650
Chem. Mater. 2005, 17, 650-655
XANES and XPS Study of Silica-Modified Titanias Prepared by the Glycothermal Method Shinji Iwamoto, Seiu Iwamoto, and Masashi Inoue* Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto UniVersity, Katusra, Kyoto 615-8510, Japan
Hisao Yoshida Department of Applied Chemistry, Graduate School of Engineering, Nagoya UniVersity, Chikusa-ku, Nagoya 464-8603, Japan
Tsunehiro Tanaka Department of Molecular Engineering, Graduate School of Engineering, Kyoto UniVersity, Katsura, Kyoto 615-8510, Japan
Koji Kagawa The Kansai Electric Power Company, Inc., 3-11-20 Nyakuoji, Amagasaki, 661-0974, Japan ReceiVed March 8, 2004. ReVised Manuscript ReceiVed NoVember 9, 2004
Silica-modified titania with the anatase structure having high thermal stabilities was prepared by the glycothermal method, and XANES and XPS techniques were applied for investigating the local structure of Si atoms in these samples. For comparison, two silica-modified titania samples were prepared by impregnation of TEOS or silica sol on titania. The surface concentration of Si atoms in the glycothermal product, as determined by XPS, was smaller than those of the reference samples prepared by the impregnation method, suggesting that in the glycothermal product silicon atoms are highly dispersed in the anatase matrix. The Si K-edge XANES spectra of the glycothermal products exhibited a main peak at 1847.1 eV due to the tetrahedrally coordinated Si atoms. However, a shoulder peak at 1849 eV was detected, which was attributed to the octahedrally coordinated Si atoms. The proportion of the latter component in the glycothermal product was larger than those in the reference samples and became large as the Si content decreased. These results clearly indicate the formation of the silica-titania solid solution. The thermal stabilization effects of Si atoms are discussed in detail.
Introduction Titanium oxide has been used as pigment, filler, membrane, catalyst support and photocatalyst and is an important material in various industrial fields. On calcination, the metastable phases, that is, anatase and brookite, transform to the thermodynamically stable rutile phase above 500 °C, which is accompanied by drastic decreases in surface area and porosity.1,2 For catalyst support, titania having a large surface area is required, and therefore the retardation of the transformation and the improvement of the thermal stability of the titania are required. Many studied have been devoted to improve the thermal stability of titania-based materials. One of the possible methods is the modification of titania with second components. Among various additives, alumina,3,4 silica,5-7 and lanthana8,9 were reported to have * To whom correspondence should be addressed. E-mail: scl.kyoto-u.ac.jp.
inoue@
(1) Montoya, A.; Viveros, T.; Domı´nguez, J. M.; Canales, L. A.; Schifter, I. Catal. Lett. 1992, 15, 207. (2) Kumar, K.-N. P.; Keizer, K.; Burggraaf, A. J. J. Mater. Sci. Lett. 1994, 13, 59. (3) Ding, X.; Liu, L.; Ma, X.; Qi, Z.; He, Y. J. Mater. Sci. Lett. 1994, 13, 462. (4) Kumar, K.-N. P. Appl. Catal., A 1994, 119, 163.
significant stabilizing effects. However, the effects of these additives are quite different according to the doping procedures and the amounts of the additives, and in many systems the real stabilization mechanisms are not clear. Another method to improve the stability is direct synthesis of a wellcrystallized titania. One of the authors has examined thermal reaction of metal alkoxides in glycol (glycothermal reaction) or other organic media and demonstrated that the number of characteristic crystalline products can be obtained directly.10-14 By applying these reaction methods, nano(5) Reddy, B. M.; Mehdi, S.; Reddy, E. P. Catal. Lett. 1993, 20, 317. (6) Yoshinaka, M.; Hirota, K.; Yamaguchi, O. J. Am. Ceram. Soc. 1997, 80, 2749. (7) Suyama, Y.; Kato, A. J. Ceram. Soc. Jpn. (Yogyo-Kyokai-Shi) 1978, 86, 31. (8) Gopalan, R.; Lin, Y. S. Ind. Eng. Chem. Res. 1995, 34, 1189. (9) LeDuc, C. A.; Campbell, J. M.; Rossin, J. A. Ind. Eng. Chem. Res. 1996, 35, 2473. (10) Inoue, M.; Tanino, H.; Kondo, Y.; Inui, T. J. Am. Ceram. Soc. 1989, 72, 3352. (11) Inoue, M.; Kominami, H.; Inui, T. J. Am. Ceram. Soc. 1990, 73, 1100. (12) Inoue, M.; Otsu, H.; Kominami, H.; Inui, T. J. Mater. Sci. Lett. 1992, 11, 269. (13) Inoue, M.; Nishikawa, T.; Nakamura, T.; Inui, T. J. Am. Ceram. Soc. 1997, 80, 2157. (14) Inoue, M.; Nishikawa, T.; Inui, T. J. Mater. Res. 1998, 13, 856.
10.1021/cm040045p CCC: $30.25 © 2005 American Chemical Society Published on Web 01/13/2005
XANES and XPS of Silica-Modified Titanias
crystalline anatases having large surface area were obtained.15,16 Recently, we have reported that the reaction of titanium tetraisopropoxide (TIP) and tetraethyl orthosilicate (TEOS) in 1,4-butanediol afforded nanocrystalline silicamodified titania with the anatase structure and that the products showed quite high thermal stabilities.17 Several methods were applied to investigate the surface property of the thus-obtained silica-modified titania, and it was shown that a significant amount (up to 10%) of silicon atoms are incorporated in titania matrix and that the incorporation of silicon atoms caused the decrease in the OH group concentration of the titania surface without modifying the essential nature of the titania surface.18 However, there still remains a question on the local structure of the silicon in the silicamodified titania prepared by the glycothermal method. X-ray absorption near edge structure (XANES) spectroscopy has been proven to be a very powerful tool for investigating the local structure of an element and was recently used in various scientific areas. However, there have been few studies on the XANES spectra of siliceous materials. This is partially because Si in the oxide system tends to occupy tetrahedrally coordinated sites ([4]Si) which present quite similar absorption spectra. Stishovite is one of the silica polymorphs and is known to have a unique octahedral Si ([6]Si) coordination. Si K- and L2,3-edge XANES of two crystalline silica materials, R-quartz and stishovite, were recently examined by Li et al.19 They reported that the absorption spectra of stishovite are shifted significantly toward higher energy as compared to those of R-quartz and suggested XANES spectra can be used for distinguishing between [4]Si and [6]Si. On this basis, the proportions of [6]Si and [4]Si in some silicophosphate-based glasses having different compositions were estimated and the local structures in these glasses were discussed.20-22 In the present work, the local structure of the silicon in the silica-modified titanias prepared by the glycothermal method was investigated by using XANES and XPS. Experimental Section Titanium tetraisopropoxide (TIP; Nacalai, 25 g) and an appropriate amount (Si/(Si+Ti) charged atomic ratio of 0-0.091) of tetraethyl orthosilicate (TEOS; Wako Pure Chemicals) were added to 100 mL of 1,4-butanediol (Wako Pure Chemicals), and this mixture was placed in a 300 mL autoclave. After the atmosphere inside the autoclave was replaced with nitrogen, the mixture was heated at a rate of 2.3 °C/min to 300 °C and kept at 300 °C for 2 h. After the assembly was cooled, the resulting powders were (15) Inoue, M.; Otsu, H.; Kominami, H.; Inui, T. Nippon Kagaku Kaishi 1991, 10, 1364. (16) Kominami, H.; Kato, J.-I.; Takada, Y.; Doushi, Y.; Ohtani, B.; Nishimoto, S.-I.; Inoue, M.; Inui, T.; Kera, Y. Catal. Lett. 1997, 46, 235. (17) Iwamoto, S.; Tanakulrungsank, W.; Inoue, M.; Koji, K.; Praserthdam, P. J. Mater. Sci. Lett. 2000, 19, 1439. (18) Iwamoto, Sh.; Iwamoto, Se.; Inoue, M.; Uemura, S.; Koji, K.; Tanakulrungsank, W.; Praserthdam, P. Ceram. Trans. 2000, 115, 643. (19) Li, D.; Bancroft, G. M.; Kasrai, M.; Fleet, M. E.; Feng, X. H.; Tan, K. H.; Yang, B. X. Solid State Commun. 1993, 87, 613. (20) Li, D.; Fleet, M. E.; Bancroft, G. M.; Kasrai, M.; Pan, Y. J. NonCryst. Solids 1995, 188, 181. (21) Fleet, M. E.; Muthupari, S.; Kasrai, M.; Prabakar, S. J. Non-Cryst. Solids 1997, 220, 85. (22) Muthupari, S.; Fleet, M. E. J. Non-Cryst. Solids 1998, 238, 259.
Chem. Mater., Vol. 17, No. 3, 2005 651 collected by centrifugation, washed with methanol or ammoniacal methanol, and air-dried. The calcination of the thus-obtained product was carried out in a box furnace in air: the product was heated at a rate of 10 °C/min to a desired temperature and kept at that temperature for 30 min. The thus-obtained samples are designated as GT(x)-y, where x is the Si/(Si+Ti) charged ratio and y is the calcination temperature. We confirmed that both of the Ti and Si alkoxides turned into the solid oxide and, therefore, the x value is the same as the bulk Si/(Si+Ti) ratio. Two reference samples having the Si/(Si+Ti) ratio of 0.091 were prepared by impregnation as follows. The titania prepared by the glycothermal method and calcined at 400 °C was used as the support. Appropriate amounts of the titania were mixed with: (1) a solution of TEOS:C2H5OH: H2O:HNO3 ) 1:10:10:0.2; (2) a solution of 30% silica sol (LudoxAS-30, DuPont). They were dried at 60 °C and calcined at the appropriate temperatures. Those samples are designated as ES(0.091)-y and SS(0.091)-y, respectively, where y is the calcination temperature. Powder X-ray diffraction (XRD) patterns were recorded on a Shimadzu XRD diffractometer using Cu KR radiation and a carbon monochromator. The crystallite size was calculated by the Scherrer equation from the half-height width of the (101) diffraction peak of anatase after correction for the instrumental broadening. The specific surface area was calculated using the BET single-point method on the basis of the nitrogen uptake measured at 77 K. X-ray photoelectron spectroscopy (XPS) spectra were obtained on an ULVAC-PHI model 5500 photoelectron spectrometer with 15 kV and 400 W Mg KR emission as the X-ray source. The charging effect was corrected by adjusting the binding energy (BE) of the main C 1s peak to 284.6 eV. Si K-edge X-ray absorption spectra were measured on BL-7A at UVSOR, Institute for Molecular Science, Okazaki, Japan, with a ring energy of 750 MeV and a stored current of 80-220 mA. Spectra were recorded at room temperature in a total electron yield mode using an InSb doublecrystal monochromator. The step angle of the monochromator scanning was 0.01°, which corresponds to about 0.2 eV at 1850 eV. The absolute energy scale was calibrated to the Si K-edge whiteline peak of a commercially available amorphous silica (Aerosil, designated as A-SiO2) at 1846.8 eV.
Results and Discussion As reported previously,17 the reaction of TIP alone in 1,4BG at 300 °C yielded ultrafine particles of anatase with the crystallite size of 17 nm and BET surface area of 91.5 m2/g. The products obtained by the addition of TEOS to the reaction system mentioned above also had the anatase structure, and no other phases were detected by XRD. With the increase in the amount of TEOS added, the crystalline size of the products decreased and BET surface area increased significantly. Table 1 shows the BET surface area and the crystal phase of the calcined samples. For GT(0), the BET surface area was relatively large after calcinations at 600 °C; however, it decreased significantly to 12.3 and 2.3 m2/g after calcinations at 800 and 1000 °C, respectively, which was accompanied by the transformation to the rutile phase. On the other hand, the silica-modified titanias preserved quite large surface areas. For example, GT(0.091) had a surface area of 157, 125, and 49.5 m2/g even after calcinations at 600, 800, and 1000 °C, respectively. The phase transformation into rutile was also significantly retarded by the addition of TEOS. For example, the samples with the Si/(Si+Ti) charged ratios of >0.038 preserved the anatase structure even after calcinations at 1000 °C.
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Iwamoto et al.
Table 1. BET Surface Area and Crystal Phase of the Silica-Modified Titanian Calcined at High Temperatures BET surface area (m2/g)
a
phasea
sample
300 °C
600 °C
800 °C
1000 °C
600 °C
800 °C
1000 °C
GT(0) GT(0.020) GT(0.038) GT(0.057) GT(0.091) ES(0.091) SS(0.091)
91.5 154 150 174 161 81.5 92.7
62.1 129 145 169 157 67.7
12.3
2.3 16.8 29.5 36.4 49.5 41.6 20.7
A A A A A A
A+R
R+A A+R A A A A A+R
96.0 112 125 60.9 43.9
A A A A A
A, anatase; R, rutile.
Table 2. Bonding Energy of Core Electrons for Silica-Modified TiO2 Samples Calcined at High Temperatures sample
Si/(Si+Ti)a
GT(0)-600 GT(0.020)-600
0.054
GT(0.038)-600
0.085
GT(0.091)-600
0.173
GT(0.091)-800
0.217
GT(0.091)-1000
0.314
ES(0.091)-600
0.247
ES(0.091)-800
0.250
ES(0.091)-1000
0.312
SS(0.091)-400
0.331
SS(0.091)-800
0.444
SS(0.091)-1000
0.444
O 1s (eV) 529.7 529.9 (84)b 531.7 (16) 530.0 (85) 531.9 (15) 530.1 (78) 532.0 (22) 530.0 (74) 532.0 (26) 529.9 (63) 531.9 (37) 529.8 (68) 532.1 (32) 529.7 (66) 531.8 (34) 529.8 (61) 531.9 (39) 529.6 (53) 531.1 (7) 533.7 (40) 529.4 (38) 530.6 (10) 533.1 (52) 529.0 (24) 530.2 (6) 531.1 (14) 532.7 (56)
Si 2p1/2 (eV)
Ti 2p3/2 (eV)
101.9
458.6 458.7
102.1
458.8
102.4
458.3
102.4
458.3
102.6
458.2
102.6
458.6
102.6
458.5
102.7
458.6
104.3
458.4
103.9
458.2
103.4
458.5
a Determined by XPS. b The number in parentheses indicates the percentage of the band.
The surface areas of ES(0.091)-300 and SS(0.091)-300 were 81.5 and 92.7 m2/g, respectively, which are almost at the same level as that of GT(0.091)-300. Although the surface areas of the ES(0.091) and SS(0.091) samples decreased with the increase in the calcination temperatures, they were still higher than that of GT(0). This means that the modification of the titania with silica by the impregnation method is also effective to improve the thermal stability. The surface areas of the ES(0.091)-1000 and SS(0.091)-1000 samples were 41.6 and 20.7 m2/g, respectively. This result suggests the order of the effectiveness of the silica modification for thermal stability is GT > ES > SS. In Table 2, the results of the XPS analysis are summarized. The surface Si/(Si+Ti) ratios measured by XPS are higher than the corresponding ratios charged to the glycothermal reaction, indicating that silicon atoms are concentrated in the surface region of the titania particles. When the samples prepared by the different methods were compared, the Si/ (Si+Ti) surface ratio of the GT(0.091) was the lowest and ES(0.091) had a slightly higher value, while that of SS(0.091) was much higher. This indicates the silicon is highly distributed in GT samples as compared to ES and SS samples
because these three samples had the same Si content. With an increase in the calcination temperature, the surface silicon concentration became larger, suggesting that Si atoms migrate toward the surface of the particles during the calcination at high temperatures. Figure 1 shows O 1s XPS spectra of GT(0.091), ES(0.091), and SS(0.091) calcined at higher temperatures. For GT(0.091) and ES(0.091), a main peak was observed at 530 eV BE together with a shoulder peak at 532 eV BE. The former peak was due to the oxygen in titania with the anatase structure, while the latter was attributed to the oxygen atoms associated with the Si-O-Ti bond.23 The proportion of the shoulder peak in GT(0.091)-600 is smaller than that of ES(0.091)-600. Because both of the samples had the same bulk Si/Ti ratio, the smaller proportion of the Si-connected oxygen atoms in the surface region also means that the silicon is distributed to deeper region in the particles of the GT(0.091). With an increase in the calcination temperature, the shoulder component became larger. This result also indicates that the migration of Si atoms toward the surface of the particles by calcinations, which agrees with the results for the surface composition. In addition to the two components mentioned above, the spectra of SS(0.091) clearly showed a large peak at 533 eV, which is attributed to the oxygen atoms in SiO2. The presence of the silica in this sample is reasonable because the SS(0.091) sample was prepared by using a silica sol as the Si source. On the contrary, the absence of the 533 eV peak in GT(0.091) and ES(0.091) indicates that the silicon atoms are preferentially attached to the Ti atoms through oxygen without forming silica particles. It should be noted that even after calcination at 1000 °C, these two samples did not exhibit the peak at 533 eV. This is clear evidence for the absence of the SiO2 cluster or particles in the surface of the GT(0.091) and ES(0.091) even after calcination at 1000 °C. The Si 2p binding energy of the samples ranged from 101.9 to 104.3 eV as shown in Table 2. The Si 2p BE for GT(0.020)-600 was the lowest, and the value increased with an increase in the amount of TEOS addition. The ES(0.091) samples had slightly higher BE, while SS(0.091) exhibited significantly higher BE as compared to those of GT(0.091) samples. The Si 2p BE for SS(0.091)-600 was 104.3 eV, which is close to that of pure silica. It was reported that in the case of SiO2 polymorphs, the Si 2p BEs of [6]Si and [4]Si were observed at different positions.27,28 However, the (23) Odenbrand, C. U. I.; Andersson, S. L. T.; Andersson, L. A. H.; Brandin, J. G. M.; Busca, G. J. Catal. 1990, 125, 541. (24) Burdett, J. K.; Hughbanks, T.; Miller, G. J.; Richardson, J. W., Jr.; Smith, J. V. J. Am. Chem. Soc. 1987, 109, 3639.
XANES and XPS of Silica-Modified Titanias
Chem. Mater., Vol. 17, No. 3, 2005 653
Figure 1. O 1s XPS spectra of the silica-modified titanias.
difference was much smaller as compared to BE shifts due to the charge transfer from other cations in mixed oxide systems. A low Si 2p BE corresponds to the decrease of the effective positive charge on the Si atoms. Because the electronegativity of Si is higher than that of Ti, the formation of the Si-O-Ti bonds causes a less effective positive charge on the Si atoms as compared to that of pure SiO2. Therefore, the observed low Si 2p BE for GT samples, especially for GT(x) with lower silica contents, can be an indication of the strong interaction of silicon atoms with the titania lattice. By calcination, the Si 2p BE for GT and ES samples slightly increased if not unchanged. On the contrary, the Si 2p BE of SS(0.091) decreased significantly, which suggests calcination formed new Si-O-Ti bonds in this sample. This result is consistent with an appearance of new components around 531 eV BE in the O 1s spectrum of SS(0.091)-1000. The Ti 2p3/2 BE of the silica-modified samples, as shown in Table 2, did not change significantly. This is probably because of the high composition of Ti in the samples of the present study. Figure 2 shows the normalized Si K-edge XANES spectra of A-SiO2 and the silica-modified titanias prepared by the glycothermal method. In the spectrum of A-SiO2, there were a strong absorption peak at 1846.8 eV and some broad absorptions in the region of 1850-1870 eV, which presents a typical feature for the tetrahedrally coordinated Si, [4]Si. The XANES spectra of GT samples, as shown in Figure 2, are quite similar to that of A-SiO2, suggesting that the silicon atoms in the GT samples have a character similar to that of [4]Si. However, some small but significant differences were observed. The peak top positions of the GT samples shifted toward the higher energy side and a shoulder peak at the higher energy side of the main peak was recognized. According to the literature,19-22,28 the [4]Si presents a strong absorption peak at 1846.8 eV, which is attributed to the (25) Stakheev, Y. A.; Shpiro, E. S.; Apijok, J. J. Phys. Chem. 1993, 97, 5668. (26) Gouma, P. I.; Mills, M. J. J. Am. Ceram. Soc. 2001, 84, 619. (27) Finster, J. Surf. Interface Anal. 1988, 12, 309. (28) Finster, J.; Klinkenberg, E. D.; Heeg, J.; Braun, W. Vacuum 1990, 41, 1586.
Figure 2. Si K-edge XANES spectra of the silica-modified titanias prepared by the glycothermal method.
transition from Si 1s to 3p-like states, and several peaks in the range between 1850 and 1865 eV, attributed to the transition from Si 1s to 3d-like states and the multiple scattering, while the [6]Si shows a main absorption peak at 1849 eV and some peaks at between 1851 and 1867 eV. Based on these features, the observed spectra of the silicamodified titanias can be explained as the combination of [6]Si and [4]Si. The shoulder peak in the spectrum of GT(0.020) is a clear indication of the presence of [6]Si. The presence of
654 Chem. Mater., Vol. 17, No. 3, 2005
Iwamoto et al. Table 3. XANES Results for Silica-Modified TiO2 Samples Calcined at High Temperatures
Figure 3. Deconvolution of the Si K-edge XANES spectra of the silicamodified titanias and simulated spectra.
octahedrally coordinated silicon atom [6]Si indicated by the XANES analysis is explained by the substitution of Ti in the anatase lattice with Si. To estimate the ratio of the [6]Si and [4]Si in the sample, curve-fitting analysis was carried out. As a control experiment, the sample holder without the samples was measured, and we observed a small absorption peak at 1844 eV. We confirmed that the origin of this peak was an impurity in the adhesive carbon tape that was used to mount the samples on the holder. This peak was small but sometimes could not be neglected, especially in the cases of the samples with low silica contents. Therefore, in the deconvolution procedure, the spectra were fitted to one common background and three Gaussian components, that is, two for the edge peaks of [4]Si and [6]Si, and one for the impurity. The peaks in the range between 1855 and 1870 eV were not employed for the quantitative analysis because the peaks for [6]Si and [4]Si in this range overlap each other and include multiple scattering contributions. The thus-deconvoluted spectra for the silicamodified titanias are shown in Figure 3. The solid line is the measured spectrum, and the dotted lines are simulated components. These spectra clearly reveal that the silicon atoms of the silica-modified titanias occupy both the tetrahedral and the octahedral sites. Peak positions and the proportion of [6]Si calculated from the peak area ratio ([6]Si/ ([4]Si+[6]Si)) in the XANES spectra are listed in Table 3. Although the proportions of the [6]Si thus obtained were not
sample
[4]Si peak position (eV)
[6]Si peak position (eV)
[6]Si peak proportion
GT(0.020)-assyn GT(0.038)-600 GT(0.057)-assyn GT(0.091)-assyn GT(0.091)-600 GT(0.091)-800 GT(0.091)-1000
1847.1 1847.1 1847.0 1847.1 1847.1 1847.1 1847.1
1849.1 1849.0 1849.1 1849.2 1849.3 1849.2 1849.5
0.29 0.20 0.22 0.11 0.14 0.12 0.05
ES(0.091)-assyn ES(0.091)-400 ES(0.091)-600 ES(0.091)-800 ES(0.091)-1000
1847.1 1847.1 1847.1 1847.1 1847.1
1849.2 1848.8 1848.9 1848.9 1849.0
0.02 0.08 0.09 0.09 0.07
SS(0.091)-assyn SS(0.091)-400 SS(0.091)-600 SS(0.091)-800 SS(0.091)-1000
1847.1 1847.1 1847.1 1847.1 1847.1
1849.0 1848.4 1848.9 1849.0 1848.8
0.05 0.06 0.04 0.06 0.06
very precise, these values for the GT samples were significantly higher than those of ES and SS samples. These results mean that the glycothermal method is suitable for incorporating the silicon atoms in the unique octahedral sites in the titania matrix. Once the anatase structure forms, the substitution of Ti with Si would be difficult because it implies the diffusion of Si from the surface and the back diffusion of Ti toward the surface with breaking of the Ti-O bond in the anatase structure. This seems to be the reason for the low proportion of [6]Si in the ES and SS samples, both of which were prepared by “postsynthesis modification”. To obtain octahedrally coordinated Si atoms, as was demonstrated in the glycothermal products, it seems to be critical that the crystallization proceeds in the presence of both Ti and Si sources. Among the GT samples, the proportion of the [6]Si was highest for the GT(0.020), and it decreased as the Si/ (Si+Ti) charged ratio increased. This tendency is because of the low stability of the [6]Si as compared to [4]Si. The amount of the Si in 6-coordinated sites is limited, and, therefore, the proportion of the [6]Si decreased with an increase in the Si contents. Figure 4 shows the structure of anatase.24 Two unit cells are depicted with a common edge along with c axis. The anatase structure is based on a closed packing of oxide ions.
Figure 4. Structure of anatase.
XANES and XPS of Silica-Modified Titanias
The TiO6 octahedron in the anatase structure is distorted in which two Ti-O bonds are longer than the other four Ti-O bonds. The titanium ions occupy one-half of the octahedral holes between the layers of oxide ions in a zigzag row. The other half of the octahedral holes is vacant, which is designated as (h) and shown by closed circles in Figure 4. The zigzag arrangement of the TiO6 octahedron in the anatase is different from the linear row arrangements in the rutile structure. According to this difference in the arrangement, the density of anatase is smaller (3.894 g/cm3) than that of rutile (4.250 g/cm3).24 This is equivalent to that the anatase structure has larger vacant sites than rutile. Because the anatase structure has a distorted oxygen packing, the (h)O6 is also a distorted octahedron, in which two of the (h)-O distances (2.8 Å, between h and oxide ion II in Figure 4) are much longer than the other four (h)-O (1.9 Å, between h and oxide ion I in Figure 4). This means that the (h)O6 octahedral hole is essentially a tetrahedral hole. The XPS data mentioned above indicate that the silicon atoms are included in the titania matrix, while the XANES results reveal that most of the silicon atoms are present in the tetrahedral sites. One possible explanation for these results is that the silicon atoms are located in some of the tetrahedral holes shown as the closed circles in Figure 4. Strain caused by incorporation of the Si atom into the tetrahedral vacant site should be considered. First, the Si-O distance, ca. 1.5 Å in the SiO4 tetrahedron, is shorter than the (h)-O distance, which indicates the Si is small enough to be incorporated. Another strain may arise because this tetrahedral vacant hole is not a regular tetrahedron. However, this can be released by changing the configuration around the Si in the titania matrix. We observed that the unit cell parameters of the products decreased by the addition of TEOS, suggesting the change of the configuration of the anatase structure. It should be noted that such relaxation of the structure would be more likely to occur at the sites near the surface of nanocrystalline titanias prepared by the glycothermal method. The insertion of Si atoms to the vacant holes causes unbalanced positive charges. This charge unbalance can be compensated for by several ways, such as the incorporation of interstitial oxide ions, the change of the valence of titanium, or the change of surface electric charge to more negative. We have reported previously that the OH group concentration on the surface of the silica-modified samples decreased by the TEOS addition.18 This result can be explained by substitution of the surface OH groups with surface oxide ions to form negative surface charge for cancellation of the positive charge caused by the insertion of the Si in the vacant holes of the anatase structure. Because the OH group concentration on the surface is directly correlated to the free energy of the surface, the insertion of Si atoms is critical for the stabilization of the silica-modified titanias. As for the stabilization mechanism for silica-modified titanias, Suyama et al. proposed that the silica coating over the titania surface interferes with the contact between the titania particles and the nucleation of rutile.7 Stakheev et al. assumed that the surface of TiO2 particles is coated by a monolayer of a titanium silicate compound.25 A similar
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coating model or a monolayer model by the addition of second components has been also proposed for other systems, for example, Al2O3-TiO23 and La2O3-TiO2.8 However, a recent work on the mechanism of anatase-to-rutile transformation by using transmission electron microscopy revealed that this polymorphic transformation starts by formation of fine lathes (plates) of rutile phase via a shear mode.26 Therefore, the presence of the surface layer on the anatase particle cannot affect the anatase-to-rutile transformation. Furthermore, if the surface is covered with the second component, the surface property would be changed. We previously investigated the surface property of the silicamodified titania by 2-propanol TPD.18 Because 2-propanol adsorbs on TiO2 and decomposes into C3H6 and H2O at ca. 200 °C while only a trace amount of 2-propanol adsorbs on SiO2, this technique is used to measure TiO2 surface area. By increasing the Si addition up to Si/(Si+Ti) ) 0.091, the amount of C3H6 desorption increased. However, the amount of C3H6 desorption from a unit surface area of the products is essentially identical. This result indicates that the silicamodified titanias prepared by the glycothermal method possessed a nature similar to that of pure titania, which means that the coating model is not applied for the present system. As has been shown in this study, the formation of silicatitania solid solution with the anatase structure seems to be critical for the stabilization of the anatase phase. Although we cannot rule out the possibility that the surface layers prevent the coarsening of the anatase particles, the presence of silicon atoms in the anatase matrix retards the change of face-centered cubic arrangement of oxide ion in anatase into hexagonal packing in rutile because this change is associated with the cleavage of a part of Ti-O bonds. The decrease of the surface energy caused by substitution of the surface OH groups with oxide ions also contributes to the thermal stability of the present products. Conclusions The XANES and XPS techniques were applied for investigating the local structure of the silica-modified titania with the anatase structure prepared by the glycothermal method. The XPS spectra revealed that the silicon atoms were highly distributed in the silica-modified titanias prepared by the glycothermal method, while the samples prepared by the impregnation with silica sol showed the presence of SiO2 cluster on the surface. The Si K-edge XANES spectra demonstrated that the silicon atoms are located mainly in the tetrahedral sites in the GT samples; however, the presence of octahedral silicon atoms was clearly shown. These results indicated that the Si atoms are inserted in the tetrahedral vacant holes of the anatase structure. Such insertion of the Si atoms caused the decrease in the surface energy of the particles and retarded the anatase-to-rutile phase transformation, thus contributing to the thermal stability of the silicamodified titanias. Acknowledgment. This work was partly supported by the Joint Studies Program of the Institute for Molecular Science, Okazaki, Japan. CM040045P
