Hierarchically Patterned Self-Cleaning Polymer Composites for Daytime Radiative Cooling

<br/>Exposure to outdoor heat stress threatens the health of humans taking outdoor physical activities, while the open nature of outdoor activities makes it impractical to use traditional electrical cooling methods. Passive daytime radiative cooling (PDRC), which uses intrinsic optical properties to achieve passive cooling without consuming electricity, has been proposed and developed to tackle outdoor cooling challenges in recent years. PDRC selectively achieves high solar reflectivity and utilizes the fact that the thermal emission can go through the sky window (in the wavelength range of 8—13 μm) into the cold universe (~3 K). However, outdoor PDRC materials are susceptible to environmental contamination, including solid microparticles and water accumulation. Such surface contaminants severely decrease the cooling efficacy, requiring routine cleaning to recover; also, for the PDRC textile worn by humans, aesthetic purposes make the daily dust cleaning necessary. In order to minimize cleaning labor, self-cleaning has been integrated with PDRC. However, commonly used methods to combine self-cleaning with PDRC materials involve adding fluorinating chemicals like polytetrafluoroethylene (PTFE) into the radiative coolers, and their fabrications are usually complex and expensive. A simple, cost-effective strategy to combine self-cleaning with radiative cooling is needed. Here, we develop a hierarchically patterned nanoporous composite (HPNC) using a facile template molding fabrication method to integrate PDRC materials with self-cleaning and antibacterial functions. The HPNC design decouples multifunctional control into different characteristic length scales that can be optimized simultaneously. Using a nanoscale porous polymer matrix and tunable particle fillers to tailor the radiative cooling properties, the HPNC samples enable &gt;7.8°C temperature reduction for outdoor cooling of the human body and 4.4°C sub-ambient cooling for building applications under intense solar irradiance. Meanwhile, a micro-scale pillar array pattern integrated into the HPNC enables superhydrophobicity with self-cleaning and anti-soiling functions to mitigate surface contamination. Moreover, the HPNC design allows for the incorporation of nanoparticles for additional functionality. Here, we embed and coat photocatalytic agents to generate effective photoinduced antibacterial effects. The scalable fabrication and multifunctional capabilities of our HPNC design offer a promising solution for practical daytime radiative cooling applications with minimal maintenance needs.