**2.1 Mechanisms of photocatalytic degradation of pollutants in water by perovskite**

The mechanisms of photocatalytic degradation of organic pollutants consists of several steps: (1) under light irradiation perovskite absorbs photon with an appropriate energy to form photoreactive species like e− and h+ ; (2) interfacial charge transfer; (3) reduction and oxidation process to form Reactive Oxidation Species; (4) degradation of organic pollutants; and (5) desorption of pollutants/intermediates from the surface of the perovskite. The reaction mechanisms of photocatalytic degradation of pollutants in water are demonstrated by Eqs. (1)–(5) [7].

$$\text{Perovskite} + \text{Light} \rightarrow \text{Perovskite} \left(\mathbf{e}\_{ab}^{-} + \mathbf{h}\_{ub}^{\*}\right) \tag{1}$$

$$\mathbf{e}\_{cb}^{-} + \mathbf{O}\_{2} \to \mathbf{O}\_{2}^{-} \tag{2}$$

*Perovskite-Based Nanomaterials and Nanocomposites for Photocatalytic Decontamination… DOI: http://dx.doi.org/10.5772/intechopen.102824*

$$\cdot \text{h}^{\*}\_{\text{ub}} + \text{H}\_{2}\text{O} / \text{OH}^{-} \rightarrow \cdot \text{OH} \tag{3}$$

$$\rm O\_2^- / \cdot OH + \rm OBr \text{amine Plottants} \rightarrow \rm Intermediateizes,} \\ CO\_2, H\_2O... \tag{4}$$

$$\text{H}\_{i\text{\textquotedbl}}^{\text{+}} + \text{Organic Pollutants} \rightarrow \text{Intermediate}, \text{CO}\_{2}, \text{H}\_{2}\text{O} \dots \tag{5}$$

Under light irradiation of perovskite, when the energy of photon is equal or larger than the perovskite band gap energy, the electrons are excited from the valence band (VB) of perovskite to the conduction band (CB), as a result of which the photoactive species (e<sup>−</sup> and h+ ) are formed. The photoexcited electrons would either reunite with holes or transfer to the surface of the perovskite, which can react with O2 to form superoxide anion radical (O2 •−), while the photogenerated holes react with water to form hydroxyl radical (• OH) at the surface of the catalyst [19]. The schematic representation of the degradation mechanism is illustrated in **Figure 2**. In this process, ·OH acts as a powerful oxidising agent that attacks the organic molecules non-selectively.

**Figure 3** shows the bandgap values, CB and VB positions, of several perovskite photocatalysts. It is apparent that pristine perovskites have the valance band potential energy (Evb) higher than the • OH/OH− redox potential, which allows for the generation of ·OH during the photocatalysis process. Nonetheless, the higher position of CB compared to that of the redox potential of O2/O2 •−, hinders the formation of O2 •− during the photocatalytic degradation process. Therefore, during the ROS production on perovskite, in order for the electrons to react with O2 and form O2 •−, the conduction band potential (Ecb) of perovskite should be more negative than the standard redox potential of O2/O2 •− (−0.33 eV vs. NHE). On the other hand, the valance band potential energy (Evb) of perovskite should be higher than standard redox potential of • OH/ OH− (+1.99 eV vs. NHE). In such case, the OH<sup>−</sup> can be oxidised by the photogenerated holes and form • OH, which can attack pollutants to convert them to nontoxic forms or completely degrade them to CO2 and H2O [20].

#### **Figure 2.**

*Schematic representation of photocatalytic degradation of organic pollutants and ROS production by perovskite (reprinted with permission from ref. [7]. Copyright © 2021, Elsevier).*

**Figure 3.**

*Band gap values of several perovskite photocatalysts (Adapted with permission from ref. [7]. Copyright © 2021, Elsevier).*

#### **2.2 Perovskite design criteria for photocatalytic degradation of organic pollutants**

The main criteria for a perovskite photocatalyst to be used in the degradation of organic pollutants in water are high capability of being activated by photons; efficiently extracting electrons for photocatalytic reaction; chemically stable; nontoxic; and cost effective. The absorption of photons the following charge generation is dependent on the physiochemical property of the perovskite and recombination.

The efficient use of solar energy still remains a great challenge. An ideal perovskite photocatalyst should have an enhanced and broaden light absorption, and capture a wide spectrum, from ultraviolet to visible light and even the near-infrared region. Therefore, it is necessary to adopt strategies that lead to optimisation of light harvesting, improving e<sup>−</sup> -h+ separation, and generating sufficient active sites on the perovskite surface for photocatalytic reaction to take place A number of strategies have been reported in the literature, such as cationic substitution, nanostructure perovskite, coating and combined perovskite-based photocatalyst systems, in which perovskite is coupled with other AOP systems. The main aim of these state-of-the-art strategies is to enhance efficient light utilisation, improve charge separation and create richer active sites on the surface of the perovskite. Narrowing band gap is usually the option for the increased light harvesting by capturing more excited photons form a wide spectrum, and consequently enhancing photocatalytic activity [21].

Once the photogenerated charges are generated and successfully migrated to the surface of perovskite, where photocatalytic reactions take place, they can still undergo surface recombination or be trapped by undesirable reactants. In the photocatalytic process the e− -h+ pairs are generated within several femtoseconds (fs) and undergo recombination within picoseconds (ps) to nanoseconds (ns), as depicted in **Figure 4**. However, the time span from the bulk to reactive sites is usually hundreds of ps, and the reaction time between the carriers and the adsorbed reactants requires nanoseconds (ns) to microseconds (μs) [22]. The lifetime of the photogenerated charges of some perovskites have been reported as BTO: 3.25 ns, STO: 2.06 ns, LFO: 3 ns and LMO: 2 ns, knowing that the reaction time to form O2 •− is several nanoseconds [22]. This implies that the relatively short lifetime of the carriers on perovskite limits their application in photocatalytic degradation of organic pollutants.

*Perovskite-Based Nanomaterials and Nanocomposites for Photocatalytic Decontamination… DOI: http://dx.doi.org/10.5772/intechopen.102824*

**Figure 4.** *Different length of time required in photocatalytic process.*

In general, photocatalytic degradation takes place on the surface of the perovskite photocatalyst. Therefore, to improve photocatalytic degradation efficiency, a good adsorption of organic pollutants on the surface of perovskite is necessary. Undoubtedly, larger surface area is required to provide higher adsorption capacity towards organic pollutants and richer active sites for photocatalytic degradation reaction. A shorter diffusion pathway of charge carriers is also expected, as it reduces chance of e− -h<sup>+</sup> recombination.
