It can be observed that silica NPs are decorated by very small
spherical ZnO NPs, whose size and aggregation depend on the zinc
loading, while the average SiO2 particle size (20–30 nm) remains
the same before (Fig. S2 in Supporting information) and after
ZnO deposition. In particular, rather homogeneously dispersed
NPs with average diameter 4–6 nm are detectable in ZSO-4.0 and
ZSO-7.7 samples (Fig. 1a and b), while surface aggregates of larger
NPs (8–10 nm in diameter) are evident at the highest ZnO concentration
(ZSO-50.2, Fig. 1d and e).