Hierarchical Microspheres-Based Composite Materials with TiO2-Coated SiO2 Combined with BaSO4 or PNIPAM for Radiative Cooling

Microspheres dispersed in composites exhibit excellent infrared emissivity for radiative cooling applications, which reflect sunlight and passively dissipate heat into space without electricity. In this study, hierarchical microspheres (HMs) with a two-tier structure, composed of SiO2, TiO2-coated SiO2, BaSO4, or PNIPAM, are incorporated into PDMS-based composites. These microspheres feature larger spheres assembled from submicrometer-scale nanoparticles and are fabricated via microfluidics to enhance radiative cooling performance. SiO2 HMs not only boost visible light reflection and exhibit structural color through a photonic stop band but also achieve an average emissivity of 97.55% in the atmospheric window. Both experimental and simulated results show that HMs enhance the emissivity performance of the composite material compared with solid SiO2 microspheres of the same diameter. Additionally, applying TiO2 coating to SiO2 HMs further increases the overall emissivity to 98.05%. Incorporating BaSO4 HMs also increased the average visible reflectivity to 96.56%, while maintaining superior infrared emissivity at 97.58%. The inclusion of PNIPAM spheres enabled temperature-responsive transmissivity, with the composite materials containing PNIPAM and SiO2 HMs preserving high infrared emissivity in the atmospheric window. These HM structures exhibit excellent solar reflectivity and thermal emission, making them effective for radiative cooling.