Z-Scheme BiOCl-Au-CdS Heterostructure with Enhanced Sunlight

Research Article pubs.acs.org/journal/ascecg

Z‑Scheme BiOCl-Au-CdS Heterostructure with Enhanced SunlightDriven Photocatalytic Activity in Degrading Water Dyes and Antibiotics Qiaoying Li,† Zhipeng Guan,† Di Wu,† Xiuge Zhao,‡ Shenyuan Bao,† Baozhu Tian,*,† and Jinlong Zhang*,† †

Key Lab for Advanced Materials and Institute of Fine Chemicals and ‡Research Center of Analysis and Test, School of Chemistry & Molecular Engineering, East China University of Science and Technology, 130 Mei long Road, Shanghai, 200237, PR China S Supporting Information *

ABSTRACT: Although semiconductor photocatalysis has made great progresses as a promising solution to solve the problem of environmental pollution, the highly efficient decomposition of organic pollutants driven by sunlight is still a challenge. Herein, we successfully constructed a Z-scheme photocatalyst BiOCl-Au-CdS for the first time by stepwise deposition of Au and CdS. It was found that the Au nanoparticles (NPs) were selectively anchored on the {001} facets of BiOCl nanosheets in the process of photoreduction while CdS NPs were further in situ deposited on Au NPs via the strong S− Au interaction. Compared to BiOCl, BiOCl-Au, and BiOCl-CdS, the Z-scheme BiOCl-Au-CdS exhibited evidently higher sunlight-driven photocatalytic activity toward the degradations of anionic dye Methyl Orange, cationic dye Rhodamine B, colorless pollutant phenol, and antibiotic sulfadiazine. The radical trapping experiments indicated that ·OH, h+, and ·O2− are the main reactive species responsible for the degradations of organic pollutants over BiOCl-Au-CdS. Based on the photoelectrochemical measurements, PL spectra, and band potential calculation, it can be concluded that the Z-scheme structure of BiOCl-Au-CdS not only retains the photogenerated electrons and holes with higher redox ability but also decreases their recombination rate. As a highly efficient sunlight driven photocatalyst, BiOCl-Au-CdS can be potentially used in environmental pollutant remediation. KEYWORDS: Z-scheme, BiOCl-Au-CdS, Photocatalytic degradation, Dye, Antibiotics

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under UV light.11,12 BiOCl possesses a unique layered structure characterized by [Bi2O2]2+ layers interleaved with double sheets of Cl ions. Theoretically, BiOCl {001} facets with a higher density of terminated oxygen atoms are inclined to form oxygen vacancies, leading to the enhanced photoactivity.13−16 However, because of the wide band gap (∼3.2 eV), BiOCl can only utilize UV light accounting for less than 5% of the solar spectrum, which severely limits its efficient utilization of solar energy. In recent years, several strategies, such as fabricating oxygen vacancies,16−18 depositing metals,19−21 and fabricating heterojunctions,22−24 have been employed to overcome this drawback. For instance, Zan et al. reported that black BiOCl with oxygen vacancies showed significantly higher photocatalytic activity than white BiOCl for Rhodamine B degradation.18 Recently, Wu et al. deposited Bi nanowires in situ on BiOCl nanosheets, which displayed enhanced photocatalytic activity and stability for NO removal under visible light irradiation.20 Li et al. found Bi2O3CO3/BiOCl heterojunction exhibited excellent photocatalytic activity toward Rhodamine B

INTRODUCTION With the rapid development of urbanization and industrialization, environmental pollution is becoming worse and worse. In the past decades, the remediation of noxious pollutants from wastewater system, such as dyes,1,2 drugs,3,4 and antibiotics,5 has always been a thorny issue that people are concerned about. For instance, by inducing the proliferation of bacterial drug resistance, antibiotics can cause considerable adverse effects on ecosystem equilibrium and human health, even with low residual activity and at a concentration as low as nanograms per liter.6 Toxic dyes also pose serious threats to biodiversity and accumulate in the human body along the food chain. Although the conventional methods, such as biological and physical adsorption have been applied in organic pollutant treatment, these techniques usually cannot eliminate these pollutants thoroughly due to their inefficiency or inevitable harmful byproducts.7,8 As an alternative, semiconductor photocatalysis has been considered as a promising solution to completely decontaminate most of the noxious substances.9,10 Among the various semiconductor materials, bismuth oxychloride (BiOCl) has been widely applied in the field of pollutant decomposition due to its excellent photocatalytic activity even better than TiO2 © 2017 American Chemical Society

Received: April 14, 2017 Revised: June 8, 2017 Published: June 27, 2017 6958

DOI: 10.1021/acssuschemeng.7b01157 ACS Sustainable Chem. Eng. 2017, 5, 6958−6968

Research Article

ACS Sustainable Chemistry & Engineering

chemical bath deposition at 60 °C for 0.5 h, the product BiOCl-AuCdS was centrifuged, rinsed with water and ethanol, and dried at 60 °C for 8 h. Synthesis of BiOCl-CdS and CdS. With the same procedures of synthesizing BiOCl-Au-CdS, BiOCl-CdS was prepared using BiOCl instead of BiOCl-Au. Pure CdS was also synthesized with the same method in the absence of BiOCl and BiOCl-Au. Characterization. X-ray diffraction (XRD) patterns of all the samples were collected in the range 5−80° (2θ) using a Rigaku D/ MAX 2550 diffractometer (Cu K radiation, λ = 1.5406 Å), operated at 40 kV and 100 mA. The morphologies were observed by transmission electron microscopy (TEM, JEM2000EX), scanning electron microscopy (SEM, Tescan Vega 3 SBH), and a Jeol JEM2100 high resolution transmission electron microscope (HR-TEM). The elemental composition of the samples was characterized by energy-dispersive X-ray spectroscopy (EDS, EDAX Genesis XM2 System). The surface elementary composition and chemical state were analyzed using a Thermo Fisher Escalab 250Xi X-ray photoelectron spectroscopy (Al Kα radiation), operated at 250 W. UV−visible diffuse reflectance spectrophotometer (DRS, Shimadzu UV-2450) was used to study the optical absorption properties of the obtained samples. Photoluminescence emission spectra were measured with a Shimadzu RF5301PC by using the 320 nm line of xenon lamp as excitation source at room temperature. The time−resolved fluorescence decay spectra were carried out by a FLS 980 spectrometer (Edinburgh Instruments) with a 360 nm LED laser as the light source. Photocatalytic Activity Measurements. The photocatalytic activities of the photocatalyst samples were evaluated toward the degradations of MO, RhB, phenol, and SD aqueous solutions under simulated sunlight irradiation. The photocatalytic reactions were conducted under simulated solar light by using a 300 W Xe lamp and a cutoff filter (AM 1.5). For each measurement, 50 mg photocatalyst was added into a quartz reactor containing 50 mL MO (20 mg/L), RhB (20 mg/L), phenol solution (10 mg/L), or SD solution (20 mg/L). Prior to the photoreaction, the suspension was stirred for 30 min in the dark to ensure the adsorption−desorption equilibrium of organic contaminant on the surface of photocatalyst. At the given time interval, the analytical sample was taken from the mixture and immediately centrifuged. The concentrations of MO and RhB were measured with a Shimadzu 2450 UV−vis spectrophotometer while those of phenol and SD were analyzed by a Shimadzu SPD-M20A high-performance liquid chromatograph (HPLC). Transient Photocurrent and Electrochemical Impedance Measurements. The transient photocurrent and electrochemical impedance measurements were performed on a Zahner electrochemical workstation by a standard three-electrode system consisting of a working electrode (FTO glass with 0.2 mg as-prepared sample with an active area of 1.5 cm2), a Pt wire as the counter electrode, and a saturated calomel electrode as the reference electrode. Transient photocurrent responses of different samples were carried out in 0.5 M Na2SO4 aqueous solution with bias potential of 50 mV under a 300 W Xe lamp with AM 1.5 cutoff filter. The electrochemical impedance spectroscopy was measured in a mixed aqueous solution containing 2.0 mM K3[Fe(CN)6], 2.0 mM K4[Fe(CN)6], and 0.5 M KCl. Photocatalytic Hydrogen Evolution. Photocatalytic H2 evolution was conducted in a top-irradiation Pyrex reaction vessel connected to a closed gas circulation and evacuation system. A 100 mg portion of photocatalysts was suspended in 100 mL of aqueous solution containing 0.25 M Na2S and 0.35 M Na2SO3 as sacrificial reagents. Prior to the irradiation, the whole system was completely vacuumized for 30 min to remove residual air in solution. A 300 W Xe lamp with an AM 1.5 cutoff filter was equipped to simulate the solar light. The evolved gases were detected by a gas chromatography (GC), equipped with a thermal conductive detector (TCD), a 5 Å molecular sieve column, and Ar carrier gas.

under visible light.24 Although the above approaches can extend the visible light response of BiOCl, the redox ability of the photogenerated electrons and holes would be decreased simultaneously. To the best of our knowledge, constructing Z-scheme semiconductor system is a feasible strategy to solve the problem, for which can maintain the redox ability as high as possible.25−27 Nevertheless, still very few works have hitherto reported the synthesis of sunlight driven BiOCl-based Zscheme photocatalysts and their applications in environmental treatment. Herein, we fabricated a novel all-solid-state Z-scheme photocatalyst BiOCl-Au-CdS for the first time, by selectively depositing Au nanoparticles (NPs) on the {001} facets of BiOCl nanosheets and further loading CdS on Au NPs. In the ternary architectures, BiOCl and CdS can respond to the sunlight, while the Au nanoparticles can improve the separation rate of photogenerated carriers with higher redox ability. The photocatalytic activities of the samples were evaluated by the degradations of anionic dye Methyl Orange (MO), cationic dye Rhodamine B (RhB), phenol, and antibiotic sulfadiazine (SD) under simulated solar light. It was found that the BiOCl-AuCdS exhibited obviously higher photocatalytic activity than BiOCl, BiOCl-Au, and BiOCl-CdS. Based on the results of photoelectrochemical measurements and radical trapping experiments, we tentatively proposed the degradation mechanism of organic pollutants over BiOCl-Au-CdS.

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EXPERIMENTAL SECTION

Materials. Cadmium acetate (Cd(CH3COO)2·2H2O, 99.0%) and bismuth nitrate pentahydrate (Bi(NO3) 3·5H2O, 99.0%) were purchased from Aladdin Industrial Corporation and Sinopharm Chemical Reagent Co., Ltd., respectively. Potassium chloride (KCl, 99.5%) and methanol (CH3OH, 99.5%) were obtained from Shanghai Lingfeng Chemical Reagent Factory. Ethanol (C2H5OH, 99.7%), chloroauric acid (HAuCl4·4H2O, 99.0%), and thiourea (NH2CSNH2, 99.0%) were obtained from Shanghai Adamas Reagent Co. Ltd. All the chemicals were analytical grade and used directly without further purification. Deionized water was used throughout all the experiments. Synthesis of Z-Scheme BiOCl-Au-CdS. Synthesis of BiOCl Nanosheets. BiOCl nanosheets with exposed {001} and {110} facets were synthesized by a modified solvothermal method, similar to the previous report.14 Typically, 1 mmol of Bi(NO3)3·5H2O and 1 mmol of KCl were added into 15 mL of distilled water and stirred for 1 h at room temperature to obtain a uniform suspension. Then, the mixture was then transferred into a 50 mL Teflon-lined steel autoclave and kept in a drying oven at 160 °C for 24 h. Finally, the resulting products was collected by centrifugation, washed with deionized water, and ethanol thoroughly, and dried at 60 °C in a vacuum oven. Synthesis of BiOCl-Au. The stepwise facets-selective photoreduction was employed to deposit Au NPs on the surface of BiOCl {001} facets. In a typical procedure, 100 mg of the as-prepared BiOCl powder was added into 30 mL of methanol/water solution (1:5 in volume). Under constant stirring, 1 mL of HAuCl4 aqueous solution (10 g/L) was injected into the above suspension in two separate times. At every turn, the suspension was stirred for 1 h in dark to achieve the preferential adsorption of Au based complex ions on BiOCl, and then exposed to a 300 W xenon arc lamp for 1 h. The product was centrifuged, rinsed with deionized water and ethanol, and dried at 60 °C for 8 h to obtain BiOCl-Au. Synthesis of BiOCl-Au-CdS. BiOCl-Au-CdS was synthesized by a chemical bath deposition method, similar to the previous literatures.26,28 In a typical procedure, 100 mg of the as-prepared BiOClAu sample and 1 mL of NH2CSNH2 aqueous solution (0.1 M) were dispersed in 10 mL of deionized water, followed by stirring for 30 min in dark to achieve the selective adsorption of NH2CSNH2 on the surface of Au NPs. Then, 1 mL of Cd(CH3COO)2·2H2O aqueous solution (0.1 M) was added to the above suspension. After the

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RESULTS AND DISCUSSION Synthetic Route, Formation Mechanism, and Morphology. The synthetic route and formation mechanism of Z6959


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BiOCl, the photogenerated electrons would be transferred from the conduction band of BiOCl to Au NPs.30,31 Therefore, when HAuCl4 was added once again, the Au ions were selectively reduced and deposited on the Au seeds, leading to the formation of Au nanoparticles on BiOCl {001} facets. Finally, the CdS NPs were deposited on Au NPs by the preferential adsorption of S2− ions and in situ deposition of Cd2+ ions. In detail, NH2CSNH2 was selectively adsorbed on the surface of Au NPs in advance, due to the strong affinity between S and Au atoms.32 When being heated, NH2CSNH2 adsorbed on Au NPs would be gradually hydrolyzed. Meanwhile, the released S2− ions react with Cd2+ ions to form CdS on the surface of Au NPs. The morphologies of the as-prepared samples were observed by SEM (Figure 1A, D), TEM (Figure 1B, F, I), FE-SEM (Figure 1E, G, H), and HR-TEM (Figure 1C, J, K). As shown in Figure 1A and B, the as-synthesized BiOCl exhibits sheetshaped morphology with a width of 2−4.5 μm and a thickness of 300−400 nm. Apparently, the surfaces of BiOCl are clean and smooth. The detailed structure of the BiOCl sample was further analyzed by HR-TEM. Figure 1C shows the HR-TEM top view of a BiOCl nanosheet. It can be seen that BiOCl nanosheet exhibits high crystallinity and clear lattice fringes. The lattice spacing of the atomic planes was measured to be 0.275 nm, consistent with the spacing of the (110) planes of BiOCl. The intersection angle between the (110) and (110) planes is 90°, indicating that the top and bottom surfaces of BiOCl nanosheet are {001} facets. On the basis the symmetries of tetragonal BiOCl, the four lateral surfaces should be {110} facets.14 After photodeposition, many well dispersed Au NPs with the size of 70−150 nm appear on the {001} facets of

scheme BiOCl-Au-CdS are illustrated in Scheme 1: First, BiOCl single-crystalline nanosheets with exposed {001} and Scheme 1. Synthetic Route and Formation Mechanism of ZScheme BiOCl-Au-CdS

{110} facets were synthesized by a modified solvothermal method, similar to the previous report.14 Subsequently, Au seeds were selectively anchored on the {001} facets of BiOCl nanosheets via a photoreduction process using HAuCl4 as the Au source and methanol as the hole scavenger. In this process, the photogenerated electrons are transferred to the BiOCl {001} facets, where the Au3+ ions are in situ reduced to Au0 seeds.29 Correspondingly, the photogenerated holes transfer to the BiOCl {110} facets and are consumed by methanol. Since the Fermi level of Au is lower than the conduction band of

Figure 1. (A) SEM, (B) TEM, and (C) HR-TEM images of BiOCl. (D) SEM image of BiOCl-Au. (E) FE-SEM and (F) TEM images of BiOCl-CdS. (G) FE-SEM image of BiOCl-Au. (H) FE-SEM, (I) TEM, and (J, K) HR-TEM images of BiOCl-Au-CdS. The inset of part K is the corresponding lattice fringes of Au. 6960


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159.0 and 164.3 eV can be assigned to Bi3+ 4f7/2 and Bi3+ 4f5/2, respectively. Similarly, the Cl 2p of BiOCl-Au-CdS also presents the characteristic double peaks at 197.7 eV for 2p3/2 and 199.3 eV for 2p1/2 (Figure 3B). As displayed in Figure 3C, the O 1s peak is wide and asymmetric, implying that the O element has at least two kinds of chemical states. According to the previous literatures,24,36 the O 1s peak can be fitted into three peaks at 529.8, 531.0, and 532.2 eV, attributed to the lattice oxygen, adsorbed −OH group and H2O, respectively. The Au 4f gives the double peaks at 83.4 and 87.2 eV (Figure 3D), confirming the existence of Au0 in the composite.28,34 In Figure 3E and F, the double peaks at 404.8 and 411.6 eV are ascribed to Cd2+ 3d while the peak at 225.9 eV corresponds to the S2− 2s, which further confirms the formation of CdS.9,37 Absorption and PL Spectra. Figure 4A shows the UV−vis diffuse reflectance spectra (DRS) of the CdS, BiOCl, BiOClAu, BiOCl-CdS, and BiOCl-Au-CdS. It can be seen that the bare BiOCl can only absorb the UV light below 360 nm and has no absorption in the visible light region. In contrast, BiOClAu exhibits a very broad absorption peak around 500 nm, attributed to the surface plasmon resonance (SPR) absorption of Au NPs loaded on the {001} facets of BiOCl sheets. Compared to BiOCl-Au, the absorption edge of BiOCl-Au-CdS exhibits a slight blue shift, which is ascribed to the strong electromagnetic coupling between Au and CdS as well as the change of surrounding environment.34 Moreover, the absorption intensity of BiOCl-Au-CdS in the visible region is slightly weaker than that of BiOCl-Au, which is because the formed CdS hinders the SPR effect of Au NPs. This result confirms that the CdS has been successfully deposited on the surface of Au NPs.38 The band gap of CdS and BiOCl were determined by the following Kubelka−Munk function (eq 1):39

BiOCl sheets, while the {110} facets still remain bald (Figure 1D, G). This phenomenon is also an evidence confirming the transmission pathway of photogenerated electrons and holes, i.e., the electrons and holes are transferred to the {001} and {110} facets, respectively, of BiOCl nanosheets. For comparison, the morphology of BiOCl-CdS was observed by FE-SEM (Figure 1E) and TEM (Figure 1F). It can be seen that the CdS nanoparticles with an average diameter about 20 nm were randomly deposited on the surface of BiOCl nanosheets. As shown in Figure 1G and H, the surface morphology of BiOClAu-CdS has no evident difference compared with that of BiOCl-Au, which is due to the low content and small size of CdS (Figure 1H). From the side view of TEM and HR-TEM images (Figure 1I, J), it can be clearly seen that the Au NPs (showing black tone) are covered by the CdS NPs (showing gray tone). In Figure 1K, the enlarged HR-TEM image displays two types of lattice fingers with the spacings of 0.23 and 0.34 nm, corresponding to the Au (111) plane and CdS (111) plane, respectively.33,34 The microstructures of BiOCl-Au-CdS were further analyzed by HR-TEM EDX mapping. From Figure S1A, it can be seen that Bi, Au, and Cd elements are coexistent in BiOCl-Au-CdS. As displayed in Figure S1C, Bi was uniformly dispersed, consistent with the existent state of Bi in BiOCl nanosheet. The Au nanoclusters in Figure S1D match well the black dots in Figure S1B, confirming that Au nanoparticles have been anchored on the surface of BiOCl nanosheets. Unfortunately, the Cd element almost cannot be distinguished (Figure S1E), due to its low content in BiOCl-Au-CdS (1.1 wt %) as well as the lower detection sensitivity compared to Au. Crystalline Structure and Surface Composition. The crystalline phases of the different samples were investigated by X-ray diffraction (XRD). As displayed in Figure 2, all the

αhν = A(hν − Eg )n /2

(1)

where α, h, hν, A, and Eg are absorption coefficient, Planck constant, light energy, a constant value, and bandgap energy, respectively. In addition, n is determined by the transition type of semiconductor. In this regard, n = 1 for direct transition, while n = 4 for indirect transition. According to the literature, the n values of CdS and BiOCl were 4 and 1, respectively.40−42 Using the plots of (αhv)2 and (αhv)1/2 versus photon energy (hv), the band gaps of CdS and BiOCl were determined to be 2.57 and 3.37 eV, respectively (Figure 4B), consistent with the previous results.40,43 As for BiOCl-CdS, the absorption curve presents a terrace in the range of 360−500 nm, resulting from the absorption of the deposited CdS. Photoluminescence (PL) emission spectroscopy has been widely applied as a useful technique to assess the efficiency of photogenerated charge carrier trapping, migration, and transfer in semiconductor photocatalysts.33 Figure 5 displays the PL emission spectra of BiOCl, BiOCl-Au, BiOCl-CdS, and BiOClAu-CdS. Compared to BiOCl, BiOCl-Au exhibits a much weaker PL intensity, which is because the deposited Au can restrain the recombination of photogenerated electrons and holes by forming Schottky barrier at the BiOCl-Au interface.44 Since the formation of heterostructure between BiOCl and CdS, BiOCl-CdS also exhibits weaker PL intensity when compared with BiOCl. Among the above samples, the ternary BiOCl-Au-CdS shows the lowest PL intensity, implying that the Z-scheme structure has the highest efficiency for the separation of photogenerated charge carriers. To further explore the

Figure 2. X-ray diffraction patterns of the as-prepared (a) BiOCl, (b) BiOCl-Au, (c) BiOCl-CdS, and (d) BiOCl-Au-CdS.

samples BiOCl, BiOCl-Au, BiOCl-CdS, and BiOCl-Au-CdS give the characteristic diffraction peaks of tetragonal phase BiOCl (JCPDS card No. 06-0249). Different to bare BiOCl, both BiOCl-Au and BiOCl-Au-CdS present two characteristic peaks at 38.2° and 44.3°, ascribed to the (111) and (200) crystal planes, respectively, of Au (JCPDS card No. 80-0019) (Figure 2b and d). No diffraction peaks assigned to CdS can be observed, probably due to its low content, high dispersity, and low crystallinity, similar to the previous reports.35 The surface elemental composition and chemical states of BiOCl-Au-CdS were analyzed by X-ray photoelectron spectroscopy (XPS). In Figure 3A, the double peaks located at 6961


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Figure 3. XPS spectra of the as-prepared BiOCl-Au-CdS: (A) Bi 4f; (B) Cl 2p; (C) O 1s; (D) Au 4f; (E) Cd 3d; (F) S 2s.

Figure 4. (A) UV−vis diffuse reflectance spectra of CdS, BiOCl, BiOCl-Au, BiOCl-CdS, and BiOCl-Au-CdS. (B) Plots of (αhv)2 (CdS) and (αhv)1/2 (BiOCl) versus photon energy (hv).

corresponding average lifetime τ and τ2 gradually increase in the order of BiOCl < BiOCl-CdS < BiOCl-Au < BiOCl-AuCdS. This result confirms that the photogenerated charge carriers possess the highest transfer efficiency in the Z-scheme structured BiOCl-Au-CdS. Photoelectrochemical Properties. The separation and transfer efficiency of photogenerated electrons and holes in the different samples were also investigated by analyzing their transient photocurrents and electrochemical impedance spectra (EIS). As displayed in Figure 6A, the photocurrents of these

transfer efficiency of photogenerated charge carriers in the different samples, we measured the PL emission decay time of BiOCl, BiOCl-Au, BiOCl-CdS, and BiOCl-Au-CdS to estimate the lifetime of photogenerated charge carriers. The fluorescence decay time of the above samples is listed in Table 1. Generally, it is considered that the fast decay lifetime (τ1) originates from the radiative emission of direct interband exciton recombination, while the much slower lifetime (τ2) is attributed to the emission decay via the indirect recombination of trapped electrons with holes.45,46 As shown in Table 1, both the 6962


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disclose the dynamics of the mobile and bound charges in the interfacial or bulk regions of semiconductors.47,48 As displayed in Figure 6B and Figure S2, regardless of whether the light is present, BiOCl-Au-CdS exhibits the smallest semicircle diameter among of all the samples, implying that this sample possesses the best electronic conductivity and the optimal carrier transfer efficiency. The above findings are in accordance with photocurrent and PL results. Photocatalytic Activity and Mechanism. To evaluate the photocatalytic activities of the different photocatalysts, anionic dye MO, cationic dye RhB, colorless pollutant phenol, and antibiotic SD were employed as the model organic pollutants for the photocatalytic degradation reactions under simulated solar light irradiation (Figure 7). As shown in Figure 7A and B, the concentrations of MO and RhB almost have no change in the absence of photocatalyst, indicating that the two dyes are stable under solar light irradiation. The bare BiOCl exhibits quite low photocatalytic activity, owing to not only its low utilization efficiency to the simulated solar light but also the high recombination rate of photogenerated charge carriers. In contrast, both the degradation rates of MO and RhB show an evident increase over BiOCl-Au when compared with those over bare BiOCl. It is well-known that the Schottky barrier between the semiconductor and noble metal can improve the separation rate of charge carriers and consequently improves the photocatalytic activity.49 Thus, the enhanced photocatalytic activity for BiOCl-Au should be attributed to the contribution of the Schottky barrier between BiOCl and Au. Similarly, the BiOCl-CdS also presents enhanced photocatalytic activity for the degradations of MO and RhB compared with bare BiOCl, which is beneficial from both the BiOCl-CdS heterojunction structure and the visible light response of CdS. Apparently, Zscheme BiOCl-Au-CdS displays the highest photocatalytic activity among all the photocatalysts. To explore the main reason that results in the superior photocatalytic activity of BiOCl-Au-CdS, we first measured the contents of CdS in BiOCl-CdS and BiOCl-Au-CdS. From Table S1, it can be seen that the content of CdS in BiOCl-CdS (1.0 wt %) is slightly lower than that in BiOCl-Au-CdS (1.1 wt %). To further explore whether the higher content of CdS in BiOCl-Au-CdS is responsible for the superior photocatalytic activity of BiOClAu-CdS, we prepared BiOCl-CdS-1 with the same procedures of synthesizing BiOCl-CdS, except for using 2-fold dosages of NH2CSNH2 and Cd(CH3COO)2·2H2O. The actual content of CdS in BiOCl-CdS-1 was measured by ICP-MS to be 1.6 wt %

Figure 5. PL spectra of BiOCl, BiOCl-Au, BiOCl-CdS, and BiOCl-AuCdS (λex = 320 nm).

Table 1. Fluorescence Decay Time of the Different Photocatalyst Samples BiOCl BiOCl-Au BiOCl-CdS BiOCl-Au-CdS

τ1 (ns)

A1 (%)

τ2 (ns)

A2 (%)

⟨τ⟩ (ns)

χ2

0.457 0.460 0.459 0.470

98.9 98.5 98.5 98.7

5.324 8.628 8.132 10.000

1.1 1.5 1.5 1.3

0.510 0.582 0.574 0.594

1.266 1.312 1.290 1.294

samples follow the order of BiOCl-Au-CdS > BiOCl-Au > BiOCl-CdS > BiOCl. Compared to bare BiOCl, all the other samples show improved photocurrents, implying that the photogenerated electrons and holes have higher transfer efficiency in these samples. However, as for the mechanisms of enhancing the photocurrent, these samples are different: For BiOCl-CdS, the enhanced photocurrent should be attributed to the heterojunction structure between BiOCl and CdS, by which the photogenerated electrons on CdS conduction band (CB) can be easily transferred to the CB of BiOCl while the holes on the valence band (VB) of BiOCl migrate to the VB of CdS. With regard to BiOCl-Au, the Au NPs are able to capture the photogenerated electrons from BiOCl CB by forming Schottky barrier, leading to the effective separation of the electron−hole pairs generated in BiOCl.21 In the case of BiOCl-Au-CdS, CdS and BiOCl can form the Z-scheme structure by Au intermediate, which can effectively improve the separation rate of the photogenerated electrons and holes with higher redox potentials.25 The electrochemical impedance spectra can

Figure 6. (A) Transient photocurrents and (B) Nyquist plots of the electrochemical impedance spectra of BiOCl, BiOCl-Au, BiOCl-CdS, and BiOCl-Au-CdS under simulated solar light irradiation (300 W Xe lamp with an AM 1.5 filter). 6963


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Figure 7. (A−D) Photocatalytic degradation curves of MO (A), RhB (B), phenol (C), and SD (D) over BiOCl, BiOCl-Au, BiOCl-CdS, and BiOClAu-CdS. All photocatalytic experiments were carried out under simulated sunlight irradiation (300 W Xe lamp with an AM 1.5 filter).

Figure 8. (A) Cyclic photocatalytic degradations of SD over BiOCl-Au-CdS. (B) XRD patterns of BiOCl-Au-CdS before and after the recycle degradation experiments. (inset) SEM image of the BiOCl-Au-CdS sample after five recycle experiments.

sulfadiazine (SD). As shown in Figure 7D, the photocatalytic activities of the samples vary in the order: BiOCl-Au-CdS > BiOCl-Au > BiOCl-CdS > BiOCl. For instance, after simulated solar light irradiation for 4 h, SD was fully decomposed over BiOCl-Au-CdS, while only 86.2%, 91.1%, and 90.2% of SD were degraded over BiOCl, BiOCl-Au, and BiOCl-CdS, respectively. For BiOCl-Au-CdS, the Z-scheme structure not only remains the photogenerated electrons and holes with higher redox ability but also decreases their recombination rate (see the subsequent photocatalytic mechanism). Since the photostability is very important to a photocatalyst for its practical applications, we further investigated the photostability of BiOCl-Au-CdS by recycle degradation experiments. From Figure 8A, it can be seen that BiOCl-Au-CdS still maintains high photocatalytic activity after five cycles (92%). The slight decline of photocataltic activity is probably come from the inevitable loss of photocatalysts during the recycle runs. To further confirm the stability of BiOCl-Au-CdS sample, we compared the XRD patterns and SEM images of the BiOClAu-CdS sample before and after photocatalytic experiments. As displayed in Figure 8B, BiOCl-Au-CdS has no obvious changes in the crystalline structure and morphology after five cycles. It is

(Table S1), which is higher than that in BiOCl-Au-CdS. However, the Z-scheme BiOCl-Au-CdS still displays higher photocatalytic activity than BiOCl-CdS-1, as shown in Figure S3. The above experiment results indicate that the main reason responsible for the superior photocatalytic activity of BiOClAu-CdS is not its higher CdS content but the Z-scheme structure. Because most of the dyes can be degraded by selfphotosensitization mechanism, here we further selected colorless phenol as a typical organic pollutant to evaluate the photocatalytic activities of the above photocatalysts, with the aim of ruling out the influence of dye-sensitization. As shown in Figure 7C, the BiOCl-Au-CdS shows superior photocatalytic performance. Phenol can be completely degraded in 100 min by employing BiOCl-Au-CdS sample as photocatalysts, while only 57.2%, 80.0%, and 73.8% of phenol can be decomposed in the case of BiOCl, BiOCl-Au, and BiOCl-CdS, respectively. Nowadays, antibiotic agents have been widely employed for treating bacterial infections. However, their vast discharge also caused a global environmental pollution and ecological destruction. Here, we also studied the photocatalytic performances of the above samples for degrading antibiotics 6964


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Figure 9. Photocatalytic degradation curves of RhB (A) and phenol (B) over BiOCl-Au-CdS in the presence of different radical scavengers including BQ (1 mM), EDTA-2Na (1 mM), and TBA (2 mM) under simulated solar light irradiation.

summarized in Table 2. By analyzing the information on the band potentials and reactive species, we can judge the migration

well-known that CdS is prone to taking place photocorrosion, which usually shortens its service life in the photocatalytic applications. Here, we compared the variation of Cd content before and after five recycle experiments by XPS analysis. As displayed in Figure S4, the intensities of Bi 4f, Au 4f, and Cd 3d before and after five recycle experiments have no evident change. The contents of Cd element in BiOCl-Au-CdS before and after recycle experiment were determined to be 3.99 and 3.61 at %, respectively (Table S2). This result indicates that CdS in BiOCl-Au-CdS is basically stable under light irradiation. For the Z-scheme BiOCl-Au-CdS, the holes of CdS and the electrons of BiOCl transfer to Au quickly and, subsequently, recombine there. Since the conduction band of CdS lacks of holes, the photocorrosion of CdS can be efficiently restrained. These experiment results demonstrate that BiOCl-Au-CdS is a photostable photocatalyst. In the photocatalytic processes, a series of reactive species, such as superoxide radical (·O2−), hole (h+), and hydroxyl radical (·OH), take part in the degradation reactions of organic contaminants. To explore the reactive species involved in the degradation of organic pollutants over Z-scheme BiOCl-AuCdS, a series of contrast experiments in the presence of different radical scavengers were performed. Here, benzoquinone (BQ) was introduced to scavenge ·O2−, EDTA-2Na for h+, and tert-butanol (TBA) for ·OH in the solution. As shown in Figure 9A and B, the degradation efficiencies of RhB and phenol are significantly prohibited after the addition of BQ, TBA, and EDTA-2Na, inferring that all the ·O2−, ·OH, and h+ are the reactive species responsible for the degradation of organic pollutants. It is well-known that the photocatalytic activity of a semiconductor is closely relative to the migration processes of the photogenerated charge carriers. To reveal the migration pathway of photogenerated charge carriers in BiOCl-Au-CdS, the CB and VB potentials of BiOCl and CdS were calculated by the following eqs (eqs 2 and 3):50 E VB = χ − Ee + 0.5Eg

Table 2. Calculation of the CB and VB Potentials of CdS and BiOCl CdS BiOCl

Eg (eV)

ECB (eV)

EVB (eV)

2.57 3.37

−0.74 0.14

1.83 3.51

direction of photogenerated electrons in BiOCl-Au-CdS. If BiOCl-Au-CdS is a traditional composite semiconductor, the photogenerated electrons would transfer from the CB of CdS to that of BiOCl. Since the CB potential of BiOCl (0.14 eV vs NHE) is more positive than E0(O2/·O2−) (−0.046 eV),41 the electrons on the CB of BiOCl cannot react with O2 to form · O2−. However, the presence of ·O2− has been proven by the radical trapping experiments (Figure 9). Therefore, we can confirm that BiOCl-Au-CdS is a Z-scheme semiconductor and the electrons for forming ·O2− come from the CB of CdS. To further confirm whether the charge transfer process in BiOCl-Au-CdS follows the Z-scheme mechanism, the photocatalytic H2 generation over BiOCl-CdS and BiOCl-Au-CdS were further performed. As shown in Figure S5, BiOCl-CdS is inactive for H2 evolution, while H2 is produced over BiOCl-AuCdS. The energy levels of CdS and BiOCl conduction bands are −0.74 and 0.14 eV (vs NHE), respectively. The above result reveals that BiOCl-CdS is a normal composite semiconductor while BiOCl-Au-CdS belongs to Z-scheme composite semiconductor. For BiOCl-CdS, the photogenerated electrons accumulate on the conduction band of BiOCl. Since the energy levels of BiOCl conduction band (0.14 eV vs NHE) is more positive than the potential for H2 production, E0(H2/H+) (0.00 eV), no H2 can be produced. In the case of BiOCl-AuCdS, the electrons accumulate on the conduction band of CdS. Because the potential of CdS conduction band is more negative than E0(H2/H+), H2 would be evolved on CdS. Therefore, the charge transfer process over BiOCl-Au-CdS follows the Zscheme mechanism. Based on the above experiment results, the degradation mechanism of organic pollutants over Z-scheme photocatalyst BiOCl-Au-CdS was proposed and illustrated in Figure 10. Under simulated solar light irradiation, the electrons are excited from the VB of BiOCl and CdS to their CB, while the holes are still on their VB. Owing to the excellent conductivity of Au joint, the electrons of BiOCl CB (with weaker reduction ability) and the holes of CdS VB (with weaker oxidation

(2)

and ECB = E VB − Eg

χ 5.05 6.33

(3)

Where EVB, ECB, are Eg are the VB potential, CB potential, and bandgap, respectively. The value of electronegativity χ for CdS is 5.05, while for BiOCl it is 6.33.42,43 Moveover, Ee is the energy of free electrons on the hydrogen scale (about 4.5 eV). The calculated band positions of CdS and BiOCl are 6965



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Research Article

AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 21 64252062. Fax: +86 21 64252062. E-mail address: [email protected] (B.T.). *E-mail: [email protected] (J.Z.). ORCID

Baozhu Tian: 0000-0001-9771-0444 Jinlong Zhang: 0000-0002-1334-6436 Notes

The authors declare no competing financial interest.

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Figure 10. Proposed Photocatalytic Mechanism of Z-Scheme BiOClAu-CdS.

ACKNOWLEDGMENTS This work has been supported by the National Natural Science Foundation of China (21573069, 21277046), the Shanghai Committee of Science and Technology (13NM1401000), and the National Basic Research Program of China (973 Program, 2013CB632403).

ability) are transferred to Au and quickly recombine there. On the contrary, the holes on BiOCl VB (with stronger oxidation ability) and the electrons on CdS CB (with stronger reduction ability) are reserved. By this means, the electrons and holes with strong redox ability can be spatially separated. Subsequently, the electrons on the CB of CdS combine with the absorbed O2 to produce ·O2−. Meanwhile, part holes on the VB of BiOCl oxidize hydroxyl to form ·OH. Finally, the reactive species ·OH, h+, and ·O2− take part in the oxidative degradation of organic pollutants.

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CONCLUSIONS

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ASSOCIATED CONTENT

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REFERENCES

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In summary, the Z-scheme photocatalyst BiOCl-Au-CdS was successfully prepared for the first time by the photoreduction and chemical bath deposition methods. In the photoreduction process, Au nuclei were produced and further grew into Au nanoparticles on the {001} facets of BiOCl nanosheets due to the oriented accumulation of photogenerated charge carriers. By the strong S−Au interaction, NH2CSNH2 molecules were selectively adsorbed onto the surface of Au NPs and further in situ reacted with Cd2+ to form CdS. Z-Scheme BiOCl-Au-CdS showed evidently higher photocatalytic activity than BiOCl, BiOCl-Au, and BiOCl-CdS toward the degradations of Methyl Orange, Rhodamine B, phenol, and sulfadiazine. Based on the photoelectrochemical measurements, PL spectra, and band potential calculation, it can be concluded that the Z-scheme structure of BiOCl-Au-CdS not only decreases the recombination rate of photogenerated electrons and holes but also makes them keep higher redox ability. The radical trapping experiments indicated that ·OH, h+, and ·O2− are the main reactive species responsible for the degradation of organic pollutants over BiOCl-Au-CdS. As a highly efficient sunlight driven photocatalyst, BiOCl-Au-CdS can be potentially used in the pollutant treatment and other photocatalytic fields. Moreover, this study also paves a strategy for fabricating other Z-scheme photocatalysts with highly solar energy efficiency.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01157. Figures S1−S5; Tables S1 and S2 (PDF) 6966


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Z-Scheme BiOCl-Au-CdS Heterostructure with Enhanced Sunlight

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