shows the time evolution of the photoexcitation (PE) and
photoluminescence (PL) spectra measured at different time periods
of ZnO colloids synthesis from the sol-gel route. As the sample is
converted from the precursor solution to the colloids the PE peak
becomes thinner and is red-shifted. This behavior is characteristic
of electron-hole excitons by the Q-dot of growing sizes. It is noteworthy
that the maximum observed for the PE peak after 5 min of
reaction for samples prepared with LiOH and NaOH occurs at a
wavelength lower (285e288 nm, Q-dot radii ¼ 1.25e1.27 nm) than
that observed for the sample hydrolyzed with KOH (300 nm, Q-dot
radii ¼ 1.37 nm). This is an evidence that the addition of KOH favors
the fast growth of Q-dot. Note that after 2 h of reaction the PE
spectrum of the sample prepared with KOH is characterized by two
peaks; the most intense occurs at 333 nmand the less intense at the
same position of the peak observed after 5 min. This behavior indicates
the presence of two families of particles, with average Q-dot
sizes of 1.37 and 1.79 nm. The peak observed at lower wavelength
appears as a shoulder both for reactions hydrolyzed with LiOH and
NaOH. Even after 18 h of reaction this shoulder is observed as a
broad tail at the left side of the main peak, irrespective of the base,
indicating the persistence of a broad size distribution of small Qdot.
As it will be discussed in the following, the formation of a
double family of Q-dot is the manifestation of the Q-dot coalescence,
while the broad size distribution is an essential condition for
the coarsening by the Ostwald-ripening mechanism.
The PL spectra is red shifted as the reaction time increases, i.e.,
the PL band maximum shifts from z499 to z560 nm as the reaction
time increases from 5 to 1080 min (18 h), for the samples
hydrolyzed with LiOH and NaOH. A faster growth of Q-dot for the
sample hydrolyzed with KOH, is confirmed by the PL band
maximum red-shifting from z503 to z565 nm. Moreover, the
FWHM (Full Width at Half Maximum) of the PL band is essentially
time invariant for the sample hydrolyzed with LiOH (z155 nm),
shows a slight increase for that hydrolyzed with NaOH (from