Abstract
Passive radiative cooling can cool objects exposed to sunlight without requiring additional energy input. However, it remains challenging to develop aesthetically pleasing coloured materials without sacrificing solar reflectance. Here we present a biomass-derived, bilayer and coloured ethyl cellulose (BCEC) coating fabricated in a single casting step through controlled drying-induced self-stratification. By adjusting the precursor concentration, we tune the thickness of the top layer to produce different colours through thin-film interference without introducing solar absorption. The hierarchically porous bottom layer provides high solar reflectance and long-wave infrared emission. The BCEC coating achieves solar reflectance of 97.0% and sub-ambient daytime radiative cooling of up to 9 °C under a solar intensity of 800 W m−2. In field tests conducted in the humid subtropical climate of Hong Kong, the BCEC coating outperforms commercially available coloured paints and fluorescence-based coloured coatings. Our one-step phase-separation approach can simplify fabrication, facilitating the practical deployment of this technology.
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Acknowledgements
This work was supported by the Research Grants Council of Hong Kong through a Collaborative Research Fund grant (C5051-22GF, D.L. and J.-G.D.), the Innovation and Technology Commission of Hong Kong through a Mainland-Hong Kong Joint Funding Scheme (MHP/162/22, D.L.), the Centre for Functional Photonics of City University of Hong Kong (D.L.), Hong Kong Branch of National Precious Metals Material Engineering Research Center (ITC Fund, D.L.), the National Natural Science Foundation of China (52578329, J.-G.D.), the National Natural Science Foundation of China (T2525033, 62134009 and 62121005, W.L.), the International Partnership Program of Chinese Academy of Sciences for Future Network (171GJHZ2023038FN, W.L.), Jilin Provincial Scientific and Technological Development Program (SKL202401020, W.L.) and the Germany Science Foundation (Mu1674/15-2, M.M.). N.B. and M.M. gratefully acknowledge the Gauss Centre for Supercomputing e.V. for providing computing time through the John von Neumann Institute for Computing (NIC) on GCS Supercomputers JUWELS/JEDI at the Jülich Supercomputing Centre (JSC). We thank B. Fei and X. Hu for their kind assistance with partial FTIR testing. We also acknowledge insightful discussions with C. Jiang, T. Xing and X. Ma and appreciate M. Pan and B. Sun for their proofreading assistance.
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D.L. and Y. Liu proposed the idea. Y. Liu prepared and optimized the coatings. N.B. and M.M. performed the particle-based simulations. Q.X. contributed to the building energy-saving calculation. J.L. assisted with the angle-resolved reflectance spectra measurement. Y. Liao and Y. Fu helped with the optical simulations. W.L. and N.Y. helped with the cooling test. Y. Liu, Y. Fang, P.C., X.X. and T.W. collaborated on conducting testing experiments and analysing data. Y. Liu wrote the original draft of the manuscript. D.L., R.Y., J-G.D., M.M. and W.L. supervised the project. All authors engaged in discussions regarding the results and made valuable comments on the paper.
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D.L. and Y. Liu are applying for patents related to the work described. The other authors declare no competing interests.
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Extended data
Extended Data Fig. 1 Cloud point experiments for phase diagram construction in Fig. 1.
(a) Schematic illustration of the experimental procedure. A series of solutions with EC: DMF mass ratios ranging from 0.5% to 20% were prepared, followed by the controlled dropwise addition of varying amounts of nonsolvent water to induce phase separation. Before each photographic recording, the solutions were mixed and heated in a 60 °C oven for 12 hours, followed by cooling to 20 °C naturally for another 12 hours. (b) Photographic documentation of the phase separation process. The transition from a clear, transparent solution to a turbid, opaque state marks the cloud point. The weight of the added water is indicated above each sample, with blue representing the point where phase separation first occurs and black for all other conditions.
Extended Data Fig. 2 Angle-resolved optical characterization of BCEC coatings.
(a) Experimental setup for angle-resolved (i) reflectance spectroscopy and (ii) optical imaging. (b) Comparison of measured and simulated angle-resolved reflectance spectra of a series of BCEC coatings with thicknesses of approximately (i) 800 µm, (ii) 580 µm, (iii) 460 µm, (iv) 430 µm, and (v) 390 µm. In the simulations, the top layer thicknesses for the five samples are 490, 390, 375, 350, and 300 nm, respectively. The reflection-mode angle-resolved optical images of the BCEC coatings are shown to the right of each spectrum. (c) (i) Schematic diagrams illustrating the measurement setup for reflection spectra collection at varying detection angles (0°–80°) under fixed incident angles of 30° and 60°. Reflection spectra collected at different detection angles (0°–80°) under fixed incident angles of (ii) 30° and (iii) 60°, spanning the wavelength range of 400–900 nm.
Extended Data Fig. 3 Scalability of the BCEC coating.
(a) Schematic diagram illustrating the fabrication process of large-sized samples. (b) BCEC coatings, from left to right: BCEC-1 at 390 µm in blue, BCEC-2 at 430 µm in yellow, and BCEC-3 at 460 µm in pink, each on a 10 cm-by-10 cm substrate. (c) Blade-coating process of the BCEC film. Top: photograph showing the wet suspension coated onto a PET substrate using a blade coater with a 15 cm wide opening. Bottom: the resulting free-standing BCEC film after drying, prepared with a blade gap of 1200 µm and an EC/DMF ratio of 1:20.
Extended Data Fig. 4 Durability of the BCEC coating.
(a-c) Schematic diagrams of durability tests under different conditions. (a) UV exposure, where the sample was continuously irradiated for 1440 hours using a 100-watt longwave (365 nm) UV lamp, (b) the sample underwent thermal cycling for 1440 hours, with each 24-hour cycle consisting of a 12-hour heating phase at 70 °C followed by a 12-hour cooling phase at 4 °C, and (c) high-humidity exposure, in which the sample was placed in moist conditions with 99.9% relative humidity in a closed chamber for 480 hours. (d-f) Solar reflectance of BCEC after durability testing under different conditions: (d) UV exposure, (e) high-low temperature cycling, and (f) high-humidity exposure. The insets present the reflection-mode optical micrographs, illustrating the distinct colour variations of the samples between their pre-test and post-test states. (g-i) LWIR emissivity of after the same durability tests: (g) UV exposure, (h) high-low temperature cycling, and (i) high-humidity exposure. BCEC-5 samples were used for UV exposure and high-low temperature cycling tests; BCEC-4 samples were used for high-humidity durability test.
Extended Data Fig. 5 Recyclability of the BCEC coating.
(a) The BCEC sample can be ground into powder, dissolved, and reprocessed into a new BCEC coating, demonstrating excellent recyclability. (b) Solar reflectance comparison between the original and refabricated BCEC-3 coatings. Insets show reflection-mode optical micrographs of the original and refabricated BCEC-3 coatings, highlighting their consistency. (c) long-wave infrared (LWIR) emissivity of the original and refabricated BCEC-3 coatings. SEM images of the (d) original and (e) refabricated BCEC-3 coatings.
Extended Data Fig. 6 Sub-ambient cooling performance of the BCEC coating in Jilin, China (43°50’53.3‘N, 125°24’28.2‘E) on October 5–6, 2023.
(a, b) Photograph (a) and schematic (b) of the outdoor cooling measurement setup. (c) Real-time temperature of the BCEC sample (BCEC-5), ambient air temperature, and corresponding weather parameters, including solar intensity, wind speed, and relative humidity during the outdoor experiment.
Extended Data Fig. 7 Comparative outdoor temperature performance of coatings on September 11, 2025.
Surface temperature measurements of BCEC, commercial (C), and fluorescent (f) coloured coatings conducted at The University of Hong Kong. The graphs include the following concurrent data: solar irradiance, relative humidity (RH), wind speed, ambient temperature, and sample-specific temperatures. No PE cover was used in the setup throughout the testing period.
Extended Data Fig. 8 Comparative outdoor temperature performance of coatings on September 13, 2025.
Surface temperature measurements of BCEC, commercial (C), and fluorescent (f) coloured coatings conducted at The University of Hong Kong. The graphs include the following concurrent data: solar irradiance, relative humidity (RH), wind speed, ambient temperature, and sample-specific temperatures. No PE cover was used in the setup throughout the testing period.
Extended Data Fig. 9 Comparative outdoor temperature performance of coatings on October 2, 2025.
Surface temperature measurements of BCEC, commercial (C), and fluorescent (f) colored coatings conducted at The University of Hong Kong. The graphs include the following concurrent data: solar irradiance, relative humidity (RH), wind speed, ambient temperature, and sample-specific temperatures. No PE cover was used in the setup throughout the testing period.
Extended Data Fig. 10 Comparative analysis of CO2 emission reduction.
(a) Annual CO2 emission reduction for cooling the stand-alone building using BCEC-green, red, and blue roof coatings across 33 cities in China, with (b) its corresponding geographical distributions; (c) annual CO2 emission reduction using commercial green, red, and blue roof paints, with (d) its corresponding geographical distributions. Basemaps in b and d from ref. 38 under a Creative Commons license CC BY 4.0.
Supplementary information
Supplementary Information (download PDF )
Supplementary Figures 1–35, Notes 1–6, Tables 1–9 and references.
Supplementary Video 1 (download MP4 )
One-step-processed BCEC at 20 °C and 60% RH, with a weight ratio of EC to DMF of 1:20.
Supplementary Video 2 (download MP4 )
Simulation of the formation process for BCEC, with a weight ratio of EC to DMF of 1:20.
Source data
Source Data Fig. 1 (download XLSX )
Unprocessed western blots and gels, statistical source data and so on.
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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
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DOI: https://doi.org/10.1038/s41560-026-02039-0
