Results and discussion
Fig. 1 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 (285–288 nm, Q-dot radii = 1.25–1.27 nm) than that observed for the sample hydrolyzed with KOH (300 nm, Q-dot radii = 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 nm and 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 Q-dot. 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.