Shell BiVO4

Research Article pubs.acs.org/journal/ascecg

A General and Facile Approach to Heterostructured Core/Shell BiVO4/BiOI p−n Junction: Room-Temperature in Situ Assembly and Highly Boosted Visible-Light Photocatalysis Hongwei Huang,*,† Ying He,† Xin Du,‡ Paul K. Chu,§ and Yihe Zhang*,† †

Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China ‡ Research Center for Bioengineering and Sensing Technology, Department of Chemistry & Biological Engineering, University of Science & Technology Beijing, Beijing 100083, China § Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China S Supporting Information *

ABSTRACT: Development of core/shell heterostructures and semiconductor p−n junctions is of great concern for environmental and energy applications. Herein, we develop a facile in situ deposition route for fabrication of a BiVO4/BiOI composite integrating both the core/shell heterostructure and semiconductor p−n junction at room temperature. In the BiVO4/BiOI core/shell heterostructure, the BiOI nanosheets are evenly assembled on the surface of the BiVO4 cores. The photocatalytic performance is evaluated by monitoring the degradation of the dye model Rhodamine B (RhB), colorless contaminant phenol, and photocurrent generation under visible-light irradiation. The heterostructured BiVO4/BiOI core/shell photocatalyst shows drastically enhanced photocatalysis properties compared to the pristine BiVO4 and BiOI. This remarkable enhancement is attributed to the intimate interfacial interactions derived from the core/shell heterostructure and formation of the p−n junction between the p-type BiOI and n-type BiVO4. Separation and transfer of photogenerated electron−hole pairs are hence greatly facilitated, thereby resulting in the improved photocatalytic performance as confirmed by electrochemical, photoelectrochemical, radicals trapping, and superoxide radical (•O2−) quantification results. Moreover, the core/shell BiVO4/BiOI also displays high photochemical stability. This work sheds new light on the construction of high-performance photocatalysts with core/shell heterostructures and matchable band structures in a simple and efficient way. KEYWORDS: Core/shell heterostructures, p−n junction, Photocatalysis, BiVO4, BiOI

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INTRODUCTION Semiconductor photocatalysts have gained considerable attention due to their diverse environmental and energy-related applications. In particular, titanium dioxide (TiO2) is a widely used photocatalyst.1,2 However, it mainly absorbs ultraviolet (UV) light due to its relatively large bandgap of 3.2 eV, thus only making use of 4% of sunlight. For the purpose of utilizing the visible-light region of the solar spectrum, visible-light sensitive photocatalysts are desired.3−6 However, the efficiency of most visible-light active photocatalysts is not satisfactory because the photogenerated charge carriers recombine easily, adversely affecting photocatalytic degradation of environmental pollutants. Bi-based compounds such as BiVO4,7 Bi2WO6,8 Bi2MoO6,9 BiOX (X = Br, I),10−12 BiFeO3,13 and CaBi2O414 show potentials as visible-light photocatalysts. These bi-based compounds have the Sillén structure, Aurivillius structure, or perovskite structure composed of alternating layers of (Bi2O2)2+ and other inorganic groups.15 BiVO4 is a member of this family © XXXX American Chemical Society

and has attracted much interest because of its relatively small band gap, high stability, and photocatalytic activity pertaining to the degradation of dye molecules and water splitting.16−18 BiVO4, a n-type semiconductor, possesses three main crystalline phases: tetragonal scheelite, monoclinic scheelite, and tetragonal zircon. Generally, the monoclinic scheelite shows the most excellent photocatalytic activity.19 But, the photocatalytic activity of individual BiVO4 is not satisfactory due to the large recombination rates of photogenerated carriers and poor adsorption capacity.20 Different types of BiVO4-based composites such as Co3O4/BiVO4,21 Cu2O/BiVO4,22 Bi2O3/BiVO4,23 BiOCl/BiVO4,24,25 and V2O5/BiVO426 are synthesized to enhance the visible-light photocatalytic efficiency. As another member of the Sillén oxide family, BiOI, is a more promising Received: August 9, 2015 Revised: October 14, 2015

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DOI: 10.1021/acssuschemeng.5b01038 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 1. Schematic crystal structure of (a) tetragonal phase BiOI and (b) monoclinic phase BiVO4. (c) XRD patterns of BiVO4, BiOI, and BiVO4/BiOI.

visible-light photocatalyst with a narrower bandgap (1.7−1.9 eV).26,27 However, BiOI suffers from some drawbacks, for instance, the weak photooxidation ability and high recombination rate of photogenated e−−h+ pairs. Construction of a heterojunction between BiOI and wide-gap semiconductor such as ZnO/ BiOI,28 BiPO4/BiOI,29 Bi2O2CO3/BiOI,30 Bi2WO6/BiOI,31 TiO2/BiOI,32 BiOBr/BiOI,33 and BiOCl/BiOI34 is an effective way to enhance the photocatalytic activity of BiOI, and it has been demonstrated that a heterostructure between a p-type semiconductor and a n-type semiconductor can significantly facilitate the charge separation and transfer due to the inducement of electric field and potential difference, thus highly promoting the photocatalytic activity. The morphological feature of the composites is a crucial factor affecting the heterostructure formation, light absorption efficiency, and charge transfer.35 In particular, the core/shell structure could maximize the interfacial area thereby furnishing a broader platform for efficient charge transfer.36,37 Owing to the unique structure-induced and enhanced optical and catalytic properties, core−shell composites have aroused considerable interest. BiOI, as a p-type semiconductor, may be used to form a p−n junction with another n-type semiconductor such as BiVO4. Hence, it might be feasible to prepare a heterostructured core/shell BiVO4/BiOI p−n junction with BiVO4 being the core and BiOI nanosheets serving as the shell. In order to improve the visible-light photocatalytic activity, the thickness of the BiOI nanosheets should be much smaller than the BiVO4 particle size, and the BiVO4 crystals must be evenly and tightly coated by BiOI nanosheets. Hence, an effective preparation method meeting the above requirements is highly desirable. On the other hand, the junctional or core−shell structured materials are usually synthesized through a hydrothermal route or beam lithography, which frequently involve long periods of time, high pressure, high temperature, and multiple steps. Therefore, development of a strategy for fabricating core/shell heterojunctions

in a facile, efficient, and economic way is still a big challenge and pursuit to worldwide researchers. In this work, the BiVO4/BiOI core/shell composite with a p−n junction structure is developed by a simple deposition− precipitation method at room temperature. RhB and phenol are used to evaluate the photocatalytic activity of BiVO4/BiOI under visible-light (λ > 420 nm) illumination. The BiVO4/BiOI core/shell photocatalyst exhibits remarkably enhanced photochemical and photoelectrochemical properties compared to individual BiVO4 and BiOI, confirming the feasibility of using core/shell p−n junctions to improve the photocatalytic activity. This study may open up a new avenue for fabrication of semiconductor heterojuctional photocatalysts with core/shell structures.

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EXPERIMENTAL SECTION

Synthsis of BiVO4/BiOI Core/Shell Heterostructure. The chemicals of analytical grade were purchased from Beijng Chemical Works and used without further purification. The pure m-BiVO4 was synthesized via a hydrothermal method. During the preparation process of m-BiVO4, Bi(NO3)3·5H2O (5 mmol) was first dissolved in the solution of 2 M HNO3 (20 mL), and 5 mmol of NH4VO3 was added to 10 mL of 1 M NaOH. Then, the above solutions were blended under constant stirring, and the pH value was adjusted to 7 by NaOH solution (1 M). After stirring for 30 min, the mixtures were transferred to a 100 mL Teflon-lined autoclave and heated at 180 °C for 12 h. The yellow precipitate was collected, washed with ethanol and distilled water three times, and dried at 80 °C for 6 h. The BiVO4/BiOI heterostructures were prepared by a simple in situ precipitation route at room temperature. Different amounts of BiVO4 were ultrasonically dispersed into 30 mL of deionized water, and 30 mL of ethylene glycol (EG) containing 1 mmol of Bi(NO3)3·5H2O was transferred to the BiVO4 suspension. One millimole KI was dissolved in deionized water (10 mL) and added drop-by-drop to the BiVO4 suspension. After stirring for 5 h, the products were collected, washed three times with ethanol and distilled water, and dried at 60 °C for 10 h. The molar ratios of V/I were 0.05, 0.10, and 0.15 by changing the amounts of BiVO4. The final BiVO4/BiOI samples were denoted B


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Figure 2. XPS spectra of the BiVO4/BiOI core/shell heterostructure (V/I-2): (a) survey, (b) Bi 4f, (c) V 2p, (d) I 3d, and (e) O 1s. as V/I-1, V/I-2, and V/I-3, respectively. For comparison, the pristine BiOI was synthesized under the same conditions without adding BiVO4. Characterization. X-ray diffraction (XRD) was conducted on the X/max-rA Advance diffractometer using Cu Kα radiation. A Varian Cary 5000 UV−vis spectrophotometer was used to record the UV−vis diffuse reflectance spectra (DRS), and the surface composition and surface states were determined by X-ray photoelectron spectroscopy (XPS) on a PerkinElmer PHI 5000C instrument. The surface morphology was examined by scanning electron microscopy (SEM) on a FEI QUANTA FEG 250 instrument. The JEM-2100 electron microscopy was used to perform transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). The photoelectrochemical and Mott−Schottky measurements were conducted on an electrochemical system (CHI-660B, China) with a three-electrode quartz cells and 0.1 M Na2SO4 as the electrolyte. The saturated calomel electrode (SCE) was the reference electrodes. A platinum wire served as the counter electrode, and the BiVO4/BiOI film depositing on ITO was the working electrode. The light intensity is measured to be 1 mW/cm2, and the voltage is 0.0 eV. Electrochemical impedance spectroscopy (EIS) was obtained over the frequency range of 1−105 Hz. The photoluminescence spectrum was conducted on a Hitachi F-4600 fluorescence spectrophotometer using a 150 W Xe lamp. Photocatalytic Activity. The photocatalytic activity was first monitored by the decomposition of RhB under visible-light illumination (λ > 420 nm, 500 W Xe lamp, 8 cm between the lamp and specimen). Thirty milligrams of the prepared samples was dispersed in 50 mL of RhB solution (0.02 mM). Prior to photocatalytic reaction, the suspensions were adequately stirred in darkness for 1 h in order to

obtain equilibrium of absorption−desorption between the prepared photocatalyst and RhB. Three milliliters of the liquid was collected and centrifuged to remove the particles at certain time intervals. UV−vis spectrophotometry (Cary 5000) was carried out to determine the concentration of RhB by measuring the absorbance at 553 nm. The phenol solution was also used to assess the photocatalytic activity of the sample. Thirty milligrams of the photocatalyst was added to 50 mL of phenol solution (0.11 mM). Prior to photocatalytic degradation, the suspension was treated in the same way to establish absorption−desorption equilibrium between the prepared photocatalyst and phenol. Three milliliters of the liquid was collected and centrifuged to remove the particles at certain time intervals. UV−vis spectrophotometry (Cary 5000) was performed to measure the concentration of RhB by recording the absorbance at 270 nm. Photogenerated Intermediates. High-performance liquid chromatography (HPLC, Lumtech) was utilized to separate and record the photogenerated intermediates in the photodegradation of phenol and RhB. The Venusil XBP-C18 (250 × 4.6 mm i. d., 5 μm)) reversed phase column was used. The mixture of methanol and water (60:40, v/v) was the mobile phase with a flow rate of 1.0 mL min−1. The intermediates in the photocatalytic degradation of phenol and RhB were identified by a Q-Exactive liquid chromatography−mass spectrometry (LC-MS) instrument. The scanned range was m/z 50−750, and positive ions were monitored. Reactive Species Detection and •O2− Quantification Experiments. We detect the active species during the photocatalytic process by adding trapping agent. Superoxide radical (•O2−), hydroxyl radical (•OH), and holes (h+) can be consumed by adding 1 mM C


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Figure 3. SEM images of (a, b) pure BiOI, (c, d) pure BiVO4, and (e, f) V/I-2. EDX mapping of V/I-2 heterostructure (g−j).

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benzoquinone (BQ), 1.0 mM iso-propanol (IPA), and 1 mM ethylene diamine tetraacetic acid disodium salt (EDTA-2Na),38,39 respectively. NBT (nitro blue tetrazolium chloride monohydrate, 0.025 mM) was herein employed to determine the amount of •O2− generated from BiVO4, BiOI, and BiVO4/BiOI. Production of •O2− can be quantitatively determined by recording the concentration of NBT on the Cary 5000 UV−vis spectrophotometer. The method was similar to the aforementioned photocatalytic activity experiments with replacement of RhB by NBT.

RESULTS AND DISCUSSION

Phase Structure. Figure 1a depicts the crystal structure of the tetragonal phase BiOI. It possesses a Sillén-related layered structure composed of [Bi2O2]2+ layers and I− ions between them. As shown in Figure 1b, the monoclinic phase BiVO4 consists of VO4 tetrahedrons and Bi−O polyhedra. Figure 1c shows the XRD patterns of BiVO4, BiOI, and BiVO4/BiOI core/shell composites. D


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Figure 4. TEM images: (a, b) pure BiOI and (c, d) V/I-2. (e) HR-TEM. (f,g) Magnified HR-TEM image of V/I-2.

heterostructures (V/I-2). The as-prepared BiOI comprises hierarchical microspheres (Figure 3a) with a diameter of about 1 μm (Figure 3b), and the flower-like BiOI microspheres are composed of a large number of BiOI nanosheets. Differently, BiVO4 consists of irregular particles (Figure 3c) with an average diameter of 1−2 μm (Figure 3d). With regard to the BiVO4/ BiOI core/shell heterostructure (Figure 3e−f), its morphology is different from those of BiVO4 particles and the uniform BiOI microspheres, which may suggest that the BiOI nanosheets are deposited on the surface of the BiVO4 particles. For further confirmation, EDX mapping of I/V-2 heterostructure was conducted. As shown in Figure 3g−j, the four constituent elements Bi, O, I, and V all can be detectable, and they are homogeneously distributed in the composite. These results imply that the uniformly core/shell structured BiVO4/BiOI are obtained. The BiVO4, BiOI, and BiVO4/BiOI core/shell heterostructures (V/I-2) are further analyzed by TEM and HR-TEM. The BiOI microspheres (Figure 4a) and self-assembled V/I-2 sample (Figure 4c,d) are consistent with SEM observation. Irregular nanosheets 50−200 nm in diameter are observed from Figure 4b, confirming that the BiOI microspheres are composed of nanosheets. Figure 4e−g display the HR-TEM image of V/I-2. It is evident that there are two sets of distinct lattice fringes in the pattern. The interplanar spacings of 0.260 and 0.361 nm correspond well to the {200} facet of BiVO4 and {002} facet of BiOI (Figure 4f,g), respectively. This evidence strongly demonstrates that the BiOI nanosheets acting as the shell are firmly assembled on the surface of BiVO4 cores.

The diffraction peaks of BiVO4 and BiOI can be indexed to the monoclinic phase BiVO4 (JCPDS File No. 14-0688) and tetragonal BiOI (JCPDS File No. 10-0445), respectively. No other peaks of impurity are observed for BiVO4 and BiOI. Because of the high content of BiOI in the BiVO4/BiOI composites, most of the diffraction peaks are indexed into that of BiOI, whereas the distinct peak at 28.8° corresponding to the (−121) plane of BiVO4 is detected from BiVO4/BiOI compared to pure BiOI. With increasing the BiVO4 content in the composites, the peak intensity of BiVO4 goes up, indicating the coexistence of both BiVO4 and BiOI phases in the BiVO4/BiOI composites. XPS Analysis. As shown in Figure 2, the BiVO4/BiOI composite (V/I-2) consists of Bi, V, I, and O. The C 1s peak at 284.8 eV is attributed to adventitious hydrocarbons generated by the XPS instrument. Figure 2b shows two peaks with strong intensity at 164.7 and 159.4 eV corresponding to Bi 4f5/2 and Bi 4f7/2, respectively. These are characteristic bands of Bi3+ in BiOI.40 The V 2p1/2 peak at 530.2 eV is associated with oxidized V in crystalline BiVO4 (Figure 2c). Figure 2d shows the I 3d spectrum. The two peaks at 619.3 and 630.8 eV are attributed to d I 3d5/2 and I 3d3/2, respectively, which is in accordance with other XPS results in BiOI.41 Figure 2e shows the O 1s peaks from the V/I-2 sample, and it can be deconvoluted into two peaks at 529.9 and 530.3 eV arising from V−O bonds in BiVO442 and Bi−O bonds in the [Bi2O2] layered structure of the BiOI.43 The XPS result validates the coexistence of BiOI and BiVO4 in the BiVO4/BiOI core/shell heterostructures. Morphology and Microstructure. Figure 3 depicts the SEM images of the BiVO4, BiOI, and BiVO4/BiOI core/shell E


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ACS Sustainable Chemistry & Engineering Scheme 1. Formation of the BiVO4/BiOI Core/Shell Structures

Figure 5. (a) UV−vis diffuse reflectance spectra and (b) bandgap energies of BiVO4 and BiOI.

Formation Process of BiVO4/BiOI Core/Shell Structures. According to the SEM and TEM results, the formation process of BiVO4/BiOI core/shell structures are proposed in Scheme 1. Bi(NO3)3 can be dissolved in ethylene glycol (Bi3+ reacting with ethylene glycol) to form a homogeneous and clear solution consisting of alkoxides ((Bi(OCH2CH2OH)2+)).44 When I− is added into the solution of alkoxides, (Bi(OCH2CH2OH)2+ reacts with I− to form the lamellar BiOI initially, and then, the primary BiOI nanoplates self-assemble to form the flowerlike BiOI microspheres in the presence of ethylene glycol and water. By adding the alkoxide solution to the BiVO4 suspension, (Bi(OCH2CH2OH)2+ is surrounded by the dispersed BiVO4 particles. When (Bi(OCH2CH2OH)2+ interacts with I−, the BiOI inoculating crystals is in situ born and absorbed on the surface of the BiVO4 particles to minimize the surface energy. These BiOI inoculating crystals then grow into BiOI plates through a dissolution−recrystallization process and finally forming the BiVO4/BiOI core/shell heterostructures.45 Optical Properties. Figure 5a displays the DRS spectra of the BiVO4, BiOI, and BiVO4/BiOI core/shell heterostructures. The absorption edge of BiVO4 is 528 nm in the visible region, whereas that of BiOI is approximately 660 nm. With regard to the BiVO4/BiOI heterostructures, the absorption edges range from 644 to 660 nm. Because of the low content of BiVO4 in the BiVO4/BiOI core/shell heterostructures, their absorption in visible-light region decreases only slightly in comparison with BiOI. As a crystalline semiconductor, the relationship of band edge and optical absorption can be determined by the equation: αhν = A(hν−Eg)n/2. Herein, hν, α, Eg and A represent the

photon energy, optical absorption coefficient, band gap, and proportionality constant, respectively.46,47 Additionally, n = 1 and 4 means that the material is direct semiconductor and indirect semiconductor, respectively. The optical transitions of BiVO4 and BiOI are direct and indirect, respectively.48−51 Hence, n values of BiVO4 and BiOI is 1 and 4, respectively. The Eg of BiVO4 is estimated from the plot of (αhv)2 versus (hv), whereas that of BiOI is calculated from te plot of (αhv)1/2 versus (hv). As shown in Figure 5b, Eg of BiVO4 and BiOI is evaluated to be 2.41 and 1.81 eV, respectively. Photocatalytic Activity. Figure 6a shows the photocatalytic activity of the BiVO4, BiOI, and BiVO4/BiOI samples by monitoring the degradation of RhB under visible light (λ > 420 nm). Almost no obvious RhB decomposition can be observed under visible-light irradiation without the photocatalyst. Pure BiVO4 shows poor photocataltic activity with only 14% of RhB degraded, but pure BiOI degrades 94% of the RhB after visible-light irradiation for 5 h. The BiVO4/BiOI core/shell composite shows enhanced photocatalytic activity compared to the pure BiVO4 and BiOI. As the BiVO4 content increased, the photocatalytic activity of BiVO4/BiOI increases initially, reaches a maximum with molar ratios of V/I 10%, and then decreases. The V/I-2 sample has the highest photocatalytic activity corresponding to the 97% degradation even after irradiation for a shorter time of 3 h. The shell of the BiOI nanosheets in the BiVO4/BiOI core/shell heterostructure plays an important role in the photocatalytic process. The BiOI nanosheets assembled on the BiVO4 particles absorb visible light to generate e−−h+ pairs to degrade the dye molecule, F


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Figure 6. (a) Photocatalytic degradation curves of RhB under visible-light irradiation. (b) Apparent rate constants for photocatalytic degradation of RhB.

while the BiVO4 core provides the p−n junction between BiVO4 and BiOI. In this way, the separation and transfer efficiency of photogenerated e−−h+ are enhanced due to the strong interfacial interaction, although the photocatalytic performance of BiVO4 is still relatively poor. The amount of generated electron−hole pairs decreases with further increase in the BiVO4 content on account of reduced visible-light absorption. Our data show that V/I-2 has the highest photocatalytic activity indicating that the optimal photocatalytic efficiency is decided by the competition between light absorption and interfacial interactions between BiVO4 and BiOI. The Langmuir−Hinshelwood (L-H) kinetics model is used to determine the pseudo-first-order kinetics as shown by the expression as follows52,53 Ln(C0/C) = kappt

(1)

where C and C0 (mol/L) represent the concentration of RhB at time t and 0, respectively, and kapp represents the apparent rate constant (h−1). Figure 6b shows the apparent rate constants of different samples. The kapp values of pure BiVO4, V/I-1, V/I-2, V/I-3, and pure BiOI are calculated to be 0.026, 0.445, 1.104, 0.612, and 0.418 h−1, respectively. V/I-2 has the largest kapp (1.104 h−1), which is about 3 and 42 times larger than those of individual BiOI and BiVO4, respectively. The photocatalytic activity of the pure BiVO4, BiOI, and BiVO4/BiOI core/shell heterostructure (V/I-2) are further evaluated by examining the degradation of phenol with visible light (λ > 420 nm). Figure 7a and b show the changes in the absorption curves of phenol from 250 to 300 nm in the presence of pure BiOI and V/I-2. As shown in Figure 7b, the intensity of absorption band of phenol at 270 nm decreases more rapidly in the presence of the BiVO4/BiOI core/shell heterostructure compared to pure BiOI (Figure 7a). Figure 7c shows the degradation curves of phenol over the pure BiVO4, BiOI, and BiVO4/BiOI core/shell heterostructure (V/I-2). Only 48.7% of the phenol is degraded by pure BiOI after 16 h visible-light irradiation, but the V/I-2 sample can degrade 63.9% of phenol under the same conditions, illustrating the activity improvement offered by the BiVO4/BiOI core/shell heterostructure. The intermediates produced during degradation process of phenol are detected by LC and LC-MS (Figure 8). The results reveal the existence of phenol, catechol, p-benzoquinone (p-BQ), resorcinol, and hydroquinone, consistent with the literature.54,55 Figure S1 shows the HPLC spectra of the intermediates generated during the photodegradation of RhB by V/I-2. No peaks are observed possibly because RhB breaks down into

Figure 7. Temporal absorption spectral patterns of phenol solution (0.11 mM) during the photodegradation process: (a) BiOI and (b) V/I-2. (c) Photocatalytic degradation curves of phenol solution under visible-light irradiation (λ > 420 nm).

smaller molecules which cannot be detected by LC and LC-MS. The stability of the BiVO4/BiOI core/shell photocatalyst is G


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Figure 8. (a) HPLC spectra of phenol degradation over V/I-2 under visible-light irradiation for 16 h. Mass spectra of (b) phenol, (c) catechol, resorcinol, and hydroquinone, and (d) p-benzoquinone.

Figure 9. Mott−Schottky curves of (a) BiOI and (b) BiVO4. Schematic diagrams of formation of p−n junction and proposed charge separation process in the BiVO4/BiOI core/shell heterostructures under visible-light irradiation.

Photocatalytic Mechanism. In order to confirm the semiconductor types of BiOI and BiVO4, Mott−Schottky (M-S) curves of as-prepared BiOI and BiVO4 are determined as shown in Figure 9a and b. The slope of linear 1/C2 potential curve of

evaluated by conducting recycling experiments. The photocatalytic activity of BiVO4/BiOI core/shell heterogeneou (V/I-2) does not decrease appreciably after five cycles (Figure S2), indicating that it is quite stable. H


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BiVO4 shifts downward until the EF of BiOI and BiVO4 reaches an equilibrium. Eventually, the conduction band bottom of BiVO4 becomes lower in energy than that of BiOI. Though absorbing visible light, the exterior BiOI nanosheers generate e−−h+ pairs. As shown in Figure 9c, the excited e− in the CB of BiOI can easily migrate to that of BiVO4, and meanwhile, the excited h+ remains in the VB of BiOI. Besides, the internal electric field also improves the migration of photogenerated electrons and holes. As a result, the p−n junction in the BiVO4/ BiOI core/shell heterostructure promotes separation of photogenerated e−−h+ pairs and reduces recombination of e− and h+, enhancing the photocatalytic activity. Photocurrent−time (PT) can reveal the interfacial generation and separation dynamics of photogenerated charges of semiconductor photocatalysts, and a larger photocurrent indicates higher electrons and holes separation efficiency.58 Figure 10 shows the PT curves of BiVO4, BiOI, and V/I-2. In comparison with BiOI and BiVO4, V/I-2 exhibits a drastically increased larger current density. The PT measurement demonstrates that separation of e−−h+ pairs in the BiVO4/BiOI core/shell heterostructure is significantly improved. It is consistent with the proposed mechanism. Electrochemical impedance spectra are obtained to elucidate the transfer and migration processes of the photoexcited e−−h+ pairs. Figure 11a shows the EIS Nynquist plots without and with irradiation. The inset in Figure 11a shows the equivalent R(RC) (RQ) circuits. In general, the radius of the arc reflects the interfacial layer resistance at the electrode surface, and

BiOI and BiVO4 are negative and positive, respectively. The result indicates that the as-prepared BiOI is a p-type semiconductor and BiVO4 is a n-type semiconductor. Hence, the enhanced photocatalytic activity observed from the BiVO4/ BiOI core/shell structures can be attributed to the fabrication of the p−n junction between BiOI and BiVO4. The valence band (VB) and conduction band (CB) of BiVO4 and BiOI are evaluated by eqs 2 and 356,57 E VB = X − E e + 0.5Eg

(2)

ECB = E VB − Eg

(3)

where Eg represents the band gap of the semiconductor, X is the electronegativity of the semiconductor calculated from the electronegativity of the constituent atoms, and Ee is the energy of free electrons on the hydrogen scale (ca. 4.5 eV). EVB and ECB of BiVO4 are calculated to be 2.75 and 0.34 eV, respectively, and those of BiOI are 2.35 and 0.54 eV, respectively. Figure Figure 9c shows the schematic diagrams of the charge separation process on the BiOI/BiVO4 p−n junction. Pure BiOI and BiVO4 have the nested band structure before contact, which cannot facilitate separation of photogenerated e−−h+ pairs. As a p-type semiconductor, the Fermi level (EF) of BiOI lies close to the VB, whereas BiVO4 is a n-type semiconductor with the EF close to its CB. When the BiOI nanosheets are assembled on BiVO4 to construct the p−n heterojunction, the energy levels of BiOI shift upward, whereas the energy band of

Figure 10. Comparison of transient photocurrent response between BiVO4, BiOI, and V/I-2 with light on/off cycles under illumination by visible light (λ > 420 nm, [Na2SO4] = 0.1 M).

Figure 12. Photoluminescence spactra of the BiOI and V/I-2 excited at 260 nm.

Figure 11. (a) EIS Nynquist plots of the BiVO4, BiOI, and V/I-2 with light on/off under irradiation by visible light ([Na2SO4] = 0.1 M). (b) Bodephase of the BiVO4, BiOI, and V/I-2. I


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Figure 13. (a) Photodegradation of RhB on V/I-2 in the presence of different scavengers. (b) Transformation percentage of NBT concentration after visible-light irradiation for 5 h (λ > 420 nm).

formation of the p−n junction between BiVO4 and BiOI. Figure 13b shows the transformation percentage of NBT (quantification experiments of •O2− production) after 5 h in visible light (λ > 420 nm). With regard to BiVO4, the transformation percentage of NBT is negligible (0.2%), suggesting that there is almost no •O2− production due to its weak ability to generate photoexcited charge carriers and fast recombination of electrons and holes. Compared to pure BiOI and BiVO4, the V/I-2 sample shows the highest transformation percentage of NBT (15%). It demonstrates that the photoexcited e− from BiOI can easily transfer to BiVO4 in p−n junctions, separating more e−−h+ pairs and reducing recombination. Furthermore, more photogenerated electrons react with O2 to produce •O2− and take part in the decomposition of RhB and phenol. This is in good agreement with the photocatalytic activity (Figures 6 and 7).

a smaller arc radius indicates higher efficiency in charge transfer.59−61 The arc radius of V/I-2 is smaller than those of BiOI and BiVO4 with and without irradiation, showing improved transfer efficiency of photoexcited e−−h+ in V/I-2 core/shell structure. In addition, lifetime of the injected electrons (τ) in the conduction band of BiVO4, BiOI, and BiVO4/BiOI core/shell heterostrcture (V/I-2) is determined through the equation τ ≈ 1/(2πf); herein, f means the inverse minimum frequency.62,63 Figure 11b shows the bode-phase of the BiVO4, BiOI, and V/I-2. The inverse minimum frequencies of BiVO4, BiOI, and V/I-2 are 7907, 9388, and 5688 Hz, respectively. Thus, the electron lifetime of V/I-2 is calculated to be 27.9 μs, which is 1.39 times (20.1 μs) and 1.64 times (17.0 μs) larger than that of the BiVO4 and BiOI, respectively. The improvement can again be attributed to that the p−n heterojunction in the BiVO4/BiOI core/shell heterostructure facilitates migration of charge carriers. In order to further prove that the p−n junction in the BiVO4/BiOI core/shell heterostructure hinders the recombination of the photoexcited e−−h+ pairs, the photoluminescence (PL) spectra of BiOI and V/I-2 have been determined (excited at 260 nm). PL spectra may provide significant evidence for monitoring the recombination efficiency of charge carriers.64,65 Figure 12 shows the PL spectra of BiOI and V/I-2 samples. It is shown that the BiVO4/BiOI core/shell heterostructure (V/I-2) shows the lower emission intensity than that of BiOI around 398 nm, indicating that the recombination rate of photoexcited e−−h+ pairs is truely hindered in BiVO4/BiOI core/ shell heterostructure. Combined with the results of photocurrent−time curves and electrochemical impedance spectra, it can be concluded that the formation of the BiVO4/BiOI core/shell structured p−n junction between the p-type BiOI and n-type BiVO4 greatly facilitates separation and depresses recombination of photogenerated e−−h+ pairs, thus highly enhancing the photocatalytic activity. To reveal the roles played by active species in the photodegradation RhB by the BiVO4/BiOI core/shell heterostructure, we use iso-propanol (IPA), ethylene diamine tetraacetic acid disodium salt (EDTA-2Na), and benzoquinone (BQ) as •OH, h+ and •O2− scavengers, respectively.66,67 Addition of IPA inhibits 5% of RhB degradation, while the inhibition efficiency of EDTA-2Na and BQ is about 55% and 72%, respectively (Figure 13a). Thee active species trapping experiments imply that •O2− is the most crucial to the photodegradation of RhB by BiVO4/BiOI. The •O2− quantification experiments are performed to understand the change in the active species after

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CONCLUSION Core/shell structured BiVO4/BiOI p−n junctions were fabricated by depositing BiOI nanosheets on BiVO4 particles. The BiVO4/ BiOI heterostructure exhibits much superior photocatalytic performance in the degradation of RhB and phenol than individual BiVO4 and BiOI. The enhanced photoactivity can be attributed to the intimate core/shell heterostructure and p−n junction formed between BiVO4 and BiOI, which greatly promotes efficient separation of the photoexcited e−−h+ pairs and charge transfer as well as depressing recombination of the charge carriers. It was corroborated by photocurrent response, electrochemical impedance spectroscopy, Bode-phase spectra, PL spectra, and active species trapping and quantification results. The materials also display good stability and recyclability. The present study demonstrates a simple room-temperature technique to prepare BiVO4/BiOI core/shell p−n junctions with improved photocatalytic activity.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01038. HPLC spectra of RhB degradation for V/I-2, and cycling runs in the photocatalytic degradation of RhB in the presence of BiVO4/BiOI heterostructure (V/I-2). (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +86-10-82322247. E-mail: [email protected]. *Tel: +86-10-82322247. E-mail: [email protected]. J


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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was jointly supported by the National Natural Science Foundations of China (Grant No. 51302251), Fundamental Research Funds for the Central Universities (2652013052 and 2652015296), National High Technology Research and Development Program (863 Program 2012AA06A109) of China, City University of Hong Kong Applied Research Grant 9667085, and Guangdong−Hong Kong Technology Cooperation Funding Scheme (TCFS) GHP/015/12SZ.

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