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Microstructural changes of microemulsion-mediated TiO2 particles during calcination
June 2001
Materials Letters 49 Ž2001. 244–249 www.elsevier.comrlocatermatlet
Microstructural changes of microemulsion-mediated TiO 2 particles during calcination Eui Jung Kim a,) , Sung-Hong Hahn b a
Department of Chemical Engineering, UniÕersity of Ulsan, Ulsan 680-749, South Korea b Department of Physics, UniÕersity of Ulsan, Ulsan 680-749, South Korea Received 10 October 2000; accepted 27 October 2000
Abstract Ultrafine titania particles were synthesized by hydrolysis of titanium tetraisoproxide ŽTTIP. in the nanodroplets of waterrNP-5rcyclohexane microemulsions. TiO 2 particles did not form at a waterrsurfactant molar ratio below 0.53 since water was mostly bound to the surfactant molecules. The particles grew largely by intra-agglomerate densification below 7008C, whereas, they grew by inter-agglomerate densification above 7008C. With raising calcination temperature, the specific surface area of the TiO 2 particles decreased rapidly, whereas, their average pore radius increased considerably due to shrinkage and densification of the agglomerates, and elimination of the minute intercrystallite pores. q 2001 Elsevier Science B.V. All rights reserved. PACS: 81.20.F Keywords: Microemulsion; Titania; Nanoparticles; Alkoxide; Microstructure; Calcination
1. Introduction TiO 2 powders possess excellent optical, dielectric, and catalytic properties resulting in industrial applications as pigments, fillers, opacifiers, and photocatalysts w1–3x. The most common procedures for the preparation of fine titania powders reported in the literature are based on the hydrolysis of titanium salts or alkoxides w4–6x. The growing scientific interest in nanosized particles with grain sizes below 100 nm has produced new synthesis methods that are capable of precise ) Corresponding author. Tel.: q82-52-259-2832; fax: q82-52259-1689. E-mail address: [email protected] ŽE.J. Kim..
control of particles to the molecular level. Various chemical methods have been proposed to prepare nanosized particles with a narrow and controllable size distribution w7–10x. Water-in-oil ŽWrO. microemulsions have been successfully employed to obtain ultrafine particles of controlled size of a variety of materials including metals, oxides, and organic polymers w11–14x. A polyoxyethylene nonylphenyl ether with an average of five oxyethylene groups per molecule ŽNP-5. was employed as a non-ionic surfactant in this work. TiO 2 nanoparticles were prepared by hydrolysis of titanium tetraisopropoxide ŽTTIP. in WrO microemulsions consisting of water, NP-5, and cyclohexane. The TiO 2 particles synthesized were characterized using FTIR, XRD, SEM, and BET
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 3 8 2 - 7
E.J. Kim, S.-H. Hahn r Materials Letters 49 (2001) 244–249
techniques. The present paper reports primarily the effect of calcination on the microstructures of the TiO 2 particles prepared in WrO microemulsions. The effect of waterrsurfactant molar ratio on the hydrolysis and condensation reactions of TTIP will also be discussed.
2. Experimental Titanium tetraisopropoxide ŽTTIP, Kanto Chemical. was used as a precursor to fabricate TiO 2 nanoparticles in reverse microemulsions. Cyclohexane ŽOriental Chemical Industries. was used as an organic solvent. NP-5 as the non-ionic surfactant was purchased from Aldrich Chemical. Isopropanol ŽOriental Chemical Industries. was used as a diluent for TTIP. All reagents were used as received without further purification. Water used in the experiments was doubly distilled and deionized. Reverse microemulsion solution was prepared by dissolving NP-5 in cyclohexane and by adding a required amount of distilled water. The water-clear appearance of the solution indicated the formation of the microemulsion. The waterrsurfactant molar ratio Ž R . was varied in the range of 0.25–6.67. TTIP was diluted in isopropanol prior to mixing with the reverse microemulsion solution to moderate the high reactivity. The hydrolysis of TTIP was carried out in a jacketed Erlenmeyer flask Ž150 ml. that was maintained at 258C by circulating a coolant with a Lauda RC20B chiller. The reaction was initiated by injecting the TTIP solution into the reverse microemulsion with stirring in nitrogen. This led to the precipitation of the TiO 2 nanoparticles within the aqueous cores of the microemulsions. In this work, the concentrations of TTIP and water were fixed at 0.04 M and 0.2 M, respectively, and the concentration of surfactant was varied to obtain a desired waterrsurfactant molar ratio. The TiO 2 particles precipitated were separated in a Hanil HA-500 centrifuge at 10,000 rpm for 10 min and were then washed with ethanol and acetone consecutively to remove organics and surfactants from the particles. The particles were then dried at 258C for about 20 h. Dried particles were calcined in a Lindberg 55342-4 furnace at temperatures above 1008C.
245
XRD patterns were obtained with a Philips PW3710 diffractometer using Cu K a radiation at 30 kV and 20 mA. The SEM photographs were taken using a Hitachi S-4200 field effect scanning electron microscope. The chemical structure of the particles was examined using an ATI Mattson Genesis Series Fourier transform infrared ŽFTIR. spectrophotometer. The specific surface area of the precipitates was measured with a Quantachrome Autosorb-1 nitrogen adsorption apparatus.
3. Results and discussion At low waterrsurfactant molar ratios Ž R . and waterrTTIP molar ratios Ž h. Že.g., R s 0.53 and h s 1., the reaction solution remained clear for 5 h after the initiation of the reaction indicating that no titania particles formed. However, at relatively high R and h Že.g., R s 2.5 and h s 5., the reaction solution became turbid, immediately after the TTIP injection implying that the formation of particles favorably occurred. If R is low, water is mostly bound to the surfactant molecules, and the mobility of the OHy ions is reduced. In addition, the interpenetrated surfactant structure of the reverse micelle at low R is such that water molecules are effectively shielded by the oxyethylene chains, which may restrict the access of TTIP to the polar domain. Accordingly, both hydrolysis and condensation are, in principle, inhibited. In addition, the number of TTIP molecules per surfactant aggregate is low, thus, nucleation is considered difficult in each reverse micelle. On the other hand, when R is relatively high, the number of free water molecules is relatively high. In this case, both hydrolysis and condensation are favored and the probability of nucleation is higher w15x. Unless stated otherwise, R and h are kept at 2.5 and 5, respectively, in the figures hereafter. Fig. 1 shows FTIR spectra for the TiO 2 particles prepared at various waterrsurfactant molar ratios Ž R . ranging from 0.53 to 6.67. Here, the waterrTTIP molar ratio Ž h. is maintained at 5. The absorption peaks appearing near 1620 and 3400 cmy1 relate to an O–H group. The peaks appearing at 2900 cmy1 and 900–1300 cmy1 are due to organic bonds. Ti`O`Ti bonds appeared in the range of 400–600 cmy1 as the result of condensation reactions. As R
246
E.J. Kim, S.-H. Hahn r Materials Letters 49 (2001) 244–249
Fig. 1. FTIR spectra for TiO 2 particles prepared at various waterrsurfactant ratios Ž R ..
was decreased, the intensities of absorption peaks due to O–H group near 1620 cmy1 reduced, whereas those of absorption peaks due to organics near 900–
1300 cmy1 increased. At a fixed water concentration, the amount of surfactant increases with decreasing with R, thus, resulting in an increase in the
Fig. 2. XRD spectra for TiO 2 particles calcined at various temperatures.
E.J. Kim, S.-H. Hahn r Materials Letters 49 (2001) 244–249 Table 1 XRD results for TiO 2 particles calcined at various temperatures for 2 h Calcination temperature Ž8C.
Crystallite size Žnm. Anatase Rutile
500 600 650 700 900
11.7 15.8 17.5 – –
– – 18.3 20.0 24.4
absorption peaks due to organic bonds. It is considered that under low R conditions, hydrolysis and condensation are relatively slow and the peaks appear at 1100 cmy1 due to unreacted alkoxide groups. Fig. 2 illustrates the XRD patterns of the particles calcined for 2 h in air at different temperatures. The
247
XRD data of the particles calcined up to 3008C indicate that they are amorphous. The particles calcined at 5008C were identified as nanocrystalline anatase. Upon increasing the temperature to 6508C, the rutile peaks appeared. This indicates that the transition from anatase to rutile has taken place. At 7008C, all the anatase peaks disappeared. If the temperature increased further to 9008C, the intensities of rutile peaks increased. The crystallite size of the particles can be determined by Scherrer’s equation w16x and is listed in Table 1. One can see that the crystallite size of the anatase phase was increased 1.5 times from 11.7 to 15.8 nm as the calcination temperature increased from 5008C to 6008C. The size of the rutile crystallites calcined at 7008C was 20.5 nm and increased to 24.4 nm at 9008C. SEM micrographs of the as-prepared and calcined TiO 2 nanoparticles are shown in Fig. 3. The calcined
Fig. 3. SEM micrographs of TiO 2 particles: Ža. as-prepared; Žb. at 5008C; Žc. 7008C; Žd. 9008C.
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E.J. Kim, S.-H. Hahn r Materials Letters 49 (2001) 244–249
powders were obtained by sintering the as-prepared TiO 2 particles at each temperature in the range of 500–9008C for 2 h in air. As shown in Fig. 3a, the as-prepared powders are composed of the secondary particles of about 30 nm in size. Here, the primary particles comprising the secondary particles cannot be identified. Fig. 3b shows that the secondary particles have been densified at 5008C and reduced in size due to shrinkage of the agglomerates and elimination of the intercrystallite pores. It is hard to identify in Fig. 3b the primary particles since grain growth may insufficiently occur at a temperature of 5008C. As the calcination temperature was increased to 7008C, the crystalline primary particles were observed clearly in the agglomerated secondary particles whose size became about 90–100 nm, resulting from inter-agglomerate densification. It is interesting to note in Fig. 3d that the size of the secondary particles increased considerably to 300–400 nm at a temperature of 9008C. Furthermore, the secondary particles became highly densified and non-spherical. However, the size of the primary particles increased just slightly relative to 7008C, which was confirmed by the XRD results. The SEM and XRD results indicate that the crystals of TiO 2 particles prepared in WrO microemulsions grew largely by intra-agglomerate densification below 7008C, whereas they
grew by inter-agglomerate densification above 7008C. It is notable in Fig. 3d that the primary particle identity still remained in the agglomerates at 9008C. The specific surface area and average pore radius of the titania particles are plotted in Fig. 4 as a function of calcination temperature. The average pore radius was roughly estimated by assuming cylindrical pore geometry w17x. It can be seen in Fig. 4 that the specific surface area decreased rapidly from 325.6 to 5.9 m2rg as the temperature was increased from 3008C to 7008C. The pore volume was also found to decrease appreciably from 2.25 = 10y7 to 7.4 = 10y8 m3rg with increasing temperature from 3008C to 7008C. The appreciable drop in pore volume with calcination temperature is attributable to elimination of the inter-agglomerate pores leading to an increase in the size of the secondary particles. The average pore radius, however, increased considerably from 1.4 to 25.1 nm by a factor of 20. Under calination at higher temperatures, the surface area becomes largely external to the agglomerates since the small pores inside the agglomerates have higher sintering rates w18x. An increase in calcination temperature facilitates shrinkage and densification of the agglomerates, and destruction of the intercrystallite pores. As a result, large dense secondary particles are formed at 9008C as illustrated above in Fig. 3d.
Fig. 4. Effect of calcination on the specific surface area and average pore radius of TiO 2 particles.
E.J. Kim, S.-H. Hahn r Materials Letters 49 (2001) 244–249
4. Conclusions TiO 2 nanoparticles were synthesized by hydrolysis of titanium tetraisoproxide ŽTTIP. in the aqueous cores of waterrNP-5rcyclohexane microemulsions. TiO 2 particles did not form at a waterrsurfactant molar ratio Ž R . below 0.53 since water is mostly bound to the surfactant molecules. When R is higher than 2.5, TiO 2 particles formed immediately after the TTIP injection since the number of free water molecules is relatively high. The crystallite size increased about twice from 11.7 to 24.4 nm with increasing temperature from 5008C to 9008C. The secondary particles, agglomerates of the primary particles, were 20–30 nm in size at 5008C and increased markedly by a factor of 10 at 9008C. The TiO 2 particles grew largely by intra-agglomerate densification below 7008C, whereas they grew by inter-agglomerate densification above 7008C. With increasing calcination temperature from 3008C to 7008C, the specific surface area of the TiO 2 particles decreased from 325.6 to 5.9 m2rg, whereas the average pore radius increased from 1.4 to 25.1 nm due to shrinkage and densification of the agglomerates, and elimination of the minute intercrystallite pores.
Acknowledgements The authors thank the University of Ulsan for financial support made to the program year of 1999.
249
References w1x A.K. Atta, P.K. Biswas, D. Ganguli, Mater. Lett. 15 Ž1992. 99. w2x Y.C. Yeh, T.Y. Tseng, D.A. Chang, J. Am. Ceram. Soc. 73 Ž1990. 1992. w3x A. Yasumori, K. Yamazaki, S. Shibata, M. Yamane, J. Ceram. Soc. Jpn. 102 Ž1994. 702. w4x H.K. Park, D.K. Kim, C.H. Kim, J. Am. Ceram. Soc. 80 Ž1997. 743. w5x J.L. Look, C.F. Zukoski, J. Am. Ceram. Soc. 75 Ž1992. 1587. w6x D. Vorkapic, T. Matsoukas, J. Am. Ceram. Soc. 81 Ž1998. 2815. w7x E. Matijevic, M. Budnic, L. Meites, J. Colloid Interface Sci. 6 Ž1977. 302. w8x E.A. Barringer, H.K. Bowen, Langmuir 1 Ž1985. 414. w9x J.H. Jean, T.A. Ring, Colloids Surf. 29 Ž1988. 273. w10x Y.T. Moon, H.K. Park, D.K. Kim, I.S. Seog, C.H. Kim, J. Am. Ceram. Soc. 78 Ž1995. 2690. w11x M.A. Lopez-Quintela, J. Rivas, J. Colloid Interface Sci. 158 ´ Ž1993. 446. w12x H. Herrig, R. Hempelmann, Mater. Lett. 27 Ž1996. 287. w13x H. Herrig, R. Hempelmann, Nanostruct. Mater. 9 Ž1997. 241. w14x F. Candau, Macromol. Symp. 92 Ž1995. 169. w15x F.J. Arriagada, K. Osseo-Asare, J. Colloid Interface Sci. 211 Ž1999. 210. w16x B.D. Cullity, Elements of X-Ray Diffraction, 2nd edn., Addison-Wesley, Reading, MA, 1978, 102. w17x J.J. Carberry, Chemical and Catalytic Reaction Engineering, McGraw-Hill, New York, 1976, 376. w18x W. Wagner, R.S. Averback, H. Hahn, W. Petry, A. Wiedenmann, J. Mater. Res. 6 Ž1991. 2193.
Materials Letters 49 Ž2001. 244–249 www.elsevier.comrlocatermatlet
Microstructural changes of microemulsion-mediated TiO 2 particles during calcination Eui Jung Kim a,) , Sung-Hong Hahn b a
Department of Chemical Engineering, UniÕersity of Ulsan, Ulsan 680-749, South Korea b Department of Physics, UniÕersity of Ulsan, Ulsan 680-749, South Korea Received 10 October 2000; accepted 27 October 2000
Abstract Ultrafine titania particles were synthesized by hydrolysis of titanium tetraisoproxide ŽTTIP. in the nanodroplets of waterrNP-5rcyclohexane microemulsions. TiO 2 particles did not form at a waterrsurfactant molar ratio below 0.53 since water was mostly bound to the surfactant molecules. The particles grew largely by intra-agglomerate densification below 7008C, whereas, they grew by inter-agglomerate densification above 7008C. With raising calcination temperature, the specific surface area of the TiO 2 particles decreased rapidly, whereas, their average pore radius increased considerably due to shrinkage and densification of the agglomerates, and elimination of the minute intercrystallite pores. q 2001 Elsevier Science B.V. All rights reserved. PACS: 81.20.F Keywords: Microemulsion; Titania; Nanoparticles; Alkoxide; Microstructure; Calcination
1. Introduction TiO 2 powders possess excellent optical, dielectric, and catalytic properties resulting in industrial applications as pigments, fillers, opacifiers, and photocatalysts w1–3x. The most common procedures for the preparation of fine titania powders reported in the literature are based on the hydrolysis of titanium salts or alkoxides w4–6x. The growing scientific interest in nanosized particles with grain sizes below 100 nm has produced new synthesis methods that are capable of precise ) Corresponding author. Tel.: q82-52-259-2832; fax: q82-52259-1689. E-mail address: [email protected] ŽE.J. Kim..
control of particles to the molecular level. Various chemical methods have been proposed to prepare nanosized particles with a narrow and controllable size distribution w7–10x. Water-in-oil ŽWrO. microemulsions have been successfully employed to obtain ultrafine particles of controlled size of a variety of materials including metals, oxides, and organic polymers w11–14x. A polyoxyethylene nonylphenyl ether with an average of five oxyethylene groups per molecule ŽNP-5. was employed as a non-ionic surfactant in this work. TiO 2 nanoparticles were prepared by hydrolysis of titanium tetraisopropoxide ŽTTIP. in WrO microemulsions consisting of water, NP-5, and cyclohexane. The TiO 2 particles synthesized were characterized using FTIR, XRD, SEM, and BET
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 3 8 2 - 7
E.J. Kim, S.-H. Hahn r Materials Letters 49 (2001) 244–249
techniques. The present paper reports primarily the effect of calcination on the microstructures of the TiO 2 particles prepared in WrO microemulsions. The effect of waterrsurfactant molar ratio on the hydrolysis and condensation reactions of TTIP will also be discussed.
2. Experimental Titanium tetraisopropoxide ŽTTIP, Kanto Chemical. was used as a precursor to fabricate TiO 2 nanoparticles in reverse microemulsions. Cyclohexane ŽOriental Chemical Industries. was used as an organic solvent. NP-5 as the non-ionic surfactant was purchased from Aldrich Chemical. Isopropanol ŽOriental Chemical Industries. was used as a diluent for TTIP. All reagents were used as received without further purification. Water used in the experiments was doubly distilled and deionized. Reverse microemulsion solution was prepared by dissolving NP-5 in cyclohexane and by adding a required amount of distilled water. The water-clear appearance of the solution indicated the formation of the microemulsion. The waterrsurfactant molar ratio Ž R . was varied in the range of 0.25–6.67. TTIP was diluted in isopropanol prior to mixing with the reverse microemulsion solution to moderate the high reactivity. The hydrolysis of TTIP was carried out in a jacketed Erlenmeyer flask Ž150 ml. that was maintained at 258C by circulating a coolant with a Lauda RC20B chiller. The reaction was initiated by injecting the TTIP solution into the reverse microemulsion with stirring in nitrogen. This led to the precipitation of the TiO 2 nanoparticles within the aqueous cores of the microemulsions. In this work, the concentrations of TTIP and water were fixed at 0.04 M and 0.2 M, respectively, and the concentration of surfactant was varied to obtain a desired waterrsurfactant molar ratio. The TiO 2 particles precipitated were separated in a Hanil HA-500 centrifuge at 10,000 rpm for 10 min and were then washed with ethanol and acetone consecutively to remove organics and surfactants from the particles. The particles were then dried at 258C for about 20 h. Dried particles were calcined in a Lindberg 55342-4 furnace at temperatures above 1008C.
245
XRD patterns were obtained with a Philips PW3710 diffractometer using Cu K a radiation at 30 kV and 20 mA. The SEM photographs were taken using a Hitachi S-4200 field effect scanning electron microscope. The chemical structure of the particles was examined using an ATI Mattson Genesis Series Fourier transform infrared ŽFTIR. spectrophotometer. The specific surface area of the precipitates was measured with a Quantachrome Autosorb-1 nitrogen adsorption apparatus.
3. Results and discussion At low waterrsurfactant molar ratios Ž R . and waterrTTIP molar ratios Ž h. Že.g., R s 0.53 and h s 1., the reaction solution remained clear for 5 h after the initiation of the reaction indicating that no titania particles formed. However, at relatively high R and h Že.g., R s 2.5 and h s 5., the reaction solution became turbid, immediately after the TTIP injection implying that the formation of particles favorably occurred. If R is low, water is mostly bound to the surfactant molecules, and the mobility of the OHy ions is reduced. In addition, the interpenetrated surfactant structure of the reverse micelle at low R is such that water molecules are effectively shielded by the oxyethylene chains, which may restrict the access of TTIP to the polar domain. Accordingly, both hydrolysis and condensation are, in principle, inhibited. In addition, the number of TTIP molecules per surfactant aggregate is low, thus, nucleation is considered difficult in each reverse micelle. On the other hand, when R is relatively high, the number of free water molecules is relatively high. In this case, both hydrolysis and condensation are favored and the probability of nucleation is higher w15x. Unless stated otherwise, R and h are kept at 2.5 and 5, respectively, in the figures hereafter. Fig. 1 shows FTIR spectra for the TiO 2 particles prepared at various waterrsurfactant molar ratios Ž R . ranging from 0.53 to 6.67. Here, the waterrTTIP molar ratio Ž h. is maintained at 5. The absorption peaks appearing near 1620 and 3400 cmy1 relate to an O–H group. The peaks appearing at 2900 cmy1 and 900–1300 cmy1 are due to organic bonds. Ti`O`Ti bonds appeared in the range of 400–600 cmy1 as the result of condensation reactions. As R
246
E.J. Kim, S.-H. Hahn r Materials Letters 49 (2001) 244–249
Fig. 1. FTIR spectra for TiO 2 particles prepared at various waterrsurfactant ratios Ž R ..
was decreased, the intensities of absorption peaks due to O–H group near 1620 cmy1 reduced, whereas those of absorption peaks due to organics near 900–
1300 cmy1 increased. At a fixed water concentration, the amount of surfactant increases with decreasing with R, thus, resulting in an increase in the
Fig. 2. XRD spectra for TiO 2 particles calcined at various temperatures.
E.J. Kim, S.-H. Hahn r Materials Letters 49 (2001) 244–249 Table 1 XRD results for TiO 2 particles calcined at various temperatures for 2 h Calcination temperature Ž8C.
Crystallite size Žnm. Anatase Rutile
500 600 650 700 900
11.7 15.8 17.5 – –
– – 18.3 20.0 24.4
absorption peaks due to organic bonds. It is considered that under low R conditions, hydrolysis and condensation are relatively slow and the peaks appear at 1100 cmy1 due to unreacted alkoxide groups. Fig. 2 illustrates the XRD patterns of the particles calcined for 2 h in air at different temperatures. The
247
XRD data of the particles calcined up to 3008C indicate that they are amorphous. The particles calcined at 5008C were identified as nanocrystalline anatase. Upon increasing the temperature to 6508C, the rutile peaks appeared. This indicates that the transition from anatase to rutile has taken place. At 7008C, all the anatase peaks disappeared. If the temperature increased further to 9008C, the intensities of rutile peaks increased. The crystallite size of the particles can be determined by Scherrer’s equation w16x and is listed in Table 1. One can see that the crystallite size of the anatase phase was increased 1.5 times from 11.7 to 15.8 nm as the calcination temperature increased from 5008C to 6008C. The size of the rutile crystallites calcined at 7008C was 20.5 nm and increased to 24.4 nm at 9008C. SEM micrographs of the as-prepared and calcined TiO 2 nanoparticles are shown in Fig. 3. The calcined
Fig. 3. SEM micrographs of TiO 2 particles: Ža. as-prepared; Žb. at 5008C; Žc. 7008C; Žd. 9008C.
248
E.J. Kim, S.-H. Hahn r Materials Letters 49 (2001) 244–249
powders were obtained by sintering the as-prepared TiO 2 particles at each temperature in the range of 500–9008C for 2 h in air. As shown in Fig. 3a, the as-prepared powders are composed of the secondary particles of about 30 nm in size. Here, the primary particles comprising the secondary particles cannot be identified. Fig. 3b shows that the secondary particles have been densified at 5008C and reduced in size due to shrinkage of the agglomerates and elimination of the intercrystallite pores. It is hard to identify in Fig. 3b the primary particles since grain growth may insufficiently occur at a temperature of 5008C. As the calcination temperature was increased to 7008C, the crystalline primary particles were observed clearly in the agglomerated secondary particles whose size became about 90–100 nm, resulting from inter-agglomerate densification. It is interesting to note in Fig. 3d that the size of the secondary particles increased considerably to 300–400 nm at a temperature of 9008C. Furthermore, the secondary particles became highly densified and non-spherical. However, the size of the primary particles increased just slightly relative to 7008C, which was confirmed by the XRD results. The SEM and XRD results indicate that the crystals of TiO 2 particles prepared in WrO microemulsions grew largely by intra-agglomerate densification below 7008C, whereas they
grew by inter-agglomerate densification above 7008C. It is notable in Fig. 3d that the primary particle identity still remained in the agglomerates at 9008C. The specific surface area and average pore radius of the titania particles are plotted in Fig. 4 as a function of calcination temperature. The average pore radius was roughly estimated by assuming cylindrical pore geometry w17x. It can be seen in Fig. 4 that the specific surface area decreased rapidly from 325.6 to 5.9 m2rg as the temperature was increased from 3008C to 7008C. The pore volume was also found to decrease appreciably from 2.25 = 10y7 to 7.4 = 10y8 m3rg with increasing temperature from 3008C to 7008C. The appreciable drop in pore volume with calcination temperature is attributable to elimination of the inter-agglomerate pores leading to an increase in the size of the secondary particles. The average pore radius, however, increased considerably from 1.4 to 25.1 nm by a factor of 20. Under calination at higher temperatures, the surface area becomes largely external to the agglomerates since the small pores inside the agglomerates have higher sintering rates w18x. An increase in calcination temperature facilitates shrinkage and densification of the agglomerates, and destruction of the intercrystallite pores. As a result, large dense secondary particles are formed at 9008C as illustrated above in Fig. 3d.
Fig. 4. Effect of calcination on the specific surface area and average pore radius of TiO 2 particles.
E.J. Kim, S.-H. Hahn r Materials Letters 49 (2001) 244–249
4. Conclusions TiO 2 nanoparticles were synthesized by hydrolysis of titanium tetraisoproxide ŽTTIP. in the aqueous cores of waterrNP-5rcyclohexane microemulsions. TiO 2 particles did not form at a waterrsurfactant molar ratio Ž R . below 0.53 since water is mostly bound to the surfactant molecules. When R is higher than 2.5, TiO 2 particles formed immediately after the TTIP injection since the number of free water molecules is relatively high. The crystallite size increased about twice from 11.7 to 24.4 nm with increasing temperature from 5008C to 9008C. The secondary particles, agglomerates of the primary particles, were 20–30 nm in size at 5008C and increased markedly by a factor of 10 at 9008C. The TiO 2 particles grew largely by intra-agglomerate densification below 7008C, whereas they grew by inter-agglomerate densification above 7008C. With increasing calcination temperature from 3008C to 7008C, the specific surface area of the TiO 2 particles decreased from 325.6 to 5.9 m2rg, whereas the average pore radius increased from 1.4 to 25.1 nm due to shrinkage and densification of the agglomerates, and elimination of the minute intercrystallite pores.
Acknowledgements The authors thank the University of Ulsan for financial support made to the program year of 1999.
249
References w1x A.K. Atta, P.K. Biswas, D. Ganguli, Mater. Lett. 15 Ž1992. 99. w2x Y.C. Yeh, T.Y. Tseng, D.A. Chang, J. Am. Ceram. Soc. 73 Ž1990. 1992. w3x A. Yasumori, K. Yamazaki, S. Shibata, M. Yamane, J. Ceram. Soc. Jpn. 102 Ž1994. 702. w4x H.K. Park, D.K. Kim, C.H. Kim, J. Am. Ceram. Soc. 80 Ž1997. 743. w5x J.L. Look, C.F. Zukoski, J. Am. Ceram. Soc. 75 Ž1992. 1587. w6x D. Vorkapic, T. Matsoukas, J. Am. Ceram. Soc. 81 Ž1998. 2815. w7x E. Matijevic, M. Budnic, L. Meites, J. Colloid Interface Sci. 6 Ž1977. 302. w8x E.A. Barringer, H.K. Bowen, Langmuir 1 Ž1985. 414. w9x J.H. Jean, T.A. Ring, Colloids Surf. 29 Ž1988. 273. w10x Y.T. Moon, H.K. Park, D.K. Kim, I.S. Seog, C.H. Kim, J. Am. Ceram. Soc. 78 Ž1995. 2690. w11x M.A. Lopez-Quintela, J. Rivas, J. Colloid Interface Sci. 158 ´ Ž1993. 446. w12x H. Herrig, R. Hempelmann, Mater. Lett. 27 Ž1996. 287. w13x H. Herrig, R. Hempelmann, Nanostruct. Mater. 9 Ž1997. 241. w14x F. Candau, Macromol. Symp. 92 Ž1995. 169. w15x F.J. Arriagada, K. Osseo-Asare, J. Colloid Interface Sci. 211 Ž1999. 210. w16x B.D. Cullity, Elements of X-Ray Diffraction, 2nd edn., Addison-Wesley, Reading, MA, 1978, 102. w17x J.J. Carberry, Chemical and Catalytic Reaction Engineering, McGraw-Hill, New York, 1976, 376. w18x W. Wagner, R.S. Averback, H. Hahn, W. Petry, A. Wiedenmann, J. Mater. Res. 6 Ž1991. 2193.