**2.2 Perovskite systems for photocatalysis**

*Perovskite and Piezoelectric Materials*

*2.1.2 Layered perovskite-related systems*

fabricating new compounds of AB(1−m)Bm

photocatalysts [44].

AI

AI

is designated as An+1BnO3n+1 or A2

new framework with different stoichiometry is A1−mAm

oxygen-deficient materials with new framework such as A1−mAm

I

photocatalysts. A common formula for DJ phase is A<sup>I</sup>

structed by one after another fluctuating layers of [Bi2O2]

Moreover, to the overall ABO3 system, further characteristic polymorphs of the perovskite system are Brownmillerite (BM) (A2B2O5) framework [39]. BM is a type of oxygen-deficient perovskite, in which the unit cell is a system of wellorganized BO4 and BO6 units. The coordination number of cations occupied by A-site was decreased to eight because of the oxygen deficiency. Perovskite (ABO3) oxides have three dissimilar ionic groups, construction for varied and possibly useful imperfection chemistry. Moreover, the partial replacement of A and B ions is permitted even though conserving the perovskite system and shortages of cations at the A-site or of oxygen anions are common [40]. The Ion-exchange method is used for the replacement of existing metal ions with similar sized or dissimilar oxidation states; then, imperfections can be announced into the system. The imperfection concentrations of perovskites could be led by doping of different cations [24]. Oxygen ion interstitials or vacancies could be formed by the replacement of B-position cations with higher or lower valence, respectively,

I

perovskite system is Brownmillerite (A2B2O5), in which one part of six oxygen atoms is eliminated. Moreover, the replacement of exciting a site cation to new cation with higher oxidation state metal ions then the formed new materials with

replacement of A-site ions with smaller oxidation state cations, consequences in

oped. Thermodynamically, the replacement of B-position vacancies in perovskite systems is not preferable due to the compact size and the high charge of B cations [42]. A-position vacancies are more detected due to the BO3 range in perovskite system forms a stable network [43]; the 12 coordinated sites can be partly absent due to bigger-size A cations. Lately, presenting suitable imperfections on top of the surface of perovskite oxides has been thoroughly examined as a means of varying the bands' position and optical properties of the starting materials. For this reason, perovskite materials afford a tremendous objective for imperfection originating to vary the photocatalytic activity of perovskite material-based

The typical formula for the furthermost recognized layered perovskite materials

[An−1BnO3n+1] (Dion-Jacobson (DJ) phase) for {100} series, (AnBnO3n+2) for {110} series and (An+1BnO3n+3) for {111}, and (Bi2O2)(An−1BnO3n+1) (Aurivillius phase) series. In these systems, n represents the number of BO6 octahedra that duration a layer, which describes the width of the layer. Typical samples of these layered systems are revealed in **Figure 1c**–**g**. For RP phases, their frameworks consist of AI

the spacing layer for the intergrowth ABO3 system. These materials hold fascinating properties such as ferroelectricity, superconductivity, magnetoresistance, and photocatalytic activity. Sr2SnO4 and Li2CaTa2O7 systems are materials of simple RP kind

 splits the perovskite-type slabs and is characteristically a monovalent alkali cation. The typical DJ kind photocatalysts are RbLnTa2O7 (n = 2) and KCa2Nb3O10 (n = 3). Associates of the AnBnO3n+2 and An+1BnO3n+3 structural sequences with dissimilar layered alignments have also been recognized in some photocatalysts like Sr2Ta2O7 and Sr5Ta4O15 (n = 4). For Aurivillius phases, their frameworks are con-

blocks. Bi2WO6 and BiMoO6 (n = 1), found as the primary ferroelectric nature for Aurivillius materials, lately have been extensively investigated as visible light

O3−δ [41]. A typical oxygen-deficient

BO3 [41]. In the case of the

BO3−x are devel-

O as

I

[An−1BnO3n+1] (n > 1), where

2+ and virtual perovskite

I

An−1BnO3n+1 (Ruddlesden-Popper (RP) phase),

**6**

photocatalysts.

A broad array of perovskite photocatalysts have been advanced for organic pollutant degradation in the presence of ultraviolet or visible-light-driven through the last two decades [45]. These typical examples and brief investigational consequences on perovskites are concise giving to their systems, then perovskite materials categorized into six groups. Precisely, ABO3-type perovskites, AA<sup>I</sup> BO3, AI ABO3, ABB<sup>I</sup> O3 and AB(ON)3-type perovskites, and AA<sup>I</sup> BBIIO3-type perovskites are listed in **Table 1**.
