Hollow silica nano and micro spheres with polystyrene templating: a mini-review

Synthesis of monodisperse hollow silica nanospheres, especially using a hard template route, has been shown to be successful, but a high yield is needed for this strategy to be used on an industrial scale. On the other hand, there is a research gap in the synthesis of hollow silica microspheres due to the popularity and easiness of the synthesis of silica nanospheres despite the larger spheres being beneficial in some fields. In this review, current trends in producing hollow silica nanospheres using hard templates, especially polystyrene, are briefly presented. Soft templates have also been used to make highly polydisperse hollow silica spheres, and complex designs have improved polydispersity. The effect of the main parameters on the coating is presented here to provide a basic understanding of the interactions between the silica and template surface in the absence or presence of surfactants. Surface charge, surface modification, parameters in the sol–gel method and interaction between the silica and templates need to be further improved to have a uniform coating and better control over the size, dispersity, wall thickness and porosity. As larger organic templates will have lower surface energy, the efficiency of the micro sphere synthesis needs to be improved. Control over the physical structure of hollow silica spheres will open up many opportunities for them to be extensively used in fields ranging from waste removal to energy storage.