Efficient Visible-Light-Driven Perovskites Photocatalysis: Design, Modification and Application

Abstract

Perovskite-based photocatalysts are oxides with the general formula ABO₃ are interesting materials that remained essential in solving a great deal of energy and environmental remediation challenges. Recent key issues for high-efficiency solar or visible light photocatalysis are the effectiveness in rapid transport to the semiconductor surface and the separation of photo-generated electron-hole pairs; thus, substantial efforts have been made to design and develop new generation of perovskite-based photocatalyst systems to improve their possible use. The present article provides an up-to-date review of recent development of perovskites-and its related materials, including titanate-based, tantalite-based, niobium-based, ferrites and others, demonstrating a remarkably rapid development and promising results in photocatalytic performance particularly in visible light-driven applications. Furthermore, the review also includes modification strategies that are commonly employed to improve the photocatalytic performance of perovskites. Finally, the summary of recent developments of perovskites-based photocatalysis for viable applications.

1 Introduction

Visible-light-induced chemical transformations or photocatalytic is a procedure of significantly speed up reactions in the presence of a photocatalyst (single-electron redox mediator) which absorbs photon energy from light irradiation to generate photoinduced electrons and holes pairs. In particular, the photocatalysis transpire through single-electron pathways, utilizing visible-light response of organic reactions for the applications of wastewater treatment and environmental protection [12, 114, 163]. This procedure is environmentally and ecologically friendly, green and produced clean energy as opposed to conventional organic reactions [83]. The process particularly refers to heterogeneous photocatalysis that has remarkable interest nowadays and mainly focuses on perovskites semiconductors that possess distinctive electronic and optical properties, as well as the ability to photogenerated electron-hole pairs [24, 152] with higher stability and potential recyclability [42, 83, 183]. Mostly semiconductors with wide band gaps i.e. TiO₂, ZnO, etc. will restrict its applicability to ultraviolet (UV) wavelengths, reflecting just 5% of the universe’s available solar light energy. Moreover, the rapid recombination of photogenerated electron-hole pairs results in a rapid dissemination of energy that decreases the performance of photocatalyst [89, 121].

Some conventional approaches to prevail over the drawbacks listed compasses the extension of the absorption to the spectrum of visible range [22, 86], and decreasing the recombination rate of the photogenerated electrons and holes [38, 156]. The solutions explored to boost photocatalytic performance include the construction of appropriate band gaps [77, 170], the employment of nanosized architechtures [183, 172, 174, 168], the implementation of interface design and facet-engineered surface [13, 52], utilization of dopants, as well as metal and non-metal co-catalysts [6, 95, 111], surface modification [54, 199] and the heterostructure construction [16, 189, 69, 101].

Amongst the most photocatalytic materials, perovskite-based catalytic (PCB) materials are auspicious photocatalytic materials with fascinating controllable physico-chemical and optoelectronic properties, such as electron mobility, redox performance, structural flexibility, efficient photocatalytic performance, long charge carrier lifetimes, excellent charge carrier mobilities, high absorbance coefficients, and uncomplicated bandgap engineering with facile fabrication route [126, 193, 194, 7, 106]. The complete replacement of cation A or B by other metals leads to variant interesting properties and alteration of the surface facets to create catalytic activity enhancement [73, 113, 127, 94]. As opposed to other visible-light active inorganic semiconductors, perovskite-type catalytic materials exhibited facile and effortless materials construction, application, and permit simple alteration of their semiconductor bandgap. In our study, we present a concise review to the current findings and noteworthy properties of perovskite materials, accompanied by the enhancement techniques and their recent representative applications for improved photocatalytic properties.

2 Overview and Design of Perovskites-Based Photocatalysts

Perovskites are materials having similar crystalline unit cell as calcium titanate (CaTiO₃), that was first discovered by Aleksevich von Perovski in 1839 [18, 118]. They have general chemical formula ABX₃, where ‘A’ and ‘B’ are cations of different ionic radii (A larger than B), and ‘X’ which is a halogen or oxygen anion, holds ‘A’ and ‘B’ together through an ionic bond [44, 65, 191]. Such different anions and various cations form oxide perovskites and halide perovskites, both of which have BX₆ octahedra in their crystal structures, with ‘A’ cation located in the interstitial voids of the neighbouring octahedra (Fig. 13.1).

Fig. 13.1

ABX₃ perovskite unit cell (Reproduced with permission from Ref. [118])

The catalytic activity of the perovskites is primarily attributed to the transition metal ion at the B-site, while their thermal endurance is mainly due to the rare earth ion at the A-site [5]. As a result of their photoelectric [141], luminescent [187], magnetic [9], and electrical properties [82], perovskites are used in the areas of biological imaging, ionic conductors, sensing, photocatalytic, electro-catalytic, information sensing and other numerous technological applications [191].

Among the perovskite materials, the perovskite oxide with the formula ABO₃ is a typical structure in inorganic chemistry [79]. It has demonstrated outstanding potential in the development of solid oxide fuel cells [207], solar cells [206], and ferroelectrics [40]. Various oxide perovskites such as titanates [109], ferrites [37], and tantalates [98] are photocatalytically active. The flexibility in the composition and structure of the oxide perovskites greatly influences their photocatalytic performance [143, 193, 194].

Previously, perovskite materials were usually prepared using the traditional solid-state reaction method, but the produced materials are mostly heterogeneous, impure, have low light absorption in the visible region, are very sensitive to temperature changes, and the excited states have short lifetimes [2, 47]. To overcome these defects, alternative methods, including sol–gel [36], freeze-drying [88], combustion synthesis [92], electrospinning method [35], sonochemical method [110], coprecipitation method [81], microemulsion method [5], glycine-nitrate route [167] and microwave-assisted method [162] have been introduced. According to Tanaka and Misono [161], the main strategies of designing perovskite catalysts for the enhancement of their catalytic activity are; (1) selection of B-site elements which principally determine its catalytic activity, (2) valency and vacancy control by the selection of A-site elements, (3) synergistic effects of mainly B-site elements, (4) enhancement of surface area via formation of fine particles or dispersion on supports, and (5) addition of precious metals with their regeneration.

The efficiency of the catalyst, including perovskite materials during photocatalytic reactions, depends on three steps: (a) photon absorption and generation of charge carriers, (b) separation and transfer of charge carriers to the active sites of the surface, and (c) consumption of photogenerated charge carriers on the active sites during redox reaction [120]. Since most perovskites have a wide bandgap, several strategies have been employed to improve their visible light absorption and enhance the separation of photogenerated charge carriers. According to Moniruddin et al. [118], the key strategies include (a) bandgap engineering to achieve suitable band edge position, (b) enhancing the separation of charge carriers by changing particle size and crystal nanostructure, (c) improving visible absorption via the use of plasmonic metal nanoparticles (Ag or Au), (d) formation of heterojunctions to enhance separation of charge carriers, and (e) introduction of ferroelectric material to capitalize on its polarization field towards photoexcited charge separation. Specifically, various photo-active perovskites catalysts will be discussed in the following sub-sections.

2.1 Titanite Perovskites

One of the classified perovskite with general formula of MTiO₃ (M = Sr, Ba, Ca, Mn, Fe, Co, Ni, Pb, Cd) known as titanate perovskites [74]. They are promising photocatalysts with structural simplicity and flexibility [125]. Although most titanates are only active under UV light because of their wide bandgap, however, those containing transition metal oxides with d⁰ and d¹⁰ orbitals, including Co²⁺, Fe²⁺, Ni²⁺, Zn²⁺, Pb²⁺, and Cd²⁺, would favour narrowing of the bandgap [21]. In general, titanates remain attractive materials in photocatalysis due to their high thermal stability and excellent resistance to photocorrosion [4, 100].

Among the titanate perovskites, SrTiO₃ happens to be the most widely studied titanate [164]. It is an n-type semiconductor posseseses an indirect bandgap between 3.1 and 3.7 eV, having a basic framework of Ti–O polyhedron as TiO₂, and remarkable charge transport properties [47, 166, 200]. Furthermore, SrTiO₃ is cheap, less toxic, and easily doped to control its electrical properties [45, 173]. Although some narrow bandgap titanates such as NiTiO₃ (2.10 eV) and CoTiO₃ (2.28 eV) are visible light active, their conduction band is below the water oxidation potential. This limitation makes them unpopular [2]. Fortunately, despite its wide bandgap, the band edge for SrTiO₃ straddles the water splitting redox potential [74].

2.2 Tantalate Perovskites

Tantalates perovskites have the general formula ATaO₃ (A = Li, Na, K), and the bandgap of lithium, sodium and potassium tantalates was found to be 4.7, 4.0 and 3.6 eV, respectively [67, 122]. Although the tantalates are only active under UV light, however, they are interesting photocatalysts due to their good quantum yield, structural feasibility and environment-friendly nature [3, 41, 184]. Moreover, the respective Ta 5d orbital was placed at a negative site as compared to titanates. This could make the tantalates paramount during a photocatalytic reaction [15].

The high photocatalytic activity of tantalates has been related to their layered structure with a corner-shared framework of TaO₆, allowing easy transport and separation of photogenerated charge carriers [208]. NaTaO₃ is the most active tantalates perovskite [153], nevertheless it needs to be modified to extend their photodetection to the visible region to harness more solar energy [164].

2.3 Vanadate Perovskites

Vanadate perovskites have the general formula RVO₃ [177]. Among the vanadate perovskites, AgVO₃ is an efficient photocatalyst with favourable morphology and nanocrystalline nature [150]. It has two crystalline phases of α-AgVO₃ and β-AgVO₃, both of which have an intense absorption in the visible light region and are strongly dependent on temperature [50, 182]. At higher temperatures beyond 200 °C, α-AgVO₃ phase starts to convert into β-AgVO₃, and the process reaches completion at 300 °C [80]. Due to its more narrow bandgap, larger structural and chemical stability, β-AgVO₃ has gained more attention and wider application than α-AgVO₃ [81]. Although the CB potential of AgVO₃ is suitable for O₂ evolution and degradation of volatile organic compounds (VOCs), it is not sufficient for H₂ evolution [26].

2.4 Niobate Perovskites

Niobate perovskites can be identified as ANbO₃ (A = Na, K, Ag, Cu) [47]. Although they are only active under UV light due to their wide bandgaps (>3 eV), they can induce both photocatalytic water splitting and oxidative degradation of organic contaminants [208].

Among the niobate perovskites, sodium vanadate (NaNbO₃) and potassium vanadate (KNbO₃) are less toxic and environmentally friendly materials and have attracted considerable interest [56]. They are both indirect bandgap semiconductors, with NaNbO₃ having a bandgap of 3.4 eV, while KNbO₃ has a slightly narrow bandgap of 3.1 eV [78]. Band structure calculation revealed that the mobilities of both charge carriers are higher in KNbO₃ than in NaNbO₃ [151]. This, in addition to better light absorption, resulted in higher photocatalytic performance by KNbO₃ compared to NaNbO₃ [151].

2.5 Ferrite Perovskites

Ferrite perovskites have been identified as AFeO₃ (A = Bi, La, Gd, etc.), with their original bandgap in the visible area [74, 164]. Among the ferrite perovskites, BiFeO₃ photocatalyst is now parallel with the famous TiO₂-based photocatalysts [43]. Apart from its narrow bandgap, other interesting features of BiFeO₃ are non-toxic nature, high chemical stability, and the coexistence of ferromagnetic and ferroelectric behaviours at room temperature [64].

2.6 Bismuthate Perovskites

Bismuthate perovskites have the general formula MBiO₃ (M = Li, Na, K, Ag), and bandgap in the order NaBiO₃ (2.53 eV) > KBiO₃ (2.04 eV) > LiBiO₃ (1.63 eV) > AgBiO₃ (0.87 eV) [132, 159]. They consist of Bi⁵⁺ with 6 s empty orbital, contributing to both the valence band top and the conduction band bottom [204]. Such a feature can narrow the bandgap and vary the band edge positions, leading to improved photocatalytic performance [97, 96].

Despite the narrow bandgap of AgBiO₃, the large radius of Ag⁺ and the strong contact between Ag atoms and O atoms hinders the free transfer of Ag⁺ ions. However, due to small ionic radii of Li⁺, Na⁺, or K⁺ and the weak interaction with [BiO₆] octahedrons, Li⁺, Na⁺, or K⁺ ions may easily transfer in the tunnelled or layered space to harness solar energy [204].

2.7 Cobaltite Perovskites

Cobaltite perovskites have the general formula ACoO₃ (A = Gd, Sm, La, Pr, Eu, etc.) [51]. Among them, LaCoO₃ is considered as a promising catalytic material, as lanthanum (La) plays a vital role in the catalytic performance due to its partly occupied 4f levels [66, 115]. At the same time, the cobalt (Co) is considered active due to its mixed-valence state, excellent electrochemical behaviour, high electrical and ionic conductivities [49, 66]. However, partial visible light-harvesting ability, a short lifetime and recombination of photogenerated charge carriers limits its practical application [66].

2.8 Nickelate Perovskites

Nickelate perovskites have the general formula RNiO₃ (R = La, Pr, Nd, etc.), and LaNiO₃ has attracted considerable attention in multiple fields [63, 140]. As a result of its outstanding optoelectronic properties, inexpensiveness, suitable bandgap (1.9 eV) and non-toxic nature, LaNiO₃ has been deemed as an interesting and hopeful visible light photocatalyst for wastewater purification [188]. However, the conduction band position of LaNiO₃ is below the H⁺/H₂ potential, and therefore electrons in the conduction band of LaNiO₃ could not be used for H₂ evolution [175].

2.9 Antimonate Perovskites

Antimonate perovskites have the general formula ASbO₃ (A = K, Ag, Cs, etc.), and AgSbO₃ has been reported as a promising material with positive response towards visible light [71]. The two main polymorphs of AgSbO₃ are the pyrochlore and ilmenite phases. The ilmenite AgSbO₃ phase was reported to show better photocatalytic performance towards the degradation of organic compounds under visible light irradiation than the pyrochlore AgSbO₃ phase [148]. However, the ilmenite AgSbO₃ phase is metastable and transforms into the stable pyrochlore AgSbO₃ phase by heat treatment under appropriate conditions [72].

The conduction band bottom of AgSbO₃ mainly consists of hybridized Ag 5 s and Sb 5 s orbitals, while the valence band top consists of hybridized Ag 4d and O 2p orbitals. The hybridization of orbitals leads to a continuous dispersion in a relatively wide energy range, resulting in high photocatalytic performance [90].

2.10 Chromite Perovskites

Chromite perovskites have the general formula RCrO₃ (R = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, etc.) [139]. Among the chromite perovskites, LaCrO₃ has been extensively examined due to its wide application in various fields, including photocatalysis [149]. It is a p-type visible light active perovskite photocatalyst with a bandgap of 2.6 eV [124]. The Cr-sites on the LaCrO₃ surface are better adsorption centres for atomic oxygen than Mn in LaMnO₃ or Fe in LaFeO₃ [169]. Such a feature in LaCrO₃ endows more favourable properties for photocatalytic applications [124].

2.11 Others

Apart from the above-discussed oxide perovskites, other oxide perovskites such as zirconates (RZrO₃ (R = Ca, Sr, and Ba)) [76], cerates (RCeO₃ (R = Ca, Sr, and Ba)) [28] and stannates [RSnO₃ (R = Ca, Sr, and Ba)] [201] also exists. However, they are barely used as photocatalysts, possibly due to their wide bandgaps [62, 70, 201].

3 Design and Modification Strategies

Solid-state technique is conventionally used to synthesize perovskites in the presence of basic salts at high thermal condition [39, 165]. Nevertheless, this method restricts the control of the particles dimension and its crystallinity [53]. On that account, an alternative method recently employed for the fabrications of tantalate perovskites, for instance, hydrothermal, [23, 25, 68, 84, 108]. solvothermal [29, 129] and polymerized complex method [178] to ensure the possibility of tuning the particle size with high surface area. It is interesting to note that, several modification strategies along with the alternative method are necessary to produce effective PCB with high photocatalytic performance. For example, the modification strategies particularly in defect engineering, doping and co-doping, sensitization, facet control and others.

The light absorption ability of the perovskite-based catalyst (PCB) was found to be enhanced when compared particularly with common wide band gap semiconductors (e.g., TiO₂), significant to the efficacy of photon and its carrier conversion. It is relevant to extend the duration of these carriers to further enhanced the photocatalytic performance. A befitting band gap is mostly crucial to optimize the absorption of light by the photocatalyst. Practically, the greater absorption in the visible range, the effectiveness of light absorption can be enhanced by tuning the band gap to be narrower [23, 25] The construction of the band structures is vital in modulating the optical and electronic properties of perovskite nanostructures for achieving multifunctional the efficacy and efficiency [123].

In particular, the perovskite material has a direct band gap for the whole visible light range, therefore, the charge carriers were effectively generated at lower energies in particularly for photocatalytic application [123, 17]. Among all, PBC has been reported perspicuous approach for tuning the band gap, by simply adjusting the defects of crystal structure, including the vacancy, impurity and the interstitial atoms. It can be useful strategy for improving its performance [128, 197, 196] and lead to reduction in band gap value, thus improving the light absorption within visible range, subsequently possessed higher photocatalytic activity [10, 48].

4 Design and Modification Strategies

One of the internal influences that profoundly impact the performance of the photocatalyst is its configuration and modification in the physical structure and composition. To be an ideal photocatalyst, the material should have narrow band gap, high charge separation efficiency and a reasonable absorption efficiency under visible light. Many photocatalytic materials suffer from wide band gap energy which couldn’t be stimulated by visible light (Zheng et al. 2015) and high recombination rate [57].

Conventional solid-state technique is commonly for synthesize perovskites perovskite materials at high temperatures [39, 165]. However, this approach limits regulation of crystallinity and particle size [53]. On that basis, for the manufacture of tantalate perovskites, hydrothermal technique currently used as an alternative strategy [23, 25, 29, 68, 84, 108, 129] and polymerized complex method [178] has been reported to ensure the possibility of tuning the particle size with better surface area. It is important to note that several modification strategies, along with the alternative method, are required to produce successful perovskite-based catalyst (PBC) with high photocatalytic efficiency. Several techniques have been developed to address these limitations, such as defect engineering, doping, heterojunction, sensitization, and other greener technique, i.e. integrated carbon-based material or biopolymers [111].

The light absorption ability of the perovskite-based catalyst was improved compared to typical large band gap semiconductors (e.g., TiO₂), as reported by Jiang and co-worker (2018). This improvement leading to the enhance the efficiency of photon to electron mobility. They proposed that it is important to increase the lifespan of these carriers to boost the overall performance of photocatalysts. An appropriate band gap is fundamentally crucial for optimizing the absorption of photocatalysts. In general, since the visible light range is greater, the absorption efficiency can also be enhanced by modulating the band gap to lower values [23, 25]. To achieve multifunctional efficacy and effectiveness, the engineering of band structures is crucial in tuning the optical properties and electronic states of perovskite nanostructures [123].

Some researcher reported the perovskite material has a direct band gap through the visible spectrum, so the charge carriers have been produced efficiently at lower energies, particularly for photocatalytic application [123, 17]. Relevant approach for tuning the band gap, by simply tuning the crystal structure defects, including the vacancy, impurity, and interstitial atoms, has been documented among the advanced features of different photocatalysts. According to [197, 196, 128], a proper tuning mechanism can be useful to enhance its efficiency and contribute to decreases in the value of the band gap, thereby increasing the light absorption in the visible spectrum and consequently allowing greater photocatalytic performance [10, 48].

4.1 Defect Engineering

The improvement in light absorption, catalytic efficiency, charge transfer and stability can be constructed by defect engineering. The defects were graded according to the defects and atomic structures of the semiconductor. In theory, structural discrepancies of photocatalysts can be classified into four such as point defects [146], line defects [131], planar defects [97, 96], and volume defects [99]. In addition, many photocatalysis with cation vacancies have been established by many researchers, and therefore significant to monitor the development of defects in perovskite materials by generating vacancies and self-doping.

For instance, one of the perovskites reported by Liu and Solhberg, [100] called strontium titanate, have been designed by defect engineering. SrTiO₃, a semiconductor of simple cubic and n-type with superior physical properties with bandgap between 3.1 and 3.3 eV. Despite the advantages, its wide optical bandgap restricts its capacity to absorb light, leaving most of the energy from solar light unused. Strategies to increase the photocatalytic potential of SrTiO₃ by controlling its bandgap to allow use of a wider visible and solar spectrum range. In this example, introducing defect or doping technique was done for the tuning of the SrTiO₃ bandgap.

Xie and co-workers [181] constructed self-doped SrTiO₃ through one-step combustion technique. The samples were treated with argon, Ti³⁺ ion vacancies and oxygen (O₂) were injected into the lattice, serving as a template to activate water molecules, helping to restore the efficacy of artificial photosynthesis. This study was able to show that these oxygen deficiencies are accommodated by inducing the gap that enables photoexcitation within the visible light region from the defect band to the conduction band, consequently encourage the adsorption for reduction of CO₂ under visible light irradiation.

In another study investigated by Luo et al. [107], they stated that the link between strontium titanate surface and its photocatalytic behaviour on CO₂ reduction. As reported, the surface of TiO₂-terminated surface is in low pH, so the electronic properties of the two Sr and Ti surfaces experienced significant different, where the Sr 4d orbital is more negative than the Ti 3d orbital in the conduction band, resulting in greater SrO-terminated surface reduction potential. The study indicates that SrTiO₃ surface-Ti-rich had the greatest potential for reducing CO₂.

Kwak and Kang [87] have attempted to comprehend the effect of the Ca: Ti ratio towards CaTiO₃ and its potential efficiency. They reported a s the molar ratio of Ti in the catalyst extends up, a reaction between the reactive sites and carbon dioxide molecules occur. Nevertheless, it is reported that an excess of Ti will prohibit both metals from having a synergistic impact. CaTiO₃ encapsulated basalt fibre was recorded as an assisted material to create the highest numbers of oxygen vacancies on the planes {001} and sufficient pores size that allowed facilitating of CO₂ as indicated by Im et al. [61]. The fiber consisting of large amounts of SiO₂ with impurities of CaO, Al₂O3, and Fe₂O₃ [27] having a role as photosensitizer to enhance photocatalytic efficiency [30, 61].

In a separate approach, Hou et al. [55] disclosed the presence of nitrogen and oxygen vacancy are able to modulate the electronic bandgap NaTaON, thereby increasing the absorption of visible light. As these vacancies able to shifts the absorption of visible light from the UV region, resulting in a reduction in the 2.18 eV bandgap. The heterojunction also increased the separation of charged and improved the durability of the materials, thus improved the activity of CO₂ reduction.

4.2 Doping or Co-doping

Doping or co-doping, which is modulated by the inclusion of a foreign element in photocatalysts, is another alteration technique. Metal impurities are introduced into the lattice of perovskite as foreign atoms in the doping technique. Doping modifies the band gap and the material’s atomic composition, thus making it possible to employ the visible light. In addition, Huang et al. [57] stated the additional energy levels can be added that help trap excitons in separate carriers while avoiding recombination. Many researchers utilized metallic elements and non- metallic elements doped perovskite [14, 102, 144, 192, 198] to modulate the band composition and electronic behaviour of studied photocatalysts.

The assortment of modifications has been investigated to broaden the photocatalyst bandgap and thus expand their photo-response to the visible light field, as reported by Samsudin and Abd Hamid [136] utilising noble metal deposition or known as doping. In other work, the significant observation by Anzai et al. [8] stated that the photocatalyst Ag-doped CaTiO₃ showed a higher generation rate and greater CO formation selectivity. In this study, Ag nanoparticles serve as unique active site within CaTiO₃ that reduced the production of H₂, and subsequently increased the creation of CO in water splitting.

Furthermore, by co-doping with La and Cr rare-earth metal, Wang et al. [172, 174, 168] successful modified hollow CaTiO₃ cubes to reduce the bandgap and boost their light-harvesting. In contrast to the small bandgap of pristine CaTiO₃, the hollow cubes exhibited stronger photocatalytic activity. This was primarily due to the improved photon interactions in La/Cr co-doping as well as the shorter charge transport paths. Another series of La/Cr perovskite compounds co-doped with ATiO₃, of which A 1⁄4 Ca, Sr and Ba were studied by Lu et al. [105] to see the differences in crystal structure as well as their optical and physicochemical properties. Critical structural contortions can be found for Ca_0.9La_0.1Ti_0.9–Cr_0.1O₃, indicating that Cr is responsible for the apparent light photoactivity. Lu et al. [105] discovered that the divergence away from the bond angle of Ti–O–Ti contributed to low catalytic efficiency as visible light absorption deteriorates. However, the existence of Ti–O–Ti bond angle ensures optimum overlap between Ti 3d orbitals and O 2p orbitals, resulting in broad band distribution and raised in charge movement for effective H₂ generations under visible light illumination.

Huang et al. [58] explored the properties of BaTiO₃ perovskite structure, by adjusting the concentrations of dopant elements such as selenium, tellurium and sulfur. These dopants substantially minimize the energy gap of BaTiO₃, thereby increasing absorption of the catalyst in the visible region. The catalytic properties of potassium tantalate, KTaO₃, studied by Chen et al. [23, 25] was doped with carbon and evaluated in the H₂ generation under sunlight illumination. From the observation, carbon-doped KTaO₃ demonstrated improved efficiency in the H₂ generation rate relative to the parent materials. In contrast, Krukowska et al. [84] investigated the role of lanthanides for KTaO₃-doped material and its efficiency towards generation of H₂. Synergistic results found between ion-doped perovskites of KTaO₃ and lanthanides and have strengthened the formation of H₂.

Depending on the crystallographic orientation, BiFeO₃ a perovskite oxide in rhombohedral unit cell and space group of R3c has known to be readily visible light active with a direct bandgap between 2.2 and 2.7 eV [138, 186]. In addition, doped or pristine ferrites materials are extensively studied in photocatalysis. For example, Yang and co-workers (2019) used Gd-doped BiFeO₃ as an efficient catalyst in generating H₂ in water splitting application. Meanwhile, Satar et al. [137] suggested that the band gap of BiFeO₃ decreased substantially in the presence of yttrium doped, increasing the percentage degradation of cationic dye, MB under sunlight irradiation. The authors claim, changes in performance are primarily due to electrons and holes being effectively produced, separated, and migrated.

4.3 Heterojunction

Heterojunction is another technique to improve the performance of the photocatalytic by increasing the efficiency of charge separation. Several studies conducted by Dutta et al. [33] and Ola and Maroto-Valer [121] showed, by bridging the semiconductor with metals or non-metals, heterostructures are produced that introduce new energy states that help to separate photogenerated charges and thereby preventing the electron from recombining. Ruzimuradov et al. [134], for instance, developed lanthanum- and N-co-doped strontium titanate-heterostructured macroporous monolithic materials with a bi-continuous morphology of titanium dioxide in visible light-active condition.

A number of works reported [11, 75] based on the effect of p-type coupled with n-type materials to form a p–n heterojunction photocatalyst in addition to doping. In specific, the efficacy of CO₂ reduction in the presence of BiFeO₃–ZnO p–n heterojunction has been investigated by Karamian and Sharifnia [75]. The author reported that the composite showed higher optical responses in the visible light spectrum with higher performance in charge separation. This is largely due to the existence of the p–n heterojunction that has supplied the CO₂ photoconversion with an excess of energetic electrons.

In another study, Bagvand et al. [11] investigated the role of ZnS for the production of n-type photocatalyst of BiFeO₃ and its efficiency effect by controlling ZnS and ZnO molar ratios. The findings demonstrated the highest efficiency of photocatalytic CO₂ reduction at the equivalent molar ratio of both ZnO, ZnS and BiFeO₃ in the experiments. They claim that p–n structure will produce localized electrical field that contributes to the transition in the opposite direction of charge carriers, leading to a reduction of the recombination rate.

4.4 Sensitization and Facet Defect

Another strategy in crafting heterojunction for improved photocatalyst efficiency is facet engineering. Facet engineering is a potent approach to intensify the crystal’s photocatalytic efficiency by inducing the creation of the facet defect by modulating the environment in which the crystal is being formed. The variation of surface energy of facets will affect the photocatalytic behaviour. There are several types of facets of the semiconductor lattice. Studies by Liu et al. [103] detected facets {110} and {101} possess low surface energy and stable, while the {001} facet has the greatest surface energy and highly reactive, respectively.

Interesting research finding by Yu et al. [190] has shown that {101} and {001} facets of high-energy TiO₂ nanocrystals exhibit different band configurations, and these co-exposed {101} and {001} facets of nanocrystals establish unique surface heterojunctions within single particles of TiO₂ that are advantageous for the rapid transfer of photo-induced electrons. In addition, building 3D/2D heterojunctions between two contact semiconductors with profound and broad facet-dependent contact areas will provide more effective gap and improved the efficiency as claimed by Cao et al. [20]. Through efficient synthesis of TiO₂/g-C3N4 heterojunction composites with a simple calcination route it resulted the photo-generated holes appear to stay in the TiO₂ valence band, while the electrons initially transfer from the conduction band of TiO₂ to the valence band of g-C3N4, which further excited the g-C3N4 VB [190].

Dye sensitization and reactive facet exposure have recently become a successful way to extend the spectrum of light reaction and prolong the lifespan of photogenerated electrons and holes. Owing to its greatest π-conjugated framework, good thermal stability, and better absorption on visible irradiation, metalloporphyrin is known as one of outstanding photosensitizer. Most of the studies by several researchers involved metalloporphyrin sensitized metal composites [85, 116, 154, 183, 104, 197, 196, 46], has been shown to effectively increase the photocatalytic activity of certain semiconductor materials. Furthermore, metalloporphyrin processes suitable energy levels as one form of organic semiconductor and can comprise effective Z-scheme heterojunction hybrid photocatalysts with n-type TiO₂. This hybrid system will prevent the possibility of recombination of photogenerated charge carriers and boost photocatalytic activity through heterojunction interfaces as claimed by Low et al. [104].

Another study from Jeyalakshmi et al. [68], the team reported that sensitization of cobalt(II) tetraphenylporphyrin controls the electronic configuration in La changed perovskite of NaTaO₃, adjusting the bandgap to be narrow bandgap. No major change in the crystal structure of the pristine NaTaO₃ nanocubes were observed, but an absorption shift to 330 nm was observed. In addition, sensitization increased the ability to minimize CO₂ and decreased the recombination rate of the charge carrier. After longer irradiation, the material has been shown to be chemically stable. In other works, Zhong et al. [202] reported the effect of exposed catalyst crystal facets on water splitting using CdSe quantum dot (QD) sensitized BaTiO₃ nanocubes. They reported that CdSe QDs located on anisotropic planes of (230) and (001) of 30-face cubic of barium titanate exhibited higher efficiency compared to isotropic (001) facets of 6-face cubic of similar perovskite.

4.5 Others

Another alternative and practical modification, by involving carbon-based materials due to their appropriate function, large surface areas, good conductivity and chemical stability, as described by Sun et al. [155], Wang et al. [172, 174, 168], Tan et al. [160]. Thus, the electron transfer of p–n heterojunctions is predicted to be strongly promoted to achieve enhanced photocatalytic behaviour when coated with a carbon film. Besides, the conductive carbon layer will also prevent p–n heterojunction nanostructure accumulation, which is helpful for the increased the stability of p–n heterojunctions.

Zhou et al. [203] reported the production of ATiO₃ hierarchical structure in 3D utilizing the natural green leaves that aimed to decrease the emission of carbon dioxide in the presence of A: 1⁄4 Sr, Ca and Pb. The porous network provided by leave vein has a wide surface area that enhances gas diffusion, thus improving overall efficiency. Furthermore, with a similar 3D structure, worked with numerous cocatalysts, such as Pt, Cu, NiOx, Au, Ag and RuO₂. The highest evolution of CO and CH₄ among all gold (Au) was observed, followed by Cu and Ag under visible light illumination.

Shi and co-workers [142] have been investigating the impact of the manufacturing approach on the catalytic efficiency of NaNbO₃ through a solid-state reaction and hydrothermal. Different morphologies were observed from those reactions. Smooth and straight nanowires produced from hydrothermal reaction while bulk particles in homogenous sizes were formed by solid-state reaction. The two methods provided perovskite with a similar bandgap of 3.4 eV and the activity was examined under UV illumination by the generation of CH4. The homogeneous nanowires with increased crystallinity and greater surface area showed better CH4 behaviour relative to pristine NaNbO₃ [142]. Further combination of NaNbO₃ nanowires with g-C3N4 steered to greater performance compared to pristine NaNbO₃ or g-C3N4 for reduction of carbon dioxide. The overlap of NaNbO₃ and C3N4 coordinated band structures increased the efficiency of photocatalytics [142].

By using rGO carbon derivative as an electron separator and transporter to improve H₂ photocatalytic performance in water splitting, Humera et al. [60] further enhanced the production of LaFeO₃. The analysis shows that the integration of rGO was able to improve the efficiency of the studied material. In addition, several researchers team like Moniruddin et al. [117] and Dong et al. [31] stated the calcination temperature and the concentration of precursors play a crucial role in improving the photocatalytic perovskite performance. The study of Moniruddin et al. [117] found the size of SrTiO₃ is greatly increased with heating temperatures up to 800 °C. The H₂ production rate increases as the temperature rises. It can be seen that with increasing precursor concentration in EtOH at 800 °C, the H₂ production rate reduces simultaneously as the size of SrTiO₃ decreases.

The effect on the size and structure of CaTiO₃ by changing the temperature, molar ratio of water to ethanol and reaction time was successfully studied by Dong et al. [31]. The shape varies from inhomogeneous structure to microspheres with longer reaction time and temperature elevation. The morphology of microspheres demonstrated greater photocatalytic activity as opposed to randomly aggregated nanosheets [31]. The growth in photocatalytic activity is largely due to the greater redox potential of CaTiO₃. The synergistic impact of morphology and visible facets in a photocatalytic system is therefore crucial to determine the efficiencies of photocatalyst [202].

5 Application of Perovskite Photocatalyst

5.1 Water Purification

Water purification is the process of discarding contaminants from water such as algae, bacteria, fungi, viruses as well as parasites as in a biological group, meanwhile in the chemical group consists of organic pollutants, inorganic pollutants, toxic metals and suspended solids. Conventional purification processes use filtration and adsorption to remove the impurities from water sources. However, there are several problems encountered, such as producing secondary waste, needing extra treatment to unclog the filter and the pollutant not completely degraded. Therefore, photocatalysis provides new insight into the water purification process. Due to the ability of photocatalyst in degrading pollutants, it overcomes all the drawbacks of conventional processes.

Figure 13.2 shows the basic mechanism of water purification by the photocatalysis process. The general photocatalysis process by perovskite photocatalyst describe as following (i) the photon from light source hit the perovskite photocatalyst surface and produce electrons and holes; the light energy must overcome the band gap in order to produce electrons and holes (ii) electron travel to the conductive band (CB), simultaneously hole travelled to valence band (VB) (iii) holes react with water molecules to produce hydroxyl ions and hydrogen ions. Further reaction of hydroxyl ions with holes produce hydroxyl radicals which are an active species to degrade the pollutants (iv) simultaneously, electrons react with electrophilic oxygen to form superoxide radical anions also one of active species. When superoxide reacts with hydrogen ions, create hydroperoxyl radicals. Two hydroperoxyl radicals react, forming the hydrogen peroxide. Hydrogen peroxide acts as fuel in producing an abundance of hydroxyl ions and hydroxyl radicals. Therefore, most of the pollutants can be degraded when they react with these active species and usually produce oxygen, carbon dioxide, water among the end products.

Fig. 13.2

Basic mechanism of perovskite photocatalysis [112]

When light source hit the perovskite photocatalyst surface,

Perovskite photocatalyst + hν → h⁺ + e⁻ (perovskite photocatalyst)

At CB,

$${\text{h}}^{ + } + {\text{H}}_{2} {\text{O}} \to {\text{OH}}^{ - } + {\text{H}}^{ + }$$

$${\text{h}}^{ + } + {\text{OH}}^{ - } \to{^\bullet } {\text{OH}}$$

Simultaneously, at VB,

$${\text{e}}^{ - } + {\text{O}}_{2} \to {\text{O}}_{2}^{ \bullet - }$$

$${\text{O}}_{2}^{ \bullet - } + {\text{H}}^{ + } \to {\text{HO}}_{2}^{ \bullet }$$

$${\text{HO}}_{2}^{ \bullet } + {\text{HO}}_{2}^{ \bullet } \to {\text{H}}_{2} {\text{O}}_{2} + {\text{O}}_{2}$$

$${\text{H}}_{2} {\text{O}}_{2} + {\text{O}}_{2}^{ \bullet - } \to{^\bullet } {\text{OH}} + {\text{OH}}^{ - } + {\text{O}}_{2}$$

$${\text{H}}_{2} {\text{O}}_{2} + {\text{e}}^{ - } \to{^\bullet } {\text{OH}} + {\text{OH}}^{ - }$$

Therefore, after generate the active species,

The pollutants + specific active species → degradation products

Absalan et al. [1] synthesis to degrade bromophenol blue wastewater. The band gap was found at 3.80 eV required visible light of 410 W halogen lamp. In 120 min, 0.05 mol perovskite photocatalyst dosage successfully degraded 82% of 10.5⁻¹⁴ mol/L bromophenol blue solution. The reaction was repeated for 3 cycles to prove the stability of that perovskite photocatalyst. Due to high crystallinity and highly photoinduced, CoTiO₃ creates electrons at CB and holes at VB after obtaining the required light source. At the surface, the same mechanism as previously stated in general perovskite photocatalysis was observed in degradation of bromophenol blue solution. Meanwhile, Fig. 13.3 shows the mechanism of BaBiO₃ perovskite photocatalyst in degradation of Rhodamine B wastewater. 0.5 g/L of perovskite photocatalyst is able to tackle 5 mg/L of Rhodamine B solution by both discoloration (83%) and mineralization (80%) within 240 min and shows high stability for 4 cycles of reaction under visible light (450 W Xenon lamp). Active species generated by excitation of BaBiO₃ (2.02 eV band gap) such as holes, hydroxyl and superoxide radicals, break the aromatic rings to form smaller molecular weight and intermediates, which further reaction produce the end products such as CO₂, NH₄⁺ and water. It proves that perovskite photocatalyst can enhance the charge mobility in the crystalline network due to high crystallinity, the low particle size promotes short diffusion length of the charge carrier to reach the photocatalyst surface and reduce the electron-hole recombination [59].

Fig. 13.3

Mechanism of BaBiO₃ perovskite photocatalyst in Rhodamine B wastewater degradation and hydrogen production [59]

Doped-perovskite photocatalysis for water purification shows a different mechanism. Usually, the doping acts as an electron trapper to avoid electron-hole recombination. Therefore, the electron acceptor such as diffuse oxygen can easily trap electrons to produce superoxide radicals and other active species to degrade pollutants. It is observed by Wang et al. [176, 171], 91.4% of 48 mg/L rhodamine B wastewater degraded within 120 min under visible light (300 W Xenon lamp) by Bi/BiOCl/ZnSn(OH)₆ doped-perovskite photocatalyst. Figure 13.4 two-part energy level for Bi/BiOCl and ZnSn(OH)₆; Firstly, light energy hit BiOCl to produce electrons at CB and holes at VB. Due to Fermi level of Bi is lower than CB of BiOCl, the electron trap at Bi-metal instead. Then, the electrons transfer to ZnSn(OH)₆ and react with diffuse oxygen to produce superoxide. The superoxide further reacts and produce hydroxyl radicals to degrade Rhodamine B molecules partly into water and CO₂. Interestingly, at dopant, due to the accumulated electrons, the same mechanism also applied to produce hydroxyl radicals and able to degrade Rhodamine B. Next, the holes created by perovskite photocatalyst act as powerful active species to directly degrade Rhodamine B molecules. Aligned with Safari et al. [135], the methylene blue molecules purified by photogenerated holes and superoxide anion radicals produced by Gd-doped NiTiO₃. The doping of gadolinium (Gd) ion helps in improving the porosity of NiTiO₃ perovskite photocatalyst by decreasing the perovskite size, improving the specific surface area and reducing the recombination of electron-hole. Thus, it enhanced the production of holes and superoxide anion radicals. Therefore, the degradation of methylene blue achieved 88.64% within 120 min.

Fig. 13.4

Doped-perovskite photocatalyst [176, 171]

$${\text{Gd}}^{3 + } + {\text{e}}^{ - } \to {\text{Gd}}^{2 + } \left( {{\text{electron}}\;{\text{trapping}}\;{\text{step}}} \right)$$

$${\text{Gd}}^{{2 + }} + {\text{O}}_{{2({\text{ads}})}} \to {\text{Gd}}^{{3 + }} + {\text{O}}_{{2({\text{ads}})}}^{{ \bullet - }} \, \left( {{\text{electron}}\;{\text{transferring}}\;{\text{step}}} \right)$$

$${\text{O}}_{{2({\text{ads}})}}^{ \bullet - } + {\text{H}}_{{({\text{ads}})}}^{ + } \to {\text{HO}}_{{2({\text{ads}})}}^{ \bullet }$$

$${\text{HO}}_{{2({\text{ads}})}}^{ \bullet } + {\text{H}}_{{({\text{ads}})}}^{ + } + {\text{e}}^{ - } \to {\text{H}}_{2} {\text{O}}_{2}$$

$${\text{H}}_{2} {\text{O}}_{{2({\text{ads}})}} + {\text{e}}^{ - } \to{^\bullet } {\text{OH}}_{{({\text{ads}})}} + {\text{OH}}_{{({\text{ads}})}}^{ - }$$

5.2 Bacteria Disinfection and Air Purification

Bacteria is a unicellular microorganism from prokaryote group. It can be divided into two types: gram-positive and gram-negative. Gram-positive bacteria have a thick cell wall made of peptidoglycan, meanwhile gram-negative bacteria have a cell wall made from outer membrane and thin peptidoglycan layer. Bacteria disinfection is physical or chemical treatment to reduce bacteria amount until obtaining desired concentration. Bacteria disinfection by photocatalysis can be achieved when the light source hits the perovskite photocatalyst to generate electrons and holes. In the CB, there are two paths to deactivate the bacteria. Powerful active species in deactivating bacteria is superoxide radicals which produce when electrons react with oxygen. In the presence of water molecules, superoxide radicals can react with it to create another active species, hydroxyl radicals to deactivate the bacteria. Meanwhile, at VB, the holes itself have the ability to deactivate the bacteria directly.

At CB,

$${\text{e}}^{ - } + {\text{O}}_{2} \to {\text{O}}_{2}^{ \bullet - }$$

$${\text{O}}_{2}^{ \bullet - } + {\text{bacteria}} \to {\text{deactivated}}\;{\text{bacteria}}$$

$${\text{O}}_{2}^{ \bullet - } + {\text{H}}_{2} {\text{O}} \to{^\bullet } {\text{OH}}$$

$$^{ \bullet } {\text{OH}} + {\text{bacteria}} \to {\text{deactivated}}\;{\text{bacteria}}$$

Simultaneously at VB,

$${\text{h}}^{ + } + {\text{bacteria}} \to {\text{deactivated}}\;{\text{bacteria}}$$

The study by Shi et al. [145] found that CuBi₂O₄/Bi₂MoO₆, a perovskite photocatalyst (Bi₂MoO₆) with heterojunction p-type semiconductor (CuBi₂O₄), disinfect Escherichia coli (E. coli) almost completely within 4 h under visible light. Figure 13.5 shows the mechanism of photocatalytic disinfection that produces the active species such as holes, superoxide radicals, and hydroxyl radicals to inactivate E. coli. The band gap of perovskite photocatalyst is 2.72 eV can highly absorb 420 nm visible light and create the holes after the electrons excite to CB. At CB of perovskite photocatalyst, the electrons accumulate and follow the same paths as the stated general mechanism to produce superoxide radical and hydroxyl radicals for bacteria deactivation. Concurrently, the holes accumulate at p-type semiconductor attack the cell membrane and cause the inactivation of E. coli. The large specific surface area of CuBi₂O₄/Bi₂MoO₆ provides a more reactive site and reaction interface between photocatalyst and the bacteria. Hence, slow recombination rate, high efficiency for separation, high utilization rate of light and large specific surface area are the valuable characteristics for photocatalyst during photo-disinfection. Meanwhile, Li et al. [93], study the application of perovskite photocatalyst for both gram-positive and gram-negative under the same factors. Under 400 nm visible light (300 W Xenon lamp) irradiation, E. coli gram-negative and Staphylococcus aureus (S. aureus) gram-positive were successfully degraded by Pb–BiFeO₃/rGO. The complete inactivation of E. coli within 30 min, meanwhile S. aureus achieved 99.7% degradation within 90 min. Gram-positive is more highly resistant due to their cell wall structure, made up of 3D spatial network structure composed of many layers of teichoic acids and peptidoglycan compare to gram-negative bacteria consists of single layer scattered structure only.

Fig. 13.5

Photocatalytic disinfection mechanism of E. coli by CuBi₂O₄/Bi₂MoO₆ [145]

Air purification is a process to kill airborne pathogens that can cause airborne disease to humans or animals such as allergies, influenza, flu and measles. The treatment reaction must be able cut-off the spread routes of pathogens either via aerosol or contaminated fluid. The perovskite photocatalyst, La_0.9MnO₃ demonstrates high oxidative ability towards influenza A virus. The illustration in Fig. 13.6 shows hemagglutinin and neuraminidase, the amino acid residue of envelope proteins on influenza virus oxidized by La_0.9MnO₃. The oxidative species might penetrate the virus and damage the genetic materials. In 15 min, almost 76% of the virus has been disinfected. Therefore, prefer features of perovskite photocatalyst as air purifier utilize visible light despite of UV light due to its carcinogenic potential to humans and animals, able to self-disinfecting the airborne pathogens without requiring external energy sources and working continuously, less harsh and also high stability.

Fig. 13.6

Deactivation of amino acids residue of envelope proteins influenza A virus by La_0.9MnO₃ perovskite photocatalyst [179]

5.3 Photocatalytic Hydrogen and Oxygen Production

Hydrogen energy is the latest renewable energy discovered by scientists in this century. In the meantime, it was developed for the purpose of hydrogen vehicles that use internal combustion engines and fuel cells that are still under progress and expected extensive study. The benefits of this approach are low greenhouse gas emissions and can obtain by various fuel sources. For energy storage, gives economic benefits due to efficient remote power systems and also reduction in production and operational costs compared to fossil fuel energy.

Photocatalytic hydrogen evolution requires electrons as major active species. Water splitting is a reversible process in hydrogen and oxygen production, so photocatalyst is introduced in the reaction in order to produce an abundance of hydrogen gas that can be collected.

Water splitting by exert energy

$$2{\text{H}}_{2} {\text{O}} + {\text{energy}} \rightleftarrows 2{\text{H}}_{2} + {\text{O}}_{2}$$

In presence of photocatalyst, at CB, water molecules reduce to form hydrogen gas

$$2{\text{H}}_{2} {\text{O}} + 2{\text{e}}^{ - } \to {\text{H}}_{2} + 2{\text{OH}}^{ - }$$

And water molecules oxidize to form oxygen gas

$$2{\text{H}}_{2} {\text{O}} + 4{\text{h}}^{ + } \to {\text{O}}_{2} + 4{\text{H}}^{ + } + 4{\text{e}}^{ - }$$

Existing research recognizes the critical role played by perovskite photocatalyst in hydrogen and oxygen production. BaBiO₃ produces 61 µmol g⁻¹ h⁻¹ hydrogen gas in 3 h under irradiated 450 W Xenon visible lamp. The water molecule is reduced to produce hydrogen gas, meanwhile it is oxidized to form oxygen gas as shown in Fig. 13.3.

Water molecule reduction to form hydrogen gas

$$2{\text{H}}_{2} {\text{O}} \to 4{\text{H}}^{ + } + 4{\text{e}}^{ - } + {\text{O}}_{2}$$

$$4{\text{H}}^{ + } + 4{\text{e}}^{ - } \to 2{\text{H}}_{2}$$

This perovskite photocatalyst synthesis with slow and good crystal growth with a less formation of defects. The defects can act as a recombination center. Thus, the high crystallinity promotes better electrons and holes mobility in the crystal network to reach the surface. Next, the particle is small in size to decrease the diffusion length of photogenerated charges transfer to the surface, short distance directly related to low resistance. It also implies low probability for electron-hole pair recombination. Furthermore, small particles contribute to high surface area and increase the adsorption capacity between perovskite photocatalyst and water molecules. Hence, the stable photocatalytic hydrogen evolution must be attributed to high crystallinity, low particle size, low recombination of electron-hole and low resistance to improve the hydrogen production.

5.4 Photocatalytic Reduction of CO₂

Almost 77% of carbon dioxide emission mainly contributed by transportation, electricity and industry was reported by the United States Environmental Protection Agency (EPA) due to large consumption of limited fossil fuels. Therefore, the endeavour to convert CO₂ into chemical fuels such as carbon monoxide, methane and methanol to solve the global energy and environmental issues. Photocatalysis has been recognized as one of promising strategies to tackle this problem. As artificial photosynthesis, photocatalysts utilize solar energy to combine with CO₂ and H₂O to produce the chemical fuels and carbon monoxide as shown in Fig. 13.7.

Fig. 13.7

List of carbon dioxide reduction reactions produce different chemical fuel products [185]

As shown in Fig. 13.8, there are two paths for CO₂ reduction either as CO or CH₄ can be achieved simultaneously during photocatalytic reactions. The influence of H⁺ ion amounts towards CO₂ molecules play an important role for the end product yields. When the light reaches SrTiO₃ perovskite photocatalyst, the electrons excite to CB leaving the holes on VB. The active species on VB react with H₂O to produce H⁺. Meanwhile, the electrons at CB transfer to the dopant, Pt and react with H⁺ to produce CO and CH₄.

Fig. 13.8

The mechanism of Pt doped SrTiO₃ perovskite photocatalyst in carbon dioxide reduction into carbon monoxide and methane [180]

$${\text{CO}}_{2} + 2{\text{H}}^{ + } + 2{\text{e}}^{ - } \to {\text{CO}} + {\text{H}}_{2} {\text{O }}\left( {{\text{E}}^{\theta } = - 0.53\;{\text{V}}} \right)$$

$${\text{CO}}_{2} + 8{\text{H}}^{ + } + 8{\text{e}}^{ - } \to {\text{CH}}_{4} + 2{\text{H}}_{2} {\text{O}}\left( {{\text{E}}^{\theta } = - 0.24\;{\text{V}}} \right)$$

Meanwhile, the morphology of perovskite photocatalysts influences light harvesting. The 3D ordered macroporous (3DOM) morphology of SrTiO₃ perovskite photocatalyst improves the light absorption by slowing the light source from reaching Pt/SrTiO₃ and separating the charge carriers. Next, the alkali earth metal, Sr in the SrTiO₃ helps the adsorption and initiate activity for CO₂ reactant due to high alkalinity of Sr. The Pt dopant helps to separate the photogenerated charges by transferring the electrons and react directly with H⁺ ions. Therefore, perovskite photocatalyst shows potential in CO₂ reduction by undergoing artificial photosynthesis.

5.5 Application of Visible-Light-Driven Perovskite as Photovoltaic Solar Cell (PSCs)

Current global power demand is increasing significantly every year aligned with the improvement of the technologies and industries need. Nowadays it was reported that the energy usage is 16TW globally and expected to increase up to 30TW in 2050 [133]. Therefore, a lot of research has been done since then with only one purpose to find an alternative source that can add-on to the current or conventional source of energy such as petroleum-based fuel. Among the various sources of energy, solar energy was found to be the most significant and promising since it is easily and readily available. This is possible by converting the solar energy to electricity. Therefore, using photovoltaic solar cells for power generation seems to be a promising way as they convert the sunlight directly into electricity.

Nowadays, the market was dominated by the crystalline silicon solar cells. However, the production of the solar cells is costly due to the expensive raw materials. Because of that, most of the researchers come up with new ideas of utilizing PV technology that has low efficiency and cost manufacturing. Despite all the facts, poor device stability and short lifetime are creating interferences in the path of commercialization of PSCs, the perovskite materials are gaining huge attention among the researchers because of their excellent PV performance, cheap raw material, and requires of easy processing parameters [205]. Furthermore, it also does not entail any complicated processing conditions, since it can be created by using simple low-cost methods such as screen printing, dual-source evaporation, spin and dip coatings techniques which can be developed on flexible substrate.

Si et al. [147] have studied the fundamentals on the absorption of PSCs. It was found that the perovskite layer tends to absorb UV and visible light bands, meanwhile, the electrode layer favours to absorb IR bands. Interestingly, plasmonic was used to create hot spots in active layer via light-flow-circulating and nonlinear absorption mechanisms so that, the light can be localized through perovskite material. Si et al. [147] also reported that the absorption of IR-band has been extended to 58.2% when the plasmonic was used in the materials.

Figure 13.9a, b shows the mesoscopic architecture and the planar heterojunction structure of PSCs, respectively. Recently, the mesoporous materials have been studied and applied as PSCs due to their high porosity and large specific surface area up to 1000 m²/g [205]. It potentially increases the light response of photosensitize material and improves the effectiveness of the instrument.

Fig. 13.9

Schematic diagram and SEM section image of a mesoscopic architecture PSCs and b planar heterojunction structure PSCs (adapted from [205]

Basically, a solar cell consists of metal electrode layer, a hole transport layer as Hole Transporting Material (HTM), a perovskite layer, a porous oxide layer, a dense electron transport layer as Electron Transporting Material (ETM), and Flourine-doped Tin Oxide (FTO) electrode. This structure arrangement not just effective to decrease the recombination rate of electrons and holes, but also provides the necessary diffusion length for the accumulation of electrons and holes effectively [34]. The TiO₂ layer in the middle layer plays a significant function such as conducting the electrons, blocking the holes and inhibits the electron-hole pairs recombination in the FTO conductive substrate. Thus, it contributes to improve the photoelectric conversion efficiency of the studied materials. Furthermore, other than TiO₂, materials like ZnO, Al₂O₃, and ZrO₂ are typically used. Meanwhile, the function of hole transport layer is to accept the generated holes and transfer them to the surface of the metal electrodes. Commonly, the hole transport material and counter electrode materials are Spiro-OMeTAD (2,2’,7,7’-Tetrakis [N,N-di(4-methoxyphenyl)amino]-9,9’-spirobifluorene) and noble metals, such as Au, Ag, and Pt, respectively.

Based on Fig. 13.9b, the distinction from the mesoscopic structure is that the planar structure has no porosity of materials between the two layers of the electron transport and the hole transport, as a result, the electron-hole pairs can be parted effectively. Additionally, it contributes enlightenment the mechanisms of light absorption and electron-hole separation, thus, enhance the versatility of device optimization for the development of highly effective and advanced coated perovskite solar cells.

Latest research by Ren et al. [130] they studied power conversion efficiency and efficient light capture and prepared the ultraviolet-ozone assisted strategy on the TiO₂ interface for perovskite solar cells. Interestingly, the modified perovskite PSCs with ultraviolet ozone TiO₂ efficiently suppress the decomposition of perovskite films under light illumination. They found that this device has better performance and remarkable stability as compared to the pristine perovskite solar cells.

5.6 Application of Visible-Light-Driven Perovskite as Photocatalytic Nitrogen Fixation

Ammonia (NH₃) is an essential and important substance in the present-day industry as the major component of production of many types of chemicals such as fertilizers and bleaching agents for the cleaning process. Conventionally it has been produced in big scale industries via Haber-Bosch reaction, which requires very high temperature ranging from 400 to 500 °C and pressure ranging from 15 to 25 Mpa [195]. Recently, numbers of research papers reported on the new and advanced techniques which are simpler and environmentally friendly known as nitrogen fixation. Previously, biological nitrogen fixation was introduced and applied in many applications. During this process, the nitrogen gas from the environment will be activated by the organism to produce NH₃ at ambient temperature and pressure, for example of the organism is nitrogenase enzymes which catalyze the reduction of N₂ to NH₃ [19]. Recently, artificial solar-powered nitrogen fixation was introduced, where the concept is by using the energy from light source such photons to excite electron which will be used to create radicals. This can be divided into two major steps; step (i) semiconductor is excited under light irradiation producing photogenerated electrons excited to the CB leaving holes in the VB. Step (ii) the photogenerated holes oxidize water to O₂ while the photogenerated electrons reduce N₂ to NH₃. Figure 13.10 shows the mechanism for the production of NH₃ by using the photocatalytic N₂ fixation technique.

Fig. 13.10

Propose mechanism for the photocatalytic N₂ reduction [195]

As reported by Li et al. [91], the bismuth molybdate nanosheet with oxygen vacancies can be prepared via NaOH etching treatment at room temperature. The function of oxygen vacancies is to boost the photocatalytic nitrogen fixation to produce ammonia under the visible light source at room temperature and ambient pressure. They found that the oxygen vacancies had improved the photogenerated charge carrier separation and improve the capability of the materials for the N₂ adsorption and activation. As a result, production of ammonia was increased up to 800 µmol g⁻¹ h⁻¹. Furthermore, these materials also show a good tolerance to the oxygen in the N₂ source for the replacement of pure N₂ with air under simulated solar conditions. The pictorial illustration of relaxed slab model of BMO sample and the Vo–BMO–OH sample surfaces and the schematic representation of photocatalytic N₂ fixation process under visible light illumination is shown in Fig. 13.11.

Fig. 13.11

Illustration of a the pictorial views of a relaxed slab model for the BMO sample and the Vo–BMO–OH sample surfaces, and b the schematic illustration of the photocatalytic N₂ fixation process on the Vo–BMO–OH sample under visible light illumination [91]

Wang et al. [176, 171], reported the importance of oxygen vacancies with abundant localized electrons, where it can assist or improve the ability of the material to capture and activate N₂. They proved that the bismuth oxybromide-based semiconductor that they have prepared produced higher NH₃ generation with rate up to 1.38 mmol h⁻¹ g⁻¹ under visible-light-induced OVs. It also resulted in the stable photoreduction of atmospheric N₂ into NH₃ in pure water, which serves as both solvent and proton source. Figure 13.12 shows that the photocatalytic N₂ fixation of the Bi₅O₇Br–NT can be divided into 4 major steps; (i) under visible light irradiation, part of the O will escape in the form of O₂ from the surface of Bi₅O₇Br–NT creating sufficient surface OVs, (ii) the N₂ is chemisorbed and activated on the OV sites, (iii) the excited electrons s injected into the activated N₂ and reduce it to NH₃ and the last step (iv) the photoinduced OVs would be refilled by seizing O atoms from water, leading to a good recovery to the original stable OV-free composition.

Fig. 13.12

Schematic illustration of the photocatalytic N₂ fixation model in which water serves as both the solvent and photon source as well as the reversible creation of light-induced OVs [176, 171]

5.7 Application of Visible-Light-Driven Perovskite Photocatalyst for Anti-fogging Glass

Fog or fogging phenomenon is the formation of small water droplets from the water condensation on the surface of glasses, goggles, camera lenses and binoculars. This is possible due to high surface tension resulting in a single droplet of water. Here, anti-fog or antifogging is very useful to avoid or to reduce the fogging phenomena, it can be done by reducing the surface tension and altering the degree of wetting by introduction of antifogging film, resulting in the super hydrophilic, non-scattering film or water instead of droplets.

Takata et al. [158], reported that they prepared the antifogging glass by coating the glass with the TiO₂ layer. It was observed that the TiO₂ has super hydrophilic properties which is very likely attracted to water other than common ability such as anti-bacterial, anti-pollution and deodorant materials. They mentioned that when the surface of TiO₂ is irradiated with UV-light could decrease the contact angle (CA) and reaches almost zero with time as illustrated in Fig. 13.13 where the incident light will not be diffracted. This finding also aligned with the statement reported by Duan et al. [32] stated that the super hydrophilic TiO₂ film reduces the CA 150° (hydrophobic TiO₂) to less than 5° (hydrophilic TiO₂). When the TiO₂ coated glass expose to the condensed water, the droplets will create or form very thin water film and become transparent, automatically, due to the self-cleaning effect it will repel oil layer and when exposed to the light source it activated and catalyzed the degradation of any possible bacteria and fungi, etc. Because of that ability, it was known that the TiO₂ has vast application as antifogging and also the antibacterial or self-cleaning assisted material.

Fig. 13.13

Illustration of water drops on a glass surface a uncoated side: hydrophobic (CA = 65°) and b coated side: hydrophilic (CA < 10°)

Nundy et al. [119] studied the performance of the super hydrophilic ZnO microstructure coating for photovoltaic and glazing applications. They reported on the wettability and photovoltaic behaviour can be affected by the different structures of ZnO by affecting the CA of each structure as shown in Fig. 13.14. From this Figure, we can see that the hydrophilic surfaces for nanorods ZnO with higher CA and super hydrophilicity of microflower and microsphere ZnO with lesser CA, respectively. This is due to the changes of crystallinity and microstructural during the synthesis of ZnO. Thus, microstructure ZnO is good for the many antifogging and photovoltaic applications.

Fig. 13.14

Contact angle measurement (CA) of a water drop before contact to the ZnO-coated surface; b nanorods ZnO, c microflowers ZnO, and d porous microspheres ZnO [119]

According to Takagi et al. [157], there are numbers of application of TiO₂ that has been studied as the sterilizer, cleaner, decomposer, and antifogging agent. This is because of the TiO₂ that has high oxidizing power when it is irradiated by solar light. They also studied the sputter deposition technique to apply the TiO₂ film on the surface of the glass or any targeted clean surface. Based on our previous studies, a lot of research papers were discussed on the potential of photocatalyst as an antifogging agent by increasing the wettability and reducing the contact angle of the surfaces especially TiO₂ and ZnO. However, best in our knowledge, studies on the perovskite structure of photocatalyst as potential antifogging glass is not yet reported or discussed elsewhere. This will become a good opportunity for the researchers to study, evaluate and conduct research on the application of perovskite photocatalyst as the antifogging agent.

6 Conclusion

This chapter revealed the theory of perovskite-based photocatalyst for various environmental remediation applications specifically under visible light and solar irradiation. Moreover, the different type of perovskite materials together with its advantages is thoroughly discussed. Its unique characteristic enables it to incorporate with other metal and non-metal nanomaterials and such modification strategies leads to an improve ability of perovskite-based photocatalyst especially in harvesting of visible or solar light and prolong the life span of the charge carriers has been further explained. The promising outcome of the fabrication perovskite materials and its development pathway for various applications for energy conversion and other environmental applications. Hence, the perovskite-based photocatalysts are a versatile material in terms of its recurrent development that enable performance enhancement and its practicality towards environment protection in the time ahead.