Cooling the globe with sustainable biomass and minerals: Hierarchical nanostructuring unlocks next-generation radiative cooling

Driven by the escalating climate crisis and intensifying extreme heatwaves, passive daytime radiative cooling (PDRC) has emerged as a disruptive, zero-energy thermal management strategy.1,2 An ideal PDRC material must simultaneously achieve ultrahigh solar reflectance (>90%) and strong mid-infrared emissivity within the atmospheric transparency window (8–13 μm).3 To date, existing PDRC materials encompassing photonic crystals, inorganic ceramics, and petrochemical-based polymers face three critical bottlenecks: energy-intensive manufacturing, dependence on non-degradable fossil resources, and inadequate mechanical strength or outdoor durability. This challenge raises a fundamental question in sustainable materials engineering: can renewable biomass and natural minerals be rationally structured to achieve efficient radiative cooling, mechanical robustness, and green manufacturing within a single material platform?

Two recent studies demonstrate that high-performance cooling materials can be engineered from wood and minerals through hierarchical nanostructuring, overcoming the limitations of conventional PDRC systems. Wu et al.4 reported a radiative cooling biocomposite from natural wood via a multi-step crystalline restructuring process involving delignification, alkali treatment, particle-solution shock, and air drying (Figure 1A). The resulting material exhibits a solar reflectance of 94.7% and mid-infrared emissivity of 90.1%, delivering a theoretical cooling power of 106 W/m2 and an outdoor sub-ambient temperature drop of 8.8°C (Figure 1C). Concurrently, Chen et al.5 constructed mineral hydroplastics from natural layered minerals (e.g., mica and kaolin) using a nonsolvent-induced phase separation (NIPS) strategy (Figure 1B). By replacing mica with calcite, the hydroplastic attains over 90% solar reflectance and excellent mid-infrared emissivity, yielding an outdoor temperature reduction of 2.6°C (Figure 1D). Furthermore, it exhibits V-0 flame retardancy with a limiting oxygen index of 37.6%, high surface hardness (0.23 GPa), and exceptional moldability. Collectively, these findings establish that integrating renewable resources with hierarchical nanostructures can effectively reconcile the long-standing trade-off between cooling efficiency and structural performance.

The underlying mechanisms reveal how multiscale structural regulation governs photon and phonon transport. Rather than acting as independent components, polymer/cellulosic matrix and mineral fillers operate as coupled phases: minerals provide strong optical scattering, infrared-active vibrations, hardness, and flame retardancy, whereas polymer or cellulose networks regulate processability, particle dispersion, interfacial adhesion, and stress transfer. In the wood-based cooler, optical and mechanical enhancement arises through complementary pathways. Delignification and alkali treatment induce the transition of cellulose Iβ to II, increasing crystallinity to 77.2%. Subsequent infiltration of SiO2 and h-BN nanoparticles creates dual-phase interfaces with refractive-index mismatches that maximize Mie scattering, while C–O–C and Si–O vibrations contribute to strong mid-infrared emissivity of 90.1%. Mechanically, capillary-driven self-densification and hydrogen-bond reconstruction yield an oriented cellulose framework in which mineral-cellulose interfaces facilitate load transfer and crack deflection. The key innovation, termed “particle-solution shock,” dynamically regulates hydration states to achieve uniform nanoparticle infiltration and robust interfacial bonding. This hierarchical integration of optical scattering centers and mechanical reinforcements effectively reconciles cooling efficiency with structural durability.