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Article

Facile Synthesis of Bi2MoO6 Microspheres Decorated by CdS Nanoparticles with Efficient Photocatalytic Removal of Levfloxacin Antibiotic

by
Shijie Li
1,*,
Yanping Liu
2,
Yunqian Long
1,*,
Liuye Mo
1,
Huiqiu Zhang
1 and
Jianshe Liu
3
1
Key Laboratory of Key Technical Factors in Zhejiang Seafood Health Hazards, Institute of Innovation & Application, Zhejiang Ocean University, Zhoushan 316022, China
2
Department of Environmental Engineering, Zhejiang Ocean University, Zhoushan 316022, China
3
State Environmental Protection Engineering Center for Pollution Treatment and Control in Textile Industry, College of Environmental Science and Engineering, Donghua University, Shanghai 201620, China
*
Authors to whom correspondence should be addressed.
Catalysts 2018, 8(10), 477; https://doi.org/10.3390/catal8100477
Submission received: 26 September 2018 / Revised: 15 October 2018 / Accepted: 18 October 2018 / Published: 19 October 2018
(This article belongs to the Special Issue Novel Heterogeneous Catalysts for Advanced Oxidation Processes (AOPs))
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Abstract

:
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.

Graphical Abstract

1. Introduction

Antibiotics accumulated in water have posed a great threat to the environmental safety and human health. In particular, levofloxacin (LEV), as a typical fluoroquinolone antibiotic, has been widely used to treat human and animal infections. As a result, LEV was excessively accumulated in aquatic environments, causing serious health-risk issues by inducing genetic exchange and the bacterial drug resistance [1,2]. Conventional technologies for the removal of antibiotics in aquatic systems have been restricted due to low efficiency, high cost and/or sophisticated instrumentation. In recent years, the semiconductor-based photocatalysis as a green oxidation technology, has made significant developments for treating water pollutions [3,4,5,6,7,8,9,10]. Bismuth (III)-based semiconductors (e.g., BiOX (X = Cl, Br, I) [11,12,13], Bi2MoO6 [14,15,16,17], and Bi2WO6 [18]) have been emerging as kinds of high-efficiency photocatalysts. Among them, Bi2MoO6 as a good visible-light-driven (VLD) photocatalyst, has been one of the focal research points in the area of photocatalysis. Unlike the popular TiO2, which only can respond to the ultraviolet light, Bi2MoO6 (Eg ≈ 2.5–2.8 eV) is active in the visible-light region. However, Bi2MoO6 still suffers from the narrow range of sunlight response and fast electron-hole recombination [15,16,17,19]. Intriguingly, the fabrication of semiconductor heterojunctions is a crucial way to obtain superior photocatalysts [20]. Thus, a multiple of Bi2MoO6-based heterojunctions have been fabricated to ameliorate the photocatalytic activity for the removal of pollutants, such as BN/Bi2MoO6 [21], Bi/Bi2MoO6 [22], C3N4/Bi2MoO6 [23,24], TiO2/Bi2MoO6 [25], etc. [26,27,28,29,30,31]. Our group has also developed some Bi2MoO6-based heterojunction photocatalysts with excellent photocatalytic properties [32,33,34,35,36]. However, the majority of them were employed to degrade traditional dyes to evaluate their activity, their application in decomposing colorless and refractory pollutants were underdeveloped.
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 Bi2MoO6 [31], it is anticipated the rationally designed CdS/Bi2MoO6 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/Bi2MoO6 have not been indicated.
Inspired by the above aspects, we designed a flower-like CdS/Bi2MoO6 for the removal of the LEV antibiotic. A facile solvothermal-precipitation-calcination method was employed to construct hierarchical CdS/Bi2MoO6 heterojunctions with tightly interfacial contact. 1.0CdS/Bi2MoO6 possesses the highest activity towards the removal of LEV antibiotic. The plausible photocatalytic mechanism for the degradation of LEV antibiotic over CdS/Bi2MoO6 was also illustrated.

2. Results

2.1. Preparation and Characterization

CdS/Bi2MoO6 heterojunctions with CdS/Bi2MoO6 molar ratios of 0.5:1, 0.75:1, 1.0:1, and 1.5:1 were fabricated and denoted as 0.5CdS/Bi2MoO6, 0.75CdS/Bi2MoO6, 1.0CdS/Bi2MoO6, and 1.5CdS/Bi2MoO6, respectively. The crystalline phases of Bi2MoO6, CdS, and the optimal sample (1.0CdS/Bi2MoO6) were examined by XRD technique (Figure 1). All the diffraction peaks for Bi2MoO6 and CdS correspond to crystal planes of orthorhombic (JCPDS 76-2388) [25,33] and (JCPDS 75-1545) [31], respectively. As for 1.0 CdS/Bi2MoO6, both the diffraction peaks belonging to Bi2MoO6 and CdS were detected in the XRD pattern, verifying the fabrication of CdS/Bi2MoO6.
The morphology of Bi2MoO6 and 1.0CdS/Bi2MoO6 was characterized by SEM and TEM analysis. As shown in Figure S1, pristine Bi2MoO6 has the shape of a microsphere with a smooth surface, in accordance with the morphology of the reported Bi2MoO6 [25]. In comparison with Bi2MoO6, the surface of 1.0CdS/Bi2MoO6 became rough due to the coating of CdS nanoparticles (size: 5–25 nm) (Figure 2a,b), such a hetero-structure favors the separation of charges. In addition, the EDX spectrum shows the co-existence of Cd, S, Bi, Mo and O elements in 1.0CdS/Bi2MoO6 (Figure 2c). The detailed microstructure of 1.0CdS/Bi2MoO6 revealed by TEM analysis (Figure 2d) shows that the surfaces of Bi2MoO6 nanoplates (size: 200 nm) are deposited by numerous of CdS nanoparticles (size: 5–25 nm). The above characterizations further verify the formation of CdS/Bi2MoO6.
The UV-Vis absorption spectra of Bi2MoO6, CdS, and 1.0CdS/Bi2MoO6 are illustrated in Figure 3. Pure Bi2MoO6 and CdS exhibit good absorption in the visible light region, consistent with the previous reports [18,37,40]. Notably, 1.0CdS/Bi2MoO6 presents better optical absorption than Bi2MoO6, indicating that 1.0CdS/Bi2MoO6 possesses a better ability to harvest sunlight.

2.2. 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 Bi2WO6 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/Bi2MoO6 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 Bi2MoO6 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/Bi2MoO6 heterojunctions show much enhanced photocatalytic activity compared to pristine CdS and Bi2MoO6. Notably, 1.0 CdS/Bi2MoO6 obtains the highest activity with 80.4% of LEV degradation within 60 min, much higher than CdS (20.6%), Bi2MoO6 (10.9%), or the mechanical mixture (25.8%), verifying the existence of synergistic interaction between CdS and Bi2MoO6. Since the introduction of CdS nanoparticles on Bi2MoO6 microspheres, the intimately contacted interfacial surface is formed for effective separation of charge carriers. However, with the excessive deposition of CdS (1.5CdS/Bi2MoO6), 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/C0) = kt. Clearly, 1.0CdS/Bi2MoO6 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), Bi2MoO6 (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/Bi2MoO6 (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/Bi2MoO6 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/Bi2MoO6 were executed. Inspiringly, 1.0CdS/Bi2MoO6 retained its initial photocatalytic activity after six successive runs (Figure 6a). Moreover, compared to the crystal phase of the fresh 1.0CdS/Bi2MoO6, no obvious phase changes (Figure 6b) of the used one was detected, illustrating that 1.0CdS/Bi2MoO6 has superior stability and reusability.

2.3. 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 O2•−) or AO (quencher of h+), reflecting that O2•− and h+ could be the dominant active species responsible for the LEV degradation over 1.0CdS/Bi2MoO6.
The bandgap (Eg) of Bi2MoO6 and CdS were calculated by the equation: (αhν) = A(hν-Eg)n/2. As presented in Figure S2, the Eg of Bi2MoO6 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 Bi2MoO6 and CdS can be obtained by the following equation: EVB = X − E0 + 0.5 Eg (1), ECB = EVB − Eg (2). Thereby, the EVB and ECB of Bi2MoO6 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 /Bi2MoO6, 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 Bi2MoO6 and 1.0CdS/Bi2MoO6 (Figure S3). Through comparing the PL emission spectra of Bi2MoO6 and 1.0CdS/Bi2MoO6 (Figure S3), it is found that 1.0CdS/Bi2MoO6 exhibits much lower fluorescence intensity compared to that of pure Bi2MoO6, indicating that the nano-junction between Bi2MoO6 and CdS greatly promotes the separation of electron-hole pairs.
On the basis of above experimental results, the interfacial charge transfer behavior of CdS/Bi2MoO6 is illustrated in Figure 8. Under visible-light illumination, electron-hole pairs can be produced in both CdS and Bi2MoO6. The photo-excited e− in the CB of CdS may rapidly migrate to that of Bi2MoO6, while the photo-excited holes in the VB of Bi2MoO6 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/Bi2MoO6. The CB potential of CdS (−0.56 eV) and is Bi2MoO6 (−0.32 eV) are more negative than E (O2/O2•−) (+0.13 eV) [46], thus the e– in the CB of CdS and Bi2MoO6 can react with O2 to generate active O2•−, further decomposing LEV antibiotic. On the other hand, the holes collected in the VB of CdS and Bi2MoO6 can directly decompose the LEV antibiotic due to their strong oxidation ability.

3. Materials and Methods

3.1. Chemicals

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

3.2. Chemicals Synthesis of Catalysts

Synthesis ofBi2MoO6: Typically, 1 mmol Bi(NO3)3·5H2O and 0.5 mmol Na2MoO4·2H2O 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/Bi2MoO6: 0.5 mmol Bi2MoO6 was suspended into 50 mL of deionized water. Then, 1mmol CdCl2·2.5H2O was dissolved in the above suspension under vigorously stirring. After that, 40 mL of Na2S (1 mmol) aqueous solution was added into the above system dropwise under magnetically stirring for 2 h. The precipitation labeled as 1.0 CdS/Bi2MoO6 was washed, dried and then calcined at 200 °C for 2 h in N2 atmosphere to obtain the catalysts. Through adjusting the amount of precursor of CdS, the samples with CdS/Bi2MoO6 molar ratios of 0.5:1, 0.75:1, and 1.5:1 are labeled as 0.5CdS/Bi2MoO6, 0.75CdS/Bi2MoO6, and 1.5CdS/Bi2MoO6, respectively. The CdS sample was also prepared by the same procedure in the absence of Bi2MoO6.

3.3. 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.

3.4. Photocatalytic Performance Tests

The photocatalytic property of CdS/Bi2MoO6 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.

4. Conclusions

In summary, CdS nanoparticles interspersed-Bi2MoO6 heterojunction photocatalytst was fabricated by a facile solvothermal-precipitation-calcination method. Due to the introduction of CdS, the photo-absorption of CdS/Bi2MoO6 and the interfacial charge separation were remarkably enhanced and promoted. By virtue of these benefits, in comparison with bare CdS and Bi2MoO6, 1.0CdS/Bi2MoO6 presents profoundly enhanced visible-light photocatalytic activity for the removal of the LEV antibiotic. Moreover, 1.0CdS/Bi2MoO6 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.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4344/8/10/477/s1, Figure S1: SEM image of pure Bi2MoO6, Figure S2: The resulting Tauc plots of CdS and Bi2MoO6, Figure S3: Photoluminescence (PL) spectra of bare Bi2MoO6 and 1.0CdS/Bi2MoO6.

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.

Funding

This work has been financially supported by the National Natural Science Foundation of China (51708504), the Public Projects of Zhejiang Province (LGN18E080003), the Science and Technology Project of Zhoushan (2017C41006), and the Qingdao Science and Technology Program (16-5-1-32-jch).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. XRD patterns of Bi2MoO6, CdS, and 1.0CdS/Bi2MoO6.
Figure 1. XRD patterns of Bi2MoO6, CdS, and 1.0CdS/Bi2MoO6.
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Figure 2. (a,b) SEM images, (c) EDX spectrum, and (d) TEM image of 1.0CdS/Bi2MoO6.
Figure 2. (a,b) SEM images, (c) EDX spectrum, and (d) TEM image of 1.0CdS/Bi2MoO6.
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Figure 3. UV-vis absorbance spectra of bare Bi2MoO6, CdS, and 1.0CdS/Bi2MoO6.
Figure 3. UV-vis absorbance spectra of bare Bi2MoO6, CdS, and 1.0CdS/Bi2MoO6.
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Figure 4. (a) The photo-degradation efficiency against LEV (20 mg L−1, 100 mL) by different catalysts (35 mg). (b) Kinetic modeling for LEV removal over different catalysts.
Figure 4. (a) The photo-degradation efficiency against LEV (20 mg L−1, 100 mL) by different catalysts (35 mg). (b) Kinetic modeling for LEV removal over different catalysts.
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Figure 5. TOC removal profile of LEV (40 mg L−1, 200 mL) over 1.0CdS/Bi2MoO6 (200 mg).
Figure 5. TOC removal profile of LEV (40 mg L−1, 200 mL) over 1.0CdS/Bi2MoO6 (200 mg).
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Figure 6. (a) Recycling tests of 1.0CdS/Bi2MoO6 for LEV degradation; (b) XRD patterns of 1.0CdS/Bi2MoO6 before and after six cycles.
Figure 6. (a) Recycling tests of 1.0CdS/Bi2MoO6 for LEV degradation; (b) XRD patterns of 1.0CdS/Bi2MoO6 before and after six cycles.
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Figure 7. Photo-degradation of LEV (20 mg L−1, 100 mL) by1.0CdS/Bi2MoO6 (35 mg) under visible light in the presence of isopropyl alcohol (IPA), ammonium oxalate (AO), and benzoquinone (BQ).
Figure 7. Photo-degradation of LEV (20 mg L−1, 100 mL) by1.0CdS/Bi2MoO6 (35 mg) under visible light in the presence of isopropyl alcohol (IPA), ammonium oxalate (AO), and benzoquinone (BQ).
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Figure 8. Photocatalytic mechanism scheme over CdS/Bi2MoO6.
Figure 8. Photocatalytic mechanism scheme over CdS/Bi2MoO6.
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MDPI and ACS Style

Li, S.; Liu, Y.; Long, Y.; Mo, L.; Zhang, H.; Liu, J. Facile Synthesis of Bi2MoO6 Microspheres Decorated by CdS Nanoparticles with Efficient Photocatalytic Removal of Levfloxacin Antibiotic. Catalysts 2018, 8, 477. https://doi.org/10.3390/catal8100477

AMA Style

Li S, Liu Y, Long Y, Mo L, Zhang H, Liu J. Facile Synthesis of Bi2MoO6 Microspheres Decorated by CdS Nanoparticles with Efficient Photocatalytic Removal of Levfloxacin Antibiotic. Catalysts. 2018; 8(10):477. https://doi.org/10.3390/catal8100477

Chicago/Turabian Style

Li, Shijie, Yanping Liu, Yunqian Long, Liuye Mo, Huiqiu Zhang, and Jianshe Liu. 2018. "Facile Synthesis of Bi2MoO6 Microspheres Decorated by CdS Nanoparticles with Efficient Photocatalytic Removal of Levfloxacin Antibiotic" Catalysts 8, no. 10: 477. https://doi.org/10.3390/catal8100477

APA Style

Li, S., Liu, Y., Long, Y., Mo, L., Zhang, H., & Liu, J. (2018). Facile Synthesis of Bi2MoO6 Microspheres Decorated by CdS Nanoparticles with Efficient Photocatalytic Removal of Levfloxacin Antibiotic. Catalysts, 8(10), 477. https://doi.org/10.3390/catal8100477

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