Hollow silica nanoparticles: A tiny pore with big dreams

Hollow nanoparticles exhibit unique functions such as thermal insulation, low dielectricity, and abnormal light scattering, especially when the inner size reaches less than twice the mean free path of air molecules (mean free path of air is 68 nm). It is easy to imagine that a nano-sized pore improves the thermal insulation performance due to less air convection flow inside. However, not only the hollow interior but also the shell microstructure (size, thickness, and density) strongly affect the functionalities. For example, a lower apparent shell density induces a higher thermal insulation probably due to phonon scattering on the siloxane bond cleavages. The best performance for the thermal conductivity of the hollow particle/polymer composite was 0.019 W/m·K, which is superior to air. The structure design of the hollow nanoparticle is quite important to produce this effect. In this review, based on important parameters, how to design the structure and wide applications of hollow nanoparticles are reviewed and a bright outlook for the future is expected.