Significant Role of Perovskite Materials for Degradation of Organic Pollutants

The advancement and the use of visible energy in ecological reparation and photodegradation of organic pollutants are being extensively investigated world-wide. Through the last two decades, great exertions have been dedicated to emerging innocuous, economical, well-organized and photostable photocatalysts for ecofriendly reparation. So far, many photocatalysts mostly based on ternary metal oxides and doped with nonmetals and metals with various systems and structures have been described. Among them, perovskite materials and their analogs (layer-type perovskites) include an emerged as semiconductor-based photocatalysts due to their flexibility and simple synthesis processes. This book chapter precisely concentrates on the overall of related perovskite materials and their associated systems; precisely on the current progress of perovskites that acts as photocatalysts and ecofriendly reparation; explores the synthesis methods and morphologies of perovskite materials; and reveals the significant tasks and outlooks on the investigation of perovskite photocatalytic applications.


Introduction
Solar energy is one of the primary sources in the field of green and pure energy that points to the power predicament and climate change task. Solar energy consumption is an ecological reconciliation, and then, the chemical change in solar is presence exhaustive, considered throughout global [1,2]. In general, solar energy is renewed into a wide range of developments, such as degradation of organic pollutants as photocatalysis, splitting of water molecules for producing clean energy, and reduction of CO 2 gas [3,4]. Consuming a similar perception, metal-oxide photocatalysis has also been widely examined for possible exertions in ecological restitution as well as the photodegradation and elimination of organic toxins in the aquatic system [5, 6], decrease of bacterial inactivation [7-9], and heavy metal ions [10][11][12]. Throughout the earlier few years, excellent applications have been dedicated to evolving well-organized, less expensive, and substantial photocatalysts, particularly those that can become active under visible light such as NaLaTiO 6 , Ag 3 PO 4 /BaTiO 3 , Pt/SrTiO 3 , SrTiO 3 -TiN, noble-metal-SrTiO 3 composites, GdCoO 3 , orthorhombic perovskites LnVO 3 and Ln 1−x Ti x VO 3 (Ln = Ce, Pr, and Nd), Ca 0.6 Ho 0.4 MnO 3 , Ce-doped BaTiO 3 , fluorinated Bi 2 WO 6 , graphitic carbon nitride-Bi 2 WO 6 , BaZrO 3−δ , CaCu 3 Ti 4 O 12 , [13][14][15][16][17][18][19][20][21][22][23][24], graphene-doped perovskite materials, and nonmetal-doped perovskites [25]. Furthermore, directed to years extended exhaustive investigation exertions on the pursuit of innovative photocatalytic systems, particularly that can produce the overall spectrum of visible-light. Out of a vast assemblage of photocatalysts, perovskite or layered-type perovskite systems and its analogs include a better candidate for capable semiconductor-based photocatalysts due to their framework easiness and versatility, excellent photostability, and systematic photocatalytic nature. In general, the ideal perovskite structure is cubic, and the formula is ABO 3 . Where A is different metal cations having charge +1 or +2 or +3 nature and B site occupies with tri or tetra and pentavalent nature, which covers the whole family of perovskite oxide materials by sensibly various metal ions at A and B locations [26], aside from a perfect cubic perovskite system, basic alteration perhaps persuaded by several cations exchange. Such framework alteration could undoubtedly vary the photophysical, optical, and photocatalytic activities of primary oxides.
Moreover, a sequence of layered-type perovskite materials contains many 2D blocks of the ABO 3 framework, which are parted by fixed blocks. The scope of formulating multicomponent perovskite systems by whichever fractional change of cations in A and B or both positions or injecting perovskite oxides into a layeredtype framework agrees scientists investigate and control the framework of crystals and the correlated electronic and photocatalytic activities of the perovskites. So far, hundreds of various types of perovskite or perovskite-based catalysts have been published, and more outstandingly, some ABO 3 -related materials became renowned with "referred" accomplishment for catalytic activities. Thus, these systems (perovskite materials) have exposed highly capable of upcoming applications on the source of applying more attempts to them. While several outstanding reviews mean that explained that perovskites performed as photocatalyst for degradation of organic pollutants [27][28][29][30], only an insufficient of them content consideration to inorganic perovskite (mostly ABO 3 -related) photocatalysts [31][32][33]. A wide range of tagging and complete attention of perovskite materials, for example, layered-type perovskite acting as photocatalysts, is relatively deficient. The purpose of this book chapter is to precise the current progress of perovskite-based photocatalysts for ecological reparation, deliberate current results, and development on perovskite oxides as catalysts, and allow a view on the upcoming investigation of perovskite materials. After a short outline on the wide-ranging structure of perovskite oxides, it was stated that perovskites act as a photocatalyst that are incorporated, arranged and explored based on preparation methods [29,34], photophysical properties based on bandgap energies, morphology-based framework and the photocatalytic activities depends on either UV or visible light energy of the semiconducting materials. Finally, this chapter is based on the current advancement and expansion of perovskite photocatalytic applications under solar energy consumption. The potential utilization, new tasks, and the research pathway will be accounted for the final part of the chapter [35].

Perovskite frameworks
The standard system of perovskite-based materials could be designated as ABO 3 , where the A and B are cations with 12-fold coordinated and 6-fold coordinated to concerning oxygen anions. Figure 1a describes the typically coordinated basic of the ABO 3 system, which consists of a 3D system, BO 6 octahedra as located at corner, and at the center, A cation are occupied. Within the ABO 3 system, the A cation usually is group I and II or a lanthanide metal, whereas the B is commonly a transition metal ion. The tolerance factor (t) = 1 calculated by using an equation t = (rA + rO)/ √ 2 (rB + rO), where r O , r A , and r B are the radii of respective ions A and B and oxygen elements for a cubic crystal structure ABO 3 perovskite system [36].
For constituting a stable perovskite, it is typically the range of t value present in between 0.75 and 1.0. The lower value of t builds a somewhat slanted perovskite framework with rhombohedral or orthorhombic symmetry. In the case of t, it is approximately 1; then, perovskite structure is an ideal cubic system at high temperatures. Even though the value of t, obtained by the size of metal ion, is a significant guide for the permanency of perovskite systems, the factor of octahedral (u) u = r B /r O and the role of the metal ions composition of A and B atoms and the coordination number of respective metals are considered [37]. Given the account of those manipulating factors and the electro-neutrality, the ABO 3 perovskite can hold a broad variety of sets of A and B by equal or dissimilar oxidation states and ionic radii. Moreover, the replacement of A or B as well as both the cations could be partly by the doping of various elements, to range the ABO 3 perovskite into a wide-ranging family of A m . The replacement of several cations into the either A or B positions could modify the structure of the original system and therefore improve the photocatalytic activities [23]. After various metal ions in perovskite oxide are doped, the optical and electronic band positions, which influence the high impact on the photocatalytic process, are modified [24].

Layered perovskite-related systems
Moreover, to the overall ABO 3 system, further characteristic polymorphs of the perovskite system are Brownmillerite (BM) (A 2 B 2 O 5 ) framework [39]. BM is a type of oxygen-deficient perovskite, in which the unit cell is a system of wellorganized BO 4 and BO 6 units. The coordination number of cations occupied by A-site was decreased to eight because of the oxygen deficiency. Perovskite (ABO 3 ) 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. series. In these systems, n represents the number of BO 6 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 A I O as the spacing layer for the intergrowth ABO 3 system. These materials hold fascinating properties such as ferroelectricity, superconductivity, magnetoresistance, and photocatalytic activity. Sr 2 SnO 4 and Li 2 CaTa 2 O 7 systems are materials of simple RP kind photocatalysts. A common formula for DJ phase is A I [A n−1 B n O 3n+1 ] (n > 1), where A I splits the perovskite-type slabs and is characteristically a monovalent alkali cation. The typical DJ kind photocatalysts are RbLnTa 2 O 7 (n = 2) and KCa 2 Nb 3 O 10 (n = 3). Associates of the A n B n O 3n+2 and A n+1 B n O 3n+3 structural sequences with dissimilar layered alignments have also been recognized in some photocatalysts like Sr 2 Ta 2 O 7 and Sr 5 Ta 4 O 15 (n = 4). For Aurivillius phases, their frameworks are constructed by one after another fluctuating layers of [Bi 2 O 2 ] 2+ and virtual perovskite blocks. Bi 2 WO 6 and BiMoO 6 (n = 1), found as the primary ferroelectric nature for Aurivillius materials, lately have been extensively investigated as visible light photocatalysts. and with wide bandgap of 4.0 eV. In order to enhance the surface area of NaTaO 3 bulk material, many investigators tried to use further synthetic ways to make nanosized particles as an additional study on the NaTaO 3 photocatalyst for degradation of organic pollutants. Kondo et al. prepared a colloidal range of NaTaO 3 nanoparticles consuming three-dimensional mesoporous carbon as a pattern, which was pretend by the colloidal arrangement of silica nanospheres. After calcining the mesoporous carbon matrix, a colloidal arrangement of NaTaO 3 nanoparticles with a range of 20 nm and a surface area of 34 m 2 g −1 was attained. C-doped NaTaO 3 material was tested for degradation of NO x under UV light [36]. Several  , and PbTiO 3 [68, 69] were also described as UV or visible light photocatalysts. Magnetic BiFeO 3 , recognized as the one of the multi-ferric perovskite materials in magnetoelectric properties, was also examined as a visible light photocatalyst for photodegradation of organic pollutants because of small bandgap energy (2.2 eV) [70][71][72][73][74][75][76][77][78][79]. In a previous account, BiFeO 3 with a bandgap of around 2.18 eV produced by a citric acid-supported sol-gel technique has revealed its visible-light-driven photocatalytic study by the disintegration of methyl orange dye [70]. The subsequent investigations on BiFeO 3 are primarily concentrated on the synthesis of new framework BiFeO 3 with various morphologies. For instance, Lin and Nan et al. prepared BiFeO 3 unvarying microspheres and microcubes by a using hydrothermal technique as revealed in Figure 2 [73].
The bandgap energies of BiFeO 3 compounds were found to be about 1.82 eV for BiFeO 3 microspheres and 2.12-2.27 eV for microcubes. This indicated that the absorption edge was moved toward the longer wavelength that is influenced by the crystal-field strength, particle size, and morphology. The microcube material showed the maximum photocatalytic degradation performance of congo red dye under visible-light irradiation due to the quite low bandgap energy. Further, a simplistic aerosol-spraying method was established for the synthesis of mesoporous BiFeO 3 hollow spheres with improved activity for the photodegradation of RhB dye and 4-chlorophenol, because of improved light absorbance ensuing from various light reflections in a hollow chamber and a very high surface area [71]. Moreover, a unusually improved water oxidation property on Au nanoparticle-filled BiFeO 3 nanowires under visible-light-driven was described [77]. The Au-BiFeO 3 hybrid system was encouraged by the electrostatic contact of negatively charged Au nanoparticles and positively charged BiFeO 3 nanowires at pH 6.0 giving to their various isoelectric points. An improved absorbance between 500 and 600 nm was found for Au/BiFeO 3 systems because of the characteristic Au surface plasmon band existing visible light region then which greater influenced in the photodegradation of organic pollutants. Also, the study of photoluminescence supported improvement of the photocatalytic property due to the effective charge transfer from BiFeO 3 to Au. Even though Ba, Ca, Mn, and Gd-doped BiFeO 3 nanomaterials have exhibited noticeable photocatalytic property for the degradation of dyes [80][81][82][83][84], several nano-based LaFeO 3 with various morphologies such as nanoparticles, nanorods, nanotubes, nanosheets, and nanospheres have also been synthesized for visible light photocatalysts for degradation of organic dyes [85][86][87][88][89][90][91][92][93]. Sodium bismuth titanate (Bi 0.5 Na 0.5 TiO 3 ) has been extensively used for ferroelectric and piezoelectric devices. It was also investigated as a UV-light photocatalyst with a bandgap energy of 3.0 eV [94][95][96][97]. Hierarchical micro/nanostructured Bi 0.5 Na 0.5 TiO 3 was produced by in situ self-assembly of Bi 0.5 Na 0.5 TiO 3 nanocrystals under precise hydrothermal conditions, through the evolution mechanism was examined in aspect means that during which the growth mechanism was studied [95]. It was anticipated that the hierarchical nanostructure was assembled through a method of nucleation and growth and accumulation of nanoparticles and following in situ dissolution-recrystallization of the microsphere type nanoparticles with extended heating period and enhanced temperature or basic settings. The 3D hierarchical Bi 0.5 Na 0.5 TiO 3 showed very high photocatalytic activity for the decomposition of methyl orange dye because of the adsorption of dye molecules and bigger surface area. The properties of Bi 0.5 Na 0.5 TiO 3 were also assessed by photocatalytic degradation of nitric oxide in the gas phase [95]. La 0.7 Sr 0.3 MnO 3 , acting as a photocatalyst, was examined for solar light-based photocatalytic decomposition of methyl orange [96][97][98]. In addition, La 0.5 Ca 0.5 NiO 3 [99], La 0.5 Ca 0.5 CoO 3−δ [100], and Sr 1−x Ba x SnO 3 (x = 0-1) [101] nanoparticles were synthesized for revealing improved photocatalytic degradation of dyes. A-site strontium-based perovskites such as SrTi 1−x Fe x O 3−δ , SrTi 0.1 Fe 0.9 O 3−δ , SrNb 0.5 Fe 0.5 O 3 , and SrCo 0.5 Fe 0.5 O 3−δ compounds were prepared through solid-state reaction and solgel approaches, and were examined for the degradation of organic pollutants under visible light irradiation [102][103][104][105]. Also, some other researchers modified A-site with lanthanum-based perovskites such as LaNi 1−x Cu x O 3 and LaFe 0.5 Ti 0.5 O 3 were confirmed as effective visible light photocatalysts for the photodegradation of p-chlorophenol [91,106,107]. The other ABB I O 3 kind photocatalysts with Ca(TiZr)O 3 [108], Ba(ZrSn)O 3 [109], Na(BiTa)O 3 [110], Na(TiCu)O 3 [111], Bi(MgFeTi)O 3 [112], and Ag(TaNb)O 3 [113] have also been studied. Related to AA I BO 3 -type perovskites, the ABB I O 3 kind system means that BI-site substitution by a different cation is another option for tuning the physicochemical or photocatalytic properties of perovskites materials as photocatalyst, due to typically the B-position cations in ABO 3 mostly regulate the position of the conduction band, moreover to construct the structure of perovskite system with oxygen atoms. The band positions of photocatalyst can be magnificently modified by sensibly coalescing dual or ternary metal cations at the B-position, or changing the ratio of several cations, which has been fine verified by the various materials as mentioned above. More studies on ABB I O 3 kind of photocatalysts are projected to show their new exhilarating photocatalytic efficiency.

Photocatalytic activity of layered perovskite materials
In the general formula of the RP phase, A n−1 A 2 I B n O 3n+1 , A and A I are alkali, alkaline earth, or rare earth metals, respectively, while B states to transition metals. A and A I cations are placed in the perovskite layer and boundary with 12-fold cuboctahedral and 9-fold coordination to the anions, respectively, whereas B cations are sited inside the perovskite system with anionic squares, octahedra, and pyramids. The tantalum-based RP phase materials have been examined as photocatalysts for degradation of organic pollutants under UV light irradiation conditions; such materials are K 2 Sr 1.5 Ta [126], and N-alkyl chain inserted H 2 CaTa 2 O 7 [127]. A series of various metals and N-doped perovskite materials were synthesized, such as Sn, Cr, Zn, V, Fe, Ni, W, and N-doped K 2 La 2 Ti 3 O 10 , for photocatalysis studies under UV and visible light irradiation [128][129][130][131][132][133]. Still, only Sn-doping efficiently decreased the bandgap energy of K 2 La 2 Ti 3 O 10 from 3.6 eV to 2.7 eV. The bandgap energy of N-doped K 2 La 2 Ti 3 O 10 was measured to be around 3.4 eV. Additional RP phase kind titanates like Sr 2 SnO 4 [134], Sr 3 Ti 2 O 7 [135], Cr-doped Sr 2 TiO 4 [136], Sr 4 Ti 3 O 10 [137], Na 2 Ca 2 Nb 4 O 13 [138], and Rh-and Ln-doped Ca 3 Ti 2 O 7 [139] have also been examined. Bi 2 WO 6 (2.8 eV) shows very high oxygen evolution efficacy than Bi 2 MoO 6 (3.0 eV) from aqueous AgNO 3 solution under visible-light-driven. Because of the appropriate bandgap energy, comparatively elevated photocatalytic performance, and good constancy, Bi 2 MO 6 materials have been thoroughly examined as the Aurivillius phase kind that acts as photocatalysts under visible light. In this connection, hundreds of publications associated to the Bi 2 MoO 6 and Bi 2 WO 6 act as photocatalysts so far reported. Most of the investigations in the reports are concentrated on the synthesis of various nanostructured Bi 2 MoO 6 and Bi 2 WO 6 as well as nanofibers, nanosheets, ordered arrays, hollow spheres, hierarchical architectures, inverse opals, and nanoplates, etc., by various synthesis techniques like solvothermal, hydrothermal, electrospinning, molten salt, thermal evaporation deposition, and microwave. All these methods of hydrothermal process have been frequently working for the controlled sizes, shapes, and morphologies of the particles. The photocatalytic properties of these perovskite materials are mostly examined by the photodegradation of organic pollutants. Moreover, the investigations on the simple Bi 2 MoO 6 and Bi 2 WO 6, doped with various metals and nonmetals such as Zn, Er, Mo, Zr, Gd, W, F, and N, into Bi 2 MoO 6 and Bi 2 WO 6 was studied for increasing the photocatalytic performance under visible light. Therefore, these Bi 2 MO 6 -based photocatalysts is not specified here, due to further full deliberations that can be shown in many reviews [140][141][142].
ABi 2 Nb 2 O 9 where A is Ca, Sr, Ba and Pb is other type of the AL-like layered perovskite material [143][144][145][146][147][148][149][150]. The bandgap energy of PbBi 2 Nb 2 O 9 is 2.88 eV and originally described as an undoped with single-phase layered-type perovskite material used as photocatalyst employed under visible light irradiation [144]. Bi 5 FeTi 3 O 15 is also Aurivillius (AL) type multi-layered nanostructured perovskite material with a low bandgap energy (2.1 eV) and also shows photocatalytic activity under visible light [151,152]. Mostly, these materials were synthesized using the hydrothermal method that has been frequently working for the controlled shapes such as flower-like hierarchical morphology, nanoplate-based, and the complete advance process from nanonet-based to nanoplate-based micro-flowers was shown. The photocatalytic activity of Bi 5 FeTi 3 O 15 was studied by the degradation of rhodamine B and acetaldehyde under visible light [151]. The La substituted Bi 5−x La x Ti 3 FeO 15 (x = 1, 2) Al-type layered materials were synthesized through hydrothermal method and these materials were used for photodegradation of rhodamine B under solarlight irradiation [153]. Among all AL-type perovskite materials, only PbBi 2 Nb 2 O 9 , Bi 2 MO 6 (M = W or Mo), and Bi 5 Ti 3 FeO 15 are very high photocatalytic active under visible-light-driven due to low bandgap energy and photostability. Another type of layered perovskite material is Dion-Jacobson phase (DJ), a simple example is CsBa 2 M 3 O 10 (M = Ta, Nb) and oxynitride crystals used for degradation of caffeine from wastewater under UVA-and visible-light-driven [154]. Similarly, another DJ phase material such means Dion-Jacobsen (DJ) as CsM 2 Nb 3 O 10 (M = Ba and Sr) and also doped with nitrogen used for photocatalysts for degradation of methylene blue [155]. Zhu et al. prepared tantalum-based {111}-layered type of perovskite material such as Ba 5 Ta 4 O 15 from hydrothermal method, which has been frequently employed for the controlled shape like hexagonal structure with nanosheets and used as photocatalyst for photodegradation of rhodamine B and gaseous formaldehyde [156]. Pola et al. synthesized a layered-type perovskite material constructed on A I A II Ti 2 O 6 (A I = Na or Ag or Cu and A II = La) structure for the photodegradation of several organic pollutants and industrial wastewater under visible-light-driven [157][158][159][160][161][162].