  Daytime radiative cooling has attracted considerable attention recently due
to its tremendous potential for passively exploiting the coldness of deep-sky
as clean and renewable energy. Many advanced materials with novel photonic
micro-nanostructures have already been developed to enable highly efficient
daytime radiative coolers, among which the flexible hierarchical porous
coatings (HPCs) are a more distinguished category. However, it is still hard to
precisely control the size distribution of the randomized pores within the
HPCs, usually resulting in a deficient solar reflection at the near-infrared
optical regime under diverse fabrication conditions of the coatings. We report
here a three-phase (i.e., air pore-phase, microsphere-phase and polymer-phase)
self-assembled hybrid porous composite coating which dramatically increases the
average solar reflectance and yields a remarkable temperature drop of ~10 degC
and 30 degC compared to the ambient circumstance and black paint, respectively,
according to the rooftop measurements. Mie theory and Monte Carlo simulations
reveal the origin of the low reflectivity of as-prepared two-phase porous HPCs,
and the optical cooling improvement of the three-phase porous composite
coatings is attributed to the newly generated interfaces possessing the high
scattering efficiency between the hierarchical pores and silica microspheres
hybridized with appropriate mass fractions. As a result, the hybrid porous
composite approach enhances the whole performance of the coatings, which
provides a promising alternative to the flexible daytime radiative cooler.
