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Bi2WO6 hierarchical composites with high visible light photocatalytic activity
Chemical Physics Letters 737 (2019) 136830
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Research paper
Fabrication of Ag/AgBr/Bi2WO6 hierarchical composites with high visible light photocatalytic activity
T
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Wei Gana, Jian Zhanga, Haihong Niub, , Lei Baoa, Hequn Haoa, Yehan Yanc, Keyue Wud, ⁎ Xucheng Fua, a
Analytical and Testing Center, West Anhui University, LuAn 237015, Anhui Prov., PR China School of Electrical and Automation Engineering, HeFei University of Technology, HeFei 230009, Anhui Prov., PR China c College of Resource Environment and Tourism Management, West Anhui University, LuAn 237015, Anhui Prov., PR China d College of Electrical and Optoelectronic Engineering, West Anhui University, LuAn 237015, Anhui Prov., PR China b
H I GH L IG H T S
of Ag/AgBr/Bi WO heterojunctions photocatalyst via oil/water self-assembly method. • Synthesis WO heterojunction can reduce the recombination of electrons and holes. • Ag/AgBr/Bi composites exhibited excellent photocatalytic activity under visible-light irradiation. • The • Holes and O are the predominant active species for Ag/AgBr/Bi WO under visible-light irradiation. 2
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A R T I C LE I N FO
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Keywords: Ag/AgBr/Bi2WO6 composites Surface plasmonic resonance Photocatalytic
Visible-light sensitive photocatalyst Ag/AgBr/Bi2WO6 was successfully synthesized by oil/water self-assembly method. The crystalline structure, morphologies and optical properties of the as-prepared samples were investigated. The results revealed that the Ag/AgBr successfully introduced into the surface of Bi2WO6. The modified of Ag/AgBr enhanced the absorption in the visible-light region, due to surface plasmonic resonance of the metallic Ag. The activities of as-prepared samples were investigated by the degradation of RhB and phenol under the visible-light. Experimental results revealed that the Ag/AgBr/Bi2WO6 composites exhibit high visiblelight photocatalytic activity compared with that of pure Ag/AgBr and Bi2WO6.
1. Introduction Due to the rapid development of urbanization, the organic pollutant is getting more and more attention [1–3]. The semiconductor photocatalysts as classic green materials have attracted great interest in the past decades due to their multi-application, safety and stable [4–6]. Nevertheless, the most widely used photocatalysts like TiO2 for organic pollutants were still far from satisfactory due to its wide band-gap and only have photocatalytic activity under the ultraviolet irradiation [7]. Therefore, the method to cover the largest proportion of the solar spectrum and exhibited high photocatalytic activity has attracted a tremendous amount of attention. The Bi2WO6 possesses an orthorhombic structure and can be excited by visible light [8]. However, pure Bi2WO6 has poor photocatalytic performance due to the fast charge recombination. Coupling with another semiconductor to form a heterojunction structure can enhance the ⁎
visible-light absorbance and improve the photocatalytic ability of Bi2WO6 [7]. Nevertheless, the disadvantages of those methods were difficult to find the right synthesis conditions [9]. At the same time, the Ag/AgCl or Ag/AgBr modified semiconductor also attracted much attention due to the SPR effect and their heterostructure, which can accelerate the separation of photoinduced charge [10]. Among them, Ag/ AgBr have received more attention due to their higher photocatalytic activity [11]. For instance, Zhang et al. prepared AgBr-Ag-Bi2WO6 via a deposition-precipitation method, which exhibited high photocatalytic activity on organic pollutants removal [12]. However, this traditional AgBr preparation method did not allow for precise control of the particle size, which significantly restricts its practical large-scale applications. Nevertheless, the oil-in-water self-assembly could effective control of photocatalyst morphology and particle size [13]. Based on the above consideration, we have fabricated photocatalyst Ag/AgBr/Bi2WO6 via an oil-in-water self-assembly method. The
Corresponding authors. E-mail address: [email protected] (X. Fu).
https://doi.org/10.1016/j.cplett.2019.136830 Received 7 August 2019; Received in revised form 4 October 2019; Accepted 7 October 2019 Available online 08 October 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
Chemical Physics Letters 737 (2019) 136830
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06-0438; Fm-3 m (2 2 5)). The morphology of Bi2WO6 was investigated by TEM and SEM. The microspheres can be discovered everywhere and the microspheres are formed by thin petals. The structure of Bi2WO6 thin petals was further investigated by the high-resolution TEM (HRTEM). In the HRTEM (Fig. 1c) image of the Bi2WO6, we can clearly found two sets of lattice fringe (Fig. 1c). The lattice spacing is found to be 0.27 nm and the angle labeled in fast Fourier transform (FFT) pattern of the same region (inset of c) was 90°. The results agree well with the lattice spacing of (2 0 0) and (0 2 0) of the orthorhombic Bi2WO6 phase. The results reflect that the Bi2WO6 sheets exposed (0 0 1) plane [15]. To further determination, we employed selected area electron diffraction (SAED) on the sheets. As displayed in Fig. 1f, the SAED can be indexed along the 001 crystallographic direction of Bi2WO6 and the sheets grow along the 100 and 010. The Ag/AgBr deposited on the surface of Bi2WO6 was shown in Fig. 1g and h, the diameter of AgBr particles ca. 10–20 nm. The inset of Fig. 1g was HRTEM images of AgBr, the lattice fringe of 0.33 nm. The result agrees well with the (1 1 1) planes of AgBr. To investigate the electronic structures of Bi2WO6 and Ag/AgBr/ Bi2WO6, the XPS were employed. As displayed, after modification of Ag/AgBr, the new existing Br and Ag peak in the survey spectrum appeared (Fig. 2a). As displayed in Fig. 2b, the characteristic peaks located at about 368.2 and 374.3 eV. It can be decomposed into four peaks, the peak at 368 eV and 374 eV for Ag 3d5/2 and Ag 3d3/2, which attributed to Ag+ in AgBr. The peaks at 374.4 and 368.7 eV could be attributed to Ag 3d5/2 and Ag 3d3/2 of metallic Ag [16]. Fig. 2c displayed the XPS spectra of Br 3d. The peaks located at 67.9 and 68.8 eV could be ascribed to the characteristic doublets of Br 3d3/2 and Br 3d1/2 [13]. The binding energy of O element in pristine Bi2WO6 (Fig. 2d) could be fitted into three peaks at 529.7, 530.5, and 531.3 eV, which assigned to Bi-O, W-O and hydroxyl groups [8,17].
morphologies, structures photoelectrochemical properties and band structure determination of as-prepared samples were characterized in detail. The activities of as-prepared samples were investigated by the degradation of RhB and phenol under the visible-light. Compared with the bare Bi2WO6 or Ag/AgBr, the Ag/AgBr/Bi2WO6 displayed enhanced catalytic activity, which is mainly related to the benefit of charge transfer processes by coupling with two semiconductors with appropriate band structure and the SPR effect of Ag. 2. Experimental section 2.1. Synthesis of the Ag/AgBr/Bi2WO6 composites The Bi2WO6 were prepared by a simple hydrothermal synthesis [13]. In brief, Bi(NO3)3·5H2O (97 mg, AR) and Na2WO4·2H2O (33 mg, AR) were mixed with deionized water (6.4 ml), acetic acid (1.6 ml, AR) and ethylene glycol (2 ml, AR). The mixture solution heated at 160 °C for 24 h in Teflon-lined autoclaves. The Ag/AgBr/Bi2WO6 hierarchical composites were facilely synthesized via a water system [14]. The 0.1 g Bi2WO6 and 0.05 g AgNO3 (AR) were dispersed in 10 ml deionized water and intensely magnetic stirring, then 3 ml NaOH (0.1 M) were added and magnetic stirring for 30 min. Subsequently, the obtained dispersion was slowly dissolved in 10 ml chloroform (AR) solution with cetyltrimethylammonium bromide (CTAB, 0.2 g, AR). The sample of Ag/AgBr was obtained by a similar method but no Bi2WO6. 2.2. Characterization The crystalline structures were obtained on X-ray diffraction (XRD). The morphologies and microstructures were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The concentrations of the pollutants were acquired by a UV–Vis absorption spectrometer (UV–Vis). The chemical environments of Ag/ AgBr/Bi2WO6 were investigated by X-ray photoelectron spectroscopy (XPS). The photoinduced electron-hole separation efficiency photocatalyst was determined by photoluminescence spectra (PL, excitation wavelength at 300 nm). The photoelectrochemical measurements were measured via an electrochemical analyzer (CHI660E). The photocurrents and photoelectrochemical measurements were performed in Na2SO4 (0.1 M, pH value was approximate 7) aqueous solution. The FTO glasses coating the samples as a working electrode and a 300 W Xe arc lamp utilized as a photo source. The Mott-Schottky plots were obtained at a frequency of 500, 1000 and 5000 Hz. The EIS were carried out in the frequency range of 0.1 to 10,000 Hz in 1 mM Fe(CN)63−/Fe (CN)64− electrolyte solution.
3.2. Photoelectrochemical properties and band structure determination For efficient utilization of sunlight, the catalysts required to display distinct absorptions in the visible region. As shown in Fig. 3(a), the absorption of Ag/AgBr/Bi2WO6 was stronger than the prime Bi2WO6 in the 300–800 nm. That was due to AgBr and the SPR effect of metallic Ag [18]. The bandgap energies of single component AgBr and Bi2WO6 could be computed from UV–vis DRS spectra based on Kubelka-Munk function. As displayed in Fig. 3(b), the bandgap of Bi2WO6 and AgBr were estimated to be 2.85 and 2.40 eV from the onset of the absorption edge, The band structures of as-prepared samples were examined by Mott-Schottky analysis (Fig. 3c and d) [19]. According to the x-intercept, the flat band potentials of AgBr and Bi2WO6 were −0.77 V and −0.54 V (SCE) or −0.53 V and −0.30 V (vis. NHE), respectively. Because of the position of conduction band potentials was deemed to be about 0.1 V below the flat band potential [20]. Therefore, the CB positions of AgBr and Bi2WO6 were −0.63 eV and −0.40 eV (vs. NHE). According to the results of UV–Vis, the VB edge was calculated to be 1.77 eV and 2.45 eV, respectively. To evaluate the electronic properties of the Ag/AgBr/Bi2WO6, we carried out EIS to characterize the as-prepared materials. As displayed, the semicircles diameters of Bi2WO6 were bigger than Ag/AgBr/ Bi2WO6, which indicated that the loading of Ag/AgBr could accelerate the separation of photoinduced charge [21,22]. The chopped light photocurrent response was employed and shown in Fig. 4b. As shown, Ag/AgBr/Bi2WO6 showed reproducible photocurrent and the current density were higher than the pure Bi2WO6, indicates that the Ag/AgBr/ Bi2WO6 has higher generation and separation efficiency for a photoinduced charge [23]. The carrier migrant property was investigated via the transient photocurrent test. The transient profile of photocurrents for Bi2WO6 and Ag/AgBr/Bi2WO6 at a constant potential of 0.2 V vs Hg/Hg2Cl2 was investigated by the following equation:
2.3. Photocatalytic performance Rhodamine (RhB) and phenol were utilized as pollutants to estimate the photocatalytic ability. The 30 mg catalyst was added in RhB (30 ml, 10 mg/L) and phenol (30 ml, 140 mg/L). Then, the suspension was magnetically stirred and ultrasonicated in the dark for 1 h. The visiblelight was obtained from a 300 W Xe lamp equipped with a filter, which provided the visible-light (λ > 420 nm). During the photocatalytic experiments, 5 ml of suspensions were obtained at a given time, and then the suspensions were centrifuged to remove the catalyst. 3. Results and discussion 3.1. Material characterization The XRD patterns of the as-synthesized samples were displayed in Fig. 1. As shown, the diffraction peaks around 28.3°, 32.9°, and 47.1° for the samples of Bi2WO6 and Ag/AgBr/Bi2WO6 are assigned to Bi2WO6 phase (JCPDS No. 73-1126; space group B2ab(41)). The peaks around 31.0°, 44.3°, and 55.1° were assigned to the AgBr (JCPDS No.
D= (It − Ist )/(Iin − Ist ) where It is the photocurrent at the time of t, Iin is the photocurrent at 2
Chemical Physics Letters 737 (2019) 136830
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Fig. 1. (a) XRD pattern of as-prepared samples, (b) and (d) was the SEM image of Bi2WO6, (c) is the HRTEM image of Bi2WO6 and inset image is corresponding FFT pattern, (e) and (g) TEM image of Bi2WO6 and Ag/AgBr/Bi2WO6, (h) HRTEM of as-synthesized Ag/AgBr/Bi2WO6, and the inset image is the HRTEM of AgBr particle, (f) and (i) is the SAED of Bi2WO6 sheet and Ag/AgBr/Bi2WO6;
t = 0, and Ist is the stationary current. The transient time constant τ, could be estimated as the time at which ln D = −1 [24]. As depicted in Fig. 4c and d, the τ of Bi2WO6 and Ag/AgBr/Bi2WO6 is 0.41 and 4 sec, which clearly showed that τ is larger in Ag/AgBr/Bi2WO6. The results reflect that the Ag/AgBr will lead to a slower charge recombination rate [25]. The separation and transfer efficiency of photoinduced electronhole was investigated by photoluminescence (PL) spectra (Fig. 5) [26]. It obviously that the Bi2WO6 and Ag/AgBr/Bi2WO6 have an emission peak locate at 469 nm, which originates from the charge-transfer transitions between the hybrid orbital of Bi 6s and O 2p (VB) to the empty W 5d orbital (CB) in the WO62- complex [27]. The Ag/AgBr/ Bi2WO6 presented lower luminescence intensity compared to the pure Bi2WO6 and AgBr. That indicated the recombination of photogenerated charge carriers was greatly inhibited by heterojunctions between the Ag/AgBr and Bi2WO6. Base on above discussion, the loading of Ag/ AgBr will lead to a reduction the recombination of electrons and holes, provided an efficient separation of photogenerated electron-hole and interface charge transfer.
be negligible. The mixtures of RhB and catalysts were stirred for 30 min in the dark to ensure adsorption-desorption equilibrium. The concentration of RhB has a decrease before the light irrigation. That may due to the surface charge of Ag/AgBr/Bi2WO6 was negative and the RhB was cationic dyes. Therefore, the negatively charged surface of catalysts will electrostatically attract dissociated cationic RhB, which caused a decrease of concentration before the light irradiation [28]. The bare Bi2WO6 and Ag/AgBr have poor photocatalytic, and only 32% and 17% RhB were degraded. Nevertheless, the Ag/AgBr/Bi2WO6 degraded ~96% RhB at the same time, which is much higher than the catalytic efficiency reported in the previous literature [29]. To further investigate the catalytic activity of as-prepared samples, the colorless phenol was employed (Fig. 6b). As expected, the bare Bi2WO6 degraded only 24% phenol within 4 h Nevertheless, Ag/AgBr/Bi2WO6 degraded 66% the phenol solution. The excellent photocatalytic activities could be ascribed to the visible-light response of and close contact between the AgBr and Bi2WO6, which could improve the separation and transfer efficiency of the photoinduced electron-hole pairs [30]. To testing the recyclability and stability, the photocatalytic degradation experiment of RhB was repeated five times using the prepared Ag/AgBr/Bi2WO6 (Fig. 6c). After five cycles, the photocatalytic activity of Ag/AgBr/ Bi2WO6 had no significant change. The crystalline phase of the used sample was also studied by XRD (Fig. 6d). The results reveal that an extremely weak diffraction peak of Ag0 appeared in the used Ag/AgBr/ Bi2WO6, which may cause by photo corrosion [31].
3.3. Photocatalytic performances To investigate the catalytic activities of the as-synthesized catalytic, we employed RhB and phenol as target contaminant. As shown in Fig. 6(a), no noticeable RhB degradation was observed without any photocatalysts, which indicated that the direct photolysis of RhB could 3
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Fig. 2. XPS spectra of as-synthesized samples, (a) the XPS survey spectrum, high resolution of (b) Ag3d, (c) Br 3d, (d) O 1s;
Fig. 3. UV–vis diffuse-reflectance spectra of samples (a) and the determined band gaps of the AgBr and Bi2WO6 (b), Mott-Schottky plots of Bi2WO6 (c) and AgBr (d) at frequencies of 500, 1000 and 5000 Hz; 4
Chemical Physics Letters 737 (2019) 136830
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Fig. 4. Nyquist plots of EIS for Bi2WO6 and Ag/AgBr/Bi2WO6 (a) and the corresponding linear sweep voltammetry scans (b) Normalized plots of the photocurrenttime dependence for the Bi2WO6(c) and Ag/AgBr/Bi2WO6 (d). Inset (c) and (d) illustrate transient photocurrent response of Bi2WO6 and Ag/AgBr/Bi2WO6;
under the visible-light irradiation and entrapped by Ag to recombine [34], and the holes migration away from Bi2WO6 and concentrate on the surface of AgBr. Due to the VB position of AgBr is lower than %OH/ H2O (2.7 eV) and redox potential of %OH/OH− (1.99 V) [35], and cannot directly oxidize H2O or OH− to %OH radicals. At the same time, the photogenerated holes remain on AgBr and Bi2WO6 could capture electrons from the dye molecules and oxidize the organic pollutants directly. In addition, some of the holes on the AgBr could oxidize Br− ions to Br0 atoms and the Br0 were also thought to act as reactive species for the oxidation of dye molecules. Base on the above discussion, photoinduced charges can fully be involved in the reaction and excellent photocatalytic performance of Ag/AgBr/Bi2WO6 is achieved.
4. Conclusions Fig. 5. PL spectra for Bi2WO6, Ag/AgBr, Ag/AgBr/Bi2WO6;
In summary, Ag/AgBr/Bi2WO6 hierarchical composites were successfully synthesized by a simple oil/water self-assembly method with Bi2WO6 as scaffolds, which procedure a uniform Ag/AgBr distribution on the surface of Bi2WO6 and provide more active sites. The Bi2WO6 flowers were assembled with ultrathin nanosheets. The top and bottom of Bi2WO6 were (0 0 1) facets and the sheets grow along the 100 and 010. The Ag/AgBr/Bi2WO6 composites exhibited the highest photocatalytic activity for the degradation of RhB and phenol compared with bare Bi2WO6 and Ag/AgBr. That was due to the loading of Ag/AgBr can increase the surface area of photocatalytic, and improves the photoinduced charge separation. Besides, the Ag/AgBr/Bi2WO6 heterojunction photocatalysts also possessed high stability under the visible-light.
3.4. Proposed mechanisms To illuminate the mechanism of the photocatalytic process, the isopropyl alcohol (IPA), ammonium oxalate (AO), and p-benzoquinone (BQ) was employed as scavenging reagent for %OH, h+, and %O2− [18]. As presented in Fig. 7a, the added IPA did not cause the apparent changes in the degradation of RhB. Nevertheless, the degradation efficiency was significantly decreased in the presence of h+ scavenger and % O2− scavengers (AO and BQ). The results showed that the h+ and % O2− were the important active species in the photocatalytic reaction. Base on the above discussion, the schematic illustration of Ag/AgBr/ Bi2WO6 were drawn and shown in Fig. 7b, which was alike to previously reported [32,33]. Under the visible-light, the Bi2WO6 and Ag/ AgBr could be excited by visible-light and produce a photoinduced charge. The excited electrons of Ag possessed enough energy and injected into the CB of AgBr. Then, the electrons can reduce the adsorbed dissolved oxygen to produce super oxygen anionic free radicals %O2− [13]. Meanwhile, the photoinduced electrons excited to CB of Bi2WO6
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 5
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Fig.6. Photocatalysis degradation RhB (a) and (b) phenol (c) the corresponding recycle test for RhB in the presence of Ag/AgBr/Bi2WO6, (d) the XRD patterns of Ag/ AgBr/Bi2WO6 after five cycles;
Fig. 7. influence of various scavengers photocatalytic activity of Ag/AgBr/Bi2WO6 toward the degradation of RhB (a); proposed photocatalytic mechanism for the degradation of Organic Dyes using Ag/AgBr/Bi2WO6 nanocomposites in the presence of visible-light (b).
Acknowledgments
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Fabrication of Ag/AgBr/Bi2WO6 hierarchical composites with high visible light photocatalytic activity
T
⁎
Wei Gana, Jian Zhanga, Haihong Niub, , Lei Baoa, Hequn Haoa, Yehan Yanc, Keyue Wud, ⁎ Xucheng Fua, a
Analytical and Testing Center, West Anhui University, LuAn 237015, Anhui Prov., PR China School of Electrical and Automation Engineering, HeFei University of Technology, HeFei 230009, Anhui Prov., PR China c College of Resource Environment and Tourism Management, West Anhui University, LuAn 237015, Anhui Prov., PR China d College of Electrical and Optoelectronic Engineering, West Anhui University, LuAn 237015, Anhui Prov., PR China b
H I GH L IG H T S
of Ag/AgBr/Bi WO heterojunctions photocatalyst via oil/water self-assembly method. • Synthesis WO heterojunction can reduce the recombination of electrons and holes. • Ag/AgBr/Bi composites exhibited excellent photocatalytic activity under visible-light irradiation. • The • Holes and O are the predominant active species for Ag/AgBr/Bi WO under visible-light irradiation. 2
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A B S T R A C T
Keywords: Ag/AgBr/Bi2WO6 composites Surface plasmonic resonance Photocatalytic
Visible-light sensitive photocatalyst Ag/AgBr/Bi2WO6 was successfully synthesized by oil/water self-assembly method. The crystalline structure, morphologies and optical properties of the as-prepared samples were investigated. The results revealed that the Ag/AgBr successfully introduced into the surface of Bi2WO6. The modified of Ag/AgBr enhanced the absorption in the visible-light region, due to surface plasmonic resonance of the metallic Ag. The activities of as-prepared samples were investigated by the degradation of RhB and phenol under the visible-light. Experimental results revealed that the Ag/AgBr/Bi2WO6 composites exhibit high visiblelight photocatalytic activity compared with that of pure Ag/AgBr and Bi2WO6.
1. Introduction Due to the rapid development of urbanization, the organic pollutant is getting more and more attention [1–3]. The semiconductor photocatalysts as classic green materials have attracted great interest in the past decades due to their multi-application, safety and stable [4–6]. Nevertheless, the most widely used photocatalysts like TiO2 for organic pollutants were still far from satisfactory due to its wide band-gap and only have photocatalytic activity under the ultraviolet irradiation [7]. Therefore, the method to cover the largest proportion of the solar spectrum and exhibited high photocatalytic activity has attracted a tremendous amount of attention. The Bi2WO6 possesses an orthorhombic structure and can be excited by visible light [8]. However, pure Bi2WO6 has poor photocatalytic performance due to the fast charge recombination. Coupling with another semiconductor to form a heterojunction structure can enhance the ⁎
visible-light absorbance and improve the photocatalytic ability of Bi2WO6 [7]. Nevertheless, the disadvantages of those methods were difficult to find the right synthesis conditions [9]. At the same time, the Ag/AgCl or Ag/AgBr modified semiconductor also attracted much attention due to the SPR effect and their heterostructure, which can accelerate the separation of photoinduced charge [10]. Among them, Ag/ AgBr have received more attention due to their higher photocatalytic activity [11]. For instance, Zhang et al. prepared AgBr-Ag-Bi2WO6 via a deposition-precipitation method, which exhibited high photocatalytic activity on organic pollutants removal [12]. However, this traditional AgBr preparation method did not allow for precise control of the particle size, which significantly restricts its practical large-scale applications. Nevertheless, the oil-in-water self-assembly could effective control of photocatalyst morphology and particle size [13]. Based on the above consideration, we have fabricated photocatalyst Ag/AgBr/Bi2WO6 via an oil-in-water self-assembly method. The
Corresponding authors. E-mail address: [email protected] (X. Fu).
https://doi.org/10.1016/j.cplett.2019.136830 Received 7 August 2019; Received in revised form 4 October 2019; Accepted 7 October 2019 Available online 08 October 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
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06-0438; Fm-3 m (2 2 5)). The morphology of Bi2WO6 was investigated by TEM and SEM. The microspheres can be discovered everywhere and the microspheres are formed by thin petals. The structure of Bi2WO6 thin petals was further investigated by the high-resolution TEM (HRTEM). In the HRTEM (Fig. 1c) image of the Bi2WO6, we can clearly found two sets of lattice fringe (Fig. 1c). The lattice spacing is found to be 0.27 nm and the angle labeled in fast Fourier transform (FFT) pattern of the same region (inset of c) was 90°. The results agree well with the lattice spacing of (2 0 0) and (0 2 0) of the orthorhombic Bi2WO6 phase. The results reflect that the Bi2WO6 sheets exposed (0 0 1) plane [15]. To further determination, we employed selected area electron diffraction (SAED) on the sheets. As displayed in Fig. 1f, the SAED can be indexed along the 001 crystallographic direction of Bi2WO6 and the sheets grow along the 100 and 010. The Ag/AgBr deposited on the surface of Bi2WO6 was shown in Fig. 1g and h, the diameter of AgBr particles ca. 10–20 nm. The inset of Fig. 1g was HRTEM images of AgBr, the lattice fringe of 0.33 nm. The result agrees well with the (1 1 1) planes of AgBr. To investigate the electronic structures of Bi2WO6 and Ag/AgBr/ Bi2WO6, the XPS were employed. As displayed, after modification of Ag/AgBr, the new existing Br and Ag peak in the survey spectrum appeared (Fig. 2a). As displayed in Fig. 2b, the characteristic peaks located at about 368.2 and 374.3 eV. It can be decomposed into four peaks, the peak at 368 eV and 374 eV for Ag 3d5/2 and Ag 3d3/2, which attributed to Ag+ in AgBr. The peaks at 374.4 and 368.7 eV could be attributed to Ag 3d5/2 and Ag 3d3/2 of metallic Ag [16]. Fig. 2c displayed the XPS spectra of Br 3d. The peaks located at 67.9 and 68.8 eV could be ascribed to the characteristic doublets of Br 3d3/2 and Br 3d1/2 [13]. The binding energy of O element in pristine Bi2WO6 (Fig. 2d) could be fitted into three peaks at 529.7, 530.5, and 531.3 eV, which assigned to Bi-O, W-O and hydroxyl groups [8,17].
morphologies, structures photoelectrochemical properties and band structure determination of as-prepared samples were characterized in detail. The activities of as-prepared samples were investigated by the degradation of RhB and phenol under the visible-light. Compared with the bare Bi2WO6 or Ag/AgBr, the Ag/AgBr/Bi2WO6 displayed enhanced catalytic activity, which is mainly related to the benefit of charge transfer processes by coupling with two semiconductors with appropriate band structure and the SPR effect of Ag. 2. Experimental section 2.1. Synthesis of the Ag/AgBr/Bi2WO6 composites The Bi2WO6 were prepared by a simple hydrothermal synthesis [13]. In brief, Bi(NO3)3·5H2O (97 mg, AR) and Na2WO4·2H2O (33 mg, AR) were mixed with deionized water (6.4 ml), acetic acid (1.6 ml, AR) and ethylene glycol (2 ml, AR). The mixture solution heated at 160 °C for 24 h in Teflon-lined autoclaves. The Ag/AgBr/Bi2WO6 hierarchical composites were facilely synthesized via a water system [14]. The 0.1 g Bi2WO6 and 0.05 g AgNO3 (AR) were dispersed in 10 ml deionized water and intensely magnetic stirring, then 3 ml NaOH (0.1 M) were added and magnetic stirring for 30 min. Subsequently, the obtained dispersion was slowly dissolved in 10 ml chloroform (AR) solution with cetyltrimethylammonium bromide (CTAB, 0.2 g, AR). The sample of Ag/AgBr was obtained by a similar method but no Bi2WO6. 2.2. Characterization The crystalline structures were obtained on X-ray diffraction (XRD). The morphologies and microstructures were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The concentrations of the pollutants were acquired by a UV–Vis absorption spectrometer (UV–Vis). The chemical environments of Ag/ AgBr/Bi2WO6 were investigated by X-ray photoelectron spectroscopy (XPS). The photoinduced electron-hole separation efficiency photocatalyst was determined by photoluminescence spectra (PL, excitation wavelength at 300 nm). The photoelectrochemical measurements were measured via an electrochemical analyzer (CHI660E). The photocurrents and photoelectrochemical measurements were performed in Na2SO4 (0.1 M, pH value was approximate 7) aqueous solution. The FTO glasses coating the samples as a working electrode and a 300 W Xe arc lamp utilized as a photo source. The Mott-Schottky plots were obtained at a frequency of 500, 1000 and 5000 Hz. The EIS were carried out in the frequency range of 0.1 to 10,000 Hz in 1 mM Fe(CN)63−/Fe (CN)64− electrolyte solution.
3.2. Photoelectrochemical properties and band structure determination For efficient utilization of sunlight, the catalysts required to display distinct absorptions in the visible region. As shown in Fig. 3(a), the absorption of Ag/AgBr/Bi2WO6 was stronger than the prime Bi2WO6 in the 300–800 nm. That was due to AgBr and the SPR effect of metallic Ag [18]. The bandgap energies of single component AgBr and Bi2WO6 could be computed from UV–vis DRS spectra based on Kubelka-Munk function. As displayed in Fig. 3(b), the bandgap of Bi2WO6 and AgBr were estimated to be 2.85 and 2.40 eV from the onset of the absorption edge, The band structures of as-prepared samples were examined by Mott-Schottky analysis (Fig. 3c and d) [19]. According to the x-intercept, the flat band potentials of AgBr and Bi2WO6 were −0.77 V and −0.54 V (SCE) or −0.53 V and −0.30 V (vis. NHE), respectively. Because of the position of conduction band potentials was deemed to be about 0.1 V below the flat band potential [20]. Therefore, the CB positions of AgBr and Bi2WO6 were −0.63 eV and −0.40 eV (vs. NHE). According to the results of UV–Vis, the VB edge was calculated to be 1.77 eV and 2.45 eV, respectively. To evaluate the electronic properties of the Ag/AgBr/Bi2WO6, we carried out EIS to characterize the as-prepared materials. As displayed, the semicircles diameters of Bi2WO6 were bigger than Ag/AgBr/ Bi2WO6, which indicated that the loading of Ag/AgBr could accelerate the separation of photoinduced charge [21,22]. The chopped light photocurrent response was employed and shown in Fig. 4b. As shown, Ag/AgBr/Bi2WO6 showed reproducible photocurrent and the current density were higher than the pure Bi2WO6, indicates that the Ag/AgBr/ Bi2WO6 has higher generation and separation efficiency for a photoinduced charge [23]. The carrier migrant property was investigated via the transient photocurrent test. The transient profile of photocurrents for Bi2WO6 and Ag/AgBr/Bi2WO6 at a constant potential of 0.2 V vs Hg/Hg2Cl2 was investigated by the following equation:
2.3. Photocatalytic performance Rhodamine (RhB) and phenol were utilized as pollutants to estimate the photocatalytic ability. The 30 mg catalyst was added in RhB (30 ml, 10 mg/L) and phenol (30 ml, 140 mg/L). Then, the suspension was magnetically stirred and ultrasonicated in the dark for 1 h. The visiblelight was obtained from a 300 W Xe lamp equipped with a filter, which provided the visible-light (λ > 420 nm). During the photocatalytic experiments, 5 ml of suspensions were obtained at a given time, and then the suspensions were centrifuged to remove the catalyst. 3. Results and discussion 3.1. Material characterization The XRD patterns of the as-synthesized samples were displayed in Fig. 1. As shown, the diffraction peaks around 28.3°, 32.9°, and 47.1° for the samples of Bi2WO6 and Ag/AgBr/Bi2WO6 are assigned to Bi2WO6 phase (JCPDS No. 73-1126; space group B2ab(41)). The peaks around 31.0°, 44.3°, and 55.1° were assigned to the AgBr (JCPDS No.
D= (It − Ist )/(Iin − Ist ) where It is the photocurrent at the time of t, Iin is the photocurrent at 2
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Fig. 1. (a) XRD pattern of as-prepared samples, (b) and (d) was the SEM image of Bi2WO6, (c) is the HRTEM image of Bi2WO6 and inset image is corresponding FFT pattern, (e) and (g) TEM image of Bi2WO6 and Ag/AgBr/Bi2WO6, (h) HRTEM of as-synthesized Ag/AgBr/Bi2WO6, and the inset image is the HRTEM of AgBr particle, (f) and (i) is the SAED of Bi2WO6 sheet and Ag/AgBr/Bi2WO6;
t = 0, and Ist is the stationary current. The transient time constant τ, could be estimated as the time at which ln D = −1 [24]. As depicted in Fig. 4c and d, the τ of Bi2WO6 and Ag/AgBr/Bi2WO6 is 0.41 and 4 sec, which clearly showed that τ is larger in Ag/AgBr/Bi2WO6. The results reflect that the Ag/AgBr will lead to a slower charge recombination rate [25]. The separation and transfer efficiency of photoinduced electronhole was investigated by photoluminescence (PL) spectra (Fig. 5) [26]. It obviously that the Bi2WO6 and Ag/AgBr/Bi2WO6 have an emission peak locate at 469 nm, which originates from the charge-transfer transitions between the hybrid orbital of Bi 6s and O 2p (VB) to the empty W 5d orbital (CB) in the WO62- complex [27]. The Ag/AgBr/ Bi2WO6 presented lower luminescence intensity compared to the pure Bi2WO6 and AgBr. That indicated the recombination of photogenerated charge carriers was greatly inhibited by heterojunctions between the Ag/AgBr and Bi2WO6. Base on above discussion, the loading of Ag/ AgBr will lead to a reduction the recombination of electrons and holes, provided an efficient separation of photogenerated electron-hole and interface charge transfer.
be negligible. The mixtures of RhB and catalysts were stirred for 30 min in the dark to ensure adsorption-desorption equilibrium. The concentration of RhB has a decrease before the light irrigation. That may due to the surface charge of Ag/AgBr/Bi2WO6 was negative and the RhB was cationic dyes. Therefore, the negatively charged surface of catalysts will electrostatically attract dissociated cationic RhB, which caused a decrease of concentration before the light irradiation [28]. The bare Bi2WO6 and Ag/AgBr have poor photocatalytic, and only 32% and 17% RhB were degraded. Nevertheless, the Ag/AgBr/Bi2WO6 degraded ~96% RhB at the same time, which is much higher than the catalytic efficiency reported in the previous literature [29]. To further investigate the catalytic activity of as-prepared samples, the colorless phenol was employed (Fig. 6b). As expected, the bare Bi2WO6 degraded only 24% phenol within 4 h Nevertheless, Ag/AgBr/Bi2WO6 degraded 66% the phenol solution. The excellent photocatalytic activities could be ascribed to the visible-light response of and close contact between the AgBr and Bi2WO6, which could improve the separation and transfer efficiency of the photoinduced electron-hole pairs [30]. To testing the recyclability and stability, the photocatalytic degradation experiment of RhB was repeated five times using the prepared Ag/AgBr/Bi2WO6 (Fig. 6c). After five cycles, the photocatalytic activity of Ag/AgBr/ Bi2WO6 had no significant change. The crystalline phase of the used sample was also studied by XRD (Fig. 6d). The results reveal that an extremely weak diffraction peak of Ag0 appeared in the used Ag/AgBr/ Bi2WO6, which may cause by photo corrosion [31].
3.3. Photocatalytic performances To investigate the catalytic activities of the as-synthesized catalytic, we employed RhB and phenol as target contaminant. As shown in Fig. 6(a), no noticeable RhB degradation was observed without any photocatalysts, which indicated that the direct photolysis of RhB could 3
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Fig. 2. XPS spectra of as-synthesized samples, (a) the XPS survey spectrum, high resolution of (b) Ag3d, (c) Br 3d, (d) O 1s;
Fig. 3. UV–vis diffuse-reflectance spectra of samples (a) and the determined band gaps of the AgBr and Bi2WO6 (b), Mott-Schottky plots of Bi2WO6 (c) and AgBr (d) at frequencies of 500, 1000 and 5000 Hz; 4
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Fig. 4. Nyquist plots of EIS for Bi2WO6 and Ag/AgBr/Bi2WO6 (a) and the corresponding linear sweep voltammetry scans (b) Normalized plots of the photocurrenttime dependence for the Bi2WO6(c) and Ag/AgBr/Bi2WO6 (d). Inset (c) and (d) illustrate transient photocurrent response of Bi2WO6 and Ag/AgBr/Bi2WO6;
under the visible-light irradiation and entrapped by Ag to recombine [34], and the holes migration away from Bi2WO6 and concentrate on the surface of AgBr. Due to the VB position of AgBr is lower than %OH/ H2O (2.7 eV) and redox potential of %OH/OH− (1.99 V) [35], and cannot directly oxidize H2O or OH− to %OH radicals. At the same time, the photogenerated holes remain on AgBr and Bi2WO6 could capture electrons from the dye molecules and oxidize the organic pollutants directly. In addition, some of the holes on the AgBr could oxidize Br− ions to Br0 atoms and the Br0 were also thought to act as reactive species for the oxidation of dye molecules. Base on the above discussion, photoinduced charges can fully be involved in the reaction and excellent photocatalytic performance of Ag/AgBr/Bi2WO6 is achieved.
4. Conclusions Fig. 5. PL spectra for Bi2WO6, Ag/AgBr, Ag/AgBr/Bi2WO6;
In summary, Ag/AgBr/Bi2WO6 hierarchical composites were successfully synthesized by a simple oil/water self-assembly method with Bi2WO6 as scaffolds, which procedure a uniform Ag/AgBr distribution on the surface of Bi2WO6 and provide more active sites. The Bi2WO6 flowers were assembled with ultrathin nanosheets. The top and bottom of Bi2WO6 were (0 0 1) facets and the sheets grow along the 100 and 010. The Ag/AgBr/Bi2WO6 composites exhibited the highest photocatalytic activity for the degradation of RhB and phenol compared with bare Bi2WO6 and Ag/AgBr. That was due to the loading of Ag/AgBr can increase the surface area of photocatalytic, and improves the photoinduced charge separation. Besides, the Ag/AgBr/Bi2WO6 heterojunction photocatalysts also possessed high stability under the visible-light.
3.4. Proposed mechanisms To illuminate the mechanism of the photocatalytic process, the isopropyl alcohol (IPA), ammonium oxalate (AO), and p-benzoquinone (BQ) was employed as scavenging reagent for %OH, h+, and %O2− [18]. As presented in Fig. 7a, the added IPA did not cause the apparent changes in the degradation of RhB. Nevertheless, the degradation efficiency was significantly decreased in the presence of h+ scavenger and % O2− scavengers (AO and BQ). The results showed that the h+ and % O2− were the important active species in the photocatalytic reaction. Base on the above discussion, the schematic illustration of Ag/AgBr/ Bi2WO6 were drawn and shown in Fig. 7b, which was alike to previously reported [32,33]. Under the visible-light, the Bi2WO6 and Ag/ AgBr could be excited by visible-light and produce a photoinduced charge. The excited electrons of Ag possessed enough energy and injected into the CB of AgBr. Then, the electrons can reduce the adsorbed dissolved oxygen to produce super oxygen anionic free radicals %O2− [13]. Meanwhile, the photoinduced electrons excited to CB of Bi2WO6
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 5
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Fig.6. Photocatalysis degradation RhB (a) and (b) phenol (c) the corresponding recycle test for RhB in the presence of Ag/AgBr/Bi2WO6, (d) the XRD patterns of Ag/ AgBr/Bi2WO6 after five cycles;
Fig. 7. influence of various scavengers photocatalytic activity of Ag/AgBr/Bi2WO6 toward the degradation of RhB (a); proposed photocatalytic mechanism for the degradation of Organic Dyes using Ag/AgBr/Bi2WO6 nanocomposites in the presence of visible-light (b).
Acknowledgments
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