Introduction

The steadily increasing waste generation levels and the need to combat them using renewable energy sources have enabled the search for materials that can be produced easily and have no side effects harming the environment [1,2,3]. As solar energy is one of the abundantly available renewable energy sources, it makes sense to utilize it to counter waste degradation by synthesizing materials that can utilize directly [4, 5]. As photocatalysis is one of the most widely researched methods for eliminating effluents from factories and the agricultural sector for their various advantages (Fig. 1), semiconductor metal oxides have always been the materials of choice for helping to generate non-toxic by-products by degrading wastes [1, 6,7,8]. But semiconductors have drawbacks: fast electron–hole recombination rate, high bandgap energy, and lower adsorption rates deter the catalytic progress [9, 10]. As one of the most crucial factors of photocatalysis is charge separation, knowledge of the crystal structure and constituent elements of the material is particularly important [11].

Fig. 1
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Properties that make a material suitable for photocatalysis

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Photocatalysis for waste degradation is a promising technology that utilizes the photo-induced redox reaction of a photocatalyst material to convert harmful pollutants into less toxic or non-toxic substances. When a photocatalyst is irradiated with light, it generates electron–hole pairs, which act as powerful oxidizing or reducing agents. These electron–hole pairs can then react with water or oxygen molecules to produce highly reactive oxygen species (ROS), such as hydroxyl radicals (•OH), superoxide radicals (•O2−), and hydrogen peroxide (H2O2) [12]. These ROS can attack and break down organic and inorganic pollutants in waste materials, converting them into simpler and less harmful compounds. One of the key advantages of photocatalysis is that it is a green and sustainable approach that does not require the use of harmful chemicals or the production of harmful by-products [13].

Additionally, photocatalysis can be used to degrade a wide range of waste materials, including organic pollutants, heavy metals, and dyes. However, one of the main challenges of photocatalysis is the selection of an appropriate photocatalyst material that is efficient and stable under the conditions required for waste degradation [14]. Another challenge is the design of the photocatalytic reactor to ensure efficient use of light energy and proper mixing of the waste material with the photocatalyst [15]. Nevertheless, photocatalysis shows great potential as an effective and sustainable solution for waste degradation.

Conventional perovskite structures with the chemical formula of ABO3 have been known to contain lead (Pb), and recent research is all focussed on replacing Pb with non-toxic materials for fuel cells, photovoltaics, memory devices, and photocatalysts [3, 16]. As the Pb-based perovskites have great power conversion efficiency, non-Pb-based perovskites must be studied to combat the instability and large energy band gap [17]. Lead-free perovskites have been synthesized by various methods, such as sol–gel, solid-state synthesis, and hydrothermal methods, to increase the chances of tuning the optical and structural properties to cater to the respective needs of the society, where perovskite oxides and halides are deemed to be effective [18,19,20]. Double perovskites are known for their flexible structural, optical, and morphological properties due to their lattice framework having a general form AAʹBBʹO6. They are more useful for hydrogen evolution due to their higher conduction band potential than simple perovskites. The existence of double metal provides structural variation, which is very promising for optical applications [21]. This mini-review introduces state-of-the-art studies that utilize lead-free double perovskites for photodegradation using visible light sources.

Double perovskites for visible light photocatalysis

Double perovskites enlarge the possible elements that can be utilized in the perovskite family and have appeared as promising proxies with elevated photocatalytic performances, as they can be more flexible regarding their bandgaps and edges [22,23,24]. Nair et al. have provided a comprehensive review of the recent research on synthesizing double perovskites and their applications [17]. There have been experimental results of lead-free perovskites showing better photocatalytic results owing to their non-toxicity. Yin et al. have provided a comprehensive review of oxide perovskites, showing improved performance in photocatalytic activities. They have summed up the typical characteristics of double perovskite oxides having higher light absorption and favorable tolerance to structural defects to be the factors that help in upgrading their photocatalytic behavior [25]. Ma et al. have commented on the adaptability of double perovskite oxides regarding their A or B site ions which makes regulation of desired properties less challenging, like the tuning of bandgap and varying their optoelectronic nature [26]. Jiang and co-workers have computed and identified stable double perovskite oxides, which can surpass existing perovskite oxides in their optoelectronic and photocatalysis significance [27]. Halder et al. have discussed how direct-to-indirect transition in bandgap structures influences the efficiency of catalytic behavior of perovskites by doping Ru in Ba2HoSbO6 [28].

Double perovskite nanoparticles have a high surface area-to-volume ratio, which makes them suitable for device-level fabrication in photocatalysis. They can be incorporated into thin films, coatings, or membranes to create high surface area photocatalytic surfaces, which can be used in waste degradation, water treatment, or air purification. Deposition techniques such as spin-coating, spray deposition, or inkjet printing can fabricate thin films or coatings [29, 30]. Incorporation into porous membranes or filters can create high surface area photocatalytic surfaces for water or air treatment [31]. Device-level fabrication of double perovskite nanoparticles is still in the early stages, and much work is needed to optimize the fabrication processes and understand their underlying mechanisms. Nonetheless, this research area has great potential for developing practical and efficient photocatalytic systems for environmental remediation and renewable energy conversion.

K2Ta2O6

Usually, most tantalate oxides prepared by the solid-state method require high temperatures, with their secondary oxides being undesirable [32]. K2Ta2O6, a pyrochlore-lattice structure, is known to be the secondary phase that forms at lower temperatures when synthesizing KTaO3, which is not desirable due to its low dielectric properties, high ferroelectric properties [33], and defect structure. It has a lower electron–phonon coupling effect, which shows enhanced charge separation, leading to better photocatalyst behavior than KTaO3 [34]. K2Ta2O6 is usually easily synthesized by hydrothermal or the sol–gel method due to the ease of not requiring elevated temperature [35,36,37,38]. Because most tantalum oxides have band gaps unsuitable for the absorbance of visible light for any optical applications, surface modification or doping K2Ta2O6 with elements like noble and rare metals have been adopted to reduce the band gap energy [39]. Such reduction in bandgap energy has helped in charge separation and generation of active species of hydroxyl and superoxide radicals, boosting the catalytic behavior. Krukowska et al. have doped Y, Yb, Ho, Pr, E rare earth metals, Au, Ag, Pt, Pd, Rh, Ru as monovalent metals and Au/Pt, Ag/Pd, Rh/R as bimetallic nanoparticles in their works, and have examined the response of K2Ta2O6 [40].

Angineni et al. have experimented with doping three transition metal ions, M2+ (M = Mn, Co, and Ni), by ion exchange method after synthesizing K2Ta2O6 by facile hydrothermal method [35]. They have observed tailing in the material despite confirming the absence of impurities [41], attributing them to lattice defects and varying metal oxidation states. All the prepared samples have cubic crystal structure agglomerated spherical shapes, ranging from 36.64 to 38.25 nm. On photodegradation of methylene blue dye with all the catalysts, 53% of the dye was degraded using the undoped K2Ta2O6, while Mn-doped K2Ta2O6 degraded 93%, Co-doped K2Ta2O6 degraded 86% and Ni-doped K2Ta2O6 69%. They have also verified that hydroxyl radicals are the reactive oxygen species by observing the addition of IPA scavenger (Fig. 2: a1–a6).

Fig. 2
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(Reprinted from [36] with permission from Elsevier. Copyright (2022) Elsevier)

a1 Schematic structure of KTO lattice, a2 Digital photographs of all catalysts, a3 Degradation curves and a4 The first-order reaction kinetics plot for degradation of MB under visible light irradiation, a5 Effects of different scavengers on the photocatalytic degradation of MB over Mn-KTO. a6 Fluorescence spectra of all catalyst suspensions in the TA solution for 120 min of irradiation time (λex = 320 nm). Reprinted from [35] with permission from Elsevier. Copyright (2022) Elsevier. a7 TEM images of KTO and Gd0075-KTO (insets show the corresponding SAED patterns). a8 The changes of temporal UV–Vis absorption spectra during the degradation of MB aqueous solution over KTO and a9 Gd0075-KTO, a10 Photocatalytic degradation profiles of MB as a function of the irradiation time under visible light over all catalysts and a11 pseudo-first-order kinetics for degradation of MB under visible light irradiation for 180 min

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Angineni et al. doped Gd3+ ions in K2Ta2O6 to comprehend how doping allows visible light photocatalytic performance [36], using methylene blue dye as the pollutant. They have shown that all doped samples have lower energy band levels than parent K2Ta2O6, with 0.075 mol% Gd3+ doped K2Ta2O6 having 4.68 eV being the least, attributed to the formation of new band energy levels. Irregular-shaped agglomerated particles having cubic morphology with a size range of 37.34 nm to 37.68 nm were synthesized. 0.075 mol% Gd3+ doped K2Ta2O6 had the least crystallite size of 35.92 nm and showed the best photodegradation, i.e., 79% MB degrading efficiency for 3 h of irradiation and maximum adsorption of 12% after 1 h (Fig. 2: a8–a9). Using scavengers to understand the nature of reactive species, it is reassuring that for K2Ta2O6, hydroxyl radicals are the dominant species contributing to the degradation process. The prepared samples also retained their stability after 5 cycles of repeated experiments, emphasizing the degradation ability of the Gd3+ doped K2Ta2O6 nanoparticles. The structural advantages of K2Ta2O6 as photocatalytic materials over other double perovskites are ongoing, as limited work is presently done.

Ba2TiMoO6

Ba2TiMoO6 double perovskite has been reported to exhibit ferroelectric hysteretic behavior at room temperature [42]. It behaves as a p-type semiconductor but manifests a decrease in the current flow when Ce3+ ions are doped in the host lattice [43] when synthesized by the solid-state reaction method. Ghrib et al. have reported well-synthesized Ce3+ doped Ba2TiMoO6 by solid-state reaction method and annealed twice to obtain homogeneous powder samples [44]. They have obtained spherical-shaped irregular grains with a size ranging from 8–22 nm, having energy levels from 3.41–3.6 eV. The maximum degradation shown by 7% Ce3+ doped Ba2TiMoO6 is almost 16% for industrial pollutant methylene blue degradation in the presence of hydrogen peroxide (H2O2). The significance of this prepared material is its ability to absorb energy in the visible electromagnetic spectrum and show evident results in 120 min. Ghrib et al. have emphasized the importance of the Ce 4f level due to doped ions, with hydroxyl radicals being the reactive species enforcing degradation (Fig. 3: a1–a6).

Fig. 3
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(Reprinted from [48] with permission from Elsevier. Copyright (2022) Elsevier)

a1 Unit cell diagram of Ce-doped Ba2TiMoO6 compound. a2 Degradation rate of the prepared samples versus time with different Ce doping percentages. a3–a6 Degradation spectra of MB with Ba2TiMoO6 catalysts for different Ce concentrations. Reprinted from [44] with permission from Elsevier. Copyright (2022) Elsevier. a7 HRTEM images of SFNO and CABB/SFNO heterostructure, respectively. Photocatalytic activity test of a8 SFNO, CABB, and CABB/SFNO heterojunction. a9 CABB/SFNO with different mass ratios. a10 Photocatalytic time courses of CO and CH4 over CABB/SFNO (1:2). a11 Recycling photocatalytic stability test of CABB/SFNO heterojunction

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Sr2FeNbO6

Sr2FeNbO6, known for its applications in cell electrode materials for solid-oxide fuel cells [45,46,47], is also efficient as a visible light n-type semiconductor photocatalyst with a bandgap of 2.04 eV [48]. This double perovskite has been synthesized by Jeong et al., with a band gap of 2.06 eV by solid-state method, and it has been proven to photo reduce and oxidize water to evolve hydrogen and oxygen, indicating its use as a visible light application material for degradation of wastes [49]. Idris et al. have successfully synthesized a heterostructure of lead-free double-perovskites Cs2AgBiBr6/Sr2FeNbO6 for photocatalytic CO2 reduction [48]. It has been observed that the lead-free double perovskites have band gaps of 2.04 eV for Sr2FeNbO6 and 2.17 eV for Cs2AgBiBr6. The heterostructure has shown photocatalytic behavior in the reaction cell saturated with pure CO2 in visible light in an ethyl acetate solution. They have prepared the heterostructure by an electrostatic self-assembly method, which has resulted in the formation of pure cubic phases for both Sr2FeNbO6 and Cs2AgBiBr6 with irregular aggregated polyhedral nanoparticles in the size range of 20–50 nm. When one compares the performance of individual photocatalysts with the heterostructure, it has been observed that the reduction rates are drastically higher for the heterostructure, implying possible application in pollutant removal. The stability of the heterostructure has also been examined over the time of 5 h for four cycles and confirmed with XRD and SEM mapping (Fig. 3: a7–a11).

La2FeTiO6

La2FeTiO6 synthesized by the sol–gel method has been reported to have still dielectric properties at room temperature to 200 °C [50]. It is a ternary metal oxide, which can be considered a possible replacement for TiO2 photocatalysts. It is placed in the same category as other double perovskites where the B-site cation is doped, resulting in the change in band gaps exhibiting varying properties. Hu et al. [51] have synthesized LaFeO3 perovskite and corresponding double perovskite La2FeTiO6 by sol–gel method to degrade p-chlorophenol in visible light. They found that the irregular polyhedron La2FeTiO6 nanostructures with a diameter of around 70 nm and crystallite size of 26.3 nm can degrade the organic effluent better than LaFeO3. La2FeTiO6 could degrade > 62% compared to LaFeO3’s degradation of 49% in visible light for 5 h. They were the first to report the superiority of La2FeTiO6 over LaFeO3 for photodegradation.

Nunes et al. have experimented with varying the amount of Ti in LaTi1−xFexO3 (where 0 ≥ x ≤ 1) and observed the changes in degrading Acid Orange 7 under ultraviolet and visible light [52]. They obtained agglomerated nanoparticles with non-homogeneous distribution and shapes. It is also observed with the increasing value of Ti and decreasing value of Fe in the prepared double perovskite, the energy band gap has steadily decreased from 3.97 eV for x = 0 to 2.2 eV for x = 1 (Fig. 4: a1), with increasing photocatalytic activity (3% in 1 h) in the presence of visible light, and high photodegradation of 47% achieved by the La2Ti2O7 particles, in 1 h under UV light (Fig. 4: a2).

Fig. 4
figure 4

(Reprinted from [57] with permission from Elsevier. Copyright (2022) Elsevier)

a1 Table Eg values and color of the LaTi1−xFexO3 oxides. a2 Absorbance removal of the AO7 solutions (Ci = 10 mg L−1) obtained with the photocatalyst powder suspensions (C = 0.5 g L−1) under UV and visible light after 1 h. Reprinted from [52] with permission from Elsevier. Copyright (2022) Elsevier. a3 Absorbance Spectra of AR-85 as a function of time for 7.5BFCO photocatalyst. a4 Percentage of degradation of Acid Red-85 dye using BiFe1−xCoxO3 (0 ≤ x ≤ 0.10) as photocatalysts. a5 Pseudo first order kinetics fitting for BiFe1−xCoxO3 (0 ≤ x ≤ 0.10). a6 Recyclability of 7.5BFCO for AR-85 degradation for three runs. a7 Schematic diagram of the possible mechanism involved for electronic energy band in BFCO samples

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BiFe1−xCoxO3

BiFeO3 is a known multiferroic material used for memory devices, piezoelectric sensors, photodetectors, and magnetoelectric devices [53, 54]. But it is known to have a low energy band gap, which makes it suitable for visible light applications [55]. In an attempt to tweak the band gap for effortless application in photocatalytic activities, cobalt has been doped in the host perovskite to observe the structural and optical changes [56]. Prasannakumara et al. have synthesized Co-doped BiFeO3 by sol–gel method and observed that with an increase in the concentration of Co ion doping, the energy band gap has steadily increased from 1.74 eV for an undoped thin film to 1.91 eV for BiFe1−xCoxO3 where x = 0.08 is the maximum doping.

Ponraj et al. have prepared BiFe1−xCoxO3 (x = 0, 0.025, 0.05, 0.075 and 0.10) and observed their photocatalytic ability by degrading Acid Red-85 dye. They have observed a decrease in crystallite size from 57 to 32.29 nm with increasing Co ion concentration with rhombohedral structure [57], attributing it to the size difference between Co3+ ions compared to Fe3+ ions. They reported that the degradation efficiency of BiFe1−xCoxO3 (x = 0.075) was the highest at 93.79% in 2 h, even higher than 1 mol% coped sample due to presence of impurities (Fig. 4: a3–a7). The prepared samples have also shown reusability for 3 cycles, with the reaction following the pseudo-first order kinetics. They have credited the improvement in photodegradation ability of the double perovskite to the increased oxygen vacancies and reduced crystallite sizes.

Conclusion

The future advancement of double perovskites for visible light photocatalysis looks very promising. Several research studies have shown that double perovskites have a wide range of potential applications in solar energy conversion, including photocatalytic degradation of pollutants, water splitting, and CO2 reduction. The unique structural and electronic properties of double perovskites make them ideal for use as photocatalysts, as they can absorb a broad range of visible and ultraviolet light and exhibit high stability and activity. In the coming years, researchers are expected to continue exploring the potential of double perovskites for visible light photocatalysis, focusing on improving their efficiency, stability, and selectivity.

One area of research that is likely to receive increased attention is the development of new doping materials to enhance the photocatalytic activity of double perovskites further. Moreover, with advancements in materials science, new synthesis routes and fabrication methods would be developed to optimize the properties of double perovskite nanomaterials, leading to their practical application in photocatalytic degradation. The synthesis of double perovskites is challenging and requires precise control over stoichiometry, temperature, and reaction time. The complex crystal structure of double perovskites makes characterization difficult and requires advanced techniques. Stability under environmental conditions, such as humidity and temperature, can also affect photocatalytic activity and practical applications. Overall, the future of double perovskites in visible light photocatalysis is very bright, and it is expected that they will play a crucial role in the development of more efficient and sustainable technologies for waste degradation and other energy conversion applications.