Size effects on the microchemistry and plasticity of Stober silica particles: A study using EFTEM, FESEM, and AFM-SEPM microscopies

Two different samples of monodisperse Stober silica particles were examined using three different microscopies: energy-filtered analytical transmission electron microscopy (EFTEM), high-resolution field-emission scanning electron microscopy(FESEM), and scanning probe microscopy, in the noncontact atomic force (AFM) and scanning electric potential microscopy (SEPM) modes. Upon drying the silica dispersions, the larger (ca. 141 nm) particles were only partially deformed by capillary adhesion, whereas the smaller particles (ca. 36 nm) were strongly deformed and closely packed into dense films of a low porosity, which is evidence of their larger plasticity, or superplasticity. Electric potential distribution maps obtained by SEPM showed a significant interparticle as well as intraparticle contrast, especially in the case of the smaller particles. Examination by electron backscattering also revealed a larger contrast among the smaller particles, thus evidencing a nonuniformity of chemical composition. The results are interpreted considering the changes in the synthetic medium and other aspects of the particle growth mechanism, and they point toward the possibility of exploiting the plasticity of the nanosized silica particles in the making of silica monoliths.