A facile green approach to the synthesis of Bi₂WO₆@V₂O₅ heterostructure and their photocatalytic activity evaluation under visible light irradiation for RhB dye removal

2.1 Synthesis of bismuth tungsten oxide (Bi₂WO₆)

Bismuth tungsten oxide nanoparticles were synthesized using the green method using Bismuth Nitrate, Azadirachta indica, and Sodium tungsten oxide. 20 g of delicately chopped Azadirachta indica leaves were added to 100 ml of water, boiled for 2 h at 100 °C, and filtered with the help of filter paper. A finely filtered solution has been obtained. 2.91 g of Bi(NO₃)₃·5H₂O and 0.99 g of Na₂WO₄·2H₂O were dissolved in 30 ml of green extract and stirred at room temperature for 20 min. A dark, homogeneous solution has been obtained. Finally, 30 ml of the homogenous solution was transferred into a Teflon-lined 50-mL autoclave and kept at 120 °C for 8 h. The processed samples were collected by centrifugation after the autoclave had cooled to room temperature, rinsed twice with ethanol and distilled water, and then dried for 12 h at 80 °C in a drying oven. Similarly, the same procedure has repeated afterward for the preparation of Bi₂WO₆ at 140 °C. The reaction equation is given below. 4Bi(NO₃)₃·5H₂O + 2Na₂WO₄·2H₂O + 2H₂O Bi₂WO₆ + 4NaOH + 12NO₂ + 3O₂

The schematic diagram is shown in Fig. 2.

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Fig. 2

Schematic diagram of synthesis of Bi₂WO_6.

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2.2 Synthesis of vanadium pentoxide (V₂O₅)

Vanadium pentaoxide (V₂O₅) was made from ammonium metavanadate using Triton X-100 as a precursor (NH₄VO₃). 19.9 g of NH₄VO₃ (BDH chemicals, England) was dissolved in 999.9 ml of Azadirachta indica leaf solution, then 0.2 g of Triton X-100 (scintillation grade) was added, and the emulsification was completed by heating at 90 °C for 60 min. To acidify the VO₃ solution, concentrated HNO₃ (35 percent, Sigma–Aldrich) was added drop by drop. The brownish-black precipitates had aged overnight. The pH of the solution was adjusted through repeated washings with water, and the last remnants on the surface were removed through washing with ethanol. The filtrates were dried for the full night at 100 °C in a vacuum chamber. Yellowish-brown V₂O₅ was created by calcining the material at 500 °C for 5 h in a furnace at a temperature with a cooling and heating rate of 25 °C/min.

2.3 Synthesis of heterostructure (Bi₂WO₆ @V₂O₅)

Synthesized vanadium pentoxide, bismuth tungsten oxide, and Azadirachta indica extract were used to synthesise heterostructures. 0.55 g of V₂O₅ were added to 30 ml of green extract in a beaker and stirred continuously at 26 °C; then, 2.09 g of Bi₂WO₆ nanoparticles were added to it. The dark, greenish solution has been obtained. Two samples of heterostructures (V₂O₅@Bi₂WO₆) have been prepared using Bi₂WO₆ (synthesized at 120 °C and 140 °C) and V₂O₅. After that, fill another beaker of 500 ml up to 100 ml with water and place on a hot plate at 80 °C. A beaker of the prepared solution was kept in the beaker of 500 ml for 3–4 h, and a very thick solution was obtained. The solution was dried at 80 °C in the oven for 4–5 h. Finally, heterostructures have been obtained. At the end of the experiments, five samples of Bi₂WO₆ were prepared at 120 °C and 140 °C. At 120 °C and 140 °C, V₂O₅ and their heterostructures Bi₂WO₆@V₂O₅ were obtained.

2.4 Characterization

An X-ray diffractometer (Rigaku Diffractometer) with X-ray wavelength (=0.154 nm) is used to look at the structural characteristics of NPs. Nicolet Instrument Corp., Madison, Wisconsin, USA, provided the Nicolet Nexus 870 FT-IR spectrometer having a detector of deuterated triglycine sulphate for FTIR investigation. An FTIR spectrophotometer simultaneously gathers high-resolution spectral information over a broad spectrum of wavelengths. The material’s morphology was revealed by scanning electron microscopy (SEM, Nova-400). A UV2102PC spectrophotometer was used to examine the optical characteristics.

2.5 Photocatalytic test

The target pollutant compounds were the pollutant dye molecules Rhodamine B (RhB). The photocatalytic experiment was performed in a beaker using a 300 W Xe light with a 400 nm cutoff filter as the visible light source. 50 ml of RhB dye solution was mixed with 20 mg of photocatalysts produced. 2 gL^-1 of catalyst load and a RhB concentration of 10 mgL^-1 were selected. The mixture was subjected to 30 min of magnetic stirring in the dark to reach an adsorption–desorption equilibrium. A transparent dye solution was created by centrifuging 4 ml of the suspensions, after which the initial concentration, C_o, was measured using a UV2102PC spectrophotometer. The Xe bulb was turned on after that. When light photons reached the suspension's surface, the photocatalytic process was started. The solution was constantly agitated with a magnetic stirrer throughout this operation. To quantify the concentration of RhB dye following light degradation, samples were taken from the slurry mixture (generally labelled as C). (C_o-C)/C_o was used to compute the deterioration efficiency. Where C_o denotes zero time and C represents experiment time (Wang, 2014).

3.1 XRD analysis

XRD examines the crystalline properties of V₂O₅, Bi₂WO₆, and Bi₂WO₆@V₂O₅ nanoparticles. The peaks of V₂O₅ at 2θ = 15.3°, 20.3°, 21.7°, 26.1°, 31°, and 32.4° with the structural plane (2 0 0), (0 1 0), (1 1 0), (4 0 0), (0 1 1), and (1 1 1) indicate the orthorhombic structure of V₂O₅, according to ICDD No. 96–101-1292, space group Pmn21 (Zhang, 2021). The characteristic diffraction peaks at 2θ = 19.04° (0 2 1), 28.08° (1 3 1), 32.21° (0 6 0), 33.89° (6 0 0), 38.7° (2 2 1), 50.75° (4 2–1), 48.74° (2 2 2), and 59.46° (12 0–5), etc., indicate the orthorhombic structure of Bi₂WO₆ according to (PDF card no. 79–2381). The peaks mentioned above are present in the heterojunctions' XRD patterns, but there is no trace of any secondary impurity phases. In heterostructures, peaks of Bi₂WO₆ and V₂O₅ are observed. No peaks related to BiO, BiV, etc., indicate the formation of a pure heterostructure of Bi₂WO₆@V₂O₅. The peak intensity of heterostructured Bi₂WO₆@V₂O₅ is increased, which shows that V₂O₅ and Bi₂WO₆ interact with each other, confirming that the heterostructure's crystallinity is improved. The high peak intensity of the heterostructure might be because the crystal nucleation center’s preferred V₂O₅ owing to the aromatic system, and then a heterostructure with a larger particle size than Bi₂WO₆ was formed (Fig. 3). Because the peak power of pure Bi₂WO₆ at 140 °C is low, the heterostructure synthesized at 140 °C had a low peak intensity.

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Fig. 3

XRD pattern of V₂O₅, Bi₂WO₆ and V₂O₅ @Bi₂WO₆ at 120 °C and 140 °C.

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3.2 FTIr

The FTIR spectra of synthesized nanostructures are shown in Fig. 4. The FTIR peaks of Bi₂WO₆ at 627.67 cm⁻¹ and 902.18 cm⁻¹ are responsible for the vibrations of bending and stretching for W–O–W and O–W–O in WO₃. For particular, the location of the W-O stretching vibrations is 644 cm⁻¹. W–O extending, Bi–O stretching, and W–O–W bridging stretching modes related to the Bi − O band positioned at 844.8 cm⁻¹ (Phuruangrat, 2014). The W = O stretching vibration is related to the peak at 950.98 cm⁻¹ (Najafi-Ashtiani, 2018). FTIR peaks of V₂O₅ are observed at 825 cm⁻¹ correspond to V–O–V stretching mode (Surya Bhaskaram et al., 2016) and at 724 cm⁻¹ and 742 cm⁻¹ show the symmetric and asymmetric stretching vibrations of V–O (Geng et al., 2014). According to research, the triple coordinated oxygen bonds (chain oxygen) asymmetric stretching vibrations and double coordinated oxygen bonds (bridge oxygen) vibration are responsible for the peak at 611 cm⁻¹ (Wu et al., 2015). The bridge V-O-V stretching is attributed to bands ranging from 700 to 900 cm⁻¹. The bands appearing at 957 cm⁻¹ are attributed to vanadly stretching modes (δ V-O) (Farahmandjou and N.J.J.o.N.R. Abaeiyan, , 2017). For heterostructures of V₂O₅@Bi₂WO₆, the peaks are observed at 627 cm⁻¹, 617 cm⁻¹, and 950.98 cm⁻¹ attributed to the vibrations of bending and stretching for O–W–O in WO₃ and W = O stretching vibration, respectively, which confirmed the presence of Bi₂WO₆. The other peaks observed at 825 cm⁻¹, 742 cm⁻¹, and 957 cm⁻¹ are attributed to the corresponding V–O–V stretching mode, the symmetric and asymmetric stretching vibrations of V–O and a vanadly stretching methods (δ V-O), respectively, which confirmed the presence of V₂O₅. There are no other impurity peaks observed. The peak intensities of heterostructured V₂O₅@Bi₂WO₆ at 120 °C are high, which shows that V₂O₅ and Bi₂WO₆ interact with each other, confirming the increased crystallinity of heterostructure.

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Fig. 4

FTIR analysis of V₂O₅, Bi₂WO₆ at 120 °C and 140 °C,V₂O₅ @Bi₂WO₆ at 120 °C and 140 °C.

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3.3 Scanning electron microscopy (SEM)

Like crystal structure, morphology is an essential part of nanoparticles that affects the dye's degradation efficiency. The morphological features of hydrothermally synthesized V₂O₅, Bi₂WO₆ at 120 °C, Bi₂WO₆ at 140 °C, V₂O₅@Bi₂WO₆ at 120 °C, and V₂O₅@Bi₂WO₆ at 140 °C nanostructures are presented in Fig. 4.

Fig. 5 represents the morphology of V₂O₅, which is composed of rod-like structures. These structures are produced by the supersaturated reactant solution process, in which tiny crystallised nuclei develop in the solution and expand until they take on the final rod-like shape. Due to improved photo-generated electron transport and collection, this shape is very effective at dye degradation (Lu, 2019). Fig. 5 shows the morphological pictures of Bi₂WO₆ samples obtained at 120 °C and 140 °C, respectively. Microspheres that resemble flowers are ready when the reaction temperature reaches 120 °C. The hierarchical self-assembly method is used to create these microspheres from thin nanosheets. A three-step process is suggested for the creation mechanism of the flower-like Bi₂WO₆ superstructures in Fig. 6 in light of the facts previously described. According to studies, even if the formation of smaller crystallites is kinetically favourable, giant crystallites are thermodynamically favourable during the first accumulation. Consequently, many nanoparticles self-aggregate into microparticles that resemble spheres during the first stage of the hydrothermal process (step 1). In order to help the transition from solid spherical microparticles to superstructure, nanoparticles continue to aggregate into spherical microparticles as hydrothermal time (2–4 h) extends (step 2 in Scheme 1). The following scenarios could account for the origin of morphological evolution. In step 2, layers that resemble perovskite (Bi₂O₂)_n²ⁿ⁺ and (WO₄)_n^2n– are alternated to form the orthorhombic Bi₂WO₆ crystal. The smaller crystallites within the solid spherical microparticles eventually dissolve due to the difference in concentration between the surface and solution, and following hydrothermal treatment, Bi₂O₂²⁺ & WO₄^2- ions are generated in the acidic aqueous solution. Numerous tiny protuberances can be seen on the surface of the solid microsphere, providing a wealth of high-energy locations for the growth of nanocrystalline structures. Due to the sphere shape of the microparticles, the Bi₂O₂²⁺ and WO₄^2- ions in the solution preferentially transfer to their faces and spontaneously form on the microscopic protuberances. Ostwald ripening, a dissolution–recrystallization phase, is a frequent occurrence in the crystal formation process (step 2).

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Fig. 5

SEM images of V₂O₅, Bi₂WO₆ & V₂O₅ @Bi₂WO₆ at 120 °C & 140 °C.

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Fig. 6

The formation mechanism of flower-like structure (Zhang, 2007).

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Bi₂WO₆ nanocrystals also prefer to form single crystals under hydrothermal conditions and develop into two-dimensional, plate-like structures because of their strong intrinsic anisotropy. Mass diffusion and Ostwald ripening cause solid spherical microparticles to become exhausted as the hydrothermal time increases, which causes a substantial amount of Bi₂WO₆ nanoplates to be produced. Contrary to published theories, which rely on hydrophobic attraction, emulsified soft or complicated templates, or both, geometric constraints of building blocks should have been a major factor in the formation of a spherical shape. Additionally, the orientated nanoplates serve as the foundation for the flower-like Bi₂WO₆ superstructures. A spherical superstructure would naturally result from the lateral contact of simple arrays of the nanoplates, which can quickly develop a curvature. Consequently, in situ-generated nanoplates finally self-organize to form Bi₂WO₆ superstructures that resemble flowers (Step 3) (Zhang, 2007). The mechanism of flower like structure is explained in Fig. 6.

Bi₂WO₆ has an altered, well-defined, and more aggregated particle size when the synthesis temperature is raised to 140 °C. In order to reduce structural flaws, the flower-like structure is converted into high-grain-size particles (Cui, 2016). SEM was used to analyse the morphology of V₂O₅ @Bi₂WO₆ at 120 °C and 140 °C, respectively. The synthesis of heterostructure (V₂O₅@Bi₂WO₆ at 120 °C) is confirmed by an enlarged SEM image of V₂O₅@Bi₂WO₆ giving microflowers that are haphazardly obsessed with nanorods. Rod-like nanostructures emerge within larger grains in the heterostructure at 140 °C. The creation of a heterostructure (V₂O₅@Bi₂WO₆ at 140 °C) was confirmed by the existence of nanorods and bigger grains. Additionally, both structures are effective at reducing dye degradation (Li, 2017).

3.4 UV visible analysis

The UV–visible spectroscopy is used to measure the bandgap of samples, as shown in Fig. 7.

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Fig. 7

Band gap of V₂O₅, Bi₂WO₆ and V₂O₅ @Bi₂WO₆ at 120 °C and 140 °C.

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The minimum amount of energy required to liberate an electron trapped in a bound state and enable conduction is known as the band gap of a semiconducting material. It is essential to study the transition of an electron. Tauc proposed the following relation for calculating band gap energy (E_g) as (Khan, 2017):

Where $\alpha$ is an absorption co-efficient, hv is photon energy, and B is a parameter of transition probability. Eg of V₂O₅ and Bi₂WO₆ at 120 °C and 140 °C, V₂O₅ and Bi₂WO₆ at 120 °C and 140 °C, respectively, are 2.6 eV, 2.65 eV, 2.74 eV, 2.32 eV, and 2.73 eV. At 120 °C and 140 °C, for example, V₂O₅ and Bi₂WO₆ are smaller than single Bi₂WO₆ and larger than V₂O₅. Due to the attraction of Bi₂WO₆ and V₂O₅, energy bands are formed between CB and VB, and the E_g of layered materials is usually reduced as the number of layers increases, with only a few omissions (Chaves, 2020). V₂O₅ and Bi₂WO₆ are both 2D layer materials. Heterostructures have more layers because the layers interact with each other. As a result, decreasing Eg is accomplished by adding more layers. The band gap of the heterostructure, which can be formed when materials are in contact with one another, is also impacted by the electric field. V₂O₅ and Bi₂WO₆′s CB layers are very close to one another. Electrons move from the semiconductor with the higher Fermi level to the other in order to balance this transfer, generating an electric field in the process (Tsymbal, et al., 2012).

The relation between a material's optical constants and how light affects a material's properties is explained. The most significant optical constants that may be computed using UV–vis spectroscopy are the dielectric constant ( $\epsilon$ ), ordinary index of refraction (n), electronegativity (ΔX*), extinction coefficient (k), and ordinary index of refraction (n).

3.4.4

3.4.4 Dielectric constant

The dielectric constant ( $\epsilon$ ) that has an inverse relationship with the polarizability of solid materials, is used to determine the polarity of a medium. The following definition gives the “ $\epsilon$ ” in terms of imaginary and real components:

(6)

$$\epsilon ={\epsilon }_r+{\epsilon }_i$$ where the following is how the “ ${\epsilon }_i$ ” and “ ${\epsilon }_r$ ” components of the dielectric constant are expressed (Amjad, 2022):

(7)

$${\epsilon }_r=n^2-k^2$$

(8)

$${\epsilon }_i=2nk$$

The light’s speed is explained by its real component, which details how much it slows down in a matter. A material's dielectric properties, or the electric field via which energy is absorbed as a result of dipole motion, are described in the imaginary part of the equation. The loss factor is determined by the ratio of ε_i and ε_r. Both the imaginary (ε_i) and real (ε_r) components of the dielectric constant are significantly influenced by photon energy (hv). The calculated ε_r for V₂O₅ and Bi₂WO₆ at 120 °C and 140 °C are 0.53, 0.50, 0.45, 0.72, and 0.45, respectively, and the calculated ε_i are 10.94, 10.86, 10.71, 11.44, and 10.72, respectively, as shown in Fig. 8. The term “decreased real part” in this context refers to a reduction in the speed of light in a medium. As a result, there is significant light scattering. The imaginary component is also an increase, which refers to a rise in light absorption. Among all samples, V₂O₅@Bi₂WO₆ at 120 °C has the highest value of ε_r, which refers to the generation of a large number of electron and hole pairs due to the high absorption of light in the material. The values of ε_r are greater than ε_i in all samples, despite this, suggesting that the values of ε_r and ε_i are changed by raising the concentrations of the various materials. (Khan, 2022). Table 1 gives the calculated optical properties.

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Fig. 8

Refractive index, extinction coefficient, optical electronegativity and dielectric constant (real & imaginary) of V₂O₅, Bi₂WO₆ and V₂O₅ @Bi₂WO₆ at 120 °C and 140 °C.

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Table 1 Summary of calculated optical parameters of samples.

Samples	Eg	n	k	ΔX*	εr	εi
V2O5	2.6	2.47	2.20	0.69	0.53	10.94
Bi2WO6 (120 °C)	2.65	2.46	2.20	0.71	0.50	10.86
V2O5 @Bi2WO6 (120 °C)	2.32	2.57	2.21	0.62	0.72	11.44
Bi2WO6 (140 °C)	2.74	2.43	2.20	0.73	0.45	10.71
V2O5 @Bi2WO6 (140 °C)	2.73	2.43	2.20	0.73	0.45	10.72

3.5 Photocatalytic performance

Fig. 9 shows the comparison of RhB dye molecule clearance against the irradiation period. Electrons from the VB are released from V₂O₅ and travel towards the CB to form electron-hole pairs when exposed to sunlight with an energy level equivalent to its Eg. Oxygen and electrons combine to form hydroxyl-free radicals. For oxidation, one can rely on cavitation's capacity to carry out a direct oxidation process or mix with water to form hydroxyl-free radicals. The relationship below explains this mechanism (Liu, 2015). (See Fig. 10).

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Fig. 9

Photocatalytic activity procedure.

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Fig. 10

Photocatalytic degradation of rhb by using v₂O₅, Bi₂WO₆ and V₂O₅ @Bi₂WO₆ at 120 °C and 140 °C.

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Bi₂WO₆ is a superior photocatalytic substance, much like V₂O₅. The band gaps of the synthesised Bi₂WO₆ at 120 °C and 140 °C were 2.65 eV and 2.74 eV, respectively. These Bi₂WO₆ decompose RhB dye very effectively and exhibit excellent light sensitivity in the visible region. RhB dye degradation efficiency was observed to be 81 % and 68 %, respectively, for materials produced at 120 °C and 140 °C. Given that this sample has strong crystallinity and shape, the efficiency at 120 °C is high. Its morphology consists of microspheres that resemble flowers, and these microspheres are made from thin nanosheets via a hierarchical self-assembly method that produces sharp edges. Flower-like microspheres are unusual in aquatic photocatalytic activity due to their fundamental characteristics, such as shorter photo-generated diffusion route distances, more efficient incidental light absorption due to excessive dispersion, simple settling, and a wide surface area. Due to the lower recombination rate, these materials' efficiency can be increased by exploiting their heterostructures (Tonda, 2018). Type II heterostructures are produced by Bi₂WO₆ and V₂O₅. Both materials' band gaps are in the visible range and span a wide range of light. Additionally, because interfaces and heterojunctions between V₂O₅ and Bi₂WO₆ occur in the heterostructures, the recombination rate is decreased. As seen in Fig. 9, the heterojunction between V₂O₅ and Bi₂WO₆ is essential for separating electron-hole pairs during photocatalysislight. Additionally, because interfaces and heterojunctions between V₂O₅ and Bi₂WO₆ occur in the heterostructures, the recombination rate is decreased. As seen in Fig. 9, the heterojunction between V₂O₅ and Bi₂WO₆ is essential for separating electron-hole pairs during photocatalysis. The number of heterojunctions is correlated with higher photocatalytic activity.

However, as shown in Fig. 9, the conduction and valence band boundaries of V₂O₅@Bi₂WO₆ and these two materials are close to one another in heterostructures. Light that has been exposed to the electrons in V₂O₅@Bi₂WO₆ jumps from VB to CB, leaving holes behind in VB. From the CB of V₂O₅ to the CB of Bi₂WO₆, electrons move downward and are caught by oxygen reduction, which produces superoxide radicals, which in turn produce oxygen and hydrogen peroxide (H₂O₂). Hydrogen peroxide is produced when the residual holes in VB undergo oxidation in the presence of water (H₂O₂). In addition, H₂O₂ breaks into two highly reactive hydroxyl radicals (OH•). OH• oxidizes organic contaminants. This substance is reduced, hence its strong photocatalytic activity.

At 120 °C, V₂O₅@Bi₂WO₆ exhibits the maximum degrading efficiency of 96.5 %, while at 140 °C, it is only 87 %. Because Bi₂WO₆ has flower-like structures and V₂O₅ has particle-like structures, the two materials combine at 120 °C to form V₂O₅@Bi₂WO₆. As previously mentioned, flower-like structures have strong photocatalytic activity. This material exhibits significant photocatalytic activity because the recombination rate is decreased in heterostructure.

To evaluate the effect of individual species (such as holes (h⁺), electrons (e^-), hydroxyl radicals (^•OH)) in the degradation of dye during the advanced oxidation process, the scavenging studies were performed. For scavenging studies, EDTA, 2-propanol, and AgNO₃, which scavenge the h⁺, ^•OH, and e^- species, respectively. This study is useful to appraise the effect of these oxidizing and reducing agents because these are the major species, which are product during the advanced oxidation process for the redox reactions. The findings revealed that the dye removal was different for scavengers as well as for the blank (without the scavenger) (Fig. 11). In case of AgNO_3, EDTA and 2-propanol, the dye removal was observed to be 85.5, 44.5 and 23 (%), respectively. In case of blank, the dye removal was 96.5 % and in case of scavenging the h⁺, ^•OH, and e^- species, the dye degradation rate of reduced, which indicates that these species have imperative role in the degradation of dye. Based on scavenging analysis, it was observed the ^•OH was the main specie, which is responsible for the oxidative removal of dyes followed by the h⁺ and e^-.

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Fig. 11

Dye removal in the presence of different scavengers.

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