**1. Introduction**

The rapid growing population, urbanisation and industrial development are the major contributors of organic pollutants in water, which have a detrimental impact on the ecosystem, and cause serious problems to the living world and environment. In order to balance the ecosystem and mitigate the huge risk caused by the persistence organic substances, the removal of organic pollutants in wastewater is paramount.

Over the past two decades, solar photocatalysis has been of particular interest for the removal and degradation of organic pollutants in wastewater. In the photocatalytic water decontamination process, the production of electron-hole (e− -h+ ) pairs via irradiation of the photocatalyst is the key step for the production of reactive oxidation species (ROS, i.e. hydroxyl radicals (• OH)), which is powerful oxidants and can

non-selectively attack organic matters, degrading them into smaller elements and finally mineralise them to H2O and CO2 [1]. Under light irradiation of a photocatalyst, photons with energy equal or greater than its band gap (Eg) are absorbed by the catalyst, resulting in the formation of an electron-hole pair. Then the photogenerated conduction band electron (eCB−) and valence band hole (hVB+) could undergo undesired recombination or participate in a series of reactions to produce highly Reactive Oxidation Species (ROS, i.e., hydroxyl radicals (• OH)) that can mineralise any organic molecule in wastewater [2]. The necessity to find photocatalysts with unique photophysical properties that can be used efficiently in the photocatalysis process has been the driving force for the development of variety of material systems to achieve an efficient removal of organic pollutants. Different types of heterogeneous semiconductors, particularly titanium dioxide (TiO2), ternary and other oxide systems, are the most widely studied materials for photocatalytic water decontamination. TiO2, which is well known for its photocatalytic properties, widely used, low-cost n-type semiconductor with (Eg) of 3.2 eV, can be used for water decontamination and water splitting and building self-cleaning facades [3]. However, the major drawback of using TiO2 in practical photocatalytic water decontamination is concerning two important aspects of its photocatalytic properties: (i) TiO2 offers low photoconversion efficiency due to undesired recombination of electrons and holes [4] and (ii) its large band gap, which can be excited only by ultraviolet light (only 4% intensity of solar radiation) [5]. In addition, compared to other advanced oxidation processes (AOPs), such as Fenton based methods; UV/Oxidant methods and electrochemical oxidation methods, which can in-situ generate ROS during water treatment, the quantum yield of TiO2 is low for photocatalytic ROS production, hindering its application in photocatalytic water decontamination [6]. Therefore, exploring novel photocatalysts that have unique photophysical properties, offer high photo-conversion efficiency and with superior photocatalytic activity in water decontamination application is of great importance.

Perovskite-based nanomaterial have attracted huge attentions as a promising photocatalysis nanomaterial for various environmental application due to their unique features such as high chemical and thermal stability; excellent electrical conductivity; and narrow band gap that can offer efficient use of solar energy, compared to other semiconductor photocatalysts. Perovskite-type oxides are complex metal oxides, with the general formula of ABO3, the structure of which is shown in **Figure 1**. General structure of perovskite oxides represents a lattice that consists of larger A cations and are alkaline rare-earth metals, which are 12 fold coordinated by oxygen atoms, and small B cations that can be a divalent or trivalent transition, within oxygen octahedra. Their high stability under aggressive conditions is attributed to the existence of transition metals in their oxidation states [8, 9]. The structure of perovskites can easily be tuned by adjusting the category and proportion of their chemical compositions, which in turn inherit them diverse and unique physicochemical properties [10]. Perovskite oxides are capable of being activated by broad solar spectra to excite e− -h+ pairs and initiate the production of ROS, which facilitate organic pollutant oxidation and mainly comprise hydroxyl-radical (• OH) and superoxide-anion radical (O2 •−) [11]. However, pure perovskites suffer from low photocatalytic efficiency, which is due to small surface area of bulk material, insufficient solar energy consumption, rapid recombination and low redox potential of e− -h+ pairs, which are unfavorable for efficient generation of reactive species [12].

The performance of perovskites in photocatalysis process is generally influenced by their structure; composition; size and shape and synthesis process. Therefore, with the aim of enhancing their photocatalytic efficiency in the degradation of

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

**Figure 1.** *ABO3-type of perovskite structure (reprinted with permission from ref. [7]. Copyright © 2021, Elsevier).*

organic pollutants, numerous studies have been carried, using various synthesis methods such as sol-gel method; hydrothermal; solvothermal; sono-chemical; microwave assisted method and co-precipitation method. In order to enhance the photocatalytic performance of perovskite, a number of strategies can be adopted, such as regulating perovskite composition through partial or full cationic substitution by certain dopant(s); rescaling its structure through downsizing or morphology alteration; hybrid modification through coating and coupling with other AOPs. It is worth pointing out that the strategy of coupling with other AOPs is beyond the scope of this chapter, therefore, no further reference will be made. The aforementioned strategies have been proven to improve perovskite's light absorption; create more active sites on the surface and inhibit e<sup>−</sup> -h<sup>+</sup> pairs recombination. By regulating perovskite composition through hetero-substitution of perovskite by hetero-valent or homo-valent cations in A and/or B site, the redox property of the perovskite is significantly improved and oxygen vacancies are increased, thereby promoting ROS generation [13]. Incorporating dopants into the lattice of perovskite, its inherent band gap can be reduced by shifting the top of its VB upward or CB downward, leading to an extended optical absorption improvement of its photocatalytic activity. Loading perovskite on substrates to obtain a hybrid nanostructure is an effective option for narrowing the band gap and optimisation of electronic structure to inhibit the recombination of e<sup>−</sup> -h<sup>+</sup> pairs. The coating strategy could address the majority of issues related to the efficient photocatalytic activity to some extent, as the coating strategy equip perovskite with an outstanding charge separation ability and strong oxidation ability. Rescaling structure and downsizing and controlling morphology of perovskites can be carried out to improve reactive sites and optimise optical absorption [14]. Smaller particle size can benefit from higher quantum efficiency due to larger accessible of reactive sites and more effective electron transfer paths. However, downsizing these particles to nanoscale increases the surface energy that prompts particles aggregation, hence elimination of the desired reactive sites and significant reduction of photocatalytic performance [15]. Controlled preparation of porous structure has been proven to equip perovskite with better optical absorption ability; increased reactive sites for photocatalytic reaction; as well as enhances the diffusion rate of organic pollutants. However, introducing pores to perovskite nanoparticles can make it physically fragile [16].

Despite intensive research studies that have been carried out on developing variety of nanoscale perovskite-based composites using different strategies, most of which with encouraging results, there is still much to be investigated. A comprehensive understanding of achieving an effective photocatalytic degradation of a wide range of organic pollutants using perovskites is highly crucial for unveiling the fundamental nature of perovskite photocatalysis for large-scale applications. In addition, to meet the requirements of designing efficient, stable and costeffective perovskite-based composite photocatalyst with an outstanding use of solar energy for actual water remediation, a fundamental study of perovskite photocatalysis using different materials and various environmental pollutants is indispensable.

This chapter provides an overview of the state-of-the-art design and synthesis strategies for perovskite-based nanomaterials and nanocomposites for efficient water remediation. Initially the principles of photocatalysis process are described, with the emphasis on the mechanisms of photocatalytic water decontamination by perovskite and highlighting its inherent challenges. An evaluation of several strategies that have been used to develop perovskite-based nanocomposites for enhanced photocatalytic degradation of organic pollutants in water is presented. Finally, the remaining challenges and perspectives for developing novel perovskite-based photocatalysts with potential large-scale application are elucidated.
