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Synthesis of chemically bonded BiOCl@Bi2WO6 microspheres with exposed (0 2 0) Bi2WO6 facets and their enhanced photocatalytic activities under visible light irradiation
Accepted Manuscript Title: Synthesis of chemically bonded BiOCl@Bi2 WO6 microspheres with exposed (020) Bi2 WO6 facets and their enhanced photocatalytic activities under visible light irradiation Author: Yongchao Ma Zhiwei Chen Dan Qu Jinsheng Shi PII: DOI: Reference:
S0169-4332(15)02827-5 http://dx.doi.org/doi:10.1016/j.apsusc.2015.11.130 APSUSC 31847
To appear in:
APSUSC
Received date: Revised date: Accepted date:
4-9-2015 13-11-2015 13-11-2015
Please cite this article as: Y. Ma, Z. Chen, D. Qu, J. Shi, Synthesis of chemically bonded BiOCl@Bi2 WO6 microspheres with exposed (020) Bi2 WO6 facets and their enhanced photocatalytic activities under visible light irradiation, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.11.130 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis of chemically bonded BiOCl@Bi2WO6 microspheres with exposed (020) Bi2WO6 facets and their enhanced photocatalytic activities under visible light irradiation
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Yongchao Maa, Zhiwei Chenb, Dan Qua, Jinsheng Shia* a
School of Life Sciences, Shandong University of Technology, Zibo 255049, People’s Republic of China
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b
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Qingdao Agricultural University, Qingdao 266109, People’s Republic of China
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* Corresponding author:
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E-mail: [email protected]; Tel: +86-532-88030161; Fax: +86-532-86080213
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Abstract Bi2WO6 photocatalysts has been extensively studied for its photocatalytic activity. However, few works have been conducted on hierarchical Bi2WO6 composite
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photocatalysts with specifically exposed facets. In this work, we report a facile
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method to synthesize BiOCl@Bi2WO6 hierarchical composite microspheres. Bi2WO6
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nanosheets with specifically exposed (020) facet were directly formed on the surface of BiOCl precursor microspheres via a controlled anion exchange route between
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BiOCl and Na2WO4. The visible-light photocatalytic activity of the BiOCl@Bi2WO6 heterojunction with exposed (020) facets (denoted as BiOCl@Bi2WO6) was
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investigated by degradation of Rhodamine B (RhB) and ciprofloxacin (CIP) aqueous solution under visible light irradiation. The experimental results indicated that the
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BiOCl@Bi2WO6 composite microsphere with intimate interfacial contacts exhibited improved efficiency for RhB photodegradation in comparison with pure BiOCl and
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Bi2WO6. The BiOCl@Bi2WO6 composite microsphere also shows high photocatalytic activity for degradation of CIP under visible light irradiation. The enhanced photocatalytic performance of BiOCl@Bi2WO6-020 hierarchical microspheres can be ascribed to the improved visible light harvesting ability, high charge separation and transfer. This work will make significant contributions toward the exploration of novel heterostructures with high potential in photocatalytic applications. Keywords: exposed facets, BiOCl@Bi2WO6, composite photocatalysts, controlled anion exchange, photocatalysis
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1. Introduction Recently, semiconductor-based photocatalysis has been attracted an increasing attention for environmental decontamination and hydrogen energy production since it
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is an environmental friendly way to take advantage of solar energy [1-3]. Traditional
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photocatalysts with wide band gap (e.g. TiO2, ZnO and SnO2) are intensively
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investigated due to their outstanding stability and suitable band gaps for photocatalytic reaction [4-6]. Unfortunately, they usually suffer from limited
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visible-light absorption due to their deep valence band that consists mostly of O2p states [7].
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Alternatively, the use of visible-light-driven semiconductor photocatalysts could effectively exploit visible light absorption for photoexcitation and further enhance the
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photocatalytic activities[8]. Up to date, Bi2WO6 has a layered structure with band gaps of about 2.75 eV, which effectively exploit visible light absorption due to the
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interaction between Bi6s and O2p orbitals at the top of the valence band[9, 10]. In recent work, Li et al. has prepared Bi2WO6 nanostructured bipyramids by the solvothermal method[11]. Hierarchical flower-like bismuth tungstate hollow spheres have been synthesized via a solvothermal process[12]. However, the photocatalytic performance of Bi2WO6 is still limited, because of its low separation efficiency of
photogenerated carriers in the photocatalytic process. To address this drawback, different methods have been studied, such as doping[13,
14],
morphology
control[15,
16],
metal
deposition[17]
and
heterostructures construction[18, 19]. Construction of a heterostrucutre photocatalyst 3
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might be an effective way to enhance the visible light photocatalytic activity of Bi2WO6[20]. In coupled semiconductors, the charge transfer from one particle to the other via interfaces is favorable for the electron-hole separation. It is well known that
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BiOCl semiconductor with wide band gap of 3.6 eV has been proved a promising
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photocatalyst because of its potential applications in wastewater treatment[21]. To
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date, some heterojunctions have been developed by coupling BiOCl with other semiconductors, such as BiOCl-Bi2S3[22], BiOCl-BiOI[23], BiOCl-Bi2WO6[24] and
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BiOCl-Co3O4[25]. The BiOCl-based composite exhibited enhanced photoactivity due to the efficient separation and transfer of photogenerated carries. On the other hand,
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surface properties are vital to a semiconductor’s photocatalytic performance, which sensitively depends on its exposed surfaces with distinct crystal facets [26, 27]. To the
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best of our knowledge, few works have been conducted on hierarchical Bi2WO6-based composite photocatalyst with specifically exposed facets.
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In this regard, we design and synthesize a novel hierarchical BiOCl@Bi2WO6
heterostructure composed of BiOCl precursor microspheres and Bi2WO6 nanoshees
with exposed (020) facets. The synthesis involves a facile ion exchange method between BiOCl precursor microspheres and Na2WO4. The excellent photocatalytic
performance of BiOCl@Bi2WO6 was evaluated by degrading RhB and CIP aqueous
solution as model pollutants under visible light (λ > 420 nm). Furthermore, the photocatalytic mechanism was also discussed. The BiOCl@Bi2WO6 heterojunction photocatalysts might be a viable alternative for photocatalytic applications. 2. Experimental section 4
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2.1 Synthesis of BiOCl precursor microspheres. All reagents were of analytical grade, purchased from Shanghai Chemical Reagent Company, and used as received without further purification. In a typical
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synthesis, 3 mmol of Bi(NO3)3‚5H2O was added slowly into an 60 mL EG solution
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containing 3 mmol of KCl. The mixture was stirred for 30 min at room temperature,
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and then poured into a 100 mL Teflon-lined stainless autoclave. The autoclave was allowed to be heated at 180 °C for 12 h, and then air cooled to room temperature. The
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resulting precipitates were collected and washed with ethanol and deionized water thoroughly and dried at 80 °C in air.
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2.2 Synthesis of mesoporous BiOCl@Bi2WO6 hierarchical microspheres The mesoporous BiOCl@Bi2WO6 hierarchical microspheres were synthesized by
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a controlled anion exchange method. Briefly, the as-prepared BiOCl was suspended into 80 mL of Na2WO4 solutions (6.25 mM) with the assistance of magnetic stirring
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for 10 min to form a homogeneous mixture. And then the resulting mixture was transferred to a 100 mL Teflon-lined stainless autoclave and maintained at 180 °C for 12 h. The autoclave was then cooled down to room temperature naturally. The product was collected by filtration and washed with deionized water and ethanol for several times, and dried in vacuum at 60 °C overnight. For comparison, the amount of Na2WO4·2H2O used in the anion exchange
process is 0.2, 1, 1.5 mmol. And the corresponding products were denoted as BiOCl/Bi2WO6-1, BiOCl/Bi2WO6-2 and Bi2WO6, respectively. For the preparation of N-doped TiO2 (N-TiO2), 1g of P25 was suspended in 5
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ethanol (10 mL). Then, urea (2 g) dissolved in 5 mL ethanol and 1 mL H2O was added into the suspension. The mixture was stirred and heated to completely evaporate the
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solvent, followed by calcination in air at 400 ℃ for 4 h[28]. 2.3 Materials Characterization
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The morphology of the as-prepared product was characterized by field-emission
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scanning electron microscopy (FESEM, JEOL, JSM-6700F) operated at an acceleration voltage of 5.0 kV. The crystalline structure of the product was analyzed
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by an X-ray diffractometer (XRD, Y-2000) with Cu Kα radiation (λ) 1.5418 Å). Transmission electron microscopy (TEM) and high-resolution transmission electron
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microscopy (HRTEM) observations were carried out on a JEOL JEM-2010 instrument in bright field and selected area electron diffraction (SAED) modes and on a HRTEM
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JEM-2010FEF instrument (operated at 200 kV). Room temperature UV-vis absorption spectrum was recorded on a TU-1901 spectrophotometer an integrating sphere
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attachment in the wavelength range of 200-800 nm. BaSO4 was used as the reflectance standard
material. The N2
adsorption-desorption isotherm and
Barrett-Joyner-Halenda (BJH) methods were analyzed on a Quantachrome NOVA 2200e analyzer.
2.4 Photocatalytic Test
The photocatalytic activities of the as-prepared products were evaluated by the degradation of RhB and CIP aqueous solution under visible light irradiation. The photocatalytic reactor (PLS-SXE 300, Beijing Perfect light Co., Ltd.), consisting of a quartz glass with a circulating water jack and a 300W Xe lamp with a 420 nm cutoff 6
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filter, was used as the visible light source. The light intensity striking the model pollutant solution was at
23 mW cm-2, as measured by a FZ-A optical Radiometer
(Photoelectric Instrument Factory of Beijing Norman University, Beijing, P. R. China).
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The optical spectrum of the 300W Xe lamp with a 420 nm cutoff filter is given in Fig.
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S1. In a typical experiment, 30 and 100 mg photocatalysts were added to an aqueous
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solution of RhB and CIP (100 mL, 10 mg L-1). Prior to irradiation, the suspensions were magnetically stirred in the dark for about 30 min to obtain a good
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adsorption-desorption equilibrium between the photocatalysts and the model pollutants under ambient conditions. At certain time intervals, 5 mL of the solution
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was taken out and centrifuged to remove the photocatalyst. The concentrations of RhB and CIP were analyzed by recording variations of the characteristic absorption
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band of 553 and 272 nm using a TU-1901 spectrophotometer, respectively. 2.5 Photoelectrochemical measurements
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The photoelectrochemical characteristics were measured in a CHI660D
electrochemical working station using a standard three-compartment cell under Xe arc lamp irradiation with 300 W. In a typical procedure, a commercial ITO glass (1.0 × 1.0 cm2) was ultrasonicated in an ethanol and distilled water bath prior to use. The
clean ITO glass substrate was then immersed in the slurry of the as-prepared photocatalyst (3 mg) and ethanol (3 ml) mixtures. The substrate was then vacuum-dried at 80 °C to eliminate ethanol and subsequently maintained at 100 °C overnight. ITO glass coated with the as-prepared samples, a piece of Pt sheet, a Ag/AgCl electrode and 0.01 M Na2SO4 were used as the working electrode, the 7
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counter electrode, the reference electrode and the electrolyte, respectively. 3. Results and discussion The as-prepared BiOCl microspheres were used as both a precursor for the
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growth of Bi2WO6 shell and a core substrate to support the shell. Previous studies
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have shown that Bi2WO6 tend to grow into nanoplates/sheets with 2D features
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because of their highly anisotropic layered structures[29, 30]. In our case, the use of BiOCl microspheres as the substrate led to a BiOCl/Bi2WO6 core/shell
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heterostructures with a hierarchical architecture.
3.1 Structure, composition and morphology of the prepared photocatalysts.
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The morphologies of the prepared products were characterized by SEM. Fig. 1a-b show the uniform BiOCl precursor microspheres with diameters ranging from
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2-4 µm, which were built by interlaced nanorods. As shown in Fig. 1c-d, the as-prepared BiOCl@Bi2WO6 composites hierarchical microspheres with the size of
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about 2-5 µm are observed. The shell of the hierarchical microsphere is composed of crossed nanosheets with the thickness of about 35 nm. The corresponding EDS is shown in Fig, 1e. It can be seen that the sample was composed of Bi, W, O, Cl elements, which verified the coexistence of both phases in the composite photocatalysts. The red arrow suggests the nature of hollow opening of the pure Bi2WO6. The effect of concentration of the WO42- anions in the reaction system on the morphologies of formed BiOCl/Bi2WO6 composites was also investigated, as shown
in Fig.S2. The morphology and structure of BiOCl@Bi2WO6 microspheres were further 8
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investigated by TEM analysis. Fig. 2a shows the spherical structure of the BiOCl@Bi2WO6 products, which is in accord with the SEM analysis (Fig. 1a). It is noteworthy that the interfacial regions are difficult to directly inspect, which can be
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explained by the following two reasons: (i) the particle size of the heterostructure is
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large; (ii) the single-crystalline Bi2WO6 nanosheets have a very good connection with
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the BiOCl core. However, the intimate interfacial contact is crucially important for effective interface electronic-structure modification and charge carrier’s transfer
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between two phases [31]. Fig. 2b shows a single Bi2WO6 nanosheet peeling off the heterostructure upon strong ultrasonic treatment with the length of around 100 nm.
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The SAED patterns (Fig. 2c), recorded from the single nanosheet in Fig. 2b, present the diffraction spots of the [010] zone axis, which confirms the single-crystal nature
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of the nanosheet. Based on the above analysis and the symmetries of Bi2WO6, the bottom and top surfaces of the Bi2WO6 nanosheet are identified as (020) facets. The
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HRTEM image taken from the marked area in Fig. 2b indicates the (200) atomic lattice spacing of 0.273 nm. Based on the length (around 200 nm) of the Bi2WO6 nanosheets in shell, it can be inferred that the diameter of BiOCl core, as indicated by the inner white circle in Fig. 2a, is nearly 1.0 µm. The high-angle annular dark field (HAADF, Fig. 3a) image and element mapping
analysis (Fig. 3b-e) of BiOCl@Bi2WO6 composite microspheres reveals the presence of W, Bi, Cl and O elements, suggesting the co-existing of both BiOCl and Bi2WO6. Furthermore, the relative location of BiOCl core and the Bi2WO6 shell can be further confirmed by cross-sectional compositional line profiles, as shown in Fig.3f-h. The 9
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comparison between line profiles of the Cl and W elements obtained along the orange wire axis (Fig. 3f) reveal that the distribution of Cl in the core and an increase of W in the surface of the composite microsphere.
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The crystal structure and purity of the as-obtained photocatalysts are examined
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by XRD analysis. As shown in Fig. 4a and c, all the diffraction peaks can be indexed
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to the tetragonal structure of BiOCl (JCPDS 06-0249) and orthorhombic structure of Bi2WO6 (JPCDS 39-0256), respectively. No other impurity peak is detected. The
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XRD pattern (Fig. 4b) of the BiOCl@Bi2WO6 composite microspheres confirms that most of the peaks are matched perfectly to those of pure BiOCl, except the peaks
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indicated by asterisks (★) can be indexed as the orthorhombic structure of Bi2WO6 (JPCDS 39-0256).
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The chemical composition and surface chemical states of the BiOCl/Bi2WO6 composite microsphere were investigated by XPS analysis, as shown in Fig. 5.
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According to the XPS observations (Fig. 5a), only C, Bi, O, W and Cl were detected in the sample. The C1s peak at around 284.6 eV can be attributed to the signal from carbon contained in the instrument and was used for calibration[32]. The characteristic spin-orbit splitting of W4f5/2 and W 4f7/2 signals is observed at
approximately 37.0 and 34.9 eV, respectively, corresponding to W6+ in Bi2WO6 (Fig. 5b) [33]. In addition, the Cl 2p high-resolution XPS spectrum showed two major peaks with binding energy at 197.1 and 198.5 eV, which could be ascribed to Cl 2p3/2 and Cl 2p1/2 of Cl-, respectively (Fig. 4c) [34]. Two strong peaks in the high-resolution
XPS spectra, at 164.4 and 159.0 eV, are assigned to Bi 4f5/2 and Bi 4f7/2, respectively, 10
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which confirms that the bismuth species in the BiOCl/Bi2WO6 composite is Bi3+ cations[35] (Fig. 5d). Based on the above analysis, the illustration of phase transition process between
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BiOCl precursors and BiOCl/Bi2WO6 heterojunction was shown in Fig. 6. In the
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formation process of BiOCl/Bi2WO6 heterojunction, the BiOCl precursor microsphere
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is employed as both the physical and chemical templates. Due to the lower solubility of Bi2WO6 relative to BiOCl, BiOCl could adopt a thermodynamically favored
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direction to transform into Bi2WO6 by reacting with WO42- ions, which is analogous to the formation of ZnS hollow nanospheres by heteroepitaxial exchange around the
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ZnO precursor nanoparticles[36, 37]. Thus, upon the introduction of WO42- followed by hydrothermal treatments, Bi2WO6 thin layer was formed around the surface of
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BiOCl precursor microspheres, during which Cl- ions were replaced by WO42- ions. On the other hand, as two Aurivillius-type oxides, BiOCl and Bi2WO6 are both
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crystallized in layered structures composed of (Bi2O2)2+ slabs interleaved with anion layers[38]. This structural similarity may be responsible for the formation of BiOCl/Bi2WO6 heterojunction with intimate contact interface, because of the
interaction between BiOCl precursor and subsequently formed Bi2WO6 component with chemical bonding[39]. Therefore, a close interfacial contact between Bi2WO6
nanosheets and BiOCl precursor was obtained, which can effectively improve the separation efficiency of photogenerated carriers. 3.2 Photocatalytic performance The photocatalytic performance of the as-prepared composite photocatalyst was 11
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evaluated using the degradation of RhB and CIP aqueous solution under visible light irradiation. As shown in Fig. 7, RhB was very stable in the absence of photocatalyst. It is interesting to note that the BiOCl@Bi2WO6-020 microspheres exhibit the highest
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photocatalytic degradation efficiency. The RhB photodegradation process was found
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to follow pseudo-first-order kinetics. Fig. S3a clearly indicates that the removal rate
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of RhB over the BiOCl@Bi2WO6-020 microspheres is much faster than that over the pure BiOCl and Bi2WO6, and the apparent rate constant k over the BiOCl, Bi2WO6
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and BiOCl@Bi2WO6 composite is 0.00254, 0.00499 and 0.01392 min−1, respectively. Moreover, the rate constant over the BiOCl@Bi2WO6 nanocomposite is more than 7.5
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times that over N-TiO2 (used as reference photocatalyst, k = 0.00186 min−1). The corresponding absorption spectra reveal a gradual decline in the intensity of the peak
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at 553 nm as exposure time increases and they eventually disappear completely after 90 min (Fig. 7B). The peak intensity in the UV region (280 nm < λ < 400 nm) also
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decreases with increasing irradiation time, which further indicates that both dye chromophore and aromatic ring have been destroyed. The photocatalytic activities of the other BiOCl/Bi2WO6 composites were also
investigated. As shown in Fig. S3b, the photodegradation efficiency increases with increasing Bi2WO6 content, but at higher Bi2WO6 concentration, the photocatalytic
activity decreases. This may be ascribed to the following reasons: with excess content of Bi2WO6 in the composite photocatalysts, more photoinduced carriers would recombine easily on the surface of Bi2WO6[40]. On the other hand, the morphology of composite photocatalysts was changed, forming loosely interfacial contact. 12
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As a broad-spectrum antibiotic agent, ciprofloxacin (CIP) has been widely used for treating bacterial infections[41]. However, the lack of treatment processes has led
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to their ubiquity in water, which may accelerate antibiotic resistance within native
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bacterial populations in the environments[42]. As a result, it is great significant to
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degrade the CIP aqueous solution. As shown in Fig. 8A, before light irradiation, about 50, 40 and 25 % CIP were adsorbed by BiOCl, Bi2WO6 and BiOCl@Bi2WO6
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photocatalyst within 30 min, respectively, which is consistent with the specific surface area analysis (As shown in Fig. S4). After irradiation of 5 h, about 50 and 65 % of CIP
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were photocatalytic degraded by Bi2WO6 and BiOCl@Bi2WO6, while BiOCl cannot be activate by visible light for its wide band gap. The corresponding absorption
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spectra of BiOCl@Bi2WO6 composite microspheres reveal a gradual decline in the intensity of the peak at 272 nm as exposure time increases (Fig. 8B).
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3.3 Mechanisms of improved photocatalytic properties. The light absorption ability of the prepared photocatalysts was investigated by
the UV-visible diffuse reflectance spectra. As shown in Fig. 9A, the absorption edge of the pure BiOCl and Bi2WO6 occur at about 360 nm and 450 nm, respectively,
which is in agreement with the previously reported result[43-45]. Notably, the ability of light absorption of the BiOCl@Bi2WO6-020 microsphere is enhanced in the wavelength range 500–700 nm. These results are attributed to the interaction between BiOCl and Bi2WO6 as well as the unique 3D hierarchical surface morphology. The large visible light trapping would endow the formation of more electron-hole pairs 13
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and
further
improve
the
photocatalytic
activity
of
BiOCl@Bi2WO6-020
microsphere[46]. According to the Kubelka–Munk function, the plot of (ahν)1/2 versus hν based on the indirect transition is shown as Fig. 9B. The band gaps of the BiOCl
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and Bi2WO6 BiOCl@Bi2WO6-020 photocatalyst are 3.22 and 2.52 eV, respectively.
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This result indicates that the BiOCl@Bi2WO6-020 microsphere has a suitable
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bandgap for photocatalytic decomposition of organic contaminants under visible light irradiation.
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To further understand the improvement of photocatalytic activity of the as-prepared photocatalysts, the photogenerated current measurements were performed,
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as shown in Fig. 10. It can be clearly observed that the BiOCl@Bi2WO6-020 microsphere exhibit enhanced photocurrents compared with the BiOCl precursors as
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well as pure Bi2WO6 photocatalyst. The noticeable improvement of photocurrent response can be ascribed to the following two reasons: (i) the formation of
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heterojunctions can provide efficient charge separation[47, 48]; (ii) Bi2WO6 has a unique layered structure, which is characterized by [Bi2O2] slabs interleaved with
perovskite-like WO42- layers[49]. This may induce the presence of internal static
electric field perpendicular to the [Bi2O2] slabs interleaved with perovskite-like WO42layers in Bi2WO6, enhancing the separation of the photogenerated elelctric-hole pairs
along the [010] direction, as schematically illustrated in Fig. S5. Fig. 11 shows the charge transfer pathway of the RhB degradation process over BiOCl/Bi2WO6 heterojunction. The band edge positions of the conduction band (CB) and valence band (VB) of Bi2WO6 and BiOCl are approximately 0.60 and 0.23 eV, 14
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3.12 and 3.45 eV, respectively (Table S1). Under the visible light irradiation (energy less than 2.95 eV), Bi2WO6 with a narrow band gap energy (Eg=2.52 eV) are photoexcited to generated electron-hole pairs, while BiOCl will be not. Meanwhile,
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electrons in the VB of Bi2WO6 could also be excited up to a higher potential edge
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(0.17 eV) due to the higher photon energy [40]. Thus, the photoinduced electron in the
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CB of Bi2WO6 could easily flow into the CB edge of BiOCl, leaving holes on the valence band of Bi2WO6. In such a way, the photogenerated electron-hole pairs can be
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efficiently separated.
Based on the above analyses, the enhanced performance of composite could be
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attributed to its unique surface and electronic properties. The microsphere morphology with layered structures allows multiple reflections of visible light, which
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can make more efficient use of the light source. Furthermore, photocatalysts in microsphere morphology can be more readily separated from the slurry system by
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filtration after the photocatalytic reaction and reused than the nanosized samples because of their large weight, weak Brownian motion and good mobility[50]. All of these advantages make the photocatalysts with layered structures appealing photocatalysts in the aqueous photocatalytic reaction. 4. Conclusions
In summary, a novel BiOCl@Bi2WO6-020 composite photocatalyst with layered structure has been synthesized. Bi2WO6 nanosheets with specifically exposed (020) facets were directly formed on the external surface of BiOCl precursor microsphere. Phase transition between the structures of BiOCl and Bi2WO6 nanosheets takes place 15
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readily by a facile anion exchange approach. Monodispersed BiOCl microspheres are first synthesized and subsequently undergone controlled chemical transformation in Na2WO4 solution under hydrothermal conditions. This is highly significant because it
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allows us to obtain delicate heterostructures. Interestingly, the molar ratio of Na2WO4
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to BiOCl precursor microspheres has a significant influence on the surface structure
microsphere
and
Bi2WO6
product,
the
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of the composite photocatalysts. Compared with the pure BiOCl precursor BiOCl@Bi2WO6-020
hierarchical
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microspheres exhibit superior photocatalytic activity for decomposition of RhB and CIP under visible light irradiation. The decomposition rate of RhB over the
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BiOCl@Bi2WO6-020 microsphere was 0.01392 min-1, which is approximately 2.79 and 5.48 times greater than that of pure Bi2WO6 and BiOCl, respectively. The
and
transfer
can
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synergistic effects of improved visible light harvesting ability, high charge separation lead
to
the
enhanced
photocatalytic
activity
of
the
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BiOCl@Bi2WO6-020 heterojunctions. The results presents in this study indicates that
the combination of morphology engineering and heterojunction construction is a useful approach for designing heterojunction photocatalysts with high charge separation efficiency. Acknowledgement
This work was supported by the National Natural Science Foundation of China (Grant No. 31071538), Natural Key Science Foundation of Shandong Province (ZR2013FB001), the Research Fund for Technology Upgrading of Large Scientific Instruments and Equipment in Shandong Province (2013SJGZ01) and Science and 16
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Technology Development Plan of Shandong Province, China (2014GNC110013) . References [1] F. Amano, A. Yamakata, K. Nogami, M. Osawa, B. Ohtani, Visible Light Responsive Pristine Metal Oxide Photocatalyst: Enhancement of Activity by Crystallization under Hydrothermal Treatment J. Am.
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Chem. Soc. 130 (2008) 17650-17651.
[2] J. Yu, X. Yu, Hydrothermal Synthesis and Photocatalytic Activity of Zinc Oxide Hollow Spheres, Environ. Sci. Technol. 42 (2008) 4902-4907.
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[3] P. Zhou, J. Yu, M. Jaroniec, All-Solid-State Z-Scheme Photocatalytic Systems, Adv. Mater. 26 (2014) 4920-4935.
[4] H. Liu, J.B. Joo, M. Dahl, L. Fu, Z. Zeng, Y. Yin, Crystallinity control of TiO2 hollow shells
us
through resin-protected calcination for enhanced photocatalytic activity, Energy Environ. Sci. 8 (2015) 286-296.
[5] S. Wang, L. Zhao, L. Bai, J. Yan, Q. Jiang, J. Lian, Enhancing photocatalytic activity of
an
disorder-engineered C/TiO2 and TiO2 nanoparticles, J. Mater. Chem. A 2 (2014) 7439-7445. [6] L.-L. Tan, W.-J. Ong, S.-P. Chai, A.R. Mohamed, Band gap engineered, oxygen-rich TiO2 for visible light induced photocatalytic reduction of CO2, Chem. Commun. 50 (2014) 6923-6926. [7] L. Ye, Y. Su, X. Jin, H. Xie, C. Zhang, Recent advances in BiOX (X = Cl, Br and I) photocatalysts:
M
synthesis, modification, facet effects and mechanisms, Environ. Sci.: Nano 1 (2014) 90-112. [8] D.J. Martin, G. Liu, S.J.A. Moniz, Y. Bi, A.M. Beale, J. Ye, J. Tang, Efficient visible driven (2015) 7808-7828.
d
photocatalyst, silver phosphate: performance, understanding and perspective, Chem. Soc. Rev. 44 [9] T. Saison, P. Gras, N. Chemin, C. Chanéac, O. Durupthy, V. Brezová, C. Colbeau-Justin, J.-P. (2013) 22656-22666.
te
Jolivet, New Insights into Bi2WO6 Properties as a Visible-Light Photocatalyst, J. Phys. Chem. C 117 [10] J. Yang, X. Wang, X. Zhao, J. Dai, S. Mo, Synthesis of Uniform Bi2WO6-Reduced Graphene
Ac ce p
Oxide Nanocomposites with Significantly Enhanced Photocatalytic Reduction Activity, J. Phys. Chem. C 119 (2015) 3068-3078.
[11] G. Zhang, Z. Hu, M. Sun, Y. Liu, L. Liu, H. Liu, C.-P. Huang, J. Qu, J. Li, Formation of Bi2WO6
Bipyramids with Vacancy Pairs for Enhanced Solar-Driven Photoactivity, Adv. Funct. Mater. 25 (2015) 3726-3734.
[12] P. Zhang, J. Zhang, A. Xie, S. Li, J. Song, Y. Shen, Hierarchical flower-like Bi2WO6 hollow microspheres: facile synthesis and excellent catalytic performance, RSC Adv. 5 (2015) 23080-23085. [13] Z. Zhang, W. Wang, Y. Zhou, Hydrothermal synthesis of a novel BiErWO6 photocatalyst with wide
spectral responsive property, Appl. Surf. Sci. 319 (2014) 250-255. [14] X. Ding, K. Zhao, L. Zhang, Enhanced photocatalytic removal of sodium pentachlorophenate with self-doped Bi2WO6 under visible light by generating more superoxide ions, Environ. Sci. Technol. 48
(2014) 5823-5831. [15] Y.-X. Zhou, L. Tong, X.-H. Zeng, X.-B. Chen, Green synthesis of flower-like Bi2WO6 microspheres as a visible-light-driven photocatalyst, New J. Chem. 38 (2014) 1973-1979. [16] R.a. He, S. Cao, P. Zhou, J. Yu, Recent advances in visible light Bi-based photocatalysts, Chin. J. Catal. 35 (2014) 989-1007. [17] D. Wang, G. Xue, Y. Zhen, F. Fu, D. Li, Monodispersed Ag nanoparticles loaded on the surface of 17
Page 17 of 22
spherical Bi2WO6 nanoarchitectures with enhanced photocatalytic activities, J. Mater. Chem. 22 (2012) 4751-4758. [18] P. Ju, P. Wang, B. Li, H. Fan, S. Ai, D. Zhang, Y. Wang, A novel calcined Bi2WO6/BiVO4 heterojunction photocatalyst with highly enhanced photocatalytic activity, Chem. Eng. J. 236 (2014) 430-437. [19] S. Girish Kumar, K.S.R. Koteswara Rao, Tungsten-based nanomaterials (WO3&Bi2WO6):
ip t
Modifications related to charge carrier transfer mechanisms and photocatalytic applications, Appl. Surf. Sci. 355 (2015) 939-958.
[20] S. Xue, Z. Wei, X. Hou, W. Xie, S. Li, X. Shang, D. He, Enhanced visible-light photocatalytic
cr
activities and mechanism insight of BiVO4/Bi2WO6 composites with virus-like structures, Appl. Surf. Sci. 355 (2015) 1107-1115.
[21] M. Gao, D. Zhang, X. Pu, M. Li, Y.M. Yu, J.J. Shim, P. Cai, S.I. Kim, H.J. Seo, Combustion
us
Synthesis of BiOCl with Tunable Percentage of Exposed {001} Facets and Enhanced Photocatalytic Properties, J. Am. Ceram. Soc. 98 (2015) 1515-1519.
[22] S. Jiang, K. Zhou, Y. Shi, S. Lo, H. Xu, Y. Hu, Z. Gui, In situ synthesis of hierarchical flower-like
an
Bi2S3/BiOCl composite with enhanced visible light photocatalytic activity, Appl. Surf. Sci. 290 (2014) 313-319.
[23] T.B. Li, G. Chen, C. Zhou, Z.Y. Shen, R.C. Jin, J.X. Sun, New photocatalyst BiOCl/BiOI composites with highly enhanced visible light photocatalytic performances, Dalton Trans. 40 (2011)
M
6751-6758.
[24] W. Yang, B. Ma, W. Wang, Y. Wen, D. Zeng, B. Shan, Enhanced photosensitized activity of a BiOCl-Bi2WO6 heterojunction by effective interfacial charge transfer, Phys. Chem. Chem. Phys. 15
d
(2013) 19387-19394.
[25] C. Tan, G. Zhu, M. Hojamberdiev, K. Okada, J. Liang, X. Luo, P. Liu, Y. Liu, Co3O4
te
nanoparticles-loaded BiOCl nanoplates with the dominant {001} facets: efficient photodegradation of organic dyes under visible light, Appl. Catal. B: Environ. 152–153 (2014) 425-436. [26] M. Pan, H. Zhang, G. Gao, L. Liu, W. Chen, Facet-Dependent Catalytic Activity of
Ac ce p
Nanosheet-Assembled Bismuth Oxyiodide Microspheres in Degradation of Bisphenol A Environ. Sci. Technol., 49 (2015) 6240-6248.
[27] J. Yu, J. Low, W. Xiao, P. Zhou, M. Jaroniec, Enhanced Photocatalytic CO2-Reduction Activity of Anatase TiO2 by Coexposed {001} and {101} Facets, J. Am. Chem. Soc. 136 (2014) 8839-8842.
[28] Z. Xiong, X.S. Zhao, Nitrogen-Doped Titanate-Anatase Core–Shell Nanobelts with Exposed {101} Anatase Facets and Enhanced Visible Light Photocatalytic Activity, J. Am. Chem. Soc. 134 (2012) 5754-5757.
[29] N. Zhang, R. Ciriminna, M. Pagliaro, Y.-J. Xu, Nanochemistry-derived Bi2WO6 nanostructures:
Towards production of sustainable chemicals and fuels induced by visible light, Chem. Soc. Rev. 43 (2014) 5276-5287. [30] Q. Sun, X. Jia, X. Wang, H. Yu, J. Yu, Facile synthesis of porous Bi2WO6 nanosheets with high photocatalytic performance, Dalton Trans. 44 (2015) 14532-14539. [31] W. Wang, J. Wang, Z. Wang, X. Wei, L. Liu, Q. Ren, W. Gao, Y. Liang, H. Shi, p-n junction CuO/BiVO4 heterogeneous nanostructures: synthesis and highly efficient visible-light photocatalytic performance, Dalton Trans. 43 (2014) 6735-6743. [32] Z. He, Y. Shi, C. Gao, L. Wen, J. Chen, S. Song, BiOCl/BiVO4 p–n Heterojunction with Enhanced
Photocatalytic Activity under Visible-Light Irradiation, J. Phys. Chem. C 118 (2013) 389-398. 18
Page 18 of 22
[33] M. Li, L. Zhang, X. Fan, Y. Zhou, M. Wu, J. Shi, Highly selective CO2 photoreduction to CO over g-C3N4/Bi2WO6 composites under visible light, J. Mater. Chem. A 3 (2015) 5189-5196. [34] J. Di, J. Xia, M. Ji, B. Wang, S. Yin, Q. Zhang, Z. Chen, H. Li, Carbon Quantum Dots Modified BiOCl Ultrathin Nanosheets with Enhanced Molecular Oxygen Activation Ability for Broad Spectrum Photocatalytic Properties and Mechanism Insight, ACS Appl. Mater. Interfaces 7 (2015) 20111-20123. [35] X. Zhang, L. Zhang, J.-S. Hu, C.-L. Pan, C.-M. Hou, Facile hydrothermal synthesis of novel
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Bi12TiO20-Bi2WO6 heterostructure photocatalyst with enhanced photocatalytic activity, Appl. Surf. Sci. 346 (2015) 33-40.
[36] H. Cheng, B. Huang, Y. Liu, Z. Wang, X. Qin, X. Zhang, Y. Dai, An anion exchange approach to
cr
Bi2WO6 hollow microspheres with efficient visible light photocatalytic reduction of CO2 to methanol, Chem. Commun. 48 (2012) 9729-9731.
[37] Y.P. Xie, Z.B. Yu, G. Liu, X.L. Ma, H.-M. Cheng, CdS-mesoporous ZnS core-shell particles for
us
efficient and stable photocatalytic hydrogen evolution under visible light, Energy Environ. Sci. 7 (2014) 1895-1901.
[38] M. Huang, Y. Yan, W. Feng, S. Weng, Z. Zheng, X. Fu, P. Liu, Controllable Tuning Various Ratios
an
of ZnO Polar Facets by Crystal Seed-Assisted Growth and Their Photocatalytic Activity, Cryst. Growth Des. 14 (2014) 2179-2186.
[39] Y.-S. Xu, Z.-J. Zhang, W.-D. Zhang, Inlay of Bi2O2CO3 nanoparticles onto Bi2WO6 nanosheets to build heterostructured photocatalysts, Dalton Trans. 43 (2014) 3660-3668.
M
[40] Z. Wu, L. Chen, C. Xing, D. Jiang, J. Xie, M. Chen, Controlled synthesis of Bi2S3/ZnS microspheres by an in situ ion-exchange process with enhanced visible light photocatalytic activity, Dalton Trans. 42 (2013) 12980-12988.
d
[41] J. Di, J. Xia, M. Ji, S. Yin, H. Li, H. Xu, Q. Zhang, H. Li, Controllable synthesis of Bi4O5Br2 ultrathin nanosheets for photocatalytic removal of ciprofloxacin and mechanism insight, J. Mater.
te
Chem. A 3 (2015) 15108-15118.
[42] T. Paul, P.L. Miller, T.J. Strathmann, Visible-Light-Mediated TiO2 Photocatalysis of Fluoroquinolone Antibacterial Agents, Environ. Sci. Technol. 41 (2007) 4720-4727.
Ac ce p
[43] D. Yue, D. Chen, Z. Wang, H. Ding, R. Zong, Y. Zhu, Enhancement of visible photocatalytic performances of a Bi2MoO6-BiOCl nanocomposite with plate-on-plate heterojunction structure, Phys.
Chem. Chem. Phys. 16 (2014) 26314-26321. [44] Z. Zhang, W. Wang, L. Wang, S. Sun, Enhancement of Visible-Light Photocatalysis by Coupling with Narrow-Band-Gap Semiconductor: A Case Study on Bi2S3/Bi2WO6, ACS Appl. Mater. Interfaces 4
(2012) 593-597.
[45] S. Shenawi-Khalil, V. Uvarov, E. Menes, I. Popov, Y. Sasson, New efficient visible light photocatalyst based on heterojunction of BiOCl–bismuth oxyhydrate, Appl. Catal. A: Gen. 413–414 (2012) 1-9.
[46] J. Di, J. Xia, M. Ji, H. Li, H. Xu, H. Li, R. Chen, The synergistic role of carbon quantum dots for the improved photocatalytic performance of Bi2MoO6, Nanoscale 7 (2015) 11433-11443. [47] Y. Shi, Y. Chen, G. Tian, H. Fu, K. Pan, J. Zhou, H. Yan, One-pot controlled synthesis of sea-urchin shaped Bi2S3/CdS hierarchical heterostructures with excellent visible light photocatalytic activity, Dalton Trans. 43 (2014) 12396-12404. [48] D. He, L. Wang, D. Xu, J. Zhai, D. Wang, T. Xie, Investigation of Photocatalytic Activities over Bi2WO6/ZnWO4 Composite under UV Light and Its Photoinduced Charge Transfer Properties, ACS Appl. Mater. Interfaces 3 (2011) 3167-3171. 19
Page 19 of 22
[49] S. Sun, W. Wang, L. Zhang, E. Gao, D. Jiang, Y. Sun, Y. Xie, Ultrathin {001}-Oriented Bismuth Tungsten Oxide Nanosheets as Highly Efficient Photocatalysts, ChemSusChem 6 (2013) 1873-1877. [50] J. Yu, X. Yu, B. Huang, X. Zhang, Y. Dai, Hydrothermal Synthesis and Visible-light Photocatalytic Activity of Novel Cage-like Ferric Oxide Hollow Spheres, Cryst. Growth Des. 9 (2009) 1474-1480.
Figure Captions
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Fig. 1 SEM images of the as-prepared samples: (a, b) BiOCl precursor microspheres,
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(c, d) hierarchical BiOCl@Bi2WO6 composites, and (f) Bi2WO6 samples; (e) the
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corresponding EDS of BiOCl@Bi2WO6 microspheres.
Fig. 2 TEM images of (a) a BiOCl@Bi2WO6 heterostructure particle and (b) a single
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Bi2WO6 nanosheet peeling off the heterostructure upon strong ultrasonic treatment. (c)
of the Bi2WO6 nanosheet in (b).
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SAED patterns recorded from the single Bi2WO6 nanosheet in (b). (d) HRTEM image
Fig. 3 (a) HAADF image and elemental mapping images of (b) Cl, (c) Bi, (d) W and
cross-sectional
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(e) O element of the BiOCl@Bi2WO6 composite microspheres; (f-h) the compositional
line
profile
of
the
as-prepared
hierarchical
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BiOCl@Bi2WO6 microspheres.
Fig. 4 The XRD patterns of the prepared samples: (a) BiOCl precursor microspheres, (b) hierarchical BiOCl@Bi2WO6 microspheres and (c) Bi2WO6. Fig. 5 XPS spectra of the as-prepared BiOCl/Bi2WO6 composite microsphere: (a) survey spectrum, (b) W4f, (c) Cl2p and (d) Bi4f. Fig. 6 Schematic illustration of the anion exchange strategy: from BiOCl precursor microspheres to prepare BiOCl@Bi2WO6 heterojunctions. Fig. 7 (A) The photocatalytic degradation of RhB solution in the presence of different photocatalysts under visible light irradiation; (B) time-dependent optical absorption 20
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spectra of RhB degradation over the BiOCl@Bi2WO6 microsphere. Fig. 8 (A) Photocatalytic degradation of CIP in the presence of (a) BiOCl, (b) Bi2WO6 and (c) BiOCl@Bi2WO6 composite microsphere; (B) time-dependent optical
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absorption spectra of CIP aqueous solution over the BiOCl@Bi2WO6 microsphere.
(a) the as-prepared BiOCl
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energy and the corresponding band gap energy of
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Fig. 9 (A) UV-vis diffuse reflectance spectra and (B) the plot of (ahν)1/2 versus photo
precursor microspheres, (b) BiOCl@Bi2WO6-020 microsphere and (c) Bi2WO6.
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Fig. 10 Transient photocurrent responses of (a) BiOCl@Bi2WO6 hierarchical heterojunction, (b) Bi2WO6 photocatalysts and (c) BiOCl precursor microspheres.
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BiOCl@Bi2WO6 heterostructure.
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Fig. 11 Energy levels and charge separation and transfer illustration of
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Graphical Abstract
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Highlights 1. BiOCl@Bi2WO6 composites were prepared via a controlled anion exchange method.
facets.
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2. The shell of composites was composed of Bi2WO6 sheets with exposed (020)
3. The BiOCl@Bi2WO6 composites showed efficient photocatalytic activity.
Ac ce p
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an
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4. A possible photocatalytic degradation mechanism is proposed.
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S0169-4332(15)02827-5 http://dx.doi.org/doi:10.1016/j.apsusc.2015.11.130 APSUSC 31847
To appear in:
APSUSC
Received date: Revised date: Accepted date:
4-9-2015 13-11-2015 13-11-2015
Please cite this article as: Y. Ma, Z. Chen, D. Qu, J. Shi, Synthesis of chemically bonded BiOCl@Bi2 WO6 microspheres with exposed (020) Bi2 WO6 facets and their enhanced photocatalytic activities under visible light irradiation, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.11.130 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis of chemically bonded BiOCl@Bi2WO6 microspheres with exposed (020) Bi2WO6 facets and their enhanced photocatalytic activities under visible light irradiation
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Yongchao Maa, Zhiwei Chenb, Dan Qua, Jinsheng Shia* a
School of Life Sciences, Shandong University of Technology, Zibo 255049, People’s Republic of China
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b
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Qingdao Agricultural University, Qingdao 266109, People’s Republic of China
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* Corresponding author:
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E-mail: [email protected]; Tel: +86-532-88030161; Fax: +86-532-86080213
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Abstract Bi2WO6 photocatalysts has been extensively studied for its photocatalytic activity. However, few works have been conducted on hierarchical Bi2WO6 composite
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photocatalysts with specifically exposed facets. In this work, we report a facile
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method to synthesize BiOCl@Bi2WO6 hierarchical composite microspheres. Bi2WO6
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nanosheets with specifically exposed (020) facet were directly formed on the surface of BiOCl precursor microspheres via a controlled anion exchange route between
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BiOCl and Na2WO4. The visible-light photocatalytic activity of the BiOCl@Bi2WO6 heterojunction with exposed (020) facets (denoted as BiOCl@Bi2WO6) was
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investigated by degradation of Rhodamine B (RhB) and ciprofloxacin (CIP) aqueous solution under visible light irradiation. The experimental results indicated that the
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BiOCl@Bi2WO6 composite microsphere with intimate interfacial contacts exhibited improved efficiency for RhB photodegradation in comparison with pure BiOCl and
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Bi2WO6. The BiOCl@Bi2WO6 composite microsphere also shows high photocatalytic activity for degradation of CIP under visible light irradiation. The enhanced photocatalytic performance of BiOCl@Bi2WO6-020 hierarchical microspheres can be ascribed to the improved visible light harvesting ability, high charge separation and transfer. This work will make significant contributions toward the exploration of novel heterostructures with high potential in photocatalytic applications. Keywords: exposed facets, BiOCl@Bi2WO6, composite photocatalysts, controlled anion exchange, photocatalysis
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1. Introduction Recently, semiconductor-based photocatalysis has been attracted an increasing attention for environmental decontamination and hydrogen energy production since it
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is an environmental friendly way to take advantage of solar energy [1-3]. Traditional
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photocatalysts with wide band gap (e.g. TiO2, ZnO and SnO2) are intensively
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investigated due to their outstanding stability and suitable band gaps for photocatalytic reaction [4-6]. Unfortunately, they usually suffer from limited
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visible-light absorption due to their deep valence band that consists mostly of O2p states [7].
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Alternatively, the use of visible-light-driven semiconductor photocatalysts could effectively exploit visible light absorption for photoexcitation and further enhance the
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photocatalytic activities[8]. Up to date, Bi2WO6 has a layered structure with band gaps of about 2.75 eV, which effectively exploit visible light absorption due to the
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interaction between Bi6s and O2p orbitals at the top of the valence band[9, 10]. In recent work, Li et al. has prepared Bi2WO6 nanostructured bipyramids by the solvothermal method[11]. Hierarchical flower-like bismuth tungstate hollow spheres have been synthesized via a solvothermal process[12]. However, the photocatalytic performance of Bi2WO6 is still limited, because of its low separation efficiency of
photogenerated carriers in the photocatalytic process. To address this drawback, different methods have been studied, such as doping[13,
14],
morphology
control[15,
16],
metal
deposition[17]
and
heterostructures construction[18, 19]. Construction of a heterostrucutre photocatalyst 3
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might be an effective way to enhance the visible light photocatalytic activity of Bi2WO6[20]. In coupled semiconductors, the charge transfer from one particle to the other via interfaces is favorable for the electron-hole separation. It is well known that
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BiOCl semiconductor with wide band gap of 3.6 eV has been proved a promising
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photocatalyst because of its potential applications in wastewater treatment[21]. To
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date, some heterojunctions have been developed by coupling BiOCl with other semiconductors, such as BiOCl-Bi2S3[22], BiOCl-BiOI[23], BiOCl-Bi2WO6[24] and
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BiOCl-Co3O4[25]. The BiOCl-based composite exhibited enhanced photoactivity due to the efficient separation and transfer of photogenerated carries. On the other hand,
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surface properties are vital to a semiconductor’s photocatalytic performance, which sensitively depends on its exposed surfaces with distinct crystal facets [26, 27]. To the
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best of our knowledge, few works have been conducted on hierarchical Bi2WO6-based composite photocatalyst with specifically exposed facets.
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In this regard, we design and synthesize a novel hierarchical BiOCl@Bi2WO6
heterostructure composed of BiOCl precursor microspheres and Bi2WO6 nanoshees
with exposed (020) facets. The synthesis involves a facile ion exchange method between BiOCl precursor microspheres and Na2WO4. The excellent photocatalytic
performance of BiOCl@Bi2WO6 was evaluated by degrading RhB and CIP aqueous
solution as model pollutants under visible light (λ > 420 nm). Furthermore, the photocatalytic mechanism was also discussed. The BiOCl@Bi2WO6 heterojunction photocatalysts might be a viable alternative for photocatalytic applications. 2. Experimental section 4
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2.1 Synthesis of BiOCl precursor microspheres. All reagents were of analytical grade, purchased from Shanghai Chemical Reagent Company, and used as received without further purification. In a typical
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synthesis, 3 mmol of Bi(NO3)3‚5H2O was added slowly into an 60 mL EG solution
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containing 3 mmol of KCl. The mixture was stirred for 30 min at room temperature,
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and then poured into a 100 mL Teflon-lined stainless autoclave. The autoclave was allowed to be heated at 180 °C for 12 h, and then air cooled to room temperature. The
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resulting precipitates were collected and washed with ethanol and deionized water thoroughly and dried at 80 °C in air.
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2.2 Synthesis of mesoporous BiOCl@Bi2WO6 hierarchical microspheres The mesoporous BiOCl@Bi2WO6 hierarchical microspheres were synthesized by
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a controlled anion exchange method. Briefly, the as-prepared BiOCl was suspended into 80 mL of Na2WO4 solutions (6.25 mM) with the assistance of magnetic stirring
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for 10 min to form a homogeneous mixture. And then the resulting mixture was transferred to a 100 mL Teflon-lined stainless autoclave and maintained at 180 °C for 12 h. The autoclave was then cooled down to room temperature naturally. The product was collected by filtration and washed with deionized water and ethanol for several times, and dried in vacuum at 60 °C overnight. For comparison, the amount of Na2WO4·2H2O used in the anion exchange
process is 0.2, 1, 1.5 mmol. And the corresponding products were denoted as BiOCl/Bi2WO6-1, BiOCl/Bi2WO6-2 and Bi2WO6, respectively. For the preparation of N-doped TiO2 (N-TiO2), 1g of P25 was suspended in 5
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ethanol (10 mL). Then, urea (2 g) dissolved in 5 mL ethanol and 1 mL H2O was added into the suspension. The mixture was stirred and heated to completely evaporate the
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solvent, followed by calcination in air at 400 ℃ for 4 h[28]. 2.3 Materials Characterization
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The morphology of the as-prepared product was characterized by field-emission
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scanning electron microscopy (FESEM, JEOL, JSM-6700F) operated at an acceleration voltage of 5.0 kV. The crystalline structure of the product was analyzed
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by an X-ray diffractometer (XRD, Y-2000) with Cu Kα radiation (λ) 1.5418 Å). Transmission electron microscopy (TEM) and high-resolution transmission electron
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microscopy (HRTEM) observations were carried out on a JEOL JEM-2010 instrument in bright field and selected area electron diffraction (SAED) modes and on a HRTEM
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JEM-2010FEF instrument (operated at 200 kV). Room temperature UV-vis absorption spectrum was recorded on a TU-1901 spectrophotometer an integrating sphere
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attachment in the wavelength range of 200-800 nm. BaSO4 was used as the reflectance standard
material. The N2
adsorption-desorption isotherm and
Barrett-Joyner-Halenda (BJH) methods were analyzed on a Quantachrome NOVA 2200e analyzer.
2.4 Photocatalytic Test
The photocatalytic activities of the as-prepared products were evaluated by the degradation of RhB and CIP aqueous solution under visible light irradiation. The photocatalytic reactor (PLS-SXE 300, Beijing Perfect light Co., Ltd.), consisting of a quartz glass with a circulating water jack and a 300W Xe lamp with a 420 nm cutoff 6
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filter, was used as the visible light source. The light intensity striking the model pollutant solution was at
23 mW cm-2, as measured by a FZ-A optical Radiometer
(Photoelectric Instrument Factory of Beijing Norman University, Beijing, P. R. China).
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The optical spectrum of the 300W Xe lamp with a 420 nm cutoff filter is given in Fig.
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S1. In a typical experiment, 30 and 100 mg photocatalysts were added to an aqueous
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solution of RhB and CIP (100 mL, 10 mg L-1). Prior to irradiation, the suspensions were magnetically stirred in the dark for about 30 min to obtain a good
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adsorption-desorption equilibrium between the photocatalysts and the model pollutants under ambient conditions. At certain time intervals, 5 mL of the solution
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was taken out and centrifuged to remove the photocatalyst. The concentrations of RhB and CIP were analyzed by recording variations of the characteristic absorption
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band of 553 and 272 nm using a TU-1901 spectrophotometer, respectively. 2.5 Photoelectrochemical measurements
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The photoelectrochemical characteristics were measured in a CHI660D
electrochemical working station using a standard three-compartment cell under Xe arc lamp irradiation with 300 W. In a typical procedure, a commercial ITO glass (1.0 × 1.0 cm2) was ultrasonicated in an ethanol and distilled water bath prior to use. The
clean ITO glass substrate was then immersed in the slurry of the as-prepared photocatalyst (3 mg) and ethanol (3 ml) mixtures. The substrate was then vacuum-dried at 80 °C to eliminate ethanol and subsequently maintained at 100 °C overnight. ITO glass coated with the as-prepared samples, a piece of Pt sheet, a Ag/AgCl electrode and 0.01 M Na2SO4 were used as the working electrode, the 7
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counter electrode, the reference electrode and the electrolyte, respectively. 3. Results and discussion The as-prepared BiOCl microspheres were used as both a precursor for the
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growth of Bi2WO6 shell and a core substrate to support the shell. Previous studies
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have shown that Bi2WO6 tend to grow into nanoplates/sheets with 2D features
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because of their highly anisotropic layered structures[29, 30]. In our case, the use of BiOCl microspheres as the substrate led to a BiOCl/Bi2WO6 core/shell
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heterostructures with a hierarchical architecture.
3.1 Structure, composition and morphology of the prepared photocatalysts.
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The morphologies of the prepared products were characterized by SEM. Fig. 1a-b show the uniform BiOCl precursor microspheres with diameters ranging from
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2-4 µm, which were built by interlaced nanorods. As shown in Fig. 1c-d, the as-prepared BiOCl@Bi2WO6 composites hierarchical microspheres with the size of
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about 2-5 µm are observed. The shell of the hierarchical microsphere is composed of crossed nanosheets with the thickness of about 35 nm. The corresponding EDS is shown in Fig, 1e. It can be seen that the sample was composed of Bi, W, O, Cl elements, which verified the coexistence of both phases in the composite photocatalysts. The red arrow suggests the nature of hollow opening of the pure Bi2WO6. The effect of concentration of the WO42- anions in the reaction system on the morphologies of formed BiOCl/Bi2WO6 composites was also investigated, as shown
in Fig.S2. The morphology and structure of BiOCl@Bi2WO6 microspheres were further 8
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investigated by TEM analysis. Fig. 2a shows the spherical structure of the BiOCl@Bi2WO6 products, which is in accord with the SEM analysis (Fig. 1a). It is noteworthy that the interfacial regions are difficult to directly inspect, which can be
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explained by the following two reasons: (i) the particle size of the heterostructure is
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large; (ii) the single-crystalline Bi2WO6 nanosheets have a very good connection with
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the BiOCl core. However, the intimate interfacial contact is crucially important for effective interface electronic-structure modification and charge carrier’s transfer
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between two phases [31]. Fig. 2b shows a single Bi2WO6 nanosheet peeling off the heterostructure upon strong ultrasonic treatment with the length of around 100 nm.
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The SAED patterns (Fig. 2c), recorded from the single nanosheet in Fig. 2b, present the diffraction spots of the [010] zone axis, which confirms the single-crystal nature
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of the nanosheet. Based on the above analysis and the symmetries of Bi2WO6, the bottom and top surfaces of the Bi2WO6 nanosheet are identified as (020) facets. The
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HRTEM image taken from the marked area in Fig. 2b indicates the (200) atomic lattice spacing of 0.273 nm. Based on the length (around 200 nm) of the Bi2WO6 nanosheets in shell, it can be inferred that the diameter of BiOCl core, as indicated by the inner white circle in Fig. 2a, is nearly 1.0 µm. The high-angle annular dark field (HAADF, Fig. 3a) image and element mapping
analysis (Fig. 3b-e) of BiOCl@Bi2WO6 composite microspheres reveals the presence of W, Bi, Cl and O elements, suggesting the co-existing of both BiOCl and Bi2WO6. Furthermore, the relative location of BiOCl core and the Bi2WO6 shell can be further confirmed by cross-sectional compositional line profiles, as shown in Fig.3f-h. The 9
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comparison between line profiles of the Cl and W elements obtained along the orange wire axis (Fig. 3f) reveal that the distribution of Cl in the core and an increase of W in the surface of the composite microsphere.
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The crystal structure and purity of the as-obtained photocatalysts are examined
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by XRD analysis. As shown in Fig. 4a and c, all the diffraction peaks can be indexed
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to the tetragonal structure of BiOCl (JCPDS 06-0249) and orthorhombic structure of Bi2WO6 (JPCDS 39-0256), respectively. No other impurity peak is detected. The
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XRD pattern (Fig. 4b) of the BiOCl@Bi2WO6 composite microspheres confirms that most of the peaks are matched perfectly to those of pure BiOCl, except the peaks
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indicated by asterisks (★) can be indexed as the orthorhombic structure of Bi2WO6 (JPCDS 39-0256).
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d
The chemical composition and surface chemical states of the BiOCl/Bi2WO6 composite microsphere were investigated by XPS analysis, as shown in Fig. 5.
Ac ce p
According to the XPS observations (Fig. 5a), only C, Bi, O, W and Cl were detected in the sample. The C1s peak at around 284.6 eV can be attributed to the signal from carbon contained in the instrument and was used for calibration[32]. The characteristic spin-orbit splitting of W4f5/2 and W 4f7/2 signals is observed at
approximately 37.0 and 34.9 eV, respectively, corresponding to W6+ in Bi2WO6 (Fig. 5b) [33]. In addition, the Cl 2p high-resolution XPS spectrum showed two major peaks with binding energy at 197.1 and 198.5 eV, which could be ascribed to Cl 2p3/2 and Cl 2p1/2 of Cl-, respectively (Fig. 4c) [34]. Two strong peaks in the high-resolution
XPS spectra, at 164.4 and 159.0 eV, are assigned to Bi 4f5/2 and Bi 4f7/2, respectively, 10
Page 10 of 22
which confirms that the bismuth species in the BiOCl/Bi2WO6 composite is Bi3+ cations[35] (Fig. 5d). Based on the above analysis, the illustration of phase transition process between
ip t
BiOCl precursors and BiOCl/Bi2WO6 heterojunction was shown in Fig. 6. In the
cr
formation process of BiOCl/Bi2WO6 heterojunction, the BiOCl precursor microsphere
us
is employed as both the physical and chemical templates. Due to the lower solubility of Bi2WO6 relative to BiOCl, BiOCl could adopt a thermodynamically favored
an
direction to transform into Bi2WO6 by reacting with WO42- ions, which is analogous to the formation of ZnS hollow nanospheres by heteroepitaxial exchange around the
M
ZnO precursor nanoparticles[36, 37]. Thus, upon the introduction of WO42- followed by hydrothermal treatments, Bi2WO6 thin layer was formed around the surface of
te
d
BiOCl precursor microspheres, during which Cl- ions were replaced by WO42- ions. On the other hand, as two Aurivillius-type oxides, BiOCl and Bi2WO6 are both
Ac ce p
crystallized in layered structures composed of (Bi2O2)2+ slabs interleaved with anion layers[38]. This structural similarity may be responsible for the formation of BiOCl/Bi2WO6 heterojunction with intimate contact interface, because of the
interaction between BiOCl precursor and subsequently formed Bi2WO6 component with chemical bonding[39]. Therefore, a close interfacial contact between Bi2WO6
nanosheets and BiOCl precursor was obtained, which can effectively improve the separation efficiency of photogenerated carriers. 3.2 Photocatalytic performance The photocatalytic performance of the as-prepared composite photocatalyst was 11
Page 11 of 22
evaluated using the degradation of RhB and CIP aqueous solution under visible light irradiation. As shown in Fig. 7, RhB was very stable in the absence of photocatalyst. It is interesting to note that the BiOCl@Bi2WO6-020 microspheres exhibit the highest
ip t
photocatalytic degradation efficiency. The RhB photodegradation process was found
cr
to follow pseudo-first-order kinetics. Fig. S3a clearly indicates that the removal rate
us
of RhB over the BiOCl@Bi2WO6-020 microspheres is much faster than that over the pure BiOCl and Bi2WO6, and the apparent rate constant k over the BiOCl, Bi2WO6
an
and BiOCl@Bi2WO6 composite is 0.00254, 0.00499 and 0.01392 min−1, respectively. Moreover, the rate constant over the BiOCl@Bi2WO6 nanocomposite is more than 7.5
M
times that over N-TiO2 (used as reference photocatalyst, k = 0.00186 min−1). The corresponding absorption spectra reveal a gradual decline in the intensity of the peak
te
d
at 553 nm as exposure time increases and they eventually disappear completely after 90 min (Fig. 7B). The peak intensity in the UV region (280 nm < λ < 400 nm) also
Ac ce p
decreases with increasing irradiation time, which further indicates that both dye chromophore and aromatic ring have been destroyed. The photocatalytic activities of the other BiOCl/Bi2WO6 composites were also
investigated. As shown in Fig. S3b, the photodegradation efficiency increases with increasing Bi2WO6 content, but at higher Bi2WO6 concentration, the photocatalytic
activity decreases. This may be ascribed to the following reasons: with excess content of Bi2WO6 in the composite photocatalysts, more photoinduced carriers would recombine easily on the surface of Bi2WO6[40]. On the other hand, the morphology of composite photocatalysts was changed, forming loosely interfacial contact. 12
Page 12 of 22
As a broad-spectrum antibiotic agent, ciprofloxacin (CIP) has been widely used for treating bacterial infections[41]. However, the lack of treatment processes has led
ip t
to their ubiquity in water, which may accelerate antibiotic resistance within native
cr
bacterial populations in the environments[42]. As a result, it is great significant to
us
degrade the CIP aqueous solution. As shown in Fig. 8A, before light irradiation, about 50, 40 and 25 % CIP were adsorbed by BiOCl, Bi2WO6 and BiOCl@Bi2WO6
an
photocatalyst within 30 min, respectively, which is consistent with the specific surface area analysis (As shown in Fig. S4). After irradiation of 5 h, about 50 and 65 % of CIP
M
were photocatalytic degraded by Bi2WO6 and BiOCl@Bi2WO6, while BiOCl cannot be activate by visible light for its wide band gap. The corresponding absorption
te
d
spectra of BiOCl@Bi2WO6 composite microspheres reveal a gradual decline in the intensity of the peak at 272 nm as exposure time increases (Fig. 8B).
Ac ce p
3.3 Mechanisms of improved photocatalytic properties. The light absorption ability of the prepared photocatalysts was investigated by
the UV-visible diffuse reflectance spectra. As shown in Fig. 9A, the absorption edge of the pure BiOCl and Bi2WO6 occur at about 360 nm and 450 nm, respectively,
which is in agreement with the previously reported result[43-45]. Notably, the ability of light absorption of the BiOCl@Bi2WO6-020 microsphere is enhanced in the wavelength range 500–700 nm. These results are attributed to the interaction between BiOCl and Bi2WO6 as well as the unique 3D hierarchical surface morphology. The large visible light trapping would endow the formation of more electron-hole pairs 13
Page 13 of 22
and
further
improve
the
photocatalytic
activity
of
BiOCl@Bi2WO6-020
microsphere[46]. According to the Kubelka–Munk function, the plot of (ahν)1/2 versus hν based on the indirect transition is shown as Fig. 9B. The band gaps of the BiOCl
ip t
and Bi2WO6 BiOCl@Bi2WO6-020 photocatalyst are 3.22 and 2.52 eV, respectively.
cr
This result indicates that the BiOCl@Bi2WO6-020 microsphere has a suitable
us
bandgap for photocatalytic decomposition of organic contaminants under visible light irradiation.
an
To further understand the improvement of photocatalytic activity of the as-prepared photocatalysts, the photogenerated current measurements were performed,
M
as shown in Fig. 10. It can be clearly observed that the BiOCl@Bi2WO6-020 microsphere exhibit enhanced photocurrents compared with the BiOCl precursors as
te
d
well as pure Bi2WO6 photocatalyst. The noticeable improvement of photocurrent response can be ascribed to the following two reasons: (i) the formation of
Ac ce p
heterojunctions can provide efficient charge separation[47, 48]; (ii) Bi2WO6 has a unique layered structure, which is characterized by [Bi2O2] slabs interleaved with
perovskite-like WO42- layers[49]. This may induce the presence of internal static
electric field perpendicular to the [Bi2O2] slabs interleaved with perovskite-like WO42layers in Bi2WO6, enhancing the separation of the photogenerated elelctric-hole pairs
along the [010] direction, as schematically illustrated in Fig. S5. Fig. 11 shows the charge transfer pathway of the RhB degradation process over BiOCl/Bi2WO6 heterojunction. The band edge positions of the conduction band (CB) and valence band (VB) of Bi2WO6 and BiOCl are approximately 0.60 and 0.23 eV, 14
Page 14 of 22
3.12 and 3.45 eV, respectively (Table S1). Under the visible light irradiation (energy less than 2.95 eV), Bi2WO6 with a narrow band gap energy (Eg=2.52 eV) are photoexcited to generated electron-hole pairs, while BiOCl will be not. Meanwhile,
ip t
electrons in the VB of Bi2WO6 could also be excited up to a higher potential edge
cr
(0.17 eV) due to the higher photon energy [40]. Thus, the photoinduced electron in the
us
CB of Bi2WO6 could easily flow into the CB edge of BiOCl, leaving holes on the valence band of Bi2WO6. In such a way, the photogenerated electron-hole pairs can be
an
efficiently separated.
Based on the above analyses, the enhanced performance of composite could be
M
attributed to its unique surface and electronic properties. The microsphere morphology with layered structures allows multiple reflections of visible light, which
te
d
can make more efficient use of the light source. Furthermore, photocatalysts in microsphere morphology can be more readily separated from the slurry system by
Ac ce p
filtration after the photocatalytic reaction and reused than the nanosized samples because of their large weight, weak Brownian motion and good mobility[50]. All of these advantages make the photocatalysts with layered structures appealing photocatalysts in the aqueous photocatalytic reaction. 4. Conclusions
In summary, a novel BiOCl@Bi2WO6-020 composite photocatalyst with layered structure has been synthesized. Bi2WO6 nanosheets with specifically exposed (020) facets were directly formed on the external surface of BiOCl precursor microsphere. Phase transition between the structures of BiOCl and Bi2WO6 nanosheets takes place 15
Page 15 of 22
readily by a facile anion exchange approach. Monodispersed BiOCl microspheres are first synthesized and subsequently undergone controlled chemical transformation in Na2WO4 solution under hydrothermal conditions. This is highly significant because it
ip t
allows us to obtain delicate heterostructures. Interestingly, the molar ratio of Na2WO4
cr
to BiOCl precursor microspheres has a significant influence on the surface structure
microsphere
and
Bi2WO6
product,
the
us
of the composite photocatalysts. Compared with the pure BiOCl precursor BiOCl@Bi2WO6-020
hierarchical
an
microspheres exhibit superior photocatalytic activity for decomposition of RhB and CIP under visible light irradiation. The decomposition rate of RhB over the
M
BiOCl@Bi2WO6-020 microsphere was 0.01392 min-1, which is approximately 2.79 and 5.48 times greater than that of pure Bi2WO6 and BiOCl, respectively. The
and
transfer
can
te
d
synergistic effects of improved visible light harvesting ability, high charge separation lead
to
the
enhanced
photocatalytic
activity
of
the
Ac ce p
BiOCl@Bi2WO6-020 heterojunctions. The results presents in this study indicates that
the combination of morphology engineering and heterojunction construction is a useful approach for designing heterojunction photocatalysts with high charge separation efficiency. Acknowledgement
This work was supported by the National Natural Science Foundation of China (Grant No. 31071538), Natural Key Science Foundation of Shandong Province (ZR2013FB001), the Research Fund for Technology Upgrading of Large Scientific Instruments and Equipment in Shandong Province (2013SJGZ01) and Science and 16
Page 16 of 22
Technology Development Plan of Shandong Province, China (2014GNC110013) . References [1] F. Amano, A. Yamakata, K. Nogami, M. Osawa, B. Ohtani, Visible Light Responsive Pristine Metal Oxide Photocatalyst: Enhancement of Activity by Crystallization under Hydrothermal Treatment J. Am.
ip t
Chem. Soc. 130 (2008) 17650-17651.
[2] J. Yu, X. Yu, Hydrothermal Synthesis and Photocatalytic Activity of Zinc Oxide Hollow Spheres, Environ. Sci. Technol. 42 (2008) 4902-4907.
cr
[3] P. Zhou, J. Yu, M. Jaroniec, All-Solid-State Z-Scheme Photocatalytic Systems, Adv. Mater. 26 (2014) 4920-4935.
[4] H. Liu, J.B. Joo, M. Dahl, L. Fu, Z. Zeng, Y. Yin, Crystallinity control of TiO2 hollow shells
us
through resin-protected calcination for enhanced photocatalytic activity, Energy Environ. Sci. 8 (2015) 286-296.
[5] S. Wang, L. Zhao, L. Bai, J. Yan, Q. Jiang, J. Lian, Enhancing photocatalytic activity of
an
disorder-engineered C/TiO2 and TiO2 nanoparticles, J. Mater. Chem. A 2 (2014) 7439-7445. [6] L.-L. Tan, W.-J. Ong, S.-P. Chai, A.R. Mohamed, Band gap engineered, oxygen-rich TiO2 for visible light induced photocatalytic reduction of CO2, Chem. Commun. 50 (2014) 6923-6926. [7] L. Ye, Y. Su, X. Jin, H. Xie, C. Zhang, Recent advances in BiOX (X = Cl, Br and I) photocatalysts:
M
synthesis, modification, facet effects and mechanisms, Environ. Sci.: Nano 1 (2014) 90-112. [8] D.J. Martin, G. Liu, S.J.A. Moniz, Y. Bi, A.M. Beale, J. Ye, J. Tang, Efficient visible driven (2015) 7808-7828.
d
photocatalyst, silver phosphate: performance, understanding and perspective, Chem. Soc. Rev. 44 [9] T. Saison, P. Gras, N. Chemin, C. Chanéac, O. Durupthy, V. Brezová, C. Colbeau-Justin, J.-P. (2013) 22656-22666.
te
Jolivet, New Insights into Bi2WO6 Properties as a Visible-Light Photocatalyst, J. Phys. Chem. C 117 [10] J. Yang, X. Wang, X. Zhao, J. Dai, S. Mo, Synthesis of Uniform Bi2WO6-Reduced Graphene
Ac ce p
Oxide Nanocomposites with Significantly Enhanced Photocatalytic Reduction Activity, J. Phys. Chem. C 119 (2015) 3068-3078.
[11] G. Zhang, Z. Hu, M. Sun, Y. Liu, L. Liu, H. Liu, C.-P. Huang, J. Qu, J. Li, Formation of Bi2WO6
Bipyramids with Vacancy Pairs for Enhanced Solar-Driven Photoactivity, Adv. Funct. Mater. 25 (2015) 3726-3734.
[12] P. Zhang, J. Zhang, A. Xie, S. Li, J. Song, Y. Shen, Hierarchical flower-like Bi2WO6 hollow microspheres: facile synthesis and excellent catalytic performance, RSC Adv. 5 (2015) 23080-23085. [13] Z. Zhang, W. Wang, Y. Zhou, Hydrothermal synthesis of a novel BiErWO6 photocatalyst with wide
spectral responsive property, Appl. Surf. Sci. 319 (2014) 250-255. [14] X. Ding, K. Zhao, L. Zhang, Enhanced photocatalytic removal of sodium pentachlorophenate with self-doped Bi2WO6 under visible light by generating more superoxide ions, Environ. Sci. Technol. 48
(2014) 5823-5831. [15] Y.-X. Zhou, L. Tong, X.-H. Zeng, X.-B. Chen, Green synthesis of flower-like Bi2WO6 microspheres as a visible-light-driven photocatalyst, New J. Chem. 38 (2014) 1973-1979. [16] R.a. He, S. Cao, P. Zhou, J. Yu, Recent advances in visible light Bi-based photocatalysts, Chin. J. Catal. 35 (2014) 989-1007. [17] D. Wang, G. Xue, Y. Zhen, F. Fu, D. Li, Monodispersed Ag nanoparticles loaded on the surface of 17
Page 17 of 22
spherical Bi2WO6 nanoarchitectures with enhanced photocatalytic activities, J. Mater. Chem. 22 (2012) 4751-4758. [18] P. Ju, P. Wang, B. Li, H. Fan, S. Ai, D. Zhang, Y. Wang, A novel calcined Bi2WO6/BiVO4 heterojunction photocatalyst with highly enhanced photocatalytic activity, Chem. Eng. J. 236 (2014) 430-437. [19] S. Girish Kumar, K.S.R. Koteswara Rao, Tungsten-based nanomaterials (WO3&Bi2WO6):
ip t
Modifications related to charge carrier transfer mechanisms and photocatalytic applications, Appl. Surf. Sci. 355 (2015) 939-958.
[20] S. Xue, Z. Wei, X. Hou, W. Xie, S. Li, X. Shang, D. He, Enhanced visible-light photocatalytic
cr
activities and mechanism insight of BiVO4/Bi2WO6 composites with virus-like structures, Appl. Surf. Sci. 355 (2015) 1107-1115.
[21] M. Gao, D. Zhang, X. Pu, M. Li, Y.M. Yu, J.J. Shim, P. Cai, S.I. Kim, H.J. Seo, Combustion
us
Synthesis of BiOCl with Tunable Percentage of Exposed {001} Facets and Enhanced Photocatalytic Properties, J. Am. Ceram. Soc. 98 (2015) 1515-1519.
[22] S. Jiang, K. Zhou, Y. Shi, S. Lo, H. Xu, Y. Hu, Z. Gui, In situ synthesis of hierarchical flower-like
an
Bi2S3/BiOCl composite with enhanced visible light photocatalytic activity, Appl. Surf. Sci. 290 (2014) 313-319.
[23] T.B. Li, G. Chen, C. Zhou, Z.Y. Shen, R.C. Jin, J.X. Sun, New photocatalyst BiOCl/BiOI composites with highly enhanced visible light photocatalytic performances, Dalton Trans. 40 (2011)
M
6751-6758.
[24] W. Yang, B. Ma, W. Wang, Y. Wen, D. Zeng, B. Shan, Enhanced photosensitized activity of a BiOCl-Bi2WO6 heterojunction by effective interfacial charge transfer, Phys. Chem. Chem. Phys. 15
d
(2013) 19387-19394.
[25] C. Tan, G. Zhu, M. Hojamberdiev, K. Okada, J. Liang, X. Luo, P. Liu, Y. Liu, Co3O4
te
nanoparticles-loaded BiOCl nanoplates with the dominant {001} facets: efficient photodegradation of organic dyes under visible light, Appl. Catal. B: Environ. 152–153 (2014) 425-436. [26] M. Pan, H. Zhang, G. Gao, L. Liu, W. Chen, Facet-Dependent Catalytic Activity of
Ac ce p
Nanosheet-Assembled Bismuth Oxyiodide Microspheres in Degradation of Bisphenol A Environ. Sci. Technol., 49 (2015) 6240-6248.
[27] J. Yu, J. Low, W. Xiao, P. Zhou, M. Jaroniec, Enhanced Photocatalytic CO2-Reduction Activity of Anatase TiO2 by Coexposed {001} and {101} Facets, J. Am. Chem. Soc. 136 (2014) 8839-8842.
[28] Z. Xiong, X.S. Zhao, Nitrogen-Doped Titanate-Anatase Core–Shell Nanobelts with Exposed {101} Anatase Facets and Enhanced Visible Light Photocatalytic Activity, J. Am. Chem. Soc. 134 (2012) 5754-5757.
[29] N. Zhang, R. Ciriminna, M. Pagliaro, Y.-J. Xu, Nanochemistry-derived Bi2WO6 nanostructures:
Towards production of sustainable chemicals and fuels induced by visible light, Chem. Soc. Rev. 43 (2014) 5276-5287. [30] Q. Sun, X. Jia, X. Wang, H. Yu, J. Yu, Facile synthesis of porous Bi2WO6 nanosheets with high photocatalytic performance, Dalton Trans. 44 (2015) 14532-14539. [31] W. Wang, J. Wang, Z. Wang, X. Wei, L. Liu, Q. Ren, W. Gao, Y. Liang, H. Shi, p-n junction CuO/BiVO4 heterogeneous nanostructures: synthesis and highly efficient visible-light photocatalytic performance, Dalton Trans. 43 (2014) 6735-6743. [32] Z. He, Y. Shi, C. Gao, L. Wen, J. Chen, S. Song, BiOCl/BiVO4 p–n Heterojunction with Enhanced
Photocatalytic Activity under Visible-Light Irradiation, J. Phys. Chem. C 118 (2013) 389-398. 18
Page 18 of 22
[33] M. Li, L. Zhang, X. Fan, Y. Zhou, M. Wu, J. Shi, Highly selective CO2 photoreduction to CO over g-C3N4/Bi2WO6 composites under visible light, J. Mater. Chem. A 3 (2015) 5189-5196. [34] J. Di, J. Xia, M. Ji, B. Wang, S. Yin, Q. Zhang, Z. Chen, H. Li, Carbon Quantum Dots Modified BiOCl Ultrathin Nanosheets with Enhanced Molecular Oxygen Activation Ability for Broad Spectrum Photocatalytic Properties and Mechanism Insight, ACS Appl. Mater. Interfaces 7 (2015) 20111-20123. [35] X. Zhang, L. Zhang, J.-S. Hu, C.-L. Pan, C.-M. Hou, Facile hydrothermal synthesis of novel
ip t
Bi12TiO20-Bi2WO6 heterostructure photocatalyst with enhanced photocatalytic activity, Appl. Surf. Sci. 346 (2015) 33-40.
[36] H. Cheng, B. Huang, Y. Liu, Z. Wang, X. Qin, X. Zhang, Y. Dai, An anion exchange approach to
cr
Bi2WO6 hollow microspheres with efficient visible light photocatalytic reduction of CO2 to methanol, Chem. Commun. 48 (2012) 9729-9731.
[37] Y.P. Xie, Z.B. Yu, G. Liu, X.L. Ma, H.-M. Cheng, CdS-mesoporous ZnS core-shell particles for
us
efficient and stable photocatalytic hydrogen evolution under visible light, Energy Environ. Sci. 7 (2014) 1895-1901.
[38] M. Huang, Y. Yan, W. Feng, S. Weng, Z. Zheng, X. Fu, P. Liu, Controllable Tuning Various Ratios
an
of ZnO Polar Facets by Crystal Seed-Assisted Growth and Their Photocatalytic Activity, Cryst. Growth Des. 14 (2014) 2179-2186.
[39] Y.-S. Xu, Z.-J. Zhang, W.-D. Zhang, Inlay of Bi2O2CO3 nanoparticles onto Bi2WO6 nanosheets to build heterostructured photocatalysts, Dalton Trans. 43 (2014) 3660-3668.
M
[40] Z. Wu, L. Chen, C. Xing, D. Jiang, J. Xie, M. Chen, Controlled synthesis of Bi2S3/ZnS microspheres by an in situ ion-exchange process with enhanced visible light photocatalytic activity, Dalton Trans. 42 (2013) 12980-12988.
d
[41] J. Di, J. Xia, M. Ji, S. Yin, H. Li, H. Xu, Q. Zhang, H. Li, Controllable synthesis of Bi4O5Br2 ultrathin nanosheets for photocatalytic removal of ciprofloxacin and mechanism insight, J. Mater.
te
Chem. A 3 (2015) 15108-15118.
[42] T. Paul, P.L. Miller, T.J. Strathmann, Visible-Light-Mediated TiO2 Photocatalysis of Fluoroquinolone Antibacterial Agents, Environ. Sci. Technol. 41 (2007) 4720-4727.
Ac ce p
[43] D. Yue, D. Chen, Z. Wang, H. Ding, R. Zong, Y. Zhu, Enhancement of visible photocatalytic performances of a Bi2MoO6-BiOCl nanocomposite with plate-on-plate heterojunction structure, Phys.
Chem. Chem. Phys. 16 (2014) 26314-26321. [44] Z. Zhang, W. Wang, L. Wang, S. Sun, Enhancement of Visible-Light Photocatalysis by Coupling with Narrow-Band-Gap Semiconductor: A Case Study on Bi2S3/Bi2WO6, ACS Appl. Mater. Interfaces 4
(2012) 593-597.
[45] S. Shenawi-Khalil, V. Uvarov, E. Menes, I. Popov, Y. Sasson, New efficient visible light photocatalyst based on heterojunction of BiOCl–bismuth oxyhydrate, Appl. Catal. A: Gen. 413–414 (2012) 1-9.
[46] J. Di, J. Xia, M. Ji, H. Li, H. Xu, H. Li, R. Chen, The synergistic role of carbon quantum dots for the improved photocatalytic performance of Bi2MoO6, Nanoscale 7 (2015) 11433-11443. [47] Y. Shi, Y. Chen, G. Tian, H. Fu, K. Pan, J. Zhou, H. Yan, One-pot controlled synthesis of sea-urchin shaped Bi2S3/CdS hierarchical heterostructures with excellent visible light photocatalytic activity, Dalton Trans. 43 (2014) 12396-12404. [48] D. He, L. Wang, D. Xu, J. Zhai, D. Wang, T. Xie, Investigation of Photocatalytic Activities over Bi2WO6/ZnWO4 Composite under UV Light and Its Photoinduced Charge Transfer Properties, ACS Appl. Mater. Interfaces 3 (2011) 3167-3171. 19
Page 19 of 22
[49] S. Sun, W. Wang, L. Zhang, E. Gao, D. Jiang, Y. Sun, Y. Xie, Ultrathin {001}-Oriented Bismuth Tungsten Oxide Nanosheets as Highly Efficient Photocatalysts, ChemSusChem 6 (2013) 1873-1877. [50] J. Yu, X. Yu, B. Huang, X. Zhang, Y. Dai, Hydrothermal Synthesis and Visible-light Photocatalytic Activity of Novel Cage-like Ferric Oxide Hollow Spheres, Cryst. Growth Des. 9 (2009) 1474-1480.
Figure Captions
ip t
Fig. 1 SEM images of the as-prepared samples: (a, b) BiOCl precursor microspheres,
cr
(c, d) hierarchical BiOCl@Bi2WO6 composites, and (f) Bi2WO6 samples; (e) the
us
corresponding EDS of BiOCl@Bi2WO6 microspheres.
Fig. 2 TEM images of (a) a BiOCl@Bi2WO6 heterostructure particle and (b) a single
an
Bi2WO6 nanosheet peeling off the heterostructure upon strong ultrasonic treatment. (c)
of the Bi2WO6 nanosheet in (b).
M
SAED patterns recorded from the single Bi2WO6 nanosheet in (b). (d) HRTEM image
Fig. 3 (a) HAADF image and elemental mapping images of (b) Cl, (c) Bi, (d) W and
cross-sectional
te
d
(e) O element of the BiOCl@Bi2WO6 composite microspheres; (f-h) the compositional
line
profile
of
the
as-prepared
hierarchical
Ac ce p
BiOCl@Bi2WO6 microspheres.
Fig. 4 The XRD patterns of the prepared samples: (a) BiOCl precursor microspheres, (b) hierarchical BiOCl@Bi2WO6 microspheres and (c) Bi2WO6. Fig. 5 XPS spectra of the as-prepared BiOCl/Bi2WO6 composite microsphere: (a) survey spectrum, (b) W4f, (c) Cl2p and (d) Bi4f. Fig. 6 Schematic illustration of the anion exchange strategy: from BiOCl precursor microspheres to prepare BiOCl@Bi2WO6 heterojunctions. Fig. 7 (A) The photocatalytic degradation of RhB solution in the presence of different photocatalysts under visible light irradiation; (B) time-dependent optical absorption 20
Page 20 of 22
spectra of RhB degradation over the BiOCl@Bi2WO6 microsphere. Fig. 8 (A) Photocatalytic degradation of CIP in the presence of (a) BiOCl, (b) Bi2WO6 and (c) BiOCl@Bi2WO6 composite microsphere; (B) time-dependent optical
ip t
absorption spectra of CIP aqueous solution over the BiOCl@Bi2WO6 microsphere.
(a) the as-prepared BiOCl
us
energy and the corresponding band gap energy of
cr
Fig. 9 (A) UV-vis diffuse reflectance spectra and (B) the plot of (ahν)1/2 versus photo
precursor microspheres, (b) BiOCl@Bi2WO6-020 microsphere and (c) Bi2WO6.
an
Fig. 10 Transient photocurrent responses of (a) BiOCl@Bi2WO6 hierarchical heterojunction, (b) Bi2WO6 photocatalysts and (c) BiOCl precursor microspheres.
te
d
BiOCl@Bi2WO6 heterostructure.
M
Fig. 11 Energy levels and charge separation and transfer illustration of
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Graphical Abstract
21
Page 21 of 22
Highlights 1. BiOCl@Bi2WO6 composites were prepared via a controlled anion exchange method.
facets.
ip t
2. The shell of composites was composed of Bi2WO6 sheets with exposed (020)
3. The BiOCl@Bi2WO6 composites showed efficient photocatalytic activity.
Ac ce p
te
d
M
an
us
cr
4. A possible photocatalytic degradation mechanism is proposed.
22
Page 22 of 22