CeO2: Mechanism insight and degradation pathways

Journal Pre-proofs The enhanced photocatalytic performance toward carbamazepine by nitrogendoped carbon dots decorated on BiOBr/CeO2 : Mechanism insight and degradation pathways Lanlan Liang, Shengwang Gao, Jianchao Zhu, Lijun Wang, Yanna Xiong, Xunfeng Xia, Liwei Yang PII: DOI: Reference:

S1385-8947(19)33014-1 https://doi.org/10.1016/j.cej.2019.123599 CEJ 123599

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

12 August 2019 5 November 2019 23 November 2019

Please cite this article as: L. Liang, S. Gao, J. Zhu, L. Wang, Y. Xiong, X. Xia, L. Yang, The enhanced photocatalytic performance toward carbamazepine by nitrogen-doped carbon dots decorated on BiOBr/CeO2 : Mechanism insight and degradation pathways, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123599

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The enhanced photocatalytic performance toward carbamazepine by nitrogen-doped carbon dots decorated on BiOBr/CeO2 : Mechanism insight and degradation pathways Lanlan Liang

a,b,

Shengwang Gao a, Jianchao Zhu a, Lijun Wang a, Yanna Xiong c,

Xunfeng Xia a*, Liwei Yang b* a

State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese

Research Academy of Environmental Sciences, Beijing 100012, China. b

School of Civil Engineering, Key Laboratory of Water Supply & Sewage

Engineering of Ministry of Housing and Urban-rural Development, Chang’an University, Xi’an 710061, China. c

Solid Waste and Chemicals Management Center, Ministry of Ecology and

Environment, Beijing 100029, China. *

Corresponding Authors

E-mail addresses: [email protected] (X. Xia); [email protected] (L. Yang)

Abstract Novel nitrogen-doped carbon dots (NCDs) modified BiOBr/CeO2 was constructed successfully via a facile method. The morphology, structure, optical and photo-electrochemical properties of the photocatalysts were characterized by various technologies. The photocatalytic performance was evaluated by the degradation of carbamazepine (CBZ) under simulated solar light irradiation. The CBZ degradation by NCDs/BiOBr/CeO2 could achieve 98% within 120 min illumination, which was much higher than BiOBr or BiOBr/CeO2. The enhanced degradation of CBZ was due to the fact that the introduction of NCDs significantly accelerated the migration and separation of the photoinduced charge carries at the tightly contacted interface. The effects of some key operating parameters were further investigated, including photocatalyst dosage, initial CBZ concentration, initial pH, coexisting inorganic anions (Cl-, NO3-, SO42-) and natural organic matters (NOM), light irradiation conditions and water matrix. Basing on the scavenging experiments and electron spin resonance (ESR), the photocatalytic mechanism was proposed for the charge separation. Furthermore, the transformation intermediates and possible degradation pathway of CBZ were investigated by liquid chromatography coupled with mass spectrometry (LC/MS) and three-dimensional excitation-emission matrix fluorescence spectra (3D EEMs). Finally, the composite showed excellent recyclability and stability. This study created new opportunities for the design of photocatalysts and degradation of recalcitrant organic contaminants in aquatic environment. Keywords: Photocatalytic; Carbamazepine; Nitrogen-doped carbon dots; Composite

1 Introduction Nowadays, the elimination of pharmaceuticals from aquatic environment have attracted increasing attention due to their potentially adverse impact on ecosystems and human health [1]. As a representative psychiatric drug, carbamazepine is continually detected in aquatic system including sewage, surface water and even drinking water, which is due to the large consumptions and stable chemical structure [2]. Moreover, the long-term exposure in CBZ solution would induce some severe symptoms even at low level [2]. However, CBZ is recalcitrant and the removal efficiency is low by the conventional wastewater treatment plants (WWTPs) [3-5], since its resistance to biological degradation. Therefore, development of effective treatment techniques is urgently required for CBZ elimination to guarantee water security. Photocatalytic degradation with efficient utilization of solar energy is regarded as one of the most promising method for organic pollutants removal, due to its high efficiency, sustainability, low toxicity and affordability [6]. Various photocatalysts have been investigated for CBZ removal, including BiVO4 [7], SnO2 [8], TiO2 [9], BiOBr [10], BiOCl [11] and so on. As a typical V-VI-VII ternary semiconductor, BiOBr has shown excellent photocatalytic performance for the contaminants elimination because of its unique layered structure, appropriate band gap and excellent stability [12], which results in high redox potential and response to visible light. However, the photocatalytic performance of single-phase materials always are

limited due to various disadvantages, including low carries mobility, rapid recombination of photoinduced electro-hole pairs and insufficient light adsorption [13]. In this view, construction of heterojunction system is considered as an efficient strategy to overcome the defects and achieve excellent photocatalytic activity. Up to now, some BiOBr-based heterojunctions with remarkable degradation activity have been synthesized, such as BiOBr/Bi2WO6 [14], BiOBr/La2TiO7 [15], BiOBr/CeO2 [16], BiOBr/g-C3N4 [17] and so on. Therein, CeO2, as inexpensive rare earth oxide, has attracted much attention because of nontoxicity, high oxygen storage ability, excellent electrical and optical properties [18; 19]. For instance, Zhang et.al prepared BiOBr/CeO2 successfully by solvothermal method, which exhibited enhanced photocatalytic degradation towards Rhodamine B (RhB) [20]. Wen et.al fabricated the p-n heterojunction with better degradation of organic pollution than pure catalyst [16]. Nevertheless, the heterojunction of BiOBr/CeO2 still has various challenges: 1) the adsorption of visible light is inefficient, which limits the photocatalytic performance; 2) the migration of photoinduced charge carries still need to be accelerated. Recently, carbon dots with size below 10 nm were combined with other photocatalysts to improve the photocatalytic performance due to their extension of the visible light adsorption and excellent electron transfer and reservoir properties including CDs/BiPO4, CDs/C3N4, CDs/BiVO4/CdS and so on [1; 21; 22], which was considered as a feasible strategy to solve the problem to some extent. Meanwhile, the electrical and optical properties of carbon dots will be tailored by doping nitrogen atoms [23; 24], which induce charge delocalization and facilitating the charge transfer

capability. Hence, we envisioned that the introduction of NCDs into heterojunction could prolong charges lifetimes and facilitate the photocatalytic degradation. To the best of our knowledge, there is no investigation about the construction of NCDs/BiOBr/CeO2 with high-performance photocatalytic degradation for recalcitrant organic contaminate. In this study, we gave a successful attempt to fabricate BiOBr/CeO2 photocatalyst modified by NCDs, which was applied for the CBZ removal under simulated solar light irradiation. The composite of NCDs/BiOBr/CeO2 exhibited significantly enhanced photocatalytic performance in comparison with pure BiOBr and BiOBr/CeO2. Moreover, the effects of some key operating parameters were examined in detail, including photocatalyst dosage, initial CBZ concentration, initial solution pH, coexisting inorganic anions (Cl-, NO3-, SO42-) and natural organic matters (NOM), light irradiation conditions and water matrix. Furthermore, the photocatalytic mechanism was investigated with the help of trapping experiments and ESR technology. This study also determined the transformation products and possible CBZ degradation pathway by LC/MS and 3D EEMs. 2 Materials and Methods 2.1 Materials Bismuth nitrate hydrate (Bi(NO3)3·5H2O), postassium bromide (KBr), cerium nitrate

hexahydrate

(Ce(NO3)3·6H2O),

ammonium

citrate

(C6H5O7(NH4)3),

ethylenediamine and ethane glycol (EG) were purchased from Sinopharm Chemical Reagent Co. Ltd., PR China. Carbamazepine was obtained from Sigma-Aldrich Co.

Methanol (HPLC grade) was obtained from Merck (Darmstadt, Germany). All other reagents and chemicals required in the experiments were obtained from Tianjin Kaitong Chemical Reagent Co, Ltd. All materials were of analytical grade and were utilized without further purification. Deionized water (DI, 18 MΩ) were obtained from a Millipore Mili-Q water purification system (Bedford, MA, USA) and used for all experimental works. 2.2 Syntheses 2.2.1 Synthesis of NCDs The NCDs were prepared by a hydrothermal method based on precious literature [25]. Specially, 5 mmol (1.216 g) of C6H5O7(NH4)3 was dissolved in 10 mL H2O containing 335 μL ethylenediamine. Then, the solution was transferred to a 25 mL Teflon-lined autoclave and maintained at 200 ℃ for 5 h. After cooling to room temperature, the dark brown suspension was dialyzed for 3 days in order to remove impurities. The obtained NCDs solution was stocked at 4 ℃

for subsequent

experiments. 2.2.2 Synthesis of CeO2 CeO2 was prepared by heating 1 g Ce(NO3)3·6H2O in muffle furnace at 535 ℃ for 3 h in ambient air condition [26]. After cooling to room temperature, the yellow product was collected and grinded into powders. 2.2.3 Synthesis of NCDs/BiOBr/CeO2 The composites were synthesized by a facile hydrolysis method. In detail, Bi(NO3)3·5H2O (5 mmol, 2.425 g) was dissolved in 40 mL ethylene glycol to form

the solution A. Meanwhile, KBr (5 mmol, 0.595 g) was dissolved into 40 mL water with certain amount of CeO2 (0.086 g, 0.172 g, 0.344g) and NCDs (0.2 mL, 0.5 mL, 1 mL, 2 mL) under magnetic stirring for 30 min and sonicating for 30 min to form the homogeneous solution B. Subsequently, the solution B was added into solution A under magnetic stirring for 3 h. Finally, the obtained samples were rinsed by DI water and absolute ethanol for several times, then dried at 70 ℃ for 13 h in a vacuum drying oven. A series of composites with different stoichiometric ratios of Bi/Ce was controlled to 5:0.5, 5:1 and 5:2. The 0.2, 0.5, 1 and 2 mL of as-prepared NCDs would be added in the process when the stoichiometric ratios of Bi/Ce was 5:1. The obtained product was labeled as NCDs-BC-0.2, NCDs-BC-0.5, NCDs-BC-1 and NCDs-BC-2, respectively. As expected, the Zeta-potential of CeO2 was positive, suggesting the Br- ions was prone to be attracted onto the surface of CeO2. The BiOBr would be in-situ deposited on the surface of CeO2. Besides, BiOBr (-19 mV) and CeO2 (43.4 mV) possessed the opposite Zeta-potential (Fig. S1) at the neutral condition during the preparation process, which implied a tightly contacted interface could be formed between CeO2 and BiOBr via an electrostatic self-assembly process. 2.3 Characterization The crystallinity of the samples was identified by XRD with D8 advance diffractometer XRD (Cu kα radiation, λ=0.15406 nm). The microstructures of the as-prepared photocatalysts were observed by scanning electron microscope SEM (Hitachi, SU8000) and transmission electron microscope TEM (Hitachi, HT7700).

The elemental compositions of the catalysts were detected by an energy dispersive spectroscopy (EDS), which was equipped on the SEM. The Fourier transform infrared (FT-IR) spectra were measured on a Nicolet spectrometer (Nexus 470, Themo Electron Corporation) with the samples embedded in potassium bromide (KBr) pellets. Raman spectral were observed on a Renishaw in plus laser Raman spectrometer with λexc=560 nm. Ultraviolet visible diffuse reflectance spectra (UV-vis DRS) were carried out on a Shimadu UV-2450 spectrophotometer from 200-900 nm (BaSO4 as reference material). Brunaure-Emmett-Teller specific surface area and average pore size were investigated using a sorption analyzer (Micromeritics, Tristar 3020II) at 77K. The surface charges of catalysts in solution with different pH values were measured by Zeta potential analyzer (NanoBrook Omni). 2.4 Photo-electrochemical measurements The electrochemical measurements of photocurrent and Electrochemical impedance spectroscopy (EIS) were tested on the CHI 660E workstation (Chen Hua Instruments) with standard three-electrode system, which employed a platinum wire as counter electrode, saturated calomel electrode as reference electrode and catalysts-based sample coated ITO glass (0.5×1 cm2) as working electrode. The time-resolved photoluminescence spectra were recorded on a model FES 920 system with an excitation wavelength of 365 nm. The electrolyte was 0.1 mol L-1 Na2SO4 aqueous solution. A 300 W Xenon lamp was used as the simulated solar light source. Photocurrent test was conducted at an initial potential of 0 V. EIS was investigated in the frequency 105-101 Hz at an alternating voltage of 5 mV.

The steady-state photoluminescense (PL) spectra were recorded on a F-7000 fluorescence spectrophotometer (Hitachi, Japan) with excitation wavelength at 354 nm. The transient time-resolved photoluminescence decay measurements (TR-PL) were detected by a Fluorolog 3-22-TCSPC luminescence spectrometer at the excitation wavelength of 365 nm. The electron spin resonance (ESR) analyses were used to determine the reactive radicals (JES-FA200, JEOL Co., Japan) by spin-trap reagent DMPO in water and methanol. 2.5 Photocatalytic activity tests The photocatalytic degradation of CBZ was evaluated under simulated solar light irradiation using a 500 W Xenon lamp (Yuming Instrument Factory, Shanghai, China). The photocatalytic tests were implemented in a TSH-GHX-VI photochemical glass reactor (Qinghan Environment Protection Technology, Jiangsu, China). Typically, 40 mg of the photocatalysts was suspended in aqueous solution (50 mL) of 5 mg/L CBZ with continuous stirring. Before illumination, the mixture was magnetically stirred in dark for 30 min to achieve the adsorption-desorption equilibrium. At given reaction time intervals, 2 mL of solution was taken out and filtered through a 0.22 μm polytetrafluroethylene membrane and the CBZ concentration was measured with high performance liquid chromatography (HPLC, Aligent 2000, USA). The liquid chromatography/mass spectrometer (LC/MS, Agilent 1260/6310, USA) was used to determine the intermediate products of CBZ degradation. Details of HPLC and LC/MS measurements are described in Text S1 and Text S2. The 3D EEMs were recorded on a fluorescence spectrophotometer

(FluoroMax-4). The total organic carbon (TOC) was examined on a TOC analyzer (Shimadzu TOC-V CPN). 3 Result and discussion 3.1 Characterization The phases and crystal structures of the as-prepared catalysts were identified by XRD (Fig. 1) For pure samples, the characteristic peaks of CeO2 located at 28.55º, 33.08º, 47.47º and 56.33º, corresponding to the (111), (200), (220) and (311) planes of CeO2 (JCPDS No:34-0394, fluorite cubic phase, a=b=c=5.411 Å). For BiOBr, the diffraction peaks at 10.90º, 25.15º, 31.69º, 32.22º, 46.20º and 57.11º were indexed to the (001), (101), (102), (110), (200) and (212) planes of BiOBr (JCPDS No:09-0303, tetragonal phase, a=b=3.926 Å, c=8.103 Å), respectively. The BiOBr/CeO2 and NCDs/BiOBr/CeO2 showed similar XRD patterns, which agreed well with the characteristic peaks of CeO2 and BiOBr, implying the successful combination between BiOBr and CeO2. However, no obvious diffraction peaks of NCDs were investigated in the composite of NCDs/BiOBr/CeO2, which could be ascribed to low crystallinity and small amount of NCDs [13; 23]. Moreover, no other diffraction peaks were detected, suggesting the high purity of the heterojunction. The morphologies of the as-synthesized catalysts were explored by SEM and TEM images (Fig. 2). It can be observed that pure BiOBr presented 3D layered flower-like microstructure with an average size of about 300-600 nm. The catalysts of CeO2 was irregular in shape and would tend to be agglomerate. As shown in Fig. 2 (c) and Fig. 2 (f), samples of BiOBr with nanosheets and 3D layered flower-like structure

both had triumphantly attached to the surface of CeO2, resulting in the hybridization of the core-shell structure for BiOBr/CeO2 composites. In addition, the small-sized NCDs (around 10 nm) were anchored on the composites in the Fig. 2 (g), suggesting that the NCDs have been coupled successfully with BiOBr/CeO2. The EDS and mapping of NCDs-BC-1 were observed in Fig. S2. The result showed that C, N, Ce, Br, O and Bi elements homogeneously distributed throughout the NCDs-BC-1, further revealing that the composites of NCDs-BC-1 were composed of NCDs, BiOBr and CeO2. The result was consistent with SEM and TEM analysis. Chemical composites and valence states of the as-prepared samples were analyzed by XPS. The element binding energy was corrected based on C 1s=284.8 eV. Fig. 3 (a) showed the XPS survey spectra of the materials, indicating the co-existence of C, N, Ce, O, Bi and Br elements in NCDs-BC-1. Fig. 3 (b-g) displayed the high resolution spectra of C 1s, N 1s, Ce 3d, O 1s, Bi 4f and Br 3d, respectively. In Fig. 3 (b), the Bi 4f spectrum of the NCDs-BC-1 presented binding energy at 165.57 eV and 159.89 eV, corresponding to Bi 4f5/2 and Bi 4f7/2 of Bi3+ [27]. For the Br 3d spectrum (Fig. 3 (c)), the characteristic peaks at 69.12 eV and 68.11 eV were belonged to Br- of Br 3d5/2 and Br 3d3/2 [28]. The O 1s spectrum of OI centered at 529.81 eV represented crystal lattice O atoms (Bi-O and Ce-O), while the OII centered at 532.47 eV was belonged to the oxygen components adsorbed on the surface of the material (H2O and OH-) [29]. In Fig. 3 (e), the Ce 3d spectra was fitted into six characteristic peaks, the binding energy of 916.38 eV, 907.17 eV and 900.95 eV were corresponded to Ce 3d3/2, and the peaks at 898.13 eV and 882.36 eV were attributed to

Ce 3d5/2, suggesting the Ce4+ chemical valence in NCDs-BC-1 [30]. The C 1s spectrum was deconvoluted into four peaks located at 284.62 eV, 285.37 eV, 286.27 eV and 288.9 eV, which were attributed to C-C of sp2, C-O, C-NH2 and O-C=O [13; 31]. In addition, the peak in N 1s spectrum located at 399.39 eV [25], which came from the NCDs in NCDs-BC-1. The XPS results indicated the coexistence of BiOBr, CeO2 and NCDs in the NCDs-BC-1 composite. UV-vis spectra were investigated to study the optical adsorption properties of as-fabricated samples. As shown in Fig. 4(a), the steep adsorption edge of pure BiOBr and CeO2 were around 440 nm and 445 nm, which both would respond to visible light. The light adsorption edge of composites was close to that of the pure catalyst. The band gap energy of the as-synthesized samples could be estimated with the following equation (Eq 1): αhv=A(hv-Eg)n/2

(1)

Where α, v, h, Eg and A are adsorption coefficient, light frequency, Plank constant, band gap and a constant, respectively. As indirect semiconductor, the value of n for BiOBr, CeO2 and the composites is 4 [32; 33]. The Eg can be estimated by extrapolation of a plot of (αhv)1/2 versus hv as presented in Fig. 4 (b). The straight line to the X axis represented the approximation of the catalysts. The Eg of BiOBr and CeO2 were around 2.67 eV and 2.51 eV, respectively. For BiOBr/CeO2 (5:1) and NCDs-BC-1, the Eg were calculated to be 2.56 eV and 2.59 eV, suggesting that the introduction of NCDs did not significantly affect the value of Eg. As reported, the valence band potential (EVB) of BiOBr and CeO2 were 3.09 eV and 2.51 eV,

respectively [10; 18]. Therefore, the conduction band potential (ECB) of BiOBr and CeO2 was around 0.42 eV and -0.15 eV. The BET surface areas of the as-fabricated samples were detected. The Barret-Joyner-Halenda diameters, volumes and BET surface area of samples were presented in Table S1. Compared with the catalyst of BiOBr (27.9595 m2/g), CeO2 showed higher surface area of 80.8493 m2/g. The BET surface areas of the core-shell composites exhibited the similar surface areas with BiOBr, which may be ascribed to that CeO2 were wrapped by BiOBr. The results and analysis of FT-IR and Raman were shown in Fig. S3. 3.2 Photocatalytic degradation of CBZ As presented in Fig. 5(a), the photocatalytic degradation of CBZ by pure BiOBr was around 60% within 120 min, and the CBZ removal by CeO2 was negligible. Compared with pristine catalysts, the composites of BiOBr/CeO2 were obviously improved, which could be ascribed to the intimate interaction between the interfacial phases. The sample of BiOBr/CeO2 (5:1) achieved 86% CBZ removal within 120 min. With the increase of CeO2 content, the degradation efficiency of the composites increased at first and then declined, which was ascribed to that excess CeO2 would lead to the decrease of active sites. The

reaction

kinetics

of

the

catalysts

was

represented

following

Langmuir-Hinshelhood equation (Eq 2). kt=-ln(Ct/Co)

(2)

Where k is the apparent reaction rate constant, t presents the irradiation time, Ct

is the concentration of CBZ at time t and Co is the initial concentration. As presented in Fig. 5(b), the photocatalytic reaction fitted well with pseudo-first-order model. It was obvious that BiOBr/CeO2 (5:1) exhibited the highest k value (0.0163 min-1), which was 1.7-fold higher than BiOBr (0.0095 min-1). With the introduction of NCDs into the system, the photocatalytic performance was further enhanced (Fig. 5(c)). However, k values did not grow directly by further increasing the content of NCDs. The appropriate amount of NCDs was critical for the degradation rate. Excess content of NCDs could impede the photocatalytic activity. The reason would be as follows: 1) Excess NCDs would reduce the photoactive sites and might act as the recombination centers on the surface of catalyst [29; 31], which inhibited the generation of reactive oxygen species (ROS); 2) Much NCDs could cause the light scattering and limit the light adsorption [34]. The k values and coefficient of determination R2 were shown in Table S2. In contrast, the composite of NCDs-BC-1 (0.0281 min-1) achieved the highest reaction rate, which was 2.95 times higher than BiOBr (0.0095 min-1) and 1.72 times higher than the composites BiOBr/CeO2 (5:1) without NCDs (0.0163 min-1). Therefore, NCDs-BC-1 was selected as the best photocatalyst to remove CBZ in the following experiments. As presented in Fig. S4, the absorbance of CBZ decreased obviously during the photocatalytic reaction, implying the high degradation performance of NCDs-BC-1. In addition, the photocatalytic performance of NCDs-BC-1 was also investigated by the degradation of RhB shown in Fig. S5. The degradation rate of NCDs-BC-1 was much higher than pure BiOBr and anatase TiO2. The 98% of RhB was removed

within 60 min. The result further indicated that the introduction of NCDs in system would enhance the photocatalytic degradation. 3.2.1 Effect of catalyst dosages To investigate the effect of NCDs-BC-1 dosage for the CBZ removal, different photocatalyst concentrations (0.2-3.0 g/L) was employed and the result was shown in Fig. 6(a). The CBZ removal within 120 min increased from 50% to 98% with the increasing dosage from 0.5 g/L to 0.8 g/L, which was attributed to the increasing active sites in the photocatalytic surface. However, the degradation rate declined gradually with the further increase of photocatalyst. The inhibition was due to the shielding of the light passage and scattering effect by the suspended particles in the photocatalytic degradation [35]. The NCDs-BC-1 dosage of 0.8 g/L achieved the best photocatalytic activity, which was chosen as the optimal dosage in the following experiments. 3.2.2 Effect of initial CBZ concentrations As depicted in Fig. 6(b), the relatively low initial CBZ concentration can achieve the high degradation efficiency. As the initial CBZ concentration increased from 3 mg/L to 20 mg/L, the k continually declined from 0.0536 min-1 to 0.0081 min-1, which was ascribed to that the reactive sites on the surface of catalyst were invariant with the determined catalyst and light irradiation intensity. Therefore, the certain amount of reactive sites was limited for the high concentration of CBZ [11]. Furthermore, the light transmission in the solution was restrained with the increase of initial CBZ, which resulted in the reduction of available photons [36]. In addition, the increased

intermediates of CBZ would compete with CBZ molecule for reactive species, resulting in the inhibition of CBZ degradation [36]. Herein, the CBZ concentration of 5 mg/L was selected for the subsequent investigation. 3.2.3 Effect of initial pH The influence of initial pH in the range of 3.10-9.20 on the CBZ degradation was investigated and was shown in Fig. 6 (c). It was observed that low pH facilitated the CBZ removal, which was in accordance with previous reports [6; 10]. The pKa values of CBZ (pKa1=2.3, pKa2=13.9) were out of the pH range used in the experiment, suggesting that initial pH had ignorable influence on the form of CBZ molecules [6]. The point of zero charge (pHpzc) for NCDs-BC-1 was around 3.43 in Fig. 6 (d), suggesting the NCDs-BC-1 surface was positively charged at pHpHpzc. With low pH (<3.43), the surface charges of catalyst were slightly positive, which was facilitated for the attraction of electron-rich aromatic nucleus or the oxygens of CBZ molecular [37]. Besides, the moiety of -CONH2 in CBZ molecule was more susceptible to be decomposed in the acid solution [11]. In alkaline conditions, the CO2 from the air and degradation process would be transformed into bicarbonate ions, which might compete with CBZ for reactive species, thus the photocatalytic degradation was restrained [38]. 3.2.4 Effect of inorganic anions and NOM Various inorganic anions (such as chloride (Cl-), nitrate (NO3-) and sulfate (SO42-)) and NOM existing in natural water could affect the photocatalytic degradation of CBZ. Fig. 7 showed the effect of different inorganic anions and NOM

to the CBZ degradation. Sodium salts (NaCl, NaNO3 and Na2SO4) were selected in the degradation process, which was due to that the cation of Na+ exhibited a negligible impact for the photocatalytic degradation. As for Cl-, the addition of chloride at low concentrations (2-5 mM) had negligible impact on the CBZ removal (shown in Fig. 7(a)). The phenomenon could be ascribed to the generation of chlorine radicals (Cl·) in the degradation process (Eqs 3-4), which could offset the effect of the consumed ·OH to some extent [11]. However, the k values gradually decreased with the further addition of Cl-. The inhibition could be due to that the excess Cl- appreciably scavenged h+ and ·OH to form Cl·, which could quench themselves (Eqs. 5-6) [39; 40]. Hence, the availability of radicals (Cl·, h+ and ·OH) were declined and the CBZ removal was impeded. Cl- + ·OH → HOCl·

(3)

HOCl· + H+ → Cl·+H2O

(4)

Cl- + h+ → Cl·

(5)

Cl·+ Cl·→Cl2

(6)

As for NO3-, the CBZ degradation was appreciably restrained with further increased concentration from 2-20 mM as shown in Fig. 7(b). The result could be ascribed to that the reactive species of h+ and O2·- were occupied by NO3- to form less reactive radicals of nitrite (·NO2) and nitrate (NO3·) (Eqs. 7-9) [41; 42]. Furthermore, the NO3- had a certain shielding effect on the UV light [43], resulting in the inhibition of photocatalytic degradation under the simulated solar light irradiation. NO3- + h+ → ·NO3

(7)

NO3- + hv → NO2- + O NO2- + O2·- + 2H+ → H2O2 + ·NO2

(8) (9)

As for SO42-, it exhibited the significant inhibition impact on the CBZ degradation as shown in Fig. 7(c). The k values reduced from 0.0282 min-1 to 0.0140 min-1 with the SO42- elevating to 20 mM. The suppression of CBZ removal was due to that SO42- restrained the adsorption of CBZ moles on the surface of NCDs-BC-1. What’s more, the reactive species of h+ and radicals could be quenched by SO42- (Eqs 10-11), leading to the reduction of availability of radicals [44]. SO42-+h+ → SO4·SO42-+·OH → SO4·-+OH-

(10) (11)

As for NOM (humid acid representing NOM in this study), it had dual effect on the CBZ removal. The presence of NOM at low concentrations (5-10 mg/L) enhanced the degradation slightly as shown in Fig. 7(d), due to the fact that humid acid was regarded as the common photosensitive organic matter in aquatic environment and was conductive to the formation of reactive species [6]. However, the humid acid showed restrained effect on the CBZ degradation in the range of 20-50 mg/L. The inhibition could be explained by the fact that humid acid could compete with photocatalyst and CBZ for photons and ROS [45]. In addition, the humid acid absorbed on the surface of NCDs-BC-1 could scavenge the h+, resulting in the reduction of degradation efficiency [46]. 3.2.5 Effect of light irradiation conditions It is essential to investigate the photocatalytic activity under simulated solar

irradiation and visible light illumination. Compared with BiOBr/CeO2 (5:1) in Fig. 7(e), the degradation of CBZ by NCDs-BC-1 was significantly improved under visible light irradiation (>420 nm), suggesting that the introduction of NCDs might promote the adsorption of visible light. It can be found that 65% CBZ was removed by NCDs-BC-1 within 120 min under visible light. While the simulated solar light irradiation (>380 nm) could result in much higher efficiency (97%), which was ascribed to the fact that short wavelength sources generated strong energy [47]. Hence, the photocatalytic catalyst of NCDs-BC-1 has promising potential under the full spectrum solar light illumination in practical application. 3.2.6 Degradation of CBZ in real waters Considering the application of NCDs-BC-1 in real water bodies, CBZ degradation was performed in various types of actual matrices, such as DI water, tap water, river and secondary effluent. The physico-chemical parameters of the different water samples were given in Table S4. The effect of water matrices on the CBZ removal was shown in Fig. 7(f). The CBZ degradation efficiency (with 120 min irradiation) of DI, tap water, river and secondary effluent achieved 93%, 40%, 35% and 22%. Compared with DI, the degradation efficiency was significantly suppressed in the real waters. Especially, the CBZ removal in the secondary effluent was the lowest. The detrimental effect was due to the following reasons: 1) The reactive species were consumed by various inorganic salts and NOM, resulting in the restraint by joint inhibition; 2) The inorganic salts, NOM and other impurity could compete with CBZ for photons and hindered the light transmission in actual water matrix [48];

3) The substance in actual water matrix might compete for reactive species, which was due to the fact that the reactive species showed nonselective behavior on pollutant [49]. 3.3 Evaluation of CBZ mineralization The degradation of total organic carbon (TOC) and the formation of inorganic ions (NO3- and NH4+) were monitored to determine the mineralization degree. The nitrogen in amine group and heterocyclic rings could be transformed to NO3- and NH4+. According to Fig. 8, the removal of TOC achieved only 20% after 12 h irradiation. Additionally, the release of NO3- and NH4+ reached 0.24 mg/L (40.6%) and 0.15 mg/L (25%). The results suggested the generation of intermediates and some of them were successfully mineralized into inorganic ions, CO2 and H2O. 3.4 Degradation mechanism and pathway 3.4.1 Reactive species The reactive species scavenging experiments were performed to determine the photocatalytic degradation mechanism with BiOBr, BiOBr/CeO2 (5:1) and NCDs-BC-1. In order to ascertain the dominant active species in the system, trapping experiments were carried out in the presence of various scavengers during the CBZ degradation. Typically, isopropanol (IPA, 10 mM), sodium oxalate (Na2C2O4, 10 mM) and 1,4-benzoquinone (BQ, 10 mM) were respectively employed as the scavengers of ·OH, h+, and ·O2- [50; 51]. As shown in Fig. 9(a), the CBZ removal by pure BiOBr declined by 43% and 65% in the presence of IPA and Na2C2O4, indicating that ·OH and h+ acted as the dominant ROS. Additionally, the presence of BQ

decreased the CBZ degradation by 15%, implying that ·O2- exerted a slight effect on CBZ removal by pure BiOBr. Compared with BiOBr, different phenomenon was observed in the system of BiOBr/CeO2 (5:1) and NCDs-BC-1. The presence of Na2C2O4 and BQ significantly reduced the CBZ degradation, while the presence of IPA suppressed the CBZ removal slightly by 9%. It is dedicated that h+ and ·O2played a crucial role in the degradation process, while the ·OH played a secondary role. In order to further study the photocatalytic degradation mechanism, the ESR spin-trap technique was carried out in the system of BiOBr, BiOBr/CeO2 (5:1) and NCDs-BC-1. The reagent of 5, 5-dimethy1-1-pyrroline N-oxide (DMPO) was employed to capture the radicals and generate testable free radical DMPO-·O2- and DMPO-·OH [52]. As presented in Fig. 9(b-c), there is no ESR signal in dark in water and methanol suspension. As illustrated in Fig. 9(b), the 4-fold characteristic peaks with intensity ratios of 1:2:2:1 (αN=14.87G αH=14.87G) for DMPO-·OH were observed under the simulated solar light irradiation in all the systems (BiOBr, BiOBr/CeO2 (5:1) and NCDs-BC-1), suggesting that ·OH was generated by the three photocatalysts [24]. Obviously, NCDs-BC-1 catalyst achieved the highest intensity signal of DMPO-·OH, indicating that NCDs-BC-1 could generate more ·OH than BiOBr and BiOBr/CeO2 (5:1). As exhibited in Fig. 9(c), signal of DMPO-·O2- were detected in three photocatalysts systems with six characteristic peaks (αN=14 G αH=10 G) under simulated solar light irradiation, implying the formation of ·O2-. Additionally, the signal of DMPO-·O2- for NCDs-BC-1 was higher than that of

BiOBr, BiOBr/CeO2 (5:1), suggesting that NCDs-BC-1 could form more ·O2- than others. Furthermore, all the results indicated that the introduction of NCDs could promote the formation of ROS, which would further facilitate the photocatalytic activity. 3.4.2 Discussion on the mechanism It is known that the main factor enhancing the degradation activity probably is the promotion of the charge carries separation and transformation on the tightly contacted interface of heterojunction [53]. To verify the migration and separation efficiency of the photoinduced charge carries, the transient photocurrent response with four on-off cycles on as-prepared samples under light irradiation were investigated (shown in Fig. 10(a)). Stronger photocurrent response revealed higher separation efficiency photogenerated charge carries. NCDs-BC-1 exhibited the highest photocurrent intensity, which was ascribed to the introduction of NCDs and formation of close connected interface between BiOBr/CeO2. Further, the EIS Nyquist plots of as-prepared samples were displayed in Fig. 10(b). The composite of NCDs-BC-1 possessed smallest radius than that of BiOBr and BiOBr/CeO2 (5:1), indicating the charge transfer efficiency of NCDs-BC-1 was the highest. It could be concluded that the existence of NCDs could boost the interfacial charge migration and separation. In addition, PL analysis was also carried out to examine the charge translation efficiency in Fig. 10(c). Generally, the lower PL intensity suggested the higher charge separation efficiency. Apparently, BiOBr presented the highest PL signal at 354 nm excitation, suffering from the intrinsic geometry structure. The PL intensity of NCDs-BC-1

exhibited lower than BiOBr/CeO2 (5:1), indicating the modification of NCDs inhibited the recombination of photogenerated electron-hole pairs. Further insight into the charge carries transfer behavior was determined by the TR-PL decays in Fig. 10(d), which were detected at 365 nm excitation. The decay curves were fitted well with tri-exponential functions with three radiation lifetime (τ1, τ2 and τ3) and different fitting coefficients (A1, A2 and A3), which were presented in Table S3. It can be seen that NCDs-BC-1 (16.78 ns) exhibited a longer average lifetime (τ) than pure BiOBr (10.53 ns) and BiOBr/CeO2 (5:1) (11.56 ns). The TR-PL result further indicated that the introduction of NCDs accelerated the migration and separation of the charge carries and retarded the recombination of photoinduced electron-hole pairs, which would enhance the photocatalytic degradation. Based on the above analysis, the mechanism for the CBZ degradation over NCDs-BC-1 composites was proposed in Scheme 1. Upon the simulated solar light irradiation, both of the BiOBr and CeO2 could be excited to yield the photoinduced electrons and hole. Then the electrons from CB of CeO2 (-0.15 eV) could efficiently jump to that of BiOBr (0.42 eV), the excited holes would be easily injected from VB of BiOBr (3.09 eV) to that of CeO2 (2.51 eV) simultaneously, which was ascribed to the reasonable type-II staggered band alignment and potential difference [54]. As a result, the electrons and holes would respectively accumulate on the CB of BiOBr and VB of CeO2. Consequently, the traditional electron-hole pairs migration way would significantly accelerate the separation efficiency of charge carries by inner electric field established on the tightly contacted interface [13]. The holes left on the VB of

CeO2 would directly oxidize the organic compound. Furthermore, the h+ could oxidize OH- group in water to produce ·OH (Eqs. 12), which can degrade organic pollution. The electrons located on the CB of BiOBr would be captured by oxygen (O2) dissolved in water to form ·O2- (Eqs. 13) or further react with H+ to yield ·OH (Eqs. 14-15). The ROS of h+, ·OH and ·O2- all participated in the photocatalytic degradation (Eq. 16). When NCDs were introduced into the photocatalytic system, more excited electrons would be easily transferred to the surface of BiOBr and the redundant electrons on BiOBr or CeO2 could be efficiently transfer to NCDs, which resulted in the enhanced separation and migration efficiency of electrons. NCDs could act as the excellent electron accepter and donors due to the conjugated π structure [55]. As a result, NCDs inhibited the recombination of photogenerated electron-hole pairs, enhancing the photocatalytic performance. h+ +OH- → ·OH

(12)

e- + O2 → ·O2-

(13)

·O2- + 2H+ + e- → H2O2 H2O2 + e- → ·OH+OHCBZ + h+/·O2-/·OH → products

(14) (15) (16)

3.4.3 Possible degradation pathway of CBZ To further determine the CBZ degradation pathway and investigate the photocatalytic mechanism, the transformation products were detected by LC/MS in positive ion mode. Seven main possible intermediates were identified on the basis of mass spectra and pertinent literatures. The detailed chemical information of the

products was summarized in Table S5 and Fig. S6. The tentative degradation pathway was proposed in Scheme 2. For path I, CBZ (m/z 237.10160) was converted to product P2 (m/z 251.08073) with the attack of ·O2- and h+ during the degradation process [6; 56], which was further oxidized to intermediate P3 (m/z 180.08023) with fragmentation and rearrangement [2]. Then, P4 (m/z 196.07510) would be obtained with the help of ·O2-. For path II, ·OH tended to attack the heterocyclic ring on CBZ molecule to form product P5 (m/z 271.06945), which was further oxidized to P6 (m/z 267.07557) with the ring contraction reaction. Subsequently, the loss of -CONH2 group and formaldehyde from P6 resulted in the formation of P7 (m/z 301.14020). For path III, the olefinic double bond on the central heterocyclic ring of CBZ would be attacked by ·O2- and ·OH, which could be further oxidized to P7 with the help of h+ [6; 57]. Thereafter, P7 underwent the ring closure and dehydration reaction to yield P6 [58]. Generally, the above transformation products would react with ROS and undergo ring cleavage reaction to generate short chain organic acid (e.g., aliphatic acid, propionoc and furmaric acid) [58], which would eventually achieve complete mineralization in the photocatalytic degradation process following the generation of CO2, H2O and inorganic ions [6]. Moreover, 3D EEMs was monitored to further investigate the photocatalytic degradation of CBZ. Fig. S7 showed that five samples were collected at different reaction time. As presented in Fig. S7(a), no fluorescence signal was detected in the initial CBZ sample, which was accordant with previous report [59]. However, after 3h illumination (Fig. S7(b)), the peak 1 located at λex/λem=(220-260 nm)/(430-480 nm)

appeared, which was attributed to the formation of fulvic acids-like organic matters [59]. With further irradiation (Fig. S7(c-e)), the peak 1 gradually weakened and another new peaks 2 emerged in the fulvic acids-like fluorescence region at λex/λem=(220-240 nm)/(380-450 nm). In addition, the intermediates of humic acid-like organic matters was generated in the degradation process, which was due to the appearance of peak 3 at λex/λem=(290-350 nm)/(380-450 nm). Those phenomena might be ascribed to the decomposition of large molecules into smaller fragments and partial mineralization of the intermediates in the degradation pathway of CBZ, including the destruction of conjugated heterocycle and aromatic moieties [36; 47; 60], Which can be seen from the LC-MS analysis. 3.5 Cycle test To evaluate the possibility of NCDs-BC-1 in practical application, the stability and reusability should be investigated. As shown in Fig. S8(a), the photocatalytic degradation efficiency still achieved 86% after 4 successive recycles. The excellent photocatalytic performance for the CBZ removal was due to the high reusability of NCDs-BC-1. In addition, to further evaluate the structural stability of NCDs-BC-1 (Fig. S8(b-c)), the XRD and Raman patterns employed in the recycled catalyst displayed that there was no apparent difference between the used and fresh sample, implying that NCDs-BC-1 owned superior recyclability and stability. Moreover, the morphology of recycled NCDs-BC-1 sample also had almost no distinct discrepancy shown in SEM images and EDS result (Fig. S9), suggesting the excellent stability of the prepared photocatalyst, which presented the great potential in actual applications.

4 Conclusion In summary, the novel composite photocatalyst of NCDs/BiOBr/CeO2 was synthesized successfully for the first time via a facile method. The optimum sample of NCDs-BC-1 exhibited the highest degradation rate at the dosage of 0.8 mg/L, which was 2.95 times higher than BiOBr and 1.72 times higher than the BiOBr/CeO2 (5:1). The significantly enhanced photocatalytic performance can be attributed to the modification of NCDs and heterojunction structure, which accelerated the migration and separation of the photoinduced charge carries at the tightly contacted interface. The radical scavenging experiments and ESR results revealed that h+, ·O2- and ·OH were ROS involved CBZ photocatalytic degradation. The catalysts of NCDs-BC-1 showed excellent reusability and stability after reusing for 6th times. With the help of LC/MS and 3D EEMs, seven intermediates were determined and three tentative degradation pathways were proposed. This study provides a new insight on the development of NCDs based composite photocatalyst in photocatalytic water purification. Acknowledgements The author acknowledges the financial support provided by the National Key Research and Development Programme (No. 2018YFC1801501) and National Key R&D Program of China (No. 2018YFE0103800). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at dol:

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List of figures

Fig. 1. XRD patterns of as-prepared samples.

Fig. 2. SEM images of (a) BiOBr; (b) CeO2; (c) N-BC-1; TEM images of (d) BiOBr; (e) CeO2; (f-g) N-BC-1.

Fig. 3. XPS spectra of as-prepared materials: (a) survey; (b) Bi 4f; (c) Br 3d; (d) O1s; (e) Ce 3d; (f) C 1s; (g) N 1s.

Fig. 4. (a) UV-vis adsorption spectra with as-prepared samples; (b) the band gap of different catalysts detemined from the (αhv)1/2 versus photon-energy.

Fig. 5. (a) Photocatalytic degradation of CBZ by as-prepared catalysts without NCDs under simulated solar light irradiation and (b) their kinetic constants; (c) the photocatalytic performance of heterojunctions with various content of NCDs and (d) the corresponding kinetic rate constants. ([CBZ]=5 mg/L)

Fig. 6. The effect of (a) photocatalyst dosages, (b) initial CBZ concentrations and (c) initial pH on the CBC removal in the presence of NCDs-BC-1 and the corresponding kinetic constant k (insert); (d) zeta potential of NCDs-BC-1.

Fig. 7. The effect of common anions and DOM on the CBZ degradation under simulated solar irradiation: (a) Cl-, (b) NO3-, (c) SO42-, (d) NOM; (e) the effect of different light irradiation conditions on the CBC removal; (f) Degradation of CBZ in actual waters. ([NCDs-BC-1]= 0.8 g/L, [CBZ]=5 mg/L)

Fig. 8. Evolution of TOC, NO3- and NH4+ during the CBZ degradation. ([NCDs-BC-1]= 0.8 g/L, [CBZ]=5 mg/L)

Fig. 9. (a) Photocatalytic degradation of CBZ by as-prepared samples in the presence of various scavengers; (b) DMPO spin-trapping ESR spectra in aqueous solution and (c) methanol with the presence of as-prepared photocatalysts under simulated solar light irradation.

Fig. 10. (a) Photocurrent response, (b) EIS, (c) PL and (d) TR-PL of the as-prepared materials.

Scheme 1. The proposed mechanism of NCDs-BC-1 under simulated solar light irradiation.

N

I

O

NH2

P1

m /Z 237

III

II O

HO

OH O

N O

OH O H

N NH2

P2 m /Z 251

O

P5

O

N

NH2 O

m/Z 271

NH 2

P7 m /Z 301 O N

P3 m /Z 180

N O

P6

O

H NH 2

O

m/Z 267

N H

P4

m/Z 196

Ring cleavage pruducts

CO2+H 2O+inorganic ions

Scheme 2. Proposed CBZ degradation pathways.

Conflict of interest

The author declares no conflict of interest.

Graphical Abstract

The enhanced photocatalytic performance toward carbamazepine by nitrogen-doped carbon dots decorated on BiOBr/CeO2 : Mechanism insight and degradation pathways

Highlight 

Novel NCDs modified BiOBr/CeO2 was constructed successfully for the first time.



The introduction of NCDs could significantly enhance the CBZ removal.



·O2- and h+ acted as the dominant reactive oxygen species in the composite.



Possible mechanism and degradation pathways were proposed.