Structural Modifications and Associated Properties of Lanthanum

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Structural Modifications and Associated Properties of Lanthanum Oxide Doped Sol-Gel Nanosized Titanium Oxide C. P. Sibu, S. Rajesh Kumar, P. Mukundan, and K. G. K. Warrier* Ceramics Division, Regional Research Laboratory, CSIR, Thiruvananthapuram 695 019, India Received October 1, 2001. Revised Manuscript Received April 30, 2002

Nanosized titanium oxide with and without addition of lanthanum oxide is prepared by the solution sol-gel method. Resultant gel precursor was characterized by thermal analysis and IR spectroscopy. The precursor gels were further calcined at various temperatures and then were subjected to measurement of the BET specific surface area, X-ray diffraction, and temperature-programmed desorption (TPD). The surface area of titanium oxide was increased from ≈1 m2/g to as high as 52 m2/g in the presence of 1% La2O3 at 700 °C. About 37% of the total pore volume was retained by La2O3 doping compared to pure titania at a calcination temperature of 700 °C. The shift of titania peak to a lower frequency region in the FTIR spectra indicates the Ti-O-La bond formation. TEM observation confirmed the nanocrystalline nature of sol-gel titania. The anatase-rutile transformation temperature increased to 850 °C in the presence of La2O3 from that of pure titania at 650 °C. The presence of La2O3 induces better surface acidity to sol-gel titania.

Introduction Titanium oxide derived from the mineral illmenite is well-known as a white pigment and as filler in pigments, rubber, and paper. In addition, titanium oxide has found applications as an electronic material1 and as gas2 and humidity sensors.3 The catalytic and photocatalytic properties are being investigated widely in the form of bulk powders, films, and membranes.4,5 Nanocrystalline titania was synthesized by chemical methods involving the sol-gel approach.6 Titanium oxides prepared via the sol-gel method exhibit comparatively high surface areas and hence have an advantage over conventional materials for potential applications as catalysts, sorbents, or electrodes.7 Titania undergoes a phase transformation from the low-temperature anatase phase to * To whom correspondence should be addressed. Fax: +91 471 490 186. E-mail: [email protected]. (1) Poznyak, S. K.; Pergushov, V. I.; Kulak, A. I.; Schlapfer, C. W. J. Phys. Chem. B 1999, 103, 1308. (2) Sharma, R. K.; Bhatnagar, M. C.; Sharma, G. L. Sens. Actuators, B: Chem. 1998, 46 (3), 194. (3) (a) Carotta, M. C.; Ferroni, M.; Gnani, D.; Guidi, V.; Merli, M.; Martinelli, G.; Casale, M. C.; Notaro, M. Sens. Actuators, B: Chem. 1999, 58 (1-3), 310. (b) Traversa, E.; Gusmano, G.; Gnappi, G.; Montenero, A. Sens. Actuators, B: Chem. 1996, 31 (1-2), 59. (c) Montesperelli, G.; Pumo, A.; Traversa, E.; Gusmano, G.; Bearzotti, A.; Montenero, A.; Gnappi, G. Sens. Actuators, B: Chem. 1995, 25 (1-3), 2, 705. (d) Azad, A. M.; Younkman, L. B.; Akbar, S. A.; Alim, M. A. J. Am. Ceram. Soc. 1994, 72 (2), 481. (4) Wang, J.-j.; Liu, X.-d.; Guan, Q.-b. Br. Ceram. Trans. 1996, 95 (6), 241. (5) (a) Chan, C. K.; Porter, J. F.; Li, Yu-Guang; Guo, W.; Chan, ChiMing. J. Am. Ceram. Soc. 1999, 82 (3), 566. (b) Watanabe, T.; Nakajima, A.; Wang, R.; Minabe, M.; Koizumi, S.; Fujishima, A.; Hashimoto, K. Thin Solid Films 1999, 351, 260. (c) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol. C: Photochem. Rev. 2000, 1, 1. (6) Luyten, J.; Goymans, J.; Smolders, C.; Vercauteren, S.; Vasant, E. F.; Laysen, R. J. Eur. Ceram. Soc. 1997, 17, 273. (7) (a) Ding, X.-z.; Liu, X.-h. Mater. Sci. Eng. A 1997, 224, 210. (b) Yoko, T.; Yuasa, A.; Kamiya, K.; Sakka, S. J. Electrochem. Soc. 1991, 138 (8), 2279.

rutile above 450 °C and is seen to extend to as high as 1000 °C depending on the method of preparation and also it is in the presence of suitable dopant oxides.8 The role of a few dopant oxides such as Fe2O3, V2O5, and CuO9 on the anatase to rutile transformation is wellreported, and certain correlation between the ionic radii of the dopants and anatase phase stability was drawn by Yang and Ferreira.10 Anatase-rutile transformation is usually followed by XRD techniques, but impedance spectral analysis was also used to study the transformation.11 Introduction of second phases such as yttria and lanthana on textural studies of anatase phase was earlier reported by Kumar et al.12 Bjorkert et al. reported the influence of La2O3 as dopant on the phase development of Al2O3:TiO2 ceramic membranes.13 In their work, lanthana was used to stabilize γ-alumina phase in Al2O3:TiO2 composite ceramic compositions. LaAl2O3 phase is reported to form at the alumina surface, which stabilized the γ-alumina phase. There (8) (a) Rajeshkumar, S.; Suresh, C.; Vasudevan, A. K.; Suja, N. R.; Mukundan, P.; Warrier, K. G. K. Mater. Lett. 1999, 38, 161. (b) Rajeshkumar, S.; Suresh, C.; Vasudevan, A. K.; Perumal, P.; Warrier, K. G. K. Trans. Ind. Ceram. Soc. 1999, 58 (5), 118, (c) Vorkapic, D.; Matscukas, T. J. Am. Ceram. Soc. 1998, 81 (11), 2815. (d) Zhang, H.; Banfield, J. F. J. Mater. Res. 2000, 15 (2), 437. (9) (a) Navı´o, J. A.; Testa, J. J.; Djedjeian, P.; Padro´n, J. R.; Rodrı´guez, D.; Litter, M. I. Appl. Catal. A: Gen. 1999, 178 (2), 191. (b) Bahranowski, K.; Janas, J.; Machej, T.; Serwicka, E. M.; Vartikian, L. A. Clay Miner. 1997, 32 (4), 665. (c) Karakitsou, K. E.; Verykios, X. E. J. Phys. Chem. 1993, 97, 1184. (d) Martin, S. T.; Morrison, C. L.; Hoffmann, M. R. J. Phys. Chem. 1994, 98, 13695. (e) Schneider, H.; Baiker, A.; Schar, V.; Waukaun, A. J. Catal. 1994, 146 (2), 545. (10) Yang, J.; Ferreira, J. M. F. Mater. Lett. 1998, 36, 320. (11) Vasudevan, A. K.; Rao, P. P.; Ghosh, S. K.; Anilkumar, G. M.; Damodaran, A. D.; Warrier, K. G. K. J. Mater. Sci. Lett. 1997, 16 (1), 8. (12) Kumar, K. N. P.; Keizer, K.; Burggraaf, A. J. Mater. Chem. 1993, 3, 141. (13) Bjorkert, D. J.; Mayappan, R.; Holland, D.; Lewis, M. H. J. Eur. Ceram. Soc. 1999, 19, 1847.

10.1021/cm010966p CCC: $22.00 © 2002 American Chemical Society Published on Web 06/25/2002

La2O3-Doped Sol-Gel Nanosized Titanium Oxide

are reports on the effects of the addition of metal ion dopants on quantum efficiency of heterogeneous photocatalysis of titanium dioxide.14 The enhanced photoactivity of titania doped by rare-earth oxides such as europium, praseodymium, and ytterbium oxides was recently reported by Ranjit et al.15 The high activity of RE oxide/TiO2 photocatalysts is attributed to the enhanced electron density imparted to the titania surface by the dopant oxides. Also, Lin and Yu16 reported the effect of the addition of Y2O3, La2O3, and CeO2 on the photocatalytic activities of titania for the oxidation of acetone. The catalytic property of V2O5/La2O3-TiO2 mixed oxide systems prepared by the coprecipitation route was reported by Benjaram et al.17 The anatase form of titania is believed to possess enhanced catalytic activity, probably because of its open structure compared to that of rutile and its high specific surface area. A few other reports on lanthanum oxide doped titania include the work of Gopalan and Lin18 and LeDuc et al.19 The former reports the evolution of pore structure and anatase phase stability as a result of the addition of La2O3. The anatase phase is stable up to 650 °C, and this is explained by a possible monolayer coverage of lanthana over titania. The report by LeDuc et al. deals also with textural stability La2O3-doped titania prepared by suspending a commercial titania catalyst in a solution of lanthanum nitrate. A doping level of 5% La2O3 was recommended for long-term thermal stability up to 650 °C. The chemical interaction between titania and lanthana has not been investigated earlier. Further, the synthesis has been carried out through a colloidal precursor route either by a solution sol-gel or through suspension of titanium oxide in a solution of lanthanum salt. No detailed study on the surface acidity has also been reported. Therefore, in the present work an attempt is made to study the synthesis of nanocrystalline titania and lanthanum oxide doped titania through a modified polymeric sol-gel route and to investigate in detail its A > R transformation as well as catalytic efficiency with a view to develop transparent titania coatings on surfaces. This study will have many applications related to the development of self-cleaning titania films, high-temperature catalysis, and nanofiltration membranes.20 Experimental Section In the present work titanium isopropoxide (Fluka, Switzerland) was used for the synthesis of titanium oxide. Lanthanum nitrate (Indian Rare Earth Ltd., India) was used for doping. Titanium isopropoxide stabilized with acetic acid usually follows a condensation pattern producing linear chains and enclosing small pores that result in high surface area. This is (14) (a) Choi, W.; Termin, A.; Hoffmann, M. R. J. Phys. Chem. 1994, 98, 13669. (b) Choi, W.; Termin, A.; Hoffmann, M. R. Angew. Chem. 1994, 106, 1148. (c) Choi, W.; Termin, A.; Hoffmann, M. R. Angew. Chem., Int. Ed. Engl. 1994, 33, 1091. (d) Lo´pez, T.; Hernandez-Ventura, J.; Go´mez, R.; Tzompantzi, F.; Sa´nchez, E.; Bokhimi, X.; Garcı´a, A. J. Mol. Catal. A: Chem. 2001, 167, 101. (15) Ranjit, K. T.; Willner; I.; Bossmann, S. H.; Braun, A. M. Environ. Sci. Technol. 2001, 35, 1544. (16) Lin. J.; Yu, J. C. J. Photochem. Photobiol. A: Chem. 1998, 116, 63. (17) Reddy, B. M.; Ganesh, I. J. Mol. Catal. A: Chem. 2001, 169 (1-2), 207. (18) Gopalan, R.; Lin, Y. S. Ind. Eng. Chem. Res. 1995, 34, 1189. (19) LeDuc, C. A.; Campbell, J. M.; Rossin, J. A. Ind. Eng. Chem. Res. 1996, 35, 2473. (20) Hyun, S. H.; Kang, B. S. J. Am. Ceram. Soc. 1996, 79, 279.

Chem. Mater., Vol. 14, No. 7, 2002 2877 because the acid-catalyzed condensation is directed preferentially toward the ends rather than the middles of the chains.21 The lanthanum oxide doped titania was prepared by a modified sol-gel process by a procedure already reported by the authors.22 In a typical experiment, 36.87 mL of titanium isopropoxide was added to 71.3 mL of glacial acetic acid (Merck, India). To this solution water was added dropwise under continuous stirring for 1 h. The titanium isopropoxide, glacial acetic acid, and water were maintained at the molar ratio 1:10:350. Lanthanum nitrate solution, in which the wt % of lanthana was calculated to 1% of the weight of titanium oxide, was then added to the acetic acid-modified, partially hydrolyzed titanium isopropoxide solution with vigorous stirring. Similar experiments were done with lanthana concentrations as 0.1, 1.0, 2.0, and 5.0 wt % of titania. Xerogels were produced by concentrating the sol over a steam bath and subsequently drying the sol at 70 °C. The xerogels were further heat-treated to 300, 400, 500, 600, 700, 800, and 900 °C for a period of 3 h in each case. The FTIR of the samples in the form of KBr mixed disks were taken using a Nicolet 560 spectrometer. The thermal decomposition of the gels were studied by TG (TG-50H Shimadzu, Japan) and DTA (DTA50H, Shimadzu, Japan) at a heating rate of 10 °C/min. The specific surface areas of the samples were measured by nitrogen adsorption measurements (BET) in a Micromeritics (Gemini 2360) surface area analyzer. TEM was performed on a JEOL 3000EX microscope operating at an accelerating voltage of 300 kV. The powder XRD patterns of calcined samples were obtained using a Philips PW1710 X-ray diffractometer in the 2θ range 20-60° using Cu KR radiation. The percentage of rutile (X) in the samples was estimated from the respective integrated peak intensities using the equation

X ) (1 + 0.8IA/IR)-1 where IA and IR are the X-ray intensities of the anatase (101) and rutile (110) peaks, respectively. Crystallite sizes were calculated from the peak widths using the Scherrer equation

Φ ) kλ/(β cos θ) where Φ is the crystallite size, k the shape factor (a value of 0.9 was used in this study), λ the X-ray radiation wavelength (1.540 Å for Cu KR), and β the line width at half-maximum height of the main intensity peak after subtraction of the equipment broadening. The temperature-programmed desorption (TPD) technique was used to measure the extent of acidic sites in a setup locally fabricated. TPD of ammonia enabled the determination of the acid strength distribution. For TPD studies, pelletized catalyst was activated at 300 °C inside the reactor under a nitrogen flow for 0.5 h. After the catalyst cooled to room temperature, ammonia was injected in the absence of the carrier gas flow and the system was allowed to attain equilibrium. The excess ammonia was flushed out by a current of nitrogen. The temperature was then raised in a stepwise manner at a linear heating rate of about 20 °C/min. The ammonia desorbed from 100 to 700 °C at intervals of 100 °C was trapped in dilute sulfuric acid solution and estimated volumetrically by back-titration with NaOH.

Results and Discussion Livage and co-workers23 reported the molecular level modification of alkoxide precursors using HOAc. HOAc is identified as an acid catalyst as well as a chelating or bridging ligand in the course of the sol-gel process. The FTIR pattern presented in Figure 1 indicates peaks (21) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: Boston, 1990. (22) Sibu, C. P.; Rajeshkumar, S.; Warrier, K. G. K. Indian Patent, Sept 2001. (23) Doeuff, S.; Henry, M.; Sanchez C.; Livage, J. J. Non-Cryst. Solids 1987, 89, 206.

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Figure 3. DTA curves of different TiO2 samples: (a) undoped TiO2; (b) 1% La2O3 doped TiO2; (c) 10% La2O3 doped TiO2. Figure 1. FTIR spectra of undoped and La2O3-doped TiO2 samples dried at 70 °C: (a) undoped; (b) 0.1% La2O3 doped TiO2; (c) 1% La2O3 doped TiO2; (d) 2% La2O3 doped TiO2; (e) 5% La2O3 doped TiO2.

Figure 2. TG curves of different TiO2 samples: (a) undoped TiO2; (b) 0.1% La2O3 doped TiO2; (c) 1% La2O3 doped TiO2; (d) 2% La2O3 doped TiO2.

corresponding to the stretching vibrations of OH groups (≈3350-3450 cm-1) and bending vibrations of adsorbed molecular water (≈1620-1635 cm-1). A large frequency separation (ν - 253 cm-1) that exists between the νsym(COO), at 1384 cm-1, and the νasym(COO), at 1637 cm-1, bands of pure titania sample reveals the presence of bridging acetate ligands. However, all the doped titania samples show a low-frequency separation (ν - 110 cm-1) consistent with chelating acetate ligands.23 All the samples show a shoulder at 1029 cm-1. This is attributed to the anatase Ti-O bond vibration,24 which is formed during gelation at room temperature itself. The undoped sample shows a peak at 512 cm-1, which is the characteristic peak of titania, but in the case of lanthana-doped samples this peak is shifted to ≈450480 cm-1, which may be due to the formation of TiO-La bond.25 The thermogravimetric curve presented in Figure 2 shows a three-step weight loss pattern for pure titania (24) Van der Marel, H. W.; Beutelspacher, H. Atlas of Infrared Spectroscopy of Clay Minerals and the Admixtures; Elsevier Scientific Publishing Company: New York, 1974. (25) Parler, C. M.; Ritter, J. A.; Amiridis, M. D. J. Non-Cryst. Solids. 2001, 279, 119.

xerogel, whereas the lanthana-doped titania shows a four-step weight loss, which indicates that the doping may have influenced certain structural changes in the precursor gel. The first step below 110 °C is due to the removal of adsorbed water. The second step of weight loss that ranges between 150 and 298 °C is attributed to the expulsion of organics that are trapped inside the pores and also due to the removal of chemisorbed water. A third step of weight loss ranging from 390 to 410 °C may be due to the removal of structural hydroxyls, which will increase the number of bridging oxygens and thus the monolithic nature of the gel, and also due to the removal of organic residues. An additional weight loss occurs in the range 320-350 °C in the doped titania, which could be due to the decomposition of nitrates that became incorporated in the gel matrix. The DTA of 1% La2O3/TiO2 presented in Figure 3 shows a corresponding exothermic peak at 305-426 °C that supports the nitrate decomposition. DTA of undoped TiO2 shows an exothermic peak with a lower enthalpy of decomposition in the above temperature range, and this could be due to the decomposition of acetate groups associated with the gel. The lower enthalpy of decomposition may be due to the structural evolution that took place with the addition of lanthanum nitrate solution. The surface area of titania is increased by 25% in the presence of La2O3 compared to that of undoped titania. Further, there is a remarkable improvement in the high-temperature stability of pores with lanthana doping. This is in agreement with the observation reported by Kumar et al.12 and Gopalan and Lin.18 However, the textural stability values observed in the present case are higher than those reported previously because of a better interaction between TiO2 and La2O3 surfaces. 1% La2O3 doped titania, after heat treatment at 700 °C, shows a surface area value of 52 m2/g while that of undoped titania under identical conditions is ≈1 m2/g (Table 1). The lanthana-doped titania retained about 37% of its initial pore volume even after calcination to 700 °C, compared to pure titania which retains as low as only 1%. This is attributed to the presence of lanthanum oxide distributed uniformly in the titania gel matrix. In addition to the effective surface coverage of lanthana on the titania particles reported by earlier studies,18 factors related to the presence of Ti-O-La bonds are seen from the shift of titania peak to a lower energy region in the FTIR spectra.25 XRD patterns of the undoped and doped titania after calcination are presented in Figure 4. While pure titania

La2O3-Doped Sol-Gel Nanosized Titanium Oxide

Figure 4. XRD patterns of different TiO2 samples: (a) undoped TiO2 aged at 70 °C; (b) 1% La2O3 doped TiO2 aged at 70 °C; (c) undoped TiO2 calcined at 700 °C; (d) 1% La2O3 doped TiO2 calcined at 700 °C; (e) undoped TiO2 calcined at 800 °C; f) 2% La2O3 doped TiO2 calcined at 800 °C. Table 1. Surface Area and XRD Results of Pure and Lanthana-Doped Titania

sample

surface pore area volume crystallite size (nm) (m2/g) (cm3/g) A R

undoped TiO2 (70 °C) undoped TiO2 (300 °C) 120.54 undoped TiO2 (500 °C) 75.59 undoped TiO2 (700 °C) 0.0879 undoped TiO2 (800 °C) 1% La2O3-TiO2 (70 °C) 1% La2O3-TiO2 (300 °C) 160.52 1% La2O3-TiO2 (500 °C) 111.02 1% La2O3-TiO2 (700 °C) 51.97 1% La2O3-TiO2 (800 °C) 2% La2O3-TiO2 (700 °C)

5.86 0.1688 0.1262 0.0021 32.58 32.58 7.08 0.2573 0.219 0.0961 13.50 27.16 11.02

0% 0% 0% 40.90 (43% R) 43.06 (92% R) 0% 0% 0% 0% 0% 0%

has a stability range of anatase phase up to 650 °C, the 1% La2O3 doped titania obtained in the present study retained nearly 100% of its anatase phase even at 800 °C. In the case of both pure and lanthana-doped titania, a hydrous anatase phase is present at the gel state itself (Figure 4). According to Ding and Liu,26 the decrease in grain size increases the rate of the grain growth process. Undoped titania gel derived at 70 °C has the smallest crystallite size (5.86 nm) compared to that of the lanthana-doped titania, which is 7.08 nm. So to minimize the surface energy, the pure titania crystallites, which are accompanied by higher free energy, should grow at a faster rate than the lanthana-doped one. The XRD results indicate the same trend. The crystallite size calculated from XRD data of samples calcined at 700 °C was 32.58 nm for undoped titania while there was a considerable reduction in size to 13.50 nm after doping with 1% La2O3. Further, with the increase of lanthana content to 2%, the crystallite size decreased to ≈11.02 nm. This reduction in crystallite size is proposed to be due to segregation of the dopant (26) Ding, X.-z.; Liu, X.-h. J. Mater. Res. 1998, 13 (9), 2556.

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cations at the grain boundary, which inhibits grain growth by restricting direct contact of grains.26 The ionic radii value27 of doped La3+ is between that of Ti4+ (0.068 nm) and O2- (0.132 nm). Therefore, the La3+ (0.1016 nm) should either replace the Ti4+ site or go to the interstitial. From the FTIR spectral analysis we found that some of the La3+ ions replace some Ti4+ ionic sites. Because of the mismatch of the ionic sizes of Ti4+ and La3+ (0.068 and 0.1016 nm, respectively), there is also a chance for the La3+ ions to go to the interstitial. The slow rate of grain growth for lanthanadoped titania compared to a pure one may also be attributed to the presence of interstitial lanthana ions. Also, the anatase phase is fairly stable up to 850 °C in the case of lanthana-doped titania. On the other hand, the A-R transformation starts as early as 650 °C in the case of pure titania. The slow rate of grain growth of the former may be due to the presence of interstitial La3+ ions, which probably became segregated in the grain boundaries of anatase titania grains and increased the diffusion barrier at the titania-titania grain contact, which is needed for the grain growth process. Also, those La3+ ions that have replaced the Ti4+ sites will have a stabilizing effect on the Ti-O bond because the more electropositive La3+ will render its electronic concentration to O2- so that it can use this increased concentration of electrons to strengthen the bonding between the less electropositive Ti4+ ions.28 This stabilization of Ti-O bond will in turn retard the A-R transformation temperature because the A-R transformation needs the breakage of Ti-O bonds,29 and in this case it is difficult to break the Ti-O bond because of the increased bond strength rendered by the La3+ ions. In the case of pure titania, the atomic mobility with respect to temperature will be higher compared to the lanthana-doped sample because of the higher probability of Ti-O bond breakage. The diffusion of atoms also will be higher for the same reason, which could increase the grain growth rate.26 It is interesting to note that the addition of even 1% lanthana could bring about this considerable improvement in properties, which is attributed to the specific synthesis route followed in this route. The result also shows the influence of A-R transformation on the grain growth;26 that is, as the A-R transformation proceeds, the grain growth of rutile phase also progresses. Table 1 shows the amount (%) of rutile phase and the corresponding crystallite sizes of undoped and La2O3-doped titania samples at temperatures of 700 and 800 °C. This result clearly indicates the influence of Ti-O bond breakage and subsequent diffusion of atoms on the crystalline growth of rutile phase. According to Mayo and co-workers,30 the presence of interagglomerate pores prevents the anatase grain growth. In our results the same trend is observed and this shows the formation of interagglomerate pores (27) Weast, R. C.; Astle, M. J. Handbook of Chemistry and Physics, 59th ed.; CRC Press: Boca Raton, FL, 1978. (28) (a) Reddy, B. M.; Chowdhury, B.; Smirniotis, P. G. Appl. Catal., A 2001, 219 (1), 53. (b) Owen, S. M.; Brooker, A. T. A Guide to Modern Inorganic Chemistry; Elsevier: New York, 1979. (29) (a) Gamboa, J. A.; Pasquevich, D. M. J. Am. Ceram. Soc. 1992, 75, 2934. (b) Shannon, R. D.; Pask, J. A. J. Am. Ceram. Soc. 1965, 48 (8), 391. (c) MacKenzie, K. J. D. Trans. J. Br. Ceram. Soc. 1975, 74 (4), 121. (d) Eppler, R. A. J. Am. Ceram. Soc. 1987, 70 (4), C64. (30) Liao, S. L.; Pae, K. D.; Mayo, W. E. Nanostruct. Mater. 1995, 5 (3), 319.

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Figure 5. (a,b) TEM images of 1% La2O3 doped TiO2 sample calcined at 400 °C. (c) TEM image of undoped titania calcined at 500 °C.

in the presence of La2O3 and the stabilization of the same at higher temperatures. It is further observed that, at a calcination temperature of 800 °C, the anatase still remained at the size of 32.58 nm while the rutile further grew to about 43.06 nm (Table 1). Transmission electron microscopic observation of typical samples of pure titania31 and that doped with La2O3 indicates the nanocrystalline nature of particles. The grain sizes of 1% La2O3 doped titania gels heated at 400 °C are shown in Figure 5a,b and that of undoped titania heated at 500 °C is shown in Figure 5c. The distribution of La2O3 in titania may be more or less in the intergranular region and increase the diffusion barrier at the anatase-anatase grain contact. The evidence for Ti-O-La linkages from FTIR data discussed earlier further supports this observation. The TPD of ammonia32 was used to characterize the acid site distribution and furthermore to obtain the quantitative amounts of acid sites in the specified temperature range. The distribution pattern can be classified into weak (desorption at 100-200 °C), medium (300 °C), and strong (400-700 °C) acid sites. From the (31) Tonejc, A. M.; Goti, M.; Grzeta, B.; Music, S.; Popovi, S.; Trojko, R.; Turkovi, A.; MuSevic, I. Mater. Sci. Eng. 1996, B40, 177. (32) (a) Arena, F.; Dario, R.; Parmaliana, A. Appl. Catal. A. 1998, 170, 127. (b) Suja, H.; Deepa, C. S.; Rani, K. S.; Sugunan, S. Appl. Catal. A, in press. (c) Li, J.; Wu, L.; Zhang, Y. Chem. Phys. Lett. 2001, 342, 249.

Table 2. TPD Results Showing the Amount of Weak and Strong Lewis Acid Sites sample

weak site

medium site

strong site

undoped TiO2 (300 °C) undoped TiO2 (700 °C) 1% La2O3 (300 °C) 1% La2O3 (700 °C) 2% La2O3 (700 °C)

0.58 3.23 1.3 2.91 3.62

0.16 0.63 0.56 0.82 1.70

0.55 1.2 1.3 2.90 3.95

TPD result, all the lanthana-doped compositions show an increased amount of total acidic sites compared to the pure titania sample heated at 300 °C (Table 2). This shows the increase in availability of adsorption sites in doped samples, making them more efficient catalysts. Further, with increasing concentration of dopants, the amount of reactive acidic sites increases. The extent of weak as well as strong acidic sites increases with increase of calcination temperature. La3+ ions, if substituted in the tetrahedral Ti4+ site, will have a net positive charge/hole and so can accommodate a lone pair of electrons.33 The increased amount of acidic sites of lanthana-doped titania is due to the influence of substituted La3+ ions in the titania matrix. La2O3-doped titania has a relatively high thermal stability and contains more of the strong acid sites, while pure titania has more of the weak acid sites. Generally, all the (33) (a) Pierre, A. C. Introduction to Sol-Gel Processing; Kluwer Academic Publishers: The Netherlands, 1998. (b) Atkinson, D.; Curthoys, G. Chem. Soc. Rev. 1979, 8, 475.

La2O3-Doped Sol-Gel Nanosized Titanium Oxide

samples show an increase in the total acidic sites with calcination temperature. This may be due to the fact that at higher temperature the physically adsorbed water and residual organics may be removed, increasing the accessible surface area.

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acidity for the La2O3-doped TiO2, indicating its possibility to be used as solid acid catalysts. The enhanced properties of titania are due to the excellent dopant distribution in a titania matrix, the presence of Ti-OLa linkage, and the increased surface acidity and nanocrystalline nature.

Conclusions Stable titania sol doped with La2O3 has been prepared starting from titanium alkoxide by a solution sol-gel process. The presence of 1 wt % of lanthanum oxide causes an increase in specific surface area of titania to as high as 160 m2/g, which decreases to 52 m2/g, upon calcining at 700 °C. On the other hand, undoped titania possessed only ≈1 m2/g after calcining at 700 °C. Further, La2O3 also enhanced the high-temperature anatase phase stability of titania. The A > R transformation temperature increased by about 200 °C in the presence of 1% La2O3. The anatase crystallite size of titania was reduced, upon calcination at 700 °C, to 13 nm from its undoped counterpart having 32 nm. Furthermore, there is a considerable increase in the surface

Acknowledgment. Mr. C. P. Sibu acknowledges the TNO Institute of Applied Physics, The Netherlands, for financial support as project fellow. Mr. C. P. Sibu and Mr. S. Rajesh Kumar acknowledge CSIR, India, for the financial support as research fellows. The authors acknowledge the assistance rendered by Prof. W. Wunderlich, Nagoya Institute of Technology, Nagoya, Japan, in providing the TEM micrographs. Supporting Information Available: Description and raw data of the temperature-programmed desorption technique (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM010966P