**3. Mechanism of photocatalysis**

In presence of light photocatalyst material shift the electron from valance band (VB) to the conduction band (CB). VB which has oxidative potential of +1.0 to +3.5 V versus the normal hydrogen electrode (NHE), and CB have a chemical potential of +0.5 to −1.5 V versus the NHE, hence they can act as reductants. **Figure 2**

**Figure 2.** *Mechanism of photocatalytic H2 production.*

#### *Hydrogen as a Clean Energy Source DOI: http://dx.doi.org/10.5772/intechopen.101536*

describes, photocatalysis method in three steps (1) migration of electrons (negative charge) from VB to CB, by leaving exact vacant holes (positive charge), (2) movement of excited holes and electrons to the surface and (3) excited electrons-holes they react with absorbed electron donors and electron acceptors for reduction and oxidation reactions. Along with proper band gap of semiconductor photocatalyst material its charge recombination rate, mobility and lifetime of electron and holes also plays a crucial role in overall hydrogen production [17].
