12866
J. Phys. Chem. C 2007, 111, 12866-12871
Synthesis of Bi2WO6 Nanoplate-Built Hierarchical Nest-like Structures with Visible-Light-Induced Photocatalytic Activity Ju Wu,†,‡ Fang Duan,† Yan Zheng,† and Yi Xie*,† School of Chemical and Material Engineering, Southern Yangtze UniVersity, Wu Xi, Jiangsu 214122, People’s Republic of China, and Chemistry and Life Science Department, West Anhui UniVersity, La An, Anhui 237015, People’s Republic of China ReceiVed: May 19, 2007; In Final Form: June 26, 2007
Bismuth tungstate hierarchical nest-like structures built by higher order nanoplate alignment have been successfully synthesized by a facile and economical method in the presence of polyvinyl pyrrolidone. The formation mechanism and effect of reaction time on the products were investigated. In addition, studies of the photocatalytic property demonstrate that the as-synthesized Bi2WO6 structures show excellent photocatalytic activity exposure to visible light irradiation. Furthermore, we first explored the electrochemical property of the Bi2WO6 nanostructure as an electrode in a lithium ion battery. Therefore, the preparation and properties studies of Bi2WO6 structures suggest potential future applications in photocatalysis by sunlight and as an electrode candidate in lithium ion batteries.
Introduction Recently, great interest in nanostructured functional materials has been focused on the morphology control of inorganic materials,1,2 such as nanotubes,3 nanowires,4,5 nanoribbons,6 diskettes,7 and hollow structures.8-10 Architectural control of nanoparticles with well-defined shapes and the alignment of nanobuilding blocks into ordered superstructures have been key issues in materials chemistry and nanotechnology.11,12 As a kind of new nanostructure with specific morphologies and higher order, hierarchical structures have been attracting researchers’ attention owing to their important role in the systematic study of structure-property relationships and their improved physical and chemical properties over their single component,13,14 which holds the advantages of both a microstructure and a nanostructure.15,16 Since the discovery of photoinduced degradation of organic compounds using Bi2WO6 (Eg ) 2.69 eV) as a photocatalyst under visible light irradiation,17-19 Bi2WO6 has received much attention as a photocatalytic material.6 Recently, Wang et al. synthesized Bi2WO6 flower-like superstructures by a facile hydrothermal process without any surfactant or template, which exhibited excellent visible-light-driven photocatalytic efficiencies for the degradation of RhB.20 For photocatalysts, the high physicochemical properties and more abundant transport paths for small molecules can be achieved by constructing organized hierarchical structures.21 Thus, special efforts to synthesize Bi2WO6 hierarchical structures with smaller sizes but a higher order is warranted. Inspired by the formation of biominerals with a special texture and morphology in nature with the existence of natural proteins, a variety of mimetic methods, especially applying polymer templates, has been designed and carried out to achieve artificial material synthesis.22 For example, Yu et al. recently reported * Corresponding author. Tel. and fax: 86-510-85917063; e-mail: yxie@ sytu.edu.cn. † Southern Yangtze University. ‡ West Anhui University.
Figure 1. XRD pattern of Bi2WO6 hierarchical net-like structures prepared at 180 °C for 24 h.
controlling the texture and morphology of the formation of CaCO3 by a double-hydrophobic block copolymer poly(ethylene glycol)-block-poly(ethylene imine)-poly(acetic acid) (PEG-bPEIPA), which preferentially adsorbed or enriched onto certain crystal faces to form oriented attachment mesocrystals.23,24 Among these polymers, polyvinyl pyrrolidone (PVP) is regarded as the simplest crystal growth modifier in the formation of inorganic ordered superstructures.25 In this work, we have successfully synthesized Bi2WO6 hierarchical nest-like structures with much smaller sizes in the presence of polymer PVP. PVP acts as a selective crystal face inhibitor to align the nanoplates, resulting in the formation of Bi2WO6 hierarchical nest-like structures, which were built by perfectly aligned nanoplates. The photocatalytic properties of the as-obtained nest-like nanostructures were investigated under visible light. To expand the applied range of Bi2WO6 besides its known applications in dielectric, ion conductive, luminescent, and photocatalyst activities,18,19 we offer the first opportunity
10.1021/jp073877u CCC: $37.00 © 2007 American Chemical Society Published on Web 08/07/2007
Bi2WO6 Nanoplate-Built Hierarchical Nest-like Structures
J. Phys. Chem. C, Vol. 111, No. 34, 2007 12867
Figure 2. XPS spectra of the Bi2WO6 nest-like structures (a) Bi 4f, (b) W 4f, and (c) O 1s.
Figure 3. Representative SEM and TEM images of Bi2WO6 hierarchical nest-like structures: (a) low magnification SEM, (b and c) high magnification SEM, (d) TEM, and (e) HRTEM. Inset is the corresponding ED pattern.
to research the potential applications of Bi2WO6 as cathode materials in lithium ion batteries enlightened by its innate layer structure character. Experimental Procedures All chemical reagents in this work were purchased from the Shanghai Chemical Company. They were of analytical grade and used without further purification. Synthesis of Bi2WO6 Hierarchical Nest-like Structures. In a typical procedure, Bi(NO3)3‚5H2O (1.212 g, 2.5 mmol) and 1 g of PVP were added to 40 mL of deionized water. Then, Na2WO4‚2H2O (0.412 g, 1.25 mmol) was added to the solution, and the mixture was vigorously stirred for 30 min to ensure that all reagents were dispersed homogeneously. The mixture was loaded into a 60 mL Teflon-lined autoclave and filled with
Figure 4. SEM images and XRD patterns of Bi2WO6 at reaction stages of (a) 1 h, (b) 2 h, (c) 3 h, (d) 6 h, and (e) 48 h and (f) XRD patterns.
deionized water up to 80% of the total volume. The autoclave was sealed, maintained at 180 °C for 24 h, and cooled to room temperature naturally. The white precipitate was collected and rinsed several times with distilled water and absolute ethanol, respectively. Then, the sample was dried in a vacuum at 60 °C for 6 h. Sample Characterization. The samples were characterized by X-ray powder diffraction (XRD) with a Japan Rigaku D/max rA X-ray diffractometer equipped with graphite monochromatized high-intensity Cu- KR radiation (λ ) 1.54178 Å), recorded with 2θ ranging from 20 to 80°. The transmission
12868 J. Phys. Chem. C, Vol. 111, No. 34, 2007 electron microscopy (TEM) images were acquired with a Hitachi Model H-800 instrument with a tungsten filament, using an accelerating voltage of 200 kV. High-resolution transmission electron microscopy (HRTEM) images and electron diffraction (ED) patterns were carried out on a JEOL-2010 transmission electron microscope at an acceleration voltage of 200 kV. The field emission scanning electron microscopy (FESEM) images were taken on a FEI Sirion-200 scanning electron microscope. X-ray photoelectron spectra (XPS) were acquired on an ESCALAB MKII with Mg KR (hν ) 1253.6 eV) as the excitation source. The binding energies obtained in the XPS spectral analysis were corrected for specimen charging by referencing C 1s to 284.5 eV. The Brunauer-Emmett-Teller (BET) surface area was determined by nitrogen adsorption (Micromeritics ASAP 2000 system). Photocatalytic Test. Photocatalytic activities of the nest-like Bi2WO6 hierarchical structures were evaluated by the degradation of rhodamine-B under visible light irradiation of a 1000 W Xe lamp with a 400 nm cutoff filter. The reaction cell was placed in a sealed black box of which the top was opened, and the cutoff filter was placed to provide visible light irradiation. In each experiment, 0.1 g of as-prepared Bi2WO6 as a photocatalyst was added into 100 mL of rhodamine-B solution (10-5 mol/L). After being dispersed in an ultrasonic bath for 5 min, the solution was stirred for one night in the dark to reach adsorption equilibrium between the catalyst and the solution and then was exposed to visible light irradiation. Concentrations of rhodamine-B were carried out using UV-vis spectra (Shimadzu UV2550) every 10 min. Electrochemical Measurements. The electrochemical properties of the Bi2WO6 hierarchical nest-like structures were investigated using the as-prepared model test cells. Test electrodes were prepared by mixing as-prepared Bi2WO6 samples (70 wt %), acetylene black (20 wt %), and polyvinylidene fluoride (PVDF) (10 wt %). A total of 10-20 mg of the mixture was pressed onto a nickel grid. The electrodes were dried at 120 °C in a vacuum furnace for 24 h before use, and lithium foil was used as the anode. The model test cells were fitted together in a glove box under an argon atmosphere. The electrolyte was a solution of 1 M LiClO4 in ethylene carbonatepropylene carbonate (1:1 mol %). For long-term cycling experiments, the cells were charged and discharged between 3 and 1 V (vs Li/Li+), using a constant current density of 0.5 mA/cm2. Electrochemical measurements were carried out using the Newware battery testing system (Newware BTS-5 V/5 mA). Results and Discussion The typical XRD patterns as shown in Figure 1 reveal the phase and purity of the as-obtained Bi2WO6 hierarchical nestlike structures. All the peaks for the sample were readily indexed to the orthorhombic phase of Bi2WO6 (JCPDS card no. 731126), with lattice constants of a ) 5.456 Å, b ) 5.436 Å, and c ) 16.426 Å (Figure 1). No characteristic peaks of the other impurities were observed. To investigate the surface compositions and chemical state of the as-prepared Bi2WO6 nest-like structures, XPS was carried out, and the results are shown in Figure 2. The binding energies obtained in the XPS analysis were corrected for specimen charging by referencing C 1s to 284.50 eV. The Bi 4f peaks of the sample appeared at ca. 163.5 and ca. 158.2 eV, which corresponded to Bi3+ according to the previous results. The W 4f peaks of the sample appeared at 36.5, 34.5, 28.1, and 25.1 eV, which was in agreement with the reported value in the literature.19 The O 1s binding energy (Figure 2C) of 529.6 eV
Wu et al. was in agreement with literature values of the bulk for Bi2WO6.17 Quantification gave the atomic ratio of Bi, W, and O, which was 2.05:1:7.30 on the basis of the areas of the Bi, W, and O peaks within experimental error. Consequently, the as-synthesized products could be determined as pure orthorhombic Bi2WO6 based on the results of XRD and XPS measurements. The morphology of the prepared samples was further investigated with FESEM, and their typical images are displayed in Figure 3. The panoramic morphology (Figure 3a) showed that the product consisted almost entirely of hierarchical nestlike structures with a diameter of 0.8-1.4 µm. Figure 3b,c shows high-magnification FESEM images of Bi2WO6 hierarchical nestlike structures from different angles. This can be vividly demonstrated by the SEM images of a hollow half-sphere fringe, from which one can clearly see that the hierarchical nest-like Bi2WO6 structures are built by two-dimensional nanoplates with a length of about 50-100 nm and a thickness of about 10 nm in a perfectly aligned manner (Figure 3b). The nanoplates are well-ordered and oriented to form a hollow half-sphere. In addition, the product was further investigated by TEM, as is shown in Figure 3d. The obvious contrast between the dark edge and the relatively bright center further confirms their hollow nature. The selective area electron diffraction (SAED) pattern that was performed on individual flat-lying nanoplates broken from hierarchical nest-like Bi2WO6 after ultrasonication (Figure 3e) reveals the well-aligned clear diffraction spots that can be indexed to the orthorhombic structure of Bi2WO6. The singlecrystalline nature and lattice parameters of the nanoplates were also confirmed by HRTEM. The spacings of the lattice fringes were found to be about 0.370 and 0.195 nm, respectively, as shown in Figure 3e, corresponding to that of the (111) and (220) planes of orthorhombic Bi2WO6. To investigate how the hierarchical nest-like structures form, several experiments through intercepting the intermediate products were performed in the different stages of 1, 2, 3, and 6 h. The intermediate products were inspected by FESEM and XRD, as shown in Figure 4. In the initial stage (1 h), the primary nanocrystals tended to aggregate together to form submicroscaled solid spheres, as presented in Figure 4a. The weak peaks are exhibited in the XRD pattern (Figure 4f), indicating its amorphous characteristics. As the reaction proceeded (2 h), the exterior of the sphere slowly cracked, and the irregular particles coexisted in the system, as seen from Figure 4b. The peaks in Figure 4f became sharp and strong, suggesting that the orthorhombic phase Bi2WO6 had crystallized at this stage. When the reaction time increased to 3 h, the solid core evacuation started at a particular region, and the flakes aligned with clearly oriented layers, pointing toward a common center, as displayed in Figure 4c. The hollow structure then gradually grew, and the irregular nanoparticles almost disappeared when the reaction time increased to 6 h (Figure 4d). Finally, when the reaction was further prolonged to 24 h, the cores in the center of the spheres dissolved completely, resulting in the formation of nest-like structures with hollow cores, as demonstrated in Figure 3. At the elongated reaction time of 48 h, the nest-like Bi2WO6 structures assembled by the compactly packed flakes were observed, as is displayed in Figure 4e. The size of the pores among the flakes decreased obviously. On the basis of the previous analysis, we proposed a reasonable mechanism for the formation of Bi2WO6 nest-like structures. It is believed that the crystal anisotropy and the faceinhibitor function of PVP play important roles in the formation of nest-like structures and that PVP could partially passivate surfaces of the crystal.26 The previous time-dependent experi-
Bi2WO6 Nanoplate-Built Hierarchical Nest-like Structures
J. Phys. Chem. C, Vol. 111, No. 34, 2007 12869
SCHEME 1: Formation Mechanism of Hierarchical Nest-like Structures Bi2WO6
Figure 5. SEM images of Bi2WO6 prepared at 180 °C for 24 h, when the amount of PVP was (a) 0 g, (b) 0.25 g, (c) 0.5 g, and (d) 2 g.
ments suggest the following evolution process: at first, when Bi(NO3)3‚5H2O and Na2WO4‚2H2O along with PVP were added to deionized water, the irregular nanoparticles formed. Then, the polymer molecule PVP anchored on nanoparticles in aqueous solution.27 Driven by the minimization of the total energy of the system and van der Waals interactions between polymer molecules,24 the small primary nanocrystals aggregated together to form submicro-scaled solid spheres. In addition, there were many free polymer molecules absorbed on the face of the submicro-scaled solid spheres.22 With prolonged hydrothermal treatment, the crystallization of Bi2WO6 began from these nucleation sites on the surface anchoring with PVP, which may provide many high-energy sites for further growth.28 Then, submicro-scaled solid spheres gradually grew inside the unstructured polymer aggregates and subsequently dissolved the nanocrystal from the inner side of the aggregates toward the outside, which led to the formation of hollow structures.11,29 In a further crystallization process, the high anisotropy characteristics of the Bi2WO6 structure were advantageous to the formation of nanoplates rather than nanoparticles. In addition, the free polymer molecules preferentially absorbed on the primary nanoplates and functioned as potential crystal face inhibitors in the system, which formed polymer interlayers on the nanoplates and benefited the formation of oriented nucleation, further leading to the construction of hierarchical nestlike structures built of well-ordered and oriented nanoflakes.12 Therefore, oriented aggregation was enhanced by the assistance of PVP. This process is illustrated in Scheme 1. To investigate the effect of reaction conditions on the formation of Bi2WO6 hierarchical nest-like structures, a series of comparative experiments was carried out through similar processes. It is obvious that the amounts of PVP in the solution profoundly affect the shape and size of the as-obtained products. When the amount of PVP was reduced to 0.5 g and the other experimental conditions were unchanged, the phase structure and morphology of Bi2WO6 remained unchanged as is shown in Figure 5c. When the quantity of PVP was reduced to 0.25 g, a small quantity of flower-like Bi2WO6 formed as is shown in Figure 5b. While the experiments were carried out without PVP, the product was the flower-like Bi2WO6 structures assembled by the out-of-order incompactly-packed nanoplates (Figure 5a). This reason may be that the growth-inhibited faces disappeared and that the probability of anisotropic growth weakened greatly when PVP was removed from the reaction system. As the quantity of PVP increased to 2 g, the phase and morphology of Bi2WO6 remained unchanged, as is shown in Figure 5d,
Figure 6. Typical UV-vis diffuse reflection spectrum of the hierarchical nest-like structures Bi2WO6.
suggesting that an appropriate amount of PVP is enough for aligning the nanoplate with clearly oriented layers. Therefore, PVP plays pivotal roles in controlling the structures of nestlike Bi2WO6 with clearly oriented layers. More in-depth studies are necessary to further understand their growth process. The optical absorption properties of a semiconductor, which are relevant to the electronic structure features, are recognized as key factors in determining its photocatalytic activity.30 The typical UV-vis diffuse reflection spectra of a good-quality Bi2WO6 hierarchical nest-like structure were measured by UVvis spectrometry (Figure 6). In Figure 6, the hierarchical nestlike structures of Bi2WO6 display photoabsorption properties from the UV light region to the visible light absorption shorter than 470 nm.19 The steep shape of the spectra indicated that the visible light absorption was due to the band gap transition.31 For a crystalline semiconductor, it was shown that the optical absorption near the band edge follows the equation ahν ) A(hν - Eg)n, where a, ν, Eg, and A are the absorption coefficient, light frequency, band gap, and constant, respectively.19 According to this equation, the value of n for Bi2WO6 was 1 from the data shown in Figure 6. The band gap of the nest-like Bi2WO6 was estimated to be 2.7 eV from the onset of the absorption edge, which is mostly close to the reported values.18 The color of the sample was yellowish, consistent with their photoabsorption spectrum.20 The BET surface area of the nest-like Bi2WO6 was 24.68 m2/g1. Clearly, the Bi2WO6 nanoplate-built hierarchical nestlike structure exhibited a much higher surface area than the solid-state reaction Bi2WO6 (0.64 m2/g1).17 It is known that Bi2WO6 has been used as a visible light photocatalyst for the
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Figure 7. (a) Absorption spectrum of the RhB solution (1.0 × 10-5 M, 100 mL) in the presence of 100 mg of nest-like Bi2WO6 under exposure to visible light. (b) Photodegradation of RhB (1.0 × 10-5 M, 100 mL) using different Bi2WO6 samples as photocatalysts. (A) With hierarchical nest-like Bi2WO6 with visible light irradiation, (B) with Bi2WO6 nanoplate with visible light, (C) with TiO2 (P-25) with visible light irradiation, (D) with hierarchical nest-like Bi2WO6 in the dark, and (E) without catalyst.
Figure 8. (a) Charge-discharge curves in intial cycle of as-prepared hierarchical nest-like Bi2WO6 at a given constant current density of 0.5 mA/cm2. (b) Cycle performances of the hierarchical nest-like Bi2WO6.
photoreductive dehalogenation of halogenated benzene derivatives and photocatalytic degradation of water pollutants.19 The as-prepared Bi2WO6 nest-like structures in our approaches have merits such as hierarchical structures with a high surface area and surface permeability and as hollow structures, which could provide ideal adsorption sites for reactant molecules. Thus, it is reasonable to expect that the as-prepared Bi2WO6 hierachical nest-like structures are ideal photocatalysts. To demonstrate the photocatalytic activity of the Bi2WO6 sample for the degradation of organic pollutants, we evaluate their photocatalytic activity toward the photodegradation of rhodamine B (RhB) at room temperature as a test reaction. Figure 7A displays the temporal evolution of the spectra during the photodegradation of RhB (initial concentration: 10-5 M, 100 mL) in the presence of the Bi2WO6 sample under visible light illumination. The absorption peaks at 553 nm corresponding to RhB diminished gradually with an absorption band shift to shorter wavelengths as the exposure time was extended. Similar hypsochromic shifts have also been observed in the RhB-Bi2WO6 system reported by Yao et al.19 In the process of photodegradation of RhB by Bi2WO6, the intense pink color of the starting RhB solution gradually faded. The adsorption peaks corresponding to RhB completely disappeared after about 105 min, suggesting the excellent photocatalytic activity of the hierarchical nest-like sample. This can be explained by their larger surface area and more capacious interspaces, which provide more active sites for the photocatalyst. To investigate the relationship of the sample’s morphology and the photocatalytic efficiency, we studied the degradation process of RhB using
different Bi2WO6 samples as photocatalysts. Figure 7B displays the results of the RhB degradation under different experimental conditions. It is clearly seen that under identical conditions, the nest-like structures exhibit superior photocatalytic activity over the samples with other morphologies. In addition, the experimental results demonstrated that the photocatalytic activity of the synthesized Bi2WO6 materials is susceptible to their morphology and sizes. Thus, such materials with interesting properties represent good candidates for further applications in various fields of nano- and microscale science and technology. For years, a worldwide research interest has been focused on searching for alternative electrode materials for lithium ion batteries to improve their energy density and safety.32 It was found that many compounds with a layer structure given an ideal host material for the intercalation and deintercalation of Li ions with flat charge-discharge curves can be used as anode materials of lithium ion batteries.33,34 But fewer reports were given on cathode materials with a flat charge-discharge plateau, which were undoubtedly more important for improving the battery performance. Thus, special effort is needed to study cathode materials having a flat charge-discharge plateau. The hierarchical nest-like Bi2WO6 with a layered structure, which will facilitate the intercalation-deintercalation of Li ions as compared to the dense bulk materials, may act as a cathode candidate for lithium ion batteries and provide the possibility of an increased diffusion coefficient with the layered structure in the Li ion batteries. Thus, we investigated the chargedischarge capabilities of the as-prepared Bi2WO6 by using the samples as the cathode in a lithium battery. Figure 8a displays
Bi2WO6 Nanoplate-Built Hierarchical Nest-like Structures the curves of voltage versus charge and discharge capacity for the cell between 3.0 and 1.0 V at a current density of 0.5 mA/ cm2 and 20 °C. Bi2WO6 nest-like structures exhibit a higher initial discharge capacity of 150.1 mAh/g, indicating that this kind of Bi2WO6 microstructure may be potentially applied as an electrode material in lithium ion batteries. In addition, the cycle performances of Bi2WO6 nest-like structures are shown in Figure 8b, which illustrates the dependence of the discharge capacity on the cycle number for the sample. The nest-like Bi2WO6 sample retains nearly 42.7% of its initial discharge capacity after 12 cycles. Note that the discharge capacity of the hierarchical nest-like structure Bi2WO6 after 12 cycles is not as ideal as expected. However, we believe that the investigations of electrochemical properties of the Bi2WO6 nanostructures acting as electrode materials in Li+ batteries may help us to understand the relationship between morphology and properties and thus inspire us to purposefully synthesize the layered nanostructures, which may act as electrode materials in Li+ batteries with good electrochemical properties. Conclusion In summary, a Bi2WO6 hierarchical nest-like structure was successfully synthesized on a large scale by a facile and economical method with the assistance of PVP. PVP plays an important role in the formation of Bi2WO6 nest-like structures. The fabricated functional semiconductor hierarchical submicrostructures exhibit a high photocatalytic activity in the degradation of RhB. It is expected that this kind of hierarchical nest-like structure Bi2WO6 could be exploited for applications in photocatalytic cleaners, optoelectronic devices, water purification, environmental cleaning, and solar energy conversion. In addition, Bi2WO6 hierarchical nest-like structures exhibit a favorable discharge capacity in a lithium ion battery, while its cycle performance still needs to be improved. This work could be of great importance in extending the potential applications of Bi2WO6, an outstanding advanced functional material. Acknowledgment. This work was financially supported by the National Nature Science Foundation (NSF) of China. References and Notes (1) Burda, C.; Chen, X. B.; Narayanan, R. et al. Chem. ReV. 2005, 105, 1025.
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