*3.4. E*ff*ects of pH Level*

The photocatalytic evolution of hydrogen is affected by the pH level of the sacrificial agent solution. However, the effect of pH on photocatalytic performance depends on the mechanism of the reaction. According to previous research by Markovskaya et al., the pH level of the sacrificial agent can be manipulated by adding NaOH or acetic acid. The addition of acetic acid initially increases the rate of hydrogen evolution, up to a sharp peak at pH 7.5. Further increasing the acetic acid causes a drop in the evolution rate. The H2 production in Na2S/Na2SO3 is described as [29]

$$\text{Na}\_2\text{S} + \text{Na}\_2\text{SO}\_3 + 2\text{H}\_2\text{O} \rightarrow \text{H}\_2 + \text{Na}\_2\text{S}\_2\text{O}\_3 + 2\text{NaOH} \tag{1}$$

According to reaction (1) of the reaction mechanism, the addition of hydroxyl ions thermodynamically impedes the H2 production reaction, while an increase in the concentration of acetic acid promotes the reaction. The dependence of the H2 generation rate on the pH level has been reported for Ni-doped CdS nanorods [100]. Both samples under pH 14.7 conditions performed better than the other two. These results suggested that the concentration of OH- is an important factor that will affect the efficiency of hydrogen generation.
