Structurally Colored Radiative Cooling Cellulosic Films

Abstract

Daytime radiative cooling (DRC) materials offer a sustainable approach to thermal management by exploiting a net positive heat transfer to deep space. While such materials typically have a white or mirror-like appearance to maximize solar reflection, extending the palette of available colors is required to promote their real-world utilization. However, the incorporation of conventional absorption-based colorants inevitably leads to solar heating, which counteracts any radiative cooling effect. In this work, we instead demonstrate efficient sub-ambient DRC (Day: -4 °C, Night: -11 °C) from a vibrant, structurally colored film prepared from naturally-derived cellulose nanocrystals (CNCs). Arising from the underlying photonic nanostructure, the film selectively reflects visible light resulting in intense, fade-resistant coloration, whilst maintaining a low solar absorption (~3%). Additionally, a high emission within the mid-infrared atmospheric transparent window (>90%) allows for significant radiative heat loss. By coating such a CNC film onto a highly-scattering, porous ethylcellulose (EC) base-layer, any sunlight that penetrates the CNC layer is backscattered by the EC layer below, achieving broadband solar reflection and vibrant structural color at the same time. Finally, we demonstrate scalable manufacturing using a commercially-relevant roll-to-roll process, which validates the potential to produce such colored radiative cooling materials at a large scale from a low-cost and sustainable feedstock.