Development of Bi₂WO₆ and Bi₂O₃ − ZnO heterostructure for enhanced photocatalytic mineralization of Bisphenol A

Due to the presence of toxic colorants, herbicides, insecticides, and other chemicals, industries are producing recalcitrant organic pollutants that pose a major threat to the environment. Bottles, food packaging, polycarbonates, and phenolic resins all are made using the organic plasticizer Bisphenol A (BPA). It interferes with the physiological processes of the brain and is a heart disease-causing agent and endocrine disruptor (Abdul et al. 2021). High levels of BPA were found in the influent (138 ng L⁻¹) and effluent (60.5−1,960 ng L⁻¹) of wastewater treatment plants (Zhang et al. 2009). It has been shown that creating effective photocatalysts is crucial for many emerging applications, such as the removal of resistant organic contaminants in wastewater and the conversion of solar energy into vital chemical feedstock (Nie et al. 2019).

Semiconductor photocatalysis offers a potential solution to the issues of energy shortages and environmental pollution. Photocatalytic degradation of organic pollutants by semiconductor photocatalysts has been a useful technology for environmental purification. However, due to the quick electron-hole recombination, the photo-efficiency of the bare semiconductor catalyst is inhibited. Therefore, it is urgent to develop highly efficient photocatalytic materials for pollutant degradation. Semiconductor oxide's physical and chemical characteristics determine its photocatalytic activity. Crystallinity, or the degree of crystallization, is one of the key determinants of the effectiveness of photocatalytic reactions because it affects the recombination of photo-excited electrons and holes at crystal lattice defects (Parvaz et al. 2021). It is also found that photo-generated charge carriers can be effectively separated inside semiconducting composite materials according to the different band gap structures of their components (Xu et al. 2009).

In the field of photocatalysis, ZnO is believed to be an efficient photocatalytic material alternative to TiO₂ because both have similar band gaps and photocatalytic mechanisms. The enhanced photocatalytic activity of the nanomaterials was ascribed to the larger surface area, increased oxygen vacancy, and the facilitation of diffusion and mass transportation of the reactant molecules (Yasin et al. 2022). The synthesis of zinc oxide nanomaterial has received great attention due to its size-dependent properties and photocatalytic applications (Kuo et al. 2007). Bismuth oxide (Bi₂O₃) is an important p-type semiconductor with four main crystallographic polymorphs denoted by α-, β-, γ-, and δ-Bi₂O. Bismuth-based oxides appear to be good candidates because they have a band gap in the visible range (Zhang et al. 2006). Bi₂O_3, with a band gap of 2.8 eV, accounts for its ability to oxidize water and possibly generate highly reactive species, such as O₂^−• and ^•OH radicals, for initiating oxidation reactions (Singh et al. 2017).

One of the effective ways to enhance electron-hole separation is to use coupled semiconductors. A heterojunction interface is built between the coupled semiconductors’ matching band potential semiconductors. In this way, the electric-field-assisted charge transport from one particle to the other via interfaces is favorable for the electron-hole separation in the coupled materials and for the consequent electron or hole abundance on the surfaces of the two semiconductors (Jiang et al. 2007). Hence, in linked semiconductors, advantages such as improvement of charge separation, increase in the lifespan of the charge carrier, and augmentation of the interfacial charge transfer efficiency to an adsorbed substrate can be realized. Cabot et al. (2004) and Hernández-Alonso et al. (2009) have also reported ZnO film/Bi₂O₃ microgrid heterojunction using the microsphere lithography technique for the degradation of methyl orange dye. Although, the wide band gap and fast recombination of electron-hole is a challenge in semiconductor metal oxides. Nowadays, more attention has been paid to decreasing the recombination ability by coupling the different semiconductors’ metal oxides to generate a heterojunction interface among different semiconductors. The present study proposed the synthesis of the coupled semiconductor containing p-n heterojunctions that decrease the recombination of electron-hole pairs. The as-synthesized heterostructures were characterized through different characterization techniques such as X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), and Fourier transfrom infrared (FTIR) photocatalytic properties of as-synthesized materials were evaluated by degradation study of Bisphenol A. Parametric analysis was carried out by considering catalytic dose (0.02–0.15 g L⁻¹), initial BPA concentration (5–20 mg L⁻¹), temperature change (5–20 °C) and solution pH changes (4–8). The kinetics analysis and reaction intermediates and final products have also been identified.

All chemicals are analytical grades. Bisphenol-A (BPA), bismuth nitrate (Bi(NO₃)₃.5H₂O), zinc nitrate (Zn(NO₃)₃.6H₂O), sodium tungstate (Na₂WO₄.2H₂O) with > 99% purity were purchased from Sigma Aldrich. Dichloromethane (CH₂Cl₂), acetic acid (CH₃COOH), hydrogen peroxide (H₂O₂), and NaHCO₃ were obtained from Hi Media, Mumbai; sulfuric acid (H₂SO₄), NaOH, and acetonitrile (>99% purity) were purchased from Fine Chemicals, New Delhi. The laboratory scale (Milli-Q Biocel) system was used for the Millipore water.

The co-precipitation method was used for the synthesis of Bi₂WO₆ and Bi₂O₃-ZnO nanocomposite heterostructure. During the preparation of the Bi₂WO₆ heterostructure, the 0.1 M Na₂WO₄.2H₂O and 0.1 M Bi(NO₃)₃.5H₂O was prepared in 100 mL DI water. The 1.0 M aqueous solutions of NaOH were dropwise added into the solution until the solution obtained a pH of 7. The resulting suspension was stirred at 80 °C for 1 h by using the magnetic stirrer. After a continuous stirring, a light-yellow suspension was formed, which was cooled at room temperature and filtered out. The yellow precipitate was collected and washed with DI water and ethanol. As obtained, precipitates were dried for 8 h in an oven at 80 °C. The dry precipitates were calcined at 550 °C for 5.0 h. During the preparation of Bi₂O₃-ZnO nanocomposite heterostructure, the 0.1 M Zn (NO₃)₃.6H₂O and Bi (NO₃)₃.5H₂O solution was prepared in 100 mL DI water, 1.0 M aqueous solution of NaOH was dropwise added into the solution until the solution obtained a pH of 7; then the resulting suspension was stirred at 80 °C for 1 h by using the magnetic stirrer. After a continuous stirring, a light-yellow suspension was formed, which was cooled at room temperature and filtered out. The yellow precipitate was collected and washed with DI water and ethanol. As obtained, precipitates were dried for 8 h in the oven at 80 °C, and the dry precipitates were calcined at 450 °C for 4.5 h. After that, both as-synthesized materials were collected and characterized.

The morphology and the structure of as-synthesized nanocomposites were examined by using several characterization techniques. The crystalline structure of as-synthesized materials was detected with powder X-ray diffraction (XRD: D8, Bruker AXS diffractometer) having Cu K_α radiation scan rate of 5° min⁻¹ at the wavelength (λ = 1.5406 Å) for 30 min within the scanning range of 10–90° and applied voltage of 40 kV. The elemental composition and morphological characteristics of as-synthesized material were determined by using field emission scanning electron microscopy (FE-SEM: EVO MA, 15 Carl Zeiss) equipped with an energy dispersive X-ray (EDX) analyzer. The Fourier transform infrared spectroscopy (FTIR: Shimadzu 8400 model) recorded the spectrum to explain the bond stretching frequency of the materials.

The BPA concentration before and after the treatment was detected by using high-performance liquid chromatography (HPLC: Waters India) equipped with a UV detector at λ_max of 276 nm for BPA and C18 column having a high-pressure pump for BPA quantification. A mixture of acetonitrile and Milli Q-water (50/50; v/v) was selected as the mobile phase in the present study at 1 mL min⁻¹ flow rate. GC-MS analysis was done by using the Perkin Elmer GC-MS (Clarus − 680) having an MS capillary column − Elite 5 with the dimension of 30 m × 0.25 mm coupled with the Clarus SQ8C MS model. 1 μL aliquots amount was injected at 493 K in split less mode and at a fixed flow rate 1.5 mL min⁻¹ of carrier helium gas. To identify the unknown peak of the sample, NIST and Wiley library are used.

C_o and C_f are the initial and final concentration change BPA in aqueous solution at t = 0 and after time t = t (min). The experiments were performed multiple times, and the averages of three replicate experiments were reported in all the results. A smaller than 5% deviation was considered for the average value of all experimental results.

Photocatalytic degradation of BPA using as-synthesized Bi₂O₃-ZnO nanocomposite was optimized by knowing the effect on major treatment parameters, including catalyst dose, initial BPA concentration, reaction temperature, and solution pH. The effect of each parameter was systematically studied as follows.

The impact of the initial concentration pollutant on the BPA degradation was studied by altering the initial BPA concentration from 5 to 20 mg L⁻¹ while keeping a constant amount of Bi₂O₃-ZnO (0.15 g L⁻¹) and H₂O₂ (1.0 mM) at pH = 7.0 (Figure 6(b)). BPA concentration of 5 mg L⁻¹ and 10 mg L⁻¹ were 100% and 90% eliminated after ∼80 min. But BPA degradation reduced from 100% to 54% when the BPA concentration was increased to 20 mg L⁻¹ because with increases in concentration, the ROS interaction with the pollutant reduced, resulting in less degradation. At a higher initial pollutant concentration, the number of free radicals and the availability of the catalyst surface to adsorb the excess pollutant molecules is lower; consequently, the pollutant removal decreases (Sharma et al. 2015b; Cherifi et al. 2019). The extra number of pollutants needs more ROS and intermediates by-products in the reaction mixture; additionally, competition of ROS between intermediates by-products and BPA molecules may describe the decreases in BPA reduction. Moreover, at higher pollutant concentrations, repulsion between imminent particles is big enough; therefore, they continue to separate from one another and happen in a dispersion state. Consequently, pollutant removal decreases with increasing the pollutant concentration.

Temperature is another important parameter in knowing the effect of BPA reduction on the Bi₂O₃-ZnO/H₂O₂ system. The BPA reduction increased with increased temperature from 5 to 20 °C. A maximum of about 72% BPA was reduced after 80 min at 5 °C. The complete BPA degradation occurred within 80 min at 20 °C (Figure 6(c)). This might depend upon the activation energy (E_a) of the molecule. Moreover, the higher temperature helps the reaction to complete more efficiently with electron-hole recombination (Chen 2009). Further increases in the temperature increase the oxidation rate of the pollutant and also reduces the adsorptive capacities of the pollutant over the surface of the photocatalyst.

As shown in Figure 6(d), the maximum BPA removal with Bi₂O₃-ZnO at a pH of 7.0 was found to be ∼ 92% because UV light activates the H₂O₂ for enhanced BPA degradation increasing the concentration of ROS at its optimal treatment condition of BPA reduction which greatly affected the three main factors, i.e., the rate of radical reaction with BPA deprotonation, conversion ROS itself at different pH, and absorption coefficient between the catalyst and BPA (Sharma et al. 2015a). The difference in initial and final pH was also tested, and it was observed that the final pH was slightly increased in most of the initial pH from acidic to alkaline, which might be ascribed to the high concentration of ROS species in the solution buffer (Qiao et al. 2021).

Scavenger test analysis was used to investigate the main reactive oxygen species that participated in BPA degradation using the Bi₂O₃-ZnO/H₂O₂ system at pH 7. Among the scavenger analysis, 1 mM of tertbutanol (t-BuOH), furfuryl alcohol (FFA), and p-benzoquinone (BQ) were used as scavengers for ^•OH, singlet oxygen (¹O₂), and superoxide anion radical (O₂^•−), respectively. The addition of a t-BuOH scavenger showed high effects on BPA degradation due to its hydrophilic nature, while BQ and FFA slightly decreased the BPA. This result suggests that ^•OH was found to be major reactive species, whereas O₂^•− and ¹O₂ played a slight role in BPA degradation with Bi₂O₃-ZnO/H₂O₂ system (Figure 8(b)).

The production of intermediates during the breakdown of BPA (m/z: 228) with the Bi₂O₃-ZnO/H₂O₂ system was identified by performing GC-MS. According to past research, the possible degradation mechanism of BPA reduction consisted of the following steps: (a) BPA hydroxylation and dehydration to generate quinone, (b) radicals breaking the C-C link connecting two benzene rings, and (c) producing phenoxyl and para-isopropenyl phenol. The attack of ^•OH radicals on an electron-rich site starts the degradation process. It is demonstrated that ^•OH radicals have a particular selectivity for attack throughout the oxidation process. Both the addition and H-abstraction reactions are considered the reaction sites for the attack of •OH radicals. When it interacts with six-membered heterocyclic compounds, hydrogenated products, and resonance-stabilized organic radicals are produced (Lee & Von Gunten 2012). Radicals such as phenol and isopropyl phenol (^•C(CH₃)₂ C₆H₄OH) are produced by the β-scission of the C-C bond of the isopropyl group by the attack of ^•OH radicals. Cyclohexanone (m/z: 228) and benzophenone (m/z: 228) are created when the BPA radical (^•CH₂(C) CH₃C₆H₆OH) is oxidized, which causes the destruction of a dimethyl group from the BPA (Takdastan et al. 2018).

It is well known that the H₂O₂ creates radical cations that then undergo hydrolysis to produce the transient ^•OH adducts that further react with aromatic compounds directly via the direct electron transfer (Guan et al. 2013). The ^•OH attack on BPA produces hydroxycyclohexadienyl radicals (m/z: 228) by electron transfer from the benzene ring (Singh et al. 2017). The quinone derivatives of monohydroxylated and hydroxylated BPA, i.e., quinone of monohydroxylated BPA (Q-MHBPA) (m/z: 247) and quinone of hydroxylated BPA (Q-DHBPA) (m/z: 252), respectively formed by the hydroxycyclohexadienyl radicals has been previously reported the similar observation during the ferrous ion-activated persulfate breakdown of BPA.

The present study shows that the Bi₂O₃-ZnO heterostructures using H₂O₂ under UV-light irradiation are a possible treatment technique for BPA elimination from the aqueous phase. It is observed that the BPA removal decreases with an increase in BPA concentration and at a higher catalyst dose. The optimum value of the catalyst dose was detected to be 0.15 gL⁻¹ for the 10 mgL⁻¹ BPA concentration. At the optimum treatment conditions, more than 90% of BPA was degraded. The enhancement of BPA degradation was formed at initial pH = 7. However, slightly less acidic and basic pH shows good results of BPA reduction than the higher acidic and alkaline pH because the conversion of H₂O₂ radicals to ^•OH and the BPA degradation increases at the selected range of solution pH. The kinetics of degradation of BPA were well fitted by the power laws model. HPLC and GC-MS results confirmed that quinone(s) and hydroxylated products are detected during the BPA reduction to proceed with the electron transfer degradation mechanism. The phenoxyl type radicals’ formation confirmed the probable degradation pathway of BPA reduction involving oxidative skeletal rearrangement, hydroxylation, dehydration, demethylation, and ring opening.

The authors are thankful to the Ministry of Education (MOE) in Taiwan, National Taiwan University (NTUCCP-110L901003, NTU-110L8807), and National Natural Science Foundation of China (Grant No. 51978654), Qing Lan Project of Jiangsu Province, and the Uttaranchal University, Dehradun, Uttarakhand, India.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.