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Apr 16, 2016 - The location of the valence band maximum (VBM) edge determines the oxidation power of the holes.(1) Therefore, the mediation of a valen...
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Mediation of Valence Band Maximum of BiOI by Cl Incorporation for Improved Oxidation Power in Photocatalysis Fan Tian, Huiping Zhao, Zan Dai, Gang Cheng, and Rong Chen* School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Xiongchu Avenue, Wuhan 430073, People’s Republic of China S Supporting Information *

ABSTRACT: Cl-incorporated BiOI nanostructures with different Cl/I molar ratios have been successfully fabricated via a facile roomtemperature method in H2O/EG mixture solvent. Combined with FTIR, Raman spectrum, XRD Rietveld refinements, and DFT calculation, the incorporated Cl ions are found mainly occupied on I sites, resulting in a hybrid valence band consisting of I 5p, O 2p, and Cl 3p orbitals, which deepens the valence band location of the materials and induces photogenerated holes with higher oxidation power. The incorporated Cl ions into the lattice also change the evolution of photogenerated electrons and holes, inducing the broadening of the bandgap but efficient production of photogenerated carriers. The enhanced oxidation power of Cl-incorporated BiOI products was evaluated by photocatalysis degradation of methylene blue, an organic dye for which pure BiOI shows negligible direct oxidation efficiency. Ascribed to the relatively low valence band maximum (VBM) and efficient generation of photoinduced electrons and holes, the incorporated products show distinct enhancement photocatalytic activities.

1. INTRODUCTION The degradation of organic compounds involving semiconductor-based photocatalysis has attracted much interest in the past decades.1−3 Generally, efficient photodegradation requires the production of active species with high oxidation power. In a specific photocatalytic system, the most powerful oxidation species are photogenerated holes, which are initiated from the separation of electron−hole pairs via photoexcitation. The dominated photogenerated electrons in the material were excitation from the valence band (VB) to the conduction band (CB), leaving holes on the valence band of the semiconductor. The location of the valence band maximum (VBM) edge determines the oxidation power of the holes.1 Therefore, the mediation of a valence band maximum (VBM) is significantly meaningful for improving the photocatalytic ability of a semiconductor. It is particularly important for the photogenerated holes-dominated photodegradation in organic wastewater treatment. Recently, bismuth-containing nanomaterials have been extensively used in photocatalysis due to it being ecological benign and able to efficiently utilize solar energy.4−17 For example, BiOX (X = Cl, Br, I) shows good photocarrier mobility and high separation efficiency of photogenerated electron−hole pairs, due to its unique layered structure interleaved with [Bi2O2] slabs and double halogen atom slabs.4,6,11,18−20 Among them, BiOI has been reported as an excellent visible-light-driven photocatalyst for direct oxidation of organic compound such as methyl orange (MO) by © 2016 American Chemical Society

photogenerated holes, which possesses the narrowest bandgap.20,21 However, direct photocatalytic oxidation of some other organic contaminants such as methylene blue (MB) over BiOI seems ambiguous,22 which might be ascribed to the relatively low highest occupied molecular orbital (HOMO) at −10.494 eV.23 The oxidation of MB requires higher oxidation power of photogenerated holes. Therefore, mediation of the VBM of BiOI to lower the VB edge is highly desired. Considering the relatively deeper VB location of BiOCl,24 in this work, we try to mediate the VBM of BiOI via Cl incorporation into the crystal lattice, realizing the enhanced photocatalytic ability of BiOI. By varying the feeding I/Cl ratio of halide precursors, different amounts of Cl-incorporated BiOI hierarchical nanostructures were prepared via a facile method at room temperature. The photocatalytic activities of the as-prepared products were evaluated by degradation of MB under visible light irradiation. By employing DFT calculations and Mott− Schottky plots, the relative VB edge locations of the as-prepared products were analyzed. Furthermore, the photogenerated electron−holes separation efficiency of the incorporated products was also investigated. This work provides a strategy to develop a new photocatalyst with high photocatalytic activity. Received: Revised: Accepted: Published: 4969

March 1, 2016 April 13, 2016 April 16, 2016 April 16, 2016 DOI: 10.1021/acs.iecr.6b00847 Ind. Eng. Chem. Res. 2016, 55, 4969−4978

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Industrial & Engineering Chemistry Research

solution (2 × 10−5 mol/L). Before irradiation, the solution was vigorously stirred in the dark for 60 min to allow the adsorption−desorption equilibrium of MB on a catalyst surface. At each given time interval, 2 mL of solution was collected and immediately centrifuged. The concentration variation of MB during the degradation was monitored by its characteristic absorption band (664 nm) using a Shimadzu UV2800 spectrophotometer. All of the measurements were carried out at room temperature. 2.5. Photoelectrochemical Measurement. The photoelectrochemical and electrochemical measurements were conducted on a CHI 660E electrochemical system (Shanghai, China) by using a standard three-electrode cell, which contained a working electrode, a platinum wire counter electrode, and a standard calomel electrode (SCE) as reference. 0.5 M Na2SO4 was used as the electrolyte. The working electrode was prepared according to the following process: 20 mg of as-prepared sample was mixed with 1 mL of DMF and 0.01 mL of Nafion solution (5%, DuPont) to form a homogeneous ink. Next, 0.1 mL of catalyst ink was dip coated on a 10 mm × 10 mm indium−tin oxide (ITO) glass electrode. The estimated loading amount of the sample is 2 mg/cm2. After drying at room temperature, the as-prepared electrode was further annealed at 80 °C for 4 h in a vacuum oven to remove the resin. Photocurrent responses of the photocatalyst as light on and off were measured at open-circuit potential, with simulated light irradiation provided by a 50 W Xe lamp. Mott− Schottky analyses were employed with potential windows ranging from −0.6 to 0 V vs Ag+/AgCl reference electrode. Electrochemical impedance spectroscopy (EIS) was carried out at the open-circuit potential in 0.5 M potassium ferricyanide solution. Samples were dip-coated into a glassy carbon working electrode. A sinusoidal AC perturbation of 5 mV was applied to the electrode over the frequency range 0.5−105 Hz. 2.6. DFT Calculation. The plane-wave-based density functional theory (PW-DFT) calculations were performed using the QUANTUM ESPRESSO package program,25 with ultrasoft pseudopotentials employed for Cl, I, O atoms and norm-conserving psedopotential for the Bi atom.26 The kinetic cutoff energies were set as 90 Ry. A 24-atom 2 × 2 × 1 super cell was used for all calculations. For incorporation model calculations, an equal number of I atoms were replaced by Cl atoms. Before band calculation, all of the atoms in the super cell were allowed to fully relax by using the Broyden−Fletcher− Goldfarb−Shanno (BFGS) method to minimalize the total energy and force.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Bismuth nitrate pentahydrate (Bi(NO3)3· 5H2O) was purchased from Aladdin (Shanghai, China). Potassium chloride (KCl), potassium iodide (KI), methylene blue (MB), and sodium sulfate (Na2SO4) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Ethylene glycol (EG) was purchased from Tianli Chemical Reagent Co., Ltd. (Tianjin, China). All of the reagents were of analytical grade and used directly without further purification. 2.2. Preparation. In a typical procedure, 0.970 g of Bi(NO3)3·5H2O (2 mmol) was dissolved into 40 mL of ethylene glycol (EG) to form solution A. 0.149 g of KI (2 mmol) was dissolved into 20 mL of deionized water to form solution B. Solution B then was slowly dropwise added into solution A within 15 min under vigorous stirring, resulting in a salmon pink suspension. After being continuously stirred for 5 min, the product was centrifuged and then washed with deionized water and ethanol five times. The sample was collected and finally dried in an oven for 24 h at 60 °C for further characterizations (BIC-1). Other samples were also prepared under identical conditions by using a KCl and KI mixture with different molar ratios or KCl as precursor, instead of KI. The detailed experimental conditions are listed in Table 1. Table 1. Synthesis Conditions for the Various Products sample

halide precursors

Bi/I/Cl

BIC-1 BIC-2 BIC-3 BIC-4 BIC-5 BIC-6

KI KI/KCl KI/KCl KI/KCl KI/KCl KCl

2:2 2:1.5:0.5 2:1:1 2:0.75:1.25 2:0.5:1.5 2:2

2.3. Characterization. Powder X-ray diffraction (XRD) was carried out on a Bruker AXS D8 Discover (Cu K = 1.5406 Å) at a scan rate of 2° min−1 in the 2θ range from 10° to 80°. Scanning electron microscopy (SEM) images were taken on a Hitachi S4800 scanning electron microscope operating at 5.0 kV. X-ray photoelectron spectroscopy (XPS) was performed on a VG Multilab 2000 photoelectron spectrometer (VG Inc.), using Al Kα radiation as the excitation source under vacuum at 2 × 10−6 Pa. All of the binding energy (BE) values were calibrated by the C 1s peak at 284.6 eV of the surface adventitious carbon. The IR spectra were recorded on a Bruker Tensor 27 FT-IR spectrometer with a spectral resolution of 4 cm−1. The Raman spectra were recorded on a Nicolet Almega XR Raman spectrometer with 532 nm laser as the light source. UV−vis diffuse reflectance spectra (DRS) were recorded on a UV−vis spectrometer (Shimadzu UV-2550) by using BaSO4 as a reference and were converted from reflection to absorbance by the Kubelka−Munk method. The Brunauer−Emmett− Teller (BET) specific surface area of the sample was analyzed by nitrogen adsorption in a Micromeritics ASAP 2020 nitrogen adsorption apparatus (U.S.). All of the as-prepared samples were degassed at 150 °C for 4 h prior to nitrogen adsorption measurements. 2.4. Photocatalytic Activity Test. Photocatalytic activities of the as-prepared samples were evaluated by the degradation of MB in aqueous solution under visible light irradiation by using a 500 W Xe lamp with a 420 nm cutoff filter. In a typical experiment, 0.01 g of sample was dispersed into 40 mL of MB

3. RESULTS AND DISCUSSION The purity and crystallinity of the as-synthesized products were examined by powder X-ray diffraction (XRD). Figure 1 shows the XRD patterns of the obtained products (BIC-1−BIC-6). The products prepared by using KI (BIC-1) and KCl (BIC-6) as halide precursor could be perfectly indexed to tetragonal BiOI (ICSD: 73-2062; a = b = 3.9840 Å, c = 9.1280 Å) and tetragonal BiOCl (ICSD: 73-2060; a = b = 3.883 Å, c = 7.3470 Å), indicative of the production of pure BiOI and BiOCl. However, the main diffraction peaks of the products obtained from the mixture of KI and KCl as halide precursors display an obvious peak shift, demonstrating the production of incorporated products. To confirm the incorporation of Cl atom to BiOI, XRD Rietveld refinements were performed for pure BiOI and Clincorporated BiOI products (BIC-1 and BIC-2). As shown in 4970

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Table 2. Cell Parameters, Structure Parameters, and Relative Cationic Occupancies Obtained after the Rietveld Refinement for BiOI (BIC-1, Top) and Cl-Doped BiOI (BIC-2, Bottom) atom

Figure 1. XRD patterns of the products prepared from different feeding ratios of Cl and I precursor at room temperature.

x/a

y/b

z/c

Biso (Å2)

Bi 0.25000 O 0.25000 I 0.25000 cell parameters: a = b atom x/a

0.25000 0.12751 0.75000 0.0000 0.25000 0.66588 = 3.991388 Å, c = 9.164334 Å y/b z/c Biso (Å2)

Bi 0.25000 O 0.25000 I 0.25000 Cl 0.25000 cell parameters: a = b

0.25000 0.13620 0.23066 0.75000 0.0000 2.13855 0.25000 0.66120 2.13855 0.25000 0.66120 2.13855 = 3.962531 Å, c = 9.079006 Å

occupancy 1 1 1 occupancy 1 1 0.8656 0.1344

the corresponding Rietveld refinements patterns in Figure 2, the refinement data of Bragg R-factors of 3.771 and 2.516

Figure 3. Raman spectra (a) and high-resolution FT-IR spectra (b) of the products prepared from different feeding ratios of Cl and I precursor at room temperature.

range of 180−40 cm−1 to investigate the halogen motion in the lattice of the products. The strong bands observed at ca. 140 and 85 cm−1 were assigned to A1g Bi−Cl external and Bi−I internal stretching modes, respectively.27,28 The two modes both show distinct shifts with the variation of I/Cl molar ratio, indicative of the incorporation of Cl ion into the lattice of BiOI. Another strong band below 60 cm−1 could be assigned to the A1g external Bi−Cl stretching mode for BiOCl or both the Eg and the A1g external Bi−I stretching modes for BiOI.27 The shifts from 46 to 58 cm−1 were observed for the products obtained from different feeding ratios of KI and KCl. The variations of Bi−O stretching modes of the as-prepared products were verified by FT-IR, as shown in Figure 3b. The A2u Bi−O stretching vibration mode of BiOI at 486.4 cm−127 shifts to high wavenumber as the Cl/I ratio increased, demonstrating the successful incorporation of Cl into the BiOI lattice. The different surface chemical composition of the asprepared BiOI, BiOCl, and Cl-incorporated BiOI was characterized by X-ray photoelectron spectra (XPS). Figure 4a shows the XPS survey spectra of BIC-1, BIC-4, and BIC-6. All of the spectra exhibit Bi, O, and C elements. The C signal comes from the adventitious carbon. Only Cl and I signals were detected in the XPS spectra of pure BiOCl (BIC-6) and BiOI (BIC-1), respectively. However, both Cl and I signals were presented in the XPS spectrum of BIC-4, demonstrating the coexistent of Cl and I element on the surface of the product. In

Figure 2. Rietveld refinement plot of BiOI fabricated from KI as a halide precursor (BIC-1, a) and Cl-incorporated BiOI fabricated from KI and KCl with a molar ratio of 3:1 (BIC-2, b).

indicate that either the BiOI unit cell or the substituted model satisfies the real structures of the as-prepared products. The final refined structure parameters are listed in Table 2. The lattice parameters were determined as a = b = 3.962531 Å and c = 9.079006 Å for BIC-2, which are smaller than that for BIC-1 with a = b = 3.991388 Å and c = 9.164334 Å, indicating that the replacement of I by Cl atoms induces narrowing of the lattice cell of the product. To further verify the incorporation of Cl into BiOI lattice, Raman and infrared spectra of the as-prepared products were further characterized. Figure 3a shows the Raman spectra in the 4971

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Figure 4. XPS spectra of BIC-1, BIC-4, and BIC-6 prepared from different halide precursors: survey spectra (a) and high-resolution spectra for Bi 4f (b), Cl 2p (c), and I 3d (d).

the high-resolution XPS spectra of BIC-4 (Figure 4b−d), it was also observed that the binding energies of Cl 2p and I 3d in BIC-4 had an obvious shift as compared to those of pure BiOI (BIC-1) and BiOCl (BIC-6). Simultaneously, the binding energies of Bi 4f5/2 and Bi 4f7/2 for the Cl-incorporated product (BIC-4) are 164.87 and 159.54 eV, which lie between the Bi 4f binding energies of BiOI (164.44 and 159.18 eV for BIC-1) and BiOCl (164.99 and 159.68 eV for BIC-6). It is because the electronegativity of Cl (3.16) is larger than that of I (2.66). The replacement of I by Cl ions would result in electrons flowing from the [Bi2O2]2+ layer to the halide layer and thus lowering the electron density around the Bi atoms, leading to a blue shift of binding energy for Bi 4f5/2 and Bi 4f7/2 as compared to that of BiOI. The introduction of Cl into the products also shows an impact on the binding energy of O. Figure S1a shows the highresolution XPS of O 1s. It was found that the spectra could be deconvoluted into two peaks at 532.6 and 530.4 eV for pure BiOI products (BIC-1), which was attributed to the oxygen in the hydroxyl group adsorbed on the products and lattice oxygen in BiOI, respectively.29 As the amount of Cl increased in the product, the binding energy of the lattice oxygen of the obtained products shows obvious blue shifts, also indicative of the successful incorporation of Cl into the lattice of BiOI. It was also found that the intensity of the hydroxyl group decreased with the increase of the Cl amount, illustrating that the incorporation of Cl suppressed the surface adsorption of the hydroxyl group. Valence band XPS shows that the introduction of Cl into BiOI lowers the location of the valence band of the products, as shown in Figure S1b. The results demonstrate that Cl ions were successfully incorporated into BiOI by replacing I ions, which was consistent with the results obtained from XRD, Raman, and FT-IR characterizations. The morphologies of the as-prepared products were characterized by scanning electron microscopy (SEM). Figure 5 shows SEM images of the products fabricated from different molar ratios of KI and KCl. Except for BIC-6 (Figure 5f), all of the products display three-dimensional hierarchical microstructures, which were assembled by tiny sheet-like nanocrystals. With the increase of the Cl/I molar ratio in the halide precursor, the product presented loose-packed structures with thin 2D sheet-like structure units, as shown in Figure 5a−d.

Figure 5. SEM images of the products fabricated from different molar ratios of KI and KCl: BIC-1 (a), BIC-2 (b), BIC-3 (c), BIC-4 (d), BIC-5 (e), and BIC-6 (f).

However, further increasing the Cl/I molar ratio in the halide precursor might promote the formation of nanocrystal with irregular sizes and morphologies, as shown in Figure 5e. Pure BiOCl products (BIC-6) exhibit a uniform sheet-like morphology with no distinct trend to form 3D hierarchical nanostructures. This phenomenon is similar to the reported results of the formation of BiOX hierarchical nanostructured in polyols.30 The specific surface areas of the as-synthesized products (BIC-1−BIC-6) were measured by using nitrogen adsorption−desorption isotherms. As shown in Figure S1, all of the isotherms could be nearly categorized as type IV with a distinct hysteresis loop observed in the range of 0.5−1.0 P/P0, indicative of mesoporous structures of the products. The pore size distribution curve (inset) illustrates that all of the samples except BIC-6 exhibit two types of pores. The narrow distributions centered at 2.5 nm are ascribed to the slit pores induced by the accumulation of nanosheets, and the wide distribution extending from several to dozens nanometers is assigned to the open pores in hierarchical nanostructures selfassembled by the sheet-like nanocrystal. A few pores large than 10 nm were observed in the distribution curve for BIC-6, demonstrating that self-assembly between sheet-like nanocrystal is not distinct in the formation of pure BiOCl. This result is consistent with the SEM observation. The calculated BET surface areas are 11.98, 20.24, 26.14, 36.01, 20.16, and 15.22 m2 g−1 for BIC-1−BIC-6, respectively, indicating that increasing the molar ratio of Cl in halide precursors not only resulted in production of incorporated products, but also shows influences on the final aggregation state of the obtained nanocrystal. The optical properties of the as-prepared products were characterized by UV−visible diffuse reflectance spectroscopy (DRS), as shown in Figure 6. It is found that all of the spectra show only one absorption edge, and the absorption edges of the products obtained from a mixed precursor of KI and KCl (BIC-2−BIC-5) show distinct blue shifts as compared to that 4972

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BiOI.24,31,32The bandgap of BiOI (BIC-1) estimated by the absorption edge of ca. 650 nm is 1.7 eV, which is in accord with the reported literature.20,21,33 The BiOCl product (BIC-6) is still only UV light responsive with a bandgap of 3.29 eV. The bandgaps of the incorporated products are estimated to be 1.82, 1.89, 1.93, and 2.04 eV for BIC-2, BIC-3, BIC-4, and BIC-5, respectively, demonstrating that the bandgap of the products could be mediated by the variation of the Cl/I molar ratio. To further investigate the locations of the valence bands of the as-prepared samples, DFT calculations and Mott−Schottky analysis were employed. Figure 7 shows the projected density of state (PDOS) for pure BiOI, Cl-corporation BiOI, and BiOCl. For pure BiOI product, the calculated conduction band and valence band are majorly contributed by I 5p, O 2p, and Bi 6p orbitals, respectively. The VBM of BiOI is calculated to be 9.36 eV (Figure 7a). For pure BiOCl products, the VBM is determined to be 8.13 eV (Figure 7d). However, the contribution of VB comes from Cl 3p, I 5p, and O 2p orbitals after the incorporation of Cl ions into BiOI lattice (Figure 7b and c). Therefore, the VBMs of Cl-incorporated samples lie between the VBMs of BiOI and BiOCl, which is found to be ranging from 9.35 to 8.20 eV, as summarized in Table S1. It was also found that the VBM value decreases with the increase of the Cl/I molar ratio. Theoretically, a low VBM value demonstrates a deeper valence band location, indicative of the higher oxidation power of the photogenerated holes. To further evaluate the oxidation power of the photogenerated holes, we also determined the flat potential for the as-prepared products in Na2SO4 solution by employing Mott−Schottky analysis,34 as shown in Figure 8. Consistent with the DFT calculation results, the flat potentials decrease with the increase of the Cl/I molar ratio. Interestingly, the Cl-incorporated products with a higher Cl/I molar ratio even exhibited a much

Figure 6. UV−visible diffuse reflectance spectroscopy (DRS) (a) and the corresponding (Ahν)1/2 versus hν plots (b) of the products fabricated from different molar ratios of KI and KCl.

of BiC-1, also demonstrating that the as-prepared samples are monophasic, and not composited material of BiOCl and

Figure 7. Projected density of states (PDOS) for pure BiOI (a), Cl-incorporated BiOI with a molar ratio of 12.5% (b), that with a molar ratio of 50% (c), and pure BiOCl (d). 4973

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Figure 8. Flat potential determination of pure BiOI (BIC-1), Cl-incorporated BiOI (BIC-2−BIC-5), and pure BiOCl (BIC-6) by Mott−Schottky plots.

degradation as compared to pure BiOI and BiOCl. Among them, BIC-4 shows the highest photocatalytic activity and could completely degrade MB within 4 h. It is believed that the incorporation of Cl into BiOI lattice varies the location of the valence band somehow; therefore, the photogenerated holes possess higher oxidation power for MB degradation. The kinetics of MB degradation are also fitted as pseudo-first-order, as shown in Figure 9b. It is found that the MB photodegradation exhibits a good linear relationship, indicating that the process follows the traditional Langmuir−Hinshelwood (L−H) mechanism.36 The apparent rate constants (kapp) for MB degradation are determined to be 0.00148, 0.00421, 0.00746, 0.01203, 0.00825, and 0.00118 min−1 for BIC-1−BIC6, respectively. To get insight into the MB degradation process, the trapping experiments of active species during photocatalysis were first performed. tert-Butyl alcohol (TBA), ammonium oxalate (AO), and CCl4 were used as •OH, h+, and e+ scavengers, respectively.37−39 As shown in Figure 10a, distinct suppression of MB degradation is only observed in the presence of hole scavenger of AO, indicating that the dominated degradation process is via direct hole oxidation. Slightly suppression of MB degradation is also observed upon the addition of CCl4, illustrating that the electron process (such as •O2−) also

higher value than pure BiOCl products, which might be ascribed to the different surface chemical environment induced by the amorphous process in growth of the products. The results provide evidence that the location of the VBM of BiOI products could be moderated by the simple incorporation of Cl ions into the lattice by replacing I ions. The photocatalytic activity of the as-prepared samples was evaluated by the degradation of methyl blue (MB) under visible light irradiation (Figure 9). It is found that only about 10% MB was degraded after 4 h visible light irradiation in the absence of photocatalyst (Figure 9a). As BiOCl product with a bandgap of 3.29 eV (BIC-6) cannot be excited under visible light irradiation, the photodegradation of MB over BIC-6 upon visible light irradiation shows negligible efficiency. Noticeably, pure BiOI products (BIC-1) also display low efficiency for MB degradation due to its low valence band location, although it has been considered an excellent visible-light-driven photocatalyst.20,21,29,35 As photodegradation of organic pollutant over BiOI was dominated as the hole oxidation process, the photogenerated holes over BiOI did not exhibit enough overpotential for the oxidation of MB, thus leading to the low photocatalytic efficiency over BIC-1. Importantly, it is found that all of the Cl-incorporated samples (BIC-2−BIC-5) exhibit remarkably enhanced photocatalytic activities for MB 4974

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Figure 10. Trapping experiments (a) and kinetics (b) for detecting the active species in the degradation.

Figure 9. Photocatalytic activities (a) and kinetics (b) for MB degradation in the presence of as-prepared products under visible light irradiation.

contributes to the degradation. Figure 10b shows the pseudofirst-order fitting of the trapping experiment data. The process also exhibits a good linear relationship in the presence of scavengers, which is in accord with direct degradation mentioned previously, demonstrating that the addition of scavenger does not affect the MB degradation process, confirming the reliability of the trapping experiment data. Therefore, the degradation of MB over the as-prepared products was ascribed to the improved oxidation power of photogenerated holes after Cl ions were incorporated, which is in accord with DFT calculations and Mott−Schottky plots. To better understand the improvement of photocatalytic activities for the incorporated products, we further explore the transient photocurrent responses, electrochemical impedance spectroscopy (EIS), transient photocurrent decay responses, and fluorescence emission properties of the as-prepared products to characterize the generation, migration, and recombination of photoinduced electrons and holes. To avoid the accumulation of electrons in electrode and obtain reproducible data, the photocurrent responses were tested in 0.5 M Na2SO4 with addition of 2 mL of H2O2 as electron depleting regent. Figure 11 shows the rapid and consistent photocurrent responses for each switch-on and -off event in multiple 20 s on−off cycles under simulated solar light irradiation in the presence of H2O2. It is worth noting that the photocurrent density of Cl-incorporated BiOI (BIC-4, ca. 0.5 μA cm−2) electrode is much higher than that of pure BiOI (BIC-1, ca. 0.04−0.05 μA cm−2) and BiOCl (BIC-6, ca.0.05− 0.06 μA cm−2). The enhanced photocurrent response of the Clincorporated BiOI sample indicates higher efficiency and lower

Figure 11. Photocurrent responses of BiOI (BIC-1), BiOCl (BIC-6), and Cl-incorporated BiOI (BIC-4) in the presence of H2O2 as electron depleting reagent in Na2SO4 solutions.

recombination rate of photogenerated electron−hole pairs in the Cl-incorporated structures. EIS was used to investigate the photogenerated charge migration process. EIS Nyquist plots of the as-prepared BiOI (BIC-1), BiOCl (BIC-6), and Clincorporated BiOI (BIC-4) are shown in Figure 12. A smaller arc radius of EIS Nyquist plot suggests an effective separation of photogenerated electron−hole pairs and fast interfacial charge transfer. The Cl-incorporated product (BIC-4) shows the smallest arc radius of EIS Nyquist plot, indicative of the most effective separation of electron−hole pairs and the fastest interfacial charge transfer. 4975

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Figure 12. EIS Nyquist plots of the as-prepared BiOI (BIC-1), BiOCl (BIC-6), and Cl-incorporated BiOI (BIC-4).

Figure 14. Fluorescence spectroscopies of the as-prepared BiOI (BIC1), BiOCl (BIC-6), and Cl-incorporated BiOI (BIC-4).

The dynamic depletion process of photoinduced electrons and holes is illustrated by photogenerated electron decay plots.40 As the photoresponses were recorded at open-circuit voltage, a fast decay of photogenerated electrons indicates a fast depletion of photoinduced carriers consumption involving nonphotocatalytic degradation of organic dyes. Figure 13 shows

of fluorescence spectroscopy. High fluorescence intensity indicates an efficient recombination process of photogenerated electron−hole pairs, which is not beneficial for improvement of photocatalytic activity. It is proposed that the incorporation of Cl into the lattice of BiOI results in a narrowing lattice cell of BiOI along the c axis, as demonstrated by XRD refinement results. It would shorten the spatial distance between [Bi2O2] slabs and halogen atoms slabs, and thus benefit the electron− hole pair recombination. It illustrates that incorporating Cl ions into the lattice of BiOI by replacing I ions not only deepens the VBM of the obtained materials, but also mediates the photocarriers evolution, inducing efficient separation of photogenerated electrons and holes and far more holes flowing to the oxidation process on the surface of materials, thus improving the overall photocatalytic efficiency for degradation of MB.

4. CONCLUSION In summary, Cl-incorporated BiOI hierarchical nanostructures were successfully synthesized by a facile EG-mediated method at room temperature. XRD, FT-IR, Raman, XPS, SEM, and DRS characterizations confirmed that the incorporated Cl ions mainly occupied the site of I ions in the lattice. A tunable bandgap and the location of VBM could be simply moderated by changing the Cl/I molar ratio. Photocatalysis activities of the as-prepared products were evaluated by degradation of methylene blue, a stable organic dye that could not be degraded over pure BiOI. As compared to the pure products, all of the Cl-incorporated BiOI shows distinct degradation performance for MB under visible light irradiation. The distinct improvement of photocatalytic activity for the incorporated products is ascribed to the suitable location of VBM, and efficient generation and separation of photoinduced electron and hole pairs. This work provides a strategy to mediate the band structure and location of valence band for BiOI, as well as a strategy to improve the photocatalytic activity by modifying the electron and hole evolution.

Figure 13. Photocurrent decays plots of the as-prepared BiOI (BIC1), BiOCl (BIC-6), and Cl-incorporated BiOI (BIC-4) in Na2SO4 solutions after simulated solar light irradiation.

the obtained decay curves. It is found that the incorporated product (BIC-4) exhibits a relatively long decay process as compared to pure BiOI (BIC-1) and BiOCl (BIC-6) samples. By fitting the decay region using a monoexponential decay relationship (y = A1*exp(−x/t1) + y0) after light-off, the obtained lifetime of photogenerated electrons for BIC-4 is 1.1514 s, which is 1.5 times higher than that of BIC-1 (0.81676 s) and 3 times higher than that of BIC-6 (0.41013 s), respectively, demonstrating a low depleting rate of photogenerated carriers by nonphotocatalytic degradation reaction for organic dyes. The recombination of photogenerated electrons and holes was recorded by fluorescence spectroscopy. Figure 14 shows the obtained fluorescence spectroscopy for BIC-1, BIC-4, and BIC-6. It is found the fluorescence emission peak of Cl-incorporated BiOI had a blue shift and the intensity increased as compared to pure BiOI (BIC-1). The blue shift of fluorescence is consistent with the DRS results of the incorporated products in that introducing Cl ions into the products resulted in broadening of the bandgap. As the observed photoemission of the products is mainly contributed by the directly photogenerated electron in the conduction band returning to its ground state, the broadening of the bandgap ascribed to the increasing Cl amount could result in a blue shift

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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/acs.iecr.6b00847. Additional detailed information including high-resolution XPS of O 1s, valence band XPS, nitrogen sorption− desorption test, and the location of valence band minimum (VBM), conduction band maximum (CBM), 4976

DOI: 10.1021/acs.iecr.6b00847 Ind. Eng. Chem. Res. 2016, 55, 4969−4978

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Industrial & Engineering Chemistry Research

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and the bandgap of BiOI, Cl-doped BiOI, and BiOCl obtained from DFT calculations (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel.: (+86)13659815698. Fax: (+86)2787195671. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21471121) and the High-Tech Industry Technology Innovation Team Training Program of the Wuhan Science and Technology Bureau (2014070504020243).

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