J. Phys. Chem. C 2008, 112, 10407–10411
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Bi2WO6 Nanocrystals with High Photocatalytic Activities under Visible Light Meng Shang, Wenzhong Wang,* Songmei Sun, Lin Zhou, and Ling Zhang 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 ReceiVed: December 20, 2007; ReVised Manuscript ReceiVed: April 25, 2008
Visible-light-induced photocatalyst Bi2WO6 with the size of ca. 30 nm has been synthesized via a facile template-free hydrothermal method. The effects of the pH values of the precursor suspensions and hydrothermal time on the photocatalytic activities of the Bi2WO6 nanoplates have been investigated in detail. The photocatalytic activity of as-prepared Bi2WO6 was about 8-10 times higher than that of the product prepared by solid-state reaction, which was evaluated by the degradation of RhB dye in water under visible light irradiation. Even under the illumination of a compact fluorescent lamp, the nanosized Bi2WO6 also exhibited high photocatalytic activity. Introduction Since the discovery of the photocatalytic splitting of water on the TiO2 electrodes by Fujishima and Honda in 1972,1 the applications on solar energy conversion and degradation of pollution by semiconductor photocatalysts have received great attention.2–10 Because of the good chemical stability, high oxidized activity, nontoxicity, and low price of the TiO2, it has been the most popular photocatalyst for environmental purification.11–13 However, the band gap of the TiO2 is 3.2 eV. It absorbs only the ultraviolet light (λ < 400 nm), which accounts for about 4% of the sunlight.14–16 In recent years, the Aurivillius family, which has the general formula Bi2An-1BnO3n+3 (A ) Ca, Sr, Ba, Pb, Bi, Na, K, and B ) Ti, Nb, Ta, Mo, W, Fe) has been investigated because of their unique properties and potential applications.17–20 In all of these compounds, Bi2WO6 is the simplest one when n ) 1, which has attracted much interests because of the ability of solving the energy and environmental problem.21–23 Zhu’s 24,25 and Yu’s groups26 have reported the preparation of Bi2WO6 nanoplates and nanoparticles and investigated their visible-light-driven photocatalytic activities. Their works revealed that Bi2WO6 could perform as excellent photocatalytic materials. It is well-known that the photocatalytic activity closely relates with the diameter size, surface areas, the efficiency of electron-hole separation, etc., of the photocatalysts.27 Thus, the synthesis of visible-light-driven Bi2WO6 photocatalysts with controlled microstructure and improved photocatalytic efficiency is a subject of considerable research interest. Herein, for the first time we report the preparation of nanosized Bi2WO6 with high photocatalytic activities using ammonium bismuth citrate (Bi(NH3)2C6H7O7 · H2O, named as CBN) by a facile template-free hydrothermal method. The photodegradation of Rhodamine B (RhB) was employed to evaluate the photocatalytic activities of Bi2WO6 under visible light illumination, using a Xe lamp (λ > 420 nm, 500 W) or even a U-type compact fluorescent lamp (433 nm < λ < 700 nm, 8 W). It is demonstrated that the nanosized Bi2WO6 * Corresponding author phone: +86-21-5241-5295; fax: +86-21-52413122, e-mail:
[email protected].
exhibit relatively excellent performance in the visible-lightdriven photocatalysis. Experimental Section All reagents were of analytical purity and were used as received from Shanghai Chemical Company. As a typical process, aqueous solutions of Bi(NH3)2C6H7O7 · H2O and Na2WO4 · 2H2O were mixed together in a 2:1 molar ratio, then the pH value of the final suspension was adjusted to about 7. The mixture was stirred for several hours at room temperature. Afterward, the suspension was added to a 50 mL, Teflon-lined autoclave up to 80% of the total volume. Then, the autoclave was sealed in a stainless steel tank and heated at 160 °C for 12-48 h with stirring. Subsequently, the autoclave was naturally cooled to room temperature. The resulting precipitates were collected, washed with deionized water and absolute ethanol, and then dried at 80 °C in air. For comparison, similar samples were also prepared using Bi(NO3)3 · 5H2O as Bi source material (the products were named t-BWO) while other steps were the same as above. Bulk Bi2WO6 was synthesized via traditional solid-state reaction (named as SSR-BWO). The X-ray diffraction (XRD) patterns of the sample was 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 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 reference. Nitrogen adsorptiondesorption measurements were conducted at 77.35 K on a Micromeritics Tristar 3000 analyzer after samples were degassed at 200 °C for 6 h. The Brunauer-Emmett-Teller (BET) surface area was estimated using 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, and a U-type compact fluorescent lamp (Philips Genie energy saving lamp, 8 W, 433 nm < λ < 700 nm)
10.1021/jp802115w CCC: $40.75 2008 American Chemical Society Published on Web 06/24/2008
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Figure 1. XRD patterns of a Bi2WO6 sample prepared by the hydrothermal method at neutral pH and 160 °C for 24 h.
was also used as a light source. In every experiment, 0.1 g of the photocatalyst was added to 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, 3 mL of 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 products, which were prepared at a neutral pH value and at 160 °C for 24 h, were investigated using XRD measurements. Figure 1 shows the XRD pattern of the Bi2WO6 sample. All diffraction peaks can be indexed to orthorhombic Bi2WO6 according to the JCPDS card No. 39-0256. After refinement, the crystal lattice parameter of Bi2WO6 was calculated as follows: a ) 5.456 Å, b ) 16.445 Å, and c ) 5.444 Å. The morphology and microstructure of Bi2WO6 samples were revealed by the TEM images (Figure 2a-c). The panoramic view shown in Figure 2a indicates that the asprepared products are composed of laminar structure with a size of ca. 30 nm. Close observation revealed by highmagnification TEM image (Figure 2b) shows most of these nanosized Bi2WO6 are nanoplate-like. As shown in Figure 2c, the shape and the scale can also be clearly observed. The shape of Bi2WO6 is like a plate that has only about 30 nm of length. The Brunauer-Emmett-Teller (BET) surface area of the Bi2WO6 sample was also estimated using N2 adsorption data. The BET surface area of the sample was ca. 51.5 m2/g, which was much higher (80-90 times) than that of the reference SSR-BWO (ca. 0.6 m2/g).23 These results demonstrated that Bi2WO6 nanocrystals with relatively smaller crystal size and larger surface area could be synthesized. Photocatalytic behavior is closely related to the particle size and surface area. For randomly generated charge carriers the average diffusion time from the bulk to the surface is given by τ ) r2π2D, where r is the grain radius and D is the diffusion coefficient of the carrier.28 If the grain radius decreases, then it will reduce the recombination opportunities of the photogenerated electron-hole pairs that could effectively move to the surface to degrade the absorbed RhB
Shang et al. molecules.29 In addition, BET surface area increases with the decrease of particle size, which was also beneficial to absorb more light and increase reaction sites. All of these factors could improve the photocatalytic performance. Therefore, to enhance the photocatalytic activity it is better to prepare photocatalysts with small particle size and high surface area, although the crystallinity, phase, structure, morphology, etc. also affected the activity. The as-prepared nanosized Bi2WO6 has small particle size and large surface area, which provide necessary elements for the high photocatalytic activity, and the experimental observations support this phenomenon. The UV-vis diffuse reflectance spectra of the assynthesized Bi2WO6 sample is shown in Figure 3. The absorption edge of the sample extended nearly to the whole spectra of visible light, which implies the possibility of high photocatalytic activity of these materials under visible light irradiation. It is well-known that the optical absorption near the band edge follows the formula Rhν ) A(hν - Eg)n/2 for a crystalline semiconductor,30 where R, h, ν, Eg, and A are the absorption coefficient, Planckʼs constant, the light frequency, the band gap, and a constant, respectively. Among them, n decides the characteristics of the transition in a semiconductor. For Bi2WO6, the value of n is 1. Therefore, the energy of the band gap could be estimated to be about 2.50 eV for Bi2WO6 nanoplates. This indicates that the nanosized Bi2WO6 has a suitable band gap for photocatalytic decomposition of organic contaminants under visible light irradiation. The color of the photocatalyst is dark yellow, as can be expected from the absorption spectra. The electronic structure of Bi2WO6 has been reported based on the DFT calculations.31 It has been reported that the valence band of the Bi2WO6 is formed by the hybrid orbitals of Bi 6s and O 2p and the conduction band of W 5d, so the band gap becomes narrower and the considerable absorption extends up to the visible region. This special electronic structure makes the valence band largely dispersed, facilitates the mobility of photoexcitated holes to the surface of the crystal, and thus is beneficial to photocatalytic oxidation of organic pollutants. The photocatalytic activities of Bi2WO6 were evaluated by the degradation of RhB dye in water under visible light irradiation (λ > 420 nm) using a 500 W Xe lamp. Under visible light irradiation, the color of RhB/Bi2WO6 suspension changes and the absorption gradually decreases, which indicates that the ethyl groups were removed; thus, RhB was degraded.24a,32 The temporal UV-vis spectral changes of RhB aqueous solution during the photocatalytic degradation reactions are shown in Figure 4. As seen in Figure 4a, when the RhB solution was irradiated with visible light (λ > 420 nm) in the presence of Bi2WO6, the main absorbance that maximized at ca. 552 nm decreased markedly with irradiation time, and it almost completely disappeared after 40 min of irradiation. Figure 4b shows the efficiencies of the photocatalytic degradation under visible light irradiation, C was the absorption of RhB at the wavelength of 552 nm and C0 was the absorption after the adsorption equilibrium on Bi2WO6 before irradiation. Blank tests (RhB without any catalyst) under visible light exhibited little photolysis. The photodegradation efficiency was only 4% after 40 min, which demonstrates that the degradation of RhB is extremely slow without a photocatalyst under visible-light illumination. The degradation of RhB with the nanosized Bi2WO6 in the dark condition for 40 min was similar to that of the blank test,
Bi2WO6 Nanocrystals with Photocatalytic Activities
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Figure 2. TEM images of as-synthesized Bi2WO6 nanoplates: (a) low-magnification, (b) high-magnification, (c) single nanoplate.
The photocatalytic activity did not exhibit any significant loss after five recycles for the photodegradation of RhB. It indicates that the Bi2WO6 nanoplates have high stability and do not photocorrode during the photocatalytic oxidation of the model pollutant molecules, which is especially important for its application. On the basis of the experimental results and observations, the possible reactions carried out are proposed as follows:
Figure 3. UV-vis diffuse reflectance spectra of the as-synthesized Bi2WO6 sample.
Bi(NO3)3 T Bi3++3NO3-
(a)
Na2WO4 T 2Na++WO42-
(b)
2Bi3+ + WO42- T Bi2WO6
(c)
-(NH3)2C6H7O73-
3+
Bi(NH3)2C6H7O7 T Bi
(d)
-(NH3)2C6H7O73-+WO42-+2H2O T Bi2WO6+
3+
which demonstrated that the absorption of RhB on the asprepared Bi2WO6 was limited after the adsorption-desorption equilibrium was reached. Although there might be photolysis for RhB,24,32 the above comparison results of the high photocatalytic degradation signified that RhB degradation in the present study was indeed through a photocatalytic process or photocatalysis together with sensitization (only activated with the existence of the Bi2WO6 photocatalyst). The experiments also showed that the photocatalytic activity of the as-prepared nanosized Bi2WO6 was much higher than that of the t-BWO (2-4 times) and SSR-BWO (8-10 times). From Figure 4c, because the crystallinity of the sample was not good enough, we can find that the photodegradation efficiency of RhB with nanosized Bi2WO6 just reaches 30% after 60 min of irradiation under the visible light when the hydrothermal time for the preparation of Bi2WO6 was 12 h. However, when the hydrothermal time was 24 h for the preparation of Bi2WO6, the photodegradation efficiency of RhB reaches nearly 100% after only 40 min of irradiation. By increasing the hydrothermal time further, the photocatalytic activities of Bi2WO6 increased a little. Figure 4d shows the different photodegradation efficiencies of RhB by the Bi2WO6 samples under the visible light illumination when the samples were prepared from the precursor suspensions with different pH values. When the pH values of the precursor suspensions are near neutral (5.3∼8.3), the photodegradation efficiencies of RhB by the nanosized Bi2WO6 were higher than those for other pH values (lower or higher) of the precursor suspensions. To confirm the stability of the high photocatalytic performance of the Bi2WO6 nanoplates, the circulating runs in the photocatalytic degradation of RhB in the presence of Bi2WO6 under visible-light (λ > 420 nm) were checked (Figure 5).
2Bi
2(NH3)2C6H7O73-+4H+ (e) Bi(NO3)3 is typically used as the Bi source in the preparation of Bi2WO6 by the hydrothermal method. As shown in Formula a, Bi3+ will be produced when Bi(NO3)3 dissolves in water, and WO42- will be produced when Na2WO6 dissolves in water (Formula b). When these two solutions are mixed, a white precipitate of Bi2WO6 will be immediately produced (Formula c). Because this precipitation process carries out naturally, the intrinsic nucleation and anisotropic growth of the Bi2WO6 is dominant in determining the microstructure of the product; thus, big particles with small surface area will be obtained. However, when Bi(NH3)2C6H7O7 instead of Bi(NO3)3 is selected as the Bi source in our case, no such precipitation was observed. The mixture became colorless and transparent under stirring when the two solutions are mixed. This could be attributed to the unique coordination of Bi(NH3)2C6H7O7 · H2O. The use of Bi(NH3)2C6H7O7 · H2O produced the complex of Bi3+ with citric anions, as shown in Formula d. Because of the coordination, Bi3+ will not directly react with WO42- to produce the precipitate Bi2WO6. This could be the reason the starting mixture containing Bi(NH3)2C6H7O7 · H2O and Na2WO4 was colorless and transparent. As the temperature increased during the hydrothermal reaction, the complex will gradually decompose to slowly release Bi3+, followed by the reaction with WO42-, as shown in Formula e. Thus, the nucleation and growth process of Bi2WO6 is tuned during the hydrothermal process, which might be similar to that in homogeneous precipitation. As a result, Bi2WO6 with smaller size and higher surface area is obtained. Because of the intrinsic anisotropic crystal structure of Bi2WO6 that is sandwiched and the possible contribution of the bismuth citrate complex that might serve as the template,33,34
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Figure 4. Photocatalytic degradation of RhB using Bi2WO6 nanoplates. (a) The changes of temporal UV-vis spectral of RhB aqueous solution; (b) comparison of t-BWO, SSR-BWO, and blank tests; (c) hydrothermal time series samples; (d) pH value series samples.
Figure 5. Cycling runs in the photocatalytic degradation of RhB in the presence of Bi2WO6 under visible-light.
Figure 6. Photocatalytic activity of Bi2WO6 under the U-type compact fluorescent lamp.
the final Bi2WO6 product tends to grow into plate-like structures. On the other hand, the photocatalytic activity is tightly related to the microstructure of the photocatalyst, such as small size and high surface area, and the as-prepared Bi2WO6 product
exhibited much higher photocatalytic activities. This may also explain why the pH value of the precursor suspensions can affect the photocatalytic activities. When the precursor suspensions are near neutral, the complex of Bi3+ with citric anions can be formed and result in the small nanoplates. However, the higher pH values may lead to the hydrolysis of Bi3+, and the lower pH values may destroy the structure of the Bi(NH3)2C6H7O7, all of which can not produce the complex of Bi3+ with citric anions. So the photodegradation efficiencies of RhB by the nanosized Bi2WO6 were higher when the pH values of the precursor suspensions were near neutral. However, the exact reason for the high photocatalytic activity of the Bi2WO6 samples need to be further investigated. An 8 W U-type compact fluorescent lamp was also used as a light source. Figure 6 demonstrates that the photocatalytic activity of Bi2WO6 is also high under this condition. After 60 min of irradiation, the photodegradation efficiency of the Bi2WO6 reached nearly 95%, which was much higher than that of the t-BWO (2-4 times). On the other hand, however, RhB photolysis (without any catalyst) was very slow, and the RhB could not be degraded in the presence of Bi2WO6 under dark conditions. These results indicate the benefit to future practical applications, because the lamp-house changed from 500 W Xe lamp to the 8 W U-type compact fluorescent lamp and the power consumption decreased sharply. In the practical application of photocatalysis, the consuming of the power is still a problem to be solved. This experiment implies this nanosized Bi2WO6 with high surface area exhibited high photocatalytic ability under indoor lighting condition, which brings the hope to its future application. Conclusions A visible-light-induced nanosized Bi2WO6 photocatalyst was hydrothermally synthesized at neutral pH values and at
Bi2WO6 Nanocrystals with Photocatalytic Activities a temperature of 160 °C for 24 h. The as-prepared nanoplates (30 nm) exhibited relatively small crystal size and large surface area. Compared with the corresponding samples prepared by solid-state reaction, the nanosized Bi2WO6 photocatalyst showed much higher (8-10 times) photocatalytic activities under 500 W Xe light (λ > 420 nm), or even under the 8 W U-type compact fluorescent lamp (433 nm < λ < 700 nm). This increases the possibility of their future application in environmental purification. Acknowledgment. We acknowledge the financial support from the Chinese Academy of Sciences and Shanghai Institute of Ceramics under the program for Recruiting Outstanding Overseas Chinese (Hundred Talents Program), the National Natural Science Foundation of China (No. 50672117), and National Basic Research Program of China (973 Program, 2007CB613302). References and Notes (1) Honda, K.; Fujishima, A. Nature 1972, 238, 37. (2) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol. C: Photochem. ReV. 2000, 1, 1. (3) Hoffmann, M. R.; Martin, S. T.; Choi, W. Chem. ReV. 1995, 95, 69. (4) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341. (5) Zhao, J. C.; Wu, T. X.; Wu, K. Q.; Oikawa, K.; Hidaka, H.; Serpone, N. EnViron. Sci. Technol. 1998, 32, 2394. (6) Choi, W.; Kim, S EnViron. Sci. Technol. 2002, 36, 2019. (7) Wong, C. C.; Chu, W. EnViron. Sci. Technol. 2003, 37, 2310. (8) Xu, Y. M.; Langford, C. H. Langmuir 2001, 17, 897. (9) Ho, W.; Yu, J.; Lin, J.; Li, P. Langmuir 2004, 20, 5865. (10) Tada, H.; Yamamoto, M.; Ito, S. Langmuir 1999, 15, 3699. (11) Carey, J. H.; Lawrence, J.; Tosine, H. M. Bull. EnViron. Contam. Toxical 1976, 16 (6), 697. (12) Frank, S. N.; Bard, A. J. J. Phys. Chem. 1977, 81, 1484.
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