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Preparation and characterization of silica-coated TiO2 nanoparticle
Colloids and Surfaces A: Physicochem. Eng. Aspects 257–258 (2005) 261–265
Preparation and characterization of silica-coated TiO2 nanoparticle Ok Kyung Park, Young Soo Kang∗ Department of Chemistry, Pukyong National University, 599-1 Daeyeon-3-dong, Nam-gu, Pusan 608-737, Korea Available online 19 November 2004
Abstract In recent years, TiO2 nanoparticle complex has been increasingly utilized in sun care products because of its UV-ray shielding performance and chemical stability. TiO2 nanoparticles from Degussa (P-25) were used as a seed for SiO2 coating using TEOS. A SiO2 -coated TiO2 nanoparticle was prepared by a sol–gel process. The high-resolution transmission electron microscope (HR-TEM), FT-IR, energy dispersion spectrum (EDS) and X-ray photoelectron spectroscopy (XPS) analyses showed that the surface of TiO2 particles was evenly coated by amorphous SiO2 layer with 10–15 nm thickness. Coated particles were measured with X-ray powder diffraction (XRD) to know the heat treatment effect on crystal structure of SiO2 layer and TiO2 seed. © 2004 Elsevier B.V. All rights reserved. Keywords: Sol–gel process; TiO2 particles; Silica coating layer
1. Introduction Fine TiO2 particles have been widely used as a white pigment in the paint, plastic and paper industries, and as absorber of UV-ray in cosmetic industry. Also, TiO2 particles are easily pulverized and the organic molecules are easily oxidized due to photodegradation by photocatalytic property of TiO2 particles, especially when they are exposed under ultraviolet, or sunlight [1]. Generally, the TiO2 particles with 100–200 nm diameter generate whitening effect, because of scattering. On the other hand, the smallest TiO2 particle with below 100 nm have transparency and maximum interception of UV light. Therefore, the crystal size of TiO2 has to be reduced below 100 nm to decrease whitening effect and to increase transparency for using as a fundamental additive material of cosmetics. In addition, to prevent a reaction with organic compounds, TiO2 surface should be covered with stable oxide layer [2]. Coating layer captures radicals to prevent oxidization of organic molecules. The hydrated oxide groups of the coating components are able to capture hydroxyl radicals produced by the electron hole pair migrating into the coating layer, thereby stabilize the TiO2 crystal [3]. Hydrous
∗
Corresponding author. Tel.: +82 51 6206379; fax: +82 51 6288147. E-mail address: [email protected] (Y.S. Kang).
0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.10.014
silica and hydrous alumina are the most commonly used coating materials of inorganic coating of TiO2 particle surface [4,5]. Hydrous silica coating can improve both the pigment weather durability and the dispersion properties. And TiO2 particles covered with zinc oxide has UV-B ray protection by absorbing. The coating of stable oxide layers has been mainly done by spraying of metal oxide precursor solution on the surface of solid particles. Unfortunately, this process results in many defects on the surface of TiO2 particles [2]. In the present paper, we introduce a new coating method of SiO2 on the TiO2 surface using phase transfer reaction between organic chloroform and aqueous water phases. Finally, evenly SiO2 -coated TiO2 nanoparticles were characterized on the structure and morphology with X-ray powder diffraction (XRD), TEM, energy dispersion spectrum (EDS), FT-IR and X-ray photoelectron spectroscopy (XPS).
2. Experimental 2.1. Methods Tetraethylorthosilicate (TEOS: Si(OC2 H5 )4 , 98%) was obtained from Aldrich. TiO2 particles were obtained from Degussa (P-25). TiO2 , water and chloroform were mixed at room temperature. TEOS and ethanol were mixed at room
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Table 1 The element compositions of SiO2 coated TiO2 nanoparticles from EDS analysis (atomic wt.%) Element atoms
The molar ratio of TiO2 to TEOS 0:1
0.2:0.8
0.4:0.6
0.5:0.5
0.6:0.4
O Ti Si
65.40 34.60 0
64.45 30.46 5.09
60.08 27.70 12.22
61.45 27.19 14.36
67.49 22.24 10.27
temperature. Two mixtures were dissolved in acetone. The molar ratio of acetone to ethanol in solution was 1:1 and chloroform was used excess as solvent and the amount of chloroform critically affected the coating time. The contents of TiO2 to TEOS were prepared as molar ratio of 0/1, 2/8, 4/6, 5/5, 6/4 and 8/2. Then mixtures were stirred at room temperature for 2 h. Following, the solvent was evaporated. The product was dried under the reduced pressure in vacuum oven and sintered at 500–1100 ◦ C as increasing temperature by 100 ◦ C interval. This is synthetic route of silica-coated TiO2 nanoparticles. 2.2. Sample characterization The products were characterized with HR-TEM and EDS to know about the morphology and composition. HR-TEM image was obtained using a Hitachi model S-2400 and a Jeol model JEM-2010. EDS was obtained using a Hitachi model, H-7500. The interfacial chemical bonding structure is checked with FT-IR. FT-IR spectra were recorded on a Perkin-Elmer Specrum 2000. Coated particles were measured with XRD to confirm the coating of silica layer on TiO2 particle. XRD spectra were collected using a Philips, X’PertMPD system. The chemical bonding structure between coating layer and the TiO2 particle surface are determined with XPS. The XPS study was performed with a VG-Scientific ESCALAB 250 spectrometer with monocromatized Al K␣ X-ray source. Fig. 1. HR-TEM images of (a) pure TiO2 (Degussa P-25) and (b) silicacoated TiO2 .
3. Results and discussion Fig. 1 shows the HR-TEM image of the silica coated TiO2 nanoparticles. The comparison of two pictures shows that TiO2 particles surface was surround by amorphous silica layer. The average diameter size of silica-coated TiO2 nanoparticles was determined as 20–50 nm. The thickness of the amorphous layer is estimated as about 10–15 nm. The atomic weight composition of TiO2 particles can be qualitatively determined by EDS, and it is shown in Fig. 2. By comparing between the EDS analysis and TEM image, the presence of silicon in the coated SiO2 layer of the surface of TiO2 particles is identified. The relative compositions on the coated TiO2 particle expressed quantitatively as atomic weight percentage (Table 1). It is concluded that amorphous layer is determined as silica coated on the surface of TiO2 par-
ticles. Fig. 3 shows that FT-IR spectra of pure and SiO2 coated TiO2 particles. In the comparison between the two spectra, an absorption peak of Ti O, Si O and Ti O Si was observed at 600–900, 1000–1200, 950 cm−1 , respectively [6,7]. The O 1s spectra of pure and SiO2 coated TiO2 particles are shown in Fig. 4. The binding energies of O 1s peaks for pure TiO2 particles are 527.6 and 529.9 eV. The intensity of peak at 527.6 eV is much higher than that at 529.9 eV. For the silicacoated TiO2 particles, the intensity of peak at 530.3 eV is higher than that at 528.1 eV in O 1s binding energy. The higher peak intensity at 530.3 eV is attributable to the silica coating on the particle surface. It indicates that the peak at 528.1 eV of O 1s in SiO2 coated TiO2 particles was resulted from a chemical shift of peak at 527.6 eV of O 1s in pure TiO2 particles. It is concluded that Ti O Si bond is formed
O.K. Park, Y.S. Kang / Colloids and Surfaces A: Physicochem. Eng. Aspects 257–258 (2005) 261–265
263
Fig. 2. EDS spectra of (a) pure TiO2 (Degussa P-25) and (b) silica-coated TiO2 .
on the surface of TiO2 particles. Since the electronegativity of Si is greater than that of Ti, O 1s peak of TiO2 particles has a chemical shift of about +0.5 eV compared to SiO2 coated TiO2 particles. Ti 2p spectra of pure and SiO2 coated TiO2
particles are shown in Fig. 5. The binding energy of Ti 2p peak of pure TiO2 particles is determined as 456.2 eV. Ti 2p peak for SiO2 coated TiO2 is 456.6 eV, +0.4 eV larger than the Ti 2p peak of pure TiO2 particles. It also indicates the
Fig. 3. FT-IR spectra of (a) pure TiO2 (Degussa P-25) and (b) silica-coated TiO2 .
Fig. 4. O 1s XPS spectra of samples (a) TiO2 and (b) silica-coated TiO2 nanoparticles.
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Fig. 5. Ti 2p XPS spectra of samples (a) TiO2 and (b) silica-coated TiO2 nanoparticles.
Ti O Si bond formation [8]. The decrease of the electron density around Ti atom results from the greater electronegativity of Si via O acting on Ti. So the shielding effect is lessened, and then the binding energy is increased. From the chemical shift of Ti 2p, O 1s peaks and FT-IR spectra, it can be concluded that silica is coated on the surface of TiO2 particles through chemical bond. Ti O Si bond forms at the interface of silica coating layer or TiO2 particles surface. In order to confirm the coating of silica layer on TiO2 particle, produced particles were annealed by increasing temperature. Fig. 6 shows that XRD patterns of silica- coated TiO2 sintered at (a) 500 ◦ C and (b) 1100 ◦ C. Seed particle of TiO2 had phase transition from anatase phase to rutile phase at above 900 ◦ C. However coated SiO2 layer did not show a phase transition with heating even at 1100 ◦ C. Fig. 7 shows XRD patterns of sintered SiO2 coated TiO2 particle at 800 ◦ C versus increasing sintering time from 1 to 11 h. This was done to know change of the phase structure of the amorphous SiO2 layer at same temperature with different sintered time. There was no phase transition of SiO2 layer and TiO2 nanoparticles with increasing sintering time. Amorphous silica layer act as a barrier which prevent TiO2 particles from heat [9,10]. Consequently, it shows that the surface of TiO2 covered uniformly by the SiO2 layer and the phase transition temperature of silica-coated TiO2 particle is higher than the phase transition temperature of pure TiO2 particles [11–13]. The increased phase transition temperature of SiO2 coated TiO2 is also attributed to possible dissolution of SiO2 in TiO2 particles and compact SiO2 layer formation. However, heating at above 1300 ◦ C, breakage of Ti O Si bonds resulting in the formation of crystallization of SiO2 [14]. Therefore, SiO2 cannot dissolve in TiO2 particle. So SiO2 coated TiO2 particle is well sintered enough at 800 ◦ C to stabilize amorphous silica layer on TiO2 surface.
Fig. 6. XRD patterns of silica-coated TiO2 sintered at (a) 500 ◦ C and (b) 1100 ◦ C. A and R represent anatase and rutile phases, respectively.
Fig. 7. XRD patterns of sintered silica-coated TiO2 for different sintering times at 800 ◦ C. A and R represent anatase and rutile phases, respectively.
O.K. Park, Y.S. Kang / Colloids and Surfaces A: Physicochem. Eng. Aspects 257–258 (2005) 261–265
4. Conclusion HR-TEM images of TiO2 particles show that TiO2 is evenly coated by amorphous SiO2 layer. As a result of the EDS analysis, amorphous layer is determined as silica. And XRD result shows that there are no phase transition of SiO2 layer with increasing heat-treatment temperature and time. Therefore, the HR-TEM, FT-IR, XPS, and EDS results show that SiO2 is evenly coated on the surface of TiO2 particles and results in the chemical bonding of Ti O Si. The average diameter size of silica-coated TiO2 was determined as 20–50 nm. And the SiO2 coating layer thickness is determined about 10–15 nm. By TEM image and EDS data, optimum molar ratio of Si:Ti is determined as 4:6 and coated particle is sintered well at 800 ◦ C because TiO2 has phase transition at above 900 ◦ C and amorphous silica layer was crystallized at above 1300 ◦ C.
Acknowledgment This work was financially supported by Korean Ministry of Science and Technology as Specific Research and De-
265
velopment Program on New Functional Chemical Materials, 2003. References [1] Y.L. Lin, T.J. Wang, Y. Jin, Powder Technol. 123 (2002) 194. [2] Y. Atou, H. Suzuki, Y. Kimura, T. Sato, T. Tanigaki, Y. Saito, C. Kaito, Physica E 16 (2003) 179. [3] European Patent Application. 91,301,183.9. [4] R.L. Decolibus, US Patent 3,928,057 (1975). [5] H.W. Jacobson, US Patent 4,416,699 (1983). [6] M.-S. Lee, G.-D. Lee, S.-S. Hong, J. Ind. Eng. Chem. 9 (5) (2003) 556. [7] W.-I. Kim, I.-K. Hong, J. Ind. Eng. Chem. 9 (6) (2003) 728. [8] M. Ramirez-Del-Solar, N. de la Rosa-Fox, L. Esquivas, J. Zarzycki, J. Non-cryst. Solids 121 (1990) 84. [9] W.-H. Cho, D.-J. Kang, S.-G. Kim, J. Mater. Sci. 38 (2003) 2619. [10] M. Machida, K. Norimoto, T. Watanabe, J. Mater. Sci. 34 (1999) 2569. [11] R.N. Viswanath, R. Vignesh, S. Ramasamy, Study of crystalline phase of TiO2 -SiO2 nanocomposite, in: Proceedings of the 59th Indian Ceramic Society Meeting, Anna University, Madras, India, 1996. [12] J.I. Murray, H.A. Wriedt, Bull. Alloy Phase Diagrams 8 (1987) 148. [13] C.N.R. Rao, Can. J. Chem. 39 (1961) 49. [14] R.N. Viswanath, S. Ramasamy, Colloids Surf. A: Physicochem. Eng. Aspects 133 (1998) 49.
Preparation and characterization of silica-coated TiO2 nanoparticle Ok Kyung Park, Young Soo Kang∗ Department of Chemistry, Pukyong National University, 599-1 Daeyeon-3-dong, Nam-gu, Pusan 608-737, Korea Available online 19 November 2004
Abstract In recent years, TiO2 nanoparticle complex has been increasingly utilized in sun care products because of its UV-ray shielding performance and chemical stability. TiO2 nanoparticles from Degussa (P-25) were used as a seed for SiO2 coating using TEOS. A SiO2 -coated TiO2 nanoparticle was prepared by a sol–gel process. The high-resolution transmission electron microscope (HR-TEM), FT-IR, energy dispersion spectrum (EDS) and X-ray photoelectron spectroscopy (XPS) analyses showed that the surface of TiO2 particles was evenly coated by amorphous SiO2 layer with 10–15 nm thickness. Coated particles were measured with X-ray powder diffraction (XRD) to know the heat treatment effect on crystal structure of SiO2 layer and TiO2 seed. © 2004 Elsevier B.V. All rights reserved. Keywords: Sol–gel process; TiO2 particles; Silica coating layer
1. Introduction Fine TiO2 particles have been widely used as a white pigment in the paint, plastic and paper industries, and as absorber of UV-ray in cosmetic industry. Also, TiO2 particles are easily pulverized and the organic molecules are easily oxidized due to photodegradation by photocatalytic property of TiO2 particles, especially when they are exposed under ultraviolet, or sunlight [1]. Generally, the TiO2 particles with 100–200 nm diameter generate whitening effect, because of scattering. On the other hand, the smallest TiO2 particle with below 100 nm have transparency and maximum interception of UV light. Therefore, the crystal size of TiO2 has to be reduced below 100 nm to decrease whitening effect and to increase transparency for using as a fundamental additive material of cosmetics. In addition, to prevent a reaction with organic compounds, TiO2 surface should be covered with stable oxide layer [2]. Coating layer captures radicals to prevent oxidization of organic molecules. The hydrated oxide groups of the coating components are able to capture hydroxyl radicals produced by the electron hole pair migrating into the coating layer, thereby stabilize the TiO2 crystal [3]. Hydrous
∗
Corresponding author. Tel.: +82 51 6206379; fax: +82 51 6288147. E-mail address: [email protected] (Y.S. Kang).
0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.10.014
silica and hydrous alumina are the most commonly used coating materials of inorganic coating of TiO2 particle surface [4,5]. Hydrous silica coating can improve both the pigment weather durability and the dispersion properties. And TiO2 particles covered with zinc oxide has UV-B ray protection by absorbing. The coating of stable oxide layers has been mainly done by spraying of metal oxide precursor solution on the surface of solid particles. Unfortunately, this process results in many defects on the surface of TiO2 particles [2]. In the present paper, we introduce a new coating method of SiO2 on the TiO2 surface using phase transfer reaction between organic chloroform and aqueous water phases. Finally, evenly SiO2 -coated TiO2 nanoparticles were characterized on the structure and morphology with X-ray powder diffraction (XRD), TEM, energy dispersion spectrum (EDS), FT-IR and X-ray photoelectron spectroscopy (XPS).
2. Experimental 2.1. Methods Tetraethylorthosilicate (TEOS: Si(OC2 H5 )4 , 98%) was obtained from Aldrich. TiO2 particles were obtained from Degussa (P-25). TiO2 , water and chloroform were mixed at room temperature. TEOS and ethanol were mixed at room
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O.K. Park, Y.S. Kang / Colloids and Surfaces A: Physicochem. Eng. Aspects 257–258 (2005) 261–265
Table 1 The element compositions of SiO2 coated TiO2 nanoparticles from EDS analysis (atomic wt.%) Element atoms
The molar ratio of TiO2 to TEOS 0:1
0.2:0.8
0.4:0.6
0.5:0.5
0.6:0.4
O Ti Si
65.40 34.60 0
64.45 30.46 5.09
60.08 27.70 12.22
61.45 27.19 14.36
67.49 22.24 10.27
temperature. Two mixtures were dissolved in acetone. The molar ratio of acetone to ethanol in solution was 1:1 and chloroform was used excess as solvent and the amount of chloroform critically affected the coating time. The contents of TiO2 to TEOS were prepared as molar ratio of 0/1, 2/8, 4/6, 5/5, 6/4 and 8/2. Then mixtures were stirred at room temperature for 2 h. Following, the solvent was evaporated. The product was dried under the reduced pressure in vacuum oven and sintered at 500–1100 ◦ C as increasing temperature by 100 ◦ C interval. This is synthetic route of silica-coated TiO2 nanoparticles. 2.2. Sample characterization The products were characterized with HR-TEM and EDS to know about the morphology and composition. HR-TEM image was obtained using a Hitachi model S-2400 and a Jeol model JEM-2010. EDS was obtained using a Hitachi model, H-7500. The interfacial chemical bonding structure is checked with FT-IR. FT-IR spectra were recorded on a Perkin-Elmer Specrum 2000. Coated particles were measured with XRD to confirm the coating of silica layer on TiO2 particle. XRD spectra were collected using a Philips, X’PertMPD system. The chemical bonding structure between coating layer and the TiO2 particle surface are determined with XPS. The XPS study was performed with a VG-Scientific ESCALAB 250 spectrometer with monocromatized Al K␣ X-ray source. Fig. 1. HR-TEM images of (a) pure TiO2 (Degussa P-25) and (b) silicacoated TiO2 .
3. Results and discussion Fig. 1 shows the HR-TEM image of the silica coated TiO2 nanoparticles. The comparison of two pictures shows that TiO2 particles surface was surround by amorphous silica layer. The average diameter size of silica-coated TiO2 nanoparticles was determined as 20–50 nm. The thickness of the amorphous layer is estimated as about 10–15 nm. The atomic weight composition of TiO2 particles can be qualitatively determined by EDS, and it is shown in Fig. 2. By comparing between the EDS analysis and TEM image, the presence of silicon in the coated SiO2 layer of the surface of TiO2 particles is identified. The relative compositions on the coated TiO2 particle expressed quantitatively as atomic weight percentage (Table 1). It is concluded that amorphous layer is determined as silica coated on the surface of TiO2 par-
ticles. Fig. 3 shows that FT-IR spectra of pure and SiO2 coated TiO2 particles. In the comparison between the two spectra, an absorption peak of Ti O, Si O and Ti O Si was observed at 600–900, 1000–1200, 950 cm−1 , respectively [6,7]. The O 1s spectra of pure and SiO2 coated TiO2 particles are shown in Fig. 4. The binding energies of O 1s peaks for pure TiO2 particles are 527.6 and 529.9 eV. The intensity of peak at 527.6 eV is much higher than that at 529.9 eV. For the silicacoated TiO2 particles, the intensity of peak at 530.3 eV is higher than that at 528.1 eV in O 1s binding energy. The higher peak intensity at 530.3 eV is attributable to the silica coating on the particle surface. It indicates that the peak at 528.1 eV of O 1s in SiO2 coated TiO2 particles was resulted from a chemical shift of peak at 527.6 eV of O 1s in pure TiO2 particles. It is concluded that Ti O Si bond is formed
O.K. Park, Y.S. Kang / Colloids and Surfaces A: Physicochem. Eng. Aspects 257–258 (2005) 261–265
263
Fig. 2. EDS spectra of (a) pure TiO2 (Degussa P-25) and (b) silica-coated TiO2 .
on the surface of TiO2 particles. Since the electronegativity of Si is greater than that of Ti, O 1s peak of TiO2 particles has a chemical shift of about +0.5 eV compared to SiO2 coated TiO2 particles. Ti 2p spectra of pure and SiO2 coated TiO2
particles are shown in Fig. 5. The binding energy of Ti 2p peak of pure TiO2 particles is determined as 456.2 eV. Ti 2p peak for SiO2 coated TiO2 is 456.6 eV, +0.4 eV larger than the Ti 2p peak of pure TiO2 particles. It also indicates the
Fig. 3. FT-IR spectra of (a) pure TiO2 (Degussa P-25) and (b) silica-coated TiO2 .
Fig. 4. O 1s XPS spectra of samples (a) TiO2 and (b) silica-coated TiO2 nanoparticles.
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O.K. Park, Y.S. Kang / Colloids and Surfaces A: Physicochem. Eng. Aspects 257–258 (2005) 261–265
Fig. 5. Ti 2p XPS spectra of samples (a) TiO2 and (b) silica-coated TiO2 nanoparticles.
Ti O Si bond formation [8]. The decrease of the electron density around Ti atom results from the greater electronegativity of Si via O acting on Ti. So the shielding effect is lessened, and then the binding energy is increased. From the chemical shift of Ti 2p, O 1s peaks and FT-IR spectra, it can be concluded that silica is coated on the surface of TiO2 particles through chemical bond. Ti O Si bond forms at the interface of silica coating layer or TiO2 particles surface. In order to confirm the coating of silica layer on TiO2 particle, produced particles were annealed by increasing temperature. Fig. 6 shows that XRD patterns of silica- coated TiO2 sintered at (a) 500 ◦ C and (b) 1100 ◦ C. Seed particle of TiO2 had phase transition from anatase phase to rutile phase at above 900 ◦ C. However coated SiO2 layer did not show a phase transition with heating even at 1100 ◦ C. Fig. 7 shows XRD patterns of sintered SiO2 coated TiO2 particle at 800 ◦ C versus increasing sintering time from 1 to 11 h. This was done to know change of the phase structure of the amorphous SiO2 layer at same temperature with different sintered time. There was no phase transition of SiO2 layer and TiO2 nanoparticles with increasing sintering time. Amorphous silica layer act as a barrier which prevent TiO2 particles from heat [9,10]. Consequently, it shows that the surface of TiO2 covered uniformly by the SiO2 layer and the phase transition temperature of silica-coated TiO2 particle is higher than the phase transition temperature of pure TiO2 particles [11–13]. The increased phase transition temperature of SiO2 coated TiO2 is also attributed to possible dissolution of SiO2 in TiO2 particles and compact SiO2 layer formation. However, heating at above 1300 ◦ C, breakage of Ti O Si bonds resulting in the formation of crystallization of SiO2 [14]. Therefore, SiO2 cannot dissolve in TiO2 particle. So SiO2 coated TiO2 particle is well sintered enough at 800 ◦ C to stabilize amorphous silica layer on TiO2 surface.
Fig. 6. XRD patterns of silica-coated TiO2 sintered at (a) 500 ◦ C and (b) 1100 ◦ C. A and R represent anatase and rutile phases, respectively.
Fig. 7. XRD patterns of sintered silica-coated TiO2 for different sintering times at 800 ◦ C. A and R represent anatase and rutile phases, respectively.
O.K. Park, Y.S. Kang / Colloids and Surfaces A: Physicochem. Eng. Aspects 257–258 (2005) 261–265
4. Conclusion HR-TEM images of TiO2 particles show that TiO2 is evenly coated by amorphous SiO2 layer. As a result of the EDS analysis, amorphous layer is determined as silica. And XRD result shows that there are no phase transition of SiO2 layer with increasing heat-treatment temperature and time. Therefore, the HR-TEM, FT-IR, XPS, and EDS results show that SiO2 is evenly coated on the surface of TiO2 particles and results in the chemical bonding of Ti O Si. The average diameter size of silica-coated TiO2 was determined as 20–50 nm. And the SiO2 coating layer thickness is determined about 10–15 nm. By TEM image and EDS data, optimum molar ratio of Si:Ti is determined as 4:6 and coated particle is sintered well at 800 ◦ C because TiO2 has phase transition at above 900 ◦ C and amorphous silica layer was crystallized at above 1300 ◦ C.
Acknowledgment This work was financially supported by Korean Ministry of Science and Technology as Specific Research and De-
265
velopment Program on New Functional Chemical Materials, 2003. References [1] Y.L. Lin, T.J. Wang, Y. Jin, Powder Technol. 123 (2002) 194. [2] Y. Atou, H. Suzuki, Y. Kimura, T. Sato, T. Tanigaki, Y. Saito, C. Kaito, Physica E 16 (2003) 179. [3] European Patent Application. 91,301,183.9. [4] R.L. Decolibus, US Patent 3,928,057 (1975). [5] H.W. Jacobson, US Patent 4,416,699 (1983). [6] M.-S. Lee, G.-D. Lee, S.-S. Hong, J. Ind. Eng. Chem. 9 (5) (2003) 556. [7] W.-I. Kim, I.-K. Hong, J. Ind. Eng. Chem. 9 (6) (2003) 728. [8] M. Ramirez-Del-Solar, N. de la Rosa-Fox, L. Esquivas, J. Zarzycki, J. Non-cryst. Solids 121 (1990) 84. [9] W.-H. Cho, D.-J. Kang, S.-G. Kim, J. Mater. Sci. 38 (2003) 2619. [10] M. Machida, K. Norimoto, T. Watanabe, J. Mater. Sci. 34 (1999) 2569. [11] R.N. Viswanath, R. Vignesh, S. Ramasamy, Study of crystalline phase of TiO2 -SiO2 nanocomposite, in: Proceedings of the 59th Indian Ceramic Society Meeting, Anna University, Madras, India, 1996. [12] J.I. Murray, H.A. Wriedt, Bull. Alloy Phase Diagrams 8 (1987) 148. [13] C.N.R. Rao, Can. J. Chem. 39 (1961) 49. [14] R.N. Viswanath, S. Ramasamy, Colloids Surf. A: Physicochem. Eng. Aspects 133 (1998) 49.