*4.1.2 Charge-transfer step*

This procedure begins with the absorption of massive amounts of light energy, which results in the excitation of positive and negative charges from the bulk to the photocatalyst surface. The efficiency of these procedures is determined by the material type, crystallinity, and particle size. High charge mobility, for example, is frequently associated with higher crystalline semiconductor materials that exhibit focused charge mobility. The adsorption and photodegradation activities of various dyes on three Cd-containing MOFs have been investigated. Anionic dyes (e.g., sulphonated) were significantly adsorbed by Cd2(4,4<sup>0</sup> -bpy)3(S2O3)2, Cd1(4,.4<sup>0</sup> )�2H2O in the dark, but not by non-cationic dyes. The presence of chromophore centers, which may also serve as recombination centers where charge carriers are trapped and quenched, causes structural deficiency in inorganic photocatalysts, resulting in significant energy loss through realized heat [77]. The charge-transfer performance of semiconductor photocatalysts with small granules is satisfactory. If the particles are too small in size, the replication process may occur due to improved surface defects.

#### *4.1.3 Surface activation step*

Charge carriers travel to the photocatalyst surface, where they can be stimulated to perform specific chemical reactions. Photogenerated charge carriers are gathered by photoelectrodes and allowed to open circuit [78]. The first step in a fairly standard photocatalytic reaction is the physical adsorption of adsorbent materials. Anionic and cationic dyes showed different surface adsorption and catalytic properties. Furthermore, dye macromolecular adsorption resulted in non-covalent weak interactions rather than radical reforms in the cadmium thiosulphate-based MOF structures. The hydroxyl radical pathway is critical in the breakdown of anthraquinonic anionic dyes, whereas a surface-controlled N-de-ethylation reaction mechanism was proposed to explain the systematic degradation of cationic dyes via sequential intermediates,

where MLCT inferred from Cd metal to HOMO as well as ligand-associated LUMO play dominant roles [79].

#### *4.1.4 Charge-carrier recombination step*

Photoluminescence (PL) spectrophotometry could be used to capture and interpret the energy released during recombination. Charge-carrier recombination accounts for the majority of energy loss in photocatalysts and PEC processes, and it remains one of the most difficult challenges to overcome [80]. Charge-transport recombination occurs in both the bulk and the surface of the photocatalyst. Reducing charge-carrier recombination from the surface and bulk phases is critical. Surface metallization with noble metals has been shown to be beneficial in charge-carrier combinations [81].

### **4.2 MOF-based photocatalytic degradation of organic pollutants**

MOFs are composed of metal-containing nodes linked by organic ligands via strong covalent bonds. When exposed to light, a few MOFs begin to behave like semiconductor materials, implying they could be useful as photocatalysts [82]. New research has demonstrated porous MOF materials to be a new type of photocatalyst. MOFs have a promising future, despite the fact that they have not been widely investigated to date. It is simple to synthesize MOFs with tunability in light absorption capacity, thereby activating appealing photocatalytic properties. Research into the application of MOFs in this area has so far been largely unexplored. In this chapter, we highlight the importance of photocatalysis in the degradation of organic pollutants into MOFs. The reaction pathway as well as the impact of external variables on electrocatalytic activity are discussed. The main issues in photocatalytic degradation and potential opportunities have been thoroughly discussed [83].

#### **4.3 Photocatalytic properties of d-block metal-based MOFs**

MOFs offer a unique opportunity for the discovery of new catalysts capable of degrading organic pollutants. More effort has been devoted to developing innovative photocatalyst materials based on MOFs. MOFs could also have potential applications in the environmentally friendly removal of organic compounds. Some d-block metalbased MOFs have good photocatalytic efficiency for organic pollutants. The d-block transition metal MOFs are important for their contributions to a variety of fields such as magnetism, catalysis, gas separation, drug delivery, and so on. The transition metal (Zn(II), Cu(II), and Cd(II)) based MOFs that have been studied as photocatalysts for photocatalytic degradation of an organic pollutant under UV, visible, or UV-vis light illumination are summarized in **Table 1** [95].

The MOF-5 is made up of Zn4O clusters that are orthogonally linked by 1,4-bdc linkers at the corners of a cubic framework structure. This MOF was discovered to have a broad absorption band in the wavelength range of 500–840 nm. MOF-5 is a highly efficient photocatalyst that would most likely succeed due to the light source [96]. MOF-5 may improve overall photocatalytic activity efficiency and photodegradation of phenol, like TiO2, could occur via a network of reactions, such as the formation of a radical cation by electron transfer from phenol to MOF (**Figure 17a**). It degraded phenol in aqueous solutions in a manner similar to commercial TiO2 and could improve overall photocatalytic activity efficiency (**Figure 17b**).

