TiO2 Heterojunction

Dec 19, 2011 - Zorn , M. E.; Tompkins , D. T.; Zeltner , W. A.; Anderson , M. A. ..... Changlin Yu , Wanqin Zhou , Jimmy C. Yu , Hong Liu , Longfu Wei...
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Efficient Degradation of Azo Dyes over Sb2S3/TiO2 Heterojunction under Visible Light Irradiation Meng Sun,† Guodong Chen,‡ Yakun Zhang,† Qin Wei,‡ Zhenmin Ma,*,† and Bin Du*,† †

School of Resources and Environment and ‡Key Laboratory of Chemical Sensing & Analysis in Universities of Shandong, School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, People's Republic of China ABSTRACT: A cost- and time-saving coupling method was developed to prepare a novel Sb2S3/TiO2 heterostructure photocatalyst. The obtained sample was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and UV vis diffuse reflectance spectroscopy (DRS). UV vis diffuse reflectance spectra showed a great shift to longer wavelengths and an extension of the absorption in the visible region for all of the samples compared to pure TiO2. The photocatalytic activity of the sample was investigated for the liquid-phase photocatalytic degradation of azo dyes under visible light irradiation. The influences of different mass ratios of Sb2S3 and sintered temperatures on the photocatalytic activity have also been investigated, and the highest efficiency is observed when calcined at 673 K with 60 wt % Sb2S3 content. Moreover, •OH has also been detected in the suspension of Sb2S3/TiO2 heterojunction under visible light irradiation, which leads to the decomposition of azo dyes.

1. INTRODUCTION In recent years, semiconductors have been used as photocatalysts to initiate photocatalytic reactions at their interface.1 Among such photocatalysts, TiO2 has been proved to be an excellent photocatalyst with its unique characteristics in band position and surface structure, as well as its extended chemical stability and nontoxicity.2 5 However, its practical application seems limited for several reasons. First, TiO2 is a semiconductor with a wide band gap (Eg ≈ 3.2 eV), and this severe disadvantage considerably limits the utilization of natural solar light or artificial visible light. Second, the high recombination rate of photoexcited electron/hole pairs result in the low photon utilization efficiency and slow photooxidation rate.6 8 To solve these problems, people have to make great effort to design novel visible light photocatalysts through modification methods, such as doping of metal ions,9,10 organic dye sensitization,11 and introduction of either anions (such as N3 , S2 , or C4 )12 17 or cations (such as vanadium and iron).18 It has been reported that one of the promising strategies is the coupling of TiO2 with other narrow band gap semiconductors capable of harvesting the photons in the visible range.12,13 The coupling of two semiconductors provides a novel approach to improve the charge separation efficiency and increase the lifetime and interfacial charge transfer rate of the charge carriers.19,20 Recently, studies have shown that the use of TiO2/WO3 and TiO2/MoO3 composites significantly enhanced the degradation rate of 1,4-dichlorobenzene.21,22 Some other TiO2 based composites, including CdS/TiO2,23 25 Bi2S3/TiO2,25 SnO2/TiO2,26 28 In2O3/TiO2,29 and ZrO2/TiO2,30 have also been studied. In this work, a narrow band gap semiconductor Sb2S3 was used to impregnate TiO2 for the purpose of extending the light absorption spectrum toward the visible region. The Sb2S3/ TiO2 heterostructure photocatalysts sintered under various temperatures were prepared in different mass proportions, and the content of Sb2S3 in the composites varied from 30 to 70 wt %. Further more, the effects of calcination temperature and Sb2S3 r 2011 American Chemical Society

content on the photocatalytic activity of Sb2S3/TiO2 composites have also been investigated.

2. EXPERIMENTAL SECTION 2.1. Materials. For the preparation of Sb2S3/TiO2 heterostructure catalyst, Potassium sulfocyanate (A.R. 98.5%), antimony trichloride (A.R. 99.5%), tartaric acid (A.R. 99.8%), Ti(OC3H7)4 (A.R. 25.0 28.0%), p-hydroxyazobenzene (A.R.), and deionized water were used without further purification. 2.2. Catalyst Preparation. The Sb2S3/TiO2 heterojunction photocatalysts were synthesized using a simple coupling method.31 In a typical procedure, SbCl3 was first dissolved in distilled water in a 250 mL round-bottom flask, and some amount of tartaric acid was added as a complexing agent to prevent the hydrolysis of SbCl3. When a transparent solution was obtained, a certain amount of KSCN was added under continuous stirring. The whole solution was then refluxed at 115 oC for 24 h. Finally, the dark brown precipitate obtained was centrifuged and washed with distilled water and absolute ethanol several times, and then dried under vacuum at 60 oC for 3 h to get Sb2S3 nanorods. A mixture of 1.3 mL of HNO3, 180 mL of H2O, and 15 mL of Ti(OC3H7)4 was peptized at 40 oC for 24 h to form a highly dispersed TiO2 colloidal solution.32 The as-prepared Sb2S3 nanorods were equably ultrasonically dispersed in a certain amount of TiO2 colloid and continuously stirred for 5 h. And, then, the mixture was dried under vacuum at 60 °C for 12 h. Finally, the resultant xerogel was heated at different temperatures for 3 h under nitrogen atmosphere and cooled to room temperature. By changing the amount of the added Sb2S3 nanorods to synthesize the Sb2S3/TiO2 heterostructure Received: November 10, 2011 Accepted: December 19, 2011 Revised: December 16, 2011 Published: December 19, 2011 2897

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Industrial & Engineering Chemistry Research catalysts with 30, 40, 50, 60, and 70 wt % Sb2S3 loadings, which were labeled by S30T, S40T, S50T, S60T, and S70T, respectively. 2.3. Characterization. The prepared Sb2S3/TiO2 heterostructure photocatalysts were characterized by X-ray diffraction (XRD) on a Bruker D8 Advance X-ray diffractometer at 40 kV and 40 mA with Ni-filtered Cu Kα radiation. The transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) were recorded on a JEOL 2010 EX microscope operated at a 200 kV accelerating voltage to observe the microstructures and morphology of the samples. To investigate the light absorption properties of photocatalysts, UV vis diffuse reflectance spectra were carried out, in the wavelength range of 200 800 nm, using a UV vis spectrophotometer (Cary 500 scan spectrophotometers, Varian, Santa Clara, CA, USA), and BaSO4 was used as a reflectance standard. The flat-band potentials (Vfb) of TiO2 and Sb2S3 were determined by an electrochemical method.33,34 And it was carried out by using conventional three-electrode cells on a PAR VMP3Multi potentiotat apparatus. TiO2 and Sb2S3 particles were deposited as a film form on a 1 cm  1 cm ITO conductive glass that served as the working electrode, while the saturated calomel electrode (SCE) and Pt were used as reference and counter electrode, respectively. The electrolyte was 0.3 M LiClO4 aqueous at pH = 3.0. The Mott Schottky plots to evaluate the flat-band potential of the semiconductor space charge region were obtained by measuring impedance spectra at a fixed frequency of 1 kHz. 2.4. Photocatalytic Activity Measurements. The photocatalytic activities of Sb2S3/TiO2 heterostructure photocatalysts were mainly evaluated by the photodegradation of p-hydroxyazobenzene (p-HAB) in aqueous solution. P-HAB, as well as methyl orange (MO), is one kind of azo dyes used industrially. A 500 W halogen lamp (Philips Electronics) placed in a cylindrical glass vessel was used as the visible light source; cold water was circulating in the vessel to avoid overheating. Two cutoff filters were used to occlude light below 420 nm and above 850 nm to ensure the photodecomposition proceed under visible light irradiation. The temperature of the suspension was maintained at 333 K using a fan blowing air to the aqueous solution. A 40 mg amount of catalyst was added into a 100 mL Pyrex glass vessel which contained 80 mL of p-HAB solution (10 ppm). The suspension was stirred in the dark for 1 h to achieve the adsorption desorption equilibrium. During irradiation, 3 mL of the suspension was sampled at given irradiation time intervals and was then centrifuged to remove the photocatalyst. The resulting degraded solution was analyzed using a Varian Cary 50 Scan UV vis spectrophotometer to record the concentration changes of p-HAB. The percentage of degradation is reported as C/C0. C is the absorption of p-HAB at each irradiated time interval of the main peak of the absorption spectrum at 347 nm. And C0 is the absorption of the initial concentration when adsorption desorption equilibrium is achieved.

3. RESULTS AND DISCUSSION 3.1. XRD Analysis. The X-ray diffraction patterns of the Sb2S3/TiO2 heterostructure catalysts with different mass ratios are shown in Figure 1. As a comparison, the XRD patterns of the as-prepared pure Sb2S3 and TiO2 are also presented. As it can be seen, the XRD patterns of the as-prepared Sb2S3 are corresponding to the orthorhombic phase of Sb2S3 (JCPDF No. 42-1393); seven distinctive peaks are found at 24.89, 25.01, 29.25, 32.35, 17.52, 47.31, and 35.52°, corresponding to (130), (310), (211),

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Figure 1. XRD patterns of the samples: (a) pure Sb2S3; (b) S70T; (c) S60T; (d) S50T; (e) S40T; (f) pure TiO2.

Figure 2. UV vis diffuse reflectance of TiO2 and Sb2S3/TiO2 composites with different Sb2S3 contents.

(221), (120), (151), and (240) crystal planes of Sb2S3, respectively, whereas all of the diffraction peaks in Figure 1f are identified as the pure anatase phase. However, in the XRD patterns of the Sb2S3/TiO2 heterostructure photocatalysts, we have found that all of the peaks observed were seemingly ascribed to Sb2S3, but no obvious peak belonging to TiO2 had been observed. The reason might be due to the fact that the peaks of TiO2 match with the reflections from the highly crystalline orthorhombic Sb2S3, making it difficult to reveal the phase of TiO2. 3.2. UV Vis Diffuse Refection Spectroscopy. Diffused reflectance spectra of the pure TiO2 and Sb2S3/TiO2 heterostructure catalysts are shown in Figure 2. It can be clearly seen that the light absorption exhibits red shifts when increasing the coupled mass ratio of Sb2S3 to TiO2. In the visible light region, the absorption intensities of Sb2S3/TiO2 composites are obviously higher than that of pure TiO2. The main absorption edge (λab) for the S60T sample is located at 675 nm, corresponding to the band gap energy of 1.84 eV. The formation of defect energy levels within the forbidden band of TiO2 and Sb2S3 would decrease the band gap energy of Sb2S3/TiO2, resulting to the red shift during the calcination process. So the composite photocatalyst Sb2S3/TiO2 can be excited by visible light (λ > 420 nm) and exhibit a higher photocatalytic activity. 2898

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Figure 5. Degradation curves of p-HAB over Sb2S3/TiO2 composite powders with different Sb2S3 contents under visible light irradiation (catalyst loading, 0.5 g/L).

Figure 3. TEM and HRTEM images of the samples: (a) TEM for pure Sb2S3; (b) HRTEM for pure Sb2S3. (inset). Corresponding FFT: (c) TEM for S60T; (d) HRTEM for S60T.

Figure 4. UV vis spectral changes of p-HAB in aqueous S60T dispersions as a function of irradiation time.

3.3. TEM Images. The morphologies of the as-synthesized Sb2S3 and Sb2S3/TiO2 composites are demonstrated in the TEM images shown in Figure 3. As shown in Figure 3a, the assynthesized Sb2S3 nanorods have diameters in the range of 50 100 nm and lengths of 100 1000 nm. A representative HRTEM image showing clear lattice fringes is shown in Figure 3b. The interlayer spacing of 0.277 nm corresponds to the (221) plane of Sb2S3. As we can see in Figure 3c, the TiO2 nanoparticles in the sizes of 10 20 nm mutually aggregated and covered the surface of Sb2S3 nanorods. We have also found that the Sb2S3 nanorods being decorated with TiO2 particles became shorter, and the reasons might due to the long-time stirring, the high-temperature calcination, or ultrasonic treatments in the preparation. The HRTEM image of the Sb2S3/TiO2 composites is shown in Figure 3d, suggesting that a tight contact was formed between TiO2 particles and Sb2S3 nanorods. The observed lattice

spacings of 0.505 and 0.352 nm correspond to the (120) and (101) planes of the orthorhombic Sb2S3 and anatase TiO2, respectively. It indicates that the coatings on the surface of Sb2S3 consist of the pure anatase grains. 3.4. Photocatalytic Activity. The photocatalytic activities of the samples were mainly evaluated by measuring the photodegradation of p-HAB aqueous solution under visible light irradiation. Temporal changes in the concentration of p-HAB were monitored by examining the variations in maximal absorption in UV vis spectra at 347 nm. To investigate the influence of Sb2S3 content on the photocatalytic activity, pure TiO2 and Sb2S3/ TiO 2 composites with different mass ratios were used to decompose p-HAB. Figure 4 displays the concentration changes of p-HAB at 347 nm as a function of irradiation time in the presence of Sb2S3/TiO2 catalyst. Obviously, it was observed that the amount of Sb2S3 in the heterojunction plays an important role. The photocatalytic activity increases monotonously as the amount of Sb2S3 increases from 30 to 60 wt %, but the photocatalytic activity decreases when the amount of Sb2S3 rises to 70 wt % (see Figure 5). The S60T sample exhibiting the best activity may result in the fact that when the mass ratio was higher than 60 wt %, the Sb2S3 nanorods were agglomerated and not well dispersed, which may hinder the contact between TiO2 and oxygen containing species in aqueous solution, leading to a negative influence on the activity of the composites. On the contrary, when the coupled mass ratio was lower than 60 wt %, the reduced visible light absorption would also lead to the decrease of photocatalytic activity. Thus, an optimal Sb2S3/ TiO2 ratio existed for the highest photocatalytic activity, and it was found to be 60 wt %. When the coupled mass ratio of Sb2S3 to TiO2 increased, the catalyst could absorb visible light more effectively, and the photogenerated electrons in the CB of Sb2S3 could transfer to that of TiO2, improving the charge separation rate effectively. However, when the coupled mass ratio of Sb2S3 to TiO2 was larger than 60 wt %, the contact surface between Sb2S3 and TiO2 particles became less good.35 On the contrary, a lager amount of Sb2S3 might increase the mean path of the electron through the Sb2S3 particles, which would favor the recombination of the photogenerated electrons and holes in the Sb2S3 particles. The S60T sample with the best activity photocatalytically degrades 89% of p-HAB after about 5 h of visible light irradiation, and the lower p-HAB removal values over 2899

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Figure 6. Degradation curves of p-HAB over S60T Sb2S3 calcined at different temperatures (catalyst loading, 0.5 g/L).

Figure 7. UV vis spectral changes of MO in aqueous S60T dispersions as a function of irradiation time.

S30T, S40T, and S50T samples are 48, 65, and 80%, respectively. It is worth mentioning that the corresponding p-HAB degradation ratio over pure Sb2S3 is only about 30%, obviously lower than those over composites. This means that the combination of Sb2S3 over TiO2 to form the heterojunction structure is an effective way to improve the photocatalytic activity. The effect of sintering temperature on the photocatalytic activity has been also investigated. As shown in Figure 6, the sample S60T sintered at different temperatures exhibits different activities, and the catalyst calcined at 673 K reveals the best photocatalytic activity. It is found that when the samples are calcined at the temperature below or above 673 K, they can only exhibit lower photocatalytic activity. As we all know, only when calcined at a high temperature can TiO2 strongly interact with Sb2S3, which is in favor of the transfer of photogenerated carriers. However, the specific surface area of Sb2S3/TiO2 may also decrease with the increased calcination temperatures, leading to the inefficient photocatalytic activity. In the photocatalytic degradation of MO, another azo dye, the sample S60T has also exhibited good photocatalytic efficiency. Nitrogen-doped TiO2 (TiO2 xNx) and CdS were also used to photocatalytically decompose MO for comparison to evaluate the photocatalytic activity of the sample S60T. As it can be seen in Figure 7, the absorption peaks of MO steadily decreased under visible light irradiation, and the peak (λ = 464 nm) nearly

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Figure 8. Degradation curves of MO over different photocatalysts under visible light irradiation (catalyst loading, 0.5 g/L).

Figure 9. Chromatogram of the MO solutions after visible light irradiation compared to the original solution.

disappeared after 5 h while a new peak (λ = 245 nm) emerged. Figure 8 displays the concentration changes of MO at 464 nm as a function of irradiation time during the degradation process in the presence of different catalysts. After irradiation for 5 h in the absence of photocatalyst, there were no obvious concentration changes of the MO solution. Similar results were obtained when the suspension containing the S60T catalyst was stirred in the dark for 5 h. The photocatalytic activity of S60T is higher than that of CdS and TiO2 xNx. After 5 h irradiation, the decomposition ratio of MO in the presence of S60T is up to 96% while CdS and TiO2 xNx can only decompose 59 and 13% of MO, respectively. Figure 9 reports the chromatogram of the MO solutions before and after visible light irradiation in the presence of S60T. Before irradiation, it had only one absorption peak of MO in the chromatogram with mass peak at m/z = 304. With the prolonged irradiation time, its intensity decreased gradually, and after about 5 h of irradiation, the peak had almost disappeared. Coupling of TiO2 with other semiconductors has been frequently investigated to improve the photocatalytic activity of TiO2 through promoting the separation ratio of photogenerated charge carriers and extending the absorption wavelength up to the visible region. A previous study showed that an extension of the wavelength range of light absorption increased the photocatalytic activity of TiO2.36 Thus, the high photocatalytic activity of Sb2S3/TiO2 compared with pure TiO2 and Sb2S3 might originate 2900

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Industrial & Engineering Chemistry Research from the unique relative band positions of these two semiconductors. Herein, Vfb of Sb2S3 and TiO2 were determined by an electrochemical method. The differences of Vfb between these semiconductors would be more reliable data than the absolute energy levels, since the Vfb is critically dependent on pH. Figure 10 displays the Mott Schottky plots for Sb2S3 and TiO2 films in 0.3 mol 3 L 1 LiClO4 solution (pH = 3.0). And it was found that the Vfb positions of Sb2S3 and TiO2 were located at 0.87 and 0.58 V, respectively. That is to say, the Vfb position of Sb2S3 is higher than that of TiO2 by 0.27 V under the same condition. The Mott Schottky plots have also revealed that both Sb2S3 and TiO2 were n-type semiconductors. And it is generally known that the conduction band potentials (ECB) of n-type semiconductors are very close to (0.1 0.2 eV more negative) the flat-band potentials.34 Thus, we could deduce that the CB position of Sb2S3 is more negative than that of TiO2. Since the CB position of TiO2 is known as 0.5 V (versus the normal hydrogen electrode (NHE)), that of Sb2S3 is estimated to 0.77 V (vs NHE). Figure 11 depicts the flat band potentials of the valence and conducting bands at pH 7 (vs NHE) for Sb2S3 and TiO2 with their band gap energy. Thermodynamic conditions for efficient electron injection from Sb2S3 to TiO2 are respected. Indeed, the CB of Sb2S3 located at 0.77 V is more cathodic than that of TiO2 ( 0.5 V). In general, the big difference of CB positions of those two semiconductors would strengthen the driving forces of electron injection. Therefore, Sb2S3 functions as a sensitizer in the photocatalytic heterojunction system, and TiO2 is a substrate. Under visible light irradiation, electrons would be generated in the VB of Sb2S3, transferred to the CB of Sb2S3, and finally injected to the CB of the coupled TiO2 due to the present electric fields between the two kinds of materials. In this case, a high concentration of electrons is obtained in the CB of TiO2. Holes generated in the VB of TiO2 transferred to the VB of Sb2S3, consequently creating a high concentration of holes in the sensitizer/electrolyte interface. This could effectively improve the charge separation efficiency, and more electrons and holes could transfer to the surfaces of the respective particles to participate in the degradation reactions, resulting in a high photocatalytic activity. Terephthalic acid photoluminescence probing technique (TA-PL) has been widely used to detect hydroxyl radicals generated in the liquid phase.28 In the suspension of S60T/TA, the hydroxyl radicals generated under visible light irradiation would be captured by terephthalic acid to form 2-hydroxylterephthalic acid. However, 2-hydroxylterephthalic acid performs a

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strong fluorescence characteristic, so, through monitoring of the fluorescence intensity changes of S60T/TA solution, we can indirectly detect the generation of hydroxyl radicals. The fluorescence spectra of S60T/TA solution irradiated by visible light for different times are shown in Figure 12; as we can see, the fluorescence intensity increases steadily with the irradiation time within 80 min. It can be concluded that hydroxyl radicals are indeed generated in the S60T suspension under visible light irradiation. The •OH radical was commonly recognized as the main reactive species responsible for the organic degradation.37 The photogenerated electrons could react with the oxygen molecule adsorbed on the surface of photocatalyst to yield O2• , which was then reacted to produce •OH. On the other

Figure 11. Proposed photodegradation pathways for pollutants in the S60T suspension.

Figure 12. •OH-trapping PL spectra of S60T/TA solution.

Figure 10. Mott Schottky plots for (a) Sb2S3 and (b) TiO2 films in 0.3 mol 3 L 2901

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Industrial & Engineering Chemistry Research hand, a partial amount of the photogenerated holes will react with water molecules to produce •OH. O2• , •OH, and photogenerated holes all could oxidize the organic pollutants absorbed on the surface of S60T to some degree. A possible mechanism for the photodegradation of organic pollutants over S60T had been proposed (as shown in Figure 11).

4. CONCLUSIONS This paper presents novel efficient Sb2S3/TiO2 heterostructure catalysts synthesized via a simple method. The XRD results showed that the Sb2S3 in the sample is orthorhombic phase, while TiO2 is anatase phase. The Sb2S3/TiO2 heterostructure catalysts exhibit strong visible light absorption due to the existence of Sb2S3 coupled in the TiO2 powders. The sample showed efficient photocatalytic activities toward azo dyes under visible light irradiation, which was higher than that of CdS, TiO2 xNx, and pure Sb2S3 nanorods. The efficient photocatalytic activity might be attributed to the synergetic effect resulting from the combination of orthorhombic Sb2S3 and anatase TiO2, and the strong visible light adsorption of Sb2S3/TiO2 heterostructure catalysts. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (Z.M.); [email protected] (B.D.). Fax: +86 531-82765969. Tel.: +86 531-82769235.

’ ACKNOWLEDGMENT This work was financially supported by the NSFC of China (Grants 21103069 and 40672158), Special Project of the National Department of Science and Technology (Grant 2009ZX07212-003), and the Scientific Research Foundation for Doctors of University of Jinan (Grants XBS1037 and XKY1043). ’ REFERENCES (1) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental applications of semiconductor photocatalysis. Chem. Rev. 1995, 95, 69–96. (2) Ding, Z.; Lu, G. Q.; Greenfield, P. F. Role of the crystallite phase of TiO2 in heterogeneous photocatalysis for phenol oxidation in water. J. Phys. Chem. B 2000, 104, 4815–4820. (3) Huang, Y.; Zheng, Z.; Ai, Z.; Zhang, L.; Fan, X.; Zou, Z. Core shell microspherical Ti1 xZrxO2 solid solutions photocatalysts directly from ultrasonic spray pyrolysis. J. Phys. Chem. B 2006, 110 19323–19328. (4) Yu, J. C.; Yu, J.; Ho, W.; Jiang, Z.; Zhang, L. Effects of F doping on the photocatalytic activity and microstructures of nanocrystalline TiO2 powders. Chem. Mater. 2002, 14, 3808–3816. (5) Ou, Y.; Lin, J.; Fang, S.; Liao, D. Study on the preparation of ultrafine mesoporous TiO2 with controllable crystalline phase and its photocatalytic activities. Catal. Commun. 2007, 8, 936–940. (6) Kang, I. C.; Zhang, Q. W.; Yin, S.; Sato, T.; Saito, F. Improvement in photocatalytic activity of TiO2 under visible irradiation through addition of N-TiO2. Environ. Sci. Technol. 2008, 42, 3622–3626. (7) Xin, B. F.; Ren, Z. Y.; Wang, P.; Liu, J.; Jing, L. Q.; Fu, H. G. Study on the mechanisms of photoinduced carriers separation and recombination for Fe3+ TiO2 photocatalysts. Appl. Surf. Sci. 2007, 253, 4390–4395. (8) Gopias, K. R.; Bohorquez, M.; Kamat, P. V. Photophysical and photochemical aspects of coupled semiconductors: Charge-transfer processes in colloidal cadmium sulfide titania and cadmium sulfide silver(I) iodide systems. J. Phys. Chem. 1990, 94, 6435–6440.

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dx.doi.org/10.1021/ie2025882 |Ind. Eng. Chem. Res. 2012, 51, 2897–2903