Ind. Eng. Chem. Res. 2009, 48, 1735–1739
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Efficient Methylene Blue Removal over Hydrothermally Synthesized Starlike BiVO4 Songmei Sun, Wenzhong Wang,* Lin Zhou, 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
Nanoplate-stacked starlike BiVO4 products have been successfully synthesized by a hydrothermal method, where a water/ethanol mixture was the solvent and ethylenediamine tetraacetic acid (EDTA) was the chelating agent. The molar ratio of EDTA to Bi3+ was found to play an important role in the formation of this morphology. These starlike BiVO4 samples exhibited a high visible-light-driven photocatalytic efficiency. For the degradation of methylene blue (MB) under visible-light irradiation (λ > 420 nm), about 91% of the MB was degraded within 25 min. This is much higher than for BiVO4 samples prepared by solid-state reaction (bulk BiVO4) and other hydrothermal synthesis methods. The reasons for the differences in the photocatalytic activities of these BiVO4 samples were further investigated. 1. Introduction Semiconductor photocatalysts have attracted extensive attention because of their potential applications to energy and environmental problems.1-4 TiO2-based materials are the most popular photocatalysts for their high photocatalytic activity and stability and have been extensively studied so far.5-8 However, TiO2 responds only to ultravisible (UV) light, which accounts for a small fraction (5%) of the solar spectrum, whereas visible light, which accounts for 43% of the solar spectrum, has not been exploited. Therefore, the development of visible-lightdriven photocatalysts has become one of the most challenging topics today. Bismuth vanadate (BiVO4), which is an effective photocatalyst for water splitting and pollutant photodegradation under visible-light irradiation, has attracted increasing attention recently.9-11 Various methods have been utilized to prepare BiVO4, including solid-state reactions,12 sonochemical routes,9 room-temperature aqueous processes,11 and hydrothermal processes.13 Facile, cost-effective, and controllable synthetic routes associated with improved visible-light-photocatalysis should be developed considering the practical application of photocatalysts in the future. Among the various pathways, hydrothermal synthesis is a soft-chemical process that is widely used in preparing many kinds of functional materials.14 The advantages of hydrothermal synthesis are that the experimental parameters such as the concentrations of reactants, the pH values, the temperature, and the reaction medium can be easily tuned to control the microstructures, and thus the properties and propertydependent applications, of the target materials. In the preparation of photocatalysts, it is essential to control their crystallinity, sizes, structures, and morphologies, as these parameters are closely related to their photocatalytic activities. For example, it has been reported that monoclinic BiVO4 has a higher photocatalytic activity than tetragonal zircon and tetragonal sheelite phases.11 Our previous study also showed that the photocatalytic activity of BiVO4 synthesized by an ultrasonic method is higher and that the photocatalytic activity is closely related to the synthetic pathway.9 Herein, we report an improved hydrothermal route, in which a water/ethanol mixture is the solvent and ethylenediamine * To whom correspondence should be addressed. E-mail: wzwang@ mail.sic.ac.cn. Tel.: +86-21-5241-5295. Fax: +86-21-5241-3122.
tetraacetic acid (EDTA) is the chelating agent. The as-prepared BiVO4 sample exhibits a novel starlike morphology. The photodegradation of methylene blue (MB) was employed to evaluate the photocatalytic activities of different BiVO4 samples under visible-light (λ > 420 nm) illumination. It is demonstrated that the as-prepared starlike BiVO4 sample shows excellent photocatalytic performance. For the degradation of MB under visible-light irradiation, about 91% of the MB is degraded within 25 min. 2. Experimental Section 2.1. Synthesis. Typically, 2.5 mmol of NH4VO3 and 0-2.5 mmol of EDTA were dissolved in 5 mL of NaOH solution (2 M) and 15 mL of NH3 · H2O solution (2 M). Meanwhile, 2.5 mmol of Bi(NO3)3 · 5H2O was dissolved in 5 mL of HNO3 (4 M). After being stirred for 10 min, this mixture was combined with the above basic NH4VO3 solution containing EDTA. Then, 5 mL of alcohol was added, and the combined mixture was transferred into a 50-mL Teflon-lined stainless steel autoclave. The autoclave was heated at 120 °C for 6 h at autogenous pressure. Then, it was cooled to room temperature. The yellow precipitates were separated by filtration, washed with deionized water and absolute alcohol several times, and then dried at 80 °C for 12 h. For comparison, a bulk BiVO4 sample was also prepared by a solid-state reaction as described in a previous study.15 2.2. Characterization. Structural features of the as-prepared samples are determined by X-ray diffraction (XRD) on an X-ray diffractometer (Rigaku D/Max 2250V) with a graphite monochromator and Cu KR radiation (λ ) 0.15418 nm) in the range of 10-80° at room temperature. The morphologies and microstructures of the products were examined by field-emission scanning electron microscopy (JEOL JSM-6700F). The UV-vis diffuse reflectance spectrum of the sample was measured with a Hitachi UV-3010PC UV-vis spectrophotometer. Nitrogen adsorption-desorption measurements were conducted at 77.35 K with a Micromeritics Tristar 3000 analyzer. The BrunauerEmmett-Teller (BET) surface area was estimated using adsorption data. 2.3. Photocatalytic Test. The photocatalytic activities of the BiVO4 samples were evaluated by the degradation of methylene blue under visible-light irradiation of a 500-W Xe lamp with a
10.1021/ie801516u CCC: $40.75 2009 American Chemical Society Published on Web 01/08/2009
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Figure 1. XRD pattern of the as-prepared starlike BiVO4 sample.
420-nm cutoff filter. Experiments were performed at ambient temperature as follows: First, 0.025 g of the as-prepared sample was added to 100 mL of 15 mg/L methylene blue solution with constant magnetic stirring. Before illumination, the solution was stirred for 12 h in the dark in order to reach the adsorptiondesorption equilibrium of methylene blue on the BiVO4 photocatalyst. The concentrations of methylene blue were monitored with a Hitachi UV-3010PC UV-vis spectrophotometer in terms of the absorbance at 664 nm during the photodegradation process. 3. Results and Discussion 3.1. Morphology and Microstructure of the Products. Figure 1 shows the XRD pattern of the as-prepared BiVO4 sample when the molar ratio of Bi3+ to EDTA was 1 to 0.75 (denoted as BiVO4-1). All diffraction peaks can be assigned to the monoclinic structure of BiVO4 (JCPDS No. 14-0688). This observation is further confirmed by the splitting of the peaks at 2θ ) 18.5°, 35°, and 46°, which is characteristic of the monoclinic structure of BiVO4.11 The relative diffraction intensity of the (040)/(-121) planes is much higher than the corresponding reference value in JCPDS 14-0688. This indicates that the BiVO4-1 sample might have anisotropic growth along the (010) plane. Figure 2 shows scanning electron microscopy (SEM) images of the as-prepared BiVO4-1 sample. As shown in Figure 2a, all of the BiVO4 products had a quasi-quadratic starlike morphology with a side length of about 1-2 µm. No other morphologies were observed, indicating a high yield of these two-dimensional structures. Higher-magnification SEM images (Figure 2b-d) showed that these starlike structures had a thickness of about 100 nm and were built from smaller nanoplates. Further information about the BiVO4-1 product was obtained from the transmission electron microscopy (TEM) images (Figure 3). They confirmed that the side lengths of the starlike BiVO4 sample were about 1-2 µm (Figure 3a,b), in agreement with the SEM images (Figure 2). Figure 3c is an enlarged TEM image of the starlike BiVO4 sample. The starlike product was composed of nanoplates with side lengths of about 100-300 nm. The selective-area electron diffraction (SAED) pattern recorded at the marked area for the [010] zone axis in the inset of Figure 3c reveals the single-crystal nature of the nanoplates and confirms that the nanoplates grew preferentially along the (010) plane. The TEM image and the SAED pattern imply that the fastest growth of the starlike crystal proceeded along the directions such as [101]. The growth along the [100] and [001] directions was slower than that along the [101] direction. It has been reported that chelators can be used for shaping crystals.16,17 In this study, EDTA was also found to play an
important role in determining the morphology. When the molar ratio of EDTA to Bi3+ was 0.75 to 1, starlike products formed (Figure 4b). However, without the addition of EDTA and with the other conditions unchanged, irregular-shaped small particles dominated (BiVO4-2, Figure 4a). When the molar ratio of EDTA to Bi3+ was increased to 1, three-dimensional regular-shaped microcrystals were obtained (BiVO4-3, Figure 4c). It is clear that the strong ligand, EDTA, is not only required to form a stable complex with Bi3+, but also acts as a capping reagent that directly affects the facet growth of the nanocrystals.18 As a strong chelator, EDTA reacts with Bi3+ to form stable Bi-EDTA complexes. The addition of EDTA can decrease the monomer concentration of Bi3+ and the nucleation and growth rate of the BiVO4 sample. Thus, the addition of EDTA to the reaction system could provide a soft growth process whereby the crystal growth is more likely to comply with the intrinsic crystal growth behavior as summarized by the Gibbs-CurieWulff theorem.19 According to the Gibbs-Curie-Wulff theorem, the growth rate of a face is inversely proportional to the atom density of the respective plane. For monoclinic BiVO4, the (010) surface has a larger atom density than the (001), (100), and (101) planes, as shown in Figure 5. Therefore, the growth along the [010] direction is slower than that along the [001], [100], and [101] directions, which leads to the formation of nanoplates along the (010) plane. The starlike crystals were stacked up from these nanoplates. On the other hand, the dissociated EDTA preferentially adsorbed on the more densely packed (010) plane, suppressed the crystal growth along the [010] direction, and thus further enhanced the anisotropic growth along the (010) plane. The influence of the adsorption of EDTA on the morphological change was similar to that found in previous studies on CeVO416 and FeOOH.17 The quasi-quadratic edge of the starlike crystals was ascribed to the similar growth speeds along the [001] and [100] directions, given that the atom densities of the (001) and (100) planes were almost the same, as shown in Figure 5. The fastest growth proceeded along the [101] direction as the (101) surface has the lowest atom density, as was also confirmed by the TEM image (Figure 3c). The vertices of the stars were along the [101] directions. In other words, the fact that the growth speed decreases in the order [101] > [100/001] . [010] might well explain the facetted outline of the starlike crystals. When the EDTA/Bi ratio increased, many more EDTA molecules could adsorb on faces other than the (010) face, such as the (100), (001), and (101) faces, resulting in a slower crystal growth process in three dimensions. The disparity in growth speeds along the different directions decreased because of the increased adsorption of EDTA on the growing faces. Under such conditions, the asgrown crystals are likely to be three-dimensional single crystals with a regular shape, as shown in Figure 4c. Based on the above analysis, both the surface adsorption of EDTA and the intrinsic crystal growth behavior play important roles in determining the final crystal morphology. 3.2. Optical Absorption Property. Diffuse reflectance spectroscopy is a useful tool for characterizing the electron states in optical materials. The UV-vis diffuse reflectance spectrum of the starlike BiVO4 sample is shown in Figure 6. The starlike BiVO4 product exhibits strong absorption in the visible range in addition to the UV range. The steep absorption edge in the visible range indicates that the absorption of visible light is due not to the transition from impurity levels but to the bandgap transition.20 For a crystalline semiconductor, the optical absorption near the band edge follows the formula21 ahν ) A(hν - Eg)n/2
(1)
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Figure 2. (a) Low- and (b) high-magnification SEM images of the as-prepared starlike sample. (c) SEM image of an individual star. (d) Further enlarged SEM image of the star.
Figure 3. TEM images of (a) the starlike sample and (b) an individual star. (c) Enlarged TEM image of the surface of the star; (inset) SAED pattern recorded at the marked area.
Figure 4. SEM images of BiVO4 samples prepared with different molar ratios of Bi3+ to EDTA: (a) 1:0 (BiVO4-2), (b) 1:0.75 (BiVO4-1), (c) 1:1 (BiVO4-3).
where R, ν, Eg, and A are the absorption coefficient, the light frequency, the band gap, and a constant, respectively. According to eq 1, the value of n for BiVO4 is 1, indicating that it is a direct band gap material. The energy of the band gap of a BiVO4 photocatalyst can thus be obtained from the plots of (Rhν)2 versus photon energy (hν), as shown in the inset of Figure 6. The value estimated from the intercept of the tangent to the plot is 2.3 eV, which is consistent with previous results.22 The BET surface areas of the BiVO4-1, BiVO4-2, BiVO4-3, and bulk BiVO4 samples are 1.07, 2.32, 1.26, and 0.24 m2/g, respectively.
3.3. Photocatalytic Activities. To study the photocatalytic activities of the starlike BiVO4 product, methylene blue (MB), with a major absorption band at 664 nm, was chosen as a model pollutant. Visible-light irradiation of an aqueous MB/BiVO4 starlike product suspension led to an apparent decrease in absorption. Figure 7 displays the temporal evolution of the spectral changes during the photodegradation of MB over starlike BiVO4 under visible-light illumination. An apparent decrease of MB absorption at 664 nm is observed. The photocatalytic activity of starlike BiVO4 samples is compared
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Figure 5. Schematic illustration of BiVO4 crystal structure viewed along different directions (1 × 1 × 1 cell) to show the structural feature of the (101), (100), (001), and (010) faces.
Figure 6. Typical diffuse-reflectance spectrum of the starlike BiVO4 products; the inset is the (ahν)2 versus hν curve.
Figure 7. UV-visible spectral changes of MB (15 mg/L) in an aqueous starlike BiVO4 product dispersion as a function of irradiation time under visible-light illumination.
sorption of MB on the starlike BiVO4 sample in the dark was also checked. After 25 min, the concentration of MB remained unchanged (Figure 8f), suggesting that the decolorization of MB by a starlike BiVO4 sample is mainly caused by photodegradation but not adsorption. For comparison, the photocatalytic property of bulk BiVO4 was also tested. After 25 min of visiblelight irradiation, the degradation rate of MB by bulk BiVO4 was only 18% (Figure 8d), evidently less efficient than that of the starlike BiVO4 sample under the same conditions. Generally, the activity of a photocatalyst increases with increasing surface area, not only because the photocatalytic reaction usually takes place on the surface, but also because the efficiency of electron-hole separation is promoted.23,24 Figure 8a-c shows the morphology dependence of the visible-light photocatalytic activity of the BiVO4 samples. The starlike sample (BiVO4-1) has a much higher photocatalytic activity than samples BiVO4-2 and BiVO4-3, even though its BET surface area is the smallest among these three samples. This high photocatalytic activity might be related to the larger atom density of the (010) lattice plane on the exposed surfaces, which favors the photocatalytic reaction.25 In addition, the photocatalytic property of monoclinic BiVO4 is related to the distortion of the BisO polyhedron.25,26 The starlike products are composed of nanoplates with relatively large distortion of the unit cell due to the large surface strain. In addition, the nanoplate structure also means that the photogenerated electrons and holes could diffuse more easily to the surface to react with MB.27 Thus, a higher photocatalytic activity is exhibited compared to the bulk material and other hydrothermally synthesized samples with different morphologies. 4. Conclusion Starlike BiVO4 products composed of nanoplates along the (010) plane were successfully synthesized by a facile hydrothermal method, in which a water/ethanol mixture is the solvent and EDTA is the chelating reagent. It was found that EDTA plays an important role in determining the morphology of the as-prepared BiVO4 samples. Photocatalytic evaluation revealed that the starlike BiVO4 sample exhibits a high photocatalytic performance. For the degradation of methylene blue (MB) under visible-light irradiation (λ > 420 nm), about 91% of the MB was degraded within 25 min. The very facile route presented here uses only common and inexpensive reagents, which might be suitable for the large-scale production of BiVO4 starlike products as a highly active visible-light-driven photocatalyst. Acknowledgment
Figure 8. Photodegradation efficiencies of MB as a function of irradiation time for different photocatalysts: (a) BiVO4-1, (b) BiVO4-3, (c) BiVO4-2, (d) bulk BiVO4, (e) MB photolysis, (f) adsorption in the dark.
with that of bulk BiVO4. The photolysis test demonstrates that the self-degradation of MB is extremely slow (Figure 8e), only 6% of MB is photolyzed after 25 min irradiation. However, with starlike BiVO4 samples as photocatalysts, 91% of the MB was decolorized after 25 min (Figure 8a), showing a high photocatalytic activity under visible-light irradiation. The ad-
This work was financially supported by the National Basic Research Program of China (973 Program, 2007CB613302) and Nanotechnology Programs of Science and Technology Commission of Shanghai Municipality (0852nm00500). Literature Cited (1) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37–38.
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ReceiVed for reView October 7, 2008 ReVised manuscript receiVed November 12, 2008 Accepted November 23, 2008 IE801516U
