissipate heat into the cold universe via electromagnetic waves through the atmospheric transparency window, thereby achieving ubiquitous cooling. Characterized by zero energy consumption and environmental compatibility, radiative cooling offers a practical and effective approach to reducing escalating cooling energy costs and mitigating global warming, which has garnered significant attention in the fields of energy and materials science. To enhance daytime radiative cooling performance, researchers have developed advanced functional materials such as photonic structures, porous polymers, polymer-dielectric composites, and hierarchical fibers/textiles. These materials optimize solar reflectance to minimize solar heat gain, and typically exhibit white or silvery appearances. However, such signal chromatic characteristics face aesthetic limitations in architectural applications and pose risks of light pollution that may harm visual health. This challenge has driven scientific exploration into colored daytime radiative cooling (CDRC) as a viable solution. However, CDRC coatings designed based on structural coloration mechanisms, such as Tamm plasmon resonance or Bragg diffraction, typically employ nanoscale layer-by-layer deposition techniques to construct multilayered nanocavity architectures. By precisely controlling the thickness and refractive index gradient of functional layers, the selective absorption of narrow-band visible spectra can be achieved. This allows specific colors to be displayed while retaining high near-infrared reflectance. However, the complex structural designs and the high precision requirements in fabrication processes hinder the large-scale application of such multilayer structural-color radiative cooling materials. Therefore, we propose a scalable bilayer-structured CDRC coating for building cooling. The designed coating comprises a spectrally selective top layer and a hierarchical porous polymer underlayer embedded with high-refractive-index dielectric nanoparticles, achieving both vivid coloration and excellent cooling performance. The inorganic colorant nanoparticles in the top layer selectively absorb solar wavelengths complementary to their displayed color, while the porous polymer underlayer strongly reflects full-spectrum solar radiation, thereby significantly reducing solar heat gain. At the same time, the high mid-infrared emissivity of the CDRC coatings can facilitate efficient heat dissipation to outer space. As a result, the CDRC coatings exhibit superior solar reflectance (particularly in the near-infrared regime) and mid-infrared emissivity compared to commercial latex paints of equivalent chromaticity. Under clear-sky conditions with an average solar irradiance of 877.1 W m–2, the developed red, yellow, and blue CDRC coatings demonstrated excellent cooling performance, exhibiting temperature reductions of 18.2, 20.0, and 14.7°C, respectively, when compared to commercial latex paints. The porous underlayer with high reflectivity is fabricated via a non-solvent induced phase separation method. This approach enables large-scale application of the precursor solution onto building facades through brushing techniques, even on irregular surfaces. Simultaneously, the colored top layer is deposited using a simple spray-coating process. Consequently, the developed colored radiative cooling coating exhibits exceptional scalability, combining architectural adaptability with manufacturing efficiency. Furthermore, the CDRC coatings demonstrate robust compatibility with diverse common substrates, including wood, glass, acrylic panels, and steel sheets, significantly expanding their application scenarios. Experimental validation confirms that formulating solutions with red, yellow, and blue inorganic nanoscale pigments enables precise chromatic tuning to meet diverse color requirements. In summary, we propose a scalable bilayer colored radiative cooling coating strategy for eco-friendly building cooling. The developed coatings retain vivid coloration while demonstrating exceptional cooling performance and high scalability, providing a practical and effective solution for green building materials that integrate aesthetic appeal with cooling functionality.