Facile Synthesis of Bi 2 MoO 6 Microspheres Decorated by CdS Nanoparticles with Efficient Photocatalytic Removal of Levfloxacin Antibiotic

Developing high-efficiency and stable visible-light-driven (VLD) photocatalysts for removal of toxic antibiotics is still a huge challenge at present. Herein, a novel CdS/Bi2MoO6 heterojunction with CdS nanoparticles decorated Bi2MoO6 microspheres has been obtained by a simple solvothermal-precipitation-calcination method. 1.0CdS/Bi2MoO6 has stronger light absorption ability and highest photocatalytic activity with levofloxacin (LEV) degradation efficiency improving 6.2 or 12.6 times compared to pristine CdS or Bi2MoO6. CdS/Bi2MoO6 is very stable during cycling tests, and no appreciable activity decline and microstructural changes are observed. Results signify that the introduction of CdS could enhance the light absorption ability and dramatically boost the separation of charge carriers, leading to the excellent photocatalytic performance of the heterojunction. This work demonstrates that flower-like CdS/ Bi2MoO6 is an excellent photocatalyst for the efficient removal of the LEV antibiotic.

CdS is known as a promising VLD photocatalyst in virtue of its favorable visible-light-response, and chemical stability [37][38][39][40].Due to the well-matched band structures of CdS and Bi 2 MoO 6 [31], it is anticipated the rationally designed CdS/Bi 2 MoO 6 can be endowed with superior photocatalytic activity for the removal of LEV antibiotic.However, reports on photocatalytic degradation of LEV antibiotic using flower-like CdS/Bi 2 MoO 6 have not been indicated.
Inspired by the above aspects, we designed a flower-like CdS/Bi 2 MoO 6 for the removal of the LEV antibiotic.A facile solvothermal-precipitation-calcination method was employed to construct hierarchical CdS/Bi 2 MoO 6 heterojunctions with tightly interfacial contact.1.0CdS/Bi 2 MoO 6 possesses the highest activity towards the removal of LEV antibiotic.The plausible photocatalytic mechanism for the degradation of LEV antibiotic over CdS/Bi 2 MoO 6 was also illustrated.

Photocatalytic Performance
A typical fluoroquinolone antibiotic LEV is a kind of persistent organic pollutants (POPS) due to its water solubility and chemical stability.Photocatalysis is a promising technique for the removal of LEV antibiotic.For example, Kaur et al. fabricated Bi 2 WO 6 nanocuboid (0.75 g/L) for almost 80% degradation of LEV (10 mg/L) within 150 min under visible light irradiation [41].In this work, the photocatalytic activity of CdS/Bi 2 MoO 6 heterojunctions was also evaluated by degrading LEV as a representative toxic antibiotic under visible light (Figure 4).No LEV degradation was detected in the blank test (without catalysts).Pure CdS or Bi 2 MoO 6 shows very limited photocatalytic activity, as only 20.6% or 10.8% of LEV was decomposed within 60 min of reaction, owing to the fast recombination of electron-hole pairs.Encouragingly, with the introduction of CdS into the composites, all of the CdS/Bi 2 MoO 6 heterojunctions show much enhanced photocatalytic activity compared to pristine CdS and Bi 2 MoO 6 .Notably, 1.0 CdS/Bi 2 MoO 6 obtains the highest activity with 80.4% of LEV degradation within 60 min, much higher than CdS (20.6%),Bi 2 MoO 6 (10.9%), or the mechanical mixture (25.8%), verifying the existence of synergistic interaction between CdS and Bi 2 MoO 6 .Since the introduction of CdS nanoparticles on Bi 2 MoO 6 microspheres, the intimately contacted interfacial surface is formed for effective separation of charge carriers.However, with the excessive deposition of CdS (1.5CdS/Bi 2 MoO 6 ), the agglomeration of CdS undermines the synergistic effect, and thus suppresses the interfacial charge transfer, resulting in the decrease of the photocatalytic activity.To make a quantitative comparison, the LEV degradation data were fitted with the pseudo-first-order model (Figure 4b), −ln(C/C 0 ) = kt.Clearly, 1.0CdS/Bi 2 MoO 6 has the largest rate constant of 0.0259 min −1 , which is approximately 6.2, 12.6 or 4.5 times higher than pristine CdS (0.0036 min −1 ), Bi 2 MoO 6 (0.0019 min −1 ), and the mixture (0.0047 min −1 ).
The mineralization of organic contaminants is pivotal in the wastewater treatment.To assess the mineralization of LEV, TOC values were recorded during the degradation of LEV (40 mg L −1 , 200 mL) by 1.0CdS/Bi 2 MoO 6 (200 mg).As depicted in Figure 5, 90.1% of TOC was removed after 2.5 h of reaction.These results indicate that 1.0CdS/Bi 2 MoO 6 could effectively decompose and mineralize the LEV antibiotic.The reusability of a photocatalyst is an important factor for the practical treatment of wastewater.Therefore, the cycling runs in LEV degradation over 1.0CdS/Bi 2 MoO 6 were executed.Inspiringly, 1.0CdS/Bi 2 MoO 6 retained its initial photocatalytic activity after six successive runs (Figure 6a).Moreover, compared to the crystal phase of the fresh 1.0CdS/Bi 2 MoO 6 , no obvious phase changes (Figure 6b) of the used one was detected, illustrating that 1.0CdS/Bi 2 MoO 6 has superior stability and reusability.

Photocatalytic Reaction Mechanism
A batch of experiments was conducted to analyze the photocatalytic reaction mechanism accounting for the degradation of the LEV antibiotic under visible light through adding various scavengers (Figure 7).The LEV degradation rate showed no obvious decline when IPA (scavenger of •OH) was introduced, indicating that •OH did not play a pivotal role.By contrast, the LEV degradation rate was substantially suppressed by BQ (quencher of O 2 •− ) or AO (quencher of h + ), reflecting that O 2 •− and h + could be the dominant active species responsible for the LEV degradation over 1.0CdS/Bi 2 MoO 6 .The bandgap (E g ) of Bi 2 MoO 6 and CdS were calculated by the equation: (αhν) = A(hν-E g ) n/2 .As presented in Figure S2, the E g of Bi 2 MoO 6 and CdS are about 2.66 eV and 2.24 eV, consistent with the reference [32,40,42].The valence band (VB) and conduction band (CB) of Bi 2 MoO 6 and CdS can be obtained by the following equation: (2).Thereby, the E VB and E CB of Bi 2 MoO 6 were −0.32 and 2.34 eV [36], while those of CdS were −0.56 and 1.68 eV [40].Apparently, a staggered type II band structure can be constructed in CdS /Bi 2 MoO 6 , beneficial to retarding the recombination of charge carriers.
Since the separation and transport rate of electron-hole pairs of a semiconductor are closely related to the fluorescence emission [29,[43][44][45], the photoluminescence (PL) technique was applied to study the transport behaviors of charge carriers in Bi 2 MoO 6 and 1.0CdS/Bi 2 MoO 6 (Figure S3).Through comparing the PL emission spectra of Bi 2 MoO 6 and 1.0CdS/Bi 2 MoO 6 (Figure S3), it is found that 1.0CdS/Bi 2 MoO 6 exhibits much lower fluorescence intensity compared to that of pure Bi 2 MoO 6 , indicating that the nano-junction between Bi 2 MoO 6 and CdS greatly promotes the separation of electron-hole pairs.
On the basis of above experimental results, the interfacial charge transfer behavior of CdS/Bi 2 MoO 6 is illustrated in Figure 8.Under visible-light illumination, electron-hole pairs can be produced in both CdS and Bi 2 MoO 6 .The photo-excited e − in the CB of CdS may rapidly migrate to that of Bi 2 MoO 6 , while the photo-excited holes in the VB of Bi 2 MoO 6 can preferably drift to that of CdS.Such an interfacial charge movement can effectively retard the electron-hole recombination, accounting for the amelioration of photocatalytic performance of CdS/Bi 2 MoO 6 .The CB potential of CdS (−0.56 eV) and is Bi 2 MoO 6 (−0.

Chemicals
All reagents were purchased from Shanghai Chemical Reagent factory (China) and used directly.

Chemicals Synthesis of Catalysts
Synthesis of Bi 2 MoO 6 : Typically, 1 mmol Bi(NO 3 ) 3 •5H 2 O and 0.5 mmol Na 2 MoO 4 •2H 2 O were added into 50 mL of ethylene glycol, and the mixture turned into clear solution with the assistance of ultrasonication for 0.5 h.After that, 30 mL of ethanol was poured into the above solution and kept stirring for 0.5 h.Subsequently, the resulting solution transferred into an autoclave with the volume of 100 mL and reacted at 160 • C for 20 h in an oven.
Synthesis of CdS/Bi 2 MoO 6 : 0.5 mmol Bi 2 MoO 6 was suspended into 50 mL of deionized water.Then, 1mmol CdCl 2 •2.5H 2 O was dissolved in the above suspension under vigorously stirring.After that, 40 mL of Na 2 S (1 mmol) aqueous solution was added into the above system dropwise under magnetically stirring for 2 h.The precipitation labeled as 1.0 CdS/Bi 2 MoO 6 was washed, dried and then calcined at 200 • C for 2 h in N 2 atmosphere to obtain the catalysts.Through adjusting the amount of precursor of CdS, the samples with CdS/Bi 2 MoO 6 molar ratios of 0.5:1, 0.75:1, and 1.5:1 are labeled as 0.5CdS/Bi 2 MoO 6 , 0.75CdS/Bi 2 MoO 6 , and 1.5CdS/Bi 2 MoO 6 , respectively.The CdS sample was also prepared by the same procedure in the absence of Bi 2 MoO 6 .

Characterization
The phases of products were characterized via X-ray diffractometer (XRD, Bruker D8 ADVANCE, Karlsruhe, Germany) equipped with mono-chromatized Cu Kα radiation.UV-Vis diffuse reflectance spectra (DRS) of products were collected using Shimadzu UV-2600 spectrophotometer with BaSO4 as the reference standard.Scanning electron microscopy (SEM, Hitachi S-4800, Tokyo, Japan) and transmission electron microscopy (TEM, JEM-2100 JEOL, Tokyo, Japan) were employed to characterize the microstructures of the samples.The corresponding elemental components were analyzed by energy-dispersive X-ray spectroscopy (EDS, Bruker Quantax 400, Berlin, Germany).Photoluminescence (PL) spectra were conducted using Hitachi RF-6000 spectrophotofluorometer with the excitation wavelength of 300 nm.

Photocatalytic Performance Tests
The photocatalytic property of CdS/Bi 2 MoO 6 was assessed by degradation of LEV antibiotic under visible-light illumination, and the light source is provided by a 300 W xenon lamp with a light filter λ > 400 nm.50 mg of catalyst was scattered in LEV (100 mL, 20 mg L −1 ) solution.The suspension was stirred in the dark for 0.5 h.Then the reaction was initiated when the lamp switches on.During the irradiation, 1.5 mL of suspension was sampled in 10 min.The LEV concentrations were analyzed by using UV-2600 spectrophotometer.Total organic carbon (TOC) of the LEV solutions during reaction was determined by using a Shimadzu TOC analyzer.

Conclusions
In summary, CdS nanoparticles interspersed-Bi 2 MoO 6 heterojunction photocatalytst was fabricated by a facile solvothermal-precipitation-calcination method.Due to the introduction of CdS, the photo-absorption of CdS/Bi 2 MoO 6 and the interfacial charge separation were remarkably enhanced and promoted.By virtue of these benefits, in comparison with bare CdS and Bi 2 MoO 6 , 1.0CdS/Bi 2 MoO 6 presents profoundly enhanced visible-light photocatalytic activity for the removal of the LEV antibiotic.Moreover, 1.0CdS/Bi 2 MoO 6 also possesses good stability and can effectively mineralize the LEV antibiotic.This work offers new insight into the design of high-performance Bi-based heterojunction photocatalysts for antibiotic wastewater treatment.
Author Contributions: S.L.: idea, and design of the paper; performed the tests; analyzed the data; wrote this paper.Y.L., Y.L., H.Z., L.M., and J.L. assisted the characterizations.

Figure 1 .
Figure 1.XRD patterns of Bi 2 MoO 6 , CdS, and 1.0CdS/Bi 2 MoO 6 .The morphology of Bi 2 MoO 6 and 1.0CdS/Bi 2 MoO 6 was characterized by SEM and TEM analysis.As shown in Figure S1, pristine Bi 2 MoO 6 has the shape of a microsphere with a smooth surface, in accordance with the morphology of the reported Bi 2 MoO 6 [25].In comparison with Bi 2 MoO 6 , the surface of 1.0CdS/Bi 2 MoO 6 became rough due to the coating of CdS nanoparticles (size: 5-25 nm)
32 eV) are more negative than E (O 2 /O 2 •− ) (+0.13 eV) [46], thus the e -in the CB of CdS and Bi 2 MoO 6 can react with O 2 to generate active O 2 •− , further decomposing LEV antibiotic.On the other hand, the holes collected in the VB of CdS and Bi 2 MoO 6 can directly decompose the LEV antibiotic due to their strong oxidation ability.