The effect of initial dye concentration on the photocatalytic process has been examined. The initial concentration of acid violet was varied from 1 ื 10−5 M to 9 ื 10−5 M. The results of these experiments are shown in Fig. 8. For the concentration of 1 ื 10−5 M the decomposition was almost complete after 120 min of illumination, whereas for the higher concentration (9 ื 10−5 M) the photocatalytic efficiency was lower (60%). However, better results can be obtained by increasing the light illumination duration. As the initial concentration of the dye is increased, more dye molecules are adsorbed onto the surface of ZnO:AC. But the adsorbed dye molecules are not degraded immediately because the intensity of the light and the catalyst amount is constant and also the light penetration is less. Hence
the production of hydroxyl and super oxide radicals are limited or reduced. Therefore, the photodegradation efficiency is reduced. Still at higher concentration of the dye, the path length was further reduced and the photodegradation was found to be negligible.