New BaTi_0.96Cu_0.02X_0.02O₃ (X = V, Nb) Photocatalysts for Dyes Effluent Remediation: Broad Visible Light Response

Abstract

The problem of industrial dyes depollution has pushed the scientific research community to identify novel photocatalysts with high performance. Herein, new photocatalysts composed of BaTiO₃, BaTi_0.96Cu_0.04O₃, BaTi_0.96Cu_0.02V_0.02O₃ and BaTi_0.96Cu_0.02Nb_0.02O₃ powders were prepared by solid-state reaction. The structural analysis of the samples confirmed the formation of the BaTiO₃ structure. The splitting of (002) and (200) planes verified the formation of the tetragonal phase. The XRD peaks shifted, and the unit cell volume expansion verified the substitution of the Ti⁴⁺ site by Cu²⁺, V⁴⁺ and Nb⁵⁺ ions. The morphological measurements showed that the addition of (Cu, V) and (Cu, Nb) ions changes the particles’ morphology of BaTiO₃, reducing its grains size. After the incorporation of (Cu, V) and (Cu, Nb) ions, the band gap of BaTiO₃ was reduced from 3.2 to 2.84 and 2.72 eV, respectively. The modification of BaTiO₃ by (Cu, Nb) ions induced superior photocatalytic properties for methyl green and methyl orange with degradation efficiencies of 97% and 94% during 60 and 90 min under sunlight irradiation, respectively. The total organic carbon results indicated that the BaTi_0.96Cu_0.02Nb_0.02O₃ catalyst has a high mineralization efficiency. In addition, it possesses a high stability during three cycles. The high photodegradation efficiency of Bi_0.96La_0.02Gd_0.02FeO₃ was related to the wide-ranging visible light absorption.

1. Introduction

Human, animals, plants and aquatic life need a clean environment and pure water supply. Many industrial activities discharge large quantities of organic dyes pollutants into water resources [1,2]. These dyes are non-biodegradable, very hazardous and carcinogenic and can cause harm to all organisms even at low concentrations [3]. The paper, textile, paint, nylon, silk, cosmetics, plastics and leather industries are the main sources of dyes waste with nearly 200,000 tons of these organic dyes being dumped into water sources annually [4]. Photocatalysis is a potential green method to solve the dyes pollution problem through using a light energy source (UV or visible light) and active photocatalysts [1,2,3,4,5]. Hydrogen production by the photocatalytic water-splitting technique has been considered as a promising methodology to produce renewable energy [6]. In the photocatalysis process, a suitable light energy was used to activate the particles of the catalysts to produce charge carriers (electron–hole pairs) which later yielded very reactive radicals such as O₂^•− and ·OH species [4,7]. These reactive radicals can attack the organic dyes molecules and mineralize them to non-harmful products (CO₂ and H₂O) [4,5,6,7,8]. The dyes depollution issue pushes the scientific research community toward the synthesis of low-cost, stable, non-toxic and highly efficient photocatalysts [9,10,11]. Numerous photocatalytic studies have been carried out on metal oxides, sulfides and perovskite materials like ZnO [12], TiO₂ [13], CuS [14] ZnS [15], Fe₂O₃ [16], WO₃ [5], g-C₃N₄ [17] and BaTiO₃ [18]. BaTiO₃-based materials were candidates as promising catalysts for the photodegradation of organic contaminants and bacterial disinfection in wastewater [19,20]. The perovskite BaTiO₃ semiconductor has a wide band gap energy varying from 3.2 to 3.4 eV [21]. Engineering the band gap energy of the BaTiO₃ material to absorb the visible light spectrum is practically useful for the photocatalytic applications [22]. Bhat et al. [22] studied the effect of Rh doping on the photocatalytic activity of BaTiO₃ material for the degradation of methylene blue dye (10 mg/L) under visible light irradiation. They found that 0.5 mol% Rh doping enhances the photocatalytic activity of BaTiO₃ toward methylene blue dye with a photodegradation efficiency of 96% during 120 min of irradiation. The Mn-doped BaTiO₃ nanotube as a photocatalyst exhibited improved visible light photocatalytic activity for methylene blue dye over 300 min [23]. The photodegradation properties of the 5% Ag-doped BaTiO₃ catalyst was investigated for 20 mg/L rhodamine B dye under visible light irradiation [24]. After 105 min, this catalyst has shown a visible light photodegradation efficiency of 79%. Cu-doped BaTiO₃ revealed photocatalytic activities of 98.2% for 10 mg/L methylene blue dye during 120 min and 99.4% for 10 mg/L rose Bengal dye within 45 min [18]. Amaechi et al. [25] showed that the 2% Fe³⁺-doped BaTiO₃ nanocatalyst has a sunlight photodegradation efficiency of 75% for 20 mg/L methyl orange. Based on our knowledge, there is no available study on the influence of (Cu, V) and (Cu, Nb) codoping on the photodegradation properties of BaTiO₃ for methyl green and methyl orange pollutants under natural sunlight and Xenon lamp photoreactor irradiation. The Cu, V and Nb elements as dopants have the ability to engineer the visible light response of BaTiO₃. These dopants possess variable oxidation states which can inhibit the recombination of the charge carriers (electron–hole pairs) through acting as trap centers. Herein, new catalysts composed of BaTiO₃, BaTi_0.96Cu_0.04O₃, BaTi_0.96Cu_0.02V_0.02O₃ and BaTi_0.96Cu_0.02Nb_0.02O₃ powders were prepared by the solid-state reaction method. The structural, morphological and optical properties of the synthesized samples were investigated and discussed. In addition, the photocatalytic properties of BaTiO₃, BaTi_0.96Cu_0.04O₃, BaTi_0.96Cu_0.02V_0.02O₃ and BaTi_0.96Cu_0.02Nb_0.02O₃ powders for the degradation of methyl green and methyl orange dyes were measured under natural sunlight and Xenon lamp photoreactor irradiation. The obtained results indicated that the BaTi_0.96Cu_0.02Nb_0.02O₃ catalyst possesses high sunlight photodegradation and mineralization efficiencies for 20 mg/L methyl green and methyl orange dyes after 60 and 90 min, respectively. In addition, the BaTi_0.96Cu_0.02Nb_0.02O₃ catalyst exhibits a high photodegradation stability for both dyes during three cycles under sunlight.

2. Results and Discussion

2.1. X-ray Diffraction (XRD) Analysis

Figure 1 demonstrates the X-ray diffraction patterns of the pure BaTiO₃, BaTi_0.96Cu_0.04O₃, BaTi_0.96Cu_0.02V_0.02O₃ and BaTi_0.96Cu_0.02Nb_0.02O₃ powders within a 2-theta angle of 10–80°. The pure BaTiO₃ sample exhibits nine X-ray diffraction peaks at 2θ values equal to 22.401°, 31.722°, 39.101°, 44.69°, 45.421°, 51.112°, 56.299°, 66.031° and 75.102°, which were indexed to (100), (110), (111), (002), (200), (201), (211), (220) and (310) crystallographic planes of a tetragonal BaTiO₃ structure (standard JCPDS, card file No. 05-0626, P4/mmm space group). The compositions of BaTi_0.96Cu_0.04O₃, BaTi_0.96Cu_0.02V_0.02O₃ and BaTi_0.96Cu_0.02Nb_0.02O₃ powders display analogous X-ray diffraction peaks to that of the pure BaTiO₃ sample, indicating also the formation of a tetragonal BaTiO₃ structure. For all samples, the splitting of (002) and (200) planes within 2θ = 44–46° (inset Figure 1) verifies the formation of the tetragonal phase and ruled out the existence of a cubic BaTiO₃ phase (both planes combine in one peak in case of a cubic BaTiO₃ structure) [26]. As presented in

Table 1. Crystallite size D, lattice parameters (a, b, c), c/a ratio, unit cell volume (V) of pure BaTiO₃, BaTi_0.96Cu_0.04O₃, BaTi_0.96Cu_0.02V_0.02O₃ and BaTi_0.96Cu_0.02Nb_0.02O₃ samples.

Samples	D (nm)	a Å	b Å	c Å	c/a	V Å3
BaTiO3	71	3.9987	3.9987	4.0111	1.0031	64.1359
BaTi0.96Cu0.04O3	61	4.0002	4.0002	4.0174	1.0038	64.2861
BaTi0.96Cu0.02V0.02O3	58	4.0008	4.0008	4.0143	1.0033	64.2535
BaTi0.96Cu0.02Nb0.02O3	56	4.0007	4.0007	4.0150	1.0035	64.2639

, the ratio of (c/a) supported the formation of the tetragonal phase for all samples. The absence of any impurities can be deduced from the non-existence of any other X-ray diffraction peaks in these patterns. The X-ray diffraction peaks of all samples have high intensities, which reflect the good crystallinity. The influence of the incorporation of Cu, (Cu, V) and (Cu, Nb) ions on the 2θ angle of the (110) and (111) planes of the BaTiO₃ structure is illustrated in Figure 2. It can be seen that both planes of BaTiO₃ were shifted to a lower 2θ angle owing to doping and dual doping by Cu, (Cu, V) and (Cu, Nb) ions. The ionic radius of the Ba²⁺ site is 1.61 Å in the XII coordination, while that of the Ti⁴⁺ site is 0.605 Å In the VI coordination. The ionic radii of Cu²⁺, V⁴⁺ and Nb⁵⁺ ions in the VI coordination are 0.73 Å, 0.58 Å and 0.64 Å, respectively. The detected shifts to a lower 2θ angle suggested the replacement of the Ti⁴⁺ site (0.605 Å) by Cu²⁺, V⁴⁺ and Nb⁵⁺ ions of larger or close ionic radii. As shown in Figure 3 and Table 1, the calculations of the unit cell volume illustrate the expansion of the lattice parameters and unit cell volume of pure BaTiO₃ after the incorporation of Cu²⁺, V⁴⁺ and Nb⁵⁺ ions, confirming the effective substitution process. These increases in the unit cell volume of BaTiO₃ can be assigned to the larger ionic radii, especially Cu²⁺ and Nb⁵⁺ ions compared to Ti⁴⁺ site (0.605 Å). The average crystallite size of the BaTiO₃, BaTi_0.96Cu_0.04O₃, BaTi_0.96Cu_0.02V_0.02O₃ and BaTi_0.96Cu_0.02Nb_0.02O₃ samples were estimated by using the Williamson Hall equation as shown below [27]:

βcosθ = (Kλ/D) + 4εsinθ

(1)

where β represents the full width at half maximum (radian), θ is the angle of the peaks in radians, K is a constant (0.9), λ is the wavelength of the X-ray radiation, D is the crystallite size and ε is the microstrain. By plotting 4sinθ (X-axis) with βcosθ (Y-axis), the average crystallite size can be estimated from the intercept of the fitting line, while the microstrain can be found from the slope. As illustrated in Figure 4, the average crystallite size of pure BaTiO₃, BaTi_0.96Cu_0.04O₃, BaTi_0.96Cu_0.02V_0.02O₃ and BaTi_0.96Cu_0.02Nb_0.02O₃ powders were estimated to be 72, 62, 58 and 56 nm, respectively.

2.2. Scanning Electron Microscope (SEM) Analysis

The scanning electron microscope (SEM) images of the synthesized pure BaTiO₃, BaTi_0.96Cu_0.04O₃, BaTi_0.96Cu_0.02V_0.02O₃ and BaTi_0.96Cu_0.02Nb_0.02O₃ powders and the effect of dopants on morphology are shown in Figure 5. The SEM image of the pure BaTiO₃ sample reveals the synthesis of coagulated large grains, which possess an asymmetrical shape with an irregular edge. The Cu doping into the BaTiO₃ lattice somewhat reduces the size of the grains, but they are still coagulated together. In contrast, the addition of the dual dopants of (Cu, V) and (Cu, Nb) ions powerfully changes the morphology of BaTiO₃ and leads to the formation of uniform semispherical particles with reduced grain size. These results point out that (Cu, V) and (Cu, Nb) ions have a strong restriction effect on the growth of the BaTiO₃ grains during the calcination process at high temperature. The energy-dispersive X-ray (EDX) investigation was carried out to analyze the elemental composition of the synthesized samples, as demonstrated in Figure 6. The patterns of the energy-dispersive X-ray spectra of BaTi_0.96Cu_0.02V_0.02O₃ and BaTi_0.96Cu_0.02Nb_0.02O₃ powders verify the presence of Ba, Ti, O, Cu, V and Nb elements and ruled out the existence of any other impurity elements. The wt. % of the dopants elements, including Cu, V and Nb ions, is close with the added values during the synthesis process.

2.3. Optical Properties and Band Gap Energy

The effective photocatalysis process is strongly related to the optical properties and the band gap energy of the used catalysts. The diffuse reflectance (DR) analysis was carried out to investigate the optical properties of the pure BaTiO₃, BaTi_0.96Cu_0.04O₃, BaTi_0.96Cu_0.02V_0.02O₃ and BaTi_0.96Cu_0.02Nb_0.02O₃ catalysts within a wavelength of 200–2400 nm, as shown in Figure 7. The reflectance spectrum (%) of the pure BaTiO₃ catalyst has a high intensity (80–90%) within 400–2400 nm, while below 400 nm, the reflectance (%) intensely decreased in a sharp manner (absorption edge), which matches with the band gap energy absorption of the pure BaTiO₃ structure. The addition of Cu²⁺, V⁴⁺ and Nb⁵⁺ ions strongly reduces the intensity of the reflectance and also shifts the absorption edge to a longer wavelength (low energy). The diffuse reflectance spectra display that Cu, (Cu, V) and (Cu, Nb) ions enhance the visible light absorption capacity of the BaTiO₃ catalyst. The Tauc and Kubelka–Munk equations were used to estimate the band gap energy of pure BaTiO₃, BaTi_0.96Cu_0.04O₃, BaTi_0.96Cu_0.02V_0.02O₃ and BaTi_0.96Cu_0.02Nb_0.02O₃ catalysts as displayed below [28]:

α = A(hν − Eg)ⁿ/hν

(2)

F (R) = (1 − R)²/2R = α/S

(3)

In both equations, the α represents the absorption coefficient, hν is the photon energy, Eg is the band gap energy, F (R) is the Kubelka–Munk function, R is the reflectance and S is the scattering coefficient. As represented in Figure 8, the plot of [F(R) hν]² with hν (photon energy) gives the band gap energy of the BaTiO₃, BaTi_0.96Cu_0.04O₃, BaTi_0.96Cu_0.02V_0.02O₃ and BaTi_0.96Cu_0.02Nb_0.02O₃ samples. The measured value of the energy band gap of the undoped BaTiO₃ catalyst was identified to be 3.2 eV, which is in agreement with the published studies [29,30]. The valence band maximum (VBM) of the BaTiO₃ structure is principally composed of Ti 3d and O 2p states, while the conduction band minimum (CBM) basically consists of Ti 3d states [30]. Owing to the incorporation of Cu, (Cu, V) and (Cu, Nb) ions, the band gap energy of the BaTiO₃ structure was reduced to 2.99 eV (λ = 414 nm), 2.84 eV (λ = 436 nm) and 2.72 eV (λ = 455 nm), respectively. The results of the optical properties prove that the dopants-treated BaTiO₃ catalysts have high visible light absorption compared to the pure one. Owing to the insertion of Cu²⁺, V⁴⁺ and Nb⁵⁺ ions, the conduction band (CB) is possibly derived from Cu-3d, V-3d or Nb-4d and Ti-3d electrons, while the valence band (VB) mostly arises from Cu-3d, V-3d or Nb-4d plus O-2p and Ti-3d states. As a result, the intense reductions of the band gap energy of BaTi_0.96Cu_0.04O₃, BaTi_0.96Cu_0.02V_0.02O₃ and BaTi_0.96Cu_0.02Nb_0.02O₃ catalysts can be assigned to the presence of impurity states below the conduction band (CB) and above the valence band (VB) due to Cu²⁺, V⁴⁺ and Nb⁵⁺ ions and the formation of oxygen vacancies for charge neutrality [18,30,31]. The sunlight spectrum has nearly 52% infrared red radiation (700–2500 nm), 43% visible light rays (400–700 nm) and 5% ultraviolet wavelengths (280–400 nm) [32]. The optical characteristics prove the suitability of the engineered band gap BaTi_0.96Cu_0.02V_0.02O₃ and BaTi_0.96Cu_0.02Nb_0.02O₃ catalysts for photocatalytic applications under visible light irradiation. In addition, the presence of long absorption states (as illustrated in Figure 8) of BaTi_0.96Cu_0.04O₃, BaTi_0.96Cu_0.02V_0.02O₃ and BaTi_0.96Cu_0.02Nb_0.02O₃ catalysts support the wide-ranging visible light absorption of the treated samples.

2.4. Depollution Properties of the Photocatalysts

The photocatalytic depollution properties of BaTiO₃, BaTi_0.96Cu_0.04O₃, BaTi_0.96Cu_0.02V_0.02O₃ and BaTi_0.96Cu_0.02Nb_0.02O₃ catalysts toward polluted water containing methyl green and methyl orange were studied under natural sunlight and Xenon lamp photoreactor irradiation. Figure 9 shows the change in absorption spectra of methyl green (20 mg/L, catalyst dose 40 mg/100 mL) in the presence of pure BaTiO₃, BaTi_0.96Cu_0.04O₃, BaTi_0.96Cu_0.02V_0.02O₃ and BaTi_0.96Cu_0.02Nb_0.02O₃ catalysts during 60 min of natural sunlight irradiation. In a sequenced manner, the pure BaTiO₃ catalyst displays low decreases in the absorbance with time with a measured efficiency of 33% during this period. The reductions of the absorbance curve are more noticeable for the BaTi_0.96Cu_0.04O₃ catalyst, and the photodegradation activity value was estimated to be 67% in the same period. In case of BaTi_0.96Cu_0.02V_0.02O₃ and BaTi_0.96Cu_0.02Nb_0.02O₃ catalysts, the absorbance curve of methyl green dye was powerfully decreased with full degradation activity of 93% and 97%, respectively. The degradation performance of these catalysts was completely assigned to the photocatalytic process, since the adsorption capacity of all samples was nearly lower than 5% as illustrated in Figure 10. For kinetic study, the rate constant of the photodegradation reactions was estimated using the non-linear least squares analysis according to [1,2]:

$$\mathrm{A}=\mathrm{X}{\mathrm{e}}^{-\mathrm{K}\mathrm{t}}+\mathrm{E}$$

(4)

where A represents the polluted molecules calculated residual, X indicates the amplitude of the process, K symbolizes the pseudo-first-order rate constant and E signifies the endpoint. As shown in Figure 11, the measured photodegradation rate constants based on non-linear least squares analysis were 0.0065 (R² = 0.98), 0.016 (R² = 0.98), 0.03 (R² = 0.96) and 0.035 min⁻¹ (R² = 0.96) for BaTiO₃, BaTi_0.96Cu_0.04O₃, BaTi_0.96Cu_0.02V_0.02O₃ and BaTi_0.96Cu_0.02Nb_0.02O₃ catalysts, respectively.

Figure 12a–d show the measurements of the reusability performance, efficient reactive radicals, total organic carbon (TOC) and photodegradation efficiency of methyl green under Xenon lamp irradiation compared to natural sunlight irradiation for BaTi_0.96Cu_0.02Nb_0.02O₃ catalyst, respectively. In the reusability test, as shown in Figure 12a, during three repeated cycles, the BaTi_0.96Cu_0.02Nb_0.02O₃ catalyst revealed high efficiencies of 97, 95 and 94% toward 20 ppm methyl green solution. The high photodegradation activity of the BaTi_0.96Cu_0.02Nb_0.02O₃ catalyst reflects the high stability for multiple uses. To analyze the reactive radicals which have the main effects during the photodegradation reaction, four trapping agents [33] including silver nitrate (AgNO₃), isopropyl alcohol (IPA), triethanolamine (TEOA) and benzoquinone (BQ) were used to capture the electrons (e⁻), hydroxyl (·OH) radicals, holes (h⁺) and superoxide (O₂^•–) species, respectively, as clarified in Figure 12b. The photodegradation activity of the BaTi_0.96Cu_0.02Nb_0.02O₃ catalyst against 20 mg/L methyl green dye was obviously decreased after the addition of TEOA and BQ substances which verifies that the holes (h⁺) and superoxide (O₂^•–) species are the basic radicals in the photo-removal process. Figure 12c displays the measurement of the total organic carbon (TOC) analysis of 20 mg/L methyl green dye for BaTi_0.96Cu_0.02Nb_0.02O₃ catalyst under sunlight irradiation. The total organic carbon analysis was carried out to investigate the mineralization behavior of methyl green dye to CO₂ and H₂O through the photocatalysis process. The obtained results prove that the rate of the total organic carbon was decreased with the sunlight irradiation time, reaching a photo-removal value of 94% after 70 min, demonstrating the complete photodegradation of methyl green dye using the BaTi_0.96Cu_0.02Nb_0.02O₃ catalyst to CO₂ and H₂O (simple products). As a result, these findings point out that the BaTi_0.96Cu_0.02Nb_0.02O₃ catalyst has a high mineralization efficiency beside the decolorization reaction. Figure 12d shows the photodegradation activity of the BaTi_0.96Cu_0.02Nb_0.02O₃ catalyst toward 20 mg/L methyl green dye under direct sunlight and Xenon lamp irradiation. Under both irradiation sources, the photodegradation activity of the BaTi_0.96Cu_0.02Nb_0.02O₃ catalyst was 97 and 99%, indicating the robust photocatalytic characteristics under the free solar energy to remove the methyl green dye in 60 min.

The photodegradation of methyl orange pollutant (MO) as an anionic organic dye was investigated using the BaTi_0.96Cu_0.02Nb_0.02O₃ catalyst (best sample) under natural sunlight irradiation. Figure 13 illustrates the change in the absorption spectra of methyl orange dye (concentration = 20 mg/L, catalyst dose = 40 mg/100 mL) in the presence of the BaTi_0.96Cu_0.02Nb_0.02O₃ catalyst during 90 min of sunlight illumination. Gradual and notable decreases of the absorption curve of methyl orange dye with time were seen to reach the final photodegradation efficiency of 94%. As shown in Figure 14a, the plot of C_t/C₀ with time for methyl orange dye degradation decreased with the sunlight irradiation. The kinetic plot of the BaTi_0.96Cu_0.02Nb_0.02O₃ catalyst for methyl orange dye reveals that the rate constant was 0.028 min⁻¹, and the degradation reaction obeys the pseudo-first-order kinetic model with an R-square value of 0.92, as shown in Figure 14b. The stability or the reusability test of the BaTi_0.96Cu_0.02Nb_0.02O₃ catalyst for 20 mg/L of methyl orange dye during three cycles proves the high photodegradation properties of this composition with final efficiencies of 94, 91 and 88%, as depicted in Figure 14c. The photocatalytic results of both dyes (methyl green and methyl orange) prove the superior photodegradation features of the BaTi_0.96Cu_0.02Nb_0.02O₃ catalyst for practical uses under natural and free sunlight energy.

The photocatalytic reaction or mechanism for the degradation of the tested organic dyes (methyl green and methyl orange) of the Bi_0.96La_0.02Gd_0.02FeO₃ catalyst essentially depends on the photo-excitation of the electrons from the top of the valence band (VB) to the bottom of the conduction band (CB) by sunlight energy. The photogenerated electrons in the conduction band (CB) attack the oxygen molecules present in solution to form the superoxide (O₂^•–) species, whereas the positive holes (h⁺) linked to the water molecules to yield the hydroxyl (·OH) radicals, respectively. The strong interaction between ·OH and O₂^•– radicals and methyl green or methyl orange produces CO₂ and H₂O, as shown below [3,4,5]:

BaTi_0.96Cu_0.02Nb_0.02O₃ (best catalyst) → natural sunlight irradiation → e⁻+ h⁺ pairs

Cu²⁺ + e⁻ → Cu⁺

Nb⁵⁺ + e⁻ → Nb⁴⁺

e⁻ + O₂ → O₂^•–

h⁺ + H₂O → ·OH + H⁺

(·OH, O₂^•–) → Methyl green and methyl orange → CO₂ and H₂O (non-harmful products)

The remarkable photodegradation efficiency of the BaTi_0.96Cu_0.02Nb_0.02O₃ catalyst can be related mainly to three factors including the high visible light absorption capacity, homogenous morphology of the grains and the efficient dual dopants separation of the charge carriers (electron–hole pairs).

Figure 15 depicts the X-ray diffraction patterns of the BaTi_0.96Cu_0.02Nb_0.02O₃ catalyst before and after the photodegradation test for methyl green dye. The X-ray diffraction peaks of both powders have a good relative intensity and indexed to the tetragonal BaTiO₃ structure, confirming the stability of the BaTi_0.96Cu_0.02Nb_0.02O₃ catalyst after the photocatalytic degradation reaction. Ahamed et al. [34] reported that Zn-doped BaTiO₃ exhibits a visible light photodegradation efficiency of 85% for 20 mg/L methylene blue in 80 min. Bhat et al. [22] showed that Rh-doped BaTiO₃ has a photoactivity of 96% for 10 ppm methylene blue in 120 min. The Mn-doped BaTiO₃ nanotube as a photocatalyst exhibited improved visible light photocatalytic activity for methylene blue dye during 300 min [23]. Ag-doped BaTiO₃ [24] possesses a visible light photodegradation efficiency of 79% for 20 mg/L rhodamine B. Amaechi et al. [25] showed that Fe-doped BaTiO₃ has a sunlight photodegradation efficiency of 75% for 20 mg/L methyl orange dye. In our study, the BaTi_0.96Cu_0.02Nb_0.02O₃ catalyst exhibited a photodegradation efficiency of 97% and 94% during 60 and 90 min under natural sunlight, respectively. This comparison illustrates the remarkable photocatalytic efficiency of the BaTi_0.96Cu_0.02Nb_0.02O₃ catalyst.

3. Preparation of Samples and Measurements

In this work, all materials were purchased from Sigma-Aldrich (Steinheim, Germany) and used as received. Barium nitrate (Ba(NO₃)₂, 99.6%), titanium dioxide (TiO₂, 99.9%), copper(II) nitrate trihydrate (Cu(NO₃)₂·3H₂O, 99.95%), ammonium monovanadate (NH₄VO₃, ≥99.0%) and niobium(V) chloride (NbCl₅, 99%) substances were used to prepare the pure BaTiO₃, BaTi_0.96Cu_0.04O₃, BaTi_0.96Cu_0.02V_0.02O₃ and BaTi_0.96Cu_0.02Nb_0.02O₃ powders by the solid-state reaction method. The used weights for each composition are summarized in

Table 2. Detailed weights of chemical substances used in the synthesis of BaTiO₃, BaTi_0.96Cu_0.04O₃, BaTi_0.96Cu_0.02V_0.02O₃ and BaTi_0.96Cu_0.02Nb_0.02O₃ catalysts.

Composition	Ba(NO3)2 (g)	TiO2 (g)	Cu(NO3)2·3H2O (g)	NH4VO3 (g)	NbCl5 (g)
BaTiO3	7.84	2.396	-	-	-
BaTi0.96Cu0.04O3	7.84	2.30	0.29	-	-
BaTi0.96Cu0.02V0.02O3	7.84	2.30	0.1449	0.07	-
BaTi0.96Cu0.02Nb0.02O3	7.84	2.30	0.1449	-	0.162

. A pure BaTiO₃ sample was prepared through good mixing of Ba(NO₃)₂ and TiO₂ (molar ratio 1:1). Then, the obtained mixture was calcined at 600 °C for 4 h, and after that, the calcination temperature was elevated to 1250 °C for 5 h and left to cool to room temperature. The BaTi_0.96Cu_0.04O₃, BaTi_0.96Cu_0.02V_0.02O₃ and BaTi_0.96Cu_0.02Nb_0.02O₃ powders were synthesized by the same method through mixing proper weights of the dopants materials (Table 2) with the Ba(NO₃)₂-TiO₂ mixture. The X-ray diffraction analysis (PANalytical X-ray diffraction equipment model X′Pert PRO), scanning electron microscope (SEM, model Quanta 250 FEG) and double-beam spectrophotometer JASCO (JASCO, V-570 UV-Vis-NIR) were used to characterize the synthesized BaTiO₃, BaTi_0.96Cu_0.04O₃, BaTi_0.96Cu_0.02V_0.02O₃ and BaTi_0.96Cu_0.02Nb_0.02O₃ compositions. The photocatalytic properties of pure BaTiO₃, BaTi_0.96Cu_0.04O₃, BaTi_0.96Cu_0.02V_0.02O₃ and BaTi_0.96Cu_0.02Nb_0.02O₃ powders for the degradation of methyl green and methyl orange dyes were measured using a 100 mL solution of methyl green (20 mg/L) and methyl orange (20 mg/L) plus 40 mg of the catalyst. The effect of adsorption was firstly tested under dark conditions, and then, the mixed solution (catalyst + dye) was exposed to a Xenon lamp or direct sunlight illumination (June, 12–2 pm). Every ten minutes, 4 mL of the exposed solution was taken and centrifuged to eliminate the particles of the powder. The efficiency of the photodegradation process was calculated via measuring the change in absorbance of the maximum peak of methyl green (633 nm) and methyl orange (464 nm). The photodegradation efficiency (PE) is expressed as follows:

PE = A_t/A₀ = C_t/C₀,

where A₀ and C₀ signify the primary absorbance and concentration of methyl green and methyl orange, whereas A_t and C_t indicate the absorbance and concentration of methyl green and methyl orange during the different irradiation times.

4. Conclusions

In this study, a new BaTi_0.96Cu_0.02Nb_0.02O₃ photocatalyst exhibits a high depollution activity for 20 mg/L methyl green and methyl orange pollutants under natural sunlight and Xenon lamp photoreactor irradiations. The solid-state reaction method was used to synthesize high-purity BaTiO₃, BaTi_0.96Cu_0.04O₃, BaTi_0.96Cu_0.02V_0.02O₃ and BaTi_0.96Cu_0.02Nb_0.02O₃ powders. The X-ray diffraction analysis of all samples proves the formation of a tetragonal perovskite BaTiO₃ structure. The SEM images demonstrate that the addition of the dual dopants of (Cu, V) and (Cu, Nb) ions to BaTiO₃ structure remarkably modifies the morphology of the particles and leads to the formation of uniform semispherical grains. The incorporation of Cu, (Cu, V) and (Cu, Nb) ions adjusts the band gap energy of the BaTiO₃ catalyst to absorb the most visible light spectrum. BaTiO₃, BaTi_0.96Cu_0.04O₃, BaTi_0.96Cu_0.02V_0.02O₃ and BaTi_0.96Cu_0.02Nb_0.02O₃ powders reveal a band gap energy of 3.2, 2.99, 2.84 and 2.72 eV with the formation of long absorption tails, respectively. The modification of BaTiO₃ by (Cu, Nb) ions induced a superior photodegradation activity for 20 mg/L methyl green and methyl orange pollutants with a total efficiency of 97% and 94% during 60 and 90 min under natural sunlight, respectively. The total organic carbon analysis indicates that the tested dyes were effectively mineralized to CO₂ and H₂O in presence of BaTi_0.96Cu_0.02Nb_0.02O₃ catalyst. In addition, BaTi_0.96Cu_0.02Nb_0.02O₃ catalyst exhibits a high stability for both dyes during three cycles.