nitrogen-doped graphene quantum dots composites with strong adsorption and effective photocatalytic activity for the degradation of antibiotic ciprofloxacin

Applied Surface Science 496 (2019) 143655

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Ultrathin BiOCl/nitrogen-doped graphene quantum dots composites with strong adsorption and effective photocatalytic activity for the degradation of antibiotic ciprofloxacin Zhigang Moua, Hui Zhanga, Zeman Liuc, Jianhua Suna, Mingshan Zhub,

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a

School of Chemistry and Environmental Engineering, Institute of Advanced Functional Materials for Energy, Jiangsu University of Technology, Changzhou 213001, China School of Environment, Jinan University, Guangzhou 510632, China c School of Energy and Power Engineering, Dalian University of Technology, Dalian 116024, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Antibiotic ciprofloxacin BiOCl Nitrogen-doped graphene quantum dots Adsorption Photocatalysis

Photocatalysis, as a green chemical technology for the removal of antibiotics, has attracted great interest recently. This study proposed a facile one-step hydrothermal process to fabricate ultrathin two dimensional (2D) BiOCl/nitrogen-doped graphene quantum dots (BiOCl/NGQDs) composites. Compared to pure BiOCl, the BiOCl/ NGQDs composites exhibited enhancement in both adsorption and photodegradation for antibiotic ciprofloxacin (CIP). The optimized content of NGQDs was 6.9% and this optimized composite showed a degradation efficiency of 82.5% within 60 min under visible-light irradiation, which was considerably better than that of pure BiOCl (34.9%). Meanwhile, most of CIP was efficiently mineralized into CO2, H2O and other inorganic products, as revealed by a total organic carbon (TOC) removal efficiency of 95.5% within 5 h. The improved photocatalytic activity was attributed to excellent adsorption capability, enhanced visible-light response and efficient separation of the photoinduced electron-hole pairs in BiOCl/NGQDs composite. The active species trap experiments and electron spin resonance revealed that %O2− and hole mainly participated in the CIP degradation process. Such an effective strategy to design ultrathin 2D composite photocatalysts would provide a new approach for application in wastewater purification.

1. Introduction Ciprofloxacin (CIP), as a second-generation of fluoroquinolone, has been extensively applied in human and veterinary drugs for treating bacterial infections [1]. A large amount of CIP was discharged into body of water as the active substance after being performed as a pharmaceutical [2]. The deliberate release of antibiotics in the environment threatens the aquatic ecosystem and human health due to their potential toxicity [3,4]. Therefore, the removal of CIP from water has a crucial influence for environmental protection. However, similar to other pharmaceutical chemicals, CIP is known to be not amenable to the conventional wastewater treatment processes [5]. Therefore, developing a powerful method to remove CIP from water is very urgent. Photocatalysis, as a green chemical process, is highly significant in solving energy and environmental problems due to its promising applications in water purification and environmental remediation with low cost and energy consumption [6,7]. Currently, due to the spatial separation of photoinduced electron–hole pairs, heterojunction

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photocatalysts can be shown to possess high photocatalytic performance [8], which provides new insight in the application of removal of antibiotic CIP pollutants [9–17]. For example, Wang et al. successfully prepared ordered mesoporous g-C3N4 by utilizing SBA-15 as template, which exhibited the enhanced photocatalytic performance for CIP degradation under simulated sunlight irradiation [9]. Gao et al. successfully fabricated Fe(III) grafted BiVO4 nanosheets by wet-impregnation method. The visible-light photodegradation rate of CIP by the Fe(III)BiVO4 composite is 4.34 times higher than that by BiVO4 [10]. However, the above reported methods for fabrication of photocatalysts are complex with multistep processes. In addition, most reported work cannot completely mineralize CIP molecules into inorganic products with low TOC removal efficiency and the intermediates of CIP in water still retain the toxicity [18]. In this regards, a facile route for preparation of photocatalysts for efficient removal of CIP is yet a great challenge. Recently, ultrathin 2D photocatalyst materials have been attracting considerable attention due to their unusual optical, electronic and

Corresponding author. E-mail addresses: [email protected] (J. Sun), [email protected] (M. Zhu).

https://doi.org/10.1016/j.apsusc.2019.143655 Received 15 May 2019; Received in revised form 20 July 2019; Accepted 11 August 2019 Available online 12 August 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

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2.2. Characterization

mechanical properties [19–22]. Compared with bulk materials, the ultrathin 2D configuration can supply large specific surface area, enhance light harvest, and reduce the migration distance of charge carriers for diverse photocatalytic applications. Recently, the ultrathin Bi-based materials have been prepared by wet-chemical synthesis methods due to its unique layered structure, the good chemical stability and improved capability of energy conversion and pollutants degradation [10,23–27]. For example, Guan et al. prepared the triple vacancy associates VBi‴VO••VBi‴ ultrathin BiOCl nanosheets via PVP-assisted solvothermal strategy, which showed outstanding solar photocatalytic efficiency for rhodamine B and phenol degradation [23]. Wang et al. prepared ultrathin C-doped Bi2MoO6 nanosheets by one-step hydrothermal route, which exhibited significantly higher visible-light photocatalytic performance toward removal of NO than that of bulk Bi2MoO6 as a result of the increasing reactive oxygen species [27]. These results indicate the ultrathin 2D bismuth-based materials or their composites can greatly improve the photocatalytic activity. Inspired by the above elaboration, herein we reported a facile onestep hydrothermal method for fabrication of ultrathin BiOCl/nitrogendoped graphene quantum dots (NGQDs) composite. Such synthetic process not only saves time and reduces the consumption of energy, but also facilitates the strong interaction between BiOCl and NGQDs. The strong interaction between BiOCl and NGQDs results in the narrowed bandgap of BiOCl nanosheets, contributing to a broadened visible-light response and efficient separation of photoinduced electron-hole pairs. Compared with pure BiOCl, ultrathin BiOCl/NGQDs composites displayed enhanced adsorption capacity and visible-light photodegradation of CIP. Moreover, the ultrathin composite led to the nearly total mineralization of CIP within 5 h based on the results of total organic carbon (TOC) removal efficiency. The result confirms the great potential of ultrathin 2D bismuth-based hybrid materials for practical antibiotic wastewater purification. This work would provide an effective way to construct highly efficient photocatalytic composite materials for quick and efficient antibiotics elimination.

The morphology of samples was performed by scanning electron microscopy (SEM) instrument (Hitachi S-4800, voltage = 10 kV) and transmission electron microscopy (TEM) instrument (JEM-2100, accelerating voltage = 200 kV). X-ray diffraction (XRD) patterns were measured on a Philips diffractometer with Ni-filtered Cu Kα radiation. The scanning rate is 1° min−1 in the 2θ range from 10 to 80°. Sample for atomic force microscopy (AFM) image was prepared by depositing a suspension onto a freshly cleaved mica substrates and causing to dry in air. Contact mode image was obtained by a Bruker multimode 8 atomic force microscope. UV–vis diffuse reflectance spectra were obtained on a Hitachi UV-3010 spectrophotometer using BaSO4 as a reference and were converted from reflection to absorbance by the Kubelka–Munk method. The photoluminescence (PL) spectra were obtained on an Edinburgh FLS920 fluorospectrophotometer with an excitation wavelength of 325 nm. X-ray photoelectron spectroscopy (XPS) measurements were taken by an AXIS Ultra DLD system (Kratos Analytical Inc.) using monochromatic Al Kα radiation. The thermogravimetric analysis (TGA) was conducted with a DSC 404F3 differential scanning calorimeter (NETZSCH, Germany) at a temperature scan rate of 10 °C min−1 in nitrogen. The FT-IR spectra were recorded on a Thermo Scientific Nicolet iS10 spectrometer. The Brunauer-Emmett-Teller (BET) surface areas of the products were calculated by nitrogen adsorption-desorption isotherms using a quantachrome QUDRASORB SI surface area and porosity analyzer. The electron spin resonance (ESR) studies were performed on a MS5000X ESR spectrometer Miniscope (Magnettech, Germany) to detect reactive species generated in the photocatalytic system with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as the radicals spin-trapped reagent under visible light irradiation. 2.3. Photoelectrochemical measurements The transient photocurrent and electrochemical impedance spectroscopy (EIS) were carried out on a CHI 660B electrochemical analyzer using 0.5 M Na2SO4 aqueous solution by a standard three-electrode system consisting of a platinum wire as the counter electrode, a saturated Ag/AgCl electrode as the reference electrode, and the sample films as the working electrodes. Transient photocurrent responses of sample films were measured with bias potential of 50 mV under visiblelight illumination (a 300 W Xe lamp with a 400 nm cut-off filter, irradiation intensity: ~105 mW cm−2). The working electrodes were fabricated as follows: 15 mg sample powder was thoroughly suspended by adding 0.2 mL DMF, and the obtained slurry was spread evenly onto 1 cm2 of conducting indium tin oxide (ITO) glass substrate to form a film with thickness around 10 μm. The spread film was dried in air and further annealed at 120 °C for 2 h to improve adhesion.

2. Experimental section 2.1. Sample preparation The ultrathin BiOCl/NGQDs composites were synthesized via a onestep hydrothermal method. In detail, 4 mmol Bi(NO3)3·5H2O and 1.6 g PVP were dissolved into 40 mL of mannitol solution (0.1 M). Then, 10 mL NaCl aqueous solution (0.4 M) was added dropwise into the above mixture under stirring. After being stirred for 30 min, 0.168 g citric acid and 0.144 g urea were dissolved in the mixture. After another 10 min of agitation, the mixture was transferred into Teflon-lined stainless steel autoclave (100 mL) and kept at 160 °C for 4 h. After cooling to room temperature, the powders were collected by filtration and washed with deionized water and ethanol thoroughly, the resulting product was dried at 50 °C for 12 h. For convenience, such obtained product was labeled as BiOCl/NGQDs(1). Notable, the mannitol in the reaction system could act as a directing agent to form square-like BiOCl nanosheets due to its long chain and polyhydroxyl of mannitol molecules [28]. Following the same procedure as above except changing the amount of citric acid and urea with 0.672 g citric acid and 0.576 g urea; 1.344 g citric acid and 1.152 g urea, the obtained products were labeled as BiOCl/NGQDs(2), BiOCl/NGQDs(3) respectively. The pure BiOCl nanosheets were prepared similar to the above procedure for the preparation of BiOCl/NGQDs, in which citric acid and urea was absent. For comparison, the pure BiOCl nanoplates were prepared similar to the procedure for the preparation of pure BiOCl nanosheets, in which PVP was absent referring to the method previously reported [23]. The pure NGQDs were prepared by a similar hydrothermal route using 0.672 g citric acid and 0.576 g urea as the staring materials referring to the method previously reported [29] with minor modification.

2.4. Adsorption and photocatalytic activity test The adsorption and photocatalytic degradation of CIP was implemented in a top-irradiation quartz vessel connected to a circulating water system to maintain at room temperature. The photocatalytic activities of the samples were investigated under the visible-light illumination same as used in the photocurrent measurement. In a typical adsorption and photocatalytic degradation reaction, 50 mg samples were added into 100 mL CIP (10 mg L−1) aqueous solution, and then stirred for 30 min in the dark to reach adsorption equilibrium between the photocatalysts and CIP pollutants prior to illumination. In the specific time interval, about 3 mL suspension was sampled and centrifugated. The residual concentrations of CIP were determined by monitoring the characteristic absorption band of 272 nm using a UV–vis spectrophotometer (Agilent, Cary-100). The amount of adsorbed CIP (μg/mg) was calculated by the formula: Adsorbed amount = [(C0 − Ct) × 0.1 × 1000] / 50, where C0 was the initial concentration (mg L−1), Ct were the detected CIP concentration at 2

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different time (mg L−1). TOC was obtained by total organic carbon analyzer (Shimadzu, Japan). The photodegradation intermediates of CIP were determined on a LC-MS system equipped with a 6410 HPLC (Agilent, USA) and a mass analyzer API 3000. A SB-C18 column (3.5 × 100 mm) from Agilent was used for chromatographic separations at 35 °C. 0.1% of formic acid aqueous solution and acetonitrile were used as mobile phases and the flow rate was 0.25 mL min−1. For further studying of the three active species (%O2−, %OH and holes,) effect on the photocatalytic performance of BiOCl/NGQDs, three different sacrificial agents (p-benzoquinone (BQ) for %O2−, disodium ethylenediaminetetraacetate (EDTA-2Na) for holes, isopropyl alcohol (IPA) for %OH) with concentration of 10 mM were added respectively into the CIP solution. 3. Results and discussion 3.1. Structure properties Fig. 2. XRD patterns of the as-prepared samples.

The pure NGQDs were obtained by a hydrothermal process using citric acid and urea as precursor materials according to the literature [29]. Ultrathin BiOCl/NGQDs composites were also prepared by onestep hydrothermal method. Moreover, the NGQDs contents can be easily tuned by adjusting the amount of citric acid and urea in the hydrothermal reaction system. The in-situ formation of BiOCl and NGQDs would ensure a strong chemical interaction between BiOCl and NGQDs. The morphology and structure of the as-prepared BiOCl/NGQDs composites were investigated by SEM, TEM and AFM images. Fig. 1a shows the SEM image of as-prepared BiOCl/NGQDs(2), which has typical 2D sheet-like morphology. The size of BiOCl/NGQDs is around 500 nm–1 μm. Furthermore, TEM image shows that NGQDs with the diameter about 5–10 nm are observed on the BiOCl surface (Fig. 1b), indicating that NGQDs have been successfully coupled with BiOCl at the nanoscale. The lattice spacing of BiOCl/NGQDs(2) observed in the HRTEM image (Fig. 1c) are 0.28 and 0.24 nm, which are corresponding to (110) crystal plane of BiOCl and (1120) crystal plane of NGQDs [12,30], respectively. The above results indicate that NGQDs are adhered to the BiOCl surface successfully. Moreover, to detect the

thickness of the as-prepared BiOCl/NGQDs(2), contact mode AFM was performed. Fig. 1d clear shows that the thickness of ultrathin nanosheet is ~3.6 nm, suggesting that ultrathin nanosheet morphology was achieved in our synthetic process. Fig. 2 displays the XRD patterns of the as-synthesized samples. For pure BiOCl, the diffraction peaks centering at 12.0°, 25.9°, 32.5°, 33.6°, 41.0°, 46.8°, 49.9°, 54.2°, 58.8°, 68.3° and 77.7° can be attributed to the diffractions of (001), (101), (110), (102), (112), (200), (113), (211), (212), (220) and (130) crystal facets of tetragonal phase BiOCl (JCPDS card no. 73-2060), respectively [31]. After loading NGQDs, the diffractions peaks of BiOCl/NGQDs(1) and BiOCl/NGQDs(2) were same as the peaks of pure BiOCl. No diffraction peaks of NGQDs could be detectable in the NGQDs/BiOCl composites, which was due to its low dosage and/or high dispersion of NGQDs in the composites [31–33]. Moreover, it is found that the diffraction peaks of BiOCl/NGQDs(3) are different from those of the two other NGQDs/BiOCl composites. The

Fig. 1. SEM, TEM and AFM images of the BiOCl/NGQDs(2): (a) SEM image; (b) low magnification TEM image; (c) high magnification TEM image; (d) AFM image and the height profile along the line. 3

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Fig. 3. XPS spectra of BiOCl and BiOCl/NGQDs(2) samples. (a) Survey of the samples; (b) Bi 4f; (c) Cl 2p; (d) O 1s; (e) C 1s; (f) N 1s.

difference includes (1) the lower peak intensity of the (101) plane; (2) the peak deformation of the (102) plane; and (3) the upshift of peak position of the (112) plane. This result indicates that the crystallization of BiOCl slightly changed with the increase overload amount of citric acid and urea in the hydrothermal reaction system. The considerable variation of pH value due to the addition of high concentrated citric acid may lead to the change of BiOCl structure [34]. XPS measurement was investigated to study the chemical composition and chemical states of the pure BiOCl and BiOCl/NGQDs(2) samples. The survey spectra (Fig. 3a) indicated the as-prepared BiOCl and BiOCl/NGQDs(2) contained all elements in the corresponding samples. The Bi 4f (Fig. 3b) for pure BiOCl revealed that the binding energies of Bi 4f7/2 and Bi 4f5/2 were observed at 158.6 and 163.9 eV, respectively, which are corresponded to Bi3+ [31,35]. Interestingly, the Bi 4f peaks in BiOCl/NGQDs(2) sample shows a tiny up-shift toward higher binding energies compared with pure BiOCl, which can be assigned to the interaction between BiOCl and NGQDs [36]. The Cl 2p (Fig. 3c) in pure BiOCl displays two main peaks at binding energies of 197.2 and 198.8 eV, which are assigned to Cl 2p3/2 and Cl 2p1/2, respectively. Same with Bi 4f, the binding energies of Cl 2p1/2 and Cl 2p3/

2 in the BiOCl/NGQDs(2) sample were also slightly higher than those of pure BiOCl. As shown in Fig. 3d, the O 1 s peak of pure BiOCl at 529.2 eV is ascribed to the BieO bond, and the peak at 530.6 eV suggests the presence of other components such as O]CeOH species adsorbed on the surface of samples [37]. The O 1s XPS spectrum for BiOCl/NGQDs(2) can be fitted with three peaks at binding energies of 529.5, 531.0 and 532.3 eV, respectively. The peak located at 529.5 eV is assigned to the BieO bond. The other two peaks at around 531.0 and 532.3 eV are assigned to the O]CeOH and CeOH, respectively [38], which mainly originate from the NGQDs in the composite since the area of the peak attributed to the O]CeOH bond substantially increased, and the new peak attributed to the CeOH bond appeared compared to pure BiOCl. Fig. 3e shows the C 1s in pure BiOCl and BiOCl/NGQDs(2) samples. For pure BiOCl, the deconvoluted C 1s XPS peaks at 284.6, 285.5 and 287.1 are mainly due to the adsorbed organic groups C]C, CeC and C]O, respectively [39]. For BiOCl/NGQDs(2), the deconvoluted C 1s XPS peaks at 284.7, 285.5, and 287.6 eV are attributed to C]C, CeC/CeN, and C]O, respectively [29]. Moreover, the area of the peak attributed to the CeC/CeN bond substantially increased compared to pure BiOCl. These results indicate the NGQDs with some

4

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the shift of the band edge [27,45]. This result suggests that NGQDs in the composite extend the visible light utilizing. PL spectra have been commonly used to reflect the recombination processes of photo-generated charge carriers for photocatalyst, in which the lower PL intensity indicates lower possibility of recombination of electron-hole pairs [46]. At 325 nm excitation, the BiOCl/NGQDs(2) displayed much lower emission intensity than that of BiOCl (Fig. 6a), which demonstrates introduction of NGQDs could effectively decrease the recombination of electron-hole pairs due to efficient electron transfer from BiOCl to NGQDs [47]. To verify the above explanation, the photocurrent responses of pure BiOCl and BiOCl/NGQDs(2) ITO electrodes were performed with on/off cycles under visible-light illumination (Fig. 6b). Firstly, BiOCl/NGQDs (2) composite shows a higher photocurrent intensity than pure BiOCl under same experimental condition. This obvious enhancement of photocurrent for BiOCl/NGQDs(2) composite suggests the more photogenerated electron-hole pairs formed due to the wide visible light absorption [31]. Secondly, the pure BiOCl (Fig. 6b inset) shows an obvious photocurrent spike at the initial time of irradiation, followed by fast decrease until a relative constant current was reached. The phenomenon indicates that the charge recombination processes are occurring [48,49]. The BiOCl/NGQDs(2) composite showed a slower decay of the photocurrent value compared with pure BiOCl, indicating that the composite have a lower charge recombination than pure BiOCl. The holes accumulating on the surface of the photocatalyst competitively recombine with electrons rather than being captured by reduced species in the electrolyte, which leading to the photocurrent decay [49]. The introduction of NGQDs could increase the formation of photogenerated electron-hole pairs and reduce their recombination rate. In addition, the BiOCl/NGQDs(2) electrode exhibited much lower resistance than pure BiOCl, as evidenced by the smaller diameter of the semicircle than that of pure BiOCl in the EIS profiles (Fig. 6c). These results confirm the introduction of highly conductive NGQDs by onestep hydrothermal method can improve interfacial charge transfer in the composite of BiOCl/NGQDs.

Fig. 4. The TGA curves of the as-prepared samples.

nitrogen-and oxygen-containing groups on the surface of BiOCl/NGQDs (2) composite. Furthermore, the N 1s XPS spectrum of BiOCl/NGQDs(2) in Fig. 3f can be fitted three peaks, which centered at 399.1, 399.7 and 400.3 eV are assigned to C]NeC bonds [40], quinonoid imine and benzenoid amine [41], respectively. The emergence of both XPS binding energies of C 1s at 285.5 eV and N 1s at 399.1 eV further verified the existence of CeN bond on the surface of BiOCl/NGQDs(2). To estimate the amounts of NGQDs in the as-prepared samples and observe their thermal stabilities, the TGA was performed under N2 conditions. As shown in Fig. 4, two major weight losses are observed in the TGA curve. The first weight loss below 250 °C is mainly due to the evaporation of water and other impurities. From 40 to 250 °C, approx. 2.6%, 5.0%, 4.9% and 8.7% weight losses for the pure BiOCl, BiOCl/ NGQDs(1), BiOCl/NGQDs(2) and BiOCl/NGQDs(3), respectively. The second weight loss from 250 to 580 °C was approx. 39.2%, 34.1%, 32.3% and 29.3% for the pure BiOCl, BiOCl/NGQDs(1), BiOCl/NGQDs (2) and BiOCl/NGQDs(3), respectively. From the TGA curves of BiOCl and BiOCl/NGQDs composites, it could be seen that both samples exhibit similar inflection points from 250 to 580 °C but different weightloss rates, which is maybe due to the different heat transfer rates of samples. The contents of NGQDs in the samples were calculated from the weight remainders after heating the samples over 800 °C. The contents of NGQDs in BiOCl/NGQDs(1), BiOCl/NGQDs(2) and BiOCl/ NGQDs(3) were about 5.1%, 6.9%, and 9.9%, respectively.

3.3. Adsorptive and photocatalytic properties To compare the adsorptive and photocatalytic performance of the as-prepared samples, the photocatalytic degradation experiments of CIP (10 mg L−1) were carried out under visible-light illumination. As shown in Fig. 7a, during the dark period, for the as-prepared photocatalysts, the adsorption rate was fast in the first 5 min with adsorption ability increasing sharply, and then decelerated gradually until reaching the adsorption-desorption equilibrium. The adsorption-desorption equilibriums were established in 30 min and the photocatalysts showed different adsorption abilities for CIP. The adsorbed amount of CIP for pure BiOCl was 2.76 μg/mg (adsorption ratio = 13.8%). With the introduction of NGQDs into BiOCl, adsorption capacity increased. They were 6.68 μg/mg (adsorption ratio = 33.4%), 11.88 μg/mg (adsorption ratio = 59.4%), and 11.90 μg/mg (adsorption ratio = 59.5%) for BiOCl/NGQDs(1), BiOCl/NGQDs(2), and BiOCl/NGQDs(3), respectively. The BET specific surface areas were calculated to be 33.1 and 60.0 m2 g−1 for pure BiOCl and BiOCl/NGQDs(2), respectively, by nitrogen adsorption-desorption analysis (Fig. S2). They both show type IV isotherms, indicating the formation of mesoporous structures due to the aggregation of the nanosheets [50]. The pore volumes of pure BiOCl and BiOCl/NGQDs(2) were 0.168 and 0.239 cm3 g−1, respectively. The introduction of NGQDs leads to the increase of BET surface area of BiOCl/NGQDs in this system, following the higher adsorption capacity for CIP pollutants [47]. In addition, NGQDs in the system can have high adsorption capacity for CIP due to the strong π-π stacking interaction between CIP molecules and aromatic domains of NGQDs [51]. Deducting the amount of adsorption, the photocatalytic performance of the as-prepared photocatalysts in degrading non-adsorbed CIP under visible-light irradiation was studied. The results were shown in

3.2. Optical spectra and photogenerated charge properties The optical properties of pure BiOCl nanosheets and BiOCl/NGQDs (2) were detected by UV–vis diffuse reflectance spectroscopy. The results were shown in Fig. 5a. The pure BiOCl nanosheets showed a clear absorption edge at ~387 nm with a trailing absorption from 387 to 466 nm. However, the pure BiOCl nanoplates showed an absorption edge at ~380 nm without the trailing absorption (Fig. S1). It is widely accepted that defect states can modify the optical properties of metal oxides due to new photoexcitation processes [42,43], so the appearance of trailing absorption and band edge red-shifting is involved in the surface defects of as-prepared BiOCl nanosheets [44]. After BiOCl nanosheets were modified with NGQDs, the BiOCl/NGQDs(2) sample exhibited a red-shift in the absorption edge (~468 nm) and wide visible-light absorption. The bandgap energy of the samples was obtained by a linear extrapolation of the reflectance. As shown in Fig. 5b, the bandgap energy of pure BiOCl nanosheets is ~3.20 eV, while the bandgap of BiOCl/NGQDs(2) decreased to ~2.65 eV. The interaction between NGQDs and BiOCl during hydrothermal treatment results in 5

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Fig. 5. (a) UV–vis diffuse reflectance spectra of pure BiOCl and BiOCl/NGQDs(2) samples. (b) Plots of (αhν)2 vs. photon energy of BiOCl and BiOCl/NGQDs(2) samples.

Fig. 6. (a) PL emission spectra and (b) photocurrent responses with on/off of the visible-light illumination (inset shows an enlarged photocurrent response of pure BiOCl) and (c) EIS of pure BiOCl and BiOCl/NGQDs(2).

adsorption capacity was recovered to the original lever, indicating the adsorbed CIP previous to photocatalytic reaction was completely degraded. The further confirmation was carried out by FT-IR analysis as shown in Fig. S3. FT-IR spectrum of CIP shows that vibration bands at 1372, 833 and 721 cm−1 correspond to in-plane bending vibration of –CH, out-of-plane bending vibration of –CH, phenyl ring out-of-plane bending vibration of –CH, respectively. Whereas in the case of CIP adsorbed BiOCl/NGQDs(2), it shows the presence of these vibration bands accompanied by 12 to 14 cm−1 shift to high wavenumbers due to the interaction of the photocatalyst and CIP molecule. For BiOCl/ NGQDs(2) sample after CIP degradation, it was apparently found that the vibration band at 1386 cm−1 of CIP adsorbed BiOCl/NGQDs(2) shifted to original position of 1372 cm−1 accompanied by a reduction of the intensity. The two other vibration bands almost vanished. The FT-IR spectrum of BiOCl/NGQDs(2) sample after CIP degradation is similar with the original BiOCl/NGQDs(2). These results indicate adsorbed CIP was completely degraded after 60 min visible-light irradiation. So the total CIP degradation efficiency (including adsorbed CIP and non-adsorbed CIP previous to photocatalytic reaction) of pure BiOCl, BiOCl/NGQDs(1), BiOCl/NGQDs(2) and BiOCl/NGQDs(3) can reach 34.9%, 57.8%, 82.5%, and 74.3%, respectively, after 60 min visible-light irradiation. To further assess the photodegradation performance of BiOCl/ NGQDs(2), the effect of initial concentration of CIP on the catalytic activity was investigated. As shown in Fig. 7c, with increasing the initial concentrations of CIP to 20 and 40 mg L−1, the adsorption capacities for CIP were 32.3% and 15.2%. Under a 300 W Xe lamp irradiation, the degradation efficiency of CIP over BiOCl/NGQDs(2) composite reaches from 90.2% to 83.6% within 120 min, respectively. The result verifies ultrathin BiOCl/NGQDs composites have great potential for the CIP removal in a fast and effective manner.

Fig. 7b. Firstly, decomposition of CIP is infinitesimal after 60 min irradiation without photocatalyst, indicating that photolysis of CIP is negligible. The pure NGQDs also show little photocatalytic activity for CIP degradation. We can find that after 60 min visible-light irradiation, BiOCl/NGQDs(2) composite showed the highest photoactivity among the different BiOCl/NGQDs composites. The CIP degradation efficiency of BiOCl/NGQDs(2) reaches 57.0%, while the pure BiOCl, BiOCl/ NGQDs(1) and BiOCl/NGQDs(3) were only 24.5%, 36.6% and 36.5%, respectively. The results indicate that proper NGQDs on BiOCl nanosheets are beneficial for the photocatalytic activity. However, too much amount of the precursors of NGQDs (citric acid and urea) during the preparation of BiOCl/NGQDs(3) composite results in not only the crystallization change of BiOCl as shown in XRD result (Fig. 2), but also excess NGQDs may block the light absorption of BiOCl. It is possible that excess NGQDs in the composite increased photoabsorption and scattering and slowed down the photogeneration at the BiOCl surface. Similar results were also found in the literatures [12,31]. The photocatalytic degradation of CIP for pure BiOCl with visible light was due to the trailing absorption in the UV–vis spectrum of ultrathin BiOCl nanosheets (Fig. 5a). Similar phenomena were occurred for the photodegradation of colorless phenol [23] and bisphenol A [31] under visible-light irradiation. Generally, the photocatalytic degradation reaction is occurred on the surface of catalysts, which the large adsorption capability ensures the potential to effectively degrade CIP from water. To verify whether the adsorbed CIP on the surface of pure BiOCl, BiOCl/NGQDs(1), BiOCl/NGQDs(2) and BiOCl/NGQDs(3) in the dark previous to photocatalytic reaction was completely degraded after the photocatalytic reaction, the reusability of the samples for CIP adsorption was checked up. The samples were obtained by centrifugation after the degradation reaction without further treatment. The result demonstrated that the 6

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Fig. 7. (a) The adsorption of CIP with different as-prepared samples. (b) Photodegradation of non-adsorbed CIP by the samples under visible-light illumination (λ ≥ 400 nm). (c) Photodegradation of CIP with different initial concentrations over BiOCl/NGQDs(2) sample. (d) Cyclic test of photodegradation of CIP over BiOCl/ NGQDs(2) sample. Photocatalyst concentration: 0.5 g L−1; initial concentration of CIP aqueous solution: 10 mg L−1.

to supplying continuous CIP to photocatalyst surface; the narrow bandgap of BiOCl/NGQDs extends the visible light utilizing; moreover, NGQDs can act as electron transfer channel, greatly suppress the recombination of photo-generated electrons and holes, which is similar to graphene, the BiOCl/graphene composite can also enhance charge transfer and photocatalytic activity due to high conductivity of graphene [53]. All of which are beneficial to the promotion of the photocatalytic activity for BiOCl/NGQDs composite. The photodegradation intermediates of CIP over BiOCl/NGQDs(2) were detected by LC-MS. The typical chromatograms after CIP degradation for 0, 30 and 60 min are shown in Fig. 8. Obviously, an ion peak for pure CIP at retention time of 10.95 min was detected in the original solution. The different ion peaks could be detected after 30 min of visible-light illumination, suggesting the formation of different intermediates. Furthermore, the intensities of the peaks significantly weakened after 60 min of visible-light illumination, suggesting the effective mineralization of CIP molecules. The corresponding mass spectra of representative intermediates after 30 min irradiation were shown in Fig. 9. By referencing relevant reports and distinguishing the intermediates, the feasible photodegradation pathway of CIP was displayed in Scheme 1. It can be discovered that the degradation pathway was ascribed to oxidative degradation of the piperazine moiety. In the first step, the piperazine ring was opened by oxidization, then producing the dialdehyde derivative with m/z of 362 (CIP-1), as reported before [11,56]; in the second step, CIP-1 eliminated C]O groups and further oxidized to CIP-2 at m/z 291. In the third step, the decarboxylation process was proceeded to achieve CIP-3 at m/z 247 and the decarbonylation process was proceed to achieve CIP-4 at m/z 263. The transformation course of CIP was in conformity with the results in the literatures [11,55]. In the last step, further oxidation would be occurred to form CIP-5 at m/z 163 with possibly proposed molecular structure.

To investigate the stability of the composite, consecutive recycling experiments of BiOCl/NGQDs(2) for the CIP photodegradation were conducted with the result shown in Fig. 7d. After each recycling run, the photocatalyst was separated by centrifugation and washed with ethanol and water, then reused for subsequent recycling run after drying at 50 °C for 12 h. The concentration of photocatalyst (0.5 g L−1) and initial concentration of CIP aqueous solution (10 mg L−1) remained constant in each run. The photodegradation rate remains nearly invariant over five consecutive cycles, suggesting that BiOCl/NGQDs(2) is stable under visible-light illumination, which is especially important for its application. It is worth noting that the degradation of CIP molecules does not imply the complete mineralization of CIP since that there are still some intermediates remaining in the solution [11,52]. TOC removal efficiency was chosen as a mineralization index to analyse CIP degradation efficiency. After 5 h visible-light irradiation, about 95.5% of the TOC were removed by using BiOCl/NGQDs(2) as photocatalyst. The result indicates that BiOCl/NGQDs(2) photocatalyst could effectively mineralize CIP molecules into CO2, H2O and other inorganic products. Accordingly, the as-prepared ultrathin BiOCl/NGQDs composites by onestep hydrothermal method showed high degradation rate and TOC removal efficiency of CIP. We also compared the degradation efficiency of the BiOCl/NGQDs(2) photocatalyst with the other Bi-based photocatalysts reported in literatures, as shown in Table 1. It is found that the degradation rate and TOC removal efficiency of the prepared ultrathin BiOCl/NGQDs(2) composite are higher than that of other photocatalysts based on an overall consideration of illumination, initial concentration of CIP and concentration of catalysts. Combining with the characterization results and adsorption study, it is reasonable to conclude that good photocatalytic activity of BiOCl/NGQDs(2) mainly benefits from its excellent adsorption capability, enhanced visible-light harvesting, and efficient charge transfer property: High adsorption can provide a synergistic effect between adsorption and photocatalysis due 7

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Table 1 Comparison of CIP degradation over various Bi-based photocatalysts. Catalyst

Illumination

Bi/Bi3NbO7 CuS/BiVO4 BiOCl@Bi2WO6 microspheres BiOBr CQDs/BiOBr nanosheets Ultrathin BiOCl/NGQDs

300 W Xe lamp with 400 cutoff filter 300 W Xe lamp with 420 cutoff filter 300 W Xe lamp with 420 cutoff filter 400 W halogen lamp with 420 cutoff filter 300 W Xe lamp with 420 cutoff filter 300 W Xe lamp with 400 cutoff filter

C0(CIP) (mg L−1)

C(catalyst) (g L−1)

Removal efficiency of CIP

TOC removal efficiency

10 10 10 5 10 10

0.5 1 1 0.5 0.3 0.5

86% in 120 min 86.7% in 90 min 65% in 5 h ~100% in 140 min ~70% in 4 h 82.5% in 60 min

53% in 180 min – – 40% in 12 h 44.3% in 4 h 95.5% in 5 h

Ref.

[52] [13] [54] [55] [12] this work

C0(CIP) denotes initial concentration of CIP. C(catalyst) denotes concentration of catalyst.

3.4. Photocatalytic mechanism To explore the photocatalytic mechanism, the ESR technique with (5,5-dimethyl-1-pyrroline-N-oxide) DMPO as the radicals spin-trapped reagent was employed to investigate the active species generated by BiOCl/NGQDs(2) under visible-light illumination (Fig. 10a and b). Firstly, there are no signals under dark condition. After 5 min visible light irradiation, typical peaks of DMPO-%O2− were detected over BiOCl/NGQDs(2). On the other hand, for DMPO-%OH species, some weak intensities peaks were found in the sample of BiOCl/NGQDs(2) after visible-light illumination. These results indicated that the concentration of species of %O2− was higher than %OH radicals during the photo-irradiated process. To further determine the generation of active species during the photodegradation process over the BiOCl/NGQDs(2) material, free radical trapping experiments were carried out under visible light irradiation (Fig. 10c). IPA, BQ, and EDTA-2Na were employed as the traps for %OH radicals, %O2− radicals, and holes, respectively [57]. When no quencher was added, the degradation rate of CIP by the BiOCl/NGQDs (2) was 82.5% within 60 min. With the adding IPA, BQ, and EDTA-2Na to the photocatalytic system, the degradation rate decreased to 75.6%, 10.5% and 9.8%, respectively. Therefore, the ESR results and free radical trapping experiments reveal that the effect on photocatalytic

Fig. 8. Typical chromatograms of CIP solution after photodegradation for 0, 30 and 60 min.

Fig. 9. MS spectra of CIP and possible intermediates. 8

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Scheme 1. Possible degradation pathways of CIP over BiOCl/NGQDs composite.

Fig. 10. (a) DMPO spin-trapping ESR spectra with BiOCl/NGQDs(2) in (a) methanol dispersion for DMPO-%O2− and (b) aqueous dispersion for DMPO-%OH. (c) The photodegradation of CIP over BiOCl/NGQDs(2) under visible-light illumination with or without the addition of IPA, BQ and EDTA-2Na within 60 min.

degradation efficiency follows the order of holes ≈%O2− > %OH. The flat-band potentials were measured by the electrochemical method to confirm the band structure of pure BiOCl and BiOCl/NGQDs (2), and the Mott-Schottky plots of pure BiOCl and BiOCl/NGQDs(2) were shown in Fig. 11. The positive slope demonstrates that they are ntype semiconductors, and the flat-band potential values of pure BiOCl and BiOCl/NGQDs(2) are −0.543 and −0.437 V vs. Ag/AgCl, respectively. Compared with pure BiOCl, the flat-band potential value of BiOCl/NGQDs(2) shifts to the positive direction, which is consistent with the reported result for BiOCl/CQDs [32]. The flat-band potential is approximately equal to the conduction band (CB) potential (ECB). The ECB values are thus estimated to be −0.343 and −0.237 eV vs. normal hydrogen electrode (NHE) for pure BiOCl and BiOCl/NGQDs(2),

respectively. The band gap energy (Eg) of the pure BiOCl and BiOCl/ NGQDs(2) are estimated to be 3.20 and 2.65 eV from the UV–vis diffuse reflectance spectra analysis, respectively. The valence band (VB) potential (EVB) can be obtained by the formula EVB = ECB + Eg. Thus, the EVB values of the pure BiOCl and BiOCl/NGQDs(2) are calculated to be 2.857 and 2.413 eV, respectively. Because the E0(O2/%O2−) is −0.046 eV (vs. NHE) [58], O2 can be reduced to %O2− for BiOCl/ NGQDs(2) composite. Based on the above experimental results, a proposed schematic mechanism of BiOCl/NGQDs composite was shown in Scheme 2. In the beginning, the CIP molecules are quickly adsorbed on the surface of BiOCl/NGQDs. The BiOCl/NGQDs composite is excited after visiblelight illumination, and then the photogenerated electrons on the CB of BiOCl migrate to the NGQDs, leading to the effective separation of electron-hole pairs. The electrons further react with solution O2 to generate %O2− and thus degrade CIP effectively. On the other hand, the photoinduced holes on the VB of BiOCl also have strong oxidative ability, in which the CIP molecules can be oxidized. 4. Conclusions We demonstrate a new one-step hydrothermal method for fabricating ultrathin BiOCl/NGQDs composites. On one hand, the enhanced BET surface area and π-π stacking interaction between CIP and NGQDs of the composite of BiOCl/NGQDs result in strong adsorption for CIP molecules. On the other hand, the results of the optical spectra, photocurrent response and EIS reveal that introduction of NGQDs could narrow the bandgap of BiOCl, efficiently promote the visible light absorption and photoinduced charge separation and transfer. The BiOCl/ NGQDs composite displayed enhanced adsorption capacity and visiblelight photoactivity for CIP degradation compared with pure BiOCl. The degradation pathway of CIP was proposed according to the determination of intermediates by LC-MS analysis. ESR result and free radicals trapping experiments confirmed the effect of active species on

Fig. 11. Mott-Schottky plots of pure BiOCl and BiOCl/NGQDs(2). 9

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Scheme 2. Schematic model for the charge separation and CIP photodegradation process over BiOCl/NGQDs photocatalyst under visible-light illumination.

photocatalytic degradation efficiency by holes and %O2−. The present work provides a simple way to synthesize highly efficient photocatalytic composite materials for eliminating antibiotics quickly and effectively and also shows the tremendous potential of ultrathin 2D bismuth-based hybrid materials in practical antibiotic wastewater purification.

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