In many cases of nanocrystal synthesis, when the diffusion rates of the two reactants are different, interesting morphologies such as hollow structures will be formed.

(a) We have Co nanospheres with 10 nm in diameter to be reacted with S to become Co₉S₈ in a solution synthesis. Imaging that the Co atoms diffuse much faster than the S atoms in Co₉S₈, what is the final morphology of Co₉S₈? It is a solid sphere or hollow sphere?

(b) If the final product Co₉S₈ is a solid sphere, then what is the diameter of the sphere? If it is a hollow sphere, then what is the thickness of the shell?

The density of Co is 8.90 g/cm³, and the density of Co₉S₈ is 5.45 g/cm³