SCIENCE CHINA Materials2026
Overcoming the aesthetic limits of radiative cooling via single-step self-stratification
The accelerating pace of urbanization and rising global temperatures have transformed reliable cooling from a luxury into a fundamental necessity for human health and economic activity. With urban populations projected to reach 66% of the global total by 2050, the energy demand for air conditioning is expected to increase by 750%. Conventional vapor-compression cooling is highly energy-intensive, accounting for approximately 17% of global electricity consumption while contributing to carbon emissions, refrigerant-related environmental concerns, and urban heat accumulation. Passive radiative cooling has emerged as a promising alternative because it dissipates heat to outer space through the atmospheric window (8–13 μm) without electricity or moving parts, offering an energy-efficient and environmentally sustainable cooling strategy. Despite its promise, effective daytime radiative cooling requires maximizing solar reflectance to minimize heat gain from solar absorption. Consequently, most radiative cooling materials appear white or silver. In recent years, researchers have proposed several strategies to overcome this aesthetic limitation. The most straightforward approach is to incorporate dyes or fluorescent pigments. Both mechanisms inevitably rely on optical absorption, resulting in parasitic heat generation that compromises cooling performance. In contrast, structural colors arise from wavelength-selective light interference or scattering by micro- or nanostructures with feature sizes comparable to the wavelength of visible light, enabling vivid coloration with minimal intrinsic absorption. Representative mechanisms include thin-film interference, diffraction gratings, and photonic crystals. Nevertheless, existing structurally colored radiative cooling materials usually require multi-step fabrication processes and specialized instruments, making large-scale production costly and time-consuming. Recently, Liu et al. reported a bilayer, colored ethyl cellulose (BCEC) coating produced in a single casting step, which significantly simplifies the fabrication process and presents a viable strategy for the practical deployment of this technology. The fabrication of BCEC involves the drying of an ethyl cellulose (EC)/N,N-dimethylformamide (DMF) solution in a water vapor environment. This induces non-solvent-induced phase separation (NIPS), driven by interactions between solute and solvent molecules. The bilayer structure forms spontaneously in a single step during the drying process: a relatively dense top surface is generated first as the DMF evaporates, after which water vapor diffuses slowly across this skin layer, initiating the NIPS process and producing the porous bottom layer. The dense top layer has a thickness of several hundred nanometers—an ideal scale for generating colors through thin-film interference. More importantly, this thickness can be conveniently and precisely tuned by adjusting the concentration of the precursor solution, making it possible to create various structural colors, including blue, yellow, red, pink, and green. In addition to thickness-dependent color tuning, the BCEC coating also exhibits angle-dependent coloration (iridescence), an intrinsic characteristic of thin-film interference, whereby the reflected peak wavelength shifts with the viewing or illumination angle. The highly porous bottom layer is responsible for the high solar reflectance, resulting from the strong scattering of light by the abundant micro- and nano-pores. The solar reflectance varies slightly with the thickness of the BCEC film; the thickest film (BCEC-5, green film) exhibits the highest solar reflectance of 0.97. Simultaneously, the intrinsic absorption derived from molecular bond vibrations, especially the C–O bond, contributes to the high the
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