Oxygen vacancy-mediated BaSn_1−xNi_xO₃ perovskite as a round-the-clock catalyst for dark and ambient degradation of azo dyes and crystal violet

Abstract

The advanced oxidation process involving reactive oxygen species (ROS) is considered a potential method for the degradation of pollutants in water. However, this process requires the application of semiconductors which, upon excitation with light generate electrons and holes on the surfaces that react with oxygen and water respectively to result in reactive oxygen species. There is a need for materials that generate ROS even in the absence of light (dark), so that the process is round the clock and economical. Towards this purpose, we have designed Ni doped BaSnO₃ perovskite to show that this system degrades many azo dyes and crystal violet in the dark and under ambient conditions. This study involves the strategic engineering of oxygen vacancies in BaSnO₃ perovskites through the controlled aliovalent substitution of Sn⁴⁺ with Ni²⁺ to enable continuous 24-hours environmental remediation. The successful incorporation of Ni into the lattice and the subsequent formation of electron-rich oxygen vacancies were confirmed by X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR), and were supported by a comprehensive suite of analytical techniques, including XRD, FE-SEM/EDS, and FT-IR. These vacancies serve as active catalytic centres, facilitating the generation of ROS in aqueous media under both dark and ambient light conditions, a mechanism further validated by comparative Nb⁵⁺ doping. Notably, Congo red dye achieved 98.4% degradation within 15 minutes with a high-rate constant of 30.7 × 10⁻² min⁻¹ in the dark. The optimised catalyst, BaSn_1−xNi_xO₃ where x = 0.2, exhibits exceptional performance, achieving ultrafast degradation. Significant mineralization efficiency, up to 82.6%, is observed for crystal violet as confirmed by total organic carbon (TOC) removal. Identification of intermediates via HR-MS coupled with ECOSAR toxicity modelling, demonstrate that the process effectively yields non-toxic products. Furthermore, ECOSAR toxicity predictions confirm that the degradation process significantly reduces the ecotoxicity of the resulting byproducts. This shift from toxic dyes to less harmful intermediates ensures the safety of the treated waste water for safe aquatic ecosystems.

The advanced oxidation process involving reactive oxygen species (ROS) is considered a potential method for the degradation of pollutants in water.

1. Introduction

The ABO₃ perovskite family is widely researched for its highly tunable physical and chemical properties across various advanced applications.¹ Perovskites generally adopt the chemical formula ABO₃, where A represents a larger cation (typically an alkaline earth or rare-earth metal), B denotes a smaller transition-metal cation, and O corresponds to oxygen anions arranged in an octahedral coordination around the B-site cation. Barium stannate (BaSnO₃) is a simple cubic perovskite oxide belonging to the alkaline-earth stannate family with the general formula MSnO₃ (M = Ca, Sr, Ba). It is a wide band gap semiconductor (∼3.15)² and consists of corner-sharing SnO₆ octahedra whose geometry is governed by the Goldschmidt tolerance factor. While BaSnO₃ maintains a stable cubic phase (∼1.02), the smaller cations in SrSnO₃ and CaSnO₃ induce transitions into orthorhombic structures.

In particular, these materials exhibit remarkable functional versatility, facilitating their integration into diverse technological domains, including photocatalysis, high-temperature superconductivity, ferroelectric circuitry, thermoelectric power generation, and spintronic magnetic media.³ In particular, photocatalysis is an advanced oxidation process (AOP) that provides an efficient and cost-effective approach for the in situ generation of highly reactive oxygen species (ROS), such as hydroxyl and superoxide radicals. These ROS play a crucial role in the rapid oxidation and mineralization of organic pollutants, making photocatalysis a promising technique for environmental remediation and wastewater treatment.⁴ Commonly investigated photocatalysts such as TiO₂, ZnO, and BiVO₄ require activation under ultraviolet or visible light to drive photocatalytic processes, these processes generally require ultraviolet or visible light for activation. From a practical standpoint, the high photon demand results in increased energy consumption, elevated operational costs, and dependence on specialized catalyst systems. To address these challenges, dark catalysis which operates in the absence of light—has emerged as a compelling solution. By functioning independently of photons, external energy sources, and chemical additives, dark catalysts represent a highly efficient and sustainable approach to AOP-based catalysis. Therefore, there is a growing need to develop catalysts capable of generating reactive oxygen species (ROS) in the absence of light, allowing advanced oxidation processes (AOPs) to operate under dark conditions. Such light-independent catalytic systems would not only overcome the limitations associated with photon-driven processes but also enable efficient pollutant degradation under ambient or low-light environments, significantly improving the feasibility of implementing AOP-based technologies in practical systems. Oxygen vacancies (OVs) are common point defects formed by the removal of lattice oxygen (O²⁻) from metal oxide frameworks under conditions such as thermal treatment and reducing atmospheres. Characterised by low formation energies, these widely studied anionic defects alter local electronic structures, encourage charge carrier localization, and boost surface reactivity. Consequently, OVs are fundamental in tuning the catalytic, electrical, and optical performance of metal oxide materials.^5,6 Recent studies have demonstrated that deliberate engineering of oxygen vacancies is an effective strategy for significantly enhancing catalytic activity.⁷ In dark catalysis, oxygen vacancies within perovskite structures provide excess electrons that react with dissolved oxygen to produce reactive oxygen species (ROS). Research involving perovskite systems such as La₂NiO₄, La₄Ni₃O₁₀, MgSrCoO, Ca_xSr_1−xCuO_3−δx = 0.75, Me_0.25Sr_0.25Cu_0.5O, CaFe_1−xNi_xO_3−δ, La_1−xSr_xCoO_3−δ, CaSrNiO₃, LaNiO_3−δ, tin doped BiFeO₃/graphene and has demonstrated that these vacancies are pivotal in driving the catalytic degradation of organic pollutants in the absence of light.^8–17 Towards dark catalysis, we have recently reported BaSn_1−xCu_xO₃ which degrades azo dyes in the absence and presence of light (day -night catalysts) through ROS generation.¹⁸ As part of our ongoing programme towards developing more such day-night catalysts, esp. based on perovskite oxides, we have explored BaSn_1−xNi_xO₃. In this study, we report for the first time the synthesis of Ni-doped BaSnO₃ as a highly efficient catalyst for the degradation of organic pollutants in the absence of light. The incorporation of Ni as a B-site dopant induces oxygen vacancies within the BaSn_1−xNi_xO₃ lattice, which are pivotal to its catalytic performance. These surface oxygen vacancies act as active sites that facilitate the generation of reactive oxygen species (ROS), enabling the effective degradation of azo dyes and triphenylmethane dyes such as crystal violet even in total darkness. Operating under ambient conditions, the catalyst ensures continuous, round-the-clock activity.

2. Experimental methods

2.1. Synthesis of Ni-doped BaSnO₃ perovskite oxides

Ni-doped BaSnO₃ (BaSn_1−xNi_xO₃) powders were prepared using a conventional solid-state synthesis method. Stoichiometric amounts of BaCO₃, SnO₂, and NiO were weighed according to the desired Ni substitution levels (x = 0.05, 0.10, 0.15, and 0.20, where x denotes the molar fraction of Ni). The precursors were thoroughly mixed and ground in an agate mortar and pestle for 30 min to ensure homogeneous blending. The resulting mixtures were calcined at 1200 °C for 12 h, followed by regrinding for 30 min to promote reaction uniformity. A second calcination step at 1200 °C for an additional 12 h was carried out to improve crystallinity and achieve phase purity. After cooling, the products were finely ground to obtain uniform powders suitable for subsequent structural and functional characterisation. BaSnO₃ is named BS, BaSn_0.95Ni_0.05O₃ as BSN05, and BaSn_0.9Ni_0.1O₃ as BSN1, BaSn_0.8Ni_0.2O₃ as BSN2.

2.2. Characterization of BaSn_1−xNi_xO₃

The synthesized BaSn_1−xNi_xO₃ samples were characterized using various analytical techniques. Powder X-ray diffraction (XRD) patterns were obtained on a PANalytical X'Pert diffractometer with Cu Kα radiation (λ = 1.5406 Å). The surface morphology and elemental distribution were examined by field-emission scanning electron microscopy (FE-SEM, Thermo Fisher FEI QUANTA 250) equipped with an energy-dispersive X-ray spectroscopy (EDS) unit, operated at 30 kV. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo Scientific K-Alpha spectrometer. Fourier transform infrared (FT-IR) spectra were recorded using a Thermo Scientific Nicolet iS10 instrument. The optical bandgap and photocatalytic degradation studies were performed with a UV-Vis-NIR spectrophotometer (JASCO V-670). Hydroxyl radical quantification and electron–hole recombination behaviour were investigated using a Hitachi F-7000 spectrofluorometer. High-resolution mass spectrometry (HR-MS) data were collected on a Waters XEVO G2-XS QToF instrument. The total organic carbon (TOC) content of degraded dye solutions was measured using a TOC-L CPN analyzer (Shimadzu).

2.3. Catalytic degradation evaluation

Azo dyes and crystal violet (CV) were used as model compound to monitor the catalytic activity of Ni doped BaSnO₃ products.

2.3.1. Catalytic measurements

The degradation of azo dyes/crystal violet was evaluated under both dark and ambient conditions. For each experiment, 100 mg of catalyst was added to 100 mL of an aqueous azo dye solution (10 mg L⁻¹) and stirred continuously. At specific intervals, 2 mL aliquots were withdrawn, centrifuged at 4000 rpm to remove the catalyst, and the absorbance of the supernatant was recorded using a UV-vis spectrophotometer. To investigate the effect of initial dye concentration, solutions were prepared at 10 ppm and 20 ppm. The effect of catalyst loading was systematically investigated by varying the dosage of Ni-doped BaSnO₃ at 25, 50, and 100 mg.

2.4. Degradation efficiency calculation

The degradation efficiency (DE) was calculated using the following equation:DE (%) = C₀ − C_t/C₀ × 100where:

• C₀ is the initial concentration of the dye.

• C_t is the residual concentration at reaction time.

3. Results and discussion

3.1. XRD analysis of BaSn_1−xNi_xO₃

Powder X-ray diffraction patterns (Fig. 1) of BaSn_1−xNi_xO₃ (x = 0, 0.05, 0.10, 0.20) products were collected using Cu Kα radiation (λ = 1.5406 Å). All samples exhibit the characteristic reflections of the cubic perovskite structure. Fig. 1 confirms the formation of single-phase BaSnO₃ with a perovskite structure and the successful incorporation of Ni without altering the crystal structure.^19,20 A summary of the main indexed reflections is provided in (110), (111), (200), (210), (211), (220), (300). The positions of the observed peaks remain unaltered, since the ionic radius Ni²⁺ (0.69 Å) ion is comparable to that of Sn⁴⁺ (0.69 Å). The compositions crystallize in cubic system with a ∼4.115(7) Å.

Fig. 1. XRD patterns of BS, BSN05, BSN1, BSN2.

3.2. FT-FTIR analysis of BaSn_1−xNi_xO₃

Fourier transform infrared (FTIR) analysis of the calcined Ni-doped BaSnO₃ samples (BS, BSN05, BSN1 and BSN2) was carried out in the wavenumber range of 400–4000 cm⁻¹, as shown in Fig. S1. All samples exhibit the characteristic absorption band of the BaSnO₃ perovskite structure. The peak at ∼618 cm⁻¹ corresponds to the symmetric stretching vibration of the SnO₆ octahedral framework.²¹ With the introduction of Ni into the Sn site, a gradual decrease in the intensity of this band is observed, which is attributed to the partial substitution of Sn⁴⁺ by Ni²⁺, resulting in distortion of the SnO₆ octahedra and weakening of the Sn–O–Sn vibration. Concurrently, a new absorption band emerges at ∼846 which is assigned to the Ni–O stretching vibration.²² The intensity of this band increases systematically from BSN05 to BSN2, providing direct evidence of the successful integration of Ni ions into the crystal lattice and the formation of Ni–O bonds. In addition, the weak absorption band located in the region of ∼1483 cm⁻¹, assigned to the bending and stretching vibrations of surface OH groups, becomes progressively more intense upon Ni incorporation.²³ This enhancement is due to the creation of oxygen vacancies (OVs) arising from the charge-compensating Ni²⁺ → Sn⁴⁺ substitution. The FTIR spectra for Ni-doped BaSnO₃ (BSN2) provide strong spectroscopic evidence for the successful incorporation of Ni into the perovskite structure.

3.3. FESEM of BaSn_1−xNi_xO₃

The FESEM analysis of the BaSnO₃ products reveals a distinct morphological evolution driven by Ni doping, transitioning from a state of high agglomeration to structured 1D growth. The pure sample, exhibits agglutinated particles (Fig. 2(a and b)). Upon Ni incorporation (x = 0.2) the particles grow into rod like structure (Fig. 2c and d). EDAX (Fig. 2e and f) confirm the elemental composition for x = 0 and x = 0.2, respectively. Ni ions in the BaSnO₃ facilitate the particle growth from bulk agglomerates to elongated rod-like structures.²⁴

Fig. 2. (a and b) FESEM image of BS, (c and d) FESEM image of BSN2, (e) EDAX of BS (f) EDAX of BSN2.

The surface morphology and grain size distribution of the synthesised BaSnO₃ (BS) and Ni-doped BaSnO₃ (BSN2) were quantitatively analysed via FE-SEM and ImageJ software. As illustrated in Fig S2(a and b), the pristine BS exhibits a relatively uniform distribution with an average particle size of 76.0 nm. Upon Ni-doping, a substantial shift in the growth kinetics is observed, with the average grain size doubling to 163.7 nm for the BSN2 composition. This significant increase in particle dimensions, fitted with a Gaussian function, suggests that the substitution of Sn⁴⁺ by Ni²⁺ induces lattice strain and promotes the coalescence of smaller grains into larger, more defined rod-like architectures.

3.4. XPS analysis of BaSn_1−xNi_xO₃

X-ray photoelectron spectroscopy (XPS) was employed to investigate the elemental composition, oxidation states, and surface chemical interactions of pure BaSnO₃ (BS) and BaSn_0.8Ni_0.2O₃ (BSN2). The XPS survey spectrum Fig. S3(a) identifies the presence of barium (Ba), tin (Sn), and oxygen (O) in both samples, while the BSN2 spectrum specifically displays a Ni 2p peak, confirming the successful incorporation of nickel into the catalyst. Analysis of the core-level splitting of peaks of Ba, illustrated in Fig. S1(b), reveals two distinct peaks corresponding to the Ba 3d_5/2 and Ba 3d_3/2 states for the BS sample, which appear at 780.06 eV and 795.35 eV, respectively, while the BSN2 sample shows a slight shift to 779.54 eV and 795.82 eV, suggesting a modified chemical environment. In the core-level splitting of peaks of Sn, shown in Fig. S3(c), the BS sample exhibits peaks at 489.68 eV and 495.18 eV, characteristic of the Sn⁴⁺ oxidation state. However, the BSN2 sample shows a significant shift in the Sn 3d_5/2 peak to 486.08 eV. Furthermore, the valence band of BS and BSN2, highlighted in Fig. S3(d) show a clear electronic shift; the valence band maximum (VBM) for BS is located at 2.29 eV, whereas the BSN2 sample displays a VBM of 1.71 eV. This 0.58 eV shift toward the Fermi level suggests a substantial reduction in the valence band energy, which likely influences the photocatalytic efficiency and charge transfer properties of the Ni-doped product. The O 1s high-resolution XPS spectra for pure BS and Ni-doped BSN2 were analysed to determine the chemical states of oxygen and the prevalence of surface defects. As shown in the deconvoluted spectra, the peaks at 530.9 eV for BS and 530.1 eV for BSN2 correspond to lattice oxygen (O_L). The peaks located at higher binding energies, specifically 533.4 eV for BS and 532.5 eV for BSN2, are attributed to chemisorbed oxygen or hydroxyl groups (O_C) on the surface. Notably, the presence of oxygen vacancies (O_V) is confirmed by the intermediate peaks centred at 531.6 eV for both samples. A comparison of the peak areas indicates that the Ni-doped BSN2 sample possesses a higher concentration of oxygen vacancies compared to the pure BS sample, as evidenced by the significantly more prominent O_V peak in the BSN2 spectrum. The percentages of O_L, O_V and O_C in BS and BSN2 are detailed in Fig. 3 and Table 1. This increase in vacancy concentration suggests that doping with nickel promotes the formation of structural defects, which can enhance the material's photocatalytic performance.

Fig. 3. XPS O 1s of BS and BSN2.

Table 1. The percentages of O_L, O_V and O_C of BS and BSN2 by XPS analysis.

Samples	OL%	OV%	OC%
BS	51.0	25.7	14.8
BSN2	34.4	42.8	22.8

3.5. EPR spectroscopy

The room-temperature continuous-wave EPR spectra of the pristine BaSnO₃ (BS) and BaSn_0.8Ni_0.2O₃ (BSN2) products are shown Fig. S4. The corresponding g-value was calculated to be 2.0034. This characteristic signal is primarily assigned to unpaired electrons trapped at oxygen vacancies and surface defect sites generated within the perovskite lattice. The intensity of EPR signal is higher for BSN2 than BS. The marked intensification of the EPR signal for the BSN2 sample confirms the successful substitution of Ni ions into the BaSnO₃ framework, creating active charge-trapping centres that facilitate enhanced interfacial charge separation and support superior catalytic performance.

3.6. Optical analysis BaSn_1−xNi_xO₃

The optical properties of the synthesized Ni-doped BaSnO₃ samples were investigated using UV-vis absorption spectroscopy (Fig. S5a). As shown in the absorbance spectra, the absorption edges exhibit a noticeable shift toward longer wavelengths (redshift) upon increasing the Ni-doping. To determine the optical band gap (E_g), Tauc plots were generated by plotting (αhν)² against photon energy (E), assuming a direct allowed transition. As shown in Fig. S5(b), the pristine BS sample exhibits a band gap of 3.15 eV, which systematically decreases with increasing dopant concentration. The band gap values are found to be 2.81 eV for BSN05, 2.70 eV for BSN1, and 2.66 eV for BSN2 Fig. S5(b). This reduction in E_g is determined using the relation.αhν = A(hν − E_g)^1/2where α is the absorption coefficient, h is Planck's constant, ν is the light frequency, A is a constant, and E_g is the band gap energy.

3.7. Catalytic degradation

3.7.1. Dark degradation

The catalytic degradation of the BSN05, BSN1 and BSN2 compositions was analyzed via UV-vis spectrophotometry across five structurally diverse organic pollutants. The selected model substrates included four azo dyes methyl orange (MO), congo red (CR), acid orangeII (AOII), and amaranth (AM) as well as the triphenylmethane dye crystal violet (CV). The catalytic performance of the samples was evaluated through dark degradation experiments and the results are shown in Fig. 4, where BSN2 exhibited no degradation of MO without stirring indicating no adsorption (Fig. S6). BSN2 demonstrated significant degradation (88.6% in 40, minutes, Fig. S7a) against the azo dye MO compared to the pristine product BS, 9% degradation in 80 minutes (Fig. S7b). This high catalytic activity of BSN2 extended to other structurally distinct dyes, achieving degradation efficiencies of 98.4% for CR in 10 minutes, 91.2% for AM in 20 minutes, 97.6% for CV in 35 minutes, and 91.2% for AOII in 40 minutes. More importantly, the degradation efficiencies of BS, BSN05, BSN1 and BSN2 against methyl orange in 40, minutes are 9%, 65.1%, 70.3% and 88.6% respectively (Table 2). The catalytic perfomance of MO, AOII, CR, AM and CV is shown in Fig. S8. The experimental deviation in percentage degradation is ±2%. These results confirm that BSN2 catalyst possesses a high density of active sites due to oxygen vacancies, which facilitate rapid dye mineralization even in the absence of light. The kinetics data for BSN2, BSN1 and BSN05 are presented in (Fig. 4(a–c) respectively. It confirms the first order kinetics of degradation of different dyes by these catalysts (Fig. 4d). Illustrates the degradation kinetics of crystal violet (CV) for BS, BSN05, BSN1, and BSN2. The degradation plots for the remaining dyes BSN2 (AOII, AM, CR, CV) are provided in the SI (Fig. S9). The rate constants are provided in Table 1, and BSN2 exhibits the highest rate constant of 30.7 × 10⁻² min⁻¹ against CR.

Fig. 4. (a) BSN2 kinetics of various dyes (MO, AM, CR, and AO). (b) BSN1 kinetics of various dyes (MO, AM, CR, and AO). (c) BSN05 kinetics of various dyes (MO, AM, CR, and AO). (d) Kinetics of CV dye for BSN05, BSN1, BSN2.

Table 2. Photocatalytic degradation percentages and rate constants of BS, BSN05, BSN1, and BSN2 under dark conditions.

Composition	Dyes	% Of degradation (min)	Kinetics (x 10−2 min−1)
BS	Methyl orange	9 (80)	0.7
	Acid orangeII	11.3 (60)	0.9
	Congo red	42.5 (60)	2.4
	Amaranth	35.7 (60)	3.9
	Crystal violet	12.2 (60)	1.1
BSN05	Methyl orange	65.1 (40)	2.8
	Acid orangeII	69.4 (40)	3.7
	Congo red	85.2 (30)	15.3
	Amaranth	55.4 (30)	7.3
	Crystal violet	62.8 (35)	4.9
BSN1	Methyl orange	70.3 (40)	3.8
	Acid orangeII	65.6 (40)	4.9
	Congo red	89.3 (30)	23.6
	Amaranth	70.5 (30)	12.1
	Crystal violet	82.2 (35)	7.16
BSN2	Methyl orange	88.6 (40)	6.3
	Acid orangeII	91.2 (40)	7.9
	Congo red	98.4 (15)	30.7
	Amaranth	91.2 (20)	14.4
	Crystal violet	97.6 (35)	10.74

To verify the role of oxygen vacancies (OVs) in catalytic activity, higher-valent cation doping was introduced as a strategy to suppress OV formation. Specifically, Nb⁵⁺ (x = 0.2) was substituted at the Sn site, resulting in a consequent reduction in surface oxygen-vacancy defects. Nb-doped BaSnO₃ samples, synthesized using the same solid-state route, was evaluated for dark catalytic activity. The degradation efficiencies for MO and AOII were found to be only 8.3% and 18.3%, respectively, confirming limited activity (Fig. S10(a and b)). This indicates that the generation of Reactive Oxygen Species (ROS) in the dark is primarily governed by OV-mediated processes. In this study, Ni²⁺ and Nb⁵⁺ were strategically chosen as dopants to systematically tune the oxygen vacancy concentration. Interestingly, the BaSn_xNi_1−xO₃ series exhibits OVs tuned by composition, with the x = 0.2 sample showing the highest catalytic activity under both dark and ambient light conditions. In contrast, BaSn_1.8Nb_0.2O₃, which contains the lowest concentration of oxygen vacancies, displayed negligible catalytic performance. The BSN2 catalyst delivers rapid, high-efficiency degradation of azo dyes, outperforming many previously reported catalysts in the literature that require significantly longer reaction times (Table S1).

3.7.2. Ambient degradation

The photocatalytic performance of the Ni-doped BaSnO₃ (BSN2) catalyst is clearly demonstrated by the time-dependent UV-vis absorption spectra. As shown in Fig. 5a, the characteristic absorption peak of Methyl Orange (MO) significantly decreases over a 40-minutes interval, resulting in a final degradation efficiency of 87.7%. Similarly, Fig. 5b illustrates the rapid degradation of Acid OrangeII (AOII), achieving an efficiency of 92.5% within the same 40-minutes timeframe. Fig. 5c shows the degradation (97.8% in 35 minutes) for crystal violet, Fig. 5d depicts the first order kinetics for the different dyes with the highest rate constant of 30.1 × 10⁻² min⁻¹ against congo red. Degradation plots of amaranth and congo red are given in Fig. S11(a) and (b). It proves that BSN2 is an efficient catalyst for the remediation of diverse organic pollutants. The experimental deviation in percentage degradation is ±2%.

Fig. 5. (a) Ambient degradation of MO dye for BSN2. (b) Ambient degradation of AOII dye for BSN2. (c) Ambient degradation curve of CV dye for BSN2. (d) Kinetics of MO, AOII, CV dye for BSN2.

3.8. Catalyst dosage

BaSn_0.8Ni_0.2O₃ catalyst dosages were varied (25, 50, and 100 mg) for MO, AOII, CR, AM and CV degradation in the dark and the results are presented in (Fig. S12). When the dosage was varied from 25 to 100 mg, the results show that for all dyes efficiency increases. 100 mg is selected as the optimized dose for maximum efficiency across tested dyes, revealing that 100 mg provides the optimal surface area and active sites density required to maximize efficiency.

3.9. Concentration of pollutant

The experimental data demonstrate a clear relationship between the initial dye concentration and the overall photocatalytic performance (Fig. S13). At a concentration of 5 ppm, the catalyst exhibited its highest activity across all pollutants, with crystal violet and congo red showing the most significant degradation. As the concentration is increased to 20 ppm a marked decline in performance occurred. This inhibitory effect was most evident for methyl orange, whereas congo red, amaranth, and acid orangeII followed a similar trend, showing reduced activity at the higher concentration. Based on these observations, 10 ppm was established as the optimal threshold, providing the best balance between pollutant load and catalytic efficiency for further investigation.

3.10. Reusability studies

To evaluate the stability and practical applicability of the Ni-doped BaSnO₃ catalyst, reusability experiments were conducted over five consecutive cycles. For each cycle, 100 mg of the Ni-doped BaSnO₃ of 10 and 20 ppm aqueous solutions of the target dye. To ensure the removal of any adsorbed intermediate species, the recovered Ni-doped BaSnO₃ was meticulously washed multiple times with deionised water. The catalyst was then dried at 80 °C for 6 h to restore its active surface. The regenerated catalyst was subsequently introduced to a fresh solution of azo dyes and CV to initiate the following cycle, repeating this protocol for a total of five cycles. The residual dye concentration for each run was measured using the same analytical procedure described previously to determine the retention of catalytic activity (Table S2).

3.11. Scavenger study

To elucidate the mechanistic pathways and identify the predominant reactive oxygen species (ROS) involved in the photocatalytic process, systematic radical quenching assays were performed for BSN2. Under baseline conditions (without scavengers), the system exhibited a robust degradation efficiency of 86.6%. The introduction of p-BQ as a superoxide radical (˙O₂⁻) scavenger resulted in a drastic attenuation of catalytic activity to 29.5%, as shown in (Fig. S14). This significant inhibition underscores that the degradation kinetics are primarily governed by the reductive pathway, where photogenerated electrons facilitate the formation of (˙O₂⁻) through dissolved oxygen reduction. In contrast, the relatively less suppression is observed with IPA (a hydroxyl radical (˙OH) quencher) and SP (a hole (h⁺)scavenger), yielding residual activities of 63.0% and 60.6% respectively, indicates that oxidative pathways involving valence band holes and ˙OH contributes less to the mineralization compared to the electron-driven super oxide species.

3.12. Detection of superoxide radical generation via NBT assay

To evaluate the generation of superoxide radicals (˙O₂⁻), BaSn_0.8Ni_0.2O₃ catalyst was tested using a Nitro Blue Tetrazolium (NBT) reduction assay (Fig. S15). The UV-vis spectra reveal a characteristic NBT absorption peak at approximately 260 nm, which undergoes a steady, time-dependent decline in intensity over 60 minutes upon exposure to the BSN2 catalyst. This marked decrease in absorbance, highlighted by the downward arrow, directly confirms the efficient production of ˙O₂⁻ radicals. The results suggest that the substitution of Ni into the BaSnO₃ framework introduces catalytically active sites related to oxygen vacancies that facilitate the reduction of molecular oxygen (O₂) into superoxide species. Notably, this ROS generation occurs in the dark, demonstrating that the BSN2 catalyst possesses superior intrinsic catalytic activity.

3.13. Analysis of ˙OH radical generation

To comprehensively map the reactive oxygen species (ROS) profile and quantify the oxidative potential of the system, terephthalic acid (TA) was utilized as a highly sensitive chemical probe to monitor the generation of hydroxyl radicals (˙OH). The intensity of the resulting fluorescence emission typically monitored at a wavelength of approximately 425 nm serves as a direct indicator of the concentration of hydroxyl radicals produced at the catalyst surface as shown in Fig. S16. The incorporation of Ni into the BaSnO₃ lattice significantly boosts the catalytic performance, extending high degradation efficiency to both azo and crystal violet dyes. This enhanced activity is attributed to the Ni dopants acting as electron traps, which suppresses charge recombination and ensures a steady supply of reactive species to mineralize diverse organic structures.

3.14. Analysis of total organic carbon (TOC) removal

To evaluate the degree of complete mineralisation versus primary degradation, total organic carbon (TOC) analysis was performed. While decolourization indicates the transformation of the dye's chromophore, TOC removal measures the actual elimination of the organic carbon backbone. The BSN2 catalyst exhibited significant mineralization capabilities under dark conditions, achieving TOC removal efficiencies of up to 82.6% for crystal violet (CV), 68.6% for methyl orange (MO), and 65.8% for acid orangeII (AOII) as shown in Fig. S17. The substantial reduction in TOC levels suggests that BSN2 does not merely convert the parent dye molecules into intermediate organic byproducts but effectively facilitates the oxidative cleavage of aromatic rings and alkyl chains. This process ultimately yields stable inorganic end-products, primarily CO₂ and H₂O.

3.15. HRMS analysis

For orangeII (AO+1) degradation. High-resolution mass spectrometry (HRMS) provides precise and detailed information on the pathways of degradation (Fig. S18–S23), which occurs through interaction with reactive oxygen species (ROS) generated by the photocatalyst. Multiple photo-intermediates were identified using HRMS analysis. As shown in (Fig. 6). Pathway 1 starts with the orangeII parent molecule (AO+1, m/z = 350), which undergoes the removal of the hydroxyl group to form AO-1 (m/z = 339). This is followed by the alteration of the naphthalene ring and loss of sulfonic functional groups to produce AO-3 (m/z = 301) and AO-5 (m/z = 216), eventually leading to the saturated hydrazine derivative AO-9 (m/z = 156). Pathway 2 begins with the direct cleavage or rearrangement of the side chains of the parent molecule to form AO-7 (m/z = 192). This intermediate undergoes further degradation of the aromatic amine structure to form AO-10 (m/z = 136), which then transitions toward smaller cyclic structures like AO-11 (m/z = 98). Pathway 3 involves the initial transformation of the sulfonic acid group on the benzene ring to form AO-2 (m/z = 316), followed by subsequent changes in the azo bond and ring saturation to produce AO-4 (m/z = 269) and AO-6 (m/z = 205). This pathway continues with the formation of naphthalene-based intermediates like AO-8 (m/z = 158) before merging into the ring-opening stage represented by AO-11 (m/z = 98). Ultimately, all these intermediates degrade further into smaller molecules and are eventually eliminated as H₂O, CO₂, O₂, and NO₃ over an extended period through complete mineralization.

Fig. 6. Proposed possible degradation pathways of acid orangeII.

In the Crystal Violet (CV) degradation process, which occurs through interaction with reactive oxygen species (ROS) generated by the photocatalyst, a total of 15 photo-intermediates were identified using HRMS analysis.^25–27 The plausible degradation mechanism follows three distinct photodegradation pathways, as shown in the Fig. S24. Pathway 1 starts with the CV cation (m/z = 372.24), which undergoes cleavage of methyl groups from the amine, forming P1 (m/z = 360.24). P1 then undergoes a ring-opening reaction with ROS, producing several intermediates, including P1-2 (m/z = 345.50), P1-3 (m/z = 262.39), P1-4 (m/z = 224.34), P1-5 (m/z = 162.13), and P1-6 (m/z = 136.22). Pathway 2 begins with the transformation of CV into P2-2 (m/z = 332.24), followed by a ring-opening reaction leading to the formation of P2-3 (m/z = 204.18), P2-4 (m/z = 178.10), P2-5 (m/z = 122.12), and P2-6 (m/z = 94.02), which further undergo cleavage in reaction with ROS. In Pathway 3, CV is broken down into intermediate P3 (m/z = 354.20), producing further intermediates P3-1 (m/z = 318.21), P3-2 (m/z = 250.10), P3-3 (m/z = 236.15), and P3-4 (m/z = 176.30). Ultimately, all the intermediates degrade further into smaller molecules and are eventually eliminated as CO₂ and H₂O over an extended period.

3.16. ECOSAR

The ECOSAR software model was used to evaluate the acute and chronic toxicity of degradation intermediates of acid orange and crystal violet (Tables S3 and S4). The analysis identified several toxic intermediates, specifically those arising from the initial cleavage of the azo bond and N-demethylation, that successfully mineralise into non-toxic products upon extended reaction. This process is driven by the oxygen vacancies within the Ni-doped BaSnO₃ lattice, which generate reactive oxygen species under dark conditions. Consequently, the results demonstrate that this catalyst effectively facilitates the complete decontamination and detoxification of both azo and triphenylmethane dyes into environmentally benign species such as CO₂ and H₂O.

3.17. Catalytic mechanism

Lakshmi Prasanna et al. reported a mechanism for the generation of ROS in the dark by nano-ZnO, suggesting that oxygen vacancies possess excess electrons that react with dissolved oxygen to form superoxide radicals.²⁸ These superoxide radicals then react with water to produce hydroperoxyl radicals, which combine to form H₂O₂ and further react with superoxide species to generate highly reactive hydroxyl radicals. This defect-mediated process facilitates the continuous degradation of toxic dye molecules into harmless end products without the need for photon irradiation, this study systematically tuned oxygen vacancy concentrations. Interestingly, the BaSn_1−xNi_xO₃ series exhibits OVs tuned by composition, with x = 0.2 showing the highest catalytic activity under both dark and ambient light conditions. This suggests that oxygen vacancy-mediated processes are the primary drivers of ROS generation in dark conditions. Fig. 7 shows mechanism of dark degradation of BaSn_0.8Ni_0.2O₃.

Fig. 7. Mechanism of dark degradation of BaSn_0.8Ni_0.2O₃.

3.17.1. Catalytic role of Ni-doped BaSnO₃

In BaSnO₃, substituting Sn⁴⁺ with Ni²⁺ creates a charge deficiency. To maintain electrical neutrality, the lattice generates oxygen vacancies (OV). These vacancies host trapped electrons that are the primary drivers of the dark reaction. Furthermore, Ni acts as a transition metal center that can facilitate electron transfer more efficiently than undoped BaSnO₃.

3.17.2. Step-by-step ROS generation mechanism

Following the pathway for Ni-doped BaSnO₃can be elaborated as follows:

Superoxide radical (˙O₂⁻) formation.

Dissolved oxygen from the aqueous solution adsorbs onto the surface of the catalyst at the vacancy sites. The excess electrons (e⁻) within these vacancies reduce the oxygen molecules.

O₂ + e⁻ → O₂⁻ 1

Formation of hydroperoxyl radicals (˙HO₂).

The newly formed superoxide radicals are unstable in water and quickly react with water molecules or protons (H⁺) to generate hydroperoxyl radicals and hydroxyl ions.

H₂O + ˙O₂⁻ → ˙HO₂ + OH⁻ 2

3.17.3. Evolution of hydrogen peroxide (H₂O₂)

Two hydroperoxyl radicals undergo a disproportionation reaction (they combine and rearrange) to form hydrogen peroxide.

2˙HO₂ → H₂O₂ + O₂ 3

3.17.4. Hydroxyl radical (˙OH) production

This is the critical final step. The H₂O₂ reacts with another superoxide radical to produce the hydroxyl radical—the most potent oxidant in the system.

H₂O₂ + ˙O₂⁻ → O₂ + OH⁻ + ˙OH 4

3.17.5. Degradation of azo dye

The ROS generated (˙O₂⁻ and ˙OH) migrate to the dye molecules adsorbed on the catalyst surface.

(1) Azo bond attack: the ˙OH radicals attack the chromophore (the –N N-azo bond), which is responsible for the dye's color.

(2) Ring opening: subsequent attacks break down aromatic rings into smaller organic acids.

(3) Mineralization: finally, the molecules are converted into harmless end products:

Azo dye + ROS (˙OH, ˙O₂⁻) → intermediates → CO₂ + H₂O 5

The Reactive Oxygen Species (ROS), superoxide (˙O₂⁻) and hydroxyl (˙OH) radicals generated at the Ni-doped BaSnO₃ migrate to the surface and react with the adsorbed dye molecule on the catalyst's active sites. The degradation process begins with N-demethylation, where the ˙OH radicals attack the methyl groups attached to the nitrogen atoms. As these methyl groups are removed, the characteristic deep color of the solution fades and degradation proceeds further leading to non-toxic products.

Dyes + ROS → demethylated products → organic acids → CO₂ + H₂O + NO₃⁻ 6

4. Conclusion

In summary, this study successfully developed an oxygen-vacancy-engineered Ni-doped BaSnO₃ system designed for the highly efficient catalytic degradation of azo dyes under both dark and ambient light conditions. By employing a high-temperature solid-state synthesis method, nickel was incorporated into the BaSnO₃ lattice at various concentrations (up to 20%), which strategically induced the formation of electron-rich oxygen vacancies. These vacancies serve as critical catalytic centres that facilitate the generation of reactive oxygen species (ROS), such as superoxide (˙O₂⁻) and hydroxyl (˙OH) radicals, by reacting with dissolved oxygen even in the complete absence of light. The optimized catalyst, BaSn_0.8Ni_0.2O₃ (BSN2), demonstrated exceptional performance, achieving ultrafast degradation of several toxic pollutants, including methyl orange (88.6%), acid orangeII (91.2%), amaranth (91.2%), congo red (98.4%) and crystal violet (97.6%) within short durations. The mineralization of these dyes was confirmed through TOC analysis, which showed significant removal efficiencies. This research highlights the significant potential of defect-engineered perovskite oxides as round-the-clock catalysts, offering a promising and energy-efficient solution for continuous environmental remediation for wastewater treatment.

Author contributions

Methodology, investigation, data curation, formal analysis, writing – original draft, R. S. conceptualization, supervision, visualization, writing – review & editing, funding acquisition, project administration, V. R.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6ra04754h.