ZnO-Bi₂O₃ Heterostructured Composite for the Photocatalytic Degradation of Orange 16 Reactive Dye: Synergistic Effect of UV Irradiation and Hydrogen Peroxide

Abstract

The development of semiconductor photocatalysts has recently witnessed notable momentum in the photocatalytic degradation of organic pollutants. ZnO is one of the most widely used photocatalysts; however, its activity is limited by the inefficient absorption of visible light and the fast electron–hole recombination. The incorporation of another metal or semiconductor with ZnO boosts its performance. In this present study, a heterostructured ZnO-Bi₂O₃ composite was synthesized via a simple co-precipitation method and was investigated for the UV-driven photocatalytic degradation of the Reactive Orange 16 (RO16), a model textile dye. The successful fabrication of ZnO-Bi₂O₃ microstructures with crystalline nature was characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX). The discoloration of the dye solution was quantified using UV–Vis spectroscopy to determine the photocatalytic efficiency. The photocatalytic activity results demonstrated that the photodegradation at ZnO-Bi₂O₃ heterojunction was more efficient and 300 and 33% faster than individual Bi₂O₃ and ZnO catalysts, respectively, an effect that is indicative of a synergistic effect. In the presence of ZnO-Bi₂O₃ particles, the UV light-driven activity for RO16 degradation was twice as high as in its absence. The influence of adding the oxidant H₂O₂ on the UV-induced photocatalytic degradation was investigated and the results revealed a two-time increase in the photocatalytic activity of ZnO-Bi₂O₃ compared to UV irradiation alone, which could be ascribed to a summative degradative effect between UV and H₂O₂. Hence, this approach holds the potential for environmentally friendly wastewater treatment.

1. Introduction

The textile industry is known for its extensive use of dyes in the manufacturing process and thus releases a large amount of wastewater containing toxic chemicals into water bodies [1]. These textile dyes are often non-biodegradable and can cause threats to aquatic ecosystems and human health [2]. For instance, the inhalation of dye particles can cause severe respiratory problems. In addition, when untreated dye effluents and other carcinogenic substances are discharged into water resources, they can contaminate drinking water sources, jeopardizing the health and well-being of communities that depend on them [3,4]. The metal complex-containing dyes can release toxic heavy metals into watercourses, posing a detrimental risk to the environment and humans. Among other textile dyes, reactive dyes are highly soluble in water, rendering their removal through conventional methods difficult [5].

It is essential to develop efficient treatment strategies. A range of conventional treatments has been investigated for the decolorization of textile effluents, including chemical oxidation [6], biodegradation, adsorption, and electrochemical degradation. These approaches have their advantages and disadvantages. For instance, although the physico-chemical methods are successful, they encounter issues with the generated sludge disposal, the formation of toxic byproducts, the high cost of operation, chemicals, and electricity [7,8]. A drawback of biodegradation is the need for a suitable environment for the microorganisms to thrive as well as the difficulty of upscaling [9]. In contrast, photodegradation provides a sustainable efficient alternative to degrading dyes under light irradiation into colorless, less hazardous waste that can be released without compromising their physicochemical properties [1,10].

Semiconductor photocatalysts are used for this photocatalytic degradation process. Among others, TiO₂ and ZnO are widely used as photocatalysts [11,12,13,14]. The advantages of photocatalysis for dye discoloration include high efficiency, no need for hazardous chemicals, being environmentally benign, being relatively cost-effective, and versatility [10,15]. However, photocatalysis has some limitations, such as the need for a light source and the possible deactivation of the photocatalyst over time. Furthermore, variables like the type of dye that is degraded, and the specific conditions of the treatment process can affect how effective photocatalysis is. Additionally, the effectiveness of photocatalysis can vary depending on factors such as the chemical composition and structure of the dye being degraded. To address these limitations, it is essential to carefully assess the specific situation and goals when choosing a method for dye degradation.

ZnO is recognized as an outstanding semiconductor in photocatalysis [16]. ZnO is an n-type semiconductor [17] with a large band gap of about 3.2 eV [15]. It is a non-toxic, cost-effective, and thermally and chemically stable material with simplicity of manufacture. For example, ZnO nanoparticles (NPs) were synthesized using an extract of Punica granatum plant and were able to degrade textile orange 16 reactive dye with an overall efficiency of 93% [16]. However, the photo-efficiency of ZnO is often limited by the inefficient absorption of visible light as well as the fast electron–hole recombination [15,18]. A promising strategy to improve ZnO photocatalytic performance is the formation of a heterojunction with other metallic or oxide semiconductors such as Bi₂O₃. This approach has proven to be effective in extending the spectral response of the hybrid system and favors charge separation, thus boosting the photocatalytic activity and stability [19,20].

Generally, improving the photocatalytic activity via synergistically combining two different metals or semiconductors has been reported as an efficient strategy [21]. For instance, Wang et al. [22] demonstrated the high efficiency of Ni-Pd/Fe₃O₄ yolk-shelled nanospheres for the catalytic reduction of N-containing dyes. These magnetic nanocomposites, with uniformly dispersed Ni and Pd, exhibit excellent recyclability and enhanced catalytic activity for various dyes compared to single metal counterparts (Ni/Fe₃O₄ and Pd/Fe₃O₄). The Au@CoP metal–semiconductor hybrid also showed improved photocatalysis performances for rhodamine 6G dye [23]. Such summative positive effect was also observed for the hybrid photocatalyst containing ZnO and β-cyclodextrin (β-CD) particles, and this composite exhibited 100% removal efficiency for ceftriaxone [24]. In this regard, integrating two oxide semiconductors to obtain optimum energy levels in the heterojunction that match the redox potentials of the involved radical generation reactions would be beneficial.

In this context, Bi₂O₃ is a p-type semiconductor [17] with a bandgap in the range of 2.1 to 3.1 eV [25,26]. Bi₂O₃ can possibly generate active radicals such as superoxide (O₂⁻) from water that could initiate dye oxidation processes. A few studies reported the synthesis of heterostructures of ZnO-Bi₂O₃ for improved photocatalytic activity against dyes. For example, ZnO with 6.8–18.9 wt% α- Bi₂O₃ nanostructures was reported, and this heterostructure demonstrated a higher discoloration rate of acid black under UV irradiation as compared to the respective single oxides [19]. Wang et al. [27] showed that a 5% molar ratio of the ZnO/α-Bi₂O₃ nanomaterial, prepared by a solid-state reaction, has the best photo-discoloration ability under sunlight for Rhodamine B, which was attributed to the effective separation of charge carriers. OH radicals were revealed as the main active species for the discoloration of this dye over this ZnO/α- Bi₂O₃. A synergistic effect in ZnO/Bi₂O₃ heterostructure was reported as a reason for the improved photodegradation performance for indigo carmine dye compared to individual ZnO and Bi₂O₃ [20]. During the photo-oxidation process, powerful oxidative radicals such as ^•OH, H₂O₂, and O₂^−• are produced from oxygen reduction and water splitting [28] and can oxidize dye molecules into lower molecular weight molecules (e.g., tiny inorganic compounds or minor aldehydes), CO₂, and water [29]. Moreover, in the presence of H₂O₂, UV radiation can be very effective in oxidizing dyes as UV (200–280 nm) causes H₂O₂ breakdown, generating ^•OH radicals that can efficiently degrade reactive dyes [30].

Motivated by some of the leading studies mentioned above, this work reports on the facile synthesis of a ZnO-Bi₂O₃ hybrid composite via a co-precipitation method. The synthesized material was characterized using the SEM, EDX, XRD, and FTIR methods. The photocatalytic performance of the prepared ZnO-Bi₂O₃ catalyst was evaluated for the Reactive Orange 16 (RO16) dye, as a model dye, under UV irradiation. Furthermore, the influence of the addition of H₂O₂ as a radical source on the dye degradation rate was assessed and the results manifested an improved degradation efficiency when UV irradiation was combined with H₂O₂ compared to only UV exposure. The kinetics of degradation was also determined. Based on the band gap energy and redox potentials of possibly involved reactions, a proposed mechanism of photocatalysis and degradation via the ZnO-Bi₂O₃ hybrid was discussed.

2. Results and Discussion

2.1. Structural and Morphological Characterization of the ZnO-Bi₂O₃ Heterojunction

2.1.1. Crystalline Structure Using XRD Analysis

The as-synthesized ZnO-Bi₂O₃ particles were systematically characterized using various techniques. The crystalline structure of the synthesized particles was determined using powder X-ray diffraction (XRD). Figure 1 compares the experimental XRD pattern of ZnO-Bi₂O₃ with the standard patterns of ZnO and Bi₂O₃. The XRD pattern of ZnO-Bi₂O₃ exhibited distinct Bragg reflections that were assigned to the crystal planes of both ZnO and Bi₂O₃. Specifically, the ZnO peaks were observed at 2θ values of 31.9°, 35.2°, 38.2°, 47.3°, 56.8°, 63.1°, 66.6°, 68.1° and 69.3° corresponding to the (100), (002), (101), (102), (110), (103), (200), (112) and (201) crystal planes, respectively [29]. The pattern is closest to the hexagonal structural of ZnO with space group P63mc (186) according to the PDF card No. 36–1451 [15]. The Bi₂O₃ peaks were observed at 2θ of 26.1°, 31.9°, 32.7°, 33.7°, 41.2°, 46.9°, 54.3°, 55.3°, 58.8°, and 66.6° corresponding to the (210), (002), (220), (102), (212), (400), (203), (421), (412), (004), and (421) crystal planes, respectively, which match the tetragonal structure of Bi₂O₃ with space group P-421c (114) (PDF card No. 27-0050) [31]. No diffraction peaks of impurities were present in the pattern. The above XRD reflections proved the presence of both ZnO and Bi₂O₃ in the prepared heterostructure ZnO-Bi₂O₃ in the study. In addition, the XRD peaks are sharp, suggesting the highly nanocrystalline nature of the ZnO-Bi₂O₃ crystallites. In addition, the crystallite size was calculated using the Debye–Scherrer equation as follows [32]:

$$D=\frac{K \lambda }{\beta \mathrm{c}\mathrm{o}\mathrm{s}\theta }$$

(1)

where D is the nanoparticle crystalline diameter, K represents the Scherrer constant and equals 0.9, λ is the X-ray wavelength (1.54 Å for copper K-α), and β denotes the full width at half maximum (FWHM). An average crystallite particle size of the ZnO-Bi₂O₃ particles of 29.6 ± 7.1 nm was obtained from the analysis of the main reflections.

2.1.2. SEM Analysis

SEM analysis was conducted to investigate the morphological characteristics and surface topography of the ZnO-Bi₂O₃ heterostructured composite. SEM images of ZnO-Bi₂O₃ at two different magnifications are depicted in Figure 2a,b. SEM images revealed the formation microstructure with nanoparticles/aggregates of irregular shape. These images illustrate the formation of porous ZnO-Bi₂O₃ particles with a size in the range of 1–25 µm that comprise smaller particles grown on bigger ones. Further, the elemental analysis was performed using energy-dispersive X-ray (EDX) spectrometry, as presented in Figure 2c. The EDX spectrum results confirmed the presence of Zn, Bi, and O elements in the sample. The appearance of a smaller signal for Bi could be attributed to the expected small content of Bi (nominally 10 mol%) in the composite, and since EDX generally probes a few µm depth of the sample, a smaller amount of Bi in the probed volume would result in a relatively smaller signal. For our application, namely the degradation of dyes, the porous structure provides a high surface area, thus enhancing the contact between the composite material and the target dye. Hence, this increased surface area would facilitate the adsorption and photocatalytic degradation of RO16 dye. The porous nature of the particles further promotes mass transfer and diffusion of the target dye, facilitating enhanced photocatalytic degradation. Thus, the composite’s unique properties, combining the photocatalytic activity of ZnO with the high surface area and catalytic capabilities of Bi₂O₃, are expected to improve degradation efficiency compared to individual components, thus creating a synergistic effect.

2.1.3. FTIR Spectroscopy

The FTIR analysis (Figure 3) of the ZnO-Bi₂O₃ photocatalyst reveals characteristic peaks that provide valuable insights into its chemical composition and functional groups. For example, the stretching vibration of Zn–O bonds was identified by a peak at 530 cm⁻¹, indicating the presence of ZnO in the composite [33]. Additionally, the peaks at 587 cm⁻¹ and 840 cm⁻¹ were assigned to the stretching vibrations of Bi–O–Bi bonds, confirming the incorporation of Bi₂O₃ into the composite structure [34]. These results are consistent with previous studies that have reported similar peak positions for Zn–O and Bi–O–Bi stretching vibrations in ZnO-Bi₂O₃ composites [33,34]. The peak at around 1405 cm⁻¹ is often associated with the stretching vibration of the Zn–O bond. This peak represents the presence of ZnO in the composite and indicates the bonding between zinc and oxygen atoms [33]. On the other hand, the peak at 1500 cm⁻¹ could be attributed to several possible vibrational modes in Bi₂O₃. For example, it might be related to the bending vibration of the O–Bi–O bond or could arise from the stretching vibrations of the Bi–OH bond or the Bi–O–Bi bond [35]. The broad peak observed at 3400 cm⁻¹ can be attributed to the stretching vibrations of OH bonds [36], indicating the presence of surface hydroxyl groups or adsorbed water molecules on the composite surface [37]. This finding infers the potential involvement of surface hydroxyl groups in the photocatalytic activity and the adsorption properties of the ZnO-Bi₂O₃ photocatalyst.

2.2. Evaluation of the Photocatalytic Activity of ZnO-Bi₂O₃

To evaluate the photocatalytic degradation and the band gap of the RO16 dye, the optical properties of this dye were initially investigated by recording the UV–Vis spectrum of a 100 ppm dye solution to identify the wavelength of maximum absorption (λ_max). Figure 4a shows the obtained spectrum, which exhibits a characteristic single strong absorption peak at a wavelength of 488 nm. Accordingly, during the photolytic and photocatalytic studies, the change in absorbance was measured at the λ_max of 488 nm to assess the degradation of the dye under different experimental conditions. Since the band gap of the ZnO-Bi₂O₃ semiconductor is key to understanding the underlying photocatalytic mechanism in this catalyst, the band gap energy (E_g) was determined using Tauc’s equation, as follows [38]:

$${\left(\alpha h\nu \right)}^{1/n}=B{(h\nu -E_g)}^n$$

(2)

where α is the molar extinction coefficient, h the Plank’s constant, ν represents the photon’s frequency, B is a constant, and n depends on the type of the electron transition. The factor n = 1/2 and 2 for the direct and indirect band gaps, respectively [15]. The linearity of the plot and the absence of concave turning point between two linear segments in the plot of (αhν)² versus hν is characteristic of a direct band gap. The E_g was determined from the x-axis intercept of the tangent of the linear part of the plot of (αhν)² versus hν, as shown in Figure 4b. The band gap energy for ZnO-Bi₂O₃ was found to be 3.13 eV, agreeing with literature values. This band gap of the heterojunction ZnO-Bi₂O₃ lies between the band gap energy of individual ZnO (3.3 eV) and Bi₂O₃ (1.8–2.7 eV), in agreement with previous reports [20,31]. These data indicate that the band gap of ZnO can be modified via the addition of Bi₂O₃.

In this section, we evaluate the degradation of RO16 dye using the ZnO-Bi₂O₃ photocatalyst under UV radiation, both with and without the addition of hydrogen peroxide as an oxidizing agent. First, the photolysis experiments were evaluated and then the photocatalysis measurements in the presence of ZnO-Bi₂O₃ were assessed. To evaluate the degradation efficiency, the change in absorbance at 490 nm over time before and after each treatment was calculated. UV radiation has proven to be an effective method for the degradation of many dyes. Therefore, RO16 solutions were subjected to UV radiation alone or with the addition of H₂O₂ and in the presence of a catalyst combined with H₂O₂. The studied UV exposure durations ranged from 20 to 100 min. The results provided valuable insights into the effectiveness of the different treatment combinations and exposure times in degrading RO16 dye. These findings thus contribute to the understanding of UV-based degradation methods for the treatment of organic dye contaminants, specifically in the context of RO16 dye solutions.

2.2.1. Effect of Sole UV Irradiation on the Photolytic Degradation of RO16 Dye

In principle, the duration of UV irradiation directly influences the degradation extent of aqueous dye solutions. A longer exposure time to UV radiation leads to a higher percentage of dye degradation, indicating a direct relationship between exposure time and degradation efficiency [30]. In this context, it is desired to have a photocatalytic system with a higher degradation rate. In the case of RO16, the initiation step of the degradation process involves the generation of hydroxyl radicals (^•OH), which trigger the conversion of dye molecules to their excited state. This process results in the degradation of the chromophoric groups of the RO16 by breaking the N-N double bond. Consequently, the dye molecules repeatedly transition to the excited state, and the hydroxyl radicals continuously attack the dye molecules, leading to their degradation. Based on this proposed mechanism, it is expected that a longer exposure period to UV radiation promotes the degradation of the dye molecules [39]. The absorbance of various concentrations (100, 150, and 200 ppm) of RO16 dye solution at 490 nm was recorded before UV irradiation (0 min) and at different UV light exposure times (20, 40, 60, 80, and 100 min), as presented in Figure 5a. It was noticed that in the absence of UV irradiation, the absorbance of the dye solution did not decrease, indicating that the dye adsorption on the photocatalyst surface is negligible. As shown in Figure 5a, one can observe a monotonic decrease in the absorbance of the dye solution over time. Consequently, the degradation percentage increased with time but reached only 18% after 100 min for 200 ppm dye (Figure 5b). A similar trend in degradation percentage was obtained for all dye concentrations but with a higher absolute degradation % for higher concentrations.

2.2.2. Effect of H₂O₂ Addition on the Degradation of RO16 under UV Irradiation

Further, the effect of adding 0.5 mL H₂O₂ on the decolorization rate of RO16 was investigated. The decomposition percentage of RO16 was slightly enhanced when H₂O₂ was present in the dye solution, reaching 22.5% after 100 min for the 200 ppm dye, as shown in Figure 6b. The enhancement was observed for all dye concentrations. It has been reported that H₂O₂ is able to absorb UV light and generate ^•OH via its photolysis [40].

The development of more ^•OH would promote the attack on the aromatic ring of RO16 and increase its decomposition rate. Moreover, species of HO₂^• can be generated from the scavenging of ^•OH radicals by H₂O₂. In the presence of H₂O₂ and UV light, the following chain reactions occur, generating highly reactive radicals [41]:

Initiation:

$${\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{O}\mathrm{H}}^{•}\to {\mathrm{H}}_2\mathrm{O}+\mathrm{H}{{\mathrm{O}}_2}^{•}$$

(3)

Propagation:

$${\mathrm{H}}_2\mathrm{O}+\mathrm{H}{{\mathrm{O}}_2}^{•}\to {\mathrm{H}}_2{\mathrm{O}}_2$$

(4)

Termination:

$${\mathrm{O}\mathrm{H}}^{•}+{\mathrm{O}\mathrm{H}}^{•}\to {\mathrm{H}}_2{\mathrm{O}}_2$$

(5)

$$\mathrm{H}{{\mathrm{O}}_2}^{•}+\mathrm{H}{{\mathrm{O}}_2}^{•}\to {\mathrm{O}}_2+{\mathrm{H}}_2{\mathrm{O}}_2$$

(6)

$${\mathrm{O}\mathrm{H}}^{•}+\mathrm{H}{{\mathrm{O}}_2}^{•}\to {\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2$$

(7)

2.2.3. Evaluating the Photocatalytic Activity of ZnO, Bi₂O₃, and ZnO-Bi₂O₃

To explore whether a synergistic effect in the synthesized ZnO-Bi₂O₃ heterojunction prevails, the photocatalytic activity of ZnO-Bi₂O₃ to RO16 dye was compared with individual ZnO and Bi₂O₃ samples, as shown in Figure 7a. Notably, under UV irradiation, Bi₂O₃ alone showed a small degradation efficiency (10%) after 100 min, whilst ZnO exhibited relatively larger degradation efficiency (27%). This observation aligns with the pH-dependent photocatalytic activity of Bi₂O₃. The limited discoloration effect of Bi₂O₃ is most likely because Bi₂O₃ exhibits the optimal decolorization performance in acidic solutions and not at neutral pH values, aligning with previous studies [42]. On the other hand, the ZnO-Bi₂O₃ composite outperformed both Bi₂O₃ and ZnO with degradation % of 29%, indicating a synergistic effect between the two components in their mixture. This result agrees with previous reports [20]. Furthermore, the degradation kinetics were studied by plotting ln(C_o/C) against time (t) for the three different photocatalysts. As displayed in Figure 7b, the data analysis of all samples revealed a linear dependence with correlation coefficients R² of 0.997, 0.994, and 0.968, respectively. This adequate linearity demonstrates first-order reaction kinetics for the degradation of the dye RO16, according to the following equation:

$$\mathit{ln}\left(\frac{C_0}{C}\right)=kt$$

(8)

where C₀ represents the initial concentration of RO16, C is the concentration at time t, and k is the reaction rate constant. The obtained rate constants were 0.001, 0.003, and 0.004 min⁻¹ for Bi₂O₃, ZnO, and ZnO-Bi₂O₃, respectively, as depicted in Figure 7b. This result demonstrates that the ZnO-Bi₂O₃ heterojunction was 300% and 33% faster than Bi₂O₃ and ZnO, respectively; an effect that is indicative of a synergistic interaction between ZnO and Bi₂O₃. These results suggest the ZnO-Bi₂O₃ composite’s potential for efficient dye degradation, and therefore, the composite was further investigated in this study.

2.2.4. Effect of RO16 Dye Concentration on the Activity of the ZnO-Bi₂O₃ Photocatalyst under UV Irradiation

To study this effect, aqueous solutions of RO16 dye with various concentrations were irradiated with UV light for different durations in the presence of 0.2 g of the ZnO-Bi₂O₃ catalyst. Figure 8 shows the recorded absorbance and the respective decolorization percentages. In the presence of ZnO-Bi₂O₃, the dye degradation was increased to 28.4% compared to 18% without a catalyst after 100 min of illumination (Figure 8b). This confirms the effectiveness of this photocatalyst in the decolorization of RO16. It is well established that the photocatalytic processes utilizing UV irradiated semiconductors offer a promising approach for the degradation of various organic compounds rather than just photolysis, resulting in the formation of CO₂, H₂O, and associated mineral acids [43]. As discussed below in Figure 9, when photons from UV light with energy equal to or greater than the band gap energy of the semiconductor strike the semiconductor particles, electrons in the conduction band and holes in the valence band are generated. These holes can oxidize water to OH^•, while the electrons can be consumed by reducible species, such as O₂, in the solution, generating O₂^−• [28]. The ^•OH, a highly reactive oxidizing reagent, is commonly responsible for the decomposition of most organic pollutants. In the case of heterostructures like ZnO-Bi₂O₃, the improved photocatalytic performance could be attributed to the extension of the absorption range to the visible light and the reduced recombination rate of the photoinduced electron–hole pairs in the semiconductor Bi₂O₃ with the introduction of ZnO into the heterostructure [44,45].

2.2.5. Effect of H₂O₂ on the Degradation Rate under UV Irradiation at ZnO-Bi₂O₃

To investigate whether a synergistic effect between UV irradiation and H₂O₂ in the presence of ZnO-Bi₂O₃ could boost the decolorization percentage, the absorbance was measured for this system, as displayed in Figure 9a. Figure 9b shows an increased degradation to 48.5% after 200 min, which is significantly higher compared to the individual components. Although the absolute degradation efficiency values did not reach 100% in the studied time, the focus of this study was to understand the synergistic effect created by combining ZnO with Bi₂O₃ and exploring the role of the H₂O₂ oxidant in the improvement of the photodegradation performance. This synergistic effect was attributed to an increased surface area, facilitating effective reactant absorption and enhanced electron–hole generation due to UV light absorption. The combination of these factors led to a more efficient degradation of the dye, highlighting the promising potential of this approach for effective pollutant removal in wastewater treatment applications.

2.2.6. Comparison of the Degradation Performance and Kinetics for RO16 under Different Reaction Conditions

Figure 10a compares the degradation percentage for a 100 ppm aqueous solution of RO16 dye under the different studied conditions. The UV light irradiation alone without H₂O₂ or the photocatalyst resulted in a degradation of only 17.8% after 100 min, indicating minimal photo-induced self-sensitized photolysis of RO16. On the other hand, the addition of the oxidant H₂O₂ to the dye solution improved the degradation percentage from 17.8% to 22.5%, still in the absence of the photocatalyst. The addition of the ZnO-Bi₂O₃ catalyst to the dye solution during UV irradiation without H₂O₂ further boosted the degradation to 28.4%. Interestingly, the most significant enhancement occurred when both the ZnO-Bi₂O₃ photocatalyst and H₂O₂ were present, achieving 48.5% degradation. The enhancement effect of H₂O₂ was much more obvious in the presence of the photocatalyst than in its absence. This could be attributed to the larger surface area of the catalyst available for more H₂O₂ and UV adsorption and thus for more generated active radicals and, accordingly, more degradation. These findings demonstrate the role of the synergistic effect between UV irradiation, the ZnO-Bi₂O₃ catalyst, and H₂O₂ in enhancing RO16 dye decolorization, rendering this approach promising in wastewater treatment and pollution remediation.

Further, the rate constants of RO16 degradation for the different studied cases were determined, as depicted in Figure 10b. The fitting of the data revealed a straight line with correlation coefficients R² in the range of 0.989 to 0.999, evidencing first-order reaction kinetics, according to Equation 8. As presented in

Table 1. Rate constant values for the degradation of RO16 dye using UV radiation, H₂O₂, and the photocatalyst ZnO-Bi₂O₃ under different conditions.

	UV without H₂O₂	UV with H₂O₂	ZnO-Bi₂O₃ +UV without H₂O₂	ZnO-Bi₂O₃ + UV with H₂O₂
k (min−1)	0.002	0.003	0.004	0.008
R2	0.989	0.999	0.997	0.998

, the obtained rate constants (k) varied with experimental conditions as follows: the highest for the combined UV radiation+ ZnO-Bi₂O₃ catalyst + H₂O₂ (0.008 min⁻¹); followed by UV radiation + ZnO-Bi₂O₃ catalyst without H₂O₂ (0.004 min⁻¹); then UV radiation with H₂O₂ (0.003 min⁻¹); and the lowest for UV radiation alone (0.002 min⁻¹). The results show that the addition of H₂O₂ doubled the degradation rate under UV irradiation. The presence of the ZnO-Bi₂O₃ photocatalyst led to at least a two-fold higher degradation efficiency than its absence, indicating the effect of the photocatalyst on the process. Hence, by combining UV, ZnO-Bi₂O₃, and H₂O₂, the highest performance was achieved due to a possible synergistic effect between the individual factors.

2.2.7. Proposed Mechanism of Photocatalytic Activity at ZnO-Bi₂O₃

In photocatalytic applications, the interface between ZnO and Bi₂O₃ plays a pivotal role in facilitating charge transfer processes and electron–hole pair separation when exposed to UV light as well as the H₂O₂ oxidant. A general proposed mechanism of the photocatalysis at ZnO-Bi₂O₃ is presented in Figure 11. The given energy values of band gaps of individual materials as well as the valence band (VB) and the conduction band (CB) in Figure 11 were sourced from the literature [15,20,46,47]. When the photocatalyst is UV-irradiated, electron–hole pairs are created in the system. Importantly, the relative energy levels at the interface of this system enable the efficient transfer of electrons from the CB of Bi₂O₃ to that of ZnO, while holes migrate from the VB of ZnO to Bi₂O₃, as shown in Figure 11. This charge transfer phenomenon predominantly occurs in the vicinity of Bi₂O₃, thus effectively mitigating recombination events and optimizing charge separation [20]. The redox potentials of expected reactions in (V) and their corresponding values on the absolute voltage scale (AVS) in (eV) are also displayed in Figure 11. Upon UV light illumination, both ZnO and Bi₂O₃ generate electron–hole pairs. The ZnO-Bi₂O₃ interface acts as a platform for the spatial separation of these photogenerated carriers. Subsequently, the separated electrons and holes are harnessed to initiate reactions with adsorbed oxygen species on the photocatalyst surface, yielding reactive oxygen species (ROS). Namely, the potential energy of holes in the valence band (3.07 V) of ZnO is sufficiently positive to drive the oxidation of water, producing hydroxyl radicals (^•OH) as follows [46]:

H₂O + h⁺ → ^•OH + H⁺    E^o = 2.31 V

(9)

Furthermore, electrons residing in the conduction band of ZnO or Bi₂O₃ can oxidize dissolved oxygen, generating superoxide radicals (O₂^•−) as follows [46,48]:

O₂ + e⁻ → O₂^•−   E^o = −0.14 V

(10)

The redox potential of this oxidation process (−0.14 V) matches the energy of the CB of Bi₂O₃ (−0.13 V). The produced superoxide may directly contribute to degradation processes of the dye or further promote hydroxyl radical formation [20]. Moreover, the addition of H₂O₂ as an oxidizing agent to the reaction solution could also contribute to the production of ^•OH and O₂^•− radicals, as discussed above [46]. The produced ROS radicals react with the dye, degrading it. It was reported that O₂^•− are the primary radicals for different dye degradation [27]. The synergy of these pathways for ^•OH and O₂^•− production would contribute to a swift degradation reaction rate and high levels of degradation, highlighting the potential of ZnO-Bi₂O₃ heterojunctions in advanced photocatalytic applications.

3. Materials and Methods

3.1. Chemicals

Potassium dichromate (K₂Cr₂O₇), acetic acid, zinc chloride (ZnCl₂), bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O), sodium hydroxide, and hydrogen peroxide were of analytical grade. RO16 dye was purchased from Sigma-Aldrich (St. Louis, MO, USA). All chemicals were used without further purification. All solutions were prepared in distilled water.

3.2. Synthesis of ZnO-Bi₂O₃ Particles

To synthesize the ZnO-Bi₂O₃ composite catalyst with 10 mol% Bi₂O₃, a simple previously reported chemical co-precipitation method was conducted with slight modification [49]. First, 0.09 M ZnCl₂ and 0.01 M (Bi(NO₃)₃·5H₂O) solutions were prepared. Both solutions were mixed and heated under constant stirring to attain 80 °C. Then, 2 mL of acetic acid was added to the heated solution and subsequently, 1M NaOH was added dropwise with vigorous stirring until pH 7 was obtained and precipitation started to occur. The solution was further stirred at 80 °C for one hour. Afterward, the solution was cooled down and the precipitate was collected, washed, and dried. Finally, the precipitate was calcined in a furnace at 400 °C for 2 h, yielding ZnO-Bi₂O₃ microparticles. This procedure has been reported to yield about 9:1 weight ratio of ZnO:Bi₂O₃ in the formed heterostructure [49].

3.3. Characterization of ZnO-Bi₂O₃ Particles

Fourier transform infrared (FTIR) spectra of the as-prepared photo-catalyst were determined using a FTIR spectrophotometer (Bruker Tensor 27) in the range of 400–4000 cm⁻¹. Powder X-ray diffraction (XRD) analysis was carried out to determine the crystallinity and crystalline structure of the synthesized material. XRD patterns of the synthesized powder were recorded on a Rigaku D/Max-IIIA diffractometer (Rigaku Co., Tokyo, Japan) with copper Kα radiation of a wavelength of 1.54051 Å. To study the morphology of the samples, scanning electron microscope (SEM) analysis was carried out using an JEOL LEO 44i instrument, which was operated at 10 kV. EDX analysis was conducted on a Nova NanoSEM 450 equipped with EDX instrument operating at 25 kV. For UV–Vis analysis, a twin beam spectrophotometer (Rayleigh-UV2601) was used.

3.4. Photocatalytic Degradation Measurements

Initially, the maximum absorbance wavelength (λ_max) of the RO16 dye was determined by recording UV–Vis spectra for 100 ppm solution. The samples were scanned in the range of 200–800 nm. The photolytic and photocatalytic activity of RO16 in aqueous solution under UV light were investigated. The photolytic measurements were conducted via the irradiation of the RO16 solution using a UV lamp with 144 watt intensity and a wavelength of 244 nm. The UV absorbance values of a portion of the solution were recorded until 100 min with a time interval of 20 min. Three concentrations of the dye aqueous solution were studied.

For the photocatalytic activity measurements, the ZnO-Bi₂O₃ catalyst (0.2 g) was added to the dye solution and the solution was investigated under the same conditions for comparison. All experiments were performed in Pyrex glass batch reactors (50 mL solution) under constant magnetic stirring. After irradiation with UV light, 0.5 mL of the dye solution was withdrawn at 0 (before irradiation), 20, 40, 60, 80 and 100 min and their absorbance at a wavelength of 490 nm were measured using a UV–Vis spectrophotometer. In addition, the effect of hydrogen peroxide on the degradation rate of RO16 was studied, where 0.5 mL H₂O₂ was added to the dye solution and the absorbance was measured under the same conditions. The degradation efficiency in % of the dye was calculated using the following equation.

$$\% Degradation=\frac{A_0-A}{A_0}\times 100=\frac{C_0-C}{C_0}\times 100$$

(11)

where A₀ and A are the absorbance of the dye solution before and after the UV irradiation of the dye solution for time t, respectively. C₀ and C represent the respective concentrations of the dye before and after the irradiation of the dye solution, respectively.

4. Conclusions

The textile industry releases hazardous chemicals, including carcinogenic reactive dyes, contributing to water pollution. Conventional degradation methods such as chemical oxidation and biodegradation have limitations in addressing this issue effectively. Photocatalysis using semiconductors provides an efficient alternative. In this study, ZnO-Bi₂O₃ composite clusters were successfully synthesized using a facile co-precipitation method and were then investigated as photocatalysts for the photocatalytic degradation of reactive Orange 16 (RO16) dye, a model textile azo dye. The presence of the ZnO-Bi₂O₃ photocatalyst in the solution doubled the decolorization rate of RO16 compared to UV irradiation alone. The degradation rate via the ZnO-Bi₂O₃ heterojunction was more efficient and around 300 and 33% faster than individual Bi₂O₃ and ZnO catalysts, respectively. Furthermore, the addition of the oxidant H₂O₂ with UV irradiation amplified the degradation rate up to two times compared to only UV light, leading to a significant improvement in the oxidation and remediation of RO16. The positive effect of H₂O₂ could be ascribed to the generation of more oxygen radicals that effectively decomposed the dye, where H₂O₂ could be decomposed to ^•OH or react with produced ^•OH, forming HO₂^• radicals. The degradation process followed first-order reaction kinetics with a rate constant of 0.008 min⁻¹ in the case of ZnO-Bi₂O₃ + UV + H₂O₂. Hence, the present study reveals the impact of combining UV radiation, H₂O₂, and the ZnO-Bi₂O₃ catalyst to achieve a synergistic effect and improved photocatalytic performance. This innovative approach holds promise for combating water pollution caused by textile industry dyes.