The intrinsic structure of spherical particles of opal

The intrinsic structure of spherical SiO₂ particles synthesized by hydrolysis of tetraethyl orthosilicate in an alcohol-water-ammonia medium was studied using transmission electron microscopy. It was established that the relatively large spherical silica particles were “tertiary” structures made up of smaller spherical particles (“ secondary” particles), which in turn consisted of even smaller primary spherical particles 5–10 nm in diameter. It was shown that, under the experimental conditions, the large SiO₂ particles can contain a central core comprising primary particles surrounded by several layers of secondary particles smaller than the core diameter.