Chemically Modified Titania Hydrolysates: Physical Properties

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Chemically Modified Titania Hydrolysates: Physical Properties P. A. Venz, J. T. Kloprogge,* and R. L. Frost Centre for Instrumental and Developmental Chemistry, Queensland University of Technology, 2 George Street, GPO Box 2434, Brisbane Q 4001, Australia Received June 25, 1999. In Final Form: February 29, 2000 Titania hydrolysates prepared from the hydrolysis of unmodified tetraisopropoxytitanium(IV) and tetraisopropoxytitanium(IV) modified before hydrolysis by replacement of the tetraisopropoxy groups via a reaction with carboxylic acids, including acetic, propanoic, and butanoic acid, are shown by FT-Raman and X-ray diffraction to be amorphous with no clear long-range ordering. The hydrolysate spectra show three broad bands around 210, 440, and 570 cm-1, which do not correspond with any known titania phase. In the modified systems, nucleophilic substitution and elimination of alkoxy groups is virtually complete. Charge separation induced by modifying ligands ensures that the removal of carboxylates is incomplete, although not occurring to the same degree in the three systems. Elimination of carboxylic acid groups is increasingly favorable with longer chain length, but the modifying groups also become more difficult to protonate. The hydrophobicity of the carboxylate groups increases with chain length, and this may also reduce the rate of nucleophilic attack by water upon carboxylate ligands. The net effect of the charge distribution and the hydrophobicity within the modified species is for the quantities of residual carboxylate to increase with the chain length of the modifying ligands. Modification also influences the particle size, the surface area, and the porosity of the hydrolysate aggregates. The rate and extent of polycondensation decreases with carboxylic acid modification and the chain length of the acid. The specific surface areas of the modified hydrolysates are reduced by the presence of residual carboxylate groups. Unreacted carboxylate groups have a significant influence upon the crystallization and phase-transformation properties of the hydrolysates. In the acetate- and propanoate-modified hydrolysates, unreacted carboxylate inhibits crystallization and phase transformation of the solids, elevating the temperatures at which these processes occur. In the butanoate-modified hydrolysates, carboxylate ligands are found in larger quantities, enhancing these processes and decreasing the crystallization and phase transformation temperatures, to the extent that the anatase f rutile phase transformation occurs at lower temperatures in butyric hydrolysates than in unmodified materials.

Introduction The main precursors in sol-gel processes are metal salts and alkoxides. The reactions of metal alkoxides have been investigated since the mid-19th century,1 but commercial applications of sol-gel-derived ceramic and glass products have appeared only within the last several decades.2,3 The volume of published research and the diversity of applications have increased dramatically in this time, particularly since the 1970s. The most widely studied and well characterized of the oxide materials is undoubtedly silica. A number of other oxides have also assumed important places in modern sol-gel science. The most prominent of these are titania, zirconia, and alumina. Titania has a wide and diverse range of applications including paint pigments, abrasives, catalysts, a variety of glass products, protective, optical, and electronic coatings, solid-state electronic devices, reinforcing fibers in composite materials, biomedical implants, and membranes and filters.4-8 * To whom correspondence should be addressed. E-mail: [email protected]. Fax: +61 7 3864 1804. Phone: +61 7 3864 1220. (1) Hench, L. L.; West, J. K. Chem. Rev. 1990, 90, 33. (2) Wood, T. E.; Dislich, H. Proceedings of the First International Symposium on Sol-Gel Science and Technology; The American Ceramic Society: Westerville, OH, 1995. (3) Dislich, H.; Hinz, P. J. Non-Cryst. Solids 1982, 48, 11. (4) Brinker, C. J.; Scherer, G. W. Sol-Gel Science. The Physics and Chemistry of Sol-Gel Processing; Academic Press: London, 1990. (5) Segal, D. Chemical synthesis of advanced ceramic materials; Cambridge University Press: Cambridge, 1989. (6) Tulloch, S. M.; Tulloch, G. E. J. Aust. Ceram. Soc. 1995, 31, 1.

Hydrous titania powders are usually produced by hydrolysis of Ti(IV) salts or alkoxides.9-27 In general, the (7) Spiccia, L.; Watkins, I. D.; West, B. O. Mater. Forum 1996, 20, 171. (8) Jones, R. W. Fundamental Principles of Sol-Gel Technology; The Institute of Metals: London, 1989. (9) Cheng, H.; Ma, J.; Zhao, Z.; Qi, L. Chem. Mater. 1995, 7, 663. (10) Busca, G.; Ramis, G.; Amores, J. M. G.; Escribano, V. S.; Piaggio, P. J. Chem. Soc., Faraday Trans. 1994, 90, 3181. (11) Georgiadou, I.; Spanos, N.; Papadopoulou, Ch.; Matralis, H.; Kordulis, Ch.; Lycourghiotis, A. Colloids Surf., A 1995, 98, 155. (12) Barringer, E. A.; Bowen, H. K. Langmuir 1985, 1, 414. (13) Jean, J. H.; Ring, T. A. Langmuir 1986, 2, 251. (14) Harris, M. T.; Byers, C. H. J. Non-Cryst. Solids 1988, 103, 49. (15) Ogihara, T.; Iizuka, M.; Yanagawa, T.; Ogata, N.; Yoshida, K. J. Mater. Sci. 1992, 27, 55. (16) Terabe, K.; Kato, K.; Miyazaki, H.; Yamaguchi, S.; Imai A.; Iguchi, Y. J. Mater. Sci. 1994, 29, 1617. (17) Barringer, E. A.; Bowen, H. K. J. Am. Ceram. Soc. 1982, 65, C-199. (18) Basca, R. R.; Gratzel, M. J. Am. Ceram. Soc. 1996, 79, 2185. (19) Gotic, M.; Ivanda, M.; Sekulic, A.; Music, S.; Popovic, S.; Turkovic, A.; Furic, K. Mater. Lett. 1996, 28, 225. (20) Music, S.; Gotic, M.; Ivanda, M.; Popovic, S.; Turkovic, A.; Trojko, R.; Sekulic, A.; Furic, K. Mater. Sci. Eng. B 1997, 47, 33. (21) Hague, D. C.; Mayo, M. J. J. Am. Ceram. Soc. 1994, 77, 1957. (22) Ocana, M.; Garcia-Ramos, J. V.; Serna, C. J. J. Am. Ceram. Soc. 1992, 75, 2010. (23) Ding, X.-Z.; Qi, Z.-Z.; He, Y.-Z. J. Mater. Sci. Lett. 1995, 14, 21. (24) Morales, B. A.; Novaro, O.; Lopez, T.; Sanchez, E.; Gomez, R. J. Mater. Res. 1995, 10, 2788. (25) Haro-Poniatowski, E.; Rodriguez-Talavera, R.; de la Cruz Heredia, M.; Cano-Corona, O.; Arroyo-Murillo, R. J. Mater. Res. 1994, 9, 2102. (26) Ogihara, T.; Ikeda, M.; Kato, M.; Mizutani, N. J. Am. Ceram. Soc. 1989, 72, 1598. (27) Hardy, A. B.; Gowda, G.; McMahon, T. J.; Riman, R. E.; Rhine, W. E.; Bowen H. K. In Ultratructure Processing of Advanced Ceramics; Mackenzie, J. D., Ulrich, D. R., Eds.; Wiley: New York, 1988; p 407.

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Chemically Modified Titania Hydrolysates

reactants in alkoxide-derived systems are diluted with alcohols to control the hydrolysis and condensation rates and produce fine, monodisperse powders.12-15,28,29 The particle size distribution, morphology, crystallization, and phase-transformation properties of the oxide powders, which are critical to the performance of the ceramic products,27,30 are largely determined by the rate and degree of hydrolysis and condensation reactions when alkoxide and water are mixed. Those reactions are in turn controlled by manipulating factors such as the alkoxy group of the alkoxide, the diluting alcohol, the hydrolysis ratio, and the catalyzing agent.12-15,24,28,29,31-33 While chemical modification of alkoxides is extensively used to control hydrolysis and condensation in polymeric sol-gel systems, this technique has found little application in particulate sol-gel processes. Some titania materials have been produced by using catalysts such as ammonium hydroxide and acetic, nitric, or hydrochloric acid, which were added to the water prior to hydrolysis.24,28 Dilution of the alkoxides in nonparent alcohols, with the possibility of alcoholysis reactions, has also been employed.21,34 However, few other modification processes have been reported for particulate sol-gel systems. Modification of alkoxides with agents such as carboxylic acids and anhydrides and β-diketones and β-ketoesters prior to hydrolysis is virtually unknown in particulate sol-gel processing. Complete reaction of all carboxylate-modifying ligands from alkoxide-derived sol-gel materials is difficult to achieve.35 It is also likely that some proportion of the alkoxy groups will remain in the hydrolysates. Likewise, 100% condensation of the alkoxide to TiO2 is unlikely when alkoxides are simply added to water,12,13,28 and some proportion of the OH-groups formed during hydrolysis are likely to remain in the hydrolysates. Raman spectroscopy is frequently used to investigate the crystalline structure of titania. The naturally occurring crystalline polymorphs of titania (anatase, rutile, and brookite) exhibit characteristic spectral features.10,22,36-41 Unlike infrared spectroscopy, Raman spectroscopy is not susceptible to signal absorption by water. Therefore, Raman techniques are very suitable for the study of wet hydrolysates, reducing the risk of chemical or physical alteration of the samples by drying or other preparation processes. Similarly, titanium alkoxides,42-46 carboxylic acids, and carboxylate compounds12,46-48 exhibit a number (28) Yoldas, B. E. J. Mater. Sci. 1986, 21, 1087. (29) Sanchez, C.; Livage, J.; Henry, M.; Babonneau, F. J. Non-Cryst. Solids 1988, 100, 65. (30) Segal, D. Chemical synthesis of advanced ceramic materials; Cambridge University Press: Cambridge, 1989. (31) Livage, J.; Henry, M.; Sanchez, C. Prog. Solid State Chem. 1988, 18, 259. (32) Sanchez, C.; Livage, J. New J. Chem. 1990, 14, 513. (33) Yoldas, B. E. J. Non-Cryst. Solids 1984, 63, 145. (34) Campbell, L. K.; Na, B. K.; Ko, E. I. Chem. Mater. 1992, 4, 1329. (35) Doeuff, S.; Henry, M.; Sanchez, C.; Livage, J. J. Non-Cryst. Solids 1987, 89, 206. (36) Ohsaka, T.; Izumi, F.; Fujiki, Y. J. Raman Spectrosc. 1978, 7, 321. (37) Porto, S. P. S.; Fleury, P. A.; Damen, T. C. Phys. Rev. 1967, 154, 522. (38) Betsch, R. J.; Park, H. L.; White, W. B. Mater. Res. Bull. 1991, 26, 613. (39) Balachandran, U.; Eror, N. G. J. Solid State Chem. 1982, 42, 276. (40) Ocana, M.; Fornes, V.; Garcia Ramos, J. V.; Serna, C. J. J. Solid State Chem. 1988, 75, 364. (41) Tompsett, G. A.; Bowmaker, G. A.; Cooney, R. P.; Metson, J. B.; Rodgers, K. A.; Seakins, J. M. J. Raman Spectrosc. 1995, 26, 57. (42) Gagliardi, C. D.; Dunuwila, D.; Berglund, K. A. Mater. Res. Soc. Symp. Proc. 1990, 180, 801. (43) Gagliardi, C. D.; Dunuwila, D.; Van Vlierberge-Torgerson, B. A.; Berglund, K. A. Mater. Res. Soc. Symp. Proc. 1992, 271, 257. (44) Laaziz, I.; Larbot, A.; Julbe, A.; Guizard, C.; Cot, L. J. Solid State Chem. 1992, 98, 393.

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of strong, distinctive infrared- and Raman-active bands useful for characterizing the organic functional groups in modified alkoxide systems. The objective of this research is to produce hydrous titania precipitates (hydrolysates) from a titanium alkoxide chemically modified with a range of short-chain carboxylic acids, to determine the predominant reaction mechanisms by which the solids are produced and the influence it may have on the surface chemistry and peptization behavior of the chemically modified hydrolysates. Experimental Methods Preparation of Modified Titania Hydrolysates. All hydrolysates were prepared from tetraisopropyl titanate (TPT, Huls Troisdorf). Acetic, propanoic, and n-butanoic acids were used as modifying agents (analytical grade, BDH Chemicals), in a ratio of 1 mol of acid per mole of TPT. The acids (21.1 g of acetic acid, 26.1 g of propanoic acid, 31.0 g of butanoic acid) were rigorously dried using standard methods prior to use and were added to TPT (100 g) at a rate of 30 cm3 per min, with constant agitation by a magnetic stirrer and a Teflon-coated stirring bar. The modified alkoxides were cooled to ambient temperature and were subsequently hydrolyzed by adding the modified alkoxides to ∼320 cm3 of deionized water (ionic conductivity < 1 µS cm-1), at a rate of 100 cm3 per min, with constant, vigorous stirring. A hydrolysis ratio of 50 mol of water per mole of TPT was obtained when using the quantities of TPT and water detailed above. The resulting hydrolysate slurry was left to stand for 30 min to allow settling of the solid particulates. The liquid phase was decanted from the solid, and the hydrolysates were agitated and washed five times with fresh deionized water (1000 cm3 per mole of TPT) in order to remove organic compounds such as 2-propanol or carboxylic acids eliminated during hydrolysis and condensation. After the fifth washing, the solid was left to stand overnight (14 h) before removal of the liquid. Unmodified hydrolysates were produced by a similar procedure, the only difference being that TPT was not chemically modified prior to hydrolysis. FT-Raman Spectroscopy. Alkoxide samples were stored in sealed sample vials and placed in the instrument sample compartment into a holder specially designed for this purpose. Hydrolysate samples were separated from the final wash and placed in the solid sample holder provided with the FT-Raman instrument. The samples were not oven-dried prior to analysis to prevent heat-induced changes. Similarly, samples were not ground or mechanically treated, as this may also alter the materials’ crystalline structure.49 FT-Raman spectra were collected with a Perkin-Elmer model 2000 near infrared/FT-Raman spectrometer, fitted with a quartz beam splitter and an indium gallium arsenide (InGaAs) detector. Rayleigh line rejection was performed with a notch filter set which, when combined with the detector response, allowed collection of Stokes-scattered radiation over an effective Raman shift range of 3400-100 cm-1. The excitation source was a watercooled Spectron Laser Systems SL301 Nd-doped YAG laser, emitting at a wavelength of 1064 nm. Alkoxide spectra were accumulated by coaddition of 250 scans. To prevent heat-induced hydrolysate sample damage during extended exposure to the excitation laser, it was important that hydrolysate spectra be accumulated as rapidly as possible. Depending upon the scattering properties of the solid samples, spectra of the moist hydrolysates were obtained over 600-800 coadded scans. The instrumental parameters used represent a (45) Kaklhana, M.; Kotaka, M.; Okamoto, M. J. Phys. Chem. 1983, 87, 2526. (46) Dollish, F. R.; Fateley, W. G.; Bentley, F. F. Characteristic Raman Frequencies of Organic Compounds; Wiley: New York, 1974. (47) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 5th ed.; Wiley-Interscience: New York, 1986. (48) Straughan, B. P.; Moore, W.; McLaughlin, R. Spectrochim. Acta 1986, 42A, 451. (49) Isobe, T.; Senna, M. J. Solid State Chem. 1991, 93, 358.

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balance between maximization of the signal-to-noise ratio and minimization of the spectrum collection time and the irradiating laser power. The instrument parameters used for collecting FT-Raman spectra of the alkoxides and hydrolysates were as follows: instrument resolution of 4 cm-1, optical path difference (OPD) velocity of 0.2 cm s-1, and laser power of 500 mW. The lack of a thermoluminescent background in the hydrolysate spectra indicates that sample heating by the laser was not an important factor under the conditions used for these studies. No discernible differences were found in preliminary experiments between spectra collected at a power of 500 mW and those collected with lower power levels. This indicates that the hydrolysates’ structures were not altered by laser-induced sample heating over the spectral collection period used in these investigations. As an optical spectrometer, the FT-Raman instrument possesses a response profile which is determined by variations with wavelength of the detector response and the transmission and reflection properties of the optical components, such as the beam splitter and the Rayleigh filters.50 The response profile, therefore, is not constant with wavelength, and spectra must be corrected if they are to be directly comparable with those collected using different instruments. To obtain the correction function, the emission profile of a calibrated blackbody source was ratioed against the response profile of the PE 2000 instrument. Raw spectra were then multiplied by this function, to give the corrected spectrum. Spectral correction was performed using the Galactic Industries Grams/32 software package. X-ray Diffraction (XRD). XRD analyses were performed on a Philips wide-angle PW 1050/25 vertical goniometer equipped with a graphite-diffracted beam monochromator. Intensity and d spacing measurements were improved by application of a selfdeveloped, computer-aided divergence slit system, enabling constant sampling area irradiation (20 mm long) at any angle of incidence. The goniometer radius was enlarged from 173 to 204 mm. The radiation applied was Co KR from a long fine-focus Co tube, operating at 40 kV and 40 mA. The samples were measured at 50% relative humidity in step scan mode with steps of 2θ ) 0.02° and a counting time of 2 s. Hydrolysate samples were dried prior to analysis by blowing dry N2 gas over the solids for several hours. BET Specific Surface Area and BJH Specific Pore Volume. The BET specific surface areas and pore volumes were obtained from nitrogen adsorption-desorption data, with a Micrometrics ASAP 2010 instrument. The Barrett-JoynerHalenda (BJH) method was used to calculate the pore size distribution and the pore volume from nitrogen desorption isotherms.

Results and Discussion Extent of Hydrolysis and Condensation in Modified Hydrolysates. Residual Organic Species. Unreacted isopropoxy and carboxylate groups are often reported to remain in polymeric gels and hydrous oxide precipitates following hydrolysis and condensation.35,42,43,51-53 Titanium alkoxides and carboxylic acids or carboxylate compounds exhibit several distinctive vibrational bands in the infrared and Raman spectra useful for characterizing the organic functional groups. Most prominent are the νsym(COO-) symmetric and νantisym(COO-) antisymmetric stretching modes, which typically exhibit bands in the 1350-1500 and 1500-1700 cm-1 regions, respectively.45-48,54-56 The most prominent band in the TPT Raman spectrum is the ν(C-O)Ti stretching mode produced by alkoxy (50) Petty, C. J.; Warnes, G. M.; Hendra, P. J.; Judkins, M. Spectrochim. Acta 1991, 47A, 1179. (51) Poncelet, O.; Robert, J.-C.; Guilment, J. Mater. Res. Soc. Symp. Proc. 1992, 271, 249. (52) van Vlierberge-Torgerson, B. A.; Dunuwila, D.; Berglund, K. A. Mater. Res. Soc. Symp. Proc. 1992, 271, 65. (53) Severin, K. G.; Ledford, J. S.; Torgerson, B. A.; Berglund, K. A. Chem. Mater. 1994, 6, 890. (54) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic Press: San Diego, CA, 1990.

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Figure 1. 1000-1700-cm-1 region of the FT-Raman spectra of washed titania hydrolysates.

groups and located at ∼1025 cm-1.42,43,57 Two other strong bands are observed in the Raman spectrum of TPT, at 1331 and 1448 cm-1. These have been assigned to symmetric and antisymmetric δ(CH3) modes of the isopropoxy groups, respectively.45,46,57 Another strong band, at 1181 cm-1, is produced by an in-phase π(CH3) rocking mode.57 A band in the Raman spectrum of TPT at 1126 cm-1 is assigned to an antisymmetric ν(CCC) stretch coupled with a ν(C-O) stretch.42,57 Symmetric and antisymmetric ν(Ti-O) stretching modes are also reported in the spectrum of TPT, at 565 and 612 cm-1, respectively.42,57 The 1000-1700-cm-1 regions of the washed hydrolysates’ FT-Raman spectra are illustrated in Figure 1. No evidence is found for the characteristic 1025 cm-1 ν(CO)Ti isopropoxy stretching mode, which produces a relatively intense band in the Raman spectrum of pure TPT.42,57 The spectral data therefore suggest that no significant quantities of unreacted isopropoxy groups remain after washing of the hydrolysates. Two bands are observed at ∼1418 and 1535 cm-1 in the spectra of the modified hydrolysates. These correspond with the νsym(COO-) and νantisym(COO-) stretching modes, respectively, of carboxylate groups in the spectra of the carboxylic acids and the modified alkoxides.46-48,54,55 This result indicates that some proportion of the carboxylate-modifying ligands remain in the hydrolysates after the reaction between modified TPT and water. The intensities of the (55) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: San Diego, CA, 1991. (56) Deacon, G. B.; Phillips, R. J. Coord. Chem. Rev. 1980, 33, 227. (57) Moran, P. D.; Bowmaker, R. A.; Cooney, R. P.; Finnie, K. S.; Bartlett, J. R.; Woolfrey, J. L. Inorg. Chem. 1998, 37, 2741.

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Table 1. Partial Charges of Functional and Leaving Groups in Carboxylic Acid-Modified Systems species

δ(Ti)

TiO(CH3COO)(OH)(H2O) TiO(CH3CH2COO)(OH)(H2O) TiO(CH3(CH2)2COO)(OH)(H2O) Ti2O3(CH3COO)2(OH2)2 Ti2O3(CH3CH2COO)2(OH2)2 Ti2O3(CH3(CH2)2COO)2(OH2)2

δ(O)

δ(OH)

δ(RCOO)

Nucleophilic Substitution of Carboxylate 0.75 -0.39 -0.19 -0.18 0.72 -0.41 -0.24 -0.01 0.70 -0.43 -0.27 0.10

Nucleophilic Substitution of Carboxylate in Dehydroxylated Species 0.76 -0.39 -0.18 0.72 -0.41 -0.03 0.70 -0.43 0.06

ν(COO-) bands increase with the alkyl chain length of the carboxylic acid modifier, indicating that the quantities of residual carboxylate are lowest in the acetate-modified hydrolysates and highest in the butanoate-modified hydrolysates. The hydrolysis, condensation, and elimination reactions in such sol-gel systems are described in terms of the partial charge model (PCM),29,32,58 which enables the partial charge residing on the metal centers, ligands, and potential entering and leaving groups in nucleophilic substitution processes to be evaluated. The calculated partial charges on RCOO- and RCOOH in model Ti(IV) sites in the evolving gel network (assuming essentially complete removal of alkoxy ligands) are summarized in Table 1. For species such as [TiO(RCOO)(OH)(OH2)] and [Ti2O3(RCOO)2(OH2)2], δ(RCOO) is negative in the acetateand propanoate-modified systems and positive in butanoate-modified systems. Formation of butanoic acid leaving groups is therefore less favorable than the equivalent process in the other modified systems. If the leaving groups cannot form in the transition state, and the likelihood of this increases with longer acid chains, then they cannot be readily eliminated by an SN2 reaction. At the same time, δ(RCOOH) values are positive in all systems. Thus, formation and removal of carboxylate by nucleophilic substitution is most favorable in acetatemodified systems and least favorable in butanoatemodified systems. This result, of course, agrees with the DTA/TGA and spectroscopic data; that is, greater quantities of modifying carboxylate ligands are retained in the hydrolysates as the chain length of the carboxylic acid increases. While the partial charge model adequately explains the properties of the modified hydrolysates in terms of the reaction mechanisms by which they are formed, it does not take into account all of the factors which influence the reactive properties of an alkoxide.29,58 In addition to the charge distribution effects described by the PCM, the contribution of hydrophobic and steric hindrance effects by alkoxy and modifying groups must also be recognized. As the size and bulkiness of the hydrophobic hydrocarbon portions of these groups increases, the approach of attacking water nucleophiles to the metal atom is increasingly inhibited. The rate and extent of reaction should decrease accordingly.29,43,51,58,59 Increasing quantities of residual carboxylate with longer carboxylate chains would also increase the hydrophobicity of the whole hydrolysate surface, not just particular sites on the particle surfaces. The hydrophobic effect would occur at all steps in the reactions of the modified alkoxide systems. The predictions of this steric hindrance effect are in accord with the trends observed in the oxide and carboxylate contents of the modified hydrolysates. Another limitation of the PCM is that while it can predict the relative ease of removal of (58) Livage, J.; Henry, M.; Sanchez, C. Prog. Solid State Chem. 1988, 18, 259. (59) Dunuwila, D. D.; Gagliardi, C. D.; Berglund, K. A. Chem. Mater. 1994, 6, 1556.

δ(H2O)

δ(RCOOH)

0.01 -0.06 -0.11

0.02 0.16 0.26

0.01 -0.07 -0.12

0.02 0.14 0.22

carboxylate groups from modified systems, it does not appear to give any quantitative estimation of the degree of carboxylate removal. Crystallization and Phase Transformation of Modified Titania Hydrolysates. Crystalline Structure of Modified Titania Hydrolysates. Titania, TiO2, has three natural crystalline modifications: anatase, rutile, and brookite.9,10,39,41,60 All of the phases consist of TiO6 octahedra but differ in the number of shared edges and corners. Anatase shares four edges and four corners, rutile two edges and six corners, and brookite three edges and five corners.9,60 Anatase is tetragonal, with two TiO2 units 19 space group.10,22,39 per unit cell, and belongs to the D4h Factor group analysis indicates that anatase has nine Raman-active vibrational modes (A1g + 2B1g + 3Eg).2,14,32,35 The ν6(Eg) band is the most prominent, being several times greater in intensity than the other anatase bands. The ν5(Eg) band is much weaker than the other anatase bands and, hence, is frequently not observed in the spectra of many titania materials. Due to the small degree of separation between the ν2(B1g) and ν3(A1g) bands, only one band is usually observed, centered at about 515 cm-1. 14 space group, and Rutile is tetragonal, belongs to the D4h has two TiO2 units in the unit cell.10,22,37,39,40 This phase has five Raman-active modes (A1g + B1g + B2g + Eu).10,22,37-41 The B1g and B2g bands are extremely weak in comparison with the other Raman bands and are frequently not observed. Brookite is orthorhombic, belongs 15 space group, has eight TiO2 units in the unit to the D2h cell, and has 36 Raman-active modes (9A1g + 9B1g + 9B2g + 9B3g).10,39,41 FT-Raman spectra of the washed hydrolysates are shown in Figure 2. The principal features of the hydrolysate spectra are three broad bands, appearing to consist of multiple components, at Raman shifts of ∼210, 440, and 570 cm-1. No significant similarities exist between the modified hydrolysate spectra and the anatase spectrum. The closest correlation between the hydrolysate spectra and the Raman spectra of known titania phases is with rutile. Allowing for the uncertainty in locating the exact band centers in the hydrolysate spectra, the 440 and 570 cm-1 bands correspond approximately with the normal positions of 447 and 612 cm-1 for the rutile Eg and A1g22,37-40,61 modes, respectively. Similarly, the band at ∼210 cm-1 corresponds roughly with a band normally located at approximately 235 cm-1, which has been variously attributed to a disorder effect, second-order scattering, or latent anharmonicity effects in rutile.22,39,40,61 Melendres et al.61 have suggested that the 235-cm-1 band does not originate from disordering in the material but from stoichiometric deficiencies in the oxide. Hydrolysate FT-Raman spectra differ from the spectrum of crystalline rutile in at least two aspects aside from band position. First, full width at half-maximum (fwhm) values (60) Mitsuhashi, T.; Kleppa, O. J. J. Am. Ceram. Soc. 1979, 62, 356. (61) Melendres, C. A.; Narayanasamy, A.; Maroni, V. A.; Siegel, R. W. J. Mater. Res. 1989, 4, 1246.

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Figure 2. 100-1100-cm-1 region of the FT-Raman spectra of washed titania hydrolysates.

Figure 3. XRD patterns of acetate-modified titania hydrolysate and the minerals rutile and anatase.

of the hydrolysate spectral bands are on the order of 100200 cm-1, while those for the Eg and A1g modes of crystalline rutile are 30-50 cm-1.49 Substantial band broadening is usually interpreted as indicative of a high degree of disordering in the materials.25,38 Relative peak intensities in the hydrolysate spectra are also significantly different from those of rutile, suggesting that the constituent structural units are distorted in comparison with those in crystalline rutile. Characterization of oxide powders is rarely performed by only one technique. While Raman spectroscopy is widely used, X-ray diffraction is extensively employed for the characterization of oxide powders. XRD patterns of the acetate-modified hydrolysate, anatase, and rutile are shown in Figure 3. The data are noisy, and the features in the hydrolysate pattern are extremely weak and broad and bear little resemblance to the XRD patterns of either anatase or rutile, although the positions of the broad hydrolysate maxima seem to coincide more with those of the anatase reflections. This suggests that there is no long-range ordering in the modified hydrolysate structures and that the solids are essentially X-ray amorphous. Similar XRD and Raman spectroscopic results for titania powder precipitates have been reported by other authors.16,22,25,61 For this reason the three bands observed in the hydrolysate FT-Raman spectra at ∼210, 440, and 570 cm-1 are frequently associated with “amorphous” titania solids in the literature. However, a number of titania materials have been reported which exhibit no characteristics corresponding to those of a crystalline titania phase, in either the Raman spectra or the XRD patterns.62,63

Thus, the hydrolysates, while sharing some Raman spectral characteristics with rutile, are clearly not highly ordered, crystalline titania. The term “rutile-like” is probably the most appropriate description for the longrange hydrolysate structure. Similar contradictions between Raman spectral data and XRD data have been reported in certain polymer systems.64 The hydrolysates are X-ray amorphous, but localized, short-range ordering occurs throughout the material.16,49,65,66 The hydrolysates lack the long-range ordering associated with the crystalline phases. Over short ranges, however, there is some resemblance to the ordering in crystalline phases of titania. Crystallization and Phase Transformation. In a previous paper67 the DTA curves of the hydrolysates, reheated after initially heating to a temperature of 450 °C and subsequently cooled, have been reported. As all water, hydroxyl species, and residual organic material have been removed by the first heat treatment, the exothermic peaks observed in the DTA curves are attributed to crystallization and phase-transformation processes. The temperatures at which these processes occur are summarized in Table 2. (62) Arsov, Lj. D.; Kormann, C.; Plieth, W. J. Raman Spectrosc. 1991, 22, 573. (63) Lottici, P. P.; Bersani, D.; Braghini, M.; Montenero, A. J. Mater. Sci. 1993, 28, 177. (64) Cutler, D. J.; Hendra, P. J.; Fraser, G. In Developments in Polymer Characterisation - 2; Dawkins, J. V., Ed.; Applied Science Publishers: London, 1980; Chapter 3. (65) Exarhos, G. J. Mater. Res. Soc. Symp. Proc. 1985, 48, 461. (66) Gopal, M.; Moberly Chan, W. J.; De Jonghe, L. C. J. Mater. Sci. 1997, 32, 6001. (67) Venz, P. A.; Frost, R. L.; Kloprogge, J. T.; Bartlett, J. R.; Woolfrey, J. L. Thermochim. Acta 2000, 346, 73.

Chemically Modified Titania Hydrolysates

Langmuir, Vol. 16, No. 11, 2000 4967

Table 2. Crystallization and Anatase-Rutile Transition Temperatures of Modified Titania Hydrolysates Based on TGA/DTA Analysis62 modifying acid

disordered/amorphous f anatase crystallization temp (°C)

anatase f rutile transformation temp (°C)

none acetic propanoic butyric

500-650 636 640 567

680 691 691 665

Crystallization of the disordered hydrolysates into anatase occurs in the 500-650 °C range. These temperatures are rather high in comparison with those for many amorphous or disordered titania materials discussed in the literature, where crystallization to anatase is frequently reported to occur in the 300-450 °C range.15,16,21-25,34,63,68-73 The temperatures for the anatase f rutile transition, between 665 and 691 °C, are more consistent with literature values, which typically occur in the 550-1050 °C range.15,21-25,34,71-74 It should be noted that the literature temperatures quoted here for crystallization and phase transformation are only the more common values and do not represent the entire range of temperatures reported. Due to the unusual TiO2 preparation method employed in this work, the crystallization and phase transformation characteristics of the hydrolysates will not necessarily be directly comparable with those of materials in the literature. In this work, material preparation differs from the usual alkoxide- or salt-based procedures in two significant respects: the alkoxide is modified with a carboxylic acid prior to hydrolysis, and in the hydrolysis step, water and alkoxide are reacted without dilution in alcohol. Several factors are known to influence the crystallization and phase-transformation properties of oxide materials. Foremost among these are the presence of organic groups remaining after reaction of an alkoxide precursor,16,21,22,70 stoichiometric defects and the extent of hydrolysis and condensation in the materials,21,23-25,70 variations in the structure and packing of the materials,21,75,76 and, in the case of anatase f rutile phase transformation, the size of the crystallites.23,39 It is proposed that the main factor influencing the crystallization and phase transformation characteristics of the modified hydrolysates is the presence of unreacted carboxylate material. In the acetic and propanoic hydrolysates, steric hindrance by the residual modifying ligands prevents long-range ordering and inhibits both the crystallization of the disordered/amorphous material into anatase and the anatase f rutile phase change.16,21,70 As the quantities of carboxylate in these two systems are similar, their crystallization and phase-transformation temperatures are essentially identical. In the butanoate-modified hydrolysate, crystallization and phase transformation are accelerated, occurring at temperatures lower even than those in the carboxylate(68) Regai, J.; Sing, K. S. W. J. Colloid Interface Sci. 1984, 101, 369. (69) Rubio, J.; Oteo, J. L.; Villegas, M.; Duran, P. J. Mater. Sci. 1997, 32, 643. (70) Iida, Y.; Furukawa, M.; Kato, K.; Morikawa, H. Appl. Spectrosc. 1997, 51, 673. (71) Chee, Y. H.; Cooney, R. P.; Howe, R. F.; van der Heide, P. A. W. J. Raman Spectrosc. 1992, 23, 161. (72) Kim, Y.-J.; Francis, L. F. J. Am. Ceram. Soc. 1993, 76, 737. (73) Selvaraj, U.; Prasadarao, A. V.; Komarneni, S.; Roy, R. J. Am. Ceram. Soc. 1992, 75, 1167. (74) Chhabra, V.; Pillai, V.; Mishra, B. K.; Morrone, A.; Shah, D. O. Langmuir 1995, 11, 3307. (75) Yoldas, B. E. J. Am. Ceram. Soc. 1982, 65, 387. (76) Yan, M. F.; Rhodes, W. W. Mater. Sci. Eng. 1983, 61, 59.

free unmodified material. Carboxylate groups, in disrupting the ordering of the materials, produce defects in the hydrolysate structure. During the organic combustion process, destruction of these defects releases stored energy, which assists in overcoming the activation energy barrier to nucleation of a crystalline phase.77 These defects would also exist in the other modified hydrolysates. As the quantities of residual butanoate groups are ∼50% greater than those of the remaining acetate and propanoate groups, the net effect in butyric hydrolysates is to decrease the energy required to induce crystallization and phase transformation. In acetate- and propanoate-modified hydrolysates, the energy released by defect destruction is more than offset by the disordering effects of the organic molecules. The rate and temperature at which organic pyrolysis occurs will also change the anatase f rutile transformation temperature of titania.23,24,70 Stoichiometric defects produced by localized Ti or O deficiencies may also reduce the energy required to induce crystallization and phase transformation.24 As discussed above, the modified hydrolysates do not consist of stoichiometric TiO2. The effect would again be most pronounced in butanoate-modified hydrolysates, as those materials are the least reactive of the systems studied. The differences between crystallization and anatase f rutile transformation temperatures range from e180 °C for unmodified hydrolysate to 51 °C for acetate- and propanoate-modified hydrolysates. Of the materials reported in the literature, the majority exhibit differences on the order of 300-400 °C. The smaller differences observed in the modified hydrolysates are attributed to retardation of the initial disordered/amorphous f anatase crystallization step by the residual carboxylate groups. In comparison with the properties of other titania materials reported in the literature, the crystallization and phase transition properties of the modified hydrolysates studied are unremarkable. The chief point of interest is that crystallization and phase transformation temperatures are influenced by the modifying carboxylate group. Of the four hydrolysates, the butanoate-modified species offers the lowest processing temperature for production of the rutile phase, while the acetate- and propanoate-modified hydrolysates require the highest processing temperatures. Surface Areas and Particle Sizes of Modified Titania Hydrolysates. The specific surface areas of the hydrolysates, as determined by the BET method, are summarized in Table 3. These values are comparable with those of many “conventional” TPT-derived titania materials reported in the literature, where the surface areas of such powders may be acetatemodified > propanoate-modified > butanoate-modified. Specific pore volumes follow a similar trend, with the exception that pore volumes are marginally higher for propanoate-modified hydrolysates than acetate-modified materials. These results must be viewed cautiously, given the preceding comments regarding the effects of residual carboxylate upon measured surface areas. This factor aside, the trends in BJH/BET ratios suggest that unmodified hydrolysates are more porous than their modified equivalents. The trends in particle size and porosity are a reflection of the reduction in reactivity imparted to the alkoxide by the modifying carboxylate ligands. Polycondensation occurs readily in unmodified systems, with condensation reactions rapidly forming networks of extensively crosslinked oxide material. Due to the rapid condensation, a very porous structure is formed.81 Hence, unmodified hydrolysates exhibit the largest BJH/BET ratios and the largest specific pore volume (Table 3). Hydrolysis and condensation occur at a slower rate in modified systems than in pure TPT, with the reactions of modified TPT producing smaller particles, at a reduced reaction rate. The slower interaggregate condensation in modified systems allows the oxide structures to arrange into configurations with denser packing and lower porosity. This effect is magnified as the chain length of the modifying acid increases and the reactivity of the modified alkoxides decreases. The DTA/TGA data also support this hypothesis, with unmodified hydrolysates exhibiting higher oxide contents than modified materials and with the quantities of residual carboxylate increasing and the oxide content decreasing with carboxylic acid chain length in the modified hydrolysates. This hypothesis is also consistent with the predictions of the partial charge model. Conclusions Physical and structural characteristics of hydrous titania precipitates prepared from tetraisopropoxy titanate chemically modified with short-chain carboxylic acids are reported for the first time. Hydrolysis of undiluted alkoxide with undiluted water, in a ratio of 50:1 water/titanium, produces solids which exhibit no evidence of crystallinity or long-range ordering, possessing only short-range, rutilelike ordering. With the experimental techniques employed (XRD and FT-Raman spectroscopy), no substantial differences are apparent between the structures of hydrolysates prepared from modified alkoxides and the structure of the hydrolysate prepared from unmodified alkoxide. The effects of carboxylic acid modification of the alkoxide are more pronounced for the other hydrolysate properties studied in this work. In all of the systems, nucleophilic (81) Bartlett, J. R.; Woolfrey, J. L. Chem. Mater. 1996, 8, 1167.

substitution and elimination of alkoxy groups is virtually complete. Charge separation induced by the modifying ligands ensures that the removal of carboxylates is not complete and does not occur to the same degree in the three systems. Elimination of carboxylic acid leaving groups is decreasingly favorable with longer chain length, but the modifying groups also become more difficult to protonate. The hydrophobicity of carboxylate groups increases with chain length, and this factor may also reduce the rate of nucleophilic attack by water upon carboxylate ligands. The net effect of charge distribution and hydrophobicity within the modified species is for the quantities of residual carboxylate to increase with the chain length of the modifying ligands. Despite the retention of carboxylate material, the yields of oxide in all systems are high and are comparable to or higher than those obtained for many alkoxide-derived oxide powders reported in the literature. Modification also influences the particle size, surface area, and porosity of the hydrolysate aggregates. The rate and extent of polycondensation decreases with carboxylic acid modification and the acid chain length of the acid. Consequently, unmodified hydrolysates have a larger aggregate size than that of the modified hydrolysates. The specific surface areas of the modified hydrolysates are reduced by the presence of residual carboxylate groups. Butanoate-modified hydrolysates therefore exhibit lower surface areas than those for any of the other materials studied here. Porosity decreases with carboxylic acid chain length, but these data may be incorrect due to the presence of carboxylate residues. Unreacted carboxylate groups have a significant influence upon the crystallization and phase-transformation properties of the hydrolysates. In the acetate- and propanoate-modified hydrolysates, unreacted carboxylate inhibits crystallization and phase transformation of the solids, elevating the temperatures at which these processes occur. In the butanoate-modified hydrolysates, carboxylate ligands are found in larger quantities, enhancing these processes and decreasing the crystallization and phasetransformation temperatures, to the extent that the anatase f rutile phase transformation occurs at lower temperatures in butyric hydrolysates than in unmodified materials. Acknowledgment. The Centre for Instrumental and Developmental Chemistry of the Queensland University of Technology is gratefully acknowledged for financial support for this project. The financial support of the Australian Institute of Nuclear Science and Engineering (AINSE) is gratefully acknowledged. J.R. Bartlett and J.L. Woolfrey are thanked for their assistance with the experimental work and their critical comments on this manuscript. LA990830U