Dear Dr. Ghomi

I think it is because of ion by ion deposition of the final compounds. Actually this process is a upward (down to up) process.

Dear 

First of all it is not necessary that the sol gel will form the nanostructured materials sometimes it can be in the range of micro also.

in the sol gel reaction there involves first hydrolysis followed by the condensation of the material, the size of the final product will depend mostly on the concentration of the precursor used and also on the surfactant (if any). like if we are using the microemulsion method that allows the particles to grow in nanoscale only.