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Synthesis and characterization of titania-coated silica fine particles by semi-batch process
Colloids and Surfaces A: Physicochem. Eng. Aspects 224 (2003) 119 /126 www.elsevier.com/locate/colsurfa
Synthesis and characterization of titania-coated silica fine particles by semi-batch process Ki Do Kim, Hyun Joo Bae, Hee Taik Kim * Department of Chemical Engineering, Hanyang University, 1271 Sa 1-dong, Sangnok-gu, Ansan-si, Gyeonggi-do 426-791, South Korea Received 14 November 2002; accepted 29 April 2003
Abstract Monodispersed, spherical titania (TiO2)-coated silica (SiO2) fine particles were synthesized by semi-batch process. By this process, SiO2 fine particles were also prepared and were used as carrier particles for the coating. The mean size of SiO2 particles was 250 nm. The reaction parameters were the concentration of tetraethylorthotitanate (TEOT), the amount of hydroxypropylcellulose (HPC) as a dispersant, the feed rate of starting solution (TEOT/HPC/EtOH) and reaction temperature. As a result, the following results were obtained; particle size of titania-coated silica particles decreased with a decreasing TEOT concentration, slow feed rate and increasing amount of HPC. In addition, it was desirable to maintain a reaction temperature at 15 /18 8C in order to obtain the monodispersed fine particles. Consequently, the optimal conditions for the preparation of titania-coated silica particles with narrow size distribution were TEOT (0.03 /0.04 M), HPC (0.001 /0.0013 g ml 1), feed rate (0.5 /0.6 ml min 1), and reaction temperature (15 / 18 8C). The samples were characterized using FT-IR, EDS, TEM and BET analyzer. The coating thickness of TiO2 particles obtained by using the above conditions was about 5 /10 nm. # 2003 Elsevier B.V. All rights reserved. Keywords: Titania (TiO2)-coated silica (SiO2); Semi-batch process; HPC; Optimal condition
1. Introduction Metal oxide fine particles such as Al2O3, SiO2 [1], Ta2O5, TiO2 [2,3], and ZrO2 are widely used in industrial applications as catalyst, ceramics, pigments, and so on. Of those metal oxide particles, titania (TiO2) and silica (SiO2) powders are used to produce paint opacifiers, catalysts, catalyst supports, ceramic membranes, fiber optics, liquid * Corresponding author. Tel.: /82-31-400-5274; fax: /8231-419-7203. E-mail address: [email protected] (H.T. Kim).
thickners, varistors, capacitors, etc. In particular, titania powders are largely used in the pigment and catalyst industries [4]. In addition, they are widely used as heterogeneous catalysts for the selective oxidation of ortho-xylene to phthalic anhydride [5,6] and for the selective catalytic reduction of NOx by ammonia in water containing gases from power stations (SCR process [7,8]). However, titania as a high surface area powder is not thermally stable and easily loses its surface area. Therefore, several groups have investigated titania coating on high surface area supports such as silica or alumina [9]. These titania coating on
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the supports has been studied for a wide variety of uses such as antifouling, antibacterial, deodorizing applications, and wet-type solar cells [10], because of their photo-catalytic properties and photovoltaic effect [11]. It is possible to increase the amount of exposed active centers by coating titania on silica support [12]. In addition, titania-coated silica spheres may be good candidates for a photonic crystal with a complete band gap in the nearinfrared and visible regions if they are made uniform in size and packed orderly in structure [13], owing to their high refractive index. Consequently, it is necessary to synthesize the nonaggregated, spherical titania-coated silica fine particles [14]. In general, methods used for coating titania on silica include impregnation, precipitation, and sol /gel techniques. Of those methods, the simplest way is impregnating the support with a solution of titanium alkoxide precursor. Accordingly, in this study, semi-batch process (that is to say, a batch process with continuous introduction of one coreactant) in which system titanium alkoxide reactants are fed into the reactor with a constant feed rate was used to impregnate the silica particles without difficulty [15]. In addition, semi-batch process is easier than batch process in controlling the size, shape, and size distribution because of short nucleation and a slow hydrolysis rate. The semi-batch method was also used to prepare the monodispersed spherical silica as carrier particles for the coating. The objectives of this work are: (1) to suggest a method for the synthesis of monodispersed spherical titania-coated silica fine particles by using semi-batch method, (2) to obtain the optimal conditions for preparing titania-coated silica fine particles with narrow particle size distribution, and (3) to confirm how TiO2 particles are uniformly coated on the silica support.
2. Experimental 2.1. Starting materials In the synthesis of the SiO2 particles, tetraethylorthosilicate (TEOS, Si(OC2H5)4, 99.9%, Aldrich
Chemical Co.), an ethanol (99.9%, Sigma Chemical Co.) solution, and water/ethanol solution with ammonium hydroxide (NH4OH, 28%, Yakuri Pure Chemicals Co.) were used. In the preparation of TiO2 particles, tetraethylorthotitanate (TEOT, Ti(OC2H5)4, technical grade, Aldrich Chemical Co.), an ethanol solution with hydroxypropylcellulose (HPC, molecular weight /100 000, Aldrich Chemical Co.) as a dispersant, and water /ethanol solution were used. The solutions were prepared in a glove box at room temperature under dry air. The humidity in the glove box was kept below a few percent. 2.2. Preparation of SiO2 particles as the support to coat Monodispersed, spherical SiO2 particles used in this study were prepared by semi-batch method, which was easier than a batch process in controlling the size, shape, and size distribution. Fig. 1(a) shows the experimental procedure for the synthesis of SiO2 particles. Firstly, 15.5 M of H2O and 1.0 M NH3 were dissolved in ethanol solution of reactor. And then, 0.2 M of TEOS solution was added at 1.5 ml min 1 of feed rate. Secondly, the mixture was aged for 90 min and was separated centrifugally at 7000 rpm for 10 min. Finally, the solution was washed and dried at 60 8C for 24 h. The silica particles with a size of 250 nm were obtained from the above experimental procedure and SEM micrograph of the particles is shown in Fig. 2. 2.3. Preparation and analysis of TiO2-coated SiO2 particles The operational parameters and desirable properties for the synthesis of titania-coated silica particles are summarized in Table 1. As shown in Table 1, four parameters affecting two properties (particle size and standard deviation of particle size) were controlled in the preparation of monodispersed titania-coated silica fine particles. TiO2 particles were coated on the monodispersed spherical silica particles prepared in this experiment and the procedure is shown in Fig. 1(b). The silica particles with 250 nm in mean particle size were
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Fig. 1. Experimental procedure for the preparation of SiO2 particles (a) and synthesis of TiO2-coated SiO2 particles (b).
Particle size, particle size distribution, shape of the particles, surface area, and the bonding state between the final particles were analyzed by laser particle size analyzer (Otsuka electronics, LPA3000, 3100, Japan), scanning electron microscope (SEM, TOPCON, SM-300, Japan), transmission electron microscope (TEM, JEOL, JEM-2010, Japan), surface area analyzer (Brunauer Emmett Teller (BET), Micrometrics, GEMINI-2375, USA), Fourier transform infrared (FT-IR, Avatar 360 E.S.P, Japan), and energy dispersion spectrum (EDS, ISIS, Oxford), respectively. Fig. 2. SEM micrograph of SiO2 particles.
sonicated in 100 ml of ethanol for 20 min and then, 0.3 M of H2O /ethanol solution was mixed. To coat the TiO2 particles, TEOT /ethanol solution with hydroxypropylcellulose (HPC) as a steric stabilizer during the precipitation of TiO2 particles was added by the micro tube pump (EYELA MP3, Japan). After the addition of TEOT solution was finished, the finial mixture was aged for 90 min and was separated centrifugally at 7000 rpm for 10 min. Finally, the solution was washed and dried at 60 8C for 24 h.
Table 1 Experimental parameters and desirable properties Parameter
Properties
TEOT (M): 0.03 /0.1 Particle size (nm): minimize HPC (g ml 1): 0 /0.002 Feed rate (ml min 1): 0.5 /5.7 Standard deviation of particle size (%): minimize Temperature (8C): 15 /62
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3. Results and discussion
3.1. Effect of TEOT concentration and amount of HPC The first part of this study focused on the effect of TEOT concentration and amount of HPC dispersant on the final particle size of TiO2 particles coated onto the surface of silica particles. A TEOT concentration was to be increased from 0.03 to 0.1 M while a H2O concentration was fixed at 0.3 M for a hydrolysis. Fig. 3(a) shows that the final particle size (diameter) increases with increasing TEOT concentration. It seems to result from the increasing amount of TEOT accelerating the hydrolysis, followed by agglomeration among the TiO2 particles being deposited onto the surface of silica
particles. Fig. 3(a) also shows that the particles with the smallest value, approximately 260 nm, are prepared at the TEOT concentration of 0.03 M. To prevent such agglomeration, an electrolyte or HPC dispersant have been used. Similarly, HPC as a dispersant exhibiting a steric repulsion effect was used in this experiment. HPC concentration was assigned to be variable from 0 to 0.002 g ml1. The effect of HPC dispersant on particle size is shown in Fig. 3(b). The final particle size decreased as amount of HPC increased. In addition, the amount of HPC had no influence on the particle size when its value exceeded 0.001 g ml1. Therefore, it can be assumed that the optimal value is approximately 0.001 /0.0013 g ml1 in consideration of a dispersant as an impurity in powders for sintering.
3.2. Effect of the feed rate and reaction temperature
Fig. 3. Effect of TEOT concentration (a) and amount of HPC (b) on particle size of titania-coated silica particles.
Fig. 4 expresses how final particle size can be influenced by a feed rate of TEOT solution and reaction temperature. As shown in Fig. 4(a), the particle size decreased with decreasing the feed rate of TEOT solution. It is based on the fact that a hydrolysis of TEOT is to be very slow as TEOT is added in a slow feed rate. Subsequently, it induces a delay of nucleus generation. As a result, the TiO2 particles can be coated onto the surface of silica particles uniformly. On the other hand, a fast feed rate induces a batch type of reaction that generates a nucleus very rapidly, followed by non-uniform depositions onto the SiO2 particles and agglomeration among the particles. Thus, the final particle size becomes large. Fig. 4(b) explains the effect of reaction temperature on final particle size. As the temperature gets higher, a nucleus tends to be generated in a very short time forming agglomerates. Accordingly, the particle size increased to 320 nm (maximum). Therefore, it is desirable to maintain the reaction temperature at 15 /18 8C (room temperature) in order to obtain the fine particles without agglomeration.
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tration than that of sample (#3) and used no dispersant, respectively. Furthermore, when a feed rate of TEOT solution increased as shown in sample (#4), there were a few agglomerations although the particle size increased. Fig. 5 shows SEM images in correspond to the results in Table 2. Based on the results abovementioned, it is better to utilize a low TEOT concentration, a large amount of HPC under a significant level, slow feed rate of TEOT solution, and reaction temperature at 15/20 8C in order to obtain monodispersed spherical TiO2-coated SiO2 fine particles. Table 3 shows the optimal conditions and related parameters. 3.4. Characterization of TiO2-coated SiO2 fine particles
Fig. 4. Effect of feed rate (a) and temperature (b) on particle size of titania-coated silica particles.
3.3. The optimal conditions for the synthesis of monodispersed spherical TiO2-coated SiO2 fine particles In order to find out optimal synthesis conditions of the titania-coated silica fine particles, three parameters (TEOT concentration, amount of HPC, and feed rate of TEOT solution) influential on the final particle size was controlled while a reaction temperature and a H2O concentration were fixed at 15 8C and 0.3 M, respectively. As a result, four samples were prepared and the comparison of particle size, size distribution, and morphology for each sample were summarized in Table 2. The sample (#3) had the smallest particle size and standard deviation value providing monodispersed fine particles, as compared with sample (#1) and (#2) which had a higher TEOT concen-
The interfacial structure, the elemental analysis, and the morphology of TiO2-coated SiO2 particles were measured by FT-IR, EDS, and TEM, respectively. The specific surface area was also checked by BET analyzer. Fig. 6 shows FT-IR spectra of TiO2-coated SiO2 particles derived by in this process. There are three major bands in the figure. First of all, an O /H stretching vibration caused common bands (/3400 cm 1) found in all samples. Besides, it can be seen that bands at / 1100 and /940 cm1 occur in the spectrum of TiO2-coated SiO2 powders. It is thought that the Si/O /Si and Ti/O /Si bonds lead to the band, respectively. Compared with the spectrum of sample (#3) and (#4), the Ti /O /Si band was not so clear for sample (#1) and (#2) due to a severe agglomeration [16]. The surface composition of sample (#3) with the narrowest particle size distribution was qualitatively determined by EDS as shown in Fig. 7. It shows that atomic and element % ratios of Ti to Si are 13.8/86.2 and 21.5/78.5, respectively. It is concluded that TiO2 particles are coated on the surface of silica particles. TEM images of TiO2 particles coated (or deposited) onto SiO2 particles are shown in Fig. 8. It shows that sample (#3) has a TiO2 thickness of approximately 5/10 nm being coated on the silica particles very uniformly (see Fig. 8(a)). In contrast, Fig. 8(b) describes sample (#1) with severe agglomeration, where TiO2 par-
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Table 2 Comparison of particle size, size distribution (S.T.D.), and morphology according to the sample code Sample code
# # # #
1 2 3 4
Parameters
Properties
TEOT (M)
HPC (g ml 1)
Feed rate (ml min 1)
Particle size (nm)
S.T.D. (%)
Sample morphology
0.10 0.03 0.03 0.03
0.001 0 0.001 0.001
0.5 0.5 0.5 2.2
851 934 258 374
19.2 54.6 3.5 9.4
Aggregated Aggregated Isolated Isolated
ticles are deposited non-uniformly and are agglomerated together. A surface area of TiO2 particles with respect to the SiO2 particles was measured by BET analyzer. Table 4 gives values of surface area expressed in
N2 absorption for each sample. All samples had higher values of surface area after the coating than that of the SiO2 particles before the coating. In particular, a severe agglomeration induced a great number of pores in sample (#1) and (#2) increas-
Fig. 5. SEM micrographs of titania-coated silica particles. (a) Sample # 1, (b) sample # 2, (c) sample # 3, (d) sample # 4.
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Table 3 Optimal synthesis conditions for preparing monodispersed spherical titania-coated silica fine particles Parameter
Range
TEOT (M) HPC (g ml 1) Feed rate (ml min 1) Temperature (8C)
0.03 /0.04 0.001 /0.0013 0.5 /0.6 15 /18
ing the amount of N2 absorption. In regard to sample (#3) composed of monodispersed fine particles, the amount of N2 absorption increased (approximately 11 m2 g1) higher than that of the SiO2 particles.
4. Conclusions Monodispersed spherical TiO2-coated SiO2 fine particles were synthesized by semi-batch process. The particle size and size distribution were controlled by changing a variety of parameters (TEOT concentration, amount of HPC, feed rate of TEOT solution, and reaction temperature). As a result, final particle size decreased with decreasing TEOT concentration, slow feed rate, and increasing
Fig. 7. EDS result of sample code # 3.
amount of HPC. In addition, it was desirable to maintain a reaction temperature at room temperature in order to obtain monodispersed fine particles. Consequently, an optimal conditions for preparing the TiO2-coated SiO2 fine particles with narrow size distribution were obtained as follows (TEOT concentration: 0.03 /0.04 M, amount of HPC: 0.001 /0.0013 g ml 1, feed rate of TEOT solution: 0.5 /0.6 ml min 1, reaction temperature: 15/18 8C). FT-IR spectra and EDS were used to observe a Ti/O /Si bonding band and to analyze the amount of Si and Ti contents,
Fig. 6. Comparison of FT-IR spectra according to the four samples (# 1 /# 4).
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Fig. 8. TEM images of coating surface of monodispersed particles ((a) */sample code # 3) and aggregated particles ((b) */sample code # 1).
Table 4 Comparison of specific surface area for each sample Sample code
Surface area (m2 g 1)
SiO2 #1 #2 #3 #4
73.3 127.4 153.4 84.3 118.7
respectively. In addition, TEM images showed that the thickness of TiO2 particles coated onto the surface of silica particles was approximately 5/ 10 nm. A surface area of TiO2 particles with respect to the SiO2 particles was also checked by BET analyzer.
References [1] K.D. Kim, H.T. Kim, J. Am. Ceram. Soc. 85 (2002) 1107.
[2] K.D. Kim, H.T. Kim, Colloids Surf. A: Physicochem. Eng. Aspects 207 (2002) 263. [3] K.D. Kim, H.T. Kim, J. Ind. Eng. Chem. 6 (2000) 212. [4] W. Buchner, G. Schliebs, G. Winter, K.H. Buchel, Industrial Inorganic Chemistry, VCH, Berlin, 1989. [5] M.S. Wainwright, N.R. Foster, Catal. Rev. Sci. Eng. 19 (1979) 211. [6] V.A. Nikolov, A. Anastasov, Ind. Eng. Chem. Res. 31 (1992) 80. [7] S.M. Cho, Chem. Eng. Prog. 90 (1994) 39. [8] P. Forzatti, L. Lietti, Heter. Chem. Rev. 93 (1993) 163. [9] R. Castillo, B. Koch, P. Ruiz, B. Delmon, J. Mater. Chem. 4 (1994) 903. [10] O. Regon, M. Gra¨tzel, Nature (London) 353 (1991) 737. [11] A. Fujishima, K. Honda, Nature (London) 238 (1972) 37. [12] A. Hanprasopwattana, S. Srinivasan, A.G. Sault, A.K. Datye, Langmuir 12 (1996) 3173. [13] D.B. Mei, P. Dong, H.Q. Li, B.Y. Cheng, D.Z. Zhang, Chin. Phys. Lett. 15 (1998) 21. [14] S. Sridhar, D.H. Rasmussen, F.P. Chen, R.K. Kannabiran, Colloids Surf. A: Physicochem. Eng. Aspects 132 (1998) 45. [15] S.K. Park, K.D. Kim, H.T. Kim, J. Ind. Eng. Chem. 6 (2000) 365. [16] J.B. Miller, L.J. Mathers, E.I. Ko, J. Mater. Chem. 5 (1995) 1759.
Synthesis and characterization of titania-coated silica fine particles by semi-batch process Ki Do Kim, Hyun Joo Bae, Hee Taik Kim * Department of Chemical Engineering, Hanyang University, 1271 Sa 1-dong, Sangnok-gu, Ansan-si, Gyeonggi-do 426-791, South Korea Received 14 November 2002; accepted 29 April 2003
Abstract Monodispersed, spherical titania (TiO2)-coated silica (SiO2) fine particles were synthesized by semi-batch process. By this process, SiO2 fine particles were also prepared and were used as carrier particles for the coating. The mean size of SiO2 particles was 250 nm. The reaction parameters were the concentration of tetraethylorthotitanate (TEOT), the amount of hydroxypropylcellulose (HPC) as a dispersant, the feed rate of starting solution (TEOT/HPC/EtOH) and reaction temperature. As a result, the following results were obtained; particle size of titania-coated silica particles decreased with a decreasing TEOT concentration, slow feed rate and increasing amount of HPC. In addition, it was desirable to maintain a reaction temperature at 15 /18 8C in order to obtain the monodispersed fine particles. Consequently, the optimal conditions for the preparation of titania-coated silica particles with narrow size distribution were TEOT (0.03 /0.04 M), HPC (0.001 /0.0013 g ml 1), feed rate (0.5 /0.6 ml min 1), and reaction temperature (15 / 18 8C). The samples were characterized using FT-IR, EDS, TEM and BET analyzer. The coating thickness of TiO2 particles obtained by using the above conditions was about 5 /10 nm. # 2003 Elsevier B.V. All rights reserved. Keywords: Titania (TiO2)-coated silica (SiO2); Semi-batch process; HPC; Optimal condition
1. Introduction Metal oxide fine particles such as Al2O3, SiO2 [1], Ta2O5, TiO2 [2,3], and ZrO2 are widely used in industrial applications as catalyst, ceramics, pigments, and so on. Of those metal oxide particles, titania (TiO2) and silica (SiO2) powders are used to produce paint opacifiers, catalysts, catalyst supports, ceramic membranes, fiber optics, liquid * Corresponding author. Tel.: /82-31-400-5274; fax: /8231-419-7203. E-mail address: [email protected] (H.T. Kim).
thickners, varistors, capacitors, etc. In particular, titania powders are largely used in the pigment and catalyst industries [4]. In addition, they are widely used as heterogeneous catalysts for the selective oxidation of ortho-xylene to phthalic anhydride [5,6] and for the selective catalytic reduction of NOx by ammonia in water containing gases from power stations (SCR process [7,8]). However, titania as a high surface area powder is not thermally stable and easily loses its surface area. Therefore, several groups have investigated titania coating on high surface area supports such as silica or alumina [9]. These titania coating on
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the supports has been studied for a wide variety of uses such as antifouling, antibacterial, deodorizing applications, and wet-type solar cells [10], because of their photo-catalytic properties and photovoltaic effect [11]. It is possible to increase the amount of exposed active centers by coating titania on silica support [12]. In addition, titania-coated silica spheres may be good candidates for a photonic crystal with a complete band gap in the nearinfrared and visible regions if they are made uniform in size and packed orderly in structure [13], owing to their high refractive index. Consequently, it is necessary to synthesize the nonaggregated, spherical titania-coated silica fine particles [14]. In general, methods used for coating titania on silica include impregnation, precipitation, and sol /gel techniques. Of those methods, the simplest way is impregnating the support with a solution of titanium alkoxide precursor. Accordingly, in this study, semi-batch process (that is to say, a batch process with continuous introduction of one coreactant) in which system titanium alkoxide reactants are fed into the reactor with a constant feed rate was used to impregnate the silica particles without difficulty [15]. In addition, semi-batch process is easier than batch process in controlling the size, shape, and size distribution because of short nucleation and a slow hydrolysis rate. The semi-batch method was also used to prepare the monodispersed spherical silica as carrier particles for the coating. The objectives of this work are: (1) to suggest a method for the synthesis of monodispersed spherical titania-coated silica fine particles by using semi-batch method, (2) to obtain the optimal conditions for preparing titania-coated silica fine particles with narrow particle size distribution, and (3) to confirm how TiO2 particles are uniformly coated on the silica support.
2. Experimental 2.1. Starting materials In the synthesis of the SiO2 particles, tetraethylorthosilicate (TEOS, Si(OC2H5)4, 99.9%, Aldrich
Chemical Co.), an ethanol (99.9%, Sigma Chemical Co.) solution, and water/ethanol solution with ammonium hydroxide (NH4OH, 28%, Yakuri Pure Chemicals Co.) were used. In the preparation of TiO2 particles, tetraethylorthotitanate (TEOT, Ti(OC2H5)4, technical grade, Aldrich Chemical Co.), an ethanol solution with hydroxypropylcellulose (HPC, molecular weight /100 000, Aldrich Chemical Co.) as a dispersant, and water /ethanol solution were used. The solutions were prepared in a glove box at room temperature under dry air. The humidity in the glove box was kept below a few percent. 2.2. Preparation of SiO2 particles as the support to coat Monodispersed, spherical SiO2 particles used in this study were prepared by semi-batch method, which was easier than a batch process in controlling the size, shape, and size distribution. Fig. 1(a) shows the experimental procedure for the synthesis of SiO2 particles. Firstly, 15.5 M of H2O and 1.0 M NH3 were dissolved in ethanol solution of reactor. And then, 0.2 M of TEOS solution was added at 1.5 ml min 1 of feed rate. Secondly, the mixture was aged for 90 min and was separated centrifugally at 7000 rpm for 10 min. Finally, the solution was washed and dried at 60 8C for 24 h. The silica particles with a size of 250 nm were obtained from the above experimental procedure and SEM micrograph of the particles is shown in Fig. 2. 2.3. Preparation and analysis of TiO2-coated SiO2 particles The operational parameters and desirable properties for the synthesis of titania-coated silica particles are summarized in Table 1. As shown in Table 1, four parameters affecting two properties (particle size and standard deviation of particle size) were controlled in the preparation of monodispersed titania-coated silica fine particles. TiO2 particles were coated on the monodispersed spherical silica particles prepared in this experiment and the procedure is shown in Fig. 1(b). The silica particles with 250 nm in mean particle size were
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Fig. 1. Experimental procedure for the preparation of SiO2 particles (a) and synthesis of TiO2-coated SiO2 particles (b).
Particle size, particle size distribution, shape of the particles, surface area, and the bonding state between the final particles were analyzed by laser particle size analyzer (Otsuka electronics, LPA3000, 3100, Japan), scanning electron microscope (SEM, TOPCON, SM-300, Japan), transmission electron microscope (TEM, JEOL, JEM-2010, Japan), surface area analyzer (Brunauer Emmett Teller (BET), Micrometrics, GEMINI-2375, USA), Fourier transform infrared (FT-IR, Avatar 360 E.S.P, Japan), and energy dispersion spectrum (EDS, ISIS, Oxford), respectively. Fig. 2. SEM micrograph of SiO2 particles.
sonicated in 100 ml of ethanol for 20 min and then, 0.3 M of H2O /ethanol solution was mixed. To coat the TiO2 particles, TEOT /ethanol solution with hydroxypropylcellulose (HPC) as a steric stabilizer during the precipitation of TiO2 particles was added by the micro tube pump (EYELA MP3, Japan). After the addition of TEOT solution was finished, the finial mixture was aged for 90 min and was separated centrifugally at 7000 rpm for 10 min. Finally, the solution was washed and dried at 60 8C for 24 h.
Table 1 Experimental parameters and desirable properties Parameter
Properties
TEOT (M): 0.03 /0.1 Particle size (nm): minimize HPC (g ml 1): 0 /0.002 Feed rate (ml min 1): 0.5 /5.7 Standard deviation of particle size (%): minimize Temperature (8C): 15 /62
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3. Results and discussion
3.1. Effect of TEOT concentration and amount of HPC The first part of this study focused on the effect of TEOT concentration and amount of HPC dispersant on the final particle size of TiO2 particles coated onto the surface of silica particles. A TEOT concentration was to be increased from 0.03 to 0.1 M while a H2O concentration was fixed at 0.3 M for a hydrolysis. Fig. 3(a) shows that the final particle size (diameter) increases with increasing TEOT concentration. It seems to result from the increasing amount of TEOT accelerating the hydrolysis, followed by agglomeration among the TiO2 particles being deposited onto the surface of silica
particles. Fig. 3(a) also shows that the particles with the smallest value, approximately 260 nm, are prepared at the TEOT concentration of 0.03 M. To prevent such agglomeration, an electrolyte or HPC dispersant have been used. Similarly, HPC as a dispersant exhibiting a steric repulsion effect was used in this experiment. HPC concentration was assigned to be variable from 0 to 0.002 g ml1. The effect of HPC dispersant on particle size is shown in Fig. 3(b). The final particle size decreased as amount of HPC increased. In addition, the amount of HPC had no influence on the particle size when its value exceeded 0.001 g ml1. Therefore, it can be assumed that the optimal value is approximately 0.001 /0.0013 g ml1 in consideration of a dispersant as an impurity in powders for sintering.
3.2. Effect of the feed rate and reaction temperature
Fig. 3. Effect of TEOT concentration (a) and amount of HPC (b) on particle size of titania-coated silica particles.
Fig. 4 expresses how final particle size can be influenced by a feed rate of TEOT solution and reaction temperature. As shown in Fig. 4(a), the particle size decreased with decreasing the feed rate of TEOT solution. It is based on the fact that a hydrolysis of TEOT is to be very slow as TEOT is added in a slow feed rate. Subsequently, it induces a delay of nucleus generation. As a result, the TiO2 particles can be coated onto the surface of silica particles uniformly. On the other hand, a fast feed rate induces a batch type of reaction that generates a nucleus very rapidly, followed by non-uniform depositions onto the SiO2 particles and agglomeration among the particles. Thus, the final particle size becomes large. Fig. 4(b) explains the effect of reaction temperature on final particle size. As the temperature gets higher, a nucleus tends to be generated in a very short time forming agglomerates. Accordingly, the particle size increased to 320 nm (maximum). Therefore, it is desirable to maintain the reaction temperature at 15 /18 8C (room temperature) in order to obtain the fine particles without agglomeration.
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tration than that of sample (#3) and used no dispersant, respectively. Furthermore, when a feed rate of TEOT solution increased as shown in sample (#4), there were a few agglomerations although the particle size increased. Fig. 5 shows SEM images in correspond to the results in Table 2. Based on the results abovementioned, it is better to utilize a low TEOT concentration, a large amount of HPC under a significant level, slow feed rate of TEOT solution, and reaction temperature at 15/20 8C in order to obtain monodispersed spherical TiO2-coated SiO2 fine particles. Table 3 shows the optimal conditions and related parameters. 3.4. Characterization of TiO2-coated SiO2 fine particles
Fig. 4. Effect of feed rate (a) and temperature (b) on particle size of titania-coated silica particles.
3.3. The optimal conditions for the synthesis of monodispersed spherical TiO2-coated SiO2 fine particles In order to find out optimal synthesis conditions of the titania-coated silica fine particles, three parameters (TEOT concentration, amount of HPC, and feed rate of TEOT solution) influential on the final particle size was controlled while a reaction temperature and a H2O concentration were fixed at 15 8C and 0.3 M, respectively. As a result, four samples were prepared and the comparison of particle size, size distribution, and morphology for each sample were summarized in Table 2. The sample (#3) had the smallest particle size and standard deviation value providing monodispersed fine particles, as compared with sample (#1) and (#2) which had a higher TEOT concen-
The interfacial structure, the elemental analysis, and the morphology of TiO2-coated SiO2 particles were measured by FT-IR, EDS, and TEM, respectively. The specific surface area was also checked by BET analyzer. Fig. 6 shows FT-IR spectra of TiO2-coated SiO2 particles derived by in this process. There are three major bands in the figure. First of all, an O /H stretching vibration caused common bands (/3400 cm 1) found in all samples. Besides, it can be seen that bands at / 1100 and /940 cm1 occur in the spectrum of TiO2-coated SiO2 powders. It is thought that the Si/O /Si and Ti/O /Si bonds lead to the band, respectively. Compared with the spectrum of sample (#3) and (#4), the Ti /O /Si band was not so clear for sample (#1) and (#2) due to a severe agglomeration [16]. The surface composition of sample (#3) with the narrowest particle size distribution was qualitatively determined by EDS as shown in Fig. 7. It shows that atomic and element % ratios of Ti to Si are 13.8/86.2 and 21.5/78.5, respectively. It is concluded that TiO2 particles are coated on the surface of silica particles. TEM images of TiO2 particles coated (or deposited) onto SiO2 particles are shown in Fig. 8. It shows that sample (#3) has a TiO2 thickness of approximately 5/10 nm being coated on the silica particles very uniformly (see Fig. 8(a)). In contrast, Fig. 8(b) describes sample (#1) with severe agglomeration, where TiO2 par-
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Table 2 Comparison of particle size, size distribution (S.T.D.), and morphology according to the sample code Sample code
# # # #
1 2 3 4
Parameters
Properties
TEOT (M)
HPC (g ml 1)
Feed rate (ml min 1)
Particle size (nm)
S.T.D. (%)
Sample morphology
0.10 0.03 0.03 0.03
0.001 0 0.001 0.001
0.5 0.5 0.5 2.2
851 934 258 374
19.2 54.6 3.5 9.4
Aggregated Aggregated Isolated Isolated
ticles are deposited non-uniformly and are agglomerated together. A surface area of TiO2 particles with respect to the SiO2 particles was measured by BET analyzer. Table 4 gives values of surface area expressed in
N2 absorption for each sample. All samples had higher values of surface area after the coating than that of the SiO2 particles before the coating. In particular, a severe agglomeration induced a great number of pores in sample (#1) and (#2) increas-
Fig. 5. SEM micrographs of titania-coated silica particles. (a) Sample # 1, (b) sample # 2, (c) sample # 3, (d) sample # 4.
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Table 3 Optimal synthesis conditions for preparing monodispersed spherical titania-coated silica fine particles Parameter
Range
TEOT (M) HPC (g ml 1) Feed rate (ml min 1) Temperature (8C)
0.03 /0.04 0.001 /0.0013 0.5 /0.6 15 /18
ing the amount of N2 absorption. In regard to sample (#3) composed of monodispersed fine particles, the amount of N2 absorption increased (approximately 11 m2 g1) higher than that of the SiO2 particles.
4. Conclusions Monodispersed spherical TiO2-coated SiO2 fine particles were synthesized by semi-batch process. The particle size and size distribution were controlled by changing a variety of parameters (TEOT concentration, amount of HPC, feed rate of TEOT solution, and reaction temperature). As a result, final particle size decreased with decreasing TEOT concentration, slow feed rate, and increasing
Fig. 7. EDS result of sample code # 3.
amount of HPC. In addition, it was desirable to maintain a reaction temperature at room temperature in order to obtain monodispersed fine particles. Consequently, an optimal conditions for preparing the TiO2-coated SiO2 fine particles with narrow size distribution were obtained as follows (TEOT concentration: 0.03 /0.04 M, amount of HPC: 0.001 /0.0013 g ml 1, feed rate of TEOT solution: 0.5 /0.6 ml min 1, reaction temperature: 15/18 8C). FT-IR spectra and EDS were used to observe a Ti/O /Si bonding band and to analyze the amount of Si and Ti contents,
Fig. 6. Comparison of FT-IR spectra according to the four samples (# 1 /# 4).
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Fig. 8. TEM images of coating surface of monodispersed particles ((a) */sample code # 3) and aggregated particles ((b) */sample code # 1).
Table 4 Comparison of specific surface area for each sample Sample code
Surface area (m2 g 1)
SiO2 #1 #2 #3 #4
73.3 127.4 153.4 84.3 118.7
respectively. In addition, TEM images showed that the thickness of TiO2 particles coated onto the surface of silica particles was approximately 5/ 10 nm. A surface area of TiO2 particles with respect to the SiO2 particles was also checked by BET analyzer.
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