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Synthesis and Characterization of Titania Prepared by Using a Photoassisted Sol-Gel Method Haimei Liu,† Wensheng Yang,† Ying Ma,† Yaan Cao,† Jiannian Yao,*,† Jing Zhang,‡ and Tiandou Hu‡ Center for Molecular Science, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, People’s Republic of China, and State Key Laboratory of Synchrotron Radiation, Institute of High Energy Physics, The Chinese Academy of Sciences, Beijing 100039, People’s Republic of China Received September 24, 2002. In Final Form: December 29, 2002 Titanium dioxide nanoparticles were prepared via a photoassisted sol-gel method in which ultraviolet light irradiation was used in the preparation process of TiO2 colloid. After characterization by X-ray diffraction, X-ray absorption near-edge structure (XANES) at the Ti K-edge, laser Raman spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy, it was found that the amorphous titania nanoparticles prepared by a photoassisted sol-gel method can be transformed into crystalline anatase phase at lower calcination temperature compared to those prepared by a conventional sol-gel method. In addition, the particle size distribution of anatase powder samples is also affected by UV illumination on the colloid. It is suggested that UV illumination can induce the formation of oxygen vacancies on the colloid and this results in the accelerated phase transition from amorphous to anatase titania.
Introduction Titanium dioxide has attracted great attention due to its high photocatalytic activity,1 excellent dielectric properties,2 and gas-sensitive properties.3 It is known that crystalline titania has three modification phases: rutile (tetragonal, P42/mnm), anatase (tetragonal, I41/amd), and brookite (orthorhombic, Pcab). It is well-known that anatase TiO2 has excellent photocatalytic activity and has been widely employed as a catalyst for decomposition of a wide variety of organic and inorganic pollutants. Many methods have been established for the synthesis of titania photocatalyst;4-9 among them the sol-gel technique is one of the most used methods. Usually the sol-gel derived precipitates are amorphous in nature, and require further annealing treatment to induce crystallization.10,11 To induce transition from amorphous to anatase phase, generally an annealing temperature higher than 300 °C is required, and this will result in the dramatic growth of the particle sizes. However, the photocatalytic activity of titania is dependent on both particle size and degree of crystallization.12 Therefore, it is meaningful to find a * To whom correspondence should be addressed. Fax: 86-1062559373. E-mail:
[email protected]. † Institute of Chemistry. ‡ Institute of High Energy Physics. (1) Hirakawa, T.; Nakaoka, Y.; Nishina, J.; Nosaka, Y. J. Phys. Chem. B 1999, 103, 4399. (2) Fukushima, K.; Yamada, I. J. Appl. Phys. 1989, 65, 619. (3) Birkefeld, L. D.; Azad, A. M.; Akbar, S. A. J. Am. Ceram. Soc. 1992, 75, 2964. (4) Morrison, P. W.; Raghavan, J. R.; Timpone, A. J.; Artelt, C. P.; Pratsinis, S. E. Chem. Mater. 1997, 9, 2702. (5) Siegel, R. W.; Ramasamy, S.; Hahn, H.; Li, Z.; Lu, T.; Gronsky, R. J. Mater. Res. 1988, 2, 1367. (6) Murakami, Y.; Matsumoto, T.; Takasu, Y. J. Phys. Chem. B 1999, 103, 1836. (7) Kazumich, Y.; James, O. J. Phys. Chem. B 1999, 103, 7781. (8) Morrison, P. W.; Ragharan, J. R.; Timpone, A. J.; Artelt, C. P.; Pratsinis, S. E. Chem. Mater. 1997, 9, 2702. (9) Zaban, A.; Aruna, S. T.; Tirosh, S.; Gregy, B. A.; Mastai, Y. J. Phys. Chem. B 2000, 104, 4130. (10) Wang, C. C.; Ying, J. Y. Chem. Mater. 1999, 11, 3113. (11) Barringer, E. A.; Bowen, H. K. J. Am. Ceram. Soc. 1982, 65, C199.
route to get phase-pure TiO2 at low calcination temperature with both small grain size and a high degree of crystallization. Recently there have been some reports concerning the application of ultraviolet irradiation in sol-gel synthesis of inorganic nanoparticles. It was found that the size distribution of semiconductor nanoparticles such as CdS, ZnO, and ZnS could be narrowed13,14 or the sol-gel derived precursor coating of SiO2, ZrO2, and SiO2-TiO2 could be made denser and crystallized at low temperatures15-17 by application of ultraviolet irradiation. Our group has prepared thin films of TiO2 by a photoassisted sol-gel method and studied their photocatalytic activities.18 In this work we investigated the effect of UV irradiation on the transition process from amorphous to anatase phase of titania nanoparticles prepared by a photoassisted solgel method. It is found that UV illumination induces the formation of oxygen deficiencies in the titania nanoparticles. As a result, the phase transformation temperature of titania from amorphous to anatase phase is decreased to as low as 100 °C. Experimental Section Materials Preparation. Titanium isopropoxide Ti(OiC3H7)4 (Acros, >98%) was used as a titanium precursor in the sol-gel process. Other chemicals and solvents were of analytical grade and used without further purification. Titania colloids were prepared according to the reference method.19 A volume of 5.0 mL of titanium isopropoxide was dissolved in 10 mL of propyl (12) Zhang, Z.; Wang, C. C.; Zakaria, R.; Ying, J. Y. J. Phys. Chem. B 1998, 102, 10871. (13) Dijken, A. V.; Janssen, A. H.; Smitsmans, M. H. P.; Meijerink, A. Chem. Mater. 1998, 10, 3513. (14) Dijken, A. V.; Vanmaekelbergh, D.; Meijerink, A. Chem. Phys. Lett. 1997, 269, 494. (15) Matsuda, A.; Matsuno, Y.; Katayama, S.; Tsuno, T.; Tohge, N.; Minami, T. J. Ceram. Soc. Jpn. 1994, 102, 330. (16) Imai, H.; Morimoto, H.; Awazu, K. Thin Solid Films 1999, 351, 91. (17) Matsuda, A.; Kogure, T.; Matsuno, Y.; Katayama, S.; Tsuno, T.; Tohge, N.; Minami, T. J. Am. Ceram. Soc. 1993, 76, 2899. (18) Guan, Z. S.; Zhang, X. T.; Ma, Y.; Cao, Y. A.; Yao, J. N. J. Mater. Res. 2001, 16, 907.
10.1021/la026600o CCC: $25.00 © 2003 American Chemical Society Published on Web 02/19/2003
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alcohol, and then the solution was dropped slowly into 40 mL of alcohol solution containing 2.5 mL of hydrochloric acid and 1.5 mL of deionized water under vigorous stirring. The solvent of TiO2 colloids was allowed to evaporate from the system at room temperature, and then the dry gel was calcined at various temperatures to obtain the powder samples. Two sets of samples were prepared, one with UV irradiation and the other without, during the sol-gel process. Photoreactor and Light Source. For preparation of photoirradiated samples, a Pyrex reactor transmitting the light with wavelength longer than 290 nm was used. The UV irradiation was provided by a 500 W high-pressure mercury lamp and passed through a 1.0 cm thick circulating-water cuvette to remove the heating IR beams. The distance between the lamp and reactor was 15 cm, and the intensity of the UV radiation reaching the reactor was measured to be about 20 mW/cm2 by a radiometer. Characterization. X-ray powder diffraction patterns were collected with Cu KR radiation (λ ) 1.5406 Å) at a scan rate of 0.02°/0.12 s. The average crystallite size, D, of the prepared powders was calculated from the Scherrer equation. XANES spectra were measured on wiggler beam line 4W1B at Beijing Synchrotron Radiation Facility (BSRF). A double crystal Si(111) was used to monochromatize X-rays from the 2.2 GeV electron storage ring with an average ring current of 80 mA. Ti K-edge absorption spectra were recorded in the transmission mode in the range from 4900 to 5200 eV at an interval of 0.5 eV. Raman spectra were taken on a Renishaw-2000 Raman spectrometer at resolution of 2 cm-1 by using the 514.5 nm line of an Ar ion laser as the excitation. XPS (X-ray photoelectron spectroscopy) measurements were carried out with a ESCA Lab 220i-XL spectrometer by using a nonmonochromated Al KR (1486.6 eV) X-ray source. All the spectra were calibrated to the binding energy of adventitious C 1s peak at 284.6 eV. For TEM (transmission electron microscope) observations, the powdered samples were dispersed in ethanol and then deposited onto carbon-coated copper grids. After solvent evaporation, the samples were observed with a JEM-200CX electron microscope at an accelerating voltage of 200 kV.
Results and Discussion Figure 1 shows the X-ray diffraction (XRD) patterns of the powder samples with and without UV irradiation after being annealed at different temperatures. The XRD pattern (Figure 1a) of the sample with UV irradiation annealed at 100 °C shows peaks attributed to {101}, {004}, {200}, {105}, {211}, and {204} reflections of the anatase (JCPDS Patterns No. 21-1272), indicating the formation of anatase phase at this annealing temperature. With increased annealing temperature, all the peaks become sharper and narrower, indicating promotion of the crystallinity degree of the sample. In case of the nonirradiated sample, no well-defined peaks could be observed in the XRD pattern when the sample was annealed at 100 and 200 °C (Figure 1b), suggesting the sample was still amorphous at this time.20,21 Therefore, it can be concluded that the application of UV irradiation during the sol-gel process can accelerate the phase transition of titania from amorphous to anatase phase. The particle sizes of the samples annealed at different temperatures calculated from the XRD data are given in Table 1. The grain sizes of the UV-irradiated samples are smaller than those without irradiation after they were annealed at the same temperature. The difference in particle sizes of two kinds of samples annealed at 100 and 200 °C are further studied by XANES spectra as shown in Figure 2. The preedge structure at (19) Nishiwaki, K.; Kakuta, N.; Ueno, A.; Nakabayashi, H. J. Catal. 1989, 118, 498. (20) Ding, X. Z.; Liu, L.; Ma, X. M.; Qi, Z. Z.; He, Y. Z. J. Mater. Sci. Lett. 1994, 13, 462. (21) Ding, X. Z.; Qi, Z. Z.; He, Y. Z. J. Mater. Sci. Lett. 1995, 14, 21.
Figure 1. XRD patterns of UV-irradiated (a) and nonirradiated (b) TiO2 powdered samples annealed at various temperatures. Table 1. Grain Size (nm) of TiO2 at Various Temperatures temp (°C)
UV irradn
nonirradn
100 200 300 400
3.6 6.0 7.2 12.7
9.8 20.3
the Ti K-edge for TiO2 features three peaks: A1, A2, and A3 (see insert). A2 and A3 originate from the 1s f 3d transition and are attributed to 1s f 2t2g and 1s f 3eg transitions in an octahedral field, respectively. A1 originates from an exciton band or a transition from 1s f 1t1g resulting from perturbation due to shakeup and shakeoff processes.22,23 The relative intensity of these preedge peaks presents the difference in particle sizes and the distortion of the octahedral TiO6 unit. The UV-irradiated samples show more intensive A2 peaks than the nonirradiated ones (see insert of Figure 2). This indicates that more Ti atoms exposed on the surface of the particles experience an anisotropic environment where a Ti atom bonds to inner and outer O atoms simultaneously and the particle sizes of the UV samples are smaller than those without irradiation.24,25 In the case of the nonirradiated samples (22) Grunes, L. A. Phys. Rev. B 1983, 27, 2111. (23) Gregor, R. B.; Lytle, F. W.; Sandstrom, D. R.; Wong, J.; Schultz, P. J. Non-Cryst. Solids 1983, 55, 27. (24) Lin, X. C.; Tijana, R.; Zhiyu, W.; Marion, C. T. J. Phys. Chem. B 1997, 101, 10688. (25) Rajh, T.; Nedeljkovic, J. M.; Chen, L. X.; Poluektov, O.; Thurnauer, M. C. J. Phys. Chem. B 1999, 103, 3515.
Titania Synthesis by Photoassisted Sol-Gel Method
Figure 2. XANES spectra of UV-irradiated TiO2 nanoparticles annealed at 100 (a) and 200 °C (b) and nonirradiated samples annealed at 100 (c) and 200 °C (d). The insert shows the amplified preedge peaks in the range from 4962 to 4972 eV.
Figure 3. Raman spectra of UV-irradiated (a) and nonirradiated TiO2 powders (b) annealed at different temperatures. The insertion shows the Raman peak shift.
(insert of Figure 2c,d), intensities of the A1 peaks increase as the particle sizes increase. These observations further confirm the XRD results that UV illumination sharpens the particle size of the powdered samples. Figure 3 shows the Raman spectra of the powder samples annealed at different temperatures. For the UVirradiated sample annealed at 100 °C, characteristic bands detected at 638, 513, and 393 cm-1 are in excellent agreement with its XRD pattern, indicating the presence of the anatase phase.26 However, for the nonirradiated samples, distinct bands corresponding to the anatase phase can only be observed when the calcination temperatures are up to 300 and 400 °C (Figure 3b). It is noted that the band attributed to the Eg vibrational mode of the UV-irradiated sample experiences a blue shift and becomes broader with decreased annealing temperature (Figure 4), which can be attributed to the stress and strain effect induced by rapid oxidation and compaction or uncontrolled oxidation leading to oxygen deficiency and nonstoichiometric TiO2.27-30 Since our results are observed with (26) Tompsett, G. A.; Bowmaker, G. A.; Cooney, T. P. J. Raman Spectrosc. 1995, 26, 57. (27) Ahonen, P. P.; Kauppinen, E. I. J. Mater. Res. 1999, 14, 3938. (28) Parker, J. C.; Siegel, R. W. J. Mater. Res. 1990, 5, 1246. (29) Poniatowski, E. H.; Talavera, R.R.; Heredia, M. C.; Corona, O. C.; Murillo, R. A. J. Mater. Res. 1994, 9, 2102.
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Figure 4. Shift of the Raman peak of UV-irradiated samples at 144 cm-1 vs calcination temperature.
uncompacted samples, mechanically induced strain and/ or stress can be excluded. The shift and broadening of the peak is more likely to be due to oxygen deficiencies induced by the application of UV irradiation during the sol-gel process. Unlike the case of small silicon particles,31,32 the shifting and broadening of the Raman peak cannot be attributed to the grain size effect although our TiO2 powdered samples are smaller than 10 nm (see Table 1) when the annealing temperature is lower than 300 °C.28 Intragrain defects induced by oxygen deficiency should be the dominant mechanism in affecting the shapes and positions of the Raman lines of the nanosized TiO2. Existence of the oxygen defects in the UV-irradiated samples is further investigated by X-ray photoelectron spectroscopy (XPS). Figure 5 shows Ti 2p and O 1s core level XPS spectra of two sets of samples calcined at 100 °C. For UV-irradiated sample the two peaks located at 458.7 and 464.5 eV can be assigned to the core levels of Ti4+ 2p3/2 and Ti4+ 2p1/2 (Figure 5a), respectively. After curve fitting, two additional peaks located at 456.0 and 461.8 eV ascribed to Ti3+ 2p3/2 and Ti3+ 2p1/2 could be identified, suggesting the presence of Ti3+ species in the irradiated sample.33,34 The O 1s core level spectrum is also asymmetric, after curve fitting; besides the main peak of O 1s located at 530.1 eV corresponding to lattice oxygen of TiO2, a shoulder peak at higher binding energy of 532.4 eV can be identified (Figure 5b). This peak should be attributed to the surface species such as Ti-OH and TiO-O- resulting from the reaction of chemisorbed water or/and oxygen molecules with Ti3+ on the sample surface.35,36 For the nonirradiated sample there were no additional peaks in Ti 2p spectrum except for the Ti4+ 2p3/2 and Ti4+ 2p1/2 (Figure 5c). The O 1s peak located at 530.2 eV (Figure 5d) can be assigned to the lattice oxygen and the O 1s peak at 531.8 eV assigned to mixed contributions from surface hydroxide and C-O.37 No O 1s peak at a higher binding energy of 532.4 eV corresponding (30) Parker, J. C.; Siegel, R. W. Appl. Phys. Lett. 1990, 57, 943. (31) Campbell, I. H.; Fauchet, P. M. Solid State Commun. 1986, 58, 739. (32) Richter, H.; Wang, Z. P.; Ley, L. Solid State Commun. 1981, 39, 625. (33) Shultz, A. N.; Jang, W.; Hetherington, W. M.; Baer, D. R.; Wang, L. Q.; Engelhard, M. H. Surf. Sci. 1995, 339, 114. (34) Sanjines, R.; Tang, H.; Berger, H.; Gozzo, F.; Margaritondo, G.; Levy, F. J. Appl. Phys. 1994, 75, 2945. (35) Go¨pel, W.; Anderson, J. A.; Frankel, D.; Jaehnig, M.; Phillips, K. Schafer, J. A.; Rocker, G. Surf. Sci. 1984, 139, 333. (36) Kurtz, R. L.; Stockbauer, R.; Madey, T. E.; Roman, E.; Desegovia, J. L. Surf. Sci. 1989, 218, 178.
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Figure 5. Ti 2p and O 1s core level spectra of UV-irradiated (a, b) and nonirradiated (c, d) samples annealed at 100 °C. Dashed lines show the curve-fitting results.
to the chemisorbed water or/and oxygen could be identified on the surface of the nonirradiated sample due to the absence of Ti3+. Therefore, it is concluded that UV irradiation induces a defect structure of oxygen vacancy and the predominant defect-induced oxidation state of titania is Ti3+ species. The defect structure is further investigated by TEM observation. Figure 6 shows the images and electron diffraction patterns of the sample annealed at 100 °C. A bright field image of the sample (Figure 6a) shows that the particles are somewhat agglomerated with the average grain size of about 10 nm, a little larger than that calculated from the XRD pattern (Table 1). The insert is selected area electron diffraction patterns of the sample indicating a tetragonally indexed structure along the [001] direction, and those strong reflections are in good agreement with the tetragonal anatase cell, implying the existence of single crystals of anatase TiO2 in the sample. Some banded texture and zigzag or stacking faults can be observed in the high-resolution transmission electron microscope (HRTEM) image of these TiO2 nanoparticles (Figure 6b). The sharp and split diffraction spots observed and extra diffraction spots and streaks appearing in the microdiffraction patterns (see insert in Figure 6b) reveal that the planar defects are arranged in a disordered manner in the nanoparticles.38,39 The crystallinity degree (37) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, D. Handbook of X-ray Photoelectron Spectroscopy; Physics Electronics Int.: Eden Prairie, MN, 1995.
of the nonirradiated sample annealed at 100 °C is further evaluated by electron diffraction patterns (Figure 6c). It is clearly shown that these particles are agglomerated with size 10-20 nm and the central halo and faint diffuse rings of the electron diffraction patterns (insert in Figure 6c) indicate that it is essentially amorphous in nature. The XRD and Raman results show that the phase transformation of amorphous to anatase is accelerated by introduction of UV irradiation in the preparation process of TiO2 colloids. The anatase phase can be induced at calcination temperature as low as 100 °C, and the distribution of particle size is also narrowed. It is expected that UV illumination can generate Ti3+ defect sites on TiO2 surface through a surface reduction process of Ti4+ to Ti3+ similar to that induced by Ar ion sputtering, electron beam exposure, and high-energy UV light.40,41 It is assumed that the surface Ti3+ defects are created by the removal of surface oxygen atoms, or most likely bridging oxygen atoms on those crystal faces that have bridging species,33 thus generating oxygen vacancies on the surface. The possible reaction to induce the formation of oxygen (38) Anderson, J. S.; Tilley, R. J. D. J. Solid State Chem. 1970, 2, 472. (39) Li, Z. Q.; Ramasamy, S.; Hahn, H.; Siegel, R. W. Mater. Lett. 1988, 6, 195. (40) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Adv. Mater. 1998, 10, 135. (41) Sakai, N.; Wang, R.; Fujishima, A.; Watanabe, T.; Kazuhito, H. Langmuir 1998, 14, 5918.
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Figure 7. UV-visible absorption spectra of UV-irradiated (a) and nonirradiated (b) TiO2 colloids.
ature. On the other hand, since the transformation involves an overall contraction or shrinking of the oxygen framework, and a cooperative movement of ions, the removal of oxygen ions might be expected to promote the transition. The oxygen vacancies form point defects in the TiO2 lattice. Under the calcination treatment, these micropoints will likely be linked to form bigger crackss planar defects.38,39 For nanosized particles, these planar defects can only be arranged randomly as observed by HRTEM (Figure 6b) and they are difficult to be detected by using X-ray diffraction. Conclusions
Figure 6. Bright field images and selected area electron diffraction (SAED) patterns of UV-irradiated (a, b) and nonirradiated (c) samples annealed at 100 °C.
vacancies is assumed to be the following:42 UV
2Ti4+ + O2-(colloid) 98 (AV) + 2Ti3+ + (1/2)O2 where AV is anion vacancies. The occurrence of this reaction is confirmed by detecting the formation of Ti3+ in the colloid. Figure 7 shows the UV-visible absorption spectral response of two different titania colloids before (b) and after UV irradiation (a). It is obvious from Figure 7a that an intense blue color develops that can be attributed to the formation of Ti3+ in the titania colloids.43 Once the oxygen vacancies are formed in the TiO2 colloid network, these vacancies can act as nucleation sites to promote the transition from amorphous to anatase phase at low calcination temper(42) Shannon, R. D.; Pask, J. A. J. Am. Ceram. Soc. 1965, 48, 391.
Nanoparticulate anatase TiO2 is obtained at calcination temperatures as low as 100 °C using a photoassisted solgel method. Compared with the conventional sol-gel method, UV irradiation on colloid lowers the phase transition temperature from amorphous to anatase phase. The oxygen vacancies induced by UV illumination on colloid accelerate the phase transformation of amorphous to anatase phase and generate small grain size of anatase. This work suggests that a photoassisted sol-gel method may be an alternative approach to prepare nanosized semiconductor oxides with high crystallinity and small grain size at relatively low calcination temperature. Moreover, it is found that our as-prepared anatase samples exhibit a new absorption band at wavelengths longer than 400 nm, which suggests the possible photocatalytic activity of these samples under visible light illumination.44 This could be very interesting and useful for preparation of photocatalysts that employ sunlight to decompose pollutants, and relevant work is being carried out now. Acknowledgment. This work was financially supported by National Science Foundation of China, Chinese Academy of Sciences, and National Research Fund for Fundamental Key Projects No. 973 (G19990330). LA026600O (43) Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. J. Phys. Chem. 1988, 92, 5196. (44) Isao, N.; Nobuaki, N.; Shuzo, K.; Tatsuhiko, I.; Shinichi, S.; Koji, T. J. Mol. Catal. A: Chem. 2000, 161, 205.
