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Bi2S3 heterojunctions: the facilitation of exposed facets of Bi2WO6 substrate
Accepted Manuscript Title: Photocatalytic activity of Bi2 WO6 /Bi2 S3 heterojunctions: the facilitation of exposed facets of Bi2 WO6 substrate Author: Long Yan Yufei Wang Huidong Shen Yu Zhang Jian Li Danjun Wang PII: DOI: Reference:
S0169-4332(16)32135-3 http://dx.doi.org/doi:10.1016/j.apsusc.2016.10.039 APSUSC 34133
To appear in:
APSUSC
Received date: Revised date: Accepted date:
28-7-2016 5-10-2016 6-10-2016
Please cite this article as: Long Yan, Yufei Wang, Huidong Shen, Yu Zhang, Jian Li, Danjun Wang, Photocatalytic activity of Bi2WO6/Bi2S3 heterojunctions: the facilitation of exposed facets of Bi2WO6 substrate, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.10.039 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Photocatalytic activity of Bi2WO6/Bi2S3 heterojunctions: the facilitation of exposed facets of Bi2WO6 substrate
Long Yana,b, Yufei Wanga, Huidong Shenb, Yu Zhangb, Jian Lia, Danjun Wangb*
a
School of Chemistry and Chemical Engineering, Yulin University, Shaanxi Key
laboratory of Low Metamorphic Coal Clean Utilization, Yulin71900, China b
School of Chemistry and Chemical Engineering, Yan’an University, Shaanxi Key
Laboratory of Chemical Reaction Engineering, Yan’an 716000, China *Correspondent author: Tel: +86 912 38911 44; Fax: +86 912 3689959; E-mail: [email protected]
Graphical Abstract
Highlights: 1. Bi2S3/Bi2WO6 hybrids with exposed (020) Bi2WO6 facets have been synthesized. 2. X-ray photoelectron spectroscopy reveals that a small amount of Bi2S3 was formed. 3. The enhanced photoactivity of hybrids is due to heterojunction and (020) facets. 4. A possible photocatalytic degradation mechanism is proposed.
Abstract Bi2S3/Bi2WO6 hybrid architectures with exposed (020) Bi2WO6 facets have been synthesized via a controlled anion exchange approach. X-ray photoelectron spectroscopy (XPS) reveals that a small amount of Bi2S3 was formed on the surface of Bi2WO6 during the anion exchange process, thus leading to the transformation from the Bi2WO6 to Bi2S3/Bi2WO6. The RhB aqueous solution was chosen as model organic pollutants to evaluate the photocatalytic activities of the Bi2S3/Bi2WO6 catalysts. Under visible light irradiation, the Bi2S3/Bi2WO6-TAA displayed the excellent visible light photoactivities compared with pure Bi2S3, Bi2WO6 and other composite
photocatalysts.
The
efficient
photocatalytic
activity
of
the
Bi2S3/Bi2WO6-TAA composite microspheres was ascribed to the constructed heterojunctions and the inner electric field caused by the exposed (020) Bi2WO6 facets. Active species trapping experiments revealed that h+ and O2•- are the main active species in the photocatalytic process. Furthermore, the as-obtained photocatalysts showed good photocatalytic activity after four recycles. The results presented in this study provide a new concept for rational design and development of high-efficient photocatalysts. Keywords: Bi2S3/Bi2WO6, heterujunciton, exposed facet, inner electric field, photocatalysis
1. Introduction Semiconductor
nanocrystals,
which
display
a
wide
range
of
novel
chemical/physical properties, have drawn ever growing interest for their applications in energy conversion[1, 2], supercapacitors[3, 4] and photocatalysis[5, 6]. In particular, the semiconductor photocatalysis has attracted intensive attention for its application in treating environment and energy issues [7, 8]. Recently, Bi2WO6, as a layered structure compound, has been extensively investigated because of its nontoxicity, stability and excellent photoactivities [9, 10]. Its crystal structure is constructed by an intergrowth of WO42- ions inserted between (Bi2O2)2+ layers. Because of the unique layered structure, the Bi6s and O2p levels can form largely dispersed hybridized valence band (VB), which favors the mobility of photogenerated holes and the oxidation reaction[11, 12]. However, the fast recombination of photoinduced electron-hole pairs limits its further practical application. Therefore, it is necessary to explore effective strategies to enhance the separation efficiency of photoexcited charge carriers, such as doping[13], noble metal deposition[14], morphology control[15] and construction heterojunctions[16]. Among them, coupling with two kinds of semiconductor into heterojunctions has been found an effective approach. Bi2S3 with narrow band gap (1.3 eV) has attracted wide attention in many significant applications [17, 18]. So far, lots of Bi2S3-based photocatalysts were prepared to enhance the charge separation and photocatalytic activity, such as Bi2S3/In2S3[19], Bi2S3/CdS[20] and Bi2S3/BiOCl composite[21]. Therefore, it is meaningful to combine both advantages of the Bi2S3 and Bi2WO6 materials in constructing Bi2S3/Bi2WO6 composites. Recently, Ye and Sun et al. synthesized 2D plate-like Bi2S3/Bi2WO6 heterojunction and explored their photocatalytic application, respectively [22, 23]. The formation of Bi2S3/Bi2WO6 heterostructures might favor the separation and transfer of photoinduced carriers, and improve the photocatalytic activity. Up to date, the morphology and crystal-facet-tuned synthesis of the semiconductor photocatalysts have attracted great attention because of the potential
application for improving the photocatalytic activities [24, 25]. For example, Zhao et al. prepared BiVO4-TiO2 heterojunctions with the {110} facet of BiVO4 and investigated its photocatalytic mechanism[26]. It was found that the heterojunction interface, which served as the channel for charge transfer, plays a crucial role in the separation of photoinduced charge carriers. Furthermore, the efficient transportation of the heterojunction interface is largely effected by the exposed facets of the photocatalysts. Thus, fabricating Bi2S3/Bi2WO6 heterojunction with exposed specific active facets is an effective way to develop excellent visible light responsive photocatalysts. Herein, a novel Bi2S3/Bi2WO6 hybrid microspheres with exposed (020) Bi2WO6 facets were synthesized by a facile anion exchange route. In addition to providing Bi3+ ions, Bi2WO6 substrates also play a role of a framework to construct the Bi2S3 /Bi2WO6 hybrid. Controlled release of S2- ions from various sulfur sources to react with Bi2WO6 by ion exchange effects the dispersion and size of Bi2S3 nanoparticles. The photoactivity of the prepared Bi2S3 /Bi2WO6 hybrid was evaluated by the degradation of RhB solution under visible light irradiation. Furthermore, the mechanism of the enhancement of photocatalytic activity of the Bi2S3 /Bi2WO6-TAA was discussed in detail. 2. Experimental section 2.1 Preparation of Bi2WO6 photocatalysts All the reagents were of analytical purity and were used as received without further purification. The nanosheet-assembled Bi2WO6 microspheres were prepared by a hydrothermal process[27]. In a typical synthesis, 4 mmol of Bi(NO3)3·5H2O and 2 mmol of Na2WO4·2H2O were added to 30 mL of deionized water under magnetic stirring, respectively. After that, the Na2WO4 aqueous solution was dropped to the Bi(NO3)3 suspension slowly. After being stirred for 30 min, the resulting precursor suspension was transferred into a Teflon-lined stainless steel autoclave (V=100 mL). The autoclave was sealed and maintained at 180 °C for 12 h and then allowed to cool to room temperature naturally. The product was filtered off, washed several times with absolute alcohol and distilled water, and finally dried at 80 °C overnight. As a
comparison, the Bi2WO6 nanoplates were also prepared and the corresponding products were denoted as P-Bi2WO6 (shown in the ESI). 2.2 Preparation of Bi2S3 /Bi2WO6 composite photocatalyst For synthesis of Bi2S3/Bi2WO6 composite microspheres, the prepared nanosheet-built Bi2WO6 microsphere was introduced to the three different 60 mL of aqueous solutions containing thiourea (2 mmol), cysteine (2 mmol) and thiacetamide (TAA, 2 mmol) with constant stirring for 3 h at room temperature, respectively. The corresponding composites were denoted as t-S/W microspheres, c-S/W microspheres and T-S/W microspheres. For comparison, the Bi2S3/Bi2WO6 nanoplate hybrids (denoted as T-S/W nanoplates) were synthesized by adding the prepared Bi2WO6 nanoplate into the TAA solution, while the other conditions remain the same to the preparation of T-S/W composite microspheres. For the control experiment, pristine Bi2S3 was prepared by reacting 4 mmol of Bi(NO3)3·5H2O and 60 mL aqueous solution of TAA (2 mmol) at room temperature. In addition, the N-doped P25 reference catalysts were synthesized based on the process as previously reported[28]. 2.3 Characterization The structure and crystallinity of the obtained samples were analyzed by powder X-ray diffraction (XRD) patterns on a Bruker AXS D8 advance powder diffractometer (Cu Kα X-ray radiation, λ = 0.154056nm). X-ray photoelectron spectroscopy (XPS) was carried out on an ESCALAB II XPS system with a monochromatic Mg Kα source and a charge neutralizer. The scanning electron microscopy (SEM) images were obtained on a Hitachi S-4800 microscope with an accelerating voltage of 7.0 kV. The transmission electron microscopy (TEM) and high resolution TEM (HRTEM) images were performed by a JEOL JEM-2100 instrument. UV–Vis diffuse reflectance spectra were collected on a spectrophotometer (UV2550, Shimadzu Corporation, Kyoto, Japan) equipped with an integrating sphere. The BaSO4 was used as a reflectance standard. Photoluminescence (PL) measurements were carried out on a fluorescence spectrophotometer (RF-5301 PC, Shimadzu, Japan) at room temperature. 2.4 Photocatalytic evaluation The photocatalytic activities of the as-prepared catalysts were evaluated by
decomposition of RhB under visible light irradiation at room temperature. The photocatalytic experiments were performed with the sample powder (30 mg) suspended in RhB (100 mL, 10-5 M) with constant stirring. Prior to visible light irradiation, the suspensions were stirred in the dark for 1 h to insure the adsorption/desorption equilibrium. The suspension was then irradiated with visible light emitted by using a 300 W Xe lamp with a 420 nm cutoff filter. The reaction temperature was kept at room temperature to prevent any thermal catalytic effect. At the given time intervals, the concentration of RhB aqueous solution was analyzed at 553 nm on a UV-Vis spectrophotometer (UV2550, Shimadzu Corporation, Kyoto, Japan). 2.5 Detection of reactive species In the process of photocatalytic degradation of target pollutants, hydroxyl radicals (•OH), electron (e-), holes (h+) and superoxide radical anions (O2•-) are the major active species. In order to determine the reactive radical species involved in the degradation of RhB, trapping experiments were performed via introducing various scavengers such as isopropyl alcohol (IPA, 10 mM), triethonoamine (TEOA, 6 mM) and 1,4-benzoquinone (BQ, 0.1 mM) into the reaction system to scavenge the •OH, h+ and O2•- species. The trapping experiment procedures were similar to that of the degradation experiment, with the various scavengers introduced separately into the aqueous substrate solution before addition of the photocatalysts.[29]
3. Results and discussion 3.1 Characterization of the prepared samples
First of all, Bi2WO6 with nanosheet-assembled architectures and nanoplates were synthesized under hydrothermal conditions. The corresponding SEM images are in shown in Fig. S2. Due to the rather lower solubility of to Bi2S3 (Ksp=1×10-91), Bi2WO6 can adopt a thermodynamically favored direction to yield Bi2S3 when it exchanges anion with S2- ions in solution[23]. The purity and phase structure of the prepared samples were examined by XRD. Fig. 1a shows the XRD pattern of the flower-like Bi2WO6 microspheres and
Bi2S3/Bi2WO6 composites. It can be observed that the XRD patterns of pure Bi2WO6 can be well indexed to the orthorhombic Bi2WO6 (JCPDS 39-0256). In the patterns of the Bi2S3/Bi2WO6 (Fig. 1b-d), however, no diffraction peaks assigned to Bi2S3 were found, which may be result from its high dispersity and low content of Bi2S3 component[30, 31]. In order to investigate the chemical states and surface composition of the Bi2S3/Bi2WO6 composites, the XPS analysis was performed out. As shown in Fig. 2, the survey spectrum shows the presence of Bi, W, O and S elements. Besides, a carbon peak at a binding energy of about 284.9-285.9 eV is clearly observed because of the carbon tape used for fixing the sample and from the adsorption of atmospheric CO2 on the sample surface.[29] Fig. 2b presents the high resolution XPS spectrum of W element. The binding energies of W4f5/2 and W4f7/2 are 37.6 eV and 35.5 eV in the oxide form of Bi2WO6, which can be attributed to the W atoms existing in a +6 oxidation state.[32] Peaks at 158.7, 161.7, 162.7 and 164.0 eV due to Bi4f7/2, S2p3/2, S2p1/2 and Bi4f5/2 can be detected, which demonstrates that the main state of Bi in the samples was +3, and that of S was -2.[33] These results illustrate the formation of Bi2S3 in the T-S/W composite microspheres. The morphologies of the as-prepared Bi2S3/Bi2WO6 hybrids are characterized by SEM analysis, as shown in Fig. 3. The as-obtained Bi2WO6 sample has a sphere-like structure, assembled from rectangular nanosheets with diameters of about 2 µm and thickness of 30 nm (Fig. S2a). Also it can be observed (Fig. S2b) that the nanosheet-assembled structures were destroyed when urea was introduced in the hydrothermal synthesis process. Moreover, it can be seen that the surface of the Bi2WO6 nanosheets are very smooth and clean. As shown in Fig. 3a-b, a large number of Bi2S3 nanoparticles were assembled on the surface of Bi2WO6 nanosheets. It is found that during the course of anion exchange reaction, the morphology of Bi2WO6 architectures was retained in the final Bi2S3/Bi2WO6 hybrid. As the relative reactivity follows the order TAA>cysteine>thiourea, the higher reactivity of the sulfur source engenders the faster growth rate of Bi2S3. TEM and SAED analyses were used to further investigate the phase structure of
the Bi2WO6 nanosheet-assembled microspheres and T-B/S heterojunctions. As shown in Fig. 4a, the Bi2WO6 sample exhibits hierarchical architectures with a size of about 2 µm built by nanosheets. In HRTEM image (Fig. 4b) of a single nanosheet, the distances between adjacent lattice fringes are measured as 2.83 and 1.98 nm, respectively. This value corresponds to the interplanar distances of Bi2WO6 (200) and (202), respectively. The set of diffraction spots can be indexed as the [010] zone axis of orthorhombic Bi2WO6, indicating that the exposed facets are characterized by (020) facets. In the HRTEM image of the T-B/S heterojunctions (Fig. 4f), It can be clearly observed that some discrete Bi2S3 nanoparticles with the size in the range of 10-15 nm are anchored on the surface of Bi2WO6 nanosheets. This result further demonstrates that the T-B/S heterojunctions was formed in the composite, which is in good agreement with the result of SEM and XPS analyses. The formation of intimate interface contact is significant for promoting the charge separation to achieve high photocatalytic activity in the coupling system. UV-vis diffuse reflectance spectra Fig. 5 presents the UV-Vis diffuse reflectance spectra of the as-prepared samples. The white Bi2WO6 nanosheet-assembled sample exhibits visible light response with an absorption edge around 450 nm, corresponding to the band gap energy of 2.75 eV. Compared to Bi2WO6 nanosheet-assembled sample, the photo absorption ability of T-Bi2S3/Bi2WO6 hybrids was enhanced in the range of 350-600 nm. In particular, the photo absorption of Bi2S3/Bi2WO6 hybrids synthesized using thiourea and cysteine are similar to that of Bi2WO6 nanosheet-assembled sample. The result indicates that the T-Bi2S3/Bi2WO6 hybrids have a suitable band gap for photocatalytic decomposition of organic pollutants under visible light illumination. Photocatalytic Activities
The photocatalytic activities of the as-prepared catalysts were demonstrated by decomposition of RhB aqueous solution under visible light illumination. As shown in Fig. 6, RhB solution kept most of its initial concentration under visible light irradiation in the absence of photocatalysts. Besides, Bi2WO6 could decompose 47.1% of RhB under irradiation for 150 min. While in the case of the Bi2WO6/Bi2S3
microspheres synthesized using TAA as the sulfur source, the photocatalytic decomposition efficiency can reach as high as about 83 % under the same conditions, showing superior photocatalytic activity as compared to that of the pure Bi 2WO6. It can also be found that the degradation rate of RhB over the as-obtained Bi2WO6/Bi2S3 composite microspheres after visible light irradiation decreased in the order of TAA > thiourea > cysteine. The aggregation of Bi2S3 particles is expected to weaken the photocatalytic activities due to the introduction of inter-particle interfaces. On the other hand, smaller Bi2S3 particles may improve the catalytic performance owing to the increase of surface area. In addition, as shown in Fig. 7B, the blue-shift in the absorption profile suggests that degradation process is a stepwise deethylation rather than photocatalytic degradation route [34]. Meanwhile, in order to investigate the photocatalytic activity of different morphologies with the same Bi2S3 content loaded on the Bi2WO6, the Bi2S3 modified Bi2WO6 nanoplate in the presence of TAA are also used to degrade the RhB. And the results are shown in Fig. 7. 46% and 87% of RhB were degraded by Bi2WO6 nanoplates and nanosheet-assembled microspheres, respectively. When the two Bi2WO6 photocatalysts were modified by the Bi2S3 prepared in the presence of TAA, the photocatalytic performances of the corresponding heterojunction were enhanced. Specially, it can be observed that the degradation efficiency of the Bi2S3/Bi2WO6 microspheres is much higher than that of the Bi2S3/Bi2WO6 nanoplates. The larger specific area of Bi2WO6 microspheres is easier and more uniform for Bi2S3 nanoparticles to than that of the Bi2WO6 nanoplates. Investigation of photocatalytic enhanced mechanism Radical and hole trapping experiments were performed to detect the main oxidative species in the photocatalytic process to reveal the photocatalytic mechanism. As is shown in Fig. 8, compared with the addition of IPA, the photocatalytic activity in the T-Bi2WO6/Bi2S3 microspheres is greatly suppressed by the addition of TEOA and BQ, respectively. This result indicates that h+ and O2•- are the main active species and predominates in oxidizing the adsorbed RhB pollutants. According to the reported literatures[35], the valence band (EVB) and conduction
band (ECB) of Bi2WO6 were 3.38 and 0.40 eV, respectively. The positions of EVB and ECB of Bi2S3 were 1.38 and -0.76 eV, respectively[20]. Band positions suggest that Bi2WO6 and Bi2S3 have the staggered energy potentials. Accordingly, the photogenerated carriers could transfer easily between the Bi2WO6 and Bi2S3 due to the band energy potential difference, which enhanced the separation efficiency of the photoinduced electron-hole pairs[36]. To understand the (020) facet-dependent enhanced photocatalytic activities of the Bi2S3/Bi2WO6-TAA composite microsphere, the crystal structure and surface atomic configurations of the (020) facets of Bi2WO6 were investigated. As shown in Fig. 9a, along the [010] direction, the [Bi2O2]2+ and WO42- induce the presence of an inner electric field, improving the separation efficiency of the photoinduced charge carriers. Furthermore, through ion exchange reaction between Bi2WO6 architectures and sulfur sources, Bi2S3 was in situ anchored on the surface of Bi2WO6 microspheres. As schematically shown in Fig. 12b, photoexcited electrons from Bi2S3 nanoparticles are more favorable to be transferred to the surface of the Bi2WO6. Therefore, the charge separation and transfer of the Bi2S3/Bi2WO6-TAA composite microsphere is high as compared to that of the Bi2WO6 nanoplate/Bi2S3. Fig. 12c shows the surface atom structure of the Bi2WO6 (020) facets. The (020) facets contains a large number of oxygen atoms, which is beneficial for the adsorption of cationic RhB solution. In order to further demonstrate the enhancement of the separation efficiency of the photoinduced charge carriers, the photoluminescence (PL) measurements of the Bi2WO6, Bi2S3/Bi2WO6-TAA and Bi2WO6 nanoplate/Bi2S3 were performed. Lower fluorescence emission intensity implies a lower electron-hole recombination rate and thus corresponds to a higher photocatalytic activity. [37] The corresponding results are demonstrated in Fig. 10. It is clearly seen that the luminescence efficiency from the Bi2S3/Bi2WO6-TAA composites is lower than that of bare Bi2WO6, indicating that the Bi2S3 nanoparticles could serve as an effective hole-accepting material, inhibiting a direct recombination of electrons and holes within the bulk Bi2WO6. In addition, as compared to the Bi2WO6 nanoplate/Bi2S3, the Bi2S3/Bi2WO6-TAA show low PL intensity, demonstrating the significant role of the properties of Bi2WO6 substrate.
Based on the above results, the exposed (020) facets of the Bi2WO6 could enhance the separation of photogenerated charge carriers because of the presence of inner electric field, which may finally improve the photocatalytic activity. Meanwhile, as shown in Fig. 11, under visible light irradiation, photogenerated electrons on the surface of Bi2S3 would transfer into the ECB of Bi2WO6 due to the more negative ECB of Bi2S3. Then, the transferred electrons reacted with molecular oxygen to form O2·-. Simultaneously, photoinduced holes of the Bi2WO6 would transfer into Bi2S3, because the EVB of the Bi2WO6 is more positive than that of Bi2S3. Thus, the fast combination of photoinduced electron-hole pairs was effectively restrained. Moreover, the highly dispersed state of Bi2S3 could suppress the formation of interfacial recombination centers. As a consequence, the separation of photogenerated electron-hole pairs is significantly improved.
4. Conclusions In summary, a novel Bi2S3/Bi2WO6 composite microspheres with exposed (020) Bi2WO6 facets were synthesized by a controlled anion exchange approach. The obtained
Bi2S3/Bi2WO6-TAA
composite
microspheres
exhibited
improved
photocatalytic activities as compared to those of the pure Bi2WO6 and Bi2S3 under visible
light
irradiation.
The
enhanced
photocatalytic
activities
of
the
Bi2S3/Bi2WO6-TAA microspheres could be attributed to the inner electric field cased by the exposed (020) facets of Bi2WO6 and the heterojunction photocatalytic system. h+ and O2•- are the main active species in the photocatalytic process and the Bi2S3/Bi2WO6-TAA microspheres showed excellent stability under visible light irradiation.
Based
on
the
efficient
and
stable
photocatalytic
activities,
Bi2S3/Bi2WO6-TAA microspheres can be regarded as promising materials for photocatalytic waste treatment. The experimental results illustrate that both the inner electric field caused by the heterojunction with matched energy levels and the exposed facet in the composite photocatalysts are beneficial for the enhancement of photoactivity, which provides a new concept for rational design and development of high-efficient photocatalysts.
Acknowledgement This work is financially supported by Key Laboratory research for the Open Fund of Shaanxi Province, China (2014SKL-DBM008).
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[32] J. Di, J. Xia, Y. Ge, H. Li, H. Ji, H. Xu, Q. Zhang, H. Li, M. Li, Novel visible-light-driven CQDs/Bi2WO6 hybrid materials with enhanced photocatalytic activity toward organic pollutants degradation and mechanism insight, Appl. Catal. B: Environ., 168–169 (2015) 51-61. [33] W. Xitao, L. Rong, W. Kang, Synthesis of ZnO@ZnS-Bi2S3 core-shell nanorod grown on reduced graphene oxide sheets and its enhanced photocatalytic performance, J. Mater. Chem. A, 2 (2014) 8304-8313. [34] D.P. Chen, W. Bowers, S.E. Skrabalak, Aerosol-Assisted Combustion Synthesis of Single-Crystalline NaSbO3 Nanoplates: A Topotactic Template for Ilmenite AgSbO3, Chem. Mater., 27 (2015) 174-180. [35] L. Zhang, X. Zhang, Y.-Q. Huang, C.-L. Pan, J.-S. Hu, C.-M. Hou, Novel Bi12ZnO20-Bi2WO6 heterostructures: facile synthesis and excellent visible-light-driven photocatalytic activities, RSC Advances, 5 (2015) 30239-30247. [36] W.J. Zhou, Y.H. Leng, D.M. Hou, H.D. Li, L.G. Li, G.Q. Li, H. Liu, S.W. Chen, Phase transformation and enhanced photocatalytic activity of S-doped Ag2O/TiO2 heterostructured nanobelts, Nanoscale, 6 (2014) 4698-4704. [37] W. Xia, C. Mei, X. Zeng, G. Fan, J. Lu, X. Meng, X. Shen, Nanoplate-Built ZnO Hollow Microspheres Decorated with Gold Nanoparticles and Their Enhanced Photocatalytic and Gas-Sensing Properties, ACS Appl. Mater. Interfaces, 7 (2015) 11824-11832.
Figure Captions Fig. 1 XRD patterns of the as-prepared samples: (a) Bi2WO6, (b-d) Bi2S3 NCs/ Bi2WO6 hybrids prepared under thiourea, cysteine and TAA, respectively. Fig. 2 XPS survey spectrum of the (a) T-S/W composite microspheres, and high-resolution spectra of (b) W, (c) Bi and (d) S. Fig. 3 FE-SEM images of the as-prepared samples: Bi2WO6/Bi2S3 hybrids prepared under (a, b) TAA, (c, d) thiourea and (e, f) L-cysteine, respectively. Fig. 4 (a, b) TEM image of Bi2WO6 nanosheet-built microspheres; and (c, d) TEM images, (e) HRTEM image and (f) SAED pattern of T-Bi2S3/Bi2WO6 microspheres. Fig. 5 UV-Vis diffuse reflectance spectra of the samples: Bi2S3/ Bi2WO6 hybrids synthesized using (a) thiourea, (b) cysteine, (c) TAA, and (d) Bi2WO6. Fig. 6 (A) Photocatalytic decomposition of RhB over different samples under visible light irradiation: (a) pristine Bi2S3, Bi2S3/Bi2WO6 microspheres synthesized using (b) cysteine, (c) pristine Bi2WO6, (d) thiourea, (e) TAA; and (B) the time-dependence adsorption spectrum of RhB over the T- Bi2S3/Bi2WO6 hybrid. Fig. 7 Photocatalytic decomposition of RhB over different samples under visible light irradiation: (a) Bi2WO6 nanoplate, (b) Bi2WO6 nanoplate/Bi2S3, (c) nanosheet-assembled Bi2WO6 microspheres and (d) T- Bi2S3/Bi2WO6 microspheres. Fig. 8 The plots of photogenerated carrier trapping in the system of photodegradation of RhB by T-W/S microspheres under visible light irradiation: (a) No scavenger, (b) Isopropyl, (c) 1,4-benzoquinone and (d) Triethanolamine. Fig. 9 (a) Structural basic units of the Bi2WO6 model. (b) Schematic illustration of the inner electric field in (020) facets of the Bi2WO6. (c) Surface atomic configurations in the (020) facets of the Bi2WO6. Fig. 10 PL spectra of (a) the used Bi2WO6, (b) Bi2S3/Bi2WO6-TAA, and (c) the Bi2WO6 nanoplate/Bi2S3 (λex=220 nm). Fig. 11 Schematic illustration of band energy positions and the charge transfer process of the T-Bi2S3/ Bi2WO6 hybrid under visible light irradiation.
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S0169-4332(16)32135-3 http://dx.doi.org/doi:10.1016/j.apsusc.2016.10.039 APSUSC 34133
To appear in:
APSUSC
Received date: Revised date: Accepted date:
28-7-2016 5-10-2016 6-10-2016
Please cite this article as: Long Yan, Yufei Wang, Huidong Shen, Yu Zhang, Jian Li, Danjun Wang, Photocatalytic activity of Bi2WO6/Bi2S3 heterojunctions: the facilitation of exposed facets of Bi2WO6 substrate, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.10.039 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Photocatalytic activity of Bi2WO6/Bi2S3 heterojunctions: the facilitation of exposed facets of Bi2WO6 substrate
Long Yana,b, Yufei Wanga, Huidong Shenb, Yu Zhangb, Jian Lia, Danjun Wangb*
a
School of Chemistry and Chemical Engineering, Yulin University, Shaanxi Key
laboratory of Low Metamorphic Coal Clean Utilization, Yulin71900, China b
School of Chemistry and Chemical Engineering, Yan’an University, Shaanxi Key
Laboratory of Chemical Reaction Engineering, Yan’an 716000, China *Correspondent author: Tel: +86 912 38911 44; Fax: +86 912 3689959; E-mail: [email protected]
Graphical Abstract
Highlights: 1. Bi2S3/Bi2WO6 hybrids with exposed (020) Bi2WO6 facets have been synthesized. 2. X-ray photoelectron spectroscopy reveals that a small amount of Bi2S3 was formed. 3. The enhanced photoactivity of hybrids is due to heterojunction and (020) facets. 4. A possible photocatalytic degradation mechanism is proposed.
Abstract Bi2S3/Bi2WO6 hybrid architectures with exposed (020) Bi2WO6 facets have been synthesized via a controlled anion exchange approach. X-ray photoelectron spectroscopy (XPS) reveals that a small amount of Bi2S3 was formed on the surface of Bi2WO6 during the anion exchange process, thus leading to the transformation from the Bi2WO6 to Bi2S3/Bi2WO6. The RhB aqueous solution was chosen as model organic pollutants to evaluate the photocatalytic activities of the Bi2S3/Bi2WO6 catalysts. Under visible light irradiation, the Bi2S3/Bi2WO6-TAA displayed the excellent visible light photoactivities compared with pure Bi2S3, Bi2WO6 and other composite
photocatalysts.
The
efficient
photocatalytic
activity
of
the
Bi2S3/Bi2WO6-TAA composite microspheres was ascribed to the constructed heterojunctions and the inner electric field caused by the exposed (020) Bi2WO6 facets. Active species trapping experiments revealed that h+ and O2•- are the main active species in the photocatalytic process. Furthermore, the as-obtained photocatalysts showed good photocatalytic activity after four recycles. The results presented in this study provide a new concept for rational design and development of high-efficient photocatalysts. Keywords: Bi2S3/Bi2WO6, heterujunciton, exposed facet, inner electric field, photocatalysis
1. Introduction Semiconductor
nanocrystals,
which
display
a
wide
range
of
novel
chemical/physical properties, have drawn ever growing interest for their applications in energy conversion[1, 2], supercapacitors[3, 4] and photocatalysis[5, 6]. In particular, the semiconductor photocatalysis has attracted intensive attention for its application in treating environment and energy issues [7, 8]. Recently, Bi2WO6, as a layered structure compound, has been extensively investigated because of its nontoxicity, stability and excellent photoactivities [9, 10]. Its crystal structure is constructed by an intergrowth of WO42- ions inserted between (Bi2O2)2+ layers. Because of the unique layered structure, the Bi6s and O2p levels can form largely dispersed hybridized valence band (VB), which favors the mobility of photogenerated holes and the oxidation reaction[11, 12]. However, the fast recombination of photoinduced electron-hole pairs limits its further practical application. Therefore, it is necessary to explore effective strategies to enhance the separation efficiency of photoexcited charge carriers, such as doping[13], noble metal deposition[14], morphology control[15] and construction heterojunctions[16]. Among them, coupling with two kinds of semiconductor into heterojunctions has been found an effective approach. Bi2S3 with narrow band gap (1.3 eV) has attracted wide attention in many significant applications [17, 18]. So far, lots of Bi2S3-based photocatalysts were prepared to enhance the charge separation and photocatalytic activity, such as Bi2S3/In2S3[19], Bi2S3/CdS[20] and Bi2S3/BiOCl composite[21]. Therefore, it is meaningful to combine both advantages of the Bi2S3 and Bi2WO6 materials in constructing Bi2S3/Bi2WO6 composites. Recently, Ye and Sun et al. synthesized 2D plate-like Bi2S3/Bi2WO6 heterojunction and explored their photocatalytic application, respectively [22, 23]. The formation of Bi2S3/Bi2WO6 heterostructures might favor the separation and transfer of photoinduced carriers, and improve the photocatalytic activity. Up to date, the morphology and crystal-facet-tuned synthesis of the semiconductor photocatalysts have attracted great attention because of the potential
application for improving the photocatalytic activities [24, 25]. For example, Zhao et al. prepared BiVO4-TiO2 heterojunctions with the {110} facet of BiVO4 and investigated its photocatalytic mechanism[26]. It was found that the heterojunction interface, which served as the channel for charge transfer, plays a crucial role in the separation of photoinduced charge carriers. Furthermore, the efficient transportation of the heterojunction interface is largely effected by the exposed facets of the photocatalysts. Thus, fabricating Bi2S3/Bi2WO6 heterojunction with exposed specific active facets is an effective way to develop excellent visible light responsive photocatalysts. Herein, a novel Bi2S3/Bi2WO6 hybrid microspheres with exposed (020) Bi2WO6 facets were synthesized by a facile anion exchange route. In addition to providing Bi3+ ions, Bi2WO6 substrates also play a role of a framework to construct the Bi2S3 /Bi2WO6 hybrid. Controlled release of S2- ions from various sulfur sources to react with Bi2WO6 by ion exchange effects the dispersion and size of Bi2S3 nanoparticles. The photoactivity of the prepared Bi2S3 /Bi2WO6 hybrid was evaluated by the degradation of RhB solution under visible light irradiation. Furthermore, the mechanism of the enhancement of photocatalytic activity of the Bi2S3 /Bi2WO6-TAA was discussed in detail. 2. Experimental section 2.1 Preparation of Bi2WO6 photocatalysts All the reagents were of analytical purity and were used as received without further purification. The nanosheet-assembled Bi2WO6 microspheres were prepared by a hydrothermal process[27]. In a typical synthesis, 4 mmol of Bi(NO3)3·5H2O and 2 mmol of Na2WO4·2H2O were added to 30 mL of deionized water under magnetic stirring, respectively. After that, the Na2WO4 aqueous solution was dropped to the Bi(NO3)3 suspension slowly. After being stirred for 30 min, the resulting precursor suspension was transferred into a Teflon-lined stainless steel autoclave (V=100 mL). The autoclave was sealed and maintained at 180 °C for 12 h and then allowed to cool to room temperature naturally. The product was filtered off, washed several times with absolute alcohol and distilled water, and finally dried at 80 °C overnight. As a
comparison, the Bi2WO6 nanoplates were also prepared and the corresponding products were denoted as P-Bi2WO6 (shown in the ESI). 2.2 Preparation of Bi2S3 /Bi2WO6 composite photocatalyst For synthesis of Bi2S3/Bi2WO6 composite microspheres, the prepared nanosheet-built Bi2WO6 microsphere was introduced to the three different 60 mL of aqueous solutions containing thiourea (2 mmol), cysteine (2 mmol) and thiacetamide (TAA, 2 mmol) with constant stirring for 3 h at room temperature, respectively. The corresponding composites were denoted as t-S/W microspheres, c-S/W microspheres and T-S/W microspheres. For comparison, the Bi2S3/Bi2WO6 nanoplate hybrids (denoted as T-S/W nanoplates) were synthesized by adding the prepared Bi2WO6 nanoplate into the TAA solution, while the other conditions remain the same to the preparation of T-S/W composite microspheres. For the control experiment, pristine Bi2S3 was prepared by reacting 4 mmol of Bi(NO3)3·5H2O and 60 mL aqueous solution of TAA (2 mmol) at room temperature. In addition, the N-doped P25 reference catalysts were synthesized based on the process as previously reported[28]. 2.3 Characterization The structure and crystallinity of the obtained samples were analyzed by powder X-ray diffraction (XRD) patterns on a Bruker AXS D8 advance powder diffractometer (Cu Kα X-ray radiation, λ = 0.154056nm). X-ray photoelectron spectroscopy (XPS) was carried out on an ESCALAB II XPS system with a monochromatic Mg Kα source and a charge neutralizer. The scanning electron microscopy (SEM) images were obtained on a Hitachi S-4800 microscope with an accelerating voltage of 7.0 kV. The transmission electron microscopy (TEM) and high resolution TEM (HRTEM) images were performed by a JEOL JEM-2100 instrument. UV–Vis diffuse reflectance spectra were collected on a spectrophotometer (UV2550, Shimadzu Corporation, Kyoto, Japan) equipped with an integrating sphere. The BaSO4 was used as a reflectance standard. Photoluminescence (PL) measurements were carried out on a fluorescence spectrophotometer (RF-5301 PC, Shimadzu, Japan) at room temperature. 2.4 Photocatalytic evaluation The photocatalytic activities of the as-prepared catalysts were evaluated by
decomposition of RhB under visible light irradiation at room temperature. The photocatalytic experiments were performed with the sample powder (30 mg) suspended in RhB (100 mL, 10-5 M) with constant stirring. Prior to visible light irradiation, the suspensions were stirred in the dark for 1 h to insure the adsorption/desorption equilibrium. The suspension was then irradiated with visible light emitted by using a 300 W Xe lamp with a 420 nm cutoff filter. The reaction temperature was kept at room temperature to prevent any thermal catalytic effect. At the given time intervals, the concentration of RhB aqueous solution was analyzed at 553 nm on a UV-Vis spectrophotometer (UV2550, Shimadzu Corporation, Kyoto, Japan). 2.5 Detection of reactive species In the process of photocatalytic degradation of target pollutants, hydroxyl radicals (•OH), electron (e-), holes (h+) and superoxide radical anions (O2•-) are the major active species. In order to determine the reactive radical species involved in the degradation of RhB, trapping experiments were performed via introducing various scavengers such as isopropyl alcohol (IPA, 10 mM), triethonoamine (TEOA, 6 mM) and 1,4-benzoquinone (BQ, 0.1 mM) into the reaction system to scavenge the •OH, h+ and O2•- species. The trapping experiment procedures were similar to that of the degradation experiment, with the various scavengers introduced separately into the aqueous substrate solution before addition of the photocatalysts.[29]
3. Results and discussion 3.1 Characterization of the prepared samples
First of all, Bi2WO6 with nanosheet-assembled architectures and nanoplates were synthesized under hydrothermal conditions. The corresponding SEM images are in shown in Fig. S2. Due to the rather lower solubility of to Bi2S3 (Ksp=1×10-91), Bi2WO6 can adopt a thermodynamically favored direction to yield Bi2S3 when it exchanges anion with S2- ions in solution[23]. The purity and phase structure of the prepared samples were examined by XRD. Fig. 1a shows the XRD pattern of the flower-like Bi2WO6 microspheres and
Bi2S3/Bi2WO6 composites. It can be observed that the XRD patterns of pure Bi2WO6 can be well indexed to the orthorhombic Bi2WO6 (JCPDS 39-0256). In the patterns of the Bi2S3/Bi2WO6 (Fig. 1b-d), however, no diffraction peaks assigned to Bi2S3 were found, which may be result from its high dispersity and low content of Bi2S3 component[30, 31]. In order to investigate the chemical states and surface composition of the Bi2S3/Bi2WO6 composites, the XPS analysis was performed out. As shown in Fig. 2, the survey spectrum shows the presence of Bi, W, O and S elements. Besides, a carbon peak at a binding energy of about 284.9-285.9 eV is clearly observed because of the carbon tape used for fixing the sample and from the adsorption of atmospheric CO2 on the sample surface.[29] Fig. 2b presents the high resolution XPS spectrum of W element. The binding energies of W4f5/2 and W4f7/2 are 37.6 eV and 35.5 eV in the oxide form of Bi2WO6, which can be attributed to the W atoms existing in a +6 oxidation state.[32] Peaks at 158.7, 161.7, 162.7 and 164.0 eV due to Bi4f7/2, S2p3/2, S2p1/2 and Bi4f5/2 can be detected, which demonstrates that the main state of Bi in the samples was +3, and that of S was -2.[33] These results illustrate the formation of Bi2S3 in the T-S/W composite microspheres. The morphologies of the as-prepared Bi2S3/Bi2WO6 hybrids are characterized by SEM analysis, as shown in Fig. 3. The as-obtained Bi2WO6 sample has a sphere-like structure, assembled from rectangular nanosheets with diameters of about 2 µm and thickness of 30 nm (Fig. S2a). Also it can be observed (Fig. S2b) that the nanosheet-assembled structures were destroyed when urea was introduced in the hydrothermal synthesis process. Moreover, it can be seen that the surface of the Bi2WO6 nanosheets are very smooth and clean. As shown in Fig. 3a-b, a large number of Bi2S3 nanoparticles were assembled on the surface of Bi2WO6 nanosheets. It is found that during the course of anion exchange reaction, the morphology of Bi2WO6 architectures was retained in the final Bi2S3/Bi2WO6 hybrid. As the relative reactivity follows the order TAA>cysteine>thiourea, the higher reactivity of the sulfur source engenders the faster growth rate of Bi2S3. TEM and SAED analyses were used to further investigate the phase structure of
the Bi2WO6 nanosheet-assembled microspheres and T-B/S heterojunctions. As shown in Fig. 4a, the Bi2WO6 sample exhibits hierarchical architectures with a size of about 2 µm built by nanosheets. In HRTEM image (Fig. 4b) of a single nanosheet, the distances between adjacent lattice fringes are measured as 2.83 and 1.98 nm, respectively. This value corresponds to the interplanar distances of Bi2WO6 (200) and (202), respectively. The set of diffraction spots can be indexed as the [010] zone axis of orthorhombic Bi2WO6, indicating that the exposed facets are characterized by (020) facets. In the HRTEM image of the T-B/S heterojunctions (Fig. 4f), It can be clearly observed that some discrete Bi2S3 nanoparticles with the size in the range of 10-15 nm are anchored on the surface of Bi2WO6 nanosheets. This result further demonstrates that the T-B/S heterojunctions was formed in the composite, which is in good agreement with the result of SEM and XPS analyses. The formation of intimate interface contact is significant for promoting the charge separation to achieve high photocatalytic activity in the coupling system. UV-vis diffuse reflectance spectra Fig. 5 presents the UV-Vis diffuse reflectance spectra of the as-prepared samples. The white Bi2WO6 nanosheet-assembled sample exhibits visible light response with an absorption edge around 450 nm, corresponding to the band gap energy of 2.75 eV. Compared to Bi2WO6 nanosheet-assembled sample, the photo absorption ability of T-Bi2S3/Bi2WO6 hybrids was enhanced in the range of 350-600 nm. In particular, the photo absorption of Bi2S3/Bi2WO6 hybrids synthesized using thiourea and cysteine are similar to that of Bi2WO6 nanosheet-assembled sample. The result indicates that the T-Bi2S3/Bi2WO6 hybrids have a suitable band gap for photocatalytic decomposition of organic pollutants under visible light illumination. Photocatalytic Activities
The photocatalytic activities of the as-prepared catalysts were demonstrated by decomposition of RhB aqueous solution under visible light illumination. As shown in Fig. 6, RhB solution kept most of its initial concentration under visible light irradiation in the absence of photocatalysts. Besides, Bi2WO6 could decompose 47.1% of RhB under irradiation for 150 min. While in the case of the Bi2WO6/Bi2S3
microspheres synthesized using TAA as the sulfur source, the photocatalytic decomposition efficiency can reach as high as about 83 % under the same conditions, showing superior photocatalytic activity as compared to that of the pure Bi 2WO6. It can also be found that the degradation rate of RhB over the as-obtained Bi2WO6/Bi2S3 composite microspheres after visible light irradiation decreased in the order of TAA > thiourea > cysteine. The aggregation of Bi2S3 particles is expected to weaken the photocatalytic activities due to the introduction of inter-particle interfaces. On the other hand, smaller Bi2S3 particles may improve the catalytic performance owing to the increase of surface area. In addition, as shown in Fig. 7B, the blue-shift in the absorption profile suggests that degradation process is a stepwise deethylation rather than photocatalytic degradation route [34]. Meanwhile, in order to investigate the photocatalytic activity of different morphologies with the same Bi2S3 content loaded on the Bi2WO6, the Bi2S3 modified Bi2WO6 nanoplate in the presence of TAA are also used to degrade the RhB. And the results are shown in Fig. 7. 46% and 87% of RhB were degraded by Bi2WO6 nanoplates and nanosheet-assembled microspheres, respectively. When the two Bi2WO6 photocatalysts were modified by the Bi2S3 prepared in the presence of TAA, the photocatalytic performances of the corresponding heterojunction were enhanced. Specially, it can be observed that the degradation efficiency of the Bi2S3/Bi2WO6 microspheres is much higher than that of the Bi2S3/Bi2WO6 nanoplates. The larger specific area of Bi2WO6 microspheres is easier and more uniform for Bi2S3 nanoparticles to than that of the Bi2WO6 nanoplates. Investigation of photocatalytic enhanced mechanism Radical and hole trapping experiments were performed to detect the main oxidative species in the photocatalytic process to reveal the photocatalytic mechanism. As is shown in Fig. 8, compared with the addition of IPA, the photocatalytic activity in the T-Bi2WO6/Bi2S3 microspheres is greatly suppressed by the addition of TEOA and BQ, respectively. This result indicates that h+ and O2•- are the main active species and predominates in oxidizing the adsorbed RhB pollutants. According to the reported literatures[35], the valence band (EVB) and conduction
band (ECB) of Bi2WO6 were 3.38 and 0.40 eV, respectively. The positions of EVB and ECB of Bi2S3 were 1.38 and -0.76 eV, respectively[20]. Band positions suggest that Bi2WO6 and Bi2S3 have the staggered energy potentials. Accordingly, the photogenerated carriers could transfer easily between the Bi2WO6 and Bi2S3 due to the band energy potential difference, which enhanced the separation efficiency of the photoinduced electron-hole pairs[36]. To understand the (020) facet-dependent enhanced photocatalytic activities of the Bi2S3/Bi2WO6-TAA composite microsphere, the crystal structure and surface atomic configurations of the (020) facets of Bi2WO6 were investigated. As shown in Fig. 9a, along the [010] direction, the [Bi2O2]2+ and WO42- induce the presence of an inner electric field, improving the separation efficiency of the photoinduced charge carriers. Furthermore, through ion exchange reaction between Bi2WO6 architectures and sulfur sources, Bi2S3 was in situ anchored on the surface of Bi2WO6 microspheres. As schematically shown in Fig. 12b, photoexcited electrons from Bi2S3 nanoparticles are more favorable to be transferred to the surface of the Bi2WO6. Therefore, the charge separation and transfer of the Bi2S3/Bi2WO6-TAA composite microsphere is high as compared to that of the Bi2WO6 nanoplate/Bi2S3. Fig. 12c shows the surface atom structure of the Bi2WO6 (020) facets. The (020) facets contains a large number of oxygen atoms, which is beneficial for the adsorption of cationic RhB solution. In order to further demonstrate the enhancement of the separation efficiency of the photoinduced charge carriers, the photoluminescence (PL) measurements of the Bi2WO6, Bi2S3/Bi2WO6-TAA and Bi2WO6 nanoplate/Bi2S3 were performed. Lower fluorescence emission intensity implies a lower electron-hole recombination rate and thus corresponds to a higher photocatalytic activity. [37] The corresponding results are demonstrated in Fig. 10. It is clearly seen that the luminescence efficiency from the Bi2S3/Bi2WO6-TAA composites is lower than that of bare Bi2WO6, indicating that the Bi2S3 nanoparticles could serve as an effective hole-accepting material, inhibiting a direct recombination of electrons and holes within the bulk Bi2WO6. In addition, as compared to the Bi2WO6 nanoplate/Bi2S3, the Bi2S3/Bi2WO6-TAA show low PL intensity, demonstrating the significant role of the properties of Bi2WO6 substrate.
Based on the above results, the exposed (020) facets of the Bi2WO6 could enhance the separation of photogenerated charge carriers because of the presence of inner electric field, which may finally improve the photocatalytic activity. Meanwhile, as shown in Fig. 11, under visible light irradiation, photogenerated electrons on the surface of Bi2S3 would transfer into the ECB of Bi2WO6 due to the more negative ECB of Bi2S3. Then, the transferred electrons reacted with molecular oxygen to form O2·-. Simultaneously, photoinduced holes of the Bi2WO6 would transfer into Bi2S3, because the EVB of the Bi2WO6 is more positive than that of Bi2S3. Thus, the fast combination of photoinduced electron-hole pairs was effectively restrained. Moreover, the highly dispersed state of Bi2S3 could suppress the formation of interfacial recombination centers. As a consequence, the separation of photogenerated electron-hole pairs is significantly improved.
4. Conclusions In summary, a novel Bi2S3/Bi2WO6 composite microspheres with exposed (020) Bi2WO6 facets were synthesized by a controlled anion exchange approach. The obtained
Bi2S3/Bi2WO6-TAA
composite
microspheres
exhibited
improved
photocatalytic activities as compared to those of the pure Bi2WO6 and Bi2S3 under visible
light
irradiation.
The
enhanced
photocatalytic
activities
of
the
Bi2S3/Bi2WO6-TAA microspheres could be attributed to the inner electric field cased by the exposed (020) facets of Bi2WO6 and the heterojunction photocatalytic system. h+ and O2•- are the main active species in the photocatalytic process and the Bi2S3/Bi2WO6-TAA microspheres showed excellent stability under visible light irradiation.
Based
on
the
efficient
and
stable
photocatalytic
activities,
Bi2S3/Bi2WO6-TAA microspheres can be regarded as promising materials for photocatalytic waste treatment. The experimental results illustrate that both the inner electric field caused by the heterojunction with matched energy levels and the exposed facet in the composite photocatalysts are beneficial for the enhancement of photoactivity, which provides a new concept for rational design and development of high-efficient photocatalysts.
Acknowledgement This work is financially supported by Key Laboratory research for the Open Fund of Shaanxi Province, China (2014SKL-DBM008).
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Figure Captions Fig. 1 XRD patterns of the as-prepared samples: (a) Bi2WO6, (b-d) Bi2S3 NCs/ Bi2WO6 hybrids prepared under thiourea, cysteine and TAA, respectively. Fig. 2 XPS survey spectrum of the (a) T-S/W composite microspheres, and high-resolution spectra of (b) W, (c) Bi and (d) S. Fig. 3 FE-SEM images of the as-prepared samples: Bi2WO6/Bi2S3 hybrids prepared under (a, b) TAA, (c, d) thiourea and (e, f) L-cysteine, respectively. Fig. 4 (a, b) TEM image of Bi2WO6 nanosheet-built microspheres; and (c, d) TEM images, (e) HRTEM image and (f) SAED pattern of T-Bi2S3/Bi2WO6 microspheres. Fig. 5 UV-Vis diffuse reflectance spectra of the samples: Bi2S3/ Bi2WO6 hybrids synthesized using (a) thiourea, (b) cysteine, (c) TAA, and (d) Bi2WO6. Fig. 6 (A) Photocatalytic decomposition of RhB over different samples under visible light irradiation: (a) pristine Bi2S3, Bi2S3/Bi2WO6 microspheres synthesized using (b) cysteine, (c) pristine Bi2WO6, (d) thiourea, (e) TAA; and (B) the time-dependence adsorption spectrum of RhB over the T- Bi2S3/Bi2WO6 hybrid. Fig. 7 Photocatalytic decomposition of RhB over different samples under visible light irradiation: (a) Bi2WO6 nanoplate, (b) Bi2WO6 nanoplate/Bi2S3, (c) nanosheet-assembled Bi2WO6 microspheres and (d) T- Bi2S3/Bi2WO6 microspheres. Fig. 8 The plots of photogenerated carrier trapping in the system of photodegradation of RhB by T-W/S microspheres under visible light irradiation: (a) No scavenger, (b) Isopropyl, (c) 1,4-benzoquinone and (d) Triethanolamine. Fig. 9 (a) Structural basic units of the Bi2WO6 model. (b) Schematic illustration of the inner electric field in (020) facets of the Bi2WO6. (c) Surface atomic configurations in the (020) facets of the Bi2WO6. Fig. 10 PL spectra of (a) the used Bi2WO6, (b) Bi2S3/Bi2WO6-TAA, and (c) the Bi2WO6 nanoplate/Bi2S3 (λex=220 nm). Fig. 11 Schematic illustration of band energy positions and the charge transfer process of the T-Bi2S3/ Bi2WO6 hybrid under visible light irradiation.
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