New Bi2WO6 Nanocages with High Visible-Light-Driven Photocatalytic Activities Prepared in Refluxing EG Meng Shang, Wenzhong Wang,* and Haolan Xu State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences 1295 DingXi Road, Shanghai 200050, P. R. China
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 2 991–996
ReceiVed July 22, 2008; ReVised Manuscript ReceiVed October 5, 2008
ABSTRACT: New Bi2WO6 nanocages were successfully synthesized with colloidal carbon as the template via a facile refluxing process in ethylene glycol (EG). The as-prepared nanocages consisted of small nanoparticles with a size of ca. 50-80 nm. By adjusting the concentration of the precursor, Bi2WO6 with different morphologies and microstructures could be obtained. The growth mechanism of such special nanocages was investigated, and EG played a key role in the formation of Bi2WO6 nanocages. The Bi2WO6 nanocages exhibited excellent visible-light-driven photocatalytic efficiency, which was increased to nearly 10 times that of products prepared by traditional solid-state reactions and commercially available TiO2. Close investigation revealed that the surface area and the porous hollow structure of the as-prepared Bi2WO6 nanocages could improve the photocatalytic activities. Moreover, the nanocages could settle naturally in 15 min, which is beneficial for separation and recycling, considering their future applications in wastewater treatment. Introduction Because of the strong correlation between the shape, size, and structure of nanostructured materials and their physical/ chemical properties, much interest has been focused on designing and preparing novel nano- and microstructured materials.1-5 Recently, increasing attention has been paid to the preparation of hollow inorganic spheres of defined structure and composition with tailored properties which have immense scientific and technological interest. These hollow nanostructures have either been found to have, or are proposed to have, diverse and fascinating applications, such as for catalysts, microcapsule reactors, chemical sensors, delivery and controlled release of drugs, building blocks in the fabrication of photonic band gap crystals, and so on.6-11 The most-applied method for the synthesis of hollow micro- or nanospheres by far is the templating of larger colloidal particles via surface precipitation of suitable inorganic molecular precursors on template cores, followed by the removal of the cores by calcination or solvent extraction.12-14 Among all the templates, colloidal carbon spheres as novel green templates have received considerable attention due to the availability of colloidal spheres with tightly controlled sizes and surface properties. These monodisperse carbon spheres inherit functional groups and have reactive surfaces, which facilitate the precipitation of metal precursors and nanoparticles.15-17 However, most of the hollow spheres fabricated by colloidal carbon were simple metal oxide because of the easy connection between the precursors and functional groups on the surface of the carbon particles.18-21 Multicomponent oxide hollow spheres are relatively difficult to obtain via these templates because most of them are produced by direct precipitation in aqueous solution which breaks down the connection. Thus it is challenging to find a way to avoid direct precipitation in the preparation of multicomponent oxide when these carbon spheres are used as hard templates. Bi2WO6, a kind of multicomponent oxide, is one of the simplest Aurivillius oxides possessing a layered structure.22,23 It has been discovered that Bi2WO6 possesses photocatalytic * Corresponding author. Tel: +86 21 5241 5295; fax: +86 21 5241 3122; e-mail:
[email protected].
activity,24,25 besides ferroelectric piezoelectricity and nonlinear dielectric susceptibility,26 which makes it an attractive material. Nowadays, photocatalytic reactions occurring under solar illumination have attracted worldwide attention. Taking sunlight into account, it is essential to develop highly effective visiblelight-driven photocatalysts.27 It is well-known that the photocatalytic activity is closely interrelated to the size, morphology, and structure of the photocatalysts.28 Our group has reported the preparation of Bi2WO6 with complex morphologies, namely, flower-, tire-, helix-, and plate-like shapes, by a facile hydrothermal process and has found out that the photocatalytic activities could be greatly improved by controlling the nanostructures, decreasing the particle sizes, etc.29 Though these are effective ways to enhance the photocatalytic activities by minimizing the particle size so as to achieve higher surface area and more active catalytic sites, it brings another negative effect. When the particle size is decreased to nanoscale, the separation and the recycling of the photocatalysts from the treated water are practical obstacles which hinder their application in industrial use even though they have high photocatalytic activity. Thus, some researchers are working on magnetic photocatalysts that can be separated from the treating system by applying an external magnetic field.30 Unfortunately, such efforts have not achieved satisfying results yet because introducing magnetic particles into the photocatalysts leads to dramatic decreases in the photocatalytic activity. The search for photocatalysts with high photocatalytic activity that can be separated easily, such as by natural settlement, is still a big challenge. Such photocatalysts would be more promising considering their future applications in remedying water pollutants. Herein, for the first time we report the fabrication of Bi2WO6 nanocages constructed by oriented nanoparticles via a simple reflux method in ethylene glycol (EG) with colloidal carbon spheres as the template. A possible formation mechanism of Bi2WO6 nanocages is proposed. Bi2WO6 with different morphologies could be obtained by adjusting the concentration of the precursor. The photodegradation of rhodamine B (RhB) was employed to evaluate the photocatalytic activities of the Bi2WO6 nanocages under visible light (λ > 420 nm) illumination. It is demonstrated that the Bi2WO6 nanocages exhibit excellent visible-light-driven photocatalytic performance. Moreover, these
10.1021/cg800799a CCC: $40.75 2009 American Chemical Society Published on Web 12/11/2008
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nanocages could settle naturally in 15 min, which would be advantageous to their future applications. Experimental Procedures All the reagents were of analytical purity and were used as received from Shanghai Chemical Company. In a typical synthesis of colloidal carbon spheres, glucose (∼4-8 g) was dissolved in deionized water (40 mL) to form a clear solution. The solution was then added into a 50 mL Teflon-lined autoclave with a stainless steel tank up to 80% of the total volume. Then the autoclave was heated at 160 °C for 10 h. Subsequently, the autoclave was cooled to room temperature naturally. The resulting precipitates were collected and washed with deionized water and absolute ethanol, then dried at 80 °C for 6 h. The synthesis of Bi2WO6 was carried out by using EG as the solvent and carbon sphere as the template. In a typical procedure, Bi(NO3)3 · 5H2O and Na2WO4 · 2H2O, in a molar ratio of 2:1, were mixed together in 100 mL of EG so as to form the solution. Then the desired amount of carbon spheres and the EG solution were added into a threenecked flask equipped with a condenser and refluxed at 160 °C for 20 h. A rinsing process involving 3-5 cycles of centrifugationwashing-redispersion was performed with either water or ethanol. The puce samples obtained were oven-dried at 80 °C for 4 h and used as the precursors of the nanocages. Carbon-free Bi2WO6 nanocages were obtained by calcining the composites at 450 °C for 1 h in air to remove the carbon core. Meanwhile, bulk Bi2WO6 powder was prepared by a traditional solid-state reaction (named SSR-Bi2WO6) according to a previous study25 for comparison with the Bi2WO6 nanocages. P25 (TiO2 nanoparticles, surface area 50 m2/g) was purchased from Degussa AG (Germany). The X-ray diffraction (XRD) patterns of the samples were measured on a D/MAX 2250V diffractometer (Rigaku, Japan) using monochromatized Cu KR (λ ) 0.15418 nm) radiation under 40 kV and 100 mA and scanning over the range of 10° e 2θ e 70°. The morphologies and microstructures of as-prepared samples were analyzed by scanning electron microscopy (SEM) (JEOL JSM-6700F) and transmission electron microscopy (TEM) (JEOL JEM-2100F, accelerating voltage 200 kV). UV-vis diffuse reflectance spectra of the samples were obtained on an UV-vis spectrophotometer (Hitachi U-3010) using BaSO4 as the reference. Nitrogen adsorption-desorption measurements were conducted at 77.35 K on a Micromeritics Tristar 3000 analyzer after the samples were degassed at 200 °C for 6 h. The BrunauerEmmett-Teller (BET) surface areas of the products were estimated using the adsorption data. Photocatalytic activities of the samples were evaluated by the photocatalytic decolorization of rhodamine-B (RhB) under visible light. A 500 W Xe lamp was used as the light source with a 420 nm cutoff filter to provide visible light irradiation. In every experiment, 0.1 g of the photocatalyst was added into 100 mL of RhB solution (10-5 mol/ L). Before illumination, the suspensions were magnetically stirred in the dark for 1 h to ensure the establishment of an adsorption-desorption equilibrium between the photocatalyst and RhB. Then the solution was exposed to visible light irradiation under magnetic stirring. At given time intervals, a 3 mL suspension was sampled and centrifuged to remove the photocatalyst particles. Then, the UV-vis adsorption spectrum of the centrifugated solution was recorded using a Hitachi U-3010 UV-vis spectrophotometer.
Results and Discussion The phase and composition of the calcined sample as well as the uncalcined sample were characterized by XRD, as shown in Figure 1. The pattern of Figure 1a indicates that the uncalcined Bi2WO6 with carbon spheres prepared in the refluxing EG is poorly crystallized. The calcination favors the formation of well-crystallized Bi2WO6 comparatively, and the diffraction peaks of this carbon-free Bi2WO6 nanocages sample agree well with those of the pure orthorhombic Bi2WO6 according to the JCPDS card no. 39-0256, as revealed in Figure 1b. The cell constants of Bi2WO6 were calculated to be a ) 5.457 Å, b ) 16.435 Å, and c ) 5.438 Å. Obviously, the calcined Bi2WO6 nanocages sample exhibits higher intensity and
Figure 1. XRD patterns of the calcined and uncalcined Bi2WO6 samples. (a) uncalcined Bi2WO6; (b) calcined Bi2WO6.
Figure 2. (a) Low-magnification TEM image of colloidal carbon nanospheres; (b) high-magnification TEM image of colloidal carbon nanospheres.
Figure 3. Typical SEM images of Bi2WO6 nanocages. (a) Overall morphology; (b) the close-up view of the nanocages.
narrower diffraction peaks in the XRD pattern compared to the uncalcined Bi2WO6 sample, which is due to the enhancement of crystallization. The carbon spheres were prepared from glucose under hydrothermal conditions at 160 °C without additives. The narrow size distributions of the final products were demonstrated by TEM observation. Figure 2a,b shows typical TEM images of such colloidal carbon nanospheres with diameters of 500-700 nm. The morphology and microstructure of the Bi2WO6 nanocages prepared by refluxing EG after calcination were revealed by the SEM images. As shown in Figure 3a, all the Bi2WO6 products have a spherical morphology, with diameters ranging from 200 to 400 nm. The close-up view of an individual sphere (Figure 3b) indicates that the Bi2WO6 sample possesses a nanocage-like appearance with a thickness of about 50 nm. Interestingly, the nanocages are in fact built by nanoparticles with an average size of about 70 nm, which can be vividly demonstrated by the SEM images (Figure 3b). These nanoparticles are aligned parallel to the surface of the nanocages with clearly oriented layers like shingles packed togother, pointing toward a common center (Figure 3b). More importantly, the
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Figure 5. The TEM images of (a) Bi2WO6 (0.5 M) loaded carbon sphere in refluxing EG at 160 °C for 20 h and (b) two-dimensional disk-like porous nanoplates of Bi2WO6 after the carbon spheres were removed; (c) the high-magnification TEM image of the porous disk-like nanoplates of Bi2WO6; (d) SEM image of the porous disk-like nanoplates of Bi2WO6.
Figure 4. (a) Bi2WO6 (2 M) loaded carbon sphere in refluxing EG at 160 °C for 20 h; (b) EDX of the Bi2WO6 loaded carbon spheres; (c) the panoramic view of Bi2WO6 nanocages; (d) and (e) The Bi2WO6 nanocages with connected center; (f) the close-up view of an individual Bi2WO6 nanocage.
unique nanocage structure with many pores and different diameter sizes, which may serve as the transport paths for small molecules or allow the transmission and multiflections of visible light within their interior cavities so as to endow these cages with greatly enhanced photocatalytic activities,31 are formed in the Bi2WO6 nanocages as shown in Figure 3. To obtain a better understanding of the formation mechanism of the Bi2WO6 nanocages, the products formed at different growth stages were collected for TEM measurements. Figure 4a shows the Bi2WO6 loaded carbon sphere in refluxing EG at 160 °C for 20 h. Owing to the poor crystallinity of this uncalcined Bi2WO6 with carbon spheres (Figure 1a), the Bi2WO6 nanoparticles were usually no larger than 20 nm decorating the surface of carbon nanospheres. Energy dispersive X-ray analysis (EDX) of the spherical core/shell structure revealed the elemental constitution of the Bi2WO6 loaded carbon spheres (Figure 4b). The four major peaks corresponded to carbon, bismuth, tungsten, and oxygen, respectively. The obtained nanocages were further investigated by means of TEM. Figure 4c displays a panoramic view of the samples after calcination at 450 °C for 1 h. The contrast between the dark edge and the pale center provides convincing evidence of the hollow structure. The samples exhibited very similar hollow sphere structures with uniform shape and size when the same carbonaceous spheres were used as the templates. The size of the Bi2WO6 nanocages
(200-400 nm) was about 40% of that of the original templates. In the calcination process, the shells become denser, and the spheres contract and cross-link to form the nanocages, which are smaller replicas of the carbon spheres.8,11,15,16 There are also some nanocages with connected centers, which may result from the softening and fusion of the carbonaceous spheres during the refluxing EG process (Figure 4d,e). The close-up view of an individual nanocage (Figure 4f) indicates that the Bi2WO6 sample possesses a hollow structure appearance with a shell thickness of around 50 nm. The nanocages are built from nanoparticles with a size of 50-80 nm, which agrees well with that revealed by the SEM images (Figure 3b). The concentration of the Bi2WO6 in the refluxing EG was tuned to a lower value (0.5 M), while the other experimental conditions were kept constant. The TEM image of the product before calcination shown in Figure 5a indicates that only some Bi2WO6 nanoparticles loaded on the surface of the carbon sphere. This is different from that of Bi2WO6 samples in Figure 4a. The interesting thing is the formation of the Bi2WO6 with two-dimensional disk-like morphology (Figure 5b) but not the three-dimensional nanocages after the carbon spheres were removed. The size of the disk-like nanoplate (200-400 nm) was also about 40% of that of the original templates, which is similar to that of the nanocages. The high-magnification TEM image of this Bi2WO6 sample was shown in Figure 5c. Clearly, the as-synthesized Bi2WO6 is also composed of small nanoparticles with the size between 50 and 80 nm, and they complect with each other to form the porous structure. The thickness of the nanoplates could be measured by the SEM image (Figure 5d), which was about 50 nm. When the concentration was tuned, the two-dimensional structure was formed. This concentration effect is different from that of the simple metal oxide, which is only the variety of the thickness of the shell.20 So far as we know, most of the hollow spheres fabricated by the colloidal carbon are simple metal oxide. Since the surface of the carbon spheres is hydrophilic and possesses a distribution
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Scheme 1. Formation Mechanism of the Bi2WO6 Nanocages
Figure 6. The experiments of natural settlement. (a) Bi2WO6 nanocages suspended in water; (b) settled naturally after 15 min.
of OH and CdO groups (Scheme 1A), the metal ions could be easily adsorbed onto the surface layer of the carbon spheres to form a composite shell. Then, in the following treatment of sonication, hydrolysis, and calcination, the metal oxide hollow spheres formed. However, the multiple-metal oxide functional materials were rarely synthesized in the form of hollow spheres via this template method. This could be ascribed to the different synthetic routes for the multiple-metal oxides, direct precipitation from the aqueous solution, which will lead to the formation of large particles. These large particles might be difficult to connect with the templates of carbon spheres,29 which favors the formation of dispersed particles rather than hollow spheres. In the aqueous solution, Bi2WO6 particles with a relative large size formed when the two reactants were mixed (formulas a-c), which could not bind the surface layer of the carbon spheres to form the hollow structure. However, on the basis of our experiments, it is found that the utilization of solvent EG favored the formation of the Bi2WO6 nanocages (formulas d and e). When Bi(NO3)3 · 5H2O and Na2WO4 · 2H2O were mixed together in EG, no precipitation was observed and the mixture was colorless and transparent under stirring, which was different
from that in the aqueous system. This might be attributed to the unique coordination of EG (formula d). The employ of EG produced the complex of M (Bi3+ or WO42-) with OH via the coordination (Scheme 1B), which could be adsorbed onto the surface layer of the carbon spheres. In the refluxing EG solution with the template of the carbon spheres, the complex gradually decomposed to release the metal ions, followed by the reaction for Bi2WO6 crystals (formula e). As a result, Bi2WO6 grains coated carbon spheres were formed (Scheme 1C, Figure 4a). In the later calcination process (Step 2 in Scheme 1), Bi2WO6 nanoparticles, the building blocks of the nanocages, grew from these small grains. Then the nanocages were formed as replicas of the carbon sphere when the concentration of the precursor was higher (Scheme 1D, as shown in Figure 4c). When the concentration of the Bi2WO6 was tuned to a lower value, though the nanoparticles connected to each other, they may not support
Figure 7. The UV-vis diffuse reflectance spectrum of the Bi2WO6 nanocages sample.
the structure as cages after calcination. Therefore, the structure collapses to form the 2D disk-like porous nanoplates (Scheme 1E, as show in Figure 5c). The Bi2WO6 nanocages are desirable due to their low density, small size, and high surface areas. Such porous structure provides efficient transport pathways to their interior voids, which is critical for catalyst, delivery, and other applications. Moreover, it is different from the general hollow sphere structure of the simple metal oxide which had only tiny pores and was suspended in the solution. This causes difficulties for the separation and recycling of these hollow spheres for industrial use. However, the large pores of the nanocages could allow the water or the dye solution get inside of the hollow spheres rapidly. Thus these nanocages not only possess enhanced photocatalytic activities, but also could settle naturally in 15 min, as shown in Figure 6. The UV-vis diffuse reflectance spectrum of the Bi2WO6 nanocages sample is shown in Figure 7. According to the spectrum, the Bi2WO6 nanocages present the photoabsorption properties from UV light to visible light shorter than 470 nm due to the band gap transition. The color of the sample is yellow, which is in accordance with its absorption spectrum. The steep shape of the spectrum indicates that the visible light absorption is not due to the transition from the impurity level but due to the band gap transition.32 After the calculation,33 the band gap (Eg) of the Bi2WO6 is estimated to be about 2.69 eV from the onset of the absorption edge (inset of Figure 7). This indicates that the Bi2WO6 nanocages have a suitable band gap for photocatalytic decomposition of organic contaminants under visible light irradiation. Rhodamine-B (RhB), a widely used dye, was selected as the model pollutant to evaluate the photocatalytic activity of the
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Figure 8. The temporal evolution of the spectra during the photodegradation of RhB mediated by the Bi2WO6 nanocages sample under visible light illumination (λ > 420 nm).
Figure 9. The photodegradation efficiencies of RhB as a function of irradiation time by different photocatalysts.
Bi2WO6 nanocages. Its characteristic absorption at about 553 nm has been used to monitor the photocatalytic degradation process.29 Figure 8 displays the temporal evolution of the spectra during the photodegradation of RhB mediated by the Bi2WO6 nanocages sample under visible light illumination (λ > 420 nm). A rapid decrease of RhB absorption at the wavelength of 553 nm is observed, and the spectral maximum shifted from 553 to 500 nm. The color of the suspension changed gradually from pink to light green, which is in agreement with the shift of the major absorption. It was well-reported that the RhB photodegradation occurred via two competitive processes: N-demethylation and the destruction of the conjugated structure.34 The sharp decrease and shift of the major absorption band within 50 min indicate that the Bi2WO6 sample exhibits excellent photocatalytic activity in the degradation of RhB due to its novel structure of nanocages. The photocatalytic performances of different photocatalysts were determined by comparing the degradation efficiency of RhB with otherwise identical conditions under visible light illumination (λ > 420 nm) (Figure 9); C was the absorption of RhB at the wavelength of 553 nm and C0 was the absorption of RhB after the adsorption equilibrium on Bi2WO6 nanocages before irradiation. Blank test (RhB without any catalyst) under visible light exhibited little photolysis. The photodegradation efficiency was only 5% after 50 min, which demonstrated that the degradation of RhB is extremely slow without a photocatalyst under visible-light illumination. The decrease of RhB with the Bi2WO6 nanocages in the dark for 1 h was similar to that of the blank test, which demonstrated that the absorption of RhB on the as-prepared Bi2WO6 was negligible after the adsorption-desorption equilibrium was reached. The photodegradation efficiency of RhB by SSR-Bi2WO6 just reached 10% after 50 min of reaction. Similarly, the commercial TiO2
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powders (P25) were obviously inactive under visible light irradiation, as reported by Asahi et al.27b It is interesting to see that in the case of the Bi2WO6 nanocages the photodegradation efficiency of RhB reached nearly 100% after 50 min irradiation only, which was increased to nearly 10 times that of the SSRBi2WO6 and the commercially available TiO2. The photocatalytic activity is affected by many factors which could cooperate with each other and enhance the photocatalytic activity. Among them the particle size and surface area are important factors.35 The BET surface area of the Bi2WO6 nanocages was estimated to be about 14.5 m2/g, which was much higher than that of the reference SSR-Bi2WO6 (0.6 m2/g).25 The high surface area brings not only more surface reached by the visible light and contacted with the RhB but also more active catalytic sites. On the other hand, for randomly generated charge carriers the average diffusion time from the bulk to the surface is given by τ ) r2/π2D, where r is the grain radius and D is the diffusion coefficient of the carrier.36 If the grain radius decreases, it will reduce the recombination opportunities of the photogenerated electron-hole pairs which could move effectively to the surface to degrade the absorbed RhB molecules.27a The Bi2WO6 nanocages are built by nanoparticles. The small size of nanoparticles is beneficial for promoting the photocatalytic efficiency because more electron-hole could be separated, transfer to the surface, and act with the organic molecules. Another contribution to the high photocatalytic activity could be the novel nanocages structure. There are plenty of pores in this structure which can be considered as transport paths for the RhB molecules. It benefits the reactant molecules to get to the reactive sites on the framework walls of the photocatalysts,whichresultsingoodphotocatalyticperformance.28b,29 Furthermore, we believe that the nanocage structure allows transmission and multiple reflections of visible light within the interior cavity; it thus utilizes the light source more efficiently and offers an improved catalytic activity.31 Conclusions In summary, Bi2WO6 nanocages were successfully prepared with colloidal carbon spheres as the template via a facile refluxing process in EG. EG was found to not only act as the reaction medium but also play an important role in the formation of Bi2WO6 nanocages. Based on the SEM, TEM, and EDX analyses, possible processes for the growth of the Bi2WO6 nanocages were proposed. It was found that the concentration of the precursor could determine the morphology of the final product. Compared with the corresponding samples prepared by the solid-state reaction and the commercially available TiO2, the Bi2WO6 nanocages exhibited higher shape-associated photocatalytic activity (nearly 10 times) in the degradation of RhB under visible-light irradiation. This work not only provides an example of shape-dependent photocatalytic properties of the Aurivillius oxides but also opens new possibilities for providing some insight into the design of cage-like hollow sphere of multiple-metal oxide semiconductors, which are desirable due to their low density, high surface area, and properties in natural settlement, separation, and recycling for future applications. Acknowledgment. We acknowledge the financial support from the National Natural Science Foundation of China (No. 50672117, No. 50732004).
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