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Jun 5, 2019 - All-solid Z-scheme Bi–BiOCl/AgCl heterojunction microspheres are successfully prepared via hydrothermal, NaBH4 reduction and chemical ...
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All-Solid Z‑Scheme Bi−BiOCl/AgCl Heterojunction Microspheres for Improved Electron−Hole Separation and Enhanced Visible LightDriven Photocatalytic Performance Meng Du,† Shiyu Zhang,† Zipeng Xing,*,† Zhenzi Li,‡ Junwei Yin,† Jinlong Zou,† Qi Zhu,*,† and Wei Zhou*,†

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Department of Environmental Science, School of Chemistry and Materials Science, Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, Heilongjiang University, Harbin 150080, P. R. China ‡ Department of Epidemiology and Biostatistics, Harbin Medical University, Harbin 150086, P. R. China S Supporting Information *

ABSTRACT: All-solid Z-scheme Bi−BiOCl/AgCl heterojunction microspheres are successfully prepared via hydrothermal, NaBH4 reduction and chemical deposition strategy. They are tested by various characterization methods, and they show that metal Bi is present after reduction and AgCl nanoparticles are successfully compounded onto BiOCl. Bi plays the role of a bridge connecting the two semiconductors of BiOCl and AgCl. All-solid Z-scheme heterojunction structures are formed successfully. The narrow band gap of the Z-scheme Bi−BiOCl/AgCl heterojunction microspheres is about 2.17 eV, which can expand the optical response range. Moreover, the photocatalytic hydrogen production rate still reaches 198.2 μmol h−1 g−1, extends the electron transport life, inhibits the recombination of electron hole pairs, and improves the photocatalytic activity.

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structure.11−13 In previous report, photocatalytic mechanism of BiOCl nanoplates was demonstrated and it exhibited excellent photocatalytic efficiency for various kinds of pollutant degradation. Dong et al.14 learned that the BiOCl nanometer has a good photocatalytic activity. However, the band gap limits the response range and is only responsive at UV, resulting that the photocatalytic ability in visible light is limited.15,16 Therefore, the photocatalytic activity of BiOCl in visible light can be improved by changing the band gap. In recent years, Dong et al. found that elemental Bi can improve the photocatalytic performance of semiconductor through the mediating effect, such as Bi/g-C3N4,17−19 Bi/ TiO2,20 Bi/BiWO6,21 Bi/(BiO)2CO3,22 Bi/BiOI23 and so on. Ding et al.24 designed Bi self-doping Bi2MoO6 and found that Bi doping could promote photogenerated charges migration and superoxide generation, thus effectively removing NO. Dong et al.25 The designed Bi−BiOCl heterojunction inhibited the electron hole pairs separation efficiency, and enhanced the degradation performance. The effective mass of elemental bismuth is lower. Moreover, Bi elements are also cheap and easily available.26

INTRODUCTION In recent decades, the most serious water pollution is industrial waste water, but conventional water treatment systems are difficult to eliminate the persistent organic pollutants (POPs), such as bisphenol A (BPA). Therefore, as an effective method to solve the global energy shortage and environmental purification, semiconductor photocatalytic technology with high efficiency and environmental protection has attracted great attention.1 Semiconductor photocatalysis has been considered to be environmentally friendly and effective technology, and its potential for solar conversion has aroused much interest.2−5 In recent years, Bi-based semiconductors, such as BiOCl, Bi2O3, Bi2MoO6, and Bi2WO6, because of their unique lamellar structure and potential photocatalytic degradation capacity, have attracted attention of many people.6 Bismuth-based semiconductors are used to degrade organic compounds such as dye contaminants and oxidize gaseous pollutants.7 To improve the BiOX activity of a single component, a variety of BiOX composites have been developed, especially bismuth oxide (BiOCl), as a novel photocatalytic environmental restoration material, has been extensively studied for its better degradation activity than TiO2 under UV irradiation.8−10 The main photocatalysis comes from the unique layered structure by [Bi2O2]2+ and double chlorine atoms. In addition, it possesses stable chemical properties, nontoxicity, indirect transition band gap, and open crystal © 2019 American Chemical Society

Received: February 27, 2019 Revised: May 24, 2019 Published: June 5, 2019 7887

DOI: 10.1021/acs.langmuir.9b00581 Langmuir 2019, 35, 7887−7895

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tubular furnace and heated under a N2 atmosphere for 1 h at 450 °C with a heating rate of 5 °C min−1. Subsequently, cooling down to room temperature under a N2 atmosphere, obtained samples were washed with DI water and absolute ethanol three times to remove the unreacted NaBH4, and dried at 60 °C. Then, the Bi self-doped BiOCl was obtained. Synthesis of Bi−BiOCl/AgCl Heterojunctions. AgNO3 (1 mmol) and Bi−BiOCl (1 mmol) were dispersed equably in 10 mL deionized water in a beaker to form suspension A, and 1 mmol KCl was dissolved in 10 mL distilled water to form solution B at the same time. Then, solution B was added into suspension A drop-by-drop with strong stirring. The mixture was continuously stirred for 0.5 h, and the Bi−BiOCl/AgCl composites were successfully achieved in this way. Finally, the samples were washed with deionized water three times, dried at 60 °C, and collected. For comparison, BiOCl/AgCl was prepared in the same way but did not add NaBH4.

On the other hand, the AgX (X = Cl, Br and I) photocatalyst is a photosensitive material,27,28 which has been widely studied because of its unusual application prospect in photocatalysis. Different catalysts, such as AgX/BiOX,29 AgI/BiOI,30−32 Ag@ AgI/ZnS,33 AgCl/ZnO,8 AgX/Ag2CO3,34 and AgBr/Ag2O,35 have been previously combined with AgX to enhance photocatalytic performance, and all show that the photocatalytic activity increased significantly after combining them with AgX. This result may be due to the formation of apparent heterojunctions between different semiconductor photocatalysts. Heterojunction can shorten the band gap and improve the response in visible light.36 The design of the Z-scheme structure can obviously improve the photocatalytic performance of the mixed system.37 For instance, a Z-doped structure g-C3N4/Au/CdS is prepared by the self-assembly process. Because of the existence of theZscheme, the redox ability of photoinduced electron−hole pairs has been significantly improved.38 Hong et al. prepared a new solid Z-scheme V2O5/g-C3N4 heterojunction by the one-step hydrothermal method. The electron spin resonance (ESR) shows that the Z-scheme system improves the separation ability of photosensitive electron−hole pairs and the degradation ability of POPs.39 Here, the hydrothermal method, NaBH4 reduction method, and chemical deposition method are used to successfully design a new type of all-solid Z-scheme Bi−BiOCl/AgCl heterojunction. Moreover, the morphology, photochemical, and degradation of Bi−BiOCl/AgCl and its mechanisms are discussed.

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RESULTS AND DISCUSSION X-ray diffraction is used to verify the peak value of the BiOCl, AgCl, BiOCl/AgCl, Bi−BiOCl, and Bi−BiOCl/AgCl (Figure 1). The diffraction peaks of BiOCl are 11.8, 25.9, 32.6, 40.9,

EXPERIMENTAL SECTION

The synthesis process of photocatalyst is shown in Scheme 1.

Scheme 1. Synthesis Schematic Diagram of All-Solid ZScheme Bi−BiOCl/AgCl Heterojunction Microspheres

Figure 1. XRD patterns of AgCl, BiOCl, BiOCl/AgCl, Bi−BiOCl, and Bi−BiOCl/AgCl, respectively.

and 46.8° and are indexed well to the (001), (011), (110), (112), and (020) plane of cubic BiOCl, respectively. The result shows that the purity of BiOCl nanoparticle is high due to the fact that no other impurity phases can be detected. Moreover, AgCl samples display diffraction peaks at around 27.9, 32.1, 54.7, and 74.5°, which correspond to the (111), (200), (222), and (420) crystal planes of AgCl, respectively.40 It should be noted that on the (111), (200), and (222) planes of AgCl, the weak diffraction peaks of BiOCl/AgCl and Bi−BiOCl/AgCl are 27.9, 32.1, and 54.7°, respectively, indicating that AgCl is loaded on BiOCl and the heterojunction is successfully formed. BiOCl/AgCl and Bi−BiOCl/AgCl possess the similar diffraction peak compare with BiOCl. It indicates that the existence of AgCl has no influence on the structure of BiOCl. Meanwhile, the peaks of Bi−BiOCl and Bi−BiOCl/AgCl are significantly weaker than those of BiOCl, which is due to oxygen vacancy (Ov) caused by the reduction of NaBH4. It is clearly seen that the diffraction peak belonging to metallic Bi cannot be observed in the Bi−BiOCl and Bi−BiOCl/AgCl because of the small amount of reduced Bi in the photocatalyst, it is difficult to be detected. Figure 2 shows the morphology and structure with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figures 2a and S1 show that the BiOCl microspheres with diameters about 5 μm are composed of nanosheets of several nanometers thick. The BiOCl/AgCl heterojunction is also characterized by SEM (Figure S2). It can

Synthesis. Synthesis of BiOCl Microspheres. The BiOCl microsphere was synthesized via the hydrothermal synthesis method. Briefly, 5 mmol Bi(NO3)3·5H2O and 5 mmol KCl were dissolved in 20 mL ethylene glycol under magnetic stirring, respectively. After 30 min, the KCl solution was slowly added to the Bi(NO3)3·5H2O solution drop-by-drop. Then, the mixture was transferred to a 50 mL Teflon-lined stainless steel autoclave after stirring for another 30 min at room temperature. Subsequently, the mixture was carried out at 160 °C for 10 h. The product was collected by centrifugal separation, washed with deionized water and absolute ethanol several times, and then dried at 60 °C overnight. The BiOCl microsphere was thus finally obtained. Synthesis of Bi Self-Doped BiOCl Microspheres. The aboveobtained BiOCl microspheres (1 mmol) were mixed with 0.4 mmol of NaBH4 to form the mixture. Then, the mixture was ground for 30 min and transferred into a porcelain boat, and afterward placed in a 7888

DOI: 10.1021/acs.langmuir.9b00581 Langmuir 2019, 35, 7887−7895

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of Bi and BiOCl increases, the surface of the sphere will have the corresponding change in which BiOCl nanosheets are increasingly blurred, and the sphere surface is more and more rough. In addition, if the reduction of Bi is too little, the morphology of BiOCl does not change, indicating that the surface plasmon resonance (SPR) effect does not appear if the reduction of Bi is not distinct and too much of it will break the structure. Therefore, 1:40 is the best Bi/BiOCl molar ratio. Moreover, as the molar ratio of Bi/BiOCl = 1:40 decreases or increases, the removal rate of BPA, 2,6-dichlorophenol, and 2,4,5-trichlorophenol decreases (Figure S7a−c), indicating that the Bi/BiOCl = 1:40 has the best performance, which also indicates that too much reduction of Bi will cause the collapse of the original structure of BiOCl that inhibit the effect of the BiOCl and AgCl formation of the heterojunction. In addition, the AgCl content in Bi−BiOCl/AgCl is 8% (Figure S5), and the content of AgCl (6, 8, and 10%) has been adjusted. As shown in Figure S6, adjusting the load of AgCl and the Bi− BiOCl microspheres does not change, indicating that the load of AgCl has no impact on the structure of Bi−BiOCl. In addition, when the AgCl content changes from 8%, with the increase or decrease of AgCl concentration, the photocatalytic effect of composite materials is not ideal in the heterojunction composite material (Figure S7d,e). The possible reason is that when the AgCl content in the composite material is 8%, it will promote the recombination of Bi−BiOCl/AgCl photoexcited electron (e−) and hole (h+) pairs. Therefore, when Bi/BiOCl = 1:40, and the content of AgCl is 8% is the best. In addition, in order to clearly understand the internal structure of Bi− BiOCl/AgCl composite products through the TEM image as shown in Figure 2c,d. Figure 2c reveals that Bi−BiOCl/AgCl is a microsphere. After zooming in, it is clear that there are many different shaped particles growing on the surface of the microsphere. The regions of yellow circles in Figure 2c are AgCl nanoparticles, indicating that AgCl has been successfully

Figure 2. SEM images of BiOCl spheres (a) and Bi−BiOCl/AgCl spheres and (b) TEM (c) and HRTEM images (d) of Bi−BiOCl/ AgCl and the corresponding elemental mapping of Bi, Cl, O, Ag (e− h), respectively.

be seen that irregular particles growing on the surface of the microsphere proves that AgCl has been on the surface of Bi− BiOCl. By contrast, Bi−BiOCl/AgCl is also a spherical structure from Figure 2b. However, Bi−BiOCl/AgCl is less smooth and has loose internal structure due to the reduction of NaBH4. At the same time, no substantial change was observed in BiOCl, indicating that the morphology of BiOCl is not affected by the loading of BiOCl and AgCl. Meanwhile, complete AgCl particles have a small size with diameters of about 1 μm (Figure S3). The SEM images of different Bi/ BiOCl molar ratios are added in Figure S4. As the molar ratio

Figure 3. UV−vis DRS (a), and determination of the indirect interband transition energies (b) of BiOCl, BiOCl/AgCl, Bi−BiOCl, Bi−BiOCl/ AgCl and AgCl, respectively. N2 adsorption−desorption isotherm curves (c) and the SKP maps (d) of pristine BiOCl (A), BiOCl/AgCl (B), Bi− BiOCl (C), and Bi−BiOCl/AgCl (D), respectively. 7889

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Figure 4. XPS spectra of O 1s (a), Cl 2p (b), Ag 3d (c), and Bi 4f (d) of Bi−BiOCl/AgCl.

Figure 5. Photocatalytic degradation of BPA by visible-light irradiation (a), 2,6-dichlorophenol by visible-light irradiation (b), 2,4,5-trichloro phenol by visible-light irradiation (c), and the fluorescence intensity in 1 h (d) of BiOCl, BiOCl/AgCl, Bi−BiOCl, and Bi−BiOCl/AgCl, respectively.

UV−vis DRS and the values of band gap energies, are shown in Figure 3, and the AgCl has an extremely weak response in both UV region and visible-light region. Moreover, BiOCl only has splendid ultraviolet absorption and BiOCl/AgCl is similar to BiOCl. After restoring the Bi element, Bi−BiOCl and Bi− BiOCl/AgCl showed a wide range of light absorption both in UV and in visible light region compared with BiOCl and BiOCl/AgCl (Figure 3a). In particular, in the visible light region, all-solid Z-scheme Bi−BiOCl/AgCl has the strongest

grown. Figure 2d shows that lattice fringes of 0.347, 0.28, and 0.325 nm belong to (101) BiOCl plane, (200) plane of AgCl, and (012) plane of Bi, respectively. Therefore, BiOCl, AgCl, and Bi are indeed complex. In order to further prove the element distribution in the catalyst Bi−BiOCl/AgCl, the SEM mapping of corresponding elements in the Bi−BiOCl/AgCl is shown in Figure 2e−h, and Bi, O, Ag, and Cl are uniformly dispersed in the Bi−BiOCl/AgCl composites that are obviously observed. 7890

DOI: 10.1021/acs.langmuir.9b00581 Langmuir 2019, 35, 7887−7895

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Figure 6. ESR signals of DMPO−•O2− (a) and DMPO−•OH (b) of BiOCl and Bi−BiOCl/AgCl under dark and visible-light illumination; effect of different scavengers on the BPA, 2,6-dichlorophenol, and 2,4,5-trichlorophenol degradation in the presence of Bi−BiOCl/AgCl under visible-light irradiation, respectively (c), and steady-state PL spectra of BiOCl, Bi−BiOCl, BiOCl/AgCl, and Bi−BiOCl/AgCl (d).

light absorption capacity, and the band gap shrinks to ∼2.17 eV. While the band gaps of BiOCl, Bi−BiOCl, Bi−BiOCl/ AgCl, and AgCl are 3.16, 3.06, 2.61, and 2.5 eV, respectively (Figure 3b). Because the reduced metal Bi has no band gap (zero band gap), which makes the band gap narrower. It indicates that Bi as the bridge and the mesoporous BiOCl/ AgCl heterostructure can obviously significantly improve visible-light response and photocatalytic activity. According to the adsorption−desorption isotherm, it is a mesoporous structure because it shows the type IV curves (Figure 3c). The specific surface areas of BiOCl, BiOCl/AgCl, Bi−BiOCl, and Bi−BiOCl/AgCl are 5.09, 7.39, 11.28, and 13.94 m2 g−1, respectively, and the trend is to increase gradually. The results show that the composite still has a mesoporous structure, indicating that the mesoporous structure will not be affected after AgCl is compounded and Bi are restored to form the Zscheme structure. Besides, Figure S8 and Table S1 show the increase in pore volume and decrease in pore diameter, which is due to the reduction of metal Bi. The performance of the work function is observed with a scanning Kelvin probe (SKP). The SKP maps of the pristine BiOCl (A), BiOCl/AgCl (B), Bi−BiOCl (C), and Bi−BiOCl/AgCl (D) are shown in Figure 3d. By comparing the four samples, the work function of Bi− BiOCl/AgCl is the lowest, indicating that the approximate Fermi level of Bi−BiOCl/AgCl is the highest, which is conducive to electron escape. To explore the surface chemical states and components of prepared Bi−BiOCl/AgCl, X-ray photoelectron spectroscopy (XPS) is used to characterize them (Figure 4). From O 1s XPS spectra (Figure 4a), the peak value is ∼530.4 eV and belongs to the bismuth−oxygen bond, and the additional peak at about 532.4 eV belongs to oxygen vacancies in Bi−BiOCl/AgCl.41 Moreover, the peaks of Cl ions in BiOCl are 1970.3 and 204.6 eV (Figure 4b). The binding energy values of Ag are about 373.0 and 366.9 eV, corresponding to Ag3/2 and Ag5/2, respectively, which are the same as the reported binding energy

values of Ag+,42 and Ag0 has no significant peak (Figure 4c), indicating the Bi−BiOCl/AgCl heterostructures are rather stable. It shows four characteristic peaks (Figure 4d), two of which are 158.7 and 164.1 eV, respectively, corresponding to Bi3+, and the other two are 156.3 and 161.5 eV, respectively, corresponding to Bi0, indicating that Bi has been successfully restored,43 the result is consistent with Figure 2d. BiOCl, BiOCl/AgCl, Bi−BiOCl, and Bi−BiOCl/AgCl are used to degrade BPA, 2,6-dichlorophenol, and 2,4,5-trichlorophenol, respectively, to test the degradation performance. There is a set of blank experiments as the benchmark (no catalyst). The blank experiment barely changed (Figure 5a−c), indicating that these three reagents do not decompose easily. In addition, adsorption capacity is measured in the dark before light reaction. Compared with original BiOCl microspheres, the photocatalytic degradation properties of BiOCl/AgCl, Bi− BiOCl, and Bi−BiOCl/AgCl are significantly improved, and Bi−BiOCl/AgCl shows the best photocatalytic activity, and the degradation rate of BPA is ∼96% in 210 minutes of visible light (Figure 5a). The rate constants (k) of BiOCl, BiOCl/AgCl, Bi−BiOCl, and Bi−BiOCl/AgCl are 0.00499, 0.00577, 0.0821, and 0.0177 min−1, respectively (Figure S9). Obviously, the value of Bi−BiOCl/AgCl is about four times of the original BiOCl. Moreover, the photocatalytic removal rates of 2,6dichlorophenol and 2,4,5-trichlorophenol for Bi−BiOCl/AgCl are ∼99 and ∼96% in 210 min of visible light, respectively (Figure 5b,c). For 2,6-dichlorophenol, the k values of BiOCl, BiOCl/AgCl, Bi−BiOCl, and Bi−BiOCl/AgCl are 0.00266, 0.00402, 0.00728, and 0.0159 min−1, respectively (Figure S10), and for 2,4,5-trichlorophenol, the k values of BiOCl, BiOCl/ AgCl, Bi−BiOCl, and Bi−BiOCl/AgCl are 0.00282, 0.00418, 0.00733, and 0.0161 min−1, respectively (Figure S11). Also, the value of Bi−BiOCl/AgCl is about six times of the original BiOCl. The results show that Bi−BiOCl/AgCl has the highest mineralization activity for phenol pollutants, and the transcendental photocatalytic efficiency of Bi−BiOCl/AgCl is 7891

DOI: 10.1021/acs.langmuir.9b00581 Langmuir 2019, 35, 7887−7895

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Figure 7. Linear sweep voltammetry (a), the Nyquist plots (b), and photocatalytic evolution of hydrogen in visible light (c) of BiOCl, BiOCl/ AgCl, Bi−BiOCl and Bi−BiOCl/AgCl, respectively, and the cycle runs for Bi−BiOCl/AgCl (d).

because the presence of heterojunction, Ov, and Bi SPR effect promotes the separation of charge carriers and improves the photocatalytic activity. Electrochemical impedance measurements are carried out to further understand photogenerated charge carrier separation. Linear sweep voltammetry is used to observe the behavior of photocatalysts. Figure 7a shows the complete I−V characteristics of Bi−BiOCl/AgCl films taken in the dark under simulated sunlight irradiation. It can be seen that the current of the three photocatalysts, BiOCl, Bi−BiOCl, and Bi−BiOCl/ AgCl, are relatively weak under dark conditions. However, the current density under simulated solar light is significantly improved compared with that in dark. It shows that the highest current is Bi−BiOCl/AgCl. EIS Nyquist plots are used to demonstrate the separation efficiency of photogenerated charge carriers. The diameter of the arc is proportional to the resistance and inversely proportional to the separation efficiency. In Figure 7b, the diameters of arc are arranged in the ascending order of Bi−BiOCl/AgCl, Bi−BiOCl, BiOCl/AgCl, and BiOCl obviously. Therefore, the separation efficiency is BiOCl < Bi−BiOCl < Bi−BiOCl/AgCl. A small radius has small impedance. Therefore, among the four materials, Bi− BiOCl/AgCl has the highest separation efficiency, which makes photocatalytic performance superior by inhibiting electron−hole pair recombination. Additionally, the radii of the black Bi−BiOCl and Bi−BiOCl/AgCl are much smaller than that of the BiOCl/AgCl, verifying the importance of Bi. The valence band (VB) positions of BiOCl and AgCl are determined by the Mott−Schottky plot (Figure S12) so as to investigate the photocatalytic enhance mechanism. When Ag/ AgCl is acted as the reference electrode, the (CB) potential of BiOCl is about 0.61 eV versus NHE. EVB can be determined by ECB and Eg. Therefore, the VB of BiOCl is about 3.7 eV versus

attributed to the Z bridge structure and the synergistic effects of BiOCl and AgCl, which can extend electron lifetime and improve performance. Figure 5d reveals the fluorescence intensity of BiOCl, BiOCl/AgCl, Bi−BiOCl, and Bi−BiOCl/ AgCl within 1 h under 420 nm cut-off filter irradiation. The intensity of the Bi−BiOCl/AgCl curve is the strongest at about 425 nm, indicating Bi−BiOCl/AgCl can make the most •OH radical. In order to explore the existence of photogenerated active species and heterojunctions, so as to better study the photocatalytic mechanism, ESR measurements and different radical scavenger tests on BiOCl and Bi−BiOCl/AgCl were carried out. It can be seen that the ESR signals of DMPO−•O2− and DMPO−•OH of BiOCl and Bi−BiOCl/ AgCl which are not observed in the dark, Bi−BiOCl/AgCl can obviously observe the signal peaks of ·O2− and •OH after 5 min of visible-light irradiation, while BiOCl still do not show a relatively obvious signal peak (Figure 6a,b). It indicates that • O2− and •OH radicals are products of Bi−BiOCl/AgCl in the photodegradation process and proved the existence of heterojunction. In addition, as Figure 6c is shown, the degradation rates of BPA, 2,6-dichlorophenol, and 2,4,5trichlorophenol with isopropanol (IPA), benzoquinone, and disodium ethylenediaminetetraacetate as capture agents of • OH, •O2−, and h+ are significantly lower than those without capture agents, while the degradation rates are decreased but not particularly significant when IPA is added, so •OH, •O2−, and h+ play an important role in the degradation process. With the reduction of Bi and the compound of AgCl, the photoluminescence (PL) intensity gradually decreased, and the PL intensity of Bi−BiOCl/AgCl is significantly lower than that of other samples (Figure 6d). It indicates that among the prepared photocatalysts, Bi−BiOCl/AgCl has the highest separation efficiency for photo-induced carriers. This is 7892

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Langmuir NHE. Similarly, the conduction band of AgCl is about −0.55 eV versus NHE because the band gap of AgCl is 2.5 eV (Figure 3b), and the VB of AgCl can be figured out about 1.95 eV versus NHE. On the other hand, the Bi−BiOCl/AgCl material is photocatalytically decomposed under visible light to produce hydrogen. In fact, the hydrogen production performance of BiOCl is not very ideal, as is shown in Figure 7c, and the photocatalytic BiOCl, Bi−BiOCl, and BiOCl/AgCl only show 45.6, 126.1, and 97.8 μmol h−1 g−1, respectively. which has been revealed in the previous studies (Qian et al., 2017; Li et al., 2018; Li et al., 2019);44−46 however, the hydrogen production rate of Bi−BiOCl/AgCl can reach 198.2 μmol h−1 g−1, five times that of the original BiOCl, suggesting that photogenerated charges have the high utilization rate and excellent photocatalytic Bi−BiOCl/AgCl recycling hydrogen production capability. After five cycles in a row (Figure 7d), Bi−BiOCl/AgCl still shows a good hydrogen production rate, indicating the high stability and conducive to the practical application. In addition, by observing the full-scale XPS before and after five cycle’s photocatalytic hydrogen evolution (Figure S13a,b), the peak strength becomes a little weaker, but the peak position does not have obvious deviation, indicating that the stability of the sample before and after the cycle is good and has not been changed. As shown in Figure S14a, after five cycles, the sphere structure of Bi−BiOCl/AgCl is still retained, and AgCl is still attached to its surface. Moreover, Figure S14b shows different lattice fringes are 0.347, 0.28, and 0.325 nm, respectively, corresponding to BiOCl(101), Bi(012), and AgCl(200), which indicate that the structure is stable. According to the XRD patterns, all the diffraction peaks in BiOCl do not change significantly after five cycles (Figure S15). Moreover, as is shown in Figure S16, after five cycles in visible light, the spherical structure is a little scattered but still intact, and AgCl is also well attached to the surface. All the above shows that the stability of Bi−BiOCl/AgCl is excellent. On the basis of the above research, a schematic illustration of visible light-driven photocatalytic mechanism of the Zscheme Bi−BiOCl/AgCl is shown in Scheme 2. It shows that

phenomenon. In the all-solid Z-scheme photocatalytic system, the photo-induced holes are transferred from BiOCl VB to the BiOCl CB, accelerating the transportation of photogenerated holes on BiOCl. Because the VB potential of BiOCl (3.7 eV vs NHE) is higher than the oxidation potential of OH−/•OH (2.40 eV vs NHE) and can react with H2O/OH− to form • OH. In addition, electrons from BiOCl CB readily enter the Bi metal and then transfer to AgCl VB, binding to the remaining holes in the AgCl VB, forming the Z-scheme structure. It greatly promotes the charge separation on AgCl. In addition, Bi in BiOCl is reduced to form Ov that can induce to (CB) of BiOCl to narrowing the band gap of BiOCl, and because of the method of chemical reduction, part of Bi also has SPR effect, thus inhibiting the recombination of photogenerated electron−hole pairs and beneficial to the separation of charge carriers. Because the CB potential of AgCl is −0.55 eV versus NHE, which is more negative than O2/•O2− (−0.33 eV vs NHE) and can react with O2 to produce •O2−. Therefore, the photocatalytic activity of Bi−BiOCl/AgCl is significantly enhanced by promoting the separation of photogenerated electron−hole pairs.

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CONCLUSIONS From what has been discussed above, the all-solid Z-scheme Bi−BiOCl/AgCl heterojunction microspheres were successfully prepared. The prepared photocatalyst formed effective heterojunctions between mesoporous BiOCl and AgCl, and oxygen vacancies were generated because of the chemical reduction method. The formed Z-scheme heterojunction exhibits a narrow band gap, in which the visible light-driven photocatalytic removal efficiencies were ∼96, 99, and 96% of BPA, 2,6-dichlorophenol, and 2,4,5-trichlorophenol, respectively. Moreover, the hydrogen production rate can reach 198.2 μmol h−1 g−1, which is five times that of the original BiOCl. The enhanced performance of Bi−BiOCl/AgCl is due to the presence of the heterojunction, which effectively reduces the band gap and promotes the separation of photo-generated electron−hole pairs. Meanwhile, the all-solid Z-scheme structure further prolonged the electron lifetime and inhibited photo-generated charge carrier recombination efficiently.

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Scheme 2. Mechanism Diagram of Z-Scheme Photocatalytic Bi−BiOCl/AgCl Driven by Visible Light

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.9b00581.

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SEM images; elemental mapping; removal rate; BJH pore-size distribution plots; BET surface areas, pore sizes, and pore volumes; variations of ln(C/C0); MottSchottky plots; full-scale XPS; and XRD patterns (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86-451-8660-8616. Fax: +86-451-8660-8240 (Z.X.). *E-mail: [email protected] (Q.Z.). *E-mail: [email protected] (W.Z.).

under visible-light irradiation, BiOCl and AgCl are respectively excited to electrons and holes. Generally, photoexcited electrons are transferred from the AgCl conductive band (CB) to the BiOCl CB, and the photoexcited holes on the valence band BiOCl VB migrate to AgCl VB. However, in this work, metal Bi can be used as the medium for electron transfer,47,48 and the traditional heterojunction is difficult to explain, while the Z-scheme mechanism conforms to this

ORCID

Zipeng Xing: 0000-0002-9429-5780 Jinlong Zou: 0000-0003-0651-761X Qi Zhu: 0000-0001-5833-7050 7893

DOI: 10.1021/acs.langmuir.9b00581 Langmuir 2019, 35, 7887−7895

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Wei Zhou: 0000-0002-2818-0408 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support of this research by the National Natural Science Foundation of China (51672073 and 21871078), the Natural Science Foundation of Heilongjiang Province (JQ2019B001, B2018010 and H2018012), the Heilongjiang Postdoctoral Startup Fund (LBH-Q14135), the University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province (UNPYSCT-2016018), and the Heilongjiang University Science Fund for Distinguished Young Scholars (JCL201802).

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DOI: 10.1021/acs.langmuir.9b00581 Langmuir 2019, 35, 7887−7895