Porous SiO₂ Hollow Spheres as a Solar Reflective Pigment for Coatings

This study starts with the synthesis of silica hollow spheres (HSs) by utilizing in situ synthesized polystyrene (PS) microspheres as the template for the deposition of a silica (SiO₂) shell, followed by a slow gasification step in air to remove the PS core. The size of HS and the thickness of the porous SiO₂ shell are tuned by varying the synthesis conditions of the PS latex and those of the sol-gel deposition, respectively. Various HS powder samples are characterized by ultraviolet-visible-near infrared (UV-vis-NIR) spectroscopy to determine their diffuse reflectance. Furthermore, they are used as the filler in an acrylic polymer matrix for the measurement of solar reflectivity on a solar spectrum reflectometer. It turns out that both cavity size and the structure of the SiO₂ shell are influential in the reflection of NIR and UV-vis light, respectively. In addition, this study examines the effect on solar reflectivity of a selected metal oxide with a SiO₂ HS. In conclusion, the cavity size of the HS has a strong impact on the reflectivity to NIR light whereas the shell itself affects the reflection of UV-blue light.