Photocatalysis and perovskite oxide-based materials: a remedy for a clean and sustainable future

The massive use of non-renewable energy resources by humankind to fulfill their energy demands is causing severe environmental issues. Photocatalysis is considered one of the potential solutions for a clean and sustainable future because of its cleanliness, inexhaustibility, efficiency, and cost-effectiveness. Significant efforts have been made to design highly proficient photocatalyst materials for various applications such as water pollutant degradation, water splitting, CO2 reduction, and nitrogen fixation. Perovskite photocatalyst materials are gained special attention due to their exceptional properties because of their flexibility in chemical composition, structure, bandgap, oxidation states, and valence states. The current review is focused on perovskite materials and their applications in photocatalysis. Special attention has been given to the structural, stoichiometric, and compositional flexibility of perovskite photocatalyst materials. The photocatalytic activity of perovskite materials in different photocatalysis applications is also discussed. Various mechanisms involved in photocatalysis application from wastewater treatment to hydrogen production are also provided. The key objective of this review is to encapsulate the role of perovskite materials in photocatalysis along with their fundamental properties to provide valuable insight for addressing future environmental challenges.


Introduction
Rapid industrialization and population growth resulted in increased energy resources utilization and release of harmful pollutants. The increased human activities are depleting the energy resources and polluting our ecosystem. The toxic gasses in the air are causing respiratory diseases, and the direct disposal of industrial and organic waste into the water reservoirs is causing water-borne diseases. Nearly 663 million people in the world have no access to clean water. The amount of CO 2 in the atmosphere is also increasing due to rapid industrialization. The increased concentration of CO 2 is causing is global warming and is the leading cause of acid rain, which is dangerous for all living things on the earth. The current decade 2021-2030, is the decade of ecosystem system restoration. Therefore, it is essential to pay attention to clean ways to tackle environmental issues. The new global challenge is the achievement of ecological sustainability. The research focus of this era is oriented toward the way to achieve the eco-friendly system. [1][2][3][4][5][6] The blissful gi of Mother Nature, the Sun is an ultimate renewable energy resource that irradiates 3.85 yotta joule (YJ) of energy yearly on the earth's surface. Sunlight is one of the best routes to deal with environmental issues owing to its cleanness and abundant availability. Sunlight can be utilized as an energy medium by photovoltaic, photoelectrochemical catalysis, or photocatalysis in daily activities. 7,8 Photocatalysis is dened as the science of employing a catalyst that uses light to speed up the chemical reaction. In photocatalysis, the photocatalyst material is used, and the energy of the light acts as a source to generate electron-hole pair. The photogenerated electron-hole pair then initiates the redox reaction at the surface of the photocatalyst. This redox reaction can be utilized to degrade water pollutants and convert abundant earth elements (H 2 O, CO 2, and N 2 ) into fuel (pure H 2 , or organic fuel like CH 4 , CH 3 OH, and NH 3 ). The conversion of water into the fuel (H 2 ) by using light and photocatalyst material is known as photocatalytic water splitting. The conversion of CO 2 into the hydrocarbons is known as CO 2 reduction, and conversion of nitrogen into NH 3 is called nitrogen xation. All these application deals with the viable way to clean the environment. The main challenge while dealing with photocatalysis is the region of operation of the photocatalyst in the solar spectrum. Most photocatalysts operate under UV radiations, which is only 5% of the sunlight reaching earth. Therefore, suitable photocatalysts are required to respond to the extensive range of the solar spectrum. [9][10][11][12][13][14][15][16][17][18] History of photocatalysis The word catalysis was rst used in 1836 by the Jöns Jakob Berzelius, derived from the Greek word "kata", meaning down or loosen. Catalysis is the process of fasting or accelerating the reaction in the presence of the catalyst. The catalyst itself doesn't participate in the reaction but increases the reaction rate. The term photocatalysis rst appeared in 1911, when Eibner studied the effect of light irradiation on transition metal oxide (ZnO) for the decolorization of Prussian blue. Photocatalysis term also appeared simultaneously in the title of the article investigating the degradation of the oxalic acid in the presence of the uranyl salts upon light irradiation. In 1921 Edward Charles Cyril Baly studied the production of formaldehyde using colloidal uranium salts and ferric hydroxides as a catalyst. Soon aer, in 1924, the Baur and the Perret at the ETH of Zurich reported the effect of light irradiation on the ZnO suspension and observed that ZnO suspension enhanced reduction of Ag + salts to Ag o upon irradiation. The mobility for the TiO 2 as a photosensitizer for the dyes was rst studied by Doodeve and Kitchener in 1938. This work reported that TiO 2 upon light irradiation decolorized the dyes, and TiO 2 remains unchanged during the process. [19][20][21][22] Many researchers from 1940 to 1972 investigated different parameters of the photocatalyst. Photocatalysis gained signicant attention when electrochemical photolysis of the water was done by using titanium dioxide under UV irradiation. The oil crisis in 1973 also changed the social and economic status of the western world. The rst time, the shortage of fossil fuels became a signicant issue, which led to an unprecedented increase in the efforts for alternative energy resources, including photocatalysis. The application of photocatalysis is not limited to energy fuels but can also clean the environment. Much attention has been given to photocatalysis during the last two decades due to its wide range of applications. 21,23 Photocatalysis has its application in the pollutant degradation and production of fuel. Material can be an excellent photocatalyst if it is responsive to light, especially (UV or visible), biologically and chemically inert, has low cost, and exhibits adsorption and absorption capacity. Normally the metal oxide semiconductors are used in the application of the photocatalyst. Perovskite materials, due to their unique optical properties, also gained importance in the application of photocatalyst during the last decade. 24,25 Perovskite oxide based materials Perovskite material was rst discovered in 1839 in the Ural Mountains by the German scientist Gustave Rose. The mineral was named perovskite aer Russian mineralogist Count Lev Aleksevish von Perovski. [26][27][28] The rst discovered perovskite material was CaTiO 3 . 29 The basic chemical formula of the single perovskite is ABX 3 , where A and B are cations while X is an anion. Normally, A is alkaline earth metals or lanthanides, B is the transition metal, and X is oxygen or any halide. The perovskite is known as perovskite oxide when the anion is oxygen and is known as perovskite halide when the anion is a halide. In single perovskite, the coordination number of A site is 6, the coordination number of B site is 12, and the coordination number of O anion is 6. In single perovskite, the perfect structure of the BO 6 octahedral connection results in the cubic lattice. [30][31][32][33] The structure of single perovskite is shown in Fig. 1.
The tolerance factor can indicate the distortion and the crystallographic structure of the perovskite material. The tolerance factor of the single perovskites ions was dened by V. M Goldschmidt in 1926. The tolerance factor of the perovskite structure tells the stability of the perovskite structure and the compatibility of the ions in the crystal structure. [34][35][36][37][38][39] The tolerance factor is a dimensionless quantity given by eqn (1). 38,40 In eqn (1), r A and r B are the ionic radii of the electropositive ions, while r X is the radius of the oxygen or halide ions. 41 The perovskite structure can be divided into octahedral hexagonal, tetragonal, and ideal cubic structures on the basis of the tolerance factor. When the tolerance factor is between 0.9 and 1, the structure is the ideal cubic structure with the length of the unit cell (a) as: 42 In non-ideal cases, the tolerance factor deviates from 1, which shows the mismatch in the A-O and B-O bond lengths. The corundum structure (a-Al 2 O 3 ) and its derivative are considered when the tolerance factor is less than 0.75. 43 The stable bixbite polymorph (a-Mn 2 O 3 ) is favored when the factor decreases. If the tolerance factor is greater than 1, the structure of perovskite is hexagonal close-packed (HCP), and [BO 6 ]octahedra share faces with the hexagonal c-axis. 30,44 The sum of the charges of cations and anion of the perovskite structure should equal one for perovskite material to be electrically neutral. Different cations having different ionic radii and valences can be doped in the structure of perovskite material by employing the method of partial substitution at A and B-site. Deciency at A and B-site cations and excess or deciency of oxygen anions can change the composition of ions and the non-stoichiometry phenomenon. 45,46 Double perovskite structures were used at a signicant scale in the 1980s. The double perovskite exhibits structure that is twice the single perovskite. The coordination number of the A and B cation is the same as that of the single perovskite. Double perovskites are generally of two types depending upon the types of the cations A 0 A 00 B 2 O 6 (double A-site) or A 2 B 0 B 00 O 6 (double Bsite). 47,48 The double perovskite strain energy depends on the charge difference between the two types of B cations (B and B 0 ). There are three ways to arrange the B-type cation based on the charge difference between B and B 0 . The arrangement is random if the charge difference is one (DQ ¼ 1). 49 The most common arrangement of the double perovskite is the rock-salt arrangement, also called elpasolite structure, in which the cations change in all three dimensions, and it dominates when the charge difference between the B and B 0 is greater than 2 (DQ > 2). Due to the difference in the charge size of the B cation in the rock salt order, the crystal symmetries are less than their single perovskite structure. Another arrangement is the layered arrangement of the B and B 0 , where the cations can change in only one dimension (Fig. 2). [50][51][52][53][54] The perovskite structures are preferred due to their structural and compositional exibility.

Structural exibility
The ideal single perovskite structure is with the high symmetry of Pm 3m. It comprises a highly exible network built up from chains of corner-sharing [BO 6 ] octahedra with A cations occupying the resulting holes with cubic octahedral symmetry. The symmetry of the structure can be transformed into the tetragonal, hexagonal, octahedral, monoclinic, triclinic, and rhombohedral structures by changing the size of cations or anions in the structure. Double perovskites generally have a double B site. Double perovskites are modied single perovskite; the high symmetry ideal single perovskite with the space group of Pm 3m is reduced to Fm3m in double perovskite. The double perovskite structure with double B site has two elements in the corner linked and has alternatively arranged BO 6 and B 0 O 6 octahedra double A site. 55,56 The tolerance factor of both single and double perovskite changes by altering the size of cations or anions in the perovskite. The resulting lattice distortion affects the dielectric, electronic, magnetic, and optoelectronic properties. The lattice distortion also inuences the movement and excitation of the photo-generated charge carrier under the inuence of light. 27,[57][58][59] The sintering temperature also affects the structure of the perovskite material. The structure generally becomes ideal cubic at high temperatures, and as the temperature goes down, the octahedral rotation in the perovskite changes to lower symmetry. 60,61 Octahedral tilting also has an indispensable role in the structural exibility of single and double perovskite structures. Octahedral tilting is the rotation along with the orthogonal symmetry of the BO 6 ; Glazier explained the octahedral tilting in 1972. Currently, there are twenty-three different perovskite structures available depending on the octahedral tilt. 62,63 The ferroelectric properties of the perovskite structure are also dependent on the octahedral tilting. Octahedral tilting breaks the centro-symmetry, which affects the ferroelectricity. The bandgap of the perovskite can also be changed by changing the octahedral tilting. Calculation of the electronic structure of the oxide perovskites on the tilted phase depicts that bandgap due to the different tilt can change up to the 0.2 eV and is even more signicant in the case of halide perovskites. 30,64,65 The large distortion in the perovskite lattice can break the structure and result in low dimension perovskite formation (1D, 2D). The distortion in the crystal structure affects the BO 6 octahedral, breaks the B-O bond, and forms a 1D/2D perovskite derivative. In 1D wires with linear or zigzag conguration, the BO 6 octahedral network can be edge-sharing, face-sharing, or corner-sharing. While in the case of a 2D stacked-layer, the BO 6 octahedra are edge-sharing. Low dimension perovskites are helpful in the application of photocatalysis due to their high surface to bulk ratio. [65][66][67] Stoichiometric and compositional exibility. The perovskite structure exhibits a high degree of stoichiometric and compositional exibility. Theoretically, 346 different kinds of ABO 3 ,  264 are experimentally investigated. The ABO 3 perovskite  structure is divided into ve groups depending upon the A and  B site charges that are A 1+ B 5+ O 3 , A 2+ B 4+ O 3 , A 3+ B 3+ O 3 , A 4+ B 2+ O 3 ,  and A 5+ B 1+ O 3 . Theoretically, there are 10 5 possible material double perovskite, out of which 10 3 double perovskites are experimentally investigated. For material to be double perovskite, the charge balance of all the charges present in the structure should be equal to 12. Double perovskite allows the elements with high valence shells to accommodate in structure, thus expanding the compositional exibility. 49,68 Perovskites' compositional exibility is oen represented by their ease of alloying on A and B sites. Normally, the mixing element in semiconductors is isovalent, while in perovskite, the mixing element is non-isovalent. The simplicity of cation mixing in perovskite materials makes it easy to alter its chemical and physical characteristics. The double perovskite structure also shows interesting properties like half-metallicity, hightemperature ferromagnetism, and many magnetic interactions. The infrastructure of the complicated oxide unit cell depends upon the electronic conguration and the atoms located at the A and B-site. The B-site atoms are considered to be more critical than A-sited cations. The size of the A 2+ cation is larger compared to the A 3+ cation. As the A 3+ cation has a smaller size, it limits the size of B-site cations. More than one thousand double perovskite materials are reported in the literature, all of which are prepared at ambient pressure. Some new materials were synthesized at high pressure. More than 720 compounds are reported as the divalent A-site compounds and 200 as trivalent A-site. 30,[69][70][71] The structural network of BO 6 octahedra can be maintained in the presence of vacancies at A, B, and O sites because of the perovskite's excellent structural and compositional exibility. Applications of the perovskite materials are broad due to these vacancies, and therefore, they carry applications in photocatalysis, photovoltaics, supercapacitors, batteries and fuel cells, etc. 30 Stability of perovskite oxide materials. The challenge to make use of perovskite photocatalyst materials come from two signicant restraints: (1) chemical instability in polar solvents and (2) vulnerability of the surface to transform with chemical species present in solution. Recent progress in this eld improved these issues and illustrated different courses of actions to make use of perovskite materials in photocatalytic processes. 72 There are two main stability issues; (1) intrinsic stability-stability issues, and (2) extrinsic stability-stability issues of perovskite. Intrinsic stability involves structural stability, electronic band structure, and thermodynamic phase stability. In contrast, extrinsic stability encompasses interaction with water molecules and pathways of degradation, thermochemical stability, light induced ion redistribution, light stability, photochemical degradation, and oxidation/photooxidation of charge-transporting layers and perovskites. 73 Perovskite material and photocatalysis. Perovskite materials, due to their structural, compositional, and stoichiometric exibility, are considered promising materials for photocatalysis (Fig. 3). Perovskites-based photocatalysts operate in UV, IR, and visible regions. There are three different sites of alteration in perovskite material which makes tuning of the bandgap in perovskite material easy. In this review, the applications of the perovskite material in photocatalysis are discussed in detail. 74,75 General reaction mechanism of photocatalysis. The light interacts with the surface of the nano photocatalyst, which generates the electron (e À ) and hole (h + ) pair. The photogenerated electron-hole pair then initiates the redox reaction at the photocatalyst's surface. There are three steps involved in the mechanism of photocatalysis. The rst one is photo-excitation, provided that the energy of the light source should be high enough to overcome the bandgap. The second step is trapping the electron-hole pairs to increase the recombination time. The recombination of photoinduced charges produces heat, which decreases the efficiency. The third and nal step is the degradation of the pollutant by the reactive species produced due to a redox reaction or the production of the fuel. [76][77][78][79] The general reaction mechanism for all the reactions is given by; 1st step: photoexcitation of photocatalyst Photo catalyst + hv / e À + h + 2nd step: trapping of electron and hole: Oragnics (pollutant) + radicals / degarded pollutant The nal step varies for each application of photocatalysis and governing equations: For water splitting: For CO 2 reduction: For nitrogen xation: The photogenerated electron-hole pair can be used in various applications depending upon the redox potential for the applications, as depicted in Fig. 4.
The selection of the photocatalyst material for the particular application is crucial and an important step to achieve better results. 80 The semiconductor material is selected by balancing the band edge potential of the semiconductor material with the reaction potential. The main hurdles in photocatalysis are the less recombination time of the photoinduced electron-hole pair and absorption wavelength corresponding to the photocatalyst material. Most materials are UV activated, and in the solar spectrum, the UV region is only 5%; therefore, for enhanced photocatalytic activity, the material bandgap should be responsive to visible and NIR radiations along with the high recombination time of electron-hole pair. 81,82 In the perovskite structure, there are three sites of the alteration (A site, B site, and O site). The doping at these sites can enhance light absorbance in the visible and NIR regions by bandgap alteration. The recombination time can also be addressed by employing the different strategies that includes tuning of bandgap and repression of electron-hole recombination. To solve both light absorption and recombination time problems, a new innovative solution known as defect engineering has been proposed. 8,[83][84][85] The strategies for tuning the bandgap are; (i) Bandgap engineering (ii) Repression of electron-hole pair recombination (iii) Defect engineering The bandgap of the various perovskite materials is given in Table 1.
Bandgap engineering. Most perovskite materials respond to UV light due to their wide-bandgap. The conduction band of perovskite materials contains d-orbital, slightly above or equal to zero eV. The valence band has the 2p orbital, due to which energy of the valence band in perovskite material is generally greater than 3 eV. The perovskite bandgap provides enough redox potential to execute different photocatalytic reactions, like H 2 generation, CO 2 reduction, contaminants degradation, and nitrogen xation. The donor and acceptor impurities in the perovskite crystal structure can adjust the bandgap. The donor and acceptor impurities in a perovskite can tune the material's bandgap. The doping at the cations or anions at different sites of perovskite structure enhances the photocatalytic activity by introducing the intra-band energy levels; the shallow and deep intraband energy levels trap the photogenerated electron-hole pair. Doping also reduces the bandgap of the perovskite material (Fig. 5). 30,95 In perovskite material, doping of the transition metals (delectrons rich cations) led to the production of donor impurities in the perovskite structure. The high atomic energy of the transition metal ensures better electron mobility without changing conduction band minima of perovskite host material. 96 The doping at the different sites of the perovskite structure changes the absorption peak of the material, thus increasing the light absorption. Doping also tailors the material's bandgap and increases the efficiency of the photocatalyst material. Incorporating a new transition energy level in the perovskite structure improves the higher wavelength absorption without changing the intrinsic absorption properties of the material. Hybridization of orbital in perovskite structure and formation heteroatom due to the doing changes the absorption spectra of material. The hybridization decreases the bandgap and therefore increases the absorption of light. Thus, the doping changes the bandgap and increases the recombination time. 83,[97][98][99] The bandgap of pure SrTiO 3 is 3.25 eV, and doping with noble metals like Ru, Rh, Au, etc., introduces the intra-band  local states, making the doped SrTiO 3 a suitable photocatalyst. 100 The Ru doping introduces the intra-band energy state between conduction band (Ti 3d) and valence band (O 2p, Sr 4p). The ionic radius of Ru is greater than Sr and nearly equals Ti; therefore, the Ru substitutes Ti atom in SrTiO 3 . Ru doping at the B site of the SrTiO 3 introduces the local states and increases the photocatalytic performance. The doping of noble metals in SrTiO 3 makes it a Vis-IR responsive photocatalyst. 86 Similarly, doping the bismuth (Bi) or copper (Cu) at A site of SrTiO 3 introduces the intra-band energy level and reduces the bandgap. The change in the absorption shi was also reported due to the change in the bandgap with doping. 101 It is reported that doping of the sulfur at the O site of the ABO 3 introduces the intraband energy level at 1.23 eV. Researchers also noted that doping the nitrogen at the O site of the perovskite material increases photocatalytic activity. 102,103 The amount and the quantity of the dopant should also be optimized to impede the recombination of the photogenerated charge carrier. Synthesis methods for preparing the perovskite material also affect the bandgap, recombination of charges, and migration properties of photocatalyst. 104,105 Repression of the photogenerated electron-hole pair recombination. The movement of photoinduced charge carriers at the photocatalyst's surface initiates the redox reaction at the surface of the photocatalyst. The recombination of charge carriers can occur either at the surface or in bulk. The recombination reduces efficiency, so it must be repressed. [106][107][108] The crucial step in photocatalysis is selecting the material, which depends upon the absorption coefficient of the photocatalyst material. The absorption coefficient of photocatalyst depends upon the bandgap and photon energy. [109][110][111] The incident light generates the electron-hole pair below the catalyst surface, known as the absorption length. The development of the space charge region takes place at the phase of the solidliquid interface, and if the absorption extent exists in the space charge region, the photoinduced electron-hole pair can be easily separated. However, in the case of the bulk material, the recombination rate will be high. 112,113 Aer separating the photo-induced charges, the next step is the migration of the charge carriers. The migration mainly depends on the material's symmetry and diffusion length. Diffusion length is the distance that photogenerated charges travel without recombining or scattering. Diffusion length is the governing factor for controlling the movement of the charges in the absence of the potential gradient. Separation of the photogenerated charges beyond the charge separation region is feasible with materials whose diffusion length is longer. The diffusion length is multiple of the lifetime of the charge carrier and diffusion constant, and it should be of high value to generate the maximum photogenerated charge carriers. [114][115][116] The limiting factor for the range of charge space region is the particle size. The width of the charge space region should not surpass the particle's radius, and therefore the use of nano-size crystals improves the photocatalytic activity. 108,117,118 The perfection in the crystal structures affects the lifetime of the photogenerated charge carrier. The potential gradient also affects the photocatalytic activity because it efficiently separates the photo-induced charges through the support interfaces between the semiconductor-semiconductor, semiconductormetal, and semiconductor-conductor surfaces, forming the heterojunctions. Therefore, it is necessary to introduce the potential gradient for attaining better efficiency of photocatalytic material. Furthermore, combining the two semiconductors with different bandgaps represses the electron-hole pair recombination. Co-catalyst can also enhance efficiency by introducing active sites and better charge transfer. 79,[119][120][121][122][123][124][125][126] Charge transfer mechanism. Charge transfer mechanisms are of different types, depending on the photocatalyst system. The coupling of two photocatalyst semiconductor materials gives the heterojunction, further categorized into three types. The heterojunction type formed by the semiconductor materials depends on their bandgap. 119,127 The three types of heterojunctions are type I, type II, and type III. The combination of the semiconductor materials forms the Z-scheme, p-n junction, and the Schottky junction based on the type of the semiconductor used (Fig. 6). 121,128,129 Type I, a heterojunction known as straddling gap heterojunction, involves one semiconductor photocatalyst (SP I) with a highly negative conduction band and highly positive valence band while the bands of the other photocatalyst semiconductor (SP II) lie within it. The drawback of type I heterojunction is that the charges accumulate on the semiconductor with a lower bandgap which is not favorable. 127,130 In type II, the conduction and valence band of the one semiconductor SPI lies above the conduction and valence band of the SPII. The charge separation of the photogenerated electron-hole pair is efficient in type II because of the electron's downward movement from the conduction band of SPII and an upward movement of the holes from SPI's valence band. 131,132 In type III heterojunction, the valence and conduction bands difference is more than type II. The photogenerated electrons of the SPI unite with the photogenerated hole of SPII. The SSPII electron and the SPI hole innate the redox reaction in the semiconductor photocatalyst. 120,130 The understanding of the Z-scheme is necessary to understand the heterojunction. In Z-scheme, two semiconductor photocatalyst materials are linked in a way that they form the letter Z. The electrons from the conduction band of the SPII trans into the valence band of the SPI. SPI acts as a reduction site, and SPII serves as an oxidation site. Due to the redox potential between the two semiconductors, the inbuilt electric eld allows efficient charge separation. The direct Z-scheme involves the contact at the interface of two semiconductor materials. In contrast, the indirect Z-scheme involves mediators that allow the charge transfer between the two semiconductor materials. The mediator can be solid or liquid. The iron-based mediators are more commonly used. [133][134][135] The heterojunction can also be formed by combining the p and n-type of the semiconductor materials. A free-electron from n-type semi-conductor material migrates toward the p-type semiconductor material upon contact. The migration of free electrons results in the formation of oppositely charged interfaces. This migration also strengthens the inbuilt electric eld at the contact of the p-n junction. The built-in electric eld facilitates the migration of the photogenerated charge carrier aer light irradiation, which increases the photocatalytic performance (Fig. 7). 136,137 Examples of semiconductor-semiconductor photocatalyst are SnO 2 -TiO 2 , CdS-TiO 2 , Bi 2 O 3 -Bi 2 WO 6 and Bi 2 WO 6 -TiO 2 , etc. 138 The increased charge separation of metal-semiconductor photocatalysts arises from the transfer of the electron across the interface of metal-semiconductor. Metal improves photocatalytic performance because it provides active sites and facilitates the separation and transfer of photoinduced charge carriers. When a metal (having a high work function) couples with semiconductor, the electrons ow from the semiconductor to the metal until their Fermi levels get aligned, resulting in upward bending of band edges in semiconductor and accompanied by the band bending, formation of Schottky barrier takes place at the metal-semiconductor interface (Fig. 8). The electron from the conduction band of the semiconductor material migrates to the co-catalyst and causes charge separation, and increases the photocatalytic activity. 139  Defect engineering. Defect engineering is a suitable and easy way to enhance the photocatalytic efficiency of perovskite materials. Substituting the different materials can introduce defects in the perovskite materials in the perovskite structure. These defects act as a site to trap the charge carriers and thus enhance the separation of charge carriers and increase the photocatalytic efficiency. These defects can be a vacancy of an extra atom that results in the formation of the line or screw dislocation. [143][144][145] In perovskite materials, three different alteration sites allow the introduction of the defects or vacancies. The doping at the A and B site cations introduces the dislocation but does not allow structural changes. The structural changes can introduce vacancies at the A site or O site. The vacancy at the B site is challenging to introduce due to the BO 6 octahedra. Furthermore, it is thermodynamically unfavorable to introduce the vacancy at the B site due to the high charge value and the small B site cation size. 146,147 The vacancies at the A site can be induced by substituting the A-site cation with a cation of the high or low valence. The subsisted single perovskite oxide formula is: A 1Àx A x BO 3 and the oxygen-decient vacancy can be created by doping the sulphur or nitrites. The second approach is doping at the B site, in which the reduction of the B site produces oxygen vacancy. In recent years, many defect-rich perovskite materials with excellent photocatalytic activity have been reported. Fig. 9 represents different strategies for bandgap tunning of perovskite materials. 30,148,149 The potential of semiconductor photocatalyst mainly depends on the electron injection capacity of the material, which is controlled by the energies of valence (E VB ) and conduction bands (E CB ). Therefore, to investigate the possible applications of semiconductors, it is crucial to have an understanding of the E CB and E VB band edges. In a semiconductor, the Fermi level (E F ) determines the electrochemical potential of electrons. At thermodynamic equilibrium, E F tells the occupation of the energy levels, but thermodynamic equilibrium can be perturbed by external bias or irradiation due to injection or photogeneration of holes and electrons. Therefore, the quasi-Fermi levels dene the non-equilibrium densities of electrons in conduction band and holes in valence band. Generally, the quasi-Fermi level for the majority carriers approximates the equilibrium E F because of the negligible increase of density of majority carriers. However, the quasi-Fermi level can be shied because of the small minority carriers at equilibrium. 150 When a semiconductor is in contact with another phase, the ionic interactions at the interface of the two phases cause an electrostatic adjustment in the material. To attain the equilibrium, especially at the semiconductor/electrolyte interface, the electrons ow from the phase of more negative E F to the other, in which the semiconductor E F matches the electrolyte E F , redox . This forms a space charge layer (SCL) in the semiconductor phase, which is associated with the upward bending of the band edges in the n-type semiconductor (Fig. 10c) and downward band bending in the p-type semiconductor (Fig. 10d). The SCL contributes to an internal electric eld in the semiconductor, where majority carriers are forced away from the interface of semiconductor/electrolyte. Such an SCL accounts for one of the three distinct double layers, in addition to the Helmholtz layer and Gouy-Chapman layer, which are commonly present at the interface of semiconductor/electrolyte. 150 Recyclability of perovskite photocatalyst. Reusability is crucial for a photocatalyst, reecting its superiority in the photocatalysis domain. It is worth mentioning that commonly available or used photocatalysts are homogeneous photocatalysts that cannot be recycled in the developed methods. Moreover, their practical applications are limited because of complex synthesis approaches, the high cost of noble metals, conditions of air-free reaction, and modest activity. Therefore, it is essential to develop an easily produced heterogeneous photocatalyst that also exhibits the property of easy catalyst separation along with recyclability. The researchers have recently started reporting heterogeneous perovskite photocatalyst in photocatalytic organic synthesis. The perovskites-catalyzed selective oxidation of benzyl alcohols to aldehydes under visible light irradiation has been reported by a group of researchers. Another group reported the photoredox CsPbBr 3catalyzed oxidative coupling of thiols to disuldes and crossdehydrogenative coupling of dialkyl H-phosphonates with tertiary amines to a-phosphoryl tertiary amines. Yan and their fellows also reported the a-acylmethylation of aldehydes starting from aldehydes and a-bromo ketones catalyzed by CsPbBr 3 under light. The same group also used CsPbBr 3 photocatalyst for the formation of C-C bond via C-H activation, formation of C-N, and C-O via N-heterocyclization, and arylesterication. A group of researchers prepared a heterogenous perovskite photocatalyst and used it at least ve times without noticeable degradation in its activity. 151,152 Applications of photocatalysis There are numerous photocatalysis applications, but during the last couple of decades, a lot of focus has been given to applications having environmental applications such as wastewater treatment, nitrogen xation, CO 2 reduction, air purication, and water splitting. All of these applications except air purication have been discussed in the current article.

Degradation of water pollutants
Rapid industrial growth, mainly the textile industry, produces increased chemical wastes in the water. Industrial chemical wastes or pollutants are known as dyes. The dyes produced by the textile industry can impede plant growth, impair photosynthesis, and its accumulation in soil disturbs the food chain. The textile dyes in water also cause genetic problems and cancer. The dyes in water aggravate the regulatory functions of the different glands in humans and can also have adverse effects, like infertility and immune suppression in humans. The traditional ways to clean the dyes in water give rise to massive sludge because of adsorbent usage, which can further cause bacterial infections and skin problems. The photocatalytic degradation of the containments in the water is more reliable than the traditional methods. Recycling the adsorbents in conventional methods generally generates secondary pollutants, and recycling adsorbents also requires harsh conditions. [153][154][155][156] The use of the photocatalyst is an eco-friendly way to degrade these water pollutants. The photocatalytic techniques utilize the advanced oxidation process (AOP) for degradation. The interaction of the light with the photocatalyst generates electronhole pair. Generally, oxygen is employed as an electron scavenger to increase the recombination time for enhanced photocatalytic activity. The redox reaction of the photogenerated electron-hole pair gives reactive species such as (OH*, OH À , H + ), and these reactive species then attack the organic pollutants and degrade them into low molecular weight products (environmentally friendly products) within no time. 153,157,158 The heterogeneous design and the self-cleaning surface of the catalyst make recyclability much easier. The reactions occur at the catalyst's surface, followed by the desorption of the materials; thus, this method is known as a sustainable solution to address the water contamination crisis. 159,160 The photocatalytic activity can not only be monitored by investigating the absorption spectra of the dye molecule but also involves the investigation of the mineralized products formed due to the cleavage of the chromosphere. The mineralized products can be investigated by measuring the chemical oxygen demand (COD) and total organic carbon (TOC). The photocatalytic reaction for degradation of the containments occurs in an aqueous environment. The irradiation of the light on the aqueous system (consisting of photocatalyst + polluted water) generates photogenerated electron-hole pair in water. The oxygen or different impurities are added as an electron scavenger to increase the recombination time. 25,104,161,162 The photogenerated electron-hole pair then migrates towards the surface of the photocatalyst and induces the reactive ions (such as OH*, O 2À *, etc.). The redox potential of the created reactive ion species depends upon the bandgap of the photocatalyst. The reactive ion species then degrade the contaminant into the smaller fragments, which are then changed into green compounds. The degradation of the contaminants takes place at the surface of the catalyst. Initially, the dye gets adsorbed at the catalyst surface, and aer the degradation product gets desorbed, the new pollutant is adsorbed, and this process continues until no contaminant is le. The overall reaction mechanism is shown in (Fig. 11). 30,76,135,163,164 Many perovskite materials are used as a photocatalyst, but titanium, bismuth, and ferrite-based single and double perovskite are widely reported due to their excellent photocatalytic properties. 165 The rst reported perovskite material CaTiO 3 exhibited photocatalytic activity and degraded pollutants. The wide bandgap of CaTiO 3 (3.0-3.5 eV) responds only to the UV light and efficiently degrade the brilliant green (BG), methyl blue (MG), and rhodamine B (RHb). 88,166 The synthesis route and the morphology of the CaTiO 3 structure change the bandgap and thus affect the photocatalytic performance for pollutant degradation. CaTiO 3 nanocuboid showed the excellent photodegradation of the RhB dye upon visible light irradiation. Zirconium (Zr) doping at the Ti site of CaTiO 3 generates oxygen vacancies, producing defects and increasing photocatalytic activity by changing the lattice structure. 167,168 The reported photocatalytic activity of the Zr doped CaTiO 3 is thirteen times greater than the un-doped CaTiO 3 . Fe-doped CaTiO 3 efficiently degraded the methyl blue using the visible light source. 166 The doping of the Fe in the CaTiO 3 increased the absorption ability of CaTiO 3 . The photocatalytic activity also depends upon the calcination temperature and the irradiation time of the light. Fe-doped CaTiO 3 showed 100% photodegradation of the methyl blue at optimum condition (temperature, irradiation time, and light source). 169  Ferrites-based perovskite (AFeO 3 , where A can be La, Bi, Ca, Sr, Gd, etc.) attracted researchers due to their low cost and small bandgap compared to the titanium-based perovskite. 171 The widely used ferrite-based perovskite is bismuth ferrite (BiFeO 3 ). The small bandgap (2.0-2.77 eV), excellent stability, and strong photoabsorption of BiFeO 3 allow the efficient photodegradation of organic dyes from textile and pharmaceutical industries under visible light. [172][173][174] The pure BiFeO 3 in the sunlight showed 69% photodegradation of methyl blue. The doping at different sites of BiFeO 3 has shown better results. Sc doped BiFeO 3 showed 100% photodegradation of the methyl blue in 3 hours under sunlight. The improved efficiency was attributed to improved ferroelectric properties due to lattice distortion produced by doping. 98 Mesh of BiFeO 3 showed 98% photodegradation of the methyl blue within 4 hours. The excellent photocatalytic activity of BiFeO 3 was due to the high interaction of the dye molecules and photocatalyst. Bi 0.90 La 0.10 Fe 0.95 -Mn 0.05 O 3 showed 97% photodegradation of Congo-red within 2 hours under the sunlight. 25 Tantalite-based perovskite is also reported for the degradation of water pollutants. Due to excellent photochemical stability, sodium tantalite (NaTaO 3 ) showed photocatalytic degradation properties. The efficiency of the NaTaO 3 based photocatalytic system is quite less due to the wide bandgap of the material. Still, the doping of nonmetals increases its efficiency by creating a local state between the conduction and valence band. The reported photodegradation of methyl blue is 95.21% by nitrogen-based NaTaO 3 photocatalyst using sunlight as an irradiation source. 175,176 Different perovskite materials used for photocatalytic pollutant degradation are given in Table  2.
Photocatalytic water splitting. Hydrogen is considered a green and clean fuel and one of the best alternatives to fossil and other non-renewable energy resources. The photocatalytic splitting of the water produces H 2 and O 2 oxygen by the fourelectron process. The overall reaction of water splitting is depicted in eqn (9).  The positive Gibbs free energy shows that with the help of some external stimuli, the reaction proceeds in the forward direction. The use of some sacricial layer (or agent) in combination with the photocatalyst material produces hydrogen and oxygen, and the reaction is known as hydrogen or oxygen evolution reaction. The process of producing hydrogen and oxygen from water by using semiconductor photocatalyst material and light as an irradiation source to innate the reaction is known as photocatalytic water splitting. [214][215][216] The photocatalytic reaction mainly depends upon the reduction potential required for water splitting. The minimum potential energy needed to convert H 2 and O 2 from water is 1.23 eV. 217 One-step photo reaction requires only one semiconductor photocatalyst material, and the overall process is depicted in Fig. 12. The use of two photocatalytic materials to form composite or heterojunction can enhance the performance of the perovskite material. Z-scheme mainly employs two materials in photocatalytic water splitting. Z-scheme is an eight electrons process. The selection of the photocatalyst depends upon the target product. In Z-scheme, the hydrogen-evolving or oxygen-evolving photocatalysts are combined, and both catalysts perform their function separately. The mediators (solid or aqueous) help transfer charge in the Z-scheme. Direct Z-scheme is a process without any mediator. The thermodynamic requirement for the water splitting can be decreased in the Zscheme by allowing only hydrogen and oxygen-evolving semiconductors. Still, the kinetics of the Z-scheme reactions are challenging, and nearly half of the hydrogen and oxygen are produced compared to a one-step reaction. [218][219][220][221] The performance of the photocatalyst can be evaluated by the quantity of H 2 evolved and the recyclability of the photocatalyst. The reaction rate is not always proportional to the use of photocatalyst mass; therefore, the preferred way is H 2 evolved per unit time (mmol h À1 ) with the mass utilized during reaction rather than writing the amount of H 2 produced in a mass normalized from (mmol h À1 g À1 ). 222,223 The intensity of the incident light for the photocatalytic performance is the number of photons collected per particle is directly related to the particle size. The reaction rate for the photocatalyst is proportional to the intensity of the incident light. In some cases, the rate of reaction increases with excitation intensity. In contrast, the increase in intensity in some reactions leads to decreased photocatalytic efficiency due to the second-order recombination of the photo-induced charges. 224 At low intensity, the concentration of the photogenerated charge carrier is insignicant compared to the concentration of intrinsic majority carriers. Thus, it can also be assumed that the concentration of minority charges varies with incident light's intensity, whereas the concentration of majority charges stays nearly constant. [225][226][227] The recombination of the photogenerated charges can be determined by the quasi-rst-order reaction with respect to the concentration of minority carriers induced by the light source. In simple words, the order of a photocatalytic reaction at a specic intensity gives an idea about the photoexcited charge carrier concentration. For comparison, a standard parameter of the apparent quantum efficiency (AQE) and the solar to hydrogen conversion (STH) are reported for measurements of photocatalytic activity. 228 Moreover, the stability of photocatalyst for real-world applications is vital as these materials need a life cycle of about ten years with an efficiency of about 5-10% STH to meet the goal   price of $ 2.00-4.00 per kg. The absorption wavelength for water splitting should be around 526 nm (2.36 eV) because below this wavelength, the photon's energy is not sufficient to achieve STH efficiency of 5-10%, considering that the reaction takes place for a lower wavelength at the AQE ¼ 1. To meet the industrial demand, the practical need for the evolution of H 2 under sunlight, the photocatalyst with the bandgap of less than 2.36 must be developed. The perovskites-based photocatalytic materials gained signicant attention for the practical application of water splitting under UV light and visible light. [231][232][233][234] Titanium-based perovskite materials are widely reported for photocatalytic water splitting. Doping and co-doping change the morphology and bandgap of SrTiO 3 , thus are making it a suitable visible light photocatalyst for water splitting reaction. The use of the Pt as a co-catalyst induces the Schottky barrier and can make the photocatalyst responsive to visible light. The presence of the Pt also provides the site for the proton reduction. The quantum yield efficiency by using the visible light as an irradiation source is 0.9% while using the UV light, the quantum yield efficiency of the system is 1.9%. The use of the other metals as Au, Ag, Fe, Ce, and Ni are also reported. [235][236][237][238][239] Alkali tantalates are the best perovskite material reported for photocatalytic water splitting when the parameters like the doping of the metal cations and morphology (nanocubes) of tantalates-based material are optimized well. The reported hydrogen evolution by KTaO 3 nanocubes is 375 mmol g À1 h À1 . The Ag doping at the surface of the KTaO 3 increases the hydrogen evolution in UV light. The irradiation of the UV light generates oscillations in the conduction band of absorbed silver and thus allows the easy excitation of the particles to the conduction band of KTaO 3 . The local electric eld of KTaO 3 was enhanced with the doping of the silver nanoparticles. The enhanced eld promotes the electron-hole pair generation, and therefore, increases the photocatalytic splitting. The reported evolution of the hydrogen in the presence of Ag as a dopant in KTaO 3 is 2072 mmol g À1 h À1 . [240][241][242][243] The lanthanum and ferritebased perovskite material are also reported. 244 Table 3 lists various perovskite materials used for water splitting.
Photocatalytic CO 2 reduction. The atmospheric concentration of CO 2 is increasing at an alarming rate due to increased human activities, deforestation, burning of fuels, and industrialization. The increased CO 2 emission is causing global warming, and for environmental sustainability, it is essential to convert CO 2 into valuable products. Mother Nature blessed humans with photosynthesis, in which chlorophyll act as a natural photocatalyst and, in the presence of sunlight, converts CO 2 into water, oxygen, and food for plants. The photocatalyst also converts CO 2 into valuable products in sunlight. 16,277,284,285 Photocatalytic CO 2 reduction requires more electrons than hydrogen evolution reaction. The end product from depending upon the number of available electrons. One of the limiting factors is the low water solubility of CO 2 . The complete reduction of the CO 2 in water also produces H 2 . The Z-scheme or heterojunction process for CO 2 reduction is reported frequently. 30,122 The rst step in CO 2 reduction involves the radical anion formation at À1.9 eV, which is impossible for most perovskite material due to less negative CB potentials of the perovskite material; therefore, high activation energy is required. The surface adsorption of CO 2 by catalyst generates the charge CO 2 dÀ which facilities the reaction to proceed. Surface engineering can enhance the CO 2 adsorption to the photocatalyst's surface and thus, increases the conversion efficiency. The low conversion efficiency of the CO 2 is considered a signicant concern in dealing with photocatalytic CO 2 reduction. 286,287 The co-catalyst can lower the activation energy resulting in enhanced photocatalytic activity. The reaction mechanism is shown in Fig. 13. In CO 2 reduction, the CO 2 gets adsorbed on the photocatalyst's surface, and electron-hole pair is produced upon irradiation. The photo-generated charges then move to the surface of the photocatalyst, and CO 2 is reduced into the valuable fuel. Aer the reduction of CO 2 by the photogenerated electrons and the formation of O 2 by photogenerated holes, the reduced and the oxidized product is formed by the desorbed hole and electron, and the cycle continues. The end product depends upon the total number of electrons used in the procedure. For a better photocatalytic activity, a signicant number of electrons should migrate towards the surface of the photocatalyst, and the bottom level of the photocatalyst's conduction band should be more negative than the redox potential of CO 2 . The water or additional sacricial reagents must consume the photogenerated holes; otherwise, holes recombine with the oxygen. The photocatalytic CO 2 reduction can be enhanced by optimizing CO 2 adsorption, charge separation, and desorption of the products. The process of CO 2 reduction in the presence of co-catalyst is shown in Fig. 14. 288,289 The CO 2 reduction by photocatalyst takes place in the presence of a water molecule, while the reaction in the gaseous or aqueous phase. The reduction potential of CO 2 should be small compared to the reduction potential of conduction band minimum and water to obtain valuable solar fuel. During the oxidation, the oxygen molecule is formed by the reaction of the water with a valence band hole.
Calculation of the AQE is used to access the performance of the perovskite material for photocatalytic CO 2 reduction. [290][291][292] Fig. 13 Photocatalytic CO 2 reduction.
The reactions are shown as: Titanium-based perovskite oxides exhibit excellent photostability for CO 2 compared to the tantalates and niobates. SrTiO 3 perovskite material is widely reported due to better charge transport properties and bandgap equivalent to TiO 2 (3.2 eV), but the poor CO 2 surface adsorption of photocatalyst (SrTiO 3 ) creates a problem. This problem can be solved by using the metal as a co-catalyst and introducing the oxygen vacancies to improve the adsorption. In the gas phase reactor using visible light, Fe doped SrTiO 3 photocatalyst with Pd (0.5 wt%) as a cocatalyst reduces CO 2 to CH 4 . The yield of the product is 421, and the CO selectivity is 84%. 16,[293][294][295] Tantalum-based perovskites are not widely reported for CO 2 reduction because of their wide bandgap. NaTaO 3 photocatalyst in combination with Au (0.5 wt%) as a co-catalyst reduces CO to CH 4 fuel in the gas phase reactor and uses UV-vis light as the irradiation source. The selectivity of CO is 96%, but the yield is small. 296 Table 4 lists perovskite materials used for CO 2 reduction.
Nitrogen xation. Nitrogen plays a vital part in building biomolecules such as amino acids, proteins, and other molecules. Earth's atmosphere has 78% of the dinitrogen (N 2 ). Dinitrogen air converts into ammonia by the process known as nitrogen xation. Plants and soil then use the ammonia formed by atmospheric nitrogen to build several processes. The mechanism to create ammonia by nitrogen is the Haber cycle. 314 At standard conditions, the ammonia formation is a thermodynamically spontaneous process (DH ¼ À92.2 kJ mol À1 ). Due to its high ionization energy of 15.58 eV and low electron affinity, the dinitrogen in the air is highly stable; therefore, the atmospheric dinitrogen in the normal conditions cannot be used to prepare the ammonia unless some catalyst or some industrial xation is used. In industry, by using iron or ruthenium as a base catalyst at high temperatures, the dinitrogen is converted into ammonia by the Haber-Borsch process. But even aer the century's advancement, the production of the indusial xation is still quite low. The NH 3 is one of the essential components in manufacturing fertilizers and hydrogen storage. Therefore, it is essential to synthesize an alternative green route to deal with energy crises and global warming. 77,315 Recently the photocatalyst material for nitrogen xation has gained much attention from researchers. The nitrogen xation by the photocatalyst utilizes only sunlight and water in the presence of the dinitrogen. The six electrons do the complete conversion of N 2 into NH 3 due to the high activation energy of the process, which makes the process completely impractical. Furthermore, direct electron transfer and proton-coupled electron transfer are not possible for the semiconductor material. Hence, photocatalyst design and surface engineering play an important role in nitrogen xation. [316][317][318] The reaction by using the photocatalyst is designed to reduce the activation energy and weakens the N^N bond. Light irradiation generates the electron-hole pair. These photogenerated charge carriers then  The nitrogen xation in the presence of the photocatalyst takes place in two ways, either by the associative way or by the dissociative way. The breakage of the N^N bond occurs in the dissociative pathway before adding the hydrogen atom as a reducing agent, which is impossible in an ambient environment. 322 In an associative pathway, breaking the N^N bond is unnecessary, so nitrogen xation is possible at ambient conditions. The nitrogen is adsorbed at the surface of the photocatalyst and then converted into the NH 3, which is then desorbed aer the breakage of the N-N bond. In the associative pathway, the hydrogenation takes place with the sideway nitrogen, which is not directly attached to the surface of the photocatalyst, which generates the NH 3 . 323,324 The remaining nitrogen attached photocatalyst's surface undergoes hydrogenation and generates NH 3 . But this route is challenging because the chemisorption of nitrogen molecules on the surface of the catalyst is not easy; therefore, surface engineering is an important parameter to enhance the photocatalyst activity for nitrogen xation. 8,321,325 The yield and the efficiency of the reaction are given by the apparent quantum efficiency (AQE) and can be calculated as; 326 AQE ¼ 6 Â number of amonia molecule evolved number of incident photons Â 100% Limited numbers of the perovskite materials are reported due to the less AQE and yield. The reported materials are SrTiO 3 , KNbO 3 , LaCoO 3, and layered double perovskite Bi 2 WO 6 . 129,[327][328][329] The efficiency of the perovskite material is less due to the selectivity of N 2 and adsorption of N 2 at the surface of the photocatalyst material; however, more research is needed in the eld of photocatalytic nitrogen xation to compete with the available industrial method of nitrogen xation (Haber cycle). 319,330 Some of the reported perovskite photocatalyst materials for nitrogen xation are listed in Table 5.  (Fig. 15). Accommodation of other cations at the A and B sites can alter the double perovskite's photophysical properties to a great extent. Among the binary oxides, only a few double perovskites exhibit visible region bandgap because of their small bandgap, such as Fe 2 O 3 , WO 6 , Bi 2 O 3 , etc. However, for hydrogen evolution, these materials have decient conduction band potential. Some materials also have low mobility of the photoexcited carriers and poor stability. Many binary oxides show efficient photocatalytic activity under UV irradiation because of their wide bandgap.
Complex compounds with a combination of 'narrow bandgap' and 'wide bandgap' materials can make use of properties of both types, and therefore, can be exploited as visible light photocatalysts. 124,125 The researchers have investigated photocatalytic activity of rare earth and bismuth-based double perovskites under visible light. Double perovskites Ba 2 XBiO 6 (X ¼ Ce, La, Nd, Pr, Eu, Sm, Dy, Gd) were synthesized and degraded MB. Researchers reported that rare-earth cation-dependent compounds (Ba 2 -SmBiO 6 , Ba 2 EuBiO 6 , and Ba 2 CeBiO 6 ) showed signicant photocatalytic activity.   333 3.13 385.0 mmol g À1 h À1 L À1 Simulated sunlight Ethanol as a sacricial layer NiS/KNbO 3 (5% NiS) 129 3.11 155.6 mmol g À1 h À1 L À1 Simulated sunlight Ethanol is used as a hole scavenger TiO 2 /SrTiO 3 /g-C 3 N 4 (ref. 125) 2.75-3.1 2192 mmol g À1 h À1 L À1 Simulated sunlight Methanol + nitrogen and ethanol as a sacricial layer CaTiO 3 (ref. 334) 3.49 236.12 mmol g À1 h À1 Natural sunlight irradiation 3D leaf-templated defective CaTiO 3 is prepared by using NaBH 4 + nitrogen environment Fig. 15 Schematic representation of a double perovskite structure (B), derived starting from a perovskite structure (A). 49 materials such as Sr 2 CoWO 6 , Ba 2 CoWO 6 , and Sr 2 NiWO 6 , Ba 2 -NiWO 6 , are reported to be stable for O 2 evolution with sacricial agents. 124,125 Although the photophysical properties of certain double perovskites also have been investigated and reported, still a lot of efforts are required before utilizing these materials on an industrial scale.

Summary
In summary, the abundantly available sun energy can be harvested through photocatalysis to deal with the concerns pertaining to the environment and humankind. This article aimed to discuss the imperative properties of perovskite materials which play vital role in photocatalysis, and will assist in understanding the fundamentals of photocatalytic mechanisms involved in designing highly efficient photocatalysts. The three altercation sites (A, B, and O sites) of perovskite materials ma them suitable for numerous applications, particularly photocatalysis. These sites help in tailoring chemical, physical, optical, and photocatalytic properties for desired photocatalytic reactions. The structural and compositional suppleness in perovskite photocatalyst strongly affects photo-generated carriers' mobility, separation, and recombination. The current article also describes the defect engineering in perovskite materials for enhanced photocatalytic performance. Recently, defect engineering gained much attention among researchers because it resulted in visible light photocatalytic activity without doping. Furthermore, surface defects provide reactive sites benecial for a process like nitrogen xation at ambient conditions. The review also provides an insight into the different applications of photocatalysis, including wastewater treatment, water splitting, CO 2 reduction, and nitrogen xation. The critical perovskite materials for each photocatalytic application are also listed with their properties in this study. Overall, it can be inferred that although perovskite oxide-based materials have exhibited signicant photocatalytic performance still, extensive challenges are ahead of their design, fabrication, cost, and efficiency for industrial and large-scale production. The biggest challenge in the industrialization of photocatalyst technology is the development of an ideal photocatalyst, which should possess four features, including high photocatalytic efficiency, a large specic surface area, full utilization of sunlight, and recyclability. Nonetheless, there is no doubt that perovskite-based oxide materials will be investigated more intensively in the coming years due to their exceptional properties and applications for a sustainable future.