Fig. 7.

Transmission electron microscopy (TEM) of silica particles. a–d fixed concentration of tetraethylorthosilicate (TEOS), ammonia solution (NH₄OH) and varied amounts of water. Fixed concentration of TEOS and water, and varied concentration of NH₄OH f 0.11 M, g 0.28 M, h 0.57 M, i 1.13 M, j 0.17 M, k 0.40 M, l 0.85 M, reprinted with permission from Greasley et al. (2016). NH₄OH concentration controls the density of the silica particles. The narrow distribution of particle size obtained at higher ammonia concentration and H₂O could accelerate TEOS hydrolysis and helps to produce larger particles