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Covalent Triazine Framework Modified BiOBr Nanoflake with Enhanced Photocatalytic Activity for Antibiotic Removal Shuai-Ru Zhu, Qi Qi, Yuan Fang, Wen-Na Zhao, Meng-Ke Wu, and Lei Han Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017

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Crystal Growth & Design

Covalent Triazine Framework Modified BiOBr Nanoflake with Enhanced Photocatalytic Activity for Antibiotic Removal

Shuai-Ru Zhu,† Qi Qi,† Yuan Fang,† Wen-Na Zhao,‡,* Meng-Ke Wu,† and Lei Han†,* †

State Key Laboratory Base of Novel Functional Materials and Preparation Science, School of Materials

Science & Chemical Engineering, Ningbo University, Ningbo 315211, China ‡

Key Laboratory for Molecular Design and Nutrition Engineering of Ningbo, Ningbo Institute of Technology,

Zhejiang University, Ningbo 315100, China

ABSTRACT: The development of photocatalysts based on covalent organic frameworks (COFs) is intriguing research due to their structural flexibility and tremendous catalytic sites, herein we demonstrate a facile strategy to prepare the composite materials combined COFs and inorganic semiconductors with enhanced photocatalytic performances. A new composite photocatalyst (BiOBr/CTF-3D) integrated 3D covalent triazine framework (CTF-3D) with 2D BiOBr nanoflake was prepared via a simple co-precipitation method. The structural characterizations demonstrated that the amorphous CTF-3D was well modified on the surfaces of BiOBr. The photocatalytic activity of the BiOBr/CTF-3D composite was evaluated by the degradation of colorless antibiotic agents, tetracycline hydrochloride (TC) and ciprofloxacin (CIP), under visible light irradiation. When the mass percent of CTF-3D was 2 %, the BiOBr/CTF-3D composite displayed the highest photocatalytic activity. The enhancement of photocatalytic performance was mainly derived from the enlarged optical adsorption range, the efficiently separated photo-generated electron-hole pairs, and the accelerated adsorption and transfer of antibiotic molecules, in the synergistically facilitating photocatalytic process. In additon, a possible photocatalytic mechanism for degrading TC by BiOBr/CTF-3D-2% was tentatively proposed. This work opens up a new strategy to improve the photocatalytic activity of traditional inorganic photocatalysts by modified with COFs materials for solving the pollution of living ecosystems. KEYWORDS: covalent triazine frameworks, BiOBr, photocatalyst, antibiotics, tetracycline hydrochloride

1. INTRODUCTION In the past decades, antibiotics are used in medical and breeding industry. More and more residues of antibiotics from hospital effluent, aquaculture and humans or live-stock medicines are widespread in aquatic ACS Paragon Plus Environment

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environment, and the antibiotic residues are now recognized as a new class of pollutants to destroy living ecosystems and a subject arising worldwide attention.1,2 Antibiotic residues in aqueous environment can potentially cause negative effects even in low concentrations, especially for tetracyclines (TCs),3 which have been widely used in human and veterinary applications against infectious diseases. The residues of TCs can induce the proliferation of drug-resistant pathogens and may cause possible risks to the human health and ecosystem through drinking water and food-chain.4,5 Due to the biotoxicity and difficult post-treatment of sorbents, the antibiotic residues are refractory to be degraded by the traditional physical adsorption and biological degradation.6 Therefore, it is very important to develop an efficient physicochemical method to remove these biotoxicity. Visible-light-driven photocatalytic degradation, a clean, energy-saving, and cost-effective technology, has been recognized as a promising treatment approach.7,8 The 2D layered BiOBr, built by interlacing [Bi2O2] slabs with double bromine slabs, exhibits excellent electrical, optical, and catalytic properties for photocatalytic energy conversion and environment remediation.7,9 However, the photocatalytic activity of individual BiOBr is always limited by the low efficiency of light absorption, slow rate of charge transfer, high recombination efficiency of the photo-induced electron–hole pairs and insufficient specific surface area. In terms of these aspects, the rational design of new BiOBr nanoflake-based composites integrating another functional material with high charge conductivity, excellent visible light absorption ability and large specific surface area, has received much attention. Hence, a number of BiOBr-based composites, such as BiOBr/semiconductors,10 BiOBr/Quantum Dots,11 BiOBr/metallic oxides,12 BiOBr/graphene-like materials13 have been reported with good chemical stability and high capability of degrading antibiotic pollutants in water under visible light irradiation. To our knowledge, the covalent organic frameworks (COFs) modified BiOBr nanocomposites have not been researched up to now. Covalent organic frameworks, as promising photocatalysts, are intriguing research due to their structural flexibility and tremendous catalytic sites.14 Particularly for covalent triazine frameworks (CTFs), a kind of typical COFs, with nitrogen-rich organic porous material, have attracted a keen attention as organic pollutants removal and heterogeneous catalysis due to unique physicochemical properties.15-25 Moreover, the π-stacked aromatic units of CTFs would be expected to promote photo-generated electron separation and charge transfer, which are preferred for photocatalysis.20,21 Thus, we considered that the catalytic cites, charge transfer ability and the large surface areas of CTFs, may endow them with potential abilities to improve the photocatalytic activity for degradation of antibiotic pollutants. In this works, we chose 1,4-dicyanobenzene as building block to construct a conjugated amorphous 3D

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CTF (CTF-3D) via the nitrile cyclotrimerization (Figure S1).15 Compared to the crystalline 2D CTF, the amorphous 3D CTF can avoid the lattice mismatch with BiOBr nanocrystals. At the same time, the triazine rings of CTF-3D were slightly broken and partially carbonized,20,26 which may result in a faster photo-generated electron separation between BiOBr and CTF-3D. Therefore, 2D BiOBr nanoflakes coupling with amorphous CTF-3D composites were synthesized for the first time via a simple co-precipitation method. Tetracycline hydrochloride (TC) and ciprofloxacin (CIP) were selected to evaluate the photocatalytic performances of as-prepared materials. The optimal weight ratio of CTF-3D and mechanism research are discussed, which provides a vital insight to detect new opportunities for other COFs materials.

2. EXPERIMENTAL SECTION 2.1. Materials. Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O), potassium bromide (KBr), potassium iodide (KI), hydrochloric acid (HCl), sodium sulphate (Na2SO4), isopropanol (IPA), ethylene glycol (EG), polyethylene glycol (PEG, 2000), 1,4-dicyanobenzene and zinc chloride (ZnCl2), tetracycline hydrochloride (TC) and ciprofloxacin (CIP) were purchased from Sinopharm, Aladdin, Aldrich and Solarbio, respectively. All chemicals were used without further purification. 2.2. Synthesis of CTF-3D. CTF-3D was synthesized following a previously described protocol.15,27 Briefly, 0.3 g 1,4-dicyanobenzene and 1.6 g ZnCl2 were transferred into a quartz ampoule under an inert atmosphere. The ampoule was evacuated, sealed and placed at 400 °C for 20 h, followed by increasing the heating to 600 °C within 30 min and kept for 20 h. The ampoule was then cooled down to room temperature and opened. The reaction mixture was subsequently grinded, washed with water to remove most of the ZnCl2, followed by stirring in diluted HCl for 15 h to remove the residual salt. Finally the material was washed with H2O and tetrahydrofuran to obtain black powder, which was dried in vacuum at 150 °C overnight. 2.3. Synthesis of BiOBr/CTF-3D Composites. Briefly, 0.5 mmol KBr and a certain amount of CTF-3D were dissolved in 5 ml deionized water and then sonicated for 30 min. 0.5 mmol Bi(NO3)3·5H2O was added to 10 mL of EG under stirring at room temperature. After Bi(NO3)3·5H2O was dissolved completely, both solutions were mixed with stirring for 20 min at room temperature. Then, the mixture was further transferred into 20 mL Teflon-lined autoclave and subsequently heated at 120 °C for 6 h. After cooling down to room temperature, the products were washed with distilled water and ethanol for several times, and dried at 60 °C for 6 h. To investigate the optimal content ratio of CTF-3D modified on the BiOBr nanoflakes, the different samples are labeled as BiOBr/CTF-3D-x%, where x is the different weight percent of CTF-3D. For comparison, pure BiOBr nanoflakes were synthesized under the above same conditions in the



absence of CTF-3D. 2.4. Materials Characterization. Powder X-ray diffraction (XRD) measurements were carried out on a Bruker D8 advance X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). The FT-IR spectrum experiments were analyzed on a Nicolet 6700 FT-IR spectrometer in the range of 400–4000 cm-1. The surface morphology and crystal structure of the as-prepared products were performed by Hitachi S-4800 field-emission scanning electron micrographs (FE-SEM) at an acceleration voltage of 10.0 kV and FEI corporation TF20 transmission electron microscopy (TEM). Thermogravimetric analysis (TGA) was detected with SII TG/DTA 7300 instrument at a heating rate of 10℃/min. UV-Vis diffuse reflectance spectra were measured by a Perkin Elmer Lambda 950 UV/Vis/NIR spectrometer using BaSO4 as reference in the wavelength of 200–800 nm. The chemical component and state were operated at Energy Dispersive Spectrometer (EDS) using a EDAX analyzer and X-ray photoelectron spectroscopy (XPS) measurements using a Thermo Fisher Scientific corporation Escalab 250Xi instrument. All binding energies were calibrated using the C 1s peak at 284.8 eV. N2 adsorption/desorption isotherms were operated with Brunauer–Emmett–Teller (BET) and pore size distributions measurements using an ASAP 2020 apparatus. The electron spin resonance (ESR) spectra were recorded on a Bruker model E580 spectrometer under Xe lamp irradiation. The radical capture agent was 5,5-dimethyl-lpyrroline-N-oxide (DMPO) dissolved in methanol (for •O2−). Total organic carbon (TOC) concentration was measured using a total organic carbon analyzer (TOC-VCPN, Shimadzu, Japan). 2.5. Photocatalytic Experiments. The photocatalytic activity of the samples was evaluated by photo-degradation of tetracycline hydrochloride (TC) and ciprofloxacin (CIP) under visible light irradiation of a 500W Xe lamp using an BILON photochemical reactor (Shanghai BILON Instrument Corporation). 40 mg of sample was dispersed into aqueous solution in a photochemical reactor containing 200 mL TC (10 mg/L) or CIP (10 mg/L). Due to strong adsorption capacity for CTF-3D, 5 mg of samples was dispersed into aqueous solution containing 200 mL TC (10 mg/L) as comparison. Experiments were carried out at 25 °C with a circulating water system to prevent thermal catalytic effects. The pH value was not adjusted when the reaction was operated. Before irradiation, the solution was ultrasonicated for 5 min and continuously magnetically stirred in the dark for 30 min to get adsorption-desorption equilibrium. At 10 min time intervals, 5 mL of suspensions were collected and centrifuged to remove the products. The pollutants concentration of the absorbance at the wavelength of 357 nm (TC) and 272 nm (CIP) were evaluated by a TU-1901 spectrometer (Beijing Purkinje General Instrument). The blank experiment was also carried out under the same processes, but no catalyst was mixed. The photo-degradation efficiency was analyzed by


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dividing C/C0, where C is the remained dye concentration and C0 is the starting dye concentration. The control experiments were studied separately with different photocatalysts added to the TC solution, which were mechanical mixtures of mass fraction of pristine CTF-3D and BiOBr was equal to the BiOBr/CTF-3D-2%. 2.6. Active Species Trapping Experiments. To explore the photocatalytic mechanism, radicals trapping experiments were further conducted to detect the reactive active species generated during the irradiation of BiOBr/CTF-3D-2% photocatalyst. 1 mM of hydroxyl radicals (•OH) scavenger (IPA), a holes (h+) scavenger (KI) were added to the TC solution. N2 is purged through TC solution to remove the dissolved O2. Otherwise, the operated method was similar to the described above-mentioned experiment of photo-degradation of pollutants. 2.7. Photoelectrochemical Measurements. The electrochemical impedance spectroscopy (EIS) and photocurrent response were analyzed on an electrochemical analyzer (CHI660E, Chenhua, Shanghai, China) using a standard three-electrode system with the samples as the working electrodes, saturated Ag/AgCl electrode as the reference electrode, and a Pt wire as the counter electrode, respectively. ITO and Ni film was used as the current collector of working electrode for measuring photocurrent response and EIS, respectively. For the fabrication of the ITO working electrodes, 5 mg photocatalyst and 1 mg PEG were mixed with 0.5 mL H2O. The mixture was coated on 1.0 cm2 ITO glass electrode and dried at 60 °C for 4 h and finally calcined at 250 °C for 4 h. For the fabrication of the Ni film working electrodes, 80 wt% of active materials, 10 wt% of acetylene black (conductive agent) and 10 wt% of vinylidene fluoride (binder) were dispersed in 1-methyl-2-pyrrolidinone to form homogeneous slurry. Then the slurry was dotted on the Ni film and dried for 24 h at room temperature. The Nyquist plots were recorded from 0.01 Hz to 100 kHz at an open circuit potential of 0.3 V and alternating current (AC) voltage amplitude of 5 mV. The electrodes were immersed in 0.5 M of Na2SO4 aqueous solution and illuminated by using a 500 W Xe lamp.

3. RESULTS AND DISCUSSION 3.1. Characterization of BiOBr/CTF-3D. The amorphous CTF-3D was prepared via the nitrile cyclotrimerization at 600 °C using ZnCl2 and 1,4-dicyanobenzene. The crystalline phases of the BiOBr, CTF-3D and BiOBr/CTF-3D-2% composites were characterized by X-ray diffraction (XRD) (Figure 1a). For CTF-3D, only a weak and broad XRD peak is obtained, which proved the amorphous structure.15 BiOBr/CTF-3D-2% composite shows the similar diffraction peaks with the pristine BiOBr. All diffraction peaks are identical matching to the structure of the



tetragonal phase BiOBr (JCPDS card No. 09-0393).7 The FT-IR spectrum of CTF-3D (Figure 1b) shows the characteristic frequencies. The bands at 1050~1270 cm‒1 can be attributed to the strong absorption of C-N stretch and the bands at 1690~1560 cm‒1 are according to the open-chain imino group because of the ring fragmentation.15,20,28 For BiOBr, the absorption bands at around 3443 and 1624 cm‒1 can be assigned as the ν(O–H) stretching and δ(O–H) bending vibrations of surface and interlayer H2O, respectively. The sharp peak at 513 cm‒1 is attributed to the Bi–O stretching mode.29 Nevertheless, it is noted that no diffraction peaks and absorption bands of CTF-3D are found in BiOBr/CTF-3D-2% at XRD and FT-IR, which may be due to its low amount and amorphous structure. (insert Figure 1) The chemical composition and surface chemical state of the as-prepared materials were characterized by the X-ray photoelectron spectroscopy (XPS). As shown in Figure 2a, the survey spectrum of BiOBr/CTF-3D-2% displays the existence of Bi, Br, O, N and C elements. The C 1s spectrum (Figure 2b) exhibits three peak at 284.9 eV, 286.1 eV and 288.5 eV corresponding to C=C, C–N and C–C of CTF-3D, respectively.

29,30

The C 1s spectrum is similar to that of CTF-3D, suggesting the existence of CTF-3D in

composite. Two peaks centered at 159.0 eV and 164.3 eV for BiOBr/CTF-3D-2% (Figure 2c) are assigned to Bi 4f7/2 and Bi 4f5/2, respectively. Figure 2d reveals that the binding energies of 398.4 eV, 399.7 eV and 400.9 eV are associated with the pyridinic nitrogen (Ni), pyrrolic nitrogen (Nii) and quaternary nitrogen (Niii) of CTF-3D, respectively, suggesting the formation of graphitized fragments.26 Meanwhile, It is also a proof that the amorphous CTF-3D structure has been synthesized successfully. In the high-resolution spectrum of Br 3d (Figure 2e), the peaks located at 68.1 eV and 69.1 eV are assigned to Br 3d5/2 and Br 3d3/2, respectively. The O 1s (Figure 2f) spectrum of BiOBr/CTF-3D-2% could be fitted into three peaks at 529.8.0 eV, 531.9 eV and 533.2 eV , which are bound up with the crystal lattice O atoms of BiOBr (Bi–O), and other components such as H2O, respectively.31,32 For CTF-3D, contained oxygen element may come from the adsorbed water molecules, which is consistent with previous reports.10 EDS analyses of BiOBr/CTF-3D-2% (Figure S2) shows peaks corresponding to the elements of Bi, Br, O, C, suggesting the CTF-3D modified on BiOBr 2D material. According to the actual content of C, the theoretical weight percentage of N element was calculated about 0.39%. Thus, the actual content of N element was not counted due to much low content. Moreover, the elemental mappings of C (Figure S3e), and N (Figure S3f) further display the elements distribution of the sample, confirming that the CTF-3D are well distributed on BiOBr nanoflakes. (insert Figure 2) The N2 adsorption/desorption isotherm of the samples is shown in Figure 3. The specific surface area


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and pore volume of pristine BiOBr, BiOBr/CTF-3D-2% and pure CTF-3D were determined to be 12.82 m2 g−1 0.058 cm3 g−1, 37.38 m2 g−1 0.092 cm3 g−1 and 2491.61 m2 g−1 1.93 cm3 g−1 respectively. Compared with pure BiOBr, BiOBr/CTF-3D-2% sample attains an increased surface area and total pore volume (Vt), which explain that CTF-3D is effectively modified on the surface of the BiOBr nanoflakes. At the same time, pore size of BiOBr/CTF-3D-2% shows slightly increasing after the introduction of CTF-3D (Figure S4 and S5). Inspired by these results, the higher surface area and proper pore volume of BiOBr/CTF-3D-2% is of great significance in accelerating the adsorption and transfer of pollutant molecules and abundant active sites to enhance the photocatalytic activity to some extent. (insert Figure 3) The surface morphology and microstructure of the obtained pure BiOBr and BiOBr/CTF-3D-x% products were measured by SEM) and TEM. It is found that pristine BiOBr is flake-like with particle sizes of approximately 150 nm (Figure S6a). By the co-precipitation method, the BiOBr nanoflakes were modified with a certain amount of CTF-3D to form BiOBr/CTF-3D-x% system. Compared with pristine BiOBr, BiOBr/CTF-3D-x% exhibit similar nanoflakes (Figure 4 and Figure S6b-f). However, more clearly observation could be found at BiOBr/CTF-3D-x% rather than pure BiOBr, demonstrating that the introduction of CTF-3D effectively could change the conductivity of the BiOBr materials. Especially for BiOBr/CTF-3D-3%, the CTF-3D on the surface of BiOBr materials can be seen, which revealing the CTF-3D and BiOBr have been coupled successfully. Furthermore, the typical TEM images (Figure 5a) of BiOBr/CTF-3D-2% further revealed that CTF-3D was dispersed into BiOBr nanoflakes due to the different transparency, which is well consistent with the SEM analysis. Figure 5b-f show the HRTEM images of the BiOBr/CTF-3D-2%, in which inter-planar lattice spacing of 0.41 nm, 0.35 nm, 0.28 nm and 0.20 nm could be corresponding to the (002), (101), (102) and (200) plane of BiOBr, respectively. Meanwhile, clear fringes with the interval of 0.38 nm, 0.36 nm and 0.26 nm may be attributed to the induced variational and exposed lattice planes of BiOBr by the introduction of CTF-3D, which further confirm the existence of the CTF-3D and BiOBr in composites. The selected area electron diffraction (SAED) pattern of BiOBr/CTF-3D-2% pattern reveals obvious diffraction rings (Figure 5g), suggesting a polycrystalline nature of this nanomaterial. (insert Figure 4) (insert Figure 5) UV-vis diffuse reflectance spectra (DRS) were used to measure the optical properties of the samples. The CTF-3D shows broad absorption in the whole visible region without distinct absorption edge, which is judged from their dark sample color (Figure 6a). The similar phenomenon can also be observed in the



previous literature.23 It is found that the BiOBr/CTF-3D-2% exhibits visible light absorption extending to 800 nm, which is further verify the formation of the CTF-3D modified on BiOBr nanoflake. In addition, the band gap energy of as-prepared catalysts can be estimated by the following equation: α(hv) = A(hv–Eg)n/2, where α, h, v, A and Eg are absorption coefficient, Planck’s constant, photon frequency, constant and energy band gap, respectively. For BiOBr, the value of n is 4 for the indirect transition. According to the tangent line of the curves (Figure 6b), the band gaps (Eg) of the pristine BiOBr and BiOBr/CTF-3D-2% are 2.85 and 2.78 eV, respectively. Thus, the decreased band gap of BiOBr/CTF-3D-2% can be beneficial to the absorption of visible light response, resulting in higher photocatalytic activity. (insert Figure 6) 3.2. Photocatalytic Performances. To evaluate the photocatalytic performances of BiOBr/CTF-3D-x% composites for antibiotic pollutants treatment, the tetracycline hydrochloride (TC) and ciprofloxacin (CIP) were selected to degrade. Moreover, the degradation of the pollutants generally can be regarded as a pseudo-first-order kinetics reaction (−ln(C/C0) = kt), where k is the rate constant. The degradation results are shown in Figure 7. TC is sable under visible light irradiation in the absence of photocatalysts. Obviously, the BiOBr/CTF-3D-2% has the best TC degradation activity (Figure 7a). For the control experiment with the mechanical mixtures of pure CTF-3D and BiOBr, although the components are the same as those in the BiOBr/CTF-3D-2% sample, there is lower photocatalytic activity than that of any BiOBr/CTF-3D composites due to weak chemical bonding within mechanical mixtures, which will be not conductive to photogenerated eletrons and holes transfer between BiOBr and CTF-3D. Meanwhile, Figure S7 shows the UV–vis absorption spectral of temporal evolution of TC solution over BiOBr/CTF-3D-2% under visible light irradiation, which shows the 90.9% degradation of TC within 50 min. In addition, BiOBr/CTF-3D-2% exhibits the optimal weight percent and has the highest degradation efficiency and rate constant among the above samples, giving a 1.86 times higher rate constant of TC degradation than pure BiOBr (Figure 7b). In order to compare with the performance of pure CTF-3D, 5 mg of samples were carried out in photocatalytic experiments due to strong adsorption capacity for CTF-3D. It can be seen from Figure 7c-d that the degradation rate constant of TC follows an order of BiOBr/CTF-3D-2% > BiOBr > CTF-3D after the same irradiation time. Therefore, it is easy to conclude that the degradation efficiency of TC can be improved in the presence of CTF-3D modified on BiOBr nanoflake systems as compared to pure CTF-3D and BiOBr. On the other hand, the blank experiment also demonstrates that the direct photolysis of CIP can be ignored. After visible light irradiation for 50 min, the photo-degradation results indicated that BiOBr/CTF-3D-2% sample for the degradation of CIP were also


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much higher than that of the pure BiOBr (Figure 7e-f). In addition, it is found that the photocatalytic activity of BiOBr/CTF-3D-2% is significantly higher than some related photocatalysts reported previously, as summed in Table S1. The excellent activity of BiOBr/CTF-3D-2% composite may make it become a valuable photocatalytic material in potential protections for ecosystem. In the practical applications of photocatalyst, good stability and reusability performance are also crucially important. As shown in Figure S8, the TC degradation efficiency decreased to around 5.4 % compared with the first run after the fourth recycling runs, suggesting the long reusable life of the BiOBr/CTF-3D-2% composite. (insert Figure 7) 3.3. Proposed Mechanism for Enhanced Photocatalytic Activity. The as-prepared samples were used as electrode materials to measure their electrochemical impedances (Figure 8a). The smaller arc radius in the high frequency region could be ascribed to charge-transfer resistance (Rct), indicating the efficient charge transfer.33,34 It can be found that BiOBr/CTF-3D-2% has a smaller radius than those of pure BiOBr and CTF-3D, implying the introduction of CTF-3D into BiOBr can improve the separation and transport efficiency of photo-generated electron-hole pairs. In order to further investigate the electrons and holes separation efficiency, the photocurrent responses were tested (Figure 8b). It’s known to all that the higher photocurrent response value of a catalyst always means the higher electron-hole separation efficiency and the higher photocatalytic activity.35 The photocurrent intensity of the BiOBr/CTF-3D-2% is nearly 3.0 times as high as that of pure BiOBr. This demonstrates that a more effective separation of photo-induced electrons and holes and a faster interfacial charge transfer occur in the BiOBr/CTF-3D-2%, resulting the higher photocatalytic activity. (insert Figure 8) (insert Figure 9) The concentration of total organic carbon (TOC) was chosen as a mineralization index of the degradation of TC. The mineralization of TC in the presence of the BiOBr/CTF-3D-2% catalyst under visible light irradiation is shown in Fig. S9. It is observed that more than 31% of total organic carbon was removed within 50 min, indicating that the TC would further mineralize into the ultimate inorganic products such as CO2 and H2O in this process. Radicals trapping experiments were further conducted to determine the reactive active species generated during the irradiation of the BiOBr/CTF-3D-2% material. During the experiments, KI was used as holes radical scavenger and isopropanol as hydroxyl radical scavenger.36,37 N2 flow excludes the dissolved O2 in solution to restrain formation of superoxide radical (•O2−).38,39 As shown in Figure 9a, it can be observed that •OH has negligible influence for photocatalytic activity, and h+ has a little influence for



photocatalytic activity. However, the degradation efficiency of RhB is greatly inhibited by N2 flow, confirming that •O2− is the main radical species in the photodegradation process. In order to investigate the superoxide radical, the ESR spectra of DMPO−•O2− adducts on BiOBr/CTF-3D-2% catalyst without TC under methanol conditions was illustrated in Figure 9b. Compared to the obvious signals detected from itself paramagnetic of sample in the dark, enhanced four characteristic peaks of DMPO−•O2− are observed under visible light irradiation.40 These proved that BiOBr/CTF-3D-2% catalyst could be efficiently excited by visible light to create photoinduced electrons, additionally reacting with adsorbed oxygen/H2O to produce •O2− on the photocatalyst surface. Therefore, the plausible mechanism could be proposed as follows and illuminated in Scheme 1. Generally, TC molecules would first be adsorbed on the surface and in the internal cavities of the BiOBr/CTF-3D-2% photocatalyst by the concentration gradient effect. Then, a more effective photo-induced electrons and holes occur in the interface of BiOBr/CTF-3D-2% under visible light. Finally, the photo-generated hole could directly oxidize TC molecules during the photocatalysis. The photo-excited electrons could reduce O2 molecules to produce the •O2− radicals that can work as a main active species to degrade TC into other products. (insert Scheme 1)

4. CONCLUSIONS In summary, we have successfully fabricated the novel BiOBr/CTF-3D composite photocatalysts by via a simple co-precipitation method. The structural characterizations demonstrated that the amorphous CTF-3D was well modified on the surfaces of BiOBr. The obtained BiOBr/CTF-3D-2% composite displays the optimal weight percent and has the highest degradation efficiency of TC compared to pure BiOBr under visible light irradiation. The trapping experiments of the active species confirmed that the •O2− was the main radical species for photocatalytic degradation of TC. The CTF-3D could cause the enlarged optical adsorption range, the efficiently separated photogenerated electron-hole pairs and the accelerated adsorption and transfer of antibiotic molecules, which synergistically account for the enhancement of photocatalytic activity. This work opens up a new strategy to improve the photocatalytic activity of traditional inorganic photocatalysts by modified with COFs materials for solving the pollution of living ecosystems.

ASSOCIATED CONTENT


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Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: xx. Some figures including SEM, EDS and mapping, UV–vis absorption spectra, pore size distributions, reusability and rate constants of samples (PDF). AUTHOR INFORMATION Corresponding Author *Email: [email protected]; [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21471086; No. 51572272), the Science and Technology Department of Zhejiang Province (No. 2017C33007), the Natural Science Foundation of Ningbo (No. 2017A610062; No. 2017A610065), and the K.C. Wong Magna Fund in Ningbo University.

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Figure 1. (a) XRD patterns and (b) FT-IR spectra of samples.

Figure 2. The XPS spectra of samples: (a) survey scan, (b) C 1s, (c) Bi 4f, (d) N 1s, (e) Br 3d and (f) O 1s.


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Figure 3. (a) and (b) N2 adsorption-desorption isotherms of different samples.

Figure 4. SEM images of BiOBr/CTF-3D-2% at different magnifications.



Figure 5. (a) TEM image, (b-f) HRTEM images and (g) corresponding SEAD result of BiOBr/CTF-3D-2% sample.


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Figure 6. (a) UV-Vis diffuse reflectance spectra. (b) The band gap energies of the samples.

Figure 7. Photocatalytic degradation of TC in the presence of different catalysts with (a) 40 mg, (c) 5 mg under visible light irradiation. (e) Photocatalytic degradation of CIP in the presence of different catalysts with 40 mg. (b,d,f) Kinetic fits of corresponding to the degradation of (a), (c), (e), respectively.



Figure 8. (a) Electrochemical impedance spectroscopy and (b) photocurrent response of samples.

Fig. 9. (a) Effects of different reactive species scavengers on the photodegradation of TC by BiOBr/CTF-3D-2% under visible-light irradiation. (b) ESR spectra of superoxide radical adducts trapped by DMPO in BiOBr/CTF-3D-2% dispersion in the dark and under visible light irradiation.

Scheme 1. Plausible mechanism for photocatalytic degradation of BiOBr/CTF-3D-2% under visible irradiation.


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"For Table of Contents Use Only"

Covalent Triazine Framework Modified BiOBr Nanoflake with Enhanced Photocatalytic Activity for Antibiotic Removal Shuai-Ru Zhu, Qi Qi, Yuan Fang, Wen-Na Zhao,* Meng-Ke Wu, and Lei Han*

A new composite photocatalyst integrated 3D covalent triazine framework with 2D BiOBr nanoflake exhibits the enhanced photocatalytic activity for the degradation of tetracycline hydrochloride and ciprofloxacin under visible light irradiation.


Covalent Triazine Framework Modified BiOBr ... - ACS Publications

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