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Sol–gel synthesis and enhanced photocatalytic activity of doped bismuth tungsten oxide composite
Materials Research Bulletin 73 (2016) 385–393
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Sol–gel synthesis and enhanced photocatalytic activity of doped bismuth tungsten oxide composite Xuetang Xua,b , Yuanxing Gea,b , Hong Wangb , Bin Lia,b , Liuhui Yub , Yanyan Liangb , Kun Chenb , Fan Wangb,* a Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China b School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 March 2015 Received in revised form 21 August 2015 Accepted 16 September 2015 Available online 21 September 2015
Pristine Bi2WO6 and Bi2WO6/Bi14W2O27 photocatalysts were synthesized by sol–gel method using Co(II) cation as dopant. The influence of Co dopant to the formation of Bi2WO6/Bi14W2O27 heterostructure composite was discussed. The photocatalytic activities of as-prepared samples were evaluated sufficiently by using rhodamine B as target organic pollutants under visible light. The as-prepared Bi2WO6/Bi14W2O27 heterostructure achieved enhanced optical absorption in the visible-light region, and exhibited much higher photocatalytic activities than that of pristine Bi2WO6. The optimum Bi/Co molar ratio and calcining temperature were also explored. The enhanced activities were attributed to the formation of heterostructure in suppressing the recombination of photo-generated carriers. The Co dopant species would participate to reduce the charge carrier recombination by acting as trapping sites for photogenerated charges. A possible photocatalytic mechanism over Bi2WO6/Bi14W2O27 heterostructure was proposed. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: A. Composites A. Interfaces A. Oxides B. Sol–gel chemistry D. Catalytic properties
1. Introduction Semiconductor-based photocatalysts have received scientific interests due to its potential applications in energy conversion and photochemical devices [1–5]. Transition metal oxide semiconductors such as TiO2, ZnO and Fe2O3 have been employed as photocatalysts due to their low cost and high chemical stability. However, most of these photocatalysts are activated under the irradiation of ultraviolet light or low activity with visible light due to their wide band gaps, which greatly hinder the practical applications. Hence, exploration of visible-light-driven photocatalysts with good activity is demanded. Among the studied visible-light-driven photocatalysts, bismuth tungstate (Bi2WO6) was emerging as a novel candidate due partly to the intrinsic polarizability induced by the Bi 6 s2 lone pair of electrons, favoring the separation of photo-generated electron–hole pairs and the transfer of these charge carriers, while its photocatalytic activity is greatly dependent on morphology, particle size, surface area, and interface structure [6–8]. Considerable efforts had been triggered to the synthesis and photocatalytic properties of the Bi2WO6
* Corresponding author. Fax: 86 771 3233718. E-mail address: [email protected] (F. Wang). http://dx.doi.org/10.1016/j.materresbull.2015.09.024 0025-5408/ ã 2015 Elsevier Ltd. All rights reserved.
photocatalyst [9–20]. Despite of these photocatalysts being effective for the degradation of organic pollutants, presently, the design of Bi2WO6 showing a high activity under visible light is still a main challenge in the field. Recently, some work in structural designing of photocatalysts, such as surface decorating with noble metals, elemental doping and formation of heterostructure, had been dedicated to increase the photocatalytic activity of Bi2WO6. Loading noble metal nanoparticles onto the surface of photocatalysts had been proven to help promoting the charge separation and light-harvesting ability, resulting in the improvement of photocatalytic performance significantly [21–24]. The formation of heterostructure with suitable banding alignment could greatly facilitate the separation of photo-generated electron/hole pairs and suppress their recombination, and hence enhance the photocatalytic activity [25–32]. However, the use of expensive noble metals or the formation of heterostructure via multi-step reaction largely limits their practical application. Actually, doping with transition-metal ions is an efficient and facile way to modify the intrinsic properties of Bi2WO6. A fundamental principle for doping process relies greatly on the matching of ionic radius and oxidation states between Bi3+/W6+ and the substitutional dopant ions. Due to the confinement of electronic states and the tendency to occupy the sites in the crystalline structure, doped photocatalysts may be
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induced new phenomena not found in pristine materials. The general doping effect of the photocatalyst shows multiple roles ranging from tuning band gap to efficient transfer of electrons due to the suppression of electron–hole recombination and generation of nonstoichiometry to target specific optoelectronic properties. Many transition-metal ions were showed individual doping effect in the photocatalytic activities of Bi2WO6 [33–39]. Additionally, the effect of foreign metal cations on the shape evolution of metal oxides had also attracted attention [40]. Mismatched doping usually results in the formation of impurity phase, which can help us understand the substitution pattern in the lattice. However, the formation of heterostructure to improve the photocatalytic activity by the introduction of doping-induced impurity phase with mismatched foreign cations is rarely reported. In the present work, we developed a facile one-step sol–gel approach to yield bismuth tungsten oxide composite heterostructure. Sol–gel method is a versatile method for the synthesis of nanostructure with high specific surface area. In addition, the internalization of foreign metal cations during the gelation step allows cations to be distributed uniformly in the host lattice. Herein, a sol–gel route for scale production of Bi2WO6 photocatalysts was presented, and the Bi2WO6/Bi14W2O27 heterostructure photocatalysts were achieved by doping Co(II) cations into Bi2WO6. These catalysts had been evaluated for their photocatalytic activity in degradation of RhB solution under visible-light irradiation. 2. Experimental 2.1. Preparation of Bi2WO6 photocatalysts All the chemicals used were analytical grade reagents without further purified. Bi2WO6 photocatalysts were synthesized via sol– gel method. In a typical procedure, citric acid (C6H8O7H2O, 1.58 g, 7.5 mmol), and Bi(NO3)35H2O (1.21 g, 2.5 mmol) were dissolved in 1 molL1 HNO3 solution to get transparent solution A. (NH4)2WO4 (0.355 g,1.25 mmol) was dissolved in hot-water to get transparent solution B. To check the influences of Co(II) cation, an amounts of Co(NO3)26H2O (Co/Bi molar ratio = 0.05) was added into the solution A. The solution B was dropwise added to solution A under vigorous stirring. After that, 5 mL of aqueous solution containing poly(ethylene glycol) (PEG-4000, 0.5 g) was added as dispersing agent. The mixed solution was just like pale-blue hydrosol. The
transparent hydrosol was slowly evaporated at 80 C until the formation of gelatin, then keeping at 120 C for 12 h. The resulting products was crushed and calcined at 450 C for 3 h to obtain pale yellow powders. The samples were denoted as P0 for pristine Bi2WO6 and P1 for the sample prepared at Co/Bi molar ratio of 0.05, respectively. 2.2. Characterization The crystalline phases of the as-prepared samples were characterized by powder X-ray diffraction (XRD, Bruker D8 advance, CuKa radiation). The morphology and microstructures of the samples were characterized by field emission scanning electron microscope (FESEM, Hitachi SU 8020). X-ray photoelectron spectroscopy (XPS, Axis Ultra, Kratos) was used to establish the valence states and content in the samples. To correct for possible changing of the materials for the X-ray irradiation, the binding energy was calibrated using the C 1 s (284.8 eV) spectrum for the hydrocarbon that remained in the XPS analysis chamber as a contaminant. The chemical analysis of samples was taken by energy dispersive X-ray analysis equipped in FESEM. BET surface area of the samples was determined by nitrogen absorption at 77 K (Nova ASAP-2020) after degassing at 120 C for 2 h. The optical diffuse reflectance spectra (DRS) were carried out with a TU1901 UV–vis spectrophotometer and BaSO4 was used as a reflectance standard. The PL emission spectra were recorded using a Fluorescence Spectrophotometer (Shimadzu, RF-5301 PC). The samples were excited at 328 nm and the emission spectra were scanned between 400 and 800 nm wavelength ranges. 2.3. Photocatalytic test Photocatalytic activities of as-obtained samples were evaluated by the degradation of RhB solution under visible light irradiation. The photo-degradation experiments were carried out in a photoreactor refrigerated by water circulation. The reaction cell was maintained at 25 C. A tungsten lamp (200 W) was used as a light source and set about 15 cm apart from the reactor. A magnetic stirrer was used to keep the solution homogenization. For each experiment, 0.1 g of as-prepared photocatalyst was added to 100 mL of RhB solution (10 mg L1). Before illumination, the solution was magnetically stirred for 30 min in darkness to ensure
Fig. 1. (a) XRD patterns of pristine Bi2WO6 and Co-doped sample after calcined at 450 C for 3 h; (b) The enlarged patterns for selected 2u ranges of pristine Bi2WO6 and Codoped sample at Co/Bi = 0.05. The peaks marked with “*” are from Bi14W2O27 phase (JCPDS card no. 85-1286),
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the adsorption–desorption equilibrium between RhB and the photocatalyst. Afterward, 1 mL of H2O2 (30%) solution was added. The solution was then exposed to the irradiation with continuous stirring. About 5 mL suspensions were collected and centrifuged for every 15 min. The change in RhB concentration was recording by measuring the absorbance at 553 nm using a Shimadzu UV2550 UV–vis spectrometer. The degradation efficiency (DE) of the catalyst was determined using the equation DE (%) = (C0 C)/ C0 100%, where C0 is the dye initial concentration (t = 0 min) and C is the dye concentration after photo-irradiation. The blank experiment (RhB photolysis) was conducted in the presence of irradiation without any photocatalyst. 3. Results and discussion Fig. 1 shows the characteristic XRD patterns of the assynthesized samples. The XRD pattern corresponding to pristine sample (Fig. 1a) is found to match well of the orthorhombic Bi2WO6 phase (JCPDS no.79-2381), indicating the single phase nature. The broad nature of the XRD peaks indicates the presence of the smaller particle size. With the introduction of Co(II) cations, the two phases of Bi2WO6 and Bi14W2O27 (JCPDS card no. 85-1286) have both been confirmed in the XRD patterns with the main phase of orthorhombic Bi2WO6 (Fig. 1a), and no peaks corresponding to cobalt oxides are observed. The positions of XRD peaks are slightly shifted to higher angle side with the addition of Co(II) (Fig. 1b). The substitution of W in WO6 octahedra by Co to form Bi2W1xCoxO6x solid solution may be taken place because of the comparable ionic radii of Co2+ (55 pm) and W6+ (60 pm). However, such substitution is debatable [41,42]. The mismatching of oxidation state between Co2+ and W6+ should be taken into account carefully. We infer that not all the stoichiometric Co is involved in the formation of solid solution, and the rest Co species are mainly oxide and/or bismuth
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cobaltates existing as a highly dispersed cluster form over the solid surface, which could not be detected by XRD. Meanwhile, the formation of Bi14W2O27 phase was related to the introduction of Co (II) into the reaction system. The peaks emerging at 2u = 27.7, 32.1, 46.0 and 54.6 are ascribed to the (111), (2 0 0), (2 2 0) and (3 11) peaks of Bi14W2O27 (Fig. 1a). The relative intensity ratio of Bi14W2O27 (111) and Bi2WO6 (1 3 1) reflects the content of Bi14W2O27 in the mixtures. The sharper peak was observed for Bi2WO6/Bi14W2O27, suggesting the increasing size of these samples. Considering the photocatalytic activity of Bi2WO6/Bi14W2O27 composite may greatly be affected by the surface structure, morphological and surface characterizations were conducted in this work. The morphology of the obtained Bi2WO6 and Bi2WO6/Bi14W2O27 composite is shown in Fig. 2. It is evident that pristine Bi2WO6 have the spherical shape with the size ranging from 30 to 50 nm (Fig. 2a). Clearly, with the introduction of Co cations, the particles are brought up irregularly while holding the spherical shape (Fig. 2b). The roughly spherical particles connect with one another to form dense aggregates. The SEM-EDS microanalysis of Bi2WO6/Bi14W2O27 composite (P1) indicates the presence of the elements of Co, Bi and W (Fig. 2c). Based on EDS analysis, the molar ratio of Bi/W is about 1.92, which correspond well to the expected atomic ratio of Bi/W for Bi2WO6 phase. The molar ratio of Co/Bi is about 0.053, consistent with the theoretical value. Fig. 3 shows the typical TEM images of the pristine Bi2WO6 (P0) and Bi2WO6/Bi14W2O27 composite (P1) particles obtained at 450 C. P0 and P1 samples are composed of irregular nanoparticles (Fig. 3a and c). The high-resolution transmission electron microscopy (HRTEM) images (Fig. 3b and d) and the selectedarea electron diffractions (SAED) of nanoparticles exhibited a polycrystalline structure. It is clear that the interplanar spacings of
Fig. 2. SEM images of (a) pristine Bi2WO6 and (b) Bi2WO6/Bi14W2O27 composite; (c) EDS result of the Bi2WO6/Bi14W2O27 composite.
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Fig. 3. TEM and HRTEM images of (a,b) pristine Bi2WO6 and (c,d) Bi2WO6/Bi14W2O27 composite. Insets are the corresponding SEAD patterns.
0.31 nm, 0.27 nm and 0.19 nm in the Fig. 3b are in accordance with the (1 3 1), (2 0 0) and (2 0 2) planes of Bi2WO6, while the interplanar spacings of 0.31 nm and 0.32 nm in the Fig. 3d correspond to the (1 3 1) plane of Bi2WO6 and (3 1 2) plane of Bi14W2O27, respectively. Based on the above results, it can be confirmed that Bi2WO6/Bi14W2O27 heterostructure was obtained in the present work. To further investigate the microstructures of the as-prepared samples, the measurements of surface area and porosity were carried out. The nitrogen adsorption/desorption isotherms and the corresponding pore size distributions of the samples are presented in Fig. 4. The isotherms and pore size distributions showed differences for pristine and doped samples. The BET surface areas
of sample P0 and P1 are 21.1 and 17.6 m2 g1 respectively, which are consistent with those of the SEM and TEM analysis. For pristine sample, the mixture of a large fraction of nanopores and a certain amount of mesopores was formed, indicating the characteristic of sol–gel product. The fraction of mesopores increases by introdution Co dopant. The difference in the porosity and surface area should have an effect on the photocatalytic properties of the corresponding samples. The large pores in the P1 sample allow the penetration of light waves and organic molecules in solution deep into the photocatalyst, which may promote the photocatalytic activity. Bi2WO6 is the simplest aurivillius-type compound with a perovskite-like structure. The crystal structure is composed of
Fig. 4. Nitrogen sorption isotherms and pore size distributions of the as-prepared samples.
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alternatively stacked bismuth oxide (Bi2O2)2+ layers and WO6 octahedron layers along the b-axis [34]. When Co2+ ion was incorporated into the crystal lattice, Co2+ substituted the B-cation W6+ in WO6 octahedron due to the similar ionic radius. To confirm the compositions, valence states of Bi and W, and the doping behavior of Co cations, X-ray photoelectron spectroscopy (XPS) was performed. For sample P0, the binding energy peaks located at 164.5 and 159.2 eV are attributed to Bi 4f5/2 and Bi 4f7/2, respectively, which reveals that the Bi species are in the form of Bi3+ (Fig. 5a). The W 4f core-level peak exhibits a well resolved doublet corresponding to W 4f5/2 and W 4f7/2 at the binding energy values of 37.7 and 35.6 eV, respectively (Fig. 5b). The splitting energy of the 4f doublet of W is 2.1 eV, indicating the valence state of W6+. For sample P1, the Bi 4f spectrum can be deconvoluted into four peaks (Fig. 5a). The strong peaks located at 164.6 and 159.2 eV are corresponding to Bi3+ in Bi2WO6, while the weak peaks located at 163.6 and 158.3 eV, exhibiting a shift toward lower binding energy, are corresponding to a surface species containing Bi2O3. This result suggests the formation of Bi2O3 or Bi14W2O27 on the surface. This conclusion is also corroborated by XRD data. The W 4f spectrum can also be deconvoluted into four peaks (Fig. 5b). The strong peaks located at 37.9 and 35.8 eV correspond to W6+ in Bi2WO6. The weak peaks are located at 36.6 and 34.5 eV. The peak shift may be due to the formation of low-valent W oxide or the alteration in the chemical coordination environment of W by substitution of Co in WO6 octahedron, suggesting the presence of
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high amount of oxygen vacancies in the lattice. In Fig. 5c, the observed binding energy peaks of Co 2p3/2 and 2p1/2 are 780.0 and 795.4 eV, respectively. Considering the slight shift in the binding energies of Co 2p3/2 for CoO (780.4 0.2 eV) and for Co3O4 (780.0 0.2 eV), the distinction between the oxidation states of Co2+ and Co3+ among Co oxides is vague [43]. The absence of prominent shake-up satellite peaks in the Co 2p spectra suggests the formation of the Co3O4 phase on the surface of the sample after calcined in air. O 1s spectra of P0 and P1 are shown in Fig. 5d. The O 1s peak is fitted into three peaks at 529.9, 530.9 and 532.2 eV. The binding energy peaks at 529.9 and 530.9 eV are ascribed to the lattice oxygen, which is related to the BiO and W O chemical bonding in the Bi2WO6 and Bi2WO6/Bi14W2O27. The binding energy peak at 532.2 eV is attributed to the surface hydroxyl oxygen. The critical step in the photocatalytic process is the photoinduced charge carrier separation, followed by the transfer of electrons and holes to the surface of the photocatalyst to induce the photocatalytic reaction. Hence, the UV–visible absorption measurements of the obtained samples were employed. The results are showed in Fig. 6a. The pristine Bi2WO6 particles display absorption properties from the regions of UV to visible light due to the intrinsic band gap transition. The valence band of the Bi2WO6 is formed by the hybrid orbital of Bi 6 s and O 2p and the conduction band of W 5 d. As a result, the considerable absorption would extend up to the visible region. Absorption edges of sample P0 occurred at about 390 nm. An obvious red shift of about 30 nm
Fig. 5. XPS spectra of (a) Bi 4f, (b) W 4f, (c) Co 2p and (d) O 1 s for pristine Bi2WO6 and Bi2WO6/Bi14W2O27 composite.
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Fig. 6. (a) Diffuse reflectance spectra and (b) Photoluminescence spectra of pristine Bi2WO6 and Bi2WO6/Bi14W2O27 composite.
in the absorption edge can be found for Bi2WO6/Bi14W2O27 composite, indicating the potential to improve photocatalytic performances under visible light. The band-gap energies are estimated to be 2.65 and 2.44 eV for P0 and P1 using the equation lg = 1240/Eg [44], respectively. In addition, the photoluminescence (PL) emission spectrum is a facile way to investigate the efficiency of charge carrier trapping, immigration and transfer, and to understand the fate of electron–hole pairs in semiconductor photocatalysts [45]. Fig. 6b shows the PL spectra of the pristine and doped samples. Pristine Bi2WO6 achieve higher PL intensity than that of Bi2WO6/Bi14W2O27, suggesting that Bi2WO6/Bi14W2O27 heterostructure formed by Co doping can efficiently inhibit the recombination of photo-generated electrons and holes. The photocatalytic activities of the pristine Bi2WO6 and Bi2WO6/Bi14W2O27 were evaluated by the degradation of RhB dye in water under visible light irradiation. Fig. 7a shows the efficiencies of the photocatalytic degradation under visible light irradiation. RhB shows a main absorption band at 553 nm. The photolysis of RhB by itself was negligible upon visible light irradiation (l > 400 nm) because of the chemical stability. In addition, photo-degradation of the dye only with the addition of H2O2 was also negligible, suggesting the role of photocatalyst for the degradation of RhB. The DE of RhB over pristine Bi2WO6 was about 65%, indicating that the activity of the obtained Bi2WO6 via
sol–gel method was barely satisfactory. In contrast, the Bi2WO6/ Bi14W2O27 composites showed a sharp increase in RhB degradation, in which approximately 90% of RhB was degraded after illumination for 60 min. The Uv–vis absorption spectra of RhB aqueous solution after different photocatalytic durations are shown in Fig. 7b. Under visible illumination, the dye is degraded in a stepwise manner with the color changing from an initial red color to a light red. The color of the dispersion diluted after 60 min of irradiation, indicating that at least the chromophoric structure of the dye was destroyed. The “surface accessibility” is crucial to obtaining the high activity of photocatalyst. The higher surface areas contribute to higher catalytic activities of photocatalysts. However, the surface area and photocatalytic activity are quite the opposite for P0 and P1, so that to account for the formation of Bi2WO6/Bi14W2O27 composites must be invoked. Nonstoichiometric bismuth tungsten oxides, such as Bi14W2O27 (7Bi2O32WO3) and Bi24WO39 (12Bi2O3WO3), were more like to occur as the intermediate product in a hydrothermal process or byproduct in thermal treatment of sol–gel precursors [17,26,46]. Otherwise, Bi24WO39 was hydrothermally formed in alkaline solution due to the precipitation of [Bi2O2]2+ and high solubility of WO42 in alkaline solution [18,19,47,48]. Interestingly, Bi14W2O27 could be converted into Bi2WO6 by selective acidic leaching [49]. However, inadequate attention had been paid to the
Fig. 7. The photodegradation efficiencies of RhB as a function of irradiation time using Bi2WO6 and Bi2WO6/Bi14W2O27 composite as photocatalysts; (b) The UV–vis spectral changes of RhB with photocatalytic time.
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Fig. 8. Effects of different scavengers on the degradation rate of RhB without (a) and with H2O2 (b) over Bi2WO6/Bi14W2O27 composite.
physiochemical properties and the photocatalytic performance of the nonstoichiometric bismuth tungsten oxides, although Wang et al. demonstrated the efficient photocatalytic activity and mineralization capacity of Bi24WO39 under simulated solar light irradiation [47]. Recently, Chen et al. reported that Bi2WO6/ Bi24WO39 composite heterojunction exhibited an enhanced photocatalytic activity compared with single Bi2WO6 or Bi24WO39 [49]. In the present work, the improved photocatalytic activity after addition Co2+ to the photocatalytic materials may be due to the formation of the Bi2WO6/Bi14W2O27 heterostructure. When Bi2WO6 and Bi14W2O27 closely joined together, an internal space charge region at the interface of Bi2WO6/Bi14W2O27 occurred after electron transfer. For semiconductor photocatalyst particles, the mean lifetime of an electron–hole pair is on the nanosecond scale, whereas charge transfer across the interface between heterojunction can be completed within picoseconds [50]. Therefore, the formation of heterostructure is certainly of much help in suppressing both the bulk and surface recombination of photogenerated carriers, as revealing by PL results. To check the main photocatalytic mechanism is important to well understand the role of heterostructure in the present work. Generally, the photocatalytic process goes into work by photogenerated electron–hole pairs. Then, with the active processes of hole and electron, the holes can react with H2O/H2O2 on the catalyst surface to yield hydroxyl radicals (OH), and electrons can be trapped by dissolved oxygen in the solution to produce the reactive superoxide radical (O2). To understand the nature of the primary active species involved in the photocatalytic process, 0.001 mol L1 of benzoquinone (BQ), 0.01 mol L1 of tert-butyl alcohol (TBA), 0.01 mol L1 of AgNO3 and 0.01 mol L1 of EDTA were used as scavengers of the superoxide radical (O2), hydroxyl radical (OH), photogenerated e and h+, respectively. Fig. 8 shows the effects of different scavengers on the photo-degradation rate. It is worthy noted that without the aid of H2O2, the photodegradation of RhB is time consuming. To speed up the reaction rate, Xe lamp (500 W) was used as light source when H2O2 was absent (Fig. 8a). After illumination for 300 min, the DEs of RhB with and without P1 were about 63% and 8%, respectively. The addition of AgNO3 slightly stimulated the photocatalytic reaction due to the weak adsorption of Ag+ on the photocatalyst surface, and the DEs of RhB over P1 was about 55%. TBA scavenger was mildly worked after addition, and the DEs of RhB over P1 was about 31%. The addition of EDTA and BQ caused significant deactivation of the P1 photocatalyst, reducing the photocatalytic activity for the DEs of RhB to 18% and 16%, respectively. These results suggested that h+
and O2 radicals were the main active species rather than OH in the RhB photocatalytic process under visible light irradiation, in according with the previous work [25]. The possible photocatalytic processes are shown in the following step: Bi2WO6/Bi14W2O27 + visible light ! Bi2WO6/Bi14W2O27 (e + h+)(1)
h+ + RhB ! degradation
(2)
O2 + e ! O2
(3)
(4)
O2 + RhB ! degradation Meanwhile, OH radicals are from an oxidative path,
h+ + H2O ! OH + H+
(5)
Fig. 9. XRD patterns of Co-doped products at different calcining temperature. (a) 400 C; (b) 450 C; (c) 500 C; and (d) 550 C.
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Fig. 10. Effects of reaction parameter onto the photocatalytic activity of Bi2WO6/Bi14W2O27 composite: (a) calcining temperature, (b) Co/Bi molar ratio.
Upon the addition of H2O2, RhB degradation on Bi2WO6/ Bi14W2O27 became fast. The positive effect of H2O2 is in agreement with the fact that H2O2 is a stronger one-electron oxidant comparing with O2. The reduction of H2O2 to OH and the reduction of O2 to H2O2 have similar standard redox potentials [8]. Then, H2O2 would compete with O2 for e on photocatalytic process, and more OH radicals can result from the reductive path, H2O2 + H+ + e ! OH + H2O
(6)
Fig. 8b shows the results of RhB degradation obtained with different scavengers under visible light in the presence of H2O2. The addition of EDTA and TBA caused significant deactivation of the photocatalyst, reducing the photocatalytic activity for the degradation rate of RhB from 90% (60 min) to 23% and 42%, respectively. This result suggests the production of OH over the irradiated Bi2WO6/Bi14W2O27 occurs through both the reductive and oxidative pathways (Eqs. (5–6)). To provide more evidence for the role of heterostructure, the photocatalytic activities of P1 obtained at elevated calcining temperatures were examined. The calcining temperatures were 400 C, 450 C, 500 C, 550 C, respectively. As shown in Fig. 9, P1 sample shows an evolution in phase composition with the increasing calcining temperature. The sample obtained at 400 C has a low degree of crystallinity and the main phase is b-Bi2O3 (JCPDS no. 78-1793). b-Bi2O3 is thermodynamically unstable phase at ambient conditions and transforms to stable a-phase at high temperature [51]. However, in the present work, the initial b-Bi2O3
phase progresses toward a mixture of Bi2WO6 and nonstoichiometric Bi14W2O27 phase with calcining temperature. As the calcining temperature increasing, the relative intensity ratio of Bi14W2O27 (111) and Bi2WO6 (111) peaks decreases, meaning the declining content of Bi14W2O27. Such results suggested that the phase composition of Bi2WO6/Bi14W2O27 composite can be tuned by the addition of Co(II) and calcining temperature, which is benefit for the adjustment of intrinsic properties and photocatalytic activity of heterostructure. Meanwhile, the difference in specific surface areas for P1 obtained at different calcining temperature is insignificant (25.1, 17.6, 16.8 and 18.8 m2 g1 at 400, 450, 500 and 550 C, respectively). Fig. 10a shows the effect of calcining temperature on the photocatalytic activity of RhB under visible light irradiation. The RhB degradation of the composite catalysts decreased in the following order: 450 C > 500 C > 400 C 550 C. Generally, the photoactivity increases in highly crystalline photocatalysts because the density of defects caused by grain boundaries, which act as recombination centres of electrons and holes, decreases with increasing crystallinity of the particles [52]. Obviously, the content of Bi14W2O27 plays a major role in photo-degradation of RhB, in which 450 C is the optimum calcining temperature to achieve appropriate content of Bi14W2O27. Moreover, Co3O4 species on the surface, which revealed by XPS, may play a constructive role. Co2+ may react with O2 adsorbed on the surface and e to generate O2. Meanwhile, e could be trapped by Co3+ with the reduction of Co3+ to Co2+, and then the Co2+ could be oxidized back to Co3+ by the adsorbed oxygen on the surface or H2O2 in the solution. The influence of Co/ Bi molar ratios onto the photocatalytic activity is shown in Fig. 10b. The photocatalyst with Co/Bi = 0.05 exhibits the best performance. However, excess Co species might cover the active sites or act as a recombination center of Bi2WO6/Bi14W2O27, which would reduce the separation efficiency of charge carriers. The stability of as-prepared Bi2WO6/Bi14W2O27 samples was evaluated by cyclic experiments. After each cycle, the used photocatalyst was washed and then dried at 80 C for 3 h. Fig. 11 shows the results of the cyclic experiment. After the photocatalytic reaction for 4 cycles, the photocatalyst still have rather high activity, indicating good stability and the potential applicability, 4. Conclusions
Fig. 11. Cycling performance of photocatalytic degradation of RhB over Bi2WO6/ Bi14W2O27 composite.
Bi2WO6/Bi14W2O27 heterostructure was successfully synthesized via a novel sol–gel method using Co(II) as dopant. Bi2WO6/Bi14W2O27 heterostructure exhibited higher photocatalytic activity than pristine Bi2WO6 for the degradation of RhB
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under visible light irradiation. This enhanced photocatalytic activity was due to the synergistic effects coming from the interfacial interactions between Bi14W2O27 and Bi2WO6, and the efficient charge carrier transfer and separation. The h+ and O2 radicals were the main active species for photocatalytic degradation of RhB over Bi2WO6/Bi14W2O27 without H2O2. By adding H2O2,OH radicals, occurred through both the reductive and oxidative pathways, became the main active species. Moreover, Co3O4 species revealed by XPS played a constructive role to generate O2. The Bi2WO6/Bi14W2O27 heterostructure exhibited good cyclic stability. This work provided a facile way to modify the electronic property of Bi2WO6 in a simple manner and to explore the photocatalytic activity enhancement mechanism of Co-doped Bi2WO6 photocatalyst. Acknowledgements This work was financial supported by National Natural Science Foundation of China (No 21063001), the Guangxi Department of Education Project (No 2013LX006), the Dean Project of Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology (No 2014K001). References [1] H.L. Wang, L.S. Zhang, Z.G. Chen, J.Q. Hu, S.J. Li, Z.H. Wang, J.S. Liu, X.C. Wang, Chem. Soc. Rev. 43 (2014) 5234–5244. [2] J. Schneider, M. Matsuoka, M. Takeuchi, J.L. Zhang, Y. Horiuchi, M. Anpo, D.W. Bahnemann, Chem. Rev. 114 (2014) 9919–9986. [3] S.G. Kumar, K.S.R.K. Rao, RSC Adv. 5 (2015) 3306–3351. [4] J. Li, Y. Yu, L.Z. Zhang, Nanoscale 6 (2014) 8473–8488. [5] A.B. Djurisic, Y.H. Leunga, A.M.C. Ng, Mater. Horiz. 1 (2014) 400–410. [6] N. Zhang, R. Ciriminna, M. Pagliaro, Y.-J. Xu, Chem. Soc. Rev. 43 (2014) 5276–5287. [7] L.S. Zhang, H.L. Wang, Z.G. Chen, P.K. Wong, J.S. Liu, Appl. Catal. B 106 (2011) 1–13. [8] J.Y. Sheng, X.J. Li, Y.M. Xu, ACS Catal. 4 (2014) 732–737. [9] Y. Zhou, Z.P. Tian, Z.Y. Zhao, Q. Liu, J.H. Kou, X.U. Chen, J. Gao, S.C. Yan, Z.G. Zou, ACS Appl. Mater. Interfaces 3 (2011) 3594–3601. [10] M. Shang, W.Z. Wang, S.M. Sun, L. Zhou, L. Zhang, J. Phys. Chem. C 112 (2008) 10407–10411. [11] X.N. Li, R.K. Huang, Y.H. Hu, Y.J. Chen, W.J. Liu, R.S. Yuan, Z.H. Li, Inorg. Chem. 51 (2012) 6245–6250. [12] X. Ding, K. Zhao, L.Z. Zhang, Environ. Sci. Technol. 48 (2014) 5823–5831. [13] T. Saison, P. Gras, Nicolas Chemin, C. Chanéac, O. Durupthy, V. Brezová, C. Colbeau-Justin, J.-P. Jolivet, J. Phys. Chem. C 117 (2013) 22656–22666. [14] Y. Yan, Y.F. Wu, Y.T. Yan, W.S. Guan, W.D. Shi, J. Phys. Chem. C 117 (2013) 20017–20028. [15] S.M. Sun, W.Z. Wang, L. Zhang, J. Phys. Chem. C 116 (2012) 19413–19418. [16] Y.-J. Liu, R. Cai, T. Fang, J.-G. Wu, A. Wei, Mater. Res. Bull. 66 (2015) 96–100. [17] G.K. Zhang, F. Lu, M. Li, J.L. Yang, X.Y. Zhang, B.B. Huang, J. Phys. Chem. Solids 71 (2010) 579–582.
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Sol–gel synthesis and enhanced photocatalytic activity of doped bismuth tungsten oxide composite Xuetang Xua,b , Yuanxing Gea,b , Hong Wangb , Bin Lia,b , Liuhui Yub , Yanyan Liangb , Kun Chenb , Fan Wangb,* a Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China b School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 March 2015 Received in revised form 21 August 2015 Accepted 16 September 2015 Available online 21 September 2015
Pristine Bi2WO6 and Bi2WO6/Bi14W2O27 photocatalysts were synthesized by sol–gel method using Co(II) cation as dopant. The influence of Co dopant to the formation of Bi2WO6/Bi14W2O27 heterostructure composite was discussed. The photocatalytic activities of as-prepared samples were evaluated sufficiently by using rhodamine B as target organic pollutants under visible light. The as-prepared Bi2WO6/Bi14W2O27 heterostructure achieved enhanced optical absorption in the visible-light region, and exhibited much higher photocatalytic activities than that of pristine Bi2WO6. The optimum Bi/Co molar ratio and calcining temperature were also explored. The enhanced activities were attributed to the formation of heterostructure in suppressing the recombination of photo-generated carriers. The Co dopant species would participate to reduce the charge carrier recombination by acting as trapping sites for photogenerated charges. A possible photocatalytic mechanism over Bi2WO6/Bi14W2O27 heterostructure was proposed. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: A. Composites A. Interfaces A. Oxides B. Sol–gel chemistry D. Catalytic properties
1. Introduction Semiconductor-based photocatalysts have received scientific interests due to its potential applications in energy conversion and photochemical devices [1–5]. Transition metal oxide semiconductors such as TiO2, ZnO and Fe2O3 have been employed as photocatalysts due to their low cost and high chemical stability. However, most of these photocatalysts are activated under the irradiation of ultraviolet light or low activity with visible light due to their wide band gaps, which greatly hinder the practical applications. Hence, exploration of visible-light-driven photocatalysts with good activity is demanded. Among the studied visible-light-driven photocatalysts, bismuth tungstate (Bi2WO6) was emerging as a novel candidate due partly to the intrinsic polarizability induced by the Bi 6 s2 lone pair of electrons, favoring the separation of photo-generated electron–hole pairs and the transfer of these charge carriers, while its photocatalytic activity is greatly dependent on morphology, particle size, surface area, and interface structure [6–8]. Considerable efforts had been triggered to the synthesis and photocatalytic properties of the Bi2WO6
* Corresponding author. Fax: 86 771 3233718. E-mail address: [email protected] (F. Wang). http://dx.doi.org/10.1016/j.materresbull.2015.09.024 0025-5408/ ã 2015 Elsevier Ltd. All rights reserved.
photocatalyst [9–20]. Despite of these photocatalysts being effective for the degradation of organic pollutants, presently, the design of Bi2WO6 showing a high activity under visible light is still a main challenge in the field. Recently, some work in structural designing of photocatalysts, such as surface decorating with noble metals, elemental doping and formation of heterostructure, had been dedicated to increase the photocatalytic activity of Bi2WO6. Loading noble metal nanoparticles onto the surface of photocatalysts had been proven to help promoting the charge separation and light-harvesting ability, resulting in the improvement of photocatalytic performance significantly [21–24]. The formation of heterostructure with suitable banding alignment could greatly facilitate the separation of photo-generated electron/hole pairs and suppress their recombination, and hence enhance the photocatalytic activity [25–32]. However, the use of expensive noble metals or the formation of heterostructure via multi-step reaction largely limits their practical application. Actually, doping with transition-metal ions is an efficient and facile way to modify the intrinsic properties of Bi2WO6. A fundamental principle for doping process relies greatly on the matching of ionic radius and oxidation states between Bi3+/W6+ and the substitutional dopant ions. Due to the confinement of electronic states and the tendency to occupy the sites in the crystalline structure, doped photocatalysts may be
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induced new phenomena not found in pristine materials. The general doping effect of the photocatalyst shows multiple roles ranging from tuning band gap to efficient transfer of electrons due to the suppression of electron–hole recombination and generation of nonstoichiometry to target specific optoelectronic properties. Many transition-metal ions were showed individual doping effect in the photocatalytic activities of Bi2WO6 [33–39]. Additionally, the effect of foreign metal cations on the shape evolution of metal oxides had also attracted attention [40]. Mismatched doping usually results in the formation of impurity phase, which can help us understand the substitution pattern in the lattice. However, the formation of heterostructure to improve the photocatalytic activity by the introduction of doping-induced impurity phase with mismatched foreign cations is rarely reported. In the present work, we developed a facile one-step sol–gel approach to yield bismuth tungsten oxide composite heterostructure. Sol–gel method is a versatile method for the synthesis of nanostructure with high specific surface area. In addition, the internalization of foreign metal cations during the gelation step allows cations to be distributed uniformly in the host lattice. Herein, a sol–gel route for scale production of Bi2WO6 photocatalysts was presented, and the Bi2WO6/Bi14W2O27 heterostructure photocatalysts were achieved by doping Co(II) cations into Bi2WO6. These catalysts had been evaluated for their photocatalytic activity in degradation of RhB solution under visible-light irradiation. 2. Experimental 2.1. Preparation of Bi2WO6 photocatalysts All the chemicals used were analytical grade reagents without further purified. Bi2WO6 photocatalysts were synthesized via sol– gel method. In a typical procedure, citric acid (C6H8O7H2O, 1.58 g, 7.5 mmol), and Bi(NO3)35H2O (1.21 g, 2.5 mmol) were dissolved in 1 molL1 HNO3 solution to get transparent solution A. (NH4)2WO4 (0.355 g,1.25 mmol) was dissolved in hot-water to get transparent solution B. To check the influences of Co(II) cation, an amounts of Co(NO3)26H2O (Co/Bi molar ratio = 0.05) was added into the solution A. The solution B was dropwise added to solution A under vigorous stirring. After that, 5 mL of aqueous solution containing poly(ethylene glycol) (PEG-4000, 0.5 g) was added as dispersing agent. The mixed solution was just like pale-blue hydrosol. The
transparent hydrosol was slowly evaporated at 80 C until the formation of gelatin, then keeping at 120 C for 12 h. The resulting products was crushed and calcined at 450 C for 3 h to obtain pale yellow powders. The samples were denoted as P0 for pristine Bi2WO6 and P1 for the sample prepared at Co/Bi molar ratio of 0.05, respectively. 2.2. Characterization The crystalline phases of the as-prepared samples were characterized by powder X-ray diffraction (XRD, Bruker D8 advance, CuKa radiation). The morphology and microstructures of the samples were characterized by field emission scanning electron microscope (FESEM, Hitachi SU 8020). X-ray photoelectron spectroscopy (XPS, Axis Ultra, Kratos) was used to establish the valence states and content in the samples. To correct for possible changing of the materials for the X-ray irradiation, the binding energy was calibrated using the C 1 s (284.8 eV) spectrum for the hydrocarbon that remained in the XPS analysis chamber as a contaminant. The chemical analysis of samples was taken by energy dispersive X-ray analysis equipped in FESEM. BET surface area of the samples was determined by nitrogen absorption at 77 K (Nova ASAP-2020) after degassing at 120 C for 2 h. The optical diffuse reflectance spectra (DRS) were carried out with a TU1901 UV–vis spectrophotometer and BaSO4 was used as a reflectance standard. The PL emission spectra were recorded using a Fluorescence Spectrophotometer (Shimadzu, RF-5301 PC). The samples were excited at 328 nm and the emission spectra were scanned between 400 and 800 nm wavelength ranges. 2.3. Photocatalytic test Photocatalytic activities of as-obtained samples were evaluated by the degradation of RhB solution under visible light irradiation. The photo-degradation experiments were carried out in a photoreactor refrigerated by water circulation. The reaction cell was maintained at 25 C. A tungsten lamp (200 W) was used as a light source and set about 15 cm apart from the reactor. A magnetic stirrer was used to keep the solution homogenization. For each experiment, 0.1 g of as-prepared photocatalyst was added to 100 mL of RhB solution (10 mg L1). Before illumination, the solution was magnetically stirred for 30 min in darkness to ensure
Fig. 1. (a) XRD patterns of pristine Bi2WO6 and Co-doped sample after calcined at 450 C for 3 h; (b) The enlarged patterns for selected 2u ranges of pristine Bi2WO6 and Codoped sample at Co/Bi = 0.05. The peaks marked with “*” are from Bi14W2O27 phase (JCPDS card no. 85-1286),
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the adsorption–desorption equilibrium between RhB and the photocatalyst. Afterward, 1 mL of H2O2 (30%) solution was added. The solution was then exposed to the irradiation with continuous stirring. About 5 mL suspensions were collected and centrifuged for every 15 min. The change in RhB concentration was recording by measuring the absorbance at 553 nm using a Shimadzu UV2550 UV–vis spectrometer. The degradation efficiency (DE) of the catalyst was determined using the equation DE (%) = (C0 C)/ C0 100%, where C0 is the dye initial concentration (t = 0 min) and C is the dye concentration after photo-irradiation. The blank experiment (RhB photolysis) was conducted in the presence of irradiation without any photocatalyst. 3. Results and discussion Fig. 1 shows the characteristic XRD patterns of the assynthesized samples. The XRD pattern corresponding to pristine sample (Fig. 1a) is found to match well of the orthorhombic Bi2WO6 phase (JCPDS no.79-2381), indicating the single phase nature. The broad nature of the XRD peaks indicates the presence of the smaller particle size. With the introduction of Co(II) cations, the two phases of Bi2WO6 and Bi14W2O27 (JCPDS card no. 85-1286) have both been confirmed in the XRD patterns with the main phase of orthorhombic Bi2WO6 (Fig. 1a), and no peaks corresponding to cobalt oxides are observed. The positions of XRD peaks are slightly shifted to higher angle side with the addition of Co(II) (Fig. 1b). The substitution of W in WO6 octahedra by Co to form Bi2W1xCoxO6x solid solution may be taken place because of the comparable ionic radii of Co2+ (55 pm) and W6+ (60 pm). However, such substitution is debatable [41,42]. The mismatching of oxidation state between Co2+ and W6+ should be taken into account carefully. We infer that not all the stoichiometric Co is involved in the formation of solid solution, and the rest Co species are mainly oxide and/or bismuth
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cobaltates existing as a highly dispersed cluster form over the solid surface, which could not be detected by XRD. Meanwhile, the formation of Bi14W2O27 phase was related to the introduction of Co (II) into the reaction system. The peaks emerging at 2u = 27.7, 32.1, 46.0 and 54.6 are ascribed to the (111), (2 0 0), (2 2 0) and (3 11) peaks of Bi14W2O27 (Fig. 1a). The relative intensity ratio of Bi14W2O27 (111) and Bi2WO6 (1 3 1) reflects the content of Bi14W2O27 in the mixtures. The sharper peak was observed for Bi2WO6/Bi14W2O27, suggesting the increasing size of these samples. Considering the photocatalytic activity of Bi2WO6/Bi14W2O27 composite may greatly be affected by the surface structure, morphological and surface characterizations were conducted in this work. The morphology of the obtained Bi2WO6 and Bi2WO6/Bi14W2O27 composite is shown in Fig. 2. It is evident that pristine Bi2WO6 have the spherical shape with the size ranging from 30 to 50 nm (Fig. 2a). Clearly, with the introduction of Co cations, the particles are brought up irregularly while holding the spherical shape (Fig. 2b). The roughly spherical particles connect with one another to form dense aggregates. The SEM-EDS microanalysis of Bi2WO6/Bi14W2O27 composite (P1) indicates the presence of the elements of Co, Bi and W (Fig. 2c). Based on EDS analysis, the molar ratio of Bi/W is about 1.92, which correspond well to the expected atomic ratio of Bi/W for Bi2WO6 phase. The molar ratio of Co/Bi is about 0.053, consistent with the theoretical value. Fig. 3 shows the typical TEM images of the pristine Bi2WO6 (P0) and Bi2WO6/Bi14W2O27 composite (P1) particles obtained at 450 C. P0 and P1 samples are composed of irregular nanoparticles (Fig. 3a and c). The high-resolution transmission electron microscopy (HRTEM) images (Fig. 3b and d) and the selectedarea electron diffractions (SAED) of nanoparticles exhibited a polycrystalline structure. It is clear that the interplanar spacings of
Fig. 2. SEM images of (a) pristine Bi2WO6 and (b) Bi2WO6/Bi14W2O27 composite; (c) EDS result of the Bi2WO6/Bi14W2O27 composite.
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Fig. 3. TEM and HRTEM images of (a,b) pristine Bi2WO6 and (c,d) Bi2WO6/Bi14W2O27 composite. Insets are the corresponding SEAD patterns.
0.31 nm, 0.27 nm and 0.19 nm in the Fig. 3b are in accordance with the (1 3 1), (2 0 0) and (2 0 2) planes of Bi2WO6, while the interplanar spacings of 0.31 nm and 0.32 nm in the Fig. 3d correspond to the (1 3 1) plane of Bi2WO6 and (3 1 2) plane of Bi14W2O27, respectively. Based on the above results, it can be confirmed that Bi2WO6/Bi14W2O27 heterostructure was obtained in the present work. To further investigate the microstructures of the as-prepared samples, the measurements of surface area and porosity were carried out. The nitrogen adsorption/desorption isotherms and the corresponding pore size distributions of the samples are presented in Fig. 4. The isotherms and pore size distributions showed differences for pristine and doped samples. The BET surface areas
of sample P0 and P1 are 21.1 and 17.6 m2 g1 respectively, which are consistent with those of the SEM and TEM analysis. For pristine sample, the mixture of a large fraction of nanopores and a certain amount of mesopores was formed, indicating the characteristic of sol–gel product. The fraction of mesopores increases by introdution Co dopant. The difference in the porosity and surface area should have an effect on the photocatalytic properties of the corresponding samples. The large pores in the P1 sample allow the penetration of light waves and organic molecules in solution deep into the photocatalyst, which may promote the photocatalytic activity. Bi2WO6 is the simplest aurivillius-type compound with a perovskite-like structure. The crystal structure is composed of
Fig. 4. Nitrogen sorption isotherms and pore size distributions of the as-prepared samples.
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alternatively stacked bismuth oxide (Bi2O2)2+ layers and WO6 octahedron layers along the b-axis [34]. When Co2+ ion was incorporated into the crystal lattice, Co2+ substituted the B-cation W6+ in WO6 octahedron due to the similar ionic radius. To confirm the compositions, valence states of Bi and W, and the doping behavior of Co cations, X-ray photoelectron spectroscopy (XPS) was performed. For sample P0, the binding energy peaks located at 164.5 and 159.2 eV are attributed to Bi 4f5/2 and Bi 4f7/2, respectively, which reveals that the Bi species are in the form of Bi3+ (Fig. 5a). The W 4f core-level peak exhibits a well resolved doublet corresponding to W 4f5/2 and W 4f7/2 at the binding energy values of 37.7 and 35.6 eV, respectively (Fig. 5b). The splitting energy of the 4f doublet of W is 2.1 eV, indicating the valence state of W6+. For sample P1, the Bi 4f spectrum can be deconvoluted into four peaks (Fig. 5a). The strong peaks located at 164.6 and 159.2 eV are corresponding to Bi3+ in Bi2WO6, while the weak peaks located at 163.6 and 158.3 eV, exhibiting a shift toward lower binding energy, are corresponding to a surface species containing Bi2O3. This result suggests the formation of Bi2O3 or Bi14W2O27 on the surface. This conclusion is also corroborated by XRD data. The W 4f spectrum can also be deconvoluted into four peaks (Fig. 5b). The strong peaks located at 37.9 and 35.8 eV correspond to W6+ in Bi2WO6. The weak peaks are located at 36.6 and 34.5 eV. The peak shift may be due to the formation of low-valent W oxide or the alteration in the chemical coordination environment of W by substitution of Co in WO6 octahedron, suggesting the presence of
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high amount of oxygen vacancies in the lattice. In Fig. 5c, the observed binding energy peaks of Co 2p3/2 and 2p1/2 are 780.0 and 795.4 eV, respectively. Considering the slight shift in the binding energies of Co 2p3/2 for CoO (780.4 0.2 eV) and for Co3O4 (780.0 0.2 eV), the distinction between the oxidation states of Co2+ and Co3+ among Co oxides is vague [43]. The absence of prominent shake-up satellite peaks in the Co 2p spectra suggests the formation of the Co3O4 phase on the surface of the sample after calcined in air. O 1s spectra of P0 and P1 are shown in Fig. 5d. The O 1s peak is fitted into three peaks at 529.9, 530.9 and 532.2 eV. The binding energy peaks at 529.9 and 530.9 eV are ascribed to the lattice oxygen, which is related to the BiO and W O chemical bonding in the Bi2WO6 and Bi2WO6/Bi14W2O27. The binding energy peak at 532.2 eV is attributed to the surface hydroxyl oxygen. The critical step in the photocatalytic process is the photoinduced charge carrier separation, followed by the transfer of electrons and holes to the surface of the photocatalyst to induce the photocatalytic reaction. Hence, the UV–visible absorption measurements of the obtained samples were employed. The results are showed in Fig. 6a. The pristine Bi2WO6 particles display absorption properties from the regions of UV to visible light due to the intrinsic band gap transition. The valence band of the Bi2WO6 is formed by the hybrid orbital of Bi 6 s and O 2p and the conduction band of W 5 d. As a result, the considerable absorption would extend up to the visible region. Absorption edges of sample P0 occurred at about 390 nm. An obvious red shift of about 30 nm
Fig. 5. XPS spectra of (a) Bi 4f, (b) W 4f, (c) Co 2p and (d) O 1 s for pristine Bi2WO6 and Bi2WO6/Bi14W2O27 composite.
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Fig. 6. (a) Diffuse reflectance spectra and (b) Photoluminescence spectra of pristine Bi2WO6 and Bi2WO6/Bi14W2O27 composite.
in the absorption edge can be found for Bi2WO6/Bi14W2O27 composite, indicating the potential to improve photocatalytic performances under visible light. The band-gap energies are estimated to be 2.65 and 2.44 eV for P0 and P1 using the equation lg = 1240/Eg [44], respectively. In addition, the photoluminescence (PL) emission spectrum is a facile way to investigate the efficiency of charge carrier trapping, immigration and transfer, and to understand the fate of electron–hole pairs in semiconductor photocatalysts [45]. Fig. 6b shows the PL spectra of the pristine and doped samples. Pristine Bi2WO6 achieve higher PL intensity than that of Bi2WO6/Bi14W2O27, suggesting that Bi2WO6/Bi14W2O27 heterostructure formed by Co doping can efficiently inhibit the recombination of photo-generated electrons and holes. The photocatalytic activities of the pristine Bi2WO6 and Bi2WO6/Bi14W2O27 were evaluated by the degradation of RhB dye in water under visible light irradiation. Fig. 7a shows the efficiencies of the photocatalytic degradation under visible light irradiation. RhB shows a main absorption band at 553 nm. The photolysis of RhB by itself was negligible upon visible light irradiation (l > 400 nm) because of the chemical stability. In addition, photo-degradation of the dye only with the addition of H2O2 was also negligible, suggesting the role of photocatalyst for the degradation of RhB. The DE of RhB over pristine Bi2WO6 was about 65%, indicating that the activity of the obtained Bi2WO6 via
sol–gel method was barely satisfactory. In contrast, the Bi2WO6/ Bi14W2O27 composites showed a sharp increase in RhB degradation, in which approximately 90% of RhB was degraded after illumination for 60 min. The Uv–vis absorption spectra of RhB aqueous solution after different photocatalytic durations are shown in Fig. 7b. Under visible illumination, the dye is degraded in a stepwise manner with the color changing from an initial red color to a light red. The color of the dispersion diluted after 60 min of irradiation, indicating that at least the chromophoric structure of the dye was destroyed. The “surface accessibility” is crucial to obtaining the high activity of photocatalyst. The higher surface areas contribute to higher catalytic activities of photocatalysts. However, the surface area and photocatalytic activity are quite the opposite for P0 and P1, so that to account for the formation of Bi2WO6/Bi14W2O27 composites must be invoked. Nonstoichiometric bismuth tungsten oxides, such as Bi14W2O27 (7Bi2O32WO3) and Bi24WO39 (12Bi2O3WO3), were more like to occur as the intermediate product in a hydrothermal process or byproduct in thermal treatment of sol–gel precursors [17,26,46]. Otherwise, Bi24WO39 was hydrothermally formed in alkaline solution due to the precipitation of [Bi2O2]2+ and high solubility of WO42 in alkaline solution [18,19,47,48]. Interestingly, Bi14W2O27 could be converted into Bi2WO6 by selective acidic leaching [49]. However, inadequate attention had been paid to the
Fig. 7. The photodegradation efficiencies of RhB as a function of irradiation time using Bi2WO6 and Bi2WO6/Bi14W2O27 composite as photocatalysts; (b) The UV–vis spectral changes of RhB with photocatalytic time.
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Fig. 8. Effects of different scavengers on the degradation rate of RhB without (a) and with H2O2 (b) over Bi2WO6/Bi14W2O27 composite.
physiochemical properties and the photocatalytic performance of the nonstoichiometric bismuth tungsten oxides, although Wang et al. demonstrated the efficient photocatalytic activity and mineralization capacity of Bi24WO39 under simulated solar light irradiation [47]. Recently, Chen et al. reported that Bi2WO6/ Bi24WO39 composite heterojunction exhibited an enhanced photocatalytic activity compared with single Bi2WO6 or Bi24WO39 [49]. In the present work, the improved photocatalytic activity after addition Co2+ to the photocatalytic materials may be due to the formation of the Bi2WO6/Bi14W2O27 heterostructure. When Bi2WO6 and Bi14W2O27 closely joined together, an internal space charge region at the interface of Bi2WO6/Bi14W2O27 occurred after electron transfer. For semiconductor photocatalyst particles, the mean lifetime of an electron–hole pair is on the nanosecond scale, whereas charge transfer across the interface between heterojunction can be completed within picoseconds [50]. Therefore, the formation of heterostructure is certainly of much help in suppressing both the bulk and surface recombination of photogenerated carriers, as revealing by PL results. To check the main photocatalytic mechanism is important to well understand the role of heterostructure in the present work. Generally, the photocatalytic process goes into work by photogenerated electron–hole pairs. Then, with the active processes of hole and electron, the holes can react with H2O/H2O2 on the catalyst surface to yield hydroxyl radicals (OH), and electrons can be trapped by dissolved oxygen in the solution to produce the reactive superoxide radical (O2). To understand the nature of the primary active species involved in the photocatalytic process, 0.001 mol L1 of benzoquinone (BQ), 0.01 mol L1 of tert-butyl alcohol (TBA), 0.01 mol L1 of AgNO3 and 0.01 mol L1 of EDTA were used as scavengers of the superoxide radical (O2), hydroxyl radical (OH), photogenerated e and h+, respectively. Fig. 8 shows the effects of different scavengers on the photo-degradation rate. It is worthy noted that without the aid of H2O2, the photodegradation of RhB is time consuming. To speed up the reaction rate, Xe lamp (500 W) was used as light source when H2O2 was absent (Fig. 8a). After illumination for 300 min, the DEs of RhB with and without P1 were about 63% and 8%, respectively. The addition of AgNO3 slightly stimulated the photocatalytic reaction due to the weak adsorption of Ag+ on the photocatalyst surface, and the DEs of RhB over P1 was about 55%. TBA scavenger was mildly worked after addition, and the DEs of RhB over P1 was about 31%. The addition of EDTA and BQ caused significant deactivation of the P1 photocatalyst, reducing the photocatalytic activity for the DEs of RhB to 18% and 16%, respectively. These results suggested that h+
and O2 radicals were the main active species rather than OH in the RhB photocatalytic process under visible light irradiation, in according with the previous work [25]. The possible photocatalytic processes are shown in the following step: Bi2WO6/Bi14W2O27 + visible light ! Bi2WO6/Bi14W2O27 (e + h+)(1)
h+ + RhB ! degradation
(2)
O2 + e ! O2
(3)
(4)
O2 + RhB ! degradation Meanwhile, OH radicals are from an oxidative path,
h+ + H2O ! OH + H+
(5)
Fig. 9. XRD patterns of Co-doped products at different calcining temperature. (a) 400 C; (b) 450 C; (c) 500 C; and (d) 550 C.
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Fig. 10. Effects of reaction parameter onto the photocatalytic activity of Bi2WO6/Bi14W2O27 composite: (a) calcining temperature, (b) Co/Bi molar ratio.
Upon the addition of H2O2, RhB degradation on Bi2WO6/ Bi14W2O27 became fast. The positive effect of H2O2 is in agreement with the fact that H2O2 is a stronger one-electron oxidant comparing with O2. The reduction of H2O2 to OH and the reduction of O2 to H2O2 have similar standard redox potentials [8]. Then, H2O2 would compete with O2 for e on photocatalytic process, and more OH radicals can result from the reductive path, H2O2 + H+ + e ! OH + H2O
(6)
Fig. 8b shows the results of RhB degradation obtained with different scavengers under visible light in the presence of H2O2. The addition of EDTA and TBA caused significant deactivation of the photocatalyst, reducing the photocatalytic activity for the degradation rate of RhB from 90% (60 min) to 23% and 42%, respectively. This result suggests the production of OH over the irradiated Bi2WO6/Bi14W2O27 occurs through both the reductive and oxidative pathways (Eqs. (5–6)). To provide more evidence for the role of heterostructure, the photocatalytic activities of P1 obtained at elevated calcining temperatures were examined. The calcining temperatures were 400 C, 450 C, 500 C, 550 C, respectively. As shown in Fig. 9, P1 sample shows an evolution in phase composition with the increasing calcining temperature. The sample obtained at 400 C has a low degree of crystallinity and the main phase is b-Bi2O3 (JCPDS no. 78-1793). b-Bi2O3 is thermodynamically unstable phase at ambient conditions and transforms to stable a-phase at high temperature [51]. However, in the present work, the initial b-Bi2O3
phase progresses toward a mixture of Bi2WO6 and nonstoichiometric Bi14W2O27 phase with calcining temperature. As the calcining temperature increasing, the relative intensity ratio of Bi14W2O27 (111) and Bi2WO6 (111) peaks decreases, meaning the declining content of Bi14W2O27. Such results suggested that the phase composition of Bi2WO6/Bi14W2O27 composite can be tuned by the addition of Co(II) and calcining temperature, which is benefit for the adjustment of intrinsic properties and photocatalytic activity of heterostructure. Meanwhile, the difference in specific surface areas for P1 obtained at different calcining temperature is insignificant (25.1, 17.6, 16.8 and 18.8 m2 g1 at 400, 450, 500 and 550 C, respectively). Fig. 10a shows the effect of calcining temperature on the photocatalytic activity of RhB under visible light irradiation. The RhB degradation of the composite catalysts decreased in the following order: 450 C > 500 C > 400 C 550 C. Generally, the photoactivity increases in highly crystalline photocatalysts because the density of defects caused by grain boundaries, which act as recombination centres of electrons and holes, decreases with increasing crystallinity of the particles [52]. Obviously, the content of Bi14W2O27 plays a major role in photo-degradation of RhB, in which 450 C is the optimum calcining temperature to achieve appropriate content of Bi14W2O27. Moreover, Co3O4 species on the surface, which revealed by XPS, may play a constructive role. Co2+ may react with O2 adsorbed on the surface and e to generate O2. Meanwhile, e could be trapped by Co3+ with the reduction of Co3+ to Co2+, and then the Co2+ could be oxidized back to Co3+ by the adsorbed oxygen on the surface or H2O2 in the solution. The influence of Co/ Bi molar ratios onto the photocatalytic activity is shown in Fig. 10b. The photocatalyst with Co/Bi = 0.05 exhibits the best performance. However, excess Co species might cover the active sites or act as a recombination center of Bi2WO6/Bi14W2O27, which would reduce the separation efficiency of charge carriers. The stability of as-prepared Bi2WO6/Bi14W2O27 samples was evaluated by cyclic experiments. After each cycle, the used photocatalyst was washed and then dried at 80 C for 3 h. Fig. 11 shows the results of the cyclic experiment. After the photocatalytic reaction for 4 cycles, the photocatalyst still have rather high activity, indicating good stability and the potential applicability, 4. Conclusions
Fig. 11. Cycling performance of photocatalytic degradation of RhB over Bi2WO6/ Bi14W2O27 composite.
Bi2WO6/Bi14W2O27 heterostructure was successfully synthesized via a novel sol–gel method using Co(II) as dopant. Bi2WO6/Bi14W2O27 heterostructure exhibited higher photocatalytic activity than pristine Bi2WO6 for the degradation of RhB
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under visible light irradiation. This enhanced photocatalytic activity was due to the synergistic effects coming from the interfacial interactions between Bi14W2O27 and Bi2WO6, and the efficient charge carrier transfer and separation. The h+ and O2 radicals were the main active species for photocatalytic degradation of RhB over Bi2WO6/Bi14W2O27 without H2O2. By adding H2O2,OH radicals, occurred through both the reductive and oxidative pathways, became the main active species. Moreover, Co3O4 species revealed by XPS played a constructive role to generate O2. The Bi2WO6/Bi14W2O27 heterostructure exhibited good cyclic stability. This work provided a facile way to modify the electronic property of Bi2WO6 in a simple manner and to explore the photocatalytic activity enhancement mechanism of Co-doped Bi2WO6 photocatalyst. Acknowledgements This work was financial supported by National Natural Science Foundation of China (No 21063001), the Guangxi Department of Education Project (No 2013LX006), the Dean Project of Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology (No 2014K001). References [1] H.L. Wang, L.S. Zhang, Z.G. Chen, J.Q. Hu, S.J. Li, Z.H. Wang, J.S. Liu, X.C. Wang, Chem. Soc. Rev. 43 (2014) 5234–5244. [2] J. Schneider, M. Matsuoka, M. Takeuchi, J.L. Zhang, Y. Horiuchi, M. Anpo, D.W. Bahnemann, Chem. Rev. 114 (2014) 9919–9986. [3] S.G. Kumar, K.S.R.K. Rao, RSC Adv. 5 (2015) 3306–3351. [4] J. Li, Y. Yu, L.Z. Zhang, Nanoscale 6 (2014) 8473–8488. [5] A.B. Djurisic, Y.H. Leunga, A.M.C. Ng, Mater. Horiz. 1 (2014) 400–410. [6] N. Zhang, R. Ciriminna, M. Pagliaro, Y.-J. Xu, Chem. Soc. Rev. 43 (2014) 5276–5287. [7] L.S. Zhang, H.L. Wang, Z.G. Chen, P.K. Wong, J.S. Liu, Appl. Catal. B 106 (2011) 1–13. [8] J.Y. Sheng, X.J. Li, Y.M. Xu, ACS Catal. 4 (2014) 732–737. [9] Y. Zhou, Z.P. Tian, Z.Y. Zhao, Q. Liu, J.H. Kou, X.U. Chen, J. Gao, S.C. Yan, Z.G. Zou, ACS Appl. Mater. Interfaces 3 (2011) 3594–3601. [10] M. Shang, W.Z. Wang, S.M. Sun, L. Zhou, L. Zhang, J. Phys. Chem. C 112 (2008) 10407–10411. [11] X.N. Li, R.K. Huang, Y.H. Hu, Y.J. Chen, W.J. Liu, R.S. Yuan, Z.H. Li, Inorg. Chem. 51 (2012) 6245–6250. [12] X. Ding, K. Zhao, L.Z. Zhang, Environ. Sci. Technol. 48 (2014) 5823–5831. [13] T. Saison, P. Gras, Nicolas Chemin, C. Chanéac, O. Durupthy, V. Brezová, C. Colbeau-Justin, J.-P. Jolivet, J. Phys. Chem. C 117 (2013) 22656–22666. [14] Y. Yan, Y.F. Wu, Y.T. Yan, W.S. Guan, W.D. Shi, J. Phys. Chem. C 117 (2013) 20017–20028. [15] S.M. Sun, W.Z. Wang, L. Zhang, J. Phys. Chem. C 116 (2012) 19413–19418. [16] Y.-J. Liu, R. Cai, T. Fang, J.-G. Wu, A. Wei, Mater. Res. Bull. 66 (2015) 96–100. [17] G.K. Zhang, F. Lu, M. Li, J.L. Yang, X.Y. Zhang, B.B. Huang, J. Phys. Chem. Solids 71 (2010) 579–582.
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