The PL spectra is red shifted as the reaction time increases, i.e., the PL band maximum shifts from ≈499 to ≈560 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 ≈503 to ≈565 nm. Moreover, the FWHM (Full Width at Half Maximum) of the PL band is essentially time invariant for the sample hydrolyzed with LiOH (≈155 nm), shows a slight increase for that hydrolyzed with NaOH (from ≈157 nm at 5 min to ≈166 nm after 1080 mim) and a pronounced increase for samples prepared with KOH (from ≈150 at 5 min to ≈171 nm after 1080 min). This feature indicates that the size distribution of the resulting Q-dot is nearly time invariant using LiOH and continually broadens in presence of KOH. As it can be seen in the pictures of samples displayed in the top of Fig. 1, the main colours of the visible spectra (excepting for the red) can be achieved by the tuning of the ZnO Q-dot size.
The effect of the nature of alkali base in the time evolution of optical properties of the colloidal system was evaluated by UV–Vis monitoring. The absorption spectra of colloids prepared with LiOH and NaOH are displayed in Fig. 2(a) and (b), respectively. The observed time evolution of the absorption spectra is similar to that reported for ZnO colloids prepared with KOH [9]. Essentially, we observe a red shift of the excitonic peak evidencing a decrease in the band gap energy due to the ZnO Q-dot size growth irrespective of the base nature. However, this evolution is faster for the first four spectra measured during the first 20 min. We note that from 5 min the amplitude of the excitonic maxima stays essentially invariant, indicating that the total amount of ZnO forming the quantum dot is kept almost constant. The first and the last recorded spectra make evident the red-shifting; from ≈306 to ≈334 nm after 500 min of LiOH base addition (Fig. 2a), while for the NaOH (Fig. 2(b)) this shift is higher reaching ≈342 nm after 500 min. For sample prepared with KOH a second regime was observed after 500 min [9], characterized by an increase in the absorption above the excitonic peak edge. This feature was attributed to the light scattering caused by the formation of large particles or aggregates, which was also evidenced by the pronounced turbidity observed with the naked eye at the end of the reaction