Recent advances in spectrally selective daytime radiative cooling materials

A groundbreaking review published in Nano-Micro Letters provides a panoramic overview of spectrally selective daytime radiative cooling (SSDRC) materials—an emerging class of engineered photonic media that promise to redefine how we cool people, buildings, and devices without consuming a single watt of electricity. Authored by Ke-Qin Zhang, Ghim Wei Ho, and Xiao-Qiao Wang from Soochow University, Jilin University, Liaoning Academy of Materials, and National University of Singapore, the review dissects the molecular fingerprints, micro-/nano-architectures, and scalable manufacturing routes that enable SSDRC materials to outperform conventional broadband emitters under peak solar irradiance.
Why This Research Matters

• Overcoming Broadband-Emitters’ Limitations: Traditional daytime radiative coolers dissipate heat across the entire mid-infrared (MIR). Inevitable re-absorption outside the 8–13 μm atmospheric transmission window (ATW) caps their net cooling power. SSDRC materials—through judicious molecular design and photonic structuring—confine emissivity to the ATW while suppressing non-ATW radiation, unlocking > 130 W m^-2 cooling power and 8 °C sub-ambient drops under 900 W m^-2 sunlight.

• Enabling More-than-Cooling Applications: Beyond static heat rejection, SSDRC fabrics and coatings integrate seamlessly with thermoelectric arrays, moisture-electric hydrogels, and triboelectric nanogenerators to create self-powered wearables, net-zero buildings, and off-grid water harvesters—meeting urgent demands for carbon-neutral thermal management.

Innovative Design and Mechanisms

• SSDRC Materials Toolbox: The review exhaustively catalogues polymers (PEO, PMP, PVDF-HFP), ceramics (SiO₂, Al₂O₃, TiO₂), and hybrid fibers/membranes/particle coatings whose intrinsic Si–O–Si, C–F, or Al–O bonds resonate exclusively within the ATW, delivering spectral selectivity ratios γ up to 2.34.

• Advanced Photonic Architectures: Multilayer Bragg stacks, Mie-scattering random pores, biomimetic cicada-wing gratings, and gradient-index metamaterials are dissected to illustrate how interference, scattering, and phonon-polariton coupling achieve > 96 % solar reflectance and > 95 % ATW emissivity while remaining visibly opaque or transparent as desired.

• 3D Integration & Flexible Form Factors: Electrospun nanofiber mats, laminated tri-layer textiles (PMP/AgNW/wool), and roll-to-roll compatible glass/Al₂O₃ coatings demonstrate wafer- to meter-scale processability, mechanical flexibility, and compatibility with existing textile, glass, and roofing industries.

Applications and Future Outlook

• Personal Thermal Management: Field tests show SSDRC vests cut skin temperature by 4 °C under noon sunlight while retaining cotton-like breathability (34 cm³ cm^-2 s^-1 air permeability) and vivid structural colors via TiO₂/SiO₂ photonic pigments.

• Building-Scale Cooling: Model houses with Al₂O₃/PES/NMP ceramic roofs maintained 5 °C lower surface temperatures and 26.8 % air-conditioning energy savings. Global simulations project 25 GJ yr^-1 reductions per building in tropical megacities.

• Energy Harvesting Symbiosis: Radiative-cooling-driven thermogalvanic ionogels delivered 0.61 V on sunny days; CNT/ PVDF-HFP fabrics produced 184.5 mV while cooling the wearer; tandem SSDRC-hydrogel devices sustained 6-day continuous moisture-electric output.

Future Research Directions

Next steps target (1) UV-/rain-/abrasion-resistant hydrophobic coatings, (2) electro/thermo-chromic VO₂ or liquid-crystal metasurfaces for seasonal tunability, and (3) AI-guided inverse design and high-throughput printing to translate lab-scale miracles into city-scale climate shields.

Conclusions
This review by Xie et al. distills a decade of progress into a single blueprint: SSDRC materials are no longer laboratory curiosities but manufacturable, multifunctional photonic engines capable of cooling humanity without warming the planet. Watch this space as researchers weave, coat, and print them into the fabric—literally—of everyday life.