One-pot Synthesis and Photocatalysis of Encapsulated TiO2 in ...
Journal of the Chinese Chemical Society, 2006, 53, 1355-1361
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One-pot Synthesis and Photocatalysis of Encapsulated TiO2 in Mesoporous SiO2
a
Yang Chuan Leea,b ( ) and Soofin Chenga* ( ) Department of Chemistry, National Taiwan University, Taipei 106, Taiwan, R.O.C. b Ching Kuo Institute of Management and Health, Keelung 203, Taiwan, R.O.C.
Nano TiO2 particles encapsulated in mesoporous SiO2 materials (abbreviated as TiO2/MS) were prepared by incorporating the nano-sized TiO2 particles in the pore-directing micelles during the cooperative assembly of silica precursor and surfactant. The materials were characterized with X-ray powder diffraction (XRD), transmission and scanning electron microcopies (TEM and SEM), N2 sorption, ICP-MS and UV-Vis spectroscopy. XRD patterns showed that titania was in the anatase phase. The results of TEM and N2 sorption studies confirmed the deposition of nano-sized particles inside the mesopores of silica material without destroying their integrity. UV-Vis spectra showed blue shift of the absorption edge for TiO2/ MS relative to bulk anatase. When used as photocatalysts, TiO2/MS showed similar activity as anatase TiO2 in photo-degradation of phenol. However, lower activity than anatase TiO2 was observed in photodegradation of lipase, implying that most of the nano-sized TiO2 were encapsulated inside the mesopores of silica. Keywords: Photocatalyst; Photodegradation; Encapsulated; Mesoporous SiO2; Reverse micelle; Microemulsion; Nano TiO2 particles.
INTRODUCTION Heterogeneous photocatalysts have shown high efficiencies in the removal of toxic and non-biodegradable pollutants commonly present in air and wastewaters.1–3 Photocatalysis over a semiconductor oxide such as TiO2 is initiated by the absorption of a photon with energy equal to or greater than the band gap of the semiconductor (3.2 eV for TiO2), producing electron-hole (e-/h+) pairs.4 The e-/h+ pairs then act as either an electron donor or acceptor for molecules in the surrounding media. In the presence of oxygen and water, OH radicals are considered to form and act as the active species to degrade the pollutants.5 Particles in nano size are considered to offer a high specific surface area for heterogeneous reactions. Unfortunately, for applications in the aqueous phase such a small particle size means high costs in filtration in order to separate the catalyst after the reaction is finished. These problems have motivated the development of supported photocatalysts in which TiO2 has been immobilized on diverse materials.6 Recent research has been focused on the preparation
of photocatalysts supported on materials with large pore sizes and high porosity. Highly ordered mesoporous SiO2 structures such as MCM-41 or SBA-15, prepared via a solgel method using surfactants as the structure directing agents have attracted great attention due to their high specific surface area (630-1040 m2g-1), pore sizes (2-30 nm) and pore volumes (0.6-2.5 cm3g-1).7–9 Two different techniques have been applied to incorporate titania into the mesoporous SiO2 materials. One was through coprecipitation (CP) of silica and titania via a sol–gel method in which the precursors of both metal oxides are hydrolyzed simultaneously.10–15 When the titanium contents were low (Si/Ti molar ratio > 20), Ti was mainly incorporated into the silica framework through isomorphous substitution of Si.10–15 Although mesoporous silica materials containing low Ti contents were catalytically active for selective oxidation of benzene to phenol10 and for epoxidation of several alkenes and styrene,11–14 photocatalytic activity under near-UV irradiation was not significant. The photocatalytic activity usually needs crystalline titania particles.16 The other synthesis route consists of impregnation (IMP) of a meso-
Dedicated to the memory of the late Professor Ho Tong-Ing. * Corresponding author. Fax: +886-2-23636359; E-mail: [email protected]
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porous SiO2 material with a titanium precursor that is subsequently hydrolyzed on the silica surface.17–20 So far, the impregnation method has been the main route used to prepare TiO2-loaded mesoporous SiO2.17–22 Photocatalytic activities of the supported TiO2 materials were shown in the degradation of gaseous toluene15 and aqueous benzene,18 chlorobenzenes,18 phenol18,19 and methylene blue.19 In the past few years, the application of photocatalytic active materials in textiles has been addressed by many research groups and companies for the purpose of removing organic pollutants in air.23,24 The photocatalyst was mixed into the textile fiber by either direct mixing or coating. However, the textile fiber itself was photo-degraded after exposure to sunlight because of the strong oxidizing power of the photocatalyst.25 In the present work, we attempt to prepare TiO2 particles encapsulated inside the pores of mesoporous silica materials by one-pot synthesis. The photocatalytic activities of the resultant material in the decomposition of the small molecule phenol and large molecule lipase were examined, and the results were compared with those over bulk anatase TiO2.
EXPERIMENTAL SECTION Synthesis of TiO2/MS Titania nano particles were prepared following the procedures mentioned in a previous report.22 Titanium tetraisopropoxide (TTIP) was hydrolyzed in the reverse micelle microemulsion (H2O/TTIP = 2) containing dioctyl sulfosuccinate sodium salt in cyclohexane. The reverse microemulsion solution was prepared by dissolving 0.0055 mol of anionic surfactant dioctyl sulfosuccinate sodium salt (98%, Acros) in a mixture of 3.9 mL cyclohexane and the required amount of distilled water. The water-clear appearance of the solution indicated the formation of the microemulsion. Then, TTIP was added in and the microemulsion solution became gel. The gel was transferred to an aqueous solution containing the amphiphilic triblock copolymer P-123 (EO20PO70EO20, Aldrich) as the pore-directing agent, and FeCl3 and NaCl salts which stabilized the micelle. The mixture was stirred at 35 °C for 1 h, followed by the addition of TEOS and stirred at 35 °C for another 48 h. The molar composition of the mixture was TEOS: Ti: FeCl3: NaCl: P123: H2O = 1.0: 0.1: 0.001: 0.03: 0.017: 233. The gel was hydrothermally heated at 90 °C for 1 day. Then, the solid product was filtered, washed with
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water, and dried at 100 °C overnight. The dried product was calcined at 560 °C for 6 h with a ramping rate of 1.1 °C/ min. For comparison, pure mesoporous SiO2 was synthesized following the same method but without the addition of TTIP. Bulk anatase TiO2 was synthesized by hydrolyzing TTIP in water, followed by filtration and calcination at 500 °C for 4 h. Characterization XRD patterns were recorded in the 2q range of 0.580° using a PANalytic X’pert Pro diffractometer with Cu Ka radiation operated at 40 mA and 45 kV. The pore structures of the samples were analyzed by nitrogen physical adsorption at liquid N2 temperature using a Micrometerics TriStar 3000 system. Prior to the experiments, samples were outgassed at 200 °C for 6-8 h under vacuum (10-3 Torr). The titanium contents of the samples were determined by using ICP-MS (Perkin-Elmer ELAN 6000) on the mixed HF/HNO3 solution-dissolved samples. Transmission electron micrographs were obtained using a Hitachi H-7100 microscope operating at an accelerating voltage of 125 kV. The morphology of the sample was examined by a Hitachi S-800 Scanning Electron Microscope. UV-Vis diffuse reflectance spectra (DRS) were taken for the drypressed disk samples using a Scan UV-Vis spectrophotometer (Hitachi 3310) equipped with an integrating sphere assembly, using BaSO4 as the reflectance reference sample. Photocatalytic activity measurements The photodegradation experiments were carried out in a quartz tubular reactor placed inside a Rayonet photochemical chamber (Model PRP-100). The stirred suspensions were illuminated by means of 128W low pressure Hg lamps with an emission wavelength of 300 nm. The catalyst in powder form, based on the same titanium equivalence as 0.01 g TiO2, was suspended in 50 mL of a 0.6 mM phenol solution or 120 ppm porcine pancreatic lipase (PPL) solution (Aldrich, 25% protein). The reactor was kept at 20 ± 2 °C with cooling water circulation during the experiments. A flow of oxygen (2 mL/min) bubbling into the reactor served as the oxidant. The outlet gases were directed through a two-stage bubbling trap containing saturated Ba(OH) 2 solution, and the CO2 yield was determined based on the weight of BaCO3 precipitated. Six molecules of CO2 should be formed by complete oxidation of one phenol molecule. The residual phenol or lipase in the solutions was analyzed by checking the absorbance at 269 nm or 258 nm, respec-
Mesoporous SiO2 Encapsulated TiO2
tively, with a UV-Vis spectrophotometer (Hitachi 3310).
RESULTS AND DISCUSSION The condition for the synthesis of a SBA-15 mesoporous molecular sieve was modified in order to incorporate high Ti loading. The method for synthesizing mesoporous silica using P123 as pore-directing agent without the addition of mineral acid was adapted.26 The main strategy of this method was to utilize the acidity self-generated in the aqueous solution of FeCl3 as the catalyst for TEOS hydrolysis. In addition, FeCl3 and NaCl also contributed to the salt effect in stabilizing the micelles and increasing the crystallographic ordering of the meso-structure. The nano TiO2 particles were prepared by hydrolysis of TTIP in the reverse micelles formed in the anion surfactant/cyclohexane solution.22,27-28 Fig. 1 shows the XRD patterns of TiO2/MS and pure mesoporous SiO2. The lowangle powder XRD patterns of the calcined samples only featured a strong diffraction peak indexed to the (100) plane of hexagonal arranged porous structure, accompanied by broad unresolved higher order reflections. These results imply that both materials possess mesostructures which however lack long-range ordering.29 The d-spacing of TiO2/MS was greater than that of pure mesoporous SiO2 (10.4 nm vs. 8.4 nm), as shown in Table 1. It is attributed to that nano TiO2 particles formed in the anion surfactant reverse micelles may be incorporated into the hydrophobic phase of the P123 micelles and cause the expansion of the micelle volume. The formation of anatase TiO2 particles was confirmed by the diffraction peaks at higher angles (2q
Fig. 1. XRD patterns of (a) TiO 2 /MS and (b) pure mesoporous SiO2.
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= 25.38°, 38.38°, 48.15°). The crystallite diameter was calculated to be 12 nm from the peak width at half maximum (pwhm) of anatase (101) peak at 2q = 25.38° using Scherrer’s equation.30 The nitrogen adsorption–desorption isotherms are illustrated in Fig. 2A. Both TiO2/MS and pure mesoporous SiO2 yielded Type IV isotherms, characterized by a steep increase in absorption volume of nitrogen at the relative pressure range around 0.45 < P/P0 < 0.75. This type of isotherm is typical for mesoporous materials having rather large pore sizes.31 The large hysteresis loop observed on the sorption isotherm of pure mesoporous SiO2 indicates that it contains ink-bottle like pore structures with a relatively narrow entrance of the pores.32 On the other hand, the hysteresis loop of TiO2/MS is less steep and covers a broader pressure range in comparison to that of pure mesoporous
Fig. 2. (A) Nitrogen adsorption-desorption isotherms and (B) BJH pore size distribution profiles of samples (a) TiO2/MS and (b) pure mesoporous SiO2.
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Table 1. Physico-chemical properties of TiO2/MS in comparison to those of pure mesoporous SiO2 Catalyst Meso SiO2 TiO2/MS
TiO2 (wt%)a
SBET (m2/g)
VP (cm3/g)
d-spacing (nm)
Db (nm)
PSD c (nm)
TiO2 cryst. diameter d (nm)
0 12
750 645
0.61 0.70
8.4 10.4
5.0 7.5
1.3 2.0
— 12
a
Analyzed by ICP-MS. BJH adsorption pore diameter. c FWHM of BJH pore size distribution profile calculated from the desorption branch of isotherm. d Obtained by Scherrer equation. b
SiO2. This indicates that TiO2/MS contains less uniformly distributed mesopore diameters than pure mesoporous SiO2. This result is confirmed by the BJH adsorption pore size distribution shown in Fig. 2B. The pore-size distribution (PSD), based on the pwhm of the profile, of TiO2/MS is 2.0 nm in comparison to 1.3 nm for pure mesoporous SiO2. The BJH pore diameter of TiO2/MS (7.5 nm) was greater than that of pure mesoporous SiO2 (5.0 nm). The larger pore size and significantly broader distribution of mesopores of TiO2/MS are attributed to the presence of titania crystallites within the mesopores. This is in contrast to the samples synthesized by the impregnation method, in which loading of titania into the meso-channels caused pore blocking and a decrease in pore size.33 However, the TiO2 crystallite diameter of 12 nm calculated from the line-broadening of the XRD peak is greater than the pore diameter of 7.5 nm, indicating that the TiO2 particles are probably incorporated in the defect sites of the mesostructure. Transmission electron micrographs of the TiO2/MS sample containing 12% TiO2 are shown in Fig. 3. Ordered mesopores in hexagonal arrangement are clearly seen. Moreover, nano-sized TiO2 particles are mostly present inside the meso-channels but many are extended to cover several pores. These results are in good agreement with those from XRD studies. The SEM image of TiO2/MS is shown in Fig. 4. Particles with irregular shapes are aggregated together and have a broad distribution of particle sizes ranging from 0.1~ 0.4 µm. The UV-Vis reflectance spectra of anatase TiO2 and calcined TiO2/MS are shown in Fig. 5. The absorption at 270 nm is due to O®Ti charge transfer, and the broad absorption at a longer wavelength is due to the excitation of electrons from the valence band to the conduction band of TiO2. The band edge of 345-350 nm observed for TiO2/MS is significantly blue-shifted from the band edge of 387 nm
for bulk anatase TiO2.34-37 The blue-shift is attributed to the quantum size effect,38 confirming that no large TiO2 crystals were formed in the TiO2/MS material. In other words, the deposition of large TiO2 particles on the external surface of mesoporous SiO2 was not likely. In order to evaluate the photocatalytic activities of TiO2/MS and to ensure that the nano TiO2 particles were encapsulated inside the mesopores, the photo-degradation of
Fig. 3. Transmission electron micrographs of sample TiO2/MS.
Mesoporous SiO2 Encapsulated TiO2
the small molecule phenol and the large molecule lipase (approx. 4.6 nm × 2.6 nm × 1.1 nm)39 were studied. The results were compared with those over anatase TiO2. Fig. 6(A) shows the results in degradation of the small molecule phenol. Both samples with the same TiO2 content showed similar photocatalytic activities. This implies that the nano TiO2 particles located inside the mesopores are as photocatalytic active as the anatase TiO2. As long as the pollutant molecules can diffuse inside the pores, they can be decomposed by photocatalysis. When the large molecule lipase was the subject of photo-degradation, TiO2/MS showed a
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much slower rate than anatase TiO2 as shown in Fig. 6(B). This is elucidated by that the slow diffusion of the large lipase molecules impedes them from contact with the nano TiO2 particles located inside the mesopores and the rate of photo-degradation is slower than that over anatase TiO2. These results prove that our synthesis method could encapsulate nano TiO2 particles inside the pores of mesoporous silica and still retain their photocatalytic activities. Several groups have reported the pathway of phenol photo-degradation using TiO2 as a photocatalyst.40-43 A UV photon(hu) which overcomes the energy gap between valence and conduction bands of TiO2 can create an electron-hole pair, as shown in equation 1. The hole then reacts with water to produce •OH radicals as shown in equation 2. In the presence of molecular oxygen, O2 can be reduced by conduction band electrons (equation 3), which in principle can also result in the formation of hydroxyl radicals (equations 4-6). This radical can react with phenol, producing a
Fig. 4. Scanning electron micrograph of sample TiO2/ MS.
Fig. 5. Diffuse reflectance UV-Vis spectra of samples (a) TiO2/MS and (b) bulk anatase TiO2.
Fig. 6. Photo-degradation rates of TiO2/MS in comparison to those of bulk anatase TiO2 (based on the same 0.01 g TiO 2 content) for (A) phenol and (B) lipase with 300 nm UV illumination.
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whole range of intermediates such as catechol (CC), hydroquinone (HQ) and benzoquinone (BQ) (equation 7). These aromatic intermediates are further oxidized by •OH radicals or holes to undergo ring cleavage and form carboxylic acids and aldehydes, which finally yield CO2 and H2O (equation 8). TiO2 + hu ® TiO2(h)+ + eTiO2(h)+ + H2O(ads) ® •OH + H+ + TiO2 O2 + e- ® •O2•O2- + •O2- + 2 H+ ® H2O2 + O2 H2O2 + •O2- ® •OH + OH- + O2 H2O2 + e- ® •OH + OH•OH + phenol ® intermediate oxygenated products (e.g., CC, HQ, BQ) TiO2(h)+ + intermediate products ® CO2 + H2O + TiO2
(1) (2) (3) (4) (5) (6) (7)
sion of dioctyl sulfosuccinate sodium salt/cyclohexane solution. The reverse microemulsion containing nano TiO2 particles were proposed to incorporate in the hydrophobic phase of the micelles formed by P123, which served as the pore-directing agent in the formation of mesoporous silica. The formation of nano TiO2 particles was proved by the blue shift of absorption in UV-Vis spectra relative to anatase TiO2 as well as the TEM photographs. When comparing the photo-degradation activities of TiO2/MS for phenol and lipase, the slower decomposition rate of lipase molecules further proved that the nano TiO2 particles in TiO2/ MS were located inside the mesopores. This material is applicable to textiles as a photocatalytic active material in the decomposition of pollutants in air without damaging the fibers.
(8) ACKNOWLEDGMENT
The role of TiO2 as a photocatalyst was proved in the present study by observing the wavelength dependence of the phenol decomposition process. Only UV photons with energy greater than the band gap energy of TiO2 (approx. 3.2 eV) are effective in the photodegradation reaction. It is also noticed that CO2 selectivity over bulk TiO2 is slightly higher than that over TiO2/MS (Fig. 6(A)). That is elucidated by that the large surface area of TiO2/MS may stabilize the oxygenated intermediates. In the case of photo-degradation of lipase, the CO2 yields over both photocatalysts were found to be negligible. The mechanisms of the oxidative cleavage of proteins are not yet fully understood.44-46 Nevertheless, the •OH radicals were presumably the active species initiating the oxidative cleavage of poly-peptide chains. The spectroscopic analysis of the aqueous solution of lipase after 2 h of irradiation over TiO2 show that the large lipase molecules were probably cleaved into peptides and the aromatic rings were oxidized.
CONCLUSIONS An effective one-pot synthesis method was developed to encapsulate nanosized TiO2 inside the mesoporous silica during the self-assembly process. The resultant material TiO2/MS contained hexagonal arranged mesopores and a high surface area. The nano TiO2 particles were prepared by hydrolysis of TTIP in the reverse micelle microemul-
Financial support from the National Science Council, Taiwan, is gratefully acknowledged.
Received July 10, 2006.
REFERENCES 1. Ollis, D. F.; Pellizzetti, E.; Serpone, N. Environ. Sci. Technol. 1991, 25, 1522. 2. Fox, M. A.; Dulay, M. Chem. Rev. 1993, 93, 341. 3. Legrini, O.; Oliveros, E.; Braun, A. M. Chem. Rev. 1993, 93, 671. 4. Linsebigler, A. L.; Yates Jr., J. T. Chem. Rev. 1995, 95, 735. 5. Anpo, M.; Chiba, K.; Tomonari, M. Bull. Chem. SOC. Jpn. 1991, 64, 543. 6. Pozzo, R. L.; Baltana’s, M. A.; Cassano, A. E. Catal. Today. 1997, 39, 219. 7. Kresge, C. T.; Leonowicz, M. E.; Beck, J. S. Nature. 1992, 359, 710. 8. Beck, J. S.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. 9. Schlenker, D.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. 10. Tanev, P. T.; Chibwe, M.; Pinnavala, T. J. Nature. 1994, 368, 321. 11. Corma, A.; Pariente, J. P. J. Chem. Soc. Chem. Commun. 1994, 147. 12. Koyano, K. A.; Tatsumi, T. Micro. Mater. 1997, 10, 259. 13. Grieken, R. V.; Mariscal, R. Catal. Today. 2000, 61, 49.
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14. Chen, Y.; Han, X.; Bao, X. Appl. Catal. A: Gen. 2004, 273, 185. 15. Kang, M.; Park, M. S. Appl. Catal. B:Environ. 2004, 53, 195. 16. Ovenstone, J.; Yanagisawa, K. Chem. Mater. 1999, 11, 2770. 17. Luan, Z.; Kevan, L. Micro. Meso. Mater. 2001, 44, 337. 18. Hsien, Y. H.; Chang, C. F.; Cheng, S. Appl. Catal. B: Environ. 2001, 31, 241. 19. Belhekar, A. A.; Awate, S. V. Anand. Catal. Commun. 2002, 3, 453. 20. Ding, H.; Sun, H.; Shan, Y. J. J. Photochem. Photobiol. A: Chem. 2005, 19, 101. 21. Chen, S. Y.; Cheng, S. Chem. Mater. 2004, 16, 4174. 22. Hong, S. S.; Lee, M. S. Catal. Today. 2003, 87, 99. 23. Yeber, M. C.; Rodriguez, J.; Freer, J. Chemosphere. 2000, 41, 1193. 24. Lei, Q.; Hinestroza, J. P. JTATM 2004, 4, 1. 25. Guillard, C.; Rousseau, A. Appl. Catal. B: Envir. 2005, 61, 58. 26. Chen, S. Y.; Cheng, S. Chem. Mater. 2004, 16, 4174. 27. Leung, R.; Hou, M. J.; Shah, D. O. Surfactant Science Series; vol. 28, Marcel Dekker: New York, 1988, p 315. 28. Pillai, V.; Shah, D. O. Industrial Application of Microemulsion; Marcel Dekker: New York, 1997, p 227. 29. Tanev, P. T.; Pinnavaia, T. J. Science 1995, 267, 865. 30. Cullity, B. D. Elements of X-Ray Diffarction; MA, 1978. 31. Zhao, D.; Feng, J.; Stucky, G. D. Science 1998, 279, 548. 32. Hwang, Y. K.; Kwonb, Y. U. Micro. Meso. Mater. 2004, 68,
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21. 33. Wittmann, G. Appl. Catal. B: Envir. 2005, 61, 47. 34. Liu, Z.; Crumbaugh, G. M.; Davis, R. J. J. Catal. 1996, 159, 83. 35. Tozzola, G.; Mantegazza, M. A.; Ranghino, G.; Petrini, G.; Bordiga, S.; Ricchiardi, G.; Lamberti, C.; Zulian, R.; Zecchina, A. J. Catal. 1998, 179, 64. 36. Geobaldo, F.; Bordiga, S.; Zecchina, A.; Giamello, E.; Leofanti, G.; Petrini, G. Catal. Lett. 1992, 16, 109. 37. Sinclair, P. E.; Sankar, G.; Richard, C.; Catlow, A.; Thomas, J. M.; Maschmeyer, T. J. Phys. Chem. B. 1997, 101, 4232. 38. Anpo, M.; Shima, T.; Kodama, S. J. Phys. Chem. 1987, 91, 4305. 39. Pignol, D.; Chapus, J. C. Chem. Phys. Lipids. 1998, 93, 123. 40. Devlin, H. R.; Harris, I. J. Ind. Eng. Chem. Fundam. 1984, 23, 387. 41. Eberlein, L.; Langford, C. H. J. Photochem. Photobiol. A: Chem. 2002, 148, 183. 42. Andersson, M.; Osterlund, L. J. Phys. Chem. B. 2002, 106, 10674. 43. Nosaka, Y.; Hirakawa, T. Langmuir. 2002, 18, 3247. 44. Muszkat, L.; Feigelson, L. J. Photochem. Photobiol. B: Biolog. 2001, 60, 32. 45. Uchida, K.; Kato, Y.; Kawakishi, S. Biochem. Biophys. Res. Commun. 1990, 169, 265. 46. Smith, G. J. J. Photochem. Photobiol. B: Biology. 1995, 27, 187.
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One-pot Synthesis and Photocatalysis of Encapsulated TiO2 in Mesoporous SiO2
a
Yang Chuan Leea,b ( ) and Soofin Chenga* ( ) Department of Chemistry, National Taiwan University, Taipei 106, Taiwan, R.O.C. b Ching Kuo Institute of Management and Health, Keelung 203, Taiwan, R.O.C.
Nano TiO2 particles encapsulated in mesoporous SiO2 materials (abbreviated as TiO2/MS) were prepared by incorporating the nano-sized TiO2 particles in the pore-directing micelles during the cooperative assembly of silica precursor and surfactant. The materials were characterized with X-ray powder diffraction (XRD), transmission and scanning electron microcopies (TEM and SEM), N2 sorption, ICP-MS and UV-Vis spectroscopy. XRD patterns showed that titania was in the anatase phase. The results of TEM and N2 sorption studies confirmed the deposition of nano-sized particles inside the mesopores of silica material without destroying their integrity. UV-Vis spectra showed blue shift of the absorption edge for TiO2/ MS relative to bulk anatase. When used as photocatalysts, TiO2/MS showed similar activity as anatase TiO2 in photo-degradation of phenol. However, lower activity than anatase TiO2 was observed in photodegradation of lipase, implying that most of the nano-sized TiO2 were encapsulated inside the mesopores of silica. Keywords: Photocatalyst; Photodegradation; Encapsulated; Mesoporous SiO2; Reverse micelle; Microemulsion; Nano TiO2 particles.
INTRODUCTION Heterogeneous photocatalysts have shown high efficiencies in the removal of toxic and non-biodegradable pollutants commonly present in air and wastewaters.1–3 Photocatalysis over a semiconductor oxide such as TiO2 is initiated by the absorption of a photon with energy equal to or greater than the band gap of the semiconductor (3.2 eV for TiO2), producing electron-hole (e-/h+) pairs.4 The e-/h+ pairs then act as either an electron donor or acceptor for molecules in the surrounding media. In the presence of oxygen and water, OH radicals are considered to form and act as the active species to degrade the pollutants.5 Particles in nano size are considered to offer a high specific surface area for heterogeneous reactions. Unfortunately, for applications in the aqueous phase such a small particle size means high costs in filtration in order to separate the catalyst after the reaction is finished. These problems have motivated the development of supported photocatalysts in which TiO2 has been immobilized on diverse materials.6 Recent research has been focused on the preparation
of photocatalysts supported on materials with large pore sizes and high porosity. Highly ordered mesoporous SiO2 structures such as MCM-41 or SBA-15, prepared via a solgel method using surfactants as the structure directing agents have attracted great attention due to their high specific surface area (630-1040 m2g-1), pore sizes (2-30 nm) and pore volumes (0.6-2.5 cm3g-1).7–9 Two different techniques have been applied to incorporate titania into the mesoporous SiO2 materials. One was through coprecipitation (CP) of silica and titania via a sol–gel method in which the precursors of both metal oxides are hydrolyzed simultaneously.10–15 When the titanium contents were low (Si/Ti molar ratio > 20), Ti was mainly incorporated into the silica framework through isomorphous substitution of Si.10–15 Although mesoporous silica materials containing low Ti contents were catalytically active for selective oxidation of benzene to phenol10 and for epoxidation of several alkenes and styrene,11–14 photocatalytic activity under near-UV irradiation was not significant. The photocatalytic activity usually needs crystalline titania particles.16 The other synthesis route consists of impregnation (IMP) of a meso-
Dedicated to the memory of the late Professor Ho Tong-Ing. * Corresponding author. Fax: +886-2-23636359; E-mail: [email protected]
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porous SiO2 material with a titanium precursor that is subsequently hydrolyzed on the silica surface.17–20 So far, the impregnation method has been the main route used to prepare TiO2-loaded mesoporous SiO2.17–22 Photocatalytic activities of the supported TiO2 materials were shown in the degradation of gaseous toluene15 and aqueous benzene,18 chlorobenzenes,18 phenol18,19 and methylene blue.19 In the past few years, the application of photocatalytic active materials in textiles has been addressed by many research groups and companies for the purpose of removing organic pollutants in air.23,24 The photocatalyst was mixed into the textile fiber by either direct mixing or coating. However, the textile fiber itself was photo-degraded after exposure to sunlight because of the strong oxidizing power of the photocatalyst.25 In the present work, we attempt to prepare TiO2 particles encapsulated inside the pores of mesoporous silica materials by one-pot synthesis. The photocatalytic activities of the resultant material in the decomposition of the small molecule phenol and large molecule lipase were examined, and the results were compared with those over bulk anatase TiO2.
EXPERIMENTAL SECTION Synthesis of TiO2/MS Titania nano particles were prepared following the procedures mentioned in a previous report.22 Titanium tetraisopropoxide (TTIP) was hydrolyzed in the reverse micelle microemulsion (H2O/TTIP = 2) containing dioctyl sulfosuccinate sodium salt in cyclohexane. The reverse microemulsion solution was prepared by dissolving 0.0055 mol of anionic surfactant dioctyl sulfosuccinate sodium salt (98%, Acros) in a mixture of 3.9 mL cyclohexane and the required amount of distilled water. The water-clear appearance of the solution indicated the formation of the microemulsion. Then, TTIP was added in and the microemulsion solution became gel. The gel was transferred to an aqueous solution containing the amphiphilic triblock copolymer P-123 (EO20PO70EO20, Aldrich) as the pore-directing agent, and FeCl3 and NaCl salts which stabilized the micelle. The mixture was stirred at 35 °C for 1 h, followed by the addition of TEOS and stirred at 35 °C for another 48 h. The molar composition of the mixture was TEOS: Ti: FeCl3: NaCl: P123: H2O = 1.0: 0.1: 0.001: 0.03: 0.017: 233. The gel was hydrothermally heated at 90 °C for 1 day. Then, the solid product was filtered, washed with
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water, and dried at 100 °C overnight. The dried product was calcined at 560 °C for 6 h with a ramping rate of 1.1 °C/ min. For comparison, pure mesoporous SiO2 was synthesized following the same method but without the addition of TTIP. Bulk anatase TiO2 was synthesized by hydrolyzing TTIP in water, followed by filtration and calcination at 500 °C for 4 h. Characterization XRD patterns were recorded in the 2q range of 0.580° using a PANalytic X’pert Pro diffractometer with Cu Ka radiation operated at 40 mA and 45 kV. The pore structures of the samples were analyzed by nitrogen physical adsorption at liquid N2 temperature using a Micrometerics TriStar 3000 system. Prior to the experiments, samples were outgassed at 200 °C for 6-8 h under vacuum (10-3 Torr). The titanium contents of the samples were determined by using ICP-MS (Perkin-Elmer ELAN 6000) on the mixed HF/HNO3 solution-dissolved samples. Transmission electron micrographs were obtained using a Hitachi H-7100 microscope operating at an accelerating voltage of 125 kV. The morphology of the sample was examined by a Hitachi S-800 Scanning Electron Microscope. UV-Vis diffuse reflectance spectra (DRS) were taken for the drypressed disk samples using a Scan UV-Vis spectrophotometer (Hitachi 3310) equipped with an integrating sphere assembly, using BaSO4 as the reflectance reference sample. Photocatalytic activity measurements The photodegradation experiments were carried out in a quartz tubular reactor placed inside a Rayonet photochemical chamber (Model PRP-100). The stirred suspensions were illuminated by means of 128W low pressure Hg lamps with an emission wavelength of 300 nm. The catalyst in powder form, based on the same titanium equivalence as 0.01 g TiO2, was suspended in 50 mL of a 0.6 mM phenol solution or 120 ppm porcine pancreatic lipase (PPL) solution (Aldrich, 25% protein). The reactor was kept at 20 ± 2 °C with cooling water circulation during the experiments. A flow of oxygen (2 mL/min) bubbling into the reactor served as the oxidant. The outlet gases were directed through a two-stage bubbling trap containing saturated Ba(OH) 2 solution, and the CO2 yield was determined based on the weight of BaCO3 precipitated. Six molecules of CO2 should be formed by complete oxidation of one phenol molecule. The residual phenol or lipase in the solutions was analyzed by checking the absorbance at 269 nm or 258 nm, respec-
Mesoporous SiO2 Encapsulated TiO2
tively, with a UV-Vis spectrophotometer (Hitachi 3310).
RESULTS AND DISCUSSION The condition for the synthesis of a SBA-15 mesoporous molecular sieve was modified in order to incorporate high Ti loading. The method for synthesizing mesoporous silica using P123 as pore-directing agent without the addition of mineral acid was adapted.26 The main strategy of this method was to utilize the acidity self-generated in the aqueous solution of FeCl3 as the catalyst for TEOS hydrolysis. In addition, FeCl3 and NaCl also contributed to the salt effect in stabilizing the micelles and increasing the crystallographic ordering of the meso-structure. The nano TiO2 particles were prepared by hydrolysis of TTIP in the reverse micelles formed in the anion surfactant/cyclohexane solution.22,27-28 Fig. 1 shows the XRD patterns of TiO2/MS and pure mesoporous SiO2. The lowangle powder XRD patterns of the calcined samples only featured a strong diffraction peak indexed to the (100) plane of hexagonal arranged porous structure, accompanied by broad unresolved higher order reflections. These results imply that both materials possess mesostructures which however lack long-range ordering.29 The d-spacing of TiO2/MS was greater than that of pure mesoporous SiO2 (10.4 nm vs. 8.4 nm), as shown in Table 1. It is attributed to that nano TiO2 particles formed in the anion surfactant reverse micelles may be incorporated into the hydrophobic phase of the P123 micelles and cause the expansion of the micelle volume. The formation of anatase TiO2 particles was confirmed by the diffraction peaks at higher angles (2q
Fig. 1. XRD patterns of (a) TiO 2 /MS and (b) pure mesoporous SiO2.
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= 25.38°, 38.38°, 48.15°). The crystallite diameter was calculated to be 12 nm from the peak width at half maximum (pwhm) of anatase (101) peak at 2q = 25.38° using Scherrer’s equation.30 The nitrogen adsorption–desorption isotherms are illustrated in Fig. 2A. Both TiO2/MS and pure mesoporous SiO2 yielded Type IV isotherms, characterized by a steep increase in absorption volume of nitrogen at the relative pressure range around 0.45 < P/P0 < 0.75. This type of isotherm is typical for mesoporous materials having rather large pore sizes.31 The large hysteresis loop observed on the sorption isotherm of pure mesoporous SiO2 indicates that it contains ink-bottle like pore structures with a relatively narrow entrance of the pores.32 On the other hand, the hysteresis loop of TiO2/MS is less steep and covers a broader pressure range in comparison to that of pure mesoporous
Fig. 2. (A) Nitrogen adsorption-desorption isotherms and (B) BJH pore size distribution profiles of samples (a) TiO2/MS and (b) pure mesoporous SiO2.
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Table 1. Physico-chemical properties of TiO2/MS in comparison to those of pure mesoporous SiO2 Catalyst Meso SiO2 TiO2/MS
TiO2 (wt%)a
SBET (m2/g)
VP (cm3/g)
d-spacing (nm)
Db (nm)
PSD c (nm)
TiO2 cryst. diameter d (nm)
0 12
750 645
0.61 0.70
8.4 10.4
5.0 7.5
1.3 2.0
— 12
a
Analyzed by ICP-MS. BJH adsorption pore diameter. c FWHM of BJH pore size distribution profile calculated from the desorption branch of isotherm. d Obtained by Scherrer equation. b
SiO2. This indicates that TiO2/MS contains less uniformly distributed mesopore diameters than pure mesoporous SiO2. This result is confirmed by the BJH adsorption pore size distribution shown in Fig. 2B. The pore-size distribution (PSD), based on the pwhm of the profile, of TiO2/MS is 2.0 nm in comparison to 1.3 nm for pure mesoporous SiO2. The BJH pore diameter of TiO2/MS (7.5 nm) was greater than that of pure mesoporous SiO2 (5.0 nm). The larger pore size and significantly broader distribution of mesopores of TiO2/MS are attributed to the presence of titania crystallites within the mesopores. This is in contrast to the samples synthesized by the impregnation method, in which loading of titania into the meso-channels caused pore blocking and a decrease in pore size.33 However, the TiO2 crystallite diameter of 12 nm calculated from the line-broadening of the XRD peak is greater than the pore diameter of 7.5 nm, indicating that the TiO2 particles are probably incorporated in the defect sites of the mesostructure. Transmission electron micrographs of the TiO2/MS sample containing 12% TiO2 are shown in Fig. 3. Ordered mesopores in hexagonal arrangement are clearly seen. Moreover, nano-sized TiO2 particles are mostly present inside the meso-channels but many are extended to cover several pores. These results are in good agreement with those from XRD studies. The SEM image of TiO2/MS is shown in Fig. 4. Particles with irregular shapes are aggregated together and have a broad distribution of particle sizes ranging from 0.1~ 0.4 µm. The UV-Vis reflectance spectra of anatase TiO2 and calcined TiO2/MS are shown in Fig. 5. The absorption at 270 nm is due to O®Ti charge transfer, and the broad absorption at a longer wavelength is due to the excitation of electrons from the valence band to the conduction band of TiO2. The band edge of 345-350 nm observed for TiO2/MS is significantly blue-shifted from the band edge of 387 nm
for bulk anatase TiO2.34-37 The blue-shift is attributed to the quantum size effect,38 confirming that no large TiO2 crystals were formed in the TiO2/MS material. In other words, the deposition of large TiO2 particles on the external surface of mesoporous SiO2 was not likely. In order to evaluate the photocatalytic activities of TiO2/MS and to ensure that the nano TiO2 particles were encapsulated inside the mesopores, the photo-degradation of
Fig. 3. Transmission electron micrographs of sample TiO2/MS.
Mesoporous SiO2 Encapsulated TiO2
the small molecule phenol and the large molecule lipase (approx. 4.6 nm × 2.6 nm × 1.1 nm)39 were studied. The results were compared with those over anatase TiO2. Fig. 6(A) shows the results in degradation of the small molecule phenol. Both samples with the same TiO2 content showed similar photocatalytic activities. This implies that the nano TiO2 particles located inside the mesopores are as photocatalytic active as the anatase TiO2. As long as the pollutant molecules can diffuse inside the pores, they can be decomposed by photocatalysis. When the large molecule lipase was the subject of photo-degradation, TiO2/MS showed a
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much slower rate than anatase TiO2 as shown in Fig. 6(B). This is elucidated by that the slow diffusion of the large lipase molecules impedes them from contact with the nano TiO2 particles located inside the mesopores and the rate of photo-degradation is slower than that over anatase TiO2. These results prove that our synthesis method could encapsulate nano TiO2 particles inside the pores of mesoporous silica and still retain their photocatalytic activities. Several groups have reported the pathway of phenol photo-degradation using TiO2 as a photocatalyst.40-43 A UV photon(hu) which overcomes the energy gap between valence and conduction bands of TiO2 can create an electron-hole pair, as shown in equation 1. The hole then reacts with water to produce •OH radicals as shown in equation 2. In the presence of molecular oxygen, O2 can be reduced by conduction band electrons (equation 3), which in principle can also result in the formation of hydroxyl radicals (equations 4-6). This radical can react with phenol, producing a
Fig. 4. Scanning electron micrograph of sample TiO2/ MS.
Fig. 5. Diffuse reflectance UV-Vis spectra of samples (a) TiO2/MS and (b) bulk anatase TiO2.
Fig. 6. Photo-degradation rates of TiO2/MS in comparison to those of bulk anatase TiO2 (based on the same 0.01 g TiO 2 content) for (A) phenol and (B) lipase with 300 nm UV illumination.
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whole range of intermediates such as catechol (CC), hydroquinone (HQ) and benzoquinone (BQ) (equation 7). These aromatic intermediates are further oxidized by •OH radicals or holes to undergo ring cleavage and form carboxylic acids and aldehydes, which finally yield CO2 and H2O (equation 8). TiO2 + hu ® TiO2(h)+ + eTiO2(h)+ + H2O(ads) ® •OH + H+ + TiO2 O2 + e- ® •O2•O2- + •O2- + 2 H+ ® H2O2 + O2 H2O2 + •O2- ® •OH + OH- + O2 H2O2 + e- ® •OH + OH•OH + phenol ® intermediate oxygenated products (e.g., CC, HQ, BQ) TiO2(h)+ + intermediate products ® CO2 + H2O + TiO2
(1) (2) (3) (4) (5) (6) (7)
sion of dioctyl sulfosuccinate sodium salt/cyclohexane solution. The reverse microemulsion containing nano TiO2 particles were proposed to incorporate in the hydrophobic phase of the micelles formed by P123, which served as the pore-directing agent in the formation of mesoporous silica. The formation of nano TiO2 particles was proved by the blue shift of absorption in UV-Vis spectra relative to anatase TiO2 as well as the TEM photographs. When comparing the photo-degradation activities of TiO2/MS for phenol and lipase, the slower decomposition rate of lipase molecules further proved that the nano TiO2 particles in TiO2/ MS were located inside the mesopores. This material is applicable to textiles as a photocatalytic active material in the decomposition of pollutants in air without damaging the fibers.
(8) ACKNOWLEDGMENT
The role of TiO2 as a photocatalyst was proved in the present study by observing the wavelength dependence of the phenol decomposition process. Only UV photons with energy greater than the band gap energy of TiO2 (approx. 3.2 eV) are effective in the photodegradation reaction. It is also noticed that CO2 selectivity over bulk TiO2 is slightly higher than that over TiO2/MS (Fig. 6(A)). That is elucidated by that the large surface area of TiO2/MS may stabilize the oxygenated intermediates. In the case of photo-degradation of lipase, the CO2 yields over both photocatalysts were found to be negligible. The mechanisms of the oxidative cleavage of proteins are not yet fully understood.44-46 Nevertheless, the •OH radicals were presumably the active species initiating the oxidative cleavage of poly-peptide chains. The spectroscopic analysis of the aqueous solution of lipase after 2 h of irradiation over TiO2 show that the large lipase molecules were probably cleaved into peptides and the aromatic rings were oxidized.
CONCLUSIONS An effective one-pot synthesis method was developed to encapsulate nanosized TiO2 inside the mesoporous silica during the self-assembly process. The resultant material TiO2/MS contained hexagonal arranged mesopores and a high surface area. The nano TiO2 particles were prepared by hydrolysis of TTIP in the reverse micelle microemul-
Financial support from the National Science Council, Taiwan, is gratefully acknowledged.
Received July 10, 2006.
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