BiOBr Ultrathin Nanosheets: In

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Nitrogen-Doped Carbon Quantum Dots/BiOBr Ultrathin Nanosheets: In Situ Strong Coupling and Improved Molecular Oxygen Activation Ability under Visible Light Irradiation Jun Di, Jiexiang Xia, Mengxia Ji, Bin Wang, Xiaowei Li, Qi Zhang, Zhigang Chen, and Huaming Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b00862 • Publication Date (Web): 23 Nov 2015 Downloaded from http://pubs.acs.org on December 5, 2015

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ACS Sustainable Chemistry & Engineering

Nitrogen-Doped Carbon Quantum Dots/BiOBr Ultrathin Nanosheets: In Situ Strong Coupling and Improved Molecular Oxygen Activation Ability under Visible Light Irradiation

Jun Di1, Jiexiang Xia1,*, Mengxia Ji1, Bin Wang1, Xiaowei Li1, Qi Zhang1,3, Zhigang Chen2, Huaming Li1,*

1

School of Chemistry and Chemical Engineering, Institute for Energy Research,

Jiangsu University, 301 Xuefu Road, Zhenjiang, 212013, P. R. China 2

School of the Environment, Jiangsu University, Zhenjiang, 212013, P. R. China

3

Hainan Provincial Key Lab of Fine Chemistry, Hainan University, Haikou 570228,

China

*Corresponding author: Tel.:+86-511-88791108; Fax: +86-511-88791108; E-mail address: [email protected]; [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT: Novel nitrogen-doped carbon quantum dots (N-CQDs)/BiOBr ultrathin nanosheets photocatalysts have been prepared via reactable ionic liquid assisted solvothermal process. The one-step formation mechanism of the N-CQDs/BiOBr ultrathin nanosheets was based on the initial formation of strong coupling between the ionic liquid and N-CQDs and subsequent result in tight junctions between N-CQDs and BiOBr with homodisperse of N-CQDs. The photocatalytic activity of the as-prepared photocatalysts was evaluated by the degradation of different pollutants under visible light irradiation such as ciprofloxacin (CIP), rhodamine B (RhB), tetracycline hydrochloride (TC), and bisphenol A (BPA). The improved photocatalytic performance of N-CQDs/BiOBr materials was ascribed to the crucial role of N-CQDs, which worked as photocenter for light harvesting, charge separation center for separating the charge carriers, and active center for degrading the pollutants. After the modification of N-CQDs, the molecular oxygen activation ability of N-CQDs/BiOBr materials was greatly enhanced. A possible photocatalytic mechanism based on the experimental results was proposed.

KEYWORDS: N-CQDs; BiOBr; Molecular oxygen; Ionic liquid; Photocatalytic

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INTRODUCTION Increasing attentions has been focused on nanomaterials for photocatalytic solar-energy conversion to identify robust new methods for water purification and environmental protection at lower cost and energy consumption.[1] The central issue of latest research was committed to the extension of the light absorption of photocatalyst to the visible light region, so as to promote the use of solar-energy. Up to now, many visible-light-response semiconductor nanomaterials have been developed successfully, such as g-C3N4, Ag3PO4, BiVO4, Bi2WO6 etc.[2-5] Recently, bismuth oxyhalides (BiOX, X = Cl, Br, and I) have been widely studied due to their excellent optical, electrical and catalytic properties.[6-12] Especially, the layered structure of BiOX, which built by interlacing [Bi2O2] slabs with double halogen slabs, resulted in the formation of self-built internal static electric fields and thus induce the outstanding photocatalytic activity.[13, 14] Among the BiOX, BiOBr was of greatly investigated due to the appropriate band gap and excellent visible light photocatalytic activity.[15-18] BiOBr materials with different morphologies such as ultrathin nanosheets,[19] flower-like structure [20] and hollow microspheres [21] have been controlled prepared. Compared with the photo-induced charge carriers generated close to the surface, these charge carriers generated inside the semiconductor will take longer time to reach the surface. Thus, the ultrathin structure materials enable the strikingly fast carrier transport from the inside to the surface, which may promote the photocatalytic activity. However, the photocatalytic acticity of the single BiOBr materials was always limited by the rapid recombination of photo-generated electron-hole pairs. In order to improve the photocatalytic acticity of BiOBr materials, several approaches have been used. For example, heteroatom doping,[22] dehalogenation,[23, 24] surface functionalization[25] and constructing hybrid materials,[26-34] among which the constructing hybrid materials was most studied and lots of hybrid materials have been prepared, such as g-C3N4/BiOBr,[26] BiOBr-BiOI,[27] Ag/AgBr/BiOBr,[28] MoS2/BiOBr,[29] BiOBr@SiO2@Fe3O4,[30] Cu2O/BiOBr,[31]

BiOBr/Bi24O31Br10,[32]

BiOBr/Bi2WO6,[33]

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and


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BiOBr-TiO2-graphene.[34] However, due to the large-size materials can hardly building perfect contact interfaces between each other, the surface defects would emerge among the interfaces. On the one hand, the inadequate contact interface enlarged the barriers for electron transport across the interface and cannot realize the effective separation of electron-hole pairs. On the other hand, certain amounts of defects formed among interfaces could serve as recombination centers of photo-generated electron-hole pairs, which may limit the photocatalytic activity. As a novel nanocarbon materials with the size less than 10 nm, carbon quantum dots (CQDs) has aroused numerous attentions due to their unique and excellent properties, such as low toxicity, biocompatibility, low cost, chemical inertness and size- and wavelength-dependent luminescence emission.[35, 36] CQDs possess amorphous to nanocrystalline cores with predominantly graphitic or turbostratic carbon (sp2 carbon) or graphene and graphene oxide sheets fused by diamond-like sp3 hybridised carbon insertions, which leading to the CQDs with electron transfer and reservoir ability.[37] Hence, the CQDs have been introduced to the photocatalysis system so as to improve the photocatalytic activity both for hydrogen generation and pollutant removal, such as CQDs/TiO2,[38-41] CQDs/C3N4,[42] CQDs/Cu2O,[43] CQDs/Ag3PO4,[44] CQDs/Bi2WO6,[45] CQDs/Bi2MoO6,[46] and CQDs/Fe3O4.[47] However, it was still highly desirable to further improve the performance of CQDs by appropriate means so as to acquire satisfactory high-efficiently photocatalytic activity of CQDs-based materials. Due to the nitrogen doping can efficiently induce charge delocalization and tune the work function of carbon, the electron transfer capability of carbon materials can be efficiently promoted.[48] Up to now, N-doped graphene, N-doped nanotube and N-doped nanofiber have been prepared and improved properties have been obtained.[49-51] Therefore, the photocatalytic performance was worth expecting if N-doped CQDs (N-CQDs) was constructed and introduced to the semiconductor system by suitable approach. In this system, the reactable ionic liquid (IL) 1-hexadecyl-3-methylimidazolium bromide ([C16mim]Br) was chosen to provide Br source and acted as bridge role to construct intimate integration of N-CQDs and BiOBr materials. The abundant 4


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oxygen-containing functional groups on the surface of N-CQDs could enable the N-CQDs connecting with IL via hydrogen bond and coulomb force interacting so as to control the homogeneous dispersion of N-CQDs and strong-coupling of N-CQDs with IL. Subsequently, the bromine ions in the IL react with bismuth nitrate to in situ form BiOBr and leading to the strong-coupling of N-CQDs and BiOBr during solvothermal treatment. The uniform dispersion of N-CQDs and strong-coupling of N-CQDs with BiOBr could give rise to the maximum interfacial contact and intimate integration between BiOBr and N-CQDs. Therefore, the excellent electron conductivity of N-CQDs can be sufficient utilized. The photocatalytic activity of the as-prepared samples was evaluated by the degradation of four different kinds of pollutants under visible light irradiation. The crucial role of N-CQDs for the enhanced photocatalytic performance was studied in details.

EXPERIMENTAL SECTION Material and Sample Preparation All the reagents were of analytical purity and were used as received. The ionic liquid [C16mim]Br (1-hexadecyl-3-methylimidazolium bromide) (99%) was purchased from Shanghai Chengjie Chemical Co. Ltd. The N-CQDs aqueous solution was synthesized according to the literature.[52] 5 mmol ammonium citrate was dissolved in 10 mL deionized water containing 335 µL ethylenediamine. The above solution was then transferred to 25 mL Teflon-lined autoclave and heat treating for 5 h at 200 °C and cooled down naturally. The obtained product was subjected to dialysis for 24 h in order to obtain the N-CQDs solution. The N-CQDs/BiOBr ultrathin nanosheets was prepared through the following steps: 1 mmol [C16mim]Br was dissolved into the mixed solution of 10 mL ethylene glycol and a certain amount of N-CQDs aqueous solution and the solution was topping up to 18 mL by H2O. Then 1 mmol Bi(NO3)3·5H2O was added in the above solution under stirring and further stirred for 30 min. The mixture was transferred into 25 mL Teflon-lined autoclave and heated at 140 oC for 12 h and then cooled down to

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room temperature naturally. The as-prepared materials were centrifuged and washed with distilled water and ethanol as well as dried in a vacuum at 50 oC. Based on the addition of 1 mL, 3 mL and 8 mL N-CQDs aqueous solution, the obtained N-CQDs/BiOBr materials was named as N-CQDs/BiOBr-1, N-CQDs/BiOBr-3 and N-CQDs/BiOBr-8. Characterization X-ray diffraction (XRD) patterns of all samples were collected in the range 10-80 (2θ) using a Shimadzu XRD-6000 diffractometer (Cu Kα radiation). The chemical compositions of the samples were analyzed using X-ray photoelectron spectroscopy (VG MultiLab 2000 system with a monochromatic Mg-Kα source). The Fourier transform (FT)-IR spectra were recorded with KBr disks containing the powder sample with the FT-IR spectrometer (Nexus 470). Raman spectroscopic measurements were performed on a Renishaw Invia Raman System with a 532 nm Nd:YAG excitation source at room temperature. Scanning electron microscopy (SEM) images were obtained on a field-emission scanning electron microscope (JEOL JSM-7001F) equipped with an energy-dispersive X-ray spectroscope (EDS) operated at an acceleration voltage of 10 kV. Transmission electron microscope (TEM) images were obtained using a JEOL JEM-2100F transmission electron microscope. The UV-vis absorption spectra were measured with a UV-vis spectrophotometer (Shimadzu UV-2450). The Brunauer-Emmett-Teller (BET) surface areas of the prepared samples were obtained from N2 adsorption-desorption isotherms using a Micromeritics TriStar II 3020 system. The PL spectra of the obtained samples were detected using a Varian Cary Eclipse spectrometer. Electrochemical and photoelectrochemical measurements were performed in a three-electrode quartz cell. 0.1 M phosphate buffered saline (pH = 7.0) and 0.1 M KCl solution containing 5 mM Fe(CN)63-/(Fe(CN)64-) was used as electrolyte solution for the photocurrent and electrochemical impedance spectra (EIS) measurement, respectively. A platinum wire was employed as a counter electrode and a saturated Ag/AgCl electrode was used as a reference electrode. The electron spin resonance (ESR) signals of radicals spin-trapped were examined on a Bruker model ESR JES-FA200 spectrometer by 6


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spin-trap reagent DMPO (Sigma Chemical Co.) in methanol and water, respectively. Photocatalytic Measurements The visible light photocatalytic activity was measured by analyzing the degradation of ciprofloxacin (CIP), rhodamine B (RhB), tetracycline hydrochloride (TC), and bisphenol A (BPA). 50 mg, 20 mg, 50 mg and 50 mg of prepared sample was added into a quartz photoreactor containing 100 mL of CIP (10 mg/L), RhB (10 mg/L), TC (20 mg/L), BPA (10 mg/L) solution, respectively. A 300 W Xe lamp equipped with a UV cutoff filter (400 nm) was used as a visible light source and the distance between the light and the reaction tube was fixed at 10 cm. The pH value was not adjusted when photocatalytic reaction was carried out. Experiments were performed at 30 oC with a circulating water system to prevent thermal catalytic effects. Aeration was conducted using an air pump to insure a constant oxygen supplying and full mixing of the solution and the photocatalysts during the photo-degradation process. The suspension was stirred in the dark for 30 min to achieve adsorption-desorption equilibrium on the catalyst surface. At certain time intervals, the dispersion was sampled (about 3 mL), and centrifuged to remove the photocatalyst. The resulting solution was analyzed by checking the maximum absorbance of the CIP, RhB, TC solution with a UV-vis spectrophotometer (Shimadzu UV-2450) at 276 nm, 553 nm and 356 nm, respectively. High performance liquid chromatography (HPLC) was applied to analyze the remnant amount of BPA. It was detected by two Varian ProStar 210 pumps, an Agilent TC-C18 column, and a Varian ProStar 325 UV-vis Detector at 230 nm. A solution of methanol and H2O in the ratio 75 : 25 (v/v) was used as the mobile phase at 1 mL min-1, and 20 µL of the sample solution was injected.

RESULTS AND DISCUSSION XRD Analysis The phases of the N-CQDs/BiOBr materials with different contents of N-CQDs were investigated by X-ray diffraction (XRD) and the result was shown in Figure 1.

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All the diffraction peaks were indexed to tetragonal phase BiOBr structure (JCPDS card No. 73-2061) with the lattice parameters of a = 3.915 Å, c= 8.076 Å. When the N-CQDs was modified on the BiOBr materials, no diffraction peak shift was observed. This indicated the formation of the N-CQDs/BiOBr materials with the present weight ratios of N-CQDs do not damage the crystal structure of BiOBr materials. The peaks with 2θ values at 10.9°, 22.0°, 25.3°, 31.8°, 32.3°, 39.4°, 46.3°, 50.8°, 56.4°, 57.3°, 66.5°, 67.6°, 71.2°, and 77.0°, can be indexed to (001), (002), (011), (012), (110), (112), (020), (014), (114), (212), (024), (220), (124), and (130) crystal planes of BiOBr, respectively. No typical diffraction peaks of N-CQDs were detected, which can be explained by the low amount of N-CQDs in the N-CQDs/BiOBr composites and similar results can also be found in previous reports.[46, 53] FT-IR Analysis The presence of N-CQDs in the as-prepared N-CQDs/BiOBr samples was determined by FT-IR spectra analysis and the result was shown in Figure 2. The absorption band at 517 cm-1 was assigned to the Bi-O stretching mode in the BiOBr materials. With respect to N-CQDs/BiOBr samples, the absorption band at 1440 cm-1 was originated from the symmetric carboxylate stretch.[54] The 1649 and 1701 cm-1 belong to the vibrational absorption band of C=O [55] and the absorption band at 1562 cm-1 was corresponding to the bending vibrations of N-H,[56] which signifying the existing of N-CQDs in the N-CQDs/BiOBr samples. The FT-IR result showed that N-CQDs and BiOBr have been coupled together successfully. XPS Analysis To analyze

the chemical composition,

XPS spectra

of BiOBr and

N-CQDs/BiOBr materials were recorded. The survey spectrum of the (Figure 3a) indicated the as-prepared N-CQDs/BiOBr sample mainly contain bismuth, oxygen, bromine, carbon, and nitrogen elements. In Figure 3b, the BiOBr sample showed binding energy at 158.9 eV for Bi 4f7/2 and 164.2 eV for Bi 4f5/2, respectively, meaning the Bi element exist in the chemical states of Bi3+. The Bi 4f peaks in N-CQDs/BiOBr shifted to 158.6 and 164.0 eV, respectively, indicating the existence of interaction between the N-CQDs and BiOBr. The peak binding energy of 529.7 and 8


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68.0 eV were ascribed to O 1s and Br 3d, respectively (Figure 3c and d). The C 1s spectrum (Figure 3e) can be fitted into three peaks located at 284.6, 286.1 and 287.6 eV, which were attributed to C-C/C=C, oxygenated carbon and nitrous carbon, respectively.[56] The peak in N 1s spectrum was located at 401.3 eV, which was ascribed to the N-H in the N-CQDs (Figure 3f).[57] The XPS analysis further indicated the N-CQDs has been modified on the BiOBr materials and intimate integration has been constructed. Raman Analysis To determine the surface microstructural difference of the as-prepared pure BiOBr and N-CQDs/BiOBr materials, the Raman investigation was carried out. Figure S1 presented the Raman spectra of the pure BiOBr and N-CQDs/BiOBr with a 532 nm laser as an excitation source. The bands centered at 57.4, 112.5 and 159.8 cm-1 of pure BiOBr belong to the A1g external, A1g internal and Eg internal Bi-Br stretching mode, respectively.[58] The D-band and G-band of carbon at 1353 and 1588 cm-1 can be seen in N-CQDs/BiOBr materials which implying the existence of N-CQDs in the N-CQDs/BiOBr.[59] From the inside figure, it can be found that the A1g external Bi-Br stretching mode shift from 57.4 cm-1 for pure BiOBr to 55.6 cm-1 for N-CQDs/BiOBr materials, which further implying the strong interaction between the N-CQDs and BiOBr. Morphology and Microstructure Analysis The morphology and microstructure of the as-synthesized pure BiOBr and N-CQDs/BiOBr materials were investigated using field-emission scanning electron microscopy (SEM). The SEM images exhibited in Figure 4a and b reveal the ultrathin nanosheets structure of pure BiOBr materials with the size about 100-400 nm. View from the side edge of the BiOBr nanosheets in Figure 4b, the thickness was determined to be about 10 nm. The Figure 4c and d displayed the SEM images of N-CQDs/BiOBr materials and similar ultrathin nanosheets morphology with about 10 nm thickness can be observed. The difference of microstructure was further investigated by transmission electron microscopy (TEM). The near transparency of the nanosheets indicated the 9



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ultrathin thickness of BiOBr materials (Figure 5a). It can be seen from the Figure 5b, the surface of BiOBr nanosheets was smooth and the thickness of nanosheets was determined to be about 12 nm, which was in consistent with the result of SEM analysis. With respect to the N-CQDs/BiOBr materials (Figure 5c and d), there have a lot of dark dots with the diameter about 6 nm can be seen. This implied the N-CQDs have been anchored on the BiOBr nanosheets successfully and intimate integration has been constructed. EDS element mapping clearly showed the elements C, N, O, Bi and Br evenly distributed in N-CQDs/BiOBr materials (Figure S2). The N-CQDs content was determined to be about 1.96 wt% in the N-CQDs/BiOBr-3 materials. Enhancement of Photocatalytic Activity The photocatalytic performance of these N-CQDs/BiOBr samples was investigated toward different pollutants photo-degradation under the visible light irradiation. The ciprofloxacin (CIP) has been employed as a broad-spectrum antibiotic agent in order to treat bacterial infections extensively. Due to the difficultly of metabolized thoroughly for CIP from in vivo, considerable fraction can be released as the active form in pharmaceutically. The plenty of employment of CIP and the short of treating processes resulting in their ubiquity in surface waters and thus may accelerate antibiotic resistance within native bacterial populations in impacted environments.[60] The N-CQDs/BiOBr materials have been used for the removal of CIP from aqueous solution under visible light irradiation. The variation of the degree of degradation (C/C0) with the time of irradiation for pure BiOBr and N-CQDs/BiOBr materials was plotted in Figure 6a. Under the visible light irradiation, negligible CIP can be degraded in the absence of photocatalyst, revealing the stability of CIP under visible light irradiation. After irradiation for 120 min, only 40.1% of CIP can be degraded by pure BiOBr. When N-CQDs was modified on the BiOBr, the photocatalytic activity of N-CQDs/BiOBr materials improved significantly. The N-CQDs/BiOBr-3 sample displayed the highest photocatalytic activity and 88% of CIP was removed after 120 min irradiation. The photo-degradation result indicated the N-CQDs play the important role for the improvement of photocatalytic activity. The widely used model pollutant rhodamine B (RhB) was employed to further evaluate the photocatalytic 10


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performance of N-CQDs/BiOBr materials, as shown in Figure 6b. The degrees of the RhB photodegradation by the N-CQDs/BiOBr materials were all higher than that of pure BiOBr. With an increase in the mass ratios of N-CQDs, the photodegradation performance of N-CQDs/BiOBr materials showed the increasing trend similar to CIP degradation. The highest photocatalytic activity was obtained for N-CQDs/BiOBr-3 materials and 96.5% of RhB can be degraded after irradiation for 30 min. The time-dependent absorption spectra for the degradation of CIP and RhB solution in the presence of the N-CQDs/BiOBr-3 sample under visible light irradiation were shown in Figure S3 and S4. The absorption at λ = 276 nm of CIP evidently decreases with the increase of irradiation time and nearly disappears after 120 min (Figure S3). The evident decrease in RhB absorption at λ = 553 nm was also observed (Figure S4), accompany with a shift in the absorption band towards the blue region, which can be attributed to the step-by-step de-ethylation process.[21] In order to study the selectivity of catalyst activity for N-CQDs/BiOBr materials, the tetracycline hydrochloride (TC) and endocrine disrupting chemical bisphenol A (BPA) were employed to further investigate the photocatalytic activity of the as-prepared materials. As shown in Figure 7a, the direct photolysis of TC was negligible. The N-CQDs/BiOBr materials exhibited better photocatalytic performance than pure BiOBr for TC degradation. After the irradiation for 120 min, 67.1% of TC can be photodegraded by N-CQDs/BiOBr-3 materials while only 38.6% of TC can be degraded by pure BiOBr, suggesting the introduction of N-CQDs was an efficient strategy for the improvement of photocatalytic activity. The time-dependent absorption spectra of TC solution in the presence of pure N-CQDs/BiOBr-3 materials was shown in Figure S5. Similar result can also be found for the degradation of BPA, as shown in Figure 7b. A 32.8% improvement of BPA degradation can be obtained by the N-CQDs modification under visible light irradiation for 3.5 h. The degradation process of BPA in the presence of pure BiOBr and N-CQDs/BiOBr materials was shown in Figure 7c and d, respectively. It can be seen that peak corresponding to BPA decreased

faster

for

N-CQDs/BiOBr

materials

than

pure

BiOBr.

These

photodegradation results of different model organic pollutants indicated that the 11



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N-CQDs modification was an effective strategy to further increase the photocatalytic performance and the N-CQDs/BiOBr materials was efficient visible-light-driven photocatalyst. Nitrogen Adsorption Analysis The nitrogen adsorption-desorption isotherms and Barrett-Joyner-Halenda (BJH) pore-size distribution plots of the BiOBr and N-CQDs/BiOBr materials were used to further study their microstructures. As shown in Figure S6, all of the isotherms of the samples correspond to type IV, which signifying the existence of mesoporous in the N-CQDs/BiOBr materials that resulted from the inter-nanosheet spacing.[61] In comparison to the pure BiOBr (0.83 m²/g), the BET surface area of the N-CQDs/BiOBr samples was larger (2.57, 6.86 and 9.55 m²/g for N-CQDs/BiOBr-1, N-CQDs/BiOBr-3 and N-CQDs/BiOBr-8). The enhanced BET surface area of N-CQDs/BiOBr materials was originated from the excellent adsorptive performance of N-CQDs. Optical and Electronic Properties The optical property of the N-CQDs/BiOBr materials was measured by using the UV-vis diffuse reflectance spectra (DRS), as shown in Figure 8a. The pure BiOBr exhibited the abrupt onset of absorption at about 420 nm which derived from the electronic transition from the valence band (VB) to the conduction band (CB) of BiOBr upon light excitation. It can be seen that the addition of different contents of N-CQDs affected the optical property of light absorption for N-CQDs/BiOBr materials significantly. There was an enhanced absorbance in the region ranging from 420 to 800 nm with the increase of N-CQDs content. A plot obtained via the transformation based on the Kubelka-Munk function versus the energy of light was shown in Figure 8b, from which the band gap values of the N-CQDs/BiOBr samples decreased with the N-CQDs contents increased. The enhanced light harvesting of N-CQDs/BiOBr materials may resulted in the formation of more photo-generated electron-hole pairs.[62] Quantum yield (Φ), defined as the number of events occurring per photon absorbed in semiconductor photocatalyst, determined the efficiency of the 12


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photocatalytic reaction. [63] In an ideal system, Φ was proportional to a simple relationship: Φ∝kct/(kct+kr), where kct represented the charge transfer rate and kr stand for the rate of charge carriers recombination. The above equation implying that the inhibition of recombination for the electron-hole pairs was vital for the improvement of the quantum yield and thus increased the photocatalytic performance. Therefore, it was importance to study the transportation and recombination processes of photogenerated electron-hole pairs to have an insight into the cause of the increased photocatalytic activity. Photoluminescence (PL) technique was employed to study the transfer and recombination processes of photo-generated charge carriers in BiOBr and N-CQDs/BiOBr materials. As shown in Figure 9, the materials have strong emission peak centered at about 468 nm under the excited at 360 nm. The N-CQDs/BiOBr materials displayed lower intensity than that of pure BiOBr and the decreased PL signal after modification of N-CQDs can be regarded as strong evidence for a remarkable decrease of the recombination rate for electron-hole pairs. This phenomenon was ascribed to highly efficient electron transfer from the BiOBr to the N-CQDs with conjugated π structure, leading to the spatial separation of the electrons and holes.[64] The photocurrent was generated by the electron transfer from the conduction band of materials to the electrodes. As shown in Figure 10a, under visible light irradiation, the photocurrent intensity for N-CQDs/BiOBr materials was much higher than that of pure BiOBr, suggesting a longer lifetime of electron-hole pair photogenerated over N-CQDs/BiOBr materials than BiOBr, which qualitatively was consistent with the result of PL analysis. Normally, the photo-generated electrons and holes would recombine quickly and lead to a relative low photocurrent intensity. When the N-CQDs was modified on the surface of BiOBr ultrathin nanosheets to build intimate integration, the photo-generated electrons could transfer from BiOBr to N-CQDs and thus result in the effective separation of photo-generated charge carriers. To get further insight into the charge carrier transfer process, the electrochemical impedance spectra (EIS) Nyquist plots has also been performed. It can be seen from Figure 10b that N-CQDs/BiOBr-3 materials showed smaller semicircle at high 13



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frequencies as compared to pure BiOBr materials. This result indicated the N-CQDs/BiOBr-3 materials possess much lower resistance than BiOBr and faster transport of charge carriers can be achieved over the N-CQDs/BiOBr-3 electrode. The highly conductive N-CQDs with intimate contaction of BiOBr nanosheets can facilitate electron transfer from BiOBr nanosheets within the whole electrode and thus decrease resistance. [65] Mechanism of Pollutant Photodegradation The main active species during the photocatalysis process was determined by using

electron

spin

resonance

(ESR)

spin-trap

technique

with

5,5-dimethyl-1-pyrroline N-oxide (DMPO), as shown in Figure 11.[66] Under visible light irradiation, the DMPO-O2•− adduct over BiOBr and N-CQDs/BiOBr was detected in methanol dispersion (Figure 11a and c). The characteristic peaks intensity of DMPO-O2•− over N-CQDs/BiOBr was much stronger than that for BiOBr, implying the N-CQDs modification greatly increase the molecular oxygen activation ability of N-CQDs/BiOBr materials. No DMPO-·OH signal was appeared which revealing the ·OH cannot be able to generated in both the BiOBr and N-CQDs/BiOBr systems (Figure 11b and d). The relative energy levels of valence band maximum (VBM) and conduction band minimum (CBM) played crucial roles in determining the redox power of photo-generated charge carriers in the photocatalysis process. Single-electron reduction of oxygen was generally considered as important pathway to produce superoxide radical by molecular oxygen trapping photo-generated electrons, where a more negative level of CBM than single-electron reduction potential of oxygen was required. To determine the energy band structure of the BiOBr, the maximum energy edge of the VB was measured by XPS valence spectra, as shown in Figure S7.[67] The total densities of states of their valence band were measured to be 1.48 eV. According to the band gap of BiOBr acquired from the DRS analysis was 2.62 eV, the CBM can be estimated to be -1.14 eV by using the formula ECB = EVB - Eg. The level of CBM of BiOBr was negative enough to allow single-electron reduction of oxygen due to the E0(O2/O2•−) was about -0.046 eV vs normal hydrogen electrode (NHE). But 14


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the level of VBM of BiOBr was not positive enough to oxidize OH- to yield ·OH since the E0(·OH/OH-) was 2.38 eV vs NHE.[68] This result was in agreement with the ESR analysis. Due to the photo-generated electron-hole pairs would be tend to recombine easily, thus only a fraction of charge carriers could produce radicals and subsequently participate in the photocatalytic process. When the N-CQDs was modified on the BiOBr to construct intimate integration, the photo-generated electrons in the CB of BiOBr would transfer to the N-CQDs and resulted in effective separation of photo-generated charge carriers. The electrons on the N-CQDs could active molecular oxygen via single-electron reduction process and thus result in producing more O2•−. Free radicals trapping experiments by using different trapping agents were performed to further determine the active species during the photodegradation process (Figure 12).[69] When the t-butanol or isopropanol (IPA) was added, the photo-degradation of RhB by N-CQDs/BiOBr-3 material was not obvious affected, revealing the ·OH was not the main reactive species during the photocatalytic reaction. When

the

benzoquinone

O2•−

(BQ,

scavenger)

and

disodium

ethylenediaminetetraacetate (EDTA-2Na, h+ scavenger) [13] was added into the reaction system, the photocatalytic activity was inhibited significantly. Based on the ESR and energy band structure analysis above mentioned and the result in the free radicals trapping experiments, the main active species were determined to be O2•− and holes in this system. According to the aforementioned analysis, the crucial role of N-CQDs for the enhanced photocatalytic activity was concluded and the photocatalytic reaction mechanism of N-CQDs/BiOBr ultrathin nanosheets was proposed. Firstly, due to the introduction of N-CQDs to the BiOBr, the light harvesting ability of the N-CQDs/BiOBr materials increased, which revealing the N-CQDs could work as photocenter for absorbing solar light. Secondly, when the visible light was irradiated to the BiOBr, the electron on the VB can be excited to the CB. Since the excellent electron transfer and collect ability of N-CQDs, the electrons can be transfer to the N-CQDs from the CB of BiOBr and effective separation of photo-generated 15



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electron-hole pairs can be obtained. Thus, the N-CQDs could act as charge separation center. Thirdly, the modification of N-CQDs on BiOBr improved the BET surface area and more pollutants can be adsorbed on the N-CQDs. Due to the O2•− was produced on the surface of N-CQDs and O2•− has been determined to be important active species for the photodegradation process in this system, the O2•− could react with the adsorbed pollutants directly and thus the N-CQDs was acted as photocatalytic reaction center for pollutant degradation.

CONCLUSIONS Novel N-CQDs/BiOBr ultrathin nanosheets photocatalysts with different N-CQDs contents have been prepared via solvothermal process. The one-step formation mechanism of the N-CQDs/BiOBr ultrathin nanosheets was based on the initial formation of strong coupling between the ionic liquid and N-CQDs, and subsequent result in tight junctions between N-CQDs and BiOBr with homodisperse of N-CQDs. After the modification of N-CQDs, the photocatalytic activity of N-CQDs/BiOBr materials for the degradation of CIP, RhB, TC and BPA greatly improved. The enhanced photocatalytic performance of N-CQDs/BiOBr was attributed to the increased light harvesting capacity, excellent electron transfer ability and improved molecular oxygen activation ability of N-CQDs. Hopefully, this facile synthesis strategy could be extended to fabricating other N-CQDs-semiconductor hybridization to obtain novel multifunctions and properties.

ASSOCIATED CONTENT Supporting Information The Raman spectroscopy of BiOBr and N-CQDs/BiOBr-3 samples, elemental mapping images of N-CQDs/BiOBr-3 materials, nitrogen absorption-desorption isotherms and valence-band XPS spectra of the BiOBr sample. This material is available free of charge via the Internet at http://pubs.acs.org.

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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 21206060, 21476098 and 21471069), Jiangsu Province (1102118C), and the Special Financial Grant from the China Postdoctoral Science Foundation (2013T60506).

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Figure Caption: Figure 1. XRD patterns of as-prepared N-CQDs/BiOBr samples Figure 2. FT-IR spectra of N-CQDs/BiOBr photocatalyst with different N-CQDs contents. Figure 3. XPS spectra of BiOBr and N-CQDs/BiOBr-3 samples: (a) survey of the sample (b) Bi 4f, (c) O 1s, (d) Br 3d, (e) C 1s and (f) N 1s. Figure 4. SEM images of (a and b) pure BiOBr and (c and d) N-CQDs/BiOBr-3 sample. Figure 5. TEM images of (a and b) pure BiOBr and (c and d) N-CQDs/BiOBr-3 sample Figure 6. Visible-light-driven photocatalytic activity of N-CQDs/BiOBr materials with different N-CQDs contents for (a) CIP and (b) RhB degradation. Figure 7. Photocatalytic degradation of (a) TC and (b) BPA in the presence of pure BiOBr and N-CQDs/BiOBr-3 samples under visible light irradiation; HPLC chromatograms of the BPA degradation of the (c) pure BiOBr and (d) N-CQDs/BiOBr materials under visible light irradiation. Figure 8. (a) UV-vis spectra of N-CQDs/BiOBr materials with different N-CQDs contents; (b) (αEphoton)1/2 vs. Ephoton curves of the as-prepared samples. Figure 9. Photoluminescence (PL) spectra of pure BiOBr and N-CQDs/BiOBr-3 materials. Figure 10. (a) Transient photocurrent response of pure BiOBr and N-CQDs/BiOBr materials under visible light irradiation; (b) Electrochemical impedance spectroscopy (EIS) of pure BiOBr and N-CQDs/BiOBr-3 materials. Figure 11. DMPO spin-trapping ESR spectra recorded with (a, b) N-CQDs/BiOBr-3 materials and (c, d) pure BiOBr photocatalyst in (a, c) methanol dispersion (for DMPO-O2•−) and (b, d) aqueous dispersion (for DMPO-·OH) under visible light irradiation. Figure 12. Trapping experiment of active species during the photocatalytic degradation of RhB over N-CQDs/BiOBr-3 material under visible light irradiation.


(012) (110)

(130)

(024) (220) (124)

(114) (212)

(014)

(020)

(112)

(002) (011)

Intensity (a.u.)

Page 26 of 35

N-CQDs/BiOBr-8

N-CQDs/BiOBr-3

N-CQDs/BiOBr-1

BiOBr 10

20

30

40

50

60

70

80

2-Theta (degree)

Figure 1

N-CQDs/BiOBr-8

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(001)


N-CQDs/BiOBr-3

N-CQDs/BiOBr-1

BiOBr 2000

1800

1600

1400

1200

1000

800

-1

Wavenumber (cm )

Figure 2


600

Page 27 of 35

(b) Bi 4f Intensity (a.u.)

Bi4f

N-CQDs/BiOBr-3

Br3d O2s

Br3p

C1s

Bi4d3 Bi4d5 N1s

O1s

Intensity (a.u.)

O KLL

Bi4p3

(a) Survey

N-CQDs/BiOBr-3

BiOBr

1000

BiOBr

800

600

400

200

0

170

168

166

Binding Energy (eV)

(c)

162

160

158

540

538

(d)

O1s

536

292

Intensity (a.u.)

N-CQDs/BiOBr-3

532

530

528

526

74

72

(f)

288

70

68

284

64

62

N 1s

N-CQDs/BiOBr-3

Intensity (a.u.) 286

66

Binding Energy (eV)

C 1s

290

152

BiOBr 534

N-CQDs/BiOBr-3

294

154

Br3d

Binding Energy (eV)

(e)

156

Binding Energy (eV)

N-CQDs/BiOBr-3

Intensity (a.u.)

164

BiOBr

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


282

280

278

410

408

406

Binding Energy (eV)

404

402

400

398

Binding Energy (eV)

Figure 3


396

394

392


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4

Figure 5


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1.0

(a)

0.8

0.6

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4

BiOBr N-CQDs/BiOBr-1 N-CQDs/BiOBr-3 N-CQDs/BiOBr-8 CIP

0.2

0.0 0

20

40

60

80

100

Irradiation time (min)


120


1.0

(b) 0.8

BiOBr N-CQDs/BiOBr-1 N-CQDs/BiOBr-3 N-CQDs/BiOBr-8 RhB

C/C0

0.6

0.4

0.2

0.0 0

10

20

30

40

50


Figure 6

1.0

(b)

1.0

(a)

0.9

0.9 0.8 0.8 0.7

C/C0

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.7

0.6

TC degradation

0.6

BPA degradation

0.5

BiOBr N-CQDs/BiOBr-3 TC

0.4 0.3 0

20

40

0.5

BiOBr N-CQDs/BiOBr-3

0.4 60

80

100

120

0.0

0.5

1.0


(c)

0

1

Reaction time/h

(d)

0 0.5 1.5 2.5 3.5

2

3

4

1.5

2.0

2.5

3.0

3.5

5

6

7

Irradiation time (h)

5

6

7

0

1

Reaction time/h 0 0.5 1.5 2.5 3.5

2

Retention time (min)

3

4

Retention time (min)

Figure 7


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1.0

(a) BiOBr N-CQDs/BiOBr-1 N-CQDs/BiOBr-3 N-CQDs/BiOBr-8

Absorbance (a.u.)

0.8

0.6

0.4

0.2

0.0 200

300

400

500

600

700

800

Wavelength (nm)

2.0

(b) 1/2

1.5

(Ephoton)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0

BiOBr N-CQDs/BiOBr-1 N-CQDs/BiOBr-3 N-CQDs/BiOBr-8

0.5

0.0 2

3

4

Ephoton (ev)

Figure 8


5

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Relative intensity (a.u.)


a b


a

b

400

450

500

550

600

Wavelength (nm)

Figure 9


650

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(a)

Photocurrent (A)

0.3

a b


b 0.2

a 0.1

0.0 0

100

Irradiation time (s)

1500

(b) 1000

-Z'' (Ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


500

BiOBr N-CQDs/BiOBr-3 0 0

500

1000

Z' (Ohm)

Figure 10


1500


(a)

318.3

318.4

(b)

superoxide radical

318.5

318.6

318.7

hydroxyl radical

Light on

Light on

Dark

Dark

318.8

318.9

318.3

318.4

318.5

B/mT

(c)

318.3

318.4

318.5

318.6

318.6

318.7

318.8

318.9

B/mT

(d)

superoxide radical

318.7

hydroxyl radical

Light on

Light on

Dark

Dark

318.8

318.9

318.3

318.4

B/mT

318.5

318.6 B/mT

Figure 11

1.0

0.8

(C0-C)/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 35

0.6

0.4

0.2

0.0

No quencher t-butanol

IPA

BQ

Figure 12


EDTA-2Na

318.7

318.8

318.9

Page 35 of 35

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TOC graphic

N-CQDs/BiOBr ultrathin nanosheets photocatalysts have been prepared with strong coupling between N-CQDs and BiOBr. The modification of N-CQDs improved the molecular oxygen activation ability under visible light irradiation.

superoxide radical

Light on

Dark

318.3

318.4

318.5

318.6 B/mT


318.7

318.8

318.9

BiOBr Ultrathin Nanosheets: In

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