Photocatalytic behavior of Ba(Sb/Ta) 2 O 6 perovskite for reduction of organic pollutants: Experimental and DFT correlation

We have synthesized closely packed hexagonal 2D plates and clustered nanoparticle morphologies of Ba(Sb/Ta) 2 O 6 (BSTO) perovskite via the polymerizable complex method for photocatalytic dye degradation activities. The BSTO crystallized in a hexagonal structure. The presence of Ba 2+ , Sb 5+ , Ta 5+ , and O 2 (cid:1) chemical states identi ﬁ ed from XPS con ﬁ rmed the formation of mixed Ba(Sb/Ta) 2 O 6 phase accompanied with a minor amount of TaO x . Furthermore, BSTO showed excellent photocatalytic activity for the degradation of various organic dyes. The kinetic studies revealed 65.9%, 77.3%, 89.8%, and 84.2%, of Crystal Violet (CV), Methylene Blue (MB), Rhodamine blue (RhB), and Methylene Orange (MO), respectively, after irradiation of 180 min without using a cocatalyst. The formation of O (cid:1) 2 and OH (cid:1) surface radicals, which are believed to facilitate the degradation of the dyes, are unveiled through ﬁ rst-principles Density Functional Theory (DFT) calculations and scavenging studies. Our results suggest that BSTO holds promise as an excellent photocatalyst with better degradation ef ﬁ ciency for various organic dyes. © 2021 The Authors. Published by Elsevier B.V. on behalf of Taiwan Institute of Chemical Engineers. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)


Introduction
Access to clean and safe drinking water is essential for human health and wellbeing, and lies at the heart of Sustainable Development Goal 6: 'i.e., ensure availability and sustainable management of water and sanitation for all.'However, human activities and pharmaceutical and textile industrial processes have introduced dangerous contaminants in water systems, raising serious health concerns.Therefore, there is an urgent need to develop efficient water treatment technologies to provide potable water for society.Various physical, chemical, and biological treatments for water purification have been explored with limited efficiencies [1,2].However, recent advancements in photocatalysis technologies have enabled the development of highly effective materials capable of degradation of various pharmaceutical compounds and the development of visible-light photocatalysts [3].Photocatalytic water treatment has many advantages over other water purification methods as it is a highly eco-friendly, low-cost, reusable, and highly efficient method.Photocatalysis has also served the needs of energy consumption and environmental pollutant remediation through various processes such as water splitting, degradation of organic pollutants, and reduction of heavy metals and CO 2 gas.
Advanced oxidation processes activate the photocatalytic material by light energy and generate electron-hole pairs responsible for generating surface hydroxyl and other radicals, which further react with organic pollutants such as dyes, herbicides, and pesticides to degrade them through the oxidation process [4].TiO 2 is the most investigated photocatalytic material, but the low adsorption of hydrophobic pollutants, high aggregation nature, complications with separation and recovery, and photocatalytic activity in only the UV region [5,6] necessitates the rational design and development of other metal oxides to gain better photocatalytic activities for dye degradation.Therefore, to overcome the limitations of TiO 2 , previous investigations have partially modified and/or replaced TiO 2 with other semiconducting oxides such as WO 3 , ZnO, SnO 2 , Fe 2 O 3, etc., [7] polymeric compounds like Polyaniline (PANI) and Polyvinylidenefluoride (PVDF), [8,9] and carbonaceous materials such as graphene, CNT, and g-C 3 N 4 , etc. [10].
In recent years, the development of new mixed metal oxides for photocatalytic degradation of various organic dyes is of prime consideration.The unique structural and electrical properties of various tantalates, niobates, antimonates, and titanates are expected to provide better stability, hence attracting attention to a large extent for the photocatalytic activity for water splitting and degradation of dyes.The corner shared octahedral arrangement of TaO 6 exhibiting a bond angle of 180°in the Ta-O-Ta structure should facilitate e À -h + pair separation and decrease their recombination rate, resulting in enhanced photocatalytic carrier generation.Therefore, the tantalates such as NaTaO 3 , KTaO On the other hand, the distorted Sb-O octahedral structure in BaSb 2 O 6 is shown to be active towards photocatalysis due to its lattice distortion in magnitude and direction [16].The antimonates such as ZnSb 2 O 6 , CdSb 2 O 6 , and BaSb 2 O 6 [17,18] are explored for the photocatalytic degradation of RhB.Recently, Chen et al. [16] reported the effect of the active surface area of BaSb 2 O 6 on the photocatalytic degradation of RhB, but H 2 O 2 was found to influence the degradation rate mostly.Moreover, the hydrogen-related defects in the hydrothermally synthesized BaTa 2 O 6 adversely affected the photocatalytic degradation of RhB in an aqueous solution under UV radiation [14].ZnSb 2 O 6 delivered 95% degradation of RhB after photocatalytic irradiation of 150 min.However, doping of Tb 3+ (1.2%) and Eu 3+ (0.8%), considered for dual functionality, did not exhibit any significant improvement in the degradation ability of ZnSb 2 O 6 [17].The doping of Ir and La has been explored to enhance the photocatalytic activities, but studies are limited to H 2 evoluation and not to dye degradation activities [19].On the other hand, the double perovskites such as Ba 2 RBiO 6 (R = Ce, Sm, Eu, Gd, Dy) delivered the bandgap in the range of 1.60 to 1.71 eV and confirmed photocatalytic dye degradation activities under visible light irradiation.Nevertheless, all those double perovskites have shown sluggish photocatalytic degradation of MB dye (i.e., ~74% in 8 h) [20].To the best of our knowledge, no research group has explored the photocatalytic activity either after combining the BaTa 2 O 6 (BTO) and BaSb 2 O 6 (BSO) or partial replacement of Sb and Ta.The distinctive larger bandgap of both BTO (i.e., 3.85 to 4.2 eV) and BSO (3.8 to 4.7 eV) [12,18,21,22] has the potential to utilize a broader spectrum for photocatalytic activity after combining or partially replacing Sb and Ta.Therefore, in the present study, we have explored the photocatalytic activity of Ba(Ta/Sb) 2 O 6 in the degradation of aqueous Methylene Orange (MO), Methylene Blue (MB), Crystal Violet (CV), and Rhodamine Blue (RhB) without the utilization of any cocatalyst.
The Ba(Sb/Ta) 2 O 6 perovskite synthesized by the polymerizable complex method is investigated for the photocatalytic degradation of MO, MB, CV, and RhB dyes at room temperature.The structural properties were investigated from X-ray diffraction.The surface morphologies and chemical composition of the BSTO were explored using field emission scanning electron microscopy (FESEM) and x-ray photoemission spectroscopy (XPS), respectively.The effect of BSTO concentration on the photocatalytic degradation of MB was investigated systematically by irradiation under a xenon lamp (300 W) and continued further for MO, CV, and RhB.The BSTO perovskite delivered the best photocatalytic activity in the degradation of RhB without using any cocatalyst.Theoretical insights into the electronic structure of the BSTO material are provided, and the active sites for the formation of O À 2 and OH À surface radicals to facilitate the degradation of the dyes are predicted.

Synthesis and characterization
The synthesis of BSTO was carried out via polymerizable complex method using barium barbonate (BaCO 3 , 99%, Sigma Aldrich), tantalum chloride (TaCl 5 , 99.99%, Sigma Aldrich), and antimony chloride (SbCl 3 , 99.95% Sigma Aldrich) as starting materials.The anhydrous methanol was used as a solvent for dissolving precursors.The stoichiometric amount of the 0.1 M BaCO 3 , 0.05 M SbCl 3, and 0.05 M TaCl 5 were dissolved in 20 ml of anhydrous methanol (CH 3 OH, 99.8%, Sigma Aldrich) at room temperature, and 0.05 M citric acid (C 6 H 8 O 7 , 99.5%, Sigma Aldrich) was added afterward under constant starring.Furthermore, the 40 ml ethylene glycol (C 2 H 6 O 2 , 99.8%, Sigma Aldrich) was added to the solution as a polymerization agent and reacted at 130°C to obtain the viscous whitish gel.The viscous gel was dried initially at 350°C and annealed further at ~1200°C for 10 h to obtain Ba(Sb/Ta) 2 O 6 (i.e., BSTO) powder.The surface morphological features of BSTO are confirmed from field emission scanning electron microscopy (FESEM, JEOL JSM-7610 F Plus).X-ray diffraction (D2-Phaser Bruker, Cu-Ka, λ = 1.5406A ) was utilized to confirm the crystal structure and identify the phase of the BSTO.The chemical states and electronic properties are confirmed using X-ray photoelectron spectroscopy (XPS, Thermo Scientific Inc. Al-Ka) with microfocus monochromated Al Ka X-rays.UV-Visible diffuse reflectance spectra recorded using a UV-Visible spectrophotometer (Shimadzu, UV-2600) in the wavelength range of 200 to 800 nm to evaluate the bandgap.Also, the dye degradation study was carried out under the xenon lamp in a chemical reactor.
The photocatalytic activities of the BSTO were analyzed by the degradation of various dyes under the xenon Lamp (λ = 100 to 1800 nm) in Photo-Chem Reactor.The photocatalytic activity was tested on MB, RhB, MO, and CV dyes.Initial optimization on the degradation activities of BSTO was carried out with variation in the concentration of the MB (i.e., 10, 20, and 30 mg/L).The dye solution was maintained in the dark for 30 min.to obtain adsorption-desorption equilibrium and further irradiated under a xenon lamp.The irradiated solution was tested under UVÀVis light at regular intervals using a UVÀVis spectrophotometer to attain maximum degradation of the dye under observation.The absorption peak at λ max signifies the dye concentration remained in solution after irradiation of UV-Visible light.The best optimization concentration (i.e., 10 mg/L for MB) was subjected further for the degradation studies of RhB, MO, and CV dyes

DFT analysis
The electronic structure calculations were performed based on density functional theory (DFT) methodology, as implemented in the Vienna Ab initio Simulation Package (VASP) [23].The interactions between the core and valence electrons were treated using the Project Augmented Wave (PAW) method [24].The electronic wave functions are expanded on a plane-wave basis set with cut-off energy of 600 eV.Geometry optimizations were performed using the conjugate-gradient algorithm until the residual HellmannÀFeynman forces on all relaxed atoms reached 10 À3 eV/A .The electronic exchange-correlation potential was calculated using the Per-dewÀBurkeÀErnzerhof (PBE) generalized gradient approximation (GGA) functional [25].Long-range vdW interactions were accounted for using the Grimme DFT-D3 scheme [26].The bulk BSTO was modeled in the hexagonal structure (P6/mmm [191]).) [27].The screened hybrid functional HSE06 [28] with an exchange value of 30% was used to predict the electronic density of states (DOS) and the bandgap.The BSTO(100) surface was used to characterize the adsorption characteristics of O 2 and H 2 O molecules.A 5 £ 5 £ 1 k-point was used for the surface calculations.Bader charge analysis [29] was used to quantify charge transfers between the BSTO(100) surface and the adsorbing molecules.

Results and discussion
The surface morphological aspects of BSTO are confirmed from FESEM images shown in Fig. 1.Fig. 1(a) illustrates that the prepared material is composed of two different morphologies, i.e., two-dimensional (2D) hexagonal plates and clusters of nanoparticles in the form of spheres.The reason for the formation of two different morphologies is unknown, but one of the possible reasons might be the segregation of Sb and Ta to form BaSb 2 O 6 and BaTa 2 O 6 independent phases.The hexagonal plates are distributed uniformly in large quantities compared to clustered nanoparticles throughout the sample (highlighted in yellow).The hexagonal plates are of size ~0.5 to 2 mm.The high-magnification FESEM images in Fig. 1(b) of the portion marked red in Fig. 1(a) confirm the formation of hexagonal 2D plates with clearly visible textural boundaries.The hexagonal plates are composed of intact stacking of 6À7 layers, each of thickness approximately < 100 nm.The clustered features are partially spherical shaped with a diameter in the range of 1 to 3 mm.Fig. 1(c) shows the high magnification FESEM image of the perfectly spherical cluster of nanoparticles from the portion marked yellow in Fig. 1(a).The spherical particle comprises irregularly shaped nanoparticles with one of the dimensions in the range of 100 to 150 nm.
The morphology as well as the lattice structure of the Ba(Sb/ Ta) 2 O 6 sample were analyzed by TEM measurements.The TEM image of the Ba(Sb/Ta) 2 O 6 sample along with the lattice fringes is shown in Fig. 2(a) and (b).It revealed the formation of a stacked structure surrounded by the clustered particles, which is identical to the FESEM studies (Fig. 1).The HRTEM image (Fig. 2(b)) clearly revealed the polycrystalline nature of the sample.Moreover, it indicated clear domain boundaries with lattice fringes having different orientations.On the basis of the calculations of the lattice spacing and the analysis of its orientation, it is found that the Ba(Sb/Ta) 2 O 6 possibly consists of combined mixed phases of BaTa 2 O 6 [lattice direction (411) with lattice spacing 0.28 nm] and BaSb 2 O 6 [lattice direction (200) with lattice spacing 0.22 nm], which are consistent with the XRD data (Fig. S1).Having similar crystal structure and matching peak positions  The crystal structure, figure print of the materials is analyzed from the XRD pattern.The XRD spectrum in Fig. S1 was recorded after annealing BSTO at 1200 °C for 10 hr, showing sharp and intense peaks, and confirmed the good crystallinity of the BSTO.The multiple peaks observed in the pattern are well indexed to the hexagonal BaTa 2 O 6 (i.e., BTO) and BaSb 2 O 6 (i.e., BSO) phases.The peaks indexed in black and blue are of BTO and BSO phases, respectively.Most of the peaks are assigned to the hexagonal BaTa 2 O 6 and BaSb 2 O 6 phases with the space groups of P6/mmm (JCPDS No.74À1321) and P-31 m (JCPDs No. 46À1494), respectively.The BTO hexagonal structure constituted of co-ordination of Ta with octahedral oxygen rings with corner and edge-sharing in the sheet-like pattern, and the Ba atom located at interstitial sites [30].The BSO phase composed of infinite sheets of (SbO 6 ) 2À octahedra with alternating layers of Ba 2+ [31] is akin to the crystal structure of BTO but only differs in the 3, 5, and 6 membered oxygen rings.However, the interchangeability between the hexagonal BTO and BSO phase in the entire crystal system may not be feasible due to their space group variation.Apart from this, two unindexed peaks observed at 38.6°and 42.1°are assigned to (202) and (212) planes of the Ta suboxide phase (i.e., TaO x ) [32].The dramatically reduced intensity of these peaks indicates the insignificant presence of the TaO x phase in the BSTO.Overall, the XRD analysis confirms that the prepared compound has arranged in the general formula of Ba(Sb/Ta) 2 O 6 with minor traces of the TaO x phase.
The electronic structure and the chemical oxidation states of the BSTO are performed using X-ray photoelectron spectroscopy (XPS).Fig. 3 shows the XPS spectra of core levels of Ba(3d), Ta(4f), Sb(3d), and O(1s) present in the BSTO.All the spectra are deconvoluted via the Voigt curve fitting function with the Shirley background.The double peak feature of Ba(3d) core levels is perfectly fitted with two peaks located (Fig. 3(a)) at binding energy (BE) of 780.2 and 795.6 eV representing the core levels of Ba 2+ (3d 5/2 ) and B 2+ (3d 3/2 ), respectively [33].The energy separation of 15.4 eV observed between Ba(3d 5/2 ) and Ba(3d 3/2 ) is assigned to the Ba 2+ oxidation state.Moreover, the XPS spectra of the Ta(4f) core levels of BSTO shows two distinct peaks identified as 'a' and 'b' located at BE of 25.9 and 27.8 eV (Fig. 3(b)), which represents the core level of the Ta(4f 7/2 ), and Ta(4f 5/2 ), respectively.The deconvolution of spectra illustrates a perfect fit for four peaks identified as aʹ, a, bʹ, and b, which are located at 25.2, 25.9, 27.4, and 27.8 eV, respectively.The peaks located at BE of 25.9 (• a), and 27.8 (• b), eV, corresponds to the Ta(4f 7/2 ) and Ta(4f 5/2 ) core level of 5+ cations [34À37] in BSTO.However, the other two weak intensity peaks observed in co-occurrence and located at BE of 25.2 (• aʹ) and 27.4 (• bʹ) eV are of the Ta(4f 7/2 ) and Ta(4f 5/2 ) core level of sub oxidized (i.e., < 4+ cations) Ta form in BSTO.Moreover, the energy separation of 1.9 eV estimated between the Ta(4f 7/2 ) (• a) and Ta(4f 5/2 ) (• b) is assigned to the Ta 5+ oxidation state, and 2.1 eV observed in the other two concurrent peaks is allotted to the suboxidized form of Ta (i.e., TaO x ).Moreover, the 9-fold higher intensity of Ta 5+ (4f) peaks (i.e., a and b) than that of sub-oxidized peaks (i.e., aʹ and bʹ) specifies the presence of very minute amount of suboxides phases of the Ta (i.e., TaO x ).Likewise, Fig. 3(c) shows the perfect fit for three distinct peaks located at BE of 530.2, 531.9, and 540.1 eV.The peaks at BE of 530.2 and 540.1 eV correspond to the Sb(3d 5/2 ) and Sb(3d 3/2 ) core levels of 5+ cations [38] in the BSTO.Moreover, the energy separation of 9.9 eV observed between the Sb(3d 5/2 ) and Sb(3d 3/2 ) core levels is akin to that identified for the Sb 5+ oxidation state [39].The peak at BE of 531.9 eV represents the O(1 s) core level of 2-anions [40À43], where the peak assigned to surface contamination is not accompanied by O(1 s) indicates the formation of oxidized BSTO phase.The utilization of SbCl 3 precursor was expected to provide the Sb 3+ oxidation state for the formation of double perovskite BSTO, but the presence of the Sb 5+ chemical state along with the Tasuboxide phase confirms the evolution of the most stable Ba (Sb/ Ta) 2 O 6 phase.This observation is akin to the XRD and TEM analysis detailed above.
UVÀVis spectra of BSTO was recorded to understand the photon dependent absorption behavior.The diffuse reflectance spectrum of BSTO was obtained using an integrated sphere detector assembly with pure BaSO 4 powder as a reference.The diffused reflectance spectra (DRS) plotted in Fig. S2 is obtained by referring to the Kubelka-Munk function, [17] F(r) = A(hn-E g ) m/2 , Where, m is constant defined from the optical transition of the material, E g is the bandgap of material, and hv is photon energy.The extrapolation of the DRS spectrum illustrated a bandgap of 3.48 eV, which is akin to the values reported for the BTO and BSO [12,18,21,22].This indicates that the UV-active BSTO has the ability to deliver the collective effect of BTO and BSO and enhance photocatalytic activities.
The various organic dyes were utilized to understand the photocatalytic dye-degradation mechanism of the BSTO.The photocatalytic reaction mechanism was optimized on the MB.Fig. 4(aÀc) shows the UVÀVis spectra recorded after the photocatalytic reaction of different amounts of MB for various reaction times.Initial optimization was performed on 10, 20, and 30 ppm MB solutions while utilizing 25 mg of BSTO.The intense absorption peak at 664 nm, accompanied by a shoulder peak at 612 nm, corresponds to the MB dyes [44] was observed after adsorption-desorption equilibrium (i.e., initial state).Another absorption peak is not observed, confirm that no intermediate reaction has happened to form dye-complexes. Fig. 4(a) shows the absorption spectra recorded at different time intervals for the photocatalytic reaction of 30 ppm MB.The absorption peak intensity is inversely proportional to the photocatalytic reaction time.The peak intensity of λ max decreased without any shifting in the peak position.This illustrates that the dye has not formed any complexes but degraded photocatalytically with an increase in the irradiation or reaction time.Moreover, Fig. 4(b) and (c) illustrate explicitly identical absorption behavior after photocatalytic reaction observed for 20 and 10 ppm MB solution, respectively.Further, to evaluate the performance of BSTO in the photocatalytic degradation of different concentrations of MB dye, the dye degradation efficiency calculated from [45] Where, C o is the absorbance (i.e., initial dye concentration) before exposure to light (i.e., t = 0), and C is a change in absorbance (i.e., dye concentration) after exposure to light (t = t).Further, we have explored the kinetics of the photocatalytic reduction of MB utilizing BSTO.The linear variation in the ln(C/C o ) with respect to the photocatalytic reaction time reveals the pseudofirst-order reaction kinetics.The pseudo-first-order contact (k) was evaluated from the equation [46].
Where, C o and C are the absorbances at time t = 0 (i.e., initial dye concentration) and t = t (i.e., dye concentration after exposure), respectively, and k is the reaction rate constant at a time interval of t (i.e., /min.).Fig. 4(e) illustrates the photocatalytic reaction timedependent linear variation in the ln(C/C o ) of various MB concentrations.The k values of 0.0073, 0.0065, and 0.0055 /min.were observed for a reduction of 10, 20, and 30 ppm MB.The larger rate constant value is observed for 10 ppm MB than 20 and 30 ppm concentration.Moreover, the correlation coefficient (R 2 ) estimated was 0.996, 0.977, and 0.976, respectively.The saturation of dye solution on the catalyst surface leads to a decrease in the active reaction sites, which might be one reason behind this [47].Moreover, the decreased penetration of light reflects the reduction of photocatalytic efficiency.We have performed further photocatalytic degradation studies on the 10 ppm concentration of various dyes in view of these observations.Fig. 5(aÀc) shows the UVÀVis spectra recorded after photocatalytic reaction of 10 ppm CV, MO, and RhB for different reaction times.The intense absorption peaks at 590, 464, and 554 nm corresponds to the CV (Fig. 5(a)), MO (Fig. 5(b)) and RhB (Fig. 5(c)) dyes, respectively, [45,48] observed after adsorption-desorption equilibrium (i.e., initial state).The nonappearance of other absorption peaks in the spectra confirms that the intermediate reaction has not formed dye-complexes.The intensity of the absorption peaks (λ max ) for all dyes has decreased with an increase in the photocatalytic reaction time, and the peak position (i.e., λ max ) has not shifted at all for CV, MO, and RhB.This corroborates that all dyes have degraded photocatalytically without forming any complexes even after increasing the irradiation time.
Fig. 5(d) shows the histogram of all studied dyes degradation efficiency estimated from the absorption spectra collected at different intervals.The time-dependent degradation behavior of the BSTO catalyst for 10 ppm CV, MO, and RhB is akin to that for MB dye.Although BSTO delivered better efficiency for the degradation of MB and MO dyes initially (i.e., < 60 min.),a better degradation response was observed for RhB after irradiation for 180 min.The degradation efficiency observed for CV, MB, RhB, and MO dyes is 65.9%, 77.3%, 89.8%, 84.2%, respectively, after 180 min.irradiation in the presence of BSTO.This specifies that the BSTO shows good photocatalytic degradation activities for CV, MB, RhB, and MB dyes.However, the maximum efficiency of 89.8% gained for 10 ppm RhB after 180 min.photocatalytic irradiation confirmed the excellent photocatalytic behavior towards RhB.Fig. 5(e) shows the irradiation time-dependent linear variation in the ln(C/C o ) of all studied dyes and revealed the pseudo-first-order reaction kinetics similar to the MB.The k values of 0.0052, 0.0073, 0.0115, and 0.0095 /min.were calculated for a reduction of 10 ppm CV, MB, RhB, and MO, respectively.The BSTO delivered a higher rate constant for RhB and a lower rate constant for CV dye.Moreover, the correlation coefficient (R 2 ) for CV, MB, RhB, and MO was 0.995, 0.996, 0.958, and 0.985, respectively.This depicts the better response of BSTO for the photocatalytic removal of 10 ppm RhB than that of CV, MO, and MB dyes.
The photocatalytic stability of the BSTO perovskites for the reduction of organic dyes is confirmed from cyclic studies.Fig. S3 shows the cyclic stability of BSTO perovskite during the photocatalytic reduction of RhB.BSTO has shown stable behavior without significant loss in photocatalytic activities for three continuous cycles of 570 min.The ~7% reduction witnessed after the third cycle might be due to the weight loss of BSTO during the recovery process.This proves the substantial stable photocatalytic cyclability of BSTO perovskite in the removal of organic dyes.
Considering that the formation of O À 2 and OH À surface radicals play an important role in the degradation of the dyes; we have carried out systematic DFT calculations to unravel the adsorption mechanism of O 2 and H 2 O species on the BSTO(100) surface.The optimized bulk  6 (e,f)).These results suggest that the BSTO(100) surface will promote the dissociation of H 2 O to form surface OH À radicals.The O À 2 and OH À radicals formed on the BSTO(100) surface are expected to react with the studied dyes, facilitating their degradation.
The distinct scavengers like methanol (MeOH), ammonium oxalate (AO), and isopropanol (IPA) were employed to reveal the impact of superoxide radicals ( ¢ O À 2 ) photogenerated holes (H + ), and hydroxyl radicals (_OH) respectively, in the photocatalytic degradation of organic dyes [49].Fig. 7(a) shows the photocatalytic reduction of RhB in the presence of MeOH, AO, and IPA.IPA revealed a more significant quenching in photocatalytic degradation of RhB and endorsed the dominance of hydroxyl radicals (_OH) in the photodegradation process.Moreover, a substantial decrease in the degradation efficiency in the presence of AO indicating the effectiveness of H + -in this process.The modest drop in the degradation efficiency for MeOH suggests the partial contribution of ¢ O À 2 .Thus, the photogenerated OH À , and H + from BSTO perovskite are substantially stimulating the photocatalytic reduction of organic dyes.Based on these results, the possible degradation mechanism for dyes adsorbed on the catalyst surface is shown in Fig. 7(b).The irradiation of BSTO under xenon lamp transfers photogenerated electrons (e À ) to the conduction band (CB), leaving holes (h + ) in the valance band (VB).The hydroxyl groups at the catalyst surface trap these holes and yield the hydroxyl radicals (_OH).However, electrons at CB produces _O 2 À radicles after reacting with dissolved oxygen molecule and further produce hydroxide radicals (HO 2 _) after protonation [50,51].These radicals dissociate the dyes into nontoxic compounds.The larger accessible surface area, the bandgap of 3.48 eV, and largely accessible surface free-electron provide a larger number of photogenerated e À Àh + pairs to boost the photocatalytic activities of BSTO.Therefore, the following photocatalytic reactions are expected for BSTO, Where, the light of sufficient photon energy (i.e., hn>E g(BSTO) ) incident on BSTO would generate e À Àh + pairs in the CB and VB, respectively (Eq. ( 3)).The sensitized H 2 O and O 2 molecules on the surface of BSTO trap holes and electrons to form hydroxyl (_OH) (Eq.( 4)) and superoxide (_O 2 ˉ) (Eq.( 5)) radicals, respectively.BSTO form _OH radicals in the presence of H + ions (Eq.( 6)) and further protonated to form H 2 O 2 (Eq.( 8)).The H 2 O 2 traps the e À and gives _OH radical (Eq.( 9)).Moreover, the trapping of e À enhances the formation of HO 2 _ radicals (Eq.( 7)), which further assists in the protonated formation of H 2 O 2 and subsequently to _OH radicals.The _OH radicals act as capable oxidizing agents to significantly degrade the dye pollutants ultimately into H 2 O and CO 2 .(Eq. ( 10)).The more significant generation of _OH radicals boosts the degradation of the dye through further oxidation.These observations are akin to our theoretical DFT predictions.The presence of more number of alkyl groups in CV than others constrains the dye solubility in water, which hampers photocatalytic degradation.Likewise, the SO 3 À withdrawing group containing dye like MO shows less reactivity towards photocatalysis.We observed the best degradation rate of BSTO towards RhB dye, which might be owing to the presence of a carboxyl group, unlike in MB [52].Overall, the perfect crystalline nature, lower bandgap, and largely accessible surface electrons of BSTO have resulted in the admirable photocatalytic degradation of organic dyes.

Conclusions
In conclusion, we have observed excellent photocatalytic activity of BSTO towards the degradation of various dyes.FESEM analysis confirms the formation of closely packed 2D hexagonal plates accompanied by clustered nanoparticles of BSTO.XPS evidenced the presence of Ba 2+ , Sb 5+ , Ta 5+ , and O 2À oxidation states and revealed the formation of Ba(Sb/Ta) 2 O 6 phase with minor traces of TaO x .The DRS revealed a bandgap of 3.48 eV for the BSTO perovskite phase.BSTO exhibited excellent photocatalytic activity for the degradation of various organic dyes.The kinetic studies of BSTO delivered photocatalytic degradation efficiency of 65.9%, 77.3%, 89.8%, and 84.2%, for the CV, MB, RhB, and MO, respectively, after irradiation of 180 min without using cocatalyst.The high ordered reaction rate of pseudo-first-order reaction kinetics indicates the excellent photocatalytic activity of BSTO.Overall, the utilization of BSTO can serve for the photocatalytic degradation of various organic dyes and may be beneficial for chemical transformation, observing the economic, environmental, and safety issues.
A 3 £ 3 £ 9 Monkhorst-Pack k-point mesh used to sample the Brillouin zone.A full unit cell relaxation yield the lattice parameters of BSTO to be a = 21.328A and c = 3.957A , which compares closely with the experimental lattice parameter of the BaTa 2 O 6 counterpart (a = 21.116A and c = 3.9157A

Fig. 1 .
Fig. 1.FESEM images of BSTO material identifying (a) stacking of 2D hexagonal nanoplates and microspheres of clustered nanoparticles (identified yellow) morphologies, (b) hexagonal 2D nanoplates stacked over one other, and (c) microsphere consisted of clustered nanoparticles identified red and yellow in Fig (a).The inset (b) shows the high magnification image of close-packed stacks of hexagon layers from the marked white region.(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

Fig. 4 (
d) shows the histogram of the MB dye degradation at different time intervals estimated from the absorption spectra.The degradation efficiency of the BSTO catalyst has increased with an increase in the photocatalytic reaction time, and maximum values are obtained after 180 min.irrespective of the concentration of MB dye.The best degradation efficiency of 77.3, 72.1, and 68.2% was achieved for 10, 20, and 30 ppm MB after 180 min photocatalytic reaction with BSTO.Thus, the highest degradation efficiency of 77.3% was obtained for 10 ppm MB dye after a photocatalytic reaction of 180 min.

Fig. 6 .
Fig. 6.(a) Optimized structure of hexagonal BSTO in a polyhedral form, (b) corresponding projected density of states, and (c) the surface structure of the (100) surface.Optimized structures of the lowest-energy adsorption geometries of (d) O 2 , (e) molecular H 2 O, and (f) dissociated H 2 O on BSTO (100) surface.

Fig. 7 .
Fig. 7. (a) Effect of distinct scavengers on the photoreduction of RhB using BSTO perovskite, (b) schematic illustration of the photocatalytic mechanism of BSTO in the degradation of organic dyes.
3 , Ba 4 Ta 2 O 9 , Ba 5 Ta 4 O 15 , BaTa 2 O 6 , BaTa 4 O 11 and Rh/Ir doped BaTa 2 O 6 [11À15] have delivered better photocatalytic activities in water splitting.Nevertheless, reports in the literature focusing on exploring their dye degradation studies are very limited.