In a breakthrough that could revolutionize energy-efficient cooling technologies, researchers have unveiled a novel biomass-derived coating capable of passive radiative cooling while offering customizable colors. This innovative material, engineered from ethyl cellulose and fabricated through a simple, one-step casting process, opens new avenues for sustainable cooling solutions that do not compromise aesthetics or performance under direct sunlight. The significance of this advancement lies in its potential to address long-standing challenges in the development of colored materials that maintain high solar reflectance—a key criterion for efficient radiative cooling.
Radiative cooling, a process by which surfaces lose heat by emitting infrared radiation to the cold outer space, has emerged as a promising strategy to curb energy consumption associated with traditional air conditioning. Materials optimized for this purpose typically require high reflectance in the solar spectrum to avoid heating during daytime, alongside strong thermal emission capabilities in the mid-infrared range. However, incorporating vivid coloration into such materials has proven difficult, as pigments usually absorb significant amounts of sunlight, thereby diminishing the cooling effect. The newly developed bilayer ethyl cellulose coating overcomes this limitation by finely tuning its structural properties to achieve both color vibrancy and solar reflectance exceeding 97%.
The key innovation in this work is the use of controlled drying-induced self-stratification during a single casting step. By carefully adjusting the concentration of the ethyl cellulose precursor solution, researchers manipulate the formation of a hierarchically structured bilayer. The top layer is meticulously engineered to have a thickness that produces specific colors via thin-film interference—an optical phenomenon where different wavelengths of light are constructively or destructively interfered as they reflect through a thin film. This approach eliminates the need for traditional coloring agents that absorb sunlight and reduce reflectance.
Beneath the colorful top layer lies a highly porous and scattering bottom layer that plays a critical role in the cooling performance. This bottom layer ensures exceptionally high reflectance across the solar spectrum, effectively minimizing solar absorption and unwanted heating during the day. Simultaneously, it exhibits strong emissivity in the long-wave infrared region, allowing thermal radiation to pass freely to outer space. This unique bilayer architecture thus harmonizes color customization with the stringent optical requirements of passive radiative cooling.
Experimental evidence demonstrated that the bilayer ethyl cellulose coating can achieve up to 9°C sub-ambient cooling under a solar irradiance of 800 W/m². This performance metric is particularly impressive considering the coating’s colored appearance, which traditionally compromises cooling efficiency. By outperforming commercially available colored paints and fluorescence-based colored coatings during field tests conducted in the humid subtropical climate of Hong Kong, the material proves its robustness and applicability in real-world, challenging environmental conditions.
Beyond exceptional cooling, the simplicity of the one-step phase-separation fabrication facilitates scalable manufacturing, which is crucial for practical deployment. Unlike multi-step or lithography-based methods common in layered thin-film materials, this approach drastically reduces complexity and costs. Moreover, the use of sustainable, biomass-derived ethyl cellulose aligns with growing demands for environmentally friendly materials in building and urban infrastructure applications.
Developing colored passive cooling materials has long been constrained by compromises between aesthetics and functionality. Traditionally, the incorporation of dyes and pigments introduced significant solar absorption, defeating the purpose of passive cooling by causing heat gain during sunlight exposure. The presented bilayer coating circumvents these issues by exploiting intrinsic optical effects rather than pigment-based coloration. This paradigm shift provides designers and architects with an unprecedented level of freedom to apply colored surfaces without sacrificing energy-saving benefits.
The hierarchical porous structure responsible for the high solar reflectance simultaneously contributes to durability and weather resistance. Porosity creates multiple scattering events that reflect incident sunlight efficiently, while the intrinsic nature of ethyl cellulose confers mechanical flexibility and environmental tolerance. These properties are vital for coatings intended for external applications on buildings, vehicles, and infrastructure where prolonged exposure to sunlight, moisture, and mechanical stresses is inevitable.
An additional merit of the approach is its broad spectrum tunability, achieved simply by controlling the top layer’s thickness during the drying process. This control enables the full color gamut via thin-film interference, from subtle pastels to vibrant hues, without any trade-off in radiative cooling performance. The seamless integration of color functionality with passive cooling technology could substantially accelerate the adoption of energy-saving surface coatings in urban areas, potentially mitigating urban heat island effects and reducing electricity demand for air conditioning.
Field testing in the subtropical climate of Hong Kong serves as a rigorous benchmark, given the region’s high humidity and intense solar radiation, which typically deteriorate radiative cooling efficacy. The bilayer ethyl cellulose coating’s superior performance in these conditions attests to its practical viability and reliability. Furthermore, the comparison with conventional colored paints and fluorescence-enhanced coatings underscores the material’s competitive advantages in both energy efficiency and appearance.
Looking forward, the integration of this innovative coating into building envelopes, vehicle exteriors, and outdoor equipment offers a new class of multifunctional materials. These could help reduce reliance on energy-consuming cooling systems, thus lowering carbon emissions and operational costs. Additionally, the bio-based nature of ethyl cellulose aligns with circular economy principles, potentially easing concerns related to material sustainability and end-of-life disposal.
In conclusion, the development of one-step-processed bilayer ethyl cellulose coatings represents a significant stride in the field of passive radiative cooling technology. By combining controlled self-stratification, thin-film interference coloration, and hierarchical porosity, researchers have crafted a material that not only achieves sub-ambient temperature reduction but also meets aesthetic and environmental standards. This discovery could catalyze widespread adoption of radiative cooling surfaces, marking a pivotal advancement in sustainable urban design and climate resilience strategies.
The synergy between advanced optical engineering and biomaterial science in this study highlights the transformative potential of interdisciplinary research. As passive cooling materials evolve, such innovations will be critical in addressing the escalating challenges posed by global warming and energy consumption. This promising work paves the way for future explorations into multifunctional coatings that harmonize energy efficiency, aesthetics, and sustainability in unprecedented ways.
Further research may delve into the long-term durability under diverse climatic conditions, large-scale manufacturability, and integration with existing building materials. Moreover, exploring other biomass-derived polymers for similar applications could widen the gamut of sustainable materials in this arena. Given the urgent demand for accessible cooling technologies worldwide, breakthroughs such as this bring us closer to cost-effective, scalable solutions that mitigate climate change effects while enriching built environments.
The implications of this study resonate beyond cooling surfaces. The principle of exploiting self-stratification and thin-film interference for property tuning can inspire innovations across photonics, energy harvesting, and sensor technologies. Ultimately, the intersection of green chemistry, nanostructured materials, and functional design embodied by this research embodies the forward path toward a sustainable and resilient technological future.
Subject of Research: Biomass-derived bilayer ethyl cellulose coatings for passive radiative cooling with full-color tunability through thin-film interference.
Article Title: One-step-processed bilayer ethyl cellulose for full-colour sub-ambient daytime radiative cooling.
Article References:
Liu, Y., Blagojevic, N., Xuan, Q. et al. One-step-processed bilayer ethyl cellulose for full-colour sub-ambient daytime radiative cooling. Nat Energy (2026). https://doi.org/10.1038/s41560-026-02039-0
