Ce and F Comodification on the Crystal Structure and Enhanced

Mirabbos Hojamberdiev , Zukhra C. Kadirova , Ehsan Zahedi , Diego Onna , María ..... Mirabbos Hojamberdiev , Zuhra C. Kadirova , Yuki Makinose , Gang...
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Ce and F Comodification on the Crystal Structure and Enhanced Photocatalytic Activity of Bi2WO6 Photocatalyst under Visible Light Irradiation Hongwei Huang,*,† Kun Liu,† Kai Chen,† Yinglei Zhang,† Yihe Zhang,*,† and Shichao Wang‡ †

National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, P. R. China ‡ Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States S Supporting Information *

ABSTRACT: The novel Ce and F codoped Bi2WO6 samples have been successfully obtained by a facile one-step hydrothermal reaction for the first time. They were characterized by X-ray diffraction patterns (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), highresolution TEM (HRTEM), X-ray photoelectron spectroscopy (XPS), and UV−vis diffuse reflectance spectra (DRS) and photoluminescence (PL) spectra. The presence of Ce3+, Ce4+, and F− dopants in Bi2WO6 was confirmed by XPS. The change of microstructure and optical band gap has also been observed after the doping of Ce and F. Under visible light, the assynthesized plate-like F−Ce−Bi2WO6 sample exhibits a much better visible-light-responsive photocatalytic performance than pure Bi2WO6 for the degradation of RhB and photocurrent (PC) generation. The mechanism of high photcatalytic activity was also suggested on the basis of the PL spectra, electrochemical impedance spectra (EIS), and active species trapping measurements. The results indicated that the synergistic effect of the Ce and F dopants is responsible for the efficient separation and migration of photoinduced charge carriers, thus resulting in the remarkably improved photocatalytic activity.

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recombination can be realized by the semiconductor fluorination,21 and the improved photocatalytic performance was also observed over other fluorinated semiconductors such as TiO2, SrTiO3, ZnWO4, and BiVO4.22−25 To achieve optimal photocatalytic activities, people turn to codoping with two elements in a photocatalyst, which can produce the synergistic effect for the efficient photodecomposition of contaminants. Recently, it was found that the codoping of B/N, Ag/B, and Nd/C can further make dramatic improvement on the photocatalytic performance of TiO2.26−28 Nevertheless, very few works of codoping modification concentrated on Bi-based photocatalytic materials, e.g., Bi2WO6 with the typically layered crystal structure. Herein, we report a novel (Ce, F)-codoped Bi 2WO6 photocatalyst by a facile one-step hydrothermal route. The Ce was selected because of its Ce4+/Ce3+ redox couple possessing the ability in transformation between Ce3+ and Ce4+ under reducing and oxidizing conditions, which can act as an electron scavenger in the photocatalytic process.29,30 Upon visible light illumination, the codoping F−Ce−Bi 2 WO 6

INTRODUCTION Semiconductor photocatalysis has become one of the most promising technologies because of its unique ability for environmental purification and energy conversion.1,2 In particular, the investigation focused on visible-light-driven photocatalytic materials has been very active as they can utilize the solar energy more efficiently than the traditional photocatalyst with wide band gap, e.g., TiO2.3 As a simplest member of the Aurivillius-type compounds, bismuth tungstate (Bi2WO6) featuring its layered configuration consisting of alternating (Bi 2 O2 )2+ layers and (WO4 ) 2− octahedral layers displays excellent photocatalytic activity for dye degradation under visible light.4−6 Nevertheless, its utilization efficiency on visible light is still too low. In addition, the fast recombination rate of photoinduced electrons and holes also limits its practical application. To settle these problems, several strategies, like noble metal deposition,7,8 heterostructure construction,9−14 ion doping,15−20 etc., have been developed. Among these, ion doping has been expected to be the most facile and efficient route to expand the visible absorption and depress the recombination of electron−hole pairs. By doping the single ions (Fe,15 Mo,16 Cu,17 Eu,18 F,19 and B20) into Bi2WO6, the photocatalytic activities were all improved. Especially, effective depression on the charge carriers © 2014 American Chemical Society

Received: March 27, 2014 Revised: June 10, 2014 Published: June 17, 2014 14379

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demonstrated a remarkably enhanced photocatalytic activity for degradation of rhodamine B (RhB). The mechanism of high photocatalytic activity was also suggested on the basis of the photoluminescence (PL) spectra and photoelectrochemical and active species trapping measurements. The highly enhanced photocatalytic efficiency observed in F−Ce codoped Bi2WO6 should be attributed to the synergistic effect of the comodification of Ce and F.

calomel electrodes (SCE) were reference electrodes, and platinum wires were utilized as the counter electrode. The working electrodes were pure Bi2WO6 and F−Ce−Bi2WO6 film coated on ITO. The electrochemical system (CHI-660B, China) was employed to record the photoelectrochemical experiment results. The photocurrent and electrochemical impedance spectra (EIS) were all measured at 0.0 V with light intensity 1 mW/cm2.

EXPERIMENTAL METHODS Synthesis. Pure Bi2WO6 and doped Bi2WO6 samples were all synthesized by a facile hydrothermal process. Bi(NO3)3· 5H2O, Ce(NO3)3·6H2O, Na2WO4·2H2O, and NaF from commercial sources are analytically pure and used as received. In a typical synthesis of F−Ce−Bi2WO6, 1 mmol of Na2WO4· 2H2O, 1.98 mmol of Bi(NO3)3·5H2O, 0.02 mmol of Ce(NO3)3·6H2O, and 1 mmol of NaF were dissolved in 100 mL of distilled water and stirred for 30 min. After that, the resulting pale-yellow suspensions were transferred into 100 mL Teflonlined stainless autoclave and kept at 180 °C for 24 h. After natural cooling, the products were filtrated, washed repeatedly with deionized water and ethanol, and then dried at 60 °C for 24 h. Similarly, the pure Bi2WO6, F−Bi2WO6, and Ce−Bi2WO6 were prepared using the same method. Characterization. Powder X-ray diffraction (XRD) patterns were recorded using an X/max-rA Advance diffractometer with Cu Kα radiation. The general morphology and microstructure of the products were obtained using a S-4800 scanning electron microscope (SEM) on a FEI QUANTA FEG 250 instrument operated at 20 kV. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) analyses were performed on a JEM-2100 electron microscope (JEOL, Japan) operating at 200 kV. UV−vis diffuse reflectance spectra (DRS) were conducted by the Varian Cary 5000 UV− vis spectrophotometer. Raman measurement was carried out by a Raman-11 spectrometer. A VGMK II X-ray photoelectron spectrometer was employed to perform the X-ray photoelectron spectroscopy (XPS). Luminescence emission spectra at room temperature were recorded using a JOBIN 10 YVON FluoroMax-3 fluorescence spectrophotometer with a 150 W Xe lamp as the excitation light source. Photocatalytic Degradation Experiment. Photocatalytic activities of pure Bi2WO6 and doped Bi2WO6 were investigated by degradation of rhodamine B (RhB) under visible light irradiation (λ > 420 nm, 1000 W). First, the photocatalysts (50 mg) were ultrasound-dispersed in an aqueous solution of RhB (50 mL, 0.01 mM). Then, the mixture of photocatalyst powder and dye solution was magnetically stirred in dark for 1 h to achieve an absorption−desorption equilibrium. Afterward, the photoreaction was started. At certain intervals, 3 mL of suspension was taken and centrifugated. The concentration of RhB was determined by measuring the UV−vis absorption based on the absorbance band of 553 nm. Active Species Trapping Experiments. In older to detect the active species generated in photocatalytic process, superoxide radical (•O2−), holes (h+), and hydroxyl radicals (•OH) were investigated by the addition of 1 mM BQ (a quencher of • O2−), 1 mM EDTA-2Na (a quencher of h+), and 1 mM IPA (a quencher of •OH), respectively.31,32 This method was similar to above photodegradation experiment only RhB was replaced. Photoelectrochemical Measurements. Photoelectrochemical tests were carried out in a three-electrode system with 0.1 M Na2SO4 solution as electrolyte solution. Saturated

RESULTS AND DISCUSSION Crystal Structure. Bi2WO6 is the simplest Aurivillius-type compound with a perovskite-like structure. The crystal structure is composed of alternatively stacked bismuth oxide (Bi2O2)2+ layers and WO6 octahedron layers along the b-axis. When Ce3+ ion was incorporated into the crstal lattice, Ce3+ will substitute the A-cation Bi3+ in (Bi2O2)2+ layers due to the same charge and the similar ionic radius of Ce3+ (103.4 pm) and Bi3+ (103 pm), and F− will replace O2− in the WO6 octahedron. As shown in Figure 1, the modified crystal structure would result in the change of iterplanar spacing, which was verified by the following X-ray diffraction (XRD) analysis.

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Figure 1. Crystal structure of F−Ce−Bi2WO6.

XRD patterns of doped and pure Bi2WO6 products are shown in Figure 2. It illustrated that the Ce and F dopants did not result in the change of crystal structure and preferential orientations from Figure 2a. As Ce3+ and Bi3+ possess the very similar ionic radius, there is no shift of the angle in the diffraction patterns of Ce doped Bi2WO6 samples. However, it can be observed that there is a slight shift to a lower 2θ value for the peak (131 lattice) of F and F−Ce doped Bi2WO6 as shown in Figure 2b. Based on Bragg’s law, the decrease in 2θ value should be attributed to the increase in lattice parameters. Besides, the cell parameters of F−Ce−Bi2WO6 and pure Bi2WO6 have determined from the XRD data, which are a = 5.456 Å, b = 16.419 Å, c = 5.424 Å for Bi2WO6 and a = 5.483 Å, b = 16.481 Å, c = 5.429 Å for F−Ce−Bi2WO6. A slightly enlargement in cell parameters was observed in F−Ce− Bi2WO6, which is also in consistent with the shift to a lower 2θ value of XRD peaks of F−Ce−Bi2WO6. These results indicated that the O atom of WO6 octahedron was truly 14380

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resolved HRTEM image from inverse FFT indicates the three sets of lattice fringes with interplanar spacing of 0.193, 0.273, and 0.315 nm, which correspond to the (202), (200), and (131) planes of orthorhombic Bi2WO6, respectively.14 Raman spectra were performed to investigate the element substitution on Bi2WO6. As shown in Figure S1, the peaks at 824 and 792 cm−1 can be assigned to the antisymmetric and symmetric Ag modes of terminal O−W−O.20 The peak at 718 cm−1 is associated with the antisymmetric bridging mode of tungstate chain. The translational mode of simultaneous motions of Bi3+ and WO66− was observed at 306 cm−1. Compared to that of Bi2WO6, the vibration bands of O−W−O at 792 and 824 cm−1 were changed in F−Ce−Bi2WO6. The peak at 824 cm−1 shifts to higher wavenumber, and the peak at 792 cm−1 shifts to lower wavenumber. It should be due to some F atoms having substituted partial oxygen atoms originally bonded with W in the WO6 lattice. No other inconsistent bands were found in the Raman spectrum of F−Ce−Bi2WO6, indicating that Ce was incorporated into the crystal lattice of Bi2WO6. No other Ce-containing phases were detected by Raman spectra, which may be due to its small amount of Ce. The presence of Ce dopants was proved by the following X-ray photoelectron spectroscopy analysis. XPS Analysis. The oxidation states and surface chemical composition of the F and Ce codoped Bi2WO6 were characterized by X-ray photoelectron spectroscopy (XPS). Typical survey XPS spectrum of F−Ce−Bi2WO6 indicates that Bi, W, O, Ce, and F elements could all be detected as shown in Figure 5a. The high resolution spectra of W 4f, Bi 4f, O 1s, F 1s, and Ce 3d are provided in Figure 5b−f. As presented in Figure 5b, the two bands at 35.4 and 37.6 eV are ascribed to W 4f7/2 and W 4f5/2, respectively.34 Compared to that of pure Bi2WO6, the binding energies of the W 4f7/2 and W 4f5/2 peaks increased about 0.2 eV. It should be ascribed to the alteration in the chemical coordination environment of W surroundings, indicating the successful substitution of F atom for O atom in WO6 octahedron. Figure 5c shows that the binding energies of Bi 4f7/2 and Bi 4f5/2 all located at 159.2 and 164.5 eV in both of F−Ce−Bi2WO6 and Bi2WO6 with difference (delta) in binding energies of 5.34 eV, respectively, which demonstrated that the coordination environment of Bi surroundings was not influenced by F substitution.35 Thus, the F atoms only replaced the O atoms of WO6 octahedron in Bi2WO6. The O 1s peaks for F−Ce−Bi2WO6 (Figure 5d) can be deconvoluted into two bands at 531.2 and 530.1 eV, which are associated with OH hydroxyl groups and oxygen species in the lattice oxygen, respectively.36 In comparison with that of pure Bi2WO6 sample, the XPS spectra of F−Ce−Bi2WO6 obviously presents a shoulder band along with the main peak of Bi 4s as shown in Figure 5e. This extra peak at 684.0 eV was observed in F−Ce− Bi2WO6 is in good accordance with the position of F 1s peak in crystal lattice.37 According to the literature, the Ce 3d spectrum could be deconvoluted into four pairs of spin−orbital bands (3d3/2 and 3d5/2 denoted as v and u, respectively): v/u, v′/u′, v″/u″, and v‴/u‴, where v/u, v″/u″, and v‴/u‴ peaks are associated with the characteristic states of Ce4+ 3d, and v′/u′ corresponds to that of Ce3+ 3d.38 Through comparison of the binding energy of Ce 3d in F−Ce−Bi2WO6 with that in Bi2WO6, the band of Ce 3d in F−Ce−Bi2WO6 can be clearly observed as shown in Figure 5f. Moreover, the Ce 3d spectra was deconvoluted into eight peaks at 882.4, 885.1, 888.6, 901.0, 903.7, 905.7, 907.5, and 916.6 eV, which corresponds to v, v′, v″, v‴, u, u′, u″, and u‴, respectively.29 The results

Figure 2. (a) XRD patterns of pure Bi2WO6, Ce−Bi2WO6, F− Bi2WO6, and F−Ce−Bi2WO6. (b) Enlargement of XRD patterns.

replaced by the F atom, resulting in the enlargement of lattice parameter.33 Furthermore, a broader peak was observed in F incorporated samples, suggesting the smaller size of these products, which is consistent with the following SEM analyses. Figure 3a−f shows the typical scanning electron microscope (SEM) images of pure Bi2WO6, 0.5% Ce−Bi2WO6, 1% Ce− Bi2WO6, 1.5% Ce−Bi2WO6, F−Bi2WO6, and F−Ce−Bi2WO6, respectively. The pure Bi2WO6 (Figure 3a) and Ce−Bi2WO6 products (Figure 3b−d) present hierarchical flower-like microstructure composed of a large number of ultrathin nanosheets, and the diameters are about several micrometers. Differently, the as-synthesized F−Bi2WO6 and F−Ce−Bi2WO6 samples clearly show the plate-like structure with size of 100−200 nm as illustrated in Figure 3e and 3f, respectively. It is obvious that F dopant can significantly influence the morphology and effectively decrease the size of the as-prepared samples. The as-obtained F−Ce−Bi2WO6 and pure Bi2WO6 products were further investigated by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). Figure 4a clearly shows the hierarchical flower-like structure of the pure Bi2WO6, and the well-dispersed nanoplates of F−Ce−Bi2WO6 sample were also confirmed from Figure 4b. The fast Fourier transform (FFT) images (Figure 4c−g) of F−Ce−Bi2WO6 were employed to investigate the crystal structure. The lattice 14381

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Figure 3. SEM images of as-prepared (a) pure Bi2WO6 (scale bar = 2 μm), (b) 0.5% Ce−Bi2WO6 (scale bar = 2 μm), (c) 1% Ce−Bi2WO6 (scale bar = 1 μm), (d) 1.5% Ce−Bi2WO6 (scale bar = 1 μm), (e) F−Bi2WO6 (scale bar = 200 nm), and (f) F−Ce−Bi2WO6 (scale bar = 200 and 100 nm for the inset).

demonstrated that both Ce3+ and Ce4+ coexist in F−Ce− Bi2WO6. Optical Properties. UV−vis diffuse reflectance spectra (DRS) of pure Bi2WO6, 1% Ce−Bi2WO6, F−Bi2WO6, and F− Ce−Bi2WO6 are displayed in Figure 6. It was found that the Ce and F dopants have different influence on the visible-light absorption of Bi2WO6. The substitution of Ce3+ for Bi3+ will lead to the blue-shift of the absorption edge, and F can result in the red-shift of the absorption edge, enhancing the light absorption in visible region. Optical band gap of Bi2WO6 could be solved by the equation αhν = A(hν − Eg)n/2, where Eg, hν, A, and α are the band gap, photonic energy, proportionality constant, and optical absorption coefficient, respectively. From which, the transition type was decided by n value (n = 4 and 1 for indirect and direct absorption, respectively). As shown in the literature, Bi2WO6 is an indirect band gap type semiconductor.19 The indirect band gaps of pure Bi2WO6, 1% Ce− Bi2WO6, F−Bi2WO6, and F−Ce−Bi2WO6 can be obtained from the chart of absorption1/2 versus energy (inset of Figure 6). Thus, the band gaps of pure Bi2WO6, 1% Ce−Bi2WO6, F− Bi2WO6, and F−Ce−Bi2WO6 were determined to be 2.77, 2.80, 2.68, and 2.71 eV, respectively. The band gap of F−Ce− Bi2WO6 is relatively narrower than that of Bi2WO6, which is of benefit to its visible-light-driven photocatalytic performance. Photocatalytic Activities. Figure 7a presents the photocatalytic activities of the obtained Ce−Bi2WO6, F−Bi2WO6, F− Ce−Bi2WO6, and pure Bi2WO6 samples in decomposition of RhB under visible light irradiation (λ > 420 nm). It was found

that all the F or Ce modified Bi2WO6 exhibit much better photocatalytic performance than pure Bi2WO6, in which only 66% of RhB has been degraded. In Ce−Bi2WO6 series, the photocatalytic activity of 1% Ce−Bi2WO6 is higher than the two others. F−Ce−Bi2WO6 displayed the highest efficiency in all the samples, and RhB was almost removed within 3 h. As shown in Figure 7b, the strongest absorbance of this solution in visible region shifted from 553 to 502 nm after 3 h irradiation, indicating the occurrence of N-demethylation and de-ethylation in the photocatalytic processes.24 In addition, in order to compare the reaction kinetics of the photodegradation process of RhB quantitatively, the pseudofirst-order model based on the Langmuir−Hinshelwood (L−H) kinetics model was applied,39,40 as shown in the following equation: ln(C0/C) = kappt

(1)

where kapp is the apparent pseudo-first-order rate constant (h−1), C0 is initial RhB concentration (mg/L), and C is the instantaneous concentration of RhB solution at time t (mol/L). The apparent rate constants of different samples for photodegradation of RhB are presented in Figure 7c. The experimental data showed the corresponding kapp values are 0.298, 0.697, 0.819, 0.463, 0.563, and 1.201 h−1 for pure Bi2WO6, 0.5% Ce−Bi2WO6, 1% Ce−Bi2WO6, 1.5% Ce− Bi2WO6, F−Bi2WO6, and F−Ce−Bi2WO6, respectively. The results demonstrated that F−Ce−Bi2WO6 exhibits the most 14382

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Figure 4. TEM images of (a) pure Bi2WO6 and (b) F−Ce−Bi2WO6. (c) HRTEM (d, f) FFT (fast Fourier transition) patterns, and (e, g) inverse FFT (fast Fourier transition) patterns of the lattice fringe of F−Ce−Bi2WO6.

of visible light adsorption, the visible-light-induced photocatalytic performance of F−Bi2WO6 would also be improved compared to pure Bi2WO6. As for the Ce3+/Ce4+ pair, it can serve as an electron scavenger and superoxide radicals (•O2−) producer shown in the following steps:

significantly enhanced photocatalytic activity, which is approximately 4 times higher than that of pure Bi2WO6. Photocatalytic Mechanism. The detection of the main active species during the photocatalytic reaction is important with respect to the elucidation of the photocatalytic mechanism. Trapping of radicals and holes in experiments involves the utilization of benzoquinone (BQ), ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), and isopropanol (IPA) as superoxide radicals (•O2−), holes (h+), and hydroxyl radicals (•OH) scavengers, respectively.41,42 Figure 8 shows the active species trapping experiment in photocatalytic process of F−Ce−Bi2WO6. It was found that the photodegradation of RhB was almost not affected through adding 1 mM IPA as quencher of •OH. In contrast, the photocatalytic activity of the F−Ce−Bi2WO6 was largely suppressed by the addition of EDTA-2Na and BQ, and the inhibition efficiencies for degradation of RhB are about 70% and 91%, respectively. Thus, it could be inferred that superoxide radicals (•O2−) and photogenerated holes (h+) serves as the main active species for the photodegradation of RhB over F−Ce−Bi2WO6 under visible light irradiation. Accordingly, the possible photocatalytic mechanism was proposed. The high photocatalytic activity of F−Ce−Bi2WO6 should be ascribed to the synergistic effect of Ce and F codoping. As shown in Figure 9, the incorporation of fluorine dopants could effectively result in a red-shift on the optical absorption edge, indicating that some energy levels were formed on the top of the valence band.43 In this case, the electron was more excited from doped F energy level to arrive at the conduction band of Bi2WO6. Owing to the enhancement

F−Ce−Bi 2WO6 + visble light → F−Ce−Bi 2WO6 (e− + h+) (2)

Ce 4 + + e− → Ce3 +

(3)

h+ + RhB → degradation

(4)

Ce3 + + O2 → •O2− + Ce 4 +

(5)

O2 + e− → •O2−

(6)

•O2− + RhB → degradation

(7)

Upon visible light irradiation, photogenerated electrons could be trapped by Ce4+, and the Ce4+ was reduced to Ce3+. Then, the Ce3+ could be oxidized back to Ce4+ by the adsorbed oxygen in this system. Meanwhile, O2 adsorbed on the surface of the photocatalyst can react with Ce3+ and e− to generate • O2−. Therefore, the presence of the Ce4+/Ce3+ pair can not only efficiently separate the electrons and holes but also result in the generation of •O2− for the degradation of the dye molecules. However, excess Ce species might cover the active sites or act as a recombination center of Bi2WO6, which would reduce the separation efficiency of charge carriers. Therefore, 1% is the optium concentration of Ce doping, and 1% Ce−F 14383

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Figure 5. XPS spectra of the F−Ce−Bi2WO6 sample: (a) survey, (b) W 4f, (c)Bi 4f, (d) O 1s, (e) F 1s, and (f) Ce 3d.

Photoluminescence (PL) spectra are usually utilized to understand the separation and transfer efficiency of photogenerated charge carrier of a photocatalyst, as PL emission mainly originates from the recombination of photogenerated holes and electrons. The lower PL intensity reflects the low recombination rate of photogenerated electrons and holes, thus the higher photocatalytic activity.44 Figure 10 displays the PL spectra of the pure Bi2WO6, 1% Ce−Bi2WO6, F−Bi2WO6, and F−Ce−Bi2WO6 at room temperature. It can be observed that the emission intensity of F−Ce−Bi2WO6 is obviously decreased comparing to other samples, indicating that it possess the highest photocatalytic activity which is in consistent with the result from photodegradation experiment. The interfacial charge transfer dynamics can also be elucidated by the photocurrent measurements, in which the generation and transfer of the photoexcited charge carrier in the photocatalytic peocess can be indirectly monitored by the photocurrent generation. Generally, the higher photocurrent indicates the higher electrons and holes separation efficiency.45 Figure 11a shows the photocurrent−time testing curves of pure Bi2WO6 and F−Ce−Bi2WO6 under visible light irradiation. Compared to pure Bi2WO6, F−Ce−Bi2WO6 exhibits an

Figure 6. UV−vis diffuse reflectance spectra and band gaps (insets) of pure Bi2WO6, Ce−Bi2WO6, F−Bi2WO6, and F−Ce−Bi2WO6.

codoped Bi2WO6 possesses the highest efficiency of charge separation. 14384

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Figure 8. Photodegradation of RhB over F−Ce−Bi2WO6 in the presence of different scavengers.

Figure 9. Schematic diagrams of the photodegradation of RhB over F−Ce−Bi2WO6 under visible light irradiation.

Figure 7. (a) Photocatalytic degradation curves of RhB under visible light irradiation. (b) Temporal absorption spectral patterns of RhB during the photodegradation process of F−Ce−Bi2WO6. (c) Apparent rate constants for the photodegradation of RhB over pure Bi2WO6, Ce−Bi2WO6, F−Bi2WO6, and F−Ce−Bi2WO6.

obviously enhanced photocurrent response. Thus, F−Ce− Bi2WO6 possesses higher separation efficiency of charge carriers than pure Bi2WO6. It is in accordance with the supposed mechanism above. The typical electrochemical impedance spectrum (EIS) is also a useful method to reflect the migration and transfer processes of photogenerated electrons and holes in a semiconductor. The interface layer resistance occurred on the surface of electrode can be indicated by the radius of the arc in the EIS spectra. The smaller the arc radius is, the higher the charge transfer efficiency is.46,47 Figure 11b shows the Nyquist plots of pure Bi2WO6 and F−Ce−Bi2WO6 with and without

Figure 10. PL spectra of pure Bi2WO6, 1% Ce−Bi2WO6, F−Bi2WO6, and F−Ce−Bi2WO6. 14385

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Figure 11. (a) Comparison of transient photocurrent responses of pure Bi2WO6 and F−Ce−Bi2WO6 under visible light irradiation (λ > 420 nm, [Na2SO4] = 0.1 M). (b) EIS Nynquist plots of pure Bi2WO6 and F−Ce−Bi2WO6 with light on/off cycles under the irradiation of visible light (λ > 420 nm, [Na2SO4] = 0.1 M).

Notes

visible light irradiation. The arc radius of F−Ce−Bi2WO6 are found to be smaller than that of pure Bi2WO6 in the cases of both before and after irradiation, implying that F−Ce−Bi2WO6 holds a stronger ability in separation and transfer of photogenerated electron−hole pairs.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundations of China (Grant No. 51302251), the Fundamental Research Funds for the Central Universities (2652013052), and the special coconstruction project of Beijing city education committee, Key Project of Chinese Ministry of Education (No. 107023).

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CONCLUSIONS In summary, Ce−F−Bi2WO6 nanoplates were successfully prepared by a facile one-step hydrothermal route. XPS results revealed the coexistence of Ce3+, Ce4+, and F− species in Bi2WO 6. Under visible light irradiation, F−Ce−Bi 2WO6 exhibits a much higher photocatalytic activity for degradation of RhB than all other doping samples, including pure Bi2WO6, Ce−Bi2WO6, and F−Bi2WO6. Based on the results of active species trapping measurements, photogenerated holes (h+) and superoxide radicals (•O2−) playing a crucial role in photodegradation of RhB over F−Ce−Bi2WO6, the photocatalytic mechanism was proposed. It was also confirmed by the PL spectra photocurrent generation and EIS measurements. The observed remarkably improved photocatalytic activity should be resulted from the synergistic effect of the codoping of Ce and F, in which the Ce3+/Ce4+ redox couple acts as an electron scavenger and F narrowed the optical band gap enhancing the visible-light absorption. This synergistic effect effectively promotes the separation and transfer of photogenerated charge carriers and simultaneously depresses the recombination of holes (h+) and electrons (e−), resulting in the high photocatalytic performance. The present results herein could not only provide a better understanding of the synergistic effect of Ce and F comodification but also be beneficial for to the design of high performance semiconductor photocatalysts.

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ASSOCIATED CONTENT

S Supporting Information *

Raman spectra of F−Ce−Bi2WO6 and Bi2WO6. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]; Tel 86-10-82332247 (H.H.). *E-mail [email protected]; Tel 86-10-82332247 (Y.Z.). 14386

dx.doi.org/10.1021/jp503025b | J. Phys. Chem. C 2014, 118, 14379−14387

The Journal of Physical Chemistry C

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