Photocatalytic Degradation Efficiency Research Articles

Constructing surface oxygen vacancy in the [Bi2O2]2+ layer defects mediated Bi2MoO6 enhanced visible light responsive photocatalytic activity.

Bi2MoO6 nanospheres with surface oxygen vacancies (SOVs) controlled by the calcination process were prepared in this study. Performance testing revealed that the Bi2MoO6-4 sample (Bi2MoO6 calcined at 350 °C for 4h) with SOVs achieved a remarkable photocatalytic degradation efficiency up to 99.16% for Rhodamine B (RhB) within 50min, which is 2.19 times higher than that of pure Bi2MoO6. The higher photocatalytic performance of the Bi2MoO6-4 sample is attributed to the SOVs' defect level located at the Bi2MoO6 bandgap, narrowing the bandgap to effectively promote the photogenerated charge separation. The promotion of photocarrier separation and electron were transferred due to the Bi-O bond breakage in the Bi2MoO6-4 [Bi2O2]2+ layer, which mediates the defect level of SOVs in the band structure. The density functional theory calculation results reveal the possible formation site of the oxygen vacancy and the vacancy-induced defect states. This study provides a new approach for fabricating new photocatalysts with surface oxygen defects.

La-supported SnO2–CaO composite catalysts for efficient malachite green degradation under UV–vis light

This study presents the development and optimization of La@SnO2–CaO composite catalysts for efficient photocatalytic degradation of malachite green dye in aqueous solutions under UV–vis light irradiation. The catalysts were prepared via conventional incipient-wetness impregnation and thoroughly characterized using advanced analytical techniques, including X-ray diffraction, Fourier transform infrared spectroscopy, UV–vis diffuse reflectance spectroscopy, N2 adsorption–desorption analysis, and scanning electron microscopy. To optimize photodegradation efficiency, the effects of three independent factors were systematically investigated using response surface methodology: Temperature, pH, and Sn/Ca molar ratio. Our results reveal optimal conditions for maximum dye degradation: pH 7, Sn/Ca molar ratio of 0.33, and a process time of 35 min, resulting in a maximum photodegradation efficiency of 98.80% for malachite green dye. Notably, visible light exhibited a more pronounced effect on dye degradation compared to UV light over time, with visible light achieving 25% greater dye removal after 60 min of illumination. Furthermore, the catalyst showed excellent recyclability, retaining 85% of its initial activity after five consecutive cycles. These findings contribute significantly to the development of sustainable methods for dye removal from wastewater and highlight the potential of La@SnO2–CaO composite catalysts in environmental remediation processes, particularly in treating textile industry effluents.

Efficient and rapid sunlight-driven photocatalytic degradation of methylene blue dye using multiferroic BiFeO3 nanoparticles

Efficient and rapid sunlight-driven photocatalytic degradation of methylene blue dye using multiferroic BiFeO3 nanoparticles

Enhancing the Light‐Driven Photocatalytic Degradation Activity of ZnO Nanoparticles With the Synergistic Incorporation of Er and Tb Elements

ABSTRACTThe existence of organic pollutants in aqueous media has become a vital issue and a critical threat to human health, where organic toxic dyes represent the major contaminants in wastewater. Over the past few years, photocatalytic techniques have garnered a lot of interest in dye removal from wastewater. In this study, a series of ErxTbxZn1 − 2xO nanophotocatalysts (where x = 0.00, 0.01, 0.03, and 0.05) were synthesized and coded as ZET0, ZET1, ZET2, and ZET3 NPs (nanoparticles). The chemical and physical characteristics of the NPs were investigated using Fourier‐transform infrared spectroscopy (FT‐IR), X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), energy‐dispersive X‐ray (EDX) spectroscopy, and ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS) techniques. Further examination by performing photocatalytic experiments, ZET1 NPs demonstrated effective Rhodamine B (RhB) dye degradation within 60 min. It was found that the kinetic rate constant values were 0.008, 0.097, 0.050, and 0.040 min−1 for ZET0, ZET1, ZET2, and ZET3 NPs, respectively. Aside from their remarkable photocatalytic degradation efficiency, these ZET1 photocatalysts are highly stable even after five consecutive cycles. In addition, the active species test revealed that the primary oxidation species involved in the photocatalytic process are holes (h+) and hydroxyl radicals (•OH), and a possible photocatalytic mechanism for degrading RhB by ZET1 photocatalysts was tentatively proposed. The enhancement of the photocatalytic degradation efficiency is due to the low recombination rate of photogenerated charge carriers, as well as a strong synergistic impact of Tb, Er, and ZnO components. Thus, the current study could offer a versatile strategy for the design of new and effective nano photocatalysts for wastewater purification in the future.

Green synthesis of N-doped-carbon dots/ZnO for enhanced photocatalytic degradation of methylene blue dye: optimization of reaction parameters.

In this research work, nitrogen-doped carbon dots (N-CDs) adorned zinc oxide nanoparticles (N-CDs/ZnO) were successfully synthesized by a simple and cost-effective solution dispersion method and later on used as photocatalyst for decontamination of aqueous methylene blue (MB) dye on irradiation of UV light (300 W, 320-400nm) at room temperature. Both the N-CDs and ZnO were prepared through green technique utilizing non-toxic, inexpensive and eco-friendly precursors, namely Foeniculum vulgare and Psidium guajava leaf extract, respectively. All the synthesized samples exhibited crystalline nature with average diameter of particle 4.42nm, 12.38nm and 14.11nm corresponding to N-CDs, ZnO and N-CDs/ZnO, respectively. Further, band gap energy value (Eg) of 3.43, 2.76 and 2.49eV for N-CDs, ZnO and N-CDs/ZnO, respectively, were obtained by using Tauc's plot. The photocatalytic capability of the sample N-CDs/ZnO was compared with bare ZnO nanoparticles, utilizing identical experimental conditions. The results demonstrated that the composite exhibited notably higher photocatalytic degradation efficiency than bare ZnO nanoparticles up to 15.54%. Lower band gap value of N-CDs/ZnO was the major factor for exhibiting this behaviour, decreasing the recombination rate and thus enhancing the efficiency. Furthermore, N-CDs/ZnO exhibited 98.17% MB degradation under optimized conditions (0.03g, 5ppm, pH 10). The resultant N-CDs/ZnO exhibited good stability and decontamination efficiency up to five cycles with efficiency loss of only 7.89%. Along with, trapping experiments was conducted to analyze the role of active species involved for deep understanding of mechanism. The order of efficiency of active constituents was observed to be: •O2- > h+ > •OH. The study analyzed the non-toxic nature of treated water, revealing normal plant growth, suggesting its potential use in irrigation of parks and roadside areas. Overall, present research work obeys the green chemistry principles with the fabrication of highly efficient, eco-friendly, cost-effective photocatalyst N-CDs/ZnO by utilizing the green precursors for the whole research work.

Efficient photocatalytic degradation of potent greenhouse gas SF6 at liquid-solid interface

Sulfur hexafluoride (SF6), widely used in industries, poses a significant threat as the strongest greenhouse gas. Current methods for degrading SF6 are hindered by high energy consumption and costs, thus there is an urgent need for a low-cost, convenient, and efficient solution. In response, we established a gas-liquid-solid three-phase (GLS) system for efficient photocatalytic SF6 degradation by Bi2O2CO3, whose efficiency was approximately 20 times that of traditional gas-solid systems. Further, possible degradation mechanisms were demonstrated through the detection of material changes in three phases, respectively. Detailed studies revealed Bi2O2CO3 can in-situ form Bi2S3 heterojunction during the reaction, facilitating carrier separation. Besides, O2 can transform to ·O2− and accelerate the photodegradation process. This approach offered a novel toolbox for SF6 degradation. We predict this method will have a wider range of applications in the field of solar-powered catalytic degradation of extremely stable fluorinated pollutants at lower costs in the future.

Target detection and simultaneous degradation of furazolidone by molecularly imprinted CoWO4/g-C3N4

In this study, a "Two birds with one stone" strategy was employed to concurrently address the challenges of electrochemical detection and photocatalytic degradation of the environmentally hazardous furazolidone (FRZ). An innovative molecularly imprinted CoWO4/g-C3N4 (i.e. MIP-CoWO4/g-C3N4) nanomaterials was constructed through a combination of hydrothermal and electrochemical polymerization techniques. The resulting MIP-CoWO4/g-C3N4 nanocomposites can be served as an exceptional sensing platform for FRZ detection, demonstrating low detection limit (2.61×10−13 mol L−1), considerable sensitivity (2.03 μA. nM−1. mm−2), excellent selectivity and decent stability. Meanwhile, the MIP-CoWO4/g-C3N4 nanocomposites exhibited remarkable photocatalytic efficiency for FRZ degradation (79.87 %). In addition, the systematic investigation into the impact of operational reaction parameters on both the electrocatalytic and photocatalytic reactions was conducted, elucidating the plausible mechanisms for both detecting and degrading FRZ. This research provides firm foundation for the development of multifunctional nanomaterials, further advancing the "integration of diagnosis and treatment" in environmental management and control.

Facile synthesis of CQD/g-C3N4 as a highly effective metal-free photocatalyst for the degradation of carmoisine and indigo carmine dye

The development of extensive dye pollution in the water ecosystem seriously endangers the health of the living organism. For effectively eradicating dyecontaminants from water bodies, photocatalysis is consideredan effective, energy-consumption, inexpensive disinfection technique. As an alternative to conventional metal-based catalysts, heterogeneous metal-free photocatalysts are more sustainable and kinder to the environment. Here, we reported the simplehydrothermalchemical production of Carbon Quantum dots (CQD)-doped graphitic carbon nitride(GCN). Analytical instruments such as XRD, SEM, FE-SEM, HR-TEM, EDX, FT-IR, thesurface area of BET analysis, photoluminescence, and UV–vis spectroscopy, zeta potentialwere used to characterize the as-prepared CQD doped GCN (CDCN). The degradation studies reveal that the CDCN catalyst displays the highest rate of degradation performance than pure GCN. Within 60 min, it shows 96 % degradation toward indigo Carmine (IC) and 93 % decomposition towards carmoisine dye (CM). Significant e−/h+ separation, an increased surface area, and a high redox potential capacity to induce charge bears may all contribute to the CDCN catalyst’s enhanced photocatalytic degradation efficiency. The efficiency of the photocatalytic process was optimized by studying and altering many variables. These include Dye concentration, catalyst concentration, and variation of the pH solution were some of these. The nanocomposite exhibited excellent stability after three successive runs of the photocatalytic procedure. According to the kinetics analysis results indicate that the photocatalytic decomposition of Indigo Carmine (IC) and carmoisine (CM) dye follows pseudo-first-order kinetics. For better photodegradation performance, a potential photocatalytic method employing several pairs of electron-hole acceptor scavengershas been put forth. Based on the positionsof the band gap and the result of the characterization, a feasible mechanismpathway for charge carriers was also presented.

Enhancement of the photocatalytic efficiency of ZnO utilizing luminescent Y2O3 particles

Abstract This study explored the superior photocatalytic performance of the nanoscale zinc oxide-yttrium oxide (ZnO-Y2O3) based composite over ZnO nanoparticles (NPs). We investigated this by following the photocatalytic degradation efficiency of methylene blue (MB). The sol–gel synthesized Y2O3 and ZnO NPs were characterized by x-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The spectral behaviour and photocatalytic efficiency of the proposed composite were investigated by UV–vis spectrophotometry, steady-state and time-resolved photoluminescence (PL) measurements, respectively. The results indicated the successful formation of Y2O3 and ZnO nanoparticles with desirable structural properties. The photocatalytic degradation efficiency of the MB solution was evaluated for different concentrations of the counterparts of the ZnO-Y2O3 composite. In particular, the ZnO-5Y2O3-based composite showed superior photocatalytic activity after 150 min of UV irradiation, achieving 98.4% degradation of the MB solution compared to the 77% degradation achieved by pure ZnO. Although Y2O3 alone does not exhibit photocatalytic activity, its combination with ZnO significantly enhances the photocatalytic performance of ZnO. This improvement was attributed to the luminescence properties of Y2O3. By elucidating this unique mechanism, the performance of photocatalytic materials can be significantly enhanced. In our study, the improvement of the photodegradation rate constant (kapp) of ZnO from 0.009 min−1 to 0.0242 min−1 demonstrated the promising photocatalytic efficiency of the ZnO-5Y2O3 based composite, opening up exciting possibilities for further applications in environmental remediation and other fields.

Synthesis of CuOQDs/g-C3N4/C Composites Via Stem Template Induction and their Photocatalytic Properties

Introduction: The synthesis of bio-templated porous CuOQDs/g-C3N4/C composites with controllable morphology and suitable energy band structure was successfully carried out via a simple thermal condensation and hydrothermal method. Method: Dicyandiamide and cadmium chloride were selected as starting materials, while Hollyhock stem was chosen as the biological template. The results indicated that, in comparison to pure g-C3N4, g-C3N4/C had a rich porous structure and better-photogenerated carrier separation efficiency. The CuOQDs were anchored evenly on the surface of the g-C3N4, thus resulting in the formation of a greater number of reactive sites. Results: The type-Z heterojunction formed between the CuOQDs and g-C3N4/C reduced the energy required for electron transition, thereby facilitating the separation of photo-generated electronhole pairs. The highest photocatalytic degradation efficiency of CuOQDs/g-C3N4/C for tetracycline (TC) was 65.1%, which was 3.3 times that of pure g-C3N4. Conclusion: In the photocatalytic process, the main reactive species is O2−. The CuOQDs/g- C3N4/C synthesized by stem induction in multi-phase heterojunction form has a stable microstructure to improve the charge separation efficiency. Further, it represents practical photocatalytic environmental protection.

Enhanced photocatalytic and antibacterial performance of CeO2-loaded carboxymethyl chitosan nanocomposites.

Carboxymethyl chitosan (CMCs) was loaded with two different concentrations of cerium dioxide (CeO2), specifically 0.3 g and 0.6 g, and compared with pure CMCs. XRD and FTIR analyses were used to evaluate the structural characteristics. The nanocomposites were examined for their photocatalytic degradation of Methylene Blue (MB) dye and antibacterial capabilities. Various parameters, including catalyst concentration, dye concentration, and pH levels, were assessed to determine the photocatalytic degradation efficiency of MB dye. The results demonstrated that the 0.6CeO2/CMCs nanocomposite outperformed the 0.3CeO2/CMCs and pure CMCs in terms of photocatalytic performance, achieving complete degradation of MB dye within 210 and 240 min after treatment with 0.3CeO2/CMCs and 0.6CeO2/CMCs nanocomposites. The agar well diffusion test was employed to assess the antibacterial activity of the nanocomposites against Staphylococcus aureus, Enterococcus faecalis, Escherichia coli, and Pseudomonas aeruginosa. The 0.6CeO2/CMCs nanocomposite exhibited the highest antibacterial effectiveness, with zones of inhibition measuring 29 ± 0.63 mm for E. coli, 28 ± 0.18 mm for P. aeruginosa, 25.5 ± 0.25 mm for S. aureus, and 23.6 ± 0.51 mm for E. faecalis. Additionally, the durability and reusability of the nanocomposites were confirmed after undergoing 5 cycles of MB dye degradation, with only a slight decrease in efficiency. The findings of this study highlight the potential of CeO₂/CMCs nanocomposites as powerful antibacterial and photocatalysts with important implications for environmental and biological applications.

A portable bifunctional platform of aluminum/alkalized carbon nitride/silver for sensitive SERS detection and efficient photocatalytic degradation of malachite green

It is necessary to construct a portable bifunctional substrate with high-repeatability and low-cost for detection and degradation of pollutants in practical applications. Herein, the alkalized carbon nitride (ACN) is assembled onto the etched aluminum sheet (EAl) by hydrogen bonding self-assembly which induces the porous structure of EAl/ACN. Subsequently, Ag nanoparticles (AgNPs) are mainly in-situ embedded in the cavities of EAl/ACN, and finally a portable bifunctional substrate of EAl/ACN/Ag is constructed. The surface-enhanced Raman scattering (SERS) response of the EAl/ACN/Ag is highly sensitive and selective for detection of malachite green (MG), and the limit for MG can reach 5.53 × 10-13 mol/L with an enhancement factor (EF) of 2.95 × 105. In addition, the EAl/ACN/Ag substrate shows excellent homogeneity, stability, recyclability and reproducibility. Moreover, the EAl/ACN/Ag substrate displays a high photocatalytic degradation efficiency of 96.4 % for MG within 50 mins even if it is reused for five cycles. Therefore, the developed portable bifunctional substrate shows bright prospects for the detection and removal of organic pollutants in the fields of industrial wastewater analysis.

Facile electrospinning-electrospray method to fabricate flexible rice straw-derived cellulose acetate/TiO2 nanofibrous membrane with excellent photocatalytic performance

A novel and facile electrospinning-electrospray (EE) method that based on electrospinning technique and simultaneous electrospray was proposed to anchor TiO2 (P25) nanoparticles on the surface of rice straw-derived cellulose acetate (CA) nanofiber, a series of EE-CA/P25 nanofibrous membranes with different P25 dosage were successfully fabricated, which were characterized in terms of SEM, TEM, FI-IR, XRD, DRS, PL, UV–vis and 3D-EMMs, etc. Results confirmed that P25 nanoparticles were anchored on the surface of CA nanofiber. For different organic dyes of Methylene blue (MB), Rhodamine B (RhB) and Methyl orange (MO), EE-CA/P25(0.05) nanofibrous membrane toward MB dye showed the best photocatalytic degradation efficiency of 99.13 % after 30 min of light exposure. For the different antibiotics of Tetracycline (TC), Ciprofloxacin (CIP) and Sulfamethoxazole (SMX), EE-CA/P25(0.05) exhibited the best photocatalytic degradation efficiency of 83.59 % for TC after 30 min of light exposure. Moreover, EE-CA/P25(0.05) exhibited a good antimicrobial efficiency of 98.42 % for E. coli, and maintained 97.49 % after 5 cycles. The reactive radical trapping experiments revealed that h+, •O2−, and •OH participated in the reaction, and h+ and •O2− played a major role in the photocatalytic degradation process. Moreover, EE-CA/P25(0.05) flexible membrane was easy to recycle and transform rice straw waste into treasure.

Photocatalytic ZnIn2S4 for 2-Mercaptobenzothiazole Degradation: Facile Synthesis, Influencing Factors and Mechanism

The development of an efficient, stable and environmentally friendly semiconductor photocatalyst is great research significance in the field of environment and energy. In this paper, ZnIn2S4 semiconductor photocatalysts with flower-like hierarchies were prepared by simple and environmentally friendly method. The influence of ZnIn2S4 dosage, pH and different inorganic salt ions on the photocatalytic degradation of 2-mercaptobenzothiazole (MBT) under visible light was investigated in detail. The results showed that the photocatalyst dosage was 0.05 g, and the photocatalytic degradation efficiency of MBT was the highest in the weakly acidic environment with pH was five. Meanwhile, the addition of some inorganic anions has a significant inhibitory effect on the photocatalytic activity of ZnIn2S4 semiconductor photocatalyst, while cations have less effect. Moreover, the ZnIn2S4 semiconductor photocatalyst had excellent stability and recyclability, and its photodegradation mechanism had been deeply studied. This work provides a theoretical and experimental foundation for exploring the different influencing factors of photocatalytic degradation of organic pollutants in wastewater by other semiconductors.

Construction and degradation mechanism of the magnetic CoFe1.95Y0.05O4/Ag/g-C3N4 Z-scheme heterojunction for enhanced photocatalytic activity

In this study, a Z-scheme heterojunction CoFe1.95Y0.05O4/Ag/g-C3N4 (CFYO/ACN) magnetic nanocomposite was prepared by hydrothermal synthesis. The composite was characterised and analysed using different characterisation tools and its photocatalytic degradation activity towards methylene blue (MB) was investigated. The results showed that the CFYO/ACN photocatalyst compared to CoFe1.95Y0.05O4 (CFYO) and Ag/g-C3N4 (ACN), the composite CFYO/ACN had the highest degradation efficiency of MB, which was up to 97% within 120 min, which was 1.26 and 1.09 times higher than that of ACN and CFYO, respectively. The enhancement of the catalytic performance of CFYO/ACN was attributed to the fact that the heterogeneous junction formation effectively inhibited the complexation of photogenerated carriers. In addition, five consecutive cyclic degradation experiments showed that CFYO/ACN exhibited efficient photocatalytic degradation, stable crystal structure, and easy recycling in the photodegradation process. Finally, the capture experiments confirmed that superoxide radicals () and hydroxyl radicals (·OH) play a major role in the degradation process. This study provides an effective strategy for the construction of efficient photocatalysts.

Highly efficient photocatalytic degradation of tetracycline antibiotic enabled by TiO2 nanodispersion

The growing pollution of antibiotics poses a significant threat to the ecological environment and human health. Photocatalysis is a promising solution for eliminating tetracycline (TC), but developing efficient photocatalysts remains a critical and challenging task. Herein, TiO2 nanodispersion is first used for the removal of TC in water. The as-prepared TiO2 nanodispersion has a uniform size of 10 nm and a large specific surface area of 198.9 m2/g, exhibiting notable adsorption capacity, exceptional photocatalytic performance, and considerable stability. Under UV light, TiO2 nanodispersion achieved 100 % degradation of TC within 60 min, using less than 1/5 of the catalyst dosage typically applied in most studies, and exhibited the highest catalytic activity, reaching 500 mgTC∙gcatalyst-1h-1. Additionally, the photocatalytic rate constant of TiO2 nanodispersion was 2.74 times higher than commercial P25. After five photocatalytic cycles, the catalyst maintained a high degradation efficiency of 94 %. Even under visible light, the degradation efficiency of TC by TiO2 nanodispersion was approximately 90 % within 20 min, which was notably higher than that of P25 (63 %). Moreover, the as-prepared TiO2 nanodispersion demonstrated outstanding photocatalytic degradation ability for other typical antibiotics (chlortetracycline, oxytetracycline, and ciprofloxacin), demonstrating its potential as an efficient photocatalyst for the treatment of antibiotic wastewater.

Efficient photocatalytic degradation of Rhodamine B dye by undoped and zinc doped zirconium dioxide NPs

Undoped and Zinc-doped Zirconium Dioxide (ZrO2) nanoparticles (NPs) were synthesized via the co-precipitation method, employing Zirconium (IV) Oxynitrate hydrate (ZrO2N2·xH2O), Zinc Sulfate (ZnSO4·7H2O), and ammonia solution as precursor materials at room temperature. A comprehensive characterization of the structural, morphological, optical, and catalytic properties of the prepared samples was conducted using various techniques. Powder X-ray diffraction (PXRD) analysis revealed that the synthesized NPs exhibited a tetragonal phase structure, with crystallite sizes decreasing as doping concentration increased. Scanning electron microscopy (SEM) images confirmed the spherical shape of the NPs. Energy-dispersive X-ray spectroscopy (EDX) verified the presence of Zr, O, and Zn elements with high purity. Raman spectroscopy corroborated the tetragonal structure of the synthesized ZrO2 NPs. Ultraviolet–visible (UV–Vis) spectroscopy studies determined the cutoff wavelengths, while Tauc plot enabled the calculation of band gap energies. Fourier transform infrared spectroscopy (FTIR) confirmed the functional groups present in the synthesized samples. Photoluminescence spectroscopy provided evidence of defects and charge carrier recombination processes. The catalytic efficacy of the synthesized ZrO2 NPs was demonstrated through the degradation of Rhodamine B dye under visible and UV light.

One-step construction of defective Fe-doped MIL-68(In)–NH2 for efficient photocatalytic degradation of bisphenol A

Defective Fe-doped MIL-68(In)–NH2 (Fe-MIL68) was successfully synthesized for photocatalytic bisphenol A (BPA) degradation in one step via a simple hydrothermal method for the first time. The presence of mesoporous structure and oxygen vacancies in Fe-doped Fe-MIL68 was confirmed using modern characterization techniques, which can significantly improve its adsorption performance and electron-hole separation ability, thus enhancing its photocatalytic activity. The results confirmed the photodegradation efficiency of Fe-MIL68 for BPA under visible light could reach 92.3 % in 100 min, which was 2.1 times higher than that of MIL-68(In)–NH2.

Two birds with one stone: Mussel-inspired anchoring strategy enabling full-spectrum utilization and superior carriers transfer capacity for efficient photocatalytic degradation and antibacterial

Two birds with one stone: Mussel-inspired anchoring strategy enabling full-spectrum utilization and superior carriers transfer capacity for efficient photocatalytic degradation and antibacterial

B doped BiVO4/Bi nanocomposites activated persulfate for efficient photocatalytic degradation of levofloxacin

B doped BiVO4/Bi nanocomposites activated persulfate for efficient photocatalytic degradation of levofloxacin