Fig. 4: Self-templated hollowing evolvement process of 3D-Sp-COFs via Ostwald ripening mechanism.

a Extinction of small particles and growth of large particles and continuous growth of large particles with dense cores and rough surfaces; b crystallites on the surface to achieve a smooth morphology; c dissolution and outmigration of inner crystallites; d formation of complete hollow spheres; e formation of symmetric core–shell hollow spheres; f formation of asymmetric core–shell hollow spheres.