5.2. The influence of the reaction time

The reaction time determines how long is the contact between light with photocatalyst and that of electrons with the reducible metal ions. The general trend observed is that the photoreduction efficiency improves as the ratio of time is further extended. Longer than the optimum time, the photoreduction is usually independent on the reaction time. In the beginning and further extension time, the contact between light and TiO2 becomes more effective, resulting in more number of electrons. Then, the extension time allows more effective contact between the electrons available with the reducible metal ions. This can enhance significantly the photoreduction. At one time, the photoreduction reaches maximum level, showing the optimum reaction time. For longer time than the optimum one, a very large amount of the products has been resulted that may prevent the contact among the reacting agents. Consequently, TiO2 is hindered to release more electrons that give no increase in the photoreduction. The other reason is the reducible ions in the solution have been completely reduced that no more ions are left in the solution. The optimum reaction time is detected to be varied: 50– 60 min and 5 h for Ag(I) [5, 10], 4 h for Cr(VI) [23], 3 h for Cu(II) [25], and 50–150 min for Hg (II) [27].
