Anchoring single-unit-cell defect-rich bismuth molybdate layers on ultrathin carbon nitride nanosheet with boosted charge transfer for efficient photocatalytic ciprofloxacin degradation

Journal of Colloid and Interface Science xxx (xxxx) xxx

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Anchoring single-unit-cell defect-rich bismuth molybdate layers on ultrathin carbon nitride nanosheet with boosted charge transfer for efficient photocatalytic ciprofloxacin degradation Bisheng Li a,b, Cui Lai a,b,⇑, Lei Qin a,b, Chengcheng Chu a,b, Mingming Zhang a,b, Shiyu Liu a,b, Xigui Liu a,b, Huan Yi a,b, Jiangfan He a,b, Ling Li a,b, Minfang Li a,b, Liang Chen c a

College of Environmental Science and Engineering, Hunan University, Changsha 410082, Hunan, PR China Key Laboratory of Environmental Biology and Pollution Control (Hunan University), Ministry of Education, Changsha 410082, Hunan, PR China c Faculty of Life Science and Technology, Central South University of Forestry and Technology, Changsha, Hunan 410004, China b

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 30 August 2019 Revised 23 October 2019 Accepted 31 October 2019 Available online xxxx Keywords: Photocatalysis 2D/2D heterostructure Charge transfer Degradation pathways Mechanism

a b s t r a c t Photocatalysis technology is regarded as a promising way for environmental remediation, but rationally designing photocatalysis system with high-speed interfacial charge transfer, sufficient photoabsorption and surface reactive sites is still a challenge. In this study, anchoring single-unit-cell defective Bi2MoO6 on ultrathin g-C3N4 to form 2D/2D heterostructure system is a triple-purpose strategy for highperformance photocatalysis. The defect structure broadens photo-responsive range. The large intimate contact interface area between two monomers promotes charges carrier transfer. The enhanced specific surface area exposes more reactive sites for mass transfer and catalytic reaction. As a result, the obtained heterostructure displays excellent photocatalytic performance for ciprofloxacin (CIP) (0.0126 min 1), which is 3.32 and 2.93 folds higher than Bi2MoO6 and g-C3N4. In addition, this heterostructure retains high-performance for actual wastewaters treatment, and it displays strong mineralization ability. And this heterojunction also exhibits excellent photostability based on cyclic experiment. Mechanism exploration reveals that hole, superoxide radical, and hydroxyl radical are chief reactive species toward CIP degradation, thereby a Z-scheme charge carrier transfer channel is proposed. In addition, the

⇑ Corresponding author at: College of Environmental Science and Engineering, Hunan University, Changsha, Hunan 410082, China. E-mail address: [email protected] (C. Lai). https://doi.org/10.1016/j.jcis.2019.10.116 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

Please cite this article as: B. Li, C. Lai, L. Qin et al., Anchoring single-unit-cell defect-rich bismuth molybdate layers on ultrathin carbon nitride nanosheet with boosted charge transfer for efficient photocatalytic ciprofloxacin degradation, Journal of Colloid and Interface Science, https://doi.org/10.1016/j. jcis.2019.10.116

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B. Li et al. / Journal of Colloid and Interface Science xxx (xxxx) xxx

intermediates and degradation pathways of CIP are tracked by liquid chromatography-triple quadrupole tandem mass spectrometry (LCMS/MS) and three-dimensional excitation-emission matrix fluorescence spectroscopy (3D EEMs). This study paves new way to design and construct atomic level 2D/2D heterojunction system for environment remediation. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction Antibiotic, one of the most significant pharmaceutical groups, has been extensively used in many fields, including physic, stockbreeding and agriculture [1–3]. Unfortunately, the abuse and misuse of antibiotics has caused lots of antibiotics in environment, which results in the appearance of antibiotic resistant bacteria and resistance genes [4]. These antibiotic resistant bacteria and resistance genes are harmful for human, which causes illnesses and deaths in the world every year [5,6]. Therefore, a regenerative, eco-friendly, and inexhaustible technology is pursued to remove antibiotics. Given the earth-abundant of sunlight, the sustainable technology (photocatalysis) with sunlight as driving force is regarded as one of the revolutionary technologies [7]. Substantial semiconductors have been developed and employed in pollution elimination and energy conversion, which include metal oxides, metal chalcogenides, and metal-free catalysts [8]. However, these conventional photocatalysts still remain some challenges such as narrow photoadsorption range, easy photoinduced charge carrier recombination, and limited exposed reactive sites [9]. Therefore, more novel and efficient photocatalysts should be developed to meet these challenges. Aurivillius phase bismuth-based photocatalysts aroused great interest owing to its non-poisonous, appropriate bandgap, and strong visible-light utilization efficiency [10]. Among them, as an Aurivillius oxide made up by alternate stacking (Bi2O2)2+ and (MoO4)2 perovskite layers, Bi2MoO6 (BMO) has become a rising star photocatalyst because of its perovskite-phase laminated structure [11]. Recently, two-dimensional (2D) materials have awakened new platform in photocatalysis application because of its large specific surface area, abbreviated photoexcited charge carrier migration distance, and exposed reactive sites [12]. And previous articles have reported that 2D Bi2MoO6 showed excellent photocatalytic performance for refractory organic pollutants [13]. Although 2D Bi2MoO6 is thought to be a promising catalyst in pollutant elimination, it also exists some bottlenecks to hinder its practical application. On the one hand, the quantum confinement effect appeared in the atomic scale BMO enhances its band gap compared to bulk material, thereby shortening solar light adsorption range [14]. On the other hand, although photoexcited electron-hole separation efficiency within UBMO has an improvement comparing with bulk materials, it still has a lot of room for promotion [15]. In addition, the prepared small sized 2D materials usually have strong mobility, which is ordinarily subjected to rigorous aggregation [16]. Thus, efficacious strategies should be proposed to solve these obstacles. Previous researches have confirmed that surface defect engineering is an effective tactic to regulate property and structure of semiconductor, thereby altering the optical properties [17]. Zhang’s group introduced defects in Sr2Bi2Nb2TiO12 nanosheets, which render it possess whole visible-spectrum absorption response ability [18]. And Zhao et al. reported that tuning defect in ultrathin TiO2 nanosheets can broaden light adsorption range from UV to visible even near-infrared light [19]. In addition, the atomic-escape energy in ultrathin 2D semiconductor is very low, the surface atom can easily abscond to produce defects [20]. Consequently, governing the thickness to form few layers or even

monolayer ultrathin BMO (UBMO) can introduce defects and thus resolve one of the nodi of poor light absorption ability. In order to accelerate the charge carrier separation, designing heterojunction could be a good choice. Thus, another ultrathin polymeric photocatalyst g-C3N4 (UCN) is selected to construct 2D/2D heterojunction owing to its excellent physicochemical properties. Although some g-C3N4/Bi2MoO6 photocatalysts have been prepared [21–23], 2D/2D atomic scale g-C3N4/Bi2MoO6 heterojunction did not report. This 2D/2D atomic scale heterojunction may possess some superiorities comparing with previous reported gC3N4/Bi2MoO6 photocatalyst. Firstly, 2D/2D heterojunction will supply large intimate contact interface area, which is more beneficial for interfacial photogenerated charges carrier separation [24– 26]. Secondly, in this 2D/2D heterojunction, UCN also can serve as support to enhance the dispersion of UBMO, thereby enhancing the specific surface area and supplying massive active sites for catalytic process [27]. Herein, an atomic level 2D/2D defective Bi2MoO6/g-C3N4 (UBN) heterostructure was fabricated via hexadecyl trimethyl ammonium bromide (CTAB) assisted solvothermal method combining with wet-impregnation process. The chemical composition, morphology structure, and photochemical properties were determined by various characterizations. Moreover, the catalytic behavior of atomic scale 2D/2D UBN was evaluated by photodegradation of CIP. The possible photodegradation pathway of CIP was also investigated by LCMS/MS and 3D EEMs. 2. Materials and methods 2.1. Materials Bismuth nitrate (Bi(NO3)3
5H2O), sodiummolybdate dehydrate (Na2MoO4
2H2O), CTAB, dicyandiamide, ammonium chloride (NH4Cl), ethylene glycol, p-benzoquinone (BQ), sodium oxalate (Na2C2O4), isopropanol (IPA), and CIP used as precursors without depuration. 2.2. Preparation 2.2.1. Synthesis of UBMO UBMO was prepared through CTAB-assisted hydrothermal method. 0.4500 g of CTAB, 0.2420 g (1 mmol) of Na2MoO4
2H2O, and 0.9701 g (2 mmol) of Bi(NO3)3
5H2O were weighed and added into 0.04 L of ultrapure water and 0.04 L of ethylene glycol and stirred for 0.5 h. The mixture was poured into 0.1 L of Teflon-lined autoclave, and placed into an oven to react at 120 °C for 24 h. When it cooled down to ambient temperature, the precipitants were obtained through filtration and washed with ultrapure water and absolute alcohol for several times until CTAB was completely removed. Ultimately, the resultant materials were dried completely for further use. 2.2.2. Synthesis of UCN The UCN was prepared through an additive-mediated synthesis [28,29]. 2 g of dicyandiamide and 10 g of NH4Cl was appended into a 0.1 L round flask which contain 0.01 L of deionized water, and the round flask retained at 80 °C for overnight to evaporate excess

Please cite this article as: B. Li, C. Lai, L. Qin et al., Anchoring single-unit-cell defect-rich bismuth molybdate layers on ultrathin carbon nitride nanosheet with boosted charge transfer for efficient photocatalytic ciprofloxacin degradation, Journal of Colloid and Interface Science, https://doi.org/10.1016/j. jcis.2019.10.116

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water for obtaining the highly mixed solid (this step plays important role in whole synthesis procedure). The obtained solid underwent calcination at 550 °C for 4 h (heating rate: 3 °C min 1). Generally, NH4Cl was used as additive material to provide a dynamic gas template, promoting UCN generation. The resultant materials were ground into powers and washed three times, and then dried overnight at 60 °C. 2.2.3. Synthesis of UBN UBN was prepared by wet-impregnation procedure. Generally, the prepared different amount of UBMO and UCN were separately added into 100 mL beaker containing methanol solution and sonicated for 2 h. Afterwards, UBMO mixed solution was dropwise appended into UCN solution, and suffering from sonication for another 2 h and magnetic stirring in a fume hood for overnight. After methanol solution was entirely evaporated, the resultant products were dried at 60 °C overnight. Ultimately, UBN with mass ratio of UBMO to UCN (1:0.2, 1:0.5, 1:1, 1:2 and 1:4) was obtained, which was recorded as UBN-0.2, UBN-0.5, UBN-1, UBN-2, and UBN-4. 2.3. Characterization The atomic and molecular structure of crystal were checked by X-ray diffraction (XRD) using D8 Advance LynxEye array detector equipped with Cu-Ka radiation (k = 0.15418 nm), and the scanning rate was 0.1° s 1 in the 2h range from 10° to 90° (Germany). The element constitution and valence state of as prepared photocatalyst were acquired through X-ray photoelectron spectroscopy (XPS) with Al-Ka X-ray (hm = 1486.6 eV) radiation (ESCALAB 250Xi, America) and binding energy was calibrated with C1s peaks at 284.8 eV. The functional group possessed by as-obtained was collected via Fourier transform infrared spectroscopy (FT-IR) (NICOLET 5700 FT-IR Spectrometer). The specific surface area and pore volume were examined by Brunauer-Emmett-Teller (BET) using ASAP2460 4MP. The thickness of photocatalyst was determined by Atomic Force Microscope (AFM) (BRUKER, ICON2-SYS). The morphologies and crystal lattice parameters were obtained through transmission electron microscopy (TEM) and high resolution TEM (HRTEM) (Tecnai G2 F20 S-TWIN TMP). The reactive oxygen species were tested by electron spin resonce (ESR) employing 5, 5-dimethyl-l-pyrroline N-oxide (DMPO) as a probe on a JES FA200 spectrometer under visible light illumination. The photoluminescence (PL) and time-resolved PL spectra (TRPL) were measured in FLs980 full-function steady-state/transient fluorescence spectrometer.

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2.4. Photocatalytic performance measurement The catalytic properties of single-unit-cell UBMO, UCN, and UBN were assessed through photodegrading CIP under visible light illumination. Before catalytic reaction, 0.1 L of CIP solution containing 0.05 g of UBMO, UCN, and UBN was stirred under dark condition for 1 h to obtain adsorption saturation state. In photodegradation process, 0.004 L of sample was withdrawn and centrifuged to remove photocatalyst at the specified time interval. The CIP concentration was measured through Shimadzu UV-vis spectrophotometer (UV-2770) with the maximum absorption peak of 277 nm. 3. Results and discussion 3.1. Structure and morphology analysis The crystalline phases of UBMO, UCN, and UBN were determined through XRD. The peaks centered at 2h of 28.42°, 32.54°, 46.68°, and 55.58° are in line with the (1 3 1), (0 0 2), (2 6 0), and (3 3 1) planes of orthorhombic BMO (Fig. 1a) [30]. It is worthwhile to note that the peak width of these peaks is wider than these in previous reported articles [31], confirming the as-obtained photocatalyst is ultrathin nanosheet. We further compare the XRD patterns of UBMO with that of BiOBr since CTAB is the precursor to prepare BiOBr, and the result shows that no diffraction peak belonged to BiOBr is detected, confirming that BiOBr does not generate (Fig. S1). However, the (0 0 2) diffraction planes (27.22°) of UCN cannot be found in UBN after combination. The appeared phenomenon can be speculated that the peak position of the (1 3 1) plane of UBMO and (0 0 2) diffraction planes of UCN is quite close (28.42° vs 27.22°), which is hard to exactly distinguish. On the other hand, the (1 3 1) planes in UBMO nanosheet is relatively wide (from 26.02° to 30.66°), it covers (0 0 2) diffraction planes of UCN completely. In order to verify the speculation, the (1 3 1) plane is amplified, it can be found that the peak position of (1 3 1) plane shifts to lower degrees with the increase of UCN content in UBN (from 28.42° to 27.86°), while (0 0 2), (2 6 0), and (3 3 1) planes have no changes, meaning that two peaks overlap and the position of the peaks is offset comparing with two monomers (Fig. 1b). The peak width continuously increases with the addition of UCN, also indicating that the (0 0 2) plane of UCN is overlapped by (3 3 1) planes of UBMO. The change of XRD pattern suggests that UBMO nanosheets and 2D/2D atomic scale heterojunction were prepared successfully.

Fig. 1. (a) The XRD pattern of UBMO, UCN, and UBN with different mass ratio of UBMO to UCN, (b) the amplified region of (1 3 1) planes of orthorhombic BMO.

Please cite this article as: B. Li, C. Lai, L. Qin et al., Anchoring single-unit-cell defect-rich bismuth molybdate layers on ultrathin carbon nitride nanosheet with boosted charge transfer for efficient photocatalytic ciprofloxacin degradation, Journal of Colloid and Interface Science, https://doi.org/10.1016/j. jcis.2019.10.116

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TEM images of UBMO, UCN, and UBN are showed in Fig. 2. The pure UBMO presents ultrathin nanosheet structure with the size of approximate 80 * 100 nm (Fig. 2a). UCN also shows nanosheet structure with some curls, which is unavoidable during the preparation process, and the size of UCN is around 1 * 2 lm (Fig. 2b). In the TEM image of UBN-1, it is observed that some small lamellae attach to the UCN surface after combination, which is ultrathin UBMO nanosheet (Fig. 2c). In HRTEM, two palpable lattice distance of 0.274 and 0.275 nm with angle of 90° are detected, which are ascribed to (2 0 0) and (0 0 2) plans of UBMO, respectively (Fig. 2d and e) [14]. Another marked interplanar spacing of 0.309 nm attributed to (0 0 1) plane of metal Bi is also found, which is because that partial Bi3+ is reduced to metal Bi by the high energy beam of TEM [32] (Fig. 2f). It is worth noting that some structural defects/disor-

ders are observed in UBMO crystal lattice (Fig. S2a). And EPR measurements also show that the characteristic signal with a g-value of 2.004 was detected for UBMO, which is ascribed to oxygen vacancies (Fig. S3) [33]. After combining UBMO with UCN to form hybrid material, the characteristic signal observed in hybrid materials, which further confirms that defects exist in the hybrid materials. The presence of defect can affect the electronic structure of UBMO, in turn, modulating its optical property [34]. EDS element mapping and EDX results show that the hybrid materials contain element Bi, Mo, O, C, and N, attesting the successful combination of UBMO and UCN (Fig. 2g-l and Fig S2b). The thicknesses of UBN-1 are determined by AFM (Fig. 3). The thickness of UCN and UBMO is approximate 2.12 and 0.76 nm, which is in line with the ultrathin g-C3N4 and monolayer Bi2MoO6 [14,35].

Fig. 2. TEM image of UBMO (a), UCN (b), and UBN-1 (c), high resolution TEM of UBN-1 (d), and corresponding enlarged high resolution TEM image (e and f), SEM of UBN-1 (g), and the corresponding EDS elemental mappings of C (h), N (i), O (j), Mo (k), and Bi (l).

Fig. 3. (a) Atomic force microscope image of UBN-1, and (b) the corresponding height profile.

Please cite this article as: B. Li, C. Lai, L. Qin et al., Anchoring single-unit-cell defect-rich bismuth molybdate layers on ultrathin carbon nitride nanosheet with boosted charge transfer for efficient photocatalytic ciprofloxacin degradation, Journal of Colloid and Interface Science, https://doi.org/10.1016/j. jcis.2019.10.116

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As displayed in Fig. 4a, the element of Bi, Mo, O, C, and N coexist in UBN as expected, while only Bi, Mo, and O exist in UBMO. However, there are traces of element Br in UBMO and UBN, this is originated from residual CTAB. In high-resolution picture, the binding energies located at 158.87 and 164.17 eV belong to Bi 4f7/2 and Bi 4f5/2, revealing Bi exist as the form of Bi3+ (Fig. 4b) [36]. Mo 3d is divided into two peaks (235.17 and 232.37 eV), which are related to Mo 3d3/2 and Mo 3d5/2 (Fig. 4c) [37,38]. As showed in Fig. 4d, three peaks at 532.17, 531.22, and 529.52 eV are originated from C@O, OAH, and lattice oxygen (BiAO and MoAO) [5]. In C 1s high resolution spectra, C 1s is split into five peaks at 293.92, 288.82,

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288.27, 286.27, and 284.72 eV. The binding energy located at 288.27 eV is ascribed to sp2 C atoms bonded to N in (NAC@N) aromatic rings (Fig. 4e) [39]. The binding energies located at 288.82, 286.27, and 284.72 eV originate from oxidized carbon CAO, sp2 CANAC, and CAC bonds. The peak at 293.92 eV refers to p-p* excitations between the graphitic layers [40,41]. As for high-resolution N 1s, the fitted peaks at around 398.77, 400.02, 401.02, and 404.47 eV stem from sp2-hybridized nitrogen (CANAC), tertiary nitrogen bonded to three carbon atoms N-(C)3, amino functional groups with a hydrogen atom (CANAH), and p-p* excitations (Fig. 4f) [42]. Unfortunately, after combining UBMO with UCN,

Fig. 4. XPS spectra of UBMO, UCN, and UBN-1: (a) survey spectrum, and high resolution of (b) Bi 4f, (c) Mo 3d, (d) O 1s, (e) C 1s, and (f) N 1s.

Please cite this article as: B. Li, C. Lai, L. Qin et al., Anchoring single-unit-cell defect-rich bismuth molybdate layers on ultrathin carbon nitride nanosheet with boosted charge transfer for efficient photocatalytic ciprofloxacin degradation, Journal of Colloid and Interface Science, https://doi.org/10.1016/j. jcis.2019.10.116

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the binding energy of Mo 3p3/2 locates at 397.97 eV, which is close to 398.87 eV of CANAC, thereby these two peaks cannot be accurately split (Fig. S4). It is found that the binding energy of Bi 4f, Mo 3d, and O 1s in UBN-1 shift to lower position compared to UBMO, while C 1s shifts to higher position compared to UCN, which reveals that electrons transfer from UBMO to the UCN [43]. The absorption peak at 810 cm 1 in FTIR of UCN pertains to breathing mode of the triazine units (Fig. 5) [44], several absorption bands at 1200–1800 cm 1 are attributed to the stretching vibrations of aromatic CAN heterocycles [23], the wide peak located at 3000–3300 cm 1 corresponds to either the absorbed water molecules or stretching mode of NAH [45]. As for UBMO FTIR spectrum, the infrared absorption peak centered at 723 cm 1 and 836 cm 1 belong to the asymmetric stretching mode of MoO6 involving vibrations of the equatorial and apical oxygen atoms [46]. The peaks situated at 1621 and 3436 cm 1 are assigned to OAH stretching and deformation vibrations of absorbed water [47]. After anchoring UBMO ultrathin nanosheet on the surface of UCN, the main absorption bands of UBMO and UCN are all appeared in UBN, testifying the successful combination of UBMO and UCN. However, the infrared absorption peak at 836 cm 1 assigned to UBMO does not appear in hybrid materials since the peak intensity of 836 cm 1 is relative weak. The result of FTIR is

in conformity with XPS, confirming the successful fabrication of UBN. To investigate the physisorption of UBMO, UCN, and UBN, BET was utilized to analyze the specific surface area and pore volume distribution. As displayed in Fig. 6a, UBMO, UCN, and UBN display the classical type IV isotherm coupling with the type H3 hysteresis loops, disclosing the mesoporous property [48,49]. The specific surface area of pristine UBMO and UCN are 31.52 and 25.46 m2 g 1, respectively. However, after immobilizing UBMO into UCN ultrathin nanosheets, the specific surface area of UBN increase with increase of UCN content, and UBN-1 has a significant enhancement (45.47 m2 g 1), the enhanced specific surface area is attributed that the presence of UCN enhances the dispersion of UBMO [50]. However, the specific surface area will decrease if the content of UCN continues to enhance, this is because that the surplus UCN decreases the specific surface area of composites because of its low specific surface area. Furthermore, pore size distributions and pore volume is exhibited in Fig. 6b and Table 1, the results present that the pore size of all photocatalysts distribute at 2–20 nm, further confirming the mesoporous properties. And the pore volume of pure UBMO and UCN are 0.12 and 0.10 cm3 g 1, while the hybrid materials arrives 0.22 cm3 g 1. The increased special surface area and pore volume expose massive reactive sites for catalytic process.

3.2. Adsorption and photocatalytic performance The adsorption capacity of UBMO, UCN, and UBN was determined under dark condition for 60 min. As displayed in Fig. S5, approximate 30.17%, 8.67%, and 22.58% of CIP can be adsorbed by UBMO, UCN, and UB within 60 min. However, according to the results of BET, the specific surface area of UBN is much larger than UBMO, while adsorption performance of UBN is lower than

Table 1 The special surface area, pore volume and pore size of UBMO, UCN, and UBN-1.

Fig. 5. The FTIR spectra of UBMO, UCN, and UBN-1.

Photocatalyst

SSA (m2/g)

PV (m3/g)

PZ (nm)

UBMO UCN UBN-0.2 UBN-0.5 UBN-1 UBN-2 UBN-4

31.52 25.46 32.19 37.42 45.47 30.61 26.83

0.10 0.12 0.13 0.20 0.22 0.18 0.10

12.72 19.09 14.56 20.53 19.25 21.94 12.55

SSA denotes special surface area, PV denotes Pore volume, and PZ denotes pore size.

Fig. 6. (a) N2 adsorption-desorption isotherm of UBMO, UCN, (b) and UNB-1, corresponding pore size distribution.

Please cite this article as: B. Li, C. Lai, L. Qin et al., Anchoring single-unit-cell defect-rich bismuth molybdate layers on ultrathin carbon nitride nanosheet with boosted charge transfer for efficient photocatalytic ciprofloxacin degradation, Journal of Colloid and Interface Science, https://doi.org/10.1016/j. jcis.2019.10.116

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UBMO, indicating that specific surface area is not the sole factor that determines adsorption ability. Furthermore, XPS result reveals that the surface of atomic scale UBMO contains a small part of Br-, which may make it negatively charged, while CIP is a positively charged species, the electrostatic attraction between UBMO and CIP may promotes adsorption capacity. Thus the Zeta potentials of UBMO, UCN, and UBN-1 were determined and the Zeta potentials of UBMO, UCN, and UBN-1 are 18.8, 24.3, and 5.38 mV, thereby electrostatic attraction is also not the main reason that determines the adsorption ability (Table S1). The highest adsorption ability of UBMO is attributed to its 2D ultrathin nanosheet structure, which contains ultrahigh proportion of surface atoms that provides more active sites to promote adsorption progress. What’s more, the as-obtained monolayer UBMO possesses open surface, which is more beneficial for pollutant adsorption [51]. In this work, the catalytic properties of prepared catalysts were tested by degrading CIP. Before irradiation, the suspension made up by photocatalyst and CIP solution was placed into a dark condition to attain the adsorption-desorption equilibrium. As displayed in Fig. 7a, no apparent photodegradation efficiency is observed in the blank experiment, revealing that the direct photolysis does not degrade CIP. The photocatalytic efficiency of CIP for pure UBMO and UCN are 35.62% and 39.52% within 120 min. However, after anchoring UBMO nanosheet on the surface of UCN, the removal

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efficiency of CIP has an improvement. With increase of the content of UCN, the photodegradation efficiency of UBN first increases and then decreases, UBN-1 possesses the best photocatalytic performance. Meanwhile, the photocatalytic performance of neat UBMO and UCN treated by the same method as the combined system also investigated, and the result shows that the treated UBMO and UCN showed similar photocatalytic performance with UBMO and UCN (did not treat by the same method as the combined system), which indicates that the conditions used to combine the materials do not dominate the improved performance (Fig. S6). The enhanced photocatlytic performance is attributed to the interaction between UBMO and UCN, which accelerates the photoexcited charge carrier separation and migration. What’s more, the pseudo first-order kinetic model is utilized to simulate the experiment results (Fig. 7b) [52,53]. The apparent rate constants of UBMO, UCN, UBN-0.2, UBN-0.5, UBN-1, UBN-2, and UBN-4 are 0.0038, 0.0063, 0.0083, 0.0126, 0.0102, 0.0070, and 0.0043, further confirming that UBM-1 possesses the best removal efficiency (Table S2). The prepared UBN-1 also displays much higher CIP degradation efficiency than that of some commercial photocatalysts, including TiO2 and ZnO (Fig. S7). Additionally, the photocatalytic performance also compared with other recently developed photocatalysts, and the result shows that the catalytic activity of 2D/2D UBN heterostructure is competitive (Table S3).

Fig. 7. (a) The photocatalytic performances of UBMO, UCN, and UBN with different mass ratio of UBMO to UCN for CIP degradation, (b) the corresponding ln (C/C0) versus the reaction time, (c) The photocatalytic performance of UBN-1 by using various water sources, (d) Cycling test for the photodegradation of CIP by UBN-1.

Please cite this article as: B. Li, C. Lai, L. Qin et al., Anchoring single-unit-cell defect-rich bismuth molybdate layers on ultrathin carbon nitride nanosheet with boosted charge transfer for efficient photocatalytic ciprofloxacin degradation, Journal of Colloid and Interface Science, https://doi.org/10.1016/j. jcis.2019.10.116

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3.3. The applicability of photocatalyst

3.4. Visible light catalytic performance enhancement investigation

As we know, there are many coexisted pollutants in practical wastewater, thus it is necessary to explore the photocatalytic activity of UBN-1 for practical wastewater treatment [54,55]. Therefore, six typical water sources are selected to carry out the photodegradation experiment. A small amount of photocatalytic efficiency reduction can be found by using lake water, river water, tap water, domestic water, and medical water as water sources (Fig. 7c). The appeared phenomenon could be accounted for by following three + – 3aspects: (1) many inorganic ions such as Cl-, SO24 , NO3, PO4 , K , + 2+ 2+ Na , Ca , and Mg coexist in the various water sources, these inorganic ions could modulate the superficial physical and chemical properties of material, thereby playing a negative role in catalytic performance; (2) the lake water, river water, domestic water, and medical water possess high turbidity, the higher the turbidity is, the more difficult it is for light to enter into liquid, thereby the quantum efficiency is low; (3) more natural organic matters exist in the different typical water, which competes with CIP for the limited reactive sites, thereby having a negative influence on catalysis (Table S4) [56]. The successive cycle experiments were implemented to evaluate the photostability of UBN-1. The degradation efficiency of UBN-1 towards CIP degradation maintains high level after six consecutive operations, indicating that UBN-1 is relative stable (Fig. 7d). Furthermore, XRD pattern of the used photocatalyst is identical with UBN-1 (Fig. S8), demonstrating that UBN-1 possesses the high photostability, which makes it have an advantage in real environment application.

The optical properties of UBMO, UCN, and UBN were checked via DRS technology. The original UCN has a photoadsorption edge at approximate 470 nm, while the adsorption edge of UBMO can run up to 550 nm and the absorption strength also has an enhancement (Fig. 8a). The light adsorption range of UBMO is larger than bulk BMO, which is irreconcilable with quantum confinement effect. The appeared phenomenon corroborates that defects exist in UBMO. The better optical properties the photocatalyst possess, the better photocatalytic performance it owns. The bandgap of different materials was determined by Kubelka-Munk equation (Supporting Information) [57–59]. The bandgaps of UBMO, UBN-0.2, UBN-0.5, UBN-1, UBN-2, UBN-4, UCN are 2.41, 2.47, 2.50, 2.54, 2.59, 2.65, and 2.69 eV, respectively (Fig. 8b and Fig. S9). It can be observed that the band gap of UBN increase with increase of UCN content. The valence band (VB) and conduction band (CB) of photocatalyst are obtained through XPS-VB technology and Mott–Schottky curve. Obviously, UBMO and UCN is n type semiconductor, thereby the flat band potential is identic with Fermi level. The flat band of UBMO and UCN are 0.03, and 0.76 V vs. Ag/AgCl (0.17 and 0.56 V vs. NHE). In addition, the energy gap between VB and Fermi level are 2.10 and 1.62 eV, which is determined by XPS-VB technology. Thereby, VB and CB of UBMO and UCN are +2.27 (+1.06) and 0.14 ( 1.63) eV (Fig. S10) [60]. TPC and EIS were thought to be the impactful tool to assess charge carrier separation and transport behavior [61]. As shown in Fig. 8c, the original UBMO and UCN exhibits weak photocurrent

Fig. 8. (a) The UV–vis DRS spectra of UBMO, UCN, and UBN with different mass ratio of UBMO to UCN, (b) the band gap of UBMO and UCN, (c) The photocurrent responses of UBMO, UCN, and UBN-1 samples, and (d) EIS spectra of UBMO, UCN, and UBN-1 under dark and visible light irradiation.

Please cite this article as: B. Li, C. Lai, L. Qin et al., Anchoring single-unit-cell defect-rich bismuth molybdate layers on ultrathin carbon nitride nanosheet with boosted charge transfer for efficient photocatalytic ciprofloxacin degradation, Journal of Colloid and Interface Science, https://doi.org/10.1016/j. jcis.2019.10.116

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density, while photocurrent density of UBN-1 has an enhancement compared with other two monomers. This is because that the interaction between UBMO and UCN greatly inhibits the recombination of photogenerated carriers. As showed in EIS image, UBN-1 possesses the smallest arc radius, indicating that UBN-1 has a lower resistance and fast electron transfer channel (Fig. 8d). As displayed in Fig. 9a, UBMO and UCN exhibit strong PL intensity at the emission wavelength of 415 and 458 nm, respectively. However, after combining UBMO and UCN, the PL intensity decreases, this is because the interaction between these two components accelerates charge carrier separation and transfer, thereby reducing the recombination population of charge carrier. In order to further discern photoinduced charge separation and transport behavior, TRPL was employed and the result was displayed in Fig. 9b-d. According to equation (Supporting Information), the average fluorescence lifetime of UBMO and UCN are 3.12 and 4.53 ns, while UBN-1 has a longer average fluorescence lifetime (5.65 ns) than other two components. The longer average fluorescence lifetime indicates that more charge carrier could participate in catalytic reaction [62]. 3.5. Mechanism exploration Reactive oxygen species play vital role in photocatalytic process and should be investigated. In this study, Na2C2O4, BQ, and IPA are separately selected as sacrificial agent of h+, O–2 and OH [63]. As displayed, a significant inhibition effect can be observed when Na2C2O4 is added into reaction system (only 44.5% of CIP is removed), indicating that hole is the important active specie in catalytic pro-

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cess (Fig. 10a). When BQ is appended into system, approximate 55.3% of CIP is degraded, revealing O–2 is also the critical active specie that determines the photocatalytic efficiency. In addition, IPA exerts some restrained effect on CIP degradation, which also suggests that OH can generate in catalytic process. ESR technique was utilized to further ascertain the formation of aforementioned active radicals. As displayed in Fig. 10b-c, when UBN-1 exposes to visible light, four obvious signals attributed to DMPO-O–2 are detected, and the intensity of signals increases with constant extension of illumination time, testifying the formation of O–2. Besides, four characteristic peaks ascribed to DMPO-OH are also found, further confirming the formation of OH. The combined action of hole, O–2, and OH determines the catalytic activity of as-prepared UBN-1. In the light of aforementioned analysis, the catalytic mechanism of UBN-1 for pollutant degradation is displayed in Scheme 1. Based on our experiment and previous articles, the direct Z-scheme mechanism will be suitable for our photocatalytic system [64–66]. Under visible light illumination, both UBMO and UCN can produce photoexcited electrons and holes owing to their suitable bandgap. The electrons in CB of UBMO recombine with the holes in the VB of UCN [67,68], and the electrons in CB of UCN and holes in UBMO are left, thereby accelerating the charge carrier transfer and retaining the high redox ability. The photoinduced electrons in CB of UCN react with O2 to generate O–2 because CB of UCN ( 1.63 eV) is more negative than the standard reduction potential of O2/O–2 ( 0.33 eV) [69]. However, the photogenerated holes in the VB of UBMO do not possess ability to oxidize water

Fig. 9. (a) The photoluminescence spectra of UBMO, UCN, and UBN-1, (b-d) and the time-resolved photoluminescence profiles of UBMO, UCN, and UBN-1.

Please cite this article as: B. Li, C. Lai, L. Qin et al., Anchoring single-unit-cell defect-rich bismuth molybdate layers on ultrathin carbon nitride nanosheet with boosted charge transfer for efficient photocatalytic ciprofloxacin degradation, Journal of Colloid and Interface Science, https://doi.org/10.1016/j. jcis.2019.10.116

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Fig. 10. (a) Effect of different quenchers on CIP photodegradation, DMPO spin-trapping ESR spectra for UBN-1 photocatalyst: (b) in aqueous dispersion for DMPO-O–2, (c) and in methanol dispersion for DMPO-OH.

to form OH, because VB of UBMO (+2.27 eV) are more negative than the standard oxidation potential of OH–/OH (+2.40 eV) [70]. But aforementioned experiment results and characterizations have affirmed that OH indeed generated. Thus, OH could be generated by photogenerated electron through multipath reaction [71]. The formed O–2, OH and hole can degrade pollutant. In addition, the reason why this 2D/2D system does not conform to type II heterojunction also is analyze. If this system conforms to type II heterojunction, the electrons in the CB of UCN flow to CB of UBMO and the holes in the VB of UBMO migrate to VB of UCN, O–2 and OH cannot be formed because of their unmatched CB potential of UBMO (0.14 vs 0.33 eV) and VB potential of UCN (+2.27 vs +2.40 eV) [72]. Thus, this photocatalytic system conforms to Z-scheme mechanism and in this face to face 2D/2D defective Z-scheme system, the photogenerated electron and hole transfer efficiency is enhanced, the photoresponse range is broadened, thereby obtaining the excellent photocatalytic performance. 3.6. Photodegradation pathways The mineralization efficiency of various materials was determined by total organic carbon measurement (TOC). As displayed in Fig. S11, the mineralization efficiency for UBMO, UCN, and UBN-1 are 19.27%, 11.73%, and 40.56%, further revealing the excellent photocatalytic performance of UBN. Significantly, the mineralization efficiency of UBN-1 is much lower than photodegradation efficiency (Fig. 7a), indicating that some intermediates are gener-

ated during the photodegradation process. To deeply explore the photodegradation process of CIP, LCMS/MS and 3D EEMs are employed. The fluorescence peak located at Excitation/Emission = 220–350/380–550 nm is observed in the original solution and adsorbed solution, which belongs to humic acid region (Fig. S12). When photocatalytic degradation by UBN-1, the peak intensity is reduced, revealing CIP is gradually degraded. Meanwhile, the new fluorescence peak centered at Excitation/ Emission = 220–260/330–400 nm is detected, indicating the small-molecule intermediates are generated during photocatalytic process. In order to discern the intermediate products and photodegradation pathway of CIP, LCMS/MS was employed (Fig. S13-16, Table S5). Based on LCMS/MS result, eighteen intermediate products are determined in the photocatalytic reaction, thereby four photodegradation pathways are proposed and displayed in Scheme 2. As for pathway 1, the original CIP is suffered from defluorination and rupture of piperazine ring to generate A1 (m/z 344), and then a carbonyl is peeled off from A1 to produce A2. In term of pathway 2, CIP firstly undergo defluorination to form A3, and then a CAH bond is substituted by C@O to produce A4. After that, A5 and A7 are generated by elimination of –C2H4 and –COOH from A4, and A6 was produced by dehydroxylation of A5 [73]. In the pathway 3, the -F group is firstly substituted by hydroxyl to generate A9, and then the dihydroquinoline group of A9 is further attacked by OH to come into being A10. Subsequently, the ring of dihydroquinoline group of A10 is opened and yield A11 [74]. The pathway 4 is the step-by-step oxidation of the piper-

Please cite this article as: B. Li, C. Lai, L. Qin et al., Anchoring single-unit-cell defect-rich bismuth molybdate layers on ultrathin carbon nitride nanosheet with boosted charge transfer for efficient photocatalytic ciprofloxacin degradation, Journal of Colloid and Interface Science, https://doi.org/10.1016/j. jcis.2019.10.116

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Scheme 1. The photocatalytic degradation mechanism scheme of UBN-1 heterojunctions under visible light irradiation (>420 nm).

Scheme 2. The possible photodegradation pathway of CIP by UBN-1 under visible light irradiation.

Please cite this article as: B. Li, C. Lai, L. Qin et al., Anchoring single-unit-cell defect-rich bismuth molybdate layers on ultrathin carbon nitride nanosheet with boosted charge transfer for efficient photocatalytic ciprofloxacin degradation, Journal of Colloid and Interface Science, https://doi.org/10.1016/j. jcis.2019.10.116

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azine side chain of CIP, which leads to the formation of dialdehyde derivative A8. And the formaldehyde group of A8 is exfoliated to yield A12. And then the other formaldehyde group and secondary amine nitrogen of A12 are abscised, which leads to the formation of A13, A14, A15 and A16. Whereafter, the -F group is fall off from A16, and A17 appear accordingly. Finally, A17 is further assaulted by active radicals to engender A18 [75]. With continuous extension of reaction time, all of these intermediate products are completely mineralized into H2O and CO2 [76]. 4. Conclusion In this work, the ultrathin 2D/2D defective UBN heterostructure photocatalyst is prepared by facile CTAB-assisted hydrothermal route coupling with simple wet-impregnation procedure. The asprepared UBN possesses several superiorities: (1) the introduction of defects in UBMO strengthen the photoresponse ability, (2) face to face 2D/2D system promotes photogenerated charge carrier separation and migration, (3) the enhanced specific surface area exposes massive reactive sites for mass transfer and catalytic process. As expected, UBN-1 possesses boosted photocatalytic performance, which is not only higher than bare UBMO and UCN, but also equal to even superior to newly developed photocatalyst. The designed photocatalyst also displays high degradation efficiency in practical wastewater treatment. Additionally, this atomic level 2D/2D heterojunction still maintains high photostability. This work well demonstrates that the ultrathin 2D/2D heterostructure can promote the catalytic activity, and many ultrathin 2D/2D photocatalysts should be developed in the future. Thus, this study not only prepares a novel photocatalyst with excellent performance, but also provides a potential strategy to develop highperformance photocatalysts for pollutant elimination. Declaration of Competing Interest The author declare that there is no conflict of interest.

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[16]

[17]

Acknowledgement [18]

This study was financially supported by the Program for the National Natural Science Foundation of China (51779090, 51408206, 41601272, 51879101, 51579098, 51521006, 51809090, 51278176, 51378190), Science and Technology Plan Project of Hunan Province (2017SK2243, 2018SK20410, 2016RS3026), the National Program for Support of Top–Notch Young Professionals of China, China (2014), the Program for New Century Excellent Talents in University (NCET-13-0186), the Program for Changjiang Scholars and Innovative Research Team in University (IRT-13R17), the Fundamental Research Funds for the Central Universities (531107050978, 531107051080, 531109200027). Appendix A. Supplementary material

[19]

[20] [21]

[22]

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Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.10.116.

[24]

References

[25]

[1] Z. Cetecioglu, B. Ince, M. Gros, S. Rodriguez-Mozaz, D. Barcelo, D. Orhon, O. Ince, Chronic impact of tetracycline on the biodegradation of an organic substrate mixture under anaerobic conditions, Water Res. 47 (2013) 2959– 2969. [2] L. Qin, G. Zeng, C. Lai, D. Huang, C. Zhang, P. Xu, T. Hu, X. Liu, M. Cheng, Y. Liu, L. Hu, Y. Zhou, A visual application of gold nanoparticles: Simple, reliable and sensitive detection of kanamycin based on hydrogen-bonding recognition, Sens. Actuat. B: Chem. 243 (2017) 946–954. [3] C. Lai, B. Li, M. Chen, G. Zeng, D. Huang, L. Qin, X. Liu, M. Cheng, J. Wan, C. Du, F. Huang, S. Liu, H. Yi, Simultaneous degradation of P-nitroaniline and electricity

[26]

[27]

[28]

generation by using a microfiltration membrane dual-chamber microbial fuel cell, Int. J. Hydrogen Energ. 43 (2018) 1749–1757. J. Pei, H. Yao, H. Wang, J. Ren, X. Yu, Comparison of ozone and thermal hydrolysis combined with anaerobic digestion for municipal and pharmaceutical waste sludge with tetracycline resistance genes, Water Res. 99 (2016) 122–128. B. Li, C. Lai, P. Xu, G. Zeng, D. Huang, L. Qin, H. Yi, M. Cheng, L. Wang, F. Huang, S. Liu, M. Zhang, Facile synthesis of bismuth oxyhalogen-based Z-scheme photocatalyst for visible-light-driven pollutant removal: Kinetics, degradation pathways and mechanism, J. Clean. Prod. 225 (2019) 898–912. L. Qin, Z. Zeng, G. Zeng, C. Lai, A. Duan, R. Xiao, D. Huang, Y. Fu, H. Yi, B. Li, X. Liu, S. Liu, M. Zhang, D. Jiang, Cooperative catalytic performance of bimetallic Ni-Au nanocatalyst for highly efficient hydrogenation of nitroaromatics and corresponding mechanism insight, Appl. Catal. B: Environ. 259 (2019) 118035. C. Lai, M. Zhang, B. Li, D. Huang, G. Zeng, L. Qin, X. Liu, H. Yi, M. Cheng, L. Li, Z. Chen, L. Chen, Fabrication of CuS/BiVO4 (040) binary heterojunction photocatalysts with enhanced photocatalytic activity for Ciprofloxacin degradation and mechanism insight, Chem. Eng. J. 358 (2019) 891–902. B. Li, C. Lai, G. Zeng, D. Huang, L. Qin, M. Zhang, M. Cheng, X. Liu, H. Yi, C. Zhou, F. Huang, S. Liu, Y. Fu, Black phosphorus, a rising star 2D nanomaterial in the post-graphene era: synthesis, properties, modifications, and photocatalysis applications, Small 15 (2019) e1804565. J. Di, C. Zhu, M. Ji, M. Duan, R. Long, C. Yan, K. Gu, J. Xiong, Y. She, J. Xia, H. Li, Z. Liu, Defect-rich Bi12O17Cl2 nanotubes self-accelerating charge separation for boosting photocatalytic CO2 reduction, Angew. Chem. Int. Edit. 130 (2018) 15063–15067. Y. Ma, Y. Jia, Z. Jiao, M. Yang, Y. Qi, Y. Bi, Hierarchical Bi2MoO6 nanosheet-built frameworks with excellent photocatalytic properties, Chem. Commun. 51 (2015) 6655. M. Wu, Y. Wang, Y. Xu, J. Ming, M. Zhou, R. Xu, Q. Fu, Y. Lei, Self-supported Bi2MoO6 nanowall for photoelectrochemical water splitting, ACS Appl. Mater. Interfaces 9 (2017) 23647–23653. Y. Sun, S. Gao, F. Lei, Y. Xie, Atomically-thin two-dimensional sheets for understanding active sites in catalysis, Chem. Soc. Rev. 44 (2015) 623–636. J. Bai, Y. Li, X. Li, L. Liu, Facile preparation of 2D Bi2MoO6 nanosheets–RGO composites with enhanced photocatalytic activity, New J. Chem. 41 (2017) 7783–7790. K. Jing, J. Xiong, N. Qin, Y. Song, L. Li, Y. Yu, S. Liang, L. Wu, Development and photocatalytic mechanism of monolayer Bi2MoO6 nanosheets for the selective oxidation of benzylic alcohols, Chem. Commun. 53 (2017) 8604–8607. Y. Li, Y.-L. Li, B. Sa, R. Ahuja, Review of two-dimensional materials for photocatalytic water splitting from a theoretical perspective, Catal. Sci. Technol. 7 (2017) 545–559. H. Yu, Y. Xue, B. Huang, L. Hui, C. Zhang, Y. Fang, Y. Liu, Y. Zhao, Y. Li, H. Liu, Y. Li, Ultrathin nanosheet of graphdiyne-supported palladium atom catalyst for efficient hydrogen production, iScience 11 (2019) 31–41. S. Gao, B. Gu, X. Jiao, Y. Sun, X. Zu, F. Yang, W. Zhu, C. Wang, Z. Feng, B. Ye, Y. Xie, Highly efficient and exceptionally durable CO2 photoreduction to methanol over freestanding defective single-unit-cell bismuth vanadate layers, J. Am. Chem. Soc. 139 (2017) 3438–3445. H. Yu, J. Li, Y. Zhang, S. Yang, K. Han, F. Dong, T. Ma, H. Huang, Three-in-one oxygen vacancies: whole visible-spectrum absorption, efficient charge separation, and surface site activation for robust CO2 photoreduction, Angew. Chem. Int. Edit 58 (2019) 3880–3884. Y. Zhao, Y. Zhao, R. Shi, B. Wang, G.I.N. Waterhouse, L.Z. Wu, C.H. Tung, T. Zhang, Tuning oxygen vacancies in ultrathin TiO2 nanosheets to boost photocatalytic nitrogen fixation up to 700 nm, Adv. Mater. 31 (2019) e1806482. J. Di, J. Xiong, H. Li, Z. Liu, Ultrathin 2D photocatalysts: Electronic-structure tailoring, hybridization, and applications, Adv. Mater. 30 (2018) 1704548. Y. Tian, F. Cheng, X. Zhang, F. Yan, B. Zhou, Z. Chen, J. Liu, F. Xi, X. Dong, Solvothermal synthesis and enhanced visible light photocatalytic activity of novel graphitic carbon nitride–Bi2MoO6 heterojunctions, Powder Technol. 267 (2014) 126–133. F. Opoku, K.K. Govender, C.G.C.E.V. Sittert, P.P. Govender, Insights into the photocatalytic mechanism of mediator-free direct Z-scheme g-C3N4/ Bi2MoO6(010) and g-C3N4/Bi2WO6(010) heterostructures: a hybrid density functional theory study, Appl. Surf. Sci. 427 (2018) 487–498. H. Li, J. Liu, W. Hou, N. Du, R. Zhang, X. Tao, Synthesis and characterization of gC3N4/Bi2MoO6 heterojunctions with enhanced visible light photocatalytic activity, Appl. Catal. B: Environ. 160–161 (2014) 89–97. B. Ye, X. Han, M. Yan, H. Zhang, F. Xi, X. Dong, J. Liu, Fabrication of metal-free two dimensional/two dimensional homojunction photocatalyst using various carbon nitride nanosheets as building blocks, J. Colloid Interface Sci. 507 (2017) 209–216. E. Liu, J. Chen, Y. Ma, J. Feng, J. Jia, J. Fan, X. Hu, Fabrication of 2D SnS2/g-C3N4 heterojunction with enhanced H2 evolution during photocatalytic water splitting, J. Colloid Interface Sci. 524 (2018) 313–324. Z. Sun, Z. Yu, Y. Liu, C. Shi, M. Zhu, A. Wang, Construction of 2D/2D BiVO4/gC3N4 nanosheet heterostructures with improved photocatalytic activity, J. Colloid Interface Sci. 533 (2019) 251–258. B. Chen, P. Li, S. Zhang, W. Zhang, X. Dong, F. Xi, J. Liu, The enhanced photocatalytic performance of Z-scheme two-dimensional/two-dimensional heterojunctions from graphitic carbon nitride nanosheets and titania nanosheets, J. Colloid Interface Sci. 478 (2016) 263–270. J. Yan, C. Zhou, P. Li, B. Chen, S. Zhang, X. Dong, F. Xi, J. Liu, Nitrogen-rich graphitic carbon nitride: Controllable nanosheet-like morphology, enhanced

Please cite this article as: B. Li, C. Lai, L. Qin et al., Anchoring single-unit-cell defect-rich bismuth molybdate layers on ultrathin carbon nitride nanosheet with boosted charge transfer for efficient photocatalytic ciprofloxacin degradation, Journal of Colloid and Interface Science, https://doi.org/10.1016/j. jcis.2019.10.116

B. Li et al. / Journal of Colloid and Interface Science xxx (xxxx) xxx

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

[50]

[51]

[52]

visible light absorption and superior photocatalytic performance, Colloids Surfaces A. 508 (2016) 257–264. X. Han, A. Yuan, C. Yao, F. Xi, J. Liu, X. Dong, Synergistic effects of phosphorous/sulfur co-doping and morphological regulation for enhanced photocatalytic performance of graphitic carbon nitride nanosheets, J. Mater. Sci. 54 (2018) 1593–1605. S. Li, S. Hu, W. Jiang, J. Zhang, K. Xu, Z. Wang, In situ construction of WO3 nanoparticles decorated Bi2MoO6 microspheres for boosting photocatalytic degradation of refractory pollutants, J. Colloid Interface Sci. 556 (2019) 335– 344. H. Shi, J. Fan, Y. Zhao, X. Hu, X. Zhang, Z. Tang, Visible light driven CuBi2O4/ Bi2MoO6 p-n heterojunction with enhanced photocatalytic inactivation of E. coli and mechanism insight, J. Hazard. Mater. 381 (2019) 121006. G. Yang, W. Miao, Z. Yuan, Z. Jiang, B. Huang, P. Wang, J. Chen, Bi quantum dots obtained via in situ photodeposition method as a new photocatalytic CO2 reduction cocatalyst instead of noble metals: Borrowing redox conversion between Bi2O3 and Bi, Appl. Catal. B: Environ. 237 (2018) 302–308. F. Lei, Y. Sun, K. Liu, S. Gao, L. Liang, B. Pan, Y. Xie, Oxygen vacancies confined in ultrathin indium oxide porous sheets for promoted visible-light water splitting, J. Am. Chem. Soc. 136 (2014) 6826–6829. J. Xie, J. Zhang, S. Li, F. Grote, X. Zhang, H. Zhang, R. Wang, Y. Lei, B. Pan, Y. Xie, Controllable disorder engineering in oxygen-incorporated MoS2 ultrathin nanosheets for efficient hydrogen evolution, J. Am. Chem. Soc. 135 (2013) 17881–17888. D. Jiang, T. Wang, Q. Xu, D. Li, S. Meng, M. Chen, Perovskite oxide ultrathin nanosheets/g-C3N4 2D–2D heterojunction photocatalysts with significantly enhanced photocatalytic activity towards the photodegradation of tetracycline, Appl. Catal. B: Environ. 201 (2017) 617–628. Y. Zhang, S.-J. Park, Au–pd bimetallic alloy nanoparticle-decorated BiPO4 nanorods for enhanced photocatalytic oxidation of trichloroethylene, J. Catal. 355 (2017) 1–10. Y. Zhang, S.-J. Park, Facile construction of MoO3@ZIF-8 core-shell nanorods for efficient photoreduction of aqueous Cr (VI), Appl. Catal. B: Environ. 240 (2019) 92–101. Y. Zhang, S.-J. Park, Formation of hollow MoO3/SnS2 heterostructured nanotubes for efficient light-driven hydrogen peroxide production, J. Mater. Chem. A 6 (2018) 20304–20312. J. Jiang, J. Yu, S. Cao, Au/PtO nanoparticle-modified g-C3N4 for plasmonenhanced photocatalytic hydrogen evolution under visible light, J. Colloid Interface Sci. 461 (2016) 56–63. J. Fan, H. Qin, S. Jiang, Mn-doped g-C3N4 composite to activate peroxymonosulfate for acetaminophen degradation: the role of superoxide anion and singlet oxygen, Chem. Eng. J. 359 (2019) 723–732. L. Li, C. Lai, F. Huang, M. Cheng, G. Zeng, D. Huang, B. Li, S. Liu, M. Zhang, L. Qin, M. Li, J. He, Y. Zhang, L. Chen, Degradation of naphthalene with magnetic biochar activate hydrogen peroxide: synergism of bio-char and Fe-Mn binary oxides, Water Res. 160 (2019) 238–248. L. Qin, D. Huang, P. Xu, G. Zeng, C. Lai, Y. Fu, H. Yi, B. Li, C. Zhang, M. Cheng, C. Zhou, X. Wen, In-situ deposition of gold nanoparticles onto polydopaminedecorated g-C3N4 for highly efficient reduction of nitroaromatics in environmental water purification, J. Colloid Interface Sci. 534 (2019) 357–369. Y. Fang, Y. Xue, L. Hui, H. Yu, Y. Liu, C. Xing, F. Lu, F. He, H. Liu, Y. Li, In situ growth of graphdiyne based heterostructure: toward efficient overall water splitting, Nano Energy 59 (2019) 591–597. Y. Zhang, J. Di, P. Ding, J. Zhao, K. Gu, X. Chen, C. Yan, S. Yin, J. Xia, H. Li, Ultrathin g-C3N4 with enriched surface carbon vacancies enables highly efficient photocatalytic nitrogen fixation, J. Colloid Interface Sci. 553 (2019) 530–539. C. Chang, L. Zhu, S. Wang, X. Chu, L. Yue, Novel mesoporous graphite carbon nitride/BiOI heterojunction for enhancing photocatalytic performance under visible-light irradiation, ACS Appl. Mater. Interfaces 6 (2014) 5083–5093. J. Ke, X. Duan, S. Luo, H. Zhang, H. Sun, J. Liu, M. Tade, S. Wang, UV-assisted construction of 3D hierarchical rGO/Bi2MoO6 composites for enhanced photocatalytic water oxidation, Chem. Eng. J. 313 (2017) 1447–1453. J. Guo, L. Shi, J. Zhao, Y. Wang, K. Tang, W. Zhang, C. Xie, X. Yuan, Enhanced visible-light photocatalytic activity of Bi2MoO6 nanoplates with heterogeneous Bi2MoO6-x@Bi2MoO6 core-shell structure, Appl. Catal. B: Environ. 224 (2018) 692–704. H. Xue, Y. Fang, L. Zeng, X. He, F. Luo, R. Liu, J. Liu, Q. Chen, M. Wei, Q. Qian, Facile synthesis of hierarchical lychee-like Zn3V3O8@C/rGO nanospheres as high-performance anodes for lithium ion batteries, J. Colloid Interface Sci. 533 (2019) 627–635. Y. Zhang, S.-J. Park, Incorporation of RuO2 into charcoal-derived carbon with controllable microporosity by CO2 activation for high-performance supercapacitor, Carbon 122 (2017) 287–297. M. Gu, U. Farooq, S. Lu, X. Zhang, Z. Qiu, Q. Sui, Degradation of trichloroethylene in aqueous solution by rGO supported nZVI catalyst under several oxic environments, J. Hazard. Mater. 349 (2018) 35–44. Y. Zhou, Y. Zhang, M. Lin, J. Long, Z. Zhang, H. Lin, J.C. Wu, X. Wang, Monolayered Bi2WO6 nanosheets mimicking heterojunction interface with open surfaces for photocatalysis, Nat. Commun. 6 (2015) 8340. X. Zhou, C. Lai, D. Huang, G. Zeng, L. Chen, L. Qin, P. Xu, M. Cheng, C. Huang, C. Zhang, C. Zhou, Preparation of water-compatible molecularly imprinted thiolfunctionalized activated titanium dioxide: Selective adsorption and efficient photodegradation of 2, 4-dinitrophenol in aqueous solution, J. Hazard. Mater. 346 (2018) 113–123.

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[53] Y. Fu, L. Qin, D. Huang, G. Zeng, C. Lai, B. Li, J. He, H. Yi, M. Zhang, M. Cheng, X. Wen, Chitosan functionalized activated coke for Au nanoparticles anchoring: Green synthesis and catalytic activities in hydrogenation of nitrophenols and azo dyes, Appl. Catal. B: Environ. 255 (2019) 117740. [54] H. Wang, Y. Wu, M. Feng, W. Tu, T. Xiao, T. Xiong, H. Ang, X. Yuan, J.W. Chew, Visible-light-driven removal of tetracycline antibiotics and reclamation of hydrogen energy from natural water matrices and wastewater by polymeric carbon nitride foam, Water Res. 144 (2018) 215–225. [55] C. Lai, S. Liu, C. Zhang, G. Zeng, D. Huang, L. Qin, X. Liu, H. Yi, R. Wang, F. Huang, B. Li, T. Hu, Electrochemical Aptasensor based on sulfur-nitrogen codoped ordered mesoporous carbon and thymine-Hg2+-thymine mismatch structure for Hg2+ detection, ACS Sensors 3 (2018) 2566–2573. [56] F. Chen, Q. Yang, Y. Zhong, H. An, J. Zhao, T. Xie, Q. Xu, X. Li, D. Wang, G. Zeng, Photo-reduction of bromate in drinking water by metallic Ag and reduced graphene oxide (RGO) jointly modified BiVO4 under visible light irradiation, Water Res. 101 (2016) 555–563. [57] H. Yi, M. Yan, D. Huang, G. Zeng, C. Lai, M. Li, X. Huo, L. Qin, S. Liu, X. Liu, B. Li, H. Wang, M. Shen, Y. Fu, X. Guo, Synergistic effect of artificial enzyme and 2D nano-structured Bi2WO6 for eco-friendly and efficient biomimetic photocatalysis, Appl. Catal. B: Environ. 250 (2019) 52–62. [58] M. Zhang, C. Lai, B. Li, D. Huang, G. Zeng, P. Xu, L. Qin, S. Liu, X. Liu, H. Yi, M. Li, C. Chu, Z. Chen, Rational design 2D/2D BiOBr/CDs/g-C3N4 Z-scheme heterojunction photocatalyst with carbon dots as solid-state electron mediators for enhanced visible and NIR photocatalytic activity: Kinetics, intermediates, and mechanism insight, J. Catal. 369 (2019) 469–481. [59] M. Zhang, C. Lai, B. Li, D. Huang, S. Liu, L. Qin, H. Yi, Y. Fu, F. Xu, M. Li, L. Li, Ultrathin oxygen-vacancy abundant WO3 decorated monolayer Bi2WO6 nanosheet: a 2D/2D heterojunction for the degradation of ciprofloxacin under visible and NIR light irradiation, J. Colloid Interface Sci. 556 (2019) 557–567. [60] C. Yao, A. Yuan, Z. Wang, H. Lei, L. Zhang, L. Guo, X. Dong, Amphiphilic twodimensional graphitic carbon nitride nanosheets for visible-light-driven phase-boundary photocatalysis, J. Mater. Chem. A 7 (2019) 13071–13079. [61] Y. Zhang, S.-J. Park, Stabilization of dispersed CuPd bimetallic alloy nanoparticles on ZIF-8 for photoreduction of Cr(VI) in aqueous solution, Chem. Eng. J. 369 (2019) 353–362. [62] Y. Yang, J. Wu, T. Xiao, Z. Tang, J. Shen, H. Li, Y. Zhou, Z. Zou, Urchin-like hierarchical CoZnAl-LDH/RGO/g-C3N4 hybrid as a Z-scheme photocatalyst for efficient and selective CO2 reduction, Appl. Catal. B: Environ. 255 (2019) 117771. [63] Y. Zhang, S.-J. Park, Bimetallic AuPd alloy nanoparticles deposited on MoO3 nanowires for enhanced visible-light driven trichloroethylene degradation, J. Catal. 361 (2018) 238–247. [64] M. Reli, I. Troppová, M. Šihor, J. Pavlovsky´, P. Praus, K. Kocˇí, Photocatalytic decomposition of N2O over g-C3N4/BiVO4 composite, Appl. Surf. Sci. 469 (2019) 181–191. [65] R. He, D. Xu, B. Cheng, J. Yu, W. Ho, Review on nanoscale Bi-based photocatalysts, Nanoscale Horiz. 3 (2018) 464–504. [66] Q. Xu, L. Zhang, J. Yu, S. Wageh, A.A. Al-Ghamdi, M. Jaroniec, Direct Z-scheme photocatalysts: principles, synthesis, and applications, Mater. Today 21 (2018) 1042–1063. [67] X. Ruan, H. Hu, G. Che, P. Zhou, C. Liu, H. Dong, Iodine ion doped bromo bismuth oxide modified bismuth germanate: a direct Z-scheme photocatalyst with enhanced visible-light photocatalytic performance, J. Colloid Interface Sci. 553 (2019) 186–196. [68] W. Yu, J. Chen, T. Shang, L. Chen, L. Gu, T. Peng, Direct Z-scheme g-C3N4/WO3 photocatalyst with atomically defined junction for H2 production, Appl. Catal. B: Environ. 219 (2017) 693–704. [69] X. Yue, X. Miao, Z. Ji, X. Shen, H. Zhou, L. Kong, G. Zhu, X. Li, S. Ali Shah, Nitrogen-doped carbon dots modified dibismuth tetraoxide microrods: a direct Z-scheme photocatalyst with excellent visible-light photocatalytic performance, J. Colloid Interface Sci. 531 (2018) 473–482. [70] R. He, J. Zhou, H. Fu, S. Zhang, C. Jiang, Room-temperature in situ fabrication of Bi2O3/g-C3N4 direct Z-scheme photocatalyst with enhanced photocatalytic activity, Appl. Surf. Sci. 430 (2018) 273–282. [71] R. He, K. Cheng, Z. Wei, S. Zhang, D. Xu, Room-temperature in situ fabrication and enhanced photocatalytic activity of direct Z-scheme BiOI/g-C3N4 photocatalyst, Appl. Surf. Sci. 465 (2019) 964–972. [72] D. Jiang, W. Ma, P. Xiao, L. Shao, D. Li, M. Chen, Enhanced photocatalytic activity of graphitic carbon nitride/carbon nanotube/Bi2WO6 ternary Z-scheme heterojunction with carbon nanotube as efficient electron mediator, J. Colloid Interface Sci. 512 (2018) 693–700. [73] A. Wang, Y. Zhang, H. Zhong, Y. Chen, X. Tian, D. Li, J. Li, Efficient mineralization of antibiotic ciprofloxacin in acid aqueous medium by a novel photoelectroFenton process using a microwave discharge electrodeless lamp irradiation, J. Hazard. Mater. 342 (2018) 364–374. [74] C. Liu, V. Nanaboina, G.V. Korshin, W. Jiang, Spectroscopic study of degradation products of ciprofloxacin, norfloxacin and lomefloxacin formed in ozonated wastewater, Water Res. 46 (2012) 5235–5246. [75] V.S. Antonin, M.C. Santos, S. Garcia-Segura, E. Brillas, Electrochemical incineration of the antibiotic ciprofloxacin in sulfate medium and synthetic urine matrix, Water Res. 83 (2015) 31–41. [76] C. Lai, F. Huang, G. Zeng, D. Huang, L. Qin, M. Cheng, C. Zhang, B. Li, H. Yi, S. Liu, L. Li, L. Chen, Fabrication of novel magnetic MnFe2O4/bio-char composite and heterogeneous photo-Fenton degradation of tetracycline in near neutral pH, Chemosphere 224 (2019) 910–921.

Please cite this article as: B. Li, C. Lai, L. Qin et al., Anchoring single-unit-cell defect-rich bismuth molybdate layers on ultrathin carbon nitride nanosheet with boosted charge transfer for efficient photocatalytic ciprofloxacin degradation, Journal of Colloid and Interface Science, https://doi.org/10.1016/j. jcis.2019.10.116