Construction of 3D sheet-packed hierarchical MoS2/BiOBr heterostructures with remarkably enhanced photocatalytic performance for tetracycline and levofloxacin degradation

In this paper, MoS2 nanosheets were prepared and deposited on BiOBr microflowers through deposition-hydrothermal strategy. MoS2 exhibited a string of nanosheets with wrinkled layer outlook, and MoS2/BiOBr composites displayed a micro-flower morphology with the diameter of 2-3 μm. Visible-light harvesting performance was significantly improved in the region of 400-600 nm for MoS2/BiOBr. The obtained MoS2/BiOBr samples exhibited tremendous enhanced catalytic activity, which could degrade 92.96% of tetracycline and 90.31% of levofloxacin within 70 min. The photo-generated holes and ⋅OH radicals played the dominant roles in the whole photocatalytic decomposition process. Based on the analysis of DRS, BET, PL, and electrochemical results, the remarkably improved photocatalytic performance may be ascribed to the synergistic effect of strong visible-light harvesting ability, enhanced BET surface area, and faster separation or transfer efficiency of photo-generated charges.


Introduction
In the last two decades, antibiotic residues were inevitably discharged into various water bodies since the antibiotics were enormously produced and extensively used for medical therapy because of its excellent activity against bacterial infection (Li et al. 2019a;Hu et al. 2021). Antibiotics polluted wastewater would provoke potential threats to the public health and ecosystem due to its toxic nature. Therefore, it is urgent to explore an efficient technology to eliminate the antibiotic residues before discharge. Among various wastewater treatment techniques, semiconductor photocatalysis has been considered as an effective approach to remove antibiotics pollutants due to its non-toxicity, cost-effective, and ambient operating condition (Chen et al. 2019;Pirhashemi et al. 2018;Sabri et al. 2021).
Bismuth oxybromide (BiOBr) is a V-VI-VII ternary oxide semiconductor material with layered tetragonal matlockite structure. The [Bi 2 O 2 ] 2+ positive slices were embedded in double Br negative slabs to yield [Br-Bi-O-Bi-O-Br] layers (Xu et al. 2014). Then [Br-Bi-O-Bi-O-Br] layers interacted together through van der Waals forces rather than closely packed, resulting in larger BET surface area and ideal absorption abilities (Xiong et al. 2014). As reported, BiOBr has been evidenced as a promising visible-light-induced photocatalyst to decompose organic effluents due to its high stability against photocorrosion, appropriate band gap (1.7-3.2 eV), and relatively excellent photocatalytic performance (Xue et al. 2014, Mao et al. 2014. Nevertheless, the low separation efficiency of photo-generated electron/hole pairs severely hindered its practical applications. Previous investigations had specified that coupling BiOBr and another semiconductor with suitable band gap to form heterojunction was an efficient strategy to promote separation rate of charge carriers (Meng and Zhang 2015;Wang et al. 2014). Up to now, various BiOBr-based heterojunctions with improved photocatalytic activities had been extensively investigated, such as BiOAc/BiOBr (Liu et al. 2021), BiOBr/BiFeWO 6 (Lu et al. 2021), BiOBr/BiOCl (Liao et al. 2021), BiSbO 4 / BiOBr (Van et al. 2022), BiOBr/C 3 N 4 (Ma et al. 2019a), and so on. The outstanding degradation performance of these photocatalysts depended on their preferable opticalabsorption characteristics, high separation efficiency of excited charge carriers and long-term stability.
Molybdenum disulfide (MoS 2 ) has a similar layered structure to BiOBr, in which Mo atoms were sandwiched covalently between S atoms to form S-Mo-S layers. Then, each formed S-Mo-S layer vertically connected together by van Der Waals interactions, which facilitated the exfoliation of 2D MoS 2 nanosheets from bulk MoS 2 . This typical layered structure not only significantly improved the utilization of active center but also greatly shortened the transfer path of photo-induced charge carriers, degradation intermediates, and products. Other excellent properties of MoS 2 includes its light absorption performance (Eg = 1.2-1.9 eV), unique physical and chemical performance, and stability against photo-corrosion. Therefore, MoS 2 was used as an ideal co-catalyst to modify C 3 N 4 Akhundi et al. 2020), CdS (Darsara et al. 2018), Bi 2 S 3 (Iqbal et al. 2022), ZnO (Benavente et al. 2018), Bi 2 O 3 (Ma et al. 2019b), and the obtained composites showed the improved photocatalytic performance for degradation organic pollutants. Significant investigations of BiOBr/MoS 2 heterojunctions have been documented for eliminating organic dyes Lee et al. 2017), where the conduction band potential was higher than that of BiOBr, and type-I band alignment was formed at BiOBr/MoS 2 interface. It promoted the charge separation and further enhanced the photocatalytic activity. The photocatalytic performance of BiOBr modified with MoS 2 also depended on the layer number in MoS 2 and the weight percentage of MoS 2 deposited on BiOBr.
Considering the unique properties of few-layered MoS 2 , in this work, few-layered MoS 2 nanosheets were fabricated firstly by a simple hydrothermal-dialysis method, and then deposited on the surface of BiOBr microflowers via a deposition-hydrothermal process. As a result, few-layer MoS 2 nanosheets coupled with BiOBr heterojunctions were successfully fabricated. The visible-light absorption capacity was enhanced, and the separation or transfer efficiency of excited charge carriers was remarkably promoted. The photocatalytic performance of as-synthesized MoS 2 /BiOBr microflowers was evaluated by degradation of tetracycline and levofloxacin antibiotics. The detailed relationship between the unique structure and the excellent catalytic performance of MoS 2 /BiOBr microflowers was explored. The possible enhanced photocatalytic mechanism was also proposed based on various experimental investigations. Furthermore, this fabrication method could be extended into a general strategy for the formation of other 2D few-layered heterojunctions.

Preparation of MoS 2 nanosheets
About 0.6 g of Na 2 MoO 4 was dissolved in deionized water with continuous ultrasonication for 10 min to obtain the transparent Na 2 MoO 4 solution, followed by the addition of CH 4 N 2 S (0.5 g). The as-obtained mixture solution was migrated into a 50-mL Teflon-lined stainless steel autoclave and kept at 190 °C for 24 h to get MoS 2 suspension. After centrifuge at 10,000 rpm for 10 min, the supernatant was transferred to a dialysis bag and kept on dialysis for 2 days. Finally, MoS 2 nanosheet dispersion was obtained for the following step.

Preparation of MoS 2 /BiOBr heterojunctions
The MoS 2 /BiOBr heterojunctions were prepared via a simple deposition-hydrothermal method. Typically, 0.729 g of CTAB was dissolved in a specified amount of anhydrous ethanol, followed by the addition of bismuth nitrate pentahydrate (0.9702 g). After ultrasonication for 30 min, 4 mL of MoS 2 nanosheets dispersion was added. The obtained suspension was added into a 100-mL Teflon-lined stainlesssteel autoclave and heated for 17 h at 160 °C. The product was cooled down to 25 °C and separated by filtration. The obtained precipitate was washed with anhydrous ethanol and dried at 60 °C to obtain MoS 2 /BiOBr samples, marked as 4-MoS 2 /BiOBr. Pure BiOBr was also synthesized using the similar procedure without the addition of MoS 2 nanosheet dispersion. By changing the volume amounts of MoS 2 nanosheet dispersion, 6-MoS 2 /BiOBr, 8-MoS 2 /BiOBr, and 10-MoS 2 /BiOBr samples were also prepared with other parameters unchanged for comparison.

Characterization of the prepared samples
Crystal structure was studied by X-ray diffractometer (D8 ADVANCE A25, Cu Kα, λ = 1.54056 Å). The morphology was investigated by scanning electron microscopy (S-3000N) and field emission transmission electron microscopy (G2F30). The surface chemical state was detected using X-ray photoelectron spectroscope (AXIS ULtrabld), operating at 3.0 × 10 −10 mbar. The optical absorption property was investigated by Lambd 950 UV-visible diffuse reflectance spectrophotometer. The photoluminescence spectra were measured using RF-6000 Fluorescence spectrophotometer.

Photocatalytic experiment
Photocatalytic capability of the samples was evaluated by elimination of tetracycline and levofloxacin antibiotics. The detailed process was described in our previous work (Ma et al. 2019a). In a typical process, under the dark condition, 10 mg of the samples was applied to degrade 40 mL tetracycline (20 mg/L) or levofloxacin (20 mg/L) solution with vigorous stirring for 20 min to obtain the equilibrium of adsorption/desorption. Then, the dispersion was continuously irradiated with a 250 W Xe lamp. Also, 4 mL suspension was sequentially collected at each 10 min interval and centrifuged through membrane filters (0.22 μm) to take away the solid photocatalyst. The obtained filtrate was transferred into a quartz cuvette to investigate the absorption spectra (200-750 nm) using a UV-3200 spectrophotometer.
The long-term stability was studied as follows. 6-MoS 2 / BiOBr heterojunctions were recycled through centrifugation after the degradation of tetracycline and levofloxacin, respectively, then rinsed with deionized water and recycled to another run.

Morphology analysis
The morphology, size, and lattice were characterized by SEM, TEM, and HRTEM techniques. The results are shown in Fig. 2. From Fig. 2 a, it is clearly observed that MoS 2 shows accumulated states of thin nanoplates with the smooth surface. The TEM in Fig. 2 b displays that MoS 2 exhibits a string of nanosheets with a wrinkled layer outlook and sufficient exposed edges, further confirming the thin size and smooth surface of MoS 2 nanosheet. The 0.947-nm lattice spacing in the HRTEM of MoS 2 (Fig. 2c) could be indexed to its (002) Fig. 2 f. An obvious tight contact boundary (marked by a yellow line) between MoS 2 and BiOBr was evidently observed, which proved that MoS 2 was closely attached to the BiOBr microstructure. It facilitated the charge separation during photocatalytic degradation process (Liu et al. 2018). Notably, clear lattice fringes with a spacing of 0.278 and 0.282 nm were obvious, which could be ascribed to (102) and (110) facet of BiOBr. Figure 3 is the elemental mapping results of 6-MoS 2 / BiOBr composites. The Bi, Br, O, Mo, and S elements were distributed in the selected district and had a good synergistic correspondence with the whole scanning region, indicating that MoS 2 were attached or immersed well into BiOBr microstructure. This result further proved that MoS 2 /BiOBr composites were successfully prepared.

XPS analysis
To further prove the interaction between MoS 2 and BiOBr in the heterojunctions and explore the chemical states of 6-MoS 2 /BiOBr, X-ray photoelectron spectroscopies (XPS) are carried out, and the results are shown in Fig. 4. Figure 4 a shows the full spectra of BiOBr and 6-MoS 2 / BiOBr. Bi, O, and Br elements could be observed in the spectrum of BiOBr. From the spectrum of 6-MoS 2 /BiOBr, Mo, and S elements were detected in addition to Bi, O, and Br, implying the successful synthesis of 6-MoS 2 /BiOBr. Figure 4 b-f are the XPS high resolution spectra of Mo 3d, S 2p, Bi 4f, O 1s, and Br 3d. From Fig. 4 b, the two strong Mo 3d signals at 235.53 and 232.29 eV could be ascribed to Mo 3d 3/2 and Mo 3d 5/2 , respectively, demonstrating the valence of molybdenum in 6-MoS 2 /BiOBr sample was +4. The XPS peaks at 164.56 and 159.2 eV shown in Fig. 4 c could be attributed to S 2p 1/2 and S 2p 3/2 (Li et al. 2018). From Fig. 4 d, two peaks at 164.4 and 159.1 eV for BiOBr sample corresponding to Bi 4f 5/2 and Bi 4f 7/2 are shifted to higher binding energies by 0.1 and 0.16 eV for 6-MoS 2 / BiOBr sample, respectively. The O 1s signal at 529.9 eV of BiOBr in Fig. 4 e could be ascribed to Bi-O bond (Li et al. 2023), which was shifted to 530.1 eV with apparent decreased intensity owning to the coupling of MoS 2 in 6-MoS 2 /BiOBr sample. Two signals with binding energies at 69.13 and 68.24 eV presented for pure BiOBr belonging to Br 3d 3/2 and Br 3d 5/2 were shifted to 69.31 and 68. 4

BET analysis
The Brunauer-Emmett-Teller (BET) surface area and porous structure of 6-MoS 2 /BiOBr heterojunction and pure BiOBr are studied based on N 2 adsorption-desorption experiments, and the results are depicted in Fig. 5. It was clear that 6-MoS 2 /BiOBr sample displayed type-II adsorptiondesorption isotherms with H4 hysteresis loop (Fig. 5a), indicating the strong adsorption between 6-MoS 2 /BiOBr and guest adsorbent. The pore size distribution was studied by the desorption data based on the Barrett-Joyner-Halenda (BJH) method. According to the inset in Fig. 5 a, 6-MoS 2 / BiOBr heterojunction shows the mesoporous nature, and the formation of pores around 2-10 nm may be attributed to the crystal growth process or the gaps between assembled nanosheets which coincided with the SEM images. Pure BiOBr exhibited type IV isotherms with H3-type hysteresis loops (Fig. 5b), and the corresponding BJH analysis (inset in Fig. 5b) revealed that most of the pore sizes fell into the range from 2 to 16 nm. The BET surface areas of 6-MoS 2 / BiOBr and pure BiOBr were about 54.9577 and 17.929 m 2 /g, respectively. After introducing MoS 2 to BiOBr, the BET surface area of BiOBr was remarkably increased. It was beneficial for the adsorption of organic molecules and provided more active sites during the photocatalytic degradation process.

Photocatalytic degradation of tetracycline and levofloxacin
The photocatalytic capabilities of BiOBr and MoS 2 /BiOBr heterostructures are investigated by photocatalytic removal of antibiotics tetracycline and levofloxacin aqueous solution, and the results are given in Fig. 6. As seen from Fig. 6 a  and b, the degradation efficiency of tetracycline and levofloxacin in the existence of BiOBr are only about 69.99% and 66.08%, respectively. Remarkable enhancement of photocatalytic performance was obtained by the MoS 2 /BiOBr heterojunctions. As shown, the photocatalytic activity of MoS 2 /BiOBr heterojunctions dramatically depended on the coupling amount of MoS 2 , and 6-MoS 2 /BiOBr exhibited the best photocatalytic efficiency, 92.96% for tetracycline and 90.31% for levofloxacin within 70 min. Higher coupling of MoS 2 reduced the photocatalytic effect due to aggregation of MoS 2 particles and charge recombination (Zarezadeh et al. 2019). Figure 6 c and d show the dependence of UV-vis absorbance for tetracycline and levofloxacin in the absence and presence of heterojunctions. The intensity of major absorption peak for both tetracycline at 370 nm and levofloxacin at 287 nm gradually diminished with prolonging the illumination time and almost disappeared after 70 min, indicating the broken of molecular structure and the conjugated π system in the tetracycline and levofloxacin molecules. This result is very important for the practical antibiotics wastewater decontamination, since the 6-MoS 2 /BiOBr heterojunctions displayed remarkable photocatalytic performance in the degradation of tetracycline and levofloxacin, the major effluents from therapeutic medicine, feed supplements, and pharmaceutical factory. Long-term stability of the photocatalysts is significant for the actual applications. To evaluate the durability of 6-MoS 2 /BiOBr heterojunctions, the recycling degradation experiments for both tetracycline and levofloxacin were examined. The detailed process was based on our reported reference (Ma et al. 2019a). Typically, 6-MoS 2 /BiOBr composites were collected by centrifugation after each run and washed with deionized water and recycled to another run. The experimental results are shown in Fig. 7. It was clear that the degradation efficiency of 6-MoS 2 /BiOBr exhibited high stability and potential applications for eliminating antibiotics pollutants, since 85% of tetracycline and 81% of levofloxacin could still be efficiently degraded after 3 runs.
In addition, in order to ensure the mineralization of the pollutants, the mineralization efficiency of 6-MoS 2 /BiOBr composites for both tetracycline and levofloxacin during the degradation reactions was monitored on an analytic Jena TOC analyzer (multi N/C 3100, Germany). From the TOC experimental data as listed in Table 1, the TOC elimination efficiencies for tetracycline and levofloxacin increase gradually with the reaction time, and nearly 55.06% of tetracycline and 63.28% of levofloxacin is mineralized within 4 h.
To investigate the migration, transfer, and separation properties of photo-generated electron/hole (e − /h + ) pairs during the photocatalytic process in the presence of BiOBr and 6-MoS 2 /BiOBr, the UV-vis diffuse reflectance spectra (DRS), photoluminescence (PL) spectroscopy, electrochemical impedance Nyquist (EIS), and transient photocurrent were employed. As evidenced in Fig. 8 a, the incorporation of MoS 2 displays a synergistic effect with significantly  . 6 Decomposition of tetracycline (a) and levofloxacin (c) under Xe lamp irradiation by BiOBr and MoS 2 /BiOBr heterojunctions and UV-vis absorption spectra of tetracycline (b) and levofloxacin (d) during the photocatalytic degradation process by 6-MoS 2 /BiOBr heterojunctions improved visible light harvesting in the region of 400-600 nm for 6-MoS 2 /BiOBr, in comparison with that of BiOBr. Furthermore, the PL intensity (Fig. 8b) and the EIS Nyquist radius (Fig. 8c) of 6-MoS 2 /BiOBr dramatically decreased, compared with that of BiOBr, implying that the recombination of e − /h + pairs in 6-MoS 2 /BiOBr was strongly inhibited (Wu et al. 2020). The transient photocurrent shown in Fig. 8 d displays that BiOBr has a weak response signal, while the heterojunction 6-MoS 2 -BiOBr exhibits stronger photocurrent than BiOBr, further proving that the efficient separation of photocarriers in 6-MoS 2 /BiOBr is promoted. The above PL, EIS, and photocurrent results were in accordance with the previous photocatalytic performance.

Detection of reactive species
To specify the main reactive species involved in tetracycline and levofloxacin photo-degradation by 6-MoS 2 /BiOBr, reactive radical capture experiments were conducted and ethylene di-ammine tetra acetic acid disodium (Na 2 EDTA), isopropanol (IPA), and benzoquinone (BQ) were employed to capture h + , ·H, and ·O 2 -, respectively. From Fig. 9 a, it could be seen that the degradation efficiency of tetracycline is dramatically suppressed from 92.96 to 54.78 and 63.23% with the addition of Na 2 EDTA and IPA in tetracycline degradation process. However, the addition of BQ had a negligible effect on tetracycline degradation. This phenomenon depicted that h + and ·OH were the main reactive reagents in tetracycline degradation, and the same results were observed in levofloxacin degradation (Fig. 9b).

Mechanism of enhanced photocatalytic performance
A possible enhanced photocatalytic mechanism for tetracycline and levofloxacin degradation over MoS 2 /BiOBr heterojunctions was proposed. Based on our previous work, the conduction band potential (E CB ) and valence band potential (E VB ) for MoS 2 is 0.33 and 1.33 eV (Ma et al. 2019b), whereas it is 0.19 and 3.17 eV for BiOBr (Ma et al. 2019a). The schematic diagram of energy band structure for MoS 2 and BiOBr is constructed and depicted in Fig. 10 a. As a p-type semiconductor, the Femi level of both MoS 2 and BiOBr are close to the valence band, and MoS 2 has a higher Femi level than BiOBr in the detached state. When MoS 2 and BiOBr were in contact (Fig. 10b), a p-p isotype   The degradation efficiency of tetracycline (a) and levofloxacin (b) over the 6-MoS 2 /BiOBr heterojunctions in the presence of scavengers heterojunction was produced. The difference in Femi levels between MoS 2 and BiOBr led to the transfer of photogenerated holes from MoS 2 to BiOBr until the Femi level of two components reached an equilibrium, producing a hole accumulation region at MoS 2 and a hole depletion region at BiOBr . Accompanied with the moving of Femi level, the whole energy band of BiOBr was elevated, while that of MoS 2 was descended. Thus, an inner electric field from MoS 2 to BiOBr was established at the interface. As irradiated by the visible-light, both MoS 2 and BiOBr could absorb photons and induce the generation of electron/ hole pairs. Driven by the inner electric field, the photo-generated electrons on CB of BiOBr are rapidly migrated to that of MoS 2 , and the holes on VB of MoS 2 are transferred to that of BiOBr. Thus, the separation of electron/hole pairs in BiOBr was promoted, and accordingly their recombination was reduced. The more efficient separation of electron/hole pairs could reduce the total voltage barrier at the interface, resulting in lower resistance and higher photocurrent, in agreement with the results in Fig. 8 c and d.
Since E CB of both MoS 2 and BiOBr were more positive than the oxidation potential of O 2 /·O 2 − (−0.33 eV vs NHE (Liu et al. 2022;Liu et al. 2017). So ·O 2 − radicals cannot be produced by both MoS 2 and BiOBr, which had been confirmed by the radical capturing experiments (Fig. 10). The E VB of MoS 2 was lower than that of the reduction potential of ·OH/OH − (1.99 eV vs NHE and ·OH/H 2 O (2.27 V vs NHE (Li et al. 2018). While the BiOBr had a more positive E VB value than 1.99 and 2.27 eV, so the OH − or H 2 O could be oxidized to ·OH radicals by BiOBr but not by MoS 2 . The ·OH radicals possess strong oxidation ability and could degrade organic molecules to small molecules. The participation of ·OH radicals in the degradation process had been approved by radical capturing experiments (Fig. 9).
In a word, the fabrication of p-p heterojunction structure at the interface of MoS 2 and BiOBr facilitates the transfer and separation of photo-generated e − /h + pairs, which could be confirmed by the diminished PL intensity, the decreased EIS Nyquist radius, and increased photocurrent signal shown in Fig. 8. The reaction procedure involved in the photo-degradation of tetracycline, and levofloxacin was depicted with the following equations:

Conclusion
In this paper, 3D sheet-packed hierarchical MoS 2 /BiOBr p-p heterojunctions were fabricated via a simple depositionhydrothermal method. In comparison with pure BiOBr, the MoS 2 /BiOBr heterojunctions demonstrated enhanced photocatalytic capability in removal of tetracycline and levofloxacin. The enhanced photocatalytic ability could be accredited to the synergistic effect of stronger photo-absorption and the fabrication of p-p heterojunction at the interface of MoS 2 and BiOBr, which facilitated the separation and migration of (1)  Fig. 10 The proposed mechanism of enhanced photocatalytic performance over MoS 2 /BiOBr heterojunctions photo-induced e − /h + pairs. Photo-generated holes and ·OH radicals acted as a key role in the whole degradation process. This MoS 2 /BiOBr heterojunction was a promising photocatalyst suitable for eliminating antibiotics in wastewater.