ARTICLE pubs.acs.org/Langmuir
Fabrication of Titanium Dioxide and Tungstophosphate Nanocomposite Films and Their Photocatalytic Degradation for Methyl Orange Ping Niu†,‡ and Jingcheng Hao*,† † ‡
Key Laboratory for Colloid and Interface Chemistry (Shandong University), Ministry of Education, Jinan 250100, P. R. China Department of Chemistry, Dezhou University, Dezhou 253023, P. R. China ABSTRACT:
Photocatalytic multilayer films with different numbers of bilayers were prepared via an electrostatic layer-by-layer (LbL) selfassembly method. These LbL films were characterized by UVvis spectroscopy, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Our results indicate that TiO2 and tungstophosphate (H3PW12O40, abbreviated as PW12) are successfully incorporated into the thin films. The as-prepared (TiO2/PW12)n films show good photocatalytic performance toward methyl orange (MO) solution at pH 2.0, which is attributed to the synergistic effect between TiO2 and PW12. The effect of experimental parameters including number of bilayers, initial concentration, and pH value of dye solution were also studied. The multilayer films can be easily recovered and reused several times with little change of degradation, indicating that they are stable under the ultraviolet (UV) irradiation. The detection of active species displays that active holes (h+) play a dominant role for MO photodegradation in the TiO2/PW12 system. Taking advantage of immobilization of catalysts on glass slides, the problem of recovery is solved. It is expected that photocatalytic multilayer films have substantial applications in industry.
’ INTRODUCTION Artificial dyes, especially azo dyes, are widely found in effluents discharged from textile industries. Industrial wastewater of azo dyes causes serious problems to aquatic environments due to their high toxicity, slow biodegradation, and potential carcinogenicity.1 A great deal of studies on the removal of dyes have been made. Physical treatments, such as adsorption, flocculation, and membrane filtration, can transfer the dyes from one phase to another and produce huge amounts of solid waste. Traditional biological treatments have been proven insufficient to remove textile dyes with strong color. Therefore, advanced oxidation process, i.e., a destructive process, has been developed, by which dye effluents can be completely degraded and mineralized.1 In past decades, TiO2 photocatalyst has been extensively studied for the degradation of organic dyes because of its low cost, innoxiousness, chemical inertness, and high photocatalytic performance.2 However, the photocatalytic efficiency of TiO2 is remarkably decreased owing to the fast recombination of r 2011 American Chemical Society
photogenerated electronhole pairs. An alternative approach to enhance separation efficiency of electronhole pairs is the addition of polyoxometalates (POMs) to TiO2. POMs, considered efficient electron acceptors, can successively transfer the photogenerated electrons from the TiO2 conduction band to the empty d orbital of theirs. Thus, the recombination of photogenerated carriers is suppressed effectively due to the synergistic effect of POMs and TiO2. Several groups have reported the synergistic effect. Choi3 and Ferry4 reported that enhanced photodegradation efficiency for methyl orange or 1,2-dichlorobenzene was obtained by incorporating POMs into TiO2 suspension. However, the industrial applications of POMs are hindered due to their high solubility. In recent years, Hu5,6 and Wu7,8 have proven the synergistic effect existed in a series of novel Received: August 14, 2011 Revised: September 30, 2011 Published: October 05, 2011 13590
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Langmuir Scheme 1. Chemical Structures of (a) PAH, (b) PSS, (c) H3PW12O40, and (d) and (e) Quinoid and Azo Structure of Methyl Orange, Respectively
photocatalysts composed of nanoporous TiO2 particles and homogeneously dispersed Keggin unit. Their studies indicated that these composites not only showed higher photocatalytic oxidation rate for organic contaminants than pure TiO2 or pure POMs, but also easily recovered through centrifugation due to the insolubility of TiO2POMs composites. The layer-by-layer self-assembly method is based on the alternate absorption of oppositely charged building blocks. It has been evolved as a powerful approach to produce different composite films on various types of substrates. In comparison with other methods, the LbL technique has significant advantages in film stability and catalyst reusability. What is more, catalysts fabricated by this technique are more readily separated, and less catalyst is required.12 In the past decade, the photocatalytic activity of TiO2 or POM hybrid films constructed by the LbL method has been extensively investigated.915 Recently, study on the POMs/TiO2 composite films constructed by the LbL method has been reported.1619 Incorporation of nanoparticles into films enhanced the photocatalytic ability for the degradation of gaseous 2-propanol or acetone1618 and photoelectrocatalytic activity for methanol.19 However, there has been no report on dye photodegradation by the TiO2/POMs nanocomposite films fabricated by the LbL method, and the role of active species produced in the TiO2/POMs system has not been illuminated until now. In this work, we fabricated the TiO2/PW12 nanocomposite films by the LbL self-assembly method and first applied them to photodegrade dye effluents. For comparison, inactive polyanions, poly(styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH), replaced the negatively and positively charged component of TiO2/PW12 films, respectively. To achieve the optimal photocatalytic efficiency, the effect of number of bilayers, initial concentration, and pH value of dye solution on photodegradation of MO was investigated. The stability of hybrid films and the role of active species involved in the photodegradation process were also discussed.
’ EXPERIMENTAL SECTION Materials. Titanium dioxide (TiO2) colloid was prepared according to the literature.20 X-ray diffraction (XRD) revealed that the TiO2 colloid was anatase with a mean particle size of 4 nm. Commercial polyelectrolytes, poly(sodium 4-styrenesulfonate) (PSS, Mw ∼70 000) and poly(allylamine hydrochloride) (PAH, Mw ∼70 000), were purchased from Sigma-Aldrich Inc., USA, and used without further treatment. H3PW12O40 3 xH2O (PW12) was purchased from Sinopharm Chemical Reagent Co., Ltd., China. Methyl orange (MO) was obtained from Jining Chemical Engineering Research Institute. Perchloric acid (HClO4) was
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Scheme 2. Formation of (TiO2/PW12)n Multilayer Film with n=2
obtained from Shanghai Jinlu Chemical Co., Ltd., China. All other chemicals were analytically pure and used as received. The typical structures of materials are presented in Scheme 1. Assembly of Multilayer Films. The solid substrates containing quartz slides, silicon wafers, and microscopic glass slides were treated according to the literature.21 The hydrophilic substrates were first immersed in PAH aqueous solution for 15 min, then rinsed by diluted HClO4 solution at pH = 2.5 and dried by N2. Subsequently, PSS was assembled in the same way as PAH. The concentration of PAH and PSS was 2.0 mg mL1 with 0.5 mol L1 NaCl and the pH was ∼2.5 adjusted by 3.0 mol L1 hydrochloride acid aqueous solution. The procedure described above was repeated once to gain a surface with uniform charge. Then, the substrates with four precursor layers were alternately dipped in positively charged TiO2 colloid (16 g L1, pH ∼2.0) for 15 min and 3 mmol L1 negative charged PW12 solution for another 15 min until the desired number of bilayers was obtained. Rinsing and drying were conducted after each deposition cycle. The multilayer films with the desired number were described as (TiO2/PW12)n, where n was the number of bilayers. For comparison, the controlled (PSS/TiO2)10 and (PAH/ PW12)10 composite films were prepared in a similar way. As for (PSS/ TiO2)10 films, the number of precursor layers was three so as to gain a final layer with TiO2 catalyst. Meanwhile, (PAH/PW12)10 films were deposited on quartz slides because PW12 was only excited by light with wavelengths less than 250 nm. The self-assembly procedure of a typical (TiO2/PW12)n film with n = 2 is illustrated in Scheme 2. Characterization of Thin Films. UVvis absorption spectra were recorded using HITACHIU-4100 spectrophotometer to monitor the growth process of multilayer films. Atomic force microscopic (AFM) images, which were acquired in air with a Digital Nanoscope IIIa Instrument operating in the tapping mode, were used to observe surface morphology of the composite films. In order to confirm the atomic composition of the films, the XPS analyses were performed on a PHI-5702 multifunctional X-ray photoelectron spectroscope, using Al Kα radiation (photon energy 1476.6 eV) as the excitation source. Measurements of Photocatalytic Activity. Photocatalytic experiments were conducted on XPA-system photochemical reactor purchased from Xujiang electromechnical plant, Nanjing of China. A 300 W medium-pressure mercury lamp (MPML, λmax = 365 nm) equipped with a double-walled quartz glass tube for water cooling was selected as light source, and the light source was located 18 cm above the dye solution. The photocatalytic decontamination of MO was evaluated in air at ambient temperature. In a typical experiment, a 20 mL MO aqueous solution was added into the culture dish. Then, six pieces of glass slides (12.7 38.1 mm2/piece) coated with films were suspended into a dye solution. The degradation reaction of MO started after the intensity of mercury lamp became stable. A decrease in MO absorbance was monitored by Shimadzu 2450 UVvis spectrophotometer, and the corresponding concentration 13591
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was calculated in light of standard curve. The degradation ratio can be estimated by the following formula degradationð%Þ ¼
Α0 Α 100% Α0
where A0 represents original absorbance of MO solution at their maximum absorption wavelength (λmax), A is residual absorbance of MO at λmax after UV light irradiation.
’ RESULTS AND DISCUSSION Characterization of Composite Films. UVvis spectroscopy was used to monitor the self-assembly process of multilayer films.
Figure 1. UVvis spectra of the (TiO2/PW12)n composite films assembled on quartz slides. Inset shows the relationship of absorbance at 253 nm versus the number of bilayers of (TiO2/PW12)n films.
Figure 1 gives the UVvis curves of the (TiO2/PW12)n composite films (with n = 2, 4, 6, 8, 10) assembled on (PAH/PSS)2 precursor film-coated quartz slides. The (PAH/PSS)2 precursor films show no absorbance in 230500 nm UVvis region. The absorbance at 253 nm, which is assigned to charge transfer from O to W atom in the Keggin unit,15 increases with TiO2 and PW12 deposition. The inset shows the absorbance at 253 nm as a function of bilayer number and an approximately linear relationship is observed. The near-linearity of the curve indicates that the amount of catalysts deposited on films is equal at each deposition cycle. The results above clearly prove the regular growth of the multilayer films and high reproducibility of the LbL method. AFM investigation was performed to observe detailed information about the surface morphology and homogeneity of multilayer films. Figure 2 presents AFM images of the (PAH/ PSS)2 precursor film and (TiO2/PW12)4 multilayer film. It can be seen from Figure 2a that the precursor film is fairly smooth and uniform with root-mean-square (rms) roughness of 1.887 nm calculated from an area of 1.0 1.0 μm2. Rms roughness increases to 4.867 nm after adsorption of TiO2 and PW12 on the precursor film as shown in Figure 2b. Film surface consists of a great deal of microdomains which are round in shape with size of about 45 nm. The formation of these small domains perhaps arises from the aggregation of TiO2 colloidal nanoparticles and PW12 nanoclusters. Furthermore, the 3-D AFM image of the (TiO2/PW12)4 multilayer film displays vertical grain structure, suggesting that the aggregated particles are uniformly distributed over the surface of substrate. The atomic composition and chemical state of the LbL films supported on single-crystal silicon slides were measured by XPS analysis. The Ti 2p and W 4f XPS lines of (TiO2/PW12)5
Figure 2. AFM images of the (a) (PAH/PSS)2 precursor film and (b) (TiO2/PW12)4 multilayer film assembled on silicon slides. 13592
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Figure 3. XPS results of the (TiO2/PW12)5 multilayer films assembled on silicon slides.
Figure 4. UVvis absorption spectra changes of MO (10 mg L1, pH = 2.0) solution in the presence of composite films: (a) (TiO2/PW12)10, (b) (PSS/TiO2)10, and (c) (PAH/PW12)10.
multilayer films are shown in Figure 3. It is clear that the multilayer films contain Ti and W elements according to the results shown in Figure 3. The binding energies (BE) of Ti 2p3/2 and Ti 2p1/2 appear at ca. 459.1 and 464.9 eV, respectively. The two peaks of Ti 2p belong to anatase TiO2 nanoparticles in multilayer films.22 The W 4f peaks, which are the characteristics of the highest oxidation state of W atom in multilayer films, appear at 35.8 eV (7/2) and 37.8 eV (5/2).23 The W 4f signals are assigned to Keggin-type POM molecules. XPS results once again confirm the existence of the TiO2 nanoparticles and PW12 polyanions in the multilayer films, which are consistent with the UVvis spectra results. Photocatalytic Performance of Composite Films. In order to evaluate the photocatalytic efficiency of the composite films, methyl orange (MO), a type of azo dye, was used as a probe for
the photocatalytic degradation. It is widely used in textiles, foodstuffs, papers, and leathers. The release to aquatic environment causes esthetic pollution and harm to human health due to its toxic, carcinogenic, and mutagenic effects. MO has two different structures, azo (yellow) and quinoid (red) forms as pH value is greater than 4.5 and less than 3.1, respectively. The maximum absorption peak of MO shifts from 463 nm under nearly neutral condition to 487 nm at pH 4.0 and to 507 nm at pH 2.0, which is due to the delocalization of lone pair electrons on the azo group. The change of MO structure at different pH not only causes red shift of the maximum absorbance peak, but also increases the peak intensity.24 In current study, the photocatalytic tests are carried out at pH 2.0 in that PW12 becomes unstable at high pH.5 Figure 4 13593
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Figure 5. Change of MO concentration (the initial concentration: 10 mg L1, pH = 2.0) using (TiO2/PW12)n composite films as catalyst.
Figure 6. Degradation of MO (initial concentration: 10 mg L1) at different pH value by (TiO2/PW12)10 composite films.
shows the UVvis absorption spectra changes of aqueous MO (10 mg mL1) solution in the presence of (TiO2/PW12)10, (PAH/PW12)10, and (PSS/TiO2)10 composite films. It can be seen from Figure 4a that the intensity of MO absorption peaks gradually decreases as irradiation time increases. At the same time, the color changes from red to nearly colorless after irradiation for 60 min, which suggests that MO is almost completely decomposed. Compared to (PAH/PW12)10 and (PSS/TiO2)10 multilayer films, the (TiO2/PW12)10 films display higher photocatalytic efficiency over the same period. The order of MO degradation efficiencies by above-mentioned photocatalysts is (TiO2/PW12)10 > (PSS/TiO2)10 > (PAH/PW12)10. Given that the amount of TiO2 assembled in (TiO2/PW12)10 film approaches that in the (PSS/TiO2)10 film,19 the higher activity may derive from a synergistic effect of TiO2 and H3PW12O40.38 PW12, an efficient electron acceptor, can successfully transfer photogenerated electrons from the TiO2 conduction band to the empty orbital of PW12 to improve the photocatalytic performance of TiO2/PW12 films. The enhancement of charge separation efficiency results in active holes and electrons having enough time to produce reactive oxygen species (ROSs). These ROSs and active holes, which also have higher reduction potential, can completely oxidize various organic contaminants to carbon dioxide and inorganic salts without selectivity. However, without the synergistic effect, the fast recombination of charge pairs cannot be avoided in pure POMs and TiO2. Therefore, the photocatalytic efficiency is inferior to the composite films consisting of these two photoactive components. It can be inferred from the above results that PW12 is of crucial importance in enhancing the photocatalytic performance of composite films. To check the effect of bilayer number of (TiO2/PW12)n films on MO photodegradation, control experiments were performed. The results are shown in Figure 5. Without UV light, degradation of MO is hardly observed in the presence of composite films (but this is not shown in here), and only 12.9% MO is degraded under UV light irradiation for 60 min in the absence of catalysts. It can be seen in Figure 5 that concentration of MO decreases with increase of bilayer number. After exposure to UV light for 60 min, the degradation of MO in the presence of (TiO2/PW12)n reaches 55.3%, 59.9%, 60.3%, 80.1%, 93.2%, and 92.1% with n = 2, 4, 6, 8, 10, and 15, respectively. It is obvious that the difference of photocatalytic activity between (TiO2/PW12)10 and (TiO2/PW12)15 composite films is negligible. So, the optimal bilayers number of (TiO2/PW12)n films is 10. It is interesting to note that the
conversion rate of MO is not linearly increased with increasing number of bilayers, which is consistent with results shown in the reference.12 The results above also account for the fact that not all the absorbed TiO2 nanoparticles and PW12 polyanions can be excited by UV light to destroy MO. In other words, the contribution to pollutant degradation is different for TiO2 and PW12 catalysts in inner layers and outer layers. The pH value of the solution plays a crucial role in the rate of photocatalytic reactions. In previous studies, researchers found the different influential tendency of pH value on the photodegradation of MO.2530 Zhang25 found that TiO2 films exhibited higher efficiency in acidic media than alkaline media on photodecomposition of MO. Similar influential tendency of pH was also observed in other systems containing TiO2,2628 but Ag-DP25 exhibited high photocatalytic performance at pH = 6.6.29 The Subramanian group30 reported that the decomposition rate of MO was higher in alkaline condition than in acid or neutral condition. Hence, the photocatalytic performance of catalysts is closely related to raw materials, preparation, and test parameters. In our study, the pH value is adjusted by HClO4 solution and not controlled in the reaction process. The photocatalytic activity of (TiO2/PW12)10 composite catalyst at different pH values is presented in Figure 6. In this section, the degradation ratio is calculated according to the absorbance of MO measured at wavelengths of 463, 487, and 507 nm, corresponding to the maximum absorption wavelength at pH = 6.4, 4.0, and 2.0, respectively. The degradation of MO is 93.2% at pH = 2.0, 54.5% at pH = 4.0, and only 10.0% at pH = 6.4. It is obviously seen that, for (TiO2/PW12)10, photocatalytic effciency decreases with increasing pH. (TiO2/PW12)10 exhibits higher degradation rate at lower pH. The presence of PW12 polyanions in the outermost layer makes the composite film surface carry a negative charge. The negatively charged surfaces can accelerate transfer rate of holes, facilitating the separation of electronhole pairs. As a consequence, the electrons have enough time to react with O2 adsorbed on catalyst surface to generate 3 O2 or H2O2 reactive oxygen species. In addition, holes can also react with adsorbed water to produce 3 OH.25 These reactive oxygen species and holes are strong oxidant, which can result in the complete degradation of MO. The decomposition of H3PW12O40 is responsible for the rather poor degradation efficiency at neutral pH. Another reason for (TiO2/PW12)10 with higher activity at acidic condition is that MO changes from azo structure to quinoid structure when the pH is below 3.1. Quinoid structure is more readily photodecomposed. 25 This fact can be proven by the fact that 13594
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Figure 7. Effect of MO initial concentration (pH = 2.0) on its degradation by (TiO2/PW12)10 catalyst. The inset is the degradation of MO under different concentrations after 60 min irradiation.
Figure 8. Influence of repetitive use on photodegradation of MO dye by (TiO2/PW12)8 films.
the degradation of MO by UV light only increases from 1.9% to 12.9% when pH value decreases from 6.4 to 2.0. The effect of initial MO concentration on degradation rate was also investigated by varying MO concentration from 5 mg L1 to 20 mg L1. It can be seen from Figure 7 that concentration of MO constantly deceases with increasing irradiation time. Degradation efficiency of MO is decreased with increasing dye concentration as shown in the inset of Figure 7. The degradation efficiency of MO is 93.2% at the initial concentration of 10 mg L1. It increases to 93.4% with 5 mg L1 MO but decreases to 84.5% and 71.5% with 15 mg L1 and 20 mg L1 MO, respectively. It is obvious that, when the concentration of dye is larger than 15 mg L1, the degradation of dye could not be effective.25 This phenomenon can be explained by the following reasons. One is that, at higher dye concentration, light transmittance of the dye solution is reduced to some extent due to the UV-screening effect of the dye itself, which in turn causes fewer incident photons to reach the catalyst surface, resulting in a decrease of degradation rate.12,25 Another reason is that, when the intensity of light source and illumination time are fixed, the amount of radicals produced is also a constant.12 Consequently, excess dyes cannot be oxidized and low degradation is obtained at higher MO concentration. For the above reasons, high concentration
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Figure 9. Effect of MeOH, i-PrOH, and I on degradation of MO (initial concentration: 10 mg L1, pH = 2.0) by (TiO2/PW12)15 films: (a) without additives, (b) with cMeOH = 10 or 100 mmol L1, (c) with ci‑PrOH = 20 or 100 mmol L1, and (d) with cKI = 20 or 100 mmol L1.
contaminants needs more time or more catalysts to be removed completely. In fact, the recovery and reusability of the catalysts is a very important issue from the view of industrial application. In our work, the long-term stability and reusability of films were checked by repetitive use of the catalysts for five cycles. The results are illustrated in Figure 8. It is obvious that, after five cycles, the degradation of MO increases from 80.1% to 88.0%, which is opposite the results reported in refs 12 and 13. However, this phenomenon was also found in photocatalytic degradation of MO by TiO2 suspension.28 It can be attributed to change of catalyst surface property and bound water produced on the reused catalysts surface.28 Further studies will be performed to clarify the surface change of catalysts after reuse. A large number of active species, such as h+, 3 OH, 3 O2, and H2O2, are involved in the photodegradation process of dyes under visible or ultraviolet light. The role of active species in the degradation process was studied extensively, yet lots of controversies still exist.3134 To clarify the roles of valence band holes and reactive oxygen species in TiO2/POM film systems, we quenched the 3 OH radicals and holes with alcohols and iodine ions, respectively. The degradation of MO in the presence of different scavengers is shown in Figure 9. If the photodegradation reaction was dominated by 3 OH radicals, the addition of methanol or isopropanol would significantly inhibit MO degradation.31,34 However, no inhibition is observed when alcohol is added, suggesting that the 3 OH radicals are not responsible for MO degradation. When iodide ions, excellent quenchers of valence band holes and 3 OH radicals,31,32 are used as a diagnostic tool to suppress the holes process, the degradation of MO is greatly decreased, as shown in Figure 9. With 20 mmol L1 KI, the degradation efficiency drops to 54.3%. However, only 30.4% MO is decomposed when the concentration of KI is 100 mmol L1. This indicated that the total contribution of 3 HO radicals to the MO degradation is 61.7%. Because the contribution of UV light is 12.9%, it is deduced that the contribution of other active species, such as 3 O2 or H2O2, is only 17.5%. This figure is obtained by subtracting the contribution of UV light (12.9%) from the total degradation in the presence of 100 mmol L1 KI (30.4%). The results above suggest that valence band holes play an overwhelming role in the MO degradation process. The kinetics of MO photodecomposition by composite films with different bilayers number was investigated. The plots of ln c0/c (where c0 stands for the initial concentration and c is the 13595
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Figure 10. Reaction kinetic study of photodegradation of MO (the initial concentration: 10 mg L1, pH = 2.0) by (a) (TiO2/PW12)n, (b) (PSS/ TiO2)10, and (PAH/PW12)10.
Table 1. Rate Constants (k) and Linear Correlation Coefficients (R2) for Degradation of MO under Different Catalysts catalyst
number of bilayers
k (min1)
R2
(TiO2/PW12)n
2
0.0135
0.9980
4
0.0156
0.9962
6
0.0153
0.9988
8 10
0.0274 0.0465
0.9964 0.9923
(PSS/TiO2)n
10
0.0187
0.9938
(PAH/PW12)n
10
0.0030
0.9983
residual concentration at time t) versus time for MO photodegradation are shown in Figure 10. The linear fit between ln c0/c and t indicates that photodecomposition of diluted MO solutions agrees well with apparent first-order kinetics. That is to say, the photocatalytic reaction fits with the LangmuirHinshelwood mechanism. The first-order rate constants (k) and the linear correlation coefficients (R2) are both listed in Table 1. It can be found that the rate constant increases with increasing number of bilayers. That means photocatalytic efficiency is enhanced. As shown in Figure 10, the degradation rate of MO solution by (TiO2/PW12)10 samples is much faster than that by (PSS/TiO2)10 and (PAH/PW12)10 catalysts under the same condition, which is ascribed to the synergistic effect of TiO2 and PW12.
’ CONCLUSIONS Photocatalytic performance of (TiO2/PW12)n composite films prepared on microscopic glass slides was evaluated by destructing MO dye wastewater. Compared with (PSS/TiO2)10 and (PAH/PW 12 )10 , (TiO 2 /PW12 )10 multilayer films show superior photocatalytic activity due to the synergistic effect between TiO2 and polyoxometalates. The photocatalytic efficiency of (TiO2/PW12)n composite films highly relies on film thickness. The pH has a great effect on the MO decomposition, and the highest degradation is obtained at pH 2.0. Investigation of the role of active species demonstrates that active holes (h+) are the predominant species in MO photodegradation process. Immobilization of catalysts by LbL method solves the problem of recovery, and films can be reused for five cycles with a slight enhancement of degradation. The greatest advantage of catalyst immobilized on microscopic glass slides can sharply curtail the cost of wastewater treatment, which is very important in practical
application. Furthmore, this method has potentially practical applications on the treatment of other texitile wastewater.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Phone/Fax: +86-531-88366074 (o).
’ ACKNOWLEDGMENT This work was financially supported by the NSFC (Grant No. 21033005), the National Basic Research Program of China (973 Program, 2009CB930103), and NFS of Shandong Province (Z2008B01). ’ REFERENCES (1) Yang, S.; Yang, X.; Shao, X.; Niu, R.; Wang, L. J. Hazard. Mater. 2011, 186, 659–666. (2) Han, F.; Kambala, V.; Srinivasan, M.; Rajarathnam, D.; Naidu, R. Appl. Catal., A 2009, 359, 25–40. (3) Yoon, M.; Chang, J.; Kim, Y.; Choi, J.; Kim, K.; Lee, S. J. Phys. Chem. B 2001, 105, 2539–2545. (4) Ozer, R.; Ferry, J. Environ. Sci. Technol. 2001, 35, 3242–3246. (5) Jin, H.; Wu, Q.; Pang, W. J. Hazard. Mater. 2007, 141, 123–127. (6) Yang, Y.; Wu, Q.; Guo, Y.; Hu, C.; Wang, E. J. Mol. Catal., A 2005, 225, 203–212. (7) Li, L.; Wu, Q.; Guo, Y.; Hu, C. Microporous Mesoporous Mater. 2005, 87, 1–9. (8) Yang, Y.; Guo, Y.; Hu, C.; Jiang, C.; Wang, E. J. Mater. Chem. 2003, 13, 1686–1694. (9) Kim, T.; Sohn, B. Appl. Surf. Sci. 2002, 201, 109–114. (10) Sohn, B.; Kim, T.; Char, K. Langmuir 2002, 18, 7770–7772. (11) Zhang, T.; Ge, L.; Wang, X.; Gu, Z. Polymer 2008, 49, 2898–2902. (12) Priya, D.; Modak, J.; Raichur, A. ACS Appl. Mater. Interfaces 2009, 1, 2684–2693. (13) Li, T.; Gao, S.; Li, F.; Cao, R. J. Colloid Interface Sci. 2009, 338, 500–505. (14) Gao, S.; Cao, R.; L€u, J.; Li, G.; Li, Y.; Yang, H. J. Mater. Chem. 2009, 19, 4157–4163. (15) You, Y.; Gao, S.; Xu, B.; Li, G.; Cao, R. J. Collid Interface Sci. 2010, 350, 562–567. (16) Yanagida, S.; Nakajima, A.; Sasaki, T.; Kameshima, Y.; Okada, K. Chem. Mater. 2008, 20, 3757–3764. (17) Yanagida, S.; Nakajima, A.; Sasaki, T.; Isobe, T.; Kameshima, Y.; Okada, K. Appl. Catal., A 2009, 366, 148–153. 13596
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ARTICLE
(18) Nakajima, A.; Koike, T.; Yanagida, S.; Isobe, T.; Kameshima, Y.; Okada, K. Appl. Catal., A 2010, 385, 130–135. (19) Sun, Z.; Xu, L.; Guo, W.; Xu, B.; Liu, S.; Li, F. J. Phys. Chem. C 2010, 114, 5211–5216. (20) Shchukin, D.; Schattka, J.; Antonietti, M.; Caruso, R. J. Phys. Chem. B 2003, 107, 952–957. (21) Fan, D.; Hao, J. J. Phys. Chem. B 2009, 113, 7513–7516. (22) Zhang, X.; Lei, L.; Zhang, J.; Chen, Q.; Bao, J.; Fang, B. Sep. Purif. Technol. 2009, 67, 50–57. (23) Huang, M.; Bi, L.; Shen, Y.; Liu, B.; Dong, S. J. Phys. Chem. B 2004, 108, 9780–9786. (24) Coutinho, C.; Gupta, V. J. Colloid Interface Sci. 2009, 333, 457–464. (25) Zhang, Y.; Wan, J.; Ke, Y. J. Hazard. Mater. 2010, 177, 750–754. (26) Chen, L.; Tsai, F.; Huang, C. J. Photochem. Photobiol., A 2005, 170, 7–14. (27) Parida, K.; Sahu, N.; Biswal, N.; Naik, B.; Pradhan, A. J. Colloid Interface Sci. 2008, 318, 231–237. (28) Al-Qaradawi, S.; Salman, S. J. Photochem. Photobiol., A 2002, 148, 161–168. (29) Devi, L.; Reddy, K. Appl. Surf. Sci. 2010, 256, 3116–3121. (30) Smith, Y.; Kar, A.; Subramanian, V. Ind. Eng. Chem. Res. 2009, 48, 10268–10276. (31) Chen, Y.; Yang, S.; Wang, K.; Lou, L. J. Photochem. Photobiol., A 2005, 172, 47–54. (32) Li, G.; Wong, K.; Zhang, X.; Hu, C.; Yu, J.; Chan, R.; Wong, P. Chemosphere 2009, 76, 1185–1191. (33) Cao, J.; Xu, B.; Luo, B.; Lin, H.; Chen, S. Appl. Surf. Sci. 2011, 257, 7083–7089. (34) Behnajady, M.; Modirshahla, N.; Shokri, M. Chemosphere 2004, 55, 129–134.
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