Preparation of hierarchical architectures of silica particles with hollow structure and nanoparticle shells: a material for the high reflectivity of UV and visible light.

Silica microcapsules (silica hollow particles) are readily prepared by a single step of the interfacial reaction method, where a W/O/W emulsion is employed effectively. This is a simple (one-step process), inexpensive approach (silica source is sodium silicate) to producing hollow silicas. The addition of NaCl to the sodium silicate solution as the inner water phase of the W/O/W emulsion plainly influenced the shell structure of the silica hollow particles. The increase of the addition of NaCl expanded the size of the mesopores in their silica shell, which reached to macropores (>50 nm). The nanoparticles in the shells of some silica hollow particles attained approximately 200-400 nm in size, which is comparable to the wavelengths of UV and visible light. According to the diffuse reflection spectra of the silica hollow particles in powder form, these particles showed the high reflection of UV and visible light, which increased with added NaCl in the preparation process of the interfacial reaction method. The reflectance of a silica hollow particle from 300 to 800 nm in wavelength was over 90%, which was significantly higher than a common solid (not hollow) silica gel. In addition, even the reflectance of UV light shorter than 300 nm in wavelength was greater than 60%. These characteristic reflections in a wide range of wavelengths were caused by both nanoparticle shells whose sizes are comparable with the wavelength of light and the hollow structures of the main micrometer-sized particles.