J. Phys. Chem. C 2007, 111, 11849-11853
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Synthesis of Well-Ordered Mesoporous Titania with Tunable Phase Content and High Photoactivity Lin Chen,* Baodian Yao, Yong Cao, and Kangnian Fan Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and InnoVatiVe Materials, Fudan UniVersity, Shanghai 200433, People’s Republic of China ReceiVed: March 15, 2007; In Final Form: June 12, 2007
Using TiCl4 and Ti(OBu)4 as the precursors and Pluronic P123 as the template, mesoporous titanium dioxide (TiO2) is successfully prepared by the nonhydrolytic evaporation-induced self-assembly (EISA) method with a three-dimensional mesostructure obtained. X-ray powder diffraction patterns (XRD) show that the pore wall is composed of both anatase and rutile phases. Transmission electron micrograph observations (TEM) reveal a well-ordered open-pore mesostructure. N2 adsorption analysis exhibits a narrow pore size distribution. Photodegradation of phenol shows that the sample has a better photoactivity than Degussa P25. Further investigations on the effect of the “acid-base pair” find that by changing the molar ratio of TiCl4/Ti(OBu)4, both the pore texture and phase composition can be well-controlled. A high content of TiCl4 is considered to favor the formation of regular mesostructures and a proper weight ratio of anatase/rutile as well, which can finally affect the photocatalytic efficiency of the sample
Introduction With the flourish of mesoporous material synthesis, much more emphasis has been stressed on the preparation of metal oxide, especially those of transition metals. Mesoporous semiconductors such as CuO, ZnO, et al. have already been synthesized and proved to be potentially applicable for practical use.1-3 Titanium dioxide (TiO2) with mesostructure is also among the most focused materials. Such characteristics as low cost, ease of handling, nontoxicity and high resistance to photoinduced decompositions have made titania one of the most promising and suitable catalysts for heterogeneous photoreactions.4,5 Since its first discovery in 1972 for photosplitting water molecules,6 TiO2 has been widely used in photocatalysis, photoelectrodes, and dye-sensitized solar cells, etc., while mesoporous titania, with its larger specific surface area, higher porosity, and multidimensional network textures, has recently attracted more and more attentions.7-9 A nonhydrolytic template-mediated sol-gel route has been reported to obtain TiO2 with regular open-pore structures.10 Organic solutions prohibit the rapid hydrolysis of Ti species, soft copolymer templates promote the formation of well-ordered octahedron titanium hydroxides, and post-thermal treatment is conducive to the removal of organic compounds. Samples obtained through the whole procedure usually possess pore structures within nanometer scales.11 In 2003, by introducing the newly developed concept of acid-base pair into the preparation of nanocrystalline semiconductors, Tian et al. successfully synthesized mesoporous TiO2 exhibiting a 2D hexagonal morphology.12 Ozin et al.13 and Sanchez et al.14 also prepared TiO2 samples with well-arranged mesostructures respectively. However, among these methods, only a single phase of anatase could be obtained and usually at the sacrifice of the ordering of mesostructure. To date, ordered mesoporous titania with rutile-phase composite has not been reported. * Corresponding author. Telephone: +86-21-61305181. Fax: +86-2165643774. E-mail:
[email protected].
Here in our report, by selecting titanium chloride (TiCl4) and titanium butoxide (Ti(OBu)4) as the precursor, in which the former being the pH “adjustor” and hydrolysis-condensation “controller”, triblock copolymer P123 serving as the template, and ethanol as the solution, ordered mesoporous titanium dioxide with a uniform pore size and high surface area was obtained. Moreover, the pore wall t is composed of a mixed-phase of both anatase and rutile. Further detailed characterizations and photocatalytic tests show that the molar ratio of TiCl4/Ti(OR)4 poses a dramatic influence on the sample’s pore structure, phase content, and photoactivity. Experiment (1) Sample Preparation. In a typical synthesis, 1.0 g of P123 was dissolved in 20 mL of ethanol; then 1 mL of TiCl4 (9.1 × 10-3 mol) and 0.78 mL of titanium butoxide (Ti(OBu)4, 2.3 × 10-3 mol) were added, and the mixed solution was further stirred for 2 h. Then the mixture was transferred to Petri dishes and underwent solvent evaporation. After gelation at 40 °C (relative humidity was controlled at 45%) for 1 day, the template was removed through calcinations at 350 °C in air for 4 h. To investigate the effect of the acid-base pair, samples of different molar ratios of TiCl4/Ti(OBu)4 were prepared for comparison, while the total amount of Ti (11.4 × 10-3 mol) was kept unchanged; the sum of the corresponding molar ratios was denoted as 2.5. (2) Characterization. The textural parameters have been measured using the BET method by N2 adsorption and desorption at 77 K in a Micromeritics TriStar system. Transmission electron micrographs (TEMs) were recorded digitally with a Gatan slow-scan charge-coupled device (CCD) camera on a JEOL 2011 electron microscope operating at 200 kV. The samples for electron microscopy were prepared by grinding and subsequently dispersing the powder in acetone and applying a drop of very dilute suspension on carbon-coated grids. The X-ray powder diffraction (XRD) of the samples was carried out on a Germany Bruker D8Advance X-ray diffractometer using nickel-
10.1021/jp072070z CCC: $37.00 © 2007 American Chemical Society Published on Web 07/26/2007
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Figure 1. XRD pattern of the typically prepared sample. Inset: Lowangle XRD pattern.
filtered Cu KR radiation with a scanning angle (2θ) of 10-80° and a voltage and current of 40 kV and 20 mA. The average size of anatase TiO2 crystallites was estimated by means of the Scherrer equation from broadening of the (101) anatase reflection. Estimation of the content of anatase is based on the following: XA ) 1/[1 + 1.265IR/IA] × 100%, where IA is the (101) peak intensity of anatase, IR is the (110) peak intensity of rutile, and 1.265 is the scattering coefficient. (3) Photoactivity Test. To test photocatalytic behavior of the as-synthesized TiO2 powders, photodegradation of phenol in water is chosen as a probe reaction. Briefly, an amount of 0.05 g of the catalyst was dispersed in 50 mL of the phenol (SCR, 99%) aqueous solution (C ) 0.06 g/L). The used source of UV irradiation was two high-pressure mercury lamps enclosed in a glass filter bulb (HPW, 150 W, Philips) whose emission consists to 93% of 365 nm radiation. The temperature of the system was controlled around 25 ( 0.5 °C by circulating cooling water. The prepared suspension was then magnetically stirred for about 30 min under the condition of oxygen bubbling (50 mL/min) in the dark to achieve the adsorption/desorption equilibrium. At regular irradiation intervals, the dispersion was sampled (ca. 1 mL), centrifuged, and subsequently filtered through a Millipore filter to remove the photocatalyst sample before the HPLC analysis (5 µm, 50 mm × 4.6 mm, Akzonobel KR100-5C18 ODS column, 50% methanol-water mobile phase, UV detector), from which the degradation yield (%) was calculated by comparing the initial content of phenol in the solution; experimental error is within (1%.
Figure 2. TEM images of the typically prepared sample viewed from the following planes: (a) [111]; (b) [110]; (c) [311]; (d) [100]. Scale bar: 50 nm. Inset: ED patterns.
mesoporous titania reported previously,12-14 whose wall structure was composed only of anatase, mixed-phase crystal textures can be clearly observed in our sample. Well-defined characteristic peaks of both anatase and rutile are labeled respectively in the pattern. The weight ratio of A (anatase) to R (rutile) is estimated to be 82:18, which is very close to that of Degussa P25 (80:20; see in Table 1). Low-angle XRD patterns exhibit several ambiguous peaks. Such a low resolution might be caused by the further crystallization of TiO2 and partial destruction of the well-ordered structures after calcinations.15,16 TEM images of the sample are illustrated in Figure 2. Regular open-pore morphologies are clearly recorded. Highly ordered mesoporous patterns are presented in large domain. Typical images of characteristic projections of the structure along various plane directions indicate an intrinsic 3D well-arranged mesostructure of our sample, which is similar to the cubic bicontinuous (Ia3hd) textures in previous literature.12 Electron diffraction patterns (Figure 2 inset) show the diffraction rings of both anatase and rutile phases, corresponding well with the XRD patterns. N2 adsorption-desorption isotherms of the calcined sample exhibits a type IV curve with a steep rise in adsorption region at 0.5-0.7 relative region, featuring a H2 hysteresis loop (Figure 3). BJH adsorption pore distribution analysis of the treated sample shows a narrow Gaussian pore size distribution, implying that the TiO2 possess very regular pore channels in the
Results (1) Preparation of Mesoporous TiO2. Figure 1 shows the XRD patterns of the 350 °C calcined sample. Unlike the
TABLE 1: Physicochemical Properties of Samples Prepared by Different Ratios of TiCl4/Ti(OBu)4 TiCl4/Ti(OBu)4 2.5/0 2.0/0.5 1.5/1.0 1.0/1.5 0.5/2.0 0/2.5 P25
SBET a (m2 g-1)
Vp a (cm3 g-1)
Dp a (nm)
D b (nm)
anatase/rutilec
degradation yieldd (%)
80.7 91.6 105 113 135 94.6 50
0.177 0.187 0.174 0.262 0.242 0.101 0.20
3.5 4.5 4.0 6.1 4.5 3.4 23.1
13.9 13.2 12.1 11.3 10.5 9.48 20.0
73:27 82:18 86:14 90:10 98:2 100:0 80:20
90.8 97.0 93.7 88.4 86.7 83.0 89.9
a BET surface area (SBET), average pore volume (Vp), and average pore diameter (Dp) estimated from nitrogen adsorption. b Average anatase crystallite size (d) evaluated from Scherrer equation. c Ratio of anatase to rutile based on XRD data. d Percent of degraded phenol after 90 min irradiation time.
Synthesis of Well-Ordered Mesoporous Titania
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Figure 3. N2 adsorption isotherms of the as-synthesized sample. Inset: Pore size distribution.
Figure 5. TEM images of samples prepared by different ratios of TiCl4/ Ti(OBu)4: (a) 2.5/0; (b) 2.0/0.5; (c) 1.0/1.5; (d) 0/2.5. Scale bar: 50 nm.
Figure 4. XRD patterns of samples prepared by different ratios of TiCl4/Ti(OBu)4. Inset: Plot of weight ratio of anatase versus TiCl4/ Titotal.
mesoporous region (4.5 nm, mean value calculated by BJH model). The estimated pore volume of the sample is 0.187 cm3 g-1, and the BET surface area is 91 m2 g-1, nearly two times larger than that of Degussa P25 (50 m2 g-1). (2) Tunable Phase Content and Pore Structure. To make further detailed investigations of the effect of the acid-base pair, different ratios of TiCl4/Ti(OBu)4 were applied to the synthetic procedure, while other reaction conditions were kept unchanged. Figure 4 presents the XRD patterns of these samples. It is interesting to find that with the change of the content of TiCl4 in the total amount of Ti precursors, the original crystal composition of the pore walls can be altered from pure anatase to a mixed phase of anatase and rutile, and the phase content can be furthermore regulated. Table 1 lists the approximate calculation of the weight ratios of A/R. It is obvious that with the increase of the amount of TiCl4, the content of anatase decreases. To gain a deeper understanding about the effect of TiCl4 taking on tuning the A/R ratios, the plot of the weight fraction of anatase versus the content of TiCl4 is shown in the inset of Figure 4. A linear relationship is found between the two factors, indicating the exact controllability of the phase composition.
Figure 6. Photodegradation of phenol by samples prepared by different ratios of TiCl4/Ti(OBu)4. Inset: Natural logarithm plot of the removal of phenol.
Figure 5 presents the TEM images of samples prepared by different molar ratios of TiCl4/Ti(OBu)4. Changes of pore structures can be apparently observed. When there is no addition of Ti(OBu)4, only wormlike structures can be found. With the increase of the amount of Ti(OBu)4, the pores of the samples change from ordered open pore textures to less regular pore channels. While without the content of TiCl4, stacks of loosely connected particles are to be formed. (3) Photocatalytic Activity. The photocatalytic degradation of phenol has been selected as a model reaction to evaluate the photoactivity of the synthesized samples. The degradation yield of phenol removal at various time intervals is illustrated in Figure 6. It is found that nearly all samples exhibit high photocatalytic efficiency. Samples prepared with a TiCl4/Ti(OBu)4 ratio of 2.0/0.5 and 1.5/1.0 present a comparable effect of P25 (almost 100% yield), while the sample of a ratio of 0/2.5 has the worst activity (91.4%). However, if the photodegradation
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TABLE 2: Kinetic Parameters of Samples for Phenol Photodegradation TiCl4/ kobs a rm b (µmol L-1 rs c (µmol L-1 ra d (µmol L-1 Ti(OBu)4 (min-1 g-1) min-1 g-1) min-1 m-2) min-1 g-1) 2.5/0 2.0/0.5 1.5/1.0 1.0/1.5 0.5/2.0 0/2.5 P25
0.506 0.740 0.578 0.450 0.426 0.388 0.478
323 472 369 287 272 247 305
3.99 5.15 3.51 2.54 3.01 2.61 6.09
4.42 5.75 4.28 3.19 2.77 2.47 3.80
a The apparent first-order reaction rate constants (kobs). b The initial reaction rates per used mass (rm). c The initial reaction rates per unit surface area (rs). d The initial reaction rates per unit anatase (ra).
SCHEME 1: Possible Mechanism of the Formation of Mesoporous TiO2 with Well-Ordered Pore Morphology and High Photoactivity
yield of 1.5 h is solely taken out for comparison, the typically prepared sample (2.0/0.5) shows the best catalytic effect, which is 7% higher than that of P25 (Table 1). To obtain a further insight of the kinetic aspect of phenol removal from aqueous solution, the plot of natural logarithm of residual phenol concentration versus UV irradiation time is presented. (Figure 6 inset) The straight line obtained shows that the degradation process follows the first-order kinetics, in accordance with what literature has reported before.17,18 To make further mathematics inferences clear, all the related kinetic parameters, such as the apparent reaction rate constants (kobs), initial reaction rates per used mass (rm), per unit surface area (rs), and per unit anatase (ra), are calculated in Table 2. It is noticeable that although the typically prepared sample has an inferior rs, the kobs, rm, and ra of the sample are found to be higher than those of P25. Discussion Nonhydrolytic evaporation-induced self-assembly (EISA) strategy19 is applied in our experiment to synthesize mesostructured TiO2. On the basis of the acid-base pair concept, TiCl4 and Ti(OBu)4 are selected and used to react under the existence of triblock copolymer template P123; we manage to obtain mesoporous TiO2 with a well-ordered pore structure. Moreover, XRD patterns show that the typically prepared sample has a mixed-phase pore wall texture, which, to our knowledge, has never been reported before. Scheme 1 illustrates the possible mechanism of the synthetic procedure. It is suggested that TiCl4 can react with EtOH to form Ti(Cl)4-x(OEt)x (x ≈ 2) species,20 resulting in a highly acid mother solution (pH ≈ 1 in our experiment). Upon the addition of Ti(OBu)4, serving as both a major titanium source and an extra oxygen donor, the acidity of the solution is reduced. Meanwhile oligomers formulated as TiXx(OH)yO2-(x+y)/2 (X ) OR or Cl; x ≈ 0.3-0.7; y ≈ 0-0.2) are obtained.21 Ti-O-Ti bridges may partially result from the condensation between Ti-Cl and Ti-OR. With the evaporation of ethanol and following thermopolymerization and calcinations to remove the template, TiO2 with a regular pore distribution is formed.
In this experiment, by tuning the ratios of TiCl4/Titotal, we are able to adjust both the pore structure and phase composition of the samples. Many factors have been discovered to affect greatly the final production of the highly organized TiO2 mesophase, such as the choice of template, the template/Ti ratio, the quantity of water in the atmosphere, the temperature during deposition, and aging, etc.14 However, during the whole process, all these above-mentioned factors are kept unchanged except the solution acidity, which is controlled by the molar ratio of TiCl4/Ti(OBu)4. Obviously too high or too low solution acidity has a detrimental effect on the final organization (Figure 5). We suggest that changes of the mother solution pH value may lead to the respective formation of hydrophobic Ti-oxo-alkoxo and hydrophilic Ti-oxo-hydroxo species.21,22 The former will hinder the proper folding of the polymers, “freezing” disordered states, while the latter will enhance the ordered template folding, creating the conditions for an adequate phase segregation at the mesoscale. Additionally, we found an approximately linear relationship between the amount of TiCl4 and the weight ratio of anatase composing the samples (Figure 4 inset). It has been widely recorded in the literature that a certain amount of HCl and counterion Cl- can favor the growth of rutile nuclei due to their weak steric effect on the TiO62- octahedron under acidic conditions.23-26 During the reaction process, by varying the amount of TiCl4, with the formation of different amounts of HCl and Cl-, we are able to obtain different quantities of anatase and rutile nuclei, which finally constitute the crystalline pore walls after calcinations. For the photocatalytic part, the typically synthesized TiO2 with a well-ordered mesostructure and a mixed phase exhibits the most photoactive behavior, even higher than that of P25 (Table 1). To gain an intrinsic understanding of the factors that regulate the sample’s photocatalytic effect, further detailed mathematic calculations are listed in Table 2. Kinetic study shows that when the sample has a faster initial reaction rate than P25, it becomes slower if averaged to per unit surface area. Notice that the phase content of the sample is similar to that of P25 (Table 1), we attribute the higher photoactivity to the wellordered 3D open-pore structure, which, combined with its relatively large surface area and pore volume, can facilitate the mass transport of the organic pollutants. By contrast, for samples prepared with a TiCl4/Ti(OBu)4 ratio of 1.0/1.5 and 0/2.5, for instance, though they possess larger surface areas, the lack of well-arranged pore channels and regular pore arrays may inhibit the flow of the phenol solution, affect the absorption/desorption process, and result in their inferior catalytic efficiencies. Moreover, by comparing the reaction rate per unit anatase, we can find that too much anatase will retard the photodegradation process, indicating that the proper amount of rutile is of certain benefit. Such a synergetic effect, also referred to as “mixedphase effect”, is supposed to efficiently prohibit the recombination of photogenerated electron-hole pairs by transferring the exited electrons from one phase to another.27,28 Therefore, both the regular open pore morphology and the biphase structure are playing crucial roles in determining the sample’s photoactivity. Conclusion In this report, a nonhydrolitic EISA route is applied and found to be effective in preparation of mesostructured TiO2. Wellordered mesoporous TiO2 with crystalline mixed-phase pore wall structure has been successfully synthesized. The molar ratio of TiCl4/Ti(OBu)4 is found to be of paramount importance in tuning not only the sample’s pore structure but also the phase composition of anatase and rutile. And both of them are
Synthesis of Well-Ordered Mesoporous Titania considered to significantly influence the sample’s photocatalytic effect. Further work is ongoing to explore the origin of the mixed phase via detailed characterization of these samples. Acknowledgment. We thank the National Natural Science Foundation of China (Grant Nos. 20473021, 20503005, 20421303, and 20407006), the National Key Basic Research Program (Grant No. 2003CB615807), and the Research Fund for the Doctoral Program of Higher Education (Grant No. 20050246071) for financial support. References and Notes (1) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (2) Kuo, C.-L.; Kuo, T.-J.; Huang, M. H. J. Phys. Chem. B 2005, 109, 20115. (3) Zhang, J.; Liu, J.; Peng, Q.; Wang, X.; Li, Y. Chem. Mater. 2006, 18, 867. (4) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (5) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol., C 2000, 1, 1. (6) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (7) Armstrong, G.; Armstrong, A. R.; Canales, J.; Bruce, P. G. Chem. Comm. 2005, 19, 2454. (8) Ma, T.; Akiyama, M.; Abe, E.; Imai, I. Nano Lett. 2005, 12, 2543. (9) Ding, B.; Kim, H.; Khil, M.; Park, S. Nanotechnology. 2003, 14, 532. (10) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Nature 1998, 396, 152.
J. Phys. Chem. C, Vol. 111, No. 32, 2007 11853 (11) Wang, T.; Yang, H.; Zhao, D. Acc. Chem. Res. 2006, 39, 423. (12) Tian, B. Z.; Liu, X.; Tu, B.; Yu, C.; Fam, J.; Wang, L.; Xie, S. H.; Stucky, G. D.; Zhao, D. Nat. Mater. 2003, 2, 159. (13) Choi, S. Y.; Mamak, M.; Coombs, Neil.; Chopra, N.; Ozin, G. A. AdV. Funct. Mater. 2004, 14, 335. (14) Crepaldi, E. L.; Soler-Illia, G. J. A. A.; Grosso, D.; Cagnol, F.; Ribot, F.; Sanchez, C. J. Am. Chem. Soc. 2003, 125, 9770. (15) Shibata, H.; Ogura, T.; Mukai, T.; Ohkubu, T.; Sakai, H.; Abe, M. J. Am. Chem. Soc. 2005, 127, 16397. (16) Wang, K.; Morris, M. A.; Holmes, J. D. Chem. Mater. 2005, 17, 1269. (17) Wei, T.-Y.; Wan, C.-C. Ind. Eng. Chem. Res. 1991, 30, 1293. (18) Ding, Z.; Liu, G. Q.; Greenfield, P. F. J. Phys. Chem. B 2000, 104, 4815. (19) Lu, Y. F.; Ganguli, R.; Drewien, C. A.; Anderson, M. T.; Brinker, C. J.; Gong, W. L.; Guo, Y. X.; Soyes, H.; Dunn, B.; Huang, M. H.; Zink, J. I. Nature 1997, 389, 364. (20) Vioux, A.; Chem. Mater. 1997, 9, 2292. (21) Blanchard, J.; Ribot, F.; Sanchez, C.; Bellot, P.-V.; Trokiner, A. J. Non-Cryst. Solids 2000, 265, 83. (22) Soler-Illia, G. J. A. A.; Sanchez, C. New J. Chem. 2000, 24, 493. (23) Yan, M.; Chen, F.; Zhang, J.; Anpo, M. J. Phys. Chem. B 2005, 109, 8673. (24) Zhang, Q.; Gao, L. Langmuir 2003, 19, 967. (25) Andersson, M.; Osterlund, L.; Ljungstrom, S.; Palmqvist, A. J. Phys. Chem. B 2002, 106, 10674. (26) Yamabi, S.; Imai, H. Chem. Mater. 2002, 14, 609. (27) Hurum, D. C.; Agrios, A. G.; Gray, K. A.; Rajh, T.; Thurnauer, M. C. J. Phys. Chem. B 2003, 107, 4545. (28) Bickley, R. I.; Gonzalez-Carreno, T.; Lees, J. S.; Palmisano, L.; Tilley, R. J. D. J. Solid State Chem. 1991, 92, 178.
