Photocatalysis

Heterogeneous photocatalysis is a process combining the absorption of light on semiconductor materials, called photocatalysts. Examples of popular photocatalysts are transition metal oxides such as TiO2, V2O5, BiVO4, etc. But how photocatalysis is operating?

The electronic structure of a semiconductor photocatalyst is composed of a valence band (VB) and a conduction band (CB) separated by the energy band gap (Eg), also called forbidden band. The activation of photocatalysis starts when the incident light (Einc = hν) has energy higher than the Eg. Indeed, for Einc > Eg, each absorbed photon results in the excitation of an electron (e-) into CB, thus leaving a hole (h+) in the VB.

Therefore, the principle of heterogeneous photocatalysis is based on the generation of e-/h+ pairs (Eq. 1). Afterward, the photogenerated charge carriers react with water and dissolved oxygen to form ROS (Eq. 2-6). Subsequently, the ROS react with the organic pollutants, thus degrading them into smaller molecules up to full mineralization. It is worth mentioning that organic pollutants can be directly degraded through photogenerated holes, which are also oxidizing species. However, such a reaction is not kinetically favored due to the strong reactivity of ROS toward organic molecules.

Semiconductor + hν →  e− + h+ (Eq. 1)

H2O + h+ →  HO• + H+ (Eq. 2)  

O2 + e− →  O2•− (Eq. 3)

O2•− + H+ =  HOO• (Eq. 4)

2HOO• →  H2O2 + O2 (Eq. 5)

H2O2 + e− (or hν) →  HO• + OH− (or HO•) (Eq. 6)

The photocatalytic process should fulfill several requirements in order to observe the above mentioned reactions. Indeed, since e- and h+ are reducing and oxidizing agents, respectively, they are used in redox reactions. These photo-induced reactions are thermodynamically feasible only if the corresponding redox potential lies within the energy band gap of the semiconductor photocatalyst. For example, the formation of O2•− and HO• (by reaction with photogenerated e- and h+, respectively) have redox potentials of -0.33 V and +2.70 V (vs. SHE), so their generation is feasible according to the energetic position and the value of Eg of the semiconductor photocatalyst. Therefore, a key parameter in photocatalysis is the selection of a suitable material (with suitable Eg). Another important parameter is the diffusion length of photogenerated charge carriers in the selected materials. Indeed, since the e-/h+ pair generation is in competition with its recombination process, the photocatalytic process is favored when the particle size of the photocatalyst is small, so the photogenerated e- and h+ can reach the surface of the material. It is important to mention that photocatalysis is a surface dependent process (Langmuir-Hinshelwood model), i.e. electrons and holes react with reactants which are adsorbed at the surface of the photocatalyst, thus highlighting the crucial role of the adsorption properties of the photocatalytic materials. Indeed, its point of zero charge (PZC) along with the pKa of the organic pollutants should be considered i.e. their electrosatic interactions.