Preparation of Yb/Bi₂WO₆ material and the photocatalytic degradation of Ofloxacin in water

Article information

Environmental Engineering Research. 2024;29(6)

Publication date (electronic) : 2024 May 9

doi : https://doi.org/10.4491/eer.2024.127

, Jing Zhang ², Jingjing Luo ¹, Dan Liu ¹, Yizhong Chen ¹, Xiaowei Lu ¹, Yalin Zeng ¹

¹School of Resources, Environment and Materials, Guangxi University, 530004, People’s Republic of China

²Guangxi Zhuang Autonomous Region Environmental Emergency and Accident Investigation Center, 530025, People’s Republic of China

^†Corresponding author: E-mail: wuls312@163.com, Tel: +86 13877151811, ORCID: 0000-0002-5973-432X

Received 2024 March 1; Revised 2024 May 6; Accepted 2024 May 7.

Abstract

In this work, ytterbium (Yb) doped Bi₂WO₆ composite (Yb/Bi₂WO₆) was prepared using a one-step hydrothermal method. The investigation focused on the photocatalytic degradation effect of Yb/Bi₂WO₆ on ofloxacin in water, with experimental conditions carefully controlled. The results revealed that under various conditions, Yb/Bi₂WO₆ exhibited excellent photocatalytic degradation performance for ofloxacin, achieving a degradation rate of 98.74% within 180 min and 93.27% after 4 cycles. These findings demonstrated that Yb/Bi₂WO₆ possessed good photocatalytic performance and cycling stability. The characterization results highlighted that the doping of Yb altered the morphology of Bi₂WO₆, narrowed the band gap, reduced photogenerated electron-hole recombination, and enhanced the photocatalytic performance of Yb/Bi₂WO₆. The free radical quenching experiments showed that superoxide radicals (•O₂⁻) played a crucial role in the photocatalytic degradation of ofloxacin by Yb/Bi₂WO₆. Overall, this work provides new insights and theoretical support for the photocatalytic degradation of antibiotics using modified bismuth-based materials doped with rare earth elements.

Abstract

Graphical Abstract

Keywords: Ofloxacin; Photocatalysis; Rare earth; Ytterbium-doped bismuth based material

1. Introduction

In recent years, antimicrobial agents have gained widespread use as emerging drugs for treating infectious diseases in both humans and animals. Additionally, they are used as growth promoters in animal husbandry and agriculture, leading to elevated antibiotic levels in the aquatic environment [1]. Moreover, interactions between antibiotics and the environment can occur, resulting in observable antibiotic effects across various sections of the ecosystem [2]. Currently, the primary techniques for antibiotic degradation in aqueous environments include biological treatment, adsorption, chlorination, and advanced oxidation. Advanced oxidation, when combined with other technological methods such as microwave [3], ultrasound [4], light [5], and electricity [6], exhibits excellent degradation capabilities. These approaches play a crucial role in addressing the environmental impact of antibiotics.

Photocatalytic technology has distinct advantages over other environmental remediation methods. It can utilize visible light efficiently, work under mild conditions [7], and is low-cost and easy to operate [8]. Photocatalytic materials for pollutant degradation under light conditions can achieve this goal without additional agents [9, 10]. Currently, Titanium dioxide (TiO₂) stands out as a common photocatalytic material in both domestic and international research, finding widespread use in photocatalysis [11]. However, like many other photocatalytic materials, TiO₂ has limitations. For instance, it possesses a large band gap, making it only excitable by ultraviolet light, and its utilization of sunlight remains low, restricting broader application [12]. Consequently, researchers such as Garg [13] and Sudhaik [14] are actively working to enhance its activity and explore alternative photocatalytic materials. In their investigations, bismuth-based materials and their composites have demonstrated stable photocatalytic performance [15]. Notably, unlike conventional photocatalysts, materials containing bismuth (Bi) elements exhibit diverse luminescent properties when exposed to light [16, 17]. As a result, scientists are increasingly focusing on developing Bi-based photocatalytic materials with coordinated features and modulation characteristics [18].

Bi-based semiconductors represent a novel class of photocatalytic materials known for their high activity, good stability, and environmentally friendly, non-toxic properties [19, 20]. Among these materials, bismuth tungstate (Bi₂WO₆) stands out as a promising bismuth semiconductor with broad applicability. However, it has certain limitations, notably a wide bandgap that restricts its effectiveness in photocatalytic pollution degradation. Specifically, the high bandgap of bismuth semiconductors limits their suitability for photocatalytic degradation of contaminants. Consequently, researchers have explored modification techniques to enhance the performance of Bi-based semiconductors. These techniques include nonionic doping [21, 22], defect modulation, crystal surface engineering [23], metal surface deposition [24], and the creation of heterojunctions [25], all of which have demonstrated efficacy in improving the photocatalytic properties of Bi-based materials. Rare earth elements, characterized by their unique electronic structure [26], play a pivotal role in this context. The incomplete filling of 4f electrons in their inner layers gives rare earths distinctive optical and magnetic properties [27], leading to diverse applications in catalysis. In this study, we evaluate the photocatalytic characteristics and mechanisms of Yb/Bi₂WO₆ for the degradation of ofloxacin (OFX) in water.

2. Experimental Section

2.1. Experimental Materials and Apparatus

Bismuth nitrate pentahydrate (Bi(NO₃)₃•5H₂O, 99%), sodium tungstate dihydrate (Na₂WO₄•2H₂O, 99.5%), ytterbium nitrate pentahydrate (Yb(NO₃)₃•5H₂O, 99.99%), and ofloxacin (C₁₈H₂₀FN₃O₄, 99%) were purchased from Shanghai Maclin Biochemical Technology Co., LTD. Sodium hydroxide (NaOH, 98%) and nitric acid (HNO₃, 65%–68%) were purchased from Chengdu Colon Chemical Co., LTD. Polyvinylpyrrolidone (PVP, AR) was purchased from Wuxi Yatai Combined Chemical Co., LTD. Sodium oxalate (Na₂C₂O₄, AR) was purchased from Guangdong Jinhua Da Chemical Reagent Co., LTD. Disodium EDTA (EDTA-2Na, 99%) was purchased from Sinopharm Group chemical reagents; Isopropyl alcohol (IPA, 99.7%) was purchased from Tianjin Kemo Chemical Reagent Co., LTD., and P-benzoquinone (p-BQ, 97%) was purchased from Shanghai Aladdin Biochemical Technology Co., LTD.

2.2. Preparation of Photocatalyst

A mmol Bi(NO₃)₃•5H₂O and B mmol Na₂WO₄•2H₂O were dissolved in HNO₃ solution (10 mL, 1 mol L⁻¹) and 10 mL deionized water, respectively, and then the two solutions were mixed and stirred on a magnetic mixer at room temperature for 60 min. In addition, C mmol Yb(NO₃)₃•5H₂O and 0.05 g PVP were added to the above mixed solution and fully stirred until completely dissolved. The pH of the mixed solution was adjusted to 4 with NaOH (1.5 mol L⁻¹), and the adjusted pH of the mixed solution was transferred to a high-pressure reactor lined with polytetrafluoroethylene, and the reaction was carried out at 160°C for 720 min. After the reaction, the product was filtered, washed repeatedly, and dried at 80°C for 360 min to obtain the final product Yb/Bi₂WO₆.

The schematic diagram of the preparation process is shown in Fig. 1, and the names of Yb/Bi₂WO₆ prepared under different conditions are shown in Table 1.

Fig. 1

Schematic of the preparation of Yb/Bi₂WO₆.

Table 1

The name of different Yb/Bi₂WO₆ (unit: mmol)

2.3. Characterization of The Photocatalyst

The composites were subjected to a series of characterizations, and the performance and mechanism of photocatalytic degradation of OFX by different Yb/Bi₂WO₆ were investigated based on the characterization results and experimental results.

The surface morphology and elemental composition of Yb/Bi₂WO₆ were imaged with scanning electron microscopy (SEM, MIRA LMS, Tescan, Czech Republic) and energy spectrometry (EDS, Xplore 30, Oxford, Britain). The crystal structure and composition of the materials before and after bimetallic strengthening were analyzed using an X-ray diffractometer (XRD, Bruker A24A10, Germany). The morphology of the forms of elements present on the surface of the samples was analyzed using an X-ray photoelectron spectrometer (XPS, Thermo Scientific K-Alpha, USA). Absorption edges of materials before and after bimetallic strengthening were analyzed by ultraviolet-visible diffuse reflectance spectroscopy (UV–Vis DRS, Shimadzu UV-3600i Plus, Japan).

2.4. Degradation Experiments

A 300 W xenon lamp was used to simulate sunlight and OFX was degraded under light irradiation to evaluate the performance of the composite photocatalytic materials.

Prepared 100 mL of 20 mg L⁻¹ OFX solution and weighed 50 mg of Yb/Bi₂WO₆ into the OFX solution. The prepared photocatalytic system was placed in a dark box for 30 min to reach the adsorption-desorption equilibrium, and then the xenon lamp was turned on and the power was adjusted to 300 w. Throughout the photoreaction process, the reaction solution was stirred continuously. After the photoreaction process started, 3 mL of the reaction solution was taken at 10 min, 15 min, 20 min, 25 min, 30 min, 60 min, 90 min, 120 min, 150 min, and 180 min. The removed reaction solution was detected by UV spectrophotometer and the degradation rate D (%) was calculated according to Eq. (1):

(1) D=C0-CtC0×100%

where C₀ is the initial OFX concentration, mg L⁻¹, C_t is the OFX concentration at moment t, mg L⁻¹.

2.5. Stability and Reusability

In practical applications, photocatalysts require good photochemical stability to facilitate subsequent recycling.

Prepared 20 mg L⁻¹ OFX solution and weighed 50 mg of Yb/Bi₂WO₆, the rest of the reaction conditions and procedure were the same as 2.4. The reacted Yb/Bi₂WO₆ was collected, centrifuged, and washed repeatedly, and then placed in an oven for drying. The dried material continued to be used for the photocatalytic degradation of OFX. This experiment was done for 4 cycles, and the photocatalytic degradation of OFX by Yb/Bi₂WO₆ was determined by measuring the absorbance using a UV spectrophotometer after each experiment.

2.6. Quenching Experiment

The effects of free radicals on the photocatalytic efficiency were analyzed by adding Sodium oxalate (Na₂C₂O₄) and Disodium EDTA (EDTA-2Na) to capture hole (h⁺), p-benzoquinone (p-BQ) to capture superoxide radical (•O₂⁻), and Isopropanol (IPA) to capture hydroxyl radical (•OH).

3. Results and Discussion

3.1. Photocatalytic Capability

3.1.1. Photocatalytic degradation of OFX

At present, the photocatalytic degradation of organic pollutants using Bi-based composites doped with rare earth elements is shown in Table 2. As can be seen from Table 2, rare-earth doped bismuth matrix composites are mostly used in the degradation of organic dyes, and the degradation experiments on antibiotics are few and the degradation effect of antibiotics is not particularly ideal. In this work, Yb/Bi₂WO₆ is prepared by a simple hydrothermal method and applied to the photocatalytic degradation of OFX in water. It provides a theoretical basis for the study of photocatalytic degradation of antibiotics in rare earth-doped bismuth matrix composites.

Table 2

Comparison of photocatalytic degradation of organic pollutants in rare earth doped bismuth matrix composites

OFX solutions with a concentration of 20 mg L⁻¹ were prepared, along with two sets of experiments: one containing 50 mg of the B₂W₁Y_0.2 and the other without any composite material input. These solutions were placed in a photocatalytic reactor under identical experimental conditions. The specific operational steps are detailed in section 2.4. The purpose of this setup was to verify that the degradation of OFX under light conditions resulted from the addition of the composite material. Notably, the reaction system containing 50 mg of B₂W₁Y_0.2 achieved nearly complete OFX degradation by the end of the reaction, with a degradation rate of 98.74%, as depicted in Fig. 2. In contrast, the concentration of OFX solution in the reaction system without the addition of B₂W₁Y_0.2 remained virtually unchanged throughout the reaction duration. Given that the sole variable in this experiment was the presence or absence of the B₂W₁Y_0.2, we can reasonably conclude that this material indeed exhibits photocatalytic degradation of OFX in the presence of light.

Fig. 2

Photocatalytic degradation of OFX by unadded B₂W₁Y_0.2 and 50 mg of B₂W₁Y_0.2 added.

3.1.2. Exploration of influencing factors

To investigate the photocatalytic degradation of OFX by the different Yb/Bi₂WO₆, as well as the photocatalytic degradation of OFX by Yb/Bi₂WO₆ under different pH, light intensity, and initial concentration of OFX.

Fig. 3(a) shows the photocatalytic degradation of OFX by B₂W₁Y_0.2, B₁W₁Y_0.1 and B₁W₂Y_0.1. From Fig. 3(a), it can be seen that B₂W₁Y_0.2 showed the best photocatalytic degradation of OFX, degrading almost all of OFX in 180 min, with a degradation rate of 98.74%; the photocatalytic degradation of OFX by B₁W₁Y_0.1 was significantly lower than that of B₂W₁Y_0.2, with a degradation rate of 78.26%; the B₁W₂Y_0.1 had a significant increase in the adsorption of OFX in the dark reaction stage, and the photocatalytic degradation of OFX was also significantly weaker than that of the other two materials, with a degradation rate of only 65.81%.

Fig. 3

Photocatalytic degradation of OFX by Yb/Bi₂WO₆ under different conditions (a) different Bi/W ratios (b) different Yb doping (c) different pH (d) different initial OFX concentrations (e) different light intensities.

As can be seen in Fig. 3(b), the photocatalytic degradation rate of the BW material with un-doped Yb was only 40% within 180 min, while the photocatalytic degradation rate of B₂W₁Y_0.1, B₂W₁Y_0.2 and B₂W₁Y_0.4 could all achieve photocatalytic degradation of OFX with the degradation rate of 96.59%, 98.74% and 98.55%. But B₂W₁Y_0.4 showed the fastest degradation rate of OFX, with a degradation rate of 88.87% of OFX at 30 min of the photoreaction, while the degradation rates of B₂W₁Y_0.1 and B₂W₁Y_0.2 were 79.81% and 84.8%.

As can be seen in Fig. 3(c), the photocatalytic degradation of OFX by B₂W₁Y_0.2 appeared to increase and then decrease with the increase of pH by varying the initial pH of OFX solution and reached the maximum value (98.74%) at pH = 7. It was shown that the pH value in the system affects the charge on the surface of the photocatalyst and the protonation state of the pollutants [34]. The degradation rate of OFX by B₂W₁Y_0.2 was 84.63% when the pH was 4, which decreased from a small decrease compared to pH = 7. At all other pH values, the degradation rate was significantly smaller than that at pH = 7. These results indicate that the B₂W₁Y_0.2 has high photocatalytic performance under neutral and weakly acidic conditions.

A total of four different initial concentrations of OFX solutions (10–50 mg L⁻¹) were set in this experiment, and all four OFX solutions were added 50 mg of B₂W₁Y_0.2. From Fig. 3(d), it can be seen that the B₂W₁Y_0.2 degraded almost all of the OFX in the four sets of experiments under the same conditions of other experimental conditions, but the degradation effect of the B₂W₁Y_0.2 on the OFX was weakened with the increase of the initial concentration of the OFX solution. This may be due to the presence of excessive OFX in the solution, which increases the pollution load in the photocatalytic system, leading to a decrease in the degradation rate of OFX by the B₂W₁Y_0.2 under the same experimental conditions.

Since the intensity of light is proportional to the output power, the power is used to indirectly represent the intensity of light in this experiment. As seen in Fig. 3(e), the degradation rate of OFX by B₂W₁Y_0.2 increases with the enhancement of light intensity, which also leads to the conclusion that light is used as a source of energy for the degradation of OFX by the composites. The stronger light intensity and higher power represent the higher energy provided by the light source to the photocatalytic degradation of OFX by B₂W₁Y_0.2 [35, 36].

3.1.3. Re-usability

To investigate the recycling properties of the materials, the reacted B₂W₁Y_0.2 were subjected to repeated centrifugation, washing, and drying in an oven at 80°C for subsequent reactions. The experiment was repeated a total of four times. As depicted in Fig. 4, the initial B₂W₁Y_0.2 achieved a photocatalytic degradation rate of 98.74% for OFX. After four repetitions of the experiment, the photocatalytic degradation of OFX by B₂W₁Y_0.2 remained high at 93.27%, with only a 5.47% reduction. These results indicate that the B₂W₁Y_0.2 exhibits a certain degree of recycling performance.

Fig. 4

Photocatalytic degradation of OFX by four cycles of B₂W₁Y_0.2.

3.2. Photocatalytic Mechanism Research

3.2.1. Photocatalytic mechanism research

To analyze the morphology of the composite photocatalytic materials, scanning electron microscopy (SEM) was employed. As depicted in Fig. 5, the Yb-undoped BW material exhibits a multilayer sheet structure, forming a nanospherical arrangement. Remarkably, this structure resembles the smooth 2D nanosheets stacked atop each other to create a 3D multilayered floral microsphere structure observed in pure-phase Bi₂WO₆, as reported in relevant literature [37]. In contrast, the B₂W₁Y_0.2 exhibits a snowflake-like morphology while retaining the spherical nanoparticle shape of the original multilayered nanosheets. These snowflakes are intricately interleaved, a phenomenon likely attributed to the Yb³⁺ doping, which inhibits the surface growth of Bi₂WO₆ and induces changes in the catalyst’s morphology.

Fig. 5

(a) BW material at 500 nm (b) B₂W₁Y_0.2 at 500 nm.

To further analyze the crystalline phase structure as well as the surface elements of the Yb/Bi₂WO₆, the materials were characterized by XRD and XPS. According to the information, crystallinity is one of the important factors affecting the photocatalytic performance of the materials [38]. As shown in Fig. S1(a), the diffraction peaks of BW, B₂W₁Y_0.1, B₂W₁Y_0.2, and B₂W₁Y_0.4 are consistent with those of PDF#26-1044, and the diffraction peaks are located at 28.58°, 33.03°, 47.31°, 56.03°, 58.36°, and 68.42°, which correspond to the (103), (200), (220), (303), (107) and (400) crystal planes of Bi₂WO₆ [39], with the highest peak diffraction peak located at 2θ = 28.586. After Yb³⁺ doping, the position of the Bi₂WO₆ diffraction peaks of each sample was not altered; rather, the peak value was reduced and gradually weakened with increasing Yb doping. It appears that Yb³⁺ doping does not create a new crystalline phase, but only alters the size of the Bi₂WO₆ grains, which can also explain why B₂W₁Y_0.1, B₂W₁Y_0.2, and B₂W₁Y_0.4 in 3.1.2 on the OFX degradation rates are different. The best crystallinity is seen in B₂W₁Y_0.2 as evidenced by the obvious presence of the distinctive Bi₂WO₆ diffraction peaks in Fig. S1(b), whereas these peaks are gradually vanishing or even nonexistent in B₁W₁Y_0.1 and B₁W₂Y_0.1. In conjunction with the slower rate of OFX degradation by B₁W₁Y_0.1 and B₁W₂Y_0.1 compared to B₂W₁Y_0.2 in 3.1.2, the slower degradation may be due to the altered internal structure of the composites reducing the photocatalytic performance. Combining with the EDS elemental mapping of the B₂W₁Y_0.2, as shown in Fig. S2(a) – (e), it can also be seen that the Yb are uniformly distributed on the surface of the B₂W₁Y_0.2, which further proves the successful doping of Yb.

To further demonstrate the successful preparation of B₂W₁Y_0.2, the composites were characterized by XPS. Based on the XPS characterization results, the material elemental composition of B₂W₁Y_0.2 before and after Yb doping as well as the chemical valence states of each element can be obtained. As can be seen in Fig. S3, the prepared B₂W₁Y_0.2 contains Bi, W, O, and Yb elements. It is proven that the elemental Bi in the B₂W₁Y_0.2 created in the current experiment exists in the form of Bi³⁺ thanks to the two significant peaks at 159.08 eV and 164.38 eV that are identified as Bi 4f_7/2 and Bi 4f_5/2, respectively, with a difference of 5.3 eV, as shown in Fig. S4(a) [40, 41]. Two peaks at 159.78 eV and 165.08 eV, meanwhile, show that Bi-O bonds are present in the B₂W₁Y_0.2. The peak binding energies of W 4f_7/2 and W 4f_5/2 are shown in Fig. S4(b), with a difference of 2.17 eV, or by the standard deviation value of W⁶⁺, indicating that W is present in the +6-valent form in the B₂W₁Y_0.2 [42]. The peaks at 530.28 eV and 532.33 eV, which are in agreement with the previous Bi 4f observations, are the O 1s signals of the Bi-O and C=O bonds, as can be shown in Fig. S4(c). The peaks at 284.88 eV and 288.46 eV in Fig. S4(d) correspond to the C 1s signals of C-C and C=O, and the peaks are consistent with the aforementioned O 1s observations. As can be seen in Fig. S4(e), Yb 4d exhibits an obvious peak at 164.68 eV, demonstrating that Yb is doped into Bi₂WO₆ in the form of Yb³⁺. In conclusion, it was further proven that B₂W₁Y_0.2 could be prepared successfully.

3.2.2. Band structure analysis

In order to verify that the Yb doping played a role in broadening the photoresponse range of B₂W₁Y_0.2, the UV-vis diffuse reflectance spectra (UV-vis) of BW and B₂W₁Y_0.2 were analyzed. As shown in Fig. S5, after Yb doping, the absorption spectra of B₂W₁Y_0.2 was shifted in the direction of small wavelengths. This shift enhances the light absorption ability of B₂W₁Y_0.2 in the visible region to some extent, making them more photoresponsive and improving the utilization of visible light compared with the undoped Yb material. The bandgap energy is calculated from Tauc formula [43] as in Eq. (2):

(2) (αhv)n=A(hv-Eg)

where α is the absorbance index, h is Planck’s constant, v is the frequency of light, Eg is the semiconductor forbidden bandwidth, A is a constant, and n is related to the type of semiconductor, with n being taken as 2 for direct bandgap type semiconductors and 1/2 for indirect type semiconductors.

In Fig. S5, it can be seen that there is a sharp decrease in the spectral curves of the BW and B₂W₁Y_0.2, indicating that the absorption of visible light is caused by the jumps in the band gap energy levels rather than the impurity energy levels [44]. The bandgap energy of B₂W₁Y_0.2 changes from 3.25 eV to 3.13 eV, indicating that the bandgap energy value can be reduced after Yb doping, which results in a reduction in the distance of the electron leaps. This improves the photo-responsibility of B₂W₁Y_0.2 and demonstrates a strong photocatalytic ability to degrade pollutants.

3.2.3. Possible photocatalytic reaction mechanism

To explore the free radicals that played a role in the photocatalytic degradation of OFX in B₂W₁Y_0.2, free radical quenching experiments were performed. Na₂C₂O₄ (10 mM), and EDTA-2Na (10 mM) were used to capture h₊, p-BQ (10 mM) was used to capture •O₂⁻, and IPA (10 mM) to capture •OH.

As shown in Fig. S6, the degradation rate of OFX was significantly decreased by the addition of p-BQ, and the degradation efficiencies of OFX were slightly decreased by the addition of Na₂C₂O₄ and EDTA-2Na, whereas the degradation rate of OFX did not undergo a significant change and only decreased by 0.1%. It can be concluded that the inhibition order of free radicals for the photocatalytic degradation of OFX in B₂W₁Y_0.2 is •O₂⁻ > h⁺ > •OH.

Combined with the above characterization, the following speculations as well as explanations for the mechanism of photocatalytic degradation of OFX by the different Yb/Bi₂WO₆ were made in this study.

Yb belongs to the lanthanide system, due to the lanthanide contraction, the rare earth ion radius decreases with the increase of atomic number [45], the ion radius La³⁺ > Ce³⁺ > ...... > Yb³⁺ > Lu³⁺. Checking the related information, we know that the ionic radius of La³⁺ is 0.115 nm [46], from which it can be concluded that the ionic radius of Bi³⁺ (0.1385 nm) [47] is larger than that of Yb³⁺, so Yb³⁺ can be doped into Bi₂WO₆ more easily [38].

When the Yb/Bi₂WO₆ are excited by light energy, photoelectrons undergo leaps and leave holes in the valence band, generating high-energy electron-hole pairs [48], which generate active groups for photocatalysis and ultimately degrade pollutants into H₂O and CO₂ [49]. Meanwhile, combined with the SEM characterization results, the doping of Yb had an inhibitory effect on the growth of Bi₂WO₆, which led to the change in the morphology of Bi₂WO₆. The XRD characterization results showed that there were differences in the crystalline phases of the generated individual Yb/Bi₂WO₆ samples, which were specifically reflected in the decreasing photocatalytic degradation rate of OFX by B₁W₁Y_0.1 and B₁W₂Y_0.1. The Bi₂WO₆ characteristic peaks of B₂W₁Y_0.1, B₂W₁Y_0.2, and B₂W₁Y_0.4 were obvious, indicating that the synthesized composites did not change the original crystal phase structure of Bi₂WO₆. Therefore, the alteration of the photocatalytic properties of the BWY composites is related to the doping of Yb. However, the characteristic peaks of obvious Yb did not appear in the XRD patterns, which may be related to the low doping amount of Yb [50]. The special electronic configuration in Yb³⁺ makes it necessary to trap an e⁻ to reach the half-full state [51]. Electrons (e⁻) in the VB of the composites are energized by visible light irradiation to be able to leapfrog to the CB, leaving behind a large amount of hole (h⁺) in the VB and retaining a large amount of e− in the CB [52]. The doping of Yb³⁺ replaces Bi³⁺, and Yb³⁺ is reduced to Yb²⁺ after getting excess e⁻, thus improving the defects of Bi₂WO₆. Yb³⁺ doped into the structure of Bi₂WO₆ captures some free electrons from the environment on its surface, and these captured e⁻ will have a redox reaction with O₂ on the surface of the composites to generate •O₂⁻ with strong oxidizing property so that Yb²⁺ is oxidized again to the stable form of Yb³⁺. Due to the conversion process of Yb³⁺ and Yb²⁺ the complexation of electrons and holes inside the composite is delayed [53], which also allows more reaction time for •O₂⁻ formation. Since the valence band potential is not positive enough, the holes cannot oxidize OH⁻ to •OH, but the holes can directly react with the pollutants [54]. The generated •O₂⁻ has a strong promotional effect on the photocatalytic process [55], which facilitates the photocatalytic degradation of composites for OFX. Combined with UV-vis mapping and Tauc mapping, the doping of Yb can shorten the band gap of the material, resulting in a further improvement of the photocatalytic performance. The possible photocatalytic reaction mechanism is shown in Fig. S7. The possible photocatalytic reactions are showed in Eq. (3) – (8):

(3) Yb/Bi2WO6+hv→e-+h+

(4) Yb3++e-→Yb2+

(5) e-+Yb/Bi2WO6+O2→·O2-

(6) Yb/Bi2WO6+·O2-+Yb2+→Yb3+

(7) ·O2-+OFX→degradation

(8) h++OFX→degradation

4. Conclusions

In summary, Yb/Bi₂WO₆ were prepared by a simple one-step hydrothermal method and applied to photocatalytic degradation of OFX in water, which provided a certain theoretical basis for the study of photocatalytic degradation of antibiotics in rare earth doped bismuth-based composites. The results showed that the doping of Yb effectively improved the defects of the pure-phase Bi₂WO₆ with large forbidden bandwidths and fast electron-hole pair compounding efficiency, which led to the improvement of the photocatalytic performance of the composites. Under the optimal experimental conditions, B₂W₁Y_0.2 degraded 98.74% of OFX within 180 min of the photocatalytic reaction, and the photocatalytic degradation rate increased by 58.74% compared with Bi₂WO₆, indicating that doping Yb³⁺ can improve the performance of the photocatalytic material. After 4 cycles, the OFX degradation rate was 94.47%, and the photocatalytic material exhibited good reusability. The characterization results indicated that the multilayered lamellar nanorodular morphology of the BW material appeared as a lamellar stacking of snowflake flake morphology after Yb doping, but the doping of Yb did not change the crystalline phase structure of BW itself, and it only changed the intensity of the characteristic peaks of Bi₂WO₆ in the composites. XPS and SEM-EDS characterization results verified the successful doping of Yb. UV-vis characterization verified that the doping of Yb can shorten the bandgap of the composites, which was favorable for the photogenerated electron leaps. The results of free radical quenching experiments indicated that superoxide radical (•O₂⁻) played a major role in the photocatalytic degradation of OFX. Since the photocatalytic degradation experiments are conducted in the environment of a single antibiotic, the actual effluent composition is more complex. In the future, photocatalytic degradation experiments will be carried out under the coexistence of multiple interfering substances, or photographs of photocatalytic data for diverse antibiotics will be further explored.

Supplementary Information

eer-2024-127-Supplementary.pdf

Notes

Author contributions

All authors contributed to the conceptualization and design of the study. B.J. (Master student) designed the study, operated the experiments, processed & analysed the data, edited the original draft, and revised the manuscript; L.S.W. (Professor) provided supervision, and checked & revised the manuscript; J.Z. (Researcher), J.J.L. (Master student), D.L. (Master student), Y.Z.C. (Master student), X.W.L. (Master student), and Y.L.Z. (Master student) made data analysis and manuscript’s checking.

^{Conflict of Interest Statement}

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Catalyst	Base material	Rare earth element	Target pollutant	Degradation	Ref.
Tm³⁺/Yb³⁺:BiVO₄/Ag₂O	BiVO₄	Tm³⁺/Yb³⁺	Rh B	86%	[28]
Tm³⁺doped Bi₂WO₆	Bi₂WO₆	Tm³⁺	Rh B	91.27%	[29]
Yb³⁺-doped BiPO₄	BiPO₄	Yb³⁺	MB	80.2%	[30]
Y-BiOCl	BiOCl	Y³⁺	TC	90.3%	[31]
La-BiOBr	BiOBr	La³⁺	CIP	48%	[32]
Ce-Bi₂O₃	Bi₂O₃	Ce³⁺	TC	89.1%	[33]
Yb/Bi₂WO₆	Bi₂WO₆	Yb³⁺	OFX	98.74%	This work

Number	A	B	C	Name
1	5	2.5	—	BW
2			0.02	B₂W₁Y_0.1
3			0.04	B₂W₁Y_0.2
4			0.08	B₂W₁Y_0.4
5	5	2.5	0.04	B₂W₁Y_0.2
6	2.5	2.5		B₁W₁Y_0.1
7	2.5	5		B₁W₂Y_0.1