ARTICLE pubs.acs.org/IECR
Porous BiOI Sonocatalysts: Hydrothermal Synthesis, Characterization, Sonocatalytic, and Kinetic Properties Limin Song,*,† Shujuan Zhang,*,‡,§ and Qingwu Wei† †
College of Environment and Chemical Engineering & State Key Laboratory of Hollow-Fiber Membrane Materials and Membrane Processes, Tianjin Polytechnic University, Tianjin 300160, P. R. China. ‡ College of Science, Tianjin University of Science & Technology, Tianjin 300457, P.R. China ABSTRACT: A series of porous BiOI nanoparticles were successfully synthesized via a hydrothermal process. The BiOI nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and UVvis absorption/ reflection spectroscopy (UVvis). The results showed that the as-prepared BiOI sonocatalyst generally had flake-like-shaped, porous particles about 100 nm in diameter. The sonocatalytic activity of BiOI was evaluated by the degradation of methylene blue (MB) in aqueous solution under ultrasonic irradiation. The effects of the catalyst dosage and initial MB concentration on the sonocatalytic degradation efficiency of MB were studied. The sonocatalytic degradation kinetics of MB was also investigated. The results showed that the degradation kinetics of MB fitted pseudofirst-order kinetics and the LangmuirHinshelwood model.
1. INTRODUCTION In the past few years, BiOX (X = Cl, Br, I) has received more and more increasing attention of scholars because of their potential applications in degrading organic compounds for environmental protection, photoelectrochemical conversion, and splitting water into H2 and O2.1 BiOI, a narrow-band semiconductor (Eg = 1.771.92 eV), is rapidly gaining popularity among many scholars given its photocatalytic applications and strong absorption in the visible-light region.2 As a photocatalyst, the capability of BiOI in the oxidative degradation of various organic dyes has been investigated in detail. Such organic dyes include methyl orange (MO), methylene blue (MB), rhodamine, and so on.15 Similar with photocatalysis, the sonocatalytic degradation of some organic dyes in aqueous solution is also an excellent method. NanoTiO2 powder is known to exhibit very high sonocatalytic activities in treating various organic dyes in wastewaters.611 However, the sonocatalytic characteristics of BiOI for treating organic dyes has not yet been reported. Recently, BiOI with different shapes, including nanosheets, flower-like structures, and so on, have been prepared in detail. Their photocatalytic activities on the degradation of organic dyes have been studied systemicly.1214 However, the synthesis of the BiOI nanoparticles with the porous structures has not yet been reported at present. In the present work, we successfully synthesized a series of porous BiOI nanoparticles via a hydrothermal process. The porous structures can provide a larger surface area so that the BiOI nanoparticles can be exposed to more active sites and enhance their catalytic activity. In addition, the sonocatalytic degradation activity of MB in aqueous solution using the as-prepared BiOI as sonocatalyst was systematically investigated. The experimental results showed that the porous BiOI nanoparticles excellently sonocatalyzed the degradation of MB. 2. EXPERIMENTAL SECTION 2.1. Chemicals. All chemicals were all of analytical reagent
grade quality and used without further purification. Deionized and doubly distilled water were used throughout this study. r 2011 American Chemical Society
2.2. Synthesis of Porous BiOI Sonocatalysts. The porous BiOI were synthesized by a hydrothermal method. First, 2.0 g (7.7 mmol) Bi(OH)3 was ultrasonic dispersed in 60 mL of deionized water, and then 2.86 g (15.4 mmol) NaI was added to the above solution under stirring. The mixture containing the Bi(OH)3 and NaI was stirred for 0.5 h. The final mixture was shifted to a stainless steel reactor and heated to 160 °C for 12 h. Products were collected by centrifugation, subsequently washed with ethanol for three times, and then dried in a vacuum oven at 60 °C for 4 h.1 2.3. Characterization of Porous BiOI Sonocatalysts. The products were characterized by using X-ray diffraction (XRD) recorded on a Rigaku D/max 2500 powder diffractometer equipped with monochromatic high-intensity CuKα radiation (λ = 1.5406 Å). UV/vis spectra were recorded on a HP8453 spectrophotometer at room temperature. The morphology and size of as-prepared products were observed by transmission electron microscopy (TEM), which was carried out on the Hitachi H-7650 transmission electron microscope. The BrunauerEmmett Teller (BET) surface areas (SBET) were measured by N2 adsorption at 196 °C using an automatic surface area and pore size analyzer (Autosorb-1-MP 1530VP). 2.4. Catalytic Activity Evaluation. The sonocatalytic activity of the porous BiOI samples under ultrasonic irradiation was evaluated by using MB as the model substrate. In a typical process, 100 mL of MB (10 mg/L) aqueous solution and 100 mg of sonocatalyst powder were mixed in an ultrasonic reactor. Prior to a sonocatalytic reaction, the sonocatalyst suspension was stirred to reach adsorption equilibrium with the sonocatalyst in darkness. The pH value of the suspension was about 7.0, and then it was irradiated with the ultrasound of 40 kHz frequency and 80 W output power under continuous stirring and bubbling (air). At a defined time interval, the concentration of MB in the Received: August 9, 2011 Accepted: December 18, 2011 Revised: December 5, 2011 Published: December 18, 2011 1193
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Figure 1. X-ray diffraction patterns of the as-synthesized BiOI sample at 160 °C for 12 h.
Figure 2. UVvis diffuse reflectance spectra of the as-synthesized BiOI sample at 160 °C for 12 h.
ultrasonic reaction was analyzed by using an UVvis spectrophotometer at 665 nm. The residual MB concentration was determined using an UVvis spectrophotometer at 665 nm. The conversion was calculated by (C0 C)/C0, where C is the concentration of the reactant after irradiation, and C0 is the concentration of the reactant after adsorption equilibrium and before the irradiation in the presence of catalyst.
3. RESULTS AND DISCUSSION 3.1. Characterization of the Porous BiOI Sonocatalysts. Figure 1 shows the XRD patterns for the porous BiOI sample prepared at 160 °C for 12 h. For the as-prepared BiOI sample, the diffraction peaks situated at 24.26°, 29.52°, 31.66°, 37.26°, 39.02°, 45.40°, 51.18°, 55.10°, 59.96°, 61.42°, 66.18°, 74.04°, and 75.18° are observed, which are due to the BiOI (101), (102), (110), (103), (004), (200), (114), (212), (213), (204), (302), and (310) lattice planes, respectively, which indicating the sample is the pure tetragonal phase of BiOI (JCPDS 10-0445, space group
Figure 3. TEM images of the as-prepared products at 160 °C for 12 h.
P4/nmm [129], the lattice constants a = b = 3.994 Å and c = 9.149 Å). Other peaks can also be attributed to the BiOI phase, and no impurities were observed. Figure 2 shows the ultravioletvisible (UVvis) reflection spectra of the as-prepared porous BiOI at 160 °C for 12 h. A strong absorption was found within the range of visible light. This finding indicated that the prepared porous BiOI had a good optical (especially visual light) absorption behavior, which may be conducive to sonocatalytic activity. 1194
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Figure 4. The temporal evolution of the spectral changes of MB solution sonodegradation process in the presence of the as-synthesized BiOI sample at 160 °C for 12 h. The initial concentration of MB is 10 mg/L. The dosage of BiOI is 1.0 g/L.
Figure 5. Sonocatalytic activities of the as-synthesized BiOI at 160 °C for 12 h with different dosage. The initial concentration of MB is 10 mg/L.
Figure 3 shows the transmission electron microscopy (TEM) image of the porous BiOI nanocrystals prepared at 160 °C for 12 h. The product particles had irregular structures as well as widths and lengths of about 80120 nm. A further amplification of the TEM image revealed that inside the particles were numerous tiny pores. These pores endowed the catalyst with larger surface areas and more exposed active sites, which helped enhance the sonocatalytic activity. 3.2. Sonocatalytic Activity of the Porous BiOI Sonocatalysts. The specific surface area of the as-synthesized BiOI sample at 160 °C for 12 h were investigated by using nitrogen adsorption. The BET specific surface area of the sample is 45.5 m2/g. Figure 4 displays the UVvis absorption spectra of the original and degraded MB solutions (100 mL of 10 mg/L MB aqueous solution and 100 mg of sonocatalyst powder). Only two absorption peaks at 290 and 665 nm were observed, which were assigned to the benzene ring and the azo bond in the MB molecule. Changes in the absorption spectra of the MB aqueous solution exposed to ultrasonic irradiation for 0180 min in the
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Figure 6. Sonocatalytic activities of the as-synthesized BiOI at 160 °C for 12 h on reaction time under ultrasonic irradiation. The initial concentration of MB is 10 mg/L.
Figure 7. Sonocatalytic activities of the as-synthesized BiOI at 160 °C for 12 h with different initial concentrations of MB. The dosage of BiOI is 1.0 g/L.
presence of the porous BiOI nanoparticles were also evident. Under ultrasonic irradiation, MB absorption at 290 and 665 nm rapidly decreased. The MB degradation rate was very fast at the beginning of the irradiation and then became slow. A rapid decrease of the two major absorption bands within 30 min indicated that the molecular structures of the porous BiOI were destroyed. The porous BiOI nanoparticles exhibited excellent sonocatalytic activity in degrading MB. The amount of catalyst is known to have a very important effect on sonocatalytic degradations. To determine the amount of porous BiOI nanoparticles most optimal for degradation, different catalyst amounts (0.5 to 1.5 g/L) were tested. Figure 5 shows that the MB degradation ratio obviously increased with the porous BiOI amounts of 0.51.5 g/L. This finding could be attributed to the exposure of more active sites in the porous BiOI nanoparticles. The degradation ratio increased up to 96% after adding 1.5 g/L activated carbon under 30 min of ultrasonic irradiation. Figure 6 shows the changes in the degradation ratio with irradiation time for a 100 mL suspension of 10 mg/L MB and 1.0 g/L 1195
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Figure 8. Sonocatalytic activities of the as-synthesized BiOI at 160 °C for 12 h for different dyes. The initial concentration of dyes is 10 mg/L. The dosage of BiOI is 1.0 g/L.
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Figure 10. The relationship between the 1/kap and the initial concentration of MB. pH = 6.9, [BiOI] = 1.0 g/L, gas flow rate = 0.15 m3/h, temperature = 23 °C, and reaction time = 30 min.
of MB molecules may weaken the catalytic degradation ability of the porous BiOI nanoparticles. To investigate the activity of different dyes in the presence of the porous BiOI nanoparticles, MB, MO, and rhodamine B (RhB) were used as reactant molecules. The volume and concentration of the dye solutions were 100 mL and 10 mg/L, respectively. The amount of the porous BiOI nanoparticles was 1.0 g/L. Figure 8 shows that the degradation rates of MO, MB, and RhB on the porous BiOI nanoparticles were 61%, 85%, and 91%, respectively. These results showed that the porous BiOI nanoparticles also excellently degraded the other dyes. 3.3. Kinetics Analysis. Regarding degradation kinetics, Figure 9 shows that MB degradation in the presence of the porous BiOI nanoparticles could be well-depicted as first-order kinetics dðCt =dtÞ ¼ kCt lnðC0 =Ct Þ ¼ kt
Figure 9. The plots of ln C0/C and reaction time with different initial concentrations of MB. pH = 6.9, [BiOI] = 1.0 g/L, gas flow rate = 0.15 m3/h, and temperature = 23 °C.
porous BiOI nanoparticles. The MB adsorption amount on the surface of the porous BiOI nanoparticles was only 11%, and the MB degradation rate without a catalyst was only 8%. These findings showed that most MB molecules were destroyed by the sonocatalytic action. In addition, the MB degradation rate using P25 was only 40% under the same conditions. In contrast, the MB degradation rate using the porous BiOI nanoparticles reached 85%, which indicated that the porous BiOI nanoparticles had a better sonocatalytic activity. A series of MB solutions with different initial concentrations ranging from 5 to 20 mg/L were used to study their influence on the degradation ratio within 30 min of ultrasonic irradiation. Figure 7 shows that the MB degradation ratio remained at about 94% in the presence of the porous BiOI nanoparticles under ultrasonic irradiation when the initial MB solution concentration was 5 mg/L. The ratio then gradually decreased with increased concentration. The ratio reached 69% when the initial MB solution concentration was 20 mg/L. Obviously, a low initial MB concentration was beneficial for MB degradation. A large number
In the two equations, Ct is the concentration of the MB aqueous solution at reaction time t, C0 is the initial MB concentration, and k is the reaction rate constant. A good linear relationship was observed between ln(C0/Ct) and irradiation time t (R2 > 0.98). The MB degradation rate constants were calculated to be 9.6 102, 6.2 102, 5.0 102, and 3.8 102 mg/L 3 min1, corresponding to the different initial concentrations of 5, 10, 15, and 20 mg/L. Apparently, a high initial MB concentration was not conducive to the degradation activity. For a heterogeneous sonocatalytic system, the sonocatalytic reaction may occur on the catalyst surface. Therefore, reactant adsorption on the catalyst surface is believed to generally follow the LangmuirHinshelwood (L-H) model r0 ¼ dC0 =dt ¼ ðkKC0 Þ=ð1 þ KC0 Þ The equation is further expressed in a linear form 1=r0 ¼ 1=k þ 1=ðkKC0 Þ The dependence of the degradation rate of MB on its initial concentration was fitted to the LangmuirHinshelwood model. 1196
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Industrial & Engineering Chemistry Research In this model, C0 is the initial MB concentration MB (mg/L), r0 is the initial sonocatalytic degradation rate (mg/L/min), K is the Langmuir adsorption constant in the sonocatalytic degradation reaction (L/mg), and k (mg/L/min) is the L-H rate constant. Figure 10 shows the good linearity (R2 > 0.98) between 1/r0 and C0. This relationship indicated that MB degradation using the porous BiOI nanoparticles were well-described by the Langmuir Hinshelwood model and was controlled by MB preadsorption.
4. CONCLUSION In summary, the porous BiOI nanoparticles were synthesized by a hydrothermal route. The as-prepared BiOI particles show good performance on the degradation of MB, MO, and RhB under ultrasonic irradiation. Sonocatalytic kinetics analysis demonstrates that the MB degradation in the presence of the porous BiOI nanoparticles could be well depicted as first-order kinetics and followed the LangmuirHinshelwood model.
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’ AUTHOR INFORMATION Corresponding Authors
*Phone/Fax: +86-22-60600658. E-mail:
[email protected] (L.S.);
[email protected] (S.Z.). Notes §
Co-first author.
’ ACKNOWLEDGMENT This work was supported by Tianjin Science & Technology Project of Research Fundation (Grant 20100206) and the National Natural Science Foundation of China (Grant 21103122). ’ REFERENCES (1) Zhang, X.; Zhang, L. Z. Electronic and band structure tuning of ternary semiconductor photocatalysts by self-doping: the case of BiOI. J. Phys. Chem. C 2010, 114, 18198–18206. (2) Li, Y. Y.; Wang, J. S. p; Yao, H. C.; Dang, L. Y.; Li, Z. J. Efficient decomposition of organic compounds and reaction mechanism with BiOI photocatalyst under visible light irradiation. J. Mol. Catal. A: Chem. 2011, 334, 116–122. (3) Gondala, M. A.; Chang, X. F.; Yamania, Z. H. UV-light induced photocatalytic decolorization of Rhodamine 6G molecules over BiOCl from aqueous solution. Chem. Eng. J. 2010, 165, 250–257. (4) Zhang, X. L.; Zhang, Z.; Xie, T. F.; Wang, D. J. Low temperature synthesis and high visible-light-induced photocatlytic activity of BiOI/TiO2 heterostructures. J. Phys. Chem. C 2009, 113, 7371–7378. (5) Zhang, X.; Ai, Z. H.; Jia, F. L.; Zhang, L. Z. Generalized one-pot synthesis, characterization, and photocatalytic activity of hierarchical BiOX (X=Cl, Br, I) nanoplate microspheres. J. Phys. Chem. C 2008, 111, 747–753. (6) Wang, J.; Guo, B. D.; Zhang, X. D.; Zhang, Z. H.; Han, J. T.; Wu, J. Sonocatalytic degradation of methyl orange in the presence of TiO2 catalysts and catalytic activity comparison of rutile and anatase. Ultrason. Sonochem. 2005, 12, 331–337. (7) Wang, J.; Ma, T.; Zhang, Z. H.; Zhang, X. D.; Jiang, Y. F.; Dong, D. B.; Zhang, P.; Li, Y. Investigation on the sonocatalytic degradation of parathion in the presence of nanometer rutile titanium dioxide (TiO2) catalyst. J. Hazard. Mater. 2006, 137, 972–980. (8) Wang, J.; Pan, Z. J.; Zhang, Z. H.; Zhang, X. D.; Wen, F. Y. Sonocatalytic degradation of methyl parathion in the presence of nanometer and ordinary anatase titanium dioxide catalysts and comparison of their sonocatalytic abilities. Ultrason. Sonochem. 2006, 13, 493–500. 1197
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