An Efficient Bismuth Tungstate Visible-Light-Driven Photocatalyst for

(J. C. Y.); [email protected] (M. K. H. L.)., †. Shanghai Normal University. , ‡. The Chinese University of Hong Kong. , §. The University of Hong ...
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Environ. Sci. Technol. 2010, 44, 4276–4281

An Efficient Bismuth Tungstate Visible-Light-Driven Photocatalyst for Breaking Down Nitric Oxide G U I S H E N G L I , * ,†,‡,§ D I E Q I N G Z H A N G , ‡ J I M M Y C . Y U , * ,‡ A N D M I C H A E L K . H . L E U N G * ,§ Department of Chemistry, Shanghai Normal University, Shanghai 200234, China, Department of Chemistry, Environmental Science Programme and Centre of Novel Functional Molecules, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China, and Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, China

Received January 10, 2010. Revised manuscript received April 24, 2010. Accepted April 29, 2010.

This paper reports a photocatalytic removal of 400 ppb level of NO in air under visible light irradiation by utilizing threedimensional (3D) hierarchical bismuth tungstate (Bi2WO6) microspheres. A facile microwave-assisted hydrothermal method involving bismuth nitrate and sodium wolframate was developed to synthesize the photocatalyst. The Bi2WO6 samples were characterized by using X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), Raman and ultraviolet-visible reflectance (UV-vis) spectroscopy. The relationship between the physicochemical property and the photocatalytic performance of the as prepared samples is discussed. The present work demonstrates that the 3D hierarchical Bi2WO6 microspheres are effective visible-light-driven photocatalytic functional materials for air purification.

Introduction Nitric oxide, a precursor of photochemical smog, is being released at an alarming rate by combustion processes and infiltration from vehicular emissions (1). This has serious implications on the environment and health of the mankind (2, 3). Nitric oxide in industrial emission can be treated by conventional techniques including physical adsorption, biofiltration, and thermal catalysis methods. However, these methods have low efficiency for pollutants at the parts per billion level, and they cannot solve the postdisposal and regeneration problems (4, 5). Recently, semiconductor photocatalysis, as a “green” technology, has been widely used to treat polluted air and water (6, 7). Outstanding stability and strong oxidation power make TiO2 the photocatalyst of choice for environmental remediation. However, TiO2 semiconductor has a relatively large band gap of 3.2 eV, corresponding to wavelengths shorter than 388 nm (8). This means only a small fraction of * Address correspondence to either author. Phone: +(852)26096268. Fax: +(852)2603-5057. E-mail: [email protected] (G. L.); [email protected]. (J. C. Y.); [email protected] (M. K. H. L.). † Shanghai Normal University. ‡ The Chinese University of Hong Kong. § The University of Hong Kong. 4276

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the solar spectrum can be utilized by TiO2. Thus, developing efficient visible-light-driven photocatalysts has become one of the most important topics in photocatalysis research (9). To date, modifying TiO2 and developing new materials are two general ways to extend the response of a photocatalyst to the visible-light range (10). Doping metals/nonmetals (11–13) and coupling with other lower band gap semiconductors (14, 15) have been effectively utilized to enhance the photoactivity under visible light irradiation. In recent years, some complex oxides with the perovskite or monoclinic structure have been found to have visible-light-driven photoactivity. These include BiVO4 (16), RbLnTa2O7 (Ln ) La, Pr, Nd, and Sm) (17), In1-xNixTaO4 (x ) 0-0.2) (18), MIn0.5Nb0.5O3 (M ) Ca, Sr, and Ba) (19), CaIn2O4 (20), and ZnIn2S4 (21). Recently, semiconducting materials of the Aurivillius oxides Bi2An-1BnO3n+3 (A ) Ca, Sr, Ba, Pb, Na, K, and B ) Ti, Nb, Ta, Mo, W, Fe) have been extensively studied because of their layer structure and unique properties (22). Among these compounds, Bi2WO6, as the simplest member of the Aurivillius family of layered perovskites, has been utilized as an excellent photocatalyst for water splitting and photodegradation of organic compounds under visible light irradiation (23–28). In spite of these advances, deftly controlled synthesis of Bi2WO6 hierarchical structures by efficient and reliable techniques is still challenge. Moreover, the photocatalytic treatment efficiency of Bi2WO6 for gaseous air pollutants has never been reported. In the present work, monodispersed 3D hierarchical Bi2WO6 microspheres were prepared with a facile and costeffective microwave-assisted hydrothermal route involving bismuth nitrate hydrate and sodium wolframate as the starting material. Herein, microwave heating supplied an economical and energy-efficient way to achieve rapid heating, faster kinetics, homogeneity, higher yield, and better reproducibility (29–33). The photooxidation of NO in air over 3D hierarchical Bi2WO6 microspheres was studied. The products were utilized to remove gaseous NO at 400 parts-per-billion level in air under visible-light irradiation. These Bi2WO6 photocatalysts exhibited very strong ability to oxidize the NO gas in air under visible-light irradiation. Importantly, these Bi2WO6 microspheres showed excellent stability and maintained a high level of photocatalytic activity after multiple reaction cycles.

Experimental Section Preparation of 3D Bi2WO6 Microspheres. In a typical procedure, Bi(NO3)3 · 5H2O (2 mmol) was dissolved in 10 mL HNO3 solution (1.2 M) under ultrasonic irradiation. To the above solution, 10 mL Na2WO4 (0.1 M) aqueous solution was added dropwise under stirring for 5 min. The volume of the as formed white suspension was adjusted to 40 mL by adding deionized water. The above solution was sealed in a Teflonlined double-walled digestion vessel. After treating the solution at 170 for 30 min using a microwave system (Ethos TC, Milestone, Italy), the vessel was then cooled to room temperature. The heating rate of the microwave system was kept at 24 °C/min from room temperature to the target temperatures. These products were collected by centrifugation, washed with deionized water and absolute ethanol, and dried in a vacuum at 100 °C for 1 h. The Bi2WO6 microspheres prepared at 170 °C with a microwave heating rate of 24 °C/ min was denoted as Bi2WO6-170-24. A microwave-induced nucleation-aggregation-ripening process (Supporting Information (SI) Scheme S1) was proposed for the synthesis of these hierarchical flower-like spheres. 10.1021/es100084a

 2010 American Chemical Society

Published on Web 05/11/2010

Characterization. The scanning electron microscopy images were recorded on a FEI Quanta 400 FEG microscope. Standard transmission electron microscopy images were recorded in a JEOL-2010F at 200 kV. The electron microscopy samples were prepared by grinding and dispersing the powder in acetone with ultrasonication for 20 s. Carboncoated copper grids were used as sample holders. X-ray diffraction patterns were recorded at room temperature in a parallel mode (ω ) 0.5°, 2θ varied from 20° to 60°) using a Bruker D8 Advance X-ray diffractometer (Cu Ka radiation, λ ) 1.5406 Å). Raman spectra were detected by a Renishaw RM3000 Micro-Raman system. The N2-sorption isotherms were recorded at 77 K in a Micromeritics ASAP 2010 instrument. All the samples were degassed at 150 °C and 10-6 Torr for 24 h prior to the measurement. The BrunauerEmmett-Teller approach was used to determine the surface area. X-ray photoelectron spectroscopy (XPS) measurement was done with a PHI Quantum 2000 XPS system with a monochromatic Al-Ka source and a charge neutralizer. All the binding energies were referenced to the C1s peak at 284.8 eV of the surface adventitious carbon. The diffuse reflectance spectra of the samples over a range of 200-800 nm were recorded by a Varian Cary 100 Scan UV-vis system equipped with a Labsphere diffuse reflectance accessory. Photocatalytic Activity Testing. The photocatalytic experiments for the removal of NO gas in air were performed at ambient temperature in a continuous flow rectangular reactor (10 H cm ×30 L cm ×15 W cm). A 300 W commercial tungsten halogen lamp (General Electric) was used as the simulated solar light source. A piece of Pyrex glass was used to cut off the UV light below 400 nm. Four minifans were used to cool the flow system. Photocatalyst (0.2 g) was coated onto a dish with a diameter of 12.0 cm. The coated dish was then pretreated at 70 °C to remove water in the suspension. The NO gas was acquired from compressed gas cylinder at a concentration of 48 ppm NO (N2 balance, BOC gas) with traceable National Institute of Stands and Technology (NIST) standard. The initial concentration of NO was diluted to about 400 ppb by the air stream supplied by a zero air generator (Thermo Environmental Inc. model 111). The desired humidity level of the NO flow was controlled at 70% (2100 ppmv) by passing the zero air streams through a humidification chamber. The gas streams were premixed completely by a gas blender and the flow rate was controlled at 4 L.min-1 by a mass flow controller. After the adsorption-desorption equilibrium among water vapor, gases and photocatalysts was achieved, the lamp was turned on. The concentration of NO was continuously measured by a chemiluminescence NO analyzer (Thermo Environmental Instruments Inc. model 42c), with a sampling rate of 0.7 L/min.

Results and Discussion The morphology of the as-synthesized Bi2WO6 was investigated by FESEM. Figure 1a shows the morphology of the product in large scale. It reveals that the product is composed of a large quantity of nearly monodispersed flower-like spheres with an average diameter of about 3.0 µm. No other morphologies can be detected, indicating a high yield of these 3D microspheres. The close-up view of the individual spheres (Figure 1b) indicates that the as prepared Bi2WO6 sample possesses a superstructure of a flower-like appearance. The hierarchical flower-like Bi2WO6 structures are built by a large quantity of two-dimensional nanoplates with a thickness of about 10.0 nm. The aggregation and/or assembly of the nanoplates may produce abundant hierarchical pores on nanoscale. High-resolution TEM (HRTEM) image (Figure 1c) was recorded at the edge of an individual Bi2WO6 sphere. It demonstrates the single-crystalline nature of the nanoplates and clarifies the particular orientation of the nanoplates.

FIGURE 1. (a) Low-magnification and (b) high-magnification SEM images of Bi2WO6-170-24 sample, (c) HRTEM image, and (d) SAED pattern of Bi2WO6-170-24 sample. The d spacings are 0.273 and 0.272 nm, which agrees well with the lattice spacing of (200) and (020) of orthorhombic Bi2WO6, respectively (26). A selected-area electron diffraction (SAED) pattern of the hierarchical flower-like Bi2WO6 spheres was also recorded (Figure 1d). It is clear that the overall assembly of Bi2WO6 has a polycrystalline nature. This indicates that the superstructure is organized by the nanocrystalline subunits. Figure 2a shows the typical XRD pattern of the as-prepared hierarchical flower-like Bi2WO6. All peaks for this sample could be indexed to the orthorhombic phase of Bi2WO6 (JCPDS card no. 73-1126), with lattice constants of a ) 5.456 Å, b ) 5.436 Å, and c ) 16.426 Å. No characteristic peaks of the other impurities were observed. The surface compositions and chemical state of the as-prepared flower-like Bi2WO6 were investigated using X-ray photoelectron spectroscopy (XPS). The results are shown in Figure 2b-d. The binding energies were referenced to the C1s peak at 284.8 eV of the surface adventitious carbon. The Bi 4f peaks of the sample appeared at ca. 163.5 and ca. 158.2 eV (Figure 2b). The W 4f peaks of the sample appeared at 36.5, 34.5, 28.1, and 25.1 eV (Figure 2c). The binding energies of both Bi 4f and W 4f were in agreement with the reported values in the literature (27). The O 1s binding energy (Figure 2d) of 529.6 eV could be attributed to the lattice oxygen in crystalline Bi2WO6 (25). The atomic ratio of Bi, W, and O was calculated to be about 2.03:1:6.94 based on the areas of the Bi, W, and O peaks within experimental error. Therefore, it is reasonable that the as-fabricated products could be determined as pure orthorhombic Bi2WO6 based on the results of XRD and XPS measurements. SI Figure S1a shows the Raman pattern of the hierarchical flower-like Bi2WO6. The peaks in the range 600-1000 cm-1 were assigned to the stretches of the W-O band. The bands at 790 and 820 cm-1 of Bi2WO6 were associated with the antisymmetric and symmetric Ag modes (polarized Raman active optical mode) of terminal O-W-O. The band at 310 cm-1 could be assigned to translational modes involving the simultaneous motions of Bi3+ and [WO4]2- (34). The Raman analysis shows that the Bi2WO6 product is well crystallized with an orthorhombic structure. These results are consistent with that of XRD and TEM measurements. UV-visible diffuse reflectance spectroscopy (DRS) was used to characterize the electronic states of the as prepared VOL. 44, NO. 11, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. (a) XRD pattern, and XPS spectra Bi 4f (b), W 4f (c), O 1s (d) of Bi2WO6-170-24. sample. SI Figure S1b represents the UV-visible absorption m2/g. Clearly, the Bi2WO6 hierarchical flower-like structure spectrum of the hierarchical flower-like Bi2WO6 sample. This exhibits a much higher surface area than the samples sample shows the photoabsorption properties from the UV prepared by the solid-state reaction (0.6 m2/g) (25) or the light region to visible light shorter than 470 nm. There is an hydrothermal reaction (20.7 m2/g). This hierarchical structure intense adsorption band with a steep edge in the visible light with meso- and macro- pores can serve as efficient transport region. Such steep shape of the spectra indicated that the paths for reactants and products in photocatalytic reactions visible light absorption band was not due to the transition (38). Meanwhile, the large surface area of Bi2WO6 not only from impurity levels but the band gap transition (35). For a provides more active sites for the degradation reaction of crystalline semiconductor, the optical absorption near the organic compounds, but also effectively promotes the band edge follows the equation Rhν ) A(hν - Eg)n (36), where separation of the electron-hole pairs, resulting in a higher R, ν, Eg, and A are the absorption coefficient, the light quantum efficiency of photocatalytic reaction (20). Light is frequency, the band gap, and a constant, respectively. Among also harvested more efficiently due to the large surface area them, n decides the characteristics of the transition in a and multiple scattering (36). semiconductor. According to the equation, the value of n for To evaluate the photocatalytic performance of these Bi2WO6 equals 1 from the data in SI Figure S1c. The band gap Bi2WO6 samples, the photo-oxidation of NO gas under visible energy of the as-prepared sample is 2.60 eV as estimated light irradiation (λ > 400 nm) in a single pass flow was used from the (ahυ)1/2 versus photon energy plots. This is in good as a photoreaction probe. Figure 3a shows the relative of NO agreement with the reported values (27). The color of the removal rate against irradiation time in the presence of sample was yellowish, consistent with their photoabsorption photocatalysts under visible-light irradiation. As shown in spectrum (26). SI Figure S1d shows the as-prepared sample Figure 3a, the NO removal rate over the Bi2WO6-170-24 has a wide pore size distribution, ranging from 4.0 to 16.0 sample reaches 52% after 30 min irradiation. The photonm, as calculated from the desorption branch of a nitrogen catalyst shows little deactivation since only 2% decrease is isotherm by the BJH method. Besides the small mesopores observed after 1 h reaction, revealing the superior photo(ca. 4.7 nm), large mesopores with a maximum pore diameter catalytic performance of the Bi2WO6-170-24 sample. The small of ca. 16.0 nm can be defined. The inset shows the decrease in activity was due to the accumulation of HNO3 corresponding nitrogen isotherm of the hierarchical Bi2WO6. on the catalyst surface, resulting in deactivation of the There are two capillary condensation steps on the N2 photocatalysts (39, 40). This was confirmed by studying the adsorption-desorption isotherm. These results suggest that effects of initial NO concentration and nitric acid pretreatthe hierarchical Bi2WO6 is composed of independently ment on the activity of the photocatalyst. As shown in SI connected mesopores. The first hysteresis loop, 0.4 < P/P0 Figure S2, the NO removal rate dropped from 52% to 30% < 0.9, of the sample is attributed to the filling of the framework when the initial NO concentration was increased from 400 confined smaller mesopores formed between intra-agto 800 ppb. Pretreating the catalyst with 2 mL of 0.5 M nitric glomerated primary particles (37). The second hysteresis loop acid drastically decreased the removal rate by 55% (SI Figure is at 0.9 < P/P0 < 1.0, corresponding to the filling of larger S2). We utilized ion chromatography (IC) to determine the textural mesopores produced by interaggregated secondary amount of NO3- on the photocatalyst after reaction (SI Figure particles (Bi2WO6 nanoplates). The BET surface area of the S3). After 90 min of photocatalytic reaction under visiblehierarchical flower-like Bi2WO6-170-24 sample is about 37.2 4278

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light, 17.3 µmol of NO3- were found on the Bi2WO6 microspheres. Since the standard redox potential of BiV/BiIII (E0 ) +1.59 eV at pH 0) is much more negative than that of OH•/ OH-(+1.99 eV) (41), the photogenerated holes on the surface of Bi2WO6 cannot react with OH-/H2O to form OH•. Thus, direct hole transfers and O2•- radical oxidation reaction mainly govern the photocatalytic oxidation of NO by Bi2WO6. As known, the potential of the holes is sufficient to generate O2 from water (24), to oxidize CHCl3 and CH3CHO (25), and to dechlorinate 4-CP (42). The oxidation of NO is mainly attributed to the direct reaction with the photogenerated holes. Besides, the OH• radicals produced from the O2•radicals may also serve as the reactive species to oxidize NO. These OH• radicals could be formed by the following reactions. + + hVB Bi2WO6 + visible light f eCB

(1)

·+ O2ads f O2ads eCB

(2)

+ O2ads + 2H+ f H2O2 2eCB

(3)

·· f HOads + OH- + O2 H2O2 + O2ads

(4)

· f HNO2 NO + HOads

(5)

· f NO2 + H2O HNO2 + HOads

(6)

· f HNO3 NO2 + HOads

(7)

·NO + O2ads f NO3

(8)

The photocatalytic performance of the Bi2WO6 samples fabricated at different temperatures was also investigated. As shown in Figure 3a, samples Bi2WO6-150-24, Bi2WO6190-24 and Bi2WO6-200-24 possess only 28, 34, and 25% NO removal rate, respectively, indicating that the microwavehydrothermal temperature has an important effect on the photocatalytic activity of Bi2WO6. The physicochemical parameters of the Bi2WO6 samples were listed in SI Table S1. For a clear quantitative comparison, we use the Langmuir-Hinshelwood model (L-H) to describe the initial rates of photocatalytic destruction of NO (43). The initial photocatalytic degradation of NO was recognized to follow masstransfer-controlled first-order-kinetics approximately as a result of low concentration target pollutants, as evidenced by the linear plot of ln (C/C0) versus photocatalytic reaction time t (Figure 3b). Figure 3c shows the relationship between initial reaction rate constant, BET surface area and NO removal rate. The initial rate constant of Bi2WO6-170-24 sample with 37 m2/g BET surface area is estimated to be

FIGURE 3. (a) Plots of the decrease in NO concentration vs irradiation time in the presence of Bi2WO6 synthesized at different temperature of 170 °C (-9-), 190 °C (-∆-), 150 °C (-b-) and 200 °C (-0-) in a single pass flow of air under visible-light irradiation. (b) Dependence of ln(C/C0) on irradiation time. (c) Relationship between initial rate constant (-O-), removal rate (-∆-) of NO and BET surface area over Bi2WO6 synthesized at different temperature. VOL. 44, NO. 11, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. (a) Plots of the decrease in NO concentration vs irradiation time in the presence of Bi2WO6 synthesized at 170 °C with a heating rate of 12 °C/min (-O-), 24 °C/min (-9-), and 36 °C/min (-b-) in a single pass flow of air under visible-light irradiation. (b) Dependence of ln(C/C0) on irradiation time. (c) The cyclability of Bi2WO6-170-24 for NO removal in a single pass flow of air. 0.0385 min-1, faster than those over Bi2WO6-190-24 and Bi2WO6-200-24 samples with lower BET surface area. These results illustrate that high surface area of Bi2WO6 is a key to the high initial rate constant, resulting a high NO removal rate. This is because high surface area is usually favorable for accelerating the diffusive transport of photogenerated holes to oxidizable species (44). Keeping other experimental conditions unchanged, only conglomeration of Bi2WO6 nanocrystals (SI Figure S4) was observed upon decreasing the reaction temperature to 150 °C. Though the Bi2WO6150-24 sample possesses the highest BET surface area, the NO removal rate is merely 28%, much lower than those of other Bi2WO6 samples. This could be due to the low crystallinity of Bi2WO6-150-24 sample, much weaker than that of the other three samples (shown in SI Figure S5). We also investigated the effect of the heating rates (12, 24, and 36 °C/min) on the NO removal rates over Bi2WO6 samples obtained at 170 °C. As shown in Figure 4a, all of the three samples exhibit high efficiency for the degradation of NO under visible-light irradiation. With a lower heating rate of 12 °C/min, the crystallinity and morphology of Bi2WO6 remained nearly unchanged as shown in SI Figures S6 and S7a. However, a higher heating rate of 36 °C/min resulted in broken flower-like Bi2WO6 microspheres (SI Figure S7b). The BET surface areas were 29.1 and 22.0 m2/g for the Bi2WO6170-12 and Bi2WO6-170-36 samples, lower than that of Bi2WO6170-24. This explains the activity differences among the samples. To test the recyclability, a sample after one trial was washed and dried for the subsequent photoreaction cycles. As shown in Figure 4c, the NO removal rate for hierarchical flower-like Bi2WO6 could be well maintained even after three cycles under visible-light irradiation. Such excellent pho4280

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tocatalytic performance is attributed to its special physicochemical properties, such as hierarchical meso/macro pore structure, large BET surface area and high crystallinity of Bi2WO6. As known, the open mesoporous/macroporous architecture with large surface area and 3D connected poresystem plays an important role in catalyst design for its being able to improve the molecular transport of reactants and products (45–47).

Acknowledgments This research was supported by the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning,the Strategic Investments Scheme administrated by The Chinese University of Hong Kong and the UDF grant from The University of Hong Kong. We also thank Prof. S. C. Lee’s group (Department of Civil and Structural Engineering of The Hong Kong Polytechnic University) for their assistance in the oxidation of NO gas measurements.

Supporting Information Available The formation mechanism and physicochemical parameters of the as-prepared Bi2WO6 samples; Raman pattern, UVvisible absorption spectra, N2-sorption isotherms and poresize distribution curves for sample Bi2WO6-170-24; NO gas removal rate of different conditions, the amount of NO3recovered by extraction with water from the Bi2WO6 microspheres and detected by IC; SEM and TEM images of Bi2WO6150-24 sample; XRD patterns for Bi2WO6 samples synthesized at different temperatures with a heating rate of 24°/min; XRD patterns and SEM images for Bi2WO6 samples synthe-

sized at 170 °C with different heating This material is available free of charge via the Internet at http://pubs.acs.org.

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