The bond interaction between SiO2 and ZnO in ZSO-X was also
studied by ATR-FTIR spectra of ZSO-X (Fig. 4). In this context, the
band at 954 cm1 which in bare SiO2 spectrum was attributed to
the ASiAOH [41] stretching vibration (Fig. 4a), in ZSO-4.0 and
ZSO-7.7 spectra gradually shifts to higher frequency (963–
965 cm1) and becomes a shoulder (Fig. 4b and c). Referring to
the spectra of zincosilicate systems [42] and of ZnO/SiO2 composites
[43,44] reported in the literature, this change may be attributed
to the co-presence of ASiOH and of the symmetric SiAOAZn
stretching modes. This suggests the formation of covalent bonds
between a number of surface silanol groups and the growing ZnO
particles in agreement with the NMR results.
The surface composition of ZSO-X was studied by XPS in comparison
with bare SiO2 treated with NaOH under the same conditions
of the ZSO-X synthesis procedure.
The survey spectrum of bare SiO2 powders treated with NaOH
showed the expected silicon and oxygen lines. The energy position
of the Si2p peak is at 103.2 eV, whereas the O1s band is centred at
532.5 eV with a small contribution (ca. 10% of the total) at 530.7 eV
(Fig. S5 in Supporting information). This latter component may be
likely assigned to the presence of SiAOH and/or SiAO groups [45]