Perovskite Material-Based Photocatalysts

Abstract

Today’s exigency to furnish more sustainable energy requires materials and expedients with enhanced or even new functionalities. Harvesting solar energy using stable, cheap, and environmentally friendly materials, for solar water splitting and dye degradation, is an attractive approach. The present chapter is focused on the recently reported perovskites oxides for the photocatalytic applications such as dye degradation, CO₂ reduction, and water splitting under UV or visible light irradiation. Material preparation and characterization are two essential parameters; former is to get materials with desired structure and physicochemical property, and latter gives the information of the textural structures and properties of the synthesized photocatalysts. In this regard, the present chapter provides an insight into information of preparation and photocatalytic application of selected ABO₃-based photocatalysts. It opens up a new viable approach for the synthesis of highly efficient perovskite-type photocatalysts for environmental remediation and energy production. The present chapter is organized into five sections: (1) a brief introduction to photocatalysis, (2) methods to tailor the photocatalytic properties of semiconductor photocatalysts, (3) a short overview of perovskite oxides as photocatalysts, (4) recent developments in enhancing the photocatalytic activity of perovskite materials, and (5) conclusions and insights.

1 Introduction

The rapid industrialization, the progression of human civilization, and escalating the use of traditional fossil fuels ensued in the extreme limits of pollution of air, water, and soil. It is observed that over the past few decades, the abusive use of pesticides, industrial chemicals, pharmaceutical and personal care products (PPCPs), organic dyes, and combustion byproducts have resulted in their undesirable accumulation in the environment. Of these, organic dyes have been extensively used in industries such as fabric, furniture, food, and paint. It is estimated that about 30–40% of these coloring agents in the form of “dye wastewater” are released (accidentally or without pretreatment) into water streams causing environmental and health problems. A few of these water-soluble dyes prevent sunlight causing damage to aquatic life. Some other dyes on exposure to sunlight decompose and generate carcinogenic byproducts. The mounting level of pollution together with the depletion of fossil fuel has forced scientists to look for new alternative energy sources and more effective mechanisms to curb pollution. Thus, the utilization of solar energy through a sustainable and cost-effective approach offers the solution to (a) reduce environmental pollution and (b) satisfy the demand for renewable energy.

Traditional methods of wastewater treatment such as coagulation, microbial degradation, adsorption of activated carbon, incineration, filtration, and sedimentation have become ineffective from an economic point of view or leave secondary pollutants. Advanced oxidation processes (AOPs), a group of established treatment methods including (i) photocatalytic oxidation using a semiconducting material as catalyst, (ii) ultrasonic cavitation, (iii) electron beam irradiation, (iv) Fenton’s reaction, and (v) reactions using O₃/H₂O₂, UV/O₃, and UV/H₂O₂ are found to be effective. The principle of AOP is to generate robust oxidizing species such as OH radicals in situ, which initiates a series of reactions that break down the dye molecule into smaller and less harmful substances, the so-called mineralization process [1,2,3]. Despite the good oxidation of organic pollutants (except method (i)), the complexity of these methods (AOPs), the high chemical consumption, and the relatively high treatment costs constitute the significant barriers for large-scale applications [2]. Hence, semiconducting material as a photocatalyst is anticipated to be both supplementary and complementary to a more conventional approach for the devastation of dangerous chemical wastes and energy production.

1.1 Heterogeneous Photocatalysis—Semiconductor Oxides as Photocatalysts

The execution of a chemical or biochemical reaction on the surface of the inorganic semiconductor is referred to as heterogeneous photocatalysis. For the fundamentals of photocatalysis, the readers can go through the excellent reviews reported [4,5,6,7,8,9]. The catalyst together with reactants/pollutants, upon irradiation of light, produce an electron (in conduction band) and hole (in valence band) leading to a series of reactions in which useful products are formed, or undesirable pollutants are decomposed to less harmful byproducts. The phase pure, well-crystallized, monodisperse, nano-sized semiconductor oxide with homogeneous morphology is ideal to be an efficient and chemically stable photocatalyst. As the abovementioned properties strongly depend on the preparative conditions of material, the selection of preparation method is essential. It is well known that the efficiency of a photocatalytic reaction or photocatalyst depends on several factors such as its crystallite size, surface area, band gap energy (E_g), recombination rate of photogenerated electron–hole pair during the photoreaction, nature of the reactant/pollutant, and nature of active sites on the catalyst, to mention a few (Fig. 1) [10]. The photocatalyst used must satisfy several functional requirements with respect to semiconducting properties: (i) suitable solar visible light absorption capacity with a band gap energy about 2.0–2.2 eV (visible light region) and band edge potentials suitable for splitting of water, (ii) capability for separating photoexcited electrons from reactive holes, (iii) minimization of energy losses related to charge transport and recombination of photoexcited electron–hole pair, (iv) chemical stability against corrosion and photo-corrosion in aqueous environment, (v) kinetically sui electron transfer properties from photocatalytic surface of water, and (vi) easy and cost-effective method of preparation.

Fig. 1

Copyright 2018. Reprinted with permission from ACS publications

Factors impacting photocatalytic activity of a single (idealized) inorganic semiconductor particle [10]

1.2 Mechanism of Semiconductor Photocatalysis

When a semiconductor absorbs light energy more than its band gap energy, it results in the promotion of the valence band electron into its conduction band. The formation of the electron (in the conduction band) and hole (in the valence band), known as “exciton” is the first step in the mechanism of semiconductor photocatalysis.

$${\text{Photocatalyst}}({\text{PC}})\, + \,h\upsilon \to \,{\text{PC}}\,\left( {h_{\text{VB}}^{ + } \, + \,e_{\text{CB}}^{ - } } \right)$$

After photoexcitation, the excited electron–hole pairs either recombine or can migrate to the surface of the semiconductor. They (electron and hole) participate in various oxidation and reduction reactions with adsorbed species such as H₂O and O₂. These oxidation and reductions are the primary reactions of photocatalytic degradation of pollutants and photocatalytic hydrogen production, respectively. In the photocatalytic water splitting reactions, the photoexcited electrons in the conduction band (CB) are responsible for generation of H₂ and OH⁻ by reacting with water. However, in order to initiate hydrogen and oxygen production, the conduction band level of the semiconductor should be more negative than the reduction potential of H₂ (H⁺/H₂) and valence band level should be more positive than the oxidation potential of O₂ (H₂O/O₂). However, the photocatalytic dye degradation mechanism is strongly affected by different model pollutants and photocatalysts. In addition, the different dominant radical species would determine the photoactivity and the decomposition path. Hence, the dye degradation mechanism is still not fully understood, even uncertain. A basic mechanism for the photocatalytic process of a semiconductor dye degradation and water splitting is presented in Figs. 2 and 3, respectively.

Fig. 2

Degradation process of pollutants using semiconductor oxides illuminated by UV light/solar light

Fig. 3

Fundamental principle of semiconductor-based photocatalytic water splitting for hydrogen and oxygen production

2 Methods to Tailor the Photocatalytic Properties of Semiconductor Photocatalysts

Since the discovery of Fujishima and Honda [11] that water can be photo-electrochemically decomposed into H₂ and O₂ using a semiconductor (TiO₂) electrode under UV irradiation, extensive works have been carried out to produce hydrogen from water splitting using a variety of semiconductor photocatalysts. Since the major part (about 45%) of sunlight belongs to visible region, it is essential that the efforts to design the semiconducting photocatalysts should be toward the utilization of this part of solar light to ignite photocatalytic reactions. Hence, control of electronic band structure of material by adjusting its elemental compositions was recognized as the effective method to extend light response range.

The methods including metal and nonmetal substitution into semiconductor materials are considered to tailor its wide band gap to narrow band gap. Metal ion substitution creates impurity levels in the forbidden band as either a donor level above the valence band or an acceptor level below the conduction band as shown in Fig. 4.

Fig. 4

a Donor level and b acceptor level formed by metal ion doping

These impurity energy levels decrease the band gap energy of the photocatalyst, thus shifting the photo-response of catalyst toward the visible region. There have been numerous reports on the modification of wide band gap photocatalysts using metal ion doping to make them visible light active [12,13,14,15,16,17].

It is also perceived in some cases that metal ion dopant can also act as a recombination center, resulting in decreased photocatalytic activities [12]. Doping of nonmetal ions such as N³⁻, S²⁻, and F⁻ is another method employed to narrow the band gap and improve the photocatalytic activity. Unlike metal ion dopants, nonmetal ion dopants are less likely to form donor levels in the forbidden band and recombination centers. Hence, anion doping is more effective to enhance the photocatalytic activity. Since the study of N-doped TiO₂ by Asahi et al., the photocatalytic studies of anion-doped semiconductors have been attracted much attention. It was found that N substitution into the metal oxide can shift its photo-response into the visible region because of the contribution of N 2p orbitals to the valence band comprised of O 2p orbitals, thereby narrowing the band gap by moving the valence band upward (Fig. 5) [18]. The method of nonmetal ion doping has been widely used to modify some UV-light-active oxide photocatalysts [19,20,21,22,23,24].

Fig. 5

New valence band formation by doping of nonmetal ions

However, there are also two momentous drawbacks observed in visible light-responsive photocatalytic reactions. First is the rapid recombination of electron/hole (e⁻/h⁺) pair. The second is low utilization of visible-light-excited high-level energy electrons (HLEEs) and limited visible light absorption of semiconductor photocatalyst. Moreover, there is an additional problem of rapid backward reaction during water splitting. These drawbacks can be overcome by the development of novel photocatalysts such as metal/nonmetal doped photocatalyst, noble metal surface-modified photocatalyst [25,26,27], dye/semiconductor-sensitized photocatalyst, and p–n heterojunction structure composite photocatalyst [28, 29].

The dopant in lattice brings defects and tunes the amount of oxygen vacancy. It is known that the photocatalyst, having lattice defects and oxygen vacancies, shows good photocatalytic properties by narrowing the band gap and suppressing recombination rate of photogenerated electron–hole [30,31,32]. When the noble metal is loaded onto the surface of the photocatalyst, the photogenerated electrons migrate to the surface of the host photocatalyst and are entrapped by the noble metal cocatalyst. Meanwhile, the photogenerated holes stay at the host photocatalyst and migrate to its surface. This results in the efficient separation of the photogenerated electrons and holes. Subsequently, the separately localized electrons and holes become involved in their roles as the reducer and oxidizer, respectively, in the photocatalytic reaction. The noble metals, such as Pt, Au, Ru, Pd, Ag, and Rh, are usually used as effective cocatalysts for photocatalytic water splitting. To date, Pt-loaded photocatalysts have the highest photocatalytic activity for hydrogen evolution from water splitting reaction under the visible light irradiation [33, 34].

The methods including sensitization with organic dyes, the coupling of either p-type and n-type semiconductor or small and large band gap semiconductors, are another ways to improve the photocatalytic properties [35,36,37]. In the dye-sensitized semiconductor photocatalysis, the dye molecule electrons excited by visible light can be injected into the conduction band of the semiconductor to initiate the catalytic reactions. In the composite photocatalyst (coupling of large band gap semiconductor with a small band gap semiconductor with a more negative conduction level), the excited electrons from conduction band of a small band gap semiconductor by the absorption of visible light can be injected into the conduction band of a large band gap semiconductor, while the photogenerated holes are trapped in a small band gap semiconductor. Thus, an effective electron–hole separation can be achieved by these methods and results in higher photocatalytic activity. However, in p–n heterojunction photocatalysis, because of the difference of band gap and position between the two different semiconductors, the strong internal electric field at the interface can initiate the transfer of photoinduced electrons to the conduction band (CB) of the n-type semiconductor and of holes to the valence band (VB) of the p-type semiconductor for the reduction and oxidation reactions, respectively [38, 39]. It has been observed that the visible-light-excited high-level energy electrons (HLEEs) of narrow band gap semiconductors such as Fe₂O₃, BiVO₄, and BiFeO₃ could transfer thermodynamically to the conduction band of TiO₂ with a high-energy level platform, leading to the extended lifetime of photogenerated charge carriers and hence to the improved visible light activities [40,41,42]. Recently, graphitic carbon nitride (g–C₃N₄) has been used as a kind of metal-free n-type semiconductor for enhancing the photocatalytic performance of materials with poor photocatalytic activity due to its unique advantages of hydrophilicity and large specific surface area [43].

3 A Short Overview of Perovskite Oxides as Photocatalysts

Among the various types of photocatalysts reported so far, the lion’s share comes from oxide semiconductors. The ease of preparation, less expensive, less toxicity, possessing tunable properties, and their utility in cyclic experiments are some of the useful features that have made to investigate oxide semiconductors extensively compared to other types of materials. Oxides such as TiO₂, ZnO, WO₃, vanadates (VO₄), and molybdates have been considerably studied as photocatalysts. Oxides belonging to perovskite family with the general formula ABO₃ have attracted the scientific community due to their remarkable properties and the wide range of applications, interesting structure, and flexible compositions. The properties include high chemical stability, catalytic properties [44], multiferroicity behavior [45], order/disorder transformations, and range of electrical and ionic conductivities that include metallic, half-metallic with spin-polarized electrical conductivity [46, 47], and superconductivity [48]. These oxides are used in SOFC technology, radioactive waste encapsulation, magnetic memory components, and dielectric resonator materials [47, 49,50,51]. These perovskite materials have a distinctive structural feature that is beneficial to photocatalytic activity. The relationship between crystal structure and energy delocalization shows that the closer to 180° the B–O–B bond angle is, the easier the excitation energy is delocalized leading to higher photoactivity [52, 53]. Thus, materials belonging to ABO₃ family with ideal perovskite structure are expected to exhibit higher photoactivity.

Perovskite oxides (ABO₃) with the structural flexibility to tolerate replacement of A- and B-sites with different ionic species have led to many novel compositions. This chapter exclusively deals with the research work published on the photocatalytic activity of ABO₃-based perovskite-type materials. Double, triple, and layered perovskites are not considered. Recently, there are fascinating review papers on perovskite materials related to their structure, synthesis, and applications in various fields. Ewelina Grabowska, reviewed the synthesis, characterization of selected perovskite oxides such as SrTiO₃, KTaO₃, NaTaO₃, KNbO₃, and NaNbO₃ and their applications in photocatalysis. Besides, the methods for enhancing the photocatalytic activity of those mentioned oxides along with the influence of different morphologies and surface properties are also discussed [54]. Guan et al. summarized the contemporary progress of perovskite materials and their photocatalytic applications in water splitting and environmental remediation, and presented the key challenges and viewpoints on the research of perovskite photocatalysts [55]. A review on photocatalytic CO₂ reduction using nano-sized perovskite oxides is reported by Sheng et al. This review correlated the physicochemical properties such as crystal structures, defect distribution, morphologies, and electronic properties to CO₂ photoreduction performance [56]. Pushkar Kanhere and Zhong Chen reviewed the perovskite materials that are classified by the B-site cations and their crystal structure, optical properties, and electronic structure accompanied by photocatalytic performance [57]. Saumyaprava et al. discussed the recent development of perovskite, layered perovskite, and their composites especially with π-conjugated carbon materials toward photocatalytic applications [58]. ABO₃-based photocatalysts for water splitting and the application of the modification scheme of ABO₃ photocatalysts in designing A_xB_yO_z photocatalysts are systematically reviewed by Jinwen Shi and Liejin Guo [59]. Wei et al. described the basic principles of water splitting reaction, photodegradation of organic dyes and solar cells as well as the requirements for efficient photocatalysts [60]. Zhu et al. reviewed the catalytic performance of perovskite oxides with different morphologies for gas-, solid-, and liquid-phase reactions [61]. Shi et al. published a review on recent progress in photocatalytic CO₂ reduction over perovskite oxides. In this review, they have discussed recent advances in the design of perovskite oxides and their derivatives for photocatalytic CO₂ reduction, placing particular emphasis on structure modulation, defect engineering, and interface construction as rational approaches for enhancing solar-driven CO₂ conversion to CH₄, CO, and other valuable oxygenates (C_xH_yO_z) [62].

4 Recent Developments in Enhancing the Photocatalytic Activity of Perovskite Materials

Although there are some review papers on the photocatalytic applications with simple ABO₃-based perovskite materials, a planned update is highly desirable to inform the latest progress, growing trends and updated summary of the current status concerning photocatalysis. Further, the rapid utilization of ABO₃ materials in photocatalytic applications requires a fresh assessment to provide a broad overview and possible future directions. Here, we have presented a review on the photocatalytic activity of lanthanum-based perovskites LaMO₃ (M = Fe, Co and Mn) and BaMO₃ (M = Zr and Sn) along with the recent developments in enhancing the photocatalytic behavior of alkali and alkaline earth metal-based perovskite materials (ATiO₃ (A = Ca, Ba, and Sr), ATaO₃ (A = Na and K), and ANbO₃ (A = Na and K)).

A succession of stable lanthanum-based perovskites, LaMO₃ (M = 3d transition metal) has been experimentally and theoretically expounded to be competent photocatalysts due to the unique crystal structure, electric conductivity, etc. Among these, LaFeO₃ has attracted a great deal of interest due to its narrow band gap, inimitable optoelectronic properties structural flexibility, and applications in removing the potential environmental pollutants. The photocatalytic performance of LaFeO₃ is low in spite of its narrow band gap energy ((2.1 eV (for dense polycrystalline samples) and 2.6 eV for powdered samples)) [63, 64] due to limited visible light absorption and the low utilization of visible-light-excited high-level energy electrons [65]. Hence, the research has been aimed to improve its photocatalytic properties by altering its surface properties (like morphology, size of the particle, and surface area) and metal/nonmetal ions doping into its lattice. LaFeO₃ nanoparticles with different morphologies have been synthesized by sol–gel method [66], sonochemical method [67], electrospinning method [68], polymeric precursor route [69], and hydrothermal method [70, 71] to improve its catalytic, magnetic, and photocatalytic properties. Shudan Li and Xianlei Wang have prepared one-dimensional LaFeO₃ fibers with different morphologies by electrospinning method. They have reported the concentration effect of acetic acid and ferric nitrate solutions on the morphology of LaFeO₃. The effects of morphology on photocatalytic activity have also been investigated [69]. The introduction of metal ion substitution in place of La/Fe in LaFeO₃ on its photocatalytic activity has also been explained [72,73,74,75,76]. Parrino et al. synthesized Cu-substituted LaFeO₃ perovskites (LaFe_1−xCu_xO_3−δ, where x = 0.05, 0.10, 0.20, and 0.40) by citrate auto-combustion method and discussed their photocatalytic activity by employing 2-propanol oxidation in the gas–solid regime as a probe reaction. They concluded that the photocatalytic activity of the Cu(II)-substituted LaFeO₃ is improved by increasing the Cu amount up to 10 mol% and slightly decreased for higher copper contents. The higher activity of Cu(II)-substituted LaFeO₃ compared to that of pristine LaFeO₃ has been related with the presence of oxygen vacancies and with the reduced electron–hole recombination [77]. It was reported that the catalytic activities of the manganese-substituted LaFeO₃ were much higher than pure LaFeO₃ because of its higher oxygen vacancies, the variable valency of Mn ions, and the strong absorption in visible light [75]. Jauher et al. reported the synthesis of LaMn_xFe_1−xO₃ (x = 0.0–0.5) and examined their H₂O₂-assisted catalytic cationic and anionic dye degradation in the presence and absence of visible light irradiation. They observed that all the Mn-substituted ferrite compositions catalyzed the degradation of dyes both in the absence and presence of visible light irradiation and stated that the catalytic activity is not influenced significantly with increasing manganese content. LaMn_0.2Fe_0.8O₃ exhibited excellent photocatalytic activity for the degradation of both cationic and anionic dyes [78]. Based on these results, Dhiman et al. studied photocatalytic properties of the transition metals doped with LaMn_0.2Fe_0.8O₃ (LaMn_0.2M_0.2Fe_0.6O₃ (M = Cr³⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺)). All LaMn_0.2M_0.2Fe_0.6O₃ compositions (except M = Cr) have shown higher photocatalytic dye degradation which is attributed to the generation of vacancy defects and smaller band gap energy values [79].

Xicai Hao and Yongcai Zhang have synthesized porous nano-LaFeO₃ through a gel combustion method at the calcination temperatures of 200–400 °C and studied the photocatalytic reduction of aqueous Cr(VI) under visible light irradiation. The LaFeO₃ prepared at lower temperature exhibited enhanced photocatalytic activity in the reduction of Cr(VI) in aqueous solution under visible light irradiation [80]. Giuseppina et al. prepared LaFeO₃ nanoparticles via solution combustion method and investigated their photocatalytic efficiency for hydrogen production from glucose aqueous solution under UV and visible light emitted by light-emitting diodes (LEDs) [81]. Stable LaFeO₃ nanoparticles with a size of 15 nm and the band gap of 1.86 eV have been prepared by a sol–gel method using reduced graphene oxide (rGO) as a template by Ren et al. [82]. They have demonstrated that LaFeO₃–rGO can function as an efficient catalyst for degradation of methylene blue (MB) or rhodamine B (RhB) under visible light irradiation. From the controlled photocatalytic experiments, it was concluded that electron transfer from the dye molecule to hole dominated the oxidation process.

To overcome the low utilization of visible-light-excited high-level energy electrons (HLEEs) and limited visible light absorption of LaFeO₃, Muhammad et al. prepared ZnO-coupled Bi-doped porous LaFeO₃ and evaluated its visible light photocatalytic performance for 2,4-dichlorophenol (2,4-DCP) degradation and CO₂ conversion. The improved visible light activity is endorsed to the improved utilization of visible-light-excited HLEEs by protracting visible light absorption via the Bi-introduced surface states and coupling ZnO to introduce a suitable high-level-energy platform for accepting electrons. The mechanism of photocatalytic degradation of 2,4-DCP over ZnO/Bi-doped porous LaFeO₃ has also been discussed [83]. Yan et al. prepared a p–n heterojunction composed of p-type LaFeO₃ and n-type g–C₃N₄ and studied its photocatalytic activity against Brilliant Blue (BB) degradation under the visible light irradiation. The improved photoactivity of g–C₃N₄ and LaFeO₃ is attributed to efficient separation of electron–hole pairs, a higher concentration of superoxide and hydroxyl radicals. Electron spin resonance (ESR) analysis and the active species trapping experimental results confirmed the Z-scheme photogenerated electron transfer mechanism integrated with the dye-sensitization effect of the photocatalytic reaction process [43]. Ibrahim et al. prepared n-type LaFeO₃ and evaluated its photoelectrochemical water splitting under simulated AM 1.5G solar illumination in view of factors influencing photocatalytic performance including the calcination temperature, the species, concentration, and the pH of the electrolyte that are evaluated [84].

A perovskite, lanthanum cobaltite (LaCoO₃), is considered as a promising catalytic material because of its appealing physicochemical properties such as electrical, ionic conductivities, and excellent electrochemical properties [85, 86]. But less visible light-harvesting, higher electron–hole recombination rates and a short lifetime of photogenerated charge carriers hindered its pragmatic applications. Feng et al. adopted sol–gel and deposition–precipitation method to get a PdO/LaCoO₃ heterojunction photocatalyst to enhance the H₂ production from formaldehyde aqueous solution without any additives at low temperature. The heterojunction structure can significantly improve the charge generation, enhance the visible light absorption, and efficiently confine the recombination of photogenerated electron–hole pairs to exhibit higher photocatalytic activity. The obtained results are demonstrated that PdO doping on the LaCoO₃ surface can effectively adjust band gap and Fermi energy levels to apply in hydrogen production from formaldehyde solution under visible light. It is found that the hydrogen production rate of 2 wt% PdO/LaCoO₃ is multiple of ten higher than pristine LaCoO₃ [87].

Minghui Wu et al. tried to prepare LaCoO₃ with propitious performance through creating oxygen vacancies in it. LaCoO₃ is prepared using natural sugarcane bagasse with tunable oxygen vacancy and enlightened the interaction between metal ions and sugarcane bagasse [88]. It has been stated that natural biomass like sugarcane bagasse and extract of Pichia pastoris GS115 may not only replace the usual chemical reagents because of the benefits of being green, environmental, and low cost but also provide ample functional groups to interact with the metal ions leading to varying in the surface properties, and even form a unique structure of the synthesized photocatalysts, which could tune the amount of oxygen vacancy and bring the lattice defects [89]. The characterization of bagasse is carried out to explicate its interaction with metal ions and role in the synthesis process. Moreover, the correlation between the amount of oxygen vacancy on the perovskite induced by biomass and the performance of photocatalytic hydrogen production is examined and reported that as-synthesized sugarcane bagasse-mediated LaCoO₃ perovskite had worthy photocatalytic performance in hydrogen production from formaldehyde solution under visible light irradiation than that prepared by conventional citric acid method [88]. Liqing et al. reported novel microbial synthesis of high efficient Cu-doped LaCoO₃ photocatalyst from Pichia pastoris GS115 for hydrogen production from formaldehyde solution under visible light irradiation. Copper doping favors the formation of impurity level and appropriate oxygen vacancy. The biomass intervention in synthesis assists the adjustment of the crystal structure and surface structure of catalyst, which makes the diffraction angle and unit cell change and simultaneously regulates surface oxygen defects. Besides, the abundant organic functional groups from biomass residue on the surface act as a photosensitizer, so that the higher photocatalytic performance is obtained for the catalyst prepared by microbial synthesis than the catalyst prepared by traditional chemical methods [90]. Jayapandi et al. prepared pristine and silver (Ag)-modified LaCoO₃ via a hydrothermal process. Silver is considered as better metal dopant because Ag can induce transport of charge carriers and acts as an effective carbon removal, better photo absorber and provide active species for oxygen activation. Ag-modified LaCoO₃ degraded methylene blue (MB) in higher rate (99% in 10 min) compared to pure LaCoO₃ (75% in 10 min) [91].

The photodegradation of methyl orange within 90 min of visible light irradiation is observed using pure LaMnO₃ nanoparticles [92]. It is well known that the porous perovskite oxides with the improved specific surface area are vital to increasing the photocatalytic activity [93, 94]. Rajesh Kumar et al. examined the significance of different morphologies of LaMnO₃ particles for the degradation of RB dye under visible light irradiation. As-prepared porous LaMnO₃ nanoparticles degraded 95% of RB dye in 60 min of irradiation time. These results imply that the porous morphology and specific surface area plays a key role in the photocatalytic performance [95]. Peisong et al. studied the photodegradation of methyl orange dye under the visible light irradiation using YbFeO₃ [96] and EuFeO₃ [97].

The structural features of BaZrO₃ like ideal cubic crystalline phase with a bond angle of 180° of the ZrO₆ octahedral sites and largely dispersed conduction band have provoked to study its photocatalytic activity. Yupeng et al. studied for the first time the photocatalytic water splitting of BaZrO₃ under the UV light irradiation [98]. Later, the same group has tried to modify the electronic structure of BaZrO₃ to enhance the evolution rate of H₂ by doping Sn⁴⁺ in Zr⁴⁺ site. They observed that BaZr_0.7Sn_0.3O₃ exhibits the highest H₂ evolution rate [99]. They have also investigated the impact of Sr²⁺ substitution on photocatalytic water splitting activity of BaSnO₃ [100]. Borse et al. switched the UV active BaSnO₃ photocatalyst into a visible-light-active photocatalyst for O₂ production through the electronic structure modification by doping Pb²⁺ for Sn²⁺ [101]. Subsequently, attempts have been made to improve the photocatalytic activity of BaZrO₃ not only by metal ion doping [102, 103] but also preparing BaZrO₃ solid solutions [ex: BaZrO₃–BaTaO₂N Solid-Solution Photocatalyst] [104] or heterojunction formation with other metal oxides [Ex: BaZrO₃/Cu₂O/Bi₂O₃] [105].

Titanium-based perovskite materials with the composition of ATiO₃ (A = Sr, Ca, and Ba) have been studied for photocatalytic applications for a long time even though having wide band gap energies due to their excellent resistance to photo-corrosion and high thermal stability [106]. Among the large number titanate perovskites, strontium titanate oxide (SrTiO₃) with an indirect band gap of 3.25 eV has been one of the most studied photocatalysts, exhibiting an unusual photocatalytic activity in various photocatalytic applications [107, 108]. Many researchers have attempted to tune the electronic structure of titanium-based perovskite oxides to obtain the improved photocatalytic activity. Here, the recent developments in enhancing the photocatalytic applications of ATiO₃ (A = Sr, Ca, and Ba) have been presented.

A series of Au/SrTiO₃ nanocomposites with different Au contents have been synthesized by Xian et al., using a photocatalytic reduction method. The photocatalytic activity of the obtained composites was investigated for the photocatalytic degradation of acid orange 7 and methyl orange dyes under simulated sunlight and visible light irradiation, and a higher photoactivity observed for Au/SrTiO₃ nanocomposite than that of pure SrTiO₃ [109]. Liu et al. successfully synthesized WO₃, nanosheets with different SrTiO3 (La, Cr) contents. The photocatalytic activity of the as-prepared samples was measured in the decomposition of methylene blue under visible light irradiation [110]. Kissa et al. synthesized SrTi_1−xRh_xO₃ photocatalyst considering different Ti/Rh ratios using a one-step hydrothermal synthesis and studied for the decomposition of methyl orange under visible light irradiation. 5% Rh-doped SrTiO₃ is shown to completely oxidize methyl orange within 30 min under visible light illumination [111]. Pure TiO₂, SrTiO₃, and SrTiO₃/TiO₂ nanocomposite were prepared by Devi et al. through a sol–gel method. The photoactivity of the as-prepared samples was evaluated by the degradation of 4-Nitrophenol (4-NP) under UV light irradiation, which revealed that the composite had improved photoactivity as compared to pure TiO₂ and SrTiO₃ [112]. Swapna et al. examined photocatalytic degradation of methylene blue under UV light with SrTiO₃ catalysts. The SrTiO₃ catalysts were prepared by a novel polymerizable sol–gel (PSG) approach for the first time [113]. SrTiO₃ powders were prepared by applying a polymeric precursor method by Silva et al. for the photocatalytic decomposition of methylene blue (MB), rhodamine B (RhB), and methyl orange (MO) dyes under UV illumination [108].

Goto et al. studied the photocatalytic decomposition of water over Al-doped SrTiO₃ (SrTiO₃:Al) catalyst under solar light irradiation [114]. The heterojunction semiconducting Au–Al/SrTiO₃ oxide was prepared by Saadetnejad et al. for hydrogen production by photocatalytic water splitting under visible light in the presence of methanol, ethanol, and isopropyl alcohol as sacrificial agents. They synthesized the Al/SrTiO₃ via solid-state reaction, while Au loading was done with the homogenous deposition–precipitation method. The results showed that the 0.25% Au–1.0% Al/SrTiO₃ composite oxide exhibited higher photocatalytic hydrogen evolution of 347 mmol/h with isopropyl alcohol as sacrificial agent [115]. Han et al. prepared Mg-modified SrTiO_x photocatalysts by solid-state method. The samples were shown excellent performance for the photocatalytic water splitting under solar light illumination and the maximum H₂ evolution activity was obtained with the Sr_1.25Mg_0.3TiO_x material [116].

Pure SrTiO₃ is not considered as a good photocatalyst because it is only active under UV irradiation, and therefore cationic/anionic dopants are often doped into SrTiO₃ to make it visible light-responsive. Kou et al. used the transition metal ions such as Co, Fe, and Ni as dopants to improve the photocatalytic CO₂ reduction to CH₄. Among the dopants, Co ions doped SrTiO₃ displays the most outstanding photocatalytic performance for CO₂ photoreduction. Photo-deposition of Pt metal ions on co-doped SrTiO₃ surface further improves the CO₂ reduction to CH₄, and the yield of CH₄ under visible light irradiation is 63.6 ppm/h [117]. Luo and co-workers reported three kinds of SrTiO₃-containing (SrTiO₃, surface-Ti-rich SrTiO₃, and Sr(OH)₂-decorated SrTiO₃) photocatalysts for CO₂ reduction with H₂O vapor under visible light irradiation. They found that the molecular oxygen and CO were only the photoreduction products and the surface-Ti-rich SrTiO₃ exhibits the highest photocatalytic activity for CO₂ reduction [118]. By taking advantage of the loading/deposition of other metal or metal oxides on SrTiO₃ semiconductors, Li et al. prepared Rh and Au co-loaded SrTiO₃ through a photo-deposition process and studied the as-prepared catalysts for synthesis of syngas (CO and H₂) from photocatalytic reduction of CO₂ with water vapor under visible light irradiation [119]. Shoji et al. prepared the strontium titanate (SrTiO₃: STO) nanorod thin films loaded with amorphous copper oxide (CuO_x) nanoclusters. They found that compared to bare STO nanorods, the loading of the CuO_x-cocatalysts onto STO nanorods improved the photocatalytic carbon dioxide (CO₂) reduction using H₂O as an electron donor into carbon monoxide (CO) [120].

It is realized that the photocatalysts with a three-dimensional (3D) porous microsphere structure exhibited a high photocatalytic performance because they possess a high specific surface area and plenty of active sites for the target reaction, absorb more light through the reflection of pore walls, and are easy to be recycled by facile mechanical filtration [121]. Yang et al. prepared porous SrTiO₃ microsphere through a sol–gel method and observed its high photocatalytic performance for the Cr(VI) reduction under UV light irradiation [122]. Later, they tried to extend the photo-response of three-dimensional (3D) porous SrTiO₃ microspheres to the visible light region, thus enhancing their visible light photocatalytic activity [123]. They prepared three-dimensional porous La-doped SrTiO₃ (LST) microspheres via a modified sol–gel method. The agarose gel/SrCO₃ microsphere and La₂O₃ were used as the template and the La resource, respectively. The morphology of as-prepared LST-0.5 (agarose gel/SrCO₃/0.5 wt% of La₂O₃) sample is shown in Fig. 6. It is observed that these microspheres exhibit a higher photocatalytic activity than the undoped SrTiO₃ sample for the reduction of Cr(VI) under visible light irradiation. They have concluded that the observed redshift of absorption edge of SrTiO₃ inhibits the recombination of photogenerated electrons and holes, leading to the higher photocatalytic activity under visible light. Among all the as-prepared LST samples, LST-0.5 reduced 84% Cr(VI) within 100 min, exhibiting the highest photocatalytic activity. It is evident from Fig. 7 that the LST-0.5 sample has an excellent reusability, which may lay a good foundation for their practical application. They have also suggested photocatalytic mechanism for the reduction of Cr(VI) by porous LST microspheres under visible light as shown in Fig. 8.

Fig. 6

Copyright 2018. Reprinted with permission from Springer Nature

a OM photo of agarose gel/SrCO₃/TiO₂/La₂O₃ composite beads; b low- and c, d high-magnification SEM photos of LST (c), outside surface; d, inner structure; e TEM photo with corresponding SAED pattern (inset); f HRTEM photo of LST [123]

Fig. 7

Copyright 2018. Reprinted with permission from Springer Nature

a Recycling test of LST-0.5 for photocatalytic reduction of Cr(VI) under visible light and b XRD patterns of LST before and after six recycling tests [123]

Fig. 8

Copyright 2018. Reprinted with permission from Springer Nature

Proposed photocatalytic mechanism for the reduction of Cr(VI) by porous LST microspheres under visible light [123]

Oxide mineral calcium titanate (CaTiO₃) was discovered in 1839 in the Ural Mountains and named for Russian mineralogist L. A. Perovski [124]. CaTiO₃ has also attracted a great deal of interest in the world of oxide electronics, not only as a functional material in electronic and piezoelectric devices but also as a semiconducting material with a band gap of ~3.5 eV. However, due to its wide band gap (3.5 eV), CaTiO₃ was rarely used for the photocatalytic applications and its photoactivity is limited to UV excitation only. In the study by Yan et al., surface disorder-engineered CaTiO₃ were applied as the semiconducting materials for photocatalytic degradation of rhodamine B (RhB) under irradiation of simulated sunlight and UV light [125]. Graphitic carbon nitride (g–C₃N₄)–CaTiO₃ heterojunction photocatalysts were synthesized by Kumar et al. for the photocatalytic decomposition of rhodamine B (RhB) under UV, visible, and natural sunlight irradiation. The CaTiO₃ (CT) nanoflakes were homogeneously deposited onto the surface of g–C₃N₄ nanosheets by using a facile mixing method [126]. Yan et al. reported a facile hydrothermal approach to synthesize CaTiO₃ nanocuboids of controlled sizes that were used for degradation of rhodamine B under simulated sunlight irradiation. The photoactivity results demonstrated that CaTiO₃ nanocuboids exhibit superior photocatalytic activity when compared with CaTiO₃ nanoparticles [127]. Han et al. prepared an orthorhombic CaTiO₃ by solid-state, sol–gel, and hydrothermal methods and studied their photocatalytic performance for the degradation of methylene blue (MB) in an aqueous solution under the irradiation of visible light [128]. The photocatalytic activity of Ag–La co-doped CaTiO₃ powder for hydrogen evolution under UV and visible light was studied by Zhang et al. The photocatalyst, Ag–La co-doped CaTiO₃, showed higher H₂ production activity than that of pure CaTiO₃ powder [129]. Im et al. synthesized CaTiO₃@basalt fiber (BF) materials through a facile dipping method. This catalyst achieved high photocatalytic activity in CO₂ reduction (CH₄ = 17.8 μmol/g, CO = 73.1 μmol/g) [130]. Similarly, Yoshida et al. studied the composite catalysts of CaTiO₃@basalt fiber (BF) materials, consisting of CaTiO₃ with various Ca/Ti ratios, for CO₂ photochemical reduction. CaTiO₃ (1.5:1)@BF, had superior CO₂ adsorption and also the best CO₂ reduction performance (CH₄ = 17.8 μmol/g and CO = 73.1 μmol/g) [131].

The photocatalytic applications of Barium titanate, BaTiO₃, are limited due to its wide band gap energy in the range of 3.7–3.8 eV. Selvarajan et al. synthesized the novel BaTiO₃–SnO₂ nanocomposites by hydrothermal and precipitation deposition method. O-Chloroaniline and methylene blue were used as the model pollutants to evaluate the photocatalytic activity of the as-synthesized samples under UV–Vis light irradiation [132]. Nageri et al. reported the successful synthesis of Mn-doped barium titanate nanotube arrays by hydrothermal method, and the photocatalytic activity of these catalysts was evaluated by studying the degradation of methylene blue (MB) dye under visible light source [133]. Thamima et al. observed the photocatalytic performance of the BaTiO₃ catalysts using various dyes such as Methylene Blue (MB), Malachite Green, and Alizarin Red S under UV light irradiation [134]. Maeda et al. synthesized rhodium-doped BaTiO₃ catalysts with five different Rh concentrations (1–5 mol%) by using a polymerized complex (PC) method to sensitize BaTiO₃ into visible light. The synthesized catalysts studied for hydrogen production by photocatalytic water splitting under visible light in the presence of methanol as an electron donor [135].

Tantalate-based semiconductor materials, such as alkali tantalates, which have unique crystal and energy band structures, are widely used in photocatalysis. Further, they possess both valence band and conduction band (CB consisting with Ta 5d orbitals at a high potential) potentials suitable for photocatalytic applications. Due to their high photosensitivity, nontoxicity, stability, and commercial availability, these catalysts scores highest among the large band gap semiconductors as a photocatalyst.

NaTaO₃ (band gap energy ~4.07 eV) has attracted extensive attention in photocatalytic applications due to its good stability and high photocatalytic activity. Moreover, NaTaO₃ is considered as a good host material for developing visible light photocatalysts, especially when doped with rare earth elements like La [136]. Wang et al. reported that W and N co-doped NaTaO₃ showed higher photocatalytic activity toward degradation of rhodamine B (RhB) under visible light irradiation than single-element-doped NaTaO₃ [137]. Lan et al. reported the photocatalytic activity of the La/Bi co-doped NaTaO₃ and pure NaTaO₃ materials for the degradation of methylene blue under visible light irradiation. The La and Bi co-doping reduces the particle size, thereby extends the absorption edge toward longer wavelength [138]. A novel sulfur-doped NaTaO₃ photocatalyst was successfully synthesized by Li et al. through a simple hydrothermal process, and its photocatalytic activity was evaluated by methyl orange (MO) and phenol degradation processes [139]. The interest in NaTaO₃ in water splitting reaction recently increased after the discovery of its photocatalytic properties as water splitting [140]. NaTaO₃-based photocatalysts doped with different rare earth metal ions such as Y, La, Ce, and Yb with a constant ratio of rare earth metal ions have been synthesized by Jana and co-workers from the solid-state method and tested their photoactivity toward H₂ production by water/methanol mixture (methanol as an electron donor) under solar light. The higher H₂ production has been obtained with the Y-doped NaTaO₃ catalyst. Further, the deposition of Pt nanoparticles as cocatalyst, over these catalysts, improves even further the water splitting activity [141]. The photocatalytic activity of La-doped and pure NaTaO₃ in the H₂ evolution under UV light was studied by Lopez-Juarez et al. The higher photocatalytic activity was observed in the 2 mol% La-doped NaTaO₃ catalysts [142].

As a functional material, potassium tantalate (KTaO₃) has good photocatalytic activity and suitable band gap energy (3.2–3.8 eV). Liu et al. found that the substitution of La for K in KTaO₃ (K_1−xLa_xO₃) can significantly improve the activity for methyl orange (MO). They demonstrated that the incorporation of La³⁺ ions in KTaO₃ host lattice led to a monotonous lattice contraction and increased the BET surface area [143]. Krukowska et al. investigated the photocatalytic activity of mono- and bimetallic noble metal nanoparticles (MNPs/BNPs) decorated on the surface of KTaO₃ using by hydrothermal reaction of KTaO₃ followed by photo-deposition of MNPs/BNPs. The photocatalytic activity was estimated considering three model reactions by measuring the decomposition efficiency of phenol solution, the removal efficiency of gaseous toluene, and amount of evolved H₂ production in the presence of formic acid solution under the presence of Vis or UV–Vis light source [144]. Reduced graphene oxide-KTaO₃ (rGO–KTaO₃) hybrid composites with various graphene contents have been prepared by Bajorowicz et al. via the hydrothermal method. The photocatalytic activity of the composite was studied by the degradation of phenol under visible light irradiation. The improved photocatalytic activity can be attributed to the photosensitizer role of graphene in the rGO–KTaO₃ composites as well as the formation of p–n heterojunctions between p-type rGO and n-type KTaO₃ [145].

For the first time, Chen et al. prepared carbon-doped KTaO₃ via a simple combination of hydrothermal and calcination processes with glucose as the carbon source and tested the obtained powders in a photocatalytic H₂ generation under simulated sunlight illumination. The excellent photocatalytic hydrogen production should be mostly ascribed to the enhanced separation efficiency of charge carriers. The H₂ production rate was higher for C-doped KTaO₃ (592 μmol/h) than pure KTaO₃ (228 μmol/h) due to the enhanced separation efficiency of charge carriers [146]. It is generally accepted that methanol is an important industrial feedstock and is derived from the photocatalytic reduction of CO₂ using tantalum-based perovskites. Xiang et al. prepared CuO-patched cubic NaTaO₃ (CNTO) photocatalysts with different Cu contents and carried out the photocatalytic reduction of CO₂ to methanol in isopropanol in a self-made batch reactor. The morphologies of pure NaTaO₃ nanocubes and nano-CuO-patched NaTaO₃ (CNTOs) were observed by SEM and TEM as shown in Figs. 9 and 10. 5 wt% CNTO was found to have the best activity (CH₃OH yield of 1302.22 μmol g/1 h) and product selectivity in converting CO₂ to CH₃OH [147]. Also, authors have proposed the mechanism for photocatalytic reduction of CO2 to methanol on CuO–NaTaO3 photocatalyst (Fig. 11).

Fig. 9

Copyright 2018. Reprinted with permission from Elsevier

SEM images: a, b pure NaTaO₃, c 2 wt% CNTO, d 5 wt% CNTO, e 7 wt% CNTO, f 10 wt% CNTO [147]

Fig. 10

Copyright 2018. Reprinted with permission from Elsevier

a TEM image and b HRTEM image of 5 wt% CNTO [147]

Fig. 11

Copyright 2018. Reprinted with permission from Elsevier

The proposed mechanism for photocatalytic reduction of CO₂ to methanol on CuO–NaTaO₃ photocatalyst [147]

Nakanishi et al. studied the photoactivity of a set of Mg-, Ca-, Sr-, Ba-, and La-doped NaTaO₃ powders samples prepared by a solid-state method, while Ni, Pd, Rh, Cu, Au, and Ru cocatalysts were loaded in situ on doped photocatalysts by photo-deposition method, in photocatalytic CO₂ reduction to form CO using water as an electron donor. The best photocatalytic CO evolution obtained with Ag-loaded doped-NaTaO₃ photocatalysts [148].

Shao et al. synthesized highly efficient potassium tantalate (KTaO₃) materials, from a single-step hydrothermal method and investigated their activities in the reduction of carbon dioxide to methanol in isopropanol under UV light irradiation. Further, they found that the photoactivity of KTaO₃ increased with the loading of different NiOx contents and 2 wt% of NiO/KTaO₃ sample exhibited highest methanol formation [149]. Li et al. synthesized KTaO₃ (KTO)-based samples by solid-state reaction (SSR-KTO) and solvothermal methods in hexane–water mixture (Hex-KTO) and ethanol (Eth-KTO) to reduce CO₂ with water under UV light irradiation. KTO prepared in hexane–water mixture sample with a nanoflake-like structure was found to have the best activity (20 times (H₂) and 7 times (CO)) and product selectivity in converting CO₂ to H₂/CO in comparison to other samples [150]. Fresno et al. compared the photocatalytic activity of NaNbO₃ and NaTaO₃ catalysts in the reduction of CO₂ under UV irradiation and found that the NaTaO₃ exhibits better photocatalytic properties regarding CO/H₂/CH₃OH/CH₄ (CO was the major product) product selectivity [151].

The niobium-based perovskite materials have been regarded as a potential candidates for a variety of photocatalytic applications compared to that of TiO₂ because the conduction band in niobates perovskite materials consists of Nb 4d which are situated at more negative potential (compared to TiO₂), thereby ensuring the separation and transfer of photoinduced charge carriers very precisely in their photocatalytic applications. NaNbO₃, possibly the most studied of the alkali niobates, is a well-known transparent semiconductor (with a band gap of 3.7 eV) with extremely technological importance. The photocatalytic activity of alkali niobates under UV radiation strongly depends on the nature of the “A” cation. If the A-site in alkali niobates is replaced by Ag ion, it extends the light absorption to visible region and improves the photocatalytic efficiency of alkali niobates. AgNbO₃ has a band gap of 2.7 eV, and thus it absorbs visible light. Wu et al. synthesized microcrystalline AgNbO₃ photocatalysts as a visible light photocatalyst for the photocatalytic decomposition of cationic dyes such as rhodamine B and methyl blue [152].

Sun et al. synthesized BiOI/NaNbO₃ p–n junction photocatalyst at low temperature by using a facile chemical bath method. The photocatalytic activity of the BiOI/NaNbO₃ composites was investigated for the degradation of MB under visible light irradiation [153]. NaNbO₃ with cubic crystal structure photocatalyst was synthesized by Chen et al. via a convenient solvothermal method and then Ru nanoparticles were added to it by photo-deposition technique. The photocatalytic activity of Ru/NaNbO₃ composite was exhibited outstanding performance for the degradation of RhB under visible light irradiation than that of naked and Ru-doped NaNbO₃ [154]. For the photodegradation of methylene blue (MB), Wang et al. fabricated cubic NaNbO₃ (c-NaNbO₃) photocatalyst at 350 °C by a surface ligand-assisted localized crystallization (SLALC) method. They found that the as-synthesized c-NaNbO₃ nanoparticles could adsorb 95% of methylene blue in 3 min and photodegrades 99.3% of methylene blue (MB) in 180 min [155]. Liu et al. investigated and compared wire- and cube-like NaNbO₃ perovskite catalysts with Pt loading for photocatalytic H₂ production and organic-pollutant degradation. It was found that after the incorporation of Pt NPs, the Pt/NaNbO₃ systems exhibited noticeably higher performance due to the better light absorption and the improved separation capability of photogenerated electron–hole pairs [156]. Liu et al. synthesized single-crystalline NaNbO₃ with wire- and cube-like morphology by a facile and eco-friendly route including a hydrothermal as well as in-site self-assembled process via optimizing thermal treatment temperature. Photocatalytic activity of as-synthesized samples was measured in the photocatalytic water splitting of H₂ under solar light irradiation. They have found that the photocatalytic activity of NaNbO₃ with wire morphology showed the highest H₂ evolution rate (330.3 μmol/g) among all the studied samples. The enhanced photoactivity of NaNbO₃ was mainly due to the good crystallinity, fewer defects, and perfect 1D nanowire morphology [157]. The research group led by Li et al. used a polymerized complex method to prepare three NaNbO₃ samples that structurally differed from each other: cubic and orthorhombic and mixed cubic–orthorhombic. The sample that consisted of mixed-phase NaNbO₃ showed the highest activity in CO₂ reduction to CH₄. The authors explained the enhanced catalytic activity was attributed to the cubic–orthorhombic surface junctions which improved the charge separation [158]. The design of composite photocatalysts, in particular with graphitic carbon (g–C₃N₄), has recently become most popular in research into the photocatalytic properties of niobate-based semiconductors. Shi et al. used this route to modify the photocatalytic properties of NaNbO₃/KNbO₃ semiconductors. They prepared g–C₃N₄/NaNbO₃ nanowire heterojunction photocatalyst via introducing polymeric g–C₃N₄ on NaNbO₃ nanowires (prepared by hydrothermal method) tested their photoactivity in reduction of CO₂ to CH₄ under visible light irradiation. They found that the g–C₃N₄/NaNbO₃ heterojunction exhibits the best photocatalytic performance than that of individual g–C₃N₄ and NaNbO₃ [159]. Similarly, Shi and his co-workers reported the photocatalytic reduction of CO₂ to CH₄ under visible light illumination, using g–C₃N₄/KNbO₃ composites as catalysts. The g–C₃N₄/KNbO₃ composite had more photocatalytic activity for CO₂ reduction to CH₄ as compared to pure g–C₃N₄ and KNbO₃ under similar reaction conditions [160].

Potassium niobate, KNbO₃, has also been reported as a UV light sensitive photocatalyst because of its wide band gap energy (3.14 eV). Raja et al. prepared Bi-doped KNbO₃ powders by the solid-state reaction by varying the ratio of K/Bi in starting materials and tested their photocatalytic activity in the degradation of methyl orange (MO) aqueous solution under UV light. They found that the photocatalytic degradation of Bi-doped KNbO₃ was 2.25-fold higher than that of pure KNbO₃ and this can due to increased BET surface area, crystallite size, and band gap alteration of the K_1−xBi_xNbO₃ [161].

Zhang et al. reported the photocatalytic H₂ evolution from aqueous methanol with a micro-cubic structured potassium niobate (KNbO₃) semiconductor system under the illumination of visible light [162]. Nitrogen doping is an effective method for modifying light absorption behavior of wide band gap semiconductors. In view of this, N-doped NaNbO₃ nanocube structured powders were obtained by Wang et al. by applying a simple hydrothermal method. The photocatalytic activity of the N-doped KNbO₃ and pure KNbO₃ has been evaluated by photocatalytic water splitting as well as degradation of four organic contaminants (rhodamine B, orange G, bisphenol A, and pentachlorophenol) under visible light irradiation [163]. Hong et al. established a visible-light-driven photocatalytic system comprised of potassium niobate microspheres and reduced graphene oxide (KNbO₃/RGO) for hydrogen evolution. The hybrid nanocomposite (KNbO₃/RGO) exhibited the highest H₂ evolution rate compared to pure potassium niobate microspheres. This enhanced photocatalytic efficiency was attributed mainly to the proficient separation of photogenerated carriers at the heterojunction of two dissimilar semiconductors [164]. We have provided a Table 1 containing the photocatalytic activity results of recently published literatures (Years: 2015–2018) on ABO₃-type photocatalysts.

Table 1 Photocatalytic activity results of recently published (Years: 2015–2018) ABO₃ type photocatalysts

5 Conclusions and Insights

Perovskite oxides with the composition ABO₃ offer ample scope in designing novel compounds by the partial substitution of metal cations in A- and B-sites, which allows the preparation of isostructural series with different physical/chemical properties. The arrangement and position of the metal ions should be taken into consideration to understand the physical properties of perovskites. From the photocatalytic point of view, these perovskite materials have a special structural feature that is beneficial to photocatalytic activity. The relationship between crystal structure and energy delocalization shows that the closer to 180° the B–O–B bond angle is, the easier the excitation energy is delocalized leading to higher photoactivity. Thus, the bond angle between metal–oxygen–metal (B–O–B) in perovskite oxide is a significant structural parameter and must be taken into account. The modification of the electronic structure of a perovskite by dopants into its lattice is an effective way to affect its photocatalytic properties. However, more efforts needed to clarify the relationship between electronic structure and photocatalytic activity to develop novel perovskites with efficient photocatalytic activity. Other than band gap energy, crystallite size, and surface area, the vacancy of a metal cation and O²⁻ anions play important roles in the photocatalytic performance of perovskites oxides. The selection of preparative method also plays a significant role in the photocatalytic performance. A traditional method like high-temperature solid-state reaction results in the destruction of pore structures, and hence too low surface areas. Hence, controlled preparation of porous perovskite materials with a high surface area is highly desirable. The strategies as discussed are widely used to enhance the photocatalytic performance of perovskites. But the literature available on the adverse effect of such methods is limited. This should be considered appropriately in future.