**4. Photocatalytic applications of MOFs**

MOFs have recently received a lot of attention in the field of photocatalytic degradation. Photocatalysis is a green technology that converts sunlight into chemical energy [70]. MOFs have a large surface area and high porosity and are ideal candidates for photovoltaics. MOF breakthroughs in organic contaminant degradation, water splitting, and CO2 photoreduction [71]. TiO2 has a large bandgap (3.2 eV), which limits its photocatalytic properties in the UV light region. This region accounts for only 5% of the spectral region, resulting in low photocatalytic activity. In the last decade, far more effort has been expended to improve light utilization by changing TiO2 via anion/cation doping or incorporating it with other metals/semiconductors [72]. These evaluations are limited to delineating the engineering premise and the photocatalysts working principle.

### **4.1 Fundamental processes**

MOF-based photocatalysis, like the vintage photodegradation phenomenon formed for conventional semiconductor photocatalysts, followed four basic steps of processes (**Figure 16**), which operate from their crystalline structure. The four fundamental operating processes in MOF-based heterogeneous catalysis are as follows [74]:

### *4.1.1 Photoexcitation step*

MOF-based photocatalysts could occur via inorganic SBU or organic ligands. MOFs generally have low absorptions in the UV-Vis region, depending on their fluorophore centers [75]. The spectra shown above are frequently associated with various π–π\* transitions of aromatic units. MOF photocatalysts exhibit a few distinct absorption coefficient bands as a result of inorganic SBUs and organic ligands having distinct molecular orbitals, as well as their lowest energy bands. Semiconductor band theory fails to describe light absorption and subsequent transitions observed in MOFs. LMCT transitions are important for understanding the mechanisms and reaction products in photocatalysis involving MOFs. Even though photo-generated holes and electrons

*Historical Developments in Synthesis Approaches and Photocatalytic Perspectives… DOI: http://dx.doi.org/10.5772/intechopen.107119*

**Figure 16.** *Fundamental steps in a traditional photocatalysis system (reproduced from Ref. [73]).*

have the same energy stages for MOF molecular orbitals (HOMO and LUMO), this is critical for understanding MOF reactions [76].
