Photocatalytic activity of PbS quantum dots sensitized flower-like Bi2WO6 for degradation of Rhodamine B under visible light irradiation

Journal of Molecular Catalysis A: Chemical 394 (2014) 309–315

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Photocatalytic activity of PbS quantum dots sensitized flower-like Bi2 WO6 for degradation of Rhodamine B under visible light irradiation Li Liu, Yafei Wang, Weijia An, Jinshan Hu, Wenquan Cui ∗ , Yinghua Liang ∗ College of Chemical Engineering, Hebei United University, Tangshan 063009, PR China

a r t i c l e

i n f o

Article history: Received 24 June 2014 Received in revised form 19 July 2014 Accepted 22 July 2014 Available online 1 August 2014 Keywords: Bi2 WO6 Quantum dots Photocatalysis PbS

a b s t r a c t A reverse-phase microemulsion process was used to prepare PbS quantum dots (PbS QDs) sensitized flower-like Bi2 WO6 . The PbS QDs possessed average diameters of 10 nm, and were tightly adhered to the surface of the Bi2 WO6 , as evidenced by characterization of the structure and composition of the resulting composite. The introduction of PbS QDs greatly improved the photocatalytic activity of the Bi2 WO6 , and the 3% PbS QDs-Bi2 WO6 composites exhibited the highest photodegradation efficiency, with almost 97% of Rhodamine B (RhB) degraded after visible light irradiation for 120 min. The enhancement of the photocatalytic activity could be attributed to the unique quantum effects of the PbS QDs, where both the utilization ratio of visible light was enhanced, and the separation of the photogenerated charge carriers at the intimately contacted interface was increased, as confirmed by the results of photoluminescence spectroscopy. Experiments using radical scavengers indicated that ?O2 − and photogenerated holes were the main reactive species. On the basis of experimental and theoretical results, a possible photocatalytic mechanism for organic pollutant degradation over PbS-Bi2 WO6 photocatalysts was proposed. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Light-responsive semiconductor materials have attracted extensive attention in photocatalytic applications such as water splitting and environmental remediation. TiO2 is the most widely investigated semiconductor due to its low cost, environmentally benign properties, and high photocatalytic performance [1–3]. However, it can only absorb ultraviolet light due to its wide bandgap. This disadvantage, coupled with its low inherent quantum efficiency limits its usefulness for practical solar applications [4,5]. The development of improved visible light responsive photocatalysts for environmental remediation has been a major objective in the field of photo-functional materials development [6–8]. Among the various photocatalysts proposed and investigated, Bi2 WO6 is a promising option due to its nontoxicity, wide solar response, high stability and good photocatalytic activity [9,10]. Of the Bi2 WO6 structures reported, nanoparticles, square nano-plates and some complicated 3D hierarchical structures such as flowerlike microspheres [11], nest-like structure [12], hollow microspheres [13] and octahedron-like assemblies have all been studied [14] and shown to have good photocatalytic performance. However, the fast recombination rate of photogenerated electron and holes in

∗ Corresponding authors. Tel.: +86 315 2592169; fax: +86 315 2592169. E-mail addresses: [email protected] (W. Cui), [email protected] (Y. Liang). http://dx.doi.org/10.1016/j.molcata.2014.07.029 1381-1169/© 2014 Elsevier B.V. All rights reserved.

Bi2 WO6 still limits its photocataytic performance [15]. Therefore, further efforts should be made to promote the charge separation efficiency and improve the visible light induced activity of Bi2 WO6 photocatalysts. The coupling of two or more semiconductors with appropriate band positions can effectively enhance their photocatalytic activity though improving the interfacial charge transfer and electron–hole pairs separation efficiency [16]. For example, Bi2 WO6 /BiOBr nanocomposites [17], Bi2 WO6 /TiO2 hierarchical hetero-structure [18] and the Bi2 O3 /Bi2 WO6 hollow microspheres [13] were reported to exhibit much improved photocatalytic performance under visible light irradiation than that of Bi2 WO6 . Alternatively, a promising method to address this problem is to use semiconductor quantum dots (QDs) to decorate Bi2 WO6 . One example of such a system is the CdS QDs [19] sensitized Bi2 WO6 photocatalyst, which was found to possess higher photocatalytic activity than pure Bi2 WO6 . The unique quantum effect of the incorporated QDs could enhance the visible light utilization rate and facilitate the separation efficiency of the photogenerated charge carriers [20]. Accordingly, in this study, lead sulphide (PbS) QDs sensitized Bi2 WO6 were synthesized via a facile method and investigated for their photocatalytic activity under visible light irradiation. To the best of our knowledge, there are no existing reports on the preparation and investigation of PbSsensitized flower-like Bi2 WO6 composite photocatalysts. In this study, we synthesized surface PbS QDs modified Bi2 WO6 composite photo-catalyst (QDs PbS-Bi2 WO6 ) by depositing PbS particles

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on the surface of Bi2 WO6 via a reverse microemulsion method. The effect of different PbS QDs content on the resulting photocatalytic activity was studied for the degradation of Rhodamine B dye under visible light, and a possible photocatalytic mechanism of QDs PbS-Bi2 WO6 was proposed. 2. Experimental 2.1. Photocatalyst synthesis All reagents were analytical grade and used without further purification. In a typical synthesis, 2.425 g (0.005 mol) of Bi(NO3 )3 ·5H2 O was dissolved in 40 mL distilled water under magnetic stirring at room temperature for 5 min. A separate solution containing 0.825 g (0.0025 mol) of Na2 WO4 ·2H2 O dissolved in 40 mL of distilled water was simultaneously prepared. Na2 WO4 aqueous solution was then added slowly to the above solution under continuous magnetic stirring for another 40 min. The resulting slurry was then transferred into a 150 mL Teflon-lined autoclave and maintained at 180 ◦ C for 12 h. The reactor was then cooled to room temperature naturally in air. The resulting yellow precipitate was collected by centrifugation, washed with distilled water, and dried in an oven at 80 ◦ C for 6 h. QDs PbS-Bi2 WO6 was synthesized by a reverse microemulsion method. Two micro-emulsions were prepared with the same proportion of surfactant (CTAB), oil phase (n-octane), co-surfactant (normal butanol) and water. One micro-emulsion was prepared with 0.01 mol/L Pb(CH3 COO)2 ·3H2 O and the other contained 0.01 mol/L Na2 S·9H2 O. As such, the lead-containing micro-emulsion and the sulfide-containing micro-emulsion were prepared with ratios of Pb and S kept constant at 1:1.5. The synthesized Bi2 WO6 powder was then added into the lead acetate micro-emulsion under vigorous stirring for 15 min. Subsequently, the solution was added dropwise to the other micro-emulsion system under vigorous stirring. The reaction was allowed to proceed for 6 h under stirring to obtain QDs PbS-Bi2 WO6 , and the system was then aged for 24 h under ambient conditions. Acetone and ethanol were then added to the homogeneous micro-emulsion, and the mixture was refrigerated at −4 ◦ C for 2 h to improve the demulsification efficiency. Finally, the product was centrifuged, washed and dried at 80 ◦ C for 12 h. 2.2. Characterization

250 mL (20 mg L−1 ) RhB solution. The dispersion was stirred for 30 min in dark to achieve adsorption–desorption equilibrium prior to irradiation. During the photocatalytic process under irradiation, 3 mL samples of the reaction suspension were withdrawn every 15 min and centrifuged at 10000 rpm for 6 min to remove the suspended particles. The collected supernatant solution was analyzed by a spectrophometer, using the characteristic absorption peak of RhB at 554 nm. The degradation efficiency (%) was calculated as follows: Degradation (%) =

c0 − c × 100% c0

where c0 is the initial concentration of RhB, and c is the timedependent concentration of dye upon irradiation. According to a simplified Langmuir–Hinshelwood (L–H) kinetic model [21], the following first order kinetic equation can be used to express the photocatalytic RhB degradation [20]: −ln

c c0

= kapp t

(2)

where c0 and c are the concentrations of dye in solution at times 0 and t, respectively, and kapp is the apparent first-order rate constant (min−1 ). 3. Result and discussion 3.1. Catalysts characterization Fig. 1 shows the X-ray diffraction patterns of the prepared PbS QDs, pure Bi2 WO6 , and PbS QDs-Bi2 WO6 . The characteristic diffraction peaks in the XRD patterns for PbS QDs were detected at 2 angles of 25.968◦ , 30.074◦ , 43.050◦ , 50.965◦ , and were attributed to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) crystal planes of PbS QDs. The results indicated that the PbS QDs sample had the crystalline structure of face-centered cubic PbS according to the standard data (JCPDS 65-0346). The prepared Bi2 WO6 possessed a high degree of crystallinity and the diffraction peaks were relatively acute with no evident impurity peaks observed, which demonstrated that the photocatalysts were well-crystallized. All of the X-ray diffraction peaks in the composite material were indexed to orthorhombic Bi2 WO6 (JCPDS 73-1126), indicating the lattice structure of Bi2 WO6 was not impacted when PbS QDs were introduced. Furthermore, upon PbS loading at a concentration of (1%), no obvious diffraction peaks assigned to the PbS QDs were observed. Crystal diffraction

The crystal structures of the catalysts were evaluated by powder X-ray diffraction (XRD, D/MAX2500PC, Cu K␣, 40 kV, 100 mA) scanning over the 5–80◦ 2 range. The morphologies of the catalysts were examined by scanning electron microscopy (SEM, Hitachi, s-4800) and transmission electron microscopy (TEM, JEM2010, 200 kV), respectively. UV–vis diffuse reflectance spectra were obtained using a Puxi, UV1901 spectrometer with BaSO4 as a reference and the chemical compositions of the catalysts were examined by energy dispersive X-ray spectroscopys (EDS, Thermo Noran 7). X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos Axis Ultra XPS system using monochromated Al K␣ radiation to explore the elements presents on the surface of the sample. To study the recombination of photoinduced charge carriers, photoluminescence spectra (PL, Hitachi F-7000, 250 nm) were measured. 2.3. Photocatalytic activity Rhodamine B (RhB) was used to assay photocatalytic activity of the QDs PbS-Bi2 WO6 . The visible light irradiation was provided in the system by a 250 W halide lamp (Philips) with a 400 nm wavelength cutoff filter. In each test, 0.1 g catalyst was added into a

(1)

Fig. 1. XRD patterns of series of PbS-Bi2 WO6 photocatalysts.

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311

Fig. 2. (a) SEM image of Bi2 WO6 ; (b) SEM images of 3% PbS-Bi2 WO6 photocatalyst; (c–f) TEM, HRTEM, SAED pattern and EDS images of 3% PbS–Bi2 WO6 composite.

peaks were found when the doping amount was 3%. It revealed the doping amount effects on the crystalline phase through the changes of the XRD patterns in Fig. 1, which could be attributed to the small amount of PbS QDs dopant and their highly dispersion in samples. The morphology and microstructure of the as-prepared samples were revealed by SEM. Fig. 2(a) displays the SEM image of Bi2 WO6 , the three dimensional Bi2 WO6 hierarchical architecture with average diameters of 3 ␮m, are constructed by numerous two dimensional (2D) interlaced nanosheets. As seen in Fig. 2(b), the PbS QDs were uniformly dispersed on the surface of Bi2 WO6 nanosheets and exhibited the similar morphologies with pure Bi2 WO6 , indicating that the microstructures of the composite samples did not change after introducing of PbS QDs, meanwhile, the nanosheet building blocks of Bi2 WO6 microsphere perform as clapboards to spatially separate PbS QDs. To further investigate the morphology and structural information, TEM was performed on the samples, and a representative image was shown in Fig. 2(c, d) for 3% QDs PbS-Bi2 WO6 . Fig. 2a presented the microsphere with a zigzag

circle consisted of sheet-like particles with the size of 50–100 nm, in accordance with the SEM images. Fig. 2b shows an enlarged fraction of the microsphere edge, clearly showed that it was composed of lots of sheets and the PbS QDs with an average diameter of about 10 nm were uniformly dispersed in the surface of Bi2 WO6 . The selected-area electron diffraction (Fig. 2e SAED) pattern for the [2 0 0] zone axis reveals its polycrystalline structure rather than well-defined single crystal and the polycrystalline structure of the nano-plate confirms that the nano-plates grow preferentially along the (2 0 0) and (0 0 2) plane. This anisotropic growth along the (2 0 0) and (0 0 2) plane may be ascribed to the crystal structure of PbS QDs and Bi2 WO6 . According to the Gibbs Curie Wulff theorem, the preferred (2 0 0) surface orientation of Bi2 WO6 platelets has already been observed in the case of individual plates and aggregated plates, forming flower-like Bi2 WO6 superstructure [22–24]. Fig. 2(f) shows the typical EDS spectrum obtained from 3% QDs PbSBi2 WO6 sample. In the spectrum, peaks associated with O, W, Bi, Pb, and S were observed. Pb and S peaks result from PbS, and Bi, W

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Fig. 3. UV–vis spectra of the samples; ((a) 5% QDs PbS-Bi2 WO6 , (b) 3% QDs PbSBi2 WO6 , (c) 1% QDs PbS-Bi2 WO6 , (d) pure Bi2 WO6 ).

and O result from Bi2 WO6 , respectively. The EDS results confirmed that the obtained product was QDs PbS-Bi2 WO6 . The absorption spectra of the as-synthesized composites were investigated and showed in Fig. 3. The pure Bi2 WO6 has photoabsorption from UV light to visible light and shows its fundamental

Fig. 4. Photoluminescence (PL) spectra of Bi2 WO6 and PbS–Bi2 WO6 samples (ex = 250 nm).

absorption edge rising at 460 nm, which can be assigned to its intrinsic band gap of 2.69 eV. The absorption intensity increased rapidly with the ratio of PbS/Bi2 WO6 from 1 to 5%, it is owing to the bad gap of PbS being 0.41 eV [25], which could absorb a majority of visible light. Compared with the pure Bi2 WO6 sample, the absorption edge of QDs PbS-Bi2 WO6 exhibited a red-shift, which

Fig. 5. Bi, Pb, O XPS spectra of different samples.

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is attributed to PbS act as a sensitizer to improve the visible light response. Based on the results of photocatalytic activities, the QDs PbS-Bi2 WO6 can be excited to produce more electron–hole pairs under the same visible light irradiation, which could result in higher photocatalytic performance. To further probe the effect of PbS QDs modification, the photoluminescence (PL) spectra carried out a study for Bi2 WO6 and QDs PbS-Bi2 WO6 , which is useful to reveal electron–hole pairs recombination. Fig. 4 shows the PL spectra of Bi2 WO6 and QDs PbS-Bi2 WO6 . There was one peak at about 385 nm for the Bi2 WO6 sample, which was attributed to the radiate recombination process of electron–hole pairs. The peaks positions of QDs PbS-Bi2 WO6 are similar with pure Bi2 WO6 samples, indicating that the interaction between PbS QDs and Bi2 WO6 is physical absorption. In addition, the emission band intensities of the spectra vary for the different doping amounts of PbS QDs and the PL intensity of 3% QDs PbS-Bi2 WO6 was the lowest. The results clearly indicate that this amount of PbS QDs was favorable for suppressing the recombination process, leading to weak recombination of the e− /h+ pairs and inhibiting high photon efficiency in the composite semiconductors. The X-ray photoelectron spectroscopy (Fig. 5 XPS) was carried out to determine the chemical composition of the 3.0% QDs PbSBi2 WO6 and the valence states of various species present therein. The XPS results illustrated that the samples contain Bi, O, Pb elements and present in Fig. 5. The spin-orbit components of Bi 4f peak are well deconvoluted by two curves at approximate 158.8 and 164.2 eV (Fig. 5 Bi), corresponding to Bi4+ in crystal structure [26]. From the high resolution XPS spectra of O1s for Bi2 WO6 , a broad peak was observed, which was subsequently deconvoluted into three peaks at 529.7 eV, 530.9 eV and 532.5 eV, corresponding to the adsorbed molecular H2 O, hydroxyl oxygen and lattice oxygen, respectively [27]. The photoelectron peaks for Pb4f were observed at 137.5 eV and 142.2 eV (Fig. 5 Pb), which can be assigned Pb2+ of PbS QDs. In QDs PbS-Bi2 WO6 , the Pb 4f7/2 peak was resolved into two bands at 136.8 eV and 138.1 eV were assigned to the Pb–S and Pb–O bonding structures, respectively. The photocatalytic activity of the composite photocatalyst was compared with pure PbS and Bi2 WO6 by photocatalytic dye degradation under visible-light irradiation ( > 420 nm). RhB was chosen as a representative model pollutant. The establishment of adsorption–desorption equilibrium was obtained under continuous stirring overnight before the degradation reaction was carried out. The absorption of an aqueous solution of RhB at a wavelength of 554 nm decreased under visible-light irradiation, which suggested an apparent decrease of RhB. The results from the photocatalytic degradation studies are shown in Fig. 6. From the dark control and blank test, it was found that the effects of adsorption and catalysis only were negligible on the degradation of RhB using the 3% QDs PbS-Bi2 WO6 alone in the absence of irradiation. The sample with 3% PbS QDs exhibited the highest photo-degraded efficiency and almost 97% of RhB was degraded after irradiation for 120 min, which was much higher than that of pure PbS and Bi2 WO6 , the reason should be correlated with the synergetic effect between PbS QDs and Bi2 WO6 could effective separate of the photogenerated charges. Meanwhile, it was clearly observed that when the loading amount was below 3%, the photocatalytic activities increased with the increase of loading amount of PbS QDs. However, when the loading amount of PbS QDs exceeded 3%, the photocatalytic activities of samples decreased as the amount of QDs PbS increased. At higher content, the nano-clusters of PbS QDs species would form overlapping agglomerates and smother the surface of Bi2 WO6 with affecting the absorption of visible light, therefore lowering the activity for RhB photo-degradation. The optimal loading amount of PbS QDs on Bi2 WO6 for improving the photocatalytic activity was 3%. This can be explained that RhB have a greater affinity for the surface area of QDs PbS-Bi2 WO6 as well as the effect of charge on

313

Fig. 6. Visible light induced photocatalytic degradation of RhB over various photocatalysts.

catalyst surface. Suitable content of PbS QDs led to fine particle dispersion on the Bi2 WO6 surface with high activity. To further evaluate the role of reactive species, a series of quenchers were used to scavenge the relevant reactive species [28]. Isopropanol (IPA) was added to the reaction system as an ?OH scavenger [29], benzoquinone [30] (BQ) was adopted to quench ?O2 − , and NaHCO3 was introduced as the scavenger for the adsorbed ?OH radical and holes [31]. The effects of scavengers on the degradation of RhB are shown in Fig. 7. It is found that the IPA did not obviously affect the Kapp of RhB degradation throughout the experiments, which demonstrates that ?OH is not the active species involved in the degradation process. However, after the addition of BQ and NaHCO3 , the Kapp decreased markedly, which shows that ?O2 − and photogenerated holes were the dominating active species for PbS–Bi2 WO6 system. The generation of ?O2 − may be via a photo-generated electron reacting directly with O2 adsorbed on the surface of the catalyst PbS-Bi2 WO6 . Besides the excellent visible-light-driven photocatalytic activity, the renewable photocatalytic activity after photocatalytic reactions is also important to the application of a photocatalyst [32]. To evaluate the stability and reusability of the PbS–Bi2 WO6 flower-like composites, the circulating runs in the photocatalytic degradation of RhB under visible light irradiation are also investigated. As shown in Fig. 8, the photocatalytic activity of the

Fig. 7. Kapp value of 3% PbS–Bi2 WO6 in the presence of various quenching species.

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Scheme 1. Schematic charge transfer process of the PbS–Bi2 WO6 hybrids exposed to visible light.

?O2 − radicals and the photogenerated holes, in according with the results of the radical scavenger experiments. 4. Conclusions

Fig. 8. Cycling runs in photocatalytic degradation of RhB in the presence of 3% PbS–Bi2 WO6 composite under visible light irradiation.

In summary, PbS QDs sensitized QDs PbS-Bi2 WO6 was prepared via a reversed-phase microemulsion method. The introduction of PbS QDs does not affect the crystal structure and the morphology of Bi2 WO6 and for the same time, the PbS QDs could greatly improve the photocatalytic activity. The 3% QDs PbS-Bi2 WO6 samples displayed the highest photodegradation efficiency and almost 97% of RhB was degraded after irradiation for 120 min. The high photocatalytic activity of the QDs PbS-Bi2 WO6 composites could be attributed to the strong coupling PbS QDs and Bi2 WO6 , which improve the visible light utilization rate and facilitated the photogenerated charge carriers at the Close contact interface. The photodegradation of RhB are mainly associated with ?O2 − radicals and photogenerated holes. Based on the energy band position and the experimental results, the photocatalytic mechanism was proposed. Acknowledgments

composites does not show obvious decrease after three catalytic recycles for the photodegradation of RhB. The fact implies that the PbS–Bi2 WO6 flower-like composites have high stability during the photocatalytic degradation of the model dye molecules, which is of advantage to their practical applications. It is well known that the driving force of charge transfer always originates from the matched band potentials between two semiconductors [33]. As depicted in Scheme 1, according to band potential calculations [34] and physical–chemical characterization [35], the conduction band (CB) edge of PbS (−1.19 eV) is more negative than that of Bi2 WO6 (+0.24 eV) and valence band (VB) edge of Bi2 WO6 (+2.94 eV) is more positive that of PbS (−0.78 eV). Upon exposure to visible light, both PbS and Bi2 WO6 are excited to generate the electron–hole pairs. Their single counterparts (flower-like Bi2 WO6 and PbS QDs) can generate the recombination of photoinduced electron–hole pairs. The electrons on the CB of QDs PbS were injected into flower-like Bi2 WO6 across the interface quickly, while the photo-generated holes on VB (Bi2 WO6 ) will not transfer to the VB (PbS), as was confirmed by the PL measurements. From the photoelectron chemistry point of view, the photo-generated electrons on the CB (PbS) can be captured by soluble O2 to yield ?O2 − radicals [36] (an oxidative species that can break down RhB), for the CB band of PbS is more negative than the standard redox potentials of O2 /?O2 − (−0.28 eV) [17]. Meanwhile, the photogenerated holes could serve as active sites responsible for RhB photodegradation, therefore, the degradation process was mainly associated with the

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